Lior Shoval

Applicability of e-beam mask inspection to EUV mask production

Ever since the 180nm technology node the semiconductor industry has been battling the sub-wavelength regime in optical lithography. During the same time development for a 13.5nm Extreme Ultraviolet [EUV] solution has been in development, which would take us back from a λ/10 to a >λ regime again - at least for one node. Add to this the potential to increase the wafer size as well, and we are at a major crossroads. The introduction of EUV has been marred by many delays, but we are finally seeing the hardware development efforts converge and multiple customers around the world embarking on this adventure. As it becomes clear that this preproduction phase will occur at or below 20nmHP, it also becomes clear that this will happen at the limiting edge of existing 19x-based patterned mask inspection technology, reaching the practical resolution limits at around 20nm HP mask densities. Resolution is coupled with sensitivity and throughput such that the extended sensitivity may come at an unreasonable throughput. Loss of resolution also badly impacts defect dispositioning, or classification, which becomes impractical. As resolution is especially critical for die to database inspection, single die masks and masks with high flare bias are at risk of not being inspectable with 19xnm based inspectors. E-Beam based mask inspection has been proposed and demonstrated as a viable technology for patterned EUV mask inspection. In this paper, we study the key questions of sensitivity and throughput, in both die-to-die and die-to-database applications. We present new results, based on a new generation of E-Beam inspection technology, which has a higher data rate at smaller spot sizes. We will demonstrate the feasibility of acceptable inspection time with EBMI. We also will discuss die-to-data-base inspection and the advantage of using E-Beam imaging for meeting future requirements of single- die EUV masks.

EUV mask defectivity study by existing DUV tools and new E-beam technology

While EUV lithography is approaching the pre-production stage, improving mask defectivity is recognized as a top challenge. The accepted strategy for EUV reticle qualification is to use a combination of a dedicated blank inspection (BI) to visualize EUV-specific multi-layer (ML) defects and patterned-mask inspection (PMI) that must be capable to meet the resolution requirements of the pattern. Actinic inspection is considered the strongest option for the blank inspection because of the limitation of optical light to visualize the nm-high distortions within the ML. Earlier publications showed that wafer inspection (WI) can potentially reveal such mask defects, This is, however, too late within the process. In addition, existing PMI and wafer inspection approaches exhibit limitations in detection capability and gaps are observed between detection of printed defects and defects detected on the mask (and the blank). We compare existing inspection solutions for detection of EUV mask defects (193nm based mask inspection and repeater analysis in a DUV wafer inspection) and present a feasibility study for use of a fast e-beam technology for mask inspection. Finally, we discuss the prospects of existing DUV tools and future e-beam technology to support EUV reticle inspection for current and future nodes.

Evaluation of novel EUV mask inspection technologies

EUV lithography is regarded as the leading technology solution for the post-ArF era. Significant progress was made in recent years in closing the gaps related to scanner technology. This progress rendered EUV mask defectivity and related infrastructure as the primary risk for EUV lithography. The smallness of mask features, the novel defectivity mechanisms associated with the multilayer reflecting coating, and the stringent constraints on both multilayer and pattern imposed by the EUV wavelength - present a major challenge to current inspection technology, which constitutes a predominant gap to EUVL production-worthiness. Here we present results from an evaluation of a DUV mask inspection system and e-beam mask inspection technology on EUV masks. On this 193nm DUV system, we studied sensitivity and contrast enhancements by resolution enhancement techniques. We studied both pattern and blank inspection. Next, we studied image formation and performance of e-beam mask inspection technology for patterned mask defects. We discuss the advantages and roadmap of DUV and EBI mask inspection solutions for blank and patterned masks.

IntenCD: an application for CD uniformity mapping of photomask and process control at maskshops

Lithographic process steps used in todays integrated circuit production require tight control of critical dimensions (CD). With new design rules dropping to 32 nm and emerging double patterning processes, parameters that were of secondary importance in previous technology generations have now become determining for the overall CD budget in the wafer fab. One of these key parameters is the intra-field mask CD uniformity (CDU) error, which is considered to consume an increasing portion of the overall CD budget for IC fabrication process. Consequently, it has become necessary to monitor and characterize CDU in both the maskshop and the wafer fab. Here, we describe the introduction of a new application for CDU monitoring into the mask making process at Samsung. The IntenCDTM application, developed by Applied Materials, is implemented on an aerial mask inspection tool. It uses transmission inspection data, which contains information about CD variation over the mask, to create a dense yet accurate CDU map of the whole mask. This CDU map is generated in parallel to the normal defect inspection run, thus adding minimal overhead to the regular inspection time. We present experimental data showing examples of mask induced CD variations from various sources such as geometry, transmission and phase variations. We show how these small variations were captured by IntenCDTM and demonstrate a high level of correlation between CD SEM analysis and IntenCDTM mapping of mask CDU. Finally, we suggest a scheme for integrating the IntenCDTM application as part of mask qualification procedure at maskshops.

Aerial imaging for FABs: productivity and yield aspects

The economy of wafer fabs is changing faster for 3x geometry requirements and below. Mask set and exposure tool costs are almost certain to increase the overall cost per die requiring manufacturers to develop productivity and yield improvements to defray the lithography cell economic burden. Lithography cell cost effectiveness can be significantly improved by increasing mask availability while reducing the amount of mask sets needed during a product life cycle. Further efficiency can be gained from reducing send-ahead wafers and qualification cycle time, and elimination of inefficient metrology. Yield is the overriding die cost modulator and is significantly more sensitive to lithography as a result of masking steps required to fabricate the integrated circuit. Thus, for productivity to increase with minimal yield risk, the sample space of reticle induced source of variations should be large, with shortest measurement acquisition time possible. This paper presents the latest introduction of mask aerial imaging technology for the fab, Aera2TM for Lithography with IntenCTM, as an enabler for efficient lithography manufacturing. IntenCD is a high throughput, high density mask-based critical dimension (CD) mapping technology, with the potential for increasing productivity and yield in a wafer production environment. Connecting IntenCD to a feed forward advance process control (APC) reduces significantly the amount of traditional CD metrology required for robust wafer CD uniformity (CDU) correction and increases wafer CD uniformity. This in turn improves the lithography process window and yield and contributes to cost reduction and cycle time reduction of new reticles qualification. Advanced mask technology has introduced a new challenge. Exposure to 193nm wavelength stimulates haze growth on the mask and imposes a regular cleaning schedule. Cleaning eventually causes mask degradation. Haze growth impacts mask CD uniformity and induce global transmission fingerprint variations. Furthermore, aggressive cleaning may damage the delicate sub-resolution assist features. IntenCD based CDU fingerprint correction can optimize the regular mask cleaning schedule, extending clean intervals therefore extending the overall mask life span. This mask availability enhancement alone reduces the amount of mask sets required during the product life cycle and potentially leads to significant savings to the fab. This mask availability enhancement alone reduces the amount of mask sets required during the product life cycle and leads to significant savings to the fab. In this paper we present three case studies from a wafer production fab and a mask shop. The data presented demonstrates clear productivity and yield enhancements. The data presented is the outcome of a range of new applications which became possible by integrating the recently introduced Applied Materials Aera2TM for Lithography aerial imaging inspection tool with the litho cluster.

Extending DUV mask inspection tool for inspecting 2xnm HP and beyond

Advanced 193nm DUV optical inspection tools that can cover 2Xnm HP node become more important and they are being tested to estimate their extendibility. We report DUV based inspection results evaluated and compared to wafer prints, as well as mask CD-SEM images in order to determine the size of printable defects that must be detected in each device node. Applied Materials® advanced Aera™ optical mask inspection tool that adapted a new optical technology enhancement was utilized to evaluate its inspection capability. The illumination conditions and pixel size were optimized to increase inspection sensitivity and reach detection requirements for not only critical defects that print on the wafer but also non-printing defects that indicate to a mask issue. Simulation was used to study suitable optical illumination conditions analyzing results to achieve the best performance for high-end EUV mask inspection toward next generation lithography.

Study of EUV mask e-beam inspection conditions for HVM

Extreme ultraviolet (EUV) e-beam patterned mask inspection (EBPMI) has been proposed by Applied Materials as a cost-effective solution for high volume manufacturing (HVM) in mask shops and fabs. Electron beam inspection technology is currently available for wafers. A recent publication described a successful sensitivity study of EUVs mask using a technology demonstration platform. Here we present a new study using extreme e-beam conditions to show the feasibility of using EBPMI in HVM. We examine potential changes in the reflectivity at the EUV wavelength after exposure to high e-beam currents, demonstrating that reflectivity does not change due to e-beam scanning. We therefore conclude that under the conditions tested, which include typical as well as extreme conditions, there is no evidence of mask damage.

Controlling phase induced CD non-uniformity effects of PSM photo-masks

Aggressive line width and other features of interest in advanced-technology node designs are achieved by using pattern-related resolution enhancement techniques (RET) coupled with mask transmission effects. Mask transmission effects, such as phase shift, are controlled by physical parameters, including mask blank material characteristics and mask architecture. In the case of advanced phase shift masks, the uniformity of transmitted phase, affected by both material properties and thickness, can become a dominant factor in achieving the final wafer CD targets. While traditional mask inspection tools are capable of detecting geometrical variation, detecting phase non-uniformity effects requires complementary, slow analytical tools. AMATs IntenCDTM is a novel application for advanced PSM masks which can be used for CD variation control in mask qualification. IntenCD captures mask CD variations in the aerial image regardless of the geometrical or physical aspect of its origin, producing a high-definition CDU map of the reticle. In this paper, we focus on a case study encountered at MP Mask where a PSM mask was sent to the fab to confirm large CD variations on a printed wafer due to mask etching process issues. Conventional defect inspection was not capable of detecting this excursion. The effect was clearly related to phase layer thickness as verified using an Atomic Force Microscope (AFM) tool. We show how the novel IntenCD application integrated into the aerial image mask inspection tool enables accurate prediction of CD variation in the aerial image due to mask phase errors.