Use and Selection of Carbon Graphite Products for the Semiconductor Industry

In the semiconductor industry, high-purity graphite is primarily used to manufacture single-crystal furnace graphite heating systems, electronic device molds, insulator sintering molds, and thyristor sintering molds.

1. Graphite Heaters
The requirements for single-crystal furnace heating systems are to ensure sufficient heat to rapidly melt silicon, germanium, and other materials, while also enabling precise and convenient temperature adjustment. Therefore, heating is typically achieved using resistance heating and high-frequency heating.

Common resistance heating methods primarily consist of a transformer and a graphite heater. Common graphite heater shapes include cup, straight, and spiral.

The heater's size, shape, and slot height are primarily determined by melt flow and crystal pulling efficiency. Its resistance must match the transformer's. The heater's inner diameter and height are selected so that the top of the graphite tray is within the heater's high-temperature zone during silicon melting and the bottom of the tray is within the heater's high-temperature zone during crystal pulling. The length of the heater's high-temperature zone is related to the length of the heater's slots. Once the heater's inner diameter and height are determined, the thickness of the heater blades is determined based on the transformer's output power.

2. Graphite Electrodes, Graphite Pillars, and Other Graphite Parts
In single-crystal furnaces, high-purity graphite is used not only for heaters but also for graphite electrodes, pillars, insulation covers, seed crystal holders, slag trays, and trays.

The principle, equipment (when using single crystal pulling) and operation process of pulling germanium single crystals by the Czochralski method are basically the same as those of pulling the above-mentioned silicon single crystals. The difference is that graphite can be used when pulling germanium single crystals, while quartz reinforcement must be installed on the graphite support when pulling silicon single crystals because silicon and graphite react at high temperatures.

3. Graphite Insulation Barrel
In zone melting, graphite insulation barrels are often used to reduce dislocation density.

The graphite barrel is clamped to the lower counter coil and heated to red by high-frequency electromagnetic induction. The radiant heat from the barrel reduces heat loss in the produced crystal, maintaining a relatively uniform temperature field and achieving the desired insulation.

The upper end of the graphite barrel must be level with or slightly higher than the lower interface of the melt zone; this is crucial for reducing dislocation density. If it is 2-3 mm below the crystallization interface, the dislocation density will rise to tens of thousands per square centimeter. If it is approximately 10 mm below the crystallization interface, the single crystal will develop numerous defects, approaching a polycrystalline state. The relative position of the graphite barrel and the crystallization interface is mainly determined by the distance between the lower counter coil and the main coil. The upper end of the graphite barrel must be level with or 0.5-1 mm higher than the lower counter coil.

The temperature of the graphite barrel should be appropriate, neither too dark nor too bright. Excessive temperature can melt the single-crystal surface or produce slip lines. The hot zone of the graphite cylinder has a certain length and a gradient from top to bottom. Therefore, a larger coil is placed appropriately below the lower counter-coil for auxiliary heating. The redness of the graphite cylinder depends on its relative position to the lower counter-coil: the higher it is, the redder it becomes, while the lower it becomes, the darker it becomes. The contact area between the graphite cylinder and the coil may darken due to the cooling water inside the coil, so the coil should only contact the graphite at a few points. At the bends of the coil, where the magnetic flux density is high, the graphite will appear particularly bright. Saw cuts should be made in the graphite at these locations to eliminate the bright spots.

Although using a graphite cylinder for heat preservation is convenient for the stable production of zone-melting single crystals with low dislocation density and has no perceptible effect on the single crystal's resistivity, careful attention should be paid to the placement of the graphite cylinder, as well as its strict handling and proper use, to avoid other adverse effects.

Because high-purity graphite has the characteristics of high temperature resistance, high purity, dimensional stability at high temperatures, and good thermal shock resistance, it is widely used in the semiconductor industry to make various sintering molds.

1. Molds for sintering electronic devices.
Graphite sintering molds are suitable for sintering the cores of various types of diodes, triodes, thyristors and other devices. Figure 4 shows some examples of sintering molds. Graphite boats are also used when pulling single crystals using the high-frequency furnace zone melting method.

2.  Various insulator sintering molds.

Graphite molds are mainly used for sintering various types of triode silicon rectifiers, thyristor sockets, capacitor insulator sintering, thick film and thin film integrated circuit insulator sintering, ultra-small relays, connectors and other components insulator sintering molds, etc. Figure 5 shows some thyristor tubes and various insulator sintering molds.

The semiconductor industry demands the highest possible purity for its graphite materials, especially for graphite components that come into direct contact with semiconductor materials, such as crucibles and sintering molds. Because impurity levels are high, the purity of the raw graphite must be strictly controlled and the ash content must be minimized through high-temperature graphitization.

The semiconductor industry also requires fine-grained graphite. Fine-grained graphite not only facilitates machining precision but also offers high high-temperature strength and minimal wear. Sintering molds, in particular, require extremely high machining precision.

Because graphite components used in the semiconductor industry (including heaters and sintering molds) must withstand repeated heating and cooling cycles, their lifespan requires excellent dimensional stability and thermal shock resistance at high temperatures. To meet these requirements, my country currently produces a range of graphite materials suitable for the semiconductor industry. For grades and performance information, please refer to JB/T 2750.

Graphite made from uncalcined petroleum coke is a fine-grained structural material with high mechanical strength. It can be used to manufacture electronic products such as glass, thin plates, disks, heaters for vacuum and high-frequency furnaces, heat shields, graphite dishes for melting pure metals, grippers (chucks) for high-temperature experimental equipment, hot-pressing dies, and filters. This material can operate at temperatures below 2500°C in inert or protective atmospheres and can operate for extended periods below 2000°C in a vacuum (10-4 to 10-5 mmHg). The properties of MIIT-8 graphite are listed. This material can be made into special-shaped products.

High-purity graphite is used in various components of semiconductor technology. It is produced by purifying ordinary structural graphite with reactive gases during the graphitization process. Purified graphite must be machined under conditions that prevent contamination of the final product. The ash content of this graphite (after purification) should not exceed 1×10-3%, the iron, aluminum, and magnesium contents should not exceed 3×10-5%, and the copper, boron, and manganese contents should not exceed 1×10-3%. These impurity limits meet the purity grade. The silicon and calcium content of this graphite should not exceed 3×10-4 (mass%). Impurity contents of high-purity and ultra-high-purity graphite are shown (in μg/g).

Industrially available, even purer structural graphite is also available. Products made from this type of graphite undergo additional purification after machining to reduce surface contamination. Impurity limits include: iron, aluminum, magnesium, boron, copper, and manganese must not exceed 1×10-3%, silicon must not exceed 3×10-3%, and titanium, nickel, chromium, and other elements must be less than 1×10-5%. The ash content of these graphites ranges from 0 to 10-4%.

Ultra-pure, high-strength graphite with a protective layer is made from ordinary fine-structured graphite that has been purified and degassed in a vacuum, followed by surface densification with pyrolytic carbon. Products made from this material (heaters, disks, graphite dishes, etc.) can be used to grow silicon thin films using the gas epitaxial growth method. Impurity levels within these products are: iron not exceeding 5×10-4%, aluminum not exceeding 2×10-4%, magnesium and copper not exceeding 5×10-5%, titanium not exceeding 1×10-4%, and nickel and cobalt not exceeding 1×10-5%.

The thickness of the densified protective layer formed by pyrolytic carbon is no greater than 2mm. A thin layer of pyrolytic graphite, no more than 0.1mm thick, can also be deposited on the surface of the densified product. Graphite products densified with pyrolytic carbon significantly reduce their permeability and gas release (gas desorption rate).

Unused graphite heating elements, insulation covers, brackets, seed crystal holders, and other graphite components must be pretreated before use to prevent contamination of semiconductor materials. This is because graphite powder may adhere to the surface of the components or penetrate the pores of the graphite during machining, and other impurities may also be introduced into the graphite pores.

One pretreatment method is to soak the components in carbon tetrachloride for several hours, then rinse with deionized water and dry them. Vacuum-heat them at operating temperature for 3-4 hours and store them for later use.

Another pretreatment method is to soak the components in mains water for 24 hours, remove them, and boil them several times with deionized water until the solution is neutral. After drying, place the components in a furnace and heat them in a vacuum (generally above 10-1 mmHg) for 1 hour. The temperature should be slightly higher than the operating temperature. After cooling, remove them and place them in a desiccant bottle for later use.

Used graphite components must be properly stored. Before reuse, first remove the surface layer with No. 0 metallographic sandpaper, then clean with deionized water and anhydrous alcohol, then dry.

Source: National Abrasive Quality Inspection System (NAQS)

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