Color-optimized image sensor

An image sensor pixel array includes a photoelectric conversion unit that has a second region in a substrate and vertically below a gate electrode of a transistor. A first region under a top surface of the substrate and above the second region supports a channel of the transistor. A color filter transmits a light via a light guide, the gate electrode and the first region to generate carriers collected by the second region. The gate electrode may be made thinner by a wet etch. An etchant for thinning the gate electrode may be introduced through an opening in an insulating film on the substrate. The light guide may be formed in the opening after the thinning. An anti-reflection stack may be formed at a bottom of the opening prior to forming the light guide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject matter disclosed generally relates to structures and methods for fabricating solid state image sensors.

2. Background Information

Photographic equipment such as digital cameras and digital camcorders may contain electronic image sensors that capture light for processing into still or video images. Electronic image sensors typically contain millions of light capturing elements such as photodiodes.

Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, photo sensors are formed in a substrate and arranged in a two-dimensional array. Image sensors typically contain millions of pixels to provide a high-resolution image.

FIG. 1shows a sectional view of a prior art solid-state image sensor1showing adjacent pixels in a CMOS type sensor, reproduced from U.S. Pat. No. 7,119,319. Each pixel has a photoelectric conversion unit2. Each conversion unit2is located adjacent to a transfer electrode3that transfers charges to a floating diffusion unit (not shown). The structure includes wires4embedded in an insulating layer5. The sensor typically includes a flattening layer6below the color filter8to compensate for top surface irregularities due to the wires4, since a flat surface is essential for conventional color filter formation by lithography. A second flattening layer10is provided above the color filter8to provide a flat surface for the formation of microlens9. The total thickness of flattening layers6and10plus the color filter8is approximately 2.0 um.

Light guides7are integrated into the sensor to guide light onto the conversion units2. The light guides7are formed of a material such as silicon nitride that has a higher index of refraction than the insulating layer5. Each light guide7has an entrance that is wider than the area adjacent to the conversion units2. The sensor1may also have a color filter8and a microlens9.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present invention relates to an image sensor supported by a substrate of a first conductivity type, comprising a first pixel configured to detect a blue light and a second pixel, where the second pixel comprises (a) a photodetector that comprises a second region of a second conductivity type in the substrate, (b) a first region of the first conductivity type being vertically above the second region and under a top interface of the substrate, (c) a first drain of a first transistor, the first drain abutting the first region, (d) a first gate of the first transistor, the first gate being vertically above the first and second regions, (e) a first color filter, the first color filter being a red color filter or a green color filter or a yellow color filter, and (f) a first light guide, the first light guide being configured to transmit a light from the first color filter to the second region via the first gate and the first region. More particularly, the first light guide uses total internal reflection at its sidewalls to prevent the light from exiting laterally. Also more particularly, the first pixel comprises (a) a second color filter, the second color filter being a blue color filter or a magenta color filter, and (b) a second light guide configured to transmit light from the second color filter to the substrate mostly without passing through a gate of a transistor.

In the first aspect, the first transistor may be a select switch or a reset switch or an output transistor. Alternately, the first transistor may be a transfer gate.

In the first aspect, it is desirable that the first gate has a thickness between 200 Angstroms and 1,000 Angstroms. It is more desirable that the first gate has a thickness between 300 Angstroms and 700 Angstroms. It is even more desirable that the first gate has a thickness within 500+/−50 Angstroms.

In the first aspect, it is desirable that the first gate is thinner than a gate of a transistor in an ADC in the image sensor.

In the first aspect, it is desirable that the first region is not deeper than 0.8 um in the substrate where the first color filter is a red color filter, and is not deeper than 0.5 um in the substrate if the first color filter is a green color filter or a yellow color filter. It is more desirable that the first region is not deeper than 0.5 um in the substrate where the first color filter is a red color filter, and is not deeper than 0.35 um in the substrate if the first color filter is a green color filter or a yellow color filter.

In the first aspect, it is desirable that the first light guide is on the first gate.

In the first aspect, the first conductivity type may be p-type and the second conductivity type may be n-type.

According to a second aspect, the present invention relates to a method for forming a pixel array of an image sensor supported by a substrate of a first conductivity type, comprising (A) forming a second region of a second conductivity type in the substrate, (B) forming a first region of the first conductivity in the substrate and vertically above the second region, (C) forming a drain of a transistor in the substrate, the drain abutting the first region, (D) forming a gate of the transistor above the substrate, the gate being vertically above the first region and adjacent to the drain; (E) forming a color filter, the color filter being a red color filter or a green color filter or a yellow color filter, and (F) forming a light guide, the light guide being configured to transmit a light from the color filter to the second region via the gate and the first region.

In the second aspect, it is desirable that the method comprises thinning the gate. It is further desirable that, after the thinning, the gate has a thickness between 200 Angstroms and 1,000 Angstroms. It is more further desirable that, after the thinning, the gate has a thickness between 300 Angstroms and 700 Angstroms. It is even more further desirable that, after the thinning, the gate has a thickness within 500+/−50 Angstroms. It is also further desirable that the gate is thinned by an etching in which an etchant is introduce through an opening in and through an insulation film over the gate, the light guide being subsequently formed in the opening. It is even further desirable that the gate is thinned by an etching in which an etchant is introduce through an opening through an insulation film over the gate, the light guide being subsequently formed in the opening. It is still even further desirable that the method further comprises forming an anti-reflection stack in the opening after the thinning and before forming the light guide. It is desirable that the opening is opened with a plasma etch first then followed by a wet etch.

In the second aspect, it is desirable that the gate is thinned using a wet etch.

In the second aspect, it is desirable that the light guide is on the gate.

In the second aspect, it is desirable that the first conductivity type is p-type and the second conductivity type is n-type.

LIST OF REFERENCE SIGNS

106=lightly doped substrate (may be p-epi)316=lower light guide (may comprise silicon nitride)130=upper light guide (may comprise silicon nitride)110=insulator (e.g. silicon oxide)111=insulator (may be a silicon oxide that further comprises boron and/or phosphorus)107=first metal (lowest metal wiring layer)108=second metal109=contact (diffusion contact or poly contact)104a=gate electrode of a transfer gate beside a light guide (may be a polysilicon gate)104b=gate electrode of a transfer gate below a light guide (may be a polysilicon gate)104c,104d=gate electrode of a transistor that is not a transfer gate (may be a polysilicon gate)114a=a blue color filter or a magenta114b=a red color filter or a green color filter or a yellow color filter that transmits light to second reg102bvia gate electrode104bor104d.120=opening in the insulation film110above gate electrode104b,104d230=anti-reflection layer (may comprise three films: two oxide films sandwiching a nitride film)231=gate oxide233=nitride liner235=gate spacer232=third anti-reflection film (may be a silicon oxide)234=second anti-reflection film (may be a silicon nitride)236=top anti-reflection film (may be a silicon oxide)52x=Pwell under a non-transfer-gate transistor (e.g. a reset switch or a select switch or an output transistor)52y=first region (medium doped p-region under a transfer gate)52z=Pwell under a non-transfer-gate transistor (e.g. a reset switch or a select switch or an output transistor) having a photodiode below53=surface p+ diffusion layer55=trench isolation region57=source/drain n+ diffusion64=barrier region (may be medium doped p-region)102a=second region of a photodiode receiving light from the color filter114a(may be medium doped n-regions)102b=second region of a photodiode receiving light from the color filter114b(may be medium doped n-regions) (102atogether with its surrounding p-regions forms a photodiode. Likewise102b.)103a=a center of a horizontal cross-section at a bottom of the second region102a.103b=a center of a horizontal cross-section at a bottom of the second region102b.168a=depletion region surrounding the second region102a.168b=depletion region surrounding the second region102b.

DETAILED DESCRIPTION

An image sensor pixel array that includes a photoelectric conversion unit supported by a substrate of a first conductivity type. The photoelectric conversion unit may be a photodiode that comprises a second region of a second conductivity type disposed in the substrate and vertically below a gate electrode of a transistor. A first region of the first conductivity and under a top surface of the substrate is disposed above the second region. The first region supports a channel of the transistor. A color filter transmits a light that penetrates through the gate electrode and the first region to generate carriers to be collected by the second region. The color filter may be a red color filter or a green color filter or a yellow color filter. The light may be transmitted to the gate electrode via a light guide. The light guide may be on top of the gate electrode. The gate electrode may be part of a transfer gate, a reset switch, a select switch or a output transistor. The gate electrode may be thinner than a gate of a transistor in a periphery circuit outside the pixel array. The gate electrode may be made thinner by means of a wet etch. An etchant for thinning the gate electrode may be introduced through an opening in an insulating film on the substrate. The light guide may be formed in the opening after the thinning. An anti-reflection stack may be formed at a bottom of the opening prior to forming the light guide.

FIG. 7illustrates an image sensor10comprising an array12of pixels14connected to a row decoder20by a group of control signals22and to a light reader circuit1by a output signals18output from the pixels14. Light reader circuit samples the output signals18from pixels14and may perform subtraction and amplification on samples of the output signals18. The pixel array12includes a color filter array that comprises color filters arrayed in two-dimensions, one color filter for each pixel14.

FIG. 4Aillustrates an example of a color filter array that may be disposed over and as part of the pixel array12.FIG. 4Ashows a Bayer primary color filter pattern that comprises a repeated two-dimensional array of a two-by-two array of color filters each having one of a green color (G), a red color (R) and a blue color (B). A pair of green color filters is disposed along one diagonal of the two-by-two array. A pair of a red color filter and a blue color filter is disposed along the other diagonal. A red color filter has negligible transmittance for wavelength of light in air less than 600 nm. A green color filter has negligible transmittance for wavelength of light in air less than 500 nm or more than 600 nm. A blue color filter has negligible transmittance for wavelength of light in air more than 500 nm. A magenta color filter has negligible admittance for wavelength of light in air between 500 nm and 600 nm. A yellow color filter has negligible admittance for wavelength of light in air less than 500 nm. A transmittance is negligible if it is less than 10%. In each of the different color filters, peak transmittance should be in excess of 50%.

FIG. 5shows a schematic for an embodiment of a pixel14of the pixel array12. The pixel14includes a photodetector100. By way of example, the photodetector100may be a photodiode. The photodetector100may be connected to a reset switch112via a transfer gate117. The photodetector100may also be coupled to a select switch114through an output (i.e. source-follower) transistor116. The transistors112,114,116,117may be field effect transistors (FETs).

A gate of the transfer gate112may be connected to a TF(n) line121. A gate of the reset switch112may be connected to a RST(n) line118. A drain node of the reset switch112may be connected to an IN line120. A gate of the select switch114may be connected to a SEL line122. A source node of the select switch114may be connected to an OUT line124. The RST(n) line118, SEL(n) line122, and TF(n) line126may be common for an entire row of pixels in the pixel array12. Likewise, the IN120and OUT124lines may be common for an entire column of pixels in the pixel array12. The RST(n) line118, SEL(n) line122and TF(n) line121are connected to the row decoder20and are part of the control lines22. The OUT(m) lines124are connected to the light reader1and are part of the vertical signal lines18.

FIG. 6illustrates a pair of pixels sharing a reset switch112, a select switch114and an output transistor116. A photodetector100aand a transfer gate117atogether form a first pixel within the pair. A photodetector100band a transfer gate117btogether form a second pixel within the pair. The first and second pixels may be located in different rows within the pixel array12. The pixel pair includes two photodetectors100a,100bconnected to a shared sense node111via transfer gates117a,117b, respectively. Transfer gates117a,117bare controlled by horizontal signals TF(n+1)121aand TF(n)121b, respectively, connected to their respective gates. A shared reset switch112connects the sense node111to the vertical IN line120under a control of a shared horizontal signal RST(n)118that is connected to a gate of the reset switch112. The reset switch112and the transfer gate117awhen turned ON together and each into a triode region by driving both the signal RST(n)118and the signal TF(n+1)121ahigh can reset the photodetector100ato a voltage transmitted by the vertical IN signal120. Likewise, the reset switch112and the transfer gate117bwhen turned ON together and each into a triode region by driving both the signal RST(n)118and the signal TF(n)121bhigh can reset the photodetector100bto a voltage transmitted by the vertical IN signal120. The RST(n) line118, SEL(n) line122and TF(n+1) line121aand TF(n) line121bare connected to the row decoder20and are part of the control lines22. The OUT(m) lines124are connected to the light reader1and are part of the vertical signal lines18.

Referring toFIG. 6, an output transistor116is connected to a vertical OUT line124via a select transistor114turned ON by horizontal signal SEL(n)122. The output transistor116and the select transistor114are shared among the two pairs of photodetectors100a,100band transfer gates117a,117b. A signal can be transmitted from photodetector100ato the vertical OUT line124by driving horizontal signals TF(n+1)121aand SEL(n)122. Likewise, a signal can be transmitted from photodetector100bto the vertical OUT line124by driving horizontal signals TF(n)121band SEL(n)122.

FIG. 2shows a sectional view of two pixels14of an embodiment of the image sensor of the instant invention where the pixels14each has a color filter114aof a color or a color filter114bof a different color. Two pixels are shown supported on a semiconductor substrate106inFIG. 2. The semiconductor substrate106may be a lightly doped semiconductor material of a first conductivity type, for example p-type. For example, substrate106may be of silicon doped with boron to the concentration between 1E15/cm3to 7E15/cm3, such as a conventional p-epi layer on a heavily doped p+ substrate (not shown).

Referring toFIG. 2, each of the two pixels is for detection of a light of a different color, as determined by the color of light transmitted by color filters114a,114b, respectively. The color filter114ahas nonnegligible transmittance for light having a wavelength in air less than 500 nm, whereas the color filter114bhas negligible transmittance for light having a wavelength in air less than 500 nm. For example, the color filter114amay be a blue color filter, or a magenta color filter, whereas the color filter114bmay be a red color filter or a green color filter or a yellow color filter.

Metal-2 wires108over the substrate106connect devices in the pixel array12with one another and/or with the row decoder20and/or light reader1. Metal-1 wires107between the metal-2 wires108and the substrate106may connect between devices in the pixel14, for example between a drain diffusion of one transistor to a polysilicon gate of another transistor, or between a device and a metal-2 wire. More wiring layers may be used. An insulating dielectric110above the substrate106supports metal-1 wires107and metal-2 wires108. The insulating dielectric110may comprise a silicon oxide. A protection film410may cover the insulating dielectric110to keep out moisture and alkali metal ions such as sodium and potassium ions. The protection film410may comprise a silicon nitride.

Referring toFIG. 2, gate electrodes104a,104b, and104care each disposed on a gate dielectric (now shown) formed on the substrate106. The gate dielectric insulates the gate electrodes104a,104b,104cfrom the substrate106. Gate electrodes104a,104band104cmay comprise a polysilicon. Gate electrodes104a,104bare each part of a transfer gate, such as a transfer gate117shown inFIG. 5or transfer gates117a,117bshown inFIG. 6. Gate electrode104bis configured to be penetrated by a light having a wavelength in air greater than 500 nm, such as green light or red light, transmitted by the color filter114band passing through a lower light guide316above the gate electrode104b. Gate electrodes104care each part of a non-transfer gate, for example any one of the reset transistor112, source-follower transistor116, and select transistor114shown inFIG. 5orFIG. 6.

Referring toFIG. 2, lower light guides316and upper light guides130are light guides arranged in cascades to transmit visible lights from the color filters114a,114b. Light guides316,130may comprise a silicon nitride, for example Si3N4. The lower light guide316on the right has a bottom on the gate electrode104bto transmit light from the color filter114bto the gate electrode104b. Between the lower light guide316on the right and the gate electrode104bmay be sandwiched an anti-reflection stack to reduce a backwards reflection of the transmitted light at the interface between the lower light guide316and the gate104b. For example, the anti-reflection stack may comprise three dielectric films (for example, oxide-nitride-oxide) or more. Each film has a thickness between 50 Angstroms and 2000 Angstroms optimized to reduce reflection for a range of light wavelengths transmitted by color filter104b. The lower light guide316on the left is disposed laterally next to the gate electrode104ato transmit light to the substrate106mostly without passing through the gate electrode104a.

A first region52yof the first conductivity type, such as p-type, is disposed under the gate oxide below the gate electrode104bto form a bulk of a transistor that comprises the gate electrode104b. The first region52ybelow the gate electrode104b, the gate electrode104bitself, and a drain diffusion57adjacent to the gate electrode104btogether form parts of a transfer gate. The first region52ymay extend from a top interface of the substrate106below the gate electrode104bto a depth of less than 0.8 μm if the color filter114bis a red color filter, more preferably less than 0.45 μm, or to a depth of less than 0.5 μm if the color filter114bis a green color filter or a yellow color filter, more preferably less than 0.35 μm. The depth of first region52ymay be shallower than conventional MOS transistors, such as ones found in the I/O cells in the periphery of the image sensor10or row decoder20or in ADC24. The shallower depth for first region52yreduces a vertical distance that light travels within the first region52y. The first region52ymay be a retrograde well having peak doping concentration between 0.1 um to 0.2 um of depth, preferably 0.13 um. The first region52ymay comprise indium with peak dopant concentration between 5E17/cm3to 5E18/cm3, preferably 3E18/cm3. A similar first region52ymay be formed below gate electrode104ato form a bulk of the transfer gate that comprises gate electrode104a.

The drain diffusion57may be a heavily doped region of a second conductivity type, such as n-type. For example, the drain diffusion57may comprise arsenic at a peak doping concentration of 1E20/cm3or higher. The drain diffusion57may be a sense node such as the sense node111ofFIG. 5orFIG. 6.

Referring toFIG. 2, a second region102bis disposed in the substrate106below the gate electrode104b. The second region102bis of the second conductivity type, for example n-type. A lateral portion of the second region102bis disposed at the opposite end (from the drain diffusion57) of the transfer gate that comprises the gate electrode104b. The lateral portion of the second region102bconnects to a buried portion of the second region102bthat is buried under the first region52ythat is vertically below the gate electrode104b. For example, the second region102bmay be formed by implanting phosphorus to form the lateral portion and the buried portion separately, each formed after one or the other of two separate masking steps. The peak doping concentration of the second region102bmay be between 1E17/cm3to 7E18/cm3. A center103b(shown as dotted circle inFIG. 2) of a horizontal section of the buried portion of the second region102bmay be vertically below the gate electrode104band may also be vertically below the first region52yas shown inFIG. 2. A depletion region extends from below where the first region52yhas a peak doping concentration of the first conductivity type into a top of the buried portion of the second region102b.

Referring toFIG. 2, unlike the second region102b, a second region102aconnected to the transfer gate that comprises the gate electrode104ahas a center103a(shown as dotted diamond shape inFIG. 2) of a horizontal section not vertically below the gate electrode104abut beside. The second region102ais of the second conductivity type, e.g. n-type, and may be formed by implanting phosphorus to a peak doping concentration between 1E16/cm3and 7E18/cm3, more preferably between 7e16/cm3and 7e17/cm3. When the transfer gate that comprises the gate electrode104aturns ON, a channel forms below the gate electrode104ato connect the second region102ato the drain diffusion57that is adjacent to the gate electrode104aand at the opposite end from the second region102a. The drain diffusion57adjacent to the gate electrode104amay be a sense node such as the sense node111ofFIG. 5orFIG. 6. The lower light guide316on the left is positioned to transmit light from the color filter114ato the substrate106instead of through the gate electrode104a. It is noted that a minor amount of light from the lower light guide may be transmitted to the gate electrode104abut this should comprise less than 10% of the light transmitted by the lower light guide, such that the lower light guide316on the left is said to be mostly transmitting light to the substrate without passing through the gate electrode114a.

The second regions102a,102bmay be isolated from a top interface of the substrate by diffusions53beside the gate electrodes104a,104bhaving a first conductivity type, e.g. p-type, and a doping concentration between 5E17 to 1E19.

The best mode of the present mention has been described above.

An alternate embodiment of the image sensor of the instant invention may use no light guides130and316over second regions102aand102bbut instead may use a conventional microlens and a color filter over each second region to focus light into the second region. In this alternate embodiment, a first microlens and a first color filter together transmit a light having a wavelength greater than 500 nm to the gate electrode104b, whereas a second microlens and a second color filter together transmit a light having a wavelength within a range of 450 nm+/−50 nm to an area of top interface of the substrate106adjacent to but not covered by gate electrode104a.

FIG. 3is a ray tracing diagram for the sectional of the image sensor shown inFIG. 2. Ray a enters the color filter114aand the light guide130on the left, reflects on sidewalls of the light guide130on the left, penetrates the second region102a, is finally absorbed in the substrate within the depletion region168a, whereupon an electron-hole pair is generated. An electron from the electron-hole pair is swept by the electric field in the depletion region168atowards the second region102awhereas a hole from the pair is repelled away into the substrate106. The accumulated charge in the second region102athus becomes more negative. Or, a light may be absorbed in the second region102a, whereupon an electron-hole pair is generated, from which a electron is held within the second region102awhereas a hole is repelled by the electric field in the depletion region168aoutwards to the substrate106. When the transfer gate that adjoins the second region102aand that comprises gate electrode104ais turned ON to form a conduction path, in this example an inversion layer in the channel at the surface of the substrate106below the gate electrode104a, between the second region102aand the drain diffusion57adjacent to the gate electrode104a, the extent of accumulated negative charges is then sensed by an amplifier circuit such as shown inFIG. 5orFIG. 6.

Referring toFIG. 3again, ray b enters the color filter114band the light guide130on the right, reflects on sidewalls of the light guide130on the right, penetrates the anti-reflection stack230, the gate electrode104band a gate-oxide beneath (not shown), penetrates the first region52y, then penetrates the second region102b, and is finally absorbed in the substrate within the depletion region168b, whereupon an electron-hole pair is generated. An electron from the electron-hole pair is swept by the electric field in the depletion region168btowards second region102bwhereas a hole from the pair is repelled away into the substrate106. The accumulated charge in the second region102bthus becomes more negative. When the transfer gate that adjoins the second region102band that comprises gate electrode104bis turned ON to form a conduction path, in this example an inversion layer in the channel at the silicon surface below the gate electrode104b, between the second region102band the n+ diffusion57adjacent to the gate electrode104b, the extent of accumulated negative charges is then sensed by an amplifier circuit such as shown in a schematic ofFIG. 5orFIG. 6.

The color filter114bmay transmit a red light or a green light or both. The color filter114bmay be a green color filter, a red color filter, or a yellow color filter. Green and red lights have wavelengths between 500 nm and 700 nm and are able to penetrate more than 1 um through silicon and polysilicon before being absorbed. Light ray b having passed through color filter114bis able to penetrate through the first region52yto generate an electron-hole pair whose electron is collected by the second region102bbelow the first region52y.

As illustrated inFIG. 3, the light ray b penetrates beyond the second region102band generates an electron-hole pair within a depletion region that surrounds the second region. Also, a light ray that passes through the color filter114bmay be absorbed in the second region102bitself, generating an electron-hole pair whose electron is retained in the second region102bwhereas the hole from the pair is repelled out to the substrate106by the electric field within the depletion region168b. Or, a light ray that passes through the color filter114bmay be absorbed in the first region52y, creates an electron-hole pair whose electron diffuses into the depletion region168band is swept by the electric field therein into the second region102b.

In an alternate embodiment, to reduce an amount of light absorbed in the gate electrode104b, the gate electrode104bmay be made thinner, at least over a portion under the lower light guide316, than gate electrode of a different transistor, for example a transistor outside the pixel array12such as one in the ADC24. For example, a conventional polysilicon gate may have a thickness about 2,000 Angstrom+/−10%. The gate electrode104bmay comprise a polysilicon. It may be made to be between 200 Angstroms and 1,000 Angstroms thick, preferably between 300 Angstroms and 700 Angstroms, more preferably within 500+/−50 Angstroms. The gate electrode104bmay be etched down from the above using wet etch after the drain diffusion57is already formed. An insulating film110, such as a silicon oxide film, may be deposited over the wafer, followed by a lithography step to form a photoresist mask over the wafer to just expose areas on the insulating film110above where gates are to be thinned, e.g. just over areas of transfer gates in the pixel array12where lower light guides316will overlap with, followed by a wet etch to form an opening to expose the top of gates, then finally a wet etch to etch the gate (e.g. polysilicon gate) down to the desired final thickness. The lithography step may be preceded by a planarization of the silicon oxide film by CMP.

As an alternative to the wet etch, a plasma etch on the insulating film110may be applied to form part of the opening followed by a wet etch to remove the residual thickness of the insulating film110, etching the opening through the insulating film110and exposing the upper areas of gates. Alternately, the entire thickness of the insulating film110may be etched through by plasma etch alone, not involving wet etch, to create the opening. The thinning of the gate104bmay be performed after a lightly-doped drain (LDD) implant for transistors in the ADC24and/or the row decoder20.

The opening may also be an opening into which lower light guide316is subsequently formed, e.g. by depositing a silicon nitride. An anti-reflection stack, such as an oxide-nitride-oxide stack comprising a lower oxide film, a middle nitride film and a top oxide film, may be deposited to the bottom of the opening prior to forming the lower light guide316in the opening.

FIG. 9A to 9Eillustrates a process for forming an opening120over the gate104b(or104d), thinning the gate104b(or104d), forming an anti-reflection stack on the gate104b(or104d), and forming lower light guide316in the opening120.FIG. 9Ashows the gate104bon a gate oxide231on the substrate106. The gate104bis flanked by spacers235. The gate104band spacers235and portions of the gate oxide231not under the gate104bor the spacers235are covered by a film of nitride liner233. An insulating film111covers the wafer to a similar height as the top of the gate104b. Insulating film111may be a silicon oxide doped with boron and phosphorus, such as BPSG. The insulating film110further covers the wafer above the insulating film111and the portion of the nitride liner233atop the gate104b. Metal wiring layers may or may not have been formed in the insulating film110at this point. The insulating film110may comprise a silicon oxide.

FIG. 9Bshows an opening120made through the insulating film110and exposes a portion of the nitride liner233above the gate104b. The opening120made be formed by plasma etch alone, or may be formed by a plasma etch followed by a wet edge. InFIG. 9C, the portion of the nitride liner233is removed, and the gate104bis etched down from above to result in a thinner gate of the desired thickness.

FIG. 9Dshows an anti-reflection stack comprising three films232,234,236are formed on the thinned gate104bat the bottom of the opening120. Top anti-reflection film236may comprise a silicon oxide. The second anti-reflection film234may comprise a silicon nitride. The third anti-reflection film232may comprise a silicon oxide. The anti-reflection stack reduces a backward reflection of light transmitted from the light guide316to the substrate106.FIG. 9Eshows light guide316is formed in the opening120after the anti-reflection stack.

FIG. 4Billustrates another example of a color filter array that may be disposed over and as part of the pixel array12.FIG. 4Bshows a green-magenta color filter pattern that comprises a repeated two-dimensional array of a two-by-two array of color filters each having one of a green color (G) and a magenta color (Mg). A pair of green color filters is disposed along one diagonal of the two-by-two array. A pair of a magenta color filters is disposed along the other diagonal. In a pixel array that incorporates the green-magenta color filter array, the green pixel (having the green color filter as color filter114b) may be formed in the manner of the pixel on the right inFIG. 2. The magenta pixel (having the magenta color filter as color filter114a) may be formed similar to the manner of the pixel on the left inFIG. 2, in addition to having an additional photodetector below the second region102aand an additional transfer gate to transfer charges from this additional photodetector.

FIG. 8shows an alternate embodiment modified from the embodiment shown inFIG. 2. The bottom portion of a second region102bofFIG. 2is modified for a red pixel or a green pixel or a yellow pixel by extending the second region102bunder a Pwell52zof a transistor having a gate104dabove the second region102band the Pwell52z. The transistor is not a transfer transistor117and may be one of the reset switches112and the select switches114and the output transistors116. The lower light guide316of the pixel overlaps the gate104dof the transistor to transmit light through the gate104dand the Pwell52zto the second region102b.

The Pwell52zmay extend from a top interface of the substrate106below the gate electrode104dto a depth of less than 0.8 μm if the color filter114bis a red color filter, more preferably less than 0.45 μm, or to a depth of less than 0.5 μm if the color filter114btransmits is a green color filter or a yellow color filter, more preferably less than 0.35 μm. The depth of Pwell52zmay be shallower than conventional MOS transistors, such as ones found in the I/O cells in the periphery of the image sensor10or row decoder20or in ADC24. The shallower depth for Pwell52zreduces a vertical distance that light travels within the Pwell52z. The Pwell52zmay be a retrograde well having peak doping concentration between 0.1 um to 0.2 um of depth, preferably 0.13 um. The Pwell52zmay comprise indium with peak dopant concentration between 5E17/cm3to 5E18/cm3, preferably 3E18/cm3.

The gate104dis preferably made thinner as described above. It is further desirable that the gate104dsatisfies one of the thickness limitations described above with respect to the thinner gate. It is also further desirable that the gate104dis thinned according to one of the methods described above for gate thinning.