Surface Modifications in Si after Rapid Thermal Annealing

Abstract

The results of an investigation of the impact of rapid thermal annealing ͑RTA͒ at two different temperatures, 750 and 1050°C, on the electrical behavior and the morphology of a Si wafer surface are presented. A remarkable degradation of the surface and subsurface regions of Si wafers is detected after the RTA: the maximum electrical damage is observed after the 750°C anneal, while surface roughening is induced by the 1050°C anneal. Several mechanisms responsible for the observed changes in the electrical and morphological wafer characteristics are discussed.Rapid thermal annealing ͑RTA͒ is currently used in microelectronics technology for a number of process steps such as dopant activation, silicidation, and reflow of dielectric layers. New interest in this technique has been prompted by the possibility for growth of very thin dielectric layers as the gate oxide of the metal oxide semiconductor ͑MOS͒ transistors. As a consequence an increasing number of studies has been devoted to investigating the impact of RTA on device performance. 1-4 A number of studies outline the importance of the changes induced in the Si substrate surface and, in particular, annealing-induced roughening. 2,3 However, the microscopic mechanisms controlling these surface modifications in the Si wafers have still to be understood.The present study is an investigation of the impact of RTA at two different temperatures, 750 and 1050°C, on the electrical behavior and the morphology of a Si wafer surface. Capacitance-voltage ͑C-V͒ and current-voltage ͑I-V͒ characteristics of Schottky diodes deposited on the as-annealed samples, minority carrier diffusion length, and surface recombination velocity measurements, electron beam induced current ͑EBIC͒, and deep level transient spectroscopy ͑DLTS͒ monitored the changes in the electrical parameters; the changes in the surface structural properties were detected by atomic force microscopy ͑AFM͒. The correlation of the electrical and structural data was used to identify some of the processes induced by thermal annealing at the wafer surface. Experimental200 mm ͑100͒ B-doped, resistivity 12 ⍀ cm, Czochralski Si wafers from MEMC, Novara, Italy, were used. The anneals at 750 and 1050°C for 60 s were performed in a commercial AG Associates heat pulse lamp system: the ambient gas was Ar. The ramp up and ramp down of the RTA cycles were of the order of 200 and 80°C/s, respectively. No surface treatment was performed on the samples prior to the RTA step.Special attention was given to the preparation of the Schottky diodes: they were realized by high purity Al evaporation on samples cut from different areas of the wafers. A purposely made Al mask was used to define the Schottky diode structure. This evaporation was performed without any previous surface treatment: no HF etching was done on the as-received surfaces in order to avoid any modification of the surface structure. Ohmic contact was made by scratching the back surface and wetting it with an Hg-In amalgam. The samples were mounted on a purposely made ho...

Cleaning of Silicon Surfaces for Nanotechnology

“…investigations have been carried out for thermally grown oxynitrides and nitrides [82] [83]. The influence of rapid thermal annealing on the electrical quality and surface roughness has been demonstrated by Castaldini et al [85].…”