SEMINAR ON MICRO-ELECTRO-MECHANICAL SYSTEMS (MEMS) A.RADHAKRISHNAN MECHANICAL ENGINEERING, CPT COLLEGE THIRUVANANTHAPURAM PART-1

2 MEMS  What is MEMS Technology ?  What are MEMS ?  Design  Fabrication  Applications

3 What is MEMS Technology?  Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through micro- fabrication technology.  While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes).  The micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.

4 What are MEMS?  Micro-electro-mechanical systems.  Very small mechanical assemblies.  Hinged plate  Hinge close-up

5 Micro-Electro-Mechanical Systems (MEMS)  MEMS promises to revolutionize nearly every product category by bringing together silicon-based microelectronics with micromachining technology, making possible the realization of complete systems-on-a-chip.  MEMS is an enabling technology allowing the development of smart products, augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators and expanding the space of possible designs and applications.

6 Micro-Electro-Mechanical Systems (MEMS)  How small exactly? The photo above shows a silicon motor together with a strand of human hair. MEMS systems are measured in micrometers (microns), that is, millionths of a meter. The diameter of human hair is about 100 microns. silicon motor together with a strand of human hair

7 Micro-Electro-Mechanical Systems (MEMS)  Microelectronic integrated circuits can be thought of as the "brains" of a system and MEMS augments this decision-making capability with "eyes" and "arms", to allow microsystems to sense and control the environment.  Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena.

8 Micro-Electro-Mechanical Systems (MEMS)  The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose. Because MEMS devices are manufactured using batch fabrication techniques similar to those used for integrated. MEMS micromotor

9 Micro-Electro-Mechanical Systems (MEMS)  Microelectromechanical systems (MEMS) consist of extremely tiny mechanical elements, often integrated together with electronic circuitry.  MEMS devices are made in a similar fashion to computer microchips. The advantage of this manufacturing process is not simply that small structures can be achieved but also that thousands or even millions of system elements can be fabricated simultaneously. This allows systems to be both highly complex and extremely low-cost.

10 What is Micro-engineering ? Microengineering refers to the technologies and practice of making three dimensional structures and devices with dimensions in the order of micrometers. The two constructional technologies of microengineering are microelectronics and micromachining. Micro-electronics, producing electronic circuitry on silicon chips, is a very well developed technology. Micromachining is the name for the techniques used to produce the structures and moving parts of microengineered devices.

11 What is Micro-engineering ? One of the main goals of Micro-engineering is to be able to integrate microelectronic circuitry into micromachined structures, to produce completely integrated systems (microsystems). Such systems could have the same advantages of low cost, reliability and small size as silicon chips produced in the microelectronics industry. When considering such small devices, a number of physical effects have different significance on the micrometer scale compared to macroscopic scales. Interest in microengineering has spawned or renewed interest in a number of areas dealing with the study of these effects on microscopic scales. This includes such topics as micromechanics, which deals with the moving parts of microengineered devices, and microfluidics, etc.

12 What is Micro-engineering ? The some of micromachining techniques that are in use / under development. Silicon micromachining is given most prominence, since this is one of the better developed micromachining techniques. Silicon is the primary substrate material used in the production microelectronic circuitry (ie, bettersilicon chips), and so is the most suitable candidate for the eventual production of microsystems. The Excimer laser is an ultraviolet laser which can be used to micro machine a number of materials without heating them, unlike many other lasers which remove material by burning or vaporising it. The Excimer laser lends itself particularly to the machining of organic materials (polymers, etc).

13 MUMPS Design Rules Separation Cut outCut in Enclose 14 mandatory design rules, 4 classes

14 Behavior  Displacement scales as linearly  Capacitance scales as size 2  Mass scales as size 3  (Forget most of what you know about electromechanical systems).

15 Design Tools  L-Edit (Tanner)  extends SPICE for mechanical analysis.  MEMCad (Microcosm).  VHDL  in the works, but not ready yet.

16 Silicon Wafer  A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuit and other micro devices.  The wafer serves as the substrate for microelectronic devices built in and over the wafer and undergoes many micro fabrication process steps such as doping or ion implantation, etching, deposition of various materials,and photolithographic patterning.

17 Fabrication  Bulk micro-machining  etch into the substrate.  Surface micro-machining  build up layers above the substrate and etch.  LIGA  deep structures.

18 Bulk micro-machining  Pressure sensing diaphragm example.  Etch with KOH (potassium hydroxide).  Etch is anisotropic.  stopped in (1,1,1) direction  SiC also stops etch and forms diaphragm.

19 Bulk micro-machined example

20 Surface micromachining  Specifically - MUMPS (Multi-User MEMS Processes).  MUMPS has been called “MOSIS (Metal Oxide Semiconductor Implementation Service) for MEMS”.  Three poly layer process derived from Berkeley Sensors and Actuators Center  Seven total material layers.  N mask steps.

21 Processing Layers (micro motor)  7 material layers/  Isolation (Nitride)  conductor (poly)  1st sacrificial (oxide)  1st structural (poly)  2nd sacrificial (oxide)  2nd structural (poly)  metal

22 Patterning Poly 0  Initial layers:  substrate/nitride/poly0/ resist.  Expose resist.  Dissolve exposed resist.  Etch poly0.  Wash remaining resist.

23 First Sacrificial (oxide) Layer  This layer will eventually be dissolved. Its purpose is to support the structural layer (poly1) above it.  Note: all layers are conformal.  Two timed etches - short for dimples, longer for anchor.

24 First structural (poly1) layer  Deposit 2  m of polysilicon.  Coat with resist, expose.  Etch with RIE (reactive ion etch).

25 2nd sacrificial (oxide) layer  Deposit 0.75  m oxide.  Two etch steps - first contacts poly1, second contacts substrate.

26 2nd structural (poly2) layer + metal  Deposit second poly layer.  Apply resist and etch (RIE).  Apply resist and etch.  Deposit metal, gold in MUMPS.

27 Released mechanism  Rinse in solvent to remove resist and overlying metal.  Soak 2 minutes in concentrated HF to dissolve sacrificial oxide layers. Micro motor

28 LIGA LIGA is a technique that can be used to produce moulds for the fabrication of micromachined components. Microengineered components can be made from a variety of materials using this technique, however it does suffer the disadvantage that currently the technique requires X-rays from a synchrotron source. LIthograpie, Galvanoformung, Abformung A quick introduction to mask design is provided following discussion of techniques and structures, rather than directly following the photolithography section. This is so that we are able to become acquainted with the concept of creating structures by sequential photolithography and machining steps first, which hopefully makes it easier to understand what mask design software is trying to achieve. synchrotron source

29 LIGA  200  m deep structures.  Coat with thick resist.  Pattern with Xrays.  Electroplate exposed area with Ni.  Machine to +-5  m.  Use titanium and Cu as sacrificial layers. LIthograpie, Galvanoformung, Abformung Deep Reactive Ion Etching

30 Photolithography Process - Steps 8) Develop inspect 5) Post-exposure bake 6) Develop7) Hard bake UV Light Mask 4) Alignment and Exposure Resist 2) Spin coat 3) Soft bake 1) Vapor prime HMDS

31  Vapour Prime  Promotes Good Photoresist- to-Wafer Adhesion.  Dehydration Bake.  Primes Wafer with Hexamethyldisilazane(HMDS). Vacuum chuck Spindle connected to spin motor To vacuum pump Photoresist dispenser Photolithography

32 MEMS DEVICES Infrared Imager (S) Inertial Measurement Unit (S) - Accelerometer - Gyros Air Velocity (S) Pressure (S) Chemical Species (S) Micromotor (A) Micromirror (A) Microbolometer, Accelerometer, Analog Device Vibration ring for gyros Micromirror/optical switch