Abstract:
A compact image sensor for imaging radiation emitted by fluorescing objects exposed to excitation light is disclosed. The compact image sensor includes a light guide defining a longitudinal axis for channeling radiation emitted by the fluorescing object; a reflective surface defined on the light guide that is oriented at an angle with respect to the longitudinal axis of the light guide to reflect the excitation light away from a detector of the image sensor; and the detector positioned at an end of the light guide for imaging radiation emitted by the fluorescing object. Also disclosed is a fluorescence imaging system for imaging radiation emitted by a fluorescing object to be imaged by compact image sensor and a method of fluorescence imaging.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of U.S. Provisional Patent Application 
         [0002]    Ser. No. 61/480,068, filed Apr. 28, 2011, which is incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0003]    The present invention relates to a fluorescence imaging system. 
       BACKGROUND OF THE INVENTION 
       [0004]    As disclosed in U.S. Pat. No. 5,943,129, which is incorporated by reference herein, in fluorescent imaging, a sample is illuminated with excitation light of one wavelength while a resulting fluorescent emission at a second, typically longer wavelength is imaged. Because the fluorescent efficiency of many samples is low, i.e. typically 1 photon of fluorescent emission or less per 100 photons of excitation, the optical imaging system must efficiently collect the weak fluorescent emission without interference from the much stronger excitation signal. The optical system must provide an efficient optical path for delivering fluorescent emission light to the imager, with little or no such path for excitation light. Typically, spectral filters, such as colored-glass or interference filters, are used to provide at least some degree of the required wavelength selectivity. Disclosed herein is a low-profile sensor device that provides an efficient optical path for delivering emission light to the imager and a method for operating the device. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0005]    The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. Included in the drawing are the following figures: 
           [0006]      FIG. 1  depicts a cross-sectional view of the image sensor, shown schematically, according to one exemplary embodiment of the invention. 
           [0007]      FIG. 2  depicts a schematic diagram of a fluorescence imaging system including the image sensor of  FIG. 1  shown in cross-section. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    This invention will now be described with reference to several embodiments selected for illustration in the drawings. It will be appreciated that the scope and spirit of the invention are not limited to the illustrated embodiments. 
         [0009]    The term “excitation light source” as used herein refers to a source of irradiance) that can provide excitation that results in fluorescent emission. Light sources can include, but are not limited to, white light, halogen lamp, lasers, solid state laser, laser diode, micro-wire laser, diode solid state lasers (DSSL), vertical-cavity surface-emitting lasers (VCSEL,), LEDs, phosphor coated LEDs, organic LEDs (OLED), thin-film electroluminescent devices (TFELD), phosphorescent OLEDs (PHOLED), inorganic-organic LEDs, LEDs using quantum dot technology, LED arrays, filament lamps, arc lamps, gas lamps, and fluorescent tubes. Light sources can have high irradiance, such as lasers, or low irradiance, such as LEDs. The different types of LEDs mentioned above can have a medium to high irradiance. 
         [0010]    The term “detector” as used herein refers to any component, portion thereof, or system of components that can detect light including a photodiode, a photodiode array, a charged coupled device (CCD), a back-side thin-cooled CCD, a front-side illuminated CCD, a CCD array, a photo-multiplier tube (PMT), a PMT array, complimentary metal-oxide semiconductor (CMOS) sensors, CMOS arrays, a charge-injection device (CID), CID arrays, etc. The detector can be adapted to relay information to a data collection device for storage, correlation, and/or manipulation of data, for example, a computer, or other signal processing system. 
         [0011]    The term “filter” as used herein refers to any electromagnetic radiation exclusion device that can operate at a particular wavelength or range of wavelengths. Filter includes optical filters. The filter material may be a pigment-based color filter or a dye-based color filter, or any combination thereof. 
         [0012]    The term “fluorescing object” as used herein refers to any biological or chemical substance with components that can be excited by excitation light to emit fluorescent light or radiation. 
         [0013]      FIG. 1  depicts a cross-sectional view of an image sensor  10 , shown schematically, according to one exemplary embodiment of the invention. The image sensor  10  generally includes one or more detectors  12  (two shown), a substrate  14  defining two countersunk apertures  15  that are each positioned above and in radial alignment with a detector  12 , a reflective layer  18  deposited on the top surface of the substrate  14  and within each aperture  15  of the substrate  14 , and a plurality of filters  20  (two shown) each of which are deposited on the reflective surfaces  16  and  17  of the reflective layer  18  and at least a portion of the top surface of the detectors  12 . 
         [0014]    The image sensor  10  comprises light guide/pipe structures  22 ( 1 ) and  22 ( 2 ), which are referred to collectively as light guide/pipe structures  22  or pixels. Each light guide/pipe structure  22  comprises co-aligned reflective surfaces  16  and  17  and the filter material  20  that is deposited on the co-aligned reflective surfaces  16  and  17 . The filters  20  are at least partially encapsulated within their respective light guides  22 . The geometry of the reflective surface  16  is defined by the geometries of the substrate aperture  15  and the lining portion  19  of the reflective layer  18 , whereas the geometry of each reflective surface  17  is dictated by the geometry of the counter sunk hole of the reflective layer  18  that forms the reflective surface  17 . 
         [0015]    The reflective surfaces  16  and  17  form acute angles with respect to a longitudinal axis ‘A’ of a respective light guide structure  22 . The angle ‘B’ of each conically-shaped reflective surface  17  may be between 5 degrees and 30 degrees, for example, with respect to the longitudinal axis ‘A’ of a respective light guide structure  22 . Alternatively, the angle ‘B’ may be between 20 degrees and 60 degrees, for example. The angle ‘C’ of each conically-shaped reflective surface  16  may be between 0 degrees and 10 degrees, for example, with respect to the longitudinal axis ‘A’ of a respective light guide structure  22 . Although not explicitly shown, the angle ‘C’ could also be between −5 degrees and 0 degrees. According to one aspect of the invention the reflective surfaces  16  and  17  are discontinuous and the angles ‘B’ and ‘C’ are different, while according to another aspect of the invention, the reflective surfaces  16  and  17  are continuous and the angles ‘B’ and ‘C’ are the same. 
         [0016]    The top portion of the reflective layer  18 , which is bounded by height h′, may be a separate component from the lining portion  19  of the reflective layer  18 . In addition to being solid metal or other reflective film(s), the top portion of the reflective layer  18  could also consist of a metallized core. 
         [0017]    The surfaces  16  and  17  of each light guide structure  22  do not have to be symmetrical about longitudinal axis A, as shown. The surface that faces the incoming light (i.e., the left-side surfaces of each light guide structure) should be angled to reflect excitation light, however, the remaining surfaces of the light guide/pipe structures  22  may not be angled, and may be vertical, for example. 
         [0018]    According to this exemplary embodiment, the detectors  12  are photodiodes, the reflective layer  18  is composed of Aluminum, the substrate  14  is composed of a dielectric material, such as Silicon Oxide, or a cross-linked polymer, and each filter  20  is composed of a red-color filter material. Those of ordinary skill in the art will recognize that other materials may be used, as described previously, without departing from the scope of the invention. 
         [0019]      FIG. 2  is a schematic diagram of a fluorescence imaging system  28  including the image sensor  10  of  FIG. 1 . The fluorescence imaging system  28  generally includes an excitation light source  30  and the sensor  10 . According to one exemplary method of operating the fluorescence imaging system  28 , the excitation light source  30  is positioned at an elevation above the top surface of the image sensor  10  and is laterally spaced from the sensor  10  and the fluorescing object  32  by a predetermined distance. The fluorescing object  32  is positioned either on or in very close proximity to the top surface of the sensor  10 . The fluorescing object  32  can be a component of a liquid or film deposited on a surface of the sensor  10 . 
         [0020]    The excitation light source  30  produces excitation light beams  34  which can be collimated. Illuminating the fluorescing object  32  with the excitation light beams  34  causes the fluorescing object  32  to fluoresce, i.e., emit radiation  36 . Some of the radiation  36  is directed vertically downwards through one or more filters  20  of the sensor  10  (see broken line arrows) and onto the top surface of one or more detectors  12  which detect the radiation  36 . The one or more detectors  12  receive and image the radiation  36  emitted from the fluorescing object  32 . The one or more detectors  12  are adapted to relay the image information to a data collection device for storage, correlation, and/or manipulation of data, for example, a computer, or other signal processing system. 
         [0021]    Because the excitation light source  30  is laterally spaced from the light guides  22  of the sensor  10  by a predetermined distance, the excitation light beams  34  produced by the light source  30  are oriented with respect to the longitudinal axis ‘A’ of the light pipes  22  by a predetermined angle ‘D.’ The angle ‘D’ may be 35 degrees to 55 degrees, for example. The value of the predetermined angle ‘D’ is defined by the respective physical locations of the excitation light source  30  and the sensor  10 . 
         [0022]    Angling the excitation light source  30  with respect to the light guides  22  by angle ‘D’ is referred to herein as angled illumination. By virtue of angled illumination, the bulk of the incoming excitation light beams  34  reflect off of the angled reflective surfaces  16  and  17  of the light guides  22  in a direction away from the sensor  10 . Thus, the detectors  12  do not detect the reflected excitation light beams  34 . Reflecting the excitation light beams  34  minimizes the ratio of excitation light to fluorescent light (i.e., radiation) that is detected by the detector  12 . This ratio is commonly referred to as the suppression ratio. 
         [0023]    In many conventional fluorescence imaging systems, the excitation light source is positioned directly above the light guides and the light guides are straight channels without angled walls. In those conventional sensor systems, the height of the filter material must be sufficiently large to filter the incoming excitation light beams in order to achieve a fixed suppression ratio requirement for a particular application. By contrast, the height ‘h’ of the filter  20 , and, consequently, the height of the sensor  10 , is minimized because the reflective surfaces  16  and  17  reflect the bulk of the unwanted incoming excitation light beams  34 . Thus, the height ‘h’ of the sensor  10  can be smaller than the height of sensors of conventional fluorescence imaging systems. The height ‘h’ of the sensor  10  can be comparatively reduced by 30-50% while achieving the same suppression ratio and signal to noise ratio as the aforementioned conventional sensor. By reducing the height ‘h’ of the sensor  10 , the pixel pitch ‘x’ (see  FIG. 1 ) can be reduced allowing to either shrink the sensor size for a given number of pixels or to increase the number of active pixels for a given sensor array size. 
         [0024]    Applications for the fluorescence imaging system  28  include, but are not limited to, any micro fluidic, lab-on-chip applications that use fluorescence spectroscopy for analysis of the properties of samples that are brought in close proximity to the entrance pupil of the image sensor pixel. 
         [0025]    According to one exemplary method of fabricating the image sensor  10  of  FIGS. 1 and 2 , the substrate  14  is first formed by a CVD oxide deposition process. The thickness of the substrate  14  may be 3-4 micrometers, for example. A lithography process patterns holes over each detector  12  or every other detector  12  or any other combinations. A dry etch process creates the apertures  15  in the substrate  14 . The etch can be adjusted to obtain any desired taper. A resist strip process is employed to remove the resist and clean the surface for better adhesion to subsequent layers. 
         [0026]    By way of a PVD deposition process, Aluminum or other metal or metals form the reflective layer  18  that is applied over the substrate  14  and within the apertures  15  of the substrate  14 . The thickness of the metal could be 0.5 microns to 2 microns, for example, on the surface of substrate  14 . The corresponding thickness of the lining portion  19  may be 300-700 Angstroms, for example, which can be adjusted by the metal deposition process. A dry etch process clears metal from the bottom of the substrate  14 . A dual etch process may be performed to clear the bottom in one step and then clear the substrate streets and bond pads with another step (for example a wet metal etch while keeping the light guides covered with resist). A single etch may also be performed with adequate etch controls. 
         [0027]    The filter layer  20  is then spin coated to fill the light guides  22  and also form an additional layer over the light guides  22  if desired. This may be done in multiple layers or in a single layer. This may be photo-definable or not. A planarization step (e.g., chemical-mechanical planarization) may be employed to planarize the filter layer. A chemical vapor deposition (CVD) oxide or other dielectric layer(s) deposition passivates the filter layer and can further tailor the optical filtering properties of the filter layer. A lithography process patterns the passivation and the filter layer. A dry etch process clears the passivation and the filter layer from the bond pads and the scribe lanes. A resist strip process removes the resist. 
         [0028]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.