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12
3Structure optimization of Spindt-type emitter
fabricated by triode high power pulsed magnetron
sputtering

45Running title: Spindt emitter optimization

6

Running Authors: Nakano et al.

7
8
9
10
11

Takeo Nakano 1,a), Hyuga Taniguchi 1, Nanako Dei 1, Makoto Ozawa 1,
Md. Suruz Mian 1, Kei Oya 1, Katsuhisa Murakami 2, and Masayoshi Nagao 2
1

Faculty of Science and Technology, Seikei University, 3-3-1 Kichijoji-Kita, Musashino, Tokyo 1808633, Japan

12
13

2

14

a) Electronic mail: nakano@st.seikei.ac.jp

National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono,
Tsukuba, Ibaraki 305-8568, Japan

15
16

Spindt-type emitters were fabricated with cavities made of Al/Mo/SiO2 using the triode

17high power pulsed magnetron sputtering method. We explored the process parameters (gas18pressure and voltage of the additional cap electrode) to optimize the sharpness of the19emitter shape. We found that the intermediate pressure and voltage were suited to obtain

20

sharp emitters. Further, we elucidated the crucial effect of the cavity dimensions, such as21

the cavity depth and hole diameter in the cavity ceiling, on the emitter shape. At a cavity

22

depth of 480 nm, the aspect ratio (AR) of the emitter increased monotonously with an

23

increase in the hole diameter. With a large hole diameter (900 nm) and even shallower

24

cavity (380 nm depth), we attempted to re-optimize the process parameters. Consequently,

25

a very sharp emitter cone structure with an AR exceeding 1.3 was obtained. The cap voltage

26

that produced the optimum AR was found to decrease for the larger-hole and shallower-

27

depth cavities. Finally, the applicability of the process for preparing a working emitter is

28

discussed.

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I. INTRODUCTION

2The Spindt-type emitter is a micro vacuum electron emitter named after C. A.

3

Spindt, who first fabricated it in 19681. The fabrication process included the vacuum4

deposition of Mo onto a small cavity with a hole in its ceiling. The hole gradually closes,

5and a Mo cone, which functions as the cathode, eventually forms inside the cavity2. During

6

this process, the incident Mo atoms should ideally be concentrated along the normal

7

direction. Otherwise, more Mo atoms are deposited on the sidewall, and the hole closes

8

early, which hampers the formation of a sharp emitter structure. To achieve this9

concentration, the evaporation source and cavity substrate must be placed far apart. Hence,10

the deposition apparatus must be large3, resulting in low efficiency of material usage and11difficulties in the fabrication of large-sized cavity arrays.

12

In contrast to vacuum evaporation, the sputter deposition process is more

13

advantageous for obtaining homogeneous coatings over large areas. However, the14conventional sputter deposition process is not suitable for Spindt-type emitter fabrication15

because most sputtered atoms are neutral and collide with ambient gases, causing their16

incidence direction to be scattered and uncontrollable. In 1995, Van Veen et al. attempted17

to apply collimated sputter deposition (CSD) for Spindt-type emitter fabrication4. This

18

technique, which was originally developed by Rossnagel5 for metallization of trench or via

19

structures for semiconductor devices6, involves the insertion of a physical collimator

20

between the target and substrate to limit the incidence angle of the deposited particles. CSD

21

can produce sharp tips, but it requires a high substrate temperature and subtle alignment

22

between the collimator array and the emitter array patterns; hence, it has not been widely

23

used.

2

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We have been trying to apply high power pulsed magnetron sputtering (HPPMS),

2

which is a variant of ionized sputtering, for the fabrication of emitter. Ionized sputtering

3

was also pioneered by Rossnagel for trench/via metallization7–9, and HPPMS was

4developed in 1999 by Kouznetsov et al.10 . HPPMS, also called HiPIMS (high power

5

impulse magnetron sputtering), involves the application of pulse waveforms of very high6discharge power and low duty ratio to achieve increased plasma density11,12. This enhances7

the ionization of sputtered atoms so that their incidence direction can be controlled by

8

introducing a potential difference between the plasma bulk and substrate.

9

In previous studies13,14, we applied the method that recently came to be known as

10“bipolar HiPIMS”15. In this process, the target voltage is set higher than the ground11

potential immediately after the high-power discharge to increase the potential of the

12

afterglow plasma. Using this technique, the emitter height was significantly improved

13

compared to that obtained using conventional magnetron sputtering. However, the emitter14

was not sufficiently sharp for practical use yet. This was because of the limited usage of15

the ionized species: the ions are subjected to the potential difference only during the16

afterglow period of the discharge.

17

Subsequently, we developed a new approach, triode HPPMS, to increase the plasma

18

potential16. The triode HPPMS added a positively biased electrode (“cap electrode”) in

19

contact with the plasma. This ensures that the plasma potential was raised during both the20

on and off periods of the discharge. Consequently, enhancement of the incidence direction

21alignment of ions could be expected.

22

We applied the triode HPPMS for the preparation of the cathode of a volcano-

23

structured double-gate Spindt-type emitter17. This kind of emitter, developed by one of the3This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
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authors18, utilized a cavity made of plastic including photoresist (resist cavity). Following

2

the emitter formation, the cavity was completely removed using an organic solvent, leaving

3

the emitters isolated on the substrate. Subsequently, double-gate electrodes can be formed

4for electron extraction and focusing.

5

After the process optimization, emitters with the best aspect ratio (AR; height per

6width) of 0.7 could be prepared at an intermediate cap electrode voltage and an

7intermediate gas pressure. Here, the alignment of ions was improved by increasing the cap

8electrode voltage, which increased the plasma potential, or by reducing gas (Ar) pressure.

9

However, under such conditions, the compressive stress induced in the deposited Mo

10overlayer caused the mechanically weak plastic cavity ceiling to bend, thus hastening the11hole closure. Therefore, the best AR value was obtained with intermediate parameters both

12

in cap voltage and in pressure.

13

In this study, we utilized a rigid cavity made of Al/Mo/SiO2 used in the original

14

Spindt-type emitter fabrication process. This rigid cavity was used to explore the effect of15

the deposition conditions on the Mo conical emitter without interference from cavity

16

deformation. We examined the impact of the cavity shape on emitter sharpness. Because17

the incoming Mo particles are not completely ionized, they should have an oblique18

component. Therefore, the effect of sidewall deposition may vary based on the cavity

19dimensions, that is, the hole diameter and cavity depth. Finally, we examined the influence

20

of gas pressure and cap voltage on the emitter shape for shallow cavities with a large hole.

21

We found that stable emitter growth was possible under low compressive stress conditions.

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II. EXPERIMENTAL

2

The Spindt-type Mo emitter was prepared on a microcavity substrate2,19. First, the3

SiO2 and Mo layers were sequentially deposited on a Si substrate by chemical vapor

4

deposition and sputtering, respectively. Then the template cavity was formed by the5lithography of the hole pattern and dry and wet etching of Mo and SiO2 layers (Fig. 1(a)).

6Subsequently, Al was deposited in the oblique direction while rotating the substrate to form

7

a sacrificial layer (Fig.1 (b)). After that, emitter material was deposited from the substrate8

normal until the hole was completely closed (Fig. 1(c)). When finishing the emitter

9fabrication process, the Al sacrificial layer was dissolved to lift off the Mo top layer (Fig.

101(d)), which results were not shown in this paper.

11

In this study, the Al layer thickness was systematically varied to investigate the12effect of cavity dimensions on the emitter shape. As seen in the cross-sectional scanning

13

electron microscope (SEM) image of such a cavity (Fig. 2), the Al layer could be quite

14

thick to obtain the significantly narrowed hole in the cavity ceiling.

15

As the emitter material, Mo was deposited using the triode HPPMS apparatus, as

16

described in our previous report17. The anode cap of the two-inch sputter gun was insulated

17

from the electrical ground and connected to a DC power supply (current capacity of 12 A)

18

to function as an additional electrode. By applying a positive “cap voltage” 𝑉𝐶 to this19

electrode, the plasma potential increased16. The HPPMS discharge was generated using a20

pulse power source (Heiwa Denki). The pulse frequency and duty ratio were set to 200 Hz

21

and 5%, respectively. The power source can be controlled only by the voltage. We adjusted

22

the voltage manually by monitoring the target current and voltage waveforms and the time-23

averaged power calculated from them. The discharge conditions were set as follows: At the5This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
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start of the discharge, the time-averaged power was adjusted to 100 W with 𝑉𝐶 = 0.

2

Thereafter, 𝑉𝐶 was increased to the desired value while keeping the time-averaged target

3

current constant.

4

The target was a Mo disk with a diameter and thickness of 50 and 3 mm,

5respectively. According to the magnetron configuration, an erosion track was formed on

6

the target as a ring, with the deepest part being 23 mm in diameter. The target-to-substrate

7(T-S) distance was 74 mm. In this experiment, the substrate was not intentionally heated8

or cooled. The deposition system was evacuated to <5 × 10−5 Pa before deposition, and9Ar gas of 5N purity was supplied. When the Ar gas flow rate was set to 10 sccm, the gas

10pressure was 0.6 Pa. Higher or lower pressures were obtained by throttling the evacuation

11

valve or reducing the gas flow rate, respectively.

12

Prior to the actual deposition experiments, a stage equipped with three quartz

13

crystal microbalance (QCM) thickness monitors was placed inside the HPPMS apparatus

14

to measure the deposition rate. The stage was approximately the same size as the substrate15

holder used in the deposition experiment. The details of the QCM stage are provided in our

16previous report20. In this study, a monitor placed at the center of the stage was used. This

17

monitor was located near the position where the substrates were placed in the deposition

18

experiments. The monitor was connected to a QCM controller (Inficon, SQM-160), and19

the variation in the deposition rate under different process conditions was recorded.

20

The emitter deposition batches were run as follows. With the discharge conditions

21

set, the target was pre-sputtered for 10 min, and then the shutter was opened. The typical22

deposition time required to completely close the hole in the ceiling of the cavity was 30–

23

90 min. In each deposition batch, a bare Si (100) substrate was placed next to the cavity

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substrate. It was treated with diluted HF, rinsed with distilled water, and then set to the2substrate holder.

3

Following Mo deposition, the cavity substrate was cleaved at the center, and the4plan view and cross-sectional images were obtained using a secondary electron microscope

5(SEM, JEOL, JSM-6510). We defined the AR of the emitter as the ratio of the half-height6of the emitter divided by the full-width at half height. We adopted this definition because7

the hem of the emitter was curved; thus, the width at the emitter bottom was difficult to8determine. The AR was determined for 4–10 emitters, and their average and standard error

9were calculated.

10

A bare Si substrate sample was also observed using cross-sectional SEM to evaluate

11

the film structure and thickness. In addition, X-ray diffraction (XRD) of the sample was

12measured by a diffractometer (Rigaku, Ultima-IV) to elucidate the Mo film orientation and13

stress condition. The X-ray source was Cu Kα, and the conventional 𝜃 − 2𝜃 mode was

14

used. In the XRD pattern of the samples within 2𝜃 = 10°– 80°, only the Mo (110) peak15

could be observed. Using the lattice plane spacing of Mo (110), the biaxial stress 𝜎 was

16

evaluated using the following relation21

17

𝜎=−

𝐸 𝑑 − 𝑑0
,
2𝜈 𝑑0

18

where 𝑑 is the measured lattice spacing of the film and 𝑑0 = 2.225 Å is the lattice

19

spacing of Mo (110) in the powder diffraction data (PDF 00-004-0809). 𝐸 = 323 GPa20

and 𝜈 = 0.31 are the Young’s modulus and Poisson’s ratio of Mo, respectively22.

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III. RESULTS AND DISCUSSION

2

A.

Effects of cap voltage and gas pressure

3

Figure 3 demonstrates the deposition rate measured by the QCM monitor at Ar gas

4pressures of 0.3 and 0.6 Pa. On increasing the cap voltage from 𝑉𝐶 = 0, the deposition rate5

increased slightly, showed a maximum at 𝑉𝐶 ≈ 20 V, and then decreased to a constant

6

value less than that at 𝑉𝐶 = 0. At other gas pressures (0.16 and 1.0 Pa), a similar tendency

7

was observed. This behavior should be due to the potential and density distribution of the8plasma, both of which are affected by the positive cap voltage. However, we do not have a

9clear explanation for it. The film thickness obtained from the cross-sectional SEM image

10

is consistent with these results.

11

We conducted the emitter fabrication experiments using a cavity with a hole12

diameter of 540 nm and depth of 280 nm, similar to the resist cavity used in the previous

13

study17. The SEM images presented in Fig. 4 indicate the influence of the cap voltage 𝑉𝐶

14

on the emitter shape prepared at an Ar gas pressure of 0.3 Pa. The best AR value was

15

1.06 ± 0.08 (𝑛 = 4) obtained at 𝑉𝐶 = 40 V (Fig. 4(b)).

16

The trend of the maximum AR occurring at an intermediate cap voltage was similar

17

to that reported in the previous study using the resist cavity. This could be explained by the18

counteracting effects of the directionality of the incident ions and the compressive stress in

19

the Mo overlayer. The increase in the plasma potential leads to an increase in the normal

20

component of the incident ion velocity, which improves the alignment of ions. However,21

the higher plasma potential increases the incident energy of ions, thereby increasing the22compressive stress in the film23. The compressive stress causes the bending of the cavity

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ceiling and the inward collapse of the Mo overlayer around the hole. Both of these hasten

2

the closure of the hole and thus obstruct the formation of a sharp emitter.

3

Figure 5 illustrates the influence of the cap voltage and Ar gas pressure on the AR.

4

As shown in Fig. 5(a), the AR reaches a maximum at the intermediate cap voltage. The

5

pressure dependence of AR at 𝑉𝐶 = 40 V, as shown in Fig. 5(b), shows that too high or

6

low a gas pressure also degrades the AR value. A higher gas pressure leads to the more

7

scattering of accelerating ions within the sheath, which leads to misalignment of ions.

8

Conversely, a lower gas pressure may improve the alignment but increase the compressive

9

stress in the film owing to the more energetic incidence. Thus, the competition between the10

incidence direction alignment and compressive stress increase occurs based also on this11

parameter.

12

The best AR value increased when the Al/Mo/SiO2 cavity was used instead of the13

resist cavity (for which the best AR was 0.7). This could be attributed to the suppression

14of cavity bending on using the rigid cavity. Notably, the gas pressure at which the AR was

15highest for the Al/Mo/SiO2 cavity was lower than that for the resist cavity. This suggests

16

that the competition described above shifted towards more directivity and higher

17

compressive stress conditions. However, the film collapses, and the degradation of the AR

18value occurs for a considerably high cap voltage. This is confirmed by the steeper collapse

19

angle of the Mo overlayer around the hole at 𝑉𝐶 = 60 V (Fig. 4(c)) in comparison to that20

at lower cap voltages ((Figs. 4(a) and 4(b)).

21

The Mo cone, observed in Fig. 4(b), is suitable for emitter operation. However,22

when we proceeded with the emitter fabrication process, delamination of the Mo overlayer

23

occurred owing to compressive stress. Accordingly, we estimated the stress using XRD.

9

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Figure 6 presents the results of XRD measurement of Mo films deposited on the bare Si

2(100) substrate at 0.3 Pa for 30 min at various anode cap voltages. Figure 6(a) shows the3wide patterns for 2𝜃 = 20°– 80∘ . The Mo films were found to be highly oriented toward4(110), which is the densest plane of the body-centered cubic crystal to which the Mo metal

5

belongs. The (110)-oriented film growth of Mo has been frequently observed in various

6

deposition methods, including conventional DC sputter deposition24 and pulsed laser

7

deposition25. Figure 6(b) shows narrow patterns around the Mo (110) peak. The vertical

8

axis of this graph was plotted on a logarithmic scale. All the peak positions were shifted to9smaller angles compared to the one corresponding to the (110) spacing of the Mo powder

10

sample (40.55°), which indicates the existence of biaxial compressive stress in the film21.

11

The shift was most prominent at 𝑉𝐶 = 40 V and was diminished at both smaller and larger

12

cap voltage values.

13

Using Eq. 1, the compressive stress in the films was calculated and plotted as

14functions of (a) 𝑉𝐶 (at 0.3 Pa) and (b) Ar pressure (at 𝑉𝐶 = 40 V), as shown in Fig. 7. The

15

film stress increased with the decrease in the gas pressure (Fig. 7(b)), whereas it reached a16

maximum at 𝑉𝐶 = 40 V(Fig. 7(a)). This shows that too high a cap voltage and plasma

17potential lead to the relaxation of the compressive stress. Windischmann summarized the18dependence of compressive stress on incident particle momentum in his review26; as the19

momentum increases, the film stress changes abruptly from tensile to compressive at a20

certain point. After the compressive stress value reaches a maximum, it relaxes in the larger

21

momentum region. He ascribed this behavior to the plastic flow of the films caused by

22

excessively high compressive stress. The results obtained from our experiment agreed with23

this tendency. With the increase in the cap voltage and subsequent increase in the particle

10

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energy and momentum, the compressive stress value initially increased, then reached a2

maximum, and finally relaxed.

3Interestingly, the Mo (110) peak became much higher at 𝑉𝐶 = 60 V, where plastic

4

flow was assumed to occur. This agrees with the discussion in a paper by Window27, in

5which the Mo film structures were studied using unbalanced magnetron sputtering with

6

substrate biasing. With increasing substrate bias, Window found that the compressive stress7

reached a maximum value, and the Mo (110) peak was enhanced. He attributed these8

phenomena to the fact that the wire drawing of Mo favors the (110) direction. During the9wire drawing process, Mo was under biaxial stress condition, similar to the film growth

10

process.

11

B.

Effect of hole size on the emitter shape12

In the course of experimental trials conducted to overcome the Mo overlayer

13

delamination as described earlier, we found that the AR of the emitter cone was also

14

strongly influenced by the cavity dimensions. Specifically, if a deeper cavity (i.e., a cavity

15with a thicker SiO2 layer) or a smaller hole was used, the AR of the fabricated emitter

16

decreased. To evaluate this effect systematically, we prepared a cavity with a shallow depth

17(thin SiO2 layer) and a large hole through the Mo layer. Then, the Al sacrificial layer was

18

obliquely deposited at various thicknesses. By increasing the Al deposition thickness, the19hole diameter was reduced by the sidewall deposition of Al. It was confirmed by observing

20

the cross-sectional SEM images of the cavities. Finally, Mo was deposited on these cavities

21

to form emitters.

22

Figure 8 shows the cross-sectional SEM images of emitters deposited onto cavities

23

with different hole diameters. The thickness of the SiO2 layer was 480 nm. The deposition11

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conditions were an Ar gas pressure of 0.3 Pa and a cap voltage of 40 V, which led to the2

optimum AR value in the experiment described in Section A. Clearly, the larger the hole3

diameter, the larger the AR value. The deposition time was 30 min for the cavities shown4

in Fig. 8(a)–(c), while for the cavity with the largest hole diameter (795 nm) (Fig. 8(d)),

5the hole could not be closed within 30 min; thus, the time was extended to 45 min.

6

The influence of hole size on the AR is shown in Fig. 9. The AR increased

7

monotonously with an increase in the hole diameter. It should be noted that the result also

8

implied that the cavity depth affected the AR value. For the cavity with a SiO2 layer

9thickness of 280 nm and a hole diameter of 580 nm, which appeared in Section A, the AR

10value was 1.06 under the same deposition conditions. On the other hand, the AR was only

110.64 at a similar hole diameter of 600 nm when the SiO2 thickness was 540 nm (Fig. 8(c)).

12This suggests that the AR also depends on the cavity depth; it becomes larger for shallower

13

cavities.

14

The results shown in Figs. 8 and 9 imply that a considerable fraction of Mo

15

particles was incident on the substrate at a tilt with respect to the normal in our experiments.

16

Such particles were deposited on the sidewall of the cavity hole, which accelerated its

17

closure. While Mo ions certainly played an important role in the process, which was

18

confirmed by the influence of the cap voltage on the AR (Figs. 4 and 5), Mo neutral atoms

19

also remained and were incident on the substrate, reflecting the effects of target–substrate20

geometry and scattering by the ambient Ar gas atoms during the transport28.

21

In our experimental setup, for the erosion track diameter of 23 mm and T-S distance22of 74 mm, the calculated line-of-sight angle from the substrate normal was 9.5°. This angle,

23

intriguingly, is close to the angle between the hem of the emitter cone and the edge of the12This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
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hole in the cavity ceiling, as indicated by the solid yellow lines in Fig. 8. Horkel et al.

2reported that the angular distribution of incident atoms (including heavy metals such as W

3or Cu) was strongly affected by the Ar gas pressure even at a low range of 0.3–0.5 Pa for

4

the T-S distance of 95 mm29. They found that the angular distribution was considerably

5

broadened with an increase in the gas pressure in this pressure range. Therefore, in our

6experiment, we assumed that a considerable amount of Mo neutral atoms impinged at

7

slanted angles.

8

The effect of the cavity dimension in such a case can be discussed in relation to the9

via-filling simulation by Smy et al.30. Considering angular distribution, they reported that10

the wider the via diameter and the shallower its depth, the better the bottom coverage. This11

can be ascribed to the difference in the solid angle from the bottom of the via to the particle

12

incidence direction. In this study, similarly, the narrower hole and the deeper cavity reduce13

the solid angle from the cavity bottom to the hole. In other words, the number of incident

14

Mo atoms decreases at the bottom of the cavity due to the geometrical restriction by the15hole edge. Hence, the effective deposition rate of Mo decreases here. Therefore, the emitter

16

cannot grow sufficiently vertically before hole closure, resulting in degraded sharpness of17

the emitter. Furthermore, the Al sacrificial layer thickness was increased for cavities with18

smaller hole diameters because the hole was narrowed by the oblique deposition of Al.

19

This may also cause the degradation of emitter sharpness.

20

C.

Influence of cap voltage for cavity with large hole21

During the experiment, concerning the effect of cavity dimension presented in

22

Section B, we noticed that the inward collapse of the Mo overlayer around the hole was23

more prominent for the larger hole (e.g., Fig. 8(d)) in comparison with that for the smaller

13

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hole (Fig. 4(b)). This can be attributed to the larger overlayer thickness required to close2

the hole of a large diameter. In this case, the accumulated compressive stress pushed the3film inward at its periphery, thereby accelerating hole closure and hampering the sharp4

emitter formation. Therefore, the optimum cap voltage may shift to a lower value to reduce5

the film stress at the expense of the directivity of the incident ions.

6

Accordingly, we re-evaluated the influence of the cap voltage dependence on the7

AR of the emitter for a shallow cavity with a large hole. In this series of emitter fabrication,8

we prepared a shallower cavity with a SiO2 layer thickness of 300 nm (note that the SiO2

9layer thickness was 480 nm for the cavities shown in Fig. 8). Figure 10 presents the results

10

for the cap voltage dependence of the AR. In these cases, 60–75 min of deposition time11

was required to close the hole owing to the large hole diameter. The AR showed a12

maximum around a cap voltage of 10–15 V. It was clearly lower than that (40 V) for the13

smaller hole case shown in Fig. 4. In addition, the maximum AR value of 1.32, which was

14

obtained at 0.3 Pa and 𝑉𝐶 = 10 V, was significantly larger than that obtained for the deeper

15

cavity, shown in Fig. 8(d).

16Figure 11 shows the emitter that yielded the maximum AR. The emitter had an

17

excellent sharp shape suitable for the effective emission of electrons and was comparable

18

to those fabricated by conventional evaporation techniques. For example, Itoh et al.

19

obtained a Mo emitter with an AR of 1.3 by electron beam (EB) evaporation31. However,20

during the fabrication of emitters by EB evaporation, an elevated substrate temperature as

21high as 200–500 ℃ is necessary to reduce the tensile film stress and obtain a sharp tip

22

apex32. In contrast, the triode HPPMS resulted in a similar AR without substrate heating.

23

This is one of the advantages of the proposed method.

14

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For the electrical emission, the radius of curvature (RoC) of the tip apex is a crucial

2

parameter. Due to the limited resolution of our SEM used in this study, we could not

3systematically determine the RoC of emitters. However, the positive correlation between

4

AR and RoC is evident, as shown in Figs. 4, 8. Among emitters shown in Fig. 4, the apex

5

was the sharpest when AR was the largest (Fig 4(b)). In Fig. 8, the keener apex of higher

6

AR emitters was also apparent. It should also be noted that several theoretical7

considerations33,34 involve AR as a parameter and show that the larger AR positively affects

8

the emission performance. In addition, in emitters fabricated by conventional vacuum9

deposition, their apex is not always ideally sharp32.

10

Unfortunately, the emitter shown in Fig. 11 does not function at this stage after the11lift-off process of the sacrificial Al layer and the Mo layer above it. This is because the gate

12

electrode (i.e., the Mo layer between the SiO2 and Al layers) is far below the emitter tip;

13

thus, the extraction field cannot be applied effectively. Nevertheless, the findings of this14

study will be of substantial help in the fabrication of Spindt-type emitters by HPPMS.

15First, it was found that, by controlling the cavity structure, sharp emitters could be

16

successfully fabricated at a cap voltage as low as 10 V, at which the strong compressive

17

stress in the Mo overlayer could be avoided. Using a cavity with an appropriate ratio of18

hole diameter to depth, which can be controlled independently by the photolithography

19

pattern and the SiO2 thickness, we can fabricate suitable height emitters aligned with the20gate electrode.

21

Second, the availability of emitters under less compressive stress conditions may

22

allow us to rethink the use of the resist cavity. That is, by preparing shallow cavities with23

a large hole using resist layers, it may be possible to fabricate isolated emitter tips on the15This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
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substrate. As another approach for preparing a freestanding emitter tip array, we may be2able to dissolve the SiO2 layer of the cavity (See Fig. 1) with structures above it. After that,3

we can fabricate the volcano-structured double-gate emitters as described in our previous

4

publication18.

5

Finally, the ionization rate of the sputtered atoms was reaffirmed as an essential

6

factor for the sharpness of the emitter. In this study, we suggested the presence of neutral

7

atoms, which were incident on the cavity obliquely and accelerated the hole closure even8

at Ar gas pressures of 0.3–0.6 Pa. If the ionization rate increases, they can be aligned by

9

the potential difference between the plasma and substrate, resulting in the formation of10

sharper emitters. It can be achieved by optimizing the discharge parameters of HPPMS or

11

exploring alternative emitter materials to Mo. We plan to take up further research in this12

direction.

16

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IV. SUMMARY

2

In this study, we considered the fabrication of a Spindt-type emitter by triode

3

HPPMS using a rigid cavity made of SiO2, Al, and Mo layers and investigated the effects

4of the Ar pressure and cap electrode voltage on the AR of the emitter. The maximum value

5of AR was obtained at the intermediate value of each, which was ascribed to the6competition between the directivity of the incident particles and compressive stress in the7overlayer. By varying the diameter of the hole in the cavity ceiling, the AR of the emitter8

was found to be larger for cavities with larger holes, suggesting the presence of a9considerable fraction of oblique incident particles, probably originating from neutral

10

sputtered atoms. For shallow cavities with a large hole diameter, the emitter with the11highest AR (1.32) was obtained under low compressive stress conditions. This opens the12possibility of fabricating emitters with excellent characteristics by tuning the cavity

13

structure.

14

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ACKNOWLEDGMENTS

2This work was partly supported by JSPS KAKENHI (Grant Number 17K05106). The

3

authors acknowledge the support of Prof. Shigeo Satokawa and Prof. Kazumasa Ohshima

4for their help with the XRD measurements. A part of this work was conducted at the AIST

5

Nano-Processing Facility, supported by “Nanotechnology Platform Program” of the

6

Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

7
8

AUTHOR DECLARATIONS

9

Conflict of Interest

10

The authors have no conflicts of interest to disclose.

1112DATA AVAILABILITY

13

The data that support the findings of this study are available from the corresponding author

14

upon reasonable request.

15

18

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References

2

1

C. A. Spindt, J. Appl. Phys. 39, 3504 (1968).

3

2

M. Nagao and T. Yoshida, Microelectron. Eng. 132, 14 (2015).

4

3

A. A. Talin, K. A. Dean, and J. E. Jaskie, Solid. State. Electron. 45, 963 (2001).

5

4

G. N. A. van Veen, B. Theunissen, K. van de Heuvel, R. Horne, and A. L. J. Burgmans,

6
7

J. Vac. Sci. Technol. B 13, 478 (1995).
5

S. M. Rossnagel, D. Mikalsen, H. Kinoshita, and J. J. Cuomo, J. Vac. Sci. Technol. A

899, 261 (1991).
6

C. Cabral, C. Lavoie, C. Murray, A. Pyzyna, and K. Rodbell, J. Vac. Sci. Technol. A

1038, 040803 (2020).

11

7

S. M. Rossnagel and J. Hopwood, Appl. Phys. Lett. 63, 3285 (1993).

12

8

U. Helmersson, M. Lattemann, J. Bohlmark, A. P. Ehiasarian, and J. T. Gudmundsson,

13

Thin Solid Films 513, 1 (2006).

14

9

15

10

J. A. Hopwood, Ionized Physical Vapor Deposition (Academic Press, San Diego, 1999).

16

V. Kouznetsov, K. Macák, J. M. Schneider, U. Helmersson, and I. Petrov, Surf. Coat.
Technol. 122, 290 (1999).

17

11

K. Sarakinos, J. Alami, and S. Konstantinidis, Surf. Coat. Technol. 204, 1661 (2010).

18

12

J. T. Gudmundsson, N. Brenning, D. Lundin, and U. Helmersson, J. Vac. Sci. Technol.

19
20

A 30, 030801 (2012).
13

21
22

Int. Disp. Work. (2012), pp. 1791–1794.
14

23
24

T. Yoshida, M. Nagao, N. Koda, T. Nishi, H. Ohsaki, and T. Nakano, in 25th Int. Vac.
Nanoelectron. Conf. (IEEE, 2012), pp. 1–2.

15

25
26

T. Yoshida, M. Nagao, T. Nishi, N. Koda, and T. Nakano, in Proc. IDW/AD ’12, 19th

J. Keraudy, R. P. B. Viloan, M. A. Raadu, N. Brenning, D. Lundin, and U. Helmersson,
Surf. Coat. Technol. 359, 433 (2019).

16

T. Nakano, T. Umahashi, and S. Baba, Jpn. J. Appl. Phys. 53, 028001 (2014).

19

This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1116/6.0002201

1

17

2
3

022204 (2017).
18

4
5T. Nakano, T. Narita, K. Oya, M. Nagao, and H. Ohsaki, J. Vac. Sci. Technol. B 35,

M. Nagao and S. Yoshizawa, in 2014 27th Int. Vac. Nanoelectron. Conf. (IEEE, 2014),
pp. 226–227.

19

6

C. A. Spindt, I. Brodie, L. Humphrey, and E.R. Westerberg, J. Appl. Phys. 47, 5248
(1976).

7

20

T. Nakano and S. Baba, Vacuum 80, 647 (2006).

8

21

M. Ohring, Materials Science of Thin Films, 2nd ed. (Academic Press, San Diego, CA,

9102002).
22

11

D. R. Lide and E. Al., editors , CRC Handbook of Chemistry and Physics, 60th ed.
(CRC, Cleavland, Ohio, 1979).

1223

H. Windischmann, Crit. Rev. Solid State Mater. Sci. 17, 547 (1992).

13

24

F. Martin, P. Muralt, and M.-A. Dubois, J. Vac. Sci. Technol. A 24, 946 (2006).

14

25

O. Fruchart, S. Jaren, and J. Rothman, Appl. Surf. Sci. 135, 218 (1998).

15

26

H. Windischmann, J. Vac. Sci. Technol. A 9, 2431 (1991).

16

27

B. Window, J. Vac. Sci. Technol. A 7, 3036 (1989).

17

28

S. Mahieu, K. Van Aeken, and D. Depla, in Reactive Sputter Deposition, edited by D.

18
19

Depla and S. Mahieu (Springer, Berlin, Heidelberg, 2008), pp. 199–227.
29

20

M. Horkel, K. Van Aeken, C. Eisenmenger-Sittner, D. Depla, S. Mahieu, and W. P.
Leroy, J. Phys. D: Appl. Phys. 43, 075302 (2010).

2130

T. Smy, K. L. Westra, and M. J. Brett, IEEE Trans. Electron Devices 37, 591 (1990).

22

31

S. Itoh, T. Watanabe, K. Ohtsu, M. Taniguchi, S. Uzawa, and N. Nishimura, J. Vac.

23
24

Sci. Technol. B 13, 487 (1995).
32

25
26
27

C. A. Spindt, I. Brodie, C. E. Holland, and P. R. Schwoebel, in Vacuum
Microelectronics, edited by W. Zhu (Wiley, New York, 2001), Chap 4.

33

D. Nicolaescu, M. Nagao, V. Filip, S. Kanemaru, and J. Itoh, J. Vac. Sci. Technol. B
21 1550 (2003).
20

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1

2

34

G. P. Kochanski, W. Zhu, Y. Goren, in Vacuum Microelectronics, edited by W. Zhu(Wiley, New York, 2001), Chap 2.

21

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FIGURE CAPTIONS

2

FIG. 1. Spindt emitter fabrication process: (a) Template cavity formation made of Mo and3

SiO2, (b) Al sacrificial layer deposition, (c) Emitter deposition from the substrate normal,

4(d) lift off by dissolving the sacrificial layer.

56FIG. 2. Cross-sectional SEM image of a Al/Mo/SiO2 cavity after the Mo emitter deposition.

7

The size of the hole in the cavity was controlled by changing the thickness of Al sacrificial

8

layer.

9
10

FIG. 3. Deposition rate of Mo films measured using QCM monitor at the center of the stage.

11
12

FIG. 4. Cross-sectional SEM images of Mo emitters fabricated inside the Al/Mo/SiO2 cavity

13

at various anode cap voltages: (a) 𝑉𝐶 = 20 V, (b) 𝑉𝐶 = 40 V, and (c) 𝑉𝐶 = 60 V. The Ar

14

gas pressure was 0.3 Pa. The scale bar of 0.5 μm length is shown at the right-bottom of

15

each figure.

16
17

FIG. 5. Aspect ratio dependence of Mo emitters (a) on the cap voltage and (b) on the Ar gas

18pressure. The cavity consisted of a hole of diameter 540 nm and SiO2 layer of thickness

19

280 nm.

20
21

FIG. 6. (a) XRD patterns of Mo films deposited on bare Si substrates at an Ar gas pressure

22of 0.3 Pa at various cap voltages for 30 min. The baseline of each XRD pattern is shifted23

by 1 × 106 . (b) Magnified pattern near the Mo (110) peak. Note that the vertical axis is in

24

logarithmic scale. The baseline is not shifted in this figure. The vertical dotted line at 2𝜃 =25

40.55∘ denotes the Mo (110) that appeared in the powder diffraction data.

26

22

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FIG. 7. Stress in Mo films deposited on Si (100) substrates. Negative value denotes2

compressive stress: (a) cap voltage dependence at Ar gas pressure of 0.3 Pa, (b) Ar gas

3pressure dependence at 𝑉𝐶 = 40 V.

45

FIG. 8. Cross-sectional SEM images of Mo emitters deposited inside the cavities with6

different hole sizes: (a) 370 nm, (b) 510 nm, (c) 600 nm, and (d) 795 nm. The SiO2 layer

7

thickness was 480 nm. The Ar gas pressure and 𝑉𝐶 during the emitter growth were 0.3 Pa

8

and 40 V, respectively. The solid yellow line was drawn from the edge of the hole with a99.5-degree tilt (see text).

1011

FIG. 9. Hole diameter dependence of the aspect ratio of the Mo emitter prepared under Ar

12gas pressure of 0.3 Pa and at 𝑉𝐶 = 40 V.

13
14

FIG. 10. Dependence of aspect ratio of Mo emitters on the Ar gas pressure and 𝑉𝐶 . The

15

cavity depth was 300 nm, and the hole diameter was 850 nm.

1617FIG. 11. Mo emitter of maximum AR value (1.32) obtained under the Ar gas pressure 0.3

18

Pa and at 𝑉𝐶 = 10 V. The cavity depth was 300 nm, and the hole diameter was 850 nm.

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FIG. 1. Spindt emitter fabrication process: (a) Template cavity formation made of Mo and
SiO2, (b) Al sacrificial layer deposition, (c) Emitter deposition from the substrate normal,(d) lift off by dissolving the sacrificial layer.

24

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FIG. 2. Cross-sectional SEM image of a Al/Mo/SiO2 cavity after the Mo emitter deposition.
The size of the hole in the cavity was controlled by changing the thickness of Al sacrificial
layer.

FIG. 3. Deposition rate of Mo films measured using QCM monitor at the center of the stage.

25

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FIG. 4. Cross-sectional SEM images of Mo emitters fabricated inside the Al/Mo/SiO2 cavity
at various anode cap voltages: (a) 𝑉𝐶 = 20 V, (b) 𝑉𝐶 = 40 V, and (c) 𝑉𝐶 = 60 V. The Ar
gas pressure was 0.3 Pa. The scale bar of 0.5 μm length is shown at the right-bottom ofeach figure.

26

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FIG. 5. Aspect ratio dependence of Mo emitters (a) on the cap voltage and (b) on the Ar gas
pressure. The cavity consisted of a hole of diameter 540 nm and SiO2 layer of thickness
280 nm.

FIG. 6. (a) XRD patterns of Mo films deposited on bare Si substrates at an Ar gas pressureof 0.3 Pa at various cap voltages for 30 min. The baseline of each XRD pattern is shiftedby 1 × 106 . (b) Magnified pattern near the Mo (110) peak. Note that the vertical axis is inlogarithmic scale. The baseline is not shifted in this figure. The vertical dotted line at 2𝜃 =40.55∘ denotes the Mo (110) that appeared in the powder diffraction data.

27

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FIG. 7. Stress in Mo films deposited on Si (100) substrates. Negative value denotes

compressive stress: (a) cap voltage dependence at Ar gas pressure of 0.3 Pa, (b) Ar gas

pressure dependence at 𝑉𝐶 = 40 V.

28

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FIG. 8. Cross-sectional SEM images of Mo emitters deposited inside the cavities withdifferent hole sizes: (a) 370 nm, (b) 510 nm, (c) 600 nm, and (d) 795 nm. The SiO2 layer
thickness was 480 nm. The Ar gas pressure and 𝑉𝐶 during the emitter growth were 0.3 Pa
and 40 V, respectively. The solid yellow line was drawn from the edge of the hole with a
9.5 degree tilt (see text).

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FIG. 9. Hole diameter dependence of the aspect ratio of the Mo emitter prepared under Ar
gas pressure of 0.3 Pa and at 𝑉𝐶 = 40 V.

FIG. 10. Dependence of aspect ratio of Mo emitters on the Ar gas pressure and 𝑉𝐶 . The
cavity depth was 300 nm, and the hole diameter was 850 nm.

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FIG. 11. Mo emitter of maximum AR value (1.32) obtained under the Ar gas pressure 0.3

Pa and at 𝑉𝐶 = 10 V. The cavity depth was 300 nm, and the hole diameter was 850 nm.

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This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
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This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
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