Spectral Interferometric Phase Retrieval Without Path Modulation
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Solution Overview
Problem
Existing optical critical dimension (OCD) metrology techniques face challenges in achieving high signal-to-noise ratio and stability due to the need for sequential measurements and high-precision machinery, which are prone to vibrations and thermal fluctuations, especially when measuring large semiconductor wafers.
Innovation Solution
A spectral interferometric system using cross-polarization and a single exposure at arbitrary z-position, combined with algorithmic methods to account for system-related effects, allows for robust spectral phase retrieval without requiring movement of optical elements, and compensates for noise and instability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sequential measurements with path length modulation are used, then phase information can be retrieved, but measurement stability deteriorates due to system drift and vibrations during the measurement sequence
Solution Approach 1:
The patent applies periodic action by using a periodically modulated reference beam that oscillates at a known frequency. This periodic modulation encodes the phase information into the time-varying interference signal, allowing phase retrieval through frequency domain analysis rather than sequential measurements. The periodic reference signal acts as a carrier that carries the phase information through the measurement process, making it immune to drift and vibrations that occur between sequential measurements.
Solution Approach 2:
The patent introduces a reference beam as an intermediary element that carries the phase information. By modulating the reference beam with a known periodic signal and interfering it with the sample beam, the phase information is transferred to the interference signal in the frequency domain. This intermediary reference signal serves as a stable carrier that protects the measurement from environmental disturbances.
2Measurement precision
If high-precision machinery is used for path length control, then measurement accuracy improves, but sensitivity to vibrations and thermal fluctuations increases
Solution Approach 1:
The patent replaces the mechanical path length modulation system with an optical modulation approach. Instead of physically moving mirrors or changing optical path lengths mechanically, the invention modulates the reference beam's phase or amplitude electronically at a fixed optical path configuration. This substitution eliminates the need for high-precision mechanical components and their inherent sensitivity to vibrations and thermal expansion.
Solution Approach 2:
The patent introduces dynamic modulation of the reference beam at a fixed optical configuration. By modulating the reference beam's properties (phase or amplitude) at a known frequency while keeping the optical paths static, the system achieves phase measurement capability without requiring mechanical movement. This dynamic optical modulation approach makes the system immune to mechanical vibrations and thermal drifts that affect stationary components.
3Measurement precision
If multiple sequential measurements are performed, then complete phase information is obtained, but measurement time increases and throughput decreases
Solution Approach 1:
The patent achieves continuous phase measurement by using a continuously modulated reference beam that provides phase information in real-time through frequency domain analysis. The periodic modulation allows the system to extract complete phase information from a continuous interference signal rather than requiring discrete sequential measurements. This continuous measurement approach maximizes throughput while maintaining complete phase information retrieval.
Solution Approach 2:
The patent uses excessive action by applying strong periodic modulation to the reference beam, creating a dominant carrier signal in the frequency domain. This excessive modulation ensures that the phase information is clearly encoded and easily extractable, allowing for rapid single-shot measurements without requiring multiple sequential acquisitions. The strong periodic signal provides robust phase information in a single measurement window.
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 enables accurate and stable spectral phase measurement with improved sensitivity, reducing the impact of transient instabilities and enhancing measurement precision in semiconductor metrology.
Implementation Method 1
Most interferometers consist of a split optical path that is recombined to form interference fringes. The interference signal from these two components is then used to extract the spectral phase.
Implementation Method 2
In some embodiments, such spectral interferometric system utilizes a cross-polarization based measurements, by using polarization filtering in the illumination and detection channels.
Data Source
AI summary
A measurement system for use in measuring parameters of a patterned sample, the system including a broadband light source, an optical system configured as an interferometric system, a detection unit, and a control unit, where the interferometric system defines illumination and detection channels having a sample arm and a reference arm having a reference reflector, and is configured for inducing an optical path difference between the sample and reference arms, the detection unit for detecting a combined light beam formed by a light beam reflected from the reflector and a light beam propagating from a sample's support, and generating measured data indicative of spectral interference pattern formed by spectral interference signatures, and the control unit for receiving the measured data and applying a model-based processing to the spectral interference pattern for determining one or more parameters of the pattern in the sample.