U.S. patent application number 15/947352 was filed with the patent office on 2018-08-16 for wafer processing with carrier extension.
This patent application is currently assigned to Veeco Instruments Inc.. The applicant listed for this patent is Veeco Instruments Inc.. Invention is credited to Eric A. Armour, Bojan Mitrovic, Ajit Paranjpe, Guanghua Wei.
Application Number | 20180230596 15/947352 |
Document ID | / |
Family ID | 45524957 |
Filed Date | 2018-08-16 |
United States Patent
Application |
20180230596 |
Kind Code |
A1 |
Mitrovic; Bojan ; et
al. |
August 16, 2018 |
WAFER PROCESSING WITH CARRIER EXTENSION
Abstract
Apparatus for treating wafers using a wafer carrier rotated
about an axis is provided with a ring which surrounds the wafer
carrier during operation. Treatment gasses directed onto a top
surface of the carrier flow outwardly away from the axis over the
carrier and over the ring, and pass downstream outside of the ring.
The outwardly flowing gasses form a boundary over the carrier and
ring. The ring helps to maintain a boundary layer of substantially
uniform thickness over the carrier, which promotes uniform
treatment of the wafers.
Inventors: |
Mitrovic; Bojan; (Somerset,
NJ) ; Wei; Guanghua; (Dayton, NJ) ; Armour;
Eric A.; (Pennington, NJ) ; Paranjpe; Ajit;
(Basking Ridge, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Veeco Instruments Inc. |
Plainview |
NY |
US |
|
|
Assignee: |
Veeco Instruments Inc.
Plainview
NY
|
Family ID: |
45524957 |
Appl. No.: |
15/947352 |
Filed: |
April 6, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15151845 |
May 11, 2016 |
9938621 |
|
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15947352 |
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13333152 |
Dec 21, 2011 |
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15151845 |
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61428250 |
Dec 30, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C30B 25/165 20130101;
C23C 16/4585 20130101; H01L 21/0262 20130101; C23C 16/4584
20130101; H01L 21/68735 20130101; H01L 21/68771 20130101; H01L
21/68764 20130101; H01L 21/0254 20130101; H01L 21/68785 20130101;
C23C 16/45508 20130101; C23C 16/45504 20130101; C23C 16/46
20130101; C30B 29/06 20130101; C23C 16/45591 20130101 |
International
Class: |
C23C 16/458 20060101
C23C016/458; C23C 16/455 20060101 C23C016/455; H01L 21/687 20060101
H01L021/687; H01L 21/02 20060101 H01L021/02; C30B 29/06 20060101
C30B029/06; C30B 25/16 20060101 C30B025/16; C23C 16/46 20060101
C23C016/46 |
Claims
1. A wafer carrier comprising a body having a circular top surface,
a peripheral surface bounding the top surface and a fitting adapted
to engage a spindle of a wafer processing reactor so that the top
surface and peripheral surface are concentric with the spindle, the
body further defining a plurality of pockets each adapted to hold a
wafer, the pockets including outer pockets adapted to hold wafers
so that portions of the wafers lie within about 5 mm of the
peripheral surface.
2. A wafer carrier as claimed in claim 2 wherein the top surface
has a diameter of about 465 mm and the pockets include at least six
pockets, each adapted to hold a wafer having a diameter of about 6
inches.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 15/151,845, filed May 11, 2016, which is a divisional of U.S.
application Ser. No. 13/333,152, filed Dec. 21, 2011, which claims
the benefit of the filing date of U.S. Provisional Application No.
61/428,250, filed Dec. 30, 2010, all of which are incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to wafer processing apparatus,
to wafer carriers for use in such processing apparatus, and to
methods of wafer processing.
[0003] Many semiconductor devices are formed by processes performed
on a substrate. The substrate typically is slab of a crystalline
material, commonly referred to as a "wafer." Typically, the wafer
is formed from a crystalline material, and is in the form of a
disc. One common process is epitaxial growth. For example, devices
formed from compound semiconductors such as III-V semiconductors
typically are formed by growing successive layers of the compound
semiconductor using metal organic chemical vapor deposition or
"MOCVD." In this process, the wafers are exposed to a combination
of gases, typically including a metal organic compound as a source
of a group III metal, and also including a source of a group V
element, which flow over the surface of the wafer while the wafer
is maintained at an elevated temperature. Typically, the metal
organic compound and group V source are combined with a carrier gas
which does not participate appreciably in the reaction as, for
example, nitrogen. One example of a III-V semiconductor is gallium
nitride, which can be formed by reaction of an organo gallium
compound and ammonia on a substrate having a suitable crystal
lattice spacing, as, for example, a sapphire wafer. Typically, the
wafer is maintained at a temperature on the order of
1000-1100.degree. C. during deposition of gallium nitride and
related compounds.
[0004] Composite devices can be fabricated by depositing numerous
layers in succession on the surface of the wafer under slightly
different reaction conditions, as, for example, additions of other
group III or group V elements to vary the crystal structure and
bandgap of the semiconductor. For example, in a gallium nitride
based semiconductor, indium, aluminum or both can be used in
varying proportion to vary the bandgap of the semiconductor. Also,
p-type or n-type dopants can be added to control the conductivity
of each layer. After all of the semiconductor layers have been
formed and, typically, after appropriate electric contacts have
been applied, the wafer is cut into individual devices. Devices
such as light-emitting diodes ("LEDs"), lasers, and other
optoelectronic devices can be fabricated in this way.
[0005] In a typical chemical vapor deposition process, numerous
wafers are held on a device commonly referred to as a wafer carrier
so that a top surface of each wafer is exposed at the top surface
of the wafer carrier. The wafer carrier is then placed into a
reaction chamber and maintained at the desired temperature while
the gas mixture flows over the surface of the wafer carrier. It is
important to maintain uniform conditions at all points on the top
surfaces of the various wafers on the carrier during the process.
Variations in process conditions can cause undesired variations in
the properties of the resulting semiconductor device. For example,
variations in the rate of deposition can cause variations in
thickness of the deposited layers, which in turn can lead to
non-uniform characteristics in the resulting devices. Thus,
considerable effort has been devoted in the art heretofore towards
maintaining uniform conditions.
[0006] One type of CVD apparatus which has been widely accepted in
the industry uses a wafer carrier in the form of a large disc with
numerous wafer-holding regions, each adapted to hold one wafer. The
wafer carrier is supported on a spindle within the reaction chamber
so that the top surface of the wafer carrier having the exposed
surfaces of the wafers faces upwardly toward a gas distribution
element. While the spindle is rotated, the gas is directed
downwardly onto the top surface of the wafer carrier and flows
across the top surface toward the periphery of the wafer carrier.
The outwardly-flowing gas forms a boundary layer covering the top
surface of the wafer carrier. The used gas flows downwardly around
the periphery of the wafer and is evacuated from the reaction
chamber through ports disposed below the wafer carrier. The wafer
carrier is maintained at the desired elevated temperature by
heating elements, typically electrical resistive heating elements
disposed below the bottom surface of the wafer carrier.
[0007] The rate of certain treatment processes, such as the growth
rate in an MOCVD process under mass-transport-limited growth
conditions, is inversely related to the boundary layer thickness.
For the case of an infinitely large carrier, theory predicts that
the rate is inversely proportional to the boundary layer thickness.
This means that for thinner boundary layers the growth rate is
higher. This reflects the fact that, as the boundary layer becomes
thinner, it takes less time for reactive moieties to diffuse
through the boundary layer to the surface of the wafer carrier and
the surfaces of the wafers. Hence, a thin and uniform diffusion
boundary layer is desirable to achieve uniform and fast deposition
rate during the MOCVD epitaxial growth. Boundary layer thickness
can be controlled by changing the rotation rate and pressure in
reactor and is inversely proportional of the square root of those
two parameters. It can also be controlled by changing the dynamic
viscosity of the gas mixture. The dynamic viscosity is a function
of fraction of different gases in the mixture as well as of carrier
and inlet temperature.
[0008] Typically, with stable flow conditions in the reactor and
with substantially uniform heating of the wafer carrier, uniform
boundary layer thickness can be achieved above the majority of the
wafer carrier surface. However, near the periphery of the wafer
carrier, the gas flow begins to change direction from radial above
the wafer carrier to the downward flow which carries the gas from
the wafer carrier to the exhaust. In the edge region of the wafer
carrier near the periphery, the boundary layer becomes thinner and
hence the process rate increases appreciably. For example, if a
wafer is positioned on the carrier with a portion of the wafer
disposed in the edge region, a chemical vapor deposition process
will form layers of uneven thickness. Thicker portions will be
formed on those parts of the wafer disposed in the edge region.
[0009] To avoid this problem, wafers are not positioned in the edge
region. Thus, the pockets or other wafer-holding features of wafer
carriers typically are provided only in the area of the wafer
carrier remote from the periphery. This limits the number and size
of wafers which can be accommodated on a carrier of a given size,
and therefore limits the productivity of the equipment and process.
Although a larger wafer carrier could accommodate more wafers,
larger carriers have significant drawbacks. Larger carriers are
more expensive, heavier and thus more difficult to handle,
particularly during movement of the carrier into and out of the
reaction chamber. Moreover, it typically is impractical to increase
the size of the wafer carriers used in existing processing
equipment.
[0010] Thus, although considerable effort has been devoted in the
art heretofore to design an optimization of such systems, still
further improvement would be desirable.
SUMMARY OF THE INVENTION
[0011] One aspect of the invention provides a reactor. The reactor
according to this aspect of the invention desirably includes a
chamber having a wall structure defining an interior surface. The
reactor preferably has a spindle disposed within the chamber and
rotatable about an upstream-to-downstream axis, the spindle being
adapted to support a wafer carrier for rotation about the axis so
that a top surface of the carrier faces in the upstream direction
at a carrier location. The reactor according to this aspect of the
invention preferably also includes a ring mounted within the
chamber, the ring having a top surface facing in the upstream
direction, the ring being constructed and arranged so that when the
reactor is in an operative condition, the ring closely surrounds
the wafer carrier supported on the spindle and the top surface of
the ring is substantially continuous with the top surface of the
carrier. The ring typically is movably mounted within the chamber,
so that it does not impede loading or unloading of carriers.
[0012] Typically, the reactor also includes a gas inlet element
communicating with the chamber upstream of the carrier location and
a gas exhaust communicating with the chamber downstream of the
carrier location. The ring typically has a peripheral surface
facing outwardly away from the axis, the ring being arranged so
that when the reactor is in an operative condition, there is a gap
between the peripheral surface of the ring and the interior surface
of the chamber. As further discussed below, during operation of the
reactor, gas discharged from the gas inlet element flows downstream
toward the wafer carrier and over the top surface of the carrier
and wafers held on the carrier, and flows outwardly over the ring.
In effect, the ring forms an extension of the carrier, so the gas
flow is similar to that which would be obtained with a carrier of
larger diameter. The boundary layer may have substantially uniform
thickness over the entire carrier, or over almost the entire
carrier, so that wafer parts or wafers can be positioned in edge
regions of the carrier.
[0013] A further aspect of the invention provides methods of
processing wafers. The method according to this aspect of the
invention desirably includes the step positioning a wafer carrier
inside a reaction chamber so that a ring within the chamber
surrounds the carrier, so that top surfaces of the carrier and ring
facing in an upstream direction are substantially continuous with
one another and so that surfaces of wafers disposed on the carrier
face in the upstream direction. The method preferably also includes
the step of directing one or more treatment gasses in a downstream
direction onto the top surfaces of the wafer carrier and wafers
while rotating the wafer carrier and wafers around an
upstream-to-downstream axis, so that the treatment gasses flow
outwardly over the top surface of the carrier and the surfaces of
the wafer, and flow outwardly from the top surface of the carrier
over the top surface of the ring. The method typically further
includes exhausting gasses from the chamber downstream from the
ring so that the gasses flowing outwardly over the top surface of
the ring pass downstream within a gap between the ring and a wall
of the chamber.
[0014] Yet another aspect of the invention provides wafer carriers.
Wafer carriers according to this aspect of the invention desirably
include a body having a circular top surface, a peripheral surface
bounding the top surface and a fitting adapted to engage a spindle
of a wafer processing reactor so that the top surface and
peripheral surface are concentric with the spindle. The body
desirably further defines a plurality of pockets each adapted to
hold a wafer, the pockets including outer pockets adapted to hold
wafers so that portions of the wafers lie within about 5 mm of the
peripheral surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] A more complete appreciation of the subject matter of the
present invention and of the various advantages thereof can be
realized by reference to the following detailed description in
which reference is made to the accompanying drawings in which:
[0016] FIG. 1 is a diagrammatic sectional view of apparatus
according to one embodiment of the invention.
[0017] FIG. 2 is a fragmentary view of the region indicated at 2 in
FIG. 1.
[0018] FIG. 3 is a fragmentary view taken along line 3-3 in FIG.
2.
[0019] FIG. 4 is a view similar to FIG. 2, but depicting a portion
of conventional apparatus according to the prior art.
[0020] FIG. 5 is a graph of predicted performance for apparatus
according to FIGS. 1-3 and for the apparatus of FIG. 4.
[0021] FIG. 6 is a view similar to FIG. 2, but depicting a portion
of apparatus according to a further embodiment of the
invention.
[0022] FIG. 7 is a diagrammatic top view of a wafer carrier
according to a further embodiment of the invention.
[0023] FIG. 8 is a view similar to FIG. 2, but depicting a portion
of apparatus according to yet another embodiment of the
invention.
[0024] FIG. 9 is a view similar to FIG. 6, but depicting a portion
of apparatus according to yet another embodiment of the
invention.
[0025] FIG. 10 is a diagrammatic view of apparatus according to yet
another embodiment of the invention.
[0026] FIG. 11 is a top plan view of the assembly of FIG. 10.
DETAILED DESCRIPTION
[0027] In describing the preferred embodiments of the subject
illustrated and to be described with respect to the drawings,
specific terminology will be used for the sake of clarity. However,
the invention is not intended to be limited to any specific terms
used herein, and it is to be understood that each specific term
includes all technical equivalents, which operate in a similar
manner to accomplish a similar purpose.
[0028] Apparatus according to one embodiment of the invention
incorporates a reaction chamber 10 having a wall structure which
incorporates a fixed wall 12 defining a generally cylindrical space
15 having a central axis 14 and an opening 16 communicating with
the interior space. As further discussed below, gas flow within the
reaction chamber during operation is generally from the region at
the top of the drawing in FIG. 1 toward the region at the bottom of
the drawing. Therefore, the direction along the axis toward the
bottom of the drawing, indicated by arrow D in FIG. 1, is referred
to herein as the "downstream" direction, and the opposite direction
denoted by arrow U is referred to herein as the "upstream"
direction.
[0029] The wall structure of the chamber further includes a
ring-like shutter 18. The shutter 18 has a central axis coincident
with the central axis 14. Shutter 18 is mounted for movement
relative to the fixed wall in the upstream and downstream
directions and is connected to a movement actuator 20. Actuator 20
is arranged to move the shutter between the operative position
illustrated in solid lines in FIG. 1 and the open position depicted
in broken lines at 18' in FIG. 1. When shutter 18 is in the
operative position, it covers opening 16. Typically, shutter 18
does not form a gas-tight seal at opening 16. Fixed wall 12 and
shutter 18 are provided with coolant passages (not shown) inside
the walls or on their exterior surfaces. The coolant passages are
connected to a coolant supply apparatus (not shown), so that the
fixed wall and shutter can be maintained at desired temperatures
during the process.
[0030] A gas inlet element 22 is provided at an upstream end of
chamber 10, towards the top of the drawing in FIG. 1. The gas inlet
element is connected to one or more sources 24 arranged to supply
one or more treatment gases. Gas inlet element 22 may be generally
conventional and may be arranged to discharge the treatment gases
in a flow directed generally in the downstream direction D. The gas
inlet element typically is arranged to discharge the treatment
gases in a pattern of discharges spaced around central axis 14 and
distributed at various radial distances from the central axis. The
gas inlet element typically is also provided with coolant channels
(not shown) for maintaining its temperature during the process.
[0031] A hollow hoop-like exhaust manifold 26 is provided adjacent
the downstream end of the chamber. The exhaust manifold has an
interior passage 28 and numerous ports 30 open to the interior of
the chamber. The interior passage 28 of the exhaust manifold, in
turn, is connected to an exhaust system 32 arranged to pump gases
out of the interior space 15 and discharge the gases to waste.
[0032] A spindle 34 is mounted to the fixed wall structure 12 for
rotation about central axis 14. Spindle 34 is connected to a rotary
drive mechanism 36. The spindle has a fitting 38 at its upstream
end. The fitting is arranged to releasably engage and hold a wafer
carrier 40 at the carrier location depicted in FIG. 1. The carrier
location is disposed downstream from gas inlet element 22, but
upstream from exhaust manifold 26. A heater 42 is disposed
downstream from the carrier location and surrounds spindle 34.
Heater 42 is supported within the chamber by supports (not shown)
fixed to the fixed wall structure 12. A circular baffle 44
surrounds the heater and extends downstream from the carrier
location. A source 45 of a heater purge gas communicates with the
space inside of baffle 44. As best seen in FIG. 2, the baffle is
dimensioned so that, when a wafer carrier 40 is mounted at the
carrier location, there is a small gap 47 between the baffle and
the carrier. During operation, the heater purge gas source 45 feeds
a purge gas, such as nitrogen, into the space within baffle 44 so
that the purge gas flows out of this space through gap 47 and
passes to exhaust 32 along with the other gas flows discussed
below. The heater purge gas prevents the treatment gas from
contacting and attacking heater 42.
[0033] An antechamber 48 communicates with opening 16 in the fixed
wall structure. Antechamber 48 is provided with a closure, such as
a gate valve element 50, schematically shown in FIG. 1. The gate
valve element is arranged to seal the antechamber and thus block
communication between antechamber 48 and the interior space 15.
Valve element 50 can be moved to a retracted position (not shown)
to allow communication between the antechamber and interior space
15. When the valve element is in the retracted position and shutter
18 is in the open position 18', a wafer carrier 40 can be removed
from its engagement with fitting 38 of the spindle and moved
through opening 16 into the antechamber using robotic handling
apparatus (not shown). A new wafer carrier 40' can be moved from
the antechamber into the reaction chamber and engaged with fitting
38 so that the new wafer carrier is positioned at the carrier
location.
[0034] A ring 52 is mounted to shutter 18 and thus positioned
within the interior space 15 of the chamber. As best seen in FIGS.
2 and 3, ring 52 has a top surface 54 facing in the upstream
direction; an outer circumferential surface 56 facing radially
outwardly, away from the central axis; and an inner surface 58
facing radially inwardly, toward the central axis. Ring 52 is
mounted to shutter 18 by struts 60 disposed around the
circumference of the chamber. One such strut is depicted in FIGS. 2
and 3. The struts are disposed below top surface 54. The outer
peripheral surface 56 of the ring is disposed radially in-board of
the adjacent surface of shutter 18, so that there is a gap 62
between the surface of the shutter and the ring. For example, in
apparatus arranged to hold a 465 mm diameter wafer carrier, the
width of gap 62 at its narrowest point may be on the order of 13
mm. Because struts 60 are relatively thin, they do not materially
obstruct gap 62. The dimensions of ring 52 and its mounting to
shutter 18 are selected so that when the shutter 18 is in an
operative condition, as shown in solid lines in FIG. 1 and as
depicted in FIG. 2, and when a wafer carrier 40 is disposed in an
operative condition and positioned at the carrier location in
engagement with fitting 38 on spindle 34, the top surface 54 of the
ring is substantially coplanar with the top or upstream surface 64
of the carrier 40. The width or radial extent of ring 52 desirably
may be about 13-15 mm, and still greater ring widths are more
desirable. In general, ring 52 should be as wide as is practicable.
Where the ring is to be fitted into existing systems originally
built without the ring, the ring width is limited by the need to
provide a gap 62 of sufficient width.
[0035] Also, ring 52 is dimensioned and mounted so that in this
operative condition, the interior surface 58 of the ring lies
adjacent the exterior peripheral surface 66 of the wafer carrier
40, leaving only a small gap 70 between the surfaces. Desirably,
gap 70 is as small as practicable, consistent with manufacturing
tolerances and allowances for differential thermal expansion of the
components. For example, gap 70 may be about 2 mm wide or less.
Preferably, the cross-sectional area of gap 70 is less than about
5% of the cross-sectional area of gap 62 between the exterior
peripheral surface of the ring and the shutter 18, as measured at
the narrowest point of gap 62.
[0036] As best seen in FIGS. 1 and 2, each wafer carrier 40 defines
numerous pockets 72, each of which is arranged to hold a wafer 74
so that a top surface of the wafer is substantially coplanar with
the top surface 64 of the carrier. Desirably, wafer carrier 40 has
a relatively sharp edge at the juncture of its top surface 64 and
peripheral surface 66, and ring 52 desirably also has sharp edges
at the juncture of its top surface with interior surface 58 and
exterior peripheral surface 56. These sharp edges desirably have
radii less than about 0.1 mm.
[0037] In operation, the apparatus is brought to the operative
condition depicted in FIGS. 1-3, with a wafer carrier 40 bearing
wafers 74 disposed on the spindle and with the shutter 18 in the
operative position shown in solid lines, so that ring 52 closely
surrounds the peripheral surface of carrier 40. Heater 42 is
actuated to bring the wafer carrier and wafers to the desired
temperature, and gas inlet element 22 is actuated to discharge
treatment gases, while rotary drive 36 is actuated to spin the
spindle 34 and wafer carrier 40 about central axis 14. The gas
discharged by gas inlet element 22 passes generally as indicated by
flow arrows F in FIG. 1. Thus, the gas passes downstream from the
inlet element towards the carrier location and flows generally
radially outwardly over the top or upstream surface of carrier 40.
The flowing gas passes outwardly beyond the periphery of the wafer
carrier and over ring 52, and then passes downwardly through the
gap 62 between the ring and the interior wall surface defined by
shutter 18. Although a minor amount of the gas passes downwardly
through gap 70, this minor amount does not substantially influence
the flow dynamics of the system. Preferably, less than about 5% of
the gas passing over the top surface of the wafer carrier passes
through gap 70, and the remainder passes through gap 62, out-board
of the ring 52. The gas continues to flow downstream towards
exhaust manifold 26 and passes into the exhaust manifold and out
from the system through exhaust system 32.
[0038] As best seen in FIG. 2, the gas flowing outwardly over top
surface 64 of the wafer carrier and over the surfaces of the wafers
74 forms a boundary layer B. Within this boundary layer, the gas
flow streamlines are nearly parallel to the top surface of the
carrier, so that the boundary layer has a substantially uniform
thickness. However, as the gas approaches the gap 62, the
streamlines converge appreciably in a region R, and the thickness
of the boundary layer decreases appreciably within this region.
However, this region is disposed over the ring 52 and not over the
wafer carrier. Therefore, the boundary layer maintains a
substantially uniform thickness over substantially the entire top
surface of the wafer carrier. This provides a substantially even
reaction rate over surfaces of wafers 74, even when the wafers 74
are disposed immediately adjacent the peripheral surface 66 of the
carrier.
[0039] After processing, shutter 18 is moved to the open 18'
configuration. Ring 52 moves with the shutter to the position shown
at 52' in FIG. 1. When the shutter is in the retracted position,
both the ring and the shutter are remote from opening 16 and do not
impede movement of wafer carriers into and out of the chamber.
[0040] FIG. 4 depicts a system identical to the system shown in
FIG. 2, but without ring 52, and using a typical wafer carrier
having an appreciable radius at the juncture between the top
surface of the carrier and the peripheral surface of the carrier.
In this system, the gas passes downstream immediately outside the
peripheral surface 66 of the wafer carrier. Thus, the streamlines
converge appreciably over the outer portion of the wafer carrier
itself. The region R of uneven boundary layer thickness extends
inwardly from the peripheral surface 66 of the wafer carrier and
covers a significant portion of the carrier top surface. Thus, if
parts of wafers 74 are positioned within the area of the carrier
covered by region R, these wafers will be subjected to uneven
growth rates. Thus, in a system without ring 52, the wafer-holding
pockets typically would be positioned differently, so as to keep
them further away from the periphery of the carrier. This, in turn,
would reduce the capacity of the wafer carrier. Stated another way,
the presence of ring 52 (FIGS. 1-3) allows placement of the wafer
carrier pockets close to the periphery of the carrier and thus
increases the capacity of the carriers. This increases the
throughput of the system, i.e., the number of wafers which can be
processed per unit time.
[0041] Moreover, placing wafers closer to the periphery of the
carrier promotes efficient use of the treatment gases. These gases
typically are expensive, high-purity materials. Typically, the
amount of each gas is determined so as to provide a constant amount
per unit area over the entire area of the wafer carrier. By placing
wafers closer to the periphery of the carrier, more of the area of
the carrier can be covered by wafers, and more of the gas will be
used to treat wafers.
[0042] The effect of the change in flow dynamics introduced by
addition of ring 52 is further shown in FIG. 5. Curve 100 in FIG. 5
represents a calculated plot of thickness versus radial position in
a chemical vapor deposition process using the reactor depicted in
FIG. 4, with no ring and with a wafer carrier having a radiused
edge. Curve 102 is a similar plot of calculated deposition
thickness in the same chemical vapor deposition process using a
reactor as shown in FIGS. 1-3, with ring 52 and with a wafer
carrier having a sharp edge at its periphery. The thickness of the
deposited layer is stated as normalized thickness, i.e., a ratio of
the thickness at each radial position to the thickness at a radial
position 190 mm from the center line. In each case, the wafer
carrier has a diameter of 465 mm, so that the peripheral surface of
the wafer carrier is disposed at a radial distance of 232.5 mm from
the central axis. The vertical line 104 at approximately 223 mm
radial distance from the center line represents the radial location
of the outer-most points of wafers on the carrier if the carrier is
configured to accommodate 54 wafers each having a diameter of two
inches. Vertical line 106 at approximately 127 mm radial distance
represents the radial location of the outer-most points on the
wafers if the carrier is arranged to accommodate 6 six-inch
diameter wafers. Curve 100 for the conventional reactor of FIG. 4
shows that, at line 104, the normalized thickness is in excess of
1.1. By comparison, curve 102 indicates a normalized thickness of
approximately 1.02 at line 104. Stated another way, if the wafer
carrier is arranged to accommodate 54 two-inch diameter wafers, the
system without the ring will yield wafers having some points with
thickness approximately 12% greater than other points on the same
wafers, whereas the system with the ring will yield wafers with
deposited layers of substantially thickness uniform to within about
2%. Further, if the wafer carrier is configured to hold 6 six-inch
diameter wafers, the system without the ring will yield wafers
having deposited layer thickness variations in excess of 40%, i.e.,
a normalized thickness of 1.4 at vertical line 106. By contrast,
curve 102 indicates a thickness variation of approximately 7% at
line 106, still within acceptable limits for many applications.
Therefore, the system with the ring can be more readily used to
process 6 six-inch wafers, using a wafer carrier of the same
diameter as the system without the ring.
[0043] Thus, the improvement afforded by the ring allows
construction of a wafer carrier having pockets close to the
periphery of the carrier, while still providing uniform treatment
of the wafers. The wafer carrier 340 depicted in FIG. 7 has a
circular body with a generally planar top surface 364 and a
peripheral surface 366. The body has a fitting 367 adapted to mate
with the spindle of a processing apparatus, such as the spindle 34
of the processing apparatus shown in FIG. 1. The fitting can be of
any configuration; for a spindle having a conical fitting 38 as
depicted in FIG. 1, the fitting of the carrier typically is a
conical opening in the bottom of the body. The wafer carrier has
wafer-holding elements in the form of pockets 372, each adapted to
hold a wafer. Each of the pockets 372 lies close to the peripheral
surface 366. Thus, the distance X between the outermost part of
each pocket and the peripheral surface 366 is less than about 5 mm.
Placement of the pockets so close to the peripheral surface has not
been acceptable heretofore due to uneven thickness. The distance
between the pocket and the peripheral surface is measured between
the pocket and the edge where the top surface joins the peripheral
surface. The body of carrier 340 may have any diameter, but
desirably has a diameter greater than 300 mm. In one example, the
carrier has a diameter of about 465 mm and has six pockets as
depicted in FIG. 7, each pocket being adapted to hold one six-inch
diameter wafer. The carrier may include a greater number of
smaller-diameter pockets, and the pockets may be arranged with only
some of the pockets disposed at the outside of the body, near the
peripheral surface.
[0044] Ring 52 also acts as a thermal barrier between the periphery
of the wafer carrier and the adjacent wall surface of the chamber
defined by shutter 18. Typically, the wafer carrier is maintained
at a temperature substantially higher than the walls of the
reactor. For example, the wafer carrier may be maintained at a
temperature on the order of 1000-1200.degree. C. or higher, whereas
the walls of the reactor may be maintained at temperatures below
100.degree. C. There is significant radiant heat transfer between
the edge of the wafer carrier and the adjacent wall surface. This
tends to make the edge region of the wafer carrier cooler than
other regions of the wafer carrier, and thus makes the wafers in
the edge region cooler as well. Such non-uniform temperature
distribution can result in non-uniform rates of reaction and
non-uniform composition of a deposited layer. Although this effect
can be counteracted to some degree by configuring the heater 42 to
apply more heat to the region of the wafer carrier near the
periphery, it is desirable to reduce this effect. Ring 52 acts as a
radiation barrier and blocks direct radiation from the peripheral
surface of the wafer carrier to the wall surface of the chamber
defined by shutter 18. This helps to maintain a uniform temperature
distribution over the wafer carrier, which in turn, promotes
uniformity of process conditions over all portions of the
wafers.
[0045] To further enhance the insulating effect of the ring, ring
52 may be provided with additional features which help to minimize
heat conduction between the interior and exterior surfaces of the
ring. For example, as seen in FIG. 6, ring 152 has a
cross-sectional shape generally in the form of an inverted U, with
a hollow interior space 153. The hollow interior space reduces heat
conduction between interior surface 158 and peripheral, or
exterior, surface 156. Preferably, the top or upstream surface 154
of the ring remains as a continuous, unbroken surface. Space 153
may be open on its downstream end, as depicted in FIG. 6, or may be
closed on its downstream end. By reducing heat conduction between
surfaces 158 and 156, the ring depicted in FIG. 6 further impedes
heat transfer between the wafer carrier 40 and shutter 18.
[0046] In a further variant, ring 52 can be formed from a plurality
of concentric rings, each of which may be formed from the same
material or from different materials. For example, the concentric
rings can be formed from refractory materials, such as graphite,
silicon carbide, and/or silicon coated graphite, as well as
refractory metals, such as molybdenum, rhenium, tungsten, niobium,
tantalum, and alloys thereof. The size and number of concentric
rings, as well as the materials that make up the ring 52, can be
varied or adjusted depending upon the type of reactor and/or
reactions that take place within the reactor. In yet another
variant, the ring 52 may incorporate a heater, such as an
electrical resistance heater. For example, where the ring is formed
as a composite of multiple rings, one or more of the multiple rings
may constitute a heating element. In one variant, the ring closest
to the peripheral surface 66 of the wafer carrier 40 may be the
heating element. Heating of the ring can be used to control the
temperature of the wafer carrier edge. The heater incorporated in
the ring can be controlled by a feedback control system (not
shown), which is sensitive to the temperature of the wafer carrier
40 near the edge as, for example, a control system using one or
more pyrometers to monitor the wafer carrier temperature. The
dimensions of rings according to these and other variants can be
selected as discussed above in connection with ring 52, (FIGS. 1
and 2). Thus, here again the ring desirably is as wide as
practicable and desirably provides the minimum practicable
clearance around the wafer carrier so as to minimize the size of
the gap between the wafer carrier and the ring.
[0047] The apparatus depicted in FIG. 8 is similar to that depicted
in FIG. 1, except that the ring 252 has an upstream or top surface
254 in the form of a portion of a cone concentric with the central
axis. Surface 254 slopes upwardly in the radially outward
direction, away from the central axis of the chamber. Stated
another way, the juncture between the upstream or top surface 254
and the outer peripheral surface 256 lies upstream of the juncture
between the upstream surface 254 and the interior surface 258 of
the ring. The vertical rise V of the top surface between interior
surface 258 and exterior surface 256 desirably is between about 1-2
mm. This upward slope helps to further suppress convergence of
streamlines in the region over the ring and over the peripheral
region of wafer carrier 40. The shutter 218 may be modified
slightly from that shown in the other figures so as provide the
same clearance C between the ring and the nearest portion of the
shutter or wall structure. In a further variant, an upwardly
sloping top surface may be provided as a surface of revolution of a
curved generatrix about the central axis. Thus, in this embodiment,
the upstream or top surface would not define a straight line, but
instead would define an upwardly sloping curved line.
[0048] Top surface 254 may alternatively slope downward in the
radially outward direction, i.e., away from the central axis of the
chamber. In yet another variation, upstream or top surface 254 may
be hemispherical in shape or may be planar with ridges or ripples
of various shapes across the surface.
[0049] Other embodiments may include a wafer carrier 40, which
itself may have a lip projecting upwardly around the periphery of
the top surface of the wafer carrier. Wafer carriers having
upwardly-sloping lips are described in U.S. Patent Application
Publication No. 2011/0215071, the disclosure of which is hereby
incorporated by reference herein. In these embodiments, the
upwardly-sloping lip of the wafer carrier may be combined with an
upwardly-sloping surface 254 of the ring 252, as hereinbefore
disclosed. Such surfaces may be arranged so that, when the wafer
carrier is mounted at the carrier location, the upwardly-sloping
surface of the carrier lip and the ring 252 are nearly continuous
with one another and cooperatively define a composite
upwardly-sloping surface. These upwardly-sloping surfaces may
provide further control of deposition rates near the edge of the
wafer carrier.
[0050] In a variant of the arrangements discussed above, structures
such as rollers and guide pins (not shown) can be mounted to ring
52 (FIGS. 1 and 2) or to baffle 44 to provide further assurance
against contact between wafer carrier 40 and ring 52. Baffle 44 can
also be further modified to include bumps or ridges on the wall
proximate to the rollers to assist in the alignment of ring 52 with
the top or upstream surface 64 of the carrier 40. In some
instances, it may be desirable to have the top or upstream surface
of ring 52 higher or lower than the top or upstream surface 64 of
the carrier 40. In other instances, it may be desirable to have
interior surface 58 of ring 52 higher or lower than the top or
upstream surface of ring 52. This can be accomplished, for example,
by having the rollers located within baffle 44 resting on the
aforementioned bumps or ridges.
[0051] In the embodiments discussed above, the ring is mounted to
the shutter. However, this is not essential. For example, the ring
can be mounted on a separate actuator, and can be moved
independently of the shutter. In still other embodiments, the wall
structure of the reactor may not include a shutter. In this case,
the ring is disposed between the location of the wafer carrier and
the fixed wall structure of the reactor. In the embodiments
discussed above, the ring is movable relative to the fixed wall
structure of the reactor, either with the shutter or independently,
so that the ring can be moved out of the way during loading and
unloading of wafer carriers from the chamber. However, this is not
essential. If the configuration of the wafer carriers and the
configuration of the elements used to move the wafer carriers into
and out of the reactor permit, the wafer carriers can be installed
and removed without moving the ring.
[0052] During CVD processes, often times film growth will occur on
the reaction chamber parts in addition to the intended substrate
surface. If not cleaned, the additional film growth on the reactor
chamber parts will affect the efficiency of the CVD process,
resulting in lower than expected yields as well as additional
maintenance on the reactor chamber. One way to remove the
additional film growth on ring 52 is to heat ring 52 to a
temperature which will flash heat off the additional film growth. A
heater configured for such purposes can be incorporated into the
ring 52, as described above.
[0053] Yet another method may be to cause vibration of ring 52.
This can be accomplished by raising and lowering ring 52
simultaneously causing the rollers in baffle 44 to roll over the
bumps or ridges located thereon. Vibration can also be accomplished
by attaching an ultrasonic transducer to the ring 52 (or a
supporting element thereof).
[0054] Yet another method to clean the additional film growth from
the ring or the shutter (for example, shutter 18) is to provide for
one or more orifices in the shutter wall facing the interior of the
reactor chamber and/or the top surface of the ring, thus allowing
non-reactive gas to flow through the orifice(s) and blast the
additional film growth off the respective surfaces.
[0055] As shown in FIG. 9 (where, with the exception of reference
numerals 300, 305, 310, 315, 320, and 325, the reference numerals
are as described above), gas inlet 300 may feed gas through gas
tube 320 which connects to and supplies the gas to orifice 325. In
this variant, orifice 325 may also exit through the wall of the
shutter 18. Moreover, as noted, one or more orifices 325 can be
placed along various positions of the wall of the shutter 18, such
orifice(s) 325 facing the interior of the reaction chamber.
[0056] A separate gas inlet 305 may also feed gas through gas tube
310, which connects to and supplies the gas to orifice 315.
However, in this variant, unlike orifice 325, orifice 315 may exit
through the top or upstream surface 154 of ring 152. As with above,
one or more orifices 315 can be placed within the top or upstream
surface 154. Additionally, the orifice(s) 315 can take the shape of
a continuous or semi-continuous slit around the circumference of
the ring 152.
[0057] Orifices 315 and 325 can be used to clean the additional
film growth on the ring (or other surfaces) in sequential or
simultaneous activation. Gases suitable for use include, for
example, H.sub.2, N.sub.2, Ar, and other inert gases. The gas(es)
can be introduced into the reactor at a temperature ranging from
about room temperature up to about 1600.degree. C.
[0058] An alternate variation to orifices 315, 325 as set forth
above, would be to have a gas tube 322 (shown in dotted lines in
FIG. 9) extend through the base plate of the reactor. Gas tube 322,
in this embodiment, may be a flexible bellows tube and may serve to
clean the additional film growth from the ring 152 and/or the
shutter 18 in a similar manner to orifices 315, 325.
[0059] Another use for the one or more orifices 315 in ring 152,
besides cleaning, may be for passing purge gas through the
orifice(s) 315 on ring 152 during the growth process. In so doing,
one may be able to adjust the height of the boundary layer in the
localized region of the purge gas by adjusting the flow rate of the
gas, which may be used to compensate for any height variations
caused by installing the ring 152. That is, if a "taller" flow
extender (ring 152) is required, i.e., one that projects above the
top planar surface 154 of the wafer carrier, a higher gas flow rate
may be used to push purge gas through the one or more orifices 315
on ring 152. Conversely, if a boundary layer closer to the "z"
plane of the wafer carrier is desired, the gas flow rate may be
decreased. By being able to adjust the efficiency of ring 152 by
raising or lowering the gas flow rate through orifice(s) 315 to
adjust the boundary layer, tool-to-tool matching is made simpler by
eliminating precise height adjustments during installation of ring
152 into MOCVD systems.
[0060] For variants of ring 152 in which the ring is formed from a
plurality of concentric rings, the rings may expand at different
rates during operation, depending on whether or not the rings are
formed of different materials. This effect may cause the additional
film growth discussed above to become dislodged from the upstream
or top surface of the ring(s).
[0061] In the embodiments discussed above, the ring, while in the
operative position, also remains stationary during processing of
the wafers. In other embodiments, the ring can be rotated around
the central axis during processing. For example, the ring may be
mounted to a spindle so that the ring can be rotated around the
central axis during processing by a separate rotary drive. A
diagrammatic view of this embodiment is shown in FIG. 10. The ring
may rotate in the same direction as the wafer carrier and spindle,
or in the opposite direction.
[0062] As shown in FIG. 10, assembly 200 has an outer spindle 134
and an inner spindle 168, which are connected to a rotary drive
mechanism 136. Inner spindle 168 may have a fitting 138 at its
upstream end. The fitting may be arranged to releasably engage and
hold a wafer carrier 440 at a carrier location similar to that
depicted in FIG. 1. Wafer carrier 440 may include numerous pockets
172, each of which is arranged to hold a wafer 174 so that the top
surface of the wafer is substantially coplanar with top surface 464
of wafer carrier 440. Outer spindle 134 may also have a fitting 238
at its upstream end. The fitting may be arranged to engage (in
other embodiments, releasably engage) support 360 to which ring 352
is mounted. The rotary drive 136 is designed to allow for
independent rotation of inner spindle 168 and outer spindle 134,
permitting the wafer carrier to rotate in the same direction as the
ring, in opposite directions, or to allow the ring to remain
stationary as the wafer carrier rotates.
[0063] Support 360 may take many shapes. In some instances, support
360 may be a susceptor or may be a series of support arms which
extend radially from outer spindle 134 to just beyond the outer
edge of wafer carrier 464, at which point ring 352 is mounted on
the support arms. Support 360 may be made from any suitable
material capable of withstanding the high temperatures inside the
reaction chamber of an MOCVD reactor and, at the same time, permit
appropriate heat transfer from heater 142 to wafer carrier 440.
Heater 142 of assembly 200 may be mounted and configured in a
manner like heater 42 described hereinabove.
[0064] FIG. 11 shows a top plan view of assembly 200 of FIG. 10,
with support 360 shown in phantom lines. Gap 170 between the
peripheral edge 466 of wafer carrier 440 and interior surface 358
of ring 352 is similar in size to that of gap 70 described
previously.
[0065] Additionally, rotary drive 136 may, apart from permitting
rotation of ring 352, allow for separate height adjustments of top
surface 464 of wafer carrier 440 in relation to top surface 564 of
ring 352. For some growth process steps, it may be beneficial to
have top surface 464 of wafer carrier 440 essentially coplanar with
top surface 564 of ring 352. In other growth process steps, it may
be beneficial to have top surface 464 of wafer carrier 440 higher
or lower than top surface 564 of ring 352. Ring 352 can also take
the shape and characteristics of ring 252 as described above.
[0066] In yet another embodiment, the ring may be disposed within
the chamber and arranged to releasably engage the outer edge of the
wafer carrier so that the ring, in effect, becomes a temporary part
of the wafer carrier during operation.
[0067] The materials of construction of the reactor elements, and
the composition of the treatment gasses, may be conventional. For
example, the wafer carrier may be formed in whole or in part from
refractory materials such as graphite, silicon carbide, and silicon
carbide coated graphite, whereas elements such as the ring may be
formed of similar materials or from refractory metals such as
molybdenum. Metals used for the ring optionally may be blackened to
increase the emissivity of the metal. The treatment gasses may be,
for example, gasses selected to react in a chemical vapor
deposition reaction or gasses selected to etch or otherwise treat
the surfaces of the wafers.
[0068] Although the invention herein has been described with
reference to particular embodiments, it is to be understood that
these embodiments are merely illustrative of the principles and
applications of the present invention. It is therefore to be
understood that numerous modifications may be made to the
illustrative embodiments and that other arrangements may be devised
without departing from the spirit and scope of the present
invention as defined by the appended claims.
* * * * *