Near infrared techniques find MEMS defects

NIR inspection systems range from manual benchtop systems used mostly in the lab to fully automated tools used on the fab production floor.

By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 9/1/2010 12:00:00 AM

Email

Detecting defects within MEMS (microelectromechanical systems) is not an easy task. The components inside these structures are a varied mix of electrical circuits, mechanical devices, and sensors. The most common defects in MEMS—subsurface cracks, particle contamination, and voids—often occur in the substrate or in the bond between the wafer and the device, so it is difficult to detect them with inspection methods that use transmitted light. Nondestructive methods for inspecting MEMS include ultrasound-based acoustic inspection, x-ray, SWIR (short-wave infrared), and NIR (near infrared), said Tom Persico, director of technology for McBain Systems.

To read past "Tech Trends" columns, go to www.tmworld.com/techtrends.

Acoustic-based inspection using ultrasonic fluid immersion is good at finding macro defects but only down to around 100 microns, said Persico. "Although x-ray easily penetrates many materials, the wavelengths are very long, and the longer the wavelength, the worse the resolution," he said. NIR inspection systems, according to Persico, are best for detecting defects down to a micron in size. NIR systems not only find subsurface defects, but also measure critical alignment marks between two die layers, between two wafer layers, or between a die layer and a wafer layer, he said. "Another use we're seeing is detecting process damage, such as from 3-D wafer-stacking processes that are still being fine-tuned."

NIR wavelengths are usually defined as starting above the visible spectrum, around 650 nm, and continuing up to wavelengths of about 1000 nm, said Persico. "Just because you can create a NIR image, however, that doesn't mean it shows you what you need to see," he said. "With highly doped parts, extra-thick materials, or rough or unpolished surfaces, you'll need to reach higher wavelengths to see through the material well enough."

Different materials may also require different wavelengths. NIR may be high enough for penetrating most silicon substrates but not for some proprietary semiconductor materials. In these cases, you need wavelengths above 1000 nm in the SWIR spectrum. "Our DDR300 NIR system actually uses both, operating between 900 nm and 1700 nm," Persico said.

NIR inspection and metrology systems range from manual benchtop systems used mostly in the lab to fully automated tools used on the fab production floor, said Persico. Automated NIR tools are somewhat new. "Some fabs add an IR camera to production equipment, but don't really optimize the tool for NIR," he said. "There are also benchtop microscopes with IR, but they still need to work in conjunction with the software, camera, motion control, and a light source for the particular application."

In the future, McBain expects to improve its NIR inspection and metrology systems with new optics for better resolution, typically on the macro side, said Persico. That means smaller pixels, higher pixel density, higher sensitivity, and lower noise. "Customers want to see the same small features in the same field of view but in a larger area, so they don't need to buy two different tools. That's because throughput is everything for semiconductor manufacturers."

AOI system targets modules, sensors The Reveal Imager Series is designed for inspection and metrology of camera modules and image sensors in back-end-of-line environments. The Reveal accurately detects particles that are 1 micron or larger, checks for process defects, and can measure and forward-feed exact sensor and pixel-array positioning. www.vitechnology.com. GigE camera adds software features Basler Vision Technologies has enhanced the software for its Runner family of GigE linescan cameras. Shading correction compensates for nonhomogeneous lighting conditions so vision application software can detect defects more easily. To create the camera's correct line-acquisition rate, the multiplier increases the frequency of an incoming trigger signal, while the divider reduces that frequency. www.baslerweb.com. HD frame grabber streams video The new HDV62 PCI Express frame grabber offers video streaming and uncompressed image acquisition in 1080p high-definition. It also provides up to 1920x1080p resolution at 60 fps, progressive scan, noise reduction, and a wide aspect ratio. An onboard FPGA and 512-Mbyte buffer are included for streaming uncompressed video and performing real-time color-space conversion. www.adlinktech.com.