A suspended piezoresistive silicon nanowire-based sensor with mechanical stress amplification

Abstract

The present invention relates to a sensor especially to a cantilever sensor comprising a surface such as cantilever beam, membranes and diaphragms (1) and experiencing a tension or a compression, a first piezoresistive gauge (2) on the surface (1) which is called silicon nano wire, a second piezoresistive gauge (3) on the surface (1) which is called silicon nano wire, and a recess (5) is provided under the silicon nano wires in such a way that the silicon nanowires act as a suspended beam and a mechanical stress amplifier (4) to amplify the induced stress in the silicon nanowires (2, 3), which also serves as a mechanism for satisfying the differential measurement scheme.

Description

[0001] A SUSPENDED PIEZORESISTIVE SILICON NANOWIRE-BASED SENSOR WITH MECHANICAL STRESS AMPLIFICATION

[0002] The present invention relates to a sensor especially to a cantilever sensor with a suspended piezoresistive silicon nanowire and a mechanical stress amplifier to maximize the silicon nanowire stress and increase the sensitivity of the sensor.

[0003] Physical sensors have broad applications in biomedical engineering, electronic devices, and the automotive industry. Physical sensors are usually designed based on cantilever beams. Cantilever beams are widely used in various physical sensors design. The working principle of the cantilever beam-based sensors is simple and straightforward. The tip of the cantilever beam is usually subjected to an external perturbation (this perturbation could be pointing a force, inertial force, acceleration, mass adsorption, temperature variation, etc.). Therefore, the cantilever beam is bent up or down. By monitoring the amplitude of the movement using different means (optical or electrical) of signal transduction, the external perturbation source could be calibrated and measured. To measure any perturbation, various techniques have been proposed so far such as capacitive, piezoelectric, optical or piezoresistive.

[0004] In the capacitive measurement method, the cantilever and the substrate are biased with a known voltage. Once the cantilever is bent, the capacitance between the cantilever and the substrate changes. The magnitude of the external perturbation can be read from the capacitance change between substrate and cantilever.

[0005] Through the piezoelectric measurement scheme, the piezoelectric effect allows the device to sense the applied force on the cantilever. One major disadvantage of these types of sensors is that they cannot be miniaturized because optical patterning is not doable on the piezoelectric materials.

[0006] Optical schemes usually are used in atomic force microscopy (AFM). Using this microscope, the topography of the surface of the materials can be probed at the atomic level. During scanning a surface, the cantilever is bent up and down according to surface topography. To measure the deflection amplitude, a laser beam is reflected off the flat top of the cantilever and changes the direction of the laser beam, which can be detected using a position-sensitive photodiode.

[0007] In piezoresistive cantilever-based sensors, a piezoresistive gauge is usually attached to the base of the cantilever. The cantilever is clamped at one end, so the maximum stress occurs at the base of the cantilever. By applying any perturbation to the tip of the cantilever, the piezoresistive gauge's resistance changes, and the current variation in the gauge can be processed through a readout circuit.

[0008] Suspended piezoresistive silicon nanowire, serve as a critical transition element from MEMS (Micro Electromechanical Systems) to NEMS (Nano Electromechanical Systems). Suspended silicon nanowires provide a broad range of advantages such as high sensitivity and high piezo resistivity.

[0009] As discussed before, cantilever beams are widely used in different technology sectors, and each of the mentioned techniques has its specific benefits and drawbacks. Thanks to the implementation of silicon nanowires, piezoresistive cantilever beam-based physical sensors are promising sensors for the next generation of highly sensitive sensors. Piezoresistive silicon nanowires demonstrate much higher sensitivity in comparison with conventional piezo resistors. Moreover, cantilever beams have a simple design suitable for various applications. In the cantilever beams that use piezoresistive material as detection scheme, the stress magnitude exerted to the piezo resistor is limited and reaches the maximum value at the surface of the cantilever. Consequently, the sensitivity of such sensors is restricted to the maximum attainable stress on the surface of the cantilever. One method to increase the sensitivity of piezoresistivebased cantilevers is decreasing the stiffness of the beam. However, this method is not preferred because it causes a considerable reduction in the cantilever's resonant frequency, which is not favorable. The use of silicon nanowires can compensate for a portion of this issue since the reduced size of the silicon nanowires localizes the stress and increases the sensitivity.

[0010] In conventional physical sensors, gauges comprising piezoresistive elements, such as piezoresistive silicon nanowires, are embedded to the upper surface of the cantilever beam. Once the tip of the cantilever beam is bent downward or upward, the upper surface experiences tension or compression, respectively. The stress induced in the gauge is equal to the stress at the upper surface of the cantilever beam. One drawback of this design is that the stress in the nanowire is limited to the magnitude of stress on the upper surface of the cantilever beam. Besides, compression and tension are not achievable simultaneously on upper surface of the cantilever. Therefore, a design is needed that not only amplifies the stress but also incorporates the differential measurement scheme.

[0011] Usually, to increase the sensitivity of the cantilever sensor, designers reduce the resonant frequency of the device, or in other words, more compliant cantilevers are used. This can be achieved through thinning of the cantilever, which is a hinder and limiting bound for devices to achieve higher performance.

[0012] To have high sensitivity and high stiffness, several innovations have been developed, for instance, in an effort of an academic publication, “Naeli, K., Brand, O., 2007. Coupling high force sensitivity and high stiffness in piezoresistive cantilevers with embedded si-nanowires. Proc. IEEE Sensors 1065-1068. https: / / doi.org / 10.1109 / ICSENS.2007.4388589”, the silicon nanowire was batch fabricated at the base of the cantilever using only one UV lithography step. Stress amplification detected was around eight times, with a reduction in stiffness by 15% compared to the conventional design.

[0013] Or in another effort in the state of the art “Wei, J., Magnani, S. and Sarro, P.M., 2012. Suspended submicron silicon-beam for high sensitivity piezoresistive force sensing cantilevers. Sensors and Actuators A: Physical, 186, pp.80-85. https: / / doi.Org / 10.1016 / j.sna.2012.02.021” , a trench (recess) is carved under neath the piezoresistive silicon nanowire to amplify stress by releasing it. The released silicon nanowire acts as an independent beam and the compressive or tensile stress localizes on the nanowire because of its small cross-sectional area. However, this design is limited, as the stress could be amplified by a maximum factor of two. Moreover, the differential measurement is not possible with this design.

[0014] The present design uses released Si nanowire along with a released amplification mechanism on the cantilever. In other words, the surface of the cantilever is modified. However, the modification of the cantilever construction is infinitesimal and negligible. Therefore, the change in the resonant frequency of the cantilever is almost negligible.

[0015] This invention aims to provide a design to amplify the stress in suspended piezoresistive silicon nano wire-based cantilever sensor or similar one-dimensional nanoscale structures used in nano wire-based physical sensors in order to increase the sensitivity.

[0016] The challenge of increasing sensitivity with a stiff cantilever is overcome through an innovative combination of the suspended stress amplification mechanism and silicon Si NW. It is important to note that this mechanism is not confined to cantilevers solely; its operational principle remains consistent for any surface experiencing tension or compression, including membranes, diaphragms, etc., used in N&MEMS technology.

[0017] The objective of the present invention is amplification of the mechanical stress in the gauges. The induced stress in the silicon nanowires can be amplified by increasing the deformation of nanowires, which leads to higher sensitivity. In the present advent, thanks to the stress amplification mechanism acting on the nanowires, the sensitivity for any physical sensor using piezoresistive nanowires will be improved dramatically.

[0018] Following objective of the present invention is to provide a differential readout measurement design. Differential readout scheme is crucial for increasing sensitivity and reducing thermal drift and non-linearities. By the help of the present invention, a pair of silicon nanowire are implemented in a way that one of them is under tension, and the other one is under compression, with the same magnitude of stress but opposite signs in tensile and compression zones, respectively. By satisfying this condition, higher sensitivity, lower thermal drift, and reduced non-linearity compared to non-differential designs could be achieved.

[0019] Mechanical stress amplification mechanisms are innovative and cannot be compared with each other, as they have different approaches in the design process. Quantitative measurements such as the stress amplification factor help researchers understand the outcome and effectiveness of such mechanisms.

[0020] The present invention has three distinct advantages over previous efforts for improving physical sensors:

[0021] • Stress amplification factor is considerably higher than previous designs in the state of the art.

[0022] • A pair of silicon nanowire on the surface of the cantilever are under stress but in an opposite manner. One of them is under tension, and the other one is under compression simultaneously, which enables differential measurement readout.

[0023] • The resonant frequency of the cantilever is not sacrificed.

[0024] The cantilever beam as a surface along with an example of the sensor at the base which is designed in order to achieve the aim of the present invention is shown in the attached figures, and from these figures;

[0025] Figure 1- is a schematic illustration of the surface as a cantilever beam and the zoomedin view of the sensor, comprising the released silicon nanowires and the mechanical stress amplifier.

[0026] Figure 2- is a schematic illustration of the zoomed-in view of the sensor region, shows the longitudinal embodiment of the mechanical stress amplifier and the Si nanowires.

[0027] Figure 3- is a schematic illustration of the cantilever beam surface shows the lateral embodiment of the mechanical stress amplifier and the Si nanowires. The elements illustrated in the figures are numbered as follows:

[0028] 1. Surface

[0029] 2. First piezoresistive gauge (Si nanowire)

[0030] 3. Second piezoresistive gauge (Si nanowire)

[0031] 4. Mechanical stress amplifier

[0032] 5. Recess

[0033] A sensor comprising a surface (1) such as cantilever beam, membranes and diaphragms and experiencing a tension or a compression, a first piezoresistive gauge (2) on the surface (1) which is called a silicon nano wire, a second piezoresistive gauge (3) on the surface (1) which is called a silicon nano wire, and a recess (5) is provided under the silicon nano wires, in such a way that the silicon nanowires are released and act as a suspended beam.

[0034] The subject of the invention sensor further comprising a mechanical stress amplifier (4) to amplify the induced stress in the silicon nanowires.

[0035] In the present invention to increase the sensitivity, suspended silicon nanowires are used. A first piezoresistive gauge (2) and a second piezoresistive gauge (3) comprising piezoresistive silicon nanowires. The embedded silicon nanowires are suspended by a recess (5) opened on the surface of the surface(l) such as a cantilever beam, membranes and diaphragms which are used in micro and nano electromechanical systems. Suspended silicon nanowires are promising elements since they feature a high sensitivity due to their small cross-sectional area and high piezo resistance coefficient. The small cross-sectional area localizes the stress in the silicon nanowire and increases the sensitivity. Furthermore, mechanical stress amplifier (4) has been used to amplify stress in the silicon nanowires. The mechanical stress amplifier (4) helps amplify the stress dramatically. Therefore, a high sensitivity due to this stress amplification could be achieved. The sensor is proposed for achieving a significant increase in stress. For this purpose, the sensor comprising the mechanical stress amplifier (4). This amplifier (4) is used to amplify induced stress in the silicon nanowires. It uses lever arms to increase deformation and, as a result, increases the strain and stress in the silicon nano wires. By increasing the stress in the first piezoresistive gauge (2) and the second piezoresistive gauge (3), higher sensitivities for the sensor are observed.

[0036] In an embodiment of the present invention, the sensor comprises a first and second piezoresistive gauges (2,3) wherein one of the gauges (2,3) is under tension, and the other one is under compression simultaneously. By the help of this, higher sensitivity, lower thermal drift, and reduced non-linearity compared to non-differential designs could be observed.

[0037] In an embodiment of the present invention, both first piezoresistive gauge (2) and second piezoresistive gauge (3) are placed longitudinally with respect to the length of the surface (1) (Figure 2). In this embodiment the mechanical stress amplifier (4) is fixed to the recess (5) with two supporting bars which have an offset with respect to each other. If a downward force is applied to the tip of the surface (1), the upper part of the surface (1) experiences a tensile stress. Therefore, a coupled force will be generated in the supports. The coupled force rotates the mechanical stress amplifier (4) in the counterclockwise direction. Therefore, first piezoresistive gauge (2) will be under uniaxial tension, and the other one (3) will be under uniaxial compression. The vice versa condition occurs when the force is upward. Using this design, differential readout could be achieved.

[0038] In another embodiment of the present invention, both first piezoresistive gauge (2) and second piezoresistive gauge (3) are placed laterally with respect to the length of the surface (1). Two gauges (2,3) are attached to the central point of the mechanical stress amplifier (4). Once the surface (1) is bent downward or upward, the amplifier (4) will be in tension or compression. In each loading, the center point of the amplifier (4) moves in the longitudinal direction of the gauges (2,3) due to compliance. Therefore, it can stretch one nanowire and compress the other one at the same time (figure 3).

[0039] In an embodiment of the present invention, the mechanical stress amplifier (4) is an arm, lever, or frame which is provided on the recess (5). The mechanical stress amplifier (4) can be connected to the surface (1) and provided over or in the recess (5) by supporting elements such as bars or rods. The mechanical stress amplifier (4) is provided over the recess (5), which makes contact with the recess (5) at a minimum of two points. In the present invention, the first piezoresistive gauge (2), the second piezoresistive gauge (3) and the mechanical stress amplifier (4) are suspended structures. Also, the support structure between the mechanical stress amplifier (4) and the surface (1) or the recess (5) is suspended. By the help of this, the amplifier (4) can rotate or deform and the stress in the nano gauges (2,3) is amplified to a considerably high value, resulting in a higher sensitivity. This increase stems from localizing the stress in the gauges (2,3) due to releasing process and the mechanical stress amplifier.

[0040] In an embodiment of the present invention, the sensor is a cantilever sensor which comprising a cantilever beam as the surface (1). In an embodiment of the present invention, the sensor is used in micro and nano electromechanical systems.

Claims

CLAIMS1. A sensor comprising a surface (1) such as cantilever beam, membranes and diaphragms and experiencing a tension or a compression, a first piezoresistive gauge(2) comprising a silicon nano wire on the surface (1), a second piezoresistive gauge(3) on the surface (1), and a recess (5) is provided under the silicon nano wires in such a way that the silicon nanowires act as a suspended beam characterized in that the sensor further comprising a mechanical stress amplifier (4) to amplify the induced stress in the silicon nanowires.

2. A sensor according to claim 1, wherein the sensor comprises two piezoresistive gauges (2,3) wherein one of the gauges (2,3) is under tension, and the other one is under compression simultaneously.

3. A sensor according to the claim 2, wherein both first piezoresistive gauge (2) and second piezoresistive gauge (3) are placed longitudinally with respect to the length of the surface(l).

4. A sensor according to the claim 2, wherein both first piezoresistive gauge (2) and second piezoresistive gauge (3) are placed laterally with respect to the length of the surface (1).

5. A sensor according to the any one of the preceding claims, wherein the mechanical stress amplifier (4) is an arm, lever, or frame which is provided on the recess (5).

6. A sensor according to claim 5, wherein the mechanical stress amplifier (4) is a lever arm and connected to the recess (5) at least two points with a support element such as bar or rod.

7. A sensor according to any one of the preceding claims, wherein the sensor is a cantilever sensor which comprising a cantilever beam as the surface (1) which is connected to a substrate at one end.

8. A sensor according to any one of the preceding claims, wherein the sensor is used in micro and nano electromechanical systems.

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