Industrial applications

Semi-conductors

Technology

The key properties of graphite to be used in the semi-conductor technology are the unusual combination of its properties and the possibility to match certain material properties with a given specification through varying raw materials and production methods.

There are several major applications for graphite in the semiconductor sector which are requiring purification to at least 5 ppm ash.

Single Crystal Growth - Silicon CarbideSingle Crystal Growth - Silicon Epitaxial Deposition Glass-To-Metal Sealing Coatable GraphitesFiber-Optics

Single Crystal Growth - Silicon Carbide:

Purified graphite with its enviable thermal properties and inertness provides a suitable constructional material in which single crystals of silicon carbide can be grown. The isomolded grades are ideal for this application.

Single Crystal Growth - Silicon:(1)Silicon is the most widely used material for the manufacture of integrated circuits and other semiconductor devices. Silicon, the second most abundant element, comprising 28 percent of the earth's crust, is a good material for circuits because its electrical properties can be precisely altered by adding controlled amounts of impurities, called dopants. One of the most essential operations in the manufacture of an integrated circuit is the conversion of this silicon into single crystal form. The most common method of converting the polycrystalline silicon to a single crystal ingot is through growing the ingot from molten silicon by the Czochralski crystal-growing process. This process involves wetting a properly-oriented seed in a melt and withdrawing it vertically to grow a crystal which might be up to 12" (300 mm) in diameter.

Graphite component

Single Crystal Growth - Silicon:(2)Graphite is extensively used in this application for a number of reasons. First, graphite has electrical properties that are necessary for good heaters. Second, it has the required thermal properties. Material is needed that can withstand temperature cycling from room temperature to above 1400°C, maintaining strength at these high temperatures. Graphite has excellent thermal conductivity, enabling it to maintain thermal equilibrium in the growing process. Third, the combination of cost, availability, machinability, and life of the graphite parts has not been improved upon by other materials. Purified fine-grain graphites are used as resistance heaters, which melt the silicon and maintain it at a temperature of approximately 1400°C. Purified isomolded superfine graphites are used as holders or crucibles to hold a quartz crucible liner containing the molten silicon. These same grades are used as a graphite pedestal which support the crucible, rotate it, and lift it as necessary. The graphite is subjected to the same thermal conditions as the crucible and also acts as a path for removal of excess heat of fusion from the puller. A graphite heat shield, surrounds the heater. This shield prevents the loss of heat outward from the crucible area. Lastly, graphite chucks are used in the polysilicon reactors. These hold filaments upon which the polysilicon is grown.

The choice of grade depends upon the size of the hot zone, and the graphite performance required.

Graphite components in CZ

Epitaxial Deposition:

Another application for graphite is epitaxial deposition, a process utilized throughout this industry for doping silicon or III - V compound substrates. Epitaxy, in silicon semiconductor technology, refers to the oriented overgrowth of one crystalline material on to another. This process is known as "vapor phase epitaxy" and uses silicon carbide coated graphite susceptors to hold the silicon wafers. Controlled doping of "P" or "N" type impurities is also possible in silicon epitaxial technology. Graphite is used in epitaxial deposition because of its high purity, electrical properties, thermal conductivity, low gas evolution, and coating characteristics. Graphites are generally superfine grain and possess CTE characteristics compatible with the coatings.

Glass-To-Metal Sealing:

In glass-to-metal sealing, a "chip" mounted to a metal lead frame must be protected, quite often by glass encapsulation. The device is mounted in a graphite fixture, heated to a desired temperature and the molten glass applied. Graphites are generally used due to their low gas evolution, high thermal conductivity, ability to be machined to very close tolerances, excellent resistance to abrasion and wear, and superior oxidation resistance.

Coatable Graphites:Coating of graphite with silicon carbide extends the applications of graphite into high temperature oxidation regimes and eliminates interaction with potentially reactive materials such as silicon.

Graphite coated with SiC

Optical micrograph ofgraphite coated with50 microns of SiC

Fiber-Optics

High grade quartz is drawn at temperatures around 1700°C to form a cable with fiber-optic qualities. In one industrial process the furnace container and heater are fabricated from high purity isomolded superfine graphite. Graphite with its combination of thermal characteristics, chemical inertness, high purity, superfine grain texture, and structural integrity at elevated temperatures is eminently suitable.

picture of graphite components for fiber optics

Graphite components for fiber optics