Anthony Garetto

Actinic review of EUV masks: first results from the AIMS EUV system integration

The EUV mask infrastructure is of key importance for a successful introduction of EUV lithography into volume production. In particular, for the production of defect free masks, actinic review of potential defect sites is required. To realize such an actinic review tool, Zeiss and the SEMATECH EUVL Mask Infrastructure consortium started a development programme for an EUV aerial image metrology system (AIMS™ EUV). In this paper, we discuss the status of the on-going system integration and show first results from the first light tests of the prototype tool.

Aerial imaging technology for photomask qualification: from a microscope to a metrology tool

Abstract Photomasks carry the structured information of the chip designs printed with lithography scanners onto wafers. These structures, for the most modern technologies, are enlarged by a factor of 4 with respect to the final circuit design, and 20–60 of these photomasks are needed for the production of a single completed chip used, for example, in computers or cell phones. Lately, designs have been reported to be on the drawing board with close to 100 of these layers. Each of these photomasks will be reproduced onto the wafer several hundred times and typically 5000–50 000 wafers will be produced with each of them. Hence, the photomasks need to be absolutely defect-free to avoid any fatal electrical shortcut in the design or drastic performance degradation. One well-known method in the semiconductor industry is to analyze the aerial image of the photomask in a dedicated tool referred to as Aerial Imaging Measurement System, which emulates the behavior of the respective lithography scanner used for the imaging of the mask. High-end lithography scanners use light with a wavelength of 193 nm and high numerical apertures (NAs) of 1.35 utilizing a water film between the last lens and the resist to be illuminated (immersion scanners). Complex illumination shapes enable the imaging of structures well below the wavelength used. Future lithography scanners will work at a wavelength of 13.5 nm [extreme ultraviolet (EUV)] and require the optical system to work with mirrors in vacuum instead of the classical lenses used in current systems. The exact behavior of these systems is emulated by the Aerial Image Measurement System (AIMS™; a Trademark of Carl Zeiss). With these systems, any position of the photomask can be imaged under the same illumination condition used by the scanners, and hence, a prediction of the printing behavior of any structure can be derived. This system is used by mask manufacturers in their process flow to review critical defects or verify defect repair success. In this paper, we give a short introduction into the lithography roadmap driving the development cycles of the AIMS systems focusing primarily on the complexity of the structures to be reviewed. Second, we describe the basic principle of the AIMS technology and how it is used. The last section is dedicated to the development of the latest generation of the AIMS for EUV, which is cofinanced by several semiconductor companies in order to close a major gap in the mask manufacturing infrastructure and the challenges to be met.

Challenging defect repair techniques for maximizing mask repair yield

In todays economic climate it is critical to improve mask yield as materials, processes and tools are more time and cost involved than ever. One way to directly improve mask yield is by reducing the number of masks scrapped due to defects which is one of the major mask yield reducing factors. The MeRiTTM MG 45, with the ability to repair both clear and opaque defects on a variety of masks, is the most comprehensive and versatile repair tool in production today. The cost of owning multiple repair tools can be reduced and time is saved when fast turnaround is required, especially when more than one defect type is present on a single mask. This paper demonstrates the ability to correct repair errors due to human mistakes and presents techniques to repair challenging production line defects with the goal of maximizing mask repair yield and cycle time reduction.

Advanced process capabilities for electron beam based photomask repair in a production environment

The cost and time associated with the production of photolithographic masks continues to grow, driven by the ever decreasing feature size, advanced mask technologies and complex resolution enhancing techniques. Thus employment of a high-resolution, comprehensive mask repair tool becomes a key element for a successful production line. The MeRiT® utilizes electron beam induced chemistry to repair both clear and opaque defects on a variety of masks and materials with the highest available resolution and edge placement precision. This paper describes the benefits of the electron beam induced technique as employed by the MeRiT® system for a production environment.

AIMS EUV first light imaging performance

Overcoming the challenges associated with photomask defectivity is one of the key aspects associated with EUV mask infrastructure. In addition to establishing specific EUV mask repair approaches, the ability to identify printable mask defects that require repair as well as to verify if a repair was successful are absolutely necessary. Such verification can only be performed by studying the repaired region using actinic light at an exact emulation of the scanner illumination conditions of the mask as can be done by the AIMSTM EUV. ZEISS, in collaboration with the SEMATECH EUVL Mask Infrastructure (EMI) consortium are currently developing the AIMSTM EUV system and have recently achieved First Light on the prototype system, a major achievement. First light results will be presented in addition to the current development status of the system.

Status of the AIMS EUV development project

The need for an actinic wavelength AIMS™ EUV tool by 2014 has been defined by SEMATECH due to the challenges associated with EUV mask manufacture and defectivity. The AIMS™ EUV development project began in June of 2011 as a collaboration between ZEISS and the SEMATECH EUVL Mask Infrastructure (EMI) consortium. The project remains on track to meet the first commercial tool shipment in September 2014. The current design status of the system after two years as well as recent progress in the prototype build will be presented.

Increasing mask yield through repair yield enhancement utilizing the MeRiT

The push toward smaller feature size at 193 nm exposure has been enabled by resolution enhancement techniques (RET) such as phase shifting technologies and optical proximity correction (OPC) which require more costly and time intensive resources to fabricate. This leads to a higher overall cost associated with each mask, making it more important than ever for the mask shop to fully utilize and improve its repair capabilities as the presence of defects on the final product is the major yield reducing factor. An increase in repair capability leads to a direct enhancement in repair yield which translates to an improvement in overall mask yield and a reduction in cycle time. The Carl Zeiss MeRiT® MG 45 provides numerous benefits over other techniques that can lead to an increase in repair yield. This paper focuses on methods utilizing the MeRiT® MG 45 that can be employed in a production environment in order to increase mask repair yield. The capability to perform multiple repairs at a single site without optical degradation enables defects that were not successfully repaired the first time to be corrected on a subsequent attempt. This not only provides operator mistakes and inexperience to be corrected for, but eliminates the need to hold up production in order to start a new mask which can cause a cascading effect down the line. Combining techniques to approach difficult partial height and combination defects that may have previously been classified as non-repairable is presented in an attempt to enable a wider range of defects to be repaired. Finally, these techniques are validated by investigating their impact in a production environment in order to increase overall mask yield and decrease cycle time.

Defect mitigation considerations for EUV photomasks

Abstract. The introduction of extreme ultraviolet (EUV) lithography into manufacturing requires changes in all aspects of the infrastructure, including the photomask. EUV reflective masks consist of a sophisticated multilayer (ML) mirror, capping layer, absorber layer, and anti-reflective coating thereby dramatically increasing the complexity of the photomask. In addition to absorber type defects similar to those the industry was forced to contend with for deep ultraviolet lithography, the complexity of the mask leads to new classes of ML defects. Furthermore, these approaches are complicated not only by the mask itself but also by unique aspects associated with the exposure of the photomask by the EUV scanner. This paper focuses on the challenges for handling defects associated with inspection, review, and repair for EUV photomasks. Blank inspection and pattern shifting, two completely new steps within the mask manufacturing process that arise from these considerations, and their relationship to mask review and repair are discussed. The impact of shadowing effects on absorber defect repair height is taken into account. The effect of mask biasing and the chief ray angle rotation due to the scanner slit arc shape will be discussed along with the implications of obtaining die-to-die references for inspection and repair. The success criteria for compensational repair of ML defects will be reviewed.

Scanner arc illumination and impact on EUV photomasks and scanner imaging

The combination of a reflective photomask with the non-telecentric illumination and arc shaped slit of the EUV scanner introduces what are known as shadowing effects. The compensation of these effects requires proper biasing of the photomask to generate the intended image on the wafer. Thus, the physical pattern on the mask ends up being noticeably different from the desired pattern to be written on the wafer. This difference has a strong dependence on both the illumination settings and the features to be printed. In this work, the impact of shadowing effects from line and space patterns with a nominal CD of 16nm at wafer was investigated with particular focus on the influence of pattern orientation and pitch, illumination pupil shape and fill (coherence) and absorber height. CD, best focus shift and contrast at best focus are utilized in detail in order to study the impact of the shadowing effects. All the simulation cases presented employ a complete scanner arc emulation, i.e. describe the impact of the azimuthal angle component of the illumination arc as in the NXE:3300 scanner and as it can be emulated by the AIMSTM EUV.

Impact of new MoSi mask compositions on processing and repair

The mask industry has recently witnessed an increasing number of new MoSi mask blank materials which are quickly replacing the older materials as the standard in high end mask shops. These new materials, including OMOG (opaque MoSi on glass) and high transmission (Hi-T) films, are driven foremost by the need to reduce feature size through resolution enhancement techniques (RET). The subject of this paper is a new low stress, Hi-T material which addresses the challenges presented by transitioning to smaller technology nodes including difficulties with pattern transfer, cleaning and repair. This material, based on currently employed MoSi films, eliminates process steps and utilizes a thinner overall substrate stack than currently used Hi-T schemes allowing an increase in critical dimension (CD) uniformity and feature resolution and more robustness due to a lower aspect ratio. While this new material is MoSi based the small compositional change requires, in some cases, a significant change in processing. Among the most impacted areas are the etch, clean and repair steps. Given the potential for defects to manifest on masks, repair is an invaluable step that can significantly impact the overall yield and lead to a reduction in cycle time1. The Carl Zeiss MeRiT® electron beam mask repair line provides the most advanced repair capabilities allowing a wide range of repairs to be performed on a number of mask types2. In a joint effort between MP Mask Technology Center LLC and Carl Zeiss SMT, this paper focuses on the benefits of the new Hi-T mask blank and the challenges it presents to the repair community. The differences between the new low stress, Hi-T material and current Hi-T technologies are presented and on site compositional analysis is performed with x-ray photoelectron spectroscopy (XPS) to illuminate the compositional differences. The development of a repair process for the new material utilizing the on-site Carl Zeiss MeRiT® MG 45 is presented along with several repairs and their AIMSTM results.