Trap Spectroscopy in Si3N4 Ultrathin Films Using Exoelectron Emission Method
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Trap Spectroscopy in Si3N4 Ultrathin Films Using Exoelectron Emission Method M. Naich, G. Rosenman, and M. Molotskii Department of Electrical Engineering-Physical Electronics, Tel Aviv University, Ramat-Aviv, 69978, Israel Ya. Roizin, Tower Semiconductor, Ltd, Israel
ABSTRACT We developed an original thermally stimulated exoelectron emission spectroscopy method (TSEE) of measurements of the activation energy Φ of electron (hole) traps in ultrathin Si3N4 films. The temperature spectra of TSEE of 50A silicon nitride films demonstrate several peaks: three low temperature peaks (T1 =373K, T2=423K, T3=498K) and a high temperature maximum at T4 ~750K. The obtained values of the energy activation are Φ1=0.82 eV, Φ2=0.93 eV, Φ3=1.09 eV, and Φ4=1.73 eV. TSEE results are shown to be consistent with Φ estimates obtained from microFLASH® two bit per cell memory transistor measurements. Electrons stored at traps with Φ4=1.73 eV explain excellent microFlash retention properties. We believe that deep traps in Silicon Nitride are Hydrogen containing centers, while Hydrogen hopping is the route cause of observed material degradation in course of TSEE measurements.
INTRODUCTION Most of nonvolatile semiconductor memory devices are based on the concept of charge storage in the floating gate of MOS transistors. The key element of the newly developed microFLASH® memory is a cell with stacked triple dielectric ONO (oxide –nitride-oxide) where the charge is stored by traps located in the intermediate silicon nitride layer. The single transistor cell has a two physical bit storage capability [1]. Additional advantages of this memory type are no erratic bit issues and very good retention/endurance performance. The concentration of traps in the silicon nitride layer of ONO structures reaches 1019-1020cm-3. The retention properties of microFLASH® two bit per cell memory are determined by lateral migration of charge trapped in silicon nitride at the opposite channel edges of the transistors and thus by the activation energy of deep traps[2]. One of the most critical problems in the improvement of the devices and material technology is the development of experimental methods for studies and characterizations of the energetic and spatial distribution of traps in silicon nitride layers. Several different methods were used for studies of electronic properties of gate and memory dielectric materials. Optical absorption trap spectroscopy[3] and discharging current transient spectroscopy methods were applied to a-Si3N4 thin films and ONO memory transistors[4]. Some important spectroscopy parameters such as electron and hole energy levels of shallow traps, density of traps, and tempt to escape frequencies were obtained from the experimental data[5]. Trap spectroscopy in ultrathin Si3N4 films and studies of the flash memory device thermal stability are performed in our work by application of Thermostimulated Exoelectron Emission (TSEE) method. The recently developed TSEE method is a new technique of energy spectroscopy of electronic states i
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