U.S. patent number RE41,214 [Application Number 11/053,683] was granted by the patent office on 2010-04-13 for bi mode ion implantation with non-parallel ion beams.
This patent grant is currently assigned to Varian Semmiconductor Equipment Associates, Inc.. Invention is credited to Joseph C. Olson, Anthony Renau.
United States Patent |
RE41,214 |
Renau , et al. |
April 13, 2010 |
Bi mode ion implantation with non-parallel ion beams
Abstract
A method for implanting ions into a workpiece, such as a
semiconductor wafer, includes the steps of generating an ion beam,
measuring an angle of non-parallelism of the ion beam, tilting the
wafer at a first angle, performing a first implant at the first
angle, tilting the wafer at a second angle, and performing a second
implant at the second angle. The first and second angles are
opposite in sign with respect to a reference direction and in
magnitude are equal to or greater than the measured angle of
non-parallelism. Preferably, the first and second implants are
controlled to provide substantially equal ion doses in the
workpiece.
Inventors: |
Renau; Anthony (West Newbury,
MA), Olson; Joseph C. (Beverly, MA) |
Assignee: |
Varian Semmiconductor Equipment
Associates, Inc. (Gloucester, MA)
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Family
ID: |
24810292 |
Appl.
No.: |
11/053,683 |
Filed: |
February 8, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
09699653 |
Oct 30, 2000 |
06573518 |
Jun 3, 2003 |
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Current U.S.
Class: |
250/492.21;
250/442.11 |
Current CPC
Class: |
H01J
37/3171 (20130101); H01J 2237/24542 (20130101); H01J
2237/24528 (20130101) |
Current International
Class: |
H01J
37/317 (20060101) |
Field of
Search: |
;250/492.21,442.11 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 926 699 |
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Jun 1999 |
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EP |
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0 975 004 |
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Jan 2000 |
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EP |
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632269223 |
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Sep 1988 |
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JP |
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03-017949 |
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Jan 1991 |
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JP |
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2001229873 |
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Aug 2001 |
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JP |
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WO 99/13488 |
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Mar 1999 |
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WO |
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WO 01/04926 |
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Jan 2001 |
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WO |
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WO 01/27968 |
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Apr 2001 |
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WO |
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Other References
JC. Olson and A. Renau, "Control of Channeling Uniformity for
Advanced Applications", 2000 International Conference on Ion
Implantation Technology Proceedings. Ion Implantation
Technology--2000, Alpbach, AUS, pp. 670-673. cited by other .
International Search Report dated Jun. 10, 2002 of International
Patent Application No. PCT/US01/31658. cited by other.
|
Primary Examiner: Berman; Jack I
Claims
What is claimed is:
1. A method for implanting ions into a workpiece, comprising the
steps of: generating an ion beam; measuring an angle of
non-parallelism of the ion beam; performing a first implant with
the workpiece oriented at a first angle; and performing a second
implant with the workpiece oriented at a second angle, wherein the
first and second angles are opposite in sign with respect to a
reference direction and in magnitude are equal to or greater than
the measured angle of non-parallelism.
2. A method as defined in claim 1 wherein the steps of performing
said first and second implants are controlled to provide
substantially equal ion doses in the workpiece.
3. A method as defined in claim 1 wherein the angle of
non-parallelism is less than about 5.degree..
4. A method as defined in claim 1 wherein the angle of
non-parallelism comprises a half angle of divergence of the ion
beam.
5. A method as defined in claim 1 wherein the angle of
non-parallelism comprises a half angle of convergence of the ion
beam.
6. A method as defined in claim 1 further comprising the step of
generating the ion beam utilizing a parallelizing device.
7. A method as defined in claim 1 further comprising the step of
generating the ion beam without utilizing a parallelizing
device.
8. A method as defined in claim 1 wherein the reference direction
comprises a direction of the ion beam at the workpiece.
9. A method as defined in claim 1 wherein the reference direction
comprises a selected implant angle relative to a direction of the
ion beam at the workpiece.
10. A method as defined in claim 1 wherein the first and second
angles are equal in magnitude.
11. A method for implanting ions into a semiconductor wafer,
comprising the steps of: generating an .[.in.]. .Iadd.ion
.Iaddend.beam; measuring an angle of non-parallelism of the ion
beam; tilting the wafer at a first angle; performing a first
implant at the first angle; tilting the wafer at a second angle;
and performing a second implant at the second angle, wherein the
first and second angles are opposite in sign with respect to a
reference direction and in magnitude are equal to or greater than
the measured angle of non-parallelism.
12. A method as defined in claim 11 wherein said first and second
implants are controlled to provide substantially equal ion doses in
the wafer.
13. A method as defined in claim 11 wherein the angle of
non-parallelism of the ion beam is less than about 5.degree..
14. A method as defined in claim 11 wherein the angle of
non-parallelism of the ion beam comprises a half angle of
divergence of the ion beam.
15. A method as defined in claim 11 wherein the angle of
non-parallelism of the ion beam comprises a half angle of
convergence of the ion beam.
16. A method as defined in claim 11 further comprising the step of
generating the ion beam utilizing a parallelizing device.
17. A method as defined in claim 11 further comprising the step of
generating the ion beam without utilizing a parallelizing
device.
18. A method as defined in claim 11 wherein the reference direction
comprises a direction of the ion beam at the wafer.
19. A method as defined in claim 11 wherein the reference direction
comprises a selected implant angle relative to a direction of the
ion beam at the wafer.
20. A method as defined in claim 11 wherein the first and second
angles are equal in magnitude.
21. Apparatus for implanting ions into a semiconductor wafer,
comprising: means for generating an ion beam; means for measuring
an angle of non-parallelism of the ion beam; means for tilting the
wafer at a first angle; means for performing a first implant at the
first angle; means for tilting the wafer at a second angle; and
means for performing a second implant at the second angle, wherein
the first and second angles are opposite in sign with respect to a
reference direction and in magnitude are equal to or greater than
the measured angle of non-parallelism.
22. A method for implanting ions into a semiconductor wafer,
comprising the steps of: generating an ion beam; tilting the wafer
at a first angle with respect to the ion beam; performing a first
implant with the wafer tilted at the first angle; tilting the wafer
at a second angle that is equal in magnitude and opposite in sign
with respect to said first angle; and performing a second implant
with the wafer tilted at the second angle.Iadd., wherein the step
of tilting the wafer at the first angle comprises tilting the wafer
at a half angle of divergence of the ion beam.Iaddend..
.[.23. A method as defined in claim 22 wherein the step of tilting
the wafer at a first angle comprises tilting the wafer at a half
angle of divergence of the ion beam..].
.[.24. A method as defined in claim 22 wherein the step of tilting
the wafer at a first angle comprises tilting the wafer at a half
angle of convergence of the ion beam..].
25. Apparatus for implanting ions into a semiconductor wafer,
comprising: an ion beam generator; a measuring system for measuring
an angle of non-parallelism of the ion beam; and a tilt mechanism
for tilting the semiconductor wafer at first and second angles,
wherein the first and second angles are opposite in sign with
respect to a reference direction and in magnitude are equal to or
greater than the measured angle of non-parallelism, wherein first
and second implants are performed at the first and second angles,
respectively.
26. Apparatus as defined in claim 25 wherein said measuring system
comprises a movable beam profiler and one or more beam
detectors.
27. Apparatus as defined in claim 25 further comprising an ion
optical element for parallelizing the ion beam.
28. Apparatus as defined in claim 25 wherein the first and second
angles are equal in magnitude.
.Iadd.29. A method for implanting ions into a workpiece,
comprising: generating an ion beam; performing a first implant with
the workpiece oriented at a first angle relative to a reference
direction; and performing a second implant with the workpiece
oriented at a second angle relative to the reference direction,
wherein the first and second angles are opposite in sign with
respect to the reference direction, and wherein the reference
direction is a selected implant angle with respect to the
workpiece..Iaddend.
.Iadd.30. A method as defined in claim 29, wherein the first and
second implants are controlled to provide substantially equal ion
doses in the workpiece..Iaddend.
.Iadd.31. A method as defined in claim 29, further comprising
generating the ion beam utilizing a parallelizing
device..Iaddend.
.Iadd.32. A method as defined in claim 29, further comprising
generating the ion beam without utilizing a parallelizing
device..Iaddend.
.Iadd.33. A method as defined in claim 29, wherein the first and
second angles are equal in magnitude..Iaddend.
.Iadd.34. A method as defined in claim 29, wherein generating an
ion beam comprises generating a scanned ion beam..Iaddend.
.Iadd.35. A method as defined in claim 29, wherein generating an
ion beam comprises generating a ribbon ion beam..Iaddend.
.Iadd.36. A method as defined in claim 30, further comprising
measuring an angle of non-parallelism of the ion beam, wherein the
magnitudes of the first and second angles are equal to or greater
than the measured angle of non-parallelism..Iaddend.
.Iadd.37. A method as defined in claim 29, wherein the first and
second angles are equal in magnitude and wherein the first and
second implants are controlled to provide substantially equal ion
doses in the workpiece..Iaddend.
.Iadd.38. A method for implanting ions into a semiconductor wafer,
comprising: generating an ion beam; tilting the wafer at a first
angle relative to a reference direction; performing a first implant
at the first angle; tilting the wafer at a second angle relative to
the reference direction; and performing a second implant at the
second angle, wherein the first and second angles are opposite in
sign with respect to the reference direction, and wherein the
reference direction is a selected implant angle with respect to the
wafer..Iaddend.
.Iadd.39. A method as defined in claim 38, wherein the first and
second implants are controlled to provide substantially equal ion
doses in the wafer..Iaddend.
.Iadd.40. A method as defined in claim 38, further comprising
generating the ion beam utilizing a parallelizing
device..Iaddend.
.Iadd.41. A method as defined in claim 38, further comprising
generating the ion beam without utilizing a parallelizing
device..Iaddend.
.Iadd.42. A method as defined in claim 38, wherein the first and
second angles are equal in magnitude..Iaddend.
.Iadd.43. A method as defined in claim 39, further comprising
measuring an angle of non-parallelism of the ion beam, wherein the
magnitudes of the first and second angles are equal to or greater
than the measured angle of non-parallelism..Iaddend.
.Iadd.44. A method as defined in claim 38, wherein the first and
second angles are equal in magnitude and wherein the first and
second implants are controlled to provide substantially equal ion
doses in the wafer..Iaddend.
.Iadd.45. Apparatus for implanting ions into a semiconductor wafer,
comprising: an ion beam generator; and a tilt mechanism for tilting
the semiconductor wafer at first and second angles, wherein the
first and second angles are opposite in sign with respect to a
reference direction, wherein first and second implants are
performed at the first and second angles, respectively, and wherein
the reference direction is a selected implant angle with respect to
the wafer..Iaddend.
.Iadd.46. Apparatus as defined in claim 45, further comprising an
ion optical element configured to parallelize the ion
beam..Iaddend.
.Iadd.47. Apparatus as defined in claim 45, further comprising a
measuring system for measuring an angle of non-parallelism of the
ion beam, wherein the magnitudes of the first and second angles are
equal to or greater than the measured angle of
non-parallelism..Iaddend.
.Iadd.48. Apparatus as defined in claim 45, wherein the first and
second angles are equal in magnitude..Iaddend.
.Iadd.49. Apparatus as defined in claim 45, wherein the ion beam
generator is configured to generate a scanned ion
beam..Iaddend.
.Iadd.50. Apparatus as defined in claim 45, wherein the ion beam
generator is configured to generate a ribbon ion beam..Iaddend.
.Iadd.51. Apparatus for implanting ions into a semiconductor wafer,
comprising: means for generating an ion beam; means for tilting the
wafer at first and second angles; and means for performing first
and second implants at the first and second angles, respectively,
wherein the first and second angles are opposite in sign with
respect to a reference direction, and wherein the reference
direction is a selected implant angle with respect to the
wafer..Iaddend.
Description
FIELD OF THE INVENTION
This invention relates to systems and methods for ion implantation
of semiconductor wafers or other workpieces and, more particularly,
to methods and apparatus for achieving uniform ion implantation
over the surface of a semiconductor wafer with non-parallel ion
beams.
BACKGROUND OF THE INVENTION
Ion implantation is a standard technique for introducing
conductivity-altering impurities into semiconductor wafers. A
desired impurity material is ionized in an ion source, the ions are
accelerated to form an ion beam of prescribed energy, and the ion
beam is directed at the surface of the wafer. The energetic ions in
the beam penetrate into the bulk of the semiconductor material and
are embedded into the crystalline lattice of the semiconductor
material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for
converting a gas or a solid material into a well-defined ion beam.
The ion beam is mass analyzed to eliminate undesired ion species,
is accelerated to a desired energy and is directed onto a target
plane. The beam is distributed over the target area by beam
scanning, by target movement or by a combination of beam scanning
and target movement. An ion implanter which utilizes a combination
of beam scanning and target movement is disclosed in U.S. Pat. No.
4,922,106 issued May 1, 1990 to Berrian et al.
The delivery of a parallel ion beam to the semiconductor wafer is
an important requirement in many applications. A parallel ion beam
is one which has parallel ion trajectories over the surface of the
semiconductor wafer. In cases where the ion beam is scanned, the
scanned beam is required to maintain parallelism over the wafer
surface. The parallel ion beam prevents channeling of incident ions
in the crystal structure of the semiconductor wafer or permits
uniform channeling in cases where channeling is desired. Typically,
a serial ion implanter is utilized when a high degree of beam
parallelism is required.
In one approach, the beam is scanned in one dimension so that it
appears to diverge from a point, referred to as the scan origin.
The scanned beam is then passed through an ion optical element
which performs focusing. The ion optical element converts the
diverging ion trajectories to parallel ion trajectories for
delivery to the semiconductor wafer. Focusing can be performed with
an angle corrector magnet or with an electrostatic lens. The angle
correction magnet produces both bending and focusing of the scanned
ion beam. Parallelism may be achieved with an electrostatic lens,
but energy contamination can be a drawback.
The output ion beam from the angle corrector magnet or other
focusing element may be parallel or may be converging or diverging,
depending on the parameters of the ion beam and the parameters of
the focusing element. When an angle corrector magnet is utilized,
parallelism can be adjusted by varying the magnetic field of the
angle corrector magnet. The angle corrector magnet typically has a
single magnetic field adjustment which varies both parallelism and
bend angle, or beam direction. It will be understood that the ion
implanter is often required to run a variety of different ion
species and ion energies. When the beam parameters are changed,
readjustment of the angle corrector magnet is required to restore
beam parallelism.
The requirement for readjustment of beam parallelism adds
complexity and delay to ion implanter operation. Furthermore, the
angle corrector magnet or other ion optical element used to produce
a parallel ion beam adds to the cost of the ion implanter and
increases the length of the ion implanter beamline.
Accordingly, there is a need for ion implantation methods and
apparatus in which the requirement for beam parallelism is relaxed,
without degrading ion implantation uniformity over the surface of
the semiconductor wafer.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, a method is provided
for implanting ions into a workpiece. The method comprises the
steps of generating an ion beam, measuring an angle of
non-parallelism of the ion beam, performing a first implant with
the workpiece oriented at a first angle, and performing a second
implant with the workpiece oriented at a second angle. The first
and second angles are opposite in sign with respect to a reference
direction and in magnitude are equal to or greater than the
measured angle of non-parallelism.
In one embodiment, the reference direction comprises the direction
of the ion beam at the workpiece. In another embodiment, the
reference direction comprises a selected implant angle relative to
the direction of the ion beam at the workpiece.
Preferably, the first and second implants are controlled to provide
substantially equal ion doses in the workpiece. The workpiece may
comprise a semiconductor wafer. In one preferred embodiment, the
first and second angles are equal in magnitude to the measured
angle of non-parallelism.
In a first embodiment, the ion beam is generated utilizing a
parallelizing device. In a second embodiment, the ion beam is
generated without utilizing a parallelizing device.
According to another aspect of the invention, a method is provided
for implanting ions into a semiconductor wafer. The method
comprises the steps of generating an ion beam, measuring an angle
of non-parallelism of the ion beam, tilting the wafer at a first
angle, performing a first implant at the first angle, tilting the
wafer at a second angle, and performing a second implant at the
second angle. The first and second angles are opposite in sign with
respect to a reference direction and in magnitude are equal to or
greater than the measured angle of non-parallelism.
According to a further aspect of the invention, apparatus is
provided for implanting ions into a semiconductor wafer. The
apparatus comprises means for generating an ion beam, means for
measuring an angle of non-parallelism of the ion beam, means for
tilting the wafer at a first angle, means for performing a first
implant at the first angle, means for tilting the wafer at a second
angle, and means for performing a second implant at the second
angle. The first and second angles are opposite in sign with
respect to a reference direction and in magnitude are equal to or
greater than the measured angle of non-parallelism.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, reference is
made to the accompanying drawings, which are incorporated herein by
reference and in which:
FIG. 1 is a schematic diagram of an ion implanter suitable for
implementing the present invention;
FIG. 2 is a schematic diagram that illustrates the operation of an
angle corrector magnet for the case of a relatively large bend
angle and converging ion trajectories;
FIG. 3 is a schematic diagram that illustrates the operation of an
angle corrector magnet for the case of a relatively small bend
angle and diverging ion trajectories;
FIG. 4 is a flow chart of an ion implantation process in accordance
with an embodiment of the invention;
FIG. 5 is a schematic diagram that illustrates bi-mode ion
implantation in accordance with an embodiment of the invention;
FIG. 6 is a table that illustrates an example of the uniformity of
implant angle obtained with bi-mode ion implantation;
FIGS. 7A and 7B are schematic diagrams that illustrate operation of
an example of a device for measuring beam parallelism;
FIGS. 8A and 8B are graphs of beam detector output as a function of
beam profile position for the beam conditions illustrated in FIGS.
.[.6.]. .Iadd.7.Iaddend.A and .[.6.]. .Iadd.7.Iaddend.B,
respectively; and
FIG. 9 is a schematic diagram that illustrates prior art ion
implantation with a non-parallel ion beam.
DETAILED DESCRIPTION
A simplified block diagram of an example of an ion implanter
suitable for incorporating the present invention is shown in FIG.
1. An ion beam generator 10 generates an ion beam of a desired
species, accelerates ions in the ion beam to desired energies,
performs mass/energy analysis of the ion beam to remove energy and
mass contaminants and supplies an energetic ion beam 12 having low
level of energy and mass contaminants. A scanning system 16, which
includes a scanner 20 and an angle corrector 24, deflects the ion
beam 12 to produce a scanned ion beam 30 having parallel or nearly
parallel ion trajectories. An end station 32 includes a platen 36
that supports a semiconductor wafer 34 or other workpiece in the
path of scanned ion beam 30 such that ions of the desired species
are implanted into the semiconductor wafer 34. The ion implanter
may include additional components well known to those skilled in
the art. For example, the end station 32 typically includes
automated wafer handling equipment for introducing wafers into the
ion implanter and for removing wafers after implantation, a dose
measuring system, an electron flood gun, etc. It will be understood
that the entire path traversed by the ion beam is evacuated during
ion implantation.
The principal components of the ion beam generator 10 include an
ion beam source 40, a source filter 42, an
acceleration/deceleration column 44 and a mass analyzer 50. The
source filter 42 is preferably positioned in close proximity to ion
beam source 40. The acceleration/deceleration column 44 is
positioned between source filter 42 and mass analyzer 50. The mass
analyzer 50 includes a dipole analyzing magnet 52 and a mask 54
having a resolving aperture 56.
The scanner 20, which may be an electrostatic scanner, deflects ion
beam 12 to produce a scanned ion beam having ion trajectories which
diverge from a scan origin 60. The scanner 20 may comprise
spaced-apart scan plates connected to a scan generator. The scan
generator applies a scan voltage waveform, such as a sawtooth
waveform, for scanning the ion beam in accordance with the electric
field between the scan plates.
Angle corrector 24 is designed to deflect ions in the scanned ion
beam to produce scanned ion beam 30 having parallel ion
trajectories. In particular, angle corrector 24 may comprise
magnetic pole pieces 26 which are spaced apart to define a gap and
a magnet coil (not shown) which is coupled to a power supply 28.
The scanned ion beam passes through the gap between the pole pieces
26 and is deflected in accordance with the magnetic field in the
gap. The magnetic field may be adjusted by varying the current
through the magnet coil. Beam scanning and beam focusing are
performed in a selected plane, such as a horizontal plane.
In the embodiment of FIG. 1, end station 32 includes a beam
parallelism and direction measuring system 80. System 80 measures
beam parallelism and direction as described below. In addition, end
station 32 includes a tilt mechanism 84 for tilting wafer support
platen 36 with respect to the scanned ion beam 30. In one
embodiment, tilt mechanism 84 may tilt wafer support platen 36 with
respect to two orthogonal axes.
Examples of operation of angle corrector 24 are shown in FIGS. 2
and 3. As shown, the pole pieces 26 of angle corrector 24 may be
wedged shaped or similarly shaped so that different ion
trajectories have different path lengths through the gap between
the pole pieces. In FIG. 2, a relatively high intensity magnetic
field is applied. The ion trajectories have a relatively large bend
angle and may be converging as they exit from angle corrector 24.
In the example of FIG. 3, a relatively low intensity magnetic field
is applied. The ion trajectories have a relatively small bend angle
and may be diverging as they exit from angle corrector 24. Thus,
scanned ion beam 30 is incident on a wafer plane 70 at a positive
angle 72 with respect to a normal to wafer plane 70 in the example
of FIG. 2 and is incident on wafer plane 70 at a negative angle 74
with respect to a normal to wafer plane 70 in the example of FIG.
3. It will be understood that parallel or nearly parallel ion
trajectories can be produced by appropriate adjustment of the
magnetic field in angle corrector 24. However in general, the
magnetic field that provides the best parallelism does not
necessarily result in normal incidence of scanned ion beam 30 on
wafer plane 70.
A flow chart of a process for implanting ions into a workpiece in
accordance with an embodiment of the invention is shown in FIG. 4.
In step 100, an ion beam is generated and is transported through
the beamline of an ion implanter. As shown in FIG. 1, ion beam 12
is generated by ion beam generator 12 and is transported through
scanner and angle corrector 24 to end station 32.
In step 102, the parallelism of the ion beam is measured at or near
the plane where the ion beam is incident on the semiconductor wafer
or other workpiece. The parallelism measurement provides an angle
of non-parallelism of the ion beam and, in particular, typically
provides a half angle of convergence or divergence of the ion beam.
The measured angle of non-parallelism represents the maximum
excursion of the ion beam trajectories from the center ray of the
ion beam. An example of a technique for measuring ion beam
parallelism is described below in connection with FIGS. 7A, 7B, 8A
and 8B. A non-parallel ion beam 130 having a half angle 132 of
divergence is shown in FIG. 5. The amount of divergence of ion beam
130 is exaggerated in FIG. 5 for purposes of illustration. Beam
parallelism is measured in the plane of scanning and focusing of
the ion beam.
Referring again to FIG. 4, the wafer is tilted at a first bi-mode
angle, +x, relative to a reference direction in step 104. As shown
in FIG. 1, tilt mechanism 84 is used to tilt wafer support platen
36 relative to scanned ion beam 30. The first bi-mode angle is
defined below. In step 106, a first implant is performed with the
water 140 tilted at the first bi-mode angle. In step 108, the wafer
is tilted at a second bi-mode angle, -y, relative to the reference
direction, as defined below. In step 110, a second implant is
performed 110 with the wafer tilted at the second bi-mode angle.
Preferably, the first implant of step 106 and the second implant of
step 110 have equal energies and doses to ensure implant uniformity
over the surface of the semiconductor wafer.
The first bi-mode angle +x and the second bi-mode angle -y are
opposite in sign relative to a reference direction and in magnitude
are equal to or greater than the measured angle of non-parallelism.
In describing the reference direction, it is useful to consider the
so-called "implant angle", which is the angle between the incident
ion beam and a normal to the wafer surface. The implant angle,
which is typically in a range of 0.degree. to 7.degree., is used to
control channeling of the energetic ions in the crystalline lattice
of the semiconductor wafer.
In the simple case where the implant angle is 0.degree., the
reference direction for the first and second bi-mode angles is the
direction of the ion beam. In this case, the wafer is tilted by
bi-mode angles of opposite sign relative to the incident ion beam,
as shown in FIG. 5. Wafer 140 is tilted by a first bi-mode angle
142 relative to wafer plane 134 for the first implant and is tilted
by a second bi-mode angle 144 relative to wafer plane 134 for the
second implant. Angles 142 and 144 are opposite in sign relative to
wafer plane 134 and in magnitude are equal to or greater than the
measured angle 132 of non-parallelism of ion beam 130. Preferably,
angles 142 and 144 are as close in magnitude as is practical to the
measured angle of non-parallelism. In a preferred embodiment,
angles 142 and 144 are equal in magnitude to the measured angle of
non-parallelism. In general however, angles 142 and 144 are not
required to have equal magnitudes.
A non-zero implant angle may be set by tilting the wafer in a
direction parallel to the plane of scanning and focusing, i.e., the
plane of FIG. 5, or may be set by tilting the wafer in a direction
orthogonal to the plane of scanning and focusing. Where the
non-zero implant angle is set by tilting the wafer in a direction
orthogonal to the plane of scanning and focusing, the reference
direction for the first and second angles is the direction of the
ion beam, as illustrated in FIG. 5 and described above. Where the
non-zero implant angle is set by tilting the wafer in a direction
parallel to the plane of scanning and focusing, the reference
direction is normal to a wafer tilted at the selected implant
angle. In this case, the first implant is performed at the first
bi-mode angle relative to the second implant angle and the second
implant is performed at the second bi-mode angle relative to the
selected implant angle. In the case where the first and second
bi-mode angles are equal in magnitude to the measured angle of
non-parallelism, the first implant is performed at the selected
implant angle plus the measured angle of non-parallelism, and the
second implant is performed at the selected implant angle minus the
measured angle of non-parallelism. For example, assume a selected
implant angle of 7.degree. and a measured angle of non-parallelism
of 1.degree.. In this example, the first bi-mode implant (step 106)
is performed at a tilt angle of 7.degree.+1.degree.=8.degree., and
the second bi-mode implant (step 110) is performed at a tilt angle
of 7.degree.-1.degree.=6.degree.. It will be understood that this
approach can be utilized with any selected implant angle and
bi-mode angles. Furthermore, the order of the first and second
implants can be reversed.
The averaging effect of a bi-mode implant in accordance with the
present invention is described with reference to FIG. 6. In the
example of FIG. 6, the desired implant angle is zero degrees, and
the non-parallel ion beam is assumed to have a measured angle of
non-parallelism of 1.degree.. In a first bi-mode implant, wafer 140
in FIG. 5 is tilted by first bi-mode angle 142 of 1.degree. with
respect to wafer plane 134. With this tilt angle, the left side of
wafer 140 is implanted at an angle of 2.degree., the center of
wafer 140 is implanted at an angle of 1.degree. and the right side
of wafer 140 is implanted at an angle of 0.degree., as summarized
in FIG. 6. In a second bi-mode implant, wafer 140 is tilted by
second bi-mode angle 144 of -1.degree. with respect to wafer plane
134. With this tilt angle, the left side of wafer 140 is implanted
at an angle of 0.degree., the center of wafer 140 is implanted at
an angle of -1.degree. and the right side of wafer 140 is implanted
at an angle of -2.degree., as summarized in FIG. 6. As further
illustrated in FIG. 6, the average of the two implants is a uniform
angle of 1.degree. across the wafer surface. The determination of
the average of the first and second bi-mode implants is based on
the assumption that implants at positive and negative angles of
equal magnitude are equivalent.
A prior art single mode implant using a non-parallel ion beam is
illustrated schematically in FIG. 9. A non-parallel ion beam 180
having an angle of non-parallelism 182 is used to implant a wafer
184 with normal incidence. The prior art implant technique shown in
FIG. 9 results in a variation in incident angle of the ion beam
over the wafer surface. In particular, for an ion beam having an
angle of non-parallelism of 1.degree., the beam has an incident
angle of +1.degree. at the left side of the wafer, an incident
angle of 0.degree. at the center of the wafer and an incident angle
of -1.degree. at the right side of the wafer. Thus, the angle of
incidence varies by 1.degree. over the wafer surface. In some
applications, such variation in angle of incidence may be
unacceptable.
An example of a technique for measuring ion beam parallelism is
described with reference to FIGS. 7A, 7B, 8A, and 8B. FIGS. 7A and
7B are schematic diagrams which illustrate the measurement of
different ion beams with a beam profiler and two beam detectors.
FIGS. 8A and 8B are graphs that illustrate the outputs of the beam
detectors as a function of profiler position.
As shown in FIGS. 7A and 7B, ion beam parallelism is measured using
a moving beam profiler 150 and spaced-apart beam detectors 152 and
154, which correspond to beam parallelism and direction measuring
system 80 (FIG. 1). Beam profiler 150 may be any element that
partially blocks the ion beam and is laterally movable relative to
the ion beam. Detectors 152 and 154, for example, may be Faraday
cups, which produce an electrical output signal in response to an
incident ion beam. As the profiler 150 is moved across the ion
beam, it blocks a portion of the ion beam and produces an ion beam
shadow. The beam shadow moves across detectors 152 and 154 and
produces output signals in the form of negative going output
current pulses.
As shown in FIG. 7A, a parallel scanned ion beam 160 has normal
incidence on a wafer plane 170. Detectors 152 and 154 produce
output pulses as shown in FIG. 8A when the profiler is positioned
in alignment with each detector. The profiler positions at which
detector output pulses are generated can be used to determine that
ion beam 160 has parallel trajectories and is normal to wafer plane
170.
Referring to FIG. 7B, a diverging ion beam 162 has normal incidence
on wafer plane 170. In this case, detector 152 produces an output
pulse as shown in FIG. 8B when profiler 150 is positioned to the
right of detector 152, and detector 154 produces an output pulse
when profiler 150 is positioned to the left of detector 154. The
profiler positions at which output pulses are generated can be used
to determine the angle of divergence of ion beam 162. In response
to a converging ion beam (not shown), detector 152 produces an
output pulse when profiler is positioned to the left of detector
152, and detector 154 produces an output pulse when profiler 150 is
positioned to the right of detector 154. The profiler positions at
which detector output pulses are generated can be used to determine
the angle of convergence of the ion beam. Additional details
regarding techniques for measuring ion beam parallelism are
provided in U.S. application Ser. No. 09/588,419, filed Jun. 6,
2000, which is hereby incorporated by reference.
It will be understood that different techniques may be used for
measuring beam parallelism within the scope of the invention. In
addition, the invention is not limited to use with a scanned ion
beam. For example, the invention may be used with a ribbon ion beam
as disclosed in U.S. Pat. No. 5,350,926, issued Sep. 27, 1994 to
White et al.
The bi-mode implant technique described above permits the
specification on beam parallelism to be relaxed without degrading
ion implantation uniformity over the surface of the semiconductor
wafer. Depending on the architecture of the ion implanter, ion
implantation uniformity may be achieved without requiring an ion
optical element to for parallelizing the ion beam. In ion implanter
architectures which include an ion optical element for
parallelizing the ion beam, the requirement for adjusting the
parallelism of the ion beam may be relaxed, in some cases
permitting a fixed parallelizing ion optical element to be
used.
While there have been shown and described what are at present
considered the preferred embodiments of the present invention, it
will be obvious to those skilled in the art that various changes
and modifications may be made therein without departing from the
scope of the invention as defined by the appended claims.
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