Abstract:
A light detector includes a semiconductor die that provides a photo sensor. An interference filter is formed on the semiconductor die and has a pass band corresponding to a wavelength of a light emitting diode to supply filtered light in the pass band to the photo sensor.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to sensing light and more particularly the use of interference filters in sensing light. 
         [0003]    2. Description of the Related Art 
         [0004]    Sensing light is used in a wide variety of applications such as proximity detectors, heart rate monitors, and pulse oximetry (oxygen levels in blood). These light detectors often work in environments having strong levels of sunlight and artificial sources. Such light can interfere with operation of the photo sensor used in the proximity detector, heart rate monitors and pulse oximetry. Accordingly, it would be desirable to improve light sensing operations for a variety of applications. 
       SUMMARY OF EMBODIMENTS OF THE INVENTION 
       [0005]    In an embodiment, an apparatus includes a semiconductor die including a photo sensor. An interference filter is formed on the semiconductor die and the interference filter has a pass band corresponding to a wavelength of a light emitting diode 
         [0006]    In another embodiment, a method includes receiving light at an interference filter formed on a semiconductor die surface. The interference filter filters the received light to allow light to pass in a pass band corresponding to a wavelength of a light emitting diode. The light is supplied to a photo sensor. 
         [0007]    In another embodiment an apparatus includes a semiconductor die including a first and second photo sensor. A first interference filter formed on the semiconductor die has a pass band corresponding to a first wavelength of a first light emitting diode. A second interference filter formed on the semiconductor die has a second pass band corresponding to a second wavelength of a second light emitting diode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
           [0009]      FIG. 1  illustrates an apparatus according to an embodiment including an interference filter and a photo sensor. 
           [0010]      FIG. 2  illustrates a high level block diagram of additional components that may be utilized in an embodiment of a proximity detector, a heart rate monitor, or a pulse oximetry detector. 
           [0011]      FIG. 3  illustrates the sunlight spectrum and operation of a 940 nm bandpass interference filter. 
           [0012]      FIG. 4  illustrates the effect of angle of incidence on operation of an interference filter. 
           [0013]      FIG. 5  illustrates a housing having an aperture to limit the angle of light incident on the interference filter. 
           [0014]      FIG. 6  illustrates an embodiment with two interference filters on a single die. 
       
    
    
       [0015]    The use of the same reference symbols in different drawings indicates similar or identical items. 
       DETAILED DESCRIPTION 
       [0016]      FIG. 1  illustrates a high level block diagram for an embodiment that may be used in a proximity detector, heart rate monitor, or a pulse oximetry detector. The embodiment illustrated in  FIG. 1  utilizes a narrow-band interference filter  101  placed on a silicon die  103 . The interference filter functions to pass light of a particular wavelength while rejecting light of other wavelengths. The light that passes through the interference filter is sensed by photo sensor  105 . The center frequency wavelength of the interference filter&#39;s pass band is chosen based on the intended LED wavelength used externally. The interference filter significantly reduces the amount of light energy having frequencies that did not originate in the LED and would otherwise be seen as noise by the photo sensor. That allows for better sensor performance under strong levels of interfering light such as sunlight. One tradeoff is that once the interference filter has been installed on the die, the device can only be used with a specific LED wavelength. However, that is not necessarily an issue since the LED wavelength choice is generally picked based on the application usage. Bond wires  108  couple the silicon die with the substrate or metal lead frame that form part of the package. Clear epoxy encapsulant  104  seals the die and helps form the package. 
         [0017]    The interference filter  101  may be formed on the die  103  by standard lithographic techniques such as etching or liftoff to form a multilayer, e.g., 50 layers providing reflection at interfaces of the various layers to provide appropriate constructive and destructive interference to achieve an interference filter having a pass band of interest. 
         [0018]    There are three main economic advantages to the integration of the interference filter onto the silicon. The first is that the same thin film processing steps used in making the silicon may be used in making the filter. Secondly, the total surface area that needs coating can be reduced, in that only the surface area of the silicon die/wafer needs to be formed as opposed to the larger surface area of a glass filter placed above the sensor. Thirdly, there is no need to handle and mount an extra component (the glass filter) in making the product. 
         [0019]    In operation LED  107 , which could be one of several LEDs, illuminates an object (not shown in  FIG. 1 ). Depending on the application the object either reflects light at its surface (proximity measurements), or returns the light after it has passed through some tissue such as in heart rate monitoring and pulse oximetry. The object returns light towards the interference filter  101 , which passes the returned light to the photo sensor  105 . 
         [0020]      FIG. 2  illustrates a high level block diagram of other portions of the detector of  FIG. 1 . As shown in  FIG. 2 , the photo sensor  105  supplies a voltage or current corresponding to the sensed light to an amplifier  201 , which in turns supplies the amplified signal corresponding to the sensed light to detector logic  203 . That logic may include a microcontroller, or other logic to implement the detection and control functions described herein. 
         [0021]    For example, in an embodiment the proximity, heart rate, or pulse oximetry detector senses light with the LED  107  off so as to obtain a background light level. The microcontroller controls the LED to turn on and light is sensed after turning on the LED. The two readings may be compared by the detection logic  203  to determine whether an object is present. If an object is present, the two readings (with LED off and on) will be different due to reflection of LED light off of the proximate object. If there is no object to reflect light, any difference between the two readings will be sufficiently small as to indicate no object. In the case of heart rate, or pulse oximetry, the measured returned light value is stored as a sample and a large number of samples are processed to derive the biological data using well known techniques. 
         [0022]    Referring to  FIG. 3 , in an embodiment, the LED  107  emits light  301  having a center frequency of near infrared (IR), e.g., 940 nanometers. The curve  301  shows typical LED signal energy density normalized to 100. Curve  303  shows the sunlight spectrum normalized to 100, and can be seen to be significantly reduced at  304 . Thus, using a sensor at that wavelength reduces possible interference from the sunlight spectrum.  FIG. 3  also shows the 0 degree flat filter response  305  of the interference filter associated with such an LED. In embodiments, the pass band of the interference filter may designed to be larger than the sum of the LED energy distribution, the variability of the LED spectrum due to manufacturing, and the variability of the interference filter due to manufacturing. In the example of the frequency response  305 , note that the 0 degree response is off center so that the off angle response is improved. Off angle filter response is discussed further below in relation to  FIG. 4 . Most of the interference at frequencies other than the pass band is removed by the interference filter rejecting light having wavelengths other than the pass band of interest. While other frequencies may be used, choosing to operate the proximity sensor at 940 nm provides good immunity to sunlight. 
         [0023]    But depending on the application, other LED wavelengths may be chosen and the interference filter is designed to correspond to the chosen LED wavelength. For example, in the usage case where the detector is used in a consumer, non-medical heart rate monitor application, an LED wavelength may be chosen for either green light, e.g., at approximately 535 nm or for yellow light, e.g., at approximately 590 nm, so as to get a maximum blood pulsation signal. For these cases, the detector may have, e.g., a 535 nm or 590 nm interference filter manufactured on-die for that usage case. Such a proximity detector would be used with LED(s) that have matching transmission wavelengths. 
         [0024]    Placing the interference filter on-die provides advantages over using after-market interference filters. Since interference filter response has angular dependencies, having the filter closer to the sensor allows system-level angular control of the light entering the system. In contrast, interference filters located off of the die need to be as large as the end product aperture, which has cost implications as well as inferior angular performance. 
         [0025]      FIG. 4  illustrates how angular performance affects operation of the interference filter. As the angle of incidence (AOI) increases, the effectiveness of the interference filter lessens. For example, for an angle of incidence of 0, 20 degrees, or 40 degrees from vertical as shown by curves  401 ,  403 , and  405  respectively, the transmission percentage is approximately 78 percent for frequencies between 880 and 940 nm. However, as shown by curves  407  and  409 , at an angle of incidence of 25 degrees the transmission percentage is approximately 72 percent at approximately 850 nm and for an angle of incidence of 60 degrees, the transmission percentage of 825 nm light is approximately 60 percent. Because the wavelength increases with the angle of incidence, the interference filter passes a higher percentage of light of other wavelengths and the wavelengths of the pass band are passed less efficiently. That contributes to a noisy signal received by the photo sensor. Thus, the interference filter tends to work best at around 0 degrees angle of incidence (AOI). At relatively large off angles, e.g., greater than 30 degrees, the interference filters tend to let in less of the desired energy and more of the undesired energy. Off angle light can either be ignored or masked through appropriate baffling techniques as shown in  FIG. 5  so that only light at an acceptable angle range reaches the filter. 
         [0026]    Thus, referring to  FIG. 1 , in embodiments, a housing  115  is provided that controls the angle of the light incident on the interference filter  101 . Referring to  FIG. 5 , the housing  115  may be used to limit the angle of incident light to ±15 degrees from vertical (line  503 ). Of course, depending on the particular application, housings with apertures providing other angles. e.g., ±30 degrees may be used. A housing structure  505  may be provided in addition to or in place of the housing structure  115 . Thus, the optical masking structure may be larger (505) or smaller ( 115 ). 
         [0027]    As discussed above, various applications may employ diodes, interference filters, and photo sensors. For example, as discussed above, a single wavelength at 940 nm may be used for proximity/distance measurements or for measuring heart rate. Of course, a single wavelength at any practical wavelength may be used for proximity/distance measurements. Near infrared may be preferred because it is invisible to humans and 940 nm may also be preferred for use with an interference filter because of the roughly 60% reduction in sunlight in that frequency area compared to adjacent wavelengths. In other applications a single green wavelength, e.g., at 535 nm, may be used for detecting heart rate at some skin locations. In still other embodiments, dual green and yellow wavelengths may be used for determining heart rate at some skin locations and some skin coloration. In still other embodiments, dual wavelengths at approximately 940 nm (near IR) and approximately 660 (deep red) may be used for pulse oximetry and heart rate measurements. 
         [0028]    Because the interference filters are integrated on the die, applications using dual wavelengths can have interference filters designed for both wavelengths formed on the same die. For example,  FIG. 6  illustrates a dual LED and sensor pair where the excitation is 940 nm and 660 nm using LEDs  601  and  602 , respectively, with filters  603  and  605  matched to 940 nm and 660 nm, respectively. Photo sensors  607  and  609  sense the light passed by their respective interference filters. Both interference filters  603  and  605  and photo sensors  607  and  609  are integrated on the same die  611  thus providing significant savings in parts and space. The ratio of the signals measured may be used using standard techniques to evaluate the oxygenation of blood. While two interference filters are shown on die  601 , some embodiments may take advantage of having the interference filters integrated on the die and utilize more than two interference filters and LED pairs. 
         [0029]    Thus, various aspects have on-die interference filters. The description of the invention set forth herein is illustrative, and is not intended to limit the scope of the invention as set forth in the following claims. Other variations and modifications of the embodiments disclosed herein, may be made based on the description set forth herein, without departing from the scope of the invention as set forth in the following claims.