Ghodssi R., Lin P., MEMS Materials and Processes Handbook

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504 D.W. Burns

Table 8.8 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

Etches slightly SiGe(poly) (<0.1 Å/s), Si

(0.2 Å/s),

(low-stress) (<0.1 Å/s);

Doesn’t signiﬁcantly etch Ag, Au, Cr, Cu, Ge(poly), Mo, Nb, Ni,

NiCr, Pd, polyimide, Pt, Si, Si(poly), Ta, TiN, TiW, Va, W; DI

water rinse [27]

14 Aluminum (Al),

deposited on silicon

30–550 Phosphoric, nitric and

acetic acids

25–75

◦

C; 40

◦

C typical (80 Å/s); Transene Aluminum Etchant

Type A; etches faster with higher etchant temperature; mask

with PR, Si

,SiO

;

Etches Cr (slightly), Cu, GaAs, Ni;

Doesn’t signiﬁcantly etch Au, NiCr, Si, Si(poly), Ta, TaN, Ti, W;

glass, Teﬂon or polypropylene tank; stir or agitate for faster etch

rates and improved uniformity; DI water rinse [94]

15 Aluminum (Al),

deposited on GaN,

GaAsP, GaP, NiCr, Si,

others

40–200 No nitric 25–50

◦

C; Transene Aluminum Etchant Type D; 40

◦

C typical

(125 Å/s); use with GaAs, GaP; mask with PR, Si

,SiO

;

Etches slightly Cr and Si

;

Doesn’t signiﬁcantly etch Au, Cu, Ni, NiCr, Si, Ta, TaN, Ti, W;

glass, Teﬂon or polypropylene tank; etch rate varies with Al

composition; DI water rinse [94]

16 Aluminum (Al),

deposited

160 Phosphoric, nitric, acetic 40

◦

C; Cyantek AL-11; agitate often; quartz tank; DI water rinse

[95]

17 Titanium (Ti), deposited H

(30%)

undiluted

50–60

◦

C; DI water rinse [45, 96]

18 Titanium (Ti), deposited H

(96%)

undiluted

◦

C; DI water rinse [45, 76]

19 Titanium (Ti), deposited H

(96%): H

(30%)

2:1

◦

C; doesn’t signiﬁcantly etch TiSi

; DI water rinse [97]

8 MEMS Wet-Etch Processes and Procedures 505

Table 8.8 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

20 Titanium (Ti), deposited H

(96%):

HF(49%):H

30:1:69

◦

C; DI water rinse [98]

21 Titanium (Ti) H

(85%)

undiluted

Warm; DI water rinse [96, 99]

22 Titanium (Ti) HCl(38%)

undiluted

Room temperature; DI water rinse [96]

23 Titanium (Ti), deposited 2000 HF(49%):H

1:9

◦

C; DHF etchant (9:1); mask with PR; DI water rinse [16, 100]

24 Titanium (Ti), deposited 180 HF(49%): H

(30%):H

1:1:20

◦

C; mask with PR; etch rate increases with higher HF

concentration;

Etches Al (25 Å/s), AlSi (2%) (40 Å/s), SiO

(2 Å/s), SiO

(PSG)

(35Å/s),W(2Å/s);

Etches slightly Si(poly) (0.5 Å/s), Si(poly, n-doped) (0.2 Å/s);

Doesn’t signiﬁcantly etch Cr, sapphire, Si, Si

(0.1 Å/s),

(low-stress) (<0.1 Å/s), TiW; DI water rinse [27, 28]

25 Titanium (Ti), deposited HF(49%): HNO

(70%)

1:200

Room temperature; DI water rinse [45]

26 Titanium (Ti), deposited HF(49%):

HNO

(70%):H

1:1:50

Room temperature; HNW etchant; DI water rinse [45, 76]

27 Titanium (Ti), deposited 3000 HF(49%):

HNO

(70%):H

1:2:7

◦

C; HNW etchant; mask with PR; DI water rinse [16, 100]

28 Titanium (Ti), deposited NH

OH(29%):

(30%):H

1:1:5

◦

C; doesn’t signiﬁcantly etch TiSi

; DI water rinse [97, 101]

506 D.W. Burns

Table 8.8 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

29 Titanium (Ti), deposited 50 Hydrochloric acid, no HF 85

◦

C; etch rate 10 Å/s at 70

◦

C; Transene titanium etchant TFTN;

mask with PR;

Etches Al, Cr, GaAs, Ti;

Etches slightly Cu, Ni;

Doesn’t signiﬁcantly etch Au, Si, Si

,SiO

,Ta,TaN,W;

polyethylene or polypropylene tank; DI water rinse [102]

30 Titanium–tungsten

(TiW) (90/10),

sputtered

(30%)

undiluted

◦

C; mask with PR;

Etches W (3 Å/s);

Doesn’t signiﬁcantly etch AlSi(2%) (0.3 Å/s), Si, Si(poly), Si

SiO

, Ti; DI water rinse [27]

31 Titanium–tungsten

(TiW), deposited

10–15 Proprietary 25–40

◦

C; Transene Ti-tungsten TiW-30; mask with PR;

Etches Cu, GaAs;

Etches slightly Ni, Ti, W;

Doesn’t signiﬁcantly etch Al, Au, Cr, Si, Si

,SiO

,Ta,TaN;

store cold; DI water rinse [103]

32 Tungsten (W), deposited 3 H

(30%)

undiluted

◦

C; mask with PR;

Etches TiW (90:10) (1 Å/s);

Etches slightly AlSi (2%) (<0.3 Å/s);

Doesn’t signiﬁcantly etch Al, Si(poly), Si

,SiO

, Ti; endpoint

difﬁcult with Al mask; DI water rinse [27, 45]

33 Tungsten (W), deposited 25 H

(30%)

undiluted

◦

C; mask with PR;

Etches Cr (18 Å/s), Ge(poly) (75 Å/s), TiW;

Etches slightly AlSi (2%) (<0.1 Å/s), SiGe(poly) (<0.1 Å/s);

Doesn’t signiﬁcantly etch Al, Parylene Type C, polyimide, Pyrex,

quartz, sapphire, Si, Si(poly), Si

,SiO

; consider ultrasonic

to remove bubbles; DI water rinse [28]

34 Tungsten (W) HCl(38%): HNO

(70%)

3:1

Hot; aqua regia; DI water rinse [104]

8 MEMS Wet-Etch Processes and Procedures 507

Table 8.8 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

35 Tungsten (W) HF(49%): HNO

(70%)

4:1

Hot; DI water rinse [104, 105]

36 Tungsten (W), deposited HF(49%): HNO

(70%)

1:1

Room temperature; DI water rinse [45, 98]

37 Tungsten (W), deposited 80–250 Ferricyanide-based 20–60

◦

C; etches 140 Å/s at 30

◦

C; Transene Tungsten Etch TFW;

mask with PR;

Etches Al, Cr, GaAs;

Etches slightly Si, SiO

;

Doesn’t signiﬁcantly etch Au, Cu, Ni, Si

, Ta, TaN, Ti; DI water

rinse [106]

508 D.W. Burns

Table 8.9 Photoresist removal processes

Material

Strip rate

(Å/s) Stripper Remarks and references

1 Organic residues N/A O

plasma Dry etch; 0.5–10 min; plasma; 300 mT, O

, 100–300 W; surface oxide may form

[108]

2 Organic residues N/A O

RIE Dry etch; 1–5 min; 10–50 mT, O

, 100–400 V bias; for most stubborn ﬁlms [108]

3 Organic residues N/A UV-Ozone 1–30 min in UV-ozone [108]

4 Photoresist (PR),

spun

>20,000

varies

Acetone 20

◦

C; etches slightly Parylene Type C (0.1 Å/s);

Doesn’t signiﬁcantly etch Ag, Al, Al

, Au, Cr, Cu, Mo, Nb, Ni, NiCr, Pd,

polyimide, Pt, Pyrex, quartz, sapphire, Si, Si(poly), Si

,SiO

,Ta,Ti,TiN,TiW,

Va, W; can be heated with loosely ﬁt lid or used ultrasonically; dissolution rate

slows signiﬁcantly if wafers have seen high temperatures above 130

◦

Cfrom

postbake cycles or dry etching; for hardened resist use piranha, O

plasma, or a

commercial stripper; chemically inert to most inorganics [27, 28]

5 Photoresist (PR),

spun

varies H

(96%): H

(30%)

2:1 to 4:1

90–140

◦

C; piranha; add peroxide to sulfuric acid when mixing; DI water rinse [109]

6 Photoresist (PR),

spun

varies H

(96%): H

(30%)

56:1

120

◦

C; high-sulfuric piranha;

Etches Ag (100 Å/s), Al (850 Å/s), AlSi (2%) (30 Å/s), Al

(3–15 Å/s), Cr

(1–3 Å/s), Cu (15 Å/s), Mo (3 Å/s), Nb (1 Å/s), Ni (65 Å/s), NiCr (15 Å/s),

polyimide (2800 Å/s), Ti (40 Å/s);

Etches slightly Ge(poly) (softens), Parylene Type C (0.4 Å/s), Pd (0.5 Å/s), Pt

(0.5 Å/s), TiW (0.1 Å/s);

Doesn’t signiﬁcantly etch Au, Pyrex, quartz, sapphire, Si, Si(poly), SiGe(poly),

,SiO

,Ta;

Heat and agitate; 5–10 min; exothermic; add H

immediately prior to use;

hydrous silicon oxide is formed on Si substrates that can be removed with DHF

etchant (10:1) for 10 s; DI water rinse [27, 28]

7 Photoresist (PR),

spun

>80

varies

Isopropanol (isopropyl

alcohol or IPA)

◦

C; strips PR less quickly than acetone or methanol;

Doesn’t signiﬁcantly etch Ag, Al, Al

, Au, Cr, Cu, Mo, Nb, Ni, NiCr, Parylene

Type C, Pd, polyimide, Pt, Pyrex, quartz, sapphire, Si, Si(poly), Si

,SiO

,Ta,

Ti,TiN,TiW,Va,W[28]

8 MEMS Wet-Etch Processes and Procedures 509

Table 8.9 (continued)

Material

Strip rate

(Å/s) Stripper Remarks and references

8 Photoresist (PR),

spun

>2500

varies

Methanol (MeOH) 20

◦

C; strips PR less quickly than acetone, can produce residues;

Doesn’t signiﬁcantly etch Ag, Al, Al

, Au, Cr, Cu, Mo, Nb, Ni, NiCr, Parylene

Type C, Pd, polyimide, Pt, Pyrex, quartz, sapphire, Si, Si(poly), Si

,SiO

,Ta,

Ti,TiN,TiW,Va,W[28]

9 Photoresist (PR),

spun

varies NH

OH(29%):

(30%):H

1:1:5

◦

C; >5 min; DI water rinse [108]

10 Photoresist (PR),

spun

varies

plasma Dry ashing; 20

◦

C; descum; 51 sccm O

in plasma with 50 W, 300 mTorr and gap of

2.6 cm; benign to inorganics [27]

11 Photoresist (PR),

spun

50 O

plasma Dry ashing or stripping; 20

◦

C; 51 sccm O

in plasma with 400 W, 300 mTorr and

gap of 2.6 cm; etches Parylene Type C (35 Å/s), polyimide (60 Å/s); benign to

inorganics [27, 28]

12 Photoresist (PR),

spun

varies O

plasma Dry ashing or stripping; plasma etcher, barrel etcher or asher [110]

13 Photoresist (PR),

spun

varies AZ300T (TMAH, NMP,

others)

◦

C; 5 min; solvent hood; ﬁnal resist strip bath; DI water rinse [111]

14 Photoresist (PR),

spun

varies EKC 830 (aminoethoxy

ethanol, n-methyl

pyrrolidone)

◦

C; 10 min at 80

◦

Cfor∼1 µm of resist; 30 min for up to 10 µm of resist; agitate

5–10 s initially and every 5 min; follow with AZ300T for 5 min at 80

◦

C; 3

quick-dump rinses; use with ultrasonic bath for liftoff processes [111]

15 Photoresist (PR),

spun

>14,000

varies

Microstrip 2001 85

◦

C; Fujiﬁlm Microstrip 2001;

Etches polyimide (85 Å/s);

Doesn’t signiﬁcantly etch graphite, Pyrex, quartz, sapphire, Si, Si(poly), Si

SiO

; use with Al, Ti, W; DI water rinse [28]

16 Photoresist (PR),

spun

varies Nanostrip (sulfuric acid,

peroxymonosulfuric acid,

hydrogen peroxide)

◦

C or higher; Cyantek Corp.; Nanostrip; >5 min per bath; multiple baths [111]

510 D.W. Burns

Table 8.10 Prelithography cleaning process (pre-spin)

Material

Strip rate

(Å/s) Stripper Remarks and references

1a Photoresist

(PR)

N/A AZ300T (NMP +

TMAH) or

PRX1165

(NMP)

◦

C; 10 min; continue with next step

[112]

1b N/A Isopropyl alcohol Room temperature; continue with next

step [112]

1c N/A DI water Rinse; N

blow dry; dehydration bake

>100

◦

C for >2 min; cool substrates

before spinning [112]

Table 8.11 Prelithography cleaning process (re-spin)

Material

Strip rate

(Å/s) Stripper Remarks and references

1 Photoresist (PR)

on wafers, not

post-baked

N/A Acetone Apply with spray bottle of acetone about

10 s until clear while spinning wafers

on coater track; acetone clears and

wafers dry while still spinning [113]

and dry photoresist removal processes. Removal rates are highly subject to the type

and thickness of the resist, previous baking cycles, and extent of exposure to chem-

ical etchants. Table 8.10 provides a prelithography cleaning process for removing

resist using heated commercial strippers. Table 8.11 presents a method for removing

photoresist from lithographically exposed wafers that have not been postbaked to

allow resist reapplication following an inadvertent or incorrect exposure step. This

technique may work with negative or positive photoresist. An alternative for positive

resists is to blanket expose the resist layer and develop the exposed resist in a stan-

dard developer. Table 8.12 provides a sequence of solvent soaks to remove organics

including spun photoresist on wafers and dried photoresist, which may also be suit-

able f or holder, tweezer, cassette, and vacuum wand cleaning [107]. For unbaked

and prebaked photoresist, the acetone and methyl alcohol soaking steps may be

reduced to 2 min each in an optional ultrasonic bath, followed by a rinse with run-

ning DI water for 30 s and drying with nitrogen [108]. Table 8.13 presents an organic

contaminant removal process using solvents and cleanroom swabs. Table 8.14

describes a particulate removal process for removal of scribing dust and debris that

may occur on wafers or substrate samples. Most of the time processes for removing

photoresist require at least 5 min in each solution, and for many, 10 min per bath is

suggested.

Wafer-cleaning processes precede nearly every furnace or deposition sequence

to ensure that contaminants and residues from wet-etch steps and other processes

are adequately removed from the wafers. These sequences are predominantly if

8 MEMS Wet-Etch Processes and Procedures 511

Table 8.12 Photoresist and organic removal sequence with solvents

Material

Strip rate

(Å/s) Stripper Remarks and references

1a Photoresist (PR)

on wafers,

partial

substrates,

holders and

tweezers

varies Acetone Room temperature; 10 min; AMI strip

sequence; solvent hood; place substrate

or tweezers in beaker with acetone and

lid and place in ultrasonic cleaner with

water; continue with next step [114]

1b N/A Methanol Room temperature; 5 min; s olvent hood;

continue with next step [114]

1c N/A Isopropyl

alcohol

Room temperature; 5 min; solvent hood;

continue with next step [114]

1d N/A DI water DI water rinse; 5 quick-dump rinse and

spin–rinse–dry or DI water spray and

blow-dry [114]

Table 8.13 Organic contaminant removal process with swabs

Material

Strip rate

(Å/s) Stripper Remarks and references

1a Photoresist (PR)

residue and

dried resist

N/A Acetone Room temperature; coat wafer surface

with acetone; scrub with a cleanroom

swab; continue with next step [115]

1b N/A Isopropyl

alcohol

Spray wafer with IPA from a squeeze

bottle to displace acetone; blow dry

with N

[115]

Table 8.14 Particulate removal process with surfactants

Material

Strip rate

(Å/s) Stripper Remarks and references

1a Particulates on

substrates

(e.g., scribe

dust)

N/A Surfactant in

DI water

Tergitol, soap, or similar surfactant with

optional ultrasonic agitation; continue

with next step [116]

1b N/A DI water Running DI water rinse; 0.5 min; N

blow-dry [116]

not completely wet, with chemical-etch steps followed by vigorous deionized water

rinsing steps and a ﬁnal spin–rinse–dry cycle with hot nitrogen. Table 8.15 presents

a standard RCA clean cycle (initially developed at RCA Laboratories) for sili-

con substrates prior to contamination-sensitive furnace and deposition operations

[117–121].

Although many variations of sequences, times, temperatures, and concentra-

tions exist, the general cleaning approach begins with a mixture of sulfuric acid

512 D.W. Burns

Table 8.15 RCA clean cycle for silicon substrates

Material

Etch rate

(Å/s) Etchant Remarks and references

1a Silicon (Si)

substrates

OH(29%):

(30%):H

1:1:5

◦

C; organic clean (a.k.a. SC-1,

standard-clean #1, or RCA-1); removes

insoluble organic contaminants and

some metals; PR previously stripped;

DI water rinse [109]

1b HCl(38%):

(30%):H

1:1:6

◦

C; ionic clean (a.k.a. SC-2,

standard-clean #2, or RCA-2); removal

of any ionic, alkaline and heavy metal

contaminants; DI water rinse;

spin–rinse–dry [109]

1c HF(49%):H

1:10

Room temperature; DHF etchant (10:1);

oxide strip; removal of thin silicon

dioxide layer and any metallic

contaminants from previous step; DI

water rinse [109]

and hydrogen peroxide at 90–120

◦

C to remove from the wafer surfaces some of

the inorganics and organics such as photoresist residue and thinly deposited oily

vapors from the air. Small particles of silica and silicon, additional organics, and

metal surface contaminants are removed in a second step with ammonia hydrox-

ide and hydrogen peroxide at about 70

◦

C, which oxidizes the wafers slightly to lift

and entrain contaminants on and near the wafer surface. A hydroﬂuoric-acid dip

at room temperature for a short time strips the thin, chemically grown oxide and

allows contaminants to be dissolved or ﬂoated away. Hydrochloric acid and hydro-

gen peroxide at about 70

◦

C in a fourth step clean the surface of metals and ionic

contaminants. The wafers are typically rinsed with DI water in a dump rinser for

5–6 cycles between each step. Ultrasonic agitation may be applied for the ammo-

nium hydroxide and HCl steps if available. The three peroxide steps are preheated

in controlled-temperature tanks, and the chemicals are generally mixed soon before

use. Note that the peroxide steps are exothermic and will cause the bath temperature

to rise when mixed or refreshed. The etchants with ammonium hydroxide and HCl

should be replaced on a regular basis, although the composition may be refreshed

with the addition of hydrogen peroxide. After the last dump rinse, the wafers are

transported into a spin–rinse–dryer where the wafers are sprayed with DI water

while spinning slowly, and then blown dry with heated nitrogen gas while spin-

ning rapidly. The wafers are transferred promptly to the corresponding furnace or

deposition system for further processing.

Table 8.16 presents an alternative process sequence for wafer cleaning cycles

prior to oxidation or anneal furnace operations. For furnace operations such as

LPCVD poly, nitride, epi deposition, or annealing that beneﬁt from a substrate

largely clear of native oxide, the order of the HF-dip is reversed as shown in

Table 8.17.

8 MEMS Wet-Etch Processes and Procedures 513

Table 8.16 Alternative clean cycle for silicon substrates (preoxidation)

Material

Etch rate

(Å/s) Etchant Remarks and references

1a Silicon (Si)

substrates

(96%):

(30%)

4:1

◦

C; 10 min; quartz hot pot; removes

trace organics; 5 QDRs; continue with

next step [122]

1b HF(49%):H

1:50

Room temperature; DHF etchant (50:1);

15–30 s Teﬂon tank; removes thin

native oxide; 5 QDRs; continue with

next step [122]

1c HCl(38%):

(30%):H

1:1:5

◦

C; 10 min; quartz hot pot; removes

trace metal ions; 5 QDRs;

spin–rinse–dry; proceed promptly to

next operation [122]

Table 8.17 Clean cycle for silicon substrates (predeposition)

Material

Etch rate

(Å/s) Etchant Remarks and references

1a Silicon (Si)

substrates

(96%):

(30%)

4:1

◦

C; 10 min; quartz hot pot; removes

trace organics; 5 QDRs; continue with

next step [122]

1b HCl(38%):

(30%):H

1:1:5

◦

C; 10 min; quartz hot pot; removes

trace metal ions; 5 QDRs; continue

with next step [122]

1c HF(49%):H

1:50

Room temperature; DHF etchant (50:1);

15–30 s Teﬂon tank; removes thin

native oxide; 5 QDRs; spin–rinse–dry;

proceed promptly to next operation

[122]

8.4.5 Examples: Wet Chemical Etching of IC-Compatible

Materials

For most IC and MEMS processes, a photoresist mask will sufﬁce for patterning

sequences involving standard materials such as oxide, nitride, silicon, polysilicon,

and metal, as depicted in Fig. 8.10. For cases where t he ﬁlm is uncommonly thick,

or when the photoresist is unable to hold up to the etchant for the required time, a

hard mask may be needed. A group of examples for the wet etching of standard IC-

compatible materials follows. These examples and others throughout this chapter

are intended to be instructive. Their accuracy should not be relied on, and their

utility should be veriﬁed by the user.