Penning Effect

A gas mixture consists of a rare gas containing impurity atoms possibly at very low concentrations. The impurity atoms have an ionization potential Vion which is lower than or equal to the metastable potential Vmeta of the minority nobles gas. The Penning effect in a 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, e.g.

Ne^∗+Ar→Ne Ar+ ⁺+e (2.8)

In a glow discharge, the Penning effect results in an increase of the ionization coefficient (Townsend first coefficient). In other hand, the breakdown potential and the cathode fall potential are decreased. The ionization and metastable energies of the rare gas are listed in Table 2.1.

Table 2.1 The ionization and metastable energies of the rare gas.

Ionization and Metastable energies

He 24.6 19.8, 20.6

Ne 21.6 16.6, 16.7

Ar 15.8 11.5, 11.7

Kr 14 9.9, 10.5

Xe 12.1 8.3, 9.4

Chapter 3

Fabrication and Measurement Instruments

3.1 Introduction

The interdigitated electrode micro-plasma device will be demonstrated in this chapter. The embodiment including the fabrication processes, technologies and instruments which are available to develop the structure of interdigitated electrode micro-plasma device will be described in the following sections. First, the semiconductor process including wet bench, furnace, spin coating, exposure, develop, sputter, lift-off and evaporate will be used. Besides, the features of the fabricated device were measured by typical semiconductor measurement systems, such as optical microscope, atomic force microscope (AFM), scanning electron microscope (SEM). In addition, the performance, such as ignition voltage, voltage margin and glow images were characterized in a specific vacuum system including charge-couple device (CCD) camera and pulse DC controller. The mentioned instruments mentioned above will be illustrated in this chapter.

3.2 Semiconductor Fabrication Process

The detail fabrication processes are listed below and the flow chart is shown in Fig. 3.1.

Metal Deposition

Barrier Rib Formation Substrate Cleaning

Silicon Oxide Growth

UV Exposure

Lift-Off

Dielectric Layer Deposition

Fabricated Device

Fig. 3.1. The flow chart of fabrication process for micro-plasma device (a) substrate cleaning (b) silicon oxide growth (c) metal deposition (d) dielectric layer deposition (e) barrier rib formation.

a. Substrate Cleaning:

First of all, silicon wafers were cleaned by RCA clean in wet bench, shown in Fig. 3.2. The RCA clean is the industry standard for removing contaminants from silicon wafer. Werner Kern developed the basic procedure at RCA (Radio Corporation of America) laboratories in 1960’s. As shown in Table 3.1, the main purpose for this procedure is removing organic residue and films from silicon wafers. The decontamination works based on sequential oxidative desorption and complexing with H2O-NH4OH-H2O2 (called “standard clean-1”, SC-1) at 75~85 °C. A second standard clean (SC-2) is often used H₂O-HCl-H₂O₂ at 75~85 °Cto further clean the surface.

SC-1 is used to remove the organic residues from silicon wafer. In the process, it oxidizes the silicon and has a thin oxide on the surface of the wafer which should be removed is a pure silicon surface is desired.

Fig. 3.2. The photograph of Wet Bench for silicon cleaning.

Table 3.1 The RCA clean procedure.

RCA Clean Steps

1. DI water rinse, 5 min

2. H²SO4 : H2O2 = 3:1 Organic Clean

3. DI water rinse, 5 min

4. HF : H²O = 1:100 Chemical oxide Strip

5. DI water rinse, 5 min

6. NH⁴OH : H2O2 : H2O = 1:4:20 (SC-1), 10 min (75~85 °C) Particle Clean 7. DI water rinse, 5 min

8. HCl : H²O2 : H2O = 1:1:6 (SC-2), 10 min (75~85 °C) Ionic Clean 9. DI water rinse, 5 min

10. HF : H²O = 1:100 Chemical oxide Strip

11. DI water rinse, 5 min

12. Spinner Dry wafers

b. Silicon Oxide Growth:

Because silicon wafer is a semiconductor, the electrode formed on the surface will influence the discharge properties without a buffer layer. In order to avoid discharge between electrode and silicon wafer, a quite thick silicon oxide (about 1 µm) was grown on the surface as a buffer layer. The buffer layer was grown at 1100 °C using both thermal wet and dry oxide by Furnace, shown in Fig. 3.3.

Fig. 3.3. The photograph of Furnace for thermal oxide.

Table 3.2 The growth of 1 µm silicon oxide for Si (100).

Thermal Oxide Growth (1100 °C)

Wet oxide 135 minutes

Dry oxide 20 minutes

c. Metal Deposition:

After growing a buffer oxide on the silicon wafer, the electrode was fabricated on the buffer oxide. Firstly, the HMDS is sprayed to increase the adhesion between substrate and photoresist. In the procedure, the positive photoresist (FH-6400) was spin coated on the surface of substrate and then was exposed UV light by Mask Aligner (MJB-3, Karl-Suss), shown in Fig. 3.4. Consequently, the pattern on the mask

was transformed to positive photoreisit after developing. The pattern was used to define the geometric feature of electrode layer. The width of the gap between each electrode and electrode are 20 µm and 15 µm, respectively. The metal electrode, chromium (Cr), was evaporated on the substrate with patterned photoresist by E-Gun Evaporation System and then the electrode layer was formed by lift-off technology.

Fig. 3.4. The photograph of Mask Aligner for photolithography.

Table 3.3 The lithography procedure for patterning the electrode.

Lithography Steps

1. Spray HMDS In a vacuum oven (150 °C)

2. Spin Coating Photoresit (FH-6400) 1^st Spin Speed: 1000 rmp, 10 sec.

2^nd Spin Speed: 3500 rmp, 40 sec.

3. Soft Bake 90 sec. at 90 °C

4. Exposure 40 sec.

5. Development 20~30 sec.

6. After Develop inspection (ADI)

7. Hard Bake 150 sec. at 120 °C

d. Dielectric Layer Deposition:

In order to make plasma by bipolar voltage waveform, the dielectric layer should be deposited on the electrode. The HfO₂ pellet which is the source material of dielectric layer was evaporated by E-Gun Evaporation System, shown in Fig. 3.5.

Before evaporating dielectric layer, the electrode pad was shadowed by the vacuum type.

Fig. 3.5. The photograph of E-Gun Evaporator System.

e. Barrier Rib Formation:

After depositing the dielectric layer (HfO2), the thick photoresist (XP SU-8 3050) was spin coated on the dielectric layer. Finally, the geometric pattern was realized by the lithographic techniques. The detail parameters are shown in Table 3.1. In the procedure, the thick photoresist (XP SU-8 3050) was spin coated on the dielectric layer. The first spin speed was 750 rmp for 25 seconds until the thick photoresist (XP SU-8 3050) reaches the edge of the substrate. The second spin speed was 3000 rmp for 40 seconds to obtain the desired thickness, referring to the attached spin speed curve in Fig. 3.6. Soft bake the coated substrate in two steps. Firstly increase the temperature from room temperature up to 65°C. Let the substrates at 65 °C for 1 min and then increase up to 90 °C for 15 min. After cooling gradually to the room temperature, the substrate was exposed by UV light for 25 seconds. Consequently, the

pattern on the mask was transformed to the thick photoresit (XP SU-8 3050) after post exposure bake (PEB). The time of post exposure bake (PEB) is 1.5 minutes at 65 °C and then 90 °C for 6 minutes. Developing 8 minutes in SU-8 Developer and rinse with Isopropanol (IPA). Once there is not any white traces the development is then finished. With temperature of 175 °C for 15 minutes, the hard bake was implemented before drying the wafer at the ambient air on a wet bench.

0

Fig. 3.6. XP SU8-3050 spin speed curve.

Table 3.4 The parameters of XP SU-8 3050 photoresist.

Product Spin Speed (rpm) Soft Bake Time Exposure Time

3.3 Measurement Instruments

After the fabrication of the interdigitated electrode micro-plasma device, the inspection will be performed to ensure that the fabricated structure agrees with the designed structure. At first, the optical microscope, atomic force microscope (AFM) and scanning electron microscope (SEM) will be introduced. In addition, a specific vacuum system is necessary to characterize the performance of the interdigitated electrode micro-plasma device after the fabrication process. Accordingly, a specific vacuum system, including charge-couple device (CCD) camera and pulse DC controller, will be illustrated in the following.

3.3.1 Atomic Force Microscope (AFM)

AFM consists of a scanning sharp tip at the end of a flexible cantilever across a sample surface while maintaining a small, constant force. The tips typically have an end radius of 2 nm to 20 nm, depending on tip type. The scanning motion is conducted by a piezoelectric tube scanner which scans the tip in a raster pattern with respect to the sample (or scans to the sample with respect to the tip). The tip-sample interaction is monitored by reflecting a laser off the back of the cantilever into a split photodiode detector. By detecting the difference in the photodetector output voltages, changes in the cantilever deflection or oscillation amplitude are determined. A schematic diagram of this mechanism is depicted in Fig. 3.7.

Fig. 3.7. Concept of AFM and the optical lever.

The two most commonly used modes of operation are contact mode AFM and tapping mode AFM, which are conducted in air or liquid environments. Contact mode AFM consists of scanning the probe across a sample surface while monitoring the change in cantilever deflection with the split photodiode detector. A feedback loop maintains a constant cantilever deflection by vertically moving the scanner to maintain a constant photodetector difference signal. The distance the scanner moves vertically at each x, y data point is stored by the computer to form the topographic image of the sample surface. This feedback loop maintains a constant force during imaging, which typically ranges between 0.1 to 100 nN.

Tapping mode AFM consists of oscillating the cantilever at its resonance frequency (typically ~300 kHz) and lightly “tapping” on the surface during scanning.

The laser deflection method is used to detect the root-mean-square (RMS) amplitude of cantilever oscillation. A feedback loop maintains a constant oscillation amplitude by moving the scanner vertically at every x, y data point. Recording this movement forms the topographical image. The advantage of tapping mode over contact mode is

that it eliminates the lateral, shear forces present in contact mode, enabling tapping mode to image soft, fragile, and adhesive surfaces without damaging them, which can be a drawback of contact mode AFM.

3.3.2 Scanning Electron Microscope (SEM)

Scanning electron microscope (SEM) is an essential instrument to measure the accuracy and fidelity of the fabricated devices, as shown in Fig. 3.8. Using a series of electromagnetic lenses to focus the accelerated electron beam, the diameter of electron beam can be converged to the dimension of 10^-3 µm. The secondary electrons are generated where the focused accelerated electrons bombard the sample. Detecting the secondary electrons can determine the location of bombardment. Simultaneously, the focusing electron beam scans the surface of sample, with the aid of scanning coil, to map the feature of measured area, as shown in Fig. 3.9. Using SEM, the feature variation of a few angstroms can be observed.

Fig. 3.8. Schematic diagram of scanning electron microscope.

In this thesis, a Hitachi S-4800 SEM was used to measure the features of the interdigitated electrode micro-plasma device. The electrode thickness, dielectric thickness, barrier rib height, aperture size of geometric pattern and cross-section of fabricated device can be accurately measured.

Fig. 3.9. Schematic diagram of Hitachi S-4800 SEM.

3.3.3 Vacuum System Setup

The specific vacuum system, illustrated in Fig. 3.10, contains the essential elements typically required to obtain high vacuum. In the designed vacuum system, there are three pumping devices, including two rotary vane vacuum pumps and the turbo molecular pump. Other components of the vacuum system, such as valves and gauges, aid the actions of these pumps. Besides, the charge-couple device (CCD) camera was adopted to capture the glow images. Accordingly, the electrical source was supplied by pulse DC controller. The details functions of the components are described below.

Fig. 3.10. The schematic diagram of the specific vacuum system.

a. Rotary Vane Pump:

The rotary vane pump's hollow body has a rotating cylinder mounted off-axis. In the rotor, two diametrically opposed radially directed vanes are spring-loaded to force contact with the pump body. Since the rotor is positioned off-axis, its motion causes the volume between the vanes and the body to vary during each half turn. The gas inlet port is so positioned that the volume behind the last vane to pass increases, allowing gas to expand into it until the next vane passes. As the volume exposed to the inlet increases, the volume trapped between the vane and the exhaust port decreases. In a single stage pump, the exhaust port has a valve connected to the atmosphere. In a two stage pump, the first stage's exhaust connects directly to the second stage's inlet. Gas exits the pump by bubbling up through the pump's oil reservoir. In either version, the gas is compressed against the final exhaust valve until it exceeds atmospheric pressure, at which point it forces open the exhaust valve and escapes. All the sliding surfaces, the bearings, and gaskets are lubricated and sealed

against gas leaks by the pump's oil. Considering the exhaust pressure illustrates an important but frequently overlooked factor of vane pump operation. With an operating temperature of, typically, 80°C and a gas pressure greater than atmospheric at the exhaust, chemical reaction between the gas and the fluid can increase when pumping corrosive gases and even explode if a hydrocarbon fluid is used when pumping 100%

oxygen.

In our designed vacuum system, two rotary vane pumps were used to be fore pumps. First one is a fore pump to vacuum chamber and the other is a fore pump to turbo molecular pump. When the chamber was exhausted below the critical pressure, the turbo molecular pump was turned on to obtain the expectative vacuum degree.

b. Turbo Molecular Pump:

A turbo pump has a stack of rotors resembling a jet engine. Each of rotors which has multiple and angled blades drive at very high speed with tangential direction. Gas molecules, hit by the underside of the angled blades, move with momentum in the direction of the higher pressure exhaust. Turbo pumps come in two basic designs. In the SNECMA design (named after a French jet-engine manufacturer) all gas enters through the main flange at the visible single-end of the pump; the Pfeiffer (named after the original turbo pump's manufacturer) double-ended design, has two rotors set-mounted on a common axle. Gas enters the pump through a right angle port between the two rotor sets and exits into an exhaust manifold connecting the two ends of the pump. For normal commercially available pumps, pumping speeds range from approximately 20 L/s to 3,000 L/s (although some turbo pumps manufactured in Russia are quoted with pumping speeds of 20,000 L/s). All gases are pumped at roughly the same rate. One pump, for example, listed with 450 L/s for nitrogen, has a pumping speed for hydrogen of 310 L/s. Turbo pumps reach full operating speed

within a few minutes of switch-on, making a separate roughing line unnecessary since the accelerating turbo can rough the chamber. The trick is to match chamber volume and the effective pumping speed, and to time the turbo pump's start so its rotational speed is high enough to prevent backstreaming when the chamber reaches 10^-1 Torr (that is, when an oil-sealed mechanical pump's relative backstreaming rate starts its rapid rise). With proper venting, a turbo pump can be entirely halted in under one minute. This slight delay before the chamber reaches atmospheric pressure and can be opened usually works for most applications. The benefit is clear. If the pump does not run while venting the chamber, we have no need for a high vacuum valve between pump and chamber. Further, in a correctly designed and operated system, the turbo pump does not allow oil vapor to backstream so that the system does not require an LN2 trap between chamber and pump. A turbo pump's high rotational speeds (some small diameter units operate at 60,000 rpm) put serious strain on the shaft bearings.

Most manufacturers now offer light-weight ceramic ball bearings with grease lubrication since this combination is both light, lowering the momentum of the bearing, and has a low bearing-lubricant vapor pressure at the pump's working temperature. More recently, manufacturers have begun offering magnetically levitated bearings which are either permanent magnets for the small sized pumps or a combination of permanent and dynamic magnetic fields, supporting the shaft without contact. Magnetically levitated pumps (such as those manufactured by Shimadzu) include non-contact bearings, resulting in a turbo pump that has true zero oil-vapor backstreaming with bearings that never wear down.

Using turbo molecular pump to obtain the expectative vacuum degree can avoid the oil contamination. The exhaust was connected to the second rotary vane pump, which produces a pressure low enough for the turbo molecular pump to work efficiently. In general, the turbo molecular pump can be a very versatile pump. It can

generate many degrees of vacuum from intermediate vacuum (~10^-4 Torr) up to ultra-high vacuum levels (~10^-10 Torr).

c. Gauge:

In order to check the vacuum degree of the chamber, two kinds of gauges were used to measure the different range of the vacuum degree.. The Baratron gauge is general purpose pressure transducers designed to provide accurate and repeatable pressure measurement in the range from 1 K Torr to as low as 0.05 Torr Full Scale (FS). However, the higher vacuum degree can be detected by ionization gauge which is used to measure the residual pressure of vacuum in the high vacuum and ultra-high vacuum ranges. Most of the ionization gauge cannot measure the higher pressures of the medium vacuum range, where the Pirani gauges, thermocouple gauges and convention gauges are use instead.

Fig. 3.11. The schematic of ionization gauge.

The ionization gauge consists of three distinct parts, shown in Fig. 3.11, including the filament, the grid and the collector. The filament is used for the

production of electrons by thermal emission. A + charge on the grid attracts the electrons away from the filament; they circulate around the grid passing through the fine structure many times until eventually they collide with the grid. Gas molecules inside the grid may collide with circulating electrons. The collision can result in the gas molecule being ionized. The collector inside the grid is - charged and attracts these + charged ions. Likewise they are repelled away from the + grid at the same time. The number of ions collected by the collector is directly proportional to the number of molecules inside the vacuum system. By this method, measuring the collected ion current gives a direct reading of the pressure. The above paragraph is a simplification of what happens. The design of the gauge head effects how efficiently electrons are produced, how long they survive, and how likely they are to collide with a molecule. Combining these factors together gives the gauge a sensitivity. As a general rule, the higher the sensitivity, the more efficient the operation of the gauge.

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d. Pulse DC Controller:

The fabricated device was excited by pulse DC controller, including a DC-pulse power controller (SPIK 2000A, SHENCHANG ELECTRIC CO., LTD) and a direct current (DC) power supply. The power supply provides the voltage to DC-pulse power controller and forms the output of bipolar voltage waveform. The bipolar voltage waveform, having the duty ratio with 20%, was applied with different frequencies between the interdigitated Chromium (Cr) electrodes which has a 20 µm gap by the pulse DC controller. Fig. 3.12 shows the photograph of the pulse DC controller set using in this thesis.

Fig. 3.12. Photograph of pulse DC controller and DC power supply.

e. Charge-Couple Device Camera:

The images of the glow dicharge from the micro-plasma device were captured by using a color charge-coupled device (CCD) camera (Watec, WAT-202D) with zoom lenes. The signal of the color charge-coupled device (CCD) cam era captured was connected to a personal computer by a capture card (UPMOST, MTV Video Capture).

Chapter 4

Experimental Results and Discussion

4.1 Introduction

The characterizations of the interdigitated electrode micro-plasma device will be presented in this chapter. The structure of interdigitated electrode micro-plasma device was fabricated agreement with the designed structure by photolithographic techniques. The interdigitated electrode micro-plasma device which has a simple fabrication process offers the advantages, such as low ignition voltage, low cost and

The characterizations of the interdigitated electrode micro-plasma device will be presented in this chapter. The structure of interdigitated electrode micro-plasma device was fabricated agreement with the designed structure by photolithographic techniques. The interdigitated electrode micro-plasma device which has a simple fabrication process offers the advantages, such as low ignition voltage, low cost and