MAPLE DIRECT WRITE: A NEW APPROACH TO FABRICATE
FERROELECTRIC THIN FILM DEVICES IN AIR AT ROOM
TEMPERATURE
J.M. Fitz-Gerald1, H.D. Wu2, A. Pique1, J.S. Horwitz1, R.C.Y.
Auyeung2, W. Chang3, W.J. Kim2, and D.B. Chrisey1
1
Naval Research Laboratory, Washington, D.C.
2
SFA, Inc., Largo, MD
3
George Washington University, Washington, D.C.
ABSTRACT
MAPLE Direct Write or MAPLE DW is a new direct writing
technique which combines some of the major positive advantages of
laser induced forward transfer and matrix assisted pulsed laser
evaporation (MAPLE). This novel laser driven direct writing
technique has demonstrated the deposition of metal, ceramic, and
polymer materials in air and at room temperature and with sub-10
micron resolution for active and passive prototype circuit elements on
planar and nonplanar substrates. Here, we have used MAPLE DW to
synthesize ferroelectric devices for tunable microwave device
applications. For ferroelectrics and the other functional materials,
MAPLE DW can be used to investigate directly the properties of
powder materials as a function of grain size, doping, and processing
technique. The best properties obtained for Ba0.5Sr0.5TiO3 and BaTiO3
films were dielectric constants of 40, with loss tangents of about 0.025
with ~ 1% tuning.
Keywords
MAPLE DW; LIFT; MAPLE; Excimer Laser; Ribbon; Grain Size;
Matrix
INTRODUCTION
The use of electronic systems has increased by orders of magnitude in
the last ten years. However, the demand for faster and smaller
products has placed an enormous emphasis on miniaturization and
increased functionality. In order to fabricate electronic assemblies
with reduced weight, volume, cost, and time, new materials and / or
methods to process them must be developed.
Until now limited progress in this area has been achieved with
surface mounted electronic components. This process is based on an
old technology that relies on a three-fold strategy of first designing,
then patterning, and finally mounting each of the system components
on a circuit board. Modifications of device performance require a time
consuming iteration of the above steps. Rapid prototyping provides a
solution to these time consuming problems. With rapid prototyping
processes, it is possible to direct write elements and components onto
any surface called for in the circuit design, essentially eliminating the
above steps and time constraints. Furthermore, the fact that these
techniques are compatible with current computer software for
integrated design and manufacture (CAD/CAM) is an added
advantage.
By utilizing the positive advantages of laser induced forward
transfer (LIFT) and matrix assisted pulsed laser evaporation
(MAPLE), a novel laser driven direct write technique has been
developed. This technique is termed MAPLE Direct Write (MAPLEDW). In this paper, we will outline the approach to the MAPLE-DW
process along with its advantages. We will show experimental results
for ferroelectric single layer capacitors, electronic multi-layer devices,
conducting lines, resistors, and in-situ laser annealing. Discussion of
device properties will be limited to the area of ferroelectric materials
and devices.
BACKGROUND
Laser Induced Forward Transfer (LIFT)
Over the past decade, many direct write techniques based on laserinduced processes have been developed for depositing materials for a
variety of applications. Among these techniques, laser induced forward
transfer (LIFT) has shown the ability to direct write metals for
interconnects and mask repair and also simple dielectric materials such
as metal oxides. LIFT was first demonstrated using metals such as Cu
and Ag over substrates such as silicon and fused silica utilizing
excimer or Nd: YAG lasers [1-2]. LIFT is a simple technique that
employs laser radiation to transfer a thin film from an optically
transparent support onto a substrate placed parallel to it. Patterning is
achieved by moving the laser beam (or substrate) or by pattern
projection. The former is a method of direct writing patterns. In order
to utilize LIFT there are several experimental requirements that can
have significant effects on the quality of the pattern or device
fabrication. The laser fluence should be adjusted so that the process is
carried out near the energy threshold to transfer only the film material.
Target films typically do not exceed a thickness of a few 1000Å.
Distances between the target film and the substrate must be controlled,
generally 25-75 µm.
Overall, LIFT has proven to be a simple technique that can be
used on a wide variety of metals, and some simple oxides, but not with
complex multi-component materials such as ferroelectrics.
Matrix Assisted Pulsed Laser Evaporation (MAPLE)
A new, laser assisted, vacuum deposition technique, known as matrix
assisted pulsed laser evaporation (MAPLE) [3] has been developed at
NRL for depositing thin, uniform layers of chemoselective polymers
[4,5,6] as well as other organic materials, such as carbohydrates [7].
This hybrid laser transfer mechanism associated with MAPLE enables
the deposition of complex organic molecules into thin films that is not
possible by conventional PLD processing.
In MAPLE, an organic compound is dissolved in a matrix
material, generally a volatile solvent such as alcohol to form a
solution. This solution is frozen to ~ 77K in the form of a laser target
(2.5 cm diameter disk, 1 cm thick). When the laser strikes the surface
of the target, it causes rapid vaporization of the solvent molecules.
Part of the thermal energy acquired by the solvent is transferred to the
organic molecules. When these molecules become exposed to the gastarget interface, they are transported into the gas phase with sufficient
kinetic energy to be desorbed from the target surface without being
denatured in the process. A film will be formed on a substrate placed
opposite to the target, while the solvent is pumped away.
MAPLE Direct Write
The MAPLE-DW technique [8] utilizes all of the advantages
associated with LIFT and MAPLE to produce a laser driven direct
write technique capable of transferring materials such as metals,
ceramics and polymers onto polymeric, metallic and ceramic
substrates at room temperature.
Overall resolution for this technique at this time is on the order
of 10µm. Since the MAPLE-DW process uses a highly focused laser
beam, it can easily be utilized for micromachining, drilling and
trimming applications, by simply removing the ribbon from the laser
path; this flexibility allows the synthesis of multi-layered structures
and patterning. Thus, MAPLE-DW is both an additive as well as
subtractive direct write process.
EXPERIMENTAL DETAILS
The key to the MAPLE-DW process is the development of a suitable
matrix containing the material to be transferred. This matrix is then
used to form a very uniform coating on the surface of a transparent
substrate, i.e., the ribbon. The matrix is chosen so that it strongly
absorbs the laser wavelength being used; generally, laser fluences
below the ablation threshold of the matrix material are used. The
purpose of the matrix is, in part, to hold the material in place until
heating from the laser pulse causes the matrix to decompose resulting
on the material being ejected from the ribbon and transferred to the
receiving substrate.
Because the laser fluence employed is selected to be lower than
the ablation threshold of the material being transferred, no
decomposition takes place, so the functionality of the transferred
material is never affected. Similarly, MAPLE-DW requires that the
ribbon be held in close proximity to the substrate. In MAPLE-DW,
the area coated per laser pulse also depends on the size of the laser
spot striking the ribbon as well as the gap between the ribbon and the
substrate. Figure 1 shows a simple schematic diagram of a MAPLEDW system.
Laser Forward
Transferred Material
UV Transparent
“Ribbon”
Pulsed Laser
Energy
Micromachined
Channel
Receiving
Substrate
Material to
be Deposited
Focusing
Objectives
Micromachined
Through-Vias
Figure 1. Schematic diagram, illustrating the basic elements of a
MAPLE-DW system. CAD/CAM control enables multi-axis motion
for complex design, features and micromachining.
This work was performed with a pulsed excimer laser (KrF mixture,
=248 nm), so fused silica quartz discs 5.0 cm diameter x 2 mm thick
were used as ribbon supports due to their low absorption at 248 nm.
Table I list the details concerning the experimental parameters for the
results to be discussed. Various substrates were used for the transfer
experiments including silicon, glass, polyimide and various types of
circuit boards such as FR-4 and Rogers RO4003.
TABLE I Experimental Device Fabrication Parameters
Devices
Transfer
Process
Material
Ribbon
Thickness
Spot Size
(µm)
Energy
mJ/cm2
Au Lines
Resistors
Capacitors
(tri-layer)
Inductor
LIFT
LIFT
MAPLE-DW
1600Å
2000 Å
3-5 µm
25
25
25
550
1500
400
3-5 µm
25
500
Interdigit.
Capacitors
MAPLE-DW
Cr, Au
Ni-Cr
BTO
(BaTiO3)
YIG
(Y3Fe5O12)
BSTO
(Ba0.5Sr0.5TiO3)
3-5 µm
200
500
MAPLE-DW
In all the transfer experiments, the coated side of the ribbon
was separated by  25 µm from the receiving substrate. Both the
substrate and ribbons were held in place using a vacuum chuck with a
x-y translation stage. The output from a KrF excimer laser was
directed through a circular aperture and then through a 10x UV grade
objective lens. By changing the aperture size, beam spots from 8 m
to 200 m were generated. The laser fluence was estimated by
averaging the total energy of the incident beam over the irradiated
area.
EXPERIMENTAL RESULTS
Conductive Lines and Resistor Elements
Using the gold coated ribbons, Au conducting lines were deposited by
the LIFT technique as shown in Figure 2(a). In order to improve the
morphology of the transferred gold it was necessary to operate at laser
fluences only slightly above the ablation threshold of the gold films.
At higher fluences, any part of the laser pulse that is not absorbed by
the Au layer on the ribbon can interact with the transferred gold over
the substrate and ablate it off the substrate. Furthermore, multiple
passes were required in order to build the gold lines to the desired
thickness of 10 µm. The average resistivity of these lines was
measured to be 75  cm at room temperature, which is about 30
times higher than that of bulk Au (2.4  cm).
The five coplanar resistors shown in Figure 2(b) were made
using nichrome ribbons. The overlap between successive passes was
optimized in order to improve the uniformity of the nichrome
structures. The final thickness of the nichrome resistors was about 10
µm. The measured resistances ranged from 65 to 190  and their
properties scaled with respect to cross section and length as expected.
The resistivity of the transferred nichrome was considerably larger
than that of bulk and is likely due to the high degree of porosity
present in the transferred nichrome as well as oxidation of the alloy
during transfer.
(a)
125 m
(b)
400 m
400 m
Figure 2. (a) Au lines deposited by LIFT on RO4003 circuit board.
The Au line width is approximately 30 µm after a final laser trimming
process, which was performed along both sides of the line, (b) shows a
micrograph of the nichrome coplanar resistors after processing.
Multi-Layer Capacitors and Inductors
For the fabrication of the capacitors and inductors a hybrid approach
was used. First, a 3 µm thick gold layer was e-beam deposited onto a
bare Rogers RO4003 substrate, which is a hydrocarbon ceramic
composite circuit board used for RF applications. The bottom
electrodes were then patterned with the laser. The ablation patterning
was performed with a 25 µm laser spot and a fluence of 3 J/cm2. Then
a ferroelectric layer consisting of BaTiO3 (BTO) in the case of the
capacitors or a ferrite layer consisting of Y3Fe5O12 (YIG) in the case of
the inductor was deposited by MAPLE-DW. Finally, the top Au
electrodes were deposited by LIFT using the same conditions
employed for making the conduction lines. Figure 3 illustrates the
above steps schematically.
1)
2)
Patterning of Botton MAPLE-DW of
Au Electrode
Dielectric
3)
LIFT of Top Au
Electrode
Figure 3. Schematic showing the fabrication steps for the parallel plate
capacitors. (1) Pattern the bottom Au electrode, (2) MAPLE-DW of
the dielectric layer, (3) LIFT of top Au electrode.
In both cases the morphology and thickness of the BTO and YIG
layers was quite uniform, and the surface roughness variations were
due primarily to the imperfections of the underlying substrate.
The parallel plate capacitors, as shown in Figure 4(a), were
evaluated from frequencies ranging from 1 MHz up to 1.8 GHz using a
HP4291A impedance analyzer. The capacitance ratio between the
large and small capacitors was close to their area ratio (4:1) as
expected with some variations attributed to non-uniformities on the
BTO transfers. Capacitance of all capacitors ranged from 2 and 40 pF
and dissipation factors between 0.11 to 0.17. These capacitors were
then annealed in a furnace at 200 °C for two hours. After the
annealing step, the capacitance’s dropped by about 40% while the
dissipation factors decreased by an order of magnitude. From these
results, the effective dielectric constant of the capacitors was estimated
to be around 25 after the annealing step.
The YIG core inductor (four turn) had a measured inductance
of 9 nH at 1 MHz. The inductor exhibited very high losses and the
effective permeability was estimated to be about 70. This result can be
attributed to the fact that the fabricated inductor had a porous YIG core
creating a large number of air gaps. Figure 4(b) shows a micrograph
of the inductor.
(a)
1.8 mm
(b)
125 m
Figure 4. (a) BaTiO3 capacitors with Au electrodes made by MAPLEDW. The larger capacitor was 1.8 mm x 1.8 mm, the other is 25%
smaller, (b) Four turn inductor with YIG core fabricated by MAPLEDW.
Interdigitated Capacitors
Interdigitated capacitors were fabricated on MgO substrates by
photolithography and metal lift-off patterning using a multi-level resist
process. To minimize the metallic losses in the interdigitated electrode
structure at microwave frequencies, a 2.5-µm thick layer of Ag,
followed by a thin Au layer, which preserves the surface for electrical
contact, was employed. A variety of device geometries were
fabricated with finger lengths varying from 40 to 80 µm, and finger
gaps varying from 3 to 9 µm with the finger width fixed at 13 µm.
Note that a small finger gap will enhance the relative dc bias electric
field strength for dielectric tuning, and a short finger length is used to
fit the pitch size of the microwave probe. A thick layer of ferroelectric
material was then deposited on top of the interdigitated electrodes
using MAPLE DW. In order to cover the large area of the electrodes a
200 µm laser spot size was used. Figure 5 shows the device structure
before and after the MAPLE DW process.
(a)
20 m
(b)
20m
Figure 5. (a) Interdigitated capacitor structure on MgO prior to
MAPLE DW processing, (b) interdigitated structure after MAPLE DW
transfer of BSTO across the electrode structures.
DISCUSSION
The microwave reflection measurements (S11) of the interdigitated
capacitors were performed using a HP 8510C vector network analyzer
and Cascade Microtech probe station with a 200-µm pitch microwave
probe. DC bias was applied to the capacitor under test through the
internal bias of the network analyzer test set. The capacitance and
quality factor (Q) were determined as a function of frequency and bias
field from the measured S11 parameters by fitting it to a parallel
resistor-capacitor model. Finally, the dielectric constant and loss
tangent (tan) of the MAPLE DW ferroelectric material were
calculated by conformal mapping techniques which convert the
interdigitated geometry to a parallel-plate capacitor structure.
Figure 6 (a) and (b) show the x-ray diffraction (XRD) patterns
of precursor powder for the BTO and BSTO ribbons, respectively.
The peak splitting of the BTO XRD patterns clearly indicate the
tetragonally distorted perovskite structure, which is the room
temperature phase of BTO. Figure 6 (b) shows no apparent splitting,
suggesting a nearly cubic room temperature phase of BSTO.
Figure 7 (a) and (b) show the dielectric constant and Q versus
frequency for the MAPLE DW (BTO) film (8.8 µm thick) with no dc
bias and with 40 V dc bias voltage. This film shows a dielectric
constant of 30, a Q of 32 (i.e., tan = 0.031) at 10 GHz, and a
dielectric tuning of ~1.5% with 135 kV/cm dc bias field.
Figure 8(a) and (b) show the dielectric constant and Q vs.
frequency for the MAPLE DW (BSTO) film (3.5 µm in thickness)
with no dc bias and with 40 V dc bias voltage. This film shows a
dielectric constant of 22, a Q of ~100 (i.e., tan ~ 0.01) at 10 GHz, and
no dielectric tuning at 80 kV/cm dc bias field.
The low dielectric constant observed for the MAPLE DW film
structures can be explained by the small grain size powders used for
the precursor materials. The dielectric constant is lower due to the
high porosity exhibited by the films, in addition to the residual
polymer that was retained during the matrix transfer.
20
(300)
(113) (311)
78
(103) (310)
76
(222)
(003)
74
(212) (221)
(202) (220)
72
(003) (300)
(112) (211)
(102) (201)
(111)
(110)
40
50
60
70
80
(311)
(310)
(300)
(221)
(210)
(220)
(211)
(200)
Ba0.5Sr0.5TiO3
(111)
30
(002) (200)
BaTiO3
(101) (110)
(001) (100)
(b)
(100)
Intensity (arb. units)
(a)
90
2q (degrees)
Figure 6. XRD patterns of (a) BTO and (b) BSTO ribbons used for
MAPLE DW.
The lowering of the dielectric constant by the porosity and residual
polymer in the film is a complicated circuit structure made up of
multiple capacitors in series and in parallel but can be accurately
predicted by the logarithmic mixing rule. The functional dependencies
of the logarithmic mixing is shown in Figure 9. It has been verified
experimentally by McNeal et al. [9] that the observed mixing behavior
of the BTO powder-polymer composite follows the logarithmic mixing
rule. Also shown in Fig. 9 are two other common dielectric mixing
rules, namely the parallel and series mixing rules.
Dielectric Constant
31
0 kV/cm
135 kV/cm
a)
30
29
5
10
15
Frequency (GHz)
50
20
b)
0 kV/cm
135 kV/cm
45
Q
40
35
30
25
5
10
15
Frequency (GHz)
20
Figure 7. (a) Dielectric constant and (b) Q-factor for the BTO film
measured from 5 GHz to 20 GHz at room temperature.
Dielectric Constant
23
0 kV/cm
80 kV/cm
22
a)
21
20
19
5
10
15
Frequency (GHz)
200
20
b)
Q
150
100
50
0 kV/cm
80 kV/cm
0
5
10
15
Frequency (GHz)
20
Figure 8. (a) Dielectric constant and (b) Q-factor for the BSTO film
measured from 45 MHz to 20 GHz at room temperature.
4
Dielectric Constant
10
Parallel
Mixing Rule
3
10
2
Logarithmic
Mixing Rule
10
Series
Mixing Rule
10
1
0
0.2
0.4
0.6
0.8
1
Volume Fraction of BTO
Figure 9. Dielectric constant of BTO powder-air composites for three
different mixing rules.
In Figure 9, we assume the BTO powder has a dielectric constant the
same as the bulk BTO ceramic of 2000 at room temperature. Figure 9
shows that to achieve a dielectric constant of 40, the volume fraction
of BTO must be 50 %.
CONCLUSIONS
We have demonstrated the efficacy of a novel direct write technique
termed MAPLE DW. This technique allows the direct writing of
various materials including metals, ceramics, and electronic materials
into device configurations for resistors, capacitors, inductors, and
conductive lines. In this paper, BSTO and BTO ferroelectric thin film
capacitor devices were fabricated using an interdigitated geometry.
Initial results show that properties of the particulate precursors namely
grain size and polymers used in ribbon fabrication, greatly affect the
performance of the devices. Dielectric constants ranged from 22-30,
with loss tangents from 30-100 at X-band. Future research will be
focused on reducing the porosity and the amount of polymer present in
the devices as well as improving the properties of the precursor
powders.
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