JPH0345178Y2 - - Google Patents

Description

[Detailed description of the invention] <Industrial application field> This invention measures the ionic activity in an electrolyte.
Regarding ISFET sensors (field effect transistor type ion sensors).

<Prior Art> This type of ISFET sensor is used in combination with a reference electrode during measurement. The reference electrode generally has an inner electrode, an internal liquid, and a liquid junction, and is immersed in the same test liquid as the sensor. As known, this reference electrode has good stability and can be said to be the most accurate as it provides a reference for measurements. However, on the other hand, various problems occur because the test liquid no longer exists between the ISFET sensor and the reference electrode when the liquid is replaced. In other words, when the test liquid no longer exists between the sensor and the reference electrode, the gate of the sensor becomes open in the air, which means that the gate electric field becomes unstable, causing gate dielectric breakdown and damage to the gate insulating film due to excessive electric field. Secondary changes may occur. This may cause problems such as the sensor becoming inoperable or the indication being deviated or drifting when the liquid is returned to the liquid state.

<Purpose and structure of the invention> The invention has been made with the above-mentioned considerations in mind, and the vicinity of the gate of the ISFET sensor where a predetermined voltage is applied to the gate via the reference electrode during measurement is
Focusing on the fact that it remains wet with the test liquid even during liquid exchange, we tested it on the same substrate on which the ISFET was formed.
In addition, by forming a metal electrode near the gate of the ISFET and applying a voltage similar to the above voltage to the metal electrode via a switch during liquid exchange, the drawbacks of the above conventional means can be overcome. This is an attempt to solve the problem.

<Example> Figure 1 is a plan view of an ISFET sensor as an example of the present invention, and Figure 2 is a side sectional view thereof.
is a sapphire substrate (SOS wafer) on which a silicon epitaxial diffusion layer 2 is formed.
A P or n channel MOSFET is installed on the substrate 1.
is formed by standard processes such as ordinary oxidation-Si island formation and impurity diffusion into the drain and source regions. 3 is the drain region, 4 is the source region, 5
is a gate. In addition, the epitaxial diffusion layer 2
A drain electrode 6 and a source electrode 7 are formed at the other end and serve as a lead wire outlet. The top of the sapphire substrate 1 except for the drain electrode 6 and the source electrode 7 is insulated with a SiO ₂ film 8 and a Si ₃ N ₄ film 9, and a liquid contact part 1 which is the tip half of the substrate is insulated.
0 is covered with a Ta ₂ O ₅ film or an ion sensitive film 11. However, this Ta ₂ O ₅ film or ion-sensitive film 11 only needs to cover at least the gate 5 portion. On the other hand, the remaining half of the substrate 1 is molded with resin 12.

Therefore, in the ISFET sensor with this configuration,
In parallel with the process of forming the MOSFET, an electrode 13 made of a noble metal such as gold is formed near the gate 5 by a thin film method such as vapor deposition or sputtering, or by a printing method, and the electrode 13 is thinned by photolithography. There is. The portion of the metal electrode 13 existing in the liquid contact portion 10 is kept from being covered by the SiO ₂ film 8, the Si ₃ N ₃ film 9, the Ta ₂ O ₅ film, or the ion-sensitive film 11. Further, a connection portion 15 for a lead wire 14 is formed at the other end of the metal electrode 13 .

It is preferable that the distance between the metal electrode 13 and the gate 5 is as close as possible, so that even when the sensor substrate 1 is taken out into the air during liquid exchange, the metal electrode 13 is also wetted by the liquid that wets the gate 5. It is necessary to be as close as possible. If the distance is such a short distance, the position and shape of the metal electrode 13 are not limited to those shown in FIG. 1, but may be, for example, as shown in FIGS. 3A and 3B. On the other hand, if the gate 5 and the metal electrode 13 are brought close to each other as described above, but if both do not get wet with liquid, a hydrophilic sol 14' can be provided to retain moisture as shown in Figure C. good.

Figure 4 shows a wiring diagram of a measurement circuit using the above ISFET sensor. In the figure, 16 is an ISFET sensor,
17 is Ag/Agcl reference electrode, 18 is test liquid, 19
is a switch. This switch 19 is operated to be turned on at least during liquid exchange. Therefore,
During liquid exchange, the ISFET sensor 16 is exposed to the air, so the gate voltage through the reference electrode 17 is
Even if V _G is no longer applied, the same gate voltage V _G is applied through the metal electrode 13 by turning on the switch 19, and the situation where the gate electric field becomes open can be avoided. In the above embodiment, only one MOSFET is formed on the Saffire substrate 1, but it is also possible to form dual MOSFETs and cancel the temperature influence of one MOSFET with the other MOSFET. , also on the same board
Thin film thermistors or diodes that correct the Nernst equation can also be formed and are included in embodiments of the present invention.

<Effects of the invention> As explained above, in the present invention, in an ISFET sensor in which a predetermined voltage is applied to the gate via the reference electrode during measurement, A metal electrode is formed near the ISFET gate, and during liquid exchange,
Since the configuration is such that a voltage similar to the above voltage is applied to the metal electrode via a switch, even if the reference electrode and the gate part of the ISFET sensor become disconnected during liquid exchange, a voltage will be formed near the gate. By applying a predetermined voltage to the gate through the metal electrode, the gate does not become open, and
Therefore, it is possible to effectively avoid gate breakdown, secondary changes in the gate insulator, etc., and desired measurements can always be performed stably.

[Brief explanation of the drawing]

Fig. 1 is a plan view of an ISFET sensor as an embodiment of the present invention, Fig. 2 is a side sectional view thereof, Fig. 3 A, B, and C are views showing other embodiments of metal electrodes, Fig. 4 is a wiring diagram of a measurement circuit using the ISFET sensor of the present invention. DESCRIPTION OF SYMBOLS 1...Substrate, 5...Gate, 13...Metal electrode, 17...Comparison electrode, 19...Switch.

Claims (1)

[Scope of utility model registration request] In the ISFET sensor, in which a predetermined voltage is applied to the gate via the reference electrode during measurement, a metal electrode is formed on the same substrate on which the ISFET is formed and in the vicinity of the ISFET gate. , an ISFET sensor configured to apply a voltage similar to the voltage to the metal electrode via a switch.