Photochemical etching

Houle F A 1989 Photochemical etching of silicon the Influence of photogenerated charge carriers Phys. Rev. B 39 10 120-32... 

Srinivasan etal.,64 in a phenomenological development, split the etch rate into thermal and photochemical components and used zeroth-order kinetics to calculate the thermal contribution to the etch rate. An averaged time-independent temperature that is proportional to the incident fluence was used to determine the kinetic rate constant. The photochemical component of the etch rate was modeled using, as previously discussed, a Beer s law relationship. The etch depth per pulse is expressed, according to this model, in the form... 

Here, L total is the depth of the etched hole per pulse and is assumed to be the sum of photochemical and photothermal contributions, Tphoto and Thermal, respectively 0Ceff is the effective photon absorption coefficient of the medium and can vary with laser emission characteristics, e g., photon density Fis the incident laser fluence Fth is the medium s threshold fluence A and F are the effective frequency factor with units of pm/pulse and the effective activation energy with units of J/cm2, respectively, for the zeroth-order thermal rate constant F0, comparable in magnitude to Fth, is important only at low fluences.64 Equation (5) is obtained after assuming that the polymer temperature T in the laser-exposed region of mass mp and the thermal rate constant k are given, respectively, as... 

From a knowledge of the absorption coefficient aeff and the threshold fluence Fth, the value of the photochemical contribution to the etch rate, Fph0to> can calculated and subtracted from the measured hole depth for any given fluence. The difference, which according to this model is the thermal etch rate Fthermai will be modeled by Eq. (5) by rewriting it as... 

Jellinek and Srinivasan,51c assuming photochemical etching, observed that for homogeneous polymers, Fth decreased in an inversely linear manner with the absorption coefficient a through the relation... 

Various fabrication schemes have emerged using this approach. A common one is the site-selective photochemical etching of an organosilane-modified surface by irradiation through a mask [86]. This results in a chemically patterned surface with active sites, where modified ONDs can be attached, surrounded by bare (irradiated) regions. 

Out of the above three methods of light-sensitive etching, the one with polarization from an external source [case (a)], called photoanodic etching, and etching in oxidizing solution [case (c)], called photochemical etching, are now of the greatest practical importance. 

Under photochemical etching, special demands should be made on the oxidizing agent in the solution. Above all, it must be reduced with the participation of conduction-band electrons alone (see above) and at potentials less negative than the potential of reducing water or H+ ions to molecular hydrogen, in order to avoid gas evolution. 

Comparison of both the methods—photoanodic and photochemical etching—shows (see Table I) that they give approximately the same results, as far as resolution is concerned. Photochemical etching is a more universal method because it is equally applicable to the treatment of both n- and p-type... 

Figure 5.4. Etch rate for PTFE at 248 nm and 300 fs pulse width. ( ) Kuper and Stuke experimental data — Baba et al. model, (--) photothermal rate, and (---) photochemical rate.

Figure 5.4. Owing to the increase in a, attributed to nonlinear absorption, the photochemical etch rate goes through a maximum around 1 J/cm. The remaining photothermal rate is plotted against ln(F/Fth)/f , as shown in the inset of Figure...

Exemplarily, Zapf et al. [145] described a manual wash coating/impregnation method, a typical procedure for the preparation of supported catalysts on microstructures. Micro structured stainless-steel plates (X2CrNiMol7 12 2 and MoTil7 12 2) structured by photochemical etching were used. The micro channels had a semi-circular cross-section and were fabricate with the dimensions 500 pm x 300 pm, 750 pm x 300 pm and 500 pm x 70 pm (width x depth). The first... 

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