1. Field of the Invention
The preferred embodiments are directed to probes of probe-based metrology instruments, and more particularly, to controlling tip-sample separation using a probe assembly including two probes adjacent to one another and having different tip heights.
2. Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically employ a probe having a tip and causing the tip to interact with the surface of a sample with appropriate forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample, which essentially involves using the probe as a sensor to detect where the sample surface is, based on its interaction with the sample. This interaction allows the AFM to detect changes in the characteristics of the sample. 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.
A typical AFM system is shown schematically in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15. Scanner 24 generates relative motion between the probe 14 and sample 22 while the probe-sample interaction is measured. In this way images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes (for example, a piezoelectric tube actuator). Alternatively, the scanner may be an assembly of multiple separate actuators. Some AFMs separate the scanner into multiple components (for example, an XY scanner that moves the sample and a separate Z-actuator that moves 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. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
In a common configuration, probe 14 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 14 at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other motion of cantilever 15. Probe 14 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 14 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 14, but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe.
Often, a selected probe 14 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14. In general, controller 20 generates control signals to maintain a relative constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
A workstation 40 is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform point selection, curve fitting, and distance determining operations, for example.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation is accomplished by moving either the sample or the probe assembly up and down relatively perpendicularly to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an “x-y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “z” direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” 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. In one mode of AFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” i.e. the force resulting from tip/sample interaction. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus or the associated technique, e.g., “atomic force microscopy.” In a recent improvement on the ubiquitous TappingMode™, called Peak Force Tapping® (PFT) Mode, feedback is based on force as measured in each oscillation cycle.
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 challenge when performing AFM is that though the imaging probe is used as a sensor to detect the surface, there are many types of experiments utilizing AFM that would benefit from being able to position the probe at a particular precise distance from the sample surface. Conventionally, the sample surface is identified by bringing the probe into contact with the sample, no matter what mode of AFM operation. Thereafter, the user has an indication of where the surface is. Ideally, knowing the exact location of the surface and being able to detect motion exactly where the tip is relative to the surface would be advantageous. However, given the range at which the tip apex needs to be positioned relative to the sample surface, as little as two nanometers or less, this task has been nearly impossible.
There are systems that employ different types of structures to identify an array of sample properties, but none have been able to achieve precise positioning of the tip-sample separation distance.
In U.S. Pat. No. 5,908,981 to Atalar et al. at Stanford University, a probe having interdigitated fingers capable of sensing different types of sample properties is provided. The issue being addressed by the Atalar et al, patent, however, is greater precision in deflection detection. Several challenges and drawbacks exist with such probes. The probes are difficult to fabricate and resolving the deflection of each finger is complex. Alternate structures, such as interferometer fibers, have been employed in AFM systems to attempt to detect the surface to help position the imaging probe, but all such solutions pose their own problems with respect to complexity, SNR, repeatability, deflection detection and ultimately precise positioning. There is one known system that is able to accurately position the probe relative to the sample. The system is maintained by IBM using a non-standard environment under highly controlled conditions, including maintaining the probe-sample relationship in a controlled environment consisting primarily of helium to minimize the effects of mechanical noise between the probe and sample. Clearly, such a system is impractical for the commercial/industrial/research applications contemplated by the preferred environments.
There was no known commercially viable instrument which was capable of detecting the motion of the imaging probe exactly where the tip is relative to the sample surface. A solution was desired.