Tungsten (W) has been used in microelectronics as a metallic contact on Si and SiC for over 40 years. Its higher mechanical and chemical resistance together with CMOS compatibility and high Silicon wettability and adhesion, make it a material of choice for the metallization of high aspect ratio patterns and through-Silicon vias (TSV) . Depending on the Si doping levels (up to about 1×1015 cm-3) the W-Si contact is usually of Schottky type . Several recipes have been developed to obtain ohmic contacts, which rely on depositing W in alloys and in high temperature annealing steps. At sufficiently high n-type doping levels (from 1018 to 1020 cm-3), tunneling induces ohmic behavior.
In the present work we show that, by high energy Pulsed Laser Deposition (PLD), W-on-n+ Si ohmic contacts can be obtained, starting from room temperature conditions. The addition of thermal annealing steps, while keeping a low thermal budget, further improves contact performance. PLD has been proven to be very effective to grow nanostructured W and W oxide structures [3,4]. Thanks to the high energy of the impinging species (about 10-100 eV per atom) implantation of atoms and sputtering phenomena are possible.
W coatings, 100 nm thick, were produced at room temperature by PLD with a laser fluence of 14.5 J cm-2. The samples deposited on Si are then annealed in N2 at 200°C, 450°C, 850°C and 1000°C, with a dwell time of 5 min. For all annealed samples, the morphology and crystallinity have been characterized by SEM and X-ray diffraction. Then, sheet resistance and contact resistance measurements have been carried out in a four-probe setup and by the transmission line method respectively. Finally electrical contact behavior has also been modelled numerically. SRIM (Stopping and Range of Ions in Matter) simulations show that PLD deposition conditions are compatible with a shallow implantation of W ions into the Si substrate, what might contribute to ohmic behavior.
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