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Polyimide microcantilever surface stress sensor using low-cost, rapidly-interchangeable, spring-loaded microprobe connections
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Microelectronic Engineering 85 (2008) 1314–1317 www.elsevier.com/locate/mee
Polyimide microcantilever surface stress sensor using low-cost, rapidly-interchangeable, spring-loaded microprobe connections R.H. Ibbotson a,b,*, R.J. Dunn a,b, V. Djakov a, P. Ko Ferrigno c,1, S.E. Huq a a
Rutherford Appleton Laboratory, Chilton, Oxon, OX11 0QX, UK b Heriot-Watt University, Edinburgh, EH14 4AS, UK c MRC Cancer Cell Unit, Hutchison/MRC Research Centre, Hills Road, Cambridge CB2 OXZ, UK Received 5 October 2007; received in revised form 14 December 2007; accepted 27 December 2007 Available online 12 January 2008
Abstract We present the fabrication of a polyimide microcantilever array with integrated gold piezoresistive strain gauges and a thick SU-8 backing for use as a surface stress sensor. The selected polyimide’s low Young’s modulus (1.8 GPa) theoretically results in a high sensitivity to changes in surface stress. Electroplated gold contact pads, 1.5 lm thick, provide a stable, repeatable surface onto which electrical connections to the fabricated device are made using spring-loaded microprobes. The piezoresistors are found to have a gauge factor of between 4 and 4.5 and a sensitivity to surface stress of 2.1 10 4 (N/m) 1. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Cantilever; Microcantilever; Polyimide; Surface stress sensor
1. Introduction Microfabricated cantilevers have been of great interest in the field of biological imaging since the invention of the atomic force microscope (AFM) [1]. More recently, cantilevers operated as static sensors, deflecting due to a change in the surface stress of a surface layer, have been used to translate biorecognition into a mechanical response [2]. These measurements have been made using microfabricated silicon cantilever arrays with tip deflections measured by an array of diode lasers [2]. The integration of piezoresistors onto AFM cantilevers [3] has enabled large arrays to be produced without the need for the alignment of external optics [4]. A Wheatstone bridge configuration is employed to detect small resistance changes in the active cantilever
* Corresponding author. Address: Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, UK. Tel.: +44 1235 445722. E-mail address: [email protected] (R.H. Ibbotson). 1 Current address: Section of Experimental Therapeutics, Leeds Institute of Molecular Medicine, St James’s University Hospital, Leeds LS9 7TF, UK.
0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.12.065
sensor. Low-cost SU-8 polymer cantilevers with integrated strain sensors have been shown to have an excellent sensitivity to surface stress which is comparable to that of piezoresistive silicon cantilevers [5]. Other polymers have been investigated [6] for their suitability and may offer further simplifications of the fabrication process and increases in sensitivity to surface stresses. Polyimides are identified as suitable polymers for microcantilevers given their low Young’s modulus, high planarity, chemical resistance and biocompatibility [7]. 2. Fabrication A similar fabrication sequence to that used by Johansson et al. [5] for SU-8 cantilever devices is followed, however polyimide and electroplated gold instead of SU-8 and electroplated nickel are used. Fabrication of the cantilever devices is performed on standard 4 in. Si wafers (Fig. 1a). Three metal layers of titanium/gold/chromium with thicknesses 5 nm/50 nm/75 nm are deposited by egun evaporation onto a clean, silicon substrate (Fig. 1b). Polyimide (Pyralin PI2562) is spin-coated onto the wafer at a
R.H. Ibbotson et al. / Microelectronic Engineering 85 (2008) 1314–1317
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Fig. 1. Process sequence showing key stages in the fabrication of the cantilever device.
speed of 5000RPM for 60 s. A post-spin hotplate bake is performed to initiate the removal of solvents from the polyimide layer. The wafer is cured from room temperature to 350 °C in an oven with an inert N2 atmosphere. This is sufficient to drive off all the solvents and results in a polyimide thickness of 0.7 lm. A 7 lm etch mask of AZ9260 is spincoated and patterned with square contact pads, which are etched through to the chromium layer beneath in a RIE O2 plasma (Fig. 1c). The remaining AZ9260 is removed in AZ100 remover and the wafer is rinsed in DI water. To remove any moisture a 2 h bake at 120 °C is performed. Lift-off lithography of the piezoresistors and contact pads is performed using LOR and JSR photoresists. The resistor layer of titanium/gold with thicknesses of 5 nm/60 nm is evaporated by egun evaporation. Lift-off is completed in acetone and JSR developer (Fig. 1d). Earlier fabrication attempts used wet etching of the gold layer, but due to the speed of the etch, repeatability is poor. A gold layer of 1.5–2.5 lm is electroplated on to provide sufficient strength to the pads for contacting by the spring-loaded microprobes. Electroplating lithography is prepared in AZ9260. The gold layer is electroplated in ECF-60 solution (Rohm-Haas) for 20 min at 76 mA/dm2 followed by 40 min at 152 mA/dm2 (Fig. 1e). Following the removal of the AZ9260, a dehydration bake at 200 °C is performed. The second polyimide layer is spin-coated on with various thicknesses; a typical spin speed for a 1.5 lm layer is 3000RPM for 60 s. The polyimide curing process is repeated for the second layer, encapsulating the piezoresistors and electroplated contact pads. A second AZ9260 etch mask is spin-coated onto the polyimide surface and patterned with cantilevers and channels. A 100 lm wide grid defines each chip and increases etchant access to the chromium sacrificial layer. Both polyimide layers are etched simultaneously in an RIE O2 plasma, reducing the number of alignment steps compared to an all SU-8 cantilever process (Fig. 1f). Any remaining AZ9260 is stripped in AZ100 remover. After a DI water rinse, a long bake at 200 °C is
performed to drive off any moisture. A 300 lm thick backing layer is created in SU-8 (Microchem 2150). Approximately 10 g of SU-8 is poured onto the wafer. Tweezers are used to spread the SU-8 evenly around the wafer. Spin-coating is performed according to Microchem’s guidelines for a 300 lm layer. The SU-8 wafer is placed on a levelled hotplate and allowed to relax for 2 h to reduce any spin-induced stresses. A soft bake of 10 min at 65 °C and 1 h 45 min at 95 °C is carried out with slow ramp rates. The SU-8 is allowed to relax at room temperature for 24 h. The wafer is patterned with a grid and channels with an exposure dose of 1500 mJ/cm2 in 5 cycles with a 45 s wait between each cycle. Post-exposure bake parameters differ from the manufacturer’s guidelines since these were found to produce a highly stressed SU-8 film, prone to delamination from the polyimide layer beneath. After a 1 h relaxation period, a 65 °C post-exposure bake is performed for 1 h 50 min. The wafer is allowed to relax for 24 h. Development is performed in EC solvent with agitation for 30 min (Fig. 1g). After a further 24 h relaxation, the cantilever chips are released by etching the sacrificial chromium layer in Chromium etchant (Transene 1002A) (Fig. 1h). This release process takes between 6 and 12 h. It has been reported that chromium and gold layers act as an enhanced sacrificial layer [9]. Higher etch speeds were evident but these were attributable to higher tensile stresses in the SU-8 layer. Released chips are rinsed carefully in DI water, dried in nitrogen and stored for use. The completed cantilever device, consisting of two full Wheatstone bridges each containing two piezoresistors integrated in cantilevers and two on-chip resistors, is shown in Figs. 2 and 3a. A pair of free-standing cantilevers with integrated piezoresistors is shown in Fig. 3b. 3. Electrical connections Creating the final electrical connection to soft polymer chips poses problems using conventional silicon intercon-
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Chip identifier Channel access well
On-chip resistor
Cantilever pair in channel
Gold tracks Contact pads
1mm
Wheatstone bridge equivalent circuit
Fig. 2. Photograph of a completed device highlighting the key features. Two full Wheatstone bridges are created on each device. The device footprint is 7.6 6 mm.
nect technology [8]. Instead, electrical connections are created simply, using general-purpose, short-travel, springloaded microprobes with a pitch of 1.3 mm (Coda-pin, PA0J Fig. 3c). Large area 1 mm2 pads, electroplated with at least 1.5 lm of gold, provide a sufficiently strong contact region for repeated contacts by the microprobes. The cantilever device requires no additional preparation beyond
the normal fabrication process and can be inserted into the electronic setup within seconds. The repeatability of the contact resistance is to within 0.5% of the piezoresistor strain gauge resistance values. The microprobes are soldered directly into a PCB which connects the device to a low voltage AC input signal and high gain amplifiers for the output signal. The Wheatstone bridge voltage change is measured using a software-based lock-in amplifier, operating at 1.1 kHz. 4. Characterisation A pinhead, mounted on a LabView controlled precision linear stage, vertically displaces the cantilever tip in 5 lm steps (Fig. 3d). Absolute resistance changes of the piezoresistor are recorded on a 6 12 digit digital multimeter (Keithley 2100). The linearity and repeatability of the piezoresistive sensor is shown in Fig. 4. A range of devices from two different wafers are found to have deflection sensitivities (DR/R per unit deflection) of between 0.01 and 0.03 ppm nm 1. This yields a gauge factor of between 4 and 4.5 for the gold piezoresistors, comparable to literature data for gold [10]. Initial experiments are performed to investigate a single cantilever’s response to surface stress changes. A titanium/ gold layer (5 nm/40 nm) is evaporated onto all 4 cantilevers on a device. The layer contains a residual stress which
Fig. 3. (a) SEM image of a completed device showing four cantilevers fabricated on a 300 lm thick SU-8 support block. Entry points for 1 mm diameter microfluidic tubing are seen in the corners. (b) Microscope image of a pair of cantilevers suspended above the SU-8 channel (390 130 lm). (c) Springloaded microprobes soldered into PCB electronics. (d) Cantilever device undergoing tip deflection characterisation.
R.H. Ibbotson et al. / Microelectronic Engineering 85 (2008) 1314–1317
oretical sensitivity to surface stress of 2.1 10 4 (N/m) 1 [10] and the change in output after etching of 56 lV, the surface stress of the gold layer is estimated as 41 MPa. This compares well to the residual stress, measured on a test wafer using a KLA Tencor long scan profiler, of 43 ± 4 MPa.
Relative resistance change
R/R (10-3)
1.4
1.2
1
0.8
5. Conclusions and outlook
0.6
Test 1 Test 2 Test 3 Test 4 Linear fit
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Tip deflection (µm)
Fig. 4. The piezoresistor’s relative resistance change measured for tip deflections of 30 lm in 5 lm steps. The linearity and repeatability of the piezoresistor response for a 390 130 lm cantilever is seen over 4 tests.
Cantilever pair 1 Cantilever pair 2
Wheatstone bridge output (μV)
60
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50
The fabrication of a polyimide cantilever surface stress sensor with integrated gold piezoresistors has been presented. Polyimide chosen for its low Young’s modulus is a good alternative to SU-8 for polymer surface stress sensing cantilevers. Low-cost spring-loaded microprobes, where the number of connections required is relatively low, are shown to be a good alternative to other interconnect techniques. Further experiments on devices with optimised layer thicknesses will be carried out in the near future. An experimental setup, which will integrate the device into a flowcell with sample flow and temperature control, is currently being designed. The device will ultimately be used as a biosensor for the detection of protein–protein interactions.
40
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Time (seconds)
Fig. 5. The Wheatstone bridge output change during the exposure of 4 cantilevers to gold etchant. Cantilever pair 1 consists of 1 gold coated cantilever and 1 uncoated cantilever. Cantilever pair 2 (control) consists of 2 uncoated cantilevers.
changes the deflection of the cantilever. In the first step of the experiment, the gold is removed from 3 of the 4 cantilevers on a device, using potassium iodide gold etchant (KI) in microcapillary tubes. In the second step, all 4 cantilevers are immersed in KI to etch gold from the remaining active cantilever. Fig. 5 shows the change in the Wheatstone bridge output signal caused by the removal of the residual stress in the gold layer and subsequent change in the active cantilever deflection. Cantilever pair 1 contains one active, gold-coated cantilever and one reference cantilever. Cantilever pair 2 (control) containing two reference cantilevers shows no response other than some drift due to the mismatch of the thermal coefficient of resistance of the piezoresistors on each cantilever pair. Based on the the-
The author acknowledges the Engineering and Physical Sciences Research Council (EPSRC) for the provision of a studentship through their Engineering Doctorate Scheme. This work was supported in part by the Technology Partnership Program, Grant No. TP0401, and in part by the European FP6 framework Contract No. 516865 – TASNANO. References [1] G. Binnig, C.F. Quate, Ch. Gerber, Phys. Rev. Lett. 56 (1986) 930– 933. [2] J. Fritz, M.K. Baller, H.P. Lang, H. Rothuizen, P. Vettiger, E. Meyer, H.-J. Gu¨ntherodt, Ch. Gerber, J.K. Gimzewski, Science 288 (2000) 316–318. [3] M. Tortonese, R.C. Barrett, C.F. Quate, Appl. Phys. Lett. 62 (1993) 834–836. [4] I.W. Rangelow et al., Microelectron. Eng. 84 (2007) 1260–1264. [5] A. Johansson, M. Calleja, P.A. Rasmussen, A. Boisen, Sens. Actuators A 123–124 (2005) 111–115. [6] R. Katragasacrificialng, W. Khalid, Y. Li, Y. Xu, Appl. Phys. Lett. 91 (2007) 083505. [7] R. Richardson, J.A. Miller, W.M. Reichert, Biomaterials 14 (1993) 627–635. [8] A. Johansson, J. Janting, P. Schultz, K. Hoppe, I.N. Hansen, A. Boisen, J. Micromech. Microeng. 16 (2006) 314–319. [9] G. Genolet, Phd thesis, EPFL, 2001. [10] J. Thaysen, A.D. Yalcßinkaya, P. Vettiger, A. Menon, J. Phys. D: Appl. Phys. 35 (2002) 698–2703.
Microelectronic Engineering 85 (2008) 1314–1317 www.elsevier.com/locate/mee
Polyimide microcantilever surface stress sensor using low-cost, rapidly-interchangeable, spring-loaded microprobe connections R.H. Ibbotson a,b,*, R.J. Dunn a,b, V. Djakov a, P. Ko Ferrigno c,1, S.E. Huq a a
Rutherford Appleton Laboratory, Chilton, Oxon, OX11 0QX, UK b Heriot-Watt University, Edinburgh, EH14 4AS, UK c MRC Cancer Cell Unit, Hutchison/MRC Research Centre, Hills Road, Cambridge CB2 OXZ, UK Received 5 October 2007; received in revised form 14 December 2007; accepted 27 December 2007 Available online 12 January 2008
Abstract We present the fabrication of a polyimide microcantilever array with integrated gold piezoresistive strain gauges and a thick SU-8 backing for use as a surface stress sensor. The selected polyimide’s low Young’s modulus (1.8 GPa) theoretically results in a high sensitivity to changes in surface stress. Electroplated gold contact pads, 1.5 lm thick, provide a stable, repeatable surface onto which electrical connections to the fabricated device are made using spring-loaded microprobes. The piezoresistors are found to have a gauge factor of between 4 and 4.5 and a sensitivity to surface stress of 2.1 10 4 (N/m) 1. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Cantilever; Microcantilever; Polyimide; Surface stress sensor
1. Introduction Microfabricated cantilevers have been of great interest in the field of biological imaging since the invention of the atomic force microscope (AFM) [1]. More recently, cantilevers operated as static sensors, deflecting due to a change in the surface stress of a surface layer, have been used to translate biorecognition into a mechanical response [2]. These measurements have been made using microfabricated silicon cantilever arrays with tip deflections measured by an array of diode lasers [2]. The integration of piezoresistors onto AFM cantilevers [3] has enabled large arrays to be produced without the need for the alignment of external optics [4]. A Wheatstone bridge configuration is employed to detect small resistance changes in the active cantilever
* Corresponding author. Address: Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, UK. Tel.: +44 1235 445722. E-mail address: [email protected] (R.H. Ibbotson). 1 Current address: Section of Experimental Therapeutics, Leeds Institute of Molecular Medicine, St James’s University Hospital, Leeds LS9 7TF, UK.
0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.12.065
sensor. Low-cost SU-8 polymer cantilevers with integrated strain sensors have been shown to have an excellent sensitivity to surface stress which is comparable to that of piezoresistive silicon cantilevers [5]. Other polymers have been investigated [6] for their suitability and may offer further simplifications of the fabrication process and increases in sensitivity to surface stresses. Polyimides are identified as suitable polymers for microcantilevers given their low Young’s modulus, high planarity, chemical resistance and biocompatibility [7]. 2. Fabrication A similar fabrication sequence to that used by Johansson et al. [5] for SU-8 cantilever devices is followed, however polyimide and electroplated gold instead of SU-8 and electroplated nickel are used. Fabrication of the cantilever devices is performed on standard 4 in. Si wafers (Fig. 1a). Three metal layers of titanium/gold/chromium with thicknesses 5 nm/50 nm/75 nm are deposited by egun evaporation onto a clean, silicon substrate (Fig. 1b). Polyimide (Pyralin PI2562) is spin-coated onto the wafer at a
R.H. Ibbotson et al. / Microelectronic Engineering 85 (2008) 1314–1317
1315
Fig. 1. Process sequence showing key stages in the fabrication of the cantilever device.
speed of 5000RPM for 60 s. A post-spin hotplate bake is performed to initiate the removal of solvents from the polyimide layer. The wafer is cured from room temperature to 350 °C in an oven with an inert N2 atmosphere. This is sufficient to drive off all the solvents and results in a polyimide thickness of 0.7 lm. A 7 lm etch mask of AZ9260 is spincoated and patterned with square contact pads, which are etched through to the chromium layer beneath in a RIE O2 plasma (Fig. 1c). The remaining AZ9260 is removed in AZ100 remover and the wafer is rinsed in DI water. To remove any moisture a 2 h bake at 120 °C is performed. Lift-off lithography of the piezoresistors and contact pads is performed using LOR and JSR photoresists. The resistor layer of titanium/gold with thicknesses of 5 nm/60 nm is evaporated by egun evaporation. Lift-off is completed in acetone and JSR developer (Fig. 1d). Earlier fabrication attempts used wet etching of the gold layer, but due to the speed of the etch, repeatability is poor. A gold layer of 1.5–2.5 lm is electroplated on to provide sufficient strength to the pads for contacting by the spring-loaded microprobes. Electroplating lithography is prepared in AZ9260. The gold layer is electroplated in ECF-60 solution (Rohm-Haas) for 20 min at 76 mA/dm2 followed by 40 min at 152 mA/dm2 (Fig. 1e). Following the removal of the AZ9260, a dehydration bake at 200 °C is performed. The second polyimide layer is spin-coated on with various thicknesses; a typical spin speed for a 1.5 lm layer is 3000RPM for 60 s. The polyimide curing process is repeated for the second layer, encapsulating the piezoresistors and electroplated contact pads. A second AZ9260 etch mask is spin-coated onto the polyimide surface and patterned with cantilevers and channels. A 100 lm wide grid defines each chip and increases etchant access to the chromium sacrificial layer. Both polyimide layers are etched simultaneously in an RIE O2 plasma, reducing the number of alignment steps compared to an all SU-8 cantilever process (Fig. 1f). Any remaining AZ9260 is stripped in AZ100 remover. After a DI water rinse, a long bake at 200 °C is
performed to drive off any moisture. A 300 lm thick backing layer is created in SU-8 (Microchem 2150). Approximately 10 g of SU-8 is poured onto the wafer. Tweezers are used to spread the SU-8 evenly around the wafer. Spin-coating is performed according to Microchem’s guidelines for a 300 lm layer. The SU-8 wafer is placed on a levelled hotplate and allowed to relax for 2 h to reduce any spin-induced stresses. A soft bake of 10 min at 65 °C and 1 h 45 min at 95 °C is carried out with slow ramp rates. The SU-8 is allowed to relax at room temperature for 24 h. The wafer is patterned with a grid and channels with an exposure dose of 1500 mJ/cm2 in 5 cycles with a 45 s wait between each cycle. Post-exposure bake parameters differ from the manufacturer’s guidelines since these were found to produce a highly stressed SU-8 film, prone to delamination from the polyimide layer beneath. After a 1 h relaxation period, a 65 °C post-exposure bake is performed for 1 h 50 min. The wafer is allowed to relax for 24 h. Development is performed in EC solvent with agitation for 30 min (Fig. 1g). After a further 24 h relaxation, the cantilever chips are released by etching the sacrificial chromium layer in Chromium etchant (Transene 1002A) (Fig. 1h). This release process takes between 6 and 12 h. It has been reported that chromium and gold layers act as an enhanced sacrificial layer [9]. Higher etch speeds were evident but these were attributable to higher tensile stresses in the SU-8 layer. Released chips are rinsed carefully in DI water, dried in nitrogen and stored for use. The completed cantilever device, consisting of two full Wheatstone bridges each containing two piezoresistors integrated in cantilevers and two on-chip resistors, is shown in Figs. 2 and 3a. A pair of free-standing cantilevers with integrated piezoresistors is shown in Fig. 3b. 3. Electrical connections Creating the final electrical connection to soft polymer chips poses problems using conventional silicon intercon-
1316
R.H. Ibbotson et al. / Microelectronic Engineering 85 (2008) 1314–1317
Chip identifier Channel access well
On-chip resistor
Cantilever pair in channel
Gold tracks Contact pads
1mm
Wheatstone bridge equivalent circuit
Fig. 2. Photograph of a completed device highlighting the key features. Two full Wheatstone bridges are created on each device. The device footprint is 7.6 6 mm.
nect technology [8]. Instead, electrical connections are created simply, using general-purpose, short-travel, springloaded microprobes with a pitch of 1.3 mm (Coda-pin, PA0J Fig. 3c). Large area 1 mm2 pads, electroplated with at least 1.5 lm of gold, provide a sufficiently strong contact region for repeated contacts by the microprobes. The cantilever device requires no additional preparation beyond
the normal fabrication process and can be inserted into the electronic setup within seconds. The repeatability of the contact resistance is to within 0.5% of the piezoresistor strain gauge resistance values. The microprobes are soldered directly into a PCB which connects the device to a low voltage AC input signal and high gain amplifiers for the output signal. The Wheatstone bridge voltage change is measured using a software-based lock-in amplifier, operating at 1.1 kHz. 4. Characterisation A pinhead, mounted on a LabView controlled precision linear stage, vertically displaces the cantilever tip in 5 lm steps (Fig. 3d). Absolute resistance changes of the piezoresistor are recorded on a 6 12 digit digital multimeter (Keithley 2100). The linearity and repeatability of the piezoresistive sensor is shown in Fig. 4. A range of devices from two different wafers are found to have deflection sensitivities (DR/R per unit deflection) of between 0.01 and 0.03 ppm nm 1. This yields a gauge factor of between 4 and 4.5 for the gold piezoresistors, comparable to literature data for gold [10]. Initial experiments are performed to investigate a single cantilever’s response to surface stress changes. A titanium/ gold layer (5 nm/40 nm) is evaporated onto all 4 cantilevers on a device. The layer contains a residual stress which
Fig. 3. (a) SEM image of a completed device showing four cantilevers fabricated on a 300 lm thick SU-8 support block. Entry points for 1 mm diameter microfluidic tubing are seen in the corners. (b) Microscope image of a pair of cantilevers suspended above the SU-8 channel (390 130 lm). (c) Springloaded microprobes soldered into PCB electronics. (d) Cantilever device undergoing tip deflection characterisation.
R.H. Ibbotson et al. / Microelectronic Engineering 85 (2008) 1314–1317
oretical sensitivity to surface stress of 2.1 10 4 (N/m) 1 [10] and the change in output after etching of 56 lV, the surface stress of the gold layer is estimated as 41 MPa. This compares well to the residual stress, measured on a test wafer using a KLA Tencor long scan profiler, of 43 ± 4 MPa.
Relative resistance change
R/R (10-3)
1.4
1.2
1
0.8
5. Conclusions and outlook
0.6
Test 1 Test 2 Test 3 Test 4 Linear fit
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5
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25
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Tip deflection (µm)
Fig. 4. The piezoresistor’s relative resistance change measured for tip deflections of 30 lm in 5 lm steps. The linearity and repeatability of the piezoresistor response for a 390 130 lm cantilever is seen over 4 tests.
Cantilever pair 1 Cantilever pair 2
Wheatstone bridge output (μV)
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The fabrication of a polyimide cantilever surface stress sensor with integrated gold piezoresistors has been presented. Polyimide chosen for its low Young’s modulus is a good alternative to SU-8 for polymer surface stress sensing cantilevers. Low-cost spring-loaded microprobes, where the number of connections required is relatively low, are shown to be a good alternative to other interconnect techniques. Further experiments on devices with optimised layer thicknesses will be carried out in the near future. An experimental setup, which will integrate the device into a flowcell with sample flow and temperature control, is currently being designed. The device will ultimately be used as a biosensor for the detection of protein–protein interactions.
40
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Fig. 5. The Wheatstone bridge output change during the exposure of 4 cantilevers to gold etchant. Cantilever pair 1 consists of 1 gold coated cantilever and 1 uncoated cantilever. Cantilever pair 2 (control) consists of 2 uncoated cantilevers.
changes the deflection of the cantilever. In the first step of the experiment, the gold is removed from 3 of the 4 cantilevers on a device, using potassium iodide gold etchant (KI) in microcapillary tubes. In the second step, all 4 cantilevers are immersed in KI to etch gold from the remaining active cantilever. Fig. 5 shows the change in the Wheatstone bridge output signal caused by the removal of the residual stress in the gold layer and subsequent change in the active cantilever deflection. Cantilever pair 1 contains one active, gold-coated cantilever and one reference cantilever. Cantilever pair 2 (control) containing two reference cantilevers shows no response other than some drift due to the mismatch of the thermal coefficient of resistance of the piezoresistors on each cantilever pair. Based on the the-
The author acknowledges the Engineering and Physical Sciences Research Council (EPSRC) for the provision of a studentship through their Engineering Doctorate Scheme. This work was supported in part by the Technology Partnership Program, Grant No. TP0401, and in part by the European FP6 framework Contract No. 516865 – TASNANO. References [1] G. Binnig, C.F. Quate, Ch. Gerber, Phys. Rev. Lett. 56 (1986) 930– 933. [2] J. Fritz, M.K. Baller, H.P. Lang, H. Rothuizen, P. Vettiger, E. Meyer, H.-J. Gu¨ntherodt, Ch. Gerber, J.K. Gimzewski, Science 288 (2000) 316–318. [3] M. Tortonese, R.C. Barrett, C.F. Quate, Appl. Phys. Lett. 62 (1993) 834–836. [4] I.W. Rangelow et al., Microelectron. Eng. 84 (2007) 1260–1264. [5] A. Johansson, M. Calleja, P.A. Rasmussen, A. Boisen, Sens. Actuators A 123–124 (2005) 111–115. [6] R. Katragasacrificialng, W. Khalid, Y. Li, Y. Xu, Appl. Phys. Lett. 91 (2007) 083505. [7] R. Richardson, J.A. Miller, W.M. Reichert, Biomaterials 14 (1993) 627–635. [8] A. Johansson, J. Janting, P. Schultz, K. Hoppe, I.N. Hansen, A. Boisen, J. Micromech. Microeng. 16 (2006) 314–319. [9] G. Genolet, Phd thesis, EPFL, 2001. [10] J. Thaysen, A.D. Yalcßinkaya, P. Vettiger, A. Menon, J. Phys. D: Appl. Phys. 35 (2002) 698–2703.