BPR Grating Calibration for Profilometer MTF Correction
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Solution Overview
Problem
Interferometric and atomic force microscopes face challenges in accurately measuring power spectral density distributions due to the unknown Modulation Transfer Function (MTF) of their instruments, which distorts higher spatial frequency measurements.
Innovation Solution
A calibration method using a Binary Pseudo-random (BPR) grating test surface, where a substrate with pseudo-randomly distributed grooves of uniform depth is used to determine the instrumental Modulation Transfer Function (MTF) by comparing experimentally measured and theoretically simulated power spectral density distributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If standard interferometric microscopy is used to measure surface topography, then sub-Angstrom rms roughness measurement capability is achieved, but the unknown Modulation Transfer Function (MTF) distorts the measured power spectral density distribution at higher spatial frequencies
Solution Approach 1:
A test surface with known pseudo-random groove pattern is introduced as an intermediary calibration standard. The groove pattern serves as a mediator between the instrument and the unknown MTF, allowing the MTF to be calculated by comparing the known input pattern with the measured output spectrum. This enables correction of the distorted spatial frequency information without sacrificing measurement precision.
Solution Approach 2:
The patent changes the parameter of the test surface by using a pseudo-random groove pattern with specifically controlled spatial frequency characteristics. This engineered pattern transforms the measurement problem into a solvable form where the MTF can be extracted as a correction factor, allowing recovery of accurate spatial frequency information from the distorted measurements.
2Ease of operation
If the Modulation Transfer Function (MTF) is not accounted for, then measurement simplicity is maintained, but the power spectral density distribution becomes distorted at higher spatial frequencies
Solution Approach 1:
The calibration test surface with its known pseudo-random groove pattern is prepared in advance before actual measurements. By performing the calibration measurement and calculating the MTF correction factor beforehand, the complex MTF accounting is simplified into a single correction step that can be applied to subsequent measurements without requiring continuous complex calculations during the measurement process.
3Measurement precision
If a calibration test surface with pseudo-random grooves is used, then the instrumental MTF can be determined and PSD spectra can be corrected, but the complexity of the calibration process increases
Solution Approach 1:
The patent creates a simplified copy of the calibration problem by using a test surface with a known pseudo-random groove pattern that replicates the essential features needed for MTF measurement. This controlled copy allows the complex MTF extraction to be performed on a known input, transforming the complex deconvolution problem into a straightforward comparison between the known pattern and measured output, thereby reducing the overall calibration process complexity.
Data Source
AI summary
The present invention provides for test surfaces and methods for calibration of surface profilometers, including interferometric and atomic force microscopes. Calibration is performed using a specially designed test surface, or the Binary Pseudo-random (BPR) grating (array). Utilizing the BPR grating (array) to measure the power spectral density (PSD) spectrum, the profilometer is calibrated by determining the instrumental modulation transfer function (IMTF).