Measuring and testing – Vibration – Sensing apparatus
Reexamination Certificate
2002-03-12
2004-02-24
Moller, Richard A. (Department: 2856)
Measuring and testing
Vibration
Sensing apparatus
C073S652000, C073S105000
Reexamination Certificate
active
06694817
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to probe-based instruments and, more particularly, relates to a method and apparatus for driving a cantilever of such an instrument using acoustic radiation pressure generated by an ultrasonic actuator.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. For example, scanning probe microscopes (SPMs) typically characterize the surface of a sample down to atomic dimensions by monitoring the interaction between the sample and a tip on the cantilever probe. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and which has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as strain gauges, capacitance sensors, etc. The probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Alternatively, some AFMs can at least selectively operate in an oscillation mode of operation such as TappingMode,™ In TappingMode™ the tip is oscillated at or near a resonant frequency of the cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
One potentially problematic characteristic of AFMs and other probe-based instruments lies in the technique employed to provide an external force to deflect or oscillate the instrument's cantilever. In an AFM, the cantilever is typically oscillated using a piezoelectric drive, often known simply as a piezo drive. Referring to
FIG. 1A
by way of example in this type of system, the typical probe
20
includes a cantilever
22
that extends outwardly from a substrate
26
coupled to a piezoelectric drive
24
via a probe mount
27
. Probe
20
also includes a tip
28
that is provided on the opposed, free end of the cantilever
20
. The piezoelectric drive
24
can be selectively excited by a signal generator
29
to move the cantilever
22
up and down relative to a sample
30
. When the instrument is configured for an oscillating mode of operation, the drive voltage is applied to the piezoelectric drive
24
to drive the cantilever
22
to oscillate at a frequency that is dependent upon the frequency of the drive voltage. This frequency is typically at or near the cantilever's resonant frequency, particularly when the instrument is operated in TappingMode™.
Such a traditional piezoelectric drive necessarily acts only on the base of the cantilever, not on the free end portion. It therefore must apply substantially greater forces to the cantilever to obtain a given deflection magnitude at the free end than it would if it were to apply forces directly to the free end or even to the body of the cantilever. This inefficiency limits the range of applications for this common type of piezo-electrically-driven probe.
For instance, the piezoelectric drive shown in
FIG. 1A
works well in air because the typical AFM cantilever can be excited to resonance in air relatively easily. This characteristic is quantified by the “quality factor” of a resonance of the cantilever. The quality factor, Q. denotes the sharpness of a cantilever's resonance curve as denoted by the ratio: f
0
/&Dgr;f, where f
0
is the resonant frequency and &Dgr;f is the bandwidth between the half-power points of the curve as reflected by the half-peak amplitude points
41
a
and
41
b
on the curve
40
in FIG.
2
A. The curve
40
demonstrates that the typical cantilever operating in air has a Q of 100-200 or even higher. The Q of a cantilever resonance is a measure of how much gain the resonance provides in an oscillating system. A resonance with a large Q can be excited to relatively large cantilever oscillation amplitudes with relatively small excitation forces. For operation in air or other gaseous environments, the cantilever typical piezoelectric drive usually has ample excitation force to drive the cantilever to produce a resonance peak
42
that is easily identified and distinguished from other, parasitic resonance peaks such as those of the mounts for the cantilever and the piezoelectric drive and the piezo drive itself (note the much smaller peaks
42
a,
42
b,
etc. denoting these parasitic resonances).
Conversely, a cantilever operated in liquid such as water has a dramatically lower Q because the liquid damps the oscillating cantilever. In fact, the typical cantilever operating in water has a Q of less than 30 and even less than 10. As a result, the typical piezoelectric drive does not have enough gain to excite the cantilever sufficiently to produce a resonance peak that is easily located and differentiated from parasitic resonances. This effect is discussed below in conjunction with FIG.
2
B.
Specialized cantilever drives are available that act along the length of the cantilever rather than only on the base. One such drive is the so-called magnetic drive. Referring to
FIG. 1B
, the typical magnetic drive system
50
has a magnetic cantilever
52
that is driven by an electromagnetic drive. The cantilever
52
has a fixed base rigidly attached to a support
54
and bears a tip
56
on its free end that interacts with sample S. The cantilever is also rendered magnetic by coating one or more of its surfaces with a magnetic layer
58
. The electromagnetic drive comprises at least one electromagnet coil
60
spaced from the cantilever
52
. The coil(s) can be energized by a controller
62
including a signal generator to impose a variable a magnetic field on the magnetic layer
58
. The magnetic field produces a torque on the cantilever
52
Degertekin F. Levent
Hadimioglu Babur B.
Onaran Abidin Guclu
Boyle Fredrickson Newholm Stein & Gratz S.C.
Georgia Tech Research Corporation
Moller Richard A.
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