MULTI-AXIS PIEZORESISTIVE FORCE SENSOR

Abstract

A microelectromechanical system (MEMS) sensor device comprising at least one microelectromechanical system sensor to characterize intracellular dynamics and behavior of a living biological cell so as to quantitatively measure the mechanical strength thereof. The microelectromechanical system sensor being responsive to mechanical force changes during said cell's contraction, migration, proliferation and differentiation.

Claims

1. A microelectromechanical system (MEMS) sensor device comprising: a microelectromechanical system sensor to characterize intracellular dynamics and behavior of a living biological cell so as to quantitatively measure a mechanical stiffness and a strength thereof, said microelectromechanical system sensor responsive to mechanical force changes during said living biological cell's contraction, migration, proliferation and differentiation; said microelectromechanical system (MEMS) sensor comprising an interaction platform in a form of a mechano-sensitive platform configured to have direct contact with said living biological cell and to measure forces generated by said living biological cell by way of measuring displacements of said interaction platform and, said microelectromechanical system (MEMS) sensor comprises strain-sensitive nanoscale elements in a form of electromechanical transducer elements configured to sense mechanical force applied by said cell in direct contact with said mechano-sensitive platform.

2. A microelectromechanical system (MEMS) sensor device as set forth in claim 1, wherein said interaction platform is a rectangular planar surface medium, an edge of which is connected with at least two strain-sensitive nanoscale elements in the form of piezoresistive nanowires.

3. A microelectromechanical system (MEMS) sensor device as set forth in claim 2, wherein said at least two strain-sensitive nanoscale elements connected with the edge of the interaction platform are connected to each other through a conductive edge line at least partially extending along said edge.

4. A microelectromechanical system (MEMS) sensor device as set forth in claim 2, wherein said at least two strain-sensitive nanoscale elements connected with the edge of said interaction platform extend between the interaction platform and a respective anchoring means associated with said edge.

5. A microelectromechanical system (MEMS) sensor device as set forth in claim 2, wherein said interaction platform is suspended by retaining springs extending between the edge of the interaction platform and a respective anchoring means associated with said edge.

6. A microelectromechanical system (MEMS) sensor device as set forth in claim 1, wherein said strain-sensitive nanoscale elements are made of doped single crystal silicon.

7. A microelectromechanical system (MEMS) sensor device as set forth in claim 5, wherein said retaining springs are configured such that a stiffness in an out-of-plane bending direction of said interaction platform is higher than that in an in-plane direction whereby the motion of the interaction platform is constrained to in-plane displacements.

8. A microelectromechanical system (MEMS) sensor device as set forth in claim 7, wherein a width of said retaining springs is configured in a decreased manner so as to obtain a lower in plane stiffness of said retaining springs and an increased magnitude of strain on the strain-sensitive nanoscale elements.

9. A microelectromechanical system (MEMS) sensor device as set forth in claim 7, wherein a height to width ratio of the retaining springs is at least greater than 2 and is preferably between 5 and 15.

10. A microelectromechanical system (MEMS) sensor device as set forth in claim 1, wherein electric current flowing through a strain-sensitive nanoscale element is measured to proportionally calculate in-plane force gradients being exerted parallel to the surface of said interaction platform.

11. A microelectromechanical system (MEMS) sensor device as set forth in claim 10, wherein a change in resistance of the strain-sensitive nanoscale element is monitored by current-voltage measurements to determine a relative change in resistance of the strain-sensitive nanoscale element and a magnitude of the force applied by a living biological cell.

12. A microelectromechanical system (MEMS) sensor device as set forth in claim 11, wherein the current flowing through the strain-sensitive nanoscale element is monitored while constant voltage is applied.

13. A microelectromechanical system (MEMS) sensor device as set forth in claim 1, wherein a direction of the applied force is determined from a ratio of the strains on strain-sensitive nanoscale elements whose longitudinal axes are perpendicular to each other.

14. A microelectromechanical system (MEMS) sensor device as set forth in claim 6, wherein the strain-sensitive nanoscale elements are oriented along directions of a plane of single crystal silicon and are doped with Boron atoms.

15. A microelectromechanical system (MEMS) sensor device as set forth in claim 5, wherein the retaining springs are provided with a folded structure disposed between two linear spring portions.

16. A microelectromechanical system (MEMS) sensor device as set forth in claim 15, wherein the folded structure disposed between two aligned spring portions comprises at least two folding lines between two linear spring portions.

17. A microelectromechanical system (MEMS) sensor device as set forth in claim 15, wherein the two linear spring portions are disposed at both ends of the folded structure and extend in an aligned manner.

18. A microelectromechanical system (MEMS) sensor device as set forth in claim 1, wherein the microelectromechanical system (MEMS) sensor device comprises an array of microelectromechanical system (MEMS) sensors to generate a 2D force vector map to conduct array type parallel time-domain multiplex analysis.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0036] Accompanying drawings are given solely for the purpose of exemplifying a multi-axis piezoresistive force sensor and a method for manufacturing the same, whose advantages over prior art were outlined above and will be explained in brief hereinafter.

[0037] The drawings are not meant to delimit the scope of protection as identified in the claims nor should they be referred to alone in an effort to interpret the scope identified in said claims without recourse to the technical disclosure in the description of the present systems and methods.

[0038] FIG. 1 demonstrates a general schematic view of a MEMS sensor comprising an interaction platform, piezoresistive nanowires, and retaining springs.

[0039] FIG. 2 provides a schematic cross-sectional view of a MEMS sensor demonstrating the cross-section of the retaining springs and position of the piezoresistive nanowires.

[0040] FIG. 3 schematically demonstrates the manner according to which direction of applied force is determined from the ratio of the strains on piezoresistive nanowires.

[0041] FIG. 4 demonstrates a general schematic view of a MEMS sensor comprising an interaction platform with the folded structure of the retaining springs.

DETAILED DESCRIPTION

[0042] The present disclosure proposes a piezoresistive sensor and array of sensors for measuring in-plane contact forces. According to the disclosure, a MEMS (Microelectromechanical system) force sensor (11) is composed of an interaction platform (12), strain-sensitive nanoscale elements (13) in the form of piezoresistive nanotubes or nanowires, and retaining springs (14). FIG. 1 shows the schematic of the device design including the components.

[0043] The interaction platform (12) is suspended by the strain-sensitive nanoscale elements (13) and retaining springs (14) which bridge between the interaction platform (12) and respective anchoring means. The interaction platform (12) can move under the action of external forces. It is the component that the biological cell interacts with and exerts forces on. The strain-sensitive nanoscale elements (13) (of silicon) are used as electromechanical transducers to measure the displacement of the interaction platform (12) due to the forces exerted on it by the biological cells. The motion of the interaction platform (12) generates mechanical stress in the strain-sensitive nanoscale elements (13). This stress induces changes in a strain-sensitive nanoscale element's (13) resistivity, and hence its electrical resistance, due to the piezoresistive properties of doped single crystal silicon. The change in resistance of the strain-sensitive nanoscale element (13) can be monitored by current-voltage (I-V) measurements, allowing the computation of the force exerted on the interaction platform (12) by the biological cells.

[0044] The function of the retaining springs (14) is to constrain the motion of the interaction platform (12) to in-plane displacements. When a force is applied to the surface of the interaction platform (12), a bending moment is generated due to the distance between the surface of the interaction platform and its center of gravity. This bending moment causes a tilting motion of the platform, and the strain-sensitive nanoscale elements (13) bend as a result. Bending of a strain-sensitive nanoscale element (13) causes a non-uniform stress distribution, which reduces the sensitivity of the piezoresistive nanowire transducers. Maintaining a uniaxial stress state (tensile or compressive) on the strain-sensitive nanoscale elements (13) is important to maximizing the sensitivity of the MEMS force sensor (11). To this end, said retaining springs (14) are used to prevent the tilting of the interaction platform (12) under applied bending moments. The retaining springs (14) are designed such that their stiffness in the out-of-plane bending direction is high while they have low stiffness in in-plane directions. The cross-sectional view demonstrated in FIG. 2 presents an exemplary design that yields retaining springs (14) with high stiffness in the out-of-plane bending direction due to a high height to width ratio. On the other hand, the stiffness of the retaining springs (14) in the in-plane directions is minimized due to the small width and a folded structure demonstrated in the schematic view in FIG. 1.

[0045] The multi-axis MEMS force sensor (11) described herein is capable of measuring the magnitude and direction of applied in-plane forces. Hence, using an array of sensors, a 2D force vector map can be generated. Current-voltage (I-V) measurements are used to determine the relative change in resistance of the strain-sensitive nanoscale elements (13) as a result of the applied force. The magnitude of the applied force can be calculated using the magnitude of the relative change in resistance. The direction of the applied force, referred to as the angle between the force vector and the positive x-axis, can be determined from the ratio of the strains on strain-sensitive nanoscale elements (13) whose longitudinal axes are perpendicular to each other (FIG. 3).

[0046] The strains on nanowire pairs that are on opposing sides of the platform (pairs of nanowires labeled as 1-3, and 2-4 in FIG. 3) are equal in magnitude but have opposite signs:

ε1=−ε3

ε2=−ε4

[0047] The angle between the direction of applied force and the positive x-direction can be calculated by:

ε2/ε1=ε4/ε3=tan θ

[0048] The overall sensitivity of the MEMS force sensor (1) depends on two major factors: First is the mechanical sensitivity of the device structure which is defined as the ratio of the magnitude of mechanical stress developed in the nanowire to the magnitude of the applied force. The second factor is the sensitivity of the piezoresistive nanowire transducers which is defined as the ratio of the relative change in resistance to the magnitude of applied strain. The MEMS force sensor (11) described herein was designed with the purpose of maximizing these two ratios.

[0049] According to the present disclosure, it is established that the mechanical sensitivity of the device structure can be optimized by minimizing the stiffness of the structure, therefore maximizing the stresses and strains on the nanowire for a given magnitude of applied force. The effect of the stiffness of the retaining springs is therefore decisive in the mechanical sensitivity of the device structure. It is established that the magnitude of strain on the nanowire increases as spring width decreases since the stiffness of the spring decreases with decreasing width.

[0050] Accordingly, the MEMS force sensor (11) device was designed to include retaining springs (14) with low width and an increased height (FIG. 3). This way, the stiffness of the springs in the in-plane directions is kept low while their stiffness in out-of-plane directions is high enough so that the springs can prevent the out-of-plane bending of the interaction platform (12) significantly. The height to width ratio of the retaining springs (14) is at least greater than 2 and is preferably 5-15.

[0051] The sensitivity of the piezoresistors (referred to as the gauge factor) is the ratio of the relative change in resistance of the nanowire to the applied strain. The gauge factor is derived from the constitutive relation describing the piezoresistive effect:

ΔR/R=πσ

[0052] Here, ΔR/R, π, and σ are the relative change in resistance, piezoresistive coefficient, and the applied stress, respectively. Incorporating Hooke's Law into this equation:

ΔR/R=(Eπ)ε

[0053] Here, E is the elastic modulus, and £ is the applied strain. The product of elastic modulus and piezoresistive coefficient is called the gauge factor, and it is defined as the sensitivity of a piezoresistor:

G=Eπ

[0054] For a doped single crystal silicon nanowire, the value of the gauge factor depends on the doping type, dopant concentration, and orientation. Regarding the doping type, the options are p-type (doped with Boron) and n-type (doped with Phosphorus or Arsenic). In terms of orientation, crystallographic directions on the (100) plane of single crystal silicon are considered since the MEMS force sensor devices are fabricated on SOI wafers with (100) silicon device layer orientation. On the (100) plane of single crystal silicon, the elastic modulus is maximum along the <110> directions. The piezoresistive coefficient of p-type doped silicon is also maximum along the <110> directions of the (100) plane. These set of material properties are found effective since it is customary in microfabrication processes to align features along <110> directions. Regarding dopant concentration, a value of 10.sup.17 cm.sup.−3 was selected for the concentration of Boron atoms since the piezoresistive coefficients decrease significantly for dopant concentrations exceeding 10.sup.18 cm.sup.−3 and dopant concentrations below 10.sup.17 cm.sup.−3 results in low conductivity and the readout signal amplitudes decrease. In conclusion, the MEMS force sensor (11) device was designed so that the piezoresistive nanowires are oriented along the <110> directions of the (100) plane of single crystal silicon and are doped with Boron atoms at a concentration of 10.sup.17 cm.sup.−3 in order to maximize the sensitivity of the piezoresistive silicon nanowire transducers.

[0055] The multi-axis MEMS force sensor (11) described herein works on the principle of piezoresistive transduction. Piezoresistive silicon nanowires are used to measure the mechanical force applied on the interaction platform (12). The applied force generates a mechanical strain on the nanowires and their resistances change due to the piezoresistive effect. This change in resistance is determined by conducting IV measurements. The current flowing through the strain-sensitive nanoscale elements (13) is monitored while applying constant voltage. The static current, which is the current measured while no force is applied, depends on the applied voltage and the nanowire resistance, based on Ohm's law. The amount of change in measured current when a force is applied to the sensor device depends on the magnitude of force and sensitivity of the device. For the MEMS force sensor (11) design disclosed herein, the predicted values of static current and current change for an applied force of 100 nN were calculated using piezoresistance theory, material properties, and strain values.

[0056] According to the disclosure, strain-sensitive nanoscale elements (13) have lengths ranging from 2 μm to 5 μm and widths ranging from 100 nm to 250 nm.

[0057] Fabrication of multi-axis MEMS devices are delineated hereinafter: The MEMS force sensors (11) are fabricated on Silicon-On-Insulator (SOI) wafers using CMOS compatible microfabrication technology. Technical specifications of the SOI wafers are shown in Table 1:

TABLE-US-00001 TABLE 1 Technical specifications of the SOI wafers used Diameter 100 mm Orientation (100) Doping type P-type (Boron) Resistivity 20 Ωcm-30 Ωcm Device layer thickness 2 ± 0.5 μm Buried oxide thickness 2 μm ± 5% Handle layer thickness 500 ± 10 μm

[0058] First, the strain-sensitive nanoscale elements (13) and metal contact pads were doped by ion implantation with Boron ions to achieve a doping concentration of 10.sup.17 cm.sup.−3. A second doping by ion implantation was performed to increase conductivity in the ohmic contact pads and unstrained parts of the nanowire bridge. These regions were doped with Boron ions to achieve a doping concentration of 10.sup.20 cm.sup.−3. After the ion implantation process is completed, a drive-in step is carried out to achieve thermally induced vertical diffusion of the implanted dopant ions through the entire device layer thickness.

[0059] Next, the sensor structure is defined by two successive silicon etch steps using reactive ion etching (RIE). First, the entire geometry is defined by RIE etching of the 2 μm thick silicon device layer. A second RIE etch step is performed to reduce the thickness of the nanowire bridge structures from 2 μm to 0.25 μm.

[0060] After the doping and silicon etching processes are completed, metallization step is carried out. The entire wafer surface is coated with a thin (˜30 nm) layer of Titanium first, and then an Aluminum film is deposited using sputtering. The metal films are patterned by photolithography and etching.

[0061] The next process is the deposition and patterning of a protection layer. The protection layer is required since metals and buried silicon dioxide need to be protected during the HF vapor release etch. Polyimide is chosen as a protection layer since it can be deposited, patterned, and removed with ease. The surface of the wafer is coated with a polyimide film and the film is patterned by photolithography to allow access to the oxide that is to be etched in the release process.

[0062] Next step is the anisotropic etching of the buried oxide with Freon RIE. This step ensures that the duration of HF vapor release is minimized since a portion of the oxide is removed and the surface area of the oxide is increased. After Freon RIE etching of the buried oxide, wafer dicing is performed to cut out the dies from the wafer.

[0063] After RIE etching of the buried oxide and dicing, HF vapor release is performed. The dies are exposed to vapor phase HF which etches the buried oxide underneath the platform, springs, and nanowires, allowing them to be suspended freely.

[0064] The final step of the microfabrication process flow is the removal of the polyimide protection layer to allow access to the metal contact pads which are going to be used to wire bond the die on a PCB substrate. The removal of the polyimide film is achieved by O.sub.2 plasma ashing.

[0065] The microelectromechanical systems (MEMS) force sensor (11) described herein utilizes strain-sensitive nanoscale elements (13) as electromechanical transducers. The strain-sensitive nanoscale elements (13) form a bridge between the interaction platform (12) and respective anchoring means. Hence, the external force applied to the interaction platform (12) is transmitted directly onto the strain-sensitive nanoscale elements (13). It is therefore to be noted that the strain-sensitive nanoscale elements (13) being directly subject to the external force ensure a more sensitive measurement. Enhancement of the sensitivity is provided particularly due to the fact that the measurement is effected not by bending of the strain-sensitive nanoscale elements (13) but by the straining activity thereof in a configuration where the retaining springs (14) substantially prevent out-of-plane bending direction movements while allowing in-plane direction movements.

[0066] It is also further to be noted that due to the fact that the measurements are directly effected by means of the strain-sensitive nanoscale elements (13), a structure dimensionally adapted to interact with a single biological cell is obtainable. The microelectromechanical system (MEMS) sensor device can also involve a plurality of MEMS force sensors (11) to interact with a single biological cell performing contact force measurements at different parts thereof or can be structured as an array of MEMS force sensors (11) interacting with a plurality of cells.

[0067] Therefore, using the strain-sensitive nanoscale elements (13) as the main sensing elements is one key aspect contrary to the situation where piezoresistors are embedded or attached on a larger mechanical structure such as a cantilever beam, a double-clamped beam, or a membrane to be then used as the mechanical sensing element. Utilizing piezoresistive nanowires as the main sensing elements enables design and microfabrication of highly miniaturized sensors capable of high-resolution force measurement. An array of highly miniaturized and high-resolution force sensors makes it possible to perform contact force measurements from multiple locations on a single biological cell. The microelectromechanical systems (MEMS) force sensor (11) described herein has dimensions of 20 μm×20 μm fabricated on a 2 μm thick Silicon layer and can be used to measure forces on the order of 100 nN.

[0068] In a nutshell, a microelectromechanical system (MEMS) sensor device comprising at least one microelectromechanical system sensor (11) to characterize intracellular dynamics and behavior of a living biological cell so as to quantitatively measure the mechanical stiffness and strength thereof, said microelectromechanical system sensor (11) being responsive to mechanical force changes during said cell's contraction, migration, proliferation and differentiation.

[0069] In one embodiment, said microelectromechanical system (MEMS) sensor comprises an interaction platform in the form of a mechano-sensitive platform to have a direct contact with said living biological cell and to measure forces generated by said cell by way of measurement of displacements of said interaction platform (12) and said microelectromechanical system (MEMS) sensor (11) comprises strain-sensitive nanoscale elements (13) in the form of electromechanical transducer elements sensing mechanical force applied by said cell in direct contact with said living biological cell.

[0070] In a further embodiment, said interaction platform (12) is a rectangular planar surface medium, each edge of which is connected with at least two strain-sensitive nanoscale elements (13) in the form of piezoresistive nanowires. The rectangular form with two nanoscale elements at each edge ensures measurement uniformity.

[0071] In a further embodiment, said at least two strain-sensitive nanoscale elements (13) connected with the edge of the interaction platform (12) are connected to each other through a conductive edge line (15) at least partially extending along said edge. The two nanoscale elements being connected ensures measurability of the mechanical force being applied.

[0072] In a further embodiment, said at least two strain-sensitive nanoscale elements (13) connected with the edge of said interaction platform (12) extend between the interaction platform (12) and a respective anchoring means associated with said edge.

[0073] In a further embodiment, said interaction platform (12) is suspended by retaining springs (14) extending between each edge of the interaction platform (12) and a respective anchoring means associated with said edge.

[0074] In a further embodiment, said strain-sensitive nanoscale elements (13) are made of doped single crystal silicon.

[0075] In a further embodiment, said retaining springs (14) are configured such that their stiffness in the out-of-plane bending direction of said interaction platform (12) is higher than that in in-plane directions whereby the motion of the interaction platform (12) is constrained to in-plane displacements.

[0076] In a further embodiment, width of said retaining springs (14) is configured in a decreased manner so as to obtain a lower in plane stiffness of said retaining springs (14) and an increased magnitude of strain on the strain-sensitive nanoscale elements (13).

[0077] In a further embodiment, height to width ratio of the retaining springs (14) is at least greater than 2 and is preferably between 5 and 15.

[0078] In a further embodiment, electric current flow through a strain-sensitive nanoscale element (13) is measured to proportionally calculate in-plane force gradients being exerted parallel to the surface of said interaction platform (12).

[0079] In a further embodiment, change in resistance of the strain-sensitive nanoscale element (13) is monitored by I-V measurements to determine the relative change in resistance of the rain-sensitive nanoscale element (13) and magnitude of the force applied by a living biological cell.

[0080] In a further embodiment, the current flowing through the strain-sensitive nanoscale element (13) is monitored while applying constant voltage.

[0081] In a further embodiment, direction of the applied force is determined from the ratio of the strains on strain-sensitive nanoscale elements (13) whose longitudinal axes are perpendicular to each other.

[0082] In a further embodiment, the strain-sensitive nanoscale elements (13) are oriented along the <110> directions of the (100) plane of single crystal silicon and are doped with Boron atoms.

[0083] In a further embodiment of the present invention, the retaining springs (14) are provided with a folded structure (16) disposed between two linear spring portions (17). The stiffness of the retaining springs (14) in the in-plane directions is minimized due to the small width and the folded structure (16) having folding lines (18).

[0084] In a further embodiment, the folded structure (16) disposed between two aligned spring portions (17) comprises at least two folding lines (18) between two linear spring portions (17).

[0085] In a further embodiment, the two linear spring portions (17) at both ends of the folded structure (16) extend in an aligned manner, contributing to the minimization if the stiffness of the retaining springs (14) in the in-plane directions.

[0086] In a further embodiment, the microelectromechanical system (MEMS) sensor device comprises an array of microelectromechanical system (MEMS) sensors (11) to generate a 2D force vector map by which array type parallel time-domain multiplex analysis is conducted.