Kerstin Jonsson

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 ...

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.

Strength of Bonded Interfaces

In this chapter, commonly used bond test methods are briefly presented together with suggestions of improvement. Integrated circuit technology and microsystem technology make good use of wafer bonding. The bond strength is considered one of the most important characteristics to determine about a bond. Several techniques for the determination of the strength of bonded interfaces exist. Bond strength measurements contribute to the understanding of chemical reactions behind the bonding process, bonding optimization, and quality assurance. Solid mechanics is the base of the models valid for bond strength measurements. For fracture to occur, it is necessary that local stress at the crack tip reaches the theoretical strength of the material and that the fracture is accompanied by global reduction of energy in the material. The most commonly used method to measure strength of wafer bonded interfaces is the double cantilever beam method. Using this method, the bonded samples are forced apart by inserting a thin blade into the bond. In the tensile test method, bonded samples are glued onto pull studs fitting a tensile strain test machine and the studs are then pulled apart, and the maximum force occurring at separation is recorded. Blister tests used in wafer bonding have regained interest lately. An internal cavity at the bond interface is pressurized pneumatically or hydrostatically until fracture occurs and the fracture pressure is a measure of the bond strength. Introduction of specially formed notch, the chevron into the bond under test has improved measurements. Chevrons have been implemented for various load configurations, such as tensile, three-point bending, and blister tests. Common to the various techniques for the determination of the strength of bonded interfaces is that good measurements can be made only with an holistic approach to the experimental setup, where every source of measurement error is addressed.

MEMS technology to achieve miniaturization, redundancy, and new functionality in space

Development of MEMS-based (Micro Electro Mechanical System) components and subsystems for space applications has been going on for at least two decades. The main driver for developing MEMS components for space is miniaturization through reduced mass, volume and power of individual components. However, the commercial breakthrough of MEMS has not occurred within the space business as it has within other branches such as the IT/telecom, the automotive industry, or other areas. In addition to miniaturization, increased redundancy and improved (or in some cases unique) performance has also been achieved by using MEMS-based components. MEMS pressure sensors integrated into the mechanical housing of another component is one example. Another example is an isolation valve which is both redundant and has an integrated particle filter on a single silicon chip weighing less than one gram. Currently there are few space missions using allowing newly developed MEMS devices onboard, but one of the exceptions is the Swedish-built Prisma satellites. One of the Prisma satellites has a MEMS-based cold gas propulsion system onboard, which contains a number of miniaturized and novel components. This paper presents the MEMS based cold gas propulsion system developed for Prisma including a number of novel components and their maiden spaceflight onboard Prisma last year.

Chapter Nineteen – Strength of Bonded Interfaces

Publisher Summary In this chapter, commonly used bond test methods are briefly presented together with suggestions of improvement. Integrated circuit technology and microsystem technology make good use of wafer bonding. The bond strength is considered one of the most important characteristics to determine about a bond. Several techniques for the determination of the strength of bonded interfaces coexist. Bond strength measurements contribute to the understanding of chemical reactions behind the bonding process, bonding optimization, and quality assurance. Solid mechanics is the base of the models valid for bond strength measurements. For fracture to occur, it is necessary that local stress at the crack tip reaches the theoretical strength of the material and that the fracture is accompanied by global reduction of energy in the material. The most commonly used method to measure strength of wafer bonded interfaces is the double cantilever beam method. Using this method, the bonded samples are forced apart by inserting a thin blade into the bond. In Tensile test method, bonded samples glued onto pull studs fitting a tensile strain test machine and studs thereafter pulled apart, and the maximum force occurring at separation is recorded. Blister tests used in wafer bonding have regained interest lately. An internal cavity at the bond interface is pressurized pneumatically or hydrostatically until fracture occurs and the fracture pressure is a measure of the bond strength. Introduction of specially formed notch, the chevron into the bond under test has improved measurements. Chevrons have been implemented for various load configurations, such as tensile, three-point bending, and blister tests. Techniques for determination of strength of bonded interfaces coexist; common to these is that good measurements only can be made with holistic approach to experimental setup, where every source of measurement error is addressed.

Demonstration of Microcoil Heaters for Microthrusters

The resilience and performance of carbon microcoil heaters made by laser-induced chemical vapor deposition (LCVD) to be used as an efficient means for increasing the specific impulse of cold/hot gas microthrusters were investigated. Two naked carbon coils and two tungsten coated carbon coils coated through LCVD were used in this experiment. Each pair of coated and uncoated carbon coils were heated resistively in a thermal cycling between 300-1173K and 973-1173K for 2 h in 0.000003 mbar and 2 bar N. The results show that at these temperatures the carbon microcoils and nitrogen propellant were compatible while the tungsten coated microcoils started degrading. It was observed that LVCD-deposited carbon and tungsten-coated carbon microcoils can withstand low to medium-high temperatures for extended periods of time during thermal cycling without showing signs of degradation.

Development of MEMS-Based Components and Subsystems for Spacecraft Propulsion

Development of a MEMS-based (Micro Electro Mechanical System) components and subsystems has been pursued at Uppsala University, Sweden since 1997. Since 2005, the continued development towards the first flight the subject MEMS products onboard a satellite in 2008 is done within the frame of NanoSpace — a company dedicated to MEMS-based products for space. Currently, two major efforts to develop MEMS-based propulsion products are ongoing. First, NanoSpace is developing a miniaturized cold gas propulsion system. The major challenge in this effort is to develop the thruster module containing four individual thrusters with the capability to deliver proportional, low noise thrust in the micro- to milli-Newton range. The thruster pod even includes valves, filters, pressure- and temperature sensors and heaters. In a future step, even control electronics and a CAN interface will be included in the thrusters pod which has the size of a golf ball and a weight of about 100 grams. A prototype of this miniaturized cold gas propulsion will be flight tested onboard the PRISMA satellite. PRISMA is an international technology demonstration program with focus on rendezvous and formation flying. It is a two satellite LEO mission with a launch scheduled to September 2008. The other major development effort underway is a MEMS-based Xenon flow control system intended for electrical propulsion systems. Using MEMS technology, a Xenon feed system including an micro isolation valve, pressure regulator, and a number of parallel flow control modules can be built with significantly reduced size and mass compared to existing systems based on conventional technology. NanoSpace is a Swedish company with the goal to be a component and subsystem supplier of MEMS-based products to space industry, based on own research and development and intellectual property rights.© 2006 ASME