Dry Etching

In wafer fabrication, etching refers to a process by which material is removed from the wafer, i.e., either from the silicon substrate itself or from any film or layer of material on the wafer.  There are two major types of etching: dry etching and wet etching. 

Dry Etching is an etching process that does not utilize any liquid chemicals or etchants to remove materials from the wafer, generating only volatile byproducts in the process. Dry etching may be accomplished by any of the following: 1) through chemical reactions that consume the material, using chemically reactive gases or plasma;  2) physical removal of the material, usually by momentum transfer; or 3) a combination of both physical removal and chemical reactions.

Plasma etching is an example of a purely chemical dry etching technique. On the other hand, physical sputtering and ion beam milling are examples of purely physical dry etching techniques.  Lastly, reactive ion etching is an example of dry etching that employs both physical and chemical processes.   

Like wet etching, dry etching also follows the resist mask patterns on the wafer, i.e., it only etches away materials that are not covered by mask material (and are therefore exposed to its etching species), while leaving areas covered by the masks almost (but not perfectly) intact.  These masks were deposited on the wafer by an earlier wafer fab step known as 'lithography.'

Plasma etching, a purely chemical dry etching technique, basically consists of the following steps:  1)  generation of reactive species in a plasma; 2)  diffusion of these species to the surface of the material being etched; 3) adsorption of these species on the surface; 4) occurrence of chemical reactions between the species and the material being etched, forming volatile byproducts; 5) desorption of the byproducts from the surface; and 6) diffusion of the desorbed byproducts into the bulk of the gas. 

Note that the desorption of the reaction byproducts from the surface of the material being plasma etched is just as important as the occurrence of the chemical reactions that consume the material.  If such desorption fails to occur, then etching can not take place even if the chemical reactions have been completed.  Thus, all the steps above must occur for the plasma etching process to be successful.

The selectivity of the species used in dry etching that employs chemical reactions is very important.  Selectivity refers to the ability of the reactive species to etch away only the material intended for removal, while leaving all other materials intact.  In particular, the species used must not attack the mask material over the material being etched as well as the material beneath it.

In general, the reactive species used in dry chemical etching must be selected so that the following criteria are met:  1) high selectivity against etching the mask material over the layer being etched; 2) high selectivity against etching the material under the layer being etched; 3) high etch rate for the material being removed; and 4) excellent etching uniformity. They should also allow a  safe, clean, and automation-ready etching process. 

Another important consideration in any etching process is its anisotropy, or property of etching in one direction only.  A completely anisotropic etching process that removes material in the vertical direction only is very desirable, since it will follow the mask patterns on the wafer very faithfully, leaving any material covered by mask material basically untouched. 

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

See Also:  Wet Etching;  Lithography/Etch;  Optical Lithography;  Electron Lithography

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