Apparatus and method of digital imaging on a semiconductor substrate
The present invention includes an active pixel sensor that detects optical energy and generates an analog output that is proportional to the optical energy. In embodiments, the active pixel sensor can be implemented in a standard CMOS process, without the need for a specialized optical process. The active pixel sensor includes a reset FET, a photo-diode, a source follower, and a current source. The photo-diode is coupled to the source of the reset FET at a discharge node. The drain of the reset FET is couple to a power supply VDD. The discharge node is also coupled to the gate input of the source follower, the output of which is coupled to output node. In embodiments, shallow trench isolation is inserted between the active devices that constitute the photo-diode, the source follower, or the current source, where the shallow trench isolation reduces leakage current between these devices. As a result, dark current is reduced and overall sensitivity is improved. This enables the active pixel sensor to be integrated on a single substrate fabricated with conventional CMOS processing.
1 . An active pixel circuit, comprising:
a reset circuit that charges a node during a reset condition;
a photo-diode that discharges said node when optical energy is received by said photo-diode; and
a source follower circuit that reproduces a voltage at said node;
wherein said reset circuit, said photo-diode, and said source follower circuit are configured on a common CMOS substrate.
2 . The active pixel circuit of claim 1 , wherein said CMOS substrate is a conventional CMOS substrate.
3 . The active pixel circuit of claim 1 , further comprising a shallow trench isolation between said photo-diode and one of said reset circuit and said source follower circuit.
4 . The active pixel circuit of claim 3 , wherein said shallow trench isolation includes an oxide filled trench.
5 . The active pixel circuit of claim 3 , wherein said shallow trench isolation retards charge carriers, thereby reducing leakage current.
6 . The active pixel circuit of claim 3 , further comprising one or more metal traces in proximity to said shallow trench isolation, said metal traces having minimal bends to reduce leakage current of said active pixel circuit.
7 . The active pixel circuit of claim 6 , wherein said metal traces are configured to reduce leakage current between active devices of said active pixel circuit.
8 . The active pixel circuit of claim 1 , wherein said CMOS substrate is fabricated with a conventional CMOS process.
9 . The active pixel circuit of claim 1 , further comprising an amplifier coupled to an output of said source follower and disposed on said substrate.
10 . The active pixel circuit of claim 9 , further comprising a shallow trench isolation between said photo-diode and said amplifier.
11 . The active pixel circuit of claim 9 , further comprising an analog-to-digital converter coupled to an output of said amplifier.
12 . The active pixel circuit of claim 11 , further comprising a shallow trench isolation between said photo-diode and said analog-to-digital converter.
13 . An active pixel circuit, comprising:
a reset circuit that charges a node during a reset condition;
a photo-diode that discharges said node when optical energy is received by said photo-diode;
a source follower circuit that reproduces a voltage at said node;
an amplifier coupled to an output of said source follower; and
an analog-to-digital converter coupled to an output of said amplifier;
said reset circuit, said photo-diode, said source follower, said amplifier, said analog-to-digital converter are configured on a common CMOS substrate; and
means for reducing leakage current from said photo-diode to any one of said reset circuit, said source follower circuit, said amplifier, and said analog-to-digital converter when said light intensity is low.
14 . The active pixel circuit of claim 13 , wherein said means for reducing includes:
a shallow trench isolation between said photo-diode and one of said reset circuit, said source follower circuit, said amplifier, and said analog-to-digital converter.
15 . The active pixel circuit of claim 14 , wherein said means for reducing includes one or more metal traces in close proximity to said shallow trench isolation, said one or more metal traces having minimal bends to reduce said leakage current of said active pixel circuit.
16 . The active pixel circuit of claim 13 , wherein metal traces associated with at least one of said reset circuit, said photo-diode, said source follower, said amplifier, and said analog-to-digital converter have minimal bends to reduce leakage current.