Etching is used in microfabrication to chemically remove layers from the surface of a wafer
during manufacturing. Etching is a critically important process module, and every wafer
undergoes many etching steps before it is complete.
For many etch steps, part of the wafer is protected from the etchant by a "masking"
material which resists etching. In some cases, the masking material is a photoresist which
has been patterned using photolithography. Other situations require a more durable mask,
such as silicon nitride.
Figures of merit If the etch is intended to make a cavity in
a material, the depth of the cavity may be controlled approximately using the etching
time and the known etch rate. More often, though, etching must entirely remove the top
layer of a multilayer structure, without damaging the underlying or masking layers. The etching
system's ability to do this depends on the ratio of etch rates in the two materials.
Some etches undercut the masking layer and form cavities with sloping sidewalls. The
distance of undercutting is called bias. Etchants with large bias are called isotropic, because
they erode the substrate equally in all directions. Modern processes greatly prefer anisotropic
etches, because they produce sharp, well-controlled features.
Etching media and technology The two fundamental types of etchants are
liquid-phase and plasma-phase. Each of these exists in several varieties.
Wet etching
The first etching processes used liquid-phase etchants. The wafer can be immersed in a bath
of etchant, which must be agitated to achieve good process control. For instance, buffered
hydrofluoric acid is used commonly to etch silicon dioxide over a silicon substrate.
Different specialised etchants can be used to characterise the surface etched.
Wet etchants are usually isotropic, which leads to large bias when etching thick films.
They also require the disposal of large amounts of toxic waste. For these reasons, they are
seldom used in state-of-the-art processes. However, the photographic developer used for
photoresist resembles wet etching. As an alternative to immersion, single wafer
machines use the Bernoulli principle to employ a gas to cushion and protect one side of the
wafer while etchant is applied to the other side. It can be done to either the front side
or back side. The etch chemistry is dispensed on the top side when in the machine and the
bottom side is not affected. This etch method is particularly effective just before "backend"
processing, where wafers are normally very much thinner after wafer backgrinding, and
very sensitive to thermal or mechanical stress. Etching a thin layer of even a few micrometres
will remove microcracks produced during backgrinding resulting in the wafer having dramatically
increased strength and flexibility without breaking.
Anisotropic wet etching
Some wet etchants etch crystalline materials at very different rates depending upon which
crystal face is exposed. In single-crystal materials, this effect can allow very high
anisotropy, as shown in the figure. The term "crystallographic etching" is synonymous with
"anisotropic etching along crystal planes". However, for some non-crystal materials like
glass, there are unconventional ways to etch in an anisotropic manner. The authors employs
multistream laminar flow that contains etching non-etching solutions to fabricate a glass
groove. The etching solution at the center is flanked by non-etching solutions and the
area contacting etching solutions is limited by the surrounding non-etching solutions.
Thereby, the direction of etching is mainly vertical to the surface of glass. The SEM
images demonstrate the breaking of conventional theoretical limit of aspect ratio and contribute
a two-fold improvement. Several anisotropic wet etchants are available
for silicon, all of them hot aqueous caustics. For instance, potassium hydroxide displays
an etch rate selectivity 400 times higher in <100> crystal directions than in <111>
directions. EDP, displays a <100>/<111> selectivity of 17X, does not etch silicon dioxide as KOH
does, and also displays high selectivity between lightly doped and heavily boron-doped silicon.
Use of these etchants on wafers that already contain CMOS integrated circuits requires
protecting the circuitry. KOH may introduce mobile potassium ions into silicon dioxide,
and EDP is highly corrosive and carcinogenic, so care is required in their use. Tetramethylammonium
hydroxide presents a safer alternative than EDP, with a 37X selectivity between {100}
and {111} planes in silicon. Etching a silicon surface through a rectangular
hole in a masking material, for example a hole in a layer of silicon nitride, creates
a pit with flat sloping {111}-oriented sidewalls and a flat-oriented bottom. The {111}-oriented
sidewalls have an angle to the surface of the wafer of:
If the etching is continued "to completion", i.e. until the flat bottom disappears, the
pit becomes a trench with a V-shaped cross section. If the original rectangle was a perfect
square, the pit when etched to completion displays a pyramidal shape.
The undercut, δ, under an edge of the masking material is given by:
,
where Rxxx is the etch rate in the direction, T is the etch time, D is the etch depth and
S is the anisotropy of the material and etchant. Different etchants have different anisotropies.
Below is a table of common anisotropic etchants for silicon:
Plasma etching
Modern VLSI processes avoid wet etching, and use plasma etching instead. Plasma etchers
can operate in several modes by adjusting the parameters of the plasma. Ordinary plasma
etching operates between 0.1 and 5 Torr. The plasma produces energetic free radicals, neutrally
charged, that react at the surface of the wafer. Since neutral particles attack the
wafer from all angles, this process is isotropic. Plasma etching can be isotropic, i.e., exhibiting
a lateral undercut rate on a patterned surface approximately the same as its downward etch
rate, or can be anisotropic, i.e., exhibiting a smaller lateral undercut rate than its downward
etch rate. Such anisotropy is maximized in deep reactive ion etching. The use of the
term anisotropy for plasma etching should not be conflated with the use of the same
term when referring to orientation-dependent etching.
The source gas for the plasma usually contains small molecules rich in chlorine or fluorine.
For instance, carbon tetrachloride etches silicon and aluminium, and trifluoromethane
etches silicon dioxide and silicon nitride. A plasma containing oxygen is used to oxidize
photoresist and facilitate its removal. Ion milling, or sputter etching, uses lower
pressures, often as low as 10−4 Torr. It bombards the wafer with energetic ions of
noble gases, often Ar+, which knock atoms from the substrate by transferring momentum.
Because the etching is performed by ions, which approach the wafer approximately from
one direction, this process is highly anisotropic. On the other hand, it tends to display poor
selectivity. Reactive-ion etching operates under conditions intermediate between sputter
and plasma etching. Deep reactive-ion etching modifies the RIE technique to produce deep,
narrow features. Common etch processes used in microfabrication
See also Chemical-Mechanical Polishing
Ingot sawing References
Jaeger, Richard C.. "Lithography". Introduction to Microelectronic Fabrication. Upper Saddle
River: Prentice Hall. ISBN 0-201-44494-1. Ibid, "Processes for MicroElectroMechanical
Systems" Inline references
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