Data processing: measuring – calibrating – or testing – Measurement system – Dimensional determination
Reexamination Certificate
2002-05-06
2004-10-26
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Dimensional determination
C702S167000
Reexamination Certificate
active
06810354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to scanning probe microscopes, and more particularly, to a method of extracting the tip shape from data obtained by a scanning probe microscope.
2. Description of Related Art
Several known probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. For example, scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip and low forces to characterize the surface of a sample down to atomic dimensions. More particularly, SPMs monitor the interaction between the sample and the tip on the cantilever of the probe. 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.
The atomic force microscope (AFM) is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and which has 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, often an optical lever system such as described 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 and/or 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. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. 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. Alternatively, some AFMs can at least selectively operate in an oscillation mode of operation such as TappingMode.™ (TappingModer™ is a trademark of the present assignee.) In oscillation mode, the tip is oscillated at or near a resonant frequency of the cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
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.
Notwithstanding the fact that scanning probe microscopes are high resolution instruments, the ultimate resolution of the data obtained by such probe-based instruments is limited by the physical characteristics of the tip of the probe itself. More particularly, there are limitations as to how small, and sharp, the tip can be made. In view of this, the tip shape is reflected in the acquired data, a problem that is exacerbated by the fact that AFMs often image very small (e.g., Angstrom scale) features. As a result, an error in the acquired data results and the corresponding accuracy of the surface image is significantly compromised. Hereinafter, the acquired SPM image will periodically be called the “dilated” image.
For some applications, this limitation may be negligible. However, for many applications, the degree of accuracy required to resolve the features of the sample surface is significantly greater, such that tip shape error is unacceptable. For instance, in the semiconductor fabrication industry, imaging features such as lines, trenches and vias with single nanometer accuracy is desired. These features may have dimensions in the range of 120 nm, and are continually getting smaller. With typical tip widths in the range of about 70 nm, the tip shape clearly introduces significant error in the data and must be removed to accurately image the sample surface.
Moreover, the aforementioned problems can be exacerbated by the fact that complex sample surface topologies require a commensurate increase in tip shape complexity to image such surfaces. For example, samples may include undercut regions where a particular x,y scan position may have multiple “Z” height values (see region “U” in
FIG. 1
, discussed in further detail below). Again, this is common in the semiconductor fabrication industry, and thus tips have been developed to allow imaging of such complex topographies. However, with the increase in tip shape complexity, there typically is a corresponding increase in error in the AFM data.
Two types of known tip shapes are illustrated in
FIGS. 1 and 2
. Note that probe tips, such as the CD tip, shown in
FIG. 2
, typically will not have the smooth symmetrical shapes illustrated in the figures. These tip shapes are merely presented as such to highlight the concepts and features of the preferred embodiment. In
FIG. 1
, a probe tip
10
of a traditional scanning probe microscope includes a parabolic, or other pointed shape that is relatively easy to characterize. Tip
10
includes a shaft
12
and a distal end
14
that although sharp is typically at least slightly rounded at its active surface
15
. During a scan (operating in an oscillating mode, for instance), tip
10
interacts with a sample surface
16
to image characteristics of that surface. Tip-sample interaction is controlled, and data is collected, via a control system (not shown) as described previously. The collected data, in turn, may be plotted to image the sample surface. Importantly, this acquired image may not accurately reflect sample surface characteristics due to, among other things, the error introduced by the shape of the pointed tip.
In addition to introducing at least some tip shape error in the acquired data, probe tip
10
is unable to image certain surfaces. In particular, although suitable for many applications, based on its shape probe tip
10
is simply unable to accurately depict vertical sidewalls and undercut regions (which often exist in semiconductor fabrication, for example) in the corresponding sample surface topography. Notably, this is due to limitations in both the tip shape and the algorithms used to control tip position.
To be able to image surface features such as vertical sidewalls and undercut regions, AFMs having more complex probe tips have been developed. In one such instrument, shown in
FIG. 2
, an AFM employs active X-Z control to follow complex surface topography using a probe tip
20
having a shaft
22
and a distal end
24
including left and right protuberances
26
,
28
, respectively, in the scan (for example X) direction. By dithering the tip in the scan direction, protuberances
26
,
28
are caused to interact with surface features such as vertical sidewalls. As a result, what be
Boyle Fredrickson Newholm Stein & Gratz S.C.
Veeco Instruments Inc.
Washburn Douglas N
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