JPS6294988A - Field effect pressure sensor - Google Patents

Abstract

PURPOSE:To obtain the high-accuracy pressure sensor by detecting the variation of the field intensity applied to a channel by flexibly deforming an upper gate electrode arranged on a gate insulating film through a cavity chamber or an strechable insulating film. CONSTITUTION:The variation of the electrostatic capacity of a capacitor composed of a lower gate electrode 7 and an upper gate electrode 6 through a cavity chamber 8 above a gate region of a field effect transistor due to a pressure is recognized as the variation of the drain current of the FET and the pressure is detected. In the field effect pressure sensor comprising a lower gate electrode, the lower gate electrode 7 is formed on a gate insulating film 5 directly so that it becomes possible to apply a D.C. voltage for operating a transistor through the lower gate electrode 7 without producing the variation of the transistor characteristics, drift of the drain current, etc. Accordingly, the transistor can be operated steadily and the pressure can be detected with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a pressure sensor capable of extracting pressure changes in the atmosphere and liquid as electrical signal output.

Pressure measurement in the atmosphere or in liquids has traditionally been carried out using mechanical devices such as Bourdon tubes, bellows, and diaphragms, which are inexpensive and easy to use. 1' = It is generally widely used because it is light. 1, ′
However, with the development of electronics technology, we can quickly respond to this! There is a need for the development of a pressure sensor that measures pressure changes as an electrical signal output.

A pressure sensor that measures pressure changes as an electrical signal output is
It can be easily connected to a data processing system, automatic measurement and automatic sterilization are easy, and direct output of measured values as electrical signals not only makes it possible to measure pressure with high precision, but also has a fast response speed. It also has the advantage of being easy to reduce in size and weight.

Therefore, various types of pressure sensors have been researched and developed as follows.

(1) A pressure sensor with a metal foil strain gauge attached to a metal diaphragm (2) A Si diaphragm type pressure sensor (t3) A pressure sensor using a piezoelectric material such as PVDF or ZnO (4) A crystal pressure sensor (5) 1 Pressure sensor using capacitance change In the conventional pressure sensor as described above, the pressure sensor (1) in which a metal foil strain gauge is attached to a metal diaphragm has a pressure sensor that distorts the metal foil as the diaphragm deforms due to pressure.
Utilizing the fact that the electrical resistance of the metal foil changes due to this distortion, it is possible to measure high pressures with a single pressure sensor, and while it has the advantage of being steeped in temperature characteristics and long-term stability, it has low sensitivity, small size, and light weight. The disadvantage is that it is difficult to convert.

(2) The Si diaphragm pressure sensor utilizes the piezoresistance effect in which the specific resistance of Si changes when pressure is applied to the Si crystal, and since it uses Sl as a material, it can be mass-produced using semiconductor technology. Although it has the advantage of being easy to integrate with peripheral circuits, it is highly temperature dependent and requires a temperature compensation circuit. A single pressure sensor has been manufactured by integrating a temperature compensation circuit and an 81 pressure sensor on the same Si substrate, but it is expensive and has the disadvantage that the element is easily damaged due to the weak mechanical strength of the S1 diaphragm. ing.

(3) Pressure sensors using piezoelectric materials such as PVDF and ZnO are pressure sensors that utilize the piezoelectric effect that generates electromotive force when these piezoelectric materials are distorted under pressure, and are small, lightweight, and have a large output. However, it has the disadvantage of low accuracy and the tendency to pick up noise due to vibrations, etc.

The crystal pressure sensor (4) utilizes the fact that the oscillation frequency of the crystal changes linearly with pressure.
It has the drawbacks of being expensive and difficult to reduce in size and weight.

(5) is a pressure sensor that detects the movement of a diaphragm as a change in capacitance.Recently, an ultra-compact capacitance variable pressure sensor using an S1 diaphragm has been developed, and a Si pressure sensor that uses the piezoresistive effect It has been pointed out that it has higher sensitivity and better stability. However, the ultra-small capacitance variable pressure sensor has a drawback that the capacitance value is very small, that is, the impedance is very high, and it is easily affected by external noise.

As described above, the pressure sensors that have been developed so far have not been satisfactory in terms of performance or cost, and have had various problems that have hindered their practical use.

In view of these points, the inventors of the present invention first applied a patent application in 1983 to create a new and useful pressure sensor that can be manufactured in an ultra-small size and at low cost using semiconductor technology by using field-effect transistors. It is proposed as No.-52602.

The field-effect pressure sensor previously proposed by the present inventors has a cavity or an insulating layer that expands and contracts under pressure on top of the gate insulating film of a field-effect transistor, and a gate electrode that can be moved and deformed under pressure is placed in the cavity. The above-mentioned gate insulating film is formed on the gate insulating film through a cavity or an insulating layer, and a gate electrode is formed on the gate insulating film through a stretchable insulating film. Due to the movable deformation, the distance between the gate electrode and the gate insulating film changes, which changes the electric field strength applied to the channel, and as a result,
The pressure is detected as a change in the drain current of a field effect transistor.

<Problems to be Solved by the Invention> As described above, the field-effect pressure sensor proposed by the present inventors is a pressure sensor that is provided on a gate insulating film via a cavity or a stretchable insulating film. When the gate electrode is movably deformed by pressure, the distance between the gate electrode and the gate insulating film changes, and the electric field strength applied to the channel changes, which is detected as a change in the drain current of the pressure field effect transistor. However, as a result of various subsequent studies, problems were discovered, including changes in the characteristics of the field effect transistor and drifting of the drain current.

That is, in the field-effect pressure sensor previously proposed by the present inventors, in order to operate the field-effect transistor, it is necessary to apply a DC voltage to the gate electrode configured as a metal diaphragm. In addition, DC voltage is also applied to the spacer, source, and drain at the bottom of the metal diaphragm, causing a problem in which the characteristics of the field effect transistor change and the drain current drifts.

The present invention was devised in view of the above points, and by improving the field-effect pressure sensor previously proposed by the present inventors, it is possible to achieve a highly efficient transistor that can operate stably. Aims to provide precision pressure sensor.

First Means to Solve the Problems〉 The field effect pressure sensor of the present invention has an insulating layer that expands and contracts under pressure in a cavity above the gate insulating film of a field effect transistor. A possible (upper) gate electrode is formed on the gate insulating film through the insulating layer in the cavity, and expands and contracts with the gate insulating film and the cavity or with the gate insulating film by pressure. The structure is such that an auxiliary (lower) gate electrode is attached to the interface of the insulating film.

Effect> With the above structure, the upper gate electrode is provided on the gate insulating film via a cavity or an expandable insulating film, and is movably deformed by pressure, so that the upper gate electrode and the lower gate electrode are As the spacing changes, the electric field strength applied to the channel changes, and as a result, the pressure is sensed by changing the drain current of the field effect transistor.

Further, by applying a DC voltage for operating the field effect transistor through the lower gate electrode, changes in characteristics of the transistor, drift of drain current, etc. do not occur.

<Example> Before explaining the examples, the operating principle of the field effect pressure sensor of the present invention will be briefly explained. In addition, the gate electrode on the top of the channel is placed directly on the gate insulating film (
[Auxiliary] In addition to being provided as a gate electrode, by providing it as an upper gate electrode on the gate insulating film through a cavity or a stretchable insulating film, the upper gate electrode is movably deformed by pressure, and the upper gate electrode and lower gate electrode are connected to each other. This method takes advantage of the fact that the electric field strength applied to the channel changes as the capacitance of the capacitors is changed.

Next, a field effect pressure sensor as an embodiment of the present invention will be described in more detail using a schematic structural diagram.

FIG. 1 is a diagram schematically showing the structure of a field-effect pressure sensor as an embodiment of the present invention.
1 is a silicon substrate, 2 is a source, 3 is a drain, 4 is a channel, 5 is a gate insulating film, 6 is an upper gate electrode, 7 is an auxiliary (lower) gate electrode, 8 is a cavity, and 9 is a pressure As shown in FIG. 1, the structure of the field-effect pressure sensor according to the present invention is as shown in FIG. Coat spacer 10 on top. Next, after removing the spacer on the gate region by etching, an upper gate electrode 6 made of a metal film is formed on the remaining spacer 10.

In the field-effect pressure sensor constructed in this way, the upper part of the gate region of the field-effect transistor is a cavity 8, and the upper gate electrode 6 made of a metal film forms a capacitor with the lower gate electrode 7. . Upper gate electrode 6
The metal film acts as a diaphragm that is movably deformed by the pressure difference between the cavity chamber 8 and the surrounding air, and when this diaphragm is displaced by the pressure difference, the static metal film consisting of the upper gate electrode 6 and the lower gate electrode The capacitance changes in response to the pressure difference, and the drain current ID changes according to the following equation.

It shows the channel length, channel width and threshold voltage of a field effect transistor element, Vo is the gate voltage, and C
m1x is a series combination of the capacitance Ci of the gate insulating film 5 and the capacitance C1 of the cavity 8, that is, the capacitance formed by the hollow interval between the lower AV part gate electrode 7 and the upper gate electrode 6. is the hybrid capacitance of , which is given by the following equation:

Here, the cavity capacitance C3av changes depending on the distance between the lower gate electrode @7 and the upper gate electrode 6 as described above, and this distance changes depending on the atmospheric pressure. Now, assume that the cavity 8 is cylindrical, Assuming that its diameter is 1, the thickness of the spacer IO is 3 μm, the diameter of the circular lower gate electrode 1 is 07, the material of the metal film diaphragm constituting the upper gate electrode 6 is copper, and the thickness is 10 μm. The capacitance of the capacitor composed of the metal film 6 and the lower gate electrode 7 changes as shown in Figure 2 due to the pressure difference between the cavity 8 and the outside atmosphere, and the capacitance when there is no pressure difference changes. Ha+
, +3. F, but the pressure of the outside atmosphere is 0.
When 2atrn increases, the diaphragm forming the upper gate electrode 6 bends toward the cavity 8, and the capacitance decreases to 1.
72 pF, which increases 1.51 times.

A field-effect pressure sensor is a state in which the capacitance of a capacitor composed of a lower gate electrode 7 and an upper gate electrode (metal film diaphragm) 6 across a cavity 8 above the gate region of a field-effect transistor changes depending on pressure. is the output change (drain current change) of the field effect transistor.
Detects pressure as Detecting pressure changes by directly measuring the capacitance of a capacitor, which changes with pressure, is difficult because the capacitance is very small, only a few F. By detecting pressure changes as changes in the drain current of a field-effect transistor, the output impedance of the element can be lowered, making it less susceptible to noise and the like, making pressure measurement easier. Furthermore, the field effect pressure sensor amplifies the change in capacitance due to the pressure of the capacitor and detects it as a change in the drain current of the field effect transistor, so it is possible to measure pressure with high sensitivity.

In addition, the pressure measurement range and sensitivity are mainly determined by the material and thickness of the metal membrane diaphragm and the size of the cavity, so by appropriately selecting the metal membrane material element structure, the measurement pressure can be adjusted from minute pressure to large pressure. The range can be set freely.

The field effect pressure sensor according to the present invention is a capacitor composed of a lower gate electrode 7 and an upper gate electrode (metal film diaphragm) 6, separated by a cavity 8 above the gate region of a field effect transistor, as shown in FIG. Pressure is detected by using changes in capacitance due to pressure as changes in the drain current of a field effect transistor. In response to this, the present inventors first proposed that the lower gate electrode (
This is a field-effect pressure sensor with a structure that does not have an auxiliary electrode (auxiliary electrode). There was a problem in that a DC voltage was also applied to the spacer 10, source 2, and drain 3 at the bottom of the field effect transistor, causing changes in the characteristics of the field effect transistor and drift of the drain current. In the field-effect pressure sensor, since the lower gate electrode 7 is formed directly on the gate insulating film 5, the DC voltage required to operate the transistor can be maintained without causing changes in transistor characteristics, drift of drain current, etc. can be applied through the lower gate electrode 7.

Note that in FIG. 1, the gate electrode is formed using a metal thin film that is easy to move and deform, but the present invention is not limited to this. It goes without saying that even if the cavity is sealed using a polymer compound to form an insulating layer that expands and contracts with pressure, and a gate electrode is formed thereon, the device can operate as a field-effect pressure sensor.

<Effects of the Invention> As described above, the field effect pressure sensor according to the present invention using a field effect transistor has the following features compared to conventional pressure sensors.

fl) Since it can be manufactured using semiconductor technology, it is possible to manufacture ultra-small devices in large quantities.

(2) Although the element shape is ultra-small, the output impedance is low and it is less susceptible to noise and the like.

(3) Since diaphragm materials and element shapes can be selected from a wide range, pressure measurement accuracy and measurement range can be freely designed.

(4) By applying a DC voltage to the lower gate electrode, the transistor can be operated stably and pressure can be detected with high accuracy.

(5) When the zero-pressure sensor is manufactured using a Si substrate, the sensor and peripheral circuits such as a signal processing circuit are integrated.
- It becomes possible to realize a chip device, and atmospheric pressure can be measured at low cost.

[Brief explanation of drawings]

FIG. 1 is a diagram schematically showing the structure of a field effect pressure sensor as an embodiment of the present invention, and FIG. FIG. 3 is a diagram showing capacitance characteristics. 1... Relicon M board, 2... Source, 3... Drain. 4... Channel, 5... Gate insulating film, 6... Upper gate electrode (diaphragm), 7... Lower gate electrode. 8... Cavity chamber, 9... Pressure, 10... Spacer.

Claims (1)

[Claims] 1. A cavity or an insulating layer that expands and contracts with pressure is provided above the gate insulating film of a field effect transistor, and a gate electrode that can be moved and deformed by pressure is connected to the gate through the cavity or the insulating layer. An auxiliary gate electrode is formed on an insulating film, and an auxiliary gate electrode is provided at the interface between the gate insulating film and a cavity, or between the gate insulating film and an insulating film that expands and contracts with pressure, and the pressure is used as a change in the drain current of the field effect transistor. A field-effect pressure sensor characterized by detecting.