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.
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;
physical removal of the material, usually by momentum transfer;
or 3) a combination of both physical removal and chemical reactions.
etching is an example of a purely chemical dry etching technique. On the
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
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
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
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.
of the species used in dry etching that employs chemical reactions is
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.
important consideration in any etching process is its
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
anisotropy is of utmost concern, then dry etching techniques that
employ physical removal of material must be considered. One such technique is
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.
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
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
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.
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.
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.
Examples of Gases Used in the RIE of Common Wafer Materials
Material to be Etched
Examples of Gases Used in the RIE
SF6; Cl2; CCl3F; etc. (w/ or w/o
doped with Si, Cu, Ti
Fluorinated plus Chlorinated Gases
(w/ or w/o oxygen)
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