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A Newsletter from Oxford Instruments Plasma Technology
IN THIS ISSUE:
Providing leading edge tools and processes to key markets worldwide
2 Two dimensional materials and heterostructures
3 Low temperature plasma
assisted ALD of conductive films
4 Collaboration proves a ‘win-win’ for TU/e and Oxford Instruments 6 Plasma Pro ® systems bought for Chinese LED Manufacture 7 Training: buy one get one half price 8 The Application of Bosch™ Deep Silicon Etch to the Manufacture of X-Ray Lenses
Discrete Devices Sensors
10 Micromachining silicon
structures on thin membranes using plasma etching
12 Power device development enabled through University of Glasgow collaboration 13 Energy savings win us a ‘Go Green’ award
13 Record numbers attend 4 th
BTNT seminar in Chennai, India
14 New etching process for
Magnetic RAM developed by Cornell and Oxford Instruments
16 Watch our informative webinars
PROCESS NEWS 1
Two dimensional materials and heterostructures
Dr Ravi Sundaram, Oxford Instruments Plasma Technology
Vapour deposition techniques have gained a lot of interest for growth of two dimensional (2D) materials 1-4 . The ability to grow large area graphene has spurred research into vapour deposition of a wide variety of atomically thin layered materials. In the recent past there has been a growing interest in studying atomic planes of these Van der Waals solids and build 2D heterostructures by stacking layers with complementary characteristics to achieve novel functionality 5 . For successful scaling up of prototypical applications demonstrated to date, technologies and processes for large area deposition of these materials need to be developed. Chemical Vapour Deposition (CVD) has been one of the most successful techniques for the large area fabrication of nanostructured materials such as graphene, carbon nanotubes and other 1D/2D nanomaterials.
The Oxford Instruments Nanofab ® is ideal for this field of research as it combines several essential features for high performance 2D material deposition: • High temperature (1200 ºC) table • Capable of handling up to 200 mm wafers • Shower head technology • Automatic load lock for wafer handling • Plasma enhancement via in chamber (parallel plate ) plasma or remote plasma via ICP • One of the key requirements for developing robust 2D materials deposition processes is the ability to efficiently deliver liquid and solid state precursors in addition to traditional gases. We offer flexible options for liquid/solid precursor delivery. Example: MoCl 5 , Mo(CO) 6 , W(CO) 6 , DTBSe, DETe etc) • Multiple precursor pod options for depositing a combination of 2D materials We have demonstrated growth of monolayer graphene and related graphene-like allotropes using this system. Recently, we have developed a thermal CVD route to synthesize hexagonal Boron Nitride (hBN) and we are currently investigating other 2D materials.
Nanofab with precursor delivery system.
Current library of 2D materials investigated to date 5 .
2 PROCESS NEWS
Low temperature plasma- assisted ALD of conductive films
Harm Knoops, Thomas Sharp, Annika Peter, Owain Thomas, and Robert Gunn, Oxford Instruments
Many applications have a limited thermal budget for processing, especially when organic materials are involved or diffusion of elements has to be minimized (e.g. dopants in electronic devices). Since ALD relies on thermally activated reactions, ALD at low temperatures can be a challenge. A paper presented at the BALD 2015 conference in Tartu, Estonia discussed low temperature plasma-assisted ALD of conductive films where the ALD of TiN and ZnO in the Oxford Instruments FlexAL ® remote plasma ALD system serve as case examples. Using a multistep ALD process as shown in Fig. 1, allows deposition of Pt at room temperature by O 2 plasma and H 2 plasma in the cycle [Mackus et al., Chem. Mater. 25, 1769 (2013)]. Similarly, this work showed that interleaving H² plasma in an ALD process for ZnO using ZnEt 2 (DEZ) and O 2 plasma, allows relatively low resistivity values, i.e., 5.7 mΩcm for a ~45 nm film at temperatures of 100°C. For deposition of metals and metal nitrides that are difficult to reduce from their oxide, exposure to oxygen should be avoided. To increase reactivity for ALD of those materials plasmas with higher ion energies can be used. Low-resistivity titanium nitride was obtained from the metal-organic precursor Ti(NMe 2 ) 4 (TDMAT) and mixed N 2 /H 2 plasma at 200°C. Through precise control of the plasma conditions we were able to achieve a resistivity of 250 µΩ∙cm at 26nm thickness and even 180 µΩ∙cm for a 74 nm film as shown in Fig. 2. TiN from TDMAT therefore offers a viable alternative for devices where chlorine-containing precursors risk device degradation. Control of plasma properties and multistep ALD processes are expected to provide the best route to deposition of conductive films by ALD.
Examples of graphene and hBN growth.
References . Li, X et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science 324, 1312-1314 (2009)  Bae, S. et al. Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nat Nanotech. 5, 574 (2010)  Ismach, A. et al. Toward the Controlled Synthesis of Hexagonal Boron Nitride Films. ACS Nano, ,6, 6378 (2012)  Zhan, Y et al. Large-Area Vapor-Phase Growth and Characterization of MoS 2 Atomic Layers on a SiO 2 Substrate. Small, 8, 966 (2012).  Geim, A.K and Grigorieva, I.V., Van der Waals heterostructures, Nature, 499,419 (2013)
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Figure 1. A schematic representation of the various steps in a multistep process. For low temperature ZnO ALD, B is an O 2 plasma and C is an H 2 plasma.
Figure 2. The film resistivity as a function of thickness for TiN deposited from TDMAT. Relatively low resistivity values are obtained for thicknesses above 20 nm.
PROCESS NEWS 3
Collaboration proves a ‘win-win’ for TU/e and Oxford Instruments
Sonja Knols, Eindhoven University of Technology
To be able to keep innovating its products, Oxford Instruments invests in close cooperation with academic research groups. One example is the Plasma and Materials Processing group of Eindhoven University and Technology. “Our cleanroom facilities are vitally important for our research” says associate professor Ageeth Bol. Ageeth Bol holds a PhD in physical chemistry and returned to academic research in 2011, after a decade of working in industry. “The presence of the cleanroom facilities here in Eindhoven certainly was a selling point for me in deciding where I wanted to go,” she says. Bol works on the development of new production processes for graphene-like, two-dimensional materials. “To be able to make these very thin layers of materials, you need to have nanometre control over the layer thickness. The Plasma and Materials Processing group I work in has in-depth knowledge of a specific method to produce these layers.” This so-called plasma-assisted atomic layer deposition (ALD) is a technique that makes use of gaseous phases of materials, which are alternately flushed onto a surface. Carefully picked precursor molecules react with specific molecular groups at the surface in a self-limiting way. “That is one of the advantages of ALD over chemical vapour deposition (CVD),” Bol explains. “With ALD, as soon as all the reactive sites are used, the reaction stops. This results in very thin and even layers.” Replacing scotch tape The group Bol is working in, is interested in optimising these type of ALD processes for a variety of applications, such as thin- film solar cells and new types of semiconductor devices. “My
personal focus lies in the usage for two-dimensional layers, such as graphene. This is important for a new generation of nanoelectronics. Even known materials start to behave differently when you force them into a 2D architecture.” At present, the most sophisticated production method for these types of structures is the ‘scotch tape method’: you stick something to the surface, and rip off a single layer of molecules. “Of course, this is not the most reproducible way to do it, and more importantly: it is certainly not scalable for industrial purposes.” Bol’s research strongly leans on the cleanroom facilities, in which Oxford Instruments play an important role. “Here in Eindhoven, we have the equipment to both fabricate and analyse devices. Ranging from lithography and deposition tools to a scanning electron microscope and X-ray photoelectron spectroscopy to characterise the layers we have deposited.” The research group uses advanced commercial tools and ones they have partly constructed themselves. “I am going to work with ALD machines from Oxford Instruments, which have been configured to meet our needs. We want to be able to play around with different parameters of the ALD process,” Bol explains. The researchers are particularly interested in relatively
4 PROCESS NEWS
low temperatures. “For CVD processes, typically temperatures of over a 1000 degrees Celsius are needed. That is often fatal for applications in semiconductors because the high temperature increases the diffusion of the atoms, which makes it harder to place them at the right spot. We want to have a process that yields materials of high quality at lower temperatures. This is especially important for the two-dimensional heterogeneous layers I am working on, since at lower temperatures less diffusion of atoms between the layers will occur.” Two-way street Supplier Oxford Instruments is heavily involved in this type of adjustments, she explains. “They help us in customising the machine for our specific needs. In fact, one of their employees works here in Eindhoven, to make sure the lines are as short as possible. This employee is technical sales specialist Harm Knoops. “For us, this cooperation is a win-win situation,” he says. “Whenever the researchers in Eindhoven publish an article on new results achieved with our machines that helps me to show other potential clients the possibilities of our equipment.” The cooperation is a genuine two-way street, he emphasises. “When their research requires new options we try to co-develop them. But also the other way around, when we have developed some new features they can test them for us. And their publications act as independent quality markers for our products.” For the ALD machines, Oxford Instruments’ main market lies in research institutes and universities. “Nothing sells better in that sort of environment than refereed publications in high impact scientific journals.”
He immediately gives an example. “Some time ago, PhD student Harald Profijt developed a method to manipulate the energy of the deposited atoms by applying an RF voltage on the substrate. Normally, in plasma assisted ALD, you need the incoming ions to have as low an energy as possible. But sometimes, the process would improve if you were to increase that energy a little, thereby increasing the mobility of the atoms at the surface. So now we can offer that option to other clients too.” Furthermore, Oxford profits from the extensive infrastructure in the cleanroom facility, he says. “At Oxford, we have a relatively small ALD lab. The university campus hosts a dedicated cleanroom with all kinds of diagnostic tooling. The research done there on ALD-processes gives us insight into what is happening inside the reactors during the deposition.” These insights also help Bol in her quest for new production processes. After receiving the prestigious Vici grant in 2014, she was recently awarded an ERC Consolidator Grant as well. With those funds, she will spend the next few years stretching the current limits of the technique: “When we can actually fabricate truly two-dimensional layers, I want to take it to the next level: stack two-dimensional layers made out of different materials. That way we can combine different properties in one device. We will have to do that at low temperatures to prevent the different layers from mixing, since that will deteriorate the quality of the materials. And if we can achieve that, it will open up a whole new field of possibilities for even smaller electronic circuits with totally new properties.”
Our thanks to TU Eindhoven for allowing us to publish this article
Image by Mikhail Ponomarev
PROCESS NEWS 5
Plasma Pro ® systems bought for Chinese LED manufacture
High tech Chinese manufacturer, Enraytek Optoelectronics Co recently purchased several leading edge Oxford Instruments Plasma Pro 800 PECVD systems for the manufacture of their High Brightness LEDs.
Enraytek’s key products are High Brightness LEDs for TV backlighting and LED lighting, with manufacturing capability covering all aspects from sapphire substrate materials, epitaxial wafers, and chips for packaging and testing. Oxford Instruments’ large capacity, open loading Plasma Pro 800 PECVD systems are industry proven for HBLED processes, allowing up to 300mm wafers, and are designed to produce the high quality and uniform dielectric films needed in LED manufacture. The Plasma Pro’s small footprint, and excellent substrate temperature control allows high performance processes and precise process control. The large electrode delivers the market leading cost of ownership demanded by the LED industry.
Says Mr.Xu Chunchao. Vice President from Enraytek, “As one of China’s foremost LED
design and manufacturing companies, cost efficient and state- of-the-art processing equipment is key for Enraytek. We chose Oxford Instruments systems following a rigorous selection process, as they could offer us the advanced technology, support and cost of ownership we require.” We manufacture the very high specification plasma deposition technology required by the LED industry, and with an already established installed base in Chinese LED companies, we are delighted that Enraytek has also chosen us as their supplier of a number of Plasma Pro 800 systems.
6 PROCESS NEWS
Training: buy one get one half price
Book one get one 1/2 price Offer ends 31/05/2016 Are you looking for advanced maintenance training or simply
training for new users to be safe and competent in the use of your Oxford Instruments tools?
We have developed a range of courses giving our customers the choice to suit you and your users different levels. Book your place on any Oxford Instruments Plasma Technology 2016 training course by 31 st May 2016 and your colleagues can book a place on the same course for half price. Visit our webpage for all course information:
www.oxinst.com/training Quote offer code: PN16
PROCESS NEWS 7
The Application of Bosch™ Deep Silicon Etch to the Manufacture of X-Ray Lenses
Katarzyna Korwin-Mikke a , Mark E McNie a , Lucia Alianelli b , a Oxford Instruments Plasma Technology b STFC Diamond Light Source, Oxfordshire, UK
The trend in X-Ray optical devices, such as refractive lenses, zone plates, curved mirrors, multilayers and multilayer Laue lenses  , is towards shrinking dimensions and/or deeper optics. Due to excellent properties, such as thermal resistance, low x-ray absorption, diamond is highly desirable material for use in many optical instruments  , however, due to cost of manufacturing, extreme hardness and resistance to chemical attack, diamond is a difficult material to realize structures suitable for x-ray lenses and this is why silicon is the leading material for x-ray lens production. Nanofocusing silicon x-ray lenses require not only high quality material but also high aspect ratio with vertical sidewalls and controlled roughness on the sidewalls to minimize aberrations and parasitic scattering
respectively. The focus of this work was to develop the process for etching silicon x-ray lenses with good profile control and smooth sidewalls in the Bosch process DSiE. To achieve high aspect ratio lenses with vertical profiles and smooth sidewalls, a short cycle time is used in conjunction with lower powers for less aggressive process conditions. The passivation is regularly refreshed to maintain sidewall integrity through rapid switching with controlled ion energies. Etching microstructures on the samples with a high silicon exposed area reduces the etch rate due to loading effects and may cause the undercut of the mask as the result of the sidewall plasma attack on isolated features (Fig.1) even when over passivated. One of the solutions to reducing the negative impact of the ions on the etched lenses is to tune the process to operate at low pressures and a low (controllable) DC bias. The other is to reduce the number of ion impacts on the sidewalls by protecting the lenses with sacrificial features that can be removed after the process. In the Oxford Instruments Applications Laboratory, etch processes for each approach were developed (i.e. with and without compensating features – Fig.2). The results gave close to vertical lens profiles (89.90°) without compensating features etched to 50µm depth with no mask undercut and controlled scallops to below 50nm (Fig.3). This process was adapted and extended to the lenses with compensation features to achieve
Figure 1. The negative impact of the ions on etched X-Ray lens structure.
Figure 3. SEM of a lens without compensation features etched to 50µm depth.
Lens without compensation features
Figure 2. Device mask patterns. Lens with compensation features
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Figure 4. SEM of a lens with compensation features etched to 75µm depth
Figure5. Plasma Pro 100 Estrelas
etch depths in excess of 70µm with smooth sidewalls (scallops <50nm), no mask undercut or sidewall damage, vertical profile (89.94°) and clean surfaces in open silicon areas (Fig.4). The X-Ray silicon lens etch processes were carried out in a Plasma Pro 100 Estrelas deep silicon etch tool (Fig.5). This  L Alianelli, K J S Sawhney, R Barrett, I Pape, A Malik M C Wilson, Optics Express (2011) vol. 19, no.12, 11120-11127  A M Malik, O J L Fox, L Alianelli, A M Korsunsky, R Stevens, I M Loader, M C Wilson, I Pape, K J S Shawhney, P W May, Journal of Micromechanics and Microengineering 23 (2013), 125018 (7pp) Adapted from work presented at the International Micro Nano Engineering (MNE) Conference, Sept. 2014 An article ‘Aberration-free short focal length x-ray lenses’ was published in Optics Letters Lucia Alianelli, 1 * Manuel Sánchez del Rio, 2 Oliver J. L. Fox 1,3 and Katarzyna Korwin-Mikke 4
latest generation deep silicon etch system can trade high rate performance for increased control to enable a very flexible set of processes from high etch rate applications (>25µm/min) to nanoscale etching. The lens processes are being transferred to a third party commercial supplier for production of the lenses going forwards. (Article published in Optics Letters (Impact Factor: 3.29). 12/2015; 40(23):5586. DOI: 10.1364/OL.40.005586) Available from: https://www.researchgate.net/publication/284519056_Aberration-free_ short_focal_length_x-ray_lenses 1 Diamond Light Source Ltd., Chilton, Didcot OX11 0DE, UK 2 European Synchrotron Radiation Facility, BP 220 38043 Grenoble Cedex, France
3 School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK 4 Oxford Instruments Plasma Technology, Yatton, BS49 4AP, UK
*Corresponding author: email@example.com
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PROCESS NEWS 9
Micromachining silicon structures on thin membranes using plasma etching
Zhong Ren, Mark E McNie, Colin C Welch, Mike Cooke Oxford Instruments
Micromachining detailed silicon structures on thin membranes is regarded as one of the primary MEMS fabrication techniques, but also one of the more difficult processes. There is a very sharp transition in chemical loading effect, as the silicon clears from the membrane layer whilst the lateral heat dissipation from etching structures on a membrane decreases significantly as the membrane is approached. The exothermic etch reaction can even cause the resist pattern and etch passivation to degrade when the associated heat is focused in a small area. There is a need for very high etch rates to remove bulk material to expose the membrane, but also a need for very precise profile control in fabricating micro- or nano-scale features. The bulk etch can use an anisotropic wet etch that is selective to the <111> crystallographic planes, such as KOH or TMAH. The wet chemistry is only suitable for low aspect ratio (AR) features and results in a characteristic slope at a 54.74º for <100> silicon wafers that limits geometric freedom and packing density  . A Bosch (or gas-chopping) deep silicon etching (DSiE) process with alternating deposition and etch steps can realize a vertical profile at high aspect-ratio (AR) with etch rates of up to 30µm/ min, and can etch arbitrary shapes as defined by the mask pattern [2, 3] with high packing density. This paper presented at MNE 2015 reports a technique which addresses these problems, and gives example results on three typical structures, using a DSiE tool ( Plasma Pro 100 Estrelas) in the Bosch process (SF 6 -C 4 F 8 chemistry). Most DSiE process tools are optimised for high rate anisotropic etch but may be limited in their ability to perform precision, lower rate processes. Such fast switching etch processes often need a significant minimum power to maintain the plasma during the transition between etching and deposition steps.
This tool can operate successfully over more than an order of magnitude of inductively coupled plasma (ICP) source power, from <500 W to >5 kW, opening the possibility of both high rate and precision, low rate processes in the same chamber. The minimum time of a single process step is 100-300 ms for removing polymer which enables a full cycle of deposition, break through and etch to be as low as 1 s. Results given here used cycle times up to 2.5s. Whilst an electrostatic chuck (ESC) clamps over the whole wafer surface in contact and to conductive layers on the membrane, where membranes are proud of the clamping plane the gap is large and so heat removal efficiency via Helium back side cooling is dramatically reduced. Thus in order to be able to etch structures on membranes without overheating and maintaining profile control, a low power process capability is also required. Three devices have been realized using this combination of high rate and precision processing: a micro-resonator high-Q cavity with a vertical profile (90º) and uniform scallop size (Fig 1&2); a fine periodic deep grating with 50:1 high AR (Fig 3&4); and a large length-width ratio cantilever on a photoresist (PR) membrane where avoiding mask overheating and not damaging the polymer film underneath the cantilever was key (Fig 5&6). Moreover, high quality and smooth sidewalls were also achieved on these etched structures.
Figure 1. Schematic section of a micro-resonator on a Si 3 N 4 membrane: back cavity by KOH etching and the top pattern defined in a silicon membrane using the Bosch process
10 PROCESS NEWS
Figure 2. Micro-resonator structure: (a) side view with vertical profile and a 60 µm etch depth; (b) sidewall roughness (scallop size) of 78nm
Figure 5. Challenges on Si membrane etch: (a) grass and striations in corners; (b) optimised etch gives striation-free and smooth sidewall; (c) PR mask burnt out during Bosch etch; (d) optimised etch avoids overheating on PR mask.
Figure 3. Schematic section of gratings in a SOI wafer: the back cavity defined by a high rate Bosch etch and fine period optical gratings realised in the device layer using a precision Bosch etch.
Figure 6. Si cantilever: (a) Illustration of cantilever fabrication in Si membrane by means wet-etching and Bosch process; (b) 5 mm length × 200 µm width × 50 µm thickness cantilever; (c) and (d) Enlarged view of the cantilever after polymer removal.
 H. Seidel, L. Csepregi, A. Heuberger, H. Baumgartel, J. Electrochem. Soc. 137 (11) (1990) 3614–3626
Figure 4. High aspect-ratio (50:1) fine period (200nm) gratings etched by a Bosch process: (a) Etch depth 5µm; (b) minimal bow at top of trenches
 F. Laermer, A. Schilp, German Patent DE-4241045, 1994  Z.Ren, M.E.McNie, Microelec. Eng. 141 (2015) 261-266
The authors would like to thank Dr. S Vollmeke (CiS Forschungsinstitut fur Mikrosensoik GmbH, Erfurt) for providing cantilever samples for etching at Oxford Instruments and permission to use the SEM images.
PROCESS NEWS 11
Power device development enabled through University of Glasgow collaboration
Oxford Instruments and the University of Glasgow (GU) have entered into a collaboration as part of a project to develop next generation GaN-on-Si power devices. The project will use an Oxford Instruments four chamber cluster system combined in a unique configuration. Under vacuum the cluster will allow device manufacturing and characterisation measurements to be performed on device interfaces and surfaces without exposure to atmosphere. System installation is currently underway in the James Watt Nanofabrication Centre at Glasgow University and development work is due to commence in March 2015. The cluster systemcombines a FlexAL ® Atomic Layer Deposition tool used for depositing very thin films of metals, oxides and nitrides using both thermal and inductively coupled plasma (ICP) ALD processes, a Plasma Pro System 100 ICP for etching of compound semiconductor materials and a Plasma Pro System 100 ICP for High-Density PECVD deposition system providing for low damage, low temperature thin films; plus the Omicron Nanoscience NanoSAM LAB, for surface sensitive chemical analysis and high resolution imaging of small (micro and nano) structures by Scanning Auger Microscopy (SAM) and Scanning Electron Microscopy (SEM). Installed, this provides a unique capability for analysis of each stage of prototype device manufacture. “This flagship project for both Glasgow University and Oxford Instruments provides an opportunity to be in the vanguard as the power semiconductor market looks to move towards GaN- on-Si.”, Comments Prof Iain Thayne, of Glasgow University, “Successful development of GaN-on-Si enables the use of low cost, large diameter substrates and easier routes to high volume manufacture of power devices. This offers significant
potential in realising smaller, cheaper, better performing and more efficient power devices, with the ensuing commercial rewards.” “Using Oxford instruments state-of-the-art equipment, the ultimate aims of this project are to produce smaller GaN-on-Si based power devices that offer better performance and levels of efficiency than those currently available.” says Dr David Haynes, Sales & Marketing Director at Oxford Instruments Plasma Technology, “This will result in commercial benefits for various applications, from electronic component power supplies in laptops to aerospace and automotive applications, in particular electric vehicles. Additionally, Glasgow University will run a second project on the cluster focussing on scalable solar thermoelectrics and photovoltaics.” Oxford Instruments will continue to support GU throughout the collaboration period, assisting with process development, and sponsoring a PhD student as part of the project, who will work on the newly installed cluster tool. This EPSRC funded project bridges the gap between fundamental research and commercial manufacture. Funding was granted to GU as part of a consortium of UK Universities (Bristol, Cambridge, Glasgow, Liverpool, Manchester, Nottingham and Sheffield) and industrial partners. This five year project will connect the very capable but currently fragmented, world-class UK GaN academic materials and device community, align them with key players in the UK academic power electronics sector, and directly link with the significant UK power semiconductor industry.
12 PROCESS NEWS
500 attendees 2 days 24 talks 30 posters Record numbers attend 4 th ‘BTNT’ seminar in Chennai, India Bringing the Nanoworld Together Plenary sessions on Day 1 were themed ‘Convergence of the Nanosciences’, with keynote Professor Dr Rudolf Gross from Walther-Meissner-Institute, Technical University of Munich. On Day 2, Thin Film Processing sessions covered fabrication of wide band gap semiconductors and latest advances in sensor fabrication. There followed talks from Nanoanalysis, Asylum Research, Nanoscience, and Andor all part of the Oxford Instruments group. These educational seminars are aimed to enhance our customers’ knowledge and keep them informed of the latest technological advances our systems have to offer. IISc Bangalore is the venue for 2016: watch out for further details on our website
Joining Go Green has inspired and challenged us to make further sustainable improvements to our business. Reducing energy consumption may seem an obvious way of being more sustainable and saving costs, but implementing this in a large organisation takes considerable planning and effort. We did just that, and our initiatives have just resulted in our company winning Bristol’s ‘Go Green’ Energy & Efficiency Best Newcomer award. We’re also working hard to implement more ‘Green’ initiatives. Energy savings win us a ‘Go Green’ award
See how we did it: www.oxinst.com/GoGreen16
PROCESS NEWS 13
New etching process for Magnetic RAM developed by Cornell and Oxford Instruments
Vince Genova, Cornell University Colin Welch, Oxford Instruments
The Cornell Nanoscale Science and Technology Facility (CNF), a leading university research facility located at Cornell University, Ithaca, NY, and Oxford Instruments Plasma Technology (OIPT), UK have collaborated... ...to develop a novel etching process targeted specifically at magnetic random-access memory MRAM based device fabrication. These results, obtained at CNF, add a significant contribution to OIPT’s large portfolio of etching processes. MRAM is a high performance, low power, low degradation, non-volatile data storage technology that some suggest gives it the potential to become a “universal memory”, able to replace SRAM, DRAM, EEPROM and flash. Etching of magnetic based materials for the development and scaling of MRAM and spintronic devices is therefore of keen interest to several leading research groups using the CNF. Vincent J. Genova, a Research Staff Member at CNF, explains the technology and the new process, “An element of MRAM consists of a magnetic tunnel junction (MTJ) and a CMOS transistor. One of the most challenging steps in MRAM fabrication is the etching of the MTJ stack. The stack typically contains a non-magnetic seed layer to promote
suffers from low etch rates, low selectivity, undesirable sidewall redeposition especially for nanoscale features, and damage to the device structure itself. Recently, several research groups have shown that chemical etching of Co, Fe, and Ni based alloys can be achieved using plasmas formed from methanol (CH 3 OH) and argon. The new CNF/OIPT process is a result of a Design of Experiment (DOE) in which the level of CH 3 OH in Ar varied, along with variations in the ICP power, bias power, and pressure. Methanol, as the principal plasma reactant, forms volatile carbonyl compounds (e.g. Ni(CO) 4 , Fe (CO) 5 , and Co 2 (CO) 8 ) at room temperature. This chemistry-based process avoids the disadvantages of purely physical milling. The antiferromagnet IrMn also etches in a methanol plasma. In addition, the selectivity over common mask materials such as Al 2 O 3 , Ta, Ti, TaN, and TiN is high, while leaving no residue on the etched devices. We demonstrated successful etching of a 41nm thick magnetic tunnel junction stack stopping on the tantalum under layer (see figure). High selectivity (>10:1) over both the Ta mask and under layer is achieved through the formation of tantalum carbide in the methanol process. CNF was pleased to announce the full facilitation of the new Plasma Pro 100 Cobra ICP etch system from Oxford Instruments Plasma Technology (OIPT) in 2015. This inductively
proper crystalline growth (e.g. Ta), an antiferromagnet such as PtMn or IrMn, a stack of alloy pinned layers (CoFeB), a tunnelling barrier such as MgO, metals such as Ru and/or Pt, and a suitable hard mask such as TiN or Ta. The problem is that magnetic materials have difficulty reacting with most chemically active plasma species to form volatile etch products, so users often have to resort to purely physical ion milling processes. However, ion milling
Scanning electron micrograph of the 41nm thick MTJ
14 PROCESS NEWS
coupled plasma (ICP) based reactive ion etch platform is configured for state of the art nanoscale etching vital to the research work of CNF. The system includes many extras that make for a highly flexible and powerful etch research tool. These include a wide range temperature (-150°C to +400°C) electrode, which greatly enhances the spectrum of materials that can be etched with volatile chemistries, low frequency electrode biasing and a vapour delivery system for methanol (CH 3 OH). This advanced methanol-based etch capability for magnetic materials is an enabling process that is now available to the researchers at CNF and to the newly formed National
Nanotechnology Coordinated Infrastructure Network (NNCI).
This research work at CNF adds a significant contribution to Oxford Instruments Plasma Technology’s extensive portfolio of etching processes, enabled through the use of our state of the art Plasma Pro 100 Cobra ICP etch system. We are delighted that our technology is assisting such a prestigious research centre achieve its fundamental research goals.
For further technical information, please contact Vincent Genova at Genova@cnf.cornell.edu or Colin Welch at firstname.lastname@example.org
PROCESS NEWS 15
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