Unequal Path Interferometer for Wafer Shape and Thickness Measurement
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Solution Overview
Problem
Current radiation-based inspection techniques, such as phase shifting interferometry, face challenges in accurately measuring the shape, thickness, and material inhomogeneity of wafers due to limitations in separating superimposed interferograms and determining refractive index distribution.
Innovation Solution
The method employs an unequal path length interferometer with a tunable laser, allowing for phase shift by changing the wavelength, and utilizes a weighted least-square fitting technique to separate phases of superimposed interferograms, enabling the creation of phase maps that represent surface shape, thickness, and refractive index inhomogeneity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase shifting interferometry is used to measure wafer parameters, then measurement accuracy is improved, but the ability to separate superimposed interferograms deteriorates
Solution Approach 1:
The patent segments the superimposed interferograms by introducing temporal modulation through vibration. Each interferogram is modulated at a distinct frequency, allowing the detector to capture composite signals that can be mathematically separated using spectral analysis or lock-in detection techniques. This segmentation in the frequency domain resolves the difficulty of separating overlapping spatial interferograms while maintaining measurement precision.
Solution Approach 2:
The patent introduces dynamic vibration to the interferometer components (mirror, beam splitter, or detector) to create time-varying phase shifts. By vibrating components at different frequencies, the static superimposed interferograms become dynamic signals with distinct temporal signatures. This dynamic approach enables separation of the interferograms through frequency-domain analysis while preserving the high measurement accuracy of phase shifting interferometry.
2Productivity
If multiple interferograms are superimposed for simultaneous measurement, then productivity is improved, but device complexity increases
Solution Approach 1:
The patent makes a single interferometer component (mirror, beam splitter, or detector) perform multiple functions by vibrating it at multiple frequencies simultaneously. This universal component generates and modulates multiple interferograms that are superimposed on the detector, enabling simultaneous measurement of multiple wafer parameters without requiring separate interferometer paths or multiple detectors, thus limiting the increase in device complexity.
Solution Approach 2:
The patent merges multiple measurement functions into a single interferometric path by superimposing multiple interferograms on one detector. By combining the measurement of different wafer parameters (thickness, shape, inhomogeneity) into a unified signal that can be separated through frequency analysis, the system achieves simultaneous multi-parameter measurement without proportionally increasing the complexity of the optical setup.
3Adaptability or versatility
If wavelength is changed to achieve phase shift, then measurement versatility is improved, but measurement time increases
Solution Approach 1:
The patent replaces gradual wavelength changes with periodic mechanical vibration of interferometer components. The vibration creates rapid, periodic phase shifts that occur continuously during the measurement cycle, eliminating the need for time-consuming wavelength tuning between measurements. This periodic mechanical action maintains measurement versatility while dramatically reducing measurement time by enabling continuous phase-shifted interferogram acquisition.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for precise measurement of multiple reflective surfaces and plate thicknesses, improving measurement repeatability and accuracy by generating phase maps that effectively separate and analyze interferograms, thereby determining surface parameters like height, thickness variation, and material inhomogeneity.
Implementation Method 1
a laser source of wavelength lambda
Implementation Method 2
Phase shifting interferometry (PSI) is a highly accurate and efficient phase measuring method
Implementation Method 3
The phase shifts between interferograms are produced by changing the optical path difference (OPD) between the measurement surface and a reference surface. The phase shifts also can be achieved by changing the wavelength of the radiation used
Data Source
AI summary
In one embodiment, an interferometer system comprises an unequal path interferometer assemble comprising; a first reference flat having a first length L1 in a first dimension, a second reference flat having a second length L2 in the first dimension, a cavity D1 defined by a distance between the first reference flat and the second reference flat, and a receptacle to receive an object in the cavity such that an optical path remains open between the first reference flat and the second reference flat, and a radiation targeting assembly to direct a collimated radiation beam to the interferometer assembly, a radiation collecting assembly to collect radiation received from the interferometer assembly, and a controller comprising logic to; vary a wavelength of the collimated radiation beam, record interferograms formed by a plurality of surfaces, extract phases of each of the interferograms for each of the plurality of surfaces to produce multiple phase maps, and determine each phase map from its corresponding interferogram, using a weighted least-square algorithm.