1. INTRODUCTION
1.4 Overview
This thesis is structured as follows. A brief introduction of gas discharge phenomena, i.e. plasma, will be described in Chapter 2. In Chapter 3, the fabrication processes of proposed device are presented. Furthermore, the experimental setup used to operate the device and characterization methods are illustrated. In Chapter 4, the collected data including current-voltage characteristic, operating voltages and voltage margin from discharges operating in argon and neon as filling gases are presented and discussed. Chapter 5 summarizes the conclusions of this thesis and discusses the possible future works.
Chapter 2
Gas Discharges
2.1 Introduction
“Gas discharge” is a phenomenon that an ionized gas with electrical discharge, i.e. plasma. The main idea for using nano-tip is to locally enhance electric field while a potential difference is applied on the electrodes. While the differential potential is applied on the device in a gaseous medium, in the high field regions free electrons can be amplified in an avalanche process. When the external electric field exceeds a threshold value, the charge multiplication develops exponentially and a gas discharge is ignited. Secondary electrons are released also from the cathode surface, not only in the gas volume.
The second chapter presents the theory of glow discharge, gas breakdown, Paschen curves and Penning effect. The described knowledge can assist reader to understand the phenomena of proposed microplasma device. A deeper theoretical study of the microplasma devices and additional information of gas discharge can be derived from [13-17].
2.2 Glow Discharges
The glow discharge is one of the most studied and widely applied gas discharges.
The name “glow discharge” is to the fact that plasma is luminous and the wavelength of the glow discharge depends on the gas used. Gas discharge (plasma) can be broadly defined as a system of electric neutrality composed of positive- and negative-charge particles. The degree of ionization may range from small as in the conventional glow discharge to very high as in the nuclear reactions. The first form of plasma observed was the positive column of the glow discharge, in which an equal number of positive ions and electrons. Gas discharge state is known as the fourth state of matter in the universe besides solid, liquid and gas state as shown in Fig. 2-1. The majority of the universe exists in a plasma state, including the stars, which are almost completely ionized with the high temperatures. Under normal state, a gas is mainly made up of uncharged particles and is almost a perfect insulator. While an electric potential of sufficient intensity is established in the gas between two electrodes, the gas can become to plasma state with an almost completely conducting state [17, 18].
Fig. 2-1. Four kinds of matter state in the universe.
The operation condition of glow discharge is typically in a gas between two electrodes with large differential potential. The voltage distribution of glow discharge process is shown in Fig. 2-2. The potential drops rapidly close to the cathode, vary slowly in the plasma, and change again close to the anode. The distribution of electric fields is restricted to sheaths at each side of the electrodes. Electrons which try to reach electrode are repulsed by the sheath fields. Electrons originating at the cathode will be accelerated, collide, transfer energy, leave by diffusion and recombination, slow by the anode. Because the electrons have enough energy to generate light by excitation collisions, the luminous glow is produced. Since a continuous loss of electrons, the steady state must be maintained with an equal degree of discharge. The electrons absorb energy from the electric field, accelerate, ionize some atoms, and the processes are continuous. Secondary emission from the cathode produces additional electrons which are very important to maintaining a sustainable discharge.
Fig. 2-2. Voltage distribution of dc glow discharge.
The sequence of layers, the distribution of brightness along the discharge tube and the potential distribution are shown in Fig. 2-3. For the conventional direct current discharge, the simplest type of glow discharge, the anode and cathode are made by metal conductor. The feature of this discharge is a layer of positive charge at the cathode, with strong field at the surface and considerable potential drop. This potential drop is named as cathode fall, and the thickness of the cathode layer is inversely proportional to the pressure of the gas state [14, 19].
Fig. 2-3. Typical dc glow discharge distribution of brightness and potential.
Aston Dark Space is a thin region to the right of the cathode with a strong electric field. This region has a negative space charge, meaning that stray initial electrons together with the secondary electrons from the cathode outnumber the ions
in this region. The electrons are too low density to excite the gas, so it appears dark.
There is an intense electric field in the cathode fall region, and the electric filed accelerates ions to the cathode. From the ion bombardment of the surface of the metal, free electron is generated and accelerated through the cathode fall region and toward the negative glow. With considerable exciting collisions, the negative glow is the most intense part of the discharge distribution. Therefore, the negative glow region is widely used in many applications such as plasma display panel (PDP). The negative glow has relatively low electric field, long, compared to the cathode glow and is the most intense on the cathode side. While the distance between electrodes is sufficiently large, an electrically neutral plasma region is formed between the cathode and anode.
In the faraday dark space region, the electron number density decreases by recombination and diffusion to the walls, the net space charge is very low, and the axial electric field is small. Right to the faraday dark space region, there is a quasi-neutral region, positive column. The electric field of the positive column is just large enough to maintain the degree of ionization at its cathode end. The positive column is a long, uniform glow, except when standing or moving striations are triggered spontaneously. The acceleration and collision processes homogeneously repeat through the complete length of the positive column and form the uniform glow discharge. Between positive column and anode sheath, the anode glow region is formed. The anode glow region is slightly brighter than the positive column region.
The anode dark space or anode sheath is a space between the anode glow and the anode. Due to electrons traveling from the positive column to the anode, there is the negative space charge in anode sheath.
The striations mentioned above means the traveling waves or stationary perturbations in the electron number density which occur in partially ionized gases. In
their usual form moving striations are propagating luminous bands which appear in positive columns. Standing striations can be easily photographed. Many apparently homogeneous partially ionized gasses in reality have moving striations. Striation is known in positive column and ionospheric plasma system, and it is associated with the wave related mechanism. Striation in typical dielectric microdischarge device predominantly occurs near the region and is basically governed by the ionization-dominated α-process [16].
Fig. 2-4. Current-voltage (I-V) characteristic of discharge.
When gas breakdown occurs, the gas state becomes conductive from non-conductive. Change in the applied voltage can cause the variation of current and voltage in the discharge. A current-voltage (I-V) characteristic for a glow discharge
can be obtained by these measured parameters. A conventional current-voltage (I-V) characteristic of discharge is shown in Fig. 2-4. At low currents, the discharge is weakly ionized, characterized by high voltages and large resistance, and is referred to as a Townsend discharge. Increasing the current will cause a decrease in voltage until a minimum is reached, and this section is the normal glow where the discharge voltage is relatively constant with current variation. The positively sloped I-V trace is the abnormal mode of the discharge. When the cathode is completely covered by the discharge, the current is increased by increasing the voltage to supply more electrons to the discharge. Further increase in the current will result in a sudden drop in voltage and transition of the discharge to an arc state.
2.3 Breakdown of gases and Paschen curve
The phenomenon of gas breakdown in discharge device is the process of transformation from a non-conducting state to a conducting state. When the applied potential is low, the gas likes a near-perfect insulator. While the associated potential reaches the breakdown potential, the considerable ionization values make the gas breakdown with a light emission. During the gas breakdown process, the electron avalanche is a critical constituent first of all. An avalanche develops in the gas with a small number of seed electrons which may appear unexpectedly, and it even can be triggered by a single electron.
Gas breakdown is essentially a threshold process which means the breakdown formed with the electric field exceeds a specific value. The applied voltage across the electrodes is gradually increased from the initial state, and there is no change of the gas state. However, a critical value of applied voltage can suddenly make the ionization rises rapidly. The threshold process is consequence of steep dependence of the atomic ionization by electron impact on field strength and by the fact that ionization, producing electron multiplication, is accompanied by mechanisms that create obstacles to the development of the avalanche. After breakdown, the plasma can be sustained with a mechanism for the generation of ions and electrons to replace the loss of those.
The electron avalanche generated in a dc field forms in time and space. To characterize the rate of ionization, an ionization coefficient α cm-1 is led in, that is, the number of ionization phenomena performed by an electron in a 1 cm path along the field. The change in the number of electrons per unit length in the avalanche is proportional to the number of electrons at x by an ionization coefficient α. The number of electrons in the avalanche grows towards the anode can be shown as the
following Equations.
where d is the electrode separation. An electron leaving the cathode generates exp(αd) 1− positive ions in the electrode separation. Each of the exp(αd) 1− positive ions knocks out γi electrons from the cathode; γi is the secondary emission coefficient for the cathode. The electronic part of the cathode current is given by the equation,
The secondary emission is caused by ions, photos and metasatable atoms. While the applied voltage between the electrodes V > Vt, the denominator ( [exp(γ αd) 1]− )
in the Equation (2.5) is negative. The negative sign presents that the current cannot be steady at V > Vt. However, the current at V < Vt with [exp(γi αd) 1]− >1 is steady and non-sustained. The transition between above two situations is
[exp( ) 1] 1
i d
γ α − = (2.6)
Equation (2.6) shows a steady self-sustained current in a homogenous electric fieldEt =V dt .
The breakdown voltage, , can be expressed as an equivalent term, and it depend on the gas, the material of the cathode, the working pressure and the discharge gap of electrodes. A conventional empirical expression of first Townsend coefficient as follow
Vt
exp( B p )
A p E
α = ⋅ − ⋅ (2.7)
where constants A and B are determined by the experimental curves, E is the electric field strength and p is the working pressure. From the combination of Equations (2.6) and (2.7), we can acquire the breakdown voltage,
( )
It is obvious that the breakdown voltage depend only on the product pd. The constants A and B are related to the Equation (2.7). A and B are normally determined
experimentally for the specific gas over a range of pressure. Consequently, the breakdown voltages of each gas depend on the pd for a specific gas and the material of electrode. The constants A and B determined with experimental method shown in Table 2-1 [13].
Table 2-1. Constants A and B for the ionization and region of applicability [13].
Gas A B E/p
1 1
cm Torr− − V cm Torr⋅ −1 −1 V cm Torr⋅ −1 −1 He 3 34 20 ~ 150
Ne 4 100 100 ~ 400 Ar 12 180 100 ~ 600 Kr 17 240 100 ~ 1000 Xe 26 350 200 ~ 800 Hg 20 370 150 ~ 600
H2 5 130 150 ~ 600
N2 12 342 100 ~ 600 CO2 20 466 500 ~ 10000 Air 15 365 100 ~ 800
Moreover, the values of secondary electron emission coefficients for tungsten and molybdenum in rare gases are shown in Fig. 2-5 [20] and the information about the value of secondary emission coefficient for metal the slow ions are shown in Table 2-2.
Fig 2-5. Secondary electron emission coefficient for the noble gas ions on clean tungsten and molybdenum.
Table 2-2. Value of secondary electron emission from metals for slow ions.
Metal Ar H2 Air N2 Ne Al 0.12 0.095 0.021 0.1 0.053
Ba 0.14 0.1 0.14
C 0.014
Cu 0.058 0.05 0.025 0.066 Fe 0.058 0.061 0.015 0.02 0.059 K 0.22 0.22 0.17 0.077 0.12 Mg 0.077 0.125 0.031 0.038 0.089 Ni 0.058 0.053 0.019 0.036 0.077 Pt 0.058 0.02 0.01 0.017 0.059
Substituting the secondary electrons emission coefficient in Table 2-1 into breakdown voltage in Equation (2.8), a set of experimental curves is acquired in Fig. 2-6 [13].
( ) V pdt
Fig. 2-6. Paschen curves: Breakdown voltage in various gases with different pd product values.
The curves in Fig. 2-6 known as Paschen curves provide the relationship between pd and the breakdown voltage. There is minimum voltage, (pd)min, for each curve. At the Paschen minimum the discharge maintains itself with a minimum cathode fall voltage Vc and minimum power dissipation. In the normal glow discharge, the current density flowing to the cathode remains approximately constant as the total current varies, as the total area of contact with the cathode increases with the total current. In the region of large pd, right side of minimum, high pressures or large gaps reveal that an electron can produce numerous ionization collisions even at high E/p
and there are many collisions with high pressure or large gap. However, in the region of left side of minimum, the possibilities of collision processes are very limited. The dimensions of microplasma devices are at small level, hence the minimum breakdown can be acquired with high working pressure up to 1 atm.
2.4 Penning effect
Electron-impact ionization is the major source for charged-particles generation in a glow discharge [18]. Moreover, another important ionization mechanism is the direct ionization by collusion with sufficiently energetic, metastable neutral particles.
This process is known as the Penning ionization and become the major mechanism with noble gases, such as Ne and Xe, are used as the gas content in plasma display panel (PDP).The probability of collisions involving excited atoms depends on the density of the excited atoms, and hence on their life time. Some excited atoms have very long lifetimes and these are known as metastable excited atoms; they arise because the selection rules forbid relaxation to the ground state, or in pratice, make such a transition rather unlikely. All the noble gases have metastable states, such as the metastable state of argon is at 11.55 eV [14].
A Penning gas mixture consists of a rare gas containing impurity atoms possibly at very low concentrations. The impurity atoms have an ionization potential which is lower than or equal to the metastable potential of the parent noble gas. The Penning effect in a Penning gas mixture is the ionization by charge transfer (charge exchange)
during collision between a metastable atom and a neutral atom which decreases the average energy to form an ion pair. When a metastable atom collides with a neutral, the neutral can become ionized if its ionization energy is less then excitation energy of the excited atom, then the process
A* + G → G+ + A + e－ (2.9)
is known as Penning Ionization and the related process
A* + G → (A G)+ + e－ (2.10)
is called Associative Ionization (AI).
In a glow discharge, this results in an increase of the ionization coefficient (Townsend first coefficient), a decrease in breakdown potential and a lowering of the cathode fall potential. Coburn and Kay (1971) demonstrated the Penning ionization effect by sputtering a europium oxide target containing a small of iron, in both neon and argon discharges. Eu+ (5.7 eV) and Fe+ (eVi=7.8 eV) were observed in both gases, but O+ (eVi=13.6 eV) was observed only in the neon discharge. Argon metastable is at 11.55 eV, whereas neon metastable is 16.62 eV [14]. Table 2-3 shows the ionization potential in penning gas mixtures of noble gases and mercury.
Table 2-3. Penning mixture of noble gases and mercury
Ionization Penning mixture
Gas First ionization energies Metastable energies Suitable penning additives
eV eV
Helium 24.6 19.8 Ar, Kr, Xe, Hg Neon 21.6 16.62 Ar, Kr, Xe, Hg
Argon 15.8 11.55 Hg
Krypton 14 9.9 Xenon 12.1 8.3 Mercury 10.4 4.7
Chapter 3
Experimental Technologies
The fabrication technologies utilized to fabricate the proposed nano-tip enhanced microplasma devices will be firstly described in this chapter. The fabrication technologies by VLSI semiconductor process include photolithography, sputtering, and lift-off technology. A very high vacuum system, which is used to prepare a vacuum environment with base vacuum level 10-7 Torr and ignite the microplasma with excepted pressure of filled gas, will be depicted then. Followed by the description of the instruments, such as electric pulsed power source set and image observation system, for characterizing current-voltage characteristics, discharge voltages, voltage margin and optical appearance of a proposed microplasma device.
3.1 Introduction
The fabrication technologies of microplasma devices are getting more and more advanced. With the development of VLSI (Very Large Scale Integration) semiconductor fabrication processes, smaller and precise structure and electrode gap becomes possible to be fabricated.
A schematic diagram of the fabricated nano-tip enhanced microplasma device structure is shown in Fig. 3-1. The structure of the nano-tip enhanced microplasma device can be separated into two parts, front panel and rear panel. Front panel part including transparent indium tin oxide (ITO) electrode is fabricated by the
photolithography process and rear panel include metal electrodes with sputtering deposition and nano-tip with specific attachment process. A vertical discharge scheme with front transparent ITO electrode and rear metal electrode has been formed in the devices. The front transparent ITO electrode has been fabricated by the photolithographic techniques and the wet etching processes. The thickness and the sheet resistance of the ITO electrode are 1000 Å and 50 Ω/
□
. The sputtering deposition method was used to deposit the rear metal electrode with a thickness of 1000 Å. After front and rear panels, a sealing process is carried out to finalize the panel type microplasma devices. In addition, a nano-tip with a tip radius below 25 nm which coated with aluminum layers of 30 nm in thickness is attached on the rear metal electrode by the silver paste (4922N, DuPont). With specific spacers, the sealing processes of exhausting tube and panels have been achieved by vacuum sealant (VARIAN) by a temperature curing process to finalize the panel type devices.The specifications of fabricated devices are offered in Table. 3-1.
Fig 3-1. Schematic diagram of the nano-tip enhanced microplasma device.
Table 3.1. Specifications of fabricated microplasma devices
The experimental arrangement designed for the measurement is schematically shown in Fig. 3-2. The experimental arrangement consists of a vacuum system, a gas filling system, a pulsed power source set and an image observation system. The vacuum system has been evacuated with residual pressure lower than 10-7 Torr by a turbomolecular pump (Leybold Vacuum). The gas filling to make the base vacuum grade from 300 to 800 Torr are neon (Ne), argon (Ar) and Ne+Ar (2%). The direct current bipolar pulsed excitation waveform, having the duty ratio with 20%, was applied with the frequency from 2 to 20 kHz between the front ITO electrode and the rear metal electrode by the pulse DC power controller (SHENCHANG ELECTRIC, SPIK 2000A).
The electrical properties of the microplasma devices, such as current-voltage characteristics and discharge voltages of microplasma, were measured with typical electrical instruments. The color charged-coupled device (CCD) images and photographs were caught by an image observation system which is composed of a color charged-coupled device (CCD) camera (Watec, WAT-202D), a zoom lens set
(Navitar, Zoom 6000), a digital camera and a personal computer. We shall describe the major features of the above mentioned technologies in this chapter.
(Navitar, Zoom 6000), a digital camera and a personal computer. We shall describe the major features of the above mentioned technologies in this chapter.