Shmoolik Mangan

What you see is what you print: aerial imaging as an optimal discriminator between printing and non-printing photomask defects

Advanced photomasks for low-k1 lithography, are prone to various defects sources: contamination, geometry, transmission, phase, etc. These defects exhibit a complex relation between the signal from an imaging detector and its print related impact, with important consequences for the performance of the detection scheme under nuisance-ubiquity conditions. We studied numerically several imaging schemes, with respect to their defect detection signal and its relation to the associated CD effect. We show that for actinic aerial imaging detection the signal is tightly correlated and linearly scaled with the induced CD variation regardless of defect source and location. Conversely, the correlation of non-actinic and/or non-aerial (high-resolution based) detection signal with printing effect is poor. Whereas the linear behavior characterizing aerial imaging is independent of the distribution of defect attributes, the statistics of non-aerial defect signal is shown to be highly sensitive to defect distribution. Such non-aerial detection schemes would generally have to compromise detection sensitivity in order to maintain a constant nuisance false alarm rate. Aerial imaging is therefore the optimal discriminator between printing and non-printing defects. The tight linear correlation between defect signal and CD effect in aerial inspection systems, allows for an optimized and effective mask inspection, suitable for all mask types and technologies. Specifically, we show here that such a tool allows a straightforward migration from 65nm node to 45nm and 32nm with double patterning, by tuning the detection threshold without being flooded by nuisance induced false alarms.

Aerial imaging for source mask optimization: mask and illumination qualification

As the semiconductor industry moved to 4X technology nodes and below, low-k1 ArF lithography approached the theoretical limits of single patterning resolution, a regime typically plagued by marginally small process windows. In order to widen the process window bottleneck, projection lithography must fully and synergistically employ all available degrees of freedom. The holistic lithography source mask optimization (SMO) methodology aims to increase the overall litho performance and achieve a robust process window for the most challenging patterns by balancing between the mask and illumination source design influences. The typical complexity of both mask and illumination source that results from a generic SMO process exceeds the current norm in the lithographic industry. In particular, the SMO literature reports on masks that fully operate as diffractive optical elements, with features that have little resemblance to the final wafer-level pattern. Additionally, SMO illumination sources are characterized by parametric or pixelated shapes and a wide range of transmission values. As a consequence of the new mask and source designs, qualifying the mask for printing and non-printing defects and accurate assessment of critical dimensions becomes one of the main mask inspection challenges. The aerial imaging technologies of Applied Materials Aera2TM mask inspection tool provide enabling solutions by separating out only the defects that matter and accurately measures aerial imaging critical dimensions. This paper presents the latest numerical and experimental SMO mask qualifications research results performed at Applied Materials with a mask containing two-dimensional DRAM production structures.

Current status of EUV mask inspection using 193nm optical inspection system in 30nm node and beyond

Extreme Ultra Violet Lithography (EUVL) is one of the most advanced patterning technologies to overcome the critical resolution limits of current ArF lithography for 30nm generation node and beyond. Since EUVL mask manufacturing process has not been fully stabilized yet, it is still suffering from many defect issues such as blank defects, defects inside multilayer causing phase defects, CD defects, LERs (Line Edge Roughness), and so on. One of the most important roles in mask manufacturing process belongs to mask inspection tools, which monitor and visualize mask features, defects and process quality for the EUVL process development. Moreover, as the portion of EUV mask production has been increased due to the EUV Pre-Production Tool (PPT) development, mask inspection technologies for EUVL become highly urgent and critical to guarantee mask quality. This paper presents a promising inspection technique for increasing the contrast of pattern imaging and defects capture rate using configurable illumination conditions in 193nm wavelength inspection tool.

Results from a novel EUV mask inspection by 193nm DUV system

The semiconductor industry recently concluded that EUV lithography is the most promising candidate to replace ArF for the 22nm half-pitch node and beyond. Significant progress was made in EUV scanner and source technology and EUV resists have achieved acceptable performance levels as well. But issues related to EUV mask inspection and defectivity remain for the most part unanswered. This gap positions EUV masks as the leading risk to the entire technology, and requires a robust solution during the introduction phase of EUVL. In this paper we present results from a EUV mask inspection system. We demonstrate optimal pattern image formation by using illumination shaping, and consider detection of various defect types that represent realistic mask defectivity scenarios. These results demonstrate that DUV-based patterned mask inspection tool can meet the requirements of the pre-production EUV phase, at 32nm half-pitch, and has adequate room to extend to production at the 22nm node.

Aerial imaging qualification and metrology for source mask optimization

As the semiconductor industry moves to 3X technology nodes and below, holistic lithography source mask optimization (SMO) methodology targets an increase in the overall litho performance with improved process windows. The typical complexity of both mask and illumination source exceeds what the lithographic industry has been accustomed to, and presents a novel challenge to mask qualification and metrology. In this paper we demonstrate the latest in aerial imaging technologies of Applied Materials Aera2TM mask inspection tool. The aerial imaging capability opens the door to a wide variety of metrological measurements analysis at aerial level and provides enabling solutions for mask and scanner qualifications. In particular, we demonstrate core and periphery DRAM pattern process window assessment and MEEF measurements, performed on an advanced test mask.

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.

EUV mask: detection studies with Aera2

The progress of optical lithography towards EUV wavelength has placed mask defectivity among major EUV program risks. Traditional mask inspection was carried in the DUV domain at 19x nm wavelength, similar to ArF lithography. As EUV mask patterns approach the 20nm half-pitch level, the resolution of DUV systems approaches its practical limits. At this limit, the lesson learned from ArF lithography is that contrast may be improved significantly by utilizing resolution enhancement techniques such as off-axis illumination shapes. Here we present an experimental study of the effects of illumination and polarization on contrast and detection. We measured a EUV patterned mask with programmed defects using Aera2 mask inspection tool at 193nm wavelength, equipped with a high NA objective. We compared the contrasts of the patterns and the defect detection signals obtained by employing 4 different illumination shapes and three polarization states: linear along x, linear along y, circular polarization. We learned that in order to achieve the best results both in terms of contrast and in terms of detection, it is most important to choose a suitable exposure conditions. In addition, a proper choice of the polarization state of the illumination can also result in some improvement.

Aera193i: aerial imaging mask inspection for immersion lithography

Advanced lithography became possible using breakthrough technologies, including phase shift masks, advanced illumination modes, aggressive OPC patterns and 193nm immersion optics. The Applied Materials Aera193 system, an at-wavelength aerial reticle inspection tool, was introduced for the 90-65nm technology nodes. In the era of immersion lithography and 55-45nm nodes, there is an increasing demand for Aerial inspection under immersion conditions. To face this demand, the Aera193i was upgraded with expanded illumination and collection optics to support up to 1.4 NA immersion conditions. Here, we describe novel Aerial inspection results under immersion conditions. We studied the detection of a variety of defect types on 55nm node phase shift masks for immersion lithography. We found that the immersion-emulation inspection was able to demonstrate a good detection line, with extremely low false alarms and nuisance call rate. We also studied the relationship between Aerial defect detection and actual defect printability by printing the same mask on wafer. We found good correlation between Aera193i detection line and actual defect printability. We also address the polarization effects under immersion NA. We demonstrate that under polarized stepper illumination the polarization effects on the image are negligible, while aerial imaging reliably emulates mask pattern polarization effects.