Lithographic Process Conditions Research Articles

Machine Learning Applied to Electron Beam Lithography to Accelerate Process Optimization of a Contact Hole Layer.

Determining the lithographic process conditions with high-resolution patterning plays a crucial role in accelerating chip manufacturing. However, lithography imaging is an extremely complex nonlinear system, and obtaining suitable process conditions requires extensive experimental attempts. This severely creates a bottleneck in optimizing and controlling the lithographic process conditions. Herein, we report a process optimization solution for a contact layer of metal oxide nanoparticle photoresists by combining electron beam lithography (EBL) experiments with machine learning. In this solution, a long short-term memory (LSTM) network and a support vector machine (SVM) model are used to establish the contact hole imaging and process condition classification models, respectively. By combining SVM with the LSTM network, the process conditions that simultaneously satisfy the requirements of the contact hole width and local critical dimension uniformity tolerance can be screened. The verification results demonstrate that the horizontal and vertical contact widths predicted by the LSTM network are highly consistent with the EBL experimental results, and the classification model shows good accuracy, providing a reference for process optimization of a contact layer.

On the origin and evolution of hotspots in multipatterning processes

Understanding the origins and propagation of defects and hotspots in patterning processes used for semiconductor fabrication is of paramount importance in managing yield. In this paper, results from physics-based simulators to model lithography and dry etch processes are presented and compared to experimental results. These models are used to study different types of hotspots and defects observed in a litho-etch-litho-etch (LELE) multipatterning process. At each pass of the LELE flow, patterns are printed into a SiO2 collecting layer using a trilayer film stack comprised of a negative tone photoresist layer, a spin-on-glass layer (SOG), and a spin-on-carbon layer (SOC). After both passes of the LELE process, the patterns in the SiO2 collecting layer will be transferred to a TiN hardmask prior to final etch into an underlying dielectric. The SOG and SiO2 layers are etched using fluorocarbon plasma, while the SOC layer is etched with an H2/N2 plasma generated in a capacitively coupled plasma source. A pinching hotspot is observed during the single litho-etch pass in a region where two features are placed very close and the image contrast is low. However, for some lithography process conditions, this hotspot is rectified by subsequent etch steps and does not always transfer as a defect into the SiO2 layer. The quenching of the hotspot occurs primarily during the etching of the SOC layer due to the aspect ratio-dependent etching (ARDE) effect. A bridging hotspot is also observed at lithography during the single litho-etch pass at high exposure doses. This hotspot, on the other hand, is exacerbated by the etch steps because of the ARDE effect. Hotspots are also identified that originate from overlay errors between photomasks exposed during first and second passes of the LELE process. The etch bias generated during etching of the SOG layer is crucial to ensure that the overlay-related hotspot does not translate to the SiO2 layer. The extent of etch bias in the SOG etch step is critical and can be tuned by adjusting the neutral to ion flux ratio during that etch step. Increasing the flux ratio improves the process window for the overlay defect; however, when the ratio is higher than approximately 20% of the nominal value, a different defect type is formed in the SOG layer due to the inverse ARDE effect that propagates downstream to the SiO2 layer.

Adaptive gradient-based source and mask co-optimization with process awareness

We develop a source and mask co-optimization framework incorporating the minimization of edge placement error (EPE) and process variability band (PV Band) into the cost function to compensate simultaneously for the image distortion and the increasingly pronounced lithographic process conditions. Explicit differentiable functions of the EPE and the PV Band are presented, and adaptive gradient methods are applied to break symmetry to escape suboptimal local minima. Dependence on the initial mask conditions is also investigated. Simulation results demonstrate the efficacy of the proposed source and mask optimization approach in pattern fidelity improvement, process robustness enhancement, and almost unaffected performance with random initial masks.

Multifactor lithographic process conditions of 3D single mode waveguide fabrication

Controlled process conditions enabled the fabrication of well-defined, high quality, and reproducible waveguides. Multifactor lithographic process conditions have been achieved in order to fabricate 3D single mode waveguide. Preheating and post-baking processes are the most important factors. Sol gel materials form an intermediate bridge between glasses and polymers which allow doping of many metal ions or functional materials to the system to extend its capabilities in high performance and active devices, such as modulation, lasing (amplifying), beam splitters, couplers, deflectors, wavelength filters and switches. It was found that the single mode or multimode waveguide can be fabricated with single step spin coating.

Thermal effects and photoluminescence of zirconium doped hybrid sol gel waveguide

Multifactor lithographic process conditions have been achieved in order to fabricate 3-D single mode waveguide. One of the key factors is the heating (preheating and post-baking) process. Good waveguides were formed with 30 min of pre-heating at 100 °C. Sol gel waveguides have low light loss (0.16 dB/cm), which was achieved with an aged sol gel slab waveguide. By carefully controlling the index difference between cladding and core, light was easily launched into 4-inch lengths of single mode 3-D ridge waveguides. As the heating time increases, the luminescence intensities increase until 65 h and ceased further post-baking process at 110 °C. The thermally produced chromophore luminescence intensity was reduced about 10% after deep UV exposure for 12 min and no further reduction was observed.

Ripple-aware optical proximity correction fragmentation for back-end-of-line designs

Accurate characterization of image rippling is critical in early detection of back-end-of-line (BEOL) patterning weakpoints, as most defects are strongly associated with excessive rippling that does not get effectively compensated by optical proximity correction (OPC). We correlate image contour with design shapes to account for design geometry-dependent rippling signature, and explore the best practice of OPC fragmentation for BEOL geometries. Specifically, we predict the optimum contour as allowed by the lithographic process and illumination conditions and locate ripple peaks, valleys, and inflection points. This allows us to identify potential process weakpoints and segment the mask accordingly to achieve the best correction results.

Simple Holographic Patterning for High‐Aspect‐Ratio Three‐Dimensional Nanostructures with Large Coverage Area

AbstractUsing the vertical standing wave phenomena commonly regarded as a deterrent in holographic lithography, multifaceted three‐dimensional (3D) nanostructures are fabricated on polymeric photoresist materials using a simple two‐beam interferometer. Large‐area 3D nanostructures with high aspect ratios (greater than 10) are readily produced using this methodology, including grating, pillar and pore patterns. Furthermore, manipulation of the lithography process conditions results in unique sidewall profiles of the nanostructures. Such 3D holographic control even produces highly porous polymer membranes composed of 3D interconnected pore networks, which resembles the 3D photonic crystal compound nanostructures that were previously attainable only with limited pattern coverage area using complex multibeam holographic lithography processes. Such well‐tailored high‐aspect‐ratio 3D nanostructures with large pattern coverage area further enable the fabrication of novel nanostructures for functionalized materials via various additive and subtractive pattern transfer techniques such as etching, deposition, and molding. In particular, direct molding followed by thermal decomposition process leads to the synthesis of hierarchical titanium oxide nanostructures of tunable 3D geometry, which would be of great significance in applications of photonic crystals, photovoltaic solar cells, and photocatalyst in water decontamination.

Studying Photoresist Type for Sub-32nm Node Dense SRAM 2nd GT Layer

With continuous shrinking of device dimensions, high performance, dense SRAM cells design has become more challenging than before, patterning quality is also more sensitive to fluctuations in lithography process. This inevitably calls for more aggressive lithography process conditions in terms of the required resolution. The tradeoff between process window optimization for random logic gates and dense SRAM is not always straightforward, and it sometimes necessitates design rule and layout modifications. In particular, patterning the small tip-to-tip gap for dense SRAM cells at gate level becomes extremely challenging. By delinking patterning of SRAM tip-to-tip from other geometries, one can optimize the patterning processes independently at the expense of cost. This can be achieved through a special double patterning technique that employs a combination of double exposure and double etch (DE2).

Advanced Lithography Simulation for Various 3-Dimensional Nano/Microstructuring Fabrications in Positive- and Negative-Tone Photoresists

Photoresist lithography has been applied to the fabrication of micro/nano devices, such as microfluidic structures, quantum dots, and photonic devices, in MEMS (micro-electro mechanical systems) and NEMS (nano-electro-mechanical systems). In particular, nano devices can be expected to present different physical phenomena due to their three-dimensional (3D) structure. The flexible 3D micro/nano fabrication technique and its process simulation have become among the major topics needed to understand nano-mechanical phenomena. For this purpose, the moving-mask technology and the lithography processes for the positive- and negative-tone photoresists were modeled. The validity of the simulation of the proposed 3D nano/microstructuring was successfully confirmed by comparing the experiment results and the simulated results. Hence, the developed model and the simulation can present and optimize photoresist characteristics and lithography process conditions due to the various 3D nano/microstructures. They could be help in the understanding of nanomaterial and mechanical phenomena.

Mechanism of Embedded Micro/Nano Channel Formation for a Negative-Tone Photoresist by Moving Mask Lithography

Photoresist lithography has been applied in MEMS (micro electro mechanical systems). A flexible 3D (three-dimensional) micro/nano fabrication technique and its process simulation tool are required for 3D MEMS. This paper presents an UV (ultraviolet) lithography process simulation for the formation of an embedded micro/nano fluidic channel in a negative-tone photoresist. For this purpose, the moving-mask technology is modeled. The simulation algorithm of the nanolithography is applied for the micro-lithography. The validity of the simulation for the proposed 3D microstructuring is successfully confirmed by a comparison between the experimental and the simulated results. Hence, modeling and simulation for the formation of various patterns of micro/nano fluidic channels in a negative-tone photoresist can be used to provide the photoresist characteristics and to optimize the lithography process conditions.

Direct optimization approach for lithographic process conditions

We present a semiautomatic procedure for the optimization of lithographic process conditions. In the regime of resolution enhancement techniques such as optical proximity correction, off-axis illumination, and phase shifting masks, the design of lithographic photomasks and illumination sources becomes increasingly intricate. Earlier "what you see is what you get" (WYSIWYG) approaches cannot be applied anymore. Instead, the employment of sophisticated design optimization tools is inevitable. In contrast to related efforts using an inverse problem formulation to address this goal, the approach presented here can be considered a direct method: mask and illumination layouts are mutually altered, evaluated, and improved, using an heuristic search algorithm. Thus, not only does this method require very little a priori knowledge of the process, it also allows for a very flexible problem formulation, enabling an easy integration of different model options or even process steps. As an underlying optimization algorithm of the presented procedure, a genetic algorithm has been used, whose flow, data representation, and basic operations are discussed briefly. Different representation types for both mask and illumination setups and the formulation of the optimization objectives are explained in detail. Various well-performing mask and illumination settings demonstrate the feasibility of the proposed approach and point out further potentials: in contrast to single problem specific approaches, this method is applicable to a great variety of different complex optimization tasks in advanced lithography.

Numerical Analyses of the Roles of Gas Phase and Liquid Phase UV Photochemistry in Conventional and Immersion 193 nm Lithography

We examine the impacts of photochemistry of air and water on resist films during lithographic exposure at 193 nm. The analysis uses a detailed quantitative kinetic model based on available literature mechanistic data and constructed with in-house simulation tools. In conventional 193 nm lithography, both oxidation of the resist polymer due to the UV photolysis of molecular oxygen, and film interaction with strong acids, formed by photo-oxidation of nitrogen and sulfur species in ambient air, have been proposed to lead to degraded resist imaging. We assess the extent to which such reactions can occur under typical lithographic process conditions, and find that while oxidation is minimal, acid deposition into the top of the resist film can be significant and can spread over distances of millimeters. Immersion lithography using 193 nm radiation utilizes a layer of highly purified, degassed water as an index-matching fluid. When water is exposed to 193 nm light, short-lived chemical intermediates are produced by two pathways, neutral and ionic. A quantitative evaluation of this photochemistry during lithographic immersion exposure shows that neither these intermediates nor acid formed by photolysis of leached photoacid generator leads to substantial changes in the composition of the water layer, so resist impacts are not likely to be significant. Organic immersion fluids may undergo significant photolysis, however there are insufficient experimental data to assess any potential impacts at this time.

Direct patterning of spin-on glass with 157nm lithography: Application to nanoscale crystal growth

Selective area homoepitaxial growth of InP in 50nm scale dense features has been demonstrated using hydrogen silsesquioxane (HSQ) as the growth mask. The HSQ growth mask was patterned lithographically using high resolution interference lithography at 157nm. Lithographic process conditions were optimized, including postapplication bake temperature, developer normality, and oxygen levels during exposure.

A New Analytical Technique for Evaluating Standing Wave Effect of Chemically Amplified Positive Resist

A new analyticai system for evaluating the standing wave effect in chemically amplified positive resist has been developed. In this paper, we describe a new system for calculating the linewidth variation due to the standing wave effect and compare simulation and experimental results. The linewidth for different resist thicknesses is calculated by changing the exposure energy at constant resist thickness. The exposure energy is determined using two series of the experimental dissolution rate data. The development effect for the different resist thicknesses is simulated by modifying the dissolution rate in the direction of resist depth. To verify this system, a two-component chemicaily amplified positive resist was used. The exposure was performed with a KrF excimer laser stepper. The results show that the newly designed system is able to rapidly evaluate the standing wave effect of resist. The system is an effective tool for optimizing resist materials and lithographic process conditions.