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A novel MOEMS based adaptive optics for X-ray focusing
Microelectronic Engineering 83 (2006) 1321–1325 www.elsevier.com/locate/mee
A novel MOEMS based adaptive optics for X-ray focusing M.Y. Al Aioubi a
a,*
, P.D. Prewett a, S.E. Huq b, V. Djakov b, A.G. Michette
c
School of Mechanical and Manufacturing Engineering., University of Birmingham, Edgbaston, Birmingham B15 2TT, UK b Central Microstructure Facility, Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, UK c Department of Physics, King’s College London, Strand, London WC2R 2LS, UK Available online 17 February 2006
Abstract This work presents the design and microfabrication of a novel X-ray micro-optical system for use in an X-ray microprobe for analysis of biological cells. A reflective microoptoelectromechanical system capable of focusing a wide range of wavelengths has been developed. The lens system consists of a pair of microfabricated optical elements, one of which has variable curvature providing a unique mechanically-actuated zoom focusing capability. Experiments have been carried out to measure the changes of the focal length (lens curvature) and for system calibration. Ó 2006 Elsevier B.V. All rights reserved. Keywords: MOEMS; Adaptive optics; Novel X-ray optics; MOXI
1. Introduction Micro-opto-electro-mechanical systems (MOEMS) are a sub-division of micro-electromechanical systems, having optical functional elements. Many applications require scanning, switching, deflecting reflecting and focusing of an optical beam. We report the design and microfabrication of a MOEMS system for X-ray analysis of biological cells. In particular, focused X-rays will be used to study the so-called Bystander Effect in which cells other than those irradiated are affected by the X-ray dose [1,2]. This requires focusing to a spot diameter on the micron scale. Our system is an integration of a flexible microlens, sensor and mechanical actuation mechanism with an electromechanical control system to set the required focal length. 2. Optics for X-ray focusing Conventional optics for focusing X-rays have low optical efficiency and are wavelength dependent. Such optics depend on the use of zone plates and/or multilayer mirrors *
Corresponding author. Tel.: +44 1235 44 5650; fax: +44 1235 44 6283. E-mail address: [email protected] (M.Y. Al Aioubi).
0167-9317/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.01.107
[3,4]. An alternative approach is grazing incidence reflective optics, which provides high efficiency, but suffers from aberration limitations. A microchannel plate, originally intended for electron multiplication, can be ‘‘slumped’’ to form a reflective polycapillary focusing element, which has been used successfully in X-ray imaging applications, such as astronomy. The plate is mechanically deformed to provide the curved surface required for focusing [4]. Even with the most advanced manufacturing techniques, channel diameters are limited to a minimum of 0.5 lm, over a few tens of millimetres and can produce focused spot sizes no smaller than 10 lm. Performance is limited by channel tilting, incorrect curvatures, waviness, diffraction and uncontrolled and limited substrate bending [5]. Recently, a microfabricated optical array for X-ray imaging (MOXI) was suggested [5,6]. The MOXI approach has similar advantages to a polycapillary system, including large aperture, high photon flux, large bandpass Dk/k and high transmission, due to low losses at grazing incidence. The basic optical element is shown schematically in Fig. 1. A series of microchannels, formed from concentric rings of an absorber/reflector such as Ni or Au, is deformed by radial actuators which also support the ring structure. A combination of a curved element and a flat
1322
M.Y. Al Aioubi et al. / Microelectronic Engineering 83 (2006) 1321–1325
Reflector, Si Support/Actuator Fig. 1. Multichannel MOXI reflective element with bimorph actuator/ supports [7].
Focal point Bent optics
Fig. 3. Schematic of MOXI with vacuum focus control.
Flat optics Central stop
X-ray paths Source
Fig. 2. A pair of curved and flat optical elements are used for focusing.
3. Design for two-dimensional focusing The design, shown schematically in Fig. 3, consists of two metal carrier rings, aligned using three pins. The first ring is used to carry two membranes, each made from Mylar of thickness 6 lm. The top membrane forms the curved optical element, consisting of a number of reflectors, microfabricated from deposited gold, in the form of symmetric rings. The bottom surface of the support ring carries a second membrane with no reflectors, which isolates the second (flat) optical element on the second mounting ring from the effect of the vacuum pressure. This also has symmetrical ring reflectors made of gold, aligned with those of the first element, using gold alignment marks. With both metal rings clamped together, as shown in Fig. 3, a finely controlled vacuum is produced using a simple mechanical piston. Mylar is chosen for its mechanical flexibility, low X-ray absorption and good chemical properties, being resistant to the chemicals used in microfabrication of the gold reflectors. Changing the vacuum in the system changes the membrane curvature, consequently changing the focal length of the X-ray focusing lens.
Fig. 4. MathCAD simulation of the membrane for different radius of curvature (from 10 to 160 mm).
Analytical study of the mechanical design was carried out using a series of equations. These equations were based on the shape geometry and Snell’s law. This model may be used for any further development. The results showed that the thickness of the gold layer may be very thin (less than 100 nm). This ensures that reflection would take place at the bent optical element. Subsequently these equations were rewritten in MathCAD code for simulation of any further development of the system. Fig. 4 is an example to show that it is possible to be used in MathCAD where Y, Y1 represent the top membranes and y2, y3 represent the bottom membrane. Then an optical design study was carried out and found that the gold thickness should be in the range 300 nm. The graph in Fig. 5 shows the X-ray attenuation length of gold. Atten Length (microns)
one, as shown in Fig. 2, forms the optical system. Thermal bimorph cantilever actuators fabricated from gold-onpolyimide provide actuator/support; performance characteristics have been reported elsewhere [7]. The new MOXI system we report here has all of the above advantages, with the ability to control the focal length precisely over a wide range. Moreover, a high magnification ratio may be obtained with a minimum spot diameter in the sub-micron regime. However, instead of the radially supported and actuated structure described above, the chosen design uses a membrane support for the gold microchannels, with curvature controlled by vacuum-derived pressure difference [6].
0.14
Au Density=19.32
0.12 0.1 0.08 0.06 0.04 0.02 0 1000
2000
3000
4000
Photon Energy [eV] Fig. 5. X-ray attenuation length of gold [9].
5000
Transmision
M.Y. Al Aioubi et al. / Microelectronic Engineering 83 (2006) 1321–1325 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1000
repeating steps 1 and 2 (j,k) and the above deposition, lithography and etch processes repeated for fabrication of the flat lens. Finally, the rings are assembled to form the inter-element vacuum space, as shown in Fig. 7. The final optical elements are symmetrically disposed gold rings with a maximum diameter of 500 lm, spaced 5 lm apart and of width 5 lm.
Thickness=15 microns
2000 3000 4000 Photon Energy [eV]
1323
5000
5. System assembly and control
Fig. 6. Transmission of Mylar [10].
4. Microfabrication of the optical elements Microfabrication of the optical elements in gold-onMylar involves the development of Mylar handling techniques, sputter coating of gold, optical lithography and wet chemical etch. A set of experiments has been carried out to optimise the fabrication process. This involved determination of the Mylar thickness, sputtering parameters for gold deposition. Different Mylar thicknesses were tested. The judgement was based on finding the minimum thickness as well as getting a proper sealing of the system. The optimal thickness of the bent optical element was found to be 6 lm. On the other hand for the flat optical element 2.5 lm was used. For 15 lm thick Mylar sheet the transmission is 93% shown in Fig. 6 at photon energy of 4.5 keV. The photon energy is selectyed as 4.5 keV. The energy is appropriate for X-ray microprobe. Another process considered is the sputtering parameter for gold deposition (set to prevent deformation of the Mylar membrane as well as burning) and optimisation of the patterning process. The fabrication sequence is as follows: Tensioning of the Mylar sheet and fixing to its carrier ring using glue (a,b), is followed by sputter coating of an adhesion layer (40 nm of chromium), followed by 300 nm of gold (c,d). Spin coating and bake-out of JSR photoresist is followed by optical lithography (e,f). Wet-etch removal of exposed gold and subsequently chromium (g,h) defines the reflecting microstructures, after which the resist is stripped. A second Mylar film is mounted on the other side of the ring by
Following the fabrication of the MOXI lens the two carrier rings (Fig. 8(a)) are assembled together inside a special jig, designed to create a vacuum inside the lens chamber – Fig. 8(b). The jig is connected to an evacuating piston and the focal length is set by controlling its distance of travel using a stepper motor. Gold resistors are patterned on the Mylar alongside the reflectors. When the membrane is curved, these resistors are strained and the measured increase in resistance provides a readout which can be calibrated to the curvature of the optical element and therefore to its focal length. The entire system is controlled by a PC using Lab ViewTM software. (Fig. 8(c)) shows an assembled MOXI. 6. System performance Fig. 9 shows the relationship between the sensor response and the focal length of the lens, obtained by measuring membrane deflection using an optical microscope (Fig. 10). The two curves (series1 and series2) represent respectively membrane deformation (focusing) and flattening (defocusing), which shows good reversibility of the
Sensor response [V]
Fig. 8. 2-D focusing MOXI.
2.7 2.5 2.3 2.1 1.9 1.7 1.5 15
25
35
45
55
65
75
85
95
105 115 125 135
Focal length [mm]
Fig. 7. Fabrication process steps.
Fig. 9. Amplified sensor response versus focal length.
M.Y. Al Aioubi et al. / Microelectronic Engineering 83 (2006) 1321–1325 3
250 200 Series1 Series2
150 100 50
Measured 2.5
Expected
2 1.5 1 0.5 0 0
0 0
1
2
3
4
5
6
7
actuation mechanism, which is expected given the elastic and mechanical properties of thin Mylar sheets [11] and relatively small deformations. The resistive sensor acts as a passive strain gauge, which output can be calibrated to the lens curvature. A number of different sensors, having different dimensions and positions on the curved lens element, were tested. The resistance variation of the centrally located sensor was measured for a range of focal lengths. At the maximum piston travel of 8 mm (corresponding to a focal length of 17 mm), the resistance change was 0.4 X [8]. The sensitivity of the system was further improved by using a 1/4 Wheatstone Bridge (Fig. 11) where one of the branches in the ‘diamond’ configuration was the piezoresistive gauge. Further enhancement is possible with incorporation of a second 1/2 Bridge) gauge on the same side of the membrane, whereas a realisation of a full bridge with four sensing elements would require more complex double-sided lithography. A further signal enhancement (by a factor of 100) was achieved with an amplifier circuit (LMC6041 CMOS Signal Micropower Op.Amp. from National Semiconduct.) (Fig. 9).The position of the sensor was found to be critical. For a sensor located near the edge of the membrane, there is a dramatic change in resistance with small change in curvature, followed by a region of little or no change for a piston travel between 2 and 7 mm (see Fig. 11). It is believed that two types of deformation, strain and elongation are responsible for the gauge changes. Whereas initial deformation is through straining and more present at the edges, the latter one is through elastic deformation, and as such,
40 35 30 25 20 15 Sensor at the edge Sensor in the center
5 0 -5
0
1
2
3
4
5
6
200
300
400
500
Fig. 12. Experimental and calculated sensor response as a function of deflection.
Fig. 10. Actuation mechanism reversibility.
10
100
Deflection [mm]
8
Piston travel distant [mm]
Sensor response [mV]
Sensor Response [V]
Membrane deflection microns
1324
7
Travel distant of the piston [mm]
Fig. 11. Sensor response at different locations using Wheatstone Bridge.
more expressed in the central area of the membrane, away from edges. It is clear from these results, that centrally located sensors provide better output for control purposes and a calibration curve plotting deflection versus sensor output has been produced (Fig. 12), showing good agreement between experiment and theory; the calculated value takes the form Y ¼ 0:7762 4:08133E4 D 7:95744E6 D2 where D and Y are, respectively, the membrane deflection and the sensor response in volts. 7. Conclusions The design, fabrication and experimental testing of a high bandpass, reflective micro-optical system for X-ray imaging (MOXI) has been completed. Its sub-micron focusing potential makes it suitable for the investigation of biological cells, including studies of the Bystander Effect. The focal length of the lens is controlled using an integrated resistive sensor and stepper motor, driven by a PC using Lab ViewTM. Experiment shows that the sensor should be to be centrally located and calibration curves for sensor output against focal length have been obtained. In the next phase, the prototype MOXI will be used in Xray imaging studies aimed ultimately at biomedical applications. References [1] K. Atkinson, Methods of efficiently focusing TiKaX-rays in lowbrightness limited-geometry systems, MRes Thesis, King’s College London, 2002. [2] C. Mothersill, C. Seymour, Int. Soc. Exp. Hematol. 31 (6) (2003) 437– 445. [3] A.G. Michette, Optical System for Soft X-rays, Plenum Press, New York, 1986. [4] I.C.E. Turcu, J.B. Dance, X-rays from Laser Plasmas: Generation and Application, Wiley, England, 1999. [5] P.D. Prewett, A.G. Michette, Proc. SPIE 4145, (2001). [6] P.D. Prewett, A.G. Michette, UK Patent Application No9927631.3, November 1999. [7] M.Y. Al Aioubi, V. Djakov, S.E. Huq, P.D. Prewett, Microelectron. Eng. 73–74 (2004) 898–903. [8] M.Y. Al Aioubi, V. Djakov, S.E. Huq, P.D. Prewett, A. Michett, Design and Fabrication of Micro Optical System for X-ray Analysis
M.Y. Al Aioubi et al. / Microelectronic Engineering 83 (2006) 1321–1325 of Biological Cells, Proceedings of SPIE, vol. 5825, Conference of Opto-Ireland 2005, 4–6 April 2005. [9] Available from:.
1325
[10] Available from:. [11] Available from: .
A novel MOEMS based adaptive optics for X-ray focusing M.Y. Al Aioubi a
a,*
, P.D. Prewett a, S.E. Huq b, V. Djakov b, A.G. Michette
c
School of Mechanical and Manufacturing Engineering., University of Birmingham, Edgbaston, Birmingham B15 2TT, UK b Central Microstructure Facility, Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, UK c Department of Physics, King’s College London, Strand, London WC2R 2LS, UK Available online 17 February 2006
Abstract This work presents the design and microfabrication of a novel X-ray micro-optical system for use in an X-ray microprobe for analysis of biological cells. A reflective microoptoelectromechanical system capable of focusing a wide range of wavelengths has been developed. The lens system consists of a pair of microfabricated optical elements, one of which has variable curvature providing a unique mechanically-actuated zoom focusing capability. Experiments have been carried out to measure the changes of the focal length (lens curvature) and for system calibration. Ó 2006 Elsevier B.V. All rights reserved. Keywords: MOEMS; Adaptive optics; Novel X-ray optics; MOXI
1. Introduction Micro-opto-electro-mechanical systems (MOEMS) are a sub-division of micro-electromechanical systems, having optical functional elements. Many applications require scanning, switching, deflecting reflecting and focusing of an optical beam. We report the design and microfabrication of a MOEMS system for X-ray analysis of biological cells. In particular, focused X-rays will be used to study the so-called Bystander Effect in which cells other than those irradiated are affected by the X-ray dose [1,2]. This requires focusing to a spot diameter on the micron scale. Our system is an integration of a flexible microlens, sensor and mechanical actuation mechanism with an electromechanical control system to set the required focal length. 2. Optics for X-ray focusing Conventional optics for focusing X-rays have low optical efficiency and are wavelength dependent. Such optics depend on the use of zone plates and/or multilayer mirrors *
Corresponding author. Tel.: +44 1235 44 5650; fax: +44 1235 44 6283. E-mail address: [email protected] (M.Y. Al Aioubi).
0167-9317/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.01.107
[3,4]. An alternative approach is grazing incidence reflective optics, which provides high efficiency, but suffers from aberration limitations. A microchannel plate, originally intended for electron multiplication, can be ‘‘slumped’’ to form a reflective polycapillary focusing element, which has been used successfully in X-ray imaging applications, such as astronomy. The plate is mechanically deformed to provide the curved surface required for focusing [4]. Even with the most advanced manufacturing techniques, channel diameters are limited to a minimum of 0.5 lm, over a few tens of millimetres and can produce focused spot sizes no smaller than 10 lm. Performance is limited by channel tilting, incorrect curvatures, waviness, diffraction and uncontrolled and limited substrate bending [5]. Recently, a microfabricated optical array for X-ray imaging (MOXI) was suggested [5,6]. The MOXI approach has similar advantages to a polycapillary system, including large aperture, high photon flux, large bandpass Dk/k and high transmission, due to low losses at grazing incidence. The basic optical element is shown schematically in Fig. 1. A series of microchannels, formed from concentric rings of an absorber/reflector such as Ni or Au, is deformed by radial actuators which also support the ring structure. A combination of a curved element and a flat
1322
M.Y. Al Aioubi et al. / Microelectronic Engineering 83 (2006) 1321–1325
Reflector, Si Support/Actuator Fig. 1. Multichannel MOXI reflective element with bimorph actuator/ supports [7].
Focal point Bent optics
Fig. 3. Schematic of MOXI with vacuum focus control.
Flat optics Central stop
X-ray paths Source
Fig. 2. A pair of curved and flat optical elements are used for focusing.
3. Design for two-dimensional focusing The design, shown schematically in Fig. 3, consists of two metal carrier rings, aligned using three pins. The first ring is used to carry two membranes, each made from Mylar of thickness 6 lm. The top membrane forms the curved optical element, consisting of a number of reflectors, microfabricated from deposited gold, in the form of symmetric rings. The bottom surface of the support ring carries a second membrane with no reflectors, which isolates the second (flat) optical element on the second mounting ring from the effect of the vacuum pressure. This also has symmetrical ring reflectors made of gold, aligned with those of the first element, using gold alignment marks. With both metal rings clamped together, as shown in Fig. 3, a finely controlled vacuum is produced using a simple mechanical piston. Mylar is chosen for its mechanical flexibility, low X-ray absorption and good chemical properties, being resistant to the chemicals used in microfabrication of the gold reflectors. Changing the vacuum in the system changes the membrane curvature, consequently changing the focal length of the X-ray focusing lens.
Fig. 4. MathCAD simulation of the membrane for different radius of curvature (from 10 to 160 mm).
Analytical study of the mechanical design was carried out using a series of equations. These equations were based on the shape geometry and Snell’s law. This model may be used for any further development. The results showed that the thickness of the gold layer may be very thin (less than 100 nm). This ensures that reflection would take place at the bent optical element. Subsequently these equations were rewritten in MathCAD code for simulation of any further development of the system. Fig. 4 is an example to show that it is possible to be used in MathCAD where Y, Y1 represent the top membranes and y2, y3 represent the bottom membrane. Then an optical design study was carried out and found that the gold thickness should be in the range 300 nm. The graph in Fig. 5 shows the X-ray attenuation length of gold. Atten Length (microns)
one, as shown in Fig. 2, forms the optical system. Thermal bimorph cantilever actuators fabricated from gold-onpolyimide provide actuator/support; performance characteristics have been reported elsewhere [7]. The new MOXI system we report here has all of the above advantages, with the ability to control the focal length precisely over a wide range. Moreover, a high magnification ratio may be obtained with a minimum spot diameter in the sub-micron regime. However, instead of the radially supported and actuated structure described above, the chosen design uses a membrane support for the gold microchannels, with curvature controlled by vacuum-derived pressure difference [6].
0.14
Au Density=19.32
0.12 0.1 0.08 0.06 0.04 0.02 0 1000
2000
3000
4000
Photon Energy [eV] Fig. 5. X-ray attenuation length of gold [9].
5000
Transmision
M.Y. Al Aioubi et al. / Microelectronic Engineering 83 (2006) 1321–1325 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1000
repeating steps 1 and 2 (j,k) and the above deposition, lithography and etch processes repeated for fabrication of the flat lens. Finally, the rings are assembled to form the inter-element vacuum space, as shown in Fig. 7. The final optical elements are symmetrically disposed gold rings with a maximum diameter of 500 lm, spaced 5 lm apart and of width 5 lm.
Thickness=15 microns
2000 3000 4000 Photon Energy [eV]
1323
5000
5. System assembly and control
Fig. 6. Transmission of Mylar [10].
4. Microfabrication of the optical elements Microfabrication of the optical elements in gold-onMylar involves the development of Mylar handling techniques, sputter coating of gold, optical lithography and wet chemical etch. A set of experiments has been carried out to optimise the fabrication process. This involved determination of the Mylar thickness, sputtering parameters for gold deposition. Different Mylar thicknesses were tested. The judgement was based on finding the minimum thickness as well as getting a proper sealing of the system. The optimal thickness of the bent optical element was found to be 6 lm. On the other hand for the flat optical element 2.5 lm was used. For 15 lm thick Mylar sheet the transmission is 93% shown in Fig. 6 at photon energy of 4.5 keV. The photon energy is selectyed as 4.5 keV. The energy is appropriate for X-ray microprobe. Another process considered is the sputtering parameter for gold deposition (set to prevent deformation of the Mylar membrane as well as burning) and optimisation of the patterning process. The fabrication sequence is as follows: Tensioning of the Mylar sheet and fixing to its carrier ring using glue (a,b), is followed by sputter coating of an adhesion layer (40 nm of chromium), followed by 300 nm of gold (c,d). Spin coating and bake-out of JSR photoresist is followed by optical lithography (e,f). Wet-etch removal of exposed gold and subsequently chromium (g,h) defines the reflecting microstructures, after which the resist is stripped. A second Mylar film is mounted on the other side of the ring by
Following the fabrication of the MOXI lens the two carrier rings (Fig. 8(a)) are assembled together inside a special jig, designed to create a vacuum inside the lens chamber – Fig. 8(b). The jig is connected to an evacuating piston and the focal length is set by controlling its distance of travel using a stepper motor. Gold resistors are patterned on the Mylar alongside the reflectors. When the membrane is curved, these resistors are strained and the measured increase in resistance provides a readout which can be calibrated to the curvature of the optical element and therefore to its focal length. The entire system is controlled by a PC using Lab ViewTM software. (Fig. 8(c)) shows an assembled MOXI. 6. System performance Fig. 9 shows the relationship between the sensor response and the focal length of the lens, obtained by measuring membrane deflection using an optical microscope (Fig. 10). The two curves (series1 and series2) represent respectively membrane deformation (focusing) and flattening (defocusing), which shows good reversibility of the
Sensor response [V]
Fig. 8. 2-D focusing MOXI.
2.7 2.5 2.3 2.1 1.9 1.7 1.5 15
25
35
45
55
65
75
85
95
105 115 125 135
Focal length [mm]
Fig. 7. Fabrication process steps.
Fig. 9. Amplified sensor response versus focal length.
M.Y. Al Aioubi et al. / Microelectronic Engineering 83 (2006) 1321–1325 3
250 200 Series1 Series2
150 100 50
Measured 2.5
Expected
2 1.5 1 0.5 0 0
0 0
1
2
3
4
5
6
7
actuation mechanism, which is expected given the elastic and mechanical properties of thin Mylar sheets [11] and relatively small deformations. The resistive sensor acts as a passive strain gauge, which output can be calibrated to the lens curvature. A number of different sensors, having different dimensions and positions on the curved lens element, were tested. The resistance variation of the centrally located sensor was measured for a range of focal lengths. At the maximum piston travel of 8 mm (corresponding to a focal length of 17 mm), the resistance change was 0.4 X [8]. The sensitivity of the system was further improved by using a 1/4 Wheatstone Bridge (Fig. 11) where one of the branches in the ‘diamond’ configuration was the piezoresistive gauge. Further enhancement is possible with incorporation of a second 1/2 Bridge) gauge on the same side of the membrane, whereas a realisation of a full bridge with four sensing elements would require more complex double-sided lithography. A further signal enhancement (by a factor of 100) was achieved with an amplifier circuit (LMC6041 CMOS Signal Micropower Op.Amp. from National Semiconduct.) (Fig. 9).The position of the sensor was found to be critical. For a sensor located near the edge of the membrane, there is a dramatic change in resistance with small change in curvature, followed by a region of little or no change for a piston travel between 2 and 7 mm (see Fig. 11). It is believed that two types of deformation, strain and elongation are responsible for the gauge changes. Whereas initial deformation is through straining and more present at the edges, the latter one is through elastic deformation, and as such,
40 35 30 25 20 15 Sensor at the edge Sensor in the center
5 0 -5
0
1
2
3
4
5
6
200
300
400
500
Fig. 12. Experimental and calculated sensor response as a function of deflection.
Fig. 10. Actuation mechanism reversibility.
10
100
Deflection [mm]
8
Piston travel distant [mm]
Sensor response [mV]
Sensor Response [V]
Membrane deflection microns
1324
7
Travel distant of the piston [mm]
Fig. 11. Sensor response at different locations using Wheatstone Bridge.
more expressed in the central area of the membrane, away from edges. It is clear from these results, that centrally located sensors provide better output for control purposes and a calibration curve plotting deflection versus sensor output has been produced (Fig. 12), showing good agreement between experiment and theory; the calculated value takes the form Y ¼ 0:7762 4:08133E4 D 7:95744E6 D2 where D and Y are, respectively, the membrane deflection and the sensor response in volts. 7. Conclusions The design, fabrication and experimental testing of a high bandpass, reflective micro-optical system for X-ray imaging (MOXI) has been completed. Its sub-micron focusing potential makes it suitable for the investigation of biological cells, including studies of the Bystander Effect. The focal length of the lens is controlled using an integrated resistive sensor and stepper motor, driven by a PC using Lab ViewTM. Experiment shows that the sensor should be to be centrally located and calibration curves for sensor output against focal length have been obtained. In the next phase, the prototype MOXI will be used in Xray imaging studies aimed ultimately at biomedical applications. References [1] K. Atkinson, Methods of efficiently focusing TiKaX-rays in lowbrightness limited-geometry systems, MRes Thesis, King’s College London, 2002. [2] C. Mothersill, C. Seymour, Int. Soc. Exp. Hematol. 31 (6) (2003) 437– 445. [3] A.G. Michette, Optical System for Soft X-rays, Plenum Press, New York, 1986. [4] I.C.E. Turcu, J.B. Dance, X-rays from Laser Plasmas: Generation and Application, Wiley, England, 1999. [5] P.D. Prewett, A.G. Michette, Proc. SPIE 4145, (2001). [6] P.D. Prewett, A.G. Michette, UK Patent Application No9927631.3, November 1999. [7] M.Y. Al Aioubi, V. Djakov, S.E. Huq, P.D. Prewett, Microelectron. Eng. 73–74 (2004) 898–903. [8] M.Y. Al Aioubi, V. Djakov, S.E. Huq, P.D. Prewett, A. Michett, Design and Fabrication of Micro Optical System for X-ray Analysis
M.Y. Al Aioubi et al. / Microelectronic Engineering 83 (2006) 1321–1325 of Biological Cells, Proceedings of SPIE, vol. 5825, Conference of Opto-Ireland 2005, 4–6 April 2005. [9] Available from:
1325
[10] Available from: