US20140264472A1 - Chemical sensor with consistent sensor surface areas - Google Patents

Abstract

In one 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 material defines an opening extending to the upper surface of the floating gate conductor, the material comprising a first dielectric underlying a second dielectric. A conductive element contacts the upper surface of the floating gate conductor and extending a distance along a sidewall of the opening.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application claims priority to U.S. Provisional Application No. 61/900,907 filed Nov. 6, 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
• The present disclosure 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, Rothberg et al., U.S. patent application Ser. No. 12/002,291 (now U.S. Pat. No. 7,948,015), filed Dec. 14, 2009, based on U.S. Prov. Pat. Appl. Nos. 60/956,324, filed Aug. 16, 2007, 60/968,748, filed Jul. 10, 2007, and 60/870,073, filed Dec. 14, 2006, 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. In addition, chemical sensor performance variation across the array results in undesirable differences in the sensor output signals, which further complicates the downstream signal processing. It is therefore desirable to provide devices including low noise chemical sensors, and methods for manufacturing such devices.
SUMMARY
• In one 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 material defining an opening extending to the upper surface of the floating gate conductor, the material comprising a first dielectric underlying a second dielectric; and a conductive element contacting the upper surface of the floating gate conductor and extending a distance along a sidewall of the opening. In an exemplary embodiment, the opening of the chemical sensor may include a lower portion within the first dielectric, and an upper portion within the second dielectric. In another embodiment, a width of the lower portion of the opening is substantially the same as a width of the upper portion. In yet another embodiment, the conductive element is conformal with a shape of the opening. In one embodiment, the conductive element extends to an upper surface of the second dielectric. In an exemplary embodiment, the conductive element includes an inner surface defining a lower portion of a reaction region for the chemical sensor, and the second dielectric includes an inner surface defining an upper portion of the opening. In an exemplary embodiment, the conductive element comprises an electrically conductive material, and an inner surface of the conductive element includes an oxide of the electrically conductive material. In another embodiment, a sensing surface of the chemical sensor includes an inner surface of the conductive element. In yet another embodiment, the chemically-sensitive field effect transistor generates a sensor signal in response to a chemical reaction occurring proximate to the conductive element.
• In one embodiment, the floating gate conductor comprises a plurality of conductors electrically coupled to one another and separated by dielectric layers, and the floating gate conductor is an uppermost conductor in the plurality of conductors.
• In another 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; forming a material defining an opening extending to the upper surface of the floating gate conductor, the material comprising a first dielectric underlying a second dielectric; and forming a conductive element contacting the upper surface of the floating gate conductor and extending a distance along a sidewall of the opening. In an exemplary embodiment, forming the material and forming the conductive element may include forming the first dielectric on the floating gate conductor, the first dielectric defining a cavity extending to the upper surface of the floating gate conductor; forming the second dielectric thereon; etching the second dielectric to expose the conductive element, thereby defining an opening; and forming the conductive element within the opening. According to another embodiment, forming the conductive element within the opening may include depositing a conductive material within the opening and on an upper surface of the first dielectric; and removing at least a portion of the conductive material from the upper surface of the second dielectric. In yet another embodiment, removing at least the portion of the conductive material may comprises depositing a layer of photoresist within the opening; and removing at least a portion of the conductive material together with the photoresist from the upper surface of the second dielectric. In one embodiment, the conductive material comprises titanium. In an exemplary embodiment, the opening is a nanowell. In an exemplary embodiment, the forming a conductive element includes depositing a conductive material conformally within the opening. In another embodiment, the conductive element includes an inner surface defining a lower portion of a reaction region for the chemical sensor, and the second dielectric includes an inner surface defining an upper portion of the opening.
• Particular aspects of one embodiment 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 a cross-sectional view of two representative chemical sensors and their corresponding reaction regions according to a first embodiment.
• FIGS. 4 to 12 illustrate stages in a manufacturing process for forming an array of chemical sensors and corresponding reaction regions according to a first embodiment.
• FIGS. 13 to 25 illustrate stages in a manufacturing process for forming an array of chemical sensors and corresponding reaction regions according to a second embodiment.
DETAILED DESCRIPTION
• A chemical detection device is described that includes 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 significantly 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 significantly reduce the signal-to-noise ratio (SNR) of the sensor output signal, which affects 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 a generally horizontal portion along the bottom surface of the reaction region, as well as a generally vertical portion extending along a sidewall of an opening containing the reaction region. The distance that the generally vertical portion extends along the sidewall is defined by the thickness of a dielectric material that forms a lower portion of the opening. The dielectric material can be deposited using a process (e.g. thin film deposition) which results in very small thickness variation across the array. In doing so, the sensor surface areas of the chemical sensors can be very well controlled, resulting in uniform chemical sensor performance across the array and thus simplifying the downstream signal processing. By extending the sensing surface in the generally vertical direction, 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 sensing surface extends a controlled distance up the sidewall, 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 aflow cell101 on anintegrated circuit device100, areference electrode108, a plurality ofreagents114 for sequencing, avalve block116, awash solution110, avalve112, afluidics controller118,lines120/122/126,passages104/109/111, awaste container106, anarray controller124, and auser interface128. Theintegrated circuit device100 includes amicrowell array107 overlying a sensor array that includes chemical sensors as described herein. Theflow cell101 includes aninlet102, anoutlet103, and aflow chamber105 defining a flow path for thereagents114 over themicrowell array107. Thereference electrode108 may be of any suitable type or shape, including a concentric cylinder with a fluid passage or a wire inserted into a lumen ofpassage111. Thereagents114 may be driven through the fluid pathways, valves, and flowcell101 by pumps, gas pressure, vacuum, or other suitable methods, and may be discarded into thewaste container106 after exiting theoutlet103 of theflow cell101. Thefluidics controller118 may control driving forces for thereagents114 and the operation ofvalve112 and valve block116 with suitable software.
• Themicrowell array107 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. Themicrowell array107 may be integrated in theintegrated circuit device100, so that themicrowell array107 and the sensor array are part of a single device or chip. Theflow cell101 may have a variety of configurations for controlling the path and flow rate ofreagents114 over themicrowell array107. Thearray controller124 provides bias voltages and timing and control signals to theintegrated circuit device100 for reading the chemical sensors of the sensor array. Thearray controller124 also provides a reference bias voltage to thereference electrode108 to bias thereagents114 flowing over themicrowell array107.
• During an experiment, thearray controller124 collects and processes output signals from the chemical sensors of the sensor array through output ports on theintegrated circuit device100 viabus127. Thearray controller124 may be a computer or other computing means. Thearray controller124 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 inFIG. 1. In the illustrated embodiment, thearray controller124 is external to theintegrated circuit device100. In some alternative embodiments, some or all of the functions performed by thearray controller124 are carried out by a controller or other data processor on theintegrated circuit device100. The values of the output signals from the chemical sensors indicate physical and/or chemical parameters of one or more reactions taking place in the corresponding reaction regions in themicrowell array107. 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. Nos. 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, each of which are incorporated by reference herein. Theuser interface128 may display information about theflow cell101 and the output signals received from chemical sensors in the sensor array on theintegrated circuit device100. Theuser interface128 may also display instrument settings and controls, and allow a user to enter or set instrument settings and controls.
• Thefluidics controller118 may control delivery of theindividual reagents114 to theflow cell101 andintegrated circuit device100 in a predetermined sequence, for predetermined durations, at predetermined flow rates. Thearray controller124 can then collect and analyze the output signals of the chemical sensors indicating chemical reactions occurring in response to the delivery of thereagents114. During the experiment, the system may also monitor and control the temperature of theintegrated circuit device100, 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 thereference electrode108 throughout an entire multi-step reaction during operation. Thevalve112 may be shut to prevent anywash solution110 from flowing intopassage109 as thereagents114 are flowing. Although the flow of wash solution may be stopped, there may still be uninterrupted fluid and electrical communication between thereference electrode108,passage109, and themicrowell array107. The distance between thereference electrode108 and the junction betweenpassages109 and111 may be selected so that little or no amount of the reagents flowing in passage109 (and possibly diffusing into passage111) reach thereference electrode108. In an exemplary embodiment, thewash solution110 may be selected as being in continuous contact with thereference electrode108, 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 theintegrated circuit device100 and flowcell101. Theintegrated circuit device100 includes themicrowell array107 of reaction regions operationally associated withsensor array205. During operation, theflow chamber105 of theflow cell101 confines areagent flow208 of delivered reagents across open ends of the reaction regions in themicrowell array107. 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 thesensor array205 are responsive to (and generate output signals related to) chemical reactions within associated reaction regions in themicrowell array107 to detect an analyte or reaction property of interest. The chemical sensors of thesensor array205 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 Schultz et al., U.S. patent application Ser. No. 12/785,667 (now U.S. Pat. No. 8,546,128), filed May 24, 2010, titled “Fluidics System for Sequential Delivery of Reagents”; Rotherberg et al., U.S. patent application Ser. No. 12/721,458 (now U.S. Pat. No. 8,306,757), filed Mar. 10, 2010, titled “Methods and Apparatus for Measuring Analytes Using Large Scale FET Arrays”; Rotherberg et al., U.S. patent application Ser. No. 12/475,311, filed May 29, 2009, titled “Methods and Apparatus for Measuring Analytes”; Rotherberg et al., U.S. patent application Ser. No. 12/474,897, filed May 29, 2009, titled “Methods and Apparatus for Measuring Analytes”; Rotherberg et al., U.S. patent application Ser. No. 12/002,781, filed Dec. 17, 2007, titled “Methods and Apparatus for Measuring Analytes Using Large Scale FET Arrays”; and U.S. patent application Ser. No. 12/474,897 (now U.S. Pat. No. 7,575,865) filed Aug. 1, 2005, titled “Methods of Amplifying and Sequencing Nucleic Acids”, each of which are 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 a first embodiment. InFIG. 3, twochemical sensors350,351 are shown, representing a small portion of a sensor array that can include millions of chemical sensors.Chemical sensor350 is coupled tocorresponding reaction region301, andchemical sensor351 is coupled tocorresponding reaction region302.Chemical sensor350 is representative of the chemical sensors in the sensor array. In the illustrated example, thechemical sensor350 is a chemically-sensitive field effect transistor (chemFET), more specifically an ion-sensitive field effect transistor (ISFET) in this example. Thechemical sensor350 includes a floatinggate structure318 having asensor plate320 coupled to thereaction region301 by an electricallyconductive element370. As can be seen inFIG. 3, thesensor plate320 is the uppermost floating gate conductor in the floatinggate structure318. In the illustrated example, the floatinggate structure318 includes multiple patterned layers of conductive material within layers ofdielectric material319.
• Thechemical sensor350 also includes asource region321 and adrain region322 within asemiconductor substrate354. Thesource region321 and thedrain region322 comprise doped semiconductor material have a conductivity type different from the conductivity type of thesubstrate354. For example, thesource region321 and thedrain region322 may comprise doped P-type semiconductor material, and the substrate may comprise doped N-type semiconductor material.Channel region323 separates thesource region321 and thedrain region322. The floatinggate structure318 overlies thechannel region323, and is separated from thesubstrate354 by agate dielectric352. Thegate dielectric352 may be for example silicon dioxide. Alternatively, other dielectrics may be used for thegate dielectric352.
• As shown inFIG. 3, thereaction region301 is within an opening having asidewall303 extending throughdielectric materials310,308 to the upper surface of thesensor plate320. Each of thedielectric materials310,308 may comprise one or more layers of material, such as silicon dioxide or silicon nitride. The opening includes alower portion314 withindielectric material308 and proximate to thesensor plate320. The opening also includes anupper portion315 within thedielectric material310 and extending from thelower portion314 to the upper surface of thedielectric material310. In the illustrated embodiment, the width of theupper portion315 of the opening is substantially the same as the width of thelower portion314 of the opening. However, depending on the material(s) and/or etch process used to create the opening, the width of theupper portion315 of the opening may be greater than the width of thelower portion314 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.
• Thelower portion314 of the opening includes the electricallyconductive element370 on thesidewall303 of thedielectric material310. In the illustrated embodiment, theinner surface371 of the electricallyconductive element370 defines a lower segment of thereaction region301. That is, there is no intervening deposited material layer between theinner surface371 of the electricallyconductive element370 and thereaction region301 for thechemical sensor350. As a result of this structure, theinner surface371 of the electricallyconductive element370 is conformal to the opening and acts as the sensing surface for thechemical sensor350. It should be understood by those skilled in the art that precise shape and dimension of the electricallyconductive element370, as with all other materials illustrated in the figures, is process dependant.
• In the illustrated embodiment, the electricallyconductive element370 is a conformal layer of material within thelower portion314 of the opening, such that the electricallyconductive element370 extends across the upper surface of thesensor plate320. In the illustrated embodiment, the electricallyconductive element370 extends beyond thelower portion314 of the opening and into theupper portion315 of the opening. The inner surface of thedielectric material310 defines an upper segment of thereaction region301. Theconductive element370 may for example extend along at least 5% of thesidewall303, at least 10%, at least 25%, at least 50%, at least 75%, or at least 85% of thesidewall303, or even extend along 99% of thesidewall303. The conformalinner surface371 of the electricallyconductive element370 allows thechemical sensor350 to have a small plan view area, while also having a sufficiently large surface area to avoid the noise issues associated with small sensing surfaces. The plan view area of thechemical sensor350 is determined in part by the width (or diameter) of thereaction region301 and can be made small, allowing for a high density array. In addition, because the sensing surface extends up thesidewall303, the sensing surface area depends upon the distance of this extension and the circumference of thereaction region301, and can be relatively large. As a result, lownoise chemical sensors350,351 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 electricallyconductive element370 may be grown which acts as a sensing material (e.g. an ion-sensitive sensing material) for thechemical sensor350. 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 one embodiment the electricallyconductive element370 may be titanium nitride, and titanium oxide or titanium oxynitride may be grown on theinner surface371 of theconductive element370 during manufacturing and/or during exposure to solutions during use. In the illustrated example, the electricallyconductive element370 is shown as a single layer of material. More generally, the electricallyconductive element370 may comprise one or more layers of a variety of electrically conductive materials, such as metals or ceramics, depending upon the embodiment. 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 theconductive element370 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, 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 embodiment.
• In operation, reactants, wash solutions, and other reagents may move in and out of thereaction region301 by adiffusion mechanism340. Thechemical sensor350 is responsive to (and generates an output signal related to) the amount of charge324 proximate to theconductive element370. The presence of charge324 in an analyte solution alters the surface potential at the interface between theconductive element370 and the analyte solution within thereaction region301. Changes in the charge324 cause changes in the voltage on the floatinggate structure318, which in turn changes in the threshold voltage of the transistor. This change in threshold voltage can be measured by measuring the current in thechannel region323 between thesource region321 and adrain region322. As a result, thechemical sensor350 can be used directly to provide a current-based output signal on an array line connected to thesource region321 or drainregion322, or indirectly with additional circuitry to provide a voltage-based output signal. Because the charge324 may be more highly concentrated near the bottom of thereaction region301, the distance that theconductive element370 extends up thesidewall303 of the opening is a tradeoff between the amplitude of the desired signal detected in response to the charge324, and the fluidic noise due to random fluctuation of charge between theconductive element370 and the analyte solution. Increasing the distance that theconductive element370 extends up thesidewall303 increases the fluidic interface area for thechemical sensor350, which acts to reduce the fluidic noise. However, due to the diffusion of charge324 out of thereaction region301, the concentration of charge324 decreases with distance from the bottom of thereaction region301. As a result, upper sidewall segments of theconductive element370 detect portions of the signal from areas having lower charge concentration, which can reduce the overall amplitude of the desired signal detected by thesensor350. In contrast, decreasing the distance theconductive element370 extends up thesidewall303 reduces the sensing surface area and thus increases the fluidic noise, but also increases the overall amplitude of the desired signal detected by thesensor350.
• For a very small sensing surface area, Applicants have found that the fluidic noise changes as a function of the sensing surface area differently than the amplitude of the desired signal. Because the SNR of the sensor output signal is the ratio of these two quantities, there is an optimal distance theconductive element370 extends along thesidewall303 at which SNR is maximum. The optimal distance can vary from embodiment to embodiment depending on the material characteristics of theconductive element370 and thedielectric material310, the volume, shape, aspect ratio (such as base width-to-well depth ratio), and other dimensional characteristics of the reaction regions, the nature of the reaction taking place, as well as the reagents, byproducts, or labeling techniques (if any) that are employed. The optimal distance may for example be determined empirically.
• As described in more detail below with respect toFIGS. 4 to 12, the distance theconductive element370 extends along thesidewall303 is defined by the etch time of the deposited layer, for example. Thedielectric material310 and electricallyconductive element370 can be etched using a timed etch process, for example, which results in selectivity of distance309 (e.g. thedistance dielectric material310 extends beyond electrically conductive element370). In doing so, the sensor surface areas of the chemical sensors can be controlled, resulting in uniform chemical sensor performance across the array and simplifying the downstream signal processing.
• In an embodiment, reactions carried out in thereaction region301 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 electricallyconductive element370. 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 thereaction region301 at the same time in order to increase the output signal generated. In an embodiment, multiple copies of an analyte may be attached to asolid phase support312, as shown inFIG. 3, either before or after deposition into thereaction region301. Thesolid phase support312 may be microparticles, nanoparticles, beads, solid or porous gels, or the like. For simplicity and ease of explanation,solid phase support312 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, 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 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's3′ 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. 4 to 12 illustrate stages in a manufacturing process for forming an array of chemical sensors and corresponding reaction regions according to a first embodiment.FIG. 4 illustrates astructure400 formed in a first stage. Thestructure400 includes the floating gate structures (e.g. floating gate structure318) for thechemical sensors350,351. Thestructure400 can be formed by depositing a layer of gate dielectric material on thesemiconductor substrate354, 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 dielectric352) 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 region321 and a drain region322) of the chemical sensors. A first layer of thedielectric material319 can then be deposited over the lowermost conductive material elements. Conductive plugs can then be formed within vias etched in the first layer ofdielectric material319 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 thedielectric material319 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 floatinggate structure318 shown inFIG. 4. Alternatively, other and/or additional techniques may be performed to form the structure. Forming thestructure400 inFIG. 4 can also include forming additional elements such as array lines (e.g. row lines, column lines, etc.) for accessing the chemical sensors, additional doped regions in thesubstrate354, and other circuitry (e.g. select switches, 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 Schultz et al., U.S. patent application Ser. No. 12/785,667 (now U.S. Pat. No. 8,546,128), filed May 24, 2010, titled “Fluidics System for Sequential Delivery of Reagents”; Rotherberg et al., U.S. patent application Ser. No. 12/721,458 (now U.S. Pat. No. 8,306,757), filed Mar. 10, 2010, titled “Methods and Apparatus for Measuring Analytes Using Large Scale FET Arrays”; Rotherberg et al., U.S. patent application Ser. No. 12/475,311, filed May 29, 2009, titled “Methods and Apparatus for Measuring Analytes”; Rotherberg et al., U.S. patent application Ser. No. 12/474,897, filed May 29, 2009, titled “Methods and Apparatus for Measuring Analytes”; Rotherberg et al., U.S. patent application Ser. No. 12/002,781, filed Dec. 17, 2007, titled “Methods and Apparatus for Measuring Analytes Using Large Scale FET Arrays”; and U.S. patent application Ser. No. 12/474,897 (now U.S. Pat. No. 7,575,865) filed Aug. 1, 2005, titled “Methods of Amplifying and Sequencing Nucleic Acids”, which were incorporated by reference in their entirety above.
• Next,dielectric material308 having a given thickness is deposited on thestructure400 illustrated inFIG. 4, resulting in the structure illustrated inFIG. 5. Thedielectric material308 comprises one or more dielectric layers of dielectric. Thedielectric material308 may be deposited using a process which results in very small variation in the thickness across the array. For example, thedielectric material308 may comprise silicon oxide and be deposited using high density plasma (HDP) deposition. Various other techniques may be used, 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. Next, thedielectric material308 of the structure inFIG. 5 is etched to formcavities600,602 extending to the upper surfaces of the floating gate structures of thechemical sensors350,351, resulting in the structure illustrated inFIG. 6. Thecavities600,602 may for example be formed by using a lithographic process to pattern a layer of photoresist on thedielectric material308 to define the locations of thecavities600,602, and then anisotropically etching thedielectric material308 using the patterned photoreist as an etch mask. The anisotropic etching of thedielectric material308 may for example be a dry etch process, such as a fluorine based Reactive Ion Etching (RIE) process. Next,dielectric material310 is formed on the structure illustrated inFIG. 6, resulting in the structure illustrated inFIG. 7. Thedielectric material310 may comprise one or more layers of deposited dielectric material, such as silicon dioxide or silicon nitride.
• Next,dielectric material310 is etched to form openings definingreaction regions301,302 extending to thesensor plate320, resulting in the structure illustrated inFIG. 8. Next, a conformal layer ofconductive material900 is deposited on the structure illustrated inFIG. 8, resulting in the structure illustrated inFIG. 9. Theconductive material900 comprises one or more layers of electrically conductive material. For example, theconductive material900 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 theconductive element370. In addition, more than one layer of conductive material may be deposited. Theconductive material900 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. In contemplated embodiments,conductive element370 may be formed to take various shapes and thicknesses that may be determined based on material and techniques described above. For example, instead of the depiction ofconductive element370 as illustrated inFIG. 3, one may imagineconductive element370 is formed such that it is conformal with the solid support (rounded to follow the shape of solid support312).
• Next,material1000 is formed on the structure illustrated inFIG. 9, resulting in the structure illustrated inFIG. 10. Thematerial1000 may comprise one or more layers of deposited dielectric material, such as silicon dioxide or silicon nitride. Alternatively,material1000 may comprise photoresist. In one embodiment, where thematerial1000 comprises photoresist, a partial etch ofmaterial1000 andconductive material900 is performed such thatdistance309 ofdielectric material310 is revealed (that is,distance309 ofsidewall303 is exposed), resulting in the structure illustrated inFIG. 11.Material1000 and electricallyconductive material900 may be etched together or separately depending on the process and/or material(s) used. For example, a partial etch may be performed using at least one of an O2 resist etch, Ar sputter breakthrough etch, and Hydrogen Bromide Titanium etch. Next,material1000 is etched to form openings definingreaction regions301,302 extending to theconductive element370,900, resulting in the structure illustrated inFIG. 12. In one embodiment, residual photoresist may need to be cleaned from the opening using techniques known to those skilled in the art, for example, O2 plasma ash.
• FIG. 13 illustrates a cross-sectional view of two representative chemical sensors and their corresponding reaction regions according to a second embodiment. The structure of the two representative chemical sensors illustrated inFIG. 13 differs in one aspect from the two representative chemical sensors illustrated inFIG. 3 in thatFIG. 13 includes vias oversensor plates320 on top of which the microwells/nanowells are built. Accordingly, fabrication for the structure inFIG. 3 is different from fabrication ofFIG. 13, as is explained in greater detail below.
• FIGS. 14-25 illustrate stages in a manufacturing process for forming an array of chemical devices and corresponding well structures according to an exemplary embodiment.FIG. 14 illustrates astructure1400 including the floating gate structures (e.g. floating gate structure318) for thechemical devices350,351. Thestructure1400 can be formed in accordance with thestructure400 described in detail above with reference toFIG. 4. As illustrated in thestructure1500 illustrated inFIG. 15, adielectric material1503 may be formed on thesensor plate320 of the field effect transistor of thechemical device350. Next, as illustrated inFIG. 16, thedielectric material1503 of thestructure1500 inFIG. 15 is etched to formopenings1618,1620 (for vias) extending to the upper surfaces of the floating gate structures of thechemical devices350,351, resulting in thestructure1600 illustrated inFIG. 16. Theopenings1618,1620 may, for example, be formed by using a lithographic process to pattern a layer of photoresist on thedielectric material1503 to define the locations of theopenings1618,1620, and then anisotropically etching thedielectric material1503 using the patterned photoreist as an etch mask. The anisotropic etching of thedielectric material1503 may, for example, be a dry etch process, such as a fluorine based Reactive Ion Etching (RIE) process. In the illustrated embodiment, theopenings1618,1620 are separated by adistance1630 and theopenings1618,1620 are of a suitable dimension for a via. For example, theseparation distance1630 may be a minimum feature size for the process (e.g. a lithographic process) used to form theopenings1618,1620. In such a case, thedistance1630 may be significantly more than thewidth1620. Next, a layer ofconductive material1704 is deposited on thestructure1600 illustrated inFIG. 16, resulting in thestructure1700 illustrated inFIG. 17.Conductive material1704 may be referred to as a conductive liner. Theconductive material1704 may comprise one or more layers of electrically conductive material. For example, theconductive material1704 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. Theconductive material1704 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.
• Next, a layer ofconductive material1805 such as tungsten, for example, is deposited on thestructure1700 illustrated inFIG. 17, resulting in thestructure1800 illustrated inFIG. 18. Theconductive material1805 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. or any other suitable techniques. Next,conductive material1704 andconductive material1805 are planarized using a Chemical Mechanical Planarization (CMP) process, for example, resulting in thestructure1900 illustrated inFIG. 19. As an optional, additional step, a via barrier liner (not shown) may be formed on the planarizedconductive material1704 andconductive material1805. For example, the via barrier liner may comprise titanium nitride.
• Next,dielectric material2006 is formed on the structure illustrated inFIG. 19, resulting in the structure illustrated inFIG. 20. Thedielectric material2006 may comprise one or more layers of deposited dielectric material, such as silicon dioxide or silicon nitride. Next,dielectric material2006 is etched to form openings extending to planarizedconductive material1704 andconductive material1805 anddielectric material1503, resulting in the structure illustrated inFIG. 21.Dielectric material1503 may be partially etched when the openings are formed such thatconductive material1704 andconductive material1805 are raised abovedielectric material1503 and protrude into the opening, as seen in the illustrated embodiment. Next, a conformal layer ofconductive material2200 is deposited on the structure illustrated inFIG. 21, resulting in the structure illustrated inFIG. 22. Theconductive material2200 comprises one or more layers of electrically conductive material. For example, theconductive material2200 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 theconductive element370. In addition, more than one layer of conductive material may be deposited. Theconductive material2200 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.
• Next,material2300 is formed on the structure illustrated inFIG. 22, resulting in the structure illustrated inFIG. 23. Thematerial2300 may comprise one or more layers of deposited dielectric material, such as silicon dioxide or silicon nitride. Alternatively,material2300 may comprise photoresist. In one embodiment, where thematerial2300 comprises photoresist, a partial etch ofmaterial2300 andconductive material2200 is performed such thatdistance1309 ofdielectric material310 is revealed (that is,distance309 ofsidewall1303 is exposed), resulting in the structure illustrated inFIG. 24.Material2300 andconductive material2200 may be etched together or separately depending on the process and/or material(s) used. For example, a partial etch may be performed using at least one of an O2 resist etch, Ar sputter breakthrough etch, and Hydrogen Bromide Titanium etch. Next,material2300 is etched to form openings definingreaction regions301,302 extending to theconductive elements370,2200, resulting in the structure illustrated inFIG. 25. In one embodiment, residual photoresist may need to be cleaned from the opening using techniques known to those skilled in the art, for example, O2 plasma ash.
• 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.

Claims (19)

What is claimed is:

1. A chemical sensor comprising:

a chemically-sensitive field effect transistor including a floating gate conductor having an upper surface;

a material defining an opening extending to the upper surface of the floating gate conductor, the material comprising a first dielectric underlying a second dielectric; and

a conductive element contacting the upper surface of the floating gate conductor and extending a distance along a sidewall of the opening.

2. The chemical sensor ofclaim 1, wherein the opening includes a lower portion within the first dielectric, and an upper portion within the second dielectric.

3. The chemical sensor ofclaim 2, wherein a width of the lower portion of the opening is substantially the same as a width of the upper portion.

4. The chemical sensor ofclaim 2, wherein the conductive element is conformal with a shape of the opening.

5. The chemical sensor ofclaim 1, wherein the conductive element extends to an upper surface of the second dielectric.

6. The chemical sensor ofclaim 1, wherein the conductive element includes an inner surface defining a lower portion of a reaction region for the chemical sensor, and the second dielectric includes an inner surface defining an upper portion of the opening.

7. The chemical sensor ofclaim 1, wherein the conductive element comprises an electrically conductive material, and an inner surface of the conductive element includes an oxide of the electrically conductive material.

8. The chemical sensor ofclaim 1, wherein a sensing surface of the chemical sensor includes an inner surface of the conductive element.

9. The chemical sensor ofclaim 1, wherein the chemically-sensitive field effect transistor generates a sensor signal in response to a chemical reaction occurring proximate to the conductive element.

10. The chemical sensor ofclaim 1, wherein the floating gate conductor comprises a plurality of conductors electrically coupled to one another and separated by dielectric layers, and the floating gate conductor is an uppermost conductor in the plurality of conductors.

11. A method for manufacturing a chemical sensor, the method comprising:

forming a chemically-sensitive field effect transistor including a floating gate conductor having an upper surface;

forming a material defining an opening extending to the upper surface of the floating gate conductor, the material comprising a first dielectric underlying a second dielectric; and

forming a conductive element contacting the upper surface of the floating gate conductor and extending a distance along a sidewall of the opening.

12. The method ofclaim 11, wherein forming the material and forming the conductive element include:

forming the first dielectric on the floating gate conductor, the first dielectric defining a cavity extending to the upper surface of the floating gate conductor;

forming the second dielectric thereon;

etching the second dielectric to expose the conductive element, thereby defining an opening; and

forming the conductive element within the opening.

13. The method ofclaim 12, wherein forming the conductive element within the opening comprises:

depositing a conductive material within the opening and on an upper surface of the first dielectric; and

removing at least a portion of the conductive material from the upper surface of the second dielectric.

14. The method ofclaim 13, wherein removing at least the portion of the conductive material comprises:

depositing a layer of photoresist within the opening and; and

removing at least a portion of the conductive material together with the photoresist from the upper surface of the second dielectric.

15. The method ofclaim 14, further comprising removing remaining photoresist.

16. The method ofclaim 11, wherein the conductive material comprises titanium.

17. The method ofclaim 11, wherein the opening is a nanowell.

18. The method ofclaim 11, wherein the forming a conductive element includes depositing a conductive material conformally within the opening.

19. The method ofclaim 11, wherein the conductive element includes an inner surface defining a lower portion of a reaction region for the chemical sensor, and the second dielectric includes an inner surface defining an upper portion of the opening

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