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Wafer Preparation Processes:

• Ingot Growth

• Wafer Type and Orientation

• Slicing, Lapping and Polishing

• Dicing

1.    Ingot Growth

Czochralski crystal growth utilized a crucible in which pieces of polycrystalline silicon have been heated to their melting point of 1415°C (Figure 1.1). The crucible containing the silicon is made of quartz (SiO₂) and is heated by either induction (RF) or thermal resistance methods. The crucible rotates during the growth process to prevent the formation of local hot or cold regions. The atmosphere around the crystal-growth apparatus or crystal puller is controlled to prevent contamination of the molten silicon. Argon is often used as the ambient gas. When the temperature of the silicon has stabilized, an arm with a piece of silicon mounted on the end is slowly lowered until it comes into contact with the surface of the molten silicon. This piece of silicon is called as the seed crystal. As the bottom of the seed crystal begins to melt in the molten silicon, the downward motion of the rod holding the silicon is reversed. As the seed crystal is slowly withdrawn from the melt, the molten silicon adhering to the crystal freezes or solidifies, taking on the crystal structure of the seed crystal. The rod continues its upward movement, forming an even large crystal. The crystal growth terminates when the silicon in the crucible is depleted. By carefully controlling the temperature of the crucible and the rotation speeds of the crucible and the rod, precise control of the diameter of the crystal is maintained. The desired impurity concentration is obtained by adding the impurities to the melt in the form of heavily doped silicon prior to crystal growth.

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2.    Wafer Type and Orientation

The silicon crystals are first ground perfectly round (if necessary), and the rotational orientation of the crystal is ascertained. The seed crystal has determined which crystal face will be presented on the wafer surface, but the rotational position of the rod determines other axes of the crystal. Since the bar of silicon is one crystal, it has preferential break or cleavage planes. It is critical for later device separation to align the circuits precisely with respect to their cleavage planes. This precise alignment is accomplished by grinding a flat along the crystal (prior to slicing) that is used as a reference during all subsequent processing steps. X-ray diffraction provides a fast and accurate method of determining the crystal orientation prior to grinding the flat.

Fig. 2.1

The crystal is usually doped p- or n-type while grown, depending on wafer specification. For coarse wafer alignment and future identification, a notch and one or more flats are ground along the length of the crystal. The primary flat is typically located on a (01) surface and is used (with a secondary flat, when present) to quickly identify the type and crystal orientation of the wafer.

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3.    Slicing, Lapping and Polishing

The silicon crystal is then sawed into thin slices of thickness 0.5-1 mm called wafers. The process of cut the silicon ingot into thin slices is called slicing as shown in Fig. 3.1. Extreme care is taken to minimize the amount of the single crystal silicon that is lost in the slicing process by using the inside diameter of a ring-shaped saw blade. The blade is coated with diamond powder to enable it to cut through the hard silicon. The slicing process leaves wafers with saw marks on both sides that must be removed. A silicon etchant is used to remove the saw marks and any accompanying damage from both sides of the wafer.

Fig. 3.1   Schematic diagram of the slicing process.

The wafers are then lapped, polished, and cleaned to remove the damage caused by slicing. Care must be taken to remove any crystal damage introduced by the slicing operation, or the damage may prevent the successful fabrication of devices. The wafers are next mounted on large circular polishing plates using either wax or a vacuum to hold them. The polishing plates are mounted on a polisher, and one side of the wafer receives a mirrorlike finish. The polishing operation uses a polishing solution that simultaneously chemically etches and mechanically polishes the wafers. The polishing pad must be tough and durable. When the wafers have reached the proper thickness rang and surface quality, the polishing plates are removed and the wafers are dismounted. The wafers are thoroughly cleaned to remove any residual contamination, and inspected to insure that wafers with imperfect surfaces are not shipped. Wafers that pass the finial inspection are ready to start on their journey to become devices (Gise and Blanchard, 1979). 

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4.    Dicing

After integrated circuit (IC) fabrication and before chip package, the wafer with well-done IC is diced into individual die with a diamond-tipped wheel, as shown in Fig. 4.1. This process is called dicing. Dicing by grinding causes fracture and cracks to the edges of the dies, which are to be removed by a chemical etching process.

Fig. 4.1   Schematic diagram of the dicing process.

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