Lewis W. Flanagin

Surface roughness development during photoresist dissolution

The minimization of nanoscale roughness in patterned images has become a priority for the process of photolithography in the production of microprocessors. In order to probe the molecular basis for surface roughness, the development of photoresist has been simulated through application of the critical-ionization model to a three-dimensional molecular lattice representation of the polymer matrix. The model was adapted to describe chemically amplified photoresists of the sort now commonly used in microlithography. Simulations of the dependence of the dissolution rate and surface roughness on the degree of polymerization, polydispersity, and fractional deprotection agree with experimental results. Changes in surface roughness are shown to correlate with the length of the experimentally observed induction period. Model predictions for the effect of void fraction and developer concentration on roughness are also presented. Observations of differences in the effect of developer concentration on top-surface and ...

Molecular model of phenolic polymer dissolution in photolithography

The resolution of photolithographic processes has advanced to the point that difficulties, such as line-edge roughness, associated with phenomena occurring at molecular length scales are becoming important. In order to control these phenomena, it is necessary to understand them. To that end, a numerical model has been used to simulate the dissolution of phenolic polymers in aqueous base. The simulation applies the Critical Ionization Model to a rectangular-lattice representation of the polymer matrix. The model has been adapted to describe the dissolution process that is responsible for photoresist function. Both continuum and molecular versions of the model are presented. The Continuum Model yields dissolution profiles that approximate the contours of the calculated spatial variations in chemical blocking (blocking profile). An algorithm has been developed to connect individual cells to form polymer chains, and to fill the lattice in a way that produces a Gaussian chain length distribution. The model employs only a single adjustable parameter, the time-step correction factor (assuming one can measure the probability of ionization once a site encounters the developer). The Molecular Model predicts a dissolution rate that decreases non-linearly with respect to degree of chemical blocking, as is observed experimentally. Dissolution profiles can be generated with the Molecular Model based either on this calculated dependence of the dissolution rate on blocking fraction or from direct application of the model to a blocking profile. The probabilistic nature of the model introduces edge roughness of the same degree as that observed experimentally.

Advancements to the critical ionization dissolution model

The microlithographic process is dependent upon the dissolution of acidic polymers in aqueous base. The fundamental mechanism that governs the dissolution of these polymers has been the subject of considerable discussion, and a number of theories have been proposed to explain this behavior. Our research group has presented the critical ionization (CI) dissolution model to explain the dissolution of phenolic polymers in aqueous base. Specifically, the model proposes that a minimum or critical fraction of ionized sites, fcrit, on a given polymer chain must be ionized in order for that chain to dissolve. The main input parameters to this model are the critical fraction of ionized sites, fcrit, and the fraction of ionized surface sites, α. In this work methods are established for measuring these parameters. A quantitative link between the CI model and experiment has been demonstrated for the dissolution rate and surface roughness dependence on polymer molecular weight. Methods for calculating α are discussed,...

Recent advances in a molecular level lithography simulation

Computer simulation of microlithography is a valuable tool for both optimization of current processes and development of advanced techniques. The capability of a computer simulation is limited by the accuracy of the physical model for the process being simulated. The post exposure bake (PEB) of a deep-ultraviolet resist is one process for which an accurate physical model does not exist. During the PEB of a deep- ultraviolet resist, mass transport of photogenerated acid allows a single acid molecule to catalyze several deprotection reactions. Unfortunately, lateral transport of acid into unexposed regions of the resist complicates control over the critical dimension of printed features. An understanding of the factors that contribute to acid mobility would allow resist manufacturers to tailor resist transport properties to their needs. Molecular level models are particularly valuable when attempting to examine mechanistic phenomena and offer the best possibility of accurately predicting lithographic performance based upon the chemical formulation of a resist. This work presents a new, molecular scale simulation of the acid generation and transport process.

Probabilistic model for the mechanism of phenolic polymer dissolution

A probabilistic model for polymer dissolution was recently presented that aims to provide a fully molecular explanation for the complex dissolution behavior of phenolic polymers such as novolac in aqueous developers. It is based on the hypothesis that a phenolic polymer, which is below the entanglement molecular weight, becomes appreciably soluble only when a certain fraction of its phenol groups are deprotonated. If the rate of dissolution of the polymer is limited by this solubility criterion rather than by mass transfer, then the dissolution rate of the polymer may be predicted from the probability of deprotonation. This hypothesis has been supported by laboratory measurements that tested the models predictions for the effect of polymer molecular weight on the minimum base concentration for development and by combinatory potentiometric and turbidimetric titrations. The model can adequately account for the observed effects of residual casting solvent and novolac/inhibitor interactions and the differential dissolution behavior between novolac and poly(hydroxystyrene). No other model for phenolic polymer dissolution predicts all of these behaviors. This evidence suggests that even in a primitive form, the probabilistic model captures the important physical elements affecting the dissolution process that are absent from models based solely on diffusion theory.

Understanding nonlinear dissolution rates in photoresists

This work focuses on understanding the dissolution phenomenon of surface inhibition, which is observed often in the development of novolac based resists. Many theories have been offered to explain this phenomenon, including a concentration gradient of resist components, oxidation of the surface, formation of a gel layer, and surface roughness effects. This work focuses on theories that propose a concentration gradient in resist components. A technique has been established to separate and analyze individual layers of thin films, and the concentration gradient in many resist components (residual solvent, low molecular weight chains, photoactive compound, density) has been compared to the observed dissolution rate. The results indicate that no significant concentration gradients exist in a 1mm novolac film, and that these hypotheses are inadequate to explain surface inhibition. Several other theories are explored, including oxidation of the surface, surface roughness effects, etc. The critical ionization dissolution model may offer an explanation for why surface inhibition is observed in novolac, but typically not in poly(p-hydroxystyrene).

Photoresist characterization for lithography simulation: IV. Processing effects on resist parameters

In the past, resist parameters (exposure and development parameters) were typically only available for a single set of processing conditions. Therefore, it has been impossible to explore the effect of processing conditions on resist performance using simulation. In this work, a statistical experimental design and response surface analysis technique was used in conjunction with our improved parameter extraction techniques to investigate the effect of processing conditions on the exposure and development parameters for a commercial i- line resist. The effect of soft bake time and temperature on the exposure parameters and the effect of soft bake temperature, soft bake time, post-exposure bake temperature, and post-exposure bake time on development parameters is discussed. Using this information, it is possible for the first time to consider optimizing resist processing conditions using lithographic simulations.