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Apparatus for Measuring Wafer Parallelism Disclosure Number: IPCOM000011176D
Publication Date: 2003-Feb-12
Document File: 7 page(s) / 1M

Publishing Venue

The Prior Art Database


Disclosed is a method that consists of an optical inspection station for measuring wafer parallelism.

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Apparatus for Measuring Wafer Parallelism

Disclosed is a method that consists of an optical inspection station for measuring wafer parallelism.


Currently, wafer parallelism is measured using Free Spectral Range (FSR) and Fourier Transform Infrared (FTIR) spectroscopy.  Both methods estimate the absolute thickness of the device under test (DUT) at each point.  The relative changes in thickness are obtained by differencing the estimates for absolute thicknesses at two points. However, this approach is indirect, and suffers from a relatively large noise term associated with the two measurements applied to the small signal of thickness difference.

General Description

The disclosed method consists of an optical inspection station containing the following parts (see Fig. 1):


  • a tunable laser and a broadband detector,


  • a broadband source, e.g. Erbium Doped Fiber Amplifier (EDFA) operated with no input signal and a tunable detector, e.g. Optical Spectrum Analyzer (OSA),


  • hardware and software for data acquisition and analysis.

An optical beam with a desired profile is created, transmitted through the DUT, and collected with the detection system.  The collected signal is a spectrum given rise from the etalon property of the DUT at one point.  A section of spectrum revealing at least one spectral peak is collected. The position of the spectral peak is extracted from the spectrum by curve fitting or peak detection.  This spectral peak information is then stored, and then the beam is translated to a new position on the DUT.  The process is repeated (see Figure 1).

A spectrum comprising enough information to estimate the absolute thickness (typically this is a spectrum with at least two spectral peaks) is additionally collected for at least one point on the DUT. Alternately, prior knowledge of absolute thickness (perhaps derived from some other test) may be entered.

The relative thickness changes from a reference point can be determined from the spectral peak positions, according to the relation


where Tref and T2 are the absolute thickness at the reference point and a second point, respectively, and lref and l2 are, respectively, the spectral peaks at the reference point and a second point of the same interference order.

 This relation can be rewritten in terms of change of thickness and change of spectral peak:

By applying this relation to each point, a two-dimensional map of wafer thickness vari...