Chapter 1 Introduction
1.4 Organization
Based on the concepts mentioned above, the objective of this thesis is to realize how the MgO thin film properties affect the PDP performance. Moreover, the PDP working mechanism and fabrication process will also be introduced. The experimental setup will be carried out firstly for the optimization and analyses; then, the investigation and comparison will be implemented to characterize the MgO thin film properties. This thesis is organized to review the basic theories and previous literatures in Chapter 2. Fabrication process and measurement instrument are described in Chapter 3. The experiments with results and discussions will be placed in Chapter 4 and following is the conclusion in Chapter 5.
Chapter 2
Principle of Plasma Display Panel
In chapter 2, the basic knowledge of PDP will be introduced, including PDP history, theory of gas discharge (plasma), fabrication process, and the key materials adopted in plasma display panels. Furthermore, the most critical material MgO thin film as a protective layer will be particularly surveyed in the final section of this chapter.
2.1 PDP History
The plasma display panel was invented at the University of Illinois at Urbana-ChampaignT (UIUC) by Donald L. Bitzer and H. Gene Slottow (shown in Fig.
2.1) in 1964 for the need for a high quality display for computer-based education,
PLATO (Programmed Logic for Automatic Teaching Op-erations) Computer System[8].
Fig. 2.1 The inventor of plasma display panel: Prof. H. Gene Slottow (left) and Prof. Donald L.
Bitzer at the University of Illinois in 1967 [8].
Looking back, it was an innovating proposal at that time but may look obvious right now. In those days, the best computers adopted vacuum tubes to achieve the desired function. The first PLATO system used a TV set and a Teletype keyboard that was connected to the University’s ILLIAC vacuum tube computer [9]. One of the key issues of this new graphics display invention was to have inherent memory so that the bulky and expensive scan converter tube memory could be eliminated. Fig. 2.2 shows an early plasma display as it was connected to the glass vacuum system used for the first generation. The first device used neon gas to generate the familiar neon orange glow.
Fig. 2.2 Early plasma panel attached to the glass vacuum system at the University of Illinois.
Arrow points to the 1-in by 1-in panel. This had the same alternating sustain voltage, neon gas, and dielectric glass insulated electrodes that have been widely used for PDP today [9].
Somehow, this vacuum system had a leak that a small amount of air would be added to the neon. The solution for this situation to prevent the leakage was to add a portion of a percent of nitrogen into the neon to achieve inherent memory.
The original monochrome panels were achieved and became popular in the early 70s. Then there followed a long period of sales decrease in the late 1970s as semiconductor memory made CRT displays relatively cheaper than plasma displays.
However, IBM introduced a 19-inch monochrome display (orange on black) which was able to show four simultaneous 3270 virtual machine (VM) terminal sessions in 1983. And late in 1992, Fujitsu announced the world's first 21-inch full color plasma display panel. A hybrid based on the plasma display created at the University of Illinois at Urbana-Champaign and NHK STRL, achieves superior brightness and good image quality. In 1997, Pioneer started the business selling the first plasma television in the markets. At that time (1922), screen sizes can only achieve about 21 inches.
Right now, the largest plasma display in the world shown at the CES (Consumer Electronics Show) in Las Vegas in 2006 made by Matsushita Electrical Industries (Panasonic) measured 103-inch.
Until quite recently the superior brightness and wider viewing angle, plasma display panels become one of the most popular forms of display for HDTV. However since that time improvements in LCD technology have closed the gap dramatically.
The lower weight, price and power consumption of LCDs have seen them make large inroads into the former plasma market.
2.1.1 Invention of Plasma Display Panel
Back to the early stage of plasma display, Fig. 2.3 shows the details of the plasma display that appear in the original patent [10]. Basically, the fundamental concept was to insulate the driving electrodes with dielectric layers and located between the electrodes and the neon gas mixture contained in cells. Two main issues should be notified. First of all, this is a pretty practical way to limit the current of the gas discharge and prevent arcs. The dielectric layer could be deposited quite uniform and at low cost. Secondly, the dielectric layer could be used to store charge on the walls, a necessary requirement for the inherent memory feature. Each pixel could store its own isolated wall charge, which allows pixels to be in either the on or the off state even when placed along a external electrode.
Fig. 2.3 The original drawings of plasma panel from the University of Illinois at Urbana Champaign (UIUC) [10].
Fig. 2.4 shows the first plasma panel with multiple pixels. This result was first
published in 1966 [11]. This was a major achievement because it was also the panelthat demonstrated the first matrix addressability [12]. The image on the panel was selectively addressed using a write pulse applied through a resistor that biased the sustain voltage to the point where a discharge would occur. This is the first time that the inventor named this device the “Plasma Display Panel.”
Fig. 2.4 Early 4 by 4 pixel panel presented in the first publication of the plasma display panel by University of Illinois in 1966. This panel was the first to have more than one pixel [11].
In 1967, another important achievement by the University of Illinois was to develop the first color plasma panel, which is shown in Fig. 2.5 [13]. The ultraviolet (UV) light generated by a xenon gas discharge to excite red and green phosphors was used in this project. There are three cells situated in this device, including one with a red phosphor, a green and a non-phosphor one. All plasma TVs produced today use the UV light from xenon gas discharge exciting the phosphors in the exact same way it was done by this early device.
Fig. 2.5 First color plasma panel was this three cell prototype with red and green color phosphors excited by a xenon gas discharge. It was developed at the University of Illinois in 1967. All of today’s color plasma TVs basically generate light in this way [13].
2.1.2 Practical Commercial Structure
Accordingly, the devices developed by the University of Illinois proved that the fundamental ideas are feasible, but they were still too fragile for public applications or commercial products. The original devices were made with three 150µm thick sheets which were commonly used for microscope cover slips. Very thin transparent gold electrodes were placed on the surface of the outer sheets, and the inner sheet had holes for each pixel, as shown on the right side of Fig. 2.6. On the other hand, torr-seal vacuum epoxy was adopted here to bond these three sheets together. One problem for this arrangement is that they could not be baked to a temperature much higher than 100°C or the epoxy would decompose. As a result, leakage, breakage, and gas contamination became the biggest issues in this condition.
In 1968, some manufacturers developed the panel shown in Fig. 2.7 having the open-cell structure shown in Fig. 2.8. [14] This structure consists of two robust 6-mm-thick substrates made of soda-lime glass and could be fabricated in the mass
production line. The thick film electrodes made of gold with glass paste was screen printed and fired on each of the substrates and then coated with a 25 µm thick-film lead–oxide-based solder-glass dielectric layer. This device could be baked under vacuum at 350°C to drive out contaminants and then filled with an all inert penning gas mixture of Ne plus 0.1% Ar. This panel was strong and the gas remained pure.
Fig. 2.6 Prof. H. Gene Slottow manually addressing a 16 by 16 pixel plasma panel, developed by University of Illinois in 1967. Magnified view on the right shows the very small 1-in by 1-in panel.
Fig. 2.7 Early open-cell structure developed by Owens-Illinois in 1968. The substrate glass was 6 mm thick and the dielectric glass layers were 25 µm thick [14].
Fig. 2.8 Open-cell structure was used in this 128 by 128 pixel plasma display developed by Owens Illinois in 1968 which measured 4-in by 4-in. It consisted of a robust 6-mm-thick glass substrate, thick film gold electrodes, and a screen printed solder glass dielectric layer [14].
Later in 1968, they developed a new type consisting of 100x100 mm area and an array of 128x128 pixels, as shown in Fig. 2.9. The front plate of this plasma display is a module for the modern PDPs. The front plate of today’s PDPs has a fundamental structure very similar to that used for the front plate. After this very significant breakthrough, the University of Illinois continued to play an important role by teaching to the industry the art of electronic addressing and sustaining [15]. The plasma display received significant recognition when it won the prestigious IR-100 Award in 1968. The 16x16 pixel panel shown in is the most beautiful example of the old micro-sheet panels at that time.
Fig. 2.9 16 by 16 pixel plasma display panel. The Industrial Research 100 Award was given to the University of Illinois in 1968 [15].
2.1.3 Key Features for TV Development A. DC Plasma Displays
In the early days before PDP was invented, direct-current (DC) PDP operated by neon negative glow had been developed for the display application. The DC- PDP was invented by Burrough Co. in the early of 1970s [16]. In those days, they used a resister to limit the discharge current instead of the capacitor. DC-PDP had a dc discharge that acted like a shift register and addressed the individual pixels in the panel. It had lower circuit costs due to the reduced number of external panel electrodes. They also developed a dc plasma memory method that did not require resistors [17]. Later on, this had been used in many future prototype color displays.
For many years until early 90s, there were existing two types of plasma displays. The type that used resistor current limiting became known as the DC-PDP. The original plasma display panel that used capacitor current limiting became known as the AC-PDP.
B. Grayscale
After successfully making plasma in the cell, how to create the desired image becomes the next topic. However, there comes a big problem for grayscale images because a pixel was either “on” or “off.” As a result, to make a half- on state becomes a critical issue. Unlike CRT using the electron gun to scan each line, PDP uses memory effect to gather wall charge for the ignition of plasma. The inherent memory of PDP has advantages for making bi-level graphic displays. By the memory eddect, PDP uses wall charge to decide the “on” or “off” state of each cell. In 1972, the grayscale problem was solved independently by Mitsubishi [18] and Hitachi. This used a technique of writing and erasing every pixel many times in a given frame so that a grayscale could be observed based on the amount of time the given pixel was on during the frame. This driving skill is well known as the Address While Display (AWD) method.
C. Full Color PDP
In 1978, NHK (Japanese Broadcast Corporation) as known as the most active group for DC-PDP revealed a high-quality full-color plasma TV panel with a 16 in diagonal
[19]. After that, NHK continued the work and keep a major force in the
development of color DC-PDP. This came as a natural consequence of their pioneering development of high-definition television (HDTV). NHK realized that high resolution image looked fantastic on a large screen because the visual acuity of the eye limits the ability to see high resolution on a small screen. For this reason, they believed that the large screen potential of the plasma display could solve this problem.However, even the future for color PDP looks very exciting in the mid 1970s, it takes more than two decades of research for the practical color PDP products to emerge.
2.2 Plasma and VUV Generation
In the following pages, the basic concept and working mechanism of the transformation from ionized gas to visible light would be introduced. PDP uses ionized gas discharge (plasma) to generate vacuum ultraviolet (VUV) to stimulate the phosphor layer. The stimulated red, green and blue phosphor layer would emit visible light though the front panel to the human eyes. Fig. 2. 10 shows the visible light emission process of plasma display panels.
Visible Light
Fig. 2. 10 The visible light emission process of plasma display panels. VUV is generated by plasma and is going to stimulate the phosphor layer to emit visible light.
2.2.1 Principle of Plasma Physics
What is plasma? The central element in a fluorescent light is plasma. Plasma is known as the fourth state of matter in the universe besides solid, liquid and gas.
Basically, a gas made up of free-flowing ions (electrically charged atoms) and electrons (negatively charged particles) is identified as plasma. As shown in Fig. 2. 11, under normal conditions, a gas is mainly made up of uncharged particles. That is, the individual gas atoms include equal numbers of positively charged particles and
electrons. The negatively charged electrons perfectly balance the positively charged protons, so the atom has a net charge of zero.
Fig. 2. 11 Plasma is recognized as the fourth state of the matter. Figure 2.11 shows the other three states in the universe includes of solid, liquid, and gas.
Plasma can be generated by introducing a lot of free electrons into the gas via establishing an electrical voltage across it. The free electrons collide with the atoms, knocking loose other electrons. With a missing electron, an atom loses its balance. It has a net positive charge, which makes it an ion. In the plasma with an electrical current running through, negatively charged particles are rushing toward the positively charged area of the plasma, and positively charged particles are rushing toward the negatively charged area. Under this circumstance, particles are constantly bumping into each other with a high frequency. These collisions excite the gas atoms into the plasma, causing them to release photons of energy. For example, xenon and neon atoms, the atoms which have been used in plasma displays release light photons when they are excited. In PDP’s case, these atoms release ultraviolet light photons
(Fig. 2. 12), which are invisible to the human eye. But ultraviolet photons can be used to excite visible light photons by stimulating the phosphor layer.
Fig. 2. 12 Where does the UV come from? Ultraviolet is emitted while a electron jumping back to the ground state of excited state.
2.2.2 Ionization and Gas Discharge
Unlike direct ionization, penning ionization is defined as the ionization that occurs through the interaction of two or more neutral gaseous species, at least one of which is internally excited. If A* denotes an excited electronic state of atom A and B the ground electronic state of another atom (or molecule), then the process
A* + B → A + B+ + e－ (2a) is known as Penning ionization (PI), and the related process
A* + B → (A B)+ + e－ (2b) is called Associative ionization (AI). Reactions of this type have long been considered by worker dealing with ionized plasmas, where order-of-magnitude estimates of rate coefficients have been sufficient for these purposes. If A* is metastable, however, it
may live long enough to be studied by molecular beam method [20].
Among those types of gas discharge, xenon and neon are the most popular gas adopted in the cells for plasma display panel. Taking Xe and Ne for example, the gas discharge process can be categorized into four parts, (I) The excitement and de-excitement of the electron impact; (II) The ionization and recombination of the electron impact; (III) Two-body heavy particle collisions; (IV) VUV radiation.
The Excitement and De-Excitement of the Electron Impact
Xe + e → Xe* + e (2-1) Ne + e → Ne* + e (2-2) Xe
*+ e → Xe + e (2-3) Ne
*+ e → Ne + e (2-4)
The Ionization and Recombination of the Electron Impact
Xe + e → Xe
++ e + e (2-5)
2.2.3 Vacuum Ultraviolet (VUV)
As described in 2.2.2, based on the different ionization phenomenon, plasma display panel would be able to emit visible light by the gas discharge and VUV generation first [21].
VUV Radiation
Xe
2*→ Xe + Xe + hυ (2-12)
Fig. 2. 13 The energy level and states of xenon gas discharge. PDP uses 147 nm and 173 nm UV light to stimulate the phosphor laer.
PDP uses the excitation and emission of the particles by emitting 147nm, 152nm and 173 nm wave length to generate the ultraviolet (UV). The energy level and states of the gas discharge used in plasma display is illustrated in Fig. 2. 13. During the UV
transportation, around 30% of the energy transferred to the kinetic energy of electrons and the other 70% transferred to the kinetic energy of charged particles. Under acceleration of electric field, the average kinetic energy of charged particles can only reach to several electron volts due to its lighter weight compared to that of electron.
This low kinetic energy of charge particle can not provide sufficient help for UV generation but transforming into heat loss. On the other hand, around 20% of the electron kinetic energy will transform into the UV light through inelastic collision. As a result, only 6% of the imported energy will be capable transformed into the UV light generation.
2.2.4 Photoluminescence
Photoluminescence is a process in which a chemical compound absorbs a photon with a wavelength in the range of visible electromagnetic radiation, transitioning to a higher electronic energy state, and then radiates a photon back out, returning to a lower energy state. The period between absorption and emission is typically extremely short, on the order of 10 nanoseconds. Under special circumstances, however, this period can be extended into minutes or hours.
Ultimately, available chemical energy states and allowed transitions between states (and therefore wavelengths of light preferentially absorbed and emitted) are determined by the rules of quantum mechanics. The simplest photoluminescence processes are resonant radiations, in which a photon of a particular wavelength is absorbed and an equivalent photon is immediately emitted. This process involves no significant internal energy transitions of the chemical substrate between absorption and emission and is extremely fast.
More interesting processes occur when the chemical substrate undergoes internal energy transitions before re-emitting the energy from the absorption event. The most
familiar such effect is fluorescence, which is also typically a fast process, but in which some of the original energy is dissipated so that the emitted light is of lower energy than that absorbed.
An even more specialized form of photoluminescence is phosphorescence, in which the energy from absorbed photons undergoes intersystem crossing into a state of higher spin multiplicity, usually a triplet state. Once the energy is trapped in the triplet state, transition back to the lower singlet energy states is quantum mechanically forbidden, meaning that it happens much more slowly than other transitions.
2.3 Driving Waveform
The most popular driving waveform goes to the address display separated (ADS) method proposed by a Japanese Company Shinoda [22]. The driving waveform used in one of the sub-fields is demonstrated in Fig. 2.14. With ADS driving method, cells are first erased by a reset step, then addressed (i.e. memory charges are deposited in the cells which need to be ON during this sub-field), and the addressed cells are turned ON during the sustain period because the previously generated wall charges in the address period served as priming seeds. The address and display periods are therefore clearly separated in the ADS method.
Moreover, the cells are addressed line by line. One voltage is applied to all the X electrodes. The reset period is used to erase space charges in the cell and set all the cells to the same initial state; the memory charges are first erased, afterward, all the cells are then turned ON with a write pulse, and then erased again. Memory charges are written on the cell when the scan pulse of a given Y electrode coincides with a
Moreover, the cells are addressed line by line. One voltage is applied to all the X electrodes. The reset period is used to erase space charges in the cell and set all the cells to the same initial state; the memory charges are first erased, afterward, all the cells are then turned ON with a write pulse, and then erased again. Memory charges are written on the cell when the scan pulse of a given Y electrode coincides with a