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Microsystems Technologies

Unit 2: Microsystems - Technologies & Methodologies

Unit Contents

• 2.1 Microsystem technologies - a short overview
• 2.2 Microsystem development flow
  • 2.2.1 Reference to Engineering MST
  • 2.2.2 Fuse newsletter (Development Technologies)
• 2.3 Microsystem design

Instructions to register with MST News

Throughout this module reference will be made to articles in MST News. Access to back issues is free but you need to register with the site first.

Go to the link:  http://www.mstnews.de/Homepage

Click on "Downloads" and follow the instructions to register (as in unit 1).

You should now be able to get to the back issues of the journal any time by following the Download link.

This will take you to a list of issues and you can select from these as before.  It may be as well to download the PDF version of the appropriate issues for easy reference.

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2.1 Microsystem technologies - a short overview

The techniques for manufacturing MST components will be covered in the following units but a short overview of the main methods will be given here.

Mechanical Machining techniques

Some, very precise, conventional machine tools can manufacture objects in at the micron level.  However, difficulties occur due to the material properties of the tool, such as elasticity, thermal effects etc.  CO₂ lasers can be used to cut and form the material but, since they do this by melting, the precision is again limited.  Finer dimensions and higher precision can be achieved through the use of Excimer lasers which operate at high frequency.  With this technique, the laser fires pulses which blast away layers of atomic thickness.  The pulses are so short that no heating or melting takes place.  The intensity of the beam can be altered to control the amount of material removed with each pulse and the depth of cut can be determined by counting the number of pulses.

The main problem with direct machining techniques is that they commonly work on one structure at a time.  This makes them suitable for prototypes but not for mass production.  However, there is still a place for such methods (particularly Eximer laser abation) in the MST portfolio of techniques.

Micromachining

As we have already established, the most useful MST manufacturing techniques have been developed from existing, microelectronics technologies.  The basic techniques were described in Unit 2 of Business Issues of Microelectronics.  You should look at this again now in order to refresh your memory on the steps involved.

These methods are based on the fundamental technique of photolithography where patterns are reproduced on the surface of the material (usually silicon) and a circuit is built up through a combination of patterning, etching, deposition, doping etc.  This results in an essentially two-dimensional structure whereas MST requires three-dimensional features.  However, the basic technique has such powerful advantages in terms of unit cost and reproducibility that it makes an excellent basis upon which to develop MST methods.  Indeed, much of MST research is focused on overcoming the restrictions of photolithographic manufacturing whilst retaining the benefits.

The term micromachining is typically used to refer to these processing techniques.  There are two main subdivisions: surface micromachining and bulk micromachining.

Surface micromachining uses common microelectronic and thin film processing to form micromechanical structures on the surface (that is, to a depth of a few microns) of the silicon (we refer to silicon but the techniques can be applied to other materials). Thin film techniques (essentially material deposition) can be employed to enhance the process capabilities.

A lot of very useful structures (such as cantilevers) can be made using surface micromachining.  But there is often a requirement for deeper, taller structures.  The big advantage is that these techniques can often be combined with a specific microelectronic process in order to produce a sensing device as part of an electronic component.

Bulk micromachining exploits the fact that silicon etches much faster in some directions than others.  This is a property of the crystal structure and can be controlled by doping the silicon with various impurities.   Two methods of etching are employed: wet etching and dry etching.

These micromachining methods can be broken down into several key processes:

• Pattern replication
• Material deposition
• Etching
• Sacrificial layer processing

LIGA

The term LIGA comes from the German for Lithography, electroplating and molding (Lithographie, Galvanformung und Abformung).  It is essentially a technique for creating a mold on the micron scale and using this to mass produce very small structures. The LIGA technique is not based on, or compatible with, silicon processing.  If it is to be combined with electronics, this must be done using a packaging technology.

Packaging Techniques

In most cases the ideal solution is to integrate the sensor or actuator on the same piece of silicon as the electronics.  This will give a true microsystem.  However, as noted above, some of the techniques for producing microsystems are often incompatible with normal silicon processing.  One solution is to combine a separate microsensor/actuator and a microchip on the same substrate using one of the packaging techniques developed for microelectronics.  This is often referred to as Hybrid or Multichip Module (MCM) technology and involves bonding of a device (or many devices) onto the same substrate which can be silicon or some other material.

It can be seen that the manufacture of microsystems can employ a variety of techniques and often a combination of methods is used.  The above list is not exhaustive.  Many other techniques are used and others are being developed.  For example, chemical and biological sensors usually employ a coating of some kind to make a FET sensitive to a particular substance.

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2.2 Microsystems development flow

Look again at the ASIC development process described in Microelectronic Technologies and Applications Section 1.2. The flow chart of diagram 1 is a typical representation of  the IC design route.  It involves a number of steps including system design, partitioning, layout and simulation with feedback loops at various stages.  Although the details of the representation may vary and the process itself will evolve, the design flow is fairly well established.

There are several reasons for this:

• The design process is tried and tested
• The manufacturing process is well understood leading to accurate modeling of the process and devices.  This impacts on the design route in the form of  established design rules and models for the chosen manufacturing process.
• Extensive and comprehensive CAD tools are available which allow design and verification.  (In many cases these tools are available as an integrated suite with comprehensive support.  The situation further improves with well established processing lines where a design team has access to the latest process data).

The opportunity for design verification is a big advantage, dramatically increasing the chances of "right first time" devices.

2.2.1 Reference to Engineering MST

This gives a view of the MST design process.  See how this differs from the ASIC case.

The main difficulties are:

• The complications arising from the possible (or probable) need to combine different technologies (and hence manufacturers)
• The lack of adequate design tools.
• In particular, note the comments on page 25.

All this makes microsystem development sound a lot more lengthy, costly and risky than that for an IC.  Especially when we consider a microsystem which effectively has an ASIC design as a subset of its overall development.  This is indeed so and is likely to remain so given the nature of the technology.  However, the issue is being addressed by the MST community, particularly by companies offering a brokerage service.

2.2.2 Fuse Newsletter

Reproduced below is an article from FUSE Newsletter No.4

Read this and compare it with the DTI Microsystems Technology 5 booklet pages 23 - 25 and with the ASIC design flow in Module 2.  In particular, look at the differences between the design routes for the two technologies and the ways in which they manage risk. Now try SAQs 1-6.

Written by H.v.d.Vlekkert of CSEM, a Swiss company specialising in MST development and supply. Reproduced with permission.

Development Technologies Introduction

The goal of the EUROPRACTICE program is to promote access to Microsystems.  A Microsystem is defined as a small (<1 cm³ typically) package containing at least a microstructure which interfaces with the non-electrical world (sensor or actuator) and an IC which provides an intelligent signal processing interface between the microstructure and the user.  By promoting access, it is intended that the number of Microsystem products manufactured in Europe will increase. 

However, manufacturing represents only the final stage of a product life cycle (as depicted in Figure 1)  The life cycle passes three main phases: technology set-up, development and production.  The technology set-up phase is the beginning of a Microsystem and, starting from an idea, results in a function demonstrator for which manufacturing is described in a cook book.  The development phase consists of two stages: product definition and product development.  The product definition stage is carried out by the customer, in co-operation with the Microsystems service provider.  The product development stage starts after the product definition phase and assumes that the Microsystem specifications have been defined and agreed upon with the customer.  The production phase starts with industrialisation followed by the production of the Microsystem.  In parallel, the customer implements market introduction, distribution and sales.

 

Figure 1: The life cycle of a microsystem

Before Microsystem manufacturing at medium to large scales can begin, a product development stage must be executed.  The goal of product development is to make industrial prototypes of the Microsystem which can be manufactured in medium to large quantities without the necessity of a redesign. 

In the past, the development stage for a Microsystem used to represent a long, costly and risky stage.  The length of a Microsystem development often spans a period of several years.  Its cost can run up to several million Euros, often with major setbacks along the project or even total failure at the end. 

To reduce these problems, CSEM has implemented a methodology for the development of Microsystems.  The methodology describes the flow of a Microsystem development to guarantee its systematic execution.  It focuses on the reuse of existing components and knowledge through the optimal use of design libraries.  The software tools are optimised to execute each part of the development stage as effectively as possible.  Check points described extensively in the methodology limit the risks of the development.

Figure 2: Microsystem Development Flow

The flow of the Microsystem development stage that has been defined in the methodology is depicted in the flow chart. 

The development begins with the system design phase in which the product specifications are partitioned over the different system components.  Simulations are performed to verify that the system will meet all specifications.  In the next stage, all components are designed in detail according to their specifications.  The results of the detailed simulations are cross-checked against the system level simulations.  When the components meet their specifications, they are fabricated and tested.  They can then be assembled to form the first prototype of the system.  This prototype is then tested extensively to gain insight in the tolerances of the system to different parameters.  When the initial prototype meets all critical specifications, the project continues with the design of the final prototype.  Minor modification to the design will be made to assure that this prototype meets all specifications. 

The experience gained with the fabrication will now also be used to optimise the final prototype so that it can be produced industrially without any further modifications.  The product specific equipment necessary for this future production will also be defined at this stage.  The final prototypes are then fabricated, tested and sent to the customer.  They can also undergo the environmental and quality tests specified in the development contract. 

The methodology for a Microsystem development is similar to the one for an ASIC with two notable difference.  The first difference is that the Microsystem methodology develops an IC, a microstructure and a package in parallel with much emphasis on their interactions during the entire development stage. The second difference is that the ASIC development methodology does not distinguish between first and industrial prototypes.  The need for this distinction in Microsystems stems from the fact that there are no standard test or assembly procedures for Micro- systems.  Therefore, the first prototype is used to optimise the test and assembly procedures for industrial production.  The resulting industrial prototype is conceived in such a way that the prototype can be produced in large quantities without the need for redesign in the industrialisation stage. 

The system design phase is very important in the Microsystem methodology.  In this phase, three different issues are addressed.  The first issue is system partitioning which distributes the customer specifications over he different components of the Microsystem.  The second issue is the choice of technologies for the different components and the verification as to whether the component specifications can be met with the technologies chosen.  The third issue is concerned with the assembly and test of the components and the system.  Given the small dimensions of a Microsystem, a test and assembly concept must be worked out during the system design. 

Throughout the entire methodology, there are checkpoints defined with the precise definition of the information which must be available.  The checkpoints are very effective in limiting the risks of the Microsystem development, since they require the evaluation of all critical aspects of the Microsystem and split the development stage into shorter, well-defined parts. 

The methodology also helps in defining the software tools needed for the Microsystem development.  The requirements of the software tools for each step of the development stage are based on the kind of information that must be available at the end of the step.  This has helped us to choose the software and implement the libraries necessary for each step.  The libraries in turn will help shorten the development time and reduce its cost, since they maximise the reuse of available components and knowledge. 

Conclusion 
The methodology described above has been implemented at CSEM and is used for all our development projects.  We are using this methodology to define software tools and libraries necessary for Microsystem design and simulation.  The libraries of existing components are currently being implemented and will be extended as more components become available. 

The result of the methodology appears to be that development projects tend to get shorter, although it is too early to reach a definitive conclusion.  The methodology has certainly helped us in discussions with potential customers, because it explains how a Microsystem development takes place and how it limits the risks of such a project.

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2.3 Microsystem Design

Let us explore the design task more thoroughly. Go back to the bullet points of section 2.2 where we list the reasons for the well established IC design flow and the difficulties in the MST case. Bearing these in mind, read the following article from the journal "MST News". (Click on the link below to reach the relevant copy of the journal and look at the paper entitled "Moving MEMS CAD Tools into the next Century"- the file is in PDF format). Read this critically now noting the following points and answer the SAQs in order to assess your understanding of the material. As you study this, compare it to what you know of the IC design task and the CAD tools available.

Note the "initial observations" made here.
Also note the suggestion that the tools will have to provide some guidance and help to the user in the area of modelling. Are there any other areas in which you think they would require help? They also suggest the use of proven intellectual property (IP) elements to reduce the Time To Market (they really mean the design time here). Can you see a difficulty with this? My own opinion is that the diversity of applications will make this difficult for MST as IP elements are likely to serve a smaller range of applications than with ICs. It is more difficult to design say a pressure sensor element for general use than say a JPEG module and the performance is more likely to be compromised. Do you agree with this?

Note the point made in the final paragraph on the inaccuracy of process parameters. Again, this is a major problem. SAQs 7, 8 & 9 refer to this paper.

Now have a look at the paper in MST News of 5/00 entitled "Towards Dedicated Design Tools for Microsystems"

The first thing to note is that the first paper was from April 1999 whereas this is dated May 2000 ie. just over a year later.

Read this paper carefully paying particular attention to the diagrams. Note the following:

• The influence attributed to "market pull".
• The four steps in standard IC design tool evolution. This is interesting as it suggests we are at a similar position to IC CAD tools in the 1980s. Do you think this is correct?
• The extra level of "system" design (c.f. IC CAD) which is suggested by figure 2.
• See the statement that "MEMS design tools do not need to handle the complexity of designs as the IC industry does. But must cover different domains and complex design flows". In other words the complexity lies in the flow and combination of flows rather than the sheer complexity of the design detail. Do you agree with this?
• Note the description of how design used to be carried out "they iterated their design between process technology runs." This means they used a prototyping, processing step as part of the design cycle. This may seem pretty drastic but IC design used to be like this in the 1980s. It used to be the rule, rather than the exception to allow for one prototyping stage. This was due to the lack of adequate simulation software. Imagine what this did to the design cycle time; design times of one year were quite common.
• Look at figure 3. Note the ad-hoc mix of tools used and note the steps of the problem-orientated approach suggested in the following paragraph.
• Look at figures 5, 6, 7 & 8. Do you think they cover all the possibilities discussed in the text? If not, where are the gaps?
• Note that Figure 5 shows a combination of fabrication simulations. These give the process models and parameters that will affect the performance of the microsystem. This is rarely included in an IC design. Have you any idea why? (The processing house will still characterise its processes to a very high degree but, since the processes are much more predictable, the results are usually taken into account in the design rules and there is no need for process modelling in the design stage).
• The author states that a behavioural model has to be deduced from the FEM results - Note the interdependence implied by this.
• The penultimate section suggests that quite a lot of co-ordinating effort is required to enable the development of comprehensive toolsets.
• Several important points are mentioned in the conclusions. In particular the need for an interdisciplinary design team. Also the need for three things: two different design flows and the link between them.
• Now note that this paper originates from the same company as the previous one. Do you think they have made much progress in the year?
  

The paper paints a fairly detailed picture of the requirements for an integrated set of design tools. Do you think this will ever come about? Can you think of any factors that would slow the development of such a toolset? The IC CAD business is largely driven by the huge amounts of money at stake. The CAD vendors have therefore a strong incentive to develop new tools and a very competitive market has resulted. For MST, the wide range of applications areas and technologies needed would mean a wide range of toolsets and a more fragmented market. This may limit the applicability of each and there may not be such an incentive for CAD vendors to get involved in this field. Until the predicted large market develops, the development of the tools may be slow.

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Updated 03.03.05 RA & RJ

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