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Description  |
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BACKGROUND OF THE INVENTION
The present invention is in the field of semiconductor manufacturing and
specifically relates to an apparatus for acquiring, holding, and releasing
a semiconductor wafer attendant to a polishing process. A typical wafer is
several inches in diameter and about 680 microns thick; one micron is 1
millionth of a meter.
The polishing process is carried out by a sophisticated,
numerically-controlled machine that eliminates the need for intervention
by the human operator of the machine.
A part of the polishing machine called the carrier remains positioned over
the wafer and presses the wafer downwardly against a flat rotating
polishing surface. In a typical polishing machine, the movement of the
carrier is programmed to acquire a wafer from a first station, to
transport the wafer to a polishing surface, to drive the wafer across the
rotating polishing surface, to transport the wafer from the polishing
surface to a second station, and to release the wafer at the second
station. In the present application, the focus is on those parts of the
apparatus that are used to acquire the wafer, to hold it during transport,
and to release the wafer.
THE PRIOR ART
The classical way of apprehending and releasing wafers is the use of a
vacuum head that includes a rigid perforated plate against which the wafer
is drawn by applying a vacuum to a plenum lying behind (above) the
perforated plate. This apparatus is analogous to a household vacuum
cleaner. The wafer is released by removing the vacuum; i.e., by restoring
atmospheric pressure to the plenum.
A companion application, filed simultaneously with the present application
and titled "Wafer Carrier for Film Planarization" deals with a different
aspect of the same polishing machine with which the present application is
concerned. The companion application describes a polishing technique for
removing material uniformly across the face of a wafer, and in particular
for uniformly reducing the thickness of a thin coating that has previously
been applied to a wafer. In that technique, the carrier is provided with a
membrane which presses down with a uniform pressure against the upper
surface of a wafer that is being polished. The uniform pressure results in
a uniform removal of material from all portions of the surface being
polished.
The use of the membrane has proven to be extremely useful in the polishing
step, however the presence of the membrane was thought to preclude the use
of a plurality of vacuum holes for picking up and releasing the wafer
before and after the polishing step, respectively. If the membrane
included a number of perforations, the pressurizing gas or fluid would
escape through the perforations during the polishing step, which could
lead to nonuniform results. The success of the membrane technique
described in the companion application depends upon the membrane being
imperforate, and this was believed to make it impossible to use a vacuum
applied to a perforated plate for apprehending and releasing the wafers.
The present application is concerned with the solution of this dilemma.
SUMMARY OF THE INVENTION
It is an objective of the present invention to provide apparatus for
apprehending a wafer from above, for holding the wafer during transport,
and for releasing the wafer at a desired destination.
It is a further objective of the present invention to provide an apparatus
including an imperforate membrane for apprehending, holding, and releasing
a wafer.
In accordance with the present invention, the upper side of an imperforate
membrane lies against the lower face of a rigid backing plate. The lower
face of the rigid backing plate includes a number of recessed areas which
all communicate with a supply line or hose that is included in the carrier
above the rigid backing plate. When a vacuum is applied to the hose, the
vacuum is communicated to the recessed areas, drawing the membrane into
the recessed areas which then serve as suction cups to hold the wafer
against the membrane. When the vacuum is released, the membrane returns to
its planar configuration because of its resiliency, thereby releasing the
wafer. The application of a small positive pressure to the hose has been
found to expedite release of the wafer.
The novel features which are believed to be characteristic of the
invention, both as to organization and method of operation, together with
further objects and advantages thereof, will be better understood from the
following description considered in connection with the accompanying
drawings in which a preferred embodiment of the invention is illustrated
by way of example. It is to be expressly understood, however, that the
drawings are for the purpose of illustration and description only and are
not intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view in cross section showing the carrier
portion of a polishing machine in accordance with the present invention;
FIG. 2 is a bottom plan view in the direction 2--2 indicated in FIG. 1,
with the membrane partially cut away;
FIG. 3 is a side elevational cross sectional view of the carrier showing
the membrane when a vacuum is applied; and,
FIG. 4 is a side elevational cross sectional view of the carrier showing
the membrane after the vacuum has been relieved.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is part of a wafer polishing machine, specifically,
the carrier. The function of the carrier is to acquire a wafer from a
first station, to transport the wafer to a polishing table, to hold the
wafer while it is being polished, to transport the wafer to a second
station, and to release the wafer at the second station. This operation is
repeated under computer control so that a large number of wafers can be
polished serially without the need for intervention by a human operator.
More specifically, the present invention relates to the acquisition,
holding, and releasing of a wafer by the carrier.
As best seen in FIG. 1, the carrier includes a ring-like part 30, a
disk-like part 32, and a floating piston 34. The floating piston 34 is
capable of limited vertical movement with respect to the ring-like part
30, and in FIG. 1 the floating piston 34 is shown at the lower limit of
its travel. The ring-like part 30 is connected to the lower end of an arm
(not shown) which serves to move the carrier laterally across the
polishing surface, which rotates about a vertical axis.
The inside diameter of the washer 38 is slightly larger than the outside
diameter of the wafer 10, so that the wafer 10 will fit loosely inside the
washer 38. In this manner the wafer is held captive within the washer 38
as the polishing surface rotates and as the carrier moves laterally. The
periphery of the lower side of the membrane 40 is bonded to the washer 38,
and then the periphery of the opposite side of the membrane is bonded to
the thicker washer-like part 39, which in turn is fastened to the
ring-like part 30 by screws, of which the screw 41 is typical.
In a preferred embodiment, the membrane is several hundred microns in
thickness and is composed of a synthetic rubber. As can be appreciated by
workers in the art, a membrane this thin is quite pliable and is utterly
lacking in stiffness.
The carrier includes a floating piston 34. As can be seen in FIG. 1, the
floating piston is movable from a lower position shown in FIG. 1, which is
determined by a shoulder on the ring-like part 30, to an upper position
determined by the disc-like part 32.
When a pressurized fluid is applied through the flexible tube 56, the
pressurized fluid flows through the passage 44 to the recessed regions in
the lower face 58 of the floating piston 34, of which the recessed region
54 is typical. The pressurized fluid urges the membrane 40 downwardly away
from the lower face 58 of the floating piston 34. At the same time, the
pressurized fluid pushes the floating piston upward; the pressure used is
great enough to overcome the weight of the floating piston, the downward
force of the compression springs, of which the spring 52 is typical, and
the friction of the O-ring 48, and therefore the floating piston is driven
upward until it encounters the disc-like part 32.
When the pressure is released so that atmospheric pressure obtains within
the flexible tube 56, the force of the springs 52 and the weight of the
floating piston 34 overcome the friction of the O-ring 48 and permit the
floating piston to return to its lower limit position in which the lower
face 58 of the floating piston bears lightly against the membrane 40.
The acquisition maneuver begins with the floating piston in its lower limit
position and with the pressure in flexible tube 56 relieved to atmospheric
pressure. An air piston 60 beneath the wafer 10 gently elevates the wafer
until it bears lightly against the lower side of the membrane 40. Next, a
vacuum is applied to the flexible tube 56, and it draws away the fluid
from the recessed regions 54 and the passage 44, and pulls the membrane
tightly against the lower face 58 of the floating piston, causing the
membrane to make a sealing contact with the lower face 58. Continued
application of the vacuum then sucks the membrane into the recessed
regions in the lower face 58. The wafer remains substantially planar, so
that reduced pressure areas are produced between the lower surface of the
membrane and the upper surface of the wafer at those areas where the
membrane has been sucked into the recessed regions. The atmospheric
pressure on the lower side of the wafer opposite the recessed regions is
therefore not countered by a corresponding pressure on the upper side of
the wafer, and the unbalanced pressure holds the wafer to its position
against the membrane. The wafer has thus been acquired, and may be held in
this position indefinitely.
Thereafter, the carrier transports the wafer to the polishing station and
the carrier is lowered until the lower side of the wafer rests on the
polishing surface.
Next, pressurized fluid is applied to the flexible tube 56. This causes the
floating piston to be driven to its upper limit position as discussed
above, and causes the membrane to exert a uniform downward pressure
against the upper face of the wafer, which in turn causes the lower face
of the wafer to push against the polishing surface 36 with a pressure that
is uniform at all portions of the lower face of the wafer. This results in
uniform removal of material from the lower face of the wafer as described
in the companion application referred to above which is incorporated by
reference herein.
After a predetermined time the rotation of the polishing surface is
stopped, and it is time for the carrier to acquire the wafer again so that
the polished wafer can be transported to a second station. The acquisition
is again accomplished by applying vacuum to the flexible tube 56. The
recessed areas act as suction cups to hold the wafer against the membrane,
which in turn lies against the lower surface of the floating piston.
When the second station has been reached, it is time for the carrier to
release the wafer, and this is accomplished by applying a lightly
pressurized fluid to the tube 56. One might think that merely restoring
atmospheric pressure to the tube 56 would be sufficient, but is has been
found that the close contact between the upper surface of the wafer and
the lower surface of the membrane limits the rate at which air can flow
between them, making release slow and unpredictable. Therefore, in the
best mode of practicing the invention, a small positive pressure is
applied to the tube 56, to free the wafer from the membrane. The release
having been effected, the carrier returns to the first station to acquire
the next wafer.
It is noted that the main reason the floating piston is movable instead of
fixed in its lower limit position is that during the polishing phase the
lower side of the floating piston would act as a rigid pressing plate
pressing against the wafer. As discussed at length in the companion
application, the use of a rigid pressing plate results in nonuniform
removal of material from the lower surface of the wafer, which is contrary
to the purpose of the polishing. However, if no polishing is to be
performed and if the apparatus is used merely to acquire, transport, and
release articles having a planar surface, then the floating piston could
be replaced by a backing plate fixed in the carrier.
FIG. 2 is a bottom plan view in the direction 2--2 indicated in FIG. 1. It
shows the pattern of the recessed areas employed in the preferred
embodiment, namely, a plurality of concentric annular recessed areas.
FIG. 3 is a side elevational cross sectional view showing how the membrane
40 is sucked into the recessed regions shown in FIG. 2 when a vacuum is
applied to the passage 44.
FIG. 4 is a side elevational cross sectional view showing the membrane
restored to its flat configuration when atmospheric pressure obtains in
the passage 44.
Thus, there has been described a wafer handling apparatus for acquiring,
holding during transport, and releasing a wafer. In the preferred
embodiment a soft imperforate synthetic rubber membrane is drawn tautly
across the lower face of a floating piston or backing plate that includes
a number of recessed areas. The recessed areas communicate with a source
of vacuum, and when the vacuum is applied, the membrane is sucked into the
recessed areas, so that each recessed area becomes a suction cup that
draws the wafer against the membrane which is drawn against the lower face
of the floating piston or backing plate. During a polishing operation, a
pressurized fluid instead of a vacuum is applied, in which case the
membrane exerts a uniform downward pressure on the upper face of the wafer
so that the entire lower face of the wafer will be polished uniformly.
The foregoing detailed description is illustrative of several embodiments
of the invention, and it is to be understood that additional embodiments
thereof will be obvious to those skilled in the art. The embodiments
described herein together with those additional embodiments are considered
to be within the scope of the invention.
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