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VI. Polysilane Thin Films for Electronic Devices

VI. Polysilane Thin Films for Electronic Devices

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Polysilane Thin Films for Electronic Devices



±V



ITO



241



0V



Al



Quartz



Polysilane



FIGURE 26. Structure of single-layer LEDs. (Reprinted from Ref. 93.)



barrier. Hattori used a PDMS film deposited onto ITO by evaporating a

PDMS source at about 300 C under high vacuum (1.0 3 1026 torr).36 An Al

top electrode with a thickness of 1000 A˚ was then evaporated in another

evaporator. Temporal exposure of the PDMS surface to air before Al evaporation was observed to induce surface oxidation and/or contamination. A

low deposition rate of 7 A˚/min was observed to ensure that the PDMS

molecules aligned normal to the substrate surface. The thickness of the film was

4000 A˚. PDHS based UV-LED was prepared by adding n-dichlorodihexyl to

sodium in n-octane at 383 K to give a 10% yield of crude PDHS

(MW 5 300,000) as a flexible hard white elastomer.110 The crude polymer was

purified by reprecipitation from benzene-ethanol. The polysilane in xylene

was then spin coated as thin films between ITO and aluminum electrodes with a

thickness of 500 nm. The EL is emitted in most cases from singlet excited states

of the emissive material, although they are generated in different ways: direct

photoexcitation for the PL and recombination of electrons and holes for the

EL.111 EL covering the whole visible spectrum can be observed by doping

polysilanes with organic dyes.97

Polarized organic LEDs of the type ITO glass/photodegraded PMPS/

phthalocyanines/Al were prepared by using PMPS films exposed to polarized

UV light.112 The device structure consisted of the hole injection ITO electrode,

the hole transporting PMPS film, the emissive phthalocyanine layer, and the Al

electrode. The EL films of phthalocyanines deposited on PMPS surfaces from

solution or vacuum evaporation showed oriented structures, which produced

polarized EL.



B. Photoconductors

Photoconductive cells have been prepared using fullerene doped

PMPS.113 With 1.6% of fullerene as dopant, its photoinduced discharge rate is

enhanced by orders of magnitude.113 Wavelength selectivities of organic photoconductive films can be obtained by varying the dopant.114 The device uses a

double-layer structure with zinc phthalocyanine/tris-8-hydroxyquinoline aluminum. The doping of polysilane can be obtained using the CT interaction

between networked polysilanes and substituents, such as alkyls, aryls or

iodine.40 With MeO-functionalized polysilanes as the starting material, networked polysilanes [MeSi(OMe)x(R)y]n have been obtained with various R



242



Photophysics and Photochemistry of Polysilanes



groups as the side chains by the action of the corresponding Grignard reagents

RMgX. Among these polysilanes, those having N,N-dialkylaminophenyl

groups as side chains exhibit electrical conductivities on the order of 1023

Scm21 upon iodine doping, which are essentially stable under oxidative conditions because of their network structures.40 Although linear polysilanes

generally show high hole mobility or photorecepting ability due to their electrical characteristics, no studies regarding such properties for network polysilanes has appeared so far. The CT between iodine and polysilane has been

used to produce photoconductor thin films.40 Organic polysilane-TiO2 thin

films prepared by a sol-gel process show lower PL quantum efficiency than PSsilica hybrid thin films115 and higher photoconductivity.116 This is explained by

the fact that the photogenerated excitons are dissociated at the PS/TiO2

nanointerface because of electron transfer from PS to TiO2 in the excited states

and/or due to photoinduced CT.



C. Photovoltaics

Fullerene/PMPS photovoltaic cells were fabricated with various concentrations of C60.117 The structure of the device is shown in Figure 27. The

ITO glass substrate with a sheet resistance of 10Ω/cm2 was cleaned by an

ultrasonic bath of isopropyl alcohol, toluene, acetone and methanol followed

by air plasma treatment. As a buffer layer, poly(3,4-ethylenedioxythiphene):

poly(4-styrenesulfonate) (PEDOT:PSS, Baytron P) was spin coated with a

thickness of 40 nm on the ITO glass substrate to decrease contact resistance

between ITO and the photoactive layer. The photoactive layer was prepared by

spin coating a C60/PMPS solution in chlorobenzene to a thickness of 80 nm on

the PEDOT:PSS buffer layer. After drying at 40 C for 12 h, a 0.6-nm LiF layer

was vacuum deposited on the photoactive layer at 1025 torr to ensure a good

ohmic contact between the metal and organic layer. A 100-m Al electrode was

then vacuum deposited. The photoactive area of the device was 4 mm2.



AI

LIF

PMPS:C60

PEDOT:PSS

ITO Glass substrate



FIGURE 27. C60/PMPS device. (Reprinted from Ref. 117.)



Polysilane Thin Films for Electronic Devices



243



Poly[p-(methylphenylsilanylene)anthrylene] (PMPSA) with anthracene

units incorporated into the backbone of poly(methylphenylsilanylene) was

synthesized.118 Schottky barrier photovoltaic cells consisting of semitransparent aluminum and phthalocyanine (H2Pc) dispersed in PMPSA were

fabricated (H2Pc-PMPSA). The conductivity of H2Pc-PMPSA films increased

by the introduction of anthracene units and annealing, which improved the

photovoltaic properties. The power conversion efficiency presented a maximum

for H2Pc-PMPSA with a copolymerization ratio of (1:1) at the annealing

temperature of 160 C for 2 h.

Quasi-solid state dye-sensitized solar cells (DSCs) have been constructed

using a new polymeric ionic fluid as the electrolyte.119 The electrolyte was

synthesized by the sol-gel route using MTMSPI1I2 as the precursor that was

made by derivatizing methylimidazolium with triethyoxysilane. Condensation

of this material in the presence of formic acid and in the absence of water led to

Si-O-Si-O-type polymerization and formation of a polysilsesquioxane-type

structure. When this material was mixed with iodine, it served as a redox

electrolyte for DSCs. The DSCs made this way are robust and easy to assemble

but their efficiency of 3.1% is relatively low. However, possible improvement

lies in modification of the organic groups attached to the polysilsesquioxane

backbone.



D. Lithography

The near-UV absorption (300À400 nm) properties of polysilanes originally attracted attention because of their possible applications as photoresists

for high-resolution lithography.46 The proposed mechanism of polydialkylsilane photodegradation by Michl46 involves two concurrent and competitive

reactions. One of these is silylene extrusion, which shortens the polymer chain

by only one unit and does not affect the molecular weight significantly, as

shown in reaction 1. The other is homolytic cleavage, which cuts the average

molecular weight approximately in half, as shown in reaction 2.



A third process proposed by Michl to be involved in photodegradation is chain

cleavage by reductive elimination according to reaction 3:



244



Photophysics and Photochemistry of Polysilanes



The degradation mechanism makes polysilanes choice materials for the

fabrication of patterns with very high resolution, because polysilanes are easily

converted to materials having completely opposite properties when exposed to

UV and high-energy beams.120 Polysilanes are dry etched at a lower rate than

usual electronic substrates such as SiO2 and silicon layers under plasma gas and

are also different from the dry-etching rates of polysiloxane layers formed by

irradiation followed by photo-oxidation of the polysilane. A special advantage

of the polysilane as photoresist materials is the fact that the Si atoms forming

the backbone are not recognized as being impurities by the Si substrate host,

while carbon or metal atoms cause serious contamination problems in the Si

layer. Bilayer resists using polysilanes became important for deep UV exposures, wavelength ,200 nm, because they eliminate the problem of penetration

depth of the light. Anti-reflection layers for deep UV lithography is another

interesting application of polysilanes, which are better than the conventional

organic anti-reflecting coatings (ARC). Advantages for polysilane ARC are

that the resist thickness can be reduced, and the ARC thickness can be

increased. The etching selectivity and optical properties make polysilanes the

most promising anti-reflection films for lithography.120 Exposure of a polymethylsilyne film with a thickness of 120À170 nm, plasma polymerized with a

240-nm light, results in an etching selectivity coefficient of 3 between the

exposed area (flux 5 100 mJ/cm2) and the unexposed area, which is a good

value for the lithographic process. The efficient photooxidation of the polysilane allows fabrication of patterns of 200 nm resolution, via the selected

production of thin oxide hard masks. The dry development has been carried

out in Cl or HBr plasmas, and the unexposed polysilane area, is etched faster

than the photooxided area, providing negative images.121À127 Polybutylsilylene

has the highest sensitivity toward HBr etching and shows photooxidation by

exposure to 193 nm. Larger substituents such as phenyl and cyclohexyl groups

decrease the sensitivity.123 Table 3 summarizes the positive and negative photoresist behavior of some polysilanes.

Copolymers between polysilanes and OH/COH bearing organic polymers

have been used for developing procedures in aqueous solutions.47,128



E. Electron Beam

Increased resolution of the LSI patterning can be obtained by e-beam

lithography. Polysilanes can be used as analogue-type positive resists, defined

as materials that have properties (thickness, solubility, Tg) developed proportionally with the UV exposure, desirable for the fabrication of optical elements by direct writing, such as masks.51 Photopolymerization using PMPS

with an average molecular mass of 1.32 3 104 and Mw/Mn 5 1.94 as a macromolecular photoradical initiator has been used to produce a polysilane-acryl

block copolymer (Mn 5 1.02 3 104, Mw/Mn 5 1.78).129 A series of thin films

were irradiated with a 50 kV e-beam. The test pattern was developed in

2-propanol and 2-buatone solution as well as reactive ion etching (RIE)



Polysilane Thin Films for Electronic Devices



245



TABLE 3. Polysilane Resists and Development Methods120



Polymer

Structure



Si



Preparation



Exposure



Development



Location



Spin coating



313



IBM/Wisconsin Unversity



248



Methylcyclohexane

Isopropylalcohol (P)

Ablation (P)



248



Ablation (P)



Sandia, N.L.



248



THF/isopropanol (P)



ATT/Sandia, N.L.



Cyclohexanol

Butylacetate

Isopropanol



IBM/University of Texas



n



CH3



Spin coating



Si



n



CH3

Bu

Si



CH3

Si



Si



Spin coating



193



Methanol,

Acetone (N)

Toluene (P)

Cl2,HBr plasma (P)



MIT/Penn State University



Plasma



193



Toluene (N)

HBr, Cl2 RIE (N)

Ablation



MIT



Spin coating



193



HBr plasma (N)



MIT/IBM/Penn State

University



ATT



n



n



n



CH3

Si



n



Plasma

Plasma



P, positive; N, negative.



248



Cl2 (N)



193



NH4F/HF (P)



193



Cl2 (N)



MIT



246



Photophysics and Photochemistry of Polysilanes





h



G



Substrate

Hydrogen

Silicon



Carbon



FIGURE 28. Structural model of the evaporated film. (Reprinted from Ref. 30.)



in fluorocarbon/O2 plasma.51 On the positive pattern, Al was then vacuum

deposited without any deformation although similar deposition of polymethylmethacralyate gave a rough surface. This result is attributed to the

higher heat resistance of the copolymer than that of polymethylmetacrylate.51

Similarly, PMPS has been used as a macromolecular photoinitiator to produce

block polysilanemethyacrylate copolymer.52 The product has been spin coated

from a toluene solution and irradiated with 50 kV electron beam with the

conclusion that the copolymer is more suitable for e-beam lithography than the

organic counterpart due to the high heat resistance, dry-etching resistance, and

good sensitivity to electron beam irradiation.

By e-beam irradiation onto a highly oriented PDMS film in which the

polymer’s backbone is perpendicular to the substrate surface, submicron patterns were obtained after the etching process.130 PDMS was synthesized by the

Wurtz-type method from dimethylchlorosilane, and oriented films were prepared by vacuum evaporation on fused silica. Figure 28 shows the structural

model of the evaporated film. The oriented character and the high density of

the film are advantageous for high resolution. The fabrication process for

obtaining submicron patterns using the oriented films is shown in Figure 29.

The first step (Fig. 29a) is the evaporation of the polysilane material and

recrystallization on the substrate. The film is then irradiated by the electron

beam (Fig. 29b). In the irradiated area of the film, C-O-C, Si-O-C, and Si-O-Si

bonds are formed between nearest-neighbor polysilane chains. Thus the



Polysilane Films for Optical Devices

(a)



(b)

Substrate



(c)

Electron

Beam

O2

O2



ne

Po

ly



sila



C

a



He



conc.ϪH2SO4



C



C O2



ter



247



C



O



O



C



O C

O C



Substrate



Substrate

Silicon backbone



CH3



FIGURE 29. Process for the fabrication of the submicron pattern. (Reprinted from

Ref. 130.)



irradiated film is hardened and has durability against etching. The specimen is

then dipped in concentrated H2SO4 so that only the irradiated part of the films

remains and forms the negative-tone pattern on the substrate surface. The

estimated dose values obtained with acceleration voltages of 15 kV, 10 kV, and

5 kV with beam currents of 200 pA, 130 pA, and 60 pA and irradiation times of

15 min are 7.2 3 1022 C/cm2, 3.3 3 1022 C/cm2, and 1.1 3 1022 C/cm2. The

electron beam could be made narrower with higher acceleration voltage so the

narrowest pattern is obtained at about 0.5 μm Patterns with irradiation times

of 20 and 30 min, with voltage and beam current kept constant at 15 kV and

200 pA, have dose values of 9.6 3 1022 C/cm2 and 1.4 3 1022 C/cm2. These

show an increase in the width of the pattern and are explained by the effects of

mechanical vibrations and backscattering of the electron.130



VII. POLYSILANE FILMS FOR OPTICAL DEVICES

Pure optical applications are found in the use of PMPS grating layers as

an alignment layer for liquid crystal cells.131 A toluene solution of Wurtzsynthesized PMPS was spin coated to 100 nm thickness on glass and ITO

substrates. Gratings were fabricated by either a holographic method or a

phase-mask method using a He-Cd laser. The sandwich-type liquid crystal cells

were prepared by parallel arrangement of two PMPS gratings on ITO substrates, as shown in Figure 30. The thickness of the liquid crystal cells was

about 19 μm and liquid crystalline compound, 4-cyano-4u-n-pentylbiphenyl

was introduced into the cells.



248



Photophysics and Photochemistry of Polysilanes



Indium tin oxide (ITO) substrate

PMPS grating

19 mm



Liquid crystal (5CB)



Grating depth : 7,13,20 nm



CN



4-cyano-4Ј-n-pentylbiphenyl



FIGURE 30. Structure of liquid crystal cells. (Reprinted from Ref. 131.)



It was found that liquid crystal molecules are aligned parallel to the

grating grooves and that even 20-nm grating depths are effective for alignment

of liquid crystal molecules. Photoinduced surface relief gratings on PMPS thin

films are thus applicable to the alignment layers for liquid crystal cells with

weak anchoring energy whose (opto)electronic properties were improved in

comparison with conventional liquid crystal display devices. The operating

voltage of liquid crystal cells with weak anchoring energy is considerably

reduced.

An array of 10-μm microlenses was fabricated from the adhesion of

an aminated silicasol on a poly[methyl(phenyl)silane-co-methyl(3,3,3-trifluoropropyl)silane] (CF3PMPS) film patterned by UV light irradiation.132 By

soaking the UV-patterned polysilane film into the sol-gel solution, a convex

xerogel layer adhered only to the UV-exposed polysilane, which was cured to

form a glass that functioned as a condensing lens.

Electrically active PMPS, known as a good hole transporting material,

and poly[bis(p-n-butylphenyl)silane] (PBPS), known as NUV EL materials,

were end grafted directly on a crystalline silicon surface.133 The cut and graft

technique used involved a one-to-one chemical reaction between a reactive

anchor built on a substrate, and an end-lithiated polysilane prepared by the

scission reaction of a Si-Si bond in polysilane using methyllithium in a polar

organic solvent. End-grafted single molecules of PMPS were observed as

dots, while end-grafted PBPS appeared as worms due to the rigidity of

the PBPS resulting from the intramolecular stacking forces between the

phenyl rings.



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