Abstract:
A sensor comprises two photodiodes sensitive to different wavelengths. The photodiodes or detectors are stacked in a vertical relationship to each other. A bandpass filter is provided to limit the wavelengths of light reaching the detectors. The photodiodes are formed of various combinations of materials such as AlGaN or InGaN, or different compositions of the same material. Charge detectors are coupled to each detector to provide a signal representative of the amount of radiation detected in their corresponding bandwidths. A biological sample is provided proximate the filter. A laser is used to illuminate the biological sample to create biofluorescence corresponding to intrinsic tryptophan of bacteria.

Description:
The Government may have rights in this invention pursuant to Contract No. N00014-00-C-0407, awarded by the Department of the Navy. 

   INCORPORATION BY REFERENCE 
   Co-pending commonly assigned U.S. patent application Ser. No. 09/275,632, to Wei Yang et al., filed Mar. 24, 1999, and entitled “BACK-ILLUMINATED HETEROJUNCTION PHOTODIODE” is hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to light sensors, and in particular to a detector for at least two wavelength bands such as infrared and visible color wavelengths. 
   BACKGROUND OF THE INVENTION 
   Optical filters are commonly used in a wide variety of applications. For example, optical filters are used to provide separate optical “channels” in optical fiber networks. Optical filters are also used to monitor the spectral emission from the power plants and engines to provide a level of combustion monitoring and control. Optical filters can also be used in biological particle identification systems to provide spectral resolution of the fluorescence needed for high levels of discrimination of biological materials. These are just a few of the many applications for optical filters. 
   Many optical filters are formed from thin films that reflect or transmit a narrow band of wavelengths. In many cases, such filters are constructed from several hundred layers of stacked narrow band filters, which collectively reflect or transmit a narrow band of wavelengths. Arrayed waveguide gratings are also commonly used. A limitation of many of these filters is that they are not wavelength tunable. That is, the operative wavelength cannot be dynamically changed during operation to select a different optical wavelength. 
   Biological or inorganic particle identification systems identify the size and class of particles in air via scattered light and intrinsic ultraviolet fluorescence measurements. These system are useful as early warning sensors in biological warfare (BW) agent attacks either in an urban area or on a battlefield. Present day systems, such as a fluorescence aerodynamic particle sizer are large and power hungry, and are not portable. Smaller and more lightweight systems are desired for both portable applications and for incorporation into larger, more complex systems for building protection and battlefield BW agent detection systems. 
   Optical filters are combined with detectors for measuring the amount of fluorescence passing through the filters. Some detectors have a limited bandwidth of detection that is not as great as the bandwidth desirably passed by the filter. There is a need for an improved detector that can detect a larger bandwidth passed by the filter. 
   SUMMARY OF THE INVENTION 
   A sensor comprises two photodiodes sensitive to different wavelengths. In one embodiment, the photodiodes or detectors are stacked in a vertical relationship to each other. A bandpass filter is provided to limit the wavelengths of light reaching the detectors. 
   In one embodiment, the detectors are sensitive to adjacent lower and higher bands of wavelengths. The wavelengths range from approximately 250 to 390 nanometers, with a first detector absorbing radiation in the range of approximately 250 to 300 nanometers, with a second stacked detector absorbing radiation in the range of approximately 290 to 390 nanometers. 
   The photodiodes are formed of various combinations of materials, the proportions of which are modified to change the wavelength they absorb. In one embodiment, the photodiodes are formed of AlGaN or InGaN. Reducing the mole fraction of Al, increases the wavelength absorbed or detected by the detector. Further increases in wavelength are obtained by using InGaN. Each of these compositions has compatible lattice constants. Each photodiode is formed of either a different material, or the same material with different compositions for different wavelengths. 
   In a further embodiment, charge detectors are coupled to each detector to provide a signal representative of the amount of radiation detected in their corresponding bandwidths. A biological sample is provided proximate the filter. A laser is used to illuminate the biological sample to create biofluorescence corresponding to intrinsic tryptophan of bacteria. 
   In one embodiment, the sensor includes a tunable bandpass filter, a pair of detectors, and readout electronics, each supported by a different substrate. The substrates are secured relative to one another to form the sensor. The readout electronics are electrically connected to one or more electrodes of the detector through, for example, one or more bump bonds. The readout electronics provide a signature of the biological sample which is compared to known signatures to identify the composition of the sample. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional representation of a sensor having a dual stacked optical detector and filter. 
       FIG. 2  is a sectional representation of the sensor of  FIG. 1  and further including a block representation of charge detectors coupled to the dual stacked optical detector. 
       FIG. 3  is a graph showing example percent transmissions of the sensor of  FIG. 1  versus incoming wavelength. 
       FIG. 4  is a graph showing lattice constant for varying mole fractions of elements comprising the dual stacked optical detector. 
       FIG. 5  is a sectional—block representation of the sensor of  FIG. 1  including a sample and laser for fluorescing the sample. 
       FIG. 6  is a cross section view illustrating formation of a first detector for the dual stacked optical detector of  FIG. 1 . 
       FIG. 7  is a graph showing the calculated percent transmission of an alternative sensor. 
       FIG. 8  is a graph showing the calculated percent transmission of a yet further alternative sensor. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
     FIG. 1  is a schematic cross-sectional side view of an illustrative tunable bandpass detector  110  in accordance with the present invention. The illustrative tunable bandpass detector  110  includes a tunable bandpass filter  112 , a multiple frequency detector  114  and readout electronics  116 , each supported by a different substrate. For example, the tunable bandpass filter  112  is supported by a first substrate  118 , the detector  114  is supported by a second substrate  120 , and the readout electronics  116  are supported by a third substrate  122 . 
   In the illustrative embodiment, the tunable bandpass filter  112  includes a Micro Electro Optical Mechanical System (MEOMS) etalon. The MEOMS includes a top plate  124  and a bottom plate  126 . The bottom plate  126  corresponds to the first substrate  118 , or other layers provided on the first substrate  118 , as desired. Both the top plate  124  and the bottom plate  126  are optionally adapted to include a reflective region. In  FIG. 1 , the top plate includes a reflective region  128 , which includes a Distributed Bragg reflector in one embodiment that includes a semiconductor and/or dielectric mirror stack. Likewise, the bottom plate  126  includes a reflective region  130 , which also includes a Distributed Bragg reflector that further includes a semiconductor and/or dielectric mirror stack. 
   The top plate  124  and the bottom plate  126  are separated by a separation gap  132  to form a Fabry-Perot cavity. To selectively tune the tunable bandpass filter  112  to a desired bandpass wavelength, the top plate is pulled toward the bottom plate  126 , which changes the separation gap  132 . The range of movement of the top plate  124  relative to the bottom plate  126  determines the spectral range of the wavelengths that can be selected. In some embodiments, a lens  134  is positioned adjacent the tunable bandpass filter  112  to help direct and/or shape the incoming light beam. The purpose of filter  112  is to provide a bandpass filter function to ensure only a limited bandwidth of light is passed through to the detector. Significant light outside the desired range of the detector would have an adverse effect on the accuracy and reliability of the detector. The bandpass filter may also be constructed in any other manner desired to provide such function. 
   In one embodiment, the top plate  124  is suspended above the bottom plate  126  by one or more supporting legs or posts  136 . In addition, one or more top electrodes  138  are mechanically coupled to the top plate  124 , and one or more bottom electrodes  140  are mechanically coupled to the bottom plate  126 . When an electric potential is applied between corresponding top electrodes  138  and bottom electrodes  140 , an electrostatic force is generated to pull the top plate  124  toward the bottom plate  126 . This changes the separation gap  132  of the Fabry-Perot cavity. In some embodiments, the electrostatic force causes the top plate  142  to deform, which provides the movement of the reflective region  128  of the top plate  124  relative to the bottom plate  126 . 
   The detector  114  is disposed adjacent the tunable bandpass filter  112 , and receives the one or more wavelengths that are passed through the tunable bandpass filter  112 . Preferably, the detector  114  is sensitive to the entire spectral range of wavelengths that can be passed through the tunable bandpass filter  112 . In an illustrative embodiment, the detector  114  is a modified form of AlGaN PIN photodiode, such as described in co-pending commonly assigned U.S. patent application Ser. No. 09/275,632, to Wei Yang et al., filed Mar. 24, 1999, and entitled “BACK-ILLUMINATED HETEROJUNCTION PHOTODIODE” which is hereby incorporated by reference at least for its teaching of the method of constructing the detector. Such detector is modified to include two stacked photodiodes  160  and  170  for detecting different wavelength energy. 
   In the embodiment shown in  FIG. 1 , the tunable bandpass filter  112  is supported by the first substrate  118 , and the detector  114  is supported by a second substrate  120 . The first and second substrates are preferably substantially transparent to the expected spectral range of wavelengths. In one illustrative embodiment, the first substrate is Pyrex and the second substrate is sapphire. The first and second substrates are secured together in a front-to-back fashion, as shown in  FIG. 1 . That is, the front side of the first substrate  118  is provided adjacent to the back side of the second substrate  120 . Bump bonds  144  or the like are used to secure the first substrate  118  to the second substrate  120 , and to make any electrical connection there between, as desired. A dielectric seal  154  is provided in one embodiment as shown to protect the tunable bandpass filter  112 . In some embodiments, the dielectric seal  154  provides a vacuum seal. Arranged in this manner, the wavelengths of interest pass, in sequence, through the first substrate  118 , the bandpass filter  112 , and the second substrate  120 , before reaching the detector  114 . 
   Readout electronics are optionally provided on a third substrate  122  or in the form of other discrete circuitry. The readout electronics are fabricated using conventional integrated circuit processing techniques. For example, the readout electronics may be fabricated using a CMOS process on a silicon substrate  122 . Metal pads may be provided to provide electrical connections to the detector  114 . In the embodiment shown in  FIG. 1 , bump bonds  146  are used to electrically connect one or more electrodes of the detector  114  to corresponding metal pads of the readout electronics. The bump bonds may also be used to secure the third substrate  122  relative to the second substrate  120 , as shown. The third substrate may be mounted to a package  150 , if desired. In the illustrative embodiment, multiple bond wires  152  are used to connect selected package pins to the readout electronics and the electrodes of the tunable bandpass filter  112 , as shown. Further bond wires are provided as needed. 
   The photodiodes  160  and  170  of the detector  114  are formed of AlGaN/InGaN, or different compositions of AlGaN or InGaN. Photodiode  160 , closer to the sapphire substrate  120  has a higher Al mole fraction and absorbs and detects short wavelengths. The second photodiode  170  is formed with a lower Al mole fraction, or a low In content and absorbs relatively longer wavelengths. 
   In one embodiment, photodiode  160  is an absorber for wavelengths of approximately 250-300 nanometers, and photodiode  170  absorbs wavelengths of 290 to 390 nanometers. Thus, radiation between 250 and 390 nanometers is detected by the detector  114 . 
   Prior to forming photodiode  160 , a buffer layer  175  is formed on the sapphire substrate  120 . This layer is approximately 500 angstrom in one embodiment and provides an epitaxial matching function to facilitate formation of the detector. The detector is formed by starting with an n doped AlxGa(1-x)N layer  177 , followed by an i doped AlyGa(1-y)N layer  178  and the photodiode layer  160  as described above. At least one n doped contact  180  is formed on layer  177 , and a further p doped contact is formed on photodiode  160  to provide contact to circuitry formed on a further substrate. A further p doped contact  190  is formed on photodiode  170  for contact to such circuitry. 
   In a further embodiment, both photodiodes are formed of AlGaN compositions. A first has a composition that is responsive to short wavelengths (210-280 nanometers) and a second photodiode has a composition that is responsive to longer wavelengths (280-360 nanometers) 
     FIG. 2  is a combined cross section/block diagram of the bandpass detector with numbering consistent with that of  FIG. 1 . The photodiodes  160  and  170  are shown coupled to charge detection circuits  210  and  220  respectively. Charge detector  210  is coupled to photodiode  160  for detection of higher frequency energy. Charge detector  220  is coupled to photodiode  170  for detection of lower frequency energy. In one embodiment, the charge detectors are integrated into third substrate  122  in  FIG. 1 . The charge detectors comprise high impedance well know integrated circuits in one embodiment. 
     FIG. 3  is a graph showing the calculated percent transmission of the tunable filter of  FIG. 1  versus incoming wavelength. The wavelength of the incoming light beam is shown across the bottom of the graph. The percentage of the incoming light that is transmitted through the bandpass filter is shown along the “y” axis for various etalon gap spacings. 
   A Pyrex absorption edge  300  is shown, which excludes wavelengths lower than approximately 260 nanometers. Three bands are shown corresponding to specific compositions of detectors. A first range indicated at 320 begins at approximately 260 nanometers and corresponds to a detector having a composition of Al.45Ga.55N T. A second range indicated at 330 begins at approximately 300 nanometers and corresponds to a detector having a composition of Al.25Ga.75N T. A third range indicated at 340 begins at approximately 380 nanometers and is comprised of In.1Ga.9N. 
   In one embodiment, the mole fractions for the above compositions are represented as Al x Ga 1-x N, Al y Ga 1-y N where y&lt;x, and InGaN or GaN. As seen above, decreasing the content or mole fraction of Al while increasing Ga results in sensitivity to higher wavelengths. Further, substituting In for Ga further increases the wavelength. 
     FIG. 4  shows a graph of different potential mole fractions for a detector. As the mole fraction is varied, the bandgap, lattice constant and sensitivity to different wavelengths shown in micrometers for each detector varies. The three substances shown in  FIG. 4  include GaN, AiN and InN. The lattice constant is shown as varying between approximately 3.2 and 3.6. This range is compatible for formation of adjacent layers of material using well known semiconductor processing techniques. SiC6H is also shown as a material for a detector, having a slightly lower lattice constant. Further materials are also within the scope of the present invention. 
   In a further embodiment, an array of detectors is formed. The array comprises spectrally tunable ultraviolet detectors that utilize a grating-less miniaturized UV spectrometer integrating a MEMS (micro-electo-mechanical systems) etalon. The array is a solid state UV detector array. In  FIG. 5 , the array is represented by a single stacked detector. Both linear and two dimensional arrays of pixels are formed in various embodiments and are useful for obtaining spectral and spatial information. 
   Multiple detectors are easily formed on a substrate in parallel. The array of detectors is tuned to the intrinsic tryptophan related to luminance, scattered light or biofluorescence spectra of organisms such as bacteria and inorganic material. Substances having proteins or amino acids emit UV radiation by fluorescence caused from a in-band source  510  such as a laser, light emitting diode, ultraviolet source, or superluminescent diode in  FIG. 5  illuminating a sample  520 . The fluorescence is indicated at  530  and is directed toward one or more filters  112  and detectors  114 , representing an array of detectors. In one example, the sample is a bacteria such as anthrax. Different samples emit a different signature that can be identified by experimentation on known samples. The detector detects signatures from unknown samples, which is then comparable to the identified signatures to identify the samples. In combination, the elements in  FIG. 5  comprise a sensor useful in detecting biological warfare substances in a very quick manner. 
     FIG. 6  shows further detail of the formation of the first detector. The second detector is formed in a similar manner on top of the first detector in one embodiment. As indicated above, the mole fractions of the elements are varied to obtain different wavelength sensitivities. 
   A cross-sectional view of the first detector is indicated at  610 . An aluminum nitride (AIN) buffer layer  614  is formed on a sapphire substrate  615 . The thickness of a substrate  615  is about 380 microns (15 mils) but may be another thickness, typically, between 200 and 500 microns (8 and 20 mils). The substrate starts out being part of a five-centimeter (two-inch) diameter wafer or other size wafer. The photo detector chip is cut at a size of 1.5 by 1.5 millimeter (mm) square. However, another convenient size is 0.3 mm by 0.3 mm. Sapphire is preferred because of its transparent characteristics to most of the UV wavelengths of interest. Materials such as silicon carbide are not transparent to all the of UV wavelengths, such as those less than 320 nanometers (nm). Buffer layer  614  is 25 nm (250 angstroms) thick but could have a thickness between 10 and 50 nm. Buffer layer  614  is for growing subsequent layer  613 . This layer  614  is transparent to the entire UV spectrum. 
   Formed on layer  614  is an n-type aluminum gallium nitride (n-Al x Ga (1-x) N) electrode layer  613 . Layer  613  is transparent to the UV spectrum and yet is electrically conductive. A p-type layer is difficult to make both conductive and transparent to UV light. Layer  613  is silicon (Si) doped with 10 17  to 10 19  atoms per cubic centimeter (cm). This layer  613  is about 1.5 microns thick but could have a thickness set between 0.25 and 20 microns. 
   Formed on layer  613  is an aluminum gallium nitride (i-Al y Ga (1-y) N) absorption layer  612 , which is not doped. This layer  612  has a built-in electric field (even without a bias) wherein the negative and positive carriers are separated as soon as they are generated, and thus gives rise to a high-level field assisted collection of carriers. Layer  612  provides high internal efficiency in that virtually all of the holes and electrons generated in this layer contribute to the current measured. A loss of carriers reduces current. UV light of selected bandwidth  618  enters from the bottom side of substrate  615  and passes up through layer  613  and is absorbed by layer  612 . wavelengths outside the selected bandwidth are passed through layer  612  and  611 . The absorption by layer  612  eliminates the diffusion limited carrier transport (i.e., current) and thus results in efficient carrier collection. Since photo generation takes place apart from the surface at the interface of layers  612  and  613 , and the interface of layer  613  and substrate  615 , surface recombination is significantly reduced or actually avoided. Layer  612  is about 0.25 micron; however, a selected thickness may be between 0.1 and 5 microns. 
   Formed on layer  612  is a p-type aluminum gallium nitride (p-Al z Ga (1-z) N) electrode layer  611 . Layer  611  is doped with magnesium (Mg) at a density of 10 17  to 10 20  atoms per cubic cm. Layer  611  is about 0.5 micron thick but could have a thickness between 0.1 and 2 microns. 
   Layers  611  and  612  are etched on one side partially down into layer  613  to a fraction of a micron. An n-contact  617  is formed on the etched portion of layer  613 . The first layer of contact  617  formed on layer  613  is about 50 nm of titanium (Ti). About 0.5 micron of aluminum is formed on the Ti. Contact  617  also serves as a contact for the second detector to be formed on top of the first detector. The aluminum layer may be up to several microns thick. On top of layer  611  is formed a p- contact  616 . First, a layer of nickel is formed on layer  611 . The thickness of the nickel may be between 10 and 500 nm. To complete contact  616 , a layer of gold, having a thickness between 0.5 and 2 microns is formed on the nickel. The gold and nickel of contacts  616  and  617 , respectively, are for bonding good electrical connections to photodiode  610 . 
   The composition portions of Al and Ga for layers  611 ,  612  and  613  are indicated by z, y and x, respectively. There is an interrelationship of portions among these three layers. For instance, z is greater or equal to y, because if z&lt;y then absorption would occur in layer  611 . Also, x&gt;y because if x≦y then the band pass of UV would be effectively eliminated by the absorption of layer  613  in that current would not be generated. 
   The second detector is formed prior to formation of contact  617 . It is masked and etched to provide access to layer  613  to form contact  617 . 
     FIG. 7  is a graph showing the calculated percent transmission of a tunable filter using two different AlxGayN detectors versus incoming wavelength. The wavelength of the incoming light beam is shown across the bottom of the graph. The percentage of the incoming light that is transmitted through the bandpass filter is shown along the “y” axis at  710 ,  720 ,  730  and  740  for various quarter wave spacings of the etalon gap. Gap dimensions are shown above the transmitted light curves. 
   In example one it is desired to cover the major fluorescence of tryptophan spectra with a dual detector shown at  750 . It is desired to cover the range from 260-360 or a fractional wavelength ratio change of 100 nm/310 nm=0.32. The etalon composed of a ZrO2/SiO2 mirror pair in the form of HLHLH will transmit the following spectra in two orders. The first detector is sensitive to the lower wavelengths (260 nm-310 nm) and transmits the longer UV wavelengths. The second detector behind the first detects the longer wavelengths (310 nm-360 nm) transmitted through the same etalon. Thus with these 2 detector signals it is possible to uniquely determine the full spectra in the 260-360 nm range. With a single detector, it is only possible to determine one of these two bands. A wider response detector (260 nm-360 nm), would not be able to discern whether the signal was from the short wave or long wave etalon transmission peak. The long wavelength response of each detector is determined by the band gap and the short wavelength response is defined by the transmission of the materials between the detector and the etalon. The band gap is defined by the Al/Ga ratio. 
     FIG. 8  is a graph showing the calculated percent transmission of a tunable filter using an AlxGayN detector stacked with an InxGayN detector to measure tryptophan and NADH profiles. The wavelength of the incoming light beam is shown across the bottom of the graph. The percentage of the incoming light that is transmitted through the bandpass filter is shown along the “y” axis at  810 ,  820 ,  830  and  840  for various quarter wave spacings of the etalon gap. Gap dimensions are shown above the transmitted light curves. 
   In the second example a wider range of wavelengths is desired. It is desired to measure the fluorescence from 2 different biological signatures. The first band is tryptophan  850  and the second band is NADH  860 . Such measurements can enhance biological discrimination. In this example two different detector materials are used to cover the wider range from 300 nm to 460 nm or a fractional wavelength change of 160 nm/380 nm=0.42. To cover this larger range it is necessary to use mirror materials that have high reflectance over a broader wavelength range. This requires using UV materials with a larger (high index/low index) ratio. Such dielectric mirror materials could be either diamond-like carbon or GaN combined with MgF2, all uv transparent. The etalon, as in example 1, transmits a number of wavelengths at each position. Once again the two different detectors are used to sort out the wavelength of the signal. From the selected curves of  FIG. 2  it can be seen that a deconvolution of the etalon position information with the signal level from the 2 detectors can provide a unique assignment of the wavelength of the transmitted signal and thus a spectra. 
   The responsivity range of each of the 2 detectors is defined by adjusting the band gap by varying either the Al/Ga or In/Ga rations. The long wavelength response is determined by the band gap and the short wavelength response is defined by the transmission of the materials between the detector and the etalon.