• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
  • 引用文献
  • 法律状态
  • 优先权
    • 专利摘要:
    The invention relates to a "front face" type image sensor comprising a substrate (22-1) of semiconductor material; an active layer (10) of semiconductor material; a matrix of photodiodes (14, 18) made in the active layer; and an insulator layer (22-2) between the substrate (22-1) and the active layer (10).
    公开号:FR3027731A1
    申请号:FR1460236
    申请日:2014-10-24
    公开日:2016-04-29
    发明作者:Didier Dutartre
    申请人:STMicroelectronics SA；
    IPC主号:
    专利说明:

    [0001] The invention relates to matrix image sensors in CMOS type 5 technology "front face" (or front-side in English). Background Figure 1 shows a schematic cross-sectional view of a front-facing CMOS image sensor pixel. In this figure and the following, the various elements of the pixel are represented with dimensions making the figures 10 intelligible and are not represented on the scale. The doping levels of the P conductivity type zones are represented by shades of gray which are all the more dark as the levels are high. The image sensor is formed in an active layer 10, often of conductivity type P, having a doping level marked P-. The layer 10 is formed on a substrate 12, often of conductivity type P. The doping level of the substrate, noted P +, is generally much higher than the doping level of the active layer 10, typically 3 decades. The layer 10 may have a thickness of between 3 and 6 μm, while the substrate may have a thickness of 780 μm. A buried zone 14 of N conductivity type, near the upper face of the layer 10, forms a photodiode with the layer 10. As shown, the portion of the layer 10 above the zone 14 may have a doping level higher than the layer 10 to ensure passivation of the upper interface. The upper face of the layer 10 carries various elements for controlling the pixel, in particular a transfer gate TG. These elements and other metallic tracks are embedded in a passivation layer 16. The pixel is laterally insulated from its neighboring pixels by trench insulators 18, generally of semiconductor oxide, which extend over the entire thickness of the the active layer 10. Alternatively, the insulation between pixels can be achieved by over-doping (type P) with respect to the layer 10, but this insulation is known to be less efficient both from an electrical point of view than from an optical point of view. The spacing between the trenches 18 defines the size of the pixels.
    [0002] In the case of a color sensor, color filters 18 are formed on the layer 16 in correspondence with the pixels. The filters 18 are generally surmounted by individual collimation lenses 20. During operation, during an integration phase, the photons absorbed in the active layer, zone 10 of the photodiode, generate electrons which are stored in the zone 14 of the photodiode. photodiode. At the end of the integration phase, the stored charge is proportional to the amount of light received by the photodiode throughout the integration phase. At the end of the integration phase, the stored charge is transferred by means of the transfer gate TG to the control elements.
    [0003] A recurring problem with this pixel structure is that one can observe the generation of carriers in the photodiode even in the absence of light, what is called a dark current. The dark current is not the same for all the pixels, or between two phases of integration of the same pixel. This phenomenon produces a visible noise in the images captured, it is particularly troublesome in low light conditions. The origins of the dark current are poorly known. An identified origin is the presence of imperfections or impurities in the semiconductor and the different interfaces between the active layer, zone 10, and the insulating materials that surround it. The semiconductor material and the insulating material are not structurally equivalent, resulting in "construction" defects at the interfaces. All these faults are electrically active. Interface faults can be neutralized by degenerating the semiconductor portion of the silicon-insulator interface. Degeneration consists of doping the semiconducting part in excess so that it has the same properties as a metal in which the generation-recombination phenomena are naturally balanced. Perfect degeneration is impossible, so defects remain, but in less quantity. In the case of the active zone 10 of the P type, the electrons generated by the interface defects diffuse towards the storage zone, zone 14. These electrons participate in the dark current of the photodiode. To limit this phenomenon, the interfaces that can present a bad surface state, such as the interface between the insulating trenches 18 and the layer 10, are neutralized. As shown, a P-type layer having a higher doping level. that the active layer 10 lines the trenches 18.
    [0004] The level of doping may be that, P +, of the substrate. Thus, the electrons generated that can diffuse to the zone 14 are fewer. Despite these measurements, a dark current remains in the "front-end" image sensors.
    [0005] Summary A "front-end" type image sensor is generally provided, comprising a substrate made of semiconductor material; an active layer of semiconductor material; a matrix of photodiodes made in the active layer; and an insulator layer between the substrate and the active layer.
    [0006] The insulating layer may be a silicon oxide layer having a thickness chosen to allow reflection of photons in the visible range. The sensor may comprise, between the insulating layer and the active layer, an intermediate layer of the same conductivity type as the active layer, having a doping level higher than that of the active layer.
    [0007] In operation, the substrate may be polarized at a potential lower than that of the active layer. The substrate and the insulating layer may be an integral part of an SOI substrate. The sensor may comprise a passivation layer on the active layer; a matrix of color filters on the passivation layer; and a collimating lens array on the filter matrix. A method of making a "front-face" image sensor may include the steps of forming an active layer on an SOI substrate; forming a matrix of photodiodes in the active layer; and arranging a matrix of color filters and collimating lenses on the active layer. The method may comprise the step of forming an intermediate layer on the insulating layer of the same conductivity type as the active layer and having a doping level higher than that of the active layer.
    [0008] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments will be set forth in the following description, given in a nonlimiting manner in relation to the appended figures among which: FIG. 1, previously described, represents a schematic sectional view of a sensor pixel, front-facing CMOS image; FIG. 2 is a schematic sectional view of a reduced dark-current pixel embodiment; and FIG. 3 is a schematic sectional view of another embodiment of a reduced dark-current pixel.
    [0009] DESCRIPTION OF EMBODIMENTS The inventor has studied the hypothesis that an electron source contributing to the dark current in a "front-face" type image sensor could also come from the substrate. Indeed, although the substrate is doped P, that is to say that the positive carriers are in the majority, there are still electrons in the NpN relationship. where np and nn are the numbers of positive and negative carriers respectively, and n is the intrinsic concentration of the semiconductor material at a given temperature. These negative carriers or electrons could under certain conditions migrate from the substrate to the active layer, even if the P doping level of the active layer is lower than that of the substrate. According to this hypothesis, the contribution of the substrate to the dark current could be suppressed by electrically isolating the active layer from the substrate. The substrate-active zone isolation can be implemented by producing the image sensor on an SOI (Silicon On Insulator) type substrate.
    [0010] FIG. 2 illustrates an image sensor pixel thus produced. This pixel may be identical in every respect to that of FIG. 1 except that the substrate is an SOI type substrate comprising a solid zone 22-1 made of P-type silicon, surmounted by a layer of silicon oxide 22. 2. The oxide layer 22-2 may have a thickness of between 10 and 200 nm. By limiting the thickness range to 100-200 nm, this layer behaves like a mirror for photons whose wavelength is around the visible spectrum. The incident photons thus reflected to the active layer contribute to the charge of the photodiode. This results in an increase in pixel sensitivity. Figure 3 illustrates a variant of the pixel of Figure 2. The layer 22-2 is used as the dielectric of a capacitor. The substrate 22-1 is biased at a potential V1 below the potential of the active zone 10 which is generally grounded. The potential V1 applied is therefore negative In this case, the active zone 10-oxide interface reacts to the potential V1 by the accumulation of the majority carriers of the active zone 10 at the interface. The potential induced by the positive charge accumulation at the active zone 10-oxide interface will be hereafter called V2 and the potential difference across the capacitor will then be V2-V1. The potential V1 to be applied depends on the thickness of the layer 22-2, so in fact the value of the capacitor. Typically V2-V1 values of 0.2 to 0.4 volts can be used for a thickness of 20 nm and values of 1.5 to 3 volts for a thickness of 150 nm. The layer 24 of FIG. 2 and the capacitor configuration of FIG. 3 are optional. These solutions serve to improve the results obtained thanks to the insulating layer 22-2 without V1 bias applied. The inventor has observed that the insulating layer 22-2 used without these options already substantially reduces the effect of the substrate on the dark current.
    权利要求:
    Claims (9)
    [0001]
    REVENDICATIONS1. "Front face" type image sensor, comprising: - a substrate (22-1) of semiconductor material; an active layer (10) of semiconductor material; and - a matrix of photodiodes (14, 18) made in the active layer; characterized in that it comprises an insulator layer (22-2) between the substrate (22-1) and the active layer (10).
    [0002]
    The sensor of claim 1, wherein the insulator layer (22-2) is a silicon oxide layer having a thickness selected to allow reflection of photons from the visible range.
    [0003]
    3. Sensor according to claim 1, comprising, between the insulating layer (22-2) and the active layer (10), an intermediate layer (24) of the same conductivity type as the active layer, having a doping level. greater than that of the active layer. 15
    [0004]
    4. The sensor of claim 1, wherein, in operation, the substrate is biased at a potential (V1) lower than that (V2) of the active layer.
    [0005]
    5. Sensor according to claim 1, wherein the substrate and the insulating layer form an integral part of an SOI substrate.
    [0006]
    The sensor of claim 1, comprising: a passivation layer (16) on the active layer; a matrix of color filters (18) on the passivation layer; and - a collimation lens array (20) on the filter array.
    [0007]
    7. A method of producing a "front face" type image sensor, comprising the steps of: forming on an SOI substrate an active layer (10); forming an array of photodiodes (14, 18) in the active layer; and- arranging a matrix (18, 20) of color filters and collimating lenses on the active layer.
    [0008]
    8. The method of claim 7, comprising the step of: - forming an intermediate layer (24) on the insulating layer, of the same conductivity type as the active layer and having a doping level higher than that of the active layer.
    [0009]
    9. The method of claim 7, comprising the following step: - polarizing the substrate at a potential (V1) less than that (V2) of the active layer.
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    同族专利:
    公开号 | 公开日
    FR3027731B1|2018-01-05|
    CN105552092A|2016-05-04|
    CN105552092B|2019-07-02|
    US20170271392A1|2017-09-21|
    US20160118431A1|2016-04-28|
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    引用文献:
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    FR3064398B1|2017-03-21|2019-06-07|Soitec|SEMICONDUCTOR TYPE STRUCTURE ON INSULATION, ESPECIALLY FOR A FRONT-SIDE TYPE IMAGE SENSOR, AND METHOD FOR MANUFACTURING SUCH STRUCTURE|
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    CN112291539A|2020-10-29|2021-01-29|维沃移动通信有限公司|Imaging device and electronic apparatus|
    法律状态:
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    2016-04-29| PLSC| Search report ready|Effective date: 20160429 |
    2016-09-21| PLFP| Fee payment|Year of fee payment: 3 |
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    2019-09-19| PLFP| Fee payment|Year of fee payment: 6 |
    2021-07-09| ST| Notification of lapse|Effective date: 20210605 |
    优先权:
    申请号 | 申请日 | 专利标题
    FR1460236|2014-10-24|
    FR1460236A|FR3027731B1|2014-10-24|2014-10-24|IMAGE SENSOR FRONT PANEL WITH REDUCED DARK CURRENT ON SOI SUBSTRATE|FR1460236A| FR3027731B1|2014-10-24|2014-10-24|IMAGE SENSOR FRONT PANEL WITH REDUCED DARK CURRENT ON SOI SUBSTRATE|
    US14/840,164| US9704903B2|2014-10-24|2015-08-31|Front-side imager having a reduced dark current on SOI substrate|
    CN201520728928.1U| CN205248275U|2014-10-24|2015-09-18|Front side image sensor|
    CN201510601251.XA| CN105552092B|2014-10-24|2015-09-18|On soi substrates with the front side imager of reduced dark current|
    US15/617,748| US20170271392A1|2014-10-24|2017-06-08|Front-Side Imager Having a Reduced Dark Current on a SOI Substrate|
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