A New MEMS Assembly Unit for Hybrid Self Micropositioning and Forced Microclamping of Submilimeter Parts

Mohsen Hamedi1,a, Milad Vismeh2,b and Parisa Salimi3,c 1,2,3 Mechanical Eng. Dept., University of Tehran, Jalal-Exp Way, Tehran, Iran

amhamedi@ut.ac.ir, bmiladvisme@ut.ac.ir, cparisasalimi@ut.ac.ir

Keywords: MEMS, Micropositioning, Microclamping, Electrothermal, Finite Element Simulation,

Abstract. In this paper we have presented an assembly unit equipped with electrothermally actuated microclamps (MCs), piezoelectric pad and rotary table to provide an environment for micropositioning and microclamping of submilimeter parts. The structural material of the system is considered to be <100> oriented Si with 20 µm thickness. Activating MCs, two approaches performed in the simulation procedure and results showed that utilizing 140 nm deposited Chromium thin layer on the U-shape structure as active material, reduces the overall input voltage and temperature in comparison with direct applying of potential difference to Si structure. To obtain more realistic results, both of these methods are simulated using finite element software in line with considering temperature-dependent thermophysical properties for structural and active material due to high operating temperatures. Design strategies and other advantages of using thin layer of chromium as active material are highlighted in the text.

Introduction Application of micro electromechanical system (MEMS) devices has become of great notice in

recent years. By devices' scaling down from macro scales to micro scales, the issues of handling, assembling and packaging of these micro components has provided a challenging environment for research efforts. During decades, several assembly methods have been proposed which can be categorized as self assembly with capillary forces [1], fluidic self assembly [2], wafer to wafer transfer [3] and manipulation with gripper arms and tweezers [4]. Parts’ uniquely positioning is specially discussed in [5] and also [6] has presented techniques for manipulation of submilimeter parts using electrothermal actuators. In addition to this, in [7] packaging at Wafer-Level is highly investigated based on uniquely orienting self assembly.

In our work we have designed a microfixturing unit in which hybrid self positioning and forced microclamping occurs so as to hold microparts after positioning. This provides them for further processes in Wafer-Level packaging method. A two degrees of freedom (d.o.f) rotary table equipped with ultrasonic vibration instruments is used to help microparts move smoothly to their positioning regions and electrothermal microclamps (MCs) are introduced here for clamping of microcomponents. Two different actuation methods for silicon MC arms are investigated in the paper and desired displacement and input voltage are obtained via finite element simulation.

MEMS Assembly Unit

The designed MEMS assembly unit consists of 2 major parts as micropositioning and microclamping to position and clamp four square parts of 415×415×30 µm dimensions. Micropositioning. Microparts are first self positioned on the unit. These microcomponents move on the substrate which is bonded on top of a steel plate. A circular piezoelectric pad is settled beneath this steel plate and by its ultrasonic vibration in frequency of 42 KHz, vibrates the plate to prevent microparts’ sticking to each other and to substrate. The vibration system is attached to a two d.o.f

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rotary table with the possibility of 20 degrees alignment. This alignment is used to guide parts to their true positions. A small magnifying camera is used on the top of the assembly unit and provides monitoring of parts’ movement with high resolution. Microclamping. As depicted in Fig. 1, the microclamping system consists of four clamps that are positioned on four sides of a square with 5.5 millimeter length. Each clamp includes two U-shape structures with anchors, thin arm, wide arm, flexure and wide arm end. Arms that benefit the advantages of standard U-shape electrothermal actuators are subject to produce in-plane rotation under the condition of applying potential difference to their anchors. Due to non-uniform joule heat generation in the structure, the thin arm heats up and expand more than the wide arm for its higher resistivity, then produces in-plane rotation. The symmetric in-plane rotation behavior of u-shape arms, trap the positioned microparts and do not let them out of the clamp (Fig. 1).

Fig. 1. Microclamping unit with its electrical connections and electrothermal u-shape arms after actuation.

Design Strategies of Microclamp Arms

Material. The principle of arms’ actuation was discussed in the previous section. Here we present some considerations for active and structural material selection by investigating two models. The structural material for both models is silicon with <100> rotation. Polysilicon, monocrystaline silicon and nickel are known materials in microfabrication processes and are suitable for wide range of applications in MEMS components. In our case study, silicon was the best candidate out of these materials due to reasons outlined here: Released Structure. There is a 10 µm gap between the lower face of clamps and substrate and 1500 µm of the overall length of microclamp arms are suspended over this gap. Thus, we have released structure with aspect ratios about 1:150. If this occurs, nickel is not an appropriate choice for its high stresses which appear during deposition that makes curled structure rather than straight one. Although these stresses are useful to make MEMS curled structures, they cannot fulfill our requirements in this study. Degradation Mechanisms. Electrothermal actuators which have identical shape to our microclamp arms, are fabricated with electroplated nickel and degradation mechanisms are investigated in [8]. Results showed that in temperatures about 513 oK (240 oC) back bending occurs in nickel structures, while expected temperatures of this phenomenon in silicon-based structures are noticeably higher than

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the mentioned value. On the other hand, plastic deformation in line with surface oxidation of nickel in high temperatures has shown to be responsible for lower output forward and backward displacement with increase in temperature. The other observation of [8] showed that because of compressive stresses in elevated temperatures and tensile stresses generated upon the removal of current, material pile up and necking takes place respectively. Integrated Fabrication. Integrated fabrication is a significant issue in manufacturing process which can reduce the overall cost of procedure. Notwithstanding the cost, the final nickel-fabricated structure in this case should be bonded to substrate via Titanium (Ti) adhesive layer and Cooper (Cu) seed layer for electroplating and this reduces the overall stability of structure in comparison with integrated silicon-based one. Dimensions. As the structure size decreases, its sensitivity to design parameters becomes more challenging. Several optimization techniques have been performed on u-shape structures to exploit the optimum dimensions so far. By applying sensitivity analysis, [9] determined that the most influencing design parameters on u-shape structure tip deflection were hot and cold arm width and flexure length. However, this was not the only clue we should have considered. In the approach presented here, the clamping unit is bonded to steel plate which works under the ultrasonic vibrations of piezoelectric pad beneath it. Since, clamps should be designed in a way that diminishes the probability of resonance which is responsible for structure destruction. To assure the clamps’ safe working, modal analysis is performed in order to find the vibrational modes and natural frequencies of the system. The first five modes and frequencies have been extracted as listed in Table 1.

Table 1. Natural frequencies of u-shape arm of a clamp from modal analysis. Number of mode 1 2 3 4 5

Frequency [KHz] 10.8 52.6 69.2 83.7 143.1

Operating frequency of the piezoelectric used here is 40 KHz. Because of the fact that vibrations start from this frequency, there is no problem with the first natural frequency which is 10.8 KHz. Clearly we see that other frequencies are higher than the value attributed to piezoelectric vibrations and so in a safe margin. Moreover, the second expanded mode which its frequency is nearer to 40 KHz between the first five modes, is expanded and shown in Fig. 2 to check for compatibility with vibrational mode of piezoelectric pad. It is interesting to highlight that according to Fig.2, not only this mode has no compatibility with piezoelectric vibrational mode to produce resonance, but also it helps the microparts better float to their target positions between clamp arms.

Fig. 2. 2nd vibration mode of clamp arm in 52.6 KHz.

By considering the guidelines mentioned above, final dimensions of microclamp arms are exploited as listed in Table 2.

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Table 2. Dimensions of microclamp arms.

Design Parameter Thin arm Wide arm Flexure Wide arm end Anchors Thickness

Length Width Length Width Length Width Length Width Length Width Value [μm] 1290 60 1100 180 190 60 150 55 340 340 20

Investigation of Two Models for Clamp Arms

Model Definitions. In this work, we have investigated two approaches for actuating clamp arms. The first one (model A) is a standard way for applying potential difference to anchors. The anchor which is connected to thin arm will accept voltage and the other anchor is electrically grounded. In this case, joule heating is produced in Si as structural and active material. The keypoint to present another actuation method is the favourable output displacement through applying lower voltage with lower input power and temperature. Therefore, the second model (model B) which comprises of Si as structural material and Cr as active material is developed for simulations. The differences of model A and B is illustrated in Fig. 3 that implies patterning of 140 nm chromium thin layer as active material on the top of the Si which serves to be structural material in model B. The Si thickness in both models is considered to be 20 μm. Since the electrical resistivity of Cr is significantly lower than Si, all the current related to input voltage passes through chromium layer in real situation and this thin layer is responsible for heating up the Si structure to produce desired displacement.

Fig. 3. Two models for actuation of clamp arms.

Finite Element Simulation

Meshing Models. As mentioned in the text, two models are selected for analysis. Meshed models are depicted in Fig. 4. In model B, because of its considerable low aspect ratio, 28699 elements were generated after meshing that increased the simulation overall time. Different colors in model B shows different materials in the model.

Fig. 4. Meshed models with single and double material for finite element simulation.

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Material Properties. Wide ranges of temperatures are obtained via simulations in this scales, thus, material models have been considered with temperature-dependant thermophysical properties from 300 oK to 1200 oK that are listed in Table 3 and Table 4.

Table 3. Thermophysical Properties for Structural and Active Material [10,11,12]. Property Si <100> Cr

Young’s modulus [GPa] 127 279 Electrical Resistivity [Ω.m] 7.5e-2 125e-9 Poisson Ratio 0.25 0.21 Thermal Expansion Coefficient [µm/mK] Table 4 4.9e-6 (at 300 K) Emissivity 0.6 0.26 Thermal Conductivity [W/mK] Table 4 Table 4 Density [Kg/m3] 2329 7190

Specific Heat [J/Kg.K] 702.24 460

Table 4.Thermal Conductivity (K) and Thermal Expansion Coefficients (α) for Cr and Si in terms of temperature [11,13,14].

T(K) Si Cr

α [µm/mK] K [W/mK] K [W/mK] 300 400 500 600 700 800 900 1000 1100 1200

2.62 3.25 3.61 3.84 4.02 4.15 4.19 4.26 4.32 4.38

156 105 80 64 52 43 36 31 28 26

93.7 90.9 86.0 80.7 76.0 71.3 68.4 65.4 63.7 61.9

Applying Boundary Conditions. The most important boundary conditions in this problem is thermal boundary condition. To increase the accuracy of estimated displacements and temperature, all three modes of heat transfer as conduction, convection and radiation are considered in both models. Due to arms’ small sizes and working in high temperatures, convection and radiation modes become significant. Convection coefficients should be calculated according to the geometry of the structure. In this work, by dividing the geometry into rectangular segments and assuming constant convection coefficient over specific temperature, these coefficients are obtained by empirical correlations for upper and lower face of horizontal hot plate and vertical plate. Fig. 5 Shows exploited coefficients in terms of temperature for segments of the arms. Validity of the model. The conditions of [8] applied on our modular model In order to validate the finite element simulation and accuracy of the model and the results of the experimental work discussed in [8] are reproduced. The results which obtained for points according to 380 oK and 460 oK in model D1 and 390 oK and 480 oK in model D2 showed results in good agreement with the corresponding values of [8].

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Fig. 5. Convection coefficients of microclamp arms in terms of temperature.

Results and Discussions

Coupled structural-thermoelectric transient analysis performed on both models. Model A converged after 10 minute while model B took 160 minutes to converge due to its considerable more elements. Tip displacement of both models are depicted in Fig. 6. The displacements are considered to range from 5 to 20 µm to compare the input voltage for models.

Fig. 6. Tip displacement versus input voltage for model A and B.

We can see that model A requires higher voltages to produce the same displacements than that of model B. In our case study, 10 µm tip displacement is enough to precisely clamp the microparts of 415×415×30 dimensions. For this magnitude of displacement, 20 and 2.8 volts are needed in model A and B respectively. Moreover, 20 volts in model A produces temperatures about 1010 oK, while

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appeared temperatures for its counterpart is 752 oK in model B. Fig. 7 shows attributed temperatures to each model in terms of applied voltage.

Fig. 7. Maximum temperatures attained in model A and B.

Lower voltage which caused lower current intensity and power and lower working temperatures made model B more suitable in our case study. Besides this, there are some advantages attributed to model B other than its lower power requirements and voltage. According to the overall structure for the clamping unit developed here and its integrated fabrication, silicon-on-insulator (SOI) wafers are the best choice. However, in case of SOI absence, we have to utilize a method for actuation and this is because of all anchors’ being electrically connected to each other via substrate. It is obvious that by patterning chromium thin layer, this connection is cut off and arms are individually actuated. Moreover, there comes a situation in which microparts are fabricated from Si that is a commonly used material in MEMS components. If this happens, microparts’ contact with clamp arms causes short circuit, whereas in case of using Cr, even the probability of this phenomenon diminishes. There are some important guidelines to choose appropriate Chromium layer thickness. Under the constant potential difference, the arms become more heated by increase in layer thickness. Changes in temperature in thicker layers are more noticeable than thinner ones; On the other hand, thicker layers increase the probability of bimetal behavior and out of plane rotation under the actuated mode; so an optimum magnitude must be patterned to meet lower voltage requirement and bimetal crossing out. There could be another model designed for higher efficiencies that can be peformed identical to NBC model discussed in [15]; this uses thermal insulators between the anchors and substrate to reduce the heat loss from anchors and in this case the dominant mode of heat transfer which occurs from anchors fades and higher efficiencies are obtained. Summary

We have developed a MEMS assembly unit for micropositioning and micoroclamping of submilimeter parts in this study. Electrothermal microclamps (ETMCs) were introduced for microclamping procedure and design strategies were investigated in detail and discussed. Then two approaches of actuation technique were proposed for the structure of arms. In model A, Si was considered as activae and structural material and in model B, 140 nm chromium thin layer was used as the active material over Si as structural material. Finite element analysis performed on both of them. Results showed that desired 10 µm tip displacement for accurate clamping of microparts in our case study, gained via applying 20 volts in model A and 2.8 volts in model B. The corresponding temperature values of these voltages appeared as 1010 oK and 752 oK for model A and B respectively. This observation together with advantages in application and fabrication of the structure, made model B more efficient in our study.

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Materials Processing Technologies 10.4028/www.scientific.net/AMR.154-155 A New MEMS Assembly Unit for Hybrid Self Micropositioning and Forced Microclamping of

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DOI References

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