Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-02-15
2001-07-10
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100, C257S447000, C257S460000
Reexamination Certificate
active
06259085
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to charge coupled devices (CCDs), and more particularly to back illuminated CCDs.
Charge coupled devices (CCDs), which were originally developed as computer memory, are widely used as imaging detectors, in fields ranging from astronomy to medicine. Incident light is converted to electronic charge in a CCD; the charge is collected in potential wells, transferred out, detected, and stored. Images are then produced from the stored data. In a CCD, the electronic circuitry and gates are formed on one side of a silicon wafer, the frontside; the other side of the wafer is the backside.
When illuminated on the frontside, absorption of incident light by the electronic circuitry reduces quantum efficiency. As an alternative, CCDs can be illuminated from the backside; however, backside illumination produces other problems. When the incident photons enter the silicon wafer, they are absorbed and produce electronic charge by the photoelectric effect. The wafer thickness must be sufficient to allow charge generation, and a depletion region should exist to transport the charge to the collecting channels. For conventional low resistivity substrates the thickness of the depletion region is limited to less than about 10 &mgr;m. Therefore, for good blue and ultraviolet response, the substrate must be extremely thin, resulting in a very fragile and expensive structure.
Therefore, a backside illuminated CCD is desired which has a thick substrate, and which has a high quantum efficiency over a broad range of wavelengths, from infrared and red to blue and ultraviolet. The CCD should be relatively simple and inexpensive to manufacture. It is desired to have a backside illuminated CCD imaging system for use in astronomy.
SUMMARY OF THE INVENTION
Accordingly it is an object of the invention to provide an improved backside illuminated CCD.
It is another object of the invention to provide a backside illuminated CCD which has a high quantum efficiency over a broad range of wavelengths.
It is also an object of the invention to provide a backside illuminted CCD which is useful for astronomical imaging applications.
The invention is a backside illuminated charge coupled device (CCD) formed of a relatively thick high resistivity photon sensitive silicon substrate, with frontside electronic circuitry, and an optically transparent backside ohmic contact for applying a backside voltage which is at least sufficient to substantially fully deplete the substrate. A bias voltage which overdepletes the substrate may also be applied. One way of applying the bias voltage to the substrate is by physically connecting the voltage source to the ohmic contact. An alternate way of applying the bias voltage to the substrate is to physically connect the voltage source to the frontside of the substrate, at a point outside the depletion region. Thus both frontside and backside contacts can be used for backside biasing to fully deplete the substrate. Also, high resistivity gaps around the CCD channels and electrically floating channel stop regions can be provided in the CCD array. The CCD array forms an imaging sensor useful in astronomy.
REFERENCES:
patent: 4570329 (1986-02-01), Paine et al.
patent: 4695861 (1987-09-01), Paine et al.
patent: 5189297 (1993-02-01), Ahlgren
patent: 5494833 (1996-02-01), Martin et al.
patent: 5541438 (1996-07-01), Martin et al.
patent: 5940685 (1999-08-01), Loomis et al.
patent: 6025585 (2000-02-01), Holland
Lee John R.
Sartorio Henry P.
The Regents of the University of California
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