Silicon Carbide Processing

Silicon carbide is an extremely hard, strong, and durable ceramic material with a melting point of 2700 degrees Celsius and colorless purity.

Reaction bonding and sintering are the two methods available to form silicon carbide, each having a significant influence on its microstructure. Reaction sintering is easy and cost-effective but suffers from low density sintering density, fragile products, orientation sensitivity when cutting, etc. Reaction sintering is more of an ideal method as its simplicity makes for efficient use while reaction sintering has greater control over final microstructure of product produced through reaction bonding or sintering whereas reaction sintering has low density sintering density but low density, fragile products and orientation sensitivity during cutting process compared with reaction sintering or reaction bonding methods of silicon carbide production.

Reaction Bonded

Reaction-bonded silicon carbide (RB SiC) is an affordable ceramic used in applications that demand wear resistance as well as corrosion, heat and other environmental influences. RB SiC is commonly found in powder metallurgy equipment and processes as well as for sintering metal materials; additionally it can also be found in chemical processing and glass manufacturing industries.

RB SiC is produced by infiltrating porous carbon or graphite preforms with liquid silicon and then infusing at very high temperatures and pressures, whereby it reacts with carbon to form additional silicon carbide resulting in a composite material with great strength, heat resistance, and hardness. This creates an extremely hard and strong ceramic that has excellent heat resistance as well as being strong and tough enough for heavy machinery use.

The manufacturing method of RB SiC involves using a silicon powder-coated substrate with either phenol resin, furfuryl alcohol resin, or epoxy resin as its bonding agent; creating a silicon carbide/carbon preform from this silicon supplying body; then contacting one surface between these materials by heating their mixture at temperatures higher than its melting point in either an inert gas environment or reaction-sintering furnace; heating again until all silicon fused together evenly throughout.

This method provides an efficient means of producing large and complex-shaped RB SiC products more cost-effectively than direct sintered silicon carbide, with lower thermal expansion coefficient and wear resistance than other methods of producing ceramics.

Lely Method

The Lely Method is an example of a vapor-condensation growth technique. To use it, one begins by placing source material, solvent pallet and seed crystal into a graphite crucible before heating in an inert argon atmosphere to high temperatures. Sublimation takes place as silicon carbide charge vapors condense at various sites along the cool cavity walls to form platelets of single crystal silicon carbide that become the seeds from which bulk crystals (boules) of silicon carbide form.

Device fabrication processes used in practical applications require silicon carbide wafers with large diameters. Unfortunately, commercial sources do not always meet the crystallographic quality required; however, Lely process offers an innovative solution by producing large crystallized boules using similar methods employed when manufacturing semiconductor chips.

A cylindrical graphite vessel with one open end and a central mandrel of 25 mms. It is lined internally with loosely piled pure silicon carbide (impurities less than 0.002%). Space within the vessel is bound by its silicon carbide lining and it is assumed that equilibrium vapour pressure between silicon carbide and silicon will always exist within this volume of space.

If the lining contains impurities that determine its conductivity during heating, and this impurity exists at an appropriate proportion (which may be periodically varied), its vapors will separate from silicon carbide and settle into the center space producing crystals with predefined conductivity types.

Physical Vapor Transport Method

Physical Vapor Transport method is utilized to cultivate silicon carbide single crystal boules of high purity and quality. The process incorporates elements from CVD for better dopant control when growing silicon carbide bulk crystals. Furthermore, the invention includes in-situ annealing of grown crystals to alleviate internal stresses within them and increase breakage ratio during production process.

Under this method, a graphite crucible is filled with high temperature raw material zone and the upper portion loaded with seed crystal. Thermal insulation layers are placed nearby the crucible for added thermal insulation.

At the middle to late stages of crystal growth, thermal insulation layers move away from the graphite crucible 5 at an appropriate speed to form an axial temperature gradient and accelerate diameter growth at an approximate pace.

Prior art PVT systems required charging a finite amount of powdered source material into the furnace at the beginning of each growth run, and once this charge had depleted it would need replenishing via time-consuming procedures. With the present invention’s revolutionary PVT method this time consuming procedure is no longer necessary and larger boules can be produced from equal volumes of initial charge compared to prior systems. PVT may even be combined with in-situ annealing to produce high performance semiconductor grade silicon carbide single crystal boules.

Single Crystal Growth

Silicon carbide production demands extreme precision and optimization for maximum useable ratio after sintering and polishing processes are complete. This is especially crucial when growing large single crystals. Larger single crystals allow for improved processing, ultimately leading to higher usable ratios post sintering/polishing process.

Conventional growth systems allow temperature distribution within a silicon carbide growth chamber to be controlled by altering the size and placement of heat loss holes within insulation materials; however, this method does not enable real-time dynamic control of inner temperature distribution – something essential when growing high quality single crystals.

This invention addresses this challenge by employing an advanced heating system to regulate the insulation layer’s position and allow dynamic control of inner temperature distribution for improved crystal quality.

Additionally, this invention also makes it possible to in-situ anneal crystals after their growth is complete. This process removes significant internal stress in grown crystals that helps decrease breakage ratio during fabrication processes and increase yield ratio of finished silicon carbide products. Furthermore, in-situ annealing reduces time required to grow larger diameter crystals and maximizes material utilization rate.