Etch Basics

When the semiconductor industry was first getting started, the way to etch a material was with chemicals, such as hydrofluoric (HF) acid. Such "wet" etches work great and are still used today in a variety of applications, but their main disadvantage is that they etch isotropically, or in all directions simultaneously. This makes it difficult to make structures or holes with sharp vertical profiles, which are necessary for optimum packing density.

The alternative is dry etching. By exposing the wafer to a plasma — a cloud of energetic ions, electrons, photons, neutral atoms and chemically reactive free radicals — it's possible to get both a chemical etch component (as in wet etch) and a physical etch component, where energetic ions and neutrals actually knock off bits of the material being etched. The chemical part of the etch is a function of the types of gases that are fed into the plasma, which of course are chosen based on the type of material to be etched (etch applications are broadly separated into silicon, dielectrics and metals). Typically, a chlorine-based chemistry is used for etching polysilicon, silicides and metals, while a fluorine-based chemistry is used for oxide and nitride etching. Etch capabilities are fine-tuned with the addition of argon, hydrogen and/or oxygen.

Neutral beams can eliminate many of the problems associated with plasma etching if beam current of sufficient magnitude for practical processing can be generated. (Source: T.P. Ma)

Typically, most etches rely heavily on a chemical etch, with just enough physical etch to create an acceptable level of anisotropy or "verticalness." The catch is that, for some materials, such as copper, the byproducts of chemical reactions are not very volatile and tend to just stay on the wafer surface. Copper, therefore, is not a good candidate for dry etch and it is why copper is patterned with a damascene approach, where holes and trenches are cut into the dielectric and then filled with copper.

What allows some parts of the wafer to be etched while others are not is a mask. Although hard masks are sometimes used (typically a dielectric like SiO₂ or Si₃N₄), the typical mask is photoresist. When a positive photoresist is exposed to light (in a lithography tool), polymers in the resist become cross-linked. Areas that were not exposed are not cross-linked and easily dissolved and washed away during the develop step (in negative resists, the areas that were exposed are removed — but most new resists are positive resists). After etch, the resist is removed, so a key challenge for photoresist manufacturers is to make a resist that can stand up well to the etch process (have good etch resistance), yet still be easily stripped after the etch is over. Selectivity of the etch — the ability to etch one material and not another — is also critical; this can be difficult when etching new low-k dielectrics, which can be organic and not hugely different than the resist itself. This is where hard masks can come into play. Etch selectivity is also critical toward the end of an etch when the underlying material becomes exposed.

A successful etch is measured by many factors. The main one, of course, is if the desired profile is achieved. Achieving features with high aspect ratios (deep but not wide) remain a challenge, particularly at very small dimensions where it becomes increasingly difficult to get etch byproducts pumped out of the bottom of a hole (all dry etch systems require a fairly high level of vacuum for the plasma to operate). Also of concern is etch rate (which largely determines throughput of the tool), uniformity, etch residue and damage, and the formation of "artifacts" — either material where you don't want it (i.e. fences around the top of a via), or lack of material where you do (i.e. faceting in corners).

One way to get around damage is to neutralize the ion beam. Recently, researchers reported on a system that produces a 98% efficient neutralized beam from negative ions by passing the ions through a grounded carbon screen (Figure). A high silicon etch rate of 6000 Å/min and an anisotropic etch profile was shown, as was the effectiveness of the neutral-beam process in eliminating charging damage.

Another exciting new development in etch is closed-loop process control. By taking measurements of the developed photoresist before etching using new optical scatterometry tools, it's possible to tune etch parameters to cancel out variances in critical dimensions or other abnormalities.