Baca A.G., Ashby C.I.H. Fabrication of GaAs Devices

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Wet etching and photolithography of GaAs and related alloys

n-typep-type

FIGURE 4.12 VBM for n-type and p-type with same Fermi energy.

of the GaAs since n-type and p-type materials can exhibit differ-

ent electrochemical potentials in contact with the same solution

(FIGURE 4.12). For a ﬁxed position of the Fermi level, it is clear

that the potential of the valence band at the surface is different.

For example, the effect of dopant type can be seen in the etching

of n-GaAs and p-GaAs by 0.3N NH

OH/0.1N H

where room

temperature rates for n-type are about 0.19 μm/min while p-type

etches at about 0.13 μm/min.

Exposure to bright light while in a wet etchant can produce pho-

togenerated electron-hole pairs. The holes in the valence band of

n-GaAs move to the surface due to the surfaceﬁeldandcanenhance

the etch rate (photochemical etching.) The photons effectively play

the same role as the chemical oxidant, such as H

. Illumination

of p-type GaAs has a minimal effect since its surface band bending

is in the opposite direction of that in n-type GaAs.

Additional doping-dependent differences can occur when a

metal is in close proximity and exposed to the etching solu-

tion. Local electrochemical effects can lead to unexpected etching

proﬁles with either enhanced or suppressed etching due to the

completion of a local galvanic cell; this is discussed further in

Section 4.7.

4.7 ELECTROLYTIC EFFECTS IN WET ETCHING

Electrochemical etching is often considered in terms of the driving

of a chemical reaction by the deliberate application of an applied

bias from an external voltage source. In the fabrication of GaAs

devices, this type of deliberate electrochemical etching does not

play a major role and will not be discussed here. Excellent books

dealing with this topic exist [10], and the reader is encouraged to

consult them should this topic be of interest.

However, unintentional electrochemical etching can occur when

a galvanic circuit is completed by the unintentional exposure of

142

Wet etching and photolithography of GaAs and related alloys

GeAuNi GeAuNi

FIGURE 4.13 Effect of etching too close to exposed contact metal in FET.

GeAu Ni

FIGURE 4.14 Rapid undercutting when approaching pn juction in HBT if

metal contact exposed to etching solution.

contact metal to the etching solution. Three examples where this

might occur are discussed here.

The ﬁrst two cases may occur when a portion of the ohmic metal

is not isolated from the etching solution with photoresist. The

dotted lines in FIGURE 4.13 represent the proﬁle of the etching

surface as the etch proceeds. When the surface proﬁle reaches

the point where separation between the etching solution and the

metal contact is of the order of 0.25 μm, a galvanic circuit through

the solution, semiconductor and metal is completed and the etch

rate rapidly and uncontrollably accelerates. This leads to rapid

trenching into the substrate.

The second example where exposed ohmic metal can cause

accelerated etching is shown in FIGURE 4.14. As the etch front

approaches the pn junction, the lateral etch rate under the GeAuNi

metal, which is normally equal to the vertical etch rate into the sub-

strate, undergoes rapid acceleration. This leads to uncontrollable

undercutting. This problem can be avoided by using a different

metallisation, such as WSi, instead of GeAuNi.

The third example is more insidious since the sample doesn’t

have to be in a solution that one would expect to be an etchant; it

occurs in DI water. A local galvanic reaction can occur where

143

Wet etching and photolithography of GaAs and related alloys

GeAuNi GeAuNi

FIGURE 4.15 Trenching in DI water with exposed metal contacts.

GeAuNi contacts the semiconductor, leading to trenching, as

shown in FIGURE 4.15. The depth of the trenching increases

with time, so it is very important to minimise rinse time in DI

water. Trenching can also be reduced by deoxygenating the water

by bubbling N

through the water (gas sparging) before rinsing.

4.8 EFFECTS OF DEFECTS AND DAMAGE

Disruption of the GaAs lattice stucture by any means will, in gen-

eral, increase the ease with which the damaged area etches. This

can be due to grown-in defects, mechanical damage, e.g. scratches,

or lattice dislocations and defects, including those generated by ion

implantation. Grown-in crystal defects tend to manifest as etch pits

with characteristic shapes. Patterned implantation can be used to

selectively enhance etching in the damaged regions. This is some-

thing to keepinmind when subsequently etching wafersemploying

implant isolation.

4.9 CONCLUSION

While wet etching has been replaced by dry etching when ver-

tical proﬁles are essential, there remain many cases where wet

etching remains useful. The electronic quality of the surface is

superior to that of dry-etched surfaces, and the higher degree of

compositional selectivity than generally available with dry pro-

cesses will continue to make wet etching the process of choice for

many applications.

REFERENCES

[1] H.K. Kuiken, J.J. Kelly, P.H.L. Notten [J. Electrochem. Soc. (USA) vol.133

(1986) p.1217–26]

[2] T. Takabe, T. Yamamoto, M. Fujii, K. Kobayashi [J. Electrochem. Soc.

(USA) vol.140 (1993) p.1169–80]

[3] C.I.H. Ashby [in Properties of GaAs, Eds. M.R. Brozel, G.E. Stillman

(Institution of Electrical Engineers, London, 1995) ch.18, p.707–16]

144

Wet etching and photolithography of GaAs and related alloys

[4] C.H. Hamann, A. Hamnett, W. Vielstich [Electrochemistry (Wiley-VCH,

New York, 1998)]

[5] M. Pourbaix [Atlas of Electrochemical Equilibria in Aqueous Solutions

Translator J. Franklin (National Association of Corrosion Engineers,

1975)]

[6] [Standard Guide for Development and Use of a Galvanic Series for Pre-

dicting Galvanic Corrosion Performance, ASTM Designation: G 82–83,

Annual Book of ASTM Standards]

[7] G.C. DeSalvo, W.F. Tseng, J. Comas [J. Electrochem. Soc.(USA) vol.139

(1992) p.831–5]

[8] M. Tong, K. Nummila, A. Ketterson, I. Adesida, C. Caneau, R. Bhat

[IEEE Electron Device Lett. (USA) vol.13 (1992) p.525–7]

[9] K. Keneﬁck [Electron. Lett. (UK) vol.21 (1985) p.558–9]

[10] J. McHardy, F. Ludwig [Electrochemistry of Semiconductors and Elec-

tronics: Processes and Device (Noyes Publications, New Jersey, 1992)]

145

Chapter 5

Dry etching of GaAs and related alloys

Chapter scope p.147

Comparison of wet and dry

etching p.147

Overview of dry etching

processes p.149

Ion-beam etching (IBE) and ion

effects in other plasma

processes p.150

Chemical dry etching p.151

Plasma etching at very low ion

energies p.152

Conventional reactive ion etching

(RIE) p.153

Halogen-based plasmas for RIE p.155

Alkane-based plasmas for RIE p.159

High-density plasma etching

(HDPE) p.160

Reactive-ion-beam etching (RIBE)

and chemically assisted ion-beam

etching (CAIBE) p.163

General issues for dry etching p.167

Etch uniformity p.167

Damage from dry etching p.168

Resists and their behaviour in dry

etching processes p.170

Advantages of Ar addition p.174

Methods for end-point

determination p.174

Plasma diagnostics for

trouble-shooting p.176

Chamber cleaning issues p.176

Effect of chamber materials p.177

Conclusion p.177

References p.178

5.1 CHAPTER SCOPE

There are ﬁve important characteristics to consider in selecting

any dry etch process. These are rate, feature proﬁles, compos-

itional selectivity, damage and uniformity across the wafer. In

general, the balance between chemical and physical contributions

to the etching for each type of process will largely determine

these characteristics. Chemical contributions generally produce

rapid rates, compositional selectivity and lower damage. However,

they also can decrease anisotropy (ratio of vertical to horizontal

etch rates) and can even produce either isotropic etching or etch

rates that depend on the relative reactivity of different crystallo-

graphic facets, as observed in wet etching. Physical sputtering

contributions generally increase anisotropy, but they also damage

the surface, degrading device performance that depends on near-

surface crystal quality. Consequently, the simultaneous goals of

an etch process that is both highly anisotropic and damage free are

frequently in conﬂict and an acceptable compromise between the

two will drive the process selection for GaAs device fabrication.

In this chapter, the many practical issues that must be considered

to successfully dry etch GaAs will be discussed.

5.2 COMPARISON OF WET AND DRY ETCHING

It is possible to etch device structures using either liquid-

phase “wet” etch processes or gas-phase “dry” processes. Both

approaches have their own advantages and disadvantages and the

best choice depends on the speciﬁcs of the application. (For advant-

ages and disadvantages of dry etching, see side note in page 149.)

Wet etch processes, in general, produce better surface elec-

tronic properties than many dry etch processes because they don’t

damage the underlying crystal lattice of the GaAs. In contrast, the

most commonly used dry processes bombard the surface with ions

having enough energy to locally disrupt the lattice, and some of

this damage may remain after the etch is completed. This problem

147

Dry etching of GaAs and related alloys

can be reduced by post-etch thermal annealing, but some damage

may remain.

Another advantage of wet etching is the ease with which it can

be used. While dry processes require gas-handling and vacuum

systems and usually complex electronic hardware to establish a

low-temperature plasma discharge, wet etching can be performed

in simple glassware on a benchtop. Chemical reagents for wet etch-

ing are also inexpensive and it is very easy to vary the chemistry.

The chemistry can be based on one of many different schemes that

oxidise the surface to yield a soluble product.

However, there are some limitations encountered with wet etch

processes. It is much harder to control the proﬁle of a feature that

has been wet-etched. The etch undercuts the mask to a depth com-

parable to the vertical etch depth. This limits the smallest critical

dimensions that can be achieved. The proﬁle may be diffusion

controlled (isotropic) or it may depend on the different etch rates

of different crystallographic faces (FIGURE 5.1). This can intro-

duce a strong dependence of the ﬁnal features on how the pattern

is aligned with the GaAs wafer’s crystal planes.

Isotropic

(100)

Crystallographic

(110)

(111)

FIGURE 5.1 Isotropic proﬁle

and crystallographic proﬁles of

wet etch.

undercut

vertical

overcut

FIGURE 5.2 Range of proﬁles

from undercut (isotropic) to

vertical to angled for RIE etching.

Wet etch rates often vary signiﬁcantly with changing temperat-

ure and with how recently the solution has been mixed. This can

make run-to-run reproducibility difﬁcult to achieve.

Dry etch processes can provide excellent proﬁle control. The

energetic ion bombardment increases the etch rate on the exposed

surface relative to those regions protected by the mask, so ver-

tical sidewalls with negligible undercutting are readily achieved.

Depending on the balance between chemical and physical con-

tributions to the dry etch, it is possible to vary the proﬁle from

isotropic/crystallographic to vertical to angled (FIGURE 5.2).

With vertical proﬁles, smaller critical dimensions are achievable.

The ion enhancement also removes the dependence on the pattern

alignment relative to the wafer crystal planes.

The primary disadvantage of dry etch processing is the degrad-

ation of electronic properties that results from both the mechanical

displacement damage and the charge-induced damage to the lattice

that are produced by ion bombardment. An additional disadvant-

age is the limited number of possible chemistries that are useful for

etching GaAs and related compounds. In general, one is restric-

ted to halogen and hydrocarbon etch chemistries since gas-phase

reactants and volatile products are required for successful dry etch-

ing. Another disadvantage of an ion-bombardment-dependent etch

is the erosion and heating of the resist by sputtering and sometimes

chemical etching. This increases the probability of ruining a device

because the resist fails before the etch process is complete.

In general, if feature proﬁle and tight control of dimensions

are less important than surface electronic quality, a wet etch

148

Dry etching of GaAs and related alloys

may be the process of choice. If vertical or controlled-angle pro-

ﬁles and small dimensions are needed, a dry etch that achieves

the best compromise between feature proﬁle and damage may be

preferred.

Advantages of dry etching

1) Good dimensional control

2) Excellent proﬁle control: vertical to

controlled angle.

Disadvantages of dry etching

1) Ion-induced surface damage

2) Limited number of chemistries

3) Resist erosion can limit depth

4) Hazardous reactant gases

5) Expensive hardware

5.3 OVERVIEW OF DRY ETCHING PROCESSES

There are several types of dry etch process. Each will be discussed

separately below, but there are many features common to several

and a thorough knowledge of the important characteristics of each

will help one select the best process for a particular application.

Types of dry etching

1) Ion-beam or sputtering

2) Chemical dry etching

3) Plasma etching

4) Reactive ion etching

5) High-density plasma etching (ECR

or ICP)

6) Reactive-ion-beam etching

7) Chemically-assisted ion-beam

etching

The simplest type is ion-beam or sputter etching. No reactive

gas is used and physical sputtering of the GaAs by the incident

high-energy ions is the only mechanism involved.

In chemical dry etching, no plasma is employed or a plasma

is remotely located without line-of-sight to the sample so no ion

bombardment is involved. A component of this will be present in

every plasma process and must be considered as a possible effect

before striking the plasma or after the plasma is quenched.

Plasma etching is used here to designate the situation where the

wafer sits within the plasma but not on a powered electrode so the

incident ion energies will usually be less than 20 eV, and typically

of the order of 10–15 eV. Highly reactive radicals, formed by dis-

sociating the source gas, react faster with the surface than what one

sees in simple chemical dry etching, but ion bombardment effects

are negligible.

Reactive ion etching (RIE) has long been the standard dry pro-

cess where reactive radicals and ions are formed and the ions are

accelerated into the wafer at energies >50 eV to produce a mechan-

ism that combines both chemical etching and physical sputtering.

The same radio frequency (RF) power supply is used to generate

the plasma and accelerate the ions into the wafer.

High-density-plasma etching (HDPE) uses two separate power

supplies with one generating the plasma and the second one provid-

ing the RF power that controls the ion bombardment energy. The

source gases are more efﬁciently dissociated than in regular RIE

(by up to two orders of magnitude) using microwaves and magnets

to establish electron cyclotron resonance (ECR) or using induct-

ively coupled RF power to generate the plasma. The latter is called

an inductivelycoupledplasma (ICP) system. HDPE providessemi-

independent control of the plasma chemical species, both ions

and neutrals, and the energy with which the ions hit the wafer

surface.

Reactive-ion-beam etching (RIBE) and chemically assisted ion-

beam etching (CAIBE) do not use an RF supply to provide the

149

Dry etching of GaAs and related alloys

incident ion energy. Ions are accelerated from a “remote” plasma

(not in direct contact with the wafer) using ion-acceleration

techniques similar to those in simple ion-beam etching. In RIBE,

a neutral gas is predissociated to provide a beam of highly reactive

radicals, such as Cl from Cl

, that strike the surface in concert with

an ion beam. In CAIBE, a beam of ions, usually Ar

or Xe

,is

directed at the wafer while it is immersed in an ambient of neutral

gas, such as Cl

. The smallest critical dimensions are achieved

with these techniques.

In the following sections, each of these dry etch techniques will

be discussed in greater detail. For a comprehensive tabulation of

etch rates for all these processes through 1995, see “Properties of

Gallium Arsenide” [1]. A useful compilation of reviews of vari-

ous aspects of plasma processing has been recently published [2].

Since ion bombardment effects are so important in RIE, HDPE,

RIBE and CAIBE, we will begin with a discussion of ion beam

etching.

5.4 ION-BEAM ETCHING (IBE) AND ION EFFECTS IN

OTHER PLASMA PROCESSES

Ion-beam etching (IBE) is essentially a momentum-transfer sput-

tering process where the kinetic energy of the incoming ions is

transferred to the surface atoms, ejecting them from the surface.

Although non-reactive IBE is not generally used to etch GaAs

devices, the sputtering process which can be studied in isolation in

IBE is a key feature of most other dry processes and requires con-

sideration whenever the ion bombardment energies exceed 40 eV,

as they usually do in RIE, HDPE, RIBE, and CAIBE.

gas

inlet

beam voltage

13.56

MHz

ion

extraction

grids

Ar plasmaAr plasmaAr plasma

FIGURE 5.3 Ion beam etching

(sputtering).

No reactive gases are used in simple IBE. A heavy inert gas such

as Ar is typically employed (FIGURE 5.3). Lack of a chemical

component to the etch means that all materials etch at similar rates,

depending only on their relative bond strength and atomic mass.

Sputtering is more efﬁcient when the mass of the incident ion is

closely matched to the mass of the sputtered atoms. Typical rates

for a wide varietyof materials range between0.5and 2.5 atoms/ion.

For 100 and 500 eV Ar

, this corresponds to a removal rate of the

order of 0.01 and 0.1 μm/min/mA/cm

, respectively. There is a

non-linear dependence of rate on ion energy but generally a linear

dependence on ion current density.

There is a pronounced dependence of sputtering rate on the ion

incidence angle with a maximum etch rate near an incidence angle

of 60 degrees, where surface atoms acquire a large momentum

component directed away from the surface. This leads to a charac-

teristic angled proﬁle. When angled proﬁles occur in plasma-based

150

Dry etching of GaAs and related alloys

etches, it is generally an indication that a sputtering component

dominates the etch.

For a given incident ion energy, IBE is the most damaging of

processes. There is appreciable disruption of the near-surface lat-

tice by the impacting ions if they exceed the atomic displacement

energy threshold for GaAs (∼40 eV). Since this damaged region is

not partially removed by chemical reaction, more damage remains

after an IBE etch than after a plasma etch at the same ion energy.

In general, the higher the ion energy in any etch process, the more

signiﬁcant the surface damage will be in degrading device per-

formance. An Ar ion energy of only 40 eV can increase a diode

ideality factor (Section 7.2.1) to 1.25 while energies of 80 and

200 eV can produce increases to 1.4–1.5 and 2.3–2.7, respect-

ively. In plasma processes where some of the damage is removed

by chemical etching, lesser device degradation will occur, but even

in these, ion energies over 100 eV generally produce some degrad-

ation and even 2 min with 60 eV ions can produce threshold voltage

shifts and transconductance decreases in FETs.

An additional effect to keep in mind is the heating of the surface

by the depositing ion energy. When it is important to avoid heating

a wafer too much, high ion energies during etching should be

avoided and, if possible, the wafer should be actively cooled during

the etch.

5.5 CHEMICAL DRY ETCHING

While device fabrication relies on plasma-based dry etching to

produce useful etch rates and the desired etch proﬁles for devices,

it is important to remember that GaAs can be etched by gas-

phase reactants through ordinary chemical reactions in the absence

of any plasma. Such reactions are isotropic or crystallograph-

ically rate-dependent, similar to wet etching reactions. Since

these reactions are thermally activated reactions of molecular spe-

cies rather than relatively temperature-independent reactions of

highly reactive atoms or other radicals, they tend to be quite

slow at room temperature. However, they can become signiﬁc-

ant at higher temperatures. Since wafers tend to heat up under

ion bombardment, temperatures over 100

◦

C may occur rather

routinely after long etches and simple thermal etching may be

appreciable. A rate of 0.01 μm/min has been observed at 100

◦

with tenths of a mTorr of Cl

and rates up to 0.2 μm/min are

possible at 300

◦

C. Consequently, letting a sample sit for exten-

ded periods without plasma but still exposed to Cl

-containing

gas mixtures may produce unintended etching whose isotropic

character could degrade device proﬁles. Leaving the wafer in the

151