Large throw magnetic microactuator

Mechatronics 18 (2008) 459–465

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Large throw magnetic microactuator A.C. Hartley, R.E. Miles *, J. Corda, N. Dimitrakopoulos School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom

a r t i c l e

i n f o

Article history: Received 2 August 2007 Accepted 28 May 2008

Keywords: Microactuators Microbeams RF switch Reluctance SU-8

a b s t r a c t The design and fabrication of an electromagnetic MEMS actuator capable of repeatedly switching over a gap length of 100 lm is presented. The device is to be used as a switch for radio-frequency (RF) circuits. It is based on a potcore type electromagnet and a mover suspended on a microbeam. The device was first modelled and optimised using a finite element analysis and then microfabricated. The ultra thick, negative photoresist, SU-8, was used extensively throughout the construction as: (i) an electroplating mould for the copper coils (up to four layers thick at 50 lm per layer), (ii) the magnetic material for the pot core and mover by loading it with various powders to produce a range of magnetic properties and (iii) the actuator beams because of its low elastic modulus. The potcore has a footprint of 0.8 mm2 and for a driving current of 1.4 A (corresponding to a power dissipation of about 80 mW) generates an initial force of 0.11 mN which is sufficient to close a 100 lm gap. In this prototype form, the device could be operated for more than 104 cycles. The maximum switching distance achieved was 200 lm. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The ultimate purpose of this work is to develop a high power radio-frequency (RF) switch with low contact resistance and high isolation in the ‘on’ and ‘off’ positions, respectively. For operation in the region of 10 GHz, the switch must have a gap of at least 100 lm to ensure effective isolation. The majority of radio-frequency MEMS switches reported to date use electrostatic actuation and there is much literature documenting their fabrication and performance – see for example the reference list in [1]. MEMS based microactuators for RF applications have certain advantages when compared to the solid state technology of pin diodes and FETs which is currently in use. For example, electrostatic switches dissipate power during the actuation process only, unlike pin switches which draw a reverse bias current even in the ‘off’ state. MEMS devices have high isolation and low insertion loss characteristics and are linear in operation. These factors reduce the need for extra components in a RF circuit. However, electrostatic devices still have limitations. The forces that can be generated are typically in the region of tens of lN. In practice, this restricts the switching distances to a few microns only which at RF results in low isolation in the ‘‘off” condition. Also, in order to achieve a low ‘‘on” resistance (and hence minimise the high frequency return loss), the switch contacts need to be small (to achieve an acceptable contact pressure) and made from a comparatively soft material. This limits their power handling abilities * Corresponding author. Tel.: +44 113 3432081; fax: +44 113 3437265. E-mail address: [email protected] (R.E. Miles). 0957-4158/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechatronics.2008.05.012

and reduces the number of switching cycles, i.e. the lifetime of the devices [2], effectively ruling them out for use in applications such as radar isolation switches which must handle up to 10 W of microwave power on a transmission line which is typically 500 lm wide. Electromagnetic actuation offers a way round these issues because far greater forces (see the following section) and hence larger switching distances and contact pressures can be achieved. The ability to switch over large distances has been demonstrated by using permanent magnets with external or integrated coils to produce the external field [3–7]. These devices have been used in microrelays and pumps but they are inherently costly in footprint area (around 10’s mm2) and have switching distances of around 10 lm rather than 100 lm as required in this work. In some cases [4,8] large forces (around 100 mN) were obtained but only when hand wound coils with a large number of turns were used. However, these are not suitable for microfabrication. Refs. [9,10] give detailed reviews and comparisons of electrostatic and electromagnetic microactuators. In this paper, we present the design and fabrication of a large throw magnetic actuator based on a classic potcore design with a footprint area of around 0.8 mm2 (1 mm diameter). The fabrication process is predominantly based on the negative photoresist SU-8 which is used for three distinct parts of the device: (i) the magnetic core, (ii) an electroplating mould for the copper coils and (iii) the microbeam material. A cross-section of the device is shown diagrammatically in Fig. 1. The switch arrangement and its RF performance based on this actuator will be presented in a subsequent paper.

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Field lines

Solid magnetic mover

Airgap, g

Coil

Static potcore

Fig. 1. Cross-sectional view of the potcore actuator.

2. Comparison between magnetic and electrostatic actuating forces In the ideal case where the reluctance of the magnetic core is zero (i.e. infinite permeability) and there is no flux fringing in the airgap, the force of magnetic attraction between pole faces of an electromagnetic actuator is given by

F em ¼

1 2 B A 2l 0

ð1Þ

where B is the flux density in the airgap, l0 is the permeability of free space (1.257  106 H/m) and A is the face area of the excited poles. In an electrostatic actuator formed of two oppositely charged conductive parallel plates separated by air, the force of attraction is given by

F es ¼

e0 2 E A 2

ð2Þ

where E is the electric field strength between the plates, e 0 is the permittivity of free space (8.854  1012 F/m) and A is the area of plates. For the same face area, A, of the magnetic and the electrostatic actuator, the ratio between the respective forces is given by

 2 F em 1 B ¼ F es l0 e 0 E

ð3Þ

Given the limitation of the breakdown electric field strength in air 3  106 V/m in the electrostatic actuator and a magnetic flux density of 0.09 T (corresponding to the maximum tolerable current density in the actuator studied in this paper) Eq. (3) gives the value for the force ratio of 81, i.e. the magnetic force is 81 times the electrostatic force for equivalent geometries. 3. Device concept and electromagnetic design optimisation The potcore actuator (Fig. 1), which has for many years been used at the macro level for latching devices and lifting magnets has been redesigned in this work to allow for planar, MEMS fabrication. The actuator magnetic core is made of soft ferromagnetic material shaped so as to capture and guide the flux across the air gap, thereby making a magnetically efficient design. It should be noted here that in this paper the actuator is driven from a current source. In the structure of Fig. 1 the flux generated by the mmf NI passes twice through the airgap, and with the assumptions stated in Section 2, the flux density in the airgap is given by

B ¼ l0

NI 2g

where g is the spacing (airgap length) between the potcore and the mover, A is the total face area of the potcore poles, N is the number of turns, I is the current and l0 is the permeability of free space. For design purposes, the diameter of the potcore was set at 1 mm and the maximum current density was limited to 103 A/ mm2. The latter was chosen on the basis of [11] in order to keep the temperature of the coil below 50 °C. The thicknesses of the coils were limited to 50 lm in order to avoid complications with their fabrication, particularly the electroplating. The actuating distance of the device, i.e. the airgap length between the potcore and the mover, was chosen to be 100 lm, a value that would give acceptable isolation in an RF switch. With these parameters fixed, the attainment of the maximum force requires optimisation of the ratio between the area of the potcore face and the area occupied by conductors. (Increasing the potcore face area, within the fixed boundary of the potcore, reduces the area available for the conductors, which in turn reduces the permissible current at a given maximum current density.) Calculations carried out using Eq. (4), for the parameters given above suggest an optimum area of the potcore face (inner and outer poles together) of 0.24 mm2 resulting in a force of 0.485 mN at a distance of 100 lm. This force is much larger than can be obtained electrostatically, but it does assume ideal conditions. In practice however, for our planar construction and large actuation distance, there will be field fringing, flux leakage in the gap and mmf drop in the material. To gain some insight into the impact of these effects on the actuator performance, the device was simulated using a software package MAGNET for magnetic field finite element analysis. Fig. 2 shows the simulated flux pattern of one side of the device cross section. The field plot shows that there is significant magnetic flux leakage in the slot between the poles of the potcore at (A), and there is also considerable field fringing between the base and mover at (B) and at the edge of the actuator (C). To reduce the reluctance of flux fringing paths, the mover diameter was increased. (This is not thought to be an increase in the footprint area, as it is theoretically possible to use the area under the extended component.) A number TM

ð4Þ

and Eq. (1) for the force, F, acting on the mover therefore becomes

F¼

l0 8g 2

A  ðNIÞ2

ð5Þ

Fig. 2. Field plot of the actuator. (N.B. The pattern is symmetrical about the actuator axis.)

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A.C. Hartley et al. / Mechatronics 18 (2008) 459–465 μr = 10

% Increase of Force on Mover

2

Pole Area = 0.0314mm 2 Pole Area = 0.126mm

16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -1 1.0

1.1

1.2

1.3

6

Pole Area = 0.0314mm 2 Pole Area = 0.126mm

1.4

1.5

2

1.6

Mover diameter/Potcore diameter Fig. 3. Percentage increase in output force as a function of the normalised mover diameter for different pole face areas and different relative permeabilities.

0.60 0.55 0.50

Force (mN)

0.45 0.40 6

0.35

μr = 10

0.30

μr = 100 μr = 10

0.25 0.20 0.15 0.10 60

80

100

120

140

160

180

ri (μm) Fig. 4. The output force generated on the mover for different inner pole radii, ri, and for various relative permeabilities.

of different pole areas for two radically different relative permeabilities (lr = 10 and lr = 106) were simulated and the results are shown in Fig. 3. The benefits of the increase in area are clear in every case. There is no optimum value for the extension, but a ratio of 1.3 between the mover and potcore diameters should be adequate for most applications because the curves tend to saturate above this value. Once the mover diameter is fixed, the pole area can be altered to give a maximum value for the force. Fig. 4 shows the simulated resulting force for the devices in the case of relative permeabilities of 10, 100 and 106. From the figure, for a relative permeability of 100 (see Section 4 below), the optimum radius is around 130 lm.

high as 1.2 T [12]. The most popular method of forming thick layers of permalloy is by electroplating, where 30 lm film thicknesses are routinely obtained. Above this level the films tend to be highly stressed and difficult to achieve consistently. In this work it was considered that layers up to 300 lm might be required and consequently a new technology was sought to achieve this. In order to overcome the above problem, we have used a magnetic epoxy (SU-8 loaded with magnetic particles) which can be spun or spread on to a substrate. The use of SU-8 as the epoxy base allows exploitation of the material’s excellent mechanical and chemical properties as well as being easily curable and compatible with the rest of the actuator. It also has the advantage that different ferromagnetic dopants of varying particle sizes can be employed leading to a range of magnetic properties for the same basic technology. Three magnetic powders were used to form the colloid: (i) permalloy, particle size 6–10 lm, (ii) Fe, particle size of 1–6 lm and (iii) a ferrite material known as 3R1 manufactured by ‘Ferrox Cube’. The 3R1 was purchased as a bulk sample and ground to a powder [13]. The magnetic and mechanical properties of the magnetic SU-8 must be such that the permeability is as high as possible while still maintaining compatibility with the rest of the actuator. In particular, fabrication requirements necessitate a material having a viscosity low enough to allow the filling of a 40 lm wide, 100 lm deep channel which is also able to withstand subsequent processing. There are therefore two main factors to consider for each powder: (i) magnetic powder concentration and (ii) the viscosity of the SU-8 used as the base epoxy. Increasing the magnetic particle concentration will increase the permeability and saturation flux density and also the viscosity. The SU-8 type will determine the viscosity of the composite for a given loading, and also the chemical resistance and mechanical strength. To measure the magnetic properties, toroidal perspex moulds were filled with the different epoxies. Each mould was then wound with a primary and secondary coil, each of 200 turns. As the current in the primary was slowly raised the induced secondary voltage was integrated to obtain the flux. The recorded values of the flux and the primary current were used to obtain the B/H curves shown in Fig. 5. There was no noticeable hysteresis in the B/H characteristics when the current was returned to zero. (For more information on this technique and the equipment and circuit used see [14].) At the maximum current density of 103 A/mm2 that the potcore coil can tolerate without overheating, the resulting flux density in the poles of the device based on the magnetic material 3R1 powder

0.6 0.5 0.4

B [T]

μr = 10

0.3 Ni Fe

0.2

3R1

4. Magnetic material considerations Research into MEMS applications has led to the creation of various materials that can be fabricated in the micro domain while still exhibiting magnetic properties comparable to their bulk counterparts. The most successful of these is probably permalloy which exhibits a relative permeability of 1000 and a saturation field as

0.1

Fe

0 0

5

10

15

20

25

(x1000) H [A/m] Fig. 5. B/H curves for the tested magnetic SU-8 samples.

30

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is around 0.09 T. In this region the other two materials have lower permeabilities (Fig. 5) and for the same current density in the potcore coil would produce lower flux densities which would result in a reduced force. This, combined with its experimentally determined ease of processing means that a composite of SU-8, EC solvent and 3R1 powder in the ratio of 2:3:20 by weight was the best choice in this work. In order to increase the heat transfer from the coil, and hence maximise the driving current, the actuators were fabricated on a mild steel substrate as shown in Fig. 6. The high thermal conductivity with the added bonus of low magnetic reluctance allows a greater current density as well as reducing the mmf drop in the potcore. An additional benefit is the simpler fabrication which results from using the substrate to provide an electrical connection to the coil. The measured magnetic properties were used with the device dimensions shown in Table 1 and current densities discussed above to design the final actuator. 5. Magnetic force estimation In the actuator arrangement illustrated in Fig. 1 the mover is suspended on an elastic beam (see Section 6 below) which is anchored at both ends. When the actuator is excited its magnetic force acts against the elastic restoring force which is being established in the beam. Fig. 7 shows (i) the computed variations of the actuator force as a function of the distance between the mover and the potcore at different values of excitation currents for an initial gap spacing of 100 lm and (ii) the restoring force in the beam. (The latter is shown by a dashed curve. Note that in this diagram the beam deflection in microns is expressed as ‘100 – gap length’.) As the air gap closes the deflection of the beam increases and, as shown in Fig. 7, the elastic restoring force (see Section 6 below) increases almost linearly with the deflection. At the same time, for any given current the magnetic force also increases but, as can be seen from Fig. 7, this force has a finite value at zero deflection and rises with an increasing gradient as the gap closes. So, for any particular initial gap spacing a minimum current is required to close the gap. This current corresponds to the curve which just touches the restoring force curve. From Fig. 7 it is estimated that for a current a little greater than 1.3 A the magnetic force always exceeds the restoring force in the beam and will consequently close the switch, i.e. reduce the gap to ‘zero’. Fig. 8 shows the current required to hold the beam at a fixed position, i.e. where magnetic and restoring forces are equal, for a range of initial gaps. As the gap closes the current required first rises as the deflection increases but then passes through a peak value corresponding to the minimum current required to close the gap. Beyond this point the current required actually reduces because of the non-linear relationship of the magnetic force with gap length. If the current was held constant at the switching level,

Table 1 Device parameters after simulation Relative permeability Potcore footprint area; diameter Inner pole diameter Copper conductor width Coil thickness Number of copper turns Mover diameter Mover thickness Actuation distance

83 at 0.09 T 0.785 mm2; 1 mm 0.24 mm 0.097 mm 0.050 mm 3 1.3 mm 0.100 mm 0.100 mm

1 I= 0.25A

0.8

I= 0.5A I= 0.75A I= 1.0

Force [mN]

462

0.6

I= 1.25A I= 1.5A Restoring force

0.4

0.2

0 0

20

40 60 Airgap length [microns]

80

100

Fig. 7. Variations of the microactuator force against the distance between potcore and mover. (Solid curves show the magnetic force for a set of constant excitation currents. The restoring force in the beam is shown by dashed curve.)

the switch would snap shut. This corresponds to movement down the left hand side of the curves which describe an unstable mechanical system. Alternatively, once the gap has been reduced to a value below that corresponding to the maxima of the curves in Fig. 8, the current required to keep the switch closing can be progressively reduced. 6. Fabrication The potcore, mover and microbeam were fabricated using photoplastic SU-8 which is a ‘high-aspect ratio’ photoresist. In MEMS, it is used for structural forms, dielectric and electroplating moulds [15]. The processing sequences are described in [16] and an outline is given in Appendix. 6.1. Potcores with coil As mentioned above, the potcores are fabricated by electroplating copper coils in micro grooves produced lithographically in

Fig. 6. Cross-section of the potcore on a steel substrate.

A.C. Hartley et al. / Mechatronics 18 (2008) 459–465

beam 5 mm long, 10 lm thick and 90 lm wide supporting a magnetic mover. The estimated resonant frequency of the microbeam and mover is 1 kHz, which may have some significance in applications where mechanical vibrations occur at this frequency. In order to have a number of devices to test, an array consisting of several actuators was fabricated. The array was not designed for simultaneous energisation and only one actuator in the array was energised at the time.

1.4 1.2 1

Current [A]

463

0.8 0.6

7. Experimental evaluation

0.4

Current vs. position for different initial gaps:from 100µm (top) to 50µm (bottom) at 10μm increments

0.2 0 0

20

40

60

80

100

Airgap length [microns] Fig. 8. Variations of the minimum current required to hold the gap at a given length for various initial positions of the beam.

The force required to produce different deflections of the beam was measured using an alpha-step surface profiler on the beam and this dependence is shown in Fig. 7 (dashed curve). To evaluate the actuator performance experimentally, the following tests were made. The actuator shown in Fig. 11 was energised from a current source with DC current that was increased in 0.2 A increments until actuation across the full gap was observed. For an initial gap length of 97 lm this occurred at a current of 1.4 A (current density of 300 A/mm2) corresponding to a power dissipation of 80 mW. A current of around 0.3 A is sufficient to hold the gap closed, corresponding to a power dissipation in the actuator of around 3 mW. The current of 1.4 A, found experimentally, compares well with the simulated value of around 1.3 A (see Fig. 7 in Section 4 above), and on this basis the simulated results for the actuation force shown in this figure can be considered realistic. The maximum initial gap length fabricated for test purposes was 196 lm, and this required a current of 3.6 A (current density of 785 A/mm2) for full actuation.

Fig. 9. Photograph of the potcore on steel substrate.

magnetic SU-8. Fig. 9 shows potcore with coil fabricated on steel substrate. 6.2. Microbeams Microbeams and membranes have found numerous applications in micro engineering but are primarily used for atomic force microscope cantilevers, micro switches and micro pumps. SU-8 was chosen for fabricating the microbeam primarily because it has a low elastic modulus. Other benefits are the low cost and batch fabrication capabilities. Fig. 10 shows a photograph of a

Fig. 11. Photograph of the potcore actuator tested.

Fig. 10. A 5-mm long SU-8 beam with magnetic mover.

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A.C. Hartley et al. / Mechatronics 18 (2008) 459–465

a

Legend Gold

b c

Steel Exposed SU-8 Unexposed SU-8

d

SU-8 10 Ferromagnetic SU-8

e

Copper

f

g

h

i

j Fig. 12. Schematic view showing the stages of coil and potcore fabrication. (N.B. The layers are not drawn to scale.)

A preliminary lifetime test was carried out using a switching circuit which supplied the actuator with 1 Hz current pulses of 50 ms duration. After more than 104 switching cycles the waveforms of voltage and current were unchanged (implying no change in inductance and coil resistance), and a visual inspection also showed no degradation in either the actuator or in the beam. The RF performance of this actuator will be presented in a later paper. 8. Conclusions A magnetically operated MEMS actuator has been designed, fabricated and characterised. The optimised potcore type electromagnet had a footprint of 0.8 mm2 and a maximum actuating distance of 200 lm requiring an initial current of 3.6 A. For a gap of 100 lm the initial actuating current was 1.4 A but a current of only 0.3 A (corresponding to a dissipation of 3 mW) was suffi-

cient to hold the gap closed. Preliminary lifetime testing shows that the device can operate for more than 104 cycles for a 100 lm gap. In the closed position, the actuator exerts a force in the mN range thus ensuring good electrical contact between switching surfaces. Further work is underway to fabricate a latching structure to reduce the power dissipation in the closed position to zero. The physical properties of SU-8 have been exploited to the full in the structure and fabrication of the magnetic and mechanical aspects of the actuator. Acknowledgements The authors wish to thank the Electromagnetic Remote Sensing Defence Technology Centre (EMRS DTC) and the School of Electronic and Electrical Engineering of Leeds University for funding this work.

A.C. Hartley et al. / Mechatronics 18 (2008) 459–465

Appendix A The fabrication process, illustrated in Fig. 12, includes the following stages: (a, b) A layer of Ti/Au  100 lm is initially deposited on the steel substrate followed by a 10 lm thick layer of SU-8(2010). The positions of the holes for the potcore are then defined photo-lithographically in this layer but not developed at this stage. (c) A second Au deposition (120–150 lm) follows which is later patterned to create the seed layer for the coil plating. (d, e) A second layer of SU-8(2035) is spun-on, baked and exposed to create 35 lm thick moulds. (f) Both SU-8 layers are developed simultaneously. (g) The inner and outer pole holes are filled in with SU-8(10). This inhibits plating in certain areas. (h) The wafer is immersed in a copper plating solution until the seed layer of the coils is plated up to the top of the moulds. In some instances lapping may be necessary to bring the features to the same level. (i) The SU-8(10) layer is removed using acetone. (j) Finally, the holes are filled with a ferromagnetic/SU-8(10) (FSU-8) mixture to form the pot core.

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