Markus Waiblinger

Repair of natural EUV reticle defects

Defects of the multi-layer (ML) mirror on a EUV reticle, so-called ML-defects, are a prime aspect why EUV mask defectivity is considered a challenge before EUV lithography can be used for the production of future node integrated circuits. The present paper addresses the possibility to mitigate the printability of these defects by repair. Repair of natural EUV mask defects is performed using the electron beam based Carl Zeiss MeRiT® repair technology and is evaluated by wafer printing on the ASML EUV Alpha Demo Tool (ADT) installed at IMEC. Both absorber defects and ML-defects are included. The success of absorber defect repair (both opaque and clear type) is illustrated. For compensation repair of ML-defects experimental proof of the technique is reported, with very encouraging results both for natural pits and bumps. In addition, simulation is used to investigate the limitations of such compensation repair, inspired by the residual printability found experimentally. As an example it was identified that alignment of the compensation repair shape with the ML-defect position requires sub-20nm accuracy. The integration of an Atomic Force Microscope (AFM) into the repair tool has been an important asset to cope with this.

Closing the gap for EUV mask repair

The EUV-photomask is used as mirror and no longer as transmissive device. In order to yield defect-free reticles, repair capability is required for defects in the absorber and for defects in the mirror. Defects can propagate between the EUV mask layers, which makes the detection and the repair complex or impossible if conventional methods are used. In this paper we give an overview of the different defect types. We discuss the EUV repair requirements including SEMinvisible multilayer defects, and demonstrate e-beam repair performance. The repairs are qualified by SEM, AFM and through-focus wafer prints. Furthermore a new repair strategy involving in-situ AFM is introduced. Successful repair is demonstrated on real defects.

EUV mask stack optimization for enhanced imaging performance

EUVL requires the use of reflective optics including a reflective mask. The reticle blank contains a reflecting multilayer, tuned for 13.5nm, and an absorber which defines the dark areas. The EUV mask is a complex optical element with many more parameters than the CD uniformity of the patterned features that impact the final wafer CDU. Peak reflectivity, centroid wavelength and absorber stack height variations need to be tightly controlled for optimum performance. Furthermore the oblique incidence of light in combination with the small wavelength compared to the mask topography causes a number of effects which are unique to EUV, such as an H-V CD offset and an orientation dependent pattern placement error. These so-called shadowing effects can be corrected by means of OPC, but also need to be considered in the mask stack design. In this paper we will show that it is possible to improve the imaging performance significantly by reducing the sensitivity to mask making variations such as capping layer thickness and absorber stack height variations. The impact of absorber stack height variations on CD and proximity effects will be determined experimentally by changing the local absorber stack height using the novel e-beam based reticle repair tool MeRiT® HR 32 from Carl Zeiss in combination with exposures on ASMLs alpha demo tool. The impact of absorber reflectivity will be shown experimentally and used to derive requirements for the reticle border around the image field, as well as possible correction techniques.

Imaging performance improvements by EUV mask stack optimization

EUVL requires the use of reflective optics including a reflective mask. The mask contains a reflecting multilayer, tuned for 13.5 nm light, and an absorber which defines the dark areas. The EUV mask itself is a complex optical element with many more parameters than just the mask CD uniformity of the patterned features that impact the final wafer CDU. One of these parameters is absorber height. It has been shown that the oblique incidence of light in combination with the small wavelength compared to the mask topography causes a so-called shadowing effect manifesting itself particularly in an HV wafer CD offset. It was also shown that this effect can be essentially decreased by reducing absorber height and, in addition, it can be corrected by means of OPC. However, reduction of absorber height has a side effect that is an increased reflectivity of a mask black border resulting in field-to-field stray light due to parasitic reflections. One of the solutions to this problem is optical process correction (OPC) at field edges. In this paper we will show experimental data obtained on ASML EUV Alpha tool illustrating the black border effect and will demonstrate that this effect can be accurately predicted by Brion Tachyon EUV model allowing for a significant cross field CD uniformity improvement with mask layout correction technique. Also we show by means of rigorous 3D simulations that it is possible to improve the imaging performance significantly by performing global optimization of mask absorber height and mask bias in order to increase exposure latitude, decrease CD sensitivity to mask making variations such as CD mask error and absorber stack height variations. By sacrificing some exposure latitude throughput of exposure tool can be increased essentially and HV mask biasing can be reduced. For four masks with different absorber thicknesses from 44 nm to 87 nm it is proven experimentally by means of the EUV Alpha tool exposures of 27 nm L/S that the absorber thickness can be tuned to maximize exposure latitude. It was also proven that dose to size grows with absorber height and optimal feature bias depends on mask absorber height.

Ebeam based mask repair as door opener for defect free EUV masks

The EUV-photomask is used as mirror and no longer as transmissive device. In order to yield defect-free reticles, repair capability is required for defects in the absorber and for defects in the mirror. Defects can propagate between the EUV mask layers, which makes the detection and the repair complex or impossible if conventional methods are used. In this paper we give an overview of the different defect types. We discuss the EUV repair requirements including SEM-invisible multilayer defects and blank defects, and demonstrate e-beam repair performance. The repairs are qualified by SEM, AFM and wafer prints. Furthermore a new repair strategy involving in-situ AFM is introduced. This new strategy is applied on natural defects and the repair quality is verified using state of the art EUV wafer printing technology.

The door opener for EUV mask repair

The EUV-photomask is used as mirror and no longer as transmissive device. In order to yield defect-free reticles, repair capability is required for defects in the absorber and for defects in the mirror. Defects can propagate between the EUV mask layers, which makes the detection and the repair complex or impossible if conventional methods are used. In this paper we give an overview of the different defect types. We discuss the EUV repair requirements including SEMinvisible multilayer defects and blank defects, and demonstrate e-beam repair performance. The repairs are qualified by SEM, AFM and through-focus wafer prints. Furthermore a new repair strategy involving in-situ AFM is introduced. We will apply this new strategy on real defects and verify the repair quality using state of the art EUV wafer printing technology.

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.

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.