Improved control of Hydrogen Depassivated Lithography (HDL) with Scanning Tunnelling Microscope ultrafast feedback loop*

The principle of scanning tunneling microscopy is based on the quantum mechanical phenomena called “tunneling” where electrons tunnel from the apex of a sharp tip to a conducting surface at a certain bias voltage. Conventionally, the STM works mostly in constant-current imaging mode, whereby a controller adjusts the tip height to keep the natural logarithm of the tunneling current constant. The controller output is then plotted to obtain the topography of the sample surface. This constant current imaging mode not only provides the topography of the surface but also gives insight into electrical and chemical properties of the sample. But, obtaining I-V curve information by this process is slow and prone to failure due to repeated freezing of the tip and  controller being disengaged.

An ultra-fast spectroscopy technique was introduced to close the loop on natural logarithm of in phase component current . This method enables the user to acquire an I-V curve for every pixel of an image simultaneously with topography image. A high-frequency sine signal generated by lock-in amplifie (LIA) is added to the bias voltage. The preamplifier output is sent to the LIA and is demodulated into in-phase and quadrature components at the fundamental frequency. The advantages of this method is that one can close the feedback loop while the bias voltage is set to zero and a cleaner topography image in the presence of flicker noise in the current signal. .

This ultrafast controller was implemented to perform HDL. The feedback loop is closed on the in-phase component of  total current for imaging and lithography. The bias voltage is -2.5 V and the modulation signal of amplitude 0.8 V, 2 kHz is added to the bias voltage. The imaging is performed for the set point of 0.5 nA/V. The tip speed is 200nm/sec for imaging. The lithography is performed for two sets of parameters i.e., at 4V, 2.5nA/V with a tip speed of 6.25 nm/sec and also at 3.5V, 2.5nA/V with a tip speed of 25nm/sec. The tip remained stable during the lithography process even at a higher tip speed during the second experiment. Consequently, we may infer that with the feedback loop control on the in-phase component of current, we can obtain stable lithography at higher currents even.