The effect of molecular weight (Mw) on line-edge roughness (LER) in ZEP resists, which are positive-tone electron-beam resists, was investigated by changing the size of the aggregates in resist films and the latent-image contrast. The LER was estimated by directly observing the pattern sidewall. It was clarified that the LER for low-Mw resist is larger than that for high-Mw resist and also that the LER for low-Mw resist is independent of the latent image contrast. On the other hand, the LER for high-Mw resist increases rapidly with decreasing contrast. These results can be explained by the relationship between the size of aggregates and the width of the transition zone between low- and high-dose regions. This strongly suggests that low-Mw resists are not necessarily advantageous in reducing LER.
To clarify the origin of the roughness generated on resist surface, we investigated the effects of molecular weight of resist, exposure dose, and developer molecular size on surface roughness by atomic force microscope observations of surface morphology of electron-beam exposed surface after development. Surface roughness is strongly influenced by the polymer aggregates appearing on the exposed resist surface after exposure and development. As the molecular weight becomes higher, surface roughness becomes larger because the aggregate size increases. On the other hand, at higher exposure doses, surface roughness gradually decreases because the number of aggregates on the surface decreases. This indicates that appearance of the aggregates due to aggregate extraction development is suppressed at higher doses. As the developer molecules become smaller, fewer aggregates appear on the surface and the surface roughness becomes small. This also indicates that the aggregate extraction development can be suppressed with small developer molecules. These results can be explained by considering the relationship between the size of free volume holes in the aggregates and the developer molecular size.
Inorganic resist such as amorphous alumina are projected as potential candidates for high resolution electron beam nanolithography; the drawbacks being its low sensitivity and tedious deposition process such as sputtering. Therefore, a spin-coatable Al2O3 resist with higher sensitivity is strongly desirable to overcome these drawbacks. In this paper, we describe the electron beam exposure characteristics of spin-coatable Al2O3 gel films prepared by reacting aluminium tri-sec-butoxide, Al(OBus)3 with chelating agents like ethylacetoacetate. The electron beam sensitivity of approximately 70nm thick Al2O3 gel films baked at 40 degrees C as well as in the no-bake condition is approximately 4mCcm-2, which is approximately 106 times higher than the sputtered alumina films. Baking at 70 degrees C seems to produce little change in the sensitivity. The Fourier transformed IR spectroscopy studies indicate that the increased sensitivity of these films is due to the rapid breakdown of chelate rings under the electron beam. This rapid breakdown of organic bonds could have resulted in the appearance of inorganic Al-O bonds which are insoluble in acetone. Indeed the spin-coatable Al2O3 resist provides high resolution negative line patterns of linewidth of about 20nm.
We investigate the origin of the line-edge roughness (LER) of line patterns of chemically amplified photo resist for the purpose of reducing size fluctuations of patterns in present and future deep-UV lithography. An atomic force microscope analysis of the pattern sidewall reveals that there are tow kinds of roughness in the LER: short-range roughness with an average period of about 50 nm and long- range roughness with an average period of about 500 nm. The short-range roughness can be identified as polymer aggregates, which are essentially formed by the base polymer in the resist film. This is because the average period of the surface roughness due to polymer aggregates observed in the base polymer films is about the same as that of the short-range roughness. In addition, it is confirmed that aggregate extraction development occurs in the photoresist. On the other hand, the long-range roughness is generated not by the base polymer only but also by the exposure process because its average period increases with the exposure dose. The origin of the long-range roughness is also discussed.
We propose a new development model (Aggregate Extraction Model), which can explain the generation process of linewidth fluctuations. This model is inherently different from some previously proposed development models in that polymer aggregates, not a single polymer, are treated as the dissolution units. We found that the polymer aggregates about 20 - 30 nm in size are naturally contained in resist films. These aggregates can be observed in the cross section of a resist film and on the lightly exposed resist pattern surface after development. A polymer aggregate dissolves more slowly than the surrounding polymer probably because the polymer density is slightly higher inside the aggregate. Once the surrounding polymer dissolves, the aggregates are extracted and float away into the developer. The polymer aggregates trapped on the pattern sidewall, on the other hand, cause the roughness of the pattern size wall surface, which is responsible for the linewidth fluctuations. We have succeeded in a direct observation of aggregate extraction during the development by AFM observations. The validity of the Aggregate Extraction Model is also discussed from the point of view of the polymer density.
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