In semiconductor manufacturing, accurate wafer orientation is critical for downstream processes such as lithography, metrology, and advanced packaging. A wafer’s notch or flat serves as the primary reference for rotational alignment, making reliable detection a cornerstone of wafer aligner technology. This article explores the most common detection methods, industry differences, and approaches for handling damaged or contaminated wafers.

1. Why Notch and Flat Detection Matters

Every wafer must enter processing equipment with a known angular orientation. Misalignment can result in:

● Overlay errors in lithography

● Mispositioning during bonding or testing

● Data mismatch in wafer mapping and sorting

A wafer aligner’s ability to accurately detect the notch or flat ensures the wafer’s 0° reference position is consistently defined.

2. Core Detection Methods

2.1 Laser Scanning Notch Detection (Laser Edge Sensor)

Laser edge sensors are widely used for notch detection in modern wafer aligners. They operate by scanning the wafer edge with a laser and measuring reflected signals to determine the presence and position of a notch.

Advantages:

● High precision and repeatability

● Works on wafers with varying reflectivity (bare silicon, oxide, or patterned wafers)

● Can detect small notches or subtle edge variations

Considerations:

● Sensitive to wafer edge damage or debris

● Requires careful calibration to maintain ±0.03° angular accuracy for high-end lithography applications

2.2 Vision-Based Notch Detection (Camera + Algorithm)

Vision systems combine cameras with image-processing algorithms to detect wafer notches. A line-scan or area-scan camera captures the wafer edge, and software analyzes contour changes to locate the reference feature.

Advantages:

● Flexible for different wafer diameters and notch shapes

● Can simultaneously detect edge defects, dirt, or other anomalies

● Easier to integrate into EFEMs or wafer sorter systems

Considerations:

● Processing speed depends on camera resolution and algorithm efficiency

● Lighting conditions must be tightly controlled to avoid false detections

3. Industry Differences: Flat vs. Notch

Different wafer diameters and historical standards have led to variations in reference features:

Modern fabs predominantly use notch detection for 300 mm wafers, while smaller wafers may still require flat detection. Aligners must be adaptable to accommodate multiple wafer sizes.

4. Handling Notch Defects and Edge Contamination

Notch or edge imperfections can interfere with detection and alignment. Common issues include:

● Notch Missing or Damaged: Laser or vision systems may fail to detect the feature. Mitigation includes pattern recognition algorithms that predict notch location based on wafer geometry.

● Contamination (Particles, Residue): Cameras or lasers can misread the edge. Cleaning wafers prior to alignment and applying multi-sensor verification improves accuracy.

● Edge Chips or Cracks: Some aligners integrate tolerance checks and can reject or flag wafers that fail edge integrity tests, preventing misalignment downstream.

Effective detection systems combine hardware robustness (sensors, cameras, lasers) with software intelligence (image processing, defect prediction) to maintain high throughput without sacrificing precision.

Conclusion

Notch and flat detection is a core capability of wafer aligners, directly influencing the accuracy and yield of semiconductor processes. Whether using laser scanning or vision-based methods, aligners must adapt to wafer size, reference type, and potential edge defects. Advanced systems not only detect the reference feature but also identify contamination or damage, ensuring wafers enter the fab with a reliable rotational orientation.

As wafer geometries shrink and process tolerances tighten, robust and flexible notch/flat detection remains a key differentiator in wafer handling and automation solutions.

Fortrend’s wafer aligner solutions combine precise laser and vision-based notch/flat detection with intelligent software to handle edge defects and contamination, ensuring wafers enter every process with reliable orientation. For fabs seeking higher yield and seamless integration with EFEMs, sorters, and lithography tools, Fortrend provides proven alignment technology and application-specific support—contact us to learn more.