Silicon Modification

P-beam modification of Si

P-beam writing in Si results in a patterned damage profile that can be used in a variety of ways. It has been used for producing micropatterned Si surfaces, controlled photoluminescence from patterned porous Si, and controlled reflectivity from patterned porous Si Bragg reflectors. Fig. 1 indicates the process used.
First, a finely focused beam of MeV ions is scanned over the Si wafer surface (Fig. 1a). The ion beam loses energy as it penetrates the semiconductor and comes to rest at a well-defined range. The stopping process damages the Si crystal by producing additional vacancies in the semiconductor [1]. A higher beam dose produces a higher vacancy concentration, so by pausing the focused beam for different amounts of time at different locations, any pattern of localized damage can be built up. The irradiated wafer is then electrochemically etched in a dilute solution of HF (Fig. 1b). An electric current is passed through the wafer, which forms porous Si at the surface. The buried regions of high vacancy concentration inhibit this formation process, so a thinner layer of porous Si is produced at the irradiated regions. A very large beam dose may reduce the etch rate to zero in the irradiated regions. The etched sample comprises patterned areas of porous Si that may emit light with greater or lesser intensity, or different wavelength, compared with the surrounding unirradiated regions. If the anodization is repeated with alternating low and high current densities then the range of wavelengths of incident light reflected from the resultant Bragg reflector depends on the irradiation dose. If the etched sample is immersed in KOH the porous Si is removed, leaving the patterned structure on the wafer surface as a three-dimensional representation of the scanned pattern and dose (Fig. 1c).

Figure 1

Figure 1(a) Patterning of p-type Si with p-beam writing; (b) electrochemical etching to selectively form patterned porous Si; and (c) removal of porous Si with dilute KOH solution to form a micromachined structure.

Si microfabrication

Many new technologies require the fabrication of precise three dimensional structures in Si [2,3]. Electrochemical etching of Si in HF is emerging as an alternative technique for micromachining because of its low cost, fast etching speed, and easy implementation. Recent results on fabricating high aspect ratio, multilevel structures in Si have been published [4-6]. Figs. 2a and 2b shows scanning electron microscopy (SEM) images of a linear waveguide with a grating at the top surface with a 1 μm period. This is designed to transmit a certain infrared (IR) wavelength along the waveguide while the grating selects which wavelengths are transmitted, depending on the periodicity. This waveguide was formed by first fabricating the waveguide using the micromachining process described above, then spinning on a layer of polymer resist and creating a grating in the polymer using a second irradiation stage with a beam spot focused to 100 nm or less. The pattern is then transferred to the Si waveguide top surface using reactive ion etching. After etching beyond the end of range of the ions, the isotropic electrochemical etching process starts to undercut the irradiated structure, so multilevel micromachined structures can be created by irradiating with two different proton energies. Regions irradiated with lower energies become undercut at a shallower etch depth, while regions irradiated with higher energy protons continue to increase in height, so multilevel freestanding microstructures can be fabricated in a single etch step. To demonstrate this, bridge structures have been irradiated with 0.5 MeV protons (range ~6 μm) and supporting pillars with 2 MeV protons (range ~48 μm). Fig. 2c shows the resultant Stonehenge-like structure after prolonged etching, in which the bridges are fully undercut and separated from the substrate but are supported by the irradiated pillars.

Figure 2

Figure 2 SEM images of (a) a linear waveguide with a grating at the top surface with a 1 μm period; and (b) at higher magnification. (c) Micromachined Stonehenge-like structure, 80 μm in diameter.

Porous Si patterning

Porous Si is of interest because of its photoluminescence (PL) and electroluminescence (EL) properties [7,8], raising the possibility of producing light-emitting devices with microelectronics compatibility. A potentially important application of porous Si is the production of combined optical/electronic devices incorporating patterned porous material directly onto a Si substrate with high spatial resolution. We have undertaken a comprehensive study of the effects of ion irradiation on a wide range of different resistivity p-type Si wafers, and recorded the resultant PL intensity and wavelength [9-11]. There are two resistivity regimes of p-type Si where PL is affected in different ways by ion irradiation: for low resistivity (~0.01 Ω.cm) wafers irradiation primarily results in a large PL increase, whereas for moderate resistivity (0.1-10 Ω.cm) wafers irradiation primarily results in a large PL wavelength red shift. A demonstration of this different behavior is shown in Figs. 3a and 3b. In low resistivity p-type Si, the irradiated dragon produces bright, red PL compared with the faint unirradiated background. In moderate resistivity Si, bright orange/red PL is produced from the dragon, whereas the background produces green PL of a similar intensity. In Fig. 3c, a range of doses have been irradiated in a continuous distribution in moderate resistivity Si, producing a gradual change in PL emission wavelength. Fig. 3d shows a PL image of the painting The Ancient of Days by William Blake, reproduced in porous Si. The dose of each region was adjusted so that the PL color emission from the porous Si resembles the original as closely as possible.

Figure 3

Fig. 3. PL images of dragons formed by irradiating a p-type Si (a) 0.02 wafer and (b) 3 wafer. (c) Concentric ring patterns in a 3 Si wafer formed by linearly increasing the dose from the outer edge to the center. (d) The Ancient of Days by William Blake, fabricated in a 500 × 500 μm2 area of 3 Si. The picture was irradiated with a dose of 5×1013 for black, 1×1013 for red, 5 × 1012 for orange and 1 × 1012 for yellow-green.[11. Teo, E. J., et al., Adv. Mater. (2006) 18, 51]


Distributed Bragg reflectors in porous Si

The refractive index of porous Si is lower than for bulk Si: it is inversely proportional to the anodization etch current density. A distributed Bragg reflector (DBR) selectively reflects a band of incident wavelengths and is easily formed in highly doped p-type Si by periodically lowering and raising the etch current density, resulting in a sequence of porous layers with alternating high and low refractive index. Fig. 4a shows an optical image of a 500 × 500 μm2 region irradiated with different overlaid scan patterns, with different doses [12]. The wafer was etched with an alternating high/low current density for 4 s per layer, with a total of 15 bilayers formed. The etch rate is progressively slowed by larger irradiation doses, resulting in thinner porous layers that reflect shorter incident wavelengths. Each dose produces a different reflected color when illuminated with white light, with red/orange colors corresponding to areas of lowest dose. The potential of this approach to form patterned arrays of color pixels and lines for display applications is shown in Fig. 4b, where vertical lines, each 10 μm wide, were irradiated to form alternating red-green-blue stripes. Figs. 4c and 4d shows examples of selectively patterning color reflective areas in which two dragon images appear in different colors, corresponding to different irradiated doses in each area.

Figure 4

Fig. 4. Optical reflection images of (a) the painting La Musique by Henri Matisse (1939) created by irradiating a 500 × 500 μm2 area. (b) Vertical lines, each 10 μm wide, irradiated to form alternating red-green-blue stripes. (c), (d) 500 × 500 μm2 areas showing two dragon images corresponding to different irradiated doses in each area. In each recorded image the sample was illuminated with white light and the reflected light was recorded for 30 s. [12. Mangaiyarkarasi, D., et al., Appl. Phys. Lett. (2006) 89, 021910]

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