Τεχνική Μικροσυστημάτων
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Μικροηλεκτρομηχανικά Συστήμτα (ΜΗΜΣ) είναι η τεχνολογία των πολύ μικρών, και συγχωνεύεται στα νανομεγέθη στα "Νανοηλεκτρομηχανικά Συστήματα" (ΝΗΜΣ) και στη Νανοτεχνολογία. Στην Ευρώπη, τα ΜΗΜΣ συναντώνται συνήθως ως Τεχνολογία Μικροσυστημάτων. Δεν πρέπει να μπερδεύονται με το υποθετικό όραμα της Μοριακής Νανοτεχνολογίας ή των Μοριακών Ηλεκτρονικών.
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[Επεξεργασία] Εισαγωγή
MEMS devices refer to mechanical components on the micrometre size and include 3D lithographic features of various geometries. They are typically manufactured using planar processing similar to semiconductor processes such as surface micromachining and/or bulk micromachining. These devices generally range in size from a micrometre (a millionth of a metre) to a millimetre (thousandth of a metre). At these size scales, a human's intuitive sense of physics do not always hold true. Due to MEMS' large surface area to volume ratio, surface effects such as electrostatics and wetting dominate volume effects such as inertia or thermal mass. They are fabricated using modified silicon fabrication technology (used to make electronics), molding and plating, wet etching (KOH, TMAH) and dry etching (RIE and DRIE), electro discharge machining (EDM), and other technologies capable of manufacturing very small devices. MEMS sometimes go by the names micromechanics, micro machines, or micro system technology (MST).
Οι εταιρείες με ισχυρή παρουσία στον τομέα των ΜΗΜΣ έχουν διάφορα μεγέθη. Οι μεγαλύτερες εταιρείες ειδικεύονται στην κατασκυή μεγάλων ποσοτήτων φθηνών εξαρτημάτων or packaged solutions for end markets όπως αυτοκίνητα, βιοϊατρική και ηλεκτρονικά. Οι επιτυχημένες μικρές εταιρείες provide value in innovative solutions and absorb the expense of custom fabrication με υψηλά περιθώρια κέρδους. In addition, και οι μεγάλες και οι μικρές εταιρείες κάνουν έρευνα ώστε να εξερευνήσουν τεχνολογίες ΜΗΜΣ. Η πολυπλοκότητα και η απόδοση των προηγμένων αισθητήρων που βασίζονται στα ΜΗΜΣ παριγράφονται με βάση τις διαφορετικές γεννιές αισθητήρων ΜΗΜΣ.
Μερικές εφαρμογές είναι οι ακόλουθες:
- Εκτυπωτές ψεκασμού, οι οποίοι χρησιμοποιούνε πιεζοηλεκτρικό ψεκασμό ή ψεκασμό φούσκας (bubble) για να αφήσουν μελάνι στο χαρτί.
- Αισθητήρες επιτάχυνσης σε σύγχρονα αυτοκίνητα για την ενεργοποίηση των αερόσακων σε περίπτωση σύγκρουσης.
- Γυροσκόπια ΜΗΜΣ χρησιμοποιούνται σε μοντέρνα αυτοκίνητα and other applications to detect yaw; e.g. to deploy a roll over bar or trigger dynamic stability control.
- Αισθητήρες πίεσης π.χ. αισθητήρες πίεσης ελαστικών, και αναλώσιμους αισθητήρες αρτηριακής πίεσης.
- Displays e.g the DMD chip in a projector based on DLP technology has on its surface several hundred thousand micromirrors.
- Optical switching technology which is used for switching technology for data communications, and is part of the emerging technology of smartdust.
Finite element analysis is an important part of MEMS design.
[Επεξεργασία] Υλικά ΜΗΜΣ
Η τεχνολογία των ΜΗΜΣ μπορεί να πραγματοποιηθεί χρησιμοποιώντας έαν αριθμό διαφορετικών υλικών και τεχνικές κατασκευής, η επιλογή των οποίων θα εξερτηθεί από τη συσκευή που θα κατασκευαστεί και τον τομέα της αγοράς μέσα στον οποίο πρέπει να λειτουργήσει.
[Επεξεργασία] Σιλικόνη
Η Σιλικόνη είναι το υλικό που χρησιμοποιείται στη δημιουργία σχεδόν όλων των ολοκληρομένων κυκλωμάτων που χρησιμοποιούνται στα καταναλωτικά ηλεκτρονικά στο σύγχρονο κόσμο. The economies of scale, ready availability of highly accurate processing and ability to incorporate electronic functionality make silicon attractive for a wide variety of MEMS applications. Silicon also has significant advantages engenderred through its material properties. In single crystal form, silicon is an almost perfect Hookean material, meaning that when it is flexed there is virtually no hysteresis and hence almost no energy dissipation. As well as making for highly repeatable motion, this also makes silicon very reliable as it suffers very little fatigue and can have service lifetimes in the range of billions to trillions of cycles without breaking. The basic techniques for producing all silicon based MEMS devices are deposition of material layers, patterning of these layers by lithography and then etching to produce the required shapes.
[Επεξεργασία] Πολυμερή
Even though the electronics industry provides an economy of scale for the silicon industry, crystaline silicon is still a complex and relatively expensive material to produce. Polymers on the other hand can be produced in huge volumes, with a great variety of material characteristics. MEMS devices can be made from polymers by processes such as injection moulding, embossing or stereolithography and are especially well suited to microfluidic applications such as disposable blood testing cartridges.
[Επεξεργασία] Μέταλλα
Μέταλλα μπορούν επίσης να χρησιμοποιηθούν στην κατασκευή κομματιών ΜΗΜΣ. Παρόλο που δεν έχουν κάποια από τα πλεονεκτήματα της σιλικόνης, όσον αφορά τις μηχανικές ιδιότητες, όταν χρησιμοποιούνται μέσα στα όριά τους, τα μέταλλα μπορούν να φανούν πολύ αξιόπιστα.
Τα μέταλλα μπορούν να deposited με electroplating, evaporation, και sputtering processes.
Συνήθως χρησιμοποιούμενα μέταλλα είναι ο Χρυσός, το Νικέλιο, το Αλουμίνιο, το Χρώμιο, το Τιτάνιο, το Tungsten, η Πλατίνα, και το Ασήμι.
[Επεξεργασία] Μέθοδοι επεξεργασίας ΜΗΜΣ
[Επεξεργασία] Επεξεργασία Καθίζησης
One of the basic building blocks in MEMS processing is the ability to deposit thin films of material. In this text we assume a thin film to have a thickness anywhere between a few nanometer to about 100 micrometer.
[Επεξεργασία] Φωτολιθογραφία
Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive material by selective exposure to a radiation source such as light. A photosensitive material is a material that experiences a change in its physical properties when exposed to a radiation source. If we selectively expose a photosensitive material to radiation (e.g. by masking some of the radiation) the pattern of the radiation on the material is transferred to the material exposed, as the properties of the exposed and unexposed regions differs.
This exposed region can then be removed or treated providing a mask for the underlying substrate. Photolithography is typically used with metal deposition, wet and dry etching.
[Επεξεργασία] Επεξεργασία Σκαψίματος
There are two basic catagories of etching process wet and dry etching Wet etching is where the material is dissolved when immersed in a chemical solution and dry etching is where the material is sputtered or dissolved using reactive ions or a vapor phase etchant.
[Επεξεργασία] Βρεγμένο Σκάψιμο
This is the simplest etching technology. All it requires is a container with a liquid solution that will dissolve the material in question. Unfortunately, there are complications since usually a mask is desired to selectively etch the material. One must find a mask that will not dissolve or at least etches much slower than the material to be patterned. Secondly, some single crystal materials, such as silicon, exhibit anisotropic etching in certain chemicals. Anisotropic etching in contrast to isotropic etching means different etch rates in different directions in the material. The classic example of this is the <111> crystal plane sidewalls that appear when etching a hole in a <100> silicon wafer in a chemical such as potassium hydroxide (KOH). The result is a pyramid shaped hole instead of a hole with rounded sidewalls with a isotropic etchant. The principle of anisotropic and isotropic wet etching is illustrated in the figure below.
[Επεξεργασία] Σκάψιμο με αντιδρόντα ιόντα
In reactive ion etching (RIE), the substrate is placed inside a reactor in which several gases are introduced. A plasma is struck in the gas mixture using an RF power source, breaking the gas molecules into ions. The ions are accelerated towards, and reacts at, the surface of the material being etched, forming another gaseous material. This is known as the chemical part of reactive ion etching. There is also a physical part which is similar in nature to the sputtering deposition process. If the ions have high enough energy, they can knock atoms out of the material to be etched without a chemical reaction. It is a very complex task to develop dry etch processes that balance chemical and physical etching, since there are many parameters to adjust. By changing the balance it is possible to influence the anisotropy of the etching, since the chemical part is isotropic and the physical part highly anisotropic the combination can form sidewalls that have shapes from rounded to vertical. A schematic of a typical reactive ion etching system is shown in the figure below.
[Επεξεργασία] Βαθύ Σκάψιμο με αντιδρόντα ιόντα
A special subclass of RIE which continues to grow rapidly in popularity is deep RIE (DRIE). In this process, etch depths of hundreds of microns can be achieved with almost vertical sidewalls. The primary technology is based on the so-called "Bosch process", named after the German company Robert Bosch which filed the original patent, where two different gas compositions are alternated in the reactor. The first gas composition creates a polymer on the surface of the substrate, and the second gas composition etches the substrate. The polymer is immediately sputtered away by the physical part of the etching, but only on the horizontal surfaces and not the sidewalls. Since the polymer only dissolves very slowly in the chemical part of the etching, it builds up on the sidewalls and protects them from etching. As a result, etching aspect ratios of 50 to 1 can be achieved. The process can easily be used to etch completely through a silicon substrate, and etch rates are 3-4 times higher than wet etching.
[Επεξεργασία] Μικρομηχανική σε μεγάλες ποσότητες
Main article: Bulk micromachining
Bulk micromachining is similar to deep etching but uses a different process to remove silicon. Bulk micromachining typically uses alkaline liquid solvents, such as potassium hydroxide, to dissolve silicon which has been left exposed by the photolithography masking step. These alkali solvents dissolve the silicon in a highly anisotropic way, with some crystallograpic orientations dissolving up to 1000 times faster than others. Such an approach is often used with very specific crystallographic orientations in the raw silicon to produce v-shaped grooves. The surface of these grooves can be atomically smooth if the etch is carried out correctly with dimensions and angles being extremely accurate.
Metals are also often used as masks for dry and wet etching other materials depending on the selectivity of the metal to the etchant.
[Επεξεργασία] Δείτε επίσης
- NEMS, Nanoelectromechanical systems are similar to MEMS but smaller
- MOEMS, Micro Opto-Electrical-Mechanical Systems, MEMS including optical elements
- IBM Millipede, a MEMS technology for non-volatile data storage of more than a terabit per square inch
- Texas Instruments pioneers of the DMD chip
- ADI one of the major early players in accelerometer development
- Lucent who developed highly advanced optical telecommunications switches
- Cantilever one of the most common forms of MEMS.
[Επεξεργασία] Εργαστήρια ΜΗΜΣ and Vendors
- U. C. Berkeley Sensor and Actuator Center
- The MEMS and Nanotechnology Exchange - A MEMS resource, fabrication and design company
- MEMS laboratory at Carnegie-Mellon
- Sensirion Inc., MEMS sensor manufacturer
