Dry etching

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In semiconductor technology and microsystem technology, the term dry etching covers a group of subtractive (removing) microstructure processes that are not based on wet chemical reactions (such as wet chemical etching , chemical mechanical polishing ). The removal of material (for example, from. Silica on silicon - wafers ) is performed either by accelerated particles (eg. Argon ion ) or using plasma activated gases. Depending on the process, chemical and physical effects are therefore used.

Classification

The dry etching processes can be classified into three groups. On the one hand the physical dry etching process, they are based on the material removal by bombardment with particles, on the other hand chemical dry etching process, they are based on a chemical reaction of a mostly plasma-activated gas. The third group, the physico-chemical dry etching process, combines processes that use both mechanisms of action and is thus able to minimize the disadvantages of the first two groups.

Physical dry etching process

In the physical dry etching process, the surface of the substrate is etched by bombarding ions, electrons or photons. The bombardment leads to the atomization of the substrate material; the processes that take place are similar to those in cathode atomization ( sputtering ), which is usually not counted as a dry etching process. The processes are named depending on the particles used. The best known and most used are: electron beam method (Engl. Electron beam ) or Laserzerstäubung (Engl. Laser vaporization ). Both are used, among other things, in photolithography (see also electron beam evaporation and laser beam evaporation ).

The etching generally takes place in high vacuum chambers in order to prevent interactions between the particle beam and the residual gas atoms (scattering, etc.). For structured samples, there are both methods based on bundling the particle beam, which etch very specifically, and large-area etching methods using a mask applied to the surface (cf. photolithography ), which protects areas not to be etched from particle bombardment.

If one considers ion etching processes, some important disadvantages of the purely physical dry etching process become apparent. They usually have a relatively low etching rate, which also has only a low material selectivity. The associated etching of the mask results in rounded edges. Furthermore, high energies are necessary for the etching so that the ions also penetrate deeper into the material. Therefore, not only is the surface etched, but deeper layers are also damaged. A further disadvantage are parasitic deposits ( redeposition ) of the etched particles on the substrate and the mask or the mask edges.

Chemical dry etching processes

(Engl. In the chemical dry etching chemical dry etching , CDE) a chemical reaction between is neutral particles / molecules (but usually radicals ) utilized and the surface of the substrate. The prerequisite for this is that the reaction product , like the starting materials used, is gaseous and volatile, for example silicon tetrafluoride (SiF 4 ) in silicon etching . Assuming a uniform supply of the etching gas, these processes are isotropic and, depending on the materials used, are sometimes highly material-selective (similar to wet chemical etching). The reactions are generally carried out in previously evacuated reactor chambers. The reaction gas is then introduced into the chamber for the process; the process pressure is approximately 100 Pa .

The etching process itself is basically as follows. The neutral atoms or molecules are passed through a plasma into the reaction chamber and flow over the substrate (e.g. silicon wafer ). There they react with the atoms on the surface. Volatile, gaseous reaction products are formed, which are sucked off by a vacuum pump.

One application in the past was the removal of photoresist by an oxygen plasma.

Physico-chemical dry etching process

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

The physical-chemical dry etching processes are combinations of physical and chemical dry etching processes. They are of great importance in the production of modern integrated circuits and micromechanical systems, since very fine and also deep structures can be produced with them. In turn, it is important for the etching process that gaseous, volatile reaction products arise.

The starting materials are usually activated or radicalized via a plasma and then passed onto the substrate for the reaction. This can be done either by convection or by electrostatic acceleration of the ions via an applied electric field. Due to the diverse possibilities of plasma generation and particle acceleration, a large number of processes, some of which are very similar, have emerged. The main ones are currently (2008) the reactive ion etching (Engl. Reactive ion etching , RIE), whose progression to reactive Ionentiefenätzen (Engl. Deep reactive ion etching , DRIE), reactive ion beam etching (Engl. Reactive ion beam etching ) and the HDP - Etching (from English high-density plasma etching ).

literature

  • Gary S. May, Simon M. Sze : Fundamentals of Semiconductor Fabrication . Wiley & Sons, 2003, ISBN 0-471-45238-6 .
  • Dietrich Widmann, Hermann Mader, Hans Friedrich: Technology of Integrated Circuits . Springer, Berlin, ISBN 3-540-66199-9 .

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