Development

Most positive tone photoresists are based upon acidic polymers.  These systems are developed in an aqueous solution of base (the most common developer solution is an aqueous solution of 0.26 N tetramethylammonium hydroxide).  The dissolution of positive tone photoresists is different from the usual concept of polymer dissolution because the species in solution (a polyion) is different from the polymer in the film (a polyol). Chemical reactions of the basic developer solution with acidic sites on the polymer chains enable dissolution of the film. Our Critical Ionization (CI) model for the dissolution of phenolic polymers in aqueous base treats photoresist dissolution as a reaction-limited process in which a critical fraction of repeat units on a given polymer chain must be ionized before the chain becomes soluble in the developer solution. The input parameters to this model are fCRIT, the critical fraction of sites on a chain that must be ionized for dissolution to occur, and a, the fraction of all polymer sites exposed to developer that are ionized at equilibrium.  fCRIT is a microscopic quantity that depends upon the chemical structure of the polymer.  a can be calculated based upon the pKa of the resist polymer and the local concentration of the developer solution.  More details on the CI model are provided elsewhere on this website.
Recently, calculations have predicted the formation of an electric double layer at the resist/developer interface. As acid sites on the polymer are ionized by the basic developer solution, a negative surface charge accumulates. This negative charge causes an accumulation of positive counterions, while depleting the negatively charged hydroxide ions from the charged surface. This depletion of base can greatly affect the dissolution process, and has been found to depend upon the local geometry of the film/developer interface.
Monte Carlo simulations of the electrostatic double layer allow us to study the influence of variables such as ion size and the influence of discrete surface charges.  We have performed such simulations for a variety of structures ranging from completely planar films to roughened planar films and even to small features.  Another advantage of this technique is that it allows us to study the dynamics of film charging after the developer is applied.
Click here to download a movie showing Monte Carlo motions of ions in the developer near a charged polymer surface (7.2MB).