Effect of ion irradiation on anodization of p-type silicon


Ion irradiation of p-type silicon results in a patterned damage profile that can be used in a variety of ways. See Figure 1 for a SRIM plot of (a) the trajectories or 1 MeV protons in silicon, and (b) the corresponding defect profile in the form of lattice displacements, or vacancies. The generation of ion induced defects in p-type silicon depends on many factors; typical defects produced are divacancies and other vacancy or impurity-related centres. Many defects act as trap levels where charge carriers undergo recombination. One major aspect of the CIBA micromachining process in p-type silicon uses high fluence ion irradiation to reduce the effective acceptor concentration in the irradiation volume to zero, resulting in a reduced anodization current by partially or fully depleting these regions.

Figure 1. (a) SRIM simulation of (top) trajectories versus depth and (bottom) 3D defect distribution for 1 MeV protons in silicon.

Figure 2 shows the effect of increasing fluence of 1 MeV protons on the direction of hole current flow around an ion irradiated line or point. For low fluence, only the end-of-range region is sufficiently depleted, so hole current bends around this region during anodization, leaving a fully crystalline buried silicon core or wire, surrounded by porous silicon above and below, depending on the anodization depth. For higher fluences the low defect density region above the core is similarly depleted and hole current is progressively excluded from the whole irradiated volume.

The underlying silicon structure may be revealed by removing the porous silicon with potassium hydroxide (KOH). The CIBA micromachining process uses this principle to fabricate a range of patterned three-dimensional microstructures in silicon. If anodization is stopped before the end-of-range depth then the silicon surface may be patterned with a wide range of feature widths and heights, Figure 2b.

Figure 2. (a) Schematic of electrochemical anodization process where end-of-range regions remain as a solid silicon core, which those regions closer to the surface are anodized. For higher fluences, the surface regions also remain unanodized. (b) By varying the fluence and irradiated width, and controlling the etch depth a variety of feature heights and geometries are produced.