Fabrication of Mo microcones for volcanostructured double-gate Spindt-type emitter
cathodes using triode high power pulsed
magnetron sputtering
Running title: Fabrication of Spindt emitter cathode by HPPMS
Running Authors: Nakano et al.

Takeo Nakano a), Tomoki Narita, Kei Oya
Seikei University, Department of Materials and Life Science, 3-3-1 Kichijoji-Kita, Musashino, Tokyo
180-8633, Japan

Masayoshi Nagao, Hisashi Ohsaki
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba,
Ibaraki 305-8568, Japana)Electronic mail: nakano@st.seikei.ac.jp

In this study, an array of Mo cones for volcano-structured Spindt-type micro electron
emitters were fabricated. A recently-developed triode high power pulsed magnetron
sputtering system was used to control the positive plasma potential and efficiently
accelerate ion species. By applying a proper positive voltage to the additional electrode,
we obtained good cone shapes with high aspect ratios in a water-cooled microcavity
structure made of two resist layers, which was previously impossible by conventional
vacuum evaporation techniques. The effects of ion acceleration on the alignment of ionsalong the normal direction, as well as on the stress in the deposited film, are discussed. The
former is important for the formation of sharp cones, while the latter is crucial for achieving
stable fabrication.

1

I. INTRODUCTION
A Spindt-type emitter is a kind of microfabricated vacuum electron emitter
originally developed by Spindt at the Stanford Research Institute.1 Each emitter consistsof a conical-structured cathode with a sharp tip to concentrate the electric field for
efficient emission of electrons. Spindt-type emitters have good electron emittercharacteristics (high current density, emission uniformity, and reliability), and are
expected to be applied in various devices, for example in the form of a field emitter array(FEA) in field emission displays.2
Conical cathodes for Spindt-type emitters have been fabricated via vacuum
evaporation of emitter material into a microcavity structure with holes of ~0.5 μm
diameter in its ceiling. Since the holes close gradually as the deposition proceeds, theemitter material forms conical structures by the end of the deposition process. In original
Spindt-type emitters, the cavity structure is made of an insulating oxide layer sandwiched
between metal layers. The bottom metal layer forms a cathode pattern, while the top
metal layer, which contains the holes, functions as gate electrode. The top surface of thecavity structure is further covered with an additional material used as a sacrificial layer inthe lift-off process applied after the emitter material deposition.
Recently, one of the authors proposed another configuration of this emitter,
named a “volcano-structured” Spindt-type emitter.3 In this process, the microcavity
structure is prepared by two resist layers: the top layer an ordinary photoresist, and thebottom layer a non-photosensitive “lift-off” resist soluble in the same solvent as the top
layer. After the gate hole pattern is exposed, the substrate is dipped into the solvent for a2

controlled period of time. Through this process, the bottom layer is isotropically etchedand the cavity structure formed. Subsequently, the emitter material is deposited to form
conical emitter structures inside the cavities. After deposition, the resist layers are
removed together with the overcoated emitter material by lift-off. Therefore, only theconical emitters are left on the substrate. The above fabrication process is summarized inFig. 1.
(1) Preparation of cavities
in double layers of resist(2) Metal deposition

Photoresist
LOR

Si

Si

(3) Deposition finished(4) Lift off resist cavity

Si

Si

FIG. 1. (Color online) Preparation process of Mo cones for volcano-structured Spindttype emitters. LOR in (1) denotes “lift-off” resist.

After preparing the emitters, the gate electrode is prepared using the etch-backprocess4 to form volcano-structured emitters. The double-gated volcano structure, which
consists of two electrodes for the extraction and focusing of electrons, can be prepared byrepeating the etch-back process. This structure has been proven to show excellent
electron emission characteristics,5,6 and is expected to be a crucial part of ultrahighsensitivity television cameras.7

3

Historically, the emitter cathode material has been deposited using vacuum
evaporation. For the effective and stable operation of the emitter at high emission current
densities, a refractory metal is the material of choice. However, refractory metals arenotorious for their very high tensile stress when deposited at low substrate temperatures8.
Therefore, to avoid delamination of the deposited material, the substrate has to be heatedto lower the tensile stress9. Since resist cavities cannot withstand such high substratetemperatures, as a compromise Ti3,10 and Ni6 emitters have been used as the cathodematerial for volcano-structured double-gate Spindt-type emitters instead. There is anotherdrawback of vacuum evaporation. It is relating to the requirement for the gradual closureof cavity holes to achieve sharp tip structures. To achieve this, the deposition flux has tobe highly aligned along the normal of the substrate, requiring a long source-to-substratedistance. As a result, the preparation system tends to be bulky, and large area FEAs are
hard to prepare.
To simultaneously overcome the above problems, we propose the use of ionizedvapor deposition.11,12 The ionized atoms can easily be accelerated by introducing a
potential drop between the plasma bulk and the substrate. By increasing the atomic
energy, various aspects of deposited films can be modified, including structure, density,
number of defects, and stress.13–15 For example, it is particularly well-known that
increasing the incident energy (or incident momentum) of depositing atoms results in thetransition of tensile to compressive stress.16,17 We can therefore expect that high tensilestress in refractory metal films can be lowered by using this technique even on low
temperature substrates.

4

In addition, the incident direction of depositing species can be aligned with thesubstrate normal by acceleration of ions in the sheath region in front of the substrate. In
fact, achieving this directivity was the original motivation for ionized physical vapor
deposition (IPVD) in order to improve the sidewall and bottom coverage in trenches andvias with high aspect ratios.11,18 The alignment of incident ions enables a shorter distance
between the source (target) and the substrate, as well as the production of larger-sized
FEAs.
We have previously attempted to fabricate Spindt-type emitter cathodes by using
a high power pulsed magnetron sputtering (HPPMS) technique. HPPMS is a variant of
IPVD that produces a high density plasma with short pulse powers to ionize sputteredatoms efficiently.19–21 The potential drop between the plasma bulk and the substrate is
generally realized by applying a negative bias to the substrate. If a positive plasma
potential can be controlled, on the other hand, a potential difference can be formed
between the plasma and a grounded substrate, which improves the design flexibility ofthe sputter deposition system. We previously proposed two techniques to achieve theabove.
One method is to introduce a positive target bias voltage during the pulse-off
period of HPPMS. It was shown that the plasma potential of the afterglow period wasraised, and the structure of the deposited films on a water-cooled room temperature
substrate could be modified.22 This technique was applied to Spindt-type emitter
fabrication, and Mo emitter structures of varying height could be prepared inside thecavities.3,9 It should be noted that DC operation using the same sputtering deposition

5

system resulted in very low structures due to the random incidence of sputtered atoms
and fast occlusion of the cavity holes.
Another method we developed more recently is the triode HPPMS system thatconsists of a chimney-like electrode on top of the sputter gun for control of the plasma
potential through the applied voltage. It was demonstrated that the plasma potential was
raised in accordance with the cap voltage during not only the afterglow period, but also
the pulse-on period. This potential difference between the plasma bulk and the substrate
throughout the discharge enabled effective use of ionized atoms. As a result, the film
structure could be modified using a significantly lower plasma potential (20-40 V)
compared with the pulse-off target bias method (~100 V).23
In the current study, we used triode HPPMS for the preparation of volcanostructured Spindt-type emitters using Mo. Several preparation conditions for Mo metal
deposition on the double-layered resist cavities were varied, such as the target to substratedistance, cap voltage, and discharge gas pressure. Obtained emitter structures wereobserved using a secondary electron microscope (SEM). Under specific conditions, sharp
emitter structures could be obtained, preferable for Spindt-type emitters.

II. EXPERIMENTAL
Microcavities with holes were fabricated on a silicon wafer using the double
layers of resists. The top photoresist layer was exposed to a hole pattern with 0.5 μm
diameter, and etched together with the bottom lift-off resist layer to form the cavities.
Details of the fabrication process are given in our previous report.10

6

Mo was deposited on top of the cavities using triode HPPMS. In a previous triode
system,23 we added a chimney-like electrode on top of the conventional sputter gun. In
this system, however, the anode cap of the magnetron sputter gun, which covers thetarget peripherally at a distance of ca. 0.5 mm from the target, was insulated from thegrounded chamber, and positive voltages Vc could be applied to it directly (Fig. 2). The
sputter gun was equipped with a Mo target of 2 inch in diameter and was located along
the symmetric axis of the sputter chamber, which was a cylinder with an inner diameterand height of 20.1 cm each. The chamber was routinely pumped down to 5×10-5 Pa prior
to the deposition.

Cooling
water
#1

#2

#3

#4

50～73 mmAnode Cap
Mo target
N

S

N

VC

FIG. 2. (Color online) Schematic of Triode high power pulsed magnetron sputtering(HPPMS) configuration. N and S in the figure denote the poles forming the magnetron.

7

The plasma discharge conditions were as follows: Ar gas pressures of 0.6-3.0 Pa,
pulse frequency of 200 Hz, and duty ratio of 5%. The cap voltage Vc was varied between
20-60 V. The time-averaged total electrical power supplied to the discharge volume (byboth the pulse power source connected to the target and the DC power supply connectedto the cap electrode) was set at 100 W using the same procedure as in our previous
report23. The plasma potential increase in accordance with Vc was checked using theLangmuir Probe (Scientific Systems, SmartProbe) before deposition experiments.
Si wafer with cavity structures were cut into substrate pieces 1 cm width and 2.54
cm length, and tightly bound to the substrate holder as shown in Fig. 2. The holder was
made of Cu and was cooled by circulating water. In most of the deposition runs, thecavity substrate was set at #2 position in Fig. 2, while a stock Si wafer substrate of thesame dimension was set at #3, mainly used to determine the deposited film thickness. The
holder was located 50 or 73 mm above the target surface. The typical deposition duration
was 90 minutes. After deposition, the resist layers were removed by dipping the sample
into an organic solvent (N-methyl-2-pyrrolidone; NMP) to obtain Mo conical structures.
Before and after the lift-off process, samples were cleaved and their cross-sections wereobserved using an SEM (JEOL, JSM-6510).

8

Plasma Potential (V)III. RESULTS AND DISCUSSION

20

10

0
0

10

20

30

Anode Cap Voltage VC (V)FIG. 3. Plasma potential vs. Anode cap voltage measured using the Langmuir probe. Gas
pressure was 1 Pa, and the potential was recorded 200 μs after ignition of the pulse
discharge using a 200 Hz frequency and 5% duty cycle.

Before depositions, plasma parameters were evaluated using time-resolved
Langmuir probe measurements. Instead of the substrate holder, the probe was inserted in
the chamber along the symmetric axis. The probe consisted of a tungsten wire of 0.38
mm diameter and 10 mm length, the remainder of which was shielded by an alumina
tube. The probe apex was located at 50 mm from the target surface. The measurement
details were the same as those described in our previous study.23 Figure 3 shows theobtained plasma potential as a function of the Vc at an Ar gas pressure of 1 Pa. The data
shown in Fig. 3 were recorded at 200 μs after plasma ignition (note that the high power
pulse duration was 250 μs). The plasma potential increased in accordance with Vc. At
lower cap voltages (Vc < 18 V), the increase of the potential was rather subtle, while it

9increased almost linearly at unity slope when the cap voltage exceeded 20 V. A similar
trend was reported with the prototype of the triode system equipped with the chimneylike electrode23. The electron density during the pulse-on period was 2-3×1017 m-3.

(a)

T-S 50 mm

5 μm

(b)

T-S 73 mm

5 μm

FIG. 4. Cross-sectional scanning electron microscope (SEM) images of Mo-deposited
samples on double resist cavity substrates at a (a) target to substrate (T-S) distance of 50
mm and a (b) T-S distance of 73 mm. The gas pressure was 1 Pa and the anode cap
voltage was 40 V.

Initially, we fixed the Ar gas pressure and pulse discharge conditions, and studied
the effects of the target to substrate (T-S) distance on the deposited emitter cathodestructure. Figure 4 shows the cross-sectional SEM images of deposited samples atdifferent T-S distances, both deposited using a cap voltage of 40 V. These images wereobserved for the samples cleaved before the lift-off process. The resist cavities in the caseof a 50 mm T-S, resist cavities were seriously deformed, as if they had melted during the10

deposition process (Fig. 4(a)). In this case, the shapes and heights of Mo cones were
found to be highly variable, which suggests random closure of cavity holes by thermal
load-induced transformation. This might be partly due to the slightly unbalanced
magnetron configuration of the sputter gun, by which we expected the spread of plasmanear the substrate surface, enhancing the directivity and energetic incidence of deposition
species.
By increasing the T-S distance, the deformation of the resist cavity was greatly
alleviated, as shown in Fig. 4(b). A certain deformation still remained, but thisdeformation seemed to occur at the cleavage stage, possibly due to stress in theovercoated Mo layer. It was also found that the uniformity of the Mo cone structures wasvery much improved. Therefore, we continued Mo deposition experiments mainly at a TS distance of 73 mm.

11

(a)
1.0 Pa VC = 20 V

1 μm

(b)
1.0 Pa VC = 40
20 V

1 μm

(c)
1.0 Pa VC = 60 V

1 μm

FIG. 5. Dependence of the Mo cone shape on the anode cap voltage.

The dependence of the Mo cone shape on the anode cap voltage was studied by
fixing the gas pressure at 1 Pa and the T-S distance at 73 mm. Figure 5 shows selected
SEM images of the cross-section of samples before the lift-off process. The structure of
the cones was also observed by SEM after the lift-off process, and the aspect ratios of thecones were measured using the SEM images. Aspect ratios were determined by dividing
the half height of the cone by the full width at the half maximum (FWHM), because therelative uncertainty in FWHM measurements was less than that using the base conediameters. Aspect ratios of 5-6 cones were measured in each sample, and the average and12standard deviation are plotted in Fig. 6. The results for samples deposited at a T-S of 50
mm and Vc of 40 V are also plotted, which shows a lower average height and poor shapeuniformity in comparison with cones prepared at a T-S of 73 mm.

Aspect Ratio

0.8
0.6
0.4
0.2

0

T-S 50 mm
T-S 73 mm
20

40

60

Anode Cap Voltage VC (V)FIG. 6. (Color online) Dependence of the aspect ratio of Mo cones on the anode cap
voltage. The gas pressure was fixed to 1 Pa.

Before entering into the detailed discussion of the effect of Vc , the growthmechanism of the emitter cones we are expecting under the triode HPPMS is described.
By increasing Vc , the larger plasma potential is achieved as demonstrated in Fig. 1. The
potential difference between the plasma bulk and the substrate is applied to the plasma
sheath formed in front of the substrate. In the HPPMS plasma, considerable part of the
sputtered Mo atoms are expected to be ionized, and accelerated by this sheath voltage.
This lead to the energetic impingement of these ions along the electric field lines, which
is formed perpendicular to the substrate surface. Therefore, the normal incidence of Mo
atoms, which is favorable for the sharp cone formation, is realized. Slanted incidence of13the atoms, which is unavoidably included in the Mo flux, result in the deposition of suchatoms on the sidewall of the cavity hole, hence the hole is gradually tightened. If thisoccurs too fast, only a low mound rather than a cone is obtained inside the cavity, which
was actually demonstrated in case of the DC sputtering3. On the other hand, if thefraction of the sidewall deposition can be controlled properly, desirable shape of the coneshould be available. In fact, in the pioneering work by Spindt utilizing the vacuum
evaporation1, two evaporation sources with different incidence angles were equipped andtheir evaporation rates were independently regulated to control the closure rate.
The sharpest conical structures (i.e., with highest average aspect ratio) could beobtained at Vc = 40 V. Lower and higher Vc values resulted in a decrease in aspect ratio.
Considering the directivity of the ionized atoms, higher plasma potentials are preferable.
At an argon gas pressure of 1 Pa, many sputtered Mo atoms reach the substrate before
thermalization24,25, and are therefore expected to retain their initial energy, which is in therange of 1-10 eV26. Since the speed of atoms is proportional to the square root of theenergy, differences in potential drop across the sheath, expected to be 10-50 V in theseexperiments, may have a significant effect on the degree of alignment of ionized metal
atoms. An increase in incident atom energy is also expected to contribute to the loweringof tensile stress, which was the original reason why vacuum evaporation could not beapplied to Mo deposition on resist cavities. However, when the energy is too high, thisfilm stress generally turns compressive,16 which can also result in destruction of thecavity structure and/or unexpected closure of holes due to deformation of the resist layer.
Since the bending torque applied to the resist layer may increase with an increase in film
thickness in cases where the film experiences biaxial compressive stress,27 holes may

14

unexpectedly close by the torque during deposition. This should result in inferior aspectratios and duller apices of the cones. This change in stress polarity, from tensile tocompressive, has been known to occur when the energy per arrival atom is a few tens ofelectron volts,13,28,29 which is within range in this study. Therefore, too high anode cap
voltages and the resultant particle energies may deteriorate the sharpness of the preparedemitter cones.

15

(a)
0.6 Pa VC = 40 V

1 μm

(b)
1.0 Pa VC = 40 V

1 μm

(c)

2.2 Pa VC = 40 V

1 μm

(d)
3.0 Pa VC = 40 V

1 μm

FIG. 7: Dependence of Mo cone shapes on Ar gas pressure.

16

Aspect Ratio

0.8
0.6
0.4
0.2
0

VC = 40 V
T-S = 73 mm
0.5

1

5

Ar gas pressure (Pa)FIG. 8. (Color online) Dependence of the aspect ratios of Mo cones on the discharge gaspressure.

The dependence of the cone shape on gas pressure is shown in Figs. 7 (SEM
images) and 8 (aspect ratios). In series of experiments, the anode cap voltage was set at
40 V, and the T-S distance at 73 mm. The observation and measurement procedures wereidentical as those used in Figs. 5 and 6. From Fig. 7, it can be seen that higher gas
pressures resulted in cones with low mound structures with gentle slopes. On the other
hand, samples prepared at pressures at or below 1 Pa were found to have sharper cones.
The latter is also confirmed by the dependence of apex ratio on gas pressure in Fig. 8.
The decrease in sharpness of the cones at higher gas pressures can be ascribed to
two reasons. First, an increase in random incidence angles of Mo ions occur at thesubstrate sheath edge, due to the collision and scattering of sputtered Mo atoms byambient Ar gas. In our previous calculations,24 the product of pressure and distance ofMo for thermalization was about 14 Pa·cm. This means that at 2 Pa, a significant part of17

Mo atoms/ions lose their initial direction from the target to the substrate after traveling 7
cm. When ions arrive at the sheath edge with oblique incidence with the energy of a few
eVs, they cannot be completely aligned to the substrate’s normal by the potential drop of
the sheath region. In the worst case, for an initial energy of 1 eV and potential drop of 30
V, the maximum final angle can be calculated as: tan-1(1/√30) ≈ 10.3°. This effect leads
to deposition of atoms on the sidewalls of cavity holes and thus their fast closure.
Secondly, higher gas pressures result in collisional scattering of Mo ions inside
the sheath. When the ions collide with neutral species and exchange charge and/or
momentum during acceleration in the sheath, they do not fully aligned along thesubstrate’s normal. In the case of Ar ion to Ar neutral, momentum transfer and charge
exchange collision cross sections are on the order of 10-18 m2 and 4-5×10-19 m2,
respectively.30 This means that at 1 Pa and 300 K, the mean free path is on the order of 410 mm. On the other hand, it was reported31 that the electron-free sheath thickness withpotential drop of 30 V was a few centimeters for plasma whose electron temperature Te
was 2.4 eV and electron density ne was 2.0×1013 m-3. This thickness scales with the
Debye length, which is proportional to √𝑇𝑒 ⁄𝑛𝑒 . When ne ≈ 1017 m-3, the sheath thickness
is about 0.2 mm. Considering that the collision mean free path is inversely proportional to
the gas pressure, collisions inside the sheath cannot be neglected and should have someeffect on the directivity of the incident ion flux. We suggest that lower gas pressures
should lead to better structures, provided that a stable pulse discharge can be maintained.
We should note again that these results were obtained at the substrate position #2
in Fig. 2 as described in the experimental section. In the preliminary stage of thisresearch, we also placed the cavity substrate at position #1 and compared the obtained18cone shapes. It was found that slightly low and slanted cone shapes were obtained at #1,
which may be due to the difference in the ratio of neutral and ionized atoms and theirfluxes. Further research is necessary for the uniformity of the cone shapes utilizing thetarget of large size, which is included in our research plan in the near future along withthe fabrication and demonstration of working emitters.

IV. SUMMARY
We have here demonstrated that Mo cones could be obtained inside resist cavities
at room temperature using a triode high power pulsed magnetron sputtering (HPPMS)
system. A positive plasma potential was confirmed, which should lead to normal
incidence and higher incident energy of depositing Mo ions. The aligned incidence was
expected to contribute to the formation of high aspect ratio cones, and the higher energy
to help the suppression of tensile film stress at room temperature. In this study, optimal
structures were obtained at an anode cap voltage Vc of 40 V at a gas pressure of 1 Pa andtarget to substrate distance of 73 mm. Higher or lower anode cap voltages, as well as
higher gas pressures, resulted in lower aspect ratios of the formed cones. In particular, it
was found that a plasma potential that was too high resulted in strong compressive stressin the deposited films, which broke or deformed the resist cavities.
In general, the formation of Mo cone structures was much improved by triode
HPPMS deposition compared with conventional vacuum evaporation and previous pulseoff voltage-modulated HPPMS deposition. The experimental results here indicate thatparameter optimization improves apex sharpness (i.e., small curvature radius) andprocess stability. This optimization can be achieved by precise control of the film stress19

using HPPMS, which may result in valuable information for many additional applications
using HPPMS.

ACKNOWLEDGMENTS
We wish to thank Mr. Shunichi Yoshizawa at the National Institute of Advanced
Industrial Science and Technology (AIST) for his assistance in fabricating the photoresist
microcavity structures. We also wish to thank Kengo Nishitani at Seikei University for
his assistance with the experiments.

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22

Figure Captions
FIG. 1. (Color online) Preparation process of Mo cones for volcano-structured Spindttype emitters. LOR in (1) denotes “lift-off” resist.

FIG. 2. (Color online) Schematic of Triode high power pulsed magnetron sputtering(HPPMS) configuration. N and S in the figure denote the poles forming the magnetron.

FIG. 3. Plasma potential vs. Anode cap voltage measured using the Langmuir probe. Gas
pressure was 1 Pa, and the potential was recorded 200 μs after ignition of the pulse
discharge using a 200 Hz frequency and 5% duty cycle.

FIG. 4. Cross-sectional scanning electron microscope (SEM) images of Mo-deposited
samples on double resist cavity substrates at a (a) target to substrate (T-S) distance of 50
mm and a (b) T-S distance of 73 mm. The gas pressure was 1 Pa and the anode cap
voltage was 40 V.

FIG. 5. Dependence of the Mo cone shape on the anode cap voltage.

FIG. 6. (Color online) Dependence of the aspect ratio of Mo cones on the anode cap
voltage. The gas pressure was fixed to 1 Pa.

FIG. 7: Dependence of Mo cone shapes on Ar gas pressure.

FIG. 8. (Color online) Dependence of the aspect ratios of Mo cones on the discharge gaspressure.

23