Matrix Assisted Pulsed Laser Evaporation Direct Write
(MAPLE DW): A New Method to Write Electronic Circuits
and Biosensor Arrays
D.B. Chrisey, A. Pique, R.C.Y. Auyeung, H.D. Wu, J.M. Fitz-Gerald, R.
Chung, R. Mohdi, D. Young, D. Weir, and B.R. Ringeisen
Naval Research Laboratory
Washington, D.C. 20375
Chrisey@ccf.nrl.navy.mil
An Article for Laser Focus World
The interaction of high repetition rate pulsed UV lasers with novel materials
results in the forward transfer of electronic materials and biomaterials into
technologically useful patterns and arrays
In the electronics industry, the continued miniaturization of individual
components and subsystems has largely overlooked mesoscale passive
devices (mesoscale devices are typically 10 m – 1 mm) and conformal
electronics due to difficulties in their fabrication as well poor performance.
Recently, developments in materials and laser-materials processing have
allowed a novel UV laser-based direct write technique, termed MAPLE DW
for Matrix Assisted Pulsed Laser Evaporation Direct Write, to simplify the
fabrication of high quality mesoscale electronic components at reduced
costs. In parallel to the materials advances, the novel technique that is being
developed at the Naval Research Laboratory (NRL), is finding unique
applications in the direct writing of biomaterials for tissue engineering and
array-based biosensor applications. When the MAPLE DW process is fully
optimized, the commercial system built by NRL’s commercial partner,
Potomac Photonics, will result in: deposition of finer features, minimal
process variation, lower prototyping and production cost, higher
manufacturing yields, decreased prototyping and production time, greater
manufacturing flexibility, and reduced capital investments.
MAPLE DW, shown schematically in Figure 1, combines the
technical approach of LIFT, or Laser Induced Forward Transfer, with the
soft transfer process of the vacuum-based MAPLE process. In this
technique, the output of a high repetition rate 355 nm UV laser is focused
through a transparent support and onto a 5-10 m matrix-based coating (see
Figure 2). The transparent support and coating is commonly referred to as
the MAPLE DW ribbon in analogy to a conventional typewriter. This
coating is transferred to the receiving substrate and, with some minimal
subsequent thermal processing, forms an adherent film with comparable
electronic properties to similar devices fabricated by typical thick film
approaches, e.g., screen printing. Electronic circuit patterns are formed by
synchronously moving the ribbon to a fresh, unexposed region and moving
the receiving substrate approximately one beam diameter.
Typical
resolutions demonstrated are < 10 m. In this way, MAPLE DW assembles
individual mesoscopic bricks of electronic material, one with each laser shot,
into the desired pattern as shown in Figure 3. When the ribbon is rapidly
changed from a metal, to a dielectric, to a metal again, parallel plate
capacitors or other 3-dimensional structures can be made. When the ribbon
is removed, the MAPLE DW system has all the attributes of a laser micro
machining. Thus, we can etch grooves or vias in the substrate, perform
surface pre-cleaning, or even surface anneal or etch individual components
to improve their performance or specification accuracy, respectively (see the
substrate in Figure 1).
There are many different CAD/CAM approaches to direct write or
transfer patterned materials including plasma-spray, laser particle guidance,
micropen, ink jet, e-beam, focused ion beam, and several novel liquid or
droplet microdispensing approaches. One theme common to all techniques
is their dependence on high quality starting materials, typically with
specially tailored chemistries and/or rheological properties. The starting
materials can include combinations of powders, nanopowders, flakes,
modified surface coatings and properties, organic precursors, binders,
vehicles, solvents, dispersants, surfactants, etc. This wide variety of
materials with applications as conductors, resistors, and capacitors are being
developed specifically for low processing temperatures (< 300 - 400C).
These will allow fabrication of passive electronic components and RF
devices with the performance of conventional thick film materials, but on
low temperature flexible substrates, e.g., plastics, paper, fabrics, etc. Figure
4 shows the spectrum of temperatures and some of the approximate chemical
and physical process temperatures relevant to direct write technologies.
Fabricating high quality crystalline materials at ~400C is nearly
impossible. Just by combining spherical powders with a narrow size
distribution, the highest possible packing density is ~74% for the facecentered cubic structure, i.e., 26% air. This amount of air reduces the
effective dielectric constant of an insulator by almost an order-of-magnitude
highlighting the importance of reducing the porosity in transferred materials.
Figure 5 shows the difficulty involved in forming a high-density coating
with spherical powders. Particle-particle bonding is even more difficult at
low temperatures. One strategy to overcome this liability is to begin with a
high density packed powder of differing particle sizes combined with
chemical precursors that form low melting point nanoparticles in situ to
chemically weld the powder together. Figure 6A shows the excellent
uniformity and packing that can be obtained by MAPLE DW. As far as can
be seen in either direction on this SEM, the optimized MAPLE DW process
has packed the particles into a dense composite. Part of optimizing the
MAPLE DW process is to optimize the relative ratios of powders,
nanoparticles and chemical precursors. In Figure 6B there is less, in this
case titania, chemical precursor present compared to 6C. Except for the
cracking due, in part, to the thermal reaction of the precursor, Figure 6C is
much more densely packed, i.e., the air between the particles (k=1 for air) is
replaced with titania (k=100). The resulting increase in effective k of the
device was nearly a factor of 2.
To further improve the electronic properties for low temperature
processing, especially of the oxide ceramics, laser surface sintering is used
to enhance particle-particle bonding and reduce porosity. Figure 7 is a
schematic diagram illustrating how for 1 m IR annealing laser pulse, a set
of dichroic optics can be used. For IR laser wavelengths (1 – 10 m) the
penetration depth for different materials is on the order of microns allowing
the benefits of higher temperature processing on thermally sensitive, but
technologically important, substrates. On the left side of Figure 4, we
assume that laser annealing will result in an increase of about 200C, for a
depth of about 3 m, and for about 100 sec in temporal extent. Comparing
this increment in temperature with the physical properties on the right side
of the figure, laser annealing should come close to allowing reacted
precursors on biological substrates and some level of bulk diffusion on
polyimide susbstrates.
Figure 8 shows a cross-section of the many different types of passive
electronic devices that can be fabricated by MAPLE DW. Simple
interconnects with conductivities close to 2 times bulk Ag, phosphor arrays
for displays, nichrome coplanar resistors or polymer thick film resistors
whose resistance can be accurately predicted over 5 decades, capacitors with
dielectric constants between 4 and 100 and with low losses (<1%), and
three-dimensional multi-turn yttrium iron garnet core inductors; all of these
devices can be rapidly made using a CAD file by MAPLE DW.
Applications or benefits of mesoscopic conformal passive electronics
fabricated include: reduced size and weight, improved performance and
robustness, and rapid prototyping for circuit modeling, design, and testing.
The individual devices shown in Figure 8 are impressive because they
demonstrate the ability to go from a concept to a working prototype in a very
short time. The current state-of-the-art for write speed is 200 mm/sec., so
individual MAPLE DW devices can be made very fast. The real
technological impact for this process is seen when multiple materials are
contained in a single CAD file representing a sophisticate “electronic
subsystem”. In Figure 9 we show an example of 600 MHz oscillator
designed, modeled, fabricated and tested at NRL on a low thermal budget,
flexible polyimide substrate. This device represents a 6 process level
electronic subsystem utilizing the unique electronic properties of 4 different
materials and it was fabricated entirely by MAPLE DW.
There are many practical and exciting applications for a technique like
MAPLE DW, but there are some that cannot be anticipated. Being able to
direct write electronic materials on living biological substrates or specimens
is a totally unexplored area. There is a need to electrically interface to the
zoological world to better understand it, but also to manipulate it. One
example is to train and use adult worker honey bees (Apis Mellifera) to
detect extremely low levels of dangerous chemicals (unexploded ordinance,
environmental toxins, etc.). In order to locate the chemicals over the several
square kilometers that the bee typically covers and not perturb is flight, an
extremely small antenna must be attached and there are unsuccessful
examples of gluing macroscopic ones onto bees. As shown in Figure 10,
with MAPLE DW we can reduce the size of antenna to its fundamental
electromagnetic limit. What is shown in this figure is an approximately 1
mm on edge fractal reflecting antenna; on the left is the CAD design, and on
the right it is written onto the abdomen of a dead honey bee. Fractal
antennas are interesting in this application because the pseudo-random bends
reduce the size and weight while providing multi-band capability. While the
abdomen is a large area, it is extremely curved and this requires that we go
back and micro-machine the surface area that should not be coated. This is
also apparent from the figure. We have also demonstrated a similar size
coating on an anestitized bee, though it was more difficult due to the
breathing through small spiracles in its abdomen.
Advances in our understanding of new materials and laser-material
interaction have driven the progress seen in the direct writing of electronic
materials, but the recent advances in the direct writing of biomaterials has
been driven by advances in the UV laser-based transfer technology; in
particular, recognizing that MAPLE DW could be an extremely gentle
transfer process. At the Naval Research Laboratory, using ribbons made of
an aqueous composite mixture patterns of viable E. coli bacteria have been
transferred onto various substrates with MAPLE DW. In Figure 11 we show
the results of the transfer spelling “NRL”. The bacterial have been labeled
with a green fluorescent protein, only visible under UV irradiation. It is
clear from this and other data not presented that MAPLE DW has the ability
to softly transfer viable biomaterials. Micron-scale patterns of viable cells,
and the methods to manufacture them, are required for next generation tissue
engineering, fabrication of cell-based microfluidic biosensor arrays, and
selective separation and culturing of microorganisms. There is no
technology capable of instantaneously writing adjacent patterns of different
viable cells from a multi-cellular ribbon or palette. It is envisioned that in
future work with MAPLE DW, we can create three-dimensional
mesoscopically-engineered structures of living cells, proteins, DNA strands
and antibodies as well as to co-fabricate electronic devices on the same
substrate to rapidly generate cell-based biosensors and bioelectronic
interfaces to probe, among other things, intercellular signaling. The
implications of Figure 11 represent a significant advance in biomaterial
processing and demonstrate the ability to use MAPLE DW to manipulate
natural systems.
Acknowledgements
We gratefully acknowledge support for this work from the Office of Naval
Research and the MICE Program in the Defense Advanced Projects
Research Agency.
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Figure Captions
Figure 1
Schematic diagram of the Matrix Assisted Pulsed Laser Direct Write (MAPLE
DW) approach to laser direct write circuit elements.
Figure 2
Temperature scale for direct writing materials by MAPLE DW. Lower
processing temperatures allow depositions on more labile and useful
substrates. Laser sintering provides reacted precursors on biomaterial
substrates and on polymer substrates like kapton it provides for some measure
of bulk sintering.
Figure 3
Basic mechanism of the MAPLE DW transfer process.
Figure 4
The interaction of the laser with the ribbon is the novelty in MAPLE DW.
Ribbons are a liability because they can be difficult to fabricate and Precursors
must transfer without significant decomposition. But the ribbon also
effectively quantizes the mater transferred making MAPLE DW coatings
highly reproducible. Each laser pulse deposits an identical mesoscopic ‘brick
of electronic material.
Figure 5
Schematic diagram of the UV MAPLE DW transfer and in situ IR laser
sintering. The benefits of in situ sintering include a smaller (thinner) region to
sinter (less organic to remove, less sensitive to the laser fluence), faster and
better alignment, less thermal stress to substrate, fewer steps and higher yield.
Figure 6
Necking of particles from precursor and/or laser sintering (particle-particle
adhesion). Optimized processing of a well-packed composite coating will
result in particle-particle adhesion or necking. Optimized processing of
resistor coatings will be more difficult to produce particle-particle adhesion
between conducting particles because there are fewer potential necking sites.
Figure 7
Fracture cross-section of optimized packing of BaTiO3 powder and precursor.
Figure 8
Under optimized MAPLE DW deposition conditions the conductivity on
kapton for 40 m wide lines processed at temperatures  300C yields lines
with 2.3 x bulk Ag resistivity.