Interface of a microfabricated scanning force sensor for combined force and position sensing

Abstract

A micro fabricated sensor for micro-mechanical and nano-mechanical testing and nano-indentation. The sensor includes a force sensing capacitive comb drive for the sensing of a force applied to a sample, a position sensing capacitive comb drive for the sensing of the position of a sample and a micro fabricated actuator to apply a load to the sample. All the sensor components mentioned above are monolithically integrated on the same silicon MEMS chip.

Claims

1. A micro-electro-mechanical system scanning force sensor for a simultaneous measurement of a force applied to a sample and a deformation of the sample, comprising: a position sensing capacitive comb drive for measuring the deformation of the sample; a force sensing capacitive comb drive for measuring the force applied to the sample, said position sensing capacitive comb drive and said force sensing capacitive comb drive enabling simultaneous force and deformation sensing; and a micro-actuator for applying a mechanical load onto the sample, said micro-actuator selected from the group consisting of an electrostatic micro-actuator and an electro-thermal micro-actuator; a fixed body; outer flexures; inner flexures; an outer movable body suspended by multiple ones of said outer flexures, said outer flexures connected between said outer movable body and said fixed body; and an inner movable body suspended by multiple ones of said inner flexures, said inner flexures connected between said inner movable body and said outer movable body.

2. The scanning force sensor according to claim 1, further comprising a sensor probe connected to said inner movable body.

3. The scanning force sensor according to claim 1, further comprising: a force sensor capacitive interface; and a position sensor capacitive interface, said force sensing capacitive comb drive and said position sensing comb drive generate two independent capacitive output signals that are interfaced by said force sensor capacitive interface and said position sensor capacitive interface.

4. The scanning force sensor according to claim 1, wherein said electrostatic micro-actuator or said electro-thermal micro-actuator are mechanically connected to said outer movable body to generate a relative motion between said outer movable body and said fixed body.

5. The scanning force sensor according to claim 4, wherein said position sensing capacitive comb drive is measuring a relative motion between said inner movable body and said fixed body.

6. The scanning force sensor according to claim 1, wherein said force sensing capacitive comb drive is measuring a relative motion between said inner movable body and said outer movable body.

7. The scanning force sensor according to claim 1, further comprising a cooling rip, said electro-thermal micro-actuator is connected to said outer movable body by said cooling rip.

8. The scanning force sensor according to claim 1, wherein said electro-thermal micro-actuator has a v-shaped geometrical amplification mechanism to increase a scanning range of the scanning force sensor.

9. The scanning force sensor according to claim 1, further comprising: a support selected from the group consisting of a printed circuit board and a chip carrier; and a sensor probe electrically connected to said printed circuit board or to said chip carrier through at least one of said outer flexures and at least one of said inner flexures.

10. The scanning force sensor according to claim 9, further comprising a multi-channel capacitive interface electronics integrated circuit; and wherein components of the scanning force sensor are mounted on said printed circuit board or on said chip carrier including said multi-channel capacitive interface electronics integrated circuit.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is an illustration of a scanning MEMS force sensor chip with electrostatic actuation according to the invention;

(2) FIG. 2 is an illustration of the scanning MEMS force sensor chip with electro-thermal actuation;

(3) FIG. 3 is an electrical schematic of an electrostatic scanning force sensor and interface electronics;

(4) FIG. 4 is an electrical schematic of an electro-thermal scanning force sensor and the interface electronics; and

(5) FIG. 5 is a block diagram of electro-thermal scanning force sensor and the interface electronics.

DETAILED DESCRIPTION OF THE INVENTION

(6) Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a MEMS based scanning force sensor 100 for micro-mechanical and nano-mechanical testing. The sensor includes a force sensing capacitive comb drive for the sensing of a force applied to a sample, a position sensing capacitive comb drive 9 sensing of the deformation of the sensor probe tip (and therefore the deformation of the sample) and a micro fabricated electrostatic actuator 6 or a micro fabricated electro-thermal actuator 7 for applying the mechanical load (force and deformation) onto the sample. All the sensor components are monolithically integrated on the same silicon MEMS chip as shown in FIG. 1.

(7) FIG. 1 shows the build-up of the scanning force sensor 100. The scanning force sensor 100 consists of the following parts: a fixed body 1; an outer movable body 2; an inner movable body 3; a sensor probe 4; multiple outer flexures 5 connecting the outer movable body 2 to the fixed body 1; multiple inner flexures 7 connecting the inner movable body 3 to the outer movable body 2; a differential position sensing capacitive comb drive 9 for measuring the position of the inner movable body 3 relative to the fixed body 1; a differential force sensing capacitive comb drive 8 for measuring the force applied to the sensor probe 4 by measuring a deflection of the inner movable body 3 relative to the outer movable body 1; and a micro fabricated electrostatic actuator 6 or an electro-thermal actuator 7 which is driving the outer movable body 2.

(8) All parts are located on the same canning force sensor 100 chip. The material of this chip is highly doped (conductive) silicon with a resistivity below 0.1 Ohm-cm. The sensor chip is fabricated by deep reactive ion etching (DRIE) of a silicon-on-insulator (SOI) wafer.

(9) The fixed body 1 is non-movable and attached to the substrate such as a chip carrier or a printed circuit board (PCB). The outer movable body 2 is elastically suspended by multiple outer flexures 5 such that it can deflect in the x-direction. The stiffness of the outer suspension in the x-direction is given by the equation:

(10) $k_o=n_{\mathrm{fo}}\frac{{\mathrm{Etw}}_o}{l_o^3}.$

(11) Where k.sub.o is a spring constant of the outer suspension, n.sub.fo is the number of outer flexures 5 in a parallel configuration, E is the Youngs Modulus of silicon, t is a wafer thickness (thickness in z-direction), w.sub.o is a width of the outer flexures 5 and l.sub.o is a length of the outer flexures 5. The restoring force Fro created by the outer flexures 5 at a deflection x is given by the equation:

(12) $F_{\mathrm{ro}}=k_{o}x=n_{\mathrm{fo}}\frac{{\mathrm{Etw}}_o}{l_o^3}x.$

(13) The outer movable body 2 is u-shaped such that the inner movable body 3 can be located inside this u. The inner movable body 3 is elastically suspended by multiple inner flexures 7 such that it can deflect in the x-direction. The stiffness of the inner flexures 7 in the x-direction is given by

(14) $k_i=n_{\mathrm{fi}}\frac{{\mathrm{Etw}}_i}{l_i^3}$
where k.sub.i is the spring constant of the inner suspension, E is the Youngs Modulus of silicon, t is the wafer thickness (thickness in z-direction), w.sub.i is the width of the inner flexures 7 and l.sub.i is the length of the inner flexures 7. The restoring force F.sub.ri created by the inner flexures 7 is given by

(15) $F_{\mathrm{ri}}=k_i{x}_f=n_{\mathrm{fi}}\frac{{\mathrm{Etw}}_i}{l_i^3}{x}_f.$

(16) The sensor probe 4 is attached by micro assembly to the inner movable body in a rigid way such as epoxy glue. Also the sensor probe 4 can be made of silicon and fabricated with the same fabrication process as the rest of the sensor. To measure the force F that is applied to the sensor probe 4 during operation, a differential position sensing capacitive comb drive 9 is used that is measuring the displacement x.sub.f between the inner movable body 3 and the outer movable body 2. The capacitance change C.sub.F is given by

(17) $C_F=\mathrm{.Math.}{A}_f\left(\frac{1}{d_{f1}-x_f}-\frac{1}{d_{f2}+x_f}\right)-\mathrm{.Math.}{A}_f\left(\frac{1}{d_{f1}+x_f}-\frac{1}{d_{f2}-x_f}\right)$
where d.sub.f1 is the air gap between the capacitor plates and d.sub.f2 is the spacing between the capacitor electrode pairs in a differential comb drive pair configuration (the buildup of differential comb drives are described in more detail in [4]),
=8.8510.sup.12C.sup.2/(Nm.sup.2)
is the permittivity of air and A.sub.f is the area of the parallel plate capacitor formed by the comb drive. The force sensor capacitive interface 21 transduces the change of capacitance C.sub.f into an analog or digital output force signal S.sub.F.

(18) The sensor probe 4 is electrically insulated from the rest of the sensor to enable testing in aqueous environments (immerse the sensor probe into liquid). This is realized by etching air gaps into the device layer of a silicon-on-insulator (SOI) wafer. Mechanically, the parts are connected by the handle layer that is separated from the device layer by a layer of silicon oxide (SiO.sub.2). Electrically, the sensor probe is connected through at least one of the inner flexures 7 and at least one of the outer flexures 5 to the fixed body 1. On the fixed body 1 there is a separate electrical pad for wire-bonding. This allows setting the sensor probe 4 to a defined electrical potential by a connector on the printed circuit board (PCB) or on the chip carrier. Alternatively, the sensor probe 4 can be used as an electrical probe also (in combination to the mechanical testing).

(19) To push the sensor probe 4 against the sample to be tested, a high resolution actuator is required. An electrostatic actuator 6 can be directly integrated into the scanning force sensor 100. The electrostatic actuator 6 is pushing the outer movable body 2 in the x-direction. The outer flexures 5 are creating the restoring force for controlling the position of the sensor probe 4. Alternatively, this scanning motion can also be generated by an electro-thermal actuator 10.

(20) Electrostatic actuation has the advantage that there is no heating of the sensor chip. By using a lateral comb drive configuration a large travel range can be achieved without the risk of the pull-in effect that is limiting the range of transverse comb drive actuators. The electrostatic driving force f.sub.e for a single finger pair is given by

(21) $f_e=\mathrm{.Math.}\frac{{\mathrm{tV}}^2}{d_a}.$

(22) For a lateral configuration, where =8.8510.sup.12C.sup.2/Nm.sup.2) is the permittivity of air, V is the driving voltage, d.sub.a, is the distance between the plates, and t is the thickness of the wafer (thickness in z-direction). The driving voltage is supplied by a voltage supply 20 that is controlled by a computer. For a set of n.sub.a capacitors, the total driving force F.sub.e is given by

(23) $F_e=n_a\mathrm{.Math.}\frac{{\mathrm{tV}}^2}{d_a}.$

(24) To electrically insulate the actuator from the rest of the sensor it is separated by an air gap. This is realized by etching air gaps into the device layer of a silicon-on-insulator (SOI) wafer. Mechanically, the parts are connected by the handle layer that is separated from the device layer by a layer of silicon oxide (SiO.sub.2).

(25) Electro-thermal actuators 10 have the advantage of a simple and mechanically stiff structure. Also, relatively large actuation forces are created during thermal expansion. To increase the actuation range one or multiple v-shaped heater designs are chosen that expand if an electrical current is put across them. A sensor configuration with a multi-beam v-shaped electro-thermal actuator 10 is shown in FIG. 2.

(26) Electro-thermal actuators 10 have the disadvantage to heat up the scanning force sensor 100. However, by integrating a cooling rip 11 which connects the outer movable body 2 to the electro-thermal actuator 10, the heating of the rest of the scanning force sensor 100, but especially the comb drives can be reduced (thermal expansion may lead to unwanted sensor drift). A computer controlled current supply 23 is used to control the electro-thermal actuator 10.

(27) For micro-mechanical and nano-mechanical testing it is important to have two independent output signals: S.sub.F for the force and S.sub.P for the position of the sensor probe 4. A multi-channel capacitive interface electronics 101 is required containing a force sensor capacitive interface 21 and a position sensor capacitive interface 22. Multi-channel capacitive interface electronics 101 as integrated circuits (ICs) are commercially available.

(28) The position of the sensor probe 4 is measured by a differential position sensing capacitive comb drive 9 which is measuring the relative deflection between the fixed body 1 and the inner movable body 3 which is connected to the sensor probe 4. With this configuration, the deformation or indentation of the sample to be tested can be measured when the sensor probe tip 4 is pushing against the sample. The position measurement works independently from the actuation (scanner) and the force sensing. No mathematical models are used to compute the position or the force.

(29) For the position measurement, a lateral differential comb drive pair is used. The differential change of capacitance C.sub.P is given by

(30) a)

(31) $C_P=\mathrm{.Math.}{A}_P\left(\frac{1}{d_{p1}-x_p}-\frac{1}{d_{p2}+x_p}\right)-\mathrm{.Math.}{A}_P\left(\frac{1}{d_{p1}+x_p}-\frac{1}{d_{p2}-x_p}\right)$
where d.sub.p1 is the air gap between the capacitor plates and d.sub.p2 is the spacing between the capacitor electrode pairs in a differential comb drive pair configuration (the buildup of differential comb drives are described in more detail in [4]). The position sensor capacitive interface 22 transduces the change of capacitance C.sub.P into an analog or digital output force signal S.sub.P.

(32) For interfacing the scanning force sensor 100 chip it is mounted on a printed circuit board (PCB) 24 or a chip carrier as illustrated in FIG. 5. Electrical contact is made by wire-bonding or a chip-flip reflow process. All electrical bonding pads of the scanning force sensor are located on the fixed body (the outer movable part and the inner movable part must not be fixed on the chip carrier or PCB). The sensor probe is overhanging the chip carrier or PCB to enable access to the sample.

(33) The scanning force sensor 100 and the multi-channel capacitive interface electronics create two independent output signals S.sub.F and S.sub.P. Therefore, multi-channel capacitive interface electronics 101 with at least two channels are required. To reduce the effect of parasitic capacitance, it is advisable to locate the multi-channel capacitive interface electronics 101 directly on the PCB 24 next to the scanning force sensor 100. Electrical pads 26 on the PCB 24 will enable the programming of the memory of interface integrated circuits (ICs). A mounting hole enables the fixation of the PCB 24 or chip carrier on a positioning unit which is used for the sensor-to-sample alignment.

(34) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 1 Fixed body 2 Outer movable body 3 Inner movable body 4 Sensor probe 5 Outer flexure 6 Electrostatic microactuator 7 Inner flexure 8 Force sensing capacitive comb drive 9 Position sensing capacitive comb drive 10 Electro-thermal microactuator 11 Cooling rip 20 Voltage supply 21 Force sensor capacitive interface 22 Position sensor capacitive interface 23 Current supply 24 Printed circuit board (PCB) 25 Mounting hole 26 Electrical pads 27 Connector 28 Wire-bonds 100 Scanning force sensor 101 Multi-channel capacitive interface electronics IC Integrated Circuit PCB Printed Circuit Board SOI Silicon On Insulator CH1 Channel 1 CH2 Channel 2

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