Method of growing diamond thin film

The present invention is directed to a method of growing thin film diamond. Since there are micro-grooves formed between internal grains of the heterogeneous substrate during lateral epitaxy growth, diamond seeds are allowed to be embedded in the micro-grooves; surface damage caused by scratching method or seeding method also can be prevented. As a result, a continuous diamond thin film with uniform thickness and high quality can be obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of growing diamond thin film, and more particularly to a method of growing diamond thin film on a heterogeneous substrate.

2. Description of the Prior Art

Diamond is considered ideal thermal dissipation materials and high frequency materials due to its high thermal conductivity. For example, it can be applied to high electron mobility transistors (HEMTs). A diamond film deposited on a heterogeneous substrate made of nitrides such as AlN/GaN or InAlN/GaN can act as a heat dissipation layer to transfer heat out of the electronic device. However, nitride substrates such as AlN or InAlN having fiat surface makes it difficult for diamond nucleation; this phenomenon has been reported in recent years. To solve this problem, rough surface of nitride substrate is usually formed by sputtering which provides more nucleation sites. Furthermore, it can be coordinated with scratching or seeding method to increase nucleation density. Nevertheless, it causes surface damage to the substrates, uneven distribution of nucleation sites, uneven thickness and rough surface of diamond films. These problems restrict industrial application of diamond.

According to conventional method of growing diamond thin film, a specimen of aluminum nitride is immersed into suspension fluid containing diamond powder for ultrasonic vibration for a period of time to generate scratches. Meanwhile, diamond powder is also embedded into scratches or absorbed on the surface of aluminum nitride to become nucleation sites. It could be seen that there is uneven distribution of diamond seeds and agglomeration of diamond power occurs on the surface of the specimen as shown inFIG. 2a. After pretreatments, a continuous diamond thin film is formed by microwave plasma chemical vapor deposition method (MPCVD). Results are shown inFIG. 2band surface of diamond film appears cauliflower-like, which is rough and irregular.

If nucleation density of diamond on the heterogeneous substrate can be increased without damaging the substrate (e.g. group III-nitride substrate) and continuous diamond film with uniform thickness and high flatness can be further obtained, wide application of diamond can be achieved.

SUMMARY OF THE INVENTION

The present invention is directed to a method of growing thin film diamond. Since there are micro-grooves formed between internal grains of the heterogeneous substrate during lateral epitaxy growth, diamond seeds are allowed to be embedded in the micro-grooves; surface damage caused by scratching method or seeding method also can be prevented. As a result, a continuous diamond thin film with uniform thickness and high quality can be obtained.

According to one embodiment of the present invention, a method of growing thin film diamond comprises: providing a carrier substrate; forming a heterogeneous substrate on the carrier substrate via epitaxial growth, wherein the heterogeneous substrate comprises multiple grains and there are multiple irregular micro-grooves formed at junction between the multiple grains on a upper surface of the heterogeneous substrate; providing multiple diamond seeds embedded into the multiple micro-grooves; and depositing a diamond thin film on the upper surface of the heterogeneous substrate.

The objective, technologies, features and advantages of the present invention will become apparent from the following description in conjunction with the accompanying drawings wherein certain embodiments of the present invention are set forth by way of illustration and example.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The detailed explanation of the present invention is described as follows. The described preferred embodiments are presented for purposes of illustrations and description, and they are not intended to limit the scope of the present invention.

Referring toFIG. 1ato1f,these are flowcharts displaying the method of diamond thin film growth. The method comprises several steps as follows: providing a carrier substrate10(as shown inFIG. 1a); forming a heterogeneous substrate20on the carrier substrate10via epitaxial growth, wherein the heterogeneous substrate20comprises multiple grains21and there are multiple irregular micro-grooves22formed at junction between the multiple grains21on a upper surface23of the heterogeneous substrate20; providing multiple diamond seeds S embedded into the multiple micro-grooves22; and depositing a diamond thin film30on the upper surface23of the heterogeneous substrate20. The “heterogeneous” substrate presented here means that the diamond thin film30is deposited on a non-diamond substrate.

Referring toFIG. 1ato1f,according to an embodiment of the present invention, the mechanism of growing the heterogeneous substrate20via epitaxial growth is described as follows. First, the multiple grains21having island-like structure are formed on the carrier substrate10(as shown inFIG. 1b) and then multiple grains21commence to grow upwards and laterally, where the multiple grains21gradually contact to each other during lateral growth to form an epitaxial layer (as shown inFIGS. 1cand1d). In one embodiment, the heterogeneous substrate20is formed on the carrier substrate10by metal organic chemical vapor deposition method (MOCVD). After growing for a period of time, because some of the multiple grains21do not fully contact to each other, there are multiple irregular micro-grooves22formed within the space between the multiple grains21on the upper surface23of the heterogeneous substrate20. These micro-grooves22can be distributed uniformly over the upper surface23of the heterogeneous substrate20(as shown inFIG. 3a) to let the diamond seeds S embedded into the micro-grooves22. (as shown inFIG. 3b). Owing to uniform distribution of the micro-grooves22on the upper surface23of the heterogeneous substrate20, the diamond seeds S also can be distributed uniformly. Besides, the micro-grooves22are naturally formed during crystal growth but not generated in a destructive way (e.g. scratching method), it does not cause damage to the heterogeneous substrate20and thus reduce inhomogeneous distribution of the diamond seeds S over the upper surface23of the heterogeneous substrate20. As a consequence, the diamond thin film30having uniform thickness, better flatness, good quality and continuous film can be obtained. In addition to MOCVD, the heterogeneous substrate20also can be deposited on the carrier substrate10by molecular beam epitaxy growth method (MBE), sputtering method or pulse laser deposition method (PLD).

Regarding selection of materials, referring toFIG. 1ato1f,the carrier substrate10and the heterogeneous substrate20can be composed of the same materials. The carrier substrate10can be selected from one of a silicon substrate, a silicon carbide substrate, an aluminum oxide substrate, an aluminum nitride (AlN) substrate, and a gallium nitride (GaN) substrate, where the silicon substrate can be {110} silicon substrate or {111} silicon substrate in cubic crystal system; the aluminum oxide substrate can be a c-plane aluminum oxide substrate, a m-plane aluminum oxide substrate, an a-plane aluminum oxide substrate or a r-plane aluminum oxide substrate. These are conventional materials of substrates applied in CVD. Accordingly, the heterogeneous substrate20growing on the carrier substrate10can be epitaxy belonging to cubic crystal system, for example, {100} epitaxy, {111} epitaxy or {110} epitaxy; it also can be c-plane epitaxy ({0001}), m-plane epitaxy ({10-10}), a-plane epitaxy ({11-20}), r-plane epitaxy ({10-12}) or semipolar epitaxy (e.g. {10-11}, {11-22} or {10-13}) in hexagonal crystal system.

Traditionally, the diamond thin film30usually grows on the heterogeneous substrate20, where materials of the heterogeneous substrate20can be nitrides selected from binary nitrides, ternary nitrides and quaternary nitrides. In one embodiment, they comprise at least one of aluminum, gallium and indium or any combination thereof. For instance, the binary nitrides comprises aluminum nitride (AlN); indium nitride (InN), or gallium nitride (GaN), and the ternary nitrides comprise aluminum gallium nitride, aluminum indium nitride, or gallium indium nitride, and the quaternary nitrides comprise aluminum gallium indium nitride. Preferably, the heterogeneous substrate20is an aluminum nitride (AlN) substrate. In addition to the nitride substrate, other substrates which have micro-grooves on the surface for diamond seeds, to be embedded in are adequate for the heterogeneous substrate20. In one embodiment, the heterogeneous substrate20is selected from a metal substrate, a ceramic substrate, a group IV compound substrate, a group III-V compound substrate, a group II-VI compound substrate and a group IV substrate.

With respect to the crystal structure of the diamond thin film30deposited on the upper surface23of the heterogeneous substrate20, it can be single crystal or polycrystalline. The diamond thin film30can be epitaxy growing on the heterogeneous substrate20. As for the surface morphology, the diamond thin film30can have a preferred orientation of <100>, <110>, or <111>.

On the other hand, according to one embodiment and referring toFIG. 1atoFIG. 1f,the way to provide multiple diamond seeds S is by immersing the heterogeneous substrate20into a suspension fluid containing multiple diamond seeds S for ultrasonic vibration so as to let the multiple diamond seeds S embedded into the multiple micro-grooves22. Ultrasonic vibration performed here is to distribute diamond seeds S homogeneously but not for generating scratches. Hence, it requires shorter time of vibration. While the diamond seeds S are imbedded into the micro-grooves22, agglomerates of the diamond seeds S may be generated on the upper surface23of the heterogeneous substrate20. In one embodiment, the dimension of the multiple diamond seeds S is smaller than that of the micro-grooves22so that agglomerates with excessive size cannot move into the micro-grooves22. Advantageously, the dimension of the diamond seeds S can be maintained uniform and benefits grain growth with average size. In one embodiment, the dimension of the multiple micro-grooves22is micron-scale, submicron-scale or nano-scale. Preferably, the dimension of the multiple micro-grooves22is less than 100 nanometers. Size of the micro-grooves22and the diamond seeds S can be formed or selected according to users' requirements.

In a preferred embodiment, the method of growing the thin film diamond further comprises using a cleaning fluid to clean the heterogeneous substrate20after providing multiple diamond seeds S embedded into the multiple micro-grooves22so as to remove multiple agglomerates of the multiple diamond seeds S formed on the upper surface23of the heterogeneous substrate20. The clean fluid comprises at least one of water, ethanol, methanol, and acetone.

The diamond thin film30can be deposited on the upper surface23of the heterogeneous substrate20by microwave plasma chemical vapor deposition method (MPCVD), hot filament chemical vapor deposition method, plasma enhanced chemical vapor deposition method (PECVD), low-pressure chemical vapor deposition method or DC plasma enhanced chemical vapor deposition method. A process gas also can be provided when the diamond thin film30is deposited on the upper surface23of the heterogeneous substrate20. The process gas comprises at least one of hydrogen, argon, carbon monoxide, carbon dioxide, alkanes, alkenes, and alkynes.

Preferred embodiments are described as follows for better explanation but not to limit the present invention.

Embodiment I

An aluminum nitride (AlN) thin film is deposited on a silicon substrate by MOCVD. There are micro-grooves with the width of 30 to 40 nm uniformly distributed on the surface of the aluminum nitride (AlN) thin film, as shown inFIG. 3a. After that, the aluminum nitride specimen is placed into the suspension fluid containing nano-diamonds for ultrasonic vibration and then cleaned by ethanol to remove agglomerates of nano-diamonds remained on the surface of the aluminum nitride thin film. As expected, only nano-diamond seeds embedded into the micro-grooves are left on the surface of the aluminum nitride, as marked by arrow inFIG. 3b.

The above-mentioned AlN specimen is then treated with MPCVD for 4 hours to grow diamond thin film thereon. By applying appropriate parameters, a diamond film with thickness of 6 μm, which is well-attached to the heterogeneous substrate, is obtained. It has better flatness and uniform thickness comparing to the diamond thin film grown on a silicon substrate by applying the same conditions described before (as shown inFIG. 4a,4b). More than this, nucleation rate and growth rate are also higher. Results of analyzing the diamond thin film by using X-ray diffractometer (as shown inFIG. 5a) and Raman spectroscopy (FIG. 5b, peak value of diamond is approx. 1332 cm−1to 1333 cm−1) display that the diamond thin film has a preferred orientation of (100) and high purity of diamond structure. Comparing to the diamond thin film with cauliflower-like morphology (as shown inFIG. 2b), the growth method of the present invention can effectively deposit continuous diamond thin films with homogeneous thickness and flatness on AlN substrates (as shown inFIGS. 4aand4b).

In conclusion, the present invention provides a method of growing thin film diamond. Since there are micro-grooves formed between internal grains of the heterogeneous substrate during lateral epitaxy growth, diamond seeds are allowed to be embedded in the micro-grooves; surface damage caused by scratching method or seeding method also can be prevented. As a result, a continuous diamond thin film with uniform thickness and high quality can be obtained.

While the invention is susceptible to various modifications and alternative forms, a specific example thereof has been shown in the drawings and is herein described in detail. It should be understood, however, that the invention is not to be limited to the particular form disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.