Optical exposure

Toshihiko Tanaka for performing the optical exposure experiments... 

Two general types of multilayer process may be distinguished, depending on whether they use oxygen reactive ion etching (RIE) (4) or optical exposure (5-11) to transfer the pattern into the resist. Optical pattern transfer may suffer from some limitation due to substrate reflections, but has an advantage in the simplicity of the equipment compared to RIE. 

Optical Exposure. Multicomponent LB films were prepared from solutions of novolac/PAC varying in concentration from 5-50 wt% PAC, and transferred at 2.5 -10 dyn/cm. The films were composed of 15 - 20 monolayers, with an average film thickness of 30 nm, as measured by ellipsometry. Exposures were performed with a Canon FP-141 4 1 stepper (primarily g-line exposure) at an exposure setting of 5.2 and with a fine line test reticle that contains line/space patterns from 20 to 1 pm (40 to 2 pm pitch). They then were then developed in 0.1 - 0.2 M KOH, depending on the PAC content The wafers received a 20 min 120°C post development bake to improve adhesion to the Cr. Finally, the Cr was etched in Cyantek CR-14 chromium etchant, and the resist and Cr images were examined by SEM. 

Practical laser processing of materials involves a batch of procedures, each of which has its own requirements preparation of the initial material (with parameters customized for the intended fabrication method), its proper optical exposure, and post-processing aimed at developing or refining the exposed material (e.g., thermal annealing, chemical development, or etching). 

In terms of beam delivery, the DLW method is based on optical microscopy, confocal microscopy [4,6,13] and laser tweezers [14] (for reviews on laser tweezers see [ 15,16]). These techniques allow for a high spatial 3D resolution of a tightly focused laser beam with optical exposure of micrometric-sized volumes via linear and nonlinear absorption. In addition, mechanical and thermal forces can be exerted upon objects as small as 10 nm molecular dipolar alignment can be controlled by polarization of light in volumes of with submicrometric cross-sections. This circumstance widens the field of applications for laser nano- and microfabrication in liquid and solid materials [17-22]. 

In an external mass-transport system, material is added to and/or removed from the sample either during or after the exposure. In such systems the mass transport typically occurs in a separate development step subsequent to the optical exposure. Clearly, external mass-transport systems cannot be used in closed volumes, but it is possible to have a spatial frequency response that extends to zero. 

The results obtained with these mixtures were rationalized on the following basis. Polymerization of the most reactive species is initiated first, raising the density in the irradiated areas. At the same time some of the less reactive species is expelled from the polymer, presumably a consequence of insolubility (or incompatibility). The diffusion rates decrease during exposure as the polymer concentration increases. After the imaging exposure a second uniform exposure can be used to polymerize all remaining monomer. The result is a sample whose chemical composition is spatially modulated in correspondence to the pattern of the optical exposure. 

The authors would like to thank Toyozi Ohshima and Ryou-ichi Narushima of Hitachi Chemicals Co., Ltd. for synthesizing the diazoniim salts, and to express our gratitude to Dr. Hiroshi Yanazawa of Hitachi Central Research Laboratory for making the optical exposure experiments used in this study. 

Lithography In order to precisely resolve the nanometer structures in microelectronics, various enhancement techniques have been applied to the current optical exposure tools that are equipped with deep UV light (193 nm wavelength). These enhancement techniques include phase-shift masks and immersion lenses (putting a liquid between final lens of the stepper and the wafer). The trade-off for the high resolution of modern steppers is an extremely small depth of focus (DOF) that is around 0.5pm over a typical field size of... 

Systems have been described that expose several pixels (4a,b) and many pixels (4c) at a time. However, even these systems are much less efficient than optical exposure systems in terms of wafer area exposed per unit time, (a) Pfeiffer, H. C. IEEE Trans. Electron Devices 1979, D-26(4), 663 (b) Moore, R. D. Caccoma, G. Pheiffer, H. C. Webber, E. Woodart, O. Electronics 1981, 54(22), 1( (c) Bohlen, H. Behringer, U. Keyser, J. Nehmiz, P. Zapka, W. Kucke, W. Solid State Technol. 1984, 27(9), 210. 

The deposition, exposure, and removal of resists involve quite a few issues of chemistry and transport that benefit from the chemical engineering perspective. For example, resists are typically deposited by a spin-on process, in which the polymer is dissolved in a solvent and poured onto a rapidly spinning wafer. With the addition of suitable viscosity thinners and careful choice of spinning speed, this mixture spreads uniformly over the wafer with simultaneous evaporation of the solvent. Detailed modeling of the simultaneous flow and evaporation presents a nontrivial transport problem. Chemical issues make their appearance during optical exposure. Positive resists contain... 

The actual production of structures deals with particles smaller than 100 nm. Up to this day, it has remained a domain of electron-beam lithography. But this follow-up technology of optical exposure is too slow for mass production, too. Therefore, diverse writing techniques are specified in view of their field of deployment. 

In the Laue method a polychromatic beam of x-rays impinges on a stationary crystal. A given reflecting plane extracts from the beam the particular wavelength which allows for constructive interference or reflection to occur. Hence, the data collection time is set only by the exposure time, without any mechanical time overhead. With an unfocussed SR beam, exposure times become of the order of a few seconds for a complete or near complete data set, in favorable cases. With the optimization of the source type and beam line optics, exposure times as short as 10 10 seconds for one Laue pattern have been achieved. 

J.M. Shaw and M. Hatzakis, Performance characteristics of diazo type photoresists under e beam and optical exposure, IEEE Trans. Electron. Dev., ED-25, p. 425 (1978). 

Fig. 3. Temperature dependence of the dark conductivity for the two a-Si H films of Fig. 1. The A lines are for after annealing and the others for after optical exposure. The numbers indicate the exposure time in minutes. [From Staebler and Wronski (1980).]...

This discussion will be limited to those lithography methods that utilize the application of external radiation (i.e., optical exposure, e-beam exposure, or X-ray exposure) and will not include imprint hthography techniques such as hot embossing. Thick-film hthography is generally used to refer to hthographic processes that utilize photoresist film thicknesses greater than 5 p, in thickness. 

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