Gas Electron Multiplier Made by Deep-Etch X-Ray Lithography

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1 Journal of the Korean Physical Society, Vol. 40, No. 5, May 2002, pp Gas Electron Multiplier Made by Deep-Etch X-Ray Lithography Ho Kyung Kim, Gyuseong Cho and Do Kyung Kim Department of Nuclear and Quantum Engineering, and Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejon Hyosung Cho Department of Medical Engineering, Research Institute of Biomedical Engineering, Yonsei University, Wonju (Received 9 November 2001) As an alternative design for a gas electron multiplier (GEM), which is a charge preamplifying device when coupled with any gas avalanche microdetector, arrays of holes having steep walls have been successfully prepared on polymethylmethacrylate (PMMA) plastic sheets of various thicknesses by using a deep-etch X-ray lithography or LIGA process. The first measurements of performance were very promising. For a 300-µm-thick LIGA device, an avalanche gain of was obtained from anode strips having the 10-µm width of the microstrip gas chamber (MSGC) used as a readout collection plane. The short-term stability of the gain was found to be constant, within 2 % change of the gain over a one and one-half hour period, and the rate capability was extended to more than 10 5 mm 2 s 1. PACS numbers: C, G Keywords: Gas avalanche detector, Gas electron multiplier, GEM, X-ray lithography, LIGA I. INTRODUCTION One of most serious problem remaining as an obstacle for the reliable use of gas avalanche microdetectors is electrical breakdown of the anode due to sparking, which causes permanent damage. Preamplification of the charges released by ionization elsewhere around microelectrodes of gas avalanche microdetectors before readout is very desirable with respect to the spark problem; moreover, it allows a safety margin in the operating voltages of the readout devices. Since the MSGC (microstrip gas chamber) was first introduced in 1987 [1], a large amount of effort has been invested in this new concept of gas avalanche microdetectors [2]. Improvements and variations of the original MSGC design have led to better detector performance. Other innovative devices have recently been proposed, such as CAT (compteur à trous) [3] and GEM (gas electron multiplier) [4], using the concept of electrodes separated by an insulator between them, resulting in intense electric field across a hole defined through the insulator. It is noted that the GEM geometry has the unique advantage that the multiplication region can be separate from the readout electrodes, which are usually very vulnerable to damage from sparking in most gas avalanche hokyung@kaist.ac.kr; Tel: ; Fax: microdetectors (MSGC, MGC, MDOT, etc.). In this charge preamplification method, the GEM is a supplement of gas avalanche microdetectors. However, the conventional GEM has a conically shaped hole in the insulating substrate (Kapton) made during wet-etching process. Because of this shape, a change in the avalanche gain with time was reported [5]. II. GAS ELECTRON MULTIPLIERS AND THEIR CHARACTERISTICS 1. Operating Principle and Various Designs A GEM is a thin insulating foil (usually a 50-µmthick Kapton) with metal cladding (typically 5-µm-thick chromium) on both sides, perforated by a regular dense matrix of holes. Typically, the holes are µm in diameter and µm in pitch [6]. Figure 1(a) shows an optical microscope photograph of a GEM, made by the gas detector development group at CERN, with a schematic cross-sectional view. Since the GEM is made by using a chemical wet-etching process, it has a conical hole-structure and a limited thickness ( 50 µm). When the potential difference is applied between the top and the bottom electrodes, an intense electric field develops through the short channel of the GEM hole

2 Gas Electron Multiplier Made by Deep-Etch X-Ray Lithography Ho Kyung Kim et al Fig. 2. Absolute GEM avalanche gains with respect to the applied GEM voltage in a gas mixture of Ar/CO 2 (70 %/30 %). Fig. 1. Optical microscope photographs of various designs of GEM: (a) the conventional GEM having double conical holes, (b) the laser-drilled GEM having nearly straight walls, and (c) the closed-end GEM. For better understanding, schematic cross-sectional views are included. The external drift field lines, defined by the drift electrode of the gas avalanche microdetector, for example the MSGC, are focused into the GEM hole. Charge generated by the primary ionization within the drift region (gas volume defined by the drift electrode and top GEM electrode) follows the drift field lines into the GEM hole and is amplified by the avalanche multiplication. Avalanche electrons that drift out of the GEM hole are, then, multiplied further by the microanode. However, all of exiting electrons may not be collected by the readout devices because a substantial fraction of them may be drawn to the bottom GEM electrode [7]. Originally, the GEM was introduced as a supplement for gas avalanche microdetectors, to serve as a charge preamplifier located between the drift cathode plane and the detector surface [8 11]. Then, the detector could be operated with a larger safety margin to get the same signal gain as that for the detector without any charge preamplifier. Due to the recent progress in fabrication technology, an avalanche gain of 10 4 was obtained by using only a GEM [12 14]. Thus, a gas avalanche microanode may be replaced by simple charge collection strips or pads. Larger avalanche gains can be obtained if thicker insulators are used for the GEM to allow a larger distance for further avalanche development. In addition, GEMs can be cascaded into multiple structures; then, the net gain will be the product of the individual gains [12 14]. For a better understanding the detector operation and for investigating the optimal design, we studied two types of GEMs: the standard GEM produced at CERN, and the laser-drilled GEM from the University of Louisville. The latter is a GEM structure realized on the 125-µmthick polyimide foil and gives nearly straight walls (8 wall inclination) as shown in Fig. 1(b) [5,15]. Anther interesting variation of the GEM is the closedend GEM [5, 16], in which the avalanche electrons are collected directly by a pad or strip covering the exit of the GEM holes: this is similar to another device called the CAT detector [3]. The design and a microphotograph of this device built at LBNL are shown in Fig. 1(c). It has a pitch of 200 µm, a hole diameter of 40 µm (in fact, the hole is a square), and 18-µm-thick insulating spacers of amorphous silicon carbide (a-si:c:h). This design may have a somewhat higher gain than the conventional GEM and can avoid the collection gap, but obviously it cannot be used as a tandem structure. 2. Performances of Various Designs Among the detector performances, the avalanche gain is the most representative. As mentioned in the above section, avalanche electrons exiting a GEM hole are partially collected at the readout devices, but some of them arrive at the bottom electrode of GEM [7]. Therefore, the absolute avalanche gain is determined by measuring both the current from the readout strips and the current from the bottom electrode of the GEM. The avalanche gain obtained only from the readout strips is effective. For three types of GEM devices discussed above, Fig. 2 shows the measurements of the absolute avalanche gains with respect to the GEM voltage for a gas mixture of Ar/CO 2 (70 %/30 %). The drift and collec-

3 -814- Journal of the Korean Physical Society, Vol. 40, No. 5, May 2002 Fig. 3. Absolute GEM avalanche gains as a function of the field strength applied across the GEM hole. This graph is a re-plot of Fig. 2. Fig. 4. Short-term gain stability for two different GEM designs, which have double conically shaped and nearly straight walls. tion fields were fixed at 4 and 6 kv/cm, respectively, for the standard and the laser-drilled GEMs (or open GEMs as compared with the closed-end GEM). For the closed-end GEM, the drift field was 2 kv/cm. During the measurements, the maximum achievable gain was determined when an unstable current was monitored. While the maximum gain of the open GEMs was 10 4, that of the closed-end GEM was only 330. The small gain of the latter is probably due to its smaller thickness (only 18 µm). A larger gain is expected for a thicker insulating spacer because a longer distance of charge travel allows a larger avalanche development. The measurement result is re-plotted in Fig. 3 as a function of the field strength applied across the GEM hole. From this figure, the effect of the channel length can be easily pointed out. To parameterize this effect, we first define the gain-slope: α ln M E, (1) where M is the avalanche gain and E is the field strength. The calculated values are 0.114, 0.237, and for the standard, the laser-drilled, and the closed-end GEM, respectively. From this result, it is found that a longer avalanche channel length gives a larger gain at the same field strength, as expected. For the reliability of the radiation detector for longterm use, drift of the operating property should be suppressed, and the effect of radiation-induced damage should be minimized. Figure 4 shows the short-term stability of the gain for two different designs of the GEM. It should be noted that the main difference is the shape of the hole walls; doubly conical and cylindrical. For the standard GEM, the gain gradually increases with time. There is about a 10 % gain shift within 1 hour. On the contrary, the laser-drilled GEM exhibits a very stable gain. The gain shift is presumably due to the charging of the GEM insulator. The presence of an insulator close to the avalanche channels introduces the possibility of charge deposition on the insulating surface, resulting in a modification of the electric field configuration and, therefore, a dynamic gain shift. The conically shaped GEM wall definitely has a large probability of surface charging. However, the GEM hole with steep walls has virtually no chance to be exposed to irradiation and, thus, no surface charging. III. GAS ELECTRON MULTIPLIER MADE BY USING THE LIGA PROCESS The laser-drilled GEM was discussed for reliable operation. However, the laser drilling technique usually leaves carbon residues on organic substrates, and tedious cleaning procedures are often necessary. Another method of avoiding charging processes in the holes is coating of the hole walls with a slightly conductive layer, as reported by the Siegen group [17]. This overcoating needs additional facilities, such as a high vacuum chamber in a plasma-enhanced chemical vapor deposition (PECVD) facility. LIGA is a microfabrication technique, which, in this case, uses polymethylmethacrylate (PMMA) as the basic material, and is ideally suited for building a microstructure having a high aspect ratio (height divided by width). Previously, the LIGA process has been used at LURE to make MSGCs with an engraved substrate, for suppression of ion accumulation [18]. The acronym LIGA originates from the German expressions for the major process steps; lithography (LIthographie), electroplating (Galvanoformung) and molding (Abformung). The LIGA process, as applied here and discussed in detail later, uses X-rays of 8 15 kev energy to expose a thick

4 Gas Electron Multiplier Made by Deep-Etch X-Ray Lithography Ho Kyung Kim et al Table 1. Dose and exposure time for various thicknesses of PMMA sheets. Exposure was done at the beamline of advanced light source at LBNL, and the current and the exposure time are for that facility. Thickness (µm) Dose (kj/cm 3 ) Top Bottom Exposure (ma min) PMMA resist layer ( µm) through a patterned mask. Modern synchrotron light sources (electron storage rings) are good sources of X-rays for this lithography step. In this study, X-rays from the Lawrence Berkeley National Laboratory (LBNL) electron storage ring, the Advanced Light Source (ALS, Beamline 3.3.1), have been used. As applied to our GEM-type device, the LIGA process is described in this study. We used PMMA, the most common X-ray resist in the LIGA process, as an insulator material for the LIGA device. Initially, the PMMA was in sheet form, and the minimum thickness of commonly available PMMA sheets is typically 1 mm. Thinner PMMA sheets may be made Fig. 5. (a) SEM photograph of the hole array in a PMMA sheet having very steep wall sides made by using the LIGA process µm 2 rectangular holes with a pitch of 300 µm are patterned on a 300-µm-thick PMMA sheet. (b) An example of the high-aspect ratio structure realized on a µm-thick PMMA sheet. by compression in a heated press, yielding reasonably good thickness uniformity and few defects. It should be noted that PMMA has a narrow distribution of high molecular weight, so the material can have a sufficiently good etch-selectivity after X-ray exposure. Based on the LIGA process, the fabrication procedures of our detectors are as follows: (1) Mask preparation. The LIGA process requires a slightly different mask preparation than used in conventional photolithography. In our mask fabrication, a 100-µm-thick 3-inch-diameter silicon wafer was used as a substrate for the thick ( 25 µm) gold-patterned absorbers. The silicon wafer was coated with a thick photoresist and patterned by using a standard photomask having hole-patterns consisting of µm 2 rectangular holes with a 300-µm pitch. The active area was mm 2. After electroplating 25-µm-thick gold absorbers onto these patterns, the photoresist was lifted off, leaving the patterned gold layer. (2) Exposure. The exposure dose was determined based on computer simulations (CXRL toolset, University of Wisconsin), considering all the spectral filtering effects of the synchrotron radiation beamline and intervening absorbing material. A minimum dose of 4 kj/cm 3 is required to expose the PMMA. The damage threshold of PMMA is approximately 15 kj/cm 3. The top to bottom dose ratio of PMMA is an important consideration in LIGA exposures due to the resist thickness; a poorly planned exposure can result in a resist which is grossly over-exposed on the top yet under-exposed on the bottom. The exposure ratio is adjusted through the addition of spectral filters and the exposure time of the PMMA. A typical LIGA exposure for a 300-µm-thick piece of PMMA is 12 hours. Table 1 shows the dose and the exposure time to be used for different thicknesses of PMMA sheets. (3) Development. After exposure, the PMMA is developed using a mixture of 2-2 butoxyelthoxyethanol, morpholine, ethoamine, and de-ionized water. This developer is at the limit of the solubility range of PMMA: below the threshold dose of 1.6 kj/cm 3, the resist has a dissolution rate of less than 0.1 µm/min. When the resist is exposed at a dose of 10 kj/cm 3, the dissolution rate in the developer is 10 µm/min, which gives an overall selectivity of better than 100:1 between the exposed and the unexposed regions of PMMA. After development, the PMMA parts are rinsed in de-ionized water and dried. A scanning electron microscope (SEM) photograph of the developed PMMA with a thickness of 300 µm is shown in Fig. 5(a). Finally, in order to introduce electrodes, a thin (25 angstroms) gold or titanium layer is deposited on the top and the bottom surfaces by using e-beam evaporation; then, a copper layer is electroplated up to 5000 angstroms. IV. EXPERIMENTAL DETAILS

5 -816- Journal of the Korean Physical Society, Vol. 40, No. 5, May 2002 Fig. 6. Schematic structure of the LIGA device coupled with a plane of collection electrodes (without gain). In our tests, we used a 200-µm-pitch MSGC for the collection electrodes. The GEM made by using the LIGA process was inserted between two electrode planes, the drift and the collection planes, which were spaced by 3 and 1 mm, respectively, from the GEM, as shown in Fig. 6. This configuration gives three distinct regions corresponding to electron drift, multiplication and charge collection. The drift plane was a thin stainless steel wire grid. An MSGC with a pitch of 200 µm was used as the collection plane, and the charge was collected without gain. In this study, the drift and the collection fields were fixed as 2 and 5 kv/cm, respectively, and the applied voltage on the LIGA device was defined as the voltage difference between the top and the bottom electrodes. In order to provide high voltage on the LIGA device safely with no spark damage, we used a single power supply with an external resistive partition network, including protective resistances. Direct application of the required voltages to each electrode with a protective resistor (typically 10 MΩ) is convenient, since it allows independent control of the various voltages and fields (drift and collection). However, a sustained spark may develop and cause irreversible damage to the GEM. A gas mixture of Ar/CO 2 (70 %/30 %) was used for the operating gases, and the source of primary ionization was an 55 Fe source. Fig. 7. Field strength along and near the wall surface (0.1- µm separation from the surface) of 50-µm-thick insulators with different surface resistivities: and Ω/square for Kapton and PMMA, respectively. Field enhancement (rabbit ears) at the conductor edges becomes very much smaller when any amount of current is allowed to flow through the insulator surface. specifying either an insulating electrostatic (ES) or conducting substrate. In the ES mode (no current flow), field enhancements, which we call rabbit ears, occur at the edges of the electrodes. These were grossly overestimated due to the finite conductivity that always exists in any insulator. Rabbit ears are greatly reduced with current flow, which establishes a uniform potential gradient on the surface of the hole. As expected from the neglect of charge flow in the gas, this result is insensitive to the surface resistivity, which was varied from to Ω/square, values that are typical of PMMA and Kapton. V. RESULTS AND DISCUSSION 1. Electric Field Analysis To understand the detector and to improve the design, we simulated the electric field by using a commercial computer code, Maxwell. For simplicity, we simulated a cylindrical GEM-like geometry with a 50-µm-thick substrate, a 100-µm-diameter hole, and a pitch of 200 µm. The drift, the collection, and the central fields in the hole were 2, 5, and 100 kv/cm, respectively. Figure 7 shows the simulated field distributions along a line parallel to the axis of the hole and 0.1 µm from the surface of the hole wall. The computer program includes the option of Fig. 8. Electron trajectories along the drift and the collection fields. The drift and the collection fields were taken to be 2 and 5 kv/cm, respectively, and the field at the center of the hole was 100 kv/cm. Field lines terminating on the top and the bottom conductors were not considered in this plot.

6 Gas Electron Multiplier Made by Deep-Etch X-Ray Lithography Ho Kyung Kim et al For a 350-µm-thick GEM, the electron trajectories (or field map) are plotted in Fig. 8. In this simulation, gas diffusion was not considered, and the electron trajectories were initiated at the drift plane. Field lines terminating on the top and the bottom electrodes are not shown in this plot. As shown in Fig. 8, all of the field lines are well concentrated in the avalanche region. 2. Measurements of Avalanche Gain Several GEMs, made by using the LIGA process, with thicknesses of 125 and 300 µm were tested. The resistance was determined by measuring the leakage current in the operating gas mixture as a function of applied voltage, which gave more than 30 GΩ. In addition, the measured capacitances were in good agreement with calculations using a dielectric constant of 2.6 for the PMMA. The measurements of the gas avalanche gain with respect to the applied voltage are plotted in Fig. 9. It is noted that in these measurements, an MSGC with the pitch of 200 µm was used as the readout plane to collect the avalanche electron signal from the LIGA device; the anodes were bunched together and connected to the pulse height analyzer (PHA), while the cathodes were grounded. Therefore, the gain reported here is based upon the electron signal obtained from the MSGC anode strips only and is a lower limit to the avalanche gain of the LIGA device. The cathode strips, which have the same potential as anode strips, collect a larger number of electrons than the anode strips. The currents to the anode and the cathode strips, and the corresponding measured gains are expected to be divided approximately by the ratio of the cathode to the anode strip width (=90 Fig. 10. Pulse-height spectrum for an 55 Fe source obtained using a GEM, made by using the LIGA process, of 300-µmthickness. The gas mixture was Ar/CO 2 (70 %/30 %). µm/10 µm=9). The pulse height spectra for different measurements appear to support this. The current of the MSGC cathodes was also measured at the same time. The maximum avalanche gain was obtained for a 300-µm-thick LIGA device running at 1,300 volts. The corresponding central field was 37 kv/cm. As compared with the conventional GEM having an effective avalanche gain of 1,000 at 500 volts per 50-µm-thick Kapton corresponding to a 100-kV/cm field strength [13], there appears to be a significant improvement in gain. This improvement could be partially explained by the longer path length available for the avalanches in the multiplication region. Figure 10 shows an example of the pulse height spectrum for the 55 Fe source obtained by using an LIGA device 300 µm in thickness. The FWHM energy resolution was 30 %, which is somewhat poor. This is probably due to the non-uniform thickness of the PMMA sheet during the compression. The best resolution obtained was 17 % with 5.9-keV X-rays. 3. Operating Stability Fig. 9. Avalanche gains measured from the pulse height on the anode strip only. The gas mixture was Ar/CO 2 (70 %/30 %). All measurements were performed with an 55 Fe source, up to voltages at which fluctuation of current became prominent. From the steep wall shape, as well as the relatively low surface resistivity, the loss of gain due to charge accumulation on the insulator surface is expected to be almost independent of the event rate. The gain change with time is shown in Fig. 11 and exhibits less than a 2 % change in gain over a one and one-half hour period. For future experiments in high-energy physics, where the detectors will be operated in an environment of very high luminosity, high rate detectors are needed. Typically, the maximum count rate that the gas avalanche microdetector can handle is limited by the occurrence of

7 -818- Journal of the Korean Physical Society, Vol. 40, No. 5, May 2002 Fig. 11. Short-term gain stability of the GEM, made by using the LIGA process, with thickness of 300 µm. The drift and the collection fields were taken to be 2 and 5 kv/cm, respectively. The applied voltage was 1060 volts. The gas mixture was Ar/CO 2 (70 %/30 %). Gas avalanche microdetectors are very vulnerable to spark damage, which is usually triggered by streamers created from the large charge multiplication in the narrow region between the anode and the cathode. The charge preamplification method using GEM has been introduced to provide a safe operating margin (reduced potential between electrodes) for the detectors and to suppress the possibility of electrical breakdown. Their performances were studied for various designs of the GEM. It was found that a longer multiplication channel length gives a larger avalanche gains. Most of all, the wall of the GEM hole should be straight for reliable operation without any gain shift. A new method, based on the deep-etch X-ray lithographic technique (or LIGA process), to make a better GEM for gas avalanche microdetectors has been discussed. The technique, as described in this study, has advantages in design flexibility, such as in thickness and pitch, and allows increased path length for gas avalanche development. There is also a reduced internal detector capacitance due to the large aspect ratio. In addition, since PMMA has a relatively low surface resistivity (10 14 Ω/square) compared with Kapton (10 16 Ω/square), the gain is expected to decrease less due to event rate and gain instabilities. Our first measurements yield very promising results; a high gain (a lower limit to 3,000), a good energy resolution (17 % of FWHM), and good stability. We have found PMMA sheets ( µm in thickness) made from impact-modified grades, which is commercially supplied from Goodfellow (Cambridge Science Park, Cambridge CB4 4DJ, England). These may show a different molecular weight distribution compared to the PMMA wafers made by us. Studies will continue for different thicknesses and qualities of PMMA, which should result in better design parameters. Fig. 12. Relative avalanche gain of the GEM, made by using the LIGA process, with a thickness of 300 µm as a function of the X-ray rate. The drift and the collection fields were taken to be 2 and 5 kv/cm, respectively. The applied voltage was 1060 volts. The gas mixture was Ar/CO 2 (70 %/30 %). space charge around the microelectrodes, screening the electric field for the electrons and causing charge recombination. Using an X-ray generator, we measured the relative avalanche gain of a GEM made by using the LIGA process as a function of the X-ray rate. As shown in Fig. 12, the rate capability extends to more than 10 5 mm 2 s 1. VI. SUMMARY ACKNOWLEDGMENTS The authors would like to thank Dr. K. Jackson at Lawrence Berkeley National Laboratory (LBNL) for his great support with the detector preparation. The authors also wish to express thanks to Drs. J. Kadyk, V. Perez-Mendez, and W. Wenzel at LBNL for useful discussions of the electric field analysis and the experimental results. REFERENCES [1] A. Oed, Nucl. Instr. Meth. A 263, 351 (1988). [2] H. K. Kim, G. Cho and H. Cho, J. Korean Phys. Soc. 39, 218 (2001). [3] F. Bartol, M. Bordessoule, G. Chaplier, M. Lemonnier and S. Megtert, J. Phys. III France 6, 337 (1996). [4] F. Sauli, Nucl. Instr. Meth. A 386, 531 (1997).

8 Gas Electron Multiplier Made by Deep-Etch X-Ray Lithography Ho Kyung Kim et al [5] H. S. Cho, J. Kadyk, S. H. Han, W. S. Hong, V. Perez- Mendez, W. Wenzel, K. Pitts, M. D. Martin and J. B. Hutchins, IEEE Trans. Nucl. Sci. 46, 306 (1999). [6] S. Bachmann, A. Bressan, L. Ropelewski, F. Sauli and D. Mormann, Nucl. Instr. Meth. A 433, 464 (1999). [7] R. Bellazzini, A. Brez, G. Gariano, L. Latronico. N. Lumb, G. Spandre, M. M. Massai, R. Raffo and M. A. Spezziga, Nucl. Instr. Meth. A 419, 429 (1998). [8] J. Benlloch, A. Bressan, C. Buttner, M. Capeans, M. Gruwe, M. Hoch, J. C. Labbe, A. Placci, L. Ropelwski, F. Sauli, A. Sharma and R. Veenhof, IEEE Tran. Nucl. Sci. 45, 234 (1998). [9] W. Beaumont, T. Beckers, J. De Troy, C. Van Dyck, O. Bouhali, F. Udo, C. Vander Velde, W. Van Doninck, P. Vanlaer and V. Zhukov, Nucl. Instr. Meth. A 419, 394 (1998). [10] Y. Benhammou, J. M. Brom, J. C. Fontaine, D. Huss, F. Jeanneau, A. Lounis, I. Ripp-Baudot and A. Zghiche, Nucl. Instr. Meth. A 419, 400 (1998). [11] R. Bellazzini, M. Bazzo, A. Brez, G. Gariano, L. Latronico. N. Lumb, M. M. Massai, A. Papanestis, R. Raffo, G. Spandre and M. A. Spezziga, Nucl. Instr. Meth. A 425, 218 (1999). [12] A. Bressan, A. Buzulutskov, L. Ropelewski, F. Sauli and L. Shekhtman, Nucl. Instr. Meth. A 423, 119 (1999). [13] A. Bressan, J. C. Labbe, P. Pagano, L. Ropelewski and F. Sauli, Nucl. Instr. Meth. A 425, 262 (1999). [14] S. Bachmann, A. Bressan, L. Ropelewski, F. Sauli, A. Sharma and D. Mormann, Nucl. Instr. Meth. A 438, 376 (1999). [15] W. K. Pitts, M. D. Martin, S. Belolipetskiy, M. Crain, J. B. Hutchins, S. Mators, K. M. Walsh and K. Solberg, Nucl. Instr. Meth. A 438, 277 (1999). [16] W. S. Hong, H. S. Cho, F. Retiere, S. Han, J. Kadyk and V. Perez-Mendez, IEEE Nuclear Science Symposium (Toronto, Nov., 1998). [17] S. Beirle, U. Werthenbach, G. Zech and T. Zeuner, Nucl. Instr. Meth. A 423, 297 (1999). [18] M. Lemonnier, A. Bahri, M. Bordessoule, F. Bartol, A. Labeque, Z. Liu, S. Megtert, M. Roulliay, M. F. Ravet, F. Rousseaux and J. Perrocheau, International Workshop on Micro-strip Gas Chambers (Legnaro, Italy, Oct. 1994).