Microengineered Quadrupole Mass Spectrometer
Richard Syms, Tom Tate, Munir Ahmad, Steve Taylor (Liverpool University)
The aim of this project is to develop and demonstrate a cheap, compact silicon-based mass-spectrometer system for use in the next generation of residual gas analysers. The mass spectrometer is being constructed in collaboration with Liverpool University. There are three main sub-assemblies: an ion source, a mass filter (a harmonically-scanned electrostatic quadrupole lens) and an ion detector. The ion source is based on a field emitter array, which emits a high current electron beam to ionise any gas atoms located the input focal plane of the quadrupole lens. The lens itself is a precision self-aligned structure, based on cylindrical electrodes mounted in anisotropically etched silicon substrates. Lens assemblies mounting 500 microns dia electrode rods with an appropriate degree of precision have already been fabricated. The detector is a CMOS electrometer located at the output plane of the lens. Preliminary demonstrations of mass selection have already been performed and resolution is improving rapidly as the filter design and construction is optimised.
|End-on view of microengineered quadrupole electrostatic lens, with 500 microns dia electrodes.|
|Overall schematic of mass spectrometer based on a micromachined quadrupole lens.|
|Mass spectrum for an argon/air/helium mixture obtained with a 3 cm long lens with 500 microns diameter electrode rods.|
Two-Dimensional Microfabricated Electrostatic Einzel Lens
Richard Syms, Laurence Michelutti, Munir Ahmad
A two-dimensional electrostatic einzel lens fabricated using microfabrication technology is described. The lens consists of cylindrical electrodes mounted in on two oxidised, silicon substrates, which are held apart by two cylindrical spacers. V-shaped grooves formed by anisotropic wet chemical etching are used to locate the electrodes and the spacers. The electrodes are metal-coated glass rods that are soldered to metal films deposited in the grooves. The lens is compatible with, and can be used as ion entrance optics for, a previously described quadrupole electrostatic lens. Data showing focusing over a range of ion masses and energies have been obtained, and the results show good agreement with a simulation of the electrostatic field and paraxial ion trajectories.
|Quadrupole mass spectrometer with integrated Einzel lens|
|Cross-sectional view of the cylindrical electrostatic lens|
|Potential distribution and ion trajectories in the cylindrical electrostatic lens.|
Array-type Microengineered Electrostatic Quadrupoles
Richard Syms, Munir Ahmad, Tom Tate, Steve Taylor (Liverpool University)
Based on the successful design of our microengineered quadrupole lens, we are currently developing an array-type quadrupole device, whose elements may either be connected either in parallel (to improve sensitivity) or driven independently (for example, as a staring array, to detect a particular set of masses). Fabrication of the array again involves relatively standard bulk micromachining; however, double-sided processing is now needed, because the electrode contacts must be made by vias passing right through the wafer. The vias are formed by anisotropic etching of single crystal silicon substrates, at the same time as the grooves that mount the electrode rods. The required metal contacts are then formed on both sides of the wafer (which is now no longer planar) by electroplating Au metal inside a mask formed in an electro-deposited resist.
|Prototype array type quadrupole lens assembly.|
|Transverse potential distribution inside a silicon V-groove-mounted quadrupole array.|
Monolithic MEMS Quadrupole Mass Spectrometer
Richard Syms, Andrew Holmes
Martin Geear, Steve Wright (Microsaic Systems)
We have demonstrated a wafer scale, batch fabrication process for constructing quadrupole mass spectrometers using microelectromechanical systems (MEMS) technology. The device is formed from two bonded silicon-on-insulator substrates, which are attached together to form a monolithic block. Deep etched features and springs formed in the outer silicon layers are used to locate cylindrical metal electrode rods, while similar features formed in the inner silicon layers are used to define integrated ion entrance and exit optics. The precision of the assembly is determined by lithography and deep etching, and by the mechanical definition of the bonded silicon layers. Mass filtering has been demonstrated, with a mass range of approx. 400 a.m.u. and a mass resolution of 1 a.m.u. at 219 a.m.u., using quadrupoles with rods of 500 micron diameter and 30 mm length, operating at 6 MHz RF frequency.
|Completed quadrupole filter assembly|
|Mass spectrum of PFTBA|
MEMS Nanospray Source
Richard Syms, Helin Zou
Max Bardwell, Marc-André Schwab (Microsaic Systems)
We have demonstrated a microengineered alignment bench for a nanospray ionisation system. The bench combines a V-groove mount for a capillary-based nano-ESI source with an extraction electrode and allows accurate axial and transverse alignment of the capillary. An input channel and plenum for nebuliser gas are also demonstrated. The structure is formed in two halves, which are assembled by stacking. Electrically conducting features are constructed by crystal plane etching, deep reactive ion etching and metallisation of silicon, while the insulating base is formed in a photopatterned plastic. Low voltage (ca 850 V) positive ion emission is demonstrated using commercial nanospray (15 m ID) capillaries, with total ion currents > 10 nA. The structures are robust, and can survive voltages > 1 kV and immersion in common solvents. Mass spectrometry has been carried out using a commercial API instrument, and high signal stability has been demonstrated.
|Nanospray source in use with a Waters ZQ mass spectrometer|
|I-V characteristics of the MEMS nanospray source.|
Catheter-based Flexible Microcoil RF Detectors for Internal MRI
Simon Taylor-Robinson, Chris Wadsworth, Wady Gedroyc, Warren Casperz, St Mary's Hospital, Paddington
|Catheter-based MRI probe|
|Catheter probe inserted into cystic duct of butchered porcine liver|
We have developed flexible catheter probes for magnetic resonance imaging (MRI) of the bile duct. The probes consist of a cytology brush modified to accept a resonant RF detector based on a spiral microcoil and hybrid integrated capacitors, and are designed for insertion into the duct via a non-magnetic endoscope during endoscopic retrograde cholangiopancreatography (ERCP). The coil must be narrow enough (< 3 mm) to pass through the biopsy channel of the endoscope and sufficiently flexible to bend through 90o to enter the duct. Coils have been fabricated as multi-turn electroplated conductors on a flexible base, and two designs formed on SU-8 and polyimide substrates been compared. We have shown that careful control of thermal load is used to obtain useable mechanical properties from SU-8, and that polyimide/SU-8 composites offer improved mechanical reliability. Good electrical performance has been demonstrated and sub-millimetre resolution has been obtained in 1H MRI experiments at 1.5 T magnetic field strength using test phantoms and in-vitro liver tissue.
|1H MR image of resolution test phantom|
|In-vitro 1H MR image of porcine cystic duct|
MEMS Expanding Coil Mechanisms
R.R.A.Syms, M.M.Ahmad, and I.R.Young
M.Ichiki, Advanced Manufacturing Research Institute, Tsukuba, Japan
We are developing a mechanism for an expanding coil designed for use in in-vivo magnetic resonance imaging. The mechanism has a small (1 mm) cross-sectional width, and is designed to pass through a tubular orifice such as the endoscope channel used for accessing a bile duct. Once in place, the coil may be deformed by controlled in plane buckling. An analytic model of the device has beeen developed using Euler bending theory and verified using ANSYS simulation. Prototypes have been fabricated in bonded silicon-on-insulator material by deep reactive ion etching and undercut. Expansion and latching have been demonstrated, and performance characteristics were found to be in good agreement with the theoretical models. A lateral expansion of 1 mm was routinely achieved on either side of a device with an initial width of 1 mm (corresponding to a three-fold expansion) using an axial compression of 400 µm.
|Principle of expanding coil mechanism|
|Prototype of coil support, fabricated by deep reactive ion etching of Si.|
|ANSYS simulation of deformed coil shape|
MEMS Helmholtz Coils for Magnetic Resonance Imaging
Richard Syms, Munir Ahmad, Ian Young, David Gilderdale
Jeff Hand, Yan Li (Hammersmith Hospital)
Miniature coils for magnetic resonance imaging and spectroscopy have been demonstrated using rectangular Helmholtz arrangement based on a pair of substrates separated by spherical alignment spacers. An optimal geometry has been developed using simple theory and verified by numerical analysis. Prototypes have been fabricated using silicon substrates shaped by anisotropic etching to form a trough to hold an internal sample and pits for use as alignment features, and carrying Cu/Au conductors fabricated by electroplating. Q-values of 11 have been obtained, and devices containing an internal proton source (‘Spenco’, a commercial burn dressing) have been developed for use as fiducial markers in 1H magnetic resonance imaging at 1.5 T. Magnetic resonance spectroscopy has also performed on the internal source, with a SNR of 85.
|Completed microcoil mounted on a printed circuit board for testing.|
|SEM view of half of a MEMS Helmholtz coil, showing spherical interconnects.|
|MR spectrum of internal proton source at 1.5 T
(Courtesy Dr David Collins, Royal Marsden Hospital)
Microengineered Needle Micro-Coils for Magnetic Resonance Spectroscopy
Richard Syms, Munir Ahmad, Ian Young, David Gilderdale,
David Collins (Royal Marsden Hospital)
We have demonstrated a process for batch fabrication of low-cost needle-shaped microcoils for magnetic resonance (MR) spectroscopy. The conductors are embedded inside a cross-section designed to avoid the signal cancellation effects that can occur with completely immersed detectors. Simple models have been developed for the sensitivity of an immersed coil and for the electrical performance of coils on silicon substrates. Conductors are fabricated on oxidised Si by electroplating metals inside a deep photoresist mould, and then capped with a thick layer of plastic. Through-wafer deep reactive ion etching is used to define needle shapes. At 63.8 MHz frequency, Q-factors obtained on Si are comparable to those on glass, and resonators based on single-turn coils have Q-factors of 14. Total immersion 1H MR imaging and spectroscopy are demonstrated in a 1.5 T magnetic field using tomato fruits. Q-factors are raised at higher frequencies (to > 30 at 255 MHz) using thick polymer isolation, and hybrid integration of additional circuitry has been demonstrated.
|Completed needle microcoils|
|1H MR spectrum of a baby plum tomato|
MEMS Tilt Sensor for Biomedical Applications
Eric Yeatman and Li Zhao
Tilt is an important parameter in many motion detection applications, including the study of human body motion, currently a topic of wide interest within biosensor design. Tracking the movement of different parts of the body can help to provide important information such as the recovery status of joint injuries, movement patterns of athletes (Fig. 1), and the sleeping patterns of insomniacs. MEMS is a good solution to overcome the limitations of conventional devices, and realize miniaturization and low cost. Although wireless body-mounted devices are the initial application target, implantable variants are also a possibility, in which case minimizing size and power consumption are naturally critical requirements. The tilt sensors introduce here are designed with low power consumption in mind.
|Fig. 1. Structure of analog tilt sensor, showing proof mass, multiple beam suspension and comb drive capacitative displacement sensor|
The device fabrication is based on bonded silicon on insulator substrates, with front and back etching to release the moving parts. An inherently digital design is introduced which can simplify the read-out circuitry, or as a combined analog-digital device can maximize the range-to-resolution ratio.
|Fig. 2. Concept of inherently digital comb drive position sensor. Each bit is detected by a set of comb fingers having corrugations of appropriate periodicity.|
|Fig. 3. SEM micrograph of comb drive structure for inherently digital tilt sensor (detail of 2nd bit sensor).|
Micromachined Refreshable Braille Cell
Stepan Lucyszyn, Jun Su Lee
We are developing a new concept for the realization of a refreshable Braille cell. The cell is based on an electrothermally controlled microactuator that exploits the hydraulic pressure obtained from the volumetric expansion of melted paraffin wax. The paraffin wax is contained within a bulk micromachined silicon container, which is sealed using an elastic diaphragm. The container is heated using gold microheaters on an underlying glass substrate. All the layers used to make up the containers are bonded together using an overglaze paste. The complete 3 x 2 dot cell has gaps between containers, to prevent unwanted actuation by means of heat leakage from adjacent containers. The prototype cell measures 7 x 8.5 x 2 mm3 and its raised dots are held in equilibrium by pulsed voltages. To maintain the dot height at 50 % of its maximum, a duty factor of more than 0.8 is used, with an average power of 0.30 W (PRF = 0.027 Hz). The total actuation time for a dot on an up/down cycle is ~ 50 seconds. The dot height increases with an increasing duty factor with a fixed PRF, and increases with decreasing PRF with a fixed duty factor. A stable maximum dot height is achieved by reducing the cooling time.
|Exploded design of micromachined Braille cell|
|Fabricated parts of micromachined Braille cell.|
|Actuation of the Braille dot after 1 minute: (a) 0 V, (b) 7 V and (c) 10 V
MEMS Seimometers for Mars
Dr. Tom Pike, Dr. Sunil Kumar, Dr. Werner Karl, Mr. Trevor Semple, Dr. Toby Hopf
The aim of this project is to develop and fabricate MEMS seismometers for the ESA ExoMars mission to Mars, scheduled for launch in 2013 and UK MoonLITE mission to Moon. The Lunar mission will deliver seismometers to a number of well-spaced sites on the planet's surface. By monitoring the arrival of seismic waves at the different sites, a model of the interior structure of Moon may be deduced. Terrestrial applications of this technology are sponsored by Kinemetrics Inc., of Pasadena, California.
|3D representation of MEMS seismometer, showing 3-wafer construction|
The seismometers consist of a large proof mass supported by an elastic suspension system. The position of the mass is read by an interdigitated capacitance sensor, and then nulled by an electromagnetic force feedback system. The complete device is fabricated as a multilayer stack, with the mass and suspension being constructed by deep reactive ion etching (DRIE) right through the central wafer. Preliminary prototypes have had high out-of-plane stiffness and low cross-axis sensitivity.
|Mass-spring system, constructed by through-wafer etching using DRIE|
|Flexure suspension system under extreme extension.|
Space Mission Links
Kumar S., "Design and Fabrication of Micromachined Silicon Suspension"PhD Thesis, Department of Electrical and Electronic Engineering, Imperial College, London UK
Pike W.T., Kumar S., "Improved design of micromachined lateral suspensions using intermediate frames", Journal of Micromechanics and Microengineering, 2007, Vol: 17, Pages: 1680 - 1694
Pike W.T., Standley I., Syms R.R.A. "Improved micro-machined suspension plate with integral proof mass for use in a seismometer or other device" US Patent Application 20050097959
Pike W.T., Standley I., Trnkoczy A. "Micro-machined accelerometer" US Patent 6,776,042
Pike W. T., Standley I. M., Banerdt W.B. “A high-sensitivity broad-band seismic sensor for shallow seismic sounding of the lunar regolith” Lunar and Planetary Science Conference XXXVI, Houston, TX, Mar. 14-18 (2005)
Blast-off for Mars mission development [theregister]
MoonLITE Project [space.com]
Quakes help hunt for Martian life [Get Pdf]
Marsquake detection sensors will take search for water underground [Get Pdf]
Sensores de terremoto ajudam a procurar vida em Marte Sensores de terremoto ajudam a procurar vida em Marte
The Microscopy Station for the Phoenix Mission to Mars
Pike (Phoenix Mission Co-Investigator), Sanjay Vijendran, Hanna Sykulska
We are involved in both the hardware and operations of the microscope station currently on its way to Mars on board NASA's Phoenix Mars Lander. Imperial provided the micromachined surfaces or 'substrates' that will hold the dust and soil of Mars for examination under atomic force and optical microscopes (see J. Microscopy 227, pp 236-245 ).
|Fig 1. One of the micromachined silicon substrates produced at Imperial and now flying aboard the Phoenix lander. An identical set of 10 of these substrates are included in the microscopy station of Phoenix.|
After a landing scheduled for May 25, 2008 we will also be part of the team that will be operating Phoenix while it digs for soil and ice near to the north pole of Mars. There is a replica of the Phoenix microscope station at Imperial which is being used as a test bed to help prepare us for imaging samples either delivered by a robot arm or collected as dust falls from the Martian atmosphere.
|Fig 2. A flight copy of the Phoenix Microscopy Station including an optical microscope, an atomic force microscope and a sample wheel translation system.|
We are busy testing the microscopes with a range of dust and soil samples found on Earth in a vacuum chamber where we can simulate some of the Martian environmental conditions like low temperature (-50 degrees Celsius) and pressure (1/1000th of normal Earth atmospheric pressure).
|Fig 3. An example of a false color-composited image taken by the Phoenix Optical Microscope of some simulant Mars soil.|
For more information see:
The University of Arizona's Phoenix mission website: http://phoenix.lpl.arizona.edu/
Tom’s BBC blog of the launch: http://news.bbc.co.uk/1/hi/sci/tech/6914836.stm
The Jet Propulsion Laboratory's Phoenix site: http://www.jpl.nasa.gov/news/phoenix/index.php