Photoresist patterns, collapse

The Effect of Adsorbed Cationic Surfactant on the Pattern Collapse of Photoresist Lines in Photolithographic Processes... 

Abstract A crucial problem in the manufacturing of high aspect ratio structures in the microchip production is the collapse of photoresist patterns caused by imbalanced capillary forces. A new concept to reduce the pattern collapse bases on the reduction of the capillary forces by adsorption of a cationic surfactant. The application of a cationic surfactant rinse step in the photolithographic process leads to a reduction of the pattern collapse. Physicochemical investigations elucidate the mechanism of surfactant adsorption... 

Fig. 1 Cross-section SEM images of free standing photoresist lines (left) and photoresist structures after pattern collapse (right).

The highest efficiency to reduce pattern collapse was found for concentrated surfactant solutions. This may, however, have serious disadvantages. Watanabe et al. [15] noticed that high surfactant concentrations melt the photoresist and amplify the pattern collapse. In contrary, Miyahara et al. [16] found that some nonionic surfactants stabilize the photoresist surfaces and prevent melting. In addition, concentrated surfactant solutions tend to foam which also might cause structure defects. Zhang et al. [17] therefore proposed a less foaming diol-type surfactant for the reduction of pattern collapse. 

It can be summarized that the addition of surfactants to the rinse liquid in many cases leads to reduced pattern collapse. There is, however, no systematic study of the performance of surfactants neither of the surfactant concentration nor of the surfactant type or chain length. None of the authors investigated the adsorption of the surfactant at the interface photoresist - fluid in more detail. Therefore the effect of surfactants to reduce pattern collapse is not really understood by now. 

On one hand it is investigated how the addition of cationic surfactants affects the pattern collapse of 193 nm photoresist lines. On the other hand, the adsorption of the surfactant on model photoresist surfaces is explored by a variety of surface chemical methods. Of special interest is how the surfactant changes the surface properties of the photoresist as surface potential and wettability. For an optimum modelling of the properties of real photoresist structures, both unexposed photoresists and photoresists that have been UV exposed, baked and developed are studied. 

In Fig. 2, top-down SEM images of photoresist structures of variable line width are shown after cationic surfactant rinse (upper row) and after conventional rinse without surfactants (lower row). The improvement is evident While without surfactant rinse the lines start to collapse at a width between 65 and 70 nm they collapse after surfactant rinse below 60 nm. Defect density measurements, too, showed a significant reduction of defects due to pattern collapse by surfactant rinse [21],... 

Dry photoresist layers. The capillary forces causing pattern collapse act on the vertical sidewall of the photoresist structures. These surfaces are formed during the development at the interface between soluble and insoluble photoresist. It is generally accepted that a photoresist with a deprotection level above 80% is soluble in the developer [3], Therefore it can be concluded that the structure sidewalls consist of a blend of more than 20% protected and less than 80% deprotected photoresist. Because of their small dimensions, the sidewalls are not accessible for most surface chemical methods. To allow an investigation, they have to be modelled by flat photoresist surfaces. To simulate the properties of the real sidewalls, these surfaces have to be processed, i.e. exposed, baked and developed. One goal of this study is to find out which of the flat... 

To understand the mechanisms that contribute to the reduction of the pattern collapse by surfactant rinse it was necessary to study the adsorption behaviour of the surfactant molecules on the photoresist surface directly in the surfactant solution. 

To estimate the influence of the surfactant adsorption on the capillary forces, the wetting tension yiv cos was calculated from the values given in Fig. 10a. The results drawn in Fig. 10b show for both measurement series a minimum of the capillary forces exactly at the concentration ceff. The capillary forces are reduced by about 20% compared to water. This confirms the hypothesis that the reduction of the pattern collapse is caused by a hydropho-bizing of photoresist processed with the threshold dose by cationic surfactant adsorption. Unfortunately the inverse ADS A method could not be applied at relative surfactant concentrations >0.2 since the bubbles became unstable due to the lower surface tension. Thus it cannot be estimated how the wetting tension evolves at higher concentrations. 

Nevertheless a good coincidence between the reduction of the capillary forces under laboratory conditions with the reduction of pattern collapse in the photolithographic process was found. This supports our hypothesis that cationic surfactants are able to reduce pattern collapse by hydrophobizing the photoresist surface. It shows also that the hydrophobizing must occur in a similar way under real conditions. Thus, flat photoresist layers processed at the threshold dose are appropriate model surfaces for the sidewalls of photoresist structures. 

Goal of the present study was to investigate how solutions of cationic surfactants affect the pattern collapse in the photolithographic sub-100 nm structuring. On one hand, the solutions were applied directly in the photolithographic process to investigate their ability to reduce the pattern collapse. On the other hand, the adsorption of the surfactant on flat model photoresist layers was studied using a variety of physicochemical characterization methods and its influence on the capillary forces was determined. 

The minimum of the capillary forces in the model system correlates with the maximum of the pattern collapse reduction in the real photolithographic process. It is concluded that the sidewalls of the photoresist structures are hydrophobized in a similar way as the model surfaces by the cationic surfactant rinse even although the conditions in the process differ from those of the characterization experiments. The resulting reduction of the capillary forces during the drying of the structures is the essential factor in the reduction of pattern collapse. 

Goldfarb, D.L., de Pablo, J.J., Nealey, P.F., Simons, J.P., Moreau, W.M. and Angelopoulos, M. (2000) Aqueous-based photoresist drying using supercritical carbon dioxide to prevent pattern collapse. /. Vacuum Sci. Technol. B, 18, 3313-3317. 

Fig. 2 Top-down SEM images (860 x 860 nm2) of photoresist structures with variable line width after surfactant rinse (a) and after conventional rinse (b). The numbers give the average line width. Small lines stand upright, broad lines represent collapsed patterns...

A fundamental difference of laterally structured thin layers of grafted (collapsed) polymer chains compared to the normal spin-coated and hence only phy-sisorbed photoresist films is their remarkable stability even after extended exposure to good solvents the burned-in pattern remains on the substrate with excellent fidelity while the spin-cast film can be washed away completely. However, the high doses of UV photons needed to etch away the polymeric material makes this a rather uneconomic process. 

Figure 9.29 Two methods for conducting lithography on a material not compatible with conventional methods. Left use of a metal layer to protect the material to be patterned from the process used to remove the photoresist. The protective metal is removed after the photoresist is removed. Otherwise the process is similar to that shown in Figure 9.28. Right ink-jet printing has been used to print materials that can be dissolved or suspended to form an ink. The figure shows the operation of a print head, (i) Ink flows into the channel in the print head against the heater and opposite the print nozzle, (ii) The heater is turned on boiling the ink locally. The resulting bubble ejects a small droplet of ink (iii) and the heater is turned off, allowing the bubble to collapse and new ink to be drawn into the print head.

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