S. Mohajerzadeh, R. Tarighat, A. Goodarzi, B. Arvan and A. Akhavan 

1-Introduction

Plasma Display Panels are among the most promising technologies for the fabrication of large-area flat panel displays. Since its proposal as a display, several configurations for pixels of both alternative current (AC-PDP) and direct current (DC PDP) have been proposed. Some of the structures come with specific features which add to the complexity of the unit and consequently the overall cost becomes higher.

At present time, the major share of the PDP market is allocated to AC-PDPs due to their acceptable performance, durability and reliability and certain technical advantages in the fabrication process. Though DC-PDPs have their own advantages over AC PDPs like less importance of process cleanliness since DC sputtering gradually cleans the impurities remained on the electrodes. The common structure for a pixel of a PDP is like what is shown in Fig.(1) where plasma is being generated between two opposite electrodes and  the ionized gas that fills the environment emits ultra-violet light. Since the generated light is mainly vacuum ultra-violet (VUV) with typical wavelengths of 147 and 175 nm, a VUV-converting layer is needed to create visible light. In these structures transparency of one of the electrodes and its holding substrates is inevitable; forcing the use of transparent conductors like ITO.

Figure 1: typical AC-PDP pixel structure.

By applying a lateral structure a different situation arises since plasma is being generated between two side-by-side electrodes and electrodes themselves do not obstruct the generated light from emitting out of pixel area. Similarity of all pixels in a PDP is an important fabrication criterion and determines the final achievable size of the panel. One of the most important parameters in each individual pixel is the spacing between electrodes. In double-plated structures where anodes and cathodes are fabricated on two separated plates, this spacing is determined by the thickness of the spacer. Such PDP structures once exposed to an external twist, may lead to considerable variations in pixel characteristics as a result of spacing changes between two parallel plates. The fabrication process introduced here uses a lateral pixel arrangement and there is no need to use transparent conductors. As a result mechanical twisting of the panel does not cause any considerable alteration of the pixel spacing and the device characteristics.

2-Fabrication process

Fig(2) illustrates  the pixel structure used in this investigation. A layer of metal on the bottom substrate (usually chromium) is patterned using standard photolithography to form one of the electrodes and a layer of a suitable insulator passivates the whole surface of the first layer. Windows are opened in the passivated surface to make connections to the underlying layer as one of the addressing line. Each of these windows will be the active area of one electrode for each pixel. To produce the other electrode a layer of metal will be deposited on insulator layer and then patterned in the form of lines in the direction perpendicular to those of the first layer (see Fig (3)). Also shown are the additional steps like sputter cleaning to remove the remaining unwanted layers from electrode surface and to reduce the turn-on voltage of each pixel.

Figure 2: The schematic of the pixel investigated in this work

3-Results and discussions

Using this technique we have successfully fabricated a monochrome 5X6 cm lateral PDP with a pixel pitch of 1mm and with green phosphor embedded glass and PET encapsulation caps. Fig. (4) shows the turn-on voltage characteristic of a pixel in Ar ambient versus pressure. A reasonable light intensity is feasible with a current about 10-20μA and typical turn on voltages range over 350-550V in Ar ambient depending on the pressure and pixel geometries. As seen from this figure, a minimum value of turn-on voltage occurs at a pressure of 300 torr.

Figure 3: Turn-on voltage versus ambient pressure.

For driving a pixel a series resistance of about 10 MΩ is used to limit the current after plasma turns on. Fig.(6) shows a picture that our PDP has generated. Some defective lines are clear in the flower’s picture which could be prevented with more accuracy in the fabrication process. This is the first picture obtained using a plasma display panel with a lateral electrode configuration.

A typical picture generated by the fabricated PDP. Picture area is about 5x6 cm and the PDP is in a chamber in proper vacuum. The connecting ports are evident in the picture.

The driving circuitry is a discrete array of vertical and horizontal switches which are derived by a computer-controlled logic circuit. In this figure the chamber that the PDP is in and the ports connecting the circuitry to PDP addressing lines are evident.

Figure 5: A line of pixels turned on in a flexible PDP. The pixel pitch is 1mm and a very high bending is externally applied. The photo has been taken in an yellow room.

Also, for the very first time a small flexible PDP is fabricated using this technique. Fig.(7) shows a line of this flexible PDP that’s on under an external bending.

4-Conclusion

A sample 5x6 cm monochrome PDP is fabricated using a novel lateral pixel structure. Also using simple external circuitry we have been able to generate the first picture on glass, although a simple line has been demonstrated on flexible PET bases. Pixel turn-on voltage-pressure characteristics have been examined. For the first time a flexible PDP has been realized and a photo showing an intentionally bent PDP with a lit line is presented. Under high curvatures no immediate change in pixel characteristics has been observed.