Photonic ring resonators used as wavelength notch filters are a promising novel solution to enable astronomical instruments to remove the signal from atmospheric OH emission in the near-infrared wavelength range. We derive design requirements from theory and finite difference time domain simulations. We find rings with radii less than 10 microns provide an adequate free spectral range for silicon nitride abd less than 3 microns for silicon. One challenge for this application is the requirement for many rings in series to suppress particular wavelengths within 0.2nm. We report progress in fabricating both silicon and silicon nitride rings for OH suppression.
Integrated optics has the potential to play a transformative role in astronomical instrumentation. It has already made a significant impact in the field of optical interferometry, through the use of planar waveguide arrays for beam combination and phase-shifting. Additionally, the potential benefits of micro-spectrographs based on array waveguide gratings have also been demonstrated.
Here we examine a new application of integrated optics, using ring resonators as notch filters to remove the signal from atmospheric OH emission lines from astronomical spectra. We also briefly discuss their use as frequency combs for wavelength calibration and as drop filters for Doppler planet searches. We discuss the theoretical requirements for ring resonators for OH suppression. We find that small radius (< 10 μm), high index contrast (Si or Si<sub>3</sub>N<sub>4</sub>) rings are necessary to provide an adequate free spectral range. The suppression depth, resolving power, and throughput for efficient OH suppression can be realised with critically coupled rings with high self-coupling coefficients.
We report on preliminary laboratory tests of our Si and Si3N4 rings and give details of their fabrication. We demonstrate high self-coupling coefficients (> 0:9) and good control over the free spectral range and wavelength separation of multi-ring devices. Current devices have Q ≈ 4000 and ≈ 10 dB suppression, which should be improved through further optimisation of the coupling coefficients. The overall prospects for the use of ring resonators in astronomical instruments is promising, provided efficient fibre-chip coupling can be achieved.
In extreme ultraviolet (EUV) lithography, 92 eV photons are used to expose photoresists. Typical EUV resists are organic-based and chemically amplified using photoacid generators (PAGs). Upon exposure, PAGs produce acids which catalyze reactions that result in changes in solubility. In EUV lithography, photo- and secondary electrons (energies of 10- 80 eV) play a large role in PAG acid-production. Several mechanisms for electron-PAG interactions (e.g. electron trapping, and hole-initiated chemistry) have been proposed. The aim of this study is to explore another mechanism – internal excitation – in which a bound PAG electron can be excited by receiving energy from another energetic electron, causing a reaction that produces acid. This paper explores the mechanism of internal excitation through the analogous process of electron-induced fluorescence, in which an electron loses energy by transferring that energy to a molecule and that molecule emits a photon rather than decomposing. We will show and quantify electron-induced fluorescence of several fluorophores in polymer films to mimic resist materials, and use this information to refine our proposed mechanism. Relationships between the molecular structure of fluorophores and fluorescent quantum yield may aid in the development of novel PAGs for EUV lithography.
Optimizing the photochemistry of extreme ultraviolet (EUV) photoresists should provide faster, more efficient resists which would lead to greater throughput in manufacturing. The fundamental reaction mechanisms in EUV resists are believed to be due to interactions with energetic electrons liberated by ionization. Identifying the likelihood (or cross section) of how these photoelectrons interact with resist components is critical to optimizing the performance of EUV resists. Chemically amplified resists utilize photoacid generators (PAGs) to improve sensitivity; measuring the cross section of electron induced decomposition of different PAGs will provide insight into developing new resist materials. To study the interactions of photoelectrons generated by EUV absorption, photoresists were exposed to electron beams at energies between 80 and 250 eV. The reactions between PAG molecules and electrons were measured using a mass spectrometer to monitor the levels of small molecules produced by PAG decomposition that outgassed from the film. Comparing the cross sections of a variety of PAG molecules can provide insight into the relationship between chemical structure and reactivity to the electrons in their environments. This research is a part of a larger SEMATECH research program to understand the fundamentals of resist exposures to help in the development of new, better performing EUV resists.
EUV photons expose photoresists by complex interactions including photoionization to create primary electrons (~80 eV), and subsequent ionization steps that create secondary electrons (10-60 eV). The mechanisms by which these electrons interact with resist components are key to optimizing the performance of EUV resists and EUV lithography as a whole. As these photoelectrons and secondary electrons are created, they deposit their energy within the resist, creating ionized atoms along the way. Because many photo- and secondary electrons can escape the resist through the surface, resists can become charged. Charging and energy deposition profiles within the resist may play a role in the sensitivity and line-edge roughness of EUV resists. In this paper, we present computational analysis of charging-influenced electron behavior in photoresists using LESiS (Low energy Electron Scattering in Solids), a software developed to understand and model electron-matter interactions. We discuss the implementation of charge and tracking and the model used to influence electron behavior. We also present the potential effects of charging on EUV and electron beam lithography by investigating secondary electron blur in charging and non-charging models.
For the past several years there have been ongoing efforts to incorporate zinc oxide (ZnO) inside polymethyl methacrylate (PMMA), in the form of nanoparticles or quantum dots, to combine their optical properties for multiple applications. We have investigated a variation of atomic layer deposition (ALD), called sequential infiltration synthesis (SiS), as an alternate method to incorporate ZnO and other oxides inside the polymer. PMMA is a well-known ebeam resist. We can expose and develop patterns useful for photonics or sensing applications first, and then convert them afterwards into a hybrid oxide material with enhanced photonic, or sensing, properties. This is much easier than micromachining films of ZnO or other similar oxides because they are difficult to etch. The amount of ZnO formed inside the polymer film is magnitudes higher than equivalent amount deposited on a flat 2D surface, and the intensity of the photoemission suggests there is an enhancement created by the polymer-ZnO interaction. Photoemission from thin films exhibit photoemission similar to intrinsic ZnO with oxygen vacancies. These vacancies can be removed by annealing the sample at 500°C in an oxygen rich environment. SiS ZnO exhibits unusual photoemission properties for thick polymer films, emitting at excitations wavelengths not found in bulk or standard ZnO. Finally we have shown that patterning the polymer and then doing SiS ZnO treatment afterwards allows modifying or manipulating the photoemission spectra. This opens the doors to novel photonic applications.
Extreme ultraviolet (EUV) photons expose photoresists by complex interactions starting with photoionization that create primary electrons (∼80 eV), followed by ionization steps that create secondary electrons (10 to 60 eV). Ultimately, these lower energy electrons interact with specific molecules in the resist that cause the chemical reactions which are responsible for changes in solubility. The mechanisms by which these electrons interact with resist components are key to optimizing the performance of EUV resists. A resist exposure chamber was built to probe the behavior of electrons within photoresists. Resists were exposed under electron beam and then developed; ellipsometry was used to identify the dependence of electron penetration depth and number of reactions on dose and energy. Additionally, our group has updated a robust software that uses a first principles-based Monte Carlo model called low-energy electron scattering in solids (LESiS) to track secondary electron production, penetration depth, and reaction mechanisms within materials-defined environments. LESiS was used to model the thickness loss experiments to validate its performance with respect to simulated electron penetration depths to inform future modeling work.
Directed self-assembly (DSA) of block copolymers (BCPs) is a rising technique for sub-20 nm patterning. To fully harness DSA capabilities for patterning, a detailed understanding of the three dimensional (3D) structure of BCPs is needed. By combining sequential infiltration synthesis (SIS) and scanning transmission electron microscopy (STEM) tomography, we have characterized the 3D structure of self-assembled and DSA BCPs films with high precision and resolution. SIS is an emerging technique for enhancing pattern transfer in BCPs through the selective growth of inorganic material in polar BCP domains. Here, Al<sub>2</sub>O<sub>3</sub> SIS was used to enhance the imaging contrast and enable tomographic characterization of BCPs with high fidelity. Moreover, by utilizing SIS for both 3D characterization and hard mask fabrication, we were able to characterize the BCP morphology as well as the alumina nanostructures that would be used for pattern transfer.
EUV photons expose photoresists by complex interactions starting with photoionization that create primary electrons (~80 eV), followed by ionization steps that create secondary electrons (10-60 eV). Ultimately, these lower energy electrons interact with specific molecules in the resist that cause the chemical reactions which are responsible for changes in solubility. The mechanisms by which these electrons interact with resist components are key to optimizing the performance of EUV resists. An electron exposure chamber was built to probe the behavior of electrons within photoresists. Upon exposure and development of a photoresist to an electron gun, ellipsometry was used to identify the dependence of electron penetration depth and number of reactions on dose and energy. Additionally, our group has updated a robust software that uses first-principles based Monte Carlo model called “LESiS”, to track secondary electron production, penetration depth, and reaction mechanisms within materials-defined environments. LESiS was used to model the thickness loss experiments to validate its performance with respect to simulated electron penetration depths to inform future modeling work.
We report our recent development in pursuing high Quality-Factor (high-Q factor) plasmonic resonances, with vertically
aligned two dimensional (2-D) periodic nanorod arrays. The 2-D vertically aligned nano-antenna array can have high-Q
resonances varying arbitrarily from near infrared to terahertz regime, as the antenna resonances of the nanorod are highly
tunable through material properties, the length of the nanorod, and the orthogonal polarization direction with respect to
the lattice surface,. The high-Q in combination with the small optical mode volume gives a very high Purcell factor,
which could potentially be applied to various enhanced nonlinear photonics or optoelectronic devices. The 'hot spots'
around the nanorods can be easily harvested as no index-matching is necessary. The resonances maintain their high-Q
factor with the change of the environmental refractive index, which is of great interest for molecular sensing.
Transparent conducting oxides (TCOs), in general, are degenerated semiconductors with large electronic band-gap. They
have been widely used for display screens, optoelectronic, photonic, and photovoltaic devices due to their unique dual
transparent and conductive properties. In this study, we report in detail a technique that we developed to fabricate single
crystal TCO nanorod arrays with controlled conductivity, height, and lattice spacing in a simple one-zone tube furnace
system. We demonstrate how novel photonic/plasmonic properties can be obtained by selecting unique combinations of
these basic parameters of the nano-rod arrays.
The simulation, fabrication and measurement of nonlinear photonic crystals (PhCs) with hexagonal symmetry in
epitaxial BaTiO<sub>3</sub> were investigated. The optical transmission properties of a PhC were simulated by a 2-D finite-difference
time domain (FDTD) method. A complete bandgap exists for both the TE and TM optical modes. The
fabricated PhC has a well-defined stop band over the spectral region of 1525 to 1575 nm. A microcavity structure
was also fabricated by incorporation of a line defect in the PhC. Transmission of the microcavity structure over the
spectral region from 1456 to 1584nm shows a well-defined 5 nm wide window at 1495nm. Simulations indicate that
the phase velocity matched PhC microcavity device of 0.5 mm long can potentially serve as modulator with a 3 dB
bandwidth of 4 THz.
Fresnel zone plates are important x-ray diffractive optics which offer a focusing resolution approaching the theoretical limit. In hard x-ray region, the refractive indices of all the materials are close to unity, which requests thick zone plate to achieve a reasonable efficiency. It makes high-resolution zone plate extremely difficult to fabricate due to its high aspect ratio. We report a LIGA-like fabrication process employing e-beam resist HSQ as the plating mold material, which is relative simply compared with traditional processes. 1-μm-thick gold zone plates with 80-nm-wide outermost zone have been fabricated with this process.
This paper describes the use of a unique combination of an environmentally stable chemically amplified photoresist (UV113, Shipley) and a copolymer of methyl styrene and chloromethyl acrylate P(MS/CMA) resist (ZEP520, Zeon), without any additional intermediate layers, in the fabrication of micro- and nanochannels. The two resists used are innocuous to each other during the designed process flow, providing flexibility, high resolution, greater throughput and ease of use. We have also determined that the maximum channel length is limited by diffusion and mass transport effects, and that sub-100 nm nanochannels can be obtained with 30 micron lengths.
We introduce a new design of tilted linear zone plates, which are named tapered tilted linear (TTL) zone plates. The purpose of the design is to increase efficiency while at the same time keeping the focal plane perpendicular to the optical path. In order to accomplish this, the zone radius and number of zones must become a function of position along the structure. Simulation work described in this paper shows improved optical performance over regular tilted linear zone plates.
An advanced Monte Carlo model and software were developed to simulate electron scattering in electron beam lithography and signal formation in scanning electron microscopy at a new level of accuracy required for lithography and metrology. The model involves generation of fast secondary and slow secondary electrons, as well as generation of volume plasmons, and electron transfer between layers with regard to the difference between work functions of layers. To track SEM detector channel, the geometry of a detector and its energy transfer function were taken into account. This advanced model was used to simulate electron trajectories, deposited energy, signal from electron detector and images in SEM. Examples of simulations are presented for electron spectra, energy deposition in 50 kV maskmaking, and signals from patterned wafers in SEM.
Proc. SPIE. 3997, Emerging Lithographic Technologies IV
KEYWORDS: Lithography, Deep ultraviolet, Polymers, Electrons, Scanning electron microscopy, Monte Carlo methods, Semiconducting wafers, Charged-particle lithography, Absorption, Chemically amplified resists
Optimization of phenolic chemically amplified resist platforms has lead to the development of new resists, capable of high throughput SCALPEL exposure. A positive resist, XP9947A, has exhibited 100 nm and 80 nm dense line resolution with good sensitivity and dose latitude. The influence of DUV absorption and 100 KV e-beam absorption to the optimization process is discussed. The nature of 100 KV e-beam absorption enables a greater freedom of resist design than encountered for DUV resists.
Proc. SPIE. 3676, Emerging Lithographic Technologies III
KEYWORDS: Signal to noise ratio, Lithography, Electron beam lithography, CMOS sensors, Scanning electron microscopy, Photomasks, Optical alignment, Semiconducting wafers, Signal detection, Charged-particle lithography
A manufacturable process for fabricating alignment marks that are compatible the SCALPEL lithography system is described. The marks were fabricated in a SiO<SUB>2</SUB>/WSi<SUB>2</SUB> structure using SCALPEL lithography and plasma processing. The positions of the marks were detected through e-beam resist in the SCALPEL proof of lithography (SPOL) tool by scanning the image of the corresponding mask mark over the wafer mark and detecting the backscattered electron (BSE) signal. Scans of 1 micrometers line-space patterns yielded mark positions that were repeatable within 20 nm 3(sigma) with a dose of 4 (mu) C/cm<SUP>2</SUP> and signal-to-noise of 32 dB. An analysis shows that the measured repeatability is consistent with a random noise limited response combined with SPOL machine factors. By using a digitally sequenced mark pattern, the capture range of the mark detection was increased to 13 micrometers while maintaining 35 nm 3(sigma) precision. Further improvements in mark detection repeatability are expected when the SCALPEL electron optics is fully optimized.
We have dramatically increased the sensitivity of a technique for the rapid inspection of EUV multilayer-coated mask blanks. In this technique an EUV sensitive resist is applied directly to a mask blank which is then flood exposed with EUV light and partially developed. Reflectivity defects in the mask blank results in mounds in a partially developed positive resists that appear as high contrast objects in a standard Nomarski microscope. The use of a higher contrast resist is shown experimentally to result in the creation of dramatically taller mounds. A simple model for the exposure and development of the resists has been developed and the predictions of the model compare well with the experimental results.
High acceleration voltage electron beam exposure is one of the possible candidates for post-optical lithography. The use of electrons, instead of photons, avoids optical related problems such as the standing wave issues. However, resists must conform to certain needs for the SCALPEL system, such as exposure in a vacuum chamber with 100kv electron beams. Taking into account the challenging requirements of high resolution, high sensitivity, low bake dependency and no outgassing, TOK has been able to develop resists to meet most of the SCALPEL system needs. However, due to the nature of chemical amplification and the PEB dependency, as is the case with DUV resist which varies for different features, we must recommend different resist for multiple features such as dense lines, isolated lines and contact holes. TOK has designed an electron beam negative resist, EN-009, which demonstrate 100nm pattern resolution. The dose to print on the SCALPEL system is 5.0(mu) C/cm<SUP>2</SUP>. The electron beam positive resist, EP-004M, has been designed for line and space patterns. The dose to print on the SCALPEL system is 8.25(mu) C/cm<SUP>2</SUP>. The processing conditions are standard, using 0.26N developer. These are the lowest exposure energies reported to date for similar resolution on this exposure tools.
We have performed experiments to study the kinetics of dissolution of the positive chemically amplified resist AZ-PF (Hoechst AG). The resist dissolution in exposed regions was shown to have non-linear time dependence, with a delay time strongly dependent on prebake and post- exposure bake conditions. Effect of the presence of a low-solubility surface layer on patterning of submicron features as well as on roughness of the developed film has been demonstrated.