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Picogram-scale Atomic Force Microscopy Probes with Integrated Nanophotonic Readout


Atomic Force Microscopy (AFM) is a nanoscale metrology technique essential for both nanoscience and nanotechnology research as well as nanoscale structure and device manufacturing. Decreasing the mass and size of AFM cantilevers both improves the speed of their response and decreases thermal mechanical noise by decreasing drag when operated in air or fluid environments. In both cases the measurement quality and throughput can be increased. Currently such miniaturization is limited by the difficulty of realizing precision measurement of the motion of such cantilevers.

NIST research has demonstrated a way to overcome this difficulty by integrating nanophotonic resonators in close proximity to extremely small cantilevers with masses below 1 pg. These allow motion measurements with a motion noise floor below 1 fm/Hz1/2 and a force noise floor of a few fN/Hz1/2 in air, and a similarly low force noise in water. High resonance frequencies enable the devices to respond on sub-microsecond time scales.

Currently such probes are economically difficult to produce in quantity, and are hard to integrate with commercial atomic force microscopes. Further development and commercialization of such probes, together with the development of the associated measurement techniques, will enable large improvements in the state of the art of AFM metrology, and its dissemination to a broader community. NIST in general, and the Center for Nanoscale Science and Technology (CNST) in particular, is interested in enabling innovative commercial research to achieve such improvement in AFM metrology, and fulfill its mission to support the U.S. nanotechnology enterprise from discovery to production by providing industry, academia, NIST, and other government agencies with access to nanoscale measurement and fabrication methods and technology.

The general goals of this subtopic are to increase the quality, availability, and throughput of high-precision AFM metrology by private sector commercialization of an innovative, NIST-developed measurement technology. In addition, this project should use small business to meet federal research and development needs, and to stimulate small business innovation in technology. The specific goals of this project are to develop nanoscale AFM cantilever probes with integrated nanophotonic cavity optomechanical high-precision readout and to develop and establish a manufacturing process that enables their mass production.

Similar to current commercial AFM cantilever probes, the new probes should be available in a variety of stiffnesses. They should be compatible with commercial AFMs and easy to exchange. They should operate in the optical telecommunication wavelengths range and allow quick and efficient coupling to commercial single-mode optical fiber excitation and detection systems with minimal mechanical alignment.

Development of such probes will meet an immediate need for these devices at NIST and enable and facilitate multiple AFM metrology research projects at CNST. Commercialization of this technology is expected to result in significant advances in the commercial state of the art in AFM instrumentation, with a further positive impact on U.S. nanotechnology enterprise as a whole.

Phase I expected results:
Demonstration of key elements of proposed probe design and fabrication process. Specifically, demonstration of the feasibility of fabricating all on-chip elements required for: 1) single-mode fiberoptic connection (such as single-mode fiber pigtailing or another optical connection approach); 2) exposing the probe tip for interaction with a sample (such as, but not necessarily, overhanging over the edge of the chip); 3) batch fabricating probe chips on 100 mm or 150 mm wafers (this may include e-beam lithography).

The demonstrated approach should be suitable for probes with modal mass below 1 pg and stiffness in the sense direction of 1 N/m to 100 N/m and optomechanical readout with a noise floor below 1 fm/Hz^0.5. Sense direction should be near-normal to the sample surface. NIST is not looking for detailed optomechanical probe structure design, but seeking to establish a clear path to batch fabrication on a wafer scale. Sufficient feasibility demonstration will require fabricating simplified test structures to demonstrate key fabrication process steps (such as overhanging the probe and/or fiber coupling), and a theoretical study specifying a detailed fabrication process flow.

Phase II expected results:
Implementation and demonstration of batch fabrication of functional AFM probes and supplying functional probes to NIST for validation. Optimization of probe’s design for improved performance. The probes shall have the modal mass below 1 pg, and optomechanical readout noise below 1 fm/Hz^0.5 with less than 50 mW incident optical power. Probes shall be available in several different designs covering a total stiffness range in the sense direction of at least 1 N/m to 100 N/m. NIST expects the awardee to have established a commercial supply of these probes at the end of the program.

NIST will provide full data on current NIST AFM probe design and performance data and details of currently used fabrication process and its implementation in the NIST CNST NanoFab, including recipes for all steps. Some of these steps are not currently compatible with batch fabrication, and NIST seeks to remedy this. NIST will provide full details of the current approach to assembly and integration of these probes into a commercial AFM instrument. NIST may be available for regular discussions/consultations regarding optomechanical design, as well as all other aspects of fabrication, as well as use of the probes in AFM. In Phase II, NIST expects to be testing the early prototype probes and final supplied probes in our fiber-optical sensing and AFM systems. NIST will implement the changes necessary to integrate and test the supplied probes in our AFM (such as custom adaptors or fiberoptic couplers, if necessary).

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