JP3711740B2 - 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]
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]
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, the semiconductor resistance layer having a uniform carrier concentration in the thickness direction, and the semiconductor resistance layer The carrier concentration is set such that the width of a depletion layer formed at the contact portion with the electrode is 10 nm or less, and the contact portion between the semiconductor resistance layer and the electrode is formed. By setting the carrier concentration to a high concentration such that the width of the depletion layer to be formed is 10 nm or less, the conductivity of the semiconductor resistance layer is increased and 1 / f noise is reduced, and the carrier of the semiconductor resistance layer is reduced. As the concentration increases, dangling bonds (unbonded hands) increase, the Fermi level hardly moves to the band edge, and the decrease in the B constant is suppressed. It is possible to improve the S / N ratio in a small size. In addition, since the carrier concentration is high, good ohmic characteristics with a good electrode can be realized, and the carrier concentration is uniform in the thickness direction. The structure is simpler than the multilayer structure provided in FIG.
[0010]
The invention of claim 2 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 a width of a depletion layer formed at a contact portion between the semiconductor resistance layer and the electrode is 10 nm or less. Since the depletion layer width can be reduced to 10 nm or less by increasing the carrier concentration, the conductivity increases as the carrier concentration of the semiconductor resistance layer increases. At the same time, 1 / f noise is reduced, and dangling bonds (unbonded hands) are increased, making it difficult for the Fermi level to move to the band edge and suppressing the decrease of the B constant. An N ratio thin film thermistor can be provided.
[0011]
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.
[0012]
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.
[0013]
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 accommodated in the chamber 22 and heated to 270 ° C. by the heater 23, for example. 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.
[0014]
In the present invention, in the thin film thermistor 10 in which the lower electrode 11a and the upper electrode 11b are in contact with both surfaces of the thermistor element 8 in the thickness direction, the contact portion between the thermistor element 8 and the lower electrode 11a, the thermistor element 8 and the upper electrode 11b. 1 / f noise is reduced by reducing the width of the depletion layer formed at both contact portions to 10 nm or less. Here, in the present embodiment, the width of the depletion layer at each contact portion is adjusted by increasing the carrier concentration in the thickness direction of the thermistor element 8 and increasing the overall carrier concentration. Therefore, changes in activation energy Ea, conductivity σ, prefactor σ ₀ ^* , and 1 / f noise with respect to the mixing ratio of the dopant gas to the source gas were measured. The results are shown in FIGS. In the present embodiment, the p-type a-SiC: H formation conditions are such that the power density of the high frequency power applied to the electrode 26b during discharge is 110 mW / cm ² and the discharge pressure is 0.9 Torr. In this embodiment, by setting the discharge power density to a relatively large value of 110 mW / cm ² , the decomposition of the gas introduced into the chamber 22 is promoted, and as a result, the prefactor σ ₀ is increased. It becomes possible.
[0015]
In FIG. 3, the horizontal axis represents the mixing ratio of diborane to monosilane, and the vertical axis represents the activation energy Ea. In FIG. 3, ♦ indicates that the mixing ratio of methane to monosilane is 0.3, ▼ indicates 0.5, ■ indicates 1.0, ● indicates 1.7, and ▲ indicates 3. The case of 0 is shown. As is apparent from FIG. 3, the activation energy Ea decreases when the mixing ratio of the dopant gas to the source gas is increased. That is, the B constant decreases when the mixing ratio of the dopant gas to the source gas is increased. Moreover, the activation energy Ea increases by increasing the mixing ratio of methane to monosilane.
[0016]
The horizontal axis of FIG. 4 indicates the mixing ratio of diborane to monosilane, and the vertical axis indicates the conductivity σ. In FIG. 4, ▼ indicates a case where the mixing ratio of methane to monosilane is 0.5, ■ indicates 1.0, and Δ indicates 3.0. As is clear from FIG. 4, the conductivity σ increases as the mixing ratio of the dopant gas to the source gas (monosilane and methane) increases.
[0017]
In FIG. 5, the horizontal axis represents the mixing ratio of diborane to monosilane, and the vertical axis represents prefactor σ ₀ ^* . In FIG. 5, ● indicates the case where the mixing ratio of methane to monosilane is 0.3, ▲ indicates 0.5, and Δ indicates 1. As is clear from FIG. 5, when the mixing ratio of the dopant gas to the source gas is increased, the prefactor σ ₀ ^* generally shows a rising tendency. However, as apparent from FIGS. 4 and 5, from the viewpoint of increasing the conductivity σ and prefactor σ ₀ ^* , it is desirable that the mixing ratio of methane to monosilane is not so high, and a desired B constant is obtained. Thus, it can be said that it is desirable to mix an appropriate amount of methane.
[0018]
In FIG. 6, the horizontal axis represents activation energy Ea, and the vertical axis represents 1 / f noise. In FIG. 6, ▲ indicates a case where the mixing ratio of diborane to monosilane is 0.25 vol%, ▪ indicates 1.0 vol%, and ● indicates 2.0 vol%. As is clear from FIG. 6, when compared with the same activation energy Ea, 1 / f noise decreases as the mixing ratio of the dopant gas to the source gas increases. Since the activation energy Ea is decreased as described above by increasing the mixing ratio of diborane to monosilane, the flow rate of methane is increased as a formation condition when viewed at the same activation energy Ea in FIG. . For example, when the mixing ratio is 0.25 vol% and the activation energy Ea is 0.42 eV, the flow rate of methane is 100 sccm, whereas the mixing ratio is 2.0% and the activation energy Ea is 0.42 eV. In this case, the flow rate of methane is increased to 170 sccm to 300 sccm.
[0019]
That is, as can be seen from FIG. 4 to FIG. 6, when the mixing ratio of the dopant gas to the source gas is increased, in other words, when the doping concentration is increased, the conductivity σ and the prefactor σ ₀ ^As 1 increases, 1 / f noise decreases. In addition, it can be said that it is desirable to mix an appropriate amount of methane so as to obtain a desired B constant. In order to increase the S / N ratio, it is desirable to increase the mixing ratio of the dopant gas to the source gas, for example, to set it to 1% or more.
[0020]
By the way, 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. 7, E _C and E _V indicate band ends (mobility ends), and E _F indicates a Fermi level.
[0021]
However, as apparent from FIG. 4, from the viewpoint of increasing the conductivity σ, it is desirable that the mixing ratio of methane to monosilane is not so high, and an appropriate amount of methane is mixed so as to obtain a desired B constant. Is desirable.
However, increasing the doping concentration of the amorphous semiconductor thin film, an increase in dangling bonds in the film (dangling bonds), since the Fermi level E _F is less likely to move to the mobility edge, reduction of B constant is suppressed The Although it is known that dangling bonds in the film are reduced by hydrogen dilution, it is possible to suppress a decrease in the B constant by suppressing the hydrogen dilution concentration.
[0022]
FIG. 8 shows IV characteristics of the thin film thermistor 10. In FIG. 8, (1) shows the case where the mixing ratio of diborane to monosilane is 0.25 vol%, and (2) shows the case where 1.0 vol%. From FIG. 8, it was confirmed that a good ohmic characteristic was obtained by setting the mixing ratio of diborane to monosilane to 1.0 vol% as compared to 0.25 vol%. By the way, when the width of the depletion layer is 10 nm or less at the metal-semiconductor contact, conduction by the tunnel of carriers occurs in addition to conduction by thermal excitation, and the conduction by this tunnel does not depend on the applied voltage, so that good ohmic characteristics are obtained. It is done. That is, in this embodiment, the width of the depletion layer formed at the contact portion between the thermistor element 8 and the lower electrode 11a is 10 nm or less by setting the mixing ratio of diborane to monosilane to 1.0 vol. Considered.
[0023]
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.
[0024]
【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, the semiconductor resistance layer having a uniform carrier concentration in the thickness direction, and the semiconductor resistance layer Since the carrier concentration is set so that the width of the depletion layer formed at the contact portion with the electrode is 10 nm or less, the width of the depletion layer formed at the contact portion between the semiconductor resistance layer and the electrode By setting the carrier concentration to a high concentration such that the thickness of the semiconductor resistance layer is 10 nm or less, the conductivity of the semiconductor resistance layer is increased, 1 / f noise is reduced, and the semiconductor concentration of the semiconductor resistance layer is increased. As ring bonds (unbonded hands) increase, the Fermi level hardly moves to the band edge, and the decrease in the B constant is suppressed, resulting in a small size and improved S / N ratio. There is an effect that can be. In addition, since the carrier concentration is high, good ohmic characteristics with a good electrode can be realized, and the carrier concentration is uniform in the thickness direction. There is an effect that the structure is simplified as compared with the multilayer structure provided in FIG.
[0025]
The invention of claim 2 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 a width of a depletion layer formed at a contact portion between the semiconductor resistance layer and the electrode is 10 nm or less. As the carrier concentration of the semiconductor resistance layer increases, the conductivity increases and 1 / f noise is reduced, and dangling bonds (unbonded hands) increase and the Fermi level is increased. Since it becomes difficult to move to the band edge and the decrease in the B constant is suppressed, there is an effect that 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 a diagram showing the relationship between the mixing ratio of the dopant gas to the source gas and the activation energy in the embodiment.
FIG. 4 is a diagram showing the relationship between the mixing ratio of the dopant gas to the source gas and the conductivity in the embodiment.
FIG. 5 is a diagram showing a relationship between a mixing ratio of a dopant gas to a source gas and a prefactor in the embodiment.
FIG. 6 is a diagram showing a relationship between activation energy and 1 / f noise in the embodiment.
FIG. 7 is an explanatory diagram of a density of states of an amorphous semiconductor thin film.
FIG. 8 is an IV characteristic diagram of the thin film thermistor in the embodiment.
[Explanation of symbols]
8 Thermistor element 10 Thin film thermistor 11a Lower electrode 11b Upper electrode

Claims (2)

A thin film thermistor in which an electrode is provided on a semiconductor resistance layer made of an amorphous semiconductor thin film, the semiconductor resistance layer having a uniform carrier concentration in a thickness direction, and at a contact portion between the semiconductor resistance layer and the electrode A thin film thermistor, wherein the carrier concentration is set so that a width of a depletion layer to be formed is 10 nm or less. A method for manufacturing 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 is formed at a carrier concentration such that the width of a depletion layer formed at a contact portion with the electrode is 10 nm or less. A method of manufacturing a thin film thermistor.

Images