Wednesday, 22 January 2014

Report of MEMS Technology

                                           REPORT OF MEMS TECHNOLOGY


                                                   TABLE OF CONTENTS



APPROVAL…………………………………………………………………………………………...I
DECLARATION………………………………………………………………………………..II
ACKNOWLEDGE……………………………………………………………………………..III
ABSTRACT…………………………………………………………………………………….IV
TABLE OF CONTENTS……………………………………………………………………….V
LIST OF FIGURE…………………………………………………………………………….VII

















                                                LIST OF FIGURE



















1          INTRODUCTION


Micro electromechanical systems (MEMS) is a technology of miniaturization that has been largely adopted  from  the integrated  circuit (IC)  industry and applied  to the miniaturization of all systems not only electrical systems but also mechanical, optical, fluid, magnetic, etc.

Micro Electromechanical systems or MEMS, represent an extraordinary technology that promises to transform whole industries and drive the next technological revolution. These devices can replace bulky actuators and sensors with micron-scale equivalent that can be produced in large quantities by fabrication processes used in integrated circuits photolithography. This reduces cost, bulk, weight and power consumption while increasing performance, production volume, and functionality by orders of magnitude. For example, one well known MEMS device is the accelerometer (its now being manufactured using mems low cost, small size, more reliability).

Furthermore, it is clear that current MEMS products are simply precursors to greater and more pervasive applications to come, including genetic and disease testing, guidance and navigation systems, power generation, RF devices( especially for cell phone technology), weapon systems, biological and chemical agent detection, and data storage. Micro mirror based optical switches have already proven their value; several start-up companies specializing in their development have already been sold to large network companies for hundreds of millions of dollars. The promise of MEMS is increasingly capturing the attention of new and old industrises alike, as more and more of their challenges are solved with MEMS.

MEMS are, in their most; basic forms, small versions of traditional electrical and mechanical devices – such as valves, pressure sensors, hinged mirrors, and gears with dimensions measured in microns – manufactured by techniques similar to those used in fabricating microprocessor chips. The first MEMS products were developed in the 1960s, when accurate hydraulic pressure sensors were needed for aircraft. Such devices were further refined in the 1980s when implemented in fuel-injected car engines to monitor intake manifold pressure.

After extensive development, todays commercial MEMS – also known as Micro System Technologies (MST), Micro Machines (MM) have proven to be more manufacturable, reliable and accurate, dollar for dollar, than their conventional counterparts. However the technical hurdles to attain these accomplishments were often costly and time- consuming, and current advances in this technology introduce newer challenges still. Because this field is till in its infancy, very little data on design, manufacturing processes or liability are common or shared.

2          FABRICATION


2.1       MATERIALS USED For the Fabrication Process


MEMS devices are fabricated using a number of materials, depending on the application requirements. One popular material is polycrystalline silicon, also called “polysilicon” or “poly”. This material is sculpted with techniques such as bulk or surface micro- machining, and Deep Reactive Ion Etching (DRIE), proving to be fairly durable for many mechanical operations. Another is nickel, which can be shaped by PMMA (a form of plexiglass) mask platng (LIGA), as well as by conventional photolithographic techniques. Other materials – such as diamond, aluminum, silicon carbide and gallium arsenide – are currently being evaluated for use in micro machines for their desirable properties; e.g., the hardness of diamond and silicon carbide. To create moveable parts, several layers are needed for structural and electrical interconnect (ground plane) purposes, with so-called “sacrificial” oxide layers in between. The current manufacturing record is five layers, making possible a variety of complex mechanical systems. These capabilities, developed over the last several years, are beginning to unlock the almost unlimited possibilities of MEMS applications.


                                   
Figure 1 Material overview                       

2.2         Fabrication Techniques


The methods used to integrate multiple patterned materials together to fabricate a completed MEMS device are just as important as the individual processes and materials themselves. Depending on the type of material used fabrication techniques are classified as:


2.2.1        Silicon Micro fabrication:

     The two most general methods of MEMS integration are:
·         Surface micro machining
·         Bulk micro machining

2.2.1.1       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 SiO2 as the sacrificial material and polysilicon as the structural material.
Simply stated, surface machining is a method of producing MEMS by depositing, patterning, and etching a sequence of thin films, typically1 µm thick. One of the most important processing steps that is required of dynamic MEMS devices is the selective removal of an underlying film, referred to as a sacrificial layer, without attacking an overlaying film, referred to as the structural layer, used to create the mechanical parts. Surface micro machining has been used to produce a wide variety of MEMS devices for many different applications. In fact, some of them are produced commercially in large volumes.
                                              
Figure 2 Process flow of surface micromachining
Advantages of surface micro machining

a)         Structures, especially thicknesses, can be smaller than 10 µm in size,
b)         The micro machined device footprint can often be much smaller than bulk wet-etched devices,
c)         It is easier to integrate electronics below surface micro-structures, and
d)         Surface microstructures generally have superior tolerance compared to bulk wet-etched devices.
The primary disadvantage is the fragility of surface microstructures to handling, particulates and condensation during manufacturing.
                                   
Figure 3 Steps of surface micromachining
                                        
Surface Micro machining is being used in commercial products such as accelerometers to trigger air bags in automobiles.

2.2.1.2       Bulk Micromachining and Wafer Bonding

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:
·         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.
·         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 SiO2 and Si3N4, and some metallic thin films such as Cr and Au (gold).

                         
Figure 4 Process flow of bulk micromachining

A drawback of wet anisotropic etching is that the microstructure geometry is defined by the internal crystalline structure of the substrate. 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

                                                                                                                                    
                                                                                                             

The development of MEMS has contributed significantly to the improvement of non-silicon micro fabrication techniques. Two prominent examples are LIGA and plastic molding from micro machined substrates.

2.2.1.1       LIGA

             LIGA is a German acronym standing for lithographie, galvanoformung (plating), and abformung (molding). However, in practice LIGA essentially stands for a process that combines extremely thick-film resists (often >1 mm) and x-ray lithography, which can pattern thick resists with high fidelity and results in vertical sidewalls. Although some applications may require only the tall patterned resist structures themselves, other applications benefit  from using the thick resist structures as plating molds (i.e., material can be quickly deposited into the mold by electroplating). A drawback to LIGA is the need for high-energy x-ray sources that are very expensive and rare.

              
Figure 5 Process flow of LIGA
The LIGA process exposes PMMA (poly methyl metha  crylate) plastic with synchrotron radiation through a mask. This is shown at the top of the Figure 1. Exposed PMMA is then washed away, leaving vertical wall structures with spectacular accuracy.  Structures a third of a millimeter high and many millimeters on a side are accurate to a few tenths of a micron.  Metal is then plated into the structure, replacing the PMMA that was washed away.  This metal piece can become the final part, or can be used as an injection mold for parts made out of a variety of plastics.
                                                 
A cheap alternative to LIGA, with nearly the same performance, has been developed. The solution is to use a special epoxy-resin-based optical   resist, called SU-8, that can be spun on in thick layers (>500 µm), patterned with commonly available.


















3          MEMS DESIGN PROCESS


There are three basic building blocks in MEMS technology, which are,
Deposition Process-the ability to deposit thin films of material on a substrate,
Lithography-to apply a patterned mask on top of the films by photolithograpic imaging.
Etching-to etch the films selectively to the mask. A MEMS process is usually a structured sequence of these operations to form actual devices.
       
Figure 6 MEMS design flow starting to end









4          PACKAGING


As with micromachining processes, many MEMS sensor-packaging techniques are the same as, or derived from, those used in the semiconductor industry. However, the mechanical requirements for a sensor package are typically much more stringent than for purely microelectronic devices. Microelectronic packages are often generic with plastic, ceramic, or metal packages being suitable for the vast majority of IC applications. For example, small stresses and strains transmitted to a microelectronics die will be tolerable as long as they stay within acceptable limits and do not affect
reliability. In the case of a MEMS physical sensor, however, such stresses and strains and other undesirable influences must be carefully controlled in order for the device to function correctly. Failure to do so, even when employing electronic compensation techniques, will reduce both the sensor performance and long-term stability.

4.1         Standard IC Packages

·         Ceramic Packages
·         Plastic Packages
·         Metal Packages
                      
Figure 7 Standard IC packeges

4.2         MEMS Mechanical Sensor Packaging


A MEMS sensor packaging must meet several requirements :
·         Protect the sensor from external influences and environmental effects. Since MEMS inherently include some microscale mechanical components, the integrity of the device must be protected against physical damage arising from mechanical shocks, vibrations, temperature cycling, and particle contamination. The electrical aspects of the device, such as the bond wires and the electrical properties of the interconnects, must also be protected against these external influences and environmental effects.

·         Protect the environment from the presence of the sensor. In addition protecting the sensor, the package must prevent the presence of the MEMS from reacting with or contaminating potentially sensitive environments. The classic examples of this are medical devices that contain packaged sensors that can be implanted or used within the body; these must be biocompatible, nontoxic, and able to withstand sterilization.

·         Provide a controlled electrical, thermal, mechanical, and/or optical interface between the sensor, its associated components, and its environment. Not only must the package protect both the sensor and its environment, it must also provide a reliable and repeatable interface for all the coupling requirements of a particular application. In the case of mechanical sensors, the interface is of fundamental  importance since, by its nature, specific mechanical coupling is essential but unwanted effects must be prevented. A simple example would be a pressure sensor where the device must be coupled in some manner to the pressure but isolated from, for example, thermally induced strains. The package must also provide reliable heat transfer to enable any heat generated to be transmitted away from the MEMS device to its environment.

In the vast majority of cases, basic plastic, metal, or ceramic packages do not satisfy these requirements. While the requirements for electrical connections and heat transfers paths on sensor packages are typically much less than in the case of most ICs, it is the mechanical interface that complicates the package design. The mechanical interface must isolate the sensor from undesirable external stresses and provide relief from residual stresses in the assembly while enabling the desired mechanical effect arising from the measurand to be coupled to the sensor. In the vast majority of practical sensor applications, each packaging solution will be developed specifically for that particular application.







 MEMS applications in various fields are as follows.

           High frequency circuits will benefit considerably from advent of the RF-MEMS technology. Electrical components such as inductors and tunable capacitors can be improved significantly compared to their integrated counter parts if they are made using MEMS technology. If the integration of such components, the performance of communication circuits will improve, while the total circuit area, power consumption and cost will be reduced. In addition, the mechanical switch, as developed by several research groups, is a key component with huge potential in various micro wave circuits. The demonstrated samples of mechanical switches have quality factors much higher than anything previously available. The MEMS technology has the potential of replacing many radial frequency components such as switches phase shifters, capacitors, inductors, and filters, used in to days mobile, communication and satellite systems. RF mems` components promise superior performance in comparison with current technologies. This also enables new functionality and system capability that are not possible with current technologies such as nano satellites.

           MEMS enabling new discoveries in science and engineering such as the polymerase chain Reaction (PCR) Microsystems for DNA amplification and identification, micro machined scanning Tunneling microscopes (STMs), Biochips for detection of hazardous chemical and biological agents, and Microsystems for high-throughput drug screening and selection.

                           Inertial sensors are mechanics sensors aiming at measuring accelerations, in the mechanics science definition. There are two categories of inertial sensors. They are, accelerometers which measures variation of rotational speed and gyroscopes which measures variation of rotational speed.

                                 
Figure 8 Capacitive accelerometer’s working diagram
                             adxl2 Photo of a microaccelerometer device
           Figure 9 Schematic of micro accelerometer, ADXL series, produced by Analog Device.
                            
                              On these diagrams, we can see a micro accelerometer device and the chip including associated electronics, made by Analog Device. This is a two axis micro accelerometer. This means it is able to measure accelerations in two directions at a time (in the directions of the plane).                                        
                              Micro accelerometers were the first MEMS device to flood the market. Micro accelerometers measure variation of translational speed. So acceleration, deceleration, even very high deceleration, like…shock! The sensor that detects a shock and launches the airbag is a micro accelerometer combined with a electronic circuit able to decide wether or not the shock was an accident or just your car passing a pothole. There are lots of applications, like navigation, micro accelerometers can help in increasing precision. There are more and more to say about micro accelerometers, they are still the spearhead of MEMS industry.

                              Micro gyroscopes are newer in the market compared to micro accelero meters. Some devices have appeared on the market for navigation application. The key point in these devices is sensitivity.


                      RF switches have been under development for years, but the commercial applications just begin to appear. The reason is the difficulty to combine high efficiency, reproducibility and reliability. RF switches will be preferred to full electronic switches on applications where security, integration capabilities, power consumption and other parameters are critical.                                
                              

                         Sports Training Devices
                         Computer Peripherals
                         Car and Personal Navigation Devices
                         Active Subwoofers


                         Earthquake Detection and Gas Shutoff
                         Machine Health
                         Shock and Tilt Sensing

5.9 Military:


                   Tanks
                   Planes
                   Equipment for Soldiers

Figure 10 Application of MEMS in various fields

6            THE FUTURE OF MEMS TECHNOLOGY


6.1  Industry Challenges


Some of the major challenges facing the MEMS industry include:


MEMS companies today have very limited access to MEMS fabrication facilities, or  foundries, for prototype and device manufacture. In addition, the majority of the organizations expected to benefit from this technology currently do not have the required capabilities and competencies to support MEMS fabrication. For example, telecommunication companies do not currently maintain micromachining facilities for the fabrication of optical switches. Affordable and receptive access to MEMS fabrication facilities is crucial for the commercialisation of MEMS.


Due to the highly integrated and interdisciplinary nature of MEMS, it is difficult to separate device design from the complexities of fabrication. Consequently, a high level of manufacturing and fabrication knowledge is necessary to design a MEMS device. Furthermore, considerable time and expense is spent during this development and subsequent prototype stage. In order to increase innovation and creativity, and reduce unnecessary ‘time-to-market’ costs, an interface should be created to separate design and fabrication. As successful device development also necessitates modelling and simulation, it is important that MEMS designers have access to adequate analytical tools. Currently, MEMS devices use older design tools and are fabricated on a ‘trial and error’ basis. Therefore, more powerful and advanced simulation and modelling tools are necessary for accurate prediction of MEMS device behaviour.


The packaging and testing of devices is probably the greatest challenge facing the MEMS industry. As previously described, MEMS packaging presents unique problems compared to traditional IC packaging in that a MEMS package typically must provide protection from an operating environment as well as enable access to it. Currently, there is no generic MEMS packaging solution, with each device requiring a specialized format. Consequently, packaging is the most expensive fabrication step and often makes up 90% (or more) of the final cost of a MEMS device.


Due to the relatively low number of commercial MEMS devices and the pace at which the  current technology is developing, standardization has been very difficult. To date, high quality control and basic forms of standardization are generally only found at multi-million dollar (or billion dollar) investment facilities. However, in 2000, progress in industry communication and knowledge sharing was made through the formation of a MEMS trade organization. Based in Pittsburgh, USA, the MEMS industry group (MEMS-IG) with founding members including Xerox, Corning, Honeywell, Intel and JDS Uniphase, grew out of study teams sponsored by DARPA that identified a need for technology roadmapping and a source for objective statistics about the MEMS industry. In addition, a MEMS industry roadmap, sponsored by the Semiconductor Equipment and Materials International organization (SEMI), has also been identified to share pre-competitive information on the processes, technology, application and markets for MEMS. This web-based organization can be found at http://www.roadmap.nl.

Several other European initiatives supported by governments and the European commission have been coordinated: Europractice (Microsystems Service for Europe), NEXUS (Network of Excellence in Multifunctional Microsystems), aimed at enhancing European industrial competitiveness in the global marketplace, and Netpack, whose role is to drive the development and use of advanced packaging and integration technologies. The networking of these smaller companies and organizations on both a European and a global scale is extremely important and necessary to lay the foundation for a formal standardization system.


The complexity and interdisciplinary nature of MEMS require educated and well-trained
scientists and engineers from a diversity of fields and backgrounds. The current numbers of
qualified MEMS-specific personnel is relatively small and certainly lower than present
industry demand. Education at graduate level is usually necessary and although the number
of universities offering MEMS-based degrees is increasing, gaining knowledge is an
expensive and time-consuming process. Therefore, in order to match the projected need for
these MEMS scientists and engineers, an efficient and lower cost education methodology is
necessary. One approach, for example, is industry-led (or driven) academic research centres
offering technology-specific programs with commercial integration, training and
technology transfer.








7          CONCLUSION


The automotive industry, motivated by the need for more efficient safety systems and the desire for enhanced performance, is the largest consumer of MEMS-based technology. In addition to accelerometers and gyroscopes, micro-sized tire pressure systems are now standard issues in new vehicles, putting MEMS pressure sensors in high demand. Such micro-sized pressure sensors can be used by physicians and surgeons in a telemetry system to measure blood pressure at a stet, allowing early detection of hypertension and restenosis. Alternatively, the detection of bio molecules can benefit most from MEMS-based biosensors. Medical applications include the detection of DNA sequences and metabolites. MEMS biosensors can also monitor several chemicals simultaneously, making them perfect for detecting toxins in the environment.

Lastly, the dynamic range of MEMS based silicon ultrasonic sensors have many advantages over existing piezoelectric sensors in non-destructive evaluation, proximity sensing and gas flow measurement. Silicon ultrasonic sensors are also very effective immersion sensors and provide improved performance in the areas of medical imaging and liquid level detection.

·         The medical, wireless technology, biotechnology, computer, automotive and aerospace industries are only a few that will benefit greatly from MEMS.
·         This 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.
·         MEMS devices are manufactured for unprecedented levels of functionality, reliability, and sophistication can be placed on a small silicon chip at a relatively low cost.
·         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 will be the indispensable factor for advancing technology in the 21st century and it promises to create entirely new categories of products.





8          REFERENCES


1. Microsensors, Muller, R.S., Howe, R.T., Senturia, S.D., Smith, R.L., and White, R.M. [Eds.], IEEE Press, New York, NY, 1991.
2. Micromechanics and MEMS: Classic and Seminal Paper to 1990, Trimmer, W.S., IEEE Press, New York, NY, 1997.
3. Journal of Microelectromechanical Systems
(http://www.ieee.org/pub_preview/mems_toc.html)
4. Journal of Micromechanics and Microengineering
5. Berkeley Sensor and Actuator Center, http://bsac.eecs.berkeley.edu
7. Trimmer, W.S., Micromechanics and MEMS: Classic and Seminal Papers to 1990, IEEE Press, New York, NY, 1997.
8. Tjerkstra, R. W., de Boer, M., Berenschot, E., Gardeniers, J.G.E., van der Berg, A., and Elwenspoek, M., Etching Technology for Microchannels, Proceedings of the 10th Annual Workshop of Micro Electro Mechanical Systems (MEMS ’97), Nagoya, Japan, Jan. 26-30, 1997, pp. 396-398.


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