Scanning probe lithography (SPL) has recently attracted much interest at the creation of nanometer-scale devices, and could be the easiest method for achieving nanometer resolution. Although atomic resolution has been successfully obtained by atomic manipulation, the conditions needed (ultrahigh vacuum or low temperature) are not well suited for many scientific and technological applications.
The aim of this paper is to investigate some of the mechanisms of STM-based modification processes in air to better impact the field of nanostructure fabrication.
The experiments were performed with a Digital Instruments Multimode SPM. With the aid of an amplifier, the current between the tip and the sample is transformed a voltage that is monitored by a TDS 320 oscilloscope. The fabricating voltage is from an arbitrary wave generator AWG2021. The scanner head was put in a glass cover in which water vapor and N2 gas were introduced, and the relative humidity could be held constant at values ranging from 10% to 90%.
In this paper, the current between the tip and the sample was monitored, and the dynamic process of fabrication is studied. Therefore, differences in fabrications with respect to a voltage pulse and a ramp are analyzed.
A series of voltage pulses of -5V and 5V for 100 ms are applied on an Au surface respectively. Both positive and negative voltage pulses can create dots, but these dots are formed randomly. Via monitoring the current between the tip and the sample, the result is analyzed. Under conditions where dots can be fabricated, the feedback loop cannot respond to the sudden increase of the current. Under conditions where dots cannot be fabricated, the feedback loop reduces the fluctuation of the current to very small values. When the voltage is increased gradually in order to make the feedback loop to respond to the adjustment to voltage in time, neither positive nor negative voltage can create dots on the sample surface. We conclude that the phenomenon above is due to the contact of the tip and the sample from the electromigration of tip material.
For pulses of -5V and 5V for 100 ms, nanodots can only be formed on Ti under conditions where the current shows substantial fluctuations, indicating tip-sample contact. If the field is applied gradually, pits are formed. We interpret pit formation as a result of field-induced oxidation. Since the time of pulses is short, electrochemical action between the tip and the sample is not effective. Moreover, experiments show that the relative humidity plays an important part in nanostructure formation on Ti.
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