CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase filing under 35 U.S.C. § 371 of PCT/GB2016/050575 filed on Mar. 4, 2016 and entitled “PHOTOEXCITATION METHOD,” the disclosure of which is incorporated herein by reference.

FIELD OF INVENTION

The invention relates to improvements in photoexcitation. Particularly, but not exclusively, it relates to methods of photolithography and composite photoresists.

BACKGROUND TO THE INVENTION

Manufacturing of nanometer scale structures has applications in a wide range of fields. Progress in the semiconductor industry is driven by reducing the minimum feature size attainable by lithography. Technological advancement is in the fields of photonics, meta-surfaces and MEMs/Sensors is similarly reliant on this Critical Dimension shrinkage.

In general, the minimum feature size of a lithographic process is limited by the wavelength of the light used. Ultraviolet (UV) photolithography is the most commonly employed patterning method, whilst the next generation of lithographic techniques (such as Extreme UV Lithography, Directed-Self-Assembling (DSA) and Nanoimprint Lithography (NIL)) are being investigated and developed. For applications requiring sub-100 nm resolution, electron-beam lithography is used almost exclusively.

The laser ablation of polymeric films was proposed as an alternative lithographic approach, allowing for direct writing applications. Despite extensive investigations, the best resolution in laser ablation processes has been limited to dimensions above 1 micron and only recently has submicron resolution been demonstrated, requiring the use of high energy, femtosecond lasers. Similarly, whilst investigations have been made regarding the applications of polymers, polyimides and dye doped systems, to laser ablation processes, the use of organic polymers as lithographic materials in laser ablation processes has been limited to low resolution applications.

In general, the resolution of laser ablation is limited by the spot size of the laser used, with spot size being proportional to the laser wavelength. Equally, the resist coating used in laser ablation lithography must typically absorb high energy, low wavelength photons in order to degrade and allow ablation of the exposed areas beneath.

The use of high energy, low wavelength lasers have a number of associated disadvantages including unwanted absorption in air (with such systems therefore requiring expensive and complicated vacuum environments), collateral damage to the sample, and ionisation and charging of certain materials. Equally, low wavelength lasers suffer from a low depth of focus which restricts the thickness of the photoresist and the depth of topography of the etched substrate below.

Accordingly, the ability to achieve high resolution lithography without the need for high energy lasers is desirable.

An object of the present invention is to mitigate some of the deficiencies of the prior art mentioned above.

SUMMARY OF THE INVENTION

In a first aspect of the invention, there is provided a method of photoexcitation comprising the steps of providing a composite, the composite comprising a first material having at least a first energy level and a second energy level, and a second material having at least a first energy level and second energy level, wherein the energy gap between the first and second energy level of the first material is less than the energy gap between the first and second energy level of the second material, and wherein the first material and second material are in communication with one another, providing a photon source configured to emit a photon, wherein the energy of the photon is equal to or greater than the energy gap of the first material but less than the energy gap of the second material, exposing the composite to the photon such that an electron is excited from the first energy level of the first material to the second energy level of the first material, and wherein said electron transfers from the second energy level of the first material to the second energy level of the second material.

Providing a composite with the energy level structure described above allows for an electron to be excited into the second energy level of the second material indirectly via a lower energy excitation in the first material. This lower energy excitation requires less energy than a direct excitation from the first energy level of the second material to the second energy level of the second material, and can be accomplished using photons with an energy lower than the energy gap of the second material. As such, the need for high energy photons (photons with an energy equal to or greater than the energy gap of the first material) is obviated, along with their associated disadvantages as noted above.

Typically the second energy level of the first material is substantially equal to the second energy level of the second material. This improves the ease of electron transmission between the first and second materials.

The term ‘energy level’ refers to the quantised, discrete values of energy of bound particles in quantum mechanical systems, typically electrons. These include the lowest energy level, otherwise known as the ground state or zero point energy. These levels may also be referred to as energy shells or simply shells. The “first energy level” described above for both the first and second material may be the ground state i.e. the zero point energy of the materials or a higher energy level. The first and second energy levels of the first and second materials do not necessarily need to be the lowest and highest energy levels associated with those materials respectively. It is however typically the case that the second energy is higher than the first energy level in a given material.

Further the ‘energy gap’, referred to herein is the energy range, or the distance in energy space between the discrete energy levels defined above. This could also be described as the difference in energy between two discrete energy levels.

Reference to ‘electron transfer’ in the specification refers to the process by which the electron in the second energy level of the first material moves to the second energy level of the second material. This may also be referred to as ‘energy transfer’, ‘photosensitization’ or the first material donating an electron, the second material receiving an electron, or the second material absorbing energy from the first material.

It is important that the first and second material are ‘in communication’ with each other. The skilled person will appreciate that if the first and second materials are isolated from one another, it is not possible for any electron transfer to occur. Therefore, the term, in communication” is intended to describe that the first and second materials are able to transmit and receive respectively an electron as described above. Accordingly, the two materials could be connected via a single interface at which the reaction can occur or may involve dispersing one material within another to maximise the contact area between the two materials.

Typically the first material further comprises one or more intervening energy levels between the first and second energy levels of the first material and wherein the energy of the photon is equal to or greater than the energy gaps between each of the energy levels of the first material but less than the energy gap of the second material.

Reference to ‘intervening energy levels’, is intended to mean energy levels located on the energy scale between the first and second energy levels of the first material.

This enables an electron to reach the second energy level of the first material by a series of smaller steps through the energy levels in the first material. Once promoted to the second energy, via the intervening energy levels, the electron can transfer from the first material to the second energy level of the second material. By providing more “rungs” to this energy level ladder, an electron can be excited up the levels by a plurality of photons each photon having a much lower energy than the energy gap between the first and second energy levels of the second material, in what is called a multi-stage photoexcitation. These lower energy photons are easier to generate and, unlike high energy photons, do not cause collateral damage to material, are not absorbed by air and have a good depth of focus.

This multi-stage photoexcitation process requires the photon flux to be above a certain threshold such that electrons in the first material can be continually promoted into higher excited states (i.e. gaining higher energy levels) until they can transfer into the second material, without decaying back down to their initial, unexcited energy state, or ground state.

The term ‘low energy photons’ or ‘lower energy photons’, is intended to describe photons having an energy less than the energy gap between the first and second energy levels of the second material. In other words, photons that are incapable of directly exciting an electron from a first energy level of the second material into a second energy level of the second material.

Typically the photon source is a laser. This allows for the provisions of a controllable source of photons of consistent energy. Further, the exposed area of the composite can be set by the spot size of the laser. Both the spot size and/or the wavelength of the laser can be varied as desired. Furthermore, the pulse duration and/or pulse amplitude of the laser can also be varied. By varying these parameters, it is possible to vary the exposure of the composite material and adjust the dimensions of the resulting features on the substrate. There is no particular limitation on the photon source used and indeed the use of any source of a focused light beam is envisaged. As discussed above, a threshold exists in the photon flux, above which the photosensitization process will commence. By using a photon source which has a Gaussian beam profile, the photon source can be tuned such that only the very centre of the beam is above this threshold. This allows for the photosensitization of an area of the second material smaller than the spot size of the photo source. In the case where the composite is a photoresist used in photolithography, this leads to an increases in the resolution of the features attainable.

Typically the laser is operated in a continuous wave mode. This provides a steady, constant and controllable stream of photons to the composite.

Typically the laser is a diode laser. Diode lasers typically require less power than most gas or chemical laser systems.

Typically the first material is an aromatic molecule. Typically a polyaromatic molecule. The inventors have found that such molecules are useful photosensitizers, having an energy level structure that makes it well suited as a first material in the claimed invention.

Typically the first material is a dye. The term “dye” is intended to refer to any compound which has an energy level structure as described above for the first material. Typically, the “dye” is an organic dye. Reference to “organic dye” is intended to cover those compounds containing a chromophore i.e. a group responsible for producing colour. Typically, these groups are aromatic regions of a molecule which absorb radiation. Although term “chromophore” typically relates to regions responsible for colour, it is also envisaged that such groups may be suitable for absorbing radiation outside the visible spectrum.

Typically the dye is a perylene or a coumarin. Typical examples of dyes include aminoanthracene or anthracene. The inventors have found that these molecules have an energy level structure that makes them well suited as a first material in the claimed invention. Reference to “perylenes” or “courmarins” herein is intended to cover the specific compound (e.g. perylene) and also those compounds including said compounds, usually as a chromophore, such as perylene-3,4,9,10-tetracarboxylic acid (“Vat Red 29”) as well as derivatives of said compounds.

The first material may be provided as particles, typically micron particles and more typically nanoparticles. Where the first material is a particle, the diameter of the particle is typically less than 2 μm, more typically less than 1 μm, even more typically less than 500 nm, and more typically still less than 100 nm. Alternatively, the first material may be provided in a molecular form, for instance as a liquid or as a solution in which the first material is dissolved. By further reducing the particle size, the resolution limit of the photoresist is further decreased and the achievable feature size is further reduced.

The ratio of the first material to the second material in the composite is not particularly limited. However, it is typically the case that the amount of second material is greater than the amount of first material. Typically, the composite comprises less than 50% by weight of the first material, more typically less than 45%, more typically less than 40%, more typically in the range 1 to 40%, more typically, 5 to 30%, more typically 5 to 10%.

Typically the second material is one of a polymer, photo acid generator, photo radical generator, photo base generator or other photodegradable material. When these materials are exposed to high energy electrons via the claimed energy transfer mechanism (or sensitised), this typically promotes a chemical change via the breaking of chemical bonds. Accordingly, the process can be used to impart electrons to very small regions of the second material thereby promoting chemical changes in very specific regions of the second material. There is no particular limitation on the material used and indeed other such molecular materials are envisaged that have bonds which can be broken using this electron transfer mechanism. Similarly mixtures of one or more polymers, photo acid generators, photo radical generators and photo base generators are also envisaged. The polymer may be a co-polymer, such as a block or random copolymer. The polymer may be functionalised to include moieties adapted to receive an electron or react on receipt of an electron from the first material.

Examples of typical polymers suitable for use in the present invention include, but are not limited to; polyacrylates, polyamides, polyimides, polyesters, polyethers, or combinations thereof.

Typically the first material is suspended in the second material. The term “suspended” as used herein is intended to cover a range of different distribution profiles. The first material may be uniformly distributed throughout the second material, or alternatively it may be located partially or continuously across a surface of the material. The first material may also be predominantly located in a particular area or plane or face of the second material. This facilitates electron transfer from the first material to the second material and defines the areas of the second material which receive an electron from the first material.

Typically the composite is a photoresist for use in a photolithography method. Further, the method is typically a photolithographic method. Typically, the photoresist is deposited over a substrate. When the second material receives an electron into its second energy level it becomes chemically differentiated from the rest of the photoresist. Depending on the desired application and the second material, this chemically differentiated material may either be developed and removed leaving the unexposed material on the substrate (in the case of a positive photoresist), remain on the substrate whilst the unexposed photoresist is removed (in the case of a negative photoresist), or preferably degrade in situ as a result of the absorbed energy and expose the underlying substrate to further processing (such as ablation by the photon source), allowing for direct laser writing of the substrate, with fewer processing steps than conventional photolithography.

In a second aspect of the invention, there is provided a photolithographic composition comprising a first material having at least a first energy level and a second energy level, and a second material having at least a first energy level and second energy level, wherein the energy gap between the first and second energy level of the first material is less than the energy gap between the first and second energy level of the second material, and wherein the first material and second material are in communication with one another, wherein the first material is provided as particles having a diameter less than 2 μm.

The advantage of such a composition over existing photoresists is that a greater degree of fidelity is achieved in modifying the second material. Existing photoresists are restricted as to the size of structures that can be achieved by the energy and wavelength of the laser which performs the ablation. By using particles of a size smaller than the wavelength of conventional ablation laser techniques, smaller structures can be achieved than with conventional photoresists.

By limiting the particle size to less than the minimum resolution of the laser, one can reduce the area in which the second material undergoes indirect photoexcitation and resulting degradation. Accordingly feature size is limited not by the laser wavelength and the corresponding spot size, but by the particle size and concentration of the first material. This leads to an increased patterning resolution and a corresponding increase in the resolution of the resulting features on the substrate.

The term ‘indirect photoexcitation’, refers to the process by which the second material gains an electron in its second energy level via a photoexcitation from the first energy level of the first material to the second energy level of the first material.

Typically the particles of the first material are suspended in the second material and are typically dispersed uniformly throughout the second material. The particles of the first material may also be predominantly located in a particular area or plane or face of the second material. This facilitates electron transfer from the particles of the first material to the second material and defines the areas of the second material which receive an electron according to the distribution of the first material.

In one embodiment, the first material be bonded to the second material. For instance, the first material may be covalently bonded to the second material. This ensures close proximity between the first material and the second material. Alternatively, the particles may be nanoparticles associated with a particular functionality of the second material. This is particularly advantageous where the second material is a polymer. By attaching the first material to a polymer, this ensures an even distribution of the first material throughout the second material and can provide for a sufficient density of the first material such that all of the second material can undergo indirect photoexcitation. Moreover, this prevents over-saturation and aggradation of the first material.

Alternatively, the second material may be doped with the first material. This enables the concentration of the first material to be precisely controlled along with its precise distribution via methods such as selective doping. By better defining the location of the first material within the second material, one can vary the location and concentration of the resulting exposed portions of the second material and the resulting features on the substrate.

Where the composition is a photoresist, it is typically the case that the photoresist is deposited over a substrate.

Other aspects of the invention will be apparent from the following description, examples and the appended claim set.

