Silicon Micromachining for Microstructures Fabrication in LMSE

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					m                 University of Ljubljana
             Faculty of Electrical Engineering
    Laboratory of Microsensor Structures and Electronics




               Silicon Micromachining
           for Microstructures Fabrication
                       in LMSE




                                                           1
   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.




                                                                                         5
                           Micromachining in LMS

Microtips for atomic force microscopy (AFM)
In AFM , topography, mechanical,
chemical and electromagnetic                                     30
properties of materials are investigated                                  apex point




                                           Underetching U [µm]
                                                                 25
with the highest spatial resolution.
                                                                 20
A microprobe with extremely sharp
                                                                 15
microtip as the most vital part is
scanned over the surface and the force                           10                                TMAH 25%
                                                                                                      T=800C
is detected via different methods.                               5
                                                                                                      T=700C
                                                                 0
                                                                      0      10    20   30    40     50   60   70

In our microtip realization,                                                       Etching time [min]
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.


                                                                                                                    6
                           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).




                                                   7
                     Micromachining in LMS

Microtips by isotropic etching and hillocks




                   Isotropic etching     spontaneous hillock

           Summarised results of etched microtips

                              Rlat          Tip         Tip Aspect
                              [µm/min]      angle [º]   ratio [h/l]
              TMAH 25%        0.85          <40         0.9-1.2
              KOH-33%         1.6           <40         0.8-1.2
              KOH-IPA         0.94          80          0.6
              TMAH-IPA        0.35          90          0.5-0.6
              Isotropic       0.6           30-60       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.

          Si3N4
          SiO2
                                       1)
                                    (11




                            (100)

     a)

          d1      d2   d3   d4


     b)
                                                    1)
                                                 (11




                                                1)
                            (100)           ( 31
     c)




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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

                                       15µm thick Si cantilevers
Resonant frequency of cantilever
             1.03 t   E
      fR 
              2 L2   


      t-beam thickness
      L-beam length
      E-young modul
      -beamdensity



                                                                                          10
                         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     E ts
                                           2

                                    1 6 t f      1
                                                    R1    R2
                                                           1
                                                                
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.                     Cross-section
Central one has integrated sensing
piezoresistor, while other two make the
device more robust against accelerations in
other directions.

                                                                 Top view
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.

                                                                 Bottom view
The accelerometer microstructure was
realised entirely by wet etching
processes.

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                           Micromachining in LMS

Telecommunications and optical applications
Microstructures found various applications
also in these fields as single optical
                                                 <110>   <110>   <100>
components for interconnections between
fibres and other active devices, optical
benches, passive or active reflecting
mirrors, active switches, beam splitters, etc.


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


        slit




                                                                         Isc [µA]
                                                                                    400
                                          
                                    20º        17º     40º   



               {110}               {111}       {212}             {311}
                                                                                    200




                                                                                      0

                                                                                          -5   0   5      10     15       20   25    30   35
Single mode fibre-NA=0.1
                                                                                                       X direction [µm]




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                        Micromachining in LMS

Characterisation setup of                                reflected image on
                                                         semitransparent
                                                         screen
reflecting mirror planes                                                                                                   light source
                                                                                                                           =632nm,
by beam image dispersion                                                                                  collimator       =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.
                                          hydrophobic surface
 AFM after RCA                             a >50º
  Ra=10-12nm                              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              Bond interface chemistry
  elastic deformation (zip)
                                                            active wafer
  Si                                                                                              14
                                                                                              1.10 H/cm
                                                                                                          2

                                                                      H              H
                                                               H                         H        15      2
                                                                                              2.10 O/cm
  Si                                    interface between
                                                               OH         OH   OH   OH   OH
                                                                                              2-3.1015H2O/cm2
       a)                                two wafers            OH         OH   OH   OH   OH         15
                                                                                              1-2.10 OH/cm
                                                                                                            2


                                                               H                         H        13
                                                                                              5.10 H/cm
                                                                                                          2

                                                                     H               H

                                                            support wafer
       b)




       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.




                           16
                                    (100) / (111) pairs                                         Overall reaction across two hydrophilic surfaces:
                           14
                                    Anneal time 60 min
  Tensile strength [MPa]




                           12

                           10                                                                   Si-OH + OH-Si Si-O-Si +H2O
                            8

                            6                                                                                             covalent bonds
                                                                                   N2
                            4
                                                                                   O2
                            2
                                                                                   vac
                            0

                                0     50   100   150   200   250   300       350    400   450
                                                                         o
                                           Annealing temperature [ C]




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                          Micromachining in LMS
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                 Bonding                        Void
larger area defects can be                   interface
recognised.




    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:                                                E-mail:
Professor Dr. Slavko Amon                 slavko.amon@fe.uni-lj.si       35
Professor Dr. Igor Medič                  igor.medic@fe.uni-lj.si        32
Assistant Professor Dr. Žarko Gorup       zarko.gorup@fe.uni-lj.si       32
Assistant Professor Dr. Andrej Levstek    andrej.levstek@fe.uni-lj.si    84
Assistant Professor Dr. Drago Resnik      drago.resnik@fe.uni-lj.si      30
Assistant Professor Dr. Danilo Vrtačnik   danilo.vrtacnik@fe.uni-lj.si   30
Senior Lecturer Niko Basarič, M. Sc.      niko.basaric@fe.uni-lj.si      33
Researcher Uroš Aljančič, M. Sc.          uros.aljancic@fe.uni-lj.si     30
Researcher Matej Možek, M. Sc             matej.mozek@fe.uni-lj.si       30
Technical Collaborator Matjaž Cvar        matjaz.cvar@fe.uni-lj.si       30
Technical Collaborator Marijan Žurga      marijan.zurga@fe.uni-lj.si     27
Phone: (+386 1) 4768 + Ext.

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