Knife-Edge Mirror Height Sensor for Semiconductor Metrology
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Solution Overview
Problem
Current 3D inspection and metrology techniques for semiconductor wafers, such as triangulation, geometric shadow, confocal microscopy, and white-light interferometry, fail to provide the necessary accuracy, precision, and throughput required for contemporary semiconductor manufacturing, especially for structures below 10 μm in height.
Innovation Solution
A system utilizing a knife-edge mirror with a reflective and anti-reflective film configuration, positioned at the focal point of the light reflected from the wafer, to differentiate between under-focused and over-focused light, coupled with sensors like photodiodes or bi-cell photodiodes to determine the focus status and calculate the height of the wafer surface, enhancing throughput and accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If white-light interferometry is used for high-resolution 3D inspection, then measurement precision is improved, but productivity deteriorates due to slow throughput
Solution Approach 1:
The patent replaces the mechanical scanning system of traditional white-light interferometers with a purely optical system. A knife-edge mirror is positioned at the focal point of the objective lens, and a single photodetector detects light intensity variations as the wafer surface moves in and out of focus. This substitution of mechanical scanning with optical detection achieves high-speed measurement (orders of magnitude faster than chopper techniques) while maintaining height measurement precision, thereby resolving the contradiction between measurement precision and productivity
Solution Approach 2:
The patent changes the measurement parameter from detecting interference patterns (which require scanning) to detecting light intensity variations at a fixed focal point. By monitoring how light intensity changes as the wafer surface moves through the focal point, the system determines height information without mechanical scanning. This parameter change enables simultaneous achievement of high precision and high throughput
2Measurement precision
If chopper technique with bi-cell photodetector is used for auto-focus control, then measurement precision is improved to better than 20 nm, but productivity deteriorates due to slow throughput
Solution Approach 1:
The patent replaces the mechanical chopper and complex bi-cell photodetector system with a simpler optical arrangement using a knife-edge mirror at the focal point and a single photodetector. The knife-edge mirror creates a sharp cutoff that converts focus position into light intensity variations detectable by the photodetector. This mechanical-to-optical substitution eliminates the throughput limitations of chopper techniques while maintaining sub-20 nm focus control precision through the sharp edge of the knife mirror
Solution Approach 2:
The patent extracts the essential function of focus detection from the complex chopper-bi-cell system and implements it through a minimal optical arrangement. By taking out only the critical element (the knife-edge at the focal point) and using a simple photodetector to measure light intensity, the system achieves the same precision as the chopper technique but at much higher speeds, resolving the productivity penalty
3Productivity
If triangulation or geometric shadow techniques are used for 3D inspection, then productivity is improved, but measurement precision deteriorates for structures below 10 μm in height
Solution Approach 1:
The patent operates at the diffraction limit (the fundamental optical resolution boundary) rather than using geometric optics approximations. By utilizing wave optics effects and positioning the knife-edge exactly at the focal point of the objective lens, the system achieves precision for sub-10 μm structures while maintaining high throughput. This dimensional transition from geometric to wave optics enables simultaneous high precision and high productivity
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 solution significantly improves the throughput, accuracy, and cost-effectiveness of 3D inspection and metrology, capable of determining wafer surface heights and detecting defects with increased precision compared to existing methods, particularly for structures below 10 μm, and can increase throughput by orders of magnitude compared to chopper techniques.
Implementation Method 1
a knife-edge mirror configured to receive light reflected from the wafer. The knife-edge mirror includes a reflective film and an anti-reflection film that are both disposed on the knife-edge mirror thereby forming a boundary between the reflective film and the anti-reflection film
Implementation Method 2
The knife-edge mirror is positioned at a focal point of the light reflected from the wafer such that the reflective film is configured to block at least some of the light reflected from the wafer
Implementation Method 3
When the light is on-focus there is zero phase shift between the reference signal from the chopper and the signals from the two channels of the bi-cell photodetector. If the light is under-focus or over-focus, then the phase of a cell is shifted negative or positive, respectively, to a reference signal
Data Source
AI summary
This semiconductor inspection and metrology system includes a knife-edge mirror configured to receive light reflected from a wafer. The knife-edge mirror is positioned at a focal point of the light reflected from the wafer such that the reflective film on the knife-edge mirror is configured to block at least some of the light reflected from the wafer. The portion of blocked light changes when the light reflected from the wafer is under-focused or over-focused. At least one sensor receives the light reflected from the wafer. Whether the light is under-focused or over-focused can be determined using a reading from the at least one sensor. A height of an illuminated region on the surface of the wafer can be determined using such a reading from the at least one sensor.