Low Temperature Refrigerators

Since the heat and light signals of the AMoRE experiment are very weak, it is impossible to measure the signals at room temperature. In order to achieve the high sensitivity and low threshold, CUP employs thermal calorimetric detection at milli-Keven temperatures as the main detection technique. The calorimeters operate well below 1 K. Refrigerators and cryostats are essential equipment not just to perform the main AMoRE experiment but also to test and characterize necessary sensors and detector modules. CUP has built up three types of refrigerators based on their base temperatures. First low temperature screening tests are done in liquid helium storage dewars and 4K refrigerators. Key parameters of metallic magnetic calorimeters (MMCs) are characterized below 1 K. Adiabatic demagnetization refrigerators are a convenient cooling system to provide 30-100 mK working temperatures. Complete detector modules including an absorber crystal and an MMC sensor with a SQUID are tested using dilution refrigerators that can realize a constant temperature down to 10 mK for long-time continuous measurement.

4K systems (dipping probes and PTR refrigerator)

Figure I-6 : 4K dipping probes

The main detector technology used in the low temperature calorimeters of AMoRE has been developed based on superconducting sensors and electronics. We use niobium-based sensors that have their superconducting transition temperature near 9 K. Basic characterization of the superconducting sensors requires frequent 4K tests. Basic screening is carried out for working devices at the temperature. Moreover, during their R&D stages, each fabrication procedure has been followed by a series of basic performance test at 4K.

The dipping probe method is the most convenient way of performing tests at 4K. One can insert a dipping probe with attached test sensors into a liquid-helium storage dewar. As shown in Figure I-6, we have made several 4 K dipping probes in our own design including vacuum conduction cooling probes. These probes are in heavy use. The transition temperatures and critical current of niobium patterns are tested with these probes. Moreover, they are used for low temperature resistivity measurement of metal films used as phonon collectors on crystals and light detectors.

Another popular 4K test system is made with a pulse-tube refrigerator (PTR); it is called a 4K system although its base temperature reaches below 3 K. It is a dry system having a large sample space. The sample holders used for the MMC and SQUID sensors of AMoRE are typically too large to be tested in a normal storage dewar with a dipping probe. Before final assembly of the detector, all of the channels of MMC/SQUID combination, with all of bonding wires, are tested in the large sample space of this 4K system. This system also has a top loading probe for a quick test of sensor devices down to 3 K in vacuum. The cooling performances of the 4K PTR system are as follows:

  • Base temperature: 2.5 K
  • Cooling power at 4 K: 0.9 W
  • Cool-down time of overall refrigerator from 300 K to 4 K: 4 hours
  • Cool-down time with top loading probe from 300 K to 4 K: around 1.5 hours

Figure I-7 shows the PTR and a measurement done in the system for one of the channels used for the present AMoRE Pilot. It is a Johnson noise measurement confirming that sensors are active with proper inductance and normal measurement circuit resistance.

Figure I-7: PTR refrigerator (left) and a Johnson noise measurement of one of photon channels done in the PTR system. This channel is being used in present Pilot system at Y2L.
Adiabatic Demagnetization refrigerator (ADR)
Figure I-8: An adiabatic demagnetization refrigerator system with lead shield

Adiabatic demagnetization refrigerators (ADRs) are a widely used refrigeration system for low temperature measurements in the 30-300 mK range. The cooling process is relatively simple. It does not require a cryogenic expert to run an ADR system. Because of adiabatic cooling, an ADR does not have a constant cooling power; it rather has a holding time in a specific temperature. It is relatively convenient system reaching below 100 mK where MMCs are active.

Our ADR (High Precise Devices, Model 103 Rainier) has a PTR with 0.7 W cooling power at 4 K. The 4K stage is often used as a test system for MMC/SQUID measurements. For mK stages, it utilizes a two-stage paramagnetic system with GGG and FAA salt pills. The cryostat of the ADR is surround by a lead radiation shield to reduce environmental gamma-ray backgrounds in detector performance tests as shown in Figure I-8. The performance figures of the ADR are as follows:

  • Base temperature of FAA stage: 25 mK
  • Holding time at 50 mK: 24 hours
  • Cool-down time of overall refrigerator from 300 K to 3 K: 15 hours
  • Cool-down time with top loading probe from 3 K to 30 mK: around 4 hours
Figure I-9 Alpha spectrum measured in an ADR. (I. Kim et al., Supercon. Sci. Tech. 30 904005 (2017))

MMCs are the essential LT detector devices used in AMoRE. The energy resolutions of MMC sensors are typically investigated in the ADR as part of sensor characterization for AMoRE applications. Using one of the MMCs made by the group, an alpha spectrometers with one of the highest energy resolutions achieved was established in the ADR. Figure I-9 shows an energy spectrum showing 0.9 keV FWHM energy resolution for an external Am-241 source. It indicates the MMC detector setup and its sensitivities are suitable for 0vBB investigation. It also showed high sensitivities at low energy regions in the few keV range. Moreover, many properties of MMC sensors such as temperature-magnetization characteristics are tested in the ADR.

Figure I-10: A 1 cc measurement setup attached to the ADR.

Because of its convenience, the ADR is used for crystal R&Ds. The heat and scintillation properties of various crystals have been tested for one of 1 cm3 detector system (Figure I-10) designed for an ADR measurement. Replacing many molybdate crystals such as CaMoO4, Na2Mo2O7, Li2MoO4, PbMoO4, and ZnMoO4, the 1 cc system attached to the ADR is being used for the crystal selection of the AMoRE-II experiment. It will be an important decision for the final stage of the project.

In addition, various R&D efforts have been performed in the ADR. Systematic vibration tests of light detectors were made. The present light detectors of the Pilot system were developed in the ADR. This was a crucial development to improve low frequency noise of the light signals. Moreover, in this ADR system, we studied a new type of light detector based on phonon-signal amplification using the Neganov-Luke effect. In this successful development, an amplification factor of 7 was obtained with 80 V applied to the amp voltage for the scintillation-light detection.

Dilution refrigerators (DRs)

(1) Dilution refrigerator for AMoRE-Pilot and AMoRE-I

A dilution refrigerator has been installed for physics runs of the AMoRE-Pilot and AMoRE-I experiments in a laboratory of the A5 tunnel at Y2L. We completed the whole cryogenic detector system, after renovating the A5 tunnel from scratch. We built a clean laboratory with dust and radon controls. In the lab space, we built a gantry system to support the cryostat of the refrigerator, the lead shields with moving motors, and a muon veto system composed of plastic scintillators. The complete AMoRE system required not just installation of a cryogenic system with dilution refrigerator in a gantry system, but also installation and management of necessary utilities for refrigeration and stable detector operations.

Presently, wirings of the cryostat were made for 12-channel measurement of AMoRE Pilot. When upgrading to AMoRE-I the number of channels will be increased to 36 for up to 18 crystal modules. From room temperature to the 4 K stage, copper-nickel (alloy 30) wires were used while NbTi wires with copper-nickel clads were chosen for low-temperature wirings below 4K connecting to the detector modules at 10 mK. Superconducting wires are carefully heatsinked to each temperature stage while connecting to the detector modules, minimizing heat load to lower temperature stages.

Figure I-11: Cooling power of the dilution refrigerator used in AMoRE-Pilot. The base temperature has been improved with better grounding after the test.

The cryostat is being used for AMoRE-Pilot and is to be used for AMoRE-I in 2018. It was designed to host about 20 detector modules at the coldest temperature stage, each with heat and light measurement channels. It has a cylindrical experimental space in 40 cm diameter and 69 cm height. The cryostat includes an additional precooling circuit using liquid nitrogen. The precooling circuit speeds the cooling of the PTR. This setup reaches 77 K in 4 days from room temperature. It takes about a week to reach 4 K from room temperature. Condensing the He3-He4 to reach the base temperature takes less than one day. Cryogenic properties of the dilution refrigerator system are listed below.

  • Base temperature: 6.5 mK
  • Cooling power in normal conditions: 2 μW at 10 mK, 19 μW at 20 mK
  • Maximum cooling power at 120 mK: 1.4 mW
  • Cool-down time of overall refrigerator including MC lead shield from 300 K to 77 K: 4 days with liquid nitrogen precooling
  • Cool-down time of overall refrigerator including mixing-chamber lead shield from 77 K to 4 K: 2 days
  • Cool-down time from 40 K to 10 mK: 8 hours

The cryostat with a dilution refrigerator unit is made of normal low temperature materials. The parts of the supporting structure and refrigerator were not selected for certain radio-purity levels. However, the copper plates of each temperature stage are made of NOSV-grade (Aurubis brand) copper. Moreover, the IR shielding cans are made of NOSV copper. To reduce the gamma radiation from the cryogenic parts above the mixing chamber (MC) plate, a total 10 cm thickness (170 kg) of low activity lead bricks are attached to the bottom of the MC plate as shown in Figure I-12.

In early stages of the Pilot measurement, we found the signals suffered from machine vibration originating from its PTR operation. We introduced two-stage vibration mitigation systems. The first vibration damping system so called mass spring damper (MSD) is made with a set of springs connecting the MC and the detector tower. Another system isolates the still plate from the 4 K plate using heavy duty springs, creating a so called spring suspended still (SSS), after replacing original connections with soft ones. Eddy current dampers provides efficient damping mechanism to reduce low frequency noise in the detector signals. With the two damping systems, the energy resolution of Pilot detectors was improved to 8.7 keV FWHM from 40 keV FWHM for 2.6 MeV gamma rays. Figure I-12 shows the assembly of the MSD and SSS system in the refrigerator.

Figure I-12: (a) Schematic view of dilution refrigerator and the detector setup in AMoRE-pilot. (b) Picture of the cryostat including MSD and the detector tower of AMoRE-pilot. (c) SSS installed between 4 K and still plate. (d) Schematics of SSS and the eddy current damper.

(2) R&D dilution refrigerators

Figure I-13: Dilution refrigerator installed in the HQ lab

One of most important decisions for AMoRE-II preparation is crystal selection. AMoRE Pilot and AMoRE-I use mainly 40Ca100MoO4 crystals. Because of the cost 40Ca, other candidate crystals are under investigation using R&D dilution refrigerators. One system was installed at the KT1 laboratory, and has recently been moved to the permanent place at the HQ laboratory as shown in Figure I-13. A systematic survey of full size molybdate crystals are to be tested in the cryostat with the proper MMC and SQUID setup. The crystal R&D does not require underground measurements. However, checks of internal background for AMoRE II will be performed in another refrigerator installed in Y2L having the same size and cooling power as the system at HQ. Installation of this system is also completed. Moreover, the underground refrigeration system is to be used for a dark-matter search R&D program. In particular, the study will target low-mass dark mater detection. The detection limit is to be investigated with crystal detectors and Si absorbers with the Neganov-Luke phonon amplification feature.

  • Base temperature: 9 mK
  • Cooling power in normal conditions: 1 μW at 10 mK, 10 μW at 20 mK
  • Maximum cooling power at 120 mK: 700 mW
  • Cool-down time of overall refrigerator including MC lead shield from 300 K to 77 K: 3 days without liquid nitrogen precooling
  • Cool-down time of overall refrigerator including MC lead shield from 77 K to 4 K: 1 day
  • Cool-down time from 40 K to 10 mK: 8 hours
Fabrication (Fab) Facility

CUP specializes in production of MMC sensors and detector modules for various applications. In particular, MMCs are used as the key technology for the thermal calorimetric detection for the AMoRE project. The heat (phonon) and light (scintillation photon) detectors are developed using cleanroom technologies. A CUP micro-fabrication facility and related equipment are needed for two main purposes. One is MMC production as these are the core sensors of the calorimetric detection of CUP projects. The other purpose is to make phonon collectors on crystal surfaces. Phonon collectors are metal films on a dielectric crystal surface, and are required for efficient heat transfer between an absorber crystal and an MMC sensor. Both of MMC production and phonon collection fabrication should be done using designated equipment with specific design and procedures.

MMC production was developed on the basis of superconducting sensor fabrication. It also requires magnetic material deposition in a high-vacuum environment and thick gold electro-deposition. MMCs are not commercially available at the moment although several world-wide institutions including IBS-CUP are capable of producing them. CUP designed optimal MMCs used in the AMoRE project to employ detector modules with an absorber crystal of 300-500 g mass. Some MMC features necessary for AMoRE project are realized only in the IBS laboratory.

AMoRE searches for extremely rare events using crystal absorbers. The radio purity of AMoRE crystals should be given special considerations during every step including the handling of raw materials, crystal growing procedures, and assembling procedures for detector modules. The crystals may not be exposed to normal laboratory conditions in which Rn gas may be adsorbed on their surface. A glove box and an air-tight transport container is required while handling crystals. A special evaporator with a series of load-lock systems is necessary to be used for phonon-collector deposition for this purpose. On the other hand, fabrication of light detectors uses similar clean-room procedures as phonon-collector deposition on the main crystal.

MMC fabrication requires 11 steps of micro fabrication in a cleanroom. We installed several pieces of fabrication equipment in established clean rooms. They are mainly used for deposition and patterning for superconductors, metals and insulators. Because the number of measurement channels needed for the AMoRE project is more than 1000, the production yield is another important parameter in the chip production line. We describe three pieces of major fabrication equipment installed in our cleanrooms as follows.

Figure I-14: CUP sputter installed in class-1000 clean room

(1) Sputters (Figure I-14)

  • Use: Metal, Superconductor deposition
  • Available targets: Nb, Au:Er, Au, Ti, Cr
  • Main features: This is the main deposition chamber for Nb superconductor, gold, and MMC sensor material (Au:Er). The main process chamber hosts five sets of DC sputtering guns. Each sputter system has its own valve on-off from the main chamber to prevent cross contamination between target materials. Two guns specified for Au:Er and Au deposition are configured for confocal co-sputtering to adjust Er concentration for an optimal condition. The substrate holder rotates the wafer, and is cooled water cooled. A load-lock camber is attached to the main chamber to maintain the high vacuum condition. An RF sputter is installed in the load-lock chamber, mainly to etch out an oxidized metal layer before processing is performed in the main chamber.
Figure I-15: E-beam evaporator designed to load 4 AMoRE crystals

(2) E-beam evaporator (Figure I-15)

  • Use: Metal (phonon-collector) deposition
  • Available source: Au, Ag, Ti, Cr
  • Main features: Thin metal layers with high electric conductivity can be fabricated in this chamber. One of important features of this e-beam evaporator is crystal handling in two step load-lock systems. The first one is a vacuum load-lock in which motor driven crystal loaders move in and out of the main chamber. The second one is a glove box connected to the vacuum load-lock. A crystal carrying container is opened in order to load crystals onto a tray in the vacuum load-lock in a controlled environment. The glove-box load lock is used to clean crystal surfaces with solvent just before phonon-collector deposition. The main chamber of the e-beam evaporator is divided with a gate valve into a source chamber and a substrate chamber. The source chamber can be open to the air, keeping the substrate chamber in vacuum while replacing or adding deposition sources. The substrate holder in the main chamber was designed to load four crystals with total mass of 2 kg. The holder rotates, and is water cooled.

(3) ICP-RIE (Inductive Coupled Plasma – Reactive Ion Etcher)

  • Use: Etching metal layers to pattering micro structures
  • Available gas: SF6, Ar, O2
  • Main features: When our detector operates at low temperatures, MMCs work by running persistent supercurrent on their chip die. The critical current of micro patterned Nb coil is one of most important parameters for successful MMC fabrication. The ICP-RIE specializes etching a thick metal layer with high vertical aspect ratio. This feature ensures a constant width of the long Nb pattern. Dense plasma is produced from a tornado coil electrode in the main chamber of the ICP-RIE. An electrostatic wafer chuck is chosen for efficient substrate cooling. Samples are loaded through a load-lock. The first pattering process of Nb film is made in the ICP-RIE chamber for the MMC fabrication.

(4) Class 100 and 1000 clean room

We established class 100 and 1000 clean rooms at the KT1 laboratory as temporary lab space available for our fabrication work. Below are pictures of the cleanrooms. We have also been setting up new clean rooms in the permanent IBS headquarter building for continuing micro fabrication.

100 class (yellow room)
1000 class
Fabrication facility
Fabrication facility
Metal thin film system Metallic magnetic calorimeter sputtering system
Radon free environment e-beam evaporator system
Pattern lithography equipment Maskless Micro Pattern Generator
Dual Focus Micro-Pattern Mask Aligner
Metal film etching equipment ICP-RIE (Inductively Coupled Plasma- Reactive Ion Etching) system
Insulation film growth equipment LT-PECVD (Low-Temperature Plasma-enhanced chemical vapor deposition)
Anodizing unit
Thick Au layer fabrication Simple electroplating unit
Chip dicing Dicing saw
Resist coating unit Spin coating system
Hot plate
Fabrication step verification 3D Measuring Laser Microscope
Optic Microscope
Collector annealing system Rapid thermal process system

(5) List of fabrication equipment

(6) Fabrication results

Example of fabrication results completed in the laboratories

Completed 3 inch wafer and an MMC die
Phonon-collector film evaporated on a crystal surface and a light detector wafer
Assembled heat sensor
Light detector assembly