Abstract:
Isolation of semiconductor based biosensors is described. The present invention is directed to prevention of undesirable influences including, but not limited to, chip leakage current. Several forms of sensor isolation and other protective means affect protection from adverse chip influences. The effect of biochemical attachment outside the sensor active region may have adverse effects on sensor performance. This potential problem is averted using protective design features.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/568,297, filed May 3, 2004, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Electronic devices fabricated with integrated circuit (IC) technologies require biases and conduits to external contacts, where such biases are applied or measured and current is conducted into and out of the electronic devices.  
         [0003]     A conductor passing over an underlying substrate is isolated from the substrate by an oxide or insulator layer. The voltage of the interconnect line creates an electric field between the conducting interconnect line and the underlying substrate. If this electric field is sufficiently large, an accumulation or inversion layer may be formed. If an inversion layer forms, there is a depletion region separating the induced channel from the underlying semi-conducting substrate. If this junction is reverse biased, the underlying induced channel connects to the drain contact and the depletion region is then reverse biased. The reversed biased depletion region delivers current to the inversion channel. The current is conducted along the channel to the drain contact and adds to the drain current. As the drain voltage changes so does the recovered depletion leakage current contribution. As the drain voltage is increased, the amount of back bias on the induced junction is increased. The depletion related volume and the amount of thermally generated leakage current are also increased. Such leakage current, if blocking means are not provided, adds to the leakage current, creating a net increase in drain current and a drain voltage dependence of the drain current magnitude. The current contribution may be substantial even at zero drain voltage if the substrate channel is reverse biased by a substrate-applied voltage. Leakage current is acquired without a drain voltage applied as long as the drain to substrate voltage biases the induced depletion regions with the thermally generated leakage current.  
         [0004]     While one solution is to increase the field oxide thickness, and this is the solution typically used by industry (thick field oxide), this solution is insufficient for the biosensor for several reasons. One reason is the desire to apply large reverse biases to the P drain and channel diode junction (with the underlying N-substrate). The arguments conceptually carry through for N-channel devices as well. P-Channel devices are used by way of example in this document since many biochemicals are negatively charged.  
         [0005]     Some improvement may be achieved by making the surface N+ surrounding the sensor, if care is taken not to form a tunneling junction. However, this solution will also fail for sufficient drain to substrate reverse biases and for some biochemical attached charge conditions.  
         [0006]     This problem is non-obvious since normal FET structures are isolated and the field oxide is designed not to invert the surface. Such inversion does not occur because a large reverse bias on the drain and channel region is normally not applied. For the biosensor, a large reverse bias is desirable at times because this can decrease the buried channel region and thus increases biosensor sensitivity or is used as a measurement parameter.  
         [0007]     A second non-obvious situation arises for any field oxide thickness. If biochemicals attached to the field region of the IC chip, the electric field has the same value inside of the field oxide regardless of the field oxide thickness. For example, a heavy coating of negatively charged biochemicals induce a P-channel on the N-substrate surface and the attendant PN junction depletion region. Thus, by simply creating a sensor condition where the amount of charge attached to the field oxide surfaces surrounding the sensor is great enough to induce a channel (negative charges may induce a P-Channel), the problem of channel and depletion formation with attendant leakage current can occur. This situation is non-obvious since IC chips do not typically have such attached charges on the field oxide, and the field oxide thickness is considered the design feature that blocks inversion of the underlying substrate. Field oxide thickness is ineffective in the attached charge chase where the Gaussian field is strong enough to induce a channel connecting to the source or drain of the sensor.  
         [0008]     Needs exist for enhanced biosensor performance and designs.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention may take several different forms to improve biosensor performance.  
         [0010]     The first form is an increase in field oxide thickness. The field oxide is increased such that the difference between the substrate voltage and overlying drain and/or source conduit voltage creates an electric field across the field oxide that is lower than that needed to form an inversion layer and a pseudo PN junction.  
         [0011]     Another form is to create P+ wells to isolate the sensor from the substrate. An epitaxial N layer is placed on a P substrate. This PN junction provides basic isolation of the sensor from the substrate. However, by placing P+ posts through the N epi layer, the N epi layer is isolated from the remainder of the substrate. A back bias exists between the P region and the portion of the N-well containing the sensor, in this example, a P-Channel device. Here the well is made as small are possible depending on the size of the actual sensor active region.  
         [0012]     Another form is to create trench isolation. The sensor is surrounded and abutted with a trench cut deep into the semiconductor substrate. A substrate contact to the underlying N region permits the reverse bias to the back gate of the sensor channel with the attendant sensitivity and sensor parameter measuring features enabled. If there is some inversion of the N surface to become a P-channel, then current cannot flow around the channel between the source S and drain D. Leakage current is reduced substantially by trench isolation, whether caused by interconnects conduits or attached biochemical charge induction. The amount of induced junction leakage is kept negligible by maintaining minimum sensor, channel, source and drain inversionable areas.  
         [0013]     Another form is mesa isolation. This form has the same function as the trench isolation but covers a much wider area.  
         [0014]     Another form is conductive shield surface protection. A structure includes a sensor region fabricated on a semiconductor substrate and a conduction region protected by insulator materials and added to the remainder of the semiconductor substrate region to protect the surface from undesirable inversion or other phenomena. The conductive layer has a pre-selected bias applied with respect to the back gate bias. The pre-selected bias is chosen to keep the surface of the semiconductor under the interconnect conductors from displaying unwanted surface effects such as inversion, depletion and related unwanted leakage currents adversely contributing to the sensor signal.  
         [0015]     An FET can acquire significant thermally generated depletion current contributions to a drain current in spite of using a very large field oxide. Field oxides are used to prevent such occurrences but it is clear that regardless of the field oxide thicknesses, such introduced erroneous drain currents may occur. The bio charge induced channel linked to the drain contact and leakage current generating depletion regions is completely independent of field oxide thickness. The field oxide approach of conventional IC chips is not adequate for certain conditions of the Silicon biosensing platform.  
         [0016]     These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1A  shows sensor isolation where the sensor is electronically isolated from the rest of the chip by the P +  posts.  
         [0018]      FIG. 1B  is a top view of the sensor isolation shown in  FIG. 1A .  
         [0019]      FIG. 2A  shows sensor isolation where trenches are provided to isolate the sensor region electronically.  
         [0020]      FIG. 2B  shows a sensor with trench isolation.  
         [0021]      FIG. 3  shows mesa isolation.  
         [0022]      FIG. 4A  shows a drain interconnect induced junction leakage current.  
         [0023]      FIG. 4B  shows channels induced by biochemical ions above a field oxide.  
         [0024]      FIG. 5  shows total sensor drain current comparison.  
         [0025]      FIG. 6A  shows a conductive protection layer.  
         [0026]      FIG. 6B  shows a top view of a protective conductor. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     The present invention may take several different forms to improve biosensor performance. The first form is an increase in field oxide thickness. The field oxide is increased such that the difference between the substrate voltage and overlying drain and/or source conduit voltage creates an electric field across the field oxide that is lower than that needed to form an inversion layer and a pseudo PN junction.  
         [0028]     Another form of improvement is to create P+ wells  13  to isolate the sensor from the substrate  17 . An epitaxial N layer  19  is placed on a P substrate  17 . A PN junction  21  provides basic isolation of the sensor from the substrate. However, by placing P+ posts  13  through the N epi layer  19 , the N epi layer  19  is isolated from the remainder of the substrate. A back bias exists between the P region and the portion of the N-well containing the sensor, in this example, a P-Channel device. Here the well is made as small are possible depending on the size of the actual sensor active region.  
         [0029]      FIG. 1A  shows sensor isolation where the sensor is electronically isolated from the rest of a chip  11  by P +  posts  13 . Reverse bias  15  of the appropriate junction, as indicated, provides isolation. An attachment region  23  includes a source P+  25  and a drain P+  7 . Depletion regions  29  are indicated with diagonal lines. An N+substrate contact  31  is located within the P+ isolation posts  13 . A conducting channel  33  runs between the source  25  and the drain  27 .  
         [0030]      FIG. 1B  is a top view of the sensor isolation shown in  FIG. 1A . The diffused P +  post  13  isolates the region of the sensor. Direct contact with the channel region  33  cannot occur without shorting the channel region.  
         [0031]     Another form of improvement is to create trench isolation. The sensor is surrounded and abutted with a trench  35  cut deep into the semiconductor substrate  17 . A substrate contact  31  to the underlying N region permits the reverse bias to the back gate of the sensor channel with the attendant sensitivity and sensor parameter measuring features enabled. If there is some inversion of the N surface to become a P-channel, then current cannot flow around the channel between the source S and drain D. Leakage current is reduced substantially by trench isolation, whether caused by interconnects conduits or attached biochemical charge induction. The amount of induced junction leakage is kept negligible by maintaining minimum sensor, channel, source and drain inversionable areas.  
         [0032]      FIG. 2A  shows sensor isolation where trenches  35  are provided to isolate the sensor region electronically.  
         [0033]      FIG. 2B  shows a sensor with trench isolation. The trench isolation surrounds the biosensor. The N+ back gate bias contact to the N-epi or implanted layer is shown. A P+ post  13  going through the N layer  19  to the underlying P substrate  17  is used to back bias the PN (epi/substrate) layer for further isolation (back biased). This approach is better than the diffused isolation P +  region. There is no shorting around the channel via any induced P channel region. Surfaces are treated to minimize surface recombination.  
         [0034]     Another form of improvement is mesa isolation. This form has the same function as the trench isolation but covers a much wider area.  
         [0035]      FIG. 3  shows mesa isolation  37 . The sensor FET region is isolated from the remainder of the N region by etching a mesa around the sensor.  
         [0036]     These methods may be used individually or in combination.  
         [0037]      FIGS. 4A and 4B  illustrate the problems with induced junction leakage currents adding to the drain current of the sensor and dependent upon the drain voltage value.  
         [0038]      FIG. 4A  shows a drain interconnect induced junction leakage current. The bias connection via a conductor over the field oxide  41  induces an inversion layer and/or depletion region  45  beneath the field oxide  41 . The depletion region has a leakage current  43  that accumulates over the entire interconnect conductor region and delivers the accumulation to the FET drain contact. The higher the drain voltage the larger the depletion region. The difference between the substrate bias (back gate FET bias) and the drain bias voltage determines the magnitude of the depletion region. Once the inversion layer is formed, the drain then provides contact to the inversion region and the bias depletes from the channel into the substrate. A drain connection conductor  49  is above the field oxide.  
         [0039]      FIG. 4B  shows channels induced by biochemical ions  51  above a field oxide  41 . In this example the attached biochemical molecules are negatively charged and attach to the surface of the field oxide. The charge induces an electric field through the field oxide  41  creating a P-channel  43  at the semiconductor surface and a resulting pseudo junction. The depletion region of the junction adds leakage current to the drain current  45 , thus introducing an error in sensor measured current. The field oxide thickness does not affect this particular biochemical charge induced substrate pseudo PN junction and attendant leakage current  47 . The substrate bias then biases the drain and pseudo junction depletion regions and both can contribute leakage current to the drain. Because of the usual very large field oxide regions such induced junction contributions to the drain current may be substantial.  
         [0040]      FIG. 5  shows total sensor drain current comparison. As the drain voltage changes so does the recovered depletion leakage current contribution. The drain current may be influenced by leakage current from an induced junction under the field oxide arising from the difference in the interconnect or drain voltage and the substrate voltage V sub . Once formed, as the drain voltage increases, the amount of induced junction leakage current increases, as shown by the solid line. If the sensor is isolated from such induced junctions, then the leakage current is blocked and the drain current is normal as represented by the dotted line. It is noted that biochemicals can induce such inversion leading to leakage current. Isolation is achieved by the methods described in the present invention.  
         [0041]     Another form of improvement is conductive shield surface protection. A structure includes a sensor region fabricated on a semiconductor substrate and a conduction region protected by insulator materials and added to the remainder of the semiconductor substrate region to protect the surface from undesirable inversion or other phenomena. The conductive layer has a pre-selected bias applied with respect to the back gate bias. The pre-selected bias is chosen to keep the surface of the semiconductor under the interconnect conductors from displaying unwanted surface effects such as inversion, depletion and related unwanted leakage currents adversely contributing to the sensor signal.  
         [0042]      FIG. 6A  shows a conductive protection layer  71 . A conductive region  71  is layered above a semiconductor  73  outside of the sensor area  61  and between insulation layers  75 . This region  61  is biased  65  with the same voltage as the back gate bias  63 . In this manner the surface is not inverted outside the sensor region  61  and undesirable leakage current from a bulk inverted region does not occur. The conductive layer may be used with device having other forms of isolation, such as trench isolation, or the conductive layer may provide protection from unwanted leakage current measurement error introduction without other sensor isolation means. An active gate attachment region  77  is above the sensor  61 .  
         [0043]      FIG. 6B  shows a top view of a protective conductor. The conductor  71 , active sensor  61  and interconnect regions of the sensor chip  11  are schematically represented. The insulator between the protective conductive region and the interconnect conductors to a source  67  and drain  69  are not shown.  
         [0044]     A partial list of these applications target molecule classes are listed in Table I by way of example. Table I is a partial list of a wide variety of chemicals that can be targeted or sensed with the performance improved biosensors. The term “receptor” is used in the most general sense. A chemical has an affinity for another chemical, a target chemical. Generally, that affinity is selective and specific to the target chemical. Antibodies are one example, nerve receptors another, drug receptors another, oligos are others.  
         [0045]     Applications of the Si based biosensor platform are described in general applications regimes, by way of example, by the list provided in Table II.  
         [0046]     While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention.  
                                 TABLE I                       Partial List of Chemical Targets                                Nucleic Acids   Oligos   Viruses   Acids           c-DNA   Bacteria and it   Bases               component parts               such as epitopes,               membranes,               proteins, Etc.           RNA   Cells (all kinds)   Chemicals                   affecting cell                   function and                   body function           DNA   Membranes   Isoelectric                   Molecules                   (conditions)           Other   Receptors   pH and pH                   influenced                   molecules       Antibodies       Proteins   Ions       Enzymes       Hormones   Toxins       BioDefense Agents       Salts and Salt   Buffering Agents               Concentrations       Pain Receptors       Insecticides   Chemical Agents       Explosives       Water Quality   Pb, Hg, and other               Monitoring   hazardous metals       Prions       Organic Chemicals   Inorganic Chemicals       Drugs       Nerve Components   Organ Components       Signaling       Surface Chemicals   Symbiotic       Chemicals           Chemicals       Buffering Solutions       Gases   Liquids       (component and       concentrations)       Membranes       IONS   Chemical Fractions       Other       Insecticides   Aerosols                  
 
         [0047]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
               
               
                 A Partial List of Sensor Applications 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Drug sensitivity 
                 Drug Efficacy 
                 Medical Diagnostics 
                 Cancer Diagnostics 
               
               
                 Proteomics 
                 Genetics 
                 Toxic Analysis 
                 Bio Defense 
               
               
                 Plant Pathogen 
                 Human Pathogen 
                 Animal Pathogen 
                 Bacteria Detection, 
               
               
                 Sensing 
                 Sensing 
                 Sensing 
                 Identification, 
               
               
                   
                   
                   
                 Characterization and 
               
               
                   
                   
                   
                 Measurement 
               
               
                 Virus Detection, 
                 Fundamental 
                 Binding Strengths of 
                 “Receptor” 
               
               
                 Identification, 
                 Biochemical 
                 molecules 
                 properties 
               
               
                 Characterization 
                 Measurements 
                 (dissociation strengths 
               
               
                   
                   
                 and affinities) 
               
               
                 Chemical 
                 Chemical Reaction 
                 Multiple targets 
                 Biochemical Load 
               
               
                 Thermodynamic 
                 Dynamics 
                 simultaneously 
               
               
                 Parameters 
               
               
                 Cells 
                 Cell chemicals 
                 Cell Dynamics 
                 Cell Division 
               
               
                 Air Quality 
                 Food Quality 
                 Crop Diseases and 
                 Water Quality 
               
               
                 Monitoring 
                 Monitoring and Safety 
                 Safety 
                 Monitoring and 
               
               
                   
                   
                   
                 Safety 
               
               
                 General 
                 Chemical 
                 Chemical 
                 Nutrition Monitoring 
               
               
                 Environmental 
                 Contamination 
                 Constituents 
                 and Diagnostics 
               
               
                 Monitoring 
               
               
                 Blood Banking 
                 Public Heath 
                 OSHA Chemical 
                 Laboratory Safety 
               
               
                   
                 Monitoring and 
                 Monitoring 
               
               
                   
                 Diagnostics 
               
               
                 Explosive Detection 
                 Forensics 
                 Chemical Properties 
                 pH Measurement 
               
               
                 and Identification 
                   
                 (e.g., isoelectric 
               
               
                   
                   
                 point) 
               
               
                 Medical Technology 
                 Materials Properties 
                 Electronic Functions 
                 Chemical Potential 
               
               
                   
                 Including 
               
               
                   
                 Thermodynamic 
               
               
                   
                 Properties