I

/

APPLICATION OF GEL PERMEATION CHRIIMATO(_APHY WITH

PHOTODIODE ARRAY DETECTION FOR MONITORIN_ OF

M ICROL ITHO[_RAPH IC PROCESSES

Ivan 8itsov

Research and Development Institute for Special Chemicals

1741 Sofia - Vladaya) Bulgaria

SUMMARY

Positive photoresiBts are widely useO in the

fabrication of integrated circuits. Recently they are

investigated as potential materials in the electron beam

microlithography. Depending on the chemical structure and

the molecular weights o constJtuent_ the photoresists

possess different sensitivity and contrast. The extent of

the chemical alterations occuring _uring the lithographic

process influences the technological behavior of these

materials as well.

In the present study an attempt is made to evaluate the

interconnection between the chemical composition of the

common photoresist and its technologic:al p_r+ormance. Gel

permeation chromatography with photodiode array detection is

used as an appropriate method �˜of th_ chemical

changes proceeding in such complex mixtures during the

lithographic exposure.

INTRODUCTION

The significant a0vance of th_ microelectl-on_cs _n the

last decade resulted in the recent production of semi-

conductor devices with Ultra L_rge Scale o_ Integration

(ULSI). Integrated circuits (ICs) in the 3-4 MB range

require technologies and appropriate mater_al_ _nabling the

0_/-. /_//' )qO_ -300-

",°

' 2

formation of topological structur_ witll _eature _izes

between 1,f_ afld (_,8 microni_.tet._,

Although introduced in the ,=arly si}'ties_ optical

lithograpiw still plays a key role in the p_'c_0_ction of Ills,

It is assumed that a decrease _n the min_n,um size

down to l_,5 micrometer u_ing single 1-_yer optical

microlithography would be ._ttained by development o �new

type_ o resolution/high sensitivity r_.,s_t syst_m_ arid

improving th_ capabilities o exposure equipment.

Allmost all commercially ava_l_lil_ outic_ll r_._i_ts are

variations o well known composition "photoacti ve

compound(PAC)/polym_:r mat.vlx(PM)" wl_ch h;_ be_._i, introduced

many decades ago, but .st:i1l meet__ th_ requirements ocontemporary i_thograp_ic processe,:s. Tl=c i¢e:yUOml_on_nts o4

the positive photoresist mi_,tures are th_ derivative o �1,2-

naphthoquinone diazide:-5-,_ui ionic acid (PAC) (I) alld i:r'_oi-

iormaldehyde resin o �novolactype (PM) (ll):

0 OH

N2 _CH2_ nS03R CH3

I []m

Th_ m_chanisw_ o_ action o _y:_tc:m i_ b_s_cl on the

photoi nduced Idol&_ 's rearangement oF d _a_',_naphthal enone

derivatives (scheme I):

0 0

_N2 hY _"-N2R R

do COOH

05R R

- 301 -

This reactzon causes an zncrease in th_ dxssolut,on rate of

the exposed areas of the phc_toresist film i_, aqueous alkali

developers - up to 1_W-20_ nm/sek. The unexposed parts of

the film dissolve at signi ylower i-ares - 0,1-_,2

nm/sek. The difference in the dissolution rates of exposed

and unexposed parts of photosensitive co,tings determines

the final imaging quality of resists.

Methods based on colligative propertie_ of positive

photoresists (optical absorption) are widely applied for

prediction and conlparison of their lithographic performance

[1]. However', investigations on the relationship between

molecular weight characte_ristics, chemicaJ structure,

composition of the photoresist constituents and final

technological performanc_ o-f the resist mi_:ture are rather

scarce [2]. On the other" hand, it was shown that the gel

permeation chromatography a,ith coupled ph_todiode array

detection would be a useful technique for simultaneous

separation and identification of component_ in the complex

photoresist mixtures [3].

]'he aim of the present study is to evaluate the

influence of the characteristic features of the photoresist

components on the lithographic properties of a common

commercially available positive photoresist using gel

permeation chromatography with photodiode i_;-ray detection

(8PC/PDA).This method is used for determination of the

oligomer composition and overall _ikol_cular weight

characteristics of PM, chemical structure and quantity of

PAC as well as _or monitoring o_ th_ cl_eff,_ca] alterations

occuring in the photoresist layer during tl_e stages of the

microlithographic process.

EXPERIMENTAL PART

Materials

Th_ novolac resins wer_ obtain_c| by kr_own methods from

m-cresol, formaldehyde (Fluka AG) and oxalic acid (Factory

-3o2-

°.

for pure reagents-Vladaya) as catalyst [4].

The photoactive compounds were prepared from 2,3,4-

trihydroxy benzophenone and 1,2-naphthoquinone diazide-5-

sufonyl chloride (Toyo Gosei Ltd) by standard synthetic

methods.

The positive photoresist ° was commercially available

product o_ Hoechst AG.

Methods

The photoresist was spin-coated on silicon wafers,

prebaked and exposed at equal conditions in clean room

enviroment. Samples were taken at each technological step by

stripping the resist layer with tetrahydrofuran (THF). All

procedures were performed at yellow light conditions and the

samples were stored in dark vials under dry nitrogen prior

to use.

The 8PC/PDA anal ysi s was performed using GPC III

apparatus equipped with an automatic injector- WISP 712, set

of four Ultrastyragel columns (1008, 500, 100, 100 _) and

990+ PDA detector (Millipore9 Waters Chromatography

Division). The separations were carrie_ out wlth THF

¢Burdick & 3ackson) as an eluent at flo_; rate _f 018 mL/min

and column temperature 45=C.

All samples were analy_ed as 015 % (W) THF solutions,

the injection volume being 012 mL.

The molecular weight characteristics of the polymers

were calculated by Waters Expert TM GPC Software (Vet.6) on

Waters M840 Data and Chromatography Control Station using

poly(styrene) standards (Polymer Laboratories Ltd.).

RESULTS AND DISCUSSION

I. Calibration procedures and system per4ormance

The plate count o column s_t is b0000. It was

determined by known procedures using o-dichlorobenzene as a

standard.

Poly(styrene) standards having molecular weights

between 30000 and 162 (monomer-initzator adduct) are used

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for calibration. The dependence "molecular weight/retention

time" is shown on Figure 1 (curve 1). The calculated

correlation coefficient assuming a polynomial regression o_

third order is _9996 with standard error of estimation

0,8178.

1

2 2

RETENTIONTIME_ (MIN)

Figure 1

The specSfic resolution o_ the column set used in this

study is determined by Bly°s equation E5]:

2 (VM.--V_b) 1

W. + W= lg(M./M=)

where V_. and V_b are retention volumes, N. and Wb are

baseline widths, and M. and Mb are peak molecular weights

(M_) of poly(styrene) standards a and b respectively. The

calculated values of R= are listed in Table I.

Table I: Specific resolution of Ultrastyragel columns

set (1080, 5_8, 108, 100 _) for poly(styrene) stanUards.

Mobile phase: THF; flow rat_: 0,8 mL/min! temperature: 45=C.

Peak Mol. 370 474 578 682 1250 2150 2950 9200

Neight

474 14,86

578 14112 11,07

683 13,64 10,71 10,30

1250 4t89 4,29 4,12 3,98

2150 2,76

2950 2,55 2_07

9200 2,10 2,01 2,21

22000 1,73 1,69 1,84 1,84

The Rm values for some pairs of standards are not calculated

because of the big differences in the molecular weights. As

expected the GPC system has the highest: resolution

capability in the range beween 100 and 3000. It is known

that the novolac resins used in the photoresist compositions

have similar molecular weights.

It has been stated that the retention time of some

novolac-like benzyl alcohol derivatives of low molecular

weight fit on the same calibration curve as styrene

oligomers and might be used in the GPL3 studies ophotoresists [3]. Since the most used materials in the

photoresist compositions are m-cresollformaldehyde poly-

condensation products, the dependence "molecular

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/'

70

weight/retention time" is investigated for oligomers of this

type (Figure 1_ curve 21. The �(�xof the

homologues series were isolated by preparative open-column

chromatography, analysed by infra-red spectroscopy and "H-

nuclear magnetic resonance for chemical identification, and

their molecular weights were determined by osmometry.

The correlation coefficient of the calibration curve is

0)9778 with standard error of estimation 0,6677. The

accuracy of calibration is lower than that for poly(styrene)

standards, the possible reasons will be discussed in the

next chapter.i

It is obvious that the poly(styrene)- and n_cre-

sol/formaldehyde standards are characterized by different

calibration curves. Thus, for the correct estimation of

molecular weights in photoresists, especially in the low

oligomer's region novolac standards should be used.

iII. Separation of novolac oligomers and PAC mixtures

It was mentioned earlier that the most common PMs used j

in positive photoresists are the cresol/aldehyde resins of

novolac type. Novolacs with different yields and molecular

weight characteristics could be prepared depending on the

reaction conditions [6]. The molecular weight increases with

the increase in the ratio formaldehyde/m-cresol from _88

trough B,? up to m_93 (Figure 2, 2-4). The polycondensation

mixtures contain homologues series of low molecular weight

products: trimer_ tetramer and higher oligomers as revealed

by spectroscopic and element analysis. It is seen that the

first peaks in the chromatograms are mixed indicating

differences in the hydrodynamic volumes of their

constituents. It could be assumed that products withi

different chain structures are co-existing in the novolac

mixture. Some of the structures for the trimer are shown on

Scheme 2.

- 306- k_

Ai,_- 61=JO °"/o' _.'I I

,w_of' ':',.v",,k/t,1 ',v \ 2

__ _t 3

_ / -ts'_7/_,.4<;'- ,"lo_cul._I_ir.'.a-r

Fi .qure 2

- 307 -

Scheme 2

OH OH OH

[_CH2, _ CH2._ o-o' trimer (]II)CH3 CH3 CH3

OH OH OH

_" %

O_CH2 CI_3CH 2_3 o-p/o-o trimer(V)CH3 OHHO

OH OH OH

CH2_ C1-12._ br0nchedtetramer {VI)

v CH3 -_H2CH3..OH3

CH3OH

It _s seen that along with the h_ghly ordered o-o"

structure (I|I) sequences 1ike LV and V migllt exist. They it

are highly stericaly hindered structur_._ _iI_cl cons_quently

have different sizes in solution. ]]ranching of the polymer

chain i_ also p_ssible starting with th_ tetraf.P_r (V_l). Al]

- 308- _j_

these factors contribute to the appearance of shoulders in

the first peaks and to a slight shift in th_ r_tm_ntion times

of the higher oligomers as well. B_cause of insufficient

resolution ir_ this region that might be the probable reason

for the lower correlation coefficient observed with the

calibration curve constructe_ by these products (cl_apter I),

The separation of phenol/formaldehyde novolac resin on

the same GPC system shows rather different picture (Figure

2_ 1). The mixture consists of oligomers with different

hydrodynamic volumes.

The molecular weight characteristics of the novolac

resins used in the photoresists strongly affect their

physical properties such as glass transition temperature

(Tm) and melting interval (T_) and there P�x�ti_etendency to

flow during the prebake step of the microlithographic

process. It has been shown that the _ina] lithographic

performance of the photoresist depends on the molecular

weight and molecular weight distribution of its PM £8-1B].

The influence of the oligomeric composition on T= has not

been investigated.

The content o_ some well resolved oligomers in the

novolac resins could be determined using the responce of the

differential re_ractometer R41_ of th_ GPC system. It is

known that for low molecular weight compounds the

sensitivity depends on differences of the re $�è (�€�indices

between eluate and eluent. So corrections T�|�the

differences in the re d�D#,�€o_ oligomers with

polymerization degree between 3 and 6 were carried out

according to [7]. The oligomeric compositions of novolacs

2-4 (Figure 2) and their glass transition temperatures are

listed in Table II.

- 309 -h...

Table II: Glass transition temperature_ (T=) of m-cre-

sol/formaldehyde novolacs depending on their composition and

molecular weight characteristics.

Novolac Oligomer content, % M. M./Mn Tm

Pm P4 Pm P_ Total =C

2 16,26 16,13 9,37 9v01 37_7 18_ 2t6 69v7

3 5t17 b,38 7_35 11,5B 3_,4 38_ 2,5 92,3

4 2_47 3,50 4,89 4_85 15,7 53BB 3,1 1_,8

The data in Table . II indicate that the oIigomeric

composition of the novolac resins also influences their

glass transition temperatures and therefore the prebake

behavior of the prepared photoresist.

The derivatives of hydroxybenzophenones an_ 1,2-diazo-

naphthalenone sul#onates used as PACs in photoresists are

prepared according to Scheme 3:

Scheme 3

0, OH 0

_C_oOH + _N 2 base

SO2Ct

00NQD 00NQDII II

VII .VIII

00NQDII

[_ C ._ONQDONQD

IX

.310 -

°_

D_pending on the ratio of the reagents, hydroxybenzophenones

with di_ Ü�8�˜�degreeso �substitutionare formed (VII-IX).

Structures VII and VIII represent an;y one of the three

possible isomers. Gel permeation chromatogram of the

reaction mixture is shown on Figure 3.

Figure 3

The three components having absorptions at 254_ 365

and 405 nm (peaks 2-4) are probably isomers of the

structures VII-IX with VIII and I_X predominating. By

-311 -

13

chemical element analyses it was determined that the overall

degree of substitution of the obtained product is 2,67.

Since the molar absorptivities of V_I_ and I_XX are quite

similar (7580 and 7388 respectively [11]) the presence of

structure IX in predominating quantities is confirmed by the

stronger refractometer- and UV-responce for peak 4.

Although the dlfferencqs in the molecular weights of

the isomers formed are rather big, the resolution attained

is not satisfactory. As the plate count value of the column

set is high enough there might be other reasons for the

observed phenomenon. Trefoans IIl et al. showed recently

that approximating the shape by an ellipsoid some of the

isomers of VII and VIII have similar effective sizes inspire

of the differences in their molecular weights (Table III].

Table III: Effective sizes of 1,2-diazonaphthalenone

adducts of 2,_,4-trihydroxybenzophenone (according to [12]).

Structure Molecular weight Semimajor axis,

a b c

7,3 b,5 5,9

VII derivatives 4&2 9,2 5,7 8,7

18,1 5,2 412

7_3 717 ?11

VIII derivatives &92 1_11 7,_ &v7

18,1 5,8 6,8

IX structure 927 18,1 9,b 7_4

Peaks I and 5 are probably due to unreacted

hydr-ox yhenz ophenone (1) and products o �azocoupl ing

reactions (5).

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. 14

III. Separation and GPCIPDA analysis of positive photo-

resist compositlons at different stages o4 the microlitho-

graphic process

The stages of the common microlithographdc process are:

I.) Spin-coating of the photoresist on the _ubstrate;

II.) Thermal treatment (prebake}; III.) Exposure; IV.) Deve-

lopment; V.) Thermal treatment (post bake); VI.) Stripping.

The major spectral changes in the photoresist layer occure

during exposure. They are presented on Figure 4. The UV

spectrum o_ the unexposed photoresist (Figure 4, 1) has

three Nell distinguished absorption maxima at 254_ 365 and

405 nm. It is seen that by increasing the exposure energy

+tom 20 mJ/cm = (2) up to 128 m_/cm = (6) the absorption in

the interval 328-48B nm is almost eliminated. Further

irradiations at higher exposure energies (38B m3/cm =) cause

small changes in the UV spectrum o_ the _-esist film.

,%

,oo,t

i_ _. . _;.;,

f.

.:, . , ,I+ ° +

."':',+. • ' . •.+++'+!+;•+,• 50 ': .3 " + ' : " ' 'I' :+'

. :'" .. ;,

4

300 400 $00

WAVELENGTH. NM

Figure 4

-313 -

t

15

The photoresi st composition during stages I-III is

checked by 8PC with multiple detection (Figure 5). The

alterations in the res15t _ilm during these stages

predetermine to a large extent the quality o4 the final

pattern _ormed. fhe UV detector responces at 254 nm: (---)-

trace; 365 nm: (- -) trace; 405 nms I-.-) trace are compared

II,

Ill,

Figure 5

-314 -

-j

la

to the responce of the differential re er:(-..-)-

trace. The gel permeation chromatograms snow that the PM of

the resist consists of three components - high molecular

weight fraction with Mp=8888, predominating medlum fraction

with Mp=3508 and low molecular w_ight fr,_ction containing

mainly oligomers with polymerization aP.grees between 3 and 7

(12,5 % of PM). From the d_,t._li,ated in Table ll it could be

: predicted that the glass transition temperature of this

_. novolac resin will be above 185mC enabling thermal treatment

; between 98 and 12_C.

I The three main parts o_ the photore_i _t are welld ist ingui shed at the contour p Iot of th_ _epar ated

photoresist mixture made by GPC/PI)A (Figure b, 1-3}. The

retention time of the PAC (2) coincides with the retention

time of peak 3 on Figure 3 (disubstituted 2,3,4-trihydroxy

benzophenone)

N2_M _ t . i . . i . . , , , _ i, i ,, , f , ,:t t,,r.. - 05,"• .'_' •_' ]°',". "" •" , %_ .I/

i . " o "

I

50( _ _ 2 3, I,

_,,_-?2_:----__-:TC':'-_.---_:__,__._.'._.__,_.........__,_ .-..........

; i 11/2;(i,,'(t _.',\"Y// •• _ "_ i

50[ 1. 2 3 II,

,-_.;_',7-_,,,,,_X._{'_-'": "" _.....'_ ' ' _ '•- _..,_-. :_. :. ,.;.;.,_=.._ __i._--., ''_Y'_:....... q,__ __-.---,__-.-.-u,_ .L___..'._,_,..,i:.._s,..............

__ "'_:-;:':. .. J'.,. ,.

............ :_...... f ..... : ........ :. • ._

,,'.._.;:t),...._..._:}0:.... -.":'..'_":.; ]. ......... 2 ....... 3 TIT.

RErENTION"TIME.,(I'IIN)'_

; Figure b

- 315 -

- b

17

It is seen that the bleaching of the phatoresist ´�˜�during

exposure observed on Figure 4 is caused by the photochemical

rearrangement (destruction) of PAC (Figure 6, Ill.).

The UV spectra of components 1-3 are shown on Figure 7.

They are used for quantitative determination of the

phot oresi st composi ti on and the changes therein during

stages I-III of the lithographic process. It is evident that

components 1 and 3 have one aria the same chemical

composition and belong most probably to the rlovQlac homolo--

I

AU_$.,.,., ..... ,.,.,.,., ....... , , ,.,., , ,.,.,., ..........

i I,

...._......._; ";...................2.....

i ' : 2#) ,.i.l v ½ . , , , • ,., , , , , , • i : • i ; , I ' ',I "

..........1 _\J/;..''? ..... I, •

2N ....... j_." ...... 1400...... 500WAVELENGTH,(_)

Fi gure 7

-316-

18

gues. Their total content in the r_sist mixture is 71% as

determined by the calibration curve "detector

responce/novolac quantity". Figures 5-7 show that the

prebake step causes only slight changes in the photoresist

composition (within 1-2%). The ratio of initial to final

optical density at 485 nm before and a4ter exposure is 1:_2

indicating that the amount of PAC decreases from 21% down

to 4 % upon exposure. It could be expected that using this

resist at appropriate irradiation energies will result in

relatively short exposure times without under-exposure and

under--development effects.

CONCLUSIONS

The results obtained show that the GPC/PDA analysis

could be applied successfully for quantitative determination

of the changes in the composition of positive photoresists

at different stages of technological treatment. The data

might be used for prediction of the lithographic performance

of the photoresist materials.

ACKNOWLEDGEMENTS

The author thanks Drs. H.U. Kuffner_ 3.F. Rutherford_

W.A. Dark and P. Nilliamson of Waters Chromatography

Division, Millipore Corporation for providing him the

possibility to perform the analyses in their laboratories.

Special thanks are due to Mr. Ts. Tsvetkov o_ Institute

for Microelectronics, Sofia for the lithographic treatment

a_ the photoresist and Dr. S.M. Miloshev of Higher Institute

_or Chemical Technology, Sofia for the assistance in

manuscript preparation.

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19

REFERENCES

I. F.H. Dilll N.P. Hornberger, PoS. Hauge and J.M.

Shaw, IEEE Trans. ED 22_ 445 (1975)

2. L.E. Stillwagon, Solid State Technol. 19851 113

3. N.A. Dark, J.Anal.PuriT. 2, 62 (1987_

4. D. Braun, H. Cherdron and N. Kernl "Praktikum der

Makromolekularen Organischen Chemie", Alfred-Hui_tig Verl.t

Heidelbergl 1971t p. 252

5. D._. _ly, 3.Polym. Sci. C, 21_ 13 (1968)

b.P.C. Novakov, V.M. Dimitrov_ S.M. Miloshev and

I. Gitsov_ Chemtronics, accepted for publication

7. W. Heltz, "Liquid Chromatography of Polymers and

Related Materials III" I J.Cazes Ed., Marcel Dekker, 1no.,

New York, 1981t p. 137

8. M. Hanabata, A. Furuta and Y. Uemura_ SPIE Proc.

&31_ 76 (1986)

9. M. Hanabata, A. Furuta and Y. Uemura, ibid. 771, 85

(1987)

lB. M. Hanabata, Y. Uetani and A. Furuta, ibid. 928,

349 (1988)

11. K. Miurat T. Ochiai, Y. Kamayama, C. Kashit S.

Uoya: M. Nakajima, A. Kawai, S. Kishimura_ ibzd. 92_, 134

(1988)

12. P. Tre_onas III_ _.K. Daniels_ M.J. Eller and A.

Zempini, ibid. 928, 203 (1988)

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