Thermal Stability of Platinum Silicide in Deep Sub-Micron Lines
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Mat. Res. Soc. Symp. Proc. Vol. 391 01995 Materials Research Society
1000A. For the patterned samples, a layer of 550A Si0 2 was first deposited by plasma-enhanced chemical vapor deposition (PECVD) and the samples were annealed at 950'C for 5 minutes in nitrogen in a rapid thermal annealing (RTA) system to densify the oxide film. PMMA was used as the lithography mask, patterns of various widths were written by focused ion beam (FIB) using a Si++ beam. These patterns were then transferred onto Si0 2 by reactive ion etching in CHF 3 /0 2 chemistry, and the PMMA film was subsequently removed. After cleaning, these samples underwent similar process for Pt deposition. Metal films with thickness of 130A, 250A structure used for electrical and 500A were studied. Fig. 1 is a schematic diagram of the measurement to extract sheet resistance. Two pads of 50x50 gin 2 in size were used for electrical probing. Lines of 70gtm in length and with identical width, which ranged from 3 gtm to 0.1 gim for different structures, were formed between the two pads. The dimension of the lines were measured from top-view scanning electron microscope (SEM) or transmission electron microscope (TEM) images. The total resistance was measured after the sample was fully processed and the sheet resistance was calculated according to the measured linewidths and the number of lines between the pads. To form silicide, the samples were annealed by a three step RTA process, i.e. at 400C, 475°C and 550TC for 30 sec each in a N2 ambient (referred to as STD hereafter). This thermal treatment has been shown to form PtSi for blanket films by Auger electron spectroscopy (AES) and X-ray diffraction measurements [4]. For patterned samples, the un-reacted metal was selectively etched in aqua regia (HCI:HNO 3 :H2 0=3:2 :1) solution. The silicided layers were subsequently annealed in N2 ambient at successively higher temperatures from 600'C to 1000°C at 50'C intervals for Fig. 1. Schematic diagram of the structure used for 2 30 sec at each temperature to test electrical probing. The contact pads are 50x50 gIm , thermal stability. the multiple lines of the same width between the pads are 70 gtm in length, and the width ranges from 3 gtm to 0.10 gtm for different structures. The stress of the blanket films was measured using a Tencor laser deflection system after each annealing step. TEM and atomic force microscope (AFM) were used to study the structural properties of the silicide films. RESULTS AND DISCUSSION Linewidth dependence of sheet resistance The sheet resistance of blanket films after annealing at different temperatures is shown in Fig. 2. The resistance decreased slightly at first after successive anneals for temperatures below 700'C, presumably because of grain growth. The relative change is more pronounced for thinner films. For the films formed from more than 500A of Pt, the variation in resistance is negligible. Details on the thickness dependence of the properties for blanket Pt silicide films have been presented elsewhere [4]. Fig. 3 shows the sheet
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