Ghodssi R., Lin P., MEMS Materials and Processes Handbook
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514 D.W. Burns
a) Photoresist Mask
b) Hard Mask
Oxide (thermal, LTO, PECVD, sputtered)
Nitride (LPCVD, PECVD, sputtered)
Silicon, Polysilicon (SCS, SOI, epi, LPCVD, sputtered)
Metal (e-beam, sputtered)
Fig. 8.10 Photoresist masks suffice for the majority of patterning and wet- or dry-etch steps for
thin films of oxides, nitrides, silicon, polysilicon, and metals, although hard masks may be used
in some cases. Photoresist masks are predominantly used for masking layers of modest thickness.
For longer and deeper etches, a carefully selected hard mask may be preferred. Although PR masks
and hard masks are options for the etching of most materials of interest, many other device and
process considerations affect the selection of masks
8.4.5.1 Example 1: Wet Etch of Low-Temperature Oxide
A 6000 Å-thick unannealed, undoped oxide deposited on all exposed surfaces of
a silicon wafer from the decomposition of silane and heterogeneous reaction with
oxygen at about 400
◦
C in an LPCVD system is coated with a patterned, 1.1 µm-
thick photoresist mask on the topside of the wafer. BHF etchant (6:1) at room
temperature etches 12 wafers in a dedicated Teflon tank and cassette at a rate of
50 Å/s. The wafers are agitated while in the etchant for the first 10 s. At 15-s inter-
vals after 1
1
/
2
min have passed, the wafers are pulled briefly out of the etchant to
inspect the wafer backsides and to determine when the oxide is gone by observing
whether the backsides pull hydrophobic whereby water does not wet the surface.
The wafers are etched for 120 s to substantially clear the oxide and another 15 s to
provide 10–15% overetch. The wafers are dump-rinsed for five cycles in DI water
and then inserted into a spin–rinse dryer for a 3-min rinse with DI water at 600 RPM
and a 4-min spin with heated nitrogen at 2000 RPM.
The wafers are removed from the dedicated cassette and placed into a run box.
One of the wafers is inspected visually in a 10x microscope to ensure narrow fea-
tures are not excessively undercut, and then measured with an optical film thickness
measurement system (e.g., NanoSpec) to ensure 100% removal of the oxide by mea-
suring the thickness of any oxide on the silicon substrate at five locations on the
wafer, showing that less than 100 Å remain at all tested locations. The photoresist
is stripped from the wafers in a dedicated cassette in a quartz tank for 20 min with
hot piranha comprising nine parts concentrated sulfuric acid (96%) and one part
hydrogen peroxide (30%) mixed 15 min earlier and heated to 120
◦
C, with assis-
tance from self-heating of the exothermic mixture. Goggles and a face shield, double
gloves, cleanroom coat, chemical apron, head mask, and booties are worn in front
8 MEMS Wet-Etch Processes and Procedures 515
of a polypropylene chemical hood with functioning exhaust and a nearby eye wash,
emergency shower, and PA system.
8.4.5.2 Example 2: Wet Etch of Silicon Nitride on Silicon
A 1200 Å-thick layer of stoichiometric LPCVD silicon nitride deposited on silicon
wafers is masked with a 5000 Å-thick layer of annealed undoped LTO and placed
in a Teflon cassette into hot phosphoric acid at a temperature of 160
◦
C for 27 min
(0.75 Å/s etch rate) in a quartz etch tank on a hot plate in a sink with a quartz reflux
system to remove the exposed nitride features on the topside and all of the nitride
on the backside of the wafers. The etch is preceded with a short dip in DHF etchant
(50:1) to remove any oxide from the nitride surface for improved etch uniformity.
8.4.5.3 Example 3: Sacrificial Etch of Deposited Polysilicon Under
a Structural Layer of Stress-Controlled Silicon Nitride
A2µm-thick layer of polysilicon is sacrificially removed from under a 10,000 Å-
thick structural layer of stress-controlled silicon nitride with a 126:5:60 volumetric
mixture of nitric acid (70% by weight), ammonium fluoride (40% by weight), and
DI water at room temperature for 33 min to free 20 µm-wide silicon nitride features
from two sides (10 µm per side at 50 Å/s etch rate), then quenched with slow and
careful insertion into a DI water overflow rinse tank for 5 min followed by careful
blow-drying with a nitrogen gun.
8.4.5.4 Example 4: Aluminum Etching over Patterned Nitride, Oxide,
and Silicon
A wafer with a 0.5 µm-thick layer of sputter-deposited aluminum with 2% sili-
con over patterned layers of silicon nitride, silicon dioxide, and contact openings is
etched with a photoresist mask and a commercial etchant at 40
◦
Cfor1min(80Å/s)
with an additional 10-s overetch using plenty of agitation and temporary withdrawal
of the wafer and Teflon cassette from the polypropylene etch tank several times in
the first 30 s, followed by dump-rinsing and spin–rinse drying.
8.4.5.5 Example 5: Junction Depth Determination for an Integrated
MEMS Device
An integrated silicon CMOS sensor die with a PECVD nitride layer over aluminum
bond pads, an underlying polysilicon layer with a TiN barrier metal atop a t hermal
oxide layer, is placed in a Teflon die holder and initially cleaned with a surfactant
(soap) in water using an ultrasonic bath for 10 min, then rinsed in running DI water
directly under a tap for 0.5 min and dried with filtered nitrogen gas from a nitrogen
516 D.W. Burns
gun to remove loosely bound particulates. This is followed by a soak sequence of
acetone, methanol, and isopropyl alcohol for 5 min each in a solvent bench and dried
with filtered N
2
to remove any s mudges and organic residue from die handling.
The die is placed in a quartz beaker with a Teflon holder on a hot plate set in a
built-in sink of a general-acid wet bench with hot phosphoric acid at 160
◦
Cfor
about 10 min to strip the 2000 Å-thick nitride (3.3 Å/s), followed by a 5-min DI
water rinse. Any remaining aluminum pads and traces are stripped in a mixture of
phosphoric acid, hydrogen peroxide, and water (8:1:1) at 35
◦
C for about two min to
clear the 1.2 µm-thick Al film (100 Å/s), and then rinsed in a beaker of DI water for
5 min. The 100 Å-thick TiN barrier metal is removed in BHF etchant (5:1) at room
temperature for about 4 min (0.4 Å/s) followed by a DI water rinse in a beaker for
5 min. The 4500 Å-thick polysilicon layer is removed in about 1 min with a wet etch
containing nitric acid, hydrofluoric acid, and water (50:1:20) at 25
◦
C (90 Å/s). The
oxide is stripped with BHF etchant (5:1) at room temperature for 15 min to clear
the 15,000 Å-thick film (17 Å/s) using the Teflon die holder and rinsed in running
DI water for 0.5 min. The junction depth i s delineated with a junction-staining etch
of HF(49%) and HNO
3
(70%) at 200:1 for 1 min [8], followed by rinsing in running
DI water for 0.5 min and blow drying with N
2
prior to microscope inspection.
8.5 Nonstandard Materials and Wet Etching
Nonstandard materials include substrates, thin films, etchants, processes, and
equipment that are actually or perceptively incompatible with IC cofabrication.
Cross-contamination is a major issue for any custom or semicustom MEMS process,
and conflict resolution is often attained only by segregation of equipment, wafers,
chemicals, and facilities. Many MEMS devices use IC processes or minor variants
thereof, however, once a nonstandard sequence is reached in a standard wafer pro-
cess flow, the wafers may be unable by policy to re-enter any portion of the standard
flow. Of particular concern are contaminants such as sodium and lifetime killers
such as gold that affect transistor performance and reliability. The seasoned MEMS
device and process designer understands these limitations and generates workable
solutions within facility, process, and design constraints. For example, a MEMS
device may use a single nonstandard wet-etch step after the completion of a 20-
mask CMOS process, and that step may be performed in a dedicated, isolated wet
bench or at an outside facility to avoid conflicts with standard CMOS processing.
Some MEMS process steps, such as an extended sacrificial etch step, may use the
same chemicals and fume hoods as a qualified CMOS sequence, although set up in
a separate etch tank in a dedicated portion of the hood. Often the MEMS steps are
relegated as close to the end of a standard CMOS or bipolar process as possible,
then finished in dedicated equipment.
The following sections present etch rates and processes for nonstandard materials
categorized as dielectrics, conductive oxides and semiconductors, metals, silicides,
plastics and polymers, and examples.
8 MEMS Wet-Etch Processes and Procedures 517
8.5.1 Nonstandard Dielectric, Semiconductor, and Metal Etching
The etch rates of nonstandard dielectrics and metals predominantly depend upon the
film material, deposition technique, type and concentration of etchant, and etchant
temperature. To a lesser extent, the etch rates depend upon stirring or agitation, the
amount of material already etched by the etchant, and pretreatment of the patterned
film such as annealing or native oxide layer removal immediately prior to etching.
A variety of premixed commercial etchants and recipes for mixing an etchant from
basic laboratory chemicals are available for nearly all dielectric and metal films.
Etch beakers, etch tanks, and wafer holders are generally Teflon or polypropylene
for room-temperature processes, with quartz or Pyrex suitable for some etchants.
The etch tanks may be heated for temperature control of the etchant. Rinsing is gen-
erally accomplished with DI water in a dump-rinser followed by a cycle through
a spin–rinse–dryer with hot N
2
. Photoresist masks are generally the most attrac-
tive, although hard masks of a metal, oxide, or other dielectric may be desirable in
some situations. Safety precautions apply to chemical handling, storage, and etching
procedures alike.
Etch rates and etchants for many nonstandard dielectrics, conductive oxides,
semimetals, semiconductors, and compound semiconductors are found in
Table 8.18, and etch rates and etchants for nonstandard metal films are found in
Table 8.19. Etch rates and etchants for standard and nonstandard metal silicides are
found in Table 8.20. Silicides can be formed by depositing the base metal through a
window in a masking oxide, then sintering and stripping the remaining metal with a
suitable metal etchant. Silicides are resistant to most wet etchants, and dry (plasma),
sputter or ion-beam etches may be used to etch them if needed. Silicides of differ-
ent phases (e.g., V
3
Si, V
5
Si
3
, and VSi
2
) may have different etch rates in a given
etchant, and some forms may prove insoluble whereas others etch readily. Many of
the silicides can be oxidized with dry oxygen or steam at elevated temperatures and
then the oxidized base metal etched accordingly [123, 124]. Reviews of s ilicides
and silicide processing can be found in the literature [21, 97, 125–128].
8.5.2 Plastic and Polymer Etching
Wet etching of thin-film plastics and polymers is strongly dependent upon the type
of polymer; t he extent of cross-linking; the chain length; the solvent concentration
in the polymer during etching; and the type, concentration, and temperature of the
etchant. Wet etchants are generally solvent or acid-based. Solvent-based processes
may use PFA, polypropylene, Pyrex, quartz, or stainless steel beakers, etch tanks,
and holders with deliberation. Agitation including ultrasonic means can increase the
etch or dissolution rate and uniformity. Safety measures and proper solvent disposal
must be observed equally. Photoresist serves as a suitable etch mask for many poly-
meric layers, particularly if baked at an elevated temperature. Deposited oxide or
metal may also serve as an etch mask, although the deposition processes for these
518 D.W. Burns
Table 8.18 Nonstandard dielectric, conductive oxide, semimetal, and semiconductor etchants and etch processes
Material
Etch rate
(Å/s) Etchant Remarks and references
1 Aluminum
antimonide (AlSb)
HCl(38%):
HNO
3
(70%)
1:1
25
◦
C; DI water rinse [129]
2 Aluminum
antimonide
(AlSb), MBE
high HF(49%):H
2
O
1:20
Room temperature; DHF etchant (20:1);
Etches GaSb;
Doesn’t significantly etch GaAs, InAs; DI water rinse [130]
3 Aluminum
antimonide (AlSb)
5 HF(49%):H
2
O:
Ethanol
1:700:7000
Room temperature; etch rate decreases with distance from PMMA
patterns;
Doesn’t significantly etch GaSb (<0.05 Å/s), InAs (<0.05 Å/s); DI
water rinse [131]
4 Aluminum
antimonide (AlSb)
HF(49%):
H
2
O
2
(30%):H
2
O
1:1:1
25
◦
C; DI water rinse [129]
5 Aluminum
antimonide (AlSb)
HF(49%):
HNO
3
(70%):Acetic
2:3:1
Room temperature; HNA etchant; faster etch rate with reduced acetic
acid; DI water rinse [132]
6 Aluminum
antimonide
(AlSb), MBE
7 MF319
(TMAH-based
developer)
undiluted
Room temperature; agitate constantly; etch rate depends on thickness;
Etches GaSb (3 Å/s), AlGaSb;
Doesn’t significantly etch InAs; DI water rinse [133]
7 Aluminum arsenide
(AlAs), MBE and
MOCVD
2500 HF(49%)
undiluted
Room temperature; HF etchant (49 wt%); etch rate increases with
increased HF concentration; 10% HF concentration recommended;
Doesn’t significantly etch Al
x
Ga
1−x
As (x < 0.5), GaAs; DI water
rinse [134]
8 Aluminum arsenide
(AlAs), MOCVD
180 HF(49%): NH
4
F
(40%):H
2
O
1:7:200
22
◦
C; etch rate for Al
x
Ga
1−x
As with x = 1.0 and 25:1 H
2
O:BHF
etchant (7:1); DI water rinse [135]
8 MEMS Wet-Etch Processes and Procedures 519
Table 8.18 (continued)
Material
Etch rate
(Å/s) Etchant Remarks and references
9 Aluminum gallium
arsenide
(AlGaAs), MBE
22 H
2
SO
4
(96%):
H
2
O
2
(30%):H
2
O
1:1:50
25
◦
C; DI water rinse [136]
10 Aluminum gallium
arsenide
(AlGaAs), MBE
45 H
2
SO
4
(96%):
H
2
O
2
(30%):H
2
O
1:4:60
Room temperature; DI water rinse [137]
11 Aluminum gallium
arsenide
(AlGaAs), MBE
26 H
3
PO
4
(85%):
H
2
O
2
(30%):H
2
O
1:1:50
25
◦
C; DI water rinse [136]
12 Aluminum gallium
arsenide
(AlGaAs)
630 HF(49%):H
2
O
1:6
Room temperature; DHF etchant (6:1); lateral undercut rate for
Al
0.7
Ga
0.3
As; add H
2
O
2
for less selectivity to GaAs; DI water rinse
[138]
13 Aluminum gallium
arsenide
(AlGaAs),
MOCVD
20 HF(49%): NH
4
F
(40%):H
2
O
1:7:24
22
◦
C; etch rate for Al
x
Ga
1−x
As with x = 0.725 and BHF etchant
(7:1):H
2
O 1:3; etch rate increases with increasing Al fraction;
doesn’t significantly etch Al
x
Ga
1−x
As with x < 0.6; use citric acid
with H
2
O
2
for Al fraction less t han 0.5; DI water rinse [ 135]
14 Aluminum gallium
arsenide
(AlGaAs), MBE
40 NH
4
OH(29%):
H
2
O
2
(30%):H
2
O
10:1:50
Room temperature; etch rate for Al
0.29
Ga
0.71
As; agitation with stirrer
can increase etch rate 3x; DI water rinse [137]
15 Aluminum gallium
indium phosphide
(AlGaInP),
OMVPE
170 H
2
SO
4
(96%)
undiluted
70
◦
C; etch rate for Al
0.35
Ga
0.15
In
0.5
P; slower for lower Al fractions;
etchant is dopant sensitive at 60
◦
C; DI water rinse [139]
520 D.W. Burns
Table 8.18 (continued)
Material
Etch rate
(Å/s) Etchant Remarks and references
16 Aluminum gallium
phosphide
(AlGaP), MBE
1.3 H
3
PO
4
(85%)
undiluted
Room temperature; etch rate for Al
0.5
Ga
0.5
P; DI water rinse [140]
17 Aluminum gallium
phosphide
(AlGaP), MBE
3 HCl(38%)
undiluted
Room temperature; etch rate for Al
0.5
Ga
0.5
P;
Etches InGaP (330 Å/s), AlInP (830 Å/s); DI water rinse [140]
18 Aluminum gallium
phosphide
(AlGaP), MBE
50 HF(49%)
undiluted
25
◦
C; HF etchant (49 wt%); etch rate for Al
0.5
Ga
0.5
P;
Etches AlInP (15 Å/s);
Doesn’t significantly etch InGaP; DI water rinse [141]
19 Aluminum gallium
indium phosphide
(AlGaInP),
OMVPE
380 HCl(38%):H
2
O
1:1
25
◦
C; etch rate for Al
0.35
Ga
0.15
In
0.5
P; slower for lower Al fractions;
dopant sensitive; DI water rinse [139]
20 Aluminum gallium
phosphide
(AlGaP), MBE
50 HCl(38%):
HNO
3
(70%):H
2
O
1:1:1
25
◦
C; etch rate for Al
0.5
Ga
0.5
P; mask with PR, black wax;
Etches AlInP, GaAs, InGaP; decrease nitric component for higher
selectivity to GaAs and lower AlGaP etch rate; decrease H
2
O
component for lower selectivity to GaAs and higher AlGaP etch
rate; stabilize mixture 30 min before use; DI water rinse [140]
21 Aluminum gallium
phosphide
(AlGaP), MBE
50 HF(49%)
undiluted
25
◦
C; HF etchant (49 wt%); etch rate for Al
0.5
Ga
0.5
P;
Etches AlInP (15 Å/s);
Doesn’t significantly etch GaAs, InGaP; DI water rinse [140]
22 Aluminum indium
arsenide (AlInAs),
MBE
HCl(38%):H
2
O
3:1
Room temperature; DI water rinse;
Doesn’t significantly etch GaAsSb [142]
23 Aluminum indium
nitride (AlInN),
MBE
15–120 AZ400K
(KOH-based PR
developer)
undiluted
20–80
◦
C; etch rate for Al
0.81
In
0.19
N; DI water rinse [143]
8 MEMS Wet-Etch Processes and Procedures 521
Table 8.18 (continued)
Material
Etch rate
(Å/s) Etchant Remarks and references
24 Aluminum indium
phosphide
(AlInP), MBE or
MOCVD
35 H
2
SO
4
(96%)
undiluted
25
◦
C; etch rate for Al
0.5
In
0.5
P;
Doesn’t significantly etch InGaP, AlGaP; DI water rinse [141]
25 Aluminum indium
phosphide
(AlInP), OMVPE
370 H
2
SO
4
(96%)
undiluted
70
◦
C; etch rate for Al
0.5
In
0.5
P; DI water rinse [139]
26 Aluminum indium
phosphide
(AlInP), MBE or
MOCVD
40 H
3
PO
4
(85%)
undiluted
25
◦
C; etch rate for Al
0.5
In
0.5
P;
Etches slightly AlGaP (1.3 Å/s);
Doesn’t significantly etch InGaP, AlGaP; DI water rinse [141]
27 Aluminum indium
phosphide
(AlInP), MBE or
MOCVD
800 HCl(38%)
undiluted
25
◦
C; etch rate for Al
0.5
In
0.5
P;
Etches InGaP (330 Å/s), AlGaP (3 Å/s); DI water rinse [141]
28 Aluminum indium
phosphide
(AlInP), OMVPE
480 HCl(38%):H
2
O
1:1
25
◦
C; etch rate for Al
0.5
In
0.5
P; DI water rinse [139]
29 Aluminum indium
phosphide
(AlInP), MBE and
MOCVD
100 HCl(38%):H
2
O
1:5
25
◦
C; etch rate for Al
0.5
In
0.5
P; etch rate decreases to 10 Å/s with
HCl:H
2
O of 1:30;
Doesn’t significantly etch GaAs (>600:1), InGaP (20:1); DI water
rinse [144]
30 Aluminum indium
phosphide
(AlInP), MBE or
MOCVD
600 HCl(38%):
HNO
3
(70%):H
2
O
1:1:1
25
◦
C; etch rate for Al
0.5
In
0.5
P; increasing HNO
3
or HCl concentration
increases etch rate; decreasing HNO
3
concentration increases
selectivity to GaAs; DI water rinse [141]
522 D.W. Burns
Table 8.18 (continued)
Material
Etch rate
(Å/s) Etchant Remarks and references
31 Aluminum indium
phosphide
(AlInP), MBE or
MOCVD
23 HNO
3
(70%)
undiluted
25
◦
C; etch rate for Al
0.5
In
0.5
P;
Etches InGaP (4 Å/s);
Doesn’t significantly etch AlGaP; DI water rinse [141]
32 Aluminum nitride
(AlN), sputtered
90 AZ400K
(KOH-based PR
developer)
undiluted
40
◦
C; unannealed film; etch rate reduces with increased crystallinity
(10x reduction for 1100
◦
C anneal); resistant to most popular
etchants except NaOH and KOH; DI water rinse [143, 145, 146]
33 Aluminum nitride
(AlN)
H
2
O
undiluted
100
◦
C; Di water rinse [21]
34 Aluminum nitride
(AlN), sputtered
300–1000 H
3
PO
4
(85%)
undiluted
80–100
◦
C; orientation dependent; DI water rinse [147]
35 Aluminum nitride
(AlN), hot pressed
1.7 HF(49%):H
2
O
1:1
57
◦
C; DHF etchant (1:1); DI water rinse [148]
36 Aluminum nitride
(AlN), hot pressed
1.3 HF(49%):
HNO
3
(70%)
1:1
57
◦
C; DI water rinse [148]
37 Aluminum nitride
(AlN), CVD
120 HNO
3
(70%)
undiluted
100
◦
C; deposition at 900
◦
C; etch rate is 30 Å/s at 75
◦
C and 1100 Å/s
at 150
◦
C; DI water rinse [149]
38 Aluminum oxide
(Al
2
O
3
, alumina,
sapphire)
25 H
2
SO
4
(96%):
H
3
PO
4
(85%)
1:1
285
◦
C; (0001) orientation; mask with sintered Cr–Pt–Cr; use platinum
holder and flask; DI water rinse [150, 151]
39 Aluminum oxide
(Al
2
O
3
), sputtered
3.8 H
3
PO
4
(85%)
undiluted
50
◦
C; as-deposited film; DI water rinse [152]
40 Aluminum oxide
(Al
2
O
3
), CVD
4H
3
PO
4
(85%)
undiluted
185
◦
C; partially crystalline film; etch rate increases with reduced
crystallinity; DI water rinse [153]
8 MEMS Wet-Etch Processes and Procedures 523
Table 8.18 (continued)
Material
Etch rate
(Å/s) Etchant Remarks and references
41 Aluminum oxide
(Al
2
O
3
), CVD
HF(49%)
undiluted
Room temperature; HF etchant (49 wt%); amorphous form; also
etches in BHF or hot phosphoric acid; doesn’t significantly etch
crystalline form; crystalline form may etch in hot phosphoric with
sulfuric acid; DI water rinse [25]
42 Aluminum oxide
(Al
2
O
3
), CVD
4NH
4
F (40%):
HF(49%)
10:1
Room temperature; BHF etchant (10:1); amorphous film; etch rate
reduces with crystallinity; crystalline films will not etch even in
concentrated HF; DI water rinse [153]
43 Aluminum oxide
(Al
2
O
3
, sapphire)
NH
4
OH(29%):
H
2
O
2
(30%):H
2
O
1:1:3
80
◦
C; substrate cleaning solution before metallization; follow with
HCl:H
2
O
2
:H
2
O 1:1:3 at 80
◦
C, each for 15 min; DI water rinse [26]
44 Aluminum oxide
(Al
2
O
3
, sapphire),
deposited
2 Ortho-phosphoric
acid
180
◦
C; Transetch-N; SiO
2
mask (120:1); silicon selectivity (120:1);
Pyrex or quartz tank; add water to replace evaporated water; DI
water rinse [58]
45 Aluminum
phosphide (AlP)
H
2
O
undiluted
Room temperature [154]
46 Antimony (Sb) H
2
SO
4
(96%)
undiluted
Hot; DI water rinse [25]
47 Antimony (Sb) HCl(38%):H
2
O
1:1
Room temperature; DI water rinse [155]
48 Antimony (Sb) HCl(38%):
H
2
O
2
(30%):H
2
O
30:5:70
Room temperature; DI water rinse [155]
49 Antimony (Sb) HCl(38%):
HNO
3
(70%)
3:1
Aqua regia; DI water rinse [25]
50 Antimony (Sb) HCl(38%):
HNO
3
(70%):H
2
O
1:1:1
Room temperature; DI water rinse [98]