JPS6342179A - Semiconductor strain detector - Google Patents

Abstract

PURPOSE:To make it possible to form a circular diaphragm and to obtain a semiconductor strain detector which is operated stably at high temperature, by using semi-insulating polycrystalline silicon as a substrate. CONSTITUTION:Four P-type piezoresistive strain gages 12 are formed on a semi-insulating polycrystalline silicon substrate 11 by implantation or diffusion of boron ions. Then, an SiO2 protecting film 13 and an SiO2 mask 14 are formed on the upper surface of the semi-insulating polycrystalline silicon substrate 11. Thereafter, the specified parts of the protecting film 13 on the piezoresistive strain gages 12 undergo photoetching by a photolithography method to form contact holes. Then a wiring part 15 made of aluminum is evaporated in a vacuum state. Thus the four strain gages are electrically interconnected so as to form a bridge. Then the mask 14 undergoes photoetching in a specified shape (circle in this case). Thereafter, semi-insulting polycrystalline silicon substrate 11 is etched with alkali. A circular diaphragm 16 is formed to construct a pressure sensor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor strain detector used as a pressure detection element constituting a pressure sensor, for example.

[Conventional technology]

Conventionally, a plurality of P-type silicon piezoresistive strain gauges constituting a bridge have been formed by impurity diffusion in an N-type single crystal silicon substrate having a thin diaphragm.
So-called diffusion type semiconductor pressure sensors are widely used.

[Problem that the invention seeks to solve]

However, in the diffusion type semiconductor pressure sensor, each strain gauge is separated from the substrate by a PN junction.
At temperatures above approximately 150°C, the leakage current at the PN junction increases rapidly, causing the bridge to become unbalanced.
There was a problem that it no longer worked as a pressure sensor.

In order to solve this problem, instead of separating each strain gauge from the substrate by a PN junction, it is possible to separate each strain gauge by a large difference in resistivity, but the resistivity of a single crystal silicon substrate is The largest one is about 100 ohms (approximately), and it is impossible to separate each gauge based on the difference in resistivity.

In addition, in the aforementioned diffusion-type semiconductor pressure sensor, the diaphragm is usually formed by anisotropic etching using an alkali such as an aqueous potassium hydroxide solution. For example, if a silicon substrate with a (110) plane orientation is used, the diaphragm will have a shape close to a hexagon, as shown in FIG. As a result of this, the symmetry of the stress in each gauge part is lost, resulting in a problem that the output is lower than that of a pressure sensor with a completely circular diaphragm. Here, the reason why a perfectly circular diaphragm cannot be obtained is because anisotropic etching is used. That is, the etching rate is the slowest (because the shape is defined by the 1111 plane).

The present invention was made in view of the above-mentioned problems, and it is possible to form a diaphragm in any shape including a circle even by using an anisotropic etching solution for single crystal silicon.
The purpose of this invention is to provide a semiconductor strain detector that maintains electrical insulation between a substrate and a semiconductor strain gauge even at high temperatures and operates stably.

[Means for solving problems]

In order to achieve the above object, the semiconductor strain detector of the present invention is characterized by comprising a semiconductor insulating polycrystalline silicon substrate having a thin-walled diaphragm, and a semiconductor strain gauge formed on the substrate. .

[Effect]

According to the above means, the semi-insulating polycrystalline silicon substrate has a sufficiently large resistivity so that the substrate and the semiconductor strain gauge can be electrically separated, and since the substrate is chemically isotropic, Even if an anisotropic etching solution for single-crystal silicon, such as a KOH aqueous solution, is used, etching is performed isotropically, making it possible to form a circular diaphragm.

〔Example〕

The present invention will be explained below with reference to embodiments shown in the drawings. 1st
The figure shows a semiconductor strain detector according to an embodiment of the present invention.
Al shows the top view, and the figure (bl shows the sectional view taken along the line A-A).

In FIG. 1, 11 has an impurity concentration of, for example, IQ16 cm-', a crystal grain size of about 1000, and I
It is a semi-insulating polycrystalline silicon substrate with a specific resistance of 10'Ω (up to). This semi-insulating polycrystalline silicon substrate 11
Next, four P-type piezo anti-strain gauges 12 are formed by boron ion implantation or diffusion. Next, on the upper surface of the semi-insulating polycrystalline silicon substrate 11, a protective film 13 made of 5 ioZ and a mask 14 for diaphragm etching made of the same (SiO2) are formed by CVD.

After that, the piezoresistive strain gauge 1 was created using photolithography.
A predetermined portion of the protective film 13 on the photo-etching layer 2 is photo-etched to form a contact hole. Further, by vacuum-depositing a wiring portion 15 made of aluminum, the four strain gauges are electrically connected to form a pre-fuji.

Next, after the mask 14 for diaphragm etching is photo-etched into a predetermined shape (circular in this example), the semi-insulating polycrystalline silicon substrate 11 is etched with potassium hydroxide (KO).
H) It is etched with an alkali such as an aqueous solution to form a circular diaphragm 16, which constitutes a pressure sensor.

Therefore, according to the above configuration, the piezoresistive strain gauge 12 is electrically isolated from the substrate by using semi-insulating polycrystalline silicon instead of using a PN junction as in conventional single-crystal silicon diffusion type pressure sensors. This is done due to the large difference in specific resistance between the silicon substrate 11 and the piezoresistive strain gauge 12,
Even at high temperatures of 150°C or higher, the insulation of each strain gauge is maintained, allowing stable operation.

Furthermore, since the substrate used in this example is semi-insulating polycrystalline silicon 11, it is a chemically isotropic substance, and a potassium hydroxide aqueous solution or the like is used to perform anisotropic etching on single crystal silicon. It becomes possible to perform isotropic etching using alkali. Therefore, a substantially completely circular diaphragm can be formed, and the degree of anger does not decrease due to the relative symmetry of stress. In addition, single crystal silicon has large anisotropy in mechanical properties, especially its fracture strength, and there are orientations (directions) with low strength, but semi-insulating polycrystalline silicon has very small single crystals with random orientations. Since it is an aggregate of particles, its mechanical properties can be considered to be completely isodirectional, and unlike single crystal silicon, there is no direction of low strength, increasing the degree of freedom in designing the diaphragm shape. There is an advantage.

Next, in FIG. 2, a piezoresistive strain gauge 22 made of a thin film layer of single crystal, polycrystalline, or amorphous silicon is formed on the main surface of the semi-insulating polycrystalline silicon substrate 11 by CVD or vacuum evaporation. FIG. 6 is a cross-sectional view of a semiconductor strain detector according to another embodiment of the present invention. Even in the case of such a structure, the same effects as the embodiment shown in FIG. 1 can be obtained.

The semi-insulating polycrystalline silicon substrate referred to in the present invention has a chemical formula of SiχOy (or SixNy), as follows: SiH4+NzO-→SiχOy.

The so-called S I Ha + CO□ −→ Stχoy is formed by the CVD method using a reaction such as
P OS (Semi4nsulating Poly
It is made of silicon only and contains almost no oxygen, and is formed by a pulling method, a casting method, an ultra-quenching method (roll method), a high-speed CVD method, etc., and its crystal grain size It is possible to apply a method in which more crystal grain boundaries exist and the specific resistance is increased by reducing the S I PO5, but it is technically difficult to form the former S I PO5 as a substrate with a predetermined thickness. In addition, 5rpos increases its specific resistance by increasing the oxygen content, but when the oxygen content is increased, the piezoresistance coefficient becomes smaller and the strain sensitivity decreases. Because there is
The latter substrate is preferable.

Furthermore, for the purpose of electrically separating the substrate and the strain gauge, the semi-insulating polycrystalline silicon substrate has a specific resistance that is normally 10-1 to 10
-3Ω·(2), it is desirable that the resistance be approximately 104Ω·around the mouth.

Furthermore, the present invention is not limited to the above two embodiments, and can be modified in various ways, for example as shown below, without departing from the spirit thereof.

(1) The protective film 13 and the mask 14 may be made of Si3N, etc., and the area occupied by the wiring portion 15 is large (
If there is no need to worry about leakage current, this protective film 13 may be omitted. By doing so, the occurrence of thermal strain can be reduced.

(2) The shape of the diaphragm 16 is not limited to a circular shape, and may be formed into any shape by taking advantage of the fact that isotropic etching is possible.

(3) In the above embodiment, a P-type piezoresistive strain gauge is used, but it goes without saying that the conductivity type may be N-type.

(4) In the above embodiment, an anisotropic etching solution such as alkali is used as the etching solution for the diaphragm 16, but it is possible to use an isotropic etching solution by using 5iiN4°Au or the like as the material of the mask 14. Good too.

〔Effect of the invention〕

As described above, according to the present invention, since semi-insulating polycrystalline silicon is used as the substrate, a circular diaphragm can be formed, and there is an effect that it is possible to provide a semiconductor strain detector that operates stably even at high temperatures. .

[Brief explanation of the drawing]

Figure 1 (al is a top view of one embodiment of the present invention, Figure 1 (b)
) is a sectional view taken along the line A-A in FIG. ...Semi-insulating polycrystalline silicon substrate, 12.22.
...P-type piezoresistive strain gauge, 16...diaphragm.

Claims (2)

[Claims] (1) A semiconductor strain detector comprising: a semi-insulating polycrystalline silicon substrate having a thin diaphragm; and a semiconductor strain gauge formed on the semi-insulating polycrystalline silicon substrate. (2) The semiconductor strain detector according to claim 1, wherein the semi-insulating polycrystalline silicon substrate has a large specific resistance by reducing its crystal grain size.