Microelectronics-Photonics Graduate Program
Research Experience for Undergraduates
Summer 2007
Final Research Report
Metal Induced Crystallization of Amorphous Silicon Using an
Electron Beam
Benjamin Newton
Dr. Hameed Naseem, Electrical Engineering
Dr. Husam Abu Safe, Electrical Engineering
Mrs. Dorinne Bower, Microeletronics-Photonics
July 26, 2007
This work was supported by National Science Foundation award EEC-0097714.
Any opinion, findings, and conclusions or recommendations expressed in this
Material are those of the author and do not necessarily reflect the views of the
National Science Foundation.
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Benjamin Newton, University of Arkansas Pine Bluff
Faculty Mentor: Dr. Hameed Naseem, Electrical Engineering
Post doc Mentor: Dr. Husam Abu Safe, Electrical Engineering
Graduate Student Mentor: Mrs. Dorinne Bower
Metal Induced Crystallization of Amorphous Silicon using an Electron Beam
Activities:
Amorphous silicon is commonly used in the production of photovoltaic cells and
thin film transistors. We investigated the effectiveness of using an electron beam in the
process of metal induced crystallization. Various annealing times, spot sizes and
accelerating voltages, were used to find the most effective method of using an electron
beam to induce crystallization.
Findings
We found that the electron beam did change the morphology of the surface of the
substrate, but that the source we used for our electron beam may not be powerful enough
to give us more defined results. The crystals that may have been produced were on the
order of nanometers and the substrate was at least a square meter of surface area. We
believe this may be the reason the XRD could not detect any crystallization although we
saw a definite change in the surface of the material. We find that an electron beam can be
used for metal induced crystallization, but the spot size of the beam or an area of multiple
spots needs to be crystallized in order for the XRD to detect crystallization.
Planned Publications
1. No planned journal publications at this time
2. Student presentation for Microelectronics and photonics on July 25,2007
Key Illustration /Figure
Figure 1 Four spots 10 microns apart created by an electron beam from top to bottom
they were 30 kv spot size 5 ,20 kv spot size 5, 10 kv spot size 4, 30 kv spot size 1. All of
the annealin times were for 30 minuts except for the last which was an hour.
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Table of Contents
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List of Figures
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Abstract
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Introduction and Background
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Research goals
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Description of Experiment
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Experimental Results and Conclusions
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Future Research Suggested by this Research
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Appendix A: List of References
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Appendix B: Intellectual Property-Commercialization Value
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Appendix C: Impact of Reu on Personal Goals and Plans
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Acknowledgements
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Figure 1 Four dots 10 microns apart created by an electron beam from top to bottom their
beam strengths and spot size are 30 kv spot size 5, 20 kv spot size 5, 10 kv spot size 4,
30 kv spotsize 1.
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Figure 2 This is the 30 kv dot spot size 5 for 30 minutes. Definite change in the surface.
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Figure 3 This is the 20kv dot spot size 4 for 30 min. There appears to be some formation
of a substance at the center of the spot.
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Figure 4 This is the 10 kv dot spot size 4 for 30 min. There appears to be a formation of
some substance at the center of the spot.
Figure 5 The first site annealed on the surface of the copper transmission electron
microscope grid. It was annealed for 15 minutes at 20kv spot size 4.
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Figure 6 Closer picture of first annealing site .
Figure 7 Second annealing site was annealed for 15 minutes at 30 kv spot size 4
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Figure 8 On the left is the crystallization and on the right is an uncrystallized portion of
the substrate from the TEM grid.
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Abstract
Amorphous silicon is commonly in the production of photovoltaic cells and thin
film transistors. The process we incorporated was aluminum induced crystallization. We
tested the effectiveness of using an electron beam in the process of aluminum induced
crystallization. Using an Environmental scanning electron microscope as our source of
electrons and incorporating various annealing times, accelerating voltages and spot sizes,
we tested the electron beam’s ability to induce crystallization.
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Introduction and Background
Metal induced crystallization (MIC) is a process used to create polycrystalline
silicon from amorphous silicon at relatively low temperatures. The process of MIC
consist of an amorphous silicon thin film that are deposited onto a substrate. The
substrate is usually glass. Then the film covered substrate is capped with a metal most
likely aluminum. This entire structure is then annealed at temperatures between 150 and
400 degrees Celsius. This causes the amorphous silicon to change into polycrystalline
silicon. Annealing in material science is a heat treatment wherein a material is altered,
causing a change in its properties. In this instance it is a change in electrical properties.
Because of rising interest in alternate renewable forms of energy, Metal induced
crystallization is now receiving much more attention from researchers. The main
application of polycrystalline silicon prepared by MIC is in the fabrication of thin film
transistors and photovoltaic cells. “ For thin film solar cells polycrystalline silicon is the
most promising material to reduce the production cost”[1]. Polycrystalline can be formed
without the use of metal but the temperature requirements are much higher and thus more
costly. “ Reduced processing temperature is the key to low cost devices”[2]. “For Al, as
low as 150 degrees Celsius crystallization temperature can be used”[3].
The common method of inducing crystallization is to anneal the entire sample. In
this experiment we hope to only anneal sections of the sample touched by the electron
beam.
Research Goals
Our goal was to prove that an electron beam could be utilized to induce
crystallization in amorphous silicon in specific spots touched without inducing
crystallization in surrounding areas. This process might require less heat might make
photovoltaic cells much cheaper in the future. According to the Department of Energy it
currently cost 6.25/watt of produced energy. We want this much lower.
Description of Experiment
A 1 cm^2 glass substrate was covered in 200 nm of amorphous silicon and then
10 nm of aluminum. The glass substrate was then placed in a scanning electron
microscope (SEM) at high vacuum and the electron beam of the SEM was then use to
anneal specific sections of the surface. The specific heat formula (c = Q/mΔT) and a spot
size vs. diameter chart from the SEM manual were used to determine the heat present at
the surface where the electron beam touched. The first annealing was sixteen specific
spots at 30 kv accelerating voltage and spot size 3 for 5 minutes per spot. The second
annealing consisted of four different spots in which 3 of the spots were for 30 minutes
and the last spot for an hour. The first three spots were 30 kv spot size 5, 20 kv spot size
5 and 10 kv spot size 4. The last spot was 30 kv spot size 1. Both the samples were then
taken to the XRD to be tested at 28.5 degrees for the existence of polycrystalline silicon
and viewed in the SEM for a change in the morphology of the surface. After this the
substrate was changed. The new substrate consisted of a copper transmission electron
microscope grid covered in 500 nm of amorphous silicon and 300 nm of aluminum. This
substrate was annealed twice. The first spot was annealed for 15 minutes at 20 kv spot
size 4. The second spot was annealed for 15 minutes at 30 kv spot size 4. The last
substrate consisted of a carbon transmission electron microscope grid covered in 500 nm
of amorphous silicon and 300 nm of aluminum. The first spot was annealed for 15
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minutes 30kv spot size 4. The remaining two spots were annealed for 30 minutes at spot
size 4.
Experimental Results and Conclusions
The sample with the sixteen spots showed no signs of a change in the morphology
of the surface or the detection of polycrystalline silicon by the XRD. The second sample
showed a definite signs of change in the morphology of the surface but did not show any
signs of polycrystalline silicon in the XRD. The third sample showed more definite signs
of a change in morphology but was still insufficiently large enough to be detected by XRay Diffraction (XRD). The transmission electron microscope substrates did show signs
of crystallization in the TEM, and they did show definite signs of change in the
morphology of the surface of the substrate.
Through the research of electron beam welding it has been found that electron
beams can exceed 25,000 degrees Celsius. We also believe that if an electron beam with
a much higher current were used it would take the beam less time to produce a change in
the surface and induce crystallization.
Future Research Suggested by this Research
We believe that if an electric gun with a high enough current were used with
multiple filaments or a mesh screen of filaments it could be used to cover the entire
surface of the substrate with these crystallized spot and dendritic trees beneath creating
highways for the electrons produced in photovoltaic reactions. Secondly, increasing the
annealing time and using the TEM beam for crystallization and immediate measurement
would be an invaluable asset.
Appendix A: List of References
1. J.H. Werner, R Bergmann, R. Brendel, Adv. Solid State Phys. 34(1995) 115
2. Maruf Hossam, Husam Abu-Safe, Hameed Naseem, Technical Digest of the
International PVSEC-14, 2004 P-33
3. M. Shahidul haque, H. A. Naseem and W. D. Brown, J. Applied Physics, vol.95 p.
3928 1994
Appendix B: Intellectual Property-Commercialization Value
The process of annealing would improve the performance of existing photovoltaic
technology and would be highly valuable. Therefore if accomplished we would seek a
patent.
Appendix C: of REU on Personal Goals and Plans
This summer has given me experience in another area of research. I now know
what area of research I would like to do for my graduate education. I also learned many
new pieces of test equipment and procedures. The past two summers in this program has
given me a real insight into graduate education. I will definitely be putting my application
in for graduate school.
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Acknowledgements:
I would like to acknowledge Mr. Vickers, Ms. Karla Clark, Dr. Husam Abu-Safe,
Dr. Hameed Naseem , Dr. Mansour Mortazavi and the micro-electronics and photonics
program for the opportunity to allow to study here for a second summer. My knowledge
and skills have been improved. Last of all, I would like to thank the National Science
Foundation for providing the funding which allows us to be able to come to this
university each summer.
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