INVERSION LAYER MOBILITY UNDER HIGH NORMAL FIELD IN NITRIDED-OXIDE MOSFETS

The inversion layer mobility µeff under effective normal field Eeff higher than 0.5 MV/cm at 298 and 82 K are studied for the first time in MOSFET’s with nanometer-range thin (reoxidized) nitrided oxides prepared by rapid thermal processing (RTP) at 900–1150°C for 15–300 s. At an operating temperature of 298 K, while/µeff of a pure oxide is degraded much faster than proportionally to [FORMULA OMITTED] when Eeff > ~0.5 MV/cm, a nitrided oxide keeps a relationship µeff ∞ [FORMULA OMITTED] up to Eeff of 1.1 MV/cm. Also at 82 K, µeff for a nitrided oxide is degraded much more slowly than that of an oxide having the −2 power Eeff dependence. As nitridation proceeds, while peak µeff degrades slowly, remarkable improvement of field-effect mobility µFE under high Eeff takes place very rapidly and then saturates. As a result, µeff under high Eeff shows a turnaround with progress of nitridation: it increases at first, reaches a maximum at a low AES-measured nitridation concentration of 2–3 at %, and then decreases gradually to a value lower than the oxide’s µeff. Thus a light nitridation is favorable from the performance improvement point of view; e.g., the µeff, at Eeff = 1 MV/cm for a typical nitrided oxide is much larger by 30 (50) % than that of a pure oxide at 298 (82) K. Accordingly, rapid nitridation achieves outstanding improvements of device and circuit performance under high gate drive VG−VT at both the temperatures; e.g., the trans-conductance gm at VG−VT = 3.5 V is improved by half an order of magnitude from that of a pure oxide. Nitridation also avoids negative gm observed at 82 K for an oxide MOSFET. These improvements remain substantially unchanged by additional reoxidation and inert annealing. Contrary to the n-channel case described above, it is found that degradations in mobility and gm with increasing Eeff and VG-VT for p-channel FET’s are enhanced by nitridation, which is recovered to some extent by the subsequent reoxidation. A model of interface states located inside the bands is proposed to explain the contrasting effects of nitridation on high-field electron and hole mobilities. © 1990 IEEE