Abstract:
A ceramic emitter substrate has a substrate body with top and bottom sides and a cavity disposed on the top side. Bonding pads are disposed within the cavity and solder pads are disposed on the bottom side. Light emitting diodes (LEDs) are electrically connected to the bonding pads. Low-resistance conductors are disposed within the ceramic substrate body so as to interconnect the bonding pads and the solder pads. The interconnect is configured so that the LEDs can be individually activated as an array via row and column drive signals applied to the solder pads.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority benefit under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 60/998,659, filed Oct. 12, 2007, titled Ceramic Emitter Substrate; and U.S. Provisional Patent Application Ser. No. 61/192,131 filed Sep. 14, 2008, titled Ceramic Emitter Substrate; all of the above applications incorporated by reference herein. 
       INCORPORATION BY REFERENCE OF COPENDING RELATED CASES 
       [0002]    The present disclosure is generally related to U.S. patent application Ser. No. 12/056,179, filed Mar. 26, 2008, titled Multiple Wavelength Optical Sensor, hereby incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Pulse oximetry systems for measuring constituents of circulating blood have gained rapid acceptance in a wide variety of medical applications, including surgical wards, intensive care and neonatal units, general wards, home care, physical training, and virtually all types of monitoring scenarios. A pulse oximetry system generally includes an optical sensor applied to a patient, a monitor for processing sensor signals and displaying results and a patient cable electrically interconnecting the sensor and the monitor. A pulse oximetry sensor has light emitting diodes (LEDs), typically one emitting a red wavelength and one emitting an infrared (IR) wavelength, and a photodiode detector. The emitters and detector are attached to a patient tissue site, such as a finger. The patient cable transmits drive signals to these emitters from the monitor, and the emitters respond to the drive signals to transmit light into the tissue site. The detector generates a signal responsive to the emitted light after attenuation by pulsatile blood flow within the tissue site. The patient cable transmits the detector signal to the monitor, which processes the signal to provide a numerical readout of physiological parameters such as oxygen saturation (SpO 2 ) and pulse rate. Advanced physiological monitoring systems utilize multiple wavelength sensors and multiple parameter monitors to provide enhanced measurement capabilities including, for example, the measurement of carboxyhemoglobin (HbCO), methemoglobin (HbMet) and total hemoglobin (Hbt). 
         [0004]    Pulse oximeters capable of reading through motion induced noise are disclosed in at least U.S. Pat. Nos. 6,770,028, 6,658,276, 6,650,917, 6,157,850, 6,002,952, 5,769,785, and 5,758,644; low noise pulse oximetry sensors are disclosed in at least U.S. Pat. Nos. 6,088,607 and 5,782,757; all of which are assigned to Masimo Corporation, Irvine, Calif. (“Masimo”) and are incorporated by reference herein. 
         [0005]    Physiological monitors and corresponding multiple wavelength optical sensors are described in at least U.S. patent application Ser. No. 11/367,013, filed Mar. 1, 2006 and entitled Multiple Wavelength Sensor Emitters and U.S. patent application Ser. No. 11/366,208, filed Mar. 1, 2006 and entitled Noninvasive Multi-Parameter Patient Monitor, both assigned to Masimo Laboratories, Irvine, Calif. (Masimo Labs) and both incorporated by reference herein. 
         [0006]    Further, physiological monitoring systems that include low noise optical sensors and pulse oximetry monitors, such as any of LNOP® adhesive or reusable sensors, SofTouch™ sensors, Hi-Fi Trauma™ or Blue™ sensors; and any of Radical®, SatShare™, Rad-9™, Rad-5™, Rad-5v™ or PPO+™ Masimo SET® pulse oximeters, are all available from Masimo. Physiological monitoring systems including multiple wavelength sensors and corresponding noninvasive blood parameter monitors, such as Rainbow™ adhesive and reusable sensors and RAD-57™ and Radical-7™ monitors for measuring SpO 2 , pulse rate, perfusion index, signal quality, HbCO and HbMet among other parameters are also available from Masimo. 
       SUMMARY OF THE INVENTION 
       [0007]      FIGS. 1A-B  illustrate a physiological monitoring system  100  capable of generating SpO 2  and in multiple wavelength configurations additional blood parameter measurements such as HbCO, HbMet and Hbt. The physiological monitoring system  100  has a monitor  110  and a sensor  150 . The sensor  150  attaches to a tissue site  1  and includes a plurality of emitters  122  capable of irradiating the tissue site with differing wavelengths of light, such as the red and infrared (IR) wavelengths utilized in pulse oximeters and, in some configurations, multiple wavelengths different than or in addition to those red and IR wavelengths. The sensor  150  also includes one or more detectors  154  capable of detecting the light after attenuation by the tissue site  1 . 
         [0008]    As shown in  FIGS. 1A-B , the monitor  110  communicates with the sensor  150  to receive one or more intensity signals indicative of one or more physiological parameters and displays the parameter values. Drivers  114  convert digital control signals into analog drive signals capable of driving sensor emitters  152 . A front-end  112  converts composite analog intensity signal(s) from light sensitive detector(s)  154  into digital data  115  input to the DSP  120 . The digital data  115  is representative of a change in the absorption of particular wavelengths of light as a function of the changes in body tissue resulting from pulsing blood. The DSP  120  may comprise a wide variety of data and/or signal processors capable of executing programs for determining physiological parameters from input data. 
         [0009]    Also shown in  FIGS. 1A-B , the instrument manager  130  may comprise one or more microcontrollers controlling system management, such as monitoring the activity of the DSP  120 . The instrument manager  130  also has a display driver  132 , an audio driver  134  and an input/output (I/O) port  138  that provides a user and/or device interface for communicating with the monitor  110 . 
         [0010]    Further shown in  FIGS. 1A-B  are one or more user I/O devices  140  including a display  142 , an audible indicator  144  and a keypad  148 . The display  142  is capable of displaying indicia representative of calculated physiological parameters such as one or more of a pulse rate (PR), signal quality and values of blood constituents in body tissue, including for example, oxygen saturation (SpO 2 ). The monitor  110  may also be capable of storing or displaying historical or trending data related to one or more of the measured parameters or combinations of the measured parameters. Displays  142  include for example readouts, colored lights or graphics generated by LEDs, LCDs or CRTs to name a few. Audible indicators  144  include, for example, tones, beeps or alarms generated by speakers or other audio transducers to name a few. The user input device  148  may include, for example, a keypad, touch screen, pointing device, voice recognition device, or the like. 
         [0011]      FIG. 2  illustrates an emitter array  200  for a multiple wavelength optical sensor having multiple emitters  210  capable of emitting light  202  having multiple wavelengths into a tissue site  1 . Row drivers  270  and column drivers  280  are electrically connected to the emitters  210  and activate one or more emitters  210  by addressing at least one row  220  and at least one column  240  of an electrical grid. In one embodiment, the emitters  210  each include a first contact  212  and a second contact  214 . The first contact  212  of a first subset  230  of emitters is in communication with a first conductor  220  of the electrical grid. The second contact  214  of a second subset  250  of emitters is in communication with a second conductor  240 . Each subset comprises at least two emitters, and at least one of the emitters of the first and second subsets  230 ,  250  are not in common. A detector  290  is capable of detecting the emitted light  202  and outputting a sensor signal  295  responsive to the emitted light  202  after attenuation by the tissue site  1 . As such, the sensor signal  295  is indicative of at least one physiological parameter corresponding to the tissue site  1 , as described above. 
         [0012]      FIG. 3  illustrates an emitter array  300  embodiment having light emitting diodes (LEDs)  301  connected within an electrical grid of n rows and m columns totaling n+m drive lines  350 ,  360 , where n and m are integers greater than one. The electrical grid minimizes the number of drive lines required to activate the LEDs  301  while preserving flexibility to selectively activate individual LEDs  301  in any sequence and multiple LEDs  301  simultaneously. The electrical grid also facilitates setting LED currents so as to control intensity at each wavelength, determining operating wavelengths and monitoring total grid current so as to limit power dissipation. The emitter array  300  is also physically configured in rows  310 . This physical organization facilitates clustering LEDs  301  according to wavelength so as to minimize pathlength variations and facilitates equalization of LED intensities. 
         [0013]    As shown in  FIG. 3 , one embodiment of an emitter array  300  comprises up to sixteen LEDs  301  configured in an electrical grid of four rows  310  and four columns  320 . Each of the four row drive lines  350  provide a common anode connection to four LEDs  301 , and each of the four column drive lines  360  provide a common cathode connection to four LEDs  301 . Thus, the sixteen LEDs  301  are driven with only eight wires, including four anode drive lines  312  and four cathode drive lines  322 . This compares favorably to conventional common anode or cathode LED configurations, which require more drive lines. 
         [0014]    Also shown in  FIG. 3 , row drivers  370  and column drivers  380  located in the monitor  110  selectively activate the LEDs  301 . In particular, row and column drivers  370 ,  380  function together as switches to Vcc and current sinks to ground, respectively, to activate LEDs and as switches to ground and Vcc, respectively, to deactivate LEDs. This push-pull drive configuration prevents parasitic current flow in deactivated LEDs. In a particular embodiment, only one row drive line  350  is switched to Vcc at a time. One to four column drive lines  360 , however, can be simultaneously switched to a current sink so as to simultaneously activate multiple LEDs within a particular row. Activation of two or more LEDs of the same wavelength facilitates intensity equalization. 
         [0015]    A ceramic emitter substrate advantageously houses, mechanically mounts and electrically interconnects an emitter array, as described with respect to  FIGS. 2-3 , above. Ceramic lends mechanical and structural precision over other substrate materials. Further, the ceramic substrate provides uniform thermal properties that allow accurate measurement of emitter temperatures utilizing a co-mounted thermistor or similar temperature responsive device. The ceramic substrate also provides a cavity which protects the emitter array and accepts encapsulants. Encapsulants may include one or more of an attenuating epoxy over selected emitter components so as to equalize emitter intensities and clear fill epoxy with or without a dispersed diffusing media, as examples. In addition, the ceramic media is multi-layered, allowing internal routing for the matrix that interconnects the emitter array. A ceramic emitter substrate incorporated into an optical sensor and also encapsulants disposed in a ceramic emitter substrate cavity are described with respect to U.S. patent application Ser. No. 12/056,179, cited above and incorporated by reference herein. 
         [0016]    In particularly advantageous embodiments, special attention is given to the ceramic substrate multi-layer conductors to achieve very low resistance. Low resistance in the emitter array interconnect minimizes the resistive heating of the substrate and corresponding spurious wavelength shifts. Also, low interconnect resistance lessens parasitic voltage drops between emitters and drivers that negatively impact available drive current. 
         [0017]    One aspect of a ceramic emitter substrate is an optical medical device that transmits optical radiation into a fleshy tissue site. The optical radiation is detected after absorption by pulsatile blood flow within the fleshy tissue site so as to compute constituents of the pulsatile blood flow. A generally rectangular-cross-sectioned ceramic body has a top side, a bottom side and an edge adjoining the sides. A cavity is defined by the ceramic body and disposed on the top side. Conductive bonding pads are disposed within the cavity. Conductive solder pads are disposed on the bottom side proximate the edge. Conductive traces and vias form an interconnect of the bonding pads and the solder pads. Light emitting diodes (LEDs) can be attached to the bonding pads and individually activated as an emitter array via row and column drive signals applied to the solder pads in order to transmit optical radiation out of the cavity. 
         [0018]    In an embodiment, the ceramic body comprises first, second, third and fourth layers. The first layer defines the top side and the cavity. The second layer underlies the first layer. The third layer underlies the second layer. A fourth layer underlies the third layer and defines the bottom side. A first portion of the bonding pads are disposed on the second layer. A second portion of the bonding pads are disposed on the third layer. LEDs are mounted to the bonding pads on the third layer and wire bonded to the bonding pads on the second layer. In a particularly advantageous embodiment, each combination of traces, vias and pads constituting a conductive path between the solder pads and the bonding pads for any one of the drive signals has a combined resistance less than about 310 milliohms. 
         [0019]    In an embodiment, a thermistor is mounted within the cavity and electrically connected to the bonding pads so that the resistance of the thermistor can be read via the solder pads and the interconnect. A portion of the third layer creates a raised partition within the cavity that separates the floor of the cavity into a first area and a second area. LEDs are mounted within the first area and the thermistor is mounted within the second area. An encapsulant may be disposed within the cavity over at least a portion of the LEDs, where the encapsulant functions as an optical filter or an optical diffuser or both. In a particularly advantageous embodiment, the ceramic body is constructed of a substantially light absorbing material so as to substantially block LED emitted optical radiation from being transmitted through the ceramic body. 
         [0020]    Another aspect of a ceramic emitter substrate comprises a ceramic body having a top side, an opposite bottom side and an edge disposed between and along the periphery of the top and bottom sides. The ceramic body has a first layer corresponding to the top side, a second layer adjacent the first layer, a third layer adjacent the second layer and a fourth layer corresponding to the bottom side. A cavity is defined by the first layer. Solder pads are disposed on the fourth layer on the bottom side proximate the edge. Bonding pads are disposed on the second layer and on the third layer. The bonding pads are accessible via the cavity. Traces are disposed on the second, third and fourth layers and vias are disposed between the second, third and fourth layers so as to interconnect the solder pads and the bonding pads. 
         [0021]    In a particularly advantageous embodiment, the traces have a substantial width relative to the area of the ceramic body sides so as to have a low resistance. In an embodiment, the resistance of any one of the traces is less than about 290 milliohms. In an embodiment, the ceramic body measures about 0.23×0.15×0.04 inches and the cavity measures about 0.18×0.10 inches. In an embodiment, the ceramic body comprises a dark material that substantially absorbs light transmitted from the light emitting diodes so as to substantially block optical leakage through the ceramic body edge and bottom side. 
         [0022]    A further aspect of a ceramic emitter substrate is a method of constructing an optical sensor having emitters that transmit optical radiation having multiple wavelengths into a tissue site and a detector that generates a sensor signal responsive to the optical radiation after absorption by the tissue site. A ceramic substrate having a top side and a bottom side is provided. A cavity is defined in the top side of the ceramic substrate. Light emitting devices are mounted within the cavity. Low-resistance conductors are routed on and within the ceramic substrate so as to transmit drive signals to the light emitting devices from a source external to the ceramic substrate. 
         [0023]    In various embodiments bonding pads are plated on the top side within the cavity. Solder pads are plated on the bottom side. The solder pads are interconnected with the bonding pads. The light emitting devices are bonded to the bonding pads so as to transmit optical radiation from the cavity in response to drive signals applied to the solder pads. In an embodiment, plating bonding pads comprises plating upper bonding pads on a second layer of the ceramic substrate, plating lower bonding pads on a third layer of the ceramic substrate and sandwiching the second layer and the third layer between a first layer of the ceramic substrate that defines the top side and the cavity and a fourth layer that defines the bottom side. In an embodiment, Interconnecting comprises disposing traces on the second, third and fourth layers, which may comprise substantially maximizing the width of each of the traces that conduct the drive signals given the number of traces and the area of the layers so as to substantially minimize the resistance of the traces. In an embodiment, traces of sufficient width are provided so that each of the traces that conduct the drive signals has a resistance less than about 290 milliohms. Solder pads, bonding pads and vias are provided so that the resistance from solder pad to bonding pad for each of the drive signals is less than about 310 milliohms. 
         [0024]    Another aspect of a ceramic emitter substrate is configured to mount in an optical sensor and to transmit optical radiation into a fleshy tissue site, the optical radiation detected after absorption by pulsatile blood flow, a signal responsive to the detected optical radiation communicated to a monitor that computes constituents of the pulsatile blood flow. The ceramic emitter substrate comprises a ceramic substrate means for housing LEDs. A solder pad means is for physically mounting and electrically interconnecting the ceramic substrate means to a sensor. Bonding pad means are for mounting and electrically interconnecting the LEDs to the ceramic substrate means. Low resistance conductive means are for interconnecting the solder pad means and the bonding pad means. 
         [0025]    In various embodiments, the ceramic substrate means comprises a first ceramic layer means for defining a cavity within the ceramic substrate means. A third ceramic layer means is for defining a device bonding area along a cavity floor. A second ceramic layer means is for defining a wire bonding area raised above the cavity floor disposed between the first and second ceramic layer means. A fourth ceramic layer means is for defining a soldering area disposed adjacent the third ceramic layer means. A first set of the bonding pad means is for mounting electrical components disposed along the device bonding area. A second set of the bonding pad means is for wiring bonding to electrical components disposed along the wire bonding area. Solder pad means are for soldering the ceramic substrate to a flexible circuit disposed along the soldering area. Low resistance conductive means are for interconnecting between the solder pad means and the first and second sets of bonding pad means. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIGS. 1A-B  are a perspective view and general block diagram of a physiological measurement system utilizing an optical sensor; 
           [0027]      FIG. 2  is a general block diagram of an emitter array for a multiple wavelength optical sensor; 
           [0028]      FIG. 3  is a schematic diagram of an emitter array; 
           [0029]      FIGS. 4A-B  are top and bottom exploded views of a multiple wavelength sensor assembly utilizing a ceramic emitter substrate; 
           [0030]      FIGS. 5A-B  are perspective and perspective cross sectional views, respectively, of a ceramic emitter substrate; 
           [0031]      FIGS. 6-14  are views of a ceramic emitter substrate embodiment; 
           [0032]      FIGS. 6A-D  are top, half-end cross sectional, bottom and half-side cross sectional views, respectively, of a ceramic emitter substrate; 
           [0033]      FIG. 7  is a plan view of ceramic emitter substrate bonding pads; 
           [0034]      FIGS. 8-11  are plan views of ceramic emitter substrate first through fourth layers; 
           [0035]      FIG. 12-13  are plan views of ceramic emitter substrate solder pads and an alumina coat layer, respectively; 
           [0036]      FIGS. 14A-C  are top, side and bottom views of an array of ceramic emitter substrates formed from a multilayer ceramic sheet; 
           [0037]      FIGS. 15-23  are views of a low-resistance ceramic emitter substrate embodiment; 
           [0038]      FIG. 15  is a resistance chart for a low-resistance ceramic emitter substrate; 
           [0039]      FIGS. 16A-F  are top, half-end cross sectional, bottom and half-side cross sectional views and top and bottom perspective views, respectively, of a low-resistance ceramic emitter substrate; 
           [0040]      FIG. 17  is a bonding plan view of a low-resistance ceramic emitter substrate; 
           [0041]      FIGS. 18-21  are plan views of first through fourth layers, respectively, for a low-resistance ceramic emitter substrate embodiment; and 
           [0042]      FIGS. 22-23  are plan views of solder pads and an alumina coat layer, respectively, for a low-resistance ceramic emitter substrate. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]      FIGS. 4A-B  illustrate an interconnect assembly  400  having a flex circuit  410 , an emitter assembly  420  and a detector assembly  490 . The flex circuit  410  mounts the emitter assembly  420  and detector assembly  490 , connects to a sensor cable  160  ( FIGS. 1A-B ) and provides electrical communications between a monitor  110  ( FIGS. 1A-B ) and emitters mounted in a ceramic emitter substrate  500 . The emitter assembly  420  has a cover  422 , a light block  424 , a ceramic emitter substrate  500 , a spacer  426  and an encapsulant  428 . The ceramic emitter substrate  500  is soldered to an emitter mount  412  on the flex circuit  410 . Similarly a ceramic-carrier detector  492  is soldered to a detector mount  414 . In particular, the emitter mount solder pads correspond to ceramic substrate solder pads  1220  ( FIG. 12 ). 
         [0044]    Advantageously, the spacer  426  and encapsulant  428  provide a relatively uniform illumination of patient tissue across all emitted wavelengths. In particular, the spacer  426  provides a gap between an emitter array mounted in the ceramic substrate  500  and a tissue site, allowing the light from each emitter to spread as it propagates to the tissue site. Further, the encapsulant  428  can be configured to diffuse or scatter emitter light from each emitter as it propagates to a tissue site. In an embodiment, the encapsulant contains glass beads in a clear silicon RTV. In an embodiment, the encapsulant also contains a filtering medium that provides pass-band characteristics according to emitted wavelengths so as to equalize intensities of the various emitters. In an embodiment, the encapsulant provides notch filter characteristics according to emitted wavelengths so as to substantially attenuate secondary emissions from one or more emitters. 
         [0045]      FIGS. 5A-B  illustrate a ceramic emitter substrate  500  having multiple layers of bonding pads, traces, vias and solder pads so as to mount and interconnect an emitter array, e.g.  300  ( FIG. 3 ). The ceramic emitter substrate  500  has a body  801  defining a cavity  802 . The cavity  802  contains LEDs  510  connected to bonding pads  900 ,  1000 . The cavity  802  also contains a thermistor  520 , the resistance of which can be measured in order to determine the bulk temperature of the LEDs  510 . The thermal characteristics of ceramic stabilize and normalize the bulk temperature of the substrate  500  so that the thermistor measurement of bulk temperature is meaningful. 
         [0046]      FIGS. 6-14  illustrate an embodiment of a ceramic emitter substrate. In particular,  FIGS. 6A-D  illustrate a ceramic emitter substrate  500  having a top side  601  and a bottom side  602 . The top side  601  has upper bonding pads  910  and lower bonding pads  1010  as described with respect to  FIG. 7 , below. The bottom side  602  has solder pads  1220 , as described with respect to  FIG. 12 , below. The ceramic emitter substrate  500  also has four layers  800 ,  900 ,  1000 ,  1100  with corresponding surfaces including bonding pads, traces, vias and solder pads, as described with respect to  FIGS. 8-11 , below. 
         [0047]      FIG. 7  illustrates upper  910  and lower  1010  bonding pads. The lower bonding pads  1010 , labeled  8  through  16 , mount and electrically connect a first side (anode or cathode) of the LEDs  510  ( FIG. 5A ) into an emitter array. Upper bonding pads  910 , labeled  1  through  7 , electrically connect a second side (cathode or anode) of the LEDs  510  ( FIG. 5A ) into the emitter array, via bonding wires  530  ( FIG. 5B ). A thermistor  520  is mounted to bonding pads  1010  labeled  17  and  18 . Plated “feed-thru” holes and other vias electrically connect the bonding pads  910 ,  1010  on the top side  601  ( FIG. 6A ) with the solder pads  1220  ( FIG. 6C ) on the bottom side  602  ( FIG. 6C ). In one embodiment, top-side  601  ( FIG. 6A ) bonding pad numbers and corresponding bottom-side  602  ( FIG. 6C ) solder pad numbers are electrically connected as shown in TABLE 1. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Connection Table 
               
             
          
           
               
                   
                 BOND PAD NO. 
                 SOLDER PAD NO. 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 1 
               
               
                   
                 6 
               
               
                   
                 11 
               
               
                   
                 2 
                 10 
               
               
                   
                 7 
               
               
                   
                 12 
               
               
                   
                 3 
                 12 
               
               
                   
                 5 
               
               
                   
                 13 
               
               
                   
                 4 
                 3 
               
               
                   
                 10 
               
               
                   
                 8 
                 2 
               
               
                   
                 9 
                 11 
               
               
                   
                 14 
                 5 
               
               
                   
                 16 
               
               
                   
                 15 
                 4 
               
               
                   
                 17 
                 9 
               
               
                   
                 18 
                 8 
               
               
                   
                   
                 6 
               
               
                   
                   
                 7 
               
               
                   
                   
                 13 
               
               
                   
                   
                 14 
               
               
                   
                   
               
             
          
         
       
     
         [0048]      FIG. 8  illustrates a first layer  800  defining the ceramic substrate top side  601  ( FIG. 6A ). The first layer  800  has a generally rectangular ceramic body  801  defining a generally rectangular cavity  802 . 
         [0049]      FIG. 9  illustrates a ceramic substrate second layer  900  proximate the first layer  800  ( FIG. 8 ). The second layer  900  has a generally rectangular body  901  having an outer perimeter coextensive with that of the first layer  801  ( FIG. 8 ). The body  901  defines a generally rectangular cavity  902  having a length less than that of the first layer cavity  801 , so as to form a shelf for the upper bonding pads  910 . The first layer body  801  extends over traces and vias  920  extending from the upper bonding pads  910 . 
         [0050]      FIG. 10  illustrates a ceramic substrate third layer  1000  proximate the second layer  900  ( FIG. 9 ). The third layer  1000  has a generally rectangular body  1001  having an outer perimeter coextensive with that of the first layer  801  ( FIG. 8 ) and second layer  901  ( FIG. 9 ). Lower bonding pads  1010  are disposed on a top surface of the third layer  1000  proximate the ceramic substrate top side  601  ( FIG. 6A ) and distal the ceramic substrate bottom side  602  ( FIG. 6C ). The bonding pads  1010  are at least substantially exposed through the first and second layer cavities  802 ,  902  ( FIGS. 8-9 ). Traces and vias  1020  are also disposed on a top surface of the third layer  1000  so as to be covered by the second layer body  901 . 
         [0051]      FIG. 11  illustrates traces and vias  1100  disposed on a top side of a fourth layer  1200  proximate the ceramic substrate top side  601  ( FIG. 6A ) and distal the ceramic substrate bottom side  602  ( FIG. 6C ). The traces and vias  1100  are wholly covered by the third layer body  1001 . 
         [0052]      FIG. 12  illustrates a ceramic substrate fourth layer  1200  proximate the third layer  1000  ( FIG. 10 ). The fourth layer  1200  has a generally rectangular body  1201  having an outer perimeter coextensive with that of the first through third layers  801 ,  901 ,  1001  ( FIGS. 8-10 ). Solder pads  1220  and traces and vias  1230  are disposed on a bottom side of the fourth layer  1200 , which is the ceramic substrate bottom side  602  ( FIG. 6C ). In an embodiment, an alumina coat  1300  ( FIG. 13 ) extends over at least a substantial portion of the bottom side  602  ( FIG. 6C ) so as to coat the traces and vias  1230  and leave exposed the solder pads  1220 . 
         [0053]      FIGS. 14A-C  illustrate top, side and bottom views of a multilayer ceramic sheet  1400  manufactured with a 6×17 matrix of ceramic emitter substrates. The multilayer ceramic sheet  1400  is sliced during manufacture so as to separate and provide 102 individual ceramic emitter substrates  500 , as described above. 
         [0054]      FIGS. 15-23  illustrate a low-resistance embodiment of a ceramic emitter substrate. Advantageously, a low-resistance ceramic emitter substrate provides multi-layer conductors (including traces, pads, contacts and vias) that are configured with respect to one or more design goals of maximizing conductor cross-sectional area (trace width×trace thickness) and minimizing trace length within the physical constraints of the ceramic substrate so as to achieve very low resistance in the interconnect between emitter drivers and the emitters. In an embodiment, contact resistance is also minimized by selection of high conductivity conductor materials. Low resistance in the emitter array interconnect minimizes the resistive heating of the substrate and corresponding spurious emitter wavelength shifts. Also, low interconnect resistance lessens parasitic voltage drops between drivers and emitters that negatively impact available drive current and, hence, emitter intensity. 
         [0055]      FIG. 15  is a conductor resistance chart for an embodiment of a low-resistance ceramic emitter substrate. Conductor design goals for this embodiment focused on maximizing conductor width and minimizing length. Conductors are a 30 μinch gold plate over 100 μinch nickel on an underlying tungsten/copper ink. In a particularly advantageous embodiment, each combination of traces, vias and pads constituting a conductive path between the ceramic emitter substrate solder pads and bonding pads for any one of the LED drive signals, described with respect to  FIGS. 2-3  above, has a combined resistance less than about 310 milliohms. Traces of sufficient width are provided so that each of the traces that conduct the drive signals has a resistance less than about 290 milliohms. 
         [0056]      FIGS. 16A-D  illustrate a ceramic emitter substrate  1600  having a top side  1601 , a bottom side  1602  and an edge  1603 . A cavity  1604  extends from the top side  1601  into the substrate body to a cavity floor  1605 . The top side  1601  has upper bonding pads  1910  and lower bonding pads  2010  as described with respect to  FIG. 17 , below. The bottom side  1602  has solder pads  2220 , as described with respect to  FIG. 22 , below. The ceramic emitter substrate  1600  also has four layers  1800 ,  1900 ,  2000 ,  2100  with corresponding surfaces including bonding pads, traces, vias and solder pads, as described with respect to  FIGS. 18-21 , below. In an embodiment, the ceramic body measures about 0.23×0.15×0.04 inches and the cavity measures about 0.18×0.10 inches. 
         [0057]      FIG. 17  illustrates upper  1910  and lower  2010  bonding pads. The lower bonding pads  2010 , labeled  8  through  16 , mount and electrically connect a first side (anode or cathode) of the LEDs  510  ( FIG. 5A ) into an emitter array. Upper bonding pads  1910 , labeled  1  through  7 , electrically connect a second side (cathode or anode) of the LEDs  510  ( FIG. 5A ) into the emitter array, via bonding wires  530  ( FIG. 5B ). A thermistor  520  ( FIG. 5A ) is mounted to bonding pads  2010  labeled  17  and  18 . Plated “feed-thru” holes and other vias electrically connect the bonding pads  1910 ,  2010  on the top side  1601  ( FIG. 16A ) with the solder pads  2220  ( FIG. 16C ) on the bottom side  1602  ( FIG. 16C ). In one embodiment, top-side  1601  ( FIG. 16A ) bonding pad numbers and corresponding bottom-side  1602  ( FIG. 16C ) solder pad numbers are electrically connected as shown in TABLE 2. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Connection Table 
               
             
          
           
               
                   
                 BOND PAD NO. 
                 SOLDER PAD NO. 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 10 
               
               
                   
                 7 
                 13 
               
               
                   
                 11 
                 14 
               
               
                   
                 2 
                 1 
               
               
                   
                 6 
               
               
                   
                 12 
               
               
                   
                 3 
                 12 
               
               
                   
                 5 
               
               
                   
                 13 
               
               
                   
                 4 
                 3 
               
               
                   
                 10 
               
               
                   
                 8 
                 4 
               
               
                   
                 9 
                 2 
               
               
                   
                 14 
                 5 
               
               
                   
                 16 
                 6 
               
               
                   
                   
                 7 
               
               
                   
                 15 
                 11 
               
               
                   
                 17 
                 9 
               
               
                   
                 18 
                 8 
               
               
                   
                   
               
             
          
         
       
     
         [0058]      FIG. 18  illustrates a first layer  1800  defining the ceramic substrate top side  1601  ( FIG. 16A ). The first layer  1800  has a generally rectangular ceramic body  1801  defining a generally rectangular cavity  1802 . 
         [0059]      FIG. 19  illustrates a ceramic substrate second layer  1900  proximate the first layer  1800  ( FIG. 18 ). The second layer  1900  has a generally rectangular body  1901  having an outer perimeter coextensive with that of the first layer  1801  ( FIG. 18 ). The body  1901  defines a generally rectangular cavity  1902  having a length less than that of the first layer cavity  1802 , so as to form a shelf for the upper bonding pads  1910 . The first layer body  1801  ( FIG. 18 ) extends over traces and vias  1920  extending from the upper bonding pads  1910 . 
         [0060]      FIG. 20  illustrates a ceramic substrate third layer  2000  proximate the second layer  1900  ( FIG. 19 ). The third layer  2000  has a generally rectangular body  2001  having an outer perimeter coextensive with that of the first layer  1801  ( FIG. 18 ) and second layer  1901  ( FIG. 19 ). Lower bonding pads  2010  are disposed on a top surface of the third layer  2000  proximate the ceramic substrate top side  1601  ( FIG. 16A ) and distal the ceramic substrate bottom side  1602  ( FIG. 16C ). The bonding pads  2010  are at least substantially exposed through the first and second layer cavities  1802 ,  1902  ( FIGS. 18-19 ). Traces and vias  2020  are also disposed on a top surface of the third layer  2000  so as to be covered by the second layer body  1901 . 
         [0061]      FIG. 21  illustrates traces and vias  2100  disposed on a top side of a fourth layer  1200  proximate the ceramic substrate top side  1601  ( FIG. 16A ) and distal the ceramic substrate bottom side  1602  ( FIG. 16C ). The traces and vias  2100  are wholly covered by the third layer body  2001 . 
         [0062]      FIG. 22  illustrates a ceramic substrate fourth layer  2200  proximate the third layer  2000  ( FIG. 20 ). The fourth layer  2200  has a generally rectangular body  2201  having an outer perimeter coextensive with that of the first through third layers  1801 ,  1901 ,  2001  ( FIGS. 18-20 ). Solder pads  2220  and traces and vias  2230  are disposed on a bottom side of the fourth layer  2200 , which is the ceramic substrate bottom side  1602  ( FIG. 16C ). In an embodiment, an alumina coat  2300  ( FIG. 23 ) extends over at least a substantial portion of the bottom side  1602  ( FIG. 16C ) so as to coat the traces and vias  2230  and leave exposed the solder pads  2220 . 
         [0063]    In an embodiment, the ceramic substrate is fabricated from a standard “green” ceramic paste with a dark additive. The resulting “black” ceramic material serves the purpose of preventing light leakage through the edges and bottom of the ceramic substrate. 
         [0064]    A ceramic emitter substrate has been disclosed in detail in connection with various embodiments. These embodiments are disclosed by way of examples only and are not to limit the scope of the claims that follow. One of ordinary skill in art will appreciate many variations and modifications.