posresred - Phenolic Resins 2 OH O C H H formaldehyde...

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R. B. Darling / EE-527 EE-527: MicroFabrication Positive Photoresists R. B. Darling / EE-527 Advantages of Positive Photoresists Most commonly used in the IC industry. Superior to negative photoresists because: They do not swell during development. They are capable of finer resolution. They are reasonably resistant to plasma processing operations. R. B. Darling / EE-527 Phenolic Resins - 1 Phenolic resins are condensation polymers of aromatic alcohols and formaldehyde. Bakelite was the first thermosetting plastic. Phenolic resins are readily cross-linked by thermal activation into rigid forms. Most phenolic resins are readily dissolved by aqueous alkaline solutions, e.g. NaOH, KOH, NH 4 OH. R. B. Darling / EE-527 Phenolic Resins - 2 OH H C H O phenol formaldehyde OH OH OH H 2 C H 2 C H 2 C bakelite H O H water + + R. B. Darling / EE-527 Phenolic Resins - 3 OH H C H O formaldehyde OH OH OH H 2 C H 2 C H 2 C H O H water + + CH 3 CH 3 CH 3 CH 3 para-cresol novolac R. B. Darling / EE-527 Important Properties of the Base Phenolic Resin average molecular weight typically in the range of 1000 to 3000 g/mole (8 to 25 repeating units in the polymer chain) dispersity of the molecular weights isomeric composition of the cresols – ortho-cresol – meta-cresol – para-cresol relative position of the methylene linkages (--CH 2 --)
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R. B. Darling / EE-527 Cresol Isomers OH OH H 2 C CH 3 CH 3 para-cresol OH OH CH 3 CH 3 ortho-cresol meta-cresol OH CH 3 OH CH 3 H 2 C H 2 C ortho-cresol resin meta-cresol resin para-cresol resin R. B. Darling / EE-527 Cresol Isomer Properties Single isomers and smaller molecular weights are desirable Manufacture of positive photoresist relies heavily upon obtaining only a single isomer of the resin, usually para- cresol. Each monomer is [C 8 H 8 O] (120.151 g/mole) Isomer Methylene Link Molecular Weight Dissolution Rate Plastic Flow Temp. ortho-cresol 3 2100 g/mole 2.7 A/sec 85 C meta-cresol 1 15000 g/mole 0.7 A/sec 73 C para-cresol 1 1600 g/mole 3.0 A/sec 119 C R. B. Darling / EE-527 Photoreaction in a Positive Photoresist O N 2 COOH + + H 2 O N 2 h ν diazonaphthaquinone (DQ) indene carboxylic acid (ICA) R. B. Darling / EE-527 Dissolution of Phenolic Resins - 1 Because of the OH groups, phenolic resins are hydrophylic and are readily dissolved by aqueous alkaline solutions.
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