U.S. patent application number 13/226590 was filed with the patent office on 2013-03-07 for ion implant apparatus and method of ion implantation.
This patent application is currently assigned to TWIN CREEKS TECHNOLOGIES, INC.. The applicant listed for this patent is Paul Eide, Joseph Gillespie, Ronald Horner, William Park, JR., Geoffrey Ryding, Takao Sakase, Theodore Smick. Invention is credited to Paul Eide, Joseph Gillespie, Ronald Horner, William Park, JR., Geoffrey Ryding, Takao Sakase, Theodore Smick.
Application Number | 20130056655 13/226590 |
Document ID | / |
Family ID | 47682804 |
Filed Date | 2013-03-07 |
United States Patent
Application |
20130056655 |
Kind Code |
A1 |
Smick; Theodore ; et
al. |
March 7, 2013 |
ION IMPLANT APPARATUS AND METHOD OF ION IMPLANTATION
Abstract
An apparatus and a method of ion implantation using a rotary
scan assembly having an axis of rotation and a periphery. A
plurality of substrate holders is distributed about the periphery,
and the substrate holders are arranged to hold respective planar
substrates. Each planar substrate has a respective geometric center
on the periphery. A beam line assembly provides a beam of ions for
implantation in the planar substrates on the holders. The beam line
assembly is arranged to direct said beam along a final beam
path.
Inventors: |
Smick; Theodore; (Essex,
MA) ; Ryding; Geoffrey; (Manchester, MA) ;
Sakase; Takao; (Rowley, MA) ; Park, JR.; William;
(Somerville, MA) ; Gillespie; Joseph; (Boston,
MA) ; Horner; Ronald; (Auburndale, MA) ; Eide;
Paul; (Stratham, NH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Smick; Theodore
Ryding; Geoffrey
Sakase; Takao
Park, JR.; William
Gillespie; Joseph
Horner; Ronald
Eide; Paul |
Essex
Manchester
Rowley
Somerville
Boston
Auburndale
Stratham |
MA
MA
MA
MA
MA
MA
NH |
US
US
US
US
US
US
US |
|
|
Assignee: |
TWIN CREEKS TECHNOLOGIES,
INC.
San Jose
CA
|
Family ID: |
47682804 |
Appl. No.: |
13/226590 |
Filed: |
September 7, 2011 |
Current U.S.
Class: |
250/492.21 |
Current CPC
Class: |
H01J 37/20 20130101;
H01J 37/3171 20130101; C23C 14/505 20130101; H01J 2237/20214
20130101; H01J 2237/201 20130101; C23C 14/48 20130101 |
Class at
Publication: |
250/492.21 |
International
Class: |
G21K 5/10 20060101
G21K005/10 |
Claims
1. Ion implant apparatus comprising: a rotary scan assembly having
an axis of rotation and a periphery; a plurality of substrate
holders distributed about said periphery, said substrate holders
being arranged to hold respective planar substrates, each having a
respective geometric center on said periphery; and a beam line
assembly to provide a beam of ions for implantation in said planar
substrates on said holders, said beam line assembly being arranged
to direct said beam along a final beam path; wherein said substrate
holders are arranged to hold said respective planar substrates at a
wafer tilt angle .alpha..sub.s and at a wafer slope angle
.beta..sub.s; wherein said beam line assembly is arranged such that
said final beam path has a beam tilt angle .alpha..sub.b and a beam
slope angle .beta..sub.b; wherein
2.degree.+.DELTA..beta..ltoreq..beta..sub.t.ltoreq.15.degree.-.DELTA..bet-
a. where .beta..sub.t=.beta..sub.b-.beta..sub.s,
2*.DELTA..beta.=(w.sub.x/.pi.d)*360.degree., w.sub.x is the
dimension of the wafer in the x-axis direction, and d is the
diameter of said periphery of the rotary scan assembly, and
F<0.5 where
F=2.alpha..sub.t*.DELTA..beta./(.beta..sub.t.sup.2-.DELTA..beta..sup.2+.a-
lpha..sub.t.sup.2), and .alpha..sub.t=.alpha..sub.b-.alpha..sub.s;
and wherein the total wafer angle .phi..ltoreq.45.degree., where
sin.sup.2.phi.=cos.sup.2(.alpha..sub.s)*sin.sup.2(.beta..sub.s)+sin.sup.2-
(.alpha..sub.s), and the total beam to substrate normal angle
.theta..ltoreq.15.degree., where
sin.sup.2.theta.=cos.sup.2(.alpha..sub.t)*sin.sup.2(.beta..sub.t)+sin.sup-
.2(.alpha..sub.t).
2. Ion implant apparatus as claimed in claim 1, wherein at least 50
substrate holders are distributed about said periphery of said
rotary scan assembly adjacent each other so that substrates on
neighbouring said holders do not obscure the ion beam, such that
w.sub.x.ltoreq..pi.d/50.
3. Ion implant apparatus as claimed in claim 1, wherein
8.degree..ltoreq..beta..sub.t.ltoreq.11.degree..
4. Ion implant apparatus as claimed in claim 3, wherein
.beta..sub.s=0.degree..
5. Ion implant apparatus as claimed in claim 1, wherein
F<0.15.
6. Ion implant apparatus as claimed in claim 5, wherein
.DELTA..beta..ltoreq..beta..sub.t/3.
7. Ion implant apparatus as claimed in claim 1, wherein
.alpha..sub.t.ltoreq.3.degree..
8. Ion implant apparatus as claimed in claim 3, wherein
.DELTA..beta.<3.5.degree., .alpha..sub.t<3.degree., and
.alpha..sub.s<12.degree..
9. Ion implant apparatus as claimed in claim 1, wherein said
substrate holders comprise respective heat sinks providing
respective support surfaces for said planar substrates, and wherein
said support surfaces face inwards towards said axis of rotation
such that said substrates are held on said support surfaces by
centrifugal force as said rotary scan assembly rotates without any
lateral restraint apart from friction between said substrates and
said support surfaces.
10. Ion implant apparatus as claimed in claim 1, wherein said beam
line assembly is arranged to produce a parallel scanned beam
directed along said final beam path, and wherein said direction of
beam scanning is orthogonal to a direction of travel of said
substrate holders through said final beam path as said rotary scan
assembly rotates.
11. Ion implant apparatus comprising: a rotary scan assembly having
an axis of rotation and a periphery; at least 30 substrate holders
distributed about said periphery, said substrate holders being
arranged to hold respective planar substrates, each having a
respective geometric center and a substrate normal at said
geometric center, wherein each said planar substrate on a
respective one of said holders subtends an angle not greater than
12.degree. about said axis of rotation; and a beam line assembly to
provide a beam of ions for implantation in said planar substrates
on said holders, said beam line assembly being arranged to direct
said beam along a final beam path, said final beam path being
defined as a line intercepting said scan assembly periphery at an
implant position, said planar substrates on said substrate holders
successively intercepting said final beam path as said rotary scan
assembly rotates; wherein said substrate holders comprise
respective heat sinks providing respective heat sinking support
surfaces for said planar substrates; wherein said support surfaces
are facing inwards towards said axis of rotation such that said
substrates are held on said support surfaces by centrifugal force
as said rotary scan assembly rotates, without any lateral restraint
apart from friction between said substrates and said support
surfaces; wherein said beam line assembly and said substrate
holders are arranged such that said final beam path is at an acute
angle not greater than 15.degree. to said substrate normal when
said geometric center of each said substrate is at said implant
position; and wherein said acute angle is formed by: a) a
predetermined beam slope rotation of said final beam path relative
to said substrate normal about an axis parallel to said axis of
rotation, said predetermined beam slope rotation being greater than
half of a subtended angle of each said substrate, and b) a
predetermined beam tilt rotation of said final beam path relative
to said substrate normal about an axis tangential to said periphery
and perpendicular to said axis of rotation, said predetermined beam
tilt rotation being in the range of zero to 10.degree..
12. Ion implantation apparatus as claimed in claim 11, wherein said
rotary scan assembly has at least 50 substrate holders distributed
about said periphery, whereby each said planar substrate on a
respective one of said substrate holders subtends an angle not
greater than 7.2.degree. about said axis of rotation.
13. Ion implantation apparatus as claimed in claim 11, wherein said
predetermined beam slope rotation is in the range 8.degree. to
11.degree..
14. Ion implantation apparatus as claimed in claim 11, wherein said
predetermined beam slope rotation is not less than one and a half
times said subtended angle.
15. Ion implantation apparatus as claimed in claim 12, wherein said
predetermined beam tilt rotation is not more than 3.degree..
16. Ion implantation apparatus as claimed in claim 11, wherein said
heat sinking support surfaces are arranged to hold said substrates
such that the respective substrate normal of each of said
substrates is at a substrate angle relative to a diameter of said
rotary scan assembly through the respective geometric center, said
substrate angle being less than 45.degree..
17. A method of ion implantation comprising the steps of: mounting
at least 30 planar substrates to be implanted about the periphery
of a rotary scan assembly, whereby each said planar substrate
subtends an angle not greater than 12.degree. about an axis of
rotation of said rotary scan assembly; directing a beam of ions to
be implanted along a final beam path to an implant position on said
periphery; and rotating said scan assembly about said axis of
rotation so that said planar substrates successively intercept said
ion beam, geometric centers of said substrates passing through said
implant position; wherein said planar substrates are mounted on
heat sinking support surfaces so as to face inwards towards said
axis of rotation such that said substrates are held on said support
surfaces by centrifugal force as said scan assembly rotates without
any lateral restraint apart from friction between said substrates
and said support surfaces; wherein said final beam path is at an
acute angle not greater than 15.degree. to a substrate normal at
said geometric center of each respective substrate when said
geometric center is at said implant position; and wherein said
acute angle is formed by: a) a predetermined beam slope rotation of
said final beam path relative to said substrate normal about an
axis parallel to said axis of rotation, said predetermined beam
slope rotation being greater than half of a subtended angle of each
said substrate, and b) a predetermined beam tilt rotation of said
final beam path relative to said substrate normal about an axis
tangential to said periphery and perpendicular to said axis of
rotation, said predetermined beam tilt rotation being in the range
zero to 10.degree..
18. A method as claimed in claim 17, wherein at least 50 said
planar substrates are mounted about the periphery of said rotary
scan assembly, whereby each said planar substrate subtends an angle
not greater than 7.2.degree. about said axis of rotation.
19. A method as claimed in claim 17, wherein said predetermined
slope rotation is in the range 8.degree. to 11.degree..
20. A method as claimed in claim 17, wherein said predetermined
slope rotation is not less than one and a half times said subtended
angle.
21. A method as claimed in claim 18, wherein said predetermined
beam tilt rotation is not more than 3.degree..
22. A method as claimed in claim 17, wherein said planar substrates
are held on said heat sinking support surfaces such that the
respective substrate normal of each of said substrates is at a
substrate angle relative to a diameter of said rotary scan assembly
through the respective geometric center, said substrate angle being
less than 15.degree..
Description
BACKGROUND OF THE INVENTION
[0001] There is increasing demand for renewable energy using
photovoltaic technology. In particular, photovoltaic cells are
commonly formed on crystalline silicon wafers which are
conventionally obtained by slicing a silicon ingot. This process,
which typically yields a silicon wafer thicker than 115 .mu.m,
wastes a substantial amount of silicon by consuming up to 50% of
the silicon body in kerf loss. The resulting wafers are also much
thicker than is needed for useful photovoltaic devices.
[0002] Thinner silicon laminae have been made by exfoliation of
film by heating after high dose ion implantation, typically with
H.sup.+ ions. However, to make useful silicon laminae by
exfoliation for photovoltaic applications, it is necessary to
implant ions at high energy, in order to create a weakness layer at
sufficient depth.
[0003] Also, in order to provide relatively high productivity, it
is desirable to employ high beam currents. Implant beams with an
ionic current of 100 mA, and energies of 1 MeV, are now being
contemplated. The effective beam power delivered to substrates
being implanted can be in the order of 100 kW or higher. The need
to prevent the substrates being heated by such high implant beam
power to excessive temperatures presents a considerable
challenge.
[0004] In a known type of ion implantation tool, a beam of ions to
be implanted is directed at substrates to be implanted (typically
silicon wafers) mounted in a batch around the periphery of a
process wheel. The process wheel or rotary scan assembly is mounted
for rotation about an axis so that the wafers on the wheel pass one
after the other through the ion beam. In this way, the power of the
ion beam can be shared between the wafers in the batch on the
process wheel. The wafers are mounted on substrate holders on the
process wheel. The substrate holders comprise a heat sinking
surface for supporting the wafer. Forced cooling of the heat
sinking surfaces is typically provided by means of water cooling
structures.
[0005] Contact between the wafers and the heat sinking support
surfaces is maintained by canting the support surfaces inwards
towards the axis of rotation, whereby the wafers are pressed by
centrifugal force against the support surfaces as the process wheel
rotates.
[0006] The effectiveness of the cooling of the wafers in such
implant apparatus using a rotary scan assembly can be dependant on
the force with which wafers are pressed against the underlying heat
sink surface. There are known ion implant apparatuses which provide
a rotary scan assembly in the form of a drum, with the wafers
mounted around the interior face of the drum, substantially facing
the axis of rotation. This arrangement maximizes the effect of
centrifugal force on the wafers to optimize wafer cooling during
the implant process.
[0007] However, such rotary drum type ion implant apparatuses have
not found favour for use when ions are to be implanted to a precise
depth in the substrate. This is because the ion beam is directed at
the wafers mounted around the interior of the rotary drum in a
final beam direction which is at a substantial angle to the axis of
rotation of the drum assembly, typically at an angle close to
90.degree.. The resulting variation in the angle between the wafer
being implanted and the implant beam, as each wafer traverses the
beam on rotation of the drum, results in a substantial amount of
ion channeling in the wafers being implanted. Ion channeling is a
known problem where ions implanted in alignment with planes or axes
of the crystal lattice of the substrate are channeled by these axes
and planes to greater depths in the substrate. As a result, depth
control of the implant process is prejudiced and a substantial
number of implanted ions penetrate to excessive depths.
[0008] This is a significant problem for the process of exfoliation
of thin wafer laminae following implantation by, for example,
H.sup.+ ions.
BRIEF SUMMARY
[0009] The invention provides ion implant apparatus comprising a
rotary scan assembly having an axis of rotation and a periphery. A
plurality of substrate holders are distributed about the periphery
and the substrate holders are arranged to hold respective planar
substrates. Each such substrate has a respective geometric center
on the periphery.
[0010] A beam line assembly provides a beam of ions for
implantation in the planar substrates on the holders and is
arranged to direct the beam along a final beam path. This final
beam path is defined as a line intersecting the periphery of the
scan assembly at an implant position. Planar substrates on the
substrate holders successively intercept the final beam path as the
rotary scan assembly rotates.
[0011] The invention is further defined by reference to a Cartesian
co-ordinate system having an origin at the above referred implant
position. An x-axis of the co-ordinate system is defined by a
tangential line which is tangential to the periphery of the rotary
scan assembly and is also perpendicular to the axis of rotation. A
y-axis is defined by a diametrical line perpendicular to and
intersecting the axis of rotation and perpendicular to the x-axis.
A z-axis is defined by an axial line parallel to the axis of
rotation.
[0012] The substrate holders are arranged to hold the respective
planar substrates at a wafer tilt angle .alpha..sub.s and at a
wafer slope angle .beta..sub.s. The wafer tilt angle .alpha..sub.s
is defined as the angle of rotation of the plane of each substrate,
when centered at the implant position, about the x-axis relative to
the x-z plane. The wafer slope angle .beta..sub.s is defined as the
angle of rotation of the substrate plane about the z-axis relative
to the x-z plane.
[0013] The beam line assembly is arranged such that the final beam
path has a beam tilt angle .alpha..sub.b and a beam slope angle
.beta..sub.b, where the beam tilt angle is defined as the angle of
rotation of an x-axis beam plane, containing the final beam path
and the x-axis, relative to the x-y plane of the co-ordinate
system, and the beam slope angle is defined as the angle of
rotation of a z-axis beam plane, containing the final beam path and
the z-axis, relative to the y-z plane of the co-ordinate
system.
[0014] In accordance with the invention,
2.degree.+.DELTA..beta..ltoreq..beta..sub.t.ltoreq.15.degree.-.DELTA..be-
ta.
where .beta..sub.t=.beta..sub.b-.beta..sub.s,
2*.DELTA..beta.=(w.sub.x/.pi.d)*360.degree.,
[0015] w.sub.x is the dimension of the wafer in the x-axis
direction, and
[0016] d is the diameter of the periphery of the rotary scan
assembly,
and F<0.5
[0017] where
F=2.alpha..sub.t*.DELTA..beta./(.beta.t.sub.2-.DELTA..beta..sup.2+.alpha.-
.sub.t.sup.2), and
[0018] .alpha..sub.t=.alpha..sub.b-.alpha..sub.s;
[0019] and
the total wafer angle .phi..ltoreq.45.degree., where
sin.sup.2.phi.=cos.sup.2(.alpha..sub.s)*sin.sup.2(.beta..sub.s)+sin.sup.2-
(.alpha..sub.s), and the total beam to substrate normal angle
.theta..ltoreq.15.degree., where sin.sup.2
.theta.=cos.sup.2(.alpha..sub.t)*sin.sup.2(.beta..sub.t)+sin.sup.2(.alpha-
..sub.t).
[0020] The invention may also provide a method of ion implantation.
In the method, a rotary scan assembly having an axis for rotation
is provided, the rotary scan assembly having a periphery with a
diameter d. A plurality of planar substrates are held on the rotary
scan assembly and are distributed about the periphery. A beam of
ions for implantation in the substrates is directed along a final
beam path which is defined as a line intersecting the scan assembly
periphery at an implant position. The rotary scan assembly is
rotated so that the planar substrates successively intercept the
final beam path. A Cartesian co-ordinate system is defined as
above. Each of the substrates has a dimension w.sub.x in the x-axis
direction and is held on the scan assembly such that the plane of
each substrate when centered at the implant position is rotated
about the x and z axis by a wafer tilt angle .alpha..sub.s and a
wafer slope angle .beta..sub.s as defined previously. Also, the
final beam path has a beam tilt angle .alpha..sub.b and a beam
slope angle .beta..sub.b as previously defined.
[0021] In accordance with the method of the invention, the values
for d, w.sub.x, .alpha..sub.s, .beta..sub.s, .alpha..sub.b and
.beta..sub.b are selected in order to fulfill the conditions set
out above.
[0022] In another aspect, the invention provides an ion implant
apparatus comprising a rotary scan assembly having an axis of
rotation and a periphery. At least 30 substrate holders are
distributed about the periphery. The substrate holders are arranged
to hold respective planar substrates, each having a respective
geometric center and a substrate normal at the geometric center.
Each planar substrate on a respective holder subtends an angle not
greater than 12.degree. about the axis of rotation.
[0023] The beam line assembly provides a beam of ions for
implantation in the planar substrates on the holders. The beam line
assembly is arranged to direct the beam along a final beam path,
which is defined as a line intersecting the scan assembly periphery
at an implant position. The planar substrates on the substrate
holders successively intercept this final beam path as the rotary
scan assembly rotates.
[0024] The substrate holders comprise respective heat sinks
providing respective heat sinking support surfaces for the planar
substrates. The support surfaces face inwards towards the axis of
rotation such that the substrates are held on the support surfaces
by centrifugal force as the rotary scan assembly rotates without
any lateral restraint apart from friction between the substrates
and the support surfaces.
[0025] The beam line assembly and the substrate holders are
arranged such that the final beam path is at an acute angle not
greater than 15.degree. to the substrate normal when the geometric
center of each substrate is at the implant position. This acute
angle is formed by:
[0026] a) a predetermined beam slope rotation of the final beam
path relative to the substrate normal about an axis parallel to the
axis of rotation, the predetermined beam slope rotation being
greater than half the subtended angle of each substrate, and
[0027] b) a predetermined beam tilt rotation of the final beam path
relative to the substrate normal about an axis tangential to the
periphery and perpendicular to the axis of rotation, the
predetermined beam tilt rotation being in the range zero to
10.degree..
[0028] The invention also provides a method of ion implantation in
which at least 30 planar substrates to be implanted are mounted
about the periphery of a rotary scan assembly, whereby each planar
substrate subtends an angle not greater than 12.degree. about an
axis of rotation of the rotary scan assembly. A beam of ions to be
implanted is directed along a final beam path to an implant
position on the periphery. The scan assembly is rotated about the
axis of rotation so that the planar substrates successively
intercept the ion beam, geometric centers of the substrates passing
through the implant position. The planar substrates are mounted on
heat sinking support surfaces so as to face inwards toward the axis
of rotation such that the substrates are held on the support
surfaces by centrifugal force as the scan assembly rotates without
any lateral restraint apart from friction between the substrates
and the support surfaces. The final beam path is at an acute angle
not greater than 15.degree. to a substrate normal at the geometric
center of each substrate when the geometric center is at the
implant position. This acute angle is formed by
[0029] a) a predetermined beam slope rotation of the final beam
path relative to the substrate normal about an axis parallel to the
axis of rotation, the predetermined beam slope rotation being
greater than half the subtended angle of each substrate, and
[0030] b) a predetermined beam tilt rotation of the final beam path
relative to the substrate normal about an axis tangential to the
periphery and perpendicular to the axis of rotation, the
predetermined beam tilt rotation being in the range zero to
10.degree..
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic plan view of ion implant apparatus
embodying the present invention.
[0032] FIG. 2 is a side view of the implant apparatus of FIG.
1.
[0033] FIG. 3 is a detailed schematic view of a substrate holder of
the implant apparatus of FIGS. 1 and 2, illustrating substrate tilt
angle and ion beam tilt angle.
[0034] FIGS. 4, 5, 6 and 7 are schematic views of a substrate wafer
illustrating substrate tilt and the substrate slope angles.
[0035] FIG. 8 is a detailed view of a substrate wafer on a
substrate holder, illustrating the effect of a combination of beam
tilt and beam slope angles.
[0036] FIG. 9 is a schematic plan view of a substrate wafer
illustrating the variation in beam twist angle relative to the
wafer.
[0037] FIG. 10 is a polar diagram illustrating the range of total
beam angles and twist angles in relation to crystal planes of the
wafer for a prior art ion implant apparatus.
[0038] FIG. 11 is a polar diagram illustrating a range of total
beam angles and twist angles for an embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0039] In one embodiment of the implant apparatus, at least 50
substrate holders are distributed about the periphery of the rotary
scan assembly adjacent to one another so that substrates on
neighbouring holders do not obscure the ion beam.
[0040] In the embodiments, the substrate holders comprise
respective heat sinks providing respective support surfaces for the
planar substrates. Then, the support surfaces face inwards toward
the axis of rotation such that the substrates are held on the
support surfaces by centrifugal force as the rotary scan assembly
rotates without any lateral restraint apart from friction between
the substrates and the support surfaces.
[0041] In an embodiment, the beam line assembly is arranged to
produce a parallel scanned beam directed along the final beam path,
the direction of beam scanning being orthogonal to a direction of
travel of the substrate holders through the final beam path as the
rotary scan assembly rotates.
[0042] In a further embodiment, the rotary scan assembly has at
least 50 substrate holders to be distributed about the periphery,
whereby each planar substrate on a respective substrate holder
subtends an angle not greater than about 7.2.degree. about the axis
of rotation. The predetermined beam slope rotation may be in the
range of about 8.degree. to about 11.degree. and may be not less
than about 1.5 times the subtended angle.
[0043] In an embodiment, the heat sinking support surfaces are
arranged to hold the substrates such that the respective substrate
normal of each substrate is at a substrate angle relative to a
diameter of the rotary scan assembly through the respective
geometric center. The substrate angle is less than about
45.degree.. In another embodiment, the substrate angle is less than
about 15.degree..
[0044] Specific applications of the ion implantation apparatus and
method include the production of laminae of crystalline
semiconductor material, such as silicon. Such silicon laminae may
be used for the production of photovoltaic cells.
[0045] Functional elements of an ion implanter embodying the
claimed invention are illustrated schematically in FIGS. 1 and 2.
The implanter comprises a rotary scan assembly 9 in the form of a
wheel or drum, comprising a peripheral rim 10 supported by spokes
11 on a central hub 12 which is mounted for rotation about an axis
13. Only three spokes 11 are shown in FIG. 1 for simplicity.
[0046] Substrate holders 14 are distributed completely around the
inside of the peripheral rim 10. FIG. 1 depicts only a few
substrate holders 14 for simplicity. The substrate holders 14 have
substrate support surfaces facing generally inward toward the axis
of rotation 13 of the rotary scan assembly drum 9. Substrate wafers
to be implanted can be held on the inwardly facing support services
of the substrate supports 14. The substrate wafers are held against
the support surfaces of the substrate holders 14 by centrifugal
force when the rotary scan assembly drum 9 is rotated. By mounting
substrate holders 14 around the inner periphery of the rotary scan
assembly drum as shown in FIGS. 1 and 2, the force, with which the
substrate wafers are pressed against the underlying support
surfaces of the substrate holders 14 by the effect of centrifugal
force, is maximized. This in turn improves the transfer of heat
from the substrate wafers to the underlying support surfaces of the
substrate holders 14, thereby facilitating excellent cooling of the
substrate wafers during implant processing.
[0047] FIG. 2 is a side view of the rotary scan assembly drum 9, so
that the axis of rotation 13 is in the plane of the paper. A beam
line assembly 20 provides a beam of ions for implantation in the
implant surfaces of the substrate wafers on the substrate holders
14. The beam of ions are directed along a final beam path 21
indicated by the dash/dot line. In FIGS. 1 and 2, the beam line
assembly 20 is illustrated only schematically. Those skilled in the
art of ion implantation are familiar with the essential components
of an implant beam line, typically including an ion source, an
accelerator assembly for accelerating ions from the source to a
desired implant energy, and a magnet assembly for directing the
implant beam onto the substrate wafers for implantation as
required. A mass selection arrangement may also be required in the
beam line assembly for selecting only the ions having the mass to
charge ratio desired for implantation from the ions extracted from
the ion source.
[0048] Referring to FIG. 1, substrate wafers on the substrate
holders 14 each have a respective geometric center. As the rotary
scan assembly drum 9 rotates, the geometric centers of the
substrate wafers on the substrate holders 14 define a peripheral
line 23. This peripheral line 23 is referred to throughout this
Specification and Claims hereto, as the periphery of the rotary
scan assembly or simply the periphery 23. As illustrated in FIGS. 1
and 2, the final beam path 21 from the beam line assembly 20 is a
line which intersects the periphery 23 at a point identified as an
implant position 22. It is apparent, therefore, that the geometric
centers of the substrate wafers on the substrate holders 14
successively pass through the defined implant position 22, at which
moment the substrate wafers are centered at the implant
position.
[0049] Generally speaking, the beam line assembly 20 directs a beam
of ions for implantation at the implant position 22 where the final
beam path 21 intercepts the periphery 23 of the rotary scan
assembly drum 9. As the rotary scan assembly drum rotates, the
substrate wafers on substrate holders 14 successively intercept the
final beam path 21. In this way, during an implant process, the
energy of the ion beam from the beam line assembly 20 is
distributed amongst the multiple substrate wafers on the substrate
holders 14 around the periphery of the rotary scan assembly drum
9.
[0050] It will be appreciated by those skilled in the art that the
ion beam from the beam line assembly 20 typically has a "foot
print" on the substrate wafer being implanted, which is smaller
than the area of the wafer. Rotation of the rotary scan assembly
drum 9 is effective to scan the beam over the substrates in the
direction of the periphery 23. Beam line assembly 20 may be
arranged to scan the ion beam itself in a scan direction which is
orthogonal to the direction of travel of substrate holders 14
through the final beam path 21 as the rotary scan assembly drum 9
rotates. This beam scanning by the beam line assembly 20 is
illustrated in FIG. 2 and can be seen more clearly in FIG. 3. The
beam line assembly 20 is effective to produce a parallel scanned
beam, which scans to and fro in the direction indicated by the
arrows 25. The beam is scanned to and fro without changing the
direction of the beam which remains parallel to the final beam path
21. The beam is scanned by the beam line assembly 20 at a scan rate
which is relatively high compared to the time taken for each
substrate wafer to traverse the final beam path 21 during rotation
of the rotary scan assembly drum 9. The ion beam is scanned
parallel in the direction of the arrows 25 to a scan width,
indicated by the length of the arrows 25 in FIG. 3, which is
comparable to the dimension of the substrate wafer on the substrate
holder 14, so that all parts of the substrate wafers are
implanted.
[0051] In another embodiment, the beam line assembly 20 may produce
a ribbon shaped beam having a footprint extending over the full
dimension of the substrate wafers. In a further embodiment, a fixed
beam extends along the final beam path 21 and two-dimensional
scanning of the substrate wafers is accompanied by additionally
translating the rotary scan assembly drum 9 to and fro parallel to
the axis of rotation.
[0052] Referring to FIGS. 1, 2 and 3, a Cartesian co-ordinate
system has been defined having an origin at the implant position 22
defined by the intersection of the final beam path 21 and the
periphery 23. An x-axis of the co-ordinate system is provided by a
tangential line forming a tangent to periphery 23. The x-axis is
also perpendicular to the axis of rotation 13. A y-axis of the
co-ordinate system is defined by a diametrical line perpendicular
to the axis of rotation 13 and also intersecting this axis, and
perpendicular to the x-axis previously defined. A z-axis is defined
by an axial line which is parallel to the axis of rotation.
[0053] Referring to FIGS. 3, 4, 5, 6 and 7, the substrate holders
14 may be arranged to support the substrate wafers so as to have a
wafer tilt angle .alpha..sub.s and a wafer slope angle
.beta..sub.s. The wafer tilt angle .alpha..sub.s is illustrated in
FIG. 3 and also in FIGS. 4 and 5. In FIG. 3, a wafer normal 30 is
defined as a line perpendicular to the implant surface of a
substrate wafer 31 on substrate holder 14. The substrate holder 14
is holding the substrate wafer 31 with the wafer normal 30 rotated
by an angle .alpha..sub.s about the x-axis (directed out of the
paper in FIG. 3). This is equivalent to rotating the plane of the
substrate wafer 31 by an angle .alpha..sub.s about the x-axis
relative to the x-z plane, as can be best seen in FIGS. 4 and
5.
[0054] FIG. 3 illustrates only a wafer tilt rotation of the
substrate wafer 31. However, it is also possible for the substrate
holders 14 to hold the substrate wafers with a wafer slope rotation
such as illustrated in FIGS. 6 and 7. As shown, each substrate
wafer may be held on its respective substrate holder so that the
plane of the wafer is rotated by an angle .beta..sub.s about the
z-axis relative to the x-z plane.
[0055] A tilt rotation of the substrate wafers, as illustrated in
FIG. 3, may be desirable to facilitate supporting the substrate
wafers on the rotary scan assembly drum 9, particularly as the
wafers are initially loaded onto the substrate supports 14 of the
drum with the drum stationary. In practice, the rotary scan
assembly drum 9 may be in a substantially horizontal plane, with
the axis of rotation 13 substantially vertical. In such an
arrangement, the defined z-axis would be pointing upwards. Then
substrate wafers 31 can be supported on the substrate holders 14
and held in position by gravity in view of the tilt rotation angle
.alpha..sub.s. A fence arrangement at the bottom edges of the
substrate holders 14 (not shown in FIG. 3) may be provided to
support the wafers when being loaded. Once all wafers are loaded on
the substrate holders 14, and the rotary scan assembly drum 9 is
spun up to operating speed, centrifugal force takes over, pressing
the substrate wafers 31 firmly against the support surfaces of the
substrate supports 14. Importantly, with sufficient spin speed, the
centrifugal force acting on the wafer substrates 31 is sufficient,
in view of the wafer tilt angle .alpha..sub.s, to overcome the
force of gravity, so that the substrate wafers 31 are being pushed
away from the retaining fences at the bottom edges of the substrate
holders 14. However, the effect of friction between the substrate
wafers 31 and the support surfaces of the substrate holders 14 is
sufficient to prevent the substrate wafers 31 from sliding off the
substrate holders 14.
[0056] The substrate holders 14 provide heat sinking support
surfaces for the substrate wafers 31. Water cooling of the
substrate holders 14 may be provided.
[0057] Referring to FIGS. 1 and 2, it can be seen that the final
beam path 21 from the beam line assembly 20 has a beam tilt angle
.alpha..sub.b about the x-axis relative to the x-y plane (refer to
FIG. 2). Such a beam tilt angle .alpha..sub.b is desirable to
enable the beam to be directed at the periphery 23 of the rotary
scan assembly drum 9, without the beam line assembly 20 interfering
with the support structure of the rotary scan assembly 9.
Generally, the beam tilt angle .alpha..sub.b is the angle of
rotation of an x-axis beam plane, containing the final beam path 21
and the x-axis, relative to the x-y plane of the co-ordinate
system.
[0058] FIG. 1 also indicates the final beam path 21 is rotated by a
beam slope angle .beta..sub.b about the z-axis relative to the y-z
plane of the co-ordinate system. In general, the beam slope angle
.beta..sub.b is the angle of rotation of a z-axis beam plane,
containing the final beam path 21 and the z-axis, relative to the
y-z plane of the co-ordinate system. In FIG. 1, the
rotation--.beta..sub.b is given a negative sign in accordance with
normal convention for rotation in a Cartesian co-ordinate
system.
[0059] The full significance of a beam slope angle .beta..sub.b
and/or a wafer slope angle .beta..sub.s will become apparent in the
discussion that follows. Whereas prior art implanters using a
rotary drum type scanning arrangement employ a combination of beam
and wafer tilt angles .alpha..sub.s and .alpha..sub.b, none of the
prior art contemplates a beam slope angle .beta..sub.b and/or a
wafer slope angle .beta..sub.s, as defined above.
[0060] In the arrangement illustrated in FIG. 3, the substrate
holder 14 is shown providing a wafer slope angle .alpha..sub.s. In
addition, the final beam path 21 is shown with a beam tilt angle
.alpha..sub.b. As a result, the final beam path 21 has a total tilt
angle .alpha..sub.t relative to the wafer normal 30, where
.alpha..sub.t=.alpha..sub.b-.alpha..sub.s. Similarly, the total
slope angle between the final beam path 21 and the wafer normal 30
is .beta..sub.t=.beta..sub.b-.beta..sub.s.
[0061] FIG. 8 illustrates the effect of a combined beam tilt angle
.alpha..sub.b and beam slope angle .beta..sub.p. The figure shows a
substrate wafer 31 aligned in the x-z plane, with its wafer normal
30 directed along the y-axis. The wafer 31 in FIG. 8 is shown
having its geometrical center at the implant position corresponding
to the origin of the Cartesian co-ordinate system. The final beam
path 21 is shown having a beam tilt rotation .alpha..sub.b about
the x-axis, and also a beam slope rotation .beta..sub.b about the
z-axis. The resulting final beam path 21 makes a total angle
.theta. relative to the substrate normal 30. The application of
simple trigonometry demonstrates that:
sin.sup.2.theta.=cos.sup.2(.alpha..sub.b)*sin.sup.2(.beta..sub.b)+sin.su-
p.2(.alpha..sub.b). (1)
[0062] It should be noted that in the case illustrated in FIG. 8,
.alpha..sub.s and .beta..sub.s are zero; there is no wafer tilt or
wafer slope. More generally:
sin.sup.2.theta.=cos.sup.2(.alpha..sub.t)*sin.sup.2(.beta..sub.t)+sin.su-
p.2(.alpha..sub.t). (2)
[0063] It can be demonstrated similarly that the total angle of the
wafer relative to the x-z plane (or the angle of the wafer normal
to the y-axis) can be expressed as .phi. where:
sin.sup.2.phi.=cos.sup.2(.alpha..sub.s)*sin.sup.2(.beta..sub.s)+sin.sup.-
2(.alpha..sub.s).
[0064] Also considering FIG. 8, the final beam path 21 projects a
line 40 onto the plane of the substrate wafer 31, and line 40 makes
an angle .gamma. to the z-axis of the co-ordinate system.
[0065] Simple trigonometry establishes the relationship:
sin 2 y = cos 2 ( .alpha. b ) sin 2 ( .beta. b ) sin 2 ( .alpha. b
) + cos 2 ( .alpha. b ) sin 2 ( .beta. b ) ( 3 ) ##EQU00001##
[0066] More generally:
sin 2 y = cos 2 ( .alpha. t ) sin 2 ( .beta. t ) sin 2 ( .alpha. t
) + cos 2 ( .alpha. t ) sin 2 ( .beta. t ) ( 4 ) ##EQU00002##
[0067] It will be understood that for real values of .alpha..sub.s
and/or .beta..sub.s the substrate wafer 31 is rotated relative to
the x-z plane. The angle .gamma. is defined as the angle between
the projection made onto the plane of the substrate wafer 31 by the
final beam path 21, relative to the line of intersection between
the wafer plane and the y-z plane of the co-ordinate system when
the wafer is in the x-z plane. Thus, the reference line for the
angle .gamma. on the wafer surface remains fixed as the wafer is
rotated about the x and/or z axes. The angle .gamma. is referred to
as the twist angle.
[0068] The twist angle is an important parameter for ion
implantation. A known problem for ion implantation is so-called
channeling. It is normally desired to ensure that ions being
implanted into a substrate are implanted to a desired depth beneath
the substrate surface corresponding to the energy of the ions in
the implant beam. If the implant direction aligns with an axis or
plane of the crystalline structure of the material of the substrate
wafer (typically silicon), then channeling can occur, which allows
implanted ions to be channeled along the axis or in the plane to
depths in excess of the desired depth. A known procedure for
minimizing channeling is to ensure that the implant beam is angled
relative to the wafer normal. The wafer normal is normally aligned
with a major crystal axis. Known implant processes arrange for the
implant beam to be angled at about 7.degree. to the wafer normal in
order to minimize channeling.
[0069] However, it is also necessary to ensure that the twist angle
.gamma. is selected to avoid implanting into crystal planes. FIGS.
10 and 11 are polar diagrams of crystal planes in crystalline
silicon. The radius from the origin at the bottom left hand corner
of each diagram represents the total angle .theta. between the beam
and the wafer normal, and the angular position corresponds to the
angle of twist .gamma. between the wafer and the projection line 40
of the final beam path 21 on the wafer plane. The bottom edge of
each diagram corresponds to the {004} crystal plane in the silicon
lattice and the diagonal line in each diagram corresponds to the
{022} crystal plane. These are the two crystal planes of primary
concern and the effective twist angle .gamma. should not coincide
with these planes, so that .gamma. should be greater than about
0.degree. and less than about 45.degree..
[0070] Substrate wafers 31 mounted on the substrate holders 14 are
rotated on the rotary scan assembly drum 9 as the drum rotates
about the axis 13. If each substrate wafer 31 on a respective
substrate holder 14 has a dimension w.sub.x (refer to FIG. 4) in
the direction of the x-axis (when the wafer is centered at the
implant point 22), and the diameter of the periphery 23 of the
rotary scan assembly drum 9 is d, then each wafer subtends an angle
2*.DELTA..beta. about the axis 13 of rotation of the drum 9, where
2*.DELTA..beta.=(w.sub.x/.pi.d)*360.degree.. 2*.DELTA..beta.
represents the amount of rotation of each substrate wafer about the
z-axis as the substrate wafer 31 traverses through the beam, or
more particularly through the final beam path 21.
[0071] The total slope angle .beta..sub.t is the angle between the
substrate wafer 31 and the final beam path 21, when the substrate
wafer 31 is centered at the implant position 22. As the substrate
wafer 31 actually traverses through the implant position, with
rotation of the drum 9, the slope angle varies from
.beta..sub.t-.DELTA..beta. to .beta..sub.t+.DELTA..beta.. This
variation in slope angle has an important effect on a criterion to
avoid channeling during implantation.
[0072] FIG. 9 is a schematic plan view of a substrate wafer 31. As
explained previously, known prior art implanters, in which the
wafers are mounted along the inside surface of a drum type rotary
scan assembly, are typically set up with only a net tilt angle
.alpha..sub.t between the final beam path 21 and wafers centered at
the implant position. In FIG. 9, line 50 represents the projection
onto the substrate wafer 31 centered at the implant position 22 of
a final beam path 21 for which .alpha..sub.t is, for example,
7.degree., and .beta..sub.t is zero. Line 51 represents the
projection of the final beam path 21 on the substrate wafer 31,
when a leading edge of substrate wafer 31 first intercepts the
final beam path 21. At this position, the substrate wafer 31 is
effectively at a wafer slope angle of -.DELTA..beta.. Line 52 on
FIG. 9, represents the projection of the final beam path 21 on the
wafer plane as the trailing edge of the substrate wafer 31 is about
to leave the final beam path 21. At this point, the substrate wafer
31 has an effective slope angle of +.DELTA..beta.. It can be seen,
therefore, that as the substrate wafer 31 passes through the final
beam path 21, from leading edge to trailing edge, the effective
wafer slope angle varies from -.DELTA..beta. through zero to
+.DELTA..beta., corresponding to the shaded region 53 on FIG.
9.
[0073] The size of the shaded region 53 is dependent on the size of
.DELTA..beta.. .DELTA..beta. is in turn dependent on the dimension
of the wafer w.sub.x relative to the periphery diameter d. In
practice, in order to maximize productivity, it is normal to locate
substrate wafers around the periphery adjacent to each other as
close as possible, without one substrate overlapping a neighbouring
substrate or obscuring the ion beam as a neighbouring substrate
passes through the implant point. Accordingly, the maximum value of
w.sub.x is:
[0074] .pi./N, where N is the number of substrate holders 14
distributed around the periphery 23 of the rotary scan assembly
drum 9.
[0075] If 60 wafers are mounted around the periphery of the drum,
the angle subtended by each wafer at the axis of rotation of the
drum is approximately 6.degree. (which equals 2.DELTA..beta.).
Referring to FIG. 9, if .DELTA..beta.=3.degree., and the preset
tilt angle between the final beam path and the beam plane
.alpha..sub.t is 7.degree., then the lines 51 and 52 are at
approximately -23.degree. and +23.degree. respectively on either
side of the line 50. Therefore, with such an arrangement, the twist
angle of the ion beam relative to the substrate wafer varies by
more than 45.degree. as each wafer passes through the beam.
Considering FIG. 10, a variation in twist angle must cause the
twist angle to pass through at least one of the major channeling
crystal planes {022} or {004} thus, introducing undesirable
channeling in the implant process. In prior art arrangements, fewer
than 60 wafers are mounted around the periphery of the drum, so
that twist angle variation is greater.
[0076] Referring to FIG. 9, line 60 represents the projection onto
the wafer plane of the final beam path angle, in the case of a beam
path which, in accordance with an embodiment of the invention, has
a preset total slope angle .beta..sub.t of about 10.degree.. In
this example, a preset tilt angle .alpha..sub.t of about 2.degree.
is also provided. The resulting twist angle .gamma. can be
calculated at about 78.5.degree. as illustrated.
[0077] Line 61 corresponds to the projection of the final beam path
as the leading edge of the wafer first intercepts the final beam
path, at which point the total slope angle is about 7.degree.,
assuming as before 60 wafers around the periphery of the drum so
that .DELTA..beta.=3.degree.. The resulting twist angle is
calculated at about 74.degree..
[0078] Line 62 represents the projection of the final beam path as
the trailing edge of the wafer is about to leave the beam path, at
which point the slope angle is about 13.degree.. The resulting
twist angle is calculated at about 81.4.degree..
[0079] With this arrangement, the twist angle of the final beam
path on the wafer plane varies by about 7.5.degree. from the
leading edge to the trailing edge of each wafer as it traverses the
beam. On the other hand, the total angle .theta. between the final
beam path and the wafer plane varies, as illustrated by the lengths
of the lines 60, 61 and 62, between about 8.degree. and about
13.5.degree.. The variation of twist angle .gamma. and absolute
beam angle .theta., as represented by lines 60, 61 and 62 on FIG.
9, is illustrated in FIG. 11 by the patch 70. The patch 70 can be
located within the polar diagram of FIG. 11 in a way that readily
avoids overlapping one of the major implant planes {004} or {022}
or symmetrically corresponding planes. Accordingly, by providing a
predetermined slope angle between the final beam path and the
wafer, an implant can be conducted with the apparatus described
whilst avoiding significant channeling.
[0080] As mentioned above, the final beam path 21 must be angled
away from the wafer normal 30 at all times during passage of the
substrate wafer 31 through the beam path, to avoid the beam
aligning with a primary crystal axis corresponding to the wafer
normal 30. Accordingly, the total angle between the final beam path
21 and the wafer plane should not be less than about 2.degree..
[0081] In one embodiment, the ion implantation apparatus described
herein may be used for the implantation of ions in a silicon
substrate in order to cause a plane of weakness thus allowing a
thin lamina of silicon to be exfoliated. The process may be
utilized for the production of thin silicon laminae for use in the
manufacture of photovoltaic solar cells. The thickness of the
exfoliated laminae should be at least about 10 microns. This may
require implant energies approaching or exceeding about 1 MeV. The
depth penetration of ions during implantation varies with the
cosine of the angle of implantation relative to the wafer normal.
Accordingly, in order to minimize the reduction in implant depth,
the angle of implantation (corresponding to angle .theta. in FIG.
8) should not exceed about 15.degree. and therefore, the maximum
slope angle as a substrate wafer traverses the ion beam should also
not be greater than about 15.degree.. These conditions lead to the
inequality:
2.degree.+.DELTA..beta..ltoreq..beta..sub.t<15.degree.-.DELTA..beta.
(5)
[0082] In FIG. 9, the variation in twist angle between lines 61 and
62 is dependent on the amount of preset tilt angle (.alpha..sub.t)
between the wafers and the final beam path. As demonstrated
previously, the twist angle .gamma. is related to .alpha..sub.t and
.beta..sub.t by equation (4) above.
[0083] In order to limit the size of the patch 70 in FIG. 11, the
total variation in twist angle, as a wafer traverses the ion beam,
should be confined to about 30.degree.. If this twist angle
variation is .DELTA..gamma. then Tan(.DELTA..gamma.)=F, where F is
approximately equal to
2.alpha..sub.t*.DELTA..beta./(.beta..sub.t.sup.2-.DELTA..beta..sup.2+.-
alpha..sub.t.sup.2). This equation for F is a reasonable
approximation for small values of .alpha..sub.t, .beta..sub.t and
.DELTA..beta.. In order to keep the variation and twist angle to
less than about 27.degree., the above function F should be less
than 0.5.
[0084] As mentioned above, the substrate holders 14 may be arranged
to support the wafer with a tilt angle .alpha..sub.s and a slope
angle .beta..sub.s. Substrate wafers 31 are mounted on the rotary
scan assembly drum 9 facing substantially inward toward the axis of
rotation of the drum. In this way, the effective centrifugal force
on the substrate wafer 31, which is pushing the substrate wafer 31
against the support surfaces of the substrate holders 14, is
maximized. It is also desirable that the substrate wafers 31 are
held on the support surfaces of the substrate holders 14 solely by
the action of centrifugal force, without any lateral restraint
apart from friction between the substrate wafers 31 and the support
surfaces of the substrate holders 14. Also, to provide heat
transfer from the substrate wafers 31 into the substrate supports
of the substrate holders 14, uniform contact pressure between the
rear surface of the substrate wafers 31 and the substrate support
surfaces of the substrate holders 14 over the whole surface area of
the wafers should be maintained. This is made easier if there are
no lateral support fences needed to hold the substrate wafers 31 in
place as the rotary scan assembly drum 9 is rotated. If the total
wafer angle .phi. exceeds about 45.degree., the contact pressure
between the wafers and the substrate support surfaces may become
less than optimum. Also, the lateral forces on the substrate wafers
31 during rotation of the rotary scan assembly drum 9 may be
sufficient to overcome the effect of friction, so that the
substrate wafers 31 may slide off of the substrate holders 14.
[0085] In the example described above, 60 substrate holders 14 are
distributed about the periphery 23 of the rotary scan assembly drum
9. The inequality (5) above, relating .beta..sub.t to
.DELTA..beta., implies a maximum value for .DELTA..beta. of about
6.5.degree.. This would correspond to a minimum of about 30
substrate holders distributed about the periphery 23 of the rotary
scan assembly drum 9, each subtending an angle of about
12.degree..
[0086] However, in order to reduce the variation in slope angle as
substrate wafers 31 pass through the beam, at least 50 substrate
holders 14 may be distributed about the periphery 23 of the rotary
scan assembly drum 9, implying the inequality:
w.sub.x.ltoreq..pi.d/50.
[0087] In practice, between 55 and 70 substrate holders 14 may be
distributed around the periphery. If more than 70 is provided,
although .DELTA..beta. is reduced, either the substrate dimension
w.sub.x must be reduced, or the drum diameter d may become
excessive for practical purposes.
[0088] With an effective value of .DELTA..beta. approximately equal
to 3.degree., the total beam angle .beta..sub.t can be set between
a minimum of about 5.degree. and a maximum of about 12.degree., for
example, 8.degree..ltoreq..beta..sub.t.ltoreq.11.degree..
[0089] In order to further confine the variation in twist angle,
function F may be less than about 0.25. In one example, function F
is less than 0.15.
[0090] In an example of implantation apparatus,
.DELTA..beta..ltoreq..beta..sub.t/3. This limits the amount of
variation in slope angle across the substrate wafer to about + or
-33%. In turn, the variation in total implant angle .theta., and
consequently also in implant depth, is limited. This is desirable
for reliable exfoliation of thin silicon laminae.
[0091] Generally, the total tilt angle .alpha..sub.t should be kept
rather small to reduce twist angle variation .DELTA..gamma.. In an
example .alpha..sub.t.ltoreq.3.degree.. In another example,
.DELTA..beta.<3.5.degree., .alpha..sub.t<3.degree., and
.alpha..sub.s<12.degree..
[0092] A variety of embodiments have been provided for clarity and
completeness. Other embodiments of the invention will be apparent
to one of ordinary skill in the art when informed by the present
specification. Whereas detailed arrangements and ranges of
parameters for implant apparatus and for an implant method have
been described herein, other arrangements and parameter settings
can be used which fall within the scope of the appended claims.
[0093] The foregoing detailed description has described only a few
of the many forms that this invention can take. For this reason
this detailed description is intended by way of illustration and
not by way of limitation. It is only the following claims,
including all equivalents, which are intended to define the scope
of the invention.
* * * * *