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Thin films of plasma polymerised organic materials and ion beam assisted deposition
Vacuum/volume Printed
in Great
38/numbers
8-l
O/pages
861 to 864/t
988
0042-207X/88$3.00+.00 Pergamon Press plc
Britain
Thin films pf plasma polymerised and ion beam assisted deposition S A R Al-Hashmi Salford
and C W Smith,
Department
of Electronic
organic
and Electrical
Engineering,
materials University
of Salford,
M5 4 WT. UK
The effect of ion beam bombardment during thin film formation by plasma polymerisation of organic materials has been investigated. The effect of low energy (200-600 eV) ion bombardment during the preparation of thin films (IO-1 000 nm) of an organic material, p yrene, by plasma polymerisation is reported. X-ray and scanning electron microscope analysis shows that the crystalline structure of the films is largely determined by the kinetic energy of the ions. Ion beam irradiation at 600 eV gives the maximum microcrystallinity without excessive re-sputtering of the material and without carbonisation which commences at 1000 eV. The characteristics of metal-insulatorsemiconductor junctions formed with pyrene films are described.
1. Introduction The work described in this paper was undertaken to repeat and extend preliminary reports from Japan, subsequently published’, on the effect of low energy ion beam bombardment on the crystalline structure of thin films of pyrene prepared by vacuum evaporation during ion bombardment at ambient temperature. The structure of the film was reported to be controlled by the kinetic energy and current density of the ions. Similar results and a theoretical explanation were subsequently reported from the USA*.
2. Experimental The apparatus used is shown in Figure 1. It was constructed inhouse during a programme of tests on the compatability of the
VIEW
various organic materials being used with vacuum techniques. Because of the high vapour pressures and toxic nature of many of the materials involved, it was decided to use an oil diffusion pump with a rotary pump protected by a zeolite trap. It was found that certain of the materials, notably anthraquinone, attacked the ‘Viton’ ring seals causing leaks to develop throughout the exposed part of the system. The following precautions were agreed with the Safety Officer for the handling of toxic materials in a vacuum system. 1. The rotary pump exhaust is to be fed into the exhaust of a fume cupboard and the fans are to be kept running when the pump is in operation. Oil mist collecting in the pipes is to be regarded as being possibly contaminated with toxic waste. 2. The materials were stored in a desiccator and kept in a refrigerator.
SAMPLE HOLDER (ADJUSTABLE)
PORT
EVAPORATION
PENNING
BOAT
.
GAUGE ION
POWER SUPPLY o-30”0-Km
POWER SUPPLY O-me” SOURCE
BOTTLE
SCHEMATIC
Figure 1. Block diagram
of the apparatus
OF
THE
ION
BEAM
ASSISTED
EVAPORATION
used for the ion beam assisted deposition
DEPOSITION
and crystallisation
APPARATUS
of thin films of pyrene. 861
S A R Al-Hashmi
and C W Smith:
Thin films
of plasma
polymerlsed
organic
materials
3. The materials were handled with gloves and spatula, care being taken to avoid inhalation of the vapour or skin contact. 4. The zeolite absorber in the trap in front of the rotary pump is to be changed regularly and disposed of as toxic waste. The pump oils are to be regarded as contaminated waste when changed. 5. The vacuum chamber is to be cleaned after each use with toluene solvent using plastic gloves. All cleaning materials are to be disposed of as toxic waste
3. Film preparation The pyrenc. which melts at 154 C. was evaporated from the tungsten boat. It was necessary to cover the boat with stainloss steel mesh to prevent spluttering from the boiling liquid. The pressure was 2 x IO-’ torr before the evaporation and 2 x IO ’ during the evaporation of the pyrene. Simultaneously with the evaporation the ion beam at energies in the range 20&l 000 eV. was directed at 45 onto the target. The current density in the ion beam was about IO /tA cm ’ and the deposition rate approximately 3 nm s- ‘. By)adjusting the beam current. the rate ofevaporation and the time of deposition, films could bc produced with any thickness from IO nm to greater than 1000 nm. The results prcscnted here were made using nitrogen ions. argon was also used but it was found that the size of the polycrystals was less. It is expected that helium will give better results than the nitrogen. 4. Experimental results The results for thin films of pyrene, approximately I Llrn in thickness, prepared in the above manner are shown in Figures 2, 3 and 4. being made at ion beam energies of 200, 300 and 600 cV, respectively. They show an increase in crystallinity as the energy is increased up to 600 eV as evidenced by the X-ray diffraction measurements and the scanning electron micrographs. The mechanism of crystal growth is probably that of rcsputtering of material out of the preferred channel directions. Above 750 eV. the crystallite sizes start to decrease; beyond
300
25”
20"
15"
10"
5"
Figure 3. X-ray diff‘raction patterns and scanning electron micrographs of pyrene lilms prepared with the ion beam energy of 300 eV.
1000 eV the films take on a darkened appearance suggesting that bond breaking is taking place with the formation of free carbon. Thin films of pyrene formed on silicon wafers. with a metal counter electrode added, have been used to produce M--I S Schottky barrier diodes. These have given reverse bias breakdown voltages of 5G-100 V across polymerised pyrene films ol 500 nm thickness. Figures 5, 6 and 7 show the diode charactcristics as measured on a curve tracer for films prepared at the above rcspectivc ion beam energies. It is seen that the rcverse bias characteristic improves as the film becomes more crystalline. These diodes were also photoscnsitivc, the largest photoeffect being on the reverse bias part of the characteristic.
1
I
30"
I
25"
I
200
I 15"
I 10"
Figure 2. X-ray diffraction patterns and scanning electron microyraphs of pyrene films prepared with the ion beam energy of200 eV. 862
1
I
5"
5
Figure 4.
X-ray diRraction patterns and scanning electron micrographs of pyrene films prepared with the ion beam energy of 600 eV
S A R Al-Hashmi and C W Smith; Thin films of plasma polymerised
organic materials
0.5mA -f
Figure 5. M-I-S
diode characteristics
obtained
with pyrene films prepared
under 200 eV ion bombardment.
_.I_
0.5mA
---I-
-II-1ov Figure 6. M-I-S
diode characteristics
obtained
with pyrene films prepared
under 300 eV ion bombardment,
863
S A R Al-Hashmi
Figure7.
5.
and C W Smith
Thin
MINI-~Sdiodechar;lcteristicaobt~tined
films
of plasma
polymerlsed
organic
aith pyrenl: films prepared
Conclusions
The present work on pyrcnc conlirms the results obtained by the Japanese group’ and results obtained on organic-on-inorganic semiconductor contact barrier diodes’. Pyrcne is not necessarily the best monomeric material from which to commence the preparation of plasma polymcriscd thin films, both from the toxic nature of the starting material and the characteristics of the MI-S diodes prepared using its polymer as the insulating layer. However. the vacuum techniques acquired have proved invaluable.
864
materials
under hOO eV ion hombnrdment
Acknowledgements This work was supported
by SERC grant No. GRXX./49534.
References
’ M Migita. Y Taniguchi, H Ishihara, M A&i, T lshiba and H Tamura, J U& P/I_vs,58, 1 157 ( 1985). ‘R M Bradley, J M E Harper and D A Smith, J rr,np/ P/I.v.\. 60, JlhO (1986). ‘S R Forrest, M L Kaplan and P H Schmidt, J tqpl P/IJS. 55, 1492 (1984).
in Great
38/numbers
8-l
O/pages
861 to 864/t
988
0042-207X/88$3.00+.00 Pergamon Press plc
Britain
Thin films pf plasma polymerised and ion beam assisted deposition S A R Al-Hashmi Salford
and C W Smith,
Department
of Electronic
organic
and Electrical
Engineering,
materials University
of Salford,
M5 4 WT. UK
The effect of ion beam bombardment during thin film formation by plasma polymerisation of organic materials has been investigated. The effect of low energy (200-600 eV) ion bombardment during the preparation of thin films (IO-1 000 nm) of an organic material, p yrene, by plasma polymerisation is reported. X-ray and scanning electron microscope analysis shows that the crystalline structure of the films is largely determined by the kinetic energy of the ions. Ion beam irradiation at 600 eV gives the maximum microcrystallinity without excessive re-sputtering of the material and without carbonisation which commences at 1000 eV. The characteristics of metal-insulatorsemiconductor junctions formed with pyrene films are described.
1. Introduction The work described in this paper was undertaken to repeat and extend preliminary reports from Japan, subsequently published’, on the effect of low energy ion beam bombardment on the crystalline structure of thin films of pyrene prepared by vacuum evaporation during ion bombardment at ambient temperature. The structure of the film was reported to be controlled by the kinetic energy and current density of the ions. Similar results and a theoretical explanation were subsequently reported from the USA*.
2. Experimental The apparatus used is shown in Figure 1. It was constructed inhouse during a programme of tests on the compatability of the
VIEW
various organic materials being used with vacuum techniques. Because of the high vapour pressures and toxic nature of many of the materials involved, it was decided to use an oil diffusion pump with a rotary pump protected by a zeolite trap. It was found that certain of the materials, notably anthraquinone, attacked the ‘Viton’ ring seals causing leaks to develop throughout the exposed part of the system. The following precautions were agreed with the Safety Officer for the handling of toxic materials in a vacuum system. 1. The rotary pump exhaust is to be fed into the exhaust of a fume cupboard and the fans are to be kept running when the pump is in operation. Oil mist collecting in the pipes is to be regarded as being possibly contaminated with toxic waste. 2. The materials were stored in a desiccator and kept in a refrigerator.
SAMPLE HOLDER (ADJUSTABLE)
PORT
EVAPORATION
PENNING
BOAT
.
GAUGE ION
POWER SUPPLY o-30”0-Km
POWER SUPPLY O-me” SOURCE
BOTTLE
SCHEMATIC
Figure 1. Block diagram
of the apparatus
OF
THE
ION
BEAM
ASSISTED
EVAPORATION
used for the ion beam assisted deposition
DEPOSITION
and crystallisation
APPARATUS
of thin films of pyrene. 861
S A R Al-Hashmi
and C W Smith:
Thin films
of plasma
polymerlsed
organic
materials
3. The materials were handled with gloves and spatula, care being taken to avoid inhalation of the vapour or skin contact. 4. The zeolite absorber in the trap in front of the rotary pump is to be changed regularly and disposed of as toxic waste. The pump oils are to be regarded as contaminated waste when changed. 5. The vacuum chamber is to be cleaned after each use with toluene solvent using plastic gloves. All cleaning materials are to be disposed of as toxic waste
3. Film preparation The pyrenc. which melts at 154 C. was evaporated from the tungsten boat. It was necessary to cover the boat with stainloss steel mesh to prevent spluttering from the boiling liquid. The pressure was 2 x IO-’ torr before the evaporation and 2 x IO ’ during the evaporation of the pyrene. Simultaneously with the evaporation the ion beam at energies in the range 20&l 000 eV. was directed at 45 onto the target. The current density in the ion beam was about IO /tA cm ’ and the deposition rate approximately 3 nm s- ‘. By)adjusting the beam current. the rate ofevaporation and the time of deposition, films could bc produced with any thickness from IO nm to greater than 1000 nm. The results prcscnted here were made using nitrogen ions. argon was also used but it was found that the size of the polycrystals was less. It is expected that helium will give better results than the nitrogen. 4. Experimental results The results for thin films of pyrene, approximately I Llrn in thickness, prepared in the above manner are shown in Figures 2, 3 and 4. being made at ion beam energies of 200, 300 and 600 cV, respectively. They show an increase in crystallinity as the energy is increased up to 600 eV as evidenced by the X-ray diffraction measurements and the scanning electron micrographs. The mechanism of crystal growth is probably that of rcsputtering of material out of the preferred channel directions. Above 750 eV. the crystallite sizes start to decrease; beyond
300
25”
20"
15"
10"
5"
Figure 3. X-ray diff‘raction patterns and scanning electron micrographs of pyrene lilms prepared with the ion beam energy of 300 eV.
1000 eV the films take on a darkened appearance suggesting that bond breaking is taking place with the formation of free carbon. Thin films of pyrene formed on silicon wafers. with a metal counter electrode added, have been used to produce M--I S Schottky barrier diodes. These have given reverse bias breakdown voltages of 5G-100 V across polymerised pyrene films ol 500 nm thickness. Figures 5, 6 and 7 show the diode charactcristics as measured on a curve tracer for films prepared at the above rcspectivc ion beam energies. It is seen that the rcverse bias characteristic improves as the film becomes more crystalline. These diodes were also photoscnsitivc, the largest photoeffect being on the reverse bias part of the characteristic.
1
I
30"
I
25"
I
200
I 15"
I 10"
Figure 2. X-ray diffraction patterns and scanning electron microyraphs of pyrene films prepared with the ion beam energy of200 eV. 862
1
I
5"
5
Figure 4.
X-ray diRraction patterns and scanning electron micrographs of pyrene films prepared with the ion beam energy of 600 eV
S A R Al-Hashmi and C W Smith; Thin films of plasma polymerised
organic materials
0.5mA -f
Figure 5. M-I-S
diode characteristics
obtained
with pyrene films prepared
under 200 eV ion bombardment.
_.I_
0.5mA
---I-
-II-1ov Figure 6. M-I-S
diode characteristics
obtained
with pyrene films prepared
under 300 eV ion bombardment,
863
S A R Al-Hashmi
Figure7.
5.
and C W Smith
Thin
MINI-~Sdiodechar;lcteristicaobt~tined
films
of plasma
polymerlsed
organic
aith pyrenl: films prepared
Conclusions
The present work on pyrcnc conlirms the results obtained by the Japanese group’ and results obtained on organic-on-inorganic semiconductor contact barrier diodes’. Pyrcne is not necessarily the best monomeric material from which to commence the preparation of plasma polymcriscd thin films, both from the toxic nature of the starting material and the characteristics of the MI-S diodes prepared using its polymer as the insulating layer. However. the vacuum techniques acquired have proved invaluable.
864
materials
under hOO eV ion hombnrdment
Acknowledgements This work was supported
by SERC grant No. GRXX./49534.
References
’ M Migita. Y Taniguchi, H Ishihara, M A&i, T lshiba and H Tamura, J U& P/I_vs,58, 1 157 ( 1985). ‘R M Bradley, J M E Harper and D A Smith, J rr,np/ P/I.v.\. 60, JlhO (1986). ‘S R Forrest, M L Kaplan and P H Schmidt, J tqpl P/IJS. 55, 1492 (1984).