Transcript Silicon Micromachining for Microstructures Fabrication

m
University of Ljubljana
Faculty of Electrical Engineering
Laboratory of Microsensor Structures and Electronics
Silicon Micromachining
for Microstructures Fabrication
in LMSE
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Laboratory of Microsensor Structures and Electronics (LMSE) is involved in research and
development of microstructures such as silicon devices, sensors, actuators and microelectromechanical
systems (MEMS). Internal properties and external characteristics of these structures are studied using
analytical and computer modelling. Technologies available in LMSE allow investigations of new
processes in the fields of mask design and fabrication, photolithography, diffusion, metallization,
depositions, cleaning, thin film processing, etching, micromachining etc. Based on these activities,
research and development of various new microstructures such as photo sensors, pressure sensors,
temperature sensors, radiation sensors, sensors for nuclear physics, actuators, nanostructures, various
3D micromechanical structures and similar, is going on. Research is supported by advanced
measurement equipment and characterisation techniques, aided by process and device modelling.
A part of research activities is involved in the field of electronic circuit theory, simulations and
applications. Harmonic balance is used as a powerful method of analysis for nonlinear dynamic circuits.
The team is also engaged with practical solutions in the field of microprocessor aided electronics. It
encompasses development of appropriate hardware and software for automatic measurements of
electronic and telecommunication equipment. Cooperation with manufacturers of professional
electronic equipment is well established. Members of LMSE are collaborating with European
universities under the framework of international projects sponsored by the EU commission.
LMSE is a free university lab, open for any kind of cooperation with other laboratories and industry.
LMSE has a well established cooperation with leading institutions all over the world. LMSE offers
complete research and development services in the field of microstructures and electronics, from
theoretical analysis to fabrication of test structures, devices and circuits, their characterisation and
optimisation.
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Micromachining in LMS - Overview
•Main activities are listed below:
Compensation of convex corners
Bossed diaphragms
Microtips for AFM and field emitters
Multilevel microstructures
Cantilevers
Accelerometer microstructures
Optical fibre aligning
Micromachined reflecting optical mirrors
Wafer bonding
Pressure sensor (low and medium range)
Smart pressure sensor approach
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Micromachining in LMS
Compensation of convex corners
It is well known that in anisotropic wet micromachining of silicon microstructures, fast
etching of high-index crystal planes ((411),(212), (323),..) inevitably occurs at convex
corners (2-3 times faster etch rate).
By utilising different shapes and/or size of compensation structures, we have been able to
mitigat this effect to a great extend. This is very important when precise and deep etched
microstructures are required (e.g. mesa structures, ridges, ..)
The degree of convex corner undercutting depends also strongly on the wet etchants.
Additives like isopropyl alcohol reduces underetching of convex corners.
Proposed compensation structures
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Micromachining in LMS
Compensation of convex corners and its applications
In case of bossed diaphragm, such as used in low-pressure measurement devices or inertial
mass there is the need for proper design of compensation structures that will occupy small
footprint and effectively compensate convex corner undercutting to depths beyond 300µm.
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Micromachining in LMS
Microtips for atomic force microscopy (AFM)
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apex point
Underetching U [µm]
In AFM , topography, mechanical,
chemical and electromagnetic
properties of materials are investigated
with the highest spatial resolution.
A microprobe with extremely sharp
microtip as the most vital part is
scanned over the surface and the force
is detected via different methods.
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TMAH 25%
T=800C
T=700C
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0
0
In our microtip realization,
phenomenon of undercutting the
convex corners is fully exploited for
achieving sharp microtips. Mask is
laterally underetched, falls off and
microtip remain with specific aspect
ratio. Further sharpening is obtained
via oxidation method.
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50
60
70
Etching time [min]
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Micromachining in LMS
Microtips for field emitter displays
Another application of microtip
application is for cold cathode
emitter tips serving as light sources
for displays .
For effective light source the angle
and sharpness of microtip are
important factors affecting the
electric field distribution and thus
the operating voltage of flat panel
displays (FPD).
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Micromachining in LMS
Microtips by isotropic etching and hillocks
Isotropic etching
spontaneous hillock
Summarised results of etched microtips
TMAH 25%
KOH-33%
KOH-IPA
TMAH-IPA
Isotropic
Rlat
[µm/min]
0.85
1.6
0.94
0.35
0.6
Tip
angle [º]
<40
<40
80
90
30-60
Tip Aspect
ratio [h/l]
0.9-1.2
0.8-1.2
0.6
0.5-0.6
0.5
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Micromachining in LMS
Multilevel microstructures
The design of microelectromechanical structures (MEMS) in bulk micromachining is in most
cases limited by mask shape and etching anisotropy of single crystal silicon planes.
To extend the variety of planes that can be obtained by wet micromachining, much effort in
LMS has been directed toward utilising several height levels with minimum set of masks
and a combination of mask and maskless etching.
(100)
(11
1)
Si3N4
SiO2
a)
d1
d2
d3
d4
(11
1)
b)
c)
(100)
1)
( 31
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Micromachining in LMS
Cantilever and bridge microstructures
These structures have many applications in sensing and in actuating devices.
When thermopiles are integrated on the cantilever (or bridge), they can detect heat
transfer, airflow, etc.
When a piezoelectric layer is deposited on the top of a cantilever or they have integrated
piezoresistors, they can sense applied force or vice-versa, they can perform as actuators
Resonant frequency of cantilever
fR 
1.03 t
2 L2
15µm thick Si cantilevers
E

t-beam thickness
L-beam length
E-young modul
-beamdensity
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Micromachining in LMS
Cantilever and bridge microstructures
70nm thick Si3N4 stress-free cantilevers
When residual stress is present in thin free-standing structures such as bridges or
membranes, they bend upward or downward, depending on material and/or combination of
layers . From the bending curvature and known dimensions of the structure, internal stress S
can be determined:
S
2
E ts
1 6 t f

1
R1
 R12

where E is Young module,  Poisson ratio, ts substrate thickness and tf thickness of thin
film producing stress on silicon, R1 curvature under stress and R2 is curvature after
removing the stress-inducing film.
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Micromachining in LMS
Aluminum cantilevers
Aluminum can play an important role as a masking material in anisotropic etching
(usually 5% TMAH-water+1.5% dissolved Si+0,6% ammonium peroxodisulfate).
By underetching method, cantilevers are obtained.
Aluminum cantilevers were fabricated by above procedure
(100µm long, 10µm wide,0.45µm thick):
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Micromachining in LMS
Micromachined accelerometer structure with piezoresisitive sensing
Inertial mass is suspended on three hinges.
Central one has integrated sensing
piezoresistor, while other two make the
device more robust against accelerations in
other directions.
Under the acceleration inertial mass is
displaced with respect to the fixed part
of microstructure, causing thereby stress
in the piezoresistor located in the central
hinge. This results in proportional
resistance change, which is detected by
the outer electric circuitry.
The accelerometer microstructure was
realised entirely by wet etching
processes.
Cross-section
Top view
Bottom view
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Micromachining in LMS
Telecommunications and optical applications
Microstructures found various applications
also in these fields as single optical
components for interconnections between
fibres and other active devices, optical
benches, passive or active reflecting
mirrors, active switches, beam splitters, etc.
<110>
<110>
<100>
Optical fibre aligning grooves
For precise alignment of optical fibres in
case of positioning or interconnections on
microoptical benches, where light sources
or detection systems can be realised
monolithically, microstructures such as
grooves are useful. By aid of wet or/and dry
etching techniques, different groove
structures can be obtained.
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Micromachining in LMS
Silicon crystal planes as reflecting optical mirrors
Optical light beam reflectors were realised on different crystal planes, micromachined out of
silicon monolithic wafer. Some important planes of interest were:
 (111) with an angle of 54,74º and very smooth surface
 (110) with an angle of 45º with respect to (100) surface,
(311) crystal planes, with an angle of 25º toward (100) surface.
Single mode
fibre guiding
632nm beam
in V groove
with 45º
mirror
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Micromachining in LMS
Reflected beam angles depending on micromachined crystal planes:
Measurements of reflected angles and dispersion of reflected light were performed by
photodiode response.
Average surface roughness Ra is in the range of 25nm and the lowest scattering was
obtained with (111) crystal planes (cca. 3º)
y
Isc
x
600
photodiode

{110}
20º
{111}

17º
{212}
40º
Isc [µA]
slit

400
{311}
200
0
Single mode fibre-NA=0.1
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X direction [µm]
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Micromachining in LMS
Characterisation setup of
reflecting mirror planes
by beam image dispersion
reflected image on
semitransparent
screen
collimator

light source
=632nm,
=1,33µm
single mode fibre
sample
xyz stage
xyz stage
xyz stage
Results:
Beam reflecting images
from crystal planes prepared
with different etchants:
KOH-IPA(100)<110>
KOH (100)<110> Al
KOH-IPA (100)<100> Al
KOH-IPA(100)<100>
Image shape is proportional
to the crystal plane roughness
TMAH (100)<110>
TMAH (100)<110>Al
TMAH-IPA(100)<110>Al
TMAH (110)<010>
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Micromachining in LMS
Low temperature bonding of silicon wafers (<400ºC)
In order to bond wafers successfully attention was paid to the following steps:
surface preparation - to obtain particle- free and hydrophilic surface. Cleaning of silicon
surface has a great impact on surface chemistry and topography.
After RCA cleaning dry wafers were immersed into hot nitric acid (HNO3), allowing
growth of a few monolayers of fresh hydrous chemical oxide, increasing roughness to
Ra=10-12nm.
AFM after RCA
Ra=10-12nm
hydrophobic surface
a >50º
after RCA cleaning
hydrophilic surface
a <10º
after forming
hydrous chemical
oxide
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Micromachining in LMS
prebonding at room temperature - two wafers were put into intimate contact in cleanroom
ambient at room temperature. We initiate bonding by locally pressing the centre region from the
top, thus enabling bonding phenomenon to propagate radially.
By doing this, we actually help to accommodate the two surfaces that suffer from nonflatness,
through elastic deformation process via attractive Van der Waals forces.
Mating of rough surfaces via
elastic deformation (zip)
Bond interface chemistry
active wafer
Si
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H
Si
a)
interface between
two wafers
H
H
2
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2.10 O/cm
2
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
2-3.1015H2O/cm2
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2
1-2.10 OH/cm
H
5.10 H/cm
H
b)
1.10 H/cm
H
H
H
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2
support wafer
c)
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Micromachining in LMS
bond annealing (strengthening) - transformation of silanol to strong siloxane bonds takes
place at elevated temperature. Bond strengthening was performed in the range from 80C to
400C, in different ambients. Bonding efficiency was characterised by quantitative analysis of
tensile strength of bond.
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Tensile strength [MPa]
Overall reaction across two hydrophilic surfaces:
(100) / (111) pairs
Anneal time 60 min
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Si-OH + OH-Si Si-O-Si +H2O
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covalent bonds
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N2
4
O2
2
vac
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400
450
o
Annealing temperature [ C]
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Bond quality characterisation
Voids at the bonding interface reduce the bond strength. Their origin could be trapped
ambient gas, particle or gaseous by-product from interface reaction.
Cross-section of bonded interface
IR transmission investigation is
performed by IR camera, model
PTC-10A. By this method only
larger area defects can be
recognised.
Void
Bonding
interface
Voids
Voids
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Micromachining in LMS
Differential pressure sensor
Four p-type resistors are diffused into the silicon membrane and connected in the
Wheatstone bridge configuration. Membrane is realised by bulk micromachining in 33%
KOH etchant at 80ºC and has thickness of 232µm.
Silicon thin membrane (25µm)
Diffused piezoresistors
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Micromachining in LMS
Differential pressure sensor characteristics
CHARACTERISTICS
Operating Pressure Range ..................................................... 0 .. 1 Bar
Overpressure (min) ...................................................................... 5 Bar
Supply Current ....................................................................... 1 .. 5 mA
Operating Temperature Range ....................................... - 40 .. +120°C
Storage Temperature Range ........................................... - 40 .. +120°C
Dimensions (W/L/H) ............................................. 1950/1950/385 µm
Input Resistance .......................................................... 390 .. 430 ohms
Output Resistance ....................................................... 390 .. 430 ohms
Sensitivity ........................................................................ 30 mV/V/Bar
Offset Voltage at Zero Pressure (max.) .................................... 5 mV/V
Temperature Coeff. of Resistance ....................................... 0,5 ohm/°C
Temperature Coeff. of Offset Voltage (max.) .................. 0,2 mmHg/°C
Offset Repeatability ........................................................... ± 0,1 mmHg
Span Repeatability .............................................................. ±0,3 mmHg
Long Term Stability of Offset and Sensitivity .................... ±0,3 mmHg
Aluminium Metalization
Relative Pressure Sensor
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Micromachining in LMS
Advanced smart pressure sensor
Smart sensors are the leading edge in advanced sensor applications. R&D activities in this
field are taking place in LMS.
The developed smart pressure sensor, in excess of standard features uses a special
calibration algorithm which minimises the offset voltage impact and compensates
temperature dependencies.
The starting point of calibration is a raw pressure sensor without any offset or
temperature compensation.
The calibration procedure also eliminates sensor nonlinearity.
Full-scale pressure range is totally adaptable to the user’s requirements
Smart pressure sensor with digital temperature compensation and in-system calibration.
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Laboratory of Microsensor Structures and Electronics (LMSE)
Phone: (+386 1) 4768 303
Fax: (+386 1) 4264 630
http://paris.fe.uni-lj.si/lms/
Head: Professor Dr. Slavko Amon
Staff:
Professor Dr. Slavko Amon
Professor Dr. Igor Medič
Assistant Professor Dr. Žarko Gorup
Assistant Professor Dr. Andrej Levstek
Assistant Professor Dr. Drago Resnik
Assistant Professor Dr. Danilo Vrtačnik
Senior Lecturer Niko Basarič, M. Sc.
Researcher Uroš Aljančič, M. Sc.
Researcher Matej Možek, M. Sc
Technical Collaborator Matjaž Cvar
Technical Collaborator Marijan Žurga
Phone: (+386 1) 4768 + Ext.
E-mail:
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
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