Selective Electron-Beam Irradiation of Metal-Oxide-Semiconductor Structures
Authors: MacDonald N. C. , Everhart T. E. 
Published: February 2004

Both transient and steady-state changes in the electrical characteristics of metal-oxide-semiconductor transistors (MOST's) produced by localized electron-beam bombardment are reported here. Typically, areas 10 μ×10 μ were bombarded on the gate of the MOST by scanning a 0.1 μ diameter electron beam over this area. The electron-beam energy was variable from 5 to 30 keV, the beam current was varied from 1 to 1000 pA, and the time to scan one frame was typically 8 msec. During bombardment with positive gate voltage VGB, the negative charge in the MOST channel induced by bombardment, QS′, was found to increase, which implies that a fixed positive charge QSB builds up in the oxide during bombardment. QS′ reached a steady-state or saturated value, QS′ (sat), after a total primary electron-beam bombardment of approximately 10−5 C/cm2. A linear relationship between QS′ (sat) and the gate voltage during bombardment VGB was observed for both n-channel and p-channel devices, for 0&lt;VGB≲3.6 V1s. This linear relationship is consistent with a fixed distance x1 between the mean position of the oxide charge QSB and the semi-conductor-oxide interface. Values of x1 ranging from 45 to 65 Å were calculated from measurements on p-channel devices, and x1 = 75 Å was calculated from measurements on n-channel devices. For VGB≳3.6 V, the QS′ (sat) vs VGB curve abruptly leveled off; this effect may be explained by electron tunneling from the semiconductor into the conduction band of the oxide. Transient measurements during bombardment with VGB&gt;0 showed that QS′ and the gate current which flowed during bombardment iGB had an exponential time dependence, with the same time constant. This time constant τc was independent of VGB, but varied with electron-beam voltage and current. The positive oxide charge QSB could be neutralized by bombarding the device with VGB&lt;0. The discharge time constant τd was always less than the charging time constant τc, and τd was a function of VGB as well as the electron-beam voltage and current. Controlled temperature measurements showed that the oxides of the devices used for these electron-bombardment experiments were free of mobile ions, that no neutralization of QSB occurred in devices heated to 150°C for several h, but partial neutralization of QSB occurred after heating at higher temperatures. The experimental results were remarkably reproducible for several trials on one device, as well as from device to device. The steady-state experimental results are explained by a constant density of traps/unit energy, Kt, which exists in the oxide near the oxide-semiconductor interface. During bombardment with VGB&gt;0, these traps become charged, such that the steady-state oxide charge density decreases exponentially away from the oxide-semiconductor interface with characteristic length x1 = [ε0x/q2Kt]1/2, The transient charging of the oxide is analyzed using a sheet-charge analysis. An additional measurable parameter results from the transient analysis, the charging time constant τc. All dependences of QS′ and oxide charging current predicted by the analysis are observed experimentally.