Plasma-assisted etching

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Plasma-assisted etching ( physical-chemical etching ) describes a group of subtractive (removing) microstructure processes in semiconductor technology . As a dry etching process , they represent an alternative structuring process to wet-chemical etching . B. of silicon dioxide on silicon - wafers is carried out using plasma activated gases and accelerated ions (. E.g., argon ion ).

General procedure

In physical-chemical etching, two etching mechanisms are used in one process, on the one hand ion bombardment of the substrate and on the other hand a chemical reaction on its surface. Physical and chemical effects are combined in the following way: Bombardment with the ionized reaction gas or other ions weakens or destroys the chemical bonds of the atoms on the surface, so that the reactive gas can react more easily and thus intensify the chemical effect in the affected areas. It does this by delivering the energy needed to activate the chemical reaction from the ion bombardment.

In addition, when using polymer passivation films - as with reactive ion deep etching - the anisotropy of the etching process of trenches and holes can be increased, because the polymer film is deposited largely evenly on the entire surface, i.e. on the substrate surface (or on the mask), in the trench floors and on the side walls of the trench. The ion bombardment, however, is highly anisotropic, so that only the bottom of the trench is bombarded by particles. There the polymer film is severely damaged, as a result of which the etching gas can attack the substrate. The side walls, on the other hand, are not damaged and the polymer film there hinders the chemical etching attack. The resulting etching attack on the floor is thus significantly higher than that on the walls, so that it occurs almost only in the direction of the impacting ions and a high anisotropy can be achieved.

variants

Reactive ion etching

Comparison of the silicon dioxide etching processes between wet chemical etching and reactive ion etching (RIE)
RIE system

The reactive ion etching ( English etching reactive ion , RIE ) is a ion-assisted Reaktivätzprozess. In contrast to ion etching, reactive ions are used here. Due to the good controllability of the etching behavior (homogeneity, etching rate, etching profile, selectivity ), RIE is a process for the production of topographical structures for micro and nano system technology. The process permits both isotropic (independent of direction) and anisotropic etching through chemical-physical removal . The etching is carried out by charged particles (ions) that were generated in a gas plasma. Appropriate masking (e.g. produced by photolithography ) of the surface gives the shape of the structures. With reactive etching, the chemical etching reaction is only triggered by the kinetic energy of the ions that strike. The volatile etching product, which is removed by the vacuum system, is created from the gas ion and the layer molecule on the surface. RIE delivers very good results even with very fine structures with dimensions well below 100 nm.

The reactive Ionentiefenätzen (engl. Deep reactive ion etching , DRIE ) is an RIE modification for Siliziumtiefenätzen by alternating etching and passivation cycles ( gas chopping , time-multiplexed etching ) for high aspect ratios , anisotropy and etching rate.

Reactive ion beam etching

The (engl. Function of the reactive ion beam etching, reactive ion beam etching , RIBE ) is similar to ion beam etching , reactive ion however also be used here. The effect is anisotropic and selective.

Alternative removal methods

Cathodic atomization - sputtering

During cathode sputtering, gas atoms are ionized in a vacuum chamber. A (mostly negative) voltage is applied to the substrate (hence the term cathode sputtering) and the ions are accelerated by the electrical field in the direction of the substrate . The kinetic energy of the ions causes atoms to be ejected from the surface. This method is not isotropic and not selective.

High selectivity would mean that there would be greater deviating removal rates for different substances (compounds). This is only the case to a limited extent. Due to the different mass of the atoms (elements), there are physical interactions ( momentum transfer ) that lead to different “sputtering rates” and the so-called preferential sputtering when processing materials with light and heavy atoms. This is almost independent of the chemical bond in the material. In contrast, there are chemically effective etching processes.

Ion beam etching

Ion beam milling or ion beam etching, not ion beam sputtering, works in a similar way to ion etching, but the ions are bundled in a beam. The effect is therefore direction-dependent ( anisotropic ) and not selective .

literature

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