Large Excimer Laser Research Articles

Ageing of laser crystallized and unhydrogenated polysilicon thin film transistors

Ageing of low temperature polysilicon Thin Film Transistors (TFTs) is reported in this study. The active layer of these high performances transistors is amorphous deposited using Low Pressure Chemical Vapor Deposition (LPCVD) technique and then laser crystallized using a single shot ECL (SSECL of SOPRA) with very large excimer laser. The drain and source regions are in-situ doped during the LPCVD deposition by using phosphine or diborane to fabricate n-type or p-type transistors respectively. These laser crystallized TFT's show poorer reliability properties than solid-phase crystallized TFT's. This poor stability is explained to originate from the high surface roughness produced by the laser crystallization, which is highlighted from Atomic Force Microscopy observations. Moreover to this conclusion, the behaviour of the threshold voltage shift ΔV T during positive and negative stresses is checked to the light of a stretched exponential law that is, as supposed, a federative law. This law is explained in hydrogenated amorphous silicon TFT's by a dispersive diffusion coefficient of hydrogen in the disordered material. Taking into account that such relation appears as sufficiently general and, particularly, can describe the behaviour of monocrystalline silicon MOSFET and un-hydrogenated polysilicon TFT's where the hydrogen cannot involved, it can be supposed that it deals with disordered materials and disordered regions in crystalline materials (interface, grain boundary, …..).

Single shot excimer laser crystallization and LPCVD silicon TFTs

One of the most interesting ways to develop good quality polysilicon films in order to fabricate thin film transistors (TFTs) on low cost glass substrates is by laser crystallization of amorphous silicon (a-Si). Crystallization technique using pulsed-ECL is characterized by films with high electrical properties but low uniformity. In this way, technology using single shot ECL with very large excimer laser (VEL) may be very promising. It is used here to crystallize undoped amorphous silicon films deposited by LPCVD which constitute the active layer of n-type thin film transistors. The performances of these TFTs are studied versus the laser fluences and the number of shots at fixed fluence. An optimum fluence is highlighted from the behaviours of the subthreshold slope S and the threshold voltage V T of these TFTs. The optimum occurs at lower fluence when the number of shots increases. The behaviour of the field effect mobility shows a maximum. This maximum occurs at higher fluence than that of the S and V T optimum. Then at low fluence, the effect of the shot number is more important on S and V T parameters, which are more linked to the insulator-silicon interface, than on the mobility. The difference is explained from the energy actually absorbed at the film surface during the laser annealing.

Single shot excimer laser crystallization of silicon films deposited by LPCVD

The quality of polysilicon obtained from the crystallization of amorphous silicon films deposited by the low pressure chemical vapor deposition technique (LPCVD) in classical furnace, is known to be high. TFT's fabricated from this material show a field effect mobility greater than 60 cm 2 V −1 s −1. To improve this quality, single shot very large excimer laser crystallization of LPCVD films is studied using characterization techniques such as atomic force microscopy observations, X-ray diffraction, Hall mobility measurements and spectroscopic ellipsometry. Undoped, highly phosphorous and boron doped, amorphous and crystallized deposited films are used to give a large overview about the possibilities of this crystallization technique. Film properties are continuously improved when the laser fluence increases. AFM shows an excellent uniformity of the film whatever the laser fluence. However, a sudden degradation happens above a fluence value. This degradation is owing to cracks certainly induced by the mismatch of thermal expansion coefficients of the substrate and the film.

Very-Low Surface Roughness in Laser Crystallized Polycrystalline Silicon

ABSTRACTExcimer laser (ECL) crystallization of silicon films on low temperature substrates is one of the most promising technique for large-area polycrystalline silicon films. Crystallization techniques using pulsed-ECL were extensively studied. They are characterized by films with high structural and electrical properties but low uniformity. In this way, the technology using single shot ECL with very large excimer laser (VEL) may be very promising. It was used for the crystallization of amorphous undoped films deposited by PECVD or by LPCVD The LPCVD technique is the most commonly used deposition technique of silicon. It presents numerous advantages and the hydrogen content in the films, known to constitute a drawback in the crystallization process leading to an increased surface roughness, is negligible (∼1 at.%). However, even with these low hydrogen content LPCVD films, high surface roughness is observed after the laser crystallization. Hence surface roughness appears as one of the major problems to be solved in the high performance TFT's realization from laser crystallized films.In this work, the reduction of the surface roughness, determined from Atomic Force Microscopy observations, is presented. This reduction originated from the use of a surface oxidation and an etch treatment to remove the oxide. The mean height of the roughness is then reduced by about 50%.Using these low surface roughness polycrystalline films, TFT's are then realized.

Double-pass versus expanding-beam concept for very large excimer laser amplifiers

In this communication, the scalability of large excimer laser amplifiers with nonsaturable absorption is examined in terms of two extraction geometries. These geometries include a single-pass expanding-beam concept, recently proposed as a means to extend the laser length beyond an absorption length product of unity, and a conventional double-pass rectilinear (nonexpanding) arrangement. It is shown that the double-pass configuration provides the same total-energy output with greater efficiency than the expanding-beam concept (assuming a constant expansion angle), but has an overall length that is smaller by a factor of ∼3.

Damage Mechanisms in Optical Materials For High-Power, Short-Wavelength Laser Systems

Damage to optical materials under intense photon irradiation has always been a major problem in the design and operation of high-energy and high-average-power lasers. In short-wavelength lasers, operating at visible and ultraviolet wavelengths, the problem appears to be especially acute; presently attainable damage thresholds seriously compromise the engineering design of laser windows and mirrors, pulsed power trains and oscillator-amplifier systems architecture. Given the present interest in ultraviolet excimer lasers and in short-pulse, high-power free-electron lasers operating at visible and shorter wavelengths, the “optical damage problem” poses a scientific and technological challenge of significantdimensions. The solution of this problem even has significant implications outside the realm of lasers, for example, in large space-borne systems (such as the Hubble Telescope) exposed to intense ultraviolet radiation.The dimensions of the problem are illustrated by the Large-Aperture krypton-fluoride laser amplifier Module (LAM) shown schematically in Figure 1. This device, now operating at the Los Alamos National Laboratory, is typical of current and planned large excimer lasers for fusion applications. The LAM has an active volume of some 2 m3, and optical surfaces (resonator mirror and windows) exceeding 1 m2 in size; the fabrication of these optical elements was the most expensive and time-consuming single item in the construction of the laser. During laser operation, a population inversion in an Ar-Kr-F2 mix ture is created through electron-beam excitation of the laser gas by two 400 kA beams of 650 keV electrons from a cold cathode discharge. The electron trajectories in the gas are constrained by a 4 kG magnetic field transverse to the optical axis produced by a pair of large Helmholtzcoils.

Expanding beam concept for building very large excimer laser amplifiers

Optical extraction efficiency from conventional lasers and amplifiers with nonsaturable intrinsic absorption is substantially below the maximum efficiency at absorption length products exceeding unity. This result is caused by the fact that high intensities in the amplifier cause saturation of gain but not absorption. In this letter, a new scalable amplifier concept is analyzed which maintains near maximum extraction efficiency as the laser length increases beyond absorption length products of unity.