WO2015190910A1

WO2015190910A1 - Flexible pressure-sensing device and process for its fabrication

Info

Publication number: WO2015190910A1
Application number: PCT/MY2015/000046
Authority: WO
Inventors: Hing Wah Lee; Daniel Chia Sheng Bien; Saat Shukri Embong
Original assignee: Mimos Bhd
Current assignee: Mimos Bhd
Priority date: 2014-06-11
Filing date: 2015-06-08
Publication date: 2015-12-17
Anticipated expiration: 2016-12-11
Also published as: MY171435A

Abstract

A pressure-sensing device (10) and a process for fabricating it is disclosed. In a preferred embodiment, a sacrificial oxide layer (11 ) is first formed onto a silicon substrate (30). A bottom polymer film (14) is then formed on oxide layer (11 ). Conductive elements including electrodes (12a, 12b) and/or contact pads (18a, 18b) are formed on the bottom polymer film (14). A graphene sheet (20) is deposited to electrically connect the electrodes (12a, 12b). A top polymer film (16) is then laid on top so that the conductive elements (12, 18) and graphene (20) are sandwiched and encapsulated in between the top and bottom polymer layers (14, 16) chosen from polyimide or poly dimethylsiloxane (PDMS). Openings (19a, 19b) on top polymer layer (16) may optionally be provided to allow for interconnection of the contact pads (18a, 18b). The sacrificial oxide (11 ) is then etched to release the completed device (10) from the substrate (30) which may be reused.

Description

Flexible pressure-sensing device

and process for its fabrication

TECHNICAL FIELD

[001] This invention generally relates to a pressure-sensing device configuration and a process for fabricating such a device. Specifically, the pressure-sensing device includes the use of nanomaterials for facilitating electrical conductity. More particularly, the fabrication process enables the resultant device to be thin and flexible to be embodied as a wearable contraption to achieve pressure or pulse sensing and monitoring.

BACKGROUND ART

[002] Film-thin or diaphragm-type of pressure-sensing devices have distinct advantageous in biomedical applications for being bendable and wearable implements, e.g. as blood pressure and heart rate measurement sensors. Fabricated as integrated semiconductor devices, its mass production also means such devices are robust and reliable yet cheap enough to be disposable. Such pressure-sensing devices are now increasingly integrated with nanomaterials such as carbon nanotubes or graphene sheets to improve sensitivity.

[003] A brief electronic and electrical properties of carbon nanomaterials such as nanotubes and graphene is discussed in the Background Art section of U.S. Patent No. 8,044,472 (Kulite Semiconductor) which proposed pressure transducing device configured with a void below the graphene and electrodes layer so that pressure may act on the thinned part of the device to enable flexure on the graphene to produce changes to electrical conductivity. Similar configuration providing a void below a thin layer, as represented in FIGURE 1 (Prior Art) - comprising FIG. 1A and FIG. 1 B - may be found in U.S. Published Applications No. 2012/0073948 (Kulite Semiconductor) and No. 2013/0270511 (IBM Corp.) discussed in Gomez- Navarro, et al. (2009) where graphene oxide (GO) and reduced graphene oxide (RGO) were used.

[004] Other types of nanomaterials are also used in pressure-sensing devices such as vertically-oriented carbon nanotubes in U.S. Patent No. 6,286,226 (Agere Systems). In International Published Application WO 2012/087065 (Sung Kyun Kwan University), arrays of pressure-sensing "cells" are created on polydimethylziloxane (PDMS) substrate so that location of pressure in the array may be registered as well. As in the case of hollow out bottom of the pressure sensing layer, the need for substrate backing means the overall device is rigid with the substrate backing.

[005] Overall flexibility achieved in a thin device or apparatus level is proposed in P.R. China Published Application No. CN102998035 (China Northeastern University). There is no drawing to disclose its configuration but the pressure sensor is described as comprising packaging films at the two outermost layers and a soft pressure volume material (to allow for pressure absorbing and thus tactile effect) in the middle. The device's copper foil electrode and lead foil electrode are packaged by polyimide film with a soft pressure volume material in between the electrodes. Thus, electrical conductivity is achieved by pressure exerted to push the respective lead and copper electrodes together across the probably dielectric soft pressure volume composite material, i.e. with graphene flakes dispersed in RTV silicon.

[006] Graphene flakes are typically small, i.e. in the range of 10 m and, hence, may not be suitable to lay over a relatively wide area such as the space between electrodes without uniform distribution problems, e.g. stacking and overlapping issues.

[007] It is thus desirable for a pressure-sensing device configuration that is thin not only in respect of electrode elements in response to pressure but also in respect of the device as a whole. It is also desirous that the pressure sensing device has sensitivity or transducing ability that varies according to the amount of pressure sensed. The thinness and flexibility of the device as a whole would enable it to be embodied as a wearable apparatus, in addition to increased sensitivity to pressure, pulse or tremor, on a subject's body as would be expected of its membrane-like configuration. A further endeavour is to enable such a pressure-sensing device to be fabricated via semiconductor techniques so that the device or apparatus are robust and sufficiently cost-effective to be disposable after use.

SUMMARY OF INVENTION

[008] In a first aspect of our invention, a flexible pressure-sensing device is disclosed as comprising a pair of electrodes sandwiched between two flexible layers, and a graphene-based sheet disposed to electrically connect the electrodes, such that the electrical connectivity between the electrodes via said graphene-based sheet varies according to the flexure of the flexible pressure-sensing device.

[009] In one embodiment, the graphene-based sheet is preferably chosen from any one of graphene, graphene oxide and like graphene-based nanomaterials. Preferably, the two sandwiching flexible layers respectively comprises a first polymer layer and a second polymer layer chosen from any one or combination of polyimide and polydimethylsiloxane (PDMS) or like materials. The graphene-based sheet may be fully encapsulated within the sandwiching flexible polymer layers. Our flexible pressure-sensing device may preferably be fabricated on a silicon or glass substrate and then released therefrom.

[010] Preferably, at least one of the electrodes is electrically connected to a contact pad and at least one of said first and second polymer layers is provided with an opening to the contact pad for interconnecting to means for processing signals from the device for pressure, pulse and/or tremor. Our pressure-sensing device may be advantageously embodied as a wearable contraption, including any one or combination of tremor, pulse and/or blood pressure sensing, monitoring and recording.

[011] In a second aspect of our invention, a process for fabricating a flexible pressure-sensing device is disclosed. Our process comprises of the steps of:

(a) forming a sacrificial oxide layer onto a silicon substrate;

(b) forming a bottom polymer film from any one of polyimide or poly dimethylsiloxane (PDMS);

(c) forming conductive elements, including electrodes and/or contact pads, on top of said bottom polymer film;

(d) depositing the graphene-based sheet to electrically connect said electrodes;

(e) forming a top polymer film from any one of polyimide or polydimethyl siloxane (PDMS) to encapsulate said conductive elements and graphene-based sheet therewithin;

(f) optionally exposing regions over contact pad on said top polymer film [for device integration interconnect]; and

(g) releasing the formed flexible pressure-sensing device from said substrate.

[012] As a second process option, step (a) may be omitted when the device is fabricated on a glass substrate. In a third option, the need for substrate as handler is omitted altogether with the bottom polymer layer serving as base for the fabrication.

[013] Where silicon substrate is used, a sacrificial oxide layer is preferably formed onto the silicon substrate by any one of thermal growth and chemical vapour deposition methods. Upon completion of fabrication, the device may be released from the silicon substrate by etching away the sacrificial oxide layer with hydrofluoric acid (HF). Preferably, the silicon substrate is reusable upon release of the pressure-sensing device in step (g).

[014] For all 3 process options, the polyimide or PDMS bottom polymer film may be laid by spin-coating followed by heat-curing. Preferably, the conductive elements are chosen from, but not limited to, aluminum, tungsten, gold, platinum or molybdenum, and may be formed by physical deposition or chemical vapour deposition followed by wet or plasma etching. The graphene- based sheet is preferably deposited by drop-casting or by spin-coating.

LIST OF ACCOMPANYING DRAWINGS

[015] The drawings accompanying this specification as listed below may provide a better understanding of our invention and its advantages when referred to in conjunction with the detailed description that follows as exemplary and non-limiting embodiments of our product and process:

[016] FIGURE 1 (Prior Art) shows a conventional configuration of pressure-sensing device with a void below thin pressure-sensing elements.

[017] FIGURE 2 illustrates a generalized embodiment of the pressure- sensing device according to our invention. [018] FIGURE 3 indicates in a schematic diagram the fabrication options in which Flowchart (a) requires a silicon or glass substrate, and Flowchart (b) does not require any substrate handler.

[019] FIGURE 4 displays a schematic step-by-step process for fabricating our pressure-sensing device on a silicon substrate.

[020] FIGURE 5 represents a schematic step-by-step process for fabricating our pressure-sensing device without a substrate handler.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[021] FIGURE 2 illustrates a simplified configuration of a general embodiment of our pressure-sensing device (10) comprising a pair of electrodes (12a, 12b) sandwiched between two flexible layers (14, 16). The electrodes (12a, 12b), as with other conductive elements and traces fabricated in the device, may be chosen from aluminium, tungsten, gold, platinum or molybdenum but not necessarily limited to these conductive metals. The two sandwiching flexible layers (14, 16) respectively comprises a first polymer layer (14) and a second polymer layer (16) chosen from any one or combination of polyimide and polydimethylsiloxane (PDMS) or like materials.

[022] A graphene-based sheet (20) is laid or disposed in a manner to electrically connect the pair of electrodes. Electrical connectivity between the electrodes via the graphene-based sheet (20) would vary with flexure of the flexible pressure-sensing device. The graphene-based sheet (20) may be chosen from any one of graphene, graphene oxide and other possible sheet-like graphene-based nanomaterials. As shown in FIG. 2, the graphene sheet (20) is disposed in such manner that it laterally connects the electrodes.

[023] The electrodes (12a, 12b) may preferably be connected to contact pads (18a, 18b) for ease of wire interconnects for circuit integration purposes. To enable the contact pad area to be accessible for wire interconnect, openings (19a, 19b) should be provided on the upper polymer layer (16) corresponding to the positions above the contact pads. Depending on the design of the pressure-sensing device and/or the apparatus, the contact pads' (18a, 18b) positions may be traced to a suitable location on the device for interconnect purposes. With the interconnect means, the electrical signals from the device may be transmitted to means for signal processing.

[024] As it will be seen from the fabrication process to be described later, the release of the completed device from the substrate backing means that openings may also be pre-planned on the lower polymer layer (14) so that such lower layer openings may render the contact pads accessible from the underside as well. The substrate used as backing for the device's fabrication may be silicon or glass and the device released therefrom upon completion.

[025] It is preferable that the graphene-based sheet (20) be fully encapsulated within the flexible polymer layers so that its interconnection between the electrodes is secured and makes the device robust. Another advantage would be that with such encapsulation, the flexibility of the polymer layers and the device as a whole may be fully transferred to flexing (as a result of pressing or bending) and/or stretching the graphene-based sheet. Thus, even when the pressure exerted on the device is a pulse from heart beat or muscle tremors, e.g. for measuring blood pressure, the device is sensitive enough to pick up such signals for sensing, monitoring and/or recording. [026] The process of fabricating the pressure-sensing device may be made with or without a substrate handler. FIGURE 3 indicates in a schematic manner both the fabrication options whereby Flowchart (a) employs handler which may be silicon or glass substrate, and Flowchart (b) which does not require any substrate handler. Briefly, in the first process (a), the sandwiching polymer layers and the graphene and electrodes are first form on a bulk substrate and then released. In the second process (b), the structures may be formed directly onto a polymer layer as substrate. The details or step-by-step process of each of the options will be described in the following.

EXAMPLE 1

Silicon substrate handler

[027] Refering to FIGURE 4, a process for fabricating a flexible pressure- sensing device having the above-described flexible polymer layers sandwiching the graphene and electrodes proposed in the foregoing may be described in a step-by-step manner in the following.

[028] In Step (a) of FIG. 4, the fabrication is conducted on a silicon or glass substrate as handler. When fabricated on a silicon substrate (30), a sacrificial oxide layer (11 ) will need to be first formed on the silicon substrate (30) before the bottom polymer film (14) is formed thereon in Step (b). This sacrificial oxide layer (11 ) may preferably be formed onto the silicon substrate (30) by any one of thermal growth and chemical vapour deposition methods.

[029] (In this specification, the term "film", "membrane" and "layer" are used interchangeable in reference to the thinness of the polymer layers (14, 16) which sandwiches the conductive elements such as the electrode elements (12, 18) ) and graphene (20) in between.) The bottom polymer film (14) may be chosen from any one of polyimide or polydimethylsiloxane (PDMS). [030] Next, the conductive elements such as the electrodes (12a, 12b) and/or contact pads (18a, 18b) , traces, etc. are formed on top of the bottom polymer film (14), as shown in Step (c). The graphene-based sheet (20) is then deposited over or in between the electrodes (12a, 12b) so as to electrically connect them in Step (d). Then, in Step (e), a top polymer film (16) chosen from any one of polyimide or polydimethylsiloxane (PDMS) is formed to encapsulate the conductive elements (12, 18) and graphene sheet (20) within. It would be preferred that the conductive elements (12, 18) and graphene (20) be encapsulated hermetically in the finished device.

[031] Depending on the design of the pressure-sensing device, contact pads (18) for device's integration interconnect may be provided connected to the electrodes (12) via traces. To enable wire interconnect, openings (19) would need to be provided on the top polymer film (16) to expose the contact pads (18). Depending on the type of polymer, i.e. either polyimide or PDMS, the openings (19) may be etched. Optionally, the contact pads (18) may be traced away from the sandwiching layers to allow for better hermetic sealing as openings on the top layer may be avoided.

[032] In the final Step (f), the sacrificial oxide layer (11 ) may be etched away to release the completed device (10). Preferably, wet chemical etching is used, such as hydrofluoric acid (HF), to etch away the oxide layer. Upon release, the silicon substrate (30) may be reused for fabrication of subsequent pressure-sensing devices.

EXAMPLE 2

Glass substrate handler

[033] Steps (b) to (f) are essentially repeated for glass (commonly known to be phosphosilicate glass, PSG, or boro-phosphosilicate glass, BPSG) in semiconductor fabrication) being used as substrate handler wherein Step (a)'s deposition of a sacrificial oxide layer may be omitted because glass itself is an oxide family material. For example, with borosilicate-based material, it can be etched using conventional oxide etching chemicals or etchants.

[034] Hence, a sacrificial oxide layer is not required for glass substrate as the polyimide is only weakly stuck onto the glass surface. The use of a HF solution will attack the weak interface between the glass and polyimide and cause the release of the completed pressure-sensing device.

EXAMPLE 3

No substrate handler

[035] Our pressure-sensing device may also be fabricated without substrate handler or carrier. The step-by-step process is represented in FIGURE 5's Steps (a) to (c).

[036] FIGURE 5 presents a schematic step-by-step process for fabricating our pressure-sensing device without a substrate handler. In Step (a), the bottom polymer layer (14) is laid and may be used as a substrate or backing itself. The conductive elements such as electrodes (12a, 12b) and/or contact pads (18a, 18b), are formed directly on top of the polymer layer (14) which may be chosen from any one of polyimide or polydimethylsiloxane (PDMS). The polyimide or PDMS sheet typically has a thickness of about 50 pm - 100 pm and should be able to hold itself as a carrier or handler during the spin-coating process. Next, the graphene-based sheet (20) is deposited to electrically connect the electrodes (12a, 12b) in Step (b).

[037] Finally, a top polymer film (16) chosen from any one of polyimide or polydimethylsiloxane (PDMS) is formed on top of the conductive elements (12, 18) and graphene (20) to encapsulate these elements therewithin. Finally, depending on the design of the pressure-sensing device, openings (19a, 19b) may be optionally formed on the top polymer layer (16) to expose the contact pads (18a, 18b) to enable device integration interconnect.

[038] For all the afore-described Examples or embodiments, the polyimide or PDMS bottom polymer film (14) may preferably be laid by spin-coating, followed by heat-curing. The conductive elements (12, 18) may be chosen from, but not limited to, aluminum, tungsten, gold, platinum or molybdenum, and may preferably be formed by physical deposition or chemical vapour deposition followed by wet or plasma etching. The graphene-based sheet (20) is deposited by drop-casting or by spin-coating. The graphene sheets envisaged to be used in our device and process are typically larger than 50 μιη in diameter on a single sheet. With the resultant device's thin and flexible configuration, our pressure-sensing device may be easily embodied as a wearable device such as a wrist-, chest- or arm-cuff, or even embodied as or in a diaphragm, for example.

[039] Apart from the aforesaid described embodiments and advantages, there are many other aspects of our invention that may be implemented in alternative forms, variations, substitution or modifications. For example, there may be other suitable flexible polymer layers apart from polyimide and PDMS that are compatible or suitable for use in a standard semiconductor fabrication environment. The electrode may also be fabricated in form of inter-digitated structure to provide for more surface area in contact with the graphene sheet. Other bulk handle substrate apart from silicon and glass may also be tried. Nanomaterial in form of sheet-like carbon nanotube mesh layer may be used as equivalent to our graphene. Such variations, alternatives, substitutes, analogs or equivalents are to be considered as falling within the letter and scope of the following claims.

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Claims

1. A flexible pressure-sensing device (10) comprising a pair of electrodes (12a, 12b) sandwiched between two flexible layers (14, 16) and a graphene- based sheet (20) disposed to electrically connect said electrodes, wherein electrical connectivity between said electrodes via said graphene-based sheet (20) varies according to the flexure of the flexible pressure-sensing device.

2. A flexible pressure-sensing device according to Claim 1 wherein the graphene-based sheet (20) is chosen from any one of graphene, graphene oxide and like graphene-based nanomaterials.

3. A flexible pressure-sensing device according to Claim 1 wherein the two sandwiching flexible layers (14, 16) respectively comprises a first polymer layer (14) and a second polymer layer (16) chosen from any one or combination of polyimide and polydimethylsiloxane (PDMS).

4. A flexible pressure-sensing device according to Claim 1 which is fabricated on a substrate chosen from any one of silicon and glass, said completed device may then be released from said substrate.

5. A flexible pressure-sensing device according to Claim 1 wherein at least one of the electrodes is electrically connected to a contact pad (18a, 18b) and at least one of said first and second polymer layers (14, 16) is provided with an opening (19a, 19b) to said contact pad.

6. A flexible pressure-sensing device according to Claim 1 wherein the graphene-based sheet (20) is fully encapsulated within the flexible polymer layers.

7. An apparatus incorporating the flexible device according to Claim 5 including interconnecting said device's contact pads (18a, 18b) via openings (19a, 19b), and including means for processing the variations in electrical signals from said device in sensing for pressure, pulse and/or tremor.

8. An apparatus according to Claim 7 embodied as a pressure-sensing wearable contraption, including any one or combination of tremor, pulse and/or blood pressure sensing, monitoring and recording.

9. A process for fabricating a flexible pressure-sensing device (10) comprising the steps of:

(a) forming a sacrificial oxide layer (11 ) onto a silicon substrate (30);

(b) forming a bottom polymer film (14) from any one of polyimide or poly dimethylsiloxane (PDMS);

(c) forming conductive elements, including electrodes (12a, 12b) and/or contact pads (18a, 18b), on top of said bottom polymer film (14);

(d) depositing the graphene-based sheet (20) to electrically connect said electrodes (12a, 12b);

(e) forming a top polymer film (16) from any one of polyimide or polydimethyl siloxane (PDMS) to encapsulate said conductive elements (12, 18) and graphene-based sheet (20) therewithin;

(g) releasing the formed flexible pressure-sensing device from said substrate.

10. A process for fabricating a flexible pressure-sensing device (10) comprising the steps of:

(a) forming a bottom polymer film (14) from any one of polyimide or poly dimethylsiloxane (PDMS) on a glass substrate;

(b) forming conductive elements, including electrodes (12a, 12b) and/or contact pads (18a, 18b), on top of said bottom polymer film (14); (c) depositing the graphene-based sheet (20) to electrically connect said electrodes (12a, 12b);

(d) forming a top polymer film (16) from any one of polyimide or polydimethyl siloxane (PDMS) to encapsulate said conductive elements (12, 18) and graphene-based sheet (20) therewithin;

(e) optionally exposing regions over contact pad (18a, 18b) on said top polymer film (16) [for device integration interconnect]; and

(f) releasing the formed flexible pressure-sensing device (10) from said substrate (30).

11. A process for fabricating a flexible pressure-sensing device comprising the steps of:

(a) forming conductive elements, including electrodes and/or contact pads, on top of a bottom polymer film (14) chosen from any one of polyimide or polydimethylsiloxane (PDMS);

(b) depositing the graphene-based sheet (20) to electrically connect said electrodes (12a, 12b);

(c) forming a top polymer film (16) from any one of polyimide or polydimethyl siloxane (PDMS) to encapsulate said conductive elements (12, 18) and graphene-based sheet (20) therewithin; and

(d) optionally exposing regions over contact pad on said top polymer film (16).

12. The process for fabricating a flexible pressure-sensing device (10) according to Claim 9 wherein the sacrificial oxide layer (11) is formed onto the silicon substrate (30) by any one of thermal growth and chemical vapour deposition methods.

13. The process for fabricating a flexible pressure-sensing device (10) according to Claim 9 wherein the silicon substrate (30) is reusable upon release of the pressure-sensing device in step (g).

14. The process for fabricating a flexible pressure-sensing device (10) according to any one of Claims 9 - 11 wherein the polyimide or PDMS bottom polymer film (14) is laid by spin-coating, followed by heat-curing.

15. The process for fabricating a flexible pressure-sensing device according to any one of Claims 9 - 11 wherein the conductive elements (12, 18) are formed by physical deposition or chemical vapour deposition followed by wet or plasma etching.

16. The process for fabricating a flexible pressure-sensing device (10) according to Claim 15 wherein the conductive elements (12, 18) are chosen from, but not limited to, aluminum, tungsten, gold, platinum or molybdenum.

17. The process for fabricating a flexible pressure-sensing device (10) according to any one of Claims 9 - 11 wherein the graphene-based sheet (20) is deposited by drop-casting or by spin-coating.

18. The process for fabricating a flexible pressure-sensing device (10) according to Claim 9 wherein the device is released from the silicon substrate (30) by etching away the sacrificial oxide layer (11) with hydrofluoric acid (HF).

19. A device incorporating a flexible pressure-sensing device fabricated with a process according to any one of Claims 9 - 18.

20. An apparatus incorporating a flexible pressure-sensing device fabricated with a process according to any one of Claims 9 - 18.

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