JPH03163532A - 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 forming an insulating film which intervenes between 1st and 2nd conductors of a hard carbon film and forming a conductor provided in contact with an under the insulating film of a zinc oxide transparent electric conductor. CONSTITUTION:The insulating film 3 which intervenes between the 1st and the 2nd conductors 2 and 4 is formed of the hard carbon film, and the conductor 2(lower conductor) provided in contact with and under the insulating film 3 is formed of the zinc oxide transparent electric conductor. In the case of forming the lower conductor 2, it is good to dope ZnO with group III, group IV or halogen element. By thus doing, resistance is lowered. Since the hard carbon film is vapor growth film, various physical properties are controlled by a film producing condition in a wide range. Thus, the design of a device is possible in the wide range and the dispersion of the characteristic of the element is reduced, then the element is made excellent in threshold voltage and withstand voltage.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a thin film two-terminal device, and more specifically, 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 active matrix systems are designed to maintain voltage by providing a switch for each pixel.

By the way, MIM (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 Ta, AQ. A metal electrode such as Ti is provided, and an oxide of the metal or Si is provided on the electrode.
Ox. It is known that an M edge film made of SiNx or the like is provided, and a metal electrode of Cr or the like is provided thereon as an upper electrode.

[Problem to be solved by the invention]

However, thin film two-terminal devices using metal oxides as insulators (insulating films)
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. 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 changed freely, 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 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 (
Current ratio L/ between positive bias and negative bias
I. ) will also cause the problem of increased variation. 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/a film two-terminal element capacitance is 10 or more, but in the case of a gold H, rII compound film, the dielectric constant Since the element capacitance is large, the element capacitance also becomes large, and thus microfabrication is required to reduce the element capacitance, that is, to reduce the element area. Furthermore, 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 an MIM element (Japanese Patent Application Laid-Open No. 61-260219) using Sun) (or SiNz) as an insulating film, the edge film is formed by a vapor phase method such as plasma CVD or sputtering, but the substrate 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 film quality tend to become non-uniform due to the substrate temperature distribution. Since the insulating film is synthesized in a gas phase, a lot of dust is generated and there are many pinholes 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 a HIM element using a hard carbon film (i-type carbon (i-C) film) as an insulating film, but the thickness of the insulating film was 20 to 100 times thinner. there were. 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, it is desirable that the film be thicker in terms 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 device which enables device design, has little variation in device characteristics, has excellent threshold voltage and breakdown voltage, and has a high yield.

A second object of the present invention is to provide a thin film two-terminal element that reduces the number of steps and costs by selecting a lower conductor material.

[Means and effects for solving the problems] In the thin film two-terminal element of the present invention, the insulating film interposed between the first conductor and the second conductor is made of a hard carbon film, and the insulating film is made of a hard carbon film. It is characterized in that the conductor (hereinafter referred to as the lower conductor) provided in contact with and below it is made of a zinc oxide transparent conductor.

Hereinafter, the thin film two-terminal element of the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a diagram showing an example of a method for manufacturing a thin film two-terminal device for a liquid crystal display device when the present invention is applied to the device.
c) shows the layer structure of the thin film two-terminal device after completion.

In the figure, l is the substrate, 2 is the lower conductor, 3 is the insulating film, and 4 is the upper conductor. In this example, the lower conductor 2 is made of zinc oxide (Z
A structural feature is that the insulating film 3 is made of a hard carbon film and is made of a transparent conductor (nO).

The method for manufacturing this thin film two-terminal device will be explained according to FIG. 1. First, hundreds to thousands of zinc oxide (ZnO) are deposited on a transparent substrate 1 made of glass, plastic, etc. by sputtering, vapor deposition, etc. Patterning and lower conductor 2
(Figure 1(a)). Note that in this example, the lower conductor 2 also serves as a pixel electrode. Here, when forming the lower conductor 2, ZnO may be doped with group Ⅰ, group Ⅰ, or halogen elements, and by doing so, the resistance can be further lowered. In particular, ZnO doped with AQ: , which is preferable because it has excellent transmittance. Next, a hard carbon film is coated with a film of 100 to 800
0 people, do you want it? Preferably, 200 to 6000, more preferably 300 to 4000, are deposited and patterned by dry etching or the like to form the insulating film 3 (FIG. 1(b)).

Finally, Pt. Ni, Ag, Cu. Au. AQ
．． Cr. Ti. W. Mo. Ta. ITO,
Hundreds to thousands of conductive thin films of ZnO:AQ, In20, SnO2, etc. are deposited by vapor deposition, sputtering, etc., and patterned to form the upper conductor 4 (Fig. 1 (C)).
). A thin film two-terminal device is obtained. Note that as the material for the upper conductor 4, Ni, Pt, and Ag are particularly excellent in terms of adhesion and long-term stability of the element.

By the way, as a transparent conductor... ITO in addition to ZnO
, In20, SnO2, etc. are known, but if they are used as a lower conductor and a hard carbon film is formed on top of them, they will be bombarded by ions or electrons present in the gas phase during the formation of the hard carbon film. Therefore, the surface becomes metal-rich,
This results in an increase in resistivity and a decrease in transmittance, impairing its function as a transparent conductor. Especially ITO or In. O,
When using carbon, In, which is rich on the surface, tends to diffuse into the hard carbon film, resulting in a problem of deterioration of the device characteristics itself. On the other hand, when ZnO is used, the above-mentioned problems do not occur, and it is highly stable against bombardment by ions or electrons. Note that the surface bonding state can be determined by, for example, xPS, and when actually measured by applying a bombardment with H2 plasma, in ITO or SnO, in addition to the bonding energy of each metal oxide, the energy of the metallic bonding of In or Sn The peak of is 1! In contrast, only oxide bonds were observed in ZnO. Of course, there are ways to avoid such problems, and a thin film two-terminal element may be fabricated using a fabrication process as shown in FIG. That is, first, I
A pixel electrode 15 made of TO, In20, SnO, etc. is formed (FIG. 2(a)). Next, the chest, Cr. Ni-C
u, Ti. Ag. Pt, Au. w, Mo. T
Several hundred conductive thin films such as a are deposited by vapor deposition, sputtering, etc.
Several thousand pieces are deposited and patterned to form the lower conductor 12 (FIG. 2(b)).

At this time, in order to protect the pixel electrode 15, patterning is performed so as to cover the entire surface of the pixel electrode 15. Next, the hard carbon film is coated by plasma CVD, ion beam method, etc.
0 to 8,000 layers are deposited and patterned by dry etching or the like to form an insulating film 13 (FIG. 2(C)). Furthermore, Pt. Ni. Ag. Cu. Ar. AQ
．． Cr. Ti, U. A conductive thin film of Mo, Ta, etc. is deposited several hundred to several thousand times by vapor deposition, sputtering, etc., and patterned to form an upper conductor] 4 (Fig. 2 (d).
)). Finally, the conductive thin film on the pixel electrode 15 is removed by etching to complete the thin film two-terminal element. However, as can be seen by comparing Figures 1 and 2, when ITO, In20=, Sn02, etc. are used as the lower conductor,
Compared to the case where ZnO is used for the lower conductor, the number of steps is significantly increased. In other words, if ZnO is used as the lower conductor, the number of steps can be significantly shortened and costs can be reduced.Furthermore, in the thin film two-terminal device of the present invention, the insulating film 3 is made of a hard material. The hard carbon film is formed of a carbon film, but the hard carbon film is a hard carbon film (i-C film,
Diamond-like carbon film, amorphous diamond film,
(also called a diamond thin film). One feature of hard carbon films is that, because they are vapor-grown films, their physical properties can be controlled over a wide range by controlling the film-forming conditions, as described below. Therefore, even though it is called an insulating film, its resistance value covers the range from semi-insulator to insulator. Kaisho 61-2602
MSI element (Metal-S
It is also positioned as an SIS element (semiconductor-insulator-semiconductor, where "semiconductor" is a device doped with impurities at a high concentration).

In addition, in order to further expand the control range of physical properties, this hard carbon film contains at least an element from group Ⅰ of the periodic table as one of the structural elements, with 5 atoms or less of the total atoms in the group Ⅰ. 35 rM% or less for elements, 5 atoms or less for group Ⅰ elements, 5 atoms or less for alkaline earth metal elements, 5 atoms or less for alkali metal elements, 5 atoms or less for nitrogen atoms, 5 atoms or less for oxygen atoms, 5 atoms or less for oxygen atoms. Less than 35 atoms of chalcogen-based elements, 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 a hard carbon film, an organic compound gas, particularly a hydrocarbon gas, is used. The phase state of these raw materials does not necessarily have to be a gas phase at room temperature and normal pressure; they can be used in either a liquid or solid phase as long as they can be vaporized through melting, evaporation, sublimation, etc. by heating or reduced pressure. be.

Regarding hydrocarbon gas as a raw material gas, for example, CH
4, C2H. C4H■. Paraffinic hydrocarbons such as C
Gases containing at least all hydrocarbons such as olefinic hydrocarbons such as 2H, diolefinic hydrocarbons, acetylenic hydrocarbons, and even aromatic hydrocarbons can be used.6 In addition, gases other than hydrocarbons, such as alcohol Any compound containing at least a carbon element, such as compounds, ketones, ethers, and esters, can be used.

The method of forming a hard carbon film from a raw material gas in the present invention is a method in which active species for film formation are formed through a plasma state generated by a plasma method using direct current, low frequency, high frequency, microwave, etc. However, 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, a method using a magnetic field effect is more preferable. Active species can also be formed by thermal decomposition at high temperatures.

In addition, it may be formed through an ionic state generated by ionization vapor deposition, ion beam vapor deposition, etc., or it may be formed from neutral particles generated by vacuum vapor deposition, sputtering, etc. Furthermore, it may be formed by a combination of these.

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

RF output: 0.1-50 W/ci Pressure: 10''-10 Torr Deposition temperature: Can be performed at room temperature to 950°C, but preferably at room temperature to 950°C
300℃.

Due to this plasma state, the raw material gas is decomposed into radicals and ions and reacts, thereby forming carbon atoms on the substrate.
A hard carbon film containing at least one of amorphous (non-quality) and microcrystalline (crystal size is from several LOA to several μm) consisting of and 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 (αhν)1/2=B(hν′−Egopt).

Amount of hydrogen in the film (CH): 290 from infrared absorption spectrum
It is determined by integrating the peak near 0 cm'' and multiplying it by the absorption cross section A. That is, CH=A-fα(ri)/ti-dri SP3/S
P'' ratio: The infrared absorption spectrum is decomposed into Gaussian functions assigned to sp3 and sp'', respectively, and determined from the area ratio.

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 thus formed shows that carbon atoms form mixed orbitals of SP3 and mixed orbitals of sp2, as shown in FIGS. 3 and 4, respectively. It has become clear that there is a mixture of interatomic bonds. The ratio of SP3 binding to SP2 binding is IR
It can be roughly estimated by separating the peaks of the spectrum. r
In the R spectrum. Although the spectra of many modes overlap in the measurement from 2800 to 3150 cm-'', the attribution of the peak corresponding to each wavenumber is clear, and the peaks are separated using a Gaussian distribution as shown in Figure 5. SP3/SP'' can be determined by calculating the area of each peak and finding the ratio.

Further, according to X, L and electron beam 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.

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) is performed at room temperature -150°C.
It has the characteristic of being able to form a high-quality hard carbon film even under relatively low temperature conditions, so it is ideal for reducing the temperature of the thin film two-terminal device manufacturing process.

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 an advantage that device characteristics can be designed freely. Furthermore, the dielectric constant of the film is 3 to 5, which is Ta20, , AQ, O, which is used in conventional MIX elements.
,, Because it is smaller than SiNx etc., when making an element with the same capacitance, the element size only needs to be larger, so it does not require as much fine processing and the yield improves (due to the driving conditions, it is different from LCD). The capacitance ratio with Element Element needs to be about 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 liquid crystal display is
From active conditions, the film thickness is 100 to 8000, and the specific resistance is 10.
Advantageously, it is in the range from 6 to 10 13 Ω·cm.

In addition, considering the margin between parking 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 6000λ or less. It is more preferable that the film thickness is 200-6000λ and the specific resistance is 5×10′-10”Ω·CII+.

The number of device defects due to pinholes in hard carbon films increases as the film thickness decreases, and becomes especially noticeable below 300λ (defect rate exceeds 1%), and the uniformity of the in-plane distribution of film thickness (The accuracy of film thickness control is limited to about 30 people, 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, a lower duty ratio (preferably 1/
1000A or less), it is more desirable that the film thickness be 4000A or less.

Taking all of these into account, the thickness of the hard carbon film is 30
It is more preferable that the resistivity is 0 to 400 OA and the specific resistance is 10 7 to 11 Ω'cm.

〔Example〕

Examples will be shown next, but the present invention is not limited thereto.

Example 1 As shown in FIG. 6, ZnO:AQ was deposited to a thickness of 1000 nm on a Pyrex transparent substrate (not shown) and then patterned to form a lower conductor (also serving as a pixel electrode) 2.
The ZnO:AQ film was formed using ZnO+AM,03(2.5
%) using a mixture target. This was done by RF magnetron sputtering method. Next, as the insulating film 3, a hard carbon film having a thickness of 800 mm was deposited by the plasma CVD method, and then patterned by dry etching. Furthermore, after depositing Ni to a thickness of 1000 nm using the Ni@EB evaporation method, the upper conductor 4 is patterned using dry etching.
A thin film two-terminal device was fabricated by applying the method. The conditions for forming the hard carbon film at this time are as follows.

Pressure: 0,035 Torr CH4 flow rate: 20 MCCM RF power: 0.2 RI/d According to the structure of Example 1, compared to the case where conventional ITO or the like is used for the pixel electrode (the formation of a protective metal film is ), it was possible to reduce the number of steps and still obtain a device with comparable characteristics. On the other hand, the same process as Example 1 (
In other words, if the pixel electrode (lower conductor) is made of, for example, ITO (without a protective metal film), the resistance value of the ITO itself increases by about three orders of magnitude after the hard carbon film is formed, and the device characteristics also change. Deviating from the normal relational expression QnIccv'V,
The symmetry of positive and negative biases also deteriorated.

Example 2 As shown in FIG. 7, ZnO:AQ was deposited to a thickness of 1500 nm on a plastic transparent substrate (not shown), and then patterned to form a lower conductor (also serving as a pixel electrode) 2.

A ZnO:l film was formed by RF magnetron sputtering in an oxygen gas atmosphere using a Zn+AQ (5%) composite target. Next, a hard carbon film was deposited as the insulating film 3 by plasma CVD method, and then Ni
was deposited to a thickness of 1,000 layers by EB evaporation, and then patterned by etching to form the upper conductor 4. Finally, the hard carbon film is patterned by dry etching,
A thin film two-terminal device was fabricated.

The conditions for forming the hard carbon film at this time are as follows.

Pressure: 0.035Torr CH4 flow rate: 10sccM RF power: 0.11+I/a# According to the structure of Example 2, in addition to the effect of Example 1, the hard carbon film and the upper conductor (Ni) are formed into a continuous film. This prevents contamination of the interface, and since the bottom conductor (ZnO:AR) is covered with a hard carbon film when etching the top conductor, it is possible to change the material or etching method without considering etching selectivity. There are advantages such as being able to choose. [Effects 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 can be processed by (1) vapor phase synthesis method such as plasma CVD method. 2) Since the film is hard and can be made thick, it is less susceptible to mechanical damage and is less prone to pin damage due to 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 has excellent uniformity in film thickness and quality, making it suitable for thin-film devices. 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 terminal element is suitable as a switching element for liquid crystal display.

Furthermore, the lower conductor of the thin-film two-terminal device of the present invention is formed of a zinc oxide transparent conductor, and this material has high plasma resistance, so it is suitable for use in a 2-terminal film. ! There is no need for a protective metal film when forming an m-film, so the process can be significantly shortened and the cost can be reduced.

[Brief explanation of the drawing]

FIG. 1 shows a typical example of a thin film two-terminal device according to the present invention (
The manufacturing process diagram of Example 1), FIG. 2, shows ITO.
In, O,. 3, 4 and 5 are diagrams for explaining the physical properties of a hard carbon film, and FIG. 6 is a diagram of a thin film two-terminal according to Example 1 of the present invention. FIG. 7 is a perspective view for explaining the structure 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. A thin film two-terminal device characterized in that the conductor is made of a zinc oxide transparent conductor.