Marie Krysak

A new inorganic EUV resist with high-etch resistance

Performance requirements for EUV resists will necessitate the development of entirely new resist platforms. As outlined in the ITRS, the new resists for EUVL must show high etch resistance (to enable pattern transfer using thinner films), improved LER and high sensitivity. A challenge in designing these new resists is the selection of molecular structures that will demonstrate superior characteristics in imaging and etch performance while maintaining minimal absorbance at EUV wavelengths. We have previously described the use of inorganic photoresists in 193 nm and e-beam lithography. These inorganic photoresists are made of HfO2 nanoparticles and have shown etch resistance that is 25 times higher than polymer resists. The high etch resistance of these materials allow the processing of very thin films (< 40 nm) and will push the resolution limits below 20 nm without pattern collapse. Additionally, the small size of the nanoparticles (< 5 nm) leads to low LER while the absorbance at EUV wavelengths is low. In this presentation we show that these inorganic resists can be applied to EUV lithography. We have successfully achieved high resolution patterning (<30 nm) with very high sensitivity and low LER.

Development of an inorganic photoresist for DUV, EUV, and electron beam imaging

The trend of ever decreasing feature sizes in subsequent lithography generations is paralleled by the need to reduce resist thickness to prevent pattern collapse. Thinner films limit the ability to transfer the pattern to the substrate during etch steps, obviating the need for a hardmask layer and thus increasing processing costs. For the 22 nm node, the critical aspect ratio will be less than 2:1, meaning 40-45 nm thick resists will be commonplace. To address this problem, we have developed new inorganic nanocomposite photoresists with significantly higher etch resistance than the usual polymer-based photoresists. Hafnium oxide nanoparticles are used as a core to build the inorganic nanocomposite into an imageable photoresist. During the sol-gel processing of nanoparticles, a variety of organic ligands can be used to control the surface chemistry of the final product. The different ligands on the surface of the nanoparticles give them unique properties, allowing these films to act as positive or negative tone photoresists for 193 nm or electron beam lithography. The development of such an inorganic resist can provide several advantages to conventional chemically amplified resist (CAR) systems. Beyond the etch resistance of the material, several other advantages exist, including improved depth of focus (DOF) and reduced line edge roughness (LER). This work will show etch data on a material that is ~3 times more etch-resistant than a PHOST standard. The refractive index of the resist at 193 nm is about 2.0, significantly improving the DOF. Imaging data, including cross-sections, will be shown for 60 nm lines/spaces (l/s) for 193 nm and e-beam lithography. Further, images and physical characteristics of the materials will be provided in both positive and negative tones for 193 nm and e-beam lithography.

Development of an inorganic nanoparticle photoresist for EUV, e-beam, and 193nm lithography

We have developed a transparent, high refractive index inorganic photoresist with significantly higher etch resistance than even the most robust polymeric resist. As feature sizes continue to decrease, film thickness must be reduced in order to prevent pattern collapse. Normally thinner films prevent sufficient pattern transfer during the etch process, creating the need for a hardmask, thus increasing production cost. Compared to PHOST, we have shown over 10 times better etch resistance. Organic photo-crosslinkable ligands have been attached to a hafnium oxide nanoparticle core to create an imageable photoresist. This resist has shown superior resolution with both E-beam and 193 nm lithography, producing sub-50 nm patterns. In addition to improved etch resistance, the inorganic photoresist exhibits a high refractive index, increasing the depth of focus (DOF). The nanoparticle size of ~ 1-2 nm has the potential to reduce line edge roughness (LER).

Oxide nanoparticle EUV resists: toward understanding the mechanism of positive and negative tone patterning

DUV, EUV and e-beam patterning of hybrid nanoparticle photoresists have been reported previously by Ober and coworkers. The present work explores the underlying mechanism that is responsible for the dual tone patterning capability of these photoresist materials. Spectroscopic results correlated with mass loss and dissolution studies suggest a ligand exchange mechanism responsible for altering the solubility between the exposed and unexposed regions.

Tightly bound ligands for hafnium nanoparticle EUV resists

Relative ligand binding energies were determined for a series of common ligand types with hafnium oxide nanoparticles, and from these results a series of novel strong binding ligands were developed. The relative equilibrium concentrations of two competing ligands bound to the nanoparticles were measured using nuclear magnetic resonance spectroscopy (NMR). For each ligand type, equilibrium constants and relative binding energies were then calculated and compared. Methane sulfonic acid was found to have the strongest binding energy, 2.0 Kcal/mol stronger than acetic acid. A group of three sulfonate ligands capable of freeradical crosslinking were made, along with three sulfonate ligands capable of creating aqueous developable nanoparticles. One of these ligands resulted in insoluble nanoparticles, however, the other two ligands resulted in nanoparticles that coated well on a silicon substrate and had dissolution rates greater than 100 nm per second.

Investigation of acid diffusion during laser spike annealing with systematically designed photoacid generators

Chemically amplified resists (CARs) are the current workhorse for photolithography, where higher resolution and smaller feature size represent a continual driving force for the semiconductor industry. As the feature size decreases to sub-30 nm, LWR and gate critical dimension (CD) control become serious concerns. In order to reach the goals in the ITRS, an unprecedented level of control of photoacid diffusion while maintaining the high resist sensitivity and resolution during image formation is required. CARs require a post exposure bake (PEB), typically performed on a hot plate at 90-150°C for 30-120 seconds, to complete the resist deprotection after photoacid generation. This bake step is a primary influence on resist performance as the time/temperature profile controls both the diffusion of photogenerated acids and the deprotection of the resist backbone. Sufficient time must be provided to achieve the level of deprotection required for the solubility switching in a developer, but the seconds timeframe of conventional hot plate PEB leads to an undesirable amount of acid diffusion. As long as the activation energy of diffusion is less than that for deprotection, higher temperatures for optimized time durations will result in reduced diffusion. However, traditional hot plate PEB cannot access times shorter than a few seconds. We utilize a laser (CO2) based scanned heating system to achieve sub-millisecond to milliseconds in heating durations with temperatures up to the thermal decomposition limit of the resist. This research is aimed at using synthetic techniques to vary the structure of the photoacid generator (PAG) in order to learn about the role of PAG size and structure on acid diffusion during sub-millisecond heating. A variety of PAGs with different anion sizes have been synthesized and tested on the CO2 laser system, and their lithographic performance and effect on acid diffusion has been studied.

Environmentally friendly patterning of thin films in linear methyl siloxanes

A number of green solvents have been explored to reduce the environmental impact of many chemical processes. Among them, linear methyl siloxanes make up a class of solvents that are low in toxicity, VOC exempt and not ozone-depleting. In addition, their unique physical properties such as low surface tension and low viscosity can mitigate several issues encountered with conventional processing solvents. In order to understand the behavior of linear methyl siloxanes as processing solvents, the solubilities of polymers and small molecular glasses are studied in this work. Using lithography as a test of solubility differences, we have successfully shown patterning of thin films in linear methyl siloxanes thereby demonstrating their utility in processing organic systems.

Environmentally friendly natural materials-based photoacid generators for next-generation photolithography

We describe the development of new triphenylsulfonium photoacid generators (TPS PAGs) with semifluorinated sulfonate anions containing glucose or other natural product groups, and their successful application to patterning sub-100 nm features using 254 nm and e-beam lithography. The TPS PAGs with functionalized octafluoro-3-oxapentanesulfonate were synthesized efficiently in high purity and high yield by utilizing simple and unique chemistries on 5-iodooctafluoro-3-oxapentanesulfonyl fluoride. The PAGs has been fully evaluated in terms of chemical properties, lithographic performance, environmental friendliness or toxicological impact. The PAGs are non-toxic and it is susceptible to chemical degradation and to microbial attack under aerobic/anaerobic conditions. These new PAGs are very attractive materials for high resolution photoresist applications and they are particularly useful in addressing the environmental concerns caused by PFOS and other perfluoroalkyl surfactants.

Solvent development processing of chemically amplified resists: chemistry, physics, and polymer science considerations

Solvent development of chemically amplified (CA), negative tone photoresists depends on several factors including molecular weight of the photoresist, the strength of polymer-solvent interactions, and the strength of polymer-polymer interactions in the undeveloped regions. Absent are the ionic interactions present in the aqueous base development of CA resists that greatly aids dissolution and image contrast. In its place, strong hydrogen bonding of the exposed photoresist leads to effective resistance to dissolution in non-polar developers. These effects are discussed in the context of Flory- Huggins theory. As part of a study of low environmental impact developers several, non-polar solvents have been investigated with negative tone, chemically amplified photoresists. These include supercritical CO2, hydrofluoroethers and silicone fluids. Each of these solvents has low surface energy, unique dissolution characteristics and is capable of developing sub-50 nm patterns. Performance aspects of these developers will be described.

All-dry processible and PAG-attached molecular glasses for improved lithographic performance

As the semiconductor industry moves forward, resolution limits are being pushed to the sub-30 nm regime. In order to meet these demands, radical new resist design and processes must be explored. We have developed a molecular glass system for all-dry processing conditions. Physical vapor deposition (PVD) has been used for film formation onto silicon wafers. PVD deposits a uniform film of controlled thickness free from impurities that are often introduced by casting solvents used in traditional spin coating methods. Thermal development is used as an alternative to processing in solvents in order to prevent resist swelling and pattern collapse by capillary forces. The deposited molecule is designed to crosslink upon E-beam irradiation without additives, and therefore form a homogeneous, single component film. PAG-attached molecular glasses have been synthesized in order to promote film homogeneity as well. By tethering PAG directly to the molecular glass core, issues such as PAG aggregation can be remedied. Acid migration, which increases blur and LER, can also be hindered.