Linear gas bearing with integral vacuum seal for use in serial process ion implantation equipment

A high vacuum ion implantation chamber has a lower wall formed with an opening accommodating a depending sleeve through which a shaft passes supporting a substrate support platform at the top and connectable to external linear and rotary drives at the bottom. The sleeve is formed with four axially spaced annular grooves each coupled to a respective vacuum pump that maintains the annular grooves at respective pressures that progressively increase for grooves further away from the vacuum chamber bottom wall. A lowermost annular groove functions as an exhaust along with the region surrounding the shaft at the bottom of the sleeve. The sleeve also includes an air inlet. The gravity forces acting upon the shaft and platform assembly are counterbalanced by the differential pressure acting over the shaft area between the high vacuum chamber and the ambient surroundings.

The present invention relates in general to gas bearings and more 
particularly concerns a novel linear gas bearing with an integral vacuum 
seal for use in serial process ion implantation equipment. The bearing is 
characterized by low friction, low particulate generation, and ability to 
maintain a good vacuum seal. 
Gas bearings are known in the art and are commercially available in a 
variety of styles, shapes and sizes. The common types of gas bearings are 
rotary, single axis linear and dual axis planar. 
Use of "differential pumping" to obtain pressure gradients between ambient 
and high vacuum environments is also known in the art. The most common 
configuration employs two back to back mechanical seals of either natural 
or synthetic rubber with a differential pressure maintained between them 
with respect to the environment on either side of the seal pair. 
Successful sealing of high vacuum to atmosphere has also been demonstrated 
by differentially pumping on grooves without mechanical seals installed in 
them, relying on the resistance of flow thru small geometries between 
grooves as the "sealing mechanism". 
The present invention relates to a combination of the techniques of gas 
bearings and differential pumping and in particular, to the use in ion 
implant equipment of a radial gas bearing with an integral, differentially 
pumped, "empty groove", type seal to obtain vacuum sealed, rotary and/or 
linear motion. 
To date, two general types of ion implant systems have evolved to meet the 
fundamental needs of semiconductor manufacturing. The "batch process" line 
of equipment is normally associated with higher dose implants and most 
typically accomplishes ion beam scanning into substrates by some 
combination of magnetic beam deflection and/or substrate mechanical 
scanning. In most, if not all, "batch" systems a number of substrates are 
sequentially passed through the ion beam at a relatively high rate of 
speed in at least one of the two scan axes. 
On the other hand, the "serial process" line of ion implant equipment is 
normally associated with medium dose implants and most typically 
accomplishes ion beam scanning by electrostatic beam deflection in both 
axes, with a single substrate held stationary at the target plane. These 
electrostatic deflection systems produce a scanned beam with trajectories 
which vary in angle of incidence across the diameter of the substrate; 
some by as much as 3.degree.. Such variations in incident angle produce 
undesirable effects with respect to the ion implant process. 
A means for producing a parallel scanned beam is described in pending 
application Ser. No. 06/849,786 filed Apr. 9, 1986, entitled ION BEAM FAST 
ALLEL SCANNING owned by the assignee of this application. The ion beam 
is rendered parallel in one axis of scan utilizing electrostatic and 
magnetic devices while a second axis of scan is achieved by linearly 
translating the substrate fully through and past the scanned beam a number 
of times in a path normal to these ion trajectories and at a constant 
velocity. 
The present invention provides low friction, low particulates, low noise, 
and high stiffness guidance to a substrate support shaft while maintaining 
a high differential pressure between the evacuated process chamber and the 
ambient environment which surrounds it. 
An important object of this invention is to provide a linear gas bearing 
which locates and guides the mechanical scan shaft with virtually no 
friction forces and provides a means for achieving a dynamic, non-contact 
high vacuum seal between the ambient environment, within which the gas 
bearing itself resides, and the high vacuum environment within which the 
working end of the shaft resides.

With reference now to the drawing and more particularly FIG. 1 thereof, 
there is shown a combined block-pictorial representation, partially in 
section, of an embodiment of the invention. The linear bearing includes a 
precision diameter shaft 10 with internal passage for fluids and rotary 
actuator (not shown) which act upon a platen 20 for maintaining desired 
substrate temperature and obtaining selected tilt angle of substrate 30 
with respect to the ion beam (not shown). Precision diameter shaft 10 is 
housed within gas bearing assembly 40 which is rigidly mounted to 
underside of vacuum chamber 50. Shaft 10 passes through a multitude of 
internal grooves 10A-10D in the gas bearing assembly 40 whose respective 
pressures are differentially staged from atmospheric to near high vacuum 
levels and maintained by vacuum pumps 60A-60D, respectively. Both bearing 
and seal are non-contact with respect to reciprocating shaft 10 and thus 
provide for an essentially friction-free and noise-free linear motion 
vacuum feedthrough. 
With mechanical scanning set up vertically as shown, gravity forces acting 
upon the free mass tend to pull the shaft/platen system downward. 
Differential pressure acting over the precision shaft area creates a 
counteracting force which tends to pull the shaft/platen system in the 
upward direction. Precision shaft diameter and moveable masses are 
selected such that a counterbalance of the opposing forces is achieved, 
leaving only forces required to accelerate and decelerate the moveable 
mass to be dealt with by the external drive system. 
As indicated in FIG. 1 internal annular grooves 10A, 10B, 10C and 10D are 
maintained at progressively increasing pressures by vacuum pumps 60A, 60B, 
60C and 60D, respectively, while pressurized gas enters at inlet 11 and 
exhausts at the bottom of assembly 40 and through annular groove 41. 
Referring to FIG. 2, there is shown an enlarged fragmentary view of the 
linear bearing partially in section helpful in understanding principles of 
operation. A high stiffness gas film 70 is maintained between scan shaft 
10 and gas bearing sleeve 40. Gas film exhausts into exhaust port 41 which 
is connected to the ambient surroundings via holes drilled radially 
outward through the gas bearing sleeve 40. A vacuum pump 60D is connected 
to pumping stage number 1 10D which establishes a pressure in said stage 
which is significantly lower than that of exhaust port 41. It can be shown 
by conventional fluid mechanics that this differential pressure promotes a 
flow of gas from port 41 to groove 10D which is proportional to the 
difference between the square of the upstream and downstream pressures, 
and also proportional to the cube of the gas film thickness. Since gas 
bearing film thicknesses are inherently very small (0.0005 inch or less), 
gas flow rates are also very small. This means that a relatively high 
differential pressure between adjacent ports can be achieved using a 
moderate size vacuum pump. A second vacuum pump 60C is connected to 
pumping stage number 2 10C; a third to pumping stage number 3 10B; and a 
fourth to pumping stage number 4 10A. 
The four sequential pumping stages create a pressure gradient along the gas 
bearing linear axis, which decreases from ambient atmosphere to chamber 
high vacuum. The pressure in the fourth stage is maintained at a point 
where the gas flow rate to the high vacuum chamber is negligibly small. In 
order to minimize additional gas flow to the high vacuum chamber during 
scan shaft actuation, the pumped groove locations are axially spread out, 
within practical limits, to cover the maximum shaft length. In addition, 
the size of the grooves in the axial direction is maximized, within 
practical limits, to increase the dwell time of gas molecules within the 
groove boundaries. 
There has been described novel apparatus and techniques for providing a 
linear gas bearing with an integral vacuum seal characterized by low 
friction, low particulates, and good sealing characteristics. It is 
evident that those skilled in the art may now make numerous uses and 
modifications of and departures from the specific apparatus and techniques 
herein disclosed without departing from the inventive concepts. 
Consequently, the invention is to be construed as embracing each and every 
novel feature and novel combination of features present in or possessed by 
the apparatus and techniques herein disclosed and limited by the spirit 
and scope of the appended claims.