|
|
News and Highlights
The 2005 Spring Meeting commenced on Monday in San Francisco with a record 12 tutorials held during the day, both in the morning and in the afternoon. The topics were varied and ranged from silicon in large-area, flexible electronics, wafer level packaging, semiconductor heterojunctions to BioMEMS to carbon nanotube-based nanotechnology to hydrogen storage materials. A special symposium aimed at new and young researchers was also conducted covering Characterization Techniques for Thin-Film Solar Cells. Thin-film Silicon Materials and Devices for large-area, flexible electronics Semiconductors used for large-area electronics include hydrogenated amorphous silicon (a-Si:H) and nano- or polycrystalline silicon (nc-Si, poly-Si). The tutorial, instructed by Sigurd Wagner (Princeton Univ.) and João Pedro Conde (Instituto Superior Tecnico, Lisbon, Portugal), covered materials growth and preparation, basic material properties, device physics, and applications. The tutorial began with an overview of silicon thin film materials. The deposition and crystallization of Si films was then described and the point was made that amorphous and nanocrystalline silicon can be directly deposited on large-area substrates. The film properties of the different types of silicon, including a-Si:H, nc-Si, and poly-Si, were then discussed. Amorphous Si has low dark conductivity, high photo-to-dark conductivity ratio, is dopable and shows effects of metastability. Nc-Si includes crystalline grains with amorphous tissue and shows a complex structure that develops with increasing thickness. In poly-Si, transport is controlled by thermionic emission over the energy barriers at grain boundaries, and the material can thus have high electron and hole mobilities.
The instructors then discussed various devices including thin film transistors (TFTs). a-Si:H transistors are the current industrial workhorse devices. Poly-Si is making inroads for industrial applications. Nc-Si is still an experimental technology but with significant potential for the future. Next, various device applications were discussed including active matrix arrays and LCDs, p-i-n photodiodes and sensors, solar cells, MEMS, microfluidics, and bioengineering. Finally, flexible and conformally shaped electronics were discussed. The tutorial concluded with a discussion of the outlook for the use of Si in large-area electronics. Clearly there is a growing range of technology-ready silicon materials even though a-Si:H dominates currently. Processing at plastic-compatible temperatures is the next major step. There are now many demonstrations of new applications including the rapid growth of flkat panel displays and image sensor arrays. There is sufficient evidence to show that a new industry is taking shape around thin-film Si. Young Scientist Tutorial on Characterization Techniques for Thin-Film Solar Cells
On Monday afternoon, a group of young researchers introduced a new concept to MRS Meeting tutorials. Daniel Abou-Ras (ETH Zurich), Jennifer Heath ( Linfield College), Xiangxin Liu ( Univ. of Toledo), and Thorsten Rissom (Hahn-Meitner-Institute Berlin) designed their tutorial for young scientists with the mission of generating open discussion and questions among young researchers. The topic addressed in this inaugural endeavor was characterization techniques of thin-film solar cells, as part of Symposium F on thin-film compound semiconductor photovoltaics. Bill Shafarman ( Univ. of Delaware) gave a brief introduction. He went over the achievements and limitations of crystal Si for solar cells, then delineated the performance and manufacturing capabilities of different kinds of thin-film solar cells—a-Si and alloys, CdTe, and CuInSe 2 and alloys. His colleagues were then available to explain how specific characterization techniques were being used to address the issues raised through thin-film PVs. The group covered transmission electron microscopy, x-ray fine structure studies, admittance measurements, and Hall-effect measurements. The tutorial generated a lot of discussions during the session fulfilling one of its major goals. Recent advances in Chemical Mechanical Planarization Technology Chemical mechanical planarization (CMP) is a critical process in the semiconductor manufacturing industry. The goal of this tutorial, which formed part of symposium W, was to describe and discuss recent advances in the field. Mike Oliver gave an overview of the technology and discussed the drivers for the industry. He then presented advances in the field as viewed by the equipment suppliers and the consumables suppliers. He showed slides sent to him by the various suppliers. CMP is now established as a key semiconductor technology. The equipment continues to evolve to be reliable and reproducible. New CMP technology is being introduced at an increasing rate. Next, Tom Tucker (Laredo Technologies) overviewed equipment requirements for CMP. There has been significant progress in optimizing CMP and post-CMP cleaning equipment. Wafer uniformity and control of CMP process time is now possible. New technologies currently allow for full automation of many CMP processes with real-time control. S.V. Babu (Clarkson Univ.) then described recent developments in slurries for Cu/Ta planarization based on work by his research group. He addressed both scientific as well as engineering challenges. Duane Boning (MIT) discussed modeling of pattern dependencies in CMP. While current efforts are aimed at understanding and modeling the process, consumables, and equipment physics and dependencies in CMP, Boning focused on the planarization of patterns, which is a key CMP application. He discussed various pattern effects and models, as well as current CMP challenges. Finally, David Evans (Sharp Labs.) discussed CMP integration into the production process. He described the various CMP processes, the chemistry and abrasive properties involved, mechanical considerations, defect generation and cleaning, and finally process control. The tutorial represented a nice introduction and beginning to symposium W on CMP. Stresses at the Micro- and Nanoscale As part of symposium O, a tutorial on stresses at the micro- and nanoscale was conducted by Ralph Spolenak (ETH-Honggerberg, Switzerland) and Thomas Buchheit (Sandia National Lab.). The overall goal of the tutorial was to provide a basic understanding of the major techniques and tools for measurements of stresses and mechanical properties. Spolenak presented the first part on measuring residual stresses in thin films and small volumes. Stresses can only be measured indirectly. He discussed substrate curvature measurements as well as diffraction methods using X-rays, each of which has its own advantages and disadvantages. Spolenak also touched upon the bulge test and Raman spectroscopy, two other techniques for measuring stresses in thin films. Thomas Buchheit presented the second half of the tutorial focusing on nanoindentation testing for measuring mechanical properties of very small volumes. He led the audience through the fundamentals of the technique including the use of Berkovich and spherical indenter tips. He discussed the various types of instrumentation. Finally, he briefly touched upon other thin film testing methods including adhesion testing, scratch testing, and the "pull-tab" test. From basic scientific studies to drug discovery, neural probes, and tissue engineering, BioMEMS is finding its place in materials research. The tutorial "Making BioMEMS devices-Materials, Fabrication, Devices," held at the start of Symposium J focused on micro- and nanofabrication, as well as biocompatibility of tiny devices that interface with living tissue. With fabrication technology matching biological length scales, bioMEMS has moved into center stage. Kevin Turner (MIT) began the tutorial with an overview of the fabrication technologies typically used in the construction of bioMEMS. He started with traditional technologies such as surface and bulk micromachining of silicon, but quickly progressed to methods for fabricating polymer microsystems and patterning cellular materials, covering a range of additive and subtractive methods. While silicon micro- and nanofabrication is quite advanced, other alternatives offer optical transparency (glass) or cost savings (polymers). Rashid Bashir (Purdue University) covered the next part of the tutorial, highlighting devices. Numerous biosensors and microfluidic systems have been developed for a range of applications from protein and DNA detection to drug delivery. As an example, microcantilevers can act as sensors by bending or resonating to reflect chemical reactions on the surface or simply mass detection. The final section of the tutorial focused on selection of materials for bioMEMS and biocompatibility, covered by David A. LaVan (Yale University). This section gave an overview of the important characteristics of materials for applications in this area and reviewed recent efforts aimed at developing materials for sensor and tissue engineering applications.
Materials for Hydrogen Storage and Production If there is going to be a “hydrogen economy,” there needs to be hydrogen, and a way to store it. Such was the focus of Tutorial GG, Materials for Hydrogen Storage and Production. Gholam-Abbas Nazri (General Motors Research and Development Center) covered critical aspects of hydrogen storage, particularly focusing on chemical absorption of hydrogen for reversible systems, but also covering physical adsorption using porous materials and composite materials combining physical and chemical processes. He emphasized solid-state hydrogen storage materials including complex chemical hydrides, metal hydrides, carbonaceous materials, porous organic and inorganic systems, and composite and hybrid materials. The challenge is achieving high storage capacity, low weight, and rapid reversible capture and release of hydrogen, so that energy efficiency can be achieved and so consumers can tolerate refueling. Currently, most hydrogen is produced from fossil fuels and other organic matter, so another challenge is efficiently extracting the hydrogen rather than turning the starting materials directly into electricity. However, other techniques for obtaining hydrogen are being studied, such as electrolysis. The tutorial concluded with Ping Chen (National University of Singapore) presenting recent discoveries and new approaches in light metal-N-H complexes, binary nitrides and imides, ternary complexes, and other systems. Some of these materials showed promising storage capacities and kinetics, with plenty of room for optimization through catalysts, additives, and other parameters. Science and Applications of Carbon Nanotubes Carbon nanotubes (CNTs), discovered 1991 by Iijima (Japan), have changed materials research drastically and have had major impact on developing new nano-based technologies and research directions. Dr. Meyyappan, a world-renown expert on CNTs from NASA AMES research center presented a very stimulating tutorial lecture on carbon nanotubes on Monday afternoon. He started his lecture with an introduction of his new textbook entitled "Carbon nanotubes: Science and Applications", M. Meyyappan (CRC Press) that was published in 2004. Dr. Meyyappan continued with an introduction into the atomic structure of CNTs, and presented a summary of the extraordinary mechanical and electrical properties of CNTs. He pointed out that the extraordinary mechanical properties of CNTs are essentially due to the strong covalent C-C bonding and the seamless hexagonal architecture. CNTs can be produced using techniques such as laser ablation, CVD techniques using transition metal catalysts, as well as arc discharge methods. Meyyappan pointed out that today’s most critical experimental challenge in the field of CNTs is to achieve control over atomic-scale properties of CNTs, as well as characterization of CNTs. For example, a major research effort is to achieve full control over chirality (determination of (m,n) parameter) in experimental production of CNTs. This is because, today, none of the techniques provides full control over the type of CNTs that are synthesized. Further critical challenges are dispersion of CNTs in host materials (e.g. in the matrix material when used in composites), industrial scale production, development of specific applications, functionalization with probe molecules, as well as assembly and integration of CNTs into signal processing and devices. Even though very exciting scientific discoveries at the laboratory scale has been made, Meyyappan pointed out that a critical issue in making CNT-based technology available on the consumer market is development of seamless integration of nano-materials and nano-devices into systems at higher length and time scale. Finally, Meyyappan summarized the state of the art of experimental techniques and results related to CNTs, and provided an excellent overview of the key challenges scientists and engineers will face in the coming decades. Despite the grand challenges and many open questions, it is likely that CNT-based technologies will play an essential role in everyday life in the coming decades.
The day of symposium tutorials ended on Monday evening with a complimentary workshop on Proposal Writing for funding from U.S. government agencies. The National Science Foundation (NSF) and the National Institutes of Health (NIH) were represented at the workshop. The key message for attendees was twofold: to research the institutes’ Web sites for grant proposal guidelines and to discuss research ideas with a program director or manager before preparing and submitting the proposal. In this way, scientists can determine the best program within the institution where their research may get funded. W. Lance Haworth, executive officer of the Division of Materials Research, described the proposal process at NSF. John Bowers of the Center for Scientific Review provided an overview of the grant proposal process at NIH, using nanoscience and biomaterials topics as supported by BECON as an example. BECON is the NIH Bioengineering Consortium. Phillip B. Messersmith, associate professor at Northwestern University, followed with a presentation from his perspective as a Principal Investigator or co-investigator who has received nine NIH grants. Messersmith is a permanent member of the Biomaterials and Biointerfaces Study Section at NIH. The presenters gave some common points of what is essential in a proposal, such as illuminating why the project should be done and why the researcher submitting the proposal should specifically be funded for it. A particular challenge in preparing a proposal for NIH resides in making the connection between materials research and the medical field. Materials researchers were highly advised to obtain help from consultants and find collaborators to supplement the materials expertise with the medical. For NSF, proposals must provide a strong educational component along with the materials research idea. Web sites of Note:
The traditional student mixer was held in the evening. As always, the event was well attended and students as well as others were able to meet and network. Students represent an important constituency in MRS and this event was a testament to that.
|
|
