Wafer Pre-Aligner Notch Window Control for Faster Alignment
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Solution Overview
Problem
Existing pre-aligners for semiconductor wafers are slow in performing pre-alignment due to the need for slow scanning velocities to detect notches, and they often require 360 degrees of rotation to center the wafer, which reduces efficiency.
Innovation Solution
A pre-aligner system that uses a rotation unit, detector, memory, and controller to rapidly align wafers by predicting the notch location based on stored data, allowing for maximum acceleration and deceleration to a notch window, then scanning at a lower velocity within that window until the notch is detected, and finally using maximum acceleration to reach the desired alignment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pre-aligner scans the wafer at maximum velocity to reduce alignment time, then productivity is improved, but measurement precision deteriorates because the detector cannot accurately detect the notch at high speeds
Solution Approach 1:
The rotation path is divided into two distinct segments: a first rotation path at maximum velocity for covering the majority of the angular range, and a second rotation path at scanning velocity for precise notch detection. This segmentation allows the system to achieve both high productivity during the majority of the rotation and high measurement precision during the critical detection phase.
Solution Approach 2:
The system dynamically adjusts the rotation velocity based on the detected position. The controller switches between maximum velocity and scanning velocity modes, creating a dynamic velocity profile that optimizes both speed and accuracy. The wafer is rotated at maximum velocity until approaching the predicted notch location, then transitions to scanning velocity for precise detection.
2Measurement precision
If the pre-aligner performs a full 360 degree rotation to locate the wafer center, then measurement precision is improved, but loss of time increases significantly
Solution Approach 1:
The system performs preliminary actions by rotating the wafer to a predicted notch location based on stored data from previous wafers. This preliminary positioning allows the detector to start scanning from a location close to the actual notch, eliminating the need for a full 360 degree rotation and significantly reducing the time required to locate the wafer center while maintaining accuracy.
Solution Approach 2:
The system uses feedback from stored notch location data to predict and guide the initial rotation position. The controller uses this feedback information to determine the optimal starting position and rotation direction, allowing the wafer to be centered much faster than a full 360 degree rotation would require.
3Productivity
If the pre-aligner rotates the wafer at maximum acceleration to reduce alignment time, then productivity is improved, but the wafer may slip on the chuck due to excessive centrifugal force
Solution Approach 1:
The rotation process is segmented into different velocity phases. Maximum acceleration is applied only during the first rotation path at maximum velocity, while the second rotation path uses reduced scanning velocity. This segmentation ensures that high centrifugal forces are applied only when necessary and for limited durations, maintaining wafer stability while achieving high overall alignment speed.
Solution Approach 2:
The system dynamically controls acceleration and velocity profiles during rotation. The controller adjusts the rotation speed and acceleration based on the detected position, applying maximum acceleration only when safe and necessary, then transitioning to lower speeds for precise positioning. This dynamic control maintains both productivity and reliability.
Data Source
AI summary
A pre-aligner for pre-aligning a wafer having a notch. The pre-aligner includes a wafer platform having a wafer receiving surface, and a drive device. A detector is provided to detect the notch, and a memory is provided to store a notch window defining a range of angles in which the notch is predicted to be located in relation to a start position. A controller performs a pre-alignment operation where the wafer is rotated from the start position to an alignment location. The controller performs the operation such that the wafer is rotated at maximum acceleration/deceleration values from the start position to a notch location detected by the detector: where the operation is limited to a maximum velocity for rotation of the wafer from the start position to a notch window; and where the operation is limited to a scanning velocity within the notch window until the notch location is detected.