JP3244249B2 - Electrode for sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor electrode having high detection sensitivity.

[0002]

2. Description of the Related Art Conventionally, electrochemical measurement has often been used for detecting trace substances in a solution. In general, there is a problem that the sensitivity is low because the size of the electrode is large. In order to improve this, attempts have been made to detect trace substances using electrodes in which a metal having a width of several μm to 10 μm and a length of several mm is arranged in a comb shape [for example, O. Niwa, M. Morit.
a, and H. Tabei, Electroanalysis, 3, 163 (1991)].
On the other hand, electrochemical gas sensors that attempt to detect gas in the atmosphere as well as analysis in solution by utilizing electrolysis that occurs by the reaction between the electrolytic solution and the gas introduced into it using such a method are also used. Had been proposed. In addition to this method, many gas sensors using oxide semiconductors, organic semiconductors, solid electrolytes, and porous ceramics have been proposed [for example, gas sensors by Masayoshi Nitta, Yoshiaki Takeda, and Yoshiyoshi Hara and their applications (Power Company)]. Gas sensors using ceramics having the properties of a semiconductor utilize changes in the electrical conductivity of ceramics caused by adsorption and desorption of various gases occurring on the surface of the ceramics. In a gas sensor using electrolyte ceramics, when there is a difference in gas concentration between two opposing surfaces, ions move in the ceramics,
Therefore, an electromotive force is generated. If the gas concentration is known on one side of the ceramics, the electromotive force can be used to detect the gas concentration on the other side. In a gas sensor using porous ceramics, when a ceramics carrier having a catalyst dispersed on its surface contacts a flammable gas in a high temperature state, the gas burns and the temperature of the ceramics rises. When a metal wire is embedded in ceramics, the resistance value of the metal wire changes. Therefore, many methods have been used such that the gas concentration is clarified by using this. In addition to these, there is a sensor utilizing a change in weight caused by gas adsorption. In any case, the problem is not enough sensitivity,
It has been extremely difficult to analyze gas concentrations on the order of ppm or less.

[0003]

In an electrochemical measurement for detecting a trace substance in a solution, the sensitivity is limited due to the large electrode size. At present, even in the case of the most highly sensitive comb-shaped electrode, since the smallest size is several μm, the sensitivity for detecting a concentration of less than 10 nmol / dm ³ even in a solution has been barely reached. Also, when this is used as a gas sensor, there is a disadvantage that the sensitivity is reduced by orders of magnitude because the amount of the gas component dissolved into the solution is extremely small. Related to the gas sensor according to the present invention is a porous ceramic sensor other than the electrochemical gas sensor utilizing the above-described electrolysis reaction. This porous ceramic sensor also has a problem that the sensitivity is not high. In addition, the porous ceramic sensor has problems in that it is difficult to optimize the size of the gas adsorption pores and to optimally control the distribution thereof. In addition, an electrochemical gas sensor using a solution requires the use of an electrolytic solution to detect the current due to the electrode reaction occurring on the electrode surface. there were. An object of the present invention is to solve the conventional problems and to provide a gas sensor having high sensitivity and excellent production controllability.

[0004]

According to the present invention, there is provided a sensor for detecting a solution or gas, comprising a multilayer film having a structure in which an insulating thin film and a metal thin film are stacked on a substrate. Various films against the substrate
By cutting at an inclined angle, the cross section of the laminated film
It is a feature of the present invention to use the metal thin film exposing as a electrode as an electrode. The present invention is characterized in the invention in that the insulator thin film and said metal thin film is laminated alternately two or more layers.

[0005]

When the electrode area is reduced in the electrochemical judgment, the diffusion state of the substance into the electrode is dominated by one-dimensional to two-dimensional and three-dimensional diffusion, depending on the electrode shape. Is also supplied to the electrode,
The amount of substance that reaches per unit time and per unit volume increases rapidly with miniaturization of electrodes. As a result, the observed current density increases significantly. When an electrode having such a small electrode area is applied to the electrochemical judgment, the current value becomes small and the iR drop becomes small because the electrode is small. For this reason, it is possible to measure a sample having a high electric resistance. Since the charging current of the electric double layer is small, measurement at high speed and high S / N ratio becomes possible. Since the diffusion is isotropic, the current density is high, and the time for a steady state is short. Features such as come to appear.

A multilayer film in which thin films are stacked can have a fine laminated structure of about 1 nm to several μm. For this reason, after arranging a plurality of metal layers at a fine interval of about 1 nm to several μm with an insulator layer interposed therebetween, the cross section of the laminated film is cut by cutting the thin film perpendicularly or at various angles. The exposed metal layer is used as an electrode, and a wiring for connecting to a voltage source is formed between these electrodes.
A minute electrode having a size of 0 nm can be easily formed. When this is used as an electrode, even in an extremely high resistance solution, the potential drop in the solution becomes small and the electrode potential can be measured more accurately. Further, since the current density is increased, it is possible to detect a substance and a concentration in a low-concentration solution. Also, with such an electrode size, even when water vapor, ethyl alcohol, ammonia gas, or other flammable gas covers the electrodes, the distance between the electrodes is extremely small. It becomes possible to detect it. By reading this current, the gas can be detected. In addition to the feature of high sensitivity due to the above-mentioned microelectrode [Koichi Aoki: Electrochemical 56, 1391A (1988)], the effect that the electrolyte is not required as compared with a normal electrochemical sensor, so that the handling is simplified. Occurs. An example will be described below. Since the electrode of the present invention only needs to have high conductivity, metals such as Al, Cu, Au, and Pt can be suitably used. Since the spacer layer to open the course of the electrode may be an insulator such as oxide oxide such as SiO _2, S
A nitride such as i ₃ N ₄ can be suitably used.

[0007]

Next, the present invention will be described with reference to examples. The embodiment is merely an example, and it goes without saying that various changes or improvements can be made without departing from the spirit of the present invention. (Example 1) Si is used as a substrate 1 and can be attached to an upper portion of a chamber with two sputter deposition sources (referred to as targets) at a lower portion of a vacuum chamber, with the substrate facing the target, and The substrate holder is set in a rotary facing type sputtering apparatus that can revolve around the target by revolving. Using Au and SiO ₂ as evaporation sources,
Ar after evacuating the inside of the vacuum chamber to the order of 10 ⁻⁸ Torr
Gas is introduced at 5 × 10 ⁻³ torr, and rf discharge is performed at a power of 200 W for the Au target and 300 W for SiO ₂ . Thereafter, the Si substrate is rotated together with the substrate holder, and the Si substrate is alternately passed over the Au target and the SiO ₂ target. In this case, the thickness of the thin film formed on the substrate has a diameter of 6 inches (15.24 cm) because the deposition rate of the sputtered particles is almost constant in the sputtering method because the substrate holder is rotating at a constant speed. It was possible to control the average value within the area to within 0.1 nm. Using this technique, 50 pairs of a 10 nm Au electrode layer and a 100 nm SiO ₂ insulating layer were formed. After cutting this multilayer film to the required size in the direction perpendicular to the plane of the thin film with a scriber, the cut surface is polished by buffing, and the electrode formation using the electrode layer in this multilayer film is performed by photolithography and reactive ion etching. went.

FIG. 1 is a schematic view of a cross-sectional structure of the formed multilayer film. In the figure, 1 indicates a substrate, 2 indicates a metal layer, and 3 indicates an insulating layer. FIG. 2 shows a cross-sectional structure of the multilayer film after oblique ion etching performed to enlarge the exposed area of the metal layer in order to perform electrode wiring connection. In the figure, 1 indicates a substrate, 2 indicates a metal layer, and 3 indicates an insulating layer. In the present invention, a method is used in which a portion unnecessary to be etched is coated with a resist, and after the etching is completed, the resist is removed. After the formation of the electrode and lead wire on the substrate, the wire was coated with a SiO ₂ thin film.

FIG. 3 shows that a part of the multilayer film is obliquely etched,
FIG. 3 schematically shows a connection state between a metal layer in a multilayer film and a lead wiring in a state where the exposed portions of the metal layer 2 and the insulating layer 3 are enlarged to facilitate connection with the lead wiring 4. .
In the figure, 1 is a substrate, 2 is a metal layer, 3 is an insulating layer, 4 is a lead wire, 5 is a contact portion, 6 is a lead wire, 7 is an electric sweeping voltage generating portion, and 8 is an insulating film for partially insulating a wiring. . FIG. 4 shows the bird's-eye view of FIG. 1 nmol of this multilayer electrode
/ Dm ^{3 in} a dopamine solution,
A voltanogram as shown in FIG. 5 was obtained, and it was confirmed that such a low concentration substance could be detected. Further, dopamine could be detected even when this concentration of dopamine coexisted with 6-fold ascorbic acid.

(Embodiment 2) As in Embodiment 1, Si is used as a substrate 1, and it is provided with two sputter deposition sources (referred to as targets) at the lower part of a vacuum chamber and the upper part of the chamber with the substrate facing the target. It is set in a rotary opposed type sputtering apparatus that can be mounted on a substrate holder and can rotate on a target by revolving a substrate holder. Pt and Si for evaporation source
The O ₂ was used. As in Example 1, a 10 nm P
A sensor electrode composed of 50 pairs of a t electrode layer and a 100 nm SiO ₂ insulating layer was formed. When dopamine was detected in the same solution as in Example 1 using this multilayer electrode, detection was possible even with a 0.5 nmol / dm ³ dopamine solution. Also, dopamine could be detected when dopamine at a concentration of 1 nmol / dm ³ coexisted with 10-fold ascorbic acid.

(Embodiment 3) Pt of 10 nm was formed on a SiO ₂ substrate in the same manner as in Embodiment 1.
A sensor electrode composed of 10 pairs of an electrode layer and a 12 nm SiO ₂ insulating layer was formed. Dopamine was detected in the same solution as in Example 1 using this multilayer electrode.
Detection was possible even with a 2 nmol / dm ³ dopamine solution. Also, dopamine was detectable when dopamine at a concentration of 1 nmol / dm ³ coexisted with 15-fold ascorbic acid.

(Embodiment 4) Pt of 10 nm was formed on a SiO ₂ substrate in the same manner as in Embodiment 1.
A sensor electrode composed of three pairs of an electrode layer and a 9 nm SiO ₂ insulating layer was formed. Using this multilayer electrode, dopamine detection was performed in the same solution as in Example 1.
Detection was possible even in a mol / dm ³ dopamine solution. Also, dopamine could be detected when dopamine at a concentration of 1 nmol / dm ³ coexisted with 30-fold ascorbic acid.

Fifth Embodiment A sensor electrode composed of 100 pairs of a 5 nm Pt electrode layer and a 20 nm SiO ₂ insulating layer is formed in the same manner as in the first embodiment. When a voltage of 2 volts was applied between the electrodes in a water vapor atmosphere, a current value of about 10 nA was obtained. It was also confirmed that as the partial pressure of water vapor was increased, the current value was also increased, and it was confirmed that the multilayer electrode structure of the present invention functions as a humidity detection sensor.

(Example 6) Observing a change in conductivity when a sensor electrode composed of 100 pairs of a prepared 5 nm Pt electrode layer and a 20 nm SiO ₂ insulating layer is left under various SO ₂ concentration conditions. The characteristics as an SO ₂ sensor were evaluated. In addition, since the conductivity of the formed thin film was very small, the applied voltage was actually changed so that the current amount was 10 ⁻¹⁰ to 10 ^−N
The current was adjusted so as to fall within the range of A, and the current change due to adsorption and desorption was measured using a microammeter. The SO ₂ concentration was controlled by changing the flow rates of dry nitrogen and SO ₂ . S
Sufficient sensitivity was obtained even when the concentration of O ₂ was 100 ppb. In addition, when the same experiment was performed by changing the electrode material to Ag or Au, the same result as that of the present example was obtained.

(Embodiment 7) A 5 nm Pt electrode layer formed in the same manner as in Embodiment 5 and 2
The characteristics as a NO ₂ sensor were evaluated by observing a change in conductivity when a sensor electrode including 100 pairs of 0-nm SiO ₂ insulating layers was left under various SO ₂ concentration conditions. The NO ₂ concentration was controlled by changing the flow rates of dry nitrogen and NO ₂ . NO ₂ concentration of 600
Also in the case of ppb, it had sufficient sensitivity. In addition, when the same experiment was performed by changing the electrode material to Cu or Ni, the same result as that of the present example was obtained.

[0016]

According to the present invention, in a sensor for detecting a solution or gas, a multilayer film having a structure in which an insulating thin film and a metal thin film are stacked on a substrate is provided.
For cutting at various angles to the substrate
By using the metal thin film with the cross section of the laminated film exposed as an electrode, the distance between the electrodes can be extremely reduced, and the electrode area can be extremely reduced.
Unlike conventional electrochemical sensors, it is a highly sensitive solution sensor. Particularly when the distance between the electrodes is less than 10 nm, the sensitivity becomes extremely high. Further, since the distance between the electrodes is extremely small, a gas component adsorbed on the electrodes can be detected by a change in resistance or the like, so that gas can be detected without a solution. Therefore, the present invention has an effect that the gas sensor can be easily handled without using a solution. According to the present invention,
The current value is small due to the small pole, and IR drop (voltage drop)
Lower) allows measurement of samples with high electrical resistance
It will work. Charge current of electric double layer formed on electrode surface
, The S / N ratio is high and the response is fast.
You. The flow of the target substance reaching the electrode becomes isotropic
Therefore, the current density is high until the steady state is reached.
Time is short. Furthermore, according to the present invention, a multilayer film is
For electrodes exposed by cutting diagonally to the
Of the target substance that arrives by diffusion (three-dimensional diffusion)
The number increases, and the target substance can be detected efficiently. Multilayer film
Cut obliquely to the substrate to expose the metal and
Adjust the electrode area according to the cutting angle for manufacturing
It is possible. As a result, the desired
An electrode for a sensor having an electrode width can be obtained. Multilayer
Manufacturing requires a lot of steps and time,
If a multi-layer film is prepared,
A multilayer electrode structure with a desired optimum electrode width
Can be obtained.

[Brief description of the drawings]

FIG. 1 shows a cross-sectional view of a multilayer electrode structure according to the present invention.

FIG. 2 is a cross-sectional structural view of a multilayer film after oblique ion etching performed to increase an exposed area of a metal layer for connecting an electrode wiring.

FIG. 3 schematically illustrates a connection state between a metal layer and a read voltage generation unit.

FIG. 4 shows the bird's-eye view of FIG. 3; (However, in order to simplify the figure, the number of metal layers and wiring parts is reduced)

FIG. 5 shows measurement results.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Metal layer 3 Insulating layer 4 Lead wiring 5 Contact part 6 Lead wire 7 Electric sweep voltage generating part 8 Insulating film for wiring partial insulation

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Shigekuni Sasaki 1-1-6 Uchisaiwai-cho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-5-2007 (JP, A) JP-A-2-19757 (JP, A) JP-A-2-140655 (JP, A) JP-A-1-223339 (JP, A) JP-A-4-130064 (JP, U) , A) (58) Fields surveyed (Int. Cl. ⁷ , DB name) G01N 27/30 G01N 27/12 G01N 27/28 331 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. A sensor for detecting a solution or a gas, comprising a multilayer film having a structure in which an insulating thin film and a metal thin film are stacked on a substrate , wherein the multilayer film is formed on a substrate by various methods.
By cutting at an angle with an appropriate inclination, it is possible to cut the laminated film.
An electrode for a sensor, wherein the metal thin film whose surface is exposed is used as an electrode. 2. A sensor electrode according to claim 1, wherein the insulator thin film and said metal thin film is laminated alternately two or more layers.