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performs several key functions. It protects the photoresist during handling, prevents sticking to the phototool during contact printing, and acts as an oxygen barrier during exposure. The optical properties of this polyester coversheet are important, especially for producing features less than 100 mm. Dry-film manufacturers have increased the optical clarity and reduced the thickness of the polyester sheet to achieve the required image quality. Thin dry films of 25 mm thickness and less generally use this high-quality coversheet. The chemical and mechanical properties of dry-film photoresists have been tailored to withstand various plating solutions and acid or ammoniacal-etching solutions. They can be grouped in terms of the processing chemistry used to develop the image:
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Aqueous-developable This group is the industry standard. Semiaqueous-developable These are used in special applications. Solvent-developable This is the original system (see Table 26.1). Since solvent development used chlorinated hydrocarbons, use of the solvent-developable photoresists has generally been abandoned.
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TABLE 26.1 Summary for Dry-Film Photoresist Types and Chemistries Photoresist type Aqueous-processable Develop solution Sodium/potassium carbonate Strip solution Sodium/potassium hydroxide Application Acidic solution: etching, plating Limited alkaline solutions: alkaline etch Alkaline solutions: etching, precious metal plating Strong alkaline: full-build electroless Cu plating
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Sodium tetraborate/ Butyl cellusolve (Methyl chloroform) Alternate to CFCs
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Chemical Composition Overview Dry-film photoresists vary in their exact chemical compositions, but all contain the following components:3
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Polymer backbone Photoactive compound Monomer Dye Additives The polymer backbone sets the fundamental solubility and chemical resistance of the system.
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On exposure, the photoactive compound absorbs light of the appropriate wavelength and subsequently reacts with the monomer to alter the solubility of the film in exposed areas. The dye changes color on exposure to provide a visible latent image of the master pattern in the film, referred to as the printout image. Although the printout image decreases the efficiency of the desired photoreaction by consuming a portion of the exposed photoactive compound, it has proven to be a valuable manufacturing aid and is used in almost all dry-film photoresists.
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Additives include adhesion promoters, flexibilizers, and other compounds that can improve a desirable property. All the ingredients function together to provide the process latitude needed for the photoresist s specific range of applications. 26.3.2 Aqueous-Processable Dry Films The majority of materials used are based on an acrylic polymer with various forms of photoreaction initiation. The initiator s absorption is designed to coincide with the major emission wavelength of the mercury arc lamps in the ultraviolet region of the spectrum at 365 nm. Many are based on the use of Michler s ketone and its subsequent sensitization of a triplet state promoted radical chain polymerization. This also explains the material s sensitivity to oxygen, an efficient triplet quencher.These materials are also very efficient at converting light energy into a chemical reaction due to the radical chain reactions; absorption of one photon of light results in many cross-links in the polymer matrix. Usual exposure doses measured as the irradiance from 330 to 405 nm to cross-link these materials functionally vary from 25 to 90 mJ/cm2 depending on the exact chemistry used and the thickness of the materials, which compares favorably to positive-acting materials used for optical lithography in semiconductor integrated circuit (IC) production, where doses of 200 to 500 mJ/cm2 are common. The images in these materials are developed in alkaline solutions of 1 percent or less by weight of sodium or potassium carbonate. The complete removal of the photoresist after pattern transfer is accomplished in 1M or greater solutions of sodium or potassium hydroxide at elevated temperatures, often with an antitarnishing additive to limit oxidation of the copper. Thus, in general, these materials exhibit excellent stability in acidic solution, but they do have varying stability to more strongly acid and alkaline solutions. In fact, there are three subclasses of materials: for acid etching of copper, for acid plating of copper, and for alkaline (ammoniacal usual) etching of copper. Increased acid stability is required for use in acid copper plating, in which the pHs of the solutions often are below 1. For ammoniacal copper etching at pH 8 to 9, increased alkaline stability is required. This latter capability illustrates the flexibility in tailoring these materials to their end use since these materials are imaged using similar alkaline chemicals: develop at pH 10.3 and strip at pH 13. Often these alkaline-stable materials are developed at high temperatures. In the past, several photoresists were formulated for exposure at a visible wavelength, 450 nm, using specialized exposure equipment either visible lasers or magnified projection printing. More recently, laser direct imaging (LDI) based on the Ar+ laser output clustered around 360 nm has been commercialized. Conventional photoresists can be exposed on LDI equipment, but higher photospeed has been key to the productivity and economics of LDI. Photoresists requiring only 10 mJ/cm2 are now commercially available for the major applications. In general, these high-speed photoresist processes are similar to conventional ones, but improvements include photospeed, sensitivity to yellow safelights, and postlamination hold.4 Novel aqueous-developable photoresists have been formulated to process in dilute acidic media. A dry film based on electrophoretic depositable (ED) materials was shown to have excellent stability in strong alkaline solutions, potentially useful for etching polyimides or for full-build electroless copper plating.5
Semiaqueous- and Solvent-Developable Dry Films Semiaqueous-developable photoresists are used when increased resistance to chemical attack is required, such as the highly alkaline solutions used in polyimide etching or gold and precious metal plating applications. Their composition is similar to the aqueous-developable photoresists, but the polymer backbone is less alkaline-soluble.This increased chemical stability provides accurate image transfer by maintaining the integrity of the surface and sidewalls and minimizing underplating. Semiaqueous-developer chemistry is slightly alkaline (sodium