JPH04132692A - Diamond thin film and production thereof - Google Patents

Abstract

PURPOSE:To obtain a diamond thin film having strong adhesiveness by forming a diamond thin film on a Si base plate through an adhering layer comprising SiC as a principal component. CONSTITUTION:Bias voltage of direct current is applied to a base plate holder to make polarity of the holder negative in film forming of a diamond thin film using a plasma CVD device, etc. Then, the surface of a Si base plate is modified to SiC or containing SiC as a principal component. Next, polarity of said bias voltage of direct current is gradually inverted from intermediate and nucleus generation of diamond is increased to form a diamond thin film. Then, an adhering layer of SiC or containing SiC as a principal component is formed between said diamond thin film and said Si base plate. Thereby, a diamond thin film is synthesized with strong adhering force.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides a diamond thin film that exhibits strong bonding properties by using an adhesive layer mainly composed of SiC, and a direct current bias method for forming the diamond thin film. The present invention relates to a method for producing a diamond thin film, in which the adhesive layer and the diamond thin film are successively formed by applying a voltage and reversing the polarity of the DC bias voltage.

[Prior art]

Due to its mechanical and electrical properties, diamond is expected to be used in a wide variety of fields, and attempts have been made to synthesize it using various methods, including high-pressure methods, impact methods, and vapor phase growth methods. Among these, the vapor phase growth method is a synthesis method that overturns the concept that high temperature and high pressure are essential for diamond synthesis. It is well known that many attempts have been made using methods such as plasma CVD and ion beam evaporation, and many success stories using diamond particles and thin films have been reported.

However, even in these successful cases, no favorable success stories have been reported in terms of adhesion to the substrate, and it is said that in many cases, the film partially or completely peels off from the substrate immediately after formation or after aging. It is being said. In addition, these exfoliation phenomena often do not always show reproducibility that allows us to infer the cause, and may be due to differences in thermal expansion coefficients, or due to diamond crystal growth growing in a tapered shape from the core on the substrate. Although there is a hypothesis that this causes compressive stress to accumulate around the grain interfaces, causing peeling from the substrate, the definitive cause has not yet been found.

Under these circumstances, a proposed solution is to provide an adhesive layer between the Si substrate and the diamond to strongly adhere the diamond thin film. For example, S is specifically proposed as an adhesive layer.
It is a material, such as iC or 5i3N4, that matches well with diamond in terms of lattice constant, or is said to be close to diamond in thermal expansion coefficient. In fact, bulk Sl
In deposition experiments on sN4, the diamond thin film is said to have shown very stable adhesion.

Considering the use as a light emitting element or device as described above, stable adhesion of a diamond thin film should be realized even on a Si substrate. There, SiC
It is an easy proposal to form an adhesive layer such as SiC or Si3N4 in the pre-stage of diamond thin film production, but introducing SiC or 5j3N4 raw material gas into the same reaction chamber before diamond thin film production is Considering the cleanliness during thin film production, this is not necessarily preferable. Furthermore, even in actual cases in which SiC or S;
The practical application of diamond thin films on l-substrates is at a standstill.

〔the purpose〕

An object of the present invention is to provide a diamond thin film on an Sl substrate, which is expected to have various applications, and a manufacturing method for putting it into practical use.

[Structure of the invention]

According to the present invention, the above object is achieved by modifying the surface of the Si substrate to SiC before forming the diamond layer.
To provide a strong adhesion force to the diamond thin film by providing an adhesive layer mainly composed of By using the plasma CVD method, which is characterized by successively forming each layer of SiC and diamond by reversing the This is achieved by

The present invention will be explained in detail below.

The present inventors have long believed that in order to put diamond into practical use as a device material, it would be necessary to be able to form a uniform, large-area film, and so we developed a magnetic field microwave plasma CVD system that applied a DC bias voltage to the substrate. Attempts have been made to form diamond thin films using this method. As a result, φ41
A diamond thin film having a uniform thickness within ±10% was obtained on a nch Si substrate. Unfortunately, however, this diamond thin film is unstable in terms of adhesion, and a phenomenon has occasionally been observed in which the diamond thin film peels off from the Si substrate surface immediately after film formation or after a period of time. As described in the prior art, SiC is said to be suitable as an adhesive layer for a diamond thin film, particularly due to its lattice constant, and the present inventors have also considered its introduction.

Regarding the film formation of the SiC layer, for example, the vapor phase growth method using Si source gas such as silane/disilane and hydrocarbons such as methane, ethylene, methanol as the raw material, graphite, Si, etc. are used as targets, and Si Many methods have been proposed, such as reactive sputtering, which involves reacting with raw materials or hydrocarbon gas. However, introducing them as they are is not preferable due to the problem of cleanliness within the reaction chamber as described above. Of course, this problem could also be solved by adopting a so-called multi-chamber system, which can guarantee the cleanliness inside the reaction chambers by using multiple reaction chambers, but the present inventors have developed a simpler device. We pursued a method that provides reproducible adhesion.

In recent research on diamond thin film synthesis using the vapor phase method, it has been pointed out that it is effective to apply a bias voltage to the substrate (or substrate holder) during film formation. (The research conducted by the present inventors in the past has also incorporated the application of bias voltage.) Among the known reports regarding the application of this bias voltage, one of particular interest is the polarity of the applied bias voltage. I can give you something. In this report, when the bias voltage applied to the substrate or substrate holder is set negative, α-81C1β-
Nucleation of SiC and diamond occurs, and conversely, when the bias voltage is set to be positive, the density of diamond nucleation increases greatly, making it possible to obtain highly crystalline diamond particles and thin films. The report also states that the cause of α, β-SiC generation when the bias voltage is set to negative is that the Si substrate surface is sputtered by accelerated carbon ions, generating Si vapor, which reacts with carbon near the surface. This is intended to form α and βSiC.

In general, in research on the synthesis of diamond thin films, the main objective has been to obtain diamond particles and thin films with higher crystallinity, but no consideration has been given to the effects of such negative bias voltages. Ta. However, the present inventors have discovered that the effect of this bias voltage is what makes it possible to transfer the SiC layer, which is preferable as an adhesive layer, to a highly crystalline diamond thin film in the same reaction chamber in a clean state without introducing any gas other than carbon source gas into the reaction chamber. They discovered that this was an important clue for continuous film formation, and through research they established it as a method for synthesizing diamond thin films with strong adhesion.

Furthermore, in the plasma CVD method described above, the present inventors promoted nucleation and suppressed nuclear growth by applying a DC bias voltage to the substrate with a pulse waveform. It was discovered that it is possible to make the diamond finer, and as a result, it is effective in alleviating the compressive stress caused by the tapered nucleus growth mentioned above and suppressing the peeling of the diamond thin film.

In the present invention, the commonly used plasma CV
Incorporate a DC power supply device that can apply a DC bias voltage, reverse the polarity of the DC bias voltage, and switch between pulse and constant voltage in the D device (high frequency discharge, microwave discharge, ECR method, etc.) You can use all of them if possible. In the present invention, a magnetic field microwave plasma CVD apparatus shown in FIG. 1 was used. This device can set the substrate (1) at 200 to +000°C and the pressure during film formation to 0°01 to 3 tOrr.
Average power introduced from microwave power source (18) 2-1
0kW microwave and external magnetic field magnet (17) (
In a high-density plasma generated by a maximum 2 kG magnetic field applied from a DC bias power supply (28")
) supplied by constant voltage or pulse (3,0~20.0
A film deposition experiment can be performed by applying a DC bias voltage (with a pulse period changeable in ms) to the substrate holder (13).

As a reaction raw material, there are no particular restrictions.
A wide variety of hydrocarbons such as methane, acetylene, methanol, acetone, etc., and gases such as methane and acetylene as they are,
Methanol, acetone, etc. can be used after being vaporized. In the present invention, for the well-known reason that a gas having a C-OH group exhibits a high film formation rate, the reaction raw material is methanol (CH,OH), which is
200 volume% (CH30H/H2 when set to 100 volume%
= 1/1), and this is set at a pressure of 0.01 to 3 during film formation.
torr, and the raw material gas supply rate can be set within a range of 30 to 600 cm3/sec.

The features of the present invention are that, as described above, when applying the DC bias voltage, the SiC layer and the diamond layer are continuously formed by reversing the polarity of the DC bias voltage, and that the application of the DC bias voltage It consists in selecting and performing pulses. First, when generating plasma with methanol to form a SiC layer, a negative constant voltage or a pulse waveform bias voltage is applied to the substrate side, and after the SiC layer deposition time has finished, the Gradually operate the polarity-switchable output adjustment switch built in the DC bias power supply (28) shown in Figure 1 to smoothly switch the bias voltage to positive, and proceed to deposit the diamond layer. This switching is a process that takes a certain amount of time as shown in FIG. The present inventors set the length of T in the figure to be 3 to IO minutes, but it goes without saying that this varies depending on the experimental equipment and various synthesis conditions.

FIG. 3 shows a diagram of the structure of the diamond thin film in the present invention. (a) shows the case where it takes 3 to 10 minutes to reverse the bias polarity, and (b) shows the case where the bias polarity is reversed instantly. In (a), in addition to the SiC layer, there is also a transition layer (3).
This makes the bond stronger. This transition layer is not a crystal grain boundary where the SiC layer and the diamond layer are clearly separated, but has a structure in which the Si concentration gradually decreases from the SiC layer to the diamond layer. 40 to 100 people, preferably 60 to 80 people
Although the thickness is appropriate, it goes without saying that these values vary depending on the synthesis conditions. On the other hand, in (b), Si
Peeling may occur from the interface between the C layer and the diamond thin film, and a reproducible high adhesion force could not be obtained. Note that α
, the β-SiC layer (2) contains fine diamond particles (5
) was found to be contained in the composition to some extent.

Hereinafter, the present invention will be explained in more detail using Examples.

Example 1j In Example 1, a diamond thin film was formed on a Si substrate under the following conditions.

・Substrate φ4 inch 5i (100) ・Substrate temperature 800°C ・Reaction chamber pressure 0.1 torr 6xlO-5
Preliminary pressure reduction was performed to torr.

・Microwave power 5kW (from SiC layer)
1 No change until Diamond.

・Magnetic field strength 4kG ・Reaction raw material methanol 50% by volume 112 dilution ・Raw material flow rate 300 cm/sec ・Bias voltage Constant voltage +50V However, the time required to switch the polarity from 50V to +50V is 5 minutes.

・Film formation time The SiC layer was formed for 30 minutes, and the diamond layer was formed for 90 minutes.

The sample obtained in this example was subjected to Raman spectroscopy to investigate the structure of the diamond thin film. The obtained char 1. is shown in FIG. 13 unique to diamonds
A sharp peak at 32 cm-' was confirmed. However, Raman spectroscopic measurements were not performed in the following examples.

"Example 2" In Example 2, the bias voltage was applied by pulses. The pulse period at that time was 8°5 μs. A diamond thin film was formed in the same manner as in Example 1 under other conditions.

Comparative Example 1j In order to compare the effects of the SiC layer, the bias voltage was fixed at a constant voltage of +50 V, and only the diamond layer was formed for 90 minutes.Other conditions were the same as in Example 1j.

"Comparative Example 2" In order to compare the polarity switching time, the polarity was switched instantly without providing any gradient in the constant voltage bias voltage. Other conditions were "the same as in Example 1j.

"Comparative Example 3" In order to compare the thickness of the SiC layer, the SiC layer was formed for 15 minutes. Other conditions were the same as in "Example 1".

"Comparative Example 4j Same < In order to compare the thickness of the SiC layer, the film formation time of the SiC layer was set to 60 minutes.Other conditions were "Example 1"
It is similar to

"Comparative Example 5j To compare the difference in pulse period, the period was set to 4゜5 m.
It was set as s. Other conditions were the same as in Example 1j.

“Comparative Example 6” In order to compare the pulse period, the period was set to IO and 5 m.
It was set as s. Other conditions were the same as in Example 1j.

The adhesive strength of the diamond layer was compared for the eight types of samples obtained as described above. However, it is difficult to quantify the adhesion force of a thin film as an absolute value. Therefore, the present inventors decided to compare the peeled areas immediately after film formation to 100 hours later for eight types of samples left in open air.

Note that ten film-forming samples were prepared and compared for the same sample, and the reproducibility of adhesion force was also examined.

The results obtained are shown in the table below.

The numerical values in the table show the value expressed as a percentage of the peeled area relative to the area of the φ41 nch-Si substrate on the top, and the standard deviation for 10 samples on the bottom. The results in the examples show that the presence of the SiC layer strengthens the adhesion, and in addition, the switching time of the bias voltage polarity and the thickness of the SiC layer play an important role in the adhesion of the diamond thin film and its reproducibility. I found out that it was working.

The effect of the pulse waveform is not clear in terms of peeling. However, when a cross section of the obtained sample was observed using a scanning electron microscope, it was found that the diamond crystals forming the film were extremely fine. A schematic cross-sectional view of the sample obtained in Example 2 is shown in FIG. When we attempted to measure the internal stress, we found that in Example 1, to
-9 ayn, but the sample with pulse waveform bias showed a significant decrease of 1 O-5 dyn.

However, no difference that seemed to be due to the pulse period could be confirmed.

〔Effect of the invention〕

As explained above, SiC
A diamond film forming technique using a continuous synthesis method in which a diamond film is continuously formed from layers can provide a high adhesion force to the diamond film.

[Brief explanation of drawings]

FIG. 1 shows a magnetic field microwave plasma CVD apparatus used in the present invention. FIG. 2 is a diagram schematically showing the change in bias voltage over time during the transition from the SiC layer to the continuous formation of the diamond film. FIG. 3 is a diagram schematically showing a cross section of a diamond thin film obtained by the present invention. FIG. 4 is a laser Raman spectroscopy pattern measured on the diamond thin film obtained in Example 1. FIG. 5 is a diagram schematically showing a cross section of a diamond thin film obtained when the bias voltage is pulsed. 1... Substrate 2... . . . . . α, β-SiC layer 3...
...Transition layer 4...Diamond layer 5... 13... 17.17 18... 19... 20... 25... 26... 27... 28...... Fine diamond particles・Substrate holder ・External magnetic field magnet) ・ ・Microwave power supply ・Reaction chamber ・Gas introduction pipe ・Valve ・Turbo molecular pump ・Roughing pump ・DC bias power supply

Claims (5)

[Claims] 1. A diamond thin film characterized in that it is synthesized with strong adhesion strength by being provided on a Si substrate via an adhesive layer. 2. The diamond thin film according to claim 1, wherein the adhesive layer to be provided between the Si substrate and the diamond thin film is SiC or a layer containing SiC as a main component. 3. When forming a diamond thin film, a DC bias voltage is applied to the substrate holder, and the polarity of the DC bias voltage is gradually reversed during film formation, thereby changing the structure of the thin film from an adhesive layer structure to diamond. A method for producing a diamond thin film. 4. A method for producing a diamond thin film according to claim 3, wherein the polarity of the DC bias voltage is negative for SiC or an adhesive layer mainly composed of SiC, and positive for a diamond thin film layer. 5. A method for producing a diamond thin film according to claim 3, wherein the DC bias voltage is applied by a rectangular pulse wave.