- Email: [email protected]
Fabrication of antenna-coupled transition edge sensors for polarimeter applications
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 559 (2006) 459–461 www.elsevier.com/locate/nima
Fabrication of antenna-coupled transition edge sensors for polarimeter applications H.G. LeDuca,, Matthew Kenyona, Peter K. Daya, Minhee Yunb, J.J. Bocka a
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove, Pasadena, CA 91109, USA b University of Pittsburgh EE Dept, Pittsburgh, PA 15260, USA Available online 4 January 2006
Abstract In this paper we describe the progress in the development of polarization sensitive superconducting, bolometric detector arrays targeted at cosmic microwave background (CMB) studies. Our implementation takes advantage of superconducting technology to reduce the complexity of the typical CMB bolometer focal plane by incorporating as many of the required elements as necessary on the detector itself. Each pixel in the array consists of a distributed dual-polarization beam forming antenna, a band pass filter, and power splitter/combiner delivering detected power to four separate superconducting transition edge sensors. This combination of elements detects the polarization of the incoming light uniquely while eliminating a large part of the cooled mass of the focal plane including feed horns, polarizers, and band pass filters. Of course this increases the fabrication complexity of the bolometric detector itself over previous implementations. We will describe our fabrication process consisting of eight front side mask levels and one back side layer designed to produce whole wafer array tiles suitable for CMB polarimeters. r 2005 Elsevier B.V. All rights reserved. PACS: 84.40.07.57.k Keywords: Detector; Bolometer; Polarimeters; Superconductor; CMB
1. Introduction Polarimeters will play a key role in the next generation of cosmic microwave background (CMB) instruments [1]. Current state-of-the-art CMB instruments based on absorber-coupled semiconductor bolometers are background limited; so improvements for next generation experiments require increased throughput, i.e. larger pixel count. These instruments are difficult to scale, which is in large part due to the readout electronics which requires an amplifier per pixel. The introduction by Irwin [2] of the voltage-biased superconducting transition edge sensors (TES) coupled to a superconducting quantum interference device (SQUID) and the subsequent development of cryogenic multiplexing based on SQUIDs has made TES sensors an attractive replacement for semiconducting bolometers. An additional constraint of absorber-coupled Corresponding author. Tel.: +1 818 354 2209; fax: +1 818 393 4540.
E-mail address: [email protected] (H.G. LeDuc). 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.12.037
bolometers is the massive cryogenically cooled fore-optics required to restrict the field of view and define the frequency band. Our approach is to replace as much of this fore optics with thin film superconducting structures such as beam forming antennas, band pass filters, power combiners and splitters. This is made possible in CMB applications because the wavelengths of interest are below the gap wavelength of conventional superconductors such as Niobium, and low-loss transmission lines can be fabricated. 2. Detector architecture Each pixel in the focal plane architecture is a dual polarization beam forming antenna similar to that described by Goldin [3]. The antenna consists of an array of coherently combined dual polarization slot antennas (see Fig. 1). This is accomplished by combining the absorbed power from each antenna element using an isophase superconducting microstrip power combining network,
ARTICLE IN PRESS 460
H.G. LeDuc et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 459–461
Fig. 1. Schematic of the beam-forcing antenna. The white crosses represent slots in a ground plane while the lines represent microstrip transmission lines. The transmission lines are terminated with a lossy normal metal section. The simplest design requires two detectors, one for each polarization. The detectors are thermally isolated using silicon nitride legs.
one for each polarization component. The absorbed power is converted to heat using lossy normal metal microstrip section terminating the superconducting microstrip feeds, which indirectly heats a TES sensor co-located on a small silicon nitride island supported and thermally isolated from the cold bath by narrow silicon nitride legs. The simplest implementation requires only two TES elements to detect both polarizations. This does not uniquely define the polarization vector; however, the introduction of a superconducting power splitter with the correct phasing allows the determination of all of the Stokes parameters with two additional detectors. A superconducting band pass filter can be added to the microstrip to control the frequency band. The pixel size and subelement pitch are set by optical considerations (see Ref. [3]). For this study we have designed polarimeters for 150 GHz center frequency and an F/4 beam. Our pixel consists of a 16 16 array of subantennas distributed evenly on a approximately 8 8 mm square. The focal plane will be built up from sub-panels of these pixels with the detector leads routed to the panel edges. An 8 8 pixel array will fit snuggly on a 100 mm wafer. 3. Baseline detector fabrication There are a number of fabrication challenges associated with this architecture. First, within each pixel there are several tens of millimeters of superconducting microstrip which form the isophase power-combining structure, all with very low microwave loss. This places a premium on dielectric quality and integrity. Second, the TES detector is subject to the processing for the additional layers required for the antenna coupling. Finally the full wafer integration required to make a single panel place constraints on process yield and uniformity. We are developing a nine
Fig. 2. Schematic of process flow. (a) Molybdenum/gold TES deposition on silicon nitride on silicon and pattern, (b) SiO protection deposition and liftoff, (c) niobium ground plane deposition (sputtering) and pattern (ICP), (d) SiO microstrip dielectric deposition and etch (ICP), (e) niobium microstrip/wire deposition (sputtering) and patterning (ICP), (f) thermal isolation including silicon nitride etch (ICP) and silicon substrate removal (DRIE). Not shown is the gold termination microstrip (liftoff) and a SiO protect (liftoff).
mask level fabrication process using projection step-andrepeat lithography to produce these detectors. A schematic of the process flow for the detector stack is shown in Fig. 2. The lithographic tools used are a Canon FPA3000 EX3 5 DUV stepper and Karl Zuss MA6 contact aligner. All front side processing was done by projection lithography. The TES in this case is a proximity stack of molybdenum and gold deposited by DC-magnetron sputtering in a UHV vacuum chamber (base pressure 4 10 8 Pa). The layer thicknesses depend on the target transition temperature. The TES patterning was a combination of ion milling (gold) and RIE in sulphur hexafluoride (molybdenum). Our baseline process uses silicon monoxide (SiO) for both the microstrip dielectric (400 nm) and protect layers for the TES and the gold absorber. These films are either thermally evaporated onto multilayer liftoff stencils consisting of photoresist over PMMA or are etched. The niobium films were deposited by DC-magnetron sputtering in a UHV deposition system with a base pressure of 4 10 8 Pa (groundplane 200 nm, microstrip 600 nm). Niobium film patterning was by dry etching using inductively coupled plasma etching with sulfur hexafluoride. The gold film used for the microstrip termination was evaporated
ARTICLE IN PRESS H.G. LeDuc et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 459–461
461
4. Process development
(a)
absorber TES
There are known deficiencies in the current baseline process which are currently being addressed. The SiO dielectric layers used in the process have been shown to have reasonably low RF-loss by Vayonakis [4]. However the dielectric step coverage is not good enough for largescale circuits. We are in the process of replacing SiO with bias sputtered SiO2. Another issue is metal coverage at steps in the topography. It is expected that this problem will be solved by controlling the step profile of the bias sputtered SiO2. Acknowledgments
(b) Fig. 3. 8 8 pixel dual polarization antenna-coupled TES polarimeter. (a) Array situated on a 100 nm wafer, (b) 50 image of a single thermistor showing the gold microstrip terminator and TES. Also shown are individual crossed slot antenna elements and parts of the power combining tree.
and patterned by liftoff. The baseline process described above has been used to fabricate 8 8 pixel detector subtiles using molybdenum/gold as the TES element. A subtile and detector details are shown in Fig. 3.
This research was performed at the Jet Propulsion Laboratory, an operating division of the California Institute of Technology, under a contract with the National Aeronautics and Space Administration. References [1] J. Zmuidzinas, P.L. Richards, Proc. IEEE 92 (10) (2004) 1597. [2] K.D. Irwin, Appl. Phys. Lett. 66 (15) (1995) 1998. [3] A. Goldin, J.J. Bock, A.E. Lange, H. LeDuc, A. Vayonakis, J. Zmuidzinas, Nucl. Instr. and Meth. A 520 (1–3) (2004) 390. [4] A. Vayonakis, C. Luo, H.G. Leduc, R. Schoelkopf, J. Zmuidzinas, LTD9 Proceedings, 2001, p. 309.
Nuclear Instruments and Methods in Physics Research A 559 (2006) 459–461 www.elsevier.com/locate/nima
Fabrication of antenna-coupled transition edge sensors for polarimeter applications H.G. LeDuca,, Matthew Kenyona, Peter K. Daya, Minhee Yunb, J.J. Bocka a
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove, Pasadena, CA 91109, USA b University of Pittsburgh EE Dept, Pittsburgh, PA 15260, USA Available online 4 January 2006
Abstract In this paper we describe the progress in the development of polarization sensitive superconducting, bolometric detector arrays targeted at cosmic microwave background (CMB) studies. Our implementation takes advantage of superconducting technology to reduce the complexity of the typical CMB bolometer focal plane by incorporating as many of the required elements as necessary on the detector itself. Each pixel in the array consists of a distributed dual-polarization beam forming antenna, a band pass filter, and power splitter/combiner delivering detected power to four separate superconducting transition edge sensors. This combination of elements detects the polarization of the incoming light uniquely while eliminating a large part of the cooled mass of the focal plane including feed horns, polarizers, and band pass filters. Of course this increases the fabrication complexity of the bolometric detector itself over previous implementations. We will describe our fabrication process consisting of eight front side mask levels and one back side layer designed to produce whole wafer array tiles suitable for CMB polarimeters. r 2005 Elsevier B.V. All rights reserved. PACS: 84.40.07.57.k Keywords: Detector; Bolometer; Polarimeters; Superconductor; CMB
1. Introduction Polarimeters will play a key role in the next generation of cosmic microwave background (CMB) instruments [1]. Current state-of-the-art CMB instruments based on absorber-coupled semiconductor bolometers are background limited; so improvements for next generation experiments require increased throughput, i.e. larger pixel count. These instruments are difficult to scale, which is in large part due to the readout electronics which requires an amplifier per pixel. The introduction by Irwin [2] of the voltage-biased superconducting transition edge sensors (TES) coupled to a superconducting quantum interference device (SQUID) and the subsequent development of cryogenic multiplexing based on SQUIDs has made TES sensors an attractive replacement for semiconducting bolometers. An additional constraint of absorber-coupled Corresponding author. Tel.: +1 818 354 2209; fax: +1 818 393 4540.
E-mail address: [email protected] (H.G. LeDuc). 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.12.037
bolometers is the massive cryogenically cooled fore-optics required to restrict the field of view and define the frequency band. Our approach is to replace as much of this fore optics with thin film superconducting structures such as beam forming antennas, band pass filters, power combiners and splitters. This is made possible in CMB applications because the wavelengths of interest are below the gap wavelength of conventional superconductors such as Niobium, and low-loss transmission lines can be fabricated. 2. Detector architecture Each pixel in the focal plane architecture is a dual polarization beam forming antenna similar to that described by Goldin [3]. The antenna consists of an array of coherently combined dual polarization slot antennas (see Fig. 1). This is accomplished by combining the absorbed power from each antenna element using an isophase superconducting microstrip power combining network,
ARTICLE IN PRESS 460
H.G. LeDuc et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 459–461
Fig. 1. Schematic of the beam-forcing antenna. The white crosses represent slots in a ground plane while the lines represent microstrip transmission lines. The transmission lines are terminated with a lossy normal metal section. The simplest design requires two detectors, one for each polarization. The detectors are thermally isolated using silicon nitride legs.
one for each polarization component. The absorbed power is converted to heat using lossy normal metal microstrip section terminating the superconducting microstrip feeds, which indirectly heats a TES sensor co-located on a small silicon nitride island supported and thermally isolated from the cold bath by narrow silicon nitride legs. The simplest implementation requires only two TES elements to detect both polarizations. This does not uniquely define the polarization vector; however, the introduction of a superconducting power splitter with the correct phasing allows the determination of all of the Stokes parameters with two additional detectors. A superconducting band pass filter can be added to the microstrip to control the frequency band. The pixel size and subelement pitch are set by optical considerations (see Ref. [3]). For this study we have designed polarimeters for 150 GHz center frequency and an F/4 beam. Our pixel consists of a 16 16 array of subantennas distributed evenly on a approximately 8 8 mm square. The focal plane will be built up from sub-panels of these pixels with the detector leads routed to the panel edges. An 8 8 pixel array will fit snuggly on a 100 mm wafer. 3. Baseline detector fabrication There are a number of fabrication challenges associated with this architecture. First, within each pixel there are several tens of millimeters of superconducting microstrip which form the isophase power-combining structure, all with very low microwave loss. This places a premium on dielectric quality and integrity. Second, the TES detector is subject to the processing for the additional layers required for the antenna coupling. Finally the full wafer integration required to make a single panel place constraints on process yield and uniformity. We are developing a nine
Fig. 2. Schematic of process flow. (a) Molybdenum/gold TES deposition on silicon nitride on silicon and pattern, (b) SiO protection deposition and liftoff, (c) niobium ground plane deposition (sputtering) and pattern (ICP), (d) SiO microstrip dielectric deposition and etch (ICP), (e) niobium microstrip/wire deposition (sputtering) and patterning (ICP), (f) thermal isolation including silicon nitride etch (ICP) and silicon substrate removal (DRIE). Not shown is the gold termination microstrip (liftoff) and a SiO protect (liftoff).
mask level fabrication process using projection step-andrepeat lithography to produce these detectors. A schematic of the process flow for the detector stack is shown in Fig. 2. The lithographic tools used are a Canon FPA3000 EX3 5 DUV stepper and Karl Zuss MA6 contact aligner. All front side processing was done by projection lithography. The TES in this case is a proximity stack of molybdenum and gold deposited by DC-magnetron sputtering in a UHV vacuum chamber (base pressure 4 10 8 Pa). The layer thicknesses depend on the target transition temperature. The TES patterning was a combination of ion milling (gold) and RIE in sulphur hexafluoride (molybdenum). Our baseline process uses silicon monoxide (SiO) for both the microstrip dielectric (400 nm) and protect layers for the TES and the gold absorber. These films are either thermally evaporated onto multilayer liftoff stencils consisting of photoresist over PMMA or are etched. The niobium films were deposited by DC-magnetron sputtering in a UHV deposition system with a base pressure of 4 10 8 Pa (groundplane 200 nm, microstrip 600 nm). Niobium film patterning was by dry etching using inductively coupled plasma etching with sulfur hexafluoride. The gold film used for the microstrip termination was evaporated
ARTICLE IN PRESS H.G. LeDuc et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 459–461
461
4. Process development
(a)
absorber TES
There are known deficiencies in the current baseline process which are currently being addressed. The SiO dielectric layers used in the process have been shown to have reasonably low RF-loss by Vayonakis [4]. However the dielectric step coverage is not good enough for largescale circuits. We are in the process of replacing SiO with bias sputtered SiO2. Another issue is metal coverage at steps in the topography. It is expected that this problem will be solved by controlling the step profile of the bias sputtered SiO2. Acknowledgments
(b) Fig. 3. 8 8 pixel dual polarization antenna-coupled TES polarimeter. (a) Array situated on a 100 nm wafer, (b) 50 image of a single thermistor showing the gold microstrip terminator and TES. Also shown are individual crossed slot antenna elements and parts of the power combining tree.
and patterned by liftoff. The baseline process described above has been used to fabricate 8 8 pixel detector subtiles using molybdenum/gold as the TES element. A subtile and detector details are shown in Fig. 3.
This research was performed at the Jet Propulsion Laboratory, an operating division of the California Institute of Technology, under a contract with the National Aeronautics and Space Administration. References [1] J. Zmuidzinas, P.L. Richards, Proc. IEEE 92 (10) (2004) 1597. [2] K.D. Irwin, Appl. Phys. Lett. 66 (15) (1995) 1998. [3] A. Goldin, J.J. Bock, A.E. Lange, H. LeDuc, A. Vayonakis, J. Zmuidzinas, Nucl. Instr. and Meth. A 520 (1–3) (2004) 390. [4] A. Vayonakis, C. Luo, H.G. Leduc, R. Schoelkopf, J. Zmuidzinas, LTD9 Proceedings, 2001, p. 309.