Technical Note ITER Bolometer sensors Introduction The manufacture of bolometer sensors currently envisaged for ITER makes use of thin-film technologies. This Technical Note describes the characteristics of the sensors and requirements on its suppliers. Experience with the manufacture of similar devices making use of thin-film technologies is a prerequisite, whereas previous experience with plasma diagnostics is not. This Technical Note also provides some historical and technical context and background. The bolometer sensor consists of a thin substrate (see Fig. 1) with on one side a deposited metal radiation absorber and on the other side thin metal resistors in a meander shape. The substrate is clamped into a housing (not shown in the figures) which also acts as a heat sink, to make a bolometer head (multi-channel assembly with heat sink, and macroscopic electrical connections). Figure 2 shows a typical lay-out of a bolometer substrate with four channels (note: 5-channel bolometer arrays are foreseen for ITER). Each sensor has two spatially separate absorbers one signal absorber exposed to radiation, and a reference absorber not exposed to radiation each of which with two resistors behind it (areas indicated in red in Fig. 2(a)); the four resistors are connected in a Wheatstone bridge (Fig. 2(b)) to determine the change in resistance as the result of heating of the reference absorber. The substrate dimensions are of the order of 20 30 mm 2, while the housing of a bolometer head is about 30 40 20 mm 3. Figure 1: Cross-section (not to scale) of a typical bolometer sensor. 1: resistor meander structure. 2: metal track for electrical connection between meander and macroscopic electrical connection. 3: substrate. 4: a support structure for substrate if the substrate is particularly thin and requires mechanical support. 5: absorber. (a) (b) Figure 2: (a) 4-channel substrate lay-out (black line indicates one channel) seen from the resistor side (absorbers are not visible). Each area indicated in red contains two resistor meanders which are connected to contact pads through tracks (indicated in blue). The cut-out shows that this particular substrate has a support structure on the rear surrounding the absorber. (b) Wheatstone-bridge schematic for one channel; the grey area indicating the two resistors corresponding to the red area in (a).
Requirements Requirements on the sensor Substrate Depending on thermal and other characteristics of the chosen material, the substrate will be 1.5 20 µm thick, either as a stand-alone thin object or as a membrane on a support structure. Substrate materials currently under particular consideration include natural mica (standalone) and silicon-nitride (the latter on a silicon support that is 100 700 µm thick). The substrate thickness and material determine the heat transfer time, which should be of the order of 100 µs, to give a time resolution of the bolometer sensors of the order 0.1 1 ms. Absorber Absorber material: gold or platinum deposited on appropriate adhesion promotor. Absorber dimension: thickness 10 15 µm (for gold a slightly thicker absorber is required than for platinum), compatible with the thermal requirements on the bolometer; covering a rectangle of about 1.5 4 mm 2. Note that the deposition of the absorber on the thin substrate must be accomplished with limited induced stresses compatible with the robustness of the sensor against repeated thermal cycling. The metal is chosen for its good absorption properties of electromagnetic radiation, considering also its transmutation cross-section under nuclear radiation. Resistors Resistor characteristics: 50 1000 nm thick; typically 30 µm wide tracks, giving resistance values of 200 1200 Ohm. Resistor material: platinum or gold. The metal is chosen for its electrical conductivity and stability, considering also its transmutation cross-section under nuclear radiation. A thicker track (500 2000 nm) of the same metal is deposited to pads where macroscopic electrical connections can be made. Intermediate layers are typically used for adhesion promotion and diffusion control of the thin resistor meanders. The stability of the resistors must be ensured up to the baking temperature (350 C). The equality of the four resistor meanders of each bolometer channel must be controlled. Adjustment of resistor meanders may be achieved through laser trimming of dedicated structures in the resistor meander with the aim of increasing the equality to significantly below 1%. The thermal coefficient of resistance (TCR) is an important parameter.
Overall and other requirements For ITER 5-channel bolometer heads are foreseen. In bolometers on present day fusion experiments, 4 channels are contained on a single foil/substrate. All channels on a single foil/substrate may also be pursued for ITER, or each channel may be cut individually. The bolometer sensor must survive prolonged exposure to the baking temperature (350 C) and thermal cycling (of the order of 1000 bake-out cycles up to 350 C). Laboratory tests may be up to 450 C. Electrical connections to the metal resistor on the substrate need to be achieved in a way that makes them robust against the mechanical stresses, as well as the high baking temperatures and thermal cycling described above. Vacuum: As the bolometer sensors will be used in a ultra-high vacuum environment, limits exist for the steady-state outgassing rate, and there are restrictions on the use of certain materials and processing steps (e.g. cleaning). Radiation hardness: Primarily a target survival neutron dose of 0.1 dpa (displacements per atom in ceramic). The radiation hardness of the prototype bolometer sensors may be tested outside the prototype manufacturing contract Foreseen requirements on the supplier for the forthcoming tendering procedure It is essential that the tenderer to the forthcoming F4E tendering procedure possesses the processing technology associated with at least one of the mentioned bolometer types (substrate: mica, silicon-nitride, or alternatives; absorber/resistor: gold and/or platinum) with appropriate quality assurance. Furthermore it is envisioned that the suppliers will be able to perform CAD design, manufacture and assembly of various sensor components, either in house or via a subcontractor. Processing Capabilities in thin-film technologies: involving substrates such as mica, silicon-nitride on silicon wafers (and associated etching away of the silicon), other ceramics or glass; deposition of metal layers, in particular gold and platinum (e.g. evaporation, sputtering, galvanic, electroplating), experience with appropriate intermediate layers for adhesion promotion and/or diffusion control, experience with control of deposited-film stresses; cutting of individual sensors or sensor arrays from wafers, as well as secure storage/shipment. Assembly Supplementary technologies/processing of the sensors, such as
o bonding of electrical contacts (e.g. laser or ultrasonic welding), o laser trimming for optimized bridge balance. Supplementary technologies/processing for assembly of bolometer head. Testing Basic factory tests: Electrical tests (e.g. measurement resistance values and corresponding temperature coefficient of resistance, capacitive coupling between contacts); Geometrical checking (geometrical measurements, e.g. with microscopy, SEM, whitelight interferometry or other); Adhesion strength of layers onto substrates (e.g. tape test); Cleanliness (e.g. visual inspection with microscope and yellow light). More-advanced tests Vacuum tests (e.g. outgassing); Heat cycling in vacuum up to 450 C; Mechanical loads, pressure and heat transients. CAD and Quality Assurance Production of manufacturing drawings as well as for masks; Production of as-built drawings; Compliance with stringent quality assurance. Intellectual property The standard conditions of F4E contracts require the contractor to grant access rights to F4E in the form of a worldwide, non-exclusive, irrevocable, royalty-free licence with the right to further sub-license and use it for any purpose. The standard conditions of F4E contracts require the contractor to grant access rights to F4E to background in the form of a worldwide, non-exclusive, irrevocable licence, with the right to further sub-license and use it for any purpose under fair and reasonable conditions (and royalty-free in certain causes in connection with earlier Euratom support). Context Fusion for Energy (F4E) has the obligation to ensure a long-term supply chain of bolometer sensors for ITER that have high performance, are robust, and will survive long in the challenging ITER environment, which is characterized by ultra-high vacuum, nuclear radiation, high operating and baking temperatures (up to 350 C), accelerations/vibrations, gas pressure transients, and radiation and particle fluxes (both prolonged exposure, and short intense pulses).
Despite significant past developments, at present there is no demonstrated solution that meets all the target requirements for ITER. F4E plans to launch an open tender procedure for one or two contract to supply bolometer prototypes. It is foreseen to tender the series manufacture of 100 150 5-channel units in approximately five years time through a separate procedure. Measures will be implemented in the contracts for the prototype-manufacture to avoid the contractors obtaining an unfair competitive advantage for the series manufacture and resulting exclusion: it is specifically intended that the contractors for the prototype manufacture can compete in the tender for series manufacture. Furthermore there will likely be a need for additional replacement units during ITER operation. Background Bolometers have been used for several decades on experimental fusion devices to measure the total radiated power [1]. A type of bolometer sensor has been developed that is well suited to this application, which is based on a thin metal absorber and resistor deposited on a thin substrate, mounted in close contact through a metal heat conduction layer (e.g. 200 nm) with a heat sink [2,3]. The dimensions are selected to optimize its performance (such as sensitivity and time constant, as well as mechanical stability). Each bolometer channel consists of a signal and reference absorber, each with two resistor meanders which are combined in a Wheatstone configuration (see Figure 2). Typically, four channels are combined in one bolometer head. The long-wavelength limit (visible or infrared range) is determined by whether the absorber is blackened or not. The short wavelength is determined by the absorber thickness. This original bolometer was developed further for hightemperature (up to 350 C) application [4] with 20-µm mica foil with 200-Ω gold resistors (of the order of 600 nm thick) and 7-µm gold absorber. Under neutron irradiation, the gold transmutes and the mica swells. Irradiation testing showed a reasonable lifetime [5]. For ITER, this lifetime should preferably be extended, and the absorber thickness should preferably be 10 15 µm to capture higher-energy radiation. Platinum on a thin silicon-nitride substrate promises higher sensitivity and could potentially be more radiation hard. Siliconnitride (1.5 µm thick) with 1200-Ω platinum resistors (300 500 nm thick) and 1.5-µm platinum absorber have been demonstrated [6]. The absorber thickness has been increased to 4.5 µm [7] and 12.5 µm [8]. However, the thin silicon-nitride substrate and thick platinum absorber have displayed issues with stability and robustness at the elevated temperatures they would experience in ITER (operating and baking temperatures of 200 350 C). Temperature stability also imposes a minimum thickness on the resistor meanders. Other issues are the
equality of all resistor meanders, as inequalities lead to bridge imbalance and drift of the measured signal, as well as the robustness of electrical contacts to thermal cycling, mechanical stress and nuclear radiation. The bolometer sensors must also be shown to be robust against certain transients in pressure and heat load, as well as electromagnetic forces. The development routes that seem most promising are: Mica substrate with gold or platinum resistors and 10 15-µm thick absorber; Silicon-nitride substrate with platinum resistors and 10 15-µm thick absorber, with controlled deposited-film stresses; although other substrates and variations in the design are not excluded. It may not be possible to develop a bolometer sensor that meets all ITER requirements on the required timescale and within the available budget. Given that it is envisaged that it should be possible to replace most bolometer sensors during the operational life of ITER, it is more important to ensure that the bolometer sensors will withstand the recurrent loads (e.g. robustness against thermal cycles, heat pulses, pressure transients and mechanical loads), than that sensitivity is augmented or that long-term degradation is guaranteed to be negligible over the entire operational life of ITER (e.g. degradation or eventual failure due to nuclear radiation exposure). The following three phases (with indicative timescale) can be distinguished for the ITER bolometer sensors: Optimization of manufacturing processes and manufacture of prototypes to demonstrate achievable performance (2015 2016) Series manufacturing (100 150 units) (approximately 2018 2021) Subsequent market (ITER replacements/upgrades and other fusion machines) References [1] L.C. Ingesson et al., Chapter 7 Tomography diagnostics: bolometry and soft x-ray detection, Fusion Sci. Technol. 53, 528 576 (2008) [2] K.F. Mast et al., A Low Noise Highly Integrated Bolometer Array for Absolute Measurement of VUV and Soft X Radiation, Rev. Sci. Instrum. 62, 744 (1991). [3] German parent patent DE 3408724 (1985/1988), with a family of European (EP 0149819), Japanese and US (US4687342) patents, expired since 2004. [4] R. Reichle et al., Bolometer for ITER, in Diagnostics for Experimental Thermonuclear Fusion Reactors (Proceedings of the International Workshop on Diagnostics for ITER, Varenna, August 28 September 1, 1995), p. 559, P.E. Stott, G. Gorini, and E. Sindoni, Eds, Plenum Press (1996).
[5] R. Reichle et al., Radiation Hardness Test of Mica Bolometers for ITER in JMTR, in Proc. 28th EPS Conf. on Controll. Fusion and Plasma Phys. (Funchal 2001), Europhysics Conference Abstracts Vol. 25A (EPS, 2001), pp. 1293 1296 [6] L. Giannone et al., Prototype of a Radiation Hard Resistive Bolometer for ITER, Plasma Phys. Control. Fusion 47, 2123 (2005). [7] H. Meister et al., Optimization of a bolometer detector for ITER based on Pt absorber on SiN membrane, Rev. Sci. Instrum. 81, 10E132 (2010) [8] H. Meister et al., Broad-band efficiency calibration of ITER bolometer prototypes using Pt absorbers on SiN membranes, Rev. Sci. Instrum. 84, 123501 (2013)