JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 6, DECEMBER 2003 761
Etch Rates for Micromachining Processing—Part II
Kirt R. Williams, Senior Member, IEEE, Kishan Gupta, Student Member, IEEE, and Matthew Wasilik
Abstract—Samples of 53 materials that are used or potentially
can be used or in the fabrication of microelectromechanical
systems and integrated circuits were prepared: single-crystal
silicon with two doping levels, polycrystalline silicon with two
doping levels, polycrystalline germanium, polycrystalline SiGe,
graphite, fused quartz, Pyrex 7740, nine other preparations of
silicon dioxide, four preparations of silicon nitride, sapphire,
two preparations of aluminum oxide, aluminum, Al/2%Si, tita-
nium, vanadium, niobium, two preparations of tantalum, two
preparations of chromium, Cr on Au, molybdenum, tungsten,
nickel, palladium, platinum, copper, silver, gold, 10 Ti/90 W, 80
Ni/20 Cr, TiN, four types of photoresist, resist pen, Parylene-C,
and spin-on polyimide. Selected samples were etched in 35
different etches: isotropic silicon etchant, potassium hydroxide,
10:1 HF, 5:1 BHF, Pad Etch 4, hot phosphoric acid, Aluminum
Etchant Type A, titanium wet etchant, CR-7 chromium etchant,
CR-14 chromium etchant, molybdenum etchant, warm hydrogen
peroxide, Copper Etchant Type CE-200, Copper Etchant APS
100, dilute aqua regia, AU-5 gold etchant, Nichrome Etchant
TFN, hot sulfuric
+
phosphoric acids, Piranha, Microstrip 2001,
acetone, methanol, isopropanol, xenon difluoride, HF
+
H
2
O
vapor, oxygen plasma, two deep reactive ion etch recipes with two
different types of wafer clamping, SF
6
plasma, SF
6
+
O
2
plasma,
CF
4
plasma, CF
4
+
O
2
plasma, and argon ion milling. The etch
rates of 620 combinations of these were measured. The etch rates
of thermal oxide in different dilutions of HF and BHF are also
reported. Sample preparation and information about the etches is
given. [1070]
Index Terms—Chemical vapor deposition (CVD), etching, evap-
oration, fabrication, materials processing, micromachining.
I. INTRODUCTION
W
HEN designing a microfabrication process, the etch rate
of each material to be etched must be known. Knowing
the etch rates of other materials that will be exposed to the etch,
such as masking films and underlying layers, enables an etch
process to be chosen for good selectivity (high ratio of etch
rate of the target material to etch rate of the other material)—if
one exists. While several large literature-review compilations
of etches that target specific materials have been made [1], [2],
these only report etch rates in some cases, and rarely have corre-
sponding selectivity information. This paper provides such in-
formation, expanding on an earlier paper [3] to give 620 etch
rates of 53 materials in 35 etches that have been used or may
Manuscript received June 3, 2003; revised October 1, 2003. Subject Editor
A. J. Ricco.
K. R. Williams was with Agilent Laboratories, Agilent Technologies, Palo
Alto, 94303 CA USA. He is currently a private consultant at 185 Willowbrook
Dr., Portola Valley, CA 94028 USA (e-mail: kirt_williams@ieee.org).
K. Gupta was with with Agilent Laboratories, Agilent Technologies, Palo
Alto, CA 94303 USA. He is now at 804 Gregory Ct., Fremont, CA 94359 USA
(e-mail: kishang@ieee.org).
M. Wasilik is with the Berkeley Sensor & Actuator Center, University of Cal-
ifornia at Berkeley, Berkeley, CA 94720-1770 USA.
Digital Object Identifier 10.1109/JMEMS.2003.820936
be used in future fabrication of microelectromechanical systems
(MEMS) and integrated circuits (ICs) (approximately 50 etch
rates measured in the earlier paper have been included in this
one). These data allow the selection of new combinations of
structural material, underlying material, and etchant for micro-
machining.
Table I summarizes the etches tested, abbreviated names for
the etches, and the target materials for each. Table II lists etch
rates of Si,Ge, SiGe, and C in the SI units of nm/min (not
/min
as in the earlier tables) [3]. Table III covers films and wafers that
are primarily silicon dioxide, produced under many different
conditions. Table IV is on silicon nitride and aluminum oxide.
Table V covers the metals Al, Ti, V, Nb, Ta, and Cr. Table VI
continues with the metals Mo, W, Ni, Pd, Pt, Cu, Ag, Au, alloys
10 Ti/90 W, 80 Ni/20 Cr, and compound TiN. Finally, Table VII
gives etch rates of organics: photoresists, a resist pen, and a
spin-on polyimide.
Section II of this paper lists the materials etched, their prepa-
ration, and some uses or potential uses in MEMS and ICs. Sec-
tion III describes the preparation and applications of the wet and
dry etches that were studied, as well as some key experimental
results. Section IV describes etch-rate measurement techniques,
and Section V discusses the results.
II. S
AMPLE PREPARATION
The preparation of the samples in the etch-rate tables is de-
scribed below, listed by the labels (in italics) used across the tops
of the tables. All coated materials were deposited on 100-mm-
diameter silicon wafers. For the isotropic silicon etchant, potas-
sium hydroxide, and a few other etches, the wafers were first
coated with LPCVD silicon nitride so that etches would not pen-
etrate into the silicon or attack the back side of the wafer.
In several cases, similar materials were prepared using dif-
ferent methods (e.g., wafer form, PECVD, LPCVD, and ion-
milled silicon dioxide; annealed and unannealed films) to study
and emphasize the effect on their etching characteristics.
Existing or potential MEMS applications are given for the
materials. Many of the materials were discussed in more detail
previously [3].
A. Silicon, Germanium, SiGe, and Carbon
(100) Si Low-Doped Wafer: Single-crystal silicon, (100) ori-
entation, phosphorus-doped n-type, resistivity of 3–40
-cm,
grown with the Czochralski (CZ method). Single-crystal silicon
is the standard starting material for bulk micromachining.
Float-Zone Si Wafer: Single-crystal silicon, (100) orienta-
tion, undoped, grown with the float-zone (FZ) method for a high
resistivity of
-cm. Float-zone wafers have been used
as substrates in RF MEMS application to reduce eddy-current
loss.
1057-7157/03$17.00 © 2003 IEEE
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762 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 6, DECEMBER 2003
TABLE I
E
TCH DESCRIPTIONS,ABBREVIATIONS, AND TARGET MATERIALS
Polysilicon LPCVD Undoped: Undoped polycrystalline sil-
icon deposited in a Tylan low-pressure chemical-vapor deposi-
tion (LPCVD) furnace with recipe SiH
sccm,
temperature
, pressure mtorr.
Deposited on a wafer with 100 nm of thermal oxide
on it to enable interferometric thickness measurements.
. Undoped poly, which has
a high sheet resistance as deposited, is the most common
structural material for surface micromachining. It can be doped
with ion implantation or by diffusing in dopant atoms from an
adjacent film (e.g., PSG, below) at high temperature.
Polysilicon LPCVD In-Situ
: An n-type, phosphorus-
doped polycrystalline silicon deposited in a Tylan LPCVD
furnace with recipe
sccm, 1.6% PH /balance
sccm, , mtorr. Deposited
on a wafer with thermal oxide on it to enable interferometric
thickness measurements.
. In situ doping gives a
conducting film, useful for thicker films and in cases in which
other considerationslimit the temperature. The deposition rateis
about
that of undoped polysilicon under similar conditions.
Poly Ge LPCVD Undoped: Undoped polycrystalline germa-
nium deposited in Tystar LPCVD furnace with recipe
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WILLIAMS et al.: ETCH RATES FOR MICROMACHINING PROCESSING—PART II 763
TABLE II
E
TCH RATES OF Si, Ge, SiGe, AND C (nm/min)
sccm, , mtorr. The polygermanium
deposition was preceded by the deposition of silicon seed layer
approximately 6 nm thick using the recipe
sccm,
, mtorr.
Germanium forms an oxide that is soluble in water. Thus,
water with a high concentration of dissolved oxygen etches ger-
manium. Hydrogen peroxide is a useful etchant for Ge, etching
faster at higher temperature.
Polygermanium has been used in surface micromachining as
a sacrificial layer in conjunction with a polycrystalline SiGe
structural layer, using warm hydrogen peroxide as the etchant
[4]. The relatively low deposition temperatures are compatible
with CMOS circuitry with aluminum interconnections.
Poly SiGe LPCVD
-Type: A p-type polycrystalline silicon-
germanium deposited in a Tystar LPCVD furnace with recipe
sccm, sccm, sccm,
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764 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 6, DECEMBER 2003
TABLE III
E
TCH RATES OF SILICON DIOXIDE (nm/min)
, mtorr. This film is approximately 48 atomic
% Ge.
Graphite Ion-Milled: Graphite ion-mill-deposited (also
known as ion-beam-deposited) in a Commonwealth Scientific
system from a graphite target with argon ions at 1250 V,
current density of about 2 mA/cm
( mA over most of a
5-inch-diameter target), chamber torr. Graphite
has had little or no use in MEMS to date. In this work, it was
found to be easily deposited and etched in silicon isotropic
etchant. It may find use as a hard mask for plasma etching due
to its low etch rate, and as a dry lubricant in MEMS.
B. Silicon Dioxide
Fused Quartz Wafer: Wafers of General Electric 124 or
NSG N fused quartz source material,
% silicon dioxide,
with amorphous structure (as opposed to true crystalline
quartz). This material is commonly referred to simply as
“quartz.” It is compatible with silicon-wafer processing steps,
and may find application as a substrate in RF MEMS as it is
not conductive, eliminating eddy-current losses.
Pyrex 7740 Wafer: Corning Pyrex 7740 glass, 81% SiO
,
13% B
O ,4%NaO, 2% Al O . Pyrex 7740 (and the very
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WILLIAMS et al.: ETCH RATES FOR MICROMACHINING PROCESSING—PART II 765
TABLE IV
E
TCH RATES OF SILICON NITRIDE AND ALUMINUM OXIDE (nm/min)
similar Borofloat glass) are used in anodic bonding to silicon
due to the high content of mobile sodium ions and to the good
match of thermal expansion rates. The large amounts of non-
silicon-dioxide “impurities” give it noticeably different etching
characteristics, etching slower in 5:1 BHF, but faster in silicon
isotropic etchant.
Thermal Oxide Wet-Grown: Silicon dioxide grown in a
Tylan atmospheric-pressure furnace with the recipe O
carrier
gas at 200 sccm, H
O vapor at a pressure just below 1 atm (the
water source is at 98
) at 1100 , and a total pressure of 1
atm, followed by a 20-min N
anneal at 1100 . .
Thermal oxide forms a conformal coating on silicon. It is
denser and etches more slowly than chemical-vapor-deposited
oxides.
Ann. LTO LPCVD Calogic: Low-temperature silicon
dioxide (LTO) deposited in a Calogic low-temperature
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