Abstract:
The present disclosure is directed to systems and methods for adjusting adhesion strength between materials during semiconductor sensor processing. One or more embodiments are directed to using various surface treatments to a substrate to adjust adhesion strength between the substrate and a polymer. In one embodiment, the surface of the substrate is roughened to decrease the adhesive strength between the substrate and the polymer. In another embodiment, the surface of the substrate is smoothed to increase the adhesive strength between the substrate and the polymer.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure is directed to systems and methods for adjusting adhesion strength between a sensor and a substrate. 
         [0003]    2. Description of the Related Art 
         [0004]    During sensor fabrication, in some types of devices the sensor is processed while attached to a substrate. The substrate acts as a carrier and provides structural and mechanical support to the sensors during subsequent processing steps. In some cases, flexible sensors are formed from a polymer film, such as polyimide, while the polymer film is attached to the substrate. 
       BRIEF SUMMARY 
       [0005]    The present disclosure is directed to systems and methods for adjusting adhesion strength between materials during semiconductor sensor processing. One or more embodiments are directed to surface treating a substrate to adjust adhesion strength between the substrate and a polymer. In one embodiment, the surface of the substrate is roughened to decrease the adhesive strength between the substrate and the polymer. In another embodiment, the surface of the substrate is smoothed to increase the adhesive strength between the substrate and the polymer. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic isometric view of a glucose sensor system in accordance with the present disclosure. 
           [0007]      FIG. 2  is close up top view of the sensor of the glucose sensor system in  FIG. 1 . 
           [0008]      FIG. 3  is a schematic top view of a substrate having a plurality of flexible sensors thereon in accordance with the present disclosure. 
           [0009]      FIG. 4  is a cross section view of the  FIG. 4 . 
           [0010]      FIG. 5  is the cross section view of the  FIG. 5  illustrating the removal of the flexible sensor. 
           [0011]      FIG. 6  is a close up view of the circle in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIGS. 1-2  collectively show a glucose sensor system  10  that includes a sensor  12  for measuring a person&#39;s glucose levels. The sensor  12  is mounted on an insertion support member  13  as is shown in  FIG. 1 . In the illustrated embodiment, the sensor  12  is a flexible bio-medical sensor, and in some embodiments may be a microelectromechanical system (MEMS) sensor. As shown in  FIG. 2 , a first end  14  of the sensor  12  includes sensing electrodes  16 . Blood is placed in contact with the sensing electrodes  16  for sensing various levels, such as glucose levels. A second end  18  of the sensor  12  includes contact pads  20  that are electrically coupled to the sensing electrodes  16  via conductive traces. The sensing electrodes  16  may comprise a metal material, and in some embodiments, the sensing electrodes  16  comprise at least one of chrome and gold. The contact pads  20  are configured to receive signals indicative of the sensed glucose levels from sensed electrodes  16  via the traces. The glucose sensor system  10  may further include a support portion  14  for supporting the insertion support member  13  and a transmitter  15  that is configured to wirelessly provide the signals indicative of the sensed glucose levels to a monitor (not shown). 
         [0013]    The sensor  12  is flexible in that it is deformable in response to a force applied thereto. In that regard, the sensor  12  may be formed from a flexible material, such as a polymer. 
         [0014]      FIG. 3  illustrates a slide  30  that includes a plurality of the flexible sensors  12  of  FIGS. 1-2  secured to a substrate  34 . The flexible sensors  12  are formed on a polymer  40  while the polymer  40  is secured to the substrate  34 . The substrate  34  provides structural and/or mechanical support to the flexible sensors  12  during subsequent processing. The flexible sensors  12  are arranged on the slide  30  in spaced relation to an outer perimeter of the slide  30 . The outer perimeter defines a cutting edge, in which the slide  30  was separated, such as by laser cutting, from adjacent slides on a wafer as is well known in the art. In some embodiments, portions of the polymer  40  may be removed from the substrate  34  at the perimeter of the slides  30  to define a cutting line. 
         [0015]      FIG. 4  shows a partial cross section view of a flexible sensor  12  on the substrate  34  as shown in  FIG. 3 . The cross section is taken adjacent to the traces in the flexible sensor  12  electrically coupling the bond pads to the sensing electrodes to simplify the image. A lower surface  36  of the polymer  40  that forms the flexible sensors  12  is secured to a surface  38  of the substrate  34 . The substrate  34  is any substrate that is suitably rigid to support the flexible sensors  12  during downstream processing. In one embodiment, the substrate  34  comprises glass, such as Alumino Silicate 1737, which is boro-aluminosilicate glass. In other embodiments, the substrate  34  comprises silicon, metal or other rigid materials. 
         [0016]    The polymer  40  used to form the flexible sensor  12  may be any polymer  40  configured to shrink during a curing or baking process. Such polymers  40  may include silicone compounds, polyimides, biocompatible solder masks, epoxy acrylate copolymers, or the like. In one embodiment, the polymer is polyimide Durimide® 116. 
         [0017]    The polymer  40  may be deposited or formed on the substrate  34 . In one embodiment, the polymer  40  is coated on the surface  38  of the substrate  34  by spin coating on the wafer. In that regard, the polymer  40  may be dispensed in a flowable form at a center portion of a wafer of which the substrate  34  is one of many that comprise the wafer; and while the wafer rotates, the polymer  40  spreads across the surface  38  of the wafer due to centripetal force. The polymer  40  is cured and the flexible sensor  12  may be formed via downstream processing. 
         [0018]    An upper surface of the flexible sensor  12  may further include an insulation layer (not shown), such as a polymer, formed thereon. It is to be understood that the insulation layer on the upper surface of the flexible sensor  12  may having openings exposing the sensing electrodes and the bond pads discussed in reference to  FIGS. 1-2 . It is to be appreciated that the insulation layer on the upper surface may be a different material or the same material as the polymer  40  that forms the flexible sensor  12  and secures the flexible sensor  12  to the substrate  34 . 
         [0019]    After processing the flexible sensor is complete, the individual flexible sensors  12  may be removed, such as peeled as is shown in  FIG. 5 , from the substrate  34  and installed in a device, such as the glucose sensor system  10  of  FIG. 1 . In order to prevent damage to the flexible sensors  12  during the removal process, adequate control over the adhesion between the polymer  40  of the flexible sensor  12  and the substrate  34  is desired. In particular, lower adhesion may be desired such that the sensor  12  may be easily removed from the substrate  34  after downstream processing steps have been completed. In other situations, however, higher adhesion may be desired such that the sensor  12  remains adhered to the substrate  34  post downstream processing. 
         [0020]    Previous ways for adjusting the adhesion strength between a polymer, such as polyimide, and a substrate are rather limiting. Methods for increasing the adhesive strength of polyimide includes adding an adhesive promoter or by curing the polyimide. However, these methods do not allow adequate control over the adhesive properties that may be needed for various applications. For instance, although adhesive promoters and curing are useful for increasing adhesive strength of the polyimide, they do not reduce the adhesive strength of the polyimide. Furthermore, the temperatures in which an adhesive material can exposed may be limited by properties of the adhesive, such as the materials curing temperature and glass transition temperature. 
         [0021]    The adhesive strength of polyimide may also be adjusted (i.e. increased or decreased) by varying the thickness of the polyimide. Although this provides some control over the adhesive strength of the polyimide, semiconductor processing design rules typically defines the thickness of the polyimide, thus preventing further adjustments to the thickness to obtain the necessary adhesive properties for a particular application. 
         [0022]    According to embodiments of the present disclosure, prior to applying the polymer  40  to the substrate  34 , the substrate  34  may be surface treated to change the surface condition of the substrate  34 . In that regard, the adhesion strength between the polymer  40  and the substrate  34  may be controlled. In the illustrated embodiment, the surface  38  of the substrate  34  is roughened as can be seen in  FIG. 5  as the flexible sensor  12  is peeled from the surface  38  of the substrate  34  and more clearly shown in the close up view in  FIG. 6 . In some embodiments, the roughened surface  38  of the substrate  34  may be achieved by etching, such as wet etching or plasma etching, the surface  38 . In one embodiment, the substrate  34  is etched with sulfuric acid. 
         [0023]    The inventors unexpectedly discovered that by changing the surface condition of the substrate  34 , different adhesion strengths between the substrate  34  and the polymer  40  can be achieved. In particular, the inventors realized that by roughening the surface  38  of the substrate  34 , adhesion strength between the substrate  34  and the polymer  40  is substantially reduced. It was also learned that by smoothing the surface  38  of the substrate  34 , the adhesion strength between the substrate  34  and the polymer  40  may be increased. 
         [0024]    One embodiment of the present disclosure will now be described. In this embodiment, the substrate is boro-aluminosilicate glass and a surface of the substrate was roughened via wet etch processing. The substrate will be referred to hereafter as “the treated substrate.” The surface of the treated substrate was roughened using a mixture of sulfuric acid, deionized water, and hydrogen peroxide, where the mixture&#39;s temperature was increased to 130° C. After treatment, the treated substrate had an average roughness value (R a ) of ˜1.0 nanometers and a root mean squared roughness value (R q ) of ˜1.33 nanometers. In this embodiment, the polyimide was applied to the surface of the treated substrate to a thickness of about 20 microns ±3, the stack was cured at a temperature of 325° C. ±10 for about 30 minutes, and flexible sensors were formed thereon. The flexible sensors were then peeled from the treated substrate at a peel speed of 1 inch per minute. The adhesion strength between the polyimide and the treated substrate was determined using the following equation: 
         [0000]    
       
         
           
             Wa 
             = 
             
               
                 
                   P 
                   θ 
                 
                  
                 
                   ( 
                   
                     1 
                     - 
                     
                       cos 
                        
                       
                           
                       
                        
                       θ 
                     
                   
                   ) 
                 
               
               w 
             
           
         
       
     
         [0000]    where, Wa=the work of detachment per unit area of bonding surface; θ=peel angle; P θ =peel force; w=width of strip being peeled. 
         [0025]    Thus, if peel angle=90° , then cos θ=0. 
         [0026]    Therefore, 
         [0000]    
       
         
           
             Wa 
             = 
             
               
                 P 
                 θ 
               
               w 
             
           
         
       
     
         [0027]    Thus, if w=1 inch, then 
         [0028]    Wa=P 90 . 
         [0029]    The peel force was measured to be 20 grams/force (gf) and was compared to the peel force measurement from a substrate that did not undergo surface treatment (referred to herein after as “untreated substrate”). The surface of the untreated substrate had a R a  value of ˜0.65 nanometers and R q  value of ˜0.94 nanometers. The peel force of the untreated substrate was 400 gf. 
         [0030]    The results, over 10 times reduction in peel force for the treated substrate over the untreated substrate, were unexpected. It is generally expected that when a flowable adhesive is applied to a roughened surface, the strength of the adhesion increases because the amount of surface area in contact with the adhesive increases. That is, the adhesive flows into the peaks and valleys making up the roughed surface and thus spreads across a greater surface area, than when the surface area is substantially smooth, without peaks and valleys. In this case, however, the adhesion strength unexpectedly decreased. The inventors discovered that because the polyimide shrank during the curing process, the amount of surface contact the polyimide had with the substrate also decreased thereby reducing the adhesion strength between the polyimide and the substrate. 
         [0031]    The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. For instance, although the described embodiments are directed to flexible sensors, it is to be understood that the sensors may also be rigid. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.