Yan Borodovsky

ILT for double exposure lithography with conventional and novel materials

Multiple paths exists to provide lithography solutions pursuant to Moores Law for next 3-5 generations of technology, yet each of those paths inevitably leads to solutions eventually requiring patterning at k1 < 0.30 and below. In this article, we explore double exposure single development lithography for k1 ≥ 0.25 (using conventional resist) and k1 < 0.25 (using new out-of-sight out-of-mind materials). For the case of k1 ≥ 0.25, we propose a novel double exposure inverse lithography technique (ILT) to split the pattern. Our algorithm is based on our earlier proposed single exposure ILT framework, and works by decomposing the aerial image (instead of the target pattern) into two parts. It also resolves the phase conflicts automatically as part of the decomposition, and the combined aerial image obtained using the estimated masks has a superior contrast. For the case of k1 < 0.25, we focus on analyzing the use of various dual patterning techniques enabled by the use of hypothetic materials with properties that allow for the violation of the linear superposition of intensities from the two exposures. We investigate the possible use of two materials: contrast enhancement layer (CEL) and two-photon absorption resists. We propose a mathematical model for CEL, define its characteristic properties, and derive fundamental bounds on the improvement in image log-slope. Simulation results demonstrate that double exposure single development lithography using CEL enables printing 80nm gratings using dry lithography. We also combine ILT, CEL, and DEL to synthesize 2-D patterns with k1 = 0.185. Finally, we discuss the viability of two-photon absorption resists for double exposure lithography.

Pixelated Phase Mask as Novel Lithography RET

Novel RET-Pixelated Phase Mask (PPM) is proposed as a novel Resolution Enhancement Technique (RET). PPM is made of pixels of various phases with lateral dimensions significantly smaller than the illuminating radiation wavelength. Such PPM with a singular choice of pixel dimensions acts as a mask with variable phase and transmission due to radiation scattering and attenuation on pixel features with the effective intensity and phase modulated by the pixel layout. Key properties of the pixelated phase masks, the steps for their practical realization, and the benefits to random logic products discussed. Wafer patterning performance and comparative functional yield results obtained for a 65nm node microprocessor patterned with PPM, as well as current PPM limitations are also presented.

Lithography strategy for 65-nm node

Intel will start high volume manufacturing (HVM) of the 65nm node in 2005. Microprocessor density and performance trends will continue to follow Moores law and cost-effective patterning solutions capable of supporting it have to be found, demonstrated and developed during 2002-2004. Given the uncertainty regarding the readiness and respective capabilities of 157nm and 193nm lithography to support 65nm technology requirements, Intel is developing both lithographic options and corresponding infrastructure with the intent to use both options in manufacturing. Development and use of dual lithographic options for a given technology node in manufacturing is not a new paradigm for Intel: whenever introduction of a new exposure wavelength presented excessive risk to the manufacturing schedule, Intel developed parallel patterning approaches in time for the manufacturing ramp. Both I-line and 248nm patterning solutions were developed and successfully used in manufacturing of the 350nm node at Intel. Similarly, 248nm and 193nm patterning solutions were fully developed for 130nm node high volume manufacturing.

Integration of pixelated phase masks for full-chip random logic layers

This work describes the advantages, tolerances and integration issues of using Pixelated Phase Masks for patterning logic interconnect layers. Pixelated Phase Masks (PPMs) can act as variable high-transmission attenuated phase shift masks where the pixelated phase configuration simultaneously optimizes OPC and SRAF generation. Thick mask effects help enable PPMs by allowing larger minimum pixel sizes and phase designs with near equal sized zero and piphase regions. PPMs with a 3-tone pixel mask (un-etched glass, etched glass, chrome) offer more flexible patterning capability compared to 2-tone pixel mask (no chrome) style but at the detriment of a more complex mask making process. We describe the issues and opportunities associated with using PPMs for patterning a 65nm generation first level metal layer of a micro-processor.

The role of strong phase shift masks in Intel's DFM infrastructure development

Intel has reported on three separate styles and applications of strong phase shift masks (PSMs) over the last decade including alt-PSM for gate patterning, alt-PSM with assist features for contact patterning and Pixelated Phase Masks (PPMs) for metal layer patterning. Each had a prominent role in Intels Design For Manufacturing (DFM) infrastructure development in terms of design rules and DFM tooling. By gradually inserting design rule changes for alt-PSM for gate patterning starting from the 130nm technology node, density and design impact were minimally effected. Alt-PSM for contact layer required development of complex methods of SRAF placement and coloring while also forcing advances in phase shift mask manufacturing infrastructure. Pixelated phase masks for metal patterning when combined with Inverse Lithography Techniques (ILTs) were successful in supporting a high level of flexibility for metal design rules including multiple feature sizes, pitches and two-dimension content.

Fabrication of defect-free full-field pixelated phase mask

Pixelated phase masks rendered from computational lithography techniques demand one generation-ahead mask technology development. In this paper, we reveal the accomplishment of fabricating Cr-less, full field, defect-free pixilated phase masks, including integration of tapeout, front-end patterning and backend defect inspection, repair, disposition and clean. This work was part of a comprehensive program within Intel which demonstrated microprocessor device yield. To pattern mask pixels with lateral sizes <100nm and vertical depth of 170nm, tapeout data management, ebeam write time management, aggressive pattern resolution scaling, etch improvement, new tool insertion and process integration were co-optimized to ensure good linearity of lateral, vertical dimensions and sidewall angle of glass pixels of arbitrary pixelated layout, including singlets, doublets, triplets, touch-corners and larger scale features of structural tones including pit/trench and pillar/mesa. The final residual systematic mask patterning imperfections were corrected and integrated upstream in the optical model and design layout. The volume of 100nm phase pixels on a full field reticle is on the order tera-scale magnitude. Multiple breakthroughs in backend mask technology were required to achieve a defect free full field mask. Specifically, integration of aerial image-based defect inspection, 3D optical model-based high resolution ebeam repair and disposition were introduced. Significant reduction of pixel mask specific defect modes, such as electro static discharge and glass pattern collapse, were executed to drive defect level down to single digit before attempt of repair. The defect printability and repair yield were verified downstream through silicon wafer print test to validate defect free mask performance.

Double-exposure materials for pitch division with 193nm lithography: requirements, results

We present the results of both theoretical and experimental investigations of materials for application either as a reversible Contrast Enhancement Layer (rCEL) or a Two-Stage PAG. The purpose of these materials is to enable Litho- Litho-Etch (LLE) patterning for Pitch Division (PD) at the 16nm logic node (2013 Manufacturing). For the rCEL, we find from modeling using an E-M solver that such a material must posses a bleaching capability equivalent to a Dill A parameter of greater than 100. This is at least a factor of ten greater than that achieved so far at 193nm by any usable organic material we have tested. In the case of the Two-Stage PAG, analytical and lithographic modeling yields a usable material process window, in terms of reversibility and two-photon vs. one-photon acid production rates (branching ratio). One class of materials, based on the cycloadduct of a tethered pair of anthracenes, has shown promise under testing at 193nm in acetonitrile. Sufficient reversibility without acid production, enabled by near-UV exposure, has been achieved. Acid production as a function of dose shows a clear quadratic component, consistent with a branching ratio greater than 1. The experimental data also supports a acid contrast value of approximately 0.05 that could in principle be obtained with this molecule under a pitch division double-exposure scenario.

Reaction kinetics of non-reciprocal photo-base generator (NRPBG)patterning

We present a simple reaction rate analysis of lithographic patterning using the Non-Reciprocal Photo Base Generation (NRPBG) scheme of Bristol (Bristol, et. al., to be published in Proceedings of the SPIE - The International Society for Optical Engineering, 2010, presentation 7639-4). Multistep reaction kinetics simulations demonstrate that the NRPBG scheme produces clear pitch division upon 193 nm double-exposure, over a range of photochemical reaction rate constants.

157-nm lithography for 100-nm generation and beyond: progress and status

157-nm has emerged as the most favorable post 193-nm lithography choice. Significant progress has been made since it was initiated at MIT-LL in 1997. Material is perhaps the most critical issue of 157nm lithography in all areas of concern: optics, resist, and mask. CaF2 is the only material currently shown to be feasible for 157-nm lens though other materials namely BaF2 are being developed as secondary material. Due to limited availability of materials in conjunction with the difficulty in developing line-narrowed F2 laser, optics design is limited. Catadioptric lens design is being considered by most major exposure tool suppliers. Meanwhile, most conventional organic materials are opaque at 157-nm. New fluorinated polymers have been discovered and currently being developed for both resist and pellicle applications. Good progress in this area has been reported at the Sematech’s First 157-nm Symposium at Dana Point, California, May 9-11, 2000. Alternative approaches are also being developed. One example is thin layer resist process using conventional chemistry to overcome high absorption issue at 157-nm. Another is the so-called hard pellicle, i.e., a ~300-um thick film of the newly developed dry-fluorine doped fused silica is used instead of the typical 1-um thin organic membrane. On the other hand, major accomplishment has been reached in the field of mask blank material to replace the existing one for 157-nm application. The new dry and fluorine-doped fused silica has been shown to have good transmittance and radiation durability. Blank and reticle making using this newly developed material have been reported to be satisfactory even with current processes. High absorption at 157-nm also leads to other requirements such as surface molecular contamination removal and system purging. Therefore, reticle handling has become critical in that reticle purging, In-Situ cleaning and ESD prevention must be considered. While recognized to be issues, possible technical solutions have been proposed. This paper will provide an overview of 157-nm lithography development. Results will be presented to show progress. Critical issues covering exposure tool, resist and mask will be discussed.

Overlay improvement through overlay modeling

The application of overlay modeling to the description of the observed and predicted overlay errors allows for the multipurpose use of the successful modeling technique in identifying and resolving overlay problems. The presented paper describes the application of overlay modeling in estimating the exposure tool alignment system sensitivity to process/tool interaction and its potential impact on overlay performance. The described methodology is applicable to the characterization of various alignment systems and its use is described in detail. Another use of the overlay model allowed us to uncover large field mismatch and translational errors due to process induced change in wafer size. This discovery prompted the development of new exposure tool capabilities to provide adequate compensation for these overlay components. It is shown that analysis of unmodeled (residual) components of overlay also provided valuable insights into the peculiarities of exposure tool and process/overlay interaction.