CMOS
07/01/1998
CMOS "down in the mouth"
CMOS technology, specifically active pixel imaging, may soon get you in and out of your dentist`s chair faster. The enabling circuit is a CMOS active pixel sensor (APS) imaging "camera-on-a chip" from Photobit, La Crescenta, CA. In product form, this plastic encapsulated chip has a green-phosphor scintillator coating that when struck by x-rays emits visible light that is conducted through a fiber optics faceplate and onto the CMOS imager (see figure). As such, it is a solid state replacement for "bit wing" dental film used for dental x-rays.
 |
Imaging configuration of Photobit`s CMOS x-ray sensor. (Source: IEEE ISSCC)
The Photobit chip is fabricated with conservative 2-?m one-poly two-metal processing. The completed chip measures 37 ? 28 mm and includes a 675 ? 900 array of photogate-type active p-channel pixels with 40-?m pixel pitch. The chip has on-chip timing, control logic, fixed-pattern noise suppression, and unique self-triggered x-ray detection that initiates image detection and readout.
Photobit`s Eric R. Fossum says, "Operation of the photogate-type pixel yields high performance - 10-MHz clock rate and 1.25 Mpixels/s. The main difference between this pixel and most pixels previously reported is its p-channel implementation with well plug. The p-channel pixel reduces collection of direct x-ray-generated signal charge in the substrate, that is x-rays not absorbed by the scintillator."
The chip`s self-triggered imaging mechanism is initiated from an event detector at each corner that detects the onset of x-ray irradiation and puts the sensor into an integration mode to capture the remainder of the signal. The four event detectors, (the number avoids shadowing by opaque material) are sampled in round-robin fashion at high sampling frequency to ensure the timely detection of x-ray irradiation on the sensor.
This circuit`s relatively large 40-?m pixel pitch is dictated by its x-ray application; it is about 10 times larger than pixel sizes in leading edge CMOS imaging technology. (One paper at the 1998 IEEE International Solid-State Circuits Conference [ISSCC] revealed 3.7 ? 3.7- ?m pixels using 0.6-?m CMOS technology. Session chair, H.-S. Philip Wong of IBM`s T.J. Watson Research Center, stated, "These are the smallest active pixels yet reported.")
Fossum says, "The advantages of CMOS APS, compared to CCD technology, is reduced sensor cost, easier production, and lower voltages and fewer cables, which are both important for the dental application because the chip is placed in the patient`s mouth." In addition, with use of CMOS technology, the circuit uses the n-well to block deep carrier collection.
Not only is this novel technology intriguing and faster, such computerized dental radiography eliminates hazardous wastes associated with conventional film processing. Further, images can be enhanced to improve diagnosis, are available faster during surgery, and can be transmitted for consultations and insurance reviews. The method also uses 10% of the x-ray dose required for film exposure. One wonders if the dentist will still leave the room?
The sensor is now in production at Photobit. Fossum says that this device has penetrated about 3% of the market, but "69% of dentists surveyed expect to buy a system in the next few years." - P.B.