ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

The Study of Etching Selectivity between InGaAs andAlGaAs in Acid Based Etching SolutionLee Hock Guan, Mohd Sazli Jusoh, Asban Dolah, Ashaari Yusof,Mohamed Razman Yahya, *Burhanuddin Yeop Majlis, MIEEE

Microelectronics UnitTelekom Research & Development Sdn Bhd

Idea Tower, UPM-MTDC, Technology Incubation Center One,Lebuh Silikon, 43400 Serdang, Selangor.

E-mail: lee@mdtm.net.my

*Institute of Microengineering and Nanoelectronics(IMEN)Universiti Kebangsaan Malaysia43600 UKM Bangi, Selangor.

Abstract The etching selectivity of AlGaAs andInGaAs was studied and analyzed. Differentchemical solutions and compositions werevaried to study the etch rate effect betweenthese two materials. The etching rates andselectivity of the H3P04:H202:H20,H2S04:H202:H20 and C6H807:H202 iscompared. The results show that theC6H807:H202 chemical solution exhibitshigher etch rate and significant selectivity incomparison with H3P04:H202:H20,H2SO4:H202:H20.

I. INTRODUCTION

High speed semiconductor devices are theessential components of digital computers,telecommunication systems, optoelectronics andadvanced electronics systems as they can handleanalog and digital signals at high frequencies andhigh bits rates. The design and development ofthese devices are vital to the continued growth ofthe high tech industries. In the processadvancements in III-V compound semiconductor,conventional wet chemical etching is still widelyused due to its cost and simple operationalmethod.

There are two common methods of wet chemicaletching GaAs p-HEMT channel and gate recess.One technique is manual iterative etching to adesired current target. This technique is timeconsuming and requires highly skilled operatorto get accurate results. The other techniqueselectivity etches the recess to an AlAs etch-stoplayer in a single etch. Uniformity as well asselectivity to the stop layer are critical to a

successful etch stop process [1]. A typical p-HEMT structure is shown in Figure I to illustratethe different material layers in a Ill-V compoundsemiconductor devices.

Figure I Typical HEMT structure

There are two categories of wet etch:nonelectrolytic and electrolytic. Nonelectrolyticetching rates can be either diffusion limited orchemical reaction limited. For diffusion limitedetches the rate is controlled by mass transport ofreactants to the surface or product from thesurface. These etches is relatively not sensitive totemperature but highly sensitive to changes in thenature and degree of agitation. For chemicalreaction limited etches the rate is controlled bychemical reactions at the GaAs surface. Theseetches tend to be anisotropic with respect tocertain crystallographic orientations, quitesensitive to temperature and relatively insensitiveto agitation. Although wet etching is not easilyapplied to large scale manufacturing, its lack ofsubsurface defect formation and relative ease ofimplementation for laboratory and small scalemanufacturing makes it a popular etchingtechnique. The basic operation of nearly all the

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

widely used wet etchants for Ill-Vsemiconductor materials relies on the principle ofoxidation followed by reduction and removal ofsurface material. For this reason, many etchantsconsist of two or more chemical components,with typically one component serving as anoxidizer and the other as a reducing orcomplexing agent for the removal of the oxidespecies. As this is an inherently electrochemicaleffect, with the charge transfer between speciesin the solution and atoms on the semiconductorsurface occurring in both oxidation and reductionstages, variants of the wet etching such as anodicand cathodic etching, as well as photochemicaletching can be applied and controlled through thedirect manipulation of the charge carrier flux [2,3].

11. EXPERIMENT

The epitaxy layers of AlGaAs and InGaAs weregrown in Molecular Beam Epitaxy (MBE). Thewafer was diced into several small samples withthe size of about 1.50 cm2. These samples werethen underwent photolithography process totransfer desired patterns as the etching patterns.The process parameters of the photolithographyare summarized in Figure 2. The height of theresist pattems were measured using TencorSurface Profiler. The average thickness of theresist patterns are around 2.Opm.

The chemicals mixture is done according to theratio set for each chemicals composition. Thechemicals composition consists of three parts; anoxidizing agent, a complexing agent and adilutant. The oxidizing and dilutant agents areusually Hydrogen Peroxide (H202) and water(H20).

Process Equipments SettingsParameters

Ist Spin 700 pmResist Spin Time 5 secCoating Coater 2nd Spin 3000 rpm

Time 25 sec

Soft Bake Oven Temp 100°CTime 90 sec

Exposure Mask Time 30 secAligner

Developing Developer Temp 20°CTime IO sec

Hard Bake Oven Temp 100'CTime 90 sec

Figure 2 Summary of the photolithography process ingate recess etching.

Phosphoric acid (H3PO4), Sulfuric acid (H2SO4)and Citric Acid (C6HsO7) were chosen as thecomplexing agent which is the etchant in thisstudy. Different chemicals ratio (by volume)were set among these chemicals to determine theetch rate in the etching of these materials. Theratios are 1:1:25, 1:1:50, 1:2:50, 2:1:50, 1:1:250and 1:1:500 for H3PO4 and H2SO4. However, forCitric acid, it was mixed with water as it is inpower form. One gram of anhydrous C6H807 ismixed with one milliliters of H20. The volume ofthe mixture was then varying accordingly withthe ratio of H202.

The samples were then dipped into the desiredmixture of chemicals and a process timer is setfor one minute. After the set time, the sampleswere rinsed under running deionized water (DI)for 20 seconds. The etching solutionstemperature was maintained at 25'C throughoutthe process. The samples were blown dry using aNitrogen blower and measured to determine thethickness that been etched.

III. RESULTS AND DISCUSSION

The materials growth parameters using the MBEsystem is shown in Figure 3. These parametersare very important as changes in the growthparameters will affect the etching profile of thematerials. The crystallographic plane for thelayers is at <100>.

Samples ID Type Parameters040708 #5 lnGaAs = 0.19040525 #3 AIGaAs X = 0.27

N(Sheet)=04051 1_#1 GGaAs;Si 4.2E+ 1 30451# N(Bulk)=

7.OE+18Figure 3 MBE growth parameters of the samples

used.

The difference in the height of the patternsbetween the initial measurements and afteretching measurements were taken and dividedover the etching time in order to get the etch rateof each chemical system. The etch rates of thematerials were summarized in Figure 4.

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Chemiical H3P04:H202:1H20Ratio AlGaAs InGaAs1:1:25 52.50 43.641:1:50 26.25 13.961:250 80.20 28.672:1:50 - 19.75 40.461:1:250 4.55 3.151:1:500 1.55 4.40_ ~~~(a)Chemical H2S04:H202:H20

Ratio AlGaAs InGaAs1:1:25 49.75 47.851:1-:50 21.68 22.351:2:50 34.14 38.352:1:50 110.10 93.751:1:250 1.77 1.811:1:500 2.28 2.29

(b)Chemical £:6-.H -8:H'2QZRato GaAs anlnGaAs i

a) H 5.20 22.351 ¢2 5.12 12.342-1 5.40 31.00i3:1 8.00 46.50

4:1 5.35- 64.00(c)

Figure 4 Etch Rates ofAlGaAs and InGaAs in(a) H3PO4, (b) H2SO4 and (c) C6H807.

The etch rates of AlGaAs and InGaAs indifferent chemical systems and compositionswere further illustrated in Figure 5, 6 and 7respectively. From the graphs, the etch rate forthese two materials is similar in H3PO4: H202:H20 solution. Thus, it is not suitable to be usedin material selectivity etching for AlGaAs andInGaAs. However in the C6H807:H202 solution,it is clearly observed the etching rates differencein these two materials.

Elch Rate, Afs Matietas Etch Rate in I-PO4:OK2090.0'80.0

70.0

60.0

50.0

400 A

30.0

20.0

10.0

.0

From Figure 5, the etch rates pattern for AlGaAsand InGaAs in H3PO4: H202: H20 solution isslightly different for these two materials. Theetch rate of AlGaAs is generally higher compareto InGaAs except in the ratio of 2:1:50 and1:1:500. On the other hand, the etch rates patternin H2SO4: H202: H20 solution (Figure 6) forthese two materials is similar where AlGaAsexhibits higher etch rates as compare to InGaAs.Comparatively, the etch rates for these twomaterials in H2SO4: H202: H20 solution is higherthan H3PO4: H202: H20 solution.

,Ns Mateas Etch Rae in IS04:H202:H

0 1 2 3 4 5 6 7I__.ASa 4 Cherical Patio

Figure 6 Etch rates of AlGaAs and InGaAs in H2SO4

60.0

50.0

40.0

30.0

20.0

10.0

Na%to, Ats NMials Ekch RteIn C6sKt,:Ih02

0 1 2 3 4 5 6AGaks + CAsherrical Paio

Figure 7 Etch rates of AlGaAs and InGaAs in C6H807

In the etching of the Ill-V compoundsemiconductor devices, the etch stop layer iswhere the etching of that particular layer issignificantly slower than other layers leading tovery accurate control of etch depth. Therefore,the selectivity etching among the Ill-V materialsis very important. Besides the etching selectivity,the process window for the etching process mustbe wide to reduce both human error andsystematic error. The etching selectivity ofAlGaAs and InGaAs in different chemicals is

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0 1 2 3 4 5 6 7+ AlGaAs * IraAs Chenical Rtio

Figure 5 Etch rates of AlGaAs and InGaAs in H3PO4

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

shown in Figure 8, 9 and 10 respectively. It isnoticeably that the selectivity between AlGaAsand InGaAs is very close in H2SO4: H202: H20solution. Hence it is not suitable to be used in aselectivity etching process. As for the H3PO4:H202: H20 solution, there are certain chemicalratios that can be used in selective etchingbetween AlGaAs and InGaAs. Among thesethree chemical solutions from this study,C6H807:H202 solution demonstrates significantetching selectivity between these two materials.The etch rate is increasing when the C6H807 ratioto H202 increased and slower if the H202 portionis increased. This can be seen by comparing theetch rate for the ratio of 1: 1, 1:2 and 2:1respectively.

Selectivity l ls EWhingSdeciin-PO H *OAEo3.00 . . .

250

200 /

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0.50

a

i 0 1afx, ; X~~~~~~~~~~~~~~~~~~IL"""" :I

I11

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0 1 2 3 4 5 6 7

LAGa4s k OicalPmQihtuoFigure 8 Etching selectivity of H3P04: H202: H20

in AIGaAs and InGaAs

sOle O* MlAO&EchingSelefin :M.H&1.40.

1.30

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-.AIc ..4- I Qun6cal MwOFigure 10 Etching selectivity of C6H807:H202

in AlGaAs and InGaAs

IV. CONCLUSION

The etching selectivity between AlGaAs andInGaAs in different chemicals is studied andanalyzed in the paper. It was found that theC6H807:H202 exhibits significant selectivitybetween these two materials, while in the H3P04:H202: H20 chemical solution, the etchingselectivity of the materials is depending on thechemical ratios. Although H2SO4: H202: H20chemical solution shows higher etch ratecompare to H3PO4: H202: H20, the etchingselectivity is very poor in this solution.

REFERENCE

[1] Frank Spooner, William Quinn, LarryHanes, Sarah Woolsey, Kim Smith, JerryMason, A Reproducible, High YieldRobust Wet Etch Etch Stop - StopProcess Using Organic Acid - PeroxideSolutions, Skyworks Solutions, Inc.Technical Report.

[2] Sorab K. Ghandhi, VLSI FabricationPrinciples - Silicon and GalliumArsenide, pg. 593-61 1, John Wiley &Sons (1994).

[3] C.Y. Chang and Francis Kai, GaAs HighSpeed Devices - Physics, Technologyand Circuit Application, pg. 119-123,John Wiley & Sons (1994).

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0 1 2 3 4 5 6 7-+-A]GaAs InGAM OtdcalI1-A l!--l

Figure 9 Etching selectivity of H2S04: H202: H20in AlGaAs and InGaAs

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