POROUS SILICON PSST-2002 Short Course Sunday 10 th March 3:00-6:00 pm by: Androula G. Nassiopoulou...

POROUS SILICONPOROUS SILICONPSST-2002 Short CoursePSST-2002 Short Course

Sunday 10th March 3:00-6:00 pm

by: Androula G. Nassiopoulouby: Androula G. Nassiopoulou

FABRICATION, PROCESSING, FABRICATION, PROCESSING,

MECHANICAL AND THERMAL PROPERTIESMECHANICAL AND THERMAL PROPERTIES

FABRICATION, PROCESSING, FABRICATION, PROCESSING,

MECHANICAL AND THERMAL PROPERTIESMECHANICAL AND THERMAL PROPERTIES

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Cross sectional view of a conventional double-tank cell

POROUS SILICON FORMATION BY POROUS SILICON FORMATION BY ELECTROCHEMICAL DISSOLUTION OF SILICON (II)ELECTROCHEMICAL DISSOLUTION OF SILICON (II)

IMEL,/NCSR Demokritos, PSST 2002, TenerifeIMEL,/NCSR Demokritos, PSST 2002, Tenerife

EFFECT OF ILLUMINATION IN POROUS SILICON EFFECT OF ILLUMINATION IN POROUS SILICON FORMATION FORMATION IN HF-WATER OR ETHANOLIC SOLUTIONS IN HF-WATER OR ETHANOLIC SOLUTIONS

(3)IMEL,/NCSR Demokritos, PSST 2002, TenerifeIMEL,/NCSR Demokritos, PSST 2002, Tenerife

For a review, see : Α. 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 illuminationneed for illumination

Effect of illumination: electron/hole pair generation holes are involved in the chemical reactions for silicon

dissolution For a doping level < ~1018cm-3 : Silicon dissolution occurs in the dark only at high voltage (>5V)

Under illumination: porous silicon formation occurs at lower potentials (<1V) (surface layer: nanoporous, underlying layer: macroporous)

For a doping level >1018cm-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)

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

STAIN ETCH POROUS SILICON FILM GROWTH STAIN ETCH POROUS SILICON FILM GROWTH


Si dissolution without electric field Chemical solution: HF/nitric acid/

water

Key component : hole (h+) generation

cathode: HNO3+3H+NO+2H20+3h+

anode : nh++Si+2H2OSiO2+4H++(4-n)e-

SiO2+6HFH2SiF6+2H2O

Above reaction: catalysed by HNO2

“incubation” period

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

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

Influence of substrate doping

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MULTILAYER STRUCTURES OF POROUS SILICON MULTILAYER STRUCTURES OF POROUS SILICON (I)(I)

IMEL/ NCSR Demokritos, PSST 2002, TenerifeIMEL/ NCSR Demokritos, PSST 2002, Tenerife

Based on the following properties:

Porosity depends on anodization current density Porosity depends on illumination parameters in n-type siliconPorosity depends strongly on doping concentrationThe silicon skeleton in the already etched structures is not affected during further processing (hole depleted)

Type I multilayersType I multilayers::

FabricationFabrication::

The poThe porroossity in the layers is ity in the layers is monitored by changing: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 PRINCIPLE OF OPERATION OF THE GAS FLOW SENSORGAS FLOW SENSOR

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Gas flow

Gas flow

T1 T2T1 = T2T1 T2T1 < T2


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POROUS SILICON FORMATION BY POROUS SILICON FORMATION BY ELECTROCHEMICAL DISSOLUTION OF SILICON (I)ELECTROCHEMICAL DISSOLUTION OF SILICON (I)

Electrochemical solution: HF-basedElectrochemical solution: HF-based

DIFFERENT ANODISATION CELLSDIFFERENT ANODISATION CELLS

Cross sectional view of a lateral anodization cell

Cross sectional view of a conventional single-tank

cellIMEL/NCSR Demokritos, PSST 2002, TenerifeIMEL/NCSR Demokritos, PSST 2002, Tenerife

MULTILAYER STRUCTURES OF POROUS SILICON MULTILAYER STRUCTURES OF POROUS SILICON (II)(II)

IMEL/NCSR Demokritos, PSST 2002, TenerifeIMEL/NCSR Demokritos, PSST 2002, Tenerife (8)

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

The layered structure is defined before anodizationThe layered structure is defined before anodization (alternate layers with different(alternate layers with different doping concentration)doping concentration)

Interface sharpness:Interface sharpness:

Type II multilayersType II multilayers::

Interference Filters

Waveguides

Porous silicon mirrors for biological applications

Interference Filters

Waveguides

Porous silicon mirrors for biological applications

APPLICATIONS OF MULTILAYER PS STRUCTURESAPPLICATIONS OF MULTILAYER PS STRUCTURES


MULTILAYER STRUCTURES OF POROUS SILICONMULTILAYER STRUCTURES OF POROUS SILICON (III) (III)


a) Type-I multilayers by varying anodisation current Current density: 6 and 104

mA/cm2

Etching time: 4.83 and 1.33 secb) Type-I multilayers by varying

the illumination density Current density: 6.4 mA/cm2

c) Type-II multilayers on epitaxially grown silicon layers with varied doping concentration of 1017 and 1019 cm-3

a) Type-I multilayers by varying anodisation current Current density: 6 and 104

mA/cm2

Etching time: 4.83 and 1.33 secb) Type-I multilayers by varying

the illumination density Current density: 6.4 mA/cm2

c) Type-II multilayers on epitaxially grown silicon layers with varied doping concentration of 1017 and 1019 cm-3

Ref: M. Thönissen and M.G.Burger in: Properties of porous silicon, edited by L.T.Canham, EMIS Datareviews Series No 18 IEE, 1997

(a)

(b)

(c)

DERIVATIZED POROUS SILICON DERIVATIZED POROUS SILICON MULTILAYERS AND BIOLOGICAL MIRRORSMULTILAYERS AND BIOLOGICAL MIRRORS

(11)Ref: L.T. Canham et al, Phys. Stat. Sol. (a) 182, 521 (2000)

IMEL/NCSR Demokritos, PSST 2002, TenerifeIMEL/NCSR Demokritos, PSST 2002, Tenerife

DRYING OF POROUS SILICON (I)DRYING OF POROUS SILICON (I)

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Crucial in order to avoid cracking Crucial in order to avoid cracking

Example :

Ref. D.Bellet in: Properties of Porous silicon

due to capillary stresses associated with the nanometric size of the pores occurs for PS layers thicker than a critical thickness hc

(hc depends on the porosity and on the surface tension of the drying liquid)

Cracking :


DRYING OF POROUS SILICON (II)DRYING OF POROUS SILICON (II)

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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: ΔΡ =2γLV/r, γLV = surface tension, r = pore radius

Example: For water γLV = 72mJ/m2 for r = 5nm ΔΡ = 30ΜPa (300 bar)

Capillary pressure: not hydrostatic, since normal air drying is out of equilibrium

Measurement of induced tensile stressesMeasurement of induced tensile stresses :Measurement of induced tensile stressesMeasurement of induced tensile stresses :

By measuring wafer curvature

Using X-ray diffraction (measuring lattice parameters)


DRYING OF POROUS SILICON (III)DRYING OF POROUS SILICON (III)

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Drying techniques to avoid cracking Drying techniques to avoid cracking

a) Water or pentane drying (pentane:lower surface tension than

water)

b) 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

c) Freeze drying • The fluid inside the pores is frozen and then sublimed under vacuum

(no interfacial tension)

(15)Ref: M. Thönissen and M.G.Burger in: Properties of porous silicon


AGEING OF POROUS SILICONAGEING OF POROUS SILICON

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It results from the reaction of It results from the reaction of the material with its the material with its environmentenvironment

It results from the reaction of It results from the reaction of the material with its the material with its environmentenvironment

In order to minimize In order to minimize storage effects:storage effects:

Intentionally oxidize PS

Isolate the internal surface by capping

Modify the internal surface

Impregnate the pores


CAPPING OF POROUS SILICON ICAPPING OF POROUS SILICON I

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Used to avoid ageingUsed to avoid ageing Used to avoid ageingUsed to avoid ageing

a) Epitaxially deposited capping layers

CoSi2 and SiGe, deposited at 600oC stabilization of

strain (Kim et al., J. Appl. Phys, 69 (1991) 2201)

Si on porous silicon. Fabrication of SOI or bond-and-etchback SOI (BESOI)

GaAs on PS: for monolithic integration of optoelectronics with Si ICs

Diamond for high temperature electronics

PbTe, for mid-infrared (3-5 μm) optoelectronic devices


CAPPING OF POROUS SILICON IICAPPING OF POROUS SILICON II

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b) 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

c) Metallic capping layers Ti or Co silicides

d) Al deposition – protection in ambient air Reduces C and O pick-up, retains F

e) Al capping – Protection during analysis avoids oxidation and carbonization of samples, and H or F desorption

f) 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

SURFACE MODIFICATION OF POROUS SILICONSURFACE MODIFICATION OF POROUS SILICON

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Surface of freshly etched porous silicon:hydrogen-passivated (SiH, SiH2, SiH3)

good electronic passivation

limited stability Surface modification :Surface modification :

OxidationOxidationAnodic, chemical, thermal

NitridationNitridationRapid thermal annealing in N2 or NH3

Organic chemical derivatisationOrganic chemical derivatisationStabilisation 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 derivatisationElectrochemical derivatisation Ref: J.N. Chazalviel et al in: Properties of Porous silicon

STABILIZATION AND FUNCTIONALIZATION VIA STABILIZATION AND FUNCTIONALIZATION VIA HYDROSILYLATION AND ELECTROGRAFTING REACTIONSHYDROSILYLATION AND ELECTROGRAFTING REACTIONS

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Substitution of the silicon hydride bonds with silicon carbon bonds

LAM (Lewis acid mediated) reaction (hydrosilylation)

Light-promoted hydrosilylation

Cathodic electrografting

Ref: J.M. Buriak, Adv. Mat. 11, 265 (1999)


ELASTIC PROPERTIES OF PSELASTIC PROPERTIES OF PS

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They differ drastically from those of bulk silicon

Young’s modulus of P.S Young’s modulus of P.S X-ray diffraction

Microechography and measurement of acoustic signature (measuring reflection and transmission parameters versus frequency)

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

Measured by

General tendency: PS is less stiff than bulk silicon (with lower Young’s modulus values)

General tendency: PS is less stiff than bulk silicon (with lower Young’s modulus values)

YOUNG’S MODULUS VALUES OF POROUS SILICONYOUNG’S MODULUS VALUES OF POROUS SILICON

(23)Ref: M. Thönissen and M.G.Burger in: Properties of porous silicon, edited by L.T.Canham, EMIS Datareviews Series No 18 IEE, 1997

THERMAL CONDUCTIVITY OF POROUS SILICONTHERMAL CONDUCTIVITY OF POROUS SILICON

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Very different from that of bulk siliconBulk silicon : 145 W m-1K-1 Porous silicon : depends on porosity

Very different from that of bulk siliconBulk silicon : 145 W m-1K-1 Porous silicon : depends on porosity

D [3]

1-10

10

-

none

65%

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)

40

[1,2][1,2]

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Temperature distribution around a heater on bulk silicon

Resistor on bulk silicon

Ref: A.G. Nassiopoulou and G. Kaltsas, Phys. Stat. Solidi (9) 182, 307 (2000).

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Resistor on PS

Temperature distribution around a

heater on porous silicon

Resistor on a free standing silicon

membrane

Temperature distribution around a heater on a free

standing silicon membrane

Ref: A.G. Nassiopoulou and G. Kaltsas, Phys. Stat. Solidi (9) 182, 307 (2000).

LOCAL FORMATION AND PATTERNING OF POROUS SILICONLOCAL FORMATION AND PATTERNING OF POROUS SILICON

IMEL/ NCSR Demokritos, PSST 2002, TenerifeIMEL/ NCSR Demokritos, PSST 2002, Tenerife (27)

Necessary in applications using monolithic integration of the corresponding devices and structures on the silicon substrate

LITHOGRAPHIC PATTERNINGLITHOGRAPHIC PATTERNING

Most commonly used masking materials: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.

Examples of local anodization through a lithographic mask Examples of local anodization through a lithographic mask

IMEL/ NCSR Demokritos, PSST 2002, TenerifeIMEL/ NCSR Demokritos, PSST 2002, Tenerife (28)

SiO2 mask:anodization time: 1 minRef: Α.Nassiopoulou et al.

Thin Solid Films 255 (1995) 329

Silicon nitride mask Ref: Α.Nassiopoulou et al

Thin Solid Films 255 (1995) 329

Polysilicon mask Ref: G.Kaltsas and A.G.Nassiopoulou,Sensors and Actuators A65(1998) 175

IMEL, NCSR Demokritos, PSST 2002, TenerifeIMEL, NCSR Demokritos, PSST 2002, TenerifeIMEL/NCSR Demokritos, PSST 2002, TenerifeIMEL/NCSR Demokritos, PSST 2002, Tenerife

LITHOGRAPHIC PATTERNING USING POLY/SiOLITHOGRAPHIC PATTERNING USING POLY/SiO22 MASK MASK

APPLICATION IN MICROMACHININGAPPLICATION IN MICROMACHINING

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Reference: G.Kaltsas and A.G.Nassiopoulou, Sensors and Actuators A65 (1998) 175

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POROUS SILICON MICROMACHININGPOROUS SILICON MICROMACHINING

Use of porous silicon as sacrificial layer for the formation of free standing membranes on top of a cavity

Examples of free standing polysilicon

membranes and cantilevers.

Examples of free standing polysilicon

membranes and cantilevers.

Ref: G. Kaltsas and A.G. Nassiopoulou, Sensors and Actuators, A65 (1998) 175-179.

DRY ETCHING OF POROUS SILICONDRY ETCHING OF POROUS SILICON

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As prepared PS layers are etched6-7 times faster compared to Si.

Typical etch rates for SF6 : RIE : 6.8 μm/min (Si:1.5μm/min) HDP : 66 μm/min (Si:10 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-SUSPENDED POROUS SILICON MICRO-HOTPLATES FOR GAS SENSORSHOTPLATES FOR GAS SENSORS

100μm

High temperatures (>400oC) can be obtained with very low energy consumption (<30mW)

Ref: C. Tsamis and A.G. Nassiopoulou, unpublished results.


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Suspended Porous Silicon membranes Suspended Porous Silicon membranes with Pt heater (60x60with Pt heater (60x60μμmm22))

P=50mWP=50mW P=57mWP=57mW

P=42mWP=42mW P=46.6mWP=46.6mW

Ref: C. Tsamis and A.G. Nassiopoulou, unpublished results.

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EXAMPLE: GAS FLOW SENSOR EXAMPLE: GAS FLOW SENSOR

Direction of gas flow

Cold thermopile contacts Hot thermopile contacts

Heating resistor

AlPads

Bulk Silicon

Porous silicon area

Ref: G. Kaltsas and A.G. Nassiopoulou, Sensors and Actuators 76 (1999) 133-138.


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Sensor characteristicsSensor characteristics

Ref: G. Kaltsas and A.G. Nassiopoulou, unpublished results

POROUS SILICON PSST-2002 Short Course Sunday 10 th March 3:00-6:00 pm by: Androula G. Nassiopoulou...

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