Anomalous Behavior in the Resistivity of N-I-N Polysilicon Resistors After Hydrogenation
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ANOMALOUS BEHAVIOR IN THE RESISTIVITY OF N-I-N POLYSILICON RESISTORS AFTER HYDROGENATION Chad B. Moore and Dieter G. Ast Dept. of Materials Science & Engineering, Bard Hall, Cornell University, Ithaca, NY 14853
ABSTRACT Undoped 20pm thick polycrystalline silicon (polysilicon) films were epitaxially grown on 0.5 jim thick polysilicon seed layers prepared by cycles of CVD depositions and oxidations. The resistivity of the epitaxial films was found to vary by a factor of 4, depending on the number of deposition/oxidation cycles used to prepare the initial seed layer. Exposing the films to a hydrogen plasma for as little as 5 seconds decreased the resistivity of n-i-n resistors by four orders of magnitude and changed their activation energy from 0.4 eV to 0.001 eV. Whereas, the resistivity of p-i-p resistors only decreased by a factor of two after a 2 hour hydrogenation with no change in the activation energy. The initial high resistivity value of the n-i-n resistors was restored by either removing 0.5 jim of polysilicon from the surface of the resistors or by a 15 minute anneal in argon at 550 °C INTRODUCTION Grain boundaries in polysilicon impede the transport of majority carriers through the formation of potential barriers, resulting in a lower mobility [1, 2]. They also act as generation sites increasing the leakage current in devices [3]. One approach to reduce these undesirable effects is to decrease the number of electronic states in the grain boundary by reconstructing the boundary into a lower energy configuration, i.e. one containing fewer 'dangling bonds' [4]. A second approach is to terminate the 'dangling bonds' with hydrogen atoms. This passivation removes the electronic state of the grain boundary from the bandgap. A few studies combining both approaches have been reported [3, 5, 6]. In this paper we report on unusual hydrogenation effects of n-i-n polysilicon resistor fabricated in reconstructed polysilicon films. Grain boundaries in polysilicon can be reconfigured through the injection of silicon self-interstitials [3, 4]. During high temperature oxidation, silicon-self interstitials are injected in front of the oxide growth front [4, 7]. These point defects, when present in excess concentration, anneal out at grain boundaries, permitting non-conservative motion of grain boundary dislocations. The increased atomic mobility permits the boundary to acquire a low energy configuration with a concomitant decrease in grain boundary electrical activity and a reduced atomic diffusivity [5]. Grain growth also occurs, reducing the total grain boundary area. Such reconstructed grain boundaries generally have a lower electrical activity in the sense that the generation current from the grain boundaries is much lower than in standard CVD polysilicon [8]. Because a high supersaturation is easier to achieve in very thin films, we chose to reconfigure thin films of sub-micron thicknesses and then use these films as templates for conventional CVD growth. PREPARATION OF POLYSILICON FILMS Substrates for all films were s
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