Bulk Micromachining is the process of sculpting features in bulk of materials
such as silicon, quartz, etc. by treatment with either orientation dependent
(anisotropic) or by orientation independent (isotropic) etchants. It is
widely used to fabricate membranes, beams, holes and grooves and other features.
In silicon micromachining, the bulk micromachining starts with a single
crystal silicon substrate (usually a (100) oriented wafer) on which a thin
film of material that is inert to chemical etchants (etch stop) is deposited.
The film is then selectively patterned photolithographically to allow the
removal of undesired portions of the etch stop film. The bulk material is
then etched using either wet chemical etching or dry etching, or both, depending
on the requirements.
i. Wet Etching (KOH Etching):
Wet etching a (100) oriented Si wafer with KOH etches
rectangular pits into the surface with the bottom of the well formed from
the (100) plane and the sides from four <111> planes. The fabrication
starts (i) with a silicon wafer which is (ii) oxidized in steam to form
a dense, thick (0.5 µ) coating of SiO2. Patterning the surface photolithographically
(iii) and selective removal of the oxide exposes a bare Si surface which
is etched with hot KOH to yield the aforementioned rectangular pits (views
parallel, perpendicular and obliquely, to the plane of the wafer in iv,
v and vi, respectively).
Etching of (100) Silicon Wafer
Deep Reactive Ion Etching (DRIE):
Dry Etching techniques (Plasma etching) are used for etching features with
variable tapering and high aspect ratio microstructures. The most common
forms of dry etching for micromachining applications are isotropic ion etching
and anisotropic deep reactive ion etching (DRIE). Deep channels and pits
(up to few tens of microns deep) with nearly vertical walls and of arbitrary
shape can be etched. Unlike anisotropic wet etching, DRIE etching is not
controlled by the relative etch rates of the silicon crystal planes.
Silicon surface micromachining uses the same equipment and processes as
the electronics semiconductor industry. This technique deposits layers of
sacrificial and structural material on the surface of a silicon wafer. As
each layer is deposited it is patterned, leaving material only where the
designer wishes. When the sacrificial material is removed, completely formed
and assembled mechanical devices are left.
The process of transferring a pattern from a photomask on to
the Silicon wafer is called photolithography. Silicon wafer is oxidized
in steam to form a dense, thick (0.5 µ) layer of SiO2,
followed by coating a thin layer of photoresist. Portions of the photoresist
are then exposed to UV radiation through photomask (quartz plate) with a
pattern. Exposed photoresist is rendered soluble and removed with a developer
solution. The photoresist pattern is used as a mask to etch the exposed
SiO2 layer. The SiO2 pattern
is then used to etch and sculpt the silicon with either wet or dry etching.
Bonding of two substrates or micromachined silicon wafers is
desired in certain processes to form complex devices and structures (such
as valves, pumps, etc). By applying heat, high electric field or pressure
or a combination of the above to two materials, a hermetic, high-strength
bond can be obtained.
Types of bonding:
· Fusion bonding (Si - Si)
· Anodic bonding (Si - Glass) and
· Adhesive, Eutetic, etc.
Dicing procedures involve slicing/cutting silicon wafers with microprocessor
controlled high speed diamond saws. Depending upon the material and requirement,
kerf may be as small as 20 µ, and dies may be diced as small as .01"
The packaging of the micro structures or devices meets the following requirements:
· Enable the user to handle the device easily
· Protects the device in harsh environments
· Prevents it from mechanical damage, chemical attack, or high temperatures