Abstract:
A photodiode has integrated shields for the rejection of noise-producing electromagnetic interference and ambient light. The electromagnetic shield forms a conductive matrix which covers the photodiode active area. The matrix is deposited as a metallization layer onto the photodiode and provides exposed portions of the active area for light detection. A pad is electrically connected to the shield to allow external termination of the shield. The ambient-light shield is in the form of a colored encapsulant surrounding the photodiode. The encapsulant provides a high-pass light transmission characteristic which passes signal light and rejects out-of-band ambient light. The photodiode is particularly advantageous for use in pulse oximetry probes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of photodiode detectors, the field of electromagnetic interference and the field of band-limiting optics. In particular, this invention relates to electromagnetic and optical shielding to reduce background noise from photodiode detectors. 
     2. Description of the Related Art 
     A photodiode is a semiconductor device which converts the photon energy of light into an electrical signal by releasing and accelerating current-conducting carriers within the semiconductor. A photodiode behaves like an ordinary signal diode, but is specialized with respect to spectral response and efficiency to optimize internally generated current derived from illumination. In applications, a photodiode is often used as a detector which is optically coupled to a light-emitting-diode (LED) emitter. Examples of such applications include solid-state relays, remote control devices, optical communications and noninvasive biomedical sensors. 
     A limitation in many photodiode applications is a background noise floor which masks the signal detected by the photodiode. A contributing factor to background noise in a photodiode detector circuit, as in most electronic circuits, is the parasitic coupling of electromagnetic interference (EMI) into the circuit. External sources of EMI vary from power lines and cellular telephones to medical devices such as diathermy, MRI and lasers. 
     Conventionally, an electromagnetic shield is utilized as an effective method of reducing the effect of EMI-induced noise. Typical shielding techniques involve surrounding potentially affected parts with a “Faraday cage” of conducting material. However, conducting materials are typically opaque to optical signals. Hence, for photodiode applications, prior art electromagnetic shields have typically consisted of optically-transparent conductive materials, such as thin film silver or silver alloy or conductive “screens” having optically transmissive openings. This is illustrated in FIG. 1, which is a cut-away view of a prior art cage  100  containing an optical detector  110 . The portions of the cage  100  within the optical path  140  between an emitter  150  and the detector  110  are constructed of a transparent or transmissive conductive material  120 . The remainder of the cage  100  is conductive material  130  which may be opaque. 
     Besides electromagnetic interference, a contributing factor to background noise in photodiode detectors is ambient light. For photodiode applications, prior art ambient light reduction techniques typically consist of placing opaque, polarized or similar light-blocking material externally around the signal optical path and external wavelength filters within the signal optical path. This is illustrated in FIG. 2, which is a cut-away view of a prior art optical enclosure  200  containing an optical detector  110 . The portion  220  of the enclosure  200  within the optical path  140  between an emitter  150  and the detector  110  is constructed of a wavelength filtering material. The remainder of the enclosure  200  is light blocking material  230 . 
     SUMMARY OF THE INVENTION 
     A particularly advantageous application of a photodiode with integrated noise shielding according to the present invention is in pulse oximetry, and in particular, as a detector in pulse oximetry probes. Pulse oximetry is the noninvasive measurement of the oxygen saturation level of arterial blood. Early detection of low blood oxygen saturation is critical because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. The use of pulse oximetry in operating rooms and critical care settings is widely accepted. 
     A pulse oximetry probe is a sensor having a photodiode which detects light projected through a capillary bed by, typically, red and infrared LED emitters. The probe is attached to a finger, for example, and connected to an instrument which measures oxygen saturation by computing the differential absorption of these two light wavelengths after transmission through the finger. A probe may also be reflective, with the emitter and detector on the same side of vascularized tissue. This is sometimes referred to as “backscatter” oximetry. The LED emitters are alternately activated by the pulse oximetry instrument which then reads voltages indicating the resulting intensities (I rd  and I ir ) detected by the photodiode, where I rd  is the detected intensity of the red light and I ir  is the detected intensity of the infrared light. A ratio of detected intensities is calculated and an arterial oxygen saturation value is empirically determined based on the ratio obtained: 
     
       
           I   rd   /I   ir =Ratio=% O 2  Saturation  
       
     
     Unfortunately, pulse oximetry probes are adversely affected by background noise generated in the photodiode detector by both EMI and ambient light. EMI-generated noise enters an unshielded detector through parasitic capacitive coupling, i.e., through the mutual capacitances that exist between any two objects. Noise from ambient light is generated by the detector when light not generated by the emitters illuminates the photodiode. A significant portion of ambient light induced noise may result from light having wavelengths outside the emitter bandwidth but within the detector bandwidth. 
     The detector output from both signal and noise sources can be represented as: 
     
       
           I   rd   /I   ir =( S   rd   +N   rd )/( S   ir   +N   ir )  
       
     
     where S rd  is the signal component of the red light, N rd  is the noise component of the red light, S ir  is the signal component of the infrared light, and N ir  is the noise component of the infrared light. If the noise level becomes large in relation to the signal, the ratio I rd /I ir  approaches 1, which corresponds to a false saturation reading of 85%. This noise problem is compounded by the critical human life mission of pulse oximetry devices. Thus, in pulse oximetry applications, there is a particular need for both EMI shielding and ambient-light shielding in order to increase the detector signal-to-noise ratio. 
     The use of conventional external noise shielding for photodiode detectors, including detectors used in pulse oximetry, has a number of drawbacks. Any practical external shielding enclosure includes openings which reduce shield effectiveness. For electromagnetic shields, shielding effectiveness (SE) can be expressed as 
     
       
           SE= 20 log(λ/2 L )  
       
     
     where λ is the interference wavelength and L the longest dimension of any opening. Thus, a mere ½ inch opening in a shield reduces shielding effectiveness beyond a minimally acceptable 20 db at frequencies as low as 1 GHz. Likewise for optical shields, small openings in opaque or wavelength filtering materials can allow noise-producing ambient light to reach the photodiode. This is particularly problematic for pulse oximetry probes, where the optical path from emitter to detector includes, for example, fingers and feet having a variety of sizes and shapes which frustrate achieving a light-tight seal. 
     In large-scale manufacturing applications, external shielding devices, both electromagnetic and optical, can add significantly to the cost of photodiode detectors, both in terms of additional parts and additional assembly steps. Conductive and optically transmissive shielding deposited directly on a photodiode substrate might overcome some limitations of external shielding but, generally, would require extra processing steps in photodiode fabrication, which would also increase final detector cost. A photodiode with integrated noise shielding according to the present invention is intended to eliminate or reduce these drawbacks encountered with conventional noise shielding techniques. 
     Another aspect of the present invention is a shielded detector which comprises a photodetector having an active area exposed to receive light. The photodetector is responsive to light of a first band of wavelengths. The shielded detector further comprises a shield deposited on at least portions of the exposed active area. In preferred embodiments, the shield comprises an electrically conductive layer deposited on at least portions of the exposed active area to provide an integrated electromagnetic shield for the photodetector. The shielded detector advantageously includes a pad portion, wherein the pad portion forms a part of the conductive layer. The shield has a low impedance path to the pad portion such that the pad portion forms an electrical path for connection to an external contact to permit external termination for the shield. In certain embodiments, the conductive layer comprises a metallization layer deposited directly on the portions of the exposed active area. The metallization layer comprises a grid which preferably forms the shield. The metallization layer also forms a second electrode for the detector. The second electrode is substantially electrically isolated from the shield. In preferred embodiments, the active area is responsive to the light of a first band of wavelengths, and the shield comprises an encapsulant covering at least the exposed active area. The encapsulant is substantially transparent to selected wavelengths within the first band of light wavelengths and is substantially attenuating to at least some other wavelengths within the first band of wavelengths. In particular embodiments, the other wavelengths within the first band of wavelengths are wavelengths below about 635 nanometers. Alternatively, the other wavelengths within the first band of wavelengths are wavelengths below about 350 nanometers. In a further alternative, the other wavelengths within the first band of wavelengths are wavelengths below about 500 nanometers. In certain preferred embodiments, the shield further comprises an optical filter which covers at least the exposed active area. The filter is substantially transparent to selected wavelengths within the first band of wavelengths and is substantially opaque to at least some other wavelengths within the first band of wavelengths. The filter advantageously comprises an encapsulant applied to the detector. The encapsulant preferably is in contact with and substantially covers the photodiode so as to form an integrated selective light filter for the photodiode. Preferably, the shielded detector further comprises an emitter which generates light of at least one selected wavelength within the first band of wavelengths. Also preferably, the photodetector comprises a generally planar photodiode, wherein the photodiode has first and second sides, the first side having the exposed active area and the second side having a first electrode for the photodiode. The shield preferably comprises a metallization layer deposited directly on at least a portion of the exposed active area. The metallization layer forms an electrically conductive grid and a second electrode for the photodiode. The second electrode is substantially electrically isolated from the grid. The shielded detector preferably includes a pad portion having a low impedance path to the grid, and the grid forms an integrated electromagnetic shield. The pad is adapted to be externally terminated through a conductor attached to the pad. Preferably, an optical filter material encapsulates the photodiode. The filter transmits light of at least selected wavelengths within the first band of wavelengths and attenuates at least some other wavelengths within the first band. 
     Another aspect of the present invention is a method of making a shielded detector having first and second sides. The method comprises the step of depositing a shield on an active area of a photodetector. Preferably, the step of depositing a shield comprises depositing a conductive grid and a shield pad for the conductive grid, wherein the active area is on the first side of the detector. The method preferably includes the further steps of depositing a first electrode for the detector on the first side and depositing a second electrode on the second side. Preferably, the grid is advantageously deposited in a pattern of cross-hatched traces disposed on exposed portions of the active area. The photodetector is mounted to provide a connection between the first and second electrodes of the photodiode with first and second electrode leads. The method preferably includes the step of bonding a first wire between the shield pad and a shield lead and the step of bonding a second wire between the electrode pad and the second electrode lead. Preferably, the active areas are responsive to light in a first band of wavelengths, and the method includes the further step of depositing an optical filter over the active area of the photodetector. The optical filter is preferably formed of an encapsulating material which substantially attenuates at least a first range of wavelengths within the first band of wavelengths and which transmits at least a second range of wavelengths within the first band of wavelengths. 
     Another aspect of the present invention is a shielded pulse oximetry probe which comprises a substrate and an emitter mounted to a first portion of the substrate. The emitter is configured to transmit light within a first band of wavelengths. A detector is mounted to a second portion of the substrate. The detector is responsive to wavelengths in the first band and to at least some wavelengths outside the first band. A shield, comprising an electromagnetic shield, is formed over the detector. Preferably, the shield further comprises an optical shield. The optical shield substantially attenuates at least a portion of the wavelengths to which the detector is responsive outside the first band and transmits at least selected wavelengths in the first band. The optical shield advantageously comprises an encapsulant which covers the detector. The photodetector preferably comprises a metallization layer, and the electromagnetic shield is preferably fabricated as an integral portion of the metallization layer. 
     Another aspect of the present invention is a photodiode detector having an integrated electromagnetic shield. The shield is a conductive layer deposited on covered portions of the photodiode active area which leaves exposed active area portions. A bonding pad is deposited as a portion of the shield to provide a low impedance path to substantially all of the shield. A conductor may be attached to the pad to provide an external shield termination. A particularly advantageous aspect of the invention is that the shield is formed during deposition of the conventional metallization layer which deposits a photodiode electrode. Thus, the shield is created by modification of a conventional metallization layer mask and requires little if any modification of the standard photodiode processing steps. Further, unlike external shields, the deposited shield requires no additional parts. An additional advantage of this integrated shield is its proximity to the photodiode component, which eliminates significant shield openings which might pass high frequency EMI. 
     Another aspect of this invention is a photodiode detector having an integrated ambient-light shield. The shield is an encapsulating material encasing the photodiode. The encapsulant has optical transmission characteristics which pass desired emitter wavelengths but filter other wavelengths that are within the response band of the detector. A particularly advantageous aspect of the integrated optical shield is that it is formed as a conventional encapsulant which protects the photodiode and retains the photodiode leads after separation from the lead frame. Thus, the shield is created by modification of the material used during a conventional encapsulation process and requires little, if any, modification to the standard photodiode fabrication steps. Further, the shield requires no additional parts, as with external shields. An additional advantage of this integrated shield is its proximity to the photodiode component, allowing for little if any ambient light leakage. 
     Another aspect of the present invention is a shielded detector which comprises a photodiode having an active area. An electrically conductive layer is deposited on covered portions of the active area and is disposed about exposed portions of the active area. The exposed portions are responsive to light. A pad portion of the conductive layer is connected by a low impedance path to substantially all of the conductive layer. The conductive layer forms an integrated electromagnetic shield for the photodiode which may be externally terminated through the pad portion. 
     Another aspect of the present invention is a shielded detector which comprises a photodiode responsive to a first band of wavelengths in optical communication with an emitter which produces a second band of wavelengths. At least a portion of the second band falls within the first band. A colored encapsulant is in contact with and substantially surrounds the photodiode. The encapsulant transmits wavelengths within the second band and blocks at least a portion of wavelengths outside the second band and within the first band. The encapsulant forms an integrated ambient-light shield for the photodiode. 
     Another aspect of the present invention is a shielded detector which comprises a generally planar photodiode having a first side and a second side. The first side has a first electrode, and the second side has an active area responsive to light within a first band of wavelengths. A metallization layer is deposited directly on the second side. The metallization layer comprises an electrically conductive grid and a second electrode. The second electrode is substantially electrically isolated from the grid. The grid is disposed around exposed portions of the active area. An emitter is operable to generate light within a second band of wavelengths. At least a portion of the second band is within the first band so that current is generated through the first and second electrodes when the exposed portions are in optical communications with the emitter. A pad portion of the grid has a low impedance path to substantially all of the grid, so that the grid functions as an integrated electromagnetic shield which may be externally terminated through a conductor attached to the pad. An optical filter material encapsulates the photodiode. The filter transmits light at wavelengths within the second band and blocks at least a portion of wavelengths within the first band and outside the second band. The filter material shields ambient light from the photodiode. 
     Another aspect of the present invention is a method of creating a shielded detector. The method comprises the step of depositing a metallization layer directly on an active area of a photodiode to form a conductive grid, a shield pad and an electrode pad. The grid is in a pattern of cross-hatched traces disposed around exposed portions of the active area. The method comprises the further steps of mounting the photodiode to provide a connection between an electrode portion of the photodiode and a first electrode lead; bonding a first wire between the shield pad and a shield lead; and bonding a second wire between the electrode pad and a second electrode lead. The method includes the further step of encapsulating the photodiode in an optically-transmissive material. Preferably, the optically-transmissive material has a filtering characteristic that attenuates light having wavelengths outside a desired frequency band to be detected. 
     Another aspect of the present invention is a shielded pulse oximetry probe which comprises a flexible circuit media. An emitter is mounted on a first portion of the media. The emitter is capable of transmitting light within a first frequency band. A detector is mounted on a second portion of the media. The detector comprises a photodiode and a surrounding encapsulant. The photodiode is at least partially responsive to light within the first frequency band. The media is configurable such that the detector is in optical communications with the emitter. A shield is fabricated as an integral portion of at least one of the photodiode metallization layer and the encapsulant. The shield reduces the amount of background noise in the detector. 
     Another aspect of the present invention is a photodiode which has integrated shields for the rejection of noise-producing electromagnetic interference and ambient light. The electromagnetic shield forms a conductive matrix which covers the photodiode active area. The matrix is deposited as a metallization layer onto the photodiode and provides exposed portions of the active area for light detection. A pad is electrically connected to the shield to allow external termination of the shield. The ambient-light shield is in the form of a colored encapsulant surrounding the photodiode. The encapsulant provides a high-pass light transmission characteristic which passes signal light and rejects out-of-band ambient light. The photodiode is particularly advantageous for use in pulse oximetry probes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in detail below in connection with the following drawing figures in which: 
     FIG. 1 is a cut-away view of a prior art “Faraday cage” external to a photodiode detector; 
     FIG. 2 is a cut-away view of prior art optical enclosure having blocking and wavelength filtering materials external to a photodiode detector; 
     FIGS. 3A and 3B are layout views of a preferred unshielded photodiode chip used in constructing a photodiode with integrated shielding according to the present invention; 
     FIG. 4A is a layout view of a photodiode chip having a transmissive-grid metallization layer which forms an integrated electromagnetic shield; 
     FIG. 4B is an enlarged view of a portion of the photodiode chip of FIG. 4A showing the relative spacing of the conductors forming the grid metallization layer; 
     FIGS. 5A and 5B illustrate a detector incorporating an encapsulated, shielded photodiode chip; 
     FIGS. 5C,  5 D and  5 E depict the light transmission characteristics for clear and colored encapsulating material; 
     FIG. 6A illustrates a photodiode detector used in a pulse oximetry probe; 
     FIGS. 6B and 6C depict a pulse oximeter probe incorporating a photodiode detector having integrated noise shielding; 
     FIGS. 7A,  7 B and  7 C are assembly diagrams of a pulse oximeter probe incorporating a photodiode detector having integrated noise shielding; and 
     FIG. 8 is a schematic illustrating the interconnection of a pulse oximeter system utilizing a photodiode detector according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 3A and 3B illustrate a preferred unshielded silicon photodiode chip used in constructing a photodiode detector with integrated noise shielding according to the present invention. The photodiode chip  300  is a planar device constructed of a layer of intrinsic-type semiconductor material sandwiched between layers of P-type and N-type semiconductor material, referred to as a PIN diode. The added intrinsic layer increases the spectral range of response of the photodiode by expanding the depletion region of the P-N junction, which then encompasses carriers released by a broader range of photon wavelengths. 
     A preferred photodiode chip is device number PD-0120C available from Opto Tech Corporation, Semiconductor Division, Hsinchu, Taiwan, R.O.C. This photodiode chip  300  is 125 mils (0.125 inch) on each side, S (FIG.  3 A), and is 12±1.5 mils thick, T (FIG.  3 B). The top side  310  of this diode  300  has an active area  320  of approximately 112×112 mils. An anti-reflective coating  325  covers the active area  320 . An aluminum alloy anode bond pad  330  which is 8 mils in diameter is deposited as a metallization layer on the side  310  of the diode  300 , preferably in one corner thereof. The back side  340  of the diode  300  has a deposited gold alloy cathode  350 . 
     FIG. 4 depicts an improved planar PIN photodiode chip  400  having a modified metallization layer which forms a conductive matrix  410  across the photodiode active area. This matrix performs as an integrated electromagnetic shield for the photodiode  400 . Advantageously, the conductive matrix is deposited on the photodiode  400  during the same process step that deposits the photodiode anode bonding pad. Thus, no additional processing steps are required to create the shield layer as compared to the unshielded photodiode depicted in FIG.  3 . 
     One shielding mechanism is the reflection of an incident electromagnetic wave by the shield surface. Reflection depends on an impedance mismatch between this incident wave and the reflecting shield surface. Shielding effectiveness (SE) is: 
     
       
           SE= 20·log| Z   w /4 ·Z   s | 
       
     
     where Z w  is the impedance of an incident wave and Z s  is the impedance of the shield in ohms/square. Thus, an effective shield has a small Z s , i.e., is highly conductive. At high frequencies, conductivity occurs only near the surface of the shield, due to skin effect. Skin depth is: 
     
       
         δ={square root over ((2/2π f +L μσ))} 
       
     
     where f is frequency of the incident electromagnetic wave, μ is permeability of the shield material and σ is conductivity of the shield material. Most of the current induced in a shield by an incident wave passes within one skin depth of the surface, and very little current goes deeper than three skin depths. Thus, above a few skin depths, the thickness of the shield material is of no consequence with respect to this reflective shielding mechanism. 
     Skin depths, in mils, of common shielding materials are: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Frequency 
                 Copper 
                 Aluminum 
                 Steel 
               
               
                   
                   
               
             
             
               
                   
                  1 MHz 
                 3 
                 3 
                 .3  
               
               
                   
                  10 MHz 
                 .8 
                 1 
                 .1  
               
               
                   
                 100 MHz 
                 .26 
                 .3 
                 .08 
               
               
                   
                  1 GHz 
                 .08 
                 .1 
                 .04 
               
               
                   
                   
               
             
          
         
       
     
     As further shown in FIG. 4, the shielding matrix of the current invention is preferably a grid composed of vacuum-sputtered aluminum traces  430 . A preferred grid pattern is a right-angled, crisscross pattern which creates alternate portions  440  of square-shaped exposed active area and metallized active area, as shown in FIG. 4. A shield bond pad  420  is located at one corner of the grid  410 . All of the grid traces are interconnected with each other and with the shield bond pad  420 . Thus, an electrical connection between the bond pad and a lead allows the entire grid  410  to be grounded via this lead. The anode bond pad  330 , is electrically isolated from the grid  410 . 
     There is a tradeoff between shield effectiveness and detector signal strength which is a function of the amount of photodiode active area which is covered by the shield grid  410 . At one extreme, if the shield is solid, Z s  is minimized and, therefore, shield effectiveness is maximized, but only minimal, if any, light can reach the photodiode. At the other extreme, the shield grid lines are thin and widely spaced, maximizing the exposed photodiode active area but decreasing shield conductivity and, hence, effectiveness. In a preferred embodiment, the effective active area of the photodiode, i.e., the active area of the photodiode which is exposed to light, is between 80% to 90% of the actual photodiode active area. That is, 80% to 90% of the photodiode active area is exposed to light. However, any coverage percentage which permits sufficient light to pass and still provide acceptable signal strength will also work. 
     One embodiment of the shield grid is dimensioned approximately 112 mils×112 mils (i.e., located over and coextensive with the active area  320  of the photodiode. As illustrated in the enlarged detail in FIG. 4A, each conductive trace has a width, A, which, in the preferred embodiment is approximately 0.55 mils. The traces are spaced apart by a spacing distance, B, which, in the preferred embodiment, is approximately 6 mils. As shown in FIG. 4A, a clearance distance, C, is provided between the anode bond pad  330  and the grid  410 . The distance C is approximately 3 mils in the preferred embodiment. The anode bond pad  330  is circular and is approximately 8 mils in diameter. The shield bond pad  420  is square and is approximately 8 mils per side. The metallized active area, being optically opaque, is not part of the effective active area of the shielded photodiode. With the foregoing dimensions, the approximate effective active area of the photodiode can be computed as follows: 
     [1] Total Active Area=112 2    
     [2] Total Area of Horizontal Metallization Lines=18×[17×(6+0.55)+0.55]×0.55 
     [3] Total Area of Vertical Line Segments=18×[17×6×0.55] 
     [4] Total Area of Removed Horizontal Line Segments=2×[2×(6+0.55)×0.55] 
     [5] Total Area of Removed Vertical Line Segments=4×[6×0.55] 
     [6] Area of Anode Bond Pad=π×4 2    
     [7] Area of Filled Inner Square of Shield Bond Pad=6 2    
     Percent of Area Covered By Metallization=([2]+[3]−[4]−[5]+[6]+[7])/[1]=(1107.81+1009.8−14.41−13.2+36+50.26)/12544=17.34% 
     Thus, the embodiment described above has an effective active area which is approximately 82.66% of the actual active area of the photodiode. 
     FIGS. 5A and 5B show the photodiode with integrated electromagnetic shield  400  packaged so as to form an encapsulated, leaded detector  500 . The chip  400  is attached to a leadframe  510  with conductive adhesive applied between the cathode side  340  of the chip  400  and the leadframe  510 . This makes an electrical connection between the photodiode cathode  350  and one lead  512  of the leadframe. An anode wire connection  520  is made between the chip anode bond pad  330  and another lead  514  of the leadframe  510 . A shield wire connection  530  is made between the shield bond pad  420  and a third lead  516  of the leadframe  510 . Preferably, the wire connections to the anode bond pad  330  and the shield bond pad  420  are gold wires. A ball bond is created on the anode bond pad  330  or the shield bond pad  420  of the diode  400 , and a stitch bond is formed on the respective lead of the leadframe  510 . The anode and shield wire connections may also be made with aluminum, copper or similar metals, and the connections can be wedge bonded. Other interconnection methods, such as TAB or flip-chip, can also be used. This detector assembly is then placed in a transfer mold which is filled with an epoxy molding compound. Other potential methods for encapsulation include pour molding, injection molding, or the dispensing of a material in liquid form which subsequently cures via a chemical reaction, the addition of heat, or exposure to radiant energy. A preferred epoxy molding compound is HYSOL® MG18, which is available from The Dexter Corporation, Electronic Materials Division, Industry, Calif. The epoxy compound is cured and deflashed to create an encapsulation  540 . The leadframe  510  is then trimmed and the leads are formed to complete the detector  500 . 
     As shown in FIGS. 5C,  5 D and  5 E, the MG18 encapsulant  540  can be purchased clear or in various colors, including light red and yellow. A colored encapsulant can advantageously be used as an integrated, ambient-light shield for a photodiode detector in applications where the signal of interest is within the passband of the color encapsulant and interfering ambient light is outside this passband. One such application is pulse oximetry, as described above. 
     As depicted in FIG. 6A, a pulse oximetry probe  602  can be attached to a finger  650 , for example, to project light through a capillary bed  658 . In a particular embodiment of the pulse oximetry probe  602 , the red LED  672  of the emitter  670  produces light centered at 660 nanometers with a bandwidth of 50 nanometers, i.e., light having wavelengths from 635 nanometers to 685 nanometers. The infrared LED  674  of the emitter  670  produces light centered at 905 nanometers. However, the photodiode detector  500  is sensitive to wavelengths as small as 450 nanometers. Thus, with the clear encapsulant shown in FIG. 5C, the detector  500  will be responsive to noise-producing ambient light which is entirely outside the band of light produced by the red LED  672 , specifically light having wavelengths in the range 450-635 nanometers. Hence, for pulse oximetry applications, a preferred encapsulant is an encapsulant which absorbs light having wavelengths in the range of 450 nanometers to 635 nanometers and which transmits light having wavelengths greater than 635 nanometers. Exemplary encapsulants meeting this criteria are the MG18 light red and the MG18 yellow epoxy molding compounds, having the transmission characteristics shown in FIG.  5 D and FIG. 5E, respectively. A most preferred encapsulant for a pulse oximetry probe  602  is the MG18 light red epoxy molding compound, having a cutoff very close to 635 nanometers (i.e., which attenuates light having wavelengths less than approximately 635 nanometers). 
     FIGS. 6B and 6C depict one embodiment of a pulse oximetry probe  602  incorporating the shielded detector. FIGS. 6B and 6C also show the attachment of the probe  602  onto the fingertip  650  of an adult patient. As shown in FIG. 6B, the probe  602  is designed to fit comfortably onto a patient&#39;s fingertip. Advantageously, the probe is also designed to be disposable. Referring to FIG. 6B, the probe has a release liner  603 , which is removed from the probe  602  to expose an adhesive surface  608  which adheres to the finger  650 . The probe  602  includes a central portion  604 , a pair of adhesive flanges  605  extending from the central portion  604 , a connector portion  610  situated between the flanges  605 , and a pair of smaller adhesive flaps  615  extending from the central portion  604  on the end of the probe  602  opposite from the connector  610 . The probe  602  further includes a connection aperture  612  formed in the connector tab  610  and an emitter aperture  620  with a light-emitting diode (LED) emitter  670  (FIG.  6 A). A flex pocket  625  is located within the central portion  604  between the emitter aperture  620  and a detector aperture  630 . The probe  602  folds at the location of the flex pocket  625  over the fingertip  650 . The detector aperture  630  allows light to pass through to a detector assembly  635  which contains a photodiode detector  500 , as described above with respect to FIGS. 5A-5E. An adult fingertip  650  is shown in phantom in FIG. 6B to illustrate the position at which the fingertip  650  would be placed within the probe  602  prior to being fastened onto the fingertip  650  for use. 
     FIG. 6C illustrates the probe  602  fastened onto the fingertip  650 . The probe  602  folds such that the flex pocket  625  aligns with the very end of the fingertip and such that adhesive flaps  605  fold downward (in the illustration of FIG. 6C) to wrap around the fingertip  650  while the adhesive flaps  615  fold upward (in the illustration of FIG. 6C) about a portion of the circumference of the fingertip  650  to provide support. When the probe  602  is folded about the fingertip  650 , the emitter aperture  620  is spaced opposite the detector assembly  635  such that light from the emitter  670  (FIG. 6A) passes through the emitter aperture  620 , through the finger  650  and is incident upon the detector assembly  635  through the detector aperture  630 . 
     FIG. 6C depicts a receiving connector portion  660  (in phantom) which engages with contacts  652  on the connector  610  to provide an electrical connection between the probe  602  and signal processing circuitry within a pulse oximeter instrument  840  (FIG.  8 ). The digital signal processing circuitry may be used to analyze the output of the detector  500  (not shown) within the assembly  635 . In one advantageous embodiment, the aperture  612  engages a tab (not shown) within the connector  660  to firmly secure the connector  660  to the probe  602 . Once the probe  602  is securely fastened to the fingertip  650  and the connector provides an electrical connection between the probe  602  and the pulse oximeter, signals are detected from the detector  500  and transmitted to the signal processing circuitry via the connector  660 . 
     FIGS. 7A-7C illustrate the assembly of the pulse oximetry probe depicted in FIGS. 6A-6C. The probe  602  is fabricated from multiple layers, including a flex circuit layer  710 , a polyester shield layer  720 , a face stock tape layer  794 , a base stock layer  792  with the releasable liner  603  (FIG.  6 B), and various pieces of pressure-sensitive adhesive (PSA). 
     Referring to FIG. 7A, a shielded flex circuit assembly  700  is formed from the flex circuit layer  710  located between folded portions of a flex circuit shield layer  720 . The flex circuit shield layer  720  is advantageously constructed from polyester laminated with a thin conductive layer, such as copper. A preferred laminated polyester is made by TECHNIMET, part number SO-2010-1-3 and has an insulator film made by Coating Sciences, part number P-341. The insulator film prevents electrical contact between flex circuit traces and the conductive layer of the flex circuit shield layer  720 . 
     A shielded detector  500  according to the present invention, which may have an integrated electromagnetic shield or an integrated ambient-light shield or both, is attached to the flex circuit  710 . Each of the three detector leads, the cathode lead  512 , the anode lead  514  and the shield lead  516 , are soldered to one of three flex circuit solder pads. In one embodiment, an encapsulated emitter  730  containing red and infrared LEDs which are connected “back-to-back” so as to share two common leads is also attached to the flex circuit  710  by soldering each of these two leads to one of two flex circuit solder pads. Other emitter configurations are also possible, such as a three-lead emitter where the red and infrared LEDs share a common anode lead but have separate cathode leads or a four-lead emitter where the LEDs have no common leads. 
     In one embodiment, a resistor  740  is also attached to the flex circuit  710 . The resistor leads are soldered to two flex circuit solder pads, connecting the resistor  740  in parallel to the emitter  730 . This resistor value provides an identifier which specifies, for example, the intended patient type (adult, neonatal, etc.) or the probe manufacturer. The resistor value can be read by a pulse oximeter connected to the probe when a voltage is applied across the emitter  730  which is less than an LED threshold voltage, thereby effectively removing the LEDs from the circuit as a current load. 
     As further shown in FIG. 7A, the polyester shield layer  720  is laminated to the flex circuit  710  by a piece of conductive PSA  712  attached to the detector end of the flex circuit  710  and by pieces of nonconductive PSA  714 ,  716  attached, respectively, to the component and non-component sides of the emitter end of the flex circuit  710 . The PSA strips bond the flex circuit shield layer  720  to both sides of the flex circuit  710  to provide a conductive EMI shield for the flex circuit  710  which covers all but the flex circuit contact fingers  718  and the optical path of the detector  500  and emitter  730 , which remain exposed. The conductive PSA  712  provides an electrical connection between a folded portion  715  of the flex circuit  710  and an uninsulated portion  722  of the conductive flex circuit shield  720 . The flex circuit folded portion  715 , in turn, is part of a shield trace on the flex circuit which provides a low impedance path to both the detector shield lead  516  and to a shield contact portion of the contact fingers  718 . 
     An optical cavity  750  is attached to the detector end of the flex circuit assembly  700  with a piece of PSA  752 . The optical cavity  750  is made from styrene in one embodiment. In one preferred embodiment, the optical cavity  750  is coated with an optical coating that is opaque to ambient light. In an alternative embodiment, the optical cavity  750  can be made from a material that is opaque to ambient light. The optical cavity  750  has a rectangular receiving receptacle  754  adapted to receive the detector end of the flex circuit assembly  700 . Advantageously, the optical cavity  750  has a wedge shape ramp  756  as part of the receptacle  754  which provides for a smooth transition for the flex circuit  710  between the surface of the base material  792 , described below with respect to FIG. 7C, and the bottom surface of the receptacle  754 . The walls of the receptacle  754  hold the flex circuit assembly  700  in position such that the attached detector aligns properly with an aperture  758  in the optical cavity  750 . Preferably, the flex circuit assembly  700  fits snugly between the side walls and against the end wall. In a preferred embodiment, the optical cavity aperture  758  is configured to be cone-shaped, cylindrical or conical. 
     A cover  770  is placed over the optical cavity  750 . The cover  770  is advantageously vacuum-formed and is cup-shaped. In a preferred embodiment, the cover  770  is made from polypropylene. A light barrier disk  760  is placed inside the cover  770  to block ambient light. Preferably, the disk  760  is made from a thin metal foil, such as aluminum foil. The cover  770  may also be made opaque to ambient light by applying a coating or by selecting a suitable construction material. The cover  770  has a flange  772  which serves as a bonding surface with the base material  792 , described below. A connector tab  780  is attached to the emitter end of the flex circuit assembly  700  with a piece of PSA  782 . The connector tab  780  is advantageously formed of ABS styrene and has an aperture  784 . 
     FIG. 7B depicts the completed flex circuit assembly  700 . As shown in FIG. 7C, the flex circuit assembly  700  is sandwiched between a base stock  792  and a face stock  794 . In one embodiment, the base stock  792  comprises Avery 5051 base material and is transparent to the emitter wavelengths. The bottom side of the base stock  792  is coated with an acrylic PSA and is provided with a thin release layer  603 , preferably made from a paper release liner or the like, as is well understood in the art. The top side of the base stock  792  is laminated with an unsupported rubber PSA, such as Coating Sciences U-224. 
     The face stock  794  is advantageously constructed from a non-woven, flexible material which is placed over the flex circuit assembly  700  and the base stock  792 . In a preferred embodiment, the face stock  794  comprises Betham part number 1107-S. The face stock  794  preferably has an aperture  795  to allow the cup portion  774  of the cover  770  to protrude through the face stock  794 . The face stock  794  covers the flange portion  772  of the cover  770 . Because the base stock  792  has PSA on the side to which the face stock  794  is applied, pressure applied to the face stock  794  bonds the face stock with the base stock. The face stock  794  may also have PSA on the side which bonds to the base stock  792 . The face stock  794  is cut such that the connector tab  780  and connector traces  718  remain exposed, forming a probe connector  798 . 
     FIG. 8 schematically represents a pulse oximeter system  800 , illustrating the cabling, interconnection and grounding for a pulse oximeter probe incorporating a photodiode with integrated noise shielding, as described above. The pulse oximeter system  800  comprises a probe  602  (described above) interconnected with a pulse oximeter instrument  840  via a patient cable  820 . The cable  820  has a first connector  822  which mates with the probe connector  798 . The cable  820  has a second connector  824  which mates with a pulse oximeter connector  842 . An embodiment of the patient cable  820  comprises a pair of signal wires  830 , an inner shield  832  surrounding the signal wires  830 , a pair of drive wires  834  and an outer shield  836  surrounding the drive wires  834  and inner shield  832 . In one embodiment, the probe connector  798  has six flex circuit connector traces  718 . The anode lead  514  and cathode lead  512  of the detector  500  are connected to two of these traces  802 , which mate to the double-shielded input wires  830  of the patient cable  820  via the first cable connector  822 . The input wires  830  are brought into the pulse oximeter instrument  840  via the second cable connector  824  and the oximeter connector  842 . The emitter  730  is also connected to two of the flex circuit connector traces  804 , which mate to the outside-shielded drive wires  834  of the patient cable  820  and which are driven by the pulse oximeter instrument  840  via the oximeter connector  842  and the second cable connector  824 . The integrated shield lead  516  of the detector  500  is connected to one of the flex circuit connector traces  806 . In one embodiment, the shield trace  806  may be connected to the patient cable inner shield  832  which, in turn, may be connected to ground  844  within the pulse oximeter instrument  840  via the second cable connector  824  and the oximeter connector  842 . In one embodiment, there is an unused probe trace  808 , and the outer shield  836  of the patient cable  820  is not connected to the probe  602 . The outer shield  836 , however, may be grounded  846  at the pulse oximeter electronics  840  via the second cable connector  824  and the oximeter connector  842 . 
     The integrated photodiode electromagnetic shield and ambient light shield and associated pulse oximeter probe have been disclosed in detail in connection with the preferred embodiments of the present invention. These embodiments are disclosed by way of examples only and are not to limit the scope of the present invention, which is defined by the claims that follow. One of ordinary skill in the art will appreciate many variations and modifications within the scope of this invention.