Fishing – trapping – and vermin destroying
Patent
1991-12-12
1993-11-30
Nelson, Peter A.
Fishing, trapping, and vermin destroying
H01L 2102
Patent
active
052665321
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Advances in microelectronics often are limited by the multitude of relatively complicated processing steps required to produce the devices. A typical example of the number of processing steps is apparent in the trench isolation technique in bulk silicon which has been investigated as a means of dielectric isolation. This technique requires etching of deep trenches between devices on the die followed by oxide growth in the trenches to form the dielectric isolation. The etching steps call for the application of a photoresist with a subsequent low temperature heat treatment. This is followed by exposure to a lamp through a mask in contact with the photoresist and development of the resist. Another heat treating step is next, then the silicon trenches are chemically etched and, lastly, the remaining photoresist subsequently is stripped from the silicon. These seven steps are typical in many standard etching techniques used in the semiconductor industry. It becomes apparent that significant savings and yield could be obtained through the more simplified procedures that might be provided by a maskless, contactless form of etching. The reduced complexity of such a procedure would eliminate the many time consuming and costly steps of the conventional etching technique.
Another particular example of the excessive number of processing steps required to produce microelectronic devices becomes apparent when noting the procedures used to fabricate a backside illuminated charge-coupled device (CCD). CCDs designed for solid-state cameras, such as camcorders, are in great demand and are widely available. They have been designed to provide adequate performance when viewing brightly illuminated scenes. However, in astronomical, scientific and military applications their spectral response, readout noise, dark current, full well-capacity and blooming characteristics are not satisfactory.
To overcome the limitations of imaging through the polysilicon gates that necessarily cover all of the sensitive pixel array, it would be desirable to illuminate the CCD from the backside if the silicon substrate were thin enough. In other words, a solution to obtaining better light sensitivity would be the thinning of the backside of the CCD to a total thickness of roughly 10 microns. The need is quite apparent for new microelectronic processing schemes to produce thin membranes such as those required for the backside illuminated CCDs. Additional features that should attend this thinning process are the creation of a smooth surface for uniform imaging, nonreflecting sidewalls for stray light rejection and large (approximately 2 m by 2 m) square cross section for optimal illumination of the active area of the array.
A conventional fabrication procedure for backside illuminated CCDs calls for chemical thinning of the silicon. However, the standard wet chemical thinning-etch procedure produces an extremely low yield process and requires the handling of fragile thin silicon membranes. Furthermore, the chemical thinning requires two processes, a deep etch using potassium hydroxide and a subsequent Dash polishing etch. The latter consists of applying a mixture of acetic, nitric and hydrofluoric acids along with a surfactant. The Dash etch process also requires additional masking to protect the frontside metalization and backside gold eutectic used for packaging. Additional cleaning and inspection steps are required to complete the thinning process. Elimination of these steps would allow further "dry" processing of the thinned die, such as laser doping or dopant activation. In addition, the minimization of the required number of processing steps always is desirable in this microelectronic processing procedure to maximize the yield and reliability while also reducing costs.
In view of the foregoing, noncontact, maskless processing is receiving widespread interest in the microelectronic industry. A variety of laser-assisted processing techniques to modify materials used in this industry are being pursued, particularly wit
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Orazi Richard J.
Russell Stephen D.
Sexton Douglas A.
Fendelman Harvey
Keough Thomas Glenn
Nelson Peter A.
The United States of America as represented by the Secretary of
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