Fundamentals of Optical Sensors and Non-Contact Precision Measurement in Semiconductor Manufacturing

In semiconductor manufacturing, it is essential to achieve both ultra-fine patterning and high-volume production. Precision measurement technologies are indispensable for this purpose. Among them, non-contact measurement methods utilizing optical sensors offer significant advantages, enabling accurate information acquisition without damaging the wafer.

High-precision measurement is required in various processes, such as wafer alignment and shape inspection. Therefore, minimizing measurement errors and risks during handling is critical. Non-contact measurement also provides major benefits for ensuring the stable supply of a larger number of products within limited timeframes.

This article organizes the fundamentals of non-contact measurement technologies using optical sensors and presents the latest applications. It also discusses practical methods for on-site utilization and key points for improving quality, serving as a valuable reference for implementing and operating these technologies in semiconductor manufacturing.

Overview of Semiconductor Manufacturing Equipment and Applications

Semiconductor manufacturing involves multiple pieces of equipment and processes working in coordination to perform precision processing and measurement at an extremely fine scale.

The manufacturing process includes various steps such as cleaning, photolithography, etching, and film deposition, each of which is highly automated. However, maintaining high accuracy requires measurement systems capable of detecting even minor misalignments or surface anomalies. In particular, non-contact precision measurement, including optical sensors, is an effective means of accurately forming micro-patterns on wafers.

Semiconductor manufacturing equipment is constantly expected to improve not only precision but also throughput. By integrating advanced measurement systems, wafer positions and shapes can be accurately determined in a short time without human intervention. This reduces unnecessary downtime, enhances yield, and ultimately contributes to product quality and productivity.

Semiconductor Manufacturing Processes and Measurement Technologies

In the exposure process, micro-patterns are transferred onto wafers using projection lenses. Even slight focal deviations can significantly impact yield. Therefore, non-contact measurement technologies using optical sensors are indispensable for accurately aligning focus and overlaying patterns. The sensors used in each process vary, including capacitive and confocal types, selected according to the target object and required accuracy.

Particularly in fine-processing steps following lithography, advanced verification is required, such as defect inspection and pattern shape assessment. Measurement methods based on reflection principles, such as laser autocollimators (tilt sensors), and interferometric techniques can easily visualize minute surface defects and height differences on wafers, making them well-suited for precision measurement. Advances in these technologies support the overall stability and reliability of semiconductor manufacturing.

Automation of Wafer Handling and Associated Challenges

With advances in automated robotic handling, opportunities for humans to physically touch wafers have drastically decreased, leading to improvements in productivity and cleanliness. However, automation does not completely eliminate the risk of cracks or warping, especially since minor impacts can accumulate during high-speed transport. By utilizing non-contact measurement, the wafer’s condition can be monitored in real time on the production line, enabling early detection of defects and facilitating rapid preventive maintenance.

Wafer Shape Measurement Solutions

The flatness and curvature of wafers directly affect device yield, making high-precision shape measurement essential.

Even slight deviations in flatness or warp can influence pattern printing or exposure accuracy in subsequent processes. Optical approaches based on reflection (tilt sensors) or interferometry are highly effective for detecting such variations. By accurately quantifying even small surface irregularities, the root causes of defects can be identified in advance, allowing for timely corrective actions.

Recently, the use of large-diameter wafers has become more common, driving demand not only for high precision but also for tools capable of measuring a wide area simultaneously. Interferometry-based systems and reflection-based optical systems increasingly offer multi-point acquisition, enabling faster and more detailed surface inspections.

Evaluation of Wafer Flatness and Warp

Laser reflection methods and interferometry are widely used to evaluate wafer flatness. Interferometric measurement can determine height variations down to the picometer scale by analyzing changes in interference fringes. Such high-precision measurements allow early detection of slight bending or warping in substrates, reducing the occurrence of defects in the production line.

3D Shape Detection and the Use of White-Light Interferometry

Three-dimensional shape detection using white-light interferometry enables high-speed, high-resolution topography measurements across the entire wafer, making it an effective tool for yield improvement. By utilizing white light, high-contrast measurements can be obtained, allowing detailed evaluation of complex surface features and patterns on the substrate. Applications are expanding across various materials and shapes, and the technology is beginning to be leveraged in precision machining fields beyond semiconductor manufacturing.

Wafer Alignment in the Lithography Process

The alignment accuracy in optical lithography directly affects semiconductor performance and yield.

Lithography is a critical process for transferring circuit patterns onto wafers, and the precision of wafer alignment performed prior to exposure significantly impacts the final quality of the chips. Non-contact optical sensor-based alignment reduces the risk of wafer damage since there is no physical contact, and it also minimizes the chance of pattern distortion.

Recently, specialized equipment capable of measuring wafer alignment rapidly and in batch has been introduced. For example, optical sensors can read multiple alignment marks simultaneously and make fine corrections relative to the mask, enabling practical high-precision alignment.

Beam Aperture Control and Precision Position Tracking

In the exposure process, precise control of the projection beam aperture is essential for maintaining the accuracy of fine patterns. During wafer alignment, it is crucial to measure the position of the illuminated area relative to alignment marks accurately and perform real-time corrections. Sub-micron tracking precision of optical sensors is a key factor supporting this entire process.

Mask Positioning and Non-Contact Measurement Technologies

Managing the position and tilt of the mask being transferred onto the wafer is also essential. Non-contact technologies such as laser interferometers, laser autocollimators (tilt sensors), and confocal sensors can detect minute mask displacements or rotations at the nanometer scale. Unlike contact-based measurement methods, these non-contact approaches allow real-time correction, minimizing the impact of small misalignments on lithography accuracy.

Measurement of Transparent Layers and Adhesive Beads

In semiconductor manufacturing, multiple transparent layers and adhesives are often stacked on wafers, making it challenging to evaluate the thickness and uniformity of each layer. Detecting interference and reflection characteristics using optical sensors is effective for accurately identifying the interfaces between multiple layers. This enables faster process optimization and root-cause analysis of defects, facilitating the mass production of high-quality devices.

Practical Examples of Precision Measurement Using Optical Sensors

Through specific applications of non-contact measurement, the advantages and operational considerations of these techniques can be highlighted.

Optical sensors are particularly effective at accurately capturing features that are difficult to observe visually, such as silicon wafer thickness and surface defects. By employing reflection or interferometric measurements, minute surface irregularities can be detected early, significantly contributing to process-specific quality control and yield improvement. Unlike conventional contact-based measurement methods, non-contact techniques greatly reduce the risk of damage and help stabilize production lines.

Additionally, systems capable of high-frequency measurements, such as laser-based or confocal chromatic methods, are well-suited for real-time online inspection. Efforts to enhance operational efficiency, including measurement of large wafers and simultaneous multi-point acquisition, are also progressing, directly supporting the advancement of semiconductor manufacturing.

Measurement of Transparent Layers and Adhesive Beads

In semiconductor manufacturing, multiple transparent layers and adhesives are often stacked on wafers, making it challenging to evaluate the thickness and uniformity of each layer. Detection of interference and reflection characteristics using optical sensors is effective for accurately identifying the interfaces between multiple layers. This enables faster process optimization and root-cause analysis of defects, supporting the mass production of high-quality devices.

Practical Examples of Precision Measurement Using Optical Sensors

In next-generation semiconductor production, the adoption of larger wafer sizes and the handling of irregularly shaped workpieces are being considered. While conventional contact-based measurements often required cutting or machining, optical sensors enable full-surface inspection without any cutting. This non-destructive approach allows for reduced processing steps while preventing damage to the product itself, earning high regard as an efficient and reliable inspection method.

Compliance with Measurement Standards and Specifications

When evaluating surface roughness and shape parameters, it is important to adhere to the standards set by international and domestic organizations. Standards such as ISO 25178-2 and JIS B 0681-2 unify measurement conditions and procedures, serving as benchmarks for sharing globally reliable data. When using optical sensors, selecting systems that comply with these standards facilitates certification acquisition and smooth quality management.

Summary

Precise and stable measurement technologies will become increasingly important as the foundation supporting the productivity and quality of advanced semiconductor devices.

Non-contact measurement using optical sensors in semiconductor manufacturing provides significant advantages across various processes, including wafer alignment and shape inspection. As devices continue to advance, the applications of these technologies are expanding, contributing to both faster inspection and improved accuracy in environments that demand strict standards and yield improvement.

Looking ahead, the adoption of larger-diameter wafers and the introduction of new materials will further complicate measurement targets and challenges. Even in such conditions, non-contact measurement technologies are expected to play a key role in improving yield and reducing risks, continuing to support cutting-edge semiconductor manufacturing processes.