WO1996010843A1 - Agencement a semi-conducteurs photosensible - Google Patents

Agencement a semi-conducteurs photosensible Download PDF

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Publication number
WO1996010843A1
WO1996010843A1 PCT/AU1995/000644 AU9500644W WO9610843A1 WO 1996010843 A1 WO1996010843 A1 WO 1996010843A1 AU 9500644 W AU9500644 W AU 9500644W WO 9610843 A1 WO9610843 A1 WO 9610843A1
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WO
WIPO (PCT)
Prior art keywords
semiconductor
photosensing
pixel
substrate
terminal
Prior art date
Application number
PCT/AU1995/000644
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English (en)
Inventor
John Frank Siliquini
Lorenzo Faraone
Original Assignee
The University Of Western Australia
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from AUPM8539A external-priority patent/AUPM853994A0/en
Priority claimed from AUPM9859A external-priority patent/AUPM985994A0/en
Application filed by The University Of Western Australia filed Critical The University Of Western Australia
Priority to AU35995/95A priority Critical patent/AU3599595A/en
Publication of WO1996010843A1 publication Critical patent/WO1996010843A1/fr

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14665—Imagers using a photoconductor layer
    • H01L27/14669—Infrared imagers
    • H01L27/1467—Infrared imagers of the hybrid type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by potential barriers, e.g. phototransistors
    • H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation
    • H01L31/1013—Devices sensitive to infrared, visible or ultraviolet radiation devices sensitive to two or more wavelengths, e.g. multi-spectrum radiation detection devices

Definitions

  • This invention relates to the field of imaging, and in particular to a photoconductor device structure suitable for two-dimensional infrared focal plane arrays.
  • IRFPAs infrared focal plane arrays
  • Hg.. Cd Te also known as HgCdTe, MCT, or mercury cadmium telluride
  • CCDs Charge Coupled Devices
  • Photoconductive Hg 1 ⁇ Cd ⁇ Te arrays are a mature technology, and have been the workhorse for high volume production of first generation sensor systems in the LWIR spectral region for the past two decades. In contrast, photovoltaic
  • Hg. Cd Te arrays are a comparatively immature technology. Although large two dimensional PV arrays have been demonstrated, these are in the short and medium wavelength infrared (SWIR and MWIR) spectral regions.
  • SWIR and MWIR short and medium wavelength infrared
  • a photosensing element comprising a plurality of pixels mounted as a two dimensional array on a suitable substrate, each said pixel including a photosensitive semiconductor and being adapted to be biased in use via a terminal connecting to said photosensitive semiconductor at an aspect thereof opposite to the aspect on which photon radiation is incident.
  • each said terminal is centrally located on each said pixel.
  • each said pixel includes a second terminal located on either side of said centrally located terminal.
  • each said pixel includes a main body of n-type compound semiconductor on top of said substrate.
  • each said pixel includes a cap layer of n type compound semiconductor on top of the said main body.
  • said main body is epitaxial.
  • said main body comprises Hg 1 ⁇ Cd ⁇ Te semiconductor, where x is approximately 0.21.
  • said cap layer comprises Hg.. Cd Te semiconductor, where x is approximately 0.25.
  • each said pixel includes a passivation layer covering the semiconductor surface (including the said cap layer).
  • the passivation layer may comprise an oxide/ZnS passivation layer.
  • each said terminal is located on top of the said passivation layer, to allow easy interconnection to a silicon chip comprising signal processing circuitry.
  • These terminals contact to the said cap layer only through suitable via holes opening through the said passivation layer.
  • each said pixel includes a plurality of semiconductor element portions located in layers on said substrate, each element portion having sensitivity to a predetermined bandwidth.
  • Preferably said predetermined bandwidths cover different wavelengths. In this manner the photo sensing element will be multi-spectral.
  • said semiconductor element portions each comprise epitaxial hetero unction layers.
  • said epitaxial heterojunction layers are separated by an electrically insulating layer and are continuously consecutive therewith.
  • said electrically insulating layer is a low doped semiconductor.
  • the starting Hg 1 ⁇ Cd ⁇ Te semiconductor material consists of epitaxially grown heterojunction layers in which a two-dimensional mosaic of photoconductors are fabricated.
  • the heterojunction layer provides high performance devices at greatly reduced power dissipation levels, while by suitably forming the contacts, allows for the high density integration of photoconductors in a two- dimensional array geometry with high fill factor.
  • the semiconductor element portions may also comprise epitaxially grown heterojunction layers.
  • the photosensing element is fabricated on a conventional single n-type epitaxial Mercury Cadmium Telluride layer on an insulating substrate.
  • indium diffused n+ "blocking" contacts can be formed at the two terminals. This type of blocking contact has also been shown to be effective in reducing the effects of minority carrier sweepout. The use of blocking contacts is a viable alternative in order to maintain high responsivities in the photoconductor.
  • the photosensing element may be connected to signal processing circuitry by forming an indium bump interconnect to each terminal.
  • This offers high performance devices at reduced power dissipation levels, and by suitably forming the contacts, allows for the high density integration of photoconductors in a two-dimensional array geometry with high fill factor.
  • a photosensing element comprising a plurality of pixels mounted as a two dimensional array on a suitable substrate, said substrate providing a surface for incident photon radiation on one side of said photosensing element, and each said pixel including a photosensitive semiconductor having a terminal connecting thereto on the opposite side of said photosensing element to said surface.
  • said terminal is centrally located on each said pixel.
  • each said pixel includes a second terminal located on either side of said centrally located terminal.
  • said terminal surmounts said pixel and each said pixel includes a second terminal either comprising said substrate or located adjacent to said substrate, in connection with each said pixel.
  • said second terminal also surmounts said pixel, adjacent to said terminal.
  • the terminal need not be centrally located.
  • said pixel includes a plurality of semiconductor element portions located in layers on said substrate, each element portion having sensitivity to a predetermined bandwidth.
  • Preferably said predetermined bandwidths cover different wavelengths. In this manner the photo sensing element will be multi-spectral.
  • said semiconductor element portions each comprise epitaxial heterojunction layers.
  • said epitaxial heterojunction layers are separated by an electrically insulating layer and are continuously consecutive therewith.
  • said electrically insulating layer is a low doped semiconductor.
  • the starting Mercury Cadmium Telluride semiconductor material consists of epitaxially grown heterojunction layers in which a two-dimensional mosaic of photoconductors are fabricated.
  • the heterojunction layer provides high performance devices at greatly reduced power dissipation levels, while the unique design allows for the high density integration of photoconductors in a two-dimensional array geometry with high fill factor.
  • the semiconductor element portions if utilised, may also comprise epitaxially grown heterojunction layers.
  • the photosensing element is fabricated on a conventional single n-type epitaxial Mercury Cadmium Telluride layer on an insulating substrate.
  • indium diffused n+ "blocking" contacts can be formed at the two terminals. This type of blocking contact has also been shown to be effective in reducing the effects of minority carrier sweepout. The use of blocking contacts is a viable alternative in order to maintain high responsivities in the photoconductor.
  • the photosensing element may be connected to signal processing circuitry by forming an indium bump interconnect to each terminal photoconductive device. This offers high performance devices at reduced power dissipation levels, and by suitably forming the contacts, allows for the high density integration of photoconductors in a two-dimensional array geometry with high fill factor.
  • a photo sensing element comprising a plurality of pixels mounted as a two dimensional array on a suitable substrate, each said pixel including a photosensitive semiconductor and being adapted to be biased in use with a voltage field extending away from the plane of said substrate.
  • each said pixel includes a first terminal located on said pixel, distal from said substrate, and a second terminal located toward, adjacent, or on said substrate away from said first terminal.
  • said substrate is electrically conductive and serves as a second terminal for each said pixel.
  • each said pixel includes a main body of n-type compound semiconductor.
  • each said pixel includes a highly doped layer of compound semiconductor between said first terminal and said main body.
  • each said pixel includes a highly doped layer of compound semiconductor between said second terminal and said main body.
  • said highly doped layer between said second terminal and said main body exhibits low free carrier absorption.
  • said main body comprises Hg.. Cd Te semiconductor, where x is approximately 0.21.
  • said main body is epitaxial.
  • said highly doped layers comprise Hg 1 ⁇ Cd ⁇ Te semiconductor, where x is approximately 0.24 to 0.25.
  • each said pixel includes a passivation layer covering the semiconductor surface.
  • the passivation layer may comprise an oxide/ZnS passivation layer.
  • said first terminals are located distal from the substrate, on top of the semiconductor material comprising each said pixel, to allow easy interconnection to a silicon chip comprising signal processing circuitry.
  • each said pixel includes a plurality of semiconductor element portions located in layers on said substrate, each element portion having sensitivity to a predetermined bandwidth.
  • Preferably said predetermined bandwidths cover different wavelengths. In this manner the photo sensing element will be multi-spectral.
  • said semiconductor element portions each comprise epitaxial heterojunction layers.
  • epitaxial heterojunction layers are separated by an electrically insulating layer and are continuously consecutive therewith.
  • said electrically insulating layer is a low doped semiconductor.
  • the starting Mercury Cadmium Telluride semiconductor material consists of epitaxially grown heterojunction layers in which a two-dimensional mosaic of photoconductors are fabricated.
  • the heterojunction layer provides high performance devices at greatly reduced power dissipation levels, while the unique design allows for the high density integration of photoconductors in a two-dimensional array geometry with high fill factor.
  • the semiconductor element portions if utilised, may also comprise epitaxially grown heterojunction layers.
  • the photosensing element is fabricated on a conventional single n-type epitaxial Mercury Cadmium Telluride layer on an insulating substrate.
  • indium diffused n+ "blocking" contacts can be formed at the two terminals. This type of blocking contact has also been shown to be effective in reducing the effects of minority carrier sweepout. The use of blocking contacts is a viable alternative in order to maintain high responsivities in the photoconductor.
  • the photosensing element may be connected to signal processing circuitry by forming an indium bump interconnect to each terminal photoconductive device. This offers high performance devices at reduced power dissipation levels, and by suitably forming the contacts, allows for the high density integration of photoconductors in a two-dimensional array geometry with high fill factor.
  • a photosensing element comprising a plurality of pixels mounted as a two dimensional array on a suitable substrate, each said pixel including a plurality of semiconductor element portions located in layers on said substrate.
  • each said semiconductor element portion comprises an epitaxial heterojunction layer.
  • epitaxial heterojunction layers are separated by an electrically insulating layer and are continuously consecutive therewith.
  • said electrically insulating layer is a low doped semiconductor.
  • each element portion has a sensitivity to a predetermined bandwidth.
  • said predetermined bandwidths cover different wavelengths. In this manner the photo sensing element will be multi-spectral.
  • a photoconductive sensing element for high density two dimensional infrared focal plane array applications comprising epitaxial heterojunction layers in which a two-dimensional mosaic of vertically biasable photoconductors are fabricated.
  • the heterojunction layer provides high performance devices at greatly reduced power dissipation levels, while the vertical design allows for the high density integration of photoconductors in a two-dimensional array geometry with high fill factor.
  • a method for fabricating a photoconductor array having at least one terminal on each pixel on the side of the substrate opposite to the side which radiation is incident.
  • a seventh aspect of the invention there is provided a method for fabricating a photoconductor array on a substrate, having a voltage field bias in each pixel extending away from the plane of said substrate.
  • Figure 1 is an illustration of a conventional (prior art) lateral photoconductor structure
  • Figure 2 is an illustration of one embodiment of a photoconductor construction according to the invention
  • Figure 3 is a side cross-section of a photoconductor construction with heterojunction contacts, according to a fourth embodiment of the invention.
  • Figure 4 is a side cross-section of a photoconductor construction with no heterojunction contacts, but with standard n-type MCT with n+ blocking contacts, according to a third embodiment of the invention.
  • Figures 5 to 8 are side cross-section views showing the production of a photoconductor array according to a second embodiment of the invention.
  • Figure 9 is a perspective view showing detail view of a hybridised photoconductor array and signal processing chip
  • Figure 10 shows a graph showing calculated responsivity as a function of device length for a vertical photoconductor with either heterojunction, n+ blocking, or ohmic contacts;
  • Figure 11 is a graph showing calculated noise voltages a function of device length for a vertical photoconductor with either heterojunction, n+ blocking, or ohmic contacts;
  • Figure 12 is a graph showing calculated detectivity as a function of device length for a vertical photoconductor with either heterojunction, n+ blocking, or ohmic contacts
  • Figure 13 is a graph showing calculated optimum length as a function of power dissipated for a vertical photoconductor with either heterojunction, n+ blocking, or ohmic contacts
  • Figure 14 is a graph showing calculated responsivity as a function of power dissipated for a vertical photoconductor with either heterojunction, n+ blocking, or ohmic contacts having device lengths of 10 ⁇ m , 14 ⁇ m and 82 ⁇ m respectively;
  • Figure 15 is a side cross-section of a photoconductor construction with heterojunction contacts, according to a fifth embodiment of the invention.
  • Figures 16 to 19 are side cross-section views showing the production of a photoconductor array according to the fifth embodiment of the invention.
  • Figure 20 is a top view of a photoconductor array according to the fifth embodiment of the invention.
  • Figure 21 is a side cross-section of a photoconductor construction with heterojunction contents, according to the sixth embodiment of the invention.
  • Figures 22 to 25 are side cross-section views showing the production of a photoconductor array according to the sixth embodiment of the invention.
  • Figure 26 is a top view of the photoconductor array according to the sixth embodiment of the invention.
  • Figure 27 is a side cross-section of a photoconductor construction with heterojunction contacts, according to a seventh embodiment of the invention
  • Figure 28 is a side cross-section of a photoconductor construction with heterojunction contacts, according to the seventh embodiment but utilising an indium bump for the common terminal of each pixel;
  • Figure 29 to 34 are side cross-section views showing the production of a photoconductor array according to the seventh embodiment of the invention.
  • Figures 35 and 36 show top views of multi-spectral photoconductor arrays having differing common configurations, according to the seventh embodiment.
  • FIG. 1 a prior art pixel is shown, in which a photosensitive semiconductor 11 is mounted on an insulating substrate 13.
  • Metal contacts 15 provide the terminals to bias the photosensitive semiconductor 11 with a voltage field having a polarity extending normal to the direction that radiation is incident.
  • the first set of embodiments of the invention comprises photosensitive elements in the form of pixels formed of photoconductor semiconductor material having terminals formed to provide in use a voltage field which is substantially transverse to, or at least across the plane of the substrate, substantially in the same direction of incident radiation.
  • the first embodiment is shown in figure 2, and comprises a photo sensing element including a pixel 17 having a photoconductor semiconductor material 19 fixed to a conducting substrate 21 which can conduct electricity and incident photons to be detected by the pixel.
  • a metal terminal 23 On top of the photoconductor semiconductor material 19 is a metal terminal 23, which provides an electrode connection to the pixel, and mounted on the light receiving face 25 on the opposite side of the pixel is a metal terminal 27.
  • the bias field is applied laterally along the plane of the semiconductor and at right angles to the incoming radiation
  • the bias field is applied axially of the incoming radiation and at right angles to the plane of the semiconductor.
  • the density and fill factor will only be limited by the photolithography and etching process used to delineate the individual detectors.
  • the array structure afforded by the configuration of the photosensing elements is suitable for hybrid flip chip mating to a silicon multiplexer.
  • This hybridisation technology is a well-established technology and in application to the invention, the front side of the detector array is aligned with the silicon multiplexer and one contact for each photoconductor is made using a previously deposited set of indium "bumps" fabricated on both the focal plane array and on the multiplexer chip.
  • this type of hybrid technology is not possible since two contacts per element are required on the upper semiconductor surface.
  • a large percentage of the area in the conventional lateral devices is taken up by contacts and metal interconnects which results in low density and low fill factors making them unsuitable for high density IRFPAs.
  • the second embodiment is the most preferred embodiment and is shown in figures 5 to 8.
  • the photosensitive element utilises the most preferred material structure for the LWIR vertical photoconductor array.
  • the starting material consists of a conducting substrate 29 that is transparent to LWIR radiation (e.g. GaAs) on which is grown a highly doped n-type layer 31 of Hg 1 Cd Te which has a slightly wider bandgap (delta x approximately 0.04) than the main body or absorbing Hg.
  • This bandgap engineered structure forms a heterojunction contact that has been shown to dramatically minimise the deleterious effects of minority carrier sweepout.
  • this layer In addition to this layer being highly doped thereby minimising the substrate/semiconductor contact resistance, it should also be relatively thick (approx. 5 ⁇ m) so as to minimise the series resistance to the photoconductor active area for situations in which an insulating substrate is used (see figures 3 and 4). It should be noted that since the array is backside illuminated, the free carrier absorption of this n+ layer 31 needs to be minimised.
  • the infrared absorbing layer 33 of n-type Hg 1 ⁇ Cd ⁇ Te of the appropriate x value and thickness is then grown.
  • Another highly doped n-type layer 35 of wider bandgap (delta x approximately 0.04) than the absorbing layer 33 of Hg 1 ⁇ Cd Te is then grown as the final layer.
  • the major fabrication steps required in order to realise the vertical photoconductor array are outlined in figures 6 to 8.
  • the procedure consists of first defining the photoconductor active or window regions and delineating each element of the array by mesa etching (e.g. by use of ion beam etching) of the semiconductor down to the underlying conducting substrate.
  • the next step is to make an electrical connection from the conducting Hg 1 ⁇ Cd ⁇ Te epitaxial layer to an external circuit in order to anodise the surface in the usual way. Because, by its very nature, the anodisation process tends to be a self regulating one in terms of thickness and insulating properties, the oxide layer formed during growth is extremely uniform over the entire exposed semiconductor surface area of the mesa structure.
  • the wafer is loaded into a vacuum evaporation system for deposition of the ZnS passivation layer 39.
  • the metal contact areas can then be defined by another photoresist process which allows the native oxide and ZnS to be etched away from the contact areas.
  • First terminals in the form of suitable metal contacts 41 or layers can then be deposited and defined using a liftoff process in order to form ohmic contacts to the Hg 1 Cd Te top mesa surface. It should be noted that this process results in a self-alignment of the passivation layer with the metal contacts. Consequently, the entire Hg 1 ⁇ Cd ⁇ Te surface is protected by either the anodic oxide/ZnS passivation layer or the multi-metal contact layer 41.
  • bumps 43 can be formed by another liftoff process for subsequent interfacing to suitable silicon read-out circuits via standard hybrid flip chip bonding.
  • Second terminals in the form of metal contacts or layers 45 are bonded to the substrate 29, between the LWIR pathways to the pixels, so as not to obscure the pixels from incoming radiation.
  • the conducting substrate 29 forms the common ohmic contact to all photoconductor elements which allows a close-packed two- dimensional mosaic of detectors to be formed on a minimum pitch. This material structure is considered optimum since it also offers the least contact and series resistance, thus maximising responsivity.
  • the photoconductor array may be produced by the use of standard fabrication processes which are compatible with existing Hg,. Cd Te photoconductor technology. It is believed that the photoconductor array may be produced by both existing or future Hg. Cd Te epitaxial growth processes.
  • the third embodiment as shown in figure 4 is such an example, wherein the array is formed on a conventional single n-type epitaxial Hg ⁇ Cd Te layer 47 on an insulating substrate 49.
  • indium diffused n+ "blocking" contacts can be formed on the top and common terminals. This type of blocking contact has also been shown to be effective in reducing the effects of minority carrier sweepout.
  • the fourth embodiment is shown in figure 3 and utilises heterolayers as used in the second embodiment (figs 5-8), formed on an insulating substrate 49.
  • the CdHgTe layers are etched down to remove the absorbing layer lying between the pixels 17, but leave intact the highly doped n+ layer 31 adjacent to the substrate 49.
  • Second terminals in the form of metal contacts 45 are then bonded to the layer 31 , at positions between the pixels .
  • the fifth embodiment is directed towards a photo sensitive element of a significantly different form of pixel construction than adopted in the preceding embodiments, although still following the general principle of element construction salient to the invention.
  • the photo sensitive element 61 comprises an insulating, photoconductive substrate 63 having an anti-reflection coating 65 on one surface for receiving incident infrared radiation, and a plurality of pixels 67 mounted as a planar two dimensional array on to the opposing surface of the substrate.
  • Each pixel of the array consists of a conventional lateral heterojunction photoconductor but rather than the two terminal contacts being on either side of the active or window area for receiving incident IR radiation, at least one contact terminal 71 is placed on the top passivated semiconductor surface 69 of each pixel, on the side opposite to the incident IR radiation. In the present embodiment, all of the terminals are located on top of the infrared detecting area of the pixel, opposite to the incident IR radiation. The detected radiation enters from the coating side of the substrate 63 into the absorbing main body semiconductor 73 and is detected by applying a suitable bias electric field between the two terminals.
  • a material structure for the LWIR lateral photoconductor array according to the fifth embodiment is shown in figures 15 to 19.
  • the starting material consists of a insulating substrate 63 (e.g. CdTe or CdZnTe) on which is grown an infrared absorbing layer of n-type Hg 1 ⁇ Cd ⁇ Te of the appropriate x value and thickness.
  • a insulating substrate 63 e.g. CdTe or CdZnTe
  • Another n-type layer of Hg 1 ⁇ Cd ⁇ Te which has a slightly wider bandgap (x approximately 0.04) than the absorbing Hg 1 ⁇ Cd ⁇ Te layer is then grown on top of the absorbing layer of Hg 1 ⁇ Cd ⁇ Te.
  • This bandgap engineered structure forms a heterojunction contact that has been shown to dramatically minimise the deleterious effects of minority carrier sweepout.
  • the major fabrication steps required in order to realise the lateral photoconductor array are outlined in figures 16 to 19.
  • the procedure consists of first defining the photoconductor active or window regions and delineating each element or the array by mesa etching (e.g. by use of ion beam etching) of the semiconductor down to the underlying insulating substrate 63.
  • the next step is to make an electrical connection from the conducting Hg. Cd Te epitaxial layer 73 to an external circuit in order to anodise the surface in the usual way. Because, by its very nature, the anodisation process tends to be a self regulating one in terms of thickness and insulating properties, the oxide layer 69 formed during growth is extremely uniform over the entire exposed semiconductor surface area of the mesa structure.
  • the wafer is loaded into a vacuum evaporation system for deposition of the ZnS passivation layer 65.
  • the metal contact areas can then be defined by another photoresist process which allows the native oxide and ZnS to be etched away from the contact areas at the required locations.
  • Suitable metal layers can then be deposited and defined using a lift-off process in order to form ohmic contacts 71 to the Hg. Cd Te top mesa surface.
  • indium "bumps" 75 can be formed by another lift-off process for subsequent interfacing to suitable silicon readout circuits via standard hybrid flip chip bonding.
  • the sixth embodiment of the invention is similar to the fifth embodiment in that the array consists of lateral heterojunction photoconductors in which all of the terminals are located on top of the infrared detecting area of the pixel, opposite to the incident IR radiation.
  • the array instead of having common metal lines (see figure 19, 71a and figure 20) running across the length of the array, common Indium bumps 77 are formed. These bumps 77, which act as metal commons for the photoconductors, may also be bump bonded to the silicon readout chip as previously described. This sort of structure will reduce the effects of series resistance on the photoconductor array. Also, it should be pointed out, that combinations of this sixth embodiment and the fifth embodiment can also be considered where the number of indium bumps used for the commons can vary from none to all photoconductors in the array.
  • the major fabrication steps required in order to realise the lateral photoconductor array are outlined in figures 22 to 25 in cross section.
  • the procedure consists of first defining the photoconductor active or window regions and delineating each element of the array by mesa etching (e.g. by use of ion beam etching) of the semiconductor down to the underlying insulating substrate.
  • the next step is to make an electrical connection from the Hg. Cd Te epitaxial layer to an external circuit in order to anodise the surface in the usual way. Because, by its very nature, the anodisation process tends to be a self regulating one in terms of thickness and insulating properties, the oxide layer formed during growth is extremely uniform over the entire exposed semiconductor surface area of the mesa structure.
  • the wafer is loaded into a vacuum evaporation system for deposition of the ZnS passivation layer.
  • the metal contact areas can then be defined by another photoresist process which allows the native oxide and ZnS to be etched away from the contact areas.
  • Suitable metal layers can then be deposited and defined using a lift-off process in order to form ohmic contacts to the Hg, ⁇ Cd Te top mesa surface.
  • indium "bumps" 77 can be formed on some or all of the common terminals by another lift-off process, for subsequent interfacing to suitable silicon readout circuits via standard hybrid flip chip bonding.
  • Figure 26 depicts a top view of the lateral photoconductor array with indium bumps 77 formed for the common terminals.
  • the seventh embodiment of the invention is similar to the fifth and sixth embodiment in that the array consists of conventional lateral heterojunction photoconductors in which all of the terminals are located on top of the infrared detecting area of the pixel, opposite to the incident IR radiation.
  • the array consists of conventional lateral heterojunction photoconductors in which all of the terminals are located on top of the infrared detecting area of the pixel, opposite to the incident IR radiation.
  • multi-layers of Hg. Cd Te are used for the specific attempt to form parallel photoconductive elements that can simultaneously detect infrared radiation in a number of wavelength regions of the electromagnetic spectrum.
  • multi-spectral photoconductor array to describe the seventh embodiment of the invention.
  • the invention covers the detection in any number of regions of the electromagnetic spectrum by suitably growing further layers of Hg. Cd Te with the desired x values and thicknesses.
  • the most preferred material structure for the tnulti- spectral lateral photoconductor array is shown in figures 27 to 36 in cross section.
  • the starting material consists of an insulating substrate 63 (eg CdTe or CdZnTe) on which is grown an infrared absorbing layer of n-type Hg 1 ⁇ Cd ⁇ Te semiconductor 79 of x value x1 and desired thickness.
  • Another n-type layer of Hg. Cd Te semiconductor 81 which has a slightly wider bandgap x2 (x2-x1 approximately equal to 0.04) than the first absorbing layer x1 is then grown on top of the Hg.
  • This bandgap engineered structure between x1 and x2 forms a heterojunction contact that has been shown to dramatically minimise the deleterious effects of minority carrier sweepout.
  • another layer of Hg. Cd Te semiconductor 83 is grown on top of layer 81.
  • This semiconductor layer 83 has x value x3 which has a sufficiently high bandgap, sufficiently low doping and sufficient thickness so as to effectively electrically isolate the layer of Hg.. ⁇ Cd ⁇ Te semiconductor 81 with x value x2 from any subsequent layers of Hg 1 ⁇ Cd ⁇ Te grown on top of this Hg 1 ⁇ Cd ⁇ Te semiconductor layer 83.
  • Te semiconductor 85 of x value x4 of appropriate thickness is grown on top of the Hg. I — A Cd ⁇ Te semiconductor layer 83.
  • Another n-type layer of Hg. I ⁇ Cd ⁇ Te semiconductor 87 which has a slightly wider bandgap x5 (x5-x4 approximately equal to 0.04) than the second absorbing layer 85 x4 is then grown on top of the Hg. Cd ⁇ Te layer 85.
  • This bandgap engineered structure between x4 and x5 forms a heterojunction contact that has been shown to dramatically minimise the deleterious effects of minority carrier sweepout.
  • the layer of n-type Hg 1 ⁇ Cd Te with x value x4 should have an x value greater than the first infrared absorbing layer of n-type Hg. Cd Te of x value x1.
  • the infrared radiation of shorter wavelength will be absorbed in the first absorbing layer.
  • the radiation with longer wavelength will pass through Hg 1 ⁇ Cd ⁇ Te layers with x values x1 , x2 and x3 and be absorbed in the second infrared absorbing layer of Hg 1 ⁇ Cd Te with x value x4.
  • the layer of Hg., Cd Te with x value x4 can also absorb radiation of the shorter wavelength
  • the layer of Hg,, Cd Te with x value x1 should be sufficiently thick so as to minimise the number of photons of shorter wavelength that pass through this layer. This will minimise the effects of crosstalk between the two wavelengths that are being detected.
  • the major fabrication steps required in order to realise the multi-spectral photoconductor array are outlined in figures 29 to 36.
  • the procedure consists of first defining the photoconductor active or window regions and delineating each element of the array by mesa etching (e.g. by use of ion beam etching) of the semiconductor down to the underlying insulating substrate (see figure 30).
  • the next step (see figure 31) is to etch ridges between adjacent photoconductors through Hg. Cd Te layers with x values x5, x4 and x3 again by a mesa etching process. This step will allow contact to be made to the layer of Hg.
  • the anodisation process tends to be a self regulating one in terms of thickness and insulating properties, the oxide layer formed during growth is extremely uniform over the entire exposed semiconductor surface area of the mesa structure.
  • the wafer is loaded into a vacuum evaporation system for deposition of the ZnS passivation layer (or any other suitable insulating layer).
  • the metal contact areas can then be defined by another photoresist process which allows the native oxide and ZnS to be etched away from the contact areas (see figure 32).
  • Suitable metal layers can then be deposited and defined using a lift-off process in order to form ohmic contacts 71 to the Hg. Cd Te layers of x value x2 and x5 and to the common areas of the photoconductors (i.e. the Hg. ⁇ Cd ⁇ Te layers of x values x2 and x5 mesa surfaces).
  • Hg. ⁇ Cd ⁇ Te layers of x values x2 and x5 mesa surfaces i.e. the Hg. ⁇ Cd ⁇ Te layers of x values x2 and x5 mesa surfaces.
  • indium “bumps” can be formed including some or all of the common terminals by another lift-off process for subsequent interfacing to suitable silicon readout circuits via standard hybrid flip chip bonding.
  • each photoconductive element has two indium bumps 75 as positive terminals for current flow (i.e.
  • FIG. 35 shows the multi-spectral photoconductor array with no indium bumps for the common terminal and instead uses common metal lines
  • figure 36 shows the multi-spectral photoconductor array with indium bumps used for the commons.
  • indium diffused n "blocking" contacts can be formed on the top and common terminals. This type of blocking contact has also been shown to be effective in reducing the effects of minority carrier sweepout.
  • IRFPA architecture- Hybrid flip-chip identical to that used for MCT PV arrays.
  • Hg. Cd Te-based IRFPA operability is essentially determined by non-uniformities in the performance of individual elements. Ignoring dimensional fluctuations, these variations in performance may be caused by preexisting spatial inhomogenities in Hg. Cd Te starting material, or may be introduced during device fabrication.
  • Hg. Cd Te arrays made using grown doublelayer junctions and mesa technology, (e.g. as used by Santa Barbara Research Center) the measured variations in responsivity uniformity are attributed to variations in junction area caused by ⁇ 1 ⁇ m fluctuations in dimensional control.
  • LWIR photovoltaic Hg.. Cd Te arrays have uniformity limits of between 3 and 8%.
  • the photoconductor Hg. Cd Te array will be uniformity limited by the same dimensional controls as these mesa PV Hg 1 ⁇ Cd ⁇ Te arrays.
  • the photoconductive Hg. Cd Te detectors are defined in the vertical dimension by some etching process (e.g. ion beam) through about 10 ⁇ m thickness of material. It is the dimensional control of this etching process that determines the active optical area of the individual elements and thus the responsivity uniformity of the photoconductor array.
  • the published values for the uniformity of conventional PC Hg. Cd Te detectors is surprisingly high at between 6 and
  • Photoconductor device performance depends mainly on the excess carrier lifetime which is determined by the spatial average over the detector volume and it would not be expected to be affected greatly by localised non-uniformities and a low density of point and native defects. This places photoconductive detectors at an intrinsic advantage over photovoltaic devices in terms of pixel yield in an array.
  • photovoltaic detectors suffer from crosstalk due to photogenerated minority carriers that diffuse laterally between adjacent devices. Since the detector elements are physically delineated by etching, this type of crosstalk is not present for the photoconductor array.
  • the photoconductor array is only limited by crosstalk due to phenomena such as optical reflection or refraction, optical scattering, diffraction of the target spot and optical system aberrations, which is also present in all IRFPAs. The majority of these effects can be controlled by suitable design considerations, and crosstalk levels of less than 0.5% should be achievable.
  • LWIR PV Hg, Cd Te detectors usually operate at zero or near zero bias voltages and thus dissipate very little power. This has been a major advantage of the photovoltaic device that has lead to their predominance in the design of IRFPAs. It has long been the disadvantage of photoconductive detectors that they dissipate excessive power and are therefore unsuitable for large IRFPA applications. It has been recently demonstrated however, that over an order of magnitude increase in voltage responsivity is achieved by using heterojunction contacts for PC Hg. Cd Te detectors over the standard design photoconductor. This equates to three orders of magnitude reduction in power dissipation of the heterojunction PC over the standard PC for a given responsivity.
  • the hybrid flip-chip architecture has been the most commonly used for interfacing two-dimensional PV Hg,, ⁇ Cd ⁇ Te detector arrays to the silicon multiplexer.
  • the electrical connection is made with indium "bumps" that provide a soft metal interconnect between each pixel and the silicon read-out electronics.
  • This arrangement facilitates the interconnection of a large number of pixels with individual preamplifiers coupled to row and column multiplexers.
  • the same well-established hybrid arrangement can be used with the proposed design photoconductor arrays. This is a significant feature of the photoconductor array design since it can use a proven and well developed silicon interface technology.
  • the R A values of LWIR PV Hg 1 ⁇ Cd ⁇ Te detectors are typically 1-100 ⁇ cm.
  • Cd Te detectors creates a difficulty in coupling them to CCD read-out circuits for IRFPA applications. This problem has already been overcome to some extent by the use of suitable buffer interfaces. Multiplexing PC Hg. Cd Te detectors with a silicon CCD with these buffer interfaces have been successfully demonstrated. The drawback, however, of using these types of buffer interfaces is the increased power dissipation in the preamplifier itself. Less power dissipated in the PC detector array means less stringent power constraints for the silicon input buffer circuits. More effort though is needed in the design of silicon CMOS compatible, low power dissipation buffer circuits for the PC Hg.
  • 2 chip size is limited to about 8 mm .
  • this corresponds to a maximum array size of 250 X 250 detector elements.
  • This density may not be achievable for planar LWIR PV Hg 1 ⁇ Cd Te detector arrays due to crosstalk restrictions on minimum detector spacing requirements.
  • arrays operating at 80K should be designed with heat loads in the order of 0.5 Watts in order to meet practical cryogenic cooling limits. If 0.25 Watts is allowed for the silicon input circuits, then 0.25 Watts can be dissipated by the photoconductor array. This therefore places a constraint on the power dissipation of each element on the array and/or how many elements can be placed in an array.
  • the array size considered for these simulations consists of 128 X 128 photoconductor elements, thus giving a power dissipation requirement of 15.2 ⁇ Watts per element.
  • the number of photons absorbed increases rapidly with increased detector lengths and is the dominant factor in determining the value of R ⁇ .
  • the number of absorbed photons begins to saturate causing R ⁇ to reach its peak value before decaying at a slow rate (inversely proportional to I ).
  • the drift length i.e. the detector length V I
  • the optimum device length for the heterojunction contact is 10 ⁇ m while for the ohmic contact device the optimum length is 82 ⁇ m .
  • Figure 11 shows the noise performance of the above vertical photoconductors as a function of detector length with the same power constraint.
  • FIG. 13 shows the optimum detector length as a function of power dissipation for a vertical photoconductor when Ohmic, blocking and heterojunction contacts are assumed. It is evident from figure 13 that the optimum detector length for a given power dissipation requirement is relatively independent of the magnitude of that power (except at high power dissipation levels) for the cases where devices have blocking and heterojunction contacts. This is the case since the blocking and heterojunction contact devices are limited by photon absorption whereas the Ohmic contact device is limited by sweepout and minority carrier recombination at the semiconductor/metal interface.
  • figures 10 and 13 show that for a 128 X 128 element array, individual photoconductor element responsivities greater than
  • figure 14 shows a plot of responsivity versus power dissipation for detector lengths of 82 ⁇ m, 14 ⁇ m and 10 ⁇ m for devices where Ohmic, blocking and heterojunction contacts are assumed, respectively.
  • higher responsivity can only be achieved at the expense of greater power dissipation.
  • the relationship is not a linear one. From figure 14 it is evident that the slope of the curve decreases with power. As we move up the power scale, a smaller gain in responsivity would be achieved for every increment in power. Hence, it is more beneficial to operate near the lower end of the power scale.
  • the present embodiments provide for a Mercury Cadmium Telluride photoconductive device structure that has been designed specifically as a sensing element for high density two-dimensional infrared focal plane array applications.
  • This structure allows for high performance devices at greatly reduced power dissipation levels, while allowing for the high density integration of photoconductors in a two dimensional array geometry with high fill factor such that hybrid bonding to silicon read-out circuits is readily achievable.
  • the invention is not limited to the specific embodiments herein described, and indeed may be applicable to other types of photosensitive semiconductor devices and methods of forming the same. Further, the invention is not limited to HgCdTe semiconductors, and a skilled addressee will understand that other semiconductors such as InSb may be suitably employed. It will be understood that photosensitive semiconductor compounds are being developed all the time, and it will be understood that these will be suitable for utilising in this invention.

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Abstract

Agencement d'éléments photosensibles et pixel destiné audit agencement, ce pixel comprenant un matériau semi-conducteur et photoconducteur (31, 33) fixé à un substrat conducteur (29) (ou, dans certaines applications, non conducteur), qui peut conduire les photons destinés à être détectés par le pixel. Une borne métallique (41), placée au-dessus du matériau photoconducteur (31), permet une connexion d'électrode avec le pixel. Lorsqu'il s'agit d'un substrat conducteur, des bornes métalliques (45) sont montées sur la face du substrat (29) recevant la lumière, du côté opposé au pixel, afin d'assurer les autres connexions de borne. La face du substrat recevant la lumière se trouve du côté opposé aux pixels, ce qui permet une densité d'implantation plus élevée des pixels, ainsi que d'autres avantages. Des bosses d'indium (43) assurent la connexion en vue du traitement des signaux. Les surfaces semi-conductrices comportent une couche de passivation (39).
PCT/AU1995/000644 1994-09-30 1995-09-29 Agencement a semi-conducteurs photosensible WO1996010843A1 (fr)

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AUPM8539A AUPM853994A0 (en) 1994-09-30 1994-09-30 Photosensitive semiconductor array
AUPM8539 1994-09-30
AUPM9859A AUPM985994A0 (en) 1994-12-05 1994-12-05 Photosensitive semiconductor array
AUPM9859 1994-12-05

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE19625235A1 (de) * 1996-06-24 1998-01-02 Abb Patent Gmbh Bewegungsmelder zur Detektion von Wärmestrahlung abgebenden, beweglichen Objekten

Citations (6)

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Publication number Priority date Publication date Assignee Title
US4549195A (en) * 1983-04-14 1985-10-22 Westinghouse Electric Corp. Heterojunction semiconductor device
US4894700A (en) * 1985-04-09 1990-01-16 Fuji Xerox Co., Ltd. Image sensor
US4961098A (en) * 1989-07-03 1990-10-02 Santa Barbara Research Center Heterojunction photodiode array
US5149956A (en) * 1991-06-12 1992-09-22 Santa Barbara Research Center Two-color radiation detector array and methods of fabricating same
US5168338A (en) * 1991-09-30 1992-12-01 Mitsubishi Denki Kabushiki Kaisha Solid-state hybrid infrared imaging device
US5189297A (en) * 1988-08-29 1993-02-23 Santa Barbara Research Center Planar double-layer heterojunction HgCdTe photodiodes and methods for fabricating same

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4549195A (en) * 1983-04-14 1985-10-22 Westinghouse Electric Corp. Heterojunction semiconductor device
US4894700A (en) * 1985-04-09 1990-01-16 Fuji Xerox Co., Ltd. Image sensor
US5189297A (en) * 1988-08-29 1993-02-23 Santa Barbara Research Center Planar double-layer heterojunction HgCdTe photodiodes and methods for fabricating same
US4961098A (en) * 1989-07-03 1990-10-02 Santa Barbara Research Center Heterojunction photodiode array
US5149956A (en) * 1991-06-12 1992-09-22 Santa Barbara Research Center Two-color radiation detector array and methods of fabricating same
US5168338A (en) * 1991-09-30 1992-12-01 Mitsubishi Denki Kabushiki Kaisha Solid-state hybrid infrared imaging device

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE19625235A1 (de) * 1996-06-24 1998-01-02 Abb Patent Gmbh Bewegungsmelder zur Detektion von Wärmestrahlung abgebenden, beweglichen Objekten

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