# Patent

## Title
Ring pixel for CMOS imagers

## Abstract

A CMOS pixel is disclosed. The CMOS pixel includes a semiconductor substrate; a sense node formed in the semiconductor substrate and positioned substantially in the center of the CMOS pixel; a transfer gate formed about the sense node; and at least one photodiode formed about the transfer gate. A reset transistor, a source follower transistor, and a row select transistor are located substantially to one side of the CMOS pixel substantially adjacent to the photodiode. The sense node is operable to be floating. An implant may be formed about the photodiode configured to step potential in a direction toward the sense node.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. provisional patent application No. 61/230,440 filed Jul. 31, 2009 and U.S. provisional patent application No. 61/358,921 filed Jun. 27, 2010, the disclosures of which are incorporated herein by reference their entirety.


FIELD OF THE INVENTION
The invention relates generally to imaging devices. More specifically, the invention relates to improving the performance of CMOS imagers through a novel CMOS pixel design.
BACKGROUND OF THE INVENTION
Generally, an image sensor is a semiconductor device for converting an optical image into an electric signal. There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices, hybrid focal plane arrays, etc. The various types of image sensors may be broadly categorized as charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) image sensors.
CCDs are often employed for image acquisition and enjoy a number of advantages which makes it attractive for many small size imaging applications. CCDs are also produced in large formats with small pixel size and they employ low noise charge domain processing techniques.
However, CCD imagers suffer from a number of disadvantages. For example, CCDs are susceptible to radiation damage; CCDs are often expensive to manufacture; CCDs require good light shielding to avoid image smear and; CCDs have a high power dissipation for large arrays. CCD imagers also have a complicated driving method and a complicated fabrication process requiring a multi-phased photo process. A control circuit, a signal processing circuit, an analog to digital (A/D) converter circuit, etc., cannot be easily integrated into a CCD chip, thereby inhibiting the use of CCDs in compact size products. While there have been some attempts to integrate on-chip signal processing with a CCD array, these attempts have not been entirely successful. CCDs also must transfer an image by linear charge transfers from pixel to pixel, requiring that the entire CCD array be read out into a memory before individual pixels or groups of pixels may be accessed and processed. This takes time. CCDs may also suffer from incomplete charge transfer from pixel to pixel during charge transfer which also results in image smear.
Because of the inherent limitations in CCD technology, there has been increased interest in CMOS imagers for possible use as low cost imaging devices. CMOS image sensors first came to the fore in relatively low-performance applications where shuttering was not required, scene dynamic range was low, and moderate to high noise levels could be tolerated. A CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, star trackers, motion detection systems, image stabilization systems and high-definition television imaging devices.
The advantages of CMOS imagers over CCD imagers are that CMOS imagers have a low voltage operation and low power consumption; CMOS imagers are compatible with integrated on-chip electronics (control logic and timing, image processing, and signal conditioning such as A/D conversion); CMOS imagers allow random access to the image data; and CMOS imagers have lower fabrication costs as compared with the conventional CCDs since standard CMOS processing techniques may be used. Additionally, CMOS imagers exhibit low power consumption because only one row of pixels at a time needs to be active during readout and there is no charge transfer (and associated switching) from pixel to pixel during image acquisition. On-chip integration of electronics is particularly advantageous because of the potential to perform many signal conditioning functions in the digital domain (versus analog signal processing) as well as to achieve a reduction in system size and cost.
FIGS. 1A and 18 are cross-sectional views of conventional CMOS pixels known as 3T pixels (for three-transistor pixel) and 5T pixels (for 3-transistor plus 2-transfer gate pixel). More precisely, the 3T pixel 10 of FIG. 1A (designated hereinafter an n—3TPPD pixel) includes three NMOS transistors 12, 14, 16 standing for a reset transistor 12, a source follower transistor 14 and a row transistor 16. The reset transistor 12 is electrically connected to a sense node 18. The sense node 18 is formed of an n+ contact 22 and a pinned photodiode 20. The pinned photodiode 20 includes a thin p-type pinning layer 26 overlying a custom n-diode implant 24, that in turn, overlies and forms a depletion region with a p-epitaxial layer 30. A p-substrate 32 underlies the p-epitaxial layer 30. A p-well 34 is formed adjacent the pinned photodiode 20 in the p-epitaxial layer 30 for isolating the n—3TPPD pixel 10 from neighboring pixels. A p+ return contact 36 is formed proximal to the other side of the pinned photodiode 20 and is held at ground potential (about 0V) for providing a return and ground reference for the n—3TPPD pixel. A p-well 38 is formed adjacent to the p+ return contact 36.
When operated, a CLOCK applied to the gate of the reset transistor 12 causes a reverse bias on the pinned photodiode 20. The source follower transistor 14 and the row transistor 16 are coupled between a drain supply VDD of about 3.3V and an output signal terminal COLUMN VIDEO. The drain of the reset transistor 12 is connected to VDD; the gate of the reset transistor 12 is connected to a RESET clock; and the source of the reset transistor is connected to the cathode of the pinned photodiode 20 so that the reset transistor 12 operates as a source follower. The source of the source follower transistor 14 is connected to the drain of the row transistor 16, and the source of the row transistor 16 is connected to output terminal COLUMN VIDEO. In applications, a plurality of such 3T pixels is coupled to the same output terminal COLUMN VIDEO. By selectively applying row address signal ROW SELECT to the gate of the selected row transistor 16, different rows may be coupled to the output terminal COLUMN VIDEO (i.e., a column bus).
The 5T pixel 40 (FIG. 1B), also known as a charge transfer pixel (designated hereinafter an n—5TPPD pixel), is similar to the 3T pixel 10 (FIG. 1A) except that the 5T pixel 40 has a transfer gate 42 coupled between the reset transistor 12 and the pinned photodiode 20 so that a sense node 44 may be created between the transfer gate 42 and the reset transistor 12. The sense node 44 may be isolated from the pinned photodiode 20. As a result, charge may be transferred from a photodetection region to the sense node 44 when a positive voltage (preferably a “logical 1” or “high” positive voltage of about 3.3V) is applied to the input TRANSFER GATE 1, where a resulting voltage is read out by the source follower transistor 14.
The 5T pixel 40 also includes a second transfer gate 46 abutting the side of the pinned photodiode 20 distal to the transfer gate 42. An n+ contact 48 is formed adjacent to the second transfer gate 44 distal to the pinned photodiode 20 and is also tied to VREF (about +3.3 volts). The second transfer gate 44 may be used as a global reset for the imager and as an antiblooming gate for preventing excess charge generated in the photodiode 20 from “blooming” through the transfer gate 42 to the sense node 44 when a voltage is applied to the input TRANSFER GATE 2 that is more positive than the transfer gate-to-sense node voltage.
Similar 4T and 6T CMOS pixel designs are also known. The n—3TPPD and n—5TPPD and similar n—4TPPD and n—6TPPD pixels are hereinafter designated as n_pixels.
High performance for both CCD and CMOS imagers implies at least very low read noise (1-4 e−), high quantum efficiency (transmission limited), deep depletion for near IR and soft x-ray charge collection efficiency (CCE) performance, low pixel cross talk (high MTF), high charge transfer efficiency (CTE), high signal-to-noise ratio for low contrast scenes and very high speed/low noise parallel readout using integrated designs. More particularly, assuming that a transfer gate barrier (or pocket) does not exist, charge transfer time (CCE) is ultimately limited by thermal diffusion. This limit applies primarily to large pixels where transfer gate fringing fields don't extend across the entire PPD region to aid in the charge transfer process.
Accordingly, what would be desirable is a CMOS pixel and resulting CMOS imager with increase speed performance by means of quick transfer of charge.
SUMMARY OF THE INVENTION
The above-described problems are addressed and a technical solution achieved in the art by providing a CMOS pixel, comprising: a semiconductor substrate; a sense node formed in the semiconductor substrate and positioned substantially in the center of the CMOS pixel; a transfer gate formed about the sense node; and at least one photodiode formed about the transfer gate. The semiconductor substrate may be of a first conductivity type and the sense node may be highly doped and of a second conductivity type. The sense node may comprise a highly doped n+ region formed in a highly doped p+ region, or a highly doped p+ region formed in a highly doped n+ region.
According to an embodiment of the present invention, the CMOS pixel may further comprise: a reset transistor of the second conductivity type in signal communication with the sense node; a source follower transistor of the second conductivity type in signal communication with the sense node; a row select transistor of the second conductivity type in signal communication with the source follower transistor. The reset transistor, the source follower transistor, and the row select transistor may be located substantially to one side of the at least one CMOS pixel substantially adjacent to the photodiode. The photodiode may be in signal communication with the sense node. The sense node is operable to be floating.
According to an embodiment of the present invention, the CMOS pixel may further comprise an implant formed about the photodiode configured to step potential in a direction toward the sense node. The implant formed about the photodiode may further comprise a buried channel and highly doped region of a first conductivity type adjacent to and formed about the buried channel.
According to an embodiment of the present invention, the CMOS pixel may further comprise driver logic formed in the semiconductor substrate. The driver logic may be at least one of an address encoder, a pixel bipolar driver, and a multiplexor.
According to an embodiment of the present invention, the CMOS pixel may further comprise at least a portion of buried oxide layer substantially underlying the semiconductor substrate and a mono layer of metal formed substantially underlying the buried oxide layer.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention may be more readily understood from the detailed description of an exemplary embodiment presented below considered in conjunction with the attached drawings and in which like reference numerals refer to similar elements and in which:
FIGS. 1A and 1B are cross-sectional views of conventional CMOS pixels known as 3T pixels (for three-transistor pixel) and 5T pixels (for 3-transistor plus 2-transfer gate pixel, respectively;
FIGS. 2A and 2B are cross-sectional views of CMOS pixels having substantially corresponding structures to the pixels of FIGS. 1A and 1B with doping conductivity types reversed, according to an embodiment of the present invention;
FIG. 3 is a cross-sectional view of a CMOS imager employing the pixels of FIGS. 2A and 2B and negative substrate bias, according to an embodiment of the present invention;
FIG. 4A is a PISCES simulation that plots potential through a p-well region as a function of device depth for the CMOS imager of FIG. 3;
FIG. 4B plots substrate current and bias showing where a limit to the amount of substrate bias that may be applied occurs for the CMOS imager of FIG. 3;
FIG. 5 is a cross-sectional view of a CMOS imager employing p_pixels and positive substrate bias, according to an embodiment of the present invention;
FIG. 6A cross-sectional view of the CMOS imager of FIG. 3 illustrating how a charge cloud (packet) travels from a back side of the imager to the front side by an electric field generated by a pixel and substrate bias, according to an embodiment of the present invention;
FIG. 6B plots potential with depth as substrate bias voltage changes from 0 to −20 V for the CMOS imager of FIG. 5A;
FIGS. 7A and 7B plot transit time and the resultant Gaussian 2σ charge cloud diameter as function of applied voltage for various fully depleted epitaxial layer thicknesses, respectively, according to an embodiment of the present invention;
FIGS. 8A and 8B plot ‘point-spread’ responses for the same thicknesses as FIG. 7B with and without −20 V substrate bias applied, respectively;
FIG. 9 is a Fe-55 x-ray image taken by a 15 μm thick front side illuminated CMOS minimal array with 8 μm pixels;
FIG. 10 shows a corresponding simulated Monte Carlo response for aforementioned CMOS imager array with and without substrate bias applied;
FIGS. 11 and 12 are similar results corresponding to FIGS. 9 and 10 for 25 μm epi silicon producing much greater charge diffusion clouds;
FIG. 13 presents a single event x-ray histogram taken from a 15 μm thick CMOS minimal array;
FIG. 14 presents simulated data for a fully depleted 5 μm sensor built on SOI epi;
FIGS. 15A and 15B show modulation transfer function (MTF) versus epi thickness and substrate bias, respectively;
FIG. 16A shows depletion depth versus epi resistivity for the CMOS imager of FIG. 3;
FIG. 16B plots epi silicon resistivity as a function of depth for 15 and 25 μm custom silicon typically used to fabricate CMOS imagers in the prior art;
FIGS. 17A and 17B present potential plots for 14 μm epi showing potential, doping and electric field profiles as the substrate bias is varied in the prior art;
FIG. 18A depicts doping concentration versus thickness along an SOI wafer employed in both the '583 patent and a high resistivity CMOS imager based on the CMOS imager of FIG. 3, according to an embodiment of the present invention;
FIG. 18B is a plot of theoretical quantum efficiency with wavelength for a variety of epi thicknesses for the high resistivity CMOS imager of FIG. 18A;
FIG. 19 is a process flow diagram illustrating exemplary steps for treating the back side of a CMOS imager employing SOI technology to produce the non-accumulated profile, according to an embodiment of the present invention;
FIG. 20 depicts a side view of a 5TPPD ring pixel, according to an embodiment of the present invention;
FIG. 21 depicts a top-down view of the ring pixel of FIG. 20, according to an embodiment of the present invention;
FIG. 22 displays the results of a PISCES simulation showing how a floating sense node naturally repels signal carriers, according to an embodiment of the present invention; and
FIG. 23 is a magnified view of FIG. 22.



It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 2A and 2B are cross-sectional views of CMOS pixels having substantially corresponding structures to the pixels of FIGS. 1A and 1B with doping conductivity types reversed, according to an embodiment of the present invention. The CMOS pixels are hereinafter designated as p—3TPPD and p—5TPPD pixels, 10′, 40′, respectively. The p—3TPPD pixel 10′ includes three PMOS transistors 12′, 14′, 16′ standing for a reset transistor 12′, a source follower transistor 14′ and a row transistor 16′. The reset transistor 12′ is electrically connected to a sense node 18′. The sense node 18′ is formed of a p+ contact 22′ and a pinned photodiode 20′. The pinned photodiode 20′ includes a thin n-type pinning layer 26′ overlying a custom p-diode implant 24′, that in turn, overlies and forms a depletion region with an n-epitaxial layer 30′. An n-substrate 32′ underlies the n-epitaxial layer 30′. An n-well 34′ is formed adjacent the pinned photodiode 20′ in the n-epitaxial layer 30′ for isolating the p—3TPPD pixel 10′ from neighboring pixels. An n+ return contact 36′ is formed proximal to the other side of the pinned photodiode 20′ and is held at “high” potential (about +3.3V) for providing a return and reference for the p—3TPPD pixel. An n-well 38′ is formed adjacent to the p+ return contact 36′.
When operated, a RESET CLOCK (about 0 V) applied to the gate of the reset transistor 12′ causes a reverse bias on the pinned photodiode 20′. The source follower transistor 14′ and the row transistor 16 are coupled between a drain supply VDD of about 0V and an output signal terminal COLUMN VIDEO. The drain of the reset transistor 12′ is connected to VDD; the gate of the reset transistor 12′ is connected to a RESET clock; and the source of the reset transistor 12′ is connected to the anode of the pinned photodiode 20′ so that the reset transistor 12′ operates as a source follower. The source of the source follower transistor 14′ is connected to the drain of the row transistor 16′, and the source of the row transistor 16′ is connected to output terminal COLUMN VIDEO. In applications, a plurality of such 3T pixels is coupled to the same output terminal COLUMN VIDEO. By selectively applying row address signal ROW SELECT to the gate of the selected row transistor 16′, different rows may be coupled to the output terminal COLUMN VIDEO (i.e., a column bus).
The 5T pixel 40′ (FIG. 2B), also known as a charge transfer pixel (designated hereinafter an n—5TPPD pixel), is similar to the 3T pixel 10′ (FIG. 2A) except that the 5T pixel 40′ has a transfer gate 42′ coupled between the reset transistor 12′ and the pinned photodiode 20′ so that a sense node 44′ may be created between the transfer gate 42′ and the reset transistor 12′. The sense node 44′ may be isolated from the pinned photodiode 20′. As a result, charge may be transferred from a photodetection region to the sense node 44′ when a “logical 0” or “low” ground potential of about 0V is applied to the input TRANSFER GATE 1, where a resulting voltage is read out by the source follower transistor 14′.
The 5T pixel 40′ also includes a second transfer gate 46′ abutting the side of the pinned photodiode 20′ distal to the transfer gate 42′. A p+ contact 48′ is formed adjacent to the second transfer gate 46′ distal to the pinned photodiode 20′ and is tied to VREF (about 0 volts). The second transfer gate 46′ may be used as a global reset for the imager and as an antiblooming gate for preventing excess charge generated in the photodiode 20′ from “blooming” through the transfer gate 42′ to the sense node 44′ when a voltage is applied to the input TRANSFER GATE 2 that is more negative than the transfer gate-to-sense node voltage.
The p—3TPPD and p—5TPPD pixels are hereinafter designated as p_pixels.
FIG. 3 is a cross-sectional view of a CMOS imager 50 (hereinafter designated as an n_imager 50) configured to have an applied negative (less than 0 V) substrate bias, according to an embodiment of the present invention. Referring now to FIGS. 1A, 1B, and 3, the n_imager 50, in a preferred embodiment shown, includes a substrate bias contact 52 having a p+ polarity and a plurality of n_pixels 54. The n_imager 50 includes driver logic 56 for the n_pixels 54. The CMOS driver logic 56 may include digital logic elements (e.g., address encoders, pixel bipolar drivers, multiplexers, etc.).
In a preferred embodiment, negative substrate bias may be applied to a portion of the front side 58 of the n_imager 50, such as the substrate bias contact 52, or it may be applied to the back side 60 of the n_imager 50, but in practice a frontside substrate contact (i.e., the substrate bias contact 52) is used.
The n_pixels 54 are separated from the CMOS driver logic 56 by a pair of n-wells 62, 64 abutting an intervening p-well 66. A p-well 68 separates the p+ substrate bias contact 52 and an n-well 70 abutting the CMOS driver logic 56 distal to the n-well 64. The n wells 62, 64 may have a separate DEEP n-well bias of about +3.3 volts applied. The n wells 64, 70 provide access contacts for biasing a deep n-well implant 72 to be described hereinbelow. The n well 62 closest to the n_pixels 54 surrounds the entire pixel region (not shown) to form a depletion region around the edge of the n_pixels 54 to ensure that the n_pixels 54 pinch off substrate bias in proximity to the p+ return contact 36 to be described hereinbelow.
The substrate bias contact 52, the n_pixels 54, and the wells 62-72 may be formed in an p-type epitaxial layer 74, preferably made of silicon, which overlies a p-type substrate 76. The epitaxial layer 74 may have a relatively high resistivity greater than about 10,000 ohm-cm. According to an embodiment of the present invention, the resistivity of the p-type epitaxial layer 74 may preferably be greater than about 100 ohm-cm, more preferably about 10,000 ohm-cm. However, substrate bias is applicable to a p-type epitaxial layer 74 having any resistivity. To achieve substantially full depletion of the p-type epitaxial layer 74, in a preferred embodiment, the p-type epitaxial layer 74 may be composed of intrinsic silicon or lightly doped (shown as p− in FIG. 2A). Since maximum depletion depth varies as the square root of the resistivity for silicon, for a resistivity of greater than about 10,000 ohm-cm, full depletion may be achieve for a depth of the p-type epitaxial layer 74 in the range of about 300 microns (and 30 microns without substrate bias). A negative voltage is applied to the bias contact 52 to deplete the p-type epitaxial layer 74 in a depletion region extending the full p-type epitaxial layer 74 thickness below each of the n_pixels 54.
In operation, charge collects below the n_pixels 54 (not shown). As the p-type epitaxial layer 74 is depleted of majority carriers (i.e., in this case holes), there is a danger that the p+ return contact 36 may short to a substrate bias depletion edge 78 shown. To prevent shorting, a sufficient potential barrier needs to be produced between the p+ return contact 36 and the substrate bias depletion edge 78 to prevent (hole) leakage current flow. The required barrier height is primarily dependent on the positive bias potential applied to the n well 62 closest to the n_pixels 54, on the width of the p-well 38 (which varies with design rules), and on the dimensions of the p+ return contact 36.
FIG. 4A presents a PISCES (Poisson and Current Continuity Equation Solver)/SUPREM (Stanford University Process Modeling) simulation that plots potential through the p_pixel's p-well 38 as a function of device depth. Reverse bias across the pinned photodiode 20 is varied (0.4, 0.8, 1.2 and 1.6 V). Note as the diode bias decreases, the barrier height under the p+ return contact 36 also decreases, signifying a limit to the amount of substrate bias that may be applied. To prevent substrate bias current from flowing, at least 250 mV of barrier voltage is preferred (i.e., about 10 kT, where k is the Boltzmann constant and T is temperature in degrees Kelvin). FIG. 4B plots substrate current and bias showing where that limit occurs.
Barrier height decreases with silicon resistivity and increases with its thickness. The width of p+ return contact 36 requires minimum design rules but should not be so small in width to cause pixel-to-pixel blooming. The n_pixels 54 should have the largest active fill factor possible for maximum substrate bias.
According to an embodiment of the present invention, the amount of substrate bias that may be applied without causing pixel ground shorting varies in inverse proportion to grounding/return area that a pixel has. As a result, the area covered by each of the p+ return contacts 36 is kept to a minimum according to standard CMOS minimum line width design rules.
As seen in FIG. 3, the CMOS driver logic 56 formed in the n_imager 50 also relies on the p+ return contact 36 for operation. It is not possible to ‘pinch off’ this circuitry from substrate bias as is done for the n_pixels 54. Instead, a biased deep n-well implant 72 is employed under the CMOS driver logic 56 to generate a sufficient barrier for substrate bias. FIG. 3 also shows that the substrate bias contact 52 is far removed from the n_pixels 54. As it turns out, the pixel depletion region does not extend to the backside of the sensor if the depletion reaches the substrate bias contact 52 first. The minimum separation dimension between the n_pixels 54 and the substrate bias contact 52 assumes that this distance is approximately equal to the thickness of the p-type epitaxial layer 74.
FIG. 5 is a cross-sectional view of a CMOS imager 50′ (hereinafter designated as a p_imager 50′) configured to have an applied positive (greater than about +3.3 V) substrate bias, according to an embodiment of the present invention. Referring now to FIGS. 2A, 2B, and 5, the p_imager 50′, in a preferred embodiment shown, includes a substrate bias contact 52′ having an n+ polarity and a plurality of p_pixels 54′. The p_imager 50′ includes driver logic 56′ for the p_pixels 54′. The CMOS driver logic 56′ may include digital logic elements (e.g., address encoders, pixel bipolar drivers, multiplexers, etc.).
In a preferred embodiment, positive substrate bias may be applied to a portion of the front side 58′ of the p_imager 50′, such as the substrate bias contact 52′, or it may be applied to the back side 60′ of the p_imager 50′, but in practice a frontside substrate contact (i.e., the substrate bias contact 52′) is used.
The p_pixels 54′ are separated from the CMOS driver logic 56′ by a p-well 62′. A p-well 68′ separates the n+ substrate bias contact 52′ from the CMOS driver logic 56′ distal to the p-well 62′. The p well 62′ may have a separate DEEP p-well bias of about 0 volts. The p wells 62′, 68′ provide access contacts for biasing a deep p-well implant 72′ to be described hereinbelow. The p well 62′ closest to the p_pixels 54′ surrounds the entire pixel region (not shown) to form a depletion region around the edge of the p_pixels 54′ to ensure that the p_pixels 54′ pinch off substrate bias in proximity to the n+ return contact 36′ to be described hereinbelow (The p-well 62′, 68′ naturally come with fabrication. A p-well is everywhere on a wafer containing at least one p_imager 50′, unless another layer is intentionally implanted. An n-well is one such layer, which is only used when required. This is the difference between n and p-well. There is never a naked silicon surface, since it must be passivated. According to an embodiment of the present invention, p-well is chosen for passivation. The same p-well passivation is applied to the n_image 50 above).
The substrate bias contact 52′, the p_pixels 54′, and the wells 62′, 68′, 72′ may be formed in an n-type epitaxial layer 74′, preferably made of silicon, which overlies an n-type substrate 76′. The epitaxial layer 74′ may have a relatively high resistivity greater than about 10,000 ohm-cm. According to an embodiment of the present invention, the resistivity of the p-type epitaxial layer 74′ may preferably be greater than about 100 ohm-cm, more preferably about 10,000 ohm-cm. However, substrate bias is applicable to a p-type epitaxial layer 74′ having any resistivity. To achieve substantially full depletion of the p-type epitaxial layer 74′, in a preferred embodiment, the p-type epitaxial layer 74′ may be composed of intrinsic silicon or lightly doped (shown as n− in FIG. 5). Since maximum depletion depth varies as the square root of the resistivity for silicon, for a resistivity of greater than about 10,000 ohm-cm, full depletion may be achieve for a depth of the n-type epitaxial layer 74′ in the range of about 300 microns (and 30 microns without substrate bias). A positive voltage (greater than about 3.3V) is applied to the bias contact 52′ to deplete the n-type epitaxial layer 74′ in a depletion region extending the full n-type epitaxial layer 74′ thickness below each of the p_pixels 54′.
In operation, charge collects below the p_pixels 54′ (not shown). As the n-type epitaxial layer 74′ is depleted of majority carriers (i.e., in this case electrons), there is a danger that the n+ return contact 36′ may short to a substrate bias depletion edge 78′ shown. To prevent shorting, a sufficient potential barrier needs to be produced between the n+ return contact 36′ and the substrate bias depletion edge 78′ to prevent (electron) leakage current flow. The required barrier height is primarily dependent on the negative bias potential applied to the p well 62′ closest to the p_pixels 54′, on the width of the n-well 38′ (which varies with design rules), and on the dimensions of the n+ return contact 36′.
CCE improvements using substrate bias are discussed hereinbelow. FIG. 6A illustrates how a charge cloud (packet) 79 travels from a back side 60 of the n_imager 50 to the front side 58 by an electric field generated by a n_pixel 54 and substrate bias for the n_imager 50 of FIG. 3. FIG. 6B plots potential with depth as substrate bias voltage changes from 0 to −20 V for the imager 50 of FIG. 3. To achieving good CCE performance in a CMOS imager, transit time needs to be minimized in moving signal charge from the back side 50 of the n_imager 50 to the target pixel's collection region without significant diffusion among neighboring pixels. Referring again to FIG. 6B, the slopes of the curves shown represent a field strength that increases with substrate bias. During transit, thermal diffusion takes place, thereby increasing the charge cloud size, which follows a Gaussian distribution of,




  
    
      
        n
        =
        
          
            
              (
              
                N
                
                  4
                  ⁢
                  π
                  ⁢
                  
                    
                  
                  ⁢
                  Dt
                
              
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              1
              /
              2
            
          
          ⁢
          
            exp
            ⁡
            
              (
              
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                    ⁢
                    Dt
                  
                
              
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        (
        1
        )
      
    
  




where N is the number of electrons per unit area, D is the diffusion coefficient for high resistivity silicon (39 cm2/s), x is distance about the center of the distribution (cm) and t is time (sec). For a fully depleted pixel, the resultant cloud diameter at the front side 58 of the n_imager 50 is only a function of sensor thickness and applied voltage:





  
    
      
        DIA
        =
        
          
            [
            
              
                
                  2
                  ⁢
                  kT
                
                q
              
              ⁢
              
                
                  x
                  THICK
                  2
                
                
                  
                    V
                    SUB
                  
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                    V
                    PIX
                  
                
              
            
            ]
          
          
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            /
            2
          
        
      
    
    
      
        (
        2
        )
      
    
  




where xTHICK is the sensor thickness, VPIX is the pixel photo diode bias, VSUB is the substrate voltage and kT/q is the thermal voltage (0.025 V at 300 K). The cloud diameter, DIA, is defined as ±1σ where 68.2% of the charge is contained in the cloud distribution (i.e., 2σ=2(2 Dt)1/2). The corresponding transit time is,





  
    
      
        tr
        =
        
          
            x
            THICK
            2
          
          
            u
            ⁡
            
              (
              
                
                  V
                  SUB
                
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                  PIX
                
              
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        (
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where μ is the silicon mobility (1500 cm2/V-s). It is important to note that the diffusion cloud size increases proportionally to sensor thickness and decreases by the square-root of applied voltage (VSUB+VPIX).

FIGS. 7A and 7B plot transit time and the resultant Gaussian 2σ charge cloud diameter as function of applied voltage for various fully depleted epi (FIG. 3) thicknesses (5, 10, 15, 20, 30, 40 and 50 μm). Usually cross talk becomes a serious performance issue when the cloud diameter grows to be comparable to the pixel size. For example, a 30 μm thick sensor with VPIX=3.3 V, VSUB=0 V applied produces a 2σ cloud diameter of 7.5 μm. For a 7.5 μm pixel this diameter still leaves 32% of the signal outside the target pixel.
FIGS. 8A and 8B plot ‘point-spread’ responses from Eq. (1) for the same thicknesses as FIG. 7B with and without −20 V substrate bias applied (VPIX=3.3 V), respectively. Note without substrate bias applied (FIG. 8A), the 8σ ‘wings’ of the point-spread remain inside an 8 μm pixel for tTHICK=10 μm. With −20 V substrate bias (FIG. 8B) the thickness may be increased to 20 μm with full charge confinement. Also shown on the VSUB=0 V plot is a PISCES simulated data point to cross check the equations above.
Fe-55 x-ray photons represent a nearly perfect point source stimulus that produces an initial 2σ cloud diameter of approximately 0.35 μm. FIG. 9 presents a Fe-55 x-ray image taken by a 15 μm thick front side illuminated CMOS minimal array (as described in J. Janesick, J. Cheng, J. Bishop, J. Andrews, J. Tower, J. Walker, M. Grygon, and T. Elliott, “CMOS minimal array”, Proc. SPIE 6295, (2006)) with 8 μm pixels. As used herein, a minimal array is defined as a CMOS imager that is stripped of many commercial CMOS features leaving only pixels, row and column address decoders and switches, pixel clock drivers and clamping analog circuitry. FIG. 10 shows a corresponding simulated Monte Carlo response for aforementioned CMOS imager array with and without substrate bias applied. Note that the simulation plots assume DMAX=8σ diffusion circles to contain all charge. The solid and dotted circles represent cloud sizes for VSUB=0 V and −20 V respectively. Ideally, the image and the simulation should contain only x-ray events that only involve the target pixel, however because of diffusion, signal charge expands into neighboring pixels. Each x-ray event produces a different cloud size depending on where in the device the x-ray is absorbed. An x-ray absorbed at the immediate front surface creates a very small point-spread equal to the initial cloud generated by the x-ray photon. Those x-rays that interact near the epi-substrate interface exhibit the largest point-spreads (corresponding to DMAX labeled in the figure). For example, for 15 μm silicon DMAX=15.2 μm for VSUB=0 V and decreases to DMAX=5.2 μm for VSUB=−20 V. FIGS. 11 and 12 are similar results for 25 μm epi silicon producing much greater charge diffusion clouds. For 25 μm silicon substrate bias is essential for an 8 μm pixel imager to achieve good CCE performance.
Even with high resistivity silicon and substrate bias that x-ray split events may still be generated. To deal with split events, split events are either discarded keeping only single pixel events, or the pixels that make up a split event are summed and degraded energy resolution is tolerated (because of the read noise increase). For example, FIG. 13 presents a single event x-ray histogram taken from a 15 μm thick CMOS minimal array. Excellent energy resolution is achieved showing low level x-ray lines from a basalt target and Fe-55 source that would not be seen unless split events were discarded by a computer. For minimum charge splitting, very thin sensors are required. For example, FIG. 14 presents simulated data for a fully depleted 5 μm sensor built on SOI epi (refer to FIG. 18 discussions). With substrate bias applied, x-ray point-spread for most events is very close to the limit of the initial cloud diameter. Curiously, ultra thin (2 μm) very small pixel (<1.5 μm) commercial BSI imagers are limited by this diameter. However, no single pixel events are observed since the initial cloud diameter is always greater than the pixel size.
The substrate biasing method of the present invention is useful for both front side and back side illuminated imagers to reduce crosstalk and to improve modulation transfer function (MTF) performance. MTF is routinely used to specify CCE performance. Diffusion MTF may be evaluated by taking the Fast Fourier Transform (FFT) of the point-spread responses shown in FIG. 8A. Performing this operation produces the results shown in FIGS. 15A and 15B. Note that spatial frequency is given in units of cycles/pixel with Nyquist being at 0.5 cycles/pixel. Also shown is the pixel MTF response assuming a 100% fill factor. For example, for VSUB=0 V plot (FIG. 15A), the diffusion and pixel MTF are 0.33 and 0.63 respectively for 30 μm thick silicon. The overall MTF is 0.20. The net MTF increases to 0.54 when a substrate bias of −20 V is applied (FIG. 15B). There is one data point produced by PISCES to check MTF results (50 μm silicon, VPIX=3.3 V and VSUB=0 V). PISCES may provide a sinusoidal stimulus at a desired optical wavelength to generate MTF directly. Agreement between equations and PISCES has been excellent.
MTF results in FIGS. 15A and 15B again show the difficulty for thick CMOS imagers to achieve high CCE. The problem is compounded by the reduction of photo diode voltage as signal charge collects. This issue is particularly critical for the n—5TPPD pixel which begin to operate with a potential that is approximately half the sense node voltage without charge (i.e., 1.5 V for 3.3 V processing). The PPD potential collapses with signal taking potential to near 0.5 V degrading CCE significantly. Therefore, substrate bias is important for scientific CMOS imagers.
The point-spread analysis presented above assumes that the p-type epitaxial layer 74 of FIG. 3 is fully depleted and produces a constant electric field throughout the p-type epitaxial layer 74. An imager fabricated with a high resistivity epitaxially grown substrate (epi) may achieve this condition and potentially deplete to 50 μm with 3.3 V bias (refer to FIG. 16A). In contrast, commercial imagers that use 5-10 ohm-cm silicon cannot achieve such a depletion depth with substrate bias (<2 μm is typical). Consequently MTF performance for commercial imagers is relatively poor for use in x-ray and near IR applications).
FIG. 16B plots epi silicon resistivity as a function of depth for 15 and 25 μm custom silicon typically used to fabricate CMOS imagers in the prior art. Note that the silicon exhibits a long epitaxial layer ‘auto doped substrate tail’ characteristic of epitaxial grown silicon. Full depletion for the epi layer may be achieved by removing some of the highly doped tail by thinning to the etch stops indicated (i.e., 13 and 23 μm respectively). FIGS. 17A and 17B present potential plots for 14 μm epi showing potential, doping and electric field profiles as the substrate bias is varied. For the epi layer silicon of FIGS. 17A and 17B, the etch stop required for full depletion and constant electric field is approximately 12 μm.
According to an embodiment of the present invention, the ‘auto doped substrate tail’ characteristic of epitaxial grown silicon may be eliminated by manufacturing a high resistivity imager employing silicon-on-insulator (SOI) technology. One version in the prior art of a back-illuminated imaging device that employs SOI technology is described in U.S. Pat. No. 7,238,583 (hereinafter “the '583 patent”). According to the '583 patent, the process of manufacturing a back-illuminated imager begins with a pre-manufactured SOI wafer comprising a mechanical substrate, a buried oxide (box) layer overlying the substrate which is highly doped, preferably with boron, and an undoped seed layer overlying the box layer. An epitaxial layer is grown overlying the seed layer at high temperature and imaging components are formed overlying the epitaxial layer. The high temperatures used during the epitaxial growth process causes the boron in the highly doped box layer to diffuse into the epitaxial layer to produce an imager with a built-in electric field which tends to drive electrons toward front side imaging components.
FIG. 18A depicts doping concentration versus thickness along an SOI wafer employed in both the '583 patent and a high resistivity imager employing substrate biasing techniques, according to an embodiment of the present invention. The SOI wafer includes a mechanical substrate layer 80, a box layer 82 overlying the substrate layer 80, and an epi layer 84 overlying the box layer 82. For a device manufactured using the process described in the '583 patent, the box layer 82 is highly doped and though a process of high temperature epitaxial growth of the epi layer 84 followed by annealing, the dopant in the box layer 82 diffuses into the epi layer 84 to produce a doping profile 86 has a maximum at an interface 90 between the box layer 82 and the epi layer 84 which decreases montonically toward the front side of the imager. A device exhibiting the doping profile 86 as manufactured using the process described in the '583 patent is called a ‘self accumulated’ profile, which exhibits an artificially exaggerated doping tail. For visible imaging applications, the substrate 80 of the '583 patent is thinned to the box layer 82.
In contrast, an imager employing SOI technology according to an embodiment of the present invention uses a non-doped box layer 82′ overlying a highly doped mechanical substrate (e.g., p+ type) and epi layer 84′ which exhibits the non-accumulated profile 90′ shown in FIG. 18A. In such circumstances, SOI is advantageous over conventional silicon because the ‘box’ oxide layer 82′ shown acts as an etch stop that makes thinning easy and precise. Also, SOI does not have the troublesome auto doped tail mentioned above. Instead high resistivity silicon with greater than about 100 ohm-cm high resistivity epi may be maintained throughout the epi 84′ to ‘box’ oxide layer 82′. Very thin imagers may be fabricated using SOI because the epi layer grown may be as thin as desired (refer to FIGS. 14 and 18B). Substrate bias may be applied to an imager employing SOI technology and having top side components (not shown) having the configuration shown in either FIG. 3 or 5 that are formed in the epi layer 84′ of FIGS. 18A and 18B). In such an imager, substrate bias may be adjusted as SOI thickness changes to maintain full depletion.
FIG. 19 is a process flow diagram illustrating exemplary steps for treating the back side of a CMOS imager employing SOI technology to produce the non-accumulated profile 90′, according to an embodiment of the present invention. At step 92, both the substrate 80′ and the box layer 82′ are removed using standard back side illumination passivation techniques. At step 94, a ‘flash oxide’ of about 15 angstroms (i.e., an ultrathin oxide layer) is grown on the epi layer 84′ via high humidity at 100 C (Flash oxide is equal to a mature native oxide after several years of growth.) According to an embodiment of the present invention, at optional step 96A, a mono-layer of metal, preferably a high work function metal, is applied to the ‘flash oxide’, known as a “flash gate.” According to still another embodiment of the present invention, at step 96B, a mono-layer of metal may be formed on the ‘flash oxide’ using a chemisorption process. ork function metal attracts electrons. Electrons in turn attract holes from the (preferably, but not limited to, boron doped) epi layer 84′. The holes ‘accumulate’ at the back surface. The hole gradient formed generates an electric field that pushes signal electrons away from the back side. At optional step 98, an anti-reflective coating may be grown underlying the mono-layer of metal.
CMOS imagers manufactured using chemisoption may have a high quantum efficiency typically in the range of about 90%. FIG. 18B plots theoretical quantum efficiency with wavelength for a variety of epi thicknesses (without anti reflection coatings applied). Note that the UV QE is not dependent on thickness as visible and near IR wavelengths are. This characteristic is attractive for semi ‘IR blind’ imagers. Ultra thin detectors are also required by commercial back side illuminated imagers with very small pixels.
Although certain embodiments of the present invention pertaining to back-illuminated imagers have employed backside treatments that include “flash gate”, chemisorption, and accumulated ultra-thin silicon-on-insulator (UTSOI), backside treatments of the present invention may also employ ion implantation with laser annealing, unaccumulated UTSOI, etc.
FIG. 20 depicts a side view of a n—5TPPD ring pixel 100, while FIG. 21 depicts a top-down view of the ring pixel 100, according to an embodiment of the present invention. The ring pixel 100 may be implemented with high resistivity semiconductor substrate 102 and substrate bias depletion. The ring pixel 100 includes a sense node 104 formed in the semiconductor substrate 102 substantially in the center of the ring pixel 100. The sense node 104 comprises a highly doped n+ region 106 formed in a highly doped p+ region 108 (hereinafter the p-well 108). A ring shaped transfer gate 110 is formed about the sense node 104. A ring shaped pinned photo diode (PPD) 102, is formed about the ring shaped transfer gate 110. An optional ring shaped implant 114 comprising a buried channel 114A and a p+ region 114B may be formed about the ring shaped PPD 112 for generating a small ‘potential step’ to help charge move towards the ring shaped transfer gate 110. The optional ring shaped implant 114 may be used for very large pixels that need this assistance. Read MOSFETS 116 abut one side of the p+ regions 114B. Referring now to FIG. 21, the ring-based architecture of the ring pixel 100 is configured so that charge (in this case electrons) is transferred from all directions originating from the ring shaped PPD 112 through the ring shaped transfer gate 110 to the central sense node 104 (indicated by the arrows 118), thereby reducing charge transfer time.
The ring pixel 100 may be substituted for the n_pixels 54 of FIG. 3 to form an n_imager. (Likewise, a ring pixel having inverted polarities and biasing voltages may be substituted for the p_pixel of FIG. 5 to for a p_imager). The ring pixel 100 is connected to substrate bias through a substrate bias contact (not shown), which is >100 um from the pixels. The substrate bias is its own return, since it references the entire substrate of the chip. Normally it is at ground (0 V) just like the p-well in which the 3 read MOSFETs are located. Certain embodiments of the present invention allow it to assume a different potential.
However, unlike sense node regions (not shown) associated with the n_pixels 54 depicted in FIG. 3, which have p+ return contacts 36, the sense node 104 of the ring pixel 100 is not grounded, but floating. However, the read MOSFETs 116 to the side of the ring pixel 100 still need to be located in a grounded p-well and may influence, just as for FIG. 3, how much substrate bias may be applied. The combination of the floating sense node 104 and the grounded p-well of the MOSFETs 116 permits the p-well 108 to have a minimum width comparable to that employed in n—3TPPD pixels. As a result, more substrate bias may be applied. Conventional CMOS pixel designs require a larger p-well width because the sense node and transfer gate are part of the same p-well that the MOSFETs share.
FIGS. 22 and 23 display the results of a PISCES simulation showing how the floating sense node 104 naturally repels signal carriers (indicated by the arrows 120) to the PPD 112, which permits the application of additional substrate bias. The signal carriers 120 (in this case electrons, e−) generated deep in the substrate 102 diffuse away from the sense node 104 into the PPD 112. Note that the carriers 120 are initially attracted toward the sense node 104 but are reflected away when they come within about 3 μm of a front side surface 122 of the ring pixel 100. At that distance, the electric fields change polarity at an interface 124 of the p-well 108, thus forcing the signal carrier 120 to the region of the PPD 112.
Referring again to FIGS. 20 and 21, to generate fringing fields at the very edges of the PPD 112, the implant 114 is employed, which ‘steps’ potential in the direction of the sense node 104 to further decrease transfer time. The ring pixel 100 may deliver high speed performance for 5TPPD pixels greater than 24 μm in thickness.
Although certain embodiments of the present invention refer to designs employing substrate bias using UTSOI technology with the bulk imager 50 of FIG. 3 or to a ring pixel 100 of FIG. 20 separately, other embodiments of the present invention (not shown) may combine ring pixels overlying a substrate whose backside treatment includes UTSOI with or without the application of substrate bias.
It is to be understood that the exemplary embodiments are merely illustrative of the invention and that many variations of the above-described embodiments may be devised by one skilled in the art without departing from the scope of the invention. It is therefore intended that all such variations be included within the scope of the following claims and their equivalents.

## Claims

1. A CMOS ring pixel, comprising:
a semiconductor substrate;
a substrate bias contact coupled to a voltage source, wherein the polarity of the voltage source depends upon the polarity of the bias contact;
a sense node formed in the semiconductor substrate and positioned substantially in the center of the ring CMOS pixel;
a ring-shaped transfer gate formed surrounding the sense node; and
at least one ring-shaped photodiode formed about the transfer gate.

2. The CMOS pixel of claim 1, wherein the semiconductor substrate is of a first conductivity type and the sense node is highly doped and of a second conductivity type.

3. The CMOS pixel of claim 2, wherein the sense node comprises a highly doped n+ region formed in a highly doped p+ region.

4. The CMOS pixel of claim 2, wherein the sense node comprises a highly doped p+ region formed in a highly doped n+ region.

5. The CMOS pixel of claim 2, further comprising:
a reset transistor of the second conductivity type in signal communication with the sense node;
a source follower transistor of the second conductivity type in signal communication with the sense node;
a row select transistor of the second conductivity type in signal communication with the source follower transistor.

6. The CMOS pixel of claim 5, wherein the reset transistor, the source follower transistor, and the row select transistor are located substantially to one side of the at least one CMOS pixel substantially adjacent to the photodiode.

7. The CMOS pixel of claim 5, wherein the photodiode is in signal communication with the sense node.

8. The CMOS pixel of claim 2, further comprising an implant formed about the photodiode configured to step potential in a direction toward the sense node.

9. The CMOS pixel of claim 8, wherein the implant formed about the photodiode further comprises a buried channel and highly doped region of a first conductivity type adjacent to and formed about the buried channel.

10. The CMOS pixel of claim 1, wherein the sense node is operable to be floating.

11. The CMOS pixel of claim 1, further comprising driver logic formed in the semiconductor substrate.

12. The CMOS pixel of claim 11, wherein the driver logic is at least one of an address encoder, a pixel bipolar driver, and a multiplexor.

13. A method for forming a ring CMOS pixel, comprising the steps of:
providing a semiconductor substrate;
forming a substrate bias contact in the semiconductor substrate and coupling the bias contact to a voltage source, wherein the polarity of the voltage source depends upon the polarity of the substrate bias;
forming a sense node in the semiconductor substrate positioned substantially in the center of the ring CMOS pixel;
forming a transfer gate in a ring shape surrounding the sense node; and
forming at least one photodiode in a ring shape about the transfer gate.

14. The method of claim 13, wherein the semiconductor substrate is of a first conductivity type and the sense node is highly doped and of a second conductivity type.

15. The method of claim 13, wherein the step of forming a sense node comprises the steps of forming a highly doped n+ region in a highly doped p+ region, wherein the highly doped p+ region is formed in the semiconductor substrate.

16. The method of claim 13, wherein the step of forming a sense node comprises the steps of forming a highly doped p+ region in a highly doped n+ region, wherein the highly doped n+ region is formed in the semiconductor substrate.

17. The method of claim 13, further comprising:
forming a reset transistor of the second conductivity type in signal communication with the sense node;
forming a source follower transistor of the second conductivity type in signal communication with the sense node;
forming a row select transistor of the second conductivity type in signal communication with the source follower transistor, wherein the reset transistor, the source follower transistor, and the row select transistor are formed in the semiconductor substrate.

18. The method of claim 17, wherein the reset transistor, the source follower transistor, and the row select transistor are formed substantially to one side of the at least one CMOS pixel substantially adjacent to the photodiode.

19. The method of claim 18, wherein the implant formed about the photodiode further comprises a buried channel and highly doped region of a first conductivity type adjacent to and formed about the buried channel.

20. The method of claim 13, wherein the photodiode is in signal communication with the sense node.

21. The method of claim 13, further comprising forming an implant about the photodiode configured to step potential in a direction toward the sense node.

22. The method of claim 13, wherein the step of providing a semiconductor substrate further comprises the steps of:
providing a silicon-on-insulator substrate including a mechanical substrate, an insulator layer substantially overlying the mechanical substrate, and a seed layer substantially overlying the insulator layer;
growing the semiconductor substrate substantially overlying the seed layer; and
removing the mechanical substrate and at least a portion of the insulator layer.

23. The method of claim 22, further comprising the steps of forming an ultrathin oxide layer substantially underlying the semiconductor substrate and forming a mono layer of metal substantially underlying the ultrathin oxide layer.

24. The method of claim 23, wherein the ultrathin oxide layer is formed by a chemisorption process.