3. EXPERIMENTAL TECHNOLOGIES
3.2 Device Fabrication
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 vapor deposition are performed in the medium vacuum range. Pressures in the very high vacuum are necessary for thin-film preparation, mass spectroscopy, crystal growth, electron microscopy, and the production of cathode ray tube. Pressure in ultra high range is required got material and surface research work [23].
The degree of vacuum state can affect the physical characteristics. Mean free path and gas molecular density are important factors in vacuum. Higher vacuum degree causes lower number of gas molecules in space and the influence in experiment from gas molecules is smaller [24].
Table 3-5. Classification of vacuum degree and related parameters.
Material of vacuum parts, selection and disposal of pump and installation of each part are important factors to make good vacuum environment. One of the most troublesome issues in vacuum state is gas release from solid materials at low pressure.
There is the slow evolution of additional gases and vapors from the interior surfaces to prolong the pumping time. The surface gas release result from several potential processes including vaporization, thermal desorption, diffusion, permeation, and electron- and ion-stimulated desorption. Fig. 3-9 shows all potential sources of gases in a vacuum system [21].
Fig. 3-9. Potential sources of gases and vapors in a vacuum system.
Gas diffusion from the wall of vacuum container contributes to the outgassing of the system and it is formed by the thermal motion of atoms or molecules. Permeation is a three steps process including adsorbing, diffusion, and desorbing and it depends on temperature and pressure. A vapor is a gas near its condensation temperature and vaporization is thermally stimulated entry of molecules into the vapor state. Vapor pressure is caused by the pressure of vaporization at specific temperature, and vapor pressure changes with different temperature and materials. A vacuum system consists of many sub-systems and parts. Sub-systems comprise chamber, vacuum gauge, vacuum pump and etc. are assembled with flange, O-ring, gasket, trap, feedthrough, valves and other elements. Leaks can be divided into real leak and virtual leak. A real leak is one that is coming into the vacuum chamber from an outside source. A virtual leak means a gas load evolving from inside the vacuum system from outgassing.
Correct vacuum sealing mechanism and suitable sealing components or materials can make the desired vacuum environment. In general, there are two seal methods between flanges, which are O-ring type and oxygen-free high conductivity copper (OFHC) gasket type. The O-ring type is only suitable for medium vacuum level and the material is elastic polymer. When the excepted vacuum level is less then 10-7 Torr or the temperature is more than 200 ºC, OFHC gasket type is used for better airtightness.
3.3.3 Setup of very high vacuum system
To have high purity gas content in our microplasma device, a very high vacuum environment is essential. The base level of the very high vacuum system in our laboratory is about 10-7 Torr. In order to achieve and maintain the base level, two mechanical rotation pumps and a turbo molecular pump have been selected.
Turbo molecular pump is using high rotation speed turbo vanes (50,000 rpm, pumping speed for N2 is 145 l/sec). The advantages of turbomolecular pump are the pumping speed is high and there is no oil contamination like the diffusion pump.
However, turbo pump is using the high rotation speed turbo vanes, and it may cause vibration issue. Some experiments which need a non-vibration environment are not suitable for turbo molecular pump such as scanning tunneling microscope (STM) and atomic force microscope (AFM). In a turbomolecular pumped system, a forepump with sufficient capacity should be chosen to keep the vane nearest the foreline in molecular flow or just in transition flow at maximum speed.
Two mechanical pumps are used in our very high vacuum (VHV) system. First one is the forepump for the vacuum chamber and the second one is the forepump for the turbomolecular pump. When the first forepump speed be large enough to keep the rough chamber pressure below the critical pressure and the turbomolecular pump is then turned on to achieve research base vacuum level. The second forepump is responsible for the rough pressure of turbomolecular pump which is separated
The detailed description of the VHV system operating procedures is provided as follow:
(1) Turn off high vacuum gauge, close the manual gate valve between turbo pump and chamber, and turn off power of all pumps.
(2) Purge with He gas, open the main window and clean
(3) Turn on two mechanical pumps, check the reading value of vacuum baratron gauge.
(4) While achieve 10-2 ~ 10-3 Torr, turn on the turbo molecule pump.
(5) While turbo pump get full speed, close the inline valve between the chamber and the foreline mechanical pump of chamber. Open the gate valve between turbo pump and chamber; turn on the high vacuum gauge.
(6) Gas filling: while the reading value of the high vacuum gauge achieve base pressure 10-7 Torr and close the gate valve between turbo pump and chamber. Inject research gas and adjust pressure with needle valve.
(7) Turn off the turbo pump, after 30 min. turn off mechanical pumps.
The nano-tip enhanced microplasma device was connected to the very high vacuum system with an adapter fitting (Ultra torr, Swagelok). Different gases or gas mixtures are injected into the vacuum system and the research pressure is adjusted with a needle valve and measured by a baratron gauge (628B, MKS). A photograph of the very high vacuum system is presented in Fig. 3-10.
Fig. 3-10. Photograph of designed VHV vacuum system.
3.4 Measurement Instruments
Discharges are excited by bipolar pulsed signal, an electric pulsed power source set including a pulse power controller (SHENCHANG ELECTRIC, SPIK 2000A; 0-1 kV, 2 kW) and a direct current power supply was used to input the waveform. The DC power supply provides the energy to pulse power controller and output the pulse power. The 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 pulsed power source. The discharge current and voltage were measured by a 1 kΩ resistor in series and a voltage probe in parallel, respectively. The photograph of the electric pulsed power source set is shown as Fig. 3-11.
Fig. 3-11. Photograph of pulsed power source set.
An image observation system including a color charged-coupled device (CCD) camera (Watec, WAT-202D), a zoom lens set (NAVITAR, Zoom 6000), a digital
camera, and a personal computer, is used to record the discharge phenomenon.
The light emitted from the glow discharge was recorded using a color charged-coupled device (CCD) camera. With a zoom lens set, the discharge was imaged on the personal computer by a capture card. When a large-scale discharge area was needed, a digital camera was utilized to record the optical appearance of the glow discharge.
Chapter 4
Results and Discussion
4.1 Introduction
In this chapter, the experimental results including electrical properties and optical appearances of nano-tip enhanced microplasma devices will be presented. The main idea for adopting nano-tip in the microplasma is to locally enhance the electric field
In this chapter, the experimental results including electrical properties and optical appearances of nano-tip enhanced microplasma devices will be presented. The main idea for adopting nano-tip in the microplasma is to locally enhance the electric field