Polycrystalline Si thin-film transistors fabricated at ≤800 °C: Effects of grain size and {110} fiber texture

Abstract

The grain size and {110} fiber texture of low-pressure chemical-vapor-deposited polycrystalline Si films were first modified using the ‘‘seed selection through ion channeling’’ process. These films were self-implanted at 200 keV, at normal incidence, to various doses (5–20×1014 cm−2), and were subsequently recrystallized at 600 °C. The as-deposited film was characterized by the smallest grain size, 0.08 μm, among all films and a weak {110} texture. The film processed with a dose of 11×1014 cm−2 had an intermediate average grain size of 1.0 μm but was characterized by the strongest {110} texture among all films. The film processed with a dose of 20×1014 cm−2, on the other hand, was characterized by the largest average grain size of 2.0 μm among all films but had no crystallographic texture. Metal-oxide-semiconductor thin-film transistors were then fabricated on these three films to examine the effects of grain size and fiber texture on device performance. Both n- and p-channel transistors were fabricated, using a self-aligned process with a maximum processing temperature of 800 °C. For both types of transistors, the as-deposited film offered the worst device performance, exhibiting the highest threshold voltages (4 and −4 V, respectively) and the lowest channel mobilities (0.4 and 0.5 cm2/V s, respectively). The 11×1014 cm−2 film offered the best device performance, exhibiting the lowest threshold voltages (1.2 and −3.0 V, respectively) and the highest channel mobilities (42 and 34 cm2/V s, respectively). The 20×1014 cm−2 film, on the other hand, exhibited equal or slightly worse threshold voltages (1.6 and −3.0 V, respectively) and channel mobilities (35 and 24 cm2/V s, respectively, than the 11×1014 cm−2 film. These results demonstrate that both the grain size and grain orientation can be important factors in determining polycrystalline transistor performance. For grain sizes in the order of 1–2 μm, a stronger {110} texture can, in fact, lead to a better transistor material than a larger average grain size.