4.6 Composition and photochemical mechanisms of photoresists  (Page 2/3)

Solvents are required to make the photoresist a liquid, which allows the resist to be spun onto a substrate. The solvents used in negative photoresists are non-polar organic solvents such as toluene, xylene, and halogenated aliphatic hydrocarbons. In positive resists, a variety of organic solvents such as ethyl cellosolve acetate, ethoxyethyl acetate, diglyme, or cyclohexanone may be used.

Photosensitizers are used to control or cause polymer reactions resulting in the photosolubilization or crosslinking of the polymer. The sensitizers may also be used to broaden or narrow the wavelength response of the photoresist. Bisazide sensitizers are used in negative photoresists while positive photoresists utilize diazonaphthoquinones. One measure of photosensitizers is their quantum efficiencies, the fraction of photons which result in photochemical reactions; the quantum efficiency of positive diazonaphthoquinone photoresist sensitizers has been measured to be 0.2 - 0.3 and the quantum efficiency of negative bis-arylazide sensitizers is in the range of 0.5 - 1.0.

Additives are also introduced into photoresists depending on the specific needs of the application. Additives may be used to increase photon absorption or to control light within the resist film. Adhesion promoters such as hexamethyldisilazane and additives to improve substrate coating are also commonly used.

Negative photoresist chemistry

The matrix resin material used in the formulation of these (negative) resists is a synthetic rubber obtained by a Ziegler-Natta polymerization of isoprene which results in the formation of poly(cis-isoprene). Acid-catalyzation of poly(cis-isoprene) produces a partially cyclized polymer material; the cyclized polymer has a higher glass transition temperature, better structural properties, and higher density. On the average, microelectronic resist polyisoprenes contain 1-3 rings per cyclic unit, with 5-20% unreacted isoprene units remaining'. The resultant material is extremely soluble in non-polar, organic solvents including toluene, xylene, and halogenated aliphatic hydrocarbons.

The condensation of para-azidobenzaldehyde with a substituted cyclohexanone produces bis-arylazide sensitizers. To maximize the absorption of a particular light source, the absorbance spectrum of the photoresist may be shifted by making structural modifications to the sensitizers; for example, by using substituted benzaldehydes, the absorption peak may be shifted to longer wavelengths. A typical bisazide-cyclized polyisoprene photoresist formulation may contain 97 parts cyclized polyisoprene to 3 parts bisazide in a (10 wt%) xylene solvent.

All negative photoresists function by cross-linking a chemically reactive polymer via a photosensitive agent that initiates the chemical cross-linking reaction. In the bisazide-cyclized polyisoprene resists, the absorption of photons by the photosensitive bisazide in the photoresist results in an insoluble crosslinked polymer. Upon exposure to light, the bisazide sensitizers decompose into nitrogen and highly reactive chemical intermediates, called nitrenes [link] . The nitrines react to produce polymer linkages and three-dimensional cross-linked structures that are less soluble in the developer solution.