Resist materials for 157nm lithography is believed to be one of the key technology for producing patterns below 70nm. Many different types of fluorine-containing polymer platforms have been energetically pursued by a number of researchers, and some of them appear to be promising in giving a high transparency that has been the essential challenge in realizing this technology. While such highly transparent polymers are the premise in achieving a good imaging, how to get sufficient etch resistance of the polymers can be of another challenge. Actually it is often reported that the etch resistance and the transparency are in trade-off relationship in many cases as a function of fluorine atom content in the polymers. Therefore how to design an etch-resistant polymer while maintaining the good transparency is still a big challenge in developing a practically usable 157nm polymer platform. One of the polymer platforms that the authors believe useful for 157nm is the polymers having hexafluoroisopropanolstyrene (HFIPS) monomer unite in their backbones. The HFIPS unit is attractive because the styrene group provides good etch resistance and hexafluoroisopropanol group (HFIP) provides an acidic molecule while implementing a transparency into the molecule. The lithographic potential of the HFIPS-based polymer system was demonstrated with the fact that a prototype resist from this system was able to print a 75nm line and space 1:1 pairs with an attenuated PSM under 0.60NA stepper exposure. A relatively thin resist thickness, 100nm, was applied due to the limited transparency of the polymer. The patterned exhibited very smooth line edge and a clear pattern definition although a slight T-topping was observed. The results imply that we should be able to achieve a similar lithographic performance with a thicker film (150~200nm), if we can further increase the transparency of the HFIPS-based polymer. The authors are pursuing the approach further aiming at this direction and are getting several new polymers that are more transparent. The paper will present some of the results from later work with such an attempt. The paper will also discuss etch resistance of the HFIPS-based polymer. The etch rates measured for the HFIPS-based polymers were only around 10% faster than standard 248nm resist, which we believe fairly good among various fluorine-containing polymers so far proposed. This was convincing that this polymer system could provide a competitive platform in the practical use. It is generally thought that the etch rate of resist films are mainly affected by their polymer compositions or structures but there are few reported on the influence of the other components in resist formulation. The authors found that the concentration of PAG and quencher influenced both etch rate and resist surface roughness after the etch in this materials system, which implied there are some more room for further etch resistance improvement.
Absorbance data on a variety of silicon-containing monomers are reported at 157nm. Choice of appropriate silicon monomers led to a second-generation bilayer resist, which showed improved transparency over the first-generation silicon-derived hydroxystyrene based resist. Increasing the overall silicon content improved its transparency and O<sub>2</sub> etch properties. The second-generation bilayer resist demonstrated 80nm resolution for dense line/space pairs. No silicon outgassing or post-exposure film loss was observed upon 157nm exposure.
Physical and lithographic properties of functionalized acetal- based polymers, newly designed bulky acetals, were investigated for the use of KrF DUV resist. The key structural design was to incorporate some functional groups into the acetal moieties in the polymers through an ether or ester linkage. The polymers were synthesized by reacting poly p- hydroxy styrene (PHS) with variety of functionalized vinyl ethers that were prepared with substitution reaction of chloroethyl vinyl ether. By selecting large moieties in size for the functional group, the polymers showed good lithographic performance even with a low level of the acetal blocking. This was advantageous for minimizing the defects that could generally be formed in image development and also for improving dry etch resistance of the resist. The ester- linked polymers showed a high dissolution discrimination which could be accounted for with dissolution inhibition induced by a molecular interaction of the ester group with photo acid generator (PAG) in the resist composition. A new class of acetal polymers having additional another acid-decomposable group in the functional group is also proposed for achieving a further improvement in lithographic property.
Structurally modified acetal resist is designed for lithography on metal layers. The acetal-based polymer used in the resist has intrinsically low post-exposure volatilization and superior metal etch resistance. This resist is designed to have excellent substrate compatibility as indicated by a foot size of less than 10 nm for 250 nm feature size. Lithographic evaluations reveal that this resists is capable of resolving sub-200 nm features on TiN substrates with high photosensitivity, dose latitude and excellent process windows for line and post features. The reduced foot size and enhanced resolution could be achieved by optimizing the resist chemistry and the processing conditions.
Lithographic properties of a variety of acetal-derivatized styrene based polymers are reported. The structural modifications in the polymers involve varying the size of the pendent acetal moiety. the lithographic performances of the resists containing structurally modified acetals were found to be superior to the conventional acetals. In the cases where the acidolysis products of the modified acetals are non-volatile alcohols, the post-exposure volatilization, film shrinkage and plasma etch resistance were found to be significantly improved.
The effect of different photoadditives in high and low activation energy resist resins on resist outgassing during lithographic exposure was studied by quartz microbalance and gas chromatography/mass spectroscopy techniques. The resist outgassing was analyzed both qualitatively and quantitatively and structure-property relationships were developed between resist outgassing and the molecular structure of photoacid generators and additives. The photoadditives examined include, aryl iodonium perfluoroalkylsulfonates, triarylsulfonium perfluoroakylsulfonates, photogenerators of sulfamic acids, 2-nitrobenzyl PAG's and doxyl derivatives.
This paper presents a discussion on the development of i- line resist chemistry as anticipated by the future technology and the needs of the new exposure tools. Commercially available high numerical aperture (NA) and low NA wide field i-line steppers stand to gain in throughput as the resist photospeed is increased. However, the advantages of developing 50 mJ/cm<SUP>2</SUP> or faster resist products capable of less than 0.3 micrometers resolution is not clear at the present time. Future development of i-line scanners and greater demands on throughput may give rise to the need for such products. Conventional novolak based/diazonaphthoquinone resist chemistry is not capable of providing such performance. Chemically amplified (CA) resist products employed in deep ultra violet (DUV) regime demonstrated the capability of this chemistry to deliver high resolution and photospeed performance. If such a performance is to be needed in i-line, then CA i-line resist chemistry would be the technology of choice. Significant resist cost reduction is possible if i-line transparent phenolic resins, such as novolaks, can be used replacing expensive poly-hydroxystyrene base polymers that are necessary to use in DUV resist products.
Chemically amplified (CA) resist systems are known to be sensitive to contamination. Environmental contaminants such as airborne amines can result in T-topping. In addition, the undesired diffusion of photogenerated acid into unexposed areas can result in linewidth slimming. To counteract these effects, amines are intentionally added to chemically amplified resist formulations. These added amines function as `buffers' or `acid traps' within the resist matrix. While the effects of strong, photogenerated acids on CA resist systems has been the focus of much research, the effects of weaker acids on these resist systems has not received as much attention. In this paper we demonstrate how the conjugate acid of some amines (amine salts) can adversely effect the lithographic performance and storage stability of CA systems. We show that salts of weak amines are sufficiently acidic at relatively low temperatures, to cause significant increases in polymer molecular weight and polydispersity. In some cases, gelation of the polymer matrix was observed. A mechanism is proposed to explain these effects. We also show how appropriate amines can provide a thermally stable salt with low acidity. Such amine additives not only improve the storage stability of the resist system but also significantly improve lithographic performance as well.
A new class of novolaks capable of self associating has been synthesized. The associating structures are resulted via extended network of hydrogen bonding. Softening temperatures of the associating novolaks are found to be 15 - 25 degree(s)C higher than their non-associating analogs. The photoresist formulated with such associating novolaks have heat deformation temperature in the range of 130 - 140 degree(s)C. Features with sub 0.35 micrometers could be resolved using i-Line exposure. Site specific hydrogen bonding in such associating novolaks is studied by NMR and molecular simulations.