Photopolymers and Photoresists for Electronic

What is photopolymers What is photoresists A polymer or plastic that undergoes a change in physical or chemical properties when exposed to light. What is photoresists photosensitive liquid polymer, used in photolithography to produce integrated circuits.

Photopolymers are imaging compositions based on polymers/oligomers/monomers which can be selectively polymerized and/or crosslinked upon imagewise exposure by light radiation such as ultra-violet light. For final use, they are made into different forms including film/sheet, liquid, solution etc. which find outlets in printing plates, photoresists, stereolithography and imaging.

Photoresists are used to make integrated circuits, flat panel displays, printed circuits, chemically milled parts, etc. A photopolymer product can be applied as a very thin coating as in liquid photoresists or formed into a large model as in a stereolithographic equipment.

There are 2 types of photoresists Positive photoresist Negative photoresist Generally, positive resist give better resolution compare to negative resist, this is because –ve resist swell during development process, hence affect the resolution.

Positive Photoresist

Methods for Positive Photoresist 1. Photomask is placed between UV source And the wafer 2. Region exposed to UV radiation become soluble Due to change in chemical structure, and when developed by developer solution, these regions are easily removed 3. Upon development, exposed areas are removed 4. Patterns form are same as those on mask

Negative resist

Methods for Negative Photoresist Pattern formed are the reversed of mask After exposure, the exposed area of negative resist absorbs the UV radiation and polymerization takes place This reaction caused crosslinking of polymer, making it insoluble to developer solution Upon development, only the unexposed area with no polymer linking reaction are washed away

Resist Requirements Solubility- in organic solvent is necessary Adhesion- good adhesion properties to various substrate Etching resistance- Sensitivity and contrast- Sensitivity, related to ability of a polymer to undergo a structural modification on irradiation. Contrast, ability of a polymer to give vertical sidewalls. Resolution (the smallest line with which can be achieved) depends on the contrast.

Resist Materials Conventional Photoresists Positive Photoresists - Consists of 2 components, Low molecular weight novolac polymer and the sensitiser (1,2-diazonaphthoquinone (DNQ))

Exposure of the resist to UV light results in photodecompositon of the sensitiser to an unstable ketocarbene This react with water to produce the base-soluble indene carboxylic acid Prevent dissolution of Novolac polymer in aqueous base

Resist Materials Conventional Photoresists Negative Photoresists - UV sensitive groups (chalcone, cinnamate, styrylacrylate, etc) are included in the main chain/side chain of polymer. - UV irradiation gives rise to crosslinking

Polymers for Electronics- Introduction Initially, electronics are dominated by traditional inorganic semiconductors, metals, and ceramics However in the last 20 years, the use of organic materials that can process electric charge has been developed

Polymers for Electronics- Introduction Starting with polymer photoresist- esp. in circuit board Conductive polymers- Additions of metallic traces in the form of catalyst Examples of conductive polymers are polyacetylene- and polyphenylene- based compound Conductive polymers are used in light emitting diodes, conductive adhesives, biosensors, etc

Conductive Polymers π-electron system – shows features in electrical transport properties Examples of π-electron system; interclated graphite, carbon-60, carbon nanotubes, charge-transfer complexes, etc