EUV pellicles guarding masks from defects and dynamic gas lock thin film windows are considered a corner stone for using EUVL in chip production. For providing such thin film products with certified features EUV actinic metrology tools are needed in the industrial supply chain. For best conservation of optical quality and throughput in the scanner, such EUV pellicles should have high transmission of > 90 % with a transmission uniformity of > 99.6 %, low reflectance of much less than 0.1 % and low scatter of EUV.
The specification for the ideal metrology tool is qualifying a full sized EUV pellicle with precision of better 0.1 % for transmission and 0.001% for reflection, the ability to quantify the scatter distribution. The measurements should be accomplished in acceptable process time and should neither add contaminate nor particulate to the product under test.
Within RI Research Instrument’s support of EUVL infrastructure by developing actinic metrology solutions, we have engaged in this task by applying our proprietary effective inband EUV metrology scheme described elsewhere.
While the first generation tool has been presented elsewhere, we now present general considerations on measurement process and tool architecture for the combined three tasks in the second generation tool. Full pellicle transmission and reflectance mapping are measured in less than 2 hours fully automatic process time (careful load-locking included). Measured transmission show precision and accuracy of better 0.1 % for a qualification pixel resolution of 300x300 µm2. Performance of the reflectance measurements is under investigation but expected to be in the range of 0.001 % precision demonstrated in proof of concept experiments in our application lab. The first generation tool is used by our customer in routine pellicle production and is constantly running with uptimes better than 95%.
With EUV lithography inserted into the semiconductor production the characterization and control of critical components like blanks, masks, pellicles, optical elements, thin film windows and photoresist is required. Needed for the supply and process chains are on one hand “traditional”, e.g. mechanical, thermal or chemical tests with extended capabilities as well as “actinic” ones, which are directly linked to the interaction of EUV radiation with the sample. Actinic tests are needed to
• measure optical properties in the spectral range which is relevant when the components are used, like reflectance, transmission, scatter
• determine or apply modifications of material when irradiated with EUV, e. g. resist exposures, degradation tests for components
• find or inspect defects and structures on optical components used in EUVL
• Mimic processes in the EUV scanner for off-line optimization.
For such actinic qualification in the individual industrial or research lab with 13.5 nm radiation only few metrology tools are readily available off-the-shelf. The industrial infrastructure for EUVL lab tools suffer from the fact that most of such tools are such specialized that only – at best – few of a kind are needed in the world, which does not deserve a classical product development.
This analysis has driven our strategy for supporting our customers mainly with tailored “one of a kind” solutions, while still reaching product like reliability by frequent reusing of already existing building blocks, tested subunits and components and general approaches and architectures.
The basis of this approach is on one hand our experience to serve our customers from “big science” worldwide with RI-engineered research installations since more than 30 year. On the other hand our EUV application experience is based on a broad source portfolio with stand-alone DPP and LPP sources for XUV together with an established supply chain in advanced EUV optics, detectors and spectral filtering.
This allows us to use long experience in design, machining and assembly of ultra-clean components in ultra-high vacuum and cleanroom applications, including ultra-clean sample handling, loading and manipulation. Our stand-alone tools offer fully automated, fail safe electronic control including custom tailored data evaluation and reporting solutions. With the special demand from each new project, we are consolidating and expanding the knowledge base.
Examples of such actinic tools for the EUVL infrastructure will be presented, as e.g. spectral reflectometers, full area inband EUV property mapping, EUV pellicle transmission and reflectance tools, microscope, defect inspection, nano-printing tools and sensitivity calibrated stand-alone resist exposers.
RI Research Instrument’s EUV pellicle transmission qualification tool EUV-PTT uses "effective inband EUV measurement" which is spectrally filtering emission of the EUV-Lamp to 2% bandwidth at 13.5 nm for measuring "as seen by the scanner". Images of about 20x20 mm<sup>2</sup> are recorded in less than 5 seconds. A full pellicle characterization with < 60 images taken is accomplished in less than one hour. The first prototype of the EUV-PTT is in production quality control since nearly one year at our customer and achieving targeted quality in measurement and cleanliness and meets uptime specifications. Amongst additional aspects to be addressed in quality control is the reflection from the pellicle, which - in worst case - may impair the printing performance in the scanner. Hence, recently, we have performed concept studies on applying the PTT technique to carbon nano tube pellicles and on measuring the reflectance of low reflectance samples are reported. The latter is heavily demanding as reflectances in the range of 0.02 % were measured with sensitivities and reproducibilities in the range of 0.002 %.
Spectral reflectometry is an established technique for actinic qualification of EUV mask and blanks. For production in the EUV scanner, however, the homogeneity of the effective inband property – e.g. reflectance - is more relevant than the spectral reflectance curves. The effective reflectance is usually extrapolated indirectly from the knowledge of R<sub>max</sub>, CWL<sub>50</sub> and FWHM, in which the efficiency is mainly proportional to R<sub>max</sub> if the spectral reflectance distribution and its central wavelength are similar to those of the scanner. This fact is accounted for in our AIMER<sup>TM</sup> metrology solution. In a single measurement AIMER registers the signal from an area of about 20×20 mm<sup>2</sup> with a typical 13.5×13.5 μm<sup>2</sup> pixel resolution. This technique is very robust and fast by irradiating the sample with a beam, which is spectrally filtered (e.g. from the Xenon EUV emission of a discharge source) to 2 % spectral bandwidth around the CWL of the scanner. With such an “inband EUV beam”, the efficiency of the sample is recorded with respect to its quality factors throughput and homogeneity “as seen by the scanner”. With multiple exposures stitched together AIMER maps the effective reflectance while being sensitive to the convoluted influence of the single parameter just as effective in scanner use. Mapping of the effective reflectance of a full EUV mask surface is accomplished in less than one hour. In this paper, we compare conventional ALS spectral reflectometry with AIMER results and show first tests of AIMER on structured EUV masks.
RI Research Instrument’s EUV pellicle transmission qualification tool EUV-PTT uses “effective inband EUV measurement” which is spectrally filtering emission of the EUV-Lamp to 2% bandwidth at 13.5 nm for measuring “as seen by the scanner”. Images of about 20*20 mm² are recorded in < 5 seconds. A full pellicle characterization with < 60 images taken is accomplished in less than one hour.
Recently, we have performed some studies on applying this technique to carbon nano tube pellicles and on measuring the reflectance of pellicles which will be reported. The latter is heavily demanding as reflectances in the range of 0.01 % were measured with sensitivities and reproducibilities in the range of 0.002 %.
The scene is set for EUV lithography high volume production. EUV masks with pellicles and scanners with dynamic gas lock thin film windows are considered a corner stone for insertion. For making the thin films usable in EUV scanners, EUV actinic metrology with high precision and accuracy is required to qualify them. One of the key features is the uniformity of the transmission at 13.5 nm through the thin film membrane to be qualified over the entire area. The specification for the required tool is qualifying a full sized EUV pellicle with an average EUV transmission of around 90 % to a uniformity of < 99.6 % – hence precision and accuracy of the process of below 0.1 % are demanded. Certainly, no particles should be added to the EUV pellicle by the qualification measurement. Within RI Research Instrument’s support of EUVL infrastructure by developing actinic metrology solutions, we have built a dedicated tool for EUV pellicle transmission characterization using the effective inband EUV scheme. The actinic measurement of the full pellicle is accomplished in less than 40 minutes. Vacuum loading and unloading consumes another 35 minutes due to the need of extreme care for such fragile samples and to avoid contamination. First images of full sized pellicles show precision and accuracy of better 0.1 % for a qualification pixel resolution of 300x300 μm<sup>2</sup>.
EUV-Lithography is preparing for high volume manufacturing. For actinic qualification of components for EUVL like mask blanks, masks, pellicles or optics, spectral resolved metrology (e.g. reflectometry) is frequently applied. With the EUV-AIME effective inband mapping reflectometry we demonstrated a proprietary new concept for full area qualification of mask blanks or masks. While spectral reflectance measurements provide unweighted information on peak reflectance (R<sub>max</sub>), central wavelength of the reflection curves (CWL<sub>50</sub>) and bandwidth (FWHM), AIMER combines those to an effective figure of merit “as effectively seen by the EUVL scanner”. Short measurement times are achieved because a single acquisition measures about 25*15 mm<sup>2</sup> in less than 5 seconds. A full (152 mm)<sup>2</sup> reticle map is acquired in less than one hour, including sample load and unload. AIMER provides intrinsic spatial resolution of (20μm)<sup>2</sup>, delivering sensitivity to larger defects or allowing for finding marks or small metrology pads on structured EUV masks. When compared to the typical 1 mm<sup>2</sup> spot measured in the spectral mode, AIMER excels in higher number of photons collected and such lower shot noise limits in reproducibility.
Most of the cutting edge EUV photoresist research and development is currently being performed at RD beamlines or pre-production scanners. However, during EUVL introduction into mass production, more economic stand-alone solutions at the suppliers are needed. Within the portfolio of actinic EUV tools we offer a variety of solutions for resist characterization (sensitivity, contrast, resolution, absorbance, degradation and stability). Together with our research partners we have building blocks for actinic EUV solutions operating at a wavelength of 13.5 nm. Our expertise is the realization of customized solutions for the industry ranging from proof of principle experiments/setups to full functional industrial tools for the quality assessment in production.