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The third generation of wide band gap semiconductor materials represented by gallium nitride (GaN) and silicon carbide (SiC) have excellent photoelectric conversion and microwave signal transmission capabilities, which can meet the needs of high frequency, high temperature, high power and radiation resistant electronic devices. Therefore, it has broad application prospects in the fields of new generation mobile communication, new energy vehicles, smart grid and LED. The comprehensive development of the third-generation semiconductor industry chain needs breakthroughs in key core technologies, constantly promote the design and innovation of devices, and solve the import dependence.

Taking silicon carbide wafer growth as an example, the graphite material and carbon-carbon composite material in the thermal field material are difficult to meet the complex atmosphere (Si, SiC?, Si?C) process at 2300℃. Not only the service life is short, different parts are replaced by one to ten furnaces, and the dialysis and volatilization of graphite at high temperatures can easily lead to crystal defects such as carbon inclusions. In order to ensure the high quality and stable growth of semiconductor crystals, and considering the cost of industrial production, the preparation of ultra-high temperature corrosion resistant ceramic coating on the surface of graphite parts will extend the life of graphite components, inhibit the migration of impurities and improve the purity of crystals. In the epitaxial growth of silicon carbide, the silicon carbide coated graphite base is usually used to carry and heat the single crystal substrate, its service life still needs to be improved, and the silicon carbide deposits on the interface need to be cleaned regularly. In contrast, tantalum carbide (TaC) coating is more resistant to corrosion atmosphere and high temperature, and is the core technology for such SiC crystals to "grow, grow thick, and grow well".

Application of TaC coated graphite parts

PART/1

Crucible, seed holder and guide ring in SiC and AIN single crystal furnace were grown by PVT method

As shown in Figure 2 [1], when physical vapor transport method (PVT) is used to prepare SiC, the seed crystal is in the relatively low temperature region, the SiC raw material is in the relatively high temperature region (above 2400 ℃), and the raw material decomposes to produce SiXCy (mainly including Si, SiC?, Si?C, etc.). The vapor phase material is transported from the high temperature region to the seed crystal in the low temperature region. Synucleate, grow and form single crystal. The thermal field materials used in this process, such as crucible, flow guide ring, seed crystal holder, should be resistant to high temperature and will not pollute SiC raw materials and SiC single crystals. Similarly, the heating elements in the growth of AlN single crystals need to be resistant to Al vapor, N? corrosion, and need to have a high eutectic temperature (and AlN) to shorten the crystal preparation period.

It was found that the SiC[2-5] and AlN[2-3] prepared by TaC coated graphite thermal field materials were cleaner, almost no carbon (oxygen, nitrogen) and other impurities, fewer edge defects, smaller resistivity in each region, and the micropore density and etching pit density were significantly reduced (after KOH etching), and the crystal quality was greatly improved. In addition, TaC crucible weight loss rate is almost zero, appearance is non-destructive, can be recycled (life up to 200h), can improve the sustainability and efficiency of such single crystal preparation.

FIG. 2. (a) Schematic diagram of SiC single crystal ingot growing device by PVT method

(b) Top TaC coated seed bracket (including SiC seed)

(c) TAC-coated graphite guide ring

PART/2

MOCVD GaN epitaxial layer growing heater

As shown in Figure 3 (a), MOCVD GaN growth is a chemical vapor deposition technology using organometrical decomposition reaction to grow thin films by vapor epitaxial growth. The temperature accuracy and uniformity in the cavity make the heater become the most important core component of MOCVD equipment. Whether the substrate can be heated quickly and uniformly for a long time (under repeated cooling), the stability at high temperature (resistance to gas corrosion) and the purity of the film will directly affect the quality of the film deposition, the thickness consistency, and the performance of the chip.

In order to improve the performance and recycling efficiency of the heater in MOCVD GaN growth system, TAC-coated graphite heater was successfully introduced. Compared with GaN epitaxial layer grown by conventional heater (using pBN coating), GaN epitaxial layer grown by TaC heater has almost the same crystal structure, thickness uniformity, intrinsic defects, impurity doping and contamination. In addition, the TaC coating has low resistivity and low surface emissivity, which can improve the efficiency and uniformity of the heater, thereby reducing power consumption and heat loss. The porosity of the coating can be adjusted by controlling the process parameters to further improve the radiation characteristics of the heater and extend its service life [5]. These advantages make TaC coated graphite heaters an excellent choice for MOCVD GaN growth systems.

FIG. 3. (a) Schematic diagram of MOCVD device for GaN epitaxial growth

(b) Molded TAC-coated graphite heater installed in MOCVD setup, excluding base and bracket (illustration showing base and bracket in heating)

(c) TAC-coated graphite heater after 17 GaN epitaxial growth. [6]

PART/3

Epitaxial coated tray

Wafer carrier is an important structural component for the preparation of SiC, AlN, GaN and other third class semiconductor wafers and epitaxial wafer growth. Most of the wafer carriers are made of graphite and coated with SiC coating to resist corrosion from process gases, with an epitaxial temperature range of 1100 to 1600 ° C, and the corrosion resistance of the protective coating plays a crucial role in the life of the wafer carrier. The results show that the corrosion rate of TaC is 6 times slower than SiC in high temperature ammonia. In high temperature hydrogen, the corrosion rate is even more than 10 times slower than SiC.

It has been proved by experiments that the trays covered with TaC show good compatibility in the blue light GaN MOCVD process and do not introduce impurities. After limited process adjustments, leds grown using TaC carriers exhibit the same performance and uniformity as conventional SiC carriers. Therefore, the service life of TAC-coated pallets is better than that of bare stone ink and SiC coated graphite pallets.

Figure 4. Wafer tray after use in GaN epitaxial grown MOCVD device (Veeco P75). The one on the left is coated with TaC and the one on the right is coated with SiC. [7]

Preparation method of common TaC coated graphite parts

PART/1

CVD (Chemical Vapor Deposition) method:

At 900-2300℃, using TaCl5 and CnHm as tantalum and carbon sources, H? as reducing atmosphere, Ar? as carrier gas, reaction deposition film. The prepared coating is compact, uniform and high purity. However, there are some problems such as complicated process, expensive cost, difficult airflow control and low deposition efficiency.

PART/2

Slurry sintering method:

The slurry containing carbon source, tantalum source, dispersant and binder is coated on the graphite and sintered at high temperature after drying. The prepared coating grows without regular orientation, has low cost and is suitable for large-scale production. It remains to be explored to achieve uniform and full coating on large graphite, eliminate support defects and enhance coating bonding force.

PART/3

Plasma spraying method:

TaC powder is melted by plasma arc at high temperature, atomized into high temperature droplets by high-speed jet, and sprayed onto the surface of graphite material. It is easy to form oxide layer under non-vacuum, and the energy consumption is large.

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TaC coated graphite parts need to be solved

PART/1

Binding force:

The thermal expansion coefficient and other physical properties between TaC and carbon materials are different, the coating bonding strength is low, it is difficult to avoid cracks, pores and thermal stress, and the coating is easy to peel off in the actual atmosphere containing rot and repeated rising and cooling process.

PART/2

Purity:

TaC coating needs to be ultra-high purity to avoid impurities and pollution under high temperature conditions, and the effective content standards and characterization standards of free carbon and intrinsic impurities on the surface and inside of the full coating need to be agreed.

PART/3

Stability:

High temperature resistance and chemical atmosphere resistance above 2300℃ are the most important indicators to test the stability of the coating. Pinholes, cracks, missing corners, and single orientation grain boundaries are easy to cause corrosive gases to penetrate and penetrate into the graphite, resulting in coating protection failure.

PART/4

Oxidation resistance:

TaC begins to oxidize to Ta2O5 when it is above 500℃, and the oxidation rate increases sharply with the increase of temperature and oxygen concentration. The surface oxidation starts from the grain boundaries and small grains, and gradually forms columnar crystals and broken crystals, resulting in a large number of gaps and holes, and oxygen infiltration intensifies until the coating is stripped. The resulting oxide layer has poor thermal conductivity and a variety of colors in appearance.

PART/5

Uniformity and roughness:

Uneven distribution of the coating surface can lead to local thermal stress concentration, increasing the risk of cracking and spalling. In addition, surface roughness directly affects the interaction between the coating and the external environment, and too high roughness easily leads to increased friction with the wafer and uneven thermal field.

PART/6

Grain size:

The uniform grain size helps the stability of the coating. If the grain size is small, the bond is not tight, and it is easy to be oxidized and corroded, resulting in a large number of cracks and holes in the grain edge, which reduces the protective performance of the coating. If the grain size is too large, it is relatively rough, and the coating is easy to flake off under thermal stress.

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Conclusion and prospect

In general, TaC coated graphite parts in the market has a huge demand and a wide range of application prospects, the current TaC coated graphite parts manufacturing mainstream is to rely on CVD TaC components. However, due to the high cost of CVD TaC production equipment and limited deposition efficiency, traditional SiC coated graphite materials have not been completely replaced. Sintering method can effectively reduce the cost of raw materials and can be adapted to compound

Mixed shape graphite parts, so as to meet the needs of more different application scenarios. At present, foreign manufacturers such as AFTech, Germany's CGT Carbon GmbH and Toyo Tanso have mature experience in TaC coating process and are the main suppliers in the domestic market. The development of TaC coated graphite parts in China is still in the initial stage of test and industrial production. In order to promote the development of the industry, it is necessary to optimize the currently used preparation methods of TaC coated graphite parts, explore new high-quality TaC coating preparation processes, and further verify and study the protection mechanism and failure mechanism of TaC coating. In addition, it is also necessary to continuously expand the application field of TaC coating, which requires continuous innovation of domestic research institutions and enterprises. With the continuous development of the domestic third-generation semiconductor market, the demand for high-performance coatings is increasing, so domestic substitution will become the future industry trend.

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