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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process and apparatus for removing deposits
such as tungsten from the backside and end edge of a semiconductor wafer.
More particularly, this invention relates to a process and apparatus for
removing such deposits with a plasma etch while retaining the
semiconductor wafer in a position adjacent to, but spaced from the
faceplate in a plasma deposition apparatus.
2. Description of the Related Art
In the processing of a wafer or substrate in the prior art, the wafer has
been spaced from a support plate at times for various reasons. For
example, Gillery U.S. Pat. No. 3,907,660 teaches the cathode sputtering of
a metal onto a glass substrate to form a transparent electroconductive
coating. An anode plate supports a series of posts arranged in
checkerboard fashion to support one or more glass sheets during the
cathode sputtering.
Whitlock et al U.S. Pat. No. 4,736,087 discloses a plasma stripping
apparatus for removing photoresist from a wafer. The apparatus is provide
with external electrodes including a cathode located near but below the
top of a reactor chamber in the apparatus and an anode located near the
bottom of the chamber but above the wafer to be stripped. On the chamber
base is a wafer support chuck having a plurality of quartz pins on which a
wafer rests during the stripping operation.
However, in the processing of semiconductor wafers, blanket deposition of a
material such as tungsten over the upper or front surface of the wafer is
sometimes carried out to form patterns of the deposited material over a
masked semiconductor wafer. If the rear surface of the wafer is not
protected, i.e., is spaced from the support plate as described above, it
will also be covered with the deposited material which may later flake
off, thereby creating undesirable particles which can interfere with
subsequent processing, resulting in a lowering of the yield.
To prevent such deposition on the backside of the wafer from occurring, it
has been the practice to clamp or seal the wafer to the backplate or
support base (susceptor) to thereby eliminate or limit exposure of the
rear surface of the semiconductor wafer. However, the materials used in
the blanket deposition still coat the side or end edge of the wafer as
well as depositing on the clamping means or clips used to secure the wafer
to the backplate. Furthermore, the use of such clamping means results in
the shielding of a portion of the front surface of the wafer, thereby
reducing the total area of the front surface of the wafer on which
integrated circuit structures may be formed.
It has previously been proposed by one of us, in Chang et al U.S. patent
application Ser. No. 337,607, now issued as U.S. Pat. No. 4,962,049, on
Oct. 9, 1990 and assigned to the assignee of this invention, to use a
plasma etch to remove unwanted deposits from the backside of a
semiconductor wafer by raising the wafer off the susceptor or backplate a
spaced distance and then forming a plasma in the gap between the wafer and
the grounded susceptor to remove the deposits from the backside of the
wafer. While this method will satisfactorily remove such unwanted backside
deposits from the wafer, some of the materials on the front side of the
wafer may also be inadvertently removed by the plasma during this removal
step.
It would, therefore, be desirable to be able to remove such undesirable
deposits from the backside and end edge of a semiconductor wafer while
inhibiting removal of any materials from the front surface of the wafer
during such a removal step.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide a method and
apparatus for removing, in a vacuum chamber, materials deposited on the
backside and end edge of a semiconductor wafer which comprises urging the
front side of the wafer against the faceplate; forming a plasma in the gap
between the backside of the wafer and the susceptor to remove materials
deposited on the backside and end edge of the wafer; and flowing into the
chamber one or more gases through a space maintained between the wafer and
the faceplate.
It is another object of this invention to provide a method and apparatus
for removing, in a vacuum chamber, materials deposited on the backside and
end edge of a semiconductor wafer which comprises urging the front side of
the wafer against the faceplate; forming a plasma in the gap between the
backside of the wafer and the susceptor to remove materials deposited on
the backside and end edge of the wafer; and flowing into the chamber,
through a space maintained between the wafer and the faceplate, one or
more gases capable of reacting with the material deposited on the backside
and end edge of the wafer.
It is yet another object of this invention to provide a method and
apparatus for removing, in a vacuum chamber, materials deposited on the
backside and end edge of a semiconductor wafer which comprises urging the
front surface of the wafer against a portion of the faceplate; forming a
plasma in the gap between the backside of the wafer and the susceptor
while flowing, in a space between the front surface of the wafer and the
faceplate, one or more gases capable of reacting with the material
deposited on the backside and end edge of the wafer; and providing spacing
means to space the front surface of the wafer from the portion of the
faceplate containing openings for such gases to permit the gases to enter
the space thereby created between the faceplate and the front surface of
the wafer to thereby inhibit the removal of materials from the front
surface of the wafer.
It is still another object of this invention to provide a method and
apparatus for removing, in a vacuum chamber, materials deposited on the
backside and end edge of a semiconductor wafer which comprises urging the
front surface of the wafer against a faceplate having a recessed central
portion containing gas passage openings to thereby create a space between
the faceplate and the front surface of the wafer; forming a plasma in the
gap between the backside of the wafer and the susceptor; flowing, through
the gas passage openings in the recessed portion of the faceplate, one or
more gases capable of reacting with the material deposited on the backside
and end edge of the wafer; and providing spacing means to space the front
surface of the wafer from the recessed portion of the faceplate to permit
the gases to enter the space thereby created between the faceplate and the
front surface of the wafer.
It is a further object of this invention to provide a method and apparatus
for removing, in a vacuum chamber, materials deposited on the backside and
end edge of a semiconductor wafer which comprises urging the front surface
of the wafer against a faceplate having a recessed central portion
containing gas passage openings to thereby create a space between the
faceplate and the front surface of the wafer, the recessed central portion
being of larger diameter than the wafer to permit flow of the gas entering
the recessed central portion through the gas passage openings to flow
around the end edges of the wafer to thereby inhibit removal of materials
from the front surface of the wafer; forming a plasma in the gap between
the backside of the wafer and the susceptor; flowing, through the gas
passage openings in the recessed portion of the faceplate, one or more
gases capable of reacting with the material deposited on the backside and
end edge of the wafer; and providing spacing means to space the front
surface of the wafer from the recessed portion of the faceplate to permit
the gases to enter the space created between the faceplate and the front
surface of the wafer.
These and other objects of the invention will be apparent from the
following description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a fragmentary vertical cross-sectional view of the apparatus used
to remove deposits from the rear surface of a semiconductor wafer.
FIG. 2 is a plan view of the modified faceplate of the invention as viewed
from below.
FIG. 3 is a plan view of the modified face plate shown in FIG. 2 with a
semiconductor wafer shown being urged against the faceplate.
FIG. 4 is a vertical cross-sectional view of the faceplate and wafer of
FIG. 3 taken along lines IV--IV showing the gap between the end edge of
the wafer and the faceplate in a recessed region where the faceplate does
not support the edge of the wafer.
FIG. 5 is a vertical cross-sectional view of the faceplate and wafer of
FIG. 3 taken along lines V--V to show a unrecessed region of the faceplate
where the edge of the wafer is supported.
FIG. 6 is a plan view of another embodiment of the modified faceplate of
the invention as viewed from below wherein the portions on the faceplate
which space the front surface of the wafer from the recessed portion of
the faceplate are not contiguous with the remaining unrecessed portion of
the surface of the faceplate to permit gases to flow around all
360.degree. of the end edges of the wafer.
FIG. 7 is a vertical cross-sectional view of the embodiment of FIG. 6 taken
along lines VII--VII.
FIG. 8 is a plan view of Yet another embodiment of the modified faceplate
of the invention as viewed from below wherein raised portions on the
faceplate space the front surface of the wafer from the surface of the
faceplate to permit the gases to flow out of the faceplate into the space
between the faceplate and the front surface of the wafer.
FIG. 9 is a vertical cross-sectional view of the embodiment of FIG. 8 taken
along lines IX--IX.
FIG. 10 is a flowsheet illustrating the process of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Turning now to FIG. 1, vacuum apparatus is generally shown at 2 which can
be used in the practice of the invention to remove deposits from the rear
surface 52 of a semiconductor wafer 50 while preventing inadvertent
removal of material from the front side or front surface 54 of wafer 50.
Vacuum apparatus 2 includes a vacuum processing chamber 4 surrounded by a
chamber wall 6 and conventionally evacuated by a pump means (not shown).
By the terms "backside" or "back surface" of wafer 50 is meant the surface
of wafer 50 which has not been subject to a fine finish or polish and on
or in which the construction of integrated circuit structures has not been
made.
By the terms "front side" or "front surface" of wafer 50 is meant that
surface of wafer 50 which has been given a fine finish polish and on or in
which have been or will be constructed integrated circuit structures.
By the term "end edge" is meant the vertical or rounded edge surfaces of
the wafer generally perpendicular to the planar surfaces of the wafer,
i.e., the front side and backside of the wafer.
Located within vacuum chamber 4 is a grounded support plate or backplate
10, sometimes also referred to as a susceptor in a plasma-assisted
chemical vacuum deposition (CVD) process, and a faceplate or "showerhead"
30 which is connected to an rf power source 90. Faceplate 30, in
accordance with the invention, is sized to a particular diameter wafer for
reasons which will be more fully explained below.
Processing gas passes through faceplate 30 into chamber 4 through openings
34 in the front surface of faceplate 30 which faces susceptor 10. Openings
34 may, in turn, be in communication with a plenum 40 connected to an
external source (or sources) of gas (not shown) through conduit 42.
The front surface of susceptor 10, in accordance with the invention, is
sized to approximate the size of wafer 50, so that when wafer 50 is
supported thereon during a normal deposition (as can be seen by the dotted
lines in FIG. 1), very little of the front surface of susceptor 10 will be
exposed, thus reducing the amount of deposits on susceptor 10 which must
be removed during the cleaning process. Preferably, the diameter of
susceptor 10 exceeds the diameter of wafer 50 by not more than about 5.08
millimeters (mm).
Susceptor or backplate 10 is further provided with a plurality of openings
or bores 16 therein which pass from the backside 12 of susceptor 10 to the
front or top surface 14 thereof. Mounted within bores 16 are removal pins
70 which may be connected by a common plate 74, beneath susceptor 10, to a
common shaft 76 connected to lowering and raising means 80 such as a fluid
power cylinder or a motor. Raising means 80 may be used to raise or lower
shaft 76, plate 74, and pins 70 when it is desired to either lower wafer
50 onto top surface 14 of susceptor 10, e.g., prior to a deposition step,
or to raise Wafer 50 from surface 14 after the deposition.
In accordance with the practice of the present invention, pins 70 are also
used to urge front surface or face 54 of wafer 50 against faceplate 30
during a cleaning or stripping step after the deposition step to remove
unwanted deposits from rear surface 52 of wafer 50 as well as to remove
deposits from susceptor 10 and other exposed surfaces within vacuum
chamber 4.
While pins 70 urge front surface 54 of wafer 50 against faceplate 30, a
plasma is ignited in the gap formed between rear surface 52 of wafer 50
and top surface 14 of susceptor 10. One or more gases, including process
gases capable of forming fluorine radicals, are then fed into chamber 4
through openings 34 in faceplate 30 to react with the deposits as will be
explained in more detail below.
To effectively remove such undesirable deposits, without, however, damaging
front surface 54 of wafer 50 on which integrated circuit structures are in
the process of being formed, it is important that front surface 54 of
wafer 50 be protected from this plasma.
In accordance with the invention, front surface 54 of wafer 50 is protected
from the plasma-assisted etchant materials by spacing wafer 50 a
predetermined distance from faceplate 30 and then flowing the incoming
gases used in the plasma through the resultant gap or space formed between
front surface 54 of wafer 50 and faceplate 30. This is accomplished by
modifying faceplate 30 so that when pins 70 urge front surface 54 of wafer
50 against faceplate 30, the desired gap between wafer 50 and faceplate 30
will always be formed with the same spacing.
Turning now to FIGS. 2-5, the bottom face of faceplate 30 is formed with a
generally circular recessed central portion 32 having a diameter slightly
larger than the diameter of of wafer 50, as best seen in FIGS. 3 and 4. It
will, therefore, be understood that a different faceplate will be used for
each size of wafer to be processed.
Recessed central portion 32 of faceplate 30 has a depth of from about 0.127
mm to about 0.508 mm (about 0.005" to about 0.020"), preferably from about
0.23 mm to about 0.28 mm (about 0.009" to about 0.11"), and most
preferably about 0.254 mm (0.010") to provide a gap of that dimension
between front surface 54 of wafer 50 and the recessed top surface 33
(FIGS. 4 and 5) of central portion 32.
To space wafer 50 from top surface 33 of recessed portion 32 of faceplate
30, segments 36 are provided on faceplate 30 which protrude into recessed
portion 32. In other words, while recessed portion 32 is generally
circular, as seen in FIGS. 2 and 3, segments 36 of faceplate 30 are not
cut away when recessed portion 32 is formed. Segments 36, therefore,
laterally protrude into recessed portion 32 sufficiently to provide a
distance between opposite segments 36 which is less than the diameter of
wafer 50. Therefore, when pins 70 are used to urge wafer 50 against
faceplate 30, front surface 54 of wafer 50 will rest on segments 36, as
shown in FIG. 3, i.e., maintaining front surface 54 of wafer 50 in the
plane of the unrecessed surface of faceplate 30.
This reproducibly forms the desired space between upper surface 54 on wafer
50 and recessed top surface 33 of faceplate 30 shown in FIGS. 4 and 5.
Usually segments 36 laterally protrude into recessed portion 32, from the
circle which generally circumscribes portion 32, a distance of from about
2.3 to about 2.54 mm (about 0.90" to about 0.10"). At least 3 such
segments 36 should be symmetrically spaced around the periphery of
recessed portion 32, about 4 to 8 of such segments being preferred, to
provide adequate support for wafer 50. Most preferably, the lateral
positioning of segments 36 in faceplate 30 will match the positioning of
pins 70 in susceptor 10 to minimize the creation of stress in wafer 50.
As discussed above, the general diameter of recessed portion 32 is larger
than the diameter of wafer 50, thus creating a gap between the sidewall 31
of recessed portion 32 and end edge 56 of wafer 50 (as shown in FIGS. 3
and 4) except in those areas where segments 36 protrude laterally into
recessed portion 32.
While wafer 50 may not always be perfectly aligned concentrically with
recessed portion 32, the gap formed between end edge 56 of wafer 50 and
sidewall 31 of recessed portion 32 will generally average from about 1.27
mm to about 1.52 mm (about 0.05" to about 0.06") to provide for passage of
gases from openings 34 which flow into the gap between front surface 54 of
wafer 50 and recessed portion 32 to flow around end edge 56 of wafer 50
and into chamber 4.
As stated earlier, the one or more gases flowing into chamber 4 include any
process gases capable of generating fluorine radicals which will then
react with the coating materials on wafer 50 and other exposed surfaces of
chamber 4 to thereby remove such unwanted deposits. Examples of such
process gases include NF.sub.3, SF.sub.6, CF.sub.4, and C.sub.2 F.sub.6.
When more than one gas is flowing into chamber 4, the mixture of gases may
further include inert or non-reactive gases such as argon, neon, or
helium.
The flow rate of such gases, which are flowed into chamber 4 through
openings 34 in faceplate 30 and through the gap created between end wall
31 of recess 32 and end edge 56 of wafer 50, may vary slightly with the
volume of chamber 4 and will vary with the area of the wafer. The flow
rate of the gase will generally be within a range from about 50 to about
300 sccm. With this amount of gas flow and this spacing between wafer 50
and faceplate 30, the plasma formed in the space between bottom surface 52
of wafer 50 and upper surface 14 of susceptor 10 will not result in
removal of any of the materials from front surface 54 of wafer 50 in
accordance with the invention. While the gases are flowing into vacuum
chamber 4, a plasma is ignited between the back surface 52 of wafer 50 and
top surface 14 of susceptor 10 using conventional rf power generation
equipment 90. The power level of the plasma should be maintained within a
range of from about 200 to about 800 watts, depending upon the area of the
wafer, during the removal of the deposits from rear surface 52 of wafer
50. For example, using a 150 mm (6") diameter wafer, a power level of
about 350 to 450 watts can be satisfactorily used.
In accordance with the invention, the process is carried out for a period
within a range of from about 30 seconds to about 5 minutes, preferably
within a range of from about 30 seconds to about 2 minutes. Usually the
process is run for at least about 1 minute because the deposits on the
surfaces in chamber 4, other than on wafer 50, usually consume much of the
process gas as it flows into chamber 4. The time period and the power
level of the plasma are related with longer time periods needed with lower
power levels.
The cleaning process is usually carried out while maintaining a pressure
within a range of from about 0.1 Torr to about 1 Torr, typically about 0.5
Torr, and at a susceptor temperature within a range of from about
450.degree. C. to about 500.degree. C., typically about 475.degree. C.
FIGS. 6 and 7 show another embodiment of the invention wherein protrusions
36 are replaced by spacer members 136 on faceplate 30' which act to space
wafer 50 from the bottom surface of faceplate 30', but which, unlike
protrusions 36, are not contiguous with the remainder of the unrecessed
surface of faceplate 30'. In this embodiment, the gases flowing in recess
32 between the front surface of wafer 50 and faceplate 30 may flow around
all 360.degree. of end edge 56 of wafer 50 to remove deposited materials
on such end edge of wafer 50.
Spacer members 136 in this embodiment space wafer 50 from the top surface
33 of recess 32 in faceplate 30 the same distance as in the previous
embodiment, i.e., from about 0.127 mm to about 0.508 mm (about 0.005" to
about 0.020"), preferably from about 0.23 mm to about 0.28 mm (about
0.009" to about 0.11"), and most preferably about 0.254 mm (0.010").
FIGS. 8 and 9 show yet another embodiment of the invention wherein
faceplate 30" is not recessed but rather has raised spacers 236 which
space wafer 50 from the bottom surface of faceplate 30" the same distance
as between wafer 50 and top surface 33 of recessed portion 32 in the
previous embodiments, i.e., from about 0.127 mm to about 0.508 mm (about
0.005" to about 0.020"), preferably from about 0.23 mm to about 0.28 mm
(about 0.009" to about 0.11"), and most preferably about 0.254 mm
(0.010").
As in the previous embodiments, process gas enters the gap between front
surface 54 of wafer 50 and surface 39 of faceplate 30" through openings 34
and this process gas then flows around the end edge 56 of wafer 50 into
the plasma in chamber 4 between rear surface 52 of wafer 50 and the top
surface 14 of susceptor 10.
To further illustrate the operation of the process and apparatus of the
invention, a 150 mm diameter silicon wafer was blanket deposited with a 1
micron coating layer of tungsten in a 6 liter plasma-assisted CVD
deposition chamber wherein the susceptor was about 157 mm (6.185") in
diameter, i.e., only slightly larger in diameter than the diameter of the
silicon wafer.
After the deposition, the process of the invention for removal of undesired
deposits on the backside and end edge of the wafer, as well as other
exposed surfaces of the vacuum chamber, was carried out. Ejection pins
located in vertical bores in the susceptor, were raised to urge the coated
wafer against the faceplate. The surface of the faceplate facing the wafer
was formed with a generally circular central recess having a depth of
about 0.254 mm and a diameter of about 153 mm (6.016") Symmetrically
spaced about the perimeter of the recessed portion of the faceplate were
six segments of the faceplate which were not recessed and which protruded
into the recessed portions by about 2.29 mm to thereby support the wafer
as the pins pressed the wafer against the faceplate.
While maintaining the silicon wafer against the faceplate, about 150 cc/min
of NF.sub.3 gas was admitted into the vacuum chamber through holes in the
top of the recessed portion of the faceplate so that the gas flowed in the
gap formed between the faceplate and the wafer by the recess and then
around the end edge of the wafer. During the removal process the
temperature of the susceptor was maintained at 475.degree. C. and the
pressure in the chamber was 0.5 Torr.
A plasma was ignited in the gap between the back surface of the wafer and
the top surface of the susceptor at a power level of 400 watts. The plasma
and gas flow were maintained for about 1 minute. The wafer was then
removed from the vacuum chamber and examined. Visual and SEM inspection
revealed no remaining deposits on the backside of the wafer except where
the pins had been in contact with the wafer. Inspection of the front
surface of the wafer by SEM revealed no ascertainable removal of
materials.
Thus, the invention provides a novel process and apparatus for the removal
of undesirable deposits from the end edge and backside or rear surface of
a semiconductor wafer without risking damage to the front surface of the
wafer by inadvertent removal of coating materials from the front surface
by portions of the plasma extending around the end edges of the wafer. By
maintaining a gap of selected distance between the front surface of the
wafer and the faceplate, and by maintaining a flow of gas through this
gap, the plasma is prevented from extending around to the front surface of
the wafer.
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