Abstract:
A method for forming a sensor is provided, together with a sensor formed according to the method. Photoresist material is deposited on a surface of the sensor, and is then patterned and etched to form an array of microlens structures. The structures are spaced close together in a predetermined pattern so that when a reflow process is performed, the structures melt and coalesce to form a barrier. The barrier defines a region for constraining or channeling the flow of reagent and analyte samples used in bio-optical sensors.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to sensors, and in particular, to a method of forming a sensor and a sensor produced in accordance with that method.  
       BACKGROUND OF THE INVENTION  
       [0002]     Bio-optical sensors detect photo-emissive chemical reactions between an analyte and a reagent. Improved bio-optical sensors have a number of different reagents to enable them to detect the presence/concentration of several analytes. During the manufacture of such sensors, it is necessary to locate and separate the reagents, and during their operation, it is necessary to guide the sample containing the analyte over the sensor sites.  
         [0003]     The sites should be mechanically isolated. Suitable structures may be patterned and etch, either on the silicon of the sensor surface or by making trenches in the silicon itself. However, these methods currently require special technology, processing and equipment which add to the manufacturing cost.  
         [0004]     When forming a structure on the surface of the sensor, materials such as polyimide are typically used because they provide good patterning and etching properties. While found in a research laboratory, these chemicals are not usually found in a production environment. The introduction of these materials requires a modification to normal production processes.  
       SUMMARY OF THE INVENTION  
       [0005]     In view of the foregoing background, an object of the invention is to provide a method for forming 3D structures reliably and inexpensively on the surface of a sensor. The method may be easily incorporated into existing manufacturing processes.  
         [0006]     According to a first aspect of the present invention, there is provided a method of forming a sensor as set out in the attached claim  1 .  
         [0007]     According to a second aspect of the present invention, there is provided a sensor as set out in the attached claim  8 . 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     Specific embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0009]      FIG. 1  shows the prior art structure and function of microlenses;  
         [0010]      FIGS. 2 and 3  show plan and cross-sectional views of prior art microlenses before a reflow step is applied;  
         [0011]      FIGS. 4 and 5  show plan and cross-sectional views of prior art microlenses after the reflow step is applied;  
         [0012]      FIGS. 6 and 7  show plan and cross-sectional views of a first embodiment of the present invention before a reflow step is applied;  
         [0013]      FIGS. 8 and 9  show plan and cross-sectional views of the first embodiment after the reflow step is applied;  
         [0014]      FIGS. 10 and 11  show plan views of a second embodiment of the present invention before and after a reflow step is applied;  
         [0015]      FIGS. 12 and 13  show plan views of a third embodiment of the present invention before and after a reflow step is applied;  
         [0016]      FIG. 14  shows a plan view of a fourth embodiment of the present invention; and  
         [0017]      FIG. 15  shows a plan view of a fifth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     Microlenses are incorporated on image sensors to overcome the sensitivity loss caused by the circuitry which blocks light.  FIG. 1  illustrates the operation of microlenses. A sensor  10  comprises circuitry  12  that overlies a substrate  14  and blocks incident light. This means that the sensor  10  is only sensitive to light at certain sensitive areas  16  of the substrate  14 . To overcome the loss of light that would normally be blocked by the circuitry  12 , a microlens  18  focuses incident light, represented by light rays  20 , onto the sensitive areas  16 .  
         [0019]     Microlenses are implemented on a large number of image sensors. They are distributed over the image array (one microlens per pixel). The technology to produce microlenses is commonly found in high-volume silicon manufacture.  
         [0020]     The microlens is formed by depositing a photoresist material on the surface of the sensor. It is patterned using photolithography (aligned to the pixel structure) and etched, resulting in a structure illustrated in  FIGS. 2 and 3 . A grid of volume portions  22  are formed, having a width W 1  and spaced apart by a distance S 1 .  FIG. 3  shows a cross-section along A-A′. Each volume portion has a height H 1 .  
         [0021]     The microlenses are typically matched to the pitch of the sensor, i.e., the sensor pixel pitch is equal to S 1 +W 1 . Typically this is between 4 μm-10 μm.  
         [0022]     The photoresist is then deformed by heating it until it melts, in what is referred to as a reflow process. This is carried out at a relatively low temperature (e.g., 200° C.)—lower than the typical manufacturing temperature for the silicon so that the silicon is undamaged. When the microlens material melts, surface tension causes it to produce a hemisphere.  FIGS. 4 and 5  illustrate the microlenses after being deformed by the heating process. Volume portions  22  have a width W 2  and are spaced apart by a distance S 2 .  FIG. 5  shows a cross-section along B-B′. Each volume portion  22  has a height H 2 .  
         [0023]     During this process, the volume and pitch between the microlenses remains unchanged. However, the shape and height does change, where W 2 &gt;W 1  and S 2 &lt;S 1 . H 1  will determine H 2 , from which the curvature and the focusing properties of the microlens are derived.  
         [0024]     The initial spacing S 1  between microlenses is critical. For construction of an efficient microlens, if S 1  is too large, the light-collecting efficiency of the microlens will be reduced. However, if S 1  is too small, two adjacent microlenses will touch and surface tension will prevent the microlens from forming correctly.  
         [0025]     The term adjacent in this context is taken to mean that two microlenses correspond to adjacent pixels on the array of the sensor. Two microlenses may be considered as being adjacent if they are the nearest neighbors, and there is a risk of them merging when they deform under heat. S 1  will ideally be as small as possible. However, for the formation of microlenses, practical values are 1 μm-2 μm.  
         [0026]     The microlens formation process can be misused to produce simple, but effective, 3-dimensional structures on the surface of the silicon. Instead of aiming to space the microlenses sufficiently far apart to prevent the merging of adjacent lenses, the microlens volume portions are deliberately formed close together so that they join up during reflow.  
         [0027]      FIGS. 6-9  illustrate a first embodiment of the present invention. As shown in  FIG. 6 , photoresist volume portions  22  are spaced apart by a distance S 3 , where S 3 &lt;S 1 , and are formed in a defined shape, namely a ring. The value of S 3  is chosen to ensure that adjacent volume portions  22  merge together during reflow. S 3  could even be made as low as zero.  
         [0028]     When S 3  is zero, a reflow step is not necessary to ensure that a barrier of photoresist material is formed for defining a region suitable for constraining bio-optical reagent or analyte sample materials. However, a reflow step can still be performed to ensure that the edges and walls of the patterned regions and channels are smoother. The increased smoothness helps to reduce resistance to the flow of bio-optical reagent or analyte sample materials.  
         [0029]     During the heating process the microlens material will melt. As the squares are closed, under the influence of gravity, the material will touch and surface tension will cause them to join up.  FIG. 8  shows the result—there is an annulus  24  of microlens material with a void  26  in the middle.  FIG. 9  shows a cross-section along line D-D′ of  FIG. 8 . The annulus  24  has a width W 3 , with the void  26  having a dimension S 4 .  
         [0030]     The void  26  shown in  FIGS. 8 and 9  allows accurate deposition of the reagent during manufacture of a bio-optical sensor. The dimension S 4  of the void  26  is independent of the width W 3  of the microlens, but is usually an integral number of pixels. A typical value for W 3  is 5 μm and for S 4  is 50 μm.  
         [0031]     In addition to forming an annulus/void, the idea can be extended to produce other shapes.  FIGS. 10 and 11  illustrate a second embodiment of the invention, where one microlens is omitted from the arrangement shown in  FIG. 6 . This produces an entrance  28  to the void  26 , producing a cup shape.  FIGS. 10 and 11  show the microlens layouts before and after reflow.  FIGS. 12 and 13  show a third embodiment for forming a channel  30 .  
         [0032]     More complex shapes can also be constructed.  FIG. 14  shows a fourth embodiment, where four annuli as shown in  FIG. 6  are combined to form a microlens structure  32  comprising four sites  34 .  
         [0033]     With this structure, four different reagents could be deposited at each of the four sites  34 . The microlens material  32  provides an effective barrier between the sites to isolate reagents located in the neighboring sites  34 . This sensor could then provide the sensing and/or detection of up to four different chemicals in the analyte.  
         [0034]      FIG. 15  shows a fifth embodiment, where two sites  36 ,  40  are connected by a channel  38 . The connection allows the analyte to flow between different sites.  
         [0035]     It will be appreciated that the regions formed by the particular shapes and formations referred to above are only a very few of a large number of regions that can be formed using the techniques of the present invention, and the present invention is in no way to be considered as being limited to these particular regions.  
         [0036]     The principles of the invention, when applied to bio-optical sensor systems, allow both accurate deposition of a reagent during manufacture of the system and also the production of channels and guides to assist the flow of the analyte during operation of the system.  
         [0037]     It is also to be recognized that the top surface of the sensor is usually formed from silicon nitride to protect the device. This material can also be patterned and etched to provide similarly defined regions that serve similar purposes as described above.  
         [0038]     The invention is compatible with existing manufacturing processes, and does not incur a cost penalty to introduce nor is its processing complex or time consuming. This reduces the overall cost of production of the sensors. Various improvements and modifications may be made to the above without departing from the scope of the invention.