Stanford Nanofabrication Facility Research Articles

(Keynote) High-Performance Carbon Nanotube Transistors for Logic Platform

Low-dimensional 1D and 2D materials hold promise as candidate channel materials for highly scaled and high-performance transistors beyond the limits of silicon-based transistors. This talk will first motivate transistors built on low-dimensional channels due to the significant speed, energy efficiency, and transistor density benefits enabled by their electronic and physical properties. This talk will focus on 1D carbon nanotube (CNT) semiconductors and describe the advanced device component process modules that have been demonstrated, explaining both the fundamental mechanisms of operation and device-level performance objectives. Single-CNT FET experimental studies give insight into the quality of (1) low resistance N- and P- contacts down to 10 nm contact length, (2) high-capacitance gate dielectrics optimized for deposition on SP2 carbon surfaces, and (3) controlled N- and P- remote dielectric doping. To achieve high current density each transistor must contain multiple CNTs, therefore we will review state-of-the-art strategies to assemble densely aligned arrays of CNT with controlled CNT spacing between 2-10 nm and uniform electronic bandgap. The final portion of the talk will highlight recent advances from our team and others to integrate the best device components together to demonstrate high-performance carbon nanotube MOSFETs. We will elaborate on the remaining challenges and provide a summary of device design tradeoffs that will help guide future studies.We are grateful for support and collaboration from TSMC Corporate Research, DoD, Stanford SystemX Alliance, Stanford Nanofabrication Facility, Stanford Nano Shared Facility, and the University of California at San Diego.

Design of a Robust Tool for Deploying Large-Area Stretchable Sensor Networks from Microscale to Macroscale.

An investigation was conducted to develop an effective automated tool to deploy micro-fabricated stretchable networks of distributed sensors onto the surface of large structures at macroscale to create “smart” structures with embedded distributed sensor networks. Integrating a large network of distributed sensors with structures has been a major challenge in the design of so-called smart structures or devices for cyber-physical applications where a large amount of usage data from structures or devices can be generated for artificial intelligence applications. Indeed, many “island-and-serpentine”-type distributed sensor networks, while promising, remain difficult to deploy. This study aims to enable such networks to be deployed in a safe, automated, and efficient way. To this end, a scissor-hinge controlled system was proposed as the basis for a deployment mechanism for such stretchable sensor networks (SSNs). A model based on a kinematic scissor-hinge mechanism was developed to simulate and design the proposed system to automatically stretch a micro-scaled square network with uniformly distributed sensor nodes. A prototype of an automatic scissor-hinge stretchable tool was constructed during the study with an array of four scissor-hinge mechanisms, each belt-driven by a single stepper motor. Two micro-fabricated SSNs from a 100 mm wafer were fabricated at the Stanford Nanofabrication Facility for this deployment study. The networks were designed to be able to cover an area 100 times their manufacturing size (from a 100 mm diameter wafer to a 1 m active area) once stretched. It was demonstrated that the proposed deployment tool could place sensor nodes in prescribed locations efficiently within a drastically shorter time than in current labor-intensive manual deployment methods.

Discernment and Denial: Nanotechnology Researchers’ Recognition of Ethical Responsibilities Related to Their Work

To what extent do nanotechnology researchers discern specific work-related ethical responsibilities that are incumbent upon them? A questionnaire was designed and administered to answer this question. Analysis of responses to 11 ethical responsibility statements (ERSs) by 213 researchers at the Stanford Nanofabrication Facility revealed widespread agreement about a number of work-related ethical responsibilities and substantial divergence in the views about several others. Explanations of this divergence are proposed. A new variable is defined that gauges the respondent’s overall level of discernment of the ethical responsibilities referenced in the ERSs. The mean discernment level score for respondents who had taken a course that included discussion of ethical issues closely related to science, technology, or engineering was significantly higher than for those who had not. Further, among respondents who had taken such a course, the mean discernment level score for those who had taken an ethics course devoted to exploration of ethical issues closely related to science, technology, or engineering was significantly higher than for those who had taken a technical science or engineering course that typically pays only fleeting attention to such issues. Implications of these findings are discussed.

Hard stamp processes for the EVG 620 full field nanoimprint system

The EVG 620 nanoimprint system is a 4 in. full wafer system suitable for university and research environments (EV Group, A-4782 Sankt Florian am Inn, Austria). By limiting the area printed and by the use of hard masks, nanoimprint lithography resolution can be extended to well below the design resolution of the system. The EVG 620 has found application primarily with soft stamps, such as those made of polydimethylsiloxane [Plachetka et al., Microelectron. Eng. 73-74, 167 (2004)]. However, soft stamps can have limited resolution and are difficult to clean, meaning they should be disposed of after a single stamp. Hard stamps made from quartz wafers have ultimate resolution limited only by the e-beam exposure system used for definition and are easy to clean in sulfuric peroxide or oxygen plasma strippers. Hard stamps have been demonstrated on the EVG 620 instrument at Stanford Nanofabrication Facility with resolution down to 20 nm. Both positive tone (PMMA, ZEP-520) and negative tone templates [hydrogen silsequioxane (HSQ)] have been fabricated. The positive tone templates require a plasma etch step, while the HSQ templates [Mancini et al., J. Vac. Sci. Technol. B 20, 2896 (2002)] can be used directly for nanoimprint. A process for creation of daughter templates from silicon master e-beam exposures has been demonstrated on the EVG 620. The silicon master with HSQ patterns is placed on the wafer stage and a quartz wafer coated with nanoimprint polymer is mounted where the template is normally placed. The resultant image on the quartz wafer is then transferred into the substrate with plasma etching, and these daughter templates can then be used for image transfer. Minimal loss of resolution has been demonstrated with this process.

Electrical Characterization and Preliminary Beam Test Results of 3D Silicon CMS Pixel Detectors

The fabrication of 3D detectors which requires bulk micromachining of columnar electrodes has been realized with advancements in MEMS technology. Since the fabrication of the first 3D prototype in Stanford Nanofabrication Facility in 1997, a significant effort has been put forth to transfer the 3D detector technology to large scale manufacturing for future high luminosity collider experiments, in which the radiation hardness will be the primary concern, and other applications such as medical imaging and X-ray imaging for molecular biology. First, alternative 3D structures, single type column (STC) and double-side double type column (DDTC) 3D detectors, were produced at FBK-irst (Trento, Italy) and CNM-Barcelona (Spain), and assessed thoroughly to improve the production technology towards the standard full-3D detectors. The 3D collaboration has been extended to include SINTEF (Norway), which is committed to small to medium scale production of active edge full-3D silicon sensors. This paper focuses on p-type 3D detectors compatible with the CMS pixel front end electronics from the second run of fabrication at SINTEF clean room facilities. The sensors that passed the wafer level electrical characterization have been bump-bonded at IZM (Germany), assembled into modules and wire-bonded for functional characterization at Purdue University. We report the leakage current characteristics, bump-bond quality, threshold, noise, and gain measurement results of these 3D modules as well as the preliminary beam test data taken at Fermi National Accelerator Laboratory.

3D active edge silicon sensors with different electrode configurations: Radiation hardness and noise performance

3D detectors, with electrodes penetrating the entire silicon wafer and active edges, were fabricated at the Stanford Nano Fabrication Facility (SNF), California, USA, with different electrode configurations. After irradiation with neutrons up to a fluence of 8.8×10 15 n eq cm −2, they were characterised using an infrared laser tuned to inject ∼2 minimum ionising particles showing signal efficiencies as high as 66% for the configuration with the shortest (56 μm) inter-electrode spacing. Sensors from the same wafer were also bump-bonded to the ATLAS FE-I3 pixel readout chip and their noise characterised. Most probable signal-to-noise ratios were calculated before and after irradiation to be as good as 38:1 after the highest irradiation level with a substrate thickness of 210 μm. These devices are promising candidates for application at the LHC such as the very forward detectors at ATLAS and CMS, the ATLAS B-Layer replacement and the general pixel upgrade. Moreover, 3D sensors could play a role in applications where high speed, high-resolution detectors are required, such as the vertex locators at the proposed Compact Linear Collider (CLIC) at CERN.

First fabrication of full 3D-detectors at SINTEF

3D-detectors, with electrodes penetrating through the entire substrates have drawn great interests for high energy physics and medical imaging applications. Since its introduction by C. Kenney et al in 1995, many laboratories have begun research on different 3D-detector structures to simplify and industrialise the fabrication process. SINTEF MiNaLab joined the 3D collaboration in 2006 and started the first 3D fabrication run in 2007. This is the first step in an effort to fabricate affordable 3D-detectors in small to medium size production volumes. The first run was fully completed in February 2008 and preliminary results are promising. Good p-n junction characteristics have been shown on selected devices at the chip level with a leakage current of less than 0.5 nA per pixel. Thus SINTEF is the second laboratory in the world after the Stanford Nanofabrication Facility that has succeeded in demonstrating full 3D-detectors with active edge. A full 3D-stacked detector system were formed by bump-bonding the detectors to the ATLAS readout electronics, and successful particle hit maps using an Am-241 source were recorded. Most modules, however, showed largely increased leakage currents after assembly, which is due to the active edge and p-spray acting as part of the total chip pn-junction and not as a depletion stop. This paper describes the first fabrication and the encountered processing issues. The preliminary measurements on both the individual detector chips and the integrated 3D-stacked modules are discussed. A new lot has now been started on p-type wafers, which offers a more robust configuration with the active edge acting as depletion stop instead of part of the pn-junction.

Marni Goldman Tribute: Contributions to Materials Science Education

AbstractThis symposium is a memorial to Dr. Marni Goldman. Although she never walked and had only limited use of her arms, Marni's academic and professional accomplishments placed her in elite company. She obtained two bachelors degrees from the University of Pennsylvania and a Ph.D. in Materials Science from the University of California at Berkeley. Even with a heavy course load, she was involved in educational outreach during her studies. She started her career as a Research Associate (Education Director) in Stanford's NSF Materials Research Science and Engineering Center on Polymer Interfaces and Macromolecular Assemblies in 2000 and retained those responsibilities until her death in 2007. During this period she rapidly added the responsibilities as Education Director for Stanford's Nanofabrication Facility and was ultimately named Associate Director of Stanford's Office of Science Outreach. Marni was a dynamo whose activities at Stanford included a large summer undergraduate internship program, a Research Experiences for Teachers program (local and national activities), a program to bring community college students (especially minority students) to the campus, public science (San Jose Tech Museum of Innovation, San Francisco Exploratorium), outreach to high schools with high minority populations, and a program with summer internships for students with disabilities. Marni's achievements are thanks in no small part to her extraordinary family, to her own intelligence and tenacity, and to a wide and loving circle of friends, drawn to her by the spirit of her determination and the unmistakable largeness of her heart.

Radiation hardness properties of full-3D active edge silicon sensors

Full-three-dimensional (3D) pixel sensors, with electrodes penetrating through the entire silicon wafer, were fabricated at the Stanford Nanofabrication Facility, Stanford, California, USA. They have 71-μm-inter-electrode spacing, active edges and a compatible geometry to the ATLAS pixel detector readout electronics. Several samples were irradiated with neutrons to different doses up to an equivalent fluence of 8.6×10 15 n 1 MeVeq cm −2. This corresponds to the integrated fluence expected after ∼5 years at the Large Hadron Collider (LHC) with a luminosity of 10 35 cm −2 s −1 at 4 cm from the interaction point, where the ATLAS B-Layer is placed. Before and after irradiation, signals were generated by a 1060 nm infrared laser calibrated to inject a charge of 14 fC. This corresponds to ∼3.5 minimum ionizing particles and should not perturb the charge status of the radiation-induced defects. After 8.6×10 15 n 1 MeVeq cm −2 the signal collected was ∼38% and corresponded to ∼7200e − for a substrate thickness of 235 μm. Signal efficiency, radiation-induced leakage current and related damage parameters are discussed here and compared with simulations. Full-3D silicon detectors with active edges are being considered for forward proton tagging at the LHC, for the ATLAS pixel B-layer replacement and for the ATLAS pixel upgrade.

The National Nanofabrication Users Network (NNUN): Potential Applicability for Bio/Biomedical Science and Technology

For several decades the US scientific community has recognized the value of pooling specialized skills and equipment in centralized user facilities. Significant efficiencies are realized within the user community by avoiding unnecessary duplication and by moving projects more quickly to completion. The user facility concept served the nanofabrication community well for over two decades at the National Nanofabrication Facility (NNF) at Cornell. In 1994, the National Science Foundation expanded the concept by integrating NNF-Cornell (Now CNF) with several other nanofabrication programs throughout the country to form the National Nanofabrication Users Network (NNUN). Within this framework of a “users” network, scientists and engineers have access to state of the art equipment and expertise.NNUN consists of two “full service” hub facilities at Cornell (the Cornell Nanofabrication Facility) and Stanford (the Stanford Nanofabrication Facility), in association with specialized facilities at Howard, Penn State and UC-Santa Barbara. The network is managed by the 5 site directors, who are responsible to NSF program management and to a Network Advisory Board.