AlGaAs devices

TECHNOLOGY

D. G. Lishan, D. J. Johnson, Y. S. Lee, B. H. Ree|fs, and R. J. Westerman

Etching in GaAs/AIGaAs devices

The challenges posed to dry etching tech-

This process offers controllable etching

niques by thin, damage-sensitive layers in

rates and high selectivity to AIGaAs while minimizing exposure to damaging high ener-

GaAs devices are well known. In this article, the development by Unaxis Semiconductors of a patent pending process is discussed.

gy ion bombardment common to many existing processes.

Frontside etching in GaAs/AIGaAs devices Successfnl fabrication of m a n y c o m p o u n d semi-

f a b r i c a t i o n s e q u e n c e of m a n y h i g h - s p e e d

c o n d u c t o r devices requires etching processes for

s e m i c o n d u c t o r devices. In particular,

thin layer structures that are controllable, selec-

h i g h e l e c t r o n mobility t r a n s i s t o r s (HEMTs)

tive, a n d low damage. Controllable in the sense

are

that etching rates result in reasonable process

s t e p d u e to t h e sensitivity of t h e s e devices to

times, selective to handle a multitude of III-V

plasma damage and etching depth. These

materials, and low damage in that material's elec-

d e m a n d i n g c o n s t r a i n t s initially led to

tronic and optical properties r e m a i n intact.

dependent o n

t h e s u c c e s s of this e t c h i n g

successful p r o c e s s flows b a s e d o n w e t

Within this class of devices, the GaAs/AIGaAs

c h e m i c a l e t c h i n g t e c h n i q u e s . But as d e v i c e

h e t e r o s t r u c t u r e system is p e r h a p s t h e most studied and applied case of these requirements.

d i m e n s i o n s c o n t i n u e to s h r i n k a n d critical d i m e n s i o n c o n t r o l b e c o m e s m o r e stringent,

T h e ability to selectively e t c h GaAs o v e r

w e t e t c h i n g is n o l o n g e r a d e q u a t e a n d dry

A1GaAs has l o n g b e e n essential in t h e

e t c h i n g b e c o m e s an e n a b l i n g technology.

Figure 1. DC bias as a function o f RF power and pressure for 13.56 and 40.68 MHz.

400 . . . . . . . . . . . . . . . . . . . . . . . . . . 13.56 MHz n 25mtorr 350300

Increasing P re s s u r ~ , , .

DC Bias (-V) 250 200-

•j•'sS ' -

150-

~..-41

25 mtorr

100 50 0 0

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20

VOL ~,5 " NO 9 - NOV/DEC 2002

40

60 80 RF Power (W)

1O0

120

TECHNOLOGY

Etching in GaAs/AIGaAs devices

700 600 500 Etching Rate 400 (A/min) 3O0 20O 100 0

I

0%

10%

3¸

F

20% 30% 40% RF Duty Cycle

I

I

50%

60%

Figure 2. GaAs etching rate as a function of RF duty cycle at 40.68 MHz.

However, t h e t r a n s i t i o n to dry e t c h i n g has n o t

are particularly s u s c e p t i b l e to d a m a g e as t h e

b e e n straightforward.

active t r a n s p o r t layer is often relatively near the

T h e early dry e t c h i n g d e v e l o p m e n t w o r k for

device surface.

GaAs/A1GaAs u s e d c h l o r i n e - b a s e d c h e m i s t r i e s

In an effort to o v e r c o m e t h e d a m a g e limitations

in capacitively c o u p l e d reactive ion e t c h e r s

of RIE b a s e d processes, device m a n u f a c t u r e r s

(RIE) o p e r a t i n g at 13.56 MHz. C h l o r i n e

t u r n e d to h i g h density plasma p r o c e s s e s s u c h as

radicals, typically from BCI 3 or SIC14, are

inductively c o u p l e d plasma (ICP) a n d e l e c t r o n

still the favored e t c h a n t due to t h e ease of

c y c l o t r o n r e s o n a n c e (ECR) r e a c t o r configura-

f o r m a t i o n of volatile p r o d u c t s , GaCI x a n d

tions. High density r e a c t o r s allow essentially

AsCI x. Selectivity to an u n d e r l y i n g A1GaAs

i n d e p e n d e n t c o n t r o l of t h e ion density a n d

e t c h i n g stop is a c h i e v e d e i t h e r t h r o u g h t h e

e n e r g y t h r o u g h the use of t w o RF p o w e r

a d d i t i o n of an o x y g e n s o u r c e (forming non-

sources. T h e s e c o n f i g u r a t i o n s allow for l o w e r

volatile A1203) or a f l u o r i n e s o u r c e (typically

ion e n e r g i e s (DC Bias < 50 Vdc) at h i g h e r ion

SF6 or SiF4) in o r d e r to form non-volatile AIF 3.

densities. W h i l e the l o w e r ion e n e r g i e s facili-

Thus, w i t h a c o n v e n t i o n a l RIE system a n d

tate low d a m a g e etching, t h e associated h i g h e r

c h e m i s t r y a n i s o t r o p i c feature profiles w i t h h i g h selectivty to u n d e r l y i n g A1GaAs could b e produced. While the p r o b l e m a p p e a r e d resolved, t h e m o r e subtle issue of damage remained. A l t h o u g h device d a m a g e c a n arise from a numb e r of sources, it is generally agreed that p l a s m a i n d u c e d damage is directly related to ion energy. Ion e n e r g y in t u r n is p r o p o r t i o n a l to t h e

plasma densities result in GaAs e t c h i n g rates in excess of 1000 •/min m a k i n g it difficult to c o n t r o l t h e e t c h i n g p r o c e s s for t h i n film (< 1000 A) applications.

Low damage While m a i n t a i n i n g t h e selectivity b e t w e e n GaAs

plasma p o t e n t i a l a n d self-induced DC bias w)lt-

and AIGaAs u s i n g t h e well u n d e r s t o o d BC13 /

age ( I Vdc I). Unfortunately, t h e h i g h ion ener-

SF6 chemistry, w e address t h e d a m a g e limita-

gies associated w i t h t h e self-induced DC bias at

tions of a c o n v e n t i o n a l 13.56 MHz RIE configu-

13.56 MHz (typically > 100 V) results in d e v i c e

ration as well as t h e e t c h i n g rate limitations of

d a m a g e w h i c h ultimately c o m p r o m i s e s d e v i c e

t h e h i g h density a p p r o a c h . The i m p r o v e d ,

p e r f o r m a n c e . Several g r o u p s have r e p o r t e d

p a t e n t - p e n d i n g Unaxis r e a c t o r uses a parallel

that e t c h i n g i n d u c e d d a m a g e is significantly

plate c o n f i g u r a t i o n p o w e r e d at 40.68 MHz.

r e d u c e d at values b e l o w 50 Vdc. HEMT devices

Figure 1 shows, as e x p e c t e d , that t h e DC bias

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Etching in GaAs/AIGaAsdevices

Figure 3. SEM cross-sections. (a) SiN mask at 50% overetch. (b) Photoresist mask at 200% overetch. Epitaxial structure is 500 ~ GaAs over 400 ~ AIGaAs.

increases w i t h RF excitation p o w e r a n d

e x a m p l e , e t c h i n g a 300 A t h i c k film at 1200

d e c r e a s e s w i t h increasing pressure. However,

a / m i n suggests an e t c h i n g time of a p p r o x i m a t e -

Figure 1 also e x h i b i t s t h e p r e d i c t e d inverse rela-

ly 15 s e c o n d s - resulting in a n a r r o w p r o c e s s

t i o n s h i p b e t w e e n DC bias a n d RF f r e q u e n c y for

w i n d o w . Slowing t h e e t c h i n g rate by simply

13.56 MHz a n d 40.68 MHz. The h i g h f r e q u e n c y

r e d u c i n g RF p o w e r levels is generally impracti-

RF excitation results in significantly l o w e r

cal since plasma stability a n d r e p r o d u c i b i l i t y

applied DC bias voltages. For e x a m p l e at 50 W

become major concerns.

RF power, t h e DC bias for 13.56 MHz is over t w i c e t h e value for t h e same p o w e r at 40.68 MHz. In c o n t r a s t to t h e 13.56 MHz DC bias curves, t h e 40.68 MHz c u r v e s s h o w a usable p r o c e s s s p a c e w i t h IVdc I < 50 V, e v e n at l o w e r pressures. The l o w e r applied voltages result in ion energies c o m p a r a b l e to low d a m a g e ICPb a s e d processes.

With t h e i n t e n t i o n of s l o w i n g t h e GaAs e t c h i n g rate to c o n t r o l l a b l e levels for t h i n film applications, t h e 40.68 MHz RF was m o d u l a t e d between high power and low power conditions over t i m e (pulsed). The e t c h i n g rate d u r i n g the high power period remains unchanged ( - 1 0 0 0 A / m i n ) w h i l e t h e e t c h i n g rate d u r i n g t h e l o w p o w e r p o r t i o n of t h e cycle is

Controllable etching rates

essentially zero. This pulsing results in an average e t c h i n g rate o v e r a n u m b e r of cycles t h a t is

In a parallel plate factor, t h e e t c h i n g rate of

d e t e r m i n e d by t h e ratio of t h e h i g h to l o w

GaAs is nearly i n d e p e n d e n t of e x c i t a t i o n fre-

p o w e r p e r i o d s (or duty cycle). Figure 2 s h o w s

quency even though the increased frequency

t h e r e l a t i o n s h i p b e t w e e n d u t y cycle (ratio of

l o w e r s t h e DC bias a n d c o n s e q u e n t l y t h e ion

h i g h p o w e r time to total cycle time) a n d GaAs

e n e r g y at t h e w a f e r surface. Since t h e e t c h i n g

e t c h i n g rate for a BCI 3 / SF6 process. It is

of GaAs in a c h l o r i n e b a s e d plasma is primarily

i m p o r t a n t to n o t e that the a p p l i e d voltage to

chemically d r i v e n (as o p p o s e d to an ion driven

t h e s u b s t r a t e was less t h a n 30 Vdc d u r i n g b o t h

m e c h a n i s m - i.e. sputtering), this result is n o t

t h e h i g h a n d l o w p o w e r p e r i o d s w h e n this data

u n e x p e c t e d . T h e c o n c e n t r a t i o n of reactive

was collected. Using a 25% duty cycle resulted

species, a n d h e n c e t h e e t c h i n g rate, is directly

in GaAs e t c h i n g rates n e a r 300 A / m i n yielding

related to t h e e x c i t a t i o n power. Thus, for a simi-

a desirable o n e m i n u t e p r o c e s s time for a

lar excitation p o w e r a n d c h e m i s t r y (BC13 / SF6)

300 ik film.

e t c h i n g rate results at 40.68 MHz w e r e consist e n t w i t h p r e v i o u s g r o u p s investigating e t c h i n g rates w i t h a 13.56 MHz RIE configuration. Both RF f r e q u e n c i e s yielded e t c h i n g rates of approximately 1200 ]k/min.

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High selectivity to AIGaAs As previously discussed, in addition to having low damage and a controllable etching rate, GaAs e t c h i n g processes for m a n y devices must also b e

In o r d e r to realize a m a n u f a c t u r a b l e p l a s m a

highly selective to A1GaAs. By using e x t e n d e d

e t c h i n g process, it is desirable to h a v e p r o c e s s

e t c h i n g times it is possible to d e t e r m i n e selectiv-

times of at least o n e m i n u t e in length. For

it3' u n d e r various conditions. Table 1 summarizes

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Etching in GaAs/AIGaAsdevices

Table 1 Etching Rate

Selectivity

GaAs

~300 A/min

GaAs:AIGaAs

>200:1

AIGaAs

-1.5 A/min

GaAs:SiN x

>20:1

GaAs:Resist

> 10:1

SiN x

14 A/min

Resist

30 A/min

t h e data for etching rates and selectivity using a

r e a c t o r is discussed. A qualitative c o m p a r i s o n

baseline GaAs etching p r o c e s s of approximately

to alternative available t e c h n i q u e s is s h o w in

300 A/rain.

Table 2.

Thus, u n d e r t h e s e specific c o n d i t i o n s t h e

The high frequency reactor described

GaAs:A1GaAs e t c h i n g selectivity is over 200:1

h e r e achieves t h e goal of l o w bias voltages

w h i c h c o m p a r e s quite favorably w i t h t h e

(<50Vdc) w h i c h significantly r e d u c e s

typical values of a p p r o x i m a t e l y 50:1 for ICP

e t c h i n g i n d u c e d damage a n d is c o m p a r a b l e

processes.

to ICP-based processes. Pulsing t h e RF r e d u c e s t h e e t c h i n g rate to t h e same o r d e r of

Profile control

m a g n i t u d e as t h e film thickness. Controllable

Feature profile is a n o t h e r i m p o r t a n t considera-

GaAs e t c h i n g rates b e t w e e n 200 A / m i n a n d

tion for GaAs frontside e t c h i n g applications.

1000 2k/min h a v e b e e n d e m o n s t r a t e d w i t h o u t

Depending on the subsequent fabrication

the addition of a p r o c e s s gas diluent. T h e s e l o w

p r o c e s s f l o w e i t h e r an a n i s o t r o p i c or control-

e t c h i n g rates are r e q u i r e d in o r d e r to o b t a i n

lably u n d e r c u t feature profile is required. Using

controllable e t c h i n g times for t h i n films (e.g.

t h e same BC13 / SF6 chemistry, b o t h SiN x a n d

gate recess definition for HEMT devices). Due

resist m a s k e d samples w e r e e t c h e d a n d cross-

to t h e l o w p r o c e s s p o w e r s u s e d in this configu-

s e c t i o n e d for SEM analysis.

ration, active cooling of t h e s u b s t r a t e is n o t required. In a d d i t i o n to allowing l o w DC bias

Figure 3 s h o w s e x a m p l e s of profiles w i t h SiN x a n d resist masks. A SiN x mask sample exhibiting a 40 n m u n d e r c u t feature profile w i t h a 50% over-etch is s h o w n in Figure 3a. T h e

operation, t h e h i g h f r e q u e n c y RF also p e r m i t s o p e r a t i o n at relatively l o w PIE p r e s s u r e s of less t h a n 20 m t o r r p r o c e s s pressures. This l o w pressure o p e r a t i o n results in highly u n i f o r m (_+4%)

u n d e r c u t increases linearly w i t h time in t h e range of 50% - 150% over-etching w i t h 100 n m of u n d e r c u t e v i d e n t at 200%. Similar experim e n t s o n resist m a s k e d samples reveal m i n i m a l u n d e r c u t e v e n w i t h an e x t e n s i v e (150%) overe t c h (see Figure 3b).

e t c h i n g rates across a 100 m m substrate. The system p e r f o r m a n c e has b e e n c h a r a c t e r i z e d for a GaAs/A1GaAs selective e t c h i n g p r o c e s s using a BC13 / SF6 c h e m i s t r y a n d selectivities are in excess of 150:1.

Author contact details:

Summary

Unaxis USA, Inc.,

In this article a low damage, selective GaAs

10050 16th St N.

e t c h i n g p r o c e s s using a BCI 3 / SF6 c h e m i s t r y in

St. P e t e r s b u r g ,

a c o m m e r c i a l l y available p u l s e d 40.68 MHz PIE

FL 3 3 7 1 6

Process

Low Damage

High Selectivity

Etching Rate Control

Profile Control

Critical Dimension

Wet

good

good

fair

poor

poor

13,56 MHz RIE

poor

good

fair

good

good

ICP

good

fair

poor

good

good

ECR

good

fair

poor

good

good

Pulsed 40.68 MHz RIE

good

good

good

good

good

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