With optical lithography prevailing into the year 2000, super-resolution processes pose a multitude of new challenges to the lithographer. Isolated to nested feature bias calls for 'pre- distorting' the photomask to compensate for proximity effects and print and etch biases in the mask and wafer manufacturing process. OPC (optical proximity correction) techniques have become a reality for sub-halfmicron lithography, and have initiated many discussions looking at the manufacturability of OPC masks. Regaining the lost DOF (depth of focus) due to ever shorter printing wavelength, and increasing yields by expanding process latitude have many IC manufacturers looking into PSMs (phase shift masks) as a viable but expensive enhancement technique for several [2-6] layers of the total [18-26] device mask set. This paper addresses manufacturability issues of various combinations of 'enhancement' masks.
Mask defect specification is derived from the needs of IC makers. It is primarily based on the wafer design rule. The specification relates to many issues that are usually defined and tested separately. In this paper we are concerned with the relation between edge defect specification and CD tolerance. The main problem with a combination of errors is how to handle the case of two errors of different kinds that are both below the specification, when considered separately, but their total change in edge position is larger than the edge defect specification. By convention, CD errors are automatically eliminated during automatic defect inspection by bias correction. Therefore, the inspection machine actually processes a database representation that matches the scanned image of the inspected die as best as possible. Thus the detection of combinations of global CD errors and edge extensions is prevented. An analysis of the problem is presented. A novel approach is suggested by which the user may optionally take into account CD errors together with edge defects during inspection. The experimental results are reported and initial conclusions are drawn.
The relation between the accuracy of the database representation and the inspection quality is discussed. In order to visualize the problems, a simplified model of an inspection machine is described. Using this model various aspects of database accuracy are presented. It is shown that some of the conventional methods for digitization of trapezoids, e.g. Brezenham method, may cause pixel-positioning errors and dropouts between figures. A better approach that is based on high precision subpixel addressing is proposed and its implication on reducing the database inaccuracy is proved.