The present application is directed to driving systems, and more particularly to a method and apparatus for driving a cantilever through the use of acoustic wave pressure generated by an ultrasonic actuator. 
Scanning probe microscopes (SPMs) represent a category of probe-based instruments designed to characterize a surface of a sample at an atomic level, through the monitoring of an interaction between a sample and a tip on a cantilever probe.  This interaction is primarily a scanning operation between the tip and sample, whereby data regarding characteristics of the surface is acquired and used to generate an image of the sample region.  The image data is commonly acquired via a raster scan of the sample. 
A particular type of SPM is known as an atomic force microscope (AFM), which functions by measuring local properties of a sample, such as height, optical absorption, magnetism or other measurable characteristic.  The resulting image will resemble an image on a television screen, as it consists of both many rows or lines of information placed one above the other. 
AFMs are designed to operate in a variety of modes, including non-contact mode, contact mode and oscillating mode, also known as a tapping mode.  In the non-contact mode, the AFM generates a topographic image from measurements based on attractive forces.  In this design, the tip does not touch the sample.  The non-contact mode does not function effectively in liquids. 
In the contact mode of operation, the AFM scans the tip across the surface of the sample, while the force of the tip on the surface of the sample is maintained at a generally constant value.  This contact operation is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample upon sensing a deflection of the cantilever as the probe is scanned horizontally across the surface. 
In the oscillating mode, the tip is made to oscillate at or near a resonant frequency of the cantilever.  The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction.  Similar to the contact mode, the feedback signals are then collected, stored and used as data to generate an image of the sample area. 
Atomic force microscopes are known to have a resolution down to the atomic level for a wide variety of surfaces.  While the general concept of an AFM is similar to that of a record player, as well as the stylus profilometer, to obtain and enable the atomic-scale resolution, AFMs have refinements, including an optical sensor which operates by reflecting a laser beam off of the cantilever.  Angular deflection of the cantilever causes an angular deflection of the laser beam.  The reflected laser beam strikes a position detector to indicate the position of the laser spot on the detector, and thus the angular deflection of the cantilever. 
An area of particular interest in designing an AFM is the mechanism employed to provide an external force to deflect or oscillate the cantilever.  In existing AFMs, the cantilever will typically be oscillated by a piezoelectric actuation. 
Traditional piezoelectric drives act on the base of the cantilever, not on a free-end portion.  Therefore, these systems must apply substantially greater forces to the cantilever to obtain a given deflection magnitude at the free end than would be required if force were applied directly to the free end or the body of the cantilever.  Such a design results in certain limitations. 
Since a typical AFM cantilever is easily excited to resonance in air, the piezoelectric drive is useful in this environment.  However, piezoelectric drives are not very useful in liquid (e.g., water) environments.  The reason for this has to do with the quality factor or Q of a resonance of the cantilever.  The quality factor, Q, denotes the sharpness of a cantilever's resonance curve.  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 typical piezoelectric drive provides ample excitation force to drive the cantilever to produce a resonance peak that is easily identified and distinguished from parasitic resonance peaks, such as those of the mounts for the cantilever and the piezoelectric drive itself. 
Conversely, a cantilever operated in liquids such as water, has a dramatically lower Q, as the liquid dampens the oscillating cantilever.  A 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. 
In view of this, specialized cantilever drives have been developed to act along the length of the cantilever rather than only on the base.  One such drive is known as a magnetic drive.  The typical magnetic drive has a magnetic cantilever that is driven by an electromagnetic force.  The cantilever has a fixed base rigidly attached to a support and bears a tip on its free end that interacts with a sample.  The cantilever is rendered magnetic by coating one or more of its surfaces with a magnetic layer.  By controlling the amplitude of the applied magnetic field, the cantilever can be deflected while the tip interacts with the sample. 
However, a magnetic drive has inherent limitations that considerably restrict its range of applications.  For instance, it requires a special magnetically-coated cantilever and cannot therefore be used in applications where the cantilever should not be coated with magnetic material.  It is also not applicable to situations where the magnetic properties of the sample and/or the environment results in undesirable deflection of the cantilever, producing errors in the measurements.  The operating ranges of the magnetic drive system are also limited. 
An acoustic drive has also been considered to drive the cantilever.  In this design, a cantilever and piezoelectric drive are mounted on a common head in a spaced-apart relationship.  The head is mounted above a fluid cell, and the cantilever extends into the fluid cell to interact with the sample in the cell.  The piezoelectric drive can be excited by a signal generator to generate acoustic waves that propagate through the glass walls of the fluid cell, through the fluid in the cell, then on to the cantilever, causing the cantilever to oscillate. 
Acoustic drives of this type have various disadvantages.  For instance, the unfocused acoustic energy will impinge on many other components of the system, such as mounts for the cantilever and the piezoelectric drive, the fluid cell, and even the fluid, exciting resonances in those components.  These resonances can be difficult to distinguish from the cantilever resonance.  The acoustic drive also has sufficient actuation force at a limited selection of operation frequencies and can be a challenge to match the cantilever resonance with the operation frequency of the acoustic actuator.  If a user selects a resonance that does not overlap with the cantilever resonance, the measurements may be unstable. 
Pending application U.S. Ser.  No.  10/456,136 (Publication No.  US 2004-0020279 A1, entitled “Method and Apparatus for the Ultrasonic Actuation of the Cantilever of a Probe-Based Instrument”; U.S. Pat.  No.  6,779,387 (U.S. Ser.  No.  10/095,850 (Publication No.  US 2003-0041657 A1)), entitled “Method and Apparatus for the Ultrasonic Actuation of the Cantilever of a Probe-Based Instrument”; and U.S. Pat.  No.  6,694,817 (U.S. Ser.  No.  10/096,367 (Publication No.  US2003-0041669A1)), entitled “Method and Apparatus for the Ultrasonic Actuation of the Cantilever of a Probe-Based Instrument”, (all claiming priority to provisional patent application Ser.  No.  60/313,911, filed Aug. 21, 2001) (commonly assigned) (all hereby incorporated by reference), describes an ultrasonic force microscope (UFM) intended to have an actuator that drives a cantilever to produce a “clean” frequency response, preferably by driving the cantilever body, rather than the base.  It is stated that by driving the body of the cantilever with an ultrasonic actuator, a much higher localized force can be achieved through the use of a traditional piezoelectric actuator.  The beam used for actuation is preferably shaped, i.e., manipulated to limit unwanted propagation and directions other than toward the cantilever, so that ultrasonic energy impinges at least primarily on the cantilever. 
Two suitable techniques for shaping the beam are listed as focusing and collimation.  The ultrasonic small diameter beams can be focused on the cantilever using a Fresnel lens or other focusing device located between the ultrasonic actuator and the cantilever.  It is noted that the Fresnel lens may focus the ultrasonic beam to a diameter of approximately 5 μm to 10 μm at a focal distance of 360 μm, where the 5 μm diameter is even smaller than the 8 μm to 12 μm diameter of most laser beams.  As a result, it is stated the lens can be used to apply a pinpoint force to the free end of the cantilever or any other point of interest along the length of the cantilever. 
It is also proposed that in an alternative embodiment, the beam may be intentionally sized larger than the cantilever to account for tolerances in alignment of the cantilever and the ultrasonic actuator.  If, for example, the cantilever is 50 microns wide, and can be reproducibly aligned within ±100 microns, an ultrasonic actuator with a beam width of 250-300 μm in the region of the cantilever could guarantee that a portion of the ultrasonic beam would always strike the cantilever. 
Thus, in the incorporated applications, the preferred embodiment purports to disclose an apparatus and procedure for providing pinpoint actuation energy to a cantilever.  It is also acknowledged that alignment errors may exist between the cantilever and actuator, whereby the pinpoint accuracy may result in the acoustic beam not impacting the cantilever. 
Misalignment of a probe may occur during manufacture or when the cantilever probe is replaced.  Particularly, it is known that in operation, the tip carried on the cantilever becomes damaged or worn and will require replacement.  Normally, the cantilever and tip come as a single unit, and the entire unit is replaced with a new cantilever/tip arrangement.  This replacement operation is a mechanical operation, and a degree of imprecision in the alignment procedure exists.  Therefore, when the cantilever/tip arrangement is inserted, and a focused pinpoint (i.e., small diameter) beam is used, misalignment may result in improper interaction between the actuator and the cantilever.  To address this issue it is proposed that the beam (i.e., the diameter) is enlarged. 
From the foregoing, it can be seen there are concerns related to use of a pinpoint acoustic beam due to misalignment issues.  A further issue, however, is that widening the beam to address misalignment causes an increase in undesirable reflections and resonance between the acoustic source and the cantilever.  These resonances can result in strong variations in the acoustic force delivered to the cantilever as the resonance conditions vary with variations in spacing between the cantilever and acoustic source.