Tuesday, March 24, 2009

Patents, history and the origins of the CMOS image sensor

Techon reports about the patent fights of Caltech regarding fundamental technologies in the area of CMOS imagers. Without knowing the details, it's difficult to form an opinion on the rightness and/or rightneousness of the lawsuit. Nevertheless, the claim done in the article is that Hitachi invented the (passive) CMOS sensor. Without entering to discuss that claim, the first information on a silicon array image sensor I have comes from the thesis of a student of professor Theuwissen, which reports the first silicon sensor being published in 1966 in a paper by M.A. Schuster and G. Strull: "A monolithic mosaic of photon sensors for solid state imaging applications", IEEE Transactions on Electron Devices, vol. ED-13, pp. 907-912, 1966. The abstract reads:

Monolithic silicon mosaics of photosensor elements have been developed for solid-state imaging applications. The physical structure, design considerations, and performance characteristics of these electrooptical devices as applied to image converter applications are discussed.

The sensing monolith consists of a square 50 by 50 mosaic of phototransistor elements on 0.010 inch centers which are interconnected both by internally diffused strips and by vapor deposited surface bars. Fifty X and fifty Y external leads provide access to any individual element Xb Ya, of the mosaic. Fabrication of this 2500 element mosaic involves the techniques of planar passivation, epitaxial growth, solid-state diffusion, and thin-film vacuum evaporation.

A discussion of sensor operation includes mechanisms of phototransistor action, electrooptical conversion efficiency, and element-to-element crosstalk minimization.

An evaluation of the electrooptical transfer characteristics of the mosaic sensor elements are presented. Uniformity of element response
is typically better than 85% for response within a 3 : 1 range and 75% for response within a 2 : 1 range. Several shades of gray can be imaged simultaneously. The mosaic dynamic range extends over 3.5 orders of incident illumination energy or five orders of output photocurrent. The minimum and maximum detectable signals are approximately 10.0 nW and 1.0 mW, respectively. Sensitivity is of the order of 10^2 to 10^8 microA/mW in the linear portion of the transfer curve.

Incidentally, these two researchers worked at the time for NASA, at which JPL lab the active pixel sensor (APS) was invented about 24 years later. The patents that CalTech owns are from NASA's JPL.

It's a tricky thing to mix patent litigation with engineering history, and I find the Techon article referenced above very disappointing in this respect. I also sense a bit of nationalism in the thesis that Hitachi started it all, but that's just me.

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.