Browse Prior Art Database

X-RAY FLUX DETECTORS

IP.com Disclosure Number: IPCOM000006194D
Original Publication Date: 1991-Jul-01
Included in the Prior Art Database: 2001-Dec-12
Document File: 2 page(s) / 115K

Publishing Venue

Motorola

Related People

Whit G. Waldo: AUTHOR

Abstract

Overlay of features in integrated circuits is a func- tion of critical dimension control and registration errors. Device functionality depends on overlay ade- quate for the design so critical dimension control receives a lot of attention from process engineers. Pattern transfer in optical lithography involves the use of light to transfer the image of a mask to the photore- sist on a wafer. The intensity and uniformity of the light source is an important parameter that is charac- terized routinely and studied carefully for its impact on critical dimension control. Optical lithography has some very mature tools to study the quality of the light source. These include real time sensors sensitive to the same wavelengths that the resist is most respon- sive to, sensors that detect at the most intense actinic wavelengths, and broadband sensors. Generally, the size of the monitoring pixel can be quite small so the exposure field can he examined in fine increments.

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MOTOROLA INC. Technical Developments Volume 13 July 1991

X-RAY FLUX DETECTORS ~

by Whit G. Waldo

  Overlay of features in integrated circuits is a func- tion of critical dimension control and registration errors. Device functionality depends on overlay ade- quate for the design so critical dimension control receives a lot of attention from process engineers. Pattern transfer in optical lithography involves the use of light to transfer the image of a mask to the photore- sist on a wafer. The intensity and uniformity of the light source is an important parameter that is charac- terized routinely and studied carefully for its impact on critical dimension control. Optical lithography has some very mature tools to study the quality of the light source. These include real time sensors sensitive to the same wavelengths that the resist is most respon- sive to, sensors that detect at the most intense actinic wavelengths, and broadband sensors. Generally, the size of the monitoring pixel can be quite small so the exposure field can he examined in fine increments.

  There are fewer convenient commercial products that can he used today with SOR X-ray monitoring, yet the importance of characterizing and ensuring the quality of the light source is just as great. Also, the importance of real-time feedback cannot be empha- sized too much. The current state-of-the-art procedure uses calibrated dosimeter film to study the flux and its variations. The dosimeter film is calibrated for optical density change to energy using an electron beam machine, which, like SOR X-rays, emits ionizing radi- ation. A sheet of this film is exposed to the beamline over the entire field. Slices of the film can be used to make photometer measurements of the optical density change with nominal exposure dose. Unfortunately, this procedure is very slow, doesn't permit the study of fine increments of the field, and suffers from relatively high gauge imprecision.

  Fortunately, there are several attractive candidates to provide the service of real time SOR X-ray moni- toring. One is the use of a scintillating crystal acting as an X-ray monitor. Such a crystal that has been widely used as a sensor in X-rays is doped calcium

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flouride, but there are several others. Scintillating crystals emit visible light ,when there is incident ioniz- ing radiation, so there ark several commercial moni- tors available to measure the crystal's emissions. A scintillating crystal ring is constructed surrounding the flange of the X-ray exposure snout. The X-ray expo- sure snout is an extension from the evacuated beam- line end at the beryllium~!window to the X-ray mask, which will be exposed; The snout is filled with helium gas, which has a low X-ray absorption cross- section. The snout is constructed to define the field edges so that the internal' field has uniform light. X- rays outside of the snout area, striking the flange, will strike the crystal and allow real-ti...