Development of an improved Kelvin probe force microscope for accurate local potential measurements on biased electronic devices

Experimental determination of the lateral resolution of surface electric potential measurements by Kelvin probe force microscopy using biased electrodes separated by a nanoscale gap and application to thin-film transistors

A method is proposed to estimate the lateral resolution of surface potential profile measurements using Kelvin probe force microscopy (KPFM) on operating electronic devices. De-embedding the measured profile from the system response is required for various applications, such as contact characterization of thin-film transistors, or local longitudinal electric field measurements. A method is developed based on the measurement of the electric potential profile of two metallic electrodes separated by a nano-gap, providing a quasi-planar configuration. The electrodes are independently biased so as to produce an abrupt and well-controlled potential step. This calibration sample is used to measure the system impulse response in various configurations. Due to the application constrains, the KPFM method employed here is based on a dual-pass mode, demonstrated to provide reliable measurements on operating electronic devices. The method is applied to two types of conductive AFM probes. Measurements are performed at different tip-to-sample heights allowing the determination of the lateral resolution of the double-pass method. Detailed description of the measurements and resolution results are given for the present KPFM configuration. The system resolution measurement technique can be extended to other KPFM modes and can be used to monitor the degradation of the tip quality during long measurement campaigns. Finally, the method is applied to the characterization of thin-film transistors, and the effects of contact edge sharpness on the device behavior is discussed. The longitudinal electric field responsible for charge injection at the source-contact edge is successfully estimated and compared for organic thin-film transistors fabricated by stencil lithography or electron-beam lithography.

Direct observation of surface charge redistribution in active nanoscale conducting channels by Kelvin Probe Force Microscopy

Surface-exposed uniformly doped silicon-on-insulator channels are fabricated to evaluate the accuracy of Kelvin Probe Force Microscopy (KPFM) measured surface potential and reveals the role of surface charge on the exposed channel operated in the ambient environment. First, the quality of the potential profile probed in the vacuum environment is assessed by the consistency of converted resistivity from KPFM result to the resistivity extracted by the other three methods. Second, in contrast to the simulated and vacuum surface potential profile and image, the ambient surface potential is bent excessively at the terminals of the channel. The excessive bending can be explained by the movement of surface charge under the drive of geometry induced strong local electric field from the channel and results in non-uniform distribution. The dynamic movement of surface charges is proved by the observation of time-dependent potential drift in the ambient measurement. The result suggests the surface charge effect should be taken into account of the measurement of the surface potential in the ambient environment and the design of charge sensitive devices whose surfaces are exposed to air or in ambient conditions in their operation.

Adaptive velocity-dependent proportional-integral controller for high-speed atomic force microscopy

High-speed atomic force microscopy (AFM) has been rapidly developed in recent years. To reduce the oscillation of the scanner, a single-tone sinusoidal wave is widely used as a scanning wave rather than a triangular wave in high-speed AFM. However, the sinusoidal wave is nonlinear, resulting in a nonconstant relative linear velocity between the sample and the tip while scanning in the x-direction. If a traditional proportional-integral controller is still used as a feedback controller in the z-direction, the control errors will be enormous. Therefore, the paper proposes a new adaptive velocity-dependent proportional-integral controller. The relationship between the proportional-integral parameters and the linear velocity is achieved by fitting the experimental results. The adaptive and traditional controllers are compared against each other in some examples. The experiments demonstrate that the adaptive controller decreases the control errors in the z-direction to a half, which provides more precise AFM images. LAY DESCRIPTION: Typically, the scanner follows a triangular waveform in fast axis (x-axis), and follows a very slow ramp signal in the slow axis (y-axis) of conventional AFM. This scanning mode can be called raster scan. However, the triangular waveform contains high-order Fourier harmonics, vibrating the scanner and distorting the image easily. In high-speed AFM, the effect of the high-order Fourier harmonics will be more severe. The above problems can be solved by replacing triangular waves with single-tone sinusoidal waveform. Therefore, the sinusoidal-raster scan and nonraster scan based on the sinusoidal waveform are widely used in high-speed atomic force microscopy. However, the nonlinearly scan path will cause a variable relative linear velocity between the sample and the tip. If a standard proportional-integral controller is still used as a feedback controller in Z direction, the control errors will be large, and this difference will be evident at high-speed scanning. Thus, the paper proposes a new adaptive velocity-dependent proportional-integral controller to solve this problem. Experiments show that the control errors obtained by using the adaptive controller is about a half of that without using it. These illustrate that the proposed method can improve the image quality of the AFM at both low and high scan speeds.