Papers

AVS1997 Session TF+FP-MoM: Thin Films and Processes for Flat Panel Displays

Monday, October 20, 1997 8:20 AM in Room B3/4

Monday Morning

Start	Invited?	Item
8:20 AM		TF+FP-MoM-1 Polycrystalline Silicon Thin Films for Active-Matrix Flat-Panel Displays T.-J. King (University of California, Berkeley) Poly-Si TFTs provide a number of advantages over their a-Si counterparts for active-matrix LCD application. This is because the carrier mobilities in poly-Si are significantly higher than those in a-Si, so that n- and p-channel devices with reasonable drive currents can be achieved in poly-Si ¹. The higher drive current allows smaller TFTs to be used as the pixel-switching elements, resulting in higher aperture ratio and lower parasitic gate-line capacitance for improved display performance ²; the capability to realize CMOS circuits allows low-power driver circuitry to be monolithically integrated with the display, for improved display-module reliability ³. Poly-Si TFT technology has found application in small, high-density LCD panels for viewfinder and projection display systems, and recently in mid-size AMLCDs for portable applications such as digital cameras. Although it is presently employed primarily in transmissive AMLCDs, it can be used for active-matrix addressing of reflective displays as well. With the trend in information display toward lightweight, low-power, rugged displays⁴, there will be a need to adapt active-matrix technology to plastic substrates in order to achieve high-performance, high-resolution, portable displays. Significant challenges exist for the development of a poly-Si TFT fabrication process which is compatible with plastic substrates: dramatic reductions in thermal-processing budget and process-module throughput are needed, concurrently with improvements in TFT performance. This paper discusses the technological approaches to surmounting these challenges and achieving high-quality poly-Si films for TFT application in compact, lightweight, low-cost high-performance display systems in the future. ¹W. G. Hawkins, IEEE Transactions on Electron Devices, Vol. 33, pp. 471-481, 1986. ²I-W. Wu, SID Digest of Technical Papers, Vol. 26, pp. 19-22, 1995. ³M. Takabatake et al., IEEE Transactions on Electron Devices, Vol. 38, pp. 1303-1309, 1991. ⁴L. W. MacDonald and A. C. Lowe, Display Systems (Wiley, New York), 1997.
9:00 AM		TF+FP-MoM-3 Characteristics of Amorphous and Polycrystalline Silicon Films Deposited at 120 °C by ECR-PECVD S.H. Bae, A.K. Kalkan, S.C. Cheng, S.J. Fonash (The Pennsylvania State University) We have investigated the characteristics of amorphous and as-deposited polycrystalline silicon (poly-Si) films deposited by electron cyclotron resonance (ECR) PECVD system. All films have been grown at 120 °C with hydrogen or argon diluted silane. Analysis with X-ray diffraction and TEM diffraction shows that the films deposited with H ₂ diluted silane contain significant amounts of crystalline phase while films grown with Ar diluted silane do not show any crystallinity. With only 3:1 H ₂ to silane ratio, the crystalline phase can be formed at 120 °C and even with the H ₂ to silane ratio increased to 13:1, there is no further increase in the crystallinity. We have also explored the effect of RF (13.5 MHz) bias to the substrate during film fabrication. According to X-ray diffraction, applying RF bias during deposition decreases the crystallinity in films. Since applying RF bias to the substrate can attract more ions from plasma to substrate rather than radicals, the presence of RF bias can not only increase order-disrupting bombardment but also change the concentration of radicals around the surface of growing films. We have found with cross-sectional TEM that the films deposited without RF bias using H ₂ diluted silane show a very uniform columnar structure whereas the films made with RF bias show a more continuous film. The films grown without RF bias have more sensitivity to buffered HF than the films deposited with RF bias. This sensitivity to buffered HF can be related to the presence of these columnar structures. With rapid thermal annealing at 700 °C, we have studied solid phase crystallization behavior of our films. We have found that amophous films deposited without RF bias display very high resistance to crystallization.
9:20 AM		TF+FP-MoM-4 Plasma Conditions for the as-grown Low Temperature Poly-Si Formation on the SiO₂ Substrate by a Sputtering and PECVD Processes M. Takeya, W.S. Park, G.S. Jong, T. Ohmi (Tohoku University, Japan) We investigated the plasma conditions for the formation of as-grown poly-Si thin film on the SiO₂ substrate with the low temperature(300°C) both PECVD(SiH₄/Ar/H₂) and Sputtering(Si-target/Ar/H₂) processes.We intended to compensate the thermal energy loss due to the low temperature processing with the low kinetic energy ion bombardment assists. Plasma parameters were measured by the tuned Langmuir probe method for 182.5MHz RF plasma. Crystallinity of the film was estimated mainly by the X-ray diffraction. Crystallization occurs by the in situ hydrogenation during the deposition both PECVD and sputtering processes. Under the same plasma condition, the PECVD processed thin films have the better crystallinity than the sputtering processed ones. In our experiment condition, ion bombardment energy was approximately 23eV. We define the normalized ion flux number(ni) and investigated the correlation between the ni and the film crystallinity. The results are as follows, the crystal size enlarges with the increasing ni up to 20 but it diminishes as the ni increases over 20. The film thickness and crystal size are 1000Å and 215Å, respectively at the optimum value of the ni in the PECVD process. Hydrogen radical is thought to be very effective for the crystallization of the film and as for the ion assist effect, there exists the optimum value of ni for a certain value of the ion bombardment energy.
9:40 AM		TF+FP-MoM-5 Vacuum Deposition Processes for Large Area, Roll-to-Roll Flexible Substrates W.B. Robbins (3M) An overview will be presented of large area, roll-to-roll vacuum deposition processes. Various issues will be reviewed specific to high rate deposition of materials onto flexible substrates in roll-to-roll form. Deposition processes to be covered include high rate non-reactive sputtering, intrinsically stable reactive sputtering and reactive evaporation. Processing techniques which produce high performance films will be described; these include web handling, web outgassing, vacuum pumping, inter-chamber isolation and gettering by deposition processes. In high rate reactive sputtering and reactive evaporation, specific requirements of flow and pressure measuring equipment useful for roll-to-roll coating will be addressed. To help control properties of deposited films, continuous monitoring of film properties can be performed on-line, in a non-contact fashion. Methods will be described to monitor the following parameters; optical transmission and reflection, electrical conductance and magnetic properties along with substrate temperature measurement. Other film properties may be derived from these measurements using a parametric technique. Such a technique as applied to the above methods will be analyzed. Adhesion performance and corrosion stability of thin films are often inadequate when deposited on polymeric web materials. General methods to improve these properties will be briefly addressed.
10:20 AM		TF+FP-MoM-7 Plasma Enhanced Chemical Vapor Deposited Silicon Nitride Thin Films Deposited at Very Low Temperatures for Thin Film Transistors on Plastic Substrates. T.M. Klein, T.A. Anderson, A.I. Chowdhury, G.N. Parsons (North Carolina State University) Plasma deposited silicon nitride (SiN_xH_y) is used as the gate dielectric in amorphous silicon thin film transistors (TFT's) in flat panel liquid crystal displays. Formation of TFT arrays on plastic substrates will require process temperatures to be reduced from 300 C to <150 C. A significant problem is the deposition of dielectric thin films with good electrical insulating properties at low temperature. We have used SiH₄, N₂, and He in a parallel plate capacitivly coupled PECVD reactor to form SiN_xH_y on 4" square substrates. To achieve low hydrogen concentrations at low temperature, N₂ was used instead of NH₃ as the N source gas. Films are characterized by transmission infrared spectroscopy and current vs. voltage electrical analysis. We find that at 250 C, N-H/Si-H bond ratios in excess of 1.5 can be achieved, indicating the N/Si ratio in the film is > 4/3, similar to films deposited from ammonia. When substrate temperature is reduced to 100 C using the same process conditions, the N-H/Si-H is reduced to 0.4, and the film becomes silicon-rich. Decreasing the partial pressure of nitrogen in the feed or by reducing the total pressure, leads to an increase the N-H/Si-H bond ratio, such that films with N/Si ratio >4/3 are deposited. The total hydrogen content at 250 C is approximately 15at.%, and increases to approximately 20 at. % at 100 C. Electrical results suggest that silicon nitride thin films with acceptable insulating properties can be deposited at <150 C, and specific electrical results will be demonstrated.
10:40 AM		TF+FP-MoM-8 Low Temperature Plasma Deposition of Silicon Nitride from Silane and Nitrogen Plasmas B.F. Hanyaloglu, E.S. Aydil (University of California, Santa Barbara) Low temperature deposition of good quality silicon nitride would enable manufacturing of thin film transistors for flat panel displays on alternative, lighter and less expensive, substrates such as plastics. The plasma enhanced chemical vapor deposition of silicon nitride films using SiH₄ and N₂ gases was investigated below 450 K in a helical resonator plasma reactor. Deposition rates, void fraction, and film composition were determined as a function of radio frequency power, operating pressure, and the substrate temperature using in situ spectroscopic ellipsometry and in situ attenuated total reflection Fourier transform infrared spectroscopy. Concentrations of H in the films bound as SiH or NH_x (x=1,2) were determined from the in situ infrared spectra. The optical properties of the deposited films were modeled using the effective medium approximation and as a mixture of silicon nitride, amorphous silicon, and voids. Film composition is determined by the relative fluxes of the silane fragments (SiH_x), H and N impinging on the surface. Silicon nitride deposited at high power and low pressure contains a relatively high concentration of H bonded to N resulting in films with high void fraction. Reducing the power or increasing the pressure reduces the void fraction in the films but results in incorporation of amorphous silicon (a-Si:H). Silane and N₂ dissociation must be balanced to deposit stoichiometric films with low void fraction and low a-Si:H incorporation.
11:00 AM		TF+FP-MoM-9 Real Time Substrate Misalignment Monitoring and Recalibration A. Hosokawa, R.E. Demaray, R. Mullapudi, M. Inagawa (Applied Komatsu Technology) Real-time substrate misalignment monitoring and automatic position recalibration was demonstrated in the Applied Komatsu Technology physical vapor deposition cluster tool. To accomplish this, accelerometers were installed at each module of the cluster tool to detect any unexpected vibration of substrate. The cluster tool consists of a load lock station, a transfer chamber, a preheat chamber and a process chamber. The substrate misalignment is monitored at the load lock station within +-0.05 mm accuracy using an optical sensor. If excessive substrate misalignment was detected at the load lock station, the accelerometer data was used to determine which specific module of the cluster tool was operating improperly. Using the monitored misalignment value at the load lock station, the specific module can be automatically recalibrated and proper operation of the cluster tool can be maintained without substrate breakage. Real-time substrate misalignment calibration avoids substrate breakage in the cluster tool, which results in an increase in the mean time between failure for the cluster tool. This technique has many practical applications for manufacturing operations in a vacuum environment.
11:20 AM		TF+FP-MoM-10 The National Technology Roadmap for Flat Panel Displays J.N. Bardsley (United States Display Consortium) Since 1993 the United States Display Consortium (USDC) has been jointly funded by the Defense Advanced Research Projects Agency and private industry to support the development of the infrastructure required for the manufacture of flat panel displays (FPDs) in the U. S., both to ensure that the displays required by the Department of Defense are available at affordable cost and to facilitate the entry of U. S. companies into this rapidly growing global industry. One of the activities of the USDC is the development of a technology roadmap. The core of the roadmap is organized at three levels, display applications, display manufacturing, and the supply of the necessary equipment and materials. The application segment is split into military and commercial chapters. The manufacturing segment covers six technologies, liquid crystal, electroluminescence, field emission, plasma, organic light emitting diodes and projection. One of the supply chapters is concerned with film processing techniques, another with materials, including thin films with desired optical, electrical, mechanical, or chemical properties. This talk will describe the goals and structure of the roadmap and will highlight the areas in which vacuum, plasma and thin film technologies are critical. Most of the funds which flow through the USDC are used for the development of new equipment and materials for the fabrication of FPDs. Examples of successful process equipment and thin film development projects will be given, including some funded by USDC. Preliminary conclusions concerning the health of the U. S. flat panel industry and suggested strategies for further growth will be given.