Time delay integration image sensors with non-destructive readout capabilities
A time delay integration image sensor may include a number of charge coupled devices (CCDs) that transfer charge in synchronization with the movement of an object being imaged. To increase the dynamic range of the image sensor, the image sensor may include circuitry configured to non-destructively sample the charge as it is transferred through the charge coupled devices. Floating gates may be included in the image sensor and may have a voltage that is proportional to the charge accumulated under the floating gates. Each floating gate may be coupled to a respective readout circuit in an additional substrate by a metal interconnect layer.
1. An image sensor comprising:
a semiconductor substrate having first and second opposing surfaces;
a plurality of floating gates formed adjacent to the first surface of the semiconductor substrate;
a plurality of readout circuits, wherein each floating gate of the plurality of floating gates is coupled to a respective readout circuit of the plurality of readout circuits; and
a plurality of additional gates formed adjacent to the first surface of the semiconductor substrate, wherein a respective subset of the plurality of additional gates is interposed between each adjacent pair of floating gates.
2. The image sensor defined in claim 1 , wherein a voltage of each floating gate is dependent upon an amount of charge accumulated in the semiconductor substrate adjacent to that floating gate.
3. The image sensor defined in claim 1 , further comprising:
gate oxide formed on the first surface, wherein the plurality of floating gates is formed on the gate oxide.
4. The image sensor defined in claim 1 , wherein the semiconductor substrate is a first semiconductor substrate, the image sensor further comprising:
a second semiconductor substrate, wherein the plurality of readout circuits is formed in the second semiconductor substrate.
5. The image sensor defined in claim 4 , further comprising:
a plurality of metal interconnect layers, wherein each metal interconnect layer is coupled between a respective floating gate and a respective readout circuit.
6. The image sensor defined in claim 5 , wherein each readout circuit comprises:
a source follower transistor coupled to the respective metal interconnect layer; and
a reset transistor coupled to the respective metal interconnect layer.
7. The image sensor defined in claim 6 , wherein each readout circuit further comprises:
a bias voltage supply terminal; and
a row select transistor, wherein the source follower transistor is coupled between the bias voltage supply terminal and the row select transistor.
8. The image sensor defined in claim 1 , wherein the semiconductor substrate includes a buried channel of a first conductivity type adjacent to the first surface of the semiconductor substrate of a second conductivity type.
9. The image sensor defined in claim 8 , further comprising:
channel stops of the second conductivity type in the semiconductor substrate of the second conductivity type, wherein each gate of the plurality of additional gates extends over multiple channel stops of the second conductivity type and wherein each floating gate of the plurality of floating gates is formed between a respective adjacent pair of channel stops of the second conductivity type.
10. The image sensor defined in claim 1 , further comprising:
a floating diffusion region in the semiconductor substrate, wherein a gate of the plurality of additional gates is configured to transfer charge to the floating diffusion region.
11. The image sensor defined in claim 10 , further comprising:
a readout circuit coupled to the floating diffusion region; and
a reset gate formed adjacent to the first surface of the semiconductor substrate, wherein the reset gate is configured to drain charge from the floating diffusion region.
12. The image sensor defined in claim 11 , wherein the floating diffusion region is formed at a periphery of the semiconductor substrate.
13. The image sensor defined in claim 10 , wherein the semiconductor substrate is a first semiconductor substrate, the image sensor further comprising:
a second semiconductor substrate, wherein the plurality of readout circuits is formed in the second semiconductor substrate.
14. The image sensor defined in claim 1 , further comprising:
a plurality of overflow drains in the semiconductor substrate, wherein each overflow drain is adjacent to a respective gate selected from the group consisting of: one of the plurality of floating gates and one of the plurality of additional gates.
15. An image sensor comprising:
a charge coupled device configured to transfer charge from a first side of the image sensor to a second side of the image sensor, wherein the charge coupled device includes a plurality of floating gates and wherein the plurality of floating gates has a varying density; and
readout circuitry configured to sample the charge multiple times as the charge is transferred from the first side of the image sensor to the second side of the image sensor.
16. The image sensor defined in claim 15 , wherein the readout circuitry comprises a plurality of readout circuits and wherein each readout circuit is coupled to a respective floating gate.
17. The image sensor defined in claim 15 , wherein the charge coupled device includes a set of intervening gates between each adjacent pair of floating gates and wherein each set of intervening gates is configured to transfer charge between the adjacent pair of floating gates.
18. An image sensor comprising:
a semiconductor substrate that includes a plurality of charge coupled devices;
a plurality of gates on the semiconductor substrate that extend across the plurality of charge coupled devices; and
a plurality of floating gates on the semiconductor substrate, wherein each floating gate of the plurality of floating gates is contained within a respective charge coupled device of the plurality of charge coupled devices and wherein a voltage of each floating gate is dependent upon an amount of charge accumulated in the semiconductor substrate adjacent to that floating gate.
19. The image sensor defined in claim 18 , further comprising:
a plurality of overflow drains in the semiconductor substrate, wherein each overflow drain is adjacent to a respective floating gate.
20. The image sensor defined in claim 18 , wherein each charge coupled device is coupled to a respective first floating diffusion output region on a first side of the image sensor and a respective second floating diffusion output region on an opposing second side of the image sensor.