One of the ultimate tasks in current nanolithography is the ability to fabricate arrays of structures with controlled size and shape, precisely positioned on a suitable surface. The race for shrinking the feature size pushes the limits of conventional lithography requiring fast, low cost, reliable and well-controlled processes of which stencilling is a promising one. The most common applications of stenciling have been patterning of printed circuit board features and interconnect technologies. Potential applications of the technique include (1) deposition on non-conventional and unstable substrate materials (i.e. bio-chemical, hydrophobic), (2) deposition of heterostructures (epitaxial, magnetic, complex oxides, piezoelectric materials) and (3) deposition of nanodevices on CMOS. Identifying, predicting and overcoming issues accompanying the nanostencil lithography is critical to the successful and timely development of this technique with wide range of potential applications.
The deposition through stencils suffers, however, from drawbacks such as the clogging of apertures and the membrane deformation due to the deposition induced stresses. Clogging occurs when the evaporated material accumulates on the membrane itself and inside the apertures. This phenomenon changes the shape of the aperture during the deposition process and leads to a distortion of the deposited pattern, eventually resulting in the complete closure of the aperture. Accumulation of material on the membrane and the subsequent deformation due to the deposition induced stress results in an increased substrate-stencil gap which is vital for pattern definition.
One of the modelling tasks within the NaPa project is to reduce the undesirable effects of these phenomena to the minimum by optimising the geometry of the stencils. It was proposed to stabilise the stencils mechanically by introducing corrugation structures/rims. Tyndall researchers are working on predicting the deposition stress induced deformation of stencils and subsequently establishing optimal corrugation geometries for various stencil designs defined by partners from EPFL, IBM and CNM. The stencils are intended for deposition of nanoresonators, for evaporating nanowires and for fabrication of three levels of interconnections to realise electrical measurement on a single molecule or nanowire. Modelling the clogging effect and subsequent prediction of stencil lifetime is also within our NaPa tasks.
Above Left: Simulated deformation of fragment of nanoresonator stencil.
Above Right: Simulated deformation of the same stencil incorporating corrugation rims. A 67% reduction in deformation is achieved.