SGIA Journal Graphic November/December 2016 - 24

This report describes how MEMS
techniques work to make inkjet printheads.
It describes new inkjet printhead models
and the advantages of using MEMS
manufacturing methods.
MEMS & Inkjet Printhead
Developments

MEMS, is technology that fabricates
small devices at the micrometer scale, i.e.
technically creating detail from a micrometer
or micron (µm) to a millimeter (mm) in
size. In practice, MEMS manufacturing
processes are yielding more densely packed
transistors, sensors and printheads with
sub-micron feature sizes smaller than 500
nanometers (nm), and thus may carry the
label NEMS, nano-electro-mechanical
systems. Photolithography, a.k.a. optical
lithography, used in some MEMS/NEMS
processing can define features as small as
10 nm.1 The ability of photolithography
and other MEMS/NEMS techniques
to produce very small parts can yield
advantages that smallness can provide,
such as greater nozzle density, improved
sensitivity, response speed and resonant
frequency performance.
MEMS is used to make nozzles for
inkjet nozzle plates, holes, manifolds
and channels that are part of inkjet head
structures. MEMS inkjet heads may use
some of the MEMS techniques including
photolithography, laser ablation, molding
and plating, modified semiconductor
fabrication technology, wet etching and dry
etching, and electro discharge machining
(EDM). 2 MEMS/NEMS technologies
machine a number of materials for inkjet
printhead manufacture including silicon,
nickel, stainless steel and other metals.
MEMS fabrication techniques3 include:
* Bulk Micromachining: A substrate
(typically silicon) is selectively etched
to yield a microstructure.4
◊ Anisotropic wet etching
(AWE) is orientation dependent
and is typically applied to a
silicon wafer creating a cavity
below the areas on the unmasked
areas of the silicon's surface,
to which the etch is applied.
The orientation of the silicon's
crystalline structure will inform
the shape of the cavity. Miller
indices describe the orientation
of crystalline structures using
numbers in parenthesis to
indicate crystalline structure
orientation configuration.5
Anisotropic wet etching of
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November/December 2016

a silicon wafer with a (100)
orientation will yield a cavity
with a trapezoidal cross-section,
with walls angled at 54.7o to the
surface of the silicon wafer. On
the other hand, anisotropic wet
etching of a silicon wafer with
a (110) orientation can yield a
cavity with vertical walls. (AWE
may also penetrate below the
mask at times as with isotropic
wet etching.) Some of the
anisotropic wet etching agents for
silicon are potassium hydroxide
(KOH), ethylenediamine
pyrocatechol (EDP), and
tetramethylammonium
hydroxide (TMAH). Anisotropic
wet etching is often used in
conjunction with Deep Reactive
Ion Etching (DRIE) with silicon
(100), where the AWE creates
the top surface etch and the
DRIE forms the vertical walls of
the etched cavity.
◊ Isotropic wet etching is not
dependent on the crystalline
orientation of the silicon
wafers. Typically, a mixture of
hydrofluoric acid, nitric acid, and
acetic acid (HNA) is the isotropic
wet etchant solvent for silicon.6
Silicon dioxide or silicon nitride
usually serves as the masking
material to resist the HNA etchant.
The isotropic wet etchant removes
the silicon both downward into
the material and laterally. It will
also typically undercut the masking
materials somewhat.
* Surface Micromachining: Several
layers of structural and sacrificial
material are deposited, typically by
vacuum techniques, and often at
high temperatures, then the sacrificial
material is etched.
* Wafer Bonding
* Deep Reactive Ion Etching (DRIE) of
Silicon7
* Deep Reactive Ion Etching (DRIE)
of Glass
* LIGA ("Lithographie
Galvanoformung Adformung" in
German): A thick photoresist is
exposed to energetic X-rays from a
synchrotron and developed, metal is
plated into cavities in the resist and
the resist is removed (the metal can
then be used for molding polymers).
* Hot Embossing
* XeF2 Dry Phase Etching
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SGIA Journal Graphic November/December 2016

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