What is the purpose of the Substrate on a Chip Wafer?

Quote- In order to provide a hospitable surface on which to support an epitaxial βSiC layer, substrates are usually made from large αSiC single crystals by sawing the crystal into wafers along a 0001 surface plane, then lapping and polishing the wafers to a smooth surface. The {0001} crystallographic surface plane provides a surface that matches the Coefficient of Thermal Expansion (CTE) of the (111) surface of βSiC. Low defect βSiC epilayers have thus been applied to single crystal αSiC.

The process of lapping and polishing single crystal αSiC substrates, however, typically cuts through a multitude of surfaces to expose surface features associated with higher surface energy than a broad, flat single surface. A surface intersecting multiple crystalline lattice is planes creates a series of physical "steps. " Multiple steps transmit three dimensional lattice information to an epitaxial layer usually causing formation of a mixture of alpha and beta polyforms in the epitaxial layer, thereby making the subject layer unsuitable for high quality devices.

Silicon carbide substrates can be used to support magnetic media in hard disk drives (U.S. Pat. No.5,623,386). The mechanical requirements for disk drive substrates rotating at up to 10,000 rotations per minute with heads gliding less than 0.025 microns above substrate surfaces surpass the mechanical requirements of chip substrates.

Magnetic media needs a substrate without surface defects such as pits that cause "dead spots" in the media or surface features exposing different lattice planes that cause variations in the magnetic media, commonly referred to as "noise. " The size of surface pits or defects on disk drive substrates which can be tolerated, however, is considerably larger than the size of surface defects which can be tolerated for high quality electronic devices like chips (approximately 2 microns for disk drive substrates versus 0.1 microns for a high quality integrated chip for example). Apart from the issue of exposing high energy lattice positions that might cause undesirable crystallographic forms, media used for disk drives is much thicker than thin film coatings used for chips and can more easily "bridge" surface defects.

Magnetic media is also different from the thin films of electrical devices in the way it functions. Magnetic media does not require electrical continuity to function. Data bits in magnetic media are magnetic domains, the polarity of which determines whether an individual bit is either a 0 or a 1 binary code.

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