Downloaded 111 times
More Related Content
Materials for MEMS
- 1. Chapter 3 Materials forMEMS & Microsystems 1
- 2. Outline 3.1 Introduction 3.2 Substratesand Wafers 3.3 Active Substrate Materials 3.4 Silicon as a Substrate Material 3.5 Silicon Compounds 3.6 Quartz 3.7 Piezoelectric Crystals 3.8 Polymers 3.9 Packaging Materials 3.10 Fabrication of Pressure Sensor. 2
- 3. 3.1 Introduction • Thecurrent technologies used in producing MEMS and microsystems are inseparable from those of microelectronics. • Many microsystems use microelectronics materials such as silicon, and gallium arsenide (GaAs) for the sensing or actuating elements. • These materials are chosen mainly because they are dimensionally stable and their microfabrication and packaging techniques are well established in microelectronics. • Other materials used for MEMS and microsystems products: • quartz and Pyrex, polymers and plastics, and ceramics―that are not commonly used in microelectronics. • Plastics and polymers — used extensively in the case of microsystems produced by the LIGA processes. 3
- 4. 3.2 Substrates andWafers • Substrate: • Micro fabricated devices are not generally freestanding (separate) devices but are usually formed over or in a thicker support substrate. • For electronic applications, semiconducting substrates such as silicon wafers can be used. For optical devices or flat panel displays, transparent substrates such as glass or quartz are common. • The substrate enables easy handling of the micro device through the many fabrication steps. Often many individual devices are made together on one substrate and then singulated into separated devices toward the end of fabrication. Examples: • Pressure sensors that convert the applied pressure to the deflection of a thin diaphragm that is an integral part of a silicon die cut from a silicon substrate. 4
- 5.
- 6.
- 7. 3.3 Active SubstrateMaterials 7
- 8. 3.4 Silicon asa Substrate Material • The Ideal Substrate for MEMS • Single-Crystal Silicon and Wafers • Crystal Structure • Mechanical Properties of Silicon 8
- 9. The Ideal Substratefor MEMS • It is mechanically stable and it can be integrated into electronics on the same substrate. • Silicon is almost an ideal structural material. It has about the same Young’s modulus as steel (about 2 × 105 MPa), but is as light as aluminum. • It has a melting point at 1400 , which is about twice as high as that of aluminum. This high℃ melting point makes silicon dimensionally stable even at elevated temperature. • Its thermal expansion coefficient is about 8 times smaller than that of steel, and is more than 10 times smaller than that of aluminum. 9
- 10.
- 11. 1106/25/18 Single-Crystal Silicon andWafers • The puller is slowly pulled up along with a continuous deposition of silicon melt onto the seed crystal. As the puller is pulled up, the deposited silicon melt condenses and a large bologna-shaped boule of single-crystal silicon several feet long is formed. • The diameter of the boules ranges from 100 mm to 300 mm. pure silicon crystal producing:
- 12.
- 13.
- 14.
- 15. Crystal Structure 15 Siliconhas basically a FCC unit cell. In a typical FCC (face centered cubic) crystal, atoms are situated at the eight corners of the cubic lattice structure, as well as at the center of each of the six faces.
- 16. 3.4.5 Mechanical Propertiesof Silicon 16 • Silicon is an elastic material with no plasticity or creep below 800 .℃ • It shows virtually no fatigue under all conceivable circumstance. These unique characteristics make it an ideal material for sensing and actuating in microsystems. • However, it is a brittle material. Therefore, undesirable brittle fracture behavior with weak resistance to impact loads needs to be considered in the design of such microsystems. • Another disadvantage of silicon substrates is that they are anisotropic. This makes accurate stress analysis of silicon structures tedious, since directional mechanical property must be included.
- 17. 3.5 Silicon Compounds 17 3.5.1 Silicon Dioxide 3.5.2 Silicon Carbide 3.5.3 Silicon Nitride 3.5.4 Polycrystalline Silicon
- 18. 1806/25/18 3.5.1 Silicon Dioxide •Principal uses: • as a thermal and electric insulator, • as a mask in the etching of silicon substrates, • as a sacrificial layer in surface micromachining. • Has much stronger resistance to most etchants than silicon. • Production: • heating silicon in an oxidant such as oxygen with or without steam. Chemical reactions for such processes • “dry” oxidation Si + O2 → SiO2 • “wet” oxidation Si + 2H2O → SiO2 + 2H2
- 19. 1906/25/18 3.5.2 Silicon carbide •Principal applications: • Its dimensional and chemical stability at high temperatures. • very strong resistance to oxidation even at very high temperatures. • Thin films of silicon carbide are often deposited over MEMS components to protect them from extreme temperature.
- 20. 2006/25/18 3.6 Quartz • Composition： Quartz is a compound of SiO2. • Characteristics and application ： • an ideal material for sensors because of its near absolute thermal dimensional stability. • used in many piezoelectric devices. • wristwatches, electronic filters, resonators. • Inexpensive. • Transparent to ultraviolet light, which is often used to detect the various species in the fluid. • Machine ： • diamond cutting • ultrasonic cutting http://baike.so.com/doc/5402201.html
- 21. 2106/25/18 3.7 Piezoelectric Crystals •Piezoelectric crystals are the solids of ceramic compounds that can produce a voltage when a mechanical force is applied between their faces. • The reverse situation, that is the application of voltage to the crystal, can also change its shape. • This unique material behavior is called the piezoelectric effect.
- 22.
- 23. 23 06/25/18 3.8 Polymers •Polymers, which include such diverse materials as plastics, adhesives, Plexiglas, and Lucite, have become increasingly popular materials for MEMS and microsystems. • Example: plastic cards approximately 150 mm wide containing over 1000 microchannel have been adopted in microfluidic electrophoretic systems by the biomedical industry. • Structure ： This type of material is made up of long chains of organic (mainly hydrocarbon) molecules. The combined molecules, i.e., polymer molecules, can be a few hundred nanometers long. • Properties ： Low mechanical strength, low melting point, and poor electrical conductivity characterize polymers. • Thermoplastics-easily formed to the desired shape for the specific product • Thermosets-have better mechanical strength and temperature resistance up to 350 .℃
- 24.
- 25. 2506/25/18 Usage and Advantages •Usage ： • Traditionally--used as insulators, sheathing, capacitor films in electric devices, and die pads in integrated circuits. • A special form, the plastics--widely used for machine and device components. • Advantages ： • Light weight • Ease in processing • Low cost of raw materials and processes for producing polymers • High corrosion resistance • High electrical resistance • High flexibility in structures • High dimensional stability
- 26. Polymers for MEMSand Microsystems 26 • Photoresist polymers are used to produce masks for creating desired patterns on substrates by photolithography. • The same photoresist polymers are used to produce the prime mold with the desired geometry of MEMS components in the LIGA process for manufacturing micro device components. • These prime molds are plated with metals such as nickel for subsequent injection molding for mass production of microcomponents.
- 27. 27 06/25/18 Conductive Polymers •For polymers to be used in certain applications in microelectronics, MEMS, and microsystems, they have to be made electrically conductive with superior dimensional stability. • Polymers have been used extensively in the packaging of MEMS, but they have also been used as substrates for some MEMS components in recent years with the successful development of techniques for controlling the electric conductivity of these materials. • Pyrolysis: • A pyro polymer can be made electrically conductive by adding an amine heated above 600 .℃ • conductivity-- 2.7 ×104 S/m>carbon. • Doping • Insertion of Conductive Fibers
- 28. 3.9 Packaging Materials 28 •Differences: • IC—protect the IC die and the interconnects from the often hostile operating environment. • microsystem—not only are the sensing or actuating elements to be protected, but they are also required to be in contact with the media that are the sources of actions. Many of these media are hostile to these elements. • Materials: wires made of noble metals, metal layers for lead wires, solders for die/constraint base attachments, etc., metals and plastics
- 29.
- 30. 3006/25/18 3.9 Packaging Materials •Applications: • aluminum or gold metal films—ohmic contacts, lead wires • plastic or stainless steel—casing • Glass—constraint bases. • tin-lead solder alloys or epoxy resins or RTV —adhesive • Copper and aluminum—metal layers(sputtered) • silicone gel or silicone oil—shield silicon diaphragm etc.
- 31. 3.10 Fabrication ofPressure Sensor : 31