JP2767514B2 - Diamond thin film and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a thin film made of carbon containing a single crystal or polycrystalline diamond as a main component (hereinafter abbreviated as a diamond thin film), and a diamond thin film obtained by the method. About.

[0002] In particular, it relates to improvement of adhesion between a substrate and a diamond thin film.

[0003]

2. Description of the Related Art A diamond thin film can be formed relatively easily by a gas phase method, and recently, diamond as an ultimate semiconductor has been attracting much attention. Also, diamond is expected to be used as a next-generation substrate material by utilizing its high thermal conductivity without directly using diamond as a semiconductor.

As a method of coating or forming such a diamond thin film on a substrate, a thermal CVD (chemical vapor deposition) method is most often used. In this thermal CVD method (hot filament CVD method), for example, as shown in FIG. 4, a reactive gas flows into a reaction vessel (31) from an inlet (32) and a current flows through a metal tungsten filament (33). In this method, a diamond thin film is synthesized by a chemical reaction on a substrate (34) heated to 400 ° C. to 1300 ° C. by heating the filament to 1500 ° C. to 3000 ° C. to emit thermoelectrons. At this time, the pressure inside the reaction vessel is 1 ~
It is maintained at a reduced pressure of about 350 Torr.

As a substrate material, there is an example in which the most ideal diamond itself is used for the substrate, but a silicon single crystal substrate, molybdenum, or the like is often used in practice because of cost and the like.

[0006]

When coating or forming a diamond thin film by the above-mentioned method, the biggest problem is the adhesion strength between the substrate and the diamond film. More or less delamination is observed. .

This is probably because the physical state of the interface, the reactivity between the substrate material and diamond, or the mechanical bonding form are complicatedly intertwined. Undoubtedly, the difference between the coefficients of thermal expansion of the materials, for example silicon and diamond, is a major cause.

[0008] This is clearly expressed by the stress remaining in the diamond thin film after film formation. When the substrate is silicon, the thermal expansion coefficient of silicon is much larger than that of diamond. It has been confirmed that a stress is applied and the stress remains in the formed film.

Therefore, in order to prevent the diamond thin film from peeling off, it is necessary to take measures such as adjusting the thermal expansion coefficient of the substrate to the same as that of diamond to reduce stress, and to devise the form of the interface (for example, an anchor effect is observed). It is considered that two methods of mechanically and firmly adhering such fine holes are particularly effective. However, such a suitable substrate material and a method of processing the substrate have not been developed so far.

The present invention provides a method for simultaneously achieving the two points for increasing the adhesion strength as described above.

[0011]

The present invention is characterized in that porous silicon is used for a substrate in order to simultaneously achieve the two points for increasing the adhesion strength as described above. That is, instead of the conventional substrate scratching process, a process of making the substrate porous by an anodizing method or the like is added.

Here, the porous silicon will be described in some detail. Porous silicon is one of the special forms of silicon and is obtained by electrolysis in a hydrofluoric acid aqueous solution using a silicon single crystal as an anode. The first discovery was relatively old in 1956, but recently, visible light luminescence was discovered, and it was discovered that it could be used for SOI (silicon-on-insulator) structures. ing.

[0013] We control the pore size of porous silicon,
Focusing on the fact that the substrate can be controlled by the specific resistance, conductivity type, and formation conditions of the substrate, the inventors decided to use this porous silicon for the production of a substrate on which an anchor effect can be expected.

[0014] In conclusion, when experiments were actually conducted, it was confirmed that a diamond thin film could be formed with good reproducibility at any comparatively large hole diameter, and the cross section was SEM.
By observing with a (scanning electron microscope), it was observed that diamond particles penetrated into the inside of the hole and exhibited an anchor effect. Although this was unexpected, it was also found that porous silicon had a great effect on stress relaxation.

This stress relaxation is based on the following reasons.
The inventors speculate. In porous silicon,
The residual silicon portion has a columnar shape, so that it can bend under externally applied force. In general, the diamond thin film is often peeled off during the cooling process because of a difference in thermal expansion coefficient between the two and compressive stress is applied to the diamond thin film and the diamond thin film cannot withstand. However, using the porous silicon of the present invention,
The columnar residual silicon portion absorbs the stress caused by the difference in thermal expansion coefficient as strain, and the stress directly applied to the diamond thin film is reduced.

The above reason is the model that we considered,
It is not clear whether or not the facts are correctly conveyed, but at least it is qualitative to understand.

In any case, by adding a step of making the silicon substrate porous and forming a diamond thin film thereon, the anchor effect and the stress relaxation are simultaneously achieved, and a thin film having extremely high adhesion can be obtained. It has become.
In the present invention, a single crystal substrate is used as a silicon substrate.
Or use polycrystalline or atypical forms
it can.

[0018]

By using porous silicon as a substrate material of a diamond thin film, the above-mentioned two points of increasing mechanical adhesion strength due to an anchor effect and relaxing stress caused by a difference in thermal expansion coefficient are considered. It became possible to achieve at the same time.

In the present invention, the type of the film forming method is not particularly limited. A general hot filament CVD method, a magnetic field plasma CVD method, a plasma jet method, or the like is also possible, and is characterized by high versatility. Also,
In the above example, the porosity forming step was performed by the anodizing method.
It has been confirmed that the same thing can be achieved by anisotropic etching by a dry process.

The following examples illustrate the present invention in more detail.

[0021]

【Example】

[Embodiment 1] In the present invention, anodization is used in the step of making the silicon substrate porous, and the magnetic field microwave CV is used for film formation.
An example using D is shown below.

First, the (100) plane of a 4-inch single crystal silicon substrate (P-type semiconductor) was used as the substrate. An aluminum electrode was formed on the back surface of the substrate by sputtering and annealing so as to form an ohmic contact.

Next, porous silicon was formed by anodization. The formation conditions are as follows.・ Silicon substrate resistance value 0.001 Ω ・ cm ・ Anode current density 80mA / cm ²・ Hydrofluoric acid concentration 25% ・ Solution temperature room temperature ・ Light irradiation energy 50mW / cm ²

After the anodization was completed, the substrate was washed with pure water, and then aluminum as a back electrode material was removed by wet etching using an aluminum etchant solution. This removal of the aluminum electrode must always be performed in the subsequent step of forming a diamond thin film, since heating at or above the melting point of aluminum is generally essential.

FIG. 1 shows an image of the porous silicon thus obtained. As can be seen from the figure, the fact that the residual silicon (43) has a columnar shape is a significant difference from the porosity obtained by other methods. In the porous membrane obtained by a general method, the void portion has a substantially spherical shape, and therefore, the remaining portion forms a three-dimensional network, and stress relaxation as in the present invention is expected. Can not.

Next, using the porous silicon obtained by the above process as a substrate, a diamond thin film was formed thereon. The film was formed by a magnetic field microwave CVD apparatus shown in FIG.

The conditions for forming this film are shown below.・ Reaction gas is methanol 50ccm hydrogen 100ccm ・ Reaction pressure 0.25Torr ・ Substrate temperature 600 ℃ ・ Treatment time 4hr ・ Microwave (2.45GHz) output 4kW ・ Magnetic field strength 875gauss or more.

FIG. 3A schematically shows a cross section of the sample after the formation of the diamond thin film according to the present invention. On the other hand, FIG. 3B shows a cross section of a sample after the formation of a diamond thin film obtained by a conventional method.

In the case of FIG. 3A, diamond grows from the upper part of the fine hole (44), and an anchor effect is expected. The point here is that not all of the micropores (44) contained in the porous silicon substrate (11) are filled with diamond, but there are gaps in some parts, and this part acts to relieve stress. That is. On the other hand, in FIG. 3B obtained by the conventional method, the diamond film (10) is in a polycrystalline state, and each crystal grows in an inverted triangular prism shape from the substrate. In the vicinity of the boundary with the silicon substrate (12), since the diamond film cannot have a sufficient contact area, the diamond film has weak adhesion strength and is easily peeled off.

Finally, as a result of the evaluation, this time, a scratch tester was used. Conventionally, peeling started with a load of 2 kg, whereas peeling did not occur up to nearly 8 kg in the one obtained by the present invention. However, it is important to note that the time for anodization is extended,
In the case of a sample having a large number of porous portions, it is necessary to adjust the point at which both the adhesion strength and the substrate strength are compatible according to the purpose because the substrate has been broken before peeling.

[Embodiment 2] In the present invention, an example is shown in which anodization is used in the step of making the silicon substrate porous and hot filament CVD is used in the film formation.

First, in Example 1, an aluminum electrode was formed as a back electrode. This is often unnecessary and intrusive in later processes. Process, film formation below the melting point of aluminum, or as a bias electrode, etc.
It does not matter if there is any usefulness, but this time it was considered unnecessary, and we decided not to form the back electrode directly. That is, in this case, anodization was performed by using mercury as an electrode and closely contacting a silicon substrate on the upper surface thereof.

The chemical conversion conditions were the same as in Example 1. After the completion, a sufficient washing was performed in pure water.

Next, using the hot filament CVD method,
A diamond thin film was formed on a substrate. A schematic of the apparatus is shown in FIG.

The film forming conditions are as follows. The reaction gas is methane 2 ccm, hydrogen 200 ccm, reaction pressure 50 Torr, substrate temperature 700 ° C., processing time 3 hours, filament temperature 2100 ° C.

After the film formation was completed, the cooling rate was sufficiently reduced to sufficiently promote the stress relaxation, and the sample was taken out after the substrate temperature reached room temperature.

The same evaluation as in Example 1 was performed, and no peeling or the like occurred up to 9 kg in the scratch tester.

Embodiment 3 In the present invention, an example in which anodization using a mercury electrode is used in the step of making the silicon substrate porous and a DC plasma jet CVD method is used in film formation will be described.

Since the processes up to anodization are exactly the same as those in the embodiment 2, they are omitted.

The film formation of the diamond thin film by the DC plasma jet CVD method was performed under the following film forming conditions.・ Reaction gas is methane 1% + hydrogen ・ Gas pressure 60Torr ・ Substrate temperature 800 ℃ ・ Treatment time 1hr

After the film formation was completed, the cooling rate was sufficiently reduced to sufficiently promote the stress relaxation, and the sample was taken out after the substrate temperature reached room temperature.

The same evaluation as in Example 1 was performed. In the scratch tester, no peeling or the like occurred up to 12 kg. However, the diamond thin film in this example was compared with those in other Examples 1 and 2. Since the film thickness is about several tens of times, it cannot be compared unconditionally, but in the past, most of the thin films obtained by the same DC plasma jet CVD method caused peeling upon cooling. On the other hand, the thin film obtained by the present invention did not have such a thing.

[0043]

As described above, by using porous silicon as the substrate material of the diamond thin film, it is possible to increase the mechanical adhesion strength due to the anchor effect and reduce the stress caused by the difference in the thermal expansion coefficient. It has become possible to achieve the above two points at the same time. In the present invention, the type of film formation method is not particularly limited, and a conventional general hot filament CVD method, a magnetic field plasma CVD method or the like can be used.
It can be achieved by using only porous silicon as a substrate in existing processes such as the plasma jet method or the plasma jet method, and is also characterized by high versatility.

According to the present invention, it is the first time that a diamond thin film having a sufficient adhesion strength can be formed on a silicon substrate. In addition to the silicon substrate, by forming a silicon film on another substrate and then making the substrate porous, the same effects as those of the present invention can be expected, and the present invention is also characterized by a very high degree of development.

[Brief description of the drawings]

FIG. 1 Image of porous silicon

FIG. 2 is an outline of a magnetic field microwave CVD apparatus.

FIG. 3 is an image of a diamond thin film according to the present invention.

FIG. 4 is an outline of a thermal CVD apparatus.

[Explanation of symbols]

 41 Porous silicon part 42 Original single crystal silicon part 43 Residual silicon (columnar) 44 Micropore 45 Silicon substrate 10 Diamond thin film 11 Porous silicon substrate 12 Single crystal silicon substrate 1,34 Silicon substrate 3 Magnet coil 2,35 Sample holder 4 Bias voltage 5,32 Reactive gas inlet 6 Microwave waveguide 31 Reaction vessel 33 Metal tungsten filament

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C23C 16/26 C23C 16/02 C30B 29/04 H01L 21/205

Claims (5)

(57) [Claims] 1. A silicon substrate at least partially made porous, the porous was in close contact with the silicon, the diamond thin film characterized in that the thin film you a diamond as the main structure is formed. 2. A method according to claim 1, wherein the porous phased sheet <br/> amplicon holes, diamond thin film characterized by having a size of 500 nm to 10 angstroms. 3. The method of claim 1, the thin film you the diamond as a main structure, a diamond thin film which is a single crystal or polycrystalline. Of 4. A silicon substrate, a step that a part or all turn into porous, that a step of forming the porous phased in close contact with the silicon you a diamond as the main structure thin film Characteristic method of making diamond thin film. 5. The method of claim 4, the method creates a diamond thin film characterized by by the step anodization to multi porosifying.