Nassir Mojarad

Evaluation of EUV resist performance with interference lithography towards 11 nm half-pitch and beyond

The performance of EUV resists is one of the main challenges for the cost-effectiveness and the introduction of EUV lithography into high-volume manufacturing. The EUV interference lithography (EUV-IL) is a simple and powerful technique to print periodic nanostructures with a resolution beyond the capabilities of other tools. In addition, the well-defined and pitch-independent aerial image of the EUV-IL provides further advantages for the analysis of resist performance. In this paper, we present evaluation of chemically-amplified resists (CAR) and inorganic resists using EUV-IL. We illustrate the performance of the tool through a reproducibility study of a baseline resist over the course of 16 months. A comparative study of the performance of different resists is presented with the aim of resolving patterns with CARs for 16 nm half-pitch (HP) and 11 nm HP. Critical dimension (CD) and line-edge roughness (LER) are evaluated as functions of dose for different process conditions. With a CAR with about 10 mJ/cm2 sensitivity, 18 nm L/S patterns are obtained with low LER and well-resolved patterns are achieved down to 16 nm HP. With another CAR of about 35 mJ/cm2 sensitivity, L/S patterns with low LER are demonstrated down to 14 nm HP. Resolved patterns are achieved down to 12 HP, demonstrating the capability of its potential towards 11 nm HP if pattern collapse mitigation can be successfully applied. With EUV-sensitive inorganic resists, patterning down to 8 nm has been realized. In summary, we show that resist platforms with reasonable sensitivities are already available for patterning at 16 nm HP, 11 nm HP, and beyond, although there is still significant progress is needed. We also show that with decreasing HP, pattern collapse becomes a crucial issue limiting the resolution and LER. Therefore resist stability, collapse mitigation, and etch resistance are some of the significant problems to be addressed in the development of resist platforms for future technology nodes.

Beyond EUV lithography: a comparative study of efficient photoresists' performance

Extreme ultraviolet (EUV) lithography at 13.5 nm is the main candidate for patterning integrated circuits and reaching sub-10-nm resolution within the next decade. Should photon-based lithography still be used for patterning smaller feature sizes, beyond EUV (BEUV) lithography at 6.x nm wavelength is an option that could potentially meet the rigid demands of the semiconductor industry. We demonstrate simultaneous characterization of the resolution, line-edge roughness, and sensitivity of distinct photoresists at BEUV and compare their properties when exposed to EUV under the same conditions. By using interference lithography at these wavelengths, we show the possibility for patterning beyond 22 nm resolution and characterize the impact of using higher energy photons on the line-edge roughness and exposure latitude. We observe high sensitivity of the photoresist performance on its chemical content and compare their overall performance using the Z-parameter criterion. Interestingly, inorganic photoresists have much better performance at BEUV, while organic chemically-amplified photoresists would need serious adaptations for being used at such wavelength. Our results have immediate implications for deeper understanding of the radiation chemistry of novel photoresists at the EUV and soft X-ray spectra.

Measuring three-dimensional interaction potentials using optical interference.

We describe the application of three-dimensional (3D) scattering interferometric (iSCAT) imaging to the measurement of spatial interaction potentials for nano-objects in solution. We study electrostatically trapped gold particles in a nanofluidic device and present details on axial particle localization in the presence of a strongly reflecting interface. Our results demonstrate high-speed (~kHz) particle tracking with subnanometer localization precision in the axial and average 2.5 nm in the lateral dimension. A comparison of the measured levitation heights of trapped particles with the calculated values for traps of various geometries reveals good agreement. Our work demonstrates that iSCAT imaging delivers label-free, high-speed and accurate 3D tracking of nano-objects conducive to probing weak and long-range interaction potentials in solution.

Enhancement of the intrinsic fluorescence of adenine using aluminum nanoparticle arrays.

This study demonstrates the metal-enhanced fluorescence of adenine using aluminum nanoparticle arrays in the deep UV range. It achieves the reproducible intensity enhancement of intrinsic fluorescence up to 80 on well-defined aluminum nanoparticle arrays at 257 nm excitation. In addition to a high signal enhancement, a strong modification of the fluorescence emission spectrum of adenine is observed. This study illustrates that the label-free detection of DNA bases and proteins that have low intrinsic fluorescence and absorption bands in the deep UV range can be facilitated using aluminum nanostructures.

EUV resists towards 11nm half-pitch

As extreme ultraviolet lithography (EUVL) prepares for its insertion into the high-volume manufacturing phase, many challenges still remain to be addressed. Among several issues, development of EUV resists with tight specifications of sensitivity (dose), resolution (HP) and line-edge roughness (LER) is required. Chemically-amplified resists (CARs) have been the major paradigm in the development of EUV resists, although several alternatives, such as molecular resists and inorganic resists, are also under development. Here we present a comparative study of the performance of CARs using the PSI’s EUV interference lithography tool, which can achieve patterning down to 7 nm HP. Also the current status of EUV resist availability towards 11 nm HP technology nodes is discussed. We show resolution down to 12 nm HP with CARs. Nevertheless, for patterning below 18 nm HP, the resolution is achieved at the expanse of sensitivity and LER. The global trend of decreasing sensitivity with increasing LER is valid across the different resists. This trade-off between resolution, LER, and sensitivity (i.e. RLS trade-off) is mainly dominated by the acid diffusion blur and remains a challenge. In addition, pattern collapse becomes a significant problem with increasing resolution. This can be partly overcome by the reducing the resist thickness, which leads to an increase in LER. Therefore, a new trade-off between pattern-collapse limited resolution and LER emerges. These two trade-offs make the progress in EUV resist development increasingly difficult.

Patterning at 6.5 nm wavelength using interference lithography

We present the results of patterning chemically-amplified and inorganic resists at 6.5 nm wavelength using interference lithography. Well-resolved patterns down to 22 nm HP are obtained. Dose-dependent line-edge roughness and critical dimensions in the resolution range of 50-22 nm half-pitch are obtained using 13.5 and 6.5 nm wavelength. The performances of the resists are compared for both cases. Increased line-edge roughness is observed for patterning 6.5 nm compared to the patterning at 13.5 nm wavelength.

Soft electrostatic trapping in nanofluidics

Trapping and manipulation of nano-objects in solution are of great interest and have emerged in a plethora of fields spanning from soft condensed matter to biophysics and medical diagnostics. We report on establishing a nanofluidic system for reliable and contact-free trapping as well as manipulation of charged nano-objects using elastic polydimethylsiloxane (PDMS)-based materials. This trapping principle is based on electrostatic repulsion between charged nanofluidic walls and confined charged objects, called geometry-induced electrostatic (GIE) trapping. With gold nanoparticles as probes, we study the performance of the devices by measuring the stiffness and potential depths of the implemented traps, and compare the results with numerical simulations. When trapping 100 nm particles, we observe potential depths of up to Q≅24 kBT that provide stable trapping for many days. Taking advantage of the soft material properties of PDMS, we actively tune the trapping strength and potential depth by elastically reducing the device channel height, which boosts the potential depth up to Q~200 kBT, providing practically permanent contact-free trapping. Due to a high-throughput and low-cost fabrication process, ease of use, and excellent trapping performance, our method provides a reliable platform for research and applications in study and manipulation of single nano-objects in fluids.

Monitoring the transformation of aliphatic and fullerene molecules by high-energy electrons using surface-enhanced Raman spectroscopy

We report on using 100 keV electrons to obtain amorphous carbon from aliphatic and fullerene molecules, and study this process by monitoring their Raman signal. In this regard, we use self-assembled monolayers of gold nanoparticles to provide high electromagnetic field enhancement, which allows the detection of the Raman signal from even nanometer-thick layers of analyte. Our results show different dynamics in the amorphization process of the two molecules, although both show suppression of their original vibrational resonances even at low exposure doses. We have also used atomic-force microscopy to evaluate the sensitivity of the C60 molecules to electron beams in forming networks of amorphized molecules that are less soluble in the development process. This method allows precise patterning possibilities as well as in situ functionalization of carbonaceous thin films in the perspective of using in electronic device applications.