Ball bearing assembly

The ball bearing assembly has a total stroke range comprising a first portion where a clearance fit is applied and a second range where an interference fit is applied. The interference stroke range is used to, for example, carry a platform where precise alignment is necessary, and deflections or other movement of the platform must be minimized. Use of the ball bearing assembly to carry a platform holding a semiconductor wafer to be probed is illustrated. After each wafer is probed, the ball cage is reset such that any migration of the ball cage is eliminated so that the clearance and interference stroke ranges return to predetermined values. Further provided are means for rotationally resetting the ball cage, providing for even wear over the entire surfaces of the shaft and housing in their working zones to increase the useful lifetime of the assembly. Also provided are means for maintaining a predetermined interference fit over an extended temperature range, even where the shaft and housing operate at different temperatures.

FIELD OF THE INVENTION 
The present invention relates to ball bearings and more particularly to a 
ball bearing assembly for movement of a platform with minimum deflection 
under various conditions. 
BACKGROUND OF THE INVENTION 
Ball bearing assemblies are used to carry loads in a variety of 
applications. For example, such an assembly may be used to raise and lower 
(or move backward and forward) a platform or other structure, and/or to 
rotate the platform or other structure. Additionally, in some applications 
both the inner and outer raceways of the assembly move reciprocally. 
Numerous configurations of ball bearing assemblies for a wide variety of 
applications are known. 
Referring to FIG. 1, ball bearing assembly 100 is shown. Ball bearing 
assembly 100 comprises shaft 101, ball cage 102 and housing 103. In 
assembly 100, shaft 101 moves in an up and down (Z) direction as shown by 
arrow 110. Shaft 101 is, for example, cylindrical, as is the inner surface 
of housing 103. Ball cage 102 is disposed in the annular space between 
shaft 101 and housing 103. Ball cage 102 retains a plurality of balls 
loosely in place circumferentially around the cage. In assembly 100, cage 
102 holds 2 `rings` of balls, 105a and 105b. In each ring there may be any 
number of balls depending upon the dimensions of the components of the 
assembly and the required support. Additionally, more than the two rings 
105a and 105b as shown in FIG. 1 may be employed. Also, it will be 
appreciated that the balls 105 do not necessarily need to be arranged in 
rings as shown. Rather, the position of the balls may be staggered, to 
provide maximum surface contact with the shaft 101. The Z movement means 
107, which could be, for example, a lead screw, is used to move the shaft 
101 up and down. At the top of shaft 101, platform 116 is shown. Platform 
116 could be used, for example, to hold a semiconductor wafer upon which 
some operation is to be performed. For example, after the semiconductor 
die have been fabricated on the wafer, it is often required to probe each 
die, before the wafer is cut to produce chips to be packaged. 
As shown in FIG. 1, the gap between shaft 101 and housing 103 is greater 
than the diameter of the balls 105. This is known as a clearance fit. The 
appropriate clearance fit is desirable because it allows the shaft to move 
within the housing 103 without undue wear. However, a problem arises in 
that unwanted motion of the shaft 101 can occur during use. Generally this 
motion is a tilting motion as shown by arrow 115. Because there is this 
unintended motion in the ball bearing assembly 100, it may be difficult to 
align precisely an object held on the platform to another object, for 
example, a semiconductor die to a probe. Additionally, even if alignment 
is completed, forces occurring during use may prevent the desired 
operation from being completed. For example, in the probing of a 
semiconductor wafer, when the outer edge of the wafer is probed a force 
shown as arrow 117 at the edge of the wafer is applied. When the force 117 
is applied, the platform 116 will tilt down where this force is applied. 
Additionally, the force may also cause X, Y and rotational motion. These 
motions will reduce the probing accuracy and may prevent the probing of 
the die. The tilting motion will prevent some of the probes from 
accurately contacting the die, and the X, Y, and rotational motion will 
cause the die to be out of alignment with the probe tips as well. 
FIG. 2A shows bearing assembly 200, which is one possible solution to this 
problem. In this case, balls 205 have a diameter that is greater than the 
annular space between shaft 201 and housing 203. In this case, the balls 
205 penetrate, to some degree, both shaft 201 and housing 203. 
Additionally, the balls 205 deform slightly. This is known as an 
interference or preload fit. In such a fit, the shaft is tightly held 
within the housing. Therefore maximum shaft stiffness is provided, which 
would minimize platform deflection under forces, such as force 117 of FIG. 
1. In FIG. 2A, the platform 216 is in an up position. FIG. 2B shows 
assembly 200 in a down position. As can be seen from the figure, as the 
shaft 201 moves up and down, ball cage 202 with balls 205 moves up and 
down as well, due to the tight fit of the balls. 
FIG. 2C shows a problem which occurs with such an arrangement over time. 
After many up and down motions ball cage 202 has a tendency to migrate in 
either an up or a down direction. In FIG. 2C ball cage 202 has migrated 
all the way to base 207. As shown, shaft 201 still has a distance 215 
which it can travel in the downward direction. As shaft 201 moves down, 
ball cage 202 cannot move with it. Because this is an interference fit, 
balls 205 cannot roll as shaft 201 moves down. In some cases, this will 
prevent shaft 201 from moving downward. In other cases, the shaft 201 can 
still move, but the balls do not roll with it. Instead, the shaft 201 
slides over the contact surface of the balls 205, with high friction. This 
results in increased wear of the shaft 201, the balls 205 and possibly the 
housing 203, and therefore a shorter life of or damage to the assembly 
200. Because of this wear, manufacturers of ball bearing assemblies 
commonly recommend a clearance fit. Typically, there will also be some 
means (e.g., platform 116) for stopping ball cage 202 near the top of 
assembly 200, so that upward migration of ball cage 202 will cause the 
same problem as downward migration. 
What is needed is a ball bearing assembly which suffers little or no 
deflection under the forces that occur during use. The assembly should 
prevent significant migration of the ball cage which leads to the problems 
described above. What is further needed is an assembly which allows for 
increased life by reducing wear of the parts. Any such assembly should 
also maintain these advantages over an extended temperature range of 
operation. 
SUMMARY OF THE INVENTION 
A preferred embodiment of the present invention comprises a ball bearing 
assembly having a total stroke range comprising a first stroke range 
wherein there is a clearance fit between the shaft and housing and a 
second stroke range wherein there is an interference fit. Means are 
provided for resetting the ball cage in relation to the shaft and housing 
such that the first and second stroke ranges are reset to predetermined 
distances. The present invention further provides for rotationally 
resetting the ball cage, such that wear of the parts is minimized. Also 
provided is a means for maintaining the design interference fit, over an 
extended temperature range, even when the shaft and housing operate at 
different temperatures. 
Other features of the present invention will be obvious from the detailed 
description, claims, and drawings herein.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
A ball bearing assembly is disclosed. In the following description, 
numerous specific details are set forth such as specific materials, 
dimensions, etc. in order to provide a thorough understanding of the 
present invention. It will be obvious, however, to one skilled in the art 
that these specific details need not be employed to practice the present 
invention. In other instances, well known materials or methods have not 
been described in detail in order to avoid unnecessarily obscuring the 
present invention. Additionally, the present invention is described in 
conjunction with its use in carrying a platform or chuck top which holds a 
semiconductor wafer to be probed. It will be understood that the present 
invention is not limited to the described use, that such use is only 
illustrative in order to aid in an understanding of the present invention, 
and that the invented ball bearing assembly may be used in many diverse 
applications. 
FIG. 3 shows a currently preferred embodiment of ball bearing assembly 300 
of the present invention. Ball bearing assembly 300 comprises shaft 301, 
ball cage 302 and housing 303. Shaft 301 is generally cylindrical, as is 
the inner surface of housing 303. Ball cage 302 is also cylindrical, 
fitting in the annular space between shaft 301 and housing 303. Although a 
cylindrical shaft 301 and inner surface of the housing 303 is shown, it 
will be appreciated that the present invention can be practiced on ball 
bearing assemblies with other shapes, for example the shaft and housing 
could be square or rectangular, with balls in a ball retainer such as cage 
302 placed on two or more sides. Such an embodiment will not have the 
rotational (theta) motion described herein. 
As stated above, ball bearing assembly 300 is used to carry a platform 
which holds a semiconductor wafer in a currently preferred embodiment. 
Shown attached to shaft 301 is platform 320 with semiconductor wafer 321 
placed thereon. Wafer 321 is typically held in place by vacuum. Shown 
above platform 320 is probe card 322 having probes 323. The tips of probes 
323 lie generally in plane 324. The Z motion means 307, which could be, 
for example, a lead screw, move shaft 301 and the attached platform 320 in 
an up and down direction 315, to contact probes 323 at plane 324. In a 
currently preferred embodiment, the inside diameter of housing 303 is 
approximately 1.9973 inches. Ball cage 302 has an inner diameter of 
approximately 1.663 inches and an outer diameter of approximately 1.957 
inches. Ball cage 302 comprises two rings of balls, 305a and 305b, 
arranged circumferentially around ball cage 302. Each ring comprises 
twenty such balls 305. The diameter of each ball 305 is approximately 3/16 
of an inch (all balls 305 for a given assembly 300 are one size selected 
from the sizes 0.1874 inch, 0.1875 inch or 0.1876 inch as will be 
described below). As can be seen from FIG. 3, shaft 301 comprises two 
sections 301a, having a diameter of approximately 1.6216 inches, and two 
sections 301b having a diameter of approximately 1.6228 inches. As shown 
in the Figure, there is a taper or a lead between the two diameters at 
each interface. As will readily be appreciated, the above dimensions are 
simply for an illustrative embodiment of the present invention. It will 
readily be appreciated that numerous different configurations and 
dimensions may be employed, within the spirit and scope of the invention 
as described and claimed herein. As will be explained in more detail 
below, range 330 shows the clearance stroke range, and range 331 shows the 
interference stroke range, with reference to the top of platform 320. 
In FIG. 3, shaft 301 is near or at the bottom of its total stroke range, 
which extends from the bottom of range 330 to the top of range 331. At 
this position, as can be seen from the Figure, the ring of balls 305a and 
305b, are at a position where the distance between the shaft 301 and the 
inner surface of housing 303 is wider than the diameter of the individual 
balls 305. That is, all balls 305 are located in a position such that they 
are adjacent to one of sections 301a. This will be the case so long as 
shaft 301 is positioned such that the top of platform 320 is within range 
330. Referring now to FIG. 4, ball bearing assembly 300 is shown after 
shaft 301 has been raised by Z motion means 307 to the end of the 
clearance stroke range 330, and the beginning of the interference stroke 
range 331. As can be seen from FIG. 4, at this point both rings of balls 
305a and 305b begin to engage sections 301b of shaft 301. At this point in 
the travel, all balls 305 are positioned such that the distance between 
the shaft 301 and the inner surface of housing 303 is less than the 
diameter of the balls 305. Therefore, at this point, ball bearing assembly 
300 begins to have an interference fit. Thus, as the platform moves 
through distance 330, ball bearing assembly 300 is in a clearance stroke 
range. After the top of distance 330 is reached, as in FIG. 4, ball 
bearing assembly 300 is in an interference stroke range. In a currently 
preferred embodiment, the clearance stroke range 330 is approximately 
0.170 inch and the interference stroke range is approximately 0.250 inch. 
These ranges can be varied as necessary to accomodate various probe cards. 
As will be readily appreciated, the ranges 330 and 331, as well as their 
relative position can be varied based upon the intended use of ball 
bearing assembly 300. 
Referring now to FIG. 5A, ball bearing assembly 300 is shown in a further 
raised position, just prior to contact with probes 323. Distance 331 shows 
the extent of interference stroke range 331. That is, so long as shaft 301 
moves such that the top of platform 320 is within range 331, assembly 300 
is in an interference range. As shown, this range extends from a distance 
below to a distance above the plane 324 of probes 323. 
The operation of assembly 300 will now be described in conjunction with the 
probing of a semiconductor wafer. As mentioned previously, a semiconductor 
wafer consists of a plurality of die. Each typically has a plurality of 
bond pads, which provide for electrical contact to the circuits on the 
die. The probe card 322 has probes 323 which contact some or all of these 
bond pads, enabling the probes to send signals to and receive signals from 
the circuits on the die. Probes 323 provide the necessary supply voltages, 
connection to ground, control signals, etc., to allow the functionality of 
each die to be tested. Typically, several bond pads lie in a row across 
two sides of each die. Therefore, each of the two probes 323 shown in the 
figure represent a row of several such probes corresponding to the bond 
pads on the die. Alternatively, the die may have bond pads surrounding the 
perimeter of the die, and probe card 322 would contain additional probes 
327 to contact these bond pads. Testing can be done at room temperature, 
as well as at lower and higher temperatures. To test at these 
temperatures, a platform 320 having a heating/cooling device therein is 
utilized. In one embodiment, the wafer 321 can be heated to temperatures 
of approximately 200.degree. C. or higher, or can be cooled to 
temperatures of approximately -65.degree. C. or lower. The testing of die 
is well known in the semiconductor art. Assembly 300 must be able to 
position the platform 320 such that the bond pads on the die are precisely 
aligned to the probes 323 in plane 324. To accomplish this, assembly 300 
must lift platform 320 and wafer 321 up to plane 324, such that the tips 
of probes 323 contact the bond pads with sufficient force to provide for 
electrical contact. In order to align the pads to the probes 323, X, Y and 
theta (rotational) adjustments must be made. 
First, the wafer 321 is placed on platform 320 when platform 320 is in a 
down position such as that shown in FIG. 3. While FIG. 3 shows platform 
320 in the fully down position, the wafer 321 can be placed on the 
platform in other positions, so long as there is sufficient clearance 
between the platform 320 and probes 323, as in a currently preferred 
embodiment. Generally, the wafer 321 is first prealigned by, for example, 
placing the flat in a predetermined location. After prealignment all that 
is needed is a slight X, Y and rotational adjustment. After loading the 
wafer 321 the platform 320 is moved up a distance. After the platform has 
gone through the clearance stroke range 330 and begins movement in 
interference stroke range 331 as shown in FIG. 4, the platform continues 
to rise until it is approximately in the position shown in FIG. 5A close 
to the plane 324 of the probes 323. At this time the tips of 323 probes 
are aligned to the bond pads of the first die to be tested. Generally, 
theta adjustment is accomplished by rotating the platform 320. X-Y 
adjustment is made by moving the platform 320 and assembly 300 on base 
306, such that the probe tips are aligned to the first die to be probed. 
Alternatively, probe card 322 can be moved. After alignment to the first 
die, the platform is brought up further, i.e. shaft 300 is moved in an 
upward direction by Z travel means 307 such that the probe tips contact 
wafer 321 as shown in FIG. 5B. After a die has been probed, platform 320 
and wafer 321 are brought down to approximately the position shown in FIG. 
5A and base 306 with its bearing assembly 300 is moved such that probes 
323 are in alignment with another die on the wafer. As is well known, the 
order of probing the die is arbitrary and can be specified by the user. 
Often, after a first die is aligned, the base 306 with bearing assembly 
300 need only be moved a specified X and Y distance to the next die, with 
no further alignment needed. Once in alignment, platform 320 is brought 
back up such that wafer 321 is again in contact with probes 323 on the 
different die, for example, as shown in FIG. 5C. 
Note that after the wafer has been loaded, and the first die is brought up 
for alignment, all motion of the platform, for example rotationally or in 
the X, Y plane to align the die or to move to other die is done while 
assembly 300 is in the interference stroke zone. As mentioned earlier, in 
this range, maximum stiffness of the shaft is provided by the interference 
fit. Therefore very tight X, Y and rotational alignment control is 
possible. Additionally, once alignment is achieved, it is maintained 
during the probing operation, even when the probe tip is at the outer edge 
of the wafer, as shown in FIG. 5C, where the Z deflection force is at its 
greatest. Therefore, this stiffness prevents to a large degree the tilting 
discussed is relational to FIG. 1, so that the probes 323 can accurately 
contact the bond pads. 
As mentioned previously, ball cages, such as ball cage 302 can migrate in 
one direction or another after repeated up-down travel. As described in 
the background section, after the ball cage has contacted a retainer, the 
shaft may be prevented from moving or will be forced to slide on the balls 
305, thereby increasing wear with possible damage to the parts of the ball 
bearing assembly. 
However, in the present invention the problem of ball cage migration is 
prevented by periodically resetting the ball bearing assembly 300. 
Typically, the migration occurring during a wafer probing operation is 
very minor. Thus, so long as the platform is reset after each wafer is 
probed, as described below, the above described problems will not occur in 
the present invention. If sufficient migration is found to occur during 
the probing of a single wafer the method described below can be performed 
as frequently as necessary. However, in most situations it will be found 
that resetting will not need to be performed any more frequently than once 
per wafer. Generally, the migration over time may be so slight that 
resetting is not needed every wafer. However, it will generally be most 
convenient to include a reset cycle between each wafer, as wafers are 
loaded and unloaded onto and from platform 320. 
Referring to FIG. 6, ball bearing assembly 300 is shown after a wafer has 
been probed and the shaft 301 has been brought part of the way down. In 
FIG. 6, there has been downward migration of the ball cage 302 during the 
probing of the wafer. Comparing FIG. 6 to FIG. 4, it can be seen that ball 
cage 302 is slightly lower in relation to the shaft in FIG. 6 than in FIG. 
4. Specifically, as shaft 301 is moving down in FIG. 6, ball cage 302 is 
contacting O-ring 345 when shaft 301 is slightly above the position it was 
in when it was rising in FIG. 4. However, after reaching the position 
shown in FIG. 6, shaft 301 will continue its downward motion to the reset 
position. As the shaft 301 lowers, cage 302 and O-ring 345 will compress 
slightly, allowing balls 305 to continue to roll, so that shaft 301 is not 
forced to slide pass the balls 305. As shaft 301 goes below the position 
shown in FIG. 6, there will again be a clearance fit, and this compression 
will be released. In a currently preferred embodiment, O-ring 345 is 
Teflon.TM. and ball cage 302 is plastic. These materials can readily 
compress a sufficient amount to absorb any downward migration that has 
occurred during the probing of a wafer, so that shaft 301 is never forced 
to slide past balls 305. Next, shaft 301 will continue its downward motion 
until it is as shown in FIG. 3. Thus, after shaft 301 is brought down, 
assembly 300 is reset to the position shown in FIG. 3. As described 
earlier, as shaft 301 is raised up it will again travel through the 
predefined clearance stroke range 330, and then again into the predefined 
interference stroke range 331. Therefore, when any downward migration 
occurs, the system will be reset such that clearance stroke range 330 and 
interference stroke range 331 are returned to their designed values. 
Referring now to FIG. 7A, as can be seen ball cage 302 has migrated to a 
slightly raised position compared with the corresponding probing positions 
shown in FIG. 5A. Again however, cage 302 is reset after each wafer. For 
example, if upward migration as shown in FIG. 7A occurred, the following 
would take place upon resetting. First shaft 301 and platform 320 would 
move downward. After this downward motion to slightly above the top of 
range 330, assembly 300 would be in a clearance fit. That is, the assembly 
300 would reach a clearance fit when the top of platform 320 is 
approximately at the level 332 shown in FIG. 7B due to the upward 
migration. As shaft 301 continues to lower, cage 302 will lower with it, 
until cage 302 resets on O-ring 345. After cage 302 reaches O-ring 345, 
its downward motion will stop. Shaft 301 will continue its downward motion 
until it is in the position shown in FIG. 3. Note that as shaft 301 
continues its downward motion after ball cage 302 has reached O-ring 345, 
it is not forced to slide on the balls since the balls are in a clearance 
fit and no load is applied on platform 320. Note that if for some reason 
ball cage 302 does not travel fully down with shaft 301, O-ring 340 will 
push it against O-ring 345 as assembly 300 resets to the position shown in 
FIG. 3. As can be seen from FIG. 3, the overall distance between 
compressed O-ring 340 and O-ring 345 is maintained, so that the ball cage 
302 is always reset to the same position relative to the shaft and 
housing. 
In the present invention, therefore, any slight upward or downward 
migration has no detrimental effect on the operation or wear of assembly 
300. As described in relation to FIG. 6, the downward migration occurring 
during the probing of a single wafer is typically much less than the 
amount of compression available in O-ring 345 and ball cage 302. 
Therefore, harmful sliding of the shaft 301 over the balls 305 cannot 
occur. Referring back to FIG. 7B, it can be seen that with upward 
migration, the upper end of the clearance range 332 is well below the 
probing range shown in FIG. 5. Therefore, there will still be an 
interference fit, as described, in the probing range. It should be noted 
that the extent of migration shown in FIGS. 6 and 7 has been exaggerated 
in order to illustrate the present invention, and less migration than 
shown occurs during the probing of a wafer. 
Referring back to FIG. 7A, rotational resetting will now be described. In 
the rotational resetting of ball bearing assembly 300, ball cage 302 is 
rotated a predetermined amount relative to shaft 301 and housing 303. In 
FIG. 7A, two screws 350 at the top of cage 302 are shown as part of ball 
cage 302. Screws 350 are present in the preferred embodiment of the 
invention. However for clarity, screws 350 are not shown in the other 
figures, but are understood to be present. Although a currently preferred 
embodiment utilizes two screws 350, it will be appreciated that any number 
of such screws 350 could be utilized. Additionally, other similar 
structures could be utilized in alternative embodiments. O-ring 340, which 
is attached to platform 320, is made of Buna-N rubber in a currently 
preferred embodiment. Because O-ring 340 is made of rubber, it is able to 
engage or "grab" screws 350, and therefore cage 302, securely. 
Referring now to FIG. 3, when assembly 300 is in the down position, cage 
302 will move rotationally with platform 320 as O-ring 340 grabs screws 
350. This rotational movement is facilitated by the fact that O-ring 345 
is made of a material with a low coefficient of friction (Teflon.TM. in a 
currently preferred embodiment) so that ball cage 302 rotates easily on 
it. In a currently preferred embodiment ball cage 302, is rotated a small 
amount (approximately 9 mils in a currently preferred embodiment) after 
each resetting of the ball cage (e.g., each wafer). In order to accomplish 
9 mils of rotation, shaft 301 must be rotated a small amount greater than 
9 mils, as some movement is lost when O-ring 340 engages cage 302. It will 
be appreciated that the exact amount of rotation after each wafer is not 
critical. After the ball cage 302 has been rotated, shaft 301 is raised 
vertically to disengage O-ring 340 from screws 350 and ball cage 302, and 
is rotated back to its original position. Although this rotational reset 
does not necessarily need to be performed after each wafer, this is a 
convenient time to do so. Therefore, over time, the balls 305 will 
generally ride evenly over all portions of shaft 301. This is important, 
because as mentioned previously, in the interference fit areas, i.e. when 
balls 305 are adjacent to regions 301b, some wear of the shaft does occur 
due to the tight compression of the materials and the stresses imposed on 
the assembly 300 during probing. However, the theta adjustment provides 
for even wear over 360.degree. of the working area of the shaft 301. It 
has been found that this theta adjustment provides for approximately an 
order of magnitude improvement in lifetime of assembly 300. Note that it 
is possible to reset the ball cage 302 relative to shaft 301 and housing 
303 due to the fact that there is a clearance fit when the assembly 300 is 
in the position shown in FIG. 3. In assembly 200 of FIG. 2, this would not 
be possible, since the ball cage 202 will always track the movement of 
shaft 201, and the relative position would be unchanged when the platform 
216 is returned to its operating position. 
As described previously the interference fit (preload) is designed to 
provide for maximum stiffness without undue wear. In determining the 
amount of preload, reference may be had to balls on diametrically opposite 
sides of the assembly, for example, the left and right sides of the view 
shown in FIGS. 3-7. Referring to the dimensions given earlier, it can be 
determined that the combined diameter of the shaft in regions 301b plus 
two times the diameter of the selected balls 305 is 0.0005 inch (0.5 mil) 
greater than the inside diameter of housing 303. There is therefore a 
total preload of 0.5 mils, or 0.25 mil per side, in the regions 301b. In 
order to maintain the desired preload, the tolerances of these dimensions 
need to be controlled to a sufficient degree. This is made easier by the 
selection of sizes for balls 305 as described below. In a currently 
preferred embodiment, the inside diameter of housing 303 is controlled to 
a tolerance of approximately .+-.0.1 mil, and the diameter of shaft 301b 
is controlled to a tolerance of approximately .+-.0.1 mil. Also in a 
currently preferred embodiment, the balls 305 are grade 25, which means 
that their dimension is controlled to 25 millionths of an inch (0.025 
mil). As mentioned previously, the size of balls 305 is selected from one 
of 0.1874 inch, 0.1875 inch or 0.1876 inch for a given assembly 300. In a 
currently preferred embodiment the housing 303 and shaft 301 are measured, 
and the ball 305 size which will result in a preload of or close to 0.5 
mil is used. For example, if the measured diameter of the shaft 301 is 
1.6227 inch, and the measured inner diameter of housing 303 is 1.9974 
inch, the 0.1876 inch size balls 305 will be used to give a 0.5 mil 
preload. As will be readily appreciated, the dimensions of the various 
parts of ball bearing assembly 300, the optimal preload, and the 
acceptable tolerances will depend upon the requirements of the user, 
including the use to which ball bearing assembly 300 will be put, desired 
cost, quality, life span, etc., and may be varied from the above. 
The above dimensions and preload are at room temperature. As is well known, 
the materials used in constructing the ball bearing assembly expand with 
increasing temperature. Therefore, the above dimensions will change at 
different temperatures. In some applications, such as in the probing 
application described herein, there may be temperature changes to which 
the assembly is subjected. Furthermore, these temperature changes may not 
be uniform throughout the assembly. As described earlier, platform 320 may 
be heated to approximately 200.degree. C. or more. Since there is thermal 
insulation between the platform 320 and shaft 301, shaft 301 does not 
reach the same temperature as platform 320. However, when platform 320 is 
heated to 200.degree. C., shaft 301 can reach temperatures as high as 
approximately 50.degree. C. near the balls 305. However, housing 303 
typically rises only to approximately 35.degree. C. in this region. Thus, 
without the teachings of the present invention as described below, the 
optimized interference fit may be outside of a specified range, for 
example, shaft 301 may expand more than the inner surface of housing 303, 
thereby leading to increased preload. 
In the present invention, this effect is overcome by choosing materials 
such that the product of the coefficient of thermal expansion times the 
expected temperature change is nearly equal for both the housing and the 
shaft. Specifically shaft 301 is made of material with a lower coefficient 
of thermal expansion than housing 303, since shaft 301 reaches a higher 
temperature than housing 303. In the currently preferred embodiment, 
housing 303 is made of A356 aluminum. Shaft 301 is made of 440C stainless 
steel. Housing 303 has a thin (approximately 40 mils thick) sleeve of 440C 
stainless steel in the regions the balls 305 contact housing 303 in order 
to provide a hard surface for balls 305 to roll against. However, the 
expansion of housing 303 is driven by the aluminum material which forms 
the far greater portion of housing 303. Also in a currently preferred 
embodiment, balls 305 are made of 440C stainless steel. The balls 305, the 
sleeves on housing 303, and shaft 301 are hardened to Rockwell C58 
minimum. 
In a currently preferred embodiment, a preload 0.42 mil (0.0042 inches) is 
desired during operation. As mentioned above, the preload of 0.5 mil is 
set at room temperature. However, under normal operation, without a hot 
platform 320, the entire assembly reaches a temperature of approximately 
30.degree. C. This increase in temperature reduces the preload to 
approximately the desired 0.42 mil, or 0.21 mil per side. When a hot 
platform is used, the shaft 301 reaches a higher temperature 
(approximately 50.degree. C.) than the housing 303 (approximately 
35.degree. C.). Without any adjustment to the materials, the preload would 
increase greatly under these circumstances. However, with the temperatures 
for the shaft 301, and housing 303 given above, and with the dimensions 
and materials described above, the preload increases only to approximately 
0.447 mil, which is acceptable. 
As will be readily appreciated, many modifications can be made to the 
embodiment shown herein, within the scope of the present invention. As 
mentioned earlier, assembly 301 can be used in many applications in 
addition to carrying a platform as described herein. Additionally, the 
number and arrangement of balls 305 can be varied from the embodiment 
described herein. As a first example, additional rings of balls such as 
305a and 305b, with an additional region 301a and 301b on shaft 301 for 
each ring, could be utilized. As with the embodiment shown in FIG. 3, the 
additional ring or rings should engage section 301b of the shaft at 
approximately the same time. The additional rings may be useful in 
providing additional stiffness and control. As a second example, two rings 
of balls could be present where one ring is shown in FIG. 3. For example, 
another ring of balls could be placed directly above the ring as 305a, 
and/or ring 305b. Again this would provide for greater stiffness and 
control. However, in order to provide for the required interference stroke 
range, the Z travel of the platform should be greater, to allow all balls 
to be within the interference stroke range 331 when probing. 
In other alternative embodiments of the invention, housing 303 moves and 
shaft 301 is stationary, or as a further embodiment shaft 301 and housing 
303 move reciprocally. In a further alternative embodiment, shaft 301 has 
a uniform diameter, and the interference and clearance fits are obtained 
by having regions in housing 303 of different inside diameter. Also, as 
mentioned earlier, numerous other configurations, besides the cylindrical 
shaft and housing described herein could be utilized in accordance with 
the present invention. All that is necessary is a region of interference 
fit in the region of travel where stiffness is required, and a region of 
clearance fit allowing for resetting of the cage with respect to the 
shaft. Means other than O-ring 340 and O-ring 345 as described herein 
could be used to reset the cage 302 upon reloading the wafer. For example, 
in other applications, shaft 301 or housing 303 could be positioned at a 
predetermined location, and some other independent means could be used to 
push or pull cage 302 to the relative location shown in FIG. 3. The 
embodiment described herein has the advantage that it allows for resetting 
both laterally and rotationally within the normal operation for which ball 
bearing assembly 300 is used. 
Thus, a novel ball bearing assembly has been described. The invented ball 
bearing allows for maximum stiffness in a certain predefined stroke 
region. The ball cage of the present invention can be reset during use, 
such that detrimental sliding over the balls does not occur, and such that 
interference fit is maintained in the desired stroke range. Additionally, 
a rotational resetting is provided for allowing for increased lifetime of 
the assembly. Finally, the present invention provides for an optimized 
interference fit over an extended temperature range, wherein the 
temperature of the shaft may be different from the temperature of the 
housing during use.