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

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584 D.W. Burns

Table 8.22 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

6 Silicon (Si), (100) 4.4 KOH:H

250 g:1000 mL

◦

C; KOH etchant (20 wt%); mask with Au/Cr, Si

,SiO

;

Etches Si(110) (6 Å/s), Si(111) (0.04 Å/s), SiO

(0.003 Å/s);

Etch rate slows for high p-type dopant levels; rough surfaces; stir

continuously; ultrasonic agitation helpful; DI water rinse [447]

7 Silicon (Si), (100) 240 KOH:H

250 g:1000 mL

◦

C; KOH etchant (20 wt%); mask with Si

,SiO

(for shallow

etches);

Etches Si(110) (370 Å/s), Si(111) (5 Å/s), SiO

(0.7 Å/s); slows for

high p-type dopant levels; near highest Si etch rate with KOH

concentration across temperature; rough surfaces; stir continuously;

DI water rinse [447]

8 Silicon (Si), (100) 180 KOH:H

1:2 by weight (1 kg

KOH pellets to 2

L water for 29%

KOH by weight;

pellets are

10–15% water)

◦

C; KOH etchant (29 wt%); PR mask fails; mask with thermal SiO

(140:1), SiO

(LTO) (110:1), Si

(>1000:1), Si

(low-stress)

(>1000:1), W, sandwich of SiO

(PECVD) and Si

,AuonCr

adhesion layer;

Etches Al (2000 Å/s), AlSi(2%) (fast), Al

(130–400 Å/s), Cr

(0.7 Å/s), Ge(poly) (high), Nb (0.5 Å/s), Pyrex (2 Å/s), quartz

(1 Å/s), Si(poly) (110–160 Å/s), Si

(PECVD) (0.1 Å/s), SiO

(1.3 Å/s), SiO

(LTO) (1.5 Å/s), SiO

(PECVD) (1–2 Å/s),

SiO

(PSG) (6 Å/s), Ta (0.5 Å/s), Ti (softens), TiW (50 Å/s), Va

(2 Å/s);

Doesn’t signiﬁcantly etch Ag, Au, Cu, Mo, Ni, Pd, Pt, sapphire,

, W; dip wafers in DHF etchant prior to nitride mask

deposition to remove native oxide and reduce undercutting; heat

and stir or pump continuously in PFA tank with recirculating pump

to reduce stratiﬁcation; add IPA to improve uniformity [447]; stops

on p

silicon; DI water rinse [27, 28]

8 MEMS Wet-Etch Processes and Procedures 585

Table 8.22 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

9 Silicon (Si), (100) 3–510 KOH:H

665 g:1000 mL

20–100

◦

C; KOH etchant (40 wt%); mask with Si

,SiO

(for

shallow etches);

Etches Si(100) 3 Å/s at 20

◦

C, 15 Å/s at 40

◦

C, 55 Å/s at 60

◦

C, 175 Å/s

at 80

◦

C, 510 Å/s at 100

◦

Etches Si(110) 5 Å/s at 20

◦

C, 22 Å/s at 40

◦

C, 83 Å/s at 60

◦

C, 275 Å/s

at 80

◦

C, 790 Å/s at 100

◦

Etches SiO

0.003 Å/s at 20

◦

C, 0.03 Å/s at 40

◦

C, 0.2 Å/s at 60

◦

1.1 Å/s at 80

◦

C, 5 Å/s at 100

◦

Near highest SiO

etch rate with KOH concentration; slows for high

p-type dopant levels; smoother surfaces; stir continuously; DI water

rinse [447]

10 Silicon (Si), (100) 2.5–480 KOH:IPA:H

250 g:200 mL:800

20–100

◦

C; KOH etchant (∼20 wt%); mask with Si

,SiO

(for

shallow etches);

Etches Si(100) 2.5 Å/s at 20

◦

C, 12 Å/s at 40

◦

C, 47 Å/s at 60

◦

160 Å/s at 80

◦

C, 480 Å/s at 100

◦

Etches Si(110) 1.3 Å/s at 20

◦

C, 5.5 Å/s at 40

◦

C, 20 Å/s at 60

◦

63 Å/s at 80

◦

C, 175 Å/s at 100

◦

Etches SiO

0.002 Å/s at 20

◦

C, 0.02 Å/s at 40

◦

C, 0.1 Å/s at 60

◦

0.7 Å/s at 80

◦

C, 3.3 Å/s at 100

◦

Smoother surfaces; lower (100) and much lower (110) etch rates with

IPA; reduced (100) to (111) selectivity; reduces undercutting of

exterior corners; DI water rinse [447]

11 Silicon (Si), (100) 175 KOH:tert-butanol:

250 g:150 mL: 850

◦

C; KOH etchant (∼23 wt%) (5 M) before addition of alcohol;

excess alcohol may ﬂoat on surface; minimal exterior corner

undercutting; small hillocks; DI water rinse [457]

586 D.W. Burns

Table 8.22 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

12 Silicon (Si), (100) 330 N

1:1

100

◦

C; hydrazine etchant (1:1); mask with SiO

,Si

;

Etches Al, Cu, polyimide, PR, Si(110) 150 Å/s, Si(111) 20 Å/s, Zn;

Doesn’t signiﬁcantly etch Ag, Au, Ta, Ti;

Smoother surfaces with increasing hydrazine concentration; reduced

Si etch rate with high p-type doping; use reﬂux reactor; be cautious

of fumes; sensitive to oxygen; dip in DHF etchant (20:1) for 20 s

prior to etch; DI water rinse [445, 458, 459]

13 Silicon (Si), (100) 65 NH

OH(29%):H

1:6

◦

C; ammonium hydroxide etchant (3.7 wt%); mask with Si

SiO

; 3:1 selectivity over (110) planes; 25:1 selectivity over (111)

planes; 8000:1 selectivity over SiO

; selectivity >8400:1 over

; rate slows with very high dopant concentrations; 8000:1

selectivity over 1.3 × 10

–3

boron-doped silicon; etches Al

(40 Å/s); mix fresh; agitate to reduce hillocks and surface

roughness; dip ﬁrst into DHF etchant; DI water rinse [460]

14 Silicon (Si), (100) 80 NH

OH(29%):H

1:2

◦

C; ammonium hydroxide etchant (9 wt%); highest etch rate versus

OH concentration; forms hillocks; agitate to reduce surface

roughness; see remarks for 3.7 wt% etchant; etches Si (45 Å/s) at 1

wt%; DI water rinse [460]

15 Silicon (Si), (100) 200 NH

OH(29%):

(30%):H

1000:3:2000

◦

C; ammonium hydroxide etchant (2.65 M); minimal hillocks and

smoother surfaces; etches slightly Si

(<0.01 Å/s), SiO

(LTO) (0.03 Å/s); doesn’t signiﬁcantly etch Al

(with at least 0.13 g Si added per liter), Au, Cr, Ta, Ti; illuminate

p-type samples for smoother surfaces; see remarks for 3.7 wt%

etchant; DI water rinse [461]

16 Silicon (Si), (100) 65 NH

OH(29%):Si:

140 mL:0.1 g: 860

◦

C; ammonium hydroxide etchant (3.7 wt%) plus dissolved Si;

doesn’t signiﬁcantly etch aluminum with prior dissolution of at

least 0.1 g of silicon per liter of etchant; see remarks for 3.7 wt%

etchant without additive; DI water rinse [460]

8 MEMS Wet-Etch Processes and Procedures 587

Table 8.22 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

17 Silicon (Si), (100) 105 TMAH (2 wt% in

◦

C; TMAH etchant (2 wt%); mask with SiO

,Si

; etches Si(111)

2 Å/s; rough surfaces and hillocks; near highest (100):(111)

selectivity; add ∼5 g Si to reduce etch rate of Al; DI water rinse

[462]

18 Silicon (Si), (100) 185 TMAH (4 wt% in

◦

C; TMAH etchant (4 wt%); mask with SiO

,Si

;etches

SiO

(0.06 Å/s); rough surfaces; near peak in (100) etch rate with

TMAH concentration; p-type Si etches slightly slower; DI water

rinse [463]

19 Silicon (Si), (100) 100 TMAH (20 wt% in

◦

C; TMAH etchant (20 wt%); etches Si(110) 185 Å/s, Si(111)

2.8 Å/s; moderately rough surface; DI water rinse [464]

20 Silicon (Si), (100) 165 TMAH (22 wt% in

◦

C; TMAH etchant (22 wt%); mask with SiO

,Si

; relatively

smooth surfaces; etches Al (160 Å/s), Si(110) (240 Å/s), Si(111)

(8 Å/s), SiO

(0.03 Å/s); etch rate reduces with high p-type doping;

DI water rinse [465]

21 Silicon (Si), (100) 75 TMAH (25 wt% in

◦

C; TMAH etchant (25 wt%); mask with SiO

,Si

; smoother

surfaces; DI water rinse [466]

22 Silicon (Si), (100) 130 TMAH (25 wt% in

◦

C; TMAH etchant (25 wt%); etches Si(110) 250 Å/s, Si(111)

4 Å/s; smoother surface; DI water rinse [464, 467]

23 Silicon (Si), (100) 73 TMAH (10 wt%)

and IPA (30 wt%)

in H

◦

C; TMAH etchant (10 wt%) etchant with IPA; mask with SiO

; smooth surfaces; DI water rinse [466]

24 Silicon (Si), (100) 72 TMAH (20 wt%)

and IPA (10 wt%)

in H

◦

C; TMAH etchant (20 wt%) with IPA; mask with SiO

,Si

;

smooth surfaces; similar etch rates for higher-order planes; DI water

rinse [466, 468]

25 Silicon (Si), (100) 165 TMAH (22 wt% in

O):Si

1000 mL:75 g

◦

C; TMAH etchant (22 wt%) predoped with Si; mask with SiO

; relatively smooth surfaces; etches slightly Al (0.05 Å/s),

SiO

(0.03 Å/s); DI water rinse [465]

588 D.W. Burns

Table 8.22 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

26 Silicon (Si), (100) 70 TMAH (25 wt% in

O):Silicic acid

1000 mL:250 g

◦

C; TMAH etchant (25 wt%) with silicic acid; mask with SiO

; etches AlSi (1%) (0.08 Å/s) (50 Å/s without additive),

Si(111) 8 Å/s, SiO

(0.03 Å/s), Si

(0.006 Å/s); dip in DHF

etchant (100:1) for 60 s prior; can use 120 g of dissolved silicon

instead of silicic acid; etch rate, surface smoothness and (100):(111)

selectivity improves with addition of 5 g pyrocatechol per liter; DI

water rinse [469]

27 Silicon (Si), (100) 100 TMAH (25 wt% in

O):Surfactant

1000 mL:5 g

◦

C; TMAH etchant (25 wt%) with surfactant; etches Si(110)

(17 Å/s), Si(111) (1.7 Å/s); smooth surfaces; surfactant is

NCW-601A from Wako, contains 30 wt% polyoxyethylene alkyl

phenyl ether; decreases (110) etch rate dramatically; replicates

exterior corners fairly well; DI water rinse [470, 471]

28 Silicon (Si), (110) 1400 Alkali-based 100

◦

C; Transene PSE-200 etchant; preferential <110> directions;

negligible <111> plane; mask with SiO

;

Doesn’t signiﬁcantly etch Ag, Au orTa; use ﬂask with reﬂux

condenser; initial dip in DHF etchant recommended; DI water rinse

[472]

8 MEMS Wet-Etch Processes and Procedures 589

P++

Fig. 8.12 A silicon wafer

with a heavily doped boron

etch stop causes the etch rate

to reduce appreciably when

the p

layer is exposed to the

etchant

Doped polysilicon ﬁlms have been shown in some cases to provide an effective etch

stop. Phosphorus-doped silicon shows a much smaller effect in these etchants, if at

all. Table 8.23 lists typical etch rate reduction ratios for heavily doped silicon. Etch

rate reductions at particular fractions and dopant levels of interest are presented. The

etch rate reduction for the doped layer is the inverse of the selectivity for the etched

material relative to the etch stop layer. Although the electrically active dopant level

is one of the major factors in etch rate reduction, etchant type, concentration, and

temperature are also important considerations.

8.6.3 Lightly Doped Silicon and Silicon–Germanium Etch Stops

Lightly doped silicon can serve as an etch stop for heavily doped silicon using par-

ticular regions of the HF–nitric–acetic acid system. Although isotropic, the etchant

can effectively stop on lightly doped n-type or p-type silicon after etching a heavily

doped p

or n

silicon layer. Etch stops based on high concentrations of germanium

in silicon are also included here, inasmuch as the etch stop material can be lightly

doped either n-type or p-type independent of the germanium level. Further remarks

are found in Table 8.24.

8.6.4 Ion-Implanted Silicon Etch Stops

Ion implantation is commonly used to locally dope silicon, and heavy dosages of

an implanted species can form adequately doped silicon or a silicon compound that

serves as an effective etch stop f or some silicon isotropic and anisotropic etchants.

For example, high dosages of boron (> 5 × 10

) implanted into silicon or

polysilicon and annealed can serve as an effective etch stop. The peak concentra-

tions can be determined and etch rate ratios estimated from the highly doped boron

) etch stop performance for various etchants (see Table 8.23). Alternatively, etch

stops have been formed in silicon substrates by ion implantation of oxygen, nitro-

gen, or carbon, then annealed to form silicon dioxide, silicon nitride, and silicon

carbide that have high selectivity to the majority of silicon etchants. Some examples

are listed in Table 8.25. Although the depth of the implanted species is limited, the

thickness above the implanted region can be augmented with an additional depo-

sition of epitaxial silicon, provided that excessive dopant diffusion does not occur

590 D.W. Burns

Table 8.23 Heavily doped silicon etch stops and processes

Material

Etch rate

reduction Etchant Remarks and references

1 Silicon (Si), (100) varies Aqueous alkali

(CsOH, KOH,

LiOH, NaOH,

RbOH)

Similar etch stop effects as KOH etchant [448]

2 Silicon (Si), (100)

<1E19 cm

–3

(p)

1.7E19 cm

–3

(p)

3.0E19 cm

–3

(p)

4.7E19 cm

–3

(p)

9.0E19 cm

–3

(p)

1.0E20 cm

–3

(p)

1.0

0.8

0.5

0.1

0.01

0.006

Ethylenediamine:

pyrocatechol:

pyrazine:H

1000 mL:160 g:

6 g:133 mL

◦

C; EDP etchant, type S; p-type dopant (boron); epi-deposited

samples; similar behavior for Si(110); selectivity decreases slightly

with increasing etchant temperature to 110

◦

C; similar behavior for

EPW (non-catalyzed) etchant [447, 448, 456]

3 Silicon (Si), (100)

<1E19 cm

–3

(p)

2.0E19 cm

–3

(p)

2.5E19 cm

–3

(p)

4.5E19 cm

–3

(p)

8.0E19 cm

–3

(p)

1.0E20 cm

–3

(p)

1.5E20 cm

–3

(p)

1.0

0.8

0.5

0.1

0.01

0.003

0.0006

Ethylenediamine:

pyrocatechol:

pyrazine:H

17 mL:3 g: 0.016 g:8

110

◦

C; EDP etchant; p-type dopant; boron diffusion-doped samples;

minimal effect on etch rate ratio of varying pyrazine concentration

and oxygen content in etchant [474]

4 Silicon (Si), (100)

<8E18 cm

–3

(p)

1.5E19 cm

–3

(p)

3.3E19 cm

–3

(p)

8.0E19 cm

–3

(p)

1.0E20 cm

–3

(p)

1.5E20 cm

–3

(p)

1.0

0.8

0.5

0.1

0.06

0.02

KOH:H

315 g:1000 mL

◦

C; KOH etchant (24 wt%); p-type dopant; similar behavior for

Si(110); selectivity decreases with increasing KOH concentration

[448]

8 MEMS Wet-Etch Processes and Procedures 591

Table 8.23 (continued)

Material

Etch rate

reduction Etchant Remarks and references

5 Silicon (Si), (100)

<8E18 cm

–3

(n)

1.5E19 cm

–3

(n)

6.0E19 cm

–3

(n)

4.0E20 cm

–3

(n)

1.0

0.8

0.5

0.2

KOH:H

110 g:1000 mL

◦

C; KOH etchant (10 wt%); n-type dopant; modest reduction in

etch rate with high P concentration [448]

6 Silicon (Si), (100)

>1.5E20 cm

–3

(p) slow

1:1

118

◦

C; 50:50 hydrazine etchant; p-type dopant; boron solid-source

diffused samples; etch rates become extremely slow above this

concentration [458]

7 Silicon (Si), (100)

1.3E20 cm

–3

(p) 0.00013

OH(29%):H

1:6

◦

C; 3.7 wt% NH

OH etchant; p-type dopant; epi-deposited samples

[460]

8 Silicon (Si), (100)

<4E18 cm

–3

(p)

2.0E19 cm

–3

(p)

7.0E19 cm

–3

(p)

1.0E20 cm

–3

(p)

2.0E20 cm

–3

(p)

2.5E20 cm

–3

(p)

1.0

0.8

0.5

0.3

0.1

0.025

TMAH (25 wt% in

◦

C; TMAH etchant (25 wt%); p-dopant; boron implanted and

gas-phase deposited; selectivity increases with decreasing etchant

temperature; minor change in selectivity with etchant concentration

[475]

9 Silicon (Si), poly

<2E18 cm

–3

(p)

4.0E18 cm

–3

(p)

2.0E19 cm

–3

(p)

1.5E20 cm

–3

(p)

4.0E20 cm

–3

(p)

1.0

0.8

0.5

0.1

0.01

TMAH (22 wt% in

◦

C; TMAH etchant (22 wt%); p-dopant; boron implanted at 30 keV

into 2000 Å thick LPCVD polysilicon through 2000 Å silicon

nitride layer, then annealed at 900

◦

C for 30 min; etch rates are

lateral through etch hole in upper nitride layer; selectivity increases

with decreasing TMAH concentration; dissolve silicon to reduce

etch rate of Al [465]

592 D.W. Burns

Table 8.24 Lightly doped silicon and silicon–germanium etch stops

Material

Etch rate

reduction Etchant Remarks and references

1 Silicon (Si), (100)

>4E18 cm

–3

(n)

<1E17 cm

–3

(n)

1.0

<0.01

HF(49%): HNO

(70%):Acetic

1:3:8

◦

C; HNA etchant; mask with Si

or thick SiO

; etches heavily

doped silicon (<0.01 cm) 100–500 Å/s, SiO

(5 Å/s); sensitive to

agitation and defects in lightly doped material (>0.068  cm); DI

water rinse [62, 444, 446, 449]

2 Silicon (Si), (100)

>9E18 cm

–3

(p)

<5E17 cm

–3

(p)

1.0

<0.01

HF(49%):

HNO

(70%):Acetic

1:3:8

◦

C; HNA etchant; see remarks for n-type substrates with same

etchant; DI water rinse [62, 444, 446, 449]

3 Si–Ge (SiGe)

<4E20 cm

–3

(Ge)

1.0E21 cm

–3

(Ge)

3.0E21 cm

–3

(Ge)

8.0E21 cm

–3

(Ge)

1.0

0.8

0.5

0.2

Ethylenediamine:

pyrocatechol:

pyrazine:H

1000 mL:160 g:

6 g:133 mL

◦

C; EDP etchant, type S; no dopant; modest reduction in etch rate

with high Ge concentration [448]

4 Si–Ge (SiGe)

1.5E22 cm

–3

(Ge) 0.0015

EDP 95

◦

C; Si

0.7

0.3

; etches Si (no Ge) 180 Å/s [476]

5 Si–Ge (SiGe)

<4E20 cm

–3

(Ge)

1.5E21 cm

–3

(Ge)

4.0E21 cm

–3

(Ge)

1.0

0.8

0.5

KOH:H

515 g:1000 mL

◦

C; KOH etchant (34 wt%); no dopant; slight reduction in etch rate

with moderate Ge concentration; Ge codoped with boron for strain

compensation (e.g., 1 ×10

−3

and 1 ×10

−3

Ge) provides

reduced etch rates similar to non-Ge samples [448]

6 Si–Ge (SiGe)

<4E20 cm

–3

(Ge)

5.0E21 cm

–3

(Ge)

7.0E21 cm

–3

(Ge)

1.0E22 cm

–3

(Ge)

1.5E22 cm

–3

(Ge)

1.0

0.1

0.05

0.01

0.005

KOH:H

515 g:1000 mL

◦

C; KOH etchant (34 wt%); no boron dopant; high reduction in etch

rate with Ge concentration 17–34%; etches Si (no Ge) at 51 Å/s

[476]

7 Si–Ge (SiGe)

1.5E22 cm

–3

(Ge) 0.0008

KOH:H

130 g:1000 mL

◦

C; KOH etchant (11.3 wt%); Si

0.7

0.3

; etches Si (no Ge) 170 Å/s

[476]

8 MEMS Wet-Etch Processes and Procedures 593

Table 8.24 (continued)

Material

Etch rate

reduction Etchant Remarks and references

8 Si–Ge (SiGe)

1.5E22 cm

–3

(Ge) 0.05

KOH:H

110 g:1000 mL

Room temperature; KOH etchant (10 wt%); Si

0.7

0.3

;etchesSi(no

Ge) 5 Å/s; etch SiGe in HF:H

:Acetic 1:2:3 etchant at 17 Å/s at

room temperature with 1000:1 selectivity to Si [84]

9 Si–Ge (SiGe)

1.5E22 cm

–3

(Ge) 0.004

TMAH(2 wt% in

◦

C; TMAH etchant (2 wt%); Si

0.7

0.3

; etches Si (no Ge) 180 Å/s;

reduced selectivity and slower Si etch rate with 25 wt% TMAH

[476]