JP3716606B2 - Thin film thermistor and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thin film thermistor using an amorphous semiconductor thin film and a method for manufacturing the same.
[0002]
[Prior art]
In recent years, development of infrared detection elements that can measure temperature in a non-contact manner has become active. As this type of infrared detection element, for example, a pyroelectric element using a pyroelectric material or a thermopile element utilizing a thermoelectromotive force has been developed.
However, the pyroelectric infrared detection element is a so-called differential detection element that absorbs infrared light as thermal energy and detects a change in the amount of charge (pyroelectric effect) that occurs as a result, and detects only the change in infrared. There was a problem that it was not possible. Further, the thermopile type infrared detection element outputs a DC voltage proportional to the temperature, but has a problem that the output is small.
[0003]
On the other hand, thermistor-type infrared detection elements are known to be able to obtain a high DC output and are suitable for miniaturization and high integration. Thermistors are widely used as temperature sensors for various devices. Yes. A thermistor constant (B constant) is known as a value representing the characteristics of the thermistor. The B constant is a value indicating the relationship between the temperature change and the resistance change, and the larger the B constant, the larger the resistance change with respect to the temperature change. Therefore, it can be said that a larger B constant is desirable for the thermistor.
[0004]
By the way, an array sensor in which a number of thermistors are two-dimensionally arranged so that the temperature distribution can be detected has been proposed. If this type of array sensor is used, the temperature distribution can be handled in the same manner as image information. . Since thermistors used for this kind of application need to be highly integrated, each thermistor must be miniaturized. However, when the volume of the thermistor element (thermistor semiconductor chip) is reduced, 1 / f noise increases in inverse proportion, resulting in a problem that the S / N ratio decreases.
[0005]
A small thermistor element used for an array sensor or the like cannot be manufactured by a conventional method of sintering a metal oxide. Therefore, an amorphous semiconductor thin film is formed on a substrate by a plasma CVD method. A manufacturing method is considered that functions as a thermistor element.
Amorphous semiconductor thin films formed by plasma CVD are used for solar cells, thin film transistors, sensors, and the like, and in particular, a-Si: H (hydrogenated amorphous Si) and a-SiC: H formed by plasma CVD. (Hydrogenated amorphous SiC) has a larger optical band gap than crystalline silicon, has a large light absorption coefficient in the visible light region, and can be easily formed into a thin film and a large area. Application fields. When forming an amorphous semiconductor thin film for use in a solar cell, the optical band gap decreases as the impurity concentration increases, so the amount of dopant gas mixed with the source gas is generally reduced. Also, when forming the thermistor element, the B constant is generally larger when the impurity concentration is lower. From this point of view, it is better that the amount of dopant gas mixed with the source gas is smaller. In addition, when forming a-SiC: H, it is common to raise a dilution rate with the hydrogen gas which dilutes source gas.
[0006]
[Problems to be solved by the invention]
However, since 1 / f noise is inversely proportional to the carrier concentration, 1 / f noise increases when the doping concentration is low. That is, if the size is reduced and the B constant is increased, the problem that the 1 / f noise increases and the S / N ratio decreases occurs.
[0007]
In order to improve the B constant, it is necessary to increase the activation energy Ea of the semiconductor resistance layer made of the amorphous semiconductor thin film. Here, the relationship between the activation energy Ea and the electrical conductivity σ is generally expressed as the following equation near room temperature.
σ = σ ₀ exp (−Ea / kT) = σ ₀ ^* exp (−Ea ₀ / kT)
Where σ ₀ is a coefficient (pre-exponential pfactor), k is a Boltzmann constant, T is an absolute temperature σ ₀ ^* is a conductivity pre-factor, and Ea ₀ is an activation energy at 0K. Further, the B constant is obtained by -Ea / k. That is, according to the above equation, if the activation energy Ea is increased to increase the B constant, the conductivity σ decreases. Therefore, in order to increase the B constant and increase the conductivity σ, prefactor σ ₀ ^* Needs to be increased. However, the conventional thin film thermistor has a small value of prefactor σ ₀ ^* , and there is a problem that if the thermistor element is downsized, the resistance value becomes too high and the measurement becomes difficult.
[0008]
Further, since the activation energy Ea changes with temperature, there is a problem that if this change is large, the prefactor σ ₀ becomes small, making it difficult to achieve both a high B constant and high conductivity.
The present invention has been made in view of the above reasons, and its purpose is to increase the B constant while reducing the 1 / f noise and the conductivity, resulting in a small size and a high S / N ratio. An object of the present invention is to provide a thin film thermistor and a method of manufacturing the same.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the invention of claim 1 is a thin film thermistor in which an electrode is provided on a semiconductor resistance layer made of an amorphous semiconductor thin film, and the semiconductor resistance layer has a Fermi level as a linear function of temperature. The proportionality factor is less than 10 times the Boltzmann constant, and the temperature dependence of the Fermi level is small and the temperature change of the activation energy is small, so that a high conductivity prefactor can be obtained. It is possible to achieve both a high B constant and a high electrical conductivity, and the 1 / f noise is reduced. As a result, the S / N ratio can be reduced and the S / N ratio can be improved.
[0010]
The invention according to claim 2 is a thin film thermistor in which an electrode is provided on a semiconductor resistance layer made of an amorphous semiconductor thin film, and the half width of the level formed between the band gaps of the semiconductor resistance layer has a Fermi level of temperature. When the linear function is used, the proportionality coefficient is a half-value width that is 10 times or less of the Boltzmann constant. The temperature dependence of the Fermi level is small and the temperature change of the activation energy is small. A prefactor of conductivity can be obtained, a high B constant and a high conductivity can be achieved at the same time, and 1 / f noise is reduced, resulting in a small size and an improved S / N ratio. .
[0011]
The invention of claim 3 is a method of manufacturing a thin film thermistor in which an electrode is provided on a semiconductor resistance layer made of an amorphous semiconductor thin film, and the proportionality coefficient when the Fermi level is a linear function of temperature is 10 times or less of the Boltzmann constant. The semiconductor resistance layer is formed so that the temperature dependence of the Fermi level is reduced and the temperature change of the activation energy is reduced, so that a high conductivity prefactor is obtained, and a high B constant is obtained. Since high conductivity is compatible and 1 / f noise is reduced, it is possible to provide a thin film thermistor with a small size and a high S / N ratio.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a cross-sectional view of an infrared detecting element using a thin film thermistor of the present invention, and a dielectric film made of a SiO ₂ / Si ₃ N ₄ film on the main surface and the back surface of a support substrate 1 made of a single crystal silicon substrate. 2 and a thin film portion 2a made of the dielectric film 2 is formed by providing a recess 6 having a V-shaped cross section from the main surface side of the support substrate 1, and a thin film is formed on the thin film portion 2a. A thermistor 10 is formed. The thin film thermistor 10 includes a lower electrode 11a made of a chromium (Cr) film formed on the thin film portion 2a, a thermistor element 8 made of a p-type a-SiC: H thin film formed on the lower electrode 11a, and a thermistor element. 8 and an upper electrode 11b made of a Cr film. An infrared absorbing film 5 is formed on the thin film thermistor 10 via a second protective film 4 made of SiON. Here, the detection part A constituted by the thin film part 2 a, the thin film thermistor 10, the infrared absorption film 5 and the like is supported on the support substrate 1 by a plurality of support beam parts 7. A first pad electrode 14a is connected to the lower electrode 11a, and a second pad electrode 14b is connected to the upper electrode 11b. Note that reference numeral 3 in FIG. 1 denotes a first protective film made of an SiO ₂ film.
[0013]
Hereinafter, the manufacturing method will be described.
First, the dielectric film 2 made of SiO ₂ / Si ₃ N ₄ film is formed on each of the main surface and the back surface of the support substrate 1 (that is, the SiO ₂ film is formed after the Si ₃ N ₄ film is formed) A Cr film is formed on the entire surface of the dielectric film 2 on the main surface side of the substrate 1 by a vapor deposition apparatus (for example, an EB vapor deposition apparatus) or the like, and a lower electrode 11a made of a Cr film having a predetermined shape is formed by a photolithography technique and an etching technique. Form. Next, a p-type a-SiC: H thin film is formed so as to cover the entire surface on the main surface side by a plasma CVD apparatus (see FIG. 2), and the p-type a-SiC: H thin film is not required by a photolithography technique and an etching technique. The thermistor element 8 is formed by removing the portion, and then an SiO ₂ film is formed so as to cover the entire main surface side of the support substrate 1, and unnecessary portions of the SiO ₂ film are removed by photolithography technique and etching technique. By removing the first protective film 3 made of SiO ₂ film. Thereafter, a Cr film is formed so as to cover the entire main surface side of the support substrate 1, and an upper electrode 11b made of a Cr film having a predetermined shape is formed by a photolithography technique and an etching technique. Thereafter, a second protective film 4 made of a SiON film is formed so as to cover the entire main surface side of the support substrate 1, and an infrared absorption film 5 is formed on the second protective film 4. Unnecessary portions of the infrared absorption film 5 are removed by an etching technique. Thereafter, a part of the second protective film 4 and the first protective film 3 is etched to form a contact hole in which a part of the surface of each of the lower electrode 11a and the upper electrode 11b is exposed, and the contact hole is buried. As described above, a metal film such as an aluminum film is formed on the entire main surface side of the support substrate 1 by a vapor deposition apparatus, and pad electrodes 14a and 14b made of the metal film are formed by removing unnecessary portions of the metal film. After that, a plurality of support beam portions 7 for supporting the detection portion A are formed apart by photolithography technology and etching technology, and the support substrate 1 is mainly formed through a slit portion formed between the support beam portions 7. The recess 6 is formed by etching from the surface side. The infrared detecting element having the structure shown in FIG. 1 is formed by the manufacturing method described above.
[0014]
FIG. 2 shows a film forming apparatus composed of the plasma CVD apparatus used for forming the p-type a-SiC: H thin film. The support substrate 1 on which the dielectric film 2 and the lower electrode 11a are formed is housed in a chamber 22 and heated to, for example, 270 ° C. by a heater 23. The chamber 22 is provided with a discharge port 24 for exhausting the inside to a vacuum and an introduction port 25 for introducing a film forming gas. The support substrate 1 is placed on the electrode 26a grounded together with the chamber 22, and a high frequency voltage (13.56 MHz) is applied from the high frequency power source 27 to the electrode 26b facing the electrode 26a. As a result, plasma due to glow discharge is generated between the electrodes 26a and 26b, the film forming gas is decomposed to generate active species, and p is formed on the main surface side of the support substrate 1 by the gas phase reaction of the active species. An a-SiC: H thin film is formed. Here, in this embodiment, a mixed gas of monosilane (SiH ₄ ) and methane (CH ₄ ) is used as a source gas, hydrogen gas (H ₂ ) is used as a gas for diluting the source gas, and H is used as a dopant gas. _Two dilutions of diborane (B ₂ H ₆ ) are used.
[0015]
The reason why the mixed gas of monosilane and methane is used as the raw material gas is that the carbon is contained in the amorphous semiconductor thin film, so that the band gap Eg shown in FIG. is there. In FIG. 3, E _C and E _V indicate band ends (mobility ends), E _F indicates Fermi level, and Ea indicates activation energy.
[0016]
FIGS. 4 to 6 show the results of measuring changes in the conductivity σ, the deposition rate, the prefactor σ ₀ ^* , and the activation energy Ea with respect to the high frequency power applied to the electrode 26b.
In FIG. 4, the horizontal axis represents high-frequency power, the left vertical axis represents conductivity σ, and the right vertical axis represents deposition rate. 450sccm all flow ○ gas 4 (the flow rate of each gas is 50 sccm of SiH _4, 200 sccm of CH _4, the H ₂ 150 sccm, 50 sccm of B ₂ H ₆ concentrations of 0.5% with H ₂ dilution) and then the conductivity of if sigma, ● the flow rate of the total flow 900 sccm (the gas 100sccm the SiH _4, 400 sccm of CH _4, 300 sccm of H _2, the concentration of H ₂ dilution of 0.5% B ₂ H ₆ Is a deposition rate when the total flow rate is 450 sccm, and ▲ is a deposition rate when the total flow rate is 900 sccm. As can be seen from FIG. 4, the deposition rate increases as the high frequency power increases, while the conductivity σ decreases. The reason why the deposition rate increases with the increase of the high frequency power is that the decomposition efficiency of the gas introduced into the chamber 22 is increased by increasing the high frequency power, and the horizontal axis in FIG. The vertical axis represents prefactor σ ₀ ^* . In FIG. 5, ● represents the conductivity when the flow rate of monosilane was 25 sccm, □ represents 50 sccm, ● represents 100 sccm, and ▲ represents 200 sccm. As is clear from FIG. 5, prefactor σ ₀ ^* increases as the high frequency power increases.
[0017]
In FIG. 6, the horizontal axis represents conductivity σ, the left vertical axis represents activation energy Ea, and the right vertical axis represents prefactor σ ₀ ^* . In FIG. 6, ▲ indicates activation energy Ea when the high frequency power is 20 W (high frequency power density is 22 mW / cm ² ), and ● indicates activation when the high frequency power is 100 W (high frequency power density is 110 mW / cm ² ). Energy Ea, Δ indicates prefactor σ ₀ ^* when the high-frequency power is 20 W, and ◯ indicates prefactor σ ₀ ^* when the high-frequency power is 100 W. In FIG. 6, the numbers in the vicinity of each measurement point indicate the flow rate ratio (CH ₄ / SiH ₄ ) of the source gas. As apparent from FIG. 6, when the high-frequency powers 20W and 100W are compared with the same conductivity σ, the activation energy Ea and the prefactor σ ₀ ^* are increased by increasing the high-frequency power. This is because the decomposition efficiency of the gas introduced into the chamber 22 is increased by increasing the high frequency power.
[0018]
That is, as can be seen from FIGS. 4 to 6 together, it is desirable to increase the high frequency power from the viewpoint of increasing the prefactor σ ₀ ^* , but from the viewpoint of increasing the conductivity, the high frequency It can be said that it is desirable not to increase the power too much, and it is desirable to set the high-frequency power so that a predetermined B constant can be obtained.
In addition, the efficiency of gas decomposition is increased by increasing the high-frequency power, and the amount of undecomposed bond species such as impurities incorporated into the a-SiC: H thin film, for example, CH _X (X = 1, 2, 3). Therefore, the half width of the impurity level in the band gap becomes narrow. In other words, in order to narrow the half width of the impurity level in the band gap, it is necessary to increase the decomposition efficiency of the source gas and reduce the undecomposed bond species in the a-SiC: H thin film.
[0019]
Incidentally, as described above, it is known that the activation energy Ea varies depending on the temperature, and is represented by the following approximate expression when the activation energy at ₀ K is Ea ₀ .
Ea = Ea ₀ + γT
Where γ is a coefficient of temperature change of the activation energy Ea, and T is an absolute temperature. Therefore, the above-described conductivity σ = σ ₀ exp (−Ea / kT) is expressed by the following equation.
σ = σ ₀ exp (−γ / k) exp (−Ea ₀ / kT) = σ ₀ ^* exp (−Ea ₀ / kT)
Here, σ ₀ is a pre-exponential factor of conductivity, k is a Boltzmann constant, and σ ₀ ^* is a measured prefactor.
Γ is
γ = γ _F + γ _G + γ _T
Is approximated by However, gamma _F is proportional coefficient when the Fermi level E _F is a linear function of temperature, gamma _G is a proportionality coefficient when the band gap Eg is the primary function of the temperature, gamma _T conduction energy Etr temperature Is a proportionality coefficient in the case of a linear function. Here, in the case of an amorphous semiconductor thin film containing impurities, γ _G and γ _T cannot take large values, but γ _F can take various values depending on the state of the film. For example, when the amorphous semiconductor thin film is in a thermal equilibrium state (for example, at 80 ° C. or higher for p-type a-Si), γ _F takes a very small value. This is because staticical by the state function of the Fermi level E _F Shift (temperature dependence of the Fermi level E _F) is canceled by the structural relaxation. However, in a frozen state (for example, when the temperature is less than 80 ° C. for p-type a-Si), structural relaxation does not occur, and γ _F can take a large value. Therefore, the change due to the temperature of the activation energy Ea in the frozen state are mainly variation of the Fermi level E _F causes.
[0020]
Further, it is known that the value of the pre-exponential factor σ ₀ is about 200. For example, when γ is 10 k,
σ ₀ ^* = σ ₀ exp (−γ / k) = 0.01
It becomes. In the experiments of the inventors, an amorphous semiconductor thin film (p-type a-SiC: H thin film) having σ ₀ ^* = σ ₀ exp (−γ / k) = 1 or more is obtained, and when σ ₀ ^* = 1. When γ is estimated, γ = 5.3k. That is, by reducing γ from 10 k to 5.3 k, the conductivity can be increased by two orders of magnitude with the same activation energy Ea. That is, by reducing γ from 10 k to 5.3 k, a thin film thermistor having a lower resistance value and higher sensitivity can be obtained. In the present embodiment, the value of prefactor σ ₀ ^* is desirably 0.01 or more, which is larger than that of a conventional thin film thermistor. In other words, to increase the conductivity σ while increasing the B constant, it is desirable that the temperature coefficient of the Fermi level E _F and 10k or less. In short, in the present invention, by increasing the prefactor σ ₀ ^* , the decrease of the B constant is suppressed and the conductivity σ is increased. Also, the increase in the conductivity σ is due to the increase in the carrier concentration in the film, so that the 1 / f noise is reduced by increasing the carrier concentration. As a result, it is possible to realize a thin film thermistor having a small size and an improved S / N ratio.
[0021]
In the embodiment described above, a mixed gas of monosilane and methane gas is used as the source gas, and diborane is used as the dopant gas. However, the source gas may be any other gas as long as it is a mixed gas of a silicon-containing gas and a carbon-containing gas. Combinations can be used. Various known materials can also be used for the dopant gas. When the thermistor element 8 made of an n-type a-SiC: H thin film is formed, for example, phosphine (PH ₃ ) is used as the dopant gas. Good.
[0022]
【The invention's effect】
The invention of claim 1 is a thin film thermistor in which an electrode is provided on a semiconductor resistance layer made of an amorphous semiconductor thin film, and the semiconductor resistance layer has a Boltzmann constant having a proportional coefficient when the Fermi level is a linear function of temperature. Since it is less than 10 times, the temperature dependence of Fermi level is small and the temperature change of activation energy is small, so a high conductivity prefactor can be obtained, and it is possible to achieve both a high B constant and a high conductivity. Since 1 / f noise is reduced, there is an effect that the S / N ratio can be improved with a small size as a result.
[0023]
The invention according to claim 2 is a thin film thermistor in which an electrode is provided on a semiconductor resistance layer made of an amorphous semiconductor thin film, and the half width of the level formed between the band gaps of the semiconductor resistance layer has a Fermi level of temperature. Since the proportionality coefficient in the case of a linear function is less than 10 times the Boltzmann constant, the temperature dependence of the Fermi level is small and the temperature change of the activation energy is small, so a high conductivity prefactor can be obtained. It is possible to achieve both a high B constant and a high conductivity, and the 1 / f noise is reduced. As a result, there is an effect that the S / N ratio can be improved with a small size.
[0024]
The invention of claim 3 is a method of manufacturing a thin film thermistor in which an electrode is provided on a semiconductor resistance layer made of an amorphous semiconductor thin film, and the proportionality coefficient when the Fermi level is a linear function of temperature is 10 times or less of the Boltzmann constant. The semiconductor resistance layer is formed so that the temperature dependence of the Fermi level is reduced, the temperature change of the activation energy is reduced, and a high conductivity prefactor is obtained, and a high B constant and a high conductivity are obtained. Since both are compatible and the 1 / f noise is reduced, it is possible to provide a thin film thermistor having a small size and a high S / N ratio.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of an infrared detecting element using a thin film thermistor of the present invention.
FIG. 2 is a schematic configuration diagram showing an apparatus for manufacturing an amorphous semiconductor thin film according to the present invention.
FIG. 3 is an explanatory diagram of a density of states of an amorphous semiconductor thin film.
FIG. 4 is a diagram showing a relationship between conductivity and deposition rate with respect to high-frequency power when an amorphous semiconductor thin film is formed in the embodiment.
FIG. 5 is a diagram showing a prefactor relationship with respect to high-frequency power when an amorphous semiconductor thin film is formed in the embodiment.
FIG. 6 is a diagram illustrating high-frequency power dependency of activation energy, conductivity, and prefactor in the embodiment.
[Explanation of symbols]
8 Thermistor element 10 Thin film thermistor 11a Lower electrode 11b Upper electrode

Claims (3)

A thin film thermistor in which an electrode is provided on a semiconductor resistance layer made of an amorphous semiconductor thin film, wherein the semiconductor resistance layer has a proportionality coefficient of 10 times or less the Boltzmann constant when the Fermi level is a linear function of temperature. A featured thin film thermistor. A thin film thermistor in which an electrode is provided on a semiconductor resistance layer made of an amorphous semiconductor thin film, and the half-value width of the level formed between the band gaps of the semiconductor resistance layer is a case where the Fermi level is a linear function of temperature. A thin film thermistor having a full width at half maximum in which the proportionality coefficient is 10 times or less of the Boltzmann constant. A method for manufacturing a thin film thermistor in which an electrode is provided on a semiconductor resistive layer made of an amorphous semiconductor thin film, wherein the proportional coefficient when the Fermi level is a linear function of temperature is not more than 10 times the Boltzmann constant. A method for manufacturing a thin film thermistor, comprising forming a layer.

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