JPH0554706B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a vibrating strain sensor that outputs strain caused by force applied to a semiconductor substrate as a frequency signal.

<Prior Art> Semiconductor strain sensors in which a resistance element is formed on a semiconductor substrate by diffusion or vapor deposition and convert force applied to the semiconductor substrate into an electrical signal are well known and have already been widely put into practical use.

In such a semiconductor strain sensor, the resistance element exhibits an analog resistance change depending on the force to be measured, but the gauge factor as an output is extremely small. For this reason, when the output signal is processed by a computer or the like, it must be amplified or A/D converted.

A sensor using a vibrating wire that obtains a frequency signal corresponding to pressure is also known, as can be seen in, for example, Japanese Patent Laid-Open No. 54-56880. This device applies a force generated on a diaphragm to a vibrating wire coupled to the diaphragm, and obtains a change in the natural frequency related to the tension of the vibrating wire as a frequency signal output.

In this device, it is necessary to connect the two so that the force generated in the diaphragm can be accurately transmitted to the vibration line, and a configuration must be devised so that changes in the environment around the vibration line will not cause changes in the natural frequency of the vibration line. There was a problem that the configuration was complicated due to the necessity of

<Problems to be Solved by the Invention> A technical problem to be solved by the present invention is to realize a vibratory strain sensor with a simple configuration that outputs a high frequency signal of a gauge factor corresponding to force.

<Means for Solving the Problems> The configuration of the present invention includes an n (or p) type semiconductor substrate 1 and a p (or n) type semiconductor substrate formed on the semiconductor substrate 1 at a predetermined distance. 1 diffusion layer 11,
11a, a vibrator 4 made of silicon which is formed by heating and recrystallizing polysilicon and having both ends in contact with the first diffusion layers 11 and 11a, and a vibrator 4 formed on both sides of the vibrator in the longitudinal direction on the substrate 1. a p (or n) type second diffusion layer 12 formed at a predetermined distance;
12a, and a transfer circuit connected to either one of the second diffusion layers 12, 12a to cause the vibrator to resonate at a resonant frequency.

<Example> Fig. 1a is an enlarged cross-sectional view of a main part showing an embodiment of a vibrating strain sensor according to the present invention, and Fig. 1b is an enlarged sectional view of a main part of a vibrating strain sensor according to an embodiment of the present invention.
FIG. 2 is an explanatory diagram showing the manufacturing process of the bridge. In FIG. 2, 1 is an n-type semiconductor substrate, and 11, 11a and 12, 12a are p ⁺ diffusion layers diffused on the semiconductor substrate 1 at predetermined intervals d and d'. 3 is a protective film made of SiO ₂ or the like, which is formed to cover a predetermined range including the p ⁺ diffusion layers (11, 11a, 12, 12a). In this case, a hole is made in the protective film 3 at a portion of the p ⁺ diffusion layer located near the center of the p + diffusion layers 11 and 11a to expose the p ⁺ diffusion layer.

Next, a band-shaped polysilicon 4 is formed on the protective film 3 and connecting the holes. That is, the polysilicon has p ⁺ diffusion layers 11 and 10a at both ends 10 and 10a.
The other portion connected to 11a faces the substrate through the thickness of the protective film 3 and is located between 12 and 12a.

Next, polysilicon is heated for a short time using a laser beam or the like to recrystallize it.

By selectively etching the protective film (SiO ₂ ) 3 with hypofluoric acid, a bridge (hereinafter referred to as a vibrator) 4 can be formed on the semiconductor substrate as shown in FIG. 1b.

According to the above configuration, the
An n-channel FET is formed with the p ⁺ diffusion layers (12, 12a) as the drain and source and the vibrator as the gate.

Figure 3 shows the circuit configuration for vibrating the above vibrator at the resonant frequency.
One end of a transfer circuit 20 in which a capacitor C and a coil L are connected in parallel is connected to the p ⁺ diffusion layer 12, and a drain voltage Vdd is connected to the other end. Then, the p ⁺ diffusion layer 12a serving as the source is grounded, and the semiconductor substrate 1
A voltage Vs is applied to the resonator 4 to insulate it from the diffusion layers 11 and 11a, and a gate voltage is applied to the vibrator 4 via a resistor R. As a result, the vibrator 4 oscillates at its natural frequency. This oscillation is caused by the following mechanism. That is, in FIG. 4, when the gate vibrates, the capacitance between the gate and the substrate (channel capacitance) changes, causing a drain current to flow. This is because an inversion layer is formed in the channel. When the channel is turned on (drain current flows), the potential directly below the vibrator 4 is considered to be approximately 1/2 (V _D −V _SS (V _SS =0).

That is, the potential directly below the vibrator 4 changes as Vs←→Vs+1/2V _D by turning the FET on and off.

This potential difference causes electrostatic attraction force to be repeatedly applied to the vibrator 4. At the resonant frequency of the vibrator 4, the phase relationship between the attraction force and the displacement causes a π/2 delay. When the center frequency of the transfer circuit 20 is set approximately equal to the center frequency of the vibrator 4, the phase relationship between the drain voltage and the drain current (in phase with the displacement of the vibrator 4) produces a phase difference of -π/2 at the resonant frequency. .
Therefore, the phase delay of the vibrator 4 is canceled, and if the Q of the vibrator 4 is sufficiently large, the system oscillates at the resonant frequency f ₀ of the vibrator 4. When this semiconductor substrate 1 is distorted due to force, the bridge 4 is also distorted,
Since the oscillation frequency changes depending on the strength of the force, the force applied to the semiconductor substrate can be measured.

The gauge factor (frequency change rate) Fg of this bridge is expressed by the following equation.

Fg=1/f・∂f/∂ε=0.118 (l/h) ² where l=length of bridge h=thickness of bridge.

For example, if l=200μm and h=1μm, Fg
is approximately 4700.

In the above embodiments, the semiconductor substrate is of n-type and a p-type diffusion layer is provided on the substrate, but the semiconductor substrate may be of p-type and an n-type diffusion layer is provided thereon. In that case, the FET shown in FIG. 3 becomes a p-channel FET.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, a vibration type with a simple configuration can output a high frequency signal of a gauge factor in response to applied force. A strain sensor can be realized.

[Brief explanation of the drawing]

FIG. 1a is an enlarged plan view of essential parts showing an embodiment of the vibration type strain sensor according to the present invention, and FIG. 1b is a plan view of FIG. 1a.
FIG. 2 is an explanatory view showing the manufacturing process of the vibrator, and FIG. 3 is a circuit diagram for exciting the vibrating strain sensor. 1... Semiconductor substrate, 2... Diffusion layer, 4... Bridge (oscillator).

Claims (1)

[Claims] 1 an n (or p) type semiconductor substrate 1 and a p type formed on the semiconductor substrate 1 at a predetermined distance.
(or n) type first diffusion layers 11, 11a, a vibrator 4 made of silicon formed by heating and recrystallizing polysilicon and having both ends in contact with the first diffusion layers 11, 11a, and the substrate 1. p (or n) type second diffusion layers 12, 12a formed on both sides of the vibrator in the longitudinal direction at a predetermined distance;
A vibrating strain sensor characterized by comprising a transfer circuit connected to either one of the second diffusion layers 12, 12a and causing the vibrator to resonate at a resonant frequency.