WO2004055880A1

WO2004055880A1 - Annealing method and device

Info

Publication number: WO2004055880A1
Application number: PCT/GB2003/005499
Authority: WO
Inventors: Paul Peter Donohue; Carl John Anthony; Michael Andrew Todd
Original assignee: Qinetiq Limited
Priority date: 2002-12-18
Filing date: 2003-12-16
Publication date: 2004-07-01
Also published as: AU2003292427A1; GB2396481A; GB0229427D0

Abstract

A method for annealing a film, such as a ferroelectric or magnetic film is described. The method comprises the steps of (i) taking a film carried on a temperature sensitive substrate, the film having a radiation absorbing structure formed thereon, and (ii) illuminating the radiation absorbing structure with radiation produced by a laser. The radiation absorbing structure is heated sufficiently by the radiation to anneal all or some of the film without exceeding the temperature budget of the temperature sensitive substrate.

Description

Annealing Method and Device.

This invention relates to a method for annealing films, and in particular to a method for annealing ferroelectric films.

Recently, there has been a considerable amount of research into the development of devices which utilise the thermal properties of ferroelectric materials. One example is the development of infrared (TR) imaging cameras based on two-dimensional arrays of ferroelectric thermal detectors. Such detectors have proved attractive due to their near ambient temperature operation.

Thermal detectors used for infra-red imaging rely on the temperature change of the sensing material due to absorption of infra-red radiation. In the case of ferroelectric materials this radiation causes a change in the electrical polarisation of the material enabling the magnitude of the change in temperature to be detected.

In order to reduce the size of the detectors, integrated ferroelectric devices have been developed in which the ferroelectric material is combined with the electronic read out circuitry in a single device. Typically, these integrated circuit (IC) devices comprise layered structures with a thin layer of ferroelectric sputtered or spin coated or otherwise deposited onto or above one or more base layers. Other examples of such integrated ferroelectric devices are thin film piezoelectric actuators and ferroelectric random access memories (FeRAM).

The combination of the ferroelectric material with the active circuitry in one package produces a more compact device than- the provision of a separate read out circuit and improves yield, reduces cost and improves performance. However, a fundamental problem with such devices is the need to deposit the ferroelectric material within a thermal budget that is compatible with the integrated circuitry not being damaged or destroyed by the elevated temperatures. It is widely recognised that exposure of an integrated circuit (e.g. a CMOS circuit) to temperatures above 450°C is a constraint on the processing of chips/materials with IC content, and this conflicts with the growth requirements of many ferroelectric layers.

A particularly important family of ferroelectric materials in use and under investigation for IR detector, actuator or FeRAM applications is the perovskites. This family include materials such as lead scandium tantalate (PST), lead zirconate titanate (PZT), barium strontium titanate (BST), lead titanate (PT) and others. For use as a ferroelectric the material layer must be in the perovskite phase. It can either be deposited directly into that phase at an elevated temperature or at a lower temperature which is then subsequently annealed into the ferroelectric perovskite phase. Layers deposited at low temperatures are generally in an amorphous, pyrochlore or other phase which is incapable of exhibiting ferroelectricity. For PST, for example, the material must be deposited at temperatures in excess of 450°C to enter the perovskite phase. Direct depositing of these materials in a perovskite phase is therefore incompatible with the temperature budgets of integrated circuitry.

One known way of providing a layer of ferroelectric material in the perovskite phase without damaging an IC provided on a base layer is to deposit the material in a non- ferroelectric state at a low temperature (say less than 450°C). The material may then be annealed using a laser to heat the layer sufficiently to convert the material into its perovskite phase. An example of such a laser annealing technique is described in US5310990.

In order to heat the ferroelectric layer sufficiently without damaging the underlying integrated Circuitry, a laser wavelength is chosen which is strongly absorbed by the ferroelectric layer. Typically the laser radiation is pulsed and the temporal width of the pulse is kept sufficiently short such that the heat diffusion length is small enough to prevent the induced heat wave from penetrating through the various layers to the IC layer.

Commercially available excimer lasers generate ultraviolet (UN) radiation of a wavelength that is strongly absorbed by typical ferroelectric layers such as PST and PZT. Typically such lasers generate pulses having relatively short pulse durations; for example around 25ns. Although such short pulses can anneal thin PST layers (up to say lOOnm in thickness), the energy density required to generate sufficient temperature at the bottom of a thick layer of PST will significantly increase the surface temperature at the top (i.e. the irradiated surface) of that layer. The surface heating effect can cause surface damage, poor crystallisation and crystal quality, poor film physical integrity and loss of stoichiometry due to evaporation of volatile components.

In addition to surface heating effects, known laser annealing techniques also lack control over the orientation of the crystalline phase that is grown during the annealing process. For thicker films, nucleation will occur as a bulk effect in a similar fashion to bulk ceramic ferroelectric materials with the consequent random, or at least mixed, orientation of the grain crystal axes.

WO 00/54317 describes an annealing process in which a temporal extender is provided to deliver a laser pulse at a slower rate than is possible with a non-extended commercially available laser source. The extended pulse increases the diffusion length through the material thus allowing thicker films to be annealed without surface damage. Although the pulse extension technique mitigates some of the problems associated with excessive surface heating in thicker films, the problems associated with nucleation and growth control remain.

According to a first aspect of the invention, a method for annealing a film comprises the steps of (i) taking a film carried on a temperature sensitive substrate, the film having a radiation absorbing structure formed thereon, and (ii) illuminating the radiation absorbing structure with radiation produced by a laser, wherein the radiation_^ absorbing structure is heated sufficiently by the radiation to anneal all or some of the film without exceeding the temperature budget of the temperature sensitive substrate. The present invention thus permits a film to be annealed without any heat induced damage of the underlying substrate. Furthermore, the use of a radiation absorbing structure to convert the incident radiation to heat minimises any damage to the surface of the film that may arise from excessive heating. The present invention is thus advantageous over the prior art techniques described above in which surface heating of the film itself is used during the annealing process. In particular, the present invention will permit films having a thickness greater than around lμm to be annealed without any significant damage to the surface of the film.

Furthermore, the present invention provides absorption which is independent of the thickness of the film. In other words, the film does not have to be a certain minimum thickness to ensure that there is sufficient absorption of the incident radiation. This is particularly advantageous in FeRAM applications where the layer of ferroelectric material used is typically very thin and would thus absorb only a small amount of any radiation directly incident on it.

Herein, the term anneal is used to mean the permanent alteration of the properties of a material by heating. For example, in the case of ferroelectric films, annealing includes converting all or some of a film from a non-ferroelectric state into a phase capable of exhibiting ferroelectricity. Annealing in the context of ferroelectric films

/ would also include improving the ferroelectricity being exhibited, for example by converting more of the film into a ferroelectric state.

A person skilled in the art would recognise that the term "film" as used herein means a deposited layer, or a plurality of deposited layers, of material. Those skilled in the art may also refer to a "film" as a '-'thin film" (noting that theterm "thin" is not typically taken to mean a film of any particular thickness) or a coating. The term film is thus used herein to describe a layer of material formed (e.g. spin coated, sputtered or grown) on a substrate as opposed to a piece of bulk material.

Conveniently, the radiation absorbing structure comprises a nucleation layer in contact with the film. The presence of a nucleation, or seeding, layer is advantageous because it helps to initiate growth of the required crystalline phase of the film which is being annealed.

Preferably, the nucleation layer comprises platinum. Alternatively, the nucleation layer may comprise a conducting oxide. For example, a layer of perovskite structure conducting oxide material such as Lanthanum Nickelate or Strontium Ruthenate may be used. Such materials can be closely lattice matched to perovskite ferroelectric materials and provide a suitable template for perovskite growth.

Advantageously, at least one layer of the radiation absorbing structure is a dielectric layer. In other words, the radiation absorbing structure includes a dielectric layer, for example on its external surface (i.e. the surface furthermost from the substrate), to maximise the amount of energy from the radiation that is retained within the radiation absorbing structure for conversion into heat. Conveniently, the dielectric layer comprises one or more layers of Silicon Dioxide.

Preferably, the dielectric layer has a thickness substantially equal to an odd multiple of a quarter of the wavelength of the radiation propagating within the dielectric layer. This increases the amount of radiation coupled into the dielectric layer and increases the heating effect that is obtained. It would be appreciated by the skilled person that, for a dielectric layer with a finite optical absorption, the maximum absorption condition would be shifted very slightly (e.g. by a few percent) from the quarter wavelength condition due to optical absorption effects.

Advantageously, the radiation absorbing structure is a resonant absorber structure. For ex--mple,*the radiation absorbing structure may comprise both a nucleation layer and a dielectric layer. In such a structure, the dielectric layer is arranged to maximise the amount of radiation absorbed by nucleation layer.

The radiation absorbing structure may also comprise an outermost metallic layer (e.g. a thin Titanium layer). A three layer stack may thus be formed that comprises a nucleation layer in contact with the film, a dielectric layer formed on the nucleation layer and an outermost metallic layer formed on the dielectric layer. The nucleation- dielectric-metal layer structure further enhances the absorption properties of the radiation absorbing structure. A variety of alternative multiple layer radiation absorbing structures could be used.

Conveniently, the method further comprises the initial step of depositing the radiation absorbing structure on the film that is carried on the temperature sensitive substrate.The actual deposition process used will depend on the type of radiation absorbing structure required. As described above, the radiation absorbing structure may comprise a single layer, or a plurality of layers.

Advantageously, the radiation absorbing structure may be formed so as to partially cover the film. In other words, a patterned radiation absorbing structure may be used to allow selective annealing of parts of the film. Regions of the film not covered by the radiation absorbing structure may or may not be masked (e.g. with a highly reflective material) as required. This arrangement overcomes the need for beam shaping (e.g. as described in US5310990) when selectively annealing. Although forming a radiation absorbing structure that partially covers the film is preferred for selective annealing, selective exposure of portions of a radiation absorbing structure may also be used.

Preferably, the film is a mixed oxide ferroelectric film. It should be noted that the mixed oxide material may not actually exhibit ferroelectric properties when initially deposited on the substrate; i.e. it may be deposited at a low temperature and hence initially be in a non-ferroelectric phase. However, the term ferroelectric film shall herein mean any film that is, or is capable of being annealed to become, ferroelectric.

Advantageously, the ferroelectric film comprises low grade deposited perovskite phase material and the annealing step improves the quality of the perovskite material. Conveniently, the film comprises material deposited substantially in the non- perovskite phase and the annealing step converts some or all of the material into the perovskite phase.

The ferroelectric film is preferably any one of lead scandium tantalate (PST), lead zirconate titanate (PZT), strontium bismuth tantalate (SBT), barium strontium titanate (BST) or lead titanate (PT).

Advantageously, the method may further comprise the step of removing some or all of the radiation absorbing structure after the film has been annealed.

Conveniently, the substrate comprises an integrated circuit which may comprise Silicon. For example, it may comprise polysilicon on a glass substrate.

Advantageously, the substrate is flexible. For example, the substrate could comprise polyamide or a plastic.

Advantageously, the step of illuminating the radiation absorbing structure with radiation produced by the laser comprises illuminating the radiation absorbing structure with a pulse of radiation. It should be noted that the pulse of radiation with which the radiation absorbing structure is illuminated may be produced by the laser in a number of ways. For example, a pulsed laser source may be used or the output from a CW laser could be intensity modulated (e.g. chopped). Alternatively, any other means of exposing a specific portion of the radiation absorbing structure to a short burst of radiation could be used. For example, a CW laser could be rapidly scanned across the radiation absorbing structure; this would also locally illuminate regions of the radiation absorbing structure with a short duration burst (i.e. a pulse) of radiation.

Preferably, the step of illuminating the radiation absorbing structure with a pulse of radiation produced by the laser is repeated a plurality of times Conveniently, an excimer laser is used to produce the pulse of radiation. Alternatively any other suitable pulsed laser source, such as a transverse excited atmospheric carbon dioxide (TEA CO2) laser, may advantageously be used to produce the pulse of radiation.

Furthermore, the pulse of radiation may advantageously be produced by a pulsed laser and pulse extension means. For example, a temporal pulse extender of the type described in WO 00/54317 could be used.

Conveniently, the film is carried by the substrate in a microbridge arrangement. In other words, the entirety of the film may not be in contact the substrate; i.e. an air gap may be provided to thermally isolate the film from the substrate.

According to a second aspect of the invention, a method of producing an infra-red detector comprises the method according to the first aspect of the invention

According to a third aspect of the invention, a method of producing a film device comprises the steps of (i) forming a film on a temperature sensitive substrate, and (ii) forming a radiation absorbing structure on the film, wherein the radiation absorbing structure is adapted such that it can be heated sufficiently by a pulse of radiation of a given wavelength to anneal all or some of the film without exceeding the temperature budget of the temperature sensitive substrate.

Conveniently, the method further comprises the step of illuminating the radiation absorbing structure with a pulse of radiation of the given wavelength.

Advantageously, the step of forming a radiation absorbing structure on the film comprises the step of depositing a nucleation layer directly on the film.

Preferably, the step of forming a radiation absorbing structure on the film comprises the step of depositing a dielectric layer. Conveniently, the radiation absorbing structure formed is a resonant absorber structure.

The film may conveniently be a mixed oxide ferroelectric film.

Preferably, the method may further comprising the step of removing some or all of the radiation absorbing structure after the film has been annealed.

According to a fourth aspect of the invention, a device incorporates a film layer annealed using the method of the first or third aspect of the invention.

According to a fifth aspect of the invention, an intermediate film device comprises a pre-annealed film carried on a substrate comprising an integrated circuit wherein the film has a radiation absorbing structure formed thereon. The intermediate film device may advantageously be used in a method according to the first aspect of the invention. The intermediate film device may advantageously be fabricated using the method of the third aspect of the invention.

Preferably, the film is a ferroelectric film.

Conveniently, the radiation absorbing structure comprises a nucleation layer in contact with the film.

The invention will now be described, by way of example only, with reference to the following drawings in which;

Figure 1 illustrates a prior art annealing process;

Figure 2 illustrates an annealing process according to the present invention;

Figure 3 shows the predicted absorption properties of a Silicon Nitride absorber layer as a function of absorber layer thickness for 10.6μm radiation; Figure 4 shows the theoretical absorption properties of a Silicon dioxide absorber layer as a function of absorber layer thickness for 10.6μm radiation;

Figure 5 shows the predicted absorption properties of a Silicon dioxide absorber layer as a function of absorber layer thickness for 10.6μm radiation using measured silicon dioxide optical properties,

Figure 6 illustrates the absorption properties of a Silicon dioxide absorber layer capped with a thin layer of titanium using measured optical properties,

Figure 7 shows the predicted absorption properties of a Silicon Nitride absorber layer as a function of absorber layer thickness for 308nm radiation; and

Figure 8 shows the intensity of radiation as a function of depth through a structure of the type described with reference to figure 7.

Referring to figure 1, a typical prior art annealing technique is illustrated. A laser pulse 2 is directed to the surface of a PST ferroelectric film 4 of thickness d which is located on a substrate 6.

The substrate 6 is formed from a number of layers, but only the uppermost electrode layer 8 is shown separately. A skilled person would recognise that the substrate 6 will comprise various adhesion and barrier layers and integrated circuitry (e.g. CMOS). As described above, it is the integrated circuitry of the substrate which is particularly temperature sensitive and may be destroyed, or damaged, if exposed to an elevated temperature. In the case of CMOS circuitry, exposure to temperatures exceeding around 450°C causes permanent damage. In other words, the substrate 6 has a temperature budget that must not be exceeded during the annealing process.

Substrates formed as so-called microbridge structures are also known. Typical microbridge structures are described in more detail with reference to figures 1 and 10 of WO 00/54317 the content of which is incorporated herein by reference thereto. In a typical microbridge structure the ferroelectric film is deposited on a substrate having a sacrificial layer which can subsequently be removed to provide an air gap. This enhances the thermal isolation of the ferroelectric film from the substrate, thereby enhancing device performance. The term substrate as used herein should thus be taken to mean anything on or above which the ferroelectric film is located.

The PST ferroelectric film 4 may be deposited on the substrate 6 using any one of a variety of known techniques. For example, chemical solution deposition, RF magnetron sputtering, metal-organic chemical vapour deposition or laser ablation. To prevent thermal damage to the substrate 6, the PST ferroelectric film is deposited on the substrate at relatively low temperatures (typically less than 450°C). The deposited PST layer is thus initially in an amorphous or pyrochlore phase that does not exhibit ferroelectricity.

The laser pulse 2 is typically produced by an excimer laser (e.g. a KrF excimer laser which produces radiation having a wavelength of 248nm) and is strongly absorbed by the PST ferroelectric film. The relatively short duration of the laser pulse results in localised heating of the ferroelectric film, primarily, at its upper surface 10, without any significant heating of the substrate 6. This localised heating causes the PST film layer to be converted from a non-ferroelectric state to a ferroelectric form; i.e. the ferroelectric film is annealed. As described in WO 00/54317, P. P. Donohue, PhD Thesis, University of Southampton, 2001 and Donohue and Todd, Integrated Ferroelectri.es, Vol. 31, pp285-296, 2000, the laser pulse duration may also be extended which can reduce surface damage when thicker films are annealed.

As described above, there are various problems associated with known laser annealing techniques. For example, it can prove difficult to provide full conversion to the required ferroelectric phase throughout the thickness of the ferroelectric film. In other words, existing techniques can only provide a limited depth of phase conversion of the film, especially in lead containing ferroelectric films. Although the conversion depth can be increased using pulse-extension, it remains limited. Another problem associated with known laser annealing techniques is the inability to control the orientation of the required crystalline phase as it is grown. For thicker films, say greater than lμm in thickness, nucleation occurs as a bulk effect in a similar fashion to bulk ceramic ferroelectric materials with the consequent random, or at least mixed, orientation of the grain crystal axes.

Referring to figure 2, an annealing technique of the present invention is illustrated. Elements common to those described with reference to figure 1 are assigned like reference numerals.

In accordance with the invention, a dual layer radiation absorbing structure 20 is located on the PST ferroelectric film 4. The radiation absorbing structure 20 comprises a first layer 22 and a second layer 24. The first layer 22 is formed from a quarter wavelength (at the laser wavelength) coating of silicon dioxide. The second layer 24 comprises a layer of Platinum that is sufficiently optically thick to prevent a significant amount of radiation passing to the underlying film 4.

The first and second layers in combination provide a structure that efficiently absorbs the incident laser radiation pulse 2, thereby heating the second layer 24 to a temperature that is sufficient to nucleate growth of the ferroelectric phase in the film. 4. The ferroelectric phase then grows downwards (i.e. away from the interface of the second layer 24 and the film 4) and preferably converts the entire thickness of the film. Using the method of the present invention thus prevents the excessive surface heating of the ferroelectric film that is often associated with prior art direct heating techniques.

In this example, the second layer is formed from Platinum that will also act as a seeding layer to aid nucleation and growth of the ferroelectric phase. The seeding effect reduces the activation energy, and hence the temperature to which the film must be heated, required to initiate growth of the ferroelectric phase. Although platinum is preferred, the second layer 24 could alternatively comprise another metal, a conducting oxide (such as lanthanum nickelate or strontium ruthenate) or an insulating oxide. The use of a material which acts as a seeding layer is preferred, but it is not essential.

The second layer 24 could alternatively comprise a material that provides control over the orientation of the growth of the ferroelectric phase. For example, a seeding function similar to that obtained using Platinum would be provided by perovskite structure conducting oxides. Such conducting oxides can also be closely lattice matched to perovskite ferroelectrics, thus providing a growth template for the required perovskite growth. This template effect provides control over the orientation of the grown ferroelectric phase. The ability to gain orientation control during ferroelectric film growth is particularly important in devices that rely on having a significant component of the polar axis of the ferroelectric in a particular direction.

It would be appreciated by the skilled person that the duration and intensity of the laser pulse 2 would be selected so as to heat the second (e.g. platinum) layer 24 sufficiently to anneal the underlying film 4 without exceeding the temperature budget of the substrate 6. If required, a laser pulse-extension technique of the type described in patent application WO00/54317 could be used.

As an approximation, the thermal diffusion length (d) within a layer can be expressed as:

where D is the diffusivity and τ is the pulse length. The diffusivity (D) is given by:

D = - (2)

P-C₀ where K is the thermal conductivity, p is the density and C_p is the heat capacity. In this manner, the preferred pulse length can be estimated for films of a given thickness.

After laser annealing, the first layer 22 could be removed using an appropriate micro-machining technique. The second layer 24 could also be removed or, if it is sufficiently conducting, all or part of it could be retained to serve as a top electrode for the ferroelectric layer. Once an insulating, or poorly conducting, second layer has been removed a high conductivity metal electrode could subsequently be deposited to provide an electrical contact.

To maximise absorption of laser radiation, the properties of the absorbing material (e.g. layer thickness) forming the first layer 22 are tailored to the particular wavelength of the laser used. Although silicon dioxide is described above, numerous alternative dielectric materials could be used (e.g. a silicon nitride etc). In fact, the first layer could comprise any material which assist absorption of the incident radiation pulse. In particular it is advantageous, although not essential, to select a material for the first layer 22 which maximises absorption of the radiation by the second layer 24; this ensures as much heat as possible is coupled in to the film.

Figure 3 shows the predicted absorption (curve 30) as a function of Si₃N₄ layer thickness for 10.6μm radiation incident on Si₃N₄ topped stack. The stack comprises an Si N layer (which is illuminated with the 10.6μm radiation) located on a O.lμm thick layer of platinum which is carried by a PST film.

It can be seen that Si₃N₄ layers of a thickness "greater than around lμm provide absorption in excess of 30%. Furthermore, such absorption is almost independent of layer thickness for any layer thickness greater than around 2μm. The thickness of the layer can thus be selected to provide the required (e.g. maximum) amount of absorption. It should be noted that the choice of Si₃N₄ layer thickness will also require consideration to be given to its heat capacity; i.e. a thicker layer will have a heat capacity that limits the temperature rise in the underlying layers. Referring to figure 4, the predicted absorption as a function of SiO₂ layer thickness for 10.6μm radiation incident on SiO₂ topped stack is shown (i.e. curve 40). This calculation uses the optical properties of SiO₂ contained in the Handbook of optical constants by E. D. Palik, Academic Press, 1991. The stack comprises an SiO₂ layer (which is illuminated with the 10.6μm radiation) located on a O.lμm thick layer of platinum which is carried by a PST film. Local absorption maxima are observed for stacks comprising SiO₂ layers around 1.8μm and 4.5μm in thickness. Again, the layer thickness can be selected to provide the desired level of absorption bearing in mind the heat capacity of the material.

Referring to figure 5, the absorption (curve 42) and reflection (curve 44) from a stack of the type described with reference to figure 4 are illustrated using the optical properties of an SiO₂ film as measured over the required wavelength range using ellipsometry. In this case, local absorption maxima are observed for stacks comprising SiO₂ layers around 1.3μm and 3.6μm in thickness. It can be seen that near total absorption is predicted from such a structure having a 3.6μm SiO₂ layer.

The data presented in figures 4 and 5 illustrates the kind of absorption levels that can be achieved using an SiO₂/Pt absorber structure. The skilled person would appreciate that the actual optical properties obtained from a device having such a structure would depend on the precise optical properties of the layers that were used. In a practical device for example, the SiO₂ layer will have optical properties that are dependent on the deposition process conditions. However, it can be seen that high absorption levels will occur across a wide range of different SiO₂ layers.

A thin metal layer (e.g. titanium) may also be added to the top surface (i.e. the surface furthest from the PST film) of a SiO₂/Pt stack of the type described above with reference to figure 5. In this manner, a Ti/SiO₂/Pt absorber structure is formed. Figure 6 shows the absorption properties (curve 46) of such a Ti/SiO2/Pt absorber stack, located on a PST film, as function of SiO₂ layer thickness. It is assumed in the optical calculation that the Ti layer is 2.7nm thick, and the platinum layer is O.lμm thick. The absorption properties of the SiOs/Pt absorber structure shown in figure 5 are re-plotted in figure 6 (again as curve 42) for comparative purposes.

It can be seen from figure 6 that, for SiO₂ layers less than lμm thick, the addition of the thin Ti layer (which is only 2.7nm thick and hence of negligible thermal mass) gives an advantage either in terms of reduced SiO₂ thickness required for a given absorption (and hence lower thermal loading) or greater absorption for a given SiO₂ thickness. For example, with 10.6μm radiation and an SiO₂ layer of 0.6μm thickness, the Ti/SiO₂/Pt structure gives around 39% absorption whereas the SiO₂/Pt structure achieves only 15%. To realise a 39% absorption in the SiO₂/Pt structure the SiO₂ thickness must be increased to 0.84μm thereby increasing the thermal loading for the same absorption level as the Ti/SiO₂/Pt structure.

A TEA CO₂ laser can be used to produce radiation having a wavelength of 10.6μm. Typical TEA CO₂ lasers can output relatively high optical powers and have a pulse duration that is controllable and can be significantly longer than typical excimer laser devices.

A standard excimer laser may also be used to illuminate the first layer, and figure 7 shows the absorption properties (curve 50) as a function of Si₃N₄ layer thickness for 308nm radiation incident on Si₃N₄ topped stack. The silicon nitride layer is located on a O.lμm thick layer of platinum which is carried on a PST film. It can be seen that the absorption maxima of the stack approaches 90% at this wavelength, however the optical power that can be output by a typical excimer laser is generally significantly lower than that produced by a typical TEA CO₂ laser.

Referring to figure 8, the intensity of 308nm radiation as a function of depth for a stack of the type described with reference to figure 7 is shown. A first region 60 corresponds to the silicon nitride layer, a second region 62 corresponds to the O.lμm platinum layer and third region 64 is the PST film. It can be seen that the majority of the radiation is absorbed at the surface of the Platinum layer (i.e. in the second region 62). In this manner, the platinum layer is heated thereby nucleating growth of the perovskite phase of the PST film.

In accordance with the teachings contained herein, the skilled person would select a laser wavelength and absorbing material(s) that are optimised for the particular film to be annealed. It would also be appreciated by a person skilled in the art that radiation absorbing structures other than the dual layer resonant structure described above could be used to implement the present invention. For example, a single layer of material that strongly absorbs the laser radiation could be employed. Alternatively, a multiple layer stack (e.g. a stack comprising three or more layers) could be used.

Although ferroelectric films are described in the above examples, it should be noted that the annealing technique of the present invention is equally applicable to other types of film. For example, the present invention could be used to anneal magnetic films etc. Alternatively, the annealing of amorphous Silicon films carried on glass or plastic substrates as used in film transistor (TFT) liquid crystal display applications would be possible.

Claims

1. A method for annealing a film comprising the steps of ;

(i) taking a film carried on a temperature sensitive substrate, the film having a radiation absorbing structure formed thereon, and

(ii) illuminating the radiation absorbing structure with radiation produced by a laser,

wherein the radiation absorbing structure is heated sufficiently by the radiation to anneal all or some of the film without exceeding the temperature budget of the temperature sensitive substrate.

2. A method according to claim 1 in which the radiation absorbing structure comprises a nucleation layer in contact with the film.

3. A method according to claim 2 in which the nucleation layer comprises platinum.

4. A method according to claim 2 in which the nucleation layer comprises conducting oxide.

5. A method according to any preceding claim in which the radiation absorbing structure comprises a plurality of layers.

6. A method according to any one of the preceding claim in which the radiation absorbing structure 6omprises a dielectric layer.

7. A method according to claim 6 in which the dielectric layer comprises one or more layers of Silicon Dioxide.

8. A method according to any one of claims 6 to 7 in which the dielectric layer has a thickness substantially equal to an odd multiple of one quarter of the wavelength of the radiation within said dielectric layer.

9. A method according to any one of claims 6 to 8 in which an outermost metallic layer is carried by the dielectric layer.

10. A method according to any one of the preceding claims in which the radiation absorbing structure is a resonant absorber structure.

11. A method according to any preceding claim wherein the radiation absorbing structure is formed so as to partially cover the film.

12. A method according to any one of the preceding claims and further comprising the initial step of depositing the radiation absorbing structure on the film that is carried on the temperature sensitive substrate.

13. A method according to any one of the preceding claims in which the film is a mixed oxide ferroelectric film.

14. A method according to claim 13 in which the ferroelectric film comprises low grade deposited perovskite phase material and the annealing step improves the quality of the perovskite material.

15. A method according to claim 13 in which the film comprises material deposited substantially in the non-perovskite phase and the anneahng step converts some or all of the material into the perovskite phase.

16. A method according to any one of claims 13 to 15 in which the ferroelectric film is any one of lead scandium tantalate (PST), lead zirconate titanate (PZT), strontium bismuth tantalate (SBT), barium strontium titanate (BST) or lead titanate (PT).

17. A method according to any one of the preceding claims and further comprising the step of removing some or all of the radiation absorbing structure after the film has been annealed.

18. A method according to any one of the preceding claims in which the substrate comprises an integrated circuit.

19. A method according to claim 18 in which the integrated circuit comprises Silicon.

20. A method according to any one of claims 1 to 18 in which the substrate is flexible.

21. A method according to any preceding claim wherein the step of illuminating the radiation absorbing structure with radiation produced by the. laser comprises illuminating the radiation absorbing structure with a pulse of radiation.

22. A method according to claim 21 wherein the step of illuminating the radiation absorbing structure with a pulse of radiation produced by the laser is repeated a plurality of times.

23. A method according to any one of claims 21 to 22 in which an excimer laser is used to produce the pulse of radiation.

24. A method according to any one claims 21 to 22 in which a transverse excited atmospheric carbon dioxide (TEA CO2) laser is used to produce the βulse of radiation.

25. A method according to any one of claims 21 to 24 wherein the pulse of radiation is produced by a pulsed laser and pulse extension means.

26. A method according to any one of the preceding claims in which the film is carried by the substrate in a microbridge arrangement.

27. A method of producing an infra-red detector comprising the method according to any one of preceding claims.

28. A method of producing a film device comprising the steps of;

(i) forming a film on a temperature sensitive substrate, and

(ii) forming a radiation absorbing structure on the film,

wherein the radiation absorbing structure is adapted such that it can be heated sufficiently by a pulse of radiation of a given wavelength to anneal all or some of the film without exceeding the temperature budget of the temperature sensitive substrate.

29. A method according to claim 28 and further comprising the step of illuminating the radiation absorbing structure with a pulse of radiation of the given wavelength.

30. A method according to any one of claims 28 to 29 in which the step of forming a radiation absorbing structure on the film comprises the step of depositing a nucleation layer directly on the film.

31. A method according to any one of claims 28 to 30 in which the step of forming a radiation absorbing structure on the film comprises the step of depositing a dielectric layer.

32. A method according to any one of claims 28 to 31 in which the radiation absorbing structure formed is a resonant absorber structure.

33. A method according to any one of claims 28 to 32 in which the film is a mixed oxide ferroelectric film.

34. A method according to any one of claims 28 to 33 and further comprising the step of removing some or all of the radiation absorbing structure after the film has been annealed.

35. A device incorporating a film layer annealed using the method as claimed in any one of the preceding claims.

36. An intermediate film device for use in the method of any one of claims 1 to 27 comprising a pre-annealed film carried on a substrate, wherein the film has a radiation absorbing structure formed thereon.

37. An intermediate film device comprising a pre-annealed film carried on a substrate comprising an integrated circuit wherein the film has a radiation absorbing structure formed thereon.

38. A device according to claim 36 or 37 wherein the film is a ferroelectric film.

39. A device according to any one of claims 36 to 38 wherein the radiation absorbing structure comprises a nucleation layer in contact with the film.