Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/34—Deposited materials, e.g. layers
  • H10P14/3402—Deposited materials, e.g. layers characterised by the chemical composition
  • H10P14/3404—Deposited materials, e.g. layers characterised by the chemical composition being Group IVA materials
  • H10P14/3411—Silicon, silicon germanium or germanium
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/34—Deposited materials, e.g. layers
  • H10P14/3402—Deposited materials, e.g. layers characterised by the chemical composition
  • H10P14/3434—Deposited materials, e.g. layers characterised by the chemical composition being oxide semiconductor materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
  • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
  • H10P14/38—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by treatments done after the formation of the materials
  • H10P14/3802—Crystallisation or recrystallisation of non-monocrystalline semiconductor materials, e.g. regrowth
  • H10P14/3808—Crystallisation or recrystallisation of non-monocrystalline semiconductor materials, e.g. regrowth using laser beams
  • H10P14/3816—Pulsed laser beam
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P34/00—Irradiation with electromagnetic or particle radiation of wafers, substrates or parts of devices
  • H10P34/40—Irradiation with electromagnetic or particle radiation of wafers, substrates or parts of devices with high-energy radiation
  • H10P34/42—Irradiation with electromagnetic or particle radiation of wafers, substrates or parts of devices with high-energy radiation with electromagnetic radiation, e.g. laser annealing

Definitions

• the invention relates to processing a target material to cause a thermally-induced change in the target material, such as crystal growth or improved uniformity.
• the invention is particularly relevant to processing semiconductor materials during fabrication of electronic devices such as displays, but is applicable to other materials.
• Polycrystalline silicon is used in large area electronics as the active and/or doped layers in thin- film transistors.
• Poly-Si may be used for example in TFTs of a display such as an LCD or AMOLED.
• a TFT consists of a source, drain and gate.
• the poly-Si gate material When integrated on a display device, the poly-Si gate material must be highly conductive.
• PV photovoltaic
• the poly-Si can be formed by solid-phase crystallization of amorphous silicon (a-Si) where the substrate (e.g. glass) can support the relatively high temperatures involved (at least 300°C).
• a-Si amorphous silicon
• the substrate e.g. glass
• the relatively high temperatures involved at least 300°C.
• substrates such as the plastic substrates that may be needed for manufacturing digital displays on flexible screens, or where other temperature sensitive components are present in the vicinity of the a-Si to be crystallized, different techniques are needed.
• laser crystallization which uses short, high intensity UV laser pulses to heat the a-Si above the melting point of silicon without melting or damaging the substrate and/or any surrounding temperature sensitive components.
• This process incorporates processes known as super lateral growth (SLG) and sequential lateral solidification (SLS).
• SSG super lateral growth
• SLS sequential lateral solidification
• the laser pulse is absorbed by the material, resulting in a change in phase by heating from an amorphous solid to a molten state, followed by rapid cooling to form a polycrystalline material.
• the duration of the laser pulse and size of the heat affected zone must be controlled carefully.
• the temperature of amorphous material must be increased significantly to enable nucleation of crystal domains.
• a-Si has a crystallization enthalpy of about 11.95 kJ/mol, with reported "bulk" crystallization temperatures in the range of 600 to 800°C.
• SLG and SLS are processes that have been optimized to create this localized temperature and to establish directional crystals on rapid cooling. By controlling the cooling of the molten silicon it is possible to form poly-Si with a controllable range of crystallite sizes.
• excimer laser sources have been used to improve the crystallinity of poly-Si materials (or convert a-Si to poly-Si).
• the lasers operate at 308nm in the UV so as to couple energy into the silicon, which is transparent at visible wavelengths.
• Long line beams e.g. 750mm x 0.030mm
• the long line beam is to ensure that large area displays (TVs, public screen etc) can be covered at the highest speed up to "Generation 10" (G10) sheets (2880mm x 3130mm).
• Excimer lasers have a low repetition rate (300-600Hz) and to effectively implement the re -crystallisation of the order of 20 laser pulses are typically required at each location. This limits the speed of processing.
• the uniformity of the beams also needs to be kept high to ensure that each transistor is annealed to the same extent. There can be about 1 billion TFT transistors dispersed throughout a typical G10 panel.
• Heat retention layers may be included to assist excimer-based laser-induced crystallization processes.
• Such heat retention layers comprise dielectric materials such as oxides and nitrides of silicon. They are deposited adjacent to the amorphous layer which is to be crystallised.
• the heat retention layers have a thickness typically of the order of one micron or greater.
• the heat retention layers remain attached to the material throughout the duration of the laser induced crystallization process.
• a patterned dielectric layer, acting as a heat retention layer may be used to preferentially assist the laser-induced crystallization process where the dielectric layer is directly adjacent to the layer which is to be crystallised.
• a dielectric layer is used because it is thought that metallic layers would inhibit the crystallization process in this context.
• MIC metal-induced crystallization
• a-Si thin film to be crystallized at temperatures as low as 140°C by annealing the a-Si while it is in contact with a metal film such as aluminium, gold or silver, which acts as a catalyst for the crystallization process.
• a metal film such as aluminium, gold or silver
• the annealing required for an MIC process is generally applied macroscopically and maintained for a much longer time than the heating associated with laser crystallization. The annealing is not performed using a laser.
• MIC generally involves: 1) bond weakening of the amorphous material at the interface with the metal, which enhances the mobility of amorphous atoms at the interface; 2) interface effects providing fast, short-circuit diffusion pathways for transportation of the amorphous atoms; and 3) interface thermodynamics, which further favour crystal growth at interfaces.
• MIC processes There are two types of MIC processes which exist currently for a-Si.
• the first of these MIC processes occurs for example where crystalline aluminium is used as the catalyst layer on a-Si. Silicon atoms preferentially wet interfaces with high angle grain boundaries of the aluminium at temperatures above 140°C. Si crystallization is nucleated when a critical thickness of the wetted a-Si layer is reached. A compressive stress arises in the aluminium layer and a tensile stress arises in the a-Si layer. As the tensile material has space to accommodate more atoms, this gradient in stress drives a diffusion process which results in aluminium grains from the aluminium layer being dissolved in the a-Si layer.
• the second of the MIC processes leads to a metal semiconductor compound being formed with a minimal lattice mismatch to the surrounding area.
• NiSi2 is formed at the crystalline nickel a-Si interface.
• Crystalline silicon precipitates from the NiSi2 phase and migrates into the a-Si material. This results in needle-like crystal structures of crystalline Si being formed. The more the process is seeded at the crystal interface, the more crystalline material is formed.
• a method of processing a target material comprising: irradiating a multilayer structure with a radiation beam, the multilayer structure comprising at least a target layer comprising the target material and an additional layer not comprising the target material, wherein: the additional layer is metallic; the target layer is irradiated through the additional layer during the irradiation of the multilayer structure; and a transfer of energy from the radiation beam to the target layer and to the additional layer is such as to cause a thermally-induced change in the target layer, the thermally-induced change comprising one or more of: crystal growth in the target material, increased carrier mobility in the target material, increased chemical stability in the target material, and increased uniformity of electrical properties in the target material.
• a method which allows thermally-induced changes to be driven efficiently in selected regions of a target material, with minimal or no impact to other regions of the target material and/or surrounding layers or nearby device structures.
• Time and energy costs are lower in comparison with alternative approaches in which the entire layer of target material is processed at the same time (e.g. by scanning an excimer laser over the surface).
• the thermally-induced changes may comprise any combination of crystal growth, increased carrier mobility, increased chemical stability, and increased uniformity of electrical properties.
• the inventors have found that the metallic nature of the additional layer promotes the thermally-induced changes (e.g. crystal growth). This was somewhat surprising given that in the context of excimer-based laser-induced crystallization metallic layers are thought to inhibit crystallization.
• the distribution of electrons in the target layer is modified by the emission of electrons from the surface of the metallic additional layer to the ambient through photoelectric and thermionic emission following the irradiation.
• the charge imbalance which results following such emission temporarily alters the electron density in both additional and target layers and thereby temporarily affects (e.g. reduces) the bond strength of the constituent atoms in the target layer.
• the temporary effect on the bond strength assists with one or more of crystal growth in the target material, increased carrier mobility in the target material, increased chemical stability in the target material, and increased uniformity of electrical properties in the target material.
• the charge neutrality of the thin multilayer structure is restored through its connection to an electrical path to ground and through its surface via the interaction with suspended ions and electrons impacting the multilayer structure from the ambient.
• the method is particularly applicable to forming poly-Si from a-Si selectively in TFT gate regions of large area electronic arrangements using for manufacturing displays, as well as in PV based devices.
• the method allows crystallinity to be increased with minimal or no impact to transparency.
• the method is also compatible with roll-to-roll or sheet-to-sheet high volume manufacturing platforms using flexible substrates, due to the very low thermal load applied to the substrate. Plastic substrates can be used without risk of melting or damage.
• the method is also applicable to other transparent conductive materials, for example for displays.
• the thermally-induced change in this context may comprise local crystallization, which may improve carrier mobility.
• the method is particularly applicable to promoting crystallization, improving carrier mobility, improving uniformity, and/or improving chemical stability, of indium gallium zinc oxide (IGZO).
• IGZO indium gallium zinc oxide
• the method is also applicable to the crystallisation of other dielectrics such as metal oxides in thin film battery technologies.
• the method is also applicable to reducing the resistance of metallic tracks, such as tracks formed using nanoparticle inks or other simple low temperature deposition processes.
• the grain size of deposited metals can be increased by matching the frequency of the radiation beam to the resonant plasmon frequency of the nanoparticle ink or grain size of the deposited metal. Efficiency can be improved by providing a well-defined particle size distribution.
• the irradiation of the multilayer structure causes detachment of at least an irradiated portion of the additional layer from the target layer after the thermally-induced change in the target layer (e.g. crystal growth) has occurred.
• the additional layer spontaneously detaches from the target material, after it has served its purpose of promoting thermally-induced change in the target material, without any additional processing steps being required.
• a fluence of the radiation beam is selected such that the thermally-induced change (e.g. crystal growth) in the target layer is achieved without any portion of the target layer entering a molten phase during the irradiation by the radiation beam.
• the avoidance of the molten state improves the quality of the crystal growth in the target material, favouring development of a uniform distribution of small grains.
• the fluence of the radiation beam is selected such that at least a portion of the target layer does enter the molten state during at least a portion of the irradiation by the radiation beam. Entry into the molten state enables larger crystallites to be formed where this is desirable.
• the target layer and the additional layer are configured such that energy from the laser radiation is transferred from the electrons of the additional layer to the lattice of the target material more quickly than energy from the laser radiation is transferred from the electrons of the additional layer to the lattice of the additional layer.
• This effect favours effective incubation of the target layer by the additional layer during the thermally-induced change (e.g. crystal growth).
• the subsequent thermalisation of the energy stored in the electron system of the additional material to the lattice of the additional material may conveniently cause detachment of the additional layer from the target layer after the thermally -induced change has occurred.
• Figure 1 schematically depicts irradiation of a multilayer structure according to an embodiment
• Figures 2(a)-(f) are optical microscopy images of a TFT pattern ((a)) and magnified views of a gate region at different stages of processing by irradiation ((b)-(f));
• FIG. 3 depicts ranges of different Fluence Regimes
• Figure 4 depicts SEM images of Si after no processing ((a)) and after processing by irradiation in different Fluence Regimes ((b)-(c));
• Figure 5 depicts Raman spectra for a-Si, nc-Si, and c-Si;
• Figure 6 depicts AFM images of silicon material after processing in each of four Fluence Regimes.
• Figure 7 depicts images illustrating processing of a multilayer structure comprising Al as a target material and Mo as an additional layer.
• Embodiments of the present disclosure relate to processing a target material to cause a desired thermally-induced change in the target material.
• the thermally -induced change typically involves crystal growth in the target material.
• the crystal growth may lead to conversion of an amorphous material to a polycrystalline material.
• the crystal growth may lead to an increase in the average size of crystallites in a target material that was already polycrystalline before the processing.
• the crystal growth is such as to reduce the resistivity, or increase the mobility of charge carriers, in the target material by reducing scattering from irregularities in the target material, such as grain boundaries or other deviations from perfect lattice periodicity.
• the thermally-induced change may comprise one or more of the following in any combination: crystal growth in the target material, increased carrier mobility in the target material, increased chemical stability in the target material, and increased uniformity of electrical properties in the target material.
• the method comprises irradiating a multilayer structure 10 with a radiation beam 5.
• the multilayer structure 10 comprises at least a target layer 2 and an additional layer 1.
• the additional layer 1 is metallic.
• the target layer 2 comprises, consists essentially of, or consists of, the target material.
• the additional layer 1 does not comprise the target material.
• the target layer 2 is irradiated through the additional layer 1 during the irradiation of the multilayer structure 10.
• the radiation beam 5 passes partially through the additional layer 1 to reach the target layer 2. Energy from the radiation beam 5 is deposited in the additional layer 1 and in the target layer 2.
• the multilayer structure 10 comprises a stack comprising the target layer 2 and the additional layer 1 on a substrate 3.
• the substrate 3 is a flexible substrate, for example formed from a plastic or a glass or a ceramic.
• the substrate 3 may, for example, be configured to be compatible with a roll-to-roll or sheet-to-sheet manufacturing process.
• One or more further layers may be provided as needed, including one or more layers on top of the additional layer 1 , between the additional layer 1 and the target layer 2, and/or between the target layer 2 and the substrate 3.
• the radiation beam 5 and multilayer structure 10 are configured (e.g. by appropriate selection of fluence and/or pulse length in the case of the radiation beam and/or by appropriate selection of materials and layer thicknesses in the case of the multilayer structure 10) such that a transfer of energy from the radiation beam 5 to the target layer 2 and to the additional layer 1 is such as to cause the desired thermally-induced change in the target material (e.g. crystal growth).
• a transfer of energy from the radiation beam 5 to the target layer 2 and to the additional layer 1 is such as to cause the desired thermally-induced change in the target material (e.g. crystal growth).
• the additional layer 1 incubates the target layer 2 and prolongs a period during which the temperature of the target material is high enough to promote thermally induced change, such as crystal growth (via solid state diffusion of atoms in the heated lattice) while at the same time allowing the peak rate of energy transfer to be kept low enough to avoid any melting of the target material during the irradiation. It is normally desirable to avoid melting because this can have a negative effect on the quality of crystalline material produced in the target layer 2 (where the aim is to promote crystallization).
• a polycrystalline structure having a sub-optimal distribution of crystallite sizes can be produced, including for example excessively large crystallites which can cause non-uniformities in structural and/or electronic properties.
• desired thermally-induced changes may benefit from entry into the molten phase, in which case a fluence of the radiation beam may be selected such that at least a portion of the target layer enters the molten state during at least a portion of the irradiation by the radiation beam.
• the radiation beam is partially absorbed by the electrons in the material of the additional layer 1. This absorption creates ballistic electrons in the additional layer 1. The excited electrons remain ballistic until they undergo scattering. Electrons can scatter elastically or inelastically with other electrons or with the lattice ions.
• the thickness of the additional layer 1 is chosen to enable ballistic electrons to propagate to the interface between the additional layer 1 and target layer 2.
• the additional layer 1 has a thickness of less than 200nm, optionally less than 150nm, optionally less than lOOnm, optionally a thickness in the range of 20-70nm.
• the ballistic electrons transfer energy to other electrons creating a hot electron gas in the additional layer 1 characterised by an electron temperature.
• the resulting hot electron gas diffuses throughout the additional layer 1 before it couples to the lattice of the additional layer 1.
• the material of the additional layer 1 and the laser parameters are selected to ensure that this time frame is relatively long (e.g. 3-50 picoseconds). Once the electrons couple to the lattice the lattice of the additional layer 1 heats and an elevated lattice temperature is generated.
• the heated electrons in the additional layer 1 exchange energy with the target material.
• the electron energy can be thought of as a wave which partially reflects and transmits through the interface between the additional layer 1 and the target layer 2.
• the excited or heated electrons in the additional layer 1 exchange energy with the target material at the interface between the two materials in a number of ways.
• the laser excited ballistic electrons can penetrate into the target material and exchange energy with the target material's electronic and lattice sub-systems through electron-electron scattering, impact ionisation and electron lattice coupling.
• hot electrons in the additional layer 1 can interact with the electrons and lattice sub-systems of the target material by electron-electron scattering and electron-phonon coupling at the interface.
• the lattice of the heated additional layer 1 can also couple energy to the lattice of the target material through phonon- phonon coupling at the interface.
• the radiation beam is also partially transmitted through the additional layer 1 to the target layer 2.
• the radiation beam thus also directly heats electrons in the target material.
• the electrons thus heated or exchanged between layers transfer energy to the lattice of the target material.
• This transfer of energy from the electrons to the lattice occurring at the interface or in the target material ideally occurs in the target material before it occurs in the additional layer 1.
• energy from the laser radiation is transferred from the electrons of the additional layer 1 to the lattice of the target material more quickly than energy from the laser radiation is transferred from the electrons of the additional layer 1 to the lattice of the additional layer 1.
• the additional layer 1 desirably detaches spontaneously from the rest of the multilayer structure 10.
• the additional layer 1 temporarily incubates the heated lattice of the target material while the additional layer 1 is present on the multilayer structure 10. Any incubation is limited in time due the detachment of the additional layer 1 which takes place within 1 microsecond to 1 millisecond after the laser pulse is incident on the multilayer structure.
• Fluence Regime I At very low fluence (referred to herein as Fluence Regime I), the additional layer 1 is eventually heated enough that it detaches from the multilayer structure 10, but the heating is not sufficient to cause significant growth of crystals in the target material in the meantime, even with the incubation effect mentioned above.
• the combination of direct absorption of the radiation beam by the target material and incubation by the additional layer 1 provides a temperature profile over time in the lattice of the target material suitable for the desired thermally-induced change (e.g. crystal growth) to occur without melting of the target material.
• the energised lattice of the target material is insufficient to disrupt the additional layer 1 at this stage (where the lattice of the additional layer 1 is still relatively cold), so the additional layer 1 remains in contact with the multilayer structure 10 and continues to incubate.
• This effect is facilitated by selection of the target material and the material of the additional layer 1 such that energy from the laser radiation is transferred from the electrons of the additional layer 1 to the lattice of the target material more quickly than energy from the laser radiation is transferred from the electrons of the additional layer 1 to the lattice of the additional layer 1.
• the target material comprises a-Si
• the material of the additional layer 1 comprises a metal, such as Mo.
• Fluence Regime III At higher fluence (referred to herein as Fluence Regime III), the coupling of electronic energy to the lattice of the additional layer 1 occurs over a shorter timescale which results in the heating of the lattice of the additional layer 1 being faster than that observed at lower fluences. This impacts the transfer of electronic energy from the additional layer 1 to the target layer 2 and thereby limits the formation of a desired thermally-induced change (e.g. a crystalline phase), for example to an outer annulus of a radiation beam in the case where a Gaussian beam is used for the radiation.
• the additional layer 1 separates from the multilayer structure 10 too quickly, reducing the electron diffusion and incubation effects discussed above.
• Fluence Regime IV At still higher fluence (referred to herein as Fluence Regime IV), the additional layer 1 is photomechanically disrupted as in Regime III but the applied fluence also melts the target material, which as discussed above can lead to suboptimal crystallization in the target material.
• Figure 2(a) depicts an optical microscopy image of a TFT pattern comprising gate regions.
• Figure 2(b) depicts a magnified microscopy image of a region corresponding to one of the gate regions after a target layer comprising a-Si has been applied over the gate region.
• Figures 2(c)-(f) depict the region of Figure 2(b) after an additional layer (comprising Mo) has been applied as a coating on the target layer and, subsequently, the resulting multilayer structure has been processed by a Gaussian radiation beam spot at different fluencies.
• the additional layer consisted of a 40nm thick layer of Mo deposited by magnetron sputtering.
• the Gaussian radiation beam spot was produced using a pulsed femtosecond laser delivering 500 fs pulses at a wavelength of 1030 nm.
• the scanning system was coupled to a machining stage (for accurate sample positioning) through a combination of reflectors and mirrors.
• a galvanometer based beam scanning system was used to scan the laser beam spot by adjusting the speed of steering mirrors.
• the laser was operated at maximum power and attenuated using a combination of half-wave plate and polarizer to keep the optimal beam shape and get higher pulse to pulse stability.
• Figure 2(c) depicts the result of irradiation at a fluence of 45 mJcm 2 (the peak fluence associated with the Gaussian beam spot).
• the fluence is high enough to cause detachment of the additional layer (Mo) in a central region of the beam spot.
• the additional layer remains in place elsewhere.
• the fluence is not high enough to cause conversion of the a-Si to poly-Si anywhere.
• Figure 2(c) thus depicts Fluence Regime I in the region where the additional layer detaches.
• Figure 2(d) depicts the result of irradiation at a fluence of 82 mJcm 2 (the peak fluence associated with the Gaussian beam spot).
• the fluence in this case causes detachment of the additional layer (Mo) over a larger region.
• crystal growth is promoted in the a-Si, leading to formation of a nanocrystalline form of poly-Si (ns-Si).
• ns-Si nanocrystalline form of poly-Si
• the region where ns-Si is observed corresponds to a region where the fluence was in Fluence Regime II and incubation by the additional layer 1 was effective to cause crystal growth in the target material.
• Figure 2(e) depicts the result of irradiation at a fluence of 145 mJcm "2 (the peak fluence associated with the Gaussian beam spot).
• the fluence in the central region corresponding to Fluence Regime III
• the rate of energy coupling is increased which means the additional layer is heated at a similar rate to the target layer.
• This heating distorts the additional layer and modifies its optical and electronic properties in the central part of the beam.
• nc-Si is formed in an annular region around the central region corresponding to a region where the fluence was in Fluence Regime II. Outside of the annular region of nc-Si the fluence drops below the level necessary to form nc-Si (corresponding to Fluence Regime I), and a-Si is again observed in an outer annular ring.
• Figure 2(f) depicts the result of irradiation at a fluence of 320 mJcm 2 (the peak fluence associated with the Gaussian beam spot).
• the fluence is so high that it causes melting of the a-Si in a central region (where the fluence was in Fluence Regime IV), leading to formation of a microcrystalline silicon phase (c-Si).
• Figure 3 depicts ranges of fluence corresponding to the four Fluence Regimes for the example implementation in which the target layer 2 comprises a-Si and the additional layer 1 comprises Mo.
• Figure 4 depicts SEM images of (a) unexposed a-Si in a gate-like region, (b) nc-Si produced by applying a relatively low energy pulse (in Fluence Regime II) through an additional layer 1 comprising Mo, and (c) c-Si produced by melting and re-solidifying Si by applying a higher energy pulse (in Fluence Regime IV).
• Figure 5 depicts Raman spectra of an a-Si layer, an nc-Si region, and a c-Si region.
• the nc- and c-Si regions were produced using a single pulse lying in Fluence Regimes II and IV respectively.
• Figure 6 depicts AFM images of silicon material after processing in each of the four Fluence Regimes.
• Fluence Regime II provides a desirable fine grained uniform distribution of crystallites.
• Fluence Regime IV provides large crystallites but less uniformity.
• Figures 2-6 discussed above, illustrate experimental results obtained with a-Si as the target material and Mo as the additional layer.
• the method is applicable to other combinations of materials.
• the other materials may comprise one or more of the following: a transparent conductive material; a dielectric material; a metal; a metal oxide; IGZO.
• Figure 7 depicts the result of applying the processing to a multilayer structure comprising Al as the target material and Mo as the additional layer.
• Figure 7(a) depicts selective removal of the Mo without apparent damage to underlying Al film.
• Figures 7(b)-(d) are AFM images showing crystal growth in the underlying Al at progressively increasing fluencies, respectively 0.4 Jem "2 , 0.8 Jem "2 , and 1.4 Jem "2 .
• Figures 7(b)-(c) depict fine grained crystal growth arising under Fluence Regime II without any melting of the Al occurring.
• Figure 7(d) depicts large grains resulting from melting of the Al at high fluence (Fluence Regime IV).
• the irradiation of the multilayer structure 10 causes detachment of at least an irradiated portion of the additional layer 1 from the target layer 2 after the thermally-induced change (e.g. crystal growth) in the target layer 2 has occurred.
• the detachment may occur because the additional layer 1 is not well adhered to the target layer 2 in the first place (e.g. because the additional layer 1 and the target layer 2 have different crystal structures and do not mix).
• the radiation beam heats the electrons in the additional layer 1 and this in turn heats the lattice of the additional layer, the lattice of the additional layer 1 will want to expand but is restrained by the target layer 2 (i.e. there is differential thermal expansion).
• the detachment is convenient because it avoids the need for separate processing to remove the additional layer 1.
• the detachment may be facilitated by appropriate selection of the material of the additional layer 1. Detachment will be favoured by choosing the material of the additional layer 1 such that it has thermal properties that are significantly different from those of the target material and/or any layers below the target material in the multilayer structure 10, for example significantly different electron-phonon coupling (e.g. with the additional layer having a longer electron- phonon coupling time), melting temperature, expansion coefficient, and/or emissivity.
• the additional layer 1 is of a material that forms a crystalline structure of different symmetry and/or significantly different lattice parameters than the target material at room temperature and pressure.
• the target material comprises silicon, which forms a diamond-like structure (comprising interpenetrating fee structures)
• the additional layer 1 may be formed from a material that adopts a bec structure at room temperatures and pressures (such as Mo).
• the additional layer 1 is applied in such a way that it only loosely adheres to the multilayer structure 10, for example using evaporation or sputtering.
• the material of the additional layer 1 may be chosen such that it does not mix with the target material in the target layer 2 or with any other layer below the target layer 2 in the multilayer structure 10.
• the additional layer 1 still contributes to thermally-induced change (e.g. crystal growth) in the target material despite being removed because of the time scales of the processes involved.
• the incubation is performed before the additional layer 1 detaches.
• the radiation beam may be a pulsed laser beam.
• a single pulse of the radiation beam is capable of causing the crystal growth in the target material. More than one pulse of radiation could, however, be applied to a given region of the target material if desired. Multiple pulses could allow further control of the crystallisation process.
• a plurality of pulses are applied to each region of the target material where crystal growth is to be promoted, wherein, in each said region, a first pulse causes crystal growth and a later pulse causes further crystal growth.
• a second timed laser pulse may be used to further energise an excited electron in a target when transferred from the additional layer to a target material consisting of a semiconductor or dielectric, for instance.
• the laser pulse duration is less than Ins, optionally less than lOOps, optionally less than 50ps, optionally less than lOps, optionally less than lps, optionally less than lOOfs, optionally less than 50fs.
• pulse durations in the 10s of picoseconds or 10s of femtoseconds may be used.
• the radiation beam comprises laser radiation in the IR, visible or UV spectra.
• the distribution of grain sizes present in the target material after processing by the method depends on the properties of the radiation beam used to irradiate the multilayer structures (particularly the fluence and duration of each pulse that is applied, and the number of pulses). As mentioned above, relatively small grains are desirable to achieve uniform mechanical and/or electrical properties.
• the grain size distribution is controlled so that an average grain size (defined as the cube root of the grain volume) is less than lOOnm, optionally less than 50nm, optionally less than 20nm.
• the additional layer 1 comprises molybdenum or tungsten.
• the target material comprises a-Si
• the additional layer comprises molybdenum or tungsten and fluence provided by a single pulse of the radiation beam is in the range of 50-125 mJcm "2
• the electrons energised in the target material when heated by the radiation transmitted through the additional layer couples to the lattice in a shorter time frame than that which occurs for electrons in the additional layer.
• such transmitted radiation may be absorbed by multiphoton or avalanche absorption, leading to rapid heating in ceramic materials such as metal oxides.

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