Spectroscopic characterization of acid mobility in chemically amplified resists (original) (raw)
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Acid diffusion in chemically amplified resist might limit the ultimate minimum half-pitch that can be achieved with high sensitivity resists unless diffusion length is reduced until new methods of sensitizing resists are found. Precise knowledge of molecular dynamics of resist materials and advanced techniques need to be developed actively for this issue. In this sense, computer simulations have become a valuable tool in terms of reducing time and costs. However, simulations are generally based on continuum or mesoscale models, which are unable to predict accurately variations at the molecular level. Deeper understanding and investigation of the coupled reaction-diffusion kinetics at the molecular scale during the post exposure bake (PEB) become crucial to achieve nanoscale features with good critical dimension (CD) control and good line-edge roughness (LER). In this work we have developed a molecular level approach for understanding of the coupled acid-catalyzed diffusion process in chemically amplified resist (CAR) systems. Here, the molecules of photoacid generator (PAG) are selected as the building blocks of a three-dimensional grid. Reaction and diffusion of the photoconverted acid molecules during the PEB step will produce resist volumes of cleaved polymers. After a certain PEB time τ, these created volumes produced by adjacent acids will almost contact each other, enabling the subsequent development of the polymer. We also determine this parameter τ by means of experiments with real resist systems and investigate the influence of the process conditions on it.
Analytical Chemistry, 2001
Chemically amplified resists (CARs) that employ acid catalysts are widely used throughout the semiconductor industry due to the need for high throughput in the lithography process. The quantum yield of the particular photoacid generator (PAG) used to generate a given acid ultimately limits the photospeed of the CAR. Determination of quantum yields of photoacid generation is therefore an important component of resist design. We report the development of an on-wafer spectrofluorometric technique for this purpose. This technique is based on one first reported by Feke et al. (J. Vac. Sci. Technol. 2000, B18, 136-139), which involves doping the resist formulations containing the candidate PAGs with a fluorescent pH indicator dye, coating one wafer per PAG, patterning the wafers with a dose ramp, and spectroscopically imaging the wafers. The response curve of each PAG is spatially and spectrally encoded in the fluorescence images of each wafer. We investigate the efficacy of coumarin 6, a dye that was introduced as an acid sensor by Pohlers et al. (Chem. Mater. 1997, 9, 3222-3230) for this application. We further apply this technique to the determination of the quantum yield of photoacid generation of four candidate PAGs for prototype 193-nm CARs. This technique is convenient, fast, robust, and nondestructive.
On-wafer photoacid determination and imaging technique for chemically amplified photoresists
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1998
A fundamental task of chemically amplified photoresists is to record the incident radiation by generating catalyst concentration gradients within the film. In many resists, the catalyst is a strong Brönsted acid which yields a latent image of pH within the exposed film. A number of mechanistic questions remain about acid generator efficiency and its mobility once generated and heated. We have developed a technique in which a pH-dependent fluorophore is incorporated into the resist ͑an undyed version of SAL 605 from the Shipley Company and similar formulations͒. The localized acid concentrations generated by exposure to x-rays are analyzed and imaged using fluorescence spectroscopy and microscopy. Initial experiments, the spectroscopic apparatus, and initial far-field imaging are reported elsewhere ͓S.
Study of acid transport using IR spectroscopy and SEM
Advances in Resist Technology and Processing XVII, 2000
The migration of acid catalyst molecules from exposed regions into unexposed regions in chemically amplified photoresists and the resulting image blur, has long been recognized as an important topic requiring close study. A fuller understanding of acid transport mechanisms occurring during the post exposure bake is important to help guide the development and formulation of photoresists capable of reliably resolving the increasingly small features required by the semiconductor industry. This paper reports the direct measurement of diffusion coefficients for perfluorobutane sulfonic (nonaflate) acid in poly(4-hydroxystyrene) at several elevated temperatures. These results show that the Fickian diffusion coefficient for nonaflate acid in poly(4-hydroxystyrene) is too small, at typical post exposure bake temperatures, to account for observed isofocal bias. Also reported is a new technique for investigating acid transport properties of photoresist films. This method uses selective silylation to decorate cleaved resist film stacks, so that the extent of acid catalyst migration can be measured directly by scanning electron microscope (SEM). Acid transport distances from the SEM method are compared to those obtained from infrared (IR) spectroscopic techniques.
Macromolecules, 2006
Neutron reflectivity and Fourier transform infrared spectroscopy measurements are used to profile the deprotection reaction-diffusion front with nanometer resolution in a model photoresist polymer using three perfluoroalkane-based photoacid generators (PAG) with varying chain lengths. As expected, the spatial extent of the deprotection reaction front increases with decreasing PAG size. Although the total extent of deprotection increases with increasing postexposure bake time for each PAG, the reaction-diffusion of deprotection does not propagate continuously into the photoresist polymer. The form of the deprotection reaction front changes because the diffusion process is affected by the changing polymer composition. The data are well described by a reactiondiffusion model that includes a simple acid-trapping term and does not require a varying PAG diffusivity. This high-resolution profiling, together with modeling, illustrates details of the coupled diffusion and deprotection reaction processes that affect lithographic performance.
Diffusivity measurements in polymers: IV. Acid diffusion in chemically amplified resists
Advances in Resist Technology and Processing XIV, 1997
Many of the strategies for sub 0.25 µm lithography depend on chemically amplified resists to provide sensitivity. For example, glass damage limits the dose that can be delivered at 193 nm, and source brightness limits the dose that can be delivered in the EUV. However, acid diffusion, an integral part of the chemical amplification process, dramatically affects the lithographic performance of chemically amplified resists [1]. The transport properties of Brønsted acids in glassy polymers have been estimated from a variety of indirect measurements [1]. We have, for the first time, directly measured the diffusion coefficients of acids in polymer films. A Quartz Crystal Microbalance (QCM) was used to make the measurements. The QCM can detect small changes in mass which is indicated by a shift in the resonant frequency of the piezoelectric quartz crystal (see the accompanying paper "Diffusivity Measurements in Polymers, Part III: Quartz Crystal Microbalance Techniques"). The experiments were conducted at different temperatures in order to establish the dependence of the diffusion coefficient on temperature. Acid diffusion in poly(hydroxystyrene) will be discussed.