[seminar 36] Scope of MEMS in Space

ABSTRACT

We present a multi-step approach for the management of MEMS quality and reliability issues for space applications.  The first step is based on a tight relationship between design activities, mission’s needs and micro characterization of technologies and materials. Due to the multiplicity of MEMS technologies, the main challenge is to get technological data concerning reliability parallel to the product development. To perform the collection of these data, test vehicles are designed on different processes in order to compare reliability and to discriminate between failures coming from design and from process.

The paper will summarize the design and technology analysis activities of MEMS.

The ultimate objective of this effort is to be able to rapidly incorporate MEMS into future space instrumentation.

INTRODUCTION

Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through micro fabrication technology. While the electronics are fabricated using integrated circuit (IC) process sequences, the micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.

Microelectronic integrated circuits can be thought of as the "brains" of a system and MEMS augments this decision-making capability with "eyes" and "arms", to allow microsystems to sense and control the environment. Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose. Because MEMS devices are manufactured using batch fabrication techniques similar to those used for integrated circuits, unprecedented levels of functionality, reliability, and sophistication can be placed on a small silicon chip at a relatively low cost.

Micro Electro Mechanical Systems (MEMS) is one of the key enabling technologies to create cost-effective, ultra-miniaturized, robust, and functionally focused spacecraft for both robotic and human exploration programs. Examples of MEMS devices at various stages of development include microgyroscope, spectrometer, microaccelerometer.

Significance OF MEMS.

MEMS promises to revolutionize nearly every product category by bringing together silicon-based microelectronics with micromachining technology, thereby, making possible the realization of complete systems-on-a-chip. MEMS is truly an enabling technology allowing the development of smart products by augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators.

Since MEMS devices are manufactured using batch fabrication techniques, similar to integrated circuits, unprecedented levels of functionality, reliability, and sophistication can be placed on a small silicon chip at a relatively low cost.

MEMS is an extremely diverse technology that potentially could significantly impact every category of commercial and military products. Already, MEMS is used for everything ranging from in-dwelling blood pressure monitoring to active suspension systems for automobiles. The nature of MEMS technology and its diversity of useful applications makes it potentially a far more pervasive technology than even integrated circuit microchips.

Fabrication Methods

1.     Bulk Micromachining

2.     LIGA

3.     Surface Micromaching

1. Bulk Micromachining :-

a.     3D structures formed by wet/dry etching of silicon substrate

b.    In wet etching chemicals are used to melt away layers of materials, much like IC fabrication

c.     High Density Plasma is used to etch the substrate in dry etching

2. LIGA

a.     Stands for X-Ray Lithography. Electroforming, and Injection Molding.

b.    X-Ray Lithography is used to pattern the substrate into a mold.

c.     Molds are filled with metal to make parts used in other MEMS.

3.  Surface Micromachining

a.     Nearly identical to IC fabrication steps

b.    Same technique used to make optical parts for MEMS

Applications of MEMS

There are numerous possible applications for MEMS. As a breakthrough technology, allowing unparallel synergy between previously unrelated field such a biology and microelectronics, many new MEMS applications will emerge, expanding beyond that which is currently identified or known. Here are a few applications of current interest::-

Automotive Industry

Accelerometers For Airbags : MEMS accelerometers are quickly replacing conventional accelerometers for crash airbag deployment systems in automobiles. MEMS technology has made it possible to integrate the accelerometers and electronics onto a single silicon chip. These MEMS accelerometers are much smaller, more functional, lighter, more reliable, and are produced for a fraction of the cost of the conventional macro-scale accelerometers elements.

Medical Industry

Sensor Systems, Monitoring Systems: The sensor, upon detecting a chemical/biological agent, would trigger a micro pump for an automatic injection of an appropriate antidote. Smaller devices are also minimally invasive, so they do less harm when used inside patients. "Micron-sized diagnostic and monitoring tools have the potential to safely take more localized measurements,"

Military Industry

Guidance Systems, Firing Failsafe, Data Acquisition

MEMS in space.

Space exploration in the coming century will emphasize cost effectiveness and highly focused mission objectives, which will result in frequent multiple missions that broaden the scope of space science and to validate new technologies on a timely basis. Micro Electro Mechanical Systems (MEMS) is one of the key enabling technologies to create cost-effective, ultra-miniaturized, robust, and functionally focused spacecraft for both robotic and human exploration programs. Examples of MEMS devices at various stages of development include micro-gyroscope, micro-seismometer, micro-hygrometer, quadruple mass spectrometer, and micro-propulsion engine. These devices, when proven successful, will serve as models for developing components and systems for new-millennium spacecraft.

NASA has a very special interest in MEMS technology.  MEMS offer the benefits of significantly reduced mass and power consumption translating directly into direct cost benefits as a result of the major decrease in size. Some of the systems utilizing MEMS devices for space applications are:

§  Micro thrusters

§  Mass Spectrometers

§  Magnetometers

§  RF Switches

§  Micro gyroscopes

The main obstacle in rapidly integrating new technologies into space systems is determining system reliability.  Reliability,  the ability of a device/system to maintain performance requirements throughout its lifetime, is a major consideration factor for making device selections for space flight applications.  Space missions can be expected to last upwards of 5 years with spacecraft subject to extreme mechanical shock, vibration, temperature, vacuum, and radiation environments.

HIGH-SENSITIVITY, LOW-POWER, LOW-COST, SOLID-STATE RADIATION MONITOR.

The ability to accurately detect and measure radiation, such as protons, neutrons, heavy ions, etc., in the natural environment of space is critical to the health of spacecraft electronics systems. Traditional equipment for detecting and monitoring radiation is bulky, consumes relatively large amounts of power, and is sometime sensitive to shock and vibration. This device is small, light-weight; accurate solid-state radiation monitor is a custom-designed Static Random Access CMOS Memory that upsets when struck by high energy particles such as protons, alpha particles, or heavy ions. Neutrons are detected by coating the detector with boron which generates alpha particles during a particle strike. The CMOS chip is 2.6 mm x 3.4 mm and requires 50 mW operating power.

MEMS-BASED XYLOPHONE MAGNETOMETER

MEMS-based xylophone magnetometer uses an alternating current to drive a micro-machined bar at its resonant frequency. The magnitude of the bar’s deflection is based on the Lorentz force produced by the current and magnetic field, which is detected optically. This device can measure local magnetic field gradients as well as spacecraft magnetic interference. This MEMS magnetometer has a linear response. By altering the drive current, the sensitivity can range from nT to T. Its dynamic range far exceeds the fluxgate magnetometer, is much smaller, and uses much less power. Piezoresistive, magneto-restrictive magnetometers, and tunnel-based magnetometers have mT to µT sensitivities. The mass of the device is 10 g, it consumes 200 mW and requires < 1 cm³.

MEMS MICRO GYROSCOPE

When fully developed the MEMS micro-gyroscope will provide the inertial reference for a spacecraft. This MEMS device is fabricated using bulk micromachining where the device has four pedals coupled to a central post that presses in response to Coriolis force that changes the pedal capacitance. The device weighs < 1 gm, requires < 1 cm3, consumes < 0.5 W, and has infinite life due to no wearing parts.

MEMS MICRO ACCELEROMETER

The MEMS micro-accelerometer is fabricated in silicon using micromachining processes. It uses a capacitance transducer and electrostatic force feedback to restrain the proof mass. The device does not require leveling and can operate in zero gravity. Its sensitivity is between 0.01 to 100 Hz with 2 ng/Ö Hz sensitivity, size is 2.5 x 2.5 x 2.0 cm3, power is 200 mW at ±5 V, and weighs 50 g.

RESEARCH FINDINGS ABOUT MEMS IN SPACE

a.     Researchers found that MEMS operate in vacuum, the environment found in space, differently than they operate in atmosphere in two ways: the voltages required for resonant operation are much lower and the energetic amplifications are much larger. The team found during testing that oscillators needed only a tenth of the voltage normally required in air.

“This is incredibly significant for space applications because instead of hundred volt supplies, which are heavier and more expensive to launch, we might be able to run space MEMS on standard low voltages of only 10 to 15 volts,”

b.    Testing also showed that the oscillators had an amplification that was hundreds of times greater. “If you whack a tuning fork, it has a high resonance, or amplification, which causes it to ring a long time,” he continues. “For MEMS that are driven at resonance, this means they will have much larger amplification while operating on less power in vacuum.”

Researchers had also worried that “stiction,” a combination of stickiness and friction, and vacuum welding, the tendency for metal parts to bond together in vacuum conditions, could be major factors in space MEMS — yet that has not been the case thus far. Water vapor and air act as lubricants for MEMS surfaces that slide on or touch each other. In vacuum, however, parts that touch lack that layer of gas between the surfaces, leading to the possibility that surfaces could exchange atoms and eventually bond. This effect most likely led to an antenna on the Galileo spacecraft being unable to open.

c.     The cost of launching payloads into space — tens of thousands of dollars per pound, depending on the launch vehicle — makes micro-electromechanical systems highly desirable for space applications.

d.    Though possible, miniaturizing many space instruments overall isn’t practical because the smaller size gives the sensor less signal, such that it receives fewer particles or photons and can’t measure the highly tenuous particle distributions or dim emissions they were intended to measure.

e.     MEMS will enable space instruments to have large aperture sizes in a flat panel shape that will be much thinner than current sensors, resulting in tremendous mass savings. MEMS devices are also highly reliable, and space instruments will use arrays of many thousands of identical MEMS. This redundancy enables an instrument that suffers failure of a small number of its devices to continue to operate at nearly full sensitivity.

ADVANTAGES of MEMS

First, MEMS is an extremely diverse technology that could significantly affect every category of commercial and military product. The nature of MEMS technology and its diversity of useful application make it potentially a far more pervasive technology than even integrated circuit microchips.

Second, MEMS blurs the distinction between complex mechanical systems and integrated circuit electronics. Historically, sensors and actuators are the most costly and unreliable part of a sensor-actuator-electronics system. MEMS technology allows these complex electromechanical systems to be manufactured using batch fabrication techniques, increasing the reliability of the sensors and actuators to equal that of integrated circuits. Yet, even though the performance of MEMS devices and systems is expected to be superior to components and systems, the price is predicted to be much lower.

Semiconductor manufacturing techniques, together with some ingenuity, lets MEMS designers put a lot of functionality into a small package eg. Implantable insulin pump.

LIMITATIONS

Unfortunately, what makes MEMS possible, also makes it expensive. The technology behind semiconductors and MEMS is capital intensive. It costs millions of dollars to set up and maintain a modern laboratory complete with the clean rooms and hardware needed to build MEMS devices. Few companies have the resources to do it.

CONCLUSION

Because of the extremely high cost of launching instrumentation into orbit, the rigors of the launch and space environments (such as vibration and radiation), and the need for extremely high-reliability devices, MEMS technology is ideally suited for space sensor needs.

The ultimate objective of this effort is to be able to rapidly incorporate MEMS into future space instrumentation. This topic solicits spacecraft technology development proposals in the area of Micro/ Nano-Sciencecraft. NASA is moving toward miniature [approximately 1-50 kilogram], low cost spacecraft playing an increasingly frequent and important role in its broad spectrum of planetary, space physics, and Earth Science missions.

FUTURE SCOPE

These devices can be affordably incorporated in highly miniaturized sensors requiring a level of manufacturing precision not possible with current macro-scale technology. Additionally, the savings in mass and power make these devices ideal for micro-scale; low-cost missions planned in future NASA programs.

Further in the future, the integration of electronics, photonics, and micromechanical functionalities into "instruments-on-a-chip" will provide the ultimate size, cost, function, and performance advantage.

There's also a perceived need for MEMS devices pushing development. "The Genome Project gave us the code that the machinery of life runs on, but now we have to figure out how that machinery runs.

MEMS is also a logical stepping stone to nanotechnology, or building devices atom by atom on the nanometer scale. (A micron is a millionth of a meter; a nanometer is a billionth of a meter.) Someone might even make a technological leap beyond MEMS and straight to nanotechnology, discovering a totally new way to do molecular manipulation rather than bulk manipulation."

REFERENCES

Aerospace Engineering Magazine (SAE )

Micro machined Transducers Sourcebook by Gregory Kovacs

www.memsnet.org

http://sbir.gsfc.nasa.gov/SBIR/sbirsttr2003/solicitation/SBIR/TOPIC_S4.html

www.NASA.gov

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