Yana Williams

Three-dimensional embedded defect detection and localization in a semi-transparent medium

The fabrication of new optical materials has many challenges that suggest the need for new metrology tools. To this purpose, the authors designed a system for localizing 10 micron embedded defects in a 10-millimeter thick semitransparent medium. The system, comprising a single camera and a motion system, uses a combination of brightfield and darkfield illumination. This paper describes the optical design and algorithm tradeoffs used to reach the desired detection and measurement characteristics using stereo photogrammetry and parallel-camera stereoscopic matching. Initial experiment results concerning defect detection and positioning, as well as analysis of computational complexity of a complete wafer inspection are presented. We concluded that parallel camera stereoscopic matching combined with darkfield illumination provides the most compatible solution to the 3D defect detection and positioning requirement, detecting 10 micron defects at a positioning accuracy of better than +/- 0.5 millimeters and at a speed of less than 3 minutes per part.

Modular sensor pack for large thickness cadmium zinc telluride (CZT) Gamma Radiation detectors

The high cost of fine spectroscopic grade CZT crystals with large gamma-ray stopping power is a limiting factor for widespread utilization in detector applications. Large volume, monolithic crystal parts suffer a lower yield because of high probability of defect opportunities. This work demonstrates a modular construction technique where volumetric scaling laws are overcome. Smaller crystal tiles are individually diced, graded and then bonded into a larger sensor package that achieves high stopping power. Choosing a common tile form factor for both healthcare and security imaging applications ensures the most efficient use of CZT wafer area by matching tile performance to application requirements. Gamma spectroscopic measurements results demonstrate that high quality detectors can be achieved in these sensor packs. The modular sensor pack is effective to circumvent the problem of low yield due to growth inhibition of a large-size, high quality crystal.

Dual-illumination NIR system for wafer level defect inspection

CdZnTe is a high efficiency, room temperature radiation detection material that has attracted great interesting in medical and security applications. CZT crystals can be grown by various methods. Particularly, CZT grown with the Transfer Heater Method (THM) method have been shown to have fewer defects and greater material uniformity. In this work, we developed a proof-of-concept dual lighting NIR imaging system that can be implemented to quickly and nondestructively screen CZT boule and wafers during the manufacturing process. The system works by imaging the defects inside CZT at a shallow depth of focus, taking a stack of images step by step at different depths through the sample. The images are then processed with in-house software, which can locate the defects at different depths, construct the 3D mapping of the defects, and provide statistical defect information. This can help with screening materials for use in detector manufacturing at an early stage, which can significantly reduce the downstream cost of detector fabrication. This inspection method can also be used to help the manufacturer understand the cause of the defect formation and ultimately improve the manufacturing process.

Electron mobility and lifetime mapping of CZT with known crystalline defects by pulsed laser excitation

A pulsed laser method is used to map the charge collection amplitude and electron mobility-lifetime product (μeτe) in Cd1−xZnxTe (x = 0.1) pieces especially selected to include regions with crystallographic defects. Additionally, ultrasound imaging is used in conjunction with X-ray diffraction in order to localize and analyze the defects. The defect regions studied include twins, misoriented secondary grains, and Te inclusions. Specific defect types were observed to degrade the local charge collection efficiency with a characteristic magnitude and spatial distribution. Coherent grain boundaries associated with the twin were less affecting than high angle, incoherent boundary associated with second grains. We consider the feasibility of in-line inspection at an early stage of device manufacture by 2D mapping of electron transport properties over large CZT wafer sizes.

Low-signature Cadmium Zinc Telluride CZT defect inspection by IR, ultrasound, etch pit density, and x-ray topography

Widespread utilization of Cadmium Zinc Telluride (CZT) in nuclear radiation detectors is currently limited by the cost of high spectroscopic quality material. Yield of devices is limited by non-uniformity in the charge collection efficiency associated crystal defects that occur during synthesis. An inspection method suitable for grading CZT parts during an early stage of device manufacturing is sought. We have implemented a combination of UT and IR imaging of CZT wafers that is successful to map sub-grain boundaries, twins and tellurium inclusions greater than 10-micron diameter. However, point defects and dislocations are below the imaging resolution of the system. It is the goal of this system to study defect density in UT and IR clear areas of CZT wafers and establish an opportunity for low signature defect mapping.

Phase shift based measurements using a pocket LCD projector

Phase shifting based measurements have been well established for use in both interferometry and structured light based measurements. The use of modern LCD, DLP or LCOS based projectors to create and shift projected patterns for use in phase shifting systems has provided new capabilities such as pattern masking, adjustable resolutions and active preprocessing, along with many challenges. Now the latest consumer projection technology has made available low cost, pocket-sized projectors, some with built in memory. These small projectors open up the possibility of mini-phase shift systems, as well as the possibility of portable measurement systems. This paper explores some of the possibilities for systems made with pocket size pattern projectors, and what some of the limitations may be that will need to be overcome. Experimental data will be presented that illustrates some of these challenges.