TAILORED PROFILE PEDESTAL FOR THERMO-ELASTICALLY STABLE COOLING OR HEATING OF SUBSTRATES
A shaped pedestal having a substantially convex profile for cooling a substrate or a substantially concave profile for heating a substrate. The tailored pedestal is formed of at least one continuous curvature segment that is generally elliptical, parabolic, or spherical in shape. The pedestal may be formed from more than one piecewise linear segments that are angled adjacent one another or stepped in a substantially convex or concave manner.
1. Field of the Invention The present invention relates to devices for heating or cooling substrates, such as silicon wafers. Specifically, the present invention relates to a tailored pedestal for heating or cooling a wafer during processing.
2. Description of Related Art
Current practices for heating and cooling wafers using a conductive media, such as gas, utilize a constant gap between the heating or cooling device and the substrate.
In the case of wafer cooling, an initial perturbation referred to as “doming” places the edge of the substrate closer to a cooling device than the center portion, as shown in
In the case of wafer heating, an initial perturbation referred to as “cupping” places the center of the substrate closer to the heating device than its edges.
In both instances, these formations are unstable, runaway conditions that lead to large thermal distortion of the substrate and highly non-uniform heat transfer. Of particular concern is the outer edge of the substrate transitions to tensile circumferential stress. If the substrate has edge defects, such as chips or cracks, the tensile stress condition may promote crack propagation and subsequent substrate breakage.
The present invention solves the problem of unstable wafer distortion during wafer heating or cooling. It also generates a biased stress state in the wafer that minimizes potential wafer breakage initiating from edge defects during heating or cooling.
SUMMARY OF THE INVENTIONBearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a tailored profile pedestal to eliminate unstable wafer distortion during wafer heating or cooling.
It is another object of the present invention to provide a tailored profile pedestal to minimize potential wafer breakage initiating from edge defects.
A further object of the invention is to provide a device for thermo-elastically stable wafer cooling and wafer heating.
A further object of this invention is to provide a device for repeatable and uniform wafer cooling.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention, which is directed to an apparatus for cooling a substrate comprising a thermally conductive, substantially convex shaped pedestal having a top surface, a center, and an edge, the pedestal top surface being in thermal contact with a lower surface of the substrate such that for a substantially flat substrate the pedestal center is initially located a closer distance to the substrate lower surface than the pedestal edge, and upon cooling activation, heat energy is transferred from the substrate to the pedestal substantially uniformly, cooling the substrate in a thermo-elastic stable manner. The pedestal top surface may comprise at least one continuous curvature segment, substantially convex in shape. The at least one continuous curvature segment may comprise a portion of an elliptical, parabolic, or spherical shape. The pedestal top surface may further include a substantially convex shape formed of more than one piecewise linear segments. The piecewise linear segments are angled relative to one another or stepped relative to one another to form the substantially convex shape. Upon the cooling activation the substantially convex shape of the pedestal top surface leads to a higher rate of cooling of a portion of the substrate lower surface closer to the pedestal center, and simultaneously to a lower rate of cooling of a portion of the substrate lower surface closer to the pedestal edge, promoting the thermo-elastic stability and compressive circumferential stress on the substrate. The thermal contact includes thermally conductive gas introduced between the pedestal top surface and the substrate lower surface. The substantially convex shape pedestal's depth of profile is determined by a gap distance between the pedestal top surface and the substrate lower surface.
In a second aspect, the present invention is directed to an apparatus for heating a substrate comprising a thermally conductive, substantially concave shaped pedestal having a top surface, a center, and an edge, the pedestal top surface being in thermal contact with a lower surface of the substrate such that for a substantially flat substrate the pedestal center is initially located a farther distance to the substrate lower surface than the pedestal edge, and upon heating activation, heat energy is transferred from the pedestal to the substrate substantially uniformly, heating the substrate in a thermo-elastic stable manner.
In a third aspect, the present invention is directed to a method for cooling a substrate having a lower surface comprising: introducing a pedestal having a center, an edge, and a substantially convex shaped top surface underneath the substrate lower surface such that for a substantially flat substrate the pedestal center is initially located a closer distance to the substrate lower surface than the pedestal edge; inserting a thermally conductive gas between the substrate lower surface and the pedestal top surface; and initiating a temperature differential between the substrate lower surface and the pedestal top surface wherein the pedestal top surface is at a lower temperature gradient than the substrate lower surface, such that heat energy is transferred from the substrate to the pedestal substantially uniformly, cooling the substrate in a thermo-elastic stable manner. The step of introducing the pedestal having the substantially convex shape top surface may comprise having the top surface formed of at least one continuous curvature segment, substantially convex in shape.
In a fourth aspect, the present invention is directed to a method for heating a substrate having a lower surface comprising: introducing a pedestal having a center, an edge, and a substantially concave shaped top surface underneath the substrate lower surface such that for a substantially flat substrate the pedestal center is initially located a further distance to the substrate lower surface than the pedestal edge; inserting a thermally conductive gas between the substrate lower surface and the pedestal top surface; and initiating a temperature differential between the substrate lower surface and the pedestal top surface wherein the pedestal top surface is at a higher temperature gradient than the substrate lower surface, such that heat energy is transferred from the pedestal to the substrate substantially uniformly, heating the substrate in a thermo-elastic stable manner.
The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:
In describing the preferred embodiment of the present invention, reference will be made herein to
Two embodiments are presented to eliminate unstable wafer distortion during wafer heating or cooling, minimize wafer cracking, and promote thermo-elastically stable wafer cooling and heating. One embodiment is directed to domed wafers during wafer cooling, and the other is directed to cupped wafers during wafer heating. The embodiments modify the surface of the cooling and heating devices, and benefit from the geometric differences between the cooling or heating device surface and the adjacent wafer surface.
In both heating and cooling conditions, the preferred profile of the shaped pedestal is such that the cooling device is generally convex and the heating device is generally concave. Different shapes will initiate different stress profiles in the substrate. The depth of the profile is determined by the desired total variation in the gap. These shapes include, but are not limited to, the cross-sectional examples depicted in
Empirical results of wafer distortion and theoretical results of tangential wafer edge “hoop” stress time histories were analyzed during cooling with helium gas at atmosphere for a 300 mm silicon wafer. Comparisons were made between a standard flat pedestal cooling device and a tailored convex shaped cooling device. In the case of the flat cooling device, the gap between the substrate and the pedestal was held at a constant 0.035 inches, and in the case of the tailored cooling device, the gap varied from 0.025 inches at the center and 0.045 inches at the edge, which is the radius length of 150 mm from the center in the spherical profile. The dimensions are such that the average gap is the same for both cooling devices. On the flat cooling device, wafers with initial domed shapes demonstrated increased distortion during cooling and were found to develop tensile edge hoop stress during cooling. However, with respect to the tailored cooling pedestal, wafers with initial domed shapes of up to 0.5 mm demonstrated decreasing or constant distortion and compressive edge hoop stress. Initially cupped wafers were stable on both devices. The cupped wafers were predicted to be more stable during cooling, showing decreasing distortion and compressive edge hoop stress. These empirical and theoretical results support the preferred design shape being more generally convex for cooling pedestals. Conversely, a domed wafer is predictably more stable during heating than a cupped wafer, supporting a preferred concave heating pedestal.
Empirical results of wafer distortion during cooling with helium gas at atmosphere is depicted in
The edge, center, and wafer average temperatures were considered when comparing a flat cooling pedestal with a convex spherically-shaped cooling pedestal for an initially 0.5 mm domed wafer as shown in
Although these predictions considered convex shaped cooling pedestals with flat or domed wafers, the converse also follows suit. Concave shaped heating pedestals have similarly expected results when comparisons are made with flat and cupped shaped wafers. The cupped wafers being unstable under heating conditions, as are the domed wafers under cooling conditions.
Additionally, a shaped pedestal will not contribute detrimentally to the cooling or heating of a flat wafer.
The analytical results are summarized in the table shown in
The present invention teaches tailored pedestals for heating and cooling wafers or substrates during processing, wherein the shape of the tailored pedestal is generally convex for cooling conditions and generally concave for heating conditions.
While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention. Thus, having described the invention, what is claimed is:
Claims
1-18. (canceled)
19. A method for cooling a substrate having a lower surface, a center area, and an edge area, the method comprising:
- introducing a pedestal having a center, an edge, and a substantially convex shaped top surface underneath said substrate lower surface such that for a substantially flat substrate said pedestal center is initially located at a closer distance to said substrate lower surface than said pedestal edge;
- inserting a thermally conductive gas between said substrate lower surface and said pedestal top surface through the gap between the pedestal edge and the substrate lower surface at such process conditions that the thermally conductive gas initiates heat transfer from; and
- said substrate lower surface to said pedestal top surface causing faster contraction of the center area than the edge area of the substrate causing substrate flattening during cooling.
20. The method of claim 19, wherein said substantially convex shape top surface comprises AT least one continuous curvature segment, substantially convex in shape.
21. The method of claim 20 wherein said at least one continuous curvature segment comprises a portion of an elliptical, parabolic, or spherical shape.
22. The method of claim 19, wherein said substantially convex shape top surface comprises more than one piecewise linear segments.
23. The method of claim 22 wherein said piecewise linear segments are angled relative to one another to form said substantially convex shape.
24. The method of claim 22 wherein said piecewise linear segments are stepped relative to one another to form said substantially convex shape.
25. A method for heating a substrate having a lower surface, a center area, and an edge area, the method comprising:
- introducing a pedestal having a center, an edge, and a substantially concave shaped top surface underneath said substrate lower surface such that for a substantially flat substrate said pedestal center is initially located at a further distance to said substrate lower surface than said pedestal edge;
- inserting a thermally conductive gas between said substrate lower surface and said pedestal top surface through the gap between the pedestal edge and the substrate lower surface at such process conditions that the thermally conductive gas initiates heat transfer from;
- said pedestal top surface to said substrate lower surface causing slower expansion of the center area than the edge area of the substrate causing substrate flattening during heating.
26. The method of claim 25, wherein said substantially concave shape top surface at least one continuous curvature segment, substantially concave in shape.
27. The method of claim 26 wherein said at least one continuous curvature segment comprises a portion of an elliptical, parabolic, or spherical shape.
28. The method of claim 25, wherein said substantially concave shape top surface comprises more than one piecewise linear segments.
29. The method of claim 28 wherein said piecewise linear segments are angled relative to one another to form said substantially concave shape.
30. The method of claim 28 wherein said piecewise linear segments are stepped relative to one another to form said substantially concave shape.
31. The method of claim 19, wherein the substrate is flatter after cooling than before cooling.
32. The method of claim 19, wherein the substrate comprises a 300-millimeter wafer distorting less than about 0.5 millimeters during cooling.
33. The method of claim 32, wherein the 300-millimeter wafer has a dome deviation of at least about 0.1 millimeter before cooling.
34. The method of claim 19, wherein the substrate comprises a 300-millimeter wafer and wherein said substrate lower surface is initially located closed to said pedestal center than to said pedestal edge by about 0.020 inches.
35. The method of claim 19, wherein the temperature of the substrate center area is within less than about 50° C. from the temperature of the substrate edge area during cooling.
Type: Application
Filed: Mar 29, 2010
Publication Date: Oct 28, 2010
Inventors: James D. Landess (San Jose, CA), James S. Templeton (San Jose, CA)
Application Number: 12/749,170
International Classification: F28F 13/00 (20060101); F28D 15/00 (20060101); F25B 29/00 (20060101);