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, SiC₂, Si₂C, etc.). The vapor phase material is transported from the high temperature region to the seed crystal in the low temperature region, forming seed nuclei, growing, and generating single crystals. 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, N₂corrosion, and need to have a high eutectic temperature (with 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 coatedseed 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

Coated susceptor for epitaxy(wafer carrier)

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.