Measurement of Energetic and Lateral Distribution of Interface State Density in Fully-Depleted Silicon on Insulator Metal-Oxide-Semiconductor Field-Effect Transistors

A new modification of the charge pumping (CP) technique based on the effect of reverse pulse bias on suppression of the geometric component is proposed in order to accurately determine the energetic and lateral distribution of interface state density (Dit) in fully-depleted silicon on insulator metal-oxide-semiconductor field-effect transistors (FD SOI MOSFETs). A compari- son of the conventional CP techniques with the proposed method is also presented. It is demonstrated that using the proposed method, the precise estimation of the energetic and lateral distribution of Dit can be simply obtained without interference from the geometry-dependent effect which often leads to the great difficulties in data interpretation in the conventional CP methods. The interface between the silicon substrate and the gate ox- ide in the active region of a metal-oxide-semiconductor field- effect transistor (MOSFET) plays a crucial role in determin- ing the device performances and affects the reliability and lifetime of the device. The measurement and characteriza- tion of the interface states are needed to understand the origin and physical properties of the interface states in a MOS sys- tem. Interface states can capture and emit charge carriers and the amount of charges in the interface states determines the device parameters. The charge pumping (CP) technique 1-3) has evolved into a sensitive and reliable method to study the interface char- acteristics in a MOSFET. The major advantage of the CP technique is that it is available for direct measurement of the interface characteristics not only in a bulk MOSFET but also for characterizing both the front and back interface properties in a SOI MOSFET. 4, 5) The experimental setup for charge pumping measurements in a SOI MOSFET is shown in Fig. 1. The gate is connected to a pulse generator. The source and drain are connected to- gether while the body is grounded through a DC ammeter. The principle of conventional CP is described in detail in refs. 2 and 3. When a gate pulse of sufficient amplitude to invert the surface is on, electrons flow to the surface from the source and drain. The surface states capture some of the in- version electrons and become negatively charged. When the pulse is off, the inversion layer electrons immediately flow back to the source and drain, while the captured ones are still retained by the interface states and recombine with the in- coming holes from the body. Similarly, when the gate surface is pulsed from accumulation to inversion, the trapped holes recombine with electrons from the source and drain. These recombinations give rise to a DC current Icp. By measuring this current, the interface state information can be obtained. A major problem of the CP measurement on MOSFETs is the so-called "geometric component" of the CP current, 6, 7) which occurs if all mobile carriers of one type, electrons (in NMOSFET) during the fall and holes during the rise of the gate pulse, are not removed rapidly enough before the arrival of the other type of carrier. The carriers that are left behind will recombine with carriers of the other type and therefore an additional component of CP current that does not involve the interface states arises. This parasitic component gives rise to an overestimation of interface state density ( Dit). The ge- ometric component is not often observed in bulk or partially- depleted (PD) SOI MOS transistors. However, in FD SOI MOSFETs, particularly in very thin film SOI structures, where the resistivity of the body region is generally high, the CP current tends to have a very large geometry-dependent component 6) resulting in an overestima- tion of Dit. A detailed discussion of the geometric effect in FD SOI MOSFETs can be read in ref. 7. When the rise and fall times are relatively long, i.e. the measurement frequency is low, this undesirable component is negligible but the sensi- tivity is severely reduced due to a low signal/noise ratio of Icp and thus the use of the CP technique is limited to only large- area devices. Therefore, the new method to suppress this component and enhance the sensitivity is strongly required as the devices are scaled down. We have already developed a new measurement technique using the reverse pulse (RP) bias to suppress the geometric component in FD SOI MOSFET. 7) In the present work, we extend this method and apply it for the accurate measurements of energetic and lateral distribu- tions of Dit in FD SOI MOSFET by the RP charge pumping method.