Amir Sagiv

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.

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.

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.

Computational inspection applied to a mask inspection system with advanced aerial imaging capability

Traditional patterned mask inspection has been off-wavelength. For the better part of the past 25years mask inspection systems never adhered to the wavelength of the exposure tools. While in the days of contact and proximity printing this was not a major issue, with the arrival of steppers and scanners and the slow migration from 436nm, 405nm, 365nm and 248nm to ultimately 193nm, on-wavelength inspection has become a necessity. At first there was the option with defect and printline review using an at-wavelength AIMS tool [Fig 1], but now the industry has moved towards Patterned Mask Inspection to be at-wavelength too. With ever decreasing wavelength, more and more materials have become opaque, and especially the 266/257nm inspection to 193nm printing wavelength has proven to be a reliability issue. The industry took a major step forward with the adoption of at-wavelength aerial inspection, a paradigm shift in mask inspection, as it uses a hardware emulation to parallel the scanners true illumination settings [Fig 2]. The technology has found wide-spread acceptance by now, and 19xnm inspection is now the industry standard.

Practical Application of Aerial Imaging Mask Inspection for memory devices

A major challenge of low-k1 microlithography that has to be addressed by any photomask defect detection strategy is the complex relation between the signal of the defect in the detector and its impact, in terms of printing errors, on the processed wafer. This non-trivial relation is immanent to the large MEEF values characterizing lithography at sub-wavelength features. One common method to work around this problem is to use aerial imaging optics which emulates the stepper exposure process. Currently available aerial inspection and review tools based on the well established fact that CD variation in the aerial image closely represents the CD variation on the wafer. Published literature explains why a defects printing effect can be captured, with high correlation, by aerial imaging optics. Here we describe a novel connection between a defects detection signal and the printed CD variation on an adjacent pattern. This connection can be exploited by aerial imaging mask inspection systems to ensure that their detection thresholds are set to detect CD variation of a given threshold. We show that under aerial imaging conditions, the defect signal and CD variation are linearly related, regardless of defects attributes, provided that the defect resides close to a patterns transition edge, or is surrounded by a dense pattern. We present experimental results, demonstrating this linear scaling between the defect signal and CD variation, and show practical application results of aerial imaging mask inspection, with implications to production mask fab.

Contact area as the intuitive definition of contact CD based on aerial image analysis

As feature sizes continue to diminish, optical lithography is driven into the extreme low-k1 regime, where the high MEEF increasingly complicates the relationship between the mask pattern and the aerial image. This is true in particular for twodimensional mask patterns, which are by nature much more complicated than patterns possessing one-dimensional symmetry. Thus, the intricacy of 2D image formation typically requires a much broader arsenal of resolution enhancement techniques over complex phase shift masks, including SRAFs and OPC, as well as exotic off-axis illumination geometries. This complexity on the mask side makes the printability effect of a random defect on a 2D pattern a field of rich and delicate phenomenology. This complexity is reflected in the dispute over the CD definition of 2D patterns: some sources use the X and Y values, while others use the contact area. Here, we argue that for compact features, for which the largest dimension is not wider than the PSF of the stepper optics, the area definition is the natural one. We study the response of the aerial image to small perturbations in mask pattern. We show that any perturbation creates an effect extending in all directions, thus affecting the area and not the size in a single direction. We also show that, irrespective of the source of perturbation, the aerial signal is proportional to the variation in the area of the printed feature. The consequence of this effect is that aerial inspection signal scales linearly with the variation of printed area of the tested feature.