U.S. patent application number 09/930022 was filed with the patent office on 2002-02-07 for wafer planarization using a uniform layer of material and method and apparatus for forming uniform layer of material used in semiconductor processing.
This patent application is currently assigned to Micron Technology Inc.. Invention is credited to Blalock, Guy T., Gordon, Brian F., Stroupe, Hugh E..
Application Number | 20020015753 09/930022 |
Document ID | / |
Family ID | 23539111 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015753 |
Kind Code |
A1 |
Blalock, Guy T. ; et
al. |
February 7, 2002 |
Wafer planarization using a uniform layer of material and method
and apparatus for forming uniform layer of material used in
semiconductor processing
Abstract
In connection with wafer planarization, an apparatus for forming
a layer of material having a substantially uniform thickness and
substantially parallel first and second major surfaces includes a
pair of pressing elements and a stop. Each of the pair of pressing
elements has a flat pressing surface. The pressing surfaces are
opposed to one another and operable to compress a quantity of the
material therebetween. The stop is positioned at least partially
between the pressing surfaces and has a thickness substantially
equal to the desired uniform thickness of the layer. The stop is
positioned to establish a spacing between the flat pressing
surfaces that is substantially equal to the thickness of the stop
and thereby to the desired uniform thickness of the layer when the
pressing elements engage the stop. As a result, engagement of the
stop by the pressing surfaces during pressing of the material forms
a layer of the material of substantially uniform thickness with
substantially parallel major surfaces formed by the flat pressing
surfaces. The layer is then used in semiconductor processing to
provide a flat surface on a layer of a substrate assembly, thereby
enhancing the planarization of the substrate assembly.
Inventors: |
Blalock, Guy T.; (Boise,
ID) ; Stroupe, Hugh E.; (Boise, ID) ; Gordon,
Brian F.; (Boise, ID) |
Correspondence
Address: |
One World Trade Center
Suite 1600
121 S.W. Salmon Street
Portland
OR
97204
US
|
Assignee: |
Micron Technology Inc.
|
Family ID: |
23539111 |
Appl. No.: |
09/930022 |
Filed: |
August 14, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09930022 |
Aug 14, 2001 |
|
|
|
09389644 |
Sep 2, 1999 |
|
|
|
Current U.S.
Class: |
425/412 ;
438/782 |
Current CPC
Class: |
B29K 2023/16 20130101;
Y10T 428/3154 20150401; B29L 2031/7728 20130101; H01L 21/67092
20130101; B29C 2035/0827 20130101; B29C 43/021 20130101; B29K
2063/00 20130101; B29L 2031/3061 20130101; B29K 2027/12
20130101 |
Class at
Publication: |
425/412 ;
438/782 |
International
Class: |
B29C 035/02; H01L
021/31; H01L 021/469 |
Claims
What is claimed is:
1. In semiconductor processing, an apparatus for compressing
material into a layer of substantially uniform thickness for use in
forming a flat surface on a substrate assembly, the material being
of a type which is capable of being heated to a temperature where
the material flows without melting, the apparatus comprising: first
and second pressing elements, each pressing element having a
respective pressing surface, the pressing surfaces being opposed to
one another; a shim positioned between the pressing surfaces, the
shim having an open center area and a stop portion having a
thickness equal to the desired uniform thickness of the layer; a
heater that heats the material to a temperature where the material
flows without melting; and a compression force applicator coupled
to each of the pair of pressing elements, the compression force
applicator moving at least a first pressing surface of the pair of
pressing surfaces toward the other of the pressing surfaces and
against the stop portion of the shim to press a quantity of the
material positioned between the pair of pressing surfaces and
within the open center area of the shim into the layer having the
substantially uniform thickness.
2. The apparatus of claim 1 wherein the compression force
applicator moves only the first pressing surface during pressing of
the material.
3. The apparatus of claim 1 wherein the compression force
applicator comprises a plurality of biasing elements coupled to the
first and second opposed pressing surfaces so as to bias the
pressing surfaces toward one another.
4. The apparatus of claim 1 wherein the compression force
applicator comprises a plurality of elongated fasteners each
interconnecting the respective pressing elements and the shim, the
pressing element which includes said first pressing surface being
slidably coupled to the elongated fasteners such that said first
pressing surface is slidable toward the other of the pressing
surfaces, and a plurality of biasing spring elements coupled to
said pressing elements to bias said first pressing surface toward
the other of the pressing surfaces.
5. An apparatus for use in forming a sheet of material for use in
semiconductor processing, comprising: an annular shim having a
border portion and a plurality of projections extending inwardly
from the border portion, the projections having a first thickness,
a shim open area positioned inwardly of the projections, overflow
material recesses being positioned between the projections and
communicating with the shim open area; and first and second
pressing surfaces positioned on opposite sides of the shim, the
first and second pressing surfaces contacting the shim at least
along the projections to compress material on at least one of the
first and second pressing surfaces within the shim open area to a
substantially uniform thickness equal to the first thickness,
wherein any material in excess of the volume defined by the shim
opening and the first and second pressing surfaces passes outwardly
from the shim opening and into the recesses.
6. The apparatus of claim 5 wherein the first and second pressing
surfaces each comprise an optical flat.
7. The apparatus of claim 6 further comprising a pressure
applicator coupled to the first and second pressing surfaces for
pressing together the first and second pressing surfaces.
8. The apparatus of claim 5 wherein the projections are of a
triangular shape.
9. The apparatus of claim 5 including an oven which receives at
least the first and second pressing surfaces and shim for heating
the material during pressing.
10. In semiconductor processing, an apparatus for forming a layer
of a material having a substantially uniform thickness and
substantially parallel first and second major surfaces, the
material being for use in producing a flat on a semiconductor, the
apparatus having: a pair of pressing elements each having a flat
pressing surface, the pressing surfaces being opposed to one
another and operable to compress a quantity of the material
therebetween; a stop positioned at least partially between the
pressing surfaces, the stop having a thickness substantially equal
to the desired uniform thickness of the layer and being positioned
to establish a spacing between the flat pressing surfaces which is
substantially equal to the thickness of the stop and thereby to the
desired uniform thickness of the layer when the pressing elements
engage the stop; and whereby engagement of the stop by the pressing
surfaces during pressing of the material forms a layer of the
material for use in producing a flat on a semiconductor, the layer
of material being of substantially uniform thickness with
substantially parallel major surfaces formed by the flat pressing
surfaces.
11. A release member for preventing a surface layer on a
semiconductor substrate assembly from adhering to a surface of a
press, the release member comprising: a body in the form of a sheet
having first and second major surfaces; the first and second major
surfaces each being within one hundred angstroms of being flat; the
first and second major surfaces being at least within five
millionths of an inch of being parallel to one another; the body
being of a material which releases from the press surface when
engaged by the press surface; and whereby when the first major
surface is pressed against the press surface and the second major
surface is positioned against the surface layer on the
semiconductor substrate assembly, the release member prevents the
layer on the semiconductor substrate assembly from adhering to the
press surface.
12. A release member according to claim 10 wherein the first and
second major surfaces are at least within fifty angstroms of being
flat.
13. A release member according to claim 11 wherein the surface
layer on the semiconductor substrate assembly contains a layer of
epoxy.
14. A sheet of a material used in producing a flat on a substrate
assembly, the sheet of material being capable of flowing without
melting when heated to a temperature below the melting point of the
material, the sheet comprising: a body having a central region with
first and second opposed major working surfaces; the first and
second major working surfaces each being within fifty angstroms of
being flat and being at least within five millionths of an inch of
being parallel to one another.
15. A sheet according to claim 14 wherein the material is
ultraviolet transmissive FEP.
16. A sheet according to claim 14 wherein the material is PTFE.
17. In an apparatus for producing a sheet of material for use in
semiconductor processing, a stop for positioning between first and
second pressing elements to limit the extent to which the pressing
elements approach one another during a pressing operation, the stop
comprising a reinforcing section of a first thickness and a stop
section of a second thickness which is less than the first
thickness; the reinforcing section comprising a closed geometric
shape which bounds a central portion of the stop; and the stop
portion extending inwardly into the central portion from the
reinforcing portion, the stop portion spanning less than the entire
area of the central portion to provide a void in the central
portion.
18. A stop according to claim 17 wherein the stop portion comprises
a plurality of projecting fingers extending inwardly into the
central portion.
19. A stop according to claim 17 wherein the stop portion is about
twenty-thousandths of an inch thick.
20. A stop according to claim 18 wherein the fingers comprise
triangular teeth extending radially inwardly into the central
portion from the annulus.
21. A stop according to claim 18 wherein each of the fingers
comprises a tooth which is triangular in shape with an apex
positioned inwardly of a base, the tooth having first and second
sides intersecting one another at the apex, the first and second
sides defining an acute angle therebetween.
22. A stop according to claim 21 wherein the acute angle is about
thirty degrees.
23. A method of forming a layer of a material on a substrate
assembly comprising: heating the material; pressing the material
between first and second flat pressing surfaces; disposing a stop
between the first and second pressing surfaces to limit the extent
to which the first and second pressing surfaces approach one
another during pressing to thereby form a layer of a substantially
uniform thickness having first and second major surfaces with the
first and second major surfaces being formed by the flat pressing
surfaces; and applying one of the first and second major surfaces
to a surface of a substrate assembly.
24. The method of claim 23 wherein the heating act comprises
heating the material until the material transitions to a plastic
state without melting the material.
25. The method of claim 23 wherein the substrate assembly has an
epoxy layer and the applying act comprises applying one of the
first and second major surfaces of the layer to a surface of the
epoxy layer of the substrate assembly.
26. A method of forming a layer of material for use in forming a
flat on a semiconductor, the material having first and second major
surfaces which are substantially parallel to one another and
substantially flat, the method comprising: pressing first and
second optical flats toward one another; disposing a shim at least
partially between the optical flats to limit the extent to which
the optical flats approach one another and to thereby establish the
uniform thickness of the layer; providing an open central region in
the shim for receiving the material to be formed onto the layer;
and flowing excess material outwardly from the central region into
pockets provided in the shim during pressing.
27. The method of claim 26 further comprising heating the
layer.
28. The method of claim 26 wherein the material is FEP and
comprising heating the layer to a temperature at which the layer
becomes plastic but remains below the melting point
temperature.
29. The method of claim 26 wherein the material is PTFE and
comprising heating the layer to a temperature at which the layer
becomes plastic but remains below the melting point
temperature.
30. The method of claim 26 further comprising removing excess
material from the peripheries of the first and second pressing
surfaces and thereafter allowing the formed layer to cool.
31. A semiconductor substrate assembly in production comprising: a
semiconductor flat comprised of semiconductor material and having a
first surface with at least one surface feature formed therein; an
epoxy layer covering the first surface and the surface feature, the
epoxy layer having a covering material engaging surface spaced from
the first surface; a cover layer overlying the covering material
engaging surface of the epoxy layer, the cover layer having first
and second major opposed surfaces with a thickness between the
first and second major opposed surfaces which is within at least
five millionths of an inch of being uniform, the first and second
major opposed surfaces being within one hundred angstroms of being
flat.
32. The semiconductor substrate assembly of claim 31 wherein the
cover layer is formed of FEP.
33. The semiconductor substrate assembly of claim 31 wherein the
cover layer is formed from PTFE.
34. The semiconductor substrate assembly of claim 31 wherein the
substantially uniform thickness of the cover layer is about
twenty-thousandth of an inch.
35. The semiconductor substrate assembly of claim 31 wherein the
first and second major surfaces are within fifty angstroms of being
flat.
36. The semiconductor substrate assembly of claim 31 wherein the
cover layer is transparent to ultraviolet radiation.
37. A method of planarizing a semiconductor wafer comprising:
applying one major working surface of a sheet of material having
first and second opposed major working surfaces to an epoxy layer
of a substrate assembly, the first and second major working
surfaces each being within fifty angstroms of being flat and being
at least within five millionths of an inch of being parallel to one
another; pressing an optical flat against the sheet of material and
substrate assembly to planarize the epoxy layer; and removing the
sheet of material.
38. A method according to claim 37 including the step of curing the
epoxy layer through the sheet of material.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to methods and apparatus for forming
a uniform layer of material for use in connection with
manufacturing a substrate assembly during semiconductor processing,
and also the layer itself. The invention also relates to a method
of planarizing a semiconductor wafer.
[0002] As used herein, "substrate" refers to the lowest layer of
semiconductor material in a semiconductor wafer, and "substrate
assembly" refers to a substrate having at least one additional
layer with structures formed thereon.
[0003] "Semiconductor flat" refers to a surface of the substrate
assembly having a precision flat surface within desired tolerances.
A significant aspect of semiconductor processing is planarization,
i.e., ensuring that the semiconductor flat and other layers are
planar within a predetermined specification.
[0004] Production methods for semiconductors are known. A
particular class of methods involves: etching or otherwise forming
desired channels or trenches in a substrate assembly surface,
applying a dielectric epoxy layer which fills the trenches over the
substrate assembly surface, using an apparatus to press the
substrate assembly having the epoxy layer to achieve desired
surface characteristics (e.g., flatness) on the epoxy layer, and
then removing the pressed substrate assembly from the apparatus for
further processing. The epoxy may be of a type which is cured with
ultraviolet radiation.
[0005] Removing the pressed substrate assembly from the apparatus
is difficult, however, because the epoxy begins bonding with the
pressing surface. Therefore, according to some methods, the epoxy
layer is first covered with a layer of a cover material before the
pressing takes place. The cover material is selected to allow easy
removal/release of the pressed substrate from the apparatus.
[0006] In addition, the cover or release member must be transparent
to the ultraviolet radiation if an epoxy of the type cured by
ultraviolet radiation is used to cure the epoxy layer beneath the
cover material. It has been previously determined that fluorinated
ethylene-propylene (FEP) can be used as the cover material. Some
types of FEP are transparent to ultraviolet radiation, and thus do
not affect the epoxy curing by ultraviolet radiation passing
through the cover.
[0007] The cover material is placed over the epoxy layer before the
substrate assembly is pressed, and thus the cover material surface
characteristics are transferred to the substrate assembly surface.
If the cover material is a uniform layer, which is defined as a
layer having parallel major (top and bottom) surfaces that are
planar, within predetermined tolerances, the pressing action
applied through the cover material will be uniformly transferred to
the epoxy layer as desired. As one result, if the cover material is
a uniform layer, the substrate assembly surface can be formed to
the same flatness as the pressing surface.
[0008] In practice, achieving a sufficiently uniform layer of a
cover material such as of FEP has not been achieved utilizing known
techniques. Because of the nature of FEP material and the desired
thickness of a typical cover (about 0.020 in. thick), the
dimensions of a FEP cover are difficult to control. For example, in
one approach where ultraviolet transmissive FEP has been heated to
a temperature below its melting point and pressed between two
optical flats during pressing, the major surfaces of the resulting
FEP layer end up significantly skewed or out of parallel from one
another. As used herein, optical flats are defined as precision
pressing surfaces, e.g., surfaces that are flat to within one
quarter of a wavelength of light.
[0009] The temperature range for processing the FEP is very narrow.
An acceptable temperature is slightly below the melting glass flow
transition point, which allows the FEP material to acquire the
surface smoothness characteristics of the optical flats. Since high
pressures are required to make the FEP surface conform to the
optical flats surfaces, at temperatures below the glass transition
point (i.e., in the plastic state), maintaining the material at a
consistent thickness is very difficult. This difficulty is due to
the uncontrolled movement of FEP material from the higher pressure
zones to the lower pressure zones at the perimeter of the pressing
mechanism. Consequently, the thickness of the layer is no longer
satisfactorily uniform.
[0010] When used as a cover layer, this non-uniformity in thickness
caused variations in the thickness of the epoxy layer.
Consequently, during subsequent semiconductor wafer processing,
involving etching through the epoxy layer, undesirable non-uniform
etching would occur because thinner portions of the epoxy layer
would be etched through first. For example, FEP sheets exhibiting
these problems had major surfaces which were flat to within about
30-35 angstroms, but which were only parallel to one another within
.+-.0.010 in., have been obtained using known processes.
[0011] Accordingly, it would be desirable to provide a method and
apparatus by which FEP and other materials used as cover layers on
a substrate assembly could be produced within desired uniform layer
specifications.
SUMMARY
[0012] Wafer planarization is enhanced utilizing a layer of
material having a substantially uniform thickness and substantially
parallel first and second major surfaces. The layer is used in
producing a flat on or planarizing a substrate assembly.
[0013] In one embodiment, an apparatus having a substantially
uniform thickness and substantially parallel first and second major
surfaces includes a pair of pressing elements and a stop. The layer
of material formed by the apparatus used in producing a flat on
semiconductors. Each of the pair of pressing elements has a flat
pressing surface. The pressing surfaces are opposed to one another
and operable to compress a quantity of the material therebetween.
The stop is positioned at least partially between the pressing
surfaces and has a thickness substantially equal to the desired
uniform thickness of the layer. The stop is positioned to establish
a spacing between the flat pressing surfaces that is substantially
equal to the thickness of the stop and thereby to the desired
uniform thickness of the layer when the pressing elements engage
the stop. As a result, engagement of the stop by the pressing
surfaces during pressing of the material forms a layer of the
material of substantially uniform thickness with substantially
parallel major surfaces formed by the flat pressing surfaces.
[0014] The apparatus can also include a heater that heats the
material to a temperature where it flows without melting. Further,
the apparatus can include a compression force applicator to move
one or both of the pressing surfaces. The compression force
applicator can include a plurality of biasing elements.
[0015] The pressing surfaces can be optical flats. The shim can
have a plurality of projections extending inwardly from the border
portion with overflow material recesses positioned between the
projections. The projections can be of a triangular shape.
[0016] In a specific example, the first and second major surfaces
of the layer are each within 100 angstroms of being flat.
Preferably, in this example, the first and major second surfaces of
the layer are at least within 0.000005 in. of being parallel to one
another. In this example, a stop portion of the shim is about 0.020
in. thick. The cover layer may also be transparent to ultraviolet
radiation.
[0017] According to an exemplary method, a layer is formed by
heating material and pressing the material between first and second
flat pressing surfaces. A stop is disposed between the first and
second pressing surfaces to limit the extent to which the first and
second pressing surfaces approach one another during pressing to
thereby form a layer of substantially uniform thickness having
first and second major surfaces with the first and major second
surfaces being formed by the flat pressing surfaces. Thereafter,
one of the first and major second surfaces of the formed layer may
be applied to a flat surface of a substrate assembly. In this
approach, the heating step may include heating the material until
the material transitions to a plastic state without melting.
[0018] The formed layer may be applied, for example, over an epoxy
layer of a substrate assembly. The assembly may then be pressed by
precision optical flats with the flatness of the optical flats
being transferred to the epoxy layer through the formed layer. The
formed layer in this case prevents the epoxy layer from adhering to
the pressing apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a side view of an apparatus for achieving a
uniform thickness of a material to be applied to a substrate.
[0020] FIG. 2 is a top view of an upper lid of the apparatus of
FIG. 1.
[0021] FIG. 3 is a bottom view of a lower lid of the apparatus of
FIG. 1.
[0022] FIG. 4 is a top view of the shim of the apparatus of FIG.
1.
[0023] FIG. 5 is a side sectional view of the shim of FIG. 4 along
the line V-V.
[0024] FIG. 6 is a magnified view of an edge portion of the shim
sectional view of FIG. 5 showing a tooth portion.
[0025] FIG. 7 is a side sectional view of the shim of FIG. 4 along
the line VII-VII and corresponding to FIG. 6, but showing an open
region.
[0026] FIG. 8 is a side sectional view of an edge portion of the
apparatus showing the upper optical flat beginning to press against
material applied on the lower optical flat with the shim between
the upper and lower optical flats, while being heated in an
oven.
[0027] FIG. 9 is a side sectional view of a portion of the
apparatus of FIG. 8 showing the apparatus after pressing is
complete with the upper and lower optical flats in contact with the
shim and the material within the shim pressed to a uniform
thickness.
[0028] FIG. 10 is a graph of time-temperature profiles showing the
temperatures of a heater element, an oven air temperature and a
representative FEP material being pressed during a heating
process.
[0029] FIG. 11 is a schematic side view of a substrate assembly
with a cover layer applied over an epoxy layer.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0030] FIG. 1 shows one form of a press assembly 100 for achieving
a desired uniform layer of a material to be applied on a substrate
assembly during manufacture. The uniform layer is used in producing
a flat on a semiconductor. The assembly 100 includes an upper lid
102, a lower lid 104, an upper optical flat 112, a lower optical
flat 114 and a stop which limits the extent to which flats 112, 114
approach one another and which may take the form of a shim 118. In
the illustrated embodiment, these components each have a generally
circular periphery, and are coaxially aligned with each other. For
clarity, the upper lid 102 and upper optical flat 112 are shown
spaced from the shim 118, lower optical flat 114 and lower lid
104.
[0031] During operation of the assembly 100, the upper optical flat
112 and the lower optical flat 114 serve as pressing elements that
are pressed together under predetermined heating conditions against
the shim 118, thereby pressing material applied on the lower
optical flat 114 within the shim 118 to a uniform thickness. As
shown in FIG. 1, a lower side of the shim 118 contacts an upper
side or pressing surface of the lower optical flat 114. A lower
side of the lower optical flat 114 contacts a supporting surface
108 of the lower lid 104.
[0032] The shim 118 may be annular or ring-shaped with projections
that extend inwardly and space the flats apart to a desired uniform
distance when engaged by the flats. The projections may comprise a
plurality of spaced apart fingers. In the specific form shown, the
fingers comprise tooth points 134 that project inwardly at
regularly spaced intervals (FIG. 4) from a border 136.
Alternatively, the shim 118 may take other forms such as being
shaped as an ellipse, triangle, square, rectangle or other closed
geometrical shape. The tooth points 134 do not span the entire
interior of the shim 118 and thus define an open center area or
void 144. Communicating with the open center area 144 are overflow
material receiving recesses, pockets or open regions 138 that lie
between adjacent tooth points 134. Material in the open center area
144 is pressed to a desired thickness B, which is equal to the
thickness of the tooth points, when the upper optical flat 112 and
the lower optical flat 114 are pressed together in a press
direction A against the tooth points 134.
[0033] As described below, excess material is pressed outward from
between the upper optical flat 112 and the lower optical flat 114
through the open regions 138. The excess material flows outward
from the open center area 144 through the open regions 138 into
areas adjacent the periphery of the first optical flat 112 and the
second optical flat 114.
[0034] The pressing action in the press direction A is achieved
through a compression force or pressure applicator. In an
illustrated embodiment, the pressing action is achieved using
elongated fasteners or bolts 120 that slidably extend through
apertures 122 in the upper lid 102 and apertures 124 in the shim
118, and are threaded into apertures 126 in the lower lid 104.
Threaded ends of the bolts 120 are received in helicoils 132
positioned within the apertures 126. The bolts 120 are each
inserted through one or more biasing elements. In the form shown,
the bolts 120 are each inserted through a pair of Belleville
washers 128, 130 oriented in a stacked back-to-back orientation to
create a pressing action when the bolts 120 are tightened. The
illustrated assembly 100 is preferably secured together by six such
bolts 120 at equally spaced intervals, but for clarity, only two
bolts 120 are shown in FIG. 1. Prior to pressing, the upper optical
flat 112 may be separated from the shim 118 by, for example,
approximately {fraction (3/16)} in.
[0035] The upper optical flat 112 and the lower optical flat 114
are cylindrically shaped and each have at least one precision
pressing surface. The pressing surfaces are preferably flat to at
least to within 100 angstroms and more preferably flat to at least
within 50 angstroms. In a specific example, these optical flats are
half-wavelength flats having a flatness of 30-35 angstroms. The
optical flats may be made of a quartz material. Although the size
of the flats may vary in a specific example, they have a diameter
of approximately 9 in. and a thickness of approximately 11/2 in.
Thus, the upper lid 102, the lower lid 104, the shim 118 and the
bolts 120 are sized accordingly.
[0036] To prevent damage to the quartz material, the upper lid 102
and the lower lid 104 may have an upper supporting surface 106 and
a lower supporting surface 108, respectively, with beveled ends
110. The edges 116 of the upper optical flat 112 and the lower
optical flat 114 are spaced outward of the beveled ends 110. As a
result, the edges 116 of the upper optical flat 112 and lower
optical flat 114 are not directly loaded during pressing.
[0037] The upper lid 102 and the lower lid 104 may be made of a
heat conducting material such as aluminum. The shim 118 may be, for
example, made of stainless steel. The Belleville washers 128, 130
may also be made of stainless steel and rated at, for example, 150
lbs.
[0038] FIG. 2 is a top view of the upper lid 102 showing its upper
surface.
[0039] FIG. 2 shows the six equally spaced apertures 122 separated
from each other by an angle E (i.e., 60.degree.). FIG. 2 also shows
the relative positions of the upper optical surface 106 and the
bevel 110 on the lower surface of the upper lid 102.
[0040] FIG. 3 is a bottom view of the lower lid 104 showing its
lower surface. Similar to the upper lid 102, FIG. 3 shows the six
equally spaced apertures 126 separated from each other by the angle
E, as well as the uniform lower support surface 108 and the bevel
110 on the upper surface of the lower lid 104. The apertures 126 of
the lower lid 104 are fitted with helicoils 132 (not shown), as
described above, for receiving threaded ends of the bolts 120.
[0041] FIG. 4 is a top view of the illustrated shim 118 showing its
upper surface with the border portion 136 from which the inwardly
projecting tooth points 134 extend. The six equally spaced
apertures 124 shown in this example extend through the border
portion or reinforcing section 136. Each tooth point 134 defines an
acute included angle F. Although variable, in the form shown, the
angle F is 30.degree.. Apexes of adjacent tooth points 134 are
separated from each other by an acute tooth point spacing angle G.
In the illustrated embodiment, the angle G. although it may be
varied, is 10.degree., and thus there are 36 tooth points 134
total. There are also 36 open regions 138 interspersed between
adjacent pairs of the tooth points 134. The major surfaces (i.e.,
the top and the bottom) of the teeth 134 are formed to be parallel
with each other within a desired tolerance. In a specific example,
this is +0/-0.000005 in.
[0042] The open central area of the shim, between the apexes of a
pair of diametrically opposed tooth points 134, is sized large
enough to result in a uniform sheet of the desired size. For
example, a circular central area having a diameter of 8.12 inches,
between the apex of a tooth and the apex of a diametrically opposed
tooth, may be used to produce a circular sheet of material having
the desired uniform thickness and flatness, which is at least eight
inches in diameter. The use of pointed teeth for the projections
facilitates the flow of material past the projections and minimizes
the possibility of non-uniformities in the sheet extending inwardly
into the central area from the teeth. Alternatively, the sheet may
be made significantly oversized, in which case non-uniformities at
the edge of the sheet may be trimmed while still having a sheet of
the desired size with the desired uniformity.
[0043] FIG. 5 is a side sectional view of the shim 118 along the
line V-V of FIG. 4. FIG. 6 is a magnified view of a right side
portion of the sectional view in region VI of the shim 118 of FIG.
5. FIG. 6 shows the extent by which the tooth points 134 extend
inwardly from the border portion 136. As also shown in FIG. 6, the
border portion 136 has a thickness H that is substantially greater
than the thickness B of the tooth points 134 extending inwardly
from the border portion 136.
[0044] FIG. 7 is a sectional view of the shim 118 along the line
VII-VII of FIG. 4 on a scale comparable to FIG. 6. FIG. 7 shows the
extent of the open regions 138 between adjacent tooth points 134,
as well as the adjacent tooth point 134' in the counterclockwise
direction.
[0045] FIG. 8 is a partial side view of a right end of the upper
optical flat 112, the lower optical fiat 114 and the shim 118. The
portion of the shim 118 shown in FIG. 8 is the same as in FIG. 7,
i.e., showing one of the open regions 138 and the adjacent tooth
point 134'. In FIG. 8, a layer 142 of cover material has been
deposited on the lower optical flat 114 and over the tooth points
134 of the shim 118, and the upper optical flat 112 and the lower
optical flat 114 are being pressed together in the direction A,
while being heated in an oven 300. As shown in FIG. 8, the layer
142 has an initial thickness C that is about two times thicker than
the desired layer thickness B.
[0046] FIG. 9 is a view similar to FIG. 8, but showing the
configuration after the upper optical flat 112 and the lower
optical flat 114 have been pressed together until stopped by the
shim 118. As shown in FIG. 9, the layer 142 has been pressed to the
thickness B uniformly, and excess material has been forced out from
between the upper optical flat 112 and the lower optical flat 114
along the path D through the open regions 138.
[0047] Assume the layer 142 is to be of FEP, and the desired
thickness B of the layer 142 is 0.020 in. To manufacture such a
layer, one specific approach is as follows:
[0048] (1) the layer 142 is initially deposited on the lower
optical flat 114 within the open center area 144 of the shim 118 to
a level about twice the desired thickness B (i.e., the starting
thickness of the FEP may be about 0.040 in.);
[0049] (2) the assembly 100 is heated in an oven to cause the layer
142 to flow, but is maintained below the melting point of FEP;
[0050] (3) a spring force in the case applied by the Belleville
washers 28, 30, press the upper optical flat 112 and the lower
optical flat 114 together, in a controlled manner;
[0051] (4) excess FEP passes outward from between the upper optical
flat 112 and the lower optical flat 114 and into the open regions
138;
[0052] (5) after the desired thickness B is reached, i.e., when the
upper optical flat 112 bears against the shim 118, the assembly 100
is allowed to cool;
[0053] (6) the excess FEP is then removed;
[0054] (7) the bolts 120 are loosened and the upper optical flat
112 and the shim 118 are raised; and
[0055] (8) the layer 142, which is a uniform layer having a
thickness B, is removed from the lower optical flat 114.
[0056] Alternatively, only the pressing surfaces, the shim 118 and
the layer 142 need to be heated to cause the layer 142 to flow.
[0057] The raw FEP is typically provided in sheets which are
normally 0.04 in. thick. These sheets are typically formed using
rollers and have significant thickness variations. Also, defects
may exist in these sheets, such as bubbles. Typically, the raw
material sheets are visually screened, and portions having bubbles
or other significant defects that are likely to show up in the
finished layer are discarded. However, minor bubbles or defects in
the raw material near the expected edges of the finished layer may
be allowed to remain as they disappear during pressing and flowing
process of making the finished layer.
[0058] FIG. 10 is one example of a time-temperature profile of
various temperatures in a pressing process in which FEP is used as
the layer 142. The curve 150 shows the temperature of a heating
element within the oven. The curve 152 shows the air temperature
within the oven. The two curves 154 represent the temperature of
the FEP as measured by thermocouples 156, 158 and 160 at the
periphery, center, and halfway between the periphery and the
center, respectively, of the lower optical flat 114 (FIG. 1).
[0059] The melting point of the specific FEP of this example is 270
C. It is desirable to heat the FEP until it transitions to a
plastic state and begins to flow, but does not melt. At point a,
following a soak of approximately 12 hours, the temperature of the
layer 142 is stabilized at about 223 C. An extended soak period is
used to prevent the possibility of overheating the layer 142 beyond
the melting point. It is also desirable to heat the upper optical
fiat 112 and the lower optical flat 114 evenly, i.e., until the
temperatures of the peripheries and the centers of the optical
flats are within 1/2 to 1 C. of each other.
[0060] After point a, the temperature of the oven is raised, as
shown in the curves 150 and 152, to increase the temperature of the
layer 142 slightly. Thereafter, the layer 142 reaches the
temperature at which the FEP flows, and the pressing takes place
until stopped by the shim 118.
[0061] In another example using PTFE as the layer 142, a
time-temperature profile similar to FIG. 10 may be used. The
melting point of one specific PTFE is approximately 317 C., and the
soak temperature is approximately 270 C. Besides these differences,
the process is generally similar to the process described above for
the layer 142 made of FEP. Of course, other temperature heating
profiles may also be used.
[0062] With the pressing complete, excess material is trimmed from
the assembly 100 near the peripheries of the upper optical flat 112
and the lower optical flat 114 such as with a dull knife.
[0063] The pressed uniform layer 142 is then allowed to cool, for
example, slowly to avoid thermal shock. In one process, the pressed
layer 142 is allowed to cool for approximately 6 hours. Over the
course of the cool down period, the layer 142 may shrink by 0.050
to 0.100 in diameter. After the cool down period is concluded, the
pressure is released, and the layer 142 is complete. The cover
layer may be removed and used in subsequent semiconductor
processing.
[0064] FIG. 11 is a schematic side view of a substrate assembly
with a cover layer. As shown in FIG. 11, the uniform layer 142 that
has been pressed to uniform thickness has been applied over an
epoxy layer 200 of a substrate assembly 202 before the substrate
assembly 202 is subsequently pressed and cured with ultraviolet
radiation. A pressing apparatus is shown schematically, in a state
separated from the substrate assembly 202, at 206. The epoxy layer
200 has been applied to fill trenches 204 in the substrate assembly
202.
[0065] With the layer 142 in place between the pressing apparatus
206 and the epoxy layer 200, the completed substrate assembly 202
is easily removed from the pressing/curing assembly (if necessary,
air can be directed between the layer 142 and the pressing surface
of the pressing apparatus 206 to facilitate removal). Because the
layer 142 is uniform (the major surfaces are substantially flat and
parallel), the precision of the pressing surface of the pressing
apparatus 206 is transferred to the epoxy layer 200 of the
substrate 202. One suitable epoxy is DEN431 Novalak resin mixed
with a solvent to achieve a desired consistency.
[0066] Although FEP is a preferred cover material for use as the
layer 142, other plastic materials that can be heated to a plastic
state without melting can also be used, with consideration of the
other requirements discussed above. One specific FEP is available
from McMaster-Carr of Los Angeles, Calif. under the catalog
designation 85375K114.
[0067] In the methods and apparatus described above, one of the
pressing surfaces remains stationary, whereas the other of the
pressing surfaces is moved. Optionally, both pressing surfaces may
be moved toward each other, as would be known to those with
ordinary skill in the art.
[0068] Having illustrated and described the principles of our
invention with reference to several preferred embodiments, it
should be apparent to those of ordinary skill in the art that the
invention may be modified in arrangement and detail without
departing from such principles. We claim as our invention all such
modifications that fall within the scope of the following
claims.
* * * * *