Application Background
In semiconductor manufacturing, wafers are typically secured to a stage using a vacuum chuck. The flatness of the chuck surface, the status of the vacuum suction, and the height difference between the wafer and the chuck directly impact processing precision and yield. Traditional contact-based measurement methods risk damaging the wafer and cannot monitor the dynamic suction process in real-time. The ST-P series laser displacement sensors utilize non-contact laser triangulation to enable in-line inspection of wafer chuck height, wafer warpage, flatness, and suction status; they are suitable for wafer inspection equipment, packaging equipment, and automated semiconductor production lines.
Product Principle and Model Selection
The ST-P series laser displacement sensors operate on the principle of laser triangulation: a laser beam is projected onto the target surface, and the reflected light is focused by a lens onto a CMOS or PSD photosensitive element. Displacement is calculated based on changes in the position of the light spot. The series offers various models, allowing for selection based on specific installation distance and precision requirements:
ST-P25: Reference distance 25 mm, measurement range ±1 mm, repeatability 0.05 μm, linearity error <±0.6 μm; suitable for high-precision, small-range wafer edge thickness measurement.
ST-P30: Reference distance 30 mm, measurement range ±5 mm, repeatability 0.15 μm, linearity error <±3 μm; suitable for standard wafer thickness and warpage inspection.
ST-P50: Reference distance 50 mm, measurement range ±10 mm, repeatability 0.25 μm, linearity error <±4 μm; balances precision and measurement range.
ST-P80: Reference distance 80 mm, measurement range ±15 mm, repeatability 0.5 μm, linearity error <±6 μm; suitable for scenarios involving significant warpage or limited installation space.
ST-P150: Reference distance 150 mm, measurement range ±40 mm, repeatability 1.2 μm, linearity error <±16 μm; suitable for long-distance or large-range inspection.
For wafer chuck height inspection, the ST-P25 or ST-P30 models are generally recommended due to their short reference distances and high precision, making them ideal for compact installation spaces. For detecting warpage over a larger area, the ST-P50 or ST-P80 models are suitable choices. The specific model selection should be determined based on the installation distance, the reflective properties of the target surface, and cycle time requirements.
Solution Recommendations
For wafer chuck height inspection, sensors can be mounted above or to the side of the chuck, aligned vertically with the measurement points. A multi-point measurement approach is recommended—such as positioning sensors at the chuck's edges and center to collect height data synchronously—with a host computer calculating flatness and warpage. The sensors support Ethernet, RS485, analog, and I/O outputs, allowing integration with PLCs or motion control platforms for real-time feedback and closed-loop control. For high-speed production lines, models with sampling frequencies up to 160 kHz are available to meet rapid inspection needs.
Installation and Commissioning Recommendations
Ensure the sensor's optical axis is perpendicular to the target surface during installation to avoid measurement errors caused by tilt. Wafer surfaces are often mirror-like or semi-transparent, which can cause multiple reflections or transmission, potentially affecting measurement stability. Testing with sample parts prior to full-scale deployment is recommended to verify sensor compatibility with different materials (e.g., silicon, glass, ceramics). For transparent wafers, adjusting the sensor angle or selecting a specific laser wavelength may be necessary. During commissioning, configure appropriate sampling frequencies and filtering parameters to balance response speed with noise suppression.
FAQ
Q: Can the sensor detect transparent wafers? A: Transparent materials may cause transmission or secondary reflections; testing based on the specific model and surface condition is required. Using a short-wavelength laser or an angled mounting configuration is recommended.
Q: How is height variation captured during the vacuum suction process? A: High-speed sampling (up to 160 kHz) allows the sensor to track height changes in real-time, with data recorded synchronously via I/O triggers or analog outputs.
Q: How is synchronized measurement with multiple sensors achieved? A: Synchronization can be achieved via Ethernet or RS485 bus triggering, or by using an external hardware trigger signal.
Summary
The ST-P series laser displacement sensors provide a non-contact, high-precision solution for measuring wafer height during vacuum chucking. Proper model selection and installation enable effective monitoring of wafer flatness, warpage, and suction status, ultimately improving semiconductor processing yields. In practical applications, sample testing should be conducted based on the characteristics of the surface being measured, and due attention should be paid to the selection of the installation angle and signal output method. The information in this document is for technical reference only; specific parameters and performance characteristics must be verified against the actual model.