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Diamond switch using new thermal actuation principle
Diamond and Related Materials 12 (2003) 418–421
Diamond switch using new thermal actuation principle P. Schmid*, F.J. Hernandez-Guillen, E. Kohn University of Ulm, Albert-Einstein-Allee 45, D-89081 Ulm, Germany
Abstract In this work, a thermally actuated diamond microswitch is presented. To realize such switches, the classical bi-metal principle has been newly applied to diamond beams. Details on the device structure and its technological realization as well as measurement results on the fabricated switches will be given. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Diamond film; Thermal properties; Actuators; Device modeling
1. Introduction Due to its outstanding material properties, the use of diamond for micro electro mechanical systems is very promising, especially in heavy duty applications. As a first device, an electrostatically actuated microswitch based on polycrystalline diamond layers has been realized in past years w1x and encouraging results even for microwave applications could be shown w2x. However, due to its high thermal conductivity and its ability to tolerate high dynamic thermal stresses, diamond is especially well suited for thermally actuated microsystems. This has already been shown in two applications, namely the diamond inkjet w3x and micro fountain w4x. Additionally, the high thermal conductivity of diamond allows operating thermal microsystems at higher speeds, since the return to equilibrium is comparably fast. Thus, a new actuation principle for diamond microswitches based on the bi-metal effect has been developed. Compared to the electrostatic actuation principle, thermal actuation of microsystems has several advantages, among these are a lower actuation voltage and a higher contact force. The main drawback of thermally actuated devices, namely the static power consumption, can be avoided by a bi-stable layout using pre-stressed structures. *Corresponding author. Tel.: q49-731-50-26177; fax: q49-73150-26155. E-mail address: [email protected] (P. Schmid).
2. Device principle The basic device consists of a free standing diamond cantilever, on top of which a second material is deposited to create a bending moment due to the different thermal expansion when heated. Because the thermal expansion coefficient of diamond is very low (f10y6 Ky1), almost every material deposited on its surface is suitable for a bi-metal layout. But since the force created by the bi-metal effect is proportional to the difference in thermal expansion coefficients and the Young’s modulus of the deposited metal, Nickel has been chosen, because its properties are optimal in this respect. By using the diamond cantilever itself as the heater element, the device layout can be very simple, because no additional material has to be used. The layout of a single anchored beam, which is bent downward when heat is applied, is shown in Fig. 1. 3. Technology and realization The basic technological steps are the growth of diamond on a sacrificial layer, the patterning of the diamond layer by reactive ion etching, the deposition of chromiumygold for electrical contacts and chromiumy nickel for the bi-metal layer and the release of the cantilevers by removal of the sacrificial layer. A brief description of this technology can be found in Ref. w1x. The conductivity of the diamond layer is obtained by doping with boron during growth. The minimum resistivity which can be obtained by this doping process is below 10 mVcm, in the case discussed below a resistiv-
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 3 9 9 - 0
P. Schmid et al. / Diamond and Related Materials 12 (2003) 418–421
419
Fig. 1. Layout of thermal microswitch.
ity of 30 mVcm has been used, because higher conductivities would require higher driving currents, which are more difficult to handle with the metalization scheme used. This resistivity leads to a resistance of the heater element of 14.5 V. The thickness of the diamond layer was 2.5 mm and to obtain operation at reasonable switching temperatures, a nickel film of comparable thickness is necessary. Thus, this film has been deposited using an electroless plating process. Selective deposition can be obtained by patterning the nickel starting layer, which has been deposited by sputtering and lift-off. To additionally prevent the deposition of nickel in other areas, these have been covered by a mask layer during plating. A micrograph of the resulting nickel structures is shown in Fig. 2. In Fig. 3 a micrograph of the realized basic switch structure is given. The bending of the switches is caused by stress in the nickel layer, which is deposited at 90 8C and thus develops thermal stress when cooled down to room temperature. The diamond layer itself shows nearly no bending due to stress, a condition which is obtained by a special doping technique. The actuation force can easily overcome the internal stresses and since the bi-metal effect creates a bending moment at the cantilever tip, the switches can be closed regardless of the intrinsic bending, which is a further advantage of this actuation technique. Especially since
Fig. 2. Selectively plated nickel structures.
for many applications an intrinsic upward bending is highly desirable, e.g. in the case of microwave switches, where this bending greatly reduces the off-state capacitance and therefore increases the off-state isolation. 4. Results The devices have been operated successfully, Fig. 4 shows a single side anchored microswitch in the open and closed state. For closing the switch, an electrical power of 120 mW is needed for a switch of 800 mm length, which corresponds to an actuation voltage of 1.3 V. This is significantly lower than the actuation voltages of electrostatic switches, which are in the order of several 10 V. Fig. 5 shows the measured deflection of the cantilever tip vs. actuation power for operation in air. It is observed, that for low heating power, the deflection is proportional to the applied power, which is expected, because the resulting temperature should also depend linearly on the generated power. This is not in the case for smaller tip deflections, since the air gap in this case
Fig. 3. Micrograph of thermal microswitches.
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P. Schmid et al. / Diamond and Related Materials 12 (2003) 418–421
Fig. 4. Thermal switch in open (left) and closed (right) state (actuated structure is in the middle).
Fig. 5. Deflection of the switch vs. applied electrical power (device geometry: length 800 mm, width 200 mm, diamond layer thickness 2.5 mm).
Fig. 6. Deflection of the switch vs. ambient temperature.
is small and a significant amount of heat flows through the air gap to the substrate thus lowering the maximum temperature and therefore the mechanical deflection. The possibility to control the switch deflection with the applied power in a wide range opens up further application, like e.g. variable capacitances or resonators. The dependence of the tip deflection on the ambient temperature has also been investigated, measurement and simulation results are given in Fig. 6. The expected linear relation can indeed be observed, the switching temperature, i.e. the temperature necessary for closing the switch, is 108 8C. After the switch is closed, an increasing force is exerted on the contact with additional heating. The expected contact force of the switch has been extracted from the simulation results to be 1.2 mN Ky1 above 108 8C. Since in the configuration described so far a static power is needed to hold the switch in its down-position and to create the contact force, this will produce a permanent power loss. To avoid this disadvantage a bistable configuration has also been developed. This layout consists of a diamond cantilever, which is anchored at both ends. In the presence of intrinsic stress the cantilever buckles, if it is thinner than a certain critical thickness. This buckling has two stable states (Fig. 7), and switching between these two states is possible by a local heating of the cantilever, if covered by a bi-metal layer. These conditions for bi-stable operation are easily fulfilled for diamond cantilevers. Fig. 8 shows simulations of the switching of such a device using a diamond
Fig. 7. Bi-stable configuration of diamond cantilever.
P. Schmid et al. / Diamond and Related Materials 12 (2003) 418–421
421
Fig. 8. Simulation of the switching event for bi-stable switch configuration.
beam from up to down and vice versa together with the temperature distribution for both cases.
nearly linear control of the deflection with the applied actuation power, other applications like variable capacitances and resonators might be possible.
5. Conclusion
References
A technology for thermally actuated microswitches based on polycrystalline diamond films has been developed. Switches have been realized and could be successfully operated with actuation voltages as low as 1.3 V. The actuation principle employed can now be incorporated e.g. in microwave switches. The static power loss needed to keep the switch in its closed position can be avoided by a bi-stable design of the switch, which has also been developed. Due to the possibility of a
w1x M. Adamschik, et al., Electrostatic Diamond Micro Switch, Tenth International Conference on Solid-State Sensors and Actuators, Transducers ‘99, Sendai, Japan, 1999, Digest of Technical Papers, vol. 2, p. 1284. w2x M. Adamschik, et al., Diamond microwave micro relay, Diamond Relat. Mater. 11 (2002) 672–676. w3x P. Gluche, et al., Novel thermal microactuator based on CVDdiamond films, IEDM, 1998. w4x A. Kaiser, et al., Diamond based injection system for spotting and synthesis in biochemistry, AIChE Annual Meeting, 2002, submitted for publication.
Diamond switch using new thermal actuation principle P. Schmid*, F.J. Hernandez-Guillen, E. Kohn University of Ulm, Albert-Einstein-Allee 45, D-89081 Ulm, Germany
Abstract In this work, a thermally actuated diamond microswitch is presented. To realize such switches, the classical bi-metal principle has been newly applied to diamond beams. Details on the device structure and its technological realization as well as measurement results on the fabricated switches will be given. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Diamond film; Thermal properties; Actuators; Device modeling
1. Introduction Due to its outstanding material properties, the use of diamond for micro electro mechanical systems is very promising, especially in heavy duty applications. As a first device, an electrostatically actuated microswitch based on polycrystalline diamond layers has been realized in past years w1x and encouraging results even for microwave applications could be shown w2x. However, due to its high thermal conductivity and its ability to tolerate high dynamic thermal stresses, diamond is especially well suited for thermally actuated microsystems. This has already been shown in two applications, namely the diamond inkjet w3x and micro fountain w4x. Additionally, the high thermal conductivity of diamond allows operating thermal microsystems at higher speeds, since the return to equilibrium is comparably fast. Thus, a new actuation principle for diamond microswitches based on the bi-metal effect has been developed. Compared to the electrostatic actuation principle, thermal actuation of microsystems has several advantages, among these are a lower actuation voltage and a higher contact force. The main drawback of thermally actuated devices, namely the static power consumption, can be avoided by a bi-stable layout using pre-stressed structures. *Corresponding author. Tel.: q49-731-50-26177; fax: q49-73150-26155. E-mail address: [email protected] (P. Schmid).
2. Device principle The basic device consists of a free standing diamond cantilever, on top of which a second material is deposited to create a bending moment due to the different thermal expansion when heated. Because the thermal expansion coefficient of diamond is very low (f10y6 Ky1), almost every material deposited on its surface is suitable for a bi-metal layout. But since the force created by the bi-metal effect is proportional to the difference in thermal expansion coefficients and the Young’s modulus of the deposited metal, Nickel has been chosen, because its properties are optimal in this respect. By using the diamond cantilever itself as the heater element, the device layout can be very simple, because no additional material has to be used. The layout of a single anchored beam, which is bent downward when heat is applied, is shown in Fig. 1. 3. Technology and realization The basic technological steps are the growth of diamond on a sacrificial layer, the patterning of the diamond layer by reactive ion etching, the deposition of chromiumygold for electrical contacts and chromiumy nickel for the bi-metal layer and the release of the cantilevers by removal of the sacrificial layer. A brief description of this technology can be found in Ref. w1x. The conductivity of the diamond layer is obtained by doping with boron during growth. The minimum resistivity which can be obtained by this doping process is below 10 mVcm, in the case discussed below a resistiv-
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 3 9 9 - 0
P. Schmid et al. / Diamond and Related Materials 12 (2003) 418–421
419
Fig. 1. Layout of thermal microswitch.
ity of 30 mVcm has been used, because higher conductivities would require higher driving currents, which are more difficult to handle with the metalization scheme used. This resistivity leads to a resistance of the heater element of 14.5 V. The thickness of the diamond layer was 2.5 mm and to obtain operation at reasonable switching temperatures, a nickel film of comparable thickness is necessary. Thus, this film has been deposited using an electroless plating process. Selective deposition can be obtained by patterning the nickel starting layer, which has been deposited by sputtering and lift-off. To additionally prevent the deposition of nickel in other areas, these have been covered by a mask layer during plating. A micrograph of the resulting nickel structures is shown in Fig. 2. In Fig. 3 a micrograph of the realized basic switch structure is given. The bending of the switches is caused by stress in the nickel layer, which is deposited at 90 8C and thus develops thermal stress when cooled down to room temperature. The diamond layer itself shows nearly no bending due to stress, a condition which is obtained by a special doping technique. The actuation force can easily overcome the internal stresses and since the bi-metal effect creates a bending moment at the cantilever tip, the switches can be closed regardless of the intrinsic bending, which is a further advantage of this actuation technique. Especially since
Fig. 2. Selectively plated nickel structures.
for many applications an intrinsic upward bending is highly desirable, e.g. in the case of microwave switches, where this bending greatly reduces the off-state capacitance and therefore increases the off-state isolation. 4. Results The devices have been operated successfully, Fig. 4 shows a single side anchored microswitch in the open and closed state. For closing the switch, an electrical power of 120 mW is needed for a switch of 800 mm length, which corresponds to an actuation voltage of 1.3 V. This is significantly lower than the actuation voltages of electrostatic switches, which are in the order of several 10 V. Fig. 5 shows the measured deflection of the cantilever tip vs. actuation power for operation in air. It is observed, that for low heating power, the deflection is proportional to the applied power, which is expected, because the resulting temperature should also depend linearly on the generated power. This is not in the case for smaller tip deflections, since the air gap in this case
Fig. 3. Micrograph of thermal microswitches.
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P. Schmid et al. / Diamond and Related Materials 12 (2003) 418–421
Fig. 4. Thermal switch in open (left) and closed (right) state (actuated structure is in the middle).
Fig. 5. Deflection of the switch vs. applied electrical power (device geometry: length 800 mm, width 200 mm, diamond layer thickness 2.5 mm).
Fig. 6. Deflection of the switch vs. ambient temperature.
is small and a significant amount of heat flows through the air gap to the substrate thus lowering the maximum temperature and therefore the mechanical deflection. The possibility to control the switch deflection with the applied power in a wide range opens up further application, like e.g. variable capacitances or resonators. The dependence of the tip deflection on the ambient temperature has also been investigated, measurement and simulation results are given in Fig. 6. The expected linear relation can indeed be observed, the switching temperature, i.e. the temperature necessary for closing the switch, is 108 8C. After the switch is closed, an increasing force is exerted on the contact with additional heating. The expected contact force of the switch has been extracted from the simulation results to be 1.2 mN Ky1 above 108 8C. Since in the configuration described so far a static power is needed to hold the switch in its down-position and to create the contact force, this will produce a permanent power loss. To avoid this disadvantage a bistable configuration has also been developed. This layout consists of a diamond cantilever, which is anchored at both ends. In the presence of intrinsic stress the cantilever buckles, if it is thinner than a certain critical thickness. This buckling has two stable states (Fig. 7), and switching between these two states is possible by a local heating of the cantilever, if covered by a bi-metal layer. These conditions for bi-stable operation are easily fulfilled for diamond cantilevers. Fig. 8 shows simulations of the switching of such a device using a diamond
Fig. 7. Bi-stable configuration of diamond cantilever.
P. Schmid et al. / Diamond and Related Materials 12 (2003) 418–421
421
Fig. 8. Simulation of the switching event for bi-stable switch configuration.
beam from up to down and vice versa together with the temperature distribution for both cases.
nearly linear control of the deflection with the applied actuation power, other applications like variable capacitances and resonators might be possible.
5. Conclusion
References
A technology for thermally actuated microswitches based on polycrystalline diamond films has been developed. Switches have been realized and could be successfully operated with actuation voltages as low as 1.3 V. The actuation principle employed can now be incorporated e.g. in microwave switches. The static power loss needed to keep the switch in its closed position can be avoided by a bi-stable design of the switch, which has also been developed. Due to the possibility of a
w1x M. Adamschik, et al., Electrostatic Diamond Micro Switch, Tenth International Conference on Solid-State Sensors and Actuators, Transducers ‘99, Sendai, Japan, 1999, Digest of Technical Papers, vol. 2, p. 1284. w2x M. Adamschik, et al., Diamond microwave micro relay, Diamond Relat. Mater. 11 (2002) 672–676. w3x P. Gluche, et al., Novel thermal microactuator based on CVDdiamond films, IEDM, 1998. w4x A. Kaiser, et al., Diamond based injection system for spotting and synthesis in biochemistry, AIChE Annual Meeting, 2002, submitted for publication.