Rapid Thermal Processing of Thin Film Electroluminescent Display Materials
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RAPID THERMAL PROCESSING OF THIN FILM ELECTROLUMINESCENT DISPLAY MATERIALS D. M. Kim*, F. Qian*, R. Solanki*, R. T. Tuenge**, and C. N. King** Department of Applied Physics and Electrical Engineering, Oregon Graduate Center, 19600 N.W. Von Neumann Drive, Beaverton, OR 97006 ** Planar Systems, Inc., 1400 N.W. Compton Drive, Beaverton, OR 97006 *
ABSTRACT Rapid thermal annealing of the electroluminescent phosphors ZnS:Mn, SrS:CeF 3 and ZnS:SmCI 3 has been examined as a function of annealing temperature (500-750°C) and time of exposure (10-120 sec.). The resulting brightness and efficiency of luminescence are correlated with the different processing conditions used. The results indicate that the brightness can be significantly improved from the value obtained with furnace annealing without causing film delamination, blistering or fatigue effect. INTRODUCTION The growing interest in flat panel displays stems from the panels being light and compact. As a result, this technology offers an attractive route by which to utilize the advantages of compactness of microelectronics on the level of instrumentation. Of all the flat panel sub-technologies, liquid crystal and thin film electroluminescent displays show most promise for full color displays. The liquid crystal display, based on rapidly maturing thin film transistor technology holds up the potential for low power color display. On the other hand, thin film electroluminescent (TFEL) device [1-4] is rugged and capable of producing high visual quality. The TFEL devices in Mn-doped ZnS have already found substantial commercial applications. A functional, matrix addressed multiple color TFEL display has been demonstrated[2]. To further improve the viewability and to realize the full color display, it is important to process higher brightness phosphor films, particularly in red and blue spectral regions. Efficient phosphor hosts for red and blue electroluminescence are alkaline earth sulfides which require higher temperature processing than the zinc sulfide host. One practical approach for this end is to post anneal conventionally evaporated films by using rapid thermal anneal (RTA). During furnace cycles above 500°C, the glass substrate softens, the indium tin oxide (ITO) conductivity decreases and the interface between the phosphor and dielectric degrades. Therefore, the RTA technique is obviously well suited for this application. In this paper we present a study of transient annealing of ZnS:Mn, SrS:CeF 3 and ZnS:SmCI 3. Here, the anneal temperature and time were methodically varied. The resulting brightness and luminescent efficiency were then correlated to the different process conditions used. The results indicate that there exists an optimal set of process conditions, capable of producing higher brightness phosphors without causing the deleterious effects mentioned earlier. DEVICE PROCESSING AND MEASUREMENT Cross-sectional view of a typical electroluminescent device is illustrated in Fig. 1. The structure consisted of the double-insulated construction, using sputtered
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