Plasma Etch (Reactive Ion Etching) is the pattern transfer process that uses chemically reactive plasma to selectively remove material through a mask — converting lithographic patterns into physical structures in silicon, dielectric, and metal films with nanometer-scale precision, where the simultaneous chemical reaction and physical ion bombardment provide the directionality (anisotropy) needed to etch vertical sidewalls, the selectivity needed to stop on underlying films, and the uniformity needed to produce identical features across the 300mm wafer.

How Plasma Etch Works

1. Plasma Generation: RF power (13.56 MHz or higher) ionizes the process gases (fluorine-based: CF₄, CHF₃, SF₆; chlorine-based: Cl₂, BCl₃, HBr) in a vacuum chamber at 1-100 mTorr. The plasma contains neutral reactive species, positive ions, electrons, and photons. 2. Chemical Component: Reactive neutral species (F, Cl radicals) diffuse isotropically to the surface and react with the target material, forming volatile products (SiF₄ from Si + F, SiCl₄ from Si + Cl). This component is isotropic (etches equally in all directions). 3. Physical Component: Positive ions (CF₃⁺, Ar⁺) are accelerated vertically by the plasma sheath voltage (50-500V) toward the wafer surface. The directional ion bombardment enhances the etch rate at horizontal surfaces (bottom of trenches) while leaving vertical surfaces (sidewalls) relatively untouched — this creates anisotropy. 4. Passivation: Polymer-forming gases (CHF₃, C₄F₈) deposit a thin passivation layer on the sidewalls, protecting them from chemical etching. The vertical ion bombardment removes passivation from horizontal surfaces, maintaining the etch rate there. This mechanism enables perfectly vertical profiles.

Selectivity

The ratio of etch rate of the target material to the etch rate of the mask or underlying film. Example: for oxide etch over silicon, selectivity of 50:1 means 50nm of oxide is removed for every 1nm of silicon loss. Selectivity is achieved by choosing chemistry that preferentially reacts with the target material while forming non-volatile products (etch stop) on the underlying film.

Critical Applications

• Fin Etch: Etching silicon fins for FinFET. Requires perfectly vertical sidewalls, <1nm width variation, and no footing at the fin base. Aspect ratio 8-10:1.
• Gate Etch: Patterning the dummy poly gate across fins. Must stop on the thin gate dielectric without damaging it. Selectivity >100:1 required.
• Contact Etch: High-aspect-ratio holes through thick dielectric to reach S/D contacts. AR up to 20:1 at 10-20nm diameter. Etch-stop on the silicide without punch-through.
• SAQP Mandrel/Spacer Etch: Multiple etch steps in the self-aligned patterning sequence, each requiring extreme selectivity and profile control.

Advanced Etch Techniques

• Atomic Layer Etching (ALE): Self-limiting etch that removes exactly one atomic layer per cycle. Adsorb a thin reactive layer, then remove it with low-energy ion bombardment. Analogous to ALD but in reverse.
• Cryogenic Etch: Cooling the wafer to −100°C or below enhances passivation and selectivity. Used for deep silicon etch (TSVs, MEMS).

Plasma Etch is the sculpting tool that gives three-dimensional form to the two-dimensional lithographic image — using the precise balance of chemistry, ion energy, and passivation to carve nanometer-scale features with the vertical walls, flat bottoms, and selective stopping that modern transistor architectures demand.