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Electrostatically actuated copper-blade microrelays
Sensors and Actuators A 100 (2002) 105–113
Electrostatically actuated copper-blade microrelays Han-Sheng Lee*, Chi H. Leung, Jenny Shi, Shih-Chia Chang Delphi Research Labs, 51786 Shelby Parkway, Shelby Twp., MI 48315, USA Received 17 June 2001; accepted 3 January 2002
Abstract Electrostatically actuated microrelays have been fabricated. The structural material, copper, is electroformed onto patterned areas. The typical dimensions of a single-blade relay are 1:88 mm 1 mm 0:01 mm (length width thickness). Microrelays with connected sectioned blades have also been fabricated, which showed improvement in lowering contact resistance. It is believed that sectioned blades not rigidly connected together have better flexibility and more contact points. Fabricated relays are capable of carrying current loads of several amperes and are able to switch 1 A resistor loads. When switching off an inductive load, the energy stored in the motor coils will discharge through the relay gap and reduce the relay service life. Using a Zener diode as a discharge path for the inductive load increased the relay operation cycles. The time delay in making and breaking the contact is approximately 50 ms. To check the mechanical fatigue property, relays were used to switch a low load, 50 mA resistor load and were able to operate for more than 1:7 106 cycles without failure. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microrelays; Electrostatically actuation
1. Introduction Conventional electromagnetic relays are actuated by passing an electric current through the coils. The force generated by the energized coils attracts the blade and makes an electric contact between two electrodes of the relay, the moving electrode (blade) and stationary electrode (contact). This type of relay has been used in the industry for decades. They are reliable, inexpensive, can carry high currents (>20 A) and have very high off-state resistance. However, they generally require a continuous current to hold the electric contact. This holding current consumes power and generates heat needlessly. The other popular type of switching device is a solid-state relay (SSR). SSRs are fast, usually have better reliability than mechanical relays and are batch manufacturable which implies low cost and uniform products. However, SSRs have lower off-state resistances than mechanical relays. They are polarity sensitive and are generally more expensive when used to carry high currents in automotive applications. Depending on the application, both mechanical relays and SSRs have their market segments. In automotive applications, there is a need for small relays to facilitate packaging and integration that is also reliable and inexpensive. These applications include switching * Corresponding author. Tel.: þ1-586-323-9466; fax: þ1-586-323-9797. E-mail address: [email protected] (H.-S. Lee).
lamps and small motors with steady-state load currents lower than 3.5 A. An all-metal, electrostatically actuated microrelay fits this category. To test the feasibility, our first attempt was to fabricate a microrelay which was tiny and could switch a 1 A resistive load. We chose copper as the relay structure material because of its high conductivity, low cost, process compatibility and the plating solutions availability. We found that the fabricated microrelays are small and can be readily integrated with the connectors or actuators they control. However, because of the low contact force, the microrelays have durability problems when handling amperes of current. Efforts are still being made to resolve these problems. Both electrostatically and magnetically actuated micromachined relays have been demonstrated to handle low currents with a high number of switching cycles [1–10]. In communication applications, Yao and Chang [1] and Zavracky et al. [2] used electrostatically actuated relays to switch a 5 mA load for more than 109 cycles. Taylor et al. showed a magnetically actuated relay that can switch a 1.2 A dc load repeatedly [3] and Wong et al. [4] showed electrostatically actuated microrelays that can carry 0.4 A. However, neither researcher mentioned the actual number of switching cycles at these loads or current levels. In this report, we will describe the fabrication of electrostatically actuated relays that are capable of switching 1 A resistor loads.
0924-4247/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 0 5 3 - 5
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1.1. Cantilever structured microrelay In an electrostatically actuated microrelay, the magnitude of the actuation voltage depends on its dimensions and structural material. Fig. 1a shows a photomicrograph of a fabricated cantilever relay and Fig. 1b shows its crosssectional view. The relay consists of a blade (moving electrode) separated from the contact terminal (stationary electrode) by an air gap with an actuator underneath the blade. With a dc voltage applied between the actuator and the top blade, an electric field induced force will bend the blade down and make an electrical connection with the lower contact. Empirically, the actuation voltage VA at which the moving electrode will snap to the stationary electrode and make the contact can be expressed as [11–13]: VA2 /
Ed3 t3 ð10e0 L4 Þ
(1)
where E, t and L are Young’s modulus, thickness and length of the blade, respectively. The d is the equivalent distance between the blade and the actuator and e0 is the permittivity of air. As shown in Fig. 1b, the blade was separated from the actuator by an air gap and an insulator, silicon nitride, d is the sum of the air gap ‘‘d’’ and the thickness of the nitride multiplied by its dielectric constant. The measured dielectric
Fig. 1. An all-metal cantilever structured microrelay: (a) a photomicrograph of a finished relay and (b) the cross-sectional view of the relay (not to scale).
constant of our deposited nitride film was seven. These physical parameters, L, d and t are shown in Fig. 1b. The length, L, is the only parameter directly related to mask dimensions, the others are material and process parameters. For example, the parameter d can be adjusted by varying the sacrificial layer thickness and the nitride thickness. The thickness of the blade, t, depends on the total electroplating time and the rate at which the blade was electroplated. The Young’s modulus, E, can be changed by the plating conditions or by plating a different material.
2. Relay fabrication Major relay fabrication steps are shown in Fig. 2. After oxidizing the silicon wafers, an adhesion promoter, Ti–W film, was sputter deposited on the wafers followed by a thin layer of gold deposition without breaking the vacuum. Wafers were then patterned to form three islands as shown in Fig. 2a. A 0.25 mm of PECVD nitride was deposited on the wafer at 350 8C and then patterned as shown in Fig. 2b. A thin layer of tungsten was then sputter deposited followed by an aluminum evaporation and photoresist (mold) coating. After patterning, the lower contact regions were opened as shown in Fig. 2c. Copper was electroplated onto the wafers to level with the mold as shown in Fig. 2d. The wafers were patterned to remove the mold except the region adjacent to the copper contact as shown in Fig. 2e. Another layer of photoresist was then spun onto the wafers as shown in Fig. 2f. The wafers were patterned to have the blade anchor regions opened followed by a sputter deposition of a thin layer of tungsten as shown in Fig. 2g. Then, the wafers were patterned to have the gold plating base exposed as shown in Fig. 2h. A thick mold (15 mm) was then spun onto the wafers and patterned to expose the blade regions as shown in Fig. 2i and j. After plating 10 mm of copper, as shown in Fig. 2k, wafers were scribed along the chip boundaries by a wafer saw. The depth of the scribed lines was about half of the wafer thickness. Afterwards, all the sacrificial materials were removed with a tungsten etchant [14]. The etchant dissolved not only tungsten, but also aluminum and photoresist (mold). The blade was released as shown in Fig. 2l. The wafers were then cleaned, tested and separated into chips. Since the wafers had been sawed halfway through after the step, in Fig. 2k, they were easily breaking into chips without damaging the released relays. The relay chip was then adhered to a TO-8 header and wire bonded. Fig. 3 shows a partially wire bonded chip where four different structured relays were on the same chip. The designed lengths of the blades were 1.28, 1.88, 1.88 and 1.80 mm for the short, long, dual actuator and bridge structure, respectively. The structure of the relay was labeled in Fig. 3. For example, the relay with a long cantilever structure was the upper left one in Fig. 3. The widths of the blade were 1, 1, 1 and 0.46 mm for the short, long, dual actuator and bridge structure, respectively.
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Fig. 2. Major processing steps.
As shown in Fig. 2l, there were two air gaps, 1 and 2. Air gap 1, 3 mm was slightly wider than air gap 2 which was 2.6 mm. These two air gaps were achieved through processing. This arrangement will help the blade make contact with the lower electrode first when the relay is activated to the
‘‘on’’ state. As shown in Fig. 2f, there was a layer of photoresist on top of the copper contact and three layers of materials on top of the actuator area: photoresist, aluminum and tungsten. The thickness of these materials determined the separation of the air gap 2 and 1, respectively.
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Fig. 3. A photomicrograph of a fabricated relay chip where four different structured relays are on the chip.
Even though, the actuation voltage is independent of the width of the cantilever, the width of the three cantilevers in this study was kept the same to minimize the complexity that could be caused by a lateral internal stress gradient in the plated blade. As shown in Fig. 3, two blade lengths were used for the three cantilever relays. Two cantilever relays had longer blades (left two) and one had a shorter blade (bottom right). The upper right relay had a bridge structure. The bridge-structured relay was anchored at ends (horizontal) and had two contacts. When activated, the middle square plate (with holes) will electrically connect the two contacts. Of the two long-blade relays, one had dual actuators, with one actuator at each side of the stationary electrode. The dual actuator structure is used to test the mechanical flexibility of the moving electrode. In the dual actuator structure, the blade is pulled down by the actuation force generated from both sides of the lower electrode (contact electrode). If the blade is flexible, the blade makes a more uniform contact laterally and perhaps increases the contact area when compared to a single actuator structure. If the step for creating spacer (Fig. 2e) were skipped, the fabricated relay could have a narrower space between the blade and the edge of lower contact as shown in Fig. 4a. In this situation there is a possibility that when the blade is making contact with the lower electrode, the blade may touch the edge of the lower electrode instead of contacting the plateau of the contact electrode. In general, it is easier to control the smoothness of the plateau than the edge of the lower electrode. Hence, the plateau is the preferred area for the blade to make contact. Fig. 4b shows a fabricated relay without skipping the step in Fig. 2e.
3. Relay properties Measured actuation voltages of the relays were 47, 19, 20 and 130 V for the short cantilever, long cantilever, dual actuator and bridge-structured relays, respectively. These voltages were measured from the relays on the same chip. For the same structure, the majority (60%) of the fabricated relays had their actuation voltages deviated from the aforementioned values by less than 2 V. From Eq. (1), the actuation voltage of a cantilever-structured relay is inversely proportional to the square of its blade length. The square of the length ratio of the long and short cantilever structured relays is ð1:88=1:28Þ2 ¼ 2:16 which is reasonably close to the inverse ratio of their measured actuation voltages, ð47=19Þ ¼ 2:47. As shown in Fig. 5a, with a voltage source connected to a 1 kO resistor load and to the stationary electrode of a long-blade cantilever relay, the switching of a relay was monitored by measuring the ‘‘output’’ voltage at the contact terminal of the relay. When the relay was electrically open, the output voltage was the same as the source voltage, 6 V. When the relay was electrically closed, the monitored voltage dropped to ground potential. Fig. 5b shows that with an applied 20 V actuation voltage (top trace), the time delay in closing the relay was 22 ms (bottom trace). After releasing the 20 V actuation voltage, Fig. 5c shows that the opening of the relay (top trace) had a time delay of 56 ms (bottom trace). With 0.5 s ‘‘on’’ and 0.5 s ‘‘off’’ 20 V pulses applied to the actuator (top trace), the responses at the contact terminal (bottom trace) are shown in Fig. 5d.
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activation voltage of the bridge structured relays were too high for our applications, we did not use the bridgestructured relays to do the test. The long-blade relays were short circuited in about 20 operation cycles. The coil of the motor in the mirror driving mechanism had an inductance, Ł, of 12 mH. For an inductive load, an induced voltage with a magnitude of Ł (d /dt), was generated by the motor coil during switchings. Here, Ł is the inductance of the motor coil and is the current through the coil during switchings. We found that the current in the motor coil increased to 0.5 A before the contact breaking. Since the time scale for making and breaking the contact is shorter than 60 ms (Fig. 5), a portion of the induced 100 V will drop across the air gap and cause arcings. The localized arcing between blade and stationary electrode can create material transfer between electrodes and shorten the relay life cycles [15].
4. Discussion When the relays were used to switch current loads of fractions of an ampere, there was a substantial reduction in the number of their operation cycles. We noticed that the following parameters affected our relay performance. 4.1. Arc
Fig. 4. A photomicrograph of the relay (a) without adding spacer during fabrication and (b) with spacer added in the fabrication.
Using pulsed actuation voltages to control the ‘‘open’’ and ‘‘close’’ of the relay, the number of operation cycles was monitored. A long-blade microrelay was activated sequentially with 0.05 s at 20 V and 0.2 s at ground voltage to switch a 50 mA resistor load. In switching this low current load, the relays were able to operate for more than 1:7 106 cycles without failure. The relays were still operational when the tests were stopped. 3.1. Switching inductive loads A long-blade cantilever microrelay was used to switch a 12 V, 5.5 W fan motor which had a very low inductance motor coil. During switching, a 1 A current surged through the microrelay. The microrelay was able to turn the fan on and off for more than 150 cycles. The relay stayed in the closed position when it failed. Long-blade cantilever microrelays were also used to switch a side mirror motor of a vehicle. Since the required
The main causes of short relay operation cycles are arc damage and welding at the contact. To check the arc damage on the relay life, we connected a Zener diode in parallel with the vehicle side mirror motor. Lockwood pointed out that the connected diode can be of much lower current capacity than the steady-state current through the motor. This is because the diode conducts only for an extremely short time [16]. When the microrelay was activated to the ‘‘on’’ state, the current through the relay went to the motor coil because the Zener diode was reversibly biased. When the relay was switched to the ‘‘off’’ state, the coil-induced voltage will forward bias the diode and make the diode electrically short the coil. The coil-stored energy went to ground through the diode. Adding the diode to the inductive load reduces the arc damage at the microrelay contact and increases the relay life cycles. The number of life cycles is the number of working cycles until the relay fails to respond the activation voltage change. Usually, the relay failed to open after releasing the activation voltage. We found that the life cycles increased to 150 cycles when a Zener diode was connected to the vehicle side mirror motor. A better approach to minimize the arc damage to microrelays is to integrate an arc suppressor with microrelays. The suppressor offers an electric path to absorb the energy spikes and also create an arcless condition for the microrelay during all swithchings [17]. With a partial protection from a Zener diode, the number of operation cycles of the microrelay was still far below the
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Fig. 5. Measured relay response time: (a) measurement setup; (b) time delay in switching ‘‘on’’, 22 ms; (c) time delay in switching ‘‘off’’, 56 ms; (d) switching a 1 kO load continuously.
low load (50 mA) test results (>1:7 106 cycles). We believe local melting at the contact area also contributed to the short operation cycles of the relay. For copper electrodes, the melting voltage is 0.43 V [15]. With 0.5 A current through the relay, if the contact resistance is higher than 0.86 O, the localized heat is enough to cause a melting problem at the contact surfaces. We found that the relay contact resistance increased to 1 O after switching the inductive motor for about 20 cycles. After this point, the relay blade stuck to the lower contact so frequently and became unusable. It is speculated that the oxidation of the copper at the contact interfaces was responsible for the increase in contact resistance.
4.2. Oxidation To check if the copper oxidation affected the relay service cycles, copper electrodes and copper covered with gold electrodes were compared. A thin layer of gold (0.1 mm) was electroplated on top of the lower contact and at the bottom of the blade in the relay fabrication. Therefore, copper was not exposed at the contact in these microrelays. Despite the fact that copper can diffuse through gold and reach the electrode surface [18], the number of service cycles of these gold covered microrelays increased from 150 to 500 cycles when the fan (low inductive load) was used as the load. To slow the copper from diffusing to the
H.-S. Lee et al. / Sensors and Actuators A 100 (2002) 105–113
contact surfaces even more, we added a diffusion barrier material, nickel, between copper and gold [19]. We changed the blade material composition by sandwiching a 2 mm nickel film between copper and gold. To minimize the possibility of blade curling due to mismatch in materials, we intentionally made the composite material symmetric in its cross-section. The new blade composed of five layers, Au (0.3 mm)/Ni (2 mm)/Cu (7.5 mm)/Ni (2 mm)/Au (0.3 mm). All the metals were electroplated at the step shown in Fig. 2k. When using this composite-blade relay to switch the fan motor, the number of operation cycles increased to 1000, a significant improvement in relay durability. We attributed the improvement to two factors; slower increase in contact resistance and larger breaking force. This is because the required activation voltage of the long-blade structured relay increased from 20 to 75 V (composite blade). Such a large increase in activation voltage was not expected because the value of Young’s modulus of nickel, 2 1012 dyn/cm2, is only 33% higher than that of copper, 1:5 1012 dyn/cm2 [20]. Besides, the relay activation voltage is proportional to the square root of Young’s modulus as shown in Eq. (1). In addition, the total thickness of nickel was about one half of the copper thickness in the composite blade and there was no significant curling in the composite blade. 4.3. Contact resistance Reducing contact resistance not only eases the welding problem but also cuts the energy loss at the contact. Several factors affect the value of contact resistance, which include contact materials, contact force and contact area (contact material deformability, surface smoothness and number of contact points) [21]. We have tried to correlate different plating solutions and plating conditions to the surface smoothness of plated material. The work is still in progress. Another method we used to reduce the contact resistance was to change the blade structure. Instead of using a single blade, multiple-blade relays were fabricated. 4.3.1. Multiple-blade relay The purpose of using multiple blades was to create sectioned blades with each section not rigidly connected to its neighbors. Connected section blades will still act as one blade and will have a better overall flexibility as opposed to one large blade piece [22]. Using multiple blades to reduce contact resistance had been proven helpful in the conventional mechanical relays [22]. Fig. 6 shows a photomicrograph of a finished four-blade microrelay. The crosssectional view of the multiple-blade relay is identical to that of Fig. 2l. The blade composed of four sections connected with small joints between them. The dimensions of the relay were 1:48 mm 1:6 mm 0:01 mm (length width thickness). Similar to the single-blade structure, the spacings were 3 and 2.6 mm for the air gap 1 and 2, respectively. The new relay which had copper–copper contact and four sectioned blades showed a contact resistance of 30 mO
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Fig. 6. A photomicrograph of a new microrelay where the blade composed of four sections connected with joints.
which was lower than the single piece blade fabricated earlier, 40 mO. The relay contact resistance was defined as the resistance at the contact interfaces and was measured by using a four-point probe resistance meter at the differential mode. A ‘‘calibration’’ relay, similar to the tested relays, was prepared by wire bond to electrically short the blade contact and stationary contact. This calibration relay was used as our zero contact resistance reference. In this arrangement the measured resistance contributed from the connection wires, bonding wires and package pins were subtracted. We connected the new relay as shown in Fig. 5a to switch a 1 A resistor load. Results are shown in Fig. 7. The top trace showed the pulsed activation voltage used, 0.08 s at 25 V followed by 1.2 s at ground potential in sequence. The middle trace showed the current switched and the bottom was the voltage measured at the ‘‘output’’ terminal (Fig. 5a). The new microrelay was able to switch 1 A load for about 300 cycles before showing the sticking problem. Even at the 1 A level, lower contact resistance helped to increase the service life of the relay. The tested relay chip was adhered to a TO-8 header, wire bonded as shown in Fig. 6 and then capped with a metal can. The can was mechanically cramped to the header. Hence, the relays were not in a controlled environment. In fabricating the multiple-blade relays, different copper plating solution was used, which contained additives for better leveling and was different from the solution used in the single-blade relay fabrication. Therefore, we do not expect that the required activation voltage for the new relays can be extrapolated from the earlier results. If the earlier plating solution results were used, the extrapolated activation voltage should be 32–35 V which is higher than the value shown in Fig. 7, i.e. 25 V. Another approach to reduce the contact welding problem is to use high melting voltage materials at the contact surfaces. The melting voltage of platinum, 0.65 V, is higher
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Fig. 7. Measured results of using a four-sectioned blade microrelay to switch a 1 A resistor load.
than that of copper, 0.43 V [23]. Higher melting voltage materials have more resistance to the contact sticking problem. Furthermore, platinum can be electroplated. It could be used at the contact surfaces. We are characterizing the properties of the electroplated platinum films and will test this approach in our relay fabrication.
5. Summary All-metal microrelays have been fabricated to switch different loads. When using the microrelays to switch a low current load (50 mA), higher than 1:7 106 operation cycles have been demonstrated. When using the microrelays to switch loads of 0.5–1 A, we found that both the arc damage and welding problem limited the relay operation life. When the microrelays were used to switch an inductive load, a vehicle side mirror in our case, the energy stored in the coils discharged through the relay air gap contributed to the low relay operation life. We increased the relay operation cycles by connecting a Zener diode in parallel to the side mirror motor to offer a discharge path for the coil-stored energy. The welding at the contact was caused by the localized heat generated at the contact points. The higher the current passing through the relay, the lower the contact resistance required. We found that instead of using one large blade, using sectioned blades connected by joints to offer better blade flexibility reduced the contact resistance. Even though, the copper had very low resistivity, the localized heat can accelerate the copper oxidation and increase the
contact resistance. We found that covering the copper with inert material, gold, increase the relay operation life. Adding a diffusion barrier between copper and gold can slow down the copper diffusion to the contact surface further and increase the microrelays service life more. We are still experimenting on plating conditions with different materials and solutions. The durability shown in these tests is not sufficient to meet automotive specifications, we believe that it can be improved by incorporating vacuum packaging techniques, adapting high melting voltage interface material and by reducing the contact resistance of the microrelays.
Acknowledgements We would like to thank Sam Lorincz, Jim Orsine and Grant Wheeler of Delphi-Packard Electric Systems and Tony Lee of Delphi Technologies Inc for their help and support to this project. References [1] J.J. Yao, M. Chang, A surface micromachined miniature switch for telecommunications applications with signal frequencies from dc up to 4 GHz, in: Proceedings of the Transducers’95, Stockholm, Sweden, 1995, pp. 384–387. [2] P.M. Zavracky, S. Majumder, N.E. McGruer, Micromechanical switches fabricated using nickel surface micromachining, IEEE, J. Microelectromech. Syst. 6 (1997) 3–9. [3] W.P. Taylor, O. Brand, M.G. Allen, Fully integrated magnetically actuated micromachined relays, IEEE, J. Microelectromech. Syst. 7 (1998) 181–191.
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[20] L.I. Maissel, R. Glang (Eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970 (Chapter 12). [21] S. Majumder, N.E. McGruer, G.G. Adams, P.M. Zavracky, R.H. Morrison, J. Krim, Study of contacts in an electrostatically actuated microswitch, Sens. Actuators A 93 (2001) 19–26. [22] J.P. Lockwood, Applying Precision Switches, Honeywell, Micro Switch Division, Freeport, IL, 1973, Chapter 3, pp. 43–44. [23] P.G. Slade (Ed.), Electrical Contacts: Principles and Applications, Marcel Dekker, New York, 1999, Chapter 1, p. 50.
Biographies Han-Sheng Lee received his PhD in Electrical Engineering from Princeton University in 1976. In the same year, he joined the GM Research Labs. He was transferred to Delphi Research Labs in 1999. His technical interests are in the general area of physics, technology and applications of sensors and semiconductor devices. Chi H. Leung received a BSc in electrical engineering and MSc and PhD in metallurgical engineering. All degrees were from the University of Michigan, Ann Arbor. He is currently, Director of R&D, AMI Doduco North America. Previously, he worked with GTE Technical Products Division and General Motor R&D Lab which later evolved into the Delphi Research Lab. His research interests are on electrical contact material for arcing contact and automotive connectors, latching relays, MEM relays and Plasma Ion Implantation. He has published 24 journal papers and is holder of 11 US patents. He is the current Chairman of the IEEE Holm Conference Operating Committee. Jenny Shi obtained her ME in Electrical Engineering from the University of Michigan in 1989 and her BSc in Material Science and Engineering from Zhejiang University in 1982. Since 1989, she has been working with General Motors Corporation’s Research Laboratories and Delphi Research Labs. Her main interest has been focused on MEMS device technology development. Shih-Chia Chang received his BSc degree in electrical engineering from National Taiwan University in 1962 and his PhD degree in physics from Brown University in 1972. He joined General Motors Research Labs in 1977 as a senior research scientist. Since 1999, he has been a senior staff research scientist at Delphi Research Labs. His research interests are in micromachining technology, microsensors and microactutaors.
Electrostatically actuated copper-blade microrelays Han-Sheng Lee*, Chi H. Leung, Jenny Shi, Shih-Chia Chang Delphi Research Labs, 51786 Shelby Parkway, Shelby Twp., MI 48315, USA Received 17 June 2001; accepted 3 January 2002
Abstract Electrostatically actuated microrelays have been fabricated. The structural material, copper, is electroformed onto patterned areas. The typical dimensions of a single-blade relay are 1:88 mm 1 mm 0:01 mm (length width thickness). Microrelays with connected sectioned blades have also been fabricated, which showed improvement in lowering contact resistance. It is believed that sectioned blades not rigidly connected together have better flexibility and more contact points. Fabricated relays are capable of carrying current loads of several amperes and are able to switch 1 A resistor loads. When switching off an inductive load, the energy stored in the motor coils will discharge through the relay gap and reduce the relay service life. Using a Zener diode as a discharge path for the inductive load increased the relay operation cycles. The time delay in making and breaking the contact is approximately 50 ms. To check the mechanical fatigue property, relays were used to switch a low load, 50 mA resistor load and were able to operate for more than 1:7 106 cycles without failure. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microrelays; Electrostatically actuation
1. Introduction Conventional electromagnetic relays are actuated by passing an electric current through the coils. The force generated by the energized coils attracts the blade and makes an electric contact between two electrodes of the relay, the moving electrode (blade) and stationary electrode (contact). This type of relay has been used in the industry for decades. They are reliable, inexpensive, can carry high currents (>20 A) and have very high off-state resistance. However, they generally require a continuous current to hold the electric contact. This holding current consumes power and generates heat needlessly. The other popular type of switching device is a solid-state relay (SSR). SSRs are fast, usually have better reliability than mechanical relays and are batch manufacturable which implies low cost and uniform products. However, SSRs have lower off-state resistances than mechanical relays. They are polarity sensitive and are generally more expensive when used to carry high currents in automotive applications. Depending on the application, both mechanical relays and SSRs have their market segments. In automotive applications, there is a need for small relays to facilitate packaging and integration that is also reliable and inexpensive. These applications include switching * Corresponding author. Tel.: þ1-586-323-9466; fax: þ1-586-323-9797. E-mail address: [email protected] (H.-S. Lee).
lamps and small motors with steady-state load currents lower than 3.5 A. An all-metal, electrostatically actuated microrelay fits this category. To test the feasibility, our first attempt was to fabricate a microrelay which was tiny and could switch a 1 A resistive load. We chose copper as the relay structure material because of its high conductivity, low cost, process compatibility and the plating solutions availability. We found that the fabricated microrelays are small and can be readily integrated with the connectors or actuators they control. However, because of the low contact force, the microrelays have durability problems when handling amperes of current. Efforts are still being made to resolve these problems. Both electrostatically and magnetically actuated micromachined relays have been demonstrated to handle low currents with a high number of switching cycles [1–10]. In communication applications, Yao and Chang [1] and Zavracky et al. [2] used electrostatically actuated relays to switch a 5 mA load for more than 109 cycles. Taylor et al. showed a magnetically actuated relay that can switch a 1.2 A dc load repeatedly [3] and Wong et al. [4] showed electrostatically actuated microrelays that can carry 0.4 A. However, neither researcher mentioned the actual number of switching cycles at these loads or current levels. In this report, we will describe the fabrication of electrostatically actuated relays that are capable of switching 1 A resistor loads.
0924-4247/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 0 5 3 - 5
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1.1. Cantilever structured microrelay In an electrostatically actuated microrelay, the magnitude of the actuation voltage depends on its dimensions and structural material. Fig. 1a shows a photomicrograph of a fabricated cantilever relay and Fig. 1b shows its crosssectional view. The relay consists of a blade (moving electrode) separated from the contact terminal (stationary electrode) by an air gap with an actuator underneath the blade. With a dc voltage applied between the actuator and the top blade, an electric field induced force will bend the blade down and make an electrical connection with the lower contact. Empirically, the actuation voltage VA at which the moving electrode will snap to the stationary electrode and make the contact can be expressed as [11–13]: VA2 /
Ed3 t3 ð10e0 L4 Þ
(1)
where E, t and L are Young’s modulus, thickness and length of the blade, respectively. The d is the equivalent distance between the blade and the actuator and e0 is the permittivity of air. As shown in Fig. 1b, the blade was separated from the actuator by an air gap and an insulator, silicon nitride, d is the sum of the air gap ‘‘d’’ and the thickness of the nitride multiplied by its dielectric constant. The measured dielectric
Fig. 1. An all-metal cantilever structured microrelay: (a) a photomicrograph of a finished relay and (b) the cross-sectional view of the relay (not to scale).
constant of our deposited nitride film was seven. These physical parameters, L, d and t are shown in Fig. 1b. The length, L, is the only parameter directly related to mask dimensions, the others are material and process parameters. For example, the parameter d can be adjusted by varying the sacrificial layer thickness and the nitride thickness. The thickness of the blade, t, depends on the total electroplating time and the rate at which the blade was electroplated. The Young’s modulus, E, can be changed by the plating conditions or by plating a different material.
2. Relay fabrication Major relay fabrication steps are shown in Fig. 2. After oxidizing the silicon wafers, an adhesion promoter, Ti–W film, was sputter deposited on the wafers followed by a thin layer of gold deposition without breaking the vacuum. Wafers were then patterned to form three islands as shown in Fig. 2a. A 0.25 mm of PECVD nitride was deposited on the wafer at 350 8C and then patterned as shown in Fig. 2b. A thin layer of tungsten was then sputter deposited followed by an aluminum evaporation and photoresist (mold) coating. After patterning, the lower contact regions were opened as shown in Fig. 2c. Copper was electroplated onto the wafers to level with the mold as shown in Fig. 2d. The wafers were patterned to remove the mold except the region adjacent to the copper contact as shown in Fig. 2e. Another layer of photoresist was then spun onto the wafers as shown in Fig. 2f. The wafers were patterned to have the blade anchor regions opened followed by a sputter deposition of a thin layer of tungsten as shown in Fig. 2g. Then, the wafers were patterned to have the gold plating base exposed as shown in Fig. 2h. A thick mold (15 mm) was then spun onto the wafers and patterned to expose the blade regions as shown in Fig. 2i and j. After plating 10 mm of copper, as shown in Fig. 2k, wafers were scribed along the chip boundaries by a wafer saw. The depth of the scribed lines was about half of the wafer thickness. Afterwards, all the sacrificial materials were removed with a tungsten etchant [14]. The etchant dissolved not only tungsten, but also aluminum and photoresist (mold). The blade was released as shown in Fig. 2l. The wafers were then cleaned, tested and separated into chips. Since the wafers had been sawed halfway through after the step, in Fig. 2k, they were easily breaking into chips without damaging the released relays. The relay chip was then adhered to a TO-8 header and wire bonded. Fig. 3 shows a partially wire bonded chip where four different structured relays were on the same chip. The designed lengths of the blades were 1.28, 1.88, 1.88 and 1.80 mm for the short, long, dual actuator and bridge structure, respectively. The structure of the relay was labeled in Fig. 3. For example, the relay with a long cantilever structure was the upper left one in Fig. 3. The widths of the blade were 1, 1, 1 and 0.46 mm for the short, long, dual actuator and bridge structure, respectively.
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Fig. 2. Major processing steps.
As shown in Fig. 2l, there were two air gaps, 1 and 2. Air gap 1, 3 mm was slightly wider than air gap 2 which was 2.6 mm. These two air gaps were achieved through processing. This arrangement will help the blade make contact with the lower electrode first when the relay is activated to the
‘‘on’’ state. As shown in Fig. 2f, there was a layer of photoresist on top of the copper contact and three layers of materials on top of the actuator area: photoresist, aluminum and tungsten. The thickness of these materials determined the separation of the air gap 2 and 1, respectively.
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Fig. 3. A photomicrograph of a fabricated relay chip where four different structured relays are on the chip.
Even though, the actuation voltage is independent of the width of the cantilever, the width of the three cantilevers in this study was kept the same to minimize the complexity that could be caused by a lateral internal stress gradient in the plated blade. As shown in Fig. 3, two blade lengths were used for the three cantilever relays. Two cantilever relays had longer blades (left two) and one had a shorter blade (bottom right). The upper right relay had a bridge structure. The bridge-structured relay was anchored at ends (horizontal) and had two contacts. When activated, the middle square plate (with holes) will electrically connect the two contacts. Of the two long-blade relays, one had dual actuators, with one actuator at each side of the stationary electrode. The dual actuator structure is used to test the mechanical flexibility of the moving electrode. In the dual actuator structure, the blade is pulled down by the actuation force generated from both sides of the lower electrode (contact electrode). If the blade is flexible, the blade makes a more uniform contact laterally and perhaps increases the contact area when compared to a single actuator structure. If the step for creating spacer (Fig. 2e) were skipped, the fabricated relay could have a narrower space between the blade and the edge of lower contact as shown in Fig. 4a. In this situation there is a possibility that when the blade is making contact with the lower electrode, the blade may touch the edge of the lower electrode instead of contacting the plateau of the contact electrode. In general, it is easier to control the smoothness of the plateau than the edge of the lower electrode. Hence, the plateau is the preferred area for the blade to make contact. Fig. 4b shows a fabricated relay without skipping the step in Fig. 2e.
3. Relay properties Measured actuation voltages of the relays were 47, 19, 20 and 130 V for the short cantilever, long cantilever, dual actuator and bridge-structured relays, respectively. These voltages were measured from the relays on the same chip. For the same structure, the majority (60%) of the fabricated relays had their actuation voltages deviated from the aforementioned values by less than 2 V. From Eq. (1), the actuation voltage of a cantilever-structured relay is inversely proportional to the square of its blade length. The square of the length ratio of the long and short cantilever structured relays is ð1:88=1:28Þ2 ¼ 2:16 which is reasonably close to the inverse ratio of their measured actuation voltages, ð47=19Þ ¼ 2:47. As shown in Fig. 5a, with a voltage source connected to a 1 kO resistor load and to the stationary electrode of a long-blade cantilever relay, the switching of a relay was monitored by measuring the ‘‘output’’ voltage at the contact terminal of the relay. When the relay was electrically open, the output voltage was the same as the source voltage, 6 V. When the relay was electrically closed, the monitored voltage dropped to ground potential. Fig. 5b shows that with an applied 20 V actuation voltage (top trace), the time delay in closing the relay was 22 ms (bottom trace). After releasing the 20 V actuation voltage, Fig. 5c shows that the opening of the relay (top trace) had a time delay of 56 ms (bottom trace). With 0.5 s ‘‘on’’ and 0.5 s ‘‘off’’ 20 V pulses applied to the actuator (top trace), the responses at the contact terminal (bottom trace) are shown in Fig. 5d.
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activation voltage of the bridge structured relays were too high for our applications, we did not use the bridgestructured relays to do the test. The long-blade relays were short circuited in about 20 operation cycles. The coil of the motor in the mirror driving mechanism had an inductance, Ł, of 12 mH. For an inductive load, an induced voltage with a magnitude of Ł (d /dt), was generated by the motor coil during switchings. Here, Ł is the inductance of the motor coil and is the current through the coil during switchings. We found that the current in the motor coil increased to 0.5 A before the contact breaking. Since the time scale for making and breaking the contact is shorter than 60 ms (Fig. 5), a portion of the induced 100 V will drop across the air gap and cause arcings. The localized arcing between blade and stationary electrode can create material transfer between electrodes and shorten the relay life cycles [15].
4. Discussion When the relays were used to switch current loads of fractions of an ampere, there was a substantial reduction in the number of their operation cycles. We noticed that the following parameters affected our relay performance. 4.1. Arc
Fig. 4. A photomicrograph of the relay (a) without adding spacer during fabrication and (b) with spacer added in the fabrication.
Using pulsed actuation voltages to control the ‘‘open’’ and ‘‘close’’ of the relay, the number of operation cycles was monitored. A long-blade microrelay was activated sequentially with 0.05 s at 20 V and 0.2 s at ground voltage to switch a 50 mA resistor load. In switching this low current load, the relays were able to operate for more than 1:7 106 cycles without failure. The relays were still operational when the tests were stopped. 3.1. Switching inductive loads A long-blade cantilever microrelay was used to switch a 12 V, 5.5 W fan motor which had a very low inductance motor coil. During switching, a 1 A current surged through the microrelay. The microrelay was able to turn the fan on and off for more than 150 cycles. The relay stayed in the closed position when it failed. Long-blade cantilever microrelays were also used to switch a side mirror motor of a vehicle. Since the required
The main causes of short relay operation cycles are arc damage and welding at the contact. To check the arc damage on the relay life, we connected a Zener diode in parallel with the vehicle side mirror motor. Lockwood pointed out that the connected diode can be of much lower current capacity than the steady-state current through the motor. This is because the diode conducts only for an extremely short time [16]. When the microrelay was activated to the ‘‘on’’ state, the current through the relay went to the motor coil because the Zener diode was reversibly biased. When the relay was switched to the ‘‘off’’ state, the coil-induced voltage will forward bias the diode and make the diode electrically short the coil. The coil-stored energy went to ground through the diode. Adding the diode to the inductive load reduces the arc damage at the microrelay contact and increases the relay life cycles. The number of life cycles is the number of working cycles until the relay fails to respond the activation voltage change. Usually, the relay failed to open after releasing the activation voltage. We found that the life cycles increased to 150 cycles when a Zener diode was connected to the vehicle side mirror motor. A better approach to minimize the arc damage to microrelays is to integrate an arc suppressor with microrelays. The suppressor offers an electric path to absorb the energy spikes and also create an arcless condition for the microrelay during all swithchings [17]. With a partial protection from a Zener diode, the number of operation cycles of the microrelay was still far below the
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Fig. 5. Measured relay response time: (a) measurement setup; (b) time delay in switching ‘‘on’’, 22 ms; (c) time delay in switching ‘‘off’’, 56 ms; (d) switching a 1 kO load continuously.
low load (50 mA) test results (>1:7 106 cycles). We believe local melting at the contact area also contributed to the short operation cycles of the relay. For copper electrodes, the melting voltage is 0.43 V [15]. With 0.5 A current through the relay, if the contact resistance is higher than 0.86 O, the localized heat is enough to cause a melting problem at the contact surfaces. We found that the relay contact resistance increased to 1 O after switching the inductive motor for about 20 cycles. After this point, the relay blade stuck to the lower contact so frequently and became unusable. It is speculated that the oxidation of the copper at the contact interfaces was responsible for the increase in contact resistance.
4.2. Oxidation To check if the copper oxidation affected the relay service cycles, copper electrodes and copper covered with gold electrodes were compared. A thin layer of gold (0.1 mm) was electroplated on top of the lower contact and at the bottom of the blade in the relay fabrication. Therefore, copper was not exposed at the contact in these microrelays. Despite the fact that copper can diffuse through gold and reach the electrode surface [18], the number of service cycles of these gold covered microrelays increased from 150 to 500 cycles when the fan (low inductive load) was used as the load. To slow the copper from diffusing to the
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contact surfaces even more, we added a diffusion barrier material, nickel, between copper and gold [19]. We changed the blade material composition by sandwiching a 2 mm nickel film between copper and gold. To minimize the possibility of blade curling due to mismatch in materials, we intentionally made the composite material symmetric in its cross-section. The new blade composed of five layers, Au (0.3 mm)/Ni (2 mm)/Cu (7.5 mm)/Ni (2 mm)/Au (0.3 mm). All the metals were electroplated at the step shown in Fig. 2k. When using this composite-blade relay to switch the fan motor, the number of operation cycles increased to 1000, a significant improvement in relay durability. We attributed the improvement to two factors; slower increase in contact resistance and larger breaking force. This is because the required activation voltage of the long-blade structured relay increased from 20 to 75 V (composite blade). Such a large increase in activation voltage was not expected because the value of Young’s modulus of nickel, 2 1012 dyn/cm2, is only 33% higher than that of copper, 1:5 1012 dyn/cm2 [20]. Besides, the relay activation voltage is proportional to the square root of Young’s modulus as shown in Eq. (1). In addition, the total thickness of nickel was about one half of the copper thickness in the composite blade and there was no significant curling in the composite blade. 4.3. Contact resistance Reducing contact resistance not only eases the welding problem but also cuts the energy loss at the contact. Several factors affect the value of contact resistance, which include contact materials, contact force and contact area (contact material deformability, surface smoothness and number of contact points) [21]. We have tried to correlate different plating solutions and plating conditions to the surface smoothness of plated material. The work is still in progress. Another method we used to reduce the contact resistance was to change the blade structure. Instead of using a single blade, multiple-blade relays were fabricated. 4.3.1. Multiple-blade relay The purpose of using multiple blades was to create sectioned blades with each section not rigidly connected to its neighbors. Connected section blades will still act as one blade and will have a better overall flexibility as opposed to one large blade piece [22]. Using multiple blades to reduce contact resistance had been proven helpful in the conventional mechanical relays [22]. Fig. 6 shows a photomicrograph of a finished four-blade microrelay. The crosssectional view of the multiple-blade relay is identical to that of Fig. 2l. The blade composed of four sections connected with small joints between them. The dimensions of the relay were 1:48 mm 1:6 mm 0:01 mm (length width thickness). Similar to the single-blade structure, the spacings were 3 and 2.6 mm for the air gap 1 and 2, respectively. The new relay which had copper–copper contact and four sectioned blades showed a contact resistance of 30 mO
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Fig. 6. A photomicrograph of a new microrelay where the blade composed of four sections connected with joints.
which was lower than the single piece blade fabricated earlier, 40 mO. The relay contact resistance was defined as the resistance at the contact interfaces and was measured by using a four-point probe resistance meter at the differential mode. A ‘‘calibration’’ relay, similar to the tested relays, was prepared by wire bond to electrically short the blade contact and stationary contact. This calibration relay was used as our zero contact resistance reference. In this arrangement the measured resistance contributed from the connection wires, bonding wires and package pins were subtracted. We connected the new relay as shown in Fig. 5a to switch a 1 A resistor load. Results are shown in Fig. 7. The top trace showed the pulsed activation voltage used, 0.08 s at 25 V followed by 1.2 s at ground potential in sequence. The middle trace showed the current switched and the bottom was the voltage measured at the ‘‘output’’ terminal (Fig. 5a). The new microrelay was able to switch 1 A load for about 300 cycles before showing the sticking problem. Even at the 1 A level, lower contact resistance helped to increase the service life of the relay. The tested relay chip was adhered to a TO-8 header, wire bonded as shown in Fig. 6 and then capped with a metal can. The can was mechanically cramped to the header. Hence, the relays were not in a controlled environment. In fabricating the multiple-blade relays, different copper plating solution was used, which contained additives for better leveling and was different from the solution used in the single-blade relay fabrication. Therefore, we do not expect that the required activation voltage for the new relays can be extrapolated from the earlier results. If the earlier plating solution results were used, the extrapolated activation voltage should be 32–35 V which is higher than the value shown in Fig. 7, i.e. 25 V. Another approach to reduce the contact welding problem is to use high melting voltage materials at the contact surfaces. The melting voltage of platinum, 0.65 V, is higher
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Fig. 7. Measured results of using a four-sectioned blade microrelay to switch a 1 A resistor load.
than that of copper, 0.43 V [23]. Higher melting voltage materials have more resistance to the contact sticking problem. Furthermore, platinum can be electroplated. It could be used at the contact surfaces. We are characterizing the properties of the electroplated platinum films and will test this approach in our relay fabrication.
5. Summary All-metal microrelays have been fabricated to switch different loads. When using the microrelays to switch a low current load (50 mA), higher than 1:7 106 operation cycles have been demonstrated. When using the microrelays to switch loads of 0.5–1 A, we found that both the arc damage and welding problem limited the relay operation life. When the microrelays were used to switch an inductive load, a vehicle side mirror in our case, the energy stored in the coils discharged through the relay air gap contributed to the low relay operation life. We increased the relay operation cycles by connecting a Zener diode in parallel to the side mirror motor to offer a discharge path for the coil-stored energy. The welding at the contact was caused by the localized heat generated at the contact points. The higher the current passing through the relay, the lower the contact resistance required. We found that instead of using one large blade, using sectioned blades connected by joints to offer better blade flexibility reduced the contact resistance. Even though, the copper had very low resistivity, the localized heat can accelerate the copper oxidation and increase the
contact resistance. We found that covering the copper with inert material, gold, increase the relay operation life. Adding a diffusion barrier between copper and gold can slow down the copper diffusion to the contact surface further and increase the microrelays service life more. We are still experimenting on plating conditions with different materials and solutions. The durability shown in these tests is not sufficient to meet automotive specifications, we believe that it can be improved by incorporating vacuum packaging techniques, adapting high melting voltage interface material and by reducing the contact resistance of the microrelays.
Acknowledgements We would like to thank Sam Lorincz, Jim Orsine and Grant Wheeler of Delphi-Packard Electric Systems and Tony Lee of Delphi Technologies Inc for their help and support to this project. References [1] J.J. Yao, M. Chang, A surface micromachined miniature switch for telecommunications applications with signal frequencies from dc up to 4 GHz, in: Proceedings of the Transducers’95, Stockholm, Sweden, 1995, pp. 384–387. [2] P.M. Zavracky, S. Majumder, N.E. McGruer, Micromechanical switches fabricated using nickel surface micromachining, IEEE, J. Microelectromech. Syst. 6 (1997) 3–9. [3] W.P. Taylor, O. Brand, M.G. Allen, Fully integrated magnetically actuated micromachined relays, IEEE, J. Microelectromech. Syst. 7 (1998) 181–191.
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Biographies Han-Sheng Lee received his PhD in Electrical Engineering from Princeton University in 1976. In the same year, he joined the GM Research Labs. He was transferred to Delphi Research Labs in 1999. His technical interests are in the general area of physics, technology and applications of sensors and semiconductor devices. Chi H. Leung received a BSc in electrical engineering and MSc and PhD in metallurgical engineering. All degrees were from the University of Michigan, Ann Arbor. He is currently, Director of R&D, AMI Doduco North America. Previously, he worked with GTE Technical Products Division and General Motor R&D Lab which later evolved into the Delphi Research Lab. His research interests are on electrical contact material for arcing contact and automotive connectors, latching relays, MEM relays and Plasma Ion Implantation. He has published 24 journal papers and is holder of 11 US patents. He is the current Chairman of the IEEE Holm Conference Operating Committee. Jenny Shi obtained her ME in Electrical Engineering from the University of Michigan in 1989 and her BSc in Material Science and Engineering from Zhejiang University in 1982. Since 1989, she has been working with General Motors Corporation’s Research Laboratories and Delphi Research Labs. Her main interest has been focused on MEMS device technology development. Shih-Chia Chang received his BSc degree in electrical engineering from National Taiwan University in 1962 and his PhD degree in physics from Brown University in 1972. He joined General Motors Research Labs in 1977 as a senior research scientist. Since 1999, he has been a senior staff research scientist at Delphi Research Labs. His research interests are in micromachining technology, microsensors and microactutaors.