Hu Xiaotang

An improved control technique for the electrochemical fabrication of scanning tunneling microscopy microtips

The electrochemical etching technique has been widely used in the fabrication of microtips. In this article, an improved feedback control technique for the dc electrochemical fabrication of scanning tunneling microscopy microtips is presented. Distinguished from the conventional control techniques, this newly developed method directly uses the etching current as the only control signal instead of comparing it with the reference current. Therefore, the control process is more accurate. The tungsten tips with radius of curvature down to 20 nm can be produced reproducibly. By varying the cutoff time of the control circuit, tips with different radii of curvature can be fabricated.

Young's modulus determination of low-k porous films by wide-band DCC/LD LSAW

Low-k interconnection is one of the key concepts in the development of high-speed ultra-large-scale integrated (ULSI) circuits. To determine the Youngs modulus of ultra thin, low hardness and fragile low-k porous films more accurately, a wideband differential confocal configured laser detected and laser-generated surface acous- tic wave (DCC/LD LSAW) detection system is developed. Based on the light deflection sensitivity detection prin- ciple, with a novel differential confocal configuration, this DCC/LD LSAW system extends the traditional laser generated surface acoustic wave (LSAW) detection systems working frequency band, making the detected SAW signals less affected by the hard substrate and providing more information about the thin porous low-k film under test. Thus it has the ability to obtain more accurate measurement results. Its detecting principle is explained and a sample of porous silica film on Si (100) is tested. A procedure of fitting an experimental SAW dispersion curve with theoretical dispersion curves was carried out in the high frequency band newly achieved by the DCC/LD LSAW system. A comparison of the measurement results of the DCC/LD LSAW with those from the traditional LSAW shows that this newly developed DCC/LD LSAW can dramatically improve the Youngs modulus measuring accuracy of such porous low-k films.

Analysis of photoconductive sampling method for micro PCSS

Via atomic force microscope (AFM)s tip induced anodic oxidization and photolithograph method, a lOOnm wide metal-insulator-metal (MIM) junction instead of an air gap was fabricated on the GaAs substrate to make a micro type photoconductive semiconductor switch (PCSS). A PCSS signal test system based on photoconductive sampling (PC) method was adopted. The PCSSs equivalent circuit mode was analyzed. The pulse width of the femtosecond laser is about 50 fs, the central wavelength is 800 nm, and the pulse energy is about lmJ with a repetition rate of 100 MHz. With this equivalent mode and experiment parameters, the pulse generators output was simulated via MATLAB. The performance of the sampling gate is analyzed, and its output charge Qs expression is explained.