Elsevier

Microelectronic Engineering

Volume 119, 1 May 2014, Pages 44-47
Microelectronic Engineering

Batch fabrication of insulated conductive scanning probe microscopy probes with reduced capacitive coupling

https://doi.org/10.1016/j.mee.2014.01.018Get rights and content

Highlights

  • Insulated conductive SPM probes for electrical measurement of soft samples in liquid.

  • Geometry of the conductor is engineered to reduce the parasitic capacitance coupling.

  • Stray capacitances from cantilever and chip have been reduced to non-detectable limit.

  • Parasitic C coupling to substrate significantly reduced compared to metallized probes.

Abstract

We report a novel fabrication process for the batch fabrication of insulated conductive scanning probe microscopy (SPM) probes for electrical and topographic characterization of soft samples in liquid media at the nanoscale. The whole SPM probe structure is insulated with a dielectric material except at the very tip end and at the contact pad area to minimize the leakage current in liquid. Additionally, the geometry of the conducting layer in the probe cantilever and substrate is engineered to reduce the parasitic capacitance coupling with the sample. The electrical characterization of the probes has shown that parasitic capacitances are significantly reduced as compared to fully metallized cantilevers.

Introduction

Since the invention of the atomic force microscope (AFM) by Binnig et al. [1], various techniques based on scanning probe microscopy (SPM) have evolved as fundamental tools in biology and nanobiotechnology [2]. Electrical SPM characterization techniques such as scanning electrochemical microscopy (SECM) [3], scanning impedance microscopy (SIM) [4] and electrostatic force microscopy (EFM) [5] should allow the study of the electrical behavior of biomolecules at the nanoscale in its native physiological environment.

Considering the small signal associated with biological samples, the SPM based electrical characterization techniques for biology should minimize the parasitic conductance and capacitance between the sample and the extended probe cantilever structure. The parasitic conductance could be minimized by insulating the whole probe with a dielectric material except at the very tip where electrical contact with the sample will be made and at the contact pad. In addition, the capacitance coupling between the sample and the probe cantilever could be minimized by engineering the width of the conductor layer in the probe cantilever. Some groups have fabricated conducting insulated probes for SECM-AFM [3], [7], [8], [9], but still no one with a reduced capacitive coupling.

With the objective of characterizing olfactory receptor proteins for the development of olfactory biosensors [6], we have developed insulated conductive SPM (C-SPM) probes for SIM characterization in liquid environment. Here we report the fabrication of probes for both DC and AC applications. These probes minimize both the leakage current in liquids and the parasitic capacitance coupling associated with the extended cantilever. We also report the characterization results of the AC probes in air. We will report the corresponding results for the DC probes elsewhere.

Section snippets

Fabrication and design

We made separate designs for DC probes (for DC electrical measurements in contact mode) and AC probes (to measure the AC response in dynamic mode). The cantilever dimensions for the DC probes range from 300 to 400 μm in length and 30–40 μm in width, with a thickness of 2 μm. These values have been defined to get probes with force constant values lower than 1 N/m, as soft probes are required to study soft biological samples in contact mode. As relatively stiff probe cantilevers are required for

Characterization results

The fabricated AC probes were tested by mounting them in commercial AFM instruments to extract their mechanical and electrical properties in air. The resonance frequency and force constant of the AC probes were in the range of 65–120 kHz and 3–20 N/m (calculated by using the Sader’s method proposed in [11]), respectively. The AC probes were first tested by measuring the topography of different samples such as carbon nanotubes, interdigitated metal electrodes, and silicon dioxide nano-structures

Conclusion

We have demonstrated the batch fabrication of conductive SPM probes insulated except at the very tip and the contact pad, for both DC and AC electrical applications in liquid media. The AC probe tips show good conductivity behavior and a good correlation between the simultaneously recorded topography and conductivity images. AC electrostatic force microscopy measurements have shown a good performance comparable to commercial probes. In addition, the capacitive coupling of the probes with a

Acknowledgements

We gratefully acknowledge the financial support from the “Bioelectronic olfactory neuron device” (BOND) project of the 7th Framework Programme of the European Union (NMP4-SL-2009-228685), and from MINECO through projects TEC2011-23600, TEC2010-16844 and Nanoselect-CSD2007-00041(Consolider-Ingenio2010 programme). We would like also to express our gratitude to all IMB-CNM clean room staff for their technical support.

References (14)

  • G. Gomila et al.

    Sens. Actuators, B

    (2006)
  • G. Villanueva et al.

    Microelectron. Eng.

    (2008)
  • G. Binnig et al.

    Phys. Rev. Lett.

    (1986)
  • D.J. Müller et al.

    Nat. Nanotechnol.

    (2008)
  • C. Kranz et al.

    Anal. Chem.

    (2001)
  • L. Fumagalli et al.

    Nano Lett.

    (2009)
  • L. Fumagalli et al.

    Nat. Mat.

    (2012)
There are more references available in the full text version of this article.
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