2. GAS DISCHARGES
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.
Fig. 3-2. Schematic diagram of the experimental arrangement.
3.2 Device fabrication
3.2.1 Substrate preparation
The proposed nano-tip microplasma device is a panel type configuration.
Therefore, indium tin oxide (ITO) coated glass and the conventional soda-lime glass were used as the substrates. Because of the essentials of transparency in the front panel, the ITO glass was selected as the front substrate. Otherwise, the rear substrate adopts a soda-lime glass to deposit metal electrodes with sputtering method. For panel type microplasma devices, the gas exhaust hole on a glass substrate should be provided. Therefore, a mechanical drilling method has been used to make the gas exhaust hole on soda-lime glass.
First step is an initial cleaning. The cleaning condition of the surface of these two glass substrates could affect the following processes and the characteristics of device.
Wet cleaning processes are necessary to obtain an ultra clean surface for subsequent fabrication. Consequently, a glass surface cleaning process has been adopted in an ultrasonic cleaner as shown in Fig. 3-3.
This cleaning process of glass surface is shown as following procedures:
(1) Rinse, clean the glass in ultrasonic cleaner with deionized (DI) water, 5 min.
(2) Rinse, clean the glass in ultrasonic cleaner with acetone (ACE), 5 min.
(3) Rinse, clean the glass in ultrasonic cleaner with isopropyl alcohol (IPA), 5 min.
(4) Rinse, clean the glass in ultrasonic cleaner with deionized (DI) water, 5 min.
(5) Rinse, clean the glass in ultrasonic cleaner with isopropyl alcohol, 5 min.
(6) Dry, dry the glass with nitrogen gas blowing.
Fig. 3-3. Ultrasonic cleaner for glass substrate cleaning.
3.2.2 ITO electrode patterning
The fabrication processes to pattern the ITO electrode include photolithography, and etching is similar to VLSI fabrication processes. We implement our fabrications in the Nano Facility Center (NFC) at National Chiao Tung University (NCTU). First we plot the electrode pattern, and then generate the pattern to a photo mask in a laser pattern generator. The third step is the VLSI lithography/etching process to produce the desired ITO electrode pattern.
Photolithography process is extensively used in Very Large Scale Integration (VLSI) or microelectromechanical (MEMS) fabrication technologies, and it can transfer the pattern of a mask to photoresist (PR). To prevent the PR peel off substrate during the developing or etching processes, hexamethyldisilazane (HMDS) is widely used in the semiconductor industry to improve photoresist adhesion. A photoresist typically consists of three components: resin, solvent and sensitizer. Resin is a binder
that provides mechanical properties to be the etching mask, sensitizer is photoactive compound that govern the sensitivity of PR, and solvent can keep the resist liquid and adjust the viscosity of PR. Because of the transparency of ITO conducting glass, we choose it to be the front panel substrate in our device. In general, the transparent conducting film for electrode in flat panel display is fabricated by sputtering with indium tin oxide (ITO) target. At the previous section, the cleaning processes of glass surface have been introduced. After cleaning, the VLSI lithography/etching processes have been carried out to fabricate the ITO electrode pattern.
The lithography/etching processes is described as follow. Firstly, HMDS layer is coated in the vacuum oven to improve photoresist adhesion after surface cleaning.
The coating of photoresist will be applied after HMDS layer. The glass is placed on a vacuum chuck in the coater and the photoresist, FH6400, is dropped onto the center of the glass. A uniform and thin photoresist layer can be coated on the glass surface after spinning the glass subtrate. The parameters of positive photoresist, FH6400, are shown in Table 3-2. Third step is exposure and the mask pattern is then transferred onto the glass by the mask aligner (MJB-3, Karl-Suss) as shown in Fig. 3-4 (a). After exposure, the exposed glass is immersed in the developer, FHD5. Therefore, the desired pattern will show up in the photoresist. Chemical wet etching process is then carried out to transfer the developed photoresist pattern to the ITO layer. Wet etching is an isotropic etching, because the etchant have a same etching rate in all directions.
Regions not covered by the photoresist are removed during the wet etching process.
The remained photoresist are then stripped by acetone (ACE) and the desired ITO pattern is represented. After developing, the completeness of ITO pattern is investigated by an optical-microscope as shown in Fig. 3-4 (b).
Table. 3-2. Parameters of FH6400 in the photolithography for ITO electrode patterning.
FH-6400
Type Positive PR
Rotation speed and time 1st: 1500 rpm, 20 sec.
2nd: 4000 rpm, 50 sec.
Soft bake 90 ° C, 80 sec.
Hard bake 120 ° C, 100 sec.
Exposure 150 sec.
Develop 90 sec.
Rinse 240 sec.
Thickness 1.5 µm
Fig. 3-4. Schematic diagram of aligner (left) and optical microscope (right).
The detailed description of the photolithography is provided as follow:
(1) HMDS and bake, coat a HMDS layer on surface and bake with 150° C in a vacuum oven.
(2) Photoresist (PR) spin coating, place the substrate on a vacuum chuck in the coater and drop appropriate PR, FH6400, on the center of the glass substrate: first step with 1000 rpm, 20 sec and second step with 4000 rpm, 50 sec.
(3) Soft bake, place it on the hot plate with 90 ° C and 80 sec.
(4) Expose, contact the mask and the glass substrate by the aligner with filter for 150 sec.
(5) Developing, develop the PR with developer, FHD5, for 90 sec.
(6) Rinse, clean the substrate with DI water for 240 sec.
(7) After Develop Investigation (ADI), check the pattern with optical microscope.
(8) Hard bake, place on the hot plate with 120 ° C and 100 sec.
In order to etch the indium tin oxide (ITO) transparent layer on glass, an etchant have to be prepared with specific recipe of mixture. The adopted mixture is HCl : HNO3 : H2O = 1 : 0.08 : 1.
The detailed description of the wet etching procedures is provided as follow:
(1) Etchant preparation, prepare the etchant with the wet etching recipe:
HCl : HNO3 : H2O = 1 : 0.08 : 1.
(2) Etching, put the sample in the prepared etchant, 2 min.
(3) Remove PR, put the sample in ultrasonic cleaner with ACE, 5 min.
(4) Dry, dry the glass with nitrogen gas blowing.
The detail processes of lithography and etching include initial cleaning, coating PR, UV exposure, develop, and chemical etching, as shown in Fig. 3-5.
Fig. 3-5. Detail processes of lithography and etching to realize the technology of ITO electrode patterning
3.2.3 Metal electrode deposition and patterning
For the rear panel substrate, a conventional soda-lime glass was selected to be the substrate of the rear panel. The two reasons are proposed. Firstly, we only need the light transmit to the front direction of panel. Moreover, the metal electrodes fabricated by the sputtering deposition provide its reflectance to enhance the exploitation of the light emission. In addition, a lift off processes including sputtering and PR striping is used to pattern the metal electrode.
The initial surface cleaning and the photolithography processes have been completed with similar procedures described in previous sections. For the photolithography processes, HMDS layer is coated to improve adhesion. Usually a thick photoresist is used in lift-off process. Therefore, a uniform thick photoresist, AZ4620, is coated onto the soda-lime glass substrate and the parameters of AZ4620 are shown in Table 3-3. Exposure process is then implemented and the mask pattern is transferred onto the glass with photoresist, AZ4620. Next step, the exposed glass is immersed in the developer, AZ300, and the desired pattern will show up in the photoresist. The sputtering deposition by a six targets magnetic sputtering apparatus is then operated to deposit a metal layer after a desired pattern from the photolithography processes.
Sputtering is a physical process that atoms from a solid target material are ejected into the gas phase due to ion bombardment of the material by energetic ions.
Sputtering is largely driven by momentum exchange between the ions and atoms in the material, due to collisions. The number of atoms from the surface per incident ion is called the sputter yield and is an important parameter of the efficiency of the sputtering process. Other things related to the sputter yield are the energy of the
atoms in the solid. The ions for the sputtering process are supplied by plasma that is ignited in the sputtering apparatus.
Table 3-3. Parameters of AZ4620 in the photolithography for lift-off.
AZ4620
Type Positive PR
Rotation speed and time 1st: 1000 rpm, 15 sec;
2nd: 3500 rpm, 30 sec.
Soft bake 90 ° C, 510 sec Exposure 490 sec.
Develop 180 sec
Rinse 240 sec
Thickness 8 µm
The detailed description of the photolithography procedures is provided as follow:
(1) HMDS prime, coat a HMDS layer on surface and bake with 150° C in a vacuum oven.
(2) Photoresist (PR) coating, place the substrate on a vacuum chuck in the coater and drop appropriate PR on the center of the substrate: first step with 1000 rpm, 15 sec and second step with 3500 rpm, 30 sec.
(3) Soft bake, place on the hot plate with 90 °C, 510 sec.
(4) Expose, contact the mask and the glass substrate by the aligner with filter, 150 sec.
(5) Developing, develop the PR with developer, AZ300, 90 sec.
(6) Rinse, clean the substrate with DI water, 240 sec.
(7) After Develop Investigation (ADI), use the microscope to investigate the electrode pattern.
(8) Hard bake, place on the hot plate with 120 ° C, 100 sec.
Table 3-4 shows the specifications of six targets magnetic sputtering apparatus and the experimental parameters.
Table. 3-4. Specifications of sputtering system and experimental parameters.
Specifications Experimental parameters Cathode 3" dia planar magnetron type Process power DC 0.1 kW
Total Power DC 2KW Thickness 1000 Å Main pump CTI Cryo pump 8 Time 615 sec Base level 3.0×10-7 Torr Deposition rate 1.62 Å/sec Rotation 15 ~ 40 rpm
Gas inlet Ar, N2
Deposition of the sputtered material tends to occur on all surfaces inside the vacuum chamber. Sputtering is used extensively in the VLSI semiconductor process to deposit thin films of various materials. Because of the low substrate temperatures used, sputtering is an ideal method deposit metals for display industry. Indium tin oxide (ITO) conducting coating layer on glass are also deposited by sputtering.
Sputtering sources are usually magnetrons that utilize strong electric and magnetic fields to trap electrons close to the target surface of the magnetron. The electrons follow helical paths around the magnetic field lines undergoing more ionizing collisions with neutrals near the target surface. The typical sputter gas is argon, a kind of noble gases. The extra argon ions created from above collisions leads to a higher deposition rate. The sputtered atoms are neutrally charged and so are unaffected by the magnetic trap [21, 22]. Fig. 3-6 shows the schematic diagram of typical magnetic sputtering mechanism.
Fig. 3-6. Schematic diagram of magnetic sputtering mechanism.
Lift-off process is then used to define the desired pattern on the soda-lime glass substrate without etching after sputtering deposition. A desired pattern is fabricated on a substrate by the pattern of photoresist as described in last section. From sputtering process chromium (Cr) film is deposited all over the substrate, covering the defined photoresist pattern and non-photoresist areas. During the lift-off process, the photoresist under the Cr film is removed with organic solvent and the Cr film is taken off. Finally, the lift-off process leaves only the deposited Cr film with desired pattern on the glass substrate.
The detailed description of the metal electrode patterning including lift-off and sputtering processes is provided as follow:
(1) Chamber vent, put the substrate with desired pattern.
(2) Change the target to chromium target and check the electric contact between the chamber and the target.
(3) Chamber pump, achieve base vacuum level to 3.0×10-7 Torr and then inject the reactive gas, argon.
(4) Set the parameters of chromium deposition: DC 0.1 kW, 615 sec.
(5) Chamber vent, take out the sample with chromium film.
(6) Chamber pump, keep the vacuum state in the chamber.
(7) Lift-off, place the sample in ultrasonic cleaner with ACE.
(8) Rinse, put the sample in ultrasonic cleaner with DI water, 5 min.
(9) Dry, dry the glass with nitrogen gas blowing.
The detail processes of rear metal electrode patterning include initial cleaning, coating PR, UV exposure, develop, sputtering and lift-off, as shown in Fig. 3-7.
Fig. 3-7. Detail processes of photolithography, sputtering and lift-off to realize the technology of metal electrode patterning
3.2.4 Panel formation
The fabrication process to finalize a panel type microplasma device by glass substrate includes panel sealing with front and rear panels and sealing of exhausting tube. First we prepare the vacuum sealant by a given recipe A : B = 3 : 1. Vacuum sealant epoxy (Torr Seal, VARIAN) as shown in Fig. 3-8 is a solvent-free sealant for high vacuum application and it can suffer vacuum level lower to 10-9 Torr, and temperatures from –45° C to 120° C. In addition, it is suitable for many materials including metal, ceramic, and glass. Gap distance between electrodes is guaranteed by specific spacers, thin soda-lime glass with a thickness of 400 µm. The panel sealing process is one that the vacuum sealant is applied on the edges of rear substrate and the front substrate is then covered on it. The pre-sealed panel is placed on the hot plate with temperature curing for 8 hours. Next step, the exhausting tube is connected to the exhausting hole on rear substrate by a same process. The above mentioned process is similar with that in plasma display panel (PDP) industry.
Fig. 3-8. Photograph of Torr seal for panel sealing.
3.3 Very High Vacuum (VHV) System
3.3.1 Vacuum technology
“Vacuum” is a space from that air or other gas has been removed. In practice, all the gas can never be removed and air is the most important gas to be pumped because it is in every system. The usual pressure unit is shown as follow.
1 atm (standard atmosphere) = 760 mm-Hg = 760 torr = 1013 mbar = 1.013×105 Pa (Pascal) = 14.7 psi
= 1.03327 kg/cm2
For convenience it is customary to divide the pressure scale blow atmospheric into several ranges and to relate phenomena and processes to them. Table. 3-5 lists the vacuum degree range. Epitaxial growth of semiconductor films takes place in the low vacuum in low vacuum range. Sputtering plasma etching and low pressure chemical
For convenience it is customary to divide the pressure scale blow atmospheric into several ranges and to relate phenomena and processes to them. Table. 3-5 lists the vacuum degree range. Epitaxial growth of semiconductor films takes place in the low vacuum in low vacuum range. Sputtering plasma etching and low pressure chemical