<title>Design considerations for a photo track monitor reticle</title>

Abstract

As I-line and deep ultraviolet (DUV) photolithography processes grow more complex, yield improvement has become more challenging and critical. Historically, expensive and time consuming monitoring of product wafers themselves has been used to monitor the health of photolithography processes and equipment. In addition, the sensitivity of the inspection scans on product wafers is usually not sufficient due to the vast amount of noise generated by underlying pattern or structures. With the advent of smaller sub-micron geometries using newer chemically amplified photoresists, a high sensitivity, easy to review and trouble-shoot monitor is essential. In order to fully understand what defects may be generated by a process or process tool, it is necessary to fully duplicate the given process on the test wafer or procedure used to monitor the defect level. That concept is called Process Induced Defects Per Wafer Pass (PIDPWP). PIDPWP requires the real and exact product process to be used in creating the defect test monitor. To monitor a photolithography process using the concept of PIDPWP, typically a coat, expose, and develop sequence is used with a selected and simplified mask, such as a diffraction grating. The role that the pattern chosen for the Photo Track Monitor (PTM) was examined. Typical structures such as diffraction grating (equal lines/spaces), contact arrays, clear and resist areas, and typical product patterns were evaluated for the types of defects that they can detect and monitor. Certain types of defects were more clearly observed depending on the structure of the pattern and the nature of the defect mechanism. Simultaneous printing of various structures on the same PTM reticle may not be allowed due to limits of the photo process, so care must be used in choosing which structures to include on the same reticle. Other factors examined included the pattern inspection by trained technicians performing the role of classification and troubleshooting when a tool goes out of control. While the inspection tools were extremely sensitive regardless of the pattern density, the defect must still be examined so that the technician can determine if the defect was of concern or not. Such factors as the resolution capability of the review optics and review SEM on the choice of the PTM reticle were examined. The results showed that no one pattern was optimal for a PTM reticle, and may require multiple fields or multiple PTM reticles to successfully monitor the photolithography process without scanning product wafers.