Clifford L. Henderson

Refractive index measurements of photoresist and antireflective coatings with Variable Angle Spectroscopic Ellipsometry

Lithography requires accurate knowledge of film thickness and refractive index (n and k) for photoresists (PR) and antireflective coatings. It is becoming increasingly necessary to track changes in refractive index over the process cycle. The refractive index can change by as much as 0.04 in both n and k simply by bleaching the film. These changes can be caused by changes in film chemistry by processing such as baking and bleaching by UV exposure. Thus, keeping track of changes in the refractive index is useful to both resist and antireflective coating manufacturers as well as the process engineer. This work uses Variable Angle Spectroscopic Ellipsometry (VASER) to determine the refractive index of photoresist and antireflective coatings over the spectral range 190 - 1700 nm. Theory, hardware, and applications of Spectroscopic Ellipsometry are discussed along with procedures used to simultaneously extract the refractive index and film thickness of photoresist and antireflective coatings. Examples of commonly used films are presented.

Photoresist characterization for lithography simulation: II. Exposure parameter measurements

Lithography simulation has become an increasingly important tool for the semiconductor industry as attempts are made to extend current lithographic technologies. The usefulness of this simulation capability has been somewhat hindered by the lack of availability of accurate modeling parameters for the various commercial resist systems. In particular, three sets of data are required to model a typical non-chemically amplified photoresist: the refractive index as a function of wavelength and exposure, the exposure or ABC parameters (the Dill parameters), and the development parameters. This work focuses on the accurate extraction of exposure parameters for non-chemically amplified resists. Previous exposure parameter extraction techniques such as those proposed by Dill involve a number of simplifying assumptions including the assumption that the refractive index of the resist does not change during exposure and that the index of the substrate is matched to the resist throughout the exposure process. However, the refractive index of the photoresist does vary during exposure as the chemical composition of the photoresist changes. A rigorous analysis technique for extracting exposure parameters which accounts for this refractive index change and other previously ignored factors has been developed. An apparatus has been constructed to perform bleaching experiments on non- chemically amplified resists and this new, rigorous analysis technique has been used to extract exposure parameters for a series of commercial resists. Some of the results of these studies are presented together with comparisons to previous parameter extraction techniques.

Modeling parameter extraction for DNQ-novolak thick film resists

Optical lithography with special thick film DNQ-novolac photoresists have been practiced for many years to fabricate microstructures that require feature heights ranging from several to hundreds of microns such as thin film magnetic heads. It is common in these thick film photoresist systems to observe interesting non-uniform profiles with narrow regions near the top surface of the film that transition into broader and more concave shapes near the bottom of the resist profile. A number of explanations have been proposed for these various observations including the formation of `dry skins at the resist surface and the presence of solvent gradients in the film which serve to modify the local development rate of the photoresist. There have been few detailed experimental studies of the development behavior of thick films resists. This has been due to part to the difficulty in studying these films with conventional dissolution rate monitors (DRMs). In general, this lack of experimental data along with other factors has made simulation and modeling of thick film resist performance difficult. As applications such as thin film head manufacturing drive to smaller features with higher aspect ratios, the need for accurate thick film simulation capability continues to grow. A new multi-wavelength DRM tool has been constructed and used in conjunction with a resist bleaching tool and rigorous parameter extraction techniques to establish exposure and development parameters for two thick film resists, AZTM 4330-RS and AZTM 9200. Simulations based on these parameters show good agreement to resist profiles for these two resists.

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.

Bleaching-induced changes in the dispersion curves of DNQ photoresists

Three sets of data are required to model a non-chemically amplified photoresist: the exposure or ABC (Dill) parameters, the development parameters, and the refractive index. The refractive index of photoresists has often been considered constant in simulations and other work. However, the refractive index can change as the chemical composition of the photoresist is modified during the exposure reaction. This work presents a study of the refractive index as a function of exposure for a series of commercial, nonchemically amplified DNQ/novolak photoresists using spectroscopic ellipsometry. It was found that the real part of the refractive index for a photoresist changes by as much as 5% at the wavelength of exposure, in part due to the resist bleaching which removes the anomalous dispersion contributions to the refractive index. A discussion of the impact of such refractive index changes on Dill parameter extraction and the simulation of standing wave patterns are presented.

Top surface imaging through silylation

Top surface imaging provides one process alternative for 193 nm lithography and is an important aspect of the EUV imaging strategy. This paper describes an effort to characterize the fundamental mechanisms underlying the silylation process. The extent of silylation has been measured as a function of exposure dose using FTIR spectroscopy. These studies demonstrate that for certain polymers, the extent of silylation has a nonlinear dependence on exposure dose. Simulation modeling has demonstrated how this nonlinearity may contribute to improved contrast. The influence of resist optical density on the shape of the silylated image has also been simulated, and these results have helped guide the design of polymers with the optimal optical density. The TSI processes function on the basis of photogenerated differences in the reactive transport of the silylating agent into and through the resist film. We are exploring alternative mechanisms for photo-induced changes in the permeability of the silylating agent. The results of these studies will be presented.

Diffusivity measurements in polymers: II. Residual casting solvent measurement by liquid scintillation counting

Simulation of the microlithographic process plays an increasingly important role in the manufacturing of integrated circuitry. Unfortunately, most lithography simulations still lack fundamental relationships that link the resist chemistry and the final resist image. This study is directed towards generating the data necessary to quantify one of these relationships, the effect of residual casting solvent on the resist image. The amount of casting solvent was measured as a function of the post apply bake temperature and time for several casting solvents directly by using liquid scintillation counting. These measurements were carried out on four identical diazonaphthoquinone-novolac resist formulations cast with different radio-labeled casting solvents (ethyl cellosolve acetate, PGMEA, diglyme, and ethyl lactate). From these data we have estimated the diffusion coefficients for the solvents and the dependence of these coefficients on temperature. These data are then convolved with dissolution parameters and Dill parameters to isolate and establish the relationships between these parameters and the post apply bake process that controls the amount of residual casting solvent.

Photoresist characterization for lithography simulation: III. Development parameter measurements

Accurate photoresist modeling parameters are required for correct lithographic simulations. In particular, three sets of data are required to model a typical non-chemically amplified resist: the refractive index as a function of wavelength and exposure, the exposure or ABC parameters (the Dill parameters), and the development parameters. This work focuses on an improved technique for the accurate extraction of development rate parameters for non-chemically amplified resists. Traditionally, the refractive index for photoresists is considered constant in simulations and other work. However, the refractive index of a photoresist varies as the chemical composition of the photoresist changes during exposure. This work presents a rigorous analysis technique for extracting development parameters from resist development rate data which accounts for this refractive index change. Development parameter measurements for a commercial resist are discussed. Comparisons of the various development rate models are made using this experimental data.

Influence of optical nonlinearities of the photoresist on the photolithographic process: applications

Using a new simulation method, the influence of refractive index changes during the bleaching of the photoresist on process parameters is investigated. For standard applications using thin resists, refractive index changes above 0.1 result in considerably modified dose latitudes, swing curves and iso- dense bias. In special applications with thick resists, the same effect occurs for much smaller refractive index changes. Optimized refractive index changes can be used for the fabrication of structures with high aspect ratios.

Some aspects of thick film resist performance and modeling

Realistic simulation of DNQ-novolac thick film resist performance requires accurate modeling of a number of steps including light propagation inside the resist and the development process. This paper addresses several advanced topics for the characterization and modeling of thick film resists including exposure induced changes in the resist refractive index and depth dependent development rate functions. Simulations performed using traditional light propagation models such as the scaled-defocus algorithm are compared to the new finite difference beam-propagation algorithm which can account for exposure induced resist refractive index changes. The impact of depth dependent development rate phenomena will also be discussed. A series of simulated profiles are compared to experimental results for two thick film resist series, AZTM AEP4000 and AZTM AE9200.