- Email: [email protected]
Infrared-transparent microstrip detectors
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 598 (2009) 84–85
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Infrared-transparent microstrip detectors M. Ferna´ndez a,,1,2, J. Duarte a, J. Gonza´lez a, S. Heinemeyer a, R. Jaramillo a, A. Lo´pez a, C. Martı´nez a, A. Ruiz a, I. Vila a, E. Cabruja b, M. Lozano b, G. Pellegrini b a b
Instituto de Fı´sica de Cantabria (IFCA), Ed. Juan Jorda, E-39005 Santander, Spain ´nica CNM-IMB, Campus Universidad Auto ´noma Barcelona, 08193 Bellaterra, Barcelona, Spain Centro Nacional de Microelectro
a r t i c l e in f o
a b s t r a c t
Available online 23 August 2008
The two main limiting factors in the accuracy of an optomechanical position monitoring system based on laser sources and photosensors are mechanical transfer between the monitored imaging sensors to the active particle tracking elements and non-straight propagation of the reference laser lines. Laser based alignment systems of Si trackers that use their own tracking detectors as photosensors are not affected by the first factor. Improving the transmittance of Si to infrared beams certainly minimizes the second one. Simulation of the passage of a light beam through a real microstrip detector and analysis of first measurements of samples are presented in this paper. & 2008 Elsevier B.V. All rights reserved.
Keywords: Tracking Strip detectors Multilayer Optical constants
1. Introduction The next generation of tracking systems, as the one envisaged for the International Linear Collider (ILC), will demand track momentum resolutions one order of magnitude better than current state-of-the-art trackers. Environmental disturbances around the detector (local temperature gradients, humidity changesy) will induce instability of the supporting structures comparable to the precision of the detectors. Independent alignment systems monitoring these changes are then needed. For the case of silicon trackers, one can take profit of the weak (but still existing) absorption of infrared light (IR) in Si and use laser beams as pseudo-tracks that traverse consecutive sensors. Based on the successful experience of AMS and CMS tracker systems [1,2], we will try to raise the transmittance of a Si microstrip detector by a further 20–30% of what was achieved until now. For that, we propose to tune the thicknesses of the different layers of the sensor such that they work as an antireflection coating for the chosen wavelength. We also propose, as a novel idea, to locally substitute Al electrodes by transparent electrodes, as indium tin oxide (ITO). In order to achieve this increase in transmittance, a thorough simulation of the propagation of a light beam through the detector has been performed (Section 2) where effects of multiple reflections in the sensor
Corresponding author. Tel.: +41 22 7670487.
E-mail address: [email protected] (M. Ferna´ndez). Work supported by the Commission of the European Communities under the 6th Framework Programme ’Structuring the European Research Area’, Contract no. RII3-026126. 2 This work was partially supported and financed by the ICTS (Integrated Nano-Microelectronics Clean Room) access within the GICSERV Programme. 1
0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.08.081
layers and diffraction by the strips are considered. In this paper (Section 3), we present the analysis of the first measurements of material samples (intrinsic Si, thermal SiO2 on Si, passivation Si3N4,y). With these measurements we achieve two goals: cross-check the optical simulation and characterize the optical constants of the materials as produced by the Instituto de Microelectro´nica de Barcelona (IMB-CNM).
2. Realistic simulation of Si microstrip detectors A silicon microstrip tracking detector is made of a stack of layers transparent or partially transparent to IR light. The interaction of light with layered media is extensively addressed in literature [3]. The first approach to study a multilayer is considering each individual layer as a perfect homogeneous planoparallel film characterized by its refraction index NðlÞ ¼ nðlÞ þ ikðlÞ and thickness. We have explicitly expressed that the refraction index is a function of the wavelength l. When the imaginary part of the refraction index is nonzero, the material will absorb light. From the typical materials employed in microstrip detectors, only Si has non-negligible absorption in the IR. If the layers are not continuous but present local features, the former theory is not applicable. Indeed, diffraction phenomena will appear if the size of the obstacle is comparable to the wavelength used. For instance, the strips of the detector, pitched every 10250 mm, are good examples of an optical diffraction grating for an incoming beam in the IR. Furthermore, since the strips are partially transparent to light we must use rigorous diffraction theories instead of the simpler Fresnel or Kirchoff approximations.
ARTICLE IN PRESS ´ndez et al. / Nuclear Instruments and Methods in Physics Research A 598 (2009) 84–85 M. Ferna
T (λ), λ tolerant design
85
0.3
0.7 0.25 0.6
λ
0.2
0.4 0.3 0.2 0.1
%T %R %A
Reflectance
T,R,A
0.5
0.15
0.1
0 1000 1020 1040 1060 1080 1100 1120 1140 λ (nm)
0.05 Fig. 1. Optical functions: T; R; A calculated using a realistic simulation of the propagation of light through a Si microstrip detector. Effects of diffraction by the strips are included. Pitch/strip width ¼ 10%.
0 1000 Our realistic simulation of the detector takes all these effects into account: change of refraction index with l, the absorption of the films and, for the first time in this kind of studies, the heterogeneity of the layers in terms of diffraction power of the strips. We show in Fig. 1 the resulting optical functions: transmittance (T), reflectance (R) and absorptance (A) calculated for the standard sensor displayed in the inlaid plot. Layer thicknesses and layout of this sensor are based on a working sensor manufactured at CNM but have been optimized to yield maximum transmittance at 1100 nm (dotted line). The optimization was achieved in two steps. First we optimized the lattice parameters (strip width and pitch) that led to maximum transmitted energy. We found that with a strip width of 10% the pitch yielded 70% transmittance. Then we fine-tuned the values of the thicknesses to achieve maximum transmittance keeping strip-width/pitch ratio fixed.
3. First measurements of Si microstrip samples Although tabulated values of the refraction index for typical materials do exist [4] it is known that the actual values depend on manufacturer, impurity level, quality of the surface and room temperature, mostly. It is then important for the simulation to have our own measured values of the optical constants of the materials. To calculate them one can use the simulation of multiple reflections on homogeneous media in the reverse direction, i.e., measure the optical functions of a layer and extract from it the values of ðn; k; dÞ as a function of the wavelength. Prior to that, we have cross-checked the simulation fitting the reflectance of a calibrated reference wafer [5] made with bands of different thickness of SiO2 on a backside-rough substrate of Si. The rough interface prevents light reaching the second Si interface to reflect specularly. This is to say that the reflectance is
1020
1040
1060
1080 λ (nm)
1100
1120
1140
Fig. 2. Measured (solid line) and calculated (dashed) reflectance of a thin film of SiO2 on a Si substrate.
independent of the thickness of the Si wafer. Fig. 2 shows (black continuous line) the measured reflectance of a SiO2 layer on the Si substrate. The red dotted line is the fitted reflectance using the calibrated SiO2 thickness (308.97 nm). Once the goodness of the simulation has been tested, we may use it to characterize samples of materials. Characterization of Si has been done on 300 mm thick double polished wafers produced by TopSiL [6]. The uniformity of the wafer was tested measuring its transmittance T at 32 different points. For each measurement the refraction index was fit to the tabulated values times a multiplicative constant, i.e. the functional form for the transmittance was Tða nt ; b kt ; d; lÞ with a; b; d fitting constants and nt ; kt the tabulated refraction indexes. The mean value (rms of the measurements) obtained for the scaling factors a; b were 0.997(0.008) and 0.964(0.006), respectively. This shows that the refraction index of the samples is very close to the tabulated values. Then we reused these values as true refraction index values and recalculated the thickness for all the measurements. The mean value obtained was 300008:8 1:2 nm. References [1] W. Wallraff, TAS status, in: AMS Tracker Meeting, Montpellier, 22–23 June 2004. [2] B. Wittmer, et al., Nucl. Instr. and Meth. A 581 (2007) 351. [3] M. Born, E. Wolf, Principles of Optics, seventh ed., Cambridge University Press, Cambridge, 1999. [4] E.D. Palik, Handbook of Optical Constants of Solids, first ed., Academic Press, New York, 15 January 1997. [5] Stepwafer SiO2 on Si, Mikropack, Germany. [6] Topsil Semiconductor Materials, Linderupvej 4, Frederikssund, Denmark.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Infrared-transparent microstrip detectors M. Ferna´ndez a,,1,2, J. Duarte a, J. Gonza´lez a, S. Heinemeyer a, R. Jaramillo a, A. Lo´pez a, C. Martı´nez a, A. Ruiz a, I. Vila a, E. Cabruja b, M. Lozano b, G. Pellegrini b a b
Instituto de Fı´sica de Cantabria (IFCA), Ed. Juan Jorda, E-39005 Santander, Spain ´nica CNM-IMB, Campus Universidad Auto ´noma Barcelona, 08193 Bellaterra, Barcelona, Spain Centro Nacional de Microelectro
a r t i c l e in f o
a b s t r a c t
Available online 23 August 2008
The two main limiting factors in the accuracy of an optomechanical position monitoring system based on laser sources and photosensors are mechanical transfer between the monitored imaging sensors to the active particle tracking elements and non-straight propagation of the reference laser lines. Laser based alignment systems of Si trackers that use their own tracking detectors as photosensors are not affected by the first factor. Improving the transmittance of Si to infrared beams certainly minimizes the second one. Simulation of the passage of a light beam through a real microstrip detector and analysis of first measurements of samples are presented in this paper. & 2008 Elsevier B.V. All rights reserved.
Keywords: Tracking Strip detectors Multilayer Optical constants
1. Introduction The next generation of tracking systems, as the one envisaged for the International Linear Collider (ILC), will demand track momentum resolutions one order of magnitude better than current state-of-the-art trackers. Environmental disturbances around the detector (local temperature gradients, humidity changesy) will induce instability of the supporting structures comparable to the precision of the detectors. Independent alignment systems monitoring these changes are then needed. For the case of silicon trackers, one can take profit of the weak (but still existing) absorption of infrared light (IR) in Si and use laser beams as pseudo-tracks that traverse consecutive sensors. Based on the successful experience of AMS and CMS tracker systems [1,2], we will try to raise the transmittance of a Si microstrip detector by a further 20–30% of what was achieved until now. For that, we propose to tune the thicknesses of the different layers of the sensor such that they work as an antireflection coating for the chosen wavelength. We also propose, as a novel idea, to locally substitute Al electrodes by transparent electrodes, as indium tin oxide (ITO). In order to achieve this increase in transmittance, a thorough simulation of the propagation of a light beam through the detector has been performed (Section 2) where effects of multiple reflections in the sensor
Corresponding author. Tel.: +41 22 7670487.
E-mail address: [email protected] (M. Ferna´ndez). Work supported by the Commission of the European Communities under the 6th Framework Programme ’Structuring the European Research Area’, Contract no. RII3-026126. 2 This work was partially supported and financed by the ICTS (Integrated Nano-Microelectronics Clean Room) access within the GICSERV Programme. 1
0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.08.081
layers and diffraction by the strips are considered. In this paper (Section 3), we present the analysis of the first measurements of material samples (intrinsic Si, thermal SiO2 on Si, passivation Si3N4,y). With these measurements we achieve two goals: cross-check the optical simulation and characterize the optical constants of the materials as produced by the Instituto de Microelectro´nica de Barcelona (IMB-CNM).
2. Realistic simulation of Si microstrip detectors A silicon microstrip tracking detector is made of a stack of layers transparent or partially transparent to IR light. The interaction of light with layered media is extensively addressed in literature [3]. The first approach to study a multilayer is considering each individual layer as a perfect homogeneous planoparallel film characterized by its refraction index NðlÞ ¼ nðlÞ þ ikðlÞ and thickness. We have explicitly expressed that the refraction index is a function of the wavelength l. When the imaginary part of the refraction index is nonzero, the material will absorb light. From the typical materials employed in microstrip detectors, only Si has non-negligible absorption in the IR. If the layers are not continuous but present local features, the former theory is not applicable. Indeed, diffraction phenomena will appear if the size of the obstacle is comparable to the wavelength used. For instance, the strips of the detector, pitched every 10250 mm, are good examples of an optical diffraction grating for an incoming beam in the IR. Furthermore, since the strips are partially transparent to light we must use rigorous diffraction theories instead of the simpler Fresnel or Kirchoff approximations.
ARTICLE IN PRESS ´ndez et al. / Nuclear Instruments and Methods in Physics Research A 598 (2009) 84–85 M. Ferna
T (λ), λ tolerant design
85
0.3
0.7 0.25 0.6
λ
0.2
0.4 0.3 0.2 0.1
%T %R %A
Reflectance
T,R,A
0.5
0.15
0.1
0 1000 1020 1040 1060 1080 1100 1120 1140 λ (nm)
0.05 Fig. 1. Optical functions: T; R; A calculated using a realistic simulation of the propagation of light through a Si microstrip detector. Effects of diffraction by the strips are included. Pitch/strip width ¼ 10%.
0 1000 Our realistic simulation of the detector takes all these effects into account: change of refraction index with l, the absorption of the films and, for the first time in this kind of studies, the heterogeneity of the layers in terms of diffraction power of the strips. We show in Fig. 1 the resulting optical functions: transmittance (T), reflectance (R) and absorptance (A) calculated for the standard sensor displayed in the inlaid plot. Layer thicknesses and layout of this sensor are based on a working sensor manufactured at CNM but have been optimized to yield maximum transmittance at 1100 nm (dotted line). The optimization was achieved in two steps. First we optimized the lattice parameters (strip width and pitch) that led to maximum transmitted energy. We found that with a strip width of 10% the pitch yielded 70% transmittance. Then we fine-tuned the values of the thicknesses to achieve maximum transmittance keeping strip-width/pitch ratio fixed.
3. First measurements of Si microstrip samples Although tabulated values of the refraction index for typical materials do exist [4] it is known that the actual values depend on manufacturer, impurity level, quality of the surface and room temperature, mostly. It is then important for the simulation to have our own measured values of the optical constants of the materials. To calculate them one can use the simulation of multiple reflections on homogeneous media in the reverse direction, i.e., measure the optical functions of a layer and extract from it the values of ðn; k; dÞ as a function of the wavelength. Prior to that, we have cross-checked the simulation fitting the reflectance of a calibrated reference wafer [5] made with bands of different thickness of SiO2 on a backside-rough substrate of Si. The rough interface prevents light reaching the second Si interface to reflect specularly. This is to say that the reflectance is
1020
1040
1060
1080 λ (nm)
1100
1120
1140
Fig. 2. Measured (solid line) and calculated (dashed) reflectance of a thin film of SiO2 on a Si substrate.
independent of the thickness of the Si wafer. Fig. 2 shows (black continuous line) the measured reflectance of a SiO2 layer on the Si substrate. The red dotted line is the fitted reflectance using the calibrated SiO2 thickness (308.97 nm). Once the goodness of the simulation has been tested, we may use it to characterize samples of materials. Characterization of Si has been done on 300 mm thick double polished wafers produced by TopSiL [6]. The uniformity of the wafer was tested measuring its transmittance T at 32 different points. For each measurement the refraction index was fit to the tabulated values times a multiplicative constant, i.e. the functional form for the transmittance was Tða nt ; b kt ; d; lÞ with a; b; d fitting constants and nt ; kt the tabulated refraction indexes. The mean value (rms of the measurements) obtained for the scaling factors a; b were 0.997(0.008) and 0.964(0.006), respectively. This shows that the refraction index of the samples is very close to the tabulated values. Then we reused these values as true refraction index values and recalculated the thickness for all the measurements. The mean value obtained was 300008:8 1:2 nm. References [1] W. Wallraff, TAS status, in: AMS Tracker Meeting, Montpellier, 22–23 June 2004. [2] B. Wittmer, et al., Nucl. Instr. and Meth. A 581 (2007) 351. [3] M. Born, E. Wolf, Principles of Optics, seventh ed., Cambridge University Press, Cambridge, 1999. [4] E.D. Palik, Handbook of Optical Constants of Solids, first ed., Academic Press, New York, 15 January 1997. [5] Stepwafer SiO2 on Si, Mikropack, Germany. [6] Topsil Semiconductor Materials, Linderupvej 4, Frederikssund, Denmark.