Bearings – Linear bearing – Fluid bearing
Reexamination Certificate
1999-11-05
2002-05-21
Footland, Lenard A. (Department: 3682)
Bearings
Linear bearing
Fluid bearing
C384S123000
Reexamination Certificate
active
06390677
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fluid bearings, vacuum chucks, and other devices and methods for producing these items. In one embodiment, the invention relates to a method of manufacturing tools and machinery that may be used during the semiconductor manufacturing process.
2. Description of Related Art
Fluid film bearings are generally formed by a pressurized film of fluid (gas or liquid) contained between two surfaces, conforming to each other with a small gap of approximately uniform thickness existing between the surfaces. These two surfaces may be referred to as the guideway and the fluid film bearing surface or plate. The shape of these members depends on the kind of kinematic constraint realized by the bearing. There are numerous types of fluid bearings, including rotary, cylindrical, flat, spherical, and conical. For example, for rotary motion about an axis, the bearing is formed by two cylindrical, conical or spherical surfaces with a small radial gap between the surfaces of the fluid film bearing plate and the guideway. The fluid film bearing of a spherical pair is free to rotate about the center of the sphere. In one embodiment, the fluid film bearing plate is the moving member and the guideway is the stationary member on which the fluid film bearing plate moves. The converse is also true. The moving member of a cylindrical pair is free to rotate about the axis of the cylinders as well as to translate along the axis.
Typically the bearing is subdivided into several areas, each one having its own bearing surface and restrictors with means for evenly distributing the pressure of the fluid film in order to maximize the load bearing capacity and to achieve optimal bearing stiffness.
Fluid film bearings are formed by either drawing the fluid into the gap by slightly wedging the entrance to the gap and using the fluid viscosity and the motion of the moving member (e.g., fluid bearing) relative to the stationary member (e.g., guideway) to draw the fluid into the gap dynamically, or by externally pressurizing the fluid and pumping it into the gap. This fluid film is delivered to the bearing gap through a pattern or system of grooves (or channels) made in one of the bearing surfaces.
Thus, fluid bearings (and vacuum chucks which are a type of fluid interface) often require a pattern, such as a pattern of grooves, to be created on a surface. An engraving machine, milling machine or stamping press is often used to manufacture a pattern, such as a pattern of grooves on an air bearing or a vacuum chuck. As a result, the patterns are slowly traced along each groove and recreated, each time the pattern is needed, by engraving or milling. This is a time-consuming and costly process. Consequently, very complicated geometries are not often used because of the cost, time and labor involved to mill or engrave such a pattern.
Another method of forming the grooves is by stamping in a stamping press. Stamping the grooves requires using a hard tool containing a protruding pattern of ridges; these ridges, when impressed into an object's surface make the impression of grooves on the surface of the object. This process deforms the object, extrudes material above the surface which must then be removed, and introduces stresses in the object which must be relieved by a heat treatment process. Moreover, if a complicated geometry is used, it is expensive even for use in mass production.
The bearing gap between the bearing's surfaces should be uniform, which usually requires that the two surfaces which are separated by the bearing gap conform to each other; that is, the surfaces should “fit” to each other as much as possible in much the same way as an idealized finger should fit into an idealized, perfectly matching glove. The pattern of grooves must be engraved, milled or stamped into the bearing surface each time the fluid bearing is made. After the grooves are created, then the surface of the fluid bearing must be lapped or ground to achieve the desired flat, cylindrical, spherical or conical shape. This is required in order to conform the one surface of the fluid bearing to the other surface. If the bearing face of a flat bearing is wavy or otherwise distorted, then the fluid bearing will not adequately support the load that is placed on it and the dynamics of the bearing will be adversely affected. Lapping is a time-and labor consuming and messy process. Because manufacturing fluid bearings is expensive and time-consuming, they are not widely used although they can be beneficial in many machines that require a smooth, straight, controlled motion, such as in positioning stages used in semiconductor equipment or precision machine tools and coordinate-measuring machines.
FIG. 1
illustrates a prior art flat pad air bearing
100
formed by an air bearing body
102
on top of a guideway
116
. The combination of the air bearing body
102
and the guideway
116
forms a fluid bearing assembly. The air bearing body
102
is made of a solid block with opening
114
in its side, which provides the air to an air duct hole
110
, then to an outlet hole
108
and finally through an orifice
106
. A groove
112
is engraved or milled in the face surface
104
, which is the surface of the air bearing body
102
that glides along the guideway
116
. Typically, the face surface is lapped to obtain a very flat surface which will conform to another flat surface. A front view of a face surface (e.g.,
104
) is shown as
200
in FIG.
2
. Three orifices
202
a
,
202
b,
202
c
are shown inserted in the face surface
200
. A simple pattern of grooves
204
a-c
has been engraved around each orifice
202
a-c.
A sill
206
is the area outside the grooves
204
. Air escaping out of the grooves
204
a-c
and past sill
206
builds up pressure, giving the bearing its load bearing capability.
An example of a prior art radially-shaped fluid bearing is shown in FIG.
3
A. The view in
FIG. 3A
of the fluid bearing is from the bearing face surface
300
that glides on a guideway. A cross-section of the fluid bearing of
FIG. 3A
is shown in FIG.
3
B. Four seats must be prepared for the four orifice inserts
303
a-
303
d
to rest in the bearing body
309
(shown in FIG.
3
B). Each orifice insert
303
a-d
is coupled to its respective groove
301
a-d.
Air is provided from the side at
307
, typically through a pneumatic fitting (not shown). In
FIG. 3B
, the orifice may have been too small to drill, so orifice inserts
303
b
and
303
d
that have pre-machined smaller orifices (
305
b
and
305
d
) are used. The smaller orifices
305
b
and
305
d
restrict the flow from the air duct
311
to a groove
301
b
and
301
d,
respectively. A better design would utilize fewer orifice inserts. But a more efficient and cost-effective design is not practically feasible in the prior art because of the cost, time and labor involved in milling, engraving or stamping grooves in a bearing surface and in lapping the surface.
While prior art techniques for producing fluid bearings or vacuum chucks have used lapping or grinding to achieve conforming surfaces, in an unrelated field, manufacturers of mirrors have used a process whereby a reflective layer is applied to a mirror substrate in such a way that the reflective layer (or layers) conforms to a flatness master.
FIG. 3C
shows an example of this process which is used to manufacture a mirror. The process
350
shown in
FIG. 3C
uses a flatness master
351
to which reflective layer
355
and releasing layer
353
are applied. These layers may be applied by known deposition techniques. The flatness master
351
is carefully lapped or ground to be as flat as possible. A mirror substrate
359
is then coated with adhesive, such as an adhesive layer
357
which is flexible before hardening. The mirror substrate
359
is then pressed against the flatness master
351
such that the adhesive layer
357
contacts the layer
355
and hardens while pressed against the layers. The arrow
361
shows the force applied against the
Blakely & Sokoloff, Taylor & Zafman
Footland Lenard A.
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