WO2006038213A2 - Millimeter wave pixel and focal plane array imaging sensors thereof - Google Patents

Millimeter wave pixel and focal plane array imaging sensors thereof Download PDF

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Publication number
WO2006038213A2
WO2006038213A2 PCT/IL2005/001062 IL2005001062W WO2006038213A2 WO 2006038213 A2 WO2006038213 A2 WO 2006038213A2 IL 2005001062 W IL2005001062 W IL 2005001062W WO 2006038213 A2 WO2006038213 A2 WO 2006038213A2
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Prior art keywords
mmw
pixel
pixels
sub
sensor
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PCT/IL2005/001062
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French (fr)
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WO2006038213A3 (en
Inventor
Yael Nemirovsky
Leslie Salem
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Bluebird Optical Mems Ltd.
Electro-Optical Research & Development Ltd.
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Publication of WO2006038213A2 publication Critical patent/WO2006038213A2/en
Publication of WO2006038213A3 publication Critical patent/WO2006038213A3/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/10—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors
    • G01J5/20—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors using resistors, thermistors or semiconductors sensitive to radiation, e.g. photoconductive devices
    • G01J5/22—Electrical features thereof
    • 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
    • H01L27/14643—Photodiode arrays; MOS imagers
    • H01L27/14649—Infrared imagers
    • 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
    • H01L31/0248—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 characterised by their semiconductor bodies
    • H01L31/0256—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 characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/0296—Inorganic materials including, apart from doping material or other impurities, only AIIBVI compounds, e.g. CdS, ZnS, HgCdTe
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L25/00—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof
    • H01L25/03—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof all the devices being of a type provided for in the same subgroup of groups H01L27/00 - H01L33/00, or in a single subclass of H10K, H10N, e.g. assemblies of rectifier diodes
    • H01L25/04—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof all the devices being of a type provided for in the same subgroup of groups H01L27/00 - H01L33/00, or in a single subclass of H10K, H10N, e.g. assemblies of rectifier diodes the devices not having separate containers
    • H01L25/041—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof all the devices being of a type provided for in the same subgroup of groups H01L27/00 - H01L33/00, or in a single subclass of H10K, H10N, e.g. assemblies of rectifier diodes the devices not having separate containers the devices being of a type provided for in group H01L31/00
    • H01L25/042—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof all the devices being of a type provided for in the same subgroup of groups H01L27/00 - H01L33/00, or in a single subclass of H10K, H10N, e.g. assemblies of rectifier diodes the devices not having separate containers the devices being of a type provided for in group H01L31/00 the devices being arranged next to each other
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L25/00—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof
    • H01L25/03—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof all the devices being of a type provided for in the same subgroup of groups H01L27/00 - H01L33/00, or in a single subclass of H10K, H10N, e.g. assemblies of rectifier diodes
    • H01L25/04—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof all the devices being of a type provided for in the same subgroup of groups H01L27/00 - H01L33/00, or in a single subclass of H10K, H10N, e.g. assemblies of rectifier diodes the devices not having separate containers
    • H01L25/065—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof all the devices being of a type provided for in the same subgroup of groups H01L27/00 - H01L33/00, or in a single subclass of H10K, H10N, e.g. assemblies of rectifier diodes the devices not having separate containers the devices being of a type provided for in group H01L27/00
    • H01L25/0655—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof all the devices being of a type provided for in the same subgroup of groups H01L27/00 - H01L33/00, or in a single subclass of H10K, H10N, e.g. assemblies of rectifier diodes the devices not having separate containers the devices being of a type provided for in group H01L27/00 the devices being arranged next to each other
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/0001—Technical content checked by a classifier
    • H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00

Definitions

  • the present invention relates generally to the detection and imaging of millimeter waves, and more specifically to the detection and imaging of millimeter waves using a focal plane array comprised of a plurality of millimeter wave pixels, each millimeter wave pixel further comprising a two-dimensional array of micro-machined thermal sensors.
  • MMW imaging arrays for a wide range of applications such as smart munitions seekers, air craft landing systems, automobile collision-avoidance radars as well as the detection of concealed weapons.
  • Most clothing and many common building materials are transparent to electromagnetic radiation at millimeter wavelength. This has motivated the development of MMW imaging systems for detection of concealed weapons.
  • Novel infrared imaging technologies use micro-machined bolometers to detect the incident radiation.
  • the incident power is deposited to an absorber element, which heats up.
  • the change in the absorber temperature is measured as a chaise in some of the image sensor's electrical properties, for example, a change in resistance.
  • These types of image sensors cannot be directly utilized in the millimeter wavelengths. The reason for this is the fact that the image sensor pixel area needs to be at least ⁇ 2 , where ⁇ is the wavelength of the radiation incident on the image sensor. Therefore, when millimeter wavelengths are to be detected, large absorbers are required, and they have a large heat capacity, a decreased sensitivity and a slow response time.
  • This difficulty can be overcome by integrating a tiny temperature sensing element with a high- efficiency lithographic antenna, known in the art as an antenna-coupled micro- bolometer.
  • the radiation is first received by an optimized lithographic antenna.
  • the radiation induces changing electrical currents to the antenna arms, which pass through a bolometer.
  • the bolometer is made as small as possible, currently by employing micromachining techniques, for faster response and enhanced sensitivity, and the antenna focuses incident power from a large area, of the order of ⁇ 2 onto the much smaller bolometer.
  • the result is an antenna-coupled micro- bolometer.
  • the device thermal time constant ⁇ is proportional to the capacitance of the bolometer, it dictates that the device's capacity should be very small. This is specifically necessary in order to ensure compatibility with the frame time of TV format where an image is to be displayed.
  • the incremental increase in temperature is then detected, for example by a change in electrical conductivity (resistive bolometers), generation of a voltage (thermocouples) or change in the electrical polarization (pyroelectric detectors).
  • resistive bolometers change in electrical conductivity
  • thermocouples change in the electrical polarization
  • pyroelectric detectors change in the electrical polarization
  • VO ⁇ is used because it has a relatively very high temperature coefficient of the order of 3%/°C.
  • the use of VO ⁇ is further described by Wood in US patent 5,450,053.
  • CMOS complementary metal-oxide semiconductor
  • the VO ⁇ technology complexity shows significant non-uniformities across the pixels and the cost of dies is still too high for many potential applications.
  • Yet another disadvantage of the larger pixels in a pixel array has to do with pixel failure, where a single pixel failure may cause the MMW sensor to be deemed useless.
  • a typical pixel size for a millimeter wave image sensor is 6 millimeters on the side, for a wavelength of 3 millimeters. In a very crude implementation of 10-by-lO pixels this requires a device of 6 centimeters on the side. The yield of such a large device is generally low because of the size of the device, and moreover, a failure of a single pixel is significant for the quality of the image.
  • the performance required from a MMW bolometer are of a G in the range of 10 "7 Watt/°K and a C of the order of 10 "9 J/°K, resulting in a ⁇ of less than 10 milliseconds.
  • a focal plane array millimeter wave image sensor composed of at least a millimeter wave pixel, the millimeter wave pixel being composed of a plurality of sub-pixels.
  • the millimeter wave pixels may be formed on a single substrate or placed on another appropriate substrate to achieve the overall size suitable for the specific image sensing application.
  • Each sub-pixel is composed of a micro-machined thermal sensor.
  • Figure 1 - is an exemplary diagram of a MMW pixel comprised of a plurality of sub- pixels in accordance with the disclosed invention
  • Figure 2 - is an exemplary photograph of an array of micro-bolometers comprising a MMW pixel, each micro-bolometer being a sub-pixel
  • Figure 3 - is an exemplary diagram of a MMW pixel comprised of a plurality of sub- pixels, each sub-pixel being a micro-bolometer
  • Figure 4 - is an exemplary TMOS transistor used as a thermal sensing element of a sub-pixel
  • Figure 5 - is an exemplary temperature coefficient of current of the thermal MOS transistor as a function of the operating gate voltage and temperature
  • Figure 6 - is an exemplary diagram of a MMW pixel comprised of a plurality of sub- pixels, each sub-pixel being a thermal MOS transistor
  • Figure 7 - is an exemplary MMW FPA imaging sensor comprised from a plurality of MMW pixels
  • Figure 8 - is an exemplary MMW system having a focusing lens and a MMW FPA imaging sensor
  • a MMW pixel is comprised of a plurality of sub-pixels, each being an uncooled thermal sensing device, and further each being capable of contributing a measurement towards the measurement value of the MMW pixel.
  • a MMW FPA imaging sensor is formed by a matrix comprised from a plurality of MMW pixels, each MMW pixel being compromised of sub-pixels, as explained in more detail herein below.
  • Fig. 1 an exemplary and non-limiting diagram of a millimeter wave (MMW) pixel 100 comprised of a plurality of sub-pixels is shown.
  • a two-dimensional (2D) array 110 comprising of "I" rows and "J" columns of sub-pixels.
  • a sub-pixel in a coordinate (i, j) is comprised of uncooled thermal sensing element, capable of detecting the energy associated with MMW radiation and contributing to the overall pixel signal.
  • Each sub-pixel may further contain circuitry to enable its sensing ability of its respective thermal sensing device.
  • thermal sensing devices are a micro-bolometer and a thermal MOS transistor, described in more detail below, but these examples should not be considered as limiting the scope of the disclosed invention, and other applicable sensing devices capable of achieving the same end-result are specifically included.
  • An analog readout circuitry 130 is coupled to the columns of array 110 allowing the reading of each of the sub-pixels, a process performed under the control of digital controller 120, capable of enabling row-by-row reading.
  • the data from analog readout circuitry 130 is accumulated in summing unit 140. Specifically, summing unit 140 sums the signals produced at each sub-pixel sensor element.
  • a typical size of a MMW pixel 100 is between 4 and 6 millimeters on the side.
  • Digital controller 120 which provides the clocking and timing signals is further capable of overcoming a failure of one or more of the sub-pixels in array 110. Specifically, a failure of a limited number of sub-pixels of the MMW pixel, may be overcome by ignoring the output result received from a faulty sub-pixel.
  • Digital controller 120 may also use summing unit 140, for the purpose of correcting the result detected by MMW pixel 100.
  • Error correction may include, but not limited to, averaging the results across fewer sub- pixels, extrapolating the expected readout of the faulty sub-pixel by evaluating several of its immediate sub-pixels, for example the 8 sub-pixels surrounding the faulty sub- pixel, or using more elaborate extrapolation techniques, including, but not limited to, weighting factors.
  • the resultant measurement is equivalent to that which is achieved by prior art solutions, however, there is no need for an antenna, there is sub-pixel redundancy, and a standard CMOS process with micro-machining post-processing may be used, all providing the desired advantages over prior art, without sacrificing accuracy and stability of the readout.
  • MMW pixel 100 further has a plurality of connecting pads 150 through which the results can be transferred for further processing as may be necessary.
  • connecting pad 154 for example for power supply V DD> and a connecting pad 152, for example for ground GND.
  • the connecting pads may be bonding pads in the case where the MMW pixel is manufactured as a stand-alone device.
  • the design of the connecting pads is such that it allows for the connection of additional MMW pixels in both row and column directions.
  • an array 110 of sub-pixels is implemented using as a sub-pixel a micro-bolometer.
  • Each micro-bolometer 210 has a pixel level circuitry 220, as well as column and row circuitry 230.
  • Each sub-pixel for example the pixel in row 2 and column 8, i.e., pixel 311 2 , 8» is a micro-bolometer, connected via appropriate circuitry to both row and column controls. This structure allows for the separate readout of each of the pixels, determining faulty pixels, and providing the output data to the summing unit 140, as explained above.
  • TMOS thermal metal-oxide semiconductor
  • Fig. 4 where an exemplary and non-limiting three dimensional array of TMOS transistors 400, used as a temperature sensing elements of sub-pixels of the MMW pixel, is shown.
  • the TMOS transistors are first created as would a regular MOS transistor, though the starting material is a silicon-on-insulator (SOI) material that has an internal buried oxide 420.
  • SOI silicon-on-insulator
  • the TMOS transistor is formed on the device side of the wafer where active silicon layer 430 is used for the formation of the drain and source of the TMOS transistor.
  • a gate 440 is formed over the active silicon area where the TMOS transistor is to be formed.
  • the drain, gate, and source are connected to metal leads 450, 460 and 470 respectively.
  • a micro-machining process takes place removing certain areas of silicon bulk 410, essentially creating a cavity 480 and causing the TMOS transistor to eventually be suspended by its metal connections to the drain, gate, and source. This release process ensures a thermal decoupling and insulation preventing one sensor from being effected by another sensor.
  • One advantage of the use of a TMOS transistor can be seen in Fig.
  • TCC temperature coefficient of current
  • TCR temperature coefficient of resistance
  • the TMOS transistor operates in deep sub-threshold voltage, and at that operating point current levels are small. While it is possible to achieve higher sensitivities, as shown in the graph of Fig. 5, for example using a V G s of 1.5V, it is not a recommended quiescent point due to the very small currents.
  • FIG. 6 where an exemplary and non-limiting MMW pixel 600, comprised of sub-pixels 611, each sub-pixel being a TMOS transistor, is shown.
  • An array 610 of sub-pixels 611 comprises a MMW pixel.
  • Each of sub-pixels 611, for example pixel 611 2i8 is comprised of at least a TMOS transistor, connected via appropriate circuitry to both row and column controls.
  • Each sub-pixel may further contain additional circuitry, such as buffers, sampling devices, and others, that may be deemed appropriate for the purpose of detecting the energy respective of the radiation of a MMW.
  • FIG. 7 where an exemplary and non-limiting MMW image sensor 700 comprised of a plurality of MMW pixels 100, is shown. On top of a substrate 790, there are mounted a plurality of MMW pixels 100, organized in "N" rows and "M” columns.
  • Each of MMW pixels 100 for example MMW pixel in coordinate (n, m), is connected to an identical neighbor MMW pixel, in coordinates (n, m-1), (n,m+l), (n-l,m), and (n+1, m), as long as "n-1" and "m-1" are greater than "0", "n+1” is no larger than "N", and "m+1” is no larger than "M”.
  • Connectivity between MMW pixels in columns is achieved through a plurality of connecting busses 770.
  • Connectivity between MMW pixels in rows is achieved through a plurality of connecting busses 760. This connectivity allows for control, selection, and readout of each of MMW pixels 100.
  • a central controller 710 is coupled to the plurality of MMW pixels 100 through connecting pads 720 for the rows and connecting pads 730 for the columns. Central controller 710 initiates the readout from each of MMW pixels 100 and is further capable of control functions of each of MMW pixels 100, including, but not limited to, testing, redundancy, and the likes.
  • Bonding pads 750 provide connectivity to external devices; bonding pad 754 provides connection to an external power supply V DD connectivity, and is further connected to the respective power supply V DD connecting pads 154 of each of MMW pixels 100.
  • Bonding pad 752 provides connection to a ground GND connectivity, and is further connected to the respective grovmd GND connecting pads 152 of each of MMW pixels 100.
  • a single SOI wafer is used to manufacture, monolithically, the entire MMW image sensor 700, and therefore substrate 790 is the actual wafer.
  • a hybrid approach is used, where substrate 790 is used for the purpose of mounting the plurality of MMW pixels 100.
  • the smaller devices will generally have a higher yield than a larger device, for reasons that are well known in the art.
  • the plurality of MMW pixels, or matrices thereof, may be placed separately on substrate 79O. Wire bonding maybe required to connect rows and columns of detached MMW pixels 100, or matrices thereof.
  • Substrate 790 may be of alumina, a printed circuit board (PCB), or the likes, to which each of the MMW pixels 100 is mounted to, or the matrices thereof.
  • Central controller 510 may also be a separate device requiring mounting and bonding on substrate 790. Once all placed on substrate 790 there is formed a hybrid MMW image sensor, allowing for higlier image resolution.
  • Fig. 8 where an exemplary and non-limiting MM ⁇ V system 800, having a focusing lens 810 and a MMW FPA imaging sensor 700, is shown. It would be advantageous to use MMW system 800 where it is necessary to eliminate the effects of radiation other than MMW radiation on the readings of MMW FPA imaging sensor 700.
  • MMW system lens 810 is coated with a selective reflective coating 820. The selectivity of the coating will enable the penetration of the MMW but not that of infrared (IR) that would affect the reading of the sub-pixels, being of the IR wavelength.
  • IR infrared
  • each MMW pixel 100-i-j is further coated with an appropriate MMW absorbing film.
  • the MMW absorbing film is deposited over the sub-pixels and serve as an absorbent of the MMW energy thereby raising the temperature of the sub-pixel.
  • the exact position of the micro-machined array of IR reflective mirrors, between lens 810 and MMW FPA imaging sensor 700, is determined by the chromatic aberration of the collecting lens that gives rise to the focal plane for IR- wavelengths to be separated by a definite distance from the MMW focal plane of the lens.
  • the MMW array is positioned at the MMW focal plane of the lens.
  • lens 810 is a chromatically corrected lens, designed to bring both DR and MMW radiations to focus at a single plane.
  • the central Airy disc for IR wavelengths is much smaller than that for MMW. It is well-known in-the-art that the diameter of the Airy disc scales linearly with the wavelength. Therefore there will be a sub-pixel of the MMW pixel that will have a much larger signal than a neighboring sub-pixels belonging to the same MMW pixel. This high energy is associated with the significantly smaller Airy disc resulting of the IR energy collected by a single sub-pixel. As in the case of a defective pixel, explained in more detail above, a pixel showing a significantly above average collection of energy may be disregarded in the summation process and hence the IR effect on the MMW pixel is avoided.

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Abstract

Shown is a focal plane array millimeter wave image sensor composed of at least a millimeter wave pixel (100) being composed of a plurality of sub-pixels (110). The millimeter wave pixels (100) may be formed on a single substrate or placed on another appropriate substrate to achieve the overall size suitable for the specific image sensing application. Each sub-pixel (110) is composed of a micro-machined thermal sensor.

Description

MILLIMETER WAVE PIXEL AND FOCAL PLANE ARRAY IMAGING SENSORS THEREOF
FIELD OF THE INVENTION
The present invention relates generally to the detection and imaging of millimeter waves, and more specifically to the detection and imaging of millimeter waves using a focal plane array comprised of a plurality of millimeter wave pixels, each millimeter wave pixel further comprising a two-dimensional array of micro-machined thermal sensors.
BACKGROUND OF THE INVENTION
There is considerable interest in the development of low-cost millimeter-wave (MMW) imaging arrays for a wide range of applications such as smart munitions seekers, air craft landing systems, automobile collision-avoidance radars as well as the detection of concealed weapons. Most clothing and many common building materials are transparent to electromagnetic radiation at millimeter wavelength. This has motivated the development of MMW imaging systems for detection of concealed weapons.
Novel infrared imaging technologies use micro-machined bolometers to detect the incident radiation. The incident power is deposited to an absorber element, which heats up. The change in the absorber temperature is measured as a chaise in some of the image sensor's electrical properties, for example, a change in resistance. These types of image sensors cannot be directly utilized in the millimeter wavelengths. The reason for this is the fact that the image sensor pixel area needs to be at least λ2, where λ is the wavelength of the radiation incident on the image sensor. Therefore, when millimeter wavelengths are to be detected, large absorbers are required, and they have a large heat capacity, a decreased sensitivity and a slow response time. This difficulty can be overcome by integrating a tiny temperature sensing element with a high- efficiency lithographic antenna, known in the art as an antenna-coupled micro- bolometer. In this type of bolometer, the radiation is first received by an optimized lithographic antenna. The radiation induces changing electrical currents to the antenna arms, which pass through a bolometer. The bolometer is made as small as possible, currently by employing micromachining techniques, for faster response and enhanced sensitivity, and the antenna focuses incident power from a large area, of the order of λ2 onto the much smaller bolometer. The result is an antenna-coupled micro- bolometer. As the device thermal time constant τ is proportional to the capacitance of the bolometer, it dictates that the device's capacity should be very small. This is specifically necessary in order to ensure compatibility with the frame time of TV format where an image is to be displayed.
One major advantage of the room-temperature antenna-coupled micro-bolometer is its seeming simplicity which holds true predominately for a single pixel. The performance of single pixels based on the antenna-coupled micro-bolometer approach is extensively discussed in published prior art. Prior art further suggests various solutions for focal-plane arrays, however, the implementation of a cost-effective focal plane array with adequate performance, has not been demonstrated. There are several difficulties, when trying to extend a single pixel into an array. Firstly the use of a plurality of antenna-coupled bolometers presents the problem of antenna crosstalk between the pixels, impacting the performance of the device. There is further a need to integrate the addressing and readout electronics with the antenna-coupled bolometers. Specifically, in solutions that impose a bottom level special consideration is required to avoiding shortening in connectivity planes. Yet another challenge lies with the fact that integrated circuits (ICs) are limited in size, generally to the exposure windows of a stepper. Even if certain stitching technologies are employed the required size for the imager may be well-beyond the size of a single wafer. Silicon micromachining technology is an excellent technology for providing miniaturized thermally isolated structures. Such thermally isolated pixels are used to sense infrared (IR) radiation by thermal detection mechanisms, in which the action of the incident radiation is to slightly increase the temperature. The incremental increase in temperature is then detected, for example by a change in electrical conductivity (resistive bolometers), generation of a voltage (thermocouples) or change in the electrical polarization (pyroelectric detectors). These effects do not require cryogenic operation as in the case of the traditional quantum photon detectors. Thus, uncooled thermal imaging focal plane arrays, based on thermal detection mechanisms, have revolutionized IR imaging and have been under extensive development since 1985. The commercial and military market for such image sensors, which can be operated without any type of cryogenic system, is enormous and hence there is an ongoing interest to improve performance and reduce cost. In one approach, the detecting area is defined by a thin membrane made of silicon nitride, upon which is deposited a thin film of vanadium oxide (VOχ). VOχ is used because it has a relatively very high temperature coefficient of the order of 3%/°C. The use of VOχ is further described by Wood in US patent 5,450,053. In spite of impressive results, this technology is much more complicated and certainly less ubiquitous compared to the well-established and commonly available complementary metal-oxide semiconductor (CMOS) technology. The VOχ technology complexity shows significant non-uniformities across the pixels and the cost of dies is still too high for many potential applications. Yet another disadvantage of the larger pixels in a pixel array has to do with pixel failure, where a single pixel failure may cause the MMW sensor to be deemed useless.
A typical pixel size for a millimeter wave image sensor is 6 millimeters on the side, for a wavelength of 3 millimeters. In a very crude implementation of 10-by-lO pixels this requires a device of 6 centimeters on the side. The yield of such a large device is generally low because of the size of the device, and moreover, a failure of a single pixel is significant for the quality of the image. The performance required from a MMW bolometer are of a G in the range of 10"7 Watt/°K and a C of the order of 10"9 J/°K, resulting in a τ of less than 10 milliseconds. Therefore, in view of the limitations of prior art solutions, there is a need for a MMW image sensor that utilizes simple and commonly available CMOS manufacturing technologies combined with established micro-machining processes, and further avoiding non-standard manufacturing processes and materials. It would be further advantageous if the solution provided a high temperature coefficient in excess of 3%/°C. It would be further advantageous if such a MMW image sensor be better immune to pixel failures and provide a superior cost performance solution. It would be further advantageous if crosstalk between pixels is to be reduced.
SUMMARY OF THE INVENTION
Disclosed is a focal plane array millimeter wave image sensor composed of at least a millimeter wave pixel, the millimeter wave pixel being composed of a plurality of sub-pixels. The millimeter wave pixels may be formed on a single substrate or placed on another appropriate substrate to achieve the overall size suitable for the specific image sensing application. Each sub-pixel is composed of a micro-machined thermal sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 - is an exemplary diagram of a MMW pixel comprised of a plurality of sub- pixels in accordance with the disclosed invention
Figure 2 - is an exemplary photograph of an array of micro-bolometers comprising a MMW pixel, each micro-bolometer being a sub-pixel
Figure 3 - is an exemplary diagram of a MMW pixel comprised of a plurality of sub- pixels, each sub-pixel being a micro-bolometer Figure 4 - is an exemplary TMOS transistor used as a thermal sensing element of a sub-pixel
Figure 5 - is an exemplary temperature coefficient of current of the thermal MOS transistor as a function of the operating gate voltage and temperature
Figure 6 - is an exemplary diagram of a MMW pixel comprised of a plurality of sub- pixels, each sub-pixel being a thermal MOS transistor
Figure 7 - is an exemplary MMW FPA imaging sensor comprised from a plurality of MMW pixels
Figure 8 - is an exemplary MMW system having a focusing lens and a MMW FPA imaging sensor
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The disclosed invention overcomes many of the limitations of prior art solutions. Specifically, an alternative to the antenna-coupled micro-bolometer for the use in millimeter wave (MMW) image sensors is provided by using an array of uncooled thermal sensing devices. Such an array may include a plurality of micro-bolometer devices, thermal MOS transistors and the likes, and as further explained below. Hence, a MMW pixel is comprised of a plurality of sub-pixels, each being an uncooled thermal sensing device, and further each being capable of contributing a measurement towards the measurement value of the MMW pixel. In accordance with the disclosed invention, a MMW FPA imaging sensor is formed by a matrix comprised from a plurality of MMW pixels, each MMW pixel being compromised of sub-pixels, as explained in more detail herein below. Reference is now made to Fig. 1 where an exemplary and non-limiting diagram of a millimeter wave (MMW) pixel 100 comprised of a plurality of sub-pixels is shown. There is shown a two-dimensional (2D) array 110 comprising of "I" rows and "J" columns of sub-pixels. A sub-pixel in a coordinate (i, j) is comprised of uncooled thermal sensing element, capable of detecting the energy associated with MMW radiation and contributing to the overall pixel signal. Each sub-pixel may further contain circuitry to enable its sensing ability of its respective thermal sensing device. Examples for thermal sensing devices are a micro-bolometer and a thermal MOS transistor, described in more detail below, but these examples should not be considered as limiting the scope of the disclosed invention, and other applicable sensing devices capable of achieving the same end-result are specifically included. An analog readout circuitry 130 is coupled to the columns of array 110 allowing the reading of each of the sub-pixels, a process performed under the control of digital controller 120, capable of enabling row-by-row reading. The data from analog readout circuitry 130 is accumulated in summing unit 140. Specifically, summing unit 140 sums the signals produced at each sub-pixel sensor element. A typical size of a MMW pixel 100 is between 4 and 6 millimeters on the side. Digital controller 120, which provides the clocking and timing signals is further capable of overcoming a failure of one or more of the sub-pixels in array 110. Specifically, a failure of a limited number of sub-pixels of the MMW pixel, may be overcome by ignoring the output result received from a faulty sub-pixel. Digital controller 120 may also use summing unit 140, for the purpose of correcting the result detected by MMW pixel 100. Error correction may include, but not limited to, averaging the results across fewer sub- pixels, extrapolating the expected readout of the faulty sub-pixel by evaluating several of its immediate sub-pixels, for example the 8 sub-pixels surrounding the faulty sub- pixel, or using more elaborate extrapolation techniques, including, but not limited to, weighting factors. The resultant measurement is equivalent to that which is achieved by prior art solutions, however, there is no need for an antenna, there is sub-pixel redundancy, and a standard CMOS process with micro-machining post-processing may be used, all providing the desired advantages over prior art, without sacrificing accuracy and stability of the readout. MMW pixel 100 further has a plurality of connecting pads 150 through which the results can be transferred for further processing as may be necessary. There are further provided a connecting pad 154, for example for power supply VDD> and a connecting pad 152, for example for ground GND. The connecting pads may be bonding pads in the case where the MMW pixel is manufactured as a stand-alone device. The design of the connecting pads is such that it allows for the connection of additional MMW pixels in both row and column directions.
In one exemplary and non-limiting embodiment of the disclosed invention an array 110 of sub-pixels is implemented using as a sub-pixel a micro-bolometer. Referring to Fig. 2 a micro-photograph of an array 200 of sub-pixel, each sub-pixel being a micro- bolometer 210, is shown. Each micro-bolometer 210 has a pixel level circuitry 220, as well as column and row circuitry 230. A schematic diagram of a MMW pixel comprised of an array of sub-pixels, where each sub-pixel is a micro-bolometer, is shown in Fig.3. Each sub-pixel, for example the pixel in row 2 and column 8, i.e., pixel 3112,8» is a micro-bolometer, connected via appropriate circuitry to both row and column controls. This structure allows for the separate readout of each of the pixels, determining faulty pixels, and providing the output data to the summing unit 140, as explained above.
In another exemplary and non-limiting embodiment of the disclosed invention there is made use of thermal metal-oxide semiconductor (TMOS) devices that are temperature sensitive, as a sub-pixel of the MMW pixel. Reference is now made to Fig. 4, where an exemplary and non-limiting three dimensional array of TMOS transistors 400, used as a temperature sensing elements of sub-pixels of the MMW pixel, is shown. The TMOS transistors are first created as would a regular MOS transistor, though the starting material is a silicon-on-insulator (SOI) material that has an internal buried oxide 420. The TMOS transistor is formed on the device side of the wafer where active silicon layer 430 is used for the formation of the drain and source of the TMOS transistor. A gate 440 is formed over the active silicon area where the TMOS transistor is to be formed. The drain, gate, and source, are connected to metal leads 450, 460 and 470 respectively. When the processing of the active side of the wafer is complete, a micro-machining process takes place removing certain areas of silicon bulk 410, essentially creating a cavity 480 and causing the TMOS transistor to eventually be suspended by its metal connections to the drain, gate, and source. This release process ensures a thermal decoupling and insulation preventing one sensor from being effected by another sensor. One advantage of the use of a TMOS transistor can be seen in Fig. 5 where a graph of the temperature coefficient of current (TCC) is shown against temperature, for different VQS voltages, applied on a PMOS type TMOS transistor. A person skilled-in-the-art would note that TCC provides an indication of the change in current due to a change in temperature and hence is similar to the temperature coefficient of resistance (TCR) of bolometers, hi the graph in Fig. 5 it is therefore easy to note that in a quiescent point of the TMOS transistor, when a VGS of 1.3V is applied TCC has a very stable value of 4%/°K. Hence, there is achieved a higher sensitivity than provided by prior art solutions, thereby increasing the accuracy of the disclosed solution. It is further noteworthy that the TMOS transistor operates in deep sub-threshold voltage, and at that operating point current levels are small. While it is possible to achieve higher sensitivities, as shown in the graph of Fig. 5, for example using a VGs of 1.5V, it is not a recommended quiescent point due to the very small currents.
Reference is now made to Fig. 6, where an exemplary and non-limiting MMW pixel 600, comprised of sub-pixels 611, each sub-pixel being a TMOS transistor, is shown. An array 610 of sub-pixels 611 comprises a MMW pixel. Each of sub-pixels 611, for example pixel 6112i8, is comprised of at least a TMOS transistor, connected via appropriate circuitry to both row and column controls. Each sub-pixel may further contain additional circuitry, such as buffers, sampling devices, and others, that may be deemed appropriate for the purpose of detecting the energy respective of the radiation of a MMW.
As noted above there is a challenge in the art to provide a MMW image sensor overcoming the limitations of prior art as described in detail above. Therefore, reference is now made to Fig. 7 where an exemplary and non-limiting MMW image sensor 700 comprised of a plurality of MMW pixels 100, is shown. On top of a substrate 790, there are mounted a plurality of MMW pixels 100, organized in "N" rows and "M" columns. Each of MMW pixels 100, for example MMW pixel in coordinate (n, m), is connected to an identical neighbor MMW pixel, in coordinates (n, m-1), (n,m+l), (n-l,m), and (n+1, m), as long as "n-1" and "m-1" are greater than "0", "n+1" is no larger than "N", and "m+1" is no larger than "M". Connectivity between MMW pixels in columns is achieved through a plurality of connecting busses 770. Connectivity between MMW pixels in rows is achieved through a plurality of connecting busses 760. This connectivity allows for control, selection, and readout of each of MMW pixels 100. A central controller 710 is coupled to the plurality of MMW pixels 100 through connecting pads 720 for the rows and connecting pads 730 for the columns. Central controller 710 initiates the readout from each of MMW pixels 100 and is further capable of control functions of each of MMW pixels 100, including, but not limited to, testing, redundancy, and the likes. Bonding pads 750 provide connectivity to external devices; bonding pad 754 provides connection to an external power supply VDD connectivity, and is further connected to the respective power supply VDD connecting pads 154 of each of MMW pixels 100. Bonding pad 752 provides connection to a ground GND connectivity, and is further connected to the respective grovmd GND connecting pads 152 of each of MMW pixels 100. In one embodiment of the disclosed invention a single SOI wafer is used to manufacture, monolithically, the entire MMW image sensor 700, and therefore substrate 790 is the actual wafer. In another embodiment, a hybrid approach is used, where substrate 790 is used for the purpose of mounting the plurality of MMW pixels 100, The hybrid approach allows the manufacture of each MMW pixel 100, or a K-by-L matrix of MMW pixels 100, where N is an integer number of K, for example N=4*K, and where L is an integer number of M, for example M=6*L, as a separate device. The smaller devices will generally have a higher yield than a larger device, for reasons that are well known in the art. The plurality of MMW pixels, or matrices thereof, may be placed separately on substrate 79O. Wire bonding maybe required to connect rows and columns of detached MMW pixels 100, or matrices thereof. Substrate 790 may be of alumina, a printed circuit board (PCB), or the likes, to which each of the MMW pixels 100 is mounted to, or the matrices thereof. Central controller 510 may also be a separate device requiring mounting and bonding on substrate 790. Once all placed on substrate 790 there is formed a hybrid MMW image sensor, allowing for higlier image resolution.
Reference is now made to Fig. 8 where an exemplary and non-limiting MMΛV system 800, having a focusing lens 810 and a MMW FPA imaging sensor 700, is shown. It would be advantageous to use MMW system 800 where it is necessary to eliminate the effects of radiation other than MMW radiation on the readings of MMW FPA imaging sensor 700. In one embodiment of the disclosed MMW system lens 810 is coated with a selective reflective coating 820. The selectivity of the coating will enable the penetration of the MMW but not that of infrared (IR) that would affect the reading of the sub-pixels, being of the IR wavelength. In yet another embodiment, and to further provide for the detection of MMW, each MMW pixel 100-i-j is further coated with an appropriate MMW absorbing film. The MMW absorbing film is deposited over the sub-pixels and serve as an absorbent of the MMW energy thereby raising the temperature of the sub-pixel. In yet another embodiment of the disclosed MMW system is positioning of an array of micro-machined IR reflective mirrors at the IR wavelength focal plane of focusing lens 810. Each IR reflective mirror being designed to insulate a corresponding sub-pixel below from absorbing IR energy. The exact position of the micro-machined array of IR reflective mirrors, between lens 810 and MMW FPA imaging sensor 700, is determined by the chromatic aberration of the collecting lens that gives rise to the focal plane for IR- wavelengths to be separated by a definite distance from the MMW focal plane of the lens. The MMW array is positioned at the MMW focal plane of the lens. As a result of the accurate positioning of the micro-machined array of IR reflective mirrors, individual sub-pixels will respond essentially to MMW energy only.
In yet another embodiment of the disclosed MMW system 800, lens 810 is a chromatically corrected lens, designed to bring both DR and MMW radiations to focus at a single plane. The central Airy disc for IR wavelengths is much smaller than that for MMW. It is well-known in-the-art that the diameter of the Airy disc scales linearly with the wavelength. Therefore there will be a sub-pixel of the MMW pixel that will have a much larger signal than a neighboring sub-pixels belonging to the same MMW pixel. This high energy is associated with the significantly smaller Airy disc resulting of the IR energy collected by a single sub-pixel. As in the case of a defective pixel, explained in more detail above, a pixel showing a significantly above average collection of energy may be disregarded in the summation process and hence the IR effect on the MMW pixel is avoided.

Claims

1. A millimeter wave (MMW) pixel device comprising:
a two-dimensional array of sub-pixels, each sub-pixel being a suspended thermal sensor;
an analog readout circuitry coupled to columns of said array of sub-pixels;
an integration circuit coupled to said analog read out circuitry for the purpose of determining the thermal energy detected by the sub-pixels of said array of sub- pixels; and,
control circuitry coupled to said array of sub-pixels, said analog read out circuitry and said integration circuit, said control circuit being capable of at least controlling the readout process of said array of sub-pixels.
2. The device of claim 1 , wherein said suspended thermal sensor is formed using at least a micro-machining process.
3. The device of claim 1, wherein said suspended thermal sensor is one of: temperature sensitive transistor, micro-bolometer.
4. The Device of claim 3, wherein said temperature sensitive transistor is a metal- oxide semiconductor (MOS) transistor.
5. The Device of claim 4, wherein said MOS transistor operates in sub-threshold gate-to-source voltage. 6. The Device of claim 1, wherein said MMW pixel is formed on a silicon-on- insulator (SOI) wafer.
7. The Device of claim 4, wherein said MOS transistor is manufactured in a complementary MOS (CMOS) manufacturing process.
8. The Device of claim 1, wherein said MMW pixel is further capable of being coupled to another MMW pixel to form a MMW image sensor comprising of a focal plane array of two or more MMW pixels.
9. The Device of claim 7, wherein said another MMW pixel may be connected to said MMW pixel to form at least one of: a row, a column.
10. The Device of claim 1, wherein said control circuitry is capable of replacing the value of a faulty sub-pixel by another value extrapolated from at least values measured on at least a neighbor sub-pixel.
11. The Device of claim 1 , wherein said thermal sensor is an uncooled thermal sensor.
20. A millimeter wave (MMW) image sensor comprising:
a( focal plane array of MMW pixels, each MMW pixel further being comprised of a two dimensional array of sub-pixels, each sub-pixel being a suspended thermal sensor, said MMW pixel further comprising respective MMW pixel control circuitry; and,
circuitry coupled to said focal plane array of MMW pixels for at least the purpose of delivering the image information detected by said MMW image sensor and for further controlling each of said MMW pixels comprising said focal plane array of MMW pixels.
21. The sensor of claim 20, wherein said suspended thermal sensor is formed using at least a micro-machining process.
22. The sensor of claim 20, wherein said suspended thermal sensor is one of: temperature sensitive transistor, micro-bolometer.
23. The sensor of claim 22, wherein said temperature sensitive transistor is a metal- oxide semiconductor (MOS) transistor.
24. The sensor of claim 23, wherein said temperature sensitive MOS transistor operates in sub-threshold gate-to-source voltage.
25. The sensor of claim 20, wherein at least said MMW pixel is formed on a silicon- on-insulator (SOI) wafer.
26. The sensor of claim 23, wherein said MOS manufacturing process is a complementary MOS (CMOS) manufacturing process.
27. The sensor of claim 20, wherein each of said MMW pixels are placed, on top of a supporting substrate.
28. The sensor of claim 27, wherein said supporting substrate is a wafer to which MMW pixels are integrated on.
29. The sensor of claim 17, wherein said supporting substrate is at least one of: alumina, printed circuit board (PCB). 30. The sensor of claim 20, wherein said circuitry coupled to said array of MMW pixels is further capable of determining at least a faulty MMW pixel.
31. The sensor of claim 30, wherein said MMW image sensor control circuitry is further capable of replacing the value of said faulty MMW pixel by another value extrapolated from at least values measured on at least a neighbor MMW pixel.
32. The sensor of claim 30, wherein said MMW pixel control circuitry is further capable of identifying at least a faulty sub-pixel.
33. The sensor of claim 32, wherein said MMW pixel control circuitry is further capable of replacing the value of said faulty sub-pixel by another value extrapolated from at least values measured on at least a neighbor sub-pixel.
40. A millimeter wave (MMW) system comprising:
an MMW image comprising a focal plane array of MMW pixels, each MMW pixel further being comprised of a two dimensional array of sub-pixels, each sub-pixel being a suspended thermal sensor, said MMW pixel further comprising respective MMW pixel control circuitry and, circuitry coupled to said focal plane array of MMW pixels for at least the purpose of delivering the image information detected by said MMW image sensor and for further controlling each of said MMW pixels comprising said focal plane array of
MMW pixels; and,
focusing means designed to direct at least MMW to at least an MMW pixel. 41. The system of claim 40, wherein said focusing means is a lens (or mirror).
42. The system of claim 41, wherein said lens is further coated with an reflective coat capable of reflecting infrared (IR) radiation.
43. The system of claim 40, wherein at least a MMW pixel is coated with a MMW absorbent material.
44. The system of claim 43, wherein said MMW absorbent material is deposited over said at least MMW pixel.
45. The system of claim 40, wherein said MMW system further comprises:
an array of IR reflective mirrors positioned between said MMW image sensor and said lens.
46. The system of claim 45, wherein said array of IR reflective mirrors is positioned at the IR focal point distance from said lens.
47. The system of claim 40, wherein said lens is capable of focusing both IR and MMW onto the same plane.
48. The system of claim 47, wherein said MMW system is further capable of discarding the reading of a sub-pixel having a significantly higher level of energy than its neighboring sub-pixels.
PCT/IL2005/001062 2004-10-04 2005-10-02 Millimeter wave pixel and focal plane array imaging sensors thereof WO2006038213A2 (en)

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