A Holographic Photomask Defect Inspection System

Abstract

The need for automatic defect inspection of photomasks has been well recognized by the semiconductor manufacturing industry. Indeed, commercial equipment for this purpose has been available for many years. The method used in the commercial equipment is based on the point-by-point comparison of one die with an adjacent die or with the data base, in a serial fashion. As the minimum features of the production integrated circuit (IC) pattern approach micrometer and submicrometer dimensions and the pattern area increases to about 150 mm in diameter, the inspection time per photomask by this method becomes formidably long, sometimes hours. To reduce the inspection time, radically different inspection methods are clearly needed. One such method is based on an optical information processing technique.1'2 This method has an inherent advantage: the IC pattern and the defect information over the entire photomask are processed simultaneously, or in parallel. However, several difficulties in the past have prevented this method from being practical. Under the sponsorship of Insystems, scientists at the University of Daytsn Research Institute were able to circumvent these difficulties by means of holography.'. Based on the successful results achieved there, Insystems undertook the task of developing an automated photo-mask inspection system, the Model 8405. This system is designed for turn-key operation in IC manufacturing facilities. Major performance specifications of the system are listed below. . Acceptable photomask sizes: standard sizes or others up to 178 mm x 178 mm (7" x 7") . Acceptable photomask thicknesses: standard thicknesses or others up to 6.4 mm (0.25") with or without pellicle . Detectable defect sizes: 0.5 um or larger with or without pellicle . Automatic inspection time: 7 minutes or less for 100 mm x 100 mm photomasks . Hologram or filter production time: approximately 6 minutes . Defect report formats: include computer printout of defect map (indicating defect types, sizes and locations), video terminal displays of holographic defect image and corresponding microscope image The purpose of this paper is to describe this system and present some preliminary results.