Silicon carbide’s crystalline structure enables its use as a ceramic chip wafer substrate for epitaxial thin films to make high quality electronic devices. Additionally, this material is widely employed at both macro and micro scale for disk drive components as well as ceramic substrates.
Sintering (the consolidation of fine SiC powders) produces this material, while various sintering aids like carbon or boron may be added for optimal sintering results.
Two-Step Growth
Modern production techniques of silicon carbide (SiC) for use in the abrasives, metallurgical, and refractory industries start with mixing pure silica sand with ground coke carbon as part of an electric resistance-type furnace and charging it with electric current, creating an electric resistance-type furnace with electric resistance-type furnace heating elements to produce carbon monoxide gas and liquid silicon which then forms into polycrystalline film when allowed to cool on substrates before crystallizing onto surfaces in layers. To produce high-quality devices it is essential that crystal grain sizes fall under device feature sizes as this ensures uniform production.
Two-step growth has been introduced as an approach to reduce the crystal defect density in these films. This involves partial melting and recrystallization, yielding smaller crystal grains. Furthermore, an accelerated crucible rotation technique may also be utilized for more uniform temperature distribution and solute concentration.
Bulk growth techniques have proven successful at producing heteroepitaxial SiC film on non-oriented Si (111) substrates using enhanced sublimation epitaxy. Unfortunately, point defects such as carbon vacancy (VC), carbon-silicon vacancy antisite (VCVSi), and Al-related defects were observed; these can be reduced by selecting an appropriate growth temperature and deposition rate.
Facet Growth
To create polycrystalline silicon carbide, amorphous Si is first melted in an LPE reactor before being transferred to an ion-beam assisted growth system for depositing on top of hexagonal silicon carbide seeds. Ion beams create a high pressure regime which can be tuned to alter both growth rate and temperature; additionally layers with different compositions may be grown by adding or subtracting silicon or carbon precursors from this high-pressure regime.
Achieve 3C-SiC heteroepitaxial growth on silicon is a complex process involving substrate etching, carbonization, and epitaxy. First the substrate is exposed to carbon precursors in a chemical-vapor deposition (CVD) process (step 1). Subsequently an ion beam implanted 3C-SiC layer will form at temperatures near silicon’s melting point (step 2) until completion.
Epitaxial layers fabricated using this material exhibit large numbers of facets with high symmetry and low strain, which is essential to developing devices using this material. To minimize this number of facets and consequently their strain, two different pillar patterns have been studied as ways of limiting cases; one being parallelepiped case wherein pillars align along [11-2] directions yielding sixfold symmetry arrangements that minimize gaps between adjacent 111-C terminated facets and reduce residual stress or equivalent strain by minimising gaps.
Annealing
An annealed silicon carbide wafer must undergo annealing in order to reorder its dislocations, which is achieved by heating the material below its recrystallization point (1700-2500 degC in a-SiC, while 1500-2500 degC for b-SiC). Rearranging dislocations resets its lattice structure, relieves internal stress, and improves crystal quality of wafers.
Soitec of Bernin near Grenoble and Mersen of Courbevoie in France have come together in a technical partnership to produce polycrystalline silicon carbide (poly-SiC) substrate materials suitable for MEMS, high temperature power semiconductor applications as well as corrosion- and oxidation-resistance. Polycrystalline substrates offer increased electrical breakdown voltage and thermal conductivity compared with monocrystalline ones while providing more corrosion and oxidation protection.
This process involves placing the a-SiC substrates into an annealing furnace and gradually heating to their target temperatures under either inert or reduction gas for 1-8 hours, then maintaining them there for 0.1-5 hours until finally cooling to room temperature slowly.
After annealing, x-ray diffraction and transmission electron microscopy demonstrate that poly-SiC has formed both amorphous Si and beta phase b-SiC nanocrystals. By raising the annealing temperature, passivation quality of beta phase b-SiC is also improved, as evidenced by its effective minority carrier lifetime being greater than 1.8 milliseconds under flow conditions of 0.5-R/silane silane.
Processing
Silicon carbide (SiC) has long been recognized for its many industrial uses, from grinding wheel grits and furnace linings to cutting tools. SiC is also considered an excellent candidate for electronic device substrates due to its excellent temperature stability, chemical resistance, and superior electrical properties; however, due to the unique mechanical requirements imposed by disk drive substrates (including magnetic media spinning at 10,000rpm and head travel of less than 0.025 microns above substrate surface), widespread adoption remains limited.
SiC substrates are produced by consolidating polycrystalline SiC mixture into large rods or blocks and consolidating into polysilicon (mc-Si), an essential raw material used for manufacturing solar photovoltaic cells and integrated circuits. Production occurs by feeding polycrystalline silicon rods into a bell-shaped reactor heated via electrical current between 1273 K and 1373 K where they decompose into polycrystalline silicon, carbon monoxide, and silicon tetrachloride.
For producing sophisticated devices, microcrystalline silica (mc-Si) is deposited onto substrates through epitaxial growth techniques to form layers with different electrical properties that will then be used in semiconductor chip production. Such devices require precise control over both epitaxial growth processes as well as various processing steps during their manufacture.
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