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Micromachined Faraday cup array using deep reactive ion etching
Sensors and Actuators A 95 (2002) 84±93
Micromachined Faraday cup array using deep reactive ion etching Robert B. Darlinga,*, Adi A. Scheidemannb, K.N. Bhatc, T.-C. Chena a
Department of Electrical Engineering, Device Electronics Laboratory, University of Washington, Box 352500, Seattle, WA 98195-2500, USA b Intelligent Ion Inc., 2815 Eastlake Ave. E, Suite 300, Seattle, WA 98102, USA c Electrical Engineering Department, Indian Institute of Technology, Chennai 600-036, India
Abstract A micromachined Faraday cup array (MFCA) has been developed using a deep reactive ion etching (DRIE) process for applications in position sensitive charged particle detection. Deeply etched trenches with vertical sidewalls create an effective ion collection structure, and the formation of MOS capacitors on the interior surfaces produces an array of low-leakage, high stability Faraday cups that can be independently addressed. Closely spaced, linear arrays of 64, 128 and 256 cups with pitches of 150 and 250 mm have been fabricated and characterized. The narrow 50 mm silicon web between each pair of Faraday cups provides electrostatic isolation with linear ®ll factors of 67 and 80%, respectively. These Faraday cup arrays enable a new generation of compact mass spectrometers which feature true multi-channel ion detection capability over a wide mass range. Since the entire array is always open to the incident ion ¯ux, no ions are lost as in scanning mass spectrometer systems, and the overall sensitivity is drastically improved by a factor that is approximately equal to the number of cups in the array. The MFCA is thus an ideal component for miniaturized mass spectrometers, ion beam pro®ling, and chemical analyzers which must handle very small samples. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion sensors; Faraday cups; Mass spectrometers; Electronic multiplexing
1. Introduction Faraday cups are the simplest and oldest of ion detectors. They are traditionally constructed as a cylindrical cup which is coaxially enclosed within a grounded outer shell, as shown in Fig. 1. The air gap between the inner cup and the outer shell forms a stable capacitance which is insensitive to minor misalignments. A suppressor ring or grid, typically held at 50 to 100 V above ground, is often used in front of the Faraday cup to repel stray electrons, to provide a ®xed target potential that is not dependent upon the charge already collected by the cup, and also to retard the backscatter of positive ions from the cup. Commercially available Faraday cups of this type are typically 5±20 mm in diameter. The principal advantages of a Faraday cup ion detector are their simplicity, ruggedness, stability with time, extremely wide dynamic range, and ability to function in nearly any level of vacuum, even up to atmospheric pressure. The sensitivity of a Faraday cup is set principally by the Johnson noise that is associated with the load resistance RL, which may also be the feedback resistor if a current-to-voltage operational ampli®er is employed [1]. Larger load resistances typically produce higher signal-to-noise ratios, since * Corresponding author. Tel.: 1-206-534-4703; fax: 1-206-543-3842. E-mail address: [email protected] (R.B. Darling).
the gain is proportional to RL and the thermal noise voltage is 1=2 proportional to RL . Load resistances of 108±1012 O are typical when implemented via an electrometer. While the only signi®cant drawback to a Faraday cup ion detector is the fact that it only has unity gain, this situation also has the advantage of presenting no shot noise contributions and allowing the Faraday cup to be used as a laboratory standard for collected charge. With proper design, Faraday cup ion detectors can measure currents as small as 10 14 A, which corresponds approximately to an ion ¯ux of 105 ions/s. These low current and high impedance levels demand that the Faraday cup be constructed with extremely low-leakage dielectrics and well designed grounding and shielding. Position sensitive ion detection is needed for many applications, including ion beam pro®ling and mass spectrometry. Previous approaches to ion beam pro®ling have included mechanical scanning of a small Faraday cup or an aperture plate, or comparatively small arrays (4±8 cups) in conjunction with reduced range mechanical scanning [2±5]. In mass spectrometry, the detector usually remains ®xed and the electric or magnetic ion separation ®elds are ramped to sweep through the desired mass range [6]. A more ideal solution would be to have a dense array of independently addressable Faraday cups, which would eliminate the need for any moving parts or complex scanning electronics. However, at present, there has not been any successful
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Fig. 1. Traditional Faraday cup with a cylindrical geometry.
device geometry which has permitted the integration of many Faraday cups into dense arrays while still achieving the necessary low-loss dielectric, stable capacitance, and electrostatic isolation. Simply miniaturizing the individual ion collection cups and placing them close together does not provide an effective design, because the cup-to-cup capacitances CCC rapidly dominate over the cup-to-ground capacitances CCG. The quality of the cup-to-ground capacitance directly determines the dynamic range, minimum signal sensitivity, and stability of a Faraday cup. Charge sharing effects between adjacent cups will introduce parasitic artifacts into the readout signal, and the size of these artifacts is approximately in proportion to the ratio of CCC/CCG. A new device geometry is presented in which deep reactive ion etching (DRIE) is used to construct a micromachined Faraday cup array (MFCA). This geometry overcomes the previously mentioned charge sharing problems by using a silicon substrate as an intervening ground plane conductor between each cup and uses a polysilicon MOS trench capacitor to produce high stability, low-leakage cup capacitances. Linear, closely spaced arrays of small Faraday cups have been previously developed for application to compact mass spectrometers for chemical analysis [7]. Using laminated shim techniques, the smallest cup pitch
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was previously limited to about 1 mm. Micromachining the Faraday cup array has now allowed the cup pitch to be reduced to the range of 0.15±0.25 mm, and conceivably smaller still in the future [8]. This increase in spatial resolution directly produces an improvement in mass resolution Dm/m for a mass spectrometer and opens up several new application areas which would only have been previously possible with larger geometry beam lines. Furthermore, each of the cups is independently electrically addressable, so that true multi-channel ion detection is achieved. Since no ion ¯ux is lost by scanning the detector mass window, the overall system sensitivity is increased by nearly a factor of N, where N is the number of cups in the array [8]. 2. Design The design of the MFCA is based upon a series of closely spaced DRIE trenches which are oxidized and then conformally coated with polysilicon to produce high stability, low-leakage MOS capacitors. The polysilicon is patterned on the top of the wafer into individual contact tabs for each cup, as shown in Fig. 2. Microfabrication of the MFCA using DRIE affords two key advantages. First, the minimum pitch of the array can be signi®cantly reduced from the limits imposed by other fabrication means, which include for example, conventional drilling and milling machining techniques, electrical discharge machining (EDM), or stacked laminate construction. Second, the DRIE process can produce nearly vertical sidewalls for the cups which is an essential feature for an effective ion trapping and charge collection structure. As an array of ion collection cups is miniaturized, and the cups thus brought more closely together, the cup-to-cup capacitances CCC can easily dominate over the desired cup-to-ground capacitances CCG. The electrostatic coupling
Fig. 2. Concept drawing of a micromachined Faraday cup array formed by deep reactive ion etching of a silicon wafer. The closely spaced trenches are oxidized and coated with polysilicon, which is then patterned into contact tabs for each cup in the array.
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between adjacent cups can be controlled by shielding each cup with a common, intervening ground conductor. An independent ground shield, however, can occupy a signi®cant space of its own, reducing the available aperture area of the array, and can present additional fabrication and interconnection dif®culties. In the present design, the silicon substrate itself is used to create a common ground shield around each Faraday cup. Using DRIE, the individual cups can be separated by a thin web whose thickness S is small in comparison to the cup aperture widths W. The ®ll factor for the array is the ratio of the open detector aperture area to the total area of the array. For a linear, one-dimensional array, the ®ll factor is F W/(W S). The DRIE process uniquely creates cups with high vertical aspect ratios, which when combined with a thin native oxide insulator and a conformally deposited polysilicon cup conductor, can integrate the cup walls, insulator, and silicon wafer ground shield within a web thickness of only a few tens of microns. An important characteristic of the MFCA is its performance in trapping incident ions without allowing the ion to backscatter out of the cup, sputter away material from the cup, or emit secondary electrons back out of the cup, any of which would alter the collected charge state of the cup. Incident ions may have a wide range of energies, with 500± 1500 eV being typical for mass spectrometry applications. At these energies, the incident ion's interaction with the cup rarely involves a single collision. The desired trapping process is for the incident ion to successively scatter off of the cup walls, dropping its momentum and kinetic energy on each collision, and ultimately becoming physisorbed onto the inner conducting surface of the cup, to which its carried charge is added. Vertical cup sidewalls provide a nearly optimal geometry for this process, provided that the
cup is suf®ciently deep for several scattering events to lower the incident kinetic energy. The specular trajectory of an incident ion is shown in Fig. 3. For a cup of depth D, the number of possible wall collisions is (D/W) tan Y, where Y is the angle off normal for the incident ion. For large depth to width ratios D/W, and also for increasingly off-normal incidence into the cup Y, the number of possible wall collisions is increased, and the ion trapping ef®ciency thus improved. While ion backscatter can be minimized by proper design of the cup walls, the shower of emitted secondary electrons must also be contained within the cup to preserve the charge collection accuracy. The present MFCA design performs well in this regard, because secondary electrons are primarily emitted normal to the struck surface, and the vertical walls of the cup tend to shower the secondary electrons against the opposite wall, as shown in Fig. 3. If the cup depth to width ratio is too small or the incident ion angle is too near normal, both the backscatter and secondary electron emission from the cup can be reduced by increasing the voltage on the suppressor electrode. Current though the suppressor electrode is usually a good measure of this net backscatter. The creation of a stable, low-leakage cup-to-ground capacitance CCG is critical to the design of the MFCA. Because the MFCA is used in a charge integrating mode, a fairly large value of CCG is required to keep the cup voltage from saturating the readout electronics. The value of CCG also needs to be larger than the parasitic interconnection capacitances of the system, which are typically several picofarads. Trench capacitors are routinely used in DRAM memory cells to provide high capacitance for a given amount of chip area [9]. The present MFCA design is, in effect, a trench capacitor, albeit on a much larger size scale.
Fig. 3. Ion capture processes within a Faraday cup.
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Fig. 4. Pictorial cross-section of the MFCA and its associated components.
With cup lengths of a few mm, cup-to-ground capacitances in the hundreds of picofarads can be readily created, since most of the capacitance comes from the sidewall area. Native SiO2 growth is used to create a uniform cup insulator, and minimum thicknesses are determined primarily by the needed breakdown voltage of the cup. A conformal cup conductor is also needed to create the inner surface of the cup, and because of the high aspect ratio of the cup, a CVD process is required. The cup conductor also needs to be thermally stable and have a low sputtering yield. Both LPCVD polysilicon and CVD tungsten are good candidates, but polysilicon was chosen because of its better availability and wider choice of etching processes. Since the incident ion ¯ux is a ¯ow of cations which will add positive charge to the cup, the silicon wafer is chosen to be n-type so that the MOS capacitor will operate in the accumulation mode, for which the capacitance per unit area will remain at a constant value of C ox eox =xox as the cup voltage increases. While the webs between adjacent cups can be created with thinner dimensions than those used, they were designed with a value of 50 mm because of the need for patterning the polysilicon into individual conductors for each cup along the top of each web. The completed structure of the MFCA is shown in Fig. 4, which includes a ®ne pitch metal screen that is positioned above the cup array to create the common suppressor grid, and a bake-out heater which is bonded to backside of the die to provide for periodic in-situ thermal desorption of the accumulated ion species. The Si/SiO2/ poly-Si construction of the MFCA allows the structure to be operated at relatively high temperatures and also easily withstand typical bake-out temperatures of 300±4008C. The high thermal conductivity of the silicon substrate is an advantage in this regard, allowing the bake-out heater to fully desorb ions on all surfaces of the cups. 3. Microfabrication The fabrication process began with n-type silicon wafers, 100 mm diameter and 400 5 mm thick, which were given
an RCA clean and an initial 500 nm thick wet oxidation. The ®rst mask was used to pattern a layer of positive photoresist for the DRIE of the Faraday cups. The DRIE process was then tuned to produce a nominal etch depth of D 300 mm, which required 110 min on the STS DRIE system. The mask set contained Faraday cup arrays of several different geometry parameters. Two cup array pitches were created, a W 100 mm cup aperture width with a S 50 mm web (150 mm pitch, 67% ®ll factor), and a W 200 mm cup aperture width with a S 50 mm web (250 mm pitch, 80% ®ll factor). Etch depth calibration pro®les for both of these pitches are shown in Fig. 5. Arrays of 64, 128, and 256 cups were created with cup lengths of L 1:0, 2.0 and 3.0 mm. After the DRIE, the wafers were cleaned for 30 min in nanostrip to remove the ¯uorocarbon residue and any remaining oxide was then removed with 10:1 BOE. A new, 150 nm thick native SiO2 layer was then grown by dry oxidation to produce a high-quality, low-leakage conformal insulator over the cup features. A conformal layer of polysilicon was then deposited by LPCVD at 6208C, 200 mTorr, and 40 sccm SiH4 to a thickness of 0.75 mm to form the Faraday cup inner conductor. This layer of polysilicon was then heavily doped with boron using a BN solid source at 10508C for 15 min to increase the conductivity of the cup conductors. The polysilicon sheet resistance was lowered by this step to 21 O/&. After the blanket oxidation, polysilicon deposition, and doping processes are completed, the polysilicon must be photolithographyically patterned and etched to electrically isolate each cup and to create an interconnection tab on the top surface of the dye. The polysilicon must be etched down to the underlying oxide on the top of each web to achieve this, as shown in Fig. 3, and the extremely nonplanar structure of the wafer makes this very challenging. Although thinner webs could have been created, the webs were speci®cally created with a minimum thickness of 50 mm to accomodate this part of the processing. Because the polysilicon surface to be etched remains level with the top of the wafer, photolithographic exposure, development, and subsequent etching are still straightforward; however,
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Fig. 5. Cross-section sections of deep reactive ion etched cups. (a) 150 mm pitch cup; (b) 250 mm pitch cup.
conformal photoresist coating is extremely dif®cult to accomplish due to the sharp edges and large depth of the DRIE trenches. As a solution to this problem, several different materials were evaluated for planarizing the wafer. Polyimides exhibited too much shrinkage upon cure to be useful for this degree of planarization, but eventually a successful procedure was developed using successive coats of thick AZ-4620 photoresist to nearly ®ll in the DRIE trenches. After contact photolithography with the second mask to expose the resist and its subsequent development, an SF6 RIE was used to etch the polysilicon down to the underlying SiO2 layer in a stripe across each web. The third mask was used to pattern another coating of AZ4620 photoresist to de®ne the electrical contact pads to the polysilicon tabs. A metalization of Al/Cr/Au (150 nm/ 15 nm/150 nm) was then deposited and lifted off, and this was then annealed at 4008C for 30 min. After the metal anneal, the wafers were diced into individual detector arrays. The die were then washed to remove dicing debris and
thoroughly cleaned in nanostrip and a ®nal oxygen plasma ashing to remove photoresist traces from deep within the cups. The resulting MFCA die are shown in Fig. 6. 4. Characterization The fabrication process thus produces a large MOS trench capacitor for each Faraday cup with the grounded silicon substrate forming an electrostatic shield around each. The cup-to-ground capacitances for the 150 mm pitch cups were 180, 345 and 498 pF for the 1.0, 2.0 and 3.0 mm lengths. For the 250 mm pitch cups, the cup-to-ground capacitances were 208, 400 and 595 pF for the 1.0, 2.0 and 3.0 mm lengths. These fairly large values of capacitance are well suited to the electronic readout circuitry, since their values are well above the packaging and interconnection parasitic levels of the system. For all of the detector arrays, the cup-to-cup capacitances were below the measurement limit of a few
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Fig. 6. Top view of overall array of 1.00 mm by 100 mm cups with a 150 mm pitch. (a) View showing polysilicon interconnect pads to the right of the cup array; (b) view of cups showing sharp internal features.
picofarads. The electrostatic shielding provided by the grounded silicon substrate webs drastically reduces this value of cup-to-cup capacitance, and thereby reduces the readout charge-sharing artifacts associated with this parasitic effect. Electrical leakage levels were also measured for each Faraday cup to ground using an electrometer. Resistance levels of over 10 GO were routinely measured, although the same values were obtained with the probe tip lifted off of the bond pad, indicating that this leakage current level may be arising from the test leads themselves. Because of the sensitivity of a MOS capacitor to impurity and structural oxide defects, additional characterization of the Faraday cup trench capacitors was performed. The interior and exterior corners of the Faraday cup are the most likely locations for the oxide integrity to be compromised. Scanning electron microscope images of these regions are
shown in Fig. 7, from which a uniform thickness, conformal SiO2 layer is apparent. The thickness and uniformity of the oxide was also checked by several cross-sectional cuts which were etched in 10:1 BOE and then imaged with an AFM. These also revealed a uniform SiO2 thickness with smooth interfaces with both the Si and polysilicon regions. 5. Packaging and electronic readout The MFCA die is packaged by means of a polyimide/ copper/polyimide ¯exible printed circuit cable. The dye is bonded on to one end of the ¯ex cable adjacent to a row of copper pads onto which each Faraday cup of the dye is connected by a bond wire. The bond wires were encapsulated in a thermal setting epoxy for handling protection.
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Fig. 7. Cross-section sections of silicon cup, SiO2 insulator, and polysilicon cup conductor. (a) Outer corner of cup; (b) interior corner of cup.
The opposite end of the ¯ex cable consists of a square ball grid array (BGA) pattern which allows it to be soldered to a mating pattern on a rigid printed circuit board containing the electronic readout circuitry. For prototyping purposes, the ¯exible cable consisted of only 64 conductors. The ¯exible cable packaging for the MFCA allows it to be easily positioned within the tight quarters of a miniature vacuum chamber, and the cable itself can be sandwiched within a ¯at surfaced o-ring seal to provide a high density, economical vacuum feedthrough. This arrangement allows the electronic readout circuitry to be placed outside of the
vacuum chamber, further reducing the interior volume in mass spectrometer applications. The electronic readout circuitry is shown in Fig. 8 in which each of the Faraday cups is directly connected to a switch of an analog multiplexer. The typically 8:1 or 16:1 multiplexer chips are cascaded to provide a suf®cient number of inputs for the overall MFCA. The appropriate switch of the analog multiplexers is determined by an address sent from a generic array logic (GAL) chip which is programmed to sequence linearly through the array and is clocked by the readout master oscillator. The output from the analog
Fig. 8. Electronic scanning readout system.
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multiplexer is fed into an integrating operational ampli®er which is successively reset by the master oscillator. For each cup in sequence, the integrator is ®rst reset, and then the analog switch is closed, creating a virtual ground for the charge in the cup to ¯ow into. The integrated charge is transferred to the feedback capacitor CF, which provides the analog output signal level of that cup in the array. The GAL chip also creates a frame synchronization pulse to mark the start of the scan through the array, and this signal can be used as an external trigger to directly display the MFCA collected charge output pro®le on an oscilloscope. The frequency of the master oscillator is made adjustable since this sets the integration time period for the MFCA and can be used to set the baseline sensitivity and saturation limits. 6. Application to a mass spectrometer The MFCA has been designed for use in a compact, linear dispersion mass spectrometer, which is shown schematically in Fig. 9 and follows a commonly used Mattauch±Herzog layout. A dc glow discharge is used to ionize the sample and the charged analyte fragments are then accelerated and focused by a dc electric ®eld. The combination of the dc electric and permanent magnetic ®eld sectors images the ionization source onto the detector plane with a position dependent upon the mass of the ion. In this geometry, the dispersed ions enter the Faraday cups at a 458 angle, which minimizes backscatter and secondary electron emission from the cup. The current state of development of this mass spectrometer is shown in Fig. 10. An ion of mass m and charge q moving with velocity v in a uniform magnetic ®eld B will be de¯ected with a radius R mv/qB. If the velocity of the ion is established prior by
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Fig. 9. Geometry of the linear dispersion mass spectrometer.
an electric acceleration ®eld, the energy of the ion will be E qV mv2 =2, where V is the acceleration potential. Thus, the position of the ion on the detector plane will be proportional to the radius of its trajectory within the magnetic sector, R 2mE1=2 =qB. However, in the present linear dispersion mass spectrometer, a uniquely designed inhomogeneous ®eld permanent magnet is used to disperse the ions onto the detector plane with their position proportional to their mass, rather than proportional to m1/2. When the MFCA is positioned onto the detector plane as shown in Fig. 10, the uniform spacing of the Faraday cups creates a constant Dm selection between adjacent cups which allows the mass spectrum to be directly read out from the MFCA using the simple scanning circuitry of Fig. 8. Most existing mass spectrometers utilize a single point ion detector and a scan through the mass spectrum range is achieved by either ramping the electric or magnetic ion optics ®elds, scanning the ®eld of a quadrupole analyzer, or mechanically moving the detector across the image plane of
Fig. 10. Current development of the compact, linear dispersion mass spectrometer.
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the output. High mass resolution is achieved by windowing only a small fraction of the overall ion ¯ux. While the detector is aligned to only that speci®c mass window, the remainder of the ionized sample is rejected and thus lost. The MFCA enables a new generation of mass spectrometers which feature true multi-channel ion detection. Since the MFCA always remains open to the full incident ion ¯ux, all mass trajectories within the detector's range are simultaneously sampled and integrated into their respective cups in the array. The entire mass spectrum is acquired simultaneously so that the overall spectrum is acquired in a much shorter time. Typically, only 1±2 ms are required to scan over a 100 amu range. Because all of the incident ions are captured by the MFCA detector, the sensitivity of the system is improved by a factor that is approximately equal to the number of cups in the array N. This improved sensitivity and scan speed are both becoming increasingly important for applications where very small samples are involved, where rapid throughput testing is required, or where continuous online monitoring is implemented [10]. 7. Conclusion A new device structure has been developed for integrating Faraday cup ion detectors into a linear, closely spaced array. DRIE is used to produce high aspect ratio trenches with narrow separator webs that function as effective ion collection traps. The incident charge is collected on low-leakage polysilicon MOS capacitors which line the interior of each cup. The grounded silicon substrate that forms the separator web also provides electrostatic shielding for each cup, making the cup-to-ground capacitance dominate over the cup-to-cup capacitance. A stable and rugged position sensitive ion detector is produced by this design which is directly applicable to mass spectrometry and ion beam pro®ling. True multi-channel ion detection is achieved with the MFCA since none of the incident ion ¯ux is rejected by scanning a mass selection window. Signi®cant improvements in sensitivity and readout speed are obtained with multi-channel detection, and the MFCA currently plays a central role in the development of a compact, linear dispersion mass spectrometer. With the mass resolution now set by the cup pitch of the MFCA relative to the dispersed distance of the ion's mass range, microfabrication techniques which can reduce the cup pitch will also shorten the required dispersed distance and lead to much more compact, high performance mass spectrometers. Acknowledgements The authors wish to thank Nadim Maluf and Bart van Drieenhuizen of Lucas NovaSensor for performing the DRIE work. Thanks are also extended to Leo L. Lam for assistance with the photolithography. This work was
sponsored by the NSF Center for Process Analytical Chemistry (CPAC) at the University of Washington. References [1] G. Scoles (Ed.), Atomic and Molecular Beam Methods, Vol. 1, Oxford University Press, New York, 1988. [2] W.N. Hammer, A.E. Michael, A technique for measuring, displaying, recording, and modifying the spatial uniformity of implanted ions, J. Appl. Phys. 47 (5) (1976) 2161±2164. [3] N. Natsuaki, K. Ohyu, T. Tokuyama, Spatial dose uniformity monitor for electrically scanned beam, Rev. Sci. Instrum. 49 (9) (1978) 1300± 1304. [4] C.O. Morain, P. O'Keeffe, S. Den, Y. Hayashi, Large diameter plasma profile monitoring using Faraday cup arrays, Meas. Sci. Tech. 4 (1993) 1484±1488. [5] R.B. Liebert, Method and Apparatus for Ion Beam Centroid Location, US Patent No. 4,724,324, 9 February 1988. [6] C.A. MacDonald (Ed.), Mass Spectrometry, McGraw-Hill, New York, 1963. [7] A.A. Scheidemann, R.B. Darling, F.J. Schumacher, A. Isakarov, Linear dispersion mass spectrometer, in: Proceedings of the Technical Digest of the 14th International Forum on Process Analytical Chemistry (IFPAC-2000); Abstract I-067, Lake Las Vegas, Nevada, 23±26 January 2000. [8] R.B. Darling, A.A. Scheidemann, K.N. Bhat, T.-C. Chen, Micromachined faraday cup array using deep reactive ion etching, in: Proceedings of the 14th IEEE International Conference on Microelectromechanical Systems (MEMS-2001), Interlaken, Switzerland, 21±25 January 2001, pp. 90±93. [9] D.A. Baglee, R.R. Doering, M. Elahy, M. Yashiro, Properties of Trench Capacitors for High Density DRAM Applications, IEDM Technical Digest, December 1985, pp. 384±387. [10] M.P. Shiha, G. Gutnikov, Development of a miniature gas chromatograph Ð mass spectrometer with a microbore capillary column and array detector, Anal. Chem. 63 (18) (1991) 2012±2016.
Biographies Robert B. Darling was born in Johnson City, TN on 15 March 1958. He received the BSEE (with highest honors), MSEE and PhD degrees in electrical engineering from the Georgia Institute of Technology in 1980, 1982 and 1985, respectively. He has held Summer positions with SperryUnivac, Bristol, TN, and Texas Instruments, Johnson City, TN, and from 1982 to 1983, he was with the Physical Sciences Division of the Georgia Technical Research Institute, Atlanta, GA. In 1985, he joined the Department of electrical engineering, University of Washington, Seattle, as an Assistant Professor. He was promoted to Associate Professor in 1990 and to Full Professor in 1999. From 1995 to 1996, he was a Visiting Associate Professor at Stanford University, Stanford, CA. Currently, he is also an Adjunct Professor of Bioengineering and Director of the Electrical Engineering Microfabrication Laboratory. His research interests include electron device physics, device modeling, microfabrication, circuit design, optoelectronics, sensors, electrochemistry, and instrumentation electronics. Dr. Darling is a senior member of the IEEE, a member of the American Physical Society, the American Vacuum Society, the Optical Society of America, and is a registered professional engineer in the State of Washington. Adi A. Scheidemann was born in Ruesselheim, close to Frankfurt, Germany in June 1957. Dr. Scheidemann received his diploma (MS) in theoretical physics from the University of Frankfurt in 1984. His thesis topic was density functional theory. Thereafter Dr. Scheidemann switched to
R.B. Darling et al. / Sensors and Actuators A 95 (2002) 84±93 experimental physics and joined the Max Planck Institute for fluid dynamics in Goettingen, Germany. He graduated in 1989 in the field of applied physics and fluid dynamics at the University in Goettingen with an experimental research project on helium nanodroplets. Post-doctoral work brought him to the University of California, Berkeley, to work on metal clusters before joining the research faculty of the Department of Chemistry at the University of Washington in 1991. Dr. Scheidemann left the UW in summer of 2000 to launch a start-up company developing miniaturized mass spectrometer systems. His research focuses on miniaturized instrumentation and nanoparticles. The later work is done in close collaboration with Prof. V. Kresin at the University of Southern California. Dr. Scheidemann is a member of the American Society for Mass Spectrometry and the American Physical Society. K.N. Bhat received the BSc degree from St. Aloysius College, Mangalore, India, in 1963, the BE degree in electrical technology from the Indian Institute of Science, Bangalore, in 1966 and the MTech degree in electrical engineering from the Indian Institute of Technology, Chennai, in 1969. He then continued on to earn the MEng degree in electrical engineering from
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Rensselaer Polytechnic, Troy, New York, in 1974, and the PhD degree in electrical engineering from the Indian Institute of Technology, Chennai, in 1978. He has been on the faculty of IIT, Chennai, since 1981, where he is presently Professor of electrical engineering and Head of the Microelectronics Laboratory. He spent 1979±1981 as a Post-doctoral Research Associate at Rensselaer Polytechnic, and more recently spent 1999 as a Visiting Professor at the University of Washington, Seattle. His research interests include microelectronic and optoelectronic devices and microelectromechanical systems (MEMS). Tai-Chang Chen is a Research Engineer in the Department of electrical engineering, University of Washington, Seattle, Washington. He received the BS degree in materials science and engineering from the National Hsing-Hua University, Taiwan, in 1988, and the MS and PhD degrees in materials science and Engineering from University of Washington, Seattle, in 1993 and 1997. He teaches courses in integrated circuit and microfabrication technologies. His present research areas include solid-state sensors, micromachined transducers and micromachining technologies.
Micromachined Faraday cup array using deep reactive ion etching Robert B. Darlinga,*, Adi A. Scheidemannb, K.N. Bhatc, T.-C. Chena a
Department of Electrical Engineering, Device Electronics Laboratory, University of Washington, Box 352500, Seattle, WA 98195-2500, USA b Intelligent Ion Inc., 2815 Eastlake Ave. E, Suite 300, Seattle, WA 98102, USA c Electrical Engineering Department, Indian Institute of Technology, Chennai 600-036, India
Abstract A micromachined Faraday cup array (MFCA) has been developed using a deep reactive ion etching (DRIE) process for applications in position sensitive charged particle detection. Deeply etched trenches with vertical sidewalls create an effective ion collection structure, and the formation of MOS capacitors on the interior surfaces produces an array of low-leakage, high stability Faraday cups that can be independently addressed. Closely spaced, linear arrays of 64, 128 and 256 cups with pitches of 150 and 250 mm have been fabricated and characterized. The narrow 50 mm silicon web between each pair of Faraday cups provides electrostatic isolation with linear ®ll factors of 67 and 80%, respectively. These Faraday cup arrays enable a new generation of compact mass spectrometers which feature true multi-channel ion detection capability over a wide mass range. Since the entire array is always open to the incident ion ¯ux, no ions are lost as in scanning mass spectrometer systems, and the overall sensitivity is drastically improved by a factor that is approximately equal to the number of cups in the array. The MFCA is thus an ideal component for miniaturized mass spectrometers, ion beam pro®ling, and chemical analyzers which must handle very small samples. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion sensors; Faraday cups; Mass spectrometers; Electronic multiplexing
1. Introduction Faraday cups are the simplest and oldest of ion detectors. They are traditionally constructed as a cylindrical cup which is coaxially enclosed within a grounded outer shell, as shown in Fig. 1. The air gap between the inner cup and the outer shell forms a stable capacitance which is insensitive to minor misalignments. A suppressor ring or grid, typically held at 50 to 100 V above ground, is often used in front of the Faraday cup to repel stray electrons, to provide a ®xed target potential that is not dependent upon the charge already collected by the cup, and also to retard the backscatter of positive ions from the cup. Commercially available Faraday cups of this type are typically 5±20 mm in diameter. The principal advantages of a Faraday cup ion detector are their simplicity, ruggedness, stability with time, extremely wide dynamic range, and ability to function in nearly any level of vacuum, even up to atmospheric pressure. The sensitivity of a Faraday cup is set principally by the Johnson noise that is associated with the load resistance RL, which may also be the feedback resistor if a current-to-voltage operational ampli®er is employed [1]. Larger load resistances typically produce higher signal-to-noise ratios, since * Corresponding author. Tel.: 1-206-534-4703; fax: 1-206-543-3842. E-mail address: [email protected] (R.B. Darling).
the gain is proportional to RL and the thermal noise voltage is 1=2 proportional to RL . Load resistances of 108±1012 O are typical when implemented via an electrometer. While the only signi®cant drawback to a Faraday cup ion detector is the fact that it only has unity gain, this situation also has the advantage of presenting no shot noise contributions and allowing the Faraday cup to be used as a laboratory standard for collected charge. With proper design, Faraday cup ion detectors can measure currents as small as 10 14 A, which corresponds approximately to an ion ¯ux of 105 ions/s. These low current and high impedance levels demand that the Faraday cup be constructed with extremely low-leakage dielectrics and well designed grounding and shielding. Position sensitive ion detection is needed for many applications, including ion beam pro®ling and mass spectrometry. Previous approaches to ion beam pro®ling have included mechanical scanning of a small Faraday cup or an aperture plate, or comparatively small arrays (4±8 cups) in conjunction with reduced range mechanical scanning [2±5]. In mass spectrometry, the detector usually remains ®xed and the electric or magnetic ion separation ®elds are ramped to sweep through the desired mass range [6]. A more ideal solution would be to have a dense array of independently addressable Faraday cups, which would eliminate the need for any moving parts or complex scanning electronics. However, at present, there has not been any successful
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 7 1 8 - X
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Fig. 1. Traditional Faraday cup with a cylindrical geometry.
device geometry which has permitted the integration of many Faraday cups into dense arrays while still achieving the necessary low-loss dielectric, stable capacitance, and electrostatic isolation. Simply miniaturizing the individual ion collection cups and placing them close together does not provide an effective design, because the cup-to-cup capacitances CCC rapidly dominate over the cup-to-ground capacitances CCG. The quality of the cup-to-ground capacitance directly determines the dynamic range, minimum signal sensitivity, and stability of a Faraday cup. Charge sharing effects between adjacent cups will introduce parasitic artifacts into the readout signal, and the size of these artifacts is approximately in proportion to the ratio of CCC/CCG. A new device geometry is presented in which deep reactive ion etching (DRIE) is used to construct a micromachined Faraday cup array (MFCA). This geometry overcomes the previously mentioned charge sharing problems by using a silicon substrate as an intervening ground plane conductor between each cup and uses a polysilicon MOS trench capacitor to produce high stability, low-leakage cup capacitances. Linear, closely spaced arrays of small Faraday cups have been previously developed for application to compact mass spectrometers for chemical analysis [7]. Using laminated shim techniques, the smallest cup pitch
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was previously limited to about 1 mm. Micromachining the Faraday cup array has now allowed the cup pitch to be reduced to the range of 0.15±0.25 mm, and conceivably smaller still in the future [8]. This increase in spatial resolution directly produces an improvement in mass resolution Dm/m for a mass spectrometer and opens up several new application areas which would only have been previously possible with larger geometry beam lines. Furthermore, each of the cups is independently electrically addressable, so that true multi-channel ion detection is achieved. Since no ion ¯ux is lost by scanning the detector mass window, the overall system sensitivity is increased by nearly a factor of N, where N is the number of cups in the array [8]. 2. Design The design of the MFCA is based upon a series of closely spaced DRIE trenches which are oxidized and then conformally coated with polysilicon to produce high stability, low-leakage MOS capacitors. The polysilicon is patterned on the top of the wafer into individual contact tabs for each cup, as shown in Fig. 2. Microfabrication of the MFCA using DRIE affords two key advantages. First, the minimum pitch of the array can be signi®cantly reduced from the limits imposed by other fabrication means, which include for example, conventional drilling and milling machining techniques, electrical discharge machining (EDM), or stacked laminate construction. Second, the DRIE process can produce nearly vertical sidewalls for the cups which is an essential feature for an effective ion trapping and charge collection structure. As an array of ion collection cups is miniaturized, and the cups thus brought more closely together, the cup-to-cup capacitances CCC can easily dominate over the desired cup-to-ground capacitances CCG. The electrostatic coupling
Fig. 2. Concept drawing of a micromachined Faraday cup array formed by deep reactive ion etching of a silicon wafer. The closely spaced trenches are oxidized and coated with polysilicon, which is then patterned into contact tabs for each cup in the array.
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between adjacent cups can be controlled by shielding each cup with a common, intervening ground conductor. An independent ground shield, however, can occupy a signi®cant space of its own, reducing the available aperture area of the array, and can present additional fabrication and interconnection dif®culties. In the present design, the silicon substrate itself is used to create a common ground shield around each Faraday cup. Using DRIE, the individual cups can be separated by a thin web whose thickness S is small in comparison to the cup aperture widths W. The ®ll factor for the array is the ratio of the open detector aperture area to the total area of the array. For a linear, one-dimensional array, the ®ll factor is F W/(W S). The DRIE process uniquely creates cups with high vertical aspect ratios, which when combined with a thin native oxide insulator and a conformally deposited polysilicon cup conductor, can integrate the cup walls, insulator, and silicon wafer ground shield within a web thickness of only a few tens of microns. An important characteristic of the MFCA is its performance in trapping incident ions without allowing the ion to backscatter out of the cup, sputter away material from the cup, or emit secondary electrons back out of the cup, any of which would alter the collected charge state of the cup. Incident ions may have a wide range of energies, with 500± 1500 eV being typical for mass spectrometry applications. At these energies, the incident ion's interaction with the cup rarely involves a single collision. The desired trapping process is for the incident ion to successively scatter off of the cup walls, dropping its momentum and kinetic energy on each collision, and ultimately becoming physisorbed onto the inner conducting surface of the cup, to which its carried charge is added. Vertical cup sidewalls provide a nearly optimal geometry for this process, provided that the
cup is suf®ciently deep for several scattering events to lower the incident kinetic energy. The specular trajectory of an incident ion is shown in Fig. 3. For a cup of depth D, the number of possible wall collisions is (D/W) tan Y, where Y is the angle off normal for the incident ion. For large depth to width ratios D/W, and also for increasingly off-normal incidence into the cup Y, the number of possible wall collisions is increased, and the ion trapping ef®ciency thus improved. While ion backscatter can be minimized by proper design of the cup walls, the shower of emitted secondary electrons must also be contained within the cup to preserve the charge collection accuracy. The present MFCA design performs well in this regard, because secondary electrons are primarily emitted normal to the struck surface, and the vertical walls of the cup tend to shower the secondary electrons against the opposite wall, as shown in Fig. 3. If the cup depth to width ratio is too small or the incident ion angle is too near normal, both the backscatter and secondary electron emission from the cup can be reduced by increasing the voltage on the suppressor electrode. Current though the suppressor electrode is usually a good measure of this net backscatter. The creation of a stable, low-leakage cup-to-ground capacitance CCG is critical to the design of the MFCA. Because the MFCA is used in a charge integrating mode, a fairly large value of CCG is required to keep the cup voltage from saturating the readout electronics. The value of CCG also needs to be larger than the parasitic interconnection capacitances of the system, which are typically several picofarads. Trench capacitors are routinely used in DRAM memory cells to provide high capacitance for a given amount of chip area [9]. The present MFCA design is, in effect, a trench capacitor, albeit on a much larger size scale.
Fig. 3. Ion capture processes within a Faraday cup.
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Fig. 4. Pictorial cross-section of the MFCA and its associated components.
With cup lengths of a few mm, cup-to-ground capacitances in the hundreds of picofarads can be readily created, since most of the capacitance comes from the sidewall area. Native SiO2 growth is used to create a uniform cup insulator, and minimum thicknesses are determined primarily by the needed breakdown voltage of the cup. A conformal cup conductor is also needed to create the inner surface of the cup, and because of the high aspect ratio of the cup, a CVD process is required. The cup conductor also needs to be thermally stable and have a low sputtering yield. Both LPCVD polysilicon and CVD tungsten are good candidates, but polysilicon was chosen because of its better availability and wider choice of etching processes. Since the incident ion ¯ux is a ¯ow of cations which will add positive charge to the cup, the silicon wafer is chosen to be n-type so that the MOS capacitor will operate in the accumulation mode, for which the capacitance per unit area will remain at a constant value of C ox eox =xox as the cup voltage increases. While the webs between adjacent cups can be created with thinner dimensions than those used, they were designed with a value of 50 mm because of the need for patterning the polysilicon into individual conductors for each cup along the top of each web. The completed structure of the MFCA is shown in Fig. 4, which includes a ®ne pitch metal screen that is positioned above the cup array to create the common suppressor grid, and a bake-out heater which is bonded to backside of the die to provide for periodic in-situ thermal desorption of the accumulated ion species. The Si/SiO2/ poly-Si construction of the MFCA allows the structure to be operated at relatively high temperatures and also easily withstand typical bake-out temperatures of 300±4008C. The high thermal conductivity of the silicon substrate is an advantage in this regard, allowing the bake-out heater to fully desorb ions on all surfaces of the cups. 3. Microfabrication The fabrication process began with n-type silicon wafers, 100 mm diameter and 400 5 mm thick, which were given
an RCA clean and an initial 500 nm thick wet oxidation. The ®rst mask was used to pattern a layer of positive photoresist for the DRIE of the Faraday cups. The DRIE process was then tuned to produce a nominal etch depth of D 300 mm, which required 110 min on the STS DRIE system. The mask set contained Faraday cup arrays of several different geometry parameters. Two cup array pitches were created, a W 100 mm cup aperture width with a S 50 mm web (150 mm pitch, 67% ®ll factor), and a W 200 mm cup aperture width with a S 50 mm web (250 mm pitch, 80% ®ll factor). Etch depth calibration pro®les for both of these pitches are shown in Fig. 5. Arrays of 64, 128, and 256 cups were created with cup lengths of L 1:0, 2.0 and 3.0 mm. After the DRIE, the wafers were cleaned for 30 min in nanostrip to remove the ¯uorocarbon residue and any remaining oxide was then removed with 10:1 BOE. A new, 150 nm thick native SiO2 layer was then grown by dry oxidation to produce a high-quality, low-leakage conformal insulator over the cup features. A conformal layer of polysilicon was then deposited by LPCVD at 6208C, 200 mTorr, and 40 sccm SiH4 to a thickness of 0.75 mm to form the Faraday cup inner conductor. This layer of polysilicon was then heavily doped with boron using a BN solid source at 10508C for 15 min to increase the conductivity of the cup conductors. The polysilicon sheet resistance was lowered by this step to 21 O/&. After the blanket oxidation, polysilicon deposition, and doping processes are completed, the polysilicon must be photolithographyically patterned and etched to electrically isolate each cup and to create an interconnection tab on the top surface of the dye. The polysilicon must be etched down to the underlying oxide on the top of each web to achieve this, as shown in Fig. 3, and the extremely nonplanar structure of the wafer makes this very challenging. Although thinner webs could have been created, the webs were speci®cally created with a minimum thickness of 50 mm to accomodate this part of the processing. Because the polysilicon surface to be etched remains level with the top of the wafer, photolithographic exposure, development, and subsequent etching are still straightforward; however,
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Fig. 5. Cross-section sections of deep reactive ion etched cups. (a) 150 mm pitch cup; (b) 250 mm pitch cup.
conformal photoresist coating is extremely dif®cult to accomplish due to the sharp edges and large depth of the DRIE trenches. As a solution to this problem, several different materials were evaluated for planarizing the wafer. Polyimides exhibited too much shrinkage upon cure to be useful for this degree of planarization, but eventually a successful procedure was developed using successive coats of thick AZ-4620 photoresist to nearly ®ll in the DRIE trenches. After contact photolithography with the second mask to expose the resist and its subsequent development, an SF6 RIE was used to etch the polysilicon down to the underlying SiO2 layer in a stripe across each web. The third mask was used to pattern another coating of AZ4620 photoresist to de®ne the electrical contact pads to the polysilicon tabs. A metalization of Al/Cr/Au (150 nm/ 15 nm/150 nm) was then deposited and lifted off, and this was then annealed at 4008C for 30 min. After the metal anneal, the wafers were diced into individual detector arrays. The die were then washed to remove dicing debris and
thoroughly cleaned in nanostrip and a ®nal oxygen plasma ashing to remove photoresist traces from deep within the cups. The resulting MFCA die are shown in Fig. 6. 4. Characterization The fabrication process thus produces a large MOS trench capacitor for each Faraday cup with the grounded silicon substrate forming an electrostatic shield around each. The cup-to-ground capacitances for the 150 mm pitch cups were 180, 345 and 498 pF for the 1.0, 2.0 and 3.0 mm lengths. For the 250 mm pitch cups, the cup-to-ground capacitances were 208, 400 and 595 pF for the 1.0, 2.0 and 3.0 mm lengths. These fairly large values of capacitance are well suited to the electronic readout circuitry, since their values are well above the packaging and interconnection parasitic levels of the system. For all of the detector arrays, the cup-to-cup capacitances were below the measurement limit of a few
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Fig. 6. Top view of overall array of 1.00 mm by 100 mm cups with a 150 mm pitch. (a) View showing polysilicon interconnect pads to the right of the cup array; (b) view of cups showing sharp internal features.
picofarads. The electrostatic shielding provided by the grounded silicon substrate webs drastically reduces this value of cup-to-cup capacitance, and thereby reduces the readout charge-sharing artifacts associated with this parasitic effect. Electrical leakage levels were also measured for each Faraday cup to ground using an electrometer. Resistance levels of over 10 GO were routinely measured, although the same values were obtained with the probe tip lifted off of the bond pad, indicating that this leakage current level may be arising from the test leads themselves. Because of the sensitivity of a MOS capacitor to impurity and structural oxide defects, additional characterization of the Faraday cup trench capacitors was performed. The interior and exterior corners of the Faraday cup are the most likely locations for the oxide integrity to be compromised. Scanning electron microscope images of these regions are
shown in Fig. 7, from which a uniform thickness, conformal SiO2 layer is apparent. The thickness and uniformity of the oxide was also checked by several cross-sectional cuts which were etched in 10:1 BOE and then imaged with an AFM. These also revealed a uniform SiO2 thickness with smooth interfaces with both the Si and polysilicon regions. 5. Packaging and electronic readout The MFCA die is packaged by means of a polyimide/ copper/polyimide ¯exible printed circuit cable. The dye is bonded on to one end of the ¯ex cable adjacent to a row of copper pads onto which each Faraday cup of the dye is connected by a bond wire. The bond wires were encapsulated in a thermal setting epoxy for handling protection.
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Fig. 7. Cross-section sections of silicon cup, SiO2 insulator, and polysilicon cup conductor. (a) Outer corner of cup; (b) interior corner of cup.
The opposite end of the ¯ex cable consists of a square ball grid array (BGA) pattern which allows it to be soldered to a mating pattern on a rigid printed circuit board containing the electronic readout circuitry. For prototyping purposes, the ¯exible cable consisted of only 64 conductors. The ¯exible cable packaging for the MFCA allows it to be easily positioned within the tight quarters of a miniature vacuum chamber, and the cable itself can be sandwiched within a ¯at surfaced o-ring seal to provide a high density, economical vacuum feedthrough. This arrangement allows the electronic readout circuitry to be placed outside of the
vacuum chamber, further reducing the interior volume in mass spectrometer applications. The electronic readout circuitry is shown in Fig. 8 in which each of the Faraday cups is directly connected to a switch of an analog multiplexer. The typically 8:1 or 16:1 multiplexer chips are cascaded to provide a suf®cient number of inputs for the overall MFCA. The appropriate switch of the analog multiplexers is determined by an address sent from a generic array logic (GAL) chip which is programmed to sequence linearly through the array and is clocked by the readout master oscillator. The output from the analog
Fig. 8. Electronic scanning readout system.
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multiplexer is fed into an integrating operational ampli®er which is successively reset by the master oscillator. For each cup in sequence, the integrator is ®rst reset, and then the analog switch is closed, creating a virtual ground for the charge in the cup to ¯ow into. The integrated charge is transferred to the feedback capacitor CF, which provides the analog output signal level of that cup in the array. The GAL chip also creates a frame synchronization pulse to mark the start of the scan through the array, and this signal can be used as an external trigger to directly display the MFCA collected charge output pro®le on an oscilloscope. The frequency of the master oscillator is made adjustable since this sets the integration time period for the MFCA and can be used to set the baseline sensitivity and saturation limits. 6. Application to a mass spectrometer The MFCA has been designed for use in a compact, linear dispersion mass spectrometer, which is shown schematically in Fig. 9 and follows a commonly used Mattauch±Herzog layout. A dc glow discharge is used to ionize the sample and the charged analyte fragments are then accelerated and focused by a dc electric ®eld. The combination of the dc electric and permanent magnetic ®eld sectors images the ionization source onto the detector plane with a position dependent upon the mass of the ion. In this geometry, the dispersed ions enter the Faraday cups at a 458 angle, which minimizes backscatter and secondary electron emission from the cup. The current state of development of this mass spectrometer is shown in Fig. 10. An ion of mass m and charge q moving with velocity v in a uniform magnetic ®eld B will be de¯ected with a radius R mv/qB. If the velocity of the ion is established prior by
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Fig. 9. Geometry of the linear dispersion mass spectrometer.
an electric acceleration ®eld, the energy of the ion will be E qV mv2 =2, where V is the acceleration potential. Thus, the position of the ion on the detector plane will be proportional to the radius of its trajectory within the magnetic sector, R 2mE1=2 =qB. However, in the present linear dispersion mass spectrometer, a uniquely designed inhomogeneous ®eld permanent magnet is used to disperse the ions onto the detector plane with their position proportional to their mass, rather than proportional to m1/2. When the MFCA is positioned onto the detector plane as shown in Fig. 10, the uniform spacing of the Faraday cups creates a constant Dm selection between adjacent cups which allows the mass spectrum to be directly read out from the MFCA using the simple scanning circuitry of Fig. 8. Most existing mass spectrometers utilize a single point ion detector and a scan through the mass spectrum range is achieved by either ramping the electric or magnetic ion optics ®elds, scanning the ®eld of a quadrupole analyzer, or mechanically moving the detector across the image plane of
Fig. 10. Current development of the compact, linear dispersion mass spectrometer.
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the output. High mass resolution is achieved by windowing only a small fraction of the overall ion ¯ux. While the detector is aligned to only that speci®c mass window, the remainder of the ionized sample is rejected and thus lost. The MFCA enables a new generation of mass spectrometers which feature true multi-channel ion detection. Since the MFCA always remains open to the full incident ion ¯ux, all mass trajectories within the detector's range are simultaneously sampled and integrated into their respective cups in the array. The entire mass spectrum is acquired simultaneously so that the overall spectrum is acquired in a much shorter time. Typically, only 1±2 ms are required to scan over a 100 amu range. Because all of the incident ions are captured by the MFCA detector, the sensitivity of the system is improved by a factor that is approximately equal to the number of cups in the array N. This improved sensitivity and scan speed are both becoming increasingly important for applications where very small samples are involved, where rapid throughput testing is required, or where continuous online monitoring is implemented [10]. 7. Conclusion A new device structure has been developed for integrating Faraday cup ion detectors into a linear, closely spaced array. DRIE is used to produce high aspect ratio trenches with narrow separator webs that function as effective ion collection traps. The incident charge is collected on low-leakage polysilicon MOS capacitors which line the interior of each cup. The grounded silicon substrate that forms the separator web also provides electrostatic shielding for each cup, making the cup-to-ground capacitance dominate over the cup-to-cup capacitance. A stable and rugged position sensitive ion detector is produced by this design which is directly applicable to mass spectrometry and ion beam pro®ling. True multi-channel ion detection is achieved with the MFCA since none of the incident ion ¯ux is rejected by scanning a mass selection window. Signi®cant improvements in sensitivity and readout speed are obtained with multi-channel detection, and the MFCA currently plays a central role in the development of a compact, linear dispersion mass spectrometer. With the mass resolution now set by the cup pitch of the MFCA relative to the dispersed distance of the ion's mass range, microfabrication techniques which can reduce the cup pitch will also shorten the required dispersed distance and lead to much more compact, high performance mass spectrometers. Acknowledgements The authors wish to thank Nadim Maluf and Bart van Drieenhuizen of Lucas NovaSensor for performing the DRIE work. Thanks are also extended to Leo L. Lam for assistance with the photolithography. This work was
sponsored by the NSF Center for Process Analytical Chemistry (CPAC) at the University of Washington. References [1] G. Scoles (Ed.), Atomic and Molecular Beam Methods, Vol. 1, Oxford University Press, New York, 1988. [2] W.N. Hammer, A.E. Michael, A technique for measuring, displaying, recording, and modifying the spatial uniformity of implanted ions, J. Appl. Phys. 47 (5) (1976) 2161±2164. [3] N. Natsuaki, K. Ohyu, T. Tokuyama, Spatial dose uniformity monitor for electrically scanned beam, Rev. Sci. Instrum. 49 (9) (1978) 1300± 1304. [4] C.O. Morain, P. O'Keeffe, S. Den, Y. Hayashi, Large diameter plasma profile monitoring using Faraday cup arrays, Meas. Sci. Tech. 4 (1993) 1484±1488. [5] R.B. Liebert, Method and Apparatus for Ion Beam Centroid Location, US Patent No. 4,724,324, 9 February 1988. [6] C.A. MacDonald (Ed.), Mass Spectrometry, McGraw-Hill, New York, 1963. [7] A.A. Scheidemann, R.B. Darling, F.J. Schumacher, A. Isakarov, Linear dispersion mass spectrometer, in: Proceedings of the Technical Digest of the 14th International Forum on Process Analytical Chemistry (IFPAC-2000); Abstract I-067, Lake Las Vegas, Nevada, 23±26 January 2000. [8] R.B. Darling, A.A. Scheidemann, K.N. Bhat, T.-C. Chen, Micromachined faraday cup array using deep reactive ion etching, in: Proceedings of the 14th IEEE International Conference on Microelectromechanical Systems (MEMS-2001), Interlaken, Switzerland, 21±25 January 2001, pp. 90±93. [9] D.A. Baglee, R.R. Doering, M. Elahy, M. Yashiro, Properties of Trench Capacitors for High Density DRAM Applications, IEDM Technical Digest, December 1985, pp. 384±387. [10] M.P. Shiha, G. Gutnikov, Development of a miniature gas chromatograph Ð mass spectrometer with a microbore capillary column and array detector, Anal. Chem. 63 (18) (1991) 2012±2016.
Biographies Robert B. Darling was born in Johnson City, TN on 15 March 1958. He received the BSEE (with highest honors), MSEE and PhD degrees in electrical engineering from the Georgia Institute of Technology in 1980, 1982 and 1985, respectively. He has held Summer positions with SperryUnivac, Bristol, TN, and Texas Instruments, Johnson City, TN, and from 1982 to 1983, he was with the Physical Sciences Division of the Georgia Technical Research Institute, Atlanta, GA. In 1985, he joined the Department of electrical engineering, University of Washington, Seattle, as an Assistant Professor. He was promoted to Associate Professor in 1990 and to Full Professor in 1999. From 1995 to 1996, he was a Visiting Associate Professor at Stanford University, Stanford, CA. Currently, he is also an Adjunct Professor of Bioengineering and Director of the Electrical Engineering Microfabrication Laboratory. His research interests include electron device physics, device modeling, microfabrication, circuit design, optoelectronics, sensors, electrochemistry, and instrumentation electronics. Dr. Darling is a senior member of the IEEE, a member of the American Physical Society, the American Vacuum Society, the Optical Society of America, and is a registered professional engineer in the State of Washington. Adi A. Scheidemann was born in Ruesselheim, close to Frankfurt, Germany in June 1957. Dr. Scheidemann received his diploma (MS) in theoretical physics from the University of Frankfurt in 1984. His thesis topic was density functional theory. Thereafter Dr. Scheidemann switched to
R.B. Darling et al. / Sensors and Actuators A 95 (2002) 84±93 experimental physics and joined the Max Planck Institute for fluid dynamics in Goettingen, Germany. He graduated in 1989 in the field of applied physics and fluid dynamics at the University in Goettingen with an experimental research project on helium nanodroplets. Post-doctoral work brought him to the University of California, Berkeley, to work on metal clusters before joining the research faculty of the Department of Chemistry at the University of Washington in 1991. Dr. Scheidemann left the UW in summer of 2000 to launch a start-up company developing miniaturized mass spectrometer systems. His research focuses on miniaturized instrumentation and nanoparticles. The later work is done in close collaboration with Prof. V. Kresin at the University of Southern California. Dr. Scheidemann is a member of the American Society for Mass Spectrometry and the American Physical Society. K.N. Bhat received the BSc degree from St. Aloysius College, Mangalore, India, in 1963, the BE degree in electrical technology from the Indian Institute of Science, Bangalore, in 1966 and the MTech degree in electrical engineering from the Indian Institute of Technology, Chennai, in 1969. He then continued on to earn the MEng degree in electrical engineering from
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Rensselaer Polytechnic, Troy, New York, in 1974, and the PhD degree in electrical engineering from the Indian Institute of Technology, Chennai, in 1978. He has been on the faculty of IIT, Chennai, since 1981, where he is presently Professor of electrical engineering and Head of the Microelectronics Laboratory. He spent 1979±1981 as a Post-doctoral Research Associate at Rensselaer Polytechnic, and more recently spent 1999 as a Visiting Professor at the University of Washington, Seattle. His research interests include microelectronic and optoelectronic devices and microelectromechanical systems (MEMS). Tai-Chang Chen is a Research Engineer in the Department of electrical engineering, University of Washington, Seattle, Washington. He received the BS degree in materials science and engineering from the National Hsing-Hua University, Taiwan, in 1988, and the MS and PhD degrees in materials science and Engineering from University of Washington, Seattle, in 1993 and 1997. He teaches courses in integrated circuit and microfabrication technologies. His present research areas include solid-state sensors, micromachined transducers and micromachining technologies.