Mark focusing system for steppers

#The invention relates to the general field of photolithography with particular reference to the problems associated with sharp focusing.

#Improvements in photolithography over the years have been responsible for many of the advances in integrated circuit technology in general. As these circuits have grown increasingly more complex, the number of different layers in the finished product has also grown considerably. Although it is general practice to planarize the surface of the wafer at regular intervals throughout the process, it is nevertheless impossible to avoid situations in which the surface onto which a given image is being focused for photolithographic purposes is quite uneven. It thus becomes necessary to devise a technique for always obtaining the “best focus”, i.e. the best compromise for that particular surface.

#Solutions to this “best focus” problem are included in commercial steppers such as the PAS-5000/50 I-line system which uses averaged information of step height over a fixed probing area of an image field to determine the focal plane and to perform field by field focusing. The focusing system comprises a focus sensor, control electronics, and actuator. It is illustrated in FIG. 1. The focus sensor 11 operates by reflecting laser light off the surface of wafer 12 (which is free to move vertically relative to projection lens 15) into cat's eye reflector 13 which sends the beam back along the same path. In effect, the sensor determines when the diameter of light spot 14, on the wafer surface, is at a minimum. Projection lens 15 is positioned to be directly above spot 14 and is both vertically and horizontally fixed relative to the focusing system in such a way that when spot 14 is at minimum diameter an image projected through the lens onto the wafer surface will be in focus there.

#FIG. 1 shows a single spot laser focusing system.

#FIG. 2 is a schematic representation of a semiconductor wafer showing an array of integrated circuit fields.

#FIG. 3 shows the wafer of FIG. 2 with the addition of a mark alignment field.

#Referring now to FIG. 3, we show there the schematic wafer representation already seen in FIG. 2 with the added inclusion of alignment mark area 31. This (and similar areas located elsewhere on the chip) are a standard feature of any chip manufacturing process, being always present in one form or another. In FIG. 4 we show an enlarged view of mark alignment area 31 showing alignment mark 41. As can be seen, most of the area occupied by the alignment mark in the alignment mark field is empty. Furthermore, the alignment mark is a straightforward pattern with a simple topography. These qualities make the alignment mark field a suitable area for detecting the behavior of focusing spot 14 (as also seen in FIG. 4).

#In order to be able to make use of the alignment mark field for focusing purposes, it is necessary to make the horizontal motion of the projection lens independent of that of the focusing system—unlike the prior art systems discussed above where the projection lens and the focusing system move together both vertically and horizontally. All motion of the wafer (actually the X-Y stage on which it sits, and which is not shown) continues to be fully independent of both the projection and focusing systems.

#The process of the present invention begins with mounting the semiconductor (silicon, germanium, gallium arsenide, etc.) wafer on an X-Y stage which is located below the projection lens and which is normally an integral part of the stepper system. The wafer is positioned under the projection lens so that the image projected by the lens will lie precisely in one of the integrated circuit fields on the surface of the wafer. In practice this would be the surface of a layer of photoresist, between about 0.9 and 3 microns thick, that is to be exposed and patterned.