U.S. patent application number 12/819784 was filed with the patent office on 2010-10-07 for system and method for reducing object deformation during a pulsed heating process.
This patent application is currently assigned to Mattson Technology, Inc.. Invention is credited to Paul Janis Timans.
Application Number | 20100252547 12/819784 |
Document ID | / |
Family ID | 37482212 |
Filed Date | 2010-10-07 |
United States Patent
Application |
20100252547 |
Kind Code |
A1 |
Timans; Paul Janis |
October 7, 2010 |
SYSTEM AND METHOD FOR REDUCING OBJECT DEFORMATION DURING A PULSED
HEATING PROCESS
Abstract
An approach for optimizing the thermal budget during a pulsed
heating process is disclosed. A heat sink or thermal transfer plate
is configured and positioned near an object, such as a
semiconductor wafer, undergoing thermal treatment. The heat sink is
configured to enhance the thermal transfer rate from the object so
that the object is rapidly brought down from the peak temperature
after an energy pulse. High thermally-conductive material may be
positioned between the plate and the object. The plate may include
protrusions, ribs, holes, recesses, and other discontinuities to
enhance heat transfer and avoid physical damage to the object
during the thermal cycle. Additionally, the optical properties of
the plate may be selected to allow for temperature measurements via
energy measurements from the plate, or to provide for a different
thermal response to the energy pulse. The plate may also allow for
pre-heating or active cooling of the wafer.
Inventors: |
Timans; Paul Janis;
(Mountain View, CA) |
Correspondence
Address: |
DORITY & MANNING, P.A.
POST OFFICE BOX 1449
GREENVILLE
SC
29602-1449
US
|
Assignee: |
Mattson Technology, Inc.
Fremont
CA
|
Family ID: |
37482212 |
Appl. No.: |
12/819784 |
Filed: |
June 21, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11443464 |
May 30, 2006 |
7745762 |
|
|
12819784 |
|
|
|
|
60686506 |
Jun 1, 2005 |
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Current U.S.
Class: |
219/390 ;
392/416; 392/418 |
Current CPC
Class: |
F27B 5/04 20130101; F27B
17/0025 20130101; H01L 21/67109 20130101; H01L 21/67115
20130101 |
Class at
Publication: |
219/390 ;
392/416; 392/418 |
International
Class: |
F27B 5/14 20060101
F27B005/14; F27D 11/00 20060101 F27D011/00 |
Claims
1-50. (canceled)
51. A thermal processing method, comprising: positioning an object
in a thermal processing chamber such that the object is adjacent to
a thermal transfer plate; pre-heating the object; and heating at
least a first location on the object by directing at least one
pulse of energy toward the first location on the object for a
duration of less than 1 second; wherein the the' Hal transfer plate
has a flexibility configured to reduce object deformation during
the thermal processing of the object.
52. The method of claim 51, wherein the thermal transfer plate has
a first region and a second region, the flexibility of the thermal
transfer plate at the first region being different than the
flexibility of the thermal transfer plate at the second region.
53. The method of claim 51, wherein the flexibility of the thermal
transfer plate is optimized to reduce object deformation during
thermal processing of the object by configuring the stiffness of
the thermal transfer plate.
54. The method of claim 51, wherein the flexibility of the thermal
transfer plate is optimized to reduce object deformation during the
thermal processing of the object by configuring the thickness of
the thermal transfer plate.
55. The method of claim 51, wherein the flexibility of the thermal
transfer plate is configured to reduce object deformation by
incorporating at least one discontinuity in the thermal transfer
plate.
56. The method of claim 55, wherein the discontinuity has a size
that is less than a thermal diffusion length associated with the
thermal process.
57. The method of claim 55, wherein the discontinuity comprises at
least one of a slit, a rib, a recess, a protrusion, or a hole.
58. The method of claim 55, wherein the discontinuity has a
dimension of about 2 mm or less.
59. The method of claim 51, wherein heating the object comprises
scanning the surface of the object with a laser.
60. The method of claim 51, wherein heating the object comprises
illuminating the object with at least one lamp.
61. The method of claim 51, wherein the thermal transfer plate is
positioned such that the plate and a surface of the object are
approximately parallel to one another and define a gap.
62. The method of claim 61, wherein the gap is partially bridged by
at least one of an offset or a protrusion.
63. The method of claim 61, wherein the distance of a first portion
of the thermal transfer plate from the object is less than the
distance of a second portion of the thermal transfer plate from the
object.
64. The method of claim 61, wherein the method comprises delivering
a gas into the gap.
65. A system for thermal treatment of an object within a chamber,
comprising: a heating arrangement configured to direct a pulse of
energy towards a surface of an object; and a thermal transfer plate
positioned adjacent to the object, the thermal transfer plate
having a flexibility that is configured to reduce object
deformation during thermal treatment of the object.
66. The system of claim 65, wherein the thermal transfer plate has
a first region and a second region, the flexibility of the thermal
transfer plate at the first region being different than the
flexibility of the thermal transfer plate at the second region.
67. The system of claim 65, wherein the thermal transfer plate has
a stiffness that is optimized to reduce object deformation during
thermal treatment of the object.
68. The system of claim 65, wherein the thermal transfer plate has
a thickness that is optimized to reduce object deformation during
thermal treatment of the object.
69. The system of claim 65, wherein the thermal transfer plate
comprises at least one discontinuity in the thermal transfer
plate.
70. The system of claim 69, wherein the discontinuity has a size
that is less than a thermal diffusion length associated with the
thermal process.
71. The system of claim 69, wherein the discontinuity comprises at
least one of a slit, a rib, a recess, a protrusion, or a hole.
72. The system of claim 69, wherein the discontinuity has a
dimension of about 2 mm or less.
73. The system of claim 65, wherein the thermal transfer plate and
the object are separated by a gap.
74. The system of claim 73, wherein the gap is partially bridged by
at least one of an offset or a protrusion.
75. The system of claim 73, wherein the distance of a first portion
of the thermal transfer plate from the object is less than the
distance of a second portion of the thermal transfer plate from the
object.
76. The system of claim 73, wherein the gap is filled with a
gas.
77. A method, comprising: preparing a thermal transfer plate for
use in a thermal processing chamber used for thermally treating an
object, the thermal transfer plate configured to be positioned
adjacent to the object, the thermal processing chamber configured
to direct at least one pulse of energy toward the object for a
duration of less than 1 second; adjusting the flexibility of the
thermal transfer plate to reduce object deformation during thermal
treatment of the object.
78. The method of claim 77, wherein adjusting the flexibility of
the thermal transfer plate comprises adjusting the stiffness of the
material used to construct the thermal transfer plate.
79. The method of claim 77, wherein adjusting the flexibility of
the thermal transfer plate comprises adjusting the thickness of the
thermal transfer plate.
80. The method of claim 77, wherein adjusting the flexibility of
the thermal transfer plate comprises incorporating a discontinuity
in thermal transfer plate.
81. The method of claim 80, wherein the discontinuity has a size
that is less than a thermal diffusion length associated with the
object.
82. The method of claim 80, wherein the discontinuity comprises at
least one of a slit, a rib, a recess, a protrusion, or a hole.
83. The method of claim 80, wherein the discontinuity has a
dimension of about 2 mm or less.
Description
CROSS-REFERENCE TO OTHER APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 60/686,506, filed Jun. 1, 2005, and hereby
incorporates by reference that provisional patent application in
its entirety.
BACKGROUND OF THE INVENTION
[0002] The production of semiconductor devices requires precise
control of material properties. Often, the production of such
devices involves controlled heating of semiconductor materials. For
instance, during the production process, semiconductor wafers (and
other devices) may be subjected to an ion implantation process or
processes that dope the wafer with impurities. After the ions have
been implanted, the crystal lattice structure of the wafer is
annealed by heating the wafer to a high temperature. Other typical
processes utilizing heat treatment include growth and deposition of
film layers, crystallization, and phase change processes.
[0003] However, heat treatment, especially at higher temperatures,
can have many undesirable side effects. For example, in an
annealing process, the high temperatures may lead to an undesired
diffusion of the dopant atoms, which will lead to unpredicted or
unwanted semiconductor properties. While such diffusion may occur
in the course of any heat treatment process, it may be especially
troublesome when a pulse of energy is used to heat-treat a
wafer.
[0004] If the pulse of energy is delivered to the wafer surface
over a time scale that is short, relative to the time that is
necessary for heat to diffuse through the thickness of the
substrate, then the surface of the wafer will become substantially
hotter than the opposite surface. For a typical silicon wafer that
is .about.775 .mu.m thick, that is at a temperature of
.about.800.degree. C., such surface heating is achieved when the
energy is absorbed within a region that is less than 200 .mu.m
below the surface of the wafer, when the pulse is of less than
.about.20 ms duration. For a large effect, the energy should be
absorbed within less than 20 .mu.m from the surface and the pulse
typically should be less than 5 ms in duration.
[0005] Immediately after the energy from the pulse is absorbed, the
heat diffuses through the thickness of the wafer, raising the
average temperature of the wafer as a result. This temperature rise
occurs over a timescale defined by the rate of heat diffusion
through the thickness of the wafer, which is typically .about.50 ms
for a wafer at 800.degree. C. that is 775 .mu.m thick, as is
typical for 300 mm diameter wafers. Hence pulsed heating produces a
rather rapid rise in the temperature of the whole of the wafer. The
subsequent evolution of the wafer temperature depends on the nature
of the heat loss from the surfaces of the wafer into the
surroundings. For example, if the back of the wafer is held in
thermal contact with a heatsink structure, heat may be conducted
away to the heatsink through the gap between the wafer and the
heatsink. Alternatively, if the wafer is only supported at a few
locations, and there are no cooler surfaces nearby, then heat may
be lost from the surfaces by convection or conduction into the gas
ambient surrounding the wafer, and by the emission of thermal
radiation.
[0006] The rate at which heat is lost from the surface into the
bulk of the wafer is typically very high, because thermal
conduction within a solid is a very rapid and efficient mechanism
of heat transfer. In contrast, transfer of heat from the wafer
surface almost inevitably involves a thermal contact resistance
that impedes the conduction of heat out of the wafer surface into a
surrounding medium, or the relatively inefficient heat transfer
mechanisms of convection or radiation. As a result, the bulk of the
wafer tends to heat up after the pulse, and the thermal exposure
(sometimes referred to as thermal budget) of the delicate
structures within the wafer may be increased to an undesirable
extent. Such thermal exposure may have deleterious effects such as
introducing excessive diffusion of dopant species that have been
introduced into the wafer. As a result there is a need to identify
improved ways of limiting the thermal exposure of the wafer.
[0007] It is often desirable to combine the pulsed heating of the
wafer surface with a second from of heating, called background
heating, that preheats the wafer prior to application of the energy
pulse. Typically such heating is useful because it enables the use
of a lower energy pulse to achieve any desired degree of heating of
the side of the wafer that is exposed to the pulsed heating
process. The magnitude of the temperature pulse associated with the
pulsed heating is also reduced, which leads to reduced stress
within the wafer, as well as reducing the magnitude of any
non-uniformity of heating associated with non-uniformity in the
delivery of the energy pulse to the wafer surface. An example of
the latter arises when the wafer is coated with materials that vary
in composition across the surface that is exposed to pulsed
heating. When such non-uniform coatings are exposed to an energy
pulse, the magnitude of the resulting temperature non-uniformity
decreases as the magnitude of the energy pulse decreases.
Furthermore the ability to preheat the wafer allows the design of
more sophisticated heating cycles, including the possibility of
ramping this background temperature up to a given value and then
applying the surface heating energy pulse. After the pulse, the
background heating can be decreased and the wafer can cool. Various
approaches are described in U.S. Pat. No. 6,849,831 and U.S. Pat.
No. 6,594,446.
[0008] The ability to cool the wafer rapidly after the pulse of
surface heating is clearly important for reducing the thermal
exposure. Methods for limiting thermal exposure and improving
cooling are described in U.S. Pat. No. 6,594,446, U.S. Pat. No.
5,561,735, U.S. patent application Ser. No. 10/629,400 filed Jul.
28, 2003, U.S. patent application Ser. No. 10/706,367 filed Nov.
12, 2003, U.S. patent application Ser. No. 10/646,144 filed Aug.
22, 2003, and U.S. patent application Ser. No. 09/527,873 filed
Mar. 17, 2000.
SUMMARY OF THE INVENTION
[0009] A method of thermally treating an object includes providing
an object in a thermal processing chamber and configuring a thermal
transfer plate such that the plate has a thermal mass (i.e. heat
capacity) that is no greater than about three times the thermal
mass of the object. The thermal transfer plate is positioned in
close thermal communication with the object and, after pre-heating,
the object is heated by directing at least one pulse of energy
toward the object for a duration of less than about 1 s. The
thermal transfer plate enhances heat transfer from the object after
the pulse has ceased by way of thermal conduction and allows for
greater heat transfer away from the object than is available by
radiative cooling of the object alone. In certain embodiments, the
thermal transfer plate has a thermal mass no greater than the
approximate thermal mass of the object.
[0010] The thermal transfer plate, also referred to as a heatsink
herein, may comprise material such as silicon, silicon carbide,
silicon nitride, silicon dioxide, aluminum oxide, sapphire, quartz,
aluminum nitride, boron nitride, aluminum oxynitride, graphite,
carbon, diamond, yttrium aluminum garnet, or other suitable
materials, including ceramics and metals.
[0011] The object can be heated by a pulsed array of lamps, by
scanning with a laser, or by other appropriate heating
methodologies. The thermal transfer plate may be positioned such
that the object rests directly upon it, or such that the plate and
the surface of the object are approximately parallel to one another
and define a gap. In certain embodiments, the gap distance is less
than about 0.2 mm.
[0012] The cooling of the object may be further enhanced by
disposing a material between the thermal transfer plate and the
object, the material being a gas in certain embodiments.
Preferably, the gas has a high thermal conductivity, and may, for
example, comprise helium.
[0013] In some embodiments, different materials may be delivered
into the gap such that the degree of thermal coupling between the
object and the thermal transfer plate differs during the thermal
treatment process.
[0014] The thermal transfer plate may be used for other portions of
the treatment process, such as by pre-heating the object by
emitting heat from the thermal transfer plate as part of the
pre-heating process.
[0015] The thermal transfer plate may comprise additional features,
such as a plurality of holes smaller than about 2 mm in diameter,
or discontinuities in the plate surface, such as slits, ribs,
recesses, protrusions, and holes that are preferably smaller than
the thermal diffusion length associated with the treatment process
in use. Furthermore, the surface of the plate can textured to
further enhance heat transfer between the plate and the object.
[0016] The thermal transfer plate may be constructed in multiple
pieces or parts, and may comprise a variety of materials or
combinations thereof. In certain embodiments, the plate may be
constructed such that it includes a material that undergoes a phase
change at a selected temperature, the temperature selected to
optimize heat transfer from the object.
[0017] The treatment process can further include augmenting the
cooling by active measures, such as directing a cooling gas at the
object or thermal transfer plate, either alone or in
combination.
[0018] As part of the thermal treatment process, the temperature of
the object may be monitored directly or indirectly by monitoring
energy emitted, reflected, transmitted, or otherwise measured from
the object and/or the thermal transfer plate. Such monitoring can
include the use of optical sensors such as pyrometers.
[0019] A system for thermal treatment of an object within a chamber
is disclosed, the system including a heating arrangement configured
to direct a pulse of energy towards a surface of an object, such as
a semiconductor wafer. The energy may be produced utilizing an
array of lamps or a scanning energy source such as a laser. The
system also includes a thermal transfer plate positioned parallel
to a surface of the object, the plate having a thermal mass no
greater than about three times the thermal mass of the object. In
other embodiments, the thermal mass, or heat capacity, of the plate
is no greater than the approximate thermal mass of the object. The
object may rest directly on the plate, or may be spaced apart from
the plate by a defined gap; in some embodiments, the gap distance
is less than about 0.2 mm.
[0020] The gap may be filled, partially or entirely, with a
material selected to enhance the thermal transfer rate between the
object and the heat sink, such as a gas with a high thermal
activity. Helium may be one such gas delivered into the gap.
[0021] The system may further comprise an additional heat source
configured to pre-heat the object. The additional heat source can
in some embodiments be a thermal transfer plate configured to
generate and emit heat, or may comprise a separate source which
pre-heats the object indirectly via the plate.
[0022] The system can further include a measurement device or
devices configured to monitor the temperature of the object based
on energy emitted, transmitted, reflected, or otherwise associated
with the object and/or the thermal transfer plate.
[0023] Embodiments of the method for thermal treatment of objects
may be applicable to processing semiconductor wafers using a pulsed
energy source, and may achieve a specific thermal profile through
selection and placement of a heat sink or thermal transfer plate
having a specified thermal mass and/or other defined thermal
characteristics. Such a method includes placing a semiconductor
wafer into a rapid thermal processing chamber, preheating the
wafer, and subjecting the wafer to at least one pulse of energy for
less than about 1 s, such that the average temperature of the
semiconductor wafer increases by a temperature difference AT
between the average temperature of the wafer just prior to the
pulse (T.sub.1) and the maximum average temperature of the wafer
after the pulse has ceased (T.sub.2). The heat transfer from the
wafer after the pulse is enhanced beyond what cooling could be
achieved otherwise. Such enhancement is obtained by placing a
thermal transfer plate in close thermal communication with the
wafer. The plate may be configured to have a thermal mass no
greater than approximately three times the thermal mass of the
wafer, and be appropriately configured and positioned so that the
average temperature of the wafer decreases from T.sub.2 by at least
50% of the value of .DELTA.T within 1.0 s.
[0024] In certain embodiments, T.sub.1 may be about at least
600.degree. C. and .DELTA.T may be at least 50.degree. C. In other
embodiments, the temperature of the wafer decreases by 50% of
.DELTA.T within 0.5 s, and in still further embodiments within 0.3
s.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] A full and enabling disclosure of the present invention,
including the best mode thereof, directed to one of ordinary skill
in the art, is set forth more particularly in the remainder of the
specification, which makes reference to the appended figures, in
which:
[0026] FIG. 1 illustrates a side view an exemplary processing
chamber including a thermal transfer plate;
[0027] FIG. 2 illustrates another side view of an exemplary
processing chamber including a thermal transfer plate;
[0028] FIG. 3 illustrates a coaxial view of an exemplary thermal
transfer plate;
[0029] FIG. 4 illustrates a coaxial view of another exemplary
thermal transfer plate;
[0030] FIGS. 5a and 5b depict side views of still further exemplary
thermal transfer plates; and
[0031] FIGS. 6-9 depict simulated temperature profiles over time
for thermal processing systems.
DETAILED DESCRIPTION OF THE INVENTION
[0032] Reference will now be made in detail to embodiments of the
present invention, one or more examples of which are illustrated in
the accompanying drawings, with like numerals representing
substantially identical structural elements. Each example is
provided by way of explanation, and not as a limitation. In fact,
it will be apparent to those skilled in the art that modifications
and variations can be made without departing from the scope or
spirit of the disclosure and claims. For instance, features
illustrated or described as part of one embodiment may be used on
another embodiment to yield a still further embodiment. Thus, it is
intended that the invention disclosed herein includes modifications
and variations as come within the scope of the appended claims and
their equivalents.
[0033] Several approaches may be of interest in the context of more
precisely controlling thermal budget during pulsed surface heating
cycles. It should be recognized that these approaches will be
useful regardless of how the pulsed heating is implemented, and
hence the methods may be used in combination with a wide variety of
heating sources, modes of processing and heating configurations.
For example they can be applied to pulsed heating with energy from
lamps, lasers, other electromagnetic energy sources (e.g. RF,
microwave, millimetre-wave, THz radiation sources), particle beams,
gas flows, plasmas, flames, chemical reactions, phase changes etc.
They can also be applied regardless of how the energy impinges on
the surface of the wafer. For example the energy may be applied to
the whole of a wafer surface, or to selected regions of the
surface. It may also be applied by scanning an energy source across
the wafer surface, or by exposing the wafer to a series of pulses
in localized regions. The pulse duration can range from 10.sup.-15
s to .about.1 s, with typical applications involving pulses with
durations between 10 .mu.s and 10 ms. The peak temperatures of the
wafer surface would lie between 50.degree. C. and 2000.degree. C.,
with typical applications involving processing between 900.degree.
C. and 1400.degree. C.
[0034] The efficiency of heat transfer from the wafer can be
increased by a variety of means. For example, if the wafer is held
in close proximity to a heatsink that is maintained at a given
temperature, then the conductive heat transfer between the wafer
and this heatsink can be improved by introducing a material with a
high thermal conductivity between the wafer and the heatsink. For
example, a good thermal bond can be introduced by a metallic
element or alloy. This is especially true for materials that are
soft and easily deform to fill in the space between the wafer
surface and the heatsink surface, thus accommodating any surface
roughness or non-planarity of either the wafer or the heatsink.
Similar benefits arise from using a material that is liquid at the
temperature of interest. Improved thermal contact can also be
achieved through applying pressure to the wafer or heatsink,
forcing them into more intimate contact. Such pressure may arise
from mechanical clamping, inertial forces, electrostatic clamping
or by gas pressure or by a vacuum chuck.
[0035] Although these approaches may improve conductive heat
transfer, they also introduce some problems. For example, many of
the best solids or liquids that could improve heat transfer between
the wafer and the heatsink, such as liquid or soft metals, may tend
to react with the wafer or contaminate it with undesirable
impurities. Furthermore, most materials may leave some residue on
the back or edges of the wafer after processing, which would have
to be removed in a cleaning process. Although cleaner materials may
be found that leave little contamination on the wafer, for example
elastomer materials, the temperature range of their use may be
quite restricted for some applications.
[0036] An alternative, discussed in more detail below, is to employ
a gas to transfer the heat. For example, by introducing a gas of
high thermal conductivity, such as a gas with a conductivity
greater than about 0.1 Wm.sup.-1K.sup.-1 into the gap, then some of
the benefits of improved heat transfer can be retained. Suitable
gases can include He, H.sub.2, D.sub.2 and mixtures thereof. Other
gases, including lower thermal conductivity gases such as N.sub.2,
O.sub.2, H.sub.2O, D.sub.2O, NH.sub.3, N.sub.2O, NO, HCl,
SiH.sub.4, Si.sub.2H.sub.6, GeH.sub.4, CH.sub.4, CF.sub.4,
C.sub.2H.sub.6, C.sub.2H.sub.2, C.sub.2H.sub.4, C.sub.3H.sub.8,
CO.sub.2, CO, NF.sub.3, Ne, Ar, Kr and Xe, could also be used, for
example in mixtures for controlling the thermal conductivity of the
mixture or to "switch" the heat transfer conditions and hence
adjust the wafer temperature. The gases may be used to affect the
heat transfer conditions between the wafer and the plate, or
between the wafer and the chamber, but they can also be used to
assist with processing the wafer. For example they can be used to
assist with oxidation, nitridation, silicidation, compound
formation, deposition of films, etching, reflow, annealing,
altering surface topography etc. They can also be used to assist
with cleaning the wafer or even other components in the process
chamber. They can be used in mixtures, including mixtures such as
O.sub.2 and H.sub.2 that can react to form water vapour, or
mixtures that can promote deposition of films, for example by
chemical vapour deposition. Small concentrations of O.sub.2, for
example, between 100 ppb and 10% may be combined with a second gas,
such as N.sub.2 or Ar, in order to help to control surface
reactions during high temperature processes, for example by forming
very thin oxide layers or by preventing thermal etching or dopant
loss from the surface of the wafer. The gases can also be combined
with chemically reactive species formed by delivering radiation or
electromagnetic energy to the processing environment. For example
radicals or ions may be formed by UV radiation or by generating a
plasma. The gases in the chamber can be at elevated pressure (e.g.
10 atmospheres), atmospheric pressure, or at reduced pressure, e.g.
down to pressures as low as 0.1 Torr. However, thermal conduction
in gases becomes inefficient at pressures below .about.1 Torr, and
the method described here will be most effective above this
pressure. The gases, their mixture and pressure can also be
optimized to provide the best mechanical damping or cushioning with
regard to the thermal deformation experienced by the wafer during a
pulsed heating process. For example, this may be done by selecting
optimal density and viscosity of the gas or gas mixture.
[0037] The heatsink itself may be made of a thermally conductive
material, such as a metal, a semiconductor or a ceramic such as
SiC. A high thermal conductivity helps to even out the temperature
across the surface of the heatsink, hence leading to a more uniform
temperature distribution across the wafer. It should be recognized
that although we refer to a "heatsink" the heatsink may also act as
a heat source for providing background heating as described above.
In this context, the heat-sinking action would be more associated
with absorbing the heat that was introduced by the pulsed heating
process, which would otherwise raise the temperature of the wafer.
The heatsink can include heating elements, for example resistive
heating elements, and these can be arranged in separate
independently controllable zones if it is desirable to tune the
temperature uniformity of the heatsink. Furthermore, the heatsink
can also include cooling channels if it is desired to extract large
amounts of heat from the heatsink structure in a controllable way.
The heatsink can contain both heating and cooling channels for
optimal control of the temperature pattern and the thermal response
of the heatsink structure. The heatsink can also contain elements
for improving the wafer support, such as a series of passages for
flowing gas or drawing vacuum. The flow of gas or application of
vacuum through such passages can assist with the wafer support as
needed. Furthermore, the heatsink can include an electrostatic
chuck or other clamping means.
[0038] Optimization of heat transfer out of the wafer may also be
useful in other ways. One difficult problem encountered when wafers
are processed by the use of energy beams scanned across the wafer
surface arises because of the thermal discontinuity at the edge of
the wafer. Because a scanned beam inevitably generates a lateral
temperature gradient parallel to the wafer surface there is always
some lateral heat flow in this direction. As a result, as the beam
approaches the edge of the wafer there is a "pile up" of heat near
the edge, which can result in very large temperature non-uniformity
or even wafer damage. One approach to improve this is to avoid
scanning to the edge of the wafer, or to mask the edge from
irradiation, but these approaches can also degrade uniformity or
limit the region that can be processed. Another approach is to
increase the scan velocity, so that the heat flow takes on a more
"one-dimensional" pattern with thermal conduction through the wafer
thickness dominating the heat flow. One criterion for ensuring
operation in the 1-D heat flow regime is to make sure that the
dwell time of the scanning beam is much smaller than the time for
lateral diffusion over the width of the beam. For a beam of width
2b, and a scan velocity of V, the dwell time is .tau..about.2b/V.
The time taken for heat diffuse across the width of this beam can
be estimated as .about.(2b).sup.2/D.sub.w, where D.sub.w is the
thermal diffusivity of the wafer at the background temperature of
interest. Hence we reach a criterion
.tau.>>(2b).sup.2/D.sub.w. This can also be recast as a
criterion relating the minimum scan speed to the dwell time,
V>>(D.sub.w/T).sup.1/2. For silicon at .about.800.degree. C.,
D.sub.w.about.10.sup.-5 m.sup.2/s, so for a heating process with a
dwell time of 1 ms, V should be greater than .about.10 cm/s. The
criterion can also be stated as a criterion on scan velocity for
any given beam width, V>>D.sub.w/(2b). For a beam width of 10
.mu.m, V should be greater than 100 cm/s, whereas for a beam width
of 200 .mu.m, V should be greater than 5 cm/s. We should note in
this discussion that many beams used for processing may not have
sharp edges, and may for example have a Gaussian intensity profile.
In such cases, beam width can be defined as a region containing the
majority of the energy of the beam. For a Gaussian beam this could
be the 1/e intensity width or the 1/e.sup.2 intensity width, for
example. Although the use of relatively high scan velocities and/or
beam widths approximates a 1-D heat flow pattern, there is a
penalty that the use of higher scan speeds or beam widths requires
higher beam power in order to achieve the same peak temperature
rise. This may be impractical, or it can limit the size of the
region that can be covered in a scan. An alternative approach is to
exploit the idea of enhancing heat transfer out from the wafer as
discussed here.
[0039] In particular, it may be useful to enhance the heat transfer
from the edge region in order to reduce the tendency for
overheating near the edge of the wafer. For example, if the wafer
is held on a heatsink that also serves to provide preheating, then
the temperature distribution and heat flow in the heatsink can be
tailored to enhance cooling at the edge of the wafer in the
position that the beam would otherwise cause overheating. Another
approach can involve providing better thermal contact between the
edge of the wafer and a heatsink structure located in close
proximity with the edge of the wafer. Any of the approaches
discussed here can be used to enhance the lateral heat transfer
from the wafer edge into a heatsink structure.
[0040] One difficulty with removing heat from the edge of the wafer
arises from the thermal expansion of the wafer. If the wafer is
loaded onto a heatsink that is used for preheating the wafer will
expand to a degree that depends on the preheat temperature
selected. Furthermore, there may be variation in the diameter of
wafers. A fixed heatsink structure may be designed to fit the size
expected for the wafer at the temperature of interest, but there is
a risk that the wafer may become too large and press against the
heatsink and break. Also there is a risk that the wafer is slightly
too small and does not make adequate contact with the heatsink
structure, causing inadequate cooling of the edge. These problems
may be alleviated by using a compressible medium between the
heatsink and the wafer, which can accommodate the size mismatch.
Alternatively the heatsink could be spring loaded so that it
maintains a constant pressure against at least the part of the
wafer where it is needed. That part may not be the whole periphery
of the wafer, since typically a scanning beam would not encounter
the whole edge of the wafer at the same time.
[0041] We should also note that it is possible that in some cases,
the reverse condition may be necessary, e.g. at locations where the
heat transfer condition can cause underheating of the wafer. Such
conditions may arise where a scan starts, because of the absence of
lateral heat transfer "ahead of the scanning beam." Such
non-uniformities may be at least partly compensated for by
adjusting the temperature or heat flow distribution in the heatsink
structure, and by employing a variable degree of heatsink at the
wafer edge as necessary.
[0042] Although such approaches may be useful when a wafer is held
on a heatsink, they are more difficult to apply to the case where
the wafer is to be preheated or cooled at a high ramp rate, because
a heatsink structure often may have a large thermal mass, which
prevents the possibility of very fast changes in the background
heating conditions. Although it is possible to provide improved
cooling in this case by various means described in the patent
applications mentioned above, or by other means such as the use of
endothermic reactions at the wafer surfaces, further improvements
are desirable.
[0043] An alternative approach is to use a much smaller thermal
mass plate-like object as a "dynamic" heatsink that is also heated
with the wafer. The wafer can sit on top of this plate, and it can
either be in close proximity to the plate, or it can be held
slightly above the plate, for example by standoffs or by a
gas-cushion etc. It is not necessary that the plate acts as the
mechanical support for the wafer, although such a configuration is
possible.
[0044] This approach of using various heatsink plates (also
referred to herein as "thermal transfer plates") will be described
in more detail, with respect to the issue of controlling the
thermal exposure of the wafer after surface heating of one side of
the wafer. In one configuration, the heatsink plate is below the
wafer and the top surface of the wafer is exposed to pulsed
heating. If the heat transfer between the heatsink plate and the
wafer is very efficient, then heat will rapidly pass from the wafer
into the support plate, which will tend to raise the temperature of
the heatsink plate while cooling the wafer.
[0045] FIG. 1 is a side view of an exemplary rapid thermal
processing (RTP) chamber including reflective walls 10, lamps 12a
and 12b, optical windows 14a and 14b, and wafer 16. Sensors 30e and
30a monitor the temperature of the wafer and the thermal transfer
plate, and may comprise, for instance, optical pyrometers
configured as known to one of ordinary skill in the art. Sensor 30c
monitors the pulse radiation incident on the wafer. Sensor 30d
monitors radiation reflected or scattered from the wafer and sensor
30b monitors radiation transmitted by the wafer. These sensors can
be photodetectors or thermal detectors. Wafer 16 is separated from
thermal transfer plate 20 by a gap 19 in this illustration, and is
surrounded at least in part by edge ring 18. The skilled artisan
will recognize that the edge ring 18 may include aspects presently
disclosed in conjunction with the thermal transfer plate 20. For
instance, ring 18 may contain cooling and/or heating features.
[0046] Depending on the composition of the system, gap 19 may vary
in size or be omitted entirely (i.e. when the wafer sits directly
on the thermal transfer plate). Gap 19 may be bridged partially or
entirely by offsets, protrusions, or other features of the thermal
transfer plate such that certain portions of the thermal transfer
plate may be closer than others. Accordingly, as discussed herein,
gap 19 refers generally to the distance between the wafer and the
thermal bulk of transfer plate 20.
[0047] The skilled artisan will appreciate that the RTP chambers
suitable for use with the present invention may include other
additional components, such as gas flow valves, inlets, exhausts
and other regulation components, and controllers such as computers
for monitoring and controlling the treatment process. For instance,
as shown in FIG. 1, the RTP chamber further includes appropriate
components for directing gas flow across the wafer and through the
gap 19.
[0048] FIG. 2 shows a side view of a second exemplary arrangement
of an RTP chamber including a thermal transfer plate 20-1
comprising gas passages 22. Additionally, a gas flow/vacuum system
is configured to direct gas through the passages 22 to and from the
gap 19. The gas flow/vacuum system may furthermore apply a vacuum
to the gap region 19 to remove gas. As will be discussed in detail
below, such gas may be used to manage the thermal conductivity
between the wafer and the thermal transfer plate. FIG. 2 also
illustrates a radiant energy source 12', rather than an array of
lamps, which is configured to heat the wafer 16 by scanning its
surface.
[0049] FIG. 3 depicts an exemplary thermal transfer plate 20-2 as
viewed coaxially, rather than from the side. Thermal transfer plate
20-2 includes gas passages 22, along with stand-off features 26,
which may comprise ribs, islands, or other protrusions of material
from the surface of the plate. Additionally, recessed regions 24
are scattered throughout the surface of the plate, some of which
surround certain gas passages 22. Recessed regions 24 may be
configured to alter cushioning effects of gasses passing through
passages 22 and into the gap 19 between the thermal transfer plate
and the wafer. Additionally, as discussed below, loading recesses
or slots 25 may be configured to allow for easier loading or
unloading of wafers by allowing clearance space for loading
apparatus, such as robotic arms.
[0050] FIG. 4 depicts another exemplary thermal transfer plate
20-3, again viewed coaxially. Plate 20-3 features gas passages 22
and a number of stand-off features 26, in this embodiment, a
plurality of ribs. The features illustrated in the plate 20-2 of
FIG. 3 and the plate 20-3 as shown in FIG. 4 may be the same on
both sides of the plate, or may differ, depending upon the thermal
characteristics desired for the RTP process. We should also note
that although the density of the holes, ribs recesses etc. is shown
as fairly even in the figures, their density can be varied across
the surface. This may be useful for varying the wafer support
positions and the damping/cushioning effect of the gas layer
between the wafer and the thermal transfer plate which may be
useful to control the deformation of the wafer and the plate in
response to the pulsed heating process.
[0051] Likewise the density of rib features and protrusions can be
varied. This may be useful for varying the degree of flexibility of
the plate, for example making a region of it more easily deformed
than a second region of it. In general we should note that the
flexibility of the plate can be optimized by selecting the
material, thickness, surface coatings and construction (including
the ribbing features and holes etc.). Varying the density of the
plates features can also help promote thermal uniformity, for
example by allowing variation in the optical properties across the
surface of the thermal plate, which can affect how it absorbs or
radiates optical or thermal energy.
[0052] The optical properties of the plate can also be varied by
applying coatings to all or parts of the plate's surfaces. For
example, thin film coatings can be applied locally to a region of
the plate to make it hotter or colder than a second region.
Likewise, variations in the features of the plate, or even the
materials used in construction of the plate, across its surface can
be used to locally adjust the thermal mass per unit area of
different regions of the plate. In the simplest case, the thermal
mass per unit area is constant, and for most designs this would be
kept similar to the thermal mass per unit area of the wafer (a
typical range might be from 0.5 to 3 times the thermal mass of the
wafer). However in some cases, the thermal mass may be higher in
order to absorb more heat, or lower to absorb less heat. Such
variations could be tuned to optimize the processing uniformity. We
may note that the thermal mass per unit area of a plate of a
thickness D.sub.plate, with a specific heat capacity C.sub.plate
and a density .rho..sub.plate is given by
.rho..sub.plateC.sub.plateD.sub.plate. Hence the thermal mass per
unit area can be conveniently varied (either locally or across the
whole of the plate) by varying any of these three parameters. It
can also be varied by using additional layers or protrusions of
other materials to vary the properties, or by introducing holes or
gaps that reduce the average local thermal mass.
[0053] FIGS. 5a and 5b show side views of still further alternative
embodiments of thermal transfer plates, in this case illustrated as
20-4 and 20-5, respectively. Both plates include a plurality of gas
passages 22, stand-off features 26, and recessed areas 24, while
FIG. 5b also includes a loading recess 25. Plates 20-4 and 20-5
further include recessed gas reservoir regions or blind holes 28,
which may further alter the cushioning effect of the plate.
Additionally, although not shown, reservoir regions 28 may be
filled with a selected material and then sealed off in order to
alter the thermal response of the transfer plate.
[0054] Operation of an RTP system using thermal transfer plates as
discussed above will now be discussed in conjunction with the
simulated temperature results shown in FIGS. 6 to 9.
[0055] FIG. 6 illustrates the concept with results from a
simulation of the temperature-time cycles experienced by a wafer
during a pulsed heating process both with and without a heatsink
plate. Curve A shows the evolution of the average wafer temperature
when it is processed without the plate. At the start of the heating
cycle the wafer temperature is ramped up to 800.degree. C., and
then the pulse of energy is applied, and immediately after this all
the heat sources are deactivated. The temperature profile shown
does not directly show the surface heating itself, but the step up
in average temperature at the point marked P indicates a relatively
rapid rise in the average temperature that occurs shortly after the
pulsed heat is delivered and diffuses through the wafer thickness.
In the example considered here the resulting rise in bulk
temperature is 100.degree. C., so that the wafer is at 900.degree.
C. just after the heat from the pulsed heating process has spread
through the thickness of the wafer. After the pulsed heating, it is
assumed that the wafer cools by thermal radiation.
[0056] Curve B shows the trend of the wafer temperature when a
heatsink plate 20 is present. In this case it was assumed that the
heatsink plate had the same thermal properties and the same
thickness as the wafer (775 .mu.m), and that the space 19 between
the wafer and the heatsink plate 20 was 100 .mu.m and that it was
filled with a medium with a thermal conductivity of 0.5
Wm.sup.-1K.sup.-1. This medium could, for example be a gas with a
fairly high thermal conductivity introduced, for example, via gas
passages 22 as shown in FIGS. 2-5, or otherwise directed into the
space 19. Curve C shows the temperature-time profile for the
heatsink plate itself, which matches that of the wafer during the
ramp up. We see from curve B that when the heatsink plate is
present, the initial cooling of the wafer from 900.degree. C. is
greatly accelerated, as a result of the rapid conduction of heat
from the wafer into the heatsink plate. Curve C shows how the
heatsink plate temperature rises as it absorbs heat from the wafer,
and then the wafer and the heatsink plate reach a common
temperature, after which point they cool at the same rate. We also
see that the radiative cooling rate of the wafer is reduced by the
heatsink plate, because the combination of the wafer and the
heatsink plate has twice the thermal mass of the wafer alone. Hence
the thermal exposure is actually greater, e.g. as the wafer cools
to temperatures below .about.800.degree. C. in this example.
[0057] Despite this, the operation of the heatsink plate may still
present advantages, because many of the undesirable processes that
occur at high temperature, such as diffusion of dopant atoms, are
thermal activated processes whose rate has an exponential
dependence on temperature. As a result rapid cooling at higher
temperatures is often much more important than the cooling rate at
lower temperatures.
[0058] For fast cooling of the wafer, it is important that there be
efficient heat transfer from the wafer to the heatsink plate.
Although some heat will be transferred by thermal radiation, the
most effective approach is to rely on thermal conduction through
the medium between the wafer and the heatsink plate. Thermal
conduction is enhanced by increasing the thermal conductivity of
the medium between the wafer and the heatsink plate and by
decreasing the size of the gap (19) between the two. FIG. 7
illustrates the effect of varying the size of the gap 19. In this
case the medium is assumed to have a thermal conductivity of 0.5
Wm.sup.-1K.sup.-1 as in FIG. 6. Curve A is the temperature-time
profile for a wafer without a heatsink plate, curve B is for the
case where the heatsink plate is 0.1 mm away from the wafer and
curve C is the case where the heatsink plate is 0.2 mm away from
the wafer. Curves D and E are the temperature cycles for the
heatsink plates when they are 0.1 or 0.2 mm away from the wafer,
respectively. FIG. 7 shows that although the initial cooling rate
is still enhanced when the plate is 0.2 mm from the wafer, the
cooling efficiency is decreased as a result of the reduced rate of
heat conduction across a larger gap.
[0059] FIG. 8 shows the effect of the choice of the thermal
conductivity of the medium between the wafer and the heatsink
plate. Curves A, B, and D are the same as for FIG. 7. Curves C and
E are the wafer and plate heating cycles respectively, for the case
where the medium between the wafer and the plate has a thermal
conductivity of only 0.05 Wm.sup.-1K.sup.-1. In this comparison we
see that introducing the heatsink plate has slowed down the cooling
rate. In order to use the heatsink plate concept in a medium with
such a low thermal conductivity, the plate would have to be brought
much closer to the wafer in order to increase the conductive
coupling.
[0060] The degree of thermal exposure experienced by the wafer will
also be affected by the properties of the heatsink plate. If the
heatsink plate is made very thin or of low heat capacity, then it
cannot absorb much of the extra heat introduced into the wafer by
the pulsed heating and it does little to limit the temperature
experienced in the wafer. FIG. 9 illustrates the issue by showing
the behaviour for a case where the heatsink plate was chosen to be
100 .mu.m thick. Curves A, B and D are the same as for FIGS. 7
& 8. Curve C is the wafer response with the heatsink plate
present, and curve E is the corresponding response of the plate
itself Introducing the plate has provided a small improvement in
the initial cooling rate, but the cooling rate becomes lower than
that of a wafer alone once the temperature has dropped below
.about.830.degree. C. The loss of cooling efficiency arises because
the thin heatsink plate heats up very rapidly as heat transfers to
it, as shown in curve E.
[0061] Although the initial cooling rate can be enhanced by using a
thick heatsink plate or one with a large heat capacity, the ability
to ramp the wafer up and down in temperature becomes limited,
because of the large thermal mass of the heatsink plate. In
practice it may be useful to accelerate the cooling rate of the
wafer and the plate above that which is possible by radiation
cooling alone. For example, a flow of gas directed at the wafer
and/or at the thermal transfer plate can increase the rate of heat
loss above that of radiation cooling alone. This gas flow can be
applied throughout the whole heating cycle, or it can be initiated
in timed relation to the preheating or pulsed heating steps. The
gas cooling can be applied by increasing the rate at which gas
flows into the chamber, or by flowing a gas that impinges directly
on the surface of the wafer and/or on the heat transfer plate. For
example a showerhead arrangement or an array of gas pipes can be
used to direct gas cooling at the surfaces of the wafer and/or the
thermal transfer plate. Alternative cooling mechanisms can also be
used for cooling either the wafer or the heat transfer plate. For
example, the cooling can be with a flow of a liquid, or a medium
that experiences a phase change or an endothermic chemical reaction
that can absorb thermal energy from the heat transfer plate or the
wafer. In some cases such cooling may be more easily applied to the
thermal heat transfer plate than the wafer, because the wafers
surfaces must be kept very clean and uncontaminated with foreign
materials. The heat transfer plate can also be designed to
accommodate such cooling features that increase the rate of heat
transfer from it and hence accelerate its cooling. The fact that
the wafer and the heat transfer plate are in close thermal
communication means that if the heat transfer plate can be cooled
rapidly, then it in turn will cool the wafer rapidly. The heat
transfer plate can even have connections that directly couple the
flow of cooling media to its surfaces or its interior. For example
it can have cooling channels within it. It can also have electrical
connections if desired, which may permit the use of thermoelectric
cooling methods. Such connections would also allow for including
electrical temperature sensors such as thermocouples or temperature
sensitive resistors. The various features of the heat transfer
plate, such as the ribs described above can also be used to enhance
its heat loss, for example by acting as cooling fins which increase
the surface area available to dissipate heat. They can also
increase the thermal emissivity and emitting area to increase the
rate of radiation heat loss.
[0062] In general the thickness or heat capacity of the heatsink
plate can be optimized with respect to the type of thermal cycle
that is desired. The nature of the optimal thermal cycle will
generally depend on the kinetics of the physical phenomena taking
place during the thermal processing. An extra degree of
optimization is possible by controlling the rate of heat transfer
between the wafer and the heatsink plate, for example by
controlling the gap between the two and the nature of the gas
ambient between the two.
[0063] In some embodiments, it is not necessary for the heatsink
plate to be ramped to the same temperature as the wafer before the
pulsed heating is applied. Indeed, the rate of heat loss from the
wafer can be increased by keeping the heatsink plate at a
relatively low temperature compared to the wafer. This may help in
reducing the need for close physical proximity between the wafer
and the heatsink plate and the need for a high thermal conductivity
medium between the two. For instance, a cooling gas may be directed
toward or through the heatsink structure, such as via channels 22.
Alternatively, the heatsink may include separate internal channels
linked to a cooling system, such as the type of structure used in
water-cooled lamps and the like. One complication with such
approaches is that it may require separate control of the
temperature of the wafer and the heatsink plate, for example by
measuring their temperatures with pyrometers and controlling the
heating and cooling sources accordingly.
[0064] If desired, the radiative properties of the heatsink plate
can be optimized to ensure a different thermal response to that of
the wafer. For example, the emissivity or absorptivity of the
heatsink plate can be optimized to be greater or smaller than those
for the wafer as desired. As noted previously, one approach for
ensuring a different thermal response between the wafer and the
plate can involve including a material within the heatsink plate
that undergoes a phase change at a predetermined temperature in
such a volume that it pins the temperature of the heatsink plate at
that predetermined temperature. For instance, such a material could
be sealed within reservoirs 28 of the heatsink structure, or could
be otherwise incorporated into a chamber or other internal
structure. Application of more heating to the plate would then
merely drive the phase change while the plate temperature remained
constant. This approach reduces the demands on the control
system.
[0065] One advantage of using a configuration where the wafer and
the heatsink plate are in close thermal contact is that the
heatsink plate can also serve as a susceptor plate. If the thermal
coupling between the wafer and the heatsink plate is strong enough,
then the temperature of the heatsink plate can be used as an
indicator of the wafer temperature. This may simplify the
temperature measurement approach used for control of the process
temperature. For example wafer back surfaces are often coated with
films that affect their emissivity. As a result of this, pyrometric
measurements of wafer temperature are often prone to error unless
extensive precautions are taken to correct for emissivity
variations. Since the heatsink plate can be made of a known
material with a well characterized emissivity, it is possible to
measure its temperature accurately with a pyrometer. If the thermal
coupling between the wafer and the heatsink plate is such that only
a small temperature difference can emerge between them during the
period before the pulsed heating is applied, then the wafer
temperature can also be closely controlled in this part of the
heating cycle. This approach provides a simple approach for very
repeatable preheating of the wafer. Even if the thermal coupling
between the thermal transfer plate and the wafer is not perfect, it
is possible to deduce the wafer temperature by measuring the
temperature of the thermal transfer plate and then estimating the
wafer temperature from a thermal model. This model can take account
of the degree of thermal contact between the wafer and the thermal
transfer plate, the nature of the medium between the wafer and the
plate, and the optical or thermal energy entering or leaving the
surfaces of the wafer and the thermal transfer plate. Estimates of
the optical energy entering or leaving the wafer can be made more
accurate through the use of measurements from sensors such as 30b,
30c and 30d.
[0066] An alternative approach for ensuring that the heatsink plate
and the wafer are at similar temperatures is through the use of
dual sided illumination, such as illustrated in FIG. 1. If the
heatsink plate and the wafer have similar thermal masses then when
they are illuminated with similar power densities from a dual sided
heating system they will tend to follow similar thermal cycles.
Lamps 12b may be controlled to pre-heat the wafer and thermal
transfer plate. This preheating can also be combined with delivery
of energy from lamps 12a. Then, a heat pulse may be provided using
lamps 12a or another appropriate energy source. A combination of
dual sided illumination and close thermal contact will provide even
closer tracking of the temperatures of the wafer and the heatsink
plate up until the pulsed heating is applied to the wafer.
[0067] One difficulty with the use of a heatsink plate arises from
the need to load the wafer onto it. Close physical proximity
prevents the use of a wafer loading mechanism that requires access
between the plate and the wafer. The wafer could be loaded onto the
plate through the use of a set of pins that pass through holes in
the heatsink plate 20. The wafer also can be loaded onto the plate
directly if the wafer is held at its edges with a mechanism that
allows the wafer to be positioned over the plate and then lowered
onto it. The heatsink plate could be adapted to facilitate such a
mechanism, such as via the loading recesses or slots 25 illustrated
in FIGS. 3 and 5b. The wafer could also be supported by an approach
that holds it from above, such as a Bernoulli pick-up approach.
Another approach can include loading the wafer onto a supporting
element that holds the wafer near its edges and then bringing the
heatsink plate into close proximity with the wafer. The component
that the wafer sits on can be very thin, so that it fits between
the heatsink plate and the wafer. Alternatively the heatsink plate
can have cut out sections so that it doesn't interfere with the
wafer support.
[0068] An alternative approach is to make the heatsink plate out of
more than one part, so that it can be "pieced together" after the
wafer is loaded onto part of it. For example, one part of the
heatsink plate may have features for supporting the wafer at
defined locations, such as the islands and ribs generally denoted
as protrusions 26 in FIGS. 2-5. The protrusions can be designed to
not interfere with the wafer loading mechanism. Once the wafer is
loaded onto the supporting portion of the heatsink plate, the
second part of the heatsink plate (and any further parts) can be
put in place to complete the heatsink assembly.
[0069] Another difficulty may arise when unloading the wafer from
the heatsink plate after processing as a result of the tendency of
the wafer and the plate to "stick together" because of the
relatively small gap between them, which impedes the flow of gas
that is necessary to pull them apart. This problem may be
alleviated by maintaining sufficient gap between the two, but that
approach may compromise the cooling efficiency. Another approach
would be to provide a series of holes in the heatsink plate which
allow gas to flow through it, such as gas flow passages 22 and
recessed areas 24 as shown in FIGS. 2-5. However, the number and
size of the holes should be small enough to not affect the
temperature uniformity on the wafer during the heating cycle. This
is possible if the size of the holes is kept small enough that
thermal diffusion can even out any temperature non-uniformity that
tends to arise in the wafer, for example as a result of uneven heat
transfer to the heatsink plate in the vicinity of the holes. This
criterion can be met by making the hole size of approximately 2 mm
or smaller, preferably less than 1 mm in size. The best uniformity
can be achieved if the features are made small compared to the
thermal diffusion length for the heating process of interest. This
rule can be applied to any features in the heatsink plate,
including those introduced to simplify loading or unloading of the
wafer or the plate. It also can apply to any gaps between parts of
any heatsink assembly that is pieced from sub-elements. It can also
apply to the dimensions of protrusions 26 on the plate that may,
for example, be used to provide stand-off features that set a gap
between the wafer and the plate. We can also note that an approach
of forming holes within the heatsink plate can also be used to
optimize the thermal mass of the plate, and to affect its radiative
properties.
[0070] Another issue that may be important arises from the tendency
of the wafer or the plate to deform during thermal processing. The
presence of the plate will affect this behaviour, and the design
can be optimized to take account of the deformation behaviour. The
use of a plate in close proximity to the wafer may reduce the
tendency of the wafer to deform as a result of the thermal stresses
generated during the heating process. The plate design can even be
optimized to reduce deformation, for example by selecting a
material that is relatively stiff or making the plate relatively
thick. Other features can be used to determine the flexibility of
the plate, such as ribs, recesses, protrusions or slits. Of course,
the nature and dimensions such features would have to be optimized
in conjunction with the thermal operation of the plate. For
example, they can be made smaller than the thermal diffusion length
associated with the process of interest, so that they do not
degrade the uniformity of the thermal processing.
[0071] The heatsink plate can be constructed of a variety of
materials, and its physical, thermal and optical properties can be
optimized. One convenient choice for the plate is silicon, on
account of its close match to the wafers properties, its good
mechanical and thermal characteristics, and its high purity. The
doping of the silicon can be selected to optimize the heat transfer
characteristics. For example, the use of heavily doped silicon,
with a resistivity below 0.1 .OMEGA.cm, can be employed in order to
make the plate relatively opaque in the near infra-red, which may
be useful if the plate's temperature is to be monitored with a
pyrometer operating at a wavelength greater than .about.1 .mu.m. In
contrast, if it is desirable to detect infra-red radiation that
either emanates from the wafer or is transmitted by the wafer, for
example to monitor the wafer temperature, then a lightly-doped
silicon plate, with a resistivity greater than 0.1 .mu.cm, which is
transparent in the near infra-red region, could be employed.
However, other materials may also be used, such as silicon carbide,
silicon dioxide, silicon nitride, aluminium oxide, sapphire,
quartz, aluminium nitride, boron nitride, aluminium oxynitride,
graphite, carbon, diamond, yttrium aluminium garnet (YAG) or other
ceramic materials of high purity. The plate may also be coated on
any of its surfaces with layers that can enhance its properties.
For example if the bulk material of the plate is not believed to be
of sufficient purity or chemical resistance to the processing
ambient, then it may be coated with a layer that serves as a
barrier to out gassing of impurities or to chemical attack.
[0072] Coatings and surface modifications may also serve to
optimize the thermal or optical properties of the plate. For
example, if a pyrometer is used for measuring the temperature of
the plate, then it may be useful to use a coating to form an
antireflection layer for the wavelength of radiation that the
pyrometer detects as a means for sensing temperature. This approach
increases the emissivity of the plate, which can help to reduce
errors in the temperature measurement that arise from reflection of
stray radiation into the pyrometer system, as well as helping to
define a high value of emissivity, which increases the signal
detected by the pyrometer and improves accuracy. Optimization of
the optical properties of the heatsink plate may also be useful in
other ways. For example, the selection of the material and coatings
can assist in defining the nature of the heating cycle. For
example, selection of a material that is at least partially
transparent to part of the radiation employed for heating would
tend to reduce the temperature it reaches during the heating cycle
relative to that of the wafer, and hence improve cooling
efficiency. Another example would be to choose a material or a
coating that increases the emissivity across the range of
wavelength where the plate emits thermal radiation. This would
increase the efficiency of the radiative cooling and can accelerate
the cooling rate.
[0073] Another aspect that may help operation of the plate in
certain conditions is preparation of the surface that faces the
wafer in a manner so that the thermal conduction into the plate is
more efficient. Under some heat transfer conditions the transfer of
heat through a gas to a surface can be sensitive to the surface
texture. In this case, it may be useful for that surface to be
textured in a manner that improves the efficiency of heat transfer.
Texturing may be performed by any conventional method that forms
grooves, protrusions, and the like.
[0074] It is appreciated by persons skilled in the art that the
present invention is not limited by what has been particularly
shown and described above. Rather, as set forth in the attached
claims, the scope of the present invention includes both
combinations and sub-combinations of various features discussed
herein, along with such variations and modifications as would occur
to a person of skill in the art.
* * * * *