The analysis and visualisation of metal microstructures

Advances in Engineering SoJbvare 28 (1997) 573-580 0 1997 Elsevier Science Limited. All rights reserved Printed in Great Britain 096%9978/97 $17.00 + 0.00

PIkSO965-9978(97)00045-8

ELSEVIER

The analysis and visualisation of metal microstructures A. E. U. Lees Senior

Lecturer,

School

of Engineering

and Manufacture,

De Mont$ort

University,

Leicester.

UK

(Received 22 June 1995; revised 5 June 1997; accepted 15 July 1997)

Micromechanics is the design and manufacture of very small three-dimensional parts. It combines electronic, mechanical, optical and other technologies into a single system and employs miniaturisation to achieve high complexity in a small volume. Micro engineered devices and components have major dimensions measured in micro- or nanometres. An overview of microengineering will be presented together with actual examples of micromachined devices. The analysis of microstructures is essentially the same as the analysis of larger components, however the tiny dimensions of these components does pose problems to some of the ‘off the shelf’ analysis systems. Some of these problems are discussed together with some simple guidelines for successful analysis. A case study example is presented detailing the finite element analysis @%A) of a novel metal microaccelerometer manufactured using the LIGA technique (Maner, A., Harsch, S. and Ehrfeld, W., Plating and Su$uce Finishing, 1988, March, 60-65). The finite element analysis software ALGOR has been used in this assessment. Special attention is given to the accuracy of results. 0 1997 Elsevier Science Limited. Key words:

ALGOR,

sensor, micromechanics,

1 INTRODUCTION

LIGA, finite element analysis.

microengineering and the processes involved in the manufacture of devices. Due to the micro scale of the components, it is envisaged that new processes and methodologies for visualisation and analysis will be required. The response of micro components subjected to mechanical loadings, such as those arising during manufacturing, handling, assembly, and operation, need to be assessed. Micromechanics is dominated currently by the use of silicon; for micromechanics to realise its full potential a much wider range of material must be investigated such as metal, plastic, ceramics, and sol-gels.4

are made using either established semiconductor technology (selective wet chemical etching of silicon) or emergent technology such as LIGA. The LIGA process is a combination of X-ray lithography, electroforming and moulding. Microengineering applications include; sensors, actuators, electrical/optical connectors, and instrumentation for minimum-invasive surgery, often with a multifunctionality not previously available.’ Few organisations have access to all the tools needed for microengineering, i.e. in the LIGA process access to a synchrotron radiation source such as the one available through the EPSRC Daresbury facility is essential. This topic is predicted by many,2 especially those in the UK’s Micro Engineering Common Interest Group (MCIG) and the EPSRC VLSI Technology Committee, to be a key enabling technology, which, in conjunction with other disciplines, will have a considerable strategic industrial importance. There is a genuine need to establish standards and design rules to facilitate the growth of micromachining; preliminary investigations will entail the scoping of Microstructures

1.1 The LIGA process During the early 198Os, researchers at the Karlsruhe Nuclear Research Centre in Germany developed a novel variation of the conventional planar process to produce non-silicon microstructures of great height.5 The technique is known by the German acronym LIGA after the process steps of Lithographie, Galvanoformung and Abformung (in English, lithography, electroforming and moulding). The basic LIGA process involves the use of a stable metal 573

A. E. U. Lees

574

MAIN PROCESS

VARIATIONS

MAIN PROCESS

VARIATIONS

Manufactureof a

. substrate

\

preparation

Formation of a sacrificial layer

anddissolveresist

1

I

Irradiate

Moulding

I

IWJJ I

conducting baseplate

I

1-z IBake

1

1 electroforming

1

Removethe resist

LocaJiaed etchmg

I

of the sacrificial layer I 4 With electrrcally conducting injection moldingplate

Metal structure via second electroforming

Fig. 1. Typical paths through the LIGA process. substrate plate, coated with an extremely thick (up to 1000 pm) layer of photoresist. After exposing and developing the resist, the resulting voids are filled with metal by using an electrodeposition process.When the resistcavities are completely filled, the remaining resist is chemically removed to leave an entirely metallic microstructure. This may then be utilised directly, or usedasa moulding tool for further production processessuch as injection moulding. In contrast with planar technologies,the microstructures produced using LIGA can have a large structural height, with height-to-width aspect ratios in the range 1OO:l to 1000:1. However, the structures still remain as projections

of a two-dimensional mask and it is for this reason that LIGA is often termed a ‘two-and-a-half-dimensional’ fabrication process. LIGA is still an immature technology and its useto fabricate microstructures cannot be describedin terms of a set sequence of well-defined process steps. Rather, it is a general basetechnology that is characterisedby the use of deep-etch lithography, electrodeposition, and a subsequent moulding process.Someof the many processvariations that are possibleusing LIGA are illustrated in Fig. 1, the particular route chosendependsupon the final product to be manufactured.

The analysis and visualisation Since an LIGA mask is large (50-70 mm diameter) in comparison with the tiny dimensions of the microstructures, there is the opportunity to prototype many structures in a single process cycle, thus allowing simultaneous fabrication of a large number of components. The most important aspect in the realisation of these LIGA microstructures is the mechanical design. It is in this phase that the key issues of structural integrity (stress, vibration, thermal behaviour), material behaviour, process tolerances and mask layout must be considered.

2 METAL

MICRO

ACCELEROMETER

2.1 Design The design of an automotive accelerometer is the result of a number of trade-offs. The sensor must be sensitive enough to measure changes in acceleration of the order of 10 mg. At the same time it must be able to withstand high shock loading during the manufacture of the vehicle and must be able to respond quickly to the accelerations to which it is subjected. Extensive modelling is required to optimise the various design criteria. The performance targets for the final design could be summarised as follows. Range Accuracy Cross-axis error Shock survival Frequency range Temp. range

t lg t 2% but -C 5% at temperature extremes 2 1% > 5ooog 0 to 50 Hz - 40to 125°C

Sub-micron geometric feature sizes and tolerances have been achieved by the LIGA process; however, for a ‘right first time’ approach feature, sizes and tolerances an order of magnitude greater would yield a higher confidence in the final design. Both designs presented here use this philosophy. When designing for LIGA deep, narrow sections should be considered very carefully. It is possible to develop and electroform deep narrow sections but it is best that they are avoided. Where devices have moving parts sharp internal and external comers will cause stress concentrations,

of metal microstructures

575

these should be replaced by fillet radii. A feature such as a 2 pm radius can be achieved; however, a larger radius would reduce stress concentrations. The initial accelerometer concept is shown in Fig. 2. The design, chosen for its simplicity, was felt to be appropriate considering the nature of the manufacturing process. The sensor was designed to perform predominantly in the zero to lg range, and it comprised a large seismic mass connected to a large fixed mass by a very thin beam, the sensor was to be bonded to a metallised alumina substrate. The dimensions of the un-packaged sensor measured 4.5 mm X 1.0 mm X 0.5 mm, the nickel was 1.0 mm thick and the alumina 0.625 mm thick. The beam was 1.0 mm X 0.5 mm X 0.02 mm. The bonding between the alumina and the nickel was to be via a silver loaded epoxy resin. The seismic mass would deflect under load, to a maximum travel of 0.01 mm, at which point it would touch the sensing element. Deflection of the seismic mass would be measured as a change in capacitance. There are several regions within the design that are subject to special tolerance requirements. The active face of the seismic mass, i.e. that face which is ‘in contact’ with the capacitive sensor, should be 10 pm above that sensor. This dimension should be maintained to within plus or minus 1 pm; a deviation would have a detrimental effect on the sensor performance. The active face and the sensing element should be flat and parallel to each other. The LIGA process yields extremely parallel and flat structural sides; however, over a 500 pm depth, a run out in the order of 1 pm is expected.

3 ANALYSIS 3.1 Investigation The size of the flexural beam was critical to the performance of the sensor. The following parameters were evaluated using the finite element analysis software ALGOR (Version 3.22): 1. the displacement of the mass with respect to the thickness of the beam;

Fig. 2. Micromachined low “g” accelerometer.

576

A. E. U. Lees Table 1. Finite element analysis material properties Poisson’s Ratio

Youngs mod. (MPa) 6000 324 300 207 000 75 800

Epoxy a Alumina Nickel Agffd b

0.33 0.22 0.36 0.33

Coeff. lin exp (K-l) 72 x lO-6 6.4 x 1O-6 13.3 x lo-6 19.5 x 1o-6

Density (kg m-3) 3750 8880 10500

a Epoxy loaded with 70% silver particulate. b Silver-palladium thick film conductor (predominantly silver). 2. 3. 4. 5.

the stress sensitivity of the maximum stress in the natural frequencies the response of the conditions.

3.2 Material

the beams due to ‘g’ loading; the beam due to overload; of the structure; structure to hostile thermal

properties

Relatively little data is available on the microscopic properties of materials, and the relationship between bulk and microscopic properties is not yet fully understood. Properties vary from source to source and in accordance with precise composition, Table 1 details the values used in the analysis. As pure nickel is a relatively poor engineering material great care should be taken over the stress levels in the component. A conservative design stress limit (yield) would be 50 MPa, however this limit would be greatly improved by alloying with other metals. 3.3 Modelling

details

Dimensions were extracted from design data in millimetres, nominal dimensions were assumed. The location spigots were neglected. Materials were assumed to be linear elastic and isotropic. ALGOR offers an estimate of the precision of results based on extrapolation from the element Gauss points. Where stresses in the beam are considered, von Mises’ stress was used as a measure of the likelihood of yielding. In order to assess model accuracy two models were investigated.

errors in model 2 would be reduced by refining the model, as large geometric (thickness) discontinuities would always be present. Model 2 was rejected as unrepresentative of the physical problem. Relatively high accuracy (25%) was predicted by Model 1 (refined mesh), yielding a high confidence in results. Model 1 was used in subsequent analyses (Fig. 3). 3.4 Displacement

and stress vs beam thickness

Four beam thicknesses were assessed; 10 pm, 20 pm, 30 pm and 40 pm. A graph of active mass displacement vs beam thickness for a Ig acceleration in the - z direction is presented in Fig. 4. A graph of Von Mises stress vs beam thickness for a lg acceleration in the - z direction is presented in Fig. 5. For the 20 pm beam, the maximum displacement for a lg acceleration in the ‘action plane’ was 5 pm at the far end of the mass and 1.1 pm at the near end, giving a mid-span displacement of 3 pm. The actual displacement would be slightly larger due to electrostatic force, however this extra displacement was neglected. The displacements were commensurate with the design requirements, i.e. the mass would not foul the sensing element, neither were the displacements too small to yield useful output. The stresses associated with this displacement were very small and did not give rise to concern. Shock loading was not considered directly (see ‘worst case analysis’ (Section 3.7)). Cross sensitivity, i.e. lg in X and lg in Y, was not evaluated. These were assumed to be second-order effects due to the considerable thickness in the third dimension.

Model 1, from isotropic four-noded 2D elements (plane strain). Model 2, from isotropic four-noded 3D plate elements (reduced shear formulation {best}). Model name

Load case Error (%)

Model 1 lgin-z Model 1 1 g in - z (redefined) Model 2 1 g in - z

Displacement (at A) mm

Stress 6’ W N/mm *

44 25

0.00497 0.00502

5.32 6.55

49

0.00794

5.68

Model 2 indicated high errors (49% overall accuracy of results). Displacements were higher than for Model 1, but stresses were comparable. It was unlikely that the high

Fig. 3. Model 1 finite element mesh (fine).

The analysis and visualisation

IOum

20um

577

of metal microstructures

30um

40um

Bum lhkknoss (microns) Fig. 4. Displacement of active mass (points A and B) vs beam thickness. 3.5 Thermal stress model In order to predict the behaviour of the silver-loaded epoxy adhesivelayer betweenthe nickel and the aluminaa thermal stressmodel was created of the passive mass.A reference temperature of 20°C and a temperaturerise (An of 100 K was applied. The model consistedof 1500 2D elementswith a symmetry plane mid-length of the passive mass. The silver-palladium thick film conductor was also modelled. Accurate material properties were not available at the time of design; silver-palladium wasaporoximated to silver, and values for silver-loaded epoxy were assimilatedfrom supplier information. Failure criteria for silver-loaded epoxy were also unavailable, however the following values for non-silver loaded epoxy provide a guideline: shearstrength for glue-line thickness of 0.01 mm = 50 N mm-* at 22°C and 20 N rnmW2at 120°C; peal strength of 10 N mm-* at 22°C and 7 N mm-* at 120°C. Peal and shearstressplotted over the glue pad length are shown Fig. 6. The stresseswere extracted from the nodes at the glue-silver interface. The

1Oum

20um

peal stresscan be seento rise dramatically at the edgesof the glue pad, although it is difficult to explain the same phenomenon as has been observed by Penado and Dropek.” The shear stress tends to zero at the junction with the free surface, further investigation using mesh refinement would demonstratethis effect. 3.6 Natural frequency vs beam thickness Vibration analysiswasperformed on Model 1 (refined) for a variety of beam thicknesses(master degrees of freedom were defined to minimise solution time). The first five vibration mode shapesare shown in Figs 7-l 1. Figure 12 shows the relationship between the first and second natural frequencies and beam thickness. 3.7 Worst case analysis The worst static loading condition the sensorwould experiencewould be when the active masswas pressedflat against

30um

Bsmnthickness(microns) Fig. 5. S equivalent (Van Mise’s stress) vs beam thickness.

40um

578

A. E. U. Lees

Fig. 6. Shear and peal stress plotted over glue pad region.

Fig. 10. Vibration modes of concept sensor: mode 4. Fig. 7. Vibration modes of concept sensor: mode 1.

Fig. 11. Vibration modes of concept sensor: mode 5. Fig. 8. Vibration modes of concept sensor: mode 2.

4 DISCUSSION There are a number of important issues arising from this investigation.

Fig. 9. Vibration modes of concept sensor: mode 3. the sensing element due to a combination of excessive g loading and electrostatic attraction. This was modelled by applying a prescribed displacement to points A and B equal to 10 pm in - 2 (Fig. 13). For the 20 pm beam this worst case situation induces von Mises’ stresses in the order of 150 MPa in the beam. This situation would be made worse under shock loading due to the mass inertia of the beam, however the mass of the beam was so small relative to the active mass that this effect was neglected.

(i) From the vibration analysis, the first natural frequency of the structure was found to be low, varying from 8 Hz for a 20 pm beam thickness to 23 Hz for a 40 pm beam thickness. The performance targets (see Section 2.1) call for a sensitive bandwidth up to 50 Hz which implies that the first resonance of the device should be much in excess of this - preferably of the order of 500 Hz. (ii) From the thermal stress analysis, the stresses (peal and shear) in the epoxy layer were found to be above the recommended maximum. This was due to the mismatch in coefficient of linear expansion between primarily the alumina and the nickel and secondarily the epoxy indicating that joint failure due to thermal cycling would be rapid. The accuracy of these results would be improved by better defined material

The analysis and visualisation

1Oum

ZOum

40um

3Oum Bamlthkknoss(-)

Fig. 12. Nickel sensor-

579

of metal microstructures

Slum

naturalfrequencyvs beamthickness.

I I

Fig. 13. Finite elementmesh.Worst caseanalysis.

properties for the epoxy layer; however, the stresses are higher than any standard epoxy could tolerate. A thicker glue layer and a more compliant structure would easethis situation. (iii) From the worst caseanalysis a stressin the nickel beam of the order of 150 MPa was predicted, such a stress would cause plastic deformation in the beam element and hence failure of the device. A conservative design stresslimit for electroplated nickel would be in the order of 50 MPa. (iv) Regarding assembly, the smallest size of silver particulate in epoxy commercially available has a particle size of 15 pm, this would preclude the proposed assembly method, in that particulate would lodge under the passive mass preventing the 10 pm gap being achieved. The low natural frequency of vibration and high silver epoxy stressesreported here would not prevent the sensor from functioning as a demonstratorbut would preclude its use in an automotive environment. The large silver particulate size however is more important and would preclude assembly. From the analytical experiences/resultsobtained for the concept design it was possible to redesign the accelerometer. A prime concern was component handling. In the

concept design plastic deformation of the thin nickel beam was a definite possibility during the pick and place stageof the assemblyprocess. A small plastic deformation in the beam would prevent the accelerometerfrom working. The final designincludes an integral cover/housing which would enablethe device to be handledwhilst protecting the fragile parts (Fig. 14). Attaching the accelerometerto the metallised substrate was inadvisable with the concept design due to the high

I

I Fig. 14. Electroreformed nickel sensor.

580

A. E. V. Lees

Fig. 17. Nickel sensorendstopdetails.

Fig. 15. Foot detailand assemblyscheme.

areasof large change in section, such as where the beam joins the mass,is time consuming. Also as the LIGA component is generally representedasone polyline on the mask, i.e. a closed line, it should be possible to automate the analysis yielding further savings in time.

5 CONCLUSION A design for a new class of nickel micro-accelerometer manufactured by the LIGA process has been developed. The designencompassesthe following features: l

Fig. 16. Configurationof beamelement. peal and shear stressesgeneratedin the silver epoxy layer during temperature cycling, also the large particulate size would have prevented the successfulbedding down of the passivemassvia the 10 pm spigots. The design detail presented in Fig. 15 removes the need for fine silver-loaded epoxy and facilitates a clean location by separatingthe location foot from the epoxy pads. Over-travel of the seismicmassis prevented in the final designby an over travel stop (Fig. 16), whereasin the concept designover-travel is prevented only by contact with the capacitive sensingelement. The over-travel stop also prevents contact between the seismic mass and the sensing element - the massis allowed to travel 6 pm before full travel. The over-travel stop is essentialif damagethrough shock loading is to be minimised. The final designavoids the low natural frequency experienced with the concept design. The length of the flectural beam was reduced to 120 pm from 1000pm and the depth from 20 to 12 pm. By doing this the first natural frequency has been elevated from 8 Hz to above 500 Hz, ensuring a flat sensitivity profile over the range of interest. The final design is more robust than the initial design becauseof the overtravel stop which limits displacement and hence stressin the flectural beam. Fillet radii have also been included at the mass/coverbeam intersectionsto smooth out the stressprofile (Fig. 17). The finite element method has been usedextensively to assist with the final design of the accelerometer. With designs such as this it may be advantageousto use the boundary element method, as extensive refinement in

l l

l

l

overtravel stops, to prevent shock load damage; integral cover, to facilitate easy assembly; good fatigue resistance through low stress amplitudes; natural frequency above 500 Hz for improved sensitivity; mechanically stable with temperature fluctuation.

Finite elementanalysishasbeen usedasa validation tool and hasproved to be both pertinent and useful in providing essentialproduct understanding.

REFERENCES 1. Tolfree, D., X-ray lithographyand LIGA at Daresbury Focus on Micromechanics, IOP Conference Proceedings, June 1993. 2. Lawes, R., Micromechanics: an Overview - Focus on Micromechanics, IOP Conference Proceedings, Central Microstructure Facility - Rutherford Appleton Laboratory, June 1993. 3. Etienne, S., Brennan, D. & Holcroft, B., Processes and their Implications for the Design of Micromachined Devices Focus on Micromechanics, IOP Conference Proceedings, Central Research Laboratories Ltd, June 1993. 4. Brite-Euram Project 3360, Study of new materials and designs for advanced micro-motors. 5. Becker, E. W., et al. Fabrication of microstructures with high aspect ratios and great structural heights by synchrotron radiation, lithography, galvanoforming and plastic moulding (LIGA process). Microelectronic Engineering, 1986, 4, 35 56. 6. Penado, F. E. & Dropek, R. K., Hercules Aerosp Co, Div Design and Anal, Magna, UT, 84044. Univ Utah, Dept Mech Engn, Salt Lake City, UT, 84112, Designing with adhesives and sealant. Numerical Design and Analysis, 477-499.