Karen D. Badger

Impact of the OMOG Substrate on 32 nm Mask OPC Inspectability, Defect Sensitivity and Mask Design Rule Restrictions

Aggressive optical proximity correction (OPC) has enabled the extension of advanced lithographic technologies to the 32nm node. The associated sub-resolution features, feature-feature spacings, and fragmented edges in the design data are difficult to reproduce on masks and even more difficult to inspect. The patterns themselves must be differentiated from defects for inspectability, while the ability to recognize small deviations must be maintained for sensitivity. This must be done without restricting necessary OPC design features. The semi-transparent nature of industry-standard 6% attenuated phase shift substrates introduces a host of problems relative to inspectable dimensions and subsequent defect sensitivities. The result is a reduction in inspectability, defect sensitivity and the inability to inspect smaller critical dimensions and OPCed features. The introduction of a binary-type attenuated phase shift film improves the ability to inspect smaller critical dimensions and smaller OPC features without loss of inspectability and sensitivity extending the capability of existing inspection hardware for 32nm ground rule masks. This paper introduces inspection characterization results for this new film, opaque MoSi on glass (referred to as OMOG in this paper) and draws a correlation between the films transmission qualities and inspectability of 32nm OPC features. The paper will further show a correlation between OPC feature size and defect sensitivity for 32nm ground rule designs. Aerial Image (AIMS) analysis will be used to identify areas where the enhanced inspection capability can be leveraged to avoid unnecessary restrictions on OPC.

Wafer Plane Inspection Evaluated for Photomask Production

Wafer Plane Inspection (WPI) is a novel approach to inspection, developed to enable high inspectability on fragmented mask features at the optimal defect sensitivity. It builds on well-established high resolution inspection capabilities to complement existing manufacturing methods. The production of defect-free photomasks is practical today only because of informed decisions on the impact of defects identified. The defect size, location and its measured printing impact can dictate that a mask is perfectly good for lithographic purposes. This inspection - verification - repair loop is timeconsuming and is predicated on the fact that detectable photomask defects do not always resolve or matter on wafer. This paper will introduce and evaluate an alternative approach that moves the mask inspection to the wafer plane. WPI uses a high NA inspection of the mask to construct a physical mask model. This mask model is used to create the mask image in the wafer plane. Finally, a threshold model is applied to enhance sensitivity to printing defects. WPI essentially eliminates the non-printing inspection stops and relaxes some of the pattern restrictions currently placed on incoming photomask designs. This paper outlines the WPI technology and explores its application to patterns and substrates representative of 32nm designs. The implications of deploying Wafer Plane Inspection will be discussed.

Generating mask inspection rules for advanced lithography

Semiconductor product designs are necessarily constrained by both the wafer and mask lithographic capabilities. When mask image sizes approach the exposure wavelength, optical and resist effects distort the printed images. Applying optical proximity correction (OPC) to design features on the mask compensates for diffraction effects. However, aggressive OPC introduces even smaller minimum features, adds notches and bulges, introduces sub-resolution assist features (SRAFs) and generally creates a more challenging mask design with respect to data handling, printing and inspection. Mask defect inspection is a critical part of the mask process, ensuring that the mask pattern matches the intended design. However, the inspection itself imposes constraints on mask patterns that can be inspected with high defect sensitivity but low nuisance defect counts. These additional restrictions are undesirable since they can reduce the effectiveness of the OPC. IBM and KLA-Tencor have developed a test mask methodology to investigate the inspectability limits of the 576 and 516 mask inspection systems. The test mask design contains a variety of rules or features that currently impose inspectability limits on the inspection tools, in a range of sizes. The design also incorporates many features essential for obtaining valid results, such as a user-friendly layout, multiple pattern orientations, and background patterns. The mask was built and inspected in IBM Burlingtons mask house. Preliminary inspection results will be presented; they underscore the importance of understanding both the inspection tool and the mask process when restricting mask design rules.

SMO Photomask Inspection in the Lithographic Plane

Source Mask Optimization (SMO) describes the co-optimization of the illumination source and mask pattern in the frequency domain. While some restrictions for manufacturable sources and masks are included in the process, the resulting photomasks do not resemble the initial designs. Some common features of SMO masks are that the line edges are heavily fragmented, the minimum design features are small and there is no one-to-one correspondence between design and mask features. When it is not possible to link a single mask feature directly to its resist counterpart, traditional concepts of mask defects no longer apply and photomask inspection emerges as a significant challenge. Aerial Plane Inspection (API) is a lithographic inspection mode that moves the detection of defects to the lithographic plane. They can be deployed to study the lithographic impact of SMO mask defects. This paper briefly reviews SMO and the lithography inspection technologies and explores their applicability to 22nm designs by presenting SMO mask inspection results. These results are compared to simulated wafer print expectations.

Impact of transmitted and reflected light inspection on mask inspectability, defect sensitivity, and mask design rule restrictions

The application of aggressive optical proximity correction (OPC) has permitted the extension of advanced lithographic technologies. OPC is also the source of challenges for the mask-maker. Sub-resolution features, small shapes between features and highly-fragmented edges in the design data are difficult to reproduce on masks and even more difficult to inspect. Since the inspection step examines every image on the mask, it is required to guarantee the total plate quality. The patterns themselves must be differentiated from defects, and the ability to recognize small deviations must be maintained. In other words, high inspectability at high defect sensitivities must be achieved simultaneously. This must be done without restricting necessary OPC designs features. Historically, transmitted light has been deployed for mask pattern inspection. Recently, the inspection challenge has been both enhanced and complicated by the introduction of reflected light pattern inspection. Reflected light reverses the image contrast of features, creating a new set of design limits. This paper introduces these new reflected inspection limits. Multiple platform capabilities will be incorporated into the study of reflected and transmitted inspection capability. The benefits and challenges of integrating a combination of transmitted and reflected light pattern inspection into manufacturing will be explored. Aerial Image Measurement System (AIMS) analysis will be used to help understand how to leverage the enhanced inspection capability while avoiding unnecessary restrictions on OPC.

High resolution mask process and substrate for 20nm and early 14nm node lithography

The lithography challenges posed by the 20 nm and 14 nm nodes continue to place strict minimum feature size requirements on photomasks. The wide spread adoption of very aggressive Optical Proximity Correction (OPC) and computational lithography techniques that are needed to maximize the lithographic process window at 20 nm and 14 nm groundrules has increased the need for sub-resolution assist features (SRAFs) down to 50 nm on the mask. In addition, the recent industry trend of migrating to use of negative tone develop and other tone inversion techniques on wafer in order to use bright field masks with better lithography process window is requiring mask makers to reduce the minimum feature size of opaque features on the reticle such as opaque SRAFs. Due to e-beam write time and pattern fidelity requirements, the increased use of bright field masks means that mask makers must focus on improving the resolution of their negative tone chemically amplified resist (NCAR) process. In this paper we will describe the development and characterization of a high resolution bright field mask process that is suitable for meeting 20 nm and early 14 nm optical lithography requirements. Work to develop and optimize use of an improved chrome hard mask material on the thin OMOG binary mask blank1 in order to resolve smaller feature sizes on the mask will be described. The improved dry etching characteristics of the new chrome hard mask material enabled the use of a very thin (down to 65 nm) NCAR resist. A comparison of the minimum feature size, linearity, and through pitch performance of different NCAR resist thicknesses will also be described. It was found that the combination of the improved mask blank and thinner NCAR could allow achievement of 50 nm opaque SRAFs on the final mask.. In addition, comparisons of the minimum feature size performance of different NCAR resist materials will be shown. A description of the optimized cleaning processes and cleaning durability of the 50 nm opaque SRAFs will be provided. Furthermore, the defect inspection results of the new high resolution mask process and substrate will be shared.

Development and Characterization of a Thinner Binary Mask Absorber for 22 nm node and Beyond

The lithography challenges posed by the 22 nm node continue to place stringent requirements on photomasks. The dimensions of the mask features continue to shrink more deeply into the sub-wavelength scale. In this regime residual mask electromagnetic field (EMF) effects due to mask topography can degrade the imaging performance of critical mask patterns by degrading the common lithography process window and by magnifying the impact of mask errors or MEEF. Based on this, an effort to reduce the mask topography effect by decreasing the thickness of the mask absorber was conducted. In this paper, we will describe the results of our effort to develop and characterize a binary mask substrate with an absorber that is approximately 20-25% thinner than the absorber on the current Opaque MoSi on Glass (OMOG) binary mask substrate. For expediency, the thin absorber development effort focused on using existing absorber materials and deposition methods. It was found that significant changes in film composition and structure were needed to obtain a substantially thinner blank while maintaining an optical density of 3.0 at 193 nm. Consequently, numerous studies to assess the mask making performance of the thinner absorber material were required and will be described. During these studies several significant mask making advantages of the thin absorber were discovered. The lower film stress and thickness of the new absorber resulted in improved mask flatness and up to a 60% reduction in process-induced mask pattern placement change. Improved cleaning durability was another benefit. Furthermore, the improved EMF performance of the thinner absorber [1] was found to have the potential to relieve mask manufacturing constraints on minimum opaque assist feature size and opaque corner to corner gap. Based on the results of evaluations performed to date, the thinner absorber has been found to be suitable for use for fabricating masks for the 22 nm node and beyond.

Shedding light on EUV mask inspection

EUV defect detectability is evaluated both through simulation and by conventional mask inspection tools at various wavelengths (13.5, 193, 257, 365, 488 and 532 nm). The simulations reveal that longer wavelength light penetrates deeper into the multilayer than shorter wavelength light, however this additional penetration does not necessarily provide an advantage over shorter wavelengths for detecting defects. Interestingly, for both blank and patterned mask inspections, each wavelength detected unique defects not seen at other wavelengths. In addition, it was confirmed that some of the defects that are detected only by longer wavelengths are printable. This study suggests that a combination of wavelengths may be the most comprehensive approach to finding printable defects as long as actinic inspection is not available.

The impact of a thinner binary mask absorber on 22nm and beyond mask inspectability and defect sensitivity

As part of 20 nm/22 nm process development, an evaluation was performed to determined the impact of Thin OMOG on mask inspection. Despite significant improvements in mask inspectability and reduced database modeling errors, thin OMOG demonstrated lower defect sensitivity as compared to Standard OMOG at the same inspection conditions (calibration, sensitivity). Stack height aside, the primary difference between standard and thin OMOG is attenuator reflectivity. It is surmised that the reduction in sensitivity is due to a lower reflected light contrast on thin-OMOG. This characteristic was noted for both 257 nm and 193 nm inspection wavelengths. In addition to the reduction in defect sensitivity, an unexpected phase interference was noted at the image edge with a 193 nm inspection wavelength, for Standard OMOG, but not for Thin OMOG. This interference, or undershoot is due in part to the low difference in reflectivity and phase between the quartz and the attenuator on the Standard OMOG substrate. This difference is more than five times greater for the Thin OMOG attenuator. The primary focus of this paper is on the characterization of thin OMOG relative to the interaction between attenuator reflectivity, image quality, database modeling and tool calibrations as they relate to mask inspectability and defect sensitivity. This paper will also address the changes required to compensate for the loss of sensitivity induced by the introduction of the thin OMOG absorber.

Minimizing tone reversal during 19x nm mask inspection: PMJ18 Best Paper (Conference Presentation)

EUV (Extreme Ultraviolet) lithography is one of the most promising techniques for imaging 5-nm node and beyond wafer features. Mask defects that matter are the ones that print during exposure at 13.5 nm wavelength. To support EUV development and production schedules, mask defectivity must be reduced to be at or near the optical defect levels. This task is complicated by the fact that actinic EUV mask inspectors are not currently available. In the absence of an actinic EUV inspection tool, all available methods for detecting and characterizing defects must be deployed. Based on extensive deployment and experience in the industry with optical masks, and on its record for reasonable throughput, 19x nm wavelength inspection is one of the strongest candidates available today, for the initial EUV mask inspection approach. However, there are several key challenges with 19x nm optical inspection of EUV masks. One such challenge is defect sensitivity. Another challenge is that EUV mask pattern image contrast changes as a function of pattern size and pitch. This is often referred to as “Tone Reversal”, and it is a phenomenon that occurs for specific features. It is essential to understand the impact of tone reversal on defect sensitivity and overall inspectability, specifically for image sizes and pitches at the point of tone reversal, and for those immediately on either side of the tone reversal. In this study, the relationship between base pattern contrast and absorber defect sensitivity will be discussed through the analysis of programmed defect macros (PDMs). We will also discuss whether we can influence the point at which tone reversal occurs and furthermore, whether that reversal point can be tailored to specific patterns sizes or pitches. We will demonstrate how inspection parameter optimization can be done to tailor 19x inspection to specific layer and specific groundrules to maximize both sensitivity and inspectability.