Tapping atomic force microscope with phase or frequency detection

Abstract

An atomic force microscope in which a probe tip is oscillated at a resonant frequency and at amplitude setpoint and scanned across the surface of a sample, which may include an adsorbed water layer on its surface, at constant amplitude in intermittent contact with the sample and changes in phase or in resonant frequency of the oscillating are measured to determine adhesion between the probe tip and the sample. The setpoint amplitude of oscillation of the probe is greater than 10 nm to assure that the energy in the lever arm is much higher than that lost in each cycle by striking the sample surface, thereby to avoid sticking of the probe tip to the sample surface. In one embodiment the probe tip is coated with an antibody or an antigen to locate corresponding antigens or antibodies on the sample as a function of detected variation in phase or frequency. In another embodiment, the frequency of oscillation of the probe tip is modulated and relative changes in phase of the oscillating probe tip observed in order to measure the damping of the oscillation due to the intermittent or constant tapping of the surface by the tip. In a further embodiment, the slope of the phase versus frequency curve is determined and outputted during translating of the oscillating probe. Force dependent sample characteristics are determined by obtaining data at different tapping amplitude setpoints and comparing the data obtained at the different tapping amplitude setpoints.

Description

BACKGROUND OF THE INVENTION1. Field of the InventionThis invention relates to an ultra-low force atomic force microscope, and particularly an improvement to the atomic force microscope described in related commonly owned U.S. patent application Ser. No. 08 / 147,571 and related U.S. Pat. Nos. 5,229,606 and 5,266,801.2. Discussion of the BackgroundAtomic Force Microscopes (AFM's) are extremely high resolution surface measuring instruments. Two types of AFM's have been made in the past, the contact mode (repulsive mode) AFM and the non-contact (attractive mode) AFM.The contact mode AFM is described in detail in U.S. Pat. No. 4,935,634 by Hansma et al, as shown in FIG. 2. This AFM operates by placing a sharp tip attached to a bendable cantilever directly on a surface and then scanning the surface laterally. The bending of the lever in response to surface height variations is monitored by a detection system. Typically, the height of the fixed end of the cantilever relative to the sample i...

AI Technical Summary

Benefits of technology

A further object of the invention is to provide a novel AFM and method to profile surfaces, including soft or sticky surfaces, at high resolution with high sensitivity and fast time response, thus overcoming the drawbacks of prior art contact and non-contact mode AFM's.

It is a further object of this invention to provide an AFM that may map magnetic or other force distributions while retaining the ability to track topography without other force components.

These and other objects are achieved according to the present invention by providing a new and improved AFM and method of operating an AFM, wherein the probe is oscillated at or near resonance or a resonant harmonic to strike the surface of the sample, so that the tip has minimal lateral motion while in contact with the surface, thus eliminating scraping and tearing. The cantilever probe is oscillated at a large amplitude, greater than 10 nm, preferably greater than 20 nm, and typically on the order of 100-200 nm, so that the energy in the lever is large enough, much higher than that lost in each oscillation cycle due to, for example, damping upon striking the same surface, so that the tip will not become stuck to the surface. The oscillation amplitude is affected by the tip striking the surface in a measurable fashion, and this limited amplitude is a direct measure of the topography of the surface. Alternatively, a feedback control can be employed to maintain the oscillation amplitude constant, and then a feedback control signal can be used to measure surface topography. The striking interaction is strong, so the sensitivity is high. The resolution approaches the contact mode microscope because the tip touches the surface. The technique can use high frequency jumps with no loss in sensitivity since the measurement of the amplitude change does not depend on frequency.

Problems solved by technology

However, for samples that are very soft or interact strongly with the tip, such as photoresist, some polymers, silicon oxides, many biological samples, and others, the contact mode has drawbacks.

The lateral shearing forces may make the measurement difficult and for soft samples may damage the sample.

Further, a stick-slip motion may cause poor resolution and distorted images.

However for many samples and applications, immersion in liquid is not of much use.

Operating in liquid requires a fluid cell and increases the complexity of using the AFM, and for industrial samples such as photoresist and silicon wafers, immersion is simply not practical.

This limitation as will be shown limits the usefulness of the non-contact method.

Although developed at essentially the same time as the contact AFM, the non-contact AFM has rarely been used outside the research environment due to problems associated with the above constraints.

These operating conditions make the possibility very likely of the tip becoming trapped in the surface fluid layer described by Hansma et al.

As can be seen from FIG. 6, there is significant hysteresis in the withdraw process, which will cause serious instability in the image data.

Moreover, because the tip must be operated above the fluid layer, the lateral resolution is inferior to the contact mode.

For measuring the frequency shift using amplitude detection, the sensitivity depends on the cantilever having a very sharp resonance peak, which in turn gives a very slow response time because undamped systems require a long time to recover from a perturbation.

Thus it can be seen that high sensitivity and fast response are very difficult to achieve with a non-contact AFM.

Furthermore, the weak force interaction places restrictions on the height at which the tip may be operated and the amplitude of oscillation.

The presence of the fluid layer near this height makes capture of a lever with a small oscillation likely, so slow time response is a serious stability problem.

For these reasons, despite their many potential advantages, non-contact AFM's have yet to be successful commercially.

This method limits the technique to conductive surfaces.

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