Capacitive Proximity Sensor with Floating Test Surface for EUV Reticle Alignment
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Solution Overview
Problem
Inspection systems face measurement errors due to spatially-varying resistance in samples, such as reticles for EUV lithography, when using typical capacitance proximity sensors, as the measured capacitance varies with the spatial position of the electrode, leading to misalignment and reduced system performance.
Innovation Solution
A capacitive proximity measurement system that includes a sensor electrode and a conductive plate parallel to the test surface, where the test surface is electrically floating, allowing for the determination of distance based on capacitance between the electrode and the test surface, as well as between the test surface and the conductive plate, reducing the impact of spatially-varying resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a typical capacitance proximity sensor is used to measure samples with spatially-varying resistance, then the sensor can detect sample position, but measurement errors occur that vary depending on the spatial position of the electrode
Solution Approach 1:
The measurement system is segmented into multiple independent capacitance measurement circuits, each measuring capacitance at different spatial locations on the sample. By dividing the measurement into multiple localized measurements rather than relying on a single electrode measurement, the system captures the spatially-varying resistance characteristics and enables accurate proximity determination despite local resistance variations.
Solution Approach 2:
The invention transitions from a single-point capacitance measurement to a multi-point spatial measurement approach. By measuring capacitance across multiple dimensions (different x-y positions on the sample surface) and combining these measurements, the system resolves the spatially-varying resistance issue by adding a spatial dimension to the measurement process.
2Device complexity
If the test surface is electrically connected to ground, then the measurement circuit is simplified, but spatially-varying resistance causes proximity errors
Solution Approach 1:
The invention introduces an intermediary measurement approach where multiple capacitance measurements are taken at different spatial locations as intermediate steps. These intermediate measurements serve as mediators that, when combined, cancel out the effects of spatially-varying resistance. The intermediary measurements allow the system to indirectly determine true proximity despite the presence of resistance variations.
Solution Approach 2:
The system changes the measurement parameters by taking capacitance measurements at multiple different spatial positions rather than at a single point. By varying the measurement location parameter and combining the results, the system transforms the measurement approach to eliminate the influence of spatially-varying resistance on the final proximity determination.
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 alignment and position control of samples with spatially-varying resistance, reducing measurement errors and improving system performance by using a capacitive proximity measurement system with a conductive plate to mitigate the effects of varying resistance.
Implementation Method 1
a measured capacitance between the electrode and the electrically-conductive top layer is directly related to the distance between the electrode and the sample
Implementation Method 2
a plate connector that may provide an electrical connection between a system ground and a conductive plate parallel to the test surface
Data Source
AI summary
A capacitive proximity measurement system may include a sensor electrode configured to be positioned proximate to a conductive measurement area on a test surface of a sample, a plate connector configured to provide an electrical connection between a system ground and a conductive plate parallel to the test surface, and a controller. A measurement circuit may be formed between the sensor electrode and the conductive plate, where the test surface is electrically floating with respect to the sensor electrode and the conductive plate. The controller may further adjust a voltage of the sensor electrode with respect to the conductive plate, determine a capacitance associated with the measurement circuit, and determine a distance between the electrode and the measurement area based on the capacitance associated with the measurement circuit.