Design Rules and Limits of Macropore Array Fabrication

Design Rules and Limits of Macropore Array Fabrication 

Not all desirable macropore array geometries can be achieved by the electrochemical etching process. This section gives the upper and lower limits for pore dimensions and a few design rules. Macropores in n-type substrates will be discussed first [Lel7]. 

The realization of a desired pore pattern requires a certain doping density of the n-type Si electrode. A good rule of thumb for the selection of an appropriate substrate is to multiply the desired pore density given (in pnT2) by 1016 and take this number as doping density (in cm-3). This dependency is shown in Fig. 6.10. A square pattern of 10 pm pitch, for example, produces a pore density of 0.01 pores pm-2, which can best be etched using a n-type substrate doping density of 1014 cm-3. 

A local variation in porosity can be produced by an inhomogeneous illumination intensity. However, any image projected on the backside of the wafer generates a smoothed-out current density distribution on the frontside, because of random diffusion of the charge carriers in the bulk. This problem can be reduced if thin wafers or illumination from the frontside is used. However, sharp lateral changes in porosity cannot be achieved. 

Arrays with pore diameters d as small as about 0.3 pm have been achieved [Lel7]. The lower limit for the pore diameter is established by breakdown, according to Fig. 8.1b, which leads to light-independent pore growth and spiking. There seems to be no upper limit for the pore diameter, because the formation of 100 pm wide pores has been shown to be feasible [Kl3]. Array porosities may range from 0.01 to close to 1. The porosity, which is controlled by the etching current, determines the ratio between pore diameter and pitch of the pore pattern. 

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