TECHNICAL FIELD

The present disclosure relates to a process for obtaining a strain sensing device with enhanced sensitivity.

The present disclosure also relates to a method for obtaining a strain sensing device; the disclosure further relates to the device with enhanced sensitivity.

The method, according to the present invention, enhances the sensitivity of a strain sensing device by localizing the inhomogeneity created by electromigration while monitoring the two-terminal resistance. Such strain sensing devices are better suited for high frequency applications because of higher gauge factors at lower sensor resistance.

BACKGROUND

Detection of very small forces and displacements in micro- and nano-electromechanical systems with integrated sensors is of paramount importance in many applications such as scanning probe microscopy, mass detection, chemical sensing, clecirometry, acoustics, inertial sensing, etc. Piezoresistive transduction is an attractive option for integrated sensing due to its simplicity and robustness.

Traditionally, piezoresistive transducers in Microelectromechanical Systems (MEMS) are made with doped semiconducting materials, silicon being the most popular. At nanoscale, metals have been shown to outperform semiconductors as piezoresistive materials [Li, M., Tang, H. X. and Roukes, M. L. Ultra-sensitive NEMS-based cantilevers for sensing, scanned probe and very high-frequency applications. Nature Nanotech. 2, 114 (2007)] due to their high carrier densities and low resistivity. Apart from the simplicity of fabrication, metallic piezoresistors are compatible with high-frequency applications due to low resistivity. Low resistance of the transducer is important to provide better impedance matching to amplifiers and read-out electronics. But applicability of metallic piezoresistors has been limited due to their low sensitivity to strain. Typical magnitude of the gauge factor (defined as the ratio of relative change in resistance to strain) in metals is 2 to 5.

Piezoresistors made from doped semiconductors provide high gauge factors but always come with high resistivities. High resistances are not desirable for Nanoclectromechanical systems (NEMS), because they generally need to operate at high frequencies. For high frequency measurements, impedance mismatch with the readout circuitry increases signal reflection. Also high transducer resistance increases susceptibility to signal degradation from parasitic reactances.

Metallic piezoresistors overcome the problem of impedance mismatch with the readout circuitry and susceptibility to signal degradation from parasitic reactances at high frequencies. Yet the sensitivity is low because of lower gauge factor.

Ultra-thin metal films possess large gauge factor. This is due to the exponential sensitivity of electrical resistance to the separation of metallic grains that charge has to tunnel through during the flow of current. This concept has been exploited to achieve very high gauge factor (-100) in thin films of functionalized gold nanoparticles [Herrmann, J. et. al. Nanopar tide films as sensitive strain gauges. Appl. Phys. Lett. 91, 183105 (2007)}. Similarly, thin films of functionalized metal nanoparticles also offer high gauge factor. However, they are constrained due to very high specific resistance, undoing the primary advantage of a metal-based piezoresistive transducer.
Hence, there is a need for a method which enhances the sensitivity of a metallic piezoresistive transducer without compromising on the low resistance.

SUMMARY

The present disclosure relates to a process for making a strain sensing device or a metallic piezoresistive transducer with increased sensitivity, comprising providing a metal film; creating a controlled inhomogeneity at a localized region on the metal film to increase gauge factor of the metal film; wherein in creating the inhomogeneity, the two-terminal resistance of the piezoresistive transducer is maintained below 1000 ohms.

The present disclosure further relates to a strain sensing device or a metallic piezoresistive transducer with increased sensitivity, comprising a metal film with a controlled inhomogeneity at a localized region on said film; wherein said controlled inhomogeneity increases gauge factor of said metal film while maintaining low resistivity.

These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the disclosure, nor is it intended to be used to limit the scope of the subject matter.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The above and other features, aspects, and advantages of the subject matter will become better understood with regard to the following description, appended claims, and accompanying drawings where:

Figure 1: Schematic representation of different components of the MEMS device.

Figure 2: Scanning electron micrograph of the MEMS device.

Figure 3: Schematic diagrams showing the current paths.

Figure 4: Piczorcsislivcly measured frequency response at different stages of clcctromigration.

Figure 5: Enhancement of sensitivity due to electromigration. Figure 6: Inhomogeneous metal film in the notch region created by electromigration

DETAILED DESCRIPTION

The present disclosure provides a process for making a strain sensing device with increased sensitivity, comprising: providing a metal film; creating a controlled inhomogeneity at a localized region on the metal film to increase gauge factor of the metal film; wherein in creating the inhomogeneity, the two-terminal resistance of the device is maintained below 1000 ohms. Two-terminal resistance is defined as the resistance of the transducer as seen by the first stage of the signal conditional circuitry, typically a low noise amplifier. It includes the resistance of the piezoresistive sensor and the contact resistances at the connecting electrodes. While other competing technologies like semiconductor based piezoresistive sensor have to be connected to metallic electrodes at some point where there will be additional contact resistance, the contact resistance can be completely eliminated by the process of the present disclosure, because the sensor is carved out of the metallic electrode itself.

The process of the present disclosure enhances the sensitivity of the strain sensing device or a metallic piezoresistive transducer by two orders of magnitude using clcctromigration.
The high gauge factors obtained by the methods of the present disclosure are comparable to or even better than that of silicon and other piezoresistive materials. The very high strain sensitivity of the present disclosure is based on non-universal percolation in inhomogeneous metal film.

In an embodiment of the present disclosure, the inhomogeneity is created in the metal film by controlled electromigration. Controlling the electromigration is very important in the process step. Controlled electromigration allows the resistance of the metal film to be maintained at permissible values. Uncontrolled electromigration damages the metal film irreversibly. It will be appreciated that the present disclosure does not rely on strain sensitivity based on electron transport by quantum mechanical tunneling.

In yet another embodiment of the present disclosure, the process, wherein the inhomogeneity can be localized in a particular region of the metal film by controlled electromigration, allows the gauge factor of the metal film to be tuned according to requirement. Controlled electromigration results in inhomogeneity at the notch in the metal film. It is the notch which restricts the inhomogeneity caused by electromigration to be localized within a region. The "notch" refers to the geometrical shape of the sensor whereas "inhomogeneity" refers to the micro-structural feature in the metal film. Notch facilitates in localizing the inhomogeneity but inhomogeneity can be created even without notch, but not localized.

Further, although the sensor in accordance with the present disclosure, is formed by a notch created in the structure for targeting the nanoscale inhomogeneity in the metal film, the inhomogeneity can be created at any desired location in the structure by appropriately patterning the metal film alone.

In a further embodiment of the present disclosure the electromigration is carried out by passing current through the metal film, while monitoring the resistance of the metal film in order to increase gauge factor of the metal film. Gauge factor is defined as the ratio of relative change in electrical resistance to the mechanical strain, which is the relative change in length. The resistance of the metal film is monitored simultaneously with the passage of current. Since electromigration changes the transport mechanism from metallic conduction to percolation, the change in electrical resistance assumes greater values for a particular mechanical strain, thereby resulting in higher gauge factor values.

In still another embodiment of the present invention the metal film used in the process can be any suitable metal that is able to sustain high passage of current such that its metallic properties are not destroyed on passage of current. Non limiting examples of such metal are gold, aluminium, copper, chromium etc. The process of the present disclosure provides for low temperature compatibility of the sensor because it is based on simple metal. The sensors, in accordance with the present invention, can be operated in very low noise mode as compared to semi-conductors because it is based on metallic material.

In another embodiment of the present disclosure, the strain sensing device obtained by the process of the present invention is a microelectromechanical system (MEMS) device or a nanoelectromechanical system (NEMS).

In an embodiment of the present disclosure, it further provides a process for creating a sensing and actuation mechanism on a structure using single process step.

In still another embodiment of the present disclosure, it provides a process for obtaining a microelectromechanical system device, said process comprising: fabricating an insulating cantilever; providing a metal film as a piezoresistive sensor ; and incorporating a notch in the piezoresistive sensor. The present invention also provides a method for enhancing the sensitivity of a metallic piezoresistive transducer in microelectromechanical system (MEMS) device. The metallic piezoresistive transducer in the MEMS device is the piezoresistive sensor for which the metal is used.

In yet another embodiment of the present disclosure it provides a process for obtaining a microelectromechanical system device, said process comprising: fabricating an insulating cantilever; providing a metal film as a piezoresistive sensor; and incorporating a notch in the piezoresistive sensor ; creating a controlled inhomogeneity at a localized region on the metal film to increase gauge factor of the metal film; wherein in creating the inhomogeneity, the two-terminal resistance of the device is maintained below 1000 ohms.

The notch in the piezoresistive sensor provides current concentration during elcctromigration. Two-terminal resistance is defined as the resistance of the transducer as seen by the first stage of the signal conditional circuitry, typically a low noise amplifier.
It includes the resistance of the piezoresistive sensor and the contact resistances at the connecting electrodes. In providing the current concentration, the notch on the piezoresistive sensor also restricts the inhomogeneity to a localized region
In still another embodiment of the present disclosure disclosure it provides a process for obtaining a microelectromechanical system device, said process comprising: fabricating an insulating cantilever; providing a metal film as a piezoresistive sensor; incorporating a notch in the piezoresistive sensor as well as on the insulating cantilever; creating a controlled inhomogeneity at a localized region on the metal film to increase gauge factor of the metal film; wherein in creating the inhomogeneity, the two-terminal resistance of the device is maintained below 1000 ohms. The notch on the insulating cantilever provides stress concentration for piezoresistive transduction. The insulating cantilever can be made out of any insulating material with good structural properties, the non limiting examples of which are silicon dioxide, silicon nitride, undoped silicon, silicon carbide, certain polymers, certain metal oxides, etc.

In a further embodiment of the present disclosure it provides a strain sensing device with increased sensitivity, comprising a metal film with a controlled inhomogeneity at a localized region on said film; wherein said controlled inhomogeneity increases gauge factor of said metal film while maintaining low resistivity.

The strain sensing device prepared from the process of the present invention results in tunable gauge factors. By tunable, it is meant that the gauge factor values can be adjusted according to the end-use applications. The tunability of the gauge factor arises from the fact that the degree of inhomogeneity can be controlled to obtain the desired value of the two terminal resistance. Due to the passage of current through the localized inhomogenized region, the degree of inhomogeneity is greater leading to higher gauge factor values. Accordingly, the gauge factor values can be tuned by monitoring the passage of current and the two-terminal resistance.

The process of the present disclosure as described in this specification provides confinement of the nanoscale inhomogeneity to a small region at the desired location.
In still another embodiment of the present disclosure it provides the strain sensing device to be microelectromechanical system device or a nanoelectromechanical system device.
In another embodiment of the present disclosure it provides the strain sensing device to be prepared by the process of the present invention. Any metal strain gauge based on the process of the present disclosure, irrespective of its shape, size and design is understood to be within the scope of the present disclosure. It is to be understood that the strain sensitivity and noise performance can be further enhanced by suitably engineering the materials used.

The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to be restrictive or to imply any limitations on the scope of the present disclosure.

EXAMPLE 1

A strain sensing device is fabricated by providing a metal film with a controlled in homogeneity at a localized region on the metal film. In creating the controlled inhomogeneity on the metal film, the two-terminal resistance of the device is maintained below 1000 ohms. The controlled inhomogeneity at a localized region on the metal film increases the gauge factor of the metal film.

The strain sensing device can be incorporated in any Microelectromechanical System or Nanoclectromechanical system.

EXAMPLE 2

This example provides the fabrication of Microelectromechanical System (MEMS) device with the metal film as the piezoresistive sensor with a notch incorporated in the metal film to restrict the inhomogeneity created by the passage of current. The present invention provides a microelectromechanical system (MEMS) device structure to demonstrate the enhancement of sensitivity of a metallic piezoresisitive transducer. The MEMS device comprises an electrothermal bimorph actuator and a metallic piezoresistive sensor, connected by a mechanical coupler.

The different components of a MEMS device are illustrated schematically in Figure 1. figure 2 shows a scanning electron microscope (SEM) image of the device. Both the actuator and the sensor are silicon dioxide cantilevers. A notch is incorporated on one (left) leg of the sensor cantilever. The notch incorporated on the sensor cantilever plays a dual role. It provides stress concentration when the cantilever is bent and it promotes electromigration by enhancing the local current density and temperature. The structure, shown in figure 1, is first uniformly coated with a 50 nm gold film by thermal evaporation. Then the actuator and sensor cantilevers are electrically isolated by melting the gold film on the coupler arm using joule heating.

EXAMPLE 3

This example provides the process for obtaining high gauge factor value of the piezoresistive sensor in the MEMS system device fabricated in Example 2

The isolation current is passed through the right leg of the sensor cantilever, coupler beam and both legs of the actuator cantilever thus avoiding any electromigration damage to the sensor notch region. The disconnected gold film on the coupler arm can be seen in Figure 2. When a heating current is passed between contacts 1 and 2 (shown in Figure 3), the actuator cantilever bends out of plane due to the differential thermal expansion pulling the sensor cantilever with it. A sinusoidal actuation current at frequency fA/2 causes the actuator cantilever to oscillate at frequency fA which is varied across its fundamental frequency f0 of flexural mode of vibration. The oscillation of the actuator cantilever couples to the sensor cantilever through the mechanical coupler beam and generates a sinusoidally varying strain in the piezoresistive sensor. Thus the resistance R between contacts 3 and 4 changes sinusoidally at frequency fA. This resistance change can be measured by biasing the sensor using a DC current and detecting the voltage signal at 1A using phase sensitive detection.

The phenomenon by which atoms in a metal migrate under the influence of a large electric current is called electromigration. To achieve sensitivity enhancement, controlled elcctromigration was carried out by passing a large DC current (current density ~ 10 A/m2) through the sensor cantilever of the MEMS device while closely monitoring R. At different values of R, the electromigration was stopped and the frequency response was measured with lower current (current density ~ 10 A/m ).

Figure 4 shows some representative graphs of piezoresistively measured frequency response of the cantilever at different stages of electromigration. Electromigration damage not only increased R, but also caused a dramatic enhancement in the sensitivity of R to actuator deflection. Since AR/R, is directly related to gauge factor of the sensor film, and the mechanical response of the device remains essentially unchanged throughout (except for a moderate change in the Q-factor, see figure 4), this large enhancement in the gold film was attributed to nanoscale inhomogeneity. The relative resistance change at resonance normalized with the Q-factor is proportional to the gauge factor of the sensor and is shown in figure 5. From this graph it was determined that the effective gauge factor has increased by a factor of 75.8 by the present invention.
The present invention and equivalent thereof have many advantages, including those which are described below.

a) The strain sensing device made by this method have huge advantage in high frequency applications over the traditionally used silicon sensors because of higher gauge factors at lower sensor resistance.

b) The method of fabricating high sensitivity strain sensing device is very simple and versatile. It can be easily integrated into a wide range of applications from gyroscopes to acoustic devices.

c) The technology is also readily scalable to Nanoelectromechanical systems (NEMS).

d) Semiconductor based piezoresistive sensor have to be connected to metallic electrodes at some point where there will be additional contact resistance, the contact resistance can be completely eliminated by the method of the present invention because the sensor is carved out of the metallic electrode itself.

Although the subject matter has been described in detail with reference to certain embodiments thereof, other embodiments are also possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein.

we claim:

1. A process for making a strain sensing device with increased sensitivity, comprising:

providing a metal film;

creating a controlled inhomogeneity at a localized region on the metal film to increase gauge factor of the metal film; wherein in creating the inhomogeneity, the two-terminal resistance of the device is maintained below 1000 ohms.

2. The process as claimed in claim 1 wherein, said creating of the inhomogeneity on the metal film is by controlled electromigration.

3. The process as claimed in claim 1 or 2, wherein said process provides tunable gauge factor of the metal film.

4. The process as claimed in claim 2 wherein, said electromigration is by passing current through the metal film, while monitoring the resistance of the metal film in order to increase gauge factor of the metal film.

5. The process as claimed in claim 1 or 2, wherein the metal is selected from gold, aluminium, copper or chromium.

6. The process as claimed in any of the preceding claims, wherein said strain sensing device is a microelectromechanical system device or a nanoelectromechanical system device.

7. A process as claimed in any of the preceding claims, for obtaining a microelectromechanical system device, said process comprising:

fabricating an insulating cantilever;

providing a metal film as a piezoresistive sensor; and

incorporating a notch in the piezoresistive sensor.

8. The process as claimed in claim 7 wherein said insulating cantilever is selected from silicon dioxide cantilever, silicon nitride cantilever, undoped silicon cantilever, or silicon carbide cantilever.

9. The process as claimed in claim 7 or 8, wherein said insulating cantilever is provided with a notch.

10. A strain sensing device with increased sensitivity, comprising a metal film with a controlled inhomogencity at a localized region on said film; wherein said controlled inhomogeneity increases gauge factor of said metal film while maintaining low resistivity.

11. The device of claim 10 wherein said device is a microelectromechanical system device or a nanoelectromechanical system device.

12. The strain sensing device prepared by a process as claimed in any of the claims 1 to 9.