Wafer carrier

A carrier for a semiconductor wafer or other substrate has an outer portion adapted for engagement by equipment for processing wafers. Interior to the outer portion a substantially planar supporting surface is provided. A retaining lip is provided above the plane in which the supporting surface lies by at least the height of a wafer of a selected size. The circumference of the retaining lip is of sufficient size and proper shape to permit the passage of a wafer of the selected size. The carrier is undercut under the lip, whereby a wafer of the selected size, when placed on the supporting surface, may be retained. In one embodiment, the carrier has a cylindrical shape. The top of the carrier includes a flat outer ring-like surface. The supporting surface and the circumference of the retaining lip are both circular.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for use in the fabrication of 
semiconductor devices and more particularly to carriers for wafers or 
other substrates for use in production of integrated circuits. 
A wafer is typically a substantially cylindrical piece of silicon, the 
height of which is very small in proportion to its diameter. Such a wafer 
normally goes through a large number of processing steps until a number of 
integrated circuits have been fabricated on the wafer. These processing 
steps require handling of the wafer by a number of different machines. 
In the manufacture of various types of integrated circuits, it is at times 
desirable to use more than one size of wafer. Six inch diameter and four 
inch diameter wafers are among those most commonly used, although a 
variety of sizes are possible. In order to change from one size to 
another, the equipment for handling the wafers must be adjusted or 
retooled. Such adjustment or retooling results in expense as well as 
periods of time when the equipment is out of use. Clips can be provided on 
processing equipment to secure wafers. However, such clips cover a fairly 
large area of the top surface of the wafer. Imperfections or particles on 
the clips can cause scratches on the wafer. 
Alternatively, separate processing facilities must be provided for 
different sizes of wafers. This alternative is also expensive. 
Objects and advantages of this invention will be evident from the detailed 
description of a preferred embodiment which follows. 
SUMMARY OF THE INVENTION 
A carrier for a substrate, for example a semiconductor wafer, includes an 
outer portion adapted for engagement by equipment for processing wafers. 
Interior to the outer portion, there is provided a substantially planar 
supporting surface on which a substrate may be supported. A retaining lip 
is provided above the plane in which the supporting surface lies by at 
least the height of a wafer of a selected size. The circumference of the 
retaining lip is of sufficient size and proper shape to permit the passage 
of a wafer of the selected size. The carrier is undercut under the 
retaining lip, whereby a wafer of the selected size placed on the 
supporting surface will be retained.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
With reference to FIGS. 1 and 2, there is shown a preferred embodiment of a 
wafer carrier 10 according to the invention. Wafer carrier 10 is 
substantially in the form of a circular cylinder. The diameter of wafer 
carrier 10 is much greater than its height. 
An annular surface 12 forms a portion of the top of wafer carrier 10. This 
annular surface 12 will be referred to as the outer top surface. The 
"outer portion" of the wafer carrier includes the perimeter and outer top 
surface 12. 
In FIG. 1, outer top surface 12 is substantially planar. A retaining lip 
20, along the interior edge of the outer top surface 12, has a 
substantially circular circumference. The carrier is undercut under the 
lip 20. The term "undercut," as used in this application, refers to 
providing an open space under a lip. As shown in FIG. 2, a surface 18 
which has substantially the form of a portion of a cone not including an 
apex, or frusto-conical form, is provided where the carrier is undercut. 
The retaining lip 20 is also the upper edge of surface 18. Surface 18 will 
be referred to as a "retaining surface." 
A substantially planar surface 16 is provided interior to outer top surface 
12. The surface 16 will be referred to as the "supporting surface." 
Supporting surface 16 is lower than the outer top surface 12. Supporting 
surface 16 is substantially parallel to outer top surface 12. The 
perimeter of supporting surface 16 coincides with a wider, lower end of 
retaining surface 18. The perimeter of supporting surface 16 is greater 
than the circumference of retaining lip 20. 
FIG. 3 shows the placement of a wafer 17 in a carrier 10. Wafer 17 rests on 
supporting surface 16. 
The dimensions of certain features of wafer carrier 10 are defined with 
respect to the dimensions of the wafer which the carrier is designed to 
hold. The height of lip 20, and at least an inner portion of retaining 
surface 18, above the supporting surface 16 must be at least slightly 
greater than the height of the wafer 17. The circumference of lip 20 must 
be of sufficient size and proper shape that a wafer may pass through the 
opening defined by lip 20. In the embodiment of FIGS. 1, 2, and 3, the 
circumference of lip 20 must be greater than the diameter of the wafer 17. 
Thus, a wafer may be placed through the aperture defined by lip 20 on 
supporting surface 16. 
When the wafer carrier is rotated about an axis lying in or parallel to the 
plane of outer top surface 12, the wafer 17 may begin to slide 
transversely across supporting surface 16. The wafer will stop sliding 
when a leading edge of the wafer contacts a portion of retaining surface 
18. The wafer 17 then rests against retaining surface 18. This is shown in 
FIG. 4. The retaining surface 18 thus functions as a transverse supporting 
member, which stops the transverse motion of the wafer. As the carrier is 
rotated further, the wafer 17 remains in place, resting against retaining 
surface 18 as shown in cross-section in FIG. 4. 
When the carrier is rotated more than 90 degrees, wafer 17 exerts an upward 
force against interior surface 18. Because retaining surface 18 extends 
over a portion of wafer 17, wafer 17 will not fall out of the carrier. It 
will be seen that the frusto-conical retaining surface 18 shown in the 
embodiment of FIGS. 1 through 4 has both the function of providing 
transverse support so that the wafer stops sliding, and the function of 
providing retention of the wafer. The combination of lip 20 and an 
undercut in the carrier under lip 20 functions to retain a wafer when the 
carrier is rotated. Depending on the design of the carrier, the carrier 
may be rotated almost 180 degrees without causing the wafer to fall out. 
Only a very small portion of the upper surface of the wafer comes in 
contact with the carrier. As a result, the possibility of the wafer being 
scratched is minimized. 
The outer diameter of the wafer carrier 10 is preferably the same as that 
of a commonly-used size of wafer. The outer diameter of wafer carrier 10 
is preferably the same as the outer diameter of the largest wafer that the 
processing equipment will accommodate. 
The form of the features of wafer carrier 10 is also defined with respect 
to the form of the wafer which the carrier is designed to hold. The shape 
of the circumference of lip 20 is preferably the same as the shape of the 
perimeter of the wafer. The shape of supporting surface 16 is preferably 
the same as the shape of the perimeter of the wafer. FIG. 5 illustrates 
the foregoing. A top view of a wafer carrier 10 is shown. Wafer 17, viewed 
from the top, has a circular perimeter, except for a flat side 40. It is 
typical for a wafer to have at least one such flat side, called the major 
flat. Retaining lip 20 of wafer carrier 10 is correspondingly circular 
when viewed from the top, except for a flat side 42 corresponding to flat 
side 40 of the wafer. Supporting surface 16 has the same shape as lip 20. 
Wafer carrier 10 is preferably designed for mating with the appropriate 
equipment. The design of a wafer of the same diameter as the wafer carrier 
may include features that facilitate engagement of the wafer by processing 
equipment. Such features might facilitate gripping of the wafer or proper 
orientation of the wafer in processing equipment. 
Features that facilitate gripping of the wafer or orientation of the wafer 
carrier may include, for example, a perimeter that is non-circular, or 
optical marks. By way of further example, the wafer carrier may include 
optical orientation marks on outer top surface 12, for orientation of the 
wafer carrier. This is illustrated in FIG. 6. An optical mark 13, 
specifically a straight line extending radially inward from the perimeter 
of wafer carrier 10, is provided on outer top surface 12. Mark 13 
facilities orientation of the wafer carrier 10. 
Ordinarily, the wafer of the largest diameter will be substantially 
circular. If so, then the wafer carrier 10 may be cylindrical, as shown in 
FIGS. 1, 2, 3 and 4. 
By way of example, and not limitation, a wafer carrier according to the 
invention may be provided having the following dimensions. The outer 
diameter is six inches, the diameter of the largest wafer which can be 
accommodated by the processing equipment. It is desired to process a 
smaller wafer having a diameter of four inches. The height of the carrier 
is approximately 0.15 centimeters. The height of lip 20 above supporting 
surface 16 is approximately 0.075 centimeters. The diameter of the 
circumference of lip 20 is approximately 10.5 centimeters. The diameter of 
supporting surface 16 is approximately 11.0 centimeters. 
In the embodiments of a wafer carrier according to the invention shown in 
FIG. 1, 2, 3, 4, 5 and 6, outer top surface 12 and retaining surface 18 
intersect to define a lip 20. There may be provided a wafer carrier 
according to the invention where a surface other than an outer top surface 
intersects a retaining surface to define retaining lip, for example the 
carrier 50 depicted in FIG. 7 and 8. Wafer carrier 50 is a substantially a 
circular cylinder. The diameter of wafer carrier 50 is much greater than 
its height. Wafer carrier 50 has a circular planar top surface 51. Annular 
member 54 projects upward from the top surface of wafer carrier 50. 
Annular member 54 is concentric with carrier 50. The portion of the top 
surface of carrier 50 exterior to annular member 54 is outer top surface 
12. As in the embodiment of FIG. 1, the outer top surface 12 is planar and 
annular. The portion of the top surface of carrier 50 interior to annular 
member 54 is supporting surface 52. Supporting surface 52 lies in a plane. 
Annular member 54 has an outer surface 57 and an inner surface 58. A 
retaining lip 56 depends inward from annular member 54. Lip 56 is defined 
by the intersection of outer surface 57 and inner surface 58 of annular 
member 54. Annular member 54 is undercut on its interior below lip 56. The 
circumference of the perimeter of supporting surface 52 is consequently 
greater than the circumference of lip 56. Inner surface 58 of annular 
member 54 has substantially the form of a portion of a cone. The 
dimensions and shape of the features of carrier 50 are defined with 
reference to the dimensions and shape of a wafer to be used with the 
carrier. The height of lip 56 above supporting surface 52 is at least 
slightly greater than the height of the wafer. The shape of the 
circumference of lip 56 is preferably the same as the shape of the 
perimeter of the wafer viewed from the top. The shape of supporting 
surface 52 is desirably the same as the shape of the perimeter of the 
wafer. In the same manner as explained above in connection with the 
embodiment of FIG. 1, the carrier 50 may be rotated about an axis lying in 
the plane of the carrier or parallel thereto without causing a wafer 
placed on supporting surface 52 to fall out. 
Another alternate preferred embodiment of a carrier according to the 
invention is shown in FIGS. 9, 10 and 11. The carrier of this embodiment 
is configured to accommodate wafers of two different sizes. Wafer carrier 
70 is shown in cross-section. Carrier 70 is substantially a circular 
cylinder. Outer top surface 12 is substantially as shown and described in 
the embodiment of FIG. 1. Outer top surface 12 is a substantially planar 
annular surface. A first retaining lip 20 of the carrier is coextensive 
with an inner edge of outer top surface 12. The carrier is undercut under 
first lip 20. A retaining surface 18, having substantially the form of a 
portion of a cone, extends downward and outward from first lip 20. 
Retaining surface 18 is as described in FIGS. 1 and 2. The lower, wider 
edge of retaining surface 18 coincides with a perimeter of a first 
supporting surface 76. First supporting surface 76 is substantially 
annular and planar. A second retaining lip 24 is provided at the interior 
edge of first supporting surface 76. The carrier is undercut beneath the 
second lip 24. A second retaining surface 30, which has substantially the 
form of a portion of a cone, is provided extending downward and outward 
from second lip 24. A lower, wider edge of second retaining surface 30 
coincides with an outer perimeter of a second supporting surface 26. 
Second supporting surface 26 is substantially planar and circular. 
As shown in FIG. 10, a wafer 17, of a first selected size, may be placed on 
first supporting surface 76. As shown in FIG. 11, alternatively, a wafer 
19, of a second selected size, may be placed on second supporting surface 
26. The size of wafer 17 is larger than the size of wafer 19. The 
dimensions and shape of carrier 70 are defined with reference to the 
dimensions and shape, or shapes, of wafers to be used with the carrier. 
The height of second lip 24 above second supporting surface 26 is at least 
slightly greater than the height of wafer 19. The shape and size of the 
circumference of lip 24 are selected so that wafer 19 may pass through the 
opening defined by lip 24. The shape of the circumference of lip 24 is 
preferably the same as the shape of the perimeter of wafer 19. The shape 
of the perimeter of second supporting surface 26 is preferably the same as 
the shape of the perimeter of wafer 19. First supporting surface 76 is 
sufficiently thick from interior edge to exterior edge that, when a wafer 
17 of a first size rests against first retaining surface 18, wafer 17 will 
remain flat, supported on first supporting surface 76. 
Additional planar annular supporting surfaces, similar to first supporting 
surface 76, may be provided. Each such annular supporting surface, on its 
outer edge, intersects a lower, wider edge of a retaining surface having 
substantially the form of a portion of a cone. Each supporting surface is 
sufficiently thick that a wafer of the proper diameter may rest thereon. 
The number of such supporting surfaces is limited by the height of the 
wafer carrier. The height differential between two supporting surfaces is 
at least slightly greater than the thickness of a wafer that is to be 
placed on the lower of the two supporting surfaces. 
FIGS. 12 and 13 depict a carrier for a semiconductor wafer in accordance 
with yet another alternate preferred embodiment of the invention. A 
carrier 110 is shown. Carrier 110 has a substantially cylindrical outline. 
An outer top surface 112 is a planar, annular surface. A bottom surface 
118 is planar and annular. An interior edge of top surface 112 is a 
retaining lip 113. Carrier 110 is undercut under lip 113 so as to provide 
a retaining surface 114 (see FIG. 13) having substantially the form of a 
portion of a cone. Lip 113 coincides with an upper, narrower edge of 
frusto-conical retaining surface 114. A lower, wider edge of retaining 
surface 114 coincides with a perimeter of a supporting surface 116. 
Supporting surface 116 lies in a plane, and is annular. Annular supporting 
surface 116 is sufficiently thick, from interior edge 120 to exterior edge 
122, that a wafer, of a selected size, supported by supporting surface 
116, and in contact at one point with retaining surface 118, will be 
supported thereon. The interior edge of supporting surface 116 defines a 
perimeter of a central cylindrical aperture 115 in carrier 110. Aperture 
115 has a top opening in supporting surface 116. Carrier 110 thus has an 
overall annular shape. This annular shape is advantageous if the carrier 
110 is heated and cooled in use. The annular shape allows the carrier 110 
to undergo thermal expansion and contraction with a minimum of distortion. 
It is preferable that the diameter of aperture 115 be a substantial 
portion of the diameter of the carrier. For example, the diameter of 
aperture 115 may be about one-half that of carrier 110. 
FIGS. 14 and 15 depict a carrier for a semiconductor wafer according to 
still another alternate preferred embodiment of the invention. Carrier 130 
is shown. Carrier 130 is substantially cylindrical. Outer top surface 132 
of carrier 130 is planar and annular. The interior edge of outer top 
surface 132 coincides with a top edge of a cylindrical vertical interior 
surface 134. A bottom edge of cylindrical interior surface 134 coincides 
with an interior edge of an annular lower surface 136. Annular lower 
surface 136 is planar and parallel to outer top surface 132. An annular 
retaining lip 137 is thus defined by an inner portion of outer top surface 
132, cylindrical interior surface 134, and annular lower surface 136. 
Carrier 130 is undercut below annular lip 137. An outer edge of annular 
lower surface 136 coincides with a top edge of a vertical cylindrical 
surface 138. A perimeter of a circular planar supporting surface 140 
coincides with a bottom edge of vertical cylindrical surface 138. 
Supporting surface 140 is parallel to outer top surface 132 and lower 
surface 136. The vertical distance from lower surface 136 of lip 137 to 
supporting surface 140 must be at least slightly greater than the height 
of a wafer of a selected size. 
A wafer of a selected size may be placed onto supporting surface 140. It 
will be seen that the diameter of a cylinder defined by cylindrical 
interior surface 134 must be greater than the diameter of a wafer of a 
selected size. When carrier 130 is rotated about an axis parallel to outer 
top surface 132, the wafer will slide transversely across supporting 
surface 140. A leading edge of the wafer will slide under lip 137. When 
the leading edge of the wafer comes in contact with vertical cylindrical 
surface 138, the wafer stops sliding transversely. Thus vertical 
cylindrical surface 138 functions to provide transverse support. When the 
rotation reaches 90 degrees from horizontal, the wafer will begin to move 
toward the top of the carrier. A portion of the wafer will come in contact 
with the lower surface 136 of lip 137. The movement of the wafer will then 
stop. Thus lip 137 serves to retain the wafer. 
It will be seen that the carrier 130 is undercut, as is the carrier 10 
shown and described in the embodiment of FIG. 2. However, the retaining 
surface 18 shown and described in the embodiment of FIG. 2, has the 
functions of both vertical cylindrical surface 138 and lower surface 136 
of lip 137 in the embodiment of FIGS. 14 and 15. Retaining surface 18, in 
the embodiment of FIG. 2, provides transverse support so that the wafer 
stops sliding when rotation of the carrier begins. Retaining surface 18 
also provides retention of the wafer when rotation of the carrier exceeds 
90 degrees. 
FIGS. 16 and 17 show a carrier for a semiconductor wafer according to 
another alternate preferred embodiment of the invention. Carrier 150 is 
depicted. Carrier 150 has a generally cylindrical outline. An outer top 
surface 152, on the top of carrier 150, is substantially planar and 
annular. A bottom surface 164 of the carrier is substantially planar and 
circular, and substantially parallel to outer top surface 152. The 
interior edge of outer top surface 152 is a retaining lip 153. Carrier 150 
is undercut below lip 153. A substantially frusto-conical retaining 
surface 154 extends downward and outward from lip 153. A substantially 
circular supporting surface 156 is provided. Supporting surface 153 is 
planar and is substantially parallel to outer top surface 152. The 
perimeter of supporting surface 156 coincides with a lower, wider edge of 
retaining surface 154. Substantially vertical cylindrical passageways 158 
are provided. Each passageway 158 has an upper opening 160 in supporting 
surface 156 and a lower opening 162. Lower openings 162 open to the 
exterior of the carrier, specifically in the bottom surface 164 of the 
carrier. If a vacuum or partial vacuum is created at bottom surface 164, a 
wafer in place in carrier 150 will be pulled down on supporting surface 
156. FIG. 16 illustrates seven passageways 158 merely for convenience of 
illustration. It may be desirable to provide more or fewer passageways. 
A wafer carrier according to the invention may be made of any suitable 
material. The material will be selected depending upon the conditions 
under which the carrier is used. For example, for low-temperature 
applications, aluminum can be used. In high-temperature application, a 
coated carbon material is preferable. A wafer carrier according to the 
invention may also be made of fused quartz. 
It will be appreciated that there are considerable variations that can be 
accomplished in an article of the invention without departing from its 
scope. As a result, although a number of preferred embodiments of an 
article of the invention have been described above, it is emphasized that 
the invention is not limited to a preferred embodiment and there exist 
other alternative embodiments that are fully encompassed within the 
invention's scope, which is intended to be limited only by the scope of 
the appended claims.