Lock-in Image Detector for Phase Measurement in Lithography
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Solution Overview
Problem
Current lithographic inspection apparatuses are limited in their ability to obtain phase information from diffraction spectra, leading to reduced throughput and accuracy in high-volume manufacturing environments, as they often require large target structures and are sensitive to process-induced variations.
Innovation Solution
An inspection apparatus and method that utilize a lock-in image detector with a radiation source and optical system to capture both amplitude and phase information of diffracted radiation by introducing a time-varying component through optical frequency shifting, allowing for precise measurement of target properties such as asymmetry, overlay, focus, and dose.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional scatterometry is used to measure target properties, then measurement capability is provided, but phase information cannot be obtained and large target structures are required
Solution Approach 1:
The patent applies periodic modulation of the illumination beam intensity to encode phase information into time-varying intensity measurements. By modulating the beam at a known frequency and detecting the modulated diffracted light, the system can recover phase information through lock-in detection techniques, transforming unmeasurable phase data into detectable time-varying intensity signals.
Solution Approach 2:
The patent introduces a modulated illumination beam as an intermediary carrier to transfer phase information. The modulation signal acts as a mediator that embeds phase data into the intensity domain, allowing phase information to be conveyed through intensity variations that can be detected by conventional intensity-sensitive detectors without requiring direct phase measurement capability.
2Productivity
If conventional scatterometry is used, then target measurement is possible, but throughput is reduced due to requirement for large targets
Solution Approach 1:
By using periodic modulation of the illumination beam, the system can extract more information from smaller targets within the same measurement time. The modulation technique enables the detection of subtle diffraction patterns from compact targets that would be insufficient for conventional unmodulated scatterometry, thereby increasing the number of targets that can be measured per unit time.
3Measurement precision
If conventional scatterometry is used, then basic target properties can be measured, but accuracy is reduced due to sensitivity to process-induced variations
Solution Approach 1:
The periodic modulation allows the system to measure target properties at multiple phases of the modulation cycle, enabling differential measurements that cancel out common-mode noise and process-induced variations. By comparing measurements taken at different modulation phases, the system can isolate true target property signals from spurious variations caused by environmental or process instabilities.
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 more accurate and efficient measurement of target properties, reducing the need for large targets and minimizing sensitivity to process variations, thereby enhancing throughput and accuracy in high-volume manufacturing.
Implementation Method 1
a collection path for collecting diffracted radiation from said target and delivering a selected portion of the diffracted radiation to the image detector
Implementation Method 2
the intensity of radiation at the image detector includes a time-varying component having a characteristic frequency corresponding to a difference between the frequencies of the diffracted radiation and the reference radiation
Implementation Method 3
at least one of said illumination path and said reference path includes a device for shifting an optical frequency of the reference radiation
Data Source
AI summary
Inspection apparatus (100) is used for measuring parameters of targets on a substrate. Coherent radiation follows an illumination path (solid rays) for illuminating target (T). A collection path (dashed rays) collects diffracted radiation from the target and delivers it to a lock-in image detector (112). A reference beam following a reference path (dotted rays). An acousto-optical modulator (108) shifts the optical frequency of the reference beam so that the intensity of radiation at the lock-in detector includes a time-varying component having a characteristic frequency corresponding to a difference between the frequencies of the diffracted radiation and the reference radiation. The lock-in image detector records two-dimensional image information representing both amplitude and phase of the time-varying component. A second reference beam with a different shift (110) follows a second reference path (dot-dash rays). Interference between the two reference beams can be used for intensity normalization.