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Dawning of the micromachine revolution

Posted: 23 Jan 2007 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS? transistor? revolution of micromachines? silicon devices? silicon?

By Benedetto Vigna and Luca Fontanella

Silicon has been associated with technological development since the latter part of the post-war period. Electronics is everywhere and silicon is the material used to create majority of today's electronic circuits and technology.

Although it has been growing in popularity, silicon's mechanical properties are less known. It can be used to build practically any mechanical object such as cogwheels, gears, levers, springs and supporting beams. In these cases, its properties often vie with those of steel (Corigliano et al., 2005), although it has so far been impossible to bring its refining processes down to the same low-cost level. The foreseeable consequence of this price difference is that we are very unlikely to see a world in which silicon is used to construct bridges or buildings.

We are however creating a world of micromachines (microelectromechanical systems or MEMS). A few millimeters in size, MEMS are actually made of silicon and are mostly constructed with the same planar and photolithographic processes used for wafers in the semiconductor industry. To make this possible, special chemical/physical processes, such as "freeing" the structures to make them mobile, have been specifically developed.

The theoretical possibility of making these microscopic structures was put forward several years ago, and was promoted by Nobel laureate Richard Feynman. In a speech he gave at the California Institute of Technology in December 1959, Feynman suggested the possibility of creating machines able to produce smaller machines, which were in turn able to produce even smaller machines, and so on down to atomic levels.

Only recently have micromechanical techniques based on silicon become available at moderately low manufacturing costs, thanks to the economies of scale commonly observed in the microelectronics industry. Thus, they can now be used in various fields.

Measuring position and vibrations
The most widespread and best known micromechanical object is the accelerometer. This device is a sensor that measures acceleration (one of the most important kinematic quantities), which reflects a body's movement and change of speed.

The micromechanical accelerometer consists of a mass suspended on a spring, which moves if subjected to acceleration. By measuring the movement of the mass, it is possible to calculate an unknown acceleration.

??ST's 3-axis accelerometer

The car airbag system was the application that initially spurred the accelerometer's development. When a vehicle crashes, its acceleration suddenly increases; and when the acceleration exceeds a defined value, the system inflates the airbags. Due to recent innovations (STMicroelectronics, 2003), accelerometers that are sensitive along all three orthogonal axes have been built into a single electronic component about 15mm3 in size and weighing only a few dozen milligrams. Since these accelerometers detect all three spatial components at the same time, they can measure the magnitude and direction of the acceleration vector. This simplifies the use of these sensors and lowers the cost of the final application.

Triaxial accelerometers may be classified into two main categories: applications that measure position in space, and those that measure vibrations. Gravity is a constant acceleration, which is uniform everywhere on earth. A triaxial accelerometer can measure the gravity vector and establish its direction with respect to a local reference system. Thus, it is possible to establish the sensor's spatial orientation unequivocally. This means that an electronic system using a sensor like this can record a static position or a movement and translate it into electrical signals that may be saved and encoded for further processing.

A variety of applications then becomes available. By putting an accelerometer inside a pen, it is now possible to record a signature. This way, an "authorized" specimen signature may be compared in real time with the one the user is signing when they make a purchase. In the case of credit card transactions, this increases security.

With accelerometers, it is now also possible to measure the activity of an individual in order to avert skeletal or muscular system "breakdowns" before they occur. An example would be the risk that an athlete may sprain a muscle during training. These traumas often occur due to fatigue or "wrong" movements. The sensor can warn that a trauma is imminent so that the training is stopped in time.

Additionally, movements that are currently assessed using qualitative methods, such as the performance of gymnasts or figure skaters, may now be measured quantitatively in terms of movement amplitude, rotational speed or compliance with a predetermined model (pattern). This makes the final assessment of the performance more objective.

ST's 3-axis digital accelerometer

In the biomedical field, another interesting use is the detection of high quantities of certain hormones or drugs. The presence of hormones and drugs may be detected by placing accelerometers on the patient's wrists. This type of analysis may be carried out both positively, for example to detect the amount of caffeine taken by the patient (increase in intensity of vibrations), and negatively, by detecting the administration of relaxants (decrease in vibrations compared to normal values). This application is already being used in the veterinary field, where an accelerometer collar is used to detect when dairy cows are on heat in order to shorten their non-productive period.

Another important application is measurement of the position and movement of prostheses. The electronic system, which drives the electric motors that move the limb, may be improved if the actual movement and position of the prosthesis are known (Canina et al., 2003). Corrections to the force, direction and quality of the movement applied to the prosthesis may be made by local electronic circuitry without the need for methods to make the patient consciously "feel" the limb.

Lab on a chip
Something quite different from accelerometers are biological analysis microlaboratories, also known as Labs-on-Chips. In this case, thanks to its biocompatibility, silicon is used not to make moving parts, but to handle minute quantities of organic liquids (picoliters, i.e. billionths of a liter), from which DNA chains may be extractedfor exampleand their fragments multiplied in order to identify the presence of predetermined gene sequences (Barlocchi, 2000). These microlaboratories consist of minute channels buried in the silicon into which the organic material is pushed, heated and left to react in order to produce the required results. All the processes needed for complete diagnosis may be created inside the silicon chip: specimen preparation, PCR (Polymerase Chain Reaction), i.e. multiplication of the DNA fragments present, and detection of gene chains.

This type of microlaboratory may be used every time it is necessary to check the presence of a DNA chain in an organic liquid, including predisposition to genetic diseases, diagnosis of infectious diseases (using the DNA or RNA of bacteria or viruses as comparison material), when evidence for the prosecution or defense is needed in criminal proceedings, to analyze food, in zoology and animal husbandry to select the most productive species, and in agriculture.

Micromotors and microsensors
Micromechanical actuators are mobile silicon systems like accelerometers. But instead of reacting to external stimuli and producing measurable data proportional to an unknown quantity, they receive a command and drive external loads. They are sometimes also called micromotors and are used to move small objects, such as microscope micromanipulators or the micromirrors in the latest-generation image projectors, or to move and switch laser rays in communication systems.

There are a few types of drug that require particularly careful dosage and administration, which depend on the transitory conditions of the patients. A particularly clear case is insulin. For this application, silicon micropumps have been developed for transdermal injections. They are driven by a control box that either includes a timer or is able to measure blood glucose concentrations in real-time and automatically administer the necessary dose at the right time as programmed by the specialist.

Due to silicon's biocompatibility, another possible application is in situ monitoring of certain physiological parameters. One example is the measurement of blood pressure in veins or arteries, with sensors implanted in the walls of the blood vessels. Silicon pressure sensors can now be as small as 0.07mm3, with performances in line with medical requirements. These sensors may be fitted with minute radio transmitters and can use energy scavenging techniques to recharge their internal microbatteries. Sensors may be implanted in the body using microinvasive (endoscopic) techniques and left in situ for several years without needing a power supply. When measurements are needed, a control box worn by the patient on their belt calls each of the sensors in the patient. Since it knows their positions, it can "map" the various pressures and calculate their distributions, gradients and changes over time. It is also possible to program alarm thresholds so that every time the pressure exceeds danger levels, signals are generated to warn the physician of an anomalous condition to be kept under observation.

Applications of the same type are possible with chemical sensors that can detect concentrations of various substances dissolved in the blood or in body tissues, such as ethyl alcohol, glucose, a few types of protein, oxygen and a few ionic species.

Beginning of the micromachines era
Many science-fiction-sounding ideas are presented daily to the international community; nearly almost always as a way to justify exorbitant requests for new financing.

We could have written about microrobots with independent decision-making capacity which, in the future, could enter the human body to "repair" vessels and/or tissues by carrying out in situ operations with "nanosurgical" techniques, without outside contact. However we preferred to give an idea of existing and not far-off technologies, whose application is taking place or is imminent.

If we look at the future from an industrial and therefore pragmatic point of view, far from what science fiction describes, it seems destined to include endless examples of silicon micromachines helping man, not just in everyday life, but also in the medical field, where the diagnostic and therapeutic possibilities seem to depict a revolution of considerable proportions.

Indeed, it seems that the era of micromachines has really begun.

? Barlocchi G., Corona P., Mastromatteo U., Villa F.F., "Silicon Micromachining for Lab on Chip," Sensors and Microsystems, Proceedings of the 5th Italian Conference, World Scientific, February 2000.
? Canina M., Rovetta A., Pasolini F., Tronconi M., Chiesa E., "Innovative System for the Accumulation of Energy of the Step in a Limb Prosthesis," Proceedings of the 11th World Congress in Mechanism and Machine Science, Tianjin, China, Aug. 18-21, 2003.
? Corigliano A., Frangi A., Comi C., De Masi B., "Propriet? Meccaniche del Polisilicio Epitassiale alla Microscala," XVI AIMETA Conference on Theoretical and Applied Mechanics, Florence, 2005.
? STMicroelectronics, "Three-Axis MEMS-based Accelerometer Targets Handheld Terminals", 3GSM Congress, Cannes, Feb. 18, 2003.

About the authors
Benedetto Vigna
is the manager of STMicroelectronics' MEMS business unit.
Luca Fontanella is the marketing manager for ST's MEMS business unit.

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