Dry Etching (Page 2 of 2): Reactive Ion Etching; Physical Sputtering

Dry Etching (Page 2 of 2)

Unfortunately, most etching techniques that employ purely chemical means to remove the material (whether through wet or dry etching) do not exhibit high anisotropy. This is because chemical reactions can and do occur in all directions. Thus, chemical reactions can attack in the horizontal direction and consume a portion of the material covered by the mask, a phenomenon known as 'undercutting.'

If maximum anisotropy is of utmost concern, then dry etching techniques that employ physical removal of material must be considered. One such technique is physical sputtering, which involves purely physical removal of material by bombarding it with highly energetic but chemically inert species or ions. These energetic ions collide with atoms of the material as they hit the material's surface, dislodging these atoms in the process.

Targeting the layer to be etched with incident ions that are perpendicular to its surface will ensure that only the material not covered by the mask will be removed. Unfortunately, such a purely physical process is also non-selective, i.e., it also attacks the mask layer covering the material being etched, since the mask is also directly hit by the bombarding species. For this reason, physical sputtering has never become popular as a dry etching technique for wafer fabrication.

A good balance between isotropy and selectivity may be achieved by employing both physical sputtering and chemical means in the same dry etching process. Reactive ion etching is one such process that involves both physical and chemical means to remove material.

Reactive ion etching (RIE), which is sometimes referred to as reactive sputter etching (RSE), consists of bombarding the material to be etched with highly energetic chemically reactive ions. Such bombardment with energetic ions dislodge atoms from the material (just like purely physical sputtering), in effect achieving material removal by sputtering.

In addition to sputter-removal, the bombarding ions used in RIE were chosen so that they will chemically react with the material being bombarded to produce highly volatile reaction byproducts that can simply be pumped out of the system. This is the reason why RIE is widely used in wafer fabrication - it achieves the required anisotropy (by means of sputter-removal) and the required selectivity (through chemical reactions). Table 1 presents some examples of the process gases usually employed in the reactive ion etching of common wafer materials.

Table 1. Examples of Gases Used in the RIE of Common Wafer Materials

Material to be Etched	Examples of Gases Used in the RIE
Polysilicon	CF₄; SF₆; Cl₂; CCl₃F; etc. (w/ or w/o oxygen)
Al; Al doped with Si, Cu, Ti	CCl₄; CCl₄+Cl₂; BCl₃; BCl₃+Cl₂
Tungsten	Fluorinated Gases
Refractory Silicides	Fluorinated plus Chlorinated Gases (w/ or w/o oxygen)
TiN; TiC	Same as Al Etch

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