Microsystems Technologies
Unit 1: Introduction to Microsystems
Unit Contents
1.1 What is a Microsystem?
You will probably come across several different definitions of a microsystem but, as pointed out in the Business Issues module, no single definition is generally accepted. A very basic definition of a microsystem might be a very finely toleranced structure. This is much too broad for our purposes and would include any precision engineered component. A more specific definition is the combination of microelectronics together with a micromachined element on the same substrate. This sounds fine but is actually quite restrictive and would exclude some of the more interesting techniques and devices emerging from this field. It will probably be useful to have a more flexible view of what constitutes a microsystem. It may also be helpful to consider the definition of the broader term: microengineering.
This question is also posed in the module Business Issues of Microelectronics unit 5. This question raises several interesting questions about how we view microelectronics in the wider sense.
Microengineering has developed over the last ten years or so and has largely arisen out of a recognition of the possibilities of using the normal microelectronics production techniques to produce things other than integrated circuits. Using batch processes which operate on hundreds or thousands of components at one time, microelectronics technologies allow the production (the mass production) of highly complex structures with feature sizes in the range from 1-100 microns. Microengineering recognises that the same, or similar, techniques can be used to fabricate very small sensors or actuators. If we combine these artifacts with some electronic signal processing at the sub-mm level, then we have a microsystem. Note that this gets round the difficulty posed by one of the above definitions in that the integration does not have to take place on the same substrate.
One of the most useful concepts is to think of the difference between a microelectronic device and a microsystem. A microelectronic component processes electrical information whereas a microsystem interacts with its environment.
It is important to realise that microelectronics technologies on their own do not provide a wide enough portfolio of processes and materials; many additional techniques are required and combinations of technologies are used together to give highly miniaturised components and systems. In particular, materials other than silicon are used. Microelectronics is essentially a two-dimensional technology, consisting of thin patterned layers. Microsystems technology(MST) often requires a third dimension.
The issue of definition is important given the potential market for microsystems and the rate at which it is expected to grow over the next decade or so. Some sources predict a market with growth rates and overall size similar to that for microelectronics over the next couple of decades. This sounds excessive, but there is no doubt that the eventual market will be huge; probably growing to some billions of dollars within the next five years (see the module on Business Issues). Indeed, in a recent survey, the European Commission has identified nearly 20,000 companies in Europe which are interested in applying microsystems technology to their products in the immediate future. See the textbook "Novel Sensors and Sensing", page 1 for growth estimates.
The specifications for a new sensor to be commercial success depends on the field of application. "Novel Sensors and Sensing" has a table of such criteria on page 2.
It is interesting to note how this technology is defined in different parts of the world. MST is largely a European term. In the US, the generally accepted term is MEMS (Micro Electro Mechanical Systems) while, in Japan, the subject is known as Micromachines.
These differences stem largely from the different evolutionary paths the technology has followed. In Europe and the U.S. the main drive has been from device engineers looking for new applications for semiconductor technologies. In Japan, the origins lie in the field of robotics and a search for increasing miniaturisation.
Microsystems technology (MST) then, is concerned with the production of new products, systems or components through the use of microengineering techniques. Such a component will incorporate sensors and/or actuators and signal processing on a microscopic scale.
This last definition will serve us well but has its limitations. Some authorities would exclude the necessity to incorporate any electronics in the system before it becomes a microsystem. And, although a photodiode is undoubtedly a sensor and an LED or semiconductor laser can be considered to be an actuator, MST sometimes excludes optoelctronic devices from its remit. We also have to take care to include new categories of component such as microfluidic devices in our definition. These may or may not have an active electronic component. One can see the need to adopt a flexible definition.
In this module we shall look at the various MST techniques, examine their capabilities and discuss some applications.
Self Assessment Questions
Question 1
Look at the various definitions of MST (both in the text and on the www
references).
Which is the most restrictive and which is the least? Write a definition
of your own.
Question 2
What are the key differences between microelectronics and microsystems?
Answer
- A microelectronic device processes electrical information whereas a microsystem interacts with its environment.
- The need for a third dimension.
You could add: - A wider range of physical dimensions (up to 100 micron).
Question 3
What is the predicted market growth for microsystems?
Answer
10% pa
Question 4
Of the pararmeters listed for an industraial sensor in the table on page 2 of "Novel Sensors and Sensing", which one might a silicon based microsystem be unable to meet?
Answer
The temprature ranges. Semiconductors cannot cope with extreme temperatures because of the effect on conductivity.
Question 5
Define the term MEMS
Answer
Micro Electro-Mechanical System
1.2 Types of microsystems
Microsystems are commonly classified by function. They can be split into four main areas:
- Sensors
- Actuators
- Microstructures and
- Integrated Microsystems
This gives rise to several possibilities:
- Sensors can be connected directly to processing electronics
- Electronics can drive microactuators
- Complete systems involving Sensors-Processing electronics-Actuators can be created
- Free-standing microstructures, which do not involve electronic functions, can be made
Reference Texts
Read pages 10 & 12 of the booklet Microsystems Technology.
This gives a basic description of each type of device.
Self Assessment Questions
Question 6
What are the main types of microsystem? Give some examples of each.
Answer
Sensors, Actuators Microstructures and Integrated microsystems (see Ref 1 pages 10 & 11 for examples).
1.3 Why have a Microsystem?
When the laser was being developed in the 1960s, it was described as "a solution
in search of a problem." Luckily, those involved in the development
(and the funding) ignored this view and kept working. The problems for
this solution came thick and fast in the 1970s and '80s and now it is difficult
to imagine a world without the laser. Applications range from industrial
to medical. In fact, the laser is now a household item, being a key
component of a compact disk player. Think about this. The chances are
that you have more than one. I have at least four in my home; two CD-ROM
drives (three if you count last year's, slow model sitting on the shelf) and
two audio CD players. One of the latter is battery powered, can be carried
round or used in the car. The key points are:
- a technology which was once seen as being of little practical use is now the basis of a huge, world-wide industry,
- the application areas are many and diverse with a requirement for a large number of types, models and variations,
- this demand can only be met be means of mass production methods - particularly when it comes to the smaller, cheaper devices.
When the original technology was being developed, it would have been very difficult to look at these early, large, power-hungry lasers, with their limited capabilities and requirement for cabinets full of drive electronics and predict that, one day, the man in the street would carry one around with him, have one in his car and a few more in his home. Not only has this happened, it has happened very quickly (within a few decades).
A virtuous circle is at work here; the emerging applications lead to the development of new devices and manufacturing techniques and the possibilities thus revealed lead to the identification of new applications which leads to further development etc. All this is driven by the, potentially enormous, financial rewards to be had. The creation of a huge consumer market for "must have" goods which did not exist previously (CD-ROMs and audio players) is a golden scenario for industry.
If anything, MST is an even more "golden" opportunity. Firstly, the origins of the technology, to a large extent, arise out of manufacturing techniques which already exist. Secondly, many of the potential markets and applications have already been identified. Indeed, some of these are already established and can be said to be mature.
The parallel with the laser industry is therefore close but not exact. In fact, some laser devices could now be considered microsystems in themselves. Lasers were a classic case of a phenomenon known as "technology push." This is where a new technology is developed and the applications and markets follow. The opposite case is where the market or need is identified first and a technology is developed (or an existing one adapted) to meet the requirement. This is called "market pull."
MST has a foot in both camps. Many of the well known "demonstrators" (for example miniature cogs, sprockets, and even motors) are of little obvious use - for the moment anyway. However, MST has responded very rapidly to some market pulls, such as the requirement for airbag triggers in the automotive industry.
There are basically two ways in which the potential of MST can be exploited:
- better ways of doing existing things
- new things that can't be done any other way
The first of these can only be true if there is some positive advantage in moving to a microsystem.
In general, there are three key advantages of microsystems over their (macro) counterparts:
- reduced size
- reduced cost
- improved performance
These may be applicable to greater or lesser degrees. They are not necessarily independent and a combination of advantages can result. This is especially so for the size and performance arguments. Let us look at this a bit more deeply.
Reduced size
This is perhaps the most obvious advantage of a microsystem. Many applications are driven, solely or largely, by considerations of space. Invasive and implantable medical devices such as catheters are an obvious example. A reduction in size also opens up the possibility of incorporating many components into one device. An example of this is the array of magnetic coils produced by the company CSEM.
Figure 1:An array of magnetic coils produced by CSEM
The microsystem need not be in the form of an array of similar components. Different components can be incorporated on the one substrate giving rise to possibilities such as the laboratory on a chip. A reduction in size brings other benefits. Smaller devices often consume less power and have a faster response (ie. improved performance).
Reduced cost
This is often a result of the cost of production derived from the processing techniques developed for microelectronics. The batch processing techniques used in microelectronics manufacture has been the key to its staggering success. The ability to make large numbers of components at once has driven costs down while functionality and performance have increased. For microsystems using similar processes, the same advantages will apply. A related benefit is that of reproducibility. Material properties and dimensions can be kept within tight limits and made uniform both within a batch and across batches. This results in predictable component characteristics which is a considerable advantage to both the system and the component designer. The microelectronics industry spends a considerable amount of time and energy improving the quality and predictability (and, in turn, reliability) of its processes. This benefits not only costs but also performance.
Improved performance
There are several reasons why a microsystem might display improved performance, mostly related to size. One is the potential to integrate the sensing element with the electronics. This means the signal does not have to travel any significant distance before being processed. Much weaker effects can thus be measured. There is also the possibility to incorporate calibration functions in the device. The size of the device also makes it less likely to interfere with its environment. A smaller sensor will be less affected by outside influences and forces (this is most obviously so for mechanical microsensors and will be discussed in Unit 3). In addition, the improved quality and reproducibility of the fabrication process will lead to improved predictability of performance. A fourth advantage exists: the ability to do things that could not be done by any other method.
Reference Texts
Read section 3.1 of the Novel Sensors and Sensing textbook.
Self Assessment Questions
Question 7
What four difficulties might microsensor design engineers have to contend with?
Answer
1. Semiconductor material may not be very sensitive to the measurand.
2. Enviromental susceptibility of the sensor.
3. Inter-disciplinary teams are needed.
4. Non-standard foundry technology is expensive.
1.4 Scaling
When we design a microsystem, we cannot simply scale down the dimensions of an existing macrodesign. We rely upon certain relationships to predict performance. As we reduce the dimensions of an object, the significance of the various parameters in these relationships changes. For example, consider an airplane. If we take the linear dimension as L, the fuel load it can carry, and hence the distance it can travel on one load, will depend on volume (L3). The drag, however, will be proportional to the surface area (L2). So, all other things being equal, if we increase the size, we will increase the distance travelled on a single load of fuel. Following the same logic, we can see that the strength of adhesion of a bond will be proportional to the area (L2) while the mass will scale by L3. A similar argument can be applied to a supporting structure and its cross sectional area so a smaller object will be more capable of supporting its own weight than a large one. Examples of this are micromechanical cantilevers which can be very long relative to their thickness and in the macro world where small animals can carry their weight with greater ease than large ones (compare an elephant to an insect).
So it is essential to have some idea of how the forces we take for granted and make use of in the macro world scale down to microsystem dimensions.
The main forces that are likely to be employed are:
- Gravitational forces
- Elastic forces
- Surface tension forces
- Electrostatic forces
- Electromagnetic forces
- Piezoelectric forces
- Thermal forces
Some of these forces will act destructively and some will be useful eg. as a source of motive power for a microactuator.
To illustrate the problem of scaling we can look at the simple spring. From Hooke’s law provided the material remains within its linear region we know that,
![\[
F = kx
\]](equations/mmt_im_eq08.gif)
Now, Young’s modulus is,
![\[
E = \frac{{stress}}{{strain}} = \frac{{\frac{F}{A}}}{{\frac{x}{l}}}
\]](equations/mmt_im_eq09.gif)
where A is the cross-sectional area and l is the original length of the spring element. Re-arranging this gives,
![\[
F = \frac{{EA}}{l}x
\]](equations/mmt_im_eq10.gif)
Comparing equations the spring constant is seen to be related to E, A and l as,
![\[
k = \frac{{EA}}{l}
\]](equations/mmt_im_eq11.gif)
Assuming that E remains constant, the effect of any changes to the dimensions of the spring can be seen by the dimensional analysis of the variables,
![\[
k = E\frac{{\left[ {L^2 } \right]}}{{\left[ L \right]}} = E \times \left[ L \right]
\]](equations/mmt_im_eq12.gif)
This shows that, if the relative proportions of the spring shape are unaltered, k will decrease proportionally with any decrease in the scale.
Self Assessment Questions
Question 8
From simple harmonic motion analysis it can be shown that for a vertically mounted spring supporting a mass m the undamped natural frequency is,
- The steady-state deflection x ?
- The undamped natural frequency
?
![\[
\omega _n = \sqrt {\frac{k}{m}}
\]](equations/mmt_im_eq13.gif)
If the spring and mass are reduced in size but retain the same relative shapes what will be the affect on:
Answer
Using the density ![\[
\rho
\]](equations/mmt_im_roh.gif)
, the mass can be written as ![\[
\rho \times volume
\]](equations/mmt_im_eq1.gif)
or dimensionally as ![\[
\rho \times \left[ L \right]^3
\]](equations/mmt_im_eq02.gif){kind=small}
.
1. The steady state deflection is then,
![\[
F = mg = kx
\]](equations/mmt_im_eq03.gif)
Substituting,
![\[
F = \rho \times \left[ L \right]^3 \times g = E\left[ L \right]x
\]](equations/mmt_im_eq04.gif)
so
![\[
x = \rho \frac{g}{E} \times \left[ L \right]^2
\]](equations/mmt_im_eq05.gif)
This shows that as the scale decreases so does x by the square of the change in the scaling factor.
2. The undamped natural frequency can be written as,
![\[
\omega _n = \sqrt {\frac{k}{{\rho \times volume}}}
\]](equations/mmt_im_eq06.gif)
This can be expressed dimensionally as,
![\[
\omega _n = \sqrt {\frac{{E\left[ L \right]}}{{\rho \times \left[ {L^3 } \right]}}} = \frac{1}{L}\sqrt {\frac{E}{\rho }}
\]](equations/mmt_im_eq07.gif)
The frequency is inversely related to the scaling factor.
On page 52 of Novel Sensors and Sensing the equations for a cantilever are
given. Cantilevers have been used in a number of accelerometer microsystem
designs. A similar analysis to that required for Question 8 below could
be done for the cantilever deflection and undamped natural frequency.
Question 9
What is the Knudsen effect and how could it impact the design of a micro-component?
Answer
See the final paragraph of section 7.2.2 of the textbook.
Suggested Reading
The physicist Richard Feynman gave a classic talk in 1959 entitled "There's Plenty of Room at the Bottom". His main theme was the possibilities and problems of making very small machines. Whilst talking about this he touched on many interesting points which are interesting from the perspective of the state of both microsystem and microelectronics technology today. You can find this on:
http://www.zyvex.com/nanotech/feynman.html
Read this and note the following, bearing in mind that this talk was given in 1959 before the creation of the semiconductor industry as we know it:
- In the section "Miniaturization by evaporation" compare what
he says in his first paragraph with what you know about current semiconductor
processing.
- Read what he says about the problems of scaling in this and the following section. Note his comment on the effect of Van der Waals attractions and compare with the Kundsen effect mentioned on page 153 of the textbook. He also hints at the possibility of creating a highly focused beam of light.
You may wish to print this paper and refer to it at the end of this module to see how Feynman fares in his predictions.
Another useful paper is "Grand In Purpose, Insignificant In Size" by William Trimmer which you can find on:
http://home.earthlink.net/~trimmerw/mems/mems_97_talk.html
This paper is very useful as a reference to others in the field and as a summary of the history of the technological process. Read this carefully. Note particularly the following:
- In the paragraph beginning "The field we are contemplating....."
he indicates that we are looking at a technology, or a series of technologies,
which have not evolved from a single point from development into manufacture
(like the spawning of the microelectronics industry from the transistor).
However, this can be considered a strength (see the paragraph which begins
"One thing giving me confidence.....").
- The sentence "Complex calculations and decisions have now become
inexpensive" sums up, in a few short words, almost the entire benefit
and reason behind the success of digital computing over the last few decades
(and, in fact, of electronics as a whole). See if you can think of an equivalent,
concise sentence to sum up the benefits of microsystems - if not now, then
when you have completed the module.
- Pay attention to his description of Surface and Bulk Micromachining and LIGA. We will look at these in more detail soon. Keep in mind his analogy of the flour & steel automobile when you come to look at Surface Micromachining
WWW Research
In this last section of Unit 1 various sources of information are listed for you to browse. These are by no means exhaustive. Material of this type is always in a state of flux and new sources may emerge than would be of benefit for this module. If you find some useful Web sites let us know and we will add them to the list.
There are a range of textbooks available. One listing is given at MEMS -- MEMS Books -- trimmer.net (TM). This is not an exhaustive list. There are two introductory booklets published by the DTI (copies are available from the AMI office if you would like these) aimed at managers and chief engineers of companies who may find microsystems useful in their product development. The following Web sites provide an in-sight into the emerging world of microsystems technology. They show agencies that are sponsored whose remit is to facilitate networking and some others offering a consultancy service. Within some of these sites you will find lists of manufacturers of various sensors or components. Browse around to get a feel for the services on offer and suppliers of microsystems.
Europractice: www.europractice.com
Europractice is a European Commission initiative. Their stated aim is,
‘… to improve the competitiveness of European industry by the
adoption of advanced electronics technologies. It is particularly aimed at microsystems.’
Visit their site at and have a look around. View the case histories listed.
Nexus : www.nexus-emsto.com/
Their site states,
NEXUS is a non profit Association headquartered in Grenoble with the aims
of providing microsystems professionals with
• access to strategic guidance through reports containing the most up-to-date
analysis of markets, technologies, applications, and long-term trends.
• information through the web portal, regular e-mail bulletins, MST News
pages, thematic workshops, and also conferences.
• high-level networking opportunities through the regular user/supplier
club meetings, and other specialist events.
• Assistance to restructure the ERA in micro and nanotechnology
Microsystems Suppliers Directory : www.microsystems-suppliers.com/
This is a straightforward company directory maintaining a database of companies
involved in the microsystems industry.
AML : www.aml.co.uk/aboutAML.htm
The home page states,
Applied Microengineering (AML), an independent company wholly owned by
its directors are leaders & one of the most experienced companies in their
field. AML was uniquely one of the first companies in the world founded (1992)
to exploit the hugely important and strategic, frontiers MEMS technology.
The AML site is of interest as an example of a UK company offering services in MST. A microsystem will often require a number of processes and techniques for its manufacture. Not all of these will be available from one source so someone (a lead contractor perhaps) will have to undertake to manage the device through these various facilities (not an easy task). Have a look at the useful links from this site. A number of companies now exist in the industry providing services such as this one and others who carry out no manufacture themselves.
KBIC Microsystems Technology KBIC - Microsystems
Technology
This is a UK based company that offers to manage projects. They state,
KBIC is a consultancy company providing neutral advice on the best means
to develop and manufacture MST/MEMS products for your application. We have
access to a wide network of established MST service providers able to design,
prototype, manufacture, test and qualify your product. We specialise in planning
and managing MST projects and can assist with technology transfer, protection
of IPR, and other commercial issues.
There is a link from this site to the design house MST Design at http://www.MST-Design.co.uk
MEMS and Nanotechnology Exchange MEMS
and Nanotechnology Exchange
This is a USA-based foundry. At their home page they state,
We provide expertise in design and fabrication services to help researchers
and companies identify and develop applications using MEMS and Nanotechnology.
At their home page there is a button MEMSnet. This is a kind of newsletter and information service. You can get directly to it using MEMS and Nanotechnology Clearinghouse
Finally you might like to receive copies of the bi-monthly journal MST|news. This is a bi-monthly e-journal which youcan register for without charge. The site is www.mstnews.de/Homepage. Then click on subscription page. Back copies can also be viewed.
updated 01.08.05, then 27.06.06 RA