JPH0325507B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a multilayer film having a structure in which a reduced product of a metal oxide is interposed between layers of silicon oxide. There have been many attempts to convert monolayer films into useful thin film materials by reducing or oxidizing them, and some of them have become useful materials in some cases. However, if the thin film is oxidized or reduced as it is, it will be accompanied by a change in volume, resulting in phenomena that are undesirable for thin film materials, such as cracking, peeling, and surface roughness. While conducting research on alternately laminated films of thin layers of silicon oxide and thin layers of various metal oxides, the inventor discovered that thin layers of silicon oxide and thin layers of gas-reducible metal oxides In alternate laminated films,
When this film is reduced with a reducing gas such as hydrogen, 1. only the metal oxide is reduced to metal or lower-order oxides 2. the original state of the film is maintained 3. the metal oxide and its reduction When the volume change between the metal oxide and the material is large, a reduced layer of the metal oxide is formed by ultrafine reduced particles.4.The reduced layer maintains its film state through oxidation. Then, they discovered that it could be made into a layer of metal oxide, etc., and arrived at a multilayer film that can replace the conventional thin film materials mentioned above. That is, the present invention provides a method for producing a multilayer film, which is characterized by gas-reducing an alternately laminated film of metal oxide and silicon oxide, which can be gas-reduced at temperatures below 500°C without destroying the multi-layer structure with silicon oxide. This is related to. Examples of metal oxides that can be easily reduced by gas include:
These include Fe ₂ O ₃ , CoO, NiO, etc. For example, these oxide thin films can reduce the degree of vacuum in the vacuum chamber to 5×10 ^-3 to 5×
10 ^-5 Torr range, oxygen pressure 5 x 10 ^-3 to 5 x
10 ^-5 Torr and evaporate the metal, and silicon oxide films can be made by evaporating SiO or SiO ₂ by vacuum evaporation, so the thickness of each film is By alternately depositing the oxides on the substrate under controlled conditions, it is possible to create an alternately laminated film of silicon oxide and metal oxide. Of course, this film can be sputtered, ion beam evaporated,
It can also be made by ion plating or the like. The composition of the silicon oxide film is SiO−SuO ₂
The composition is often between
Expressed as SiO _x . Hydrogen reduction is generally sufficient for reduction, 200℃,
Fe ₂ O ₃ becomes Fe ₃ O ₄ and CoO becomes Co in about 1 hour of reduction.
In addition, NiO is reduced to Ni. With this level of heat treatment, the film state is maintained very well. The present invention will be described in more detail below with reference to Examples. Example 1 The apparatus used to form the alternately laminated film was a vacuum evaporation machine as shown in FIG. A substrate 1 is placed above, and a heater 2 is provided for heating the substrate, or a water cooling tube is provided for cooling the substrate. There are two evaporation sources 31 and 32 below.
Evaporate SiO and Co, respectively. First, the whole thing is 5x
Evaporate SiO under a high vacuum of about 10 ^-6 Torr.
^After forming the _SiO
Co is evaporated to form a CoO film. After that, Co and SiO are evaporated alternately in the same way, and CoO and
SiO _x forms a periodically stacked ivy film. Glass was used as the substrate 1, and the vapor deposition was performed while cooling it with water. The film thickness was measured using a crystal vibrating film thickness meter 6, and was controlled to a predetermined film thickness. Also
Both CoO and SiO were deposited at a deposition rate of about 1 Å/sec. Figure 2a is an X-ray diffraction diagram of an alternately laminated film of 135 Å CoO - 15 Å SiO _x repeated 10 times to a total film thickness of 1500 Å. Here, diffraction peaks appear around 2θ = 1.2° and 1.7°, but these are diffraction peaks (d = 147 Å) whose period is the sum of the film thicknesses of CoO and SiO _x ,
Around 2θ=1.2°, when calculated from 2dsinθ=n〓, n
=2, and around 1.7° corresponds to the peak of n=3. The value of 147 Å calculated from the secondary peak is almost the same as the total of 150 Å for CoO and SiO set in the experiment. This is, as expected, CoO−
This means that the SiO _x film repeats periodically.
When this film is reduced in hydrogen at 200°C for 1 hour, the transparent film changes to a black film. The X-ray diffraction of this film is shown in Figure 2b. Similarly, a diffraction peak appears at a low angle, and from this the period is calculated to be 144 Å. The period maintains the original period within the error range. Electron diffraction analysis of this film revealed that it had been reduced to Co metal. Even if it is reduced to Co in this way, the film remains the original CoO−
The reason why it maintains the same period _as the SiO _x film is that SiO _x forms the skeleton of the entire film, and _SiO You will be supporting them. Therefore, we used an electron microscope to examine the state of Co metal when CoO sandwiched between SiO _x films is reduced. first
Create a film of 200ÅSiO _x −100ÅCoO−30ÅSiO _x ,
A sample was prepared by reducing this with hydrogen. Electron micrographs of these films are shown in FIG. FIG. 3a is a photograph before reduction, and it shows that the film is a continuous film and contains crystal grains of about 80 to 100 Å.
After reduction, as shown in b, the particles are ultrafine particles with a size of 100 to 140 Å, and it can be seen that they are not connected to each other and are separated. When changing from CoO to Co, the volume is approximately halved, but since the distance between the SiO _x films does not change, it is thought that the space created by _this space exists in the form of fine particles. Therefore, the present invention can be effectively used as a method for producing ultrafine metal particles. Varying the CoO film thickness from 25 Å to 200 Å,
Alternately laminated films were made with SiO _x constant at 15 Å, and 200 Å
Magnetic measurements were carried out on the film which had been subjected to hydrogen reduction for 1 hour at °C.
FIG. 4 shows a plot of the coercive force Hc against the Co film thickness (corresponding to the thickness of one layer of original CoO). When the film thickness is less than 40 Å, Hc is zero, and when it is more than 50 Å, Hc appears and increases as the film thickness increases up to 800 Oe at 160 Å. If the film thickness is less than 40 Å, the reduced
Co becomes ultrafine particles and therefore becomes superparamagnetic, so Hc is considered to be zero. FIG. 5 shows the results of magnetic measurements of the same sample as above at liquid nitrogen temperature. A film with a thickness of 40 Å showed superparamagnetism at room temperature, but at 77 K it showed superparamagnetism of 800 Oe.
It is clear that this particle is ferromagnetic. This is probably the first sample in which superparamagnetic particles existed stably in air for a long time. Furthermore, the Hc value of Co particles at room temperature is 800 Oe, which is close to the highest value to date. These reduced films are electrical insulators, which are Co
This shows that the particles are independent from each other and are not continuous. Ultrafine particles are generated in the reduced film, and the particle size can be controlled by changing the CoO film thickness, and the particles are well dispersed. Generally, ultrafine metal particles have durability problems such as oxidation, but this material is stable against oxidation because the metal particles are sandwiched between SiO _x . Therefore, industrial applications as ultrafine particles can be expected. Example 2 An alternately laminated film of NiO-SiO _x was obtained in the same manner as in Example 1 except that Ni was used instead of Co. This film A
If it is reduced at 200℃ for 1 hour, it becomes Ni-SiO _x film B,
According to X-ray diffraction, the same periodicity as the original film is maintained. Furthermore, a Ni−SiO _x multilayer film was added at 50°C1.
It was confirmed that when heated in air for a period of time, it changed to the original NiO-SiO _x multilayer film C, and that similar results were obtained regarding periodicity. Table 1 shows the results of examining the reflectance, absorption, and transmittance of these films using laser light at a wavelength of 830 nm. The sample is 80ÅNiO−15ÅSiO _x
This film is made by laminating 10 times.

[Table] NiO−SiO _x made by reoxidizing the reduced sample
Sample C is very similar to starting sample A, but the reflectance is somewhat reduced, probably because the surface is a little rough. B is completely different from A and C in that the reflectance and absorption are increased and the transmittance is significantly decreased. The optical property of B is that it has a high absorption rate, so it can be applied as an optical recording material. In fact, when recording with 10mW laser light, the pulse width
Enough writing is possible even at 2 μsec, and the area where the light hits becomes a light-transmissive type. The part that was clearly exposed to light is Ni.
It is thought that NiO was oxidized and changed to NiO. Example 3 Next, Fe was evaporated instead of Co in Example 1.
Fe ₂ O ₃ was generated, and a Fe ₂ O ₃ -SiO _x alternately laminated film A was made. When this film is reduced at 200℃ for 1 hour, Fe ₃ O ₄
-Changes to SiO _x multilayer film B. Since the volume change (ΔV/ _VFe2O3 ) from Fe ₂ O ₃ to Fe ₃ O ₄ is only 3.5%, no change in period due to reduction is observed at all. It is also a good electrical conductor, indicating that Fe ₃ O ₄ is in the form of a thin film rather than fine particles. Fe ₃ O ₄ is a ferromagnetic material, and the results of examining changes in magnetism depending on film thickness are shown in FIG. 6th
The figure shows the change in the Mössbauer spectrum depending on the Fe ₃ O ₄ film thickness. Below 40 Å, the spectrum shows superparamagnetism, and above 80 Å, it shows a normal Fe ₃ O ₄ spectrum. FIG. 7 shows the saturation magnetization of this film. Below 40 Å, the saturation magnetization is extremely small due to superparamagnetism and increases as the film thickness increases. At 120 Å, the saturation magnetization is 60 emu/g,
Although it is still smaller than normal Fe ₃ O ₄ , the superparamagnetic component has almost disappeared. As described above, it is clear that hydrogen reduction at 200°C completely changes to Fe ₃ O ₄ . This is further heated to 250℃
It immediately changes to Fe ₂ O ₃ when heated in air to . It is thought that the change is rapid because it is a very thin film. In this way, the film becomes Fe ₂ O ₃ →Fe ₃ O ₄ →
Easily changes to Fe ₂ O ₃ and becomes red and transparent →
Changes from black opaque to red transparent. Table 2 shows the optical properties with 830 nm light. The sample is 80Å Fe ₂ O ₃
-15Å SiO _x is laminated 10 times.

[Table] When Fe ₃ O 4 becomes Fe 3 O ₄ , the absorption rate increases significantly.
Although it is slightly lower than 54.84%, it is close to 40%.
On the other hand, the transmittance decreases considerably. For optical recording, the Fe ₃ O _{4 -SiO}
Since it changes, it can be fully utilized. As described above, when thin films are stacked in multiple layers by sandwiching oxides between SiO _x , the films will be reduced and oxidized at a rate previously unimaginable. Moreover, the film remains intact despite such chemical changes, and furthermore, near room temperature, _SiO It has become clear that it prevents things from changing over time. The fact that it is highly reactive at high temperatures and inactive at low temperatures is thought to be mainly due to the characteristics of SiO _x . Since SiO _x does not directly affect functionality, it is better to keep it as thin as possible, so in the examples so far, the basic thickness is 15 Å, which is considered to be the minimum film thickness necessary to form a continuous film. . Perhaps because of such a thin film, oxidation occurs through SiO _x at temperatures near 200°C.
Reduction reaction becomes easy. Therefore, if this point is utilized, it is possible to use it as a gas sensor. i.e. in hydrogen at 200℃
The Fe ₂ O ₃ -SiO _x multilayer film is reduced, and the reduction increases the electrical conductivity of the film by more than 10 ⁶ orders of magnitude, making industrial applications as gas sensors possible. Although typical metal oxides have been specifically explained above, examples of metal oxides that can be easily reduced by gas according to the present invention include those illustrated below in addition to the above. Metal oxide reduction product Co ₃ O ₄ → CoO Cu ₂ O→ Cu CuO→ Cu ₂ O V ₂ O ₅ → VO ₂ Mn ₂ O ₃ → Mn ₃ O ₄ MoO ₃ → MoO ₂ WO ₃ → WO ₂ or more books The invention is useful in providing new materials with distinctive electrical, magnetic, and optical properties.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the vacuum evaporation machine, Figure 2 a is CoO
-X-ray diffraction diagram of SiO _x alternately laminated film, Figure 2b is Co
-X-ray diffraction diagram of SiO _x multilayer film, Figure 3a is SiO _x
(200 Å)-CoO (100 Å) _-SiO
Figure 5 is a graph showing the relationship between coercive force and Co film thickness at room temperature in a SiO _x multilayer film. Figure 5 is a graph showing the same relationship at liquid nitrogen temperature for the same multilayer film. Figure 6 is
The Mössbauer spectrum of the Fe ₃ O ₄ film thickness in a Fe ₃ O ₄ -SiO _x multilayer film at room temperature. Figure 7 shows the relationship between the Fe ₃ O ₄ film thickness and saturation magnetization in the same multilayer film at room temperature. The graphs shown in FIG. In Figure 1, 1...substrate, 2...heater, 3
1, 32... Evaporation source, 4... Exhaust port, 5... Gas inlet, 6... Quartz crystal film thickness meter.

Claims (1)

[Claims] 1 Metal oxide CoO, which can be reduced to gas at temperatures below 500℃ without destroying the multilayer structure with silicon oxide.
Co ₃ O ₄ , Cu ₂ O, CuO, Fe ₂ O ₃ , Fe ₃ O ₄ , NiO,
A method for producing a multilayer film, which comprises gas-reducing an alternately laminated film of V ₂ O ₅ , Mn ₂ O ₃ , MoO ₃ or WO ₃ and silicon oxide.