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

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494 D.W. Burns

Table 8.6 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and References

7 Silicon nitride

(Si

), LPCVD

stochiometric

0.2 HF(49%):H

1:10

◦

C; DHF etchant (10:1); mask with PR, Si(poly) or W; etches SiO

(>4 Å/s), TiW (1 Å/s); DI water rinse [27]

8 Silicon nitride

(Si

), PECVD

1–5 HF(49%):H

1:10

◦

C; DHF etchant (10:1); DI water rinse [56]

9 Silicon nitride

(Si

), CVD

HF(49%):

HNO

(70%)

3:10

◦

C; etches Si rapidly [57]

10 Silicon nitride

(Si

), LPCVD

stochiometric

0.15 HF(49%): NH

(40%)

1:5

◦

C; BHF etchant (5:1); mask with PR or Si(poly);

Etches SiO

(>15 Å/s), Ti (fast), TiW (17 Å/s), W (<0.3 Å/s);

Doesn’t etch Au, Cr, Nb, Pd, Pt; nitride etch rate decreases with

increasing refractive index; DI water rinse [27]

11 Silicon nitride

(Si

), PECVD

low index

10 HF(49%): NH

(40%)

1:5

◦

C; BHF etchant (5:1); mask with PR or Si(poly); see remarks for

stoichiometric Si

with same etchant [28]

12 Silicon nitride

(Si

), PECVD

high index

1.3 HF(49%): NH

(40%)

1:5

◦

C; BHF etchant (5:1); mask with PR or Si(poly); see remarks for

stoichiometric Si

with same etchant [28]

13 Silicon nitride

(Si

), deposited

2 Ortho-phosphoric

acid

180

◦

C; Transetch-N; no PR; mask with SiO

(125:1); silicon

selectivity (125:1); Pyrex or quartz tank; add water to replace

evaporated water; DI water rinse [58]

8 MEMS Wet-Etch Processes and Procedures 495

Dry etching of polysilicon ﬁlms with a photoresist mask is quite prevalent in

IC and MEMS processing, in part because the thickness of the deposited polysil-

icon ﬁlms is typically less than a few micrometers, which makes dry etching cost

effective. Wet etching of polysilicon with anisotropic wet etchants provides similar

results to isotropic etches, because the grain structure of polysilicon has little long-

range order and only individual grains are subject to the anisotropy. Wet etching

of polysilicon may be preferred over dry etching for clear-ﬁeld features with small

aspect ratios and wide lines, along with the need to remove the polysilicon layer con-

currently from the backside of the wafer. Isotropic etchants for silicon or polysilicon

include nitric acid and hydroﬂuoric acid mixed with water or acetic acid to reach the

desired concentration [60–62]. The nitric acid r eacts with the exposed silicon sur-

face to form silicon dioxide, which is promptly etched away by the hydroﬂuoric

acid. Ammonium ﬂuoride can substitute for the hydroﬂuoric acid, because HF is

formed from the nitric acid [27]. Typical masks for wet or dry etching of polysili-

con include photoresist for small etch depths, and oxide, nitride, or metal for deeper

etches. Typical etch chemistries and etch rates for isotropic etching of single-crystal

silicon, polycrystalline silicon, and germanium are listed in Table 8.7. Some of the

etchants exhibit modest anisotropy.

8.4.3 Standard Metal Etching

Thin-ﬁlm metal layers serve important roles in IC and MEMS processing to help

create substrate contacts, electrical interconnections, electrodes, reﬂective struc-

tures, and hard masks for underlying layers. Etchants for metals are generally

acid-based and are available in various concentrations and mixtures. The etch rates

are primarily dependent on the acid concentration, type of metal, and etch tem-

perature, and secondarily dependent on alloy content in the metal layer, agitation,

etchant exhaustion, and storage time before use, particularly if the etchant con-

tains a decaying component such as hydrogen peroxide. PFA and polypropylene

cassettes and holders are standard equipment for the handling and transport of

wafers. Deionized water serves as a suitable rinse liquid in a dump rinser, followed

by nitrogen blow-drying or the passing of the substrates through a spin–rinse–dry

cycle. Because metals are of particular concern in shortening carrier recombination

lifetimes of semiconductor components, precautions should be made to separate

metal-etch facilities and the labware used for clean wafer processing.

Photoresist masks dominate metal patterning sequences, although deposited

oxides, nitrides, and other metals may also serve as suitable etch masks. Veriﬁcation

of etch completion can often be done visually and reinforced with microscope

inspection because one is able observe the dramatic shift in reﬂectivity when metal

is removed and underlying layers become exposed. Proﬁlometry can be used for

validating etch completion by measuring step heights, however, noncontact ellip-

sometry or spectrometry will reveal the thickness of the underlying material to

validate removal of the metal layer.

496 D.W. Burns

Table 8.7 Germanium, silicon, and polysilicon isotropic etchants and etch processes

Material

Etch rate

(Å/s) Etchant Remarks and references

1 Germanium (Ge),

LPCVD poly

77 H

(30%)

undiluted

◦

C; mask with photoresist, Si

,SiO

(LTO);

Etches Cr (20 Å/s), TiW, W (25 Å/s);

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

(<0.1 Å/s);

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

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

,SiO

; DI water rinse;

Doesn’t signiﬁcantly etch poly-SiGe with Ge fraction less than 0.7

[28, 63]

2 Germanium (Ge), (111) 5 H

(30%):H

1:5

◦

C; (111) surface; DI water rinse [64]

3 Germanium (Ge), (100) 40 H

(85%):

(30%):H

1:6:3

◦

C; etches Ge(110) 50 Å/s, Ge(111) 30 Å/s; DI water rinse [65]

4 Germanium (Ge), (100) 1800 HF(49%): H

(30%)

1:1

◦

C; (100) direction; etches faster in (111) (3500 Å/s) and (110)

directions (6500 Å/s); orientation-dependent; agitate; DI water

rinse [66]

5 Germanium (Ge), (111) 4200 HF(49%): H

(30%)

2:1

◦

C; Ge(111) surface; Ge(100) etches slower, Ge(110) etches

faster; highly selective to silicon; DI water rinse [64, 67, 68]

6 Germanium (Ge), (111) 330 HF(49%): H

(30%):H

1:2:10

◦

C; Ge(111) surface; DI water rinse [67]

7 Germanium (Ge), (100) 700 HF(49%): H

(30%):H

1:1:4

◦

C; (100) surface; preferential etchant; fastest for (110) surface

and low doping; DI water rinse [64]

8 Germanium (Ge),

single-crystal

4000 HF(49%): HNO

(70%)

1:9

Room temperature; rates are approximate; DI water rinse [69]

9 Germanium (Ge),

single-crystal

HF(49%):

HNO

(70%):Acetic

3:5:3

Room temperature; HNA etchant; also CP-4A etchant; etch rate

increases with CP-4 etchant (Camp #4 etchant) with ∼0.5%

bromine [26]); DI water rinse [70]

8 MEMS Wet-Etch Processes and Procedures 497

Table 8.7 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

10 Germanium (Ge), (100) 2100 HF(49%):

HNO

(70%):Acetic

18:8:5

Room temperature; HNA etchant; agitate to remove bubbles and

reduce hillock formation; DI water rinse [71]

11 Germanium (Ge), (100) 4300 HF(49%):

HNO

(70%):Acetic

18:8:5

Room temperature; HNA etchant; agitate to remove bubbles and

reduce hillock formation; DI water rinse [71]

12 Germanium (Ge) HNO

(70%)

undiluted

Warm; DI water rinse [72]

13 Germanium (Ge),

LPCVD poly

150 HNO

(70%): NH

(40%):H

126:5:60

◦

C; mask with nitride; etches thermal oxide (1.5 Å/s), deposited

oxide (2–65 Å/s); DI water rinse; see remarks for single-crystal

siliconwithsameetchant[28]

14 Germanium (Ge), bulk 35 NH

OH(29%):

(30%):H

1:6:48

Room temperature; DI water rinse [73]

15 Germanium (Ge) 250 Nitric, HF, and acetic 20

◦

C; Transene RSE-563; mask with SiO

; isotropic; DI water

rinse [74]

16 Silicon (Si),

single-crystal

460–830 BrF

vapor, reduced

pressure

Room temperature; BrF

vapor etchant; pulsed etching; 1 min per

pulse; 500 mTorr; etch selectivities > 1000 for oxide, hard-baked

resist, Al, Cu, Au, Ni; slightly etches Si

(400:1 to

800:1 selectivity) [75]

17 Silicon (Si) HF(49%): HNO

(70%)

1:4

◦

C; DI water rinse [53]

18 Silicon (Si), poly 700 HF(49%): HNO

(70%)

1:3

Room temperature; DI water rinse [76]

19 Silicon (Si),

single-crystal

40–8000 HF(49%): HNO

(70%)

2:98 to 34:66

◦

C; with stirring and sample rotation; etch rate increases with

increasing HF; rate reduces with use; DI water rinse [77]

498 D.W. Burns

Table 8.7 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

20 Silicon (Si), poly 90 HF(49%):

HNO

(70%):H

2:70:28

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

21 Silicon (Si),

single-crystal

1200–

>27,000

HF(49%):

HNO

(70%):H

varies

◦

C; HNW etchant; rate increases with agitation; rate decreases

with use; substituting water for acetic acid reduces etch rate; DI

water rinse [61, 78]

22 Silicon (Si), poly 25 HF(49%):

HNO

(70%):Acetic

1:20:20

Room temperature; HNA etchant; stir constantly; slightly etches

oxide; mix fresh; DI water rinse [45]

23 Silicon (Si),

single-crystal

5800 HF(49%):

HNO

(70%):Acetic

3:5:3

Room temperature; HNA etchant; also CP-4A etchant; fast

chemical polish; DI water rinse [22]

24 Silicon (Si),

single-crystal

1200–

>80,000

HF(49%):

HNO

(70%):Acetic

varies

◦

C; HNA etchant; mask with Si

,SiO

, black wax (room

temperature), etches 600–3200 Å/s at 2:1:3 HNO

:HF:Acetic;

rate increases with agitation; rate decreases with use; rate

decreases with light doping; DI water rinse [61, 78]

25 Silicon (Si),

single-crystal

25 HNO

(70%): NH

(40%):H

126:5:60

◦

C; mask with PR cautiously, SiO

(15:1), Si

(750:1),

(low-stress) (500:1);

Etches Ag (8 Å/s), Al (10 Å/s), AlSi (2%) (65 Å/s), Al

(2–17 Å/s), Cr (1.5 Å/s), Cu (6 Å/s), Ge(poly) (150 Å/s),

graphite (10 Å/s), Mo (1800 Å/s), Nb (13 Å/s), Ni (3.5 Å/s),

Pyrex (20 Å/s), quartz (2 Å/s), Si

(PECVD) (2–10 Å/s),

Si(poly) (15–50 Å/s), SiGe(poly) (90 Å/s), SiO

(LTO) (2 Å/s),

SiO

(PECVD) (4–17 Å/s), SiO

(PSG) (30–65 Å/s), Ta (1 Å/s),

Ti (50 Å/s), TiW (4 Å/s), Va (1600 Å/s), W (2 Å/s);

8 MEMS Wet-Etch Processes and Procedures 499

Table 8.7 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

Etches slightly sapphire (0.1 Å/s);

Doesn’t signiﬁcantly etch Au, Parylene Type C, Pd, polyimide; Pt,

TiN; mix several hours before use; agitate for improved

uniformity; etch rate decreases with use; DI water rinse [27, 28]

26 Silicon (Si), LPCVD

poly

15–50 HNO

(70%): NH

(40%):H

126:5:60

◦

C; doping-level dependent; see remarks for single-crystal

silicon and same etchant [28]

27 Silicon (Si), deposited

amorphous

1.2 NH

OH(29%):H

1:10

◦

C; DI water rinse [79]

28 Silicon (Si),

single-crystal

165 SF

vapor

1:1000

1060

◦

C; SF

vapor etchant [80]

29 Silicon (Si),

single-crystal

77 XeF

vapor, reduced

pressure

◦

C; XeF

vapor etchant; 2.6 Torr; mask with Al (>1000:1), PR,

(35:1), Si

(low-stress) (>1000:1), SiO

(>1000:1);

Etches Ge, Mo, poly (30 Å/s), Nb, Ta, TiW, SiGe, Ti (5 Å/s), Va,

W(13Å/s)

Doesn’t signiﬁcantly etch Al, AlSi (2%), polyimide, quartz, SiO

ZnO [81];

Doesn’t etch acrylic, Al, Ga, Ni, photoresist, Si

,SiC,Pt[82];

strip native oxide prior with BHF etchant (10:1); etch rate is

strongly feature a nd load dependent [27, 28]

30 Silicon (Si), LPCVD

poly

30 XeF

vapor, reduced

pressure

Room temperature; XeF

vapor etchant; see remarks for single

crystal silicon above [27]

31 Silicon (Si) 100 Nitric and HF 25

◦

C; Transene RSE-200; SiO

mask; isotropic; DI water rinse

[83]

32 Silicon–germanium

(SiGe), (100), epi

17 HF(49%):

(30%):Acetic

1:2:3

Room temperature; Si

0.7

0.3

; etches slightly Si (no Ge)

∼0.02 Å/s; DI water rinse [84]

500 D.W. Burns

Table 8.7 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

33 Silicon–germanium

(SiGe), (100), MBE

18 HF(49%):

(30%):Acetic

1:2:3

◦

C; p-type Si

0.6

0.4

; etches slightly Si (<0.02 Å/s); n-doped

0.6

0.4

has faster etch rate (38 Å/s for 1 ×10

−3

Sb);

higher Ge fraction increases etch rate (860 Å/s for 100% Ge

with 50,000:1 selectivity over Si); wait 100–180 min after

mixing before etching; similar effects with H

O replacing acetic

acid though slower; DI water rinse [85]

34 Silicon–germanium

(SiGe), (100), MBE

3.5 HF(49%):

HNO

(70%):H

5:40:20

◦

C; HNW etchant; Si

0.6

0.4

; etches slightly Si (0.27 Å/s); etch

rates increase with higher HF or HNO

concentrations; etch

selectivity increases with lower HNO

concentration; DI water

rinse [86]

35 Silicon–germanium

(SiGe), (100), MBE

6.8 HF(49%):

HNO

(70%):H

1:350:300

◦

C; HNW etchant; Si

0.7

0.3

; etches slightly Si (0.07 Å/s);

substituting acetic acid for H

O doubles etch rates for SiGe and

Si while retaining 100:1 selectivity; DI water rinse [87]

36 Silicon–germanium

(SiGe), LPCVD poly

90 HNO

(70%): NH

(40%):H

126:5:60

◦

C; see remarks for single-crystal silicon and same e tchant [28]

37 Silicon–germanium

(SiGe), RTCVD poly

3.3 NH

OH(29%):

(30%):H

1:1:5

◦

C; Si

0.6

0.4

; etches slightly Si-poly (0.1 Å/s), SiO

(0.03 Å/s); n-doped Si

0.6

0.4

increases etch rate; p-doped

0.6

0.4

decreases etch rate; higher Ge fraction increases etch

rate (1100 Å/s for 100% Ge-poly); DI water rinse [88]

8 MEMS Wet-Etch Processes and Procedures 501

Standard metals, as used in this chapter, include aluminum, titanium, and tung-

sten. Etchants with nitric and phosphoric acids convert exposed aluminum to

aluminum oxide with the nitric acid, and then etch the oxide layer with the phospho-

ric acid. Faster etch rates are generally achieved by elevating the etchant temperature

or increasing the etchant concentration. Table 8.8 contains etchants and etch rates

for various standard metal layers.

8.4.4 Photoresist Removal Techniques and Wafer Cleaning

Processes

Removal of patterned photoresist occurs multiple times in nearly every IC or MEMS

process and often follows immediately after the etching of an underlying ﬁlm.

Although freshly made and premixed commercial strippers form the majority of PR

stripping processes, plasma-stripping of resist in a low-pressure (e.g., 300 mTorr)

oxygen plasma often presents a clean, fast, and efﬁcient way of removing resist from

the fronts and backsides of wafers with thin photoresist residue or thick, hard-baked

photoresist that has been exposed to harsh etchants or high-energy plasmas.

Freshly mixed resist strippers with sulfuric acid and hydrogen peroxide

(described as “piranha” by those who have experienced a drop or two of this self-

heating combination on exposed skin) also provide a quick way to remove resist

from wafers. Wet stripping of resists that cause effervescent bubble formation can

lift and ﬂoat wafers into undesirable positions, invoking the need for aggressive agi-

tation and pre-emptive withdrawal and reinsertion of a wafer holder or cassette to

knock off aggregated bubbles on wafers, particularly those wafers with thick single-

or double-sided resist. Organic solvents such as acetone can effectively remove

resist prior to postbaking, although they can be ineffective after hardening and expo-

sure of the resist to plasma etching. Liftoff techniques remove the resist and undercut

overlying metal ﬁlms, often with ultrasonically agitated acetone in a metal tank.

Wafer cleaning cycles prior to oxidation cycles or LPCVD depositions will

remove some residual resist, however, it is expected that the photoresist stripping

sequence will remove all of the resist prior to wafer cleaning. PFA or quartz beakers

and holders are generally used for wet stripping of resist, which can take place at

elevated temperatures up to about 120

◦

C. For elevated-temperature acidic strippers,

rinsing of wafers immediately after resist removal must be done carefully to avoid

spitting of the acid and water when the two meet on the hot wafer surfaces. Dump

rinsing after the strip step effectively dilutes and removes the acid, base, or solvent,

followed by a spin–rinse–dry cycle or use of a nitrogen gun to blow-dry the wafers.

Visual validation of resist removal can be augmented with microscope inspection

for residual resist that may show as thin, redeposited sheets of hardened resist on

underlying wafer features.

The tables given below present relevant procedures for photoresist removal.

Although not exhaustive, they provide the MEMS and integrated MEMS developer

with information on common resist-related operations with chemicals commonly

available in a cleanroom processing environment. Table 8.9 lists a range of wet

502 D.W. Burns

Table 8.8 Standard metal (Al, Ti, W) etchants and etch processes

Material

Etch rate

(Å/s) Etchant Remarks and references

1 Aluminum (Al),

deposited

870 H

(96%): H

(30%)

56:1

120

◦

C; high-sulfuric piranha;

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

(3–15 Å/s), Cr

(1–3 Å/s), C u (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), Pt

(0.5 Å/s), TiW (0.1 Å/s);

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

SiGe(poly), Si

,SiO

, Ta; exothermic; add H

immediately prior to use; DI water rinse [27, 28]

2 Aluminum (Al),

deposited

(96%):

HCl(38%):H

1:1:1

20–50

◦

C; doesn’t signiﬁcantly etch Si, SiO

; DI water rinse [45]

3 Aluminum (Al),

deposited

33 H

(85%)

undiluted

◦

C; etches slightly Si, SiO

; DI water rinse [45]

4 Aluminum (Al),

deposited

100 H

(85%):

(30%):H

8:1:1

◦

C; Krumm etchant; hardbake PR for improved etch resistance;

etch rate increases to 120 Å/s at 50

◦

C, 300 Å/s at 80

◦

C; also

etches GaAs; DI water rinse [26, 89]

5 Aluminum (Al) 25 H

(85%): HNO

(70%)

16:1

◦

C; add water to reduce etch rate; agitate; DI water rinse [26]

6 Aluminum (Al),

evaporated

15 H

(85%):

HNO

(70%):H

16:1:4

Room temperature; DI water rinse [90]

7 Aluminum (Al),

evaporated

(85%):

HNO

(70%):Acetic:

4:1:4:1

◦

C; PAN etchant (phosphoric-acetic-nitric acids); etch rate

increases with temperature; DI water rinse [91]

8 MEMS Wet-Etch Processes and Procedures 503

Table 8.8 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

8 Aluminum (Al),

deposited

15–50 H

(85%):

HNO

(70%):Acetic:

16:1:1:2

35–45

◦

C; PAN etchant; faster etch rate with higher temperature;

rate varies with Cu and Si in Al alloy; clear H

bubbles with

agitation; DI water rinse [30]

9 Aluminum (Al),

deposited on GaAs

HCl(38%):H

1:2

◦

C; can be used with GaAs; DI water rinse [8]

10 Aluminum (Al),

sputtered

1400 HCl(38%):

(30%):H

8:1:1

◦

C; etch rate increases to 1700 Å/s at 50

◦

C; hardbake PR for

improved etch resistance; DI water rinse [89]

11 Aluminum (Al),

deposited

100 HCl(38%):

HNO

(70%):H

3:1:2

◦

C; mask with PR;

Etches Ag, Au (115 Å/s), AlSi (2%), Cu (100 Å/s), Mo (110 Å/s),

Ni (17 Å/s), Pd (65 Å/s);

Etches slightly Al

(0.2 Å/s), SiO

(PECVD) (0.1 Å/s), Pt

(0.6 Å/s), Ta (0.3 Å/s), Ti (0.1 Å/s), TiW (0.6 Å/s), W (0.9 Å/s);

Doesn’t signiﬁcantly etch Cr, Nb, polyimide, quartz, sapphire, Si,

Si(poly), Si

,SiO

(LTO), SiO

(PSG); water added to reduce

PR attack; self-heating; DI water rinse [28]

12 Aluminum (Al) 4000–8000 HCl(38%):

HNO

(70%):H

10:1:9

◦

C; Al and Al alloys; DI water rinse [92]

13 Aluminum (Al),

deposited

40 HF(49%):H

1:10

◦

C; DHF etchant (10:1); etch rate for AlSi (2%); rate drops for

high HF concentrations (0.7 Å/s for HF etchant (49 wt%),

0.4 Å/s for anhydrous HF [93]);maskwithPR;

Etches SiO

(4 Å/s), SiO

-LTO (6 Å/s), SiO

-PSG (80–250 Å/s),

Ti (180 Å/s), TiW (1 Å/s);