Wafer Thickness Measurement Using Synchronized Wavelength Sweeping
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Solution Overview
Problem
Existing non-contact thickness measurement devices for semiconductor wafers using wavelength sweeping-type laser light sources suffer from mechanical errors due to polygon mirrors, leading to inaccurate thickness measurements.
Innovation Solution
A non-contact apparatus utilizing a monolithic wavelength sweeping semiconductor laser light source that eliminates mechanical operation units, enabling high-accuracy thickness measurement by controlling the laser source to oscillate with a linear sweep wavelength profile.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a mechanical operation unit such as a polygon mirror is used to obtain sweep wavelength, then wavelength sweeping is achieved, but slight distortion on the mirror surface causes shift in reflected light and wavelength, and mechanical vibration generates additional wavelength shift
Solution Approach 1:
The patent replaces the mechanical polygon mirror system with an acousto-optic modulator (AOM) that uses acoustic waves to diffract and sweep the laser wavelength. This substitution eliminates mechanical moving parts, distortion issues, and vibrations, thereby maintaining wavelength sweeping capability while dramatically improving measurement precision by removing the sources of wavelength shift.
Solution Approach 2:
The patent introduces an acousto-optic modulator as an intermediary device between the laser source and the measurement system. The AOM uses sound waves as a mediator to control the optical path and achieve wavelength sweeping without mechanical movement, thus eliminating the direct connection between mechanical operation and optical wavelength that causes measurement errors.
2Ease of operation
If mechanical vibration is present from the operation unit, then wavelength sweeping operation is maintained, but the vibration results in shift of the wavelength to be swept
Solution Approach 1:
The patent replaces mechanical vibration-based wavelength sweeping with an acousto-optic mechanism that uses controlled acoustic waves in a stationary medium. This eliminates unwanted mechanical vibrations while maintaining the ability to sweep wavelengths through controlled acoustic modulation, thereby improving wavelength stability without sacrificing operational capability.
3Productivity
If high-speed Fourier transform is used for signal processing, then measurement speed is improved, but center frequency shift due to wavelength shift generates accidental error in thickness
Solution Approach 1:
By replacing the mechanical wavelength sweeping system with an acousto-optic modulator, the patent eliminates the wavelength shifts that cause center frequency errors in Fourier transform analysis. This allows high-speed Fourier transform processing to be used for rapid measurement while maintaining high accuracy, as the input signal frequency remains stable without mechanical vibration-induced shifts.
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
The apparatus achieves precise and accurate thickness measurements with minimal accidental error, as the monolithic laser source ensures stable wavelength sweeping, synchronized with high-speed Fourier transform analysis.
Implementation Method 1
a laser source, a laser control unit that controls the laser source, and a processor that is configured to cause the laser control unit to control the laser source so as to oscillate laser light having a wavelength that changes with a setting profile relative to time
Implementation Method 2
an optical system that guides and emits the laser light onto a measurement portion of a wafer a thickness of which is desired to be obtained
Implementation Method 3
a detection unit that detects an interference light signal of reflected light or transmitted light obtained from the measurement portion
Data Source
AI summary
A non-contact apparatus for measuring wafer thickness includes a monolithic wavelength sweeping semiconductor laser light source having a laser source, a laser control unit that controls the laser source, and a processor to control the laser source to oscillate laser light having a wavelength that changes with a setting profile relative to time; an optical system that guides and emits the laser light onto a wafer; a detection unit that detects an interference light signal of reflected light; an A/D converter that converts the interference light signal detected by the detection unit into a digital signal; and a calculation unit that calculates a thickness of the wafer by analyzing the digital signal from the A/D converter. The processor causes the laser control unit to operate with a clock signal, and to oscillate laser light that performs wavelength-sweeping with the setting profile relative to the time, from the laser source. The A/D converts the interference light signal by generating a sampling clock in synchronization with the clock signal or directly using the clock signal as a sampling clock.