Photo-annealing in Metal Oxide Sensors

Abstract

We disclose herein a method of annealing a composition to produce a film for a sensing device, the composition comprising at least one metal oxide material, the method comprising: depositing the composition on one side of a substrate; providing a source of electromagnetic radiation in a proximity to the composition; exposing a surface of the composition to a first dose of electromagnetic radiation, wherein the first dose comprises a first property which induces annealing of the composition; exposing the surface of the composition to a second dose of electromagnetic radiation, wherein the second dose comprises a second property which induces annealing of the composition, wherein the first property is substantially the same or different to the second property.

Claims

1. A method of annealing a composition to produce a film for a sensing device, the composition comprising at least one metal oxide material, the method comprising: depositing the composition on one side of a substrate; providing a source of electromagnetic radiation in a proximity to the composition; exposing a surface of the composition to a first dose of electromagnetic radiation, wherein the first dose comprises a first property that induces annealing of the composition; and exposing the surface of the composition to a second dose of electromagnetic radiation, wherein the second dose comprises a second property that induces annealing of the composition and wherein the first property is substantially the same or different to the second property.

2. The method of claim 1, wherein the electromagnetic radiation of the first and second doses comprise electromagnetic radiation in a range of any of: UV, visible, near infra-red, and far infra-red spectra.

3. The method of claim 1, wherein the first and second properties of the first and second doses relate to frequencies of electromagnetic radiation.

4. The method of claim 3, wherein either or both of the first and second properties are chosen such that the frequencies of electromagnetic radiation complement an absorption spectrum of the at least one metal oxide.

5. The method of claim 1, wherein the first and second properties of the first and second doses relate to lengths of exposure time.

6. The method of claim 1, wherein at least one of first and second properties of the first and second doses is chosen to functionalize an aspect of chemistry of the surface of composition.

7. The method of claim 1, wherein the composition comprises a first metal oxide material and a second metal oxide material, wherein the first and second metal oxide materials possess different first and second absorption spectra.

8. The method of claim 7, further comprising choosing the first and second absorption spectra of the first and second doses of radiation to complement the absorption spectra of the first and second metal oxide materials.

9. The method of claim 7, wherein the first dose of radiation induces a rate of annealing in the first metal oxide that is higher relative to a rate of annealing induced in the second metal oxide by the first dose, and wherein the second dose of radiation induces a rate of annealing in the second metal oxide that is higher relative to a rate of annealing induced in the first metal oxide by the second dose.

10. The method of claim 9, wherein: the higher rate of annealing in the first metal oxides corresponds to a higher temperature only in a local region of the first metal oxides; and the higher rate of annealing in the second metal oxides corresponds to a higher temperature only in a local region of the second metal oxides.

11. The method of claim 1, wherein an area of the surface of composition exposed is the same between the first and second doses of radiation.

12. The method of claim 1, wherein the surface of the composition comprises a first region and a second region, wherein the first and second regions are separated.

13. The method of claim 12, further comprising: during the exposing the surface of the composition to the first dose of electromagnetic radiation, providing a first mask before the source of electromagnetic radiation, wherein the first mask allows transmission of radiation to the first region of the composition.

14. The method of claim 12, further comprising: during exposing the surface of the composition to the second dose of electromagnetic radiation, providing a second mask before the source of electromagnetic radiation, wherein the second mask allows transmission of radiation to the second region of the composition.

15. The method of claim 1, further comprising providing either or both of the first dose and second dose of radiation in a plurality of pulses, wherein the pulses are interspersed with intervals of non-exposure.

16. The method of claim 1, wherein exposing the surface of the composition to either or both of the first and second doses causes the composition to reach a temperature between 400 and 1500 degree centigrade.

17. The method of claim 1, wherein a total exposure time is less than about 10 milliseconds.

18. The method of claim 1, wherein the composition further comprises a polymer, wherein either or both of the first and second dose causes decomposition of the polymer to a gaseous by-product.

19. The method of claim 1, further comprising providing a second source of electromagnetic radiation in a proximity to the composition, wherein the second source provides the second dose of electromagnetic radiation having the associated second property.

20. A method of manufacturing a sensing device, wherein the sensing device comprises a film comprising at least one metal oxide, the method comprising: providing a precursor composition comprising at least one metal oxide and an organic additive; depositing the precursor composition onto a substrate; exposing a surface of the precursor composition to a first dose of electromagnetic radiation, wherein the first dose has a first property that induces annealing of the precursor composition; and exposing the surface of the precursor composition to a second dose of electromagnetic radiation, wherein the second dose has a second property that induces annealing of the precursor composition, wherein the first property is different than the second property; wherein the film produced as a result of the first and second doses of electromagnetic radiation comprises a structure porous to gas.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0034] Some preferred embodiments of the disclosure will now be described by way of example only and with reference to the accompanying drawings, in which:

[0035] FIG. 1 shows a schematic cross-section of a sensing device with two metal oxide sensing regions under irradiation;

[0036] FIG. 2 shows an example exposure profile comprising a packet of radiation with a sequence of light pulses as achieved by turning the light source on and off;

[0037] FIGS. 3a and 3b show a plan view and cross sectional schematic, respectively, of a shadow mask method;

[0038] FIG. 4 shows data containing emission spectra for a lamp driven at three different driving voltages which control intensity and peak of the bandwidth;

[0039] FIG. 5 shows example absorption spectra for a doped SnO.sub.2 based metal oxide;

[0040] FIG. 6 is a graph with a comparison of film resistance before and after a single 200 s photo annealing pulse obtained by driving the source lamp at various voltages;

[0041] FIG. 7 shows cross-sectional scanning electron microscope (SEM) images of electrically annealed and photo-annealed metal oxide films;

[0042] FIG. 8 shows specific data of resistance profiles of gas sensing devices produced by electrically annealed and photo-annealed during a looped gas test;

[0043] FIG. 9 shows three examples of simulated volumetric (i.e., bulk) temperatures profiles during photo-annealing, after a metal oxide film is exposed to a packet of radiation with increasing number of pulses;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] FIG. 1 shows a schematic cross-section view of a sensing device 100, bearing two metal oxide sensing regions 104a, 104b disposed on top of a dielectric/insulating substrate 106, 108, and contact pads 107. The substrate is disposed on further handles 110, which may comprise silicon. The device is shown under irradiation used to anneal the metal oxide (MOX) sensing regions 104a, 104b. The substrate may comprise silicon, which may generally be crystalline silicon. T1, T2 and T3 have to be intended as the characteristic temperatures achieved on the region in correspondence of the contact pads, the metal oxide materials 104a and 104b. It will be understood that the handles 110 are actually back etched substrates forming a cavity portion between two handles or etched substrates 110. The dielectric region 108 over the etched substrate becomes a dielectric membrane, particularly for this disclosure, it will be understood that the dielectric region 108 directly over the etched cavity portion between two handles/substrates 110 forms the dielectric membrane.

[0045] The metal oxide areas 104a, 104b are disposed over electrodes 105a and 105b, respectively. The electrodes may be used to sense the presence of certain gases. Therefore, the annealed metal oxide regions 104a, 104b may be porous such that they allow the transport of gas from the environment to the electrodes 105a, 105b. The sensing electrodes 105a, 105b may be interdigitated electrodes (IDE), and may be made of a range of suitable electrode materials, e.g. gold. In this example, the electrodes 105a, 105b are disposed above the dielectric membrane 108. In the same way that two MOX material may be present in each of 104a and 104b, two different electrode configurations may be present in each of 104a and 104b. Alternatively, sensing electrodes may also be present above or below the membrane in contact with the sensing material.

[0046] The MOx compositions/film 104a, 104b may define part of a sensing structure. The metal oxide comprise in the sensing structure may comprise a material such as tin oxide, tungsten oxide, zinc oxide, chromium oxide, cerium oxide and indium oxide. The metal oxide material may be pure or doped with other metals. The sensing structure may be a porous layer. The sensing structure may be a gas sensitive layer. The sensing composition forming the sensing structure may be deposited using a technique selected from a group including: screen printing, sputtering, chemical vapour deposition (CVD), atomic layer deposition (ALD), ink-jet, drop coating and flame spray pyrolysis.

[0047] The electrodes 105a, 105b of the can be made of titanium nitride (TiN), tungsten, titanium tungsten (TiW), gold or platinum. TiN, TiW and tungsten are complementary metal-oxide-semiconductor (CMOS) usable materials and thus when these materials are used as electrodes 105a, 105b they can be manufactured within the CMOS process. Gold and platinum are not CMOS compatible and thus when these materials are used as the electrodes 105a, 105b are generally made outside the CMOS processing steps using a post-CMOS process.

[0048] The irradiation 102 is generally in the UV (for example, 10 nm to 400 nm) to visible range (for example, 380 nm to 740 nm) of the electromagnetic spectrum. The wavelengths of light comprised in the exposure/irradiation are may be substantially uniform, i.e. comprising substantially a single wavelength. Alternatively, the exposure/photons may comprise photons having a range of wavelengths. A peak intensity of the irradiation spectrum may overlap with a peak absorption of a metal oxide material comprised in the sensing areas 104a, 104b.

[0049] FIG. 2 shows an example exposure profile 200 comprising a packet of radiation. As mentioned, the radiation generally in the UV to visible range of the electromagnetic spectrum. However, in some examples the radiation may comprise photons in the near-IR region.

[0050] An exposure, or dose, of radiation may comprise a plurality of individual pulses 206 having a particular pulse length. The plurality of pulses may make up a pulse envelope 202, or packet 202, of radiation. The individual pluses are separated by a corresponding plurality of intervals 208 having an interval length. The pulse has an intensity 204 which can be controlled by the light source voltage for instance.

[0051] It is further possible to define a duty cycle, which is the proportion (in units of time) of the total cumulative length of exposure time, compared to the total cumulative time of the overall envelope length (i.e. total annealing time). For example, a pulse length may be 200 ps, with 5 pulses used at a duty cycle of 40%. This results in an overall envelope 202 annealing time length of 2.5 ms, since a duty cycle of 40% defines that the interval length is 300 s in this example.

[0052] In other example, many more pulses than 5 may be used, and the skilled person will further appreciate that various different patterns of envelopes may be used during an annealing process; for example, to tailor an envelope to a particular metal oxide material.

[0053] FIG. 3a shows a plan view of a shadow mask method 300, in which at least one mask 302, 304 may be used to selectively block light/radiation from reached certain metal oxide sensing regions 306, 308. FIG. 3b similarly shows a cross-sectional schematic of a shadow mask method using a shadow mask 302, 304, in conjunction with the device shown in FIG. 1.

[0054] For example, a device may comprise two different metal oxide materials, e.g. one MOx material 104a in a first region 306 and another MOX material 104b in a second region 308. Advantageously, when using photo-annealing, each MOX 104a, 104b and region 306, 308 may readily be subjected to a different spectrum of photo-radiation in order to tailor the MOX material to an emission spectrum of light. For example, a spectrum of radiation in a dose may be chosen to maximise the efficiency of the absorption into the material. This method is not possible when using equilibrium annealing processes such as oven annealing processes, since it is almost impossible in that case to exclude the annealing of a particular MOx region.

[0055] One way of irradiating and annealing the surface of a device with two different doses comprising two different emission spectra, is to use a photomask 302, 304. For example, masks 302 may be placed over 306 such that a first dose exposes only the three of the metal oxide regions, and omits the upper right MOX composition/region of 306. Another exposure may be performed where mask 304 is used, optionally using a different spectrum of light which may be better suited to annealing the upper right MOX region 308. Thus, in the other exposure, irradiation will only occur onto the upper right MOX region.

[0056] Masks may be substantially opaque such that no light or radiation is transmitted through it, except where deliberate gaps spaces are provided as in FIG. 3.

[0057] It will be understood that masks are not necessarily needed in order to provide two (or more) exposures of radiation to a MOX sensors comprising multiple MOX materials. For example, two exposures comprising different wavelengths (e.g. different ranges of electromagnetic radiation) may be directed at a substrate bearing two different sensing regions 306, 308 each having a different MOX composition 104a, 104b. One exposure may be tuned to induce annealing only in the first MOX material 104a, and the other exposure may be tuned (e.g. be chosen to have specific wavelengths of light) specifically to induce annealing only in the other/second MOX materials 104b. In other words, each dose of light/radiation may be substantially better absorbed by only one of the MOX materials, which in turn results in the heating and annealing of only one MOX material at a time. The different exposures comprising different ranges of wavelengths of light may be provided by two different light sources, e.g. two lamps. For example, Ushio Halogen lamps may be used to provide a light source in the Visible to near IR range. Xenon flash lamps may be used to provide exposures of light in the near-UV to visible range.

[0058] Additionally, it will be appreciated by the skilled person that any number of different exposures corresponding to said any number of MOX materials may performed. Similarly, this may correspond to having an arbitrary number of different shadow masks. For example, an environmental sensing device may be required to detect 4 gases, wherein 4 different MOX compositions and/or regions are used, thus requiring a third and fourth dose of electromagnetic region specifically directed at annealing each of the third and fourth species of MOX, respectively.

[0059] FIG. 4 shows data 400 containing emission spectra for a lamp driven at three different voltages. In this example, photons comprising a range of wavelengths are shown, where a maximum intensity of wavelength is generally seen towards the blue/violet and UV range just below 500 nm. Emission spectrum 402 relates to a lamp driven at 800 V; spectrum 404 relates to 600 V; and spectrum 406 relates to 400 V.

[0060] FIG. 5 shows example absorption spectra 500 for a doped SnO.sub.2 based metal oxide. It can be seen that an area of maximum absorbance 502 occurs for this MOX material in the region below 1000 nm, and particularly in the region below 700 nm. Thus, it will be appreciated that an exposure containing light with a spectrum 402, 404, 406 (such as in the FIG. 4 graph 400) will be efficiently absorbed into this doped SnO.sub.2 based metal oxide. For example, the doped metal oxide may be a Pd-Pt-SnO.sub.2 MOX material.

[0061] Thus, is should be appreciated that, by tuning the emissions spectrum, e.g., by controlling lamp voltages as in 402, 404, 406, or by using a cut-off filter in between the light source and the sample, may result in an effective overlap with the absorption spectra of a MOX material, a highly efficient annealing method may be implemented. In more detail, a very high energy density exposure can be implemented, which in turn allows for very short pulses (below 1 ms) to be used. Therefore, photo-annealing allows for rapid annealing processing of MOX films, and a much higher through-put in manufacture of devices which required annealed MOX films.

[0062] An energy density from a photo-annealing method may thus, advantageously, be much greater than an IR-lamp annealing method. For example, a full cycle using an IR lamp annealing method (such as rapid temperature processing, RTP) may produce an energy density of 0.07 kW cm.sup.2 with pulse length of at least 0.2 s. By contrast, an energy density of a photo-annealing method may be up to about 35 kW cm.sup.2, where pulse lengths of only about 25 s-10 ms may be required. Increasingly efficient annealing is especially achievable by maximising an overlap between an emission spectrum of a lamp and absorption of a MOX material.

[0063] The energy density of the infrared lamps is thus much lower (e.g., 0.07 kW/cm.sup.2) than for instance the one offered by a photo-annealing lamp, e.g. a xenon lamps (e.g., 35 kW/cm.sup.2). In addition, absorption of infrared radiation by metal oxide is lower than absorption of radiation within the visible and UV range. Thus, annealing by IR-lamps affords a less effective method than photo-annealing.

[0064] As a result of the very short pulse times achievable by using photo-annealing, and due to the high energy density available, very high annealing temperatures of the MOX surface/composition may be achieved. Moreover, because this high temperature of annealing may be achieved in a short period of time, active substrate cooling is not necessary (as it may be with IR-lamp methods). Thus, the equipment may generally be simpler, and faster, than for IR-lamp or RTP annealing processes. Electrodes 105, for example gold IDE (interdigitated electrodes) may be particularly susceptible to damage due to high temperatures. Therefore, photo-annealing provides a safer means of annealing MOX sensing regions 104a, 104b, because lower temperatures may be maintained in the immediately regions (105, 106, 107, 108) underneath the MOX layer 104a, 104b.

[0065] FIG. 6 is a graph with a comparison of film resistance before and after a photo annealing step. A single 200 s pulse was used to produce the graph with the lamp driven at increasing voltages thus resulting in larger energy density (i.e., intensity) and larger shift towards UV. The left-hand bars 602 at each voltage level correspond to devices before photo-annealing, and the right-hand bars 604 correspond to devices after photo-annealing. It can be seen that the resistance of a MOX film is dramatically reduced after a single photo-annealing pulse.

[0066] FIG. 7 shows cross-sectional plan view scanning electron microscope (SEM) images 700 of annealed metal oxide films. The lower images correspond to higher resolution images of the upper images.

[0067] The lighter layer on the upper surface, with multiples pores of varying sizes, is the annealed MOX film. MOX films 702, 704 of the right hand side correspond to films which have been photo-annealed. MOX films 706, 708 on the left have been annealed using a combination of RTP (i.e. using an IR lamp), and an electrical annealing step.

[0068] Generally, an advantage of the high temperatures and rapid heating accessible with photo-annealing, the sintering of the MOX film may be improved during annealing. That is, sintering may be improved relative to annealing using conventional annealing method, for example which may have thermal equilibrium and prolonged heating. The porosity of the MOX film may generally be tuned using photo-annealing. For example, micro-porosity in a post-annealed MOX film may be reduced using photo-annealing. Thus, photo-annealed films 702 and 704 comprise fewer micro-pores than conventionally annealed MOX films 706, 708.

[0069] Moreover, due to the improved/reduced properties of micro-porosity, the structure of the MOX film may be advantageously strengthened using Photo-annealing. The reduction in micro-pores may be caused by the rapid annealing, in which materials used in a precursor composition of the MOX film are rapidly decomposed during annealing. Decomposition may comprise formation of gases, e.g. CO.sub.2 and H.sub.2O.

[0070] FIG. 8 shows specific data 800 of resistance profiles of gas sensing devices during a looped gas test. The resistance response of a sensing region comprised within the MOX is shown, corresponding to various gases: methane, NO.sub.2, Acetone, toluene, CO, ethanol, and Hydrogen. The gas sequence is repeated over time.

[0071] The resistance can be seen to reduce depending on increasing cycle number, for devices (upper lines, 802) which are annealed using conventional electrical and RTP annealing. This overall change, e.g. reduction, in resistance may be defined as drift. However, devices annealed using photo-annealing (804, lower lines) generally show much less drift. Therefore, there is evidence that photo-annealing provides an advantageously stable advice, with greater reliability, and which exhibits less variation with time.

[0072] As mentioned, various components underlying the MOX sensors may be sensitive to high temperatures, such as interdigitates electrodes, for example made of gold. Damage of an IDE due to excess heat may cause undesirable drift to occur in a device after successive cycles. Advantageously, use of photo-annealing in a MOX device 100 containing such sensitive electrode 105a, 105b may mitigate potential damage and drift in devices. This is result of the rapid heating of the MOX surface, and corresponding rapid heat dissipation of heat to the surrounding surface.

[0073] FIG. 9 shows three examples of simulated profiles of the volumetric (bulk) temperatures 900 during photo-annealing, after a metal oxide film has been exposed to a pulse of radiation. Each enveloped was based on a series of pulse of light, with pulse duration of 200 s, duty cycle 40%, and pulse interval length 300 s. A voltage of 460 V was used to power the lamp providing the source of light. Further data is tabulated below:

TABLE-US-00001 Energy Surface Volumetric Envelope Number density/mJ temperature/ temperature/ FIG. length/ms of pulses cm.sup.2 C. C. 902 2.5 5 3.16 1030 425 904 10 20 9.57 2420 1190 906 50 100 19.4 2580 1610

[0074] Thus, it can be seen that very high annealing temperatures may be achieved with only very short pulses. The rapid decay of the temperatures profiles 902, 904, 906 also has the advantage that heat is dissipated very quickly away from the MOX film. Therefore, heating of an adjacent or underlying substrate is minimised, and so no active substrate cooling is needed. Moreover, this is advantageous in examples where the substrate contains heat-sensitive materials. For example, the bulk Aluminium oxidises at above around 380 C., which may exclude its use in a substrate when conventional annealing techniques are used. However, due to rapid heat dissipation of photo-annealing, aluminium may safely be used in substrates.

[0075] As illustrated in FIG. 9, and in the SEM images of FIG. 7, a state of MOX can be reached that is not accessible in an equilibration process (such as oven annealing). For instance, a higher temperature can be achieved using photo-annealing (although for a short time) than using RTP (using pulses of IR radiation and IR-lamps), or oven annealing. In the latter processes MOX temperature is in a thermal equilibrium with the surrounding environment under given operational conditions.

[0076] Using light allows further advantages to be achieved in addition to efficient annealing. For example, particular wavelengths of light may be used in order to tune the surface chemistry of a MOX composition, for instance changing the concentration of hydroxyl groups and thus wettability and electrical conductivity of the metal oxide. In addition, UV light may enhance the decomposition of polymer additives by photo-catalysis. For example, light e.g. UV light may induce production of excitons (e.g. electron-hole pairs) in a MOX material. Photo-generated holes may migrate towards the surface reacting, with the polymer additives i.e., oxidizing, and further enhancing the thermal decomposition of these molecules. In other examples oxygen vacancies may be altered in the MOX film, which again may modulate the response of a gas sensing device to certain gases. Generally, it will be appreciated that the versatility of the photo-annealing process in general, an in particular the ability to functionalise a surface of a MOX sensing region, allows to tune a sensor's response to particular gases.

Index of Figure References:

[0077]

TABLE-US-00002 FIG. reference Corresponding language 100 Model structure 102 Photons, irradiation, exposure 104a, 104b Sensing material, metal oxide film, composition 105a, 104b Electrodes, sensing electrodes, interdigitated electrodes 106, 108 Insulating/dielectric material 107 Contact pads 110 (Silicon) substrate, handles 200 Exposure profile, packet 202 Pulse envelope length 204 Lamp voltage, pulse intensity 206 Light pulse length 208 Pulse interval/duty cycle 300 Shadow mask method 302, 304 Shadow mask 306, 308 Metal oxide films, sensing material 400 Emission spectra 402, 404, 406 Voltage dependent emission profiles 500 Absorption spectrum 502 High absorbance region, UV region 504 Low absorbance region, IR region 600 Annealing comparison 602 Before photo-annealing 604 After photo-annealing 700 Annealing morphologies SEM 702, 704 Photon-annealing morphology/micro-porosity 706, 708 RTP + electrical annealing morphology/micro- porosity 800, 802, 804 Cycle vs resistance: Comparison of photo- annealing method 900 Example annealing results 902, 904, 906 Pulse-dependent annealing temperatures

[0078] The skilled person will understand that in the preceding description and appended claims, positional terms such as top, above, overlap, under, lateral, etc. are made with reference to conceptual illustrations of a device, such as those showing standard cross-sectional perspectives and those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to a device when in an orientation as shown in the accompanying drawings.

[0079] Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.