Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are instruments which typically use a sharp tip to characterize the surface of a sample down to nanoscale dimensions. The term nanoscale as used for purposes of this invention refers to dimensions smaller than one micrometer. SPMs monitor the interaction between the sample and the probe tip. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular site on the sample, and a corresponding map of the site can be generated. Because of their resolution and versatility, SPMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research. In other applications, SPM systems may be used for measuring nanomechanical properties of a sample.
The probe of a typical SPM includes a very small cantilever fixed to a support at its base and having a sharp probe tip extending from 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 such as an optical lever system as described, for example, 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, the probe, or a combination of both. The instrument is thus capable of measuring the topography or other surface properties or nanomechanical properties of the sample.
SPMs may be configured to operate in a variety of modes, including modes for measuring, imaging, or otherwise inspecting a surface, and modes for measuring nanomechanical properties of a sample. In a contact mode operation, the microscope typically scans the tip across the surface of the sample while maintaining a constant probe-sample interaction force. In an oscillation mode of operation, sometimes referred to as tapping mode, the tip of the SPM is oscillated while interacting with the sample at or near a resonant frequency of the cantilever of the probe. The amplitude or phase angle of this oscillation is affected by the probe-sample interaction, and changes in the oscillation are sensed.
As the probe is scanned over the surface of the sample, a probe positioning control system monitors the interaction of the probe with the sample surface such as, for example, deflection of the cantilever (in the case of contact mode), or changes in the oscillation amplitude or phase angle (in the case of oscillating mode). The control system adjusts the probe's position (or average position in the case of oscillating mode) relative to the sample to maintain a constant probe-sample interaction. The position adjustment thus tracks the topography of the sample. In this way, the data associated with the position adjustment can be stored, and processed into data that characterizes the sample. This data can be used to construct an image of the inspected sample's surface, or to make certain measurements of selected surface features (such as, for example, a height of the feature).
The probe position adjustment is effected by a cantilever positioning actuator that is driven by a driving circuit. Various technologies for cantilever actuators are known, including piezoelectric and magnetic transducers. The driving circuit generates a probe positioning signal, and amplifies the probe positioning signal to produce a driving signal that is applied to the actuator. The driving signal continuously repositions the probe's separation distance from the sample to track an arbitrary topography of the sample's surface. Accordingly, the driving signal has a bandwidth from zero hertz to a frequency associated with the maximum operating bandwidth of the SPM, which corresponds to the maximum speed at which the probe can track the topography of the surface of the sample.
To increase the speed at which the sample can be inspected, the bandwidth of the driving signal must be increased commensurately. Achieving a high driving signal bandwidth presents a number of challenges. These include generating the driving power at high frequencies and maintaining stable operation over the operating bandwidth. At the higher operating frequencies, the transfer function of the actuator driving system exhibits a roll-off in gain and greater phase shift between the probe positioning signal and the cantilever. As a practical matter, the cantilever control system must be able to drive the actuator effectively, and remain stable over the operating bandwidth of the SPM. Accordingly, the actuator driving system must not produce a phase shift of 180 degrees or more while the gain is greater than unity in order to avoid a positive feedback scenario.
Typically, designers of SPM systems are constrained by a phase budget that must be met by the combination of every subsystem and component involved in the driving and control of the SPM in order to not exceed a phase shift of 180 degrees. Contributors to phase offset include the cantilever itself, the actuator, the driving signal amplifier, and the probe positioning control system, which includes components such as a displacement sensor, an analog-to-digital converter, a demodulator, and an RMS amplifier.
Conventional amplifiers driving piezoelectric actuators typically have an internal feedback network that includes frequency compensation to ensure stability of the amplifier circuit in the form of a separate feedback loop that is nested within the actuator driving control system. This frequency compensation internal to the driving amplifier circuit substantially reduces the amplifier's gain at the high frequencies and introduces additional phase delay, thereby restricting the amplifier's ability to drive the actuator at those high frequencies, and presenting a greater contribution to the phase offset of the actuator control system.
In SPM systems, a common cause of instability in the driving signal amplifier is the actuator's reactive loading characteristics on the driving signal amplifier. For instance, piezoelectric actuators present a capacitive load that can vary widely over frequency. Under certain conditions, the reactive nature of this capacitive load can cause uncontrolled oscillations in the amplifier circuit. To ensure that the amplifier circuit remains stable, a load-isolating resistance is typically placed at the output of the amplifier. While the load-isolating resistance beneficially stabilizes the amplifier, it does so at the cost of introducing another phase offset into the cantilever control system, further limiting the overall operating bandwidth.
Conventional general-purpose amplifiers and amplifier stabilization techniques, which are not adapted specifically for SPM applications, suffer from these, and various other shortcomings when applied to high-speed SPM applications. Accordingly, an actuator-driving amplifier suitable for high-speed SPM applications is needed that provides stable, high-gain, high-bandwidth performance while consuming as little of the SPM cantilever control loop phase budget as possible.