Feb 2, 2009 - Micromechanical System Design for targeting a specific class ..... separation of components, the sharp edges near breaking points of each ...
Modular Microoptics assembly system with locking mechanism fabricated by one layer silicon ICP process
by Nataraja Sekhar Y MST Technologist µ‐bes Team, MST Design Lab I Department of Microsystems Engineering University of Freiburg Microbes team, MST Design Lab I, IMTEK, University of Freiburg
Modular Microoptics assembly system with locking mechanism fabricated by one layer silicon ICP process by
µbes Team Robert W Becker (Project Manager) Shankar KTR (Model, CAD, Website) Oscar Cota (Mask Layout, CAD) Nataraja Sekhar Y (MST Technologist, Assembly, Project report) Abhishek Ojha (Industrial View, Poster) Submitted to MST Design Lab, Department of Microsystems Engineering University of Freiburg
2nd Feb 2009, Freiburg Microbes team, MST Design Lab I, IMTEK, University of Freiburg
Contents Chapters
Page No
1. Project Plan(Budget & Time)
1
2. Introduction & Objective
2
3. Fabrication Process
4
4. Design task
5
5. Mask Layout & Assembly
11
6. Results and Discussion
13
7. Acknowledgement
17
8. Bibliography
18
i
Figures
Page No
1. Project time plan
1
2. FRDPARRC
3
3. Silicon ICP Process for fabrication of designs on Si wafer
5
4. Pillar concept
6
5. Design component list
7 & 8
6. 3Ddiagram of Model A assembly
9
7. 3D diagram of Model B assembly
11
8. Mask Layout for both the models
12
9. Fabricated wafer
13
10. Step by step assembly process of basic tower
14
11. Etching variation in the crosses of ground plate
14
12. Etching variations in pin and its effects
15
13. Model A assembly
15
14. Model B assembly
16
15. Schematic representation of micro‐interferometer
17
ii
Abstract The following report describes the methodology and results employed in a Micromechanical System Design for targeting a specific class assignment. Mechanical parts such as building blocks, compliant hinges and optical device holders were designed to build a LEGO‐like‐system and manufactured out of a Silicon Wafer by means of Silicon ICP process. Using the FRDPARRC analysis, two designs were obtained as variations of the same idea. The geometry of the parts was optimized to achieve as minimum size, best assembly and maximum stability as possible. With micro‐optics as an application focus, the parts were assembled to build an interferometer, although it can be assembled into another configuration. The system proved to function correctly with a total size of 3.8 cm. Results show promising for micro‐optical applications.
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1.
Project Plan
The project plan states the time and resources available in order to solve the assigned task (see page number3). 1.1
Budget Plan
Cost of Mask Layout (half mask) Double – polished wafer (3 half wafers) Cost of Silicon ICP process for Fabrication Other Costs Total Budget of Project
1.2
Time Plan
= 175 € = 60 € = 240 € = 40 € ‐‐‐‐‐‐‐‐‐‐‐‐‐‐ = 515 € ‐‐‐‐‐‐‐‐‐‐‐‐‐‐
Time was allotted proportionately to each stage in the project by taking into consideration of project tasks such as design, fabrication, assembly, report and presentation. Each of the five team members was given with sufficient time schedule in weeks to fulfil his task and towards achieving the project assignment. For Individual tasks please refer our site [4]
Figure 1 – Project time plan 1
2.
Introduction
Microfabrication or micromachining refers to the fabrication of devices with at least some of their dimensions in the micrometer range. In the early years, this discipline was almost exclusively based on thin and thick film processes and materials borrowed from Integrated Circuits (IC) fabrication labs. So the importance was on UV lithography, single‐crystal Si, and polycrystalline Si for mechanical applications such as pressure sensors etc. By the end of 20th century, as the application of micromachining broadened, emphasis shifted to a more all‐inclusive view of micromanufacturing methods like micromolding, drop delivery systems, wire electro‐discharge machining (WEDM), laser machining, ion‐ and e‐beam machining, and computer numerically controlled (CNC) ultra fine diamond milling etc [1]. In recent years, the applications of microfabrication or microsystems technology spread to the advanced research of diversified fields such as in the development of micro‐devices for biomedical research towards diagnostics, drug delivery, neural prosthetics, tissue engineering and finally in minimum invasive surgeries [2]. Particularly micro‐optical applications quickly transferred from advanced research labs to commercial market in endoscopy, laser systems, sensor systems and light sources. Further, the microsystem technology touches common life through optical fibres, digital image projectors, LCD (liquid crystal display) and TFT LCDs (Thin film transistors) etc. Last but not least, this advanced technology also helpful in marine science, forestry and as a key aspect of space research. So it is obvious that the development and fabrication of micro‐optical devices are of increasing importance in the field of data‐ and telecommunication networks capable of transmitting multimedia signals with high bit rates. Miniature optical sensors such as spectrometers and interferometers are another example for rapidly growing markets with a wide range of applications in biotechnology, chemistry, pharmacy, environmental technology, and automation, to convey only the most obvious. Various technologies are used for the development and fabrication of such devices. However, the success of the resulting product heavily depends on its price. Therefore, the manufacturing technique and miniaturization of devices are at least as important as the product itself [3]. “By fabrication we naturally mean more than the production of a system that allows mass‐production of the product. Among the criteria are also the pros and cons of the material to be used, automated assembly aspects, the compatibility with existing components or systems etc.” In the present study, we designed and fabricated a hinge and clip building system at the micro scale. Further, we proved the design pertinence by developing an advanced optical system using hinge and clipping system. We have also described the pros and cons of our model for future evolution, and explained the extensile of our hinge and clip system for developing diversified optical systems like advanced spectrophotometers, and scanners etc. 2
2.1.
Objective
The project assignment is as follows, To create a micromachined clip that pairs with itself at 90o so that two micromachined clips can be interconnected without breaking. The clip should be as compact as possible To design a set of parts based on these clips. To equip some parts with holders for ball lenses, and others with at least one further optical function (e.g., prism, lens, grating, Fresnel lens, mirror etc) To create a base plate from which the construction with manufactured parts can start. To create a hinge structure that would allow a part containing an optical component to be moved parallel to one of the construction axes by a significant distance. 2.2.
Project idea
Designs of the hinge and clip building system were developed considering the FRDPARRC* analysis shown in Figure 2. FRDPARRC is a systematic model development and decision making procedure that expresses the functional requirements, evaluation of design parameters, their risks and possible countermeasures. The goal is to develop the design with minimal risk.
Figure 2 – FRDPARRC * Functional‐Requirements‐Design Parameters‐Analysis‐References‐Risks‐Countermeasures. 3
After analyzing the functional requirements, design and assembly parameters, we preferred to develop two models with similar principle, viz. design A and design B. Major parameters considered for the optimization of design were miniaturization, stability and assembly, while both the designs diversified in range of miniaturization and several key elements, discussed in chapter 4. Utmost care has been taken while developing the designs. The designs were fabricated using a particular “Silicon ICP Process” using single mask. We will start the next section with a brief explanation of this important fabrication process, followed by the project designs, mask layout, assembly of he system with their subsequent results. The fabrication process is described in the first place, because of its importance to understand the design constraints.
3.
Fabrication Process
The aim of this section is to state the design constraints generated by the manufacturing process. The process used for the fabrication of designed parts was “Silicon ICP Process” using single mask and Si wafer. A wafer is a thin slice of semiconductor material, such as silicon, serves as the substrate for micro level devices built in and over the wafer and undergoes many microfabrication process steps such as mask preparation (discussed in chapter 5), cleaning the wafer, doping or ion implantation or metal oxidising, resist development, photolithographic patterning, etching, deposition of various materials etc. Over the years, each of the above process steps were developed sophisticatedly blended with high precision and accuracy for less than micro meter scaled designs. In few words, there are several cleaning methods to remove chemical and organic material from the surface of wafer, Positive and Negative resist development methods using mask, baking process of applied resist following soft chemical treatment to expose the etching area on Si wafer etc. There are different dry and wet etching methods in usage at commercial scale in IC (Integrated Circuits) and microsystems industries. So each step is crucial to achieve the final design and assembly. Improper cleaning leads to improper resist development and undesired etching. Figure 2 describes the complete fabrication process in brief. 3.1.1. Constraints generated by “Si ICP Process”
The following constraints were observed for the designs: 1. Components must be flat extruded profiles. (Single mask process) 2. Thickness of all components is 380µm. (Characteristics of the wafer) 3. The minimum feature size is 100µm (Etching process) 4. The basic length unit for the design becomes the thickness T=380µm. 5. Parts need a tolerance of 20µm. (Etching process) 4
Figure 3 – Silicon ICP Process for fabrication of designs on Si wafer With this brief introduction to fabrication process and associated constraints, we describe the development of designs in next section (i.e. chapter 4). The designs fit well into requirements and achieved certain precision for applications in optical microsystems and also for other microsystem devices.
4.
Design task
This section describes the development of two designs ‘A’ and ‘B’ towards assembly and functional aspects. In the first place we will explain the whole task using design ‘A’. Then the design ‘B’ would be described by comparing the appropriate variations with previous design. Initially we discuss about the principle of design followed by miniaturization and dimensional aspects. 5
The complete list of individual components used for design ‘A’ is shown in Figure 5A, associated with corresponding measurements or dimensions. For any additional information regarding these components please see supplementary information provided with the report [4]. The 2D and 3D CAD drawings were generated using an evaluation copy of Alibre Design v11 Tool [5]. The complete 3D diagram of design ‘A' is shown in Figure 6. 4.1.
Design of hinge and clip system
The first task is to create a micromachined clip that pairs with itself at 90o so that two micromachined clips can be interconnected without breaking. Further the clip should be as compact as possible. After repeated considerations of several perpendicularly aligned components, we finalised with the components shown in the Figure 5A (a, b and c). The functional parameter of b and c are similar. They pair perpendicularly with the base plate as shown in Figure 4a. Though several different components can be arranged in right angled directions, these particular pieces shown in ‘a’, ‘b’ and ‘c’ of Figure 4A has a partial lock and key mechanism. Common problem with miniaturized components is their assembly and automation. In the current design, the base pillars and the main pillars could be easily arranged as shown in Figure 4a. With a very flexible arrangement shown in figure b, the pillars could be easily aligned or raised with base plate and finally it was locked with specially designed pins. Without the support of pin the whole system is precarious, so the design of pin makes a major role in the development of 90o clip arrangement as shown in Figure–4b and 4c. The arrangement shown in Figure 4b was supported by a ground plate shown in Figure 6. This way of arrangement in vertical and horizontal directions could be continued as per the need and is stable. The total size of the design ‘A’ is 2.8 cm and dimensions of individual components were shown in Figure 5A. The smallest component in the whole design is the pin used for locking the pillars and plate, which is 120 µm in length and 48 µm in width.
a)
c)
b)
Figure 4 – chain concept a) Pillars connected to the plates in the tower. b) Analogy between tower connections and chain links connections. c) Ends of pillars are the half of the total height and leave a slot for the bridge. 6
Figure 5A Design ‘A’ Component List (All measurements in mm)
b
a
d
c
e f
g
h i
j
l k
7
Figure 5B Model B Component List (All measurements in mm)
a
b
c
d e
f
g h
i k
j
l
M
8
Elastic behaviour of the pin The special design of the pin was shown in the Figure 5A‘d’. The clip was meant to work as a cantilever beam under deflection. When fixed in the position, it would push the nearby wall of the cross in ground plate and prevent itself from disjointing. The pin was given a selected deflection (40µm) to calculate the length of this beam in order to minimize stress in the material. The pin was designed in such a way that they hold a very good tolerance and provide good stability to aligned base plate and pillars. As the pins were made up of silicon, the tolerance and other important physical parameters were calculated accordingly. The calculation of cantilever beam for pin was made as follows, For a cantilever beam loaded at its free end, with a force F, the maximum stress is generated at the fixed end. The maximum stress, σmax = Mmax/Izz × (h /2) The maximum deflection produced, w = FL³/3EIzz Maximum moment, Mmax = FL So wmax = 2 x σmax × L²/ 3Eh The deflection is expected to take care of the process tolerance of ± 20 µm. Beam dimensions are h = 100 µm and b = 380µm. Maximum shear stress, σmax = 500MPa Modulus of elasticity of silicon, E = 60GPa So we get L= 848 µm. So the length of beam, L is set as 1 mm in the design.
Figure 6 – 3Ddiagram of design ‘A’ 9
Optical Components and Bifurcation into 2 models Further task was extended to the design of components to support the desired optical components such as for ball lenses, prisms, mirrors, gratings etc. In this context, we chose the arrangement of Interferometer as a functional aspect to prove the pertinence of designed hinge and clip system. For each design, different holders were designed according to the plates. These included a beam splitter holder, a ball lens holder and a mirror holder. While design ‘A’ achieved minimal size, design ‘B’ allowed more freedom in designing their geometry. The optical system holders for design ‘A’ were shown in Figure 5A ‘e’, ’f’, ’g’, ’h’ and ‘i’. They can be easily located on the hinge and clip system. Interferometer arrangement Interferometer arrangement was achieved by developing appropriate base plates as shown in sub figures ‘e’ to ‘i’ of Figure 5A. The components of the interferometer designed in the current project were optical fibre for guiding the light rays, ball lens for projection, a beam splitter and mirror. Appropriate ground plates for the interferometer components were designed to support and hold the basic tower structures. Further the whole arrangement and pillars were placed at required distances to achieve the interferometer function. The ground plate arrangement was shown in sub figure ‘l’ of Figure 5A. The final design task was to create a hinge structure that would allow a part containing an optical component to be moved parallel to one of the construction axed by a significant distance. The ability of microsystem to perform a considerable movement is a complex requirement under fabrication constraints. Several physical parameters contact forces, weight and properties of silicon made this task complicated. Many solutions didn’t fit into the requirements as they made the whole set up unstable and further, the process and assembly tolerances of silicon aggravated this. However, important note is the considerable movement need to be achieved in microsystem would be few micro meters or little vibrations, which could obviously show effect on diverged light rays. This condition gave a little room to design the apt system. A bridge arranged with a set of spring structures could produce considerable vibration, however this particular function should be proved practically, so this topic is kept for analysing once again after assembly (i.e. chapter 6). For now, the bridge alignment is a little complicated process, where it should be locked by two main pillars and the mirror holder is made to link with the centre of the bridge in between set of springs. The whole arrangement should be assembled so carefully that no component should break, considering the tolerance limits of silicon. All that designed so far fit well for the required or expected functions. The final graphical assembly of the designed system or interferometer was shown in Figure–6. 10
4.2.
Design ‘B’ in detail
Design A is little complex and the variations in small dimensions during the fabrication, may cause total system failure. So we have developed design B with little flexibility. Design B was divided into two divisions, finally connected by a lock and bridge. The list of components for design B were shown in Figure 5B and final model is shown in Figure 7. The component ‘f’ in Figure 3B is ground plate, which could be clamped with similar base plates as many as we need. Unlike in design ‘A’, here we planned to build one layer of tower. This reduced the complexity of model assembly. A bridge supporting the mirror was used to connect two base plates and was designed for considerable movement. Further changes were ball lens and beam splitter holders. Special Y shaped and U shaped holders were used to improve the support for lenses. Pin concept was continued in this model. Several pins were designed to reduce the risk of model failure, shown in Figure 5B ‘c’, ‘j’ and ‘i’. The total length of design ’B’ is 3.8 cm, is 1cm bigger than design ‘A’. The pin structures were developedo with similar calculations and diversified in shapes.
Figure 7 – 3D diagram of Model B assembly The optical components for interferometer like mirrors were obtained from silicon wafer.
5.
Mask Layout and Assembly
In simple words, the mask could be explained as a stencil used to repeatedly generate a desired pattern on resist‐coated wafers. In use, a photomask is nearly optically flat glass, transparent to deep UV with an absorber pattern metal (e.g., an 800 Å thick chromium layer). It is directly placed in direct contact with a photoresist‐coated surface, and the wafer is exposed to UV light. The mask layout can be transferred directly on to semiconductor like Si surface. The masks, making direct contact with the substrate are called contact masks (hard contact). There are different kinds of masks 11
and their applications. Non contacted masks are called as soft masks, which are raised from 10 to 90 µm above the wafer. In the present assignment, mask layout is an important stage to get the designed components from the silicon wafer and to assemble the system. Care was taken while drawing the mask layout. There are few standard tools to generate mask layout. In the current study, the mask layout was generated using CleWin 4 tools [6]. The assignment parameters for mask layout are as follows. The wafer will be 380 micrometers (380 µm) thick. Furthermore, we will have half a wafer space for our design. 2mm of space along the middle straight line and little space of 10mm at the edge of 100mm diameter wafer was left according to instruction to facilitate the fabrication, also for breaking and collecting the components. Based upon several other parameters, the designs were transferred onto the surface of mask area. Care was taken while designing break points and outer surfaces of the designs were curved to avoid the breaks during fabrication as shown in Figure 8. Care has been taken while designing the breaking points to avoid the damage to components. Sufficient trench of 100 µm was used to avoid fabrication damages.
Figure 8 – Mask Layout included with both the models b) Wafer working area [2] c) Special features for layout. d) Breaking point close‐up 12
6.
Results and Discussion
In overall, the assembly of design ‘B’ was accomplished with minimal troubles and the assembly of design ‘A’ was unsuccessful and focused light on several hidden factors to be accommodated during the design, fabrication and assembly stages. The basic principle of locking mechanism is successful, as proved by model B. The following section explained the problems faced in design ‘A’ followed by the success in design ‘B’. 6.1. Breaking points of components The fabricated wafer was shown in Figure 9. Individual components for each model were collected by breaking them using surgical blade number 11 under microscope. Though we took utmost care, some of the components were damaged completely during breaking. For example, the pin structure shown in Figure 5B ‘c’ was complete failure. The breaking points were designed at the middle of the structure and it leads to the breakage of whole into two halves. Except this particular pin, breaking points for remaining components were designed better and collected easily. Another problem in breaking points is, it leads to sharp edges near breakages and disturbed the assembly process. Some of the collected components were shown in Figure 13b. Following the separation of components, the sharp edges near breaking points of each component were carefully removed. Finally, components for both the designs were collected.
Figure 9 – Fabricated wafer containing Model A and Model B together
6.2.
Tower assembly
The common principle shared by both the designs was hinge and clip system used for tower assembly, though the design is stable and extensible, it should be constructed only in particular sequence shown in Figure 10. Any other initiation than Figure 10 is not suitable to assemble the structure. During the assembly of design ‘A’, 13
we faced a major challenge, as we discussed previously regarding the dimensional risks during fabrication. The pillar structures couldn’t be located in base plates as shown in Figure 4. The width of pillar doesn’t match the trench in the crosses of ground plate. Several attempts using different pillars result the same. Further the pin structure in design ‘A’ affected the assembly process. The variations in the sizes of pin leads to lose holding of plate and pillar in few crosses of ground plate and become tight and couldn’t be placed in few crosses. Though it is the problem for the design ‘A’, the assembly of design ‘B’ were completed successfully.
Figure 10 – Step by step assembly process of basic tower for both models. 6.3.
Etching problems
As we discussed previously that the design ‘A’ calls for precise and accurate dimensions of components during fabrication process. When we observed the wafer under high resolution microscope, it revealed several etching problems. Some of them shown in Figure 11 and Figure 12. The inside etching patterns of crosses were disturbing. Some of the crosses were short etched and doesn’t help to fit the pillars and pins together. Further some crosses of ground plate were so loose that the pillars and pins disjointed with little vibrations in surface. In contrast, assembly of model ‘B’ is smooth, as it was designed to overcome the etching variations.
Figure 11 – Etching variation in the crosses of ground plate. 14
The pin in design ‘A’ was also affected collectively due etching problems in ground plate and pillars. The pin structure depends entirely on the tolerance of material and possible maximum deflection when inserted in positions. Being the smallest structure in whole design, some pins were short etched as shown in Figure 12c and 12d, others were broken during assembly as shown in Figure 12a and 12b. Finally the pillars were also not matching with the tranches in crosses.
Figure 12 – Etching variations in pin and its effects 6.4.
Partial success of design ‘A’
In spite of many problems, we could able to assemble small structures proving the possibility of principle as shown in Figure 13a.
Figure 13 – a) Assembled parts of design ‘A’, b) Separated parts of model A from Si wafer. 15
6.5.
Assembly of design ‘B’ accomplished
Figure 14 shows the completely assembled micro interferometer. In spite of partial success with design ‘A’, the design ‘B’ fabricated using the same process is complete as it was designed to overcome the etching problems. The pin structure in design ‘B’ succeeded in holding the structures stable.
Figure 14 – Complete assembly of design ‘B’ (length of the system is approximately 3.5 to 4.5 cm) Final aspect of the whole task is to prove the suitability of designs for optical microsystems. The interferometer arrangement was successfully accomplished as shown in Figure 15. Figure 15a shows a schematic representation of micro interferometer. Figure 15b shows the close view of micro‐interferometer arrangement. Providing a coherent light source through an optical fibre towards ball lens is possible and the beam splitter and mirrors or other components of any optical microsystem can be achieved simply by altering the dimensions in the hinge and clip system we provided in chapter 4. Further, the principle or system developed here is extensible to any optical microsystem applications. The design faced considerable problem during bridge assembly, because of vision problems in horizontal view and need to be redesigned. Considerable movement can be achieved from the spring structure provided with in bridge, as it proved to be stable and generate considerable deflection in incoming light 16
rays with small vibration. The mirror component attached was displaced with small vibrations. The assembly of whole structure with skilled hand under microscope may take approximately 5hrs. In spite of success in holder design, they took considerable time to assemble with complexity. The bridge arrangement should be changed to make it easy to assemble.
Figure 15 – Schematic representation of micro‐interferometer from top view Summarizing the whole task, provided with a suitable fabrication process, it is possible to accomplish the assembly of design ‘A’. The design ‘B’ should be refined further to replace the complex assembly process. The principle developed throughout the process proved to be suitable for optical microsystems and also extensible for advanced optical microsystems like Interferometers, Spectrophotometers, scanners etc.
7.
Future prospects
The design described above proves the success in basic principle and faced several rectifiable problems in dimensions, miniaturization and assembly process. Further whole design need to be refined with the above principle and the complexity in assembly process should be reduced. The system proves stable and flexible to be utilized as subsystems in optical micro‐devices with appropriate changes.
8.
Acknowledgement
We gratefully acknowledge Prof. Dr. Jan Korvink, whole staff of Laboratory of Simulation, Department of Microsystems Engineering (IMTEK) for their immense support, encouragement and co‐operation.
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9.
Bibliography
[1]
Madou MJ (2002). Fundamentals of Microfabrication: The Science of Miniaturization. 2E, CNC.
[2]
Paolo Dario et al (2000). Micro‐systems in biomedical applications. J. Micromech. Microeng, 10 235‐244.
[3]
Ehrfeld Wolfgang et al (1998). Novel concepts and technologies for manufacturing optical microdevices Proc. SPIE, Vol. 3573, 442.
[4]
http://sites.google.com/site/microbes05/downloads
[5]
Alibre Inc., 2350 Campbell Creek Blvd, Suite 100, Richardson, TX 75082
[6]
CleWin 4 Tool, Phoenix software for micro and nano technologies, PhoeniX Software GMBH, Dortmund.
[7]
http://sites.google.com/site/mstdesignlaboratory/notes
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