Autofocus Control for Substrate Inspection via Zigzag Scanning
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Solution Overview
Problem
In substrate inspection apparatuses, the existing autofocus systems face challenges with accurate focus control due to delays in feedback control and interference from diffracted and scattered light, leading to indistinct images and longer inspection times, especially with finer patterns.
Innovation Solution
The apparatus employs a zigzag scanning motion to acquire defect detection and focus control data in parallel, using a focus control signal generated from smoothed data to minimize the impact of diffracted and scattered light, and adjusts the focus control signal for subsequent scan lines based on data from earlier scans, allowing for more accurate and efficient focus control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If feedback control is used to control the focal point position, then the focus control responds to mask deflection, but delay occurs in the servo system limiting accurate focus control
Solution Approach 1:
The patent measures the height distribution of the mask surface in advance to create a three-dimensional topographical map before inspection. This pre-measured data is then used to control the objective lens position during inspection, eliminating the need for real-time feedback control and its associated delays.
2Reliability
If focus error signal is used for feedback control, then focus control is performed, but diffracted light and scattered light appear as noise causing the focal point to deviate from the mask surface
Solution Approach 1:
The patent extracts only the necessary height information from the light signals by using a height distribution measurement method that is less sensitive to diffracted and scattered light. Instead of relying on focus error signals that are heavily contaminated by these noise components, the system measures surface height directly through a different optical approach.
3Reliability
If three dimensional topographical map is produced ahead of inspection, then accurate focus control is achieved, but twice scanning operations are necessary degrading inspection throughput
Solution Approach 1:
The patent merges the height distribution measurement and defect inspection processes into a single scanning operation. By simultaneously acquiring both height data and defect data during one pass over the mask, the system eliminates the need for a separate pre-scanning step to create a topographical map, thereby maintaining high inspection throughput while achieving accurate focus control.
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 focus control with reduced interference from diffracted and scattered light, improving inspection throughput and accuracy even with complex patterns, while maintaining inspection efficiency.
Implementation Method 1
an objective lens for focusing transmitted light through the substrate or reflected light by the substrate
Implementation Method 2
a focus detector for detecting the deviation between the focal point of the objective lens and the surface of the photomask by for example an astigmatic method
Data Source
AI summary
Substrate inspection apparatus, in which the acquisition of the inspection data for a defect and the acquisition of the focus data of the objective lens are performed in parallel, includes an autofocus apparatus for controlling position of the objective lens along its optical axis. The autofocus apparatus includes a focus error detection unit and a focus control signal generation unit for generating a focus control signal for controlling the position of the objective lens for each scan line using a focus data signal composed of an objective position signal or the objective position signal to which a focus error signal is added. When “i” is assumed as a positive integer and “m” is as a natural number, the focus data signal which was acquired during the scanning period of i-th scan line is used to produce the focus control signal used to scan the (i+2m)-th scan line.