Glenn H. Chapman

A monolithic Hough transform processor based on restructurable VLSI

The implementation of a Hough transform processor using a wafer-scale-integration technology, restructurable VLSI circuit is described. The Hough transform is typically used as a grouping operation in an image processing sequence. The transform discussed here groups pixels in order to extract linear features. This calculation is realized with a wafer-scale processor that allows a complete line extraction system to be integrated on a single PC board. Also discussed is the use of the CAD tools that allowed this processor to be realized without incurring silicon layout and processing overhead. >

A wafer-scale digital integrator using restructurable VSLI

Wafer-scale integration has been demonstrated by fabricating a digital integrator on a monolithic 20-cm2silicon chip, the first laser-restructured digital logic system. Large-area integration is accomplished by laser programming of metal interconnect for defect avoidance. This paper describes the technology for laser welding and cutting, the design methodology and CAD tools developed for wafer-scale integration, and the integrator itself.

Angular domain imaging of objects within highly scattering media using silicon micromachined collimating arrays

Optical imaging of objects within highly scattering media, such as tissue, requires the detection of ballistic/quasi-ballistic photons through these media. Recent works have used phase/coherence domain or time domain tomography (femtosecond laser pulses) to detect the shortest path photons through scattering media. This work explores an alternative, angular domain imaging, which uses collimation detection capabilities of small acceptance angle devices to extract photons emitted aligned closely to a laser source. It employs a high aspect ratio, micromachined collimating detector array fabricated by high-resolution silicon surface micromachining. Consider a linear collimating array of very high aspect ratio (200: 1) containing 51/spl times/1000 /spl mu/m etched channels with 102-/spl mu/m spacing over a 10-mm silicon width. With precise array alignment to a laser source, unscattered light passes directly through the channels to the charge coupled device detector and the channel walls absorb the scattered light at angles >0.29/spl deg/. Objects within a scattering medium were scanned quickly with a computer-controlled Z axis table. High-resolution images of 100-/spl mu/m-wide lines and spaces were detected at scattered-to-ballistic ratios of 5/spl times/10/sup 5/: 1, with objects located near the middle of the sample seen at even higher levels. At >5/spl times/10/sup 6/: 1 ratios, a uniform background of scattered illumination degrades the image contrast unless recovered by background subtraction. Monte Carlo simulation programs designed to test the angular domain imaging concept showed that the collimator detects the shortest path length photons, as in other optical tomography methods. Furthermore, the collimator acts as an optical filter to remove scattered light while preserving the image resolution. Simulations suggest smaller channels and longer arrays could enhance detection by >100.

Prototype laser-activated bimetallic thermal resist for microfabrication

The Thermal Resist Enhanced Optical Lithography (TREOL) process models an optical system to double device resolution by exploiting non-reciprocal laser activated processes. A possible prototype thermal resist consists of stacked bismuth on indium layers sputter deposited on a glass/quartz substrate with thickness ratios matching the eutectic alloy (Bi 53%). Laser radiation locally melts the metals which alloy upon cooling. BiIn resist is relatively wavelength insensitive because its UV optical characteristics vary modestly. Reflection and energy absorption/cc calculations indicate the best arrangement is a 30-45-nm total thickness bilayer with bismuth on indium. Exposing the highly absorbing BiIn with CW argon (514/488 nm) or 4-ns Nd:YAG pulses at 533 nm (40 mJ/cm2 for 300-nm thick) and 266 nm transforms the resist to a weakly absorbing alloy with a visually identifiable pattern. 30-nm thick converted film transmission changes from 1.0OD to 0.35OD (830-350 nm) until a 350-nm absorption edge. Profilometry and SEM showed no signs of ablation or oxide growth in exposed areas. The resist was developed with HNO3:CH3COOH:H2O etch, preferentially removing unexposed areas, leaving written patterns of alloyed lines seen both in profilometry and SEM images. Thus BiIn forms a complete thermal alloying resist with selectively etched exposed patterns that can be stripped in an HCl:H2O2:H2O bath.

Laser Restructurable Technology and Design

The Restructurable VLSI project at MIT Lincoln Laboratory has developed a design methodology, new technology, and CAD tools for WSI. Six wafer scale systems have been fabricated and three of much larger size are being designed. Figure 1 shows one of these packaged WS circuits. The accomplishments and current research status of this project, which was conceived in 1979 [1], are described in this chapter.

The technology of laser formed interactions for wafer scale integration

Restructurable VLSI wafer-scale circuits have been built using two methods, both using laser energy to create low resistance connections between bus lines on already existing circuits. In one technique verticle connections of about 10 Omega are made up from top metal, through silicon nitride, to first metal lines. The other involves melting of silicon in the gap between two implant regions, with the lateral diffusion of dopants creating connections of about 100 Omega . Details of the linking structures, their characteristics, and the apparatus used to interconnect them are described.<<ETX>>

A self-correcting active pixel camera

Digital cameras on-a-chip are becoming more common and are expected to be used in many industrial and consumer products. With the size of the CMOS active pixel-array implemented in such chips increasing to 512/spl times/512 and beyond, the possibility of degradation in the reliability of the chip over time must be a factor in the chip design. In digital circuits, a commonly used technique for reliability or yield enhancement is the incorporation of redundancy (e.g., adding redundant rows and columns in large memory ICs). Very limited attempts have been directed towards fault-tolerance in analog circuits, mainly due to the very high level of irregularity in their design. Since active pixel arrays have a regular structure, they are amenable to reliability enhancement through a limited amount of added redundancy. The purpose of this paper is to investigate the advantages of incorporating some fault-tolerance methods, including redundancy, into the design of an active pixel sensor array.

Laser‐formed connections using polyimide

Electrical connections have been formed in a new lateral link structure which uses polyimide in the gap between, and overlapping, two aluminum electrodes. An argon ion laser, with a pulse width of 1 ms and power levels of about 2 W, was used to locally graphitize the polyimide. One kilohm connections were formed reliably in links ranging in width between 4 and 15 μm and gap length between 2 and 5 μm. This technique is the simplest yet proposed for restructuring the connections on an integrated circuit, after fabrication and test, in order to incorporate redundancy for yield improvement.

Laser-induced diode linking for wafer-scale integration

Diodes formed by ion implantation and diffusion in a conventional CMOS process are positioned such that when desired they may be used to obtain an electrical link between two otherwise separate sections of the integrated circuit. Electrical connections so obtained enable the realization of wafer-scale ICs as demonstrated in recent applications. They theory of laser-beam application to silicon is discussed and it is shown how the various beam and substrate parameters effect the properties of the diode links. Particular attention is paid to the important issue of the reflectivity from the composite system. Careful analytical examinations of the resulting molten zone properties have been performed to fully qualify the use of laser radiation in this technology. Both scanning electron microscopy and secondary-ion mass spectrometry were used to examine such parameters as the lateral and in-depth extension of the molten zone. In addition, electrical measurements were carried out. The results for the various observables compare well with the theoretical predictions. >

Image contrast enhancement in angular domain optical imaging of turbid media

Imaging structures within a turbid medium using Angular Domain Imaging (ADI) employs an angular filter array to separate weakly scattered photons from those that are highly scattered. At high scattering coefficients, ADI contrast declines due to the large fraction of non-uniform background scattered light still within the acceptance angle. This paper demonstrates various methods to enhance the image contrast in ADI. Experiments where a wedge prism was used to deviate the laser source so that scattered photons could be imaged and subtracted from the image obtained by standard ADI provided the greatest improvement in image contrast.