Abstract:
A high performance microbolometer in which a pixel contains the material VOx wherein x of VOx is set at a value to adjust a thermal coefficient of resistance to a selected value between 0.005 and 0.05.

Description:
This Appln is a Div. of Ser. No. 08/770,894 filed Dec. 31, 1996. 
    
    
     The present invention pertains to microbolometer sensors and particularly to detector material for microbolometers. More particularly, the invention pertains to a particular detector material which is fabrication from a special ion beam sputter deposition process. The U.S. Government has certain rights in the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     A major factor in the sensitivity of a bolometer is the TCR (thermal coefficient of resistance) of the detector material. The overall NETD sensitivity of the bolometer also depends on the noise level. Previous bolometer materials are typically high TCR metals with a TCR in the range from 0.003 to 0.004. These materials have low noise but also have low TCR. Since the metals are reflectors, they also degrade the absorbance properties of the detector. Materials which undergo a phase transitions (i.e., Mott transition) can have a very high TCR&#39;s in the transition region but can suffer from a number of problems. First, the latent heat accompanying the phase change for these materials may significantly decrease the sensitivity of the detector. Second, most switching material can be produced in only one form without additional doping, which defines the material resistance and TCR. Further, the temperature range over which the transition occurs is typically very small requiring tight temperature control of the operation. Finally, the films must be produced in crystalline form which requires high temperature depositions. 
     SUMMARY OF THE INVENTION 
     The present invention is peculiar vanadium oxide (VOx/ABx) (i.e., VO x /AB x ) detector material and process that is used to make that material. The x of VOx is a value fitting for the pixel being sputter deposited by the present process and is not necessarily a specific digit such as “2”, but may be between 1 and 2.5. That material is deposited as part of a pixel for a high performance microbolometer. The material is deposited by an ion beam sputtering with control of the deposition process leading to a flexible detector process for microbolometer detectors. These detector materials have optical, electrical, and thermal properties compatible with high performance detectors but which can be readily modified to suit individual requirements of an array design. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 reveals a TCR comparison of various materials, including VOx (ABx). 
     FIG. 2 is a graph showing electrical characteristics of a high TCR VOx detector film. 
     FIG. 3 exhibits determination of VOx resistance by control of the deposition environment. 
     FIGS. 4 a  and  4   b  are a schematic of the deposition system. 
     FIG. 5 is a schematic of the deposition process flow. 
     FIGS. 6 a  and  6   b  show several stages of the deposited wafer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     VOx deposited by a controlled ion beam sputtering process has produced bolometer detector films with a wide range of performance properties which has led to a flexible detector manufacturing process. Some of the advantages of the present detector film materials of the invention which is the class of these materials and the processing used to make these materials, are noted. Bolometer film materials with high TCR&#39;s ranging from 0.005 to &gt;0.03, such as 0.05, have been demonstrated. For a given resistivity, these materials are higher performing in microbolometers than any known material except for a few crystalline materials (which have not been produced on pixels), as shown in FIG.  1 . FIG. 1 is a graph showing a comparison of the TCR versus conductivity of various materials relative to ABx (i.e., VOx). This class of materials has a well behaved relationship between resistance and TCR given by the relationship TCR=A+B*log((rho) which, for example, may be (TCR=0.03+0.01*log((rho)). A wide range of pixel resistance and TCR properties are possible by using different resistivity materials (obtained by slight modifications to the deposition process) combined with different detector patterns and thicknesses. These electrical properties for any particular VOx film are well behaved and characterized over a wide range of temperatures and are not limited to a narrow transition region, as indicated by FIG.  2 . For any particular VOx film, resistance is given by LnR=A+(B/T) and the TCR which is defined as 1/R dR/dT is therefore TCR=B/T 2 . FIG. 2 exhibits the electrical characteristics of a high TCR VOx detector film. These characteristics reveal a material having a well-behaved operation over a wide temperature range. Resistance levels are in the proper range to permit high current bolometer operation for optimal responsivity. The films are amorphous and exhibit no latent heat effects unlike the phase transition effects in VO2. The films are stable after annealing if not taken to a higher temperature. The resistance change on annealing is well-defined and can be corrected for by changes to the initial deposition conditions. 1/f noise levels are defined by V noise =V bias {square root over (k/f)}. k values as low as 10 −12  to 10 −14  make 1/f noise contributions to total noise very small. Noise levels are close to Johnson noise limited values. 
     (U) The optical properties of VOx are compatible with high absorbance in the detector. The thermal mass of VOx, the thermal property of importance, is comparable to the major pixel material, Si3N4 (i.e., Si 3 N 4 ). These VOx films have a high TCR over a range of thicknesses from as low as a few hundred Angstroms to as thick as 1500 Angstroms. This material of the films is compatible with microbolometer properties. The VOx material properties can readily be modified by a simple change in the ion beam sputter deposition environment of the process of the present invention, as revealed by FIG.  3 . FIG. 3 is a graph that shows determination of VOx resistance by control of the ion beam sputter deposition environment such as the gas control level. The present ion beam sputtering provides sufficient control of the oxidation process to permit non-stoichiometric formation of VOx films. In other reactive deposition techniques, the oxidation process tends to proceed to completion forming only stoichiometric material. The ion beam sputter deposition is a lower temperature deposition process. This means that added flexibility in the patterning of VOx films can be achieved via liftoff processing which entails the use of photoresist during deposition. 
     The method of the present invention is a process  18  of FIG. 5, performed in conjunction with deposition  10  of FIG. 4 a,  which is capable of making the above-noted VOx material. Circular silicon wafers  11  with substrates containing electronic circuits and pixel lower layers  12  which are coated with an approximately 5000 Angstrom Si3N4 layer  13 , are loaded into a five-wafer carousel  19  (of FIGS. 4 a  and  4   b ) through port  15  of deposition apparatus  10 . Also, photoresist  74  may be on wafer  17  which defines pixels. At present, each wafer  11  has a diameter  14  of four inches. Wafer  11  loading is step  16  of process  18  flowchart. System  10  is first calibrated according to calibration step  20  of process  18 , which involves pumping the system down to 2×10 −7  Torr pressure. The vacuum system is capable of pumping to base pressure less than 1×10 −7  Torr with a throughput of better than an eight inch two-stage cyropump  23  via valve  24 . System  10  is pumped down by opening valve  21  which is left open during the process. system  10  may be warmed up at step  25  with lamps  23 . A residual gas analyser (RGA)  22  is warmed up at step  26  before the process calibration. RGA  22  is connected to chamber  27  and has a quadrupole probe which has an electron multiplier (EM) detector in a turbo-pumped sampling manifold to allow for operation in the 10-7 Torr pressure environment. RGA  22  may be connected to either upper chamber  31  or lower chamber  27 . RGA  22  has an analog output. 
     Silicon wafers  11  may be anything with active electronics on it such as CMOS or bipolar devices  73  or a sacrifical layer  73  of FIGS. 6 a  and  6   b,  or other kinds of detector components up to the Si3N4 layer  13 , photoresist  74  and/or VOx layer  35  deposition FIGS. 6 a  and  6   b.  RGA  22  valve  28  is opened and argon flow is set to approximately 3 scc/m at stage  29  of process  18 . RGA  22  is calibrated via an adjustment of its gain to get a standard RGA argon reading at step  30 . RGA  22  is a closed source device. It is connected to chamber  27  in FIG. 4 a.  The output of RGA  22  detects a spectrum of species of gas constituents and the detected gases are identified. RGA  22  is pumped out so that the pressure in RGA  22  is lower than chamber  27 , and the ion species have to go through a small orifice at valve  23 . RGA  22  is made by UTI Inc. With RGA  22  valve open, the argon gas flow reading is set for calibrating RGA  22  to a known argon peak level. 
     (U) At step  32 , the argon gas flow is set for gun  33  and hollow cathode neutralizer (HCN)  34 . Gun  33  uses argon as a sputtering gas. Ion beam  35  is neutralized by beam  40  of HCN  34 . The ion gun is from Ion Tech Inc. 
     A target on target holder  36  is positioned relative to gun  33  beam  35  so that vanadium is selected as a target material  46 . The other side of target holder has SiO2 as a selectable target material  47 . The target material is set at a 45 to 55 degree angle relative to the direction of beam  35 . The center of the target material is at a distance of 7 and ⅝ inches from the beam  35  exit of gun  33 . Argon gas flow to gun  33  is set with MFC  37  to a 2.5 scc/m operating level, and to HCN  34  set with MFC  39  to a 3.5 scc/m operating level. Xenon (Xe) may also be used in place of Argon (Ar). Cooling water from source  41  is turned on to gun  33  via line  42  and to target holder  36  via line  43 . At step  38 , an ion gun power supply  44  is turned on and the initial gun start-up parameters are set to 20 mA at 1 kV. The ion gun source is turned on and the source stabilizes. Beam  35  of ion gun  33  is turned on. The power supply voltages are adjusted. There is plasma in gun  33  and ion beam  35  is generated or accelerated through grids of gun  35 . 
     At next step  48 , target material  46  is pre-sputtered with no oxygen. Pre-heat system quartz lamps  45  are turned off if on. Target material  46  is pre-sputtered for 240 seconds at the low power of 20 mA at 1 kV. During the next 240 seconds, target  46  is presputtered at the medium power of 35 mA at 1.5 kV. Presputter continues for the next 120 seconds at the high power of 50 mA at 2 kV, which concludes system calibration stage  20 . The presputter without oxygen is for cleaning target  46  for ten minutes or so. 
     The following stage  50  begins with step  49  of pre-sputter with an oxygen ramp to condition target  46  to a desired RGA  22  level. The oxygen goes up by little steps and then to larger steps as one measures the oxygen within system  18 . This not cleaning target  46  but conditioning target  46  to the desired RGA  22  level. One increases the oxygen to get a character profile for film  35  based on past experience and RGA  22  is set at an arbitrary level called y. The beam  35  power is kept at 50 mA at 2 kV and controller  53  of MFC  51  sets the oxygen flow to chamber  27  via tube  52 , to 0.5 scc/m for 90 seconds. Then the oxygen flow is increased by 0.1 scc/m for 90 seconds. The latter is repeated until the 32 AMU partial pressure increase is equal to or greater than ten times over the previous partial pressure. The operating level setpoint is based on the partial pressure rise of the previous step which is about midpoint of the last 32 AMU partial pressure increase. The precise location of the operating setpoint determines the resistance and TCR. The 32 AMU partial pressure setpoint is entered into controller  53 , which determines the level of oxygen flow for VOx film  35  deposition. One increases the flow of oxygen in an incremental way until O 2  is at a point where the RGA outputs an O 2  signal. One measures the mass and monitors the RGA O 2 . Ion gun  33  is run at a set level with a fixed voltage and current. Monitoring of RGA  22  of 32 AMU is done at controller  53  where the flow is adjusted to achieve the starting level. A computer processor  56  may be used at step  59  to monitor RGA  22  and adjust the oxygen flow via controller  53  to achieve starting condition or level. 
     At step  54 , rotation of wafer substrate  11  is started. A control loop is started with a presputter for 300 seconds at the setpoint of controller  53 . A shutter  55  is opened at step  58  after system  10  has stabilized. A timer  57  is started with the time determined by a desired thickness, of which the deposition rate is approximately at 25 Angstroms per second. RGA  22  may be monitored and oxygen flow adjusted at step  60  during deposition step  61 . The center of sputtered target  46  with its surface at 45 to 55 degrees relative to and 12 inches from the to-be-deposited surface of wafer  11 , is aligned at one inch from the center of wafer  11 . After the desired thickness is achieved, then shutter  55  is closed at step  62 . Then carousel  19  is turned at step  63  for the next wafer  11  to be coated, the control loop starts with the presputter at step  49  of deposition stage  20 , and goes through the same steps of the process for the previous wafer  11  deposition. After all the subtrates  11  of carousel are deposited, the control loop of stage  20  is stopped. The oxygen MFC  51  is set to zero scc/m, RGA  22  sample valve  28  is closed, ion beam  33  is turned off or to stand-by mode, and RGA  22  is turned off. 
     Process  10  is a low temperature process which does not go over 100 degrees C. which would cause photoresist  74  to harden. Typically this process is performed at about 80 degrees C. or less. 
     Process  18  for wafers  11  moves on to stage  64  for SiO2 deposition. At step  65 , target holder  36  is rotated so that the surface of target  47  will be at a 45 degree angle relative to the direction at the center of ion beam  35  when it is turned on, and the oxygen flow is set to 2.0 scc/m at MFC  51 . First wafer  11  is rotated in by carousel  19  as step  66 . Ion gun  33  is turned on at 50 mA at 2 kV to presputter target  47  for 300 seconds. Shutter  55  is opened and timer  57  is started. Timer  57  is set to a time period to attain a desired thickness of SiO2 on wafer  11  at a deposition rate of 0.33 Angstrom per second at step  67 . Then shutter  55  is closed. Next wafer  11  is rotated to by carousel  19  for SiO2 deposition at step  68 , and steps for depositing SiO2 on previous wafer  11  are followed. After the last wafer  11  is coated with deposition of SiO2, the system  10  is shut down at step  69  of stage  64 . Ion beam  35  is turned off, MFC  51  is set to 0.0 scc/m, ion beam source  33  is turned off, and substrate rotator  70  and carousel  19  are turned off. The one should wait and let system  10  cool down for 45 minutes. After cool-down, Hivac valve  24  is closed. and system  10  is vented with dry N2 from supply  71  through valve  72 . One may open system  10  when it is at atmospheric pressure and remove wafers  11 . To start process  18  over with another set of substrates or wafers  11 , one introduces wafers  11  into upper chamber  31  through port  15  and close system  10 . Quartz lamps  45  are turned on to a preset level to yield 80 degree C. temperatures in system  10 . N2 vent gas valve  72  is turned off and MFC  37  and MFC  39  are set to 0.0 scc/m. Next pump down with cyropump  23  and open Hivac valve  24 . Check for leaks and go through process  18  as indicated above. The values of the parameters and settings of the above-noted embodiments are by example only, but could vary from case to case. 
     From wafer  11  processed by system  10 , is made microbolometer pixels  77  of FIGS. 6 a  and  6   b.  On wafer  11 , prior to system  10  depositions of VOx and SiO2, may be Si substrate  12  covered with a device layer  73 . On layer  73  is pixel Si3N4 material layer  13 . Pixels  77  are defined with a photoresist mask  74 . Then, VOx layer  35  and SiO2 layer  75  are respectively deposited on wafer  11  as indicated above and revealed in FIG. 6 a.  The next step is to chemically remove photoresist  74  and a portion of layers  35  and  75  formed on and over photoresist layer is likewise removed, resulting in pixel  77  as shown in FIG. 6 b.  A via or hole  78  is etched through layer  75  for electrical contact which is made with a nichrome (NiCr) strip  76  formed by depositing and patterning on a small portion of pixel  77 . NiCr  76  forms the contact to the VOx portion of pixel  77 . VOx layer  35  covers a major portion of pixel  77 . An additional Si3N4 layer may be formed on pixel  77  of FIG. 6 b  for more protection. Layer  75  is about 200 Angstroms. If layer  75  were not put on prior to placement of contact  76 , then the VOx portion of the pixel would be degraded due to the electrical degradation of VOx during presputter of the film prior to Si3N4 and subsequent NiCr deposition of strip  76 . Layer  35  may range from 200 to 2000 Angstroms depending on the desired TCR. Layer  13  is about 500 Angstroms. But if an Si3N4 layer  80  is formed on layer  75 , then NiCr strip would go through layer  80  and layer  75  to contact VOx layer  35 .