U.S.A.
1
Anvik Corporation, 6 Skyline Dr., Hawthorne, NY 10532, U.S.A.
(Received February 2, 2006; accepted June 30, 2006; published online October 24, 2006)
Microcavity plasma devices with circular, crescent or, for example, trapezoidal cross-section microcavities (characteristic
dimension d ¼ 30 {100
m
m), produced by excimer laser ablation and overcoated with a silicon nitride barrier lm, have been
fabricated in Ni/30
m
m polyimide/3
m
m Cu layered substrates. 12
12 arrays of devices with cylindrical microcavities
100
m
m in diameter exhibit turn-on voltages of 255 270 V
rms
for a Ne pressure of 700 Torr and a sinusoidal excitation voltage
having a frequency of 5 20 kHz. All of the device designs explored to date operate in the abnormal glow region, and an
increase of 15 20% in the ignition voltage for these arrays is observed when pd is raised from 4 to 5 Torr cm. Tests in which
the arrays were intentionally damaged or photoablation parameters were altered from the optimal values show the
microplasma devices to be extraordinarily robust and insensitive to the cross-sectional shape of the microcavity.
[DOI:
10.1143/JJAP.45.8221
]
KEYWORDS: microplasma, photoablation, exible arrays, microcavities
1.
Introduction
Although the rst plasma devices with a sub-mm cathode
aperture were reported by White in 1959,
1)
a concerted eort
to pursue the properties of plasmas conned to cavities with
characteristic dimensions <500 mm began roughly a decade
ago. In the intervening years, it has become evident that such
microplasmas have properties attractive for applications as
diverse as the destruction of volatile organic compounds
2,3)
and the ecient generation of deep-ultraviolet (UV) or
vacuum ultraviolet (VUV) radiation.
4)
Details concerning
these and other emerging applications of microplasma
devices can be found in ref. 5.
Commercial adoption of microplasma technology will, to
a signicant degree, hinge on the development of device
structures that are robust and yet are fabricated in inexpensive
materials by processes amenable to large volume production
methods such as roll-to-roll processing. However, it is
customary in the literature of electronic and optical devices,
in general, to report structures that have been optimized,
leaving unanswered the question of potential degradations in
performance arising from the variations in processing con-
ditions that inevitably occur in manufacturing.
This paper presents the results of experiments character-
izing a microplasma device structure that has proven to be
exceptionally robust, despite processing conditions that are
far from optimal. With a dielectric barrier and microcavities
produced in thin polyimide layers on a Cu substrate by
excimer laser photoablation, these devices can be fabricated
on large sheets by roll-to-roll processing and yet are
insensitive to the microcavity cross-section.
2.
Device Structure and Fabrication Process
A cross-sectional diagram (not to scale) of the generalized
microcavity plasma device structure adopted for these
experiments is illustrated in Fig. 1. The exible and
inexpensive substrate for these devices comprises 3 5 mm
of Cu on
30 mm of Kapton. After micromachining in the
polyimide lm the device cavity of the desired cross-
sectional geometry by excimer laser (KrF) ablation, a 1.5 mm
thick silicon nitride lm was deposited within the cavity and
onto the surrounding Kapton surface by plasma-enhanced
chemical vapor deposition. Subsequently, a 0.1 0.15 mm
thick Ni lm was deposited around the perimeter of each
microcavity by electron beam evaporation and, for a number
of the microcavity plasma devices, a Ni screen electrode
6)
of
600 mesh and 17 mm in thickness was also bonded to the
device, as indicated in Fig. 1.
All of the microcavities were ablated with an Anvik Corp.
photolithography system, of which a portion is shown
schematically in Fig. 2. The deep ultraviolet ( ¼ 248:4 nm)
radiation from the excimer laser, which was operated at a
pulse repetition frequency (PRF) of 200 Hz, was directed
into an illumination system that both homogenizes the beam
and alters its cross-section. Specically, the rectangular
cross-section of the beam produced by the laser is trans-
formed into a hexagon-shaped radiation pattern with an
intensity prole that is uniform to within 5%. After passing
through the mask, the homogenized beam is imaged onto the
substrate with a 1 : 1 projection lens. For an energy uence
at the polymer/Cu surface of 200 400 mJ cm
2
, ablating
30 mm of polyimide requires no more than 200 laser pulses
and thus the laser dwell time per microcavity is typically
0.5 1 s. For most of the devices fabricated to date, the
ablation process was terminated at the polyimide/Cu inter-
face but, as suggested by Fig. 1, the microcavity for several
devices extended a short distance (11.5 mm) into the Cu
lm. It should also be emphasized that a system closely
related to that of Fig. 2 is the Anvik HexScan 3100 system
which is designed for roll-to-roll processing. Consequently,
although the experimental results reported here are associ-
ated with arrays fabricated on substrate areas as large as
100 cm
2
, the structure of Fig. 1 is designed to facilitate the
transition to high speed production of microcavity plasma
arrays in large exible sheets.
E-mail address: jgeden@uiuc.edu
y
Present address: Department of Electrical and Computer Engineering,
University of Illinois, 1406 W. Green St., Urbana, IL 61801, U.S.A.
z
Present address: Light Age, Inc., 500 Apgar Drive, Somerset, NJ 08873,
U.S.A.
Japanese Journal of Applied Physics
Vol. 45, No. 10B, 2006, pp. 82218224
#2006 The Japan Society of Applied Physics
8221 This system produces cavities of exceptional uniformity,
and clean sidewalls. Figure 3 presents two photographs of
completed arrays. At left, the exibility of this thin array
structure is evident. A 12 device segment (3
4) of a larger
array of Ni/silicon nitride/polyimide/Cu devices with
100 mm dia. cavities is shown by the optical micrograph on
the right side of Fig. 3.
Before leaving this section, it should be noted that the
structure of Fig. 1 is similar to that of the DC-driven metal/
polymer/metal devices rst described by Park et al.
7)
and
subsequently applied by Sankaran and Giapis
8)
to the etching
of Si. As will be evident later, the incorporation of the
dielectric barrier improves dramatically the ruggedness and
lifetime of these devices.
3.
Experimental Results and Discussion
3.1
Optical and electrical characteristics
An optical micrograph of four 6
6 arrays comprising
devices having the structure of Fig. 1 is presented in Fig. 4.
Shown operating in 500 Torr of Ne and driven by a 20 kHz
sinusoidal AC driving voltage, these devices were fabricated
in a 30 mm polyimide/3 mm Cu substrate and the 100 mm dia.
microcavities have a 200 mm pitch. Each microplasma
device produces a diuse glow but extension of the plasma
over a portion of the surface of the array is evident between
several of the devices in the subarrays. As one would expect,
this eect vanishes at lower current levels and optimization
of the microcavity aspect ratio (depth-to-diameter) and
dielectric thickness is expected to conne the microplasmas
within the cavities at higher current levels.
Figure 5 shows the voltagecurrent (VI) characteristics
for the array of Figs. 3 and 4. These data were acquired at a
Ne pressure (p
Ne
) of 700 Torr and sinusoidal driving voltage
frequencies of 5, 10, and 20 kHz. Current values were
inferred from the voltage generated across a small (710
)
resistor in series with the array. The lower set of (vertical)
arrows in the gure indicates the voltage (240
5 V
rms
) at
which the rst device in the array ignites for each excitation
frequency. The higher voltages denoted by the upper set of
(horizontal) arrows in Fig. 5 represent the all on voltage
Fig. 1.
Diagram (not to scale) of the microplasma
device structure in cross-section. A number of single
devices and large arrays were also fabricated with-
out the screen electrode. The thicknesses of the
silicon nitride and Ni lms were 1.5 mm and 0.1
0.15 mm, respectively.
Fig. 2.
Schematic diagram of the projection imaging, excimer laser photoablation system with which the microcavities were produced.
Throughout these experiments, the laser wavelength was 248.4 nm (KrF).
100
µm
Fig. 3.
Photographs of: (left) an array of microplasma devices, illustrating the exibility of the structure; (right) 3
4 segment of Ni/
silicon nitride/polyimide/Cu microcavity plasma devices that is a portion of a larger array. The cylindrical microcavities are 100 mm
in dia. and the apparent asymmetry of several of the devices shown is an artifact of the microscope and the area size being imaged.
Jpn. J. Appl. Phys., Vol. 45, No. 10B (2006)
S.-J. P
ARK
et al.
8222 which is measured to be 265
10 V
rms
. For driving voltages
below the ignition value, the array exhibits a large capacitive
impedance [ 1 M
at 10 kHz or C
array
¼ ð
17
1Þ pF]
which is to be expected in view of the metal lm/polymer/
metal layered structure into which the microcavities are
fabricated. Patterning the electrodes will undoubtedly lower
the array capacitance considerably. Notice that the impe-
dance of the array changes almost imperceptibly above
ignition for the 5 kHz data whereas, at a driving frequency of
20 kHz, a rapid decline in the overall impedance (device
structure plus plasma) is evident above the array ignition
voltage as the current is increased. Although the VI
characteristics have, to date, been examined over a small
range in current (<1 mA), it is clear that once th