Application Background
During semiconductor manufacturing, wafers may develop warpage, bending deformation, or edge height differences after multiple processes. These deformations directly affect subsequent lithography, etching, and packaging quality. Traditional contact measurement methods risk damaging the wafer and are inefficient. The ST-P series laser displacement sensors use laser triangulation for non-contact measurement, enabling real-time acquisition of wafer surface displacement data to evaluate warpage and flatness. They are suitable for pre- and post-handling, in-process, and final inspection stages.
Product Principle and Selection Guide
The ST-P series laser displacement sensors operate on the laser triangulation principle: a laser beam is projected onto the target surface, and the reflected light is imaged onto a CMOS/PSD array via a lens. The displacement is calculated from the position change of the light spot. The series offers multiple models to meet different measurement distances and accuracy requirements:
- ST-P25: Reference distance 25 mm, measurement range ±1 mm, repeatability 0.05 μm, linearity error < ±0.6 μm. Ideal for ultra-precision wafer warpage measurement.
- ST-P30: Reference distance 30 mm, measurement range ±5 mm, repeatability 0.15 μm, linearity error < ±3 μm. Suitable for general wafer height difference measurement.
- ST-P50: Reference distance 50 mm, measurement range ±10 mm, repeatability 0.25 μm, linearity error < ±4 μm.
- ST-P80: Reference distance 80 mm, measurement range ±15 mm, repeatability 0.5 μm, linearity error < ±6 μm.
- ST-P150: Reference distance 150 mm, measurement range ±40 mm, repeatability 1.2 μm, linearity error < ±16 μm.
When selecting a model, consider installation space, required accuracy, and wafer surface characteristics (e.g., reflectivity, transparency, color). For mirror-like wafers, use a diffuse reflection type or adjust the mounting angle to avoid specular reflection interference. The maximum sampling frequency reaches 160 kHz, supporting high-speed inline inspection.
Solution Recommendations
A typical measurement setup involves installing multiple ST-P series sensors on the wafer chuck or along the handling path to form an array that measures heights at multiple points on the wafer surface. The warpage (e.g., maximum height difference) is calculated by comparing displacement data from different points. Sensor outputs include Ethernet, RS485, analog, and IO signals, which can be connected to PLCs, host computers, or motion control platforms. For high-speed production lines, Ethernet output is recommended for real-time data transmission.
Installation and Commissioning Tips
- Ensure the laser beam is perpendicular or at a small incident angle to the wafer surface to avoid spot distortion due to excessive tilt.
- Set the installation distance strictly according to the model's reference distance; deviation may affect linearity.
- For transparent wafers or those with thin films, perform sample testing to verify measurement stability.
- When using multiple sensors synchronously, consider trigger synchronization or software timestamp alignment.
Frequently Asked Questions
Q: Does wafer surface reflectivity affect measurement?
A: Specular reflection may cause excessive light intensity or signal loss. It is recommended to use a diffuse reflection sensor or adjust the mounting angle. In some cases, surface pretreatment (e.g., powder coating) may be applied, but care must be taken to avoid contamination. Always test with actual samples before deployment.
Q: Can the measurement accuracy meet wafer warpage requirements?
A: The ST-P25 offers repeatability of 0.05 μm and linearity error < ±0.6 μm, which meets most wafer warpage inspection needs. However, the actual accuracy depends on the specific model, installation conditions, and target surface characteristics. Confirm with sample testing.
Conclusion
The ST-P series laser displacement sensors provide a non-contact, high-precision, and high-speed solution for wafer warpage and deformation measurement. Selection should consider measurement distance, accuracy, and surface properties, and be validated through sample testing. With multiple output options, the series can be easily integrated into semiconductor automation lines for inline quality control.