Scanning Near-Field Ultrasound Holography (SNFUH) (23061)
The invention is a revolutionary approach which provides non-invasive nanoscale imaging capabilities for deeply buried and embedded structures in physical sciences, engineering systems as well as biological structures under physiologically viable conditions. The invention has operated in near-contact mode, which has enabled it to image biological structures.
The technique, termed: Scanning near field Ultrasound holography (SNFUH) synergistically integrates three disparate approaches: a unique combination of scanning probe microscope platform (which enjoys excellent lateral and vertical resolution) coupled to micro-scale ultrasound source and detection (which facilitates "looking" deeper into structures, section-by-section) and a novel holography approach (to enhance phase resolution and phase coupling in imaging).
The unique capabilities of SNFUH over any existing non-destructive imaging techniques include the following:
Subsurface flaw imaging in nano- and micro-composites, MEMS, CMOS, and heterostructures. In-vitro non-destructive imaging of biopolymers, biomaterials and biological structures: e.g. viewing cell membrance or implant-bio interface, under physiologically viable conditions. Voiding and subsurface defects in microelectronics stacks, such as low-K dielectric materials and interconnects. Stress migration and defect analysis in 3D interconnects and MEMS. Dopant profiling and modulus mapping in non-contact mode of diverse materials. Non-invasive monitoring of molecular markers/tags -signal pathways, at nanoscale resolution. In SNFUH, a high frequency (on the order of hundreds of Mega-Hertz) acoustic wave is launched from the bottom of the specimen, while another wave is launched on the AFM cantilever. These acoustic waves are mixed together through our novel development of SNFUH electronic module which includes in-house combination of various filters, mixers, feedback electronics and electronic components to obtain the desired information on fundamental resonances and their related harmonics, which are monitored by the AFM tip, which itself acts as an antenna for both phase and amplitude of scattered specimen acoustic waves. As the specimen acoustic wave gets perturbed by buried features, especially its phase, the local surface acoustic waves are very effectively monitored by the AFM tip. Thus, within the near-field regime (which enjoys superb lateral and vertical resolution), the acoustic wave (which is non-destructive and sensitive to mechanical/elastic variation in its "e;path") is fully analyzed, point-by-point, by the AFM acoustic antenna in terms of phase and amplitude. Thus, as the specimen is scanned across, a pictorial representation of acoustic wave’s perturbation is fully recorded and displayed, to offer "quantitative" account of internal microstructure of the specimen.
The SNFUH system is operational in the linear and near-contact regime of tip-sample interaction and proves very effective for in-vitro imaging of biological cells and tissues, which has been made possible with the SNFUH electronic module. The earlier literature and patents report the non-linear tip-sample interaction to measure mechanical mapping of structures. Prior approaches have several major drawbacks. For example, out-of-plane vibrations create non-linear tip sample interactions, which result in very hard elastic contact with the sample surface. Non-linear approaches yield poor surface/sub-surface phase contrast and most of the contrast will arise from surface, rather than embedded/buried features. Moreover, none of the prior approaches are suitable for soft polymeric and biological materials. These multiple innovations for mode of SNFUH operations have been achieved through the novel electronic module and software developed at Northwestern University. We are pursuing a massively parallel implementation for imaging large-scale structures (inches).
We believe SNFUH bridges the critical length-scale gap of 10-100+ nm scale, for non-destructive imaging needs for embedded or buried features in diverse materials systems.
The invention is described in further detail at Science: 7 October 2005; 310: 89-92.
APPLICATIONS: The applications of this instrument are numerous and represent areas of critical and immediate needs in current/future generation microelectronics, especially as an advanced nanoscale surface and sub-surface metrology tool-set. Further, it naturally provides crucial imaging needs for Nanoelectronics, Microsystems (MEMS), and Nanotechnology, in general, specially biomolecular interconnects and BioMEMS. Lastly, it will provide in-vitro imaging of biological structures without having to "open-up" internal structures. By combining the nanometer-scale spatial resolution of conventional SPMs with the sub-surface imaging capabilities, this invention will fill a critical need in characterizing the surface defects, structures with high resolution and will have further potential for developing nanoscale non-invasive 3D tomography.
STAGE OF DEVELOPMENT: Northwestern has demonstrated the proof-of-feasibility of the Scanning Near field Ultrasound Holography (SNFUH) invention in near contact and contact mode with product frequencies for the following structures and devices: (1) Investigating mechanical uniformity and process-induced mechanical modification of materials in integrated circuit (IC) structures and MEMS; (2) Real-time in-vitro biological imaging of red blood cells infected with malaria parasites; (3) voiding in copper interconnects and (iv) Non-invasive monitoring of nanoparticles buried under polymeric films.
Such capabilities would complement current cross-sectional imaging techniques such as SEM-EDS, TEM-EDS, TEM-EELS, and ex situ STM to investigate the nanomechanics and subsurface imaging of material interfaces, the uniformity of conformally deposited coatings, and mechanical defects in multilayer structures
Northwestern University seeks a partner to commercialize this invention. Patent issued U.S. Patent 7,448,269
Vinayak Dravid and Gajendra Shekhawat
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