WO2018050965A1

WO2018050965A1 - An accelerometer device and method for manufacturing the accelerometer device

Info

Publication number: WO2018050965A1
Application number: PCT/FI2017/050652
Authority: WO
Inventors: Xuefeng Song; Pertti Hakonen
Original assignee: Aalto Korkeakoulusaatio sr
Current assignee: Aalto Korkeakoulusaatio sr
Priority date: 2016-09-14
Filing date: 2017-09-13
Publication date: 2018-03-22
Anticipated expiration: 2019-03-14

Abstract

An accelerometer (100) comprises a cantilever anchored to a substrate (11), and a source electrode (15), drain electrode (16) and a gate electrode (17) arranged on the substrate (11). The drain electrode (16) and the cantilever (12) are arranged so that a tunneling gap (20) is formed between the drain electrode (16) and the cantilever (12) in order to allow tunneling current over the tunneling gap (20). The tunneling current is maintained over the tunneling gap constant by controlling the gate voltage applied on the gate electrode (17) so that electrostatic force between the gate electrode (17) and the cantilever (12) forces the cantilever in relation to the substrate in a position so that the tunneling current is maintained constant. The gate voltage applied on the gate electrode (17) is proportional to the acceleration, whereupon the acceleration is determined based on the gate voltage applied.

Description

AN ACCELEROMETER DEVICE AND METHOD FOR MANUFACTURING THE ACCELEROMETER DEVICE

TECHNICAL FIELD OF THE INVENTION The invention relates to an accelerometer device for determining acceleration introduced to the accelerometer device and manufacturing method for manufacturing the accelerometer device. In particularly the invention relates to tunneling accelerometers.

BACKGROUND OF THE INVENTION

Accelerometers are widely used in various scenarios in modern world, e. g., seismology, transportation, aerospace, navigation, automation, industry, security, and consumer electronics. Wide bandwidth, high sensitivity, small footprint MEMS (Micro Electro-Mechanical System) accelerometers are favored in many applications, but such high-end accelerometers are far from mature for large scale industrial deployment. Quantum tunneling effect of electrons between adjacent electrodes has been known to be extremely sensitive to the tunneling gap, which makes it a plausible candidate for miniaturized sensors of displacement and acceleration. Unlike the capacitive accelerometers where the minimum size is limited by stray capacitance, or the piezoelectric ones where high voltage is required, tunneling based accelerometers are superior on miniaturization and integration with semiconductor circuits. However, besides all the proven advantages on bandwidth, sensitivity, miniaturization and integration, the tunneling current based accelerometers do suffer from difficult fabrication and long-term instability in their commercialization. There is thus a need for a tunneling accelerometer which is small, sensitive, easy to fabricate, and reliable.

SUMMARY OF THE INVENTION An object of the invention is to alleviate and eliminate the problems relating to the known prior art. Especially the object of the invention is to provide an accelerometer which is small, sensitive, easy to fabricate, and reliable. The object of the invention can be achieved by the features of independent claims.

The invention relates to an accelerometer device according to claim 1. In addition the invention relates to a manufacturing method for manufacturing the accelerometer device according to claim 16.

According to an embodiment of the invention an accelerometer device for determining acceleration introduced to the accelerometer device comprises a substrate having first and second surfaces. In addition there is provided a cantilever having first and second ends, wherein the cantilever is anchored to the substrate via the first end and wherein the second end is a free end. The free end is moved towards and away from the first surface of the substrate when acceleration is introduced.

The device also comprises a source electrode, drain electrode and a gate electrode arranged on the substrate, advantageously on the first surface of the substrate being faced against the cantilever. The source electrode and the first end of the cantilever are arranged in a galvanic contact with each other. The source electrode is preferably made of Pd, Pt, or Au in order to provide good galvanic or Ohmic contact with the cantilever.

The drain electrode and the cantilever are arranged so that a tunneling gap is formed between the drain electrode and the cantilever in order to allow tunneling current from the drain electrode to the cantilever over the tunneling gap. In addition bias voltage (typically from 0.01 V to 10 V) is also provided from the drain electrode to the source electrode and again to the cantilever so that the tunneling current from the drain electrode (16) to the source electrode via the cantilever has a net flow.

The tunneling current is advantageously maintained over the tunneling gap in a chosen constant value, typically in the range of 0.1 nA to 10 nA through a feedback circuit by controlling the gate voltage on the gate electrode. By this electrostatic force between the gate electrode and the cantilever is applied to maintain the gap constant, so to force, advantageously to pull, the second end of the cantilever in relation to the substrate in a position so that the tunneling current (electron flow) due to tunneling effect between the drain electrode and the cantilever is maintained constant and thus maintaining the tunneling gap between the cantilever and the drain electrode. The equilibrium position is balanced between the electrostatic pulling force and the spring-restoring force of the cantilever. The tunneling gap should be small enough, typically from 0.1 nm to 10 nm, so that the electrons tunnel through the gap. The gate voltage applied on the gate electrode is thus proportional to the acceleration introduced, and the acceleration can be determined by determining the gate voltage applied on the gate electrode needed to maintain the tunneling gap and thus the tunneling current constant.

According to embodiments of the invention the cantilever is either a mono- or multilayer cantilever. The cantilever advantageously comprises carbon material, such as graphene or diamond like carbon (DLC), which offers high reliability due to the crystalline nature of graphene or DLC.

In addition the substrate may also comprise a stamp, advantageously a polymer stamp, for anchoring the cantilever to the substrate. The cantilever is advantageously anchored to the substrate via the first end of the cantilever, and it can be anchored either directly by the stamp or the first end of the cantilever can be arranged between the stamp and the source electrode thereby providing the galvanic contact between the source electrode (and the cantilever via the first end of the cantilever. The cantilever may also comprise a proof mass, but it is to be noted that this is an optional feature. The proof mass may comprise e.g. Pt, Pd, or Au, the weight of which can be varied to achieve different bandwidth-sensitivity specifications. In general, lower proof mass weight means higher bandwidth and less sensitivity; whereas higher proof mass weight means lower bandwidth and more sensitivity. The source electrode may comprise Pd, Pt, or Au in order to provide good galvanic contact. The drain electrode may have a shape a portion, such as an edge portion, which can be arranged to form an actual contact with the cantilever and may comprise W or Pt-lr or conductive carbon material. According to an advantageous embodiments a suspended graphene cantilever is stamp-transferred onto a set of electrodes (source, drain, and gate), and a tunneling current from drain to source via the graphene cantilever is maintained by a feedback voltage on the gate electrode. The polymer assisted stamp-transfer process is simple and substrate-insensitive. The cantilever made of crystalline graphene (mono- or multi- layer) is mechanically, electrically and chemically robust, which guarantees long- term stability and reliability of the sensor.

The device can be composed of a mono- or multi- layer graphene cantilever with an optional extra proof mass deposit on it, a polymer stamp used to transfer and hold the graphene cantilever, a set of electrodes (source, drain, and gate) on a substrate, and a necessary feedback circuit to maintain a constant tunneling current with a controlled electrostatic attractive force between the gate electrode and the cantilever. The acceleration causes an inertial force on the cantilever due to the mass of the cantilever and the optional extra proof mass. The inertial force is balanced by the electrostatic force caused by the controlled voltage on the gate electrode, so that the tunneling gap between the cantilever and the drain electrode is remained constant, which results in a constant tunneling current under a given bias voltage between the drain and the source. In this closed-loop sensor mechanism, the acceleration is transduced to the feedback gate voltage.

The present invention offers advantages over the known prior art. The accelerometer device according to the embodiments of the invention has e.g. wider bandwidth, smaller size, high sensitivity, and it is easy to integrate with semiconductor circuit when compared e.g. to a capacitive accelerometers or the piezoelectric ones, which are bulky and in addition there an expensive crystal is required. In addition the integration of the capacitive accelerometers or the piezoelectric ones with a semiconductor circuit is not easy.

The exemplary embodiments presented in this text are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this text as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific example embodiments when read in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which:

Figure 1 illustrates an exemplary accelerometer device design for determining acceleration according to an advantageous embodiment of the invention,

Figure 2 illustrates an exemplary accelerometer device design for determining acceleration according to another advantageous embodiment of the invention,

Figure 3 illustrates an exemplary manufacturing method for manufacturing the accelerometer device according to an advantageous embodiment of the invention, and Figure 4 illustrates an exemplary cantilever of the accelerometer device according to an advantageous embodiment of the invention,

DETAILED DESCRIPTION

Figure 1 illustrates an exemplary accelerometer device design 100 for determining acceleration according to an advantageous embodiment of the invention, where the device 100 is an assembly of two main parts that can be made separately. One part is composed of a solid substrate 1 1 having first and second surfaces 1 1A, 1 1 B, and a set of electrodes, i.e., the source electrode 15, the drain electrode 16, and the gate electrode 17 advantageously arranged on the first surface 1 1 A. The width of the electrodes 15-17 is typically from 0.1 μιτι to 50 μιτι, the height of the source electrode 15 is preferably lower than that of drain electrode 16 and gate electrode 17, but all typically in the range from 10 nm to 500 nm. The distance between the electrodes may vary in different designs, but typically all are in the range from 0.1 μιτι to 50 μιτι. The other part is composed of a polymer stamp 14, a mono- or multilayer graphene cantilever 12, and an optional extra proof mass 13. The accelerometer is assembled by placing the polymer stamp 14 onto the substrate 1 1 in such a way that the root of the cantilever 12 is anchored on the substrate 1 1 under the polymer stamp 14, while the arm of the cantilever 12 is bent away from the substrate surface 1 1A due to the contact with the source electrode 15. A gate voltage is applied on the gate electrode 17, so that the electrostatic force between the electrode 17 and the cantilever 12 is attractive force and will force (pull) the cantilever towards the substrate until a tunneling gap 20 between the cantilever 12 and the edge of the drain electrode 16 is small enough (typically from 0.1 nm to 10 nm) for the electrons to tunnel through the tunneling gap 20. A small bias voltage (typically from 0.01 V to 10 V) is applied from the drain electrode 16 to the source electrode 15, so that the tunneling current has a net flow from the drain electrode 16 to the source electrode 15. This tunneling current is maintained to a chosen constant value (typically from 0.1 nA to 10 nA) through a feedback circuit (not shown) by controlling the gate voltage on the gate electrode 17. The feedback circuit can be a simple PID controller or a circuit with more comprehensive features such as temperature compensation. Depending on the target application, such a feedback circuit can be built in the form of a PCB module, an ASIC chip, or even integrated on the same substrate 1 1. The materials used for the electrodes 15-17 can be different metals. In order to form an Ohmic or galvanic contact with the cantilever 12, the source electrode 15 is preferably made of Pd, Pt, or Au. Hard metals (preferably W or Pt-lr) or conductive carbon materials are preferred for the edge or other contact portion of the drain electrode 16 for better long-term stability. The edge of the drain electrode 16 can be designed to be a straight line or various curves. The optional proof mass 13 can be any inert materials (preferably Pt, Pd, or Au) that stick well on the cantilever 12, and the weight can be designed easily to achieve different bandwidth-sensitivity specifications. In general, lower proof mass weight means higher bandwidth and less sensitivity; higher proof mass weight means lower bandwidth and more sensitivity. The polymer stamp 14 is made of photosensitive polymers (preferably commercial photoresists from AZ series or SU-8 series) or e- beam sensitive polymers (preferably commercial PMMA or MMA-MAA copolymer), so that it can be patterned with photolithography or e-beam lithography. The graphene cantilever 12 can be either monolayer or multilayer, from any graphene sources (e.g., exfoliated flakes, CVD wafers). The profile of the cantilever 12 can be patterned into any desired shapes by oxygen plasma etching, as explained below in Figure 3.

Figure 2 illustrates an exemplary accelerometer device design 100 for determining acceleration according to another advantageous embodiment of the invention, which is a variant version of the first embodiment shown in Figure 1. The substrate is marked as 21 having first and second surfaces 21 A, 21 B. The mono- or multilayer graphene cantilever is marked as 22. The optional proof mass is marked as 23. The polymer stamp is marked as 24. The source, drain, gate electrodes are marked as 25, 26, 27, respectively. Most of the above description, numbers and features for the first embodiment are still valid. The differences in relation to the device illustrated in Figure 1 are as follow. The height of the source electrode 25 is higher than the drain electrode 26 and the gate electrode 27. The root or the first end 22A of the cantilever 22 is anchored not on the substrate, but on the source electrode 25. The order of the electrodes is different, now with the gate electrode 27 in between the source electrode 25 and the drain electrode 26. And the edge for electron tunneling is on the right of the source electrode 26. However, the principle of functioning of the device is similar and analogous with the device illustrated in Figure 1. Figure 3 illustrates an exemplary manufacturing method for manufacturing the accelerometer device 100 of the polymer stamp with graphene cantilever and proof mass according to an advantageous embodiment of the invention. As noted in connection with the Figures 1 and 2, the accelerometer 100 in the embodiment is an assembly of two main parts that are made separately. The fabrication of the substrate with electrodes on it can be done with normal photolithography and lift-off.

In the manufacturing example, a mono- or multi-layer graphene flake 37 is located on a substrate 31 with a sacrificial layer 32. The graphene flake 37 may come from exfoliated natural graphite, transferred CVD graphene, or original CVD graphene grown on Ni coated Si. The graphene flake 37 is then cut to a designed cantilever shape (not necessarily rectangular but also other shapes can also be utilized) via photolithography or e-beam lithography followed by oxygen plasma etching. The Au proof mass 38 is deposit on the cantilever 37 via another photolithography or e-beam lithography followed by standard lift-off. The photo resist or e-beam resist 33 is spun coat and then patterned into the designed shape shown in Fig 3. The stamp 36 has a pick-up hole 34 in it, and a few weak links 35 around it. The width and length of the stamp 36 is typically in the range from 5 μιτι to 500 μιτι, with a thickness typically in the range from 0.1 m to 10 μιτι. The diameter of the hole 34 is typically in the range from 1 μιτι to 50 μιτι. When the sacrificial layer 32 is dissolved, the stamp 36 can be picked up from the whole membrane 33 by breaking the weak links 35. The graphene cantilever 37 and the proof mass 38 are then transferred with the stamp 36 onto the target substrate 1 1 in Figure 1 or 21 in Figure 2 to form the final accelerometer devices 100 as described above. The transfer can be done using a fine glass tip attached to a micromanipulator, for example.

As described above, the shape of the graphene cantilever is designable and can vary. Two more advantageous and exemplary embodiments on the cantilever design are shown in Figure 4. The cantilever 471 is perforated with an array of holes 491 , so that the spring constant and mass of the cantilever 471 can be reduced. The cantilever 472 is in addition round- edged to reduce the risk of electron emission or corona discharge on sharp corners. Similar to the other embodiments above, the extra proof mass 481 and 482 are optional, and the polymer 461 and 462 are parts of the stamps in connection with the cantilevers 471 and 472, respectively. The invention has been explained above with reference to the aforementioned embodiments, and several advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the spirit and scope of the inventive thought and the following patent claims. In particularly it is to be noted that the materials and shapes and dimensions described with the embodiments of Figures 1 -4 are only examples and that the features compatible with other features of different embodiments can be naturally mixed with each other even if not described in more details in this particular document. In addition the features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated.

Claims

1. An accelerometer device (100) for determining acceleration introduced to the accelerometer device (100),

wherein the device (100) comprises:

- a substrate (1 1 , 21 ) having first and second surfaces (1 1 A, 21 A, 1 1 B, 21 B),

- a cantilever (12, 22) having first and second ends (12A, 22A, 12B,

22B), wherein the cantilever is anchored to the substrate (1 1 , 21 ) via the first end (12A, 22A) and wherein the second end (12B, 22B) is a free end, which is configured to be tend to move towards and away from the first surface (1 1 A, 21 A) of the substrate (1 1 , 21 ) due to the acceleration introduced,

- a source electrode (15, 25), drain electrode (16, 26) and a gate electrode (17, 27) arranged on the substrate (1 1 , 21 ),

wherein

- the source electrode (15, 25) and the cantilever (12, 22) is arranged in a galvanic contact with each other,

- the drain electrode (16, 26) and the cantilever (12, 22) is arranged so that a tunneling gap (20) is formed between the drain electrode (16, 26) and the cantilever (12, 22) in order to allow tunneling current from the drain electrode (16, 26) to the cantilever (12, 22) over the tunneling gap (20), and

- the device (100) is configured to provide bias voltage from the drain electrode (16, 26) to the source electrode (15, 25) and again to the cantilever (12, 22) so that the tunneling current from the drain electrode (16, 26) to the source electrode (15, 25) via the cantilever (12, 22) has a net flow from the drain electrode (16, 26) to the source electrode (15, 25),

whereupon

- the device is configured to maintain the tunneling current over the tunneling gap (20) from the drain electrode (16, 26) to the source electrode (15, 25) via the cantilever (12, 22) constant by controlling gate voltage applied on the gate electrode (17, 27) so that electrostatic force between the gate electrode (17, 27) and the cantilever (12, 22) forces the second end (12B, 22B) of the cantilever in relation to the substrate in a position so that the tunneling current due to tunneling effect between the drain electrode (16, 26) and the cantilever (12, 22) is maintained constant, where the gate voltage applied on the gate electrode (17, 27) is proportional to the acceleration introduced, and

- the device is configured to determine the gate voltage applied on the gate electrode (17, 27) needed to maintain the tunneling current constant and determine the acceleration based on the gate voltage applied.

2. A device (100) of claim 1 , wherein the cantilever (12, 22) is a mono- or multilayer cantilever and comprises carbon material, such as graphene or diamond like carbon (DLC).

3. A device (100) of any previous claims, wherein the substrate (11 , 21 ) comprises a stamp (14, 24) for anchoring the cantilever (12, 22) to the substrate (1 1 , 21 ) via the first end (12A, 22A) of the cantilever (12, 22).

4. A device (100) of claim 3, wherein the first end (22A) of the cantilever (22) is arranged between the stamp (24) and the source electrode (25).

5. A device (100) of any previous claims 3-4, wherein the gate electrode (27) is arranged on the substrate (21 ) between the source electrode (25) and the drain electrode (26).

6. A device (100) of any previous claims 3-5, wherein the second end (22B) of the cantilever (22) is arranged to form said tunneling gap (20) between the drain electrode (26) and the cantilever (22).

7. A device (100) of claim 3, wherein the drain electrode (16) is arranged on the substrate (1 1 ) between the source electrode (15) and the gate electrode (17).

8. A device (100) of claim 7, wherein a first portion next to the first end (12A) of the cantilever (12) is arranged to form the galvanic contact with the source electrode (15), and the second end (12B) of the cantilever (12) is arranged to form the electrostatic force between the gate electrode (17) and the cantilever (12) and a middle portion of the cantilever (12) between the first end (12A) and the second end (12B) is configured to form said tunneling gap (20) between the drain electrode (16) and the cantilever (12).

9. A device (100) of any previous claims, wherein the cantilever (12, 22) comprises a proof mass (13, 23).

10. A device (100) of any previous claims, wherein the source electrode (15, 25) comprises Pd, Pt, or Au; and wherein an edge portion of the drain electrode (16, 26) to be contacted with the cantilever (12, 22) comprises W or Pt-lr or conductive carbon material; and wherein the proof mass (13, 23) of claim 9 comprises Pt, Pd, or Au.

1 1. A device (100) of any previous claims, wherein the device is configured to maintain the tunneling current to a predetermined constant value through a feedback circuit by controlling the gate voltage on the gate electrode (17, 27).

12. A device (100) of any previous claims, wherein the predetermined constant value is in the range of 0.1 nA to 10 nA.

13. A device (100) of any previous claims, wherein the device is configured to provide the bias voltage, where the bias voltage is in the range of 0.01 V to 10 V.

14. A device (100) of any previous claims, wherein the tunneling gap (20) between the cantilever (12, 22) and the drain electrode (16, 26) is in the range of 0.1 nm to 10 nm.

15. A device (100) of any previous claims, wherein the cantilever (12, 22, 471 ) is perforated with holes (491 ); and/or wherein the cantilever (12, 22, 471 , 472) has a round-edged portion.

16. A manufacturing method for manufacturing the accelerometer device (100) according to any of previous claims, wherein the manufacturing method comprises steps of:

- providing a mono- or multilayer cantilever flake (37) on a first substrate

(31 ) comprising a sacrificial layer (32),

- modifying the cantilever flake (37) to a designed cantilever shape (12),

- providing a stamp (14, 24, 36) for anchoring the cantilever (12, 22) to the first substrate (1 1 ),

- providing photoresist or e-beam resist material coat (33) and patterning said coat (33) into a designed shape to cover at least the stamp (14, 24, 36) portion surrounding the stamp (14, 24, 36), and leaving at least one link (35) between the stamp (14, 24, 36) and portion surrounding the stamp (14, 24, 36) ,

- dissolving the sacrificial layer (32) from the are not coated by the photoresist or e-beam resist material coat (33), and breaking said at least one link (35), and

- removing the stamp (14, 24, 36) with the cantilever (12, 22, 37) and transferring with the stamp (36) onto a target substrate (1 1 , 21 ) to form said accelerometer device according to any of previous claims.

17. A method of claim 16, wherein the cantilever flake (37) is cut to the designed cantilever shape (12, 22) via a first photolithography or e-beam lithography followed by oxygen plasma etching.

18. A method of any claims 16-17, wherein a proof mass (13, 23, 38) is deposited on the cantilever flake (37) via a second photolithography or e- beam lithography followed by a lift-off.

19. A method of any claims 16-18, wherein cantilever flake (37) is a graphene cantilever flake (37).