1. Introduction
1.1 Background of research
An electronic device is used to manipulate and control electrical signals, and there
are numerous types of electronic devices. The main types of electronic devices are
introduced as follows[1, 2]. Resistors are employed to limit the flow of current in circuits and are used for
voltage division, current control, voltage drops, and other purposes. Capacitors are
devices that can store and dissipate electrical energy. They allow or filter temporal
changes in electrical signals by temporarily storing charge and controlling current
flow. Inductors are electronic devices that generate a magnetic field to interfere
with current. They store magnetic energy and allow or filter temporal changes in current.
Diodes are semiconductor devices applied for current rectification, protection, and
optical communication[3]. They allow current to flow in only one direction. Lastly, transistors are semiconductor
devices used for switching and amplification. They can amplify or control current
or voltage signals[4]. Additionally, there are various types of sensors that detect physical changes and
convert them into electrical signals. These sensors can detect temperature, pressure,
illumination, and sound[5-8].
These electronic devices are typically made of the following materials. First and
foremost is the semiconductor, which has the greatest influence on the degree of integration
and operation speed of the electronic device. Semiconductors are materials whose electrical
characteristics change when electrical signals are controlled and manipulated. Silicon
is a commonly used material in memory or system semiconductors[9]. Additionally, materials like gallium arsenide and gallium nitride are applied in
optoelectronic devices[10, 11]. Second, to connect various components and transmit signals and power within an electronic
device, connection lines are made of metal, typically copper or aluminum[12]. Moreover, metal pads are used as connection points to transfer signals and power
from the electronic device to an external circuit. Third, there are insulating materials
used as protective layers for electronic devices, safeguarding them from physical
damage or environmental factors. Silicon oxide or aluminum oxide are commonly used
materials for this purpose[13, 14]. The equipment used to manufacture these semiconductor devices is based on vacuum
deposition. Vacuum deposition is a process where solid materials are directly deposited
to form thin films without converting them into gases or liquids. This process prevents
the formation of impurities or other ions, thereby enhancing the purity and characteristics
of the deposited film. Equipment components used in this process include a deposition
source, vacuum chamber, low vacuum, and high vacuum pumps[15]. Main technologies include thermal evaporation, chemical vapor deposition, and sputtering
methods. Among these, sputtering is mainly used for forming metal films. Sputtering
has a disadvantage of requiring a chamber and waiting time to the desired vacuum level.
To address this, a method using a load-lock system to shorten the process time is
applied, but deposition is still conducted in a vacuum atmosphere.
This study investigated research focused on depositing titanium (Ti) onto glass using
atmospheric plasma equipment. Building upon the history of the research team, who
have already conducted related research, the equipment was remodeled, allowing progress
toward the possibility of thin film deposition under atmospheric pressure conditions[16].
1.2 Experimental details
In a vacuum, when high-energy argon plasma is accelerated and collides with the target,
particles from the target are separated and deposited onto the sample. However, atmospheric
pressure plasma equipment can form argon plasma, but acceleration is not possible.
Even if the equipment is modified to force acceleration, forming the sample is challenging
due to the short moving distance of the separated particles in the atmospheric pressure
atmosphere. To overcome this, a new concept of etching the target was applied to generate
a large number of Ti particles and deposit them on the glass. Additionally, the plasma-generating
equipment was modified to concentrate the plasma in the direction of the sample. The
modifications made to the plasma head are currently in the process of being patented.
The experimental method for this study is as follows. First, high heat resistance
glass is cut into a size of approximately 5cm × 2.5cm. To change the glass surface
from hydrophobic to hydrophilic, the sample is treated with plasma in an oxygen atmosphere
for about 30s. The experiment used a 4-inch Ti target for sputtering, as seen in Fig. 1(a), and the glass was placed on this target as shown in Fig. 1(b). CF4 gas, which can etch Ti, is supplied to eject a large number of particles from
the target. If the argon (Ar) supply is less than 10 liters per minute (lpm), the
generated argon plasma decreases the decomposition of CF4 gas. If CF4 is supplied
less than 200 standard cubic centimeters per minute (sccm), the etching rate of the
Ti target significantly decreases, as explained below. Therefore, Ar was supplied
at a rate of 10 lpm, and CF4 was supplied at 200 sccm. The argon plasma generated
with 13.56 MHz radio frequency (RF) and 150 W power was transformed into a pink reactive
gas by reacting CF4 with Ti, as shown in Fig. 1(d).
As explained above, the equipment head was modified to direct the flow of plasma toward
the sample (glass). To ensure safety, the experimental equipment was placed inside
a transparent acrylic box, and an exhaust line was connected to expel the reacted
gases from the box as in Fig. 1(c). The height between the lowest part of the plasma head and the glass is 3mm, and
the process time is 10 min.
Fig. 1. Images of (a) 4-inch sputter target, (b) Glass placed on the target, (c) Experimental
equipment, and (d) Etching the target by generating plasma
2. Results and Discussion
Fig. 2 shows an image observed at low magnification using scanning electron microscopy (SEM)
equipment after cutting the area indicated by the red dotted line in Fig. 1(b). The positions on the image are marked from 1 to 21, with Position 1 being the closest
to the region where plasma is formed to etch the target, and Position 21 being the
farthest. The colors in the image indicate variations in thickness with other components,
and these areas are labeled with numbers in Fig. 2 for detailed analysis. The distance from Position 1 to Position 21 was approximately
2.5cm, and it was observed that the etched materials were moved this distance and
deposited on the sample. Although this result was achieved with the concept of etching
rather than deposition, it is an important finding that indicates that a desired film
can be deposited even in an atmospheric pressure atmosphere if a significant number
of Ti particles can be separated from the target.
For a more detailed analysis, Fig. 3 shows the image magnified 30,000 times with SEM equipment at different locations.
In Fig. 2, the surface conditions at Positions 2, 3, 4, and 5 are almost the same, so only
the image at Position 5 is shown in Fig. 3(b). Similarly, Positions 7 and 8 had a similar shape, so only the SEM results of Position
8 are presented in Fig. 3(d). A peculiar phenomenon is observed at Positions 9 and 10 (Fig. 3(e)) and Position 11 (Fig. 3(f)), where different morphologies are evident. However, similar images were observed
at Positions 9, 10, 12, and 13, with region 12 exhibiting a significantly wide area,
composed of elongated particles.
Fig. 2. Low-magnification SEM image of the surface of the film-deposited sample
Fig. 3. High-magnification SEM images of the film formed at the Positions (a) 1, (b)
5, (c) 6, (d) 8, (e) 10, and (f) 11 of the sample shown in Fig. 2
Fig. 4 is a picture magnified 30,000 times for each position, as shown in Fig. 3, starting from Position 14 of the sample in Fig. 2. Similar images were found at Positions 14 and 15, and similar shapes were also evident
at Positions 17 and 18. From these results, two facts can be derived: First, as the
distance from the area where the plasma is formed increases, only small-sized particles
are observed, as evident in Fig. 4(b), 4(c), 4(d), and 4(e). Second, the irregular plate shape indicated by the arrow in
Fig. 4(a) becomes smaller in Fig. 4(b) and Fig. 4(c), eventually disappearing from the surface in Fig. 4(d) and Fig. 4(e).
Energy-dispersive X-ray spectroscopy (EDX) was utilized to investigate the causes
and elemental components of the observed phenomena. Fig. 5 shows the component analysis results in the areas marked as 1 and 2 in Fig. 3(a). As the experimental substrate is glass, the detected substrate components include
silicon (Si), oxygen (O), sodium (Na), aluminum (Al), and potassium (K). The key components
of interest are fluorine (F) and titanium (Ti). The detected elemental components
are identical to those in Ref. 16.
In Fig. 5(a), it was not easy to distinguish the Ti component from oxygen. Therefore, the data
was converted to atomic content and presented in Table 1. At both Points (1 and 2), the atomic concentration of Ti exceeds 5%, and the concentration
of F is also significantly high, at 50%. These data imply that the film contains a
considerable amount of F, which contributes to the film exhibiting insulator properties.
When depositing a Ti film in a vacuum environment, the Ti element content is 99% or
higher[17].
Fig. 4. High-magnification SEM images of the film formed at the Positions (a) 14,
(b) 16, (c) 18, (d) 19, and (e) 21 of the sample shown in Fig. 2
Fig. 5. EDS analysis results in the areas marked as (a) 1 and (b) 2 in Fig. 3(a)
Table 1. Concentration data of EDS in the areas marked as Positions (a) 1 and (b)
2 in Fig. 3(a)
|
Atom (%)
|
C-K
|
O-K
|
F-K
|
Na-K
|
Al-K
|
Si-K
|
K-K
|
Ti-K
|
|
Point 1
|
7.57
|
37.91
|
17.77
|
7.86
|
2.02
|
17.08
|
3.18
|
6.61
|
|
Point 2
|
5.26
|
19.42
|
50.77
|
8.25
|
0.92
|
1.06
|
1.74
|
12.59
|
Additionally, component analysis in Fig. 4(a) was performed, and the results are presented in Fig. 6 as follows. The fluorine concentration appears to be high at Point 1, and the specific
concentration values were confirmed through analysis, as shown in Table 2. The concentration of fluorine at Point 1 is approximately three times higher than
at Point 2, while the concentration of Ti is reduced by half. This indicates that
the irregular plate shapes observed in Fig. 4(a) are primarily formed by the element F. Based on these results, the fluorine concentration
in Figs. 4(d) and 4(e) is expected to be quite low.
Fig. 6. EDS analysis results in the areas marked as (a) 1 and (b) 2 in Fig. 4(a)
Table 2. Concentration data of EDS in the areas marked as (a) 1 and (b) 2 in Fig. 4(a)
|
Atom (%)
|
C-K
|
O-K
|
F-K
|
Na-K
|
Al-K
|
Si-K
|
K-K
|
Ti-K
|
|
Point 1
|
6.99
|
15.68
|
43.89
|
9.92
|
4.83
|
6.11
|
5.19
|
7.39
|
|
Point 2
|
4.06
|
35.59
|
15.92
|
8.58
|
1.16
|
16.86
|
1.23
|
16.60
|
To investigate this assumption, Fig. 7 shows the EDS analysis results obtained by irradiating three locations in Fig. 4(e), which are the furthest areas from where the plasma is formed. In the analyzed area,
no fluorine concentration or only a small amount of fluorine was observed, and the
amount of Ti also decreased. Elemental concentration fractions are further detailed
in Table 3.
Fig. 7. EDS analysis results in the areas marked as (a) 1, (b) 2, and (c) 3 in Fig. 4(e)
The relatively higher oxygen and silicon concentrations compared to the data in Tables
1 and 2 indicate that the thin film thickness is low. Only 0.69% fluorine was found
at Position 2, and the Ti concentration was 1.59% higher than in other regions. This
observation serves as an important basis for demonstrating the potential of forming
a thin film consisting solely of Ti using atmospheric pressure plasma technology,
as presented in this study. Furthermore, it is observed that the concentration of
carbon is significantly reduced, and the concentration is similar regardless of the
location.
Since Ti is a conductive material, the surface resistance was measured using 4-point
probe equipment. Up to Position 19 in Fig. 2, the resistance value showed infinity (1.44 × 106 Ω/cm2 value with the equipment), indicating non-conductive or insulating behavior. However,
at position 21, the value reduced by about 1/5 to 2.84 × 105 Ω/cm2. These reduced resistance values suggest the possibility of forming a Ti thin film
with insulating characteristics using the etching technology with atmospheric plasma.
Although the conductivity characteristics of a perfect Ti thin film were not achieved,
the surface resistance results still indicate a potential for forming a Ti thin film
by applying the etching technology using atmospheric plasma.
Table 3. Concentration data of EDS in the areas marked as (a) 1, (b) 2, and (c) 3
in Fig. 4(e)
|
Atom (%)
|
C-K
|
O-K
|
F-K
|
Na-K
|
Al-K
|
Si-K
|
K-K
|
Ti-K
|
|
Point 1
|
0.13
|
59.91
|
|
5.77
|
1.82
|
28.68
|
2.50
|
1.19
|
|
Point 2
|
0.23
|
58.12
|
0.69
|
6.26
|
2.13
|
28.37
|
2.61
|
1.59
|
|
Point 3
|
0.21
|
59.14
|
|
5.70
|
1.94
|
29.10
|
2.45
|
1.46
|
3. Conclusion
In the semiconductor or display field, the deposition of metals or insulators onto
substrates typically requires film formation in a vacuum atmosphere. However, in this
study, a novel method was proposed to coat Ti, a metal with specific characteristics,
onto glass using an atmospheric pressure plasma method.
In this method, the reactive gas is generated by supplying argon and CF4 gas to atmospheric
pressure plasma equipment, and the by-products resulting from the etching of the sputter
target are deposited on the glass. For this purpose, modifications were made to the
head that generates plasma in the equipment. The deposited film was divided into 21
locations based on color and subjected to examination for surface shape, elemental
composition, and surface resistance. Several areas exhibited similar surface topography.
Although the concentration varied depending on the location, the elements Si, O, Na,
Al, K, F, Ti, and C were detected.
Since CF4 was used as the etching gas, the changes in the positional concentration
of C, Ti, and F elements were investigated. F was hardly detected in the region farthest
from the location where the plasma was generated. Additionally, the concentration
of C also decreased, and surface resistance values were measured.
In conclusion, the possibility of depositing a Ti metal thin film using the etching
technology with atmospheric pressure plasma equipment was confirmed in this study.