2.6.4 Advanced Resist Systems

As already mentioned the application of DQN resists is limited to UV wavelengths like I-line because of strong absorption beginning at approximately 250 nm. With a carefully designed resist process, however, it is possible to extend their usage to KrF-illumination at a wavelength 248 nm. Several enhancement techniques to a conventional resist process are necessary for that. All methods have in common that the complexity of the resist process increases significantly. The most important and promising approaches are described below.

Contrast Enhancement layer. In this method a conventional photoresist is coated after the softbake with a material that is nominally opaque at the used wavelength. The typically 0.4-0.5 $\mu$m thin film is referred to as contrast enhancement layer (CEL) [25]. During exposure the CEL undergoes a bleaching reaction that renders it transparent in the exposed regions. Therefore the exposure time approximately increases by a factor of three. Next, the CEL is stripped and the conventional photoresist is developed. The big advantage lies in the transfer of the mask into the CEL that is in intimate contact with the resist. Hence, the modulation or contrast of the conventional resist is significantly enhanced. The CEL approach can almost halve the k₁ parameter in (2.1), i.e., the achievable resolution W is doubled. This method is also called top surface imaging. Top surface imaging is particularly important for DUV applications since the optical sources may be less intense and the matrix material of the resists tend to absorb the radiation.

Antireflective and Antiscattering Coating. Due to the different refractive indices of resist and substrate material light is reflected back and interferes with incoming waves. Thereby a standing wave pattern is produced. Imaging above nonplanar specular metals can even cause exposure of nominally unexposed parts because of light scattering. Both effects, standing waves and reflective notching, make linewidth control hardly possible and dramatically degenerate image quality. As already mentioned post-exposure bake can somehow smoothen the resulting variations of the development rate, antireflective (ARC) or antiscattering (ASC) coating try to avoid both disturbing phenomena. A typically 0.1-0.2 $\mu$m thin, highly absorbing polymer film is deposited between the reflective substrate and the photoresist. Adhesion problems usually do not occur. Before spinning on the photoresist the dissolution property of the polymer film has to be thermally adjusted. A stringent control of this bake step is crucial due to the high temperature sensitivity of the polymers. To avoid any remaining parts of the ARC/ASC after stripping a short oxygen plasma etch is performed. The many additional steps increase the resist process complexity considerable. An alternative strategy to avoid scattering is the addition of non-bleaching dyes to the resist to reduce the range. This approach suffers from the ``desired'' increased absorption and is therefore not attractive for DUV wavelengths.

Multilayer Resist Systems. Both methods described above introduce a second material layer and therefore belong, strictly speaking, already to multilayer resist systems. However, multilayer resist systems commonly refer to a different approach. The first layer, directly on top of the wafer is chosen to be thick enough to locally planarize the entire topography. On top of this planar layer one or two thin films are spinned on, depending on whether a bilayer or trilayer approach is chosen. In the bilayer method polymethyl methacrylate (PMMA) is often chosen for the thick film. PMMA is a DUV resist and will be described in more detail in a subsequent paragraph. The thin layer is a conventional photoresist. The pattern is first transferred to the upper layer by a common UV process, whereby the PMMA layer also works as ARC/ASC. Then, a DUV exposure is performed. The already defined pattern in the conventional resist acts now as contact mask as it is opaque for DUV wavelengths. The contrast is considerably enhanced. Problems can arise from the interface between the planarizing PMMA layer and the conventional photoresist. A possible solution is the introduction of a thin third layer in between. A common material for that is silicon dioxide that can be deposited either by sputtering or spin-on glass techniques. The top-most layer, i.e., the conventional photoresist, is processed as usual. The pattern transfer to the two lower layers is then accomplished by anisotropic plasma etching, which enables an almost vertical pattern transfer. Reflective notching is totally avoided.

Chemically Amplified Resists. A totally different mechanism is exploited in the case of chemically amplified resists (CARs). An additional photoactive compound commonly called photoacid generator (PAG) is added to the matrix and photosensitizer. The PAG dissolves into a strong acid when exposed to light. A post-exposure bake is required to thermally induce a chemical reaction. This may be the activation of a crosslinking agent for a negative resist or the deblocking of the polymer resin for a positive resist. The acid acts as a catalyzer, so that it is hardly consumed by the reaction. For example, one molecule PAG might trigger 500 to 1000 chemical reactions. This results in a dramatic increase in quantum efficiency and sensitivity. Exposure doses as low as 10 mJ/cm² have been reported, i.e., a reduction by a factor of 10 or more is achieved as compared to conventional photoresists. While the use of CARs in UV lithography can be thought of as a manufacturing enhancement, they are indispensable for DUV wavelengths. A serious problem arises from surface evaporation of the generated acid and from environmental contamination by airborne chemicals in the time period after exposure and until the end of the post-exposure bake. Various methods have been developed to address these problems including the use of resist top-coating, filtration of clean-room air, the addition of weak organic acid filters to wafer tracks, and chemical treatments, such as additives. Chemical amplification is one of the most promising resist technologies for DUV applications.

DUV Resists. For 193 nm ArF-lithography the only possible way is to provide totally new DUV resist materials. One choice is polymethyl methacrylate (PMMA) as already mentioned. PMMA consists of a long chain polymer that is compressed or ``rolled up.'' Under DUV exposure the long chains break up and the resulting shorter molecules dissolve more easily in a developer. Although PMMA is often used to demonstrate ultimate resolution in lithography research, it has two primary drawbacks that limits its utility for the factory floor manufacturing. The first is the very low plasma etch tolerance, lower in fact than most of the films to be etched. Unless a very thick layer is applied, the PMMA resist disappears more quickly than the intended film does. Furthermore, the dissociation of the PMMA changes the etch chemistry and frequently leads to polymeric deposition on the wafer surface. The other primary limitation is its very low sensitivity. Exposure doses have to be greater than 200 mJ/cm². Such high doses cannot be achieved with ArF-excimer lasers. Various sensitizers can be added to PMMA that additionally increase etch resistivity. A second possibility to enhance sensitivity is to expose the wafer at an elevated temperature, e.g., 140 ^oC, with the benefit of a higher contrast. Because of the many problems related to PMMA intensive research is performed to find different DUV resist materials.

A promising approach relies on polyhydroxystyrene (PHS). The basic idea is to find a balance between a hydrophobic and hydrophilic material, that can be developed in an aqueous solution. To find the desired balance is extremely critical, because resists that are too hydrophobic will suffer from surface inhibition and insoluble residues, whereas too hydrophilic resists cause unacceptable high erosion rates in unexposed regions. Dry developable resists constitute another category of new resist materials. This approach tries to realize the advantages of multilayer resist systems in one single resist layer. The image resides in a very thin layer on top because of the high absorption at DUV wavelengths. Now a resist material is required that enables an anisotropic plasma etch transfer of this image into the regions below. Thus the top layer must be highly resistant to such plasma processes. Resists containing silicon are very attractive since they form silicon dioxide that protects the underlying resist parts in an oxygen plasma. Polymeric silicon materials, also called polysilynes, exhibit the desired properties and can easily be fabricated.