Abstract:
A spectrum of electromagnetic radiation is detected by spatially dispersing radiation of varying wavelengths onto micromechanical sensors. As the micromechanical sensors absorb radiation, the sensors bend and/or undergo a shift in the resonance characteristics. The device can be used as a spectrometer or a temperature sensing device. A temperature sensor using micromechanical sensors can accurately and quickly measure the temperature of a remote object by sensing a spectrum of infrared radiation emitted by the object. The temperature sensor can measure temperature without knowing the emissivity of the object or the distance of the object from the detector.

Description:
[0001] This invention was made with Government support under contract DE-AC05-96OR22464 awarded by the U.S. Department of Energy to Lockheed Martin Energy Systems, Inc. and the Government has certain rights in this invention. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to the field of measuring and testing, and more specifically, to the detection of electromagnetic radiation using micromechanical sensors.  
         BACKGROUND OF THE INVENTION  
         [0003]    Miniature electromagnetic radiation detectors are needed for a variety of applications. For example, miniature spectrometers are needed for field analysis and analyzing small quantities of samples and miniature infrared detectors are needed for measuring temperature in tight locations. Considerable difficulties are encountered, however, when attempting to miniaturize existing detectors.  
           [0004]    One particularly useful application for a miniature radiation detector is for use as a temperature sensor. Every object emits infrared radiation which varies in intensity as a function of wavelength. The emitted infrared radiation spectrum is characteristic of the object&#39;s temperature. The temperature of an object can be determined by detecting the emitted infrared radiation. However, determining an accurate temperature of the object based on its emitted infrared radiation is a challenging problem when the emissivity of the object and the distance of the object from the detector is not known.  
           [0005]    Most currently available devices produce a signal based on the intensity of the incident infrared radiation, without correcting for emissivity and distance of the temperature source from the infrared detector. Hotter objects that are far way can appear as cooler objects with respect to relatively colder objects at shorter distances.  
           [0006]    One way of measuring absolute temperature is by measuring the intensity of infrared radiation at different wavelengths, and then correlating the intensity values to a temperature using a well known method such as that described in U.S. Pat. Nos. 5,118,200 or 5,326,173.  
           [0007]    One way of measuring infrared radiation at multiple wavelengths is by placing different filters in front of the infrared detector. By interchanging the filters, the intensity of infrared radiation at various wavelengths can be calculated. This, however, can be slow due to the time needed for the mechanical interchange of different filters.  
           [0008]    What is needed is a temperature detector than can be made very small, and can measure the temperature of an object accurately and quickly without knowing the emissivity of the object or its distance from the detector.  
         SUMMARY OF THE INVENTION  
         [0009]    An object of the present invention is to provide a detector which is capable of detecting a broad spectrum of electromagnetic radiation.  
           [0010]    Another object of the present invention is to provide a detector which capable of being miniaturized while detecting electromagnetic radiation with picojoule sensitivity.  
           [0011]    Still another object of the present invention is to provide a temperature detector that can measure the temperature of an object without knowing the emissivity of the object or the distance of the object from the detector.  
           [0012]    These and other objects of the invention are met by providing an apparatus and method for detecting radiation comprising a dispersive element which spatially disperses radiation, at least one cantilever in a path of the spatially dispersed radiation, wherein the cantilever has at least one physical property affected by the spatially dispersed radiation.  
           [0013]    The dispersive element may include a lens, a prism, a mirror, or a grating. For a temperature detector, the cantilever would respond to infrared radiation. The temperature can be determined based on the infrared radiation spectrum. The cantilevers may remain stationary or may be moved sequentially to a plurality of locations, wherein a measure of radiation is performed at more than one location. Alternatively, the dispersive element may be moved or rotated to change the angle of the dispersed radiation, wherein a measure of radiation is performed after a movement of the dispersive element.  
           [0014]    The cantilevers may be arranged in a fixed array of cantilevers, wherein each cantilever detects spatially dispersed radiation dispersed at a different angle by the dispersive element.  
           [0015]    Another embodiment of the invention is for use as a spectrophotometer. Radiation from a radiation source may be transmitted through a substance before entering the radiation dispersive element. In this case the cantilevers&#39; response would represent a radiation absorption spectrum of the substance.  
           [0016]    Alternatively the radiation from the radiation source may reflected off a substance before entering the radiation dispersive element. In this case the cantilevers&#39; response would indicate a radiation reflectance spectrum of the substance.  
           [0017]    In another specific embodiment of the invention, the cantilevers respond to spatially dispersed radiation at a focal point along a principal axis of a lens. The cantilever is approximately the same size as the diameter of a beam waist for a spatially dispersed radiation beam of a specific wavelength.  
           [0018]    In another specific embodiment of the invention, the detector further comprises an aperture, wherein the aperture has a diameter approximately the same size as the diameter of a beam waist for a radiation beam of a desired wavelength. The aperture transmits the radiation of the desired wavelength, while blocking radiation of desired wavelengths. A cantilever may be scanned along the principal axis of a lens along with the aperture.  
           [0019]    Other objects, advantages, and salient features will be more apparent when considered with the following detailed description and drawing that are provided to facilitate the understanding of the subject invention without any limitation thereto. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0020]    [0020]FIG. 1 is a schematic view of a radiation detector utilizing an array of cantilevers and a prism according to an embodiment of the present invention.  
         [0021]    [0021]FIG. 2 is an enlarged, perspective view of an individual cantilever.  
         [0022]    [0022]FIG. 3 is a schematic view of an embodiment of a radiation detector utilizing a lens and a microcantilever array.  
         [0023]    [0023]FIG. 4 is a schematic view of a radiation detector utilizing an aperture.  
         [0024]    [0024]FIG. 5 is a schematic view of an embodiment of the present invention for use as a spectrophotometer. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure and to the figures of the drawing, where like reference characters designate like or similar elements. In accordance with an embodiment of the invention, radiation detection over a range of wavelengths is based upon absorption of radiation to cause physical movement and changes in the mechanical resonance of a microcantilever.  
         [0026]    Referring to FIG. 1, a detector according to the present invention is generally referred to by the numeral  10 . A radiation source  12  outputs radiation  14  which impinges upon dispersive element  16 . Dispersive element  16  spatially disperses incident radiation  14 . Dispersive element  16  may be a prism, a lens, a diffraction grating, or other element which spatially disperses incident radiation. Dispersive element  16  may also include a combination of elements such as a combination of lenses, mirrors, prisms, and/or gratings.  
         [0027]    The radiation outputted from dispersive element  16  impinges upon microcantilever array  22  comprised of individual microcantilevers which respond to incident radiation. The description of a microcantilever which detects electromagnetic and nuclear radiation and methods for detection of microcantilever response are the subject of U.S. Pat. No. 5,445,008 and copending U.S. patent application Ser. No. 08/588,484 (filed Jan. 18, 1996), which are incorporated by reference herein.  
         [0028]    [0028]FIG. 1 shows two exemplary rays outputted from dispersive element  16 : rays  18  and  20 . Ray  18  has wavelength λ 1  and ray  20  has wavelength λ 2 . Ray  18  impinges upon individual microcantilever  26 , which consequently responds to the intensity of radiation of wavelength λ 1 . Output ray  20  impinges upon individual microcantilever  24 , which consequently responds to the intensity of radiation of wavelength λ 2 .  
         [0029]    By detecting the response of the individual microcantilevers to the impinging radiation, the intensity of the radiation over a range of wavelengths can be measured, and hence allows one to measure the intensity spectrum of the radiation source  30 , and obtain the shape of the radiation intensity profile.  
         [0030]    The microcantilever array  22  may be a one, two, or three dimensional array of microcantilevers. As an alternative to a fixed array of microcantilevers  22 , the detector  10  may instead utilize one or more microcantilevers which are moved sequentially to different positions or scanned along an axis to detect the intensity of radiation of different wavelengths. For example, a single microcantilever could be moved to the position occupied by individual microcantilever  26  in FIG. 1, to measure the intensity of radiation with wavelength λ 1 , and subsequently the same microcantilever could then be moved to the position occupied by microcantilever  24 , to measure radiation of wavelength λ 2 . Alternatively, one or more microcantilevers can remain fixed in one location, while the dispersive element is moved or rotated to change the angle of the dispersed radiation.  
         [0031]    Referring to FIG. 2, one form of a microcantilever radiation sensor is generally referred to by the numeral  30 . The sensor  30  includes a microcantilever  32  connected at its proximal end to, and extending outwardly from, a base  36 . The microcantilever is coated with one or more coating materials  34  that react to electromagnetic radiation. As the coatings on the microcantilever absorb electromagnetic radiation, the microcantilever bends, and/or undergoes a shift in resonance frequency.  
         [0032]    The primary advantages of using microcantilevers is their very high sensitivity, since microcantilever motion can be detected with subnanometer precision, and the ability to fabricate microcantilevers into a multi-element sensor array. Microcantilever elements that are made bimetallic or bimaterial are extremely sensitive to changes in temperature and undergo bending due to differential thermal expansions of different members of the bimaterial system. The sensitivity of a bimaterial cantilever can be increased by choosing the members of the bimaterial system such that the differential thermal expansion is optimum. This can be easily achieved by coating a silicon microcantilever with a metal overlayer. Using such an arrangement, temperature changes as small as 10 −6 ° C. or heat changes on the order of a femto-Joule can be detected by measuring the changes in the cantilever bending.  
         [0033]    Coating one side of a microcantilever with a different material, such as metal film, makes the microcantilever sensitive to temperature variations due to the bimetallic or bimaterial effect resulting in cantilever bending. The bending of the microcantilever is proportional to the heat energy absorbed by the microcantilever. The maximum microcantilever deflection, z max , due to differential stress induced by incident heat energy on the bimaterial cantilever is given by:  
               z   max     =       5   4                (       t   1     +     t   2       )          l   3           (         λ   1          t   1       +       λ   2          t   2         )          wt   2   2         ·         (       α   1     -     α   2       )          (     dQ   /   dt     )           4        (     1   +       t   1   2     /     t   2   2         )       +       1   /     t   1              t   2          (       6        t   1   2       +       E   1            t   2   2     /     E   2           )         +       E   1            t   1   3     /     E   2            t   2   3                       (   1   )                               
 
         [0034]    Where dQ/dt is the incident heat energy,  1  and w are the length and width of the microcantilever, respectively, t 1  and t 2  are the thicknesses of the two layers, λ 1  and λ 2  are the thermal conductivities, α 1  and α 2  are the thermal expansion coefficients, and E 1  and E 2  are the Young&#39;s moduli of elasticity of the two layers.  
         [0035]    In addition to bending, the microcantilever can also respond to changes in temperature by a shift in resonance frequency. The resonance frequency, f, of an oscillating cantilever can be expressed as:  
             f   =       1     2      π              k     m   *                   (   2   )                               
 
         [0036]    where k is the spring constant of the lever and m* is the effective mass of the microcantilever.  
         [0037]    The spring constant of a microcantilever can change due to changes in heat. This can be due to surface stress as in the case of bimaterial effect or changes in physical dimensions. The change in spring constant δk of the cantilever can be calculated from the bending of the cantilever as follows:  
               δ                 k     =       π   2        n          (       δ                   s   1       -     δ                   s   2         )       4        n   1                   (   3   )                               
 
         [0038]    where δs 1  and δs 2  are the stresses on the cantilever surfaces and n is a constant and n 1  is a geometrical constant.  
         [0039]    Since the spring constant of a microcantilever is related to physical dimensions, the resonance frequency can also change due to changes in dimensions. The resonance frequency of a cantilever is directly proportional to the square root of the width and cube root of the thickness. The resonance frequency varies inversely as the cube root of length.  
         [0040]    The bending of a cantilever can be measured with sub-angstrom resolution using various techniques. Examples include: (1) detecting changes in intensity of a reflected beam of a laser diode focused at the end of the microcantilever using a position sensitive detector, (2) detecting the variation in the piezoresistance of a boron implanted channel in a silicon microcantilevers, (3) detecting changes in capacitance between microcantilever and a fixed surface, and (4) detecting variation in the piezoelectric voltage of piezoelectric film on a microcantilever. The need for an optical set up can be eliminated by using one of the electrical detection schemes discussed above. The resonance frequency variation of the microcantilever can be detected using the same techniques discussed above.  
         [0041]    The invention shown in FIG. 1 is particularly useful to measure the spectrum of infrared radiation due to the large refractive and dispersive properties of certain materials in the infrared region. The detector  10  can measure the temperature of an object by measuring the infrared radiation spectrum emitted by that object. Since the intensity spectrum over a range of wavelengths can be measured, the peak of the infrared profile can be determined, and the temperature of the object can be determined using a well-known method without knowing the emissivity of the object.  
         [0042]    [0042]FIG. 3 depicts an embodiment of the present invention which utilizes a lens  50  as the dispersive element. As shown in FIG. 3, lens  50  refracts incoming parallel radiation to various focal points along the principal axis  62 . The location of the focal point varies as a function of wavelength of the incoming radiation. For an aberrant, convex-concave, refracting lens with refractive index n(λ) and with radii of curvature R 1  and R 2 , the focal length f(λ) is given by:  
               f        (   λ   )       =       1       n        (   λ   )       -   1                R   1          R   2           R   1     +     R   2                   (   4   )                               
 
         [0043]    where n is the refractive index, and R 1  and R 2  are the radii of curvature of the lens. Focal point f(λ) refers to the focal point for incident radiation of wavelength λ. The distance between focal lengths for radiation of different wavelengths can also be calculated using the above equation.  
         [0044]    [0044]FIG. 3 depicts exemplary rays  42  and  44 , which both have a wavelength λ 1 . Exemplary rays  46  and  48  both have a wavelength λ 2 . The lens refracts rays  46  and  48  into focal point  54  and the lens refracts rays  42  and  44  into focal point  60 . By positioning the microcantilever array  52  along the principal axis  62  such that individual microcantilever  56  is located at focal point  54 , then individual microcantilever  56  will respond to impinging rays  46  and  48 , and consequently measure the intensity of radiation of wavelength λ 2 . Similarly, microcantilever  58  will measure the intensity of impinging rays  42  and  44 , with wavelength λ 1 .  
         [0045]    If the difference between wavelengths λ 1  and λ 2  is small, then focal points  54  and  60  will be close together on the principal axis  62 . For smaller Δ=λ 1 −λ 2 , focal points  54  and  60  will be closer together. For very small Δ, it may be difficult to distinguish separate signals for λ 1  and λ 2 . The ability of the microcantilever detector to distinguish separate signals when Δ is small improves when the microcantilevers are more finely spaced, but worsens with a larger focus spot size of the radiation.  
         [0046]    One method for improving the ability of the detector to distinguish signals with a small Δ, is to use an aperture located at the focal point. FIG. 4 depicts two beams of radiation,  76  and  78 , passing through lens  50 . Photons of wavelength λ 2  form beam  76  while photons of wavelength λ 1  form beam  78 . The beam  76  is the narrowest at the beam waist  70 . The photons of wavelength λ 2  pass through the beam waist  70 . Similarly, photons of wavelength λ 1  form beam  78  and pass through beam waist  72 .  
         [0047]    To best detect the intensity of a radiation signal with wavelength λ 2 , a microcantilever should be positioned at beam waist  70 , and the size of the detector should be approximately equal to the diameter of the beam waist. In this way it can minimize the effect from other wavelengths. An aperture  74  with a diameter approximately equal to the diameter of the beam waist  70  may be placed at beam waist  70 , so that most of the radiation passing through the aperture  74  will be due to radiation of wavelength λ 2 . Similarly, an aperture with a diameter approximately equal to the diameter of beam waist  72  may be placed at beam waist  72 , and most of the radiation passing through that aperture will be due to radiation of wavelength λ 1 . The diameter of the aperture should approximately equal the diameter of the beam waist, which is given by:  
             w   =           2      λ     π          (       f        (   λ   )       D     )                   for                     f        (   λ   )       /   D          1             (   5   )                               
 
         [0048]    where w is the diameter of the beam waist, D is the diameter of the lens, and f(λ) is the wavelength dependent focus of the lens.  
         [0049]    The aperture  74  and a microcantilever may be joined to form a detector assembly and then scanned along the principal axis  62 . By sampling the radiation intensity as it scans along the principal axis  62 , it can measure the intensity profile of the source. The curve may be plotted by recording data points along the principal axis  62 . In the case of infrared radiation, the peak of the profile can be used to calculate the temperature of the source.  
         [0050]    The system can be further optimized by designing the lens such that focal points for wavelengths of interest are sufficiently separated along the principal axis  62 . From Equation (4) it is clear that the focal-length variation depends on refractive index n and radii of curvature of the lens R 1  and R 2 . Therefore, focal distances may be adjusted by appropriate selection of these parameters.  
         [0051]    One application for the present invention is for use as a spectrophotometer as shown in FIG. 5. A spectrophotometer measures the transmission or reflectance of radiation as a function of wavelength, permitting accurate analysis of color. FIG. 5 depicts an exemplary spectrophotometer  80 . A sample  82  of a gas, a liquid, or any material which partially transmits radiation is placed between the radiation source  12  and the dispersive element  16 . As the radiation passes through sample  82 , the attenuation of the transmitted radiation will vary as a function of radiation wavelength. Microcantilever array  22  thus can measure the spectrum profile of the transmitted radiation, and hence determine the absorption characteristics of the sample.  
         [0052]    A reference spectrum can be generated by measuring the microcantilever response without the sample present. The difference between the spectrum with the sample present and the spectrum without the sample present represents the absolute absorption spectrum for the sample.  
         [0053]    In an alternative spectrophotometer arrangement, the sample  82  may be placed between dispersive element  16  and the microcantilever array  22 . The sample  82  may also be placed directly on the microcantilever array. If a single microcantilever is used instead of an array of microcantilevers, then the sample can be placed on the microcantilever as it is scanned across the radiation.  
         [0054]    The lens configuration in FIG. 3 can also be used as a spectrophotometer. The sample can be placed in a stationary position on either side of the lens, or can be placed on the microcantilever array  52 . If an arrangement is used where a microcantilever is attached to an aperture and scanned along the principal axis of the lens, then the sample may be placed directly on the microcantilever.  
         [0055]    In an alternate embodiment of a spectrophotometer, instead of transmitting the radiation through a sample, the radiation may be reflected from a sample by the use of an appropriate optical arrangement. The radiation measured by the detector then represents the reflectance characteristics rather than the absorption characteristics of the sample.  
         [0056]    While several particular forms of the invention have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention.