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POROUS SILICON PSST-2002 Short Course Sunday 10th
March 300-600 pm
FABRICATION, PROCESSING, MECHANICAL
AND THERMAL PROPERTIES
by Androula G. Nassiopoulou
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POROUS SILICON FORMATION BY ELECTROCHEMICAL
DISSOLUTION OF SILICON (II)
Cross sectional view of a conventional
double-tank cell
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EFFECT OF ILLUMINATION IN POROUS SILICON
FORMATION IN HF-WATER OR ETHANOLIC SOLUTIONS
For a review, see ?. Halimaoui in Properties
of porous silicon, edited by L.T.Canhan EMIS
DATAREVIEWS series No 18 IEE 1997

• ? Anodization of p-type silicon ? in HF-water
  or ethanolic solutions
• ? Anodization of n-type silicon ? in above
  solutions need for illumination
• Effect of illumination ? electron/hole
  pair generation
• ? holes are involved in the chemical
  reactions for silicon
• dissolution
• For a doping level lt 1018cm-3 ? Silicon
  dissolution occurs in the dark only at high
  voltage (gt5V)
• ? Under illumination porous silicon
  formation occurs at lower potentials (lt1V)
  (surface layer nanoporous, underlying layer
  macroporous)
• For a doping level gt1018cm-3 ? porous
  silicon formation mesoporous even in the dark
  (holes generated by electric field induced
  avalanch breakthrough

Using specially designed electrolytes
macroporous silicon formation on n-type silicon
without any illumination is possible (current
bursts model) Ref H.Föll et al., Physica Status
Solidi (1) 182,7 (2000)
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STAIN ETCH POROUS SILICON FILM GROWTH
References G.Di Francia et al, J.Appl.
Phys. Vol 77 (1995) p 3549 Di Francia,
Solid State Communications, vol 96 (1995) 79
Noguchi et al., Appl. Phys. Lett. 62(12)
1993, 1429

• Obtained films
• In general non-uniform (due to random anodic and
  cathodic sites)
• With Al-150 to 200 nm thick on Si ? instantaneous
  reaction of silicon with HF/HNO2/H2O etchant, due
  to the reaction of Al and HNO3 to provide holes
  (selective formation) of PS)
• Use of sonication during stain etching
• thicker PS films
• more rough PS surface
• Simple illumination of Si in 50 HF with HeNe
  laser porous silicon formation

• Si dissolution without electric
• field
•  
• Chemical solution HF/nitric acid/
  water
•  Key component hole (h) generation
• cathode HNO33H?NO2H203h
• anode nhSi2H2O?SiO24H(4-n)e-
• SiO26HF?H2SiF62H2O
•  
• Above reaction catalysed by HNO2 ?
  incubation period

Influence of substrate doping

•  
• It influences the incubation time
•     p-type silicon incubation time increases
  with substrate resistivity
• n-type silicon
  decreased with increasing resistivity

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MULTILAYER STRUCTURES OF POROUS SILICON (I)

• Based on the following properties
•  
• Porosity depends on anodization current density
• Porosity depends on illumination parameters in
  n-type
• silicon
• Porosity depends strongly on doping concentration
• The silicon skeleton in the already etched
  structures is
• not affected during further processing (hole
  depleted)

• The porosity in the layers is monitored by
  changing
• The anodization current density
• The illumination parameters in
• n-doped substrates

Ref M. Thönissen and M.G.Burger in Properties
of porous silicon, edited by L.T.Canham, EMIS
Datareviews Series No 18 IEE, 1997
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PRINCIPLE OF OPERATION OF THE GAS FLOW SENSOR
T1 T2
T1 lt T2
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POROUS SILICON FORMATION BY ELECTROCHEMICAL
DISSOLUTION OF SILICON (I)
Electrochemical solution HF-based DIFFERENT
ANODISATION CELLS
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MULTILAYER STRUCTURES OF POROUS SILICON (II)
Type II multilayers
The layered structure is defined before
anodization (alternate layers with different
doping concentration)

• In type-I multilayers ? given by the transition
  of the anodization current and its effect
  on etching.Transition zone below 15
  nm is achieved.
• In type-II multilayers ? given by the epitaxy

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APPLICATIONS OF MULTILAYER PS STRUCTURES

• Interference Filters
• Waveguides
• Porous silicon mirrors for biological
• applications

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MULTILAYER STRUCTURES OF POROUS SILICON (III)

• Type-I multilayers by varying anodisation current
• Current density 6 and 104 mA/cm2
• Etching time 4.83 and 1.33 sec
• Type-I multilayers by varying the illumination
  density
• Current density 6.4 mA/cm2
• Type-II multilayers on epitaxially grown silicon
  layers with varied doping concentration of 1017
  and 1019 cm-3

(a)
(b)
(c)
Ref M. Thönissen and M.G.Burger in Properties
of porous silicon, edited by L.T.Canham, EMIS
Datareviews Series No 18 IEE, 1997
(10)
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DERIVATIZED POROUS SILICON MULTILAYERS AND
BIOLOGICAL MIRRORS
(11)
Ref L.T. Canham et al, Phys. Stat. Sol. (a) 182,
521 (2000)
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DRYING OF POROUS SILICON (I)
Crucial in order to avoid cracking
Example
Ref. D.Bellet in Properties of Porous silicon
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DRYING OF POROUS SILICON (II)
Origin of cracking ? evaporation of the pore
liquid gives rise to capillary tension
Maximum capillary stress ? at the critical
point when the menisci enter the pores
Induced pressure ?? 2?LV/r, ?LV surface
tension, r pore radius   Example For water
?LV 72mJ/m2 ? for r 5nm ? ?? 30?Pa (300
bar) Capillary pressure not hydrostatic, since
normal air drying is out of equilibrium
Measurement of induced tensile stresses
By measuring wafer curvature
Using X-ray diffraction (measuring lattice
parameters)
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DRYING OF POROUS SILICON (III)
Drying techniques to avoid cracking

• Water or pentane drying
• (pentanelower surface tension than water)

• Supercritical drying
• Most efficient drying method (L.T. Canham et al.
  Nature (UK) Vol 368 (1994) p133)
• Used fluid CO2, drying above the critical point
  (40oC, 163 bar)
• Result ultrahigh porosity films
• Freeze drying
• The fluid inside the pores is frozen and then
  sublimed under vacuum (no interfacial tension)

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(15)
Ref M. Thönissen and M.G.Burger in Properties
of porous silicon
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AGEING OF POROUS SILICON
It results from the reaction of the material with
its environment
Intentionally oxidize PS
Isolate the internal surface by capping
Modify the internal surface
In order to minimize storage effects
Impregnate the pores
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CAPPING OF POROUS SILICON I
Used to avoid ageing
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CAPPING OF POROUS SILICON II

• Organic/polymeric capping layers
• Paraffin on the surface of PS (Tischler et al).
  Short term stabilization of PL
• Capping with conducting polymers, as plyaniline,
  polypyrrole
• Polymer within the pores
• Metallic capping layers
• Ti or Co silicides
• Al deposition protection in ambient air
• Reduces C and O pick-up, retains F
• Al capping Protection during analysis
• avoids oxidation and carbonization of samples,
  and H or F desorption
• Dielectric capping
• CVD deposited SiO2 on medium porosity Si ?
  minimizes ion-beam induced ageing
• Ion-implanted O or N, or PECVD-deposited SiO2,
  Si3N4
• No result on PL stabilization

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SURFACE MODIFICATION OF POROUS SILICON
Surface of freshly etched porous
silicon hydrogen-passivated (SiH, SiH2, SiH3)
good electronic passivation
Surface modification
limited stability

• Oxidation
• Anodic, chemical, thermal
• Nitridation
• Rapid thermal annealing in N2 or NH3
• Organic chemical derivatisation
• Stabilisation by organic groups, process stopped
  at a monolayer
• Surface covered with SiH and SiCH2CH3 upon
  dissociative adsorption
• of diethylsilace (Dillon et al. (1992)
• Grafting of trimethylsiloxy groups. Substitution
  of - H with - OSi (CH3)3
• Anderson et al 1993)
• Thermal derivatisation with alcohols (Hory et
  al. 1995, Kim et al. 1997)
• Grafting of alkoxy groups (Li et al. 1994)
• Electrochemical derivatisation

Ref J.N. Chazalviel et al in Properties of
Porous silicon
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STABILIZATION AND FUNCTIONALIZATION VIA
HYDROSILYLATION AND ELECTROGRAFTING REACTIONS
Substitution of the silicon hydride bonds with
silicon carbon bonds

• LAM (Lewis acid mediated) reaction
• (hydrosilylation)
• Light-promoted hydrosilylation
• Cathodic electrografting

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Ref J.M. Buriak, Adv. Mat. 11, 265 (1999)
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ELASTIC PROPERTIES OF PS
They differ drastically from those of bulk
silicon
X-ray diffraction
Youngs modulus of P.S
Microechography and measurement of acoustic
signature (measuring reflection and transmission
parameters versus frequency)
Measured by
Nanoindentation investigation (Nanoindenter it
measures force and displacement as an indentation
is performed on the material using a very low
load)
Brillouin scattering used to investigate the
surface acoustic waves on a PS-layer
General tendency PS is less stiff than bulk
silicon (with lower Youngs modulus values)
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YOUNGS MODULUS VALUES OF POROUS SILICON
Ref M. Thönissen and M.G.Burger in Properties
of porous silicon, edited by L.T.Canham, EMIS
Datareviews Series No 18 IEE, 1997
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THERMAL CONDUCTIVITY OF POROUS SILICON
Very different from that of bulk silicon Bulk
silicon 145 W m-1K-1 Porous silicon
depends on porosity
1,2
1,2
D 3


1-10
10
-
none
65
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1.2 (3)

1. A. Drost et al. Sens. Mat. (Japan), vol 7 (1995)
   p 111
2. W.Lang et al. Mater. Res. Soc. Symp. Proc. (USA)
   vol. 358 (1995) p561
3. A.G.Nassiopoulou et al. Phys. St. Sol. (a) 182,
   307 (2000)

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Temperature distribution around a heater on bulk
silicon
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Ref A.G. Nassiopoulou and G. Kaltsas, Phys.
Stat. Solidi (9) 182, 307 (2000).
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Resistor on a free standing silicon membrane
(26)
Ref A.G. Nassiopoulou and G. Kaltsas, Phys.
Stat. Solidi (9) 182, 307 (2000).
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LOCAL FORMATION AND PATTERNING OF POROUS SILICON
Necessary in applications using monolithic
integration of the corresponding devices and
structures on the silicon substrate
LITHOGRAPHIC PATTERNING

• Most commonly used masking materials
• Photo resists
• Common photoresists (AZ5214) withstand etching
  solutions only for short anodization time.
• Use of SU8 photoresist long anodization time
  (V.V. Starkov et al (this Conference))
• Silicon dioxide For anodization times of a few
  minutes
• Stoichiometric silicon nitride or silicon
  carbide
• Resistant to the anodization solution but they
  show problems related to stress effects and
  cracking
• Non-stroichiometric nitride, deposited by LPCVD ?
  good mask
• Double layer of polysilicon/SiO2
• Perfect mask for porous silicon micromachining.

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Examples of local anodization through a
lithographic mask
Silicon nitride mask Ref ?.Nassiopoulou et
al Thin Solid Films 255 (1995) 329
SiO2 maskanodization time 1 min Ref
?.Nassiopoulou et al. Thin Solid Films 255 (1995)
329
Polysilicon mask Ref G.Kaltsas and
A.G.Nassiopoulou, Sensors and Actuators A65(1998)
175
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LITHOGRAPHIC PATTERNING USING POLY/SiO2 MASK
APPLICATION IN MICROMACHINING
Reference G.Kaltsas and A.G.Nassiopoulou,
Sensors and Actuators A65 (1998) 175
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POROUS SILICON MICROMACHINING
Use of porous silicon as sacrificial layer for
the formation of free standing membranes on top
of a cavity
Ref G. Kaltsas and A.G. Nassiopoulou, Sensors
and Actuators, A65 (1998) 175-179.
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DRY ETCHING OF POROUS SILICON

• As prepared PS layers are etched
• 6-7 times faster compared to Si.
• Typical etch rates for SF6
• RIE 6.8 µm/min (Si1.5µm/min)
• HDP 66 µm/min (Si10 µm/min)
• Etching rate depends on
• The porosity
• Aging of the layer.
• Thermal treatment.
• The etch rate of thermally treated PS layers is
  significantly smaller than that of freshly etched
  PS.
• HDP 0.33 µm/min

Ref A. Tserepi et al, PSST 2002, abstract book
page 187.
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SUSPENDED POROUS SILICON MICRO-HOTPLATES FOR GAS
SENSORS
High temperatures (gt400oC) can be obtained with
very low energy consumption (lt30mW)
Ref C. Tsamis and A.G. Nassiopoulou, unpublished
results.
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Suspended Porous Silicon membranes with Pt heater
(60x60µm2)
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Ref C. Tsamis and A.G. Nassiopoulou, unpublished
results.
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EXAMPLE GAS FLOW SENSOR
Ref G. Kaltsas and A.G. Nassiopoulou, Sensors
and Actuators 76 (1999) 133-138.
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Sensor characteristics
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Ref G. Kaltsas and A.G. Nassiopoulou,
unpublished results