TDI line scan, or Time Delay Integration line scan, is an advanced imaging technology that combines the benefits of line scan sensors with the signal amplification of multiple exposure stages. Unlike conventional line scan cameras that capture a single line per trigger, TDI line scan integrates multiple exposures of the same moving object as it passes the sensor. This process significantly enhances the signal-to-noise ratio (SNR) without sacrificing speed, making it ideal for low-light, high-speed, and high-resolution industrial inspection applications such as wafer inspection, flat panel display testing, and document scanning.

1、TDI line scan camera working principle

The working principle of a TDI line scan camera is rooted in the concept of time delay integration, where multiple linear sensor arrays are arranged in parallel, each acting as an individual exposure stage. As the object moves continuously past the sensor, the photogenerated charge from each pixel is transferred to the next stage at a rate synchronized with the object's velocity. This process accumulates the signal from the same point on the object across dozens or even hundreds of stages, effectively multiplying the exposure time without requiring the object to stop. For example, a 256-stage TDI sensor can increase the effective integration time by a factor of 256 compared to a single-line sensor, dramatically boosting the SNR. The charge packets are then read out at the end of the TDI chain, producing a high-quality, low-noise line image. This mechanism requires precise timing synchronization between the sensor readout and the motion of the object, often achieved through encoders or external trigger signals. The key advantage is that the camera can operate at very high line rates—up to hundreds of thousands of lines per second—while maintaining excellent sensitivity, which is impossible with conventional line scan cameras in low-light conditions. Modern TDI line scan cameras often use CMOS technology, which offers lower power consumption and faster readout speeds compared to traditional CCD TDI sensors. The integration of advanced noise reduction algorithms further enhances image quality, making TDI line scan the preferred choice for demanding inspection tasks where contrast and detail are critical.

2、TDI line scan vs CCD line scan

When comparing TDI line scan versus CCD line scan, the fundamental difference lies in the exposure mechanism and the resulting image quality. A standard CCD line scan camera uses a single linear array of pixels, capturing one line of the object at a time. The exposure time is determined by the line rate and the object velocity, and any increase in sensitivity requires either slowing down the object or using brighter illumination. In contrast, TDI line scan cameras use multiple stages to integrate the signal over time, allowing for much higher sensitivity at the same line rate. This makes TDI superior in low-light applications where illumination is limited or where high-speed operation is mandatory. For instance, in semiconductor wafer inspection, where defects must be detected at extremely high throughput, TDI line scan can achieve SNR levels that are unattainable with standard CCD line scan. Additionally, TDI sensors generally offer better dynamic range and lower noise because the signal is averaged over multiple stages, reducing random noise components. However, CCD line scan cameras have advantages in simpler system design and lower cost, as they do not require the complex timing and synchronization circuitry of TDI. CCD line scan also tends to have higher pixel resolution in some configurations, though TDI sensors are catching up with advances in pixel pitch technology. Another critical distinction is that TDI line scan is more tolerant of motion blur and vibration because the charge transfer is inherently synchronized with the object movement. In contrast, CCD line scan can suffer from smearing if the object speed fluctuates. For applications requiring extreme sensitivity and high speed, such as aerial imaging or medical film digitization, TDI line scan is the clear winner, while CCD line scan remains adequate for simpler, well-lit inspection tasks.

3、TDI line scan sensor pixel size

The pixel size of a TDI line scan sensor is a critical parameter that directly influences the resolution, sensitivity, and field of view of the imaging system. Typical TDI line scan sensors offer pixel sizes ranging from 5 micrometers to 14 micrometers, with 7 micrometers and 10 micrometers being common choices for industrial applications. Smaller pixel sizes, such as 5 micrometers, provide higher spatial resolution, allowing the camera to detect finer defects on objects like semiconductor wafers or printed circuit boards. However, smaller pixels also collect less light per pixel, which can reduce SNR unless the TDI stages compensate through integration. Larger pixels, such as 14 micrometers, offer better light collection and higher sensitivity, making them suitable for low-light or high-speed applications where resolution is less critical. The choice of pixel size also affects the maximum line rate and the overall system cost, as smaller pixels require more precise optics and tighter tolerances. In TDI sensors, the pixel size is designed to match the optical resolution of the lens and the required pixel density on the object. For example, a 10 micrometer pixel used with a 1x magnification lens will capture a 10 micrometer area on the object, which is adequate for many machine vision tasks. Advanced TDI line scan sensors now offer variable pixel binning modes, where multiple pixels can be combined to effectively increase pixel size and sensitivity at the cost of resolution. This flexibility allows system integrators to optimize the sensor for different inspection scenarios. Additionally, the pixel architecture—whether front-illuminated or back-illuminated—affects quantum efficiency and dark current, with back-illuminated designs offering superior low-light performance. When selecting a TDI line scan sensor, engineers must balance pixel size against the required line rate, object speed, and illumination budget to achieve optimal image quality.

4、TDI line scan applications in semiconductor

TDI line scan technology has become indispensable in the semiconductor industry, where it is used for wafer inspection, reticle inspection, and packaging quality control. In wafer inspection, TDI line scan cameras capture high-resolution images of silicon wafers to detect defects such as particles, scratches, pattern errors, and overlay misalignments. The high sensitivity of TDI allows inspection at very high throughput rates, often exceeding 200 wafers per hour, while maintaining the ability to detect sub-micrometer defects. For example, a 256-stage TDI sensor can achieve an SNR improvement of over 20 dB compared to a standard line scan camera, enabling reliable detection of low-contrast defects on reflective wafer surfaces. In reticle and photomask inspection, TDI line scan is used to image the patterns that define integrated circuits, where any defect can be catastrophic. The ability to operate with narrow bandpass illumination and low light levels makes TDI ideal for this application. Additionally, in advanced packaging processes such as flip-chip and wafer-level packaging, TDI line scan cameras inspect solder bumps, underfill materials, and die attachment for voids or misalignment. The technology is also employed in metrology applications, where precise dimensional measurements of critical features are required. The combination of high resolution, high speed, and low noise makes TDI line scan the gold standard for semiconductor inspection, displacing older CCD-based systems. As semiconductor nodes shrink to 3 nanometers and below, the need for even higher sensitivity and resolution drives continuous innovation in TDI sensor design, including the development of multi-spectral TDI sensors that can detect defects across different wavelengths.

5、TDI line scan speed and resolution

The speed and resolution of TDI line scan cameras are tightly coupled parameters that define the system's performance envelope. Line speed, measured in lines per second (kHz), determines how fast the camera can capture images as the object moves. Modern TDI line scan cameras can achieve line rates from 50 kHz up to 250 kHz or higher, depending on the number of TDI stages and the readout architecture. For example, a 128-stage TDI sensor operating at 100 kHz can inspect objects moving at speeds of several meters per second, making it suitable for high-volume manufacturing lines. Resolution, on the other hand, is determined by the number of pixels in the linear array, which can range from 1024 pixels to 16384 pixels or more. Higher pixel counts allow for wider fields of view or finer pixel sampling on the object. However, increasing resolution typically reduces the maximum line rate due to data bandwidth limitations. To balance speed and resolution, many TDI cameras offer multiple operating modes, such as high-speed mode with reduced TDI stages or high-resolution mode with slower line rates. The effective resolution also depends on the optical magnification and the pixel size, as discussed earlier. For instance, a 12,000-pixel TDI sensor with 5 micrometer pixels can achieve a resolution of 5 micrometers per pixel on the object, which is sufficient for detecting defects smaller than 10 micrometers. In practice, the trade-off between speed and resolution must be optimized for each application. For high-speed web inspection of materials like paper or film, speed is prioritized, while for semiconductor inspection, resolution is paramount. Advanced TDI line scan systems also incorporate features like region-of-interest readout and multi-line output to further enhance throughput without sacrificing image quality.

The five key aspects of TDI line scan technology—working principle, comparison with CCD line scan, sensor pixel size, semiconductor applications, and speed-resolution trade-offs—collectively illustrate why this imaging method is so powerful. TDI line scan cameras leverage time delay integration to overcome the sensitivity limitations of conventional line scan sensors, enabling high-speed, high-resolution inspection in challenging environments. The working principle relies on synchronized charge transfer across multiple stages, which dramatically improves signal-to-noise ratio. When compared to CCD line scan, TDI offers superior low-light performance and motion tolerance, though at higher system complexity. Sensor pixel size is a critical design choice that balances resolution and sensitivity, with smaller pixels offering finer detail but requiring more light. In semiconductor applications, TDI line scan excels at wafer and reticle inspection, detecting sub-micrometer defects at high throughput. Finally, the interplay between speed and resolution defines the practical limits of the system, requiring careful optimization for each use case. Understanding these elements is essential for engineers and system integrators looking to implement TDI line scan in their inspection workflows.

This article has provided a comprehensive overview of TDI line scan technology, covering its fundamental principles, comparative advantages, critical sensor parameters, leading applications, and performance characteristics. Whether you are designing a new machine vision system or upgrading an existing inspection line, TDI line scan offers a proven solution for achieving high-quality images at high speeds. The ability to integrate multiple exposures without slowing down the process makes TDI uniquely suited for modern manufacturing demands. As sensor technology continues to evolve, with advances in CMOS TDI, higher pixel counts, and improved quantum efficiency, the applications of TDI line scan will expand further into fields such as biomedical imaging, document digitization, and remote sensing. By leveraging the insights presented here, you can make informed decisions about integrating TDI line scan into your next project, ensuring optimal performance and reliability.