Summary

The introduction to PDP history and fundamental working mechanism are introduced in this Chapter. Literature surveys point out the key features and state the critical issues of plasma display panels. Among those features and characteristics, MgO thin film as a protective layer is considered as one of the most important issues.

In the following chapter, experimental setup will be designed to modify the effect of MgO thin film on PDP performance. E-Beam evaporation would be adopted to deposit the MgO thin film on the dielectric layer. Flow of fabrication process would also be introduced in Chapter 3.

Chapter 3

Fabrication and Measurement Instrument

Plasma display panels (PDPs) can be made in large sizes and are expected to be the best candidates for wall-mount displays that are too big to be made using CRTs (i.e., 40-inch and larger). However, broadcasting of high-definition TV (HDTV) will start when digital TV broadcasting begins, and resolutions exceeding one million pixels will be required even for standard TV units.

3.1 Introduction

The markets for large area PDPs larger than 40 inch diagonal size have been expanded rapidly. The large area flat panel display is demanded for a HDTV and a digital TV which displays a real and life-size image in addition to the traditional TV image because of the development of the both computer and network technologies.

This propagation includes the three-color PDP in 1989, the 21-inch diagonal color PDP with 260 thousand colors in 1993, and the 42 inch diagonal full color PDP with 16.7 million colors in 1996. These results have opened the dream of a wall hanging TV. Looking back a history of color PDP, a development of the 21 inch PDP was the most important step in which the most essential technologies for large area PDPs have been completed. The commercialized 42 inch and 50 inch PDPs have been developed based on several technologies [26－

29]. This chapter will describe the manufacturing

technologies for modern color PDPs in the markets.

Before introducing the flow of PDP fabrication process, fundamental PDP structure would be reviewed firstly. The practically developed T-shaped PDP structure is shown in Fig. 3.1. Paired parallel display data electrodes, sustain electrode

X and scan electrode Y, are formed on the front glass substrate. Each display electrode is composed of a transparent SnO2 (ITO) and a narrow bus electrode of multi-layered Cr, Cu and Cr. They are capable to emit a luminance effectively through the transparent electrode and reduce the electrode resistance. These electrodes are covered with a dielectric layer which is made of low melting glass materials. This layer is covered by another protective layer－MgO thin film. Besides, striped address electrodes A are arranged on the rear substrate.

Fig. 3.1 The typical T-shaped structure which has been widely used in PDP products.

Striped type barrier ribs are on both side of the address electrodes to separate the adjacent discharge cells and to eliminate the optical cross-talk between each cell.

Three primary color phosphor materials for red, blue, and green colors are deposited in the neighboring channels made by the ribs to cover both of the side wall of the ribs and the dielectric layer. The structure has realized good performances such as a high luminance, a high luminous efficiency and a wide viewing angle.

Phosphor materials are BaMgAl14O23:Eu for blue, (Y.Ga)BO3:Eu for red，and Zn2SiO4: Mn for green. The substrates are assembled onto each other with about 120

µm gap. A Ne + Xe gas mixture is introduced between the cell gap. The fabrication

process is also simple enough to mass-produce so the PDP has advantages such as a low cost process and easiness to manufacture large area panels and high resolution panels.

3.2 Fabrication Process of Plasma Display Panel

Figure 3.2 shows the basic PDP fabrication process flow chart. First of all, the

front plate process would be introduced. The transparent conductive ITO film is deposited onto the front glass panel. The multiple paired data electrodes are deposited by photolithography technology. As shown in the figure, the metal electrode film consists of a Cr/Cu/Cr multi-layer is sputtered on these transparent ITO electrodes.

The bus electrode is also formed by photolithography technology. Besides, these electrodes are covered by a frit glass layer with screen printing method and then fired to about 600° C to turn out the dielectric layer. The seal glass layer with a width of about 3 mm is adhesive surrounded outside the display area with a pre-firing process.

Additionally, MgO protective layer is evaporated on the dielectric layer over the display area of inside of the seal layer.

Fig. 3.2 The basic fabrication process of plasma display panel [30].

Following are the rear plate fabrication processes. A small hole of a diameter of about 1 mm is drilled on a corner of the rear plate. The Ag address electrodes are printed and fired. The frit glass is printed on the electrodes in the display area and then fired about 600° C to form the dielectric layer. The barrier ribs are made by sandblasting the frit glass on both sides of the address electrodes and then fire. The red, green, and blue phosphors are printed inside of the channel between the barrier ribs. Each color phosphors are printed simultaneously and repeated three times, and then dried. The rear plate is completed with these processes.

Both front and rear plates are assembled and fixed with clips. The assembled plates are fired to melt the seal layer and the plates are glued to integrate the panel.

For the gas filling process, the panel is connected to an evacuation gas-filling system through the evacuating glass tube. After the baking step, the discharge gas mixture is then filled in. Finally, the whole PDP fabrication is completed after cutting off the evacuating tube. The driving pulse is applied to the panel and discharges are ignited in every discharge cells to reduce and make stable the operating voltage, which is known as the aging process.

3.2.1 Front Plate Fabrication I. Glass Substrate

The most famous glass substrate company in PDP business goes to Asahi Glass Co., Ltd. It is reported that the number of PDP television sets is forecast to rise to 9 million units in 2007 from 1.4 million units in 2003, demonstrating annual growth of 60% during this period.

Asahi Glass started its PDP glass substrate production in 1996 at the Kansai Plant, becoming the world's pioneer in the field, and since then the Company's PD200 model has become a de facto standard. The PD200 now accounts for 90% of the

global market for PDP glass substrate products. This glass substrate material for PDP has been developed to prevent the distortion and the shrinkage of the glass substrate in the firing processes of high temperature. The high strain point glass has about 100° C higher strain point comparing to the conventional soda-lime glass. This eliminated the distortion and reduced the shrinkage in the process and then made it possible to construct the process with a large process margin.

II. ITO Electrode

The ITO (Indium Tin Oxide) or SnO2 is designed for the use in AC-PDP. It is used for a transparent electrode in the front panel in order not to block the light transmission. ITO has excellent conductivity and good transparency but worse heat and etching resistance. Fig. 3.3 illustrates the processes for ITO and SnO2 formation in the front panel fabrication.

Fig. 3.3 Processes for ITO and SnO2 formation in the front panel fabrication [30].

The ITO film is made by a sputtering or an ion- plating method. It is patterned by photolithography processes. A photo resist is usually coated with a roll-coating machine because of it’s the large area. The expose uses a proximity method with a vertically supporting mechanics to prevent the bending of the glass substrate. As an exchange of a large photo mask is also a big issue, so the direct exposure machine is developed and practically used. The ITO film is etched with a solution of hydrochloric acid with small amount of nitric acid. The performances of the conductivity, transparency, etching ability, the adherence with Cr, resistance to the etching solution of Cr and resistance to the reaction with the dielectric layer while firing should be concerned to optimize an ITO films. Although SnO2 has advantages on the stability of the film quality and conductivity over ITO, the etching parameters and conditions are too difficult to control. The film is now patterned with the liftoff process with heat resistant photo -resist.

III. Bus Electrode

Bus electrode is also known as auxiliary electrode. Since alarming heat would be released during the gas discharge and lead to the increasing resistance of transparent electrode, bus electrode is used to reduce the electric resistance of the transparent display electrodes. Ag and Cr/Cu/Cr are currently used to be the bus electrode. An Ag electrode is made with a printing method or a photolithography with photosensitive paste including Ag and frit glass as shown in Fig. 3.4. The Cr/Cu/Cr is made with sputtering method and patterned with photolithography method. Bus electrode is contributive to control and stabilize the gas discharge and enhance the conductivity, so it is considered as an auxiliary electrode.

Fig. 3.4 An Ag electrode is made with a printing method or a photolithography process with photosensitive paste including Ag and frit glass [30].

IV. Dielectric Layer

Dielectric layer has good voltage resistance and is able to capture charges to achieve the memory effect. Dielectric layer is made by forming the layer of the low melting frit glass composed of and then fired at about 600° C to make transparent glass layer. It is important to make a uniform dielectric layer in the PDP fabrication process. The dielectric layer of a front substrate should have high transparency due to affect to the display performance. There are some methods to make the frit glass layer, such as screen-printing, slot coating, roll coating, and green sheet [30]. The most common method is screen-printing and shown in Fig. 3.5.

The glass layer is made by the way that the paste including frit glass and organic solution is deposited on the substrate through the mesh of screen. It is important to keep a sufficient time for leveling after printing to eliminate the roughness due to the mesh because the thickness uniformity affects on the differences in the discharge voltage of the cells. Beside screen printing method, two technologies are introduced.

The slot coating method is the way that the glass paste is deposited through the slot of

thickness around 10 µm. The roll coating method is the way that once the layer of a paste is made on the roll and then transcribed on the substrate. Both methods do not use the screen mesh. There however remains an issue of that if the drying condition is controlled insufficiently, the uniformity is not adequate and the crack of the layer appears. The green sheet method is the way that a dried layer made on the base film is put on the substrate and then fired. Although this also realizes a good uniformity, a sufficient degas while firing is required because much organic binder is included in the glass film.

Fig. 3.5 Screen printing method [30].

V. MgO Protective Layer

It is believed that the protective layer is the one of the most key element to realize a good performance. At the early stage of PDP development, even though a lot of materials were investigated for the protection function, but MgO was then found to be the most appropriate material for a protecting layer [31]. Other materials were never successful because MgO combines several unique characteristics. The performances requested for the protecting layer includes high secondary electron emission, high sputtering resistance, high transparency, non-conductivity, high stability in the PDP fabrication processes

As a result, MgO becomes the most appropriate materials for these requirements.

For the mass production, MgO layer is usually formed by electron beam evaporation method. E-beam evaporation can provide stable result and the cost is quite acceptable.

There are several new methods have been investigated in order to achieve more efficiently production, such as ion plating, reactive sputtering, plasma treatment, etc.

3.2.2 Rear Plate Fabrication I. Address Electrode

The address electrode process is almost same as the bus electrode in the front plate process. Since address electrode is located in the rear plate, there is no need to be transparent. Ag is currently adopted for the address electrode material. An Ag electrode is made with a printing method or a photolithography with photosensitive paste including Ag and frit glass. Just like bus electrode, address electrode is contributive to control and stabilize the gas discharge and enhance the conductivity.

II. Barrier Ribs

Barrier ribs are arranged and located to a spacer supporting front panel and separating each cell. The formation of barrier rib is one of the most unique processes in PDP fabrication and significantly effect could be seen on the cost. The thickness and width of the rib is around 150 µm and 70 µm, respectively. Although the printing technologies were used at first to make barrier ribs, the sandblasting method then took over due to the accuracy formation and excellent structure shape.

The sandblasting has advantage to get the high accurate barrier rib due to use the photolithography.

Fig. 3.6 shows the process for sandblasting method. A thick frit

glass layer is formed on the display area, and then covered with a dry photo-sensitive elastic film. The film is exposed through the film with barrier rib pattern and then

developed. The small hard particles are then sprayed out in a high pressure. Even though the frit glass layer of the area without elastic layer is cut, the area of the layer coated by the elastic film is remained because the particles would be reflected.

Fig. 3.6 Typically sandblasting method used to form the barrier ribs [30].

As shown in Fig. 3.6, the covered film is removed and then fired to complete the barrier ribs. The subject of the sandblasting process is that the 70 % of the materials formed on the plate are thrown away finally and then material cost is expensive. The recycle system of the material should be developed. In the photosensitive paste (PS) method, the thick PS layer composed of photosensitive material and frit glass is used.

As the glasses in the PS layers reflect and disturb to make accurate barrier rib, thick layer is not possible at once. The processes of formation of PS layer and exposition are repeated two or three times and then fired to form the barrier rib.

III. Phosphor Layer

While depositing phosphor on the barrier ribs, it is important to prevent the cross printing. The printing method for phosphor layers is shown in Fig. 3.7. It is the most practical and efficient method so far in the industry. The phosphor formation process is unique because the phosphor layer is deposited on the barrier ribs and sidewall inside of the channel. The required tolerance of the phosphor deposition with the printing is not severe comparing to the printing processes for electrodes and barrier rib. That is because that as the phosphor paste is filled in the channel through the screen, the pattern of the screen can be designed narrower than the channel width. The filled paste is then dried. The drying condition is important to make uniform phosphor layers. The thickness of the phosphors can be determined by the composition of the phosphors in the paste. The phosphor layer deposited is shown in Fig. 3.8.

Fig. 3.7 Screen printing method used to deposit phosphor layer on the barrier ribs [30].

Fig. 3.8 Phosphor layer on the barrier ribs with red, green, and blue color [30].

3.2.3 Panel Assembling

After the front and rear panel process, it needs assembling, evacuation and gas filling to complete the whole fabrication process. The front and rear plates are assembled to align the display electrodes and address electrode in orthogonal. As shown in Fig. 3.9, the plates are pre-fixed with clips. The evacuating tube is placed on the hole with the frit paste and PCB. After the assembling, it is fired about 400° C to glue the plate each other by melting the seal layer. The panel is connected with an evacuation and gas filling system through the evacuating tube. The panel is placed inside of the furnace and baked out about at 350° C to evacuate the adsorbed gases on the surface of MgO, phosphors, barrier ribs and dielectric layers. After the sufficient baking, the panel is cooled down to the room temperature and then discharge gas is introduced to the designed pressure. Finally, the PDP is completed after cutting off the evacuating tube. As this process is one of the most important processes to decide the characteristics of PDP, the temperature while evacuation, evacuation system without impurity, and purity of filling gas should be controlled carefully. After the assembling, aging test is required to stabilize the panel discharge condition in order to provide normalized panel performance.

Fig. 3.9 Assembling process of front panel and rear panel. It needs to fire up to 400 degree C to seal the panel [30].

3.3 Experimental Setup

It is believed that PDP performance is strongly influenced by the surface characteristics of MgO thin films because the film is exposed to the plasma directly.

Gas discharge and ion bombardment would sputter the surface structure and affect the panel properties. In this study, the deposition parameters are optimized and the relationship between the density of the MgO films and the properties of the AC-PDP is investigated. The influence of MgO density and surface morphology on panel properties has been examined with a accelerated life aging test.

The experiments in this study have been carried out to characterize the properties of twelve, 46-inch WVGA type plasma display panels. In order to characterize the effect of MgO thin film on PDP panel performance, several parameters would be chosen to make different formation MgO thin films. The experimental PDP modules are fabricated by ChungHwa Picture Tube (CPT) Co.

Table 3.1 The evaporation parameter of MgO thin film by E-Beam evaporation in this study.

MgO thin film as a protective layer in experimental PDP modules were deposited by electron beam evaporation. The deposition parameters are categorized in Table 3.1.

The MgO films were deposited at one of several value of O2 flow rates (10, 40, 50

sccm), and electron beam current (550, 600, 650 mA). Moreover, different film thickness (5000, 7500, 10000 Å) were selected to examine the experiments and the temperature in the chamber during the evaporation is controlled from 220 to 240˚C.

After the panel fabrication, the bonding process required to attach the specific driving circuits on the back of rear panel. Afterward, each experimental PDP module was operated in an accelerated aging environment. In this study, the aging process requires a frequency three times higher than the conventional driving waveform and all of the panels were operated continuously prolonged to 1500 hours for the data recording.

Generally, the conventional PDP manufactured by CPT uses 40 sccm O2 flow rate and 600 mA electron beam current to form a 7500 nm thick MgO thin film.

Accordingly, the evaporated parameters were designed to investigate the details and panel properties difference between conventional ones and experimental ones. It is believed that varied oxygen flow rate during the Mg and O atom formation can change the stoichiometery of the MgO thin films [32^－

34]. We believe that this change

Accordingly, the evaporated parameters were designed to investigate the details and panel properties difference between conventional ones and experimental ones. It is believed that varied oxygen flow rate during the Mg and O atom formation can change the stoichiometery of the MgO thin films [32^－

34]. We believe that this change

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