Nanoscale Patterning of UHV Clean Si(001)R. Linklater and J. Nogami

Dept. of Materials Science and Engineering, University of Toronto, Toronto, ON, Canada

IntroductionThis work is focused on establishing a repeatable experimental procedure for patterned deposition of metal onto a semiconductor substrate in UHV. This is achieved through the use of a self-aligning nanostencil, or shadow mask with nanoscale apertures, brought to within a small distance of the substrate. Thermally evaporated metal reproduces nanostencil features on the substrate.Shadow effects of the nanostencil edge were examined with STM and results show interesting diffusion behaviour of deposited metal on the semiconductor surface.Investigation of the deposited metal features was carried out by SEM and STM.

References[1] M. Deshmukh et al, Appl. Phys. Lett. 75 (1999) 1631[2] M. Aquilar et al, Surface Science 482-485 (2001), 935-939[3] J. Nogami et al, J. Vac. Sci. Technol. B 12 (1994) 2090[4] A. Zangwill, Physics At Surfaces, Cambridge University Press 1989

ISBN 0521347521

E-mail: ronnie.linklater@utoronto.ca http://www.utoronto.ca/msestm/

Deposition ModelDeposition model based on a line of sight geometry. Although this neglects any surface diffusion of the deposited metal, a model of this type has been shown to be a reasonable approximation in published literature1.Feature resolution is limited by the d/L ratio. We use contact alignment of the nanostencil to the substrate to achieve a small d. The length L is fixed in the experimental chamber. The edge of the nanostencil creates a metal gradient with three distinct regions; 1) a metal film, 2) a penumbra and 3) an umbra. The penumbra width is also controlled by the d/L ratio. By measuring the penumbra width wp we can estimate the nanostencil to substrate separation.In our system the source to nanostencil separation is L = 225mm, the source size ws= 2mm and the smallest apertures are typically wa = 70-100nm. Thus the mask to substrate separation should be d ~ 10-15μm, to limit the feature spreading to ≤ 50nm.

sp wLdw =

( )asaf wwLdww ++=

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Height of gold monolayer islands is 150pm (empty states STM) seen in the image and profile at right.Gold has diffused more than the 500nm of umbra imaged.Gold metal coverage estimated with software and graphed versus distance (at right).Linear fit (MS Office Excel, R2 = 0.932).Comparing the coverage profile with the roughness profile shows that coverage of ~0.5ML occurs near maximal roughness in umbra region, as would be expected for a single atomic layer film4.

Penumbra region is a transition between the thick gold film and the umbra. It is partially shadowed by the nanostencil and shows decreasing gold island size and thicknessRMS roughness is proportional to the metal thickness above 1ML2 and is measured in sections along the image. Resulting profile quantifies the three shadow regions and gives the penumbra width (200nm).Penumbra region displays a significant linear slope by roughness profiling. The width is predicted in the deposition model and can be used to find the separation distance, d.Penumbra width, wp = 200nm d = 22.5μm, best achieved is d = 15μm

PenumbraUmbra

Composite image of the gold gradient formed by edge of the nanostencil. 10 STM images, each 100 x 100nm.Three shadow regions:1. FULL GOLD FILM, constant gold thickness, not covered by nanostencil2. PENUMBRA, area partially covered by nanostencil edge, displays decreasing gold

coverage from full thickness to a 1ML band3. UMBRA, area completely covered by nanostencil, Au diffuses into umbra and forms

small islands of 1ML height

Total Image Span 766nm

Gold Gradient Direction 750nm

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Gold Gradient Imaged with STM

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Region of gold diffusion. Gold forms 1ML islands elongated perpendicular to the Si(001) dimer rows, visible in the composite image. This morphology agrees with literature3.

Linear coverage profile of gold over Si(001)

Full Gold Film

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Thick gold film was deposited outside the nanostencil edge. In this region the gold mounds display a plateau morphology shown in the 50 x 50nm STM image and line proflie, taken at an area far from the shadow boundary. Gold mounds maintain this plateau shape up to the onset of the penumbra as seen in the composite image. Full film thickness is ~2nm

50x50nm STM image of full gold film and indicated line profile

Nanoscale PatterningSub-micron patterning experiment.

Pressure ~ 10-5 Torr. Thus the Si(001) was not UHV clean and a native oxide was present. 1.6nm of Silver deposited.Ag clustering, not uniform dot as expected on clean Si(001)Ag Dot size: 560 x 660nm compared to Nanostencil Holes: 450 x 560nmSpreading of ~ 100nm gives d = 15μm.

STM image of Au islands with line profile

ConclusionsIn this work we have demonstrated and observed:

• Sub micron patterning of features on a UHV clean semiconductor surface• Simple mechanical alignment of nanostencil to substrate to approximately 10μm with entirely optical viewing• Significant metal diffusion which may ultimately limit the overall resolution of shadowmask patterning of semiconductors

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Roughness profile showing the three shadow regions

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3D rendering of an STM image of the penumbra region (200 x 200nm)

SEM images of nanostencil features. Apertures at right are 450 x 560nm.

Silver dot created with nanostencil above. Dot size 560 x 660nm

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Edge of stencil shadow creates three regions

Line of sight model of feature definition

2nm of gold deposited on UHV clean Si(001)Only one Si(001) terrace imaged, no Si step edges

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