JP5541530B2 - Ion sensor and ion concentration measuring method - Google Patents

Description

  The present invention relates to an ion sensor or the like that measures the ion concentration of a liquid to be measured based on the output of a working electrode and the output of a reference electrode. In particular, the present invention relates to a pH sensor among ion sensors.

  Japanese Patent Application Laid-Open No. 9-005290 discloses a glass electrode pH sensor that uses a glass electrode that functions as a working electrode to measure the pH of a liquid to be measured by detecting a potential difference generated inside and outside the glass film. ing. In the pH sensor disclosed in Japanese Patent Laid-Open No. 9-005290, an internal electrode is disposed inside a glass film, and a reference electrode is provided in an internal liquid such as a potassium chloride solution (KCI solution) filled in the holder. Be placed. A liquid junction ceramic is attached to the holder. At the time of pH measurement, the glass film and the liquid junction ceramic are immersed in the liquid to be measured. At this time, the internal electrode of the working electrode is electrically connected to the inner wall of the glass film via the internal liquid. On the other hand, the reference electrode is electrically connected to the outer wall of the glass film via the internal liquid, the liquid junction ceramic, and the liquid to be measured. Therefore, the pH of the liquid to be measured can be measured by detecting the potential difference between the reference electrode and the internal electrode of the working electrode.

  Japanese Unexamined Patent Application Publication No. 2009-236687 includes first and second ion-sensitive field effect transistors (ISFETs), which differ in sensitivity (Nernst responsiveness) to both measurement target ions. An ion sensor is disclosed.

JP 9-005290 A JP 2007-78373 A JP 2009-236687 A

  However, in the glass electrode type pH sensor, contamination of the liquid to be measured due to leakage of the internal liquid in the holder in which the reference electrode is accommodated to the liquid to be measured, state change due to water evaporation of the internal liquid, Problems such as crystallization may occur.

  Further, in the ion sensor disclosed in Japanese Patent Application Laid-Open No. 2009-236687, a self-assembled monolayer (SAM) is used for the ion sensitive part and the ion insensitive part of the ISFET. For this reason, physical / chemical instability of the self-assembled monolayer becomes a problem in a high-temperature and high-pressure state of a chemical synthesis plant or a strong acid or strong alkali process of a semiconductor manufacturing process, which makes it difficult to use. In bioprocesses that handle biological materials such as proteins, protein and the like adsorb to SAM, making accurate measurement difficult.

  The objective of this invention is providing the ion sensor etc. which can be used also in a severe environment while solving the malfunction of a reference pole.

The ion sensor of the present invention includes a reference electrode and a working electrode, and the ion sensor measures the ion concentration of the liquid to be measured based on the outputs of the reference electrode and the working electrode. A channel field effect transistor, the working electrode is composed of a second p-channel field effect transistor, a termination of a semiconductor surface of a gate portion of the second p-channel field effect transistor, and the first p-channel field effect Unlike the termination of the semiconductor surface of the gate portion of the transistor, the semiconductor surface of the first p-channel field effect transistor has diamond, and the semiconductor surface of the gate portion of the first p-channel field effect transistor is hydrogen. A diamond that has been terminated and all hydrogen terminated, or the first p Semiconductor surface of the gate portion of Yaneru field effect transistor in which a portion of the hydrogen termination a diamond hydrogen termination treatment is performed is replaced with fluorine termination.

  The working electrode is composed of a second p-channel field effect transistor, and the sensitivity of the gate portion of the second p-channel field effect transistor to the measurement target ion at the end of the semiconductor surface is the first p-channel field effect transistor. The sensitivity to the measurement target ion at the end of the semiconductor surface of the gate portion is made higher.

  According to this ion sensor, since the semiconductor surface of the first p-channel field effect transistor is made of diamond, the problem of the reference electrode is solved, and the ion sensor can be used in severe environments such as high temperature and high pressure. Can be obtained.

  The semiconductor surface of the gate portion of the first p-channel field effect transistor may be a hydrogen ion insensitive termination in which part of the hydrogen termination of diamond subjected to hydrogen termination treatment is replaced with an oxygen termination or a fluorine termination.

Sp constituting the semiconductor surface of the gate portion of the first p-channel field effect transistor
The content of three-bond crystals may be greater than the content of sp2-bond crystals.
The content of sp3-bonded crystals is preferably larger than the content of sp2-bonded crystals. That is, the diamond constituting the diamond surface is most preferably a carbon structure composed of sp3 bonded crystals, but suitable characteristics can be obtained even if sp2 bonded crystals are included. When sp2-bonded crystals are included, a higher value of (content of sp3-bonded crystals) / (content of sp2-bonded crystals) provides better characteristics.
The working electrode and the reference electrode may be provided to face each other.

The semiconductor surface of the gate portion of the second p-channel field effect transistor may have diamond with hydrogen termination.
The semiconductor surface of the gate portion of the second p-channel field effect transistor may be an ion-sensitive termination in which part of the hydrogen termination of diamond subjected to hydrogen termination treatment is replaced with an amino termination or an oxygen termination.

  The measurement target ion may be a hydrogen ion.

  The pH sensor may include a temperature sensor that detects the temperature of the p-channel field effect transistor.

The ion concentration measurement method of the present invention comprises the steps of bringing a measured liquid into contact with a reference electrode and a working electrode, and measuring an ion concentration of the measured liquid based on outputs of the reference electrode and the working electrode. In the ion concentration measuring method, the reference electrode is configured by a first p-channel field effect transistor, the working electrode is configured by a second p-channel field effect transistor, and the gate of the second p-channel field effect transistor. The semiconductor surface termination is different from the semiconductor surface termination of the first p-channel field effect transistor, the semiconductor surface has diamond, and the first p-channel field effect transistor gate portion ones of the semiconductor surface is all hydrogen termination a diamond hydrogen termination process is performed, also The semiconductor surface of the gate portion of the first p-channel field effect transistor in which a portion of the hydrogen termination a diamond hydrogen termination treatment is performed is replaced with fluorine termination.

  According to this ion concentration measurement method, the semiconductor surface of the first p-channel field effect transistor is composed of diamond, so that the problem of the reference electrode is solved and it can be used even in severe environments such as high temperature and high pressure. An ion sensor can be obtained.

  According to the ion sensor of the present invention, the semiconductor surface of the first p-channel field effect transistor is made of diamond, so that the problem of the reference electrode is solved and it can be used in severe environments such as high temperature and high pressure. An ion sensor can be obtained.

  According to the ion concentration measuring method of the present invention, since the semiconductor surface of the first p-channel field effect transistor is composed of diamond, the problem of the reference electrode is solved and the semiconductor device can be used in severe environments such as high temperature and high pressure. A possible ion sensor can be obtained.

Sectional drawing which shows the structure of the pH sensor of 1st Example of this invention. The top view which shows the shape of the drain of a reference pole and working electrode of the Example of FIG. 1, and a source | sauce. The circuit example which measures pH of the to-be-measured liquid 3 using the Example of FIG. Drain-source characteristics of the embodiment of FIG. PH / voltage characteristics of the embodiment of FIG. The flowchart which shows the film-forming process of the diamond thin film of the Example of FIG. The figure which shows typically the termination | terminus state of the diamond thin film surface. (A) is a terminal state of as-grown diamond, (b) is a terminal state of diamond subjected to hydrogenation termination treatment, (c) is a terminal state of diamond subjected to partial amino termination treatment, and (d) is partial fluorine. Termination state of diamond after termination. The characteristic view which shows the relationship between hydrogen ion sensitivity and oxygen termination substitution degree. Sectional drawing which shows the pH sensor of 2nd Example of this invention. Sectional drawing which shows the structure of the reference pole of 3rd Example of this invention. The top view seen from the IX-IX line direction of the Example of FIG.

  Hereinafter, an embodiment in which an ion sensor according to the present invention is applied to a pH sensor will be described.

  FIG. 1 is a cross-sectional view showing a configuration of a pH sensor according to a first embodiment of the present invention. FIG. 2 shows a drain 13 (23) and a source 14 (of a reference electrode 1 and a working electrode 2 of the embodiment of FIG. It is a top view which shows the shape of 24).

  As shown in FIGS. 1 and 2, the pH sensor as the ion sensor of this embodiment uses a diamond ISFET as a reference electrode 1 and a working electrode 2, respectively.

  As shown in FIGS. 1 and 2, the reference electrode 1 includes a silicon wafer 11 having a diamond thin film 12 formed on the surface thereof, a drain 13 formed on the surface of the diamond thin film 12, and a drain 13 formed on the surface of the diamond thin film 12. And a protective film 15 covering the drain 13 and the source 14. In this pH sensor, a region sandwiched between the drain 13 and the source 14 functions as the gate 10. The drain 13, the source 14, and the gate 10 form a p-channel field effect transistor and form an ion sensitive field effect transistor. Moreover, since it has a diamond thin film, it is also called a diamond ISFET.

  Arbitrary numerical values can be applied to the sizes of electrodes and intervals of the drain 13 and the source 14. For example, the distance α between the drain 13 and the source 14 in FIG. 2 is 10 to 1000 μm, the width of the ISFET portion of the source 14 (drain 13 ISFET portion width) β: 0.01 to 50 mm, source 14 length (drain 13 length) γ: 5 to 50 mm, source 14 width (drain 13 width) δ: 5 to 100 mm Is preferable.

The surface of the diamond thin film 12 at the gate 10 portion of the reference electrode 1 has a stable potential in a hydrogen ion concentration range of 1.0 × 10 ⁻¹ mol / L to 1.0 × 10 ⁻¹⁴ mol / L, or The termination element is controlled so that the potential stability is maintained to such an extent that ion sensitivity does not become a practical problem.

As a diamond surface where the potential is stable in the range of hydrogen ion concentration of 1.0 × 10 ⁻¹ mol / L to 1.0 × 10 ⁻¹⁴ mol / L, or ion sensitivity does not become a practical problem, hydrogen is used. Diamond with hydrogen density increased by plasma treatment, diamond with hydrogen terminated diamond partially oxygen terminated, diamond with hydrogen terminated diamond partially oxygen terminated, diamond with hydrogen terminated diamond partially fluorine terminated, etc. Available.

  As shown in FIG. 1, the working electrode 2 is provided to face the reference electrode 1. As shown in FIGS. 1 and 2, the working electrode 2 includes a silicon wafer 21 having a diamond thin film 22 formed on the surface thereof, a drain 23 formed on the surface of the diamond thin film 22, and a drain 23 formed on the surface of the diamond thin film 22. And a protective film 25 that covers the drain 23 and the source 24. In this pH sensor, a region sandwiched between the drain 23 and the source 24 functions as the gate 20.

  It is desirable that the drain 23 and the source 24 have the same shape as the drain 13 and the source 14 of the reference electrode 1 shown in FIG. The size of electrodes and intervals of the drain 23 and the source 24 may be different from that of the reference electrode 1, but in this case also, α: 10 to 1000 μm, β: 0.01 to 50 mm, and γ: 5 to 50 mm in FIG. , Δ: The range of 5 to 100 mm is preferable.

The surface of the diamond thin film 22 in the gate 20 portion of the reference electrode 2 has a potential depending on the pH value in a hydrogen ion concentration range of 1.0 × 10 ⁻¹ mol / L to 1.0 × 10 ⁻¹⁴ mol / L. The terminal element is controlled so as to respond linearly or nonlinearly.

A diamond surface in which the termination element is controlled so that the potential responds linearly or nonlinearly depending on pH in the range of hydrogen ion concentration of 1.0 × 10 ⁻¹ mol / L to 1.0 × 10 ⁻¹⁴ mol / L. May be diamond that is partially hydrogen-terminated with diamond whose hydrogen density has been increased by hydrogen plasma treatment so that the hydrogen ion-sensitive terminal ends, or diamond that is replaced with an amino terminal end.

In the second p-channel field effect transistor of the working electrode 2, the sensitivity (Nernst response) to the measurement target ion at the end of the semiconductor surface of the gate 20 portion is the voltage of the working electrode 2 with respect to the ion concentration.
The sensitivity (Nernst responsiveness) to the measurement target ion at the end of the semiconductor surface of the gate 10 portion in the first p-channel field effect transistor of the reference electrode 1 is a voltage of the reference electrode 2 with respect to the ion concentration.

  The sensitivity to the measurement target ions at the end of the semiconductor surface of the gate 20 portion is made higher than the sensitivity to the measurement target ions at the end of the semiconductor surface of the gate 10 portion.

  Next, the operation of the pH sensor will be described.

  As shown in FIG. 1, the measured liquid 3 contacts the surface of the diamond thin film 12 in the region of the gate 10 sandwiched between the drain 13 and the source 14 of the reference electrode 1. On the other hand, due to the presence of the protective film 15, the liquid 3 to be measured does not directly contact the drain 13 and the source 14.

  In addition, in the region of the gate 20 sandwiched between the drain 23 and the source 24 of the working electrode 2, the liquid 3 to be measured contacts the surface of the diamond thin film 22. On the other hand, due to the presence of the protective film 25, the liquid 3 to be measured does not directly contact the drain 23 and the source 24.

  The potential of the liquid 3 to be measured is controlled by a pseudo reference electrode (not shown) that contacts the liquid 3 to be measured. By applying a potential to the pseudo reference electrode, the potential is applied to the region of the gate 10 sandwiched between the drain 13 and the source 14 of the reference electrode 1 via the measured solution 3 and at the same time, via the measured solution 3. Thus, the potential is applied to the region of the gate 20 sandwiched between the drain 23 and the source 24 of the working electrode 2. In this manner, the potential of the gate 10 of the reference electrode 1 and the potential of the gate 20 of the working electrode 2 are controlled by the voltage applied to the pseudo reference electrode.

  FIG. 3 is a circuit example for measuring the pH of the liquid 3 to be measured using the embodiment of FIG. A predetermined voltage V is applied to the pseudo reference electrode (G). The pseudo reference electrode (G) is in contact with the liquid to be measured 3 (not shown). A constant current source and a buffer are connected to the drain (D) of the reference electrode 1, and a buffer is connected to the source (S) of the reference electrode 1. The output 1 is connected to the source (S) of the reference electrode 1 through a buffer, and is connected to the drain (D) of the reference electrode 1 through a resistor and the buffer. A constant current source and a buffer are connected to the drain (D) of the working electrode 2, and a buffer is connected to the source (S) of the working electrode 2. The output 2 is connected to the source (S) of the working electrode 2 through a buffer, and is connected to the drain (D) of the working electrode 2 through a resistor and a buffer.

  In the circuit example of FIG. 3, when a voltage V (to ground potential) is applied to the liquid 3 to be measured via the pseudo reference electrode (G), the potentials are respectively applied to the gate 10 of the reference electrode 1 and the gate 20 of the working electrode 2. Arise.

  In the circuit of FIG. 3, a voltage based on the drain (D) of the reference electrode 1 and the source (S) of the reference electrode 1 is generated at the output 1, and the drain (D) of the working electrode 2 and the working electrode 2 are generated at the output 2. A voltage based on the source (S) of is generated. The difference between the voltage value of output 1 and the voltage value of output 2 has a correlation with the pH of the liquid 3 to be measured.

  FIG. 4 shows drain / source characteristics of the embodiment of FIG. The horizontal axis represents the voltage −Vds (V) between the drain 13 (23) and the source 14 (24), and the vertical axis represents the current −Ids (A) between the drain 13 (23) and the source 14 (24). is there. When the voltage -Vgs of the pseudo reference electrode (G) is constant at -α (V), the current -Ids (A) / voltage -Vds (V) characteristics, and the voltage -Vgs of the pseudo reference electrode (G) is -β The current-Ids (A) / voltage-Vds (V) characteristics when (V) is constant, and the current -Ids (A) when the voltage -Vgs of the pseudo reference electrode (G) is constant at -γ (V). ) -Voltage-Vds (V) characteristics. When α (V)> β (V)> γ (V), characteristic curve when voltage −Vgs = −α (V)> characteristic curve when voltage −Vgs = −β (V)> voltage −Vgs = − It becomes a characteristic curve at γ (V).

  When the pH value of the measured liquid 3 increases, the characteristic curve of FIG. 4 moves upward, and when the pH value of the measured liquid 3 decreases, the characteristic curve of FIG. 4 moves downward.

  FIG. 5 shows pH / voltage characteristics of the embodiment of FIG. The horizontal axis represents the pH value of the liquid 3 to be measured, and the vertical axis represents the voltage Vgs of the pseudo reference electrode (G).

  When the current Ids (A) between the drain 13 (23) and the source 14 (24) is constant, the voltage Vgs of the pseudo reference electrode (G) decreases as the pH value of the liquid 3 to be measured increases. . If this characteristic is used, the pH value of the liquid 3 to be measured is calculated from the value of the voltage Vgs of the pseudo reference electrode (G).

  Note that, under specific conditions, when the pH value of the liquid 3 to be measured increases, the voltage Vgs of the pseudo reference electrode (G) may increase. Further, under certain conditions, when the pH value of the liquid 3 to be measured increases, the voltage Vgs of the pseudo reference electrode (G) may change nonlinearly. Even in this case, the value of the voltage Vgs of the pseudo reference electrode (G) is correlated with the pH value of the measured liquid 3, and the pH value of the measured liquid 3 is calculated from the value of the voltage Vgs of the pseudo reference electrode (G). it can.

  Next, a process for forming the diamond thin film 12 and the diamond thin film 22 will be described.

  FIG. 6 is a flowchart showing a diamond thin film forming process of the embodiment of FIG.

  In step S1 of FIG. 6, the surfaces of the silicon wafers 11 and 21 are polished. In order to improve the adhesion between the silicon wafers 11 and 21 and the diamond layer, it is desirable that the arithmetic average roughness Ra is 0.1 to 15 μm and the maximum height Rz is 1 to 100 μm.

  Next, in step S2, nucleation of diamond powder is performed.

  In this step, a diamond nucleation process is performed on the surfaces of the polished silicon wafers 11 and 21 in order to grow a uniform diamond layer. As a nucleation method, a solution containing diamond fine particles may be applied to the surfaces of the silicon wafers 11 and 21 by an ultrasonic method, a dipping method, or other methods, followed by solvent drying.

  Next, in step S3, a diamond film forming process is performed.

  In this step, a diamond film is formed by a hot filament CVD method. A carbon source (for example, a low molecular organic compound such as methane, alcohol, or acetone) is supplied to the filament together with hydrogen gas or the like. The filament is heated to a temperature range where carbon radicals and the like are generated (for example, 1800 to 2800 ° C.), and the silicon wafer 11 is disposed so as to be in a temperature range (for example, 750 to 950 ° C.) where diamond is deposited in this atmosphere. To do. The supply rate of the mixed gas depends on the size of the reaction vessel, but the pressure is preferably 15 to 760 Torr. A diamond fine particle layer having a particle size of usually 0.001 to 2 μm is deposited on the silicon wafer. The thickness of the diamond fine particle layer can be adjusted by the deposition time, but is preferably 0.5 to 20 μm from the viewpoint of economy.

  Next, in step S4, hydrogen termination is performed on the as-grown diamond.

  In this step, a terminal other than hydrogen (for example, carbon terminal or oxygen terminal) after the diamond film formation is replaced with a hydrogen terminal to obtain a high-density hydrogen terminal. As a method for the high-density hydrogen termination treatment, any of a treatment with a hydrofluoric acid aqueous solution, a hydrogen plasma treatment, a heat treatment in a hydrogen atmosphere, a hydrogen radical treatment, and a cathode reduction method can be selected. Two or more methods may be combined to increase the efficiency of the hydrogen termination treatment.

As the hydrogen plasma treatment, for example, the hydrogen density at the end of diamond can be increased under the treatment conditions of 1 kw, H ₂ -flow 400 sccm, and plasma irradiation time 5 hours. As the cathode reduction method, for example, a method of applying a voltage of about −1.8 V to a conductive diamond electrode in an as-grown state and immersing it in a 0.1 M sulfuric acid aqueous solution (H ₂ SO ₄ ) for 30 minutes or more. Can be used.

  Note that the step S1, the step S2, and the step S4 may be omitted.

  Regarding the qualitative quantification of hydrogen termination on the diamond surface produced through the above steps, for example, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometer (SIMS), Fourier transform infrared spectrophotometer (FT-IR) Etc.) can be inspected by a conventionally known analysis method.

  Next, an example of a process for manufacturing a diamond ISFET on the silicon wafers 11 and 21 on which the diamond thin films 12 and 22 are formed will be described.

  First, the surfaces of the diamond thin films 12 and 22 are partially oxygen-terminated. In this step, a resist is spin-coated on the surfaces of the diamond thin films 12 and 22, and the resist is patterned by exposure and development. Thereafter, only the exposed regions of the diamond thin films 12 and 22 are selectively oxygen-terminated by oxygen RIE, and the resist is removed by irradiation with a solvent and ultrasonic waves. In this step, the region of the gate 10, the lower region of the drain 13, and the lower region of the source 14 sandwiched between the drain 13 and the source 14 where the diamond thin film 12 of the reference electrode 1 is in contact with the liquid to be measured No oxygen termination. The region other than the region of the gate 10 and the region other than the lower region of the drain 13 and the region other than the lower region of the source 14 is oxygen-terminated.

  Next, a resist is spin-coated on the surfaces of the diamond thin films 12 and 22, and the resist is patterned by exposure and development. Thereafter, Au / Ti sputtering is performed and lift-off is performed, whereby an Au / Ti thin film having a pattern shown in FIG. 2 is formed on the diamond thin films 12 and 22. Thereby, drains 13 and 23 and sources 14 and 24 are formed.

  Next, a resist to be the protective films 15 and 25 is spin-coated on the substrate on which the diamond thin films 12 and 22 and the Au / Ti thin film are formed, and the resist is patterned by exposure and development. In the resist removal region, the diamond thin films 12 and 22 are exposed. The gates 10 and 20 between the drains 13 and 23 and the sources 14 and 24 correspond to this region, and the liquid 3 to be measured directly contacts the diamond thin films 12 and 22 in this region.

  In the above embodiment, an example in which a silicon wafer is used as the substrate has been described, but the material of the substrate is arbitrary.

  Moreover, the method of carrying a diamond film on a board | substrate is not limited to said method, Arbitrary things can be used. As a typical film forming method, a vapor phase synthesis method can be used, and examples of the vapor phase synthesis method include a CVD (chemical vapor deposition) method, a physical vapor deposition (PVD) method, and a plasma jet method. Examples of the CVD method include a hot filament CVD method and a microwave plasma CVD method.

In addition, regardless of which diamond film forming method is used, the synthesized diamond layer is polycrystalline, and amorphous carbon and graphite components may remain in the diamond layer. Amorphous carbon or graphite component from the viewpoint of the stability of the diamond layer is preferably lesser, in Raman spectroscopic analysis, and around 1332 cm ^-1 attributable to the diamond peak present in (1321～1352Cm range of ^-1) intensity I (D) , the ratio I (D) / I of the peak intensity near 1580 cm ^-1 attributable to the G band of graphite (range ^{1560~1600cm -1) I (G) (} G) is not less than 1, the content of diamond It is preferable that the content be higher than the graphite content.

  Instead of forming a diamond thin film on a substrate, a self-supporting diamond bulk body may be used without using a substrate such as Si or carbon.

  7A and 7B are diagrams schematically showing the termination state of the surface of the diamond thin film 12, where FIG. 7A shows the termination state of as-grown diamond, and FIG. 7B shows the hydrogenation-terminated diamond. FIG. 7C shows the terminal state, and FIG. 7D shows the terminal state of the diamond subjected to the partial fluorine termination, and FIG. 7D shows the terminal state of the diamond subjected to the partial fluorine termination.

  The diamond thin film 12 subjected to the hydrogen termination treatment is disposed at the gate 10 portion of the reference electrode 1 as long as the gate portion of the reference electrode has a diamond surface having a hydrogen ion insensitive termination. The hydrogen termination treatment is not limited. For example, diamond with hydrogen termination (FIG. 7 (b)), diamond in which part of the hydrogen termination of hydrogen-terminated diamond is replaced with a hydrogen ion insensitive element, for example, partially fluorine-terminated diamond (FIG. 7). (D)), partially oxygen-terminated diamond, and the like.

  In addition, as the hydrogen-terminated diamond at the gate portion of the working electrode, a diamond obtained by replacing a part of the hydrogen-terminated diamond with a hydrogen-terminated element, for example, a partially amino-terminated diamond (FIG. 7 ( c)) and partially oxygen-terminated diamonds can be used.

  FIG. 8 is a characteristic diagram showing the relationship between hydrogen ion sensitivity and the degree of oxygen termination substitution, with the vertical axis representing the hydrogen ion sensitivity and the horizontal axis representing the degree of oxygen termination substitution. The oxygen-terminated substitution degree here is expressed by the following formula.

  Degree of oxygen-terminated substitution = A / (A + B)

However, A: “Carbon number of diamond surface with oxygen termination”, B: “Carbon number of diamond surface with no oxygen termination”
It is.

  An oxygen-terminated substitution degree of 0% means a diamond surface without oxygen termination, and an oxygen-termination substitution degree of 100% means a diamond surface with only oxygen termination. For example, diamond subjected to hydrogen termination treatment has an oxygen termination substitution degree close to 0%.

  As shown in FIG. 8, the sensitivity to hydrogen ions increases as the degree of oxygen-terminated substitution increases from 0%, but eventually decreases and becomes almost zero when the degree of oxygen-terminated substitution exceeds a certain value. . Therefore, in the gate portion of the reference electrode, for example, an oxygen-terminated substitution degree in the range of a% or less, or in the range of b% or more, in which ion insensitivity is obtained in FIG. 8 is selected. In the gate portion of the working electrode, for example, an oxygen-terminated substitution degree in a range of a% or more and b% or less that can obtain ion sensitivity in FIG. 8 is selected.

  As described above, according to the present invention, by using the ISFET of the termination controlled diamond having the liquid electrolyte as the measurement liquid as a gate as the reference electrode and the working electrode, the internal liquid excellent in high temperature and high pressure and acid and alkali resistance. A pH sensor having an unnecessary reference electrode can be obtained.

  This solves the problems of internal liquid leakage and deterioration over time, which are the problems of the conventional internal liquid-containing reference electrode, as well as the strong acid and strong alkaline conditions in the semiconductor production process of chemical synthesis plants, and bio-related materials such as proteins. A pH sensor that enables accurate measurement in a bioprocess that handles substances can be provided, and contributes to visualization of the pH value of a production process.

  As a kind of diamond used for the diamond ISFET in the present invention, a single crystal may be used in addition to the exemplified polycrystalline diamond. In addition to conductive diamond (doped diamond; polycrystal, single crystal), diamond-like carbon, conductive diamond-like carbon (doped diamond-like carbon), ECR sputtered carbon, RF sputtered carbon, carbon nanotube, fullerene, carbon A simple substance such as a nanotube or a conductive carbon material containing them as a main component can be used. A structure having a high sp3 bond crystal content ratio (sp3 / sp2 ratio) to sp2 bond crystal content, such as diamond, ECR sputtered carbon, and diamond-like carbon, is more preferable. Diamond having the highest crystal composition ratio is most suitable when the present invention is carried out.

  When carrying a conductive diamond film (doped diamond), any method uses a mixed gas of hydrogen gas and a carbon source as a diamond raw material, but the valence is different in order to impart conductivity to diamond. A trace amount of an element (dopant) may be added. As a dopant, boron, phosphorus, and nitrogen are preferable, and a preferable content rate is 1 to 100,000 ppm, and more preferably 100 to 10,000 ppm.

  FIG. 9 is a sectional view showing a pH sensor according to a second embodiment of the present invention. In this embodiment, a reference electrode and a working electrode are formed on a common substrate.

  As shown in FIG. 9, the reference electrode 101 and the working electrode 102 are formed on a common silicon wafer 100. The functions of the reference electrode 101 and the working electrode 102 correspond to the functions of the reference electrode 1 and the working electrode 2 shown in FIGS. 1 to 2, respectively, and the pH can be measured by the same principle.

  As shown in FIG. 9, the reference electrode 101 has a diamond thin film 112 formed on the surface of the silicon wafer 11, a drain 113 formed on the surface of the diamond thin film 112, and a drain 113 facing the surface of the diamond thin film 112. And a protective film 150 that covers the drain 113 and the source 114. A region sandwiched between the drain 113 and the source 114 functions as the gate 110.

The surface of the diamond thin film 112 at the gate 110 portion of the reference electrode 101 has a stable potential in a hydrogen ion concentration range of 1.0 × 10 ⁻¹ mol / L to 1.0 × 10 ⁻¹⁴ mol / L, or The termination element is controlled so that the potential stability is maintained to such an extent that ion sensitivity does not become a practical problem.

  As shown in FIG. 9, the working electrode 102 is opposed to the diamond thin film 122 formed on the surface of the silicon wafer 100, the drain 123 formed on the surface of the diamond thin film 122, and the drain 123 on the surface of the diamond thin film 122. The drain 123 and the source 124 are covered with a protective film 150. A region sandwiched between the drain 123 and the source 124 functions as the gate 120.

  It is desirable that the drain 123 and the source 124 have the same shape as the drain 113 and the source 114 of the reference electrode 101.

The surface of the diamond thin film 122 at the gate 120 portion of the reference electrode 102 has a potential depending on the pH value in a hydrogen ion concentration range of 1.0 × 10 ⁻¹ mol / L to 1.0 × 10 ⁻¹⁴ mol / L. The terminal element is controlled so as to respond linearly or nonlinearly.

  The termination elements on the surfaces of the diamond thin film 112 and the diamond thin film 122 can be controlled by an arbitrary process. For example, an as-grown diamond thin film as the same layer constituting the diamond thin film 112 and the diamond thin film 122 is formed. Thereafter, a desired ion-sensitive distribution can be obtained by performing a termination process for each region.

  When the pH measurement is performed by the pH sensor shown in FIG. 9, as shown in FIG. 9, the liquid to be measured is applied to the surface of the diamond thin film 112 in the region of the gate 110 sandwiched between the drain 113 and the source 114 of the reference electrode 101. 3 contacts. On the other hand, due to the presence of the protective film 150, the measured liquid 3 does not directly contact the drain 113 and the source 114.

  In addition, in the region of the gate 120 sandwiched between the drain 123 and the source 124 of the working electrode 102, the liquid 3 to be measured contacts the surface of the diamond thin film 122. On the other hand, due to the presence of the protective film 150, the measured liquid 3 does not contact the drain 123 and the source 124 directly.

  The pH of the liquid 3 to be measured is the operating characteristics of the reference electrode 101 and the working electrode 102 such as a difference in voltage / current characteristics between the drain and source of the reference electrode 101 and the working electrode 102 and a gate voltage under a certain condition. It is calculated based on the difference in operating state. For example, the circuit shown in FIG. 3 can be used.

  Next, an example in which temperature compensation of a diamond ISFET is performed will be described with reference to FIGS. A temperature sensor may be provided to compensate for the effect of temperature on ion sensitivity.

  The configuration described below can be applied to both the reference electrode and the working electrode.

FIG. 10 is a cross-sectional view showing the configuration of the reference electrode of the third embodiment of the present invention. In this embodiment, a thermistor 5 is provided.
FIG. 11 is a plan view of the embodiment of FIG. 10 as viewed from the IX-IX line direction. 10 and 11, the same elements as those in FIGS. 1 and 2 are denoted by the same reference numerals.

  As shown in FIGS. 10 and 11, the thermistor 5 as a temperature sensor is formed on the silicon wear 11 of the reference electrode 1 </ b> A, and the temperature characteristics of the ISFET that constitutes the reference electrode 1 </ b> A based on the resistance value change of the thermistor 5. Compensated. In this case, an accurate pH can always be measured regardless of the temperature of the liquid to be measured. The output value of the working electrode can be similarly temperature-compensated based on the resistance value change of the thermistor 5.

  As described above, according to the ion sensor of the present invention, since the reference electrode and the working electrode are configured as p-channel field effect transistors, problems such as leakage of internal liquid and deterioration with time in the reference electrode can be solved. In addition, the present invention can be applied to semiconductor manufacturing processes in chemical synthesis plants under strong acid and strong alkali conditions, and bioprocesses that handle biologically related substances such as proteins.

  The scope of application of the present invention is not limited to the above embodiment. The present invention can be widely applied to an ion sensor or the like that includes a reference electrode and a working electrode and measures the ion concentration of the liquid to be measured based on the working electrode and the output of the working electrode.

1,101 Reference electrode (p-channel field effect transistor)
2,102 Working electrode (p-channel field effect transistor)
3 Liquid to be measured 10, 101 Gate 11, 21 Silicon wafer 20, 220 Gate 12, 112 Diamond thin film (semiconductor surface)
22,122 Diamond thin film (semiconductor surface)
13, 23 Drain 14, 24 Source 15 Protective film 25 Protective film 100 Silicon wafer 110 Gate 120 Gate 113 Drain 123 Drain 114 Source 124 Source 150 Protective film 5 Thermistor

Claims (8)

In an ion sensor that includes a reference electrode and a working electrode, and measures an ion concentration of a liquid to be measured based on outputs of the reference electrode and the working electrode.
The reference electrode comprises a first p-channel field effect transistor;
The working electrode is constituted by a second p-channel field effect transistor,
The termination of the semiconductor surface of the gate portion of the second p-channel field effect transistor is different from the termination of the semiconductor surface of the gate portion of the first p-channel field effect transistor,
The semiconductor surface of the first p-channel field effect transistor comprises diamond;
The semiconductor surface of the gate portion of the first p-channel field effect transistor is a hydrogen-terminated diamond that is all hydrogen-terminated, or the semiconductor of the gate portion of the first p-channel field effect transistor. An ion sensor characterized in that the surface is diamond which has been subjected to hydrogen termination treatment, and a part of the hydrogen termination is replaced with a fluorine termination .   2. The ion sensor according to claim 1, wherein the content of sp3 bonded crystals constituting the semiconductor surface of the gate portion of the first p-channel field effect transistor is larger than the content of sp2 bonded crystals. The working electrode and the reference electrode are provided facing each other.
The ion sensor according to claim 1. 2. The ion sensor according to claim 1, wherein a semiconductor surface of a gate portion of the second p-channel field effect transistor has diamond subjected to hydrogen termination.   The semiconductor surface of the gate portion of the second p-channel field effect transistor is an ion sensitive termination in which part of the hydrogen termination of diamond subjected to hydrogen termination is replaced with an amino termination or an oxygen termination. 5. The ion sensor according to any one of 4 above.   The ion sensor according to claim 1, wherein the measurement target ion is a hydrogen ion. A temperature sensor for detecting the temperature of the p-channel field effect transistor;
It is a pH sensor, The ion sensor of any one of Claims 1-6 characterized by the above-mentioned. Contacting a liquid to be measured with a reference electrode and a working electrode;
Measuring an ion concentration of the liquid to be measured based on outputs of the reference electrode and the working electrode;
In an ion concentration measurement method comprising:
The reference electrode comprises a first p-channel field effect transistor;
The working electrode is constituted by a second p-channel field effect transistor,
The termination of the semiconductor surface of the gate portion of the second p-channel field effect transistor is different from the termination of the semiconductor surface of the gate portion of the first p-channel field effect transistor,
The semiconductor surface has diamond;
The semiconductor surface of the gate portion of the first p-channel field effect transistor is a hydrogen-terminated diamond that is all hydrogen-terminated, or the semiconductor of the gate portion of the first p-channel field effect transistor. A method for measuring an ion concentration, characterized in that the surface is diamond that has been subjected to hydrogen termination treatment, wherein a part of the hydrogen termination is replaced with a fluorine termination .

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