Gaetano L Episcopo Introduction to MEMS
What are MEMS? Micro Electro Mechanichal Systems MEMS are integrated devices, or systems of devices, with microscopic parts, such as: Mechanical Parts Electrical Parts MEMS devices have typical sizes from micrometer to centimeter with individual features of a few micrometers or less. 2
Acronyms Micro Electro Mechanichal Systems Small size, microfabricated structures Electrical signal / control (IN/OUT) Mechanical functionality (IN/OUT) Structures, devices, systems control In Europe and USA, the acronym MST (Micro-System Technology) is also used. 3
MEMS: Definitions MEMS is an engineering discipline that studies the design and fabrication of micrometer to centimeter scale mechanical systems. MEMS devices are in widespread use, and are often referred to as solid state sensor and actuators, or solid state transducers. MEMS fabrication is commonly referred to as Micromachining. MEMS design is often referred to as micro-systems Engineering. 4
MEMS: Why? An effort to miniaturize sensors and actuators for the purposes of: Reducing size, weight, energy consumption, and fabrication cost Integrating micromachines and microelectronics on the same chip Replacing electronics with mechanical equivalent In many cases, obtain better device performance than macro equivalent Making small things is new and cool, but not always the best solution 5
MEMS: Properties Micro Electro Mechanichal Systems: Integrated microdevices or systems combining electrical and mechanical components. Fabrication using integrated circuits (ICs) compatible batch-processing techniques and silicon-based technologies. Size from micrometers to millimeters. Sensing, computation and actuation onto a single silicon die. Combination of two or more of the following: electrical, mechanical, optical, chemical, biological, magnetic or other properties, integrated onto a single or multichip hybrid 6
MEMS allow us to create artificial systems that are on the same scale and functionality as insects. MEMS: Dimensions 7
MEMS: scale of objects 8
Sensing Processing - Actuation The combination of Sensors and Actuators with Integrated Circuits completes a loop allowing completely interactive systems. INPUTS Eyes and ears PROCESSING Brains OUTPUTS Hands and mouth Sensors Circuits Actuators 9
Sensing Processing - Actuation The combination of Sensors and Actuators with Integrated Circuits completes a loop allowing completely interactive systems. Physical event Sensor Processing Actuation Physical response Micro-Electro-Mechanical System 10
MEMS: Products Microwave and Wireless Switches Filters Components Power sensors Pressure MAP sensors Microphones Medical and Biological Lab on a Chip DNA analysis Chem/Bio Detection Drug Delivery Ink Jet Printers Thermal Ink-Jets Inertial Accelerometers Gyroscopes Optics Projection Displays Laser Printers Switching Networks Tunable Lasers Filters 11
MEMS: Industrial applications Automotive Industry: pressure sensors (engine oil pressure, vacuum pressure, fuel injection pressure, tire pressure, stored air bag pressure), accelerometers (triggering of air bag, locking seat belt) and temperature sensors (to monitor oil, antifreeze and air temperature) Controls for Industry or Home: sensors to measure external environment and actuators for adjustment Instrumentation and Control Industry uses MEMS devices which sense pressure, temperature, acceleration and proximity. 12
MEMS: Manufactures 13
Applications: Automotive 14
Applications: Medicine 15
MEMS: Biotechnology Examples of MEMS applications in Biotechnology: Polymerase Chain Reaction (PCR) microsystems for DNA amplification and identification; Enzyme linked immunosorbent assay (ELISA); Capillary electrophoresis; Electroporation; Micromachined Scanning Tunneling Microscopes (STMs); Biochips for detection of hazardous chemical and biological agents. Microsystems for high-throughput drug screening and selection. 16
Applications: Aeronautics Pressure sensor belt on jet planes 17
Applications: Gyroscopes Micromachined gyroscopes applications. 18
Applications: Nintendo Wii 19
Applications: smart-phone 20
MEMS: Resources Books: Microsensors Principles and applications J. W. Gardner 1994, Wiley Micromechanics and MEMS Classic and Seminal Papers to 1990 Edited by William S.Trimmer 1997, IEEE PRESS Fundamentals of Microfabrication M. Madou 1997, CRC Press LLC Micromachined Transducers Sourcebook G. T. A. Kovacs 1998, McGraw-Hill Micro Mechanical Systems Principles and Technology Handbook of Sensors and Actuators, Vol.6 T. Fukuda and W. Menz 1998, ELSEVIER Scientific Journals: IEEE/ASME, Journal of Microelectromechanical Systems Elsevier, Sensors and Actuators IoP, Journal of Micromechanics and Microengineering IEEE, Sensors Journal International Scientific Conferences: IEEE MEMS SPIE Smart Structures and Systems Smart electronics and MEMS Eurosensors IEEE Sensors Websites: http://www.memsnet.org http://www.memscap.com 21
MEMS: Fabrication MEMS are fabricated using integrated circuits (ICs) compatible batch-processing techniques and silicon-based technologies. 22
MEMS fabrication: Advantages 23
MEMS and Microelectronics (IC) Interactions with the environment: Microelectronics generally interacts with "information" such as signals or streams of electrons; MEMS interacts with a wide variety of physical quantities (fluid, acceleration, optics, electromagnetic waves, etc.). Structural dimensionality: Microelectronic systems are "predominantly" two-dimensional (the structure is built in a layer thinner than the thickness of the entire substrate). MEMS are inherently three-dimensional (Some properties are developed for more than 100 micrometers in depth). Fabrication technology: Although Microsystems were originally developed on the same silicon substrates for microelectronic circuits, today other substrates and different technologies are adopted. 24
MEMS: Fabrication 25
MEMS: Fabrication MEMS fabrication techniques can be categorized as follows: Surface Micromachining (structures made from single or multiple films that are patterned) Bulk Micromachining (structures made from chemically etched bulk material) Micromolding (structures made using molds, stereo lithography, milling) Patterning and shaping, in the above techniques, is usually accomplishedthrough: Photolithography Chemical Etching 26
Silicon as structural material The monocrystalline silicon structure has a face-centered cubic (diamondlattice, lattice constant = 5.43 Å). Each atom is placed at the center of a tetrahedron and is associated with covalent bond, four equidistant atoms. To allow easy identification of the crystallographic planes, "Miller Indices" (inverse of the intersection between the plane and the unit vectors of the reference system) are used. The surface of the silicon wafer is contained in a three main floors. Conventions for the recognition of the type of substrate are adopted. 27
Other materials for MEMS Polycrystalline Silicon or Polysilicon: used as both a strucutral material and conductive material, piezo-resistive properties. Silicon Dioxide (SiO 2 ): used as an electrical isolant and in some cases as a structural or sacrificial layer. Silicon Nitride (Si 3 N 4 ): used as an electrical isolant and in some cases as a structural or sacrificial layer. Metals (Alluminium, Platinum, Gold, Nickel, Tungsten, etc ): used as an electrical conductors, optical reflective, thermo-mechanical transducer and in some cases as structural materials. PZT: used for piezo-electrical conversions. 28
Silicon micro-fabrication processes Classification by type of material Metal (Al, Au, Cu, Ti, Ni, Pt, etc.) Non-metal inorganic material (SiO 2, Si 3 N 4, SiC, compound nitride materials) Classification by material addiction/subtraction Deposition process Photoresist Metal evaporation Oxidation Sputtering CVD Anisotropic etching Isotropic etching Bulk/film etching Sacrifical Classification by process temperature Examples: Oxidation:950-110 C Annealing: 850-110 C Si 3 N 4 LPCVD: 780 C Polysilicon LPCVD: 580-650 C Low temp. oxide LPCVD: about 550 C Plasma deposition: about 350 C Spin-coating: room temperature Classification by size: Thin film (thickness < 10 μm) Thick film Bulk 29
Deposition processes Addictive processes to deposit material on a layer: Spin-on films Polyimide (PI), Photoresist (PR) Spin-on glass (SOG) Physical Vapor Deposition (PVD) Evaporation Sputtering Chemical Vapor Deposition (CVD) Thermal Oxidation (wet/dry) CVD (Atmopheric Pressure) Low Pressure CVD (LPCVD) Plasma Enhanced CVD (PECVD) 30
Deposition processes: Spin-on films Spin-on films or Spin Casting Thin film material is dissolved in a volatile liquid solvent, spin coated onto a substrate to form films due to centrifugal force. Examples: Polyimide (PI), Photoresist (PR) Spin-on glass (SOG) 31
Deposition processes: PVD Physical Vapor Deposition (PVD) Evaporation The material to be deposited is heated by resistive, inductive, or electron beam and led to gaseous state. Condensation of the evaporated gas, on high-vacuum chambers, induces the deposition of the layer. Sputtering High-energy ion beams (plasma) are used to remove atoms from the surface of the source material (material to be deposited) to create a layer. 32
Deposition processes: CVD Chemical Vapor Deposition (CVD) Deposition on a substrate of a solid layer as a result of a chemical reaction between the substrate material and the gases in the atmosphere. Thermal Oxidation (wet/dry) CVD (Atmopheric Pressure) Low Pressure CVD (LPCVD) Plasma Enhanced CVD (PECVD) CVD - LPCDV PECVD 33
Photolithography Technique of transferring a geometric two-dimensional pattern to a surface. It is divided into six phases: (a) uniform deposition of a layer of structural material where to tranfer the pattern, (b) deposition of photoresist, (c) selective exposure to UV light through mask layout, (d) selective removal of the photoresist (e) selective removal of structural material, (f) final removal of residual photoresist. 34
Photolithography: Layout Masks Layout masks define the geometry / pattern and size of the device. The type of photoresist (positive/negative) used in the process of photolithography affects the definition of the layout masks. Positive Photoresist: becomes soluble in exposed areas the material is etched in the areas covered by the mask transferred pattern is the same of the mask. Negative Photoresist: become insoluble in the exposed areas material is excavated in the areas uncovered by the mask transferred pattern is the negative of the mask. 35
Substraction processes Etching process: process of selectively removing material The selectivity is determined by using appropriate photolithographic masks together with masking materials. The selectivity with respect to the materials is characterized by the etch-rate (rate etching, for a given chemical etchant, is characteristic of the material). Classification of etching processes By depth: Surface etching (removal of thin films from the surface of the wafer) Bulk etching (removal of part of the substrate) By surface: Top (etch from the upper surface of the wafer) Bottom (etch from the bottom surface of the wafer) By etch-rate direction: Isotropic Anisotropic 36
Wet etch by KOH Dry etch by RIE Substraction processes: wet/dry etch Wet etch: removal by chemical attack (by KOH or TMAH) in the liquid phase. Dry etch: material removal occurs by reaction with a gas in "vapor-phase" or "plasma-phase" etching. 37
Sacrificial etching Sacrificial layer Substrate 1 4 Polysilicon 2 5 3 6 38
Tipical micro-fabrication process Starting Silicon Bulk Substrate Starting silicon substrate for supporting purpose Material Deposition Deposition of material with a defined thickness Photolitography Two-dimensional patterning of the previously deposited material Final Etching Final etching to realize suspended structures 39
Surface Micromachining 40
Bulk Micromachining Example on Silicon-On-Insulator (SOI) substrate 41