Method for removing particulate matter

A method for controlling the effective velocity of a spray of cleaning material in order to remove particulate matter and other contamination from a surface of a workpiece. The surface under treatment is moved relative to the position of a sprayhead in order to increase or reduce the velocity at which the spray impacts the surface. In one embodiment, the workpiece is mounted on a disk-shaped support plate. The sprayhead tracks across the support plate to direct the spray over the entire workpiece. The support plate is rotated towards the sprayhead to increase the effective spray velocity, and away from the sprayhead to decrease the effective spray velocity. Another embodiment of the invention adjusts the speed of rotation of the support plate in such a manner that the spray maintains a constant effective velocity as the sprayhead is moved radially across the support plate.

FIELD OF THE INVENTION 
The present invention relates to ultra cleaning methods in general and, in 
particular, to methods of removing microscopic particles with accelerated 
sprays. 
BACKGROUND OF THE INVENTION 
In recent years, a great deal of attention has been given to improving 
techniques for cleaning surfaces. Surfaces are often cleaned to remove 
contamination in the form of film-like processing residues (cutting or 
lubricating oils, mold release materials, salts and oils from finger 
prints received in handling, etc.). Film-type residues are typically 
removed by solvent processes, using fluid baths, sprays, vapor cleaners or 
other methods and apparatus well known and used commercially. Such solvent 
processes also remove other types of contamination in the form of 
particulate materials that may include metal, ceramic or polymeric 
fragments created during a manufacturing process, or deposited by 
environmental (e.g., airborne) contamination. 
In certain applications, the need for improved levels of cleanliness have 
become more stringent, and traditional methods of cleaning, such as the 
use of solvent baths, have proven unable to provide satisfactory results. 
This fact is particularly true in the electronics industry where effective 
removal of submicron particulate contamination greatly affects the yield 
of high resolution electronic devices such as integrated circuits as well 
as a number of other products of commercial importance. 
As the size of a particle to be removed from a surface decreases, the 
removal of such particles becomes increasingly difficult. For this reason, 
conventional approaches that make use of solvent dips or washes, or fluid 
streams, lose effectiveness as particle size falls substantially below 1 
micrometer (micron or .mu.m). The relative force of adhesion of these 
particles rises exponentially as the particle size decreases. Table I, 
below, reports relative adhesion force as a function of particle size. 
TABLE I 
______________________________________ 
Particle Adhesion of Glass Beads on a Glass Slide 
Particle Size 
Relative Force to displace 
(.mu.m) (gravitational units) 
______________________________________ 
100 510 
50 2,159 
10 57,716 
1 674,600 
0.1 749,552,300 
______________________________________ 
Source: "Particulate Removal with Dense CO.sub.2 Fluids," D. Zhang, D. B. 
Kittelson, B. Y. H. Liu, 1992, presented by McHardy at the Third 
International Workshop on Solvent Substitution, Phoenix, Ariz., 1992. 
As the force necessary to break the combined adhesion and binding charges 
of a particle rises, the amount of force that can be transferred to the 
particle by a fluid stream remains constant due to the fixed 
cross-sectional area of the particle. Additionally, boundary layer effects 
near tile surface, taking the form of laminar flows of the cleaning gases 
or fluids over the particles, further isolate microscopic particles from 
removal. Thus, as the particle size decreases, the ability to displace 
such particles with fluid streams falls off to the point that it becomes 
nearly impossible to remove microscopic particulates by spraying the 
particles with streams of solvent. 
In recognition of the ineffective physics of high pressure gas or solvent 
streams for particulate removal, other investigators have used the kinetic 
energy of droplet sprays or finely divided solids to remove particulates 
by means of momentum transfer. Such droplets typically comprise water or 
CO.sub.2 snow sprays that are directed with a specialized nozzle onto the 
workpiece being cleaned. Often high pressure driving gases are used to 
accelerate the snow spray to a sufficient velocity to clean the workpiece. 
These cleaning methods are similar to methods wherein naturally occurring 
sand or manufactured abrasive grit is used as a blasting agent for 
descaling and cleaning purposes. The principles involved in heavy cleaning 
or stripping applications consist of supplying sufficient kinetic energy 
of impact to the blasting agent in order to exceed the adhesive or 
cohesive strength of the material being removed or abraded and are well 
known to those skilled in the art. The use of sublimable or phase-changing 
materials (e.g., water or dry ice) instead of sand or grit allow for 
easier cleanup and eliminate residues of hard particles, which could 
damage products in later use. 
These aggressive blasting applications, which can produce heavy material 
abrasion or wear, lie at one end of a continuum of related cleaning 
processes. At the other end of the continuum are fine sprays of liquid 
and/or finely divided solid matter, which carry modest levels of kinetic 
energy. Fine sprays of comparatively low energy, particularly with liquids 
or low hardness materials such as ice or CO.sub.2 snow, cause little or no 
damage to surfaces, and are known to be effective for gentle cleaning. 
However, as shown earlier in Table I, the binding forces for very finely 
divided matter disposed on a surface increase exponentially as the size of 
the particles decreases. With some materials, the binding energies of fine 
matter on surfaces begin to approach the cohesive strength of the 
underlying matter under treatment. As impinging sprays are made more 
aggressive by enlargement of the droplet mass or velocity (or both) in 
attempts to improve submicron particulate removal efficiency, the 
threshold level for damage to the underlying material is approached. 
One difficulty of cleaning methods that do not control blast particle size 
closely (e.g., single-stage phase transformation at an orifice) is that a 
range of droplet sizes and mixed phases are present. A substantial amount 
of the sprayed material is too fine and fugitive to have any effect, 
yielding low efficiency, and calling for higher and higher pressures for 
accelerating gases. At the same time, the presence of solid or liquid 
phase material in larger masses may produce damage to the surfaces being 
cleaned with high driving pressures. 
The use of high driving gas pressures also accelerates the sublimation of 
solid phase CO.sub.2, providing low efficiency in the use of the material 
as a cleaning agent. In addition, the use of a driving gas such as high 
pressure air or nitrogen adds complexity, and creates additional 
opportunities for the introduction of impurities. 
To summarize the existing art, the velocity of spray or blasting particles 
has been varied using the following known methods: 
1. Use of air or another driving gas to accelerate pellets or sprayed 
matter, in a manner analogous to sand blasting. 
2. Use of the dynamics of liquid spray from siphon-type bottles, wherein 
the driving force may be the vapor pressure of the liquefied or 
pressurized gas in the storage cylinder. Siphon pressure may be augmented 
by an additional head pressure (e.g., as in supercritical fluid extraction 
grade CO.sub.2, which is supplied with a 2,000 psi head of helium). 
3. Airless sprayers to deliver high speed droplets. 
4. Centrifugal force to accelerate pellets of CO.sub.2 ice so that pellets 
can be ejected from a centrifuge at high velocity. 
All of these methods suffer from one of two problems. Either the particles 
sprayed on the workpiece are so hard that the workpiece may be damaged or, 
if the sprays are softer, the equipment required to accelerate the spray 
are dangerous and a possible source of contamination. Thus, there is a 
need for a new method of controlling the kinetic energy of cleaning sprays 
impinging on surfaces that alleviates some of the shortcomings in the art, 
as now known and practiced. 
SUMMARY OF THE INVENTION 
The present invention is a method and apparatus for increasing the 
effective velocity of a cleaning material directed at an object to be 
cleaned. The method comprises the steps of directing an amount of cleaning 
material at the object and moving the object relative to the cleaning 
material to change the velocity at which the cleaning material impacts the 
surface of the object. 
In one embodiment of the invention, the objects to be cleaned are mounted 
on a support disk. A sprayhead produces a spray of cleaning material that 
is directed onto the support plate. The sprayhead moves along the radius 
of the support plate to direct the spray onto the entire surface of the 
object to be cleaned. The support plate is rotated counter to the 
direction of the spray to increase the effective velocity of the spray as 
it impacts the objects mounted on the support plate. In a second 
embodiment of the present invention, one or more position sensors detect 
the position of the sprayhead. A microcontroller circuit varies the speed 
at which the support plate rotates as a function of the position of the 
sprayhead. The speed of rotation can be adjusted so that the effective 
velocity of the spray remains constant as the sprayhead moves radially 
across the support plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates, in a simplified fashion, the cleaning method according 
to the present invention. A workpiece 10 to be cleaned is impinged by a 
spray of cleaning material 20 delivered from a sprayhead 22. The amount of 
kinetic energy contained in the spray droplets is a function of the mass 
of the spray droplets and their velocity. As indicated above, the problem 
with prior art spray cleaning systems is that it is difficult to impart 
enough kinetic energy to the spray to dislodge microscopic contaminants 
from the workpiece. Because the size of the droplets tend to remain fairly 
uniform, the only way to increase the kinetic energy of the spray is to 
increase the spray velocity. However, even with high pressure gas 
accelerators, the maximum spray velocity obtainable in a typical prior art 
spray cleaning system is on the order of 145 feet per second. The present 
invention increases the effective kinetic energy of the spray by moving 
the workpiece in relation to the spray. 
In the present invention, the relative velocity of the spray is controlled 
by moving the workpiece under treatment in a direction 12 that is counter 
to the direction of the spray, or in a direction 14 that is the same as 
the direction of the spray. If the workpiece is moved in a direction 12 
toward the spray, the effective velocity of the spray is increased. On the 
other hand, if the workpiece is moved in a direction 14 away from the 
spray, the effective velocity of the spray is decreased. The sprayhead 22 
is maintained at an angle .alpha. to the surface of the workpiece 10. If 
the spray has a velocity V1, and the workpiece is moving toward the spray 
at a velocity V2, then the effective velocity of the spray, in a direction 
horizontal to the surface of the workpiece, is given by: 
EQU V1Cos(.alpha.)+V2, (1) 
As can be seen from this equation, a greater effective velocity is obtained 
when the angle .alpha. is small. 
FIG. 2 shows a first embodiment of the present invention used to clean a 
plurality of silicon wafers 30. The silicon wafers 30 are removably 
affixed around an outer perimeter of a support plate 40 by any of a number 
of suitable mechanisms such as a vacuum chuck (not shown). The support 
plate 40 is rotated about a center point 42 by a motor (also not shown). A 
solvent or spray material, preferably comprising liquid CO.sub.2, is 
supplied through a hollow tube 50 to a metering valve 52, where the 
solvent material is conditioned by a sprayhead 54 and ejected as a spray 
56. The spray 56 is directed onto the spinning wafers 30 at an angle less 
than 90.degree. to the surface of the wafers. The sprayhead is movable in 
a direction 58 from an inner diameter 44 to an outer diameter that is 
established by the perimeter of the support plate 40. 
The support plate 40 is rotated by the motor in a direction 60 counter to 
the direction of the spray 56. The rotation of the support plate in the 
direction 60 provides an enhancement of the relative velocity of the spray 
droplets or elements and the surface of the wafers 30 under treatment. If 
the direction of support plate rotation were reversed such that the 
support plate is rotated in the same direction as the spray, the relative 
velocity of the droplets or spray elements and the surface of the wafers 
under treatment would be reduced. 
By way of illustration, assume the support plate 40 shown in FIG. 2 has a 
diameter of 24 inches and the silicon wafers 30 each have an 8-inch 
diameter. If the support plate 40 is rotated at 3600 rpm the linear 
velocity of the support plate at a position four inches from the center 
point 42 of the support plate would be about 126 fps (3.84 m/sec). At a 
fixed rate of rotation, the linear velocity increases radially outward, 
reaching a maximum of linear velocity of about 377 fps (11.49 m/sec) at 
the perimeter of the support plate 40. 
In Table 2, below, the effect of rotating the support plate 40 counter to 
the direction of the spray 56 is illustrated, based on a rotational speed 
of 3600 rpm and a spray velocity of 146 fps (4.45 m/sec). 
TABLE 2 
______________________________________ 
Spray or Blast Particle Velocity Enhancement 
Relative Velocity 
Radial Spray Surface of Spray and 
Location Velocity Velocity Surface 
(inches) (fps) (fps) (fps) 
______________________________________ 
4 146 126 272 
5 146 157 303 
6 146 188 334 
7 146 220 366 
8 146 251 397 
9 146 283 429 
10 146 314 460 
11 146 346 492 
12 146 377 523 
______________________________________ 
Referring to Table 2, the spray velocity of 146 fps is enhanced by adding 
the opposing velocity of the rotating work (assuming a low angle of the 
spray with respect to the surface of the wafers) at various positions 
radially outward from the center point 42. Given the fixed rotational rate 
of 3600 rpm, the relative velocity at spray/surface impact ranges from 272 
fps at a position 4 inches from the center of the support plate to 523 fps 
at the perimeter of the support plate. This provides a percent increase of 
92% at the 4-inch radial range, and a maximum increase of 258% at the 
12-inch radial location, in relative velocity of spray/surface impact, as 
compared with the static spray of 146 fps. 
By reversing the direction of rotation or relocating the sprayhead and its 
direction of traverse, the silicon wafers 30 disposed on the support plate 
may be moved away from the sprayhead, traveling in substantially the same 
direction as the spray. In this situation, the linear velocity of the 
support plate is subtracted from the velocity of the spray to reduce the 
velocity of the spray relative to the surface of the silicon wafers 30 
under treatment. 
The present invention allows the kinetic energy of a cleaning spray to be 
increased without the need for expensive and potentially dangerous high 
pressure accelerators. This increased kinetic energy can dislodge much 
smaller particles on the workpieces to be cleaned than were able to be 
removed using prior art blast or snow cleaning systems. 
Optimum velocity relationships with some products under treatment may be 
obtained by locating the workpieces within an annulus rotated at a 
constant speed. So long as the rotational speed and positioning of the 
work and sprayhead lie within an acceptable range and produce an effective 
range of relative velocities, this simple approach will be acceptable. 
In other applications, where the difference between impact kinetics for 
desired cleaning and the damage threshold for the material being cleaned 
is small, it may be necessary to more closely control the relative 
velocity of the spray and surface under treatment. This problem can be 
addressed by providing a constant linear velocity of the surface under the 
sprayhead, regardless of the radial location of the sprayhead. This 
outcome is readily achieved by varying the rotational speed of the support 
plate as a function of the radial location of the sprayhead. 
FIG. 3 is a block diagram of a variable velocity cleaning system according 
to a second embodiment of the present invention. As with the cleaning 
system described above, a cleaning solution such as liquid CO.sub.2 is 
stored in reservoir 51 and delivered under pressure through a tube 50 to a 
metering valve 52. The cleaning solution is discharged from the sprayhead 
54 in the form of either a spray or a snow onto the surface of the support 
plate 40. Objects to be cleaned may be secured onto the support plate 40 
by a plurality of conventional vacuum chucks that are driven by a vacuum 
unit 45. The sprayhead 54 moves radially across the face of the support 
plate 40 on a track 70. The track 70 is a motor driven, threaded rod or 
other type of linkage mechanism that allows for the precise positioning of 
the sprayhead. The details of the track 70 are well known to those of 
ordinary skill in the art and therefore need not be discussed further. 
One or more position sensors 72 sense the position of the sprayhead. 
Suitable position sensors are optical, inductive, capacitive, resistive, 
and numerous other types well known in the art. The sensors produce an 
analog electrical signal that is proportional to the radial position of 
the sprayhead. This analog signal is transmitted over a lead 74 to a 
position sensor conditioning circuit. The conditioning circuit amplifies 
the signal produced by the sensors and converts the analog signal into a 
digital signal that can be interpreted by a computer. 
The output of the position sensor conditioning circuit is transmitted over 
a lead 78 to a microcontroller circuit 80. The microcontroller circuit 
includes a microprocessor that is programmed to read the output signal of 
the position sensor conditioning circuit and to produce a motor drive 
signal on a lead 84 that changes the speed of a variable speed motor 88. 
The microcontroller circuit 80 changes the speed of the motor 88 so that 
the effective velocity of the spray remains constant at the point where 
the spray impacts the surface of the support plate 40 regardless of the 
radial position of the sprayhead 54. 
In the example of the silicon wafer cleaning method above, we may select 
3,600 rpm at the 4-inch inner radius as producing a hypothetically ideal 
linear velocity of 126 fps. If we wish to maintain this ideal constant 
linear velocity of the surface under treatment as the sprayhead traverses 
from the inner 4-inch radius to a maximum radius of 12 inches at the 
perimeter of the support plate, then the speed of rotation must be reduced 
incrementally from 3,600 rpm to 1,200 rpm. 
For any increasingly large radial position, the requisite speed change is 
directly and inversely proportional to the radius, if the intent is to 
obtain a constant linear velocity. The ratio of fastest to slowest speed 
required to maintain the constant linear velocity is on the order of 3:1. 
In addition to the microcontroller 80 described above, variable 
transmissions, rheostatic motor controllers, and variable-speed air motors 
can be used to achieve the desired variation in rotational speed. 
Additionally, instead of using externally mounted position sensors, 
information regarding the required rotational speed as a function of the 
position of the sprayhead may be applied directly on the support plate in 
the form of a digital encoding, as is done with laser-read audio disks. 
The encoding could be read by an optical sensor disposed on the sprayhead 
and used to vary the speed of rotation accordingly. Alternatively, the 
position of the sprayhead may be mechanically read by a cam and cam 
follower arrangement, through an electrical analog device such as a 
potentiometer, or by numerous other means well known to those of ordinary 
skill in the art. 
As will be apparent to those skilled in the art, the microcontroller 80 
could be programmed to vary the effective velocity of the spray in 
relation to the position of the sprayhead in other ways besides providing 
a constant spray velocity. For example, the spray could be made to have a 
greater effective velocity towards the center of the support plate 40 and 
a slower speed towards the perimeter, etc. The particular effective spray 
velocity desired will often depend on the material to be cleaned. 
In instances where the surface to be cleaned is a cylinder, a third 
embodiment of the present invention is shown in FIG. 4. Here, a 
cylindrical surface 100 to be cleaned is rotated in a direction 102, 
counter to the direction of the spray 56 from the sprayhead 54. The 
cylinder is mounted in a chuck (not shown) and rotated by a conventional 
motor similar to a lathe. The cleaning spray supply is controlled by a 
metering valve 52, or otherwise established by the use of fixed orifices 
or other rate-limiting devices. In order to clean the entire cylindrical 
surface, the sprayhead 54 is moved along a direction 55 parallel to the 
major axis of the cylindrical surface 100 under treatment. 
As with the first embodiment of the present invention described above, 
spinning the cylinder in FIG. 4 in a direction 102 counter to the 
direction of the spray has the effect of adding the rotational velocity of 
the exterior surface of the cylinder to the spray velocity to increase the 
effective velocity of the spray, as compared to spraying a fixed surface. 
Conversely, rotating the cylinder 100 in the same direction as the spray 
direction has the effect of reducing the effective velocity of the spray 
relative to the surface under treatment. 
FIG. 5 shows another embodiment of the present invention used to clean the 
interior of a cylindrical surface. Here, a hollow cylinder or tube 110 is 
mounted in a conventional chuck (not shown) and rotated by a motor (also 
not shown) in a direction 112. Traveling within the tube along its major 
axis is a disk-shaped sprayhead 114, which provides one or more sprays 116 
exiting tangentially in a direction counter to the direction of the tube 
rotation. The sprayhead is supported by a hollow pipe 120, and the solvent 
to the sprayhead is supplied through a lumen 122 in the pipe 120. As an 
additional, but not essential, possibility, the sprayhead 114 and its 
supporting pipe may be rotated in a direction 124 counter to the rotation 
of the tube under treatment to further increase the effective velocity of 
the sprays 116. 
As with other embodiments of the present invention, rotation of the 
cylinder in a direction counter to the direction of the spray increases 
the effective velocity of the spray that impacts the interior surface of 
the cylinder. Similarly, rotation of the cylinder in the same direction as 
the direction of the spray reduces the effective velocity of the spray. 
The foregoing illustrated embodiments are meant to illustrate practical 
applications of the invention, and are not in any way intended to limit 
other possible embodiments or uses which will readily suggest themselves 
to those familiar with the art. 
Other embodiments, not shown, might include a ring-shaped or fly-cutter 
geometry sprayhead carrier, which could be rotated above the work and 
tilted to provide coverage of convex or concave spherical surfaces of 
revolution, in ways analogous to abrasive ring or cup-type curve 
generators used in the optical trade. 
In many applications, relative movement of the sprayhead and work may be 
readily achieved by moving the sprayhead rather than the surface, or by 
moving both the sprayhead and the surface under treatment. In addition to 
rotating or spinning motions discussed above, raster-type linear scanning 
can be achieved by moving the workpiece, the sprayhead or both.