doi: 10.1149/1.21317051978, Volume 125, Issue 9, Pages 1510-1514.J. Electrochem. Soc. 

 Yoshifumi Mori and Naozo Watanabe 

?O? , for GaAs and Its Kinetics2??H?−?2?O?2??H?−?4?PO?3A New Etching Solution System, ?H?

serviceEmail alerting

  click herethe box at the top right corner of the article or Receive free email alerts when new articles cite this article - sign up in

http://jes.ecsdl.org/subscriptions go to: Journal of The Electrochemical SocietyTo subscribe to

© 1978 ECS - The Electrochemical Society

A New Etching Solution System, H PO,-H O -H O,

for GaAs and Its Kinetics Yoshifumi Mori and Naozo Watanabe

Sony Corporation Research Center, 174 Fu~itsuka-cho, Hodogaya-ku, Yokohama 240, Japan

ABSTRACT

A new solution system consisting of H3PO4, H202, and H20 was found useful for etching GaAs wafers. This solution system can be divided into four regions a-d, according to etching characteristics. The boundaries be- tween the regions are given by a rfiole ratio (m �9 r) of H202 to HsPO4 of about 2.3 and a mole fraction (m �9 5) of H20 of about 0.9 at room temperature. Rate- l imit ing processes are: a, adsorption of H202 ( m . r <_ 2.3, m . ] ~ > 0.9)', b,

diffusion of H202 (m �9 r <~ 2.3, m �9 ] < 0.9) ; c, dissolution of oxidized products

( m - r > 2.3, m - ] <_ 0.9); and d, adsorption of H~PO~ ( ~ - r _ > 2.3,

m . ] > 0.9). Solutions in region a have a reproducible etching rate of 0.01- 0.1 ~m/min, which is useful for MESFET processing. Crystallographic etching is also available with solutions in region c.

In the fabrication of microwave GaAs devices with sufficient rel iabi l i ty and reproducibili ty, the thickness of the active layer must be controlled. Depth profiles of impur i ty concentrat ions and mobil i ty must be mea- sured accurately to correlate device characteristics with mater ia l properties. An etching technology using electrochemical reactions such as an anodic oxidation, followed by dissolving the oxide (1-3) or an anodic dissolution (4) has been proven a useful way for ac- curate GaAs dissolution. An etching technology using chemical reactions with sufficient accuracy and repro- ducibil i ty however, has not yet been established.

GaAs field-effect transistors are usual ly fabricated by a lift-off and self-align techniques (5) in which the gate regions of the active layer are etched ac- curately through windows in protective masks. For this purpose, the chemical etching is more favorable technology than the electrochemical etching because the former way does not need ohmic contacts. How- ever, in the selective etching through windows in protective masks, an anomaly of etching rate such as enhanced etching rate near the edge of the masks (6) is undesirable.

We have found that etching rates of less than 0.1 #m/ra in without this anomaly are reproducibly a t - t a i n a b l e using a new solution consisting of H~PO4, H202, and H20 within a specific composition range. The etching rate is reproducible to wi thin an accuracy of several tens of angstroms.

There is a specific composition range in which the etching rate depends strongly on crystallographic ori- entation. We report in this paper the etching charac- teristics of GaAs in this new solution and its etching kinetics.

Exper imenta l The solution was prepared by mixing 85 w e i g h t

percent (w/o) H~PO4, 30 w/o H202, and deionized water. The etching studies were general ly performed on Te-doped n - type (100), (110), ( l l l ) A , and ( I l l ) B - oriented GaAs wafers with net donor concentrat ion of ~10 ~s cm -~. Etching rate was not affected by all kinds of dopants and by conductivities of wafers. Photoresist AZ-1350 and sputtered SiO2 films were used as masks for selective etching. Etching was carried out in a tem- pera ture range of 0~176 without s t i r r ing and i l - lumina t ion with temperature control of __.0.5~ The etched depth was measured from a step height be- tween etched and unetched surfaces using an in ter fer - ence microscope.

Key words: etching, gallium arsenic, phosphoric acid, kinetics.

Results Compositio~ dependence o~ etching rate.--Figure 1

is a t e rnary diagram of the exper imental results on a (100) surface obtained at 30~ The basic components in the three corners in this figure correspond to H20, 85 w/o H~PO4, and 30 w/o H202. Concentrat ions are expressed by volume percent. The numbers wi thin the t r iangle of the figure refer to etching rates in ~m/min. For a given i~20 content, a ma x i mum rate of etching is at tained for a mole ratio of H202 to t-I~PO4 (re.r) of about 2.3. The t e rnary diagram of the etching ra te can roughly be divided into four regions a, b, c, and d corresponding to specific etching characteristics. The etched depth depends l inear ly on etching t ime for various surface orientations in regions a, c, and d, as shown in Fig. 2. The rate in region b, on the other hand, depends roughly l inear ly on the square root of etching time. The etching rate in region b is considered to be l imited by the diffusion of I-I202 in th e so lut ion , The increase in the viscosity for increasing the HaPO4 concentrat ion supports this mechanism (1, 2). Figure 3 shows the dependence of the etching rate on the c o n -

% ~" "A-iY - ~ k T ~ r " ~ ' \ ~ " e O .o ./\, .-

i ) b t/ \i I' I I I

/ \ / \l i /IX ! VXx. \,X'xXX i I l i / \ l / I I / I _ \ I "(:)#.~.

100 86 ) 40 20 o ~0 �9 , t % 30 C

85'~I~/oH3PO 4 H 3 P O 4 v o . ( 1 0 0 )

Fig. 1. Dashed lines are constant etching rate contours on (100) surface as a function of the composition of the solution. The num- bers represent etching rates in p.m/min obtained at 30~ Solid lines divide ternary diagram in 4 regions: a, b, c, and d.

1510

Vol. 125, No. 9 E T C H I N G S O L U T I O N S Y S T E M 1511

/

a(3:1:50)

3 //~7 . /

0 20 40 60

E 2 =L

1

"o

1D r-16 u

- i - J hJ

12

_ b ( 7 : 3 : 3) ,$%~

I I ~ 0 4 8 12

Etch ing

o ( 1 0 0 ) , �9 (110) ,

4 C1(1: 9 : 21/0/)..~/

, / // (

I I I 0 20 40 60

0 4 8 12 Time (min. ) 24 ~

A(111)A , A(111)B

Fig. 2. Relation between etched depth and etching time for vari- ous surfaces. Composition of each region is expressed by volume ratio of 85 w/o HsPO4 to 30 w/o H202 to H20.

E

..~ 1.0

S

~0.1 x: u w

I

0.01

H3PO 4 concentrat ion (mol /o) 0.1 1.0 10.0

I t I I I I 1 I I I I I

/ ] f l ~ j ~ O I I t I I I I

0.1 t.0 10. 0

H202 concen t ra t i on

I I I

30~ [ i

( mol Io )

Fig. 3. Composition dependence of etching rate on the concert- tration of H202, for regions a and b and of H3PO4 for regions c and d. Open circles correspond to [H202/H3PO4] mole ratio (rn �9 r) _-- 9.1 and closed circles and triangles correspond to 1.02 and 0.43, respectively. Concentrations are expressed by mole per- cent~.

centra t ions of minor i ty reactants , H202 and I-~PO4, in solut ions ob ta ined on the (100) surface, i.e., regions a and b, and c and d, respect ively. Concentrat ions are expressed by mole percent . The abscissa is scaled b y the percentages of minor i ty reactants , tha t is, H202 in regions a and b, and HsPO4 in regions c and d.

Data in region a l ie on an ident ica l s t ra igh t l ine for two series of solut ion having different m.r's, but they differ in region b. This fact indicates tha t the e tching ra te in region a is de te rmined b y and is p ropor t iona l

to H202 (minor i ty reac tant ) concentrat ion. A fiat- bot tomed hole is a lways obtained. In region d, the ra te is p ropor t iona l to the square of H3PO4 concentrat ion. The etched surface f requen t ly becomes cloudy due to format ion of an oxidized film wi th smal l pits. The ra te tends to sa tura te in region c. V-shaped and r e - verse mesa - shaped holes are c lear ly observed, as wi l l be shown later .

Temperature dependence of the etching rate.--Tem- pera tu re dependences of the e tching ra te for var ious surface or ientat ions in each region are shown in Fig. 4. Values of ac t iva t ion energies a re also given in the figure. The act ivat ion energy is the same in regions a and d and takes a va lue of about 10 kca l /mole , i nde - penden t of surface orientat ion. In regions b and c, however, i t depends on the surface or ientat ion. These resul t s suggest tha t the r a t e - d e t e r m i n i n g process is different in each region, as is discussed later .

Slow and reproducible etching.--Stable etching is a t - t a inab le using the solut ion in region a. The etching ra te in this region is independen t of H~PO4 concentra t ion and of etching time. The ac t iva t ion energy is i nde - pendent of surface orientat ion. F igure 5 shows changes of etching ra tes on repea ted use of etching solutions. Ver t ica l scales re fer to the etching ra te normal ized to the ini t ia l rate. In the H2SO4-H202-H20 system, the e tching ra te was not s table a f te r about 100 min f rom its prepara t ion . I t sometimes increased by 50% of the in i t ia l va lue and then decreased g r adua l l y a f te rward . F luc tua t ion of the etching ra te for r epea ted use is the la rges t in this system. The ra te decreased g r adua l l y in the KOH-H202-H20 system. Etching was not a t t a ina - b le af ter a s torage t ime of a few months in H2SO4 and KOH systems. In the H3PO4-H202-H20 system, on the other hand, a s low etching ra te wi th excel lent r eproduc ib i l i ty was ma in ta ined over severa l months. In the H2SO4 system, hea t evolves upon mix ing the reactants and water . The solut ion is essent ia l ly un- stable. Bubbles a re genera ted by decomposi t ion o f H202 on the surface of a GaAs wafer whi le etching in the KOH system. The concentra t ion of H202 changes wi th r epea ted use. The mix ing of reac tan t s is non- exo thermic in the H3PO4 system. H~O2 is not exces-

_,~, a ( 3 : 1 : 5 0 )

- - ' ~ 0.1

!o,,0o, \% Ea = 10.0 Kco.l/m.~ " " ~

e (110),- 10.0 ",,, ~,~, �9 ~(111)A;-'10.0 \,. \';\

rr ~ �9 B'19"6 I `-~ I 3.2 3.4 3.6

b ( 7 : 3 : 3 ) ~ 1.C 10

o -5.7 "

zx -9.6 �9 - 4 . 9 \ A

d ( 1 : 9 : 2 1 0 )

~10.0

I b I _ \ 3.2 3.4 3.6

- ~ ' I k c ( 1 : 9 : 1 )

%-69 \ - �9 - 6 . 7 - - , \ _ z~-10.0 \

�9 - 6 . 2 . k ~

I I I \ 1 [ I I I z2 3.4 3.6 (lO r) 3.2 3.4 3.6 (lO r)

Fig, 4. Temperature dependence of etching rote on various sur- face orientations.

1512 J. Electrochem. Soc.: S O L I D - S T A T E SCIENCE AND TECHNOLOGY S e p t e m b e r 1978

% LLI

IL l

(1,

r r

"(3 0., N

E O Z

1.1

1.C

0 .9 - i 1.1 / ~ . ~/

1.1 ! - -

~ 85%H3P04:30%H202 :H20=(3i!:52

E.Ro = 0.080 (l,~m/min.)

95%H2SO 4 : 30"%H202 : H20 = 3:1 : 50

E.R 0 = 0 .066

10"/o KOH : 30W/oH202 =10:1 0 . 9 - -

\ E.Ro = 0.121

10 102 103 Etching Time (rain.)

Fig. 5. Change of etching rates on repeated use of etching so- lutions obtained at 20~C. Every time etching was done for 5 min.

sively consumed in the etching reaction (no bubbles were observed). This is one reason why the I-I3PO4 system is the most stable of the three solutions.

Crystallographic etching.--The ratios of etching rates on the ( l l l ) B to ( l l l ) A surfaces as a function of I-I202 concentrat ion at three different temperatures are shown in Fig. 6. Tempera ture dependence of this ratio is larger for larger H20~ concentrations. This is a result of the difference in the temperature dependences of the etching rate for different surface orientations in each region (see Fig. 4). Crystallographic etching can be performed by control of tempera ture and of solu- t ion composition in region c. Figure 7 shows pictures of the (011) cleavage plane of the selectively etched (100) surface using a photoresist mask (AZ-1350). The upper pictures are for the case where m.r is 3.05 and the mole fraction of H20 (re.f) is 0.89. The lower pictures are for the cases of 9.15 and 0.83, re- spectively. Correlation between the etched shape and the etched depth in the case of (011) cleavage plane can be seen in Fig. 8. The etching tempera ture and etching time (t) are given in each photograph.

Discussion We formulated a rate equat ion for GaAs etching to

determine a ra te- l imi t ing process in each region. The etching process was assumed to consist of the following

-- /4} =Q

o

B ~ he"

r u

UJ tO

i

I I I I I 1 1 1

E-

1 I _,- -_OoC_

! / I I #

7

[ ] l i t I 10 -3 10-2

1"~02 M o l e F r ( z c t i o n

.," _;cca~_24o C. // / 1:18:1

/ 1:9:1 "] " ~ , % ? : $ : $

"~] . . . . 40oC_

1 1 1 1 I ] 10-1

Fig. 6. Ratio of etching rate an (111)B to (111)A surface as a function of mole fraction of H20~. Volume ratios of 85 w/o H3P04, 30 w/o H202 to H20 are shown in the figure.

five steps: (i) adsorption of m molecules of H202 on an active center; (ii) formation of oxidized products; (iii) adsorption of n molecules of I-I~PO4 on oxidized product; (iv) dissolution of oxidized products; and (v) desorption of dissolved complex into solution.

The reaction processes are assumed to proceed a t specific active centers on a GaAs surface. The active centers are in different states as the reaction proceeds through the above-ment ioned five steps. The total number of active centers in these states is assumed to be constant. Solving the simultaneous rate equations for the five reaction steps, the etching rate, R, under a steady state is obtained as follows

1/R = 1/kl[H202] m +1/k2[HsP04] n + 1/k3 [1]

where kl and k2 are rate coefficients for the steps (i) and (iii), and 1/k3 is the summat ion of inverse rate coefficients of each of the other steps. These parameters include temperature, activation energy, part i t ion func- tions of the activated species, and the reaction fre- quency factor. For di luted solutions, the k3 term can be neglected. Exponents m and n can be determined from exper imental data of the concentrat ion depen- dence of etching rate, as was shown in Fig. 3

m _ l . 0

n ~ 2 . 0

Equat ion [1] is then reduced to the following simple relat ion

[H202] [H~PO4] 2 R _~ [2]

[H~O2]/k2 + [H~PO4] 2/kz

This equation corresponds to the case where the ad- sorption steps of reactants, (i) and (iii), are rate limiting. Equat ion [2] is reduced to the relation, R kl[H202], when [H202] < < [H~PO4]. This corre- sponds to the rate equation for the H202 adsorption process. R is equal to k2[H~PO4] 2 when [H202] > > [H~PO4], which corresponds to the rate of H3PO4 ad- sorption. The HsPO4 adsorption is bimolecular.

Curve fitting was carried out using Eq. [2], which is shown in ~'ig. 9. Concentrat ions in this ternary diagram are expressed by mole percent. Solid and dashed lines represent calculated and experimental results. The calculated results in most parts of regions a, d, and a par t of region c agree well with the ex- per imental results. The etching rate in these regions can be considered to be l imited by the adsorption processes. Agreement is not good in region b and in the concentrated portion of region c. The ra te- l imi t ing process in region b was presumed to be the diffusion of H202 in the solution. Therefore, the disagreement is a na tura l consequence. Disagreement in region c may be caused by the contr ibut ion of other surface reaction processes such as dissolution of oxidized prod- ucts. It was concluded that the ra te - l imi t ing processes in regions a to d are the adsorption of H202 on the active centers of GaAs surface, diffusion of H202 in solution, dissolution of oxidized products, and the ad- sorption of H3PO4 on the oxidized products, respec- tively. The results are summarized in Table I.

A general concept for GaAs etching is given through the above study. The etching rate is determined by effective densities of reactants and of active centers on the GaAs surface, when the ra te - l imi t ing process is the adsorption of a reactant in diluted solutions. The temperature dependence of the etching rate on various surface orientations must be equal. In con- centrated solutions, the ra te- l imi t ing process is the formation and/or dissolution of oxidized products. The etching rate is affected by the surface activities of GaAs. Temperature dependence of etching rate is different on various orientations.

Summary A newly found solution, which consists of I-I3PO4,

H202, and H20, is presented as a GaAs etchant. This

VoL 125, No. 9 ETCH I N G SOLUTION SYSTEM 1513

Fig. 7. Pictures of (011) cleav- age plane of selectively etched (100) surface in the solution in region c.

Fig. 8. Correlation between the etched shape and etched depth. Pictures are (01T) cleav- age plane.

solution system was divided into four regions a-d. The boundaries be tween the regions are given by a mole ratio of H202 to H3PO4 0 n ' r ) of about 2.3 and a mole fraction of H20 (re.S) of about 0.9 at room temperature. The activation energy in diluted solu- tions: regions a, ~r~.r ~< 2.3 and m'5 >~ 0.9; and d, m.r ~ 2.3 and m'5 >~ 0.9, is independent of crystallo- graphic orientation. The ra te - l imi t ing process is deter- mined as the adsorption of minor i ty reactant in a solu-

tion, that is H202 in region a and H3PO4 in region d. We confirmed that the solutions in region a were useful for slow, stable, and accurate GaAs etching.

The ra te - l imi t ing process is the surface reaction, such as dissolution of oxidized products in concen- trated solutions: in region c, m . r >~ 2.3 and m-] <~ 0.9. A solution composition in this region c is useful for crystallographic etching, The etching rate is affected by the surface activities.

Table I. Summary of the experiments in Fig. 7 and 8

Region a b e d

>2.3 H~O2/HsPO~ ~2.3 ~2.3 > 2.3 Mole ratio H~O >0.9 ~0.9 ~0.9 ~0 .9 Mole frac. Depth vs. time ~t ~t:/2 ~t oct

Ea (surf. orient.) Ind. Dep. Dep. Ind. Rate-limiting process H~O~ adsorption H~O~ diffusion Surface reaction H~PO, adsorption

Useful for Accurate reproduc 4 Sub. surface Preferential etching m ible slow etch treatment

1514 J. Electrochem. Soc.: S O L I D - S T A T E SCIENCE AND TECHNOLOGY September 19 78

H20

/ % / l \

I l l / t o.7Z,.6 3. I e

30 ~ (IO0)

Fig. 9. Comparison between the calculated (solid lines) and the experimental (dashed lines) results on etching rate as a function of composition of solution. Compositions ore expressed by mole percent. (Curve fitting was carried out using Eq. [2].)

Acknowledgment We wish to thank Miss I. Sugawara for her ass i s t -

a n c e throughout these experiments. We also e x p r e s s our appreciation to Mr. T. Yamada, Dr. K. Morizane, and Messrs. M. Inoue and I-L Kawai for their va luable discuss ion .

Manuscript submit ted Sept. 12, 1977; revised manu- script received April 3, 1978. This was Paper 90 pre- sented at the Philadelphia, Pennsylvania , Meeting of the Society, May 8-13, 1977.

Any discussion of this paper will appear in a Discus- sion Section to be published in the June 1979 JOURNAL. All discussions for the June 1979 Discussion Section should be submit ted by Feb, 1, 1979.

Publication costs o] this article were assisted by Sony Corporation.

REFERENCES 1. R. A. Logan, B. Schwartz, and W. J. Sundburg, This

Journal, 120, 1385 (1973). 2. D. L. Rode, B. Schwartz, and J. V. DiLorenzo,

Solid-State Electron., 17, 1119 (1974). 3. J. C. Verplanke and R. P. Tijburg, This Journal,

124, 802 (1977). 4. T. Ambridge and M. M. Faktor, Inst. Phys. Conf.,

Ser. 24, 320 (1975). 5. C. A. Liechti, IEEE Trans. Microwave Theory

Tech., mtt-24, 279 (1976). 6. D. W. Shaw, This Journal, 113, 958 (1966). 7. S. Iida and K. Ito, ibid., 118, 768 (1971).

Improved Theoretical Predictions for the Steam

Oxidation of Silicon at any Elevation

Don L. Kendall ,* Marcelo Garcla C., and Mariano Aceves

Instituto NacionaI de Astrof~sica, Optica y ElectrSnica, Pueb~a, Pueb~a, Mexico

ABSTRACT

The steam oxidation of {111} Si at an altitude of 2160m is studied be- tween 800 ~ and 1200~ The oxidation time to obtain a given thickness is s'hown to be 30% longer than that required near sea level. The data are in general agreement with the theoretical formulation of Deal and Grove (1) who s'howed that the oxide thickness X in #m could be related to the time of oxidation t in minutes by the expression X 2 -{- AX -~ Bt for oxidation in steam. It is found that this expression is valid over a wide range of tempera- ture and pressure for values of A ~ 6.2 • 10 -6 exp (1.16 eV/kT) and B = 6.7(P/760) exp (--0.776 eV/kT), where A is in ~m, B is in ~m2/min, and P is in Torr. These values lead to thickness corrections of about 10% at 900~ and 6% at I000~ as compared to the original formulation of Deal and Grove.

The work of Deal and Grove (1) on the oxidation of silicon in dry and wet oxygen is widely accepted as the s tandard at atmospheric pressure at sea level, namely 760 Tort. We have repeated these experiments over a wide range of temperatures for steam oxidation at an al t i tude of 2160m where the atmospheric pres- sure is only 586 Tort.

As expected, the same type of behavior is ob- served in these experiments as in those of Deal and Grove; namely the thickness is proport ional to time at short times and to the square root of t ime at long times. However, by obtaining data over a wide range of temperatures, we show that the mathematical pre- dictions of Deal and Grove are not precise enough to allow extrapolat ion to temperatures below about IO50~

In view of the importance of obtaining precise thickness estimates over a wide range of temperatures,

Key words: silicon, steam oxidation, reduced pressure, process control.

for example, for computer-aided process simulations, we have calculated new activation energies and pre- exponentials for the steam oxidation of silicon. By suitable ad jus tment of the pre-exponentials , these expressions can be used at any al t i tude and with any value of atmospheric pressure.

Experimental Chemically polished silicon slices grown by the

Czochralski method were used in these experiments. They were n-type, the resistivity was 5 ~-cm, and the orientat ion was {111}.

The silicon slices were placed horizontal ly on a quartz boat dur ing the oxidation. Steam was pro- vided to the slices by boiling deionized water. Con- densation in the connecting tubing was avoided by wrapping the tubing with heat ing tape. The labora- tory is in Tonantzint la , Puebla, near the city of Puebla, Mexico, where the al t i tude is approximately 2160m. The atmospheric pressure at this al t i tude ]s 586 Tort.