The PolyMEMS INAOE module for surface micromachining has been developed for the fabrication of electrostatic and electrothermal (Joule effect) sensors and actuators. In this module the designer can choose up to 3 Poly silicon layers and aluminum as electrical interconnecting material. A micromechanical test chip has been fabricated which includes the following. a) Micro test structures for residual stress measurement; cantilever beams, clamped-clamped beams, ring-and-beam structures, diamond-and-beam structures, rotation beams, Vernier gauges, cantilever spirals, double-clamped microgauge, and b) Actuators; torsion and bending mirrors, resonators, single two-arms Joule structures (STA), chevron-like Joule arrays, capacitive array for accelerometers. In this work we are presenting the measured residual stress on our process, by using the clamped-clamped beam and ring-and-beam arrays. The measured compressive stress is in the 21-26 MPa range for both types of microgauges. A maximum typical value for this tensile stress is 50 MPa, which is higher than that obtained in our experimental procedure. From this residual stress measurement technique and other mechanical testing routines we can conclude the following: the thermal load, the polysilicon microstructure, and the releasing technique; all of them result in a reliable process for the fabrication of dynamic and static polysilicon microstructures.

1. PolyMEMS INAOE, a Surface Micromachining Fabrication Module
and the Development of Microstructures for Residual Stress
Analysis
A. Alanís, D. Díaz, C. Reyes. C. Zúñiga, A. Torres, P. Rosales, J. Molina, J. Hidalga, M. Linares, M.
Aceves, and W. Calleja
Laboratorio de Microelectrónica and Centro de Diseño de MEMS, INAOE
A. P. 51, C. P. 72000, Puebla, México
wcalleja@inaoep.mx
Abstract
The PolyMEMS INAOE module for surface micromachining has been developed for the fabrication of
electrostatic and electrothermal (Joule effect) sensors and actuators. In this module the designer can
choose up to 3 Polysilicon layers and aluminum as electrical interconnecting material. A
micromechanical test chip has been fabricated which includes the following. a) Micro test structures
for residual stress measurement; cantilever beams, clamped-clamped beams, ring-and-beam
structures, diamond-and-beam structures, rotation beams, Vernier gauges, cantilever spirals, double-
clamped microgauge, and b) Actuators; torsion and bending mirrors, resonators, single two-arms
Joule structures (STA), chevron-like Joule arrays, capacitive array for accelerometers. In this work we
are presenting the measured residual stress on our process, by using the clamped-clamped beam
and ring-and-beam arrays. The measured compressive stress is in the 21-26 MPa range for both
types of microgauges. A maximum typical value for this tensile stress is 50 MPa, which is higher than
that obtained in our experimental procedure. From this residual stress measurement technique and
other mechanical testing routines we can conclude the following: the thermal load, the polysilicon
microstructure, and the releasing technique; all of them result in a reliable process for the fabrication
of dynamic and static polysilicon microstructures.
Keywords: Surface Micromachining, Polysilicon, MEMS, Sensors and Actuators, Residual Stress,
Release Etching.
Introduction
Surface micromachining technology,
with Polysilicon (Poly) films as prime material, is
a key tool for the development of a wide variety
of MicroElectroMechanical Systems (MEMS,
MicroSystems), for medical, automotive,
communications, and some other commercial
applications. Now Microsystem technology
offers new and ever varied developments based
on the great capability of silicon-based
Microelectronics.
Regarding conventional electronic
circuits, oscillators based upon the mechanical
vibrations of crystals such a quartz resonators
have long been ubiquitous in electronics, as a
result of their simplicity and excellent reliability
for frequency control applications. Passive
mechanical silicon resonators generate a
damped response to impulsive stimuli and
require continuous a.c. signal, these small
devices have shown applications in precision
timekeeping [1], communications [2] and
sensing [3]. This distinguishes from bulky active
quartz oscillators, which are characterized by a
spontaneous self-sustaining periodic signal (~10
MHz) requiring power from d.c. sources.
Over the past years, there has been
several considerable efforts to miniaturize such
mechanical resonators, in order to integrate
them on-chip with electronic components to add
frequency-selection and tuning elements [4, 5].
In particular, it is desirable to realize highly
accurate and stable clocks or frequency
references with integrated, chip based systems
using miniaturized resonant devices.
Recently, an autonomous and self-
sustaining nanoelectromechanical oscillator that
generates continuous ultrahigh-frequency
signals (428 MHz) when powered by a steady
d.c. source. This prototype is developed using a
very thin single-crystal silicon carbide (SiC) film
[6]. Single-crystal materials are characterized by
a very low defect density.
In contrast, polycrystalline materials are
produced with high-density defects and a poor
reproducibility from system to system, and the
most critical feature is their structural
dependence on thermal treatments which finally
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2. lead to residual stress in the fabricated
micromechanisms. Hence it is desirable to get
the best control on the mechanical properties of
polysilicon films as a requirement to fabricate
reliable and submicron sized which could attain
some frequency oscillation approaching to that
seen in single crystalline microstructures.
Residual stress in thin films (and
microstructures) is a major concern for the
operation and the reliability of sensors and
actuators. Several methods have been
proposed to extract the residual stress in
polycrystalline films and associated materials
like silicon nitride (Si3N4), phosfosilicate glass
(PSG), and silicon oxide (SiO2) films. Residual
stress can be compressive, which makes the
film expand parallel to the surface, or tensile,
which makes the film shrink. The use of stress
values extracted from the literature is not
enough since the properties of polycrystalline
and amorphous materials can change from
laboratory to laboratory as well as between two
processes. It is therefore essential the
measuring and the control of the stress
separately in each layer that form the
microstructure for designing reliable
micromechanical devices.
The development of a thin film
polysilicon micromachining fabrication module,
the design and computer simulation of static and
dynamic microstructures, the electrical and
mechanical testing, and some applications are
described in this paper.
Experimental
The Poly films were deposited using a
Low-Pressure Chemical Vapor Deposition
(LPCVD) system, at 650 °C using silane (SiH4)
as reactive gas. After deposition the Poly films
were doped to degeneracy (n
++
) in a diffusion
furnace using phosphine at 1,000 ºC. After that
Poly films were thermally oxidized (Poly-SiO2)
at 900 ºC, for selective etching purposes. The
Poly microstructures were developed using
alternatively a potassium hydroxide (KOH)
aqueous solution or a Reactive Ion Etching
technique with a gas mixture based on chlorine
(Cl). This procedure is repeated for each Poly
layer during the fabrication process.
The fabrication module follows three
basic steps: a) A phosphosilicate glass film
(PSG) is CVD deposited, usually 1.0 µm-thick as
a sacrificial layer, then the film is densified
(RPSG) at 1000 °C in H20 and N2 ambient for
40 minutes; b) The Poly film (2.0 µm-thick) is
deposited, patterned and etched; and c) The
microstructures are released using a
(NH4F:CH3COOH:H2O) 33% aqueous solution
[7].
The thermal load for the full fabrication
module with three poly films is shown in Fig. 1.
Fig 1 Thermal load for the surface micromachining module.
Note the steps at 900 °C for growing the Poly-SiO2 films.
The microstructures were fabricated
using Czochralsky-grown (0 0 1), N-type, ρ=2-5
Ω-cm, 2-inches silicon wafers. This type of
substrate was selected because we are aiming
to the fabrication of Microsystems through the
integration of the surface micromachining
technique and a 0.8 µm BiCMOS Si technology
[8, 9]. This Microsystem technology is named
PolyMEMS INAOE.
Following the development of this
surface micromachining module all the
insulating and conducting films are listed next
including the typical thickness in microns.
Thermal SiO2 0.2
LPCVD Si3N4 0.1
Poly 1 0.5
APCVD PSG 1 2.0
Poly 2 2.0
APCVD PSG 2 2.0
Poly 3 1.0
APCVD PSG 3 1.0
Aluminum 2.0
These materials are listed according
with the development of the fabrication process.
The fabrication module is monitored
using the chip PolyMEMS 3, see Fig. 2 (last
page). This chip was designed taking in
consideration the following 9 lithography steps:
Poly 1
Base Poly 2
Contacts 1
Poly2
Base Poly 3
Poly 3
Contacts 2
Aluminum
Releasing
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3. This is a partially planarized surface
micromachining module which allows the
fabrication of several microstructures like that
shown in Fig. 3.
A
B
Fig. 3. A) Torsional micromirrors, and B) Zigzag beams for
lithography resolution analysis.
The PolyMEMS 3 chip was designed including
three main groups of microstructures which are
listed next:
Electrical testing
Electromechanical Actuators
Mechanical testing
The electrical testing devices are
intended for monitoring the electrical properties
of the poly films and the electrical contacts
Aluminum/Poly. In the electromechanical
actuators the following microstructures are
included: inertial sensors, resonators, torsional
and bending mirrors, single two-arms Joule
structures (STA), chevron-like Joule arrays,
capacitive array for accelerometers etc.
The mechanical testing group is
composed by structures mainly designed for
analyzing the residual stress in the poly films.
This block is arranged by Ring-and-Beam
structures, Diamond-and-Beam structures,
Vernier Gauges, Rotation Beams, Cantilever
Spirals, Cantilever beams, and Clamped-
Clamped beams. The test structures and their
overall dimensions were designed according the
lithography capabilities of our current facilities
and their operation mode.
The Ring-and-Beam, Diamond-and-
Beam, and clamped-Clamped Beam
arrangements are intended for analyzing tensile
stress in the x, y plane, the analysis is based
on the measurement of the buckling induced in
the crossbar. The Spiral (see Fig. 4) and
Cantilever Beam arrangements are intended for
analyzing residual stress (+/-) in the z axis. The
Vernier Gauge is mainly utilized for the
calculation of tensile and compressive internal
stress [10, 11].
Fig. 4. Spiral cantilever beam showing a very low residual
stress when is fabricated with a well-controlled thermal
budget.
Release etching.
When surface micromachined structures are
combined with on-chip circuitry, the presence of
aluminum interconnect causes several problems
related with metal corrosion and eventually
contamination, due to the use of hydrofluoric
acid (HF) for releasing the structures. Pure HF
and aqueous solution are the most common
etchants for PSG films during the releasing of
the poly microstructures. For this specific
condition all the metal films are protected with a
photoresistive film and then leading to a longer
releasing procedure because the need to
continuous baking of the resist for recovering
their mechanical integrity.
As an alternative procedure we have
used an ammonium fluoride based-solution
(NH4F) at 50 C° which no shows corrosion in the
aluminum films, but this PSG etching
mechanism is occurring at a lower rate (4:1)
than that seen with HF aqueous solutions. In our
experimental procedure we are alternatively
using an NH4F aqueous solution
(CH3COOH:NH4F:H2O) 33% at 50 °C [7], which
shows an excellent etching mechanism as
shown in Fig. 5, this micrograph shows the
evolution of the lateral etching below some
clamped-clamped beams.
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4. Fig. 5. Evolution of the lateral etching below some clamped-
clamped beams using NH4F aqueous solution, the central
mesa-like regions are the remaining PSG film.
Residual Stress Analysis
In this section the residual stress on 2.0
µm-thick Poly films doped during 70 minutes at
1000 °C (see Fig. 1) is presented. Additionally,
the residual stress is analyzed under the
influence of another thermal treatment, 1000 °C,
30 minutes in nitrogen. In the surface
micromachining module this additional thermal
treatment is utilized for reflow of the PSG film
(RPSG). In this sense it is known that after
deposition and thermal doping of Poly films, if
the film is additionally annealed some
crystallization is promoted and then a higher
temperature annealing leads to lower the
residual stress [12-14].
In the residual stress analysis, first a
clamped-clamped beam array is utilized and
then a ring-and-beam array and a diamond-and-
beam array are utilized for the same calculation.
Fig. 6 shows a SEM micrograph of the clamped-
clamped beam array. Fig. 6A shows a sample
after doping, the structure marked in red is the
reference for illustrating the buckling effect, in
this case the critical beam length Lcr=200 µm
and the width is 50 µm. Fig. 6B shows a sample
with the additional thermal treatment, the
structure marked in red is the reference for
illustrating the buckling effect, in this case the
critical beam length Lcr=400µm and the width is
10µm. Equations (1) and (2) are utilized to
calculate the residual strain e and residual
stress s respectively [11, 15].
2
2
3






=
cr
L
z
π
ε (1)
E
ε
σ = (2)
where E is the Young’s modulus and z is the
beam thickness. According the thermal load for
the poly films we are taking E=160GPa [16],
then for the only-doped poly films the residual
strain is
4
10
94
.
4 −
= x
ε and the residual stress
is MPa
79
−
≈
σ (compressive). For the
samples additionally annealed
4
10
28
.
1 −
= x
ε
and MPa
21
−
≈
σ , which shows a very low
compressive residual stress, according to that
stated in the literature [17].
A
B
Fig. 6. Clamped-Clamped Beam Array utilized to calculate
the residual stress in Poly films. a) After doping, the film is
no additionally thermal treated. B) After doping, the film is
additionally thermally treated.
Discussion
The measured tensile stress is in the
21-26 MPa range for the three types of
microgauges: clamped-clamped beams, ring-
and-beam, and diamond-and-beam structures.
A maximum allowable typical value for this
tensile stress is 50 MPa [11], which is higher
than that obtained in our experimental
procedure. This low residual stress is evidenced
if we carefully analyze the imperceptible lateral
shift in the Vernier gauge (Fig 7). Finally, this
surface micromachining module is utilized to
516

5. fabricate bending polysilicon micromirrors, see
Fig. 8 (to be published).
From this described low level residual
stress and based on other mechanical testing
routines (see Fig. 4) we can conclude that the
thermal load, the polysilicon film microstructure,
and the releasing technique, result in a reliable
process for the fabrication of dynamic and static
polysilicon sensor and actuators.
Fig. 7. Vernier gauge showing no evidenced lateral shift as
a measure of a very low residual stress.
Fig. 8. Bending-type micromirrors fabricated with two
polysilicon layers.
Conclusions
The PolyMEMS INAOE supported by a
surface micromachining module has been
developed for the fabrication of electrostatic and
electrothermal (Joule effect) sensors and
actuators. In this module the designer can
choose up to 3 Poly layers and aluminum as
electrical interconnecting material. A
micromechanical test chip has been fabricated
which includes the following. a)
Micromechanical test structures for residual
stress measurement; cantilever beams,
clamped-clamped beams, ring-and-beam
structures, diamond-and-beam structures,
rotation beams, Vernier gauges, cantilever
spirals, double-clamped microgauge, and b)
Actuators; torsion and bending mirrors,
resonators, single two-arms Joule structures
(STA), chevron-like Joule arrays, capacitive
array for accelerometers.
In this work we are presenting the
measured residual stress on our process, by
using the clamped-clamped beam, ring-and-
beam, and diamond-and-beam arrays. The
measured compressive stress is in the 21-26
MPa range for three types of microgauges. A
maximum typical value for this tensile stress is
50 MPa, which is higher than that obtained in
our experimental procedure. From this residual
stress measurement and other mechanical
testing routines we can conclude that the
thermal load, the polysilicon film microstructure,
and the releasing technique, result in a reliable
process for the fabrication of dynamic and static
polysilicon sensor and actuators.
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6. A
B
Fig. 2 PolyMEMS INAOE, electromechanical test chip. A) The lay-out is featured by 9 levels. B) SEM picture, note this image is
mirror-projected.
518