High quality-factor quartz tuning fork glass probe used in tapping mode atomic force microscopy for surface profile measurement

Abstract

This paper presents a high quality-factor (Q-factor) quartz tuning fork (QTF) with a glass probe attached, used in frequency modulation tapping mode atomic force microscopy (AFM) for the surface profile metrology of micro and nanostructures. Unlike conventionally used QTFs, which have tungsten or platinum probes for tapping mode AFM, and suffer from a low Q-factor influenced by the relatively large mass of the probe, the glass probe, which has a lower density, increases the Q-factor of the QTF probe unit allowing it to obtain better measurement sensitivity. In addition, the process of attaching the probe to the QTF with epoxy resin, which is necessary for tapping mode AFM, is also optimized to further improve the Q-factor of the QTF glass probe. The Q-factor of the optimized QTF glass probe unit is demonstrated to be very close to that of a bare QTF without a probe attached. To verify the effectiveness and the advantages of the optimized QTF glass probe unit, the probe unit is integrated into a home-built tapping mode AFM for conducting surface profile measurements of micro and nanostructures. A blazed grating with fine tool marks of 100 nm, a microprism sheet with a vertical amplitude of 25 µm and a Fresnel lens with a steep slope of 90 degrees are used as measurement specimens. From the measurement results, it is demonstrated that the optimized QTF glass probe unit can achieve higher sensitivity as well as better stability than conventional probes in the measurement of micro and nanostructures.

Resonance and sensing characteristics of quartz tuning fork for force sensor

“…In the spring model, let the spring constant be 𝑘, and the mass 𝑚, then the eigenfrequency 𝑓 of spring mechanical vibration is [11] :…”

“…QTF has been widely used as a force sensor in atomic force microscopy (AFM) [7][8][9][10][11][12][13][14][15][16][17] . AFM usually utilises a micro-cantilever probe for laser detecting in its measurement and scanning imaging.…”

“…The force sensor based on QTF in AFM has been studied a lot. Using classical physical theory, models and formulas, physical parameters such as resonance frequency and Q-factor of mechanical vibration of QTF can be analysed and calculated [10][11][12][13][14][15] . Ideally, the two prongs of a QTF are perfectly symmetrical.…”

“…They have a significant negative impact on sensing performance. Previous researches show that by reducing the additional mass [11] on the QTF prongs or rebalancing these additional mass [11,16,17] on the QTF prongs, the symmetry of the QTF can be restored to some extent, thereby improving its Qfactor and sensing characteristics.…”

“…The impact of additional mass and mass rebalance on QTF force sensors has been studied previously [11,16,17] . However, the influence of additional mass and its position on the mechanical resonance and sensing characteristics of QTF has not been systematically studied.…”