An MD&DI January 1997 Feature Article
Highly innovative microfabrication techniques have emerged from the
laboratory environment during the last decade, creating a new method for
developing and producing microstructures and tiny microsystems. What began about
20 years ago with the three-dimensional micromachining of silicon wafers has
since become a technology that holds much promise for the medical device
industry. Today, thousands of pressure or acceleration sensors can be batch
processed from a single silicon wafer, and numerous applications for
microfabrication techniques have been identified, including sensing or actuating
principles for mechanical, optical, or fluidic functions. For instance,
structures with micron features and tolerances in the submicron range are being
used in optical systems as waveguides, switches, or connectors, and as
read-write heads in miniaturized disk drives, and microstructured orifices are
used for ink-jet printing and fuel injection applications.
The medical industry has certainly benefited from spinoffs from other
industries. For example, disposable micromachined sensors are now being used to
monitor blood pressure in a patient's IV line or to support treatment after a
brain trauma. Among the medical applications that involve microfabrication and
assembly techniques are drug-delivery systems that use micropumps or flow
restrictors to precisely administer medicine over time, microneedles that are
used as medical implants to stimulate nerves or as ultrasharp lancets for
less-painful blood sampling, and micronozzles that are key in atomizing aerosols
with droplets that are several microns in diameter and allow accurate control of
metered dose inhalation. Additional applications are automated in vitro
diagnostic systems that use disposable microstructures for defining capillary
flow paths, mixing structures, reaction chambers, and structures that support
their assembly. Flexible endoscopes or catheters developed for brain surgery
also use microstructured components that can integrate multiple sensing and
working functions. With these sorts of advances, it seems the real impact of new
applications for microfabrication technology is just beginning to be realized.
Microfabrication techniques are no longer limited to silicon machining.
Techniques used today range from various types of laser machining to UV
lithography to newer techniques like high-aspect-ratio micromolding. Determining
which method or combination of methods to use for a particular application
depends on a variety of factors, several of which are listed in Table I (below) along
with current micromachining techniques.
||Materials Typically Used
||Smallest Lateral Aspect (µm)||Aspect Ratio (height/
|Surface Roughness (µm)||Design Freedom||Prototyping (Simple Geometries)||Prototyping (Complex Geometries)||Mass Fabrication (Simple Geometries)||Mass Fabrication (Complex Geometries)|
|Plastics, metals||< 1
||< 15||> 0.20||1||3, 4||1, 2||1-3||1|
(x-ray lithography, electroform-
|Plastics, metals||< 1||< 15 (for molding)||0.02-0.03||2||3, 4||2, 3||2,
graphy (and electroforming)
|Plastics, metals||2||< 5||> 0.03||2||2||1, 2||2||2
|< 1||< 40||> 0.03||3||2||4||1||2, 3|
|Dry etching||Silicon, metals, plastics, ceramics||< 1||< 10||> 0.03||3||2||3, 4||2, 3||2, 3|
|Excimer laser||Metals, polymers||1||< 10||> 0.1||2, 3||1, 2||1, 2||2||2, 3|
|Metals, polymers, ceramics||25||<
|> 0.2||4||1||4, 5||1||3, 4|
|Metals||40||< 3||> 0.1||3, 4||2||3, 4||3||5|
|Metals, plastics||20||< 5||> 0.1||3, 4||2||3||3||5|
|*Includes LIGA or different
micromachining techniques that can be combined to generate the tool insert for
the succeeding molding step.
Table I. Micromachining techniques with corresponding information
regarding their effectiveness in various practical applications. (Scale for
columns 6-10: 1 = very effective; 5 = ineffective.)
High-Aspect-Ratio Microreplication (HARM). HARM is a process that
involves micromachining as a tooling step followed by injection molding or
embossing and, if required, by electroforming to replicate microstructures in
metal from molded parts. It is one of the most attractive technologies for
replicating microstructures at a high performance-to-cost ratio. In this
approach, a microstructured preform is defined in a polymer or soft metal and is
replicated by electroforming into a tool insert. This insert, or an array of
inserts, is then used in the succeeding molding step.
Products micromachined with this technique include fluidic structures such
as molded orifice plates for ink-jet printing and microchannel plates for
disposable assays used in various diagnostic applications. The materials that
can be used are electroformable metals and plastics, including polysulfone,
acrylate, polycarbonate, polyimide, and styrene. Additional materials that
customers may suggest need to be qualified by tests.
The most challenging features to manufacture with any technique are
high-aspect-ratio microstructures with structural aspects that can be as small
as a few microns in the two axes of the plane and up to several hundred microns
deep. Molding an array of several hundred thousand posts or holes with a minimum
post diameter of 2 µm and a structural height starting at 20 µm isn't
an easy task. Doing it consistently in high volume while maintaining quality is
the most challenging part, and success is mostly determined by the precision of
the tool inserts.
LIGA. An important tooling and replication method for high-aspect-ratio microstructures is called LIGA, which is a German acronym for
deep-etch lithography, electroforming, and molding. The technique employs x-ray
synchrotron radiation to expose thick acrylic resist (polymethylmethacrylate)
under a lithographic mask (see Figure 1 below). The exposed areas are chemically
dissolved and, in areas where the material is removed, metal is electroformed,
thereby defining the tool insert for the succeeding molding step.
Figure 1. The LIGA technique.
Other Combined Techniques. Other microreplication techniques can be
combined to generate a preform for the tool insert. These include laser
ablation, multiple-step optical (UV) lithography, and mechanical micromachining,
which includes electrodischarge machining (EDM) and diamond milling. EDM uses a
spark erosion technique, while diamond milling uses highly accurate, preshaped
diamond geometries. This mix of techniques provides the freedom to develop and
design geometries for a wide range of customer-specific design requirements.
Designs may include stepped features, parallel lines, and tapered or curved
slopes. Additionally, special alignment helps and interconnecting bridging
structures can be integrated to interface with conventional industrial
assembling and handling techniques.
The molding process itself is also demanding since it is difficult to fill
the small, high-aspect-ratio features without leaving cavities. After the
ejection of the molded parts, an additional step can be added by electroforming
metal into cavities to replicate the structure in metal, or to define
electrically conductive areas. Different from conventional tooling and molding,
the microreplication techniques require a more extensive feasibility and design
phase up front in order to avoid high costs for multiple redesigns.
Bulk and Surface Machining. Two micromachining techniques often
used are bulk machining and surface machining. Bulk machining is a subtractive
process that uses wet anisotropic etchingwhich depends on the crystal
orientation of siliconor a dry etching method such as reactive ion etching
(RIE). Materials typically used for wet etching are silicon and quartz, while
dry etching is generally used with silicon, metals, plastics, and ceramics.
Typical features for sensing or fluidic structures that can be created using
bulk machining include geometries such as membranes, beams, holes, or grooves.
In addition to bulk machining, surface machiningwhich is an additive process
used to deposit several layers onto a silicon wafer, including sacrificial
layers that are then selectively etchedcan be used to combine different layers
to add sensing functions such as measuring temperature, magnetic fields, or
While silicon is a well-known material preferred in applications that
combine its electrical performance as a semiconductor with its excellent
mechanical properties (for instance, high tensile strength, hardness,
elasticity, and low density), limiting factors can include lengthy processing
times and the relatively high cost of the substrate if a large area is required.
In some medical applications, such as with micropumps or valves, the brittleness
of the material may limit its usefulness as well.
Laser Machining. The first use of lasers in industrial
manufacturing processes began more than 25 years ago. Today, lasers are
increasingly used for precise welding and cutting, and for structuring many
polymers, metals, and especially hard materials. Recent technological advances
have significantly improved laser performance, reliability, and cost. Better
optics and the development of a line-narrowed microlithographic excimer laser,
for example, have increased the precision and flexibility in combining various
geometries. Applications for laser machining vary, ranging from uses in
prototyping to drilling holes in flow restrictors and in catheters for liquid or
The integration of internal halogen generators, higher pulse rates, and
improved corrosion-resistant materials have pushed the processing costs of
excimer laser equipment to less than $1 for 250,000 laser pulses. A new
application for ink-jet printing has just recently become available in which 100
to 300 nozzles with hole diameters as low as 20 µm are drilled in a field
of 8 X 20 mm with precisely defined hole shapes and submicron tolerances.
A good overview of laser manufacturing is provided in Ronald Schaeffer's
article in the November 1996 issue of MD&DI. The article addresses
the advantages associated with laser machining and also details the most common
industrial lasers available: carbon dioxide (CO2),
solid-state (Nd:YAG), and excimer lasers. While these techniques are not
specifically addressed here, they have been included in Table I.
The demand for diagnostic and analytical equipment that is smaller and
performs faster is a major growth area in today's industry that involves
microstructure technology. A fast and reliable response is essential, for
instance, to measure blood parameters in emergency situations and during
surgery, and to test for drugs of abuse at a crime scene. In point-of-care
situations such as in a physician's office or in home-care applications, the use
of on-site tests is currently limited because of the complexity of test
procedures or the limited reliability of existing disposable tests. This is why
tests for infectious diseases, therapeutic drugs, drugs of abuse, immunology,
allergy, and tumors are typically performed in centralized laboratories on
advanced instruments operated by skilled personnel.
The types of diagnostic testing mentioned above often require sophisticated
instrumentation and multiple-step procedures, including sample dilution,
variable incubation times, and wash steps. Most of these functions can be
incorporated by microfabrication techniques that already have the ability to
improve the performance of disposable assays. New tests are currently being
developed in which microreplicated capillary structures are used to improve the
performance of disposable assays for the exam- ination of human fluids.
Extremely precise geometries allow for exact control of the volume flow, the
timing of reactions between the sample and predeposited reagents, the separation
of cells, and the mixing of different substances.
This postage-stamp-size spectrometer is microreplicated in polymer as a complete optical bench and mounted on a chip.
Some additional advantages of microfabrication result from the drastically
reduced sample and reagent consumption, the low-power operation feasible with
microfluid components, and the reduction of analysis time resulting from the
short diffusion zones inside the miniaturized fluid system. Other developments
include applications such as cell counting, cell separation, and the emerging
field of DNA sequencing. DNA bases, for example, can be incorporated into
microstructured capillaries where they are mixed with buffers, reagents, and a
torturous post matrix to help filter the fragments. Electric current sent
through a gel pulls the DNA fragments along, and different fragment sizes can
then be separated by the speed of movement into visible groups. The fragments
that are tagged with a visible dye can be finally distinguished with a closely
located miniaturized spectrometer.
Micromachining and microreplication techniques have been qualified for
high-series production in many applications, providing the key to integrating
the intelligence of electronic circuits with sensing and actuating functions to
produce complete microsytems. The advantages are evident not only in the
reduction of size, but also in the increase of functional performance and
reliability, and a unit-cost reduction in high-volume batch processing. Because
of high capital investments for expensive manufacturing equipment,
microstructures or microsystems are successful in applications where the
performance-to-cost ratio of the system increases significantly and high
production volumes offset the up-front investments. And as the medical and
diagnostic industry continues to benefit from breakthroughs in other application
areas, it is expected to be one of the fastest growing areas for
Photos courtesy of American Laubscher Corp.