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Description  |
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BRIEF SUMMARY OF THE INVENTION
This invention concerns cleaning workpieces by ultrasonic energy and, more
specifically, has reference to cleaning by ultrosonic energy delicate flat
workpieces, such as semiconductor wafers, used in the manufacture of
electronic integrated circuits. The use of ultrasonic energy in
conjunction with a solvent for cleaning workpieces is well established in
the art. Cleaning apparatus of this type have been described, for
instance, in U.S. Pat. No. 2,845,077, dated July 29, 1958; U.S. Pat. No.
3,293,456, dated Dec. 20, 1966; U.S. Pat. No. 3,318,578, dated May 9,
1967; U.S. Pat. No. 3,651,352, dated Mar. 21, 1972; and in "Ultrasonic
Engineering" (book), John Wiley & Sons, New York, N.Y. (1965), pp. 130 to
143.
In typical prior art devices, a metal container or tank is filled with a
suitable solvent and the workpiece to be cleaned is immersed in the
solvent. The container or tank is provided with one or more ultrasonic
transducers which responsive to energization with high frequency energy,
produce cavitation in the solvent which action scrubs the workpiece clean
by dislodging and removing contaminants adhering to the workpiece surface.
Such cleaning occurs also in normally hidden recesses along the workpiece
surface. For instance, when cleaning medical instruments, cleaning is
achieved in crevices and between overlapping hinged portions. The solvent
is selected depending upon the contaminant and such solvents may comprise
aqueous or fluorocarbon solutions and the like, all as is known to those
skilled in the art.
The present invention is particularly suited for cleaning delicate
workpieces, specifically flat wafer like objects which require a high
degree of cleanliness. As stated heretofore, this applies quite
specifically to semiconductor wafers which are processed to produce highly
complex integrated circuits used in the electronic industry. These wafers
must not only be free from contaminants and fingerprints, but also all
traces of the solvent must be removed after cleaning. In the past, the
wafer has been placed on a rotating shaft so that the wafer rotates in an
horizontal plane. As the wafer rotates, the top surface of the wafer to be
cleaned is wetted with a suitable solvent and a scrubbing brush is caused
to engage the top surface to dislodge contaminants and provide a cleaned
surface. It will be apparent that such physical scrubbing by bristles is
undesireable, especially when cleaning articles of the type described,
since such brushing may cause physical damage to the surface, for
instance, scratches resulting from contact with the bristles. Moreover,
the brushes may become charged with hard foreign material which
subsequently scratches the workpiece surface. Finally, the brush is
subject to wear and may need to be replaced without such replacement being
done by operating personnel, thereby producing insufficiently cleaned
workpieces. While in some applications the brush is replaced by an
abrasive cloth, substantially the same disadvantages remain. Various still
further disadvantages of cleaning by mechanical friction processes will
readily be apparent to those skilled in the art.
In the present invention, the mechanical contact scrubbing of the wafer is
replaced by ultrasonic cleaning which provides cleaning of the workpiece
without physical contact.
To this end, the workpiece to be cleaned, in accordance with the present
invention, is rotated upon a shaft and a relatively thin film of solvent
is caused to overflow the surface of the workpiece while ultrasonic energy
is applied to the liquid film. The ultrasonic energy applied to the
solvent causes intense cleaning of the workpiece surface and dislodging of
contaminants and debris, the latter being flushed by the flowing solvent
film. When a clean surface has been attained, the flow of solvent is shut
off and the shaft is rotated at a high speed, causing the workpiece to
spin for effecting drying of the workpiece by centrifugal force. The dry
and clean workpiece is then removed from the shaft and processed further.
This method overcomes the shortcomings and disadvantages of the prior art.
One of the principal objects of this invention is therefore the provision
of a new and improved method for cleaning delicate workpieces.
Another object of this invention is the provision of a new method for
cleaning delicate, wafer like workpieces by ultrasonic energy.
Another important object of this invention is the provision of a new method
for cleaning delicate flat workpieces utilizing a flowing film of solvent
overlying the workpiece surface to be cleaned, and the use of ultrasonic
energy coupled through such film to the workpiece surface for dislodging
contaminants adhering to the workpieces surface.
A further object of this invention is the provision of a method for
efficiently cleaning flat semiconductor wafers as used in the electronics
industry, the cleaning being accomplished without mechanical scrubbing or
engagement of the workpiece surface.
Further and still other objects of this invention will be more clearly
apparent by reference to the following description when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a typical embodiment of the present
apparatus;
FIG. 2 is an elevational view, partly in section, of the apparatus shown in
FIG. 1, and
FIG. 3 is a schematic electrical circuit diagram showing the operation of
the various components forming the electrical circuit.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the figures and FIGS. 1 and 2 in particular, there is
shown a stationary support 11 which supports an open vessel 12. A
workpiece 14 to be cleaned is disposed in a horizontal plane and rests
with its underside on an O-ring gasket 16 which is disposed in an annular
groove of a bushing 18. The bushing 18 is fitted upon a rotatable shaft 20
which is sealed liquid tight with vessel 12 by means of a gasket 21. The
shaft is fitted at its lower end with a pulley 22 and is journalled in a
U-shaped housing 24. A motor 26 via a belt 28 is adapted to rotate the
shaft 20 and, hence, the workpiece 14 resting upon gasket 16 of bushing
18. It will be apparent later that the motor 26, in the preferred example,
is a two-speed motor.
The shaft 20 is provided also with an internal bore 30 which leads to a
similar bore 32 in the housing 24, to a hose 34 and to a vacuum pump 36.
By operating the vacuum pump 36, the wafer 14 is held against the bushing
18, thereby avoiding mechanical clamping means which would have to engage
the rim or the top surface of the wafer 14 to retain the workpiece 14 upon
the shaft 20 during its rotation.
Suitably selected solvent is dispensed from a pump 40 via conduit 42 upon
the exposed top surface of the workpiece 14 and after the solvent flows
over the surface, it is collected in a drain 44 of the vessel 12, fed to
drain hose 46, and is returned to the pump 40 for recirculation. For the
sake of simplicity, a separate solvent reservoir and filter have not been
shown.
The above described arrangement is a fairly standard unit, its components
being incorporated in a scrubber device manufactured by Macronetics
Corporation of Sunnyvale, Calif. It will be apparent that the described
mechanical items can take various other shapes and forms and may be
constructed in different embodiments as is well within the skill of
persons working in the respective art.
Referring still to FIGS. 1 and 2, there is shown a flat, electroacoustic
transducer means 50 comprising in the preferred example, a piezoelectric
wafer 51 of circular shape contained within a metal housing 52. The space
between the piezoelectric wafer 51 and the housing 52 is filled by epoxy
resin 54 as is well known in the construction of ultrasonic transducers.
The housing 52 is mounted to a tubing 56 which contains internally a pair
of electrical conductors 58 for providing electrical high frequency energy
from a generator 60 to the piezoelectric wafer 51. In a typical example,
the piezoelectric wafer 51 is dimensioned to be energized with a frequency
of 70 kHz which renders the piezoelectric wafer resonant. It will be
apparent that, depending on the dimensions of the piezoelectric wafer,
other frequencies will be required to cause the transducer means 50 to
become resonant, but generally a a frequency in the range from 20 kHz to
100 kHz will be the preferred range. The tubing 56 is mounted through a
plate 70 and to a block 71 which is pivotally coupled via pin 73 to a
stationary structure 72. Responsive to the energizing of a solenoid 74, a
linkage mechanism 76 causes the transducer means 50 to swing upward and
assume the position shown by the dashed lines in FIG. 1. A screw 77
adapted to contact the plate 70 stops the downward motion of the
transducer means 50 when the solenoid is deenergized and thereby regulates
the spacing between the front face of the ultrasonic transducer means and
the surface of the workpiece 14. In order to obtain optimum cleaning
results, the surface of the transducer means should be in parallel
alignment with the flat workpiece surface.
DESCRIPTION OF THE OPERATION
Operation of the present apparatus may be visualized by the following
description.
With the solenoid 74, see also FIG. 3, energized causing the transducer
means 50 to be in the raised position, a wafer 14 to be cleaned is placed
on the bushing 18. Next, the vacuum pump 36 is energized for causing a
vacuum to be pulled in the bore 30, thereby retaining the workpiece on the
shaft 20. Next, the motor 26 is energized at its low speed, typically at
100 rpm, causing the workpiece 14 to rotate. Too high a rotational speed
produces excessive tangential velocity upon the solvent accompanied by
poor cleaning results. Upon rotation of the wafer 14, the solvent pump 40
is actuated and a valve 80 disposed in the solvent conduit, not shown in
FIGS. 1 and 2, is opened thereby permitting solvent to flow from conduit
42 in a film across the top surface of the rotating workpiece 14. Next,
the transducer means 50 is lowered to be disposed above the workpiece 14
by deenergizing the solenoid 74. With the liquid film overflowing the
workpiece surface, the electrical high frequency generator 60 is energized
causing the transducer element 51 to be resonant and produce cavitation in
the relatively thin solvent film flowing continuously across the workpiece
surface. Preferably, the solvent film is relatively thin, typically 0.040
inch (1 mm) or less. A thicker film up to 1/4 inch (6 mm) is acceptable
also except that a greater amount of ultrasonic energy will be required.
It will be apparent that the thinner the film, the more ultrasonic energy
reaches the workpiece surface and the lower the power requirement. As the
ultrasonic energy dislodges the contaminants from the workpiece surface,
the flowing film removes the contamination and debris from the workpiece
surface. Moreover, as the wafer rotates all surface portions of the wafer
become exposed to the ultrasonic energy and the transducer does not need
to be of the same diameter as the wafer, it being of slightly larger
diameter than the radius of the circular workpiece.
After cleaning has been accomplished, typically a period from five to 30
seconds, the pump 40 is shut off and valve 80 closed. This shuts off
solvent flow. Also the generator 60 is shut off at this time and most
suitably the solenoid 74 is energized in order to raise the transducer
means away from the workpiece 14. Next, the motor 26 is turned to its high
speed, for instance 5,000 rpm, causing rapid spinning of the workpiece to
cause solvent overlying the workpiece surfaces to become driven off by
centrifugal force. After this drying action has been completed, typically
a period of only ten seconds, the motor 26 is stopped and the vacuum pump
36 is stopped. When the workpiece stands still and the vacuum has been
dissipated by itself or an additional vacuum bleed valve, not shown, has
been actuated the workpiece 14 is removed from the apparatus which now is
ready for the receipt of a new workpiece.
The sequencing described heretofore, as will be apparent to those skilled
in the art, can be accomplished manually, but if desired a control device
100, in the form of a simple cam operated motor driven timing device, may
be substituted. Moreover, the sequence described above can be varied to
some extent without affecting the cleaning process. For instance, a rinse
cycle during which water flows across the wafer to remove solvent residue
may be added prior to drying.
It should be noted that the effective cleaning action is caused primarily
by the combination of a thin flowing film of solvent to which ultrasonic
energy is applied while the workpiece is in motion. Due to the combination
of this cleaning action in conjunction with spin drying, manual contact
with the workpiece is avoided, thus providing superior results and
precluding surface scratches and other materials to be introduced upon the
delicate workpiece surface as is detrimental when the workpiece becomes a
part of a delicate electronic circuit product.
In an alternative embodiment, the transducer means 50 is made to be
substantially of the same diameter or of a larger diameter than the wafer
14 for covering the entire surface. The wafer then is cleaned while
stationary with cleaning solvent supplied to the space between the
transducer means and the wafer surface. As described, the wafer is rotated
for spin drying, thus requiring only a single-speed motor 26.
While the above described embodiment shows a recirculation arrangement for
the cleaning solvent, it is apparent that the surface of the wafer may be
flushed with solvent which subsequently is drained from the apparatus.
While there has been described and illustrated a specific embodiment of the
present invention and several modifications have been indicated, it will
be apparent to those skilled in the art that various changes and
modifications may be made therein without deviating from the broad
principle and spirit of the present invention which shall be limited only
by the scope of the appended claims.
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