Nanofabrication using a stencil mask

We have developed a technique to fabricate nanostructures by the evaporation of metal through a stencil pattern etched in a free-standing silicon nitride membrane. Collimated evaporation through the mask gives metal dots less than 15 nm in diameter and lines 15-20 nm wide. The primary motivation for developing this technique being the fabrication of nanometer-scale structures in ultra-high vaccum systems, where the use of resist in not feasible.

People involved

Mandar Deshmukh and Dan Ralph

Motivation

The usual procedures for fabricating nm-scale devices, using electron-beam, x-ray, or other forms of lithography, involve exposing a pattern in a polymer-resist layer applied directly to a sample substrate. However, there exist several circumstances in which it will be convenient to do away with resist on the sample; instead completing the pattern formation process separately to make a free-standing stencil, which will be used to deposit patterned material on the substrate. By this means, lithography could be performed on substrate material which would be damaged by the chemical and thermal stresses encountered during resist application and baking. Another application is for fabrication of nanostructures on ultrahigh vaccum surfaces, for experiments such as studies of atomic diffusion.

Mask fabrication

The stencil masks are made by adapting a procedure that has been employed previously as part of a process for making metal point-contacts [1] and tunneling devices [2]. The freely suspended silicon nitride membranes are fabricated by wet KOH etch of patterned silicon-nitride coated silicon wafers. Electron-beam lithography is then performed to write an array of holes on the membrane. The pattern is then transferred onto the membrane by a RIE etch, leading to formation of holes in the membrane. The etching conditions are such that the hole is bowl-shaped, and the final orifice is bowl shaped, and the final orifice is smaller than the original pattern formed by lithography [2]. Fig. 1(a) displays a STEM (Scanning Transmission Electron Microscope) bright-field image of a single 5-nm-diameter hole in an array, looking through the membrane. The gradual darkening in the region surrounding the hole is due to the taper on the sides of the bowl-shaped hole.


Fig. 1a :STEM ( Scanning Transmission Electron Microscope) image of a 5-nm-diameter hole in the silicon nitride membrane. Fig.1b: STEM image of a dot fabricated by evaporating metal through a hole similar to one shown in Fig. 1a. Fig. 1c: STEM image of a 15-20 nm wide line shaped orifice etched in a silicon nitride membrane. Fig. 1d: STEM image of two lines made by depositing material through line shaped orifice.

Evaporation

The geometry for deposition is shown in Fig. 2. The source of evaporation is a pinhole source with an orifice of 1mm. This leads to deposition of metal along line of sight. The separation between the mask and the substrate is d=1.5 microns and the distance between the stage and the pinhole source is 30 cm.


Fig. 2: schematic of the evaporation geometry.

Fig. 1b shows the bright-field plan view STEM image metal dot 15 nm in diameter made by depositing 10 nm of erbium (Er) through the membrane. Fig. 1c shows a section of a 4 microns long line-shaped orifice in a stencil membrane, and in Fig. 1d we see images of two 10-nm thick Er lines deposited through such a hole. Both the deposited lines and the stencil orifice have widths 15-20 nm. Our investigation indicates that 30 nm of Er can be evaporated through a 20-nm-diameter holes before they can clog whereas 65 nm of metal can be evaporated through 40 nm holes.

Making devices

We have also fabricated electrical devices using this technique. We have accomplished this by making a stencil pattern that contains both line on the 25-nm scale and electrodes on the 10's of micron scale. Fig. 3 shows the SEM image of such a device fabricated. The inset shows the SEM image of the mask that was used to make the device using three angle evaporations.


Fig. 3: SEM image of a wire contacted by three angle evaporation. Inset shows the mask used for fabrication.

References

  1. K. S. Ralls, R. A. Buhrman, and R. C. Tiberio, Appl. Phys. Lett.,55, 2459 (1989).
  2. D. C. Ralph, C. T. Black, and M. Tinkham, Phys. Rev. Lett., 74 , 3241 (1995).

Last updated: 2001-11-29

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