Surface Machining

Figure 1 below shows cross-section SEMs of lines created with different proton fluences after anodization. The wafer was etched with an alternating high/low current density for 4 seconds per layer, with a total of 15 bilayers formed so that the progression of the etch front with depth can be easily followed. The etch rate progressively slows with higher irradiation fluences, resulting in thinner porous layers and a surface step after removal of the porous silicon, the height of which can be tuned.

Figure 1. SEMs of lines created with proton fluences of (a) 5x1014/cm2, (b) 1x1015/cm2, (c) 2.5x1015/cm2, (d) 7.5x1015/cm2. The lines are 4 um wide, the linewidth is arrowed in (a) where it is least obvious.

This is the basic principle underlying the ability of the CIBA process to machine arbitrary shape surface patterns by locally varying the fluence, see figure 2 below.

Figure 2. Examples of surface machined structures

Figure 2.(a) shows a SEM image of checkerboard silicon surface pattern having many different heights. This was fabricated with irradiating two sets of crossing bars, each with fluences of 2, 4, 6, 8 and 10×1015 protons cm−2 using 2 MeV protons. Figure 2(b) shows SEM images of a linear waveguide with a grating at the top surface with a 1 μm period made with this process. 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 CIBA micromachining process. Then a layer of polymer resist is spun on and a grating created in the polymer using a second irradiation stage with a ion beam spot focused to 100 nm or less. The pattern is then transferred to the Si waveguide top surface using reactive ion etching. Figure 2(c) shows optical and SEM images of concave silicon surfaces, (c) optical image of an array of concave surfaces with a 20 μm diameter. These can be used as concave DBR mirrors – when illuminated with white light they reflect and focus one particular wavelength.  Figure 2(d) shows a SEM image of transferred binary amplitude pattern in silicon irradiated with 600 keV helium ions at 5×1014 ions/cm2, along with a  grey-scale hologram reconstructed from it.