|
Nanopositioners are designed to move objects in the sub-microscopic ranges of motion necessary for research work in nanotechnology. Unlike motor driven microscope stages or precision machine tools, the nanopositioner has a very limited range of motion and a correspondingly high position resolution. Total range of motion is typically limited to a few microns up to a couple of hundred microns. Absolute position can be resolved down to less than 1 nanometer (1/1000 of a micron). At this level of physical precision, care must be taken to prevent thermal expansion from adversely affecting the positioning accuracy.
Aluminum Nanopositioning Stages
Nanopositioning stages are constructed from a variety of materials to take advantage of their thermal characteristics. While aluminum alloys are inexpensive and easily machined, they exhibit a fairly high coefficient of thermal expansion (2.3 x 10-5 mm/mm per ºC). If a positioning application requires nanometer level resolution, temperature changes and time become important factors. In general, if the entire measurement or procedure can be completed in a “short” timeframe (i.e. minutes vs. hours) then average laboratory heating/cooling systems provide adequate temperature control and aluminum nanopositioners can be used.
Titanium Nanopositioner Stages
If the positioning accuracy must be maintained over a prolonged time and temperatures cannot be maintained accurately, use of alternative materials may provide the necessary positioning stability. Titanium’s coefficient of thermal expansion (0.9 x 10-5 mm/mm per ºC) represents a significant improvement over aluminum alloys and offers the additional benefit of higher strength –improving nanopositioner performance.
Invar and Super Invar Nanopositioner Stages
Further enhancing thermal performance requires use of the specialized alloys, invar and super-invar. Invar is an iron/nickel alloy which exhibits a coefficient of thermal expansion less than one tenth that of aluminum (approx. 1.1 x 10-6 mm/mm per ºC). While expensive and difficult to machine, it improves the thermal performance of a nanopositioning stage by an order of magnitude. The next step along the path to thermal stability is super-invar. Super-invar’s coefficient of thermal expansion is approximately half that of standard invar but, again, is expensive and difficult to machine. Due to their high iron and nickel content, both invar and super-invar are highly magnetic and are limited to applications where their magnetic characteristics are not a problem.
Experimental Assembly Design Considerations
It should be noted that the complete experimental assembly must be analyzed for thermal expansion issues. For example, optical microscopes are often constructed out of aluminum and temperature induced dimensional changes in the microscope will not be removed simply by using a nanopositioning stage constructed out of titanium or invar. If actuation speed is a priority, the relatively high density of invar and super invar needs to be taken into account in the nanopositioner design. Ideally, high speed nanopositioning stages are constructed in a manner to minimize the mass of the moving assembly and use of low density materials, such as aluminum or titanium, is preferred.
Atmospheric Design Considerations
In addition to thermal expansion issues, other environmental conditions impose their own limitations on acceptable materials for nanopositioners. Placement of the nanopositioner inside a scanning electron microscope requires the nanopositioner to be completely non-magnetic. For these applications, aluminum and titanium are ideal. Use of nanopositioners inside vacuum chambers requires specialized construction to avoid materials prone to outgassing. Anodized aluminum is generally avoided in ultra high vacuum applications due to surface retention of water and other impurities.
|