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Fabrication of large NbSi bolometer arrays for CMB applications
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 559 (2006) 554–556 www.elsevier.com/locate/nima
Fabrication of large NbSi bolometer arrays for CMB applications M. Ukibea,b, B. Belierc, Ph. Camusd,, C. Dobreab, L. Dumoulinb, B. Fernandezd, T. Fournierd, O. Guillaudine, S. Marnierosb, S.J.C. Yatesb a
AIST, Tsukuba Central 2, Tsukuba, Ibaraki 305-8568, Japan b CNRS-CSNSM, Bat 104, Orsay Campus F-91405, France c CNRS-IEF, Bat 220, Orsay Campus F-91405, France d CNRS-CRTBT, 25 avenue des Martyrs, Grenoble F-38042, France e CNRS-LPSC, 53 avenue des Martyrs, Grenoble F-38042, France
Abstract Future cosmic microwave background experiments for high-resolution anisotropy mapping and polarisation detection require large arrays of bolometers at low temperature. We have developed a process to build arrays of antenna-coupled bolometers for that purpose. With adjustment of the Nbx Si1x alloy composition, the array can be made of high impedance or superconductive (TES) sensors. r 2006 Elsevier B.V. All rights reserved. PACS: 07.57.Kp Keywords: Bolometer array; Far-infrared; Astronomy
1. Motivation Fully sampled arrays of bolometers are required to increase the sensitivity of modern millimeter instruments in cosmic microwave background (CMB) astrophysics [1]. At long wavelengths (1–3 mm), a bolometer needs to be cooled to a temperature less than 200 mK and requires a large collecting surface per detector. Dense arrays of antennacoupled bolometers for imaging applications at 100 GHz and room temperature have been proposed in Ref. [2]. At low temperature, one obvious advantage is that the sensitive bolometer can be made significantly smaller than the wavelength. The Nbx Si1x bolometers have been studied previously [2]. An interesting property of the NbSi binary alloy is that the sensor can be made either high impedance or superconducting by modifying the Nb molar fraction. With the developed co-evaporation technique, the alloy
Corresponding author. Tel.: +33 4 76889047; fax: +33 4 76885060.
E-mail address: [email protected] (Ph. Camus). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.12.069
has the properties of an Anderson insulator for xo9%. With x412% it is a superconductor whose T c can be adjusted between 50 mK and 1 K. The present fabrication process utilises the high impedance of the composition range.
2. The antenna array design Bolometers are superior to heterodyne techniques in broadband applications ðDf =f 410%Þ. One classical planar antenna design is the bow–tie geometry. In order to make a compact array for imaging, a solution was proposed in Ref. [3], using a dissipative element at the centre of the antenna. The two sides of the antenna are linked by the dissipator where the optical power is converted to heat. The dissipator is deposited on a thin membrane thermally insulated from the substrate. With adequate electrical insulation, a thermometer can be deposited at the centre of the membrane. The detector element is then replicated on a triangular basis to form a regular array and sample the focal plane.
ARTICLE IN PRESS M. Ukibe et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 554–556
555
100 90 ABSORPTION (%)
80 70 60 50 40 30 20 10 0 0
50
100 FREQUENCY (GHz)
150
200
Fig. 1. Absorption efficiency of a planar wave normal to the antenna; the dotted line is for vacuum and t ¼ 500 mm ðl ¼ 2 mmÞ; the thin line is for a Si substrate with t ¼ 500 mm and the thick one for t ¼ 150 mm. The dissipator is a pure 300 O resistance. Fig. 2. Fabrication steps of the membrane.
In the design, the main questions are: (1) the bandwidth of the detector, (2) the required dissipator impedance, and (3) the antenna radiation pattern for the matching to the front optics. In order to obtain a high absorption efficiency, a reflective plane is placed at a distance t ¼ l=4 from the antenna plane. A detailed computation was used on an infinite array (HFSS software). The desirable broadband property is shown in Fig. 1 for the vacuum case. If the substrate is modelled, the main resonant mode frequency is pffiffi reduced by the dielectric optical thickness ðt Þ. By reducing the substrate thickness from 525 to 150 mm, we can move the main mode from 45 to 150 GHz, but complex substrate modes are excited. 3. The array fabrication In the first step the material for the membrane, antenna and thermometers are deposited. Then, a deep etching process is used to remove the silicon under the detectors. 3.1. The trilayer membrane technique An overview of the membrane fabrication process is shown in Fig. 2. A low-stressed trilayer film is composed of SiO2 =SiNx =SiO2 (thickness 200/230/100 nm). The successive layers of SiO2 and SiNx are made with a plasma-CVD deposition in a mixture of SiH4/N2O and SiH4/NH3 gases, respectively. The residual stress is compensated between the materials and is less than 50 MPa (Fig. 2a). The sensor device (Section 3.2) is then deposited on top of the trilayer (Fig. 2b). A 10 mm thick positive resist defines the membrane pattern on the rear side. In order to keep the wafer temperature lower than 100 1C during the etching process, the front side is
Fig. 3. Antenna-coupled NbSi bolometer; the size of the Pd dissipator and the NbSi thermometer are 200 mm 300 mm and 300 mm 300 mm, respectively; the Al antenna is composed of two trapezoids of 200, 1000 mm side and 700 mm height; the membrane size is 1 1 mm2 .
bonded by a thermal grease to a cooled 4 in. silicon wafer (Fig. 2c). The silicon is etched by deep reactive ion etching (DRIE) (Fig. 2d). We need to avoid over-etching of the bottom SiO2 layer of the membrane. The etching selectivity (speed ratio between Si and SiO2) varies with the Si etching speed from 100 at 10 mm= min to 400 at 5 mm= min. So the last step of the etching process uses a high selectivity recipe. The device wafer is then separated from the support wafer. 3.2. The NbSi antenna-coupled bolometer A schematic cross-section of the device is shown in Fig. 3. The dissipator resistance has to be matched to the impedance of the antenna. A thin Pd or Ir film of 10 nm
ARTICLE IN PRESS 556
M. Ukibe et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 554–556
Another low-stressed SiNx 100 nm thick film is deposited for the electrical insulation. The noise performance of the high impedance NbSi films depends on the electrode thickness. Electrodes are made by lift-off after evaporation of the 10-nm-thick Nb lead layer with an angle of 10 between the device wafer and the Nb target. The NbSi film with a thickness of 100 nm is coevaporated by irradiating the Nb and the Si targets simultaneously. Acknowledgements
Fig. 4. Small-scale array prototype of 204 bolometers; the antenna distance is 2 mm to sample an image at l ¼ 2 mm with a f =2 optics.
This work has been funded by R&D CNES, PNC and INSU programs. S.J.C. Yates would like to thank the European Union Research Training Network on Applied Cryodetectors (HPRN-CT-2002-00322). References
can be adapted to the 50–500 O impedance range. The antenna is made of 40-nm-thick superconductive Al or Nb to minimise the thermal conductance on the membrane (Fig. 4).
[1] M.J. Griffin, J.J. Bock, W.K. Gear, Appl. Opt. 41 (31) (2002) 6543. [2] Ph. Camus, L. Berge´, L. Dumoulin, S. Marnieros, J.P. Torre, Nucl. Instr. and Meth. A 444 (2000) 419. [3] O. Savry, Thesis at the Institut National Polytechnique de Grenoble, 2001.
Nuclear Instruments and Methods in Physics Research A 559 (2006) 554–556 www.elsevier.com/locate/nima
Fabrication of large NbSi bolometer arrays for CMB applications M. Ukibea,b, B. Belierc, Ph. Camusd,, C. Dobreab, L. Dumoulinb, B. Fernandezd, T. Fournierd, O. Guillaudine, S. Marnierosb, S.J.C. Yatesb a
AIST, Tsukuba Central 2, Tsukuba, Ibaraki 305-8568, Japan b CNRS-CSNSM, Bat 104, Orsay Campus F-91405, France c CNRS-IEF, Bat 220, Orsay Campus F-91405, France d CNRS-CRTBT, 25 avenue des Martyrs, Grenoble F-38042, France e CNRS-LPSC, 53 avenue des Martyrs, Grenoble F-38042, France
Abstract Future cosmic microwave background experiments for high-resolution anisotropy mapping and polarisation detection require large arrays of bolometers at low temperature. We have developed a process to build arrays of antenna-coupled bolometers for that purpose. With adjustment of the Nbx Si1x alloy composition, the array can be made of high impedance or superconductive (TES) sensors. r 2006 Elsevier B.V. All rights reserved. PACS: 07.57.Kp Keywords: Bolometer array; Far-infrared; Astronomy
1. Motivation Fully sampled arrays of bolometers are required to increase the sensitivity of modern millimeter instruments in cosmic microwave background (CMB) astrophysics [1]. At long wavelengths (1–3 mm), a bolometer needs to be cooled to a temperature less than 200 mK and requires a large collecting surface per detector. Dense arrays of antennacoupled bolometers for imaging applications at 100 GHz and room temperature have been proposed in Ref. [2]. At low temperature, one obvious advantage is that the sensitive bolometer can be made significantly smaller than the wavelength. The Nbx Si1x bolometers have been studied previously [2]. An interesting property of the NbSi binary alloy is that the sensor can be made either high impedance or superconducting by modifying the Nb molar fraction. With the developed co-evaporation technique, the alloy
Corresponding author. Tel.: +33 4 76889047; fax: +33 4 76885060.
E-mail address: [email protected] (Ph. Camus). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.12.069
has the properties of an Anderson insulator for xo9%. With x412% it is a superconductor whose T c can be adjusted between 50 mK and 1 K. The present fabrication process utilises the high impedance of the composition range.
2. The antenna array design Bolometers are superior to heterodyne techniques in broadband applications ðDf =f 410%Þ. One classical planar antenna design is the bow–tie geometry. In order to make a compact array for imaging, a solution was proposed in Ref. [3], using a dissipative element at the centre of the antenna. The two sides of the antenna are linked by the dissipator where the optical power is converted to heat. The dissipator is deposited on a thin membrane thermally insulated from the substrate. With adequate electrical insulation, a thermometer can be deposited at the centre of the membrane. The detector element is then replicated on a triangular basis to form a regular array and sample the focal plane.
ARTICLE IN PRESS M. Ukibe et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 554–556
555
100 90 ABSORPTION (%)
80 70 60 50 40 30 20 10 0 0
50
100 FREQUENCY (GHz)
150
200
Fig. 1. Absorption efficiency of a planar wave normal to the antenna; the dotted line is for vacuum and t ¼ 500 mm ðl ¼ 2 mmÞ; the thin line is for a Si substrate with t ¼ 500 mm and the thick one for t ¼ 150 mm. The dissipator is a pure 300 O resistance. Fig. 2. Fabrication steps of the membrane.
In the design, the main questions are: (1) the bandwidth of the detector, (2) the required dissipator impedance, and (3) the antenna radiation pattern for the matching to the front optics. In order to obtain a high absorption efficiency, a reflective plane is placed at a distance t ¼ l=4 from the antenna plane. A detailed computation was used on an infinite array (HFSS software). The desirable broadband property is shown in Fig. 1 for the vacuum case. If the substrate is modelled, the main resonant mode frequency is pffiffi reduced by the dielectric optical thickness ðt Þ. By reducing the substrate thickness from 525 to 150 mm, we can move the main mode from 45 to 150 GHz, but complex substrate modes are excited. 3. The array fabrication In the first step the material for the membrane, antenna and thermometers are deposited. Then, a deep etching process is used to remove the silicon under the detectors. 3.1. The trilayer membrane technique An overview of the membrane fabrication process is shown in Fig. 2. A low-stressed trilayer film is composed of SiO2 =SiNx =SiO2 (thickness 200/230/100 nm). The successive layers of SiO2 and SiNx are made with a plasma-CVD deposition in a mixture of SiH4/N2O and SiH4/NH3 gases, respectively. The residual stress is compensated between the materials and is less than 50 MPa (Fig. 2a). The sensor device (Section 3.2) is then deposited on top of the trilayer (Fig. 2b). A 10 mm thick positive resist defines the membrane pattern on the rear side. In order to keep the wafer temperature lower than 100 1C during the etching process, the front side is
Fig. 3. Antenna-coupled NbSi bolometer; the size of the Pd dissipator and the NbSi thermometer are 200 mm 300 mm and 300 mm 300 mm, respectively; the Al antenna is composed of two trapezoids of 200, 1000 mm side and 700 mm height; the membrane size is 1 1 mm2 .
bonded by a thermal grease to a cooled 4 in. silicon wafer (Fig. 2c). The silicon is etched by deep reactive ion etching (DRIE) (Fig. 2d). We need to avoid over-etching of the bottom SiO2 layer of the membrane. The etching selectivity (speed ratio between Si and SiO2) varies with the Si etching speed from 100 at 10 mm= min to 400 at 5 mm= min. So the last step of the etching process uses a high selectivity recipe. The device wafer is then separated from the support wafer. 3.2. The NbSi antenna-coupled bolometer A schematic cross-section of the device is shown in Fig. 3. The dissipator resistance has to be matched to the impedance of the antenna. A thin Pd or Ir film of 10 nm
ARTICLE IN PRESS 556
M. Ukibe et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 554–556
Another low-stressed SiNx 100 nm thick film is deposited for the electrical insulation. The noise performance of the high impedance NbSi films depends on the electrode thickness. Electrodes are made by lift-off after evaporation of the 10-nm-thick Nb lead layer with an angle of 10 between the device wafer and the Nb target. The NbSi film with a thickness of 100 nm is coevaporated by irradiating the Nb and the Si targets simultaneously. Acknowledgements
Fig. 4. Small-scale array prototype of 204 bolometers; the antenna distance is 2 mm to sample an image at l ¼ 2 mm with a f =2 optics.
This work has been funded by R&D CNES, PNC and INSU programs. S.J.C. Yates would like to thank the European Union Research Training Network on Applied Cryodetectors (HPRN-CT-2002-00322). References
can be adapted to the 50–500 O impedance range. The antenna is made of 40-nm-thick superconductive Al or Nb to minimise the thermal conductance on the membrane (Fig. 4).
[1] M.J. Griffin, J.J. Bock, W.K. Gear, Appl. Opt. 41 (31) (2002) 6543. [2] Ph. Camus, L. Berge´, L. Dumoulin, S. Marnieros, J.P. Torre, Nucl. Instr. and Meth. A 444 (2000) 419. [3] O. Savry, Thesis at the Institut National Polytechnique de Grenoble, 2001.