Back side illuminated image sensors with back side charge storage
View Patent ↗A back side illuminated image sensor may be provided with an array of image sensor pixels. Each image sensor pixel may include a substrate having a front surface and a back surface. The image sensor pixels may have a charge storage region formed at the back surface and a charge readout node formed at the front surface of the substrate. The image sensor pixels may receive image light at the back surface of the substrate. Photo-generated charge may be accumulated at the charge storage region during a charge integration cycle. Upon completion of the charge integration cycle, a transfer gate formed at the front surface may be pulsed high to move the charge from the charge storage region to the charge readout node. The charge may be converted to a voltage at the charge readout node and may be read out using a rolling shutter readout mode.
1. An image sensor pixel comprising:
a substrate having opposing front and back surfaces, wherein the substrate comprises a p-type doped substrate;
a charge storage region formed at the back surface of the substrate, wherein the charge storage region is configured to accumulate photo-generated charge, and wherein the charge storage region comprises an n-type doping layer; and
a charge readout node formed at the front surface of the substrate, wherein the charge readout node is configured to receive the accumulated photo-generated charge from the charge storage region, wherein the charge readout node comprises an n-type doped floating diffusion region, and wherein only p-type doped silicon is interposed between the charge storage region and the charge readout node.
2. The image sensor pixel defined in claim 1 , further comprising:
a transfer gate formed at the front surface of the substrate, wherein the transfer gate is configured to transfer the accumulated photo-generated charge from the charge storage region to the charge readout node.
3. The image sensor pixel defined in claim 2 , wherein the photo-generated charge accumulated at the charge storage region comprises electrons.
4. The image sensor pixel defined in claim 2 , wherein the photo-generated charge accumulated at the charge storage region comprises holes.
5. The image sensor pixel defined in claim 2 , further comprising:
a blooming drain region formed at the front surface of the substrate; and
a blooming control gate formed at the front surface of the substrate, wherein the blooming control gate is configured to transfer overflow charge from the charge storage region to the blooming drain region.
6. The image sensor pixel defined in claim 5 , wherein the image sensor pixel is coupled to additional image sensor pixels and wherein the blooming control gate is shared between the image sensor pixel and the additional image sensor pixels.
7. The image sensor pixel defined in claim 1 , wherein the n-type doped floating diffusion region is formed at the front surface of the substrate.
8. The image sensor defined in claim 7 , further comprising:
analog-to-digital converter circuitry formed at the front surface of the substrate and coupled to the floating diffusion node, wherein the analog-to-digital converter circuitry is configured to convert an analog image signal received from the floating diffusion node into a digital image signal.
9. The image sensor pixel defined in claim 7 , further comprising:
a source follower transistor formed at the front surface of the substrate;
an addressing transistor formed at the front surface of the substrate; and
a reset transistor formed at the front surface of the substrate.
10. The image sensor pixel defined in claim 9 , wherein the source follower transistor has a source and a drain and wherein the addressing transistor is connected in series with the drain of the source follower transistor.
11. The image sensor pixel defined in claim 9 , wherein the source follower transistor has a source and a drain and wherein the addressing transistor is connected in series with the source of the source follower transistor.
12. The image sensor pixel defined in claim 1 , further comprising:
a color filter, wherein the color filter is formed over the back surface of the substrate.
13. The image sensor pixel defined in claim 12 , further comprising:
a microlens, wherein the microlens is formed over the back surface of the substrate.
14. A system, comprising:
a central processing unit;
memory;
input-output circuitry; and
an imaging device, wherein the imaging device comprises a pixel array having at least one pixel circuit, the at least one pixel circuit comprising:
a substrate having opposing front and back surfaces;
a charge storage region formed at the back surface of the substrate, wherein the charge storage region is configured to accumulate photo-generated charge;
a charge readout node formed at the front surface of the substrate, wherein the charge readout node is configured to receive the accumulated photo-generated charge photo from the charge storage region and wherein the charge readout node is configured to convert the accumulated photo-generated charge to a voltage;
a charge transfer gate formed at the front surface of the substrate adjacent to the charge readout node, wherein the charge transfer gate is configured to transfer the accumulated photo-generated charge from the charge storage region to the charge readout node;
a blooming drain region formed at the front surface of the substrate; and
a blooming control gate formed at the front surface of the substrate adjacent to the charge transfer gate, wherein the blooming control gate is configured to transfer overflow charge from the charge storage region to the blooming drain region while photo-generated charge is being accumulated at the charge storage region, and wherein the charge readout node comprises a floating diffusion node and wherein the charge transfer gate is configured to transfer the accumulated photo-generated charge from the charge storage region to the floating diffusion node while the blooming control gate is biased low.
15. An image sensor pixel comprising:
a substrate having opposing front and back surfaces, wherein the substrate comprises an n-type doped substrate;
a charge storage region formed at the back surface of the substrate, wherein the charge storage region is configured to accumulate photo-generated charge, and wherein the charge storage region comprises a p-type doping layer; and
a charge readout node formed at the front surface of the substrate, wherein the charge readout node is configured to receive the accumulated photo-generated charge from the charge storage region, wherein the charge readout node comprises a p-type doped floating diffusion region, and wherein only n-type doped silicon is interposed between the charge storage region and the charge readout node.