Extreme Ultraviolet Reticle Research Articles

Improved ptychographic inspection of EUV reticles via inclusion of prior information.

The development of actinic mask metrology tools represents one of the major challenges to be addressed on the roadmap of extreme ultraviolet (EUV) lithography. Technological advancements in EUV lithography result in the possibility to print increasingly fine and highly resolved structures on a silicon wafer; however, the presence of fine-scale defects, interspersed in the printable mask layout, may lead to defective wafer prints. Hence, the development of actinic methods for review of potential defect sites becomes paramount. Here, we report on a ptychographic algorithm that makes use of prior information about the object to be retrieved, generated by means of rigorous computations, to improve the detectability of defects whose dimensions are of the order of the wavelength. The comprehensive study demonstrates that the inclusion of prior information as a regularizer in the ptychographic optimization problem results in a higher reconstruction quality and an improved robustness to noise with respect to the standard ptychographic iterative engine (PIE). We show that the proposed method decreases the number of scan positions necessary to retrieve a high-quality image and relaxes requirements in terms of signal-to-noise ratio (SNR). The results are further compared with state-of-the-art total variation-based ptychographic imaging.

RESCAN: an actinic lensless microscope for defect inspection of EUV reticles

Actinic mask defect inspection is an essential process step for the implementation of extreme ultraviolet (EUV) lithography in high-volume manufacturing. The main challenges for any mask defect inspection platform are resolution, sensitivity, and throughput. The reflective-mode EUV mask scanning lensless imaging microscope (RESCAN) is being developed to provide actinic patterned mask inspection capabilities for defects and patterns with high resolution and high throughput for node 7 and beyond. Namely, the goal of the RESCAN project is to develop a tool capable of inspecting an EUV reticle in 7 h and detect mask defects down to a size of 10 nm×10 nm. The lensless imaging concept allows overcoming the resolution limitations due to the numerical aperture and lens aberrations of conventional mask imaging systems. With the increasing availability of computational power and the refinement of iterative phase reconstruction algorithms, lensless imaging became a powerful tool to synthesize the complex amplitude of the reticle image providing us also with extremely valuable information about phase and mask three-dimensional effects. Here, we present a brief description of the current prototype of the RESCAN platform and illustrate a few experimental examples of programmed defect detection.

EUV mask defectivity – a process of increasing control toward HVM

Abstract This article covers the various aspects of defectivity of a typical mask used for extreme ultra-violet (EUV) lithography. The focus of the present article is on those aspects that are more specific for EUV lithography. A prime type of defect that fully falls under the really EUV-specific category consists of the so-called multilayer defects (ML-defects): these defects relate to the ML mirror on the mask, which makes it reflective. While not specific, particle contamination plays a special role in EUV lithography and includes two aspects: both front- and backside of an EUV mask have peculiarities beyond historical deep-UV lithography. Frontside particles can print when they exceed a critical size and, while solvable as in (deep)-UV lithography by means of a pellicle, there are specific challenges. Backside particles can distort a clamped EUV reticle, and the resulting local non-flatness of the reticle may result in focus and overlay errors on the wafer. A last aspect that requires special attention for EUVL can be categorized under reticle degradation by extensive use. That includes the high-volume manufacturing-oriented conditions of high source power needed to obtain competitive throughput. For each aspect of EUV mask defectivity, and hence each type of defect, the paper reviews how the community tackles them and how their possible impact on the result of wafer printing with a given EUV reticle is minimized. This includes a summary of the authors’ own contribution to related learning and developments. Finally, a personal interpretation is given of what are the remaining open items before a workable or full solution can be considered in place.

Reduction of airborne molecular contamination on an extreme ultraviolet reticle dual pod using clean dry air purging technology

SEMI Standard E152-0214 requires that detailed physical structure and interface analysis would be carried out for the design of extreme ultraviolet (EUV) pods. However, specific requirements for the structure's design, performance characteristics and testing and evaluation methodology are not included in the aforementioned document. Also it is impractical to incorporate the existing immersion lithography technology in the design of the new EUV pod. Few previous studies appear to have reported the related research.Clean dry air (CDA) purging has been proved to be an effective method to retard the haze formation on wafers in the wafer EUV pod, which is caused by deposition of airborne molecular contamination (AMCs) on wafer surfaces.Chemiluminescence (CL) spectroscopy and Photoionization detector (PID) techniques were used to measure AMCs and evaluate the purging effect of varying numbers of inlet and outlet arrangements in the EUV pod. Among these, the 2 inlets and 2 outlets arrangement was found to be the most efficient, providing a purging flow rate of 16l per minute (LPM).

Photoluminescence-based detection of particle contamination on extreme ultraviolet reticles.

Here, we propose a comparison-free inspection technique to detect particle contamination on the reticle of extreme ultraviolet (EUV) lithography systems, based on the photoluminescence spectral characteristics of the contaminant particles and their elemental composition. We have analyzed the spectra from different particles found on reticles in EUV lithographic systems and have determined the minimum detectable particle size: 25 nm for organic particles and 100 nm for Al particles. Stainless steel coatings (50 nm thick and 50 × 50 μm(2) in area) exhibit detectable photoluminescence, and the estimated minimum detectable particle is 2 μm.

Actinic imaging and evaluation of phase structures on extreme ultraviolet lithography masks

The authors describe the implementation of a phase-retrieval algorithm to reconstruct the phase and complex amplitude of structures on extreme ultraviolet (EUV) lithography masks. Many native defects commonly found on EUV reticles are difficult to detect and review accurately because they have a strong phase component. Understanding the complex amplitude of mask features is essential for predictive modeling of defect printability and defect repair. Besides printing in a stepper, the most accurate way to characterize such defects is with actinic inspection, performed at the design, EUV wavelength. Phase defect and phase structures show a distinct through-focus behavior that enables qualitative evaluation of the object phase from two or more high-resolution intensity measurements. For the first time, the phase of structures and defects on EUV masks were quantitatively reconstructed based on aerial image measurements, using a modified version of a phase-retrieval algorithm developed to test optical phase shifting reticles.

Photon flux requirements for extreme ultraviolet reticle imaging in the 22- and 16-nm nodes

Extreme UV (EUV)-wavelength actinic microscopy yields detailed information about EUV mask patterns, architectures, defects, and the performance of defect repair strategies without the complications of photoresist imaging.To understand the pattern measurement limits of EUV mask microscopy, we investigate the effects of shot noise on aerial image linewidth measurements in the 22- and 16-nm lithography generations. Using a simple model we probe the influence of photon shot noise on measured, apparent line roughness, and find general flux density requirements independent of the specific EUV microscope configurations. Analysis reveals the trade-offs between photon energy density, effective pixel dimension on the CCD, and image log slope (ILS). We find that shot-noise-induced linewidth roughness (LWR) varies inversely with the square root of the photon energy density and is proportional to the magnification ratio. While high magnification is necessary for adequate spatial resolution, for a given flux density, higher magnification ratios have diminishing benefits. We find that to achieve an LWR (3) value of 5% of linewidth for dense, 88-nm mask features with a 2.52 normalized ILS value (image log-slope, ILS, equal to 28.6/µm) and 13.5-nm effective pixel width (1000 × magnification ratio), a peak photon flux of approximately 1400 photons/pixel per exposure is required.

Distortion of chucked extreme ultraviolet reticles from entrapped particles

Successful imaging of patterns with critical dimensions less than 45nm with extreme ultraviolet lithography (EUVL) requires stringent controls on all sources of image placement (IP) errors. Among the potential sources of IP error is the mechanical distortion of the patterned mask when mounted in the exposure tool. An EUVL reticle can exhibit both in-plane distortion and out-of-plane distortion due to the presence of debris lodged between the mask and the electrostatic chuck. Even particles with a compressed height as small as 100nm have the potential to consume a significant portion of the IP error budget. To alleviate this problem, a thorough understanding of the response of the reticle∕particle∕chuck system during electrostatic chucking is essential. This article describes experimental indentation testing to characterize relevant nanoscale material properties and the subsequent use of the data in finite element models that simulate the system response under typical chucking conditions.

Predicting the dynamic response of an extreme ultraviolet reticle during exposure scanning

Extreme ultraviolet (EUV) masks are expected to experience up to 6g’s of acceleration and achieve speeds as high as 500mm∕s while mounted in the exposure tool. Therefore, it is critical that the mounting technique employed to hold the mask in position prevents the mask from slipping under these conditions. An electrostatic pin chuck must generate adequate pressure between the mask and chuck, as well as a sufficient amount of friction. To ensure the success of such a mount design, three-dimensional finite element (FE) models have been created to predict the conditions necessary to prevent mask slippage during exposure scanning. The FE modeling is used to predict the influence of the initial substrate backside surface shape on mask flatness when mounted in the chuck, as well as the effects of friction on the relative displacement between the reticle and chuck. Simulation results indicate that for typical EUV mask substrates, coefficients of friction greater than 0.2 are necessary to prevent bulk displacement of the mask during the acceleration duty cycle. Larger coefficients may be required to limit local in-plane displacements, depending upon the initial backside shape.

Reticle’s contribution to critical dimension control and overlay in extreme-ultraviolet lithography

In order to design an exposure tool for extreme-ultraviolet (EUV) lithography that adheres to technology roadmap requirements, we translated overall system performance requirements to system design specifications by setting up detailed error budgets for critical dimension (CD) control and overlay. In this article we concentrate on the EUV reticle contributions, including surface flatness, hydrocarbon contamination, multilayer d-spacing variations, and mounting in the writer and exposure tool. It is determined that in order to meet the CD-control budget, the reticle multilayer d-spacing variation must be limited to 0.25%, whereas the thickness variation of the hydrocarbon growth should be less than 1.5 nm. The overlay budget requires that the reticle flatness within the site area must be better than ±25 nm, both on the patterned side and on the back side. Also, the chuck of the exposure tool must be flat within ±25 nm. To avoid in-plane pattern distortion, the flatness of the chuck in the e-beam writer must be identical to that of the chuck in the exposure tool, and hence also must be flat within ±25 nm.

Demonstration of pattern transfer into sub-100 nm polysilicon line/space features patterned with extreme ultraviolet lithography

In two separate experiments, we have successfully demonstrated the transfer of dense- and loose-pitch line/space (L/S) photoresist features, patterned with extreme ultraviolet (EUV) lithography, into an underlying hard mask material. In both experiments, a deep-UV photoresist (∼90 nm thick) was spin cast in bilayer format onto a hard mask (50–90 nm thick) and was subsequently exposed to EUV radiation using a 10× reduction EUV exposure system. The EUV reticle was fabricated at Motorola (Tempe, AZ) using a subtractive process with Ta-based absorbers on Mo/Si multilayer mask blanks. In the first set of experiments, following the EUV exposures, the L/S patterns were transferred first into a SiO2 hard mask (60 nm thick) using a reactive ion etch (RIE), and then into polysilicon (350 nm thick) using a triode-coupled plasma RIE etcher at the University of California, Berkeley, microfabrication facilities. The latter etch process, which produced steep (>85°) sidewalls, employed a HBr/Cl chemistry with a large (>10:1) etch selectivity of polysilicon to silicon dioxide. In the second set of experiments, hard mask films of SiON (50 nm thick) and SiO2 (87 nm thick) were used. A RIE was performed at Motorola using a halogen gas chemistry that resulted in a hard mask-to-photoresist etch selectivity >3:1 and sidewall profile angles ⩾85°. Line edge roughness (LER) and linewidth critical dimension (CD) measurements were performed using Sandia’s GORA© CD digital image analysis software. Low LER values (6–9 nm, 3σ, one side) and good CD linearity (better than 10%) were demonstrated for the final pattern-transferred dense polysilicon L/S features from 80 to 175 nm. In addition, pattern transfer (into polysilicon) of loose-pitch (1:2) L/S features with CDs⩾60 nm was demonstrated.