Lars Stenmark

A Hybrid Cold Gas Microthruster System for Spacecraft

A hybrid cold gas microthruster system suitable for low Δv applications on spacecraft have been developed. Microelectromechanical system (MEMS) components together with fine-mechanics form the micr ...

Titanium–aluminum–nitride coatings for satellite temperature control

Abstract Intense solar irradiation, radiative cooling to outer space, and internal heat generation determine the equilibrium temperature of a spacecraft. The balance between the solar absorption and thermal emittance of the surface is therefore crucial, in particular for autonomous parts directly exposed to the solar radiation and thermally insulated from the main thermal mass of the spacecraft. The material composition but also the coating thickness are found to influence the equilibrium temperature of an object in space. In this paper we report on a systematic search for a suitable composition and thickness of TixAlyNz alloy coatings prepared by reactive, unbalanced magnetron sputtering from targets consisting of differently sized titanium and aluminum sectors. The films were deposited on glass, glassy carbon, aluminum sheet metal, and on sputtered aluminum and TixAl(1−x) films on glass. The stoichiometry and sheet resistance of the films was determined with Rutherford backscattering and four-point probe measurements respectively. Reflectance spectra for the visible and infrared spectral ranges were used to obtain average solar absorptance and thermal emittance values used in model calculations of the equilibrium temperature. Neglecting internal heat contributions, the lowest calculated equilibrium temperature in orbit around the Earth, 32.5°C, was obtained for a 505-nm-thick Ti0.14Al0.47N0.40-film.

Weibull Fracture Probability for Silicon Wafer Bond Evaluation

This work presents a strategy toward determining the mechanical reliability of bonded silicon microsystems. The fracture strength of a bond has been examined using burst tests and Weibull statistic ...

Chip mounting and interconnection in multi-chip modules for space applications

The objective of this study is to develop processing knowledge and technology of three-dimensional multi-chip modules for space applications. The aim is to test the reliability as well as to optimize the different process steps. Bisbenzocyclobuthene (BCB) was chosen as the interdielectric material as it fulfills the requirements for space qualification and process compatibility. Integrated circuit chips have been mounted recessed in silicon using BCB as glue. Metal interconnects of thin-film aluminum are isolated with spin-on deposited BCB as an interlayer dielectric (ILD). Two layers of BCB, total thickness 30 µm, are needed to achieve chip edge step coverage. Via interconnections, minimum dimensions 100×100 µm2 have successfully been patterned in a 30 µm thick BCB layer. Measuring I-V characteristics has validated the connections.

Oxygen plasma wafer bonding evaluated by the Weibull fracture probability method

In this paper, the oxygen plasma bonding process for fusion bonded silicon wafers has been characterized by a new approach. The mechanical reliability of bonded microstructures was determined using burst tests and Weibull statistic analyses. The fracture characteristic of the bonded system is considered to depend on the stress distribution, the defect distribution and the fracture surface energy at the bond. Using Weibull theory, it is possible to extract the Weibull modulus m and the mean fracture uniform tensile stress per unit length, ?fc, from the measured data. These quantities make it possible to compare the joint defect distribution and the fracture surface energy at the bonded interface for the processing conditions under observation. These experiments also demonstrate that it is possible to distinguish between these quantities under certain conditions. The fracture probability for different annealing temperatures has been evaluated and found to agree with previous results from surface energy measurements. It is shown that the bond fracture probability increases with annealing times in the range of 10-100?h. The saturated bond strength value is considerably enhanced by oxygen plasma activation prior to bonding. In this study, plasma activations at room temperature and 300??C compare to chemical activations in hot nitric acid annealed at 120??C and 700??C, respectively. The tendency to form voids at elevated temperatures, e.g.?300??C, is increased by the oxygen plasma treatment. If the surface energy is considered to be homogeneous over the bonded interface, the Weibull modulus m is an indirect measure of the defect distribution, low m values indicate a wide spectrum of defect types, whereas a high m value narrows the defect distribution responsible for fracture. The Weibull modulus m is shown to be valuable for evaluation of the bonded interface. It is demonstrated that a more scattered defect distribution emerges for in situ bonded wafers as compared to ex situ, and annealing at 300??C for 90?h as compared to room-temperature storage. However, the defect distribution becomes increasingly more narrow with storage time. These variations may be due to either changes in microcracks or void configuration or inhomogeneities in the fracture surface energy over the bond interface.

The electrothermal feasibility of carbon microcoil heaters for cold/hot gas microthrusters

With the miniaturization of spacecraft the need for efficient, accurate and low-weight attitude control systems is becoming evident. To this end, the cold/hot gas microthruster system of this paper incorporates carbon microcoils—deposited via laser-induced chemical vapor deposition—for heating the propellant gas (nitrogen) before the nozzle inlet. By increasing the temperature of the propellant gas for such a system, the specific impulse (Isp) of the microthruster will increase. The benefits of a higher Isp are lower propellant mass, higher thrust and shorter burning times. Therefore, the feasibility of achieving this increase with the carbon microcoils is investigated. The carbon microcoils have been characterized experimentally with respect to their electrothermal performance, i.e. resistance, temperature, parasitic heat losses and degradation in ambient. The resulting heat losses from the heater and the heated gas have been estimated through a combination of experiments, numerical simulation and approximate analytical expressions. At high powers, degradation of the carbon material leads to coil failure in ambient where trace oxygen was present. Thus, the next generation of carbon microcoils to be tested will have a protective coating to extend their lifetime. Theoretical modeling showed that an increase in the propellant gas temperature from 300 to 1200 K and a corresponding two-fold increase in the Isp can be achieved if 1.0 W of power is supplied to each coil in a three-coil thruster. These simulation results show that if the coils are capable of dissipating 1 W of heat at 1700 K coil temperature, the doubling of the Isp may be achieved. Comparing to the electrothermal characterization results we find that the carbon coils can survive at 1700 K if protected, and that they can be expected to reach 1700 K at power below 1 W.

Micromachined S-band patch antenna with reduced dielectric constant

A generic dielectric constant reduction method for silicon antenna substrates is presented in detail along with a process description to produce functional dielectric layers for planar antennas. Virtually any dielectric constant below 11.9 down to 3.8 aimed for in this paper can be produced. Very small honeycomb cells with wall thickness of 16 /spl mu/m and inner wall length of 86.6 /spl mu/m is etched down using deep reactive ion etch (DRIE) to 475 /spl mu/m depth in each of two 525 /spl mu/m 4-inch high ohmic wafers. These two wafers are bonded together with the etched side of both wafers facing each other. A volumetric averaging yields an average dielectric constant of 3.8 for the two bonded wafers. By adjusting the etch depth, different dielectric constants for bonded pairs of silicon wafers are attainable. A demonstration of the concept has been physically realized showing an increase in the resonance frequency of a simple coaxial-fed disk-patch antenna with a simulated resonance frequency of 2.5 GHz.

NanoSpace-1: The impacts of the first Swedish nanosatellite on spacecaft architecture and design

Abstract The impacts of the NanoSpace satellite program and the NanoSpace-1 spacecraft are expected to qualify highly advanced Micro Systems Technologies (MST) for space use. Furthermore, changes in systems design and the introduction of Multifunctional Micro Systems (MMS) modules reduce the mass and volume by orders of magnitude. An MMS module incorporates several features that are very capable and can work fully in parallel. The total spacecraft weight is 7kg and is constructed around innovative designs for thermal management, structure, RF-MEMS, Controller Area Network (CAN), and for Attitude and Guidance control. Several NS-1 subsystems are designed with innovative MST technologies. Many features are packed into 3D-Multichip Modules (3D-MCM). A new systems design philosophy with a high level of autonomy and. distributed intelligence is used to increase the spacecraft modularity. In order to determine what conclusions that are NS-1 specific and which that are general in nature, a design method is studied and applied to the MMS modules. The traditional bottom-up approach to system assembly, where each device is separately developed, verified, and qualified is abandoned in favor of the integrated approach, where parallel parts of devices are simultaneously assembled to from the entire system.

Thin film coatings with variable emittance

The temperature of a nanosatellite in orbit varies strongly as it goes into earth shadow or solar radiation. A variable emittance panel built with an electrochromic material, could improve the temperature control by providing an adaptive thermal control. The active function is due to an electrochromic layer, WO3, deposited by sputtering. Intercalation of Li+ leads to a change in the electron configuration, which modulates the radiation properties. Thin WO3 films were deposited onto Indium Tin Oxide (ITO) coated glass by reactive dc sputtering. IR measurements were done in a Perkin-Elmer 983 spectrophotometer. The IR emittance modulation, (Delta) (epsilon) of WO3 films, deposited on ITO coated glass, has been investigated. The emittance, (epsilon) , is computed from the reflectance, at 25 degrees C. Initial results show emittances in the range from 0.2 to 0.5 crystalline film has (Delta) (epsilon) equals 0.12, and the amorphous film has (Delta) (epsilon) equals 0.3. IR properties of WO3 need to be further investigated as well as the construction of a whole device.

Development of a MOEMS sun sensor for space applications

The paper presents the development of a miniaturized sun sensor for sun angle detection in space applications. The sun sensor has a field of view (FoV) of greater than 2/spl pi/ sr and a resolution of approximately 1 degree in elevation and azimuth angle; thus, it is a coarse sun sensor with the advantage of having a large field of view. Key elements of this sun sensor are its curved shape, the photosensitive layer consisting of copper indium gallium diselenide (CIGS), the transparent conductive layer consisting of thin molybdenum or aluminium doped zinc oxide, and its integrated design. The sun sensor will be one of the sensors in the attitude determination and control system (ADCS) of a nano-satellite.