Frequency [MHz]
6.3 Magnetic Actuation
6.4.3 Piezoelectric driven Response In Air
When this drive technique is used in air, the expected broad resonance of the damped response function is convolved with resonances in the sample holder assembly. This is expected due to the implementation commonly employed in the AFM community7 in which the entire holder must vibrate, as opposed to local actuation of the device. The data are shown in Fig. 6.4.B. A rough fit to the data gives a quality factor of ~10, consistent with the value of 12 obtained for theoretical calculations based on the cantilever geometry.
0 100000 200000 300000 400000 500000 600000 0.0
0.2 0.4
Signal2 (µV)2
f [kHz]
Fig. 6.4.B Piezoelectrically driven resonance in Air
The fine structure is consistently observed with this method of drive in air and arises from the response of the sample holder which also vibrates under this method of drive.
Due to this fine structure the Lorentzian fit to the data is only presented as a guide. The quality factor for the fit shown is 10, consistent with the value of 12 obtained for theoretical calculations based on the cantilever geometry.
6.5 Directions for future experiments
Each of the methods of drive discussed in this chapter must be examined critically in light of the goal of using these methods of actuation for microfluidic based sensors. The first method, that of actuation by heating, is useful as a quick method for characterizing the device before immersing it in fluid. Whether it will prove useful in some form in fluid remains to be determined since the high thermal conductance of the fluidic environment permits only substantially reduced heating.
The limitations of the magnetic drive were presented in the text, namely the difficulty in achieving a sufficiently large actuation force, particularly in light of the separation imposed by the presence of the microfluidics. It is possible that these problems might be overcome, possibly through using an integrated coil that could be placed inside the microfluidics or by use of thinner glass, stronger coils and tip magnets, or some combination of the above. However, it is clear that any such solution will require extensive engineering while maintaining a number of drawbacks not shared by the on chip piezoelectric actuation discussed below. Namely:
• in its present form the magnetic drive is not localized to a particular cantilever but extends over all cantilevers in a given via,
• the drive is difficult to quantify and reproduce since it depends on the exact cantilever-coil separation, and
• it is difficult to combine magnetic drive with optical detection.
the final detection scheme, it is important for confirming the results of early measurements.
The (off-chip) piezoelectric drive technique used here is also not intended for measurements in fluid. (Although it might serve a purpose as an “ultrasound” to dislodge particles that become stuck and serve as a supplement to diffusion for bringing the analyte in contact with the cantilever). However, the extension of piezoelectric drive to on-chip actuation at the cantilever legs would be exciting both because the attainable drive forces are in the necessary range and because this would be a method of drive in which the cantilevers are individually addressable.
Here we perform a very crude (order of magnitude) estimate of the drive force for this form of actuation. The stress induced by a piezoelectric actuator is given by epE, where ep~1C/m2 for typical piezoelectric materials. So for an area of 10 µm2 and a force of 100pN (representative of typical antibody-antigen binding forces8,9) an electric field of 10V/m is required. Across the 100nm device thickness this corresponds to 1 µV. Clearly much higher forces could be attained while maintaining a tolerable voltage across the piezo actuator. This calculation is for a uniform force across a plate, and while the extension to a cantilever is non-trivial, this calculation suggests that the method of drive shows promise.
In order to minimize cross-talk the piezoelectric actuators should be located on separate legs from the piezoresistive sensors (Fig. 6.5.A). As mentioned in the introduction to section 6.4, the fabrication procedure for combining piezoelectric actuation and piezoresistive sensing is non-trivial (due to the number of fabrication steps involved). A new technology which would open the way to purely piezoelectric sensing and actuation is currently under development by Soitec (from whom we currently purchase SOI). Soitec is developing the techniques to fabricate new types of bonded wafers, including Gallium Nitride- On –Insulator wafers, referring to a three layer material comprised of a silicon handle layer, a buried oxide layer, and a thin Gallium nitride transducer layer. This would allow the fabrication of purely piezoelectric devices with our existing technology (the material would be fully compatible with top side electrode patterning and the DRIE plus oxide release suspension); the only new processing required would be the etching of the GaN itself, recipes for which are readily available in the literature.10 Many of the greatest difficulties with current MEMS applications of GaN is the difficulty of selective etching and stop layers (currently accomplished using n-doped material as a stop layer for etching p-doped material).11 This would no longer be a concern in an implementation using a bried oxide layer.
Combining piezoelectric drive and sensing, allowing for the use of a single material, would greatly simplify the fabrication process.
Fig. 6.5.A Conceptual schematic of device with piezoelectric actuation
For electrical isolation the piezoelectric actuators would be located on separate legs from the piezoresistive sensors.
Piezoelectric Actuator
6.6 References
1 M. Barbic, J.J. Mock, A.P. Gray, and S. Schultz, Appl. Phys. Lett., p. 1897-1899 (2001).
2 Allegheny Ludlum Steel Corporation, Electrical Materials Handbook, Pittsburg, Pa, pVII-19 (1961).
3 D. Jiles, Introduction to Magnetism and Magnetic Materials, Chapman and Hall, London, p. 294 (1991).
4 I.S. Grant and W.R. Phillips, Electromagnetism, John Wiley and Sons, Chichester, p.
197 (1990).
5 J. D. Jackson, Classical Electrodynamics, John Wiley and Sons, New York, p. 183 (1999).
6 http://sorcerer.ucsd.edu/es160/lecture8/web6/node20.html
7 T.E. Schäffer, J.P. Cleveland, ,F. Ohnesorge, D.A. Walters, and P.K. Hansma , J.
Appl. Phys. 80, 3622-3627 (1996).
8 U. Dammer, M. Hegner, D. Anselmetti, P. Wagner, M. Dreier, W. Huber, and H.-J.
Güntherodt, Biophys. J. 70, 2437-2441 (1996).
9 P. Hinterdorfer, W. Baumgartner, H.J. Gruber, K. Schilcher, and H. Schindler, Proc.
Natl. Acad. Sci. USA. 93, 3477-3481 (1996).
10 M. Minsky, ,M. White, and E. Hu, Appl. Phys. Lett. 68, 1531 (1996).
11 R.P. Strittmatter, R.A. Beach, and T.C. McGill, Appl. Phys. Lett. 78, 3226-3228 (2001).
7 Measurements in Liquid Ambients
7.1 Overview
In this chapter we discuss actively-driven fluid-based piezoresistive sensors (passive sensors for the detection of Brownian fluctuations are discussed in chapter 8). For these experiments the microfluidics are used to deliver pulsatory fluid flow, and the cantilever motion is subsequently monitored through the piezoresistive read-out. This method of actuation has been used to characterize the piezoresistive read-out in fluid and to perform preliminary cellular detection experiments.
Most of this thesis has been concerned with silicon based piezoresistive devices.
Silicon and germanium have an advantage over metallic piezoresistive elements for many sensing applications due to their higher gauge factor (more than 20x greater at our doping level and as much as 60x greater for lower doping levels in silicon). Despite this higher gauge factor, metallic piezoresistive devices dominate the market of commercial strain gauges. For most applications the drawbacks to silicon devices include cost, fragility (silicon is more brittle than gold), and the greater temperature coefficient of resistance of silicon1. Silicon devices remain important for applications where maximum possible sensitivity can be critical, however, we will see in chapter 8 (where we investigate piezoresistive sensors for the detection of Brownian fluctuations in fluid) that even for applications which require high levels of sensitivity, for certain types of measurements the benefits of working with metallic resistors; particularly the lower resistance, absence of 1/f noise, and fabrication advantages for making thin devices, outweigh the drawbacks
of a lower gauge factor. In this section we present data for both silicon and gold piezoresistive sensors.
The fabrication of silicon-based piezoresistive sensors was discussed in Chapter 3.
The gold stress sensors are fabricated by the same procedure, except before defining the cantilever by e-beam lithography a gold conducting path is patterned by e-beam lithography and deposited by liftoff along the region that will be the device legs. An SEM micrograph of a gold piezoresistive device is shown in Fig. 7.1.A.
Fig. 7.1.A Metallic piezoresistive cantilever
The conducting path is through the gold piezoresistive elements patterned on the device surface. The gold on the device legs is 25nm in thickness and 100nm in width. Thicker gold is patterned to the edge of the device ledge to reduce the electrode resistance.
7.2 Piezoresistive Cantilevers as Flow Meters
In this chapter we present experiments performed on piezoresistive micro- cantilevers embedded in microfluidics. The fabrication procedure for microfluidic embedded devices was discussed in section 3.1.5. A typical device is shown in Fig.
7.2.A. The microfluidic valves are operated using a voltage controlled pressure switch purchased from Fluidigm, Inc. This allowed an adjustable pressure (a pressure of between 21-24 PSI was used) to be applied to the control valves. The valves could be opened using a computer controlled voltage to the Fluidigm controller. In the case of fluidic drive, an oscillating voltage of square wave form was supplied to the Fludigm controller via an Agilent 33250A function generator, allowing the valve of interest (generally the output valve, although other valves also worked for this technique) to be opened and closed at known frequency.