JPS62147335A - Electric field effect type pressure sensor - Google Patents

Abstract

PURPOSE:To make it possible to suppress the change in output due to the change in temp. or a secular change, by detecting the pressure difference between a measuring atmosphere and a cavity chamber as the change in the drain current of an electric field effect transistor. CONSTITUTION:A lower gate electrode 7 is formed on the gate insulating film 5 of an electric field effect transistor and a spacer 11 is further applied to said electrode 7. Next, the spacer on the gate region is removed by etching and an upper gate electrode 6 comprising a metal film is subsequently formed on the residual spacer 11. A fine aperture 9 is preliminarily opened to a silicon substrate 1 to communicate a cavity chamber 8 with the outside. By the above mentioned constitution, a pressure sensor grasps such a state that the electrostatic capacity of a condenser constituted of a lower gate electrode 7 and an upper electrode 6 changes by pressure difference 10 across the cavity chamber 8 above the gate region of the electric field effect transistor as the output change of the electric field effect transistor, that is, the change in a drain current to detect pressure. By this method, the output change of a sensor by temp. change or a secular change can be suppressed.

Description

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

<Prior art> Bourdon tubes,
The t-pressure is measured using mechanical changes in bellows, diaphragms, etc., and it is generally widely used because it is cheap and easy. However, with the development of electronics technology, there is a demand for the development of a pressure sensor that measures pressure changes as an electrical signal output in order to respond quickly to this trend.

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 control are easy, and direct output of measured values as electric signals not only enables pressure measurement with high accuracy 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) Pressure sensor with a metal foil strain gauge attached to a metal diaphragm (21Si diaphragm type pressure sensor + 3+ Pressure sensor using piezoelectric material such as PVDF, ZnO, etc.) (4) Crystal pressure sensor (5) Using capacitance change Pressure sensor In the conventional pressure sensor as described above, in the pressure sensor (1) in which a metal foil strain gauge is attached to a metal diaphragm, the metal foil is distorted when the diaphragm is deformed by pressure, and this distortion causes the electrical Pressure sensors that utilize changes in resistance have the advantage of being able to measure high pressures and have excellent temperature characteristics and long-term stability, but have the disadvantages of low sensitivity and being difficult to miniaturize. are doing.

(2) 81 diaphragm type pressure sensor is S i fa
It utilizes the piezoresistance effect in which the specific resistance of Si changes by applying pressure to the crystal, and since it is used as a SiK material, it has the advantage of being mass-produced using semiconductor technology and easy to integrate with peripheral circuits. , the temperature dependence is large and a temperature compensation circuit is required. Temperature compensation circuit and Si
A pressure sensor in which the pressure sensor is integrated with the same -S1 substrate has also been manufactured, but it has the disadvantage that it is expensive and the element is easily damaged because the mechanical strength of the Si diaphragm is weak.

(3) Pressure sensors using piezoelectric materials such as PVDF and Z no. Although it has advantages such as high output, it has low accuracy and tends to pick up noise due to vibration etc.

The crystal pressure sensor (4) uses the fact that the oscillation frequency of the crystal changes linearly with pressure, but
It has the drawbacks of being expensive and difficult to make small and bulky.

(No. 51 is a pressure sensor that detects the movement of a diaphragm as a change in the electrostatic field.Recently, an ultra-compact capacitance variable pressure sensor using a Si diaphragm has been developed, and it is better than a Si pressure sensor using a piezoresistive effect. It has been pointed out that it has high sensitivity and good stability.However, ultra-compact capacitance variable pressure sensors have a very small capacitance value, which means they have a very high impedance and are susceptible to external noise. However, it has some drawbacks.

As described above, the pressure sensors that have been developed so far have not been satisfactory in terms of performance or price, and have had various problems when put into practical use.

In view of these points, Ganto Ota filed a special patent application in 1983 for a new pressure sensor for seawater use, which could be manufactured ultra-small and inexpensively using semiconductor technology by using VC field-effect transistors. It is proposed as No.-52602.

The field-effect pressure sensor proposed earlier by Ganto Motoyama has a cavity above the gate insulating film of a field-effect transistor, and the gate electrode, which can be moved and deformed by pressure, is connected to the gate insulator through the cavity. When the gate electrode, which is provided on the gate insulating film through a cavity, is movably deformed by pressure, the distance between the gate electrode and the gate insulating film changes, and the voltage is applied to the channel. As a result, the pressure is detected as a change in the drain current of the field effect transistor.

<Problems to be solved by the invention> As mentioned above, the field-effect pressure sensor proposed by Mizu et al. As a result, the distance between the gate electrode and the gate insulating film changes and the electric field strength applied to the channel changes, and as a result, the pressure is detected as a change in the drain current of the field effect transistor. As a result of the study, problems were found, such as relatively large output changes due to temperature changes and aging.

That is, the field-effect pressure sensor previously proposed by Ganto Ohta is an absolute pressure sensor that maintains a cavity at a constant pressure or vacuum due to its structure.

This absolute pressure type field effect pressure sensor maintains a cavity at a constant pressure or vacuum, and utilizes the movable deformation of a metal film diaphragm on top of the gate insulating film of a field effect transistor due to pressure changes in the outside atmosphere. It is. In such an absolute pressure sensor, the cavity must be kept at a constant pressure or vacuum at all times without being affected by aging, so it is necessary to make the cavity completely airtight. Therefore, the spacer material must be carefully selected, and the adhesion between the spacer and the metal film diaphragm must be sufficiently high.

Furthermore, if the sealed cavity is configured to have a constant pressure, there is a problem in that the pressure fluctuates greatly depending on the temperature of the cavity, and the metal membrane diaphragm is displaced due to temperature changes, causing a change in the output of the field effect transistor. Ta.

The present invention was devised in view of these points, and by improving the field-effect pressure sensor previously proposed by Mizu, it suppresses output changes due to temperature changes, aging, etc. as much as possible. The object of the present invention is to provide a new and useful differential pressure type field effect pressure sensor.

Means for Solving the Problems> In order to achieve the above object, an uninvented field effect pressure sensor has a cavity completely submerged above the gate insulating film of a field effect transistor using a semiconductor substrate. A gate electrode movable and deformable by pressure is formed above the chamber, and a paper bill communicating between the cavity chamber and the outside is formed on the semiconductor substrate.

<Function> With the above configuration, the gate electrode provided on the gate insulating film through the cavity is movably deformed due to the pressure difference between the measurement atmosphere and the cavity communicating with the outside air, which is the reference atmosphere. As a result, the electric field strength applied to the channel changes, and as a result, the pressure difference is detected as a change in the drain current of the field effect transistor.

In addition, since the semiconductor substrate is provided with pores to communicate between the cavity and the outside to create a differential pressure type pressure sensor, pressure is always applied to the gate electrode, which forms a diaphragm, like an absolute pressure type pressure sensor. Since the diaphragm displaces only when a pressure difference occurs without any pressure being applied, there is no distortion in the diaphragm or the spacer that forms the cavity under normal conditions.As a result, changes in temperature, aging, etc. This results in a long-life sensor that does not cause any damage.

<Example> Before explaining the examples, the operating principle of the field effect pressure sensor of the present invention will be briefly explained. change depending on the electric field, and
The gate electrode at the top of the channel is provided directly on the gate insulating film as a lower (auxiliary) gate insulating film, and a cavity is formed on the gate insulating film that communicates with the outside through a pore? By providing the upper gate electrode through the capacitor, the upper gate electrode is movably deformed due to the pressure difference between the measurement atmosphere and the reference atmosphere, which is a hollow chamber that communicates with the outside air. The capacitance changes,
This takes advantage of the fact that the electric field strength applied to the channel changes.

Next, a one-field effect type pressure sensor as an embodiment of the invention will be explained 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.
■ 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 (diaphragm), 7 is an auxiliary (lower) gate electrode, 8 is a cavity, 9 is a silicon substrate It is a pore that communicates between the outside formed by IK and the cavity chamber 8, and IO is a pore that communicates between the measurement atmosphere and the cavity chamber 8.
It shows the differential pressure between

As shown in FIG. 1, the structure of the field-effect pressure sensor according to the present invention is such that an auxiliary electrode (lower gate electrode) 7 is entirely formed on the gate insulating film 5 of the field-effect transistor, and a spacer 11 is further formed on the auxiliary electrode (lower gate electrode) 7. 'r coat. 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 (ζ 11).Furthermore, a pore 9 is previously opened in the silicon substrate 1. In addition, the cavity chamber 8 is configured to communicate with the reference atmosphere (exterior).

In the field-effect pressure sensor configured in this manner, the upper part of the gate insulating film 5 of the field-effect transistor is a cavity 8, and the upper gate electrode 6 and lower gate electrode 7 made of a metal film form a capacitor. Do it. The cavity chamber 8 and the reference atmosphere are in communication through all the pores 9 made in the silicon substrate, and the upper gate electrode 6 made of a metal film on the upper part of the cavity chamber 8 is caused by the pressure difference between the measurement atmosphere and the reference atmosphere. The metal membrane diaphragm is movably changed. When this diaphragm is displaced due to a pressure difference, the capacitance formed by the upper gate electrode 6 and the lower gate electrode 7, which are metal films, changes in response to the pressure difference, and the drain current ID changes according to the following equation. become.

Here, μ is the carrier mobility, L, W, and VTH are the channel/l/length, channel width, and threshold voltage of the field effect transistor element, respectively, vG is the gate voltage, and C
ml is a case where the capacitance C1 of the gate insulating film 5 and the capacitance Ccav of the cavity chamber 8, that is, the capacitance formed by the hollow space between the lower gate electrode 7 and the upper electrode 6 are coupled in series. There is a hybrid capacitor of , which is given by the following equation.

Here, the cavity capacitance C6aV 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 pressure difference between the measurement atmosphere and the reference atmosphere.

In a field-effect pressure sensor, the capacitance of a capacitor composed of a lower gate electrode 7 and an upper gate electrode (metal film diaphragm) 6 across a cavity chamber 8 above the gate M region of a field-effect transistor changes depending on the pressure difference. The pressure is detected as a change in the output of a field effect transistor (a change in drain current). It is difficult to detect pressure changes by directly measuring the capacitance of a capacitor, which changes depending on the pressure difference, because the capacitance is very small, only a few PF. 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 difference between the capacitors 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. The small pressure measurement range and sensitivity are mainly determined by the material and thickness of the metal membrane diaphragm and the cavity size Kj, so by appropriately selecting the metal membrane material element structure, it is possible to measure pressures from minute to large. You can freely set the measurement pressure range up to

As shown in FIG. 1, a field-effect pressure sensor according to an uninvented embodiment is composed of a lower gate electrode 7 and an upper gate electrode (metallic diaphragm) 6, separated by a cavity 8 above the gate region of a field-effect transistor. The capacitance of the capacitor changes depending on the pressure, and the pressure is detected as a change in the drain current of two field effect transistors.

In the field effect pressure sensor having the lower gate electrode as described above, the lower gate electrode 7 is directly connected to the gate insulating film 5.
By forming A on the lower gate electrode 7, it becomes possible to apply a DC voltage for operating the transistor through the lower gate electrode 7 without causing changes in transistor characteristics, drift of drain current, etc.

Further, as described above, the uninvented field effect pressure sensor has a cavity 8 which is connected to the cavity 8 through the pore 9 opened in the silicon substrate 1.
Since the reference atmosphere is in communication with the reference atmosphere, there is no need to maintain the cavity 8 at a constant pressure or vacuum, and high airtightness of the cavity is not required as in the case of an absolute pressure sensor. Furthermore, the pressure within the cavity 8 is no longer significantly affected by temperature changes.

The metal film serving as the upper gate electrode 6 acts as a diaphragm that is movably deformed by the pressure difference between the reference atmosphere communicating with the cavity chamber 8 and the measurement atmosphere acting on the upper gate electrode 6.
When the diaphragm deforms due to the pressure difference, the static volume composed of the metal film diaphragm and the lower gate electrode 7 changes in response to the pressure difference.

When this capacitance changes in response to the pressure difference, the pressure is detected as a change in the output of the field effect transistor. Note that by using a differential pressure type, pressure does not always apply to the diaphragm like with absolute pressure type pressure sensors, and the diaphragm only displaces when a pressure difference occurs, so there is no possibility of distortion occurring in the diaphragm or spacer. do not have.

<Effects of the Invention> As described above, the differential pressure type field effect pressure sensor according to the present invention has the following advantages when compared with the absolute pressure type field effect type pressure sensor previously proposed by Ganto Motoyama. have

■ Differential pressure type pressure sensors have a cavity that communicates with the reference atmosphere through pores, so there is no need to maintain a constant pressure or vacuum in the cavity, and it is not necessary to maintain complete airtightness of the cavity or between the spacer and metal membrane diaphragm. Youth is not required.

(2) Since the cavity communicates with the reference atmosphere, there is little effect on pressure fluctuations due to temperature changes within the cavity, and therefore there are few changes in the movement of the metal membrane diaphragm due to the temperature change IC.

■ Unlike absolute pressure sensors, no pressure is applied to the metal membrane diaphragm, and changes in diaphragm movement are observed only when a pressure difference occurs between the cavity chamber and the measurement atmosphere. It is possible to produce a long-life sensor without distortion.

[Brief explanation of drawings]

FIG. 1 is a diagram schematically showing the structure of a differential pressure field effect pressure sensor as an embodiment of the present invention. l: silicon substrate, 2: source, 8 drains, 4: channel, 5: gate insulating film, 6: upper gate electrode (diaphragm), 7: lower gate electrode, 8: cavity, 9: pore, 10: difference Pressure, 11 Ni spacer 0

Claims (1)

[Claims] 1. A cavity is provided above the gate insulating film of a field effect transistor using a semiconductor substrate, a gate electrode movable and deformable by pressure is formed on the cavity, and the cavity and the outside are connected to each other. A field-effect pressure field-effect type pressure device, characterized in that a pore is formed in the semiconductor substrate to communicate with the semiconductor substrate, and the pressure difference between the measurement atmosphere and the cavity chamber is detected as a change in the drain current of a field-effect transistor. sensor.