Excimer Laser Crystallisation of Poly-Si TFTs for AMLCDs
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Excimer Laser Crystallisation of Poly-Si TFTs for AMLCDs S D Brotherton, D J McCulloch, J P Gowers, J R Ayres, C A Fisher and F W Rohlfing Philips Research Laboratories, Cross Oak Lane, Redhill, Surrey, RH1 5HA, England ABSTRACT There is interest in reducing the shot number in the poly-Si laser crystallisation process in order to improve its throughput. Two distinct shot number dependent effects have been identified, which are both laser intensity dependent. The critical laser energy density is that which causes full film melt-through, and the major issue occurs at energies greater than this, where there is a considerable degradation in device uniformity with reducing shot number. The cause of this is non-uniform recovery of the full-melt-through fine grain poly-Si, and it is demonstrated that by extending the trailing edge of the beam, the material uniformity at reduced shot number can be improved. For energies less than this, the issue is not so much uniformity, as a general degradation in overall device properties with reducing shot number, which has been correlated with reducing grain size. In more demanding, future applications (such as system-on-panel), it will be necessary to improve circuit performance and approach that of current MOSFET devices. This will require short channel, self-aligned (SA) TFTs, and some of the issues with this architecture, particularly lateral ion implantation damage beneath the gate edge and drain field relief are discussed. 1. INTRODUCTION Excimer laser crystallisation is now the preferred technique for the formation of thin films of poly-Si which are used as the active layer in poly-Si TFTs. Whilst this technique, under optimum conditions, is able to form very high performance TFTs [1], there remain a number of issues in its application to commercial production. The broad objectives in applying the technique in a production environment are the formation of high quality films in a reproducible manner, with a large process window, and the maintenance of good uniformity under conditions of high throughput. Unfortunately, these conditions may be mutually exclusive: for instance, the highest quality and most uniform material is most easily produced with a large number of laser shots, but the high shot number would result in low throughput. Hence, in order to optimise these potentially conflicting requirements, it is necessary to develop a good understanding of the crystallisation process and to identify the key factors determining the resulting material properties. In this paper we discuss the main causes of non-uniformity in material which has been crystallised by a line beam excimer laser. The fundamental issues with the laser and beam shaping optics are: a) the pulse to pulse variation in output intensity from the laser, which, under critical conditions, can lead to large variations in material properties in the sweep direction of the beam, b) the beam profile along its short axis, which will influence the impact of (a),
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c) non-uniformities of the beam intensity along its
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