Phase Shift Mask Research Articles

Advancements and Challenges in Photomask Technologies for Semiconductor Lithography: A Comparative Review of Binary, PhaseShift, and EUV Masks

This report investigates essential photomask technologies in semiconductor manufacturing and their developments. It outlines the principles and uses of binary photomasks, attenuated phase shift masks, and extreme ultraviolet (EUV) photomasks in lithography systems. With technological advancements, masks have evolved to tackle the demand for smaller feature sizes. Despite these advancements, each mask type has specific applications and constraints. The report also touches on other mask types, including ternary, chromeless phase lithography (CPL), and silicon-containing masks, and anticipates future challenges and directions in photomask technology

Silicon nanocavity with a quality factor of 6.7 million fabricated by a CMOS-compatible process.

Here, we report on the increase of the quality-factors of photonic crystal nanocavities fabricated by a CMOS-compatible process. We fabricated nanocavities with the same cavity design but used either a binary photomask or a phase-shift photomask in the photolithography step to assess the impact of the photomask-type on the fabrication accuracy of the air holes. We characterized 62 cavities using time-resolved measurements and the best cavity had a quality-factor of 6.65 × 106. All cavities exhibited a quality-factor larger than 2 million and the overall average was 3.25 × 106. While the estimated magnitude of the scattering loss due to the air hole variations in the 33 cavities fabricated with the phase-shift photomask was slightly lower than that in the 29 cavities fabricated with binary photomask, the phase-shift photomask did not provide a significant improvement in the fabrication accuracy. On average, the scattering loss in these samples is more than 3 times larger than that of nanocavities fabricated using electron-beam lithography, which indicates room for further improvement.

Informatics-based computational lithography for phase-shifting mask optimization.

Phase-shifting mask (PSM) is widely used in semiconductor fabrication to improve the resolution and image fidelity of optical lithography process. However, the theoretical image fidelity limit of the PSM lithography process is not fully understood. In this paper, the information transmission mechanism of PSM layout in optical lithography system is unveiled from the perspective of information theory. First, an information channel model is established to depict the information transfer of PSM layout in optical lithography system. Based on the law of information entropy, the optimal mutual information (OMI) of the PSM lithography process is derived, and the theoretical lower bound of the PSM image error is obtained. Finally, an informatics-based computational lithography approach is proposed to optimize the PSM, which achieves higher image fidelity compared to the traditional optimization algorithm. The benefit of PSM over the binary lithography mask is also discussed from the information theoretical aspects.

Characterization of electrostatic discharge threshold voltage of phase-shift mask reticle

A reticle is a stencil used in lithography process for forming integrated circuit (IC) on silicon substrate. It consists of a thin (100 nm) coating of masking metallic patterned (features) with critical dimension (CD) of nanometers on a thicker quartz substrate. The features can be damaged by electrostatic discharge (ESD) when exposed to the environment electrostatic charge and caused deformed IC and eventually device difunctional. Semiconductor equipment materials industry (SEMI) standard established the allowable electrostatic charge on reticle based on the characterization of ESD threshold voltage on binary reticle. However, there is another type of reticle which is phase-shift mask (PSM), has not been characterized for its ESD threshold voltage. A direct current (DC) voltage is applied directly to the structures with CD of 80 nm, 110 nm, and 160 nm. The surface current is recorded at all levels of stress from 1 to 100 V. The current–voltage (IV) curve and physical inspection results for each cell are then reviewed and classified. The results yielded which no electric field induced migration (EFM) defect and breakdown voltage occurred at any of the structures. The cathode’s metal work function has been identified as the factor that influences the PSM reticle ESD threshold voltage.

Proximal Algorithms for Discrete-Level Phase-Shifting Mask Design with Application to Optogenetics

This work studies the problem of designing computer-generated holograms using phase-shifting masks limited to represent only a small number of discrete phase levels. This problem has various applications, notably in the emerging field of optogenetics and lithography. A novel regularized cost function is proposed for the problem at hand that penalizes the unfeasible phase levels. Since the proposed cost function is non-smooth, we derive proper proximal gradient algorithms for its minimization. Simulation results, considering an optogenetics application, demonstrate that the proposed proximal gradient algorithm yields better performance as compared to other algorithms proposed in the literature.

Stochastic side-lobe printing in EUV lithography: a simulation study

Background: Extreme ultraviolet (EUV) phase shifting masks (PSMs) may be introduced at the N3 node as alternative to double patterning or high-NA exposures. To be optically advantageous, PSMs may have to come with phase different from 180 deg and high transmission. Aim: The main concern is how such exposures could be affected by stochastic side-lobe printing. Approach: We present a simulation study where we compare stochastic distributions for contact layers with different mask approaches: binary mask, PSM with and without assist features, and TriTone masks. The purpose of this work is to evaluate whether PSMs in EUV lithography may become a valuable option to further extend single-exposure EUV at NA = 0.33. Results: We identify the best exposure conditions for lowest stochastic defectivity for each of the masks considered and perform a large-scale study at these conditions. We report CD and placement error distribution parameters, along with frequencies of observed defects. Conclusions: We find that PSMs are more limited in range of CDs that can be printed without stochastic defects, compared with binary masks. When compared at target CD that is optimal for PSM, proposed phase shift masks did not exhibit a clear advantage, suggesting careful consideration of stochastic defects will be needed during future process development.

Optimal phase shift mask and multilayer stack with the evaluation of imaging performance and process latitude in extreme ultraviolet high numerical aperture

In high numerical aperture (NA) extreme ultraviolet lithography, which is used to implement a finer linewidth of 10 nm or lower, serious problems arise in patterning as the NA increases. To alleviate such problems, a thin absorber and a multilayer with good reflective efficiency and improved pattern quality are required. To develop an effective EUV photomask for the commercialization of high-NA systems, we determined the optimal ruthenium (Ru)/silicon (Si) multilayer structure using a phase-shift mask (PSM) absorber. A Ru/Si multilayer using PSM as an absorber has a smaller best-focus range and placement error compared to the molybdenum (Mo)/silicon (Si) multilayer. At the same time, it provides improved image contrast, enabling more stable patterning. Even when the number of layers of the Ru/Si multilayer was reduced, it was confirmed that the reflectance efficiency and image quality were maintained.

Effects of mask pattern transmission on ArF lithographic performance in contact hole patterning

Even with the increase in need for next-generation lithography, immersion ArF lithography is still applied to the majority of critical layers. However, as circuit designs shrink, conventional 6% phase-shift mask (PSM) will become difficult to meet the lithography requirements for dense dot pattern compared to dense line pattern. To enhance immersion ArF lithographic performances for dot pattern, high-transmission PSM (High-T PSM) is attracting attention because the transmission of PSM has a significant impact on lithographic performances. From results of transmission dependency evaluated by mask three-dimensional (3D) simulation, it was found that 30% transmission has the best lithographic performances for dense dot. Based on these results, mask blank and mask making process for the new 30% PSM were developed. The results showed good cross-section profile, mask pattern resolution, and defect repairability. In addition, the durability against chemical cleaning and ArF irradiation were also improved. Wafer printability test using negative tone development demonstrated that new PSM has advantages in process window and mask error enhancement factor for dense dots (holes on wafer). Finally, the potential for further application of new PSM was investigated by mask 3D simulation. The results showed that new PSM has lithographic benefits not only for dense dots but also other patterns.

Mask-absorber optimization: the next phase

We continue our work on the physics of mask-topography-induced phase effects in imaging using extreme ultraviolet (EUV) lithography, and specifically how these effects can be mitigated by alternative mask absorbers. We present a semianalytical model to calculate the mask-topography-induced phase offset and study its trend throughout the entire material space at 13.5-nm wavelength. We demonstrate that the model is in good agreement with 3D rigorous simulations. Using the model, we explain why the previously demonstrated phase shift close to 1.2π works optimally for EUV imaging. We show a low refractive index mask absorber (n < 0.91) is crucial for good mask 3D mitigation. We demonstrate the importance of mask bias and incident angle for imaging with an optimized attenuated phase-shift mask (PSM), which makes good source-mask optimization indispensable. We present the lithographic performance of alternative mask absorbers including a high-k mask, and a low- and high-transmission attenuated PSM for a few basic use cases, confirming the lithographic gain that can be obtained by mask-absorber optimization.

Sub-Resolution Assist Feature in Attenuated Phase-Shift Mask for Extreme Ultraviolet Lithography

Sub-Resolution Assist Feature in Attenuated Phase-Shift Mask for Extreme Ultraviolet Lithography

Phase Shift Mask to Compensate for Mask 3D Effect in High-Numerical-Aperture Extreme Ultraviolet Lithography

Phase Shift Mask to Compensate for Mask 3D Effect in High-Numerical-Aperture Extreme Ultraviolet Lithography

Coherent scattering microscopy as an effective inspection tool for analyzing performance of phase shift mask.

The imaging performance of a half-tone phase shift mask (PSM) has been analyzed using coherent scattering microscopy (CSM), which allows analysis of the actinic characteristics of an extreme ultraviolet (EUV) mask such as its reflectivity, diffraction efficiency, and phase information. This paper presents the 1st experimental result showing the effect of 180° phase difference between the absorber and reflector in EUV mask. This reveals that a PSM offers a 46% improvement in 1st/0th diffraction efficiency and 14% improvement in image contrast when compared to a binary intensity mask (BIM). The horizontal-vertical critical dimension (H-V CD) bias is also reduced by 1.37 nm at 22 nm line and space (L/S) patterns. Since the performance of PSM can be evaluated without a wafer patterning process, CSM is expected to be a useful inspection tool for the development of novel EUV masks.

Micro‐Optics for Photolithography

AbstractPhotolithography is the engine that empowered semiconductor industry to reduce the minimum feature size of the components of a microchip from some 50 microns in the 1960s to below 14 nanometers today. Diffractive and refractive micro‐optical components play a decisive role in modern photolithography systems, e. g. for laser line width narrowing, laser beam shaping (customized illumination), as phase‐shift masks (PSM), for optical proximity correction (OPC), and for diffraction‐based overlay (DBO). Wafer‐based manufacturing of high‐quality micro‐optics and their importance for photolithography will be explained.

Patterning of self-assembled monolayers by phase-shifting mask and its applications in large-scale assembly of nanowires

A nonselective micropatterning method of self-assembled monolayers (SAMs) based on laser and phase-shifting mask (PSM) is demonstrated. Laser beam is spatially modulated by a PSM, and periodic SAM patterns are generated sequentially through thermal desorption. Patterned wettability is achieved with alternating hydrophilic/hydrophobic stripes on octadecyltrichlorosilane monolayers. The substrate is then used to assemble CdS semiconductor nanowires (NWs) from a solution, obtaining well-aligned NWs in one step. Our results show valuably the application potential of this technique in engineering SAMs for integration of functional devices.

Attenuated phase-shift mask for mitigation of photon shot noise effect in contact hole pattern for extreme ultraviolet lithography

In extreme ultraviolet lithography (EUVL), insufficient light source power is the biggest concern for high-volume manufacturing. Additionally, the photon shot noise (PSN) effect is believed to be the main source of degradation in various aspects of imaging performance. In this study, we propose an attenuated phase-shift mask (PSM) as a solution to both of these issues, yielding improved mask performance for the printing of small contact hole (C/H) patterns. Our PSM shows superior imaging performance over that of a binary intensity mask. We speculate that the stochastic imaging characteristics are improved by the enhanced diffraction efficiency of the PSM.

Resolution enhancement for advanced mask aligner lithography using phase-shifting photomasks

The application of the phase-shift method allows a significant resolution enhancement for proximity lithography in mask aligners. Typically a resolution of 3 µm (half-pitch) at a proximity distance of 30 µm is achieved utilizing binary photomasks. By using an alternating aperture phase shift photomask (AAPSM), a resolution of 1.5 µm (half-pitch) for non-periodic lines and spaces pattern was demonstrated at 30 µm proximity gap. In a second attempt a diffractive photomask design for an elbow pattern having a half-pitch of 2 µm was developed with an iterative design algorithm. The photomask was fabricated by electron-beam lithography and consists of binary amplitude and phase levels.

Surface modification of a MoSiON phase shift mask to reduce critical dimension variation after exposure to a 193-nm ArF excimer laser

Abstract Introduction of a MoSi-based phase shift mask (PSM) improves photolithography resolution by causing light to shift phase by 180° thus canceling the overlap. However, when MoSiON PSM was exposed to an ArF excimer laser ( λ = 193 nm), a significant increase in patterned critical dimension (CD) was observed. It was confirmed that the CD increase resulted from oxidation progression into the MoSiON layer. In this study, N 2 O or NH 3 plasma treatment and thermal annealing in NH 3 effectively suppressed CD variation after ArF laser exposure. While the compositional ratio of Si, N, O, and Mo elements in the MoSiON layer was not changed, an increase in oxygen content only in the top 5 nm was observed. Therefore, it is concluded that slight oxidation of the top surface of MoSiON PSM by introducing either N 2 O or NH 3 plasma treatment or thermal annealing in NH 3 suppresses an increase in the patterned CD of MoSiON PSM after exposure to a 193-nm ArF excimer laser.

Sidewall profile engineering for the reduction of cut exposures in self-aligned pitch division patterning

As 193-nm immersion lithography will likely be required to be extended beyond 40-nm half-pitch, multiple patterning lithography will become a necessity in that scenario. We present a cost-effective approach for double patterning with extendibility to sub-10-nm half-pitch division, which is a very promising candidate for advanced logic nodes. Spacers on sufficiently sloped sidewalls directly transferred from a low-contrast photoresist profile can be removed by anisotropic etching. Alternatively, spacer gaps for defining trenches may be prevented from penetrating to the substrate by the use of sloped sidewalls. These sloped sidewalls are defined by attenuated phase-shift mask features, which impart phase shifts other than 180 deg or 0 deg. Loop trimming and sidewall spacer definition are accomplished in a single photomask. In addition, there is now an extra ability to define random, arbitrary breaks in the spacer-defined pattern, without using an extra exposure for specified cuts. In this way, a single exposure using a modified attenuated phase-shift photomask, followed by a low-contrast development process around the sensitivity limit, is sufficient to pattern regularly arranged spacer-defined lines at fixed pitch while including some predetermined line cut locations.

Comparing the Advantage and Disadvantage Between OMOG and PSM Masks for 20nm Node

As 193 nm immersion lithography moves toward sub 20 nm half pitch, a new type of mask known as opaque MoSi on glass (OMOG) has been introduced to overcome the shortcomings of the well-established phase shift mask (PSM). The OMOG gives smaller Mask Enhanced Error Factor (MEEF), better Mask CDU and registration but it also suffers some problems such as poor Line Width Roughness (LWR), lower image contrast. In this paper, we gave a series of experiments to compare the OMOG mask with PSM mask. We checked the thru pitch FEM result for both types of mask. And we have tried some methods to improve contrast and process window. The results showed very promising. The disadvantages for OMOG mask would cause some serious problems. PSM mask can give better LWR and 2D pattern profile but is also suffered higher MEEF and CD control issue.

Efficient and Robust Synthesis of Phase-Shifting Masks in Optical Lithography

Phase-shifting masks (PSMs) are extensively used in semiconductor industry to push the limit of optical lithography. Their synthesis is generally performed via an optimization method under the nominal process condition, and the synthesis process is time-consuming. In this work, we apply a statistical strategy to optimize the average performance with respect to process fluctuations to enhance the robustness of the synthesized PSM pattern, and employ a cascadic multigrid algorithm to improve its computational efficiency. Moreover, we investigate and discuss the impact of the initial pattern on the synthesized result.