High quality mask storage in an Advanced Logic-Fab

Transcription

1 High quality mask storage in an Advanced Logic-Fab Carmen Jähnert and Silvio Fritsche Infineon Technologies Dresden GmbH PO Box , D-01079, Dresden, Germany Abstract High efficient mask logistics as well as safe and high quality mask storage are essential requirements within an advanced lithography area of a modern logic waferfab. Fast operational availability of the required masks at the exposure tool with excellent mask condition requires a safe mask handling, safeguarding of high mask quality over the whole mask usage time without any quality degradation and an intelligent mask logistics. One big challenge is the prevention of haze on high advanced phase shift masks used in a high volume production line for some thousands of 248nm or 193nm exposures. In 2008 Infineon Dresden qualified a customer specific developed semi-bare mask storage system from DMS- Dynamic Micro Systems in combination with a high advanced mask handling and an interconnected complex logistic system. This high-capacity mask storage system DMS M for more than 3000 masks with fully automated mask and box handling as well as full-blown XCDA purge has been developed and adapted to the Infineon Lithotoollandscape using Nikon and SMIF reticle cases. Advanced features for ESD safety and mask security, mask tracking via RFID and interactions with the exposure tools were developed and implemented. The stocker is remote controlled by the icada-rsm system, ordering of the requested mask directly from the affected exposure tool allows fast access. This paper discusses the advantages and challenges for this approach as well as the practical experience gained during the implementation of the new system which improves the fab performance with respect to mask quality, security and throughput. Especially the realization of an extremely low and stable humidity level in addition with a well controlled air flow at each mask surface, preventing masks from haze degradation and particle contamination, turns out to be a notable technical achievement. The longterm stability of haze critical masks has been improved significantly. Relevant environmental parameters like temperature, humidity, AMC (Airborne Molecular Contamination) and particles are controlled online within the system and monitored via the Cleanroom Monitoring System and icada RSM. The storage system is well conditioned, based on a fine adjusted heating and cooling concept whereby the desired temperature and humidity values are kept very stable even under high frequent mask transactions. The in-house developed RFID system and traceability of masks within the Infineon Dresden Lithotool landscape is a new and complex logistics improvement, decoupling masks from boxes, saving costs and time and reducing particles. The presented hardware and software solution shows how the potential of automation and improved production efficiency can be increased by such adapted systems even in a mature 200mm waferfab. Keywords: mask storage, haze prevention, fab mask logistics, automation

2 1. Introduction Fast wafer cycle times, throughput, cost efficiency and customer satisfaction are main areas of interest in a waferfab. Within the lithography departments the attention is aligned to the lithography process with the available platforms of exposure and resist processes, controlled by the established metrology procedures. Typically, the masks are given as naturally available resources, always available at the point of need. With introduction of shorter exposure wavelength at 193nm and the usage of phase shift masks some years ago, many fabs suffered from mask degradation issues due to haze which can be generated on a structured mask surface after some 193nm exposures. Mask availability, free of defects at the point of use, was no longer a common case. Masks are sensitive key enablers for lithography processes as well. Prevention of haze issues on masks in combination with high frequent inspection procedures became hot topics of waferfabs [1] to protect the produced wafer from repeating killer defects and yield loss. Based on intensive investigations by mask makers and users on the nature and mechanisms of haze formation the main root causes and contributors have been identified. Besides to inappropriate mask cleaning procedures with higher risk of residual sulfuric or ammonia surface ion contamination and outgassing effects of the applied pellicles or glue types, also environmental contamination within the usage area of the mask can induce mask degradation by crystal growth over time. Even when the virgin mask is delivered clean without any ion residues and with low outgassing pellicles this mask can degrade over time if it is exposed to a high concentration of sulfate or amine levels in the surrounding atmosphere at exposure or storage place. In order to avoid haze degradation issues on a mask, storing them under purge will be the most effective way to ensure long term mask quality, preventing yield drops, line stops for mask cleans and additional costs for mask repells and high-frequent inspections. Implementing such a purging mask storage system for some thousands of masks in a high volume, multi-product wafer fab, like Infineon Dresden with the given Litho-tool landscape, was a challenging and interesting task for both tool vendor and customer. At this point there was no commercial stocker system available satisfying all requirements of the Infineon Dresden fab. The presented stocker system has been developed by the company DMS in close cooperation with MCRT GmbH, implementing their long experience in the field of air flow concepts and mini-environment solutions. It was a big challenge to develop a purging system for a huge air volume which is working stable and reliable in a production environment with high frequent mask exchanges each day. The stocker communication with the fab reticle management system has been implemented by icada and the stocker is remote controlled by RSM. Some new efficient logistic solutions have been developed, decreasing the operator effort and the contamination risk, an innovative RFID solution was implemented for Nikon reticle cases. In addition the mask ordering directly from the exposure tool is now possible. The prototype of the new stocker was installed in After some improvements and optimizations of the purge methodology as well as of the hardware and software solutions the tool has been released for production at the beginning of Up to now the implemented system is running very stable in production and ensures high mask quality and fast availability. This paper will show some technical solutions of the new system and discuss the approach and the experience of mask storage under XCDA (Extreme Clean Dry Air ) purge.

3 2. Fab requirements with respect to masks The mask is one of the four key contributors to a lithography process like wafer, exposure light and resist material. Thousands of productive masks need to be managed, handled and stored within a modern logic waferfab. Each mask must be and stay defect-free and well in specification for long lifetimes up to some years and many thousands of exposures. Up to exposed wafers is a realistic number for a 193nm gate layer of an automotive product, running more than 6 years in production. To avoid mask-driven repeating defects on wafer causing yield loss and line stops for mask clean or repell, each type of mask degradation over time like haze, contamination, defects, pellicle- or ESD damage must be avoided. Each mask has to be in a perfect condition at the point of use. Rejects from the exposure tool due to contamination on backglas or pellicle need to be reduced to a minimum. To ensure this high quality in particular for the expensive high end masks, the following measures need to be implemented in a modern waferfab: - safe and clean mask storage under purge - safe and clean mask handling and cleaning - ESD protection in all mask handling and storing areas - well controlled environments for all mask related areas by installation of AMC filters within the air flow With respect to fab logistics and throughput a fast availability of the required mask for exposure, clear transparency of the mask status and location as well as short transportation times are essential requirements. The operator effort to handle the mask should be as low as possible, even in a multi-tool lithography landscape like at Infineon Dresden. Automated routines are the preferred solutions. The usage of the correct mask for the given lot, product and layer has to be ensured as well. Mask mix-up needs to be avoided. Reject of the mask from the exposure tool due to wrong barcode reading result during loading should be minimized. Within the Infineon Litho-tool landscape the correct loading of the required mask into the correct box-type ( Nikon or ASML ) with the correct orientation is a fundamental requirement to save time and confusions. Special secure requirements for secure mask logistics, e.g. authorized operator access and continuous mask location tracking, need to be established as well. In summary, high quality mask storage and logistic are essential premises to run an efficient and advanced logic waferfab. Driven by the request to purge haze critical masks in combination with the need to expand reticle storage capacity for the increased product starts and volume ramp Infineon Dresden looked for a new advanced mask storage system addressing most of the existing requirements, able to handle both, Nikon-boxes and SMIF-Pods, and applying XCDA purge. As no state of the art stocker system was available with such a configuration, DMS developed in close cooperation with MCRT GmbH a prototype storage system adapted to the Infineon application needs.

4 3. Mask storage under purge 3.1. Haze experiences and purge requirements With introduction of 193nm exposure and halftone phase shift masks 10 years ago Infineon Dresden observed some mask degradation issues at a very early state and adjusted the mask requalification strategy focused on masks with increased haze risk. An intensive investigation and analysis was started together with our mask vendor Toppan photomasks to understand the root cause and haze mechanism for the common haze type of ammonia sulfate. The following main contributors / observations were identified, resulting in immediately installed containment actions: 1. Mask Cleaning Process Sulfate and amine ion residues on mask surface seeds for ammoniasulfate formation, crystal growth under exposure energy at 248nm /193nm wavelength, attenuated half-tone phase shift masks with high clear field area preferred affected Actions: implementation of low-sulfate and sulfate-free cleans by the mask vendor implementation of contamination specification for sulfate and amine ion levels on the mask surfaces after final clean 2. Pellicle Outgassing Pellicle, adhesive, glue and pellicle-frame may serve as supplier for chemical components Main impact: outgassing of membrane, glue and pellicle frame after aluminium eloxation Actions: stop usage of high outgassing pellicle types repells of affected masks usage of pellicles with low outgassing membranes and frames only 3. Fab environment Airborne Molecular Contamination Airborne sulfate or amine ions may adsorb or diffuse on mask surface chemical reaction with ion residues on mask surface under exposure at 248nm /193nm Longterm mask storage test with witness-control plates by IFD confirmed this assumption Actions: installation of AMC filters for make-up air of exposure tools and over each mask handling-, inspection- and storage area and stockers installation of sulfate-filtration for air inlet to Infineon cleanroom weather monitoring for outside sulfate level including alarms mask storage under purge recommended Purge tests with haze-risky masks have been executed by Infineon Dresden using both media N 2 and XCDA at very low humidity and different flow parameter. It was shown, that mask lifetime until haze degradation was doubled. A mask with ion residues due to insufficient cleaning process or outgassing pellicle material degrades after storage under purge as well, but clearly later than usual.

5 It has been verified, that a nitrogen purge is not applicable for masks with 6,3mm pellicle stand off due to the large pellicle-bow up to 1mm height, which is generated by the Osmose-effect after taking off the mask from purge media and exposing with ambient air. Even after some hours recovery and usage of vented pellicles the pellicle-bow was clearly visible with the risk of pellicle damage while loading the mask into the exposure tool. Furthermore a peridoc pellicle bowing effect generates a mechanical stress on the interface between pellicle membrane and frame with the increased risk of pellicle damage under common mask handlings and cleaning procedures within the fab. Masks, that have been purged with XCDA showed the same good haze resistance like nitrogen-purged masks but without any pellicle bowing effect. For this reason and as the N 2 Purge might be a health risk for operators, Infineon Dresden decided to order a mask storage system with XCDA purge Advanced Mask Storage at IFD New Features / Advantages The system DMS M Semi Bare Mask Storage System is a compact mask storage system with low footprint divided into 3 main areas : reticle storage carousel triple carousel handling and transfer area buffer area with loadports for 8 Nikon-boxes and 1 SMIF-RSP It has an overall capacity of 3038 masks. The whole system is completely purged with XCDA protecting the stored masks from airborne contamination and haze. Purching such a huge volume with stable conditions even under high-frequent usage was a big challenge and no common solution was available at that time. The masks are stored in semi-bares within the carousels, therefore the pellicle is well protected by the semi-bare cover against mechanical damage. The backglas side is less prone to particles due to face-down storing. One advantage of mask storing in semi-bares instead of bare mask storage is the possibility of a manual removal of a required mask within the semi-bare modul in case of stocker down without risk of pellicle or backside damage. The complete mask handling is done automatically by 2 robot systems to protect masks from contamination and damage : box handling in buffer area by Bosch linear robot mask and semi-bare handling in handling and storage area by Kuka robot The robot handling flows, gripper positions and teaching points were fine adjusted, the sensor concept was optimized with respect to Infineon Dresden needs. Mask insert is done via Nikon or SMIF loadport. Afterwards the mask is removed from the Nikonbox and transferred onto the barcode-reading and rotation station, were the mask is identified via barcode and orientated correctly. On the semi-bare station the inserted bare mask is placed face and pellicle down into a prepared empty semi- bare holder and transferred via Kuka robot into a free storage slot in the carousel. Empty Nikon-boxes are stored within the buffer waiting for another mask to unload. All prepared masks for unload are stored in the buffer within the appropriate ordered case as well. Empty semi-bares are always stored within the storage carousel, to keep them clean and purged.

6 Mask unload follows the backward procedure and can be activated either directly from the stocker GUI or instructed from the exposure tool to pre-store the mask into the Nikon / SMIF-box within the buffer until unload command. Each mask has an unique identification within the storage system, based on the tool-type barcode red at the insert step, whereby reticles are always stored in the stocker with the same orientation (ASML). During unload the mask will be orientated correctly depending on the ordering litho-tool with the suitable box type. Masks, unloaded into Nikon boxes, are always rotated 180. The stocker is remote controlled via icada RSM, production interactions are activated via web-based user interface, whereat service interactions are started directly from the stocker GUI ( features and advantages see chapter 4.1.). One big improvement is the application of RFID-technique for box and mask identification instead of box labels (see chapter 4.2.) Buffer for full and empty Nikon and SMIF Pods Load Ports RFID reader Figure 1 : DMS M Semi Bare Full Automated Mask Storage System Front side with load ports and buffer system

7 Figure 2 : Loadports for Nikon boxes (left side) and SMIF-pod (right side) Kuka Robot for mask and semi-bare handling Mask stored face/pellicle down in a semi-bare Figure 3 : DMS M Semi Bare Full Automated Mask Storage System Back-side-view with Kuka robot and storage carousel with semi-bares

8 Quad Ion Bars Nikon Port Opener Barcode Reading & Rotation Station Semi-Bare Station Figure 4: DMS M Handling area with Kuka robot SMIF Port Opener 3.3. Airflow and monitoring concept As pointed out before, the demand to purge such a huge volume and complex system including all internal installations and during dynamic processing is a very challenging mission. The system should safely and reliable protect the stored and handled masks from haze degradation particle contamination mechanical damage ESD damage Within the storage carousel and the handling area the ISO class 2 is specified to protect the handled and stored bare masks from particle contamination. Within the buffer area the requirements for ISO class 3 must be fulfilled. To avoid adsorption of airborne molecules during mask storage with the high risk of haze formation at mask exposure the mask surfaces should be protected reliable against AMC by purging with extreme clean dry air ( XCDA). It is required, that each mask within the carousel should get the same constant condition for airflow, humidity, temperature and particle protection. The purging air needs to be free of particles and chemical contaminants like amines, sulfates and organica. Extremely low relative humidity << 10 % within the storage and handling area is requested in order to avoid chemical reactions catalyzed by humidity on the mask surface.

9 The airflow over each mask surface needs to be well controlled and limited to maximum 1m/s to avoid electrostatic charging under such dry conditions, even under handling movements. Furthermore the temperature within the whole system has to stay stable at 23 +/-0,5 C. To avoid particle penetration from outside, an overpressure of the system to the surrounding cleanroom is necessary. The handling area and loadports need to be sucked for particles. All these complex and advanced requirements were discussed and implemented by the supplier DMS in combination with the expertise from MCRT GmbH. According to their comprehensive know how in the field of cleanroom technology and minienvironment solutions [2] a complex airflow system including monitoring concept has been developed, installed and adopted in field successfully. Figure 5 : DMS M Airflow scheme and monitoring points The system is equipped with a complete air circulation by continuous addition of 10% fresh air filtered for AMC at air inlet. The circulation air is streaming upwards outside the storage carousel and from top of the carousel downwards and lateral over each mask, than extracted to the outlet air drain. The handling area, loadports and buffer area are purged similarly. Requested low humidity is realized by two redundant drying systems of Munters type, installed in the basement.

10 Filterfan units in combination with intelligent design of the storage carousel facilitate a constant air flow within the storage carousel and overpressure to the cleanroom. Cooling systems within the air circulation channels were installed to reach the requested temperature level. After some configuration modifications and flow adjustments the following parameters could be realized within the storage system fulfilling the specifications finally : climate : RH (relative humidity): < 5% (<<10%) temperature : 23+/-0,5 C differential pressure : stocker & handlings area to fab: 3 Pa buffer to fab: 0,5 Pa air flow velocity : < 1m/s The system is continuously monitored for the main parameters humidity, temperature, particle contamination and AMC ( NH3, SO2) implemented into the Infineon Dresden parameter control system (space system). In case of deviations alarms are triggered from the icada RSM and space. Over the last two years the system performed very stable without any critical excursions. Figure 6 : AMC and particle monitoring within the mask storage system

11 Figure 7 : Space-chart for amine- and sulfate-monitoring Monitoring points: AMC2 carousel-stocker & AMC3 outlet air stocker Figure 8: Space chart for particle-monitoring, testpoint inlet-air stocker Particle excursion due to damaged fan-unit stable performance after exchange of fan-unit

12 Figure 9 : Humidity and temperature monitoring data, 4 weeks end of 2011 charts from RSM process data monitoring 3.4. ESD Protection Dry air and low humidity environment create an increased risk for electrostatic charging / discharging for the stored and handled masks [3], [4]. Therefore a crucial risk assessment for all materials used within the storage system and potential ESD hazards was done, supported and carried out by the qualified expertise from Estion GmbH. Electrostatic charging was measured at the potential hotspots for static and dynamic conditions. Based on this ESD assessment some issues were detected, which started some improvements. Materials within the buffer and loadport area were changed to antistatic-type materials. Besides to the aero bars on top of the storage carousel and at the vertical walls of the interface between storage and handling area, additional Quad ion bars were implemented above the semi bare station and the Nikon / SMIF loadports, where the masks are moving faster and the air velocity over the mask is at a higher level. The airflow conditioning of the system (FFUs, exhaust, flow amount) in combination with the dynamic motion sequences of the Kuka has been optimized (speed, flows) to achieve the recommended airflow limit of maximum 1m/s at any mask position within the storage carousel and while handling into boxes. The measured discharging times are well in specification finally, as well in the storage area (carousel top, middle, down ) and on the handlings modules ( semi-bare station, rotation station, loadports ).

13 3.5. Lessons learned with installation driving system optimizations As this new stocker system is a prototype with very complex configuration and requirements various experiences and learnings were done with installation and initiation of this new system: The airflow concept and differential pressure distribution was completely readjusted. To reach the required low humidity level within the tool the sealing performance of the air channels of the drying systems in the basement as well as the sealing of the covers and doors were improved significantly. Three additional cooling units were installed within the circulating air channels to achieve the required temperature of 23 +/-0,5 C constantly. The exhaust of the handling area below the loadports was boosted to ensure ISO class 2 specification. The hole storage carousel and semi-bares were cleaned up and preconditioned by purging before loading the system with masks. Some constructive improvements were adopted to the loadport design, semi-bare design, Kuka-gripper and sensoric concept. The motion sequences of the Kuka and Bosch robots were optimized with respect to speed and safety. Workflows were paralleled to reduce load and unload times. Some more problems were identified and solved within the first months of production on hardware and on software side. Finally the system meets all specifications and is working very stable and reliable for more than two years now, even under high mask loading and high-frequent daily usage. 4. Efficient mask logistics 4.1. New software and logistics solutions Due to the complex fab requirements as a result of the litho-tool mix with Canon and ASML steppers and scanners the new storage system must be able to handle both Nikon and SMIF pods in parallel and interact with different barcode types, pellicles and mask orientations. Mismatch of mask and mask ID must be avoided, clear transparency of each mask location and status is necessary for a fast and secure access. The system is integrated via icada RSM and remote controlled. Many advanced features have been developed by the experts from DMS and icada during the implementation of the new system. Main procedures, like unload to buffer or loadport are started via the icada host with a Web-based interface. Automated box selection and correct mask orientation in the box belonging to the ordering exposure tool was implemented. Reticles are always stored with the same orientation based on the barcode location. Depending on the ordering exposure tool and box type the mask is oriented correctly into the reticle box. Reticles can be assigned to specific storage areas in the stocker library due to storage groups.

14 Advanced lot management by the operator and fast access to the required mask is possible through prereservation, unloading of the next required masks in advance into the buffer and fast unload from buffer at point of use. The ordering of the requested mask can be done directly from the exposure tool PC via RSM-Web interface saving the operator time. Short mask unload times from buffer <20 sec are typical. Parallelization of workflows for mask insert, unload and reservation with priority for unload was implemented as well as intelligent parallel workflows of the two robots. Event based single slot control and single mask tracking is realized. Secure-mask requirements like complete event-based location tracking, authorized user access, alarm management and some secure-logistics simplifications are fulfilled. The RSM Web interface is designed operatorfriendly. History tracking and monitoring of the main stocker parameters humidity and temperature are managed as process variables in RSM, including notifications in case of specification violations. Based on the semi-bare storage and buffer concept the mask is decoupled from the box. Multi-use of reticle boxes is possible saving numbers and costs for reticle boxes significantly. In combination with the event-based tracking of each mask- and box usage and location within the stocker, exposure tools and inspection tools, an intelligent counter and forecast system could be integrated to manage mask inspection and box cleaning in an efficient manner. Figure 10: RSM Web interface, Remote control of DMS reticle stocker

15 Figure 11 : Workflow Request reticle from stocker Left :reticle selection, Right : exposure- tool Selection Figure 12 : RSM stocker interface, unload from buffer presenting mask-id and loadport status

16 4.2. RFID Box logistics The identification of each single mask is done by barcode reading on the barcode reading and rotation station as 1 st step of the mask insertion procedure. Under this unique identification, named tool-type barcode, the mask is stored in the stocker and the RSM database. With removal of the mask from the reticle-case they are decoupled and the empty box is stored within the buffer until usage for another mask unload. By this solution without dedicated mask-to-box mapping a cost saving multi-box usage with a relatively low stock of reticle cases is possible. The challenge was the technical realization of a fast and clean method providing a correct correlation between mask and box with each unload procedure from the stocker and a good visualization of the mask ID at each application step. Labeling the reticle cases with printed barcode of the inserted mask was the common method for older bare stockers. This method includes the risk of particle generation as well as a high risk for mask permutations. For this new storage system a new application of RFID identification has been developed and implemented in cooperation between experts from Infineon, DMS and icada. The used Nikon and SMIF boxes are equipped with small RFIDs having a unique number to mark and localize each box within the buffer. With each unload of a mask from the stocker into an empty box the mask barcode delivered from the RSM database is written into the box-rfid at the loadport before take off, with each loading of a new mask the RFID content is overwritten. From this time on the mask is strongly dedicated to this reticle box until next insertion into the stocker. The mask barcode from the RFID is readable at each point of time during the use and shown immediately - all exposure tools, stockers and mask inspections tools are equipped with RFID readers. This labelfree RFID solution gives an advantage for clean and fast mask logistics with low operator effort in the fab. It is working very functional and reliable for many years now. RFID Canon stepper Figure 13: RFID Reader exposure tool Nikonbox marker and RFID-tag

17 5. Experiences / Outlook Storage of haze-critical masks under XCDA purge avoids mask degradation and haze, ensuring high mask quality and increased life times Up to now masks that are stored under XCDA purge conditions are not affected by haze issues Airflow, humidity and temperature are well controlled and stable over long usage times even under high frequent mask exchanges within such a huge purged system No issues with ESD or particle contamination after three years of stocker usage observed No long term degradation effects of the stored masks, pellicles, adhesive or back glass, induced by dry-air purge, were observed Robots, drying and cooling systems performing stable, including the monitoring systems RFID logistics with stocker and icada RSM was implemented and works reliable Mask IDs at exposure tools, chosen boxes and orientations were always correct Outlook: The full automated mask transport from the stocker to the exposure tool is in preparation to reduce operator effort and time for mask handling. A load port is already prepared, the transportation concept has been developed and the tests with the designed transportation boxes will be started soon. 6. Summary The installation and implementation of a new prototype of purged storage system was an interesting but also challenging task for all involved parties: tool vendor, sub-contractor and customer. Many optimizations and adjustments were applied during the installation and evaluation phase. Finally, all requirements and specifications were fulfilled and the stocker system is working stable with a high mask load under daily business conditions for more than three years now. The purge system is running robustly under continuous monitoring. The new storage system leads to a safe and clean storing of the sensitive photomasks, preventing them from haze and contamination by applying XCDA purge with very low humidity, elongating the mask lifetimes at a high quality. The masks are well protected from mechanical damage by automated handling robots and safe storing conditions within semi-bares. Intelligent logistic procedures, using stocker integration via icada RSM and RFID box handling system enable fast and precise access to the required masks for exposure as well as efficient mask quality management procedures.

18 Acknowledgement The authors gratefully thank Mr. Udo Widmann from DMS-Dynamic Micro Systems and Dr. Michael Rüb, Mr. Peter Stark from MCRT GmbH for installation, evaluation and optimization of the new system until production release. Many thanks to the very committed and constructive work from the icada counterparts Mr. Tobias Ferber, Mr. Alexander Zschach and Mr. Peter Schnebel for implementing efficient workflows and software features. Another big thank to Mr. Thomas Sebald from Estion GmbH for the highly qualified ESD consulting and assessment. Last, we warmly thank our colleagues from Infineon Cleanroom Service Mr. Christoph Hocke and Mrs. Tina- Maria Kaden for supporting us during tool acceptance and implementation of the monitoring system as well as our Infineon litho experts Mr. Rainer Husse and Mr. Steffen Habel for their excellent support for system hookup, installation and service support. References [1] K. Bhattacharyya, M.Eickhoff, Mark Ma D.D.S, S.Pas A reticle quality management strategy in wafer fabs addressing progressive mask defect growth problem at low k1 lithography, Proc. of SPIE 5853 (2005) [2] M.Dobler, M.Rüb, T.Billen, Minienvironment solutions : special concepts for masksystems Proc. of SPIE 7985 (2011), EMCL2011 [3] G.Rider, Protection of Reticles Against Damage from Field-Induced Electrostatic Discharge [4] T.Sebald, Don t kill Canaries!: introducing a new test-device to assess the electrostatic risk-potential to photomasks, PMJ 2008, 712