JP2742019B2 - Polarized gas detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polar gas detector, and more particularly, to a polar gas detector using porous silicon.

[0002]

2. Description of the Related Art As a gas detector for detecting the type and concentration of a gas, for example, a gas detector using an oxide semiconductor such as SnO ₂ , ZnO, or TiO ₂ is known. The resistance changes when it comes into contact with the reducing gas in a high-temperature atmosphere of about ° C, and the type and concentration of the gas are detected from the amount of change in the resistance. As a non-electrical resistance gas detector, one having a MOSFET structure is also known. In this type of gas detector, an ion-sensitive film such as Pb is provided as a gate electrode, and the reaction of the film with respect to a gas such as hydrogen is performed. , The change in the threshold voltage of the MOSFET is detected, and the gas concentration is measured.

By the way, in the gas detector having the above-described configuration, first, in the former case, since the measurement is performed in a high-temperature atmosphere, measurement cannot be performed at room temperature, and the target gas is also limited. Also, in the latter case, the target gas is similarly limited, and cannot be applied to detection of a polar gas such as alcohol. Therefore, recently, a technique that enables measurement at room temperature and uses porous silicon for detecting a polar gas has attracted attention, and various R & D has been conducted.

A gas detector having a porous silicon layer formed on the upper surface side of a p-type silicon substrate was announced at the 47th lecture of the Japan Society of Applied Physics held in October 1986. According to the materials of this lecture, when the porous silicon layer adsorbs the polar gas, the capacitance of the detector becomes about 10%.
It has been reported that the electrical resistance decreases to about 1/20 with the increase of 0 times, and it is suggested that the gas type and concentration can be detected at room temperature by utilizing such a change.

However, according to studies by the present inventors, the gas detector presented at this lecture had the following technical problems.

[0006]

That is, the electric conductivity of the porous silicon formed on the p-type silicon substrate in the gas detector having the above-described structure is described in detail in the materials distributed at the lecture. Although there is no
94 American Instituteof P
hsics ”, Vol. 64, no. 4,24Janu
ary (pages 481 to 483) describes a detailed experimental report.

According to this experimental report, the electrical conductivity of porous silicon formed on a p-type silicon substrate is very small, and even when a voltage of about 100 V is applied, only a current of about 25 nA flows. At a low applied voltage and current change rate, it could not be used as a practical gas detector. The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a practical polar gas detector capable of obtaining a large current change with a low applied voltage. It is in. For other purposes,
An object of the present invention is to provide a polar gas detector that can be easily manufactured by using a widely used semiconductor manufacturing technique.

[0008]

In order to achieve the above object, the present invention provides an n-type or p-type silicon substrate, a p-type or n-type semiconductor layer formed on the silicon substrate, A porous silicon layer formed so as to be in contact with the semiconductor layer.

A plurality of the semiconductor layers are formed on the silicon substrate so as to be separated from each other, and a plurality of the semiconductor layers can be connected in parallel. In addition, at least one pair of the semiconductor layers is formed on the silicon substrate at a predetermined interval, and the silicon substrate and the pair of semiconductor layers may have a three-pole structure.

[0010]

According to the polar gas detector having the above-described structure, the n-type or p-type silicon substrate, the p-type or n-type semiconductor layer formed on the silicon substrate, the silicon substrate and the semiconductor layer, respectively. Since it has a porous silicon layer formed so as to be in contact with it, as apparent from the experimental results described below, at a low applied voltage of about several volts,
A large current change on the order of μA or mA can be obtained.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. 1 and 2 show a polar gas detector according to a first embodiment of the present invention.
The polar gas detector shown in the figure is a p-type silicon substrate 1 in which a silicon single crystal substrate is doped with an impurity such as B.
And a pair of n-type semiconductor layers 2 and 2 formed so as to face the p-type silicon substrate 1, and porous silicon formed between the p-type silicon substrate 1 and the n-type semiconductor layers 2 and 2. And a layer 3.

As shown in FIG. 2, a pair of n-type semiconductor layers 2 and 2 have a long side of W ₁ and a short side of W ₁ at a predetermined interval L. _Two identical rectangles are formed. The porous silicon layer 3 is a p-type silicon substrate 1
And the n-type semiconductor layers 2 and 2 are formed so as to come into contact with the long and short sides, respectively, on the four circumferences of each of the n-type semiconductor layers 2 and 2. It is embedded inside.

Then, the back side of the p-type silicon substrate 1 and
Aluminum electrode layers 4, 4 are provided on the surface side of the n-type semiconductor layers 2, 2. A method for manufacturing a polar gas detector having such a structure will be described with reference to FIG. The p-type silicon substrate 1 is, for example, a p-type silicon (100), a wafer A having a specific resistance of about 10 to 20 Ωcm.
Is used.

Then, oxidation and phosphorus diffusion are performed on the wafer A using standard integrated circuit technology to form n-type semiconductor layers 2 and 2 and to provide a pn junction (FIG. 3B). reference). Next, after removing the oxide films B formed on the front surface, aluminum C for the electrode layer 4 is vacuum-deposited on the back surface side of the wafer A. Next, after covering the surfaces of the n-type semiconductor layers 2 and 2 with the resist D, anodizing treatment is performed in an HF solution. In this process, except for the regions (n-type semiconductor layers 2 and 2) covered with the resist D, the surface of the wafer A
The porous silicon layer 3 is formed by making it porous. Then, low-temperature heat treatment is performed at 300 ° C. for stabilization of the porous silicon layer 3 (see FIG. 3C).

Next, after removing the resist D, each n
When aluminum D is deposited on the mold semiconductor layers 2 and 2 to form the electrode layer 4, the gas detector having the configuration shown in FIG. 1 is obtained. Here, the details of the anodizing treatment will be described. Porous silicon usually has a pore size of several nm to several μm.
Silicon containing a large number of holes. In the anodizing treatment for forming such porous silicon, a solution obtained by diluting a hydrofluoric acid aqueous solution (HF) with pure water, alcohol, or the like is used.

When anodizing the wafer A, the wafer A is set on the lower end side of a reactor F as shown in FIG. Is installed. Then, the reaction is performed by filling the reactor F with the diluted HF solution and applying a DC voltage in which the wafer A side is positive and the platinum electrode G side is negative.

The structure and thickness of the porous layer obtained by such anodizing treatment depend on the conductivity of the wafer A used,
It depends on resistance, crystal direction, temperature of HF solution, HF concentration, current density, light irradiation, and the like. The following table shows the anodizing conditions employed to obtain the gas detector of the present invention.

[Table 1] 5 to 10 show measurement results of a performance test performed to confirm the operation and effect of the polar gas detector having the structure shown in FIG. In the performance test at this time, as shown in FIG. 1, a variable voltage power supply ev was used.
Is connected to the electrode 4 on one n-type semiconductor layer 2 and the-side is connected to the electrode 4 on the other n-type semiconductor layer 2, and the electrode 4 of the p-type silicon substrate 1 is connected to the power supply ev. , And a three-pole structure similar to that of the transistor.

Note that such a connection state is based on MOSFET
The connection state between the source, the drain and the substrate is the same except for the gate terminal. Then, the voltage of the power supply ev is gradually and continuously increased, and at that time, the n-type semiconductor layers 2, 2
The current flowing between them was measured. The dimensions and shape of the gas detector used at this time were such that the distance L between the n-type semiconductor layers 2 and 30 was 30 to 40 μm, and the length W ₁ of the long side of the n-type semiconductor layers 2 and 2 was 500 to 2100 μm. The thickness of the porous silicon layer 3 was about 0.3 to 0.6 μm.

The measurement results shown in FIG. 5 are obtained by measuring the gas detector in the atmosphere (when no gas is applied). The current between the n-type semiconductor layers 2 and 2 is sufficiently low. The vessel is
It turns out that it is in an off state. FIG. 6 shows the measurement results when saturated vapor of ethanol was supplied to a gas measuring instrument using nitrogen as a carrier gas. As is apparent from this figure, when the voltage between the n-type semiconductor layers 2 and 2 becomes 2 V or more, the flowing current increases and the gas detector is turned on.

The current flowing with the increase in voltage monotonously increased, and no saturation tendency was observed. The characteristics are stable in about 1 minute after the gas is supplied, and 1-2
It returned to the initial state in about a minute. 7 to 10 show the respective measurement results when the measurement target gas is a saturated vapor of acetone, methanol, trichlene, and thinner. In these test results, the increase in current was due to ethanol>
When the current flowing between the n-type semiconductor layers 2 and 2 was measured at a predetermined applied voltage in the order of methanol>acetone> thinner, it was confirmed that the type of gas could be specified. In addition,
In the case of cinna and trichlene, the increase in the current flowing at an applied voltage of 5 V was 10 μA or less, but the value was much larger than that of the above-mentioned conventional detector, and the practicality was sufficiently recognized. Can be

In this performance test, a prototype having the structure shown in FIG. 1 and having no porous silicon layer 3 was produced.
A similar test was performed, but no change in current was observed. That is, it was found that the porous silicon layer 3 plays an important role in gas detection. The change in the current depends on the time of the anodizing treatment, that is, the thickness, the pore diameter, and the density of the porous silicon layer 3. Were prepared, and the current-voltage characteristics of each were measured.

FIG. 11 shows the measurement results. As is apparent from FIG. 11, the longer the anodizing treatment time, that is, the larger the thickness of the porous silicon layer 3, the larger the current. The change was large, and it was found that the porous silicon layer 3 preferably had a certain thickness.
At this time, the thickness of the porous silicon layer 3 of the sample was about 0.6 μm for the 2-minute processing and about 0.3 μm for the 1-minute processing.

The current also depends on the dimensions (L, W _1, W ₂ ) of the n-type semiconductor layers 2, 2. In particular, the current value increases as the length W ₂ in the longitudinal direction increases. Make sure that. In the first embodiment shown in FIG.
Even when the electrical connection between the silicon substrate 1 and the power supply ev is removed, a current response similar to that of the first embodiment is observed, and even when the n-type semiconductor layers 2 are connected to each other in a two-pole state. It has been confirmed that it has a gas detection function.

FIG. 12 shows a polar gas detector according to a second embodiment of the present invention. Only the features of the second embodiment will be described below. The polar gas detector shown in FIG. 1 includes an n-type silicon substrate 5, a pair of p-type semiconductor layers 6 and 6 formed so as to face the n-type silicon substrate 5, An electrode layer 4 is formed on each of the substrate 5 and the semiconductor layer 6. The porous silicon layer 3 a is formed between the mold semiconductor layers 6 and 6.

That is, the gas detector of the second embodiment is
As compared with the first embodiment, the pn junction is replaced with an pn junction. A performance test similar to that of the first embodiment was performed on the polar gas detector having such a configuration. In the gas detector of this embodiment, the characteristics were stabilized in about 3 minutes after the introduction of the saturated gas, and did not return to the initial state even after several hours after the gas was stopped. The tendency was similar to that of the first embodiment.

In the gas detector of the second embodiment,
Light irradiation was performed during the anodizing treatment to form the porous silicon layer 3a. FIG. 13 shows a third embodiment of the polar gas detector according to the present invention. Only the features of the third embodiment will be described below. The gas detector shown in FIG.
A p-type silicon substrate 1 in which a silicon single crystal substrate is doped with an impurity such as B, one n-type semiconductor layer 2 formed on the p-type silicon substrate 1, a p-type silicon substrate 1 and an n-type semiconductor layer 2; Porous silicon layer 3 formed between
It is composed of

That is, the gas detector of this embodiment has the first
The gas detector of the embodiment has a structure in which only one pn junction is taken out. FIG. 14 shows the gas detector of the third embodiment as shown in FIG.
Are connected, and the current-voltage characteristics are measured. In FIG. 14, current-voltage characteristics are measured in the atmosphere and in saturated steam of ethanol, respectively.
When ethanol is supplied, the current increases as the voltage increases. Although the magnitude of the current is different from that of the first embodiment, the current change is sufficiently detected at a low voltage. be able to.

FIG. 15 shows a fourth embodiment of the polar gas detector according to the present invention. Only the features of the fourth embodiment will be described below. The gas detector shown in FIG. 1 includes a p-type silicon substrate 1 in which a silicon single crystal substrate is doped with an impurity such as B, and a plurality of n-type semiconductor layers 2, 2, 2 formed on the p-type silicon substrate 1. And a porous silicon layer 3 formed between the p-type silicon substrate 1 and the n-type semiconductor layer 2
It is composed of

The respective n-type semiconductor layers 2, 2, 2 are connected in parallel. That is, the gas detector of this embodiment has a structure in which two or more pn junctions are formed in the gas detector of the first embodiment, and these are connected in parallel in a two-pole structure. I have. FIG. 16 shows a gas detector according to the fourth embodiment, as shown in FIG.
5 is a measurement result when v is connected and the current-voltage characteristic is measured. In FIG. 16, current-voltage characteristics are measured in the atmosphere and in a saturated vapor of ethanol, respectively.
When ethanol is supplied, the current increases as the voltage increases.

In this case, when the number of pn junctions connected in parallel is increased, the current increases accordingly. That is, if the area of the pn junction doubles with an applied voltage of 5 V or more, the result is that the current also increases approximately twice, and if the pn junctions are connected in parallel, a larger current change can be obtained. It turns out. As described above, according to the polar gas detector according to the present invention, a current change of the order of mA occurs in the case of a three-pole structure at an applied voltage of about several V, and in the case of a two-pole structure, , And a current change on the order of μA, which is sufficiently practical for this type of gas detector utilizing conventional porous silicon.

Next, the operation mechanism of the polar gas detector according to the present invention will be described. The operation mechanism of the gas detection function of porous silicon is not necessarily clear, but the present inventors infer as follows. The effective surface area of porous silicon is very large (（200 m ² / cm ³ ), and the surface is made of Si—O, Si
It is said that -H, Si-F bonds exist. When a polar gas is supplied to such a surface, dipoles of gas molecules are adsorbed by weak electrostatic attraction. In this case, it is considered that the OH group and the hydrogen bond of the polar molecule play an important role in the adsorption.

The adsorption of the polar molecule is as follows.
This causes a change in the potential of the crystal surface, and this change in the potential induces a charge at the porous-crystal interface, and in the case of the first and second embodiments, the charge induces a charge at the porous-crystal interface. It is considered that a current flows between the mold semiconductor layers. In this case, an increase in current is observed in both the pnp3 pole structure (first embodiment) and the pnp3 pole structure (second embodiment), and the characteristics are different from those of a normal MOSFET. As the voltage increased, the current increased monotonously, and did not show a saturation characteristic.

As a mechanism of the current increase, first, the adsorbed molecules induce electric charges at the porous-crystal interface to form an inversion layer between a pair of n-type or p-type semiconductor layers, and the current flowing therewith is considered. It is also conceivable that leakage current due to induced charge flows from the side surface of one of the reverse-biased pn or np junctions. In the gas detector according to the present invention, it is considered that the latter leakage current greatly contributes to the current increase. That is, in the third embodiment shown in FIG. 13, as shown in FIG. 14, according to the current-voltage characteristics of the reverse-biased pn junction, the current starts to flow when the applied voltage is around 2 V, and then increases monotonously. From this fact, it is assumed that the increase in the current of the gas detector having the three-electrode structure is based on the leakage current due to the induced charge.

In the first and second embodiments, n
Although an example in which one pn3 pole structure and one pnp3 pole structure are provided has been illustrated, the present invention is not limited to this, and a plurality of pnp3 pole structures may be connected in parallel. Also, in the third and fourth embodiments, it is also possible to use an np junction instead of a pn junction.

[0035]

As described above in detail in the embodiments,
According to the polar gas detector of the present invention, a large current change of μA or mA can be obtained at a low applied voltage of about several volts, which is practical. Further, the gas detector of the present invention has an advantage that it can be easily manufactured by using an existing semiconductor manufacturing technology widely used.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a polar gas detector according to a first embodiment of the present invention.

FIG. 2 is a top view of FIG.

FIG. 3 is an explanatory sectional view showing an example of a method for manufacturing the gas detector shown in FIG. 1 in the order of steps;

FIG. 4 is an explanatory view of an anodizing treatment when forming a porous silicon layer in the manufacturing process of FIG. 3;

FIG. 5 is a current-voltage characteristic diagram of the gas detector of the first embodiment shown in FIG. 1 in the atmosphere.

6 is a current-voltage characteristic diagram of the gas detector of the first embodiment shown in FIG. 1 in saturated ethanol vapor.

FIG. 7 is a current-voltage characteristic diagram of the gas detector of the first embodiment shown in FIG. 1 in acetone-saturated steam.

FIG. 8 is a current-voltage characteristic diagram of the gas detector of the first embodiment shown in FIG. 1 in methanol-saturated steam.

FIG. 9 is a current-voltage characteristic diagram of the gas detector of the first embodiment shown in FIG. 1 in trichlene-saturated steam.

FIG. 10 is a current-voltage characteristic diagram in thinner saturated steam of the gas detector of the first embodiment shown in FIG.

11 is a current-voltage characteristic diagram in ethanol-saturated steam when the thickness of the porous silicon layer of the gas detector of the first embodiment shown in FIG. 1 is changed.

FIG. 12 is a sectional view showing a second embodiment of the polar gas detector according to the present invention.

FIG. 13 is a sectional view showing a third embodiment of the polar gas detector according to the present invention.

FIG. 14 is a diagram showing current-voltage characteristics of the gas detector shown in FIG.

FIG. 15 is a sectional view showing a polar gas detector according to a fourth embodiment of the present invention.

16 is a diagram showing current-voltage characteristics of the gas detector shown in FIG.

[Explanation of symbols]

 Reference Signs List 1 p-type silicon substrate 2 n-type semiconductor layer 3 porous silicon layer 4 electrode layer 5 n-type silicon substrate 6 p-type semiconductor layer

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuhito Watanabe 1-17-14 Shinjuku, Katsushika-ku, Tokyo (56) References JP-A-61-218932 (JP, A) JP-A-2-10146 (JP, A)

Claims (3)

(57) [Claims] 1. An n-type or p-type silicon substrate, a p-type or n-type semiconductor layer formed on the silicon substrate, and a porous layer formed so as to be in contact with the silicon substrate and the semiconductor layer, respectively. A polar gas detector characterized by having a porous silicon layer. 2. The polar gas detector according to claim 1, wherein a plurality of the semiconductor layers are formed on the silicon substrate so as to be separated from each other, and a plurality of the semiconductor layers are connected in parallel. . 3. The semiconductor layer according to claim 1, wherein at least one pair of the semiconductor layers is formed on the silicon substrate at a predetermined interval, and the silicon substrate and the pair of semiconductor layers have a three-pole structure. A polar gas detector as described.