Radiant energy – Inspection of solids or liquids by charged particles – Methods
Reexamination Certificate
2000-04-28
2001-07-24
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Methods
Reexamination Certificate
active
06265718
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to scanning probe microscopes, which are used to obtain extremely detailed analyses of the topography and other characteristics of samples such as semiconductor devices and data storage media and, more particularly, to scanning probe microscopes which are classified as scanning force microscopes or scanning tunneling microscopes.
BACKGROUND OF THE INVENTION
Definitions
“Scanning probe microscope” (SPM) means an instrument which provides a microscopic analysis of the topographical features or other characteristics of a surface by causing a probe to scan the surface. It refers to a class of instruments which employ a technique of mapping the spatial distribution of a surface property, by localizing the influence of the property to a small probe. The probe moves relative to the sample and measures the change in the property or follows constant contours of the property. Depending on the type of SPM, the probe either contacts or rides slightly (up to a few hundred Angstroms) above the surface to be analyzed. Scanning probe microscopes include devices such as scanning force microscopes (SFMs), scanning tunneling microscopes (STMs), scanning acoustic microscopes, scanning capacitance microscopes, magnetic force microscopes, scanning thermal microscopes, scanning optical microscopes, and scanning ion-conductive microscopes.
“Probe” means the element of an SPM which rides on or over the surface of the sample and acts as the sensing point for surface interactions. In an SFM the probe includes a flexible cantilever and a microscopic tip which projects from an end of the cantilever. In an STM the probe includes a sharp metallic tip which is capable of sustaining a tunneling current with the surface of the sample. This current can be measured and maintained by means of sensitive actuators and amplifying electronics. In a combined SFM/STM the probe includes a cantilever and tip which are conductive, and the cantilever deflection and the tunneling current are measured simultaneously.
“Cantilever” means the portion of the probe of an SFM which deflects slightly in response to forces acting on the tip, allowing a deflection sensor to generate an error signal as the probe scans the surface of the sample.
“Tip” in an SFM means the microscopic projection from one end of the cantilever which rides on or slightly above the surface of the sample. In an STM, “tip” refers to the metallic tip.
“Package” means an assembly which includes the cantilever and tip, a chip from which the cantilever projects, and may include a plate on which the chip is mounted.
“Scanning Force Microscope” SFM (sometimes referred to as Atomic Force Microscope) means an SPM which senses the topography of a surface by detecting the deflection of a cantilever as the sample is scanned. An SFM may operate in a contacting mode, in which the tip of the probe is in contact with the sample surface, or a non-contacting mode, in which the tip is maintained at a spacing of about 50 Å or greater above the sample surface. The cantilever deflects in response to electrostatic, magnetic, van der Waals or other forces between the tip and surface. In these cases, the deflection of the cantilever from which the tip projects is measured.
“Scanning Tunneling Microscope” (STM) means an SPM in which a tunneling current flows between the probe and the sample surface, from which it is separated by approximately 1-10 Å. The magnitude of the tunneling current is highly sensitive to changes in the spacing between the probe and sample. STMs are normally operated in a constant current mode, wherein changes in the tunneling current are detected as an error signal. A feedback loop used this signal to send a correction signal to a transducer element to adjust the spacing between the probe and sample and thereby maintain a constant tunneling current. An STM may also be operated in a constant height mode, wherein the probe is maintained at a constant height so that the probe-sample gap is not controlled, and variations in the tunneling current are detected.
“Kinematic mounting” means a technique of removably mounting a rigid object relative to another rigid object so as to yield a very accurate, reproducible positioning of the objects with respect to each other. The position of the first object is defined by six points of contact on the second. These six points must not over or under constrain the position of the first object. In one common form of kinematic mounting, three balls on the first object contact a conical depression, a slot (or groove) and a flat contact zone, respectively, on the second object. Alternatively, the three balls fit snugly within three slots formed at 120° angles to one another on the second object. The foregoing are only examples; numerous other kinematic mounting arrangements are possible. According to the principles of kinematic mounting, which are well known in the mechanical arts, six points of contact between the two objects are required to establish a kinematic mounting arrangement. For example, in the first illustration given above, the first ball makes contact at three points on the conical surface (because of inherent surface imperfections, a continuous contact around the cone will not occur), two points in the slot, and one point on the flat surface, giving it a total of six contact points. In the second illustration, each ball contacts points on either side of the slot into which it first.
THE PRIOR ART
Scanning probe microscopes (SPMs) are used to obtain extremely detailed analyses of the topographical or other features of a surface, with sensitivities extending down to the scale of individual atoms and molecules. Several components are common to practically all scanning probe microscopes. The essential component of the microscope is a tiny probe positioned in very close proximity to a sample surface and providing a measurement of its topography or some other physical parameter, with a resolution that is determined primarily by the shape of the tip and its proximity to the surface. In a scanning force microscope (SFM), the probe includes a tip which projects from the end of a cantilever. Typically, the tip is very sharp to achieve maximum lateral resolution by confining the force interaction to the end of the tip. A deflection sensor detects the deflection of the cantilever and generates a deflection signal, which is then compared with a desired or reference deflection signal. The reference signal is then subtracted from the deflection signal to obtain an error signal, which is delivered to a controller. There are several types of deflection sensors. One type uses an optical interferometer as described in an article by D. Rugar et al.,
Review of Scientific Instruments,
Vol. 59, p. 2337 (1988). Most commercial SFMs, however, employ a laser beam which is reflected from the back of the cantilever and use a photodetector to sense the angular movement of the beam as the cantilever is deflected. The probe (cantilever and tip) and deflection sensor are normally housed in a unit referred to as a head, which also contains circuitry for preamplifying the signals generated by the deflection sensor before they are passed to a controller. An image is formed by scanning the sample with respect to the probe in a raster pattern, recording data at successive points in the scan, and displaying the data on a video display. The development of scanning (or atomic) force microscopy is described in articles by G. Binnig et al.,
Europhys. Lett.,
Vol. 3, p. 1281 (1987), and T. R. Albrecht et al.,
J. Vac. Sci. Technology,
A6, p. 271 (1988). The development of the cantilever for SFMs is described in an article by T. R. Albrecht et al., entitled “Microfabricated Cantilever Stylus for Atomic Force Microscopy”.
J. Vac. Sci. Technol., A
8, p. 3386 (1990). Other types of SPMs, such as scanning capacitance or scanning magnetic force microscopes, also use similar deflection sensors.
A scanning tunneling microscope (STM) is similar to an SFM in overall structure, but the probe consists of a sh
Park Sang-il
Smith Ian R.
Nguyen Kiet T.
ThermoMicroscopes, Corp.
Weitz David J.
Wilson Sonsini Goodrich & Rosati
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