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Automatic Integrated Circuit Mask Defect Detection System

IP.com Disclosure Number: IPCOM000080382D
Original Publication Date: 1973-Dec-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 3 page(s) / 55K

Publishing Venue

IBM

Related People

Wohl, RJ: AUTHOR

Abstract

This optical arrangement is useful for comparing a master and a copy of any print or artwork for quality control or screening in the printing, casting or integrated circuit fabricating arts.

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Automatic Integrated Circuit Mask Defect Detection System

This optical arrangement is useful for comparing a master and a copy of any print or artwork for quality control or screening in the printing, casting or integrated circuit fabricating arts.

The optical images of a master 10 and a copy 12, as shown in Fig. 1, are superimposed onto a rearview projection screen 14, appropriate X-Y motion being provided in conventional form for effecting superposition. A pair of light sources shown here as lamps 16, 18 are alternately turned on and off by a control circuit 20. Light from the lamp 16 passes through a condensing lens arrangement 22, a beam splitting device 26, a projection lens 28 and through a fresnel mirror 30 to the screen 14. Light from the lamp 18 passes through a condensing lens system 14 and the copy 12, is reflected off the beam splitting device 26, through the projection lens system 28 and the fresnel mirror 30 to the screen 14.

The fresnel mirror also diverts part of the light through a lens system 32 to a photosensitive device 34. Electric current from the photosensitive device 34 is applied to an electric filter 36, sharply tuned to the "flicker" frequency (or the frequency of light varying, due to switching by the control circuit 20). The output of the filter 36 is applied to an amplifying circuit 38 and delivered at output terminals 40, for operating an alarm or automatic circuitry or the like.

If there is no defect (either added or subtracted), the photosensitive device 34 is steadily illuminated. However, if the copy 12 differs from the master 10, there will be a component of light varying at the alternation or "flicker" frequency, for example, 10 KHz. Any 10 KHz component passing through the sharply tuned filter 36 will deliver a potential at the terminals 40, Which will sound an alarm or operate other circuitry. In a simple application, the operator, on noting an alarm, slows the light alternation or flicker to below the fusion frequency of the human eye, which is about 60 Hz. Any blinking light in the field on the screen 14 will indicate the discrepancy between the copy 12 and the master 10.

Automatic location of the discrepancy is comtemplated by the use of an image dissector tube, instead of or in conjunction with the screen 14. The image of the copy tube, whereby a counterpart image of variable-electron density is formed at an image plane within the tube. The output terminals 40 are connected to circuitry causing the deflection components of the image dissector tube to begin a raster scan.

Each unit of the electron image is then scanned by the secondary emission multiplier located within the image dissector tube. The output of the multiplier is passed through a narrow-pass filter as before, so that only the flicker frequency is effective. In this arrangement, the flicker frequency is of the order of 100 KHz so that standard TV receiver components may be used for image deflection, with a l...