Radiant energy – Inspection of solids or liquids by charged particles
Reexamination Certificate
2000-04-07
2002-09-17
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
C250S307000, C073S105000
Reexamination Certificate
active
06452170
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning force microscope used for studying surface properties of materials on size scales ranging from the angstrom to the micron level.
2. Description of Related Art
Scanning force microscopes (SFM), also referred to as atomic force microscopes (AFM), are known for their use in a broad range of fields where high resolution information regarding the surface region of a sample is desired. Some types of SFMs utilize a small probe comprising a tip attached to the free end of a flexible cantilever for probing the surface of a sample. The tip of the probe is sharp and may either contact the sample or sense the sample without direct contact. The position of the tip is normally determined in all modes of operation of the machine. This position is usually obtained by measuring the angular deflection of the cantilever to which the tip is attached. The cantilever tip assembly is conventionally modeled as a mechanical simple harmonic oscillator (SHO) having an effective mass and effective spring constant.
The length of the cantilever is generally less than 300 &mgr;m. Forces between the tip of the probe and the sample surface cause the cantilever to deflect (i.e., bend), and a detector measures the cantilever deflection as the tip is scanned over the sample, or the sample is moved under the tip. The measured cantilever deflections can be used to generate constant force contours that are related to the surface topography. SFMs can be used to study solids, liquids, insulators, semiconductors or electrical conductors.
In addition to imaging, SFMs are used to measure forces of interaction between the probe tip and the surface. This is accomplished by performing a force-distance measurement. Conventional SFMs measure the position of the tip and the position of the sample. The value of a single spring constant associated with the elastic properties of the cantilever is determined experimentally. In a static calibration, the deflection caused by known loads applied to the cantilever is measured, and the constant is obtained from the deflection. In a dynamic calibration, the shift in the lowest resonance frequency of the cantilever is measured for different mass loads, and the constant is derived from that frequency shift. The spring constant is then used to convert cantilever deflections into forces.
The forces that contribute to the deflection of the SFM cantilever can be divided into two categories: repulsive and attractive. The repulsive force that typically dominates at very short range (tip-to-sample separation <0.3 nm) is the strong core repulsive force. At larger separations, the tip-to-sample force arises from a number of physical phenomena such as electrostatics, magneto-statics and surface tension. One important long range force that affects all SFMs is the electrostatic force commonly referred to as the Van Der Waals force. The variation of the total force, including the Van Der Waals force, upon the distance between the tip and the sample depends on whether the distance is within the contact region or the non-contact region. In the contact region, the cantilever is held less than a few tenths of a nano-meter from the sample surface, and the total inter-atomic force between the cantilever and the sample is repulsive. In the non-contact region, the cantilever is held on the order of one to ten nano-meters from the sample surface, and the inter-atomic force between the cantilever and the sample can be either attractive or repulsive. The significance of these two forces can be illustrated with some examples. The repulsive force is responsible for keeping individual elements of systems dispersed, such as keeping red blood cells separate and preventing coagulating of the blood in blood vessels. The attractive force is responsible, for example, for the attachment of drugs to the proper receptors, so that the drugs can have effect.
When used as imaging tools, SFMs operate in one of two modes: variable tip position or constant tip position. In the variable tip position mode, forces between tip and sample are allowed to alter the Z-axis position of the tip. The point at which the tip probes the sample surface is raster scanned (the tip and sample surface move with respect to one another in an X-Y plane) while the position of the tip (along the Z direction) is recorded. In this manner, a series of positional data point sets (x,y,z) are obtained. In the constant tip position mode, the Z position of the tip is maintained fixed during the raster scan, by applying forces to the cantilever through a piezoelectric actuator. In this mode, the Z portion of the positional data point (x,y,z) is obtained by measuring the piezoelectric voltage necessary to maintain a constant separation.
Cantilever based SFMs utilize three distinct sub-modes of operation which can be performed in either the constant tip position mode or the variable tip position mode. These sub-modes are contact, intermittent contact, and non-contact. In contact-SFM, also known as repulsive-SFM, the probe tip makes physical contact with the sample (i.e., the tip is brought close enough to the sample surface so that the dominant repulsive force is the strong core force). The tip is attached to the free end of a cantilever having a spring constant lower than the effective spring constant holding the atoms of the sample together. As the scanner gently traces the tip across the sample (or the sample moves under the tip), the contact force causes the cantilever to bend to accommodate changes in sample topography. The Z position of the cantilever is typically measured using optical techniques. The most common method involves the use of an optic lever, consisting of a laser beam reflected by the surface of the cantilever onto a position-sensitive photo-detector (PSPD). As the cantilever bends, the position of the laser beam on the detector shifts, indicating the bending of the beam, which is approximately equal to the change in the Z-displacement of the free end of the cantilever. Other methods to detect the cantilever deflection are known, and include optical interference, a tunneling microscope, the use of a cantilever fabricated from a piezo-electric material, or a magnetic pickup system.
An SFM can also be operated in a mode where the tip is not in direct contact with the sample surface (i.e., where the dominant force is not the strong core repulsion). The simplest non-contact mode of operation places the tip far enough above the surface so that the structural stiffness of the cantilever at the equilibrium position is sufficient to counter the sum of all attractive forces. The tip-to-sample separation (usually a few nano-meters) must be small enough so that the force field generated by the sample is sufficient to measurably deflect the cantilever. The sample is then moved towards the tip, and the tip displacement is recorded as in the variable contact mode technique. This is the only conventional non-contact mode to work in fluid, but it is difficult to implement.
Another non-contact technique involves oscillating the cantilever near its resonant frequency. The tip-to-sample distance is then reduced until the existence of tip-to-sample forces causes a shift in the resonant frequency of the cantilever. Either the amplitude of vibration at the original resonant frequency is measured or the shift in phase between the driving signal and the cantilever oscillation is measured. A major shortcoming of the oscillating non-contact mode is that it provides lower lateral resolution than the contact mode. Generally, lateral resolution around 10 nano-meters is obtained.
Non-contact SFM is desirable because it provides a means for measuring sample topography with no contact between the tip and the sample and thus causes minimal damage to the sample. It is desirable to have the highest possible resonant frequency so that physically meaningful averages can be taken at reasonable raster scanning rates. Typically, cantilevers with spring constants around 100 N/m havi
Eppell Steve
Zypman Fredy R.
Nguyen Kiet T.
Patent Law Offices of Heath W. Hoglund
University of Puerto Rico
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