Ορολογία στην Ξένη ΓλώσσαΕνότητα: Microelectromechanical systems (MEMS)

Παναγιώτης Τσατσαρός

Τμήμα Μηχ. Αυτοματισμού ΤΕ

ΕΛΛΗΝΙΚΗ ΔΗΜΟΚΡΑΤΙΑΑνώτατο Εκπαιδευτικό Ίδρυμα ΠειραιάΤεχνολογικού Τομέα

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Χρηματοδότηση

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• Το έργο «Ανοικτά Ακαδημαϊκά Μαθήματα στο Ανώτατο Εκπαιδευτικό Ίδρυμα Πειραιά Τεχνολογικού Τομέα» έχει χρηματοδοτήσει μόνο την αναδιαμόρφωση του εκπαιδευτικού υλικού.

• Το έργο υλοποιείται στο πλαίσιο του Επιχειρησιακού Προγράμματος «Εκπαίδευση και Δια Βίου Μάθηση» και συγχρηματοδοτείται από την Ευρωπαϊκή Ένωση (Ευρωπαϊκό Κοινωνικό Ταμείο) και από εθνικούς πόρους.

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1. Σκοποί ενότητας...................................................................................................4

2. Περιεχόμενα ενότητας..........................................................................................4

3. Introduction...........................................................................................................5

4. Historical Background...........................................................................................5

5. Fabrication Technologies.....................................................................................7

5.1 IC Fabrication.................................................................................................8

5.1.1 Fabrication Technologies, IC Fabrication: Sentence completion...........9

5.2 Bulk Micromachining and Wafer Bonding......................................................9

5.3 Surface Micromachining..............................................................................10

5.4 Micromolding................................................................................................11

6. Bulk Micromachining, Wafer Bonding, Surface micromachining, Micromolding: Open questions..........................................................................................................11

7. Applications of MEMS........................................................................................12

7.1 Pressure Sensors........................................................................................12

7.2 Accelerometers............................................................................................12

7.3 Inertial Sensors............................................................................................12

7.4 Microengines................................................................................................13

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1. Σκοποί ενότητας

The aims of this unit are to:

Provide authentic text and vocabulary specific to the needs of students of Automation Engineering

Encourage students to combine their knowledge of English with their technical knowledge

Enable students to differentiate between slightly different words and notions Enable students to perceive time sequence and time relations Enable students to cope with reading comprehension through skimming and

scanning techniques Encourage students to construct whole sentences and compile full meaning

2. Περιεχόμενα ενότητας

Definition, examples, Historical background and developments Fabrication technologies: miniaturization, multiplicity, microelectronics MEMs and IC Fabrication Bulk Micromachining: anisotropic etching, etch masks and etch-stop

techniques Wafer Bonding Surface Micromachining Micromolding Applications: pressure sensors, accelerometers, inertial sensors,

microengines

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3. Introduction

Underline the correct word between the two options given in italics.

Microelectromechanical systems (MEMS) are small (integrating/ integrated) devices or systems that [combine/compose) electrical and mechanical components. They (range/rate) (on/in) size from the sub micrometer (or sub micron) level to the millimeter level and there can be any number, from a few to millions, in a particular system. MEMS (expand/extend) the fabrication techniques developed for the integrated circuit industry to add mechanical elements such as beams, gears, diaphragms, and springs to devices.

Examples of MEMS device applications (include/contain) inkjet-printer cartridges, accelerometers, miniature robots, microengines, locks, inertial sensors, microtransmissions, micromirrors, micro actuators, optical scanners, fluid pumps, transducers, and chemical, pressure and flow sensors. New applications are (immersing/emerging) as the existing technology is applied (to/for) the miniaturization and integration of (compatible/conventional) devices.

These systems can sense, control, and activate mechanical processes (on/at) the micro scale, and function (individually/individual) or in arrays to generate effects on the macro scale. The micro fabrication technology (activates/enables) fabrication of large arrays of devices, which individually perform simple tasks, (but/and) in combination can accomplish complicated functions.

MEMS are not about any one application or device, nor are they (designed/defined) by a single fabrication process or limited to a few materials. They are a fabrication approach that (carries/conveys) the advantages of miniaturization, multiple components, and microelectronics to the design and (construction/structure) of integrated electromechanical systems. MEMS are not only about miniaturization of mechanical systems; they are also a new paradigm for (designing/designed) mechanical devices and systems.

The MEMS industry has an (calculated/estimated) $10 billion market, and with a projected 10-20% annual growth (rate/rating), it is estimated to have a $34 billion market in 2012 [Because of the significant impact that MEMS can have on the commercial and defense markets, industry and the federal government have both (taken/given) a special interest in their development.

4. Historical Background

Choose the correct verb form between the two given in italics

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The invention of the transistor at Bell Telephone Laboratories in 1947 (sparked/has sparked) a fast-(grown/growing) microelectronic technology. Jack Kilby of Texas Instruments (build/built) the first integrated circuit (IC) in 1958 using germanium (Ge) devices. It (consists/consisted) of one transistor, three resistors, and one capacitor. The IC was (implemented/implementing) on a sliver of Ge that was (glued/gluing) on a glass slide. Later that same year Robert Noyce of Fairchild Semiconductor (announced/was announcing) the development of a planar double- diffused Si IC. The complete transition from the original Ge transistors with (growing/grown) and (alloying/alloyed) junctions to silicon (Si) planar double- [diffused/diffusing) devices (took/has taken) about 10 years. The success of Si as an electronic material was due partly to its wide availability from silicon dioxide (Si02) (sand), (resulting/resulted) in potentially lower material costs relative to other semiconductors.

Since 1970, the complexity of ICs (has doubled/doubled) every two to three years. The minimum dimension of manufactured devices and ICs (has decreased/decreased) from 20 microns to the sub micron levels of today. Current ultra-large-scale-integration (ULSI) technology (enables/enabled) the fabrication of more than 10 million transistors and capacitors on a typical chip.

IC fabrication is dependent upon sensors to provide input from the surrounding environment, just as control systems need actuators (also (referred/referring) to as transducers) in order to carry out their desired functions. Due to the availability of sand as a material, much effort was put into (develop/developing) Si processing and characterization tools. These tools are now (being used/used) to (advance/advancing) transducer technology. Today's IC technology far (outstrips/has outstripped) the original sensors and actuators in performance, size, and cost.

Attention in this area was first (focusing/focused) on microsensor (i.e., microfabricated sensor) development. The first microsensor, which has also been the most successful, was the Si pressure sensor. In 1954 it was (discovering/discovered) that the piezoresistive effect in Ge and Si (has/had) the potential to produce Ge and Si strain gauges with a gauge factor (i.e., instrument sensitivity) 10 to 20 times greater than those (basing/based) on metal films. As a result, Si strain gauges (began/had begun) to be (developing/developed ) commercially in 1958. The first high-volume pressure sensor was (marketed/marketing) by National Semiconductor in 1974. This sensor (includes/included) a temperature controller for constant- temperature operation. Improvements in this technology since then (have included/have been including) the utilization of ion implantation for (improving/improved) control of the piezoresistor fabrication. Si pressure sensors are now a billion-dollar industry.Around 1982, the term micromachining (come/came) into use to designate the fabrication of micromechanical parts (such as pressure-sensor diaphragms or accelerometer suspension beams) for Si microsensors. The micromechanical parts were (fabricating/fabricated) by selectively etching areas of the Si substrate away in order to leave behind the desired geometries. Isotropic etching of Si was (developed/developing) in the early 1960s for transistor fabrication. Anisotropic

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etching of Si then came about in 1967. Various etch-stop techniques were subsequently developed to provide further process flexibility.

These techniques also (form/formed) the basis of the bulk micromachining processing techniques. Bulk micromachining designates the point at which the bulk of the Si substrate is (etched/etching) away to leave behind the desired micromechanical elements. Bulk micromachining (has remained/is remaining) a powerful technique for the fabrication of micromechanical elements. However, the need for flexibility in device design and performance improvement (has motivated/is motivating) the development of new concepts and techniques for micromachining.

Among these is the sacrificial layer technique, first demonstrated in 1965 by Nathanson and Wickstrom, in which a layer of material (is deposited/is being deposited) between structural layers for mechanical separation and isolation. This layer {is removed/is being removed) during the release etch to free the structural layers and to allow mechanical devices to move relative to the substrate. A layer is releasable when a sacrificial layer (has separated/separates) it from the substrate. The application of the sacrificial layer technique to micromachining in 1985 (gave/has given) rise to surface micromachining, in which the Si substrate is primarily used as a mechanical support upon which the micromechanical elements (are/are being) fabricated.

Prior to 1987, these micromechanical structures were (limiting/limited) in motion. During 1987-1988, a turning point (was reached/had been reached) in micromachining when, for the first time, techniques for integrated fabrication of mechanisms (i.e. rigid bodies connected by joints for transmitting, controlling, or constraining relative movement) on Si (were/were being) demonstrated. During a series of three separate workshops on microdynamics held in 1987, the term MEMS was (coining/coined). Equivalent terms for MEMS are microsystems (preferred in Europe) and micromachines (preferred in Japan).

5. Fabrication Technologies

Read the following text and then complete the sentences with appropriate information in the right form:

The three characteristic features of MEMS fabrication technologies are miniaturization, multiplicity, and microelectronics. Miniaturization enables the production of compact, quick-response devices. Multiplicity refers to the batch fabrication inherent in semiconductor processing, which allows thousands or millions of components to be easily and concurrently fabricated. Microelectronics provides the intelligence to MEMS and allows the monolithic merger of sensors, actuators, and logic to build closed-loop feedback components and systems. The successful miniaturization and multiplicity of traditional electronics systems would not have been possible without IC fabrication technology. Therefore, IC fabrication technology, or microfabrication, has so far been the primary enabling technology for the

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development of MEMS. Microfabrication provides a powerful tool for batch processing and miniaturization of mechanical systems into a dimensional domain not accessible by conventional (machining) techniques. Furthermore, microfabrication provides an opportunity for integration of mechanical systems with electronics to develop high-performance closed-loop-controlled MEMS.

Advances in IC technology in the last decade have brought about corresponding progress in MEMS fabrication processes. Manufacturing processes allow for the monolithic integration of microelectromechanical structures with driving, controlling, and signal-processing electronics. This integration promises to improve the performance of micromechanical devices as well as reduce the cost of manufacturing, packaging, and instrumenting these devices [7].

5.1 IC Fabrication

Any discussion of MEMS requires a basic understanding of IC fabrication technology, or microfabrication, the primary enabling technology for the development of MEMS. The major steps in IC fabrication technology are film growth, doping, lithography, etching, dicing, and packaging.

Film growth: Usually, a polished Si wafer is used as the substrate, on which a thin film is grown. The film, which may be epitaxial Si, Si02, silicon nitride (Si3N4), polycrystalline Si {polysilicon), or metal, is used to build both active or passive components and interconnections between circuits.

Doping: To modulate the properties of the device layer, a low and controllable level of an atomic impurity may be introduced into the layer by thermal diffusion or ion implantation.

Lithography: A pattern on a mask is then transferred to the film by means of a photosensitive (i.e., light sensitive) chemical known as a photoresist. The process of pattern generation and transfer is called photolithography. A typical mask consists of a glass plate coated with a patterned chromium (Cr) film.

Etching: Next is the selective removal of unwanted regions of a film or substrate for pattern delineation. Wet chemical etching or dry etching may be used. Etch-mask materials are used at various stages in the removal process to selectively prevent those portions of the material from being etched. These materials include Si02, Si3N4, and hard-baked photoresist.

Dicing: The finished wafer is sawed or machined into small squares, or dice, from which electronic components can be made.

Packaging: The individual sections are then packaged, a process that involves physically locating, connecting, and protecting a device or component. MEMS design is strongly coupled to the packaging requirements, which in turn are dictated by the application environment.Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim

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veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.

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5.1.1 Fabrication Technologies, IC Fabrication: Sentence completion

1. Compact, quick-response devices are ....................................................................2. Thousands or millions of components can be easily fabricated by..........................3. The successful miniaturization and multiplicity of traditional electronics systems

would not have been possible if...............................................................................4. By using microfabrication, mechanical systems can................................................5. Progress in MEMS fabrication processes has.........................................................6. Building active and passive components requires...................................................7. The introduction of a low and controllable level of an atomic impurity allows

……………………………………………………………………………………………...8. The function of the photoresist is……………………………………………………….9. Pattern delineation is achieved by...........................................................................10. Physically locating, connecting, and protecting a device or component constitute

……………………………………………………………………………………………...

5.2 Bulk Micromachining and Wafer Bonding

Read the following passage and answer the questions at the end

Bulk micromachining is an extension of IC technology for the fabrication of 3D structures. Bulk micromachining of Si uses wet- and dry-etching techniques in conjunction with etch masks and etch stops to sculpt micromechanical devices from the Si substrate. The two key capabilities that make bulk micromachining a viable technology are:

1) Anisotropic etchants of Si, such as ethylene-diamine and pyrocatechol (EDP), potassium hydroxide (KOH), and hydrazine (N2H4). These preferentially etch single crystal Si along given crystal planes.

2) Etch masks and etch-stop techniques that can be used with Si anisotropic etchants to selectively prevent regions of Si from being etched. Good etch masks are provided by Si02 and Si3N4, and some metallic thin films such as Cr and Au (gold).

A drawback of wet anisotropic etching is that the microstructure geometry is defined by the internal crystalline structure of the substrate. Consequently, fabricating multiple, interconnected micromechanical structures of free-form geometry is often difficult or impossible. Two additional processing techniques have extended the range of traditional bulk micromachining technology: deep anisotropic dry etching and wafer bonding. Reactive gas plasmas can perform deep anisotropic dry etching of Si wafers, up to a depth of a few hundred microns, while maintaining smooth vertical sidewall profiles. The other technology, wafer bonding, permits a Si substrate to be attached to another substrate, typically Si or glass. Used in combination, anisotropic etching and wafer bonding techniques can construct 3D complex microstructures such as microvalves and micropumps.

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5.3 Surface Micromachining

Surface micromachining enables the fabrication of complex multicomponent integrated micromechanical structures that would not be possible with traditional bulk micromachining. This technique encases specific structural parts of a device in layers of a sacrificial material during the fabrication process. The substrate wafer is used primarily as a mechanical support on which multiple alternating layers of structural and sacrificial material are deposited and patterned to realize micromechanical structures. The sacrificial material is then dissolved in a chemical etchant that does not attack the structural parts. The most widely used surface micromachining technique, polysilicon surface micromachining, uses Si02 as the sacrificial material and polysilicon as the structural material.

At the University of Wisconsin at Madison, polysilicon surface micromachining research started in the early 1980s in an effort to create high-precision micro pressure sensors. The control of the internal stresses of a thin film is important for the fabrication of microelectromechanical structures. The microelectronic fabrication industry typically grows polysilicon, silicon nitride, and silicon dioxide films using recipes that minimize time. Unfortunately, a deposition process that is optimized to speed does not always create a low internal stress film. In fact, most of these films have internal stresses that are highly compressive (tending to contract). A freestanding plate of highly compressive polysilicon that is held at all its edges will buckle (i.e., collapse or give way). This is highly undesirable. The solution is to modify the film deposition process to control the internal stress by making it stress-free or slightly tensile.

One way to do this is to dope the film with boron, phosphorus, or arsenic. However, a doped polysilicon film is conductive, and this property may interfere with the mechanical devices incorporated electronics. Another problem with doped polysilicon is that it is roughened by hydrofluoric acid (HF), which is commonly used to free sections of the final mechanical device from the substrate. Rough polysilicon has different mechanical properties than smooth polysilicon. Therefore, the amount of roughening must be taken into account when designing the mechanical parts of the micro device.

A better way to control the stress in polysilicon is through post annealing, which involves the deposition of pure, fine-grained, compressive (i.e., can be compressed) polysilicon. Annealing the polysilicon after deposition at elevated temperatures can change the film to be stress-free or tensile. The annealing temperature sets the film's final stress. After this, electronics can then be incorporated into polysilicon films through selective doping, and hydrofluoric acid will not change the mechanical properties of the material.

Deposition temperature and the film's silicon to nitride ratio can control the stress of a silicon nitride (Si3N4) film. The films can be deposited in compression, stress-free, or in tension [6].

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Deposition temperature and post annealing can control silicon dioxide (Si02) film stress. Because it is difficult to control the stress of Si02 accurately, Si02 is typically not used as a mechanical material by itself, but as electronicisolation or as a sacrificial layer under polysilicon.

5.4 Micromolding

In the micromolding process, microstructures are fabricated using molds to define the deposition of the structural layer. The structural material is deposited only in those areas constituting the microdevice structure, in contrast to bulk and surface micromachining, which feature blanket deposition of the structural material followed by etching to realize the final device geometry. After the structural layer deposition, the mold is dissolved in a chemical etchant that does not attack the structural material. One of the most prominent micromolding processes is the LICA process. LICA is a German acronym standing for lithographie, galvanoformung, und abformung (lithography, electroplating, and molding). This process can be used for the manufacture of hiqh-aspect-ratio 3D microstructures in a wide variety of materials, such as metals, polymers, ceramics, and glasses. Photosensitive polyimides are also used for fabricating plating molds. The photolithography process is similar to conventional photolithography, except that polyimide works as a negative resist.

6. Bulk Micromachining, Wafer Bonding, Surface micromachining, Micromolding: Open questions

1. How is bulk micromachining made possible?2. How is that fabricating multiple, interconnected structures of free-form geometry

is often difficult or impossible?3. How can 3D complex microstructures be constructed?4. What are multiple alternating layers of structural and artificial material deposited

on?5. What did polysilicon surface micromachining research aim at?6. How are internal film stresses important in the fabrication of MEM structures?7. How does doping the film with boron, phosphorus, or arsenic help?8. What factor should be specially considered in film doping?9. What is the deposition of pure, fine-grained, compressive polysilicon called?

What is the point in using it?10. How is micromolding different from bulk and surface micromachining?11. What is another advantage of micromolding?12. Which process is mostly useful for manufacturing high-aspect- ratio 3D

microstructures?

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7. Applications of MEMS

Here are some examples of MEMS technology. Try to fill in the blanks with your own words or phrases.

7.1 Pressure SensorsMEMS pressure microsensors typically have a flexible diaphragm that deforms_________ the presence of a pressure difference. The deformation is converted_________ an electrical signal appearing_________ the sensor output. A pressure sensor can be used_________ sense the absolute air pressure_________ the intake manifold of an automobile engine; so that the amount of fuel required _________ each engine cylinder can be computed. _________ this example, piezoresistors sliver of Ge are patterned_________ the edges of a region where a silicon diaphragm will be micromachined. The substrate is etched _________ create the diaphragm. The sensor die is then bonded _________ a glass substrate, creating a sealed vacuum cavity_________ the diaphragm. The die is mounted _________ a package, where the topside of the diaphragm is exposed to the environment. The change in ambient pressure forces the downward deformation of the diaphragm, resulting_________ a change of resistance of the piezoresistors. _________ -chip electronics measure the resistance change, which causes a corresponding voltage signal_________ appear_________ the output pin of the sensor package.

7.2 Accelerometers

Accelerometers are acceleration sensors. An inertial mass suspended _________ springs is acted_________ by acceleration forces that cause the mass to be deflected_________ its initial position. This deflection is converted _________ an electrical signal, which appears_________ the sensor output. The application of MEMS technology_________ accelerometers is a relatively new development.

7.3 Inertial Sensors

Inertial sensors are a type of accelerometer and are one of the principal commercial products that utilize surface micromachining. They are used_________ airbag-deployment sensors_________ automobiles, and_________ tilt or shock sensors. The application of these accelerometers to inertial measurement units (IMUs) is limited_________ the need to manually align and assemble them _________ three-axis systems, and_________ the resulting alignment tolerances, their lack of_________ -chip analog-to-digital conversion circuitry, and their lower limit of sensitivity. A three-axis force-balanced accelerometer has been designed _________ the University of California, Berkeley, _________ overcome some of these limitations. The accelerometer was designed_________ the integrated MEMS/CMOS technology. This technology involves a manufacturing technique where a single-level

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(plus a second electrical interconnect level) polysilicon micromachining process is integrated_________ 1.25-micron CMOS.

7.4 Microengines

A three-level polysilicon micromachining process has enabled the fabrication of devices_________ increased degrees of complexity. The process includes three movable levels of polysilicon, each separated_________ a sacrificial oxide layer, plus a stationary level. Operation of the small gears_________ rotational speeds greater than 300,000 rpm has been demonstrated. Microengines can be used _________ drive the wheels of microcombination locks. Thev can also be used _________combination _________a microtransmission _________drive a pop-up mirror_________ _________a plane. This device is known _________a micromirror.

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