FIELD

This invention relates generally to techniques for fabricating electronic devices, in particular on a flexible substrate, and to fabricated devices such as sensors and (opto)electronic devices. BACKGROUND

Layered materials such as graphene, transition metal dichalcogenides (TMDs) offer advantages for (opto)electronic devices but fabrication of (opto)electronic devices using layered materials can be difficult and costly.

Nanoscale, 2017, 9, 6998, “Inkjet printed highly transparent and flexible graphene micro supercapacitors”, Delekta et al., describes a technique in which silver ink is used as a hard mask for a thin film of graphene flakes. Adv. Mater. 2015, 27, 599-626, “Inkjet Technology for Crystalline Silicon Photovoltaics“ , Stuwe et al., describes ink jet printing of hot melt masks. WO2011/031949 describes fabrication of a graphene device in which a graphene layer is patterned using conventional patterning techniques. Further background prior art can be found in: US2007/0148336; US2005/0074963; US2014/0299741 ; WO2017/032850; and US2013/0248380. SUMMARY

The invention is set out in the claims.

In one aspect there is described a method of fabricating an electronic device on a device substrate. The method may comprise providing a layer of a first layered material on a device substrate, wherein the first layered material is grown using chemical vapour deposition (CVD). The method may further comprise inkjet printing a pattern of mask material over the layer of first layered material on the substrate to define a mask. The mask may be considered to define an inkjet printed sacrificial material pattern. The method may further comprise dry etching the layer of first layered material using the mask to define a corresponding pattern of first layered material on the substrate for the electronic device. The method may further comprise removing the mask. The method may further comprise completing fabrication of the electronic device.

Implementations of this technique have an advantage that electronic devices with improved electrical performance may be obtained. The technique is also compatible with fabrication of electronic devices on large area and/or flexible substrates with good resolution (e.g. ~50 pm). The technique also facilitates electronic device manufacture with reduced cost and time as compared with conventional fabrication techniques and may use less costly equipment. Implementations of the technique may be used for high throughput e.g. roll-to-roll electronic device fabrication on a flexible e.g. polymer substrate.

In some implementations the first layered material may be grown on an initial substrate e.g. a metal e.g. copper substrate for single layer graphene (SLG), and then transferred to the device substrate, e.g. using a wet transfer technique. In some other implementations the first layered material may be grown directly on the device substrate e.g. using PECVD (Plasma-enhanced CVD). The dry etching may comprise reactive ion etching (RIE); the device substrate may be a flexible/polymer substrate e.g. comprising polyimide.

In implementations the first layered material comprises a semiconductor. In some other implementations the layered material may comprise a semi-metal or insulator material. The first layered material may comprise an element e.g. it may be a layered allotrope of an element, such as graphene. Alternatively the first layered material may comprise a compound such as a transition metal dichalcogenide (TMD), or the electrical insulator hexagonal boron nitride, h-BN (which has a hexagonal lattice structure with alternating boron and nitrogen atoms in “A” and “B” triangular sub-lattices). A TMD may have the general formula MX ₂ where M is a transition metal such as Mo or W and X is a chalcogen atom (S, Se or Te), for example MoS ₂ or WS ₂ . TMD materials can be useful because they have strong optical absorption.

The first layered material may have a crystal structure comprising layers or sheets of atoms. For example, the first layered material may have a crystal structure consisting of a single layer or sheet of atoms. The layer of the first layered material may comprise one or a few such layers or sheets of atoms, e.g. it may have in the range 1-20 stacked 1-atom thick layers. Thus the first layered material may be a 2d (two-dimensional) material. The layer of the first layered material may comprise a layered heterostructure e.g. a vertical heterostructure.

The method may further comprise forming one or more contacts each having an electrical connection to the pattern of first layered material on the substrate. Completing fabrication of the electronic device may include using the one or more contacts as one or more electrical connections to the device. For example, in some implementations the contacts comprise source and drain contacts to the channel of a field-effect transistor defined by the pattern of first layered material. Forming the one or more electrodes may comprise inkjet printing an electrode (contact) material, e.g. a metal, over the pattern in the layer of first layered material.

In implementations the mask material comprises a water-soluble polymer, e.g. PVP (polyvinylpyrrolidone). Removing the mask may then comprise applying a water-based (lift-off) process. This can facilitate achieving improved electrical performance as water is less likely than some other solvents to leave behind material which may generate defects, traps or residuals. It also has a benefit of being non-toxic and water does not have a high boiling point. The method may further comprise inkjet printing a layer of a second active material over the pattern of first layered material using an ink comprising the second active material. The second active material may comprise, for example, an active nanomaterial such as a colloidal quantum dot material, a colloidal nanoplatelet material, a perovskite material, or a liquid phase exfoliated (LPE) layered material. For example, in some implementations the second active material may comprise a second layered material, in particular a second layered semiconductor material such as black phosphorous (BP), also known as phosphorene. Optionally one or more further layers of active material may be printed over the second active material. In some implementations the ink has a boiling point of less than 300°C, more preferably less than 200°C and still more preferably less than 100°C. This facilitates rapid drying and hence can reduce degradation due to oxidation of the second active material. The ink may have or consists of a carrier fluid comprising (dry) isopropyl alcohol (IPA) and/or wherein a process of inkjet printing the ink may have an inverse Ohnesorge number between 1 and 14; or greater than 10; or greater than 14. Where the second active material comprises a second layered material (which may also be a 2d material) the ink may comprise flakes of the second layered material.

The device substrate may be a flexible substrate e.g. a textile, polymer, fabric, or paper substrate. In particular where the device substrate is a textile substrate e.g. a polymer textile substrate, the method further comprising depositing a planarizing layer on the device substrate prior to providing the layer of first layered material on the device substrate. In such cases the method may comprise providing two layers of the first layered material on the device substrate prior to inkjet printing the pattern of mask material e.g. by transferring a first layer of the layered material from the initial substrate onto the device substrate and then a further layer of the layered material from the initial substrate onto the device substrate. This can improve the electrical performance of the electronic device. A layer of material may then be deposited over the channel. The method may further comprise inkjet printing one or more encapsulating layers over the layer of second active material. In some implementations e.g. where the substrate is textile or fabric substrate, two layers of encapsulating material may be printed over the layer of second active material, a first layer of a first encapsulating material e.g. an encapsulating material comprising a poly(p-xylylene)-based polymer such as Parylene™, and a second layer of a second, different encapsulating material e.g. a second polymer such as PDMS (polydimethylsiloxane).

The method may comprise fabricating multiple electronic devices as a continuous roll- to-roll process by drawing the flexible substrate from a first roll, performing the above described method steps, and then rolling the fabricated electronic devices up onto a second roll.

A method as described above may be used to fabricate any sort of passive or active electronic devices including, by way of example, transistors, diodes, resistors and capacitors. The fabricated electronic device may be a sensor device. Such a sensor device may include one or more sensor layers; for example the second active material may provide a light or chemical e.g. liquid or gas sensing layer for the device. The fabricated electronic device may be a light emitting device; for example the second active material may comprise a light-emitting material. The fabricated electronic device may be a solar cell; for example the second active material may comprise a light-harvesting material. In some implementations the electronic device is a photosensitive or light-emitting device, e.g. a photovoltaic device or light-emitting diode. Then the layer of first layered material may have a transmission coefficient of greater than 50% over a range of wavelengths from at least 200nm or 500nm to 1000nm, 2000nm, 3000nm or 4000 nm. The mask may define a photoactive region of the device.

In some implementations the electronic device comprises a field-effect transistor sensor, the one or more contacts comprise drain and source contacts of the field-effect transistor, the pattern of first layered material on the device substrate defines a channel of the field- effect transistor, and the second active material comprises a sensing layer over the channel such as a light or gas sensing layer. The field-effect transistor may also include one or more gates such as a back gate, top gate or one or more side gates (e.g. to one or both sides of the channel). In use the gate(s) may be biased e.g. to maximize a sensitivity of the field-effect transistor sensor to a target such as a wavelength or light or chemical.

The above described method may also be used to form a patterned transparent conductive region e.g. a patterned electrode region for an electronic device. Such a patterned electrode may thus combine high electrical conductivity, thermal conductivity, chemical stability and high transparency.

Thus in another aspect there is described a method of forming a patterned transparent conductive region on a device substrate, comprising providing a layer of a first layered semiconductor material on a device substrate, wherein the first layered semiconductor material is grown using CVD. The method may further comprise inkjet printing a pattern of mask material over the layer of first layered semiconductor material on the substrate to define a mask. The method may further comprise dry etching the layer of first layered semiconductor material using the mask to define a corresponding pattern of first layered material on the substrate for the electronic device. The method may further comprise removing the mask to leave the pattern of first layered semiconductor material on the substrate. Further features of this method may be as previously described. There is also provided an electronic device fabricated by a method as described above.

Thus, there is also described a mechanically bendable (flexible) electronic device comprising a flexible device substrate; a layer defining a pattern of a first layered material on the device substrate; a layer of a second active material over the pattern of first layered material; and one or more contacts i.e. electrodes having an electrical connection to the pattern of the first layered material. DRAWINGS

These and other aspects of the invention will now be further described byway of example only, with reference to the accompanying Figures, in which: Figure 1 shows a process for fabricating an electronic device;

Figures 2a and 2b show, respectively, a schematic diagram and an SEM (scanning electron microscope) image of an example electronic device fabricated using the process of Figure 1 ;

Figures 3a and 3b illustrate the performance of the example electronic device of Figure 2; and

Figures 4a to 4d show, respectively, example electronic devices fabricated on a textile substrate, an SEM image of an example electronic device fabricated on a textile substrate, a washing machine, and a graph showing performance of the example electronic device as a function of a number of washing cycles, generated using the washing machine of Figure 4c. Like elements are indicated by like reference numerals.

DESCRIPTION Figure 1 shows an example process for fabricating an electronic device. In this example the device is fabricated on a substrate 100 comprising a layer of oxide (Si0 ₂ ) 102 on silicon 104. The process begins by growing a layer of layered material using CVD (chemical vapour deposition). For example, a layer of SLG (single layer graphene) 106 may be grown on a copper substrate according to a known process. In other examples the CVD grown layer may comprise bilayer or multilayer graphene, or a layer of another layered material allotrope or compound. Alternatively, a layer of layered material such as SLG may be purchased, e.g. on a metal substrate or grown directly on top of the substrates.

The layered material, e.g. SLG 106 is then transferred to a surface of the substrate 100, to provide the structure of Figure 1 step (1). In an example wet transfer process PMMA (polymethyl methacrylate) is spin coated onto a layer of SLG on copper, and then the copper/SLG/PMMA is left on a solution of ammonium persulfate in deionized (Dl) water to etch away the copper, cleaned with Dl water, transferred onto substrate 100, left to dry, and then the PMMA is removed with acetone and IPA. A similar technique may be used with a polymer or textile substrate. Alternatively, the layered material may be grown on substrate 100 e.g. using PECVD to grow SLG on polyimide or another polymer.

A pattern of mask material 108 is then inkjet printed over the layer of layered material e.g. SLG, to define a mask, as shown in Figure 1 step (2). The mask material may comprise PVP, and this may be dissolved in water and/or IPA to make an ink for printing. For example, 5mg PVP may be dissolved in 5mg IPA. The formulation may be adjusted for stable jetting and appropriate substrate wetting.

The layered material is then dry etched e.g. using reactive ion etching (RIE), using the PVP as a mask to protect the layered material beneath, Figure 1 step (3). The PVP is then removed e.g. by gently adding droplets of water, to leave a corresponding pattern 110 of layered material, e.g. SLG, on the substrate, as shown in Figure 1 step (4). The pattern 110 of layered material e.g. SLG may form an active layer of an electronic device e.g. a channel of a field-effect transistor.

Contacts 112 are then provided for the pattern 110 of layered material by inkjet printing an electrode material, e.g. by inkjet printing silver ink, Figure 1 step (5). Afterwards the printed electrode material may be cured e.g. at around 100°C for 2 hours. The result is a patterned layer of layered material on a substrate, which may be a flexible substrate. Fabrication of the electronic device may then be completed in an appropriate manner for the device. For example, the patterned layer may serve as a (transparent) conductive electrode in an active or a passive device e.g. a light emitting diode, photovoltaic device, or capacitor or resistor; or the patterned layer may be an active layer in an active semiconductor device such as a transistor.

In some devices, e.g. some graphene-based devices, a patterned graphene layer may be gated by one or more gate electrodes. For example, the silicon 104 of substrate 100 may provide a back gate voltage, or a separate electrically conductive layer may be provided for a back gate voltage of a non-conductive substrate. Also or instead the patterning process may also be used to define one or more side gates for biasing the device, or these may be inkjet printed in a similar manner to the contacts 112.

Figure 2 shows one example of an electronic device which may be fabricated using the above process. In this example a layer of a second active material is inkjet-printed over the patterned layer of layered material. More particularly the second active material is a second layered material, in the example BP, also known as phosphorene.

The example device of Figure 2 comprises a phototransistor 200. Referring to Figure 2a, a layer of BP 206 has been deposited over the patterned SLG by inkjet printing. Contacts 112a,b provide respective source and drain connections to a channel 204 of the phototransistor, which is sensitive to light 208 as described later. Figure 2b shows an SEM image of the phototransistor 200.

In the example the BP is obtained by LPE (a liquid phase exfoliation) and the resulting flakes are made into an ink for printing. For example BP crystals may be ground in a mortar, the powder mixed with IPA and the mixture sonicated for some hours then centrifuged to sediment the unexfoliated flakes. The supernatant may be used to make the ink.

The stable jetting of individual droplets of ink from a nozzle depends on the ink viscosity h (mPa.s), surface tension g (mN/m), and density p (g/cm ³ ), and on the nozzle diameter D (pm). The jettability of an ink is characterized by the dimensionless inverse Ohnesorge number Z, where Z = (grO)Ί/h. In general, it is considered that if Z<1 the ink will not jet and if Z> 14 the ink tends to jet secondary droplets. However, counter-intuitively it has been found that inkjet printing of layered materials is possible with Z>10 or Z>14.

Thus, it has been found that the ink for depositing LPE BP may use anhydrous IPA as a solvent i.e. carrier fluid. This has a low boiling point of <300°C, more preferably <200 °C and still more preferably <100 °C which allows a shorter drying time with consequential reduced risk of oxidation/degradation of the BP. The average maximum-lateral- dimension of a flake of layered material may be less than 1/10, 1/20 or 1/50 of a nozzle lateral dimension (diameter) to inhibit clogging and the accumulation of deposits at the nozzle edge. The flake size may be measured using e.g. STEM (scanning transmission electron microscopy).

In one example, the following parameters were used for the BP ink: h º 0.6 mPa.s, g º 26mNm ^~1 , p º 0.8 gcm ^~3 , D = 22 pm, average maximum-lateral-dimension of a flake less than Imth; giving Z º 35. In one example corresponding parameters for the PVP ink were h = 1.3 mPa.s, g = 69mNm ^~1 , p = 1 gcm ^~3 , D — 22pm giving Z = 30.

Although an example process has been described using LPE BP, a similar approach may be used for printing a layer of another layered material e.g. comprising colloidal nanoplatelets and/or perovskites, colloidal quantum dots, other LPE layered materials, or other layered materials in general.

Following deposition of the layer of e.g. BP the device may be sealed, e.g. under vacuum, by deposition of a Parylene C coating.

Figure 3 illustrates performance of the example phototransistor 200 of Figure 2, illustrating the potential for broadband photodetection (e.g. from visible to infrared with wavelength > 2 pm). Figure 3a shows a set of curves illustrating the change in drain current (at a source-drain voltage of 0.5V) for a range of incident optical power from ~ pW to 612m1K at 488nm.

Not shown in Figure 2, a back gate connection to the silicon layer 104 of the substrate may be biased to a gate voltage (V _BG ) which adjusts the response of the phototransistor 200. Controlling the gate voltage controls the SLG carrier density and hence responsivity of the structure. Deposition of the BP shifts the Dirac point, V _D , of the SLG from e.g. around 10V to around -7V due to transfer of electrons from the BP to SLG. Illuminating the channel 204 shifts the Dirac point to higher gate voltages and the drain current increases. Figure 3b shows the photocurrent through the channel (I _llght - I _dark ) for the same range of incident optical power, at V _BG = -2QV and for a range of source-drain voltages. The photoconductive gain is of order 10 ⁶ over a wavelength range of at least ultraviolet to 2700nm. The good performance illustrated in Figure 3 illustrates, for an example fabricated device, that the techniques described herein can be used to obtain large area electronic devices with good electrical performance.

Figure 4 illustrates that the techniques described herein can be used to fabricate electronic devices with robust performance on flexible, in particular textile substrates.

As an example, a phototransistor of the type described above was fabricated on a polyester fabric. To fabricate a device on a textile (fabric) substrate, the substrate may first be planarized by depositing, e.g., a layer of polyurethane (PU). For example, a layer of PU may be rod coated onto the fabric substrate to smooth the surface.

Fabrication may then proceed in a similar way to that previously described. However, two layers of the first layered material, i.e. SLG, may be provided prior to inkjet printing the pattern of mask material to reduce the risk of cracking. A second layer of encapsulation, e.g. of PDMS, may be provided for additional protection if the textile is to be washed, for additional protection against water and mechanical stress e.g. bending and stretching.

Figure 4a shows an example of polyester fabric on which an array of photodetectors has been fabricated. Each photodetector is similar to the previously described phototransistor except that no back (or side) gate is provided. Figure 4b shows an SEM image of one such photodetector. As previously, the photodetector may be operated at a drain source voltage of less than 1V, showing Ohmic contact and can exhibit a photoconductive gain under incident light. The fabric bearing the photodetectors was subjected to multiple washing cycles in the tumble washing machine of Figure 4c. Figure 4d shows a graph illustrating the photocurrent in a photodetector after washing (according to industrial standards) as a ratio to the photocurrent in the photodetector prior to washing. After washing the photocurrent drops to around half its original value but from then on decreases only slowly. This demonstrates that devices with good performance can fabricated on a textile substrate using the methods described herein, thus facilitating applications for wearable electronic devices.

Applications for the techniques described herein include the fabrication of electronic devices for health and well-being monitoring. For example the described techniques may be used to fabricate an array of photoconductive photodetectors on rigid substrate for a high-resolution, broadband image sensor and e.g. for a digital camera that is sensitive to ultraviolet, visible and infrared light. Moreover, the technique can be used to fabricate a sensor for sensing one or more of: heart rate, respiration rate, blood pulse oxygenation, and exposure to UV radiation from the sun. Such sensors may be used for tracking vital signs for fitness and health monitoring. Other applications include fabricating devices for energy harvesting such as photovoltaic devices (solar cells), e.g. on wearable or flexible substrates.

The techniques described herein may be used to fabricate a light source e.g. LED and a photodetector on the same substrate e.g. on a textile or fabric substrate; the light source and photodetector may be integrated into a single device, potentially a large area device. Such an approach may be used to fabricate a textile or fabric-based photoplethysmography (PPG) system e.g. for cardiac pulse measurement or other tracking or analysis of changes in the volume of blood vessels.

In another example a light source e.g. LED may be fabricated in the vicinity of photodetector, both on a textile substrate for a “smart textile”. A reflection or absorption spectrum may be captured from light from the light source reflected from or absorbed by the skin surface (depending upon whether the source and detector are on the same side or opposite sides of a body/skin region). The captured optical absorption/reflectance spectra may then be analysed and e.g. correlated to cardiac activity such as heart rate and/or waveform. In some applications multiple light source - photodetector pairs may be provided on a textile or fabric e.g. to capture optical reflection or absorption data from multiple different areas of the body surface. These data may be combined e.g. to provide a more accurate estimate of cardiac activity using statistical techniques.

The techniques described herein can provide a scalable device fabrication process for mass-production of low-cost devices.

No doubt many effective alternatives will occur to the skilled person. The invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.