CircuiTree - December 2008 - (Page 16) Tech Talk By Karl Dietz Fine Line Etching Revisited – Part A revious Tech Talk articles1,2 looked at the limitation of forming fine line circuits due to undesirable lateral etching (undercut), not just in the print and etch process but also in pattern plating, panel plate/tent and etch, and even in semiadditive processing. It appears that neither photoresist resolution nor plating capabilities but rather etching presents the ultimate limit for these traditional circuitizing techniques. This column looks at ways to extend the subtractive technology to achieve finer features. Some technical advances described here have found commercial acceptance whereas others don’t appear to have gained traction. There are several problem areas associated with etching, some interrelated, some independent (see Figure 1). Benefits from process remedies may be seen in one or more of these problem areas. The first problem area, shown in Figure 1, is addressed here in Part A, and the latter three will be dealt with in Part B. Addressing etch undercut first (see Figure 2), we have to acknowledge that wet etching is basically isotropic, and all techniques we apply to promote downward etching do not completely eliminate lateral etching. LP-Chemie’s impulse process3 creates spray pulses of etchant droplets with high vertical impingement force onto the copper surface that easily penetrate an etch inhibitor layer (banking agent) at the bottom of the etch channel but leave the inhibitor layer on the etch channel sidewalls largely undisturbed. This technique results in enhanced downward etching (i.e., a more favorable etch factor) and higher etch speed. The pulses are generated mechanically by the rapid rotation of a coaxial pipe sleeve with slots around the spray bar. This process is combined with Pill’s top side suction bar approach that addresses topto-bottom etch non-uniformity due to puddling.4 It has been reported that controlling acid etchants at very low free acid nor- P mality improves the etch factor.5 These findings are in line with earlier work that showed that using NaCl instead of HCl as a source of chloride ions in acid etchers, to complex copper salts and keep them Etching Problems • Undesirable Lateral Etching (Etch Undercut) • Top to Bottom Variation Due to Puddling Effect • Etch Variation Due to Obstructions in the Way of the Spray Pattern • Etch Rate Differences Due to Circuit Pattern Layout Figure 1 Problem Areas in Etching from precipitating, results in an improved etch factor. Different etch chemistries result in slightly different etch factors. Alkaline etching typically yields a less favorable etch factor than acid etchant. Some studies show a better etch factor with ferric chloride than with cupric chloride, but results are not always conclusive. The study of T. Yamamoto et al.6 (see Figure 3) shows the beneficial effect of wider etch channels and thinner resist but gives no clear distinction between ferric and cupric chloride. The study in Reference 6 has shown that the etch factor deteriorates as the etch channel aspect ratio increases and as the Reducing Undesirable Lateral Etching (Etch Undercut) • Use of Pulsed Sprays and Banking Agent (Impulse Process, L.P. Chemie, Germany) • Use of Low Free Acid Normality Etch Chemistry • Use of Ferric Chloride Etchant (vs Alkali or Cupric Chloride Etch) • Going to More Favorable Etch Channel Aspect Ratio, By • Using Thinner Copper • Using Thinner (Liquid) Resist in Print and Etch • Moving Away from the Traditional Panel Plate/Tent and Etch Process that Requires Resist to • Electrophoretically Deposited (ED) Resist (e.g., Nippon Paint, Kansai Paint), or • Laser Ablatable Immersion Tin Resist (Siemens), or • Laser Ablatable (Non-Photodefinable) Organic Resist Coating (Atotech) Figure 2 Approaches to Reduce Lateral Etching Figure 3 Relationship Between Etch Factor, Etch Channel Width, and Resist Thickness for Three Etchants (Source: Oak Mitsui) 16 December 2008 • circuitree.com http://www.circuitree.com
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