JPH04245229A - Thin film two-terminal element - Google Patents

Abstract

PURPOSE:To equip a two-terminal element to operate for a long time stably by forming an insulation layer for restricting the active part of the element so as to be adjoining to insulation layer in order to restrict the active part. CONSTITUTION:As a thin film for lower electrode, an electroconductive film of Al, Ta, etc., is formed on a transparent base board 1 made of glass, plastic, etc., followed by patterning to form a lower electrode 2. As an insulation layer 3, Si, Nx film, or hard carbonic film is formed, followed by patterning. Another insulation layer 5 is formed by a method similar to that for the first named insulation layer 3, followed by patterning process which should generate patterns restricting the active part of an MIM(Metal Insulator Metal) element. As an upper electrode 4, further, an electroconductive thin film of Pt, Ni., etc., is formed, followed by patterning process.

Description

[Detailed description of the invention]

0001

TECHNICAL FIELD The present invention relates to a thin film two-terminal device, and more particularly, a thin film two-terminal device useful as a switching device suitably used in flat panel displays such as office automation equipment and TVs, and particularly a thin film two-terminal device useful as a switching device in a liquid crystal display device. Regarding terminal elements.

0002

2. Description of the Related Art There is a strong demand for the use of large-area liquid crystal panels in office automation equipment terminals and liquid crystal TVs, and for this reason, in the active matrix system, a switch is provided for each pixel to maintain voltage. By the way, one of the switches is MIM (Metal Insulat).
or Metal) element, MSM (Metal Sem
metal) are often used. This is because the thin film two-terminal element exhibits nonlinear current-voltage characteristics that are good for switching. The conventional thin film two-terminal device has a metal electrode such as Ta, Al, Ti, etc. as a lower electrode on an insulating substrate such as a glass substrate, and an oxide of the metal or SiOx
, an insulating layer or a semiconductor layer made of SiNx, etc. is provided,
Further, it is known that a metal electrode such as Al or Cr is provided thereon as an upper electrode. Schematic diagrams of an MIM element having such a structure are shown in FIGS. 4(a) and 4(b). (
a) is a perspective view, (b) is a cross-sectional view, 1 is a substrate, 2 is a lower electrode (transparent electrode), 3 is an insulating layer (for example, SiNx)
, 4 is an upper electrode. As is clear from FIG. 4(b), since the cross-sectional shape of the lower electrode 2 has a rectangular outer shape, the insulating layer 3 formed by a vapor phase synthesis method (for example, plasma CVD method) is formed on the side portion of the lower electrode 2. However, there are problems in that the film thickness is thinner or non-uniform than on the upper surface. Therefore, when voltage is applied, especially when applied continuously for a long period of time, poor characteristics or short circuits are likely to occur in that part. Furthermore, since the lower electrode 2 has corners, electric field concentration occurs there, resulting in the same drawbacks as described above. Since the active part of the MIM element is a part where the lower electrode 2 and the upper electrode 4 overlap, its area includes variations in both the line width of the lower electrode and the line width of the upper electrode. That is, each variation becomes a cause of variation in characteristics of the MIM element. The above are disadvantages in terms of the structure of the electrodes in conventional MIM elements, but there are also disadvantages in the material of the insulating layer. In other words, in the case of an MIM element whose insulating layer is an anodic oxide film, (1)
Since the insulating layer is limited to the anodic oxide film of the lower electrode, it is impossible to control its physical properties and, by extension, the characteristics of the MIM element.(2) Heat treatment at about 300 to 500°C is required. (3) Due to its high dielectric constant, when used as a switching element in a liquid crystal display device, the substrate material used is limited to one with high heat resistance.
Due to the constraint that M element capacitance)/(liquid crystal capacitance)<1/10, it is necessary to reduce the element area, and it has drawbacks such as requiring advanced microfabrication. On the other hand, SiNx,S
Since those using insulating layers (or semi-insulating layers) such as iOx are formed by vapor phase synthesis, they do not have the drawbacks mentioned in (1) above, and have the advantage that physical properties can be controlled over a wide range. . However, since the film forming temperature is as high as about 300° C., there are problems such as limitations on substrate materials and difficulty in forming a film with uniform characteristics over a large area. In addition, there is also the problem that pinholes are likely to occur due to dust and the like, resulting in a lower yield.

0003

[Object] The first object of the present invention is to eliminate the structural defects of electrodes in conventional MIM elements and to provide an MIM element that operates stably for a long time. A second object of the present invention is to eliminate the disadvantages of the material of the insulating layer in conventional MIM elements, and to provide a material suitable for use in liquid crystal display devices in particular.
An object of the present invention is to provide a low-cost and highly reliable MIM element.

0004

[Structure] The present invention includes a substrate, a first electrode formed on the substrate, an insulating layer formed on the first electrode, and a second electrode formed on the insulating layer. The present invention relates to a thin film two-terminal element comprising an electrode and an active part limiting insulating layer formed adjacent to the insulating layer to limit the active part of the element. FIG. 1 shows a cross-sectional view for explaining a thin film two-terminal device according to the present invention. First, a thin film of Al, Ta, Ti, Cr, Ni, Cu, Au, A, etc. for the lower electrode is formed on a transparent substrate 1 such as a glass substrate, a plastic substrate, or a plastic film.
g, W, Mo, Pt, ITO, ZnO:Al, In2O
3. A conductive thin film such as SnO3 is formed by a method such as sputtering or vapor deposition, and is patterned into a predetermined pattern to form the lower electrode 2. Then, SiN is used as the insulating layer 3.
x film, hard carbon film, etc. are formed to a thickness of several hundred to several thousand angstroms by plasma CVD, ion beam, sputtering, etc., and then formed into a predetermined pattern by wet etching, dry etching, or lift-off. pattern. Next, an insulating layer 5 is formed in the same manner as the insulating layer 3, and patterned by an etching method or a lift-off method to form a pattern that limits the active part of the MIM element. Finally, as the upper electrode 4, Pt, Ni, Ta, T
i, Cr, Cu, Au, Ag, W, Mo, ITO, Zn
A conductive thin film of O:Al, In2O3, SnO3, etc. is formed by a method such as sputtering or vapor deposition, and patterned into a predetermined pattern. Thus, MI according to the present invention
In the M element, the active part of the element is limited by the patterned second insulating layer 5, so the thin part of the insulating layer on the side part of the lower electrode may cause short circuit or dielectric breakdown. The corners of the lower electrode can be removed from the active part of the device. As a result, it can operate stably for a long time, and the first object of the present invention can be achieved. on the other hand,
In order to solve various problems related to the materials of conventional insulating films, which is the second object of the present invention, it is preferable to use a hard carbon film as the insulating film. This insulating film is a hard carbon film (also known as i-C film, diamond-like carbon film, amorphous diamond film, or diamond thin film) that contains carbon atoms and hydrogen atoms as main structure-forming elements and at least one of amorphous and microcrystalline materials. ). One feature of the hard carbon film is that since it is a vapor-phase grown film, its various physical properties can be controlled over a wide range by changing the film formation conditions, as will be described later. Therefore, even though it is an insulating film, its resistance value covers a range from a semi-insulator to an insulator, and in this sense, the MIM element of the present invention is similar to the MSI element (Metel- Semi-Insulat
or). An organic compound gas, especially a hydrocarbon gas, is used to form the hard carbon film. The phase state of these raw materials does not necessarily have to be a gas phase at normal temperature and 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 raw material gas, for example, CH4, C2H
6, paraffinic hydrocarbons such as C3H8, C4H10,
Gases containing at least all hydrocarbons such as acetylenic hydrocarbons such as C2H4, olefinic hydrocarbons, diolefinic hydrocarbons, and even aromatic hydrocarbons can be used. Furthermore, other than hydrocarbons, such as alcohols, ketones, ethers, esters, CO, C
Any compound containing at least carbon element, such as O2, 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 the film-forming active species is formed through a plasma state generated by a plasma method using direct current, low frequency, high frequency, microwave, etc. However, since the deposition is performed under low pressure for the purpose of increasing the area, improving uniformity, and 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 may be formed from neutral particles generated by vacuum vapor deposition, sputtering, etc. However, it may also be formed by a combination of these. An example of the deposition conditions for the hard carbon film produced in this manner is as follows in the case of the plasma CVD method. RF output: 0.1 to 50 W/cm2 Pressure: 1/103 to 10 Torr Deposition temperature: Room temperature to 950°C Due to this plasma state, the raw material gas is decomposed into radicals and ions and reacts, thereby forming carbon atoms C on the substrate.
A hard carbon film containing at least one of amorphous (amorphous) and microcrystalline (crystal size is several tens of angstroms to several μm) is deposited. Further, Table 1 shows various properties of the hard carbon film.

[0005]

[Table 1]

Note) Measuring method: Specific resistance (ρ): Obtained from the IV characteristics using a coplanar cell. Optical bandgap (Egopt): Obtain the absorption coefficient (α) from the spectral characteristics and determine from the relationship shown in Equation 4.

[0007]

[Math 1]

Amount of hydrogen in the film [C(H)]: Obtained by integrating the peak near 2900/cm from the infrared absorption spectrum and multiplying it by the absorption cross section A. That is, [C(H)]=A・∫α(v)/v・dvSP3/SP
2 ratio: The infrared absorption spectrum is decomposed into Gaussian functions assigned to SP3 and SP2, respectively, and determined from the area ratio. Vickers hardness (H): Based on a 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 the carbon atoms have SP3 hybrid orbitals and SP2 hybrid orbitals, as shown in FIGS. 5 and 6, respectively.
It has become clear that there are interatomic bonds that form a hybrid orbital. The ratio of SP3 bonds to SP2 bonds can be approximately estimated by peak-separating the IR spectrum. 2800-3150/cm for IR spectrum
Although the spectra of many modes overlap and are measured,
The attribution of peaks corresponding to each wave number is clear, and if we perform peak separation using a Gaussian distribution as shown in Figure 7, calculate the area of each peak, and find the ratio, we can obtain SP.
3/SP2 can be known. Moreover, according to X-ray and electron diffraction analysis, it has been found that it is in an amorphous state (a-C:H) and/or an amorphous state containing microcrystalline grains of about 50 Å to several μm. In the case of the plasma CVD method, which is generally suitable for mass production, the lower the RF output, the higher the specific resistance and hardness of the film, and the lower the pressure, the longer the life of active species, so lowering the substrate temperature and increasing the The area can be made uniform, and the specific resistance 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 has the characteristic that it can form a hard carbon film of good quality even under relatively low temperature conditions of about room temperature to 150° C., so it is optimal for lowering the temperature of the MIM element 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. Furthermore, 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 2 to 6, which is Ta2O, which is used in conventional MIM devices.
5.Since it is smaller than Al2O3 and SiNx, when making an element with the same capacitance, the element size only needs to be larger, so it does not require much fine processing and the yield improves (because of the driving conditions, LCD The capacitance ratio of C(LCD)/C(MIM) = approximately 10:1 is required between C(LCD) and MIM element. Furthermore, since the film has high hardness, there is less damage caused by the rubbing process during encapsulation of the liquid crystal material, which also improves the yield. Considering the above points, by using a hard carbon film, it is possible to achieve low cost, gradation (colorization), and high density LC.
D can be realized. Furthermore, in addition to carbon atoms and hydrogen atoms, this hard carbon film contains elements of group III, group IV, and V of the periodic table, alkali metal elements, alkaline earth metal elements, nitrogen atoms, and oxygen elements. , a chalcogen-based element, or a halogen atom as a constituent element. If a group III element of the periodic table, a group V element, an alkali metal element, an alkaline earth metal element, nitrogen atom, or oxygen atom is introduced as one of the constituent elements, the thickness of the hard carbon film is non-doped. It can be made about 2 to 3 times thicker than that of 100%, and this can prevent the occurrence of pinholes during device fabrication and dramatically improve the mechanical strength of the device. Further, in the case of a nitrogen atom or an oxygen atom, the same effect as in the case of a group IV element of the periodic table as described below can be obtained. Similarly, devices incorporating Group IV elements of the periodic table, chalcogen elements, or halogen elements dramatically improve the stability of the hard carbon film and improve the hardness of the film, resulting in highly reliable elements. can be made. These effects can be obtained because Group IV elements and chalcogen elements reduce the active double bonds present in the hard carbon film, and in the case of halogen elements, 1) abstraction for hydrogen The reaction promotes the decomposition of the raw material gas and reduces dangling bonds in the film, and 2) during the film formation process, the halogen element X pulls out hydrogen in the C-H bond and replaces it, forming a C-X bond. This is because it enters the film and increases the bond energy (the bond energy between C-H and between C-X is larger for C-X). In order to use these elements as constituent elements of the film, in addition to hydrocarbon gas and hydrogen as raw material gases, group III elements, group IV elements, and group V elements of the periodic table must be added as dopants to the film. In order to contain elements, alkali metal elements, alkaline earth metal elements, nitrogen atoms, oxygen atoms, chalcogen elements, or halogen elements, compounds (or molecules) containing these elements or atoms (hereinafter referred to as "other (also called "compound")
gas is used. Examples of compounds containing Group III elements of the periodic table include B(OC2H5)3, B2
H6, BCl3, BBr3, BF3, Al(O-i-C
3H7)3, (CH3)3Al, (C2H5)3Al,
(i-C4H9)3Al,AlCl3,Ga(O-i-
C3H7)3, (CH3)3Ga, (C2H5)3Ga
, GaCl3, GaBr3, (O-i-C3H7)3I
n, (C2H5)3In, etc. Examples of compounds containing Group IV elements of the periodic table include Si3H6, (C2H
5) 3SiH, SiF4, SiH2Cl2, SiCl4
,Si(OCH3)4,Si(OC2H5)4,Si(
OC3H7)4,GeCl4,GeH4,Ge(OC2
H5)4,Ge(C2H5)4,(CH3)4Sn,(
C2H5)4Sn, SnCl4, etc. periodic table V
Examples of compounds containing group elements include PH3, PF3,
PF5, PCl2F3, PCl3, PCl2F, PBr
3, PO(OCH3)3, P(C2H5)3, POCl
3, AsH3, AsCl3, AsBr3, AsF3, A
sF5, AsCl3, SbH3, SbF3, SbCl3
, Sb(OC2H5)3, etc. Examples of compounds containing alkali metal atoms include LiO-i-C3H7,N
There are aO-i-C3H7, KO-i-C3H7, etc. Examples of compounds containing alkaline earth metal atoms include Ca
(OC2H5)3, Mg(OC2H5)2, (C2H5
)2Mg etc. Examples of compounds containing nitrogen atoms include nitrogen gas, inorganic compounds such as ammonia, amino groups,
Examples include organic compounds having a functional group such as a cyano group and a nitrogen-containing heterocycle. Examples of compounds containing oxygen atoms include oxygen gas, ozone, water (steam), hydrogen peroxide, carbon monoxide, carbon dioxide, suboxide, nitrogen monoxide, nitrogen dioxide, dinitrogen trioxide, and dinitrogen pentoxide. , inorganic compounds such as nitrogen trioxide, hydroxyl groups, aldehyde groups, acyl groups, ketone groups, nitro groups, nitroso groups, sulfone groups, ether bonds,
Examples include organic compounds having functional groups or bonds such as ester bonds, peptide bonds, and oxygen-containing heterocycles, and metal alkoxides. Examples of compounds containing chalcogen elements include H2S, (CH3)(CH2)4S(
CH2)4CH3,CH2=CHCH2SCH2CH=
There are CH2, C2H5SC2H5, C2H5SCH3, thiophene, H2Se, (C2H5)2Se, H2Te, etc. Examples of compounds containing halogen elements include fluorine, chlorine, bromine, iodine, hydrogen fluoride, carbon fluoride, chlorine fluoride, bromine fluoride, iodine fluoride, hydrogen chloride, bromine chloride, iodine chloride, and hydrogen bromide. , inorganic compounds such as iodine bromide and hydrogen iodide, and organic compounds such as alkyl halides, aryl halides, halogenated styrene, halogenated polymethylene, and haloform. The amount of these impurities is usually 5 at.% based on the total constituent atoms for Group III elements of the periodic table.
Hereinafter, the amount of Group IV elements is 35 atomic % or less, the amount of Group V elements is 5 atomic % or less, the amount of alkali metal elements is 5 atomic % or less, and the amount of alkaline earth metal elements is 5 atomic %. Hereinafter, the amount of nitrogen atoms is 5 at % or less, the amount of oxygen atoms is 5 at % or less, the amount of chalcogen elements is 35 at % or less, and the amount of halogen elements is 35 at % or less. Note that the amounts of these elements or atoms can be measured by a conventional method of elemental analysis, for example, Auger analysis. Further, this amount can be adjusted by adjusting the amount of other compounds contained in the source gas, film forming conditions, etc. It is advantageous for a hard carbon film suitable as a liquid crystal driving MIM element to have a film thickness in a range of 100 to 8000 Å and a specific resistance in a range of 10 6 to 10 13 Ω·cm in view of driving conditions. In addition, considering the margin between drive voltage and withstand voltage (dielectric breakdown voltage), it is desirable that the film thickness be 200 Å or more, and color unevenness due to the step difference (cell gap difference) between the pixel part and the thin film two-terminal element part should be avoided. In order to prevent this from becoming a practical problem, it is desirable that the film thickness be 6000 Å or less, so the thickness of the hard carbon film should be 200 to 6000 Å,
More preferably, the specific resistance is 5×10 6 to 10 13 Ω·cm. The number of device defects due to pinholes in a hard carbon film increases as the film thickness decreases, and becomes especially noticeable below 300 Å (defect rate exceeds 1%);
Uniformity of in-plane distribution of film thickness (and therefore uniformity of device characteristics)
(The accuracy of film thickness control is limited to about 30 Å, and the variation in film thickness exceeds 10%). Therefore, it is more desirable that the film thickness is 300 Å or more. In addition, in order to make the hard carbon film less likely to peel off due to stress, and to lower the duty ratio (preferably 1/1000
(below), it is more desirable that the film thickness be 4000 Å or less. Taking these into consideration, the thickness of the hard carbon film is 300 to 4000 Å, and the specific resistivity is 10.
More preferably, it is 7 to 1011 Ω·cm. The insulating layer 5 for limiting the active part can be formed of a hard carbon film similarly to the insulating layer 3. Patterning of the insulating layer 5 for limiting the active part of the element can be performed using, for example, a lift-off method. Further, the insulating layer 5 can also be formed of photosensitive resin. For example, by using a polyimide-based photosensitive resin, a fine pattern of the polyimide insulating layer can be obtained by simply performing a photolithography process directly on the photosensitive resin without using a photoresist. This shortens the manufacturing process of the device, leading to improved device stability and yield. In addition, in the conventional structure, the area of the active part of the MIM element is the overlapped part of the lower electrode and the upper electrode, but in the structure according to the present invention, the area of the active part is determined by the insulating layer for limiting the active part. Variations in area and further variations in device characteristics can be suppressed.

0010

EXAMPLES The present invention will be explained in more detail based on examples. Example 1 An example will be explained with reference to FIG. Using Pyrex glass as the substrate 1, an MIM element was manufactured as follows. First, Al was deposited on the pixel electrode of the substrate 1 to a thickness of 100% by vapor deposition.
After depositing to a thickness of 0 Å, the lower electrode 2 was formed by patterning. Next, a hard carbon film serving as the insulating layer 3 was formed by plasma CVD. In this embodiment, a parallel plate type plasma CVD apparatus is used. A source gas containing a mixture of CH4 and hydrogen is introduced into the device, and approximately 13.56 MH
A hard carbon film 3 having a thickness of 800 Å was deposited on the substrate 1 and the Al electrode layer 2 by applying a high frequency electric field of z to cause the source gas to decompose into radicals and ions and cause a reaction. Furthermore, the insulating layer 5 for limiting the active part of the device was formed by a lift-off method in the following manner. First, a positive pattern of the active part of the device was formed using a photoresist through a photolithography process. On top of this, an active part limiting insulating layer 5 is made of a hard carbon film similarly to the insulating layer 3, and plasma carbon
A film with a thickness of 1000 Å was formed by the VD method. The insulating layer of the active part of the device was lifted off, and a pattern of the insulating layer 5 for limiting the active part was formed. Further, Ni was deposited to a thickness of 1000 Å by vapor deposition, and then patterned to form the upper electrode 4. Example 2 Using the same materials and method as in Example 1, up to the lower electrode 2 was formed. Next, as shown in FIG. 2, an active part limiting insulating layer 5 made of a hard carbon film with a thickness of 1000 Å was formed by the plasma CVD method in the same manner as in Example 1, and patterned into a predetermined pattern. Next, a hard carbon film as an insulating layer 3 was deposited on this to a thickness of 800 Å. Further, Ni was deposited to a thickness of 1000 Å by vapor deposition, and then patterned to form the upper electrode 4. Example 3 Example 3 will be described based on FIG. 3. Active part limiting insulating layer 5
The structure is completely the same as that of Example 1 except for the materials and manufacturing method. The active part limiting insulating layer 5 of this embodiment is made of "Photonice" (Toray Industries, Inc.), which is a polyimide photosensitive resin.
(manufactured by) was used. "Photonice" UR-3100 was applied with a spinner onto the substrate on which the insulating layer 3 was formed, and after exposure, development, rinsing, and patterning, heat treatment was performed to form the active part limiting insulating layer 5. The insulating layer 5 thus formed was strong, smooth, and had excellent insulating properties and dielectric properties.

[0011]

[Effect] According to the present invention, since the active part limiting insulating layer is formed adjacent to the insulating layer to limit the active part of the element, it is highly reliable and stable for a long time. In addition to operation, it has the effect of reducing variations in device characteristics. Further, when a hard carbon film is used as the insulating layer, the following characteristics are obtained. (1) Since it is produced by a vapor phase synthesis method such as plasma CVD, the physical properties can be controlled over a wide range depending on 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 into a thick film, it is less susceptible to mechanical damage, and it is expected that pinholes will be reduced by making the film thicker. (3) Since a high-quality film can be formed even at low temperatures near room temperature, there are no restrictions on the substrate material. (4) It has excellent uniformity in film thickness and film quality, making it suitable for thin film devices. (5) Since the dielectric constant is low, advanced microfabrication technology is not required, which is advantageous for increasing the area of the device. Therefore, an MIM element using such an insulating layer is suitable as a switching element for liquid crystal display.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing an MIM element according to Example 1 of the present invention.

FIG. 2 is a cross-sectional view showing an MIM device according to Example 2 of the present invention.

FIG. 3 is a cross-sectional view showing an MIM device according to Example 3 of the present invention.

FIG. 4 (a) and (b) are respectively conventional MIM
FIG. 2 is a perspective view and a cross-sectional view showing the element.

FIG. 5 is a spectrum diagram showing the results of an IR absorption analysis of the hard carbon film used for the insulating layer of the MIM element of the present invention.

FIG. 6 is a spectrum diagram showing the results of Raman spectroscopy analysis of the hard carbon film used as the insulating layer of the thin film two-terminal device of the present invention.

FIG. 7 is a diagram showing a Gaussian distribution of the IR spectrum of the hard carbon film.

[Explanation of symbols]

1 Board 2 Lower electrode 3 Insulating layer 4 Upper electrode 5 Insulating layer for active part limitation

Claims (2)

[Claims] 1. A substrate; a first substrate formed on the substrate;
In a thin film two-terminal device consisting of an electrode, an insulating layer formed on the first electrode, and a second electrode formed on the insulating layer, the insulating layer is used to limit the active part of the device. A thin film two-terminal device characterized by having an active part limiting insulating layer formed adjacent to the active part limiting insulating layer. 2. The thin film two-terminal device according to claim 1, wherein at least one of the insulating layer and the active part limiting insulating layer is a hard carbon film.