JPH08261979A - Electrode for sensor - Google Patents

Abstract

PURPOSE: To make an electrode area very small and improve sensitivity by forming a multi-layer film sensor laminated with an insulator thin film and a metal thin film alternately into two or more layers on a substrate. CONSTITUTION: A multi-layer film laminated with 50 pairs of an Au metal layer 2 of 10nm and a SiO2 insulating layer 3 of 10nm alternately is formed on a SiO2 substrate 1, for example, it is cut to the required size, the cut face is polished, and a liquid or gas detecting sensor using the metal layers 2 as electrodes is formed. Since the electrodes (metal layers) 2 of this sensor are small, the current value is small, the iR drop is small, and a sample having high electric resistance can be measured at a high S-N ratio because of a small charging current of the electric double layer. The substance is isotropically diffused to the electrodes 2, the current density is high, and stationary state can be recovered in a short time.

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 been widely 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 by 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 [eg O. Niwa, M. Morit.
a, and H. Tabei, Electroanalysis, 3, 163 (1991)].
On the other hand, according to such a method, not only the analysis in the solution by utilizing the electrolysis that occurs in the reaction between the gas and the electrolytic solution using the liquid, but also the electrochemical gas sensor for detecting the gas in the atmosphere Was proposed. In addition to this method, many gas sensors using oxide semiconductors, organic semiconductors, solid electrolytes, porous ceramics, etc. have been proposed [eg, Masayoshi Nitta, Yoshiaki Takeda, Mikichi Harumaru and their applications (power. Company)]. A gas sensor using a ceramic having a semiconductor property utilizes a change in electric conductivity of the ceramic due to adsorption and desorption of various gases on the surface of the ceramic. 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, electromotive force is generated. If the gas concentration on one side of the ceramic is known, this electromotive force can be used to detect the gas concentration on the other side. In a gas sensor using porous ceramics, when a ceramic carrier having a catalyst dispersed on its surface comes into contact with a combustible gas in a high temperature state, the gas burns and the temperature of the ceramic rises. If a metal wire is embedded inside the ceramics, the resistance value of the metal wire changes, so many methods are used, such as using this to clarify the gas concentration. Other than these, there are sensors that utilize the weight change caused by gas adsorption. In any case, the problem is that the sensitivity is not sufficient,
It has been extremely difficult to analyze the gas concentration on the order of ppm or less.

[0003]

In the electrochemical measurement for detecting a trace substance in a solution, the electrode size is large and the sensitivity is limited. At present, even the comb-shaped electrode, which is the most sensitive electrode, has the smallest size of several μm, so that the sensitivity for detecting the concentration below 10 nmol / dm ³ was barely reached even in the solution. In addition, even when this is used as a gas sensor, the amount of the gas component that melts into the solution is extremely small, so that there is a drawback in that the sensitivity deteriorates by an order of magnitude. Related to the gas sensor according to the present invention is a porous ceramics sensor in addition to the electrochemical gas sensor utilizing the above electrolysis reaction. This porous ceramics sensor also has a problem that the sensitivity is not high. In addition to this, the porous ceramics sensor has a problem that it is difficult to optimize the size of the gas adsorption pores and to control the distribution thereof optimally. In addition, an electrochemical gas sensor that uses a solution also requires the use of an electrolytic solution to detect an electric current due to an electrode reaction that occurs on the electrode surface, so that it is difficult to control because the evaporation and absorption of water in the electrolytic solution occurs. 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 controllability of production.

[0004]

To achieve the above object, the present invention provides a sensor for detecting a solution or a gas,
The invention is characterized in that it is composed of a multilayer film having a structure in which an insulator thin film and a metal thin film are stacked on a substrate, and the metal thin film is used as an electrode. Furthermore, the present invention is characterized in that the insulator thin film and the metal thin film are alternately laminated in two or more layers.

[0005]

When the electrode area is reduced in the electrochemical judgment, the diffusion state of the substance into the electrode becomes dominant from one-dimensional to two-dimensional and three-dimensional diffusion, that is, depending on the electrode shape. Substance is supplied to the electrode,
The amount of substance that reaches per unit time and unit volume increases rapidly with the miniaturization of electrodes. As a result, the observed current density is significantly increased. When such an electrode having a small electrode area is applied to the electrochemical determination, the current value becomes small and the iR drop becomes small because the electrode is small. Therefore, it becomes possible to measure a sample having high electric resistance. Since the charging current of the electric double layer becomes small, S /
Measurement with a high N ratio becomes possible. Since the diffusion becomes isotropic, the current density is high and the time to reach a steady state is short. And other features will appear.

On the other hand, 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, a plurality of metal layers are arranged at a fine interval of about 1 nm to several μm with an insulator layer sandwiched therebetween, and then cut at a vertical angle to the thin film surface or at an angle with various inclinations to form a cross section of the laminated film. When the exposed metal layer is used as an electrode and a wiring for connecting to a voltage source is formed between these electrodes, it is about 1 nm to several 1
It is possible to easily form a minute electrode having a size of 000 nm. If this is used as an electrode, the potential drop in the solution becomes small even in an extremely high resistance solution, and the electrode potential can be measured more accurately. Moreover, since the current density is increased, it is possible to detect the substance in the low-concentration solution and to detect the concentration. Also, with such an electrode size, water vapor,
Even when ethyl alcohol, ammonia gas or other combustible gas covers the electrode, the electrode interval is extremely small, so that gas molecules adsorbed on the electrode surface can be electrically detected by resistance change or the like. Gas can be detected by reading this current. In addition to the feature of high sensitivity due to the above-mentioned fine electrodes [Koichi Aoki: Electrochemistry 56, 1391A (1988)], an electrolytic solution is not required as compared with the usual electrochemical type sensor, and thus the handling is easy. Occurs. The example is shown below. A metal such as Al, Cu, Au or Pt can be preferably used for the electrode of the present invention as long as it has high conductivity. Since the spacer layer between the electrodes may be an insulator such as an oxide, an oxide such as SiO ₂
A nitride such as Si ₃ N ₄ can be preferably 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 can be used as the substrate 1, and this can be mounted on the upper part of the chamber with two sputter deposition sources (called targets) in the lower part of the vacuum chamber and the substrate facing the target. The substrate holder is set on a rotary opposed-type sputtering apparatus that can revolve around the target and rotate on the target. Au and SiO for evaporation source
₂ was used, Ar gas was introduced at 5 × 10 ⁻³ torr after exhausting the vacuum chamber to the 10 ⁻⁸ Torr level, Au target was 200 W, SiO ₂ was 300 W and r
f discharge is performed. After that, 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, since the substrate holder is rotating at a constant speed and the deposition rate of sputtered particles is almost constant in the sputtering method, the thickness of the thin film formed on the substrate is 6 inches (15.24c).
m) The average value could be controlled within a range of 0.1 nm within the diameter area. Using this technique, A of 10 nm
Fifty pairs of the u electrode layer and the 100 nm SiO ₂ insulating layer were formed. After cutting this multilayer film to the required size in the direction perpendicular to the thin film plane with a scriber, after polishing the cut surface by buffing, the electrode formation using the electrode layer in this multilayer film is performed by photolithography and reactive ion etching. went.

FIG. 1 shows a schematic view of the cross-sectional structure of the formed multilayer film. In the figure, 1 is a substrate, 2 is a metal layer, and 3 is an insulating layer. FIG. 2 shows a cross-sectional structure of the multilayer film after oblique ion etching for expanding the exposed area of the metal layer for connecting the electrode wiring. In the figure, 1 is a substrate, 2 is a metal layer, and 3 is an insulating layer. In the present invention, a method is used in which a portion not requiring etching is coated with a resist and the resist is removed after the etching is completed. After the electrodes on the substrate and the lead-only wiring portion were prepared, the wiring portion was coated with a SiO ₂ thin film.

In FIG. 3, a part of the multilayer film is obliquely etched,
1 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 the 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 sweep reference voltage generating portion, and 8 is an insulating film for insulating a wiring portion. . FIG. 4 shows a bird's-eye view of FIG. 1 nmol of this multilayer electrode
When put into a dopamine solution of / dm ³ and measured,
A voltanomogram 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 amount of ascorbic acid.

(Embodiment 2) As in Embodiment 1, Si is used as the substrate 1, which has two sputter deposition sources (referred to as targets) in the lower part of the vacuum chamber and the substrate facing the target and the upper part of the chamber. The substrate holder is set on a rotary opposed type sputtering apparatus which can be mounted on the target and can rotate on the target by revolving the substrate holder. Pt and SiO ₂ were used as vapor deposition sources. As in Example 1, a 10 nm Pt electrode layer and a 100 nm Si layer were formed.
A sensor electrode having 50 pairs of O ₂ insulating layers was formed.
When dopamine detection was carried out in the same solution as in Example 1 using this multilayer film electrode, detection was possible even in a dopamine solution of 0.5 nmol / dm ³ . Also, 1
Dopamine could be detected even when the concentration of nmol / dm ³ coexisted with 10 times ascorbic acid.

Example 3 As in Example 1, SiO
10 nm Pt electrode layer and 12 nm SiO ₂ on ₂ substrates
A sensor electrode having 10 pairs of insulating layers was formed. When dopamine detection was carried out in the same solution as in Example 1 using this multilayer electrode, detection was possible even in a dopamine solution of 0.2 nmol / dm ³ . Also, 1 nm
Dopamine could be detected even when the concentration of ol / dm ³ coexisted with ascorbic acid in a 15-fold amount.

(Embodiment 4) As in Embodiment 1, SiO
_A pair of 10 nm Pt electrode layers and 3 nm 9 nm SiO ₂ insulating layers were formed as sensor electrodes on _two substrates. When dopamine detection was carried out in the same solution as in Example 1 using this multilayer film electrode, detection was possible even in a dopamine solution of 0.1 nmol / dm ³ . In addition, 1 nmol
Even when a concentration of dopamine of / dm ³ coexists with 30 times ascorbic acid, dopamine could be detected.

(Embodiment 5) As in Embodiment 1, 5 nm
Sensor electrode consisting of 100 pairs of Pt electrode layer of 20 nm and SiO ₂ insulating layer of 20 nm is formed, and the multi-layered film electrode is placed in an atmosphere of 1 atm in a 40% steam atmosphere and a voltage of 2 V is applied between the electrodes. As a result, a current value of about 10 nA was obtained. It was also confirmed that the current value increased as the partial pressure of water vapor was increased, and it was confirmed that the multilayer film type electrode structure of the present invention functions as a humidity detection sensor.

(Embodiment 6) To observe the change in conductivity when the prepared sensor electrode consisting of 100 pairs of 5 nm Pt electrode layer and 20 nm SiO ₂ insulating layer was left under various SO ₂ concentration conditions. The characteristics of the SO ₂ sensor were evaluated by. Since the conductivity of the prepared thin film was very small, the applied voltage was actually changed to adjust the current amount to fall within the range of 10 ⁻¹⁰ to 10 ^−NA , and the current change due to adsorption / desorption was changed. It measured using the microammeter. S
The O ₂ concentration was controlled by changing the flow rates of dry nitrogen and SO ₂ . It had sufficient sensitivity even when the SO ₂ concentration was 100 ppb. The electrode material is Ag or Au.
Even when the same experiment was performed by changing to, the same result as this example was obtained.

[0015] (Example 7) SiO ₂ insulating layer of Pt electrode layer and 20nm of 5nm prepared in the same manner as in Example 5 is 1
By observing the change in conductivity when the sensor electrode consisting of 00 pairs was left under various SO ₂ concentration conditions,
The characteristics as an O ₂ sensor were evaluated. NO ₂ concentration is
It was controlled by changing the flow rates of dry nitrogen and NO ₂ . It had a sufficient sensitivity even when the concentration of NO ₂ was 600 ppb. Even when the same experiment was performed by changing the electrode material to Cu or Ni, the same result as that of this 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 insulator thin film and a metal thin film are stacked on a substrate is used, and the metal thin film is used as an electrode. Since the distance between the electrodes can be made extremely small and the electrode area can be made extremely small, it becomes a highly sensitive solution sensor unlike conventional electrochemical sensors. In particular, 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, the gas component adsorbed on the electrodes can be detected by a resistance change or the like, so that the gas can be detected without a solution. Therefore, the gas sensor has an effect of requiring no solution and being easy to handle.

[Brief description of drawings]

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 for expanding an exposed area of a metal layer for connecting electrode wiring.

FIG. 3 is a schematic diagram showing a connection state between a metal layer and a lead voltage generating section.

FIG. 4 shows a bird's-eye view of FIG.

FIG. 5 shows measurement results.

[Explanation of symbols]

 1 Substrate 2 Metal layer 3 Insulating layer 4 Lead wiring 5 Contact part 6 Lead wire 7 Electric sweep reference voltage generation part 8 Insulation film for wiring part insulation

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigenkuni Sasaki 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. A sensor for detecting a solution or gas, which comprises a multilayer film having a structure in which an insulator thin film and a metal thin film are stacked on a substrate, and the metal thin film is used as an electrode. . 2. The insulator thin film and the metal thin film are alternately formed into two.
The sensor electrode according to claim 1, wherein the electrode is formed by stacking more than one layer.