A Study Of Injection Conditions In The Substrate Hot Electron Induced Degradation Of n-MOSFETs

The dependence of homogeneous n3fOSFET degradation on the energy of the electrons impinging at the interface is a controversial issue not yet completely clarified [1,2]. In this work we address the issue by comparing experiments on nMOSFET degradation due to optically generated substrate hot electrons (SHE [3]) with accurate simulations of the gate current ( I G ) and the electron e n e r e distribution ( E E D ) a t the Si Si02 interface. Differently from previous works, we focus on the range of small injected charges (Qrnl), where injection conditions are expected to play a fundamentai role. The results suggest that in its earliest stages degradation is mainly due to electrons with energy below the Si Si02 energy barrier ( E B ) . The degradation experiments were performed on LDD nMOS transistors featuring 2 pm gate length. 2OOA oxide thickness and 6 x 1016cm-3 surface doping concentration. Fig 2 shows the measured equivalent trapped charge density ( Q T ) due to the injection of up to lo1’ c h a r g e s l m ’ ; as observed in [2], QT increases rapidly for increasing oxide field (Foe), while no significant dependence is found with respect to changes in the substrate voltage ( ISs ) , hence in the electron energy. However, for small injected charges, and in spite of some spread in the da ta due to unavoidable differences between devices, the trapped charge (Fig.3) and the density of surface states (proportional to the shift in the charge pumping current AIcp, Fig.4) exhibit a decreasing dependence on V . s not observed in previous works [1,2]. Such a dependence becomes vanishingly small a t large QInl (i.e. in the range explored in [1,2]), probably due to the changes induced by the corresponding trapped charge on the internal field distribution. To interpret the observed phenomena the SHE experiment has been simulated by means of an injection model already proven to accurately account for hot electron injection and gate current over a wide range of Po,, VSS and doping profiles [4,5). The program is based on the iterative technique of solving the Boltzmann transport equation and implements the electron transport model of [SI. which provides a satisfactory approximation of the silicon energy bands up t o 3.4 el/ . IG is calculated adding the contributions of Shottky emission above the Si Si02 energy barrier ( E B = 3.1 eV) and of tunneling. This latter term is computed, consistently with the calculated E E D . by means of the Fowler-Nordheim expression of the tunneling probability. The model does not include charge trapping or surface state generation phenomena. Consistently with this simplification we restricted our analysis to the early stages of device degradation, when Q,,,, is SO small that the corresponding trapped charge has not substantially changed the field distribution inside the device. The injection model was tested by comparing measured and calculated values of the injection probability from silicon into silicon dioxide [3,5]; sample results demonstrating the accuracy of the model in predicting the gate current are reported in Fig 5. As for the interpretation of the experiments, Fig.6 shows the E E D of the overall and injected electrons for two different VSB normalized to reflect a condition of equal injected charge. As can be seen, the number of injected electrons below w 3 eV decreases for increasing V . 5 , thus exhibiting the same bias dependence as the observed degradation This strongly suggests that , in the beginning, degradation is mainly due to electrons with energy below rz 3 e[/ . Fig.7 compares the calculated increase in the number of electrons injected from different energy levels with the increase exhibited by the trapped charge (A) and the density of surface states (0 ) for constant values of the injected charge (Qinj < 1Ols charges/cm’). As can be seen the measured degradation increases as much as the number of electrons injected a t r.z. 2.8 2.9 eV, thus suggesting that the initial degradation may be preferentially due to those electrons (see Fig.6) tunneling through the top of the energy barrier. This physical picture is confirmed by the da ta in Fig 8, referring to different oxide fields in the range below the threshold for substantial generation of oxide traps. As For is increased, tunneling becomes possible from lower energy levels; correspondingly, the increase of the trapped charge and the density of surface states resembles more closely that exhibited by the number of electrons injected from progressively lower energy levels. In summary, degradation experiments performed a t very small values of the injected charge have been interpreted by nieans of a n accurate injection model; the results indicate that in the first steps of de\ ice degradation the damage is mainly due t o electrons with energy below EB and, in particular, t o those injected through the top of the energy barrier.