Evanescent wave assist features for optical projection lithography

Evanescent Wave Assist Features (EWAFs) are features that are sensitive to near-field radiation that modify diffracted order intensities from photomask patterns. In implementations studied in this thesis, the EWAFs increase a transmitting feature\u27s image contrast and Normalized Image Log Slope (NILS). In this way, the EWAFs are a way to improve image fidelity for high-resolution features. The assist features consist of local, buried grooves located around transmitting mask regions. These grooves reside in otherwise unused areas, since they are located under or on top of opaque mask absorber regions. In these buried locations, they are not optically visible to the lithographic system in a traditional sense. Designs are explored for both top-surface and bottom-surface EWAFs on 1-D and 2-D layouts. Using EWAFs, 27% image contrast improvements have been shown on contact layouts, as well as best-case image contrast improvements of over 2X on 1-D slot-type mask layouts. Dependence of EWAF effect on mask absorber material and bottom-surface relief shape is studied, as well as polarization sensitivity and the role of Surface Plasmon Polaritons (SPP). TM polarized light creates a normal-component field enhancement that amplifies surface waves across suitably conductive absorbers. These waves can then interact with bottom-surface EWAF grooves, and convert to propagating based on grating action. The converted orders may then interact with standard transmitted orders from a transmission feature, resulting in enhancement or suppression, depending on EWAF tone, pitch regime, and illumination angle. A demonstration EWAF sample, as well as a reference sample with no grooves, was fabricated at the RIT SMFL and tested using a Variable Angle Spectroscopic Ellipsometer (VASE). Accounting for pitch deviations during fabrication, as well as lateral inter-layer alignment offsets gives diffracted order responses that agree with SPP resonances observed in the samples at normal incidence and diffracted order enhancement factors that agree with simulation

Design and Fabrication of a Micromechanical Pressure Sensor

A Microelectromechanical (MEMS) pressure sensor was designed, fabricated, and tested. Photomasks were designed for the project and built in house at RIT. The masks included designs for three separate device designs: devices to be fabricated with a KOH bulk etch, devices to be fabricated with an Surface Technology Systems (STS) Deep Reactive Ion Etch (DRIE), and a third set of scaled device designs for use with the STS DRIE process. Devices were tested in house, and the ideal design was determined. The most sensitive device, which had a resistor L/W of 10, demonstrated a voltage differential of 39 mV

Evanescent Eave Imaging in Optical Lithograpy

New applications of evanescent imaging for microlithography are introduced. The use of evanescent wave lithography (EWL) has been employed for 26nm resolution at 1.85NA using a 193nm ArF excimer laser wavelength to record images in a photoresist with a refractive index of 1.71. Additionally, a photomask enhancement effect is described using evanescent wave assist features (EWAF) to take advantage of the coupling of the evanescent energy bound at the substrate-absorber surface, enhancing the transmission of a mask opening through coupled interference

Gray Assist Bar OPC

Assist bar Optical Proximity Correction (OPC) has been demonstrated to increase across pitch performance and depth-of- focus of semi-dense to isolated lines1. As the sub-resolution assist feature (SRAF) or assist bar’s size increases, so does its desired lithographic effect, as well as its undesired printability. In other words, when large assist features are required at isolated pitches, the assist features may print. A frequency-preserving assist bar solution is the most preferred one, but difficult to realize for opaque assist features due to printability. The concept of frequency-preserving Gray Assist Bar OPC has been introduced as a method to extend imaging performance for small features across a wide rage of duty ratios3. In this paper, we will present the experimental validation of this concept. The Gray Assist Bar mask was manufactured using a two-level lithography process, and the optical properties have been characterized using a Woollam VUV VASE system. Additional metrology was performed using an AFM (SNP9000) and CD SEM (KLA8250XR). Exposures on a 0.75NA 193nm scanner clearly show the expected effects. The use of the Gray Assist Bar features reduces the through pitch critical dimension (CD) variations significantly and can hence be regarded as an ‘Optical Proximity Correction’. The isofocal inflection point of aerial images is shifted in cases with Gray Assist Bars, resulting in flatter bossung curves and a larger depth of focus (DOF) for the various features through pitch at their target size. This results in larger overlapping process windows. The Gray Assist Bars has also shown a very low printability, even with aggressive off-axis illumination (OAI) settings

Study of Air Bubble Induced Light Scattering Effect On Image Quality in 193 nm Immersion Lithography

As an emerging technique, immersion lithography offers the capability of reducing critical dimensions by increasing numerical aperture (NA) due to the higher refractive indices of immersion liquids than that of air. Among the candidates for immersion liquids, water appears to be an excellent choice due to its high transparency at a wavelength of 193 nm, as well as its immediate availability and low processing cost. However, in the process of forming a water fluid layer between the resist and lens surfaces, air bubbles are often created due to the high surface tension of water. The presence of air bubbles in the immersion layer will degrade the image quality because of the inhomogeneity induced light scattering in the optical path. Therefore, it is essential to understand the air bubble induced light scattering effect on image quality. Analysis by geometrical optics indicates that the total reflection of light causes the enhancement of scattering in the region where the scattering angle is less than the critical scattering angle, which is 92 degrees at 193 nm. Based on Mie theory, numerical evaluation of scattering due to air bubbles, polystyrene spheres and PMMA spheres was conducted for TE, TM or unpolarized incident light. Comparison of the scattering patterns shows that the polystyrene spheres and air bubbles resemble each other with respect to scattering properties. Hence polystyrene spheres are used to mimic air bubbles in studies of lithographic imaging of “bubbles” in immersion water. In direct interference lithography, it is found that polystyrene spheres (2 μm in diameter) 0.3 mm away from the resist surface would not image, while for interferometric lithography at 0.5NA, this distance is estimated to be 1.3 mm. Surprisingly, polystyrene spheres in diameter of 0.5 μm (which is 5 times larger than the interferometric line-width) will not image. It is proposed that “bubbles” are repelled from contact with the resist film by surface tension. The scatter of exposure light can be characterized as “flare”. This work shows that microbubbles are not a technical barrier to immersion lithography

Approaching the Numerical Aperture of Water - Immersion Lithography at 193nm

As immersion nanolithography gains acceptance for next generation device applications, experimental data becomes increasingly important. The behavior of resist materials, fluids, coatings, sources, and optical components in the presence of a water immersion media presents conditions unique compared to convention “dry” lithography. Several groups have initiated fundamental studies into the imaging, fluids, contamination, and integration issues involved with water immersion lithography at 193nm. This paper will present the status and results of the next stage of the development efforts carried out at RIT. The status of two systems are presented; a small field projection microstepper utilizing a 1.05 catadioptric immersion objective lens and a 0.50 to 1.26NA interferometric immersion exposure system based on a compact Talbot prism lens design. Results of the fundamental resolution limits of resist materials and of imaging optics are presented. Additionally, an exploration into the benefits of increasing the refractive index of water is addressed through the use of sulfate and phosphate additives. The potential of KrF, 248nm immersion lithography is also presented with experimental resist imaging results

Immersion Microlithography at 193 nm with a Talbot Prism Interferometer

A Talbot interference immersion lithography system that uses a compact prism is presented. The use of a compact prism allows the formation of a fluid layer between the optics and the image plane, enhancing the resolution. The reduced dimensions of the system alleviate coherence requirements placed on the source, allowing the use of a compact ArF excimer laser. Photoresist patterns with a half-pitch of 45 nm were formed at an effective NA of 1.05. In addition, a variable-NA immersion interference system was used to achieve an effective NA of 1.25. The smallest half-pitch of the photoresist pattern produced with this system was 38 nm

Sensitive polysulfone based chain scissioning resists for 193 nm lithography

Chain scissioning resists do not require addition of photoacid generators to function. Previously reported chain scissioning polysulfone resists were able to achieve enhanced sensitivity by incorporation of absorbing repeat units, but these groups also inhibited the depolymerization reaction, which could further enhance sensitivity. Here we report the development of sensitive polysulfone chain scissioning resists for 193 nm that are able to undergo depolymerization. The effect of depolymerization of LER is also discussed. These polymers underwent CD shrinkage upon overdose, which may be useful for double patterning processes

Development and Evaluation of a 193nm Immersion Generation-Three Fluid Candidates

The need to extend 193nm immersion lithography necessitates the development of a third generation (Gen-3) of high refractive index (RI) fluids that will enable approximately 1.7 numerical aperture (NA) imaging. A multi-pronged approach was taken to develop these materials. One approach investigated the highest-index organic thus far discovered. The second approach used a very high refractive index nanoparticle to make a nanocomposite fluid. This report will describe the chemistry of the best Gen-3 fluid candidates and the systematic approach to their identification and synthesis. Images obtained with the Gen-3 fluid candidates will also be presented for a NA ≥ 1.7

Development and Evaluation of a 193nm Immersion Generation-Three Fluid Candidates

The need to extend 193nm immersion lithography necessitates the development of a third generation (Gen-3) of high refractive index (RI) fluids that will enable approximately 1.7 numerical aperture (NA) imaging. A multi-pronged approach was taken to develop these materials. One approach investigated the highest-index organic thus far discovered. The second approach used a very high refractive index nanoparticle to make a nanocomposite fluid. This report will describe the chemistry of the best Gen-3 fluid candidates and the systematic approach to their identification and synthesis. Images obtained with the Gen-3 fluid candidates will also be presented for a NA ≥ 1.7