Micro-system design laboratory: the essence of engineering is design Jan G. Korvink Department of Microsystems Engineering – IMTEK and Freiburg Institute for Advanced Studies – FRIAS University of Freiburg, Germany (
[email protected])
Abstract This paper describes a relatively new Master's-level laboratory course in micro-system design, offered since two years at the University of Freiburg. The aim was to create a course that renders the following premises true: 1. The essence of micro-engineering, which is the act of designing micro-system product solutions, can be taught in a tightly organised two semester course. 2. Such a course should have student assignments which possess no single correct (or even necessarily known) answer. 3. The coursework should focus on creative engineering processes, yet build upon the scientific foundations of the degree program, so as to create opportunities for students to experience the exhilaration of starting a new endeavour through the creation of a unique product solution. 4. Students should work through a complete development cycle, from analysing a customer briefing, through creative conception of novel solutions, to manufacturing, assembly, prototype testing and reporting, i.e., students should learn by doing. 5. Attending and presenting such a course should be both educational and enjoyable. It is believed that the positive experience of running the course is relevant to design educators from other engineering disciplines. The aim of the paper is to communicate the concept to other practitioners. Keywords: Design course, microsystem technology, MEMS, laboratory course
1. INTRODUCTION A balanced engineering curriculum requires many varied components, such as theoretical courses that cover the fundamentals of technical science, practical laboratory courses that teach experimental methods, design exercises that practise the act of engineering, for example by planning and manufacturing prototype engineering systems, and workspace skills such as project management, quality control, or presentation skills. There appears to be a general agreement on this aspect among responsible planners, so that differences between curricula mainly address the needs of particular disciplines (such as the special requirements of e.g. mechanical, electrical, civil, mining, or chemical engineering). But in addition to established practise, it is thought that an engineering education also requires the introduction of design coursework that is completely devoid of predetermined or correct answers. Such uncertainty on the way to a solution echoes the engineering reality that exists outside of the classroom, a reality where engineers are expected to create new products and business processes in a highly competitive atmosphere. This paper discusses a two semester design course that is centrally organised around this aspect, during which student teams collaborate to solve engineering design, manufacturing and assembly assignment. The course was specially planned for students taking a Master's degree in microsystems engineering. It is hoped that the organisational principles are nevertheless interesting and useful to a wider engineering community. The course runs for the first two semesters of our German-language M.Sc. and English-language M.S.E. degree programs, and these two segments are referred to as microsystem design laboratory I and II (or DL I and DL II in abbreviated notation). The microsystem design laboratory course differs from laboratory courses based on the well-known 2.07 (now named 2.007, Fundamentals of design) course offered for over 4 decades at MIT [1], often considered to be the root of all design courses, whose assignments are less specialised, whose setting is an
end-of-semester competition, and whose students are younger and therefore have less theoretical background. It also goes beyond the scheme described in [2], where an interdisciplinary MEMS education curriculum is presented, but where the design work is placed in a highly predetermined framework. In [3] a nanotechnology and MEMS course is described that places the focus on systems design and control theory. There is no doubt about the value of such an approach, (and indeed IMTEK offers such a course as part of the Bachelor curriculum,) but careful analysis shows that it does focus on electrical engineering issues. In contrast, for the course presented here, a multidisciplinary end product is targeted, and creativity techniques are exercised. 2. PROJECT ASSIGNMENTS FOR MICROSYSTEM DESIGN LABORATORY I The primary goal of the design laboratory course, from a microsystems viewpoint, is to get the students used to thinking about and making very small machines. Nevertheless, in order to avoid too much complexity at the start of the course, and to steer away from systems design which would require an electrical engineering focus, it was decided to concentrate on kinematic micro-assembly design. The idea that has resulted is to combine design-formovement with systems design, and to combine functional design with design-for-manufacture and design-forassembly. Taken together, these requirements have resulted in the following two exemplary topics: 1. Building block system. Create a Lego®-like system of micro-parts that can be snapped together to build larger objects. The system should be as general as possible. Demonstrate the system by creating a microoptical bench which implements a nontrivial optical function. 2. Kinematic system. Based on a base plate with defined rectangular raster, create kinematic units that implement low order kinematic functions (translation, rotation, etc.) and that can be jointed together to form more complex kinematic chains.
The projects always have a generality requirement, which draws the design work into a generic direction, and a demonstrator requirement, which calls for specific solutions. This tension between generality and specialisation is useful, for generality demands simplicity, and many small interlocking parts, and specificity requires complicated, large, specialised parts. The compromise usually lies in between the stated extrema, and experience has proven this to be a very productive tension. What makes the assignment hard is that students have to design monolithic silicon parts that click together. Silicon has a useful modulus of elasticity close to that of construction steel, but it is a crystalline material and therefore suffers from the initiation of cracks at stress concentrations. So designing is a 2D affair, but with much attention to detail (rounded corners, part shaping, deformation limiters, and so on). Inspiration for manufacturing approaches are taken from two books [4], [5], and three exemplary papers [6], [7], [8]. Reference [4] presents a detailed explanation of the engineering design process, and [5] is a standard reference on manufacturing processes for MEMS. The papers [6], [7], [8] broadly address micro-assembly processes, and some of the techniques presented therein are used as inspiration for the assignments used in the design laboratory I course.
Figure 1. Microsystem design laboratory II project briefing (2008 & 2009).
3. PROJECT ASSIGNMENTS FOR MICROSYSTEM DESIGN LABORATORY II The second part of the course provides the student teams (now only 3 persons) with more organisational freedom. In addition, the requirements are set higher, and the assignments are planned to be closer to real-world
applications. The available technologies are freed up as compared with DL I. Students teams act as innovators creating intellectual property (IP,) followed by prototyping, for an imaginary client company. This is similar to the way firms like Zühlke [9], Debiotech [10] and perhaps Idealab [11] operate. One important complexity increment for DL II is the co-existence and co-design of two 2-dimensional design files (at least 2 out of: lithography mask, milling path, laser cutting path, printed circuit board (PCB) layout). The other requirement is to target at least one fabrication process that is both low cost and commercially available. Currently, the choice has fallen on the following manufacturing processes: 1. Soft lithography using SU8-on-silicon moulds and PDMS processing to create micro-fluidic channels and pneumatically actuated membranes. 2. Commercially available flex-stiff PCB substrates are manufactured at a provider company and are combined with Ultraviolet laser cutting or computer-controlled (CNC) precision milling. 3. Ultraviolet laser cutting of plastics, together with a lamination process. In DL II more than one design topic is provided, and the number of teams per topic is limited. This gives the students more possibilities to pursue their particular interests. An exemplary list of topics are: 1. Cell catcher. Based on a published paper [12], to create an improved cell handler. 2. Small search robot. Based on a published milli-robot [13], to create a low cost robot platform for earthquake victims. 3. Wind energy. To create a wind generator for low energy applications in remote regions. The students are provided the assignments in the form of a glossy brochure, the design briefing illustrated in Figure 1, where nontechnical paragraphs (visions) describe the nature of the products desired by the hypothetical clients. The text is of course a far cry from a precise engineering specification, and extracting information and constructing a precise assignment is one of the first tasks that the teams are faced with. 4. DESIGN METHODOLOGY AND ASSIGNMENT FORMAT The "Fredpark" or FRDPARRC methodology established by Alexander Slocum at MIT [1] is closely followed (although probably a number of other structured methods, see e.g. [14], would be appropriate as well). The letters FRDPARRC are an acronym for the column headings of a table that structures and guides creative engineering work. The column headings stand for: 1. Functional Requirements – what does the client want? 2. Design Parameters – how could the functional requirement be fulfilled? 3. Analysis – what is the rationale for the proposed solution? 4. References – what supports the proposed solution? 5. Risks – what can go wrong? 6. Countermeasures – what is the alternative plan? The method appears to be effective, because it not only instils a positive attitude, e.g. by helping students to ask the right questions at the appropriate stage of design, but it also appears to efficiently separate the phases characterised by the phrases "what does the client really demand of us", "let us think up unconventional approaches", "let us now get practical and refine our design", etc. It was instructive to apply the methodology to the design of the laboratory course itself, as indicated by Table 1. The assignment is presented in a form that more closely resembles the reality of an open marketplace. Students are posed the design challenge in a purposefully ambiguous manner, with many open questions begging for an explanation. The goal is to underline the need for students to take the responsibility to extract the "true" assignment from these statements. Typically clients will have some sort of a vision of what they want, a feeling for how it should be done. Students should learn how to listen to the wishes of the client, and to analyse out the true constraints and functional requirements, and separate these from implied constraints that artificially restrict thinking. Much care is invested to design the assignment so that it fulfils these requirements. 5. STUDENT TEAMS, ROLES, AND EVALUATION Students of DL I are organised into teams of five members. At the start of semester, when the teams form, they assign team members with the following roles and associated tasks:
1. Project manager. Creates a project plan (Gantt chart). Manages teamwork and discussions. Reports to staff on a weekly basis. Ensures that the team keeps all deadlines. This student is evaluated on the quality of the project planning. 2. Mask lay-outer. Creates the lithography mask layout of the team's design. The student is evaluated on the quality of the mask layout. 3. Industrial designer. Focuses on industrial design issues, such as usability. Creates a poster of the project work. This student is evaluated on the poster quality. 4. MST technologist. Focuses on the manufacturing technology and design rules. Creates the design report. This student is evaluated on the report quality. 5. Model builder. Focuses on building large prototypes of the design for group evaluation of concepts. Maintains a web page. This student is evaluated on the quality of the web page.
Functional requirements
Design parameters
Analysis
References
Risks
Countermeasures
Educational
New ideas, new responsibilities, design is focus
The content of other engineering curricula.
Design literature. MIT 2.07 & 2.007 course notes [2]
Too much new, too much close to other courses
Keep design methodology in foreground
Manageable
# required staff, exact event plan, budget, WWW
TA can handle 2 teams of 5
Course at MIT and other Mech. Eng. universities
Too much work. Too many staff required
Plan well. Estimate time. Increase staff.
MST relevant
MST solution, cleanroom manufacturing
Process is Si ICP etch, one mask; MST kinematics
Many processes are described in [3]
MST is broad and complex, hard for students
Fix technology to a few steps. Clarify feasibility
Team-oriented
Small teams with clear member roles
Mix M.S.E and M.Sc. students
Creativity techniques + the dream team are described in [4]
One does all, lazy students pass anyway
Give marks for roles, and for result. 60/40
Problem solving focus
No preconceived solution, handson experience
Prototype items; CNC milling, laser cutting?
The importance of model building in other creative areas.
Students wait for my solution; they avoid thinking
Reiterate idea weekly during entire course.
Interdisciplinary
Project must require more than one technique
Process, mechanics, systems
Current hot topics in research and start-ups.
Focus is on one area, losing interest of group
Select fields well according to current interest
Part of a larger whole
DL II follows from DL I
Increase responsibility, expectations, complexity
Becomes too complex, continuity. difficult
Plan well. Join planning of both courses
Learn entire design cycle
Phases include all steps, shift of focus
Add deadlines/ deliverables at end of each phase
Course is not completed in required time
Give milestones, mile-pebbles, deliverables
Inspirational & entrepreneurial
Dynamic rhythm, fun topics, revelations
Draw from product design, Start-ups
Students lose momentum, lack of attendance
Keep topics fresh, short, focused, with time to think
Idealab, Smit
Table 1. FRDPARRC table used to plan microsystem design laboratory I & II. Each student maintains a hand-written notebook where solutions and "patent-able" ideas are sketched, where information is collected, and where proof of participation in all activities are recorded. Each team member may contribute ideas to any aspect of the project, and the notebook bears witness to this effort. Thus, the notebooks are evaluated and contribute to the individual mark of each student. The teams submit a lithography mask design, a report and a poster at given deadlines, and then present their project with an oral overhead presentation followed by a short poster presentation and demo of the assembled microsystem. For the quality of each of these components they are assigned a team-wide mark. The final mark is the sum of a student's individual performance and the team's performance. A summary is given in Table 2.
PERSONAL Role
PERSONAL Notebook
TEAM Report
TEAM Poster
TEAM Prototype
TEAM Presentation
30
10
15
15
15
15
Table 2. Typical student mark weights (in %) for microsystem design laboratory I components.
6. PROTOTYPE COURSE PLAN The course is planned as a set of activities and deadlines, with goals for the instructors and the students, see Table 3. Based on 15 weeks, the table specifies the plan for DL I. The instructor and student goals are quite different, since instructors aim to teach in tandem with the pace of learning, gradually revealing details about the design requirements, whereas students, in general, wish to complete a design, and to remain productive.
Week
Topic
instructor goal
Student goal
Deadline
1
The design process
How to design
Get team organised
2
Creating ideas
How to get inspiration
Create connector ideas
3
Assembly systems & transcending flatland
Achieve 3d with 2d parts
Create parts system
4
Compliant kinematics Guest talk on ICP design rules
Avoid hinges at the micro-scale
Create a prototype model
5
Mask layout laboratory
Teach the layout tool
Redesign for process
6
Reverse engineering
Learn from others
Simplify concept
7
Assembly & microscopy laboratory. Guest talk on industrial design.
Influence design for assembly
Create first mask layout
8
Mask design feedback
Avoid obvious mistakes
Make deadline
9
Drawing, documenting, reporting
What to tell where
Poster design
10
Report writing
How to write a report
Overhead design
11
Ethics in design Guest talk on professional responsibilities
Not just solving the problem
Report design
12
Assembly laboratory I
Time for assembly
Build system
Poster
13
Assembly laboratory II
Time for assembly
Photograph system
Report
14
Project presentation
Evaluation
Transmit ideas
Overheads
15
Project presentation
Evaluation
Transmit ideas
Prototype models
Lithography mask design
Table 3. Event plan for microsystem design laboratory I.
7. MAINTAINING MOMENTUM AND INSPIRATION A number of measures have been taken to captivate the imagination of the students, to keep their momentum level high, and to maintain motivation throughout the semester: 1. Pace. The pace of the course is considerable, necessitating that timing and ordering is continuously adjusted to maintain it close to the needs of the students. 2. Competition. A healthy competitive spirit exists, as teams veer for the best design solution, yet learn to help each other in gaining access to resources. A prize is awarded to the team with the best design.
3. Bonding. Student teams choose a corporate identity, and keep their ideas confidential from other teams until the design deadline. 4. Inspiration. Invited industrial experts (for example, an industrial designer, an innovation manager, a silicon technologist, an application scientist) are engaged to hold presentations and discuss designs with the teams. 5. Haptic. The CNC milled or 3D printed prototypes produced early on in the project provide not only haptic feedback, but represent a first level of motivating success. 6. Success. The most inspiring aspect for the students, as far as can be determined, is the chance to manufacture something small. For most of the engineering students, the opportunity to "engineer" appears to be an important goal. In addition, the entire course with documentation and interaction is supported by a bilingual website, see Figure 2. Whereas in itself this is nothing really new, the website is maintained as a space to visit regularly, a space to find answers to organisational questions, and resources, but also a space to find inspiration.
A further measure has been the creation of a course flyer aimed at Bachelor-level students in the semester before they will attend the course. Its goal is to advertise the course, to answer initial questions and to clarify initial uncertainties.
Figure 2. Pages from the class website at http://www.imtek.uni-freiburg.de/simulation/dl .
8. SELECTED RESULTS The non-public student team websites organise each group's work by providing a repository for design files, a corporate identity for the group, and an online forum for their discussions. At the same time, staff can regularly access the websites to monitor progress of the groups, and to identify any arising difficulties. Figure 3 shows some images of the student websites.
Figure 3. Pages of some student team websites from microsystem design laboratory I 2008/2009. From left to right the teams are: Rudis MST Fabrik; MicroBricks, CliX.
The DL I tasks result in an assembly system made of monolithic silicon, etched by inductively coupled plasma (ICP, often referred to as the Bosch process) from 380 µm thick 100 mm diameter silicon wafers. Student solutions vary from systems using a few special parts, to many simpler pieces. To demonstrate success, students have to prototype the kinematics of an optical bench. An exemplary set of solutions is shown in Figure 4.
The DL II results are taken from the cell catcher projects. A total of 19 unique designs were created and tested by the students. Most student groups used the COMSOL finite element simulation workbench [15] to model the laminar flow profiles around their cell catcher geometries. Simulation enabled design optimisation and subsequently a reasonable success rate for cell isolation. Illustrations of various designs are shown in Figure 5. 9. TIMING IS EVERYTHING At Freiburg University an academic year is split into two 15 week semesters. All courses are expected to be single semester packages. These are the most important time constraints. The planning strategy has been to initially plan backwards from the end of semester, and to start with the timing of the manufacturing. For example, in the project where students design silicon parts that are etched from 10 cm diameter silicon wafers, if 2 weeks are allocated for the student presentations, 2 weeks for assembly laboratory, 2 weeks for cleanroom micro-manufacture, and a week for mask production (which is done by a commercial vendor), then on has to plan the design deadline 7 weeks from the end of semester. (Other disciplines and manufacturing technologies will require similar considerations.) This is about halfway through the class, and implies that any design background material can only be presented during the first 8 weeks of semester, which is subsequently planned in a bottom-up manner. The tasks to be completed during the second half of semester focus on documentation and on assembly. A typical semester plan for DL I, with the weekly topics, instructor goals, student tasks and hard deadlines, is shown in table 3.
Figure 4. Micro-optical bench assembly systems produced by 6 of the DL I student teams in 2008/2009. Team images of (1) assembled optical benches and (2) typical parts are marked by a letter of the alphabet as follows: (A) Microbricks; (B) Rudis MST Fabrik; (C) Micobes; (D) DARAF Silicon BAM; (E) MICS; (F) Build it smart.
Given these constraints, it is essential to ramp the students up to full speed in the most efficient manner that is both practicable and affordable. One of the challenges (to course planners) of modern times is posed by the mobility of students within Europe; in our case the challenge especially arises because of the heterogeneous backgrounds that beginning Masters degree students have. Currently, students arrive at the start of the course in one of two broad categories: those that have a Bachelor degree in microsystems engineering or with relevant industrial experience (and so e.g. know what a cleanroom is, and a photolithographic mask, and how silicon is etched, among other things) and those that do not (and so need a considerable amount of filling in of these details so as to be able to complete their design assignments). Of the many ideas on how to equalise these two groups, (e.g., adding additional classes beforehand, providing additional information material, or by splitting students into two streams,) experience indicates that an efficient method has been to form mixed groups of students. The student teams spend a fair amount time together, discussing, developing ideas, and seem to be quite efficient in supporting each other in getting to par on the essential background ideas. The method has another positive side-effect, in that foreign students appear to make friends early on during their studies, and so are experienced to be more efficiently assimilated. It is speculated that the local students, apparently proud to help, learn to respect and even enjoy the international atmosphere, and additionally gain an opportunity to practise their English.
a)
b)
e)
c)
d)
Figure 5. Cell catchers, approximately 5 mm by 10 mm: a) design; b) & c) layout; d) FEM flow simulation streamline plot; e) successful realisation and testing with glass beads of 40 µm diameter.
Conclusions It appears that MST Design laboratory I & II has verified the premises stated at the beginning of this article. The two semester course works well, mainly because it is tightly organised, with the organisational burden sensibly divided between students and staff. The lack of a single correct answer to the assignment confuses students at the start, but then results in a creative phase with unique solutions. Students move from scribbles to calculations and simulations, and simultaneously a team identity emerges as students identify with their creation. To date, every team has successfully completed the course, albeit with varying degree of stress towards the deadline, and ingenuity in the final product. Student feedback is positive to a high degree, with few exceptions. As an instructor, teaching microsystem design laboratory has been a positive experience. The bundled creative enthusiasm of a large group of capable young people is certainly quite impressive. Seeing projects start from an empty slate and progress, in parallel, to a wide range of different interpretations and solutions, often with ingenuity, is personally very rewarding indeed. So one can certainly conclude that it is a rewarding experience. All the more, the pressure that is experienced to ensure that everything works just "right" is considerable and should not be underestimated. In addition, the course places strong demands on the exercise staff, who all have to carry the spirit of the course whilst interacting with the students during the exercise, and help to ensure that the manufacturing happens on time. Thus, having a capable and motivated team of teaching assistants is an essential conclusion. And doing a dry run with the technology during the preceding lecture break helps the instructor team to clarify the challenges that students will be faced with during their project. Another important conclusion is related to the micro-manufacturing step. Major manufacturing difficulties have yet to be experienced, but problems could arise at any time (e.g., a machine could break down). It is hard to have
a plan B ready for such an in-eventuality, and the most important backup is currently represented by co-operation relationships with other fabrication facilities, mostly dependent on good-will. Substantial costs are involved in running and equipping such a course, and for micro-technological fabrication these have to be invested carefully. A major conclusion, though, is related to the design of the course. It has been learned that keeping the manufacturing in the realm of known processes, utilising only a few manufacturing steps, and keeping well away from design rule limits is not only wise, but quite educational in itself. From numerical optimisation research it is suspected that heavily-constrained problems are very hard to solve. In this realm, however, the human brain appears to excel and, given enough talent and preparation, it can even become ingenious. Experience indicates that students respond well to the challenges posed to them. With known processes, students can completely focus on learning the benefit of rational design. The added benefit of a successful manufacturing process step is of course its motivational effect on students and staff alike. Acknowledgements I wish to express my sincere gratitude to those IMTEK staff members who have helped me to run the course. Their engaged involvement has greatly enriched the experience of the students. From my own research group, Dr. Patrick Smith and his team Dr. Laura Del Tin, Mr. Dario Mager, Mr. Emmanuel Bouendeu, Mr. Dirk Strohmeier and Mrs. Ute Löffelmann have helped to co-ordinate and run the course. Dr. Michael Wandt and his team of the Cleanroom Service Centre have ensured that the silicon processing was done seamlessly and on time. Dr. Vlad Badilita and Mr. Florian Schneider of the Micro-actuators group have helped us extensively with the SU-8 and PDMS processing. My sincere thanks goes to the guest speakers, Mr. Stefan Lippert, Dr. Patrick Ruther, and Mr. Dieter Schaudel. Mr. Schaudel as chairman of the FAM (www.fam.uni-freiburg.de) is also thanked for donating the student prizes. I wish to thank the IMTEK colleagues who evaluated the student contributions. Finally, my appreciation goes to the engaged students whose hard work makes the course so rewarding for me: 1. Student team Microbricks consists of: Moritz Stürmer, Markus Spothelfer, Philip Dautel, Ralf Zimmermann, Christian Böhle 2. Student team Rudis MST Fabrik consists of: Mark Keller, Ralph Müller, Bülent Kanat, Alexander Bär, Rainer Schiller 3. Student team Micobes consists of: Robert W. Beck, Oscar F. Cota, Abishek Ojha, Sekhar Nataraja Y., Shankar Karanilam Thundiparambu Ravindran 4. Student team DARAF Silicon BAM consists of: Damir Pfau, Alex Hoch, Roman Keding, Aron Guttowski, Fabian Kohler 5. Student team MICS consists of: Manuel Rainmann, Tobias Burger, Stanislaw Munt, Max Schellenberg, Hagen Meyer 6. Student team Build it smart consists of: Dongzhe Yue, Chen Nai-Hsuan, Andreas Müller, Szymon Pacak, Nan Wang 7. Student team CliX consists of: Johannes Domke, Jonas Handwerker, Michael Badeja, Sebastian Kiss, Marius Clad, Nirdesh Woja References [1] A. Slocum, Fundamentals of Design course 2.007 website (On 27 December 2008 at http:// pergatory.mit.edu/2.007/resources/FUNdaMENTALS.html) [2]
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[3]
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[6]
M. B. Cohn, K. F. Böhringer, J. M. Novorolski, A. Singh, C. G. Keller, K. Y. Goldberg, R. T. Howe, Microassembly Technologies for MEMS. SPIE Conference on Micro-machining and Micro-fabrication Process Technology IV, pp. 2-16, Santa Clara, CA, September 21-22, 1998
[7]
Tahhan, Isam N.; Zhuang, Yan; Boehringer, Karl F.; Pister, Kristofer S.; Goldberg, Kenneth Y., MEMS fixtures for handling and assembly of micro-parts, Proc. SPIE Vol. 3876, p. 129-139, Micro-machined Devices and Components V, Patrick J. French; Eric Peeters; Ed
[8]
Syms, R.R.A.; Yeatman, E.M.; Bright, V.M.; Whitesides, G.M., Surface tension-powered self-assembly of microstructures–the state-of-the-art, Journal of Microelectromechanical Systems, Volume 12, Issue 4, Aug. 2003 Page(s): 387 - 417
[9]
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