TRANSPARENT SU-8 THREE-AXIS MICRO STRAIN GAUGE FORCE SENSING PILLAR ARRAYS FOR BIOLOGICAL APPLICATIONS N. Klejwa1*, N. Harjee1*, R. Kwon2, S.M. Coulthard2, and B.L. Pruitt2 1 Department of Electrical Engineering, Stanford University, Stanford, U.S.A. (Tel : +1-650-736-7148/+1-650-725-0307; E-mail:
[email protected],
[email protected]) 2 Department of Mechanical Engineering, Stanford University, Stanford, CA, USA Abstract: This paper presents three-axis micro strain gauge force-sensitive pillar arrays constructed of multiple layers of SU-8 and metal on quartz substrates to create transparent sensors for use in standard inverted microscopes. The sensor meets specific requirements for measuring tactile sensitivity and forces exerted during locomotion by small organisms such as fruit flies (Drosophila melanogaster), including: 1µN force sensitivity, >25µN range, and bio-compatibility. By virtue of its three-axis capability, this sensor has flexible applications for biological and non-biological force sensing. Keywords: SU-8, Force Sensor, Strain Gauge 1. INTRODUCTION. Forces on the micro-organism level are of interest in determining interactions that govern touch sensation, mechanical transduction, organism development, and locomotion. In previous works, poly-dimethylsiloxane (PDMS) has been molded into a “flexible carpet” or “forest” of microneedles to measure biological forces in two-axes via optical measurement of needle tip displacement (Fig.1) [1-2].
The goal for this work was to fabricate transparent devices enabling simultaneous visual observation and force measurement while using calibrated, electrical three-axis force sensing for continuous, synchronous data acquisition. To achieve these goals, we developed a sensor composed of SU-8 and metal layers on a quartz substrate. 2. DEVICE OVERVIEW The device consists of a 2x2 sensor array, with each three-axis sensor consisting of four fixedguided cantilever arms in a “plus” configuration with a vertical pillar extending from the central suspended junction (Fig.2).
Pillar Fig. 1 C. Elegans in PDMS pillars (left) [2] Drosophila on SU-8 pillars (right) Silicon micromachining has been used to create one-axis force sensors on a planar surface to measure cellular traction and adhesion forces [3]. Additionally, polyurethane “artificial haircells” have been created with carbon-impregnated force sensitive resistors for two-axis sensing on both glass and polyimide substrates [4]. *These authors contributed equally.
Strain Gauge
Tension Force
Cantilever
Compression Anchor Fig. 2 Schematic of a single sensor (left) and FEM simulation showing strain distribution under an applied force (right). Forces acting at the pillar tip induce strains at the fixed ends of the cantilevers which are sensed by metal strain gauges. With four cantilevers,
differential measurements can be taken in the two in-plane axes simultaneously and converted to force measurements using the device spring constant. Out-of-plane measurements can be made by measurement of the average strain sensed by all four strain gauges. Surrounding the sensitive pillars are arrays of non-sensitive pillars to facilitate organism locomotion and large square spacer blocks to aid in vertical device alignment. 3. FABRICATION Devices were fabricated on a quartz substrate by first sputtering a semi-transparent 50Å Cr / 250Å Au adhesion layer. This layer is critical in preventing delamination of SU-8 from the substrate during the final hydrogen fluoride (HF) release etch. For the 5µm cantilever arm layer, SU-8 2005 was spun at 1750 RPM, then hot plate pre-exposure baked (65˚C→95˚C @10˚C/min, 1min @95˚C, 95˚C→70˚C @10˚C/min) (Fig. 3a). Temperature ramping is necessary to reduce stress and eliminate cracking along sharp features. ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy (a) ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;;yyyyyyyyyyyyyyyy yyyy ;;;;;;;;;;;;;;;;yyy ;;;yyyyyyyyyyyyyyy ;;;;;;;;;;;;;;;yyyyy ;;;;;
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Quartz
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Exposed SU-8
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Unexposed SU-8 Fig. 3 Complete device fabrication process
The SU-8 cantilever arms were patterned with 9 cycles of 4sec exposure and 30sec rest intervals using an energy of 15 mW/cm2 on a Karl Suss MA-6 aligner. The wafers were post exposure baked (1min @65˚C, 1min @95˚C), developed, then hard-baked (65˚C→150˚C @5˚C/min, 5min @150˚C, 150˚C→70˚C @5˚C/min). The SU-8 masked the wet etch of the Cr/Au adhesion layer (Fig. 3b). To prevent bond pad delamination during wire-bonding, the SU-8 surface was roughened by a 60sec O2 plasma etch. A 100Å Cr / 1000Å Au thin-film layer was then evaporated, patterned, and wet etched to form strain gauges and bond pads (Fig. 3c). The pillars and spacer blocks were formed from five 70µm layers of SU8 2035. Each layer was spun at 3000 RPM and baked (3min @65˚C, 6min @95˚C). Once all layers were deposited, the wafer was baked (180min @95˚C) to fully harden the SU-8. The pillar and spacer layer was patterned using 20 cycles of 6sec each with 30sec rest intervals, then post-exposure baked (65˚C→95˚C @2˚C/min, 15min @95˚C, 95˚C→70˚C @2˚C/min). Wafers were then diamond sawed halfway through from the back, creating score marks (Fig. 3d). The SU-8 was developed for 45min with constant agitation then hard-baked (65˚C→150˚C @5˚C/min, 60min @150˚C, 150˚C→70˚C @5˚C/min). The dice were separated along saw lines and released individually by etching in 49% HF for 35min to fully undercut the SU-8 cantilever arms (Fig. 3e). No SU-8 delamination occurred due to the Cr/Au adhesion layer. As a final step, the devices were glued to a transparent package and wire-bonded (Fig. 3f). Completed devices are shown in Fig. 4. 45 µm
Sensitive Pillars Fig. 4 Fabricated sensor from top (optical micrograph) (left). Sensitive pillars, non-sensitive pillars, and spacers (optical microscope) (right)
4. MEASUREMENTS To characterize the devices, strain gauges for each axis were connected in a balanced Wheatstone bridge configuration with 10,000x low-noise differential amplification (INA103). The output voltage Vout can be approximated from the bridge bias voltage Vin, amplifier gain A, strain gauge Poisson ratio υ, applied force F, pillar height h, cantilever arm Young’s modulus E (4 GPa [5]), width w, and thickness t: (1)
A piezoresistive silicon cantilever was mounted on a piezoelectric actuator and used to apply a calibrated force to the sensitive pillar tip, as in Barlian [6] (Fig. 5).
Output Voltage (V)
10 5
Output Voltage (V)
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Y-Axis Sensor
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FEM Simulation
One non-ideal characteristic of long cantilever devices is an inverted voltage vs. force curve for small forces. When a small force is applied, tension develops in the cantilever opposite the direction of applied force, while compression develops in the cantilever in the direction of applied force generating an inverted output voltage (Fig. 7). Large Force
Small Force
X-axis Sensor
0
T -5
Y-Axis Sensor
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Fig. 6 On- and off-axis output voltage vs. applied force for sensor with 500x45x5µm cantilever arms
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FEM Simulation
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3V A(1 + 2ν ) Fh . ≅ in 4 Ewt 2
Output Voltage (V)
Vout
0.34±0.07 V/µN (n=3), off-axis rejection >10:1, and good linearity with the amplifier circuit saturating at ±10V. For large applied forces (>50µN), hysteresis was observed, but none was measured for moderate forces (
