System and method for optimizing wafer flatness at high rotational speeds

Description

FIELD OF THE INVENTION

The present invention relates to the field of optical profilometers and more particularly to the field of wafer inspection machines.

BACKGROUND OF THE INVENTION

Wafer inspection machines typically permit a wafer to be placed on top of a rotating chuck with rotation speeds that exceed 4000 revolutions per minute being common. In some conventional systems the wafer lies flat upon a receiving surface in the wafer inspection machine which contacts substantially the entire wafer on one side. When the wafer spins the wafer remains flat. One problem with this system is that by contacting substantially the entire wafer on one side there is an increased likelihood of contaminants being introduced to the wafer on the contacting side. In addition, any contaminant caught between the two contacting surfaces will cause a deformation of the wafer surface, jeopardizing the accuracy of the defect inspection, especially for optical inspection systems.

In another conventional system, a chuck includes a vacuum corridor whose ends receive the wafer. The wafer contacts the chuck near the wafer's outer edge. The wafer is held in place at high rotational speeds (in excess of 3000 rpm) by supplying vacuum to this corridor. A problem with this system is that at high rotation speeds the air pressure above the wafer is less than the air pressure below the wafer which results in the wafer bowing upwards. Such bowing can prevent the wafer inspection machine from properly inspecting the wafer because, for example, the bowing may result in the wafer being out of focus in some areas which may prevent defects from being detected in these areas.

One conventional solution involves using a sensor on the chuck to actively measure the distance the chuck to the center of the wafer as the chuck rotates. Using this information a pressure regulator modifies the pressure beneath the wafer to reduce the bowing of the wafer. The problem with this solution is that it adds cost and complexity to the system.

What is needed is a system and method for modifying the vacuum chuck to automatically reduce the bow of an object placed thereupon without requiring any feedback mechanism.

SUMMARY OF THE INVENTION

The present invention is a chuck having a vacuum groove that is capable of holding a wafer as the chuck rotates on a spindle. As the chuck rotates the air pressure above the center of the wafer is reduced. In order to reduce the bowing of the wafer that can result from this low pressure area above the wafer, the present invention introduces venturi holes in the chuck which reduces the air pressure in the area below the wafer. In order to prevent the air pressure in the area below the wafer from decreasing too much, the present invention uses air inlet holes to balance the affect of the venturi holes in order to substantially balance the air pressure above and below the wafer which results in significantly less bowing of the wafer when compared to conventional systems. The present invention accomplishes this without requiring sensors or other active measuring devices for reducing the bowing of the wafer.

The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a chuck with a vacuum groove according to one embodiment of the present invention with the chuck holding a wafer.

FIG. 2 is an illustration is a top view of a chuck with a vacuum groove according to one embodiment of the present invention.

FIG. 3a is an illustration depicting the bowing of a wafer at a high revolution rate when the air pressure above the center of the wafer is less than the air pressure below the center of the wafer.

FIG. 3b is an illustration depicting the lack of bowing of a wafer when the air pressure above the center of the wafer is approximately equal to the air pressure below the center of the wafer.

FIG. 4 is an table illustrating experimental results for one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention is now described with reference to the figures where like reference numbers indicate identical or functionally similar elements. Also in the figures, the left most digit of each reference number corresponds to the figure in which the reference number is first used.

Reference in the specification to “one embodiment” or to “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.

In addition, the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.

FIG. 1 is a cross-sectional view of a chuck 102 having a vacuum groove 104 according to one embodiment of the present invention with the chuck shown securing a wafer 110. A spindle 100 rotates and is coupled to the chuck 102 that includes a vacuum groove 104 that receives a wafer 110. The wafer can be thin film disk, a semiconductor wafer, a compound semiconductor wafer, a glass wafer or a plastic disk for example. FIG. 2 is an illustration is a top view of a chuck 102 with a vacuum groove 104 according to one embodiment of the present invention. In FIG. 2, the wafer is not shown. In this perspective the vacuum groove is shown to be circular and has a radius that is smaller than the radius of the wafer 110 it receives. In one embodiment, the radius of the vacuum groove is approximately 85 millimeters (mm). In one embodiment the width of the vacuum groove is 0.9 mm and the vacuum groove includes two ridges (150) of the vacuum groove (the inner ridge and the outer ridge) that provide enough contact area with the wafer 110 to prevent it from shearing off the chuck when the chuck and wafer are rotating at high rates. In alternate embodiments the shape and radius can differ and, in some embodiments, the vacuum groove can be discontinuous.

The air pressure in the vacuum groove is reduced in order to hold the wafer 110 on the chuck 102 while the chuck 102 is rotating. As described above, a problem with conventional systems is that, as the wafer rotates, the air pressure above P₁ the wafer is reduced which causes the wafer to bow. FIG. 3a is an illustration depicting the bowing of a wafer at a high rate of revolution when the pressure above P₁ the center of the wafer is less than the pressure below P₂ the center of the wafer.

In contrast, FIG. 3b is an illustration depicting the lack of bowing of a wafer when the air pressure above P₁ the center of the wafer is approximately equal to the air pressure below P₂ the center of the wafer.

The present invention reduces bowing of the wafer by reducing the pressure below P₂ the wafer so that is more aligned with the pressure above P₁ the wafer. In conventional systems, the center of a wafer may bow well in excess of 30 micrometers (μm). Such a large bow may cause the device reading the wafer to not work properly. For example, devices for inspecting a wafer, such as the C2, which is commercially available from Candela Instruments, Inc. in Fremont, Calif., images a wafer by scanning a laser across the rotating surface of the wafer. The depth of focus of the inspection optics in the C2 is approximately 30 μm. Therefore, bowing that exceeds this focus depth may cause a problem during wafer inspection since parts of the wafer will be out of focus and defects or other characteristics of the wafer in the bowed areas may not be properly detected unless the optics are refocused.

The present invention uses the principle that if the air pressure on both sides of the wafer is equalized during a scan, then the wafer will not deflect or bow. In the present invention, the rotation of the chuck can be harnessed to create lower pressure beneath P₂ the wafer by introducing venturi holes 106 on the side wall and the floor plate of the chuck. These holes act as small venturis that suck air out of the area 112 below the wafer. However, even the smallest of venturi holes 106 will cause a large air flow rate out of the area 112 below the wafer which reduces the air pressure and may cause the air pressure in the area 112 below the wafer to become substantially lower than the air pressure in the area 114 above the wafer which causes the wafer to bow downward. In addition, such bowing may cause the wafer 110 to lose its seal with the vacuum groove 104 which may result in the wafer 110 detaching itself from the chuck 102.

In order to compensate for this ‘negative’ deflection, the present invention introduces inlet holes 108 that, in one embodiment, are positioned closer toward the center of the chuck as compared to the venturi holes 106. The high linear velocity at the outer edge of the chuck enables the outer holes [106] to act as venture holes, sucking air out of the chamber. In order to compensate for the air flowing out, larger holes have to be made closer to the center of the chuck, enabling air to flow into the chamber. The number of inlet holes 108 and venturi holes 106 can vary depending upon a variety of factors including the type of wafer, the thickness of the wafer and the anticipated rotation rate. In one embodiment, numerous inlet holes 108 and venturi holes 106 are available to be used and the user can cover or block those that are not necessary for the particular wafer 110 that will be tested. The inlet holes 108 reduce the effect of the venturi holes 106 and result in a substantial reduction in the bowing or deflection of the wafer at high rates of rotation.

FIG. 2 illustrates a configuration of the inlet holes 108 and venturi holes 106 on a chuck designed to receive 8 inch (or 200 mm) diameter wafers.

FIG. 4 is a table illustrating experimental results for one embodiment of the present invention using the configuration illustrated in FIG. 2. In this experiment three different wafers were used including a 1.1 mm thick glass wafer, a 0.7 mm thick glass wafer and a 0.725 mm thick silicon wafer. The present invention maintained the bowing to well under 10 μm for each of these various types of wafers at a rotational speed of 5000 revolutions per minute (rpm). This reduction in bowing occurs because the air pressure in the area below the wafer 112 substantially matches the air pressure in the area 114 above the wafer. The horizontal axis represents the distance from the center of the wafer in millimeters while the vertical axis represents the bowing of the wafer in microns. The nature of the air flow through the holes in the chuck results in the wafer bowing up closer to the point of contact with the chuck.

The present invention does not require the use of sensors or gas delivery systems or other real-time feedback to adjust the pressure in the area 112 below the wafer. Instead the present invention uses the combination of pre-set venturi holes 106 and air inlet holes 108 to significantly reduce the bowing (deflection) of the wafer during high rotational speeds when compared to conventional systems.

While particular embodiments and applications of the present invention have been illustrated and described herein, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes, and variations may be made in the arrangement, operation, and details of the methods and apparatuses of the present invention without departing from the spirit and scope of the invention as it is defined in the appended claims.