Direct Mask Overlay Inspection

Direct mask overlay inspectionLiang -Choo Hsia and Lo -Soun SuMask Engineering, General Technology DivisionIBM Corporation, East Fishkill, New York 12533AbstractIn this paper, we present a mask inspection methodology and procedure that involvesdirect X -Y measurements. A group of dice is selected for overlay measurement; four measure-ment targets were laid out in the kerf of each die. The measured coordinates are then fit-ted to either a "historical" grid, which reflects the individual tool bias, or to an idealgrid squares fashion. Measurements are done using a Nikon X -Y laser interferometric mea-surement system, which provides a reference grid. The stability of the measurement systemis essential. We then apply appropriate statistics to the residual after the fit to deter-mine the overlay performance.Statistical methods play an important role in the product disposition. The acceptancecriterion is, however, a compromise between the cost for mask making and the final deviceyield. In order to satisfy the demand on mask houses for quality of masks and high volume,mixing lithographic tools in mask making has become more popular, in particular, mixingoptical and E -beam tools. In this paper, we also discuss the inspection procedure formixing different lithographic tools.IntroductionThe trend towards denser circuits, smaller device geometries, and larger wafer size hasmade mask overlay inspection most critical in VLSI manufacture. Misregistration problemscan give rise to large yield losses. Therefore, a reliable mask overlay inspection pro-cedure is essential. The measurements must be accurate and repeatable. On the other hand,the advent of projection aligner has dramatically increased the working life of a mask andhas made the number of masks needed per level decrease continuously. The near future maysee a situation where one to two masks suffice for each masking level, even for high volumeVLSI device manufacture. Therefore, it will be more economical to inspect each mask verycarefully. In particular, for bipolar device fabrication, which requires more than 16lithographic steps, the quality of each photomask has great impact on yield losses.As improvements have been made over the years, E -beam lithographic tools have beengradually accepted by mask makers for 1X mask making. Compared with optical mask making,E -beam mask making has the advantages of design flexibility, quick turnaround time, betterresolution, and perhaps better placement accuracy. However, E -beam mask making is expen-sive and the equipment is difficult to maintain. A cost -effective approach lies in mixingE -beam with optical mask making. The overlay matchability between the two, therefore,must first be guaranteed. In this paper, we present an inspection methodology and proced-ure for solving this problem.