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A SThM probe positioned over a chip

UNIVERSITY of GLASGOW

Department of Electronic & Electrical Engineering

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The AFM & Hyperlithography Group


The AFM and Hyperlithography group was founded in 1990 as a subgroup in the Nanoelectronics Research Centre at Glasgow University. From an initial interest in the use of advanced microscopy techniques to investigate lithographically defined nanostructures, the group rapidly became concerned with the use of lithography as a technique for the realisation of advanced scientific instrumentation.

Building on the long-standing expertise at Glasgow in the area of electron-beam lithography, the first strand of the work is concerned with the functionalisation of AFM probes. In this work, the AFM itself is used primarily as a vehicle to allow a lithographically defined sensor to be positioned close to, or in contact with a sample. The AFM also provides the normal AFM map of the sample which may be used to locate the region of interest. On the end of the conventional AFM tip is situated the sensor. Since this is "drawn" using a wide area electron-beam lithography system, the sensor may be very small. The processes used to define the sensor are derived from conventional resist-based nanolithography. Hence a complete wafer of probes may be defined at a time. Also, because the processes used are based on a conventional resist coating / exposure / development / pattern transfer methodology, several layers of lithography may be aligned and stacked together to produce complicated sensors with good yield. Over 16 years, these processes have been developed to the point of technological maturity, allowing the production of wafers of thermometer - heaters, near-field optical apertures, magnetometers, electrochemical probes, nanowire tips and scanning capacitance microscope probes and so on, with critical dimensions down to 50nm. All probes have a tip and cantilever, to allow convenient integration with a conventional AFM system. Lithographic definition of the "active" element of such probes is useful, since the sensor is defined in pure (electronic grade) materials with precisely defined, arbitrary shapes. Such probes tend to be reproducible and stable, and they are much cheaper to make than probes which are fabricated using "handicraft". The use of arbitrary shapes is useful, for example in near-field optical probes, where shaped apertures may be defined which have better optical throughput than simple round apertures having the same resolution. A more recent innovation has been the use of conventional silicon VLSI as a substrate for the fabrication of scanning capacitance microscope probes. In this case the high cost of the substrate is mitigated by the greatly improved performance offered by a monolithic solution. The functionalised AFM processes are a complete toolbox for the batch fabrication of "made to measure" nanosensors.

Hyperlithography is concerned with the desire to drive the size of structures down to the level at which lithographic objects (which come on a wafer and have convenient external connections) become commensurate with natural objects, such as large molecules or the wavelength of an electron in a metal. The resolution of conventional electron-beam lithography is typically around 10nm for an isolated shape. This is limited by the mechanical properties of the resist, surface diffusion of evaporated metals and other defects of the pattern transfer process, as well as the size of the electron beam used to write the structure, the properties of the resist, shot noise in the exposure process and so on. Fortunately, only the size of the atom limits the accuracy with which a structure may be positioned on a wafer, or how thick a layer of deposited material may be made. Hyperlithography uses this loophole to allow the definition of very small gaps and wires, with significant dimensions down to 1nm or below. As an example, two wires may reliably be positioned on a solid substrate at a separation which is small enough to allow electron tunneling to occur. Unlike such a gap realised in a scanning tunneling microscope, however, the gap is intrinsically stable over an indefinite time and may be incorporated into larger systems as a component, or transferred from lab to lab to allow many different measurements to be made on the same entity.

 

A SThM probe positioned over a chip

A SThM probe positioned over a chip