COMPLEMENTARY METAL-OXIDE-SEMICONDUCTOR IMAGE SENSORS WITH SUBMICRON PIXELS AND PUNCH THROUGH CHARGE TRANSFER
A backside illuminated (BSI) complementary metal-oxide-semiconductor (CMOS) image sensor array may operate in a rolling shutter scanning mode. The CMOS image sensor may use chip stacking technology with chip-to-chip electrical connections between the top image sensing chip and carrier chip pixel circuits. Each chip-to-chip connection may electrically connect groups of pixels at floating diffusion nodes to a readout circuit. This arrangement allows for small, submicron sized pixels to be formed while using bonding pads that have a larger size. The top light sensing chip pixels do not have transfer gates for lateral charge transfer from photodiodes to the floating diffusion regions. The charge transfer from the photodiode regions is accomplished using a charge punch through technique in a vertical direction. This type of arrangement allows for submicron size pixels in a very large sensor array.
1 . An image sensor comprising:
first and second semiconductor substrates;
an array of imaging pixels arranged in rows and columns, wherein each imaging pixel comprises:
a photodiode in the first semiconductor substrate;
a floating diffusion region in the first semiconductor substrate; and
a storage gate at a surface of the first semiconductor substrate;
a plurality of floating diffusion signal lines, wherein each floating diffusion signal line is coupled to a plurality of floating diffusion regions in a single column; and
a plurality of metal interconnect layers, wherein each metal interconnect layer electrically connects a respective floating diffusion signal line to a respective readout circuit in the second semiconductor substrate.
2 . The image sensor defined in claim 1 , wherein the first semiconductor substrate includes a plurality of isolation regions and wherein each isolation region is interposed between two respective photodiodes.
3 . The image sensor defined in claim 2 , wherein each one of the isolation regions is a deep trench isolation region that includes hafnium oxide, wherein the surface of the first semiconductor substrate is a front surface, and wherein an additional layer of hafnium oxide is formed at a back surface of the first semiconductor substrate.
4 . The image sensor defined in claim 1 , wherein no charge transferring transistors that laterally transfer charge between the photodiode and the floating diffusion region are included at the surface of the first semiconductor substrate.
5 . The image sensor defined in claim 1 , wherein charge is transferred from each photodiode to a respective floating diffusion region in a vertical direction using a charge punch through process.
6 . The image sensor defined in claim 5 , wherein the charge punch through process comprises applying a punch through signal to the respective floating diffusion region through a punch through capacitor in the second semiconductor substrate.
7 . The image sensor defined in claim 1 , wherein each metal interconnect layer comprises a hybrid bond that includes a first bond pad that is shorted to a respective floating diffusion signal line and a second bond pad that is shorted to a respective readout circuit in the second semiconductor substrate.
8 . The image sensor defined in claim 1 , wherein storage gates for all but one imaging pixel coupled to a given floating diffusion signal line are configured to store charge under the storage gates while the one remaining imaging pixel coupled to the given floating diffusion signal line is configured to store charge at the photodiode for a punch through charge transfer.
9 . The image sensor defined in claim 1 , wherein each readout circuit in the second semiconductor substrate comprises:
an inverting negative feedback amplifier transistor;
a feedback capacitor that sets an amplifier gain for the inverting negative feedback amplifier transistor;
a reset transistor; and
a current source biasing transistor.
10 . The image sensor defined in claim 9 , wherein each readout circuit in the second semiconductor substrate comprises:
a source follower transistor;
a column sense line; and
an addressing transistor that connects an output of the inverting negative feedback amplifier transistor through the source follower and the addressing transistor to the column sense line.
11 . The image sensor defined in claim 10 , wherein each readout circuit in the second semiconductor substrate comprises:
a p-channel transistor current source that biases the inverting negative feedback amplifier transistor drain, wherein the p-channel transistor is configured to bias the floating diffusion node to a reference potential through the reset transistor during a charge integration period causing overflow charge from the imaging pixel to be drained.
12 . The image sensor defined in claim 11 , wherein the inverting negative feedback amplifier transistor is configured to be turned off by biasing a source terminal of the inverting negative feedback amplifier transistor to a positive potential.
13 . An image sensor comprising:
an image sensing chip comprising an array of imaging pixels that include photodiodes and floating diffusion regions;
a carrier chip including a plurality of readout circuits; and
a plurality of metal interconnect layers between the image sensing chip and the carrier chip, wherein each metal interconnect layer electrically connects a group of floating diffusion regions to a respective readout circuit of the plurality of readout circuits.
14 . The image sensor defined in claim 13 , wherein each imaging pixel includes a storage gate.
15 . The image sensor defined in claim 13 , wherein each metal interconnect layer is formed form a hybrid bond between a first bond pad on the image sensing chip and a second bond pad on the carrier chip, wherein each imaging pixel has a first maximum lateral dimension, and wherein each one of the first bond pads has a second maximum lateral dimension that is greater than the first maximum lateral dimension.
16 . The image sensor defined in claim 13 , wherein a length and a width of each imaging pixel are both less than 1 micron.
17 . The image sensor defined in claim 13 , wherein a punch through signal is configured to be applied to a respective floating diffusion region through the metal interconnect layer to transfer charge to that floating diffusion region.
18 . An image sensor comprising:
an array of imaging pixels arranged in rows and columns, wherein each imaging pixel comprises:
a photodiode formed in a semiconductor substrate;
a floating diffusion region; and
a storage gate at a surface of the semiconductor substrate, wherein all but one storage gate in a given column of imaging pixels are configured to store charge under the respective storage gates away from respective photodiodes while the one storage gate in the given column transfers charge from a respective photodiode to a respective floating diffusion region.
19 . The image sensor defined in claim 18 , further comprising:
a plurality of signal lines, wherein each signal line is shorted to multiple floating diffusion regions in a respective column of imaging pixels.
20 . The image sensor defined in claim 19 , further comprising:
a plurality of hybrid bonds, wherein each hybrid bond is formed between a respective signal line and a readout circuit in an additional semiconductor substrate.