Variable Fringe Interferometry for Transparent Wafer Defect Detection
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Solution Overview
Problem
Current inspection techniques are inadequate for detecting defects on partially or fully transparent wafers, as they cannot distinguish between surface and internal defects, and existing systems like the Candela™ are difficult to focus, leading to unreliable results.
Innovation Solution
A method and system using a rotating wafer with an interferometric device emitting quasi-collimated light beams to create a measurement volume with variable inter-fringe distances, allowing for the detection of defects through Doppler frequency analysis, which determines the defect's position in the radial direction and thickness of the wafer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If confocal detection is used to inspect transparent wafers, then measurement sensitivity can be improved, but the system becomes difficult to focus and positioning accuracy requirements increase
Solution Approach 1:
The patent replaces the mechanical confocal detection system with a laser Doppler velocimetry-based interferometric system. Instead of using confocal microscopy mechanics that require precise focusing and positioning, the invention uses optical interference fringes created by two intersecting laser beams to detect defects through Doppler frequency shifts, eliminating the need for complex focusing mechanisms and high positioning accuracy.
Solution Approach 2:
The patent changes the detection parameter from confocal intensity measurement to Doppler frequency measurement. By measuring the frequency shift of scattered light caused by defect motion through interference fringes, the system achieves high sensitivity without requiring precise mechanical focusing, as frequency measurement is less sensitive to positioning errors than intensity-based confocal detection.
2Reliability
If traditional inspection techniques are used on transparent wafers, then surface defects can be detected, but it is impossible to determine whether defects are on the surface or inside the wafer
Solution Approach 1:
The patent introduces a spatial dimension through variable inter-fringe distance across the measurement volume. By creating interference fringes with spacing that varies with depth (non-parallel fringes), the system encodes the z-position (depth) information into the Doppler frequency measurement, allowing differentiation between surface and internal defects while maintaining reliable defect detection.
3Measurement precision
If variable inter-fringe distance is used in the measurement volume, then defect position determination accuracy is improved, but the interferometric device complexity increases
Solution Approach 1:
The patent uses curved (spheroidal) wavefronts instead of planar wavefronts in the interferometric setup. By using spherical or cylindrical wavefronts from point sources or line sources, the interference fringes naturally have variable spacing that provides depth encoding, achieving position determination accuracy without requiring complex adjustable optical elements.
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 enhances sensitivity and repeatability by accurately locating defects on transparent wafers, providing higher resolution than traditional methods and avoiding the focusing challenges of confocal techniques.
Implementation Method 1
an interferometric device 30 coupled with the light source and arranged facing the surface S of the wafer 2 for inspection... comprising two branches for dividing the beam originating from the light source into two incident beams. At the output of the optical waveguide, the two branches are oriented in relation to one another so as to form, at the intersection between the two beams, a measurement volume comprising a plurality of parallel fringes.
Implementation Method 2
The presence of a defect on the surface of the wafer is indicated, when this defect crosses the interference fringes, by the scattering of a Doppler burst measured by the detection module. A Doppler burst is a signal that has a double frequency component: a low-frequency component, forming the envelope of the signal, corresponding to the mean light intensity scattered by the defect, and a high-frequency component, corresponding to the Doppler frequency containing the information on the velocity of the defect.
Implementation Method 3
emitting, from a light source coupled with an interferometric device, two quasi-collimated incident light beams, so as to form, at the intersection between the two beams, a measurement volume containing interference fringes extending transversally to the path of rotation of the wafer and having a variable inter-fringe distance within the measurement volume
Implementation Method 4
capturing the collected light and emitting an electrical signal representing the variation in the light intensity of the collected light as a function of time
Data Source
AI summary
An electronic wafer inspecting method includes:rotating the wavelength transparent wafer,emitting, from a light source coupled with an interferometric device, two light beams, to form, a measurement volume and having a variable inter-fringe distance within the volume, a time signature of a defect intersecting the measurement volume depending on an inter-fringe distance where the defect intersects the volume,the device and the wafer arranged so that the measurement volume extends into a wafer region,collecting the light scattered by the wafer region,emitting a signal representing the variation in the intensity of the collected light per time,detecting in the signal, a frequency of the intensity, the frequency being the time of the passage of a defect through the measurement volume,determining, based on the value of the inter-fringe distance at the location where the defect passes, the position of the defect.