Patent Publication Number: US-2021164932-A1

Title: Method of forming ion sensors

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims benefit of U.S. Provisional Application No. 62/719,576, filed Aug. 17, 2018, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure, in general, relates to methods of forming ion sensors and sensors formed thereby. 
     BACKGROUND 
     In industries as diverse as environmental monitoring and genetic sequencing, there is increasing interest in utilizing small-scale sensors to detect changes in ion concentration, such as changes in hydrogen or hydronium ion concentrations indicative of pH. For environmental monitoring, a change in ion concentration in a river may indicate a new mining operation upstream, or may indicate the seasonal change in a lake or the influx of brackish water along coastal tributaries. Further, small-scale ion sensors can be used in biochemical applications, such as genetic sequencing, in which an increase in hydrogen or hydronium ion concentration can indicate the incorporation of a nucleotide on an extending polynucleotide. In each case, the quality of the signal emanating from the ion sensor influences the accuracy of a measurement. 
     In particular, semiconductor processing used in the formation of such ion sensors influences signal-to-noise ratios and offset potentials associated with the sensors, either weakening the signal relative to environmental noise or creating an offset that makes it difficult to measure or detect the signal. Such issues are particularly pronounced when working with an array of sensors associated with microwells. In an array of sensors, offset variability among sensors in the array leads to difficulty in signal processing. As such, the semiconductor processes that are used to form sensor surfaces can adversely affect the performance of the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
         FIG. 1  includes an illustration of an example system for genetic sequencing utilizing an array of ion sensors. 
         FIG. 2  includes an illustration of an example sensor and flow cell. 
         FIG. 3  includes an illustration of an example sensor. 
         FIG. 4  includes a block flow diagram illustrating an example method for forming and ion sensor. 
         FIG. 5 ,  FIG. 6 ,  FIG. 7 , and  FIG. 8  include illustrations of workpieces and a process for forming ion sensors. 
         FIG. 9  includes a block flow diagram illustrating an example method for forming and ion sensor. 
         FIG. 10 ,  FIG. 11 ,  FIG. 12 ,  FIG. 13 , and  FIG. 14  include illustrations of workpieces during a process for the formation of an ion sensor. 
     
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DETAILED DESCRIPTION 
     In an example embodiment, a process for forming an ion sensitive sensor includes etching an insulation material to form a well, exposing a sensor pad and removing photoresist used in the etching process. The process further includes annealing the ion sensor pad in a hydrogen containing atmosphere and annealing in an oxygen containing atmosphere. Optionally, the sensor pad is disposed at a bottom of a microwell. In an alternative example, a conductive layer can extend over the sensor pad and at least a portion of the sidewalls of the microwell, extending the sensor surface area. 
     In an example, the ion sensors can be used for genetic sequencing.  FIG. 1  illustrates a block diagram of components of a system for nucleic acid sequencing according to an example 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 for the reagents  114  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, vacuum, 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 reaction regions, 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 . 
     In operation, 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 . In the illustrated embodiment, the array controller  124  is external to the integrated circuit device  100 . In some alternative embodiments, some or all of the functions performed by the array controller  124  are carried out by a controller or other data processor on the integrated circuit device  100 . The values of the output signals from the chemical sensors indicate physical or chemical parameters of one or more reactions taking place in the corresponding reaction regions in the microwell array  107 . 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 some embodiments, 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 then 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 ) reaches the reference electrode  108 . In an example 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 cross-sectional and expanded views of a portion of the integrated circuit device  100  and flow cell  101 . The integrated circuit device  100  includes the microwell array  107  of reaction regions operationally associated with sensor array  205 . 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 related to) 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). 
     Provided herein is a device for detecting a reaction. The reaction may be localized to a reaction region and multiple reactions of the same type may occur in the same reaction region. The reaction that may occur may be a chemical reaction that results in the detection of a reaction by-product or the detection of a signal indicating a reaction. A sensor may be located in proximity to the reaction region and may detect the reaction by-product or the signal. The sensor may be a CMOS type of sensor. In some embodiments, the sensor may detect a hydrogen ion, hydronium ion, hydroxide ion, or the release of pyrophosphate. In some embodiments, the sensor may detect the presence of a charged probe molecule. 
       FIG. 3  illustrates a representative reaction region  301  and a chemical sensor  314 . The reaction region may be an opening such as a well, depression, or channel. Alternatively, the reaction region may be an area where any suitable reaction takes place. A sensor array may have millions of these chemical sensors  314  and reaction regions  301 . The chemical sensor  314  may be a chemically-sensitive field effect transistor (chemFET), or more specifically an ion-sensitive field effect transistor (ISFET). The chemical sensor  314  includes a floating gate structure  318  having a sensor plate  320  coupled to a reaction region  301  via an electrically conductive layer within the reaction region  301 . The floating gate structure  318  may include multiple layers of conductive material within layers of dielectric material or may include a single layer of conductive material within a single layer of dielectric material  311 . The chemical sensor may include a source  321  and a drain  322  located within the substrate. The source  321  and the drain  322  include doped semiconductor material having a conductivity type different from the conductivity type of the substrate. For example, the source  321  and the drain  322  may comprise doped P-type semiconductor material, and the substrate may comprise doped N-type semiconductor material. A channel  323  separates the source  321  and the drain  322 . The floating gate structure  318  overlies the channel region  323 , and is separated from the substrate by a gate dielectric  352 . The gate dielectric  352  may be for example silicon dioxide. Alternatively, other dielectrics may be used for the gate dielectric  352 . 
     As shown in  FIG. 3 , the reaction region  301  is within an opening extending through dielectric materials  310  to the upper surface of the sensor plate  320 . The dielectric material  310  may comprise one or more layers of material, such as silicon dioxide or silicon nitride. The opening also includes an upper portion  315  within the dielectric material  310  and extends from the chemical sensor  314  to the upper surface of the dielectric material  310 . In some embodiments, the width of the upper portion of the opening is substantially the same as the width of the lower portion of the reaction region. Alternatively, depending on the material(s) or etch process used to create the opening, the width of the upper portion of the opening may be greater than the width of the lower portion of the opening, or vice versa. The opening may for example have a circular cross-section. Alternatively, the opening may be non-circular. For example, the cross-section may be square, rectangular, hexagonal, or irregularly shaped. The dimensions of the openings, and their pitch, can vary from embodiment to embodiment. 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, but, optionally, at least 0.001 micrometers, such as at least 0.01 micrometers. 
     In some embodiments, during manufacturing or operation of the device, an electrically conductive material is formed as part of the sensor, and a thin oxide of the material of the electrically conductive material may be grown or deposited which acts as a sensing material (e.g. an ion-sensitive sensing material) for the chemical sensor. Whether an oxide is formed depends on the conductive material, the manufacturing processes performed, and the conditions under which the device is operated. For example, in some embodiments the electrically conductive element may be titanium nitride, and titanium oxide or titanium oxynitride may be grown on the inner surface of the conductive material during manufacturing or during exposure to solutions during use. The electrically conductive element may comprise one or more layers of a variety of electrically conductive materials, such as metals or ceramics. The conductive material can be for example a metallic material or alloy thereof, or can be a ceramic material, or a combination thereof. An example metallic material includes one of aluminum, copper, nickel, titanium, silver, gold, platinum, hafnium, lanthanum, tantalum, tungsten, iridium, zirconium, palladium, or a combination thereof. An example 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 is deposited on the conductive element and within the openings. The sensing material may comprise one or more of a variety of different materials to facilitate sensitivity to particular ions. For example, metal oxides such as zinc 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 embodiment. 
     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  314  is responsive to (and generates an output signal related to) the amount of charge  324  proximate to the sensor plate  320 . The presence of charge  324  in an analyte solution alters the surface potential at the interface between the sensor plate  320  and the analyte solution within the reaction region  301 . Changes in the charge  324  cause changes in the voltage on the floating gate structure  318 , which in turn changes the threshold voltage of the transistor. This change in threshold voltage can be measured by measuring the current in the channel region  323  between the source  321  and a drain  322 . As a result, the chemical sensor  314  can be used directly to provide a current-based output signal on an array line connected to the source  321  or drain  322 , or indirectly with additional circuitry to provide a voltage-based output signal. 
     In some embodiments, 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 sensor plate  320  or any other materials or coatings that may be placed on the sensor plate to increase sensitivity. 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 some embodiments, multiple copies of an analyte may be attached to a solid phase support  312 , as shown in  FIG. 3 , either before or after deposition into the reaction region  301 . The solid phase support  312  may be a particle, microparticle, nanoparticle, or bead. The solid phase support may be solid or porous or may be a gel, or a combination thereof. The solid support may be a structure located in the middle of the reaction region. Alternatively, the solid support may be located at the bottom of the reaction region. For a nucleic acid analyte, multiple, connected copies may be made by rolling circle amplification (RCA), exponential RCA, Recombinase Polymerase Amplification (RPA), Polymerase Chain Reaction amplification (PCR), emulsion PCR amplification, or like techniques, to produce an amplicon without the need of a solid support. 
     In various example embodiments, the methods and systems described herein may advantageously be used to process 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 detection method 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 deoxynucleotide 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. 
       FIG. 4  illustrates an example method  400  for forming an ion sensor. The method  400  includes depositing one or more insulator layers over a substrate and sensors pads of electronics formed on or in the substrate, as illustrated at block  402 . The one or more insulator layers can be deposited or grown, for example, using chemical vapor deposition, such as plasma-enhanced chemical vapor deposition. In an example, the insulator layer can include silicon dioxide, silicon nitride, or a low temperature oxide formed, for example, from TEOS. In a further example, layers can be formed of silicon oxide, followed by a layer of silicon nitride disposed over the layer of silicon oxide, and optionally a layer of low temperature oxide formed from TEOS disposed over the silicon nitride layer. 
     As illustrated at block  404 , a photoresist layer can be deposited and patterned to allow an opening to form over the sensor pad extending through the insulator material. For example, as illustrated in  FIG. 5 , sensor pads  504  can be disposed in or on a substrate  502 . In an example, the substrate  502  can include a silicon-based substrate. In another example, substrate can include a gallium arsenide substrate or a sapphire substrate. The sensor pads can be formed of zinc, copper, aluminum, tantalum, titanium, tungsten, gold, silver, oxides thereof, nitrides thereof, or combinations thereof. In an example, the sensor pads can include titanium. In another example, the sensors pads can include a conductive ceramic, such as titanium nitride. One or more insulator layers  506  can be deposited over the substrate  502  and the sensor pads  504 , and a photoresist layer  508  can be coated over the insulator layer or layers  506 . In particular, the insulator layer or layers  506  can be formed by chemical vapor deposition. Optionally, the photoresist layer  508  can be formed by spin coating. 
     Returning to  FIG. 4 , the insulator layers and resist can be etched, as illustrated at block  406 . The process can include a wet etch, a plasma etch, or a combination thereof. In particular, the process can include a plasma etch, such as a fluorine containing plasma etch. In another example, the wet etch can include a bromine etch or a hydrogen fluoride etch. In a particular example, a plasma etch can be followed by a wet etch and wash. Following etching, the photoresist can be removed such as through ashing, as illustrated at block  408 . Often, a native oxide forms as a result of the etching and ashing processes. For example, as illustrated in  FIG. 6 , microwells  610  are formed through the insulator materials  506  to expose the sensor pads  504 . On the surface of the sensor pads  504 , a native oxide  612  can form. Typically, such an oxide is non-uniform and varies in both thickness and quality across the surface of the sensor pad  504 . 
     Returning to  FIG. 4 , the sensor pads can be annealed in a hydrogen containing atmosphere, as illustrated at block  410 . Annealing can be performed at a temperature in a range of 300° C. to 500° C., such as a range of 400° C. to 450° C. As illustrated in  FIG. 7 , the surface  714  of the sensor pads  504  are free of native oxide and can be relatively uniform in thickness and composition. 
     Optionally, the surface can be further etched to remove oxygen depleted regions on the surface of the sensor pad, as illustrated at  412 . For example, surface oxides and optionally oxygen depleted regions within the surface are removed through the optional surface etch. In an example, the surface etch includes a vapor hydrogen fluoride etch (VHF etch). 
     In addition, the sensor pads can be annealed an oxygen containing atmosphere at a temperature of less than 400° C., as illustrated at block  414 . For example, the sensor pads can be exposed to temperatures in a range of 290° C. to 380° C., such as a range of 300° C. to 370° C., a range of 330° C. to 360° C., or a range of 345° C. to 355° C. Selection of temperature has been found to affect the performance of the sensors. In an example, the sensor pads can be exposed to the oxygen containing atmosphere for a period in a range of 5 minutes to 120 minutes, such as a range of 10 minutes to 80 minutes, a range of 15 minutes to 60 minutes, or a range of 15 minutes to 45 minutes. As illustrated in  FIG. 8 , a uniform oxide layer  814  is disposed over the sensor pad  514 . Such a uniform oxide layer  814  provides for consistent performance of an array or across an array of sensors. 
     In another example, an additional conductive layer can be disposed over the sensor pad and optionally, extend at least a portion along the walls of the microwell. For example,  FIG. 9  includes a method  900  for forming the additional conductive layer. Following the etching and ashing processes used to form the microwell over the sensor pad, the surface of the sensor pad is optionally sputtered using a noble gas, such as argon, as illustrated at block  902 . Such optional sputtering can prepare the surface of the sensor pad for contact with the conductive layer. For example, a native oxide can be substantially removed from the surface of the sensor pad. In an example, the sputtering can be performed at a power in a range of 100 W to 400 W, such as a range of 250 W to 350 W. In particular, the sputter is sufficient to remove 1-10 nanometers of oxide. 
     Following the argon sputter, a conformal layer can be deposited over the sensor pad and insulator layers, as illustrated at block  904 . For example, the conformal layer can be deposited using sputtering technique. The conformal conductive layer can, for example, be formed from titanium, tantalum, hafnium, tungsten, aluminum, copper, gold, silver, or any combination thereof. In an example, the conformal layer is formed of titanium. In another example, the conformal layer is tantalum. 
     Following deposition of the conformal layer, a photoresist layer can be deposited and pattern, as illustrated at block  906 . For example, the photoresist layer can be spin coated over the conformal conductive layer, forming photoresist within the wells and over the surface of the substrate. For example, as illustrated in  FIG. 10 , a conformal layer  1016  is deposited over the insulation layer  506  and the sensor pads  504 . A photoresist layer  1018  is deposited over the conformal layer  1016  and enters the microwells  610 . 
     Returning to  FIG. 9 , the photoresist can be etched to an endpoint, as illustrated at block  908 . In an example, the process may use endpoint detection to detect exposure of the conformal layer  1016 . For example, as illustrated in  FIG. 11 , the photoresist  1018  is etched to expose portions of the conformal conductive layer  1016  disposed on top of the insulating layer  506 , while protecting the portions of the conformal layer  1016  disposed within the wells  610 . The etch can be a wet etch, a plasma etch, or a chemical mechanic planarization process. 
     As illustrated at block  910 , the conformal layer can be etched. For example, conformal layer can be etched using a wet etch process, a plasma etch process, or a chemical mechanical polishing process. In a particular example, the conformal layers are etched using a plasma process. Further, the photoresist can be removed, as illustrated at block  912 , such as through ashing. As illustrated in  FIG. 12 , the etching and ashing process can leave a non-uniform native oxide layer  1220  over a surface of the conductive layer  1216  formed from the conformal conductive layer  1016 . 
     As illustrated at block  914 , the sensor surface can be annealed in a hydrogen containing atmosphere. Such annealing can deplete oxygen from regions of the sensor pad or conformal layer, including thin native oxide. Optionally, the surface can be etched to remove the oxygen depleted regions, as illustrated at block  916  providing for a uniform surface over the conductive layer  1216 . In particular, the etch is sufficient to remove  1 - 10  nanometers. For example, as illustrated in  FIG. 13 , a uniform surface  1322  is formed over the conductive layer  1216 . 
     Returning to  FIG. 9 , the surface can be further annealed in an oxygen containing atmosphere, as illustrated block  918 . For example, the sensor pads can be exposed to temperatures of less than 400° C., such as in a range of 290° C. to 380° C., a range of 300° C. to 370° C., a range of 330° C. to 360° C., or a range of 345° C. to 355° C. As illustrated in  FIG. 14 , a uniform oxide layer  1424  can be formed over the conductive layer  1216 , extending the sensor surface area of the sensor pad  504 . 
     In a first embodiment, a method for manufacturing a sensor includes etching an insulator layer disposed over a substrate to define an opening exposing a sensor surface of a sensor disposed on the substrate. A native oxide forming on the sensor surface. The method further includes annealing the sensor surface in a hydrogen containing atmosphere and annealing the senor surface in an oxygen atmosphere at a temperature of not greater than 400° C. 
     In an example of the first embodiment, annealing the sensor surface in an oxygen containing atmosphere includes annealing at a temperature in a range of 290° C. to 380° C. For example, the temperature is in a range of 300° C. to 370° C., such as in a range of 330° C. to 360° C. 
     In another example of the first embodiment and the above examples, annealing the sensor surface in an oxygen containing atmosphere includes annealing for a period in a range of 5 minutes to 120 minutes. For example, the period is in a range of 10 minutes to 80 minutes, such as in a range of 15 minutes to 60 minutes. 
     In a further example of the first embodiment and the above examples, annealing in a hydrogen containing atmosphere includes annealing at a temperature in a range of 350° C. to 500° C. For example, annealing in the hydrogen containing atmosphere includes annealing at a temperature in a range of 400° C. to 450° C. 
     In an additional example of the first embodiment and the above examples, the method further includes etching the sensor surface following annealing in a hydrogen containing atmosphere and before annealing in an oxygen containing atmosphere. For example, etching the sensor surface following annealing in the hydrogen containing atmosphere includes etching in a vapor hydrogen fluoride atmosphere. 
     In another example of the first embodiment and the above examples, etching includes a plasma etch, a wet etch, or a combination thereof. For example, the plasma etch includes a fluorine containing plasma etch. 
     In a further example of the first embodiment and the above examples, the method further includes depositing the insulator layer by chemical vapor deposition. For example, the insulator layer includes silicon dioxide, silicon nitride, a silicon oxide formed from tetraethylortho silicate, or a combination thereof. 
     In an additional example of the first embodiment and the above examples, the sensor is formed from zinc, copper, aluminum, tantalum, titanium, tungsten, gold, silver, oxides thereof, nitrides thereof, or combinations thereof. 
     In a second embodiment, a method for manufacturing a sensor includes applying a conformal conductive layer over an insulator layer and in an opening exposing a sensor surface of a sensor disposed on a substrate and removing the conformal conductive layer from an upper surface of the insulator to form conductive cup structures disposed in the openings and contacting the sensor surface. A native oxide forms on the conductive cup structure. The method further includes annealing the conductive cup structure in a hydrogen containing atmosphere and annealing the conductive cup structure in an oxygen atmosphere at a temperature of not greater than 400° C. 
     In an example of the second embodiment, applying the conformal conductive layer includes sputtering a conductive material. For example, the conductive material comprises titanium, tantalum, hafnium, tungsten, aluminum, copper, gold, silver, or any combination thereof. 
     In another example of the second embodiment and the above examples, removing the conformal conductive layer includes coating the conformal conductive layer with a photoresist layer. In an example, the method further includes etching the photoresist and conformal conductive layer to remove the conductive layer from the upper surface of the insulator and removing the photoresist. In a further example, the method includes planarizing to remove the conformal conductive layer from the upper surface of the insulator and removing the photoresist. 
     In a further example of the second embodiment and the above examples, the method further includes supporting the sensor surface with a noble gas prior to applying the conformal conductive coating. For example, sputtering with a noble gas include sputtering with argon. In an example, sputtering includes sputtering at a power in a range of 100 W to 400 W. 
     In an additional example of the second embodiment and the above examples, annealing the conductive cup structure in an oxygen containing atmosphere includes annealing at a temperature in a range of 290° C. to 380° C. For example, the temperature is in a range of 300° C. to 370° C., such as in a range of 330° C. to 360° C. 
     In another example of the second embodiment and the above examples, annealing the conductive cup structure in an oxygen containing atmosphere includes annealing for a period in a range of 5 minutes to 120 minutes. For example, the period is in a range of 10 minutes to 80 minutes, such as in a range of 15 minutes to 60 minutes. 
     In a further example of the second embodiment and the above examples, annealing in a hydrogen containing atmosphere includes annealing at a temperature in a range of 350° C. to 500° C. For example, annealing in the hydrogen containing atmosphere includes annealing at a temperature in a range of 400° C. to 450° C. 
     In an additional example of the second embodiment and the above examples, the method further includes etching the conductive cup structure following annealing in a hydrogen containing atmosphere and before annealing in an oxygen containing atmosphere. 
     In another example of the second embodiment and the above examples, etching the conductive cup structure following annealing in the hydrogen containing atmosphere includes etching in a vapor hydrogen fluoride atmosphere. 
     In a further example of the second embodiment and the above examples, the sensor is formed from zinc, copper, aluminum, tantalum, titanium, tungsten, gold, silver, oxides thereof, nitrides thereof, or combinations thereof. 
     In a third embodiment, an apparatus includes a substrate including circuitry formed on or in the substrate, the circuitry including an array of sensors, each sensor has a sensor surface associated with an opening and formed by the method of any one of the above embodiments and examples. 
     Note that not all of the activities described above in the general description or the examples are required, that a portion of a specific activity may not be required, and that one or more further activities may be performed in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed. 
     In the foregoing specification, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive-or and not to an exclusive-or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     Also, the use of “a” or “an” are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. 
     After reading the specification, skilled artisans will appreciate that certain features are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range.