Patent Publication Number: US-9835585-B2

Title: Chemical sensor with protruded sensor surface

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/868,947 filed Aug. 22, 2013 and 61/790,866 filed Mar. 15, 2013, the entire contents of which are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This disclosure, in general, relates to sensors for chemical analysis, and to methods for manufacturing such sensors. 
     BACKGROUND 
     A variety of types of chemical sensors have been used in the detection of chemical processes. One type is a chemically-sensitive field effect transistor (chemFET). A chemFET includes a source and a drain separated by a channel region, and a chemically sensitive area coupled to the channel region. The operation of the chemFET is based on the modulation of channel conductance, caused by changes in charge at the sensitive area due to a chemical reaction occurring nearby. The modulation of the channel conductance changes the threshold voltage of the chemFET, which can be measured to detect and/or determine characteristics of the chemical reaction. The threshold voltage may, for example, be measured by applying appropriate bias voltages to the source and drain, and measuring a resulting current flowing through the chemFET. As another example, the threshold voltage may be measured by driving a known current through the chemFET, and measuring a resulting voltage at the source or drain. 
     An ion-sensitive field effect transistor (ISFET) is a type of chemFET that includes an ion-sensitive layer at the sensitive area. The presence of ions in an analyte solution alters the surface potential at the interface between the ion-sensitive layer and the analyte solution, due to the protonation or deprotonation of surface charge groups caused by the ions present in the analyte solution. The change in surface potential at the sensitive area of the ISFET affects the threshold voltage of the device, which can be measured to indicate the presence and/or concentration of ions within the solution. Arrays of ISFETs may be used for monitoring chemical reactions, such as DNA sequencing reactions, based on the detection of ions present, generated, or used during the reactions. See, for example, U.S. Pat. No. 7,948,015 to Rothberg et al., which is incorporated by reference herein in its entirety. More generally, large arrays of chemFETs or other types of chemical sensors may be employed to detect and measure static and/or dynamic amounts or concentrations of a variety of analytes (e.g. hydrogen ions, other ions, compounds, etc.) in a variety of processes. The processes may, for example, be biological or chemical reactions, cell or tissue cultures or monitoring neural activity, nucleic acid sequencing, etc. 
     An issue that arises in the operation of large scale chemical sensor arrays is the susceptibility of the sensor output signals to noise. Specifically, the noise affects the accuracy of the downstream signal processing used to determine the characteristics of the chemical and/or biological process being detected by the sensors. It is therefore desirable to provide devices including low noise chemical sensors, and methods for manufacturing such devices. 
     SUMMARY 
     In one exemplary embodiment, a chemical sensor is described. The chemical sensor includes a chemically-sensitive field effect transistor including a floating gate conductor having an upper surface. A conductive element may protrude from the upper surface of the floating gate conductor. A dielectric material defines a reaction region. The reaction region may overly and extend below an upper surface of the conductive element. According to one embodiment, the conductive element may have a width less than a width of the reaction region. According to another embodiment, the upper surface of the conductive element may be below an upper surface of the dielectric material. In another embodiment, the dielectric material may include a first layer and a second layer on the first layer, and the conductive element may extend from the upper surface of the floating gate conductor a distance defined by a thickness of the first layer. According to one embodiment, the conductive element may comprise an electrically conductive material, and an upper surface of the conductive element may include an oxide of the electrically conductive material. In one embodiment, the chemical sensor further includes a layer of sensing material on the conductive element. The sensing material may comprise a metal-oxide. The sensing material is sensitive to hydrogen ions. According to another embodiment, the chemically-sensitive field effect transistor may include a floating gate structure comprising a plurality of conductors electrically coupled to one another and separated by dielectric layers, and the floating gate conductor may be an uppermost conductor in the plurality of conductors. In another embodiment, the chemically-sensitive field effect transistor may generate a sensor signal in response to a chemical reaction occurring proximate to the conductive element. In yet another embodiment, the chemical sensor further includes a microfluidic structure in fluid flow communication with the chemically-sensitive field effect transistor, and arranged to deliver analytes for sequencing. 
     In another exemplary embodiment, a method for manufacturing a chemical sensor is described. The method includes forming a chemically-sensitive field effect transistor including a floating gate conductor having an upper surface. The method further includes forming a conductive element protruding from the upper surface of the floating gate conductor. The method further includes forming a dielectric material defining a reaction region overlying and extending below an upper surface of the conductive element. According to one embodiment, the forming the dielectric material and forming the conductive element includes forming a first dielectric material on the upper surface of the floating gate, the first dielectric defining a cavity extending to the upper surface of the floating gate conductor; forming the conductive element within the cavity; forming a second dielectric material on the conductive element and the first dielectric; and etching the first and second dielectric material to expose the conductive element and define the reaction region. In another embodiment, the forming the conductive element within the cavity may include depositing a conductive material on an upper surface of the first dielectric and filling the cavity; and removing at least a portion of the conductive from the upper surface of the dielectric. In one embodiment, the removing at least the portion of the conductive material may comprise performing a planarization process to expose the upper surface of the first dielectric. According to one embodiment, a sensing surface of the chemical sensor may include the upper surface of the conductive element. According to another embodiment, the sensing surface of the chemical sensor further includes an outer surface of the conductive element. In another embodiment, the dielectric material contacts a lower portion of an outer surface of the conductive element. 
     Particular aspects of one more implementations of the subject matter described in this specification are set forth in the drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of components of a system for nucleic acid sequencing according to an exemplary embodiment. 
         FIG. 2  illustrates a cross-sectional view of a portion of the integrated circuit device and flow cell according to an exemplary embodiment. 
         FIG. 3  illustrates cross-sectional of representative chemical sensors and corresponding reaction regions according to an exemplary embodiment. 
         FIG. 4  illustrates cross-sectional of representative chemical sensors and corresponding reaction regions according to another exemplary embodiment. 
         FIGS. 5 to 11  illustrate stages in a manufacturing process for forming an array of chemical sensors and corresponding well structures according to an exemplary embodiment. 
         FIGS. 12 to 14  illustrate stages in a manufacturing process for forming an array of chemical sensors and corresponding well structures according to another exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A chemical detection device are described that include low noise chemical sensors, such as chemically-sensitive field effect transistors (chemFETs), for detecting chemical reactions within overlying, operationally associated reaction regions. Reducing the plan or top view area (or footprint) of individual chemical sensors and the overlying reaction regions allows for higher density devices. However, as the dimensions of the chemical sensors are reduced, Applicants have found that a corresponding reduction in the sensing surface area of the sensors can impact performance. For example, for chemical sensors having sensing surfaces defined at the bottom of the reaction regions, reducing the plan view dimensions (e.g. the width or diameter) of the reaction regions results in a similar reduction in the sensing surface areas. Applicants have found that as the sensing surface area is reduced to technology limits, fluidic noise due to the random fluctuation of charge on the sensing surface contributes to an increasing proportion of the total variation in sensing surface potential. This can reduce the signal-to-noise ratio (SNR) of the sensor output signal, which may affect the accuracy of the downstream signal processing used to determine the characteristics of the chemical and/or biological process being detected by the sensor. Chemical sensors described herein have sensing surface areas which are not limited to a two-dimensional area at the bottom of the reaction regions. In embodiments described herein, the sensing surface of the chemical sensor includes conductive element protruding from the upper surface of the floating gate conductor into an opening. By extending the conductive element into the opening, the chemical sensor can have a small footprint, while also having a sufficiently large sensing surface area to avoid the noise issues associated with small sensing surfaces. The footprint of a chemical sensor is determined in part by the width (e.g. diameter) of the overlying reaction region and can be made small, allowing for a high density array. In addition, because the reaction region overlies and extends below an upper surface of the conductive element, the sensing surface area can be relatively large. As a result, low noise chemical sensors can be provided in a high density array, such that the characteristics of reactions can be accurately detected. 
       FIG. 1  illustrates a block diagram of components of a system for nucleic acid sequencing according to an exemplary embodiment. The components include a flow cell  101  on an integrated circuit device  100 , a reference electrode  108 , a plurality of reagents  114  for sequencing, a valve block  116 , a wash solution  110 , a valve  112 , a fluidics controller  118 , lines  120 / 122 / 126 , passages  104 / 109 / 111 , a waste container  106 , an array controller  124 , and a user interface  128 . The integrated circuit device  100  includes a microwell array  107  overlying a sensor array that includes chemical sensors as described herein. The flow cell  101  includes an inlet  102 , an outlet  103 , and a flow chamber  105  defining a flow path of reagents over the microwell array  107 . The reference electrode  108  may be of any suitable type or shape, including a concentric cylinder with a fluid passage or a wire inserted into a lumen of passage  111 . The reagents  114  may be driven through the fluid pathways, valves, and flow cell  101  by pumps, gas pressure, or other suitable methods, and may be discarded into the waste container  106  after exiting the outlet  103  of the flow cell  101 . The fluidics controller  118  may control driving forces for the reagents  114  and the operation of valve  112  and valve block  116  with suitable software. The microwell array  107  includes an array of reaction regions as described herein, also referred to herein as microwells, which are operationally associated with corresponding chemical sensors in the sensor array. For example, each reaction region may be coupled to a chemical sensor suitable for detecting an analyte or reaction property of interest within that reaction region. The microwell array  107  may be integrated in the integrated circuit device  100 , so that the microwell array  107  and the sensor array are part of a single device or chip. The flow cell  101  may have a variety of configurations for controlling the path and flow rate of reagents  114  over the microwell array  107 . The array controller  124  provides bias voltages and timing and control signals to the integrated circuit device  100  for reading the chemical sensors of the sensor array. The array controller  124  also provides a reference bias voltage to the reference electrode  108  to bias the reagents  114  flowing over the microwell array  107 . 
     During an experiment, the array controller  124  collects and processes output signals from the chemical sensors of the sensor array through output ports on the integrated circuit device  100  via bus  127 . The array controller  124  may be a computer or other computing means. The array controller  124  may include memory for storage of data and software applications, a processor for accessing data and executing applications, and components that facilitate communication with the various components of the system in  FIG. 1 . The values of the output signals of the chemical sensors indicate physical and/or chemical parameters of one or more reactions taking place in the corresponding reaction regions in the microwell array  107 . For example, in an exemplary embodiment, the values of the output signals may be processed using the techniques disclosed in Rearick et al., U.S. patent application Ser. No. 13/339,846, filed Dec. 29, 2011, based on U.S. Prov. Pat. Appl. No. 61/428,743, filed Dec. 30, 2010, and 61/429,328, filed Jan. 3, 2011, and in Hubbell, U.S. patent application Ser. No. 13/339,753, filed Dec. 29, 2011, based on U.S. Prov. Pat. Appl. No. 61/428,097, filed Dec. 29, 2010, which are all incorporated by reference herein in their entirety. The user interface  128  may display information about the flow cell  101  and the output signals received from chemical sensors in the sensor array on the integrated circuit device  100 . The user interface  128  may also display instrument settings and controls, and allow a user to enter or set instrument settings and controls. 
     In an exemplary embodiment, during the experiment the fluidics controller  118  may control delivery of the individual reagents  114  to the flow cell  101  and integrated circuit device  100  in a predetermined sequence, for predetermined durations, at predetermined flow rates. The array controller  124  can collect and analyze the output signals of the chemical sensors indicating chemical reactions occurring in response to the delivery of the reagents  114 . During the experiment, the system may also monitor and control the temperature of the integrated circuit device  100 , so that reactions take place and measurements are made at a known predetermined temperature. The system may be configured to let a single fluid or reagent contact the reference electrode  108  throughout an entire multi-step reaction during operation. The valve  112  may be shut to prevent any wash solution  110  from flowing into passage  109  as the reagents  114  are flowing. Although the flow of wash solution may be stopped, there may still be uninterrupted fluid and electrical communication between the reference electrode  108 , passage  109 , and the microwell array  107 . The distance between the reference electrode  108  and the junction between passages  109  and  111  may be selected so that little or no amount of the reagents flowing in passage  109  and possibly diffusing into passage  111  reach the reference electrode  108 . In an exemplary embodiment, the wash solution  110  may be selected as being in continuous contact with the reference electrode  108 , which may be especially useful for multi-step reactions using frequent wash steps. 
       FIG. 2  illustrates a cross-sectional view of a portion of the integrated circuit device  100  and flow cell  101 . During operation, the flow chamber  105  of the flow cell  101  confines a reagent flow  208  of delivered reagents across open ends of the reaction regions in the microwell array  107 . The volume, shape, aspect ratio (such as base width-to-well depth ratio), and other dimensional characteristics of the reaction regions may be selected based on the nature of the reaction taking place, as well as the reagents, byproducts, or labeling techniques (if any) that are employed. The chemical sensors of the sensor array  205  are responsive to (and generate output signals) chemical reactions within associated reaction regions in the microwell array  107  to detect an analyte or reaction property of interest. The chemical sensors of the sensor array  205  may, for example, be chemically sensitive field-effect transistors (chemFETs), such as ion-sensitive field effect transistors (ISFETs). Examples of chemical sensors and array configurations that may be used in embodiments are described in U.S. Patent Application Publication No. 2010/0300559, No. 2010/0197507, No. 2010/0301398, No. 2010/0300895, No. 2010/0137143, and No. 2009/0026082, and U.S. Pat. No. 7,575,865, which are all incorporated by reference herein in their entirety. 
       FIG. 3  illustrates a cross-sectional view of two representative chemical sensors and their corresponding reaction regions according to an exemplary embodiment. In  FIG. 3 , two chemical sensors  350 ,  351  are shown, representing a small portion of a sensor array that can include millions of chemical sensors. Chemical sensor  350  is coupled to corresponding reaction region  301 , and chemical sensor  351  is coupled to corresponding reaction region  302 . Chemical sensor  350  is representative of the chemical sensors in the sensor array. In the illustrated example, the chemical sensor  350  is a chemically-sensitive field effect transistor (chemFET), more specifically an ion-sensitive field effect transistor (ISFET) in this example. The chemical sensor  350  includes a floating gate structure  318  having a sensor plate  320  coupled to the reaction region  301  by an electrically conductive element  370 . As is illustrated in  FIG. 3 , the sensor plate  320  is the uppermost floating gate conductor in the floating gate structure  318 . In the illustrated example, the floating gate structure  318  includes multiple patterned layers of conductive material within layers of dielectric material  319 . The chemical sensor  350  also includes a source region  321  and a drain region  322  within a semiconductor substrate  354 . The source region  321  and the drain region  322  comprise doped semiconductor material having a conductivity type different from the conductivity type of the substrate  354 . For example, the source region  321  and the drain region  322  may comprise doped P-type semiconductor material, and the substrate may comprise doped N-type semiconductor material. Channel region  323  separates the source region  321  and the drain region  322 . The floating gate structure  318  overlies the channel region  323 , and is separated from the substrate  354  by a gate dielectric  352 . The gate dielectric  352  may be silicon dioxide, for example. Alternatively, other dielectrics may be used for the gate dielectric  352 . 
     As shown in  FIG. 3 , the dielectric material defines the reaction region  301  which may be within opening created by the absence of the dielectric material. The dielectric material  308  may comprise one or more layers of material, such as silicon dioxide or silicon nitride. The dimensions of the openings, and their pitch, can vary from implementation to implementation. In some embodiments, the openings can have a characteristic diameter, defined as the square root of 4 times the plan view cross-sectional area (A) divided by Pi (e.g., sqrt(4*A/π), of not greater than 5 micrometers, such as not greater than 3.5 micrometers, not greater than 2.0 micrometers, not greater than 1.6 micrometers, not greater than 1.0 micrometers, not greater than 0.8 micrometers, not greater than 0.6 micrometers, not greater than 0.4 micrometers, not greater than 0.2 micrometers or even not greater than 0.1 micrometers. The chemical sensor  350  includes a conductive element  370  protruding from the upper surface of the floating gate conductor  377  into the opening as illustrated in  FIG. 3 . In the illustrated embodiment, dielectric materials  308 / 308 ′ define a reaction region  301 . Dielectric materials  308  and  308 ′ may comprise an upper layer  315  and a lower layer  314 , respectively. The reaction region  301  may overly and extend below an upper surface  371  of the conductive element  370 . The conductive element  370  may have a width narrower than a width of the reaction region. The conductive element  370  may have a height less than the height of the dielectric material defining the reaction region. As a result of this structure, the conductive element  370  is shaped like a post and the upper surface  371  of the conductive element  370  acts as the sensing surface for the chemical sensor  350 . The conductive element as discussed throughout the disclosure may be formed in various shapes (width, height, etc.) depending on the materials/etch techniques/fabrication processes etc. used during the manufacture process. In addition, because the electrically conductive element  370  protrudes into the opening in the dielectric material, the sensing surface area of the chemical sensor  350  is not limited by the surface area of the opening. The electrically conductive element  370  may comprise one or more of a variety of different materials to facilitate sensitivity to particular ions (e.g. hydrogen ions). 
     According to one embodiment, the conductive element may have a width W less than a width W′ of the reaction region. According to another embodiment, the upper surface  371  of the conductive element  370  may be below an upper surface  371 ′ of the dielectric material  308 / 308 ′. In another embodiment, the dielectric material may include a first layer  308  and a second layer  308 ′ on the first layer  308 , and the conductive element  370  may extend from the upper surface of the floating gate conductor  377  a distance defined by a thickness  609  of the first layer  308 . According to one embodiment, the conductive element may comprise an electrically conductive material, and an upper surface of the conductive element may include an oxide of the electrically conductive material. In one embodiment, the chemical sensor further includes a layer of sensing material on the conductive element. The sensing material may comprise a metal-oxide. The sensing material is sensitive to hydrogen ions. According to another embodiment, the chemically-sensitive field effect transistor may include a floating gate structure comprising a plurality of conductors electrically coupled to one another and separated by dielectric layers, and the floating gate conductor may be an uppermost conductor in the plurality of conductors. In another embodiment, the chemically-sensitive field effect transistor may generate a sensor signal in response to a chemical reaction occurring proximate to the conductive element. In yet another embodiment, the chemical sensor further includes a microfluidic structure in fluid flow communication with the chemically-sensitive field effect transistor, and arranged to deliver analytes for sequencing. 
     The post-shaped electrically conductive element  370  provides the chemical sensor  350  with a small plan view area, while also providing a sufficiently large surface area to avoid the noise issues associated with small sensing surfaces. The plan view area of the chemical sensor is determined in part by the width (or diameter) of the reaction region  301  and can be made small, allowing for a high density array. In addition, because the reaction region  301  overlies and extends below the upper surface  371  of the conductive element  370 , the sensing surface area depends upon the depth and the circumference of the reaction region  301 , as well as the distance that the electrically conductive element  370  extends into the opening, and can be relatively large. As a result, low noise chemical sensors  350 ,  351  can be provided in a high density array, such that the characteristics of reactions can be accurately detected. 
     During manufacturing and/or operation of the device, a thin oxide of the material of the electrically conductive element  370  may be grown on the upper surface  371  which acts as a sensing material (e.g. an ion-sensitive sensing material) for the chemical sensor  350 . For example, in one embodiment the electrically conductive element  370  may be titanium nitride, and titanium oxide or titanium oxynitride may be grown on the upper surface  371  during manufacturing and/or during exposure to solutions during use. Whether an oxide is formed depends on the conductive material, the manufacturing processes performed, and the conditions under which the device is operated. In the illustrated example, the electrically conductive element  370  is shown as a single layer of material. More generally, the electrically conductive element  370  may comprise one or more layers of a variety of electrically conductive materials, such as metals or ceramics, depending upon the implementation. The conductive material can be, for example, a metallic material or alloy thereof, or can be a ceramic material, or a combination thereof. An exemplary metallic material includes one of aluminum, copper, nickel, titanium, silver, gold, platinum, hafnium, lanthanum, tantalum, tungsten, iridium, zirconium, palladium, or a combination thereof. An exemplary ceramic material includes one of titanium nitride, titanium aluminum nitride, titanium oxynitride, tantalum nitride, or a combination thereof. In some alternative embodiments, an additional conformal sensing material (not shown) is deposited on the upper surface  371  of the electrically conductive element  370 . The sensing material may comprise one or more of a variety of different materials to facilitate sensitivity to particular ions. For example, silicon nitride or silicon oxynitride, as well as metal oxides such as silicon oxide, aluminum or tantalum oxides, generally provide sensitivity to hydrogen ions, whereas sensing materials comprising polyvinyl chloride containing valinomycin provide sensitivity to potassium ions. Materials sensitive to other ions such as sodium, silver, iron, bromine, iodine, calcium, and nitrate may also be used, depending upon the implementation. 
     Referring again to  FIG. 3 , in operation, reactants, wash solutions, and other reagents may move in and out of the reaction region  301  by a diffusion mechanism  340 . The chemical sensor  350  is responsive to (and generates an output signal related to) the amount of a charge  324  proximate to the electrically conductive element  370 . The presence of charge  324  in an analyte solution alters the surface potential at the interface between the analyte solution and the upper surface  371  of the electrically conductive element  370 , due to the protonation or deprotonation of surface charge groups caused by the ions present in the analyte solution. Changes in the charge  324  cause changes in the voltage on the floating gate structure  318 , which in turn changes in the threshold voltage of the transistor of the chemical sensor  350 . This change in threshold voltage can be measured by measuring the current in the channel region  323  between the source region  321  and a drain region  322 . As a result, the chemical sensor  350  can be used directly to provide a current-based output signal on an array line connected to the source region  321  or drain region  322 , or indirectly with additional circuitry to provide a voltage-based output signal. As described in more detail below with respect to  FIGS. 5-14 , the dielectric material has an upper layer  315  and a lower layer  314  and the conductive element  370  protrudes from the upper surface  371  of the floating gate conductor a distance defined by a thickness  609  of the lower layer  314 . Because the charge  324  may be more highly concentrated near the bottom of the reaction region  301 , in some embodiments variations in the dimensions of this extension may have a significant effect on the amplitude of the signal detected in response to the charge  324 . In an embodiment, reactions carried out in the reaction region  301  can be analytical reactions to identify or determine characteristics or properties of an analyte of interest. Such reactions can generate directly or indirectly byproducts that affect the amount of charge adjacent to the electrically conductive element  370 . If such byproducts are produced in small amounts or rapidly decay or react with other constituents, multiple copies of the same analyte may be analyzed in the reaction region  301  at the same time in order to increase the output signal generated. In an embodiment, multiple copies of an analyte may be attached to a solid phase support  312 , either before or after deposition into the reaction region  301 . The solid phase support  312  may be microparticles, nanoparticles, beads, solid or porous comprising gels, or the like. For simplicity and ease of explanation, solid phase support  312  is also referred herein as a particle. For a nucleic acid analyte, multiple, connected copies may be made by rolling circle amplification (RCA), exponential RCA, or like techniques, to produce an amplicon without the need of a solid support. 
       FIG. 4  illustrates a chemical sensor according to an alternate embodiment. As illustrated in  FIG. 4 , the chemical sensors  450 ,  451  include a conductive element  470  protruding from the upper surface of the floating gate conductor into the opening. In the illustrated embodiment, the dielectric material (i.e.  408  and  408 ′) defines a reaction region  401 . The dielectric material may comprise an upper layer  415  and a lower layer  414 . The reaction region  401  may overly and extend below an upper surface  471  of the conductive element  470 . For example, the conductive material  470  may be a layer of titanium nitride, or a layer of titanium. Alternatively, other and/or additional conductive materials may be used, such as those described above with reference to the electrically conductive element. In addition, more than one layer of conductive material may be deposited. The conductive material  470  may be deposited using various techniques, such as sputtering, reactive sputtering, atomic layer deposition (ALD), low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), metal organic chemical vapor deposition (MOCVD), etc. The solid phase support may be of varied size, as would be understood by one of ordinary skill in the art. In the illustrated embodiment, solid phase support  412  is larger than solid support  412  illustrated in  FIG. 4 . The dielectric material has an upper layer  415  and a lower layer  414  and the conductive element  470  protrudes from the upper surface  471  of the floating gate conductor a distance defined by a thickness  609 ′ of the lower layer  414 . Because the charge  424  may be more highly concentrated near the bottom of the reaction region  401 , in some embodiments variations in the dimensions of this extension may have a significant effect on the amplitude of the signal detected in response to the charge  424 . 
     In various exemplary embodiments, the methods, systems, and computer readable media described herein may advantageously be used to process and/or analyze data and signals obtained from electronic or charged-based nucleic acid sequencing. In electronic or charged-based sequencing (such as, pH-based sequencing), a nucleotide incorporation event may be determined by detecting ions (e.g., hydrogen ions) that are generated as natural by-products of polymerase-catalyzed nucleotide extension reactions. This may be used to sequence a sample or template nucleic acid, which may be a fragment of a nucleic acid sequence of interest, for example, and which may be directly or indirectly attached as a clonal population to a solid support, such as a particle, microparticle, bead, etc. The sample or template nucleic acid may be operably associated to a primer and polymerase and may be subjected to repeated cycles or “flows” of deoxynucleoside triphosphate (“dNTP”) addition (which may be referred to herein as “nucleotide flows” from which nucleotide incorporations may result) and washing. The primer may be annealed to the sample or template so that the primer&#39;s  3 ′ end can be extended by a polymerase whenever dNTPs complementary to the next base in the template are added. Then, based on the known sequence of nucleotide flows and on measured output signals of the chemical sensors indicative of ion concentration during each nucleotide flow, the identity of the type, sequence and number of nucleotide(s) associated with a sample nucleic acid present in a reaction region coupled to a chemical sensor can be determined. 
       FIGS. 5-11  illustrate stages in a manufacturing process for forming an array of chemical sensors and corresponding well structures according to an exemplary embodiment.  FIG. 5  illustrates a structure  500  including the floating gate structures (e.g. floating gate structure  318 ) for the chemical sensors  350 ,  351 . The structure  500  can be formed by depositing a layer of gate dielectric material on the semiconductor substrate  354 , and depositing a layer of polysilicon (or other electrically conductive material) on the layer of gate dielectric material. The layer of polysilicon and the layer gate dielectric material can then be etched using an etch mask to form the gate dielectric elements (e.g. gate dielectric  352 ) and the lowermost conductive material element of the floating gate structures. Following formation of an ion-implantation mask, ion implantation can then be performed to form the source and drain regions (e.g. source region  321  and a drain region  322 ) of the chemical sensors. A first layer of the dielectric material  319  can be deposited over the lowermost conductive material elements. Conductive plugs can then be formed within vias etched in the first layer of dielectric material  319  to contact the lowermost conductive material elements of the floating gate structures. A layer of conductive material can then be deposited on the first layer of the dielectric material  319  and patterned to form second conductive material elements electrically connected to the conductive plugs. This process can then be repeated multiple times to form the completed floating gate structure  318  shown in  FIG. 5 . Alternatively, other and/or additional techniques may be performed to form the structure. Forming the structure  500  in  FIG. 5  can also include forming additional elements such as array lines (e.g. word lines, bit lines, etc.) for accessing the chemical sensors, additional doped regions in the substrate  354 , and other circuitry (e.g. access circuitry, bias circuitry etc.) used to operate the chemical sensors, depending upon the device and array configuration in which the chemical sensors described herein are implemented. In some embodiments, the elements of the structure may, for example, be manufactured using techniques described in U.S. Patent Application Publication No. 2010/0300559, No. 2010/0197507, No. 2010/0301398, No. 2010/0300895, No. 2010/0137143, and No. 2009/0026082, and U.S. Pat. No. 7,575,865, each of which were incorporated by reference in their entirety above. 
     As illustrated in the structure  600  illustrated in  FIG. 6 , a dielectric material  308  may be formed on the sensor plate  320  of the field effect transistor of the chemical sensor  350 . The thickness of the dielectric material  308  at this stage may be defined by a distance  609 . This thickness is one way to control the height and shape of the conductive element. Next, as illustrated in  FIG. 7 , the dielectric material  308  of the structure  600  in  FIG. 6  is etched to form openings  710 ,  712  extending to the upper surfaces of the floating gate structures of the chemical sensors  350 ,  351 , resulting in the structure  700  illustrated in  FIG. 7 . The openings  710 ,  712  may, for example, be formed by using a lithographic process to pattern a layer of photoresist on the dielectric material  308  to define the locations of the openings  710 ,  712 , and then anisotropically etching the dielectric material  308  using the patterned photoreist as an etch mask. The anisotropic etching of the dielectric material  308  may, for example, be a dry etch process, such as a fluorine based Reactive Ion Etching (RIE) process. In the illustrated embodiment, the openings  710 ,  712  are separated by a distance  730  that is equal to their width  720 . Alternatively, the separation distance  730  between adjacent openings may be less than the width  720 . For example, the separation distance  730  may be a minimum feature size for the process (e.g. a lithographic process) used to form the openings  710 ,  712 . In such a case, the distance  730  may be significantly less than the width  720 . Next, a layer of conductive material  870  is deposited on the structure  700  illustrated in  FIG. 7 , resulting in the structure  800  illustrated in  FIG. 8 . The conductive material  870  comprises one or more layers of electrically conductive material. For example, the conductive material  870  may be a layer of titanium nitride, or a layer of titanium. Alternatively, other and/or additional conductive materials may be used, such as those described above with reference to the electrically conductive element. In addition, more than one layer of conductive material may be deposited. The conductive material  870  may be deposited using various techniques, such as sputtering, reactive sputtering, atomic layer deposition (ALD), low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), metal organic chemical vapor deposition (MOCVD), etc. For simplicity, the conductive material  870  is shown as planar; however there may be divots in the conductive material  870  above the openings, for example. Next, the conductive material  870  is etched using a Chemical Mechanical Planarization (CMP) process, for example, resulting in the structure  900  illustrated in  FIG. 9 . The etching process planarizes conductive material  870  and the dielectric material  308 . Next, a second dielectric material  308 ′ may be formed on the planarized conductive material  870  and dielectric material  308 , resulting in the structure  1000  illustrated in  FIG. 10 . For example, the second dielectric material  308 ′ may be Tetraethyl orthosilicate, (TEOS) or silicon dioxide. Next, the dielectric materials  308  and  308 ′ and conductive material  870  of the structure  1000  in  FIG. 10  is etched to form openings  1110 ,  1112  extending to the upper surfaces of the floating gate structures of the chemical sensors  350 ,  351 , resulting in the structure  1100  illustrated in  FIG. 11 . The etching process may be a timed etch process, for example. 
       FIGS. 12-14  illustrate stages in a manufacturing process for forming an array of chemical sensors and corresponding well structures according to an exemplary embodiment.  FIG. 12  illustrates an alternate method of forming the structure  1400  illustrated in  FIG. 14 . Beginning with the structure  600  illustrated in  FIG. 6 , dielectric material  308  may be reduced and/or planarized by using a Chemical Mechanical Planarization (CMP) process or an etch back process (or by another suitable technique described above) and a material  1212  suitable for use with an etch stop technique (for example, Silicon Nitride) may be formed thereon, resulting in the structure illustrated in  FIG. 12 . Next, a dielectric material  1313  (for example, Tetraethyl orthosilicate, (TEOS)) may be formed over material  1212 , resulting in the structure  1300  illustrated in  FIG. 13 . Thereafter, openings extending to the upper surfaces of the floating gate structures of the chemical sensors  350 ,  351  may be formed in the dielectric material, as discussed above with reference to  FIGS. 6 and 7 . A layer of conductive material  1370  is deposited within the openings, as discussed above with reference to  FIGS. 7 and 8 . For example, the conductive material  1370  may be a layer of titanium nitride, or a layer of titanium. Alternatively, other and/or additional conductive materials may be used, such as those described above with reference to the electrically conductive element  370 . In addition, more than one layer of conductive material may be deposited. The conductive material  1370  may be deposited using various techniques, such as sputtering, reactive sputtering, atomic layer deposition (ALD), low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), metal organic chemical vapor deposition (MOCVD), etc. The conductive material  1370  and dielectric material  1313  are etched using a Chemical Mechanical Planarization (CMP) process as discussed above (or by another suitable technique described above) with reference to  FIGS. 8 and 9 . Once the conductive material  1370  and the dielectric material  1313  (for example, TEOS, or those described above) have been planarized, a dielectric material  1313 ′ (for example, TEOS, or those disclosed above) is formed on the planarized surface. Next, the dielectric materials  1313  and  1313 ′ and conductive material  1370  are etched using an etch stop technique to form openings  1110 ,  1112  extending to the material  1212 . The conductive element  1370  extends to the upper surfaces of the floating gate structures of the chemical sensors  350 ,  351 , resulting in the structure  1400  illustrated in  FIG. 14 . 
     While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.