JPS5883244A - Gas sensitive membrane - Google Patents

Abstract

PURPOSE:To obtain a gas sensitive membrane which can detect the concns. of gases and alcohol with high sensitivity by optimizing the thickness of thin films. CONSTITUTION:On account of the O<-> ions adsorbed on a surface, carriers (electrons) cannot enter a thin film 2 for the distance delta from the surface thereof. In other words, the effective channel thickness Lc for carriers is D-2delta. When the ethyl alcohol existing on the film surface reacts with the adsorbed O<-> ions, the O<-> ions desorb as water from the film and applies carriers (e) into the film; at the same time, the effect as scattering centers annihilates; therefore, the effective channel thickness Lc is partially equal to D in the inside and with an increase in the concn. of the ethyl alcohol, mobility muH increases as well. If the thickness Lc is O while no ethyl alcohol exists, the change in muH attains maximum and as the Lc increases, that is, as the D increases, the muH decreases. If the thickness D is set within a range of 1-2 times the Debye length delta, the gas sensitivity is made higher by about >=80% than that in the case of the film thickness thickner than the same.

Description

[Detailed description of the invention]

The present invention provides a gas-sensitive membrane that interacts with external agents such as gas and alcohol and can detect the concentration of gas and alcohol with high sensitivity. The present invention is based on the discovery that the sensitivity of a gas sensor can be increased by optimizing the thickness of a thin film used as a gas-sensitive film. 21, - In general, the electrical conductivity σ of a substance is expressed as σ2n8μ (1) (where 嗗 is unit charge) using carrier, a concentration n, and carrier mobility μ. In order to clarify the operating mechanism of the thin film gas sensor, σ was separated into n and μ by Hall effect measurements. Figure 1 shows σ, μ, and alcohol concentration [0].
It shows the change in n. Figure 1 (ILI is for a tin oxide thin film with a film thickness of about 4oλ, and Figure 1('b) is a film with a film thickness of 10
This is the case of a tin oxide thin film with a thickness of about 0oλ. These thin films were all produced by a reactive RF spark method using tin as a target. That is, the argon pressure in the Pelger was set to about 0.6 Pa, the oxygen pressure was set to c), and the alkali-free glass substrate with its surface polished was heated to 160 Pa.
Sputter tin while heated to about ℃. R at this time
When the F power is 200 W, a tin oxide thin film is formed on the substrate at a rate of about 60 per minute. After this, 360 in the air
Heat treatment for 2 hours at -°C is performed to stabilize the properties. 1st
In the case of Figure (IL), the sputtering time is 37 seconds, and the sputtering time is 60 seconds, and in the case of Figure 1 (BL), the sputtering time is 20 minutes. was set to 260°C.The conductivity σ of each sample was determined by the ethyl alcohol concentration [
C] increases linearly on a logarithmic graph. That is, σ=αCO]N @) (here α, N
is a constant. ) can be expressed as In the case of a thin film with a thickness of about 40 people as shown in Figure 1 (IL), H is 1, and Figure 1 (b)
In the case of a thin film with a film thickness of about 1000 people, N:;
It is 0.5. This Hall effect measurement made it very clear that the mechanism of conductivity change due to ethyl alcohol adsorption differs greatly depending on the thickness of the thin film. The carrier concentration n increases as the ethyl alcohol concentration [C) increases. And its slope is C5. That is, △(ioi n )/△(logcc])=o, 5
p). The increase in carrier concentration induced by gas adsorption is determined solely by the chemical reaction between the solid tin oxide and the gas ethyl alcohol. Therefore, if measured under the same conditions (ambient temperature, operating temperature, etc.), △(logn/△(log[C]
) are naturally the same. In normal air, oxygen is adsorbed on the membrane surface in a manner determined by the operating temperature of the membrane. As will be described later, this oxygen adsorption state is expressed by the following equilibrium equation. 02 (g1802 (ad)+! 02” (IL
(134:!O-(IL(1)1”' 02-(ad)
402- (lat) +4 where fl, &(1, lat are gas and adsorption, respectively. They represent each state of the lattice, and e represents a conduction electron. At an operating temperature of 260°C, the adsorption state of oxygen is 0-. The reaction between 0− adsorbed on the membrane surface and ethyl alcohol molecules can be expressed as follows: j 02H50H+ O−d 0T1sOHO+H20+
e (@b 62.- That is, at an operating temperature of 260°C, ethyl alcohol molecules adsorbed on the membrane surface or in the atmosphere are oxidized by oxygen ions adsorbed on the membrane surface and converted to acetaldehyde. At this time, the electrons released from the oxygen ions return to the membrane and the carrier concentration increases.The validity of this reaction has been confirmed by mass spectrometry measurements.In the equilibrium state of the reaction shown by equation (6), From the law of mass action, kf[C2H5OH]O-] = kb[CHsCHO
][HzO')[e](@.If the ethyl alcohol concentration is lower than the surface adsorbed oxygen ion concentration, (
In equation 6), the oxygen ion concentration [0-] can be considered to be constant. Furthermore, since there are enough water molecules in the air, [H20] is also considered to be constant. (From @formula [0HsC;
HO]=[e] (7). +61? (63t From equation (7) and the above assumption, [e
l = [C2H5OH] (@). In other words, the carrier concentration increases in proportion to the square root of the ethyl alcohol concentration. This analytical result is in good agreement with the results shown in Figure 1. On the other hand, the mobility μm The dependence on ethyl alcohol concentration [C] is significantly different between the case of a thin film consisting of 40 people and the case of a thin film of 1000 people.In other words, when the film thickness is 40 people (the first In figure (a), △C1ogtx
)/△(7!Og[(]):;0.61 film thickness is 1000
In the case of humans (Figure 1 (b)), △(log/-m)/△
(log[c])-0. This person (l o g t
t, VMlotf-C8) is largely dependent on the thickness of the thin film. Now, to summarize the above experimental results here, the electrical conductivity σ of the film is expressed as σ=n,%,μ (1) as shown in equation (1). Both sides of this equation (1) are expressed as alcohol concentration [
C], we get The first term on the right side of equation (9) is independent of the type of film and its manufacturing conditions, and is approximately 0.6 when the operating temperature of the film is 250°C. The second term on the right side of equation (9) is strongly related to the properties of the film. In the case of a thin film of about 40 people, the thickness is about q6, but in the case of a thin film of 1000 people, it is 71-mi○. For these reasons (the left side of Equation 91 is 1 for a thin film with an optimal film thickness, and becomes a value smaller than 1 when the film thickness is thicker than the optimal value. In other words, the high gas sensitivity characteristic of a thin film is This is due to changes in the mobility of the film due to gas adsorption, and the amount of change in mobility in a thin film due to gas adsorption is closely related to the film thickness. △(log
tsi)/△(log

The film thickness dependence of the value [0] is shown. When the film thickness is 3C) to 40 people, △(lOgJH)/△(
The value of zogccp ) shows the maximum value and rapidly decreases as the film thickness increases. The film thickness on IooAll is Δ
Ilog~]/△(7!ogCC)) Q value is almost 0
It is. In the atmosphere, oxygen is negatively charged and adsorbed on the surface of the tin oxide thin film. Chan et al. concluded based on 1cSR measurements that when the operating temperature is in the range of 150°C to 260°C, this adsorbed oxygen is in the form of 0-, and at higher temperatures it becomes 02-. CS, C6Chang: J, Vac. SCJ&Techn07! , 17 (1980) 3661
00
'' (on becomes the scattering center of carriers (electrons) moving in the film. The distance over which this ion greatly influences the movement of carriers can be expressed by the Debye length shown in equation (1o). Here, ε is the dielectric constant ('= 13.5X8.85X10"
η1 is Boltzmann constant (=1.38×1σ23J10
K) The operating temperature n is the true carrier concentration. As the oxygen partial pressure in the atmosphere increases and the number of adsorbed 0- ions increases, the area where carrier movement is hindered increases, and eventually a continuous band with a thickness of δ is formed from the surface. It is considered to be. This situation is schematically shown in FIG. 3 (&). As shown in the figure, due to the 0-ions adsorbed on the surface, δ
Carriers (electrons) cannot enter between a distance of . In other words, the effective channel thickness LO for the carrier is expressed by equation (11) as shown in Table 9. Lc==D-2δ (11) Now, the carrier concentration found from the measurement results of the Hall effect is 2 × 1
0"(Im-', so if the film is composed at 260"C (D operating temperature i, To = = 523 °K, n =. 2, OX 1 d 8 crn-' is substituted into equation (10). δ: ; can be calculated as 4'0 people. In a sample with a film thickness of about 40 people, Lc is almost O and is very narrow. Figure 3 ('b) shows the ethyl existing on the surface of the ultrafine particle film or in the atmosphere. This figure schematically shows the case where alcohol reacts with adsorbed 0- ions. In the case of ILI, the effective channel thickness Lc that allows carriers to move freely in the membrane is Lc = n-26 (11 ), but as shown in fb) in the same figure, when ethyl alcohol is present and the reaction shown in equation 6) occurs, the O- ions adsorbed on the thin film surface become water and desorb from the film. At this time, carriers are provided into the film and at the same time, the conventional effect as a scattering center also disappears.Therefore, the effective channel thickness Lc is partially reduced by D(12)S. As the ethyl alcohol concentration increases, as shown in equation (3), the carrier concentration becomes △(logl/△(jlog[c))=o,s
It increases at the rate of (3). In other words, in the oxygen ion concentration -
] is △(60g[0-)3/△(Jog[C])=-o, t
s (13). Therefore, the effective channel thickness Lc in the channel region of the membrane is Lc:? The partial sum that becomes D is △(logum)/△(Jog [C) 3 = o, c
s (14). As a result, the mobility μ
H also increases. The above equations (13) and (14) hold true in the absence of ethyl alcohol, that is, in the case where the effective channel thickness Lc is 0 in FIG. 3 (&). At this time, the change in Lc due to gas adsorption, that is, the change in μm, becomes maximum. As Lc increases, 11z'-: As D increases, △(7!og
pH)/△(Jog[C)) o value is small < fil. For a thin film with a film thickness of about 4 degrees, δ is 40 degrees from equation (10) at an operating temperature of 250°C, so LC:l:0, and △(JogμH)/△ClogC(]) is shown in Figure 1 ( a
) shows the maximum value of approximately 0.6. When the film thickness reaches 1000 layers, the effective channel thickness L
c is Lc;D (4-
19), so as shown in Figure 1 tb+, △(lo
gμV△(lOgC,C]) becomes 20. That is, in Figure 2, △(140gpH)/△(
βogcc]) is 0.4 or more (gas sensitivity increases by more than 80% compared to the normally used thin film of several hundred A19).
It can be seen that it is below i. As mentioned above, by setting the thickness of the thin gas-sensitive film in the range of 1 to 2 times the Debye length, gas sensitivity can be improved by about 80% or more compared to the case where the film thickness is greater. It turns out that it can be done. Although we have described the reaction between tin oxide and ethyl alcohol, this model also applies to the reaction of other metal oxide thin films such as zinc oxide and copper oxide with other reducing gases such as hydrogen and isobutane. Needless to say, it can also be applied to As described above, the present invention provides a gas-sensitive membrane with extremely high sensitivity.

[Brief explanation of the drawing]

Figure 1 (&) and (bl are diagrams showing the changes in electrical conductivity, carrier concentration, and mobility of a gas-sensitive film made of a thin tin oxide film, respectively, with respect to alcohol concentration. A diagram showing the film thickness dependence of the magnitude of change in alcohol concentration, Figure 3 + A diagram schematically showing the state in which a reducing gas reacts with acid accumulated ions adsorbed on the ILI membrane surface. 1...Glass substrate, 2-...Tin oxide thin film, 3...13... Electrode. Figure 111 (α)

Claims (2)

[Claims] (1) A gas sensitive sensor comprising a thin film whose resistance value changes when one surface comes into contact with a gas atmosphere, and the thickness of the thin film is set within a range of 1 to 2 times the Debye length. film. (2) The gas-sensitive film according to claim 1, wherein the thin film is made of a metal oxide semiconductor that exhibits n-type conductivity with respect to reducing gas.