Abstract:
A bio-optical sensor has a surface provided with an array of sensing pixels and calibration pixels. The sensing and calibration pixels are arranged in an interleaved fashion. The sensing and calibration pixels may be interleaved 1:1, or they may be arranged in interleaved blocks. The image plane receives an analyte and a reagent that reacts with the analyte to produce light. The sensing pixels generate signals as a function of the light produced.

Description:
FIELD OF THE INVENTION 
   The present invention relates to bi-optical sensors, and in particular, to a light-sensitive semiconductor device for detecting and measuring light emitted by the reaction of a reagent with a biological sample. 
   BACKGROUND OF THE INVENTION 
   In known bio-optical sensors, the reaction takes place on a surface of the semiconductor device which is an image surface divided into pixels. The light produced by reactions of this nature is small, and accordingly, the signal produced by any pixel of the device is also small. The signal is frequently less than other effects such as dark current (leakage current) from the pixel and voltage offsets. Therefore, a calibration/cancellation scheme may be necessary to increase the sensitivity of the system. 
   In the related field of solid state image sensors, there are a number of known calibration techniques. In image sensors, it may be necessary to have a continuous image plane on which the image is formed. Calibration techniques involve either the use of dark frame cancellation or the use of special calibration pixels. 
   In dark frame cancellation, a dark reference frame is taken and the resulting signal output is subtracted from the image frame. The dark reference frame is usually taken with the same exposure. The integration time is the same as the image but no light is impinged on the sensor, either by use of a shutter or by turning off the scene illumination. 
   When calibration pixels are used, they are provided at the edge of the sensor. The calibration pixels are usually in the form of a single row or column, since it is necessary to have a continuous image surface. 
   In current bio-optical sensors, a dark image is acquired before the analyte and reagents are deposited on the sensor. This calibration image is used during detection and processing of the photo-signal. This means that there is a time difference between the acquisition of the dark reference frame and the detection and processing of the sensor signal. During this time there may be changes in the conditions on the device, e.g., operating voltage and temperature may change due to a low battery, a change in the ambient temperature, or self-heating due to power dissipation. Consequently, the calibration signal is not an accurate representation of the dark signal at the relevant time. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a bio-optical sensor having a more accurate calibration signal. A more accurate calibration signal increases system sensitivity and enables the system to function with less analyte, less reagent, or in a shorter time. 
   Accordingly, the invention provides a bio-optical sensor comprising a semiconductor substrate having an image plane formed as an array of pixels. The image plane may receive thereon an analyte and a reagent which reacts with the analyte to produce light. The pixels may comprise sensing pixels which generate signals which are a function of light emitted by the reaction, and calibration pixels which are not exposed to the light. The calibration pixels may be interleaved with the sensing pixels. 
   Preferred features and other advantages of the invention will be apparent from the following description and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: 
       FIG. 1  is a schematic plan view of the image area in accordance with the invention; 
       FIGS. 2 ,  3  and  4  are schematic plan views of further embodiments of the image area in accordance with the invention; 
       FIG. 5  illustrates a general case of the image area in accordance with the invention; and 
       FIG. 6  is a graph showing the relationship between the size of pixel blocks and spatial efficiency in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a straightforward form of the invention in which the image surface is divided into sensing pixels  10  and calibration pixels  12  which are interleaved on a 1:1 basis, i.e., in a checker board fashion. Each of the pixels  10 ,  12  is an imaging pixel of a well-known type, such as a 3-transistor or 4-transistor pixel based on CMOS technology. The calibration pixels  12  are shielded from light by a suitable mask, which may be printed on top of the array or may be formed by selective metallization during fabrication, for example. When a metal mask is used, there is preferably a layer separated from the readout electronics to reduce parasitic capacitance. 
   Alternatives to metallization for forming the opaque layer include silicided gate oxide, and superposition of color filters, i.e., overlaying red green and blue filters to give black. 
   It is preferable that the border pixels situated at the edge of the sensor are not used, either for sensing or calibration. The border pixels have neighboring pixels on less than four sides, whereas the other pixels have neighboring pixels on all four sides. Also, practical issues with the fabrication processing of the sensor cause variations in the size of the patterned features that will be exacerbated at the edges. These factors change the analog performance of the border pixels at the edges, and thus the border pixels are best ignored. 
   In the arrangement of  FIG. 1 , the entire image surface may be covered with analyte and reagent since it would be difficult to physically contain a liquid system to single pixel areas. This has the disadvantage that only 50% of the analyte and reagent is available to the sensing pixels, while the quantities of both are usually limited by problems obtaining a sample and the costs of the reagent. 
   This problem can be addressed by dividing the surface into sensitive regions and calibration regions. This allows the analyte and reagent to be applied only to the sensitive regions. 
     FIG. 2  shows an interleaving scheme using 2×2 blocks of pixels. However, interleaving in blocks does pose problems. It is reasonable to assume that an edge pixel of a block will have a response significantly different to interior pixels, and should be discarded. Thus, the  FIG. 2  array may not be practicable.  FIG. 3  shows an array interleaved in blocks of 3×3 in which, if the edge pixels are not used, only 1/9 of the surface area will be effective.  FIG. 4  shows an array interleaved in 4×4 blocks in which ¼ of the area will be effective if edge pixels are not used. 
     FIG. 5  shows the general case where the sensor has X (horizontal) by Y (vertical) pixels arranged in blocks of M×N pixels. Each block therefore has (M−2)×(N−2) useful pixels. The graph of  FIG. 6  shows the percentage of useful pixels for different block sizes, assuming square blocks with M=N. 
   If we define spatial efficiency=100×(No. of useful pixels/total no. of pixels), then  FIG. 6  shows that with block sizes of 6×6 or less the spatial efficiency is less than 50%, i.e., worse than the straightforward 1×1 interleave form. For 7×7 blocks, spatial efficiency is greater than 50%, i.e., there is an improvement over the 1×1 form. 
   The graph also illustrates the point of diminishing returns. With 20×20 pixels, the efficiency is 80% and increases only slowly from this point. The most useful block size is likely to lie in the range of 20-30 pixels. 
   The foregoing embodiments show the blocks of sensing and calibration pixels distributed in a common-centered manner, that is, in such a way that the “center of gravity” of the two types is in a common location. This is the preferred manner, although other patterns of interleaving may be used. 
   Likewise, the preferred embodiments have equal numbers of sensing and calibration pixels, but the proportion of calibration pixels could be reduced while still benefiting from the underlying concept. 
   A typical method of operating the sensor is as follows: 
   1. Obtain image with no analyte/reagent present and no light produced: Idark(x,y); 
   2. Separate the image data into two images, pixel data Pdark(x,y) and calibration data “Cdark(x,y); 
   3. Add the analyte/reagent and obtain an image with light Ilight(x,y); 
   4. Separate this into two images, pixel data Plight(x,y) and calibration data Clight(x,y); 
   5. The uncompensated image is then calculated by Plight(x,y)−Pdark(x,y) (on a pixel basis); 
   6. The compensation signal is calculated from the calibration pixels as fnCal(Clight(x,y), Cdark(x,y)); and 
   7. Compute compensated image Output(x,y)={Plight(x,y)−Pdark(x,y)}×fnCal(x,y). 
   In the simplest case, fnCal could be linear, i.e., fnCal(x,y)=Cdark(x,y)/Clight(x,y). This is suitable where the error source changes linearly. However, the main use for this technique is to correct for temperature where the dark current rises exponentially with temperature. The calibration function can represent this, e.g., fnCal(x,y)=log(Cdark(x,y)/Clight(x,y)). 
   Depending on the design of the sense node, other errors may be significant and require a change to the calibration function. This can be computed arithmetically or determined empirically, and incorporated in a look-up table.