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The Importance of Variable Wafer Tilt for Defect Classification by Sheldon Moll, Ph.D., Consultant
The scanning electron microscope-based Defect Review Tool (DRT) has now become an essential component of a suite of instruments dedicated to Statistical Process Control in the modern fab. More recently, the tool has transitioned from an engineering-type instrument — requiring full operator attention — to a true, automated in-line monitor. The DRT supplements the laser scan or light optical image-based inspectors (defect detection tools) by performing an operator-free Automatic Defect Classification (ADC) of individual defects. It is clear that the DRT must yield highly accurate defect classifications in order to maintain high in-line yields.
F i g u r e 1. Photoresist 45 º t i l t .
F i g u re 2. Photoresist 0 º t i l t .
F i g u re 3. Photores ist 0 º t i l t shado w perspective.
Defect review tools are available today with wafer handling stages allowing variable wafer tilt, no tilt at all, or only a fixed, nonadjustable tilt. The purpose of the following information is to demonstrate the imaging improvement and consequent classification accuracy benefits which derive from a DRT wafer stage allowing fully variable tilting. Three-dimensional perspective
A natural attribute of the SEM is its ability to form magnified images of three-dimensional objects which not only appear “natural” to the human eye, but also allow a direct, quantitative measure of their physical dimensions. An understanding of the height dimension of an object can be determined directly from a tilted view. A top-down, or untilted view, does not directly reveal the magnitude 36
Autumn 1999
Yield Management Solutions
of the object’s “z” — the height dimension of the posts. For example, a cylinder viewed end-on appears to be a simple circle and can falsely be indentified as such. Figure 1 is a SEM micrograph of a group of photoresist structures destined, after further processing, to become the vias of an IC. The post-like structures are readily apparent in this image taken with the wafer tilted 45º with respect to the electron beam. Figure 2 shows the same structures with the electron beam “looking” top-down — a zero tilt orientation. The posts appear only as reticulated ovals and the image yields no indication of their vertical profile. It must be understood that SEM images appear as if viewed from the direction of the scanning incident beam. However, from the standpoint of dark and light areas and shadows in the image, the electron detector (or detectors) serves the eye only as the illumination source.