Radiant energy – Inspection of solids or liquids by charged particles
Reexamination Certificate
2002-03-01
2004-01-06
Berman, Jack (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
C250S442110, C073S105000
Reexamination Certificate
active
06674074
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to an improved scanning probe microscope for use in scanning microscope and lithography applications.
BACKGROUND OF THE INVENTION
Scanning probe microscopes (SPMs) are a family of instruments used for studying surface properties of materials from the atomic to the micron level. The atomic force microscope (AFM) probes the surface of a sample with a sharp tip, a couple of microns long and often less than 100 Å in diameter. The tip is located at the free end of a cantilever that is typically 100 to 200 &mgr;m long. Forces between the tip and the sample surface cause the cantilever to bend, or deflect. A detector measures the cantilever deflection as the tip is scanned over the sample, or the sample is scanned under the tip. The measured cantilever deflections allow a computer to generate a map of surface topography. Several forces typically contribute to the deflection of an AFM cantilever including repulsive forces arising from interatomic forces between the cantilever and the sample and attractive van der Waals forces. Additionally, the deflection of the AFM cantilever is minimized through the feedback of the AFM.
In contact AFM mode, an AFM tip makes soft “physical contact” with the sample. The tip is attached to the end of a cantilever with a low 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 under the tip), the contact force causes the cantilever to bend to accommodate changes in topography of the scanned sample. In AFM this means that when the cantilever pushes the tip against the sample, the cantilever bends rather than forcing the atoms of the tip closer to the atoms of the sample. Even if you design a very stiff cantilever to exert large forces on the sample, the interatomic separation between the tip and sample atoms is unlikely to decrease much. Instead, the sample surface is likely to deform.
In addition to the repulsive and attractive forces described above, two other forces are generally present during contact AFM operation: a capillary force exerted by the thin water layer often present in an ambient environment, and the force exerted by the cantilever itself. The capillary force arises due to the thickness of the meniscus, which is a function of humidity, and acts to hold the tip in contact with the surface. The magnitude of the capillary force depends upon the tip-to-sample separation. The force exerted by the cantilever is like the force of a compressed spring. The magnitude and sign (repulsive or attractive) of the cantilever force depends upon the deflection of the cantilever and upon its spring constant. As long as the tip is in contact with the sample, the capillary force should be constant because the distance between the tip and the sample is virtually incompressible assuming that the water layer is reasonably homogeneous. The variable force in contact AFM is the force exerted by the cantilever. The total force that the tip exerts on the sample is the sum of the capillary plus cantilever forces, and will be enhanced or offset by the repulsive and attractive forces discussed above. The magnitude of the total force exerted on the sample varies. Control of the surrounding atmosphere while conducting SPM is very important from a standpoint of both imaging and manipulating a scanned surface. Relative humidity, temperature and the presence of other vapors affects every day operation. Furthermore, surface manipulation techniques, such as Dip Pen Nanolithography (DPN) (R. D. Piner, J. Shu, S. Hong, C. A. Mirkin, “Dip-Pen” Nanolithography, Science, 283, 661-663, 1999), Meniscus Force Nanografting (MFN) (described in Provisional Patent Application No. 60/243,168 incorporated herein by reference in its entirety) and High Force Nanografting (HFN) (Schwartz, P. V. Langmuir (2001) 17:5971, incorporated herein by reference in its entirety) depend strongly on gas constituents, water vapor, temperature, and possibly vapors of other liquids. Thus, a need exists for a device that allows complete control over the atmospheric conditions at the SPM probe/substrate interface.
Most AFMs currently on the market detect the position of the cantilever with optical techniques. In the most common scheme, a laser beam bounces off the back of the cantilever onto a position-sensitive photodetector (PSPD). As the cantilever bends, the position of the laser beam on the detector shifts. The PSPD itself can measure displacements of light as small as 10 Å. The ratio of the path length between the cantilever and the detector to the length of the cantilever itself produces a mechanical amplification. As a result, the system can detect sub-angstrom vertical movement of the cantilever tip. In constant-force mode, the deflection of the cantilever can be used as input to a feedback circuit that moves the scanner up and down, responding to the topography by keeping the cantilever deflection constant and generating an image from the scanner's motion. Constant-force mode is generally preferred for most applications, but the laser must be reflectd from the back of the AFM cantilever onto the photodiode. Depending on the sample scanned or the cantilever used (such as twisted cantilevers), the laser may not be reflected into the sensing photodiode. Therefore, a need exists for a innovation that will allow the use of different tips and different surfaces while adjusting the AFM tip cartridge to assure the reflection of the laser from the back of the AFM cantilever onto the photodiode.
Normally an SPM is used to image a surface without damaging it in any way. However, an AFM can be used to modify the surface deliberately by applying excessive force to the tip. Nanolithography techniques are disclosed in pending application Ser. No. 09/477,997 filed Jan. 5, 2000, the complete disclosure of which is incorporated herein by reference.
All commercial SPMs now include optical microscopes to help monitor the tip-to-sample approach and to select the areas of interest on the sample surface. An optical microscope enables positioning the tip quickly and accurately, exactly where the user wishes to take an SPM image. Additionally, if the very rough or oddly shaped samples (for example geological samples) or cross sections of any kind that require landing the tip on a narrow edge are to be imaged, an optical microscope is indispensable for positioning of the cantilevered tip. However, the link between the sample stage and the objective of the optical microscope is very long, resulting in a very loose mechanical connection. The optical microscope oscillates with respect to the sample causing the image to shake and resulting captured images to be blurry. Thus, a need exists for a means that allows for much higher resolution images by preventing relative movement of between the optical microscope and the AFM stage.
Under normal operating conditions, an AFM tip may last for a couple of days, so changing the probe tip is a regular occurrence. However, the chips upon which cantilevers are mounted are very small and can be unwieldy to handle. New designs permit pre-aligned, pre-mounted probes to be changed with minimal alignment of the beam-bounce detection system although it is often still difficult to gain access to the tip. Thus, there exits a need for an easier means of gaining access to the AFM tip and cantilever.
SUMMARY OF THE INVENTION
One aspect of the present invention provides a device having a scanning probe microscope tip, a stage for holding a substrate and an atmosphere control enclosure defining an enclosed space. The atmosphere control enclosure surrounds the scanning probe microscope tip and the stage providing an enclosure in which the temperature, humidity and gas composition can be controlled.
Another aspect of the present invention provides a method of nanolithography including providing a substrate, providing a scanning probe microscope tip, and providing an atmosphere control enclosure defining an enclosed space. Th
Berman Jack
Northwestern University
Sheridan Ross P.C
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