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Method for detecting top-side scratches on wafers

IP.com Disclosure Number: IPCOM000127910D
Publication Date: 2005-Sep-14
Document File: 6 page(s) / 103K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for detecting top-side scratches on wafers. Benefits include improved functionality and improved line yield.

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Method for detecting top-side scratches on wafers

Disclosed is a method for detecting top-side scratches on wafers. Benefits include improved functionality and improved line yield.

Background

              Very light top-side scratching can occur on wafers during fabrication due to wafer handling issues with process tools. In each process generation, feature sizes are reduced to improve transistor density, speed, and die size. As a result, the device structures on these wafers have become increasingly fragile and prone to mechanical damage. For example, there have been several excursions related to thin fibers attached to a robot end effector, possibly from a wipe used to clean the robot during regularly scheduled preventative maintenance (PM). Fibers from the wipe, often invisible to the naked eye, may transfer to the bottom of the end effector, hang down, and scratch the top of wafers as the robot passes over them while taking wafers in/out of the front-opening unified pods (FOUPs) (see Figure 1).

              A conventional defect detection monitor cycles a bare silicon wafer through a fab process tool. The wafer is inspected for particles, scratches, and defects in a separate defect metrology tool by shining a laser on it as oblique incidence. Defects on the wafer scatter the laser light into a photodetector (see Figure 2).

              Tool-level checks, such as particle monitors and visual inspection, are typically unable to detect subtle failure modes, such as fiber excursion. Historically, only in-line pattern-defect inspection on production wafers has provided containment.

General description

              The disclosed method includes a grating pattern printed in photoresist material on bare silicon wafers using conventional lithography techniques. The result is a mechanically fragile substrate that is extremely sensitive to top-side scratches so it detects wafer-handling issues with fabrication (fab) equipment. When a handling issue is suspected with a tool, the grating wafers can be cycled through the tool as a monitor. After cycling, the wafers are inspected for damage by shining a laser on them.  Light scattered at angles not due to diffraction from the grating indicates a problem.

              The feature size of the lines used in the grating pattern must be small. Typically, less than 200 nm provides sufficient mechanical sensitivity to issues, such as a thin fiber hanging off the end of a robot end-effector that is dragging over the top of a wafer.

              The geometry of the laser and photodetectors used to inspect the grating wafers for damage must be selected so that the detected scattered light is due mainly to damage, not diffraction. The grating pitch must be optimized relative to the laser wavelength.

Advantages

              The disclosed method provides advantages, including:
•             Improved functionality due to providing a tool-level monitor for detecting ultralight top-side wafer scratches
•             Improved line yie...