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

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574 D.W. Burns

Table 8.21 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

32 Polyethylene glycol

(PEG)

undiluted

Room temperature; also dissolves in acetone and alcohol [404]

33 Polyethylene

terephthalate,

(PET), Mylar



4200 H

(96%):

Phenol:H

115:76:9 by

weight

110

◦

C; nonstandard chemical; DI water rinse [405]

34 Polyimide (PI,

Kapton



adhesive-backed

tape

Acetone

undiluted

Apply tape with tweezers on top of PR; peel tape off manually or soak

in acetone to remove [406]

35 Polyimide (PI) 150 CF

/Ar/O

plasma

5:10:30 sccm

Dry etch; 300 mT; 180 W; etches Si

(9 Å/s), oxide-LTO (9 Å/s), Si

(3 Å/s); doesn’t signiﬁcantly etch Al [45]

36 Polyimide–Kevlar

(PI–Kevlar



PCB substrates

(96%)

undiluted

185

◦

C; DI water rinse [407]

37 Polyimide (PI) H

(96%):

(30%)

7:1

◦

C; Caro’s acid; DI water rinse [45]

38 Polyimide (PI), spun 2800 H

(96%):

(30%)

50:1

120

◦

C; add fresh peroxide; DI water rinse [28]

39 Polyimide (PI), spun 1 HF(49%): NH

(40%)

1:5

◦

C; BHF etchant (5:1); DI water rinse [28]

40 Polyimide (PI) HNO

(70%)

undiluted

◦

C; DI water rinse [45]

8 MEMS Wet-Etch Processes and Procedures 575

Table 8.21 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

41 Polyimide (PI) 650 O

plasma Dry etch; lateral etch rate; doesn’t signiﬁcantly etch SiC [408]

42 Polyimide (PI,

Kapton



)

55–295 Proprietary 40–60

◦

C; Transene Kapton polymide ﬁlm etchant; mask with PR or

Riston [409]

43 Polymer, residues H

(96%):

(30%)

2:1 to 4:1

90–140

◦

C; general organics; DI water rinse [109]

44 Polymer, residues HF(49%):H

1:10 to 1:200

◦

C; DHF etchant; can undercut organics; DI water rinse [109, 410]

45 Polymer, residues NH

OH(29%):

(30%):H

1:1:5

40–75

◦

C; general organics; DI water rinse [109]

46 Polymer, sidewall NH

OH(29%):

(30%)

5:1

120

◦

C; polymer-dependent; can etch Si; DI water rinse [411]

47 Polymer, sidewall H

(96%):

(30%)

4:1

120

◦

C; polymer-dependent; DI water rinse [411]

48 Polymethylmeth-

acrylate

(PMMA)

Acetone

undiluted

Room temperature; may use ultrasonic agitation; DI water or

isopropyl alcohol rinse [412]

49 Polymethylmeth-

acrylate

(PMMA)

Chloroform

undiluted

Room temperature; nonstandard solvent; solvent bonding of

PMMA–PMMA [413]

50 Polymethylmeth-

acrylate

(PMMA)

DCM

undiluted

Room temperature; nonstandard solvent; DI water or isopropyl

alcohol rinse [414]

576 D.W. Burns

Table 8.21 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

51 Polymethylmeth-

acrylate

(PMMA)

NMP

undiluted

Room temperature; nonstandard solvent; consider MEK, THF or

others [415]

52 Polyparaxylylene

(Parylene C)

80 O

RIE

120 sccm

Dry etch; 400 W; 200 mTorr [416, 417]

53 Polypropylene (PP) 0.3 CrO

600 g:1000 mL

◦

C; chromic acid, 6 M solution; nonstandard chemical; DI water

rinse [418]

54 Polypropylene (PP) DCM

undiluted

◦

C; DI water or isopropyl alcohol rinse [419]

55 Polypropylene (PP) HCl(38%):

HNO

(70%)

3:1

Warm; aqua regia; DI water rinse [420]

56 Polystyrene (PS),

sheet ﬁlm

Acetone

undiluted

Room temperature; DI water rinse [394]

57 Polystyrene (PS),

spun

Toluene

undiluted

Room temperature; nonstandard solvent; doesn’t signiﬁcantly etch

SU-8; DI water rinse [421]

58 Polysulfone (PSF) NMP

undiluted

Room temperature; nonstandard solvent; DI water or isopropyl

alcohol rinse [422]

59 Polytetraﬂuoro-

ethylene (PTFE,

Teﬂon), substrate

160 Ar IBE Ion beam etch; Ar

ions; 500 eV; 0.5 mA/cm

,[423]

60 Polytetraﬂuoro-

ethylene (PTFE,

Teﬂon), ﬁlm

Benzoin dianon:

Potassium tert-

butoxide:Me

0.27 g:1.0 g: 35

◦

C; nonstandard chemicals; surface modiﬁcation; strip with KClO

in H

; DI water or isopropyl alcohol rinse [424, 425]

8 MEMS Wet-Etch Processes and Procedures 577

Table 8.21 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

61 Polytetraﬂuoro-

ethylene (PTFE,

Teﬂon), sheet ﬁlm

Sodium, naphthalene 50

◦

C; FluoroEtch



, Acton Technologies; non-standard chemical;

surface treatment; not for cross-linked ﬂuoropolymers; IPA and DI

water rinse [426, 427]

62 Polytetraﬂuoro-

ethylene (PTFE,

Teﬂon), ﬁlm

THF

undiluted

Warm; nonstandard solvent; surface modiﬁcation; DI water or

isopropyl alcohol rinse [424]

63 Polyurethane (PU) Acetone Room temperature; causes swelling then weight loss with 65

◦

bakeout [428, 429]

64 Polyvinyl chloride

(PVC)

MEK

undiluted

Room temperature; nonstandard solvent; DI water or isopropyl

alcohol rinse [430]

65 Polyvinyl chloride

(PVC)

THF

undiluted

Room temperature; nonstandard solvent; DI water or isopropyl

alcohol rinse [431]

66 Polyvinylidene

ﬂuoride (PVDF)

HNO

(70%)

undiluted

Elevated temperature; DI water rinse [432]

67 Polyvinylidene

ﬂuoride (PVDF)

NMP

undiluted

Room temperature; nonstandard solvent; DI water or isopropyl

alcohol rinse [422]

68 Residues, inorganic H

undiluted

Room temperature; water-soluble inorganics such as salts,

electrolytes, acidic residues, alkaline residues [45]

69 Residues, organic H

(96%):

(30%)

7:3

Elevated temperature; stubborn organics; exothermic; stir and mix

carefully; do not store; consider O

plasma; DI water rinse [45]

70 Residues, organic Isopropyl alcohol,

Acetone

undiluted

Room temperature; soluble organics; DI water rinse [45]

71 Silicone, gel Isopropyl alcohol

undiluted

Room temperature; effective for silicone gels; DI water rinse [45]

72 Silicone HNO

(70%)

undiluted

Room temperature soak; can heat to boiling for faster removal;

consider commercial strippers; DI water rinse [45]

578 D.W. Burns

Table 8.21 (continued)

Material

Etch rate

(Å/s) Etchant Remarks and references

73 Silicon rubber

(RTV),

encapsulant

530 TMAH (24 wt% in

methanol):IPA

1:10

◦

C; IPA rinse [433]

74 SU-8 (PR), spun and

uncured

Acetone

undiluted

Room temperature; DI water rinse [434]

75 SU-8 (PR), spun H

(96%):

(30%)

50:1

◦

C; DI water rinse [435]

76 SU-8 (PR), spun Methyl chloride,

phenol, and

organic acids

Room temperature; Miller–Stephenson MS-111 epoxy stripping

agent, nonstandard solvent; DI water rinse [436]

77 SU-8 (PR), spun 165 NMP

undiluted

Hot; nonstandard solvent; DI water rinse [421]

8 MEMS Wet-Etch Processes and Procedures 579

aCF

(20:80) or SF

(10:90) plasma, or by using a piranha solution of sul-

furic acid and hydrogen peroxide (4:1) at 120

◦

C followed by a DI-water rinse and

spin dry [376, 437, 438].

8.5.3.2 Example 2: COC Patterning and Solvent Bonding

A homogeneous, trilaminate microﬂuidic device for DNA sequencing is formed

from three layers of cyclo–oleﬁn copolymer (COC) by hot-embossing (above the

glass-transition temperature) 50 µm-deep features from a 50 µm-thick, negative-

tone dry-resist pattern on a 200 µm-thick PET backing sheet into a middle layer

of 130 µm-thick COC. An upper layer of COC is solvent-bonded to the middle

layer by pretreating the bonding surface of the upper COC ﬁlm with a 1:12 mix-

ture of hexadecane in isopropyl alcohol (IPA), removing the excess liquid, allowing

the solvent mixture to dry while penetrating and plasticizing the COC surface, then

warm-laminating (below the glass-transition temperature) the two layers t ogether

to form covered channels. After punching features through the two layers to access

the channels, a lower layer of COC is treated with the solvent mixture in a similar

manner and warm-laminated to the previously laminated middle layer to seal off

one side of the access ports. A blade may be used to trim the tri-layer laminate to

size, and ﬁttings may be epoxied over the ports [378].

8.5.3.3 Example 3: LIGA Mold Removal

A 120 µm-thick layer of PMMA is cast and polymerized on a silicon wafer hav-

ing a 500 Å deposited nickel seed layer on top of a 3.0 µm deposited oxide layer.

The PMMA is exposed with an X-ray source using an X-ray mask, developed in a

commercial developer, and plated with nickel from a nickel sulfamate plating bath.

The PMMA mold is stripped in acetone, re-exposed brieﬂy from the X-ray source

without a mask, and redeveloped to remove residual PMMA. The nickel seed layer

is removed in the open areas with a dilute solution of nitric acid and water (1:20) for

1 min. The sacriﬁcial oxide layer is removed under narrower nickel features by etch-

ing in BHF etchant (5:1) for 15 min and the wafer with freestanding nickel actuator

features is rinsed carefully in DI water [414, 439].

8.6 Anisotropic Silicon Etching and Silicon Etch Stops

Anisotropic etching is direction-dependent etching, stemming from crystalline char-

acteristics of the material being etched or from induced anisotropies in the etch

process itself. The effect has been studied extensively and used for a variety of

MEMS devices including pressure sensors, accelerometers, angular rate sensors,

microphones, microﬂuidic devices, inkjet nozzles, through-wafer vias, and cap

wafers. Crystallographically dependent anisotropic etchants have etch rates that are

dependent on the crystal orientation of the substrate or thin ﬁlm. Isotropic etchants

are blind to the crystallography and etch in all directions at approximately the same

580 D.W. Burns

rate, resulting in appreciable undercutting of a selective masking layer with a lateral

distance close to the etched depth.

Anisotropic etches, with carefully aligned masking patterns, can result in min-

imal undercutting although the etches generally produce pronounced ﬂanks. A

single-crystal silicon substrate, with its diamond crystal lattice, provides certain

planes that etch quickly and other planes that etch slowly in anisotropic etchants

such as potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH),

ethylene-diamine pyrocatecol (EDP), ammonium hydroxide (NH

OH), sodium

hydroxide (NaOH), hydrazine (N

), and others. Surfaces with exposed {100}

planes such as the topside or bottomside of a (100) wafer etch quickly in these

etchants, whereas (111) sidewalls at 54.74

◦

(the sidewalls are closer to the vertical

than 45

◦

) etch appreciably more slowly. For example, a large open-square feature

in a hard mask on the wafer surface will result in a cavity with a square bottom

and tapered sides whereas a small open feature r esult in a pointed, self-stopping

inverted pyramidal cavity. Rectangular patterns produce rectangular cavity bottoms

with tapered sidewalls or a v-groove trench for small geometries. Silicon wafers

with (111) surfaces etch minimally and are not often used for anisotropic etching.

Wafers with (110) and more exotic orientations allow trenches with nearly verti-

cal sidewalls, however, they don’t allow rectangular-bottomed cavities. Geometrical

features such as exterior (concave) corners result in oddly faceted structures that

may require specially conﬁgured undercutting features placed on the mask or

etchant additives to achieve the desired etched shape with minimal loss of pattern

ﬁdelity. Deeply etched cavities may show pillowing effects where the anisotropic

etchant etches slightly more quickly near the sides of the cavity than in the middle,

resulting in a somewhat thicker center region.

The primary use of anisotropic etching is to form diaphragms, cavities, or

trenches that may require signiﬁcant removal of the substrate top or bottom. Etched

structures such as diaphragms fabricated using a timed etch can be difﬁcult to pro-

duce repeatedly, particularly when the ﬁnal diaphragm thickness is a small fraction

of the total wafer thickness. For example, a small 1% variation in the etch rate can

result in a nearly 10% variation in the feature thickness when the target diaphragm

thickness is 1/10 of the substrate thickness. Automatic etch stops that allow signif-

icant overetching to accommodate etch rate variations are often used to surmount

this problem. Etch stops such as heavily boron-doped silicon layers or a buried oxide

layer in a silicon-on-insulator (SOI) wafer are built in, and are more robust compared

to electrochemical etch stops that require externally connected voltage supplies.

Etch-resistant layers such as patterned oxide or silicon nitride are commonly used

to transfer patterns into the substrate and to protect active devices and peripheral

portions of each die during the etch sequence. Anisotropic wet etchants generally

require elevated etch-bath temperatures with careful control over the etchant con-

centration and a watchful eye for etchant exhaustion. Although anisotropic etching

remains prevalent for many devices, surface micromachining with thin-ﬁlm sac-

riﬁcial layers or dry etching of the substrate with high-speed deep reactive ion

etchers (D-RIE) are used for many contemporary designs. The following sections

8 MEMS Wet-Etch Processes and Procedures 581

and tables provide more detailed information on etch rates for silicon with vari-

ous anisotropic etchants, important types of passive and active etch stops, and a

few examples.

8.6.1 Anisotropic Etching of Silicon

Anisotropic etching of silicon strongly depends upon wafer orientation and pat-

terned features including shape, size, orientation, and corner positions. Figure 8.11

depicts several representative geometries, including a large square, a long rectan-

gle, a small circle, and an annular square to illustrate typical cavity geometries that

result in anisotropic etching of a (100) silicon wafer. Long rectangular features form

trenches with elongated bottoms or self-terminating v-grooves for narrow features

when aligned along a <110> direction. Features such as small squares, circles, and

small mask defects tend to form self-limiting pyramidal pits. Features with exterior

corners such as a square in a square result in undercutting of the internal corners

as higher-order crystalline planes become exposed, each with its own chemistry-

dependent etch rate that can be higher than the (100) etch rate. Area permitting,

enlarged corner-compensation features can be placed on the exterior corners to pro-

duce the desired feature as the etch process reaches completion, although these

approaches place additional sensitivity on the etch rate and etch time. Certain addi-

tives may also slow the etching of the higher-order planes and eliminate the need

for corner compensation features. One may notice that rotational misalignment of

the mask features can result in excessive sidewall undercutting. Manufactured ori-

entation ﬂats and notches can identify the crystal orientation to about 0.5

◦

or so,

Fig. 8.11 Basic patterns in a masking layer on the surface of a (100) silicon wafer, followed

by anisotropic etching into the substrate. (a) A large square pattern results in a smaller square-

bottomed cavity with 54.736

◦

(cos

–1

(1/sqrt(3))) ﬂanks. (b) A long rectangular feature results in a

self-limiting trench or v-groove. (c) A small circular feature or mask defect results in a self-limiting

pyramidal pit. (d) A patterned square within a square results in ﬂanks along the outer walls and

near the centers of the inner walls, whereas the corners of the inside square will preferentially

undercut as higher-order planes become exposed

582 D.W. Burns

however, devices requiring more concise alignment may beneﬁt from initially form-

ing one or more long, narrow v-grooves on each wafer, then aligning to the groove

edges. Devices with deep cavities may have signiﬁcant projected-area loss due to the

sloped sidewall ﬂanks and the wafers may become quite fragile as they are etched,

prompting special handling protocols and ﬁxtures.

The etch rates of silicon with anisotropic etchants depend primarily upon the

exposed crystal faces, etchant type, etchant temperature, and etchant concentration.

To a lesser extent, the etch rate depends upon the amount of agitation or stirring,

amount of exposed area, etchant exhaustion, substrate doping, and etchant addi-

tives. Approximate etch rates of several standard anisotropic etchants including

KOH, TMAH, NH

OH, EDP, and hydrazine are listed in Table 8.22. Ammonium

hydroxide, although much slower than KOH or EDP etchants, is generally a standard

cleanroom chemical and should be considered, particularly for etching shallower

topside features. Sodium hydroxide etchants and others may present contamination

concerns. Wafers etched in KOH, TMAH, or NH

OH etchants can be cleaned suf-

ﬁciently to allow continued processing in standard IC processing equipment. EDP

etchants stop better than KOH on abrupt heavily doped p

layers and have an appre-

ciably higher etch selectivity to oxide, although they present some handling and

discarding concerns. Most of the anisotropic silicon etchants will attack standard

aluminum pads and traces, however, TMAH and NH

OH etchants can be predoped

with silicon powder or silicic acid to provide an anisotropic etchant with high resis-

tance to standard aluminum metallurgy. Many excellent articles and summaries have

been written on anisotropic etching of silicon, and the reader is encouraged to con-

sult these publications [440–448] and books [78, 449–455] on the subject for more

detailed information.

8.6.2 Heavily Doped Silicon Etch Stops

Silicon with high concentrations of substitutional elemental boron has a substan-

tially slower etch rate in most anisotropic etchants and can be used as an effective

etch stop, as illustrated in Fig. 8.12. Alkaline anisotropic etchants such as CsOH,

KOH, LiOH, NaOH, and RbOH exhibit a strong reduction in the etch rate for high

boron concentrations in silicon exceeding about 2 ×10

−3

. Organic anisotropic

etchants such as EDP and TMAH, ammonium-hydroxide etchants, and hydrazine

etchants show similar effects. Boron concentrations in excess of 1 × 10

−3

can

cause an etch rate reduction of 100 or more [473].

Highly doped p

silicon can be formed in a silicon wafer using solid source dif-

fusion of boron from a boron–nitride wafer at elevated temperatures, from gaseous

deposition and diff usion of boron, from incorporation of high levels of boron dopant

during epitaxial silicon growth, during crystal pulling of silicon from a melt with a

high concentration of boron, or from a high dosage of ion-implanted boron. The

heavily boron-doped silicon is under high tensile stress that may result in the for-

mation of slip planes visible to the eye as plaid-type patterns, although germanium

may be codoped with the boron to provide strain compensation and reduce slippage.

8 MEMS Wet-Etch Processes and Procedures 583

Table 8.22 Anisotropic silicon etchants and etch processes

Material

Etch rate

(Å/s) Etchant Remarks and references

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

(CsOH, KOH,

LiOH, NaOH,

RbOH)

Less commonly used anisotropic e tchants except for KOH

2 Silicon (Si), (100) 140 Ethylenediamine:

pyrocatechol:H

680 mL:120 g: 320

110

◦

C; EPW etchant; mask with Ag, Au, Cr, Cu, Si

,SiO

,Ta;

Etches Si(110) 83 Å/s; Si(111) 8 Å/s;

Doesn’t signiﬁcantly etch Ge, SiO

(0.05 Å/s); reduced etch rate for

SiGe (>5% Ge); use quartz reﬂux system; sensitive to oxygen;

reduced etch rate with high p-type doping; DI water rinse [442]

3 Silicon (Si), (100) 225 Ethylenediamine:

pyrocatechol:

pyrazine:H

750 mL:240 g:

4.5 g:240 mL

115

◦

C; EDP etchant, type F; less sensitive to oxygen; residues at

lower temperatures; DI water rinse [456]

4 Silicon (Si), (100) 40 Ethylenediamine:

pyrocatechol:

pyrazine:H

880 mL:140 g:

5.3 g:120 mL

◦

C; EDP etchant, type S;

Etches Si(110) 53 Å/s, Si(111) 0.6 Å/s, SiO

(0.001 Å/s); etches

Si(100) 125 Å/s at 115

◦

C, 90 Å/s at 105

◦

C, 70 Å/s at 95

◦

C, 13 Å/s

at 50

◦

Fewer residues at lower temperatures; low oxygen sensitivity; reduced

etch rate with high p-type doping; stir continuously; DI water rinse

[447, 456]

5 Silicon (Si), (100) 70 Ethylenediamine-

based

100

◦

C; Transene PSE-300 etchant; preferential <100> directions;

negligible <111> plane; mask with SiO

; etches Al, Cu;

Doesn’t signiﬁcantly etch Ag, Au or Ta; use quartz ﬂask with reﬂux

condenser; initial dip in DHF recommended; DI water rinse [472]