Ablation of Neat PTFE

As noted earlier, many polymers absorb well in the UV and VUV region of the energy spectrum with successful surface modification occurring when these materials are exposed to sufficient photon intensity of appropriate wavelength. In the early 1980s, Kawarmura et and Srinivasan and Mayne-Banton 

If a given wavelength is assumed, the interaction between the laser s photons and the irradiated medium is governed by the latter s molecular structure (chromophores) and the chromophores arrangement with respect to one another. A useful relationship for identifying the interaction between low-intensity radiation and an absorbing medium is Beer s law. 

Although Eq. (2) is not adequate to fully describe ablation processes, the absorption coefficient is still a useful parameter as it provides a quantitative evaluation of the level of interaction between a given medium and photons of a specific wavelength and, to a first-order approximation, their propensity to laser ablation. 5 different materials can have different chemical compositions and structural arrangements, it is not surprising that they have different absorption coefficients and thus can exhibit different ablation characteristics. This is indeed found to be the case. For instance, PMMA is readily structured at 193 run ( 193 2 X 10 cm I) although it is essentially transparent and unaffected at 308 nm (a 308 1 x 10 cm i). However, for an aromatic polyimide such as the system whose dianhydride/diamine components are pyromelletic dianhy-dride/oxydianiline (PMDA-ODA), ablation occurs readily not only at 193 nm 

Srinivasan et al., 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 tliat 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 die form 

L total is the depth of the etched hole per pulse and is assumed to be the sum of photochemical and photothermal contributions, Lphoto and L thermal. respectively tteff 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 ft 11 is the medium s threshold fluence Ai and E are the effective frequency factor with units of pm/pulse and the effective activation energy with units of J/crn, respectively, for the zeroth-order thermal rate constant Fg, comparable in magnitude to Fjh, is important only at low fluences. Equation (5) is obtained after assuming that the polymer temperature T in the laser-exposed region of mass trip and the thermal rate constant k are given, respectively, as 

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The threshold fluence decreases with increasing dopant concentration and for the lowest concentration where ablation is observed (0.2% polyimide) the threshold fluence is about 0.7 J/cm2 at 248 nm and about 0.9 J/cm2 at 308 nm. Additionally, at fluences around 10 J/cm2, the maximum measured etching rates at 248 and 308 nm are about 3 and 6 pm/pulsc, respectively. While the etching rate for the 248 nm ablation of the 0.2% polyimide-doped sample has begun to saturate, the corresponding curve for the 308-nm ablation is still increasing. In comparison, the threshold fluence for the ablation of neat PTFE using 300 fs... 

Figure 5.21 a shows an SEM micrograph (in cross section) of a feature ablated in doped PTFE, specifically 0.5% polyimide, at 12 J/cm2. The ablated feature is well defined and exhibits a smooth wall profile, typical of all blends having more than 0.1% (wt/wt) polyimide. The sidewall profiles of the less heavily doped blends are extremely vertical, having less taper than typically observed for more heavily doped PTFE films, e.g., 1.0 and 5.0% (Figures 5.21b and 5.21c, respectively) or Upilex-S polyimide, (Figure 5.21d). Ablation rates for a variety of PI-PTFE blends [0.2-5% polyimide (wt/wt) and neat polyimide] at 248 nm and 308 nm are shown in Figures 5.22 and 5.23, respectively.78...

Dopant-induced ablation is of great significance because it provides the opportunity to process materials that do not exhibit desirable excimer laser structuring characteristics, i.e., micro-feature formation, but offer other outstanding properties, e.g., low e,. Like the matrix polymers of the initial doping investigations, neat PTFE is transparent to conventional excimer laser emissions. FIFE, whose chemical repeat unit is... 

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