Figure 7 is a planar view of a focal plane array utilizing a large number of detector pixel structures in accordance with the present invention; and. Figure 8 is a block and schematic illustration of an electrical circuit for each EQWIP pixel structure in accordance with the present invention.
Detector structures with metal gratings are shown in Figure of this article. These are shown in Figures 1 and 2 herein. Reflected zero order diffractive mode radiation is indicated by arrows Trapped higher order diffractive mode radiation is indicated by arrows A metal diffractive grating 38 is joined to the MQW region.
Because of quantum mechanical selection rules an MQW only absorbs radiation through modes in which a component of the electric field is perpendicular to the MQW layers in the MQW region The purpose of the grating 38 is to increase the absorption of radiation by diffracting that radiation, as indicated by arrow 22, by the grating 38 to produce trapped diffracting radiation modes shown by the arrows 26 with electric field component perpendicular to the MQW region.
The arrows 26 represent trapped radiation. Thus, the addition of the metal grating 38 increases the absorption of infrared radiation by the QWIP A QWIP 40, similar to that shown in Figure 1, has incident mode radiation indicated by arrow 22, reflected zero order diffractive mode radiation indicated by arrows 24 and trapped higher order diffractive mode radiation indicated by arrows The Aluminum to Gallium alloy ratio, x, is typically 0.
A sequence of processing steps for fabricating a detector in accordance with the present invention is shown in Figures 3 A, 3B, 3C and 3D. An Al 6 Ga. As etch stop layer 64 is formed on the surface of the substrate The layer 64 has a preferable thickness of 1,A. The contact 66 has a preferable thickness of 0.
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A multiple quantum well MQW structure 68 is formed over the contact Structure 68 comprises a plurality of material layers which make up the MQW. The detailed characteristics of structure 68 are shown in Table I below. Contact 70 has a preferred thickness of 0. The doping for contacts 66 and 70 is shown in Table I. Referring to Figure 3B, openings 76 and 78 are formed by etching through the contact 70 and MQW structure Referring to Figure 3C, an In indium bump 80 is bonded to the reflector 72 and to a contact terminal not shown of a silicon readout integrated circuit ROIC The bump 80 is formed by a photoresist image reversal process followed by In layer deposition and liftoff.
A wicked epoxy 83 provides a physical bond between the ROIC 81, the bump 80 and the remaining exposed structure of the detector Rogatto, Editor, Chapter 5,. Further processing of the detector 60 is shown in Figure 3C. The structure shown in Figure 3C is inverted from that shown in Figure 3B. The bulk of the substrate 62 is removed by lapping and the remainder is removed by etching down to the etch stop layer The layer 64 itself is then removed.
AlGaAs/GaAs Quantum Well Infrared Photodetectors
Contact 66 is thinned from 0. Referring to Figure 3D, a conventional resist mask is applied to the surface of the layer 66 and an etch operation is performed. The etch extends through the layer 66, the MQW structure 68 down to the contact This produces elongate structures 82, 84, 86, 88, 90 and In a similar fashion, elongate structure 86 comprises a contact 66B which is in physical and electrical contact with an elongate MQW element 68B, structure 88 comprises a contact 66C over an MQW element 68C and structure 90 comprises a contact 66D over an MQW element. The elements 84, 86, 88 and 90 are spaced in such a manner as to form a diffraction grating for the infrared radiation to be received by the detector The etching process shown in Figure 3D also produces structures 82 and Referring to Figure 4A, there is shown a dimensioned layout for a detector in accordance with the present invention.
This represents one pixel element within an array of such elements. The detector includes elements , , , and which are formed in the same manner as the elements 84, 86, 88 and 90 shown in Figure 3D. Each of the elements has a width of 1. This physical configuration is tuned for optimum response to LWIR radiation, which is in the The ohmic contact is fabricated on the surface of contact before the formation of reflector The ohmic contact provides a good ohmic connection between the contact and the reflector A further embodiment of a detector in accordance with the present invention is shown in Figure 4B.
This is essentially the same as the detector shown in Figure 4A, but the etching operation which formed the elements , is continued until the etching extends through a portion of the lower contact , but not the entire thickness of the contact This etching operation produces elements , , , and The width and spacing of these elements is the same as the width and spacing of the dements shown in Figure 4A. The patterned contact is positioned on the surface of a metal reflector , which is essentially the same as the reflector shown in Figure 4A.
A further embodiment of a detector in accordance with the present invention is shown in Figure 4C. This is the same as the detector shown in Figure 4A but the etching operation which formed the elements is continued until the etching extends through the lower contact except for one region. A region A is not removed. This etching produces dements , , , and The width and spacing of these elements is approximately the same as the width and spacing of the dements shown in Figure 4A. The sensitivity of a detector to a given wavdength of infrared radiation is related to the width and spacing of the MQW elements.
The ohmic contact provides a good ohmic connection between the region A, which is connected electrically to all of the other lower contacts, such as C, and the reflector Figures 4A, 4B and 4C represent section views of the elongate dements. Referring now to Figure 5, there is shown a focal plane detector array which includes a plurality of detectors as described above arranged in an array. A view of the detector array as indicated by the arrows corresponds substantially to the views shown in Figures 3D and 4A.
The detector array includes detector pixel structures , , , , and Each pixel structure has a corresponding In bump such as bump for pixel structure , bump for pixel structure , bump for pixel structure and bump for pixel structure Each of the bumps is connected to a corresponding terminal not shown of a ROIC A wicked epoxy bonds the pixel structures to the ROIC The pixel structure is described in detail.
Quantum well infrared photodetector - Wikipedia
The remaining pixel structures have a similar configuration. Elongate segments in pixel structure include segments E, F, G and H. The region between the bumps is filled with wicked epoxy , such as shown in Figures 3C and 3D. This is essentially the same as shown in Figures 3D and 4A. A planar contact is formed in a position between the MQW structure and a contact reflector. The pixd structures , , , , and have respective lower second contacts A, B, C, D, E and F.
Reflector A is physically and dectrically in contact with bump Reflectors AD correspond to reflector 72 in Figure 3D. Slots , , , and are etched into the structure of detector to electrically isolate each pixel structure. Each of these slots extends upward from the region occupied by the bumps up to the lower surface of the contact These slots are filled with the epoxy , which is electrically nonconductive. Each of the pixel structures , , , , and has 25 sections 5 by 5 and has generally at the base of one section a rectangular, planar ohmic contact, such as ohmic contact shown for pixd structure The ohmic contact can extend to have an area greater than that of one section.
A similar ohmic contact is provided at a similar location in each of the other pixd structures. The detector array is fabricated by use of the materials and steps described in reference to Figures 3A-3D. Parameters for the detector array , as shown in Figure 5, are set out in Table I below. The top contact is connected to through-conductors on the periphery outside the pixel structures. These through-conductors extend down to In bumps, as described above, to terminals of the underlying ROIC.
The detector array shown in Figure 5 is non-polarized because the MQW diffractive grating dements are transverse to each other within each pixd structure.
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Therefore, all polarizations of incident infrared radiation are received. Referring to Figure 6, there is shown a polarized detector array which is similar to the detector array shown in Figure 5, but has dongate diffractive MQW dements extending in only one direction. The detector array includes detector pad structures , , , , and Bump connects pixel structure to a terminal not shown of ROIC In a similar fashion, there is provided a bump for pixel structure , a bump for pixel structure and a bump for pixd structure There is a corresponding bump for each of the pixds within the detector array Each pixel structure therefore generates a different signal which is provided to the ROIC The space between the bumps is filled with wicked epoxy for bonding the surrounding elements together.
The pixel structure has elongate contacts A and B. Below the contact , there is a diffractive MQW structure similar to the structure shown in Figure 5 and the structure 68 shown in Figure 3D. This consists of contact A for structure , contact B for structure , contact C for structure , contact D for structure , contact E for structure and contact F for structure Contact corresponds to the contact 70 shown in Figures 3A-3D.
The contacts A-F are separated by slots , , , and Pixd structure has reflector A, structure has reflector B, structure has reflector C and pixd structure has reflector D. As described for detector array in Figure 5, each of the reflectors provides an dectrical connection to the corresponding bump beneath the pixd structure. Each reflector also functions as an infrared radiation reflector. Each of the pixel structures , , , , and has five longitudinal sections and one longitudinal section thereof is provided with an ohmic contact, such as ohmic contact shown for pixel structure and contact shown for pixel structure Each ohmic contact is formed between the corresponding lower contact, such as contact E for pixel structure and the underlying reflector C.
The detector array shown in Figure 6 has dongate MQW elements running in only one direction. It is therefore sensitive to only one polarity of infrared radiation. Referring to Figure 7, there is shown a focal plane array which comprises a plurality of pixel structures, as previously described in accordance with the present invention.
This array preferably has pixel structures horizontally and pixel structures vertically. One pixel structure , as an example, corresponds to pixel structure described in reference to Figure 5. Each of the pixd structures within the array has a separate dectrical output signal so that a complete image having x dements can be produced. The electrical connection of pixd structures to an ROIC as described for the present invention is illustrated in Figure 8.
Reference is made to the detector array described in Figure 5. For a preferred embodiment this is a "direct injection" ROIC. Transistor functions as a preamplifier. The amplified signal produced by transistor is integrated by a capacitor The integrated signal is provided to a multiplexer not shown which selectively samples the signal produced by each of the pixel structures throughout the array.
The MQW element is provided with the noted bias voltage through the transistor and intervening conductors to the MQW element When infrared radiation is absorbed by the diffractive MQW element , carriers are produced which change the conductivity of the MQW element. This changes the output signal from the element. This change in signal corresponds to the received infrared radiation.
The collection of all of the signals from the pixel dements produces an image representative of the received infrared radiation. Their combined work covers a significant share of QWIP research that has been conducted worldwide. Harald Schneider , Hui C. Photoconductive QWIP.
Photovoltaic QWIP. Optical Coupling Miscellaneous Effects.
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Related Structures and Devices. Conclusions and outlook. References Quantum well infrared photodetectors: physics and applications Harald Schneider , H.