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C h a p t e r

P R O C E S S I N G O F E P IT A X IA L H E T E R O S T R U C T U R E

D E V I C E S

Patrick Fay

Depar tm en t o f E l ec t r ica l Eng ineer ing Un iversi ty o f No t re Dame No t re Dame

Indiana USA

Ilesanmi desida

Depar tm en t o f E l ec t r ica l and C ompu ter Eng ineer ing Un iversi ty o f Il li no is a t

Urbana -Cham paign Urbana I l linois USA

on t en t s

2 .1 . Introdu ctio n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.2. ProcessingTechniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

2.3. DeviceP rocessing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

2 .4 . In teg ratio n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

2 .5 . C onclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

2 . 1 . I N T R O D U C T I O N

D r a m a t i c a d v a n c e s i n w i r e l e s s a n d h i g h - s p e e d c o m m u n i c a t i o n n e t w o r k s h a v e b e e n d r i v e n

d i r ec t l y by h i gh - s peed dev i ce t e chno l ogy t ha t i s ba s ed on ep i t ax i a l he t e r os t r uc t u r e

advances . Thes e dev i ce s i nc l ude he t e r o j unc t i on b i po l a r t r ans i s t o r s ( H B Ts ) , h i gh - e l ec t r on

m o bi l i t y t r ans i s t o r s ( H E M Ts ) , he t e r os t r uc t u r e f i e l d - e f f ec t t r ans i s t o r s ( H FE Ts ) , pho t o -

de t ec t o r s , and s em i cond uc t o r l a s e r d i odes . P r og r e s s i n t he a r ea o f u l t r a - h i gh s pe ed c i r cu i t s

h a s b e e n r e m a r k a b l e ; d i g it a l c ir c u it s w i t h c l o c k r at e s o f u p t o 8 0 G b / s h a v e b e e n

dem ons t r a t ed f o r u l t r a - h i gh b i t - r a t e da t a ne t w or ks , ana l og - t o - d i g i t a l ( A / D ) conve r t e r s w i t h

s a m p l e r a t e s o f s e v e r al g i g a s a m p l e s p e r s e c o n d h a v e b e e n f a b ri c a te d , a n d l o w - n o i s e a n d

h i gh - pow er am pl i f i e r s ope r a t i ona l a t m i l l i m e t e r - w ave f r equenc i e s have a l s o been

d e m o n s t r a t e d . U l t r a - h i g h - s p e e d o p t o e l e c tr o n i c s y s t e m s b a s e d o n m e t a l - s e m i c o n d u c t o r -

m e t a l p h o t o d e t e c t o rs ( M S M - P D s ) [ 1] a n d p - i- n p h o t o d i o d e s [ 2, 3] h a v e b e e n d e v e l o p e d

t h a t d e p e n d o n t h e p e r f o r m a n c e a c h i e v a b le w i t h h e t e r o s tr u c t u r e d e v i c e s. T h e a c h i e v e m e n t

o f t he s e r e s u lt s , a s w e l l a s t hos e t o com e , i s heav i l y depe nden t upo n t he av a i l ab i l it y o f

econo m i ca l , h i gh - y i e l d p r oces s i ng t e ch n i ques f o r f ab r i ca t i on o f ep i t ax i a l he t e r os t r uc t u r e

dev i ce s . A r ev i ew o f p r oces s e s f o r I I I - V he t e r os t r uc t u r e dev i ce f ab r i ca t i on i s unde r t ak en ,

s t a r t ing w i t h a d i s cus s i on o f e ach o f t he m a j o r e l em e n t s o f dev i ce f ab r i ca t ion , i nc l ud i ng

ISBN 0-12-762870-3/$35.00

Handbook of Thin Film Devices edited by Mau rice H. Francom be

Volume 1: Hetero-Structures or H igh Performance Devices

Copyright 9 2000 by Academic Press

All fights of reproduction in any form reserved.

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FAY AND ADESIDA

lithography, we t and dry etching, metallization for electrical contacts, and ion im plantation

for device isolation. The direct application of each of these techniques is then discussed

with respect to the fabrication of HBTs, H FET s, and lasers, followed by a discussion o f the

integration o f these devices into larger subsystems including m onolithic microwav e

integrated circuits (MMICs) and optoelectronic integrated circuits (OEICs).

2 .2 . P R O C E S S I N G T E C H N I Q U E S

2 2 1 L i thography

A critical aspect of any integrated-circuit fabrication technology, regardless of material

system, is the need for adequate lithographic techniq ues to define circuit features. O ptical,

electron-beam, x-ray, and focused ion-beam lithographic techniques have all been

demonstrated, with many novel permutations and modifications of each basic technique

presented in the literature. A lthough all of these method s have their own relative merits an d

difficulties, the two most common techniques for processing III-V microelectronic devices

are optical and electron-beam lithographies. Some o f the features o f these techniques that

have led to their dominance include: i) the ability to define patterns with high resolution

over large areas; ii) the ability to m aintain tigh t layer-to-layer overlay tolerances; iii) the

advantage of well-developed resists and exposure tool technology; iv) little or no device

damage during lithographic processing; and v) the ability to support their use

economically. A br ief overview of the fundam entals o f each of these two lithographic

techniques follows.

2 2 1 1 Opt ica lLithography

Optical lithography is the mainstay of conventional silicon integrated circuit processing,

and it has steadily advanced o ver the past 20 years. The oft-quoted M oore's Law , in which

semiconductor packing density is increased by 50% every 18 months, has proven to be a

remarkably reliable predictor of lithographic technology. Despite many predictions of its

ultimate demise due to fundamental physical limits, optical lithography has continued to

advance in capability. The primary figures of merit that quantify the performance of an

optical lithography system consists o f two interrelated parameters, resolution and depth o f

focus. Due to the fundamental limitations imposed on conventional lithography systems by

diffraction, resolution can be expressed by the Rayleigh criterion

0.612

r = (2.1)

N A

whe re r is the resolution, 2 is the wavelength o f the incident light, and NA is the num erical

aperture of the exposure tool. In this expression, it has been assumed that the smallest

feature that can be resolved is given by the spacing between the maximu m and f irst minima

of the Airy function that results from imaging a point source on the substrate in the

presence of diffraction. In practice, the factor of 0.61 in Eq. 2.1 is often considered a

process-dependent parameter, and it is typically in the range from 0.5-0.7 as determined

experimentally [4] . Depth of focus (DOF), w hich is mo st conveniently def ined as the

displacem ent from the focal plane that is required to blur a focused spot to the size of

the resolution limit r, is given by

2

D O F = (2.2)

2 N A 2

Both of these factors impose separate limits on the resolution achievable with o ptical

lithography, a nd provide explanation for the Si IC industry's trend of decreasing 2 and

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PROCESSING OF EPITAXIAL HETEROSTRU CTURE DEVICES

inc reas ing NA. Advances in op t i ca l l i thography , inc lud ing adopt ion of deep u l t rav io le t

(DUV) l i thography sources based on m ercury l am ps or ArF exc im er l a se r sources a t

248nm and 193nm , re spec t ive ly , a re expec ted to t rans fe r in to I I I -V fabr ica t ion

technology . Desp i te the d ram at ic d i f ference in the s ca le o f p roduc t ion be tw een cur ren t

I I I -V and S i t echnolog ies , the exce l l en t re so lu t ion a f fo rded by s ta te -of - the -a r t op t i ca l

l i thography , w i th p ro jec ted re so lu t ion o f 0 . 13 ~ tm or be t te r , m ay w e l l p rove to be

econom ica l ly v iab le fo r advanced I I I -V fabr ica tion fac i l it i e s in the fu tu re . The re a re m any

is sues s t i l l to be re so lved fo r S i dev ice fabr ica t ion us ing DUV l i thography , inc lud ing i t s

s ens i t iv i ty to a i rborne chem ica l con tam inan t s and de lays in p roces s f low [5] . However , i t is

expec ted tha t these obs tac le s wi l l be surm ounted , and the re su l t s app l ied wi th re la t ive ly

sm al le r deve lopm ent e f fo r t s to com pound sem iconduc tor dev ice fabr ica t ion .

P hotores i s t s fo r op t i ca l l i thography fo r m id-UV (g- th rough i - l ine ) have com m only

cons i s ted o f novolac -based re s ins wi th p ropr ie ta ry inh ib i to rs and photoac t ive agen t s add ed

to p rov ide the requ i red s ens i t iv i ty and con t ra s t in re sponse to op t i ca l exposure . As such ,

bo th p os i t ive - and nega t ive - tone re s i s ts a re wide ly ava i l ab le , wi th va ry ing re so lu t ions and

sens i t ivi t ies . In general , pos i t ive res is ts possess superior resolut ion, while negat ive res is ts

m ay suf fe r f rom fea tu re swe l l ing and d i s to r t ion dur ing deve lopm ent . In a l l ca ses ,

pho tores i s t p roces s ing typ ica l ly requ i re s t igh t con t ro l no t on ly o f exposure pa ram ete rs such

as dose bu t a l so o f the t em pera tu re and dura t ion o f p re - and pos t -exposure bake p roces ses

in o rde r to reduce the appea rance o f s t and ing-wave pa t t e rns in the re s i s t and dr ive the

photoca ta lys i s reac t ions wi th in the re s i s t . S eve ra l DUV res i s t s based on hydroxys tyrene

have been in t roduced . T he f i r st o f these p roduced on a com m erc ia l s ca le was IBM s AP EX

pos i t ive re si s t, wi th m ore recen t in t roduc t ion o f pos i t ive re si s t s (Hoechs t A Z D X-46 , Ol in

A R C H a n d I B M K R S a n d E S C A P ) , a n d n e g a t i v e r e s i s t s ( I B M C G R a n d S h i p l e y S N R ) .

Wi th the adven t o f DUV l i thography , desp i t e the fac t tha t the under ly ing chem is t ry o f

DUV res is ts is fundamental ly different from that of i - l ine res is ts (for a review, see [6]) ,

f rom the pe rspec t ive o f the p roces s e ng inee r these re s i s ts behave qu i te s im i lar ly , a lbe i t wi th

s ignificant ly t ighter proces s tolerances . Th ese t ighter tolerances , as well as DU V res is t s

s ens i t iv i ty to ex te rna l fac to rs such a s a i rborne con tam inan t s , a re due no t o n ly to the change

in chem is t ry bu t a l so to the fac t tha t the m in im um fea ture s i zes a re so m uch sm a l le r wi th

DUV l i thography than wi th i - l ine .

2.2.1.2. Elec tron eam L i thography

Elec t ron beam l i thography i s a t echnology tha t com plem ents op t i ca l l i thography tha t i s

used in the fabr ica t ion o f h igh-pe r form ance c om pou nd sem iconduc to r dev ices . To da te , it

has no t been econom ica l to use d i rec t -wr i t e e lec t ron beam l i thography fo r com m erc ia l S i

UL S I p rodu c t ion due to i ts inhe ren t ly low th roughput ; the ro le o f d i rec t-wr i t e e lec t ron

beam l i thography in th i s a rena i s in m ask m aking . S ign i f i can t e f fo r t s a re cur ren t ly be ing

m ade to deve lop the S CALP EL techn ique [7 ] fo r pa ra l l e l e l ec t ron beam exposures a t h igh

reso lu t ion (< 0 . 1 ~ tm ) fo r S i m anufac tur ing .

Di rec t -wr i t e e lec t ron beam l i thograph y i s cur ren t ly v iab le fo r I I I -V fabr ica tion because

of the p rem ium p laced on dev ice pe r form ance . T he advan tage o f e lec t ron-beam

l i thography i s i t s supe r io r re so lu t ion com pared to op t i ca l l ighography . Wi th op t im ized

exposure m e thods and the use o f h igh re so lu t ion res i s ts such a s po ly m e thy l m e thac ry la te

(P M MA ), re so lu t ion o f le s s than 10 nm i s pos s ib le . Th i s d ram at ic im prov em en t in

reso lu t ion com pared to op t i ca l l i thograph y i s a d i rect consequence o f the fundam enta l

d i f ference in ope ra t iona l p r inc ip le s o f e lec t ron-beam l i thography and op t ica l l ithography;

ra the r than expo s ing the re s i s t wi th l igh t o f a pa r t icu la r wave leng th , the s am ple i s exposed

to e lec t rons in a des i red pa t t e rn . Because the de Brog l ie wave leng th o f e lec t rons in the

ene rgy range gene ra l ly used fo r l i thography i s o rde rs o f m agni tude sm a l le r than typ ica l

op t i ca l wave leng ths , d i f f rac t ion e f fec t s a re neg l ig ib le and exce l l en t re so lu t ion can be

achieved.

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There a re , however , two s ign i f i can t inconveniences a s soc ia ted wi th e lec t ron-beam

l i thography . The f i r s t i s tha t the th roughput o f even com m erc ia l e l ec t ron-beam exposure

too l s in wafe rs /h our i s m uc h lower than tha t o f m o dern o p t i ca l s teppe rs . The s econd i s

tha t , desp i t e the in t r insica l ly h igh re so lu t ion o f the e lec t ron-beam based l i thograph ic

proces s , s eve ra l phys ica l e f fec t s l im i t the ac tua l ach ievab le re so lu t ion under norm a l

m ic roe lec t ron ic fabr ica tion condi t ions . The so-ca l l ed p rox im i ty e f fect i s one o f these

l im i t ing fac to rs . Ar i s ing f rom e lec t ron s ca t te r ing , the m a in obse rvab le consequence o f th is

e f fect i s a degrada t ion in re so lu t ion and a coopera t ive exposure be tw een ad jacen t fea tu res

tha t can l im i t how c lose ly packed the l i thograph ic fea tu res can be . E lec t rons s ca t te r bo th in

the re s i s t a s they t rave l towards the subs t ra te and backwards a f t e r s ca t t e r ing f rom the

subs t ra te . As these two com ponents a re m arked ly d i f fe ren t , a double -Gauss ian m ode l fo r

the beam prof i l e i s typ ica l ly em ployed fo r m ode l ing the p rox im i ty e f fec t [8 ] . S eve ra l

approaches to ove rcom ing the de le te r ious e f fect s o f the p rox im i ty e f fec t inc lude : i ) ca re fu l

ad jus tm ent o f the e lec t ron exposure dose ; i i) use o f ve ry h igh beam vol tage 50- 100 keV

to t rans fer the fo rward and backsca t t e red e lec t ron prof i le s to m ore nea r ly norm a l ang les o f

inc idence ); i ii ) use o f p roces ses wi th re s i s t s o f ve ry h igh con t ra s t ; iv ) a l t e ra tion o f the s i ze

and shape o f a pa t t e rn to be exposed in o rde r to com pensa te fo r the p rox im i ty e f fec t shape

b ias ing) ; and v) com p ensa t ion fo r p rox im i ty e ffec ts th rough m odula t ion o f the dose a s a

func t ion o f pos i t ion wi th in the pa t t ern . B oth o f the l a s t two a l t e rna tives a re com p ensa t ion

ra the r than m i t iga t ion s t ra teg ie s and gene ra l ly requ i re sophis t i ca ted sof tware m ode l ing of

the s ca t t e r ing p roces ses to num er ica l ly com pute the geom et r i c pa t t e rn o r dose p rof i l e tha t

i s requ i red to ach ieve the des i red pa t t e rn on the wafe r . These com puta t ions a re typ ica l ly

ve ry in tens ive and requ i re l a rge am ou nts o f p roces s ing t im e fo r a l l bu t the s im ples t o f

pa t t e rns . Othe r phys ica l obs tac le s tha t can po ten t i a l ly l im i t the re so lu t ion o f an e lec t ron-

beam l i thography sys tem inc lude chrom at ic d i spe rs ion , beam space -cha rge e f fec t s and

othe rs , bu t these e f fec t s a re a l l com m on to h igh- re so lu t ion e lec t ron-beam m ic roscopy as

we l l a s to l i thography . S am ple and re s i s t cha rge accum ula t ion a l so degrades l i thograph ic

pa t t e rn f ide l i ty by in t roduc ing un in ten t iona l e lec t ros ta ti c de f lec t ion o f the inc iden t b eam .

The interes ted reade r is referred to Brew er [9] for t reatm ent of these issues .

In gene ra l , e l ec t ron-beam res i s t is fundam enta l ly d i f fe ren t f rom photores i s t; ins tead o f

the pho toac t ive agen t s and inh ib i to rs , m os t nonam pl i f i ed e lec t ron-beam res i s t s re ly on

e lec t ron-beam induced po lym er ic cha in s c i s s ion or cha in c ros s - l ink ing to c rea te the

exposure reac t ion . O ne of the m os t com m o n and m os t m a ture e lec t ron beam l i thography

res is t s in use i s P MM A. In norm a l usage , the re s is t i s pos i t ive tone so tha t exposed reg ions

undergo po lym er cha in s c i s s ion , re su l t ing in sm a l le r m olecu la r we igh t m olecu les tha t can

be m ore eas i ly washed away in the deve lope r so lven t . A typ ica l deve lope r so lven t fo r

P M M A is a m ix ture o f isopropyl a lcohol IP A) and m e thy l i sobu ty l ke tone MIB K) , wi th

vo lum et r i c ra t ios o f be tween 5 :1 and 1 :1 be ing co m m on. F igure 2 .1 shows the ob ta ined

sens i t iv i ty /con t ra s t cha rac ter i s ti c fo r 50 K and 950 K m olecu la r we igh t P M M A deve loped

in 3 :1 IP A:M IBK . S ens i t iv i ty i s de f ined as the m in im um dose ex t rapo la ted f rom the l inea r

reg ion o f the g raph) requ i red to co m ple te ly rem ove the ex posed re s i s t a ft e r deve lopm en t

for po s i t ive res is t ) . C ontras t , ~,, is defined by

1

- I ~ Z J / )l'lo g ''2 D l ( 2 . 3 )

where D2 and D1 are as shown in the f igure; D2 is the sens i t ivi ty. F igure 2.1 i l lus tra tes a

con t ra s t o f 5 .9 fo r 950 K P M MA , w hich i s typ ica l o f h igh-con t ra s t p roces ses . Al tho ugh

P M M A has exce l l en t in t rins ic re so lu t ion o f < 10nm [10], i t su f fers f rom ra the r poor d ry

e tch re s i s t ance and re la t ive ly l a rge exposure dose requ i rem ents , re su l t ing in long

l i thography wr i t e t im es . Al though fundam enta l e lec t ron-dose s t a t i s t i c s d ic ta te tha t h ighe r

re so lu t ion re s i s t s requ i re l a rge r doses fo r com ple te deve lopm ent , the re a re o the r re s i s t s

wi th be t t e r s ens i t iv i ty than P M MA . A n exam ple i s ZEP -520 , w hich i s a pos i t ive e lect ron-

b e a m r e si s t f ro m N i p p o n Z e o n C o . t h a t h a s r e s o lu t i o n l im i t s a p p r o ac h i n g t h a t o f P M M A

58

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PROCESSING OF EPITAXIAL HETEROSTRUCTURE DEVICES

0.9

o 0 8

~az 0.7 D 1

... 0.6

.~.

o 0 5

r D 2

~ 0.4

N

0.3

o o .2 . 5 o : i

Z 0.1 ~ 950K PMMA

1 1 1 i w . I . . . . . . .

10 100 1000

Dose (~C/cm 2)

Fig. 2. 1. Sensitivity/contrastcurve for 50K and 9 50K PMM A and 3:1 IPA:MIBK developer. The

contrast is give n by the slope of the linear portion of the curv e, and the sensitivity is defined as D2.

w i th 10x sma l l e r se ns i t i v i t y a nd a pp rox ima te ly 2 .5 t ime s be t t e r d ry e t c h r e s i s t a nc e [11 ,

12] . In addi t ion to resis ts based on cha in sc ission for exposure , nega t ive- tone resis ts tha t

r e ly on re s i s t c ro ss - l i nk ing a re a l so a va i l a b l e . The c he mic a l ly -a mpl i f i e d Sh ip l e y SA L

fa mi ly o f r e si s t s ( e .g . , SA L 601 ) i s a mon g the se . I t s re so lu t ion o f a pp rox im a te ly 50 nm i s

in fe r io r t o t ha t o f PM M A [13 ], bu t t h i s novo la c r e s in -ba se d re s i s t o f fer s goo d d ry e t c h

resis tance and smal l D2, resul t ing in short wri te t imes.

Mul t ip l e - l a ye r r e s i s t s t ruc tu re s a re a l so u se d e i t he r to fu r the r e nha nc e l i ne w id th c on t ro l

or to provide spec ia l -purpose device fea tures. In the f i rs t ca tegory , b i layer resis ts have been

show n to p rov ide e xc e l l e n t l i ne w id th c on t ro l on nonp la na r subs t r a t e s ; i n t h i s c a se , t he

bot tom layer i s essent ia l ly a p lanar iz ing layer , whi le the th in top layer se rves to def ine the

l i t hog ra ph ic f e a tu re . D e ve lopme n t c a n the n be pe r fo rme d in tw o s t e ps ; t he f i r s t s t e p

de ve lops t he e xp ose d fe a tu re s i n t he t op l a yer , a nd a se l e c tive de ve lope r o r o xyge n p l a sma

e tc h i s u se d to impre ss t he se f e a tu re s i n to t he bo t tom l a ye r . A s imi l a r a pp roa c h ha s be e n

be e n de mons t ra t e d fo r improv ing l i t hog ra ph ic r e so lu t ion . A s po lyme r i c r e s i s t s ha ve

c ons ide ra b ly low e r a ve ra ge a tomic numbe r t ha n I I I -V subs t ra t e ma te r i a l s , e l e c t ron

sca t te r ing is less severe in resis t than in the subst ra te . Consequent ly , proximity e ffec ts a re

re duc e d by u s ing a t h in t op r e s i s t l a ye r t o de f ine the ge ome t ry w h i l e a t h i c k unde r l a ye r

se rve s a s a n in t e rme d ia t e subs t r a t e be tw e e n the se mic o nduc to r a nd the t op r e s is t .

A spe c i a l -pu rpose de v ic e f e a tu re t ha t ha s be e n show n to be ve ry a dva n ta ge ous fo r

e p i t a x ia l he t e ro s t ruc tu re de v ic e pe r fo rm a nc e a nd c a n be e f f ic i e n tly r e a l iz e d u s ing mu l t ip l e -

l a ye r r e s i s t s t ruc tu re s i s t he c ommon mush room- o r T- sha pe d me ta l ga t e fo r H EMTs.

F igu re 2 .2 (a ) show s the r e s i s t p ro f i l e ob ta ine d f rom a n e xposu re o f a mu l t i l a ye r r e s i s t

s t ru c t ur e c o n s is t i ng o f , f ro m t h e b o t t o m , 9 5 0 K P M M A , c o p o l y m e r ( P ( M M A - M A A ) ) , a n d

50 K P M M A re s i s ts a nd de ve lope d in 3 :1 IP A :M IB K . The re su l t i ng ga t e s truc tu re a f te r

me ta l l i z a t ion a nd l i f t -o f f is show n in F igu re 2 .2 (b ) . A sho r t ga t e l e ng th i s a c h ie ve d a t t he

ga te footpr in t , whi le low ga te resis tance (due to the la rge c ross-sec t iona l a rea) i s

s imu l t a ne ous ly a c h ie ve d. B y e x te ns ion o f t hi s ge ne ra l i de a , f e a tu re s suc h a s ga t e l e ng th ,

r e c e ss t r e nc h w id th a nd pos i t i on , a nd T-ga t e sha pe a nd w id th ha ve be e n de mo ns t ra t e d to be

inde pe nde n t ly c on t ro l la b l e w i th a s ing le e xposu re s t e p by p rope r a d ju s tme n t o f e l e c tron -

be a m e xposu re a nd se l e c t ive de ve lopme n t [14 ] .

2 2 2 E t c h i n g

In t he f a b r i c a tion o f e p i ta x i a l he t e ro s t ruc tu re de v ic e s , pe rha ps no s ing le t ype o f p roc e ss ing

ha s a l a rge r impa c t on f ina l de v ic e pe r fo rma nc e tha n e t c h ing . E tc h ing f inds ma ny use s i n

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FAY AND ADESIDA

Fig. 2.2. a) Resist profile for trilayer T-shaped gate. b) Ga te cross section aider metallization for T-

shaped gate.

the fabrication of devices, as it is frequently employed: i) to isolate devices from each

other; ii) to gate recess and adjust channel current and threshold voltage in HFETs; iii) to

expose the various layers in a heterostructure device for contacting; and iv) to reduce

parasitic capacitances for improved device performance. Several issues have direct impact

on the viabi l i ty of any etch process for use in contemporary heterostructure device

fabrication. These include the accuracy and controllability of etch depths, uniformity of the

etch both across identical features on the wafer as well as across features of wide ly varying

dimensions, repeatab ility from wafe r to wafer a nd lot to lot, selectivity of the etch rate with

respect to the various materials contained in the heterostructure layer stack, etch sidewall

profile, and the degree of anisotropy of the etch including the effects of crystal orientation,

fideli ty of the pat tern t ransfer between the masking material and the etched sem iconductor

pattern, and any etching-induced damage and its relation to final device performance.

Among those issues, the unique requirements of a particular device will dictate the relative

importance of each, and in many cases wil l suggest a natural type of process required to

mee t them. O f the etching processes available, mos t fall into two separate and in many

ways complementary categories , wet etching in aqueous chemical solut ions and dry

etching in plasmas. These two cases will be treated separately, with their comparative

merits and limitations delineated in what follows.

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PR O C ESSIN G O F EPITA X IA L H ETER O STR U C TU R E D EV IC ES

2 2 2 1 Wet Etching

A l though w e t e t c h ing i s no t e a s i l y a pp l i e d to l a rge - sc a l e ma nufa c tu r ing , i t s l a c k o f

subsu r fa c e de fe c t fo rma t ion a nd re l a t i ve e a se o f imp le me n ta t ion fo r l a bo ra to ry a nd sma l l -

sc a l e ma nu fa c tu r ing m a ke s i t a popu la r e t c h ing t e c hn ique . The ba s i c op e ra t ion o f ne a r ly al l

o f t he w ide ly u se d w e t e t c ha n t s fo r I I I -V se mic on duc to r m a te r i a ls r e l i e s on the p r inc ip l e o f

ox ida t ion fo l low e d by re duc t ion a nd re mova l o f su r fa c e ma te r i a l . Fo r t h i s r e a son , ma ny

e tc ha n t s c ons i s t o f tw o o r more c he m ic a l c ompone n t s , w i th t yp i c a l ly one c om pone n t

se rv ing a s a n ox id i z e r a nd the o the r a s a r e duc ing o r c om ple x ing a ge n t fo r r e mova l o f the

ox ide spe c i e s . A s th i s i s a n inhe re n t ly e l e c t roc he mic a l e f fec t , w i th c ha rge t r a ns fe r be tw e e n

spe c i e s i n so lu t ion a nd a toms on the se mic onduc to r su r fa c e oc c u r r ing in bo th ox ida t ion

a nd re duc t ion s t a ge s , va ri a n t s o f w e t e t c h ing suc h a s a nod ic a nd c a thod ic e t c h ing , a s w e l l

a s pho toc he mic a l e t c h ing c a n be a pp l i e d a nd c on t ro l l e d th rough the d i r e c t ma n ipu la t ion o f

charge carr ie r f lux .

In a dd i t i on to t he t yp ic a l ly tw o-s t e p se m ic onduc to r d i s so lu t ion r e a c t ion , t he phys i c a l

p roc e ss gove rn ing e t c h ing a l so c ons i s ts o f tw o pha se s . F o r r e mova l o f ma te r i a l , a c t ive

e t c ha n t spe c i e s mus t be de l ive re d to t he se mic onduc to r su r fa c e , t he subse que n t tw o-s t a ge

e t c h ing re a c t ion mus t oc c u r , a nd the r e a c t ion p roduc t s mus t be r e move d f rom the su r fa c e

to ma ke w a y fo r fu r the r r e a c t ion / re mo va l c yc l e s to t a ke p l a c e. In ge ne ra l , t he e t c h r a te c a n

be l im i t e d by e i t he r the de l ive ry (o r r e mova l ) o f re a c t a n t spe c i e s o r t he se que n t i a l ox ida t ion

a nd re duc t ion r e a c t ions . W hic h o f t he se tw o p roc e sse s l im i t s the r a t e o f e t c h ing fo r a

pa r t i c u l a r e t c ha n t ha s a s ign i f ic a n t impa c t on the ove ra l l pe r fo rma nc e a nd su i t a b il i t y o f the

e t c h fo r a g ive n p roc ess . E t c h ing l im i t e d by the f i r st o f the se m e c ha n i sm s i s sa id to be

d i f fu s ion - ra t e l im i t e d , w h i l e e t c h ing l im i t e d by the se c ond me c ha n i sm i s sa id t o be

reac t ion-ra te l imi ted . In genera l , e tches tha t a re d i f fusion-ra te l imi ted ac t as pol ish ing

e tc he s fo r l a rge e t ch a re a s , bu t su f fe r f rom s ign i f i c an t e t c h a n i so t ropy n e a r ma ske d fe a tu res

and are of ten d i ff icul t to contro l due to the sensi t iv i ty of the e tch ra te to type and ra te o f

agi ta t ion during e tching. As i t i s of ten d i ff icul t to ensure tha t a l l points on a sample a re

sub je c t e d to t he sa me a g i t a t i on c ond i t i ons f rom w a fe r t o w a fe r a nd lo t t o l o t, e t c h ing ba se d

on d i f fu s ion - l imi t e d p roc e sse s c a n su f fe r f rom s ign i f i c a n tly nonun i fo rmi ty . E tc he s t ha t a re

l imi ted by surface reac t ion k ine t ics , on the o ther hand, a re genera l ly insensi t ive to agi ta t ion

bu t ha ve a n A r rhe n ius de pe nde nc e o f e t c h ra t e on t e mpe ra tu re , a nd so r e qu i re c a re fu l

mon i to r ing a nd c on t ro l o f so lu t ion t e mpe ra tu re fo r a c c u ra t e e t c h r a te c on t ro l. F o r bo th

diffusion- and reac t ion-ra te l imi ted e tches, in contrast to the s i tua t ion for s i l icon, e tching of

I I I -V se mic onduc t ing ma te r i a l s i s o f t e n s t rong ly c ry s t a l l og ra ph ic a nd thus i nhe re n t ly

a n i so t rop ic . T h i s e f fec t a r ise s f ro m the c omp o tmd n a tu re o f t he se se mic onduc to r s ; t he

c he m is t ry a ssoc i a t e d w i th t he r e mova l o f a g roup I I I a tom i s s ign i fi c a n t ly d i f f e re n t f rom

the r e mova l o f a g roup V a tom. A s a c onse q ue nc e , i t i s typ i c a l fo r w e t e t c h s ide w a l l

prof i les to evolve towa rds (11 1) p lanes [ 15 , 16] , as these surfaces a re e i ther pure ly g roup

I I I o r g roup V , de pe nd ing on su r fa c e po la r i t y . N o t a l l e t c he s r e su l t i n e xpose d (111 )

su r fa c e s , how e ve r ; e t c he s ha ve be e n ide n t i f i e d tha t e t c h p re fe re n t i a l l y t o h ighe r - inde x

planes such as (332) [17] .

B e c a use the e t c h ing o f I I I -V s r c ons i s t s e sse n t i a l ly o f a l t e rna t ely

ox id i z ing a nd re duc ing the su r fa c e , a ny c he mic a l spe c i e s c a pa b le o f i nduc ing e i the r o f

the se e f fe c t s i s a po te n t i a l r e a c t a n t . A s a r e su l t , a g re a t ma ny c he mic a l sy s t e ms ha ve be e n

inve s t iga te d fo r t he e t c h ing o f I I I -V s , w i th r e a c t a n t s r a ng ing f rom the s t rong ino rga n ic

a c id s , suc h a s su l fu r i c , hyd roc h lo r i c a nd phosphor i c , t h rough w e a ke r o rga n ic a c id s ,

i nc lud ing c i t r i c , suc c in i c a nd a c e t i c a c id s , t o ba se s i nc lud ing a mmonium hydrox ide a nd

po ta ss ium hydrox ide . M a ny o f t he a c id ic so lu t ions i nc lude hyd roge n pe rox ide a s the

ox id i z ing a ge n t , w i th t he a c id se rv ing a s t he r e duc ing a nd c omple x ing a ge n t fo r su r fa c e

a tom de so rp t ion . R e p re se n ta t ive e xa m ple s o f w e t e t c ha n t s pub l i she d in t he l i t e ra tu re fo r

se mic onduc to r s ba se d in t he G a A s- a nd InP-ba se d ma te r i a l sy s t e ms a re l i s t e d in Ta b le 2 .1 .

This i s not a comprehensive l i s t ; i t i s in tended to se rve only as a s ta r t ing point and to

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Table 2.1. Wet Chemica l Etchants for III-V Com poun d Semiconductors

Material Etchan t Etch rate Selectivity

Comments Reference ,

G a A s

H 2 S O 4 : H 2 0 2 : H 2 0 : : l: 1 0 : 2 0 0 0 3 0 n m / m i n @ 23 ~

N H 4 O H : H 2 02 : H 2 0 : : 3 : 1 : 5 0

3 : 1 : 1 5 0

6 0 0 n m / m i n @ 2 0 ~

8 0 n m / m i n @ 2 0 ~

C A : H 2 0 2 : : 4 :1 5 3 4 n m / m i n

InP HC 1 12 gm / mi n @ 25 ~

HC I : H20: : 3 :1 600 nm/ m i n @ 20 ~

HC I : H3PO4: C H3O H: : 1 : 1 : 1 100n m/ m i n @ 20 ~

1-3 B r : C H3OH > 12 gm/ m i n @ 25 ~

G a N ( n -t y pe ) 0 . 04 M K O H a n d H g a rc l a m p > 3 0 0 n m / m i n @ 5 0 m W / c m 2

(wavelength = 365 nm) l ight intensity

A1N AZ 400 K - - ,30nm/ m i n @ 36 ~

(KOH-based developer)

InGa As H3PO4 : H202 : H20: : 1 : 1 : 8 470 nm / mi n

: : 3 8

132 nm / mi n @ 20 ~

InA1As H3PO 4: Hz Oz : H20 : : 1 : 1 : 38 210 nm / mi n @ 20 ~

InGa P m HC I : 1 H20 for m = 0 .6 3 nm/ m i n

for m = 1 .0 27 nm/ m i n

@ 23 ~

GaAs AlxGal_xAs C A1 (pH = 6 .5 ) : H 202: : 50 : 1

125 nm/ m i n

C A : H 2 0 2 : : 4 : 1 3 6 0 n m / m i n @ 2 0 ~

4 2 0 n m / m i n @ 2 0 ~

C A : H 2 0 2 : : 3 :1 6 3 n m / m i n @ 2 2 ~

AlxGal_xAs GaAs

K I : H z S O 4 : :I : I ( p H = 0 . 9 ) f o r x = 0 . 3 1 6 5 n m /m i n

@ 3 ~

InA1As / InGa As HC I : H20: : 3 :1 648 nm/ m i n

I n G a A s /I n A 1 A s S A ( p H - - 5 ) : H 2 0 2 : : 1 5 : 2 5 5 n m / m i n

C A: H2 02 : : 1 : 1 120n m/ m i n @ 20 ~

InGa A s / InP C A: H2 02 : : 10 :1 120 nm / mi n @ 28 ~

H 2 S O 4 : H 2 0 : H 2 0 : : l : l : 1 0 1 8 6 0 n m / m i n @ 3 8 ~

InA1As / InP C A: H 202: : 10 :1 60 nm/ m i n @ 20 ~

H 3 P O 4 : H 2 0 2 : H 2 0 : : l : l :3 8 2 1 0 n m / m i n @ 2 0 ~

I n P / I n G a A s H B r : C H 3 C O O H : : 1 :1 8 8 0 n m / m i n @ 5 ~

Selective over

GaN, InN,

A1203 ,Ga As

Very selective to

GaAs, A1GaAs

for x = 0.15

for x = 0 .3

for x = 0.3

for x = 0.15

for x = 0.32

330

2000

69

25

400

Very selective

160

Very selective

Very selective

80

120

105

50

Mesa e tching and

gate recess

Mesa e tching

Mesa e tching and

gate recess

Mesa e tching

[19]

[20]

[21]

Mesa e tching

Via hole [22]

Photoenh anced [ 18]

wet e tching

Reaction-rate [23]

limited etching, etch

rate crystall ine

qual i ty dependent

Nonse lec t ive ga te recess [24]

[25]

Nonselec t ive ga te recess [25]

Strong ly HC1 [ 19]

concent ra t ion

dependent ; HBT

fabrication

HFE T fabrica tion [26]

Selective gate recess

for HE MT

100 DB R laser fabrica t ion

Selective etchant

HEMT fabrica t ion

HEMT fabrica t ion

HEMT fabrica t ion

[28]

[27]

[291

[30]

[3 ]

[321

[32]

[32]

a Data from authors ' l abora tory

CA1 = 1 g sol id c i tr i c ac id in 10 0ml water; NHaO H for pH cont rol

CA - 100 g sol id c i tr i c ac id in 100 ml of DI w ater

SA = 200 g crysta l succ inic ac id in 1 l i t e r DI water; NH4O H for pH cont rol

KI = 28 g of KI, 16.25 g o f Ia and 25 ml DI water; solut ion s tabi l i zed for 3 days

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PR O C ESSIN G O F EPITA X IA L H ETER O STR U C TU R E D EV IC ES

i l lu s t r at e t he va r i e ty o f e t c ha n t s a va i l ab l e a nd some impor t a n t e t c ha n t f e a tu re s . A l so

inc lude d on the l i s t a re som e e t c ha n t s fo r A 1N a nd G a N . In t he c a se o f G a N , e t c h ing w a s

a c h ie ve d u s ing the pho toe l e c t roc he mic a l PEC ) w e t e t c h ing me thod , i n w h ic h u l t r a v io le t

U V ) ra d i a t ion i s d i r e c te d a t t he sa mp le i n a n e l e c t roc he mic a l c e l l [ 18 ] . The P EC me tho d

ha s de m ons t ra t e d ve r sa t i l i t y in t he e t c h ing o f n - type G a N . V a r ious su r fac e morph o log ie s ,

a s w e l l a s et c h a n i so t ropy , c a n be ob ta ine d de pe nd ing on the i n t e ns i ty o f the i nc ide n t U V

ra d ia t ion a nd the c on c e n t ra t ion o f t he e t cha n t . F igu re 2 .3 show s a h igh ly a n i so t rop ic p ro f i le

in G a N ob ta ine d by e t c h ing in K O H so lu t ion [18 ] .

O ne pa r t i c u l a r ly u se fu l f e a tu re i n t he f a b r ic a t ion o f H FE Ts a nd H B T s i s t he a va i l ab i l i ty

o f bo th nonse l e c t ive a nd se l e c tive w e t e t c ha n ts . In pa r t ic u l a r , e t c h ing to e xpo se the t h in

e p i t a x ia l ba se o f a n H B T o r fo r gat e r e c e ss i n a n H FET re qu i re s ve ry p re c i se e t c h de p th

c on t ro l . Th i s i s o f t e n be s t done th rough the u se o f a n e t c ha n t t ha t e t c he s ve ry w e l l t he

ma te r i a l to be r e move d bu t doe s no t e t c h o r e tc he s muc h mo re s low ly ) t he ma te r i a l to be

l e f t i n p l a c e . A n e xa mple o f a se l e c t ive e t c ha n t fo r A 1G a A s a nd G a A s i s t he c i t r i c

a c id /hy d rog e n pe rox ide sy s t e m. D e pe n d ing on the r e l a t ive a moun t s o f hyd rog e n pe rox ide

and c i t r ic ac id in the solu t ion and the A1 mo le f rac t ion , the e tch ra tes of GaA s and A 1GaAs

ma y be a d ju s t e d , a nd e i the r se l e c t ive o r non -se l e c t ive e t c h ing c a n be a c h ie ve d . F igu re

2 .4 a ) show s the e t c h r a t e fo r G a A s a nd fo r se ve ra l mo le f r a c tions o f A 1G a A s a s a func t ion

o f e t c ha n t c ompos i t i on , w h i l e F ig . 2 .4 b ) show s the se l e c t iv i ty ob ta ine d . Se l e c t iv i ty as h igh

as 260 for GaA s o ver A10.45Ga0.55As is obta ined for a ra tio o f 4 :1 c i t r ic ac id to H2 02, and

se l e c t iv i ty o f 1450 w a s ob ta ine d fo r G a A s ove r A lA s [33 ] . U se o f se l e c t ive e t c ha n t s suc h

as th is in conjunc t ion wi th e tch stop layers in he te rost ruc tures can lead to very accura te

contro l of e tch depths. Figure 2 .5 show s e tch depth vs e tch dura t ion for a 4 :1 c i t r ic

a c id :H 20 2 e t c h on a he t e ro s t ruc tu re c ons i s t ing o f 300 nm o f G a A s , a 3 -nm A lA s e t c h s top

la yer , a nd a n a dd i t i ona l 300 nm o f G a A s . O nc e the t op l a ye r o f G a A s i s r e move d , t he e t c h

de p th r e ma ins e sse n t i a ll y i nde pe nde n t o f e t c h time fo r ove r 10 min . T h i s a pp roa c h a l low s

ve ry a c c u ra t e a nd re pe a t a b l e e t c h de p ths t o be a c h ie ve d w i th ve ry r e l a xe d to l e ra nc e s on

e tch t ime.

2 2 2 2 Dry Etching

In a dd i t i on to r e mova l o f ma te r ia l u s ing w e t e t c h ing , a va ri e ty o f d ry -e t c h ing t e c hn ique s

e x i s t t ha t ha ve s ign i fi c a n t a dva n ta ge s ove r w e t e t c h ing fo r f a b r i c a t ion o f h igh -pe r fo rma nc e

c ompound se mic onduc to r de v ic e s . In ge ne ra l , fo r d ry e t c h p roc e ss ing , t he e t c ha n t s a re

in t roduc e d in to a va c uum c ha m be r a s ga se ous spe c i es , a nd a re t yp i c a l ly ion i z e d o r

p rom ote d to e xc i t e d e ne rgy s t a t e s by the a pp l ic a t ion o f r a d io f r e que nc y R F) o r mic row a ve

ra d ia t ion to fo rm p la sma s . The a dva n ta ge s o f d ry e t ch ing c om pa re d to w e t e t ch p roc e sse s

inc lude: i ) d i re c t c on t ro l ove r t he de g re e a nd type o f a n i so t ropy o f t he e t c ha n t , r a ng ing

from near i so t ropic e tch prof i les to ver t ica l s idewal ls ; i i ) the abi l i ty to in i t ia te e tching in

ve ry sma l l a nd ve ry l a rge f e a tu re s s imu l t a ne ous ly s inc e the t yp ic a l w e t -e t c h p rob le ms o f

su r fa c e w e t t a b i l i t y a nd bubb le fo rma t ion a re e l imina t e d ; i i i ) improve d un i fo rmi ty a c ross

the w a fe r ; iv ) a re duc t ion in t he vo lume o f w a s t e byp roduc t s ; a n d v ) e a sy a da p ta t ion to

p roc e ss a u toma t ion . D i sa dva n ta ge s i nc lude loa d ing e f fe c t s w he re by pa t t e rn f e a tu re s o f

va r ious s i z e s e t c h a t d i f f e re n t r a t e s a nd the h igh c os t o f d ry e t c h ing e qu ipme n t .

Se ve ra l ma jo r e qu ipme n t c on f igu ra t ions ha ve w ide a pp l i c a t ion in d ry e t c h ing . Th e mos t

c ommon d ry e t c h p roc e sse s i nc lude p l a sma e t c h ing , c onve n t iona l r e a c t ive i on e t c h ing

R IE) , el e c t ron c yc lo t ron re sona nc e EC R ) R IE , a nd induc t ive ly c oup le d p l a sma IC P)

R IE . In a dd i t i on to t he se ma jo r c a t e go r i e s , t he re a re ma ny poss ib l e c ombina t ions a nd

mod i f i c a t ions t o t he ba s i c r e a c to r s t ruc tu re fo r spe c i a l i z e d a pp l i c a t ions . F igu re 2 .6 a ) - e )

show s sc he ma t i c a l ly t he e xpe r ime n ta l c on f igu ra t ions r e qu i re d to imp le me n t p l a sma e t c h ,

c onve n t iona l R IE , EC R -R IE, IC P-R IE , a nd c he mic a l ly a ss i s t e d ion be a m e t c h ing

C A IB E) . The m a in d i f f e re n t ia t i ng f a c to rs be tw e e n e a c h o f t he se e t c h ing c on f igu ra tions

a re the d e ns i ty o f t he p l a sma c re a t e d a nd the e ne rge t i c a nd d i re c t iona l d i s t r i bu t ions o f the

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Fig. 2.3. Highly anisotropic GaN etch profile obtained by photoelectrochemical etching in KOH.

Fig. 2.4. a) Etch rate for GaAs and A1GaAs vs citric acid compositi on and A1 mole fraction, b)

selectivity of GaAs vs A1GaAs as a function of etch solution composition.

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0.9

0.8

0.1

0.7

~ 0 . 6

v

0.5

0

0.4

0

- 0.3

0.2-

1 ! 9

0

0

m

PROCESSING OF EPITAXIAL HETEROSTRUCTU RE DEVICES

. . . . I . . . . I . . . . I . . . . I . . . . I I . . . . I . . . .

100 200 300 400 500 600 700 800

E t c h T i m e ( s )

F i g . 2. 5 . E t c h d e p t h v s t i m e f o r 4 1 c i t r ic a c i d : H 2 0 2 s o l u t i o n e t c h i n g a 3 0 0 n m G a A s / 3 n m

A 1 A s / 3 0 0 n m G a A s h e t e r o s t r u c t u r e .

ionized etchant radicals . As shown in Fig. 2.6(a) , in plasma etching the samples are placed

on the ground electrode and as a consequence the plasma sel f-bias that i s generated

accelerates ions in the plasma away from the sample. As a result , the etching in this case is

a lmost ent i re ly chemical in nature , and is facil i ta ted only b y the exci ted nature of the

etchant species in the plasma. This leads to isotropic or crys ta llographic e tching d epending

on which comb inat ion o f e tchant and sem iconduc tor is being co ns idered. In a l l of the

varie t ies of RIE, the samples are placed on the e lect rode driven with RF energy. This

configurat ion resul ts in ions being accelerated towards the sample , and thus int roduces a

physical or sput ter ing comp onen t of e tching in addi t ion to the e tching due to the chemical

act ion of the exci ted radicals in the plasma.

The ECR-RIE and ICP-RIE tools are high dens i ty plasma sys tems that ut i l ize enhanced

metho ds of ionizat ion to generate h igh ion dens i t ies (> 1011 cm -3) in com parison w ith

convent ional RIE w here the ion dens i ty is an order of magni tude lower. For the ECR -RIE

and ICP-RIE methods , the RF power for control l ing the ion f lux and for f ixing the ion

energy are independent . This deco upl ing a l lows for the del ivery o f large ion f luxes a t low

energies (or biases) onto samples . The higher vacuum sus ta ined in the ECR-RIE and ICP-

RIE chambers provides di rect ional i ty to the ions even at low energies and thus a ids

anisotropy in e tching.

Other pract ical ion-ass is ted e tching m ethods are based on the ion mil l ing technique and

these are react ive ion beam etching (RIBE), chemical ly ass is ted ion beam etching

(CAIBE) (a lso cal led ion beam ass is ted e tching (IBAE)), and radical beam ion beam

etching (RBIBE). These methods , as in the RIE techniques , couple the physical e tching of

energet ic ions with chemical e tching o f neutra ls and exci ted neutra ls . The RIB E me thod

involves generat ing react ive chemical species such as C1+ and C12+ in an ion source and

direct ing the beam onto a sample . F or CAIB E, a beam of an inert ion such as Ar + is

directed onto a samp le located in an am bient react ive neutra l gas such as C12. The R BIB E

meth od is s imilar to CAIB E bu t the ambient neutra l gas here is predissociated to provide a

beam of highly react ive radicals .

The chemis t ri e s for the dry e t ch ing of I I I -V comp ound semiconduc tors can be d iv ided

into categories based on the primary e tchant gases [34] . Typical chemis t r ies for RIE-type

etching of III -V s are bas ed on: i) C12; ii) CH4; ii i) Br2; and iv) 12. Specific gase s utilized

under these categories wi th some com men ts are l is ted in Table 2.2. M any o f these gases

are usual ly mixed with Ar, He, N, or O for di lut ion, control of e tch ra tes , and in some

cases, for eas ier igni tion o f the plasma. Typical e tch ra tes in som e o f these chem is t r ies are

l is ted in Table 2.3 for the convent ional RIE technique. These ra tes can be used only as

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Fig. 2.6. Dry etcher configurations. a) plasm a etching, b) RIE, c) ECR-RIE , d) ICP-RIE, e) CAIBE.

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P R O C E S S IN G O F E P I T A X IA L H E T E R O S T R U C T U R E D E V I C E S

Table 2.2. Typical Gas Chemistries for RIE of III- V Semiconductors

Chemist ry Comments

a) C12-Based

C12, BC13, SIC14, PC13 CC12F2

b) CH4-Based

CH4, C 2H6, C3H8

c) Br2-Based

Br2, HBr, IBr, CF3Br

d) I2-Based

I2, HI, IC1, IBr

Addi t ions of He, N, Ar may be necessary

Difficult etchants for In-based compounds

Addi t ions of H2, He, A r for d i lu t ion and stabi l ity

Polymer deposi t ion in chamber and on mask

Difficult to use resist masks

Group V depletion can lead to rough surfaces

Corrosive to gas lines

Chamber maintenance more f requent

Iodide compoun d deposi t io in

Chamber maintenance more f requent

Table 2 .3 . Typical Etch Rates for Selected I I I -V Compou nds Using

React ive Ion Etching

Etch rate

Mater ial Chemist ry Bias vol tage nm/m in) Reference

GaA s C12 300 V 250 [35]

C12/Ar 100 V 2000 [36]

BC13/Ar 750 [37]

SiC14/A r 100 V 500 [36]

CC12F2/Ar 750 [38]

CH4/H2 600 V 40 [39]

InP SiCI4 /Ar 150 [42]

HB r 130 V 10 [41 ]

CH4/H2 480 V 75 [40]

GaSb C12/Ar 100 V 550 [36]

CC12F2/O2 390 V 180 [42]

SiC14/Ar 100 V 80 [36]

CH6/H2 430 V 25 [42]

GaN SIC14/At 400 V 51 [43]

BC13/Ar 48 [44]

HB r 400 V 50 [45]

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FAY AND ADESIDA

guides because m a te r ia l qua l i ty and e tch ing appa ra tus can d i f fe r s ign i f i can t ly f rom

labora to ry to l abora to ry . The invo la t i l i ty o f g roup V f luor ides p rec ludes pu re ly f luor ina ted

gases a s p r im ary e tchan t s fo r I I I -V m a te r ia ls . Th i s does , however , pe rm i t the fo rm a t ion o f

d ie lect r ic m asks on I I I -V s us ing F -con ta in ing gases such a s CF 4, NF 3 an d S F 6 .

The e tch ra te s o f GaAs in the chem is t r i e s l i s t ed in Tab le 2 . 2 a re p rac t i ca l in t e rm s of

app l ica t ions , a l though the m os t co m m only em ployed proces ses a re based on BC13 or

S iCI4 . These gases have the ab i l i ty to ge t t e r wa te r vapor and to read i ly a t t ack the n a t ive

ox ides o f I I I -V s , w hich a l lows e tch ing to p roceed im m edia te ly wi thou t any dead or

incuba t ion pe r iod . The e tch ing produc t s fo r GaAs and A1GaAs in C12-based gases a re

GaClx , A1Clx , and A sClx. The vo la t i l i ty o f these p roduc t s i s a ided by ion bom bardm ent .

The e tch ing o f GaAs and an y com p os i t ion o f AlxGa l_xAs proceeds a t equa l e tch ra te s in

C12, BC13 and S IC14 p la sm as under idea l condi t ions w hen no w a te r vapor i s p resen t . T he

presence o f wa te r vapor p ro m otes the con t inuous fo rm a t ion o f ox ides (e .g . , A1Ox) , which

can s ign i f i can t ly lower the e tch ra te s o f AlxGa l_xAs in a com par i son wi th GaA s . Wh ere i t

i s des i red to con t ro l l ab ly e tch A1GaAs a t lower e tch ra te s than GaAs , CC12F 2 or o the r

ch lor ine -based gases m ixed wi th a f luor ina ted gas can be u t i l i zed . Exam ples o f such gas

m ix tures a re S iC14/S iF 4 and BC13/S F 6. These m ix tures can be used in any ra tio to ach ieve

des ired select ivi t ies . The f luorine in the plasma promotes the formation of involat i le A1F3

on the A1GaAs sur face , which ac t s to s low down or s top the e tch ing . Due to ozone

dep le t ion conce rns CC12F 2 i s no longe r co m m only used fo r e tch ing .

Because the chlorides of In are re la t ively involat i le i t is diff icul t to e tch In-containing

I I I -V com pounds in ch lor ine d i s cha rges . However , i t i s pos s ib le to enhance the vo la t i l i ty

of ind ium ch lor ides by hea t ing the sam ple above 150~ to p rom ote desorp t ion o f

invo la t i l e p roduc t s . The m e thane chem is t ry CH4/H2 was in t roduced to surm ount the

l im i ta t ions o f ch lor ine chem is t ry in the e tch ing of In -con ta in ing com poun ds [46] . The

m ethy l rad ica l CH3 gene ra ted f rom CH4 rem oves the g roup I I I spec ie s (M) a s

m e ta l lo rgan ic com pounds , (CH3)xMy, whi le H2 rem oves g roup V e lem ents a s hydr ides .

Etch ra tes for a l l III-V materia ls in this gas mixture ( i .e . , CH4/H2) are low. To increase

e tch ra te s , h ighe r a lkenes such a s C2H6 and C3H 8 a re u t i l ized ins tead o f CH4. The add i t ion

of At to C H4/H 2 a l so inc reases e tch ra te s due to enhanced ion ac t iv i t ie s . Al thoug h

ext rem ely sm ooth sur faces can be ob ta ined in m e thane -based p la sm as , the m a jor p rob lem

assoc ia ted wi th these m ix tures i s the depos i t ion o f a po lym er f i lm wi th in the reac tor

cham ber , on m as k sur faces , and a l so on the sur face o f the s am ple . L ow CH4 :H2 ra t ios can

be u t i l i zed to m in im ize po lym er depos i t ion ; however , the exces s H2 can cause s eve re

dep le t ion o f the g roup V e lem ent , re su l ting in nons to ich iom et r i c and ro ugh e tched

sur faces . Anothe r m e thod used to m in im ize po lym er depos i t ion i s the add i t ion o f sm a l l

a m o u n t s o f 2 to the CH4/H2 m ix ture . Addi t iona l ly , the po lym er fo rm ed can be rem oved

af te r e tch ing in m os t cases us ing O2 p la sm a . The con t ro l ach ievab le in us ing CH4/H2 i s

dem o ns t ra ted by the h igh ly an i so t rop ic InP e tched prof il e s in F ig . 2 . 7 , which w as ob ta ined

us ing a cyc l i c CH4/H2 e tch /O2 c lean proces s [47]. The s l igh t ly rough e tched sur face i s

due to the h igh ly In - r i ch s to ich iom et ry re su l t ing f rom the dep le t ion o f P due to the

form a t ion of vo la t i l e hydr ides wi th hydrogen in the p la sm a .

T h e a d v e n t o f h i g h p l a s m a d e n s i t y s y s t em s ( E C R - R I E a n d I C P - R I E ) h a s m a d e p o s s i b le

the h igh ra te e tch ing of In -con ta in ing m a te r ia ls in ch lor ina ted gases a t room tem pera ture .

These re su l t s have been a t t r ibu ted to the h igh spu t te r - induced deso rp t ion o f low vo la t i l i ty

InClx e tch products before the surface is encapsulated [48]. I t is a lso poss ible that the

sur face o f the e tched s am ple i s loca l ly e leva ted to h ighe r t em p era tu res , which a l so

prom otes desorp t ion .

Brz -based and I2 -based d i s cha rges have a l so been app l ied to the e tch ing of va r ious In -

con ta in ing com pounds , and to o the r I I I -V m a te r ia l s [49] . Hydrogen brom ide has been

used to e tch InGaAs and has dem ons t ra ted h igh e tch ing s e lec t iv i ty ove r InA1As [50] . A

m ore de l ibe ra te gas m ix ture des igned to ach ieve h ighe r e tch s e lec t iv i t i e s fo r InGaAs ove r

InA1As is S iC14 /S iF 4/HB r [41 ] . Nov e l gas m ix tures such a s IBr / Ar and IC1/A r have been

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PROCESSING OF EPITAXIAL HETEROSTRUCTUR E DEVICES

Fig. 2.7 . An 80-nm period InP grating formed by alternateCH4 H2and

2

RIE.

applied to the etching of diverse samples, such as GaAs, A1GaAs, GaS b, InP, InGaAs , and

InSb in ECR -RIE reactors [49]. The advan tage of these halogens is that the boiling points

of volatile iodine and brom ine co mpo unds of group III elements e.g., GaIx, InIx, GaBr~,

and InBr~) have values similar to those o f volatile gro up V elements e.g., PI~ and PBrx). In

contrast to the chlorides, this means that preferential losses of group V elements are

minim ized and the formation of nonstoichiom etric etched surfaces is prevented. In

practice, however, care mus t be taken because Br2- and I2-based gases are corrosive to gas

lines.

Wide bandgap GaN and re lated compoun ds have high bond energies and are therefore

inert and chemically stable, preventing conventional wet etching in common chemicals.

External energies are essential to initiating and sustaining etching for these materials. The

various forms of etching ranging from CAIBE to ICP-RIE have been utilized in etching

GaN, A1GaN, InGaN, and InA1N. Figure 2.8 a) shows the etch rate of GaN as a function o f

plasm a self-bias voltage using RIE in SIC14 plasma [43]. Etch rates were found to be

invariant with respect to chamber pressure as shown in the figure but highly dependent on

the self-bias voltage. Ho wever, the etch rates at voltages as high as 400 V are less than

55 nm /mi n. The highest etch rates for GaN and related materials have been demo nstrated

using Cl2-based discharges in ICP-RIE reactors. Etch rate curves for the ICP-RIE of GaN

and AlxGal_~N in a C12/Ar gas mixture are shown in Fig. 2.8 b). The decrease in etch rate

for increasing A1N concentration is due to the increasing bon ding energy. Aniso tropic etch

profiles useful for GaN laser facets have also been demonstrated. However, more

development work at low ion energies suitable for the fabrication of GaN FETs and HBTs

is still needed.

An important issue with any dry etching technique is that of damage to the

semiconductor material. Due to the ion bombardment that occurs in all varieties of RIE

and other ion-assisted etching methods, the semiconductor is damaged by collisions with

energetic ions. The damag e can be manifested in the form of nonstoichiom etric regions,

defect aggregates, and po int defects [34]. The electrical effects of these etching-indu ced

defects include compensation of dopants, a decrease in carrier mobility, and an increase in

flicker noise. An additional effect that is peculiar to com pou nd s emicond uctors is that, due

to the unequal masses of the sem iconduc tor constituents, the local stoichiometry of the

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FAY AND ADESIDA

6 0

5O

o

so

o

lO

1 0 0 2 0 0 3 0 0 4 0 0

P l a s m a B i a s V o l t a g e - V )

a)

6 0 0

500 -

E 4 0 0 -

. , m

E

E

3 0 0 -

t~

iv

t -

o

2 0 0 -

k

100 -

GaN

---O-- AIo.~oGao.9oN

AIo.soGao.5oN

AIo.72Gao.2aN

AIN

o ~ l - . . . . . . . .

i I I I

0 50 100 150 200 250 300

S u b s t r a t e B i a s V o l t a g e ( V )

fo)

Fig. 2.8. a) Etch rate of GaN as a function of plasma self-bias voltage for SIC14 RIE. b) Etch rate of

GaN and A1GaN in C12 ArICP-RIE.

sem i cond uc t o r can be a l t e r ed , lead i ng t o r eg i ons t ha t a r e ove r l y r ich i n g roup I I I a t oms , f o r

exam pl e , and t hus a l t e r i ng t he p rope r t i e s o f t he ma t e r i a l subs t an t i a l ly . T h i s e f f ec t i s o f ten

obse rv ed a t t he su r f ace o f R IE -p rocess ed m a t e r i a l , and e l ec t r ica l l y man i f e s t s i t s e l f a s

a l t e r ed S ch o t t ky ba r r i e r he i gh t s [ 51 ] . An exam pl e o f the e l ec t r i ca l cha rac t e r i s t ic s o f a P -

dep l e t ed InP e t ched su r f ace is shown i n F ig . 2 .9 a ) , b ) . Af t e r CH4/ H 2 R IE , t he sampl e

w a s c o a t e d w i t h A u f o l l o w i n g v a r i o u s s u r f a c e t r e a t m e n t s i n H C 1 t o r e m o v e v a r y i n g

a m o u n t s o f m a t e r i a l f ro m t h e s u r f a ce . T h e a s - e t c h e d s a m p l e i s s e e n to b e o h m i c b e c a u s e

t he Au i s i n con t ac t w i t h t he In - r i ch su r f ace . T h e su r f ace co n t ac t was s t i ll ohmi c a f t e r 5 nm

o f m a t e r i a l w a s r e m o v e d . S c h o t t k y c h a r a c t er i st i cs w e r e o b s e r v e d a f te r w e t e t c h i n g b e y o n d

12 nm i n dep t h , w i t h a r ecove ry o f cha rac t e r i s t ic s equ i va l en t t o the une t ch ed con t ro l

s a m p l e r e s t o r e d o n l y a f t e r 3 0 n m h a d b e e n r e m o v e d . T h i s P d e p l e t i o n a l s o h a s

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PROCESSING OF EPITAXIAL HETEROSTRUCTURE DEVICES

5 n m ,

0 . 0 2

o.o5

r

~ 0 0 1 2 n m ~

0 -0 .1

0 o . 1 0 . 2 - 6 - 4 - 2 0

B i a s v o l t s ) B i a s v o l t s )

a ) b )

Fig. 2.9. a) Forward and b) reverse current-voltage characteristics of InP/Au contacts afte r

CH4 H2

RIE for several post-RIE surface wet-etch treatments.

consequences for optical propert ies of the etched material and the dam aged surface mu st

be removed before regrowth or device processing can proceed.

Assessm ent of material damage due to etching can be performed in man y ways. These

include mobil i ty measurements using the Van der Pauw technique, resis t iv i ty measure-

ments using the TLM technique, carrier concentrat ion profi l ing using CV measurement,

surface dam age and s toichiometry using tradi tional surface analysis such as Auger electron

spectroscopy and x-ray photoelectron spectroscopy, as well as electrical ly through

Schottky barrier height measurments . Approximate damage depth profi les may be

obtained through photoluminescence measurements of mult iple chirped quantum-well

heterostructures [52]. The creat ion of ion-induc ed dam age can s ignificantly impact the

performance of devices , and thus the desire to l imit these effects imposes constraints on

al lowable plasma self-bias and amo unt of overetch used. The reduction of plasma self-bias

at h igh plasma densi t ies that is possible with ECR- and ICP-RIE permits both high etch

rates due to high plasma densi ty) and comparat ively li t tle damage due to low ion

bombardm ent energy) .

2 2 3 M e t a l l iz a t io n

Formation of high-quali ty contacts , both ohmic and Schottky, is essential for the

real izat ion of compound semiconductor microelectronic devices . Contacts with ohmic

current-voltage characteris t ics are required for the real izat ion of virtual ly al l sem iconduc-

tor devices, including heterojunction bipolar t ransis tors HBTs), h igh electron mobil i ty

transis tors HEM Ts) and heterostructure field-effect transis tors HFET s), and laser diodes.

Additionally, contacts with rectifying Scho ttky) characteristics are required for the

fabricat ion of HFETs and HEMTs, as well as metal-semiconductor-metal photodetectors

and Schottky diode-based mixers . These contacts are fabricated by dep osi t ing the contact

metal material onto the semiconductor using ei ther sputtering or evaporat ion processes .

The issue as to which of these deposi t ion techniques is preferable depend s o n several

factors . From a microelectronic fabricat ion engineering perspect ive, the main difference

between sputtering- and eva porat ion-based metal l ization processes centers not around the

metal l izat ion i tself but arou nd the processing required to pat tern the m etal in to the desired

geometry. Sputter deposi t ion of metal over the ent ire wafer surface fol lowed by

photol i thography and etching is the predominant method of metal pat terning for Si

device processing. However, th is is not the case for devices on III-V epitaxial

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FAY AND ADESIDA

he te ros t ruc tu re s . A n a l t e ma t ive a pp roa c h , i n w h ic h the pho to l i t hog ra phy i s pe r fo rme d f i r s t

fo l low e d by me ta l de pos i t i on u s ing e va po ra t ion a nd the n l i ft o f f, i s more c om mo nly u se d in

e p i t a x i a l I I I -V de v ic e p roc e ss ing . C ompa re d to spu t t e r de pos i t i on , e va po ra t ion su f fe r s

f rom poor s t e p -e dge c ove ra ge be c a use the f i lm doe s no t c on fo rm to su r fa c e topo logy bu t

instead is inc ident f rom a single d i rec t ion and thus is subjec t to shadowing by ta l l surface

fea tures on the wafer . An ad di t iona l i ssue tha t a r ises in the depo si t ion o f mul t ip le -

c ons t i t ue n t me ta l f i lms suc h a s a l l oys is t he p re se rva t ion o f a l l oy s to i c h iome t ry in t he

de pos i t i on p roc e ss . S inc e spu t t e r de pos i t i on r e l ie s on bom ba rdm e n t o f a sou rc e ma te r i al

t a rge t w i th i ne r t ga se ous spe c i e s ( suc h a s A r ) , t he s to i c h iome t ry o f t he t a rge t a nd the

de pos i t e d m e ta l a re be t t e r p re se rve d . In e va po ra t ion f rom a l loys, how e ve r , t he c ons t i t ue n t

w i th t he h ighe r va po r p re ssu re e va po ra t e s f rom the me l t more r e a d i ly , r e su l t i ng in

va r i a t i ons i n a l l oy c ompos i t i on th roughou t t he e va po ra t ion p roc e ss a s t he sou rc e

c ompos i t i on c ha nge s . Th i s c a n r e su l t i n d i f f i c u l t i n a c h ie v ing re pe a t a b i l i t y i n p roc e ss ing .

One solu t ion to th is d i f f icul ty wi th ev apora t io n of a l loys is to evapora te the ent i re source

c ha rge du r ing the e va po ra t ion . A l though the c omp os i t i on o f the e va po ra t e d f i lm w i l l no t be

un i fo rm th rough ou t t he t h i c kne ss o f t he f i lm , t he a ve ra ge c omp os i t i on o f t he e n t ir e f i lm

w i l l ma tc h tha t o f t he sou rc e be c a use the e n t ir e c ha rge ha s be e n de pos i t e d . Th i s g loba l

s to i c h iom e t ry i s su f f ic i e n t fo r ma ny a pp l ic a t ions.

The fo rma t ion o f r e c t ify ing S c ho t tky c on ta c t s i s t he na tu ral r e su l t o f de pos i t i ng me ta l

on a c l e a n se m ic onduc to r su r fac e . D ue to t he d i ff e re nc e be tw e e n the me ta l w ork func t ion ,

~bm, a nd the se m ic onduc to r w ork func t ion , q5 , t he e ne rgy ba nds in the se m ic onduc to r m us t

be nd , a nd a ba r r i e r t o c ha rge t r a ns fe r be tw e e n the me ta l a nd the se mic onduc to r i s

e s t a b li she d . W i th in some v e ry s imp le a pp rox ima t ions , t he he igh t o f t he ba r r i e r to e l e c t rons

t r a ve l ing f rom the se mic onduc to r t o t he me ta l i s ~ m - - ~ s while those e lec t rons t rave l ing

f rom the me ta l i n to t he se mic onduc to r mus t ove rc ome a ba r r i e r o f ~bb = ~ m - Z The

obv ious a sym me t ry be tw e e n the se ba r r i e r s g ive s r i se t o t he r e c ti fy ing p rope r t ie s o f t he se

c on ta c t s . C on ta c t s so fo rme d a re u se d a s ga t e s i n H FETs a nd H EMTs a nd the i r r e c t i fy ing

p rope r t i e s a re u se d e xp l i c it l y i n the fo rm a t ion o f S c ho t tky d iode s fo r sw i t c he s a n d mixe r s .

The p r ima ry pa ra me te r s o f i n t e re s t i n t he fo rma t ion o f Sc ho t tky c on ta c t s i s t he Sc ho t tky

barr ie r he ight q5b, the idea l i ty fac tor n , and the reverse leakage current o f the junc t io ns.

F rom the se s imp le e xp re ss ions , one w ou ld e xpe c t t ha t t he ba r r i e r he igh t c ou ld be e a s i ly

a d jus t e d by jud ic ious se l e c t ion o f c on ta c t me ta ll i z a t ion th ro ugh se l e c tion o f a n a pp rop r i a t e

me ta l w ork func t ion . D ue to t he p re se nc e o f s ta t e s on the se mic onduc to r su r fa c e tha t se rve

to p in t he Fe rmi l e ve l a t a p re -de t e rmine d e ne rgy l e ve l , t h i s be ha v io r i s no t obse rve d fo r

ma ny im por t a n t I I I - V se mic onduc to r s . T a b le 2 .4 i l lu s t r at e s t yp ic a l Sc ho t tky ba r r i e r

he igh t s ob ta ine d fo r a num be r o f d if f e ren t me ta l s on the (100 ) su r fa c es o f G a A s [53] a nd

InA1As [54 -57] , a nd the (0001) surface o f GaN [58] . Fro m th is table , i t i s c lear tha t

Table 2. 4. Selected Schottky Barrier Heights for Comm on

Metals on (100) GaAs, (100 ) InA1As, and (0001) GaN

Metal GaAs [53] InA1As GaN [58]

AI 0.76 0.670 [56] Ohmic

Au 0.92 0.699 [55] 0.88

Cr 0.81 0.6 [56] 0.52

Ni 0.82 0.99

Pt 0.99 0.74 6 [54] 1.08

Ti 0.83 0.59 [56] 0.6

Pd 0.93 0.698 [54] 0.92

W 0.80 Slightly rectifying

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PROCESSING OF EPITAXIAL HETEROSTRUCT URE DEVICES

al though barr ier height is correla ted to metal work funct ion on GaN and to some extent

InA1As, the barr ier height is nearly invariant as a funct ion of metal dep osi ted on GaA s due

to the very large surface s ta te dens i ty on (100) GaAs.

Opt imized ohmic contacts are essent ia l to the performan ce o f microelectronic devices .

Paras i tics created by nonideal oh mic contacts can con tr ibute to degrad at ion of dc and high

frequency p erformance, as wel l as heat ing o f devices . Despi te the s imple d escript ion of the

electr ical character is t ics of ohmic contact behavior , the deta i ls underlying ohmic contact

format ion are s t rongly depende nt on the m icroscopic s t ructure o f the contact layer. For the

format ion of high-qual i ty oh mic con tacts, two bas ic s t ra tegies are widely appl ied. The f i rs t

of these strategies, the alloyed or sintered contact, relies on liquid- or solid-pha se reactions

between the contact metal l izat ion and the semiconductor surface to form a highly doped

surface semiconductor layer and/or interfacia l layer of intermetal l ic narrow bandgap

materia l. The second ma jor approach cons is ts of varia t ions on the theme of epi taxial ly

grow ing a special ohmic contact layer of highly doped, narrow ban dgap materia l especial ly

to faci l i ta te a nonal loyed ohmic contact . For both a l loyed and nonal loyed ohmic contacts ,

the required l inearity of the current-vol tage character is tic is obtained by eng ineering the

surface Schot tky contact barr ier to be as smal l as poss ible . This is brought about by both

increas ing the surface doping with i ts a t tendant reduct ion in barr ier thickness and thus

increased tunnel ing probabi l ity, and reducing the bandgap of the surface semicon duct ing

material, thus reducing ~bb itself.

Al loyed ohmic contacts have been real ized us ing a number of contact metal l izat ions .

Mo st of these cons is t of a metal (often Au) an d a dopan t such as Ge for contact to n-type

materia l and Be or Zn for contact to p-typ e materia l . Nickel is of ten a lso incorporated into

the contact structure as well, as it has been demonstrated to facilitate diffusion of Ge into

GaAs [59]. Contacts based on AuGe, AuZn, or AuBe are typical ly real ized by evaporat ion

from eutect ic o f a full evaporat ion charge onto the wafer in order to en sure that the global

s toichiometry of the metal l ization matches that of the source. Fol lowing pat tern fo rmat ion

by l i f toff , the ohmic contact i s formed by thermal anneal ing in a furnace or rapid thermal

processor (RTP) in e i ther an inert ambient (Ar or He are typical) or forming gas (15% H2,

85% N2). In this approach, the surface doping level a t ta ined by the Ge, Be, or Zn doping

should exceed 2 x 1019 cm -3 in order to develop a tunnel ing-do minated Schot tky contact

with suff ic iently l inear current-vol tage character is tics . O ne l imita t ion of the AuG e and

AuGe/Ni metal l izat ions that has been widely observed is that they form a so-cal led

spikin g contact. This can resul t in subs tant ia l nonu niform ity both in depth of doping

and interfacial layer penetra t ion as wel l as the spat ia l compo si t ion of the contact , leading to

nonuniform current dens i ty across the contact area . In addi t ion to the scal ing l imits that

this imposes o n contact areas due to inh omog enei ty, these contacts are not thermal ly s table

and suffer from degradat ion with subsequent thermal process ing or high-temperature

device operation [60].

One approach to avoid the diff icul t ies associated with these nonhomogenei t ies for

ohmic contacts to GaAs includes replacing the AuGe-based contact metal l izat ion with a

s tack of metals cons is t ing of Pd, Ge, and Au. As with the AuG e-based contacts , af ter

evaporat ion of the contact material the contact is s intered a t 35 0-4 50 ~ to effect the Ge

diffus ion. This metal l izat ion has proven to be more thermal ly s table and to produce a

smooth surface morphology, and has t rans la ted into improved device re l iabi l i ty for high-

temperature operat ion [61 ] . An al ternat ive a pproach cons is ts of us ing near-no ble t rans i t ion

metal contacts (e .g. , Ti , P t ) that are normal ly associated with Schot tky contact format ion,

but ensuring that the surface doping concentra t ion o f the sem iconducto r is suff ic ient ly high

that a tunneling contact is established. In addition, in some processes the contact is further

ass is ted by R TP in order to form a thin interfacia l layer (< 40 nm ) o f narrow b andgap

intermetallic pha ses [62]. Specific co ntact resistanc es in the r ange of 10 -6 to

3 x 10 -8 f~ cm 2 are obtainable for h ighly doped materia l us ing this m ethod. High-

temperature device operation requires stil l more inert, stable contacts; for this purpose,

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FAY AND ADESIDA

r e f r a c t o r y c o n t a c t s b a s e d o n W S i , G e W S i , W N , W S i N , W T i N , W P t A g f o r G a A s a n d I n P

ha ve be e n inve s t iga t e d . The se c on ta c t s ha ve be e n show n to be ine r t up to t e mpe ra tu re s o f

700 ~ w i th spe c if i c c on ta c t r e s i s t a nc e s o f 10 -6 -1 0 -7 ~ c m 2 on h igh ly dope d ma te r i a l .

The se c ond a pp roa c h to ohmic c on ta c t fo rma t ion tha t doe s no t r e ly on a l l oy ing o r

s in t e r ing to a c h ie ve oh mic c on ta c t i s the e xp l i c i t inc lu s ion o f na r row ba ndga p , h igh ly

dop ed su rface layers in the epi taxia l he te rost ruc tur e for the expl ic i t purp ose of fac i li ta t ing

ohm ic co ntac t form at ion. On InP, use o f n + la t t ice-m atched In0.53Ga0.47As has bee n show n

to r e su l t i n h igh -qua l i t y c on ta c ts , w i th t yp i c a l c on ta c t r e s i s t anc e s o f 0 .07 -0 .1 f~ m m in

typ ic a l H EM T he te ros t ruc tu re s [63] . The u se o f a dva nc e d he t e roe p i t a xy o f re l a t e d l aye r s o f

l a t t i c e -misma tc he d InG a A s a nd InA s on G a A s [64 -66 ] a l l ow s the u se o f r e f ra c to ry M o-

ba se d c on ta c t s t ha t ha ve improve d the rma l s t a b i l i t y w h i l e s t i l l ma in t a in ing h igh qua l i t y

c on ta c t . A n a dd i t i ona l a pp roa c h to ohmic fo rma t ion tha t doe s no t r e qu i re a nne a l ing o r

s in t e r ing i s t he u se o f s imp le Sc ho t tky c on ta c t s on de ge ne ra t e ly dope d se mic onduc to r

ma te r i a l . The e x t re me ly th in ba r r i e r l a ye r t ha t r e su l t s f rom the spa t i a l l y r a p id be nd ing o f

the e ne rgy ba nds p roduc e s c on ta c t s i n w h ic h the c u r re n t i s p r ima r i ly c onduc te d th rough

tunne l ing (o r f i e ld -e miss ion ) t h roug h the ba r r i e r , r a the r t ha n the rmion ic e miss ion ove r t he

ba r r i e r , t hus g re a t ly r e duc ing re c t i f i c a t ion . Th i s a pp roa c h i s w ide ly u se d to c on ta c t t he

he a v i ly dope d ba se , e mi t t e r , a nd subc o l l e c to r l a ye r s i n H B T fa b r i c a t ion [ e .g . , 67 ] .

Fo rm a t ion o f ohmic c on ta c t s on G a N a nd re l a te d c om pounds i s s ti l l a sub je c t o f i n te nse

s tudy . O hm ic c on ta c t s fo r bo th n - type a nd p - type G a N a re o f t he a l l oye d type . The ba s i c

me ta l l i z a t ion sc he me fo r n - type ohmic c on ta c t s i n t he se ma te r i a l s i nvo lve s T i a nd A 1 .

S inc e the a nne a l ing te mpe ra tu re c a n ra nge f rom 600-9 50 ~ ox ida t ion o f A 1 c a n

c o n s ti tu t e a p ro b l e m . T h e r e f o r e o h m i c m e t a ll i za t io n s o f T i / A 1 / N i / A u a n d T i / A 1 / T i / A u

ha ve be e n a dop te d to a l l ev i a t e th i s p rob le m [68] . The be s t n - type ohm ic c on ta c t r e s i s t iv i t y

repo rted to da te i s 8 .9 x 10 -8 f~ cm 2. I t has prov en m ore d i ff icul t to achieve p- ty pe ohm ic

due to t he w e l l -know n p - typ e dop ing p rob le m s in G a N . Typ ic a l ho l e c onc e n t ra t i ons o f

a pp rox im a te ly 5 x 1017 c m -3 a nd mob i l i t i e s o f < 10 c m e /V s a re s ti ll too low to a c h ie ve

e xc e l l e n t c on ta c t s . N o tw i th s t a nd ing th i s , ma ny c on ta c t me ta l l i z a t ions ha ve be e n

i n v e st i ga t e d o n p - t y p e G a N . T h e s e in c l u d e N i / A u , P d / A u , P t / N i / A u , N i / P t / A u , a n d

Ta /T i . The be s t c on ta c t r e s i s t iv i t y ob ta ine d so f a r i s 3 x 10 -5 f~ c m 2 fo r t he Ta /T i

me ta l l i z a t ion [69 ] . The de t a i l e d mic ros t ruc tu ra l me c ha n i sms fo r ohmic c on ta c t s i n t he se

mater ia ls a re ye t to be comple te ly c la r i f ied .

2 2 4 Io n Imp la nt a t io n

A l thoug h ion imp la n ta t ion i s t he domina n t me thod o f dop ing in a dva nc e d S i de v ic e

p roc e ss ing a nd ha s be e n u se d e x t e ns ive ly i n G a A s MESFET fa b r i c a t ion , dop ing v i a i on

implanta t ion in epi taxia l he te rost ruc ture devices i s ra re ly used. For epi taxia l he te ro-

s t ruc tu re s , i t i s muc h more c ommon to pe r fo rm the r e qu i re d dop ing du r ing the

he te ros t ruc tu re g row th , a nd thus e l imina t e a p roc e ss ing s t e p . A p rob le m w i th i on

imp la n ta t ion fo r dop ing in I I I -V se mic onduc to r s i s t ha t e l e c t r i c a l a c t iva t ion i s no t

a u toma t i c e ve n w he n imp la n te d spe c i e s a re p romote d in to a pp rop r i a t e subs t i t u t i ona l s i t e s

by a nne a l ing [70 ] . The so l id - s t a t e r e g row th p roc e ss e nge nde re d by a nne a l ing a f t e r

imp la n ta t ion i s more c ompl i c a t e d due to t he b ina r y / t e m a ry na tu re o f I I I -V s ; c ry s t a l

de fe c t s suc h a s tw inn ing a nd s t a c k ing fa u l t s oc c u r fo r h igh imp la n ta t ion dose s more

re a d i ly i n I I I -V s t ha n in s i l ic on . The lo ss o r de p le t ion o f g roup V e l e me n t s a t mode ra t e

t e mpe ra tu re s a l so me a ns tha t I I I -V su r fa c e s mus t be p ro t e c t e d du r ing a nne a l ing . In

a dd i t ion , i on imp la n ta t ion o f c e r t a in spe c i e s m a y p rom ote in s t a b i l i ty a t he t e ro s t ruc tu re

in t e r fa c e s a t h igh t e m pe ra tu re s . T h i s i n s t a b i l it y i s t he ba s i s fo r t he impur i ty - induc e d l a ye r

d i so rde r ing phe nome non tha t i s u se d in t he f a b r i c a t ion o f some op t i c a l de v ic e s .

The m ore c o mm on usa ge fo r i on imp la n ta t ion in e p it a x i a l he t e ro s t ruc tu re de v ic e s i s fo r

i so l at i on . A dva n ta g e s o f a n ion imp la n ta t ion -ba se d i so l a ti on sc he me c o mpa re d to me sa

i so l a t ion i s improve d p l a na r i ty , l a c k o f e xpose d su r fa c e s a nd thus a po te n ti a l de c re a se i n

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PR O C ESSIN G O F EPITA X IA L H ETER O STR U C TU R E D EV IC ES

surface s ta tes , and in some cases improved iso la t ion and s idega t ing immunity . There a re ,

how e ve r , s ome l imi t a t ions a nd po te n t i a l c onc e rns w i th r e s pe c t t o i on - imp la n ta t ion -ba s e d

isola t ion . On e such e ffec t is the thermal s tab i l i ty of the iso la t ion . Since im planta t io n-ba sed

i s o la tion s c he m e s a re o f t e n ba s e d a t l e a st i n pa r t on da ma ge - in duc e d c a r r i e r de p le t ion , a ny

e le va te d t e mpe ra tu re s tha t t e nd to a nne a l ou t t he imp la n t da ma ge c a n de g ra de the qua l i t y

o f t he i s o l at ion . A n o the r l im i t a t ion i s one o f ma te r i a l l im i ts ; imp la n t -ba s e d i s o l a tion ha s

be e n de mo ns t ra t e d to be e f fe c t ive in G a A s , A 1G a A s , InP , a nd InA 1A s p rov ide d tha t t he

c ond i t ions a re p rope r ly op t imiz e d , bu t due to the i r s ma l l ba ndga ps i t i s ge ne ra l ly no t

s uc c e s s fu l fo r InA s a nd i s poo r fo r InG a A s l a t t i c e -ma tc he d to InP .

Imp la n t -ba s e d i s o l a t ion s c he me s fo r I I I -V s e mic onduc to r s f a l l i n to tw o b roa d

c a te go r i e s , da ma ge -ba s e d a nd c ompe ns a t ion -ba s e d . In a da ma ge -ba s e d i s o l a t ion p roc e s s ,

the imp la n ta t ion i s i n t e nde d to ge ne ra t e de e p l e ve l s a nd t r a pp ing s t a t e s due to induc e d

la t ti c e da ma ge . Typ ic a l imp la n te d s pe c ies fo r t h i s t ype o f im p la n t a re p ro tons , de u te r ium,

oxyge n , a nd bo ron . Fo r G a A s -ba s e d i s o l a tion imp la n t s , t he dos e s a re typ ic a l ly in the r a nge

o f 1014-1015 c m -2 fo r p ro ton im p la n ta t ion , w i th oxyge n a nd bo ron re q u i t ing on the o rde r

o f t en t ime s s m a l l e r dos e to a c h ie ve the s a me l e ve l o f da ma g e due to the h ighe r i on ma s s e s

o f t he s e s pe c ies . In ge ne ra l , t h i s t ype o f da m a ge -ba s e d i s o l a tion s t r a te gy i s t he rma l ly s t a b le

on ly to the t e mpe ra tu re a t w h ic h the imp la n t d a ma ge be g ins to a nne a l ou t . Th i s i s t yp ic a l ly

45 0 - 47 0 ~ fo r B imp la n t s [71 , 72 ] t o 600 ~ fo r mu l t ip l e imp la n t s i n to G a A s [73 -75 ]

and In0.asA10.52As [76] , and as low as 2 00 ~ for In0 .53Ga0.a7As [77]. An addi t iona l

c ons ide ra t ion w i th d a ma g e -ba s e d i s o l at ion imp la n t s i s t he r e d i st r ibu t ion o f t he imp la n t

s pe c ie s w i th s ubs e que n t t he rma l p roc e s s ing o r h igh t e mpe ra tu re de v ic e ope ra t ion . T ime -

de pe nde n t ga in in imp la n t - i s o l a t e d H B T s , fo r e xa mple , ha s be e n a t t r i bu te d to t ime -va ry ing

pa s s iva t ion o f c a rbon a c c e p to r s i n the ba s e due to the mob i l e hyd rog e n ions . Imp la n t

iso la t ion with He has been shown to subs tant ia l ly a l levia te th is d i ff icul ty [78] .

The inc re a s e in ma te r i a l r e s i s t i v i ty fo r da ma ge -ba s e d i s o l a t ion i s no t a mono ton ic a l ly

increas ing func t ion o f dose , and so an opt imal dose fo r i so la t ion exis ts . Iso la t ion shee t

res is tances of 10 ~~ f~/[--] have b een dem ons tra ted for opt im ized B implan ts in to

G a A s / InG a A s pH EMT he te ros t ruc tu re s [72 ] . A t dos e s a bove the op t imum, c onduc t ion

c a n be e nha nc e d fo r s e ve ra l r e a s ons . The de fe c t de ns i ty c a n be c ome ve ry l a rge a t h igh

doses , leading to conduct ion due to carr ie rs hopping from one defec t s ta te to the next . In

a dd i t ion , i n s ome ma te r i a l s t he de fe c t e ne rgy l e ve l s a re no t de e p in the ba ndga p bu t l i e

re la t ive ly c lose to the ban d edges , a nd so a t e leva ted doses a subs tant ia l nu mb er of these

defec t leve ls can become e lec t r ica l ly ac t ive and ac tua l ly increase the carr ie r concentra t ion

a bove the p re - imp la n te d va lue . Th i s i s pa r t i c u la r ly p rob le ma