Example Use of Milliner Indices

EXAMPLE USE OF MILLINER INDICES

Wafer

Wafers are thin slices of semiconducting material widely used in the production of electronics and micro mechanical devices. In electronics, wafer (also called slice substrate) is the crystalline silicon (C - Si) used to manufacture integrated circuits and solar cells. (Fig.1.36)

For example, the silicon wafer is cut from the silicon ingot which is grown by crystal growth techniques.

Generally, wafers are available with diameters from 25.4 mm (1 inch) to 300 mm (11.8 inch)

WAFER SURFACE ORIENTATION

The wafer surface is preoriented along a specific crystallographic plane. A precise surface orientation is critical in certain device processing steps. The orientation of crystalline materials determines their optical, electrical and mechanical properties. (Fig. 1.37)

The growth plane of the crystalline silicon orientations are described using Miller indices such as (100), (110), (111), (211), etc. The different growth planes and orientations have different

arrangement of the atoms or lattice.

The crystallographic orientations most commonly used in silicon wafer are (100), (110) and (111).

Wafer flat and Notches

In order to identity a reference direction within the surface plane of a wafer (i) Flat (ii) Notch is grounded at the periphery at the pr of wafer.

WAFER FLATS

A wafer has a flat cut on one or more sides at the periphery to indicate crystal direction as shown in fig. The large and small values of flat cut are are typically set by wafer manufacturers. In certain wafer double flat cuts are also made at the periphery.

Primary Flat:

The flat of long length appearing in the circumference of the wafer. The primary flat has a specific crystallographic orientation relative to the wafer surface. (Fig. 1.37)

Secondary Flat:

The flat of short length appearing in the circumference of the wafer. This flat cut denotes type of wafer (either p type or n type).

WAFER NOTCH

Instead of flat, a notch can also be used to indicate crystallographic orientation of the wafer surface as shown in fig. A notch a V-shaped of wafer. Usually, in silicon wafers the notch indicates (011) direction of silicon.

Importance

The position and orientation of a semiconductor wafer is critical during the wafer fabrication process. This is achieved by monitoring a notch on the wafer to understand the wafer's orientation through each step.

The facing surface is polished and etched yielding a damage-free, mirror-like finish. The figure explains the use of Miller indices in the (100) plane and (011) direction designations.