Masking With Matched Sets

MASKING WITH MATCHED SETSA. H. DePuyIBM System Products DivisionEssex Junction, Vermont 05452AbstractThe emphasis today in wafer fabrication is to continually improve device yields.This is usually accomplished at the expense of the mask maker, who is consistently beingrequested to provide masks of higher and higher quality. Higher mask quality is also de-sired due to the long -life masks, especially when used with a proximity or projectionprinting system. A mask inspection and usage scheme has been developed, based on an age -old inspection technique, whereby higher device yields can be realized without paying theprice of increasing mask quality.IntroductionFabrication of semiconductor devices involves a series of selective diffusions and aninterconnecting layer of selective metallization. These areas are selected by usingphotographic masking operations. Because masks cannot be produced free from defects,these masks contribute to a loss in chip yield.The defects are caused by either the mask process or the raw materials and occur ran-domly throughout the mask. With today's large chips and repeated masking operations,mask defects are being given increasing attention. The yield loss due to mask defectscan be reduced if a "lot" of wafers is exposed to a set of masks, selected in such a waythat as many as possible of the good (defect -free) chip locations at all the levels fallon one another. Such a set of masks is called a "matched set."Present PracticeThe present procedure for semiconductor device masking operations is to select themask for the first masking operation from an existing inventory of first masks, expose itto a lot of resist - coated wafers, complete the required process steps; and, when readyfor the next masking operation, select the second mask, again from the inventory of sec-ond masks, and continue the process steps; always selecting the masks from the availablemask inventory when it is time for the next masking operation.In this mode of operation, there are continal demands on the mask fabricator forhigher quality masks. In the operation of a stable device process, device process en-gineers will usually attempt to obtain an increase in their device yield, by requestinghigher quality masks, instead of making changes to the device process. These requestsfor higher quality masks provide the mask makers little choice. They can change theirinspection plans to get the higher quality mask, for which they will suffer a yield loss,or they can attempt to improve the process, in order to consistently produce the higherquality.It should be emphasized that selecting masks at random from a mask inventory resultsin both a lower chip yield and a lower mask fabrication yield."Matched Sets"Instead of masking a lot of wafers to masks, selected at random from the mask inven-tory, one can achieve a higher device yield if the wafers are exposed to a "matched set"of masks. A "matched set" is a set or group of masks (one for each masking operation)selected in such a way that the defective chip location at each masking operation alignwith one another.By using the scheme of "Masking with Matched Sets," higher device yields can also beachieved without impacting the mask builder. Matched sets are depicted by Figure 1.