JPH03163533A - Thin film two-terminal element - Google Patents

Abstract

PURPOSE:To enhance the degree of freedom in design, to make film thickness and film quality uniform and to have the advantage of making the area of an element large by constituting an insulating film which intervenes between 1st and 2nd conductors of a hard carbon film and making a conductor provided in contact with and under the insulating film contain Al as a principal component element. CONSTITUTION:In a thin film two-terminal element obtained by making the insulating film 2 intervene between the 1st and the 2nd conductors 1 and 3, the insulating film 2 is constituted of the hard carbon film and the conductor 1(Hereinafter referred to as a lower conductor) provided in contact with and under the insulating film 2 is made to contain at least Al as the principal component element. Since the hard carbon film is a vapor growth film, the various physical properties of the film are controlled in a wide range by a film producing condition. By selecting a material containing Al as the principal component element as the material of the lower conductor, the element is made excellent especially in the aspect of pressure proof and yield(difficulty in being peeled off). Thus, the design of the device is possible in the wide range and the dispersion of the characteristic of the element is reduced. Furthermore, the element is made excellent in threshold voltage and pressure proof and the yield is improved.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a thin film two-terminal device, and more specifically, to a switching device that can be suitably used in flat panel displays for office automation equipment, TVs, etc., especially liquid crystal displays. The present invention relates to a thin film two-terminal device useful as a switching device for devices.

[Conventional technology]

There is a strong demand for the use of large-area liquid crystal panels for office automation equipment terminals and liquid crystal TVs, and for this reason, the active matrix method is designed to maintain voltage by providing a switch for each pixel.

By the way, one of the switches is MIN (Meta
l Insulator Metal) elements are often used. This is because the thin film two-terminal element exhibits nonlinear current-voltage characteristics that are good for switching.

Conventional thin film two-terminal devices have a Ta. AQ- A metal electrode such as Ti is provided, and an oxide of the metal or Si
Ox. It is known that an insulating film made of SiNx or the like is provided, and a rib or a gold rA electrode made of Cr or the like is provided thereon as an upper electrode.

[Problem to be solved by the invention]

However, a thin film two-terminal device using a metal oxide as an insulator (insulating film)
No. 2689, No. 62-62333, etc.), the insulating film is formed by anodic oxidation or thermal oxidation of the lower electrode, so the process is complicated and requires high-temperature heat treatment (no. In addition, the film controllability (uniformity and reproducibility of film quality and thickness) is poor, and the substrate is limited to heat-resistant materials. Since the insulating film is made of a metal oxide with fixed physical properties, the device material and device characteristics cannot be freely changed, and the degree of freedom in design is limited. This means that it is difficult to design and manufacture a device that fully satisfies the specifications of a liquid crystal display device incorporating a thin film two-terminal element. In addition, if the film controllability is poor in this way, the current (I) and voltage (v) characteristics as element characteristics, especially the IV characteristics and the symmetry of the IV characteristics (
A problem also arises in that the current ratio A/E +) varies greatly between the positive bias and the negative bias.

In addition, when using a thin film two-terminal element for a liquid crystal display (LCD), it is generally desirable that the ratio of liquid crystal part capacitance to thin film two-terminal element capacitance be 10 or more, but in the case of metal oxide films, the dielectric constant is large. The element capacitance also increases, and therefore microfabrication is required to reduce the element capacitance, that is, to reduce the element area. In this case, there is also the problem that the insulating film is mechanically damaged during the rubbing process or the like during the filling of the liquid crystal material, resulting in a reduction in yield in combination with microfabrication.

On the other hand, in the case of the Hijia element (Japanese Patent Application Laid-Open No. 61-260219) using SiOz or SUN as the insulating film, the insulating film is formed by a vapor phase method such as plasma CVD or sputtering. Since a temperature of about 300°C is usually required, low-cost substrates cannot be used, and when increasing the area, the film thickness and quality tend to become non-uniform due to the substrate temperature distribution. Since the insulating film is bonded in a gas phase, a lot of dust is generated and there are many bottle holes in the film, which reduces the yield of devices.Furthermore, the film stress is large, causing film peeling, and this This also reduces the yield of devices.

The present inventors previously proposed an MIM element using a hard carbon film (i-type carbon (i-C) film) as an insulating film, but the thickness of the insulating film was as thin as 20 to 100λ. Ta. In the case of this insulating film (i-C film), the conduction mechanism is tunnel conduction, and it is rather suitable for applications as ultra-thin film elements such as high-speed switches and tunnel light emission. However, when applied to liquid crystal display devices and the like, a thicker film is preferable from the viewpoints of breakdown voltage, yield (defect rate), uniformity of device characteristics, and threshold voltage.

The first object of the present invention is to use a low dielectric constant insulating film (hard carbon film) that can be formed at a relatively low temperature and in a simple process, and has excellent film controllability and mechanical strength. It is an object of the present invention to provide a thin film two-terminal element that allows device design in the same manner, has less variation in element characteristics, has excellent threshold voltage and breakdown voltage, and has a high yield.

Furthermore, the second object of the present invention is to provide a thin film structure that, by selecting the lower conductor material, has a small leakage current, a wide margin of characteristics suitable for liquid crystal driving, and higher device reliability and yield. An object of the present invention is to provide a terminal element.

[Means and effects for solving the problems] The thin film two-terminal element of the present invention is a thin film two-terminal element in which an insulating film is interposed between a first conductor and a second conductor, in which the insulating film is It is characterized in that a conductor (hereinafter referred to as a lower conductor) made of a hard carbon film and provided in contact with and below the insulating film contains at least legs as a main constituent element.

The thin film two-terminal element of the present invention will be described in detail below with reference to the drawings. Figure 1 is a perspective view showing an example of the element configuration when the present invention is applied to a thin film two-terminal element of a liquid crystal display element. Electrode 4
A part of the pixel electrode 4 is formed on a substrate (not shown) on which the pixel electrode 4 is formed.
A lower conductor 1 is formed so as to overlap with the upper surface of the lower conductor 1, and an insulating film 2 and an upper conductor 3 are formed on the lower conductor 1 as shown in the figure, thereby forming a thin film two-terminal element. The structural feature of this thin film two-terminal element is that the insulating film 3 is formed from a hard carbon film, and the lower conductor 1 is formed from a material containing at least carbon as a main constituent element.

As described above, the lower conductor 1 is formed to a thickness of several hundred to several thousand layers using a material containing at least AQ as a main constituent element by vapor deposition, sputtering, or the like. Insulating film 2
For example, a hard carbon film formed by the method described below is 100%
It is used with a film thickness of ~8,000, preferably 300 to 4,000. Is the upper conductor 3 an example? Pt. Ni. A
g. Cu, Au, IQ, Cr, Ti. W, Ao
．． Ta, ITO, ZnO: [1. In20,,S
It is formed to a thickness of several hundred to several thousand layers using a conductive material such as in■ by a vapor deposition method, a sputtering method, or the like.

In the thin film two-terminal device of the present invention, the insulating film 3 interposed between both conductors 1 and 2 is formed of a hard carbon film, and the hard carbon film is a material that absorbs carbon atoms and hydrogen atoms and forms the main structure. It consists of a hard carbon film (also called an i-C film, a diamond-like carbon film, an amorphous diamond film, or a diamond thin film) containing at least one of non-quality and microcrystalline elements.

One feature of the hard carbon film is that it is a vapor-phase grown film.
As will be described later, the various physical properties can be controlled over a wide range by controlling the film forming conditions. Therefore, even though it is called an insulating film, its resistance value covers the range from semi-insulator to insulator. Kaisho 61-26
MSI element (Metal
-Semi-Insulator) and SIS element (semiconductor-insulator-semiconductor, where the "semiconductor" is a device doped with impurities at a high concentration).

In addition, in order to further widen the control range of physical properties, this hard carbon film contains at least 5 atoms z or less of an element from group Ⅰ of the periodic table as one of the constituent elements, and z or less of an element from group Ⅰ of the periodic table. 35 atoms 2 or less, same group V element as 5yX
2 or less atoms, alkaline earth metal elements 5 atoms or less, alkali metal elements 5 atoms 2, nitrogen atoms 5 atoms or less, oxygen atoms 5yA atoms or less 2, chalcogen-based elements 35 atoms or less 2, or halogen The element may be contained in an amount of 35 atoms/2 or less.

The amounts of these elements or atoms can be measured by conventional methods of elemental analysis, such as Auger analysis. Further, the amount can be adjusted by adjusting the amount of other compounds contained in the raw material gas and certain film conditions.

In order to form such hard carbon films, organic compound gases, especially hydrocarbon gases, are used.The phase state of these raw materials does not necessarily have to be a gas phase at room temperature and normal pressure, but can be melted or evaporated by heating or reduced pressure. , can it be vaporized through sublimation, etc.? For example, it can be used in either liquid or solid phase.

Regarding hydrocarbon gas as a raw material gas, for example, CH
4,C,}I,,C4H■. Gases containing at least all hydrocarbons such as paraffinic hydrocarbons such as, olefinic hydrocarbons such as C2H4, diolefinic hydrocarbons, acetylenic hydrocarbons, and even aromatic hydrocarbons can be used.

In addition, compounds other than hydrocarbons, such as alcohols, ketones, ethers, and esters, can be used as long as they contain at least the carbon element. In the present invention, a method for forming a hard carbon film from a raw material gas includes a method in which a certain film active species is formed through a plasma state generated by a plasma method using direct current, low frequency, high frequency, microwave, etc. Although preferred, a method using a magnetic field effect is more preferred in order to perform deposition under low pressure for the purpose of increasing the area, improving uniformity, and/or forming a film at a low temperature. Active species can also be formed by thermal decomposition at high temperatures. In addition, it may be formed through an ionic state produced by ionization vapor deposition, ion beam vapor deposition, etc., or may be formed from neutral particles generated by vacuum vapor deposition, sputtering, etc. , or a combination thereof.

An example of the deposition conditions for the hard carbon film produced in this manner is approximately as follows in the case of the plasma CVD method.

RF output: 0.1-50 W/cJ Pressure: 10'-3-10 Torr Deposition temperature: It can be carried out at room temperature to 950°C, but preferably room temperature to 300°C. This plasma state allows the source gas to become radicals. By decomposing and reacting with ions, carbon atoms C
A hard carbon film containing at least one of amorphous (non-quality) and microcrystalline (crystal size is several tens of micrometers to several micrometers) consisting of hydrogen atoms H is deposited. Table 1 shows the properties of the hard carbon film.

Table 1 Note) Measurement method; Specific resistance (ρ): Determined from the IV characteristics of a coplanar cell.

Optical band gap (Egopt): Obtain the absorption coefficient (α) from the spectral characteristics and determine from the relationship (αhv)1/2=B(hy'-Egopt).

Hydrogen content in the film (CM): 290 from infrared absorption spectrum
It is determined by integrating the peak near 0 cm-1 and multiplying by the absorption cross section @A. That is, CH:A-fα(v)/w・dv SP3/SP” ratio: infrared absorption spectrum, sp”, s
It is decomposed into Gaussian functions respectively attributed to p2 and determined from the area ratio thereof.

Vickers hardness (H): Based on micro Vickers meter.

Refractive index (n): by ellipsometer.

Defect density: Based on ESR.

As a result of analysis by IR absorption method and Raman spectroscopy, the hard carbon film formed in this way shows that the carbon atoms have an sp" hybrid orbital and an sp" hybrid orbital, as shown in FIGS. 2 and 3, respectively. It has become clear that there is a mixture of interatomic bonds that formed . The ratio of SPJ binding to SP2 binding is IR
It can be roughly estimated by separating the peaks of the spectrum. I
The R spectrum is measured with many mode spectra overlapping at 2800 to 3150 cm-1, but the attribution of the peak corresponding to each wave number has been clarified, and is determined by the Gaussian distribution shown in Figure 4. SP3/SP'' can be determined by performing peak separation, calculating the area of each peak, and finding the ratio.

Further, according to X-ray and electron diffraction analysis, the hard carbon film is in an amorphous state (a-C:H) and/or an amorphous state containing microcrystalline grains of several tens of micrometers to several micrometers. It turns out that there is something.

In the case of plasma CVD method, which is generally suitable for mass production,
The lower the RF output, the higher the specific resistance and hardness of the membrane, and the lower the pressure, the longer the life of active species.
The substrate temperature is becoming lower and more uniform over a large area, and resistivity and hardness tend to increase. Furthermore, since the plasma density decreases at low pressures, methods using magnetic field confinement effects are particularly effective in increasing resistivity. Furthermore, this method (plasma CVD method) has the characteristic that it can form a hard carbon film of good quality even under relatively low temperature conditions of room temperature to 150°C, so it can be used at low temperatures in the thin film two-terminal device manufacturing process. It is ideal for

Therefore, the degree of freedom in selecting the substrate material to be used is increased, and the substrate temperature can be easily controlled, so that a uniform film can be obtained over a large area.

As shown in Table 1, the structure and physical properties of the hard carbon film can be controlled over a wide range, so there is the advantage that device characteristics can be designed freely. Furthermore, the dielectric constant of the film is 3 to 5, which is compared to Ta20, IAfl, which are used in conventional MIM devices.
Because it is smaller than O, SiNx, etc., when manufacturing an element with the same capacitance, the element size can be large, so it does not require as much fine processing and improves yield (due to driving conditions, LCD The capacitance ratio between CLCD and MI element is required to be about 1 (CLCD:CMIM"lO:1).

Furthermore, since the film has high hardness, there is little damage caused by the rubbing process during encapsulation of the liquid crystal material, which also improves yield.

A hard carbon film suitable as a thin film two-terminal element for driving a liquid crystal is
The film thickness is 100-8000A due to the turbulence conditions. Specific resistance is 1
Advantageously, it is in the range 0'-1013 Ω·cffI.

Furthermore, considering the margin between drive voltage and withstand voltage (dielectric breakdown voltage), it is desirable that the film thickness is 200 or more.
In addition, in order to prevent color unevenness caused by the level difference (cell gap difference) between the pixel part and the thin film two-terminal element part from becoming a practical problem, it is desirable that the film thickness be 6000 or less. More preferably, the film thickness is 200-6000 cm, and the specific resistance is 5X 10'-10"Ω"cm.

The number of device defects due to pinholes in the hard carbon film increases as the film thickness decreases, and becomes especially noticeable when the number of people is less than 300 (defect rate exceeds 1%), and the in-plane distribution of the film thickness increases. The film thickness should not exceed 300 Ω because uniformity (and therefore uniformity of device characteristics) cannot be ensured (the accuracy of film thickness control is limited to about 30 A, and the variation in film thickness exceeds 10%). More desirable.

In addition, in order to make it difficult for the hard carbon film to peel off due to stress, and to lower the duty ratio (preferably 1/
1,000 λ or less), it is more desirable that the film thickness is 4,000 λ or less.

Taking all of these into consideration, the thickness of the hard carbon film is 30
0-4000 input, specific resistance is 10'-1011Ω・cII
More preferably, it is 1.

By the way, as a result of intensive studies on the relationship between the material of the lower conductor provided under and in contact with the insulating film (hard carbon film) and the physical properties of the hard carbon film, the inventors have determined the selection of the material for the lower conductor. The physical properties of the hard carbon film differ depending on the type of film, and if a material containing at least ribs as a main constituent element is selected as the lower conductor material, the I-V characteristics of the device (characteristics particularly suitable for use in switching devices for liquid crystal parking), pressure resistance,
It has been found that it is particularly excellent in terms of yield (difficulty in peeling off), etc. Table 2 is a table showing a comparison of various characteristics of hard carbon films and devices made of various lower conductor materials. Note that the element configuration was as shown in FIG.

Table 2 From Table 2, it can be seen that, as the element characteristics, IlIon (ON current) is the smallest and the withstand voltage is the highest.

A small Ion means that the specific resistance of the hard carbon film is large, and has the following advantages: (1) a small leakage current, and (2) a wide margin for characteristics (κ, β) suitable for liquid crystal swelling.

Here, an explanation will be added regarding ■ using FIG. 5.

FIG. 5 is a conceptual diagram showing the appropriate range of liquid crystal egg movement determined by simulation. Although the values and widths shift depending on the driving conditions, they are similar in shape. κ and β are coefficients in the Poole-Frenkel conduction equation representing the IV characteristics of the thin film two-terminal device of the present invention, and are expressed by the following relational expression.

I=κexp(βvt72)
-(1) I: Current V: Applied voltage κ: Conductivity coefficient β
: Poole-Frenkel coefficient n: Carrier density μ: Carrier mobility q: Electron charge Φ: Trap depth ρ: Specific resistance d
: Thickness of insulating film (person) k: Boltzmann's constant T: Ambient temperature ε, : Permittivity of insulating film From equation (2), the larger the specific resistance, the smaller κ becomes. Therefore, from Figure 5, the characteristics are suitable for driving. It can be seen that the margin becomes wider. Furthermore, the high withstand voltage has advantages such as (a) high reliability of the element, and (c) suitability for higher duty because the driving voltage can be increased.

Such device characteristics can be related to the amount of hydrogen in the hard carbon film. In other words, when the lower conductor material is AQ, the amount of hydrogen in the film is the highest and the dangling bonds of carbon atoms are effectively terminated, resulting in a lower defect density.
It is thought that as a result, the specific resistance increases and the withstand voltage also increases. Furthermore, i shows good results in terms of resistance to peeling (Cr also has good results) and has the advantage of high yield.

Note that the lower conductor is not limited to Afl alone, and the same effect can be obtained by containing other elements such as SL, Cu, etc. as necessary. [Example] Next, Examples will be shown, but the present invention is not limited thereto.

Example 1 As shown in FIG. 1, an ITO transparent conductive thin film was deposited on a Pyrex transparent substrate (not shown) to a thickness of several hundred to several thousand λ by sputtering, and then patterned to form a pixel electrode 4. Intimidated. Next, a thin film two-terminal device was fabricated as follows. First, after depositing the liver to a thickness of IOOOA by vapor deposition method,
The lower conductor 1 was formed by patterning. After 800 hard carbon films were deposited thereon by the plasma CVD method, they were patterned by dry etching to form the N edge film 2. Furthermore, Ni was deposited on this layer to a thickness of 1,000 layers by EB evaporation, and then patterned to form the upper conductor 3. The conditions for forming the hard carbon film at this time are as follows.

Pressure: 0,035 Torr CH4 flow rate: 10 sccn RF power: 0.2 V/a# Example 2 A thin film two-terminal device as shown in FIG. 6 was fabricated as follows. First, an AQ thin film having a thickness of IOOOA was formed on a plastic transparent substrate 5 by vapor deposition and patterned to form a lower conductor 1. A hard carbon film having a thickness of 1,100 mm was deposited thereon as an insulating film 2 by plasma CVD, and then patterned by dry etching. Furthermore, a 1,000-layer thick ITO film is coated on top of it using the EB evaporation method.
The upper conductor 3 was formed by patterning by etching. The conditions for forming the hard carbon film at this time were as follows. Pressure: 0,035 Torr CH4 flow rate: 20 sccm RF power: 0. IV/ail According to the structure of Example 1 or 2, the element characteristics (■-■
The characteristics (or the symmetry of the IV characteristics) were good, the breakdown voltage was 20 V or more, and there was no problem in stability for continuous operation. On the other hand, when Ni was used as the lower conductor, the withstand voltage was as low as 1Ov or less in the same configuration. Also C
It has been found that when r is used, the device characteristics become abnormal (accompanied by deterioration of the film) in a short period of time (several minutes to several tens of minutes) during continuous operation.

〔Effect of the invention〕

In the thin film two-terminal device of the present invention, the insulating film interposed between the first conductor and the second conductor is a hard carbon film, and this film is manufactured by 1) a vapor phase synthesis method such as plasma CVD method; , the physical properties can be controlled over a wide range by changing the film formation conditions, and therefore there is a large degree of freedom in device design. 2) Since it is hard and can be made thick, it is less susceptible to mechanical damage, and it is expected that pinholes will be reduced due to the thicker film. 3) It is possible to form a high-quality film even at low temperatures near room temperature, so there are no restrictions on the substrate material. 4) It is suitable for thin-film devices because it has excellent uniformity in film thickness and film quality. , 5) Since the dielectric constant is low, advanced microfabrication technology is not required, and therefore it is advantageous for increasing the area of the device. The device is suitable as a switching device for liquid crystal display.

Furthermore, since the lower conductor of the thin film two-terminal device of the present invention contains at least AQ as a main constituent element, 1) the leakage current is small, and a certain level 1 has a margin of characteristics suitable for liquid crystal display. 2) It has a high withstand voltage, making it suitable for high-duty applications as it increases the reliability of the element or increases the swinging voltage. 3) The yield rate is high because the hard carbon film is difficult to peel off.

[Brief explanation of the drawing]

FIG. 1 shows a typical example of a thin film two-terminal device according to the present invention (
A perspective view of Example 1), FIG. 2, FIG. 3, and FIG. 4 are diagrams for explaining the physical properties of the hard carbon film, and FIG. 5 is a conceptual diagram showing the appropriate range of liquid crystal intoxication determined by simulation. 6th
The figure is a cross-sectional view for explaining the configuration of a thin film two-terminal device according to Example 2 of the present invention.

Claims (1)

[Claims] (1) In a thin film two-terminal element in which an insulating film is interposed between a first conductor and a second conductor, the insulating film is made of a hard carbon film, and the insulating film is provided in contact with and below the insulating film. 1. A thin film two-terminal device characterized in that a conductor containing at least Al as a main constituent element.