Given the impracticality of using full-size masks for many analytical instruments due to their size and cost constraints, a critical first step towards advancing novel metrology for mask degradation was to develop and validate a dicing procedure suitable for (EUV-)exposed samples. Small markers were engraved prior to the dicing to facilitate precise navigation to locations of interest in the metrology tools, and allow correlation of metrology results with both each other and the EUV dose. Our investigation revealed no discernible changes induced by the dicing process, as confirmed by light microscopy, XPS and AFM analysis. Two samples were used in this study: a multilayer blank and an absorber blank. Once these blanks were diced, novel methods for mask degradation were tested on thermally degraded mask samples: IR-AFM for detection of (near-)surface morphological and chemical changes. XPS-depth profiling with Al Kα (conventional) and Ag Lα (hard) x-ray sources was used to study the sub-surface and the multilayer below the absorber. We show that IR-AFM analysis produces clear signals, but data interpretation is challenging, and its sensitivity to degradation seems limited, possibly as a result of hydrocarbon contamination. For hard XPS (HAXPES), we found that the signal-to-noise ratio in our instrument is too low to detect the changes induced by thermal annealing. XPS depth profiling on the other hand has very clear potential. Upon annealing, it revealed several changes within the sample, both close to the surface and deeper in the samples.
A parallelism is reported between reticle lifetime experiments undertaken on TNO’s EBL2 platform and wafer printing on the ASML NXE EUV scanner installed at imec. EBL2 mimics reticle impact due to exposure of ten thousand wafers in NXE representative conditions in less than a day. In-situ X-ray Photoelectron Spectroscopy (XPS) has shown that a local high-dose EUV exposure removes surface carbon and reduces ruthenium oxide to ruthenium. These effects not only happen at the directly exposed location, but equally centimeters away. Repeating XPS after a period of reticle storage outside of the vacuum, revealed regrowth of such contamination layer and re-oxidation of ruthenium. This learning based on EBL2 explains a small but significant trend noticed in critical dimension measurement results on wafer through a batch of wafers exposed on NXE, depending on the prior storage conditions of the reticle. During first exposures following reticle entry into vacuum reticle storage effects become gradually undone. Both storage-induced mask contamination effects are shown to build-up beyond one month. Local effects of the high-dose EUV exposure remain measurable by EUV reflectometry after several weeks of storage in air.
The EUV BeamLine 2 (EBL2) is being used to expose samples to EUV radiation for optics and mask lifetime testing. Before and after exposure the samples can be analyzed in-situ by X-ray photoelectron spectroscopy (XPS). During exposure the samples can be monitored in real-time by an imaging ellipsometer. We report on the development of two additional real-time diagnostic systems that further extend the capabilities of the EBL2 system, a thermal imaging system and an EUV reflectometer. The thermal imaging system monitors the sample surface radiation during accelerated lifetime tests and the EUV reflectometer is able to monitor the sample reflectivity in real-time. These diagnostics systems will allow for a more efficient use of the EBL2 beam-time and therefore speed up the development of EUV optics suitable for high source power and high NA imaging.
Adoption of EUV lithography for high-volume production is accelerating. TNO has been involved in lifetime studies from the beginning of the EUV alpha demo tools. One of the facilities for these studies is the EUV Beam Line (EBL1) designed and installed at TNO, in close cooperation with Carl Zeiss. There was a desire to improve on the performance of EBL1 in terms of source power and intensity, and in handling of full size EUV photomasks. For this purpose TNO has invested in the realization of a second EUV Beam Line: EBL2. EBL2 makes use of a tin fueled (USHIO) source in order to have a similar pulse length, shape and spectrum as an EUV scanner of ASML. Samples can be exposed to various doses/intensities of EUV light. Various process gasses can be introduced in a broad range of partial pressures and also sample temperature can be controlled. In-situ ellipsometry and in-situ X-ray Photoelectric Spectroscopy (XPS) is available to track surface changes/modifications. In this presentation we will discuss the capabilities of this unique research facility which is open for external customers studying the influence of EUV radiation on mirrors, sensors, fiducials, pellicles and EUV photomasks. We will discuss in this presentation parts of the validation studies and the experience we gained over the past year by running the setup for external customers.
TNO has built EBL2, an EUV exposure facility equipped with an in vacuo X-ray photoelectron spectroscopy setup (XPS) and an in-situ ellipsometer. EBL2 enables lifetime testing of EUV optics, photomasks, pellicles and related components under development in relevant EUV scanner and source conditions, which was previously not available to industry. This lifetime testing can help the industry to prepare for high volume production using EUV lithography by bringing forward information about material behavior which facilitates the development cycle. This paper describes an EUV photomask lifetime test performed at EBL2. The mask was exposed to different EUV doses under a controlled gas and temperature environment. To investigate how EUV light interacts with the mask, various analysis techniques were applied before and after EUV exposure. In-situ XPS was used to investigate elemental compositions of the mask surface. An ex-situ critical dimension scanning electron microscope (CD-SEM) and an atomic force microscope (AFM) were used to explore the impact of EUV light on critical dimensions (CD) and feature profiles. In addition, EUV reflectometry (EUVR) was used to investigate the change of reflectivity after EUV exposures. The exposure conditions are reported, as well as an analysis of the effects observed.
Novel absorber materials are being developed to improve EUV-reticle imaging performance for the next generations of EUV lithography tools. TNO, together with ASML, has developed a compatibility assessment for novel absorber materials, which addresses the risk that exposure of incompatible materials to EUV-radiation and EUV-plasma conditions results in contamination of the optics in the EUV lithography tools. The assessment is divided in two stages to optimize the efficiency of the procedure. Most contamination risks can be addressed cost-efficiently in the first stage with existing vacuum and plasma test facilities. Novel absorber materials can thus be assessed in an early stage of their development without the immediate need for more expensive EUV testing. This stage of the compatibility assessment was executed with an EUV reticle piece with a TaN-based absorber, and results are presented. The TaN-based absorber showed no compatibility issues, as expected. This test procedure now sets the baseline for testing novel absorber materials. 96.000 exposures can be performed in a NXE 3400 EUV lithography tool with a 300W source with absorber materials that successfully passed the first stage of the compatibility assessment. Assuming 96 exposures per wafer, this equals 1000 wafers. Absorber materials that passed the first stage may proceed to the second stage: an accelerated EUV test exposure in the EUV Beamline 2 (EBL2). Each material will be exposed to an EUV-dose equivalent to about half a year of reticle exposure in the NXE 3400 lithography tool with a 300W source. This test is in preparation and expected to be available in the second quarter of 2019.
EBL2 is TNO's platform for EUV exposure testing and surface analysis. EBL2 is capable of generating conditions relevant to EUV mask operation at all foreseen source power nodes. The authors describe how TNO performs a customized (accelerated) lifetime test on EUV masks. The required gas, EUV, and thermal parameters will be considered, and related to simulated and measured performance of EBL2. This approach can also be applied to EUV pellicles and optics.
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