IR Spectral Interferometry for Back-Side Reflection Separation
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Solution Overview
Problem
Existing optical metrology techniques face challenges in accurately separating and interpreting spectral interferometric signals from the top and bottom portions of patterned structures, particularly in silicon wafers, due to contamination from back-side reflections, which affect data interpretation and are not effectively removable using conventional methods.
Innovation Solution
A novel technique that extracts the spectral amplitude and phase of the top portion of a sample by analyzing the variation of signal intensity with optical path difference, independent of interferometric signals from the bottom portion, using IR-based spectral interferometry and a control system to process raw measured data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If normal incidence mode using IR illumination is used, then hardware simplicity and simulation simplicity are improved, but contamination from back-side reflections increases
Solution Approach 1:
The patent segments the measured signal into distinct components: the desired top-surface reflection signal and the harmful back-side reflection signal. By analyzing the spectral interferometric data at different optical path differences, the method separates the signal from the sample's top portion from the parasitic signal from the bottom portion, allowing independent processing of each component.
Solution Approach 2:
The patent extracts the harmful back-side reflection contribution from the measured signal by algorithmically removing it based on the limited temporal coherence of the interferometer. This extraction allows the desired top-surface signal to be isolated and processed without contamination from the back-side reflections.
2Measurement precision
If algorithmic removal of wafer back-side reflections is used, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent performs preliminary actions by measuring the spectral interferometric signal at multiple different optical path differences before final analysis. This preliminary data collection enables subsequent algorithmic separation of the back-side reflection contribution from the top-surface signal, improving measurement precision through systematic data gathering.
Solution Approach 2:
The method uses feedback from the measured spectral interferometric signals at different optical path differences to iteratively refine the separation of back-side reflection contributions. By analyzing how the signal changes with optical path difference, the system feedback-adjusts the algorithmic removal process to achieve accurate signal separation.
3Object-affected harmful factors
If confocal optical layout is used, then out-of-focus contributions are reduced, but sensitivity to focus errors increases
Solution Approach 1:
The patent replaces the mechanical confocal optical layout with a spectral interferometric approach that uses optical path difference variation instead of mechanical focus adjustment. By measuring at multiple optical path differences and using algorithmic separation, the system achieves similar signal separation without the mechanical complexity and focus sensitivity issues of confocal systems.
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
Enables direct determination of the spectral amplitude and phase of the top portion of the sample, effectively removing contamination from back-side reflections and maintaining high spectral quality in metrology measurements.
Implementation Method 1
Spectral Interferometry is used... electromagnetic field scattered from a structure also contains spectral phase information
Implementation Method 2
Extending the wavelength range of the scatterometry measurements towards IR range provides multiple benefits, in particular the ability to penetrate materials which are opaque in the Vis-UV range
Implementation Method 3
incident light is not absorbed within the structure but rather continues to propagate inside the structure and is partially reflected back from the bottom (back side)
Data Source
AI summary
A measurement system for use in optical metrology, the measurement system which includes a control system configured and operable to carry out the following: (i) receive raw measured data generated by a measurement unit that is configured and operable for performing normal-incidence spectral interferometric measurements on a sample and generating the raw measured data indicative of spectral interferometric signals measured on the sample for a number of different optical path differences (OPDs) between sample and reference arms using infrared wavelengths; (ii) extract, from the raw measured data, a portion of spectral interferometric signals describing variation of signal intensity with a change of an optical path difference OPD during interferometric measurements, said portion of the spectral interferometric signals being independent of interferometric signals returned from a bottom portion of the sample in response to said illuminating electromagnetic field; and (iii) directly determine, from the extracted portion of the spectral interferometric signals, both spectral amplitude and phase of reflection of the illuminating electromagnetic field from the top portion of the sample, thereby enabling to determine a measured spectral signature characterizing the top portion of the sample.