Evaluation of a force sensor based on a quartz tuning fork for operation at low temperatures and ultrahigh vacuum

doi:10.1016/S0169-4332(01)00976-X

Applied Surface Science

Volume 188, Issues 3–4, 28 March 2002, Pages 445-449

https://doi.org/10.1016/S0169-4332(01)00976-X Get rights and content

Abstract

The noise performance of the force sensor is crucial for optimizing the resolution in non-contact atomic force microscopy. Sensing forces in vacuum and low temperatures is even more demanding than at ambient conditions. Here we analyze the noise performance and the sensitivity of a force sensor based on a quartz tuning fork of which one of the prongs is fixed (qPlus sensor). The noise characteristic of the qPlus sensor, optical and piezoresistive detection schemes at room temperature are compared and the qPlus sensor is investigated at low temperatures. The frequency variation of quartz tuning forks as a function of temperature is experimentally determined for the temperature range from 4 to 300 K.

Introduction

Since Binnig et al. [1] introduced the atomic force microscope (AFM) in 1986, many different operation modes have been developed for this instrument. From a pure static contact mode to the dynamic non-contact mode with large amplitudes which reaches true atomic resolution on reactive surfaces like the Si(1 1 1) (7×7) [2], the resolution has been steadily increased. The noise characteristics and the sensitivity of an atomic force sensor are responsible for the quality of the measurements in non-contact atomic force microscopy. Typically, a force sensor for AFM consists of a cantilever and a deflection measurement scheme. The experimental deflection noise is determined by both the cantilever and the deflection measurement scheme.

In principle, two different kinds of detection systems can be distinguished:

•
self-sensing devices such as piezoresistive [3] or piezoelectric cantilevers [4];
•
cantilevers with an external deflection measurement scheme such as a tunneling tip [1], the beam deflection technique [5], [6] or the interferometric technique [7].

While operation in vacuum and low temperatures has been demonstrated with the tunneling [8] and the interferometric techniques [9], [10], [11], [12], cantilevers with self-sensing capability are particularly desirable for operation in vacuum and low temperature because of the great difficulties involved with the alignment of a tunneling tip, optical fiber or laser beam in these environments. Low-temperature operation of piezoresistive cantilevers has been demonstrated by Volodin and Van Haesendonk [13], and piezoelectric sensors have been operated at low temperature by Rychen et al. [14].

Excellent resolution at room temperature has already been demonstrated with the qPlus sensor [15] using optimized imaging parameters [16]. Because thermal drift is strongly reduced and the bandwidth can be reduced greatly, even better resolution is expected at low temperatures. In addition to working with stiff cantilevers, it is important to use small amplitudes in the Angström range to achieve this resolution [17]. To realize such small amplitudes in a controlled manner, the noise of the force detection unit must be as small as possible. The spectral deflection noise density is a quantity for noise calibration and expressed by the ratio between the deflection noise and the bandwidth.

Section snippets

Measurement at room temperature

As described, the deflection of a cantilever can be measured by optical, piezoresistive and piezoelectric means. Ultimately, all systems convert the cantilever deflection into a voltage. For the qPlus sensor, Fig. 1 shows the sensor and its measurement circuit necessary to convert the deflection into an output voltage. For the measurements presented here, the operational amplifier AD711 [18] and a 60 MΩ feedback resistor is used. Because the amplification is proportional to R and the Johnson

Measurement at low temperatures

At low temperatures, we expect a reduced thermal noise because the dominant contribution to the white noise is due to the Johnson noise of the feedback resistor of the current-to-voltage converter. In addition, Eq. (3) predicts an increased sensitivity because d₁₂ rises about 10% and the Young’s modulus rises about 1% if the temperature decreases from 300 to 5 K [19]. Both the effects lead to a decreasing amplitude noise density of the qPlus sensor.

First of all we have determined the

Summary and conclusion

We have shown that the qPlus sensor functions at low temperatures with an even better noise performance than at room temperature. Because the spectral deflection noise density decreases with temperature, an AFM equipped with the qPlus sensor is expected to achieve at low temperatures an even better resolution than at 300 K.

The variation of eigenfrequency as a function of temperature has been determined experimentally for a temperature range from 4 to 300 K. As shown in Fig. 3, the temperature

Acknowledgements

This work is supported by the BMBF (Project No. 13N6918).

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Evaluation of a force sensor based on a quartz tuning fork for operation at low temperatures and ultrahigh vacuum

Abstract

Introduction

Section snippets

Measurement at room temperature

Measurement at low temperatures

Summary and conclusion

Acknowledgements

Appl. Surf. Sci.

Phys. Rev. Lett.

Science

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

J. Appl. Phys.

J. Vac. Sci. Technol. B

Rev. Sci. Instrum.