In the last decades, a very large effort has been made to measure, with high sensitivity, the intensity and spectral contents of millimetric (mm) and submillimetric (submm) light from the Universe. Today this picture is in the way to be routinely completed by polarization measurements that give access to previously hidden processes, for example the traces of primordial gravitational waves in the case of CMB (mainly mm), or the effect of magnetic field for star formation mechanisms (submm and mm optical ranges).
The classical way to measure the light polarization is to split the two components by a polarizer grid and record intensities with two conjugated detection setups. This approach implies the deployment of a complex instrument system, very sensitive to external constraints (vibrations, alinement, thermal expansion…), or internal ones: determine low degrees of polarization implies a large increase in sensitivity when compared with intensity measurements.
The need of detector arrays, with in pixel polarization measurement capabilities, has been well understood for years: all the complexity being reported at the focal plane level. Subsequently, the instrument integration, verification and tests procedure is considerately alleviated, specially for space applications.
All silicon bolometer arrays using the same micromachining techniques than the Herschel PACS modules are well suited for this type of development. New thermometers doped for 50 mK operations permit to achieve, with a new design, sensitivities close to the aW/√Hz. It is based on all-legs bolometers (ALB), where the absorbing, insulating and thermometric functions are made by the same suspended silicon structure. This ALB structure, with in this case a spiral design, permits to separate the absorption of the two electromagnetic components of the light polarization. Each pixel consists of four bolometer divided in two pairs, each sensitive to one direction of polarization. This permits to combine the bolometer bridges in a fully differential global structure with a Wheatstone bridge arrangement. Total intensity and polarization unbalance are available directly at the detector level, thanks to a cold readout circuit integrated in the detector structure. This combination of functions is achieved by above IC manufacture techniques (IC for Integrated Circuit).
All these developments take place in the prospect of the joint JAXA-ESA SPICA project, to equip a 1344 pixels polarimetric and imaging camera covering three spectral bands (100, 200 and 350 µm).
CEA has a long history of customizing optoelectronic components for space and astronomy applications. Based on this expertise, we are undertaking the development of cooled silicon bolometers for millimetre-wave (mm-wave) polarization detection in the next generation of space astronomy missions such as SPICA. Silicon bolometer technology has been demonstrated successfully in space conditions through the Herschel mission. There are many benefits of this technology such as the use of a simple and low-power read-out circuit that can be integrated below the detector array in an above-IC (Integrated Circuit) integration scheme. The advanced integration in a large array and the fabrication process based on microelectronics techniques are key challenges for these developments. This work presents the early results on the design, the fabrication and the first characterization of an innovative pixel for mm-wave polarization detection. The aim is to have an adapted absorption around λ=1.5 mm. These bolometers are composed of an absorbing layer and a thermometer, which are thermally insulated from the substrate. To increase the sensitivity, these detectors are working at very low temperature typically between 50 and 100 mK. The suspended thermometer is made of silicon implanted with Phosphorus and Boron species, and we optimized the design to have a high sensitivity with a 3D Variable Range Hopping conduction (Efros law) and a low 1/f noise at low temperature. The heat capacity of the bolometer is optimized by using a meander shape of the thermometer together with superconducting Ti/TiN thin films for the electromagnetic wave absorption. This sensor is implemented on a standard SOI substrate. Measurements of test structures at room temperature, and first results at very low temperature have been performed to evaluate the electrical performances of the fabricated detectors. The mechanical behaviour of released structures, including pixels with a pitch of 1200μm and 600μm, is presented and discussed.
In the field of uncooled Long Wave Infra Red (LWIR) imaging, CMOS compatible bolometers technology is being more and more popular, exhibiting precise temperature measurement at moderate cost. The price of this technology is proportional to the number of components produced per wafer, leading to a shrinkage of the pixel. Enhancing the resolution level of the focal plane array (FPA) requires an improvement of the point spread function (PSF) of the optical system, leading to more and more complex aspheric lenses, and an increased cost of imaging systems. We propose to add a sub-wavelength blade to the existing parts of the imaging system to ease the overall improvement of the image quality in applications with a constraint budget. The main function of such a subwavelength blade should be to control the phase of the light into an optical system to compensate optical aberrations. A cost effective solution consists to make such devices using microelectronics based collective fabrication process. The main difficulty is to predict the subwavelength blade behavior within an optical system that is to say combining millimeter sized optical components that are modeled using ray-tracing or electromagnetic simulations. In this paper we present the results obtained from an effort to simulate, fabricate and characterize all-dielectric subwavelength blade. In an imaging system, our devices will have to deal with non-flat wavefronts. Our method is based on Fourier Modal Method and Angular Spectrum Method to simulate subwavelength optics into such an optical system. Finally, we have compared our simulations results to experiments on basic examples, like spherical aberration correction of a commercial lens.
As conducting polymer based devices, organic electrochemical transistors (OECTs) are suited for printing process. The convenience of the screen-printing techniques allowed us to design and fabricate OECTs with a selected design and using different gate material. Depending on the material used, we were able to tune the transistor for different biological application. Ag/AgCl gate provided transistor with good transconductance, and electrochemical sensitivity to pH was provided by polyaniline ink. Finally, we validate the enzymatic sensing of glucose and lactate with a Poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) gate often used due to its biocompatible properties. The screen-printing process allowed us to fabricate a large amount of devices in a short period of time, using only commercially available grades of ink, showing by this way the possible transfer to industrial purpose.
The IXO/XMS instrument baseline is an array of TES sensors. Alternatively, we are now developing a μ-
calorimeter array based on Silicon doped sensors. Our strength stands in a very low power consumption at 50
mK, allowing more than 4000 readout channels in the limited power budget of the IXO/XMS cryostat, for a
Field of View as large as 6'x6' square while keeping the same spectral resolution. In parallel, we develop the
cold (2-4K) frontend electronics based on High Electron Mobility Transistors (GaAlAs/GaAs) and SiGe ASIC
electronics to readout, amplify and multiplex the signals. We present the status of our development and our
current design study.
Several successful development programs have been conducted on Infra-Red bolometer arrays at the French
Atomic Energy Commission (CEA-LETI Grenoble), in collaboration with the CEA-Sap (Saclay); taking
advantage of this background, we are now developing an X-ray spectro-imaging camera for next generation
space astronomy missions, using silicon technology. We have developed monolithic silicon micro-calorimeters
based on implanted thermistors. These micro-calorimeter arrays will be used for future space missions. A 8×8
array prototype consisting of a grid of 64 suspended pixels on SOI (Silicon On Insulator) has been created. Each
pixel of this array detector is made of a tantalum (Ta) absorber and is bonded, by means of an indium bump
hybridization process, to a silicon thermistor. The absorber array is bound to the thermistor array in a collective
process step. The fabrication process of our detector involves a combination of standard silicon technologies
such as Si bulk micromachining techniques, based on deposition, photolithography and plasma etching steps.
Finally, we present the results of measurements performed on the different building elements and processes that
are required to create a detector array up to 32*32 pixels in size.