JPH07114288B2 - Semiconductor strain measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor strain measuring device that converts physical strain into an electric signal and outputs the electric signal.

[Prior Art] Conventionally, a load or force applied to a solid, or a liquid,
Various mechanical-electrical converters that detect a mechanical quantity such as the pressure of a fluid such as a gas and convert it into an electric quantity are known.

There is a semiconductor strain measuring device as one of such mechanical-electrical converters, and with the recent development of IC technology, it has been widely adopted in various fields.

Here, this type of semiconductor strain measuring device is a piezoresistive element of a conductivity type (P type or N type) different from that of the semiconductor single crystal substrate in a strain generation region where strain occurs in accordance with a force such as a pressure of the semiconductor single crystal substrate. It is composed of a diffused structure. This semiconductor strain measuring device converts the physical strain generated in the semiconductor single crystal substrate into an electric signal by utilizing the piezoresistive effect in the piezoresistive element (the effect that the resistance value changes according to the stress). Can be output. As described above, the semiconductor strain measurement device can directly output the detected strain as an electric signal and can make the entire device extremely small, and thus is widely used for local strain measurement and pressure measurement. It is becoming popular.

For example, in the field of automobiles, it is very important to accurately measure the pressure of each part in the engine in order to control combustion efficiently in the engine. Since the semiconductor strain measuring device can be formed in a small size as described above, it is considered to be very suitable for the measurement in this way, and it is proposed to use the semiconductor strain measuring device for pressure measurement of each part in the engine. ing.

[Problems to be Solved by the Invention] However, the conventional semiconductor strain measuring device has a problem that measurement accuracy is drastically lowered in a high temperature region, and measurement becomes impossible. That is, in the conventional semiconductor strain measuring apparatus, when the temperature of the apparatus exceeds 275 ° C., a non-negligible leakage current is generated between the semiconductor substrate of different conductivity type and the piezoresistive element, which makes measurement impossible.

On the other hand, when measuring the pressure inside the engine, the measuring device may reach about 300 ° C at the highest temperature. Therefore, for this measurement, consider a semiconductor strain measuring device that is stable at 300 ° C or higher.
It is required to be able to measure up to a high temperature range of 50 ° C or higher. Therefore, in the conventional semiconductor strain measuring device,
It was necessary to provide equipment such as cooling so that the temperature of the device would not exceed the measurement limit temperature. However, when the cooling equipment is provided, the apparatus becomes large in size, and additional wiring and piping are required, so that there is a problem that the degree of freedom of arrangement is significantly limited.

OBJECT OF THE INVENTION The present invention has been made to solve the above problems, and an object thereof is to provide a semiconductor strain measuring device capable of performing stable strain measurement even in a high temperature environment of 350 ° C. or higher. To do.

[Means for Solving the Problems] According to the invention described in claim (1), a semiconductor single crystal substrate and physical strain generated on the semiconductor single crystal substrate and generated in the semiconductor single crystal substrate are used as electric signals. And a piezoresistive element for outputting, wherein one of the semiconductor single crystal substrate and the piezoresistive element is P-type and the other is N-type, the surface impurity concentration N1 of the piezoresistive element, and the semiconductor single crystal The ratio N1 / N2 to the impurity concentration N2 of the substrate is set to 10 ² or more, the area A of the PN junction where the semiconductor single crystal substrate and the piezoresistive element are in contact, and the impurity concentration N2 of the semiconductor single crystal substrate. The ratio A / N2 is set to 5 × 10 ⁻²² cm ⁵ or less.

The invention according to claim (2) is characterized in that the surface impurity concentration N1 of the piezoresistive element is in the range of 2.4 × 10 ²⁰ to 4 × 10 ²⁰ cm ⁻³ .

Here, the semiconductor single crystal substrate is formed of, for example, single crystal silicon. When the semiconductor single crystal is N type, the piezoresistive element is P type, and when the semiconductor single crystal is P type, the piezoresistive element is N type. Further, the piezoresistive element is formed on a semiconductor single crystal substrate by diffusion of impurities (phosphorus P for N type, boron B for P type, etc.).

[Operation] The semiconductor strain measuring device according to the present invention has the above-described configuration and outputs the physical strain of the piezoresistive element as an electric signal. The principle of generation of this electric signal is that the piezoresistive element causes a resistance change depending on the presence or absence of strain. Then, the semiconductor strain measuring apparatus outputs the change in the resistance value of the piezoresistive element as an electric signal. For example, applying an appropriate voltage from the outside to the piezoresistive element,
The strain can be detected by measuring the change in the output voltage when the strain occurs in this state.

Here, in the semiconductor strain measuring apparatus according to the present invention, in order to realize strain measurement in a high temperature environment, manufacturing conditions of a semiconductor substrate and a piezoresistive element having different conductivity types are optimized. That is, the semiconductor substrate and the piezoresistive element are formed in the regions having opposite conductivity types, and the PN
Insulated and separated by joining. Then, this blocks the current flowing between the piezoresistive element and the substrate. However, the insulation separation due to the PN junction deteriorates as the temperature rises, and leakage current occurs. Then, this leakage current becomes an error factor of the measured value at high temperature. Therefore,
In the present invention, by reducing the leakage current at the PN junction, it is possible to perform measurement with little error even at high temperatures.

For example, in a strain measuring device, a voltage is applied to a piezoresistive element, and a measurement current Ig is flown there. In this state, if the piezoresistive element and the substrate are completely insulated from each other, no leakage current will occur at the PN junction and no error factor will occur. At around room temperature, the leakage current is of the order of [nA], and it can be considered that the leakage current is completely isolated.

However, the leakage current of the PN junction has a large positive temperature dependence. Therefore, when the leakage current in this PN junction was examined, it was found that the leakage current Ir of the PN junction was expressed by the following equation.

Ir = ni ² · (q · Dp / Lp) · (A / Nd) + ni ² · (q · Dp / Ln)
(A / Na) where ni is the intrinsic impurity concentration, q is the elementary charge, D is the diffusion coefficient of minority carriers, L is the diffusion length of small carriers, N is the concentration of impurities (donors and acceptors), and A is PN junction area. The subscript n indicates an electron, and the subscript p indicates a hole.

If the substrate is N type and the piezoresistive element is P type, Nd represents the impurity concentration of the substrate and Na represents the impurity concentration of the piezoresistive element.

Examining the temperature dependence of each parameter, the parameter having the largest temperature dependence is the intrinsic impurity concentration ni, which tends to increase exponentially with increasing temperature. The temperature dependence of the intrinsic impurity concentration ni is unique to the material and cannot be controlled by the manufacturing process of the semiconductor strain measuring device.

Further, q is an elementary charge, which has no temperature dependence.
Furthermore, L and D have much smaller temperature dependence than ni, and these L and D are parameters that depend on the material and the manufacturing process, but if the material is determined, their values are almost determined. Is. Therefore, it is considered that these L and D are parameters that cannot be easily controlled in the manufacturing process.

On the other hand, the concentration N of impurities (donor and acceptor) is a parameter that can be easily controlled in the manufacturing process, although it has no temperature dependence by itself. Further, the PN junction area A also has no temperature dependence, but can be easily controlled in the manufacturing process.

Thus, it is considered that the temperature dependence of the leakage current Ir is governed by the temperature dependence of the intrinsic impurity concentration ni. However, the temperature dependence of this intrinsic impurity concentration ni cannot be controlled.

Therefore, in the present invention, the leakage current Ir at the PN junction is
In order to reduce the above, attention is paid to N and A, which are controllable parameters in a manufacturing process that does not itself have temperature dependence, and the following improvements are made.

First, by increasing the ratio of the impurity concentration Nd (N2) of the substrate and the surface impurity concentration Na (N1) of the piezoresistive element, a small value can be obtained so that the second term can be ignored with respect to the first term in the above equation. To do. That is, the second term can be substantially omitted by setting the condition of Na / Nd ≧ 10 ² .

Therefore, the leakage current Ir is expressed by the following equation.

Ir = ni ² · (q · Dp / Lp) · (A / Nd) Next, in the equation omitting this second term, pay attention to the term of (A / Nd) and derive the condition for reducing the leakage current. It was

That is, by setting the range of (A / Nd) ≦ 5 × 10 ⁻²² [cm ⁵ ], we succeeded in reducing the leakage current of the PN junction by one digit or more compared with the conventional one.

Furthermore, the surface impurity concentration Na of the piezoresistive element is 2.4 × 10
^By setting it in the range of ²⁰ to 4 × 10 ²⁰ , it is possible to sufficiently reduce the temperature change characteristics of the bridge output when the bridge circuit composed of piezoresistive elements is driven by a constant current source, and to measure strain at high temperatures. Is possible.

[Effects of the Invention] As described above, according to the semiconductor strain measuring apparatus of the present invention, the leakage current flowing through the PN junction between the piezoresistive element and the substrate can be effectively reduced even in a high temperature region, and the temperature of 350 ° C.
Accurate strain measurement can be performed even in the above high temperature region.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing the structure of the embodiment, and FIG. 2 is a plan view thereof. In the figure, the substrate 10 is formed of single crystal silicon. An insulating layer 12 made of silicon dioxide is provided on the surface of the substrate 10. The insulating layer 12 has a function as a surface protective film.

Then, on the upper surface of the semiconductor substrate 10, a piezoresistive element 14 formed by diffusion of impurities is integrally formed with the substrate. Since the resistance value of the piezoresistive element 14 changes in response to physical strain, the strain can be output as an electric signal by passing a current there.

In this example, four piezoresistive elements 14 are formed on the substrate surface so as to draw a square, and the four piezoresistive elements 14 form a bridge circuit. .
Further, aluminum electrodes 16 are connected to the four corners of the four piezoresistive elements which correspond to both ends thereof. That is, the aluminum electrode 16 penetrates the insulating layer 12 and is in direct contact with the piezoresistive element 14. In this way, when the piezoresistive element (diffusion layer) 14 is used as a bridge circuit and an appropriate voltage is applied from the outside through the aluminum electrode 16, the bridge output changes when the piezoresistive element 14 is distorted in this state. The strain can be detected by measuring

The piezoresistive element 14 is formed by diffusing impurities from the surface of the substrate. For example, as shown in FIG. 3, a region having a depth of about 2 μm from the surface of the substrate 10 is the piezoresistive element 14.

In the normal case, each of the piezoresistive elements 14 is set to about 1 kΩ, and a voltage of about 3 V is applied to the bridge formed by the piezoresistive elements 14. Also, the substrate 10
Is an N-type containing phosphorus P of 10 ¹⁵ to 10 ¹⁸ cm ^-3 as an impurity,
The piezoresistive element can be a P type in which boron B is diffused by 10 ²⁰ cm ^-3 .

Example 1 FIG. 4 shows Example 1 as a temperature characteristic of a semiconductor strain measuring apparatus according to the present invention. In this case the PN junction area is 10 ^-4 cm
^2. The impurity concentration of the substrate 10 is set to 2 × 10 ¹⁸ cm ^-3 . Therefore, A / Nd is 5 × 10 ^-23 cm ⁵ . And the piezoresistive element in this semiconductor strain measuring device
The rate of change of resistance with respect to temperature 14 increases linearly up to about 400 ° C. as shown in FIG. 4 (A). And 400 ℃
The rate of change in resistance starts to decrease when the temperature exceeds 10 ° C.
This characteristic is related to the temperature characteristic of the leakage current Ir at the PN junction surface, and with respect to the measured current Ig flowing through the piezoresistive element 14, if the leakage current Ir of the PN junction reaches a non-negligible order, it is affected by it. The apparent resistance is reduced. Therefore, at temperatures above this point (peak point),
The semiconductor strain measuring device cannot detect the change in the resistance of the piezoresistive element accurately, and the peak point corresponds to the use limit temperature.

When viewed from the sensitivity change rate of the semiconductor strain measuring apparatus in FIG. 4 (B), the change in the sensitivity change rate is minute in the temperature region until the peak point is reached. However, when the peak point is reached, the sensitivity drops sharply. It is understood that the temperature at which the sensitivity changes abruptly is the use limit temperature.

From this experimental result, according to the apparatus of Example 1, about 400
It can be seen that stable strain measurement is possible up to ℃.

Example 2 Next, FIG. 5 shows the experimental results of an example in which A / Nd was further reduced. In this embodiment, stable strain measurement up to about 450 ° C. is possible.

That is, the peak point of the resistance change rate shown in FIG. 5 (A) is approximately 450 ° C., and the sensitivity change rate shown in FIG. 5 (B) is also 4
Stable up to 50 ° C.

Further, FIG. 6 shows the leakage current Ir in the device of the second embodiment.
Shows the temperature characteristics of. Thus, according to the second embodiment, 45
It is understood that the leakage current Ir is 10 ⁻⁴ [A] at a high temperature of 0 ° C. and is suppressed to about 3% with respect to the measured current Ig (3 × 10 ⁻³ [A]).

Next, FIG. 7 shows the experimental results for examining how the above-mentioned peak temperature (usage limit temperature) changes when the value of A / Nd is changed. According to the figure, the value of A / Nd should be 5 in order to keep the service temperature limit above 350 ° C.
It can be seen that it is necessary to set it to × 10 ^-22 cm ⁵ or less.

Furthermore, the surface impurity concentration of the piezoresistive element 14 is set to 2.4 × 10.
^By setting it in the range of ²⁰ to 4 × 10 ²⁰ cm ⁻³ , it is possible to effectively suppress the temperature dependence of the output when the bridge circuit thus formed is driven by a constant current source.

Although the substrate of the semiconductor strain measuring device is of N type and the piezoresistive element is of P type in the above-described embodiments, the present invention is not limited to this, and the substrate is of P type and the piezoresistive element of N type. The same can be applied to the case of a mold.

[Brief description of drawings]

FIG. 1 is a front sectional view of an embodiment of the semiconductor strain measuring apparatus according to the present invention, FIG. 2 is a plan view of the same embodiment, and FIG. 3 is a characteristic showing the impurity concentration in the depth direction of the same embodiment. FIGS. 4 (A) and 4 (B) are characteristic diagrams showing the relationship between temperature and change in resistivity and sensitivity in Example 1, and FIGS. 5 (A) and 5 (B) are resistance to temperature in Example 2. FIG. 6 is a characteristic diagram showing rate change and sensitivity change, FIG. 6 is a characteristic diagram showing temperature dependence of the leakage current Ir of Example 2, and FIG. 7 is a characteristic diagram showing a relationship between A / Nd and use limit temperature. 10 ... Substrate 12 ... Silicon oxide 14 ... Piezoresistive element 16 ... Amyl electrode 18 ... PN junction surface

Claims (2)

[Claims] 1. A semiconductor single crystal substrate comprising: a semiconductor single crystal substrate; and a piezoresistive element which is formed on the semiconductor single crystal substrate and outputs a physical strain generated in the semiconductor single crystal substrate as an electric signal. One of the substrate and the piezoresistive element is P-type and the other is N-type, and the ratio N1 / N2 of the surface impurity concentration N1 of the piezoresistive element and the impurity concentration N2 of the semiconductor single crystal substrate is 10 ² In addition to the above, the semiconductor single crystal substrate and the piezoresistive element are in contact with each other.
Area A of PN junction and impurity concentration of the semiconductor single crystal substrate
A semiconductor strain measuring device characterized by having a ratio A / N2 with N2 of 5 × 10 ⁻²² cm ⁵ or less. 2. The device according to claim 1, wherein the surface impurity concentration N1 of the piezoresistive element is 2.4 × 10 ²⁰ to
A semiconductor strain measuring device having a range of 4 × 10 ²⁰ cm ^-3 .