Application of Picosecond Ultrasonic Technology for CMOS Image Sensor

Picosecond Ultrasonic Technology (PULSE™ Technology) has been widely used in metal, >span class="NormalTextRun SCXW222830104 BCX9">-film metrology because of its unique advantages, such as being a rapid, non-contact and non-destructive technology with the capability for simultaneous multiple-layer measurement. In this paper, we describe the advantages of PULSE Technology in the manufacturing of backside illumination (BSI) CMOS image sensors (CIS). In a front-side illuminated (FSI) sensor, the light reaches the photo diode active region through the passivation, metallization and inter-dielectric layers. The coupling of light from the front side of the sensor results in a loss of light, which, in turn, results in a reduction in quantum efficiency (QE). >span class="NormalTextRun SCXW222830104 BCX9"> contributes to a significant amount of crosstalk, resulting in reduction of the sensor’s signal-to-noise ratio (SNR). A BSI sensor contains the same components as a FSI sensor, but the sensor’s metals are located behind the photodiode. Deposition and dry etching to >span class="NormalTextRun SCXW222830104 BCX9">span class="NormalTextRun SCXW222830104 BCX9">tungsten (W) film grid at the physical boundaries of each pixel is a common practice for traditional manufacturers. Chemical mechanical polishing (CMP) prior to dry etching provides better top-profile control of >span class="NormalTextRun SCXW222830104 BCX9">W grid film, resulting in a better SNR and performance. For BSI application, >span class="NormalTextRun SCXW222830104 BCX9">adoption of PULSE Technology for inline W thickness measurements has proven to be key for device-level process control and yield improvement. Picosecond ultrasonic measurements are rapid and provide excellent repeatability and long-tern stability, making it possible to achieve the high-sampling rate required in a high-volume manufacturing environment.