Tin vapor control for better EUV collection efficiency. He said ionic debris can be controlled via magnetic field, and proposed controlling neutral debris with laser resonant ionization of Sn.
Scaling of lasers to high power will need 25 kW CO2 laser modules. One of the toughest challenges in developing such lasers is windows, although diamond windows may be the answer.
Archive for October, 2012
The 2012 Source Workshop was held Oct. 8-11 in Dublin, Ireland, in the Clinton Auditorium on the campus of University College Dublin. This is the industry’s largest annual gathering of EUV and soft X-ray source experts, who took the opportunity to discuss the latest results from their labs.
A keynote talk was given by Akira Endo of Waseda University and the HiLASE project. He focused on identifying technology areas that need immediate development to enable current sources of 100 to 250 W. These areas include droplet generation at 150 kHz via electrostatic acceleration; 500 W solid state lasers with picosecond pulses and mJ energy for pre- pulse; and the ability to focus on 10 µm droplets. He also outlined a roadmap for 1000 W source at 13.5 nm and 6.x nm.
Dr. Endo also identified other important focus areas, including:
Vadim Banine of ASML (the EUVL scanner-maker that recently acquired EUV source supplier Cymer) outlined the state of source technology and a path to 1000 W sources. He also listed top areas that need R&D work to enable power scaling for sources utilizing discharge-produced plasma (DPP) and laser-produced plasma (LPP). Describing the current status of tin-based LPP and DPP sources, Dr. Banine said Sn LPP has demonstrated 50 W average power at 80% duty cycle, along with scaling to 158 W at 3% duty cycle.
As for Sn DPP, Jeroen Jonkers of Ushio said 74 W power is possible today at intermediate focus (IF) in burst mode for a one-hour run. Dr. Jonkers elaborated on this data and presented a development area that may allow DPP to scale to 250 W. Konstantin, et al. presented results on ISAN’s new DPP design to potentially scale up to even higher power than Ushio’s design. Such concepts need to be further investigated to enable power scaling of DPP-based sources.
Although the roadmaps for plasma sources are rather clear, we know that the goals for 1000 W scaling of EUV sources are not easily attainable. After all, suppliers are still working hard to ready 100 W sources with reliable performance.
It was noted in various presentations that scaling of power for beyond EUV (BEUV) sources may be even harder. The potential of free electron laser (FEL) based sources for delivering 1.7 kW of BEUV photons at 6.x nm was discussed in a paper by Diana Tuerke of Carl Zeiss. Source design was presented for a 3 MHz, 1.7 kW, and facility costing 200 M Euro, with annual operational cost of 20 M Euro.
Coherent sources are typically not used for lithography due to large loss in the process of making the beam incoherent. However, it was very interesting to hear Zeiss mention that they developed an invention allowing them to use all coherent light without loss! This was exciting news indeed, as it may further open doors to the feasibility of coherent sources for lithography.
Highlights of workshop
Ulrich Mueller of Carl Zeiss presented the source requirements for mask defect AIMS tools. For their tools, high source stability is required : <0.3% for position and <3.5 % for energy in pulse to pulse. Sources will need brightness of > 30W/mm2sr with a target of 100/mm2sr. Currently they have sources of 8 W/mm2sr for tool development.
Klaus Bergmann of ILT showed the champion data for his Xe DPP source for metrology, with brightness of 21 W/mm2sr, operation frequency of 3.3 KHz, conversion efficiency (CE) of 0.35 and source radius of 155 mm for 20 kW input. He sees potential scaling to >50 W/mm2sr with a maximum limiting value of 71 W/mm2sr.
Steve Horne of Energetiq proposed a 100 W/mm2sr high frequency xenon Z-pinch DPP source for mask metrology. He thinks that the physics can be tested in six months – and if successful, the system can be built within 2 years. Cost of this new system would be similar to that of the present system in the field.
Paul Sheridan of NewLambda Technology described his LPP source as having a CE >1% at 45 degree viewing angle, and source size of 250 x 400 mm2 at intermediate focus (IF). For 1,000 hours of operation, he measured 80 W/mm2sr brightness. For his source, at stability at IF is 7% in position and 8% in size.
Larissa Juschkin of RWTH presented theoretical calculations for estimating source brightness requirements for EUV Microscopes.
Sergey Zakharov of EPPRA revealed a "plasma lens" design for the capillary discharge Xe DPP source. The workings of this discharge produced focused EUV beam had been the subject of speculation in the past, and Sergey finally described it for us!
Igor Makhotkin from FOM Institute DIFFER provided the results of using BEUV optics to support EUV lithography at 6.x nm. Reflectivity for LaN/B based multilayer mirrors, the material of choice for BEUV optics, was reported to be 53.6% for normal incidence and 175 periods of multilayers. So far, this is the highest experimental value reported for these mirrors.
Leonid Sjmaenok of PhysTeX presented Zr filters with one pass transmission of 84% for 25 nm thickness and 80 mm aperture frames. Such filters, he noted, now have no more than a 2 degree max deviation from flatness. These filters will be key elements key to controlling out of band radiation and debris in sources.
For BEUV lithography sources, Takeshi Higashiguchi of Utsunomiya University proposed a mixed complex target of Gd and Tb for 6.x nm photons. He proposed punch out targets (mist) for Gd, as droplet generation is very difficult due to the high melting point of Gd. He also suggested phosphorus as a candidate material for BEUV sources.
Soft X-ray (SXR) sources
The Workshop has been successful in bringing together a large gathering of source experts by inviting technologists from the EUV (13.5 nm), BEUV (6.x nm) and Soft X-ray (~ 1- 50 nm) regions. Due to a lack of funding for research on EUV sources (despite its being the #1 issue in EUVL), many source experts now work on non-lithography applications of EUV and SXR sources. In widening the scope of the Workshop, we were able to attract EUV source experts who could give us good insights on EUV source development, even though their work may not be focused on lithography.
Two of the keynote presenters for the Workshop were global leaders in soft X-ray source technology. They focused their talks on SXR sources and their potential for non-lithographic applications. Prof. Jorge Rocca of Colorado State University talked about desktop EUV laser and its applications. Prof. Alan Michette of King’s College London discussed biological applications of soft X-ray sources.
The Workshop also had many other excellent, oral presentations and poster sessions on SXR sources and their applications.
M. Selin of KTH Royal Institute revealed a high-brightness liquid-jet laser-plasma source that enables 10 second exposure for water-window cryo microscopy. He claimed that its brightness of 1.5 x 1012 photons/ (second x mm2 x mrad2 x line), is the highest operating in a lab today. He said the 10s exposures that are now possible with the new system make this microscope comparable to microscopy based on early synchrotron sources.
James Evans of Pacific Northwest National Lab and University of California at Davis presented "Whole Cell Cryogenic Soft X-ray Tomography" with a laboratory light source from Energetiq. He pointed out that soft X-ray tomography of whole cells is now available commercially and said he is working on improved zone plates to get better resolution. A new standalone, higher brightness non- plasma source with a small footprint is planned. I think such non-plasma based sources may have potential EUVL applications, and I plan to investigate the feasibility of such sources.
The 2012 Source Workshop succeeded in its objective to bring together more than 80 source R&D experts for discussions and updates. We came away with a list of topics that need focus for scaling sources for current and future generation technology. The virtual lack of sales pitches may have induced participants to let down their guard a bit among colleagues to acknowledge the problems we still face today, while celebrating the progress made since last year’s Workshop in Dublin. The proceedings of this workshop are available for download at www.euvlitho.com. If your business is EUV or SXR sources, you won’t want to miss our next Source Workshop on Nov. 4-7, 2013, in Dublin.
In order to bring EUVL scanners into high volume manufacturing (HVM) of computer chips, its throughput of 10 wafers per hour (WPH) needs to increase. That brings up three questions: how much do we need to increase the current throughput for HVM insertion, what needs to be done to increase throughput, and how quickly can this increase be achieved?
Throughput of EUVL scanner for HVM insertion
Imaging by EUVL scanner offers a higher k1 value than is available from 193 immersion (193i) based lithography. A higher k1 value results in better imaging and lower lithography process complexity, hence the attraction of EUVL as an optical projection lithography. Today, 193i scanners are used in a double pattering process to print the smallest features needed in HVM. Toward 14 nm and smaller nodes, if EUVL is not ready, chipmakers will need to use quadruple patterning with 193i scanners, combined with increased optical proximity correction (OPC) and design rules restrictions to print increasingly smaller features. This is not an attractive option for chipmakers, hence their increasing emphasis on EUVL readiness. As manufacturers evaluate available technology, switching from double patterning-based 193i to EUVL, throughput is most often mentioned as the criterion for evaluation.
As printing of circuits is a sequential process, in double patterning (DP) the same wafer is exposed twice in a 193i scanner. Between the two exposures, there are many additional processing steps to enable the DP process. Hence, we need less than 50% throughput from an EUVL scanner (as compared to 193i scanners) to achieve a given feature size. In the case of quadruple pattering, an EUVL scanner needs less than 25% throughput to compare with an immersion scanner due to the four exposures. After accounting for the additional processes of deposition, etch, ash and metrology, the equivalent throughput of an EUVL scanner may become less than 40% and 20% to compete with double and quadruple patterning, respectively. Thus, to match the throughput of a 200 WPH 193i scanner for DP process , we need less than 80 WPH and 40 WPH from an EUVL scanner. This is an important point, as it’s often said in press that an EUVL scanner must reach the throughput of a 193i scanner to be considered equal. (Cost of Ownership wise, the Lithography team of the International Technology Roadmap for Semiconductors (ITRS) has already shown that EUVL is more cost-effective than 193i DP for next generation lithography (NGL) ).
How to increase EUVL scanner throughput
Of course, EUVL scanners still need to boost their throughput numbers from the current 10 WPH. For economic reasons, it’s best to have throughput as high as possible from an EUVL scanner. Although much focus is placed on sources for improving throughput, other things can be done to increase the productivity of an EUVL scanner.
To better understand the challenge, let’s start with a model that estimates throughput of an EUVL scanner for 1) a given source power, 2) scanner parameters, and 3) reflection/transmission efficiency of various components . EUVL scanners are not very efficient in transferring photons from source to wafer. Hence, in addition to increasing the number of photons available to the scanner, we can also work to increase its transmission. It is important to note the relationship of scanner throughput to scanner’s overhead time and resist sensitivity.  For example, for 50 W of source power at intermediate focus (IF), 20 mJ resist will allow 30 WPH while 10 mJ resist will allow 55 WPH. For a 10 mJ resist at 80 WPH, we need 115 W of power for 18 s overhead time, while for the 10 s overhead time we need only 50 W of power! 
There are additional factors that can help increase throughput. By decreasing the resist sensitivity to out-of-band radiation, the need for spectral purity filters may be eliminated. Reflection of mask as well as effective reflection of optics can be increased as well. Optical throughput of the NXE3300B is supposed to be 50% more than the NXE3100  so there is already progress in increasing scanner throughput.
EUV sources are a difficult challenge due to the inherent complexity of reliable and repeatable generation of high temperature plasma of 40 eV for a production environment. Current EUV source conversion efficiency (CE) is only 2 % (i.e., 2% of input energy is converted into EUV photons). Of these photons, only about 10% can be collected due to the limitations of collector optics, debris mitigation and spectral purity filters. We need improvement in each of these areas to enable higher power and increased throughput. CE of 5.5 % has been demonstrated recently, larger collectors are being developed and debris mitigation techniques will continue to improve – all allowing more photons to reach the wafer.
How higher scanner throughput can be achieved quickly
There is no magic bullet, so lots of innovative solutions are needed to lessen various loss factors to reach 100 W of source power. Beyond that, we may need different approaches to key source components such as fuel delivery. In meetings at the 2012 International Workshop on EUV and Soft X-Ray Sources (Dublin, Ireland, October 8-11), the largest annual gathering of EUV source experts, we can expect discussion on some of these key topics in EUV source development. The workshop will include:
· Several papers on how to increase CE of sources for both EUV and beyond BEUV ( 6.x nm) LPP sources
· New designs to allow higher power DPP sources
· Data on the latest SPF of up to 80% transmission, improved collector optics, and other topics
I look forward to seeing the latest results from the industry’s source experts and will report them on this site.
In summary, I expect a rather slow but steady increase in EUV source power, and I’m still on record as predicting enough throughput by 2014 to allow adoption of EUVL scanners for HVM by leading chipmakers.
1. Lithography Chapter, International Technology Roadmap for Semiconductors (2009).
2. Chapter 3, “EUV Source Technology,” in EUV Lithography, Vivek Bakshi (Editor), SPIE Press 2008, for discussion of a general throughput model for an EUVL scanner.
3. Rudy Peters, ASML presentation at the 2011 EUVL Symposium.