SIMULATION OF DRY ETCHED LINE - EDGE PROFILES

bY

John L. Reynolds

Memorandum No. UCB/ERL M81/2

12 January 1981

ELECTRONICS RESEARCH LABORATORY

Col lege o f Engineer ing U n i v e r s i t y o f C a l i f o r n i a , Berkeley

94720

Research sponsored by t h e Nat ional Science Foundation under Gra ECS77-14600-A01, Hughes A i r c r a f t , INTEL, S ignet ics , Hewlett-Pac and I B M Corporat ion.

ABSTRACT

Computer simulation of plasma etched two-dimensional profi

explored. The model divides the process into three rate compor

isotropic or chemically reactive etching, anisotropic or direcl

etching, and orientationally dependent ion beam etching. The t

extends the user-oriented computer program for Simulation And F

of Profiles in Lithography and Etching (SAMPLE) which simulate:

time evolution of line-edge profiles by advancing nodes on a :

Applications here include residue left at a step using anisotrc

and undercut produced using arsenic implanted poly-silicon layc

The reported research completed the author's Master of Sc' requirements for the University of California at Berkeley, Dect Although it is not an official copy, this has been reproduced t UNIX work processor TROFF.

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TABLE OF CONTENTS

I. Introduction. 11. Theory.

A. Distinctions. B. Mechanisms.

1. Isotropic. 2. Directional. 3. Shadowing.

C. Ion milling.

A. B. Isotropic arid diretional components. C. Ion milling analytics. D. Subroutine summary. Comparison of simulation with experiment. A. B. Two distinct step process. C. Combination process.

A. B. Arsenic implanted enhanced etching. References. Appendix A: Keywords and default values. Appendix B: Common block description. Appendix C: Fortran code.

111. Implementation. String point model and etchrate information.

IV. Multilayer process with analytic solution.

V. Applications. Anisotropic etching over a nonuniform layer.

1

I. Introduction.

As device sizes shrink etched profiles and shapes become increasingly important tion. Dry etching techniques are particularly popular in producing small di achieve VLSI technologies. They range from chemical, isotropic processes, using re1 reactors, to directional, physical processes, in diode or sputter etching systems. affords anisotropy in etching for optimal profile control and pattern transfer. By a within the range, computer simulation of the time evolution of line edge profi cially useful tool in exploring and understanding various degrees of anisotro processes.

Numerical algorithms suitable for simulating ion milled and sputtered profiles Carter et. ai. [13,14,151 and Ducommun et al. [24,251. Lehmann [401 and Bay1 modelling of other mechanisms in ion milling. Mathematical expressions d linear profile features in anisotropic etching are given by Jinno and Inomata I3 [12l. The case of reactive ion etching is considered in a cell dissol Viswanathan (731.

This report generalizes a string development model [33,511 to the case of ani ing. The algorithm is intended for use in the user oriented program for Modelling of Profiles in Lithography and Etching (SAMPLE} [52,541. The SAMPLE simulates the exposure and development of positive photoresists [ reported here extend the capability to two-dimensional etched structures. to dry etching processes as those reported by Mogab and Harshbarged [64,651, and Coburn and Winters[21,22]. Basically, the routines divide th into isotropic and anisotropic components. Each component is chara etchrate information.

Section I1 discusses the various types of etching techniques and mechani I11 describes the implementation of the string development model in dry ples are presented for comparison in section IV and further exploratory in V. Finally, the appendices explain the use of the etch routines, and th

2

TABLE I: ETCHING CHARACTERISTICS .= thickness

I

adhesion profile

Wet Chemical

...

crucial ...

undercut

circular cross section lines narrow

good

bath

none none varies liquid

Substrate

overetch

profile

Plasma Reactive Ion Ion Millirg

... depends on depends an etchrates etchrate!;

... ... ...

... depends on critical etchrates in facetirg

undercut flexi bie substantial mist erosion .

> 45O controllable controllave I

lines narrow lines narrow lines vaer slightly with resi: t

good controllable very poo-

barrel reactor parallel plate ion mill or reactor sputtere?.

< lOOW 100-300W > 200q

1 .1-20T 10- 1 OOmT .2-20m

free radicals free radicals and ions ions sensi tive sensitive must be co led

in plasma in plasma (usually inhrt)

lines vs spaces

selectivity

Operating Conditions

chamber

power pressure

temperature etchant

3

tion bombarding the surface enhances etching by speeding the chemical reaction anc. residue 149,681. The added physical interaction allows etching to be anisotropic, the direction of the radiation flux. This reduces undercutting and allows more prof Planar systems operated at moderate power and low pressure demonstrate this type 0:

At the physical extreme of etching there are sputtering and ion milling. Inert ions (e.g. Ar ) physically remove chunks of the exposed material [8,111. This leads sidewalls, faceting and surface damage. To achieve useful etchrates with such a must be carried out at very low pressures with the ions accelerated through electric fields. In ion milling, the ions are neutralized before striking the substrate. tralization distinguishes it from sputter etching and allows orientational flexibility sin

+

In a barrel reactor, all ions are electrically shielded so that only free fluorine present. The etching then proceeds isotropically, wherever there is exposed absorbed and silicon tetrafluoride removed by vacuum pumping. Any plasma that redeposit slow the etchrate without interrupting the isotropic isotropic etching conditions apply to other combinations of etchgas and and loading effects may make the etch rate nonuniform, but these

In the case of plasma assisted etching several mechanisms

secondary. Isotropic etching is identical to wet etching with ideal mask adhesion, too

due to the chemistry of the plasma discharge some isotropic tant mechanism, ions in the discharge enhance etching in lines. Consider Figure 1 , for the radiation flux shown, Va etchrate and V,, is the physically enhanced rate. The

removing primarily in

le control. etching.

accelerated to steep

process, it extremely high

The neu- :e the sub-

4

remove rate limiting residue and also speed any chemical reaction. The ratio:

defines a measure of the anisotropy. For A = 0 the process is completely isotropic lar profiles, and for A = 1 the process is anisotropic yielding vertical sidewalls.

The enhancement of the etchrate by radiation allows a directionality in etching ure 2). An orientation or angular dependence is then representative of reactive The normal incident rate (Le. in the flux’s direction) is the largest, diminishing to ing incidence. This is effectively modelled as a cosine angular dependence (unl complicated sputter etching with faceting angles and maxima away from normal

(Va) etching is apparent in shadowed or unexposed areas, no matter what the mechanism occurs most often while etching non-uniform surfaces at obliq1.e incidence.

The third mechanism or ion effect is shadowing. Again, only the backgrotnd,

vb

vo + vb A -

with circu-

(see Fig- ion etching.

zero at glanc- ke the more

incidence).

isotropic orie.itation. This

angles of

The next step is to build these characteristic etching components into a rithm. The algorithm, in this case, is intended for use in the user oriented prograr? tion And Modelling of Profiles in Lithography and Etching (SAMPLE [50,53,541). model I331 advances nodes along a string to simulate the time evolution of a two profile. The simulation follows the exposure and development of positive stored rate information to determine the advancement of the string points. Similarly, for the etching routines is to use rate information to advance the string nodes to etching processes.

The plasma etching process is divided into two adjustable components; anisotropic, each characterized by empirical etch rates, Va and Vb, defined in F isotropic component Va models, for example, a chemically reactive etch which profiles with undercut and circular cross sections. The anisotropic component directional dependence and shadowing effect caused by the more physical, ion-enhanced

simulation algo- for Simula-

The original dimensional

photoresist, using the goal

simulate dry

sotropic and gure 1. The

results in contains a

effects

5

in dry etching.

A piecewise linear string of points represents the two-dimensional line e rate of advancement, or etch rate, of these points depends on their position Figure 4 shows a magnified segment of the string model. Under isotropi string-points are advanced at a constant rate Vo, in the direction of the perp of the adjacent segments. To model the directional mechanism in reactive i are advanced in the direction of the radiation flux or electric field lines. A d calculated from the angular information and a specialized rate advances the this unit vector, similar to the isotropic advancement. Alternatively, the a can be looked at as a rate proportional to the cosine of the angle between flux d surface normal (the perpendicular bisector of adjacent segments). Both way etching according to the etchant flux that would strike the exposed surface.

To incorporate the shadowing mechanism, the positions of all points respect to a line parallel to the radiation flux. Points that are shaded by advanced with only the isotropic, background rate. Hence, two adjustable ra shadowing calculation simulate the three mechanisms described in dry etching.

The user specifies the etchrates in plasma etching directly for simul each layer, a single rate is interpreted as the isotropic chemical component (if specified) is taken as vb , the directional etch rate. The degree of an trolled by adjusting their ratio. When using directional etching, the inci specified by an angle, which is assumed to be 0' (normal incidence) if inputs a single time, or a time range with a number of outputs, for etchi ered structure is modelled by arranging and naming material layers with The etching simulation can follow the lithographic routines directly, or a may be created and etching continued from it.

The ion milling or sputtering process is defined by a single, more com licated com- ponent. It uses the same advancing algorithm and shadowing mechanism as direc ional etching. In addition, average angular information is taken from adjacent segments and th incident ion beam. The simulation uses this information to determine a rate according to he sputtering yield [24,13,42,691

i 4

2 4 I S ( 6 ) = So(+lp)[AcosO + Bcos 6 + Ccos 61

where So, A, B, and C characterize the sputtering yield of the material being io etched. p is the atomic density of the layer and 4 is the current density of the ion flux Faceting is automatic due to the maximum etch rate at the facet angle.

The ion milling etch rates and durations are specified similar to those i Also, an ion flux density is specified along with the angle of incidence. Analyti specified along with other material parameters for each layer when simulating ion

checking and delooping routines to manage the string [531. points remain equitably spaced along the string; deleting points that come points where segments advance apart. Subroutine DELOOP eliminates

In addition to the advancement, rate and shadowing subroutines, the Subroutine

6

should cross itself during advancement.

There are also input routines for setting the initial line-edge profile (STPRFL) continuing from a developed profile (COMPAT). Output routines include a message ar subroutine (ETHMSG), a plotting subroutine (PLTLPR) , a numerical printing (PRTNUM), and a card punching routine (PLTDGT) with coordinates of the string poir

Figure 7 illustrates the individual etching components and the output format of dimensional profile. In a). a nonerodible squared mask protects the isotropically etch strate at three different times in the etching process. The previously defined anisotropic is 0. Characteristically the profile shows undercut and quartercircular cross sections. with the same profile Figure 7b). shows the profiles of a completely directionally etch1:d strate, A = 1, at the same three times. Here there is no undercutting and a vertical s In general an etching process is a combination of these two specified components. In 7c). the substrate is etched with a 50% mixture of the two, that is A = 1/2.

IV. COMPARISON OF SIMULATION WITH EXPERIMENT.

In order to verify the accuracy of the numerical algorithm, a number of cases are lated for which there are known analytical solutions. Figure 8 shows profiles at various during the purely anisotropic etch of a three layer structure with an initially tapered, mask. The taper angles of each layer are analytically related by the mask taper and [341 by:

v,

7

and or d error routine ts.

he two :d sub- ratio A

Beginning sub-

dewall. Figure

simu- times

erodible exhrates

other processing conditions, including the extreme cases of anisotropic and completely i otropic etching, may be simulated as well. i

Two examples of dry etching processes are presented below to illustrate the usefu ess of computer simulation in understanding experimental observation. The first case consi rs the anisotropic etching of a layer of isotropically deposited (by CVD, Chemical Vapour Dep sition, technique) material on a non-uniform second layer. It is often observed (see Figure 11 that a residue remains near the bottom of a step under anisotropic etching conditions [21]. A imula- tion of this problem is shown in Figure 12 for a 0.7 p m step covered with a 0.7 p m lay . The step angle is varied 30' to 90' in the figures. In all cases the etchrate selectivity i l O : l , filmsubstrate. The thickness of the deposited layer vertically etches such that the ma .i imum residue occurs at the concave corner of the step. Figure 13 shows the results of several imula- tions with no over etch. For more gradual steps the normalized residue, or overetch f factor, converges to (sed - 1) independent of the deposited thickness. For steep steps it appdoaches H/T, the step height divided by the deposited thickness. At intermediate values the ov r etch factor is reduced due to the curvature of the deposited profile.

The second example characterizes the enhancement of isotropic plasma etch ng by arsenic ion implantation. Figure 14 shows micrographs of etched polysilicon layers. The urface has been implanted with various arsenic doses. The etched profile depends on the am unt of As implantation that the layer underwent. In Figure 15 a 0.1 p m layer of poly silicon h s been implanted, then covered with an additional 0.5 p m of polysilicon. The composite is lasma etched in a barrel reactor resulting in the stepped profiles shown. The implanted layer ' both cases, etches more quickly leading to various degrees of undercut. The profiles in Fi re 14 were etched 15 minutes, at 15 Watts, 10 mTorr, 7OoC in CF4/S0/o02, and those in Figur 15 30 minutes, at 15 Watts, 10 mTorr, 70' C in CF4/5%02. All implants were done at 30 Ke ~ . Fig- ure 16 plots the experimental stepped taper angle for the various implant doses. The l a r k the dose the more gradual is the step.

2 The use of stepped polysilicon layers in E IC [62,631 requires allowance for a lertain amount of undercutting. Simulation can be used to study the amount of undercutting which must be tolerated. The process is modelled by layers of different etchrates depending n the implant dose. For example, Figure 17 shows the simulation of the time evolution1 of an implanted buried layer structure. It agrees quite closely with the time development in plasma etching, in Figure 18. Some second order effects become evident at the implante#/non- implanted interface, Figure 19, which the model does not include. The model does, ho ever,

Each of the above applications can be incorporated into the SAMPLE program ia the TRIAL statement. There is a special additive rate function, SPECRT, which allows the ser to specify his own special rates within a material layer. A piecewise linear profile speci s the nonuniform second layer and can be used when plasma etching follows evaporation and I eposi-

clearly show the undercut necessary to produce the stepped profile.

tion.

1 V. APPLICATIONS.

I:

1

8

I

REFERENCES

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J.of the

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on Electron

R r d E g 31,

lids-Part

3".

angular

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[ l l

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(21

Theory

bombard-

CF4 and

[31

[41

151

[6 1 [71

[81

[91

I101

[ l l l

[I21

[151

[161

I171

[181

I191

[201

[211

[221

I231

[241

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[261

[271

9

[281

E291

[301

[311

[321

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W.R.Hudson. "Ion beam texturing". J. VacSci. Tech. 14, no. 1 (1977) 286-289.

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[S21

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A.R.Neureuther, M.M.O'Toole, W.G.Oldham, & S.N.Nandgaonkar. "Use of simulation to optimize prtjection printing profiles". 153rd meeting of the ECS, Seattle, (May 21-26, 1978).

I

M.M.O'Toole. "Simulation of optically formed image profiles in positive resist". Ph.D. Thesis. Ele Research Laboratory, University of California, Berkeley. (June 1979)

10

I541 W.G.Oldham, S.N.Nandgaonkar, A.R.Neureuther, & M.M.O'Toole. "A General simulator for VLSI phy and etching processes, Part I--Application to projection lithography". Truns. on Electron Devices (April 1979) 717-722.

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~

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[SSl

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[601

[611

structures".

position

[621

[631

[641

[661

[671

1681

1691

I701

[711

[741

11

[APPENDIX A]

KEYWORD DESCRIPTIONS

Notes on symbols and conventions:

UPPER CASE BOLD indicates keywords and commands. lower case italics indicates numerical values. UPPER CASE ROMAN indicates common block variable names [ 1 bracketed variables or words indicates optional. { 1 braced variables or words indicates alternatives. ( ) parenthesized words specify units of the numerical values.

Inputting a profile without running the develop machine.

(LINE } dimension (in microns) MODEL CONTOURS {SPACE} dimension (in microns)

{EDGE }

LINE inputs line of width dimension centered in WINDOW. SPACE inputs a space of width dimension centered in WINDOW. EDGE inputs a falling edge centered in WINDOW

MODEL CONTOURS (x,z)z,~x,z~2’. . .~x,z~ (in microns). P The coordinates (x,zIi are the turning points in a piecewise linear profile. Associated common block variables in /PROF /

IPVAR- 1 signifies LINE IPVAR = 2 signifies SPACE IPVAR = 3 signifies EDGE DIMEN = dimension

PXZ(D = (x ,z)~ IPLAST-p. (p< 12)

Inputting horizontal dimensions.

WINDOW OUTPUT x1 xz (in microns)

xz is the horizontal width of the output window. x2 is the distance from the left side of the window to the edge of the mask. applicable to the etching routines. Associated common block variables in /ETHWND/

HORWIN=xl

Inputting vertical dimensions as layer thicknesses.

It is not

12

(NITRIDE]

(RESIST] (SPECIAL]

LAYER layer#n {OXIDE) thickness (in microns)

luyer#n places the layer, # n = 0 means substrate, 1 the next layer and thr number is the topmost layer. NITRIDE, OXIDE, RESIST, SPECIAL also specify the layer, especially c( default values and material parameters. thickness is the layer thickness in microns. Associated common block variable in /THKNES/

THICK(NMLYRS-#n) = thickness

Inputting etching parameters.

greatest

icerning

13

Plasma Etching:

n is the number of outputs. Each output is equally spaced in time within the

Associated common block variables in /ETCHTM/. to ' 2

MODEL layer#n ETCH R I [R2/ (in microns/min) ISOURCE ETCH 8 (in degrees)]

luyer#n indicates the layer to which the etching parameters refer. RI is the isotropic, nondirectional etchrate in microns/minute. If onl] etching is to be preformed this is the only number needed. R2 is the illuminated, directional etchrate in microns/minute. 8 is the angle in degrees of the incident radiation flux when directional being used. 0' means direct incidence and positive angles from the left. Associated common block variables in /ISOETH/ & /ILETCH/.

RISOTR(NMLYRS-#n) = RI RTISO (NMLYRS-# n) = R 1

RTNORM(NMLYRS-#n) =R2 DANGLE-8

range r I

Ion Milling and Sputtering:

MODEL layer#n IONETCH So(#n) [p(#n) A ( # d B.#n) C(# SOURCE IONETCH 8 (in degrees) 4 (in ions/cm )

isotropic

:tching is

91

So is the sputter etch rate at normal incidence in l/seconds for layer n. 2 p is the density of layer n in ions/cm .

8 is the angle of flux incidence in degrees. 2 4 is the flux density of the incoming ion beam in ions/cm .

A(#n), B(#n), and C(#n) are the analytic coefficients that model th$ angular dependent sputtering yield as a cosine power series. Associated common block variables in /SPUTYD/:

ASPUT(NMLYRS-#n) = A (#d BSPUT (NMLY RS- # n) B (# n) CSPUT(NMLYRS-#n) = C(#d DENS(NMLYRS-#n) = p (#n)

SPTY DO (NMLY RS- # n) = So (# n) PHI-4

DANGLE=^

Inputting the etching times.

14

EHTMl = fI

NMEHTM = n EHTM2 = ‘2

Instructions for printing and plotting.

PLOT IDPI

PRINT (DEBUG] (CONTOURS)

PLOT plots the output contours on the lineprinter. PLOT DP punches cards with the contours coordinates for a digital plotter. PRINT DEBUG prints diagnostics concerning string point model. PRINT CONTOURS prints the coordinates of the string points numerically. Associated common block variables.

INDVAR(2) =O(off),l (on) PLOT DP INDVAR(5) =O(off), 1 (on) PRINT DEBUG INDVAR(6) =O(off),l (on) PRINT CONTOURS INDVAR(7) =O(off),l (on) PLOT

Accuracy

ACCURACY k

k is an integer between 0 and 5 , where 5 demands the most accuracy and 0 tl is the default value. Associated common block variable in /INDCAT/.

INDVAR(3) = k

Running the etch routines.

(DESCUM) RUN {ETCH]

(IONETCH]

DESCUM simulates descumming the resist layer isotropically. ETCH simulates piasma etching. IONETCH simulates ion milling and sputtering. Associated common block variable in /INDCAT/.

INDVAR(4) = 0 no RUN command. = 1 RUN ETCH with one etchrate specified. = 2 RUN ETCH with two etchrates specified. = 3 RUN DESCUM = 4 RUN IONETCH = 5 , 6 using TRIAL statement.

: least. 2

15

DEFAULTS AND TYPICAL VALUES

The consistency of the default values arises from the assumptions that all parameters i

represent RESIST and it’s defaults, 2 SPECIAL (polycrystaline silicon), 3 NITRIDE (silicl

OXIDE (silicon dioxide), and 5 SUBSTRATE ( < 100 > silicon).

Variable 1 thickh) 1 risotr (n)

Plasma etching defaults

r tnormh) I rtiso(n)

Units

Material

1 RESIST

2 SPECIAL

3 NITRIDE

4 OXIDE

microns microns/minute microns/minute microns/minute

thickness isotropic non-direc tional directional

rate rate

0.800 0.0060 0.0005 0.0010

0.600 0.3000 0.0300 0.0360

0.300 0.0300 0.0120 0.0430

0.080 0.0125 0.0060 0.0390

5 SUBSTRATE 1 0.200* 1 0.2040 I 0.0300 I 0.0360 I I I I I

* This is not the actual thickness of the substrate. but the amount that will appear in the ou

dexed by 1

1 nitride), 4

)Ut plot.

16

Variable

indvar (1)

indvar (2)

indvar (3)

indvar (4)

indvar ( 5 )

indvar(6)

indvar (7)

ipvar

dimen

horwin

nmlyrs I

dangle

phi

ehtml

ehtm2

nmehtm

General defaults

Description

type of profile

card punch

accuracy

type of etch

diagnostics

print coordinates

line print profiles

profile = line

linewidth

window size

number of layers

angle of incidence

ion flux

initial etch time

final etch time

number of profiles

Value

1

1 .o 4.0

5

Units

...

...

...

...

...

...

...

...

microns

microns

...

0.0

.32

degrees 2 milli Amp/cm

400.

1000.

4

seconds

seconds

...

17

Material

AZ1350

Poly

NITRIDE

OXIDE

SILICON

Gold

GaAs

Aluminum

Titanium

2000.

4272.

81 7.

959.

4212.

53 10

1460

33500

595.

density

3.00

4.98

1.48

2.66

4.99

5.89

1.54

6.02

5.66

Typical values in ion milling

A

15.0806

7.8776

7.8776

10.7000

3.2696

1.7660

6.7702

3.4142

10.6300

B

- 19.932 1

-4.8938

-4.8938

-11.7000

13.1059

-0.2589

-6.1548

0.7574

- 14.2 138

C

5.8515

-1.9838

-1.9838

2.0000

-15.3755

-0.5072

0.3846

-3.1716

4.5837

max angle

65

52

52

60

60

30

5 5

45

65

-T m x:norm

3.00

2.71

2.71

2.55

3.95

1.05

1.90

2.00

2.10

18

TRIAL CARD SUMMARY FOR ETCHING ROUTINES

comments

initializes type of etching desired.

TRIAL # keywords

DESCUM ETCH

IONETCH E'LANARIZE

run etch routines

save profile in iounit

arguments

jgpe [rates]

SAVE

trial 78

trial 79 i< 0 nmlyrs ilayer thickness

specifies numbers of layers. thickness of layer in microns according to ilayer (substrate = 0)

LAYER

trial 80 jwpe [rates] less specific than 78 I trial 81 ilayer sputydO A B C density material parameters for ilayer

specified as sputtering coefficients, maximum sputtering angle and ratio at maximum, or etchrate table at incremental angles.

I 1 MODEL

ilyr so max ma:norm dens

ilayer 0 inc rates

e kl trial 82 set source angle and ion flux (in milliAmps)

~ SOURCE

trial 83 dose energy where special As-implanted layer at a depth of where.

~~~ ~ ~

accuracy and diagnostics. qCCURACY

or profile type & dimension. set piecewise linear profile @ONTOURS

trial 84

trial 85

window I

1 horizontal window dimension WINDOW

plotter coordinates 1 PLOTDP lineprinter plot PLOT lineprinter print PRINT reset profile

I

I

trial 86

trial 87 [i2 i7 i6 ill

trial 88 etch time specification TIME ETCH tl [tend nstepsl

cirupe/ trial 89

trial 90 iounit

trial 91 iounit load profile from iounit I LOAD

trial 94 ilayer s i g m a sigmaz [coef where]

surface diffusion specification. 1 trial 95 ilayer thick rate forced addition of additive rate.

19

[APPENDIX BI

/DEVFLG/

COMMON BLOCK DESCRIPTION

Develop flags are common to the Controller and to Subroutines CHKR and DE'

Common blocks that are common to the photoresist

developing routines.

OOP.

IDEVFL(5)- a one dimensional array containing the flags for machine DELOC signifies [yes], 0 [no].

IDEVFL(1) (=INDVAR(5)] prints the number of string points and other diagnostics from CHKR and DELOOP. (cf keyword PRINT DEBUG; consistency with INDVAR ( 5 ) is insured in controller)

/DVELPl/

Development block 1 is internal to Subroutines RIETCH, CHKR, DELOOP, ADJUST, RIEADV, SHADOW, ETHMSG, PLTDGT, PRTNUM, PLTLPR, DIST and PROJ.

) and CHK

STPRFL, and funk

XZ(450)- complex array containing the x and z positions of the 450 possible stribg-points.

XMAX- the real maximum value of x.

ZMAX-- the real maximum value of z; the summation of the layer thicknesses. ~

I NPTS-- the current number of string-points.

CXZL- the normalized left endpoint direction of etching.

CXZR- the normalized right endpoint direction of etching.

NADCHK- the number of advances (calls on Subroutine RIEADV) between cbecks (cab Subroutine CHKR) .

NCKOUT-- the number of checks between outputs, {thus the number of ad ances be&

I

~

outputs is NADCHK*NCKOUT) . .1 I /DVELP2/

20

Development block 2 is internal to Subroutines RIETCH, CHKR, DELOOP and RIEA

TADV- the current time increment between advances (in RIEADV)

TCHK- the current time increment between checks.

TTOT- the total current etching time during the execution of RIETCH.

IFLAG- flag set in RIEADV that indicates some segments are long or too short. 1

then called from RIETCH.

SMAXX- the maximum x string segment length allowed by CHKR.

SMINX- the minimum x string segment length allowed by CHKR.

SMAXZ- the maximum z string segment length allowed by CHKR.

/IO1 /

Input/output block 1 is common to all subroutines and Controller. The block is decid top level controller for handling input/output in a uniform way. It contains the I

names for logical input/output unit numbers.

IPRINT- the correct character to access the printer with the FORTRAN I

"WRITE(IPRINT,###)".

IPUNCH- the correct character to access the card puncher with the FORTRAN I

"WRITE(IPUNCH,###) 'I.

/THKNES/

MNLYRS- the maximum number of layers allowed in the problem. The first layer is layer; the last is the substrate. Four intermediate layers are allowed.

NMLYRS- the number of layers in the current problem.

THICK(6)- a one dimensional array containing the thicknesses of the various layers ir characterized by etchrates RTNORM, RTISO, RISOTR, and SPTYDO. THICK(NM1 the amount of the substrate that that is shown in the output. {cf keyword LAYER]

Common blocks initialized in the etching simulation

v.

IKR is

in the nbollic

:ement

:ement

: resist

iicrons RS) is

/ETCHTM'/

21

Etch times is common to the Controller and Subroutines RIETCH, ETHMSG, PL PRTNUM, and PLTLPR.

MXNEHT- maximum number of timed etched outputs allowed in a single p Currently, it is 20.

NMEHTM- number of output profiles specified for a problem. (cf keyword TIME ETC

NCOEHT- index of the current output profile.

EHTMl-- the etching time in seconds of the first output profile. (cf keyword TIME ET(

EHTM2-- the etching time in seconds of the last output profile. (cf keyword TIME ETC

EHTM- the etching time in seconds of the current output profile.

EHTMST- the time step between output profiles.

/ETHPAR/

Etching parameters is common to Subroutines RIETCH, PLTLPR, and STPRFL.

NSTPTS- number of points in the initial profile created by STPRFL; the number ii

with accuracy.

DELTXE- the horizontal spacing of points in the initial profile created by STPRFL.

DELTZE- the vertical spacing of points in the initial profile created by STPRFL.

/ETHWND/

Etch window is common to the Controller and to the RIETCH, STPRFL, COMPAT, PI PLTDGT, and PRTNUM Subroutines.

HORWIN- the x (horizontal) window width in microns; it is set by the Controller. ( word WINDOW OUTPUT}

VERWIN- the z (vertical) window length in microns; it is the summation of tl. thicknesses for etching routines and the photoresist thickness in descumming.

TDGT,

oblem.

0

HI

-I)

xeases

TLPR,

f. key-

: layer

/ILETCH/

Illuminated etching coefficients is common to the Controller, Functions RIERT, RISORP, and Subroutines RIETCH and ETHMSG.

RTNORM(6)-- the one dimensional array specifying the directional etchrates at lorma1 incidence for each layer. (cf. keyword MODEL layer# ETCH]

I

I

I RTISO(6)-- the one dimensional array specifying the background, isotropic etchrates

DANGLE-- angle in degrees of the incident radiation flux. (cf. keyword MODEL

layer. (cf. keyword MODEL layer# ETCH)

ETCH theta)

22

/INDCAT/

Indicator variables are common to the Controller and Subroutines RIETCH, RII PLTLPR, AIMPLT, and STEP.

INDVAR(6) -- The one dimensional array specifying the different options for the etchii tines.

INDVAR(1)--type of profile input.

0

1

2

stop with the development of the resist.

continue etching from the developed resist profile.

create an initial profile by specifying coordinates for a piecewise linear profi keyword MODEL CONTOURS]

read the string points from a tape, cards or file, and continue etching. (cf k 3 LOAD}

INDVAR(2)--cards for the etched profile. (cf. keyword PLOT DP}

0 no cards punched.

1 cards punched in the format /4 corners of output graph/ num outputs/number of points in output/coordinates of each string point/

INDVAR (3) --accuracy (cf keyword ACCURACY #)

0-5 0 being the least accurate, and 5 being the most accurate. INDVAR(3) c number of initial points, spacing between points, and the number of advan output.

INDVAR(4)--type of etching. (cf keywords such as ETCH, IONETCH, DESCUM or TF

0

1 isotropic or wet etching.

2

3 descum of the photoresist.

4

5 6

no etching or descum after resist development.

reactive ion or plasma assisted etching.

ion milling or sputter etching.

special etching feature with regions of different etchrate.

etching over a non-uniform second layer.

:ADV,

g rou-

e. (cf.

:word

ier of

mtrols :es per

IAL 1

INDVAR(5)--diagnostics as points are added and deleted. [= IDEVFL(1)l (cf. keyword RINT P I

0 no diagnostics. ~

DEBUG}

1 diagnostics from CHKR.

INDVAR(6) --coordinates of string point contours are printed numerically. (cf. keyword $RINT CONTOURS] I 0 Off .

23

1 coordinates printed numerically.

word PLOT] INDVAR(7) -- line printer plots the contours with coordinates marked as alphabetics.

0 off.

1 line printed plot.

/ISOETH/

Isotropic etching components are common to the Controller, Subroutines ETHM AIMPLT, and Function SPECRT.

RISOTR(6) -- a one dimensional array specifying the isotropic (non-directional) etck each layer. (cf. keyword MODEL layer# ETCH}

/PROF /

Profile information is common to the Controller and Subroutines STPRFL, ETHM, STEP.

PXZ(12)- a complex array of the twelve possible (x,z) coordinates used in creating th piecewise linear profile. (cf. keyword MODEL CONTOURS)

IPLAST- the number of coordinate pairs which set the initial profile. (cf. keyword CONTOURS)

IPVAR-- the indicator variable for default and preset profiles. (cf. keyword MODE TOURS]

0

1

2

3

4 A specially saved profile.

All profile turning points must be specified.

A line of width DIMEN centered in CPWIND

A space of width DIMEN centered in CPWIND

A step with the falling edge centered in CPWIND

DIMEN-- The dimension of the feature indicated by IPVAR. (cf. keyword MODE TOURS}

/SHADE /

Shade, which contains information about the direction of the radiation flux, is internal routines RIETCH, RIEADV, SHADOW, ADJUST, ETHMSG and Functions DIST and

SLOPE-- the slope of a line parallel to the radiation flux.

FACTOR-- -/(SLOPE**2 + 1)

ANGLE- the angle of incidence in radians of radiation flux.

:f. key-

G and

'ate for

G, and

initial,

IODEL

CON-

CON-

to Sub- PROJ.

24

MSHADE(450)-- a one dimensional array of flags specifying the exposure of a string- means in shadow, 0 exposed.

/SPUTYD/

SPUTYD contains information about ion milling and sputtering and is common to t troller , Subroutines RIETCH, ETHMSG. and function RIMRATE.

ASPUT(6)-- a one dimensional array specifying the coefficient in the sputtering yield S(O)=AcosO+Bcos 8+Ccos 8, for each layer. (cf. keyword MODEL layer# IONETC€

BSPUT (6) -- a one dimensional array specifying the coefficient in the sputtering yield S(O> =AcosO +Bcos20+Ccos40, for each layer. (cf. keyword MODEL layer# IONETCI

CSPUT(6)-- a one dimensional array specifying the coefficient in the sputtering yield S ( 6 ) = AcosO +Bcos20 +Ccos40, for each layer. (cf. keyword MODEL layer# IONETCE

SPTYDO- a one dimensional array specifying the sputtering yield at normal incidence each layer in l/seconds. {cf. keyword MODEL layer# IONETCH]

DENS(6)- a one dimensional array specifying the density of each layer material in atoi

2 4

(cf. keyword MODEL layer# IONETCH) 2 PHI- the incident ion flux density in milliamperes/cm .

IONETCH theta phi) (cf. keyword MODEL S

Common blocks inserted for special purposes and

TRIAL runs

/IMPLNT/

Implant information is common to TRIAL, Subroutines AIMPLT, ETHMSG, and I SPECRT.

QCM2-- the implant dose in fluence/cm**2.

DOSKEV- the implant energy in KeV.

RANGE-- the average depth of penetration of implanted ions.

STNDEV- the standard deviation in the implantation depth.

DNSCM3- the density in ions/cm**3 of ions in the implanted material.

/SPCIAL/

25

lint; 1

Con-

.alytic,

dytic,

lalytic,

IO, for

2 dcm .

URCE

nction

Special information block 1 is common to TRIAL, Subroutines ETHMSG and AIMPLl Function SPECRT.

IFLSPC- flag identifying the position of the implanted layer (surface = 1 or buried = 2)

RTSPEC(6)- a one dimensional array specifying the etchrates within the regions defin RLAYER(6).

/SPC2 /

Special information block 2 is common to TRIAL, Subroutines ETHMSG and AIMPLl Function SPECRT.

RLAYER(6)- The thicknesses in microns of regions (within material layers) of v etchrates (RTSPEC(6)).

NLAYER- the number of regions of differing etchrates.

BURLYR-- the thickness in microns of the buried layer with a different etchrate.

/NUNFRM/

Step information is common to TRIAL, Subroutines ETHMSG and STEP, and all rate tions.

STPSLP- slope of the piecewise linear non-uniform second layer.

XINCPT-- x-intercept of the non-uniform second layer.

GAMMA-- angle the non-uniform layer makes with the horizontal.

LSTEP- flag specifying a non-uniform layer in rate functions.

PI-- 3.141 5926 ...

, and

:d by

, and

irious

func-

26

FIGURE 1 : ISOTROPIC AND DIRECTIONAL ETCHRATES A S THEY APP

A MASKED SURFACE, A ) , I S O T R O P I C ON S I X Y T

B). D I R E C T I ~ N A L AND ISOTROPIC ON EXPOSED SURFACE,

AND C), IS3TROPIC ON SHADOWED SURFACE,

c

I I

> I t

\ > /

W 0 7 W Q 7 W Q, W c3

-r 3 0 cn w a

1. I u t- w

I - w 0

w o

A I

0 U

r"L I - L s I I a

W / I

, I

I NC I D E N T i3EAM

F I G U R E 5: SHADOWED POINTS ALONG THE STRING,

0 0 I1

1-Q C D I i

I

\ b:g' -- I- z 0 c. I

I

I

F I G U R E 7 ,

0 . 8 p m

60s I

9 0 s

1.2 p m 120s * .

A )

B)

E T C H R A T E io0 SECOND i

60s

90s

120s ,

I S O T R O P I C E T C H

1.1_

LL

C I 1

c p € a In

0 .LL.

- 7 . . . . . .

I c

I

a

a z

z 0

PHOTO R E S

Si 02( 0.7p\

\ Poly-Si L O p m

Mogab 81 Harsh- barqer, Electronics,

August 31,1978

-~ - ~ - ~- - -~ ___ - -

I S O T R O P I C U N D E R C U T FOLLOWED B Y A N I S O T R O P I C E T C H

R E A C T I V E I O N E T C H OF P O L Y - S i ( 0 . 5 p m ) ---DEP(>SITED O V E R S O 2 S T E P

0 0

FIGURE 12 ( C O N ' T ) .

30'

I S9TROP I C DEPOS I T I ON FOLLOlJED BY AN I SOTROP I C ETCH I iJG

F I G U R E 12 (CON'T),

I SOTROP I C DEP3S I T I OI FOLLOWED BY ANISOTROPIC ETCH

.- a

h

I- *

TR T

2.0

.6

I .2

0.8

0.4

0.0

FIGURE 13,

f i

S e c 8 - 1

+ /+-+ /

N O R M A L I Z E D R E S I D U E V E R S U S S T E P T A P E R

N O I M P L A N T IO l5 / c m

I M - P L A S M A ETC-HED P R O F I L E S F O R A s P L A N T E D S U R F A C E L A Y E R S

L..

t

d - 0

Ln

- X

Cn Ty W t- a

I I I

n W

Ty 3

-

m

a W I- - L

I n

a

I I 0 0

I 0

I I I 0 0 0 0 (0

Ln 0 Ln 0 Ln Ln 0 * Kf- m

F I G U R E 1 7 ,

t

IO 0.6p.m

c + a . 1 I I d

S I M U L A T I O N O F B U R I E D L A Y E R I M P L A N T A N D P L A S M A E T C H

I 5 m i n 30 min

N

E 0 \ M 0 - x

n

F

i