Matrix Assisted Laser Transfer of Electronic Materials for Direct Write
Applications
R.C.Y. AUYEUNG*, H.D. WU*, R. MODI**, A. PIQUÉ, J.M. FITZ-GERALD, H.D. YOUNG*, S.
LAKEOU*, R. CHUNG† and D.B. CHRISEY
Naval Research Laboratory, Code 6372, Washington DC 20375
*SFA Inc., 9315 Largo Drive West, Largo, MD 20774
**George Washington University, Department of Mechanical and Aerospace
Engineering, Washington DC 20052
†
Geo-Centers, Inc., 1801 Rockville Pike, Rockville, MD 20852
A novel laser-based direct-write technique, called Matrix Assisted Pulsed Laser Evaporation Direct
Write (MAPLE-DW), has been developed for the rapid prototyping of electronic devices. MAPLEDW is a maskless deposition process operating under ambient conditions which allows for the rapid
fabrication of complex patterns of electronic materials. The technique utilizes a laser transparent
substrate with one side coated with a matrix of the materials of interest mixed with an organic vehicle.
The laser is focussed through the transparent substrate onto the matrix coating which aids in
transferring the materials of interest to an acceptor substrate placed parallel to the matrix surface. With
MAPLE-DW, diverse materials including metals, dielectrics, ferroelectrics, ferrites and polymers have
been transferred onto various acceptor substrates. The capability for laser-modifying the surface of the
acceptor substrate and laser-post-processing the transferred material has been demonstrated as well.
This simple yet powerful technique has been used to fabricate passive thin film electronic components
such as resistors, capacitors and metal lines with good functional properties. An overview of these key
results along with a discussion of their materials and properties characterization will be presented.
Keywords: direct write, laser forward transfer, matrix assisted pulsed laser evaporation (MAPLE),
matrix assisted pulsed laser evaporation direct write (MAPLE-DW), rapid prototyping, electronics
devices.
1. Introduction
There is a strong need in industry for rapid prototyping
and manufacturing of passive electronic components on
various substrates in the mescoscopic regime (micron to mm
range). This capability is required in order to fabricate
smaller and more versatile electronics devices, to iteratively
measure the performance of circuits too difficult to model
and to quickly design and test circuits without a timeconsuming photolithographic mask process.
Direct-write technologies provide a flexible, maskless
and efficient technique for depositing a wide variety of
materials under ambient conditions which can be easily
integrated into a CAD/CAM system. Direct-write methods
do not compete directly with photolithography in
manufacturing sub-micron size components, but are
intended to supplant current surface mount technologies in
the mesoscopic regime.
Various direct-write technologies have been developed
in the last few years. Some of these techniques include
micropen, inkjet, plasma spray, focused ion beam, e-beam
and other liquid microdispensing approaches. Laser-based
direct-write techniques1 include Laser Induced Forward
Transfer (LIFT), laser CVD, laser particle guidance and
Matrix Assisted Pulsed Laser Evaporation Direct Write
(MAPLE-DW).
In this paper, the principles of the MAPLE-DW
technique will be described and results from some of the
fabricated structures will be presented.
2. Background
Ever since the invention of the laser in 1960, much
research has been directed towards the interaction of laser
radiation with materials. Its high coherence, directionality
and brightness properties make laser radiation ideal for
materials processing applications. These same properties
also make it ideal for direct-write applications where a
1
highly controlled source of energy with well-defined spatial
qualities can be used to process materials in a specific
location and time interval. By choosing the correct laser
wavelength, fluence and optical beam delivery system, many
types of materials can be deposited at room temperature
under ambient conditions with feature sizes in the micron
range. Various laser-based direct-write techniques have
been used to deposit metals2 and dielectrics3 onto various
substrates with varying degrees of success for each.
A promising new direct-write technique that allows the
deposition of almost any kind of material onto any substrate
type and shape is the MAPLE-DW process4. This technique
combines aspects of both the MAPLE5 and LIFT6 processes
into a simple but versatile direct-write technology. In the
MAPLE process, large and fragile organic molecules to be
deposited into thin film form are first dissolved in a dilute
(usually frozen) matrix target. Then, a low fluence UV laser
beam strikes the target to gently warm the solvent matrix to
gently release the organic molecule while the now volatile
solvent molecules are removed by vacuum pumping. This
pulsed thermal process heats the surface to below the
decomposition temperature of the organic molecules while
the solvent matrix is heated to its evaporation point. The
combined action of the evaporating matrix desorbs the large
species intact and deposits them as uniform thin films.
LIFT is a simple pyrolytic direct-write technique where
focused laser radiation vaporizes a thin film coating
(<2000 Å) on one side of an optically transparent support.
The vaporized material is transferred onto a substrate placed
adjacent to this coating. Compared to MAPLE, LIFT
requires a higher laser fluence because it must remove the
thin film from its support by physical vapor deposition. For
this reason and to maintain good lateral resolution, the thin
film coating on the support is usually less than 1000 Å thick.
Thicker depositions would require time-consuming iterative
steps. Because the laser radiation tends to atomize the thin
coating, LIFT is best suited for metals since multicomponent
crystalline materials would not be able to be transferred
intact. Other disadvantages of LIFT include poor spatial
resolution, adhesion and morphology.
MAPLE-DW takes the experimental support of LIFT
and combines it with the matrix desorption mechanism of
MAPLE into a versatile direct-write process that takes place
at room temperature. The support is transparent at the laser
wavelength and consists of a coating of the materials of
interest dispersed in a matrix. The matrix can consist of
organic solvents, binders, dyes or other components, which
aid in the transfer and particle bonding process. Ideally,
when the laser beam strikes this coating and transfers it to
the substrate, most of the matrix components should be
removed, easily decomposed or evaporated after the
transfer. An important benefit of MAPLE-DW is that unlike
other approaches, this technique does not change the
properties of the material of interest after the transfer.7
Another
advantage
of
MAPLE-DW,
which
distinguishes it from other techniques, is that it can operate
additively or subtractively. The presence of the laser beam
allows in-situ, pre- or post-processing of the transferred
material and substrate. For example, the laser can be used
to pre-clean the substrate surface, micromachine vias or
channels, trim circuit elements, or sinter the deposited
material.
The MAPLE-DW process has been used in this work to
successfully deposit metal lines, resistors and capacitor
structures with good properties on various substrates.
3. Experimental Procedure
The experimental setup used in MAPLE-DW is very
similar to that used in micromachining or the LIFT process.
A UV laser operating at 355 nm with a 5 ns pulsewidth is
focused onto a UV fused silica disk which acts as the
optically transparent support. This disk is coated on one
side with the material of interest and its corresponding
matrix. The disk is placed with its coating side adjacent to
the receiving substrate to within tens of microns gap
separation. The entire disk and substrate is placed on a
computer-controlled X-Y table which is synchronized to the
output of the laser. The optical delivery system can
generate UV focal spots from 10 to 300 micron diameter
and fluences over 5 J/cm2. The UV beam energies were
monitored continuously during each deposition by an in-line
beam-splitter and focal burn patterns were recorded on
polyimide or UV photosensitive film. A secondary IR
Nd:YAG laser operating in the free-running mode provides
1.06 µm pulses at 10-40 Hz with a 50 µs width for postprocessing the transferred material. The table speed can be
adjusted so that the IR focal spots overlap accordingly to
give the required sintering dwell times.
Materials were transferred onto various substrates such
as glass, alumina and polyimide. By optimizing the
deposition parameters, deposited materials showed good
morphology, linewidth and functional properties. Metal
lines were transferred by MAPLE-DW between two
photolithographically patterned Au electrode pads. An ebeam deposited gold line served as a reference on each
sample. A standard 4—point probe DC measurement was
used to characterize the resistivity of these lines. Resistor
lines were transferred by direct-write between two Au
electrode pads separated by 700 µm and its electrical
properties characterized by both a 4-point probe
measurement and an HP4291B impedance analyzer.
Dielectrics were evaluated by transferring a (~0.7x0.8 mm)
pad of material over a previously patterned interdigitated
capacitor structure (IDC). This structure allowed quick
characterization of dielectric material properties and avoids
the complication of depositing extra metal pad layers as
would be found in a parallel-plate capacitor structure. The
capacitance and loss tangent values of the capacitors were
measured on an HP4284A LCR meter up to 1 MHz and on
the impedance analyzer from 1 MHz to 1.8 GHz. Values
measured at 1 MHz agreed within error on both systems and
2
provide confidence in our measurement technique.
All samples were evaluated by optical and scanning
electron microscopy (SEM). Both surface and fracture
cross-sectional SEM’s were performed on the different
substrates.
4. Results and Discussion
Thickness (m)
3
2
1
0
Fig. 2 SEM fracture cross-section of Ag line on alumina
processed by a 1.06 m laser.
is not unique to polymer resistors as it is also observed in
cermet resistor material formulations8. The oscillations at
the highest frequencies are due to resonance effects from the
test leads and sample during the experiment. Note the good
agreement between the measured sheet resistance values at 1
MHz and those actually specified for the starting material.
This agreement demonstrates again that the MAPLE-DW
process does not significantly alter the intrinsic properties of
the material after transfer.
The dielectric material barium titanate (BTO) was
transferred onto pre-fabricated IDC’s on polyimide and
alumina substrates. As shown in Fig. 4, an SEM crosssection of the transfer shows that a high packing density was
achieved.
The measured capacitance and loss tangent of a BTO
IDC from 1 MHz to 1.8 GHz are shown in Fig. 5. The
measured dielectric constant r of 70 and loss tangent of 4 %
(after correcting for the substrate) at 1 MHz is significantly
below the corresponding values for the powder starting
material. Due to the high dependence of the dielectric
constant on the porosity of the dielectric material9, it is
possible that the air volume fraction in the BTO transfer is
still sufficient to dramatically lower the r value.
Sheet Resistance @ 16 (/sq)
Silver lines were transferred onto polyimide substrates
and after a furnace anneal at 300 C to react the metal
precursor, the resistivities were measured to be 1.1 to 1.6X
bulk Ag. A photograph of the line as well as profilometer
scans across 6 different locations along a 7 mm-long
segment are shown in Fig. 1. The lineshape definition is
good with minimal debris as deposited. Only a single laser
pass was used to deposit these 2.5 µm (average) thick lines
and the 40-µm linewidth was comparable to the laser spot
size. Silver lines were also deposited onto alumina and
glass substrates with somewhat higher resistivities. A scotch
tape test of the Ag line on glass left it intact which
demonstrates the excellent adhesion of the transferred
material.
Laser sintering was attempted on some of the higher
resistivity silver lines with mixed results. At low levels of
IR power absorption, only the surface silver layer showed
signs of annealing or melting as shown in Fig. 2. When the
IR laser power was increased, the silver line tended to
“bead” or eventually delaminate from the substrate surface.
The thermal diffusion depth for silver is over 10 m for a
1 s laser pulse so it is possible that sufficient porosity
remains in the cross-section preventing efficient thermal
conduction of the heat from the surface layer to the entire
volume.
Polymer resistor lines were transferred between two Au
pads on alumina substrates. By changing the formulations
of the starting resistor material, nearly 4 orders of magnitude
change in the sheet resistance was obtained in the transferred
lines as shown in Fig. 3. The drop-off at higher frequencies
50
100
Scan Length (m)
150
200
Fig. 1 Profilometer scans across a 7 mm-long Ag line
segment deposited on polyimide. The inset shows an
optical micrograph of the 40 m wide line.
10 5
100 k/sq
10
4
10 k/sq
10 3 1 k/sq
10 2
100 /sq
10 1 10 /sq
10 0
101
102
Frequency (MHz)
103
104
Fig. 3 Frequency behavior of sheet resistance of 5
different resistor lines made by MAPLE-DW.
3
r

0.1
70
0.08
65
0.06
60
0.04
55
0.02
50
0
Fig.4 SEM fracture cross-section of BTO deposited on
an interdigitated capacitor finger on polyimide.
0.5
1
1.5
Frequency (GHz)
2
tan
75
0
Fig.5 Frequency dependence of the dielectric
constant and loss tangent of a BTO IDC
deposited by MAPLE-DW on alumina.
5. Summary
In conclusion, we have demonstrated the versatility and
strength of the MAPLE-DW technique as a rapid directwrite technology in depositing good-quality electronics
materials under ambient environmental conditions. Silver
lines with bulk-like conductivities have been deposited onto
polyimide with a short furnace treatment as the only postprocessing step. Resistor lines with sheet resistances of 4
decades have been obtained by only changing the
composition of the starting material. Barium titanate
compositions have been transferred as pads and their
measured dielectric constants and loss show good promise as
useful capacitor structures.
In addition, we have shown that MAPLE-DW can
deposit different types of materials onto various substrates
such as polyimide, metals, glass and alumina.
The
usefulness of this technique has been strongly demonstrated
in this paper with high-quality material properties obtained
directly with a simple transfer and furnace treatment step.
No other pre- or post-processing procedures were used.
Improvements in the materials formulations as well as a
deeper understanding of the mechanism of the MAPLE-DW
process should bring further success of this technology as a
rapid prototyping tool.
6. Acknowledgements
We gratefully acknowledge support for this work by the
DARPA-MICE program and the Office of Naval Research.
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