Johan Köhler

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 ...

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 ...

Fabrication and mechanical characterization of LCVD-deposited carbon micro-springs

Thirty-eight batches of carbon micro-springs were fabricated using laser-assisted chemical vapor deposition (LCVD) at laser powers and ethylene pressures ranging between 300 mW and 700 mW and 400 mbar and 900 mbar, respectively. A scanning electron microscope was used to determine geometrical parameters such as wire diameter, outside coil diameter, spacing, and pitch. The spring rates for nine batches of springs - chosen from the extremes and middle of the ranges - were determined. With the spring rate values, trends regarding the modulus of rigidity and the modulus of elasticity of the carbon material as a function of laser power and precursor pressure were deduced.

Demonstration of a single use microsystem valve for high gas pressure applications

Demonstrated and characterized here is a single use valve developed for high-pressure applications. Incorporated within the single use valve is a particle filter. The filter serves to remove any particle debris created by the activation process. The valve is solder sealed to be leakage proof. The solder is remelted to obtain activation of the valve. Local heater elements are incorporated on the valve surface together with solder wetting pads. The gas mass flow through the device was evaluated prior to sealing and after activation. The valve was functional at pressures of 100 bar, and opened in less than 10 s with an applied power of 13 W.

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.

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.

A solder sealing method for paraffin-filled microcavities

Demonstrated and investigated here is a method to seal microfluidic systems by soldering. As a particularly difficult case of growing importance, the sealing of openings contaminated with paraffin wax was studied. Solder paste, screen printed on a metallized silicon substrate, was melted locally through application of 6.5?10 V to a 5 ? copper film resistor for a few seconds and was found able to drive an intermediate layer of paraffin away and seal a 0.2 mm diameter circular via by wetting a surrounding copper pad. Although verified to be robust, the process did result in failing seals on excessive heating because of consumption of the pads. Correctly performed, the technique provided a seal at least withstanding a pressure of 8 bar for 8 h at 85 ?C.

MEMS enablement and analysis of the miniature autonomous submersible explorer

The miniature autonomous submersible explorer (MASE) was designed as a vehicle for astrobiology science by Behar et al. . This paper focuses on the MASE concept and extrapolates a future design based on microelectromechanical systems (MEMS), multifunctional microsystems (MMS), and three-dimensional multichip modules (3-D-MCM). Miniaturization of the electronics increases the payload volumes and power capabilities significantly and this is the main rationale for pursuing extreme miniaturization. The original MASE vehicle accommodated 1-2 instruments while the MEMS enhanced miniature autonomous submersible explorer (MEMSEMASE) accommodates up to six instruments. It is shown that the occupied area of the electronics components is reduced eight times, and the volume 25 times. The vehicle is shaped as a tube with 5 cm in diameter and 20 cm in length and can support 8 W continuously over 5 h. The maximum range is 25 km while the typical onboard instrumentation is conductivity, temperature, depth (CTD), and a high resolution camera. An optical fiber is used for bidirectional communication with the vessel. The goal of this enriched concept is to present an extremely miniaturized submersible design. The vehicle volume is defined to fit inside host vehicles with the goal of future deployment on Europa, oceans on Earth, and bore holes. The paper will focus on showing how electronics can be densely packed into micromachined silicon modules and how these can be designed and interconnected theoretically.

Silicon fusion bond interfaces resilient to wet anisotropic etchants

The bond interface in silicon microsystems is sensitive to the subjection to wet anisotropic etchants. Fusion bond interfaces of bonded wafers resilient to potassium hydroxide or tetramethyl ammonium hydroxide etching are obtained using wafers of oxidized silicon bonded to oxidized silicon, where the bond oxide is removed by trifluoromethane plasma etching. Other investigated bond configurations initiate severe damages during etching.