JPS6340847A - Gas detection - Google Patents

Abstract

PURPOSE:To facilitate the detection of various gases, by arranging an element in which electrodes are formed on the surface of a porous silicon layer of a monocrystalline silicon substrate with the porous silicon layer on one surface thereof and on the surface of the monocrystalline silicon substrate as the back thereof respectively. CONSTITUTION:A lead in which an electrode 12 is formed on a part of the surface of a porous silicon layer 11 is connected to one surface of a monocrystalline silicon substrate 10. On the other hand, the surface of the monocrystalline silicon substrate 10 as the back thereof undergoes an anode chemical conversion treatment to form an element. The element thus formed is measured to show how it reacts in atmospheres of various gasses. The measurement was conducted in vacuum and in a saturated vapor of water or alcohol and the results are compared. The capacity of a sample porous silicon layer formed at a thickness of 22mum is 28pF in vacuum whereas it increases significantly to 180pF in the saturated alcohol vapor and 2,900pF in the insaturated water vapor. This confirmed that the element reacts with the gas to increase the capacity thereof noticeably.

Description

[Detailed description of the invention] [Field of industrial application] The present invention relates to a method for detecting nine gases using a gas sensor that reacts with various gases, and in particular a method for detecting gases using porous silicon. It is related to.

[Prior art and its problems]

There are various materials and structures for gas sensors. Among these, those using semiconductor substrates such as silicon are attracting attention because they have the advantage of being able to integrate amplifiers and other components and can be made smaller.

However, in order to use it as a gas sensor, there have been problems such as a complicated structure and the need to use biochemical substances, and there have been few that have been satisfactory in terms of cost, etc.

Therefore, the inventor proposed to obtain a gas sensor using porous silicon (Japanese Patent Application No. 61-42966). This has a simple structure in which electrodes are formed on an element that combines single crystal silicon and porous silicon. but,
The method of forming the electrode, reaction and recovery time, etc. were not always satisfactory.

〔the purpose〕

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a gas detection method using porous silicon that can undergo reaction and recovery in a short time.

Another object of the present invention is to provide a gas detection method in which the electrodes of the element used can be easily formed and stable characteristics can be obtained.

[Means for solving problems]

The present invention achieves the above object by applying an alternating current bias instead of a direct current bias to an element formed of porous silicon and detecting a change in the capacitance.

That is, - a single crystal silicon substrate having a porous silicon layer on its front side, and an element in which a layer F is formed on the surface of the porous silicon layer and the surface of the single crystal silicon substrate on the back side thereof are placed in a gas atmosphere; The feature is that gas is detected by changes in the capacitance of the element.

Furthermore, by detecting this change in capacitance together with a change in frequency, detection can be facilitated.

[Implementation row]

The present invention was developed in the process of researching the properties of porous silicone having a large surface area, and utilizes the fact that the electrical properties of porous silicon change to I due to attached gas. There are some aspects of this principle that have not yet been completely elucidated.

Additionally, although there are some unknowns regarding the properties of porous silicon itself, changes in electrical properties were confirmed with sufficient reproducibility. The implementation sequence will be explained below.

FIG. 1 is a front sectional rIIJ view showing one row of sensor sections used in the present invention.

Porous silicon Nj is formed on one surface of the single crystal silicon substrate 10.
11 is formed on a portion of the surface of the porous silicon layer 110. 12 is formed and a lead wire is connected thereto. On the other hand, an electric conductor 13 is formed on the entire surface in contact with the single crystal silicon substrate 10 on the back side.

Porous silicon layer 11 is formed by subjecting the surface of single crystal silicon substrate 100 to anodization treatment.

P-type nest crystal silicon substrate doped with boron (ρ=90
・One surface of the wound) was anodized in a hydrogen fluoride (HF) solution. Although the thickness of the porous silicon layer is determined by the conditions of the anodization treatment, the thickness was set to 7 to 45 μm for K in which the present invention is carried out.

'TjL pole 13 may be made of any material, and can be formed of K such as aluminum. The electrode 12 formed on the surface of the porous silicon layer 110 is not formed over the entire surface but in a 10×10
It was formed only in the center of the surface of m.

As the material of this electrode 12, vacuum-deposited aluminum, ultrasonic soldered Pb-8n-Zn-Cd alloy, or sputtered gold pattern was used.

We measured how the elements formed as described above react in various gas atmospheres. The measurements were performed when the water tank was placed in a vacuum state and when the water tank was placed in saturated alcohol vapor.

In a sample in which the porous silicon layer was formed with a thickness of 22 μm, the capacitance in vacuum at 24.5°C was 28 pF, but in saturated alcohol vapor the capacitance was 180 pF,
In saturated steam, the capacity increased significantly to 2900 pF. In addition, the porous silica rarely had a value of 30 pF in a vacuum at 22°C, but increased to 2700 pF in saturated steam.

It was confirmed that the responses of both types of K increased significantly in response to the gas. Note that this container is measured using LC.
When the resistance formed in parallel with the capacitance was measured using an R meter, it was found that the resistance had decreased from 20MΩ to α5MΩ, which is approximately 20 minutes lower. This decrease in resistance is thought to be related to the increase in capacity, but the detailed mechanism has not been elucidated.

The difference in the material of the electrode 12 formed on the porous silicon layer 11 had almost no effect on the change in the characteristics described above.

FIG. 2 shows a row in which an element and an oscillator using TLO71CP (manufactured by TI) are combined in order to detect the above-described change in capacitance as a change in frequency. Sen? The element 20 is connected to an oscillation circuit 21, and its output is connected to a frequency meter 22. This oscillation circuit is not limited to this row, and the configuration of the pond is also shown in Figure 2, which shows the change in oscillation frequency when the sensor element is placed in dry air and saturated steam at air temperature. This is Figure 5. Curve 31 shows the change in oscillation frequency when placed in dry air to saturated steam.
sb indicates that the frequency drops rapidly in a short period of time. After about 40 seconds, the frequency becomes stable. This results in a noticeable change in frequency within a few seconds. A curve 52 shows the state of recovery when this element is placed in dry air again. It was completely recovered in about 60 seconds, which shows that both the response and the recovery are short.

FIG. 4 shows the relationship between capacity and frequency when the oscillation circuit shown in FIG. 2 is used, and shows that the oscillation frequency decreases as the capacitance between the ring electrodes increases.

In the column shown in Figure 2, a frequency meter is used for measurement, but
If this is replaced with a speaker or buzzer, the sound can be changed and detected. Since the frequency changes rapidly in a short period of time, the change can be easily noticed.

The effect was confirmed not only for the above-mentioned alcohol and water but also for acetic acid, acetone, etc. as reacting gases. Most of the experiments conducted showed a reaction in an atmosphere of polar molecules.

In the measurement, it was also confirmed that the response and recovery times were shorter when comparing in air and in atmospheric gas than in vacuum and in atmospheric gas.

For example, when placed alternately in air and acetone gas, an immediate change occurred.

〔effect〕

According to the present invention, a silicon substrate provided with a porous silicon layer having a simple structure can be used as a senna element,
It can detect various gases.

In addition, response and recovery are possible in an extremely short time, providing a practical gas detection method.

Furthermore, it has the advantage of being a versatile detection method as it reacts with a wide variety of gases.

[Brief explanation of drawings]

Fig. 1 is a front sectional view of an example of a sensor element used in the present invention, Fig. 2 is a circuit diagram showing the first aspect of the measurement method, Fig. 3 is an explanatory diagram showing the response situation, and Fig. 4 is the capacitance and frequency. An explanatory diagram showing the relationship between. 10... Single crystal silicon substrate. 11... Porous silico y h +12.13.
・・・・・・Electrode theory

Claims (2)

[Claims] (1) An element having electrodes formed on the surface of the porous silicon layer and the surface of the single crystal silicon substrate on the back side of a single crystal silicon substrate having a porous silicon layer on one surface is placed in a gas atmosphere, and the element is placed in a gas atmosphere. A gas detection method characterized by detecting gas by a change in capacitance between electrodes. (2) The gas detection method according to claim 1, wherein the gas is detected by using the change in capacitance as a change in frequency.