U.S. patent application number 12/269276 was filed with the patent office on 2009-05-14 for particle beam assisted modification of thin film materials.
This patent application is currently assigned to VARIAN SEMICONDUCTOR EQUIPMENT ASSOCIATES, INC.. Invention is credited to Rajesh Dorai, Jonathan G. ENGLAND, Ludovic Godet.
Application Number | 20090124064 12/269276 |
Document ID | / |
Family ID | 40624096 |
Filed Date | 2009-05-14 |
United States Patent
Application |
20090124064 |
Kind Code |
A1 |
ENGLAND; Jonathan G. ; et
al. |
May 14, 2009 |
PARTICLE BEAM ASSISTED MODIFICATION OF THIN FILM MATERIALS
Abstract
Several examples of a method for processing a substrate are
disclosed. In a particular embodiment, the method may include:
disposing a substrate having an upper surface and a lower surface
on a platen contained in a chamber; generating a plasma containing
a plurality of charged particles above the upper surface of the
substrate, the plasma having a cross sectional area equal to or
greater than a surface area of the upper surface of the substrate;
applying a first bias voltage to the substrate to attract the
charged particles toward the upper surface of the substrate;
introducing the charged particles to a region extending under
entire upper surface of the substrate; and initiating,
concurrently, a first phase transformation in the region from the
amorphous phase to a crystalline phase.
Inventors: |
ENGLAND; Jonathan G.;
(Horsham, GB) ; Dorai; Rajesh; (Woburn, MA)
; Godet; Ludovic; (North Reading, MA) |
Correspondence
Address: |
VARIAN SEMICONDUCTOR EQUIPMENT ASSC., INC.
35 DORY RD.
GLOUCESTER
MA
01930-2297
US
|
Assignee: |
VARIAN SEMICONDUCTOR EQUIPMENT
ASSOCIATES, INC.
Gloucester
MA
|
Family ID: |
40624096 |
Appl. No.: |
12/269276 |
Filed: |
November 12, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60987629 |
Nov 13, 2007 |
|
|
|
60987667 |
Nov 13, 2007 |
|
|
|
60987650 |
Nov 13, 2007 |
|
|
|
Current U.S.
Class: |
438/486 ;
257/E21.135 |
Current CPC
Class: |
H01L 31/182 20130101;
H01L 21/02532 20130101; H01L 21/02689 20130101; C30B 1/023
20130101; H01L 31/0747 20130101; H01L 21/02422 20130101; Y02P
70/521 20151101; Y02P 70/50 20151101; H01L 31/202 20130101; H01L
21/0237 20130101; C30B 29/06 20130101; H01L 21/268 20130101; Y02E
10/546 20130101 |
Class at
Publication: |
438/486 ;
257/E21.135 |
International
Class: |
H01L 21/20 20060101
H01L021/20 |
Claims
1. A method for processing a substrate in an amorphous phase, the
method comprising: disposing a substrate having an upper surface
and a lower surface on a platen contained in a chamber; generating
a plasma containing a plurality of charged particles above the
upper surface of the substrate, the plasma having a cross sectional
area equal to or greater than a surface area of the upper surface
of the substrate; applying a first bias voltage to the substrate to
attract the charged particles toward the upper surface of the
substrate; introducing the charged particles to a region extending
under entire upper surface of the substrate; and initiating,
concurrently, a first phase transformation in the region from the
amorphous phase to a crystalline phase.
2. The method of claim 1, wherein the charged particles are
introduced to the region at a rate of approximately
5.times.10.sup.14 particles/cm.sup.2 sec or greater.
3. The method of claim 1, further comprising: adjusting the bias
applied to the substrate whilst the introducing of the particles to
the first region.
4. The method of claim 3, wherein the adjusting comprises
increasing the bias.
5. The method of claim 1, further comprising: adjusting temperature
of the substrate prior to introducing the particles to the
region.
6. The method of claim 5, wherein the adjusting the temperature
comprises decreasing the temperature.
7. The method of claim 1, wherein the charged particles comprises
one or more species selected from a group consisting of: He, Ne,
Ar, Kr, Xe, and Rn.
8. The method of claim 1, wherein the charged particles comprises
molecular ions.
9. The method of claim 1, wherein the charged particles comprises
Ga ions.
10. The method of claim 1, wherein the region comprises Si and
wherein the charged particles comprises one or more species
selected from a group consisting of C and Ge ions, so as to convert
the region to a stressed region.
11. The method of claim 1, wherein the region comprises a material
selected from Group IV elements and wherein the charged particles
comprises one or more species chosen from a group consisting of B,
Ga, In, P, As, Sb, and Bi, so as to change an electrical property
of the region.
12. The method of claim 1, wherein the region comprises a material
selected from Group IV elements and wherein the charged particles
comprises one or more species chosen from a group consisting of Yb,
Ti, Zr, Hf, Pd, Pt, and Al, so as to change a bandgap property of
the region.
13. The method of claim 1, wherein the region comprises a material
selected from Group IV elements and the charged particles comprises
one or more species chosen from a group consisting of C containing
ions, Si containing ions, Ge containing ions, F containing ions, N
containing ions, H containing ions, He containing ions, Sn
containing ions, and Pb containing ions, so as to prevent changing
of an electrical property of the region.
14. The method of claim 1, wherein the region comprises a material
selected from Group IV elements and the charged particles comprises
metallic ions so as to increase a rate of the transformation of the
region from the amorphous phase to the crystalline phase.
15. The method of claim 15, the metallic ions comprises Ni
ions.
16. The method of claim 1, further comprising: transforming phase
of entire region from the amorphous phase to the crystalline phase,
the region being less than entire substrate.
17. The method of claim 17, wherein the region comprises at least
one crystal.
18. The method of claim 18, further comprising: extending a
boundary of the at least one crystal beyond the region of the
substrate until a phase of the entire substrate is transformed to
the crystalline phase.
Description
PRIORITY
[0001] This application claims priority to a Provisional
Application No. 60/987629 titled "Particle Beam Assisted
Modification of Thin Film Materials" and filed on Nov. 13, 2007; a
Provisional Application No.: 60/987667 titled "Particle Beam
Assisted Modification of Thin Film Materials" and filed on Nov. 13,
2007; and a Provisional Application 60/987,650 titled "Particle
Beam Assisted Modification of Thin Film Materials" and filed on
Nov. 13, 2007, each of which is incorporated in entirety by
reference.
RELATED APPLICATIONS
[0002] This application is related to co-pending application Ser.
No. ______ titled "Particle Beam Assisted Modification of Thin Film
Materials" and filed on ______, and co-pending application Ser. No.
______ titled "Particle Beam Assisted Modification of Thin Film
Materials" and filed on ______. Each of the co-pending applications
are incorporated in entirety by reference.
FIELD
[0003] This disclosure relates to a system and technique for
processing a substrate, more particularly, to a system and
technique for forming a substrate crystalline phase.
BACKGROUND
[0004] The widespread adoption of emerging technologies such as
flat panel displays (FPD) and solar cells depends on the ability to
manufacture electrical devices on low cost substrates. In
manufacturing FPD, pixels of a typical low cost flat panel display
(FPD), are switched by thin film transistors (TFT) which may be
typically manufactured on thin (-50 nm thick) films of amorphous
silicon deposited on inert, glass substrates. However, improved
FPDs demand better performing pixel TFTs, and it may be
advantageous to manufacture high performance control electronics
directly onto the panel. One advantage may be to eliminate the need
for costly and potentially unreliable connections between the panel
and external control circuitry.
[0005] Current FPDs contain a layer of Si that is deposited onto
the glass panel of the display via a low temperature deposition
process such as sputtering, evaporation, plasma enhanced chemical
vapor deposition (PECVD), or low pressure chemical vapor deposition
(LPCVD) process. Such low temperature processes are desirable, as
the panel used to manufacture FPD tends to be amorphous and has
glass transition temperature of approximately 600.degree. C. If
manufactured above 600.degree. C., the panel may have a non-uniform
or uneven structure or surface. Higher temperature tolerant glass
panels such as quartz or sapphire panel exist; however, the high
cost of such glasses discourages their use. Further cost reduction
would be possible if cheaper, lower temperature tolerant glass or
plastic panels could be used.
[0006] The low temperature deposition process, however, does not
yield optimal Si film. As known in the art, solid Si has three
common phases: amorphous, poly-crystalline, and mono-crystalline
phases. If Si is deposited at low temperature, the deposited Si
film tends to be in an amorphous phase. The channels of thin film
transistors based on amorphous Si film may have lower mobility
compared to those on either poly-crystalline Si or mono-crystalline
Si films.
[0007] To obtain a polycrystalline or mono-crystalline Si layer,
the panel may undergo further processes to convert the amorphous Si
film to either polycrystalline or mono-crystalline film. To obtain
a panel with poly-crystalline Si film, the panel may undergo an
excimer laser annealing (ELA) process. An example of the ELA
process may be found in more detail in U.S. Pat. No. 5,766,989. To
obtain a panel with larger crystals, the panel may undergo a
process known as Sequential Lateral Solidification ("SLS") process.
An example of SLS process may be found in U.S. Pat. No. 6,322,625.
Although ELA and SLS processes may result in a panel with
mono-crystalline or poly-crystalline Si thin film, each process is
not without disadvantages. For example, excimer lasers used in both
processes may be expensive to operate, resulting in an expensive
TFT. In addition, the duty cycle may not be optimum for the best
conversion of amorphous Si into crystalline Si. Further, the
excimer laser may have pulse-to-pulse variations and spatial
non-uniformity in the delivered power which may affect the
uniformity of the processes. There may also be intra-pulse
non-uniformity which may be caused by for example,
self-interference of the beam. Such inter-pulse and intra-pulse
non-uniformity may result in Si films with non-uniform
crystals.
[0008] As such, new methods and apparatus for particle processing
for the cost effective and production worthy manufacture of high
quality crystalline materials on low temperature substrates are
needed.
SUMMARY
[0009] Several examples of a method for processing a substrate are
disclosed. In a particular embodiment, the method may include:
disposing a substrate having an upper surface and a lower surface
on a platen contained in a chamber; generating a plasma containing
a plurality of charged particles above the upper surface of the
substrate, the plasma having a cross sectional area equal to or
greater than a surface area of the upper surface of the substrate;
applying a first bias voltage to the substrate to attract the
charged particles toward the upper surface of the substrate;
introducing the charged particles to a region extending under
entire upper surface of the substrate; and initiating,
concurrently, a first phase transformation in the region from the
amorphous phase to a crystalline phase.
[0010] The present disclosure will now be described in more detail
with reference to exemplary embodiments thereof as shown in the
accompanying drawings. While the present disclosure is described
below with reference to exemplary embodiments, it should be
understood that the present disclosure is not limited thereto.
Those of ordinary skill in the art having access to the teachings
herein will recognize additional implementations, modifications,
and embodiments, as well as other fields of use, which are within
the scope of the present disclosure as described herein, and with
respect to which the present disclosure may be of significant
utility.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] In order to facilitate a fuller understanding of the present
disclosure, reference is now made to the accompanying drawings, in
which like features are referenced with like numerals. These
figures should not be construed as limiting the present disclosure,
but are intended to be exemplary only.
[0012] FIG. 1 is a block diagram of various mechanisms through
which amorphous material may transform into crystalline
material.
[0013] FIG. 2 shows a graph of the depth of Ar ions introduced to a
Si substrate according to one embodiment of the present
disclosure.
[0014] FIG. 3 shows block diagram of a system for processing a
substrate according to one embodiment of the present
disclosure.
[0015] FIG. 4 shows a block diagram of a particular exemplary
system of the system shown in FIG. 3.
[0016] FIG. 5 shows a block diagram of another system for
processing a substrate according to another embodiment of the
present disclosure.
[0017] FIG. 6 shows a graph comparing the temperature of a
substrate irradiated with a laser beam or a particle beam.
[0018] FIG. 7 shows a graph of the temperature of a substrate
irradiated with a focused particle beams according to another
embodiment of the present disclosure.
[0019] FIG. 8A-8B show a method that can be incorporated into
manufacturing of a solar cell according to another embodiment of
the present disclosure.
[0020] FIG. 9A-9C show another method that can be incorporated into
manufacturing of a solar cell according to another embodiment of
the present disclosure.
[0021] FIG. 10A-10D show another method that can be incorporated
into manufacturing of a solar cell according to another embodiment
of the present disclosure.
DETAILED DESCRIPTION
[0022] To overcome the above-identified and other deficiencies of
existing laser-based thin film materials processing, several
embodiments of particle based processing are disclosed. The
particle-based processing may be advantageous as it may promote
non-equilibrium processes. In addition, particle parameters may be
controlled with much more precision than parameters of the laser.
Examples of the particle parameters may include spatial parameters
(such as beam size and current densities), particle flux (and/or
beam current), particles species, and particle dose etc. . . .
[0023] In the present disclosure, several embodiments are disclosed
in context to a beamline ion implantation system and a plasma based
substrate processing system such as, for example, a plasma assisted
doping (PLAD) system or plasma immersion ion implantation (PIII)
system. However, those of ordinary skill in the art should
recognize that the present disclosure may be equally applicable to
other systems including other types of particle based system. The
term "particles" used herein may refer to sub-atomic, atomic, or
molecular particles, charged or neutral. For example, the particles
may be protons; ions, atomic or molecular; or gas clusters.
[0024] In the present disclosure, several embodiments are described
in context to a substrate. The substrate may be a wafer (e.g. Si
wafer) or a substrate comprising a plurality of films. In addition,
the substrate may be an elemental substrate containing only one
element (e.g. Si wafer or metal foil); a compound substrate
containing more than one element (e.g. SiGe, SiC, InTe, GaAs, InP,
GaInAs, GaInP; CdTe; CdS; and combinations of (Cu, Ag and/or Au)
with (Al, Ga, and/or In) and (S, Se and/or Te) such as CuInGaSe,
CuInSe2, other group III-V semiconductors and other group II-VI
compounds); and/or an alloy substrate. The material contained in
the substrate may be metal, semiconductor, and/or insulator (e.g.
glass, Polyethylene terephthalate (PET), sapphire, and quartz).
Further, the substrate may be a thin film substrate containing
multiple layers (e.g. SOI). If the substrate comprises multiple
layers, at least one of the layers may be a semiconducting film or
a metallic film, whereas another one of the films may be an
insulator. The semiconducting or metallic film may be disposed on a
single insulating film or, alternatively, interposed between a
plurality of insulating films. Conversely, the insulating film may
be disposed on a single semiconducting or metallic film or,
alternatively, interposed between multiple semiconducting or
metallic films or both.
Phase Transformation
[0025] The most rapid mechanism for crystallization of thin
amorphous layers is solid phase epitaxial re-growth (SPER). In
SPER, amorphous Si may transform to crystalline Si by extending an
underlying, pre-existing, extensive crystal layer. This scenario is
commonly encountered during annealing of a surface layer of a
crystalline Si wafer after it has been amorphized by ion
implantation. The present disclosure relate to processing an
amorphous substrate in which an extensive pre-existing lattice does
not exist and which phase transformation occur via crystal
nucleation prior to the growth of the crystals. Referring to FIG.
1, there is shown a block diagram of various mechanisms through
which a material without extensive pre-existing lattice may
transform from an amorphous phase into a crystalline phase. As
known in the art, the crystalline phase may be categorized as a
poly-crystalline phase or a mono-crystalline phase. The
poly-crystalline phase may sometimes be further subdivided into
different categories (such as multi-, micro-, nano-crystalline etc)
depending on the crystal size. However, such a distinction may not
be important in the context of this disclosure, and may not be
necessary to describe FIG. 1. Accordingly, these phases may be
referred herein collectively as a crystalline phase.
[0026] As illustrated in FIG. 1, the phase transformation from the
amorphous phase to a crystalline phase may occur via various
mechanisms. For example, the transformation may occur via melting
and solidification mechanism 100a and solid phase crystallization
(SPC) transformation mechanism 100b. In the melting and
solidification mechanism 100a and SPC mechanism, the transformation
may occur via nucleation of crystallites and growth of the
crystallites. In the SPER mechanism, the transformation may occur
by growth on the extensive pre-existing crystal lattice.
[0027] In the melting and solidification mechanism 100a, energy in
the form of radiation, heat, or kinetic energy, may be introduced
to a portion of the amorphous substrate and melt the portion. If
the condition of the molten region is adequate to induce nucleation
(e.g. supercooling), crystals may nucleate as described by the
classical nucleation theory. The crystals may nucleate via two
schemes. The crystals may nucleate heterogeneously on pre-existing
seeds. The pre-existing seeds may be grain boundaries of
pre-existing crystals that did not melt upon introduction of the
energy. The pre-exiting seeds may also be the boundary between the
molten region and adjacent solid region. If the pre-existing seeds
are absent, the crystals may nucleate homogeneously. Upon
nucleation, the crystals may grow until the growth is halted.
[0028] In the solid phase transformation mechanism 100b, the phase
transformation may occur despite the absence of the melting. For
example, crystals may nucleate in the region introduced with
energy, and the nucleation may be followed by the growth of the
nucleated crystals. As in the case of the melt process, nucleation
during SPC can occur heterogeneously if pre-existing seeds exist,
or homogeneously if such seeds are absent.
Particle Assisted Processes
[0029] In the present disclosure, particles may be introduced to a
substrate to induce the phase transformation. The phase
transformation may be that from the amorphous phase to one of the
polycrystalline and/or mono-crystalline phases. In addition, the
phase transformation may occur via nucleation and growth of the
crystals. To induce the transformation, the particles may be
introduced near the upper surface of the substrate, the lower
surface of the substrate, or a region between the upper and lower
surfaces, or a combination thereof. If the substrate comprises two
or more different materials, the particles may be introduced to a
region near the interface of the different materials.
Particle Species
[0030] Numerous types of particles may be introduced to induce the
phase transformation. For example, the particles that are
chemically and/or electrically inert with respect to the substrate
may be used. However, chemically and/or electrically active
material may also be used. As noted above, the particles may be
charged or neutral sub-atomic particles, atomic particles, or
molecular particles, or a combination thereof. In some embodiments,
molecular particles are preferred. In other embodiments, cluster
particles are preferred. Molecular and cluster particles may be
preferred as they may be introduced to the substrate at much higher
dose and energy. In particular, molecular and cluster particles
introduced to a substrate may disintegrate on impact, and the
kinetic energy of the particles may be shared in the ratio of the
atomic masses of the particle atoms. The overlapping collision
cascades may achieve result similar to introduction of atomic
particles at much higher dose rate. Due to their greater mass, the
molecular particles may also be introduced to the substrate at much
higher energy. The generation of atomic and molecular species in
implanters, PLAD and PIII will be familiar to those skilled in the
art. A detailed description of the generation of cluster particles
may be found in U.S. Pat. No. 5,459,326, which is incorporated in
entirety by reference.
[0031] The choice of the particles introduced to the substrate may
also depend on the effect of the particles on the substrate. Some
characteristics and illustrative examples are shown in Table 1.
TABLE-US-00001 TABLE 1 Some possible choices of ion species
Characteristic Example species Electrically Ge, Si, C, F, N H, He,
Sn, Pb, inactive in hydrocarbon molecules, molecules silicon
containing C and two or more other elements, hydrides of silicon
such as tetra-silane, molecules containing Si and two or more other
elements Dopants B, P, As, Sb, In, Ga, Sb, Bi, Shallow Junction C,
F Co-implant species Amorphizing Noble Gases (including He, Xe),
Ge, Si Strain producing Ge, C Bandgap Yb, Ti, Hf, Zr, Pd, Pt, Al
engineering Passivating H, D Defect Pinning N Crystallization Ni,
metals catalysts
Depth and Energy
[0032] When the particles are introduced to the substrate, the
kinetic energy of the particles may be transferred to the
substrate. The magnitude of the transferred kinetic energy may
depend on the size, mass, and energy of the particles. For example,
heavy ions introduced to a substrate may experience more nuclear
stopping than lighter ions. When the particles lose their kinetic
energy via the nuclear stopping mechanism, the mechanism tends to
form defects such as, for example, dangling bonds, vacancies, and
di-vacancies, whose presence may enhance the crystallization
process. At the same time, kinetic energy transferred to the
substrate via electronic stopping may cause crystallization.
[0033] Depending on the energy of the particles, the location of
the particles delivery, and the properties of the substrate (e.g.
thermal conductivity, heat capacity and melting temperature of the
substrate), nucleation of crystals may be initiated at the upper
surface of the substrate; the lower surface of the substrate; the
region between the upper and lower surfaces; or near the interface
of different materials. Thereafter, the phase transformation may
continue in a direction away from the location where the
transformation is initiated.
[0034] Unlike the radiation based phase transformation, energy
deposited to the substrate via the particle introduction may peak
at the surface or, alternatively, below the surface. In addition,
the particles may be introduced to the substrate at a constant
energy. Alternatively, the particles may be introduced at varied
energies. For example, the energy of the particles introduced to
the substrate may change while the particles are being introduced.
The change in the energy may occur continuously or in a sequence.
If a beam-line particle system is used, the particle energy may be
changed during the particle introduction using acceleration or
deceleration voltage associated with beam-line systems described
herein. If PLAD, PIII, or other plasma based system is used, the
energy may be changed during the introduction by varying the
voltage applied to the substrate.
[0035] Referring to FIG. 2, there is a graph showing depth and
energy of particles introduced to a substrate, according to one
embodiment of the present disclosure. In the present embodiment, Ar
ions are implanted into Si thin film. As shown in FIG. 2, the
points joined by the line represent the average range of the Ar
ions and the vertical error bars represent the straggle in depth.
The total range of all ions can then be estimated by the sum of the
average range plus a multiple (one or more) of the straggle. If the
Ar ions were required to be contained within a surface layer of Si
of known depth, the maximum energy may be estimated from this
curve. The inset chart is a larger representation of the low energy
scale of the main chart.
Spatial and Temporal Profile
[0036] In addition to the energy, the spatial and temporal profile
of the particles may be controlled. For example, the particles may
be introduced as a particle beam, and the beam may have constant or
varied beam current density (i.e. number of particles in a
predetermined area for a predetermined time). The current density
may be adjusted by changing the particle current and/or beam size;
the beam dwell time by controlling the beam and/or substrate
scanning speeds and/or pulse length; and the beam duty cycle (e.g.
time between beam pulses or return time if the beam and/or
substrate are scanned).
[0037] In the present disclosure, the particles may be introduced
to the substrate continuously or periodically in sequence. If the
particles are introduced as a particle beam, the beam may have
various shapes. For example, the particles may be introduced as a
ribbon beam or a spot beam. The ribbon beam may have a ribbon shape
or a shape where the dimension of the beam along one direction is
larger than along another direction. Such a ribbon beam may be
wider than the substrate or, alternatively, narrower than the
substrate. The spot beam, meanwhile, may have a dimension smaller
than the substrate. If used, the spot beam may be scanned, either
magnetically or electrostatically at a rate of approximately
100-1000 Hz, to resemble the ribbon beam.
[0038] If the cross section of the beam, whether a ribbon beam or a
spot beam, is smaller and does not cover the entire surface area of
the substrate, the beam may be additionally scanned relative to the
substrate. For example, the beam may be scanned along 2 directions,
along the width direction and length direction, such that the
particles may be introduced to the entire surface of the substrate.
In the present disclosure, such scanning may be achieved by
translating the substrate along the length and width directions
relative to a stationary beam or by translating the beam along the
length and width directions relative to the stationary substrate.
By controlling the rate of the relative scanning of the beam and/or
the substrate, the phase transformation of the substrate may be
controlled.
[0039] In addition, the particle beam introduced to the substrate
may be a focused beam or a non-focused beam. In addition, the
particles beam may be uniform or non-uniform along its cross
section. For example, a ribbon beam may have a higher current
density at its leading edge followed by a trailing edge having a
lower current density, or vice versa. The non-uniform beam may have
other intensity profiles. It is believed that a non-uniform beam
may enhance the nucleation process and the growth process. For
example, the non-uniform beam may have an intense leading edge to
initiate nucleation, followed by a less intense trailing edge. For
example, the high density portion of the beam may initiate the
phase transformation by melting the substrate, and the low density
portion of the beam may enhance the extent of the transformation by
controlling the solidification of the molten region.
[0040] Further, more than one beam may be operated and introduced
to the substrate either simultaneously or sequentially. If more
than one beam is used the beam may be introduced to the entire
width and/or length of the substrate at one time.
Direction
[0041] The particle assisted phase transformation may have some
advantage in orienting the crystal growth and/or crystal shapes. In
the present disclosure, the particles may be introduced to the
substrate at various angles. Introduction of the particles at
various angles may be achieved by the tilting the substrate
relative to the beam and/or the beam may be tilted relative to the
substrate.
[0042] In one embodiment, the particles may be introduced to the
substrate at 0.degree. (i.e. perpendicular to the surface of the
substrate). The particles introduced at 0.degree. may
preferentially destroy {200} grain boundaries that may limit
electrical conductivity in FPDs. Alternatively, the particles may
be introduced at other angle, for example, 7.degree..
Substrate Condition
[0043] In addition to the parameters of the particles, the
conditions of the substrate may be adjusted before, during, or
after introduction of the particles. For example, the temperature
of the substrate may be adjusted. In one embodiment, the substrate
may be heated to, for example, approximately 500.degree. C. prior
to or during the introduction of the particles. Heating the
substrate may mitigate thermal shock caused by the particle beam.
In addition, heating the substrate may induce formation of larger
crystals. For example, heating the substrate may cause the region
introduced with the particles to cool at a slower rate (as this
region may largely loose its energy through conduction into the
substrate).
[0044] The crystallization may be enhanced if the substrate were
cooled below room temperature. For example, the substrate may be
cooled to a temperature ranging from about 0.degree. C. to about
-100.degree. C. In addition, cooling the substrate may prevent the
structure of the insulating film from being unstable.
[0045] When the particles are introduced to the substrate, the
substrate may be in vacuum or at atmospheric pressure. The vacuum
pressure may be higher than those usually associated with ion
implantation (i.e. pressure higher than 10-4 mbar) to reduce pump
cost.
Exemplary Systems
[0046] Referring to FIG. 3, there is shown a block diagram of an
exemplary system 300 for processing a substrate according to one
embodiment of the present disclosure. The system 300 may be a
beam-line particle system 300. The system 300 may comprise an ion
source 302; an extraction system 304; an acceleration system 306;
optional beam manipulation components 308; and a neutralization
system 310. In addition, the system 300 may comprise an end station
312 communicating with the neutralization system 310. The end
station 312 may comprise a window 314 and one or more loadlocks 316
and 318. Within the end station 312, a platen that supports a
substrate 322 may be positioned. In addition, one or more of
substrate translation, cooling and/or heating sub-system 324 may be
disposed in the end station 312.
[0047] In the present disclosure, the ion source 302 may be a
Bernas type, with indirectly heated cathode. Those of ordinary
skill in the art will recognize that the ion source 302 may also be
other types of ion source. Meanwhile, the extraction system 304 may
be a single slit or, alternatively, a multiple slit extraction
system 304. The extraction system 304 may be translatable in one or
more orthogonal directions. In addition, the extraction system 304
may be designed to extract a temporally constant beam current. In
addition, the extraction system 304 may extract the particle
continuously or intermittently. The extraction system 304 may also
focus the particle beam or beamlets to allow a desirable spatial
and/or temporal beam profiles. The particles beam extracted via the
extraction system 304 may have energy of approximately 80 keV.
[0048] If higher energy is required, the system 300 may include an
acceleration system 306 that may accelerate the particle beam. The
system 300 may also include one or more additional, optional beam
manipulation components 308 to filter, scan, and shape the
particles to a particle beam. As illustrated in FIG. 4, a specific
example of the system 300, the optional beam manipulation
components 308 may include a first magnet analyzer 406, a first
deceleration (D1) stage 408, a second magnet analyzer 410, and a
second deceleration and a second deceleration (D2) stage 412. In
the present disclosure, the first magnet analyzer 406, a
substantially dipole magnet, may filter the particles based on the
particles' mass and energy such that particles of undesired mass
and/or energy will not pass through the magnet analyzer 406.
Meanwhile, the second magnet analyzer 410, another substantially
dipole magnet, may be configured to collimate the particles into a
particle beam having desired shape (e.g. ribbon) and/or dimension.
D1 and D2 deceleration stages 410 and 412 may manipulate the energy
of the particles passing through such that the particles may be
introduced to the substrate at a desired energy. In one embodiment,
the D1 and/or D2 may be segmented lenses capable of minimizing the
space charge effect and maintaining spatial integrity of the
beam.
[0049] Although not shown, the beam manipulation components may
also include one or more substantially quadrupole magnets or einzel
lenses to focus the beam. Further, the beam manipulation components
may also include magnetic multipoles or rods such as described in
U.S. Pat. Nos. 6,933,507 and 5,350,926 to control the uniformity of
the beam profile.
[0050] Returning to FIG. 3, the charge neutralization system 310,
according to the present embodiment, may also be included to reduce
charge build-up in the substrate 322. The charge neutralization
system 310 may be one or more systems of hot filament, or
microwave, or rf driven type, such as that described in U.S. patent
application Ser. No. 11/376850. Alternatively, the charge
neutralization system 310 may be an electron source.
[0051] In the end station, the environment around the substrate may
be controlled in order to prevent, for example, deposition of other
materials on the substrate or to promote passivation to enhance the
crystallization process. To control the environment, the end
station 312 may include a thin foil window or a differentially
pumped aperture 314, through which the particles may enter, and one
or more loadlocks 316 and 318, through which the substrate may be
admitted. The loadlocks 316 and 318 may be replaced by one or more
differentially pumped stages through which the substrate may be
admitted.
[0052] The end station 312 may also contain substrate movement,
cooling, and heating subsystem 324. Examples of sub-system 324 may
include a chiller, a heat source, a roplat capable of
translating/rotating the substrate along several axes. Specific
examples of the chiller may be found in U.S. patent application
Ser. No. 11/504,367, 11/525,878, and 11/733,445, each of which is
incorporated by reference in entirety. Specific examples of the
heat source may be a laser, flash lamp, platen providing fluid
heating, resistive heat source, or those described in U.S. patent
application Ser. Nos. 11/770,220 and 11/778,335, each of which is
incorporated by reference in entirety.
[0053] To monitor the process and substrate parameters/conditions,
one or more parameters/conditions measuring components may also be
included near the substrate 322. Such components may be coupled to
one or more controllers, and the controllers may control the
parameters/conditions of the substrate and/or the particles based
upon the signals from the measuring components.
[0054] Referring to FIG. 5, there is shown another exemplary system
for processing a substrate according to another embodiment of the
present disclosure. In particular, the system 500 may be a PLAD,
PIII system, or other plasma based substrate processing system.
PLAD system 500 may comprise a chamber 501 including top section
502 and a lower section 504. The top section 502 may include a
first dielectric section 506 that extends in a generally horizontal
direction and a second dielectric section 508 that extends in a
generally vertical direction. In one embodiment, each dielectric
section 506 and 508 may be ceramic that is chemically resistant and
that has good thermal properties. The top section 502 may also
include at least one or more antennas 510 and 512. The one or more
antennas 510 and 512 may be, for example, a horizontal antenna 510
and/or a vertical antenna 512. In one embodiment, the horizontal
antenna 510 may be a planar coil having multiple windings, whereas
the vertical antenna 512 may be a helical coil of multiple
windings. At least one of the antennas 510 and 512 may be
electrically coupled to a power supply 514 via an impedance
matching network 516.
[0055] On the lower section 504 of the system 500, a platen 520 and
a substrate 522 supported by the platen 520 may be positioned at a
predetermined height below the top section 502. However, it is also
contemplated that the platen 502 ad the substrate 522 may be
positioned in the top section 502. A bias voltage power supply 524
may be electrically coupled to the platen 520 to DC or RF bias the
platen 520.
[0056] In operation, at least one of the antennas 510 and 512 may
be RF or DC powered by the power source 514. If only one of the
antennas 510 and 512 is RF or DC powered, the other one of the
antennas 510 and 512 may be a parasitic antenna. The other one of
the antennas 510 and 512 may be a parasitic antenna as it is not
electrically coupled to the power source 514. Instead, the other
one of the antennas 510 and 512 is magnetically coupled to the
antenna that is powered by the power source 514. Alternatively,
both of the antennas 510 and 512 may be powered by the power source
514 with an RF current. Thereafter, at least one of the antennas
510 and 512 induces the RF currents into the system 500 via the
first and second dielectric sections 506 and 508. The
electromagnetic fields induced by the RF currents may covert the
gas contained in the system 500 into plasma. Meanwhile, the bias
voltage power supply 524 may bias the platen 520 to attract the
charged particles in the plasma to the substrate 522. Additional
details of the system 500 may be found in U.S. patent application
Ser. No. 11/766984; application publication No. 2005/0205211;
application publication No. 2005/0205212, and application
publication No. 2007/0170867, each of which is incorporated in
entirety by reference.
Optional Components
[0057] In addition to the components described above, the exemplary
systems 300-500 may optionally include one or more masks between
the particle source (e.g. ion source or plasma) and the substrate.
If included, the mask may preferentially be a carbon (C) based
material, Si based material (e.g. SiC), or refractory metal, such
as W or Ta, containing material. However, other materials may also
be used. Such a mask may have one or more aperture having various
shapes including chevron shape to control the shape of the beam
incident on the substrate.
FPD
[0058] Hereinafter, description of several applications of the
particle induced phase transformation is provided. As noted above,
the particles may be introduced into a Si layer of a thin film
substrate to induce the phase transformation from the amorphous to
the crystalline phase. For purpose of clarity, a comparison of the
particle induced phase transformation is made with the ELA
process.
[0059] In the present embodiment, the particles may be directed to
an FPD having an amorphous Si film of about 500 .ANG. thick
disposed on an insulating film. The insulating film may be, for
example, amorphous glass or Corning 1737 glass having a thickness
of about 0.7 mm, quartz, plastic, or sapphire. However, those of
ordinary skill in the art will recognize that other types of
insulating film may also be used.
[0060] In ELA process, a single laser pulse may deliver an energy
pulse of 360 mJ/cm.sup.2 in a 12 nanosecond long pulse. This
equates to a power density of 3.times.10.sup.10 W/m.sup.2. If an Ar
ion beam is directed to the Si film, the beam may have an energy of
20 keV. With such energy, all of the directed Ar ions may not
penetrate the substrate beyond the Si layer (see FIG. 2). If a
ribbon shaped Ar particle beam is used, the beam may be assumed to
have dimensions of 300 mm wide by 0.1 mm tall. With a beam current
of 25 mA, this implies a power density of 1.7.times.10.sup.7
W/m.sup.2.
[0061] In ELA process, the laser beam incident on the substrate may
heat the Si layer to 1000.degree. C., near the melting temperature
of amorphous Si. Upon incidence, the laser beam may initiate at
least a partial melting of Si layer. The thermal diffusivity for Si
is relatively high, varying between .about.1 cm.sup.2/sec at room
temperature and 0.1 cm.sup.2/sec at 1400K. Hence, even if the laser
energy is absorbed in the top few nm of the Si surface, absent any
latent heat effects, there may be a very small temperature gradient
within the Si layer. Heat may diffuse from the Si into the glass.
The diffusivity for the glass is small (.about.0.005 cm2/s over a
large temperature range), and so a large thermal gradient may exist
across the thick glass layer. The results of the model shown in
FIG. 6, calculate that the glass even within 0.1 .mu.m of the Si,
does not reach above 500.degree. C.
[0062] As the particle beam has a lower power density, the exposure
time needed to deposit sufficient energy to heat the Si film may be
higher (80 ms) compared to the laser (12 ns). In addition, as the
heat deposited to the substrate via the particles may be lost to
the insulating via thermal conduction, more energy may be needed to
heat the Si film sufficiently. Under these assumptions, the
insulating film within 50 .mu.m of the Si may be heated above
600.degree. C. Nevertheless, sufficient amount of the insulating
may not be heated above its glass transition (or melting)
temperature such that these conditions may be acceptable.
[0063] If the height of the ribbon beam were to increase to 1 mm,
it may take approximately 2.4 seconds to sufficiently heat the Si
film, in which time the peak temperature of the bottom of glass may
reach 600.degree. C. This example, compared to the 0.1 mm case in
FIG. 7, demonstrates the need to keep the power density of the beam
as high as possible. This may be achieved by maintaining the beam
area as small as possible, increasing beam current, and/or
increasing the beam energy. The mass of the ion species may also be
increased. The use of a molecular particle beam may be desirable as
it allows the use of higher beam energies. At the same time, the
higher beam energy may reduce additional detrimental effects such
as space charge blow-up that may otherwise limit the beam currents
and the beam focusing.
[0064] The particle beam irradiation may retain the solid Si in the
amorphous phase, allowing melting to occur at 1300K. Crystalline Si
does not melt until 1683K. Therefore if the amorphous Si undergoes
crystallization before melting commences, more power may be
required to completely melt the material. Also, liquid Si may
reflect a portion of the laser radiation and so coupling power into
the bulk of the Si may be difficult once the Si surface has melted.
The presence of a particle beam during the cooling and
crystallization phase may influence the production of high quality
material.
Thin Film Solar Cell
[0065] The particle induced phase transformation described in the
present disclosure may also be applied to manufacture of thin film
solar cells. As known in the art, amorphous Si is a direct band gap
material and may absorb light more efficiently than crystalline Si,
an indirect band gap material. In addition, amorphous Si absorbs
more light in the visible spectrum than crystalline Si. However,
amorphous Si has lower electrical conductivity. As such, amorphous
Si may preferably transform incident radiation to electrical
current, whereas crystalline Si may preferably transfer the
generated electrical current. Accordingly, the solar cell,
according to the present embodiment, may preferably have a layer of
amorphous Si above another layer of crystalline Si. Incident
radiation at visible wavelengths may be efficiently converted into
photocurrent in the amorphous Si. Light not converted in the
amorphous layer (including infra-red radiation) may be converted
into photocurrent in the crystalline Si.
[0066] Referring to FIG. 8, there is shown a process that may be
incorporated in preparing a substrate according to another
embodiment of the present disclosure. In the present embodiment,
the substrate may be a thin film solar cell with crystalline and
amorphous layers. In another embodiment, the substrate may be a
semiconducting layer of a FPD that is disposed on an insulating
layer (not shown). As illustrated in FIG. 8A, an amorphous Si layer
802 may be deposited onto a glass layer (not shown). The Si layer
802 may have thickness of 1.5 .mu.m, whereas the glass layer may
have thickness of 3 mm. The particles 804 having a predetermined
dose and energy may then be introduced to the amorphous Si layer
802. As illustrated in FIG. 8B, the particles 804 may be introduced
below the surface of Si layer to crystallize a lower portion of Si
layer 802, without inducing crystallization of the upper portion of
amorphous Si layer 802. The resulting substrate may be used in a
solar cell having an amorphous Si layer 802 disposed on the crystal
Si layer 806.
[0067] Referring to FIG. 9, there is shown a process that may be
incorporated in preparing a substrate according to another
embodiment of the present disclosure. In the present embodiment,
the substrate may be a thin film solar cell with crystalline and
amorphous layers. In another embodiment, the substrate may be a
semiconducting layer of a FPD that is disposed on an insulating
layer (not shown). As illustrated in FIG. 9A, an amorphous Si layer
902 may be deposited onto a glass layer (not shown). Thereafter,
particles 904 having a predetermined dose and energy may be
introduced to the amorphous Si layer 902 to crystallize the entire
Si layer 906 (FIG. 9B). As illustrated in FIG. 9C, a plurality of
particles of second species 908, energy, and dose may be introduced
to the substrate to amorphize a layer near the surface of the
crystalline Si layer. The resulting solar cell may have an
amorphous top Si layer 904 and a crystalline lower Si layer
902.
[0068] Referring to FIG. 10, there is shown a process that may be
incorporated in preparing a substrate according to another
embodiment of the present disclosure. In the present embodiment,
the substrate may be a thin film solar cell with crystalline and
amorphous layers. In another embodiment, the substrate may be a
semiconducting layer of a FPD that is disposed on an insulating
layer (not shown). As illustrated in FIG. 10A, an amorphous Si
layer 1002 may be deposited onto a glass layer (not shown).
Thereafter, particles 1004 having a predetermined dose and energy
may be introduced to the amorphous Si layer 1002 to crystallize a
sub-layer 1006 within the Si layer 1002 (FIG. 10B). Although FIG.
10B illustrate a sub-layer disposed near the upper surface of the
Si layer 1002, those of ordinary skill in the art should recognize
that the sub-layer 1006 may be positioned near the upper surface,
near the lower surface, or anywhere between the upper surface and
the lower surface of Si layer 1002.
[0069] After forming the crystalline sub-layer 1006, one or more of
the crystals in the sub-layer 1006 may be grown away from the
sub-layer 1006 until the entire Si layer 1002 may be crystallized.
The crystals may be grown via one of furnace annealing, rapid
thermal annealing (RTA), flashlamp annealing, and laser annealing.
Alternatively, the crystals may be grown by particle assisted
annealing. For example, the same or another types of particles (not
shown) having another predetermined dose and/or another
predetermined energy to the region below the crystallized sub-layer
to extend the grain boundary of one or more crystals toward the
lower surface of the substrate. In the process, the entire Si layer
1002 may contain one or more crystals having grain boundaries that
extend in a vertical direction. The present embodiment may also
include an optional amorphizing step to amorphize a portion of the
newly crystallized Si layer 1006. For example, the particles 1010
may then be introduced to the newly crystallized Si layer 1002 to
amorphize at least a portion of the newly crystallized Si layer
1002 (FIG. 10D) to form an amorphous sub-layer 1012. In the present
disclosure, the particles introduced to the newly crystallize Si
layer 1002 the same particles as those used to crystallize the
previous amorphous Si layer 1002. Alternatively, the the particles
introduced to the newly crystallize Si layer 1002 may be different
from those used to crystallize the previous amorphous Si layer
1002. The above process may be used to crystallize a thick
amorphous Si layer.
[0070] The particle induced phase transformation may also be used
to manufacture an efficient polycrystalline Si solar cell. The
grain boundaries of crystals may be efficient sites for gettering
impurities, such as metal contaminants. In addition, grain boundary
may serve as a barrier for charge carriers' mobility, inhibiting
the carriers from traveling through the boundary. Accordingly,
polycrystalline solar cells having multiple crystals, thus multiple
grain boundaries, may have relatively low electrical conductivity
if the grain boundaries are located across the path of the charge
carriers. In the polycrystalline solar cells, electrical current
generated at the upper surface must be transported to contact
areas, which are generally located at the lower surface of the
solar cell. If the grain boundaries in the polycrystalline solar
cells are positioned across the path of the charge carriers, the
efficiency of the solar cells may be lowered. As such, it may be
desirable to manufacture polycrystalline solar cells having grain
boundaries oriented in parallel manner relative to the path of the
charge carriers.
[0071] To manufacture an efficient polycrystalline solar cell, an
amorphous Si substrate may be prepared. Thereafter, the upper
surface of the Si layer may be crystallized, and the crystals may
grow downward per solid phase epitaxial regrowth (SPER). The ion
energy may be chosen so that the power density delivered to the
substrate may be maximized. This may correspond to an energy of
between 40 to 100 kev, where typical ion beam systems can extract
the maximum beam currents from an ion source and where space charge
effects are reduced for beam transport and focusing. Such an ion
beam may cause crystallization near the surface of the silicon
which in turn may seed SPER downwards until the whole layer is
crystallized. The SPER may take place as part of the beam induced
crystallization step, or in a further annealing step that may use
one or more of furnace, RTA, flashlamp, laser or other annealing
methods. The resulting substrate will likely to have crystals with
vertically extending grain boundaries. Thereafter, particles of
second species, energy, and dose may be introduced to the substrate
to amorphize a layer near the surface of the polycrystalline
substrate. The solar cell may then have a structure of amorphous Si
layer above vertically extending polycrystalline Si layer. As noted
above, such a solar cell will likely to convert radiation energy to
electrical energy more efficiently, and, at the same time,
transport the converted electrical energy more efficiently.
[0072] In the present disclosure, the size and orientation of the
boundaries may be influenced by the choice of the particle beam
conditions used to assist the crystallization of the top layer.
Phosphorous may be a favorable species as it is a good getter
species, and may be the dopant of choice for the solar cell. The
direction of implant may be chosen to influence the grain
orientation. The whole active layer may be implanted, or the
surface layer may be implanted to create a top crystalline surface
with few voids, and the rest of the substrate may be regrown by
SPER.
[0073] The terms and expressions which have been employed herein
are used as terms of description and not of limitation, and there
is no intention, in the use of such terms and expressions, of
excluding any equivalents of the features shown and described (or
portions thereof), and it is recognized that various modifications
are possible within the scope of the claims. Other modifications,
variations, and alternatives are also possible. Accordingly, the
foregoing description is by way of example only and is not intended
as limiting. What is claimed is any feature detailed herein.
* * * * *