Diffractive optics calibrator: measurement of etching variations for binary computer-generated holograms

Area of Science:

• Optics and Photonics
• Metrology and Measurement Science
• Nanofabrication and Lithography

Background:

• Computer-generated holograms (CGHs) are critical for advanced optical systems.
• Precise control over CGH fabrication, particularly etching parameters, is essential for achieving desired optical performance.
• Existing metrology methods may lack the sensitivity or specificity required for detailed etching variation analysis.

Purpose of the Study:

• To introduce a novel device, the Diffractive Optics Calibrator (DOC), for quantitative measurement of etching variations in CGHs.
• To establish a method for obtaining local etching parameters (duty cycle, depth, period) from diffraction patterns.
• To demonstrate the DOC's capability in identifying parameter variations causing sub-nanometer phase errors and its application in quality control and calibration.

Main Methods:

• Utilizing a Diffractive Optics Calibrator (DOC) to capture far-field diffraction patterns from CGHs.
• Fitting the captured intensity distribution to a parametric model to extract local etching parameters.
• Analyzing the sensitivity of each parameter to fabrication variations.
• Validating system performance using a phase shift Fizeau interferometer.

Main Results:

• The DOC can measure etching parameter variations causing 1 nm peak-to-valley phase errors in CGH-generated wavefronts.
• Local etching parameters like duty cycle, etching depth, and grating period are accurately determined.
• System performance is verified, demonstrating the device's reliability for quality control.
• The DOC can generate induced phase error maps for calibrating measurements of free-form aspheric surfaces to 1 nm RMS accuracy.

Conclusions:

• The Diffractive Optics Calibrator (DOC) is an effective tool for quality control of CGHs, enabling precise measurement of etching variations.
• The device provides valuable data for evaluating fabrication performance and guiding future CGH design.
• DOC facilitates essential calibration for high-accuracy metrology of complex optical surfaces.