Large-Area Pulsed Laser Deposition (PLD)
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ABSTRACT PLD of uniform thin films on 4"-wafers has been realized by the integration of a PLDsource into a commercial MBE-system. Thickness homogeneity over the total substrate area is obtained by precise spatial control of the plasma plumes excited simultaneously at 4 adjacent target locations. Computer controlled motion of a cylindrical target with respect to the stationary focal spots of the laser beams, has to provide (a) a suitable deflection of the plume axis and (b) a uniform target erosion. Process control was investigated by computer simulations. The efficiency of the developed technique for large area coating is illustrated by the preparation of DLC thin film specimens and their ellipsometric characterization. INTRODUCTION Most of the industrial process lines for the synthesis of electronic devices are capable of handling substrates of diameters - 4". However excellent properties of thin films synthesized by PLD were mainly achieved on small areas only, typically 1 cm'. Consequently, an upscaling of PLD was necessary to provide uniform coatings at large area substrates. Earlier work on large-area PLD was published by Greer [1] in 1989. His arrangement involves a rotating substrate, a large-diameter counterrotating target, and a scan of the laser beam at the target surface. Substrates of 1" diameter were homogeneously coated by the so called off-axis PLD of Erington and lanno [2] in 1990. This technology utilizes an offset of the plume to the rotating substrate without laser beam scanning. Nonuniformities could be kept < ± 5 % over a substrate area of 2" by this method [6]. An improvement of this
procedure was suggested by Eddy [3] and Smith [4]. Additional to the rotation a translation of the substrate holder was introduced. They succeeded in the deposition of thin films having maximum deviations of ± 6 % over 5" substrates. Computer controlled substrate motion is provided. Greer achieved a homogeneous deposition at 3-diameter substrates (nonuniformities < ± 6 %) by a refinement of his method [5]. These developments were initiated by the demands for perfect films of high-temperature superconductor (YBa 2Cu3 O 7 .). Substrate motion is necessary for all the technologies considered above. This could be a disadvantage for in-situ thin film characterization like ellipsometry, RHEED or X-ray diffraction as applied in the preparation of semiconductor films or X-ray multilayers. In the technology proposed by the present authors, substrate motion is optional. Film uniformity is achieved only by the control of the ablation plume. Knowledge about the particle distribution in the plume is necessary to optimize the deposition process for every large-area PLD technology. The angular distribution of the particle flux I(a) is suitably described by a cosine power function: 1(4) = C cos(oX),
83 Mat. Res. Soc. Symp. Proc. Vol. 382 01995 Materials Research Society
(1)
where C and n contain the information on intensity and width of the plume, respectively. The shape exponent n sensitively depends on laser beam parameter
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