Range modulated implants for image sensors
Image sensors may include a plurality of photodiodes. The photodiodes may be isolated from each other using isolations regions formed from p-well or n-well implants. Deep and narrow isolation regions may be formed using a multi-step process that selectively places implants at desired depths in a substrate. If desired, the multi-step process may include only one photolithographic patterning step, which in turn can help reduce costs, fabrication time, and alignment errors. The process may include passing ions through a stack of alternating layers of material such as alternating layers of oxide and nitride. After each implant, a layer in the stack may be removed and ions may be passed through the layers remaining in the stack to form an implant at a different depth in the substrate.
1. A method of forming an image sensor with isolation regions in an image sensor substrate, comprising:
depositing a plurality of layers onto an upper surface of the image sensor substrate, wherein the plurality of layers comprises at least a first layer formed from a first material, a second layer formed from a second material, a third layer formed from a third material, and a fourth layer formed from a fourth material, and wherein the first layer is interposed between the upper surface of the image sensor substrate and the second layer;
implanting a first set of ions into the image sensor substrate through the first and second layers;
after implanting the first set of ions into the image sensor substrate, removing the second layer;
after removing the second layer, implanting a second set of ions into the image sensor substrate through the first layer;
implanting a third set of ions into the image sensor substrate through the first, second, and third layers; and
implanting a fourth set of ions into the image sensor substrate through the first, second, third, and fourth layers.
2. The method defined in claim 1 wherein removing the second layer comprises dry etching the second layer.
3. The method defined in claim 1 further comprising:
removing the third layer before implanting the first set of ions into the image sensor substrate through the first and second layers.
4. The method defined in claim 3 wherein removing the third layer comprises dry etching the third layer.
5. The method defined in claim 1 further comprising:
removing the fourth layer before implanting the third set of ions into the image sensor substrate through the first, second, and third layers.
6. The method defined in claim 1 wherein the second set of ions that are implanted into the image sensor substrate through the first layer are implanted at a first depth in the image sensor substrate, wherein the first set of ions that are implanted into the image sensor substrate through the first and second layers are implanted at a second depth in the image sensor substrate, wherein the third set of ions that are implanted into the image sensor substrate through the first, second, and third layers are implanted at a third depth in the image sensor substrate, and wherein the fourth set of ions that are implanted into the image sensor substrate through the first, second, third, and fourth layers are implanted at a fourth depth in the image sensor substrate.
7. The method defined in claim 6 wherein the third depth is greater than the fourth depth, wherein the second depth is greater than the third depth, and wherein the first depth is greater than the second depth.
8. The method defined in claim 1 wherein the first and second materials are selected from the group consisting of: oxide, nitride, and silicon nitride.
9. A method of forming isolation regions in an image sensor pixel array having a substrate with an upper surface, comprising:
implanting a first set of ions into a first region of the substrate through first and second layers of material, wherein the first layer of material is different from the second layer of material, and wherein the first layer of material is interposed between the upper surface of the substrate and the second layer of material;
after implanting the first set of ions into the first region, removing the second layer of material;
after removing the second layer of material, implanting a second set of ions into a second region of the substrate through the first layer of material, wherein the second region is below the first region and contacting the first region;
implanting a third set of ions into the substrate through the first layer, the second layer, and a third layer; and
implanting a fourth set of ions into the substrate through the first layer, the second layer, the third layer, and a fourth layer.
10. The method defined in claim 9 wherein the first and second layers of material are formed from materials selected from the group consisting of: oxide, nitride, and silicon nitride.
11. The method defined in claim 9 further comprising:
forming a photodiode in the substrate, wherein the first and second sets of ions isolate the photodiode from an adjacent photodiode.
12. The method defined in claim 9 further comprising:
after implanting the second set of ions, removing the first layer of material.
13. The method defined in claim 12 wherein removing the first layer of material comprises dry etching the first layer of material, the method further comprising:
after dry etching the first layer of material, removing an etch stop layer from the substrate.
14. A method of forming implants in a substrate having an upper surface, comprising:
implanting a first set of ions in the substrate by passing the first set of ions through a plurality of layers that are formed on and over the upper surface of the substrate, wherein the first set of ions are implanted at a first depth in the substrate;
after implanting the first set of ions in the substrate, removing an uppermost layer in the plurality of layers from the substrate;
implanting a second set of ions in the substrate by passing the second set of ions through a first group of layers that remain on the substrate after removing the uppermost layer, wherein the second set of ions are implanted at a second depth in the substrate and wherein the second depth is greater than the first depth;
after implanting the second set of ions, removing an additional layer from the first group of layers that remain on the substrate; and
implanting a third set of ions in the substrate by passing the third set of ions through a second group of layers that remain on the substrate after removing the additional layer, wherein the third set of ions are implanted at a third depth in the substrate and wherein the third depth is greater than the second depth.
15. The method defined in claim 14 further comprising:
depositing an etch stop layer onto the substrate;
depositing the plurality of layers onto the substrate over the etch stop layer; and
depositing and patterning a layer of photoresist on the plurality of layers to form a plurality of openings in the layer of photoresist.
16. The method defined in claim 15 further comprising:
etching the plurality of layers up to the etch stop layer to remove portions of the plurality of layers that overlap the openings in the layer of photoresist; and
after etching the plurality of layers up to the etch stop layer, removing the layer of photoresist from the substrate.
17. The method defined in claim 14 wherein the plurality of layers includes at least two materials selected from the group consisting of: oxide, silicon nitride, and nitride.