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Three-dimensional micromachining of silicon using focused high-energy ion beams

There are many techniques available for two-dimensional machining and patterning of semiconductor surfaces for optoelectronic and microelectronic applications. These mainly use photolithography, electron beam or X-ray lithography or reactive ion etching. Smooth surfaces and high-aspect ratio sidewalls can be produced, but they are all limited to creating a single etch depth per processing stage. Multiple depths of etching and inclined or curved surfaces require a sequence of repeated processes which is both complex and expensive.

We have recently developed a means of using finely-focused, high-energy ion beams to pattern semiconductor wafer surfaces. A high-energy beam of hydrogen or helium ions, focused to a spot size as small as 50 nm, selectively damages the semiconductor lattice in the irradiated regions. This damage acts as an electrical barrier during subsequent electrochemical etching of the semiconductor surface, so the un-irradiated regions are preferentially removed. The thickness of removed material at a particular location depends on the irradiated ion dose, so multiple level structures can be fabricated with a single irradiation step simply by accurately controlling the local beam dose at each scan position. We demonstrate the use of this fabrication technique to produce a variety of high-aspect ratio and multilevel structures.

Figure 1. Structure irradiated with several different doses.

Figure 1 shows a test structure fabricated in silicon that comprises a 200 micrometer square scanned region, containing an irradiated cross and with smaller irradiated areas on each cross arm. The raised portions of the structure were produced using higher beam doses than the surrounding large cross. The effect of “over-etching” this wafer can be seen as under-cutting of the structure around the outer edge of the cross. This is where the etched depth of the porous silicon is greater than the implanted ion range. Etching stops being anisotropic and starts etching in all directions, enabling the formation of cantilever structures.

Figure 2.  Array of high aspect-ratio pillars obtained by single spot irradiations.
(Inset picture shows the profile of the pillars.)

Figure 2 shows a SEM of a uniform array of closely packed, high aspect-ratio pillars obtained by single spot irradiations of a focused proton beam. The pillars are 4.5 mm high with a diameter of 0.6 mm, and a periodicity of 2 mm. Such a periodic array of submicron diameter pillars is potentially important for the fabrication of photonic crystals.

Multilevel structures can be created by exposing the sample with two different proton energies. Since the structure irradiated with lower energy has a shorter range, it will begin to undercut at a shallower etch depth. In this way, we can fabricate multi-level free-standing microstructures in a single etch step, rather than the multiple processing steps required in conventional lithography technique. To demonstrate this capability, a bridge structure was irradiated with 0.5 MeV protons, and two supporting pillars with 2 MeV protons. Figure 3(a) shows the evolution of the etching process that led to the formation of a free-standing bridge. Initially, etching occurs in all regions except that irradiated portions. As the depth of etching goes beyond the penetration range of the 0.5 MeV protons (~6 µm), undercutting of the bridge starts to occur. At an etch depth of 14 µm, the SEM picture in figure 3(b) shows that the bridge is fully undercut and separates from the substrate. It remains supported by the two pillars irradiated by higher energy. The structure of a free-standing bridge is formed after further etching to 25 µm below the surface (Figure 3(c)). Undercutting does not occur at the pillars as the range of 2 MeV protons is 48 µm.

Figure  3. (a) Evolution of the double-energy irradiated structure with etching depth.
(b) At etch depth of 14 µm, the bridge starts to separate from the substrate.
(c) The bridge is completely free-standing at an etch depth of 25 µm.

In conclusion, it is clear that this technique opens up new possibilities for precise 3D microfabrication of silicon in a direct and flexible way.

This work is based on experimental work done by Dr E.J. Teo, who is a post-doctoral fellow working with A/Prof. D.J. Blackwood.

Contact Person : Assoc Prof D. J Blackwood

E-mail: msedjb@nus.edu.sg

Tel : 6874 6289
Fax: 6776 3604