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Description  |
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This invention relates in general to semiconductor processing and, more
particularly, to a method for subdividing a semiconductor wafer into
pellets.
It is almost the universal practice to form relatively small semiconductor
devices in large numbers on wafers of silicon or other semiconductor
materials and then to divide the wafers into a plurality of pellets, each
of which is a semiconductor device. In this way, simultaneous processing
of a great number of devices may be accomplished with attendant saving in
time and expanse. The subdivision of wafers, which wafers may be quite
thin and brittle, has heretofore been accomplished in a number of ways,
each of which suffers from one or more disadvantages. Quite commonly,
wafers are scribed along lines which define the boundaries of the
individual pellets, the scribing serving to weaken the wafer along the
scribed lines, and the wafers are then broken into individual pellets.
This breaking is oftentimes accomplished by placing the scrived wafer on a
resilient surface and deforming the wafer, for example, by rolling a rigid
member over the surface opposite the resilient surface to fracture the
wafer along the scribed lines. This method, while relatively inexpensive
and readily performed, suffers from the disadvantage that the pellets
themselves are oftentimes subjected to stresses, during the fracturing
process, which adversely affect the characteristics of the completed
devices. Similarly, where it is desirable, for example, to passivate
certain portions of the wafers, and especially the edges thereof, with
brittle passivating materials such as glass or the like, undesirable
degradation, for example, cracking of the passivation, oftentimes occurs
during the breaking of the wafer into pellets.
In an attempt to partially solve these and other associated problems, it
has been suggested to saw rather than scribe the wafer prior to dividing
by breaking. One prior art method for dividing wafers into pellets
involves sawing partially through the wafer and then completely dividing
the wafer into pellets by breaking the partially sawed wafer as has been
hereinabove described in conjunction with scribing. This method produces
somewhat less stress on the pellets than the foregoing scribing method of
division, but is still not an optimum technique.
Sawing completely through the wafer has been recognized as a partial
solution to at least some of these problems. In the past, sawing all the
way through the wafer has been accomplished by mounting the wafer on a
support material such as graphite, with an intermediate adhesive layer
such as wax, and then sawing completely through the wafer and at least
partially into the wax for dividing the wafer into pellets. It has been a
disadvantage of this method that, first, it is difficult to provide
adequate adhesion of the pellets to the support therefor, both during and
after the sawing operation; and second, it is difficult to both readily
remove the pellets from the adhesive and to do so without damaging them.
Generally, wax residue must be removed from the pellets by dissolving it
with an appropriate solvent. This requires that the pellets be
subsequently handled in what is commonly referred to as the loose pellet
form, that is, not conveniently mounted to any support, but rather loose
in a container. This type of handling suffers from several disadvantages.
It is costly, it is time consuming, it increases the likelihood of
damaging the pellets during operations subsequent to dividing the wafers,
it requires that the pellets be individually oriented for further
processing, and, not the least significant, it exposes the pellets to
damage by the solvents used to remove the wax residue.
Accordingly, it is an object of this invention to provide an improved
method for dividing a semiconductor wafer into pellets which overcomes the
disadvantages of prior art methods.
It is another object of this invention to provide an improved method for
subdividing a semiconductor wafer into pellets which improves the quality
of the pellets produced by reducing the stresses created in the pellets
during the dividing operation.
It is yet another object of this invention to provide an improved method
for subdividing a semiconductor wafer into pellets which allows for the
subsequent automatic or semiautomatic handling of the pellets by virtue of
the fact that they are not loose, but rather are removably mounted on a
carrier which is readily compatible with subsequent processing by
automatic equipment.
It is still another object of this invention to provide an improved method
for subdividing a semiconductor wafer into pellets which does not require
any potentially, pellet-damaging solvent to remove adhesive materials
subsequent to dividing.
It is a further object of this invention to provide an improved method for
subdividing a semiconductor wafer into pellets which allows the wafer to
be sawed all the way through so as to eliminate the necessity for breaking
the wafer into pellets.
Briefly stated, and in accordance with a presently preferred embodiment of
the invention, a semiconductor wafer is divided into a plurality of
pellets by applying a strippable vinyl coating to the wafer, adhering the
coated wafer to a membrane, sawing all the way through the wafer and at
least partially through said coating, and removing the pellets from the
membrane.
In accordance with one aspect of the invention, the strippable coating may
be a resinous vinyl coating which is readily applied to the wafer in
liquid form, and then cured. Advantageously, the liquid coating may be
simultaneously evenly distributed over the surface of the wafer, and cured
by spinning the wafer after the application of the coating in liquid form.
The wafer may be readily attached to the membrane by the application of
heat to the juxtaposed combination of membrane and coated wafer, and the
combination then sawed. After sawing, the individual pellets remain
attached to the membrane and in the same orientation as they had when in
wafer form. Subsequent removal of the pellets and mounting thereof is
readily accomplished by automatic or semiautomatic methods.
The features of the invention which are believed to be novel are pointed
out with particularity in the appended claims. The invention itself,
however, both as to its organization and method of operation together with
further objects and advantages thereof may best be understood by reference
to the following description taken in conjunction with the accompanying
drawing in which:
FIG. 1 is a view of a prior art method for dividing a semiconductor wafer
into pellets;
FIG. 2 is another view of a prior art method for dividing a wafer into
pellets;
FIG. 3 is a view of yet another method for dividing a semiconductor wafer
into pellets in accordance with the prior art;
FIG. 4 is a view of a semiconductor wafer having a plurality of devices
therein and coated with a coupling material in accordance with the
teachings of this invention;
FIG. 5 is a view of means for mounting the coated wafer to a membrane in
accordance with this invention.
FIG. 6 is a view of a wafer mounted to a membrane on a fixture ready for
sawing in accordance with this invention;
FIG. 7 is a view of the sawing of a wafer;
FIG. 8 is a view of a wafer after division into pellets by sawing while
remaining attached to a membrane in accordance with this invention;
FIG. 9 is an enlarged view of a portion of FIG. 8 showing the depth of saw
cut with particularity.
FIGS. 10 and 11 are diagrammatic views of a method and apparatus for
removing pellets from a membrane in accordance with this invention.
Referring now to FIG. 1, there is illustrated a prior art method for
dividing a semiconductor wafer including a plurality of semiconductor
devices therein into a plurality of pellets each of which comprises a
single device. A wafer 10 includes a plurality of regions 12 therein
forming a like plurality of semiconductor devices upon division. While
relatively simple devices are illustrated in FIG. 1 and in the remaining
Figures, for purpose of enabling a ready understanding of both the prior
art and the improvement of this invention, it will be understood that the
invention is widely applicable to dividing into pellets, wafers including
devices ranging from the most simple two-region diodes to integrated
circuits of vast complexity including many thousands of elements.
Wafer 10 is initially scribed, as for example, by laser scribbing, to
weaken the crystal along intersecting lines which will be the lines along
which fracture of the wafer will subsequently occur. Typically, after
scribing, the wafer is placed upon resilient mat 14 and rolled by member
16 to cause fracture to take place along lines 18. The individual pellets
may be picked up by any convenient means and further processing thereof
accomplished.
In accordance with preferred pelletizing according to the prior art as
described by U.S. Pat. No. 3,918,150, the wafer may advantageously be
mounted to a membrane before dividing and the membrane stretched after
dividing to separate the pellets and to maintain them attached to and
usefully oriented on the membrane for subsequent removal.
The method of FIG. 1 has become widely employed in the semiconductor
industry despite the several disadvantages thereof in certain applications
as hereinabove identified.
FIG. 2 illustrates another prior art method for dividing wafers into
pellets which attempts to eliminate some of the problems associated with
the method of FIG. 1. A wafer is sawed part of the way through as
indicated by slots 22. Since the wafer remains intact, the sawing is
easily accomplished by holding it in any well known manner as, for
example, by vacuum chuck or the like, and sawing.
The same saws as have heretofore been utilized for dividing wafers in
accordance with the above method and the method hereinbelow described may
be utilized in accordance with the improved method of the instant
invention.
After sawing, the wafer is inverted and placed on resilient mat 14 which
may be of the same characteristics as like-numbered mat 14 of FIG. 1. A
roller 16 is employed in a similar manner as like-numbered roller 16 of
FIG. 1 to fracture wafer 20 into a plurality of pellets. While the rolling
of wafer 20 and of wafer 10 of FIG. 1 are illustrated as being
accomplished from opposite sides with respect to the devices formed
therein, it will be understood by those skilled in the art that consistent
with not damaging the devices formed in the wafers, rolling and the
attendant scribing and partial sawing operations may be performed from
either side of the wafer.
FIG. 3 illustrates a wafer 34 which has already been divided into pellets,
attached to a support 26 by an adhesive layer 28. In accordance with this
prior art method for dividing a wafer into pellets, wax is oftentimes
utilized as an adhesive. Wax is usefully employed insofar as it is readily
applied in a relatively thick layer for permitting the sawing of the wafer
into completely separate pellets. Wax, however, adheres tenaciously to
pellets 24 after dividing and the pellets may not be directly utilized
after revoval from support 26 following sawing. A wax residue remains on
the pellets and they must be cleaned prior to mounting or further use.
Accordingly, a solvent is typically employed to remove the wax residue
from pellets 24. It has long been a problem to identify a solvent which
completely removes wax from the pellets without damaging them. Still
further, even if the pellets could be completely cleansed of the wax
residue after sawing, they are, nevertheless, no longer held in place on a
carrier such that they can be automatically handled for subsequent
processing. They must be individually handled as loose pellets at
consequent increase in the complexity and cost of handling.
FIG. 4 illustrates the first step in an improved method for dividing a
wafer into pellets in accordance with this invention. It will be
understood that prior to the step illustrated in FIG. 5, the wafer has
undergone the required processes for forming therein a plurality of
semiconductor devices as, for example, the steps of masking, diffusion,
crystal growth, annealing, and the like, as well as testing if desired. In
accordance with one embodiment of the invention, the individual devices
may be tested while the wafer is intact, marked, or the relevant results
otherwise stored; and the pellets, which remain in the same relative
positions after sawing, sorted according to the test results.
After the foregoing processing, wafer 30 is provided with a layer of
ductile coating material 32 on one surface thereof. Preferably, coating 32
is applied to the side of wafer 30 opposite the side on which devices are
formed. Conventionally, one side of a semiconductor pellet is adapted as,
for example, by the provision of a metal layer (not shown) thereon, to be
directly connected to an electrical contact. Coating layer 32 is
preferably applied to the metal layer. One advantageous method for
applying layer 32 is to apply the coating material, for example, with a
dropper, in a liquid form, and then to evenly distribute it over the
surface of the wafer by spinning the wafer as for example on fixture 34 to
which wafer 30 may be temporarily attached by adhesive holding layer 33,
or the like. Adhesive layer 33 may conveniently be double-sided adhesive
tape which may be readily replaced as required to avoid contaminating
wafer 30. Spinning the wafer both uniformly distributes the coating over
the surface of the wafer and cures the coating. Preferably, a final
thickness of coating on the order of 1-2 mils is desired for providing
both secure wafer and pellet holding to a membrane and ready removal of
the pellets after pelletizing.
The coating 32 is preferably a ductile material such as a synthetic vinyl
resin having proper stretchability and the other properties hereinafter
set forth. Typical examples of suitable resins include: Coveriac SC267,
SC250L, SC249, SC3808, and SC515, all manufactured by Spraylat
Corporation, Mount Vernon, N.Y., These and other suitable coatings are
commonly known in the art as strippable coatings due to their
characteristic of forming a deformable plastic layer upon curing which is
readily removable from the surface to which it is applied by peeling the
coating from the surface, or in the case of the instant invention, by
peeling the pellet from the coated membrane. The characteristics of the
coating and the membrane are similar, and the coating is to be considered
not as an adhesive which sticks the wafer to the membrane, but rather as a
material which is of like character with the membrane and, which, upon
heating of the combination, becomes part thereof, adhering more
tenaciously to the membrane than to the wafer, or subsequently, to the
pellets. When the pellets are removed from the membrane, the coating does
not adhere to the pellets, and it is not necessary to perform any
additional steps for the removal of residue from the pellets.
While the method of this invention is widely applicable, certain surfaces
are to be avoided for achieving good holding of the pellets to the
membrane. For example, exceptionally smooth, highly polished surfaces have
been found to provide, in combination with the strippable coating,
inadequate holding strength between the wafer and the membrane to insure
that the pellets are not loosened during sawing and, therefore, likely to
fall off after being completely separated. Accordingly, it is unnecessary
and, in fact undesirable, to provide a highly polished surface on the
wafer prior to attaching the wafer to the membrane. A suitable surface is
provided, for example, by the application of a conductive metal contact
as, for example, gold, by the process of vacuum deposition followed by
alloying or sintering. It is emphasized that the requirement for a surface
which is not polished to a high degree does not impose an additional
burden on those desiring to practice this invention, but, rather, it eases
the requirements on the degree of smoothness of the metal contact
oftentimes found on semiconductor devices of the type to which this
invention is directed. The techniques commonly used for preparing the
surfaces of silicon wafers for the application of metal layers are well
known. Conventionally, the silicon surface is lapped or sandblasted prior
to the deposition thereon by a vacuum deposition process of a gold or
other suitable metal layer. Vacuum deposition as used herein includes
similar processes such as electron beam evaporative plating, other types
of evaporative plating, such as those wherein a coated filament is heated
to create a metal vapor, and any of the other types of plating commonly
employed in the art. After plating, the coated wafer is oftentimes heated
or sintered to alloy the metal layer to the silicon wafer. In some cases,
more than one metal layer may be provided, as exemplified by the commonly
utilized combination of chromium, nickel, and silver.
While examples of typical electrode structures and methods for the
formation thereof have been hereinabove set forth, it is to be understood
that no particular structure or method is required in conjunction with the
techniques of this invention, but rather the invention is widely
applicable and tolerant of a wide variety of contacts essentially as
described, so long as a surface is provided having a degree of roughness
such that a strippable coating will adhere thereto.
FIG. 5 illustrates the attaching of coated wafer to membrane 40. Wafer 30
is temporarily held on vacuum fixture 35. Preferably, protective member 37
which may conveniently be a soft paper member is placed between wafer 30
and fixture 35 for preventing damage to the surface of the wafer. Membrane
40 is placed in contact with coating 32. A second soft paper element 39 is
placed on the opposite surface of membrane 40 from wafer 30. Heat is
applied by heated element 41 to the combination of wafer 30 and membrane
40 through paper element 39. Paper element 39 is useful in subsequent
steps as will be described. Membrane 40 is bonded to coated wafer 30, a
stronger bond being created between the coating and the membrane than
between the coating and the wafer. Due to the similarity of composition of
the coating and the membrane, the nature of the bond between the coating
and the membrane is not of the nature of an adhesive bond, but rather a
continuous layer of vinyl material is formed including the membrane and
the previously applied coating layer.
FIG. 6 illustrates the coated wafer mounted to membrane 40 which in turn is
mounted on a carrying frame 36 for easy handling. The membrane is
preferably of a plastic material which is of carefully controlled and
uniform thickness so as to permit sawing to a preset depth without cutting
through the membrane. More preferably, the membrane is a vinyl membrane
which is at least slightly stretchable for mounting to frame 36 by a
convenient means exemplified by O-ring 38. A preferred membrane for use
with the processes of this invention is available from the Semiconductor
Equipment Corporation, Newbury Part, Calif. and, is known in the
semiconductor industry as "Hugle Membrane". The membrane is stretchable,
is manufactured of vinyl, and is available in a variety of thicknesses, 4
mil thick membrane being usefully employed herein.
As has been hereinabove described, the wafer 30 has been coated with layer
32 of strippable coating. The coated wafer may be stored for a convenient
time prior to mounting on membrane 40. Mounting is accomplished by
physically placing the membrane and the coated wafer in face-to-face
relationship, the coated surface of the wafer abutting the surface of the
membrane, and applying heat to the opposite side of the wafer. Heat may be
applied for example, by placing a heated member against the surface of the
membrane either directly, or through a thin paper element, or the like, to
prevent the heat source from sticking to the membrane. The wafer may be
attached to the membrane prior to mounting the combination on a frame 36.
As mounted to membrane 40, and the combination mounted on frame 36, the
wafer is prepared for sawing. While frame 36 is illustrated as being a
ring-shaped frame, which frame is well suited to subsequent automated or
semiautomated pellet removal, other types of support for the combination
of the membrane and the wafer may be employed. For example, a solid
support having small holes therethrough for communicating a vacuum from
the lower surface to the upper surface may be used. Where a hollow
ring-shaped frame is used, membrane 40 is preferably slightly stretched
for insuring that a relatively flat surface is obtained. Ring 38 serves
two purposes: it holds membrane 40 firmly onto the frame 36 and it
prevents the edges of the membrane from coming in contact with the saw.
As has been hereinabove described, sawing of wafers per se and in fact
sawing all the way through wafers are known in the art. Saws are available
which have been used in the past in conjunction with the aforementioned
wax mounted sawing method which are suitable for use in conjunction with
this invention. Such saws are manufactured by Micro Automation, Inc.,
Santa Clara, CA, Model 1000 Programmed Dicing Saw; DAI-ICHI SEITOSHO Co.,
Ltd., Tokyo, Japan, Model DAD 2 H Automatic Scriber Dicer; and Tempress
Division of Sola Basic Industries, Model 602 Dicing Saw. These saws
exemplify the characteristics which are required to practice this
invention, and are not meant to embody an exclusive list. They share the
common characteristics of being accurately controlled as to depth of cut,
direction of cut, speed of cut, and have the ability to cut silicon or
other semiconductor materials.
FIG. 7 illustrates the sawing of wafer 30 by saw 42. The wafer is mounted
to the membrane as described above and the combination is presented to the
saw by temporarily mounting on a chuck 44 by convenient means as, for
example, by vacuum holding means 46. The actual cutting of wafer 30 may be
performed either by moving the saw relative to the fixed-chuck-mounted
wafer, or by holding the saw fixed and moving the wafer. The first of
these techniques is preferred for sawing all of the necessary cuts for
dividing the wafer in one direction. Rotation of the wafer with respect to
the saw is then accomplished by rotating the chuck precisely 90 degrees so
that the remaining cuts may be made. Several of the saws mentioned above
have the capability for performing the foregoing operations automatically.
FIG. 8 illustrates in more detail the wafer after sawing and also the
mounting of the wafer for sawing. Ring 36 fits over upper portion 47 of
chuck 46 and is held in place thereon by a vacuum which is communicated to
the membrane through passages 49 and which is evenly distributed by paper
element previously applied. Also facilitates the removal of the membrane
from the vacuum chuck. It has been found that without paper element the
membrane tends to stick to the smooth surface of the chuck.
The depth of cut made by saw 42 is at least all the way through wafer 30
and preferably at least partially through membrane 40. In an exemplary
embodiment of this invention wherein excellent results are obtained, a
membrane thickness of 4 mils is employed, a strippable coating of 1 mil
thickness is used to mount the wafer to the membrane, and any desired
wafer thickness may be used. A saw cut is made which extends completely
through both the wafer and the coating and 1 mil into the membrane. It has
been found that the membrane is not adversely affected by this slight
cutting insofar as its ability to securely hold the pellets is concerned
and the pellets are found to be both completely divided and readily
removed from the membrane as will hereinbelow be described.
Saw 42 preferably is a high speed diamond blade saw having a blade width
between about 1.0 and 1.1 mils so that the spacing between pellets on a
semiconductor wafer may be small and the waste represented thereby reduced
to the extent possible.
FIGS. 10 and 11 depict apparaus for removing the separated pellets from
membrane 40 after sawing. It is emphasized that pellets after sawing are
retained on membrane 40 in the same orientation that they had on the wafer
and in the same relative position. This allows for test of the pellets
while still in wafer form with sorting accomplished after pelletizing as a
function of the earlier test results. Pellet pick up may be accomplished
in a number of ways. For example, a vacuum pick-up arm 52 may be employed
to lift the individual pellets from the membrane with an optional pointed
member 54 being employed to raise the pellet to be lifted above the level
of the other pellets while at the same time partially separating the
pellet from the membrane as illustrated in FIG. 10. Alternatively, the
pellets may be lifted by a fixture which grasps the pellets at the edges
thereof. After removal of the pellets from membrane 40 by pick-up arm 52,
the pellets may conveniently be deposited in receptacle 56 according to a
sorting procedure accomplished either at the time of removal from the
membrane or at some earlier time. In order to retain the advantages of the
instant invention, the pellets are preferably maintained in the same
respective orientation by the pick-up arm so that subsequent mountdown may
be accomplished by automatic means without reorienting where desired.
While the invention has been illustrated in connection with a presently
preferred embodiment thereof, it will be understood by those skilled in
the art that certain modifications and changes may be made without
departing from the true spirit and scope of the invention. For example,
while the drawings and the accompanying description have described a
sawing method in conjunction with square pellets, other straight-sided
pellets may be accommodated by the method taught herein along with
arbitrarily-shaped, curved pellets where the constraints of the particular
saw which is selected allow. Accordingly, these and other modifications
and changes which are apparent to those skilled in the art are intended to
be covered by the appended claims.
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