Selective Electron-Beam Irradiation of Metal-Oxide-Semiconductor Structures

Abstract

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 V_GB, the negative charge in the MOST channel induced by bombardment, Q_S′, was found to increase, which implies that a fixed positive charge Q_SB builds up in the oxide during bombardment. Q_S′ reached a steady‐state or saturated value, Q_S′ (sat), after a total primary electron‐beam bombardment of approximately 10⁻⁵ C/cm². A linear relationship between Q_S′ (sat) and the gate voltage during bombardment V_GB was observed for both n‐channel and p‐channel devices, for ${\text{0<V}}_{\mathrm{GB}}\text{≲3.6\hspace{0.17em}}\mathrm{V}\text{1s}$ . This linear relationship is consistent with a fixed distance x₁ between the mean position of the oxide charge Q_SB 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 $V_{\mathrm{GB}}\text{≳3.6\hspace{0.17em}}\mathrm{V},$ the Q_S′ (sat) vs V_GB 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 V_GB>0 showed that Q_S′ and the gate current which flowed during bombardment i_GB had an exponential time dependence, with the same time constant. This time constant τ_c was independent of V_GB, but varied with electron‐beam voltage and current. The positive oxide charge Q_SB could be neutralized by bombarding the device with V_GB<0. The discharge time constant τ_d was always less than the charging time constant τ_c, and τ_d was a function of V_GB 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 Q_SB occurred in devices heated to 150°C for several h, but partial neutralization of Q_SB 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, K_t, which exists in the oxide near the oxide‐semiconductor interface. During bombardment with V_GB>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 = [ε_0x/q²K_t]^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 τ_c. All dependences of Q_S′ and oxide charging current predicted by the analysis are observed experimentally.