“Dry” (plasma) etching is used for circuit-defining steps, while “wet” etching (using chemical baths) is used mainly to clean wafers. Dry etching is one of the most frequently used processes in semiconductor manufacturing. Before etching begins, a wafer is coated with photoresist or a hard mask (usually oxide or nitride) and exposed to a circuit pattern during photolithography. Etching removes material only from the pattern traces. This sequence of patterning and etching is repeated multiple times during the chip making process.

Etch processes are referred to as conductor etch, dielectric etch, or polysilicon etch to indicate the types of films they are remove from the wafer. For example, dielectric etch is involved when an oxide layer is etched to leave “oxide isolators” separating devices from each other; polysilicon etch is used to create the gate in a transistor; dielectric etch is employed to etch via holes and trenches for metal conductive paths; and metal etch removes aluminum, tungsten, or copper layers to reveal the pattern of circuitry at progressively higher levels of the device structure.

Plasma etching is performed by applying electromagnetic energy [typically radio frequency (RF)] to a gas containing a chemically reactive element, such as fluorine or chlorine. The plasma releases positively charged ions that bombard the wafer to remove (etch) materials and chemically reactive free radicals that react with the etched material to form volatile or nonvolatile byproducts. The electric charge of the ions directs them vertically toward the wafer. This produces the almost vertical etch profiles essential for the miniscule features in today’s densely packed chip designs. Typically, high etch rates (amount of material removed in a given time) are desirable.

Process chemistries differ depending on the types of films to be etched. Those used in dielectric etch applications are typically fluorine-based. Silicon and metal etch use chlorine-based chemistries. A specific etch step may be performed on one or more film layers. When multiple layers are involved and also when the etch process must stop precisely on a particular layer without damaging it, the selectivity of the process becomes important. Selectivity is the ratio of two etch rates: the rate for the layer to be removed and the rate for the layer to be protected (e.g. mask or stop layer). Higher selectivities are usually desirable.

In reactive ion etching (RIE), described above, the objective is to optimize the balance between physical and chemical etching such that physical bombardment (etch rate) is sufficient to remove the requisite material while appropriate chemical reactions occur to form either easily exhausted volatile byproducts or protective deposits on the remainder (selectivity and profile control). Magnetically enhanced RIE can aid processing by increasing ion density without increasing ion energy (which can damage the wafer).

Ideally, the etch rate is the same (uniform) at all points on a wafer. The degree to which it might vary at different points on the wafer is known as non-uniformity (or microloading) and is usually expressed as a percentage. Minimizing non-uniformity and microloading are important objectives in etching.

Source: Applied Materials

Additional Reading

Moving atomic layer etch from lab to fab

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