Papers

AVS1998 Session TF-ThM: Thin Films for Flat Panel Applications

Thursday, November 5, 1998 8:20 AM in Room 310

Thursday Morning

Start	Invited?	Item
8:20 AM		TF-ThM-1 New Dry Etch Applications for Amorphous TFTs in Flat Panel Displays(FPD) W.W. Yao (dpiX; A Xerox New Enterprise Company) Key goals driving FPD process development are: 1. large panel size; 2. Low cost; 3. Low power consumption. Recent introduction of new process technologies especially in the dry etch area are key enablers for new display architecture. Increase in display size with longer gate and data lines is driving the switch to aluminum metal with its lower resistivity to reduce the RC time delay. Hillock free Aluminum gate metal is enabled by tapered Al dry etching and high rate PECVD process with short time-temeprature cycle. Cost reduction has focused on productivity improvement but new 3rd generataion tools has renewed interest recently in reduced mask count display architecture. ITO pixel is etched using very strong acids and dictates its placement in the process architecture. New ITO dry etch removes process constraints and allows placement of the ITO on top of the passivation dielectric. Power consumption is dominated by backlight intensity and can be reduced by higher aperture ratio pixel design. The ITO pixel dimension is increased if the TFT size and its assoicated coupling capacitance is shrinked through the use of selective n+ etch in intrinsic silicon.
9:00 AM		TF-ThM-3 Field Emission and Photo Emission from Si Micro Tip Arrays Coated with Bias-Grown Diamond Films M.Q. Ding, A.R. Krauss, O. Auciello, D.M. Gruen, T.D. Corrigan (Argonne National Laboratory); M.E. Kordesch (Ohio University); D. Temple, D. Palmer, G.E. McGuire (MCNC) A considerable improvement in the properties of field and photoelectron emission from ungated Si micro tip arrays coated with bias-grown diamond thin films is reported. Prior to loading into a MPCVD reactor (ASTeX PDS-17), the microtip arrays were ultrasonically treated in a 0.1 µmm diamond suspension. Diamond films were grown in a CH₄-H₂-N₂ plasma at a substrate temperature of 800 °C and a negative bias of -150 V. The film had a complete coverage over the tip arrays as observed from secondary electron microscopy (SEM). Electron emission characteristics were measured in two different systems: field emission current-field (IF) measuring apparatus and photoelectron emission microscopy (PEEM). IF measurements showed a very low turn-on electric field with a threshold fields of 1.5 V/µmm (vs 40 V/µmm for uncoated tip arrays), and a current density of 1 mA/cm2 at around 4 V/µmm. In the PEEM studies, UV light from a mercury arc lamp was used to excite photoelectrons. While the lamp was on, the PEEM revealed a uniform and bright photoelectron emission image of the coated tip arrays in contrast to that of the uncoated arrays. When the lamp was off, stable field electron emission images of the coated tip arrays were also seen with a reasonable brightness at an electric field 5.6 V/µmm, whereas images of uncoated arrays could hardly be discerned and emission was unstable. Such a remarkable improvement in both field emission and photoemission properties, as compared to those of the uncoated arrays, indicates that the microtip arrays coated with bias-grown diamond films are promising for applications such as flat panel displays. A possible mechanism for the improvement will be discussed. This work is supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-39.
9:20 AM		TF-ThM-4 Hydrogen Concentration Distribution in Plasma Deposited Hydrogenated Amorphous Silicon and Silicon Nitride Films B.F. Hanyaloglu, D.C. Marra, E.S. Aydil (University of California, Santa Barbara) Understanding H distribution in plasma deposited hydrogenated amorphous silicon and silicon nitride (a-SiN:H) films as well as at the interfaces of these films is important for manufacturing of thin film transistors (TFTs) for flat panel display applications. Hydrogen concentration distribution and bonding in plasma deposited a-Si:H and a-SiN:H thin films were studied using in situ multiple internal reflection Fourier transform infrared spectroscopy in conjunction with in situ spectroscopic ellipsometry. The infrared spectra as a function of time were recorded both during deposition of the film and during etching with CF₄ plasma. The hydrogen concentrations as a function of depth below the film surface were obtained from the spectra recorded during the deposition and the etching experiments. Analysis of the spectra shows that the H distribution in a-Si:H and a-SiN:H films is surprisingly complex and far from uniform. The a-Si:H film consists of a very thin H rich layer at the surface that is primarily composed of SiH₂ and SiH₃. This H-rich surface layer is followed by a few 100 Angstrom thick subsurface region that is depleted in H compared to the bulk film. The bulk a-Si:H film grows beneath these two layers, which move up and stay at the surface during deposition. In a-Si:H deposition, there is evidence that H penetrates into the film through a process other than simple diffusion. We have also investigated H distribution and bonding in a-SiN:H films deposited on a-Si:H and a-Si:H films on a-SiN:H in order to study how the deposition sequence alters the chemical composition and structure of the a-Si:H/a-SiN:H interfaces. Implications of these experimental results on the differences in the peformance of top-gate and bottom-gate configuration TFTs will be discussed.
9:40 AM		TF-ThM-5 Poly-Si Thin Film Transistors Fabricated on Low Temperature Plastic Substrates P.G. Carey, P.M. Smith, P. Wickbolt, S.D. Theiss (Lawrence Livermore National Laboratory) Flat panel displays made on plastic substrates are envisioned for use in certain commercial and military systems because they are more rugged and lightweight than displays made on glass substrates. High information content can be attained for such displays using active matrix arrays of thin film transistors (TFTs). In this talk the fabrication of poly-Si TFTs on flexible plastic substrates will by discussed. Plastic substrates pose severe temperature constraints on the fabrication process. To overcome these constraints, our group at LLNL has used low temperature (<150C) silicon, oxide, and aluminum thin film deposition steps and pulsed excimer laser processing to perform the TFT channel crystallization and the source/drain doping.
10:20 AM		TF-ThM-7 Stability of Very Low Temperature Amorphous Silicon Thin Film Transistors on Flexible Plastic Substrates C.S. Yang, L.L. Smith, C.B. Arthur, G.N. Parsons (North Carolina State University) Active matrix transistor arrays on transparent plastic substrates will enable new high resolution flexible and rugged large area electronic display systems, including liquid crystal displays (LCDs), and organic light emitting displays (OLEDs). Hydrogenated amorphous silicon thin film transistors (TFTs) for active matrix LCDs are currently formed on glass substrates using temperatures in excess of 250°C. Lower temperature processes are of interest for TFTs on plastics, but stability of low temperature TFTs has not been reported. In this presentation, we will describe low temperature (<125°C) plasma deposition processes for amorphous silicon and silicon nitride, including atomic force microscopy (AFM) analysis of substrate surfaces, nucleation, and film growth. We will also describe the fabrication of bottom gate inverted staggered TFTs on 7 mil (190 µm) polyethylene terephthalate (PET) substrates. Low-temperature devices showed linear mobilities >0.3 cm²/V-s and off currents <2x10^-12 A, with on/off ratios ~10⁷. We will also show results of stability tests under dc electrical stress, and compare the low T devices with conventional TFTs formed at higher temperatures. We find that when electrical stress (V_g=25 V, V_sd=10 V) is applied for 60 seconds to a device formed at 250°C, the threshold voltage on a I_d vs V_g plot shifts from 1.5 V to approximately 3.0 V. Upon negative applied gate bias, the drain current onset is observed to shift to more negative voltages, consistent with state creation in the amorphous silicon layer near the amorphous silicon/silicon nitride interface. For the device formed at 110°C, a larger initial threshold of 5.0 V is observed. However, after 60 s dc stress, the threshold shifts to 7.0 V, so that the change in threshold is similar for the two devices. These results indicate that amorphous silicon is a viable material for stable active matrix array electronics on low temperature plastic substrates.
10:40 AM		TF-ThM-8 Advanced Deposition Technique for Producing Thin Films of Polycrystalline Silicon J.B.O. Caughman, D.B. Beach, G.L. Bell (Oak Ridge National Laboratory) An improved plasma enhanced chemical vapor deposition technique has been demonstrated for depositing poly-crystalline silicon thin films for flat panel display applications. The technique combines a high density radio frequency (rf) inductively coupled plasma source with downstream gas injection that has resulted in device quality films deposited at high rates. Unlike conventional rf reactors, inductively coupled sources have high plasma and atomic species density along with low ion energies hitting the film surface during growth. Our system uses a planar induction coil for the plasma coupling that creates a dense hydrogen plasma over a large area (30 cm diameter). The hydrogen plasma serves as a source of large quantities of atomic hydrogen that aid in the deposition process. For better control of the plasma chemistry, silane (100%) is injected downstream, where the precursors needed for film growth are separated from the ionization region. The films are deposited on quartz samples on a heated substrate (< 400 degrees C). The power coupling mechanism (inductive vs. capacitive coupling) has been analyzed by using an rf sensor (located after the matching network) to determine processing conditions favorable for polysilicon growth. Deposition rates increase substantially with the amount of inductive power coupling and reach values of 60-80 nm/min. Increasing the coupled power beyond 1 kW eventually leads to a decrease in the net deposition rate, possibly due to increased etching of the deposited film by the hydrogen. Deposition results show that the conductivity (10^-7 S/cm) and the crystallinity (>80% based on the Raman spectrum) of the films are good. Analysis of the X-ray diffraction spectrum shows a highly preferred grain orientation in the <110> plane. Details of the deposition conditions and the power coupling mechanism will be discussed. ¹Research sponsored by the Laboratory Directed Research and Development Program of ORNL, managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under contract no. DE-AC05-96OR22464.
11:00 AM		TF-ThM-9 Polycrystalline Silicon Films Deposited Directly on Glass by Reactive Magnetron Sputtering Using a Microcrystalline Silicon Nucleation Layer D.S. Kim (Samsung Advanced Institute of Technology, Korea); J.E. Gerbi, J.R. Abelson (University of Illinois, Urbana-Champaign) We investigate the microstructure of polycrystalline silicon (px-Si) thin films which are deposited directly onto glass substrates using reactive magnetron sputtering in a single-pumpdown, two step growth process. px-Si films are of technological interest as thin film transistors in flat panel displays and as absorber layers in solar cells. In the first step, we deposit a microcrystalline Si (µmc-Si:H) film <20 nm thick by sputtering a Si target in (Ar + H₂) at 230 °C. The large hydrogen flux produces µmc-Si:H with no detectable interfacial layer of amorphous silicon, due in part to subsurface implantation effects. The substrate temperature is then to 400 °C and px-Si is deposited by sputtering in Ar only. The uc-Si:H acts as a nucleation layer such that the px-Si growth reaches steady-state within 1 grain diameter; in the absence of this nucleation layer, the first 0.3-0.5 µmm of "px-Si" growth at 400 °C is amorphous silicon. We characterize the nucleation, crystallinity, and grain size of the px-Si films using real-time spectroscopic ellipsometry during growth, and using post-deposition TEM, X-ray diffraction, and Raman spectroscopy. By optimizing the growth parameters of the nucleation layer, including hydrogen partial pressure, thickness, and the use of alternating deposition and atomic hydrogen exposure, px-Si films with [110] preferred orientation and grain diameter > 400 Å can easily be obtained. We will also report preliminary electrical characterizations in both the as-deposited and post-hydrogenated states.