Atomic force microscope for profiling high aspect ratio samples

Details Atomic force microscope for profiling high aspect ratio samples Atomic force microscope for profiling high aspect ratio samples

1999-09-30

2002-12-03

Anderson, Bruce (Department: 2881)

Radiant energy

Inspection of solids or liquids by charged particles

C250S307000

Reexamination Certificate

active

06489611

ABSTRACT:

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
BACKGROUND OF THE INVENTION
Obtaining images of high aspect ratio structures poses certain challenges. High aspect ratio structures are those where one characteristic dimension is more pronounced than the others. Examples of these type of structures include probes of atomic force and scanning tunnelling microscopes, field emission probes, micro-indenters and Micro Electro-Mechanical systems (MEM'S) structures. Such high aspect ratio structures have typical heights on the order of tens of micrometers and tip radii in the range of tens of nanometers. Further, these structures may or may not be conductive.
In imaging such structures, it is sometimes desirable to image the side walls of the structure and to measure the radius of the tip of the structure in a non-destructive manner. Desired image resolutions can be on the order of 1 nm in the vertical (normal to the surface) direction and 10 nm in the lateral direction. Such imaging criteria prevent the use of certain types of profilometry methods.
In one type of conventional stylus profilometer, a stylus with a sharp tip is mechanically dragged along the sample surface. The deflection of the hinged stylus arm is measured and recorded as the surface profile. The use of a hinged stylus arm allows measurement of very rough surfaces, for example those with peak-to-peak heights greater than 1 mm. Probe-to-surface contact forces range from 10
−3
N to 10
−6
N. However, since the hinged stylus arm is partially supported by the stylus itself, physical rigidity limits the minimum stylus tip radius and hence the lateral resolution to about 0.1 mm.
In optical profilometry, many different optical phenomena (such as interference and internal reflection) can be utilized. The most popular technique is based on phase-measuring interferometry, in which a light beam reflecting off the sample surface is interfered with a phase-varied reference beam. The surface profile is deduced from the resulting fringe patterns. With a collimated light beam and a large photodetector array, the entire surface can be profiled simultaneously. This and other conventional optical profilometry methods are limited in lateral resolution by the minimum focussing spot size of about 0.5 &mgr;m (for visible light). In addition, measurement values are dependent on the surface reflectivity of the material being profiled.
Currently, only the recently developed scanning probe microscopes can meet a 10 nm lateral resolution requirement. In these microscopes, an atomically sharp (or nearly so) tip at a very close spacing to the sample surface is moved over the surface using a piezoactuator. One type of scanning probe microscope is the atomic force microscope (AFM), which measures the topography of a surface with a probe that has a very sharp tip. A probe assembly includes a cantilever beam from which the probe, or microstylus extends. The probe terminates at the probe tip having a typical tip radius of less than 0.1 &mgr;m. The probe typically has a length on the order of a couple of micrometers and the cantilever beam typically has a length between 100 &mgr;m and 200 &mgr;m.
As is illustrated in
FIG. 1
, the AFM can operate in two different regimes, contact and non-contact, depending on the spacing maintained between the probe and sample. In the contact regime, the probe is kept some angstroms from the sample surface and the interactions are mainly repulsive. In the non-contact regime, the spacing between the probe and the sample surface is from tens to hundreds of angstroms and the interactions are attractive, mainly due to the long range van der Waals forces.
In a contact mode atomic force microscope, the probe is moved relative to the surface of a sample and deflection of the cantilever is measured to provide a measure of the surface topography. More particularly, a laser beam is directed toward, and reflects off the back surface of the cantilever to impinge upon a sensor, such as a photodetector array. The electrical output signals of the photodetector array provide a topographical image of the sample surface and, further, provide feedback signals to a fine motion actuator, sometimes provided in the form of a piezoelectric actuator. In a constant force contact AFM, the fine motion actuator is responsive to the feedback signals for maintaining a substantially constant force between the probe tip and the sample, such as forces on the order of 10
−8
N to 10
−11
N.
Initial contact between the probe and the sample is conventionally achieved with the assistance of a camera located above the sample. The probe and sample are visualized with the camera and, once the probe is positioned at a desired area of the sample, the user actuates a coarse motion actuator which moves the probe into contact with the sample surface. Generally, the coarse motion actuator has a relatively large vertical range, such as on the order of 2-10 centimeters.
Contact atomic force microscopy offers high lateral and vertical resolutions, such as less than 1 nm vertical resolution and less than 50 nm lateral resolution. Further, since the contact AFM relies on contact forces rather than on magnetic or electric surface effects, advantageously the contact AFM can be used to profile conductive and non-conductive samples. However, the maximum surface roughness that can be profiled is much less than that of conventional stylus profilometers which use a linear variable differential transducer (LVDT).
In the non-contact atomic force microscope, long range van der Waals forces are measured by vibrating the cantilever near its resonance frequency and detecting the change in the vibrational amplitude of a laser beam reflected off the cantilever due to a change in the force gradient caused by changes in the surface profile. The non-contact atomic force microscope offers non-invasive profiling. However, the technique has some disadvantages when compared to contact atomic force microscopy. First, van der Waals forces are hard-to-measure weak forces, rendering the microscope more susceptible to noise. Secondly, the probe tip must be maintained at a fixed height above the sample, typically on the order of a few nanometers, and the feedback control necessary to maintain this spacing must operate slowly to avoid crashing the probe tip on the sample. Thirdly, since the tip is always floating above the surface, the effective tip radius is increased and hence the achievable lateral resolution is decreased.
BRIEF SUMMARY OF THE INVENTION
According to the invention, methods and apparatus utilizing contact atomic force microscopy are provided for profiling both conductive and non-conductive samples having high aspect ratio structures, with a lateral resolution on the order of 10 nm and a vertical resolution on the order of 1 nm. Conventional atomic force microscopes are typically used to provide a topographical image of relatively flat surfaces and certain problems arise when using atomic force microscopy to profile high aspect ratio features. As a result, high aspect ratio structures are most typically imaged under a Scanning Electron Microscope (SEM). However, AFM imaging is more desirable because the topographic data retrieved is already in numerical format, whereas SEM pictures must be interpreted, based on the image contrast.
Various aspects of the present invention address and overcome the problems faced when using an AFM for profiling high aspect ratio structures. In conventional AFMs, a user controllable coarse motion actuator is used to bring the probe into initial contact with a desired area of the sample surface with the assistance of a camera. Although the camera facilitates landing the probe in a desired area of the sample, due to the extremely small dimensions of the probe tip and high aspect ratio features and also due to the practical resolution limitations of the camera, this technique is not generally capable of reliably landing the probe tip on a high aspect ratio feature to be profiled. If the probe tip initially la

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