WO2007142610A1

WO2007142610A1 - Method and device for selective etching of composite materials by laser ablation

Info

Publication number: WO2007142610A1
Application number: PCT/SI2007/000025
Authority: WO
Inventors: Uros Cvelbar; Miran Mozetic; Slobodan Milosevic; Niksa Krstulovic
Original assignee: Institut 'jozef Stefan'
Priority date: 2006-06-06
Filing date: 2007-05-22
Publication date: 2007-12-13
Also published as: SI22288A

Abstract

The invention relates to a method for surface treatment of composite materials with a polymer matrix and a device for accomplishing selective etching of the surface of the composite. The method is based on the surface treatment of the composite with a stream of energy- particles, preferably a laser beam, which is known as laser ablation. The polymer is partially or completely removed from the surface, while the fillers remain virtually untouched. Such modifications bring about essential changes in surface properties, primarily in the adhesiveness and the porosity of various coatings on the composite.

Description

Method and Device for Selective Etching of Composite Materials by Laser Ablation

The present invention relates to a method and a device for selective etching of composite materials by laser ablation, which is to say, a method for treating the surface of composite materials having a polymer matrix, and a device for accomplishing selective etching of the surface. The method is based on irradiating the surface of the composite with a pulse stream of energy particles from a laser beam, the process being known as laser ablation. During the treatment with selected pulse parameters or additional sources of energy particles and/or gases, which allow higher densities of interacting particles to be achieved on the surface, the composition and the morphology of the surface region of the composite are modified. The polymer is partially or entirely removed from the surface, so that only fillers remain thereon. The said process is referred to as selective etching of the composite. Such modifications of the surface bring about essential changes in surface properties, primarily in the adhesiveness and the porosity of various coatings on the composite.

Description of the problem

Composite materials with a polymer matrix (hereinafter referred to as composites) are widespread in several fields of industry. Composites consist of a polymer matrix, wherein various particles called fillers are distributed in an arbitrary arrangement. Fillers may be organic or inorganic particles, and may be of various shapes and sizes. The properties of a given composite depend on the type of polymer matrix, the type of fillers and their concentration, and the distribution (and sometimes the orientation) of the fillers within the polymer matrix. The surface properties of the composites (their composition, morphology, surface tension) in turn depend primarily on the properties of the polymer matrix, given that, as a rule, the polymer covers the fillers completely. Examples may be found in composite coatings, such as various kinds of color coatings containing diverse fillers. Selective treatment of the surface, on the other hand, allows the polymer to be removed from the surface, allowing the surface to be characterized for certain properties, e.g. for the quality and homogeneity of the distribution of the fillers within the polymer matrix. Frequently there is a desire to adjust the surface properties of the composite for an increased adhesiveness of the composite to a coating of a given material, to be applied subsequently. In such cases it is most convenient to remove the polymer from the surface, so that only uncovered fillers remain thereon, to which the coating will adhere in a stronger fashion.

The polymer may be removed off the surface in various ways, for example by means of chemical etching, or by removing it mechanically. Both the said methods are environment-unfriendly, of low quality, or inconveniently priced, and, above all, impractical for the purpose of achieving localized etching. Plasma etching is more environment-friendly, although it does not ensure selective removal of material locally, in confined areas, and is more indicated for selective etching of larger surfaces. A problem occurs, however, when selective material removal is to be carried out evenly in particular spots or in holes, which is a standard problem in the fabrication of microelectronic devices.

Description of the Related Art

The traditional method employed for etching polymers from the surface of composites is wet chemical etching. The technique is used, for example, in surface treatment of superconductive composites (US6214249 and WO02071462) or prior to metal plating (US6080836) . The polymer may also be removed from the surface of the composite by oxidation. Flame treatment, corona discharge treatment, or wet process chemical etching may be used for the purpose. Polymer etching is widely used in microelectronics, for example in the production of integrated circuits. For this purpose, multistage processes may be employed, such as high frequency discharge etching, i.e. plasma etching (US2002055263, US5705428 and US2002125207) . Plasma etching of polymers is also used for treating polyethylene (UHMPE) fibers/vinyl ester resin composites in order to promote the surface wettability thereof. In this case, plasma etching is a stage of the production process, wherein the wettability of the surface of the fibers can be improved to a degree enabling them to be subsequently impregnated with vinyl ester resins (US5221431) . Selective plasma etching of composites may be found in DE10320483, wherein the surface is treated with radicals of reactive gases.

Laser light is employed in many applications for removing material. More specifically, laser ablation or a combination thereof with other types of sources is utilized. A combination of laser light technology and - A - electrical discharge, wherein laser plasma is created between the specimen and the anode, and then utilized for treating the material, is found in JP11197947. Generally, laser ablation is widely used in printing (JP10291319, US5836249, JP9118017, US6165687) and lithography (DE19817756) where composite materials are employed. Deposition of materials by means of laser ablation is also known in the art and used for creating composites (JP5179429, WO2004042785) or for depositing material thereon (DE3915261, FR2816756) . Laser ablation is used in medicine, mainly for treating tissues (WO03101529, US5807379), or for catheter ablation (US2003199755, US6701176, WO09638193) . With this method, nanostructure materials such as carbon nanotubes may be fabricated (JP2003054922) , materials may be sintered (US2004081573) , and compact films, to be utilized for example as optical waveguides, may be synthesized by treating polymers (US5106211). The said method is also frequently used for removing material from two-layer composites, when the underlayer is transparent to laser light ( JP2001300749) .

Selective laser ablation is used in the fabrication of microelectronic or semiconductor devices, wherein the surface is patterned (EP0542656, US5348609, WO9803271) , covered (WO9719269) , or the material is removed by means of selective ablation of the metallic layer or the polymer material thereon (US6057173, US4568409, US5035918, US5348609, EP0337658) . Selective removal is used in the fabrication of integrated electro-optical devices (US2003048974, US5281798) in the case of multilayer composites, or in the fabrication of optical devices for shaping the polycrystalline diamond (US5500157, EP0618043) . It is also used for marking and opening containers (WO0168460) . In semiconductor manufacturing, laser-assisted etching is used in halocarbon ambients (gases) , wherein the etching efficiency is improved (US5348609) . Selective etching of the surface layers of the substrate may also be carried out by means of a flashlight having sufficient intensity for ablation, where selectivity is ensured by utilizing light of different wavelengths (US5281798), the process being employed for removing paint from the surfaces of aircraft without damaging or degrading the underlying substrate, typically consisting of lightweight aluminum.

The majority of the published efforts are concerned with multilayer composites as used in the semiconductor and microelectronic industry. There are no data available in the patent documents regarding selective laser ablation or selective surface treatment with a combined stream of energy particles for composites with a polymer matrix that are not multilayered, but comprise fillers distributed arbitrarily. In a paper published by K. C. A. Crane and J. R. Brown in J. Phys . D: App. Phys . , 14 (1981), p. 2341, the authors describe laser ablation on a composite fiber having an epoxy matrix, however without achieving process selectivity. Partial selective removal of the polymer matrix PEEK is achieved by P. E. Dyer et al. in their article published in J. Mater. Res. 5 (1992), p. 1152, where the energies of the laser beam are too high and don't activate the surface.

It is the object of the present invention to disclose a method and a device for selective etching of composite materials by means of laser ablation, which is to say, a method for treating the surface of composite materials having a polymer matrix, and a device for ensuring selective removal of the polymer from the surface and for additionally activating the surface.

In accordance with the invention, the said object is achieved by a method and a device for selective etching of composite materials with a polymer matrix by means of laser ablation as per the independent patent claims. According to the invention, the polymer of the composite material is partially or completely removed from the surface, so that only fillers remain on the surface. This brings about an essential alteration in the surface properties of the composite, primarily in the adhesiveness and the porosity of various coatings on the composite.

Description of the Inventive Solution

The invention comprises a method and a device for treating composites. The surface of the composites is subjected to a stream of energy particles. Photons and ions may be used as energy particles. During the treatment, the composition and the morphology of the surface region of the composite are modified due to the interaction of the energy particles. Owing to the selective interaction, the polymer is partially or completely removed from the surface, so that only fillers, generally untouched by the process, remain on the surface. Selective interaction mainly results from quick local heating, a quasi-equilibrium state with high particle densities being obtained at the border between the surface and the vacuum. Induced by the laser pulse, concentrations of charged particles and even atoms appear in the localized area, additionally etching, breaking surface bonds, while polar groups are created if gases are additionally induced. The phenomenon is known as surface activation. Because of the etching, the roughness of the surface is increased as well. Such surface alterations in turn lead to essential changes in surface properties, particularly in the adhesiveness and the porosity of various coatings on the composite.

The invention shall hereinafter be described with reference to a preferred embodiment thereof and the appended drawings, representing:

Figure 1: a schematic view of a lateral cross-section of a composite with a polymer matrix and with two types of fillers;

Figure 2: a SEM image of the surface of a composite with a polymer matrix prior to treatment;

Figure 3: roughness analysis of the untreated specimen of the composite of Figure 1;

Figure 4 : a SEM image of the surface of a composite with a polymer matrix after being treated with a laser stream;

Figure 5: roughness analysis of the specimen of the composite, treated with a laser stream of Figure 4;

Figure 6: a schematic representation of the device for treating a composite with a stream of energy particles.

In Figure 1, a typical composite is schematically represented. In general, a composite comprises a polymer matrix, in which different fillers 2 and 3 are dispersed. The surface of the composite is typically covered with a polymer layer, which is particularly typical of composites fabricated with the sintering process at an elevated temperature. The surface properties of such composites (such as morphology or surface tension) depend on the type of polymer matrix, and not on the type of fillers. Given that the polymer is softened when compacted at elevated temperatures, the surface of the composite is relatively smooth. The surface energy of the composite, however, does not correspond to the surface energy of the polymer which is typically quite low, i.e. lower than 40mN/m. Such polymers are, for example, paraffin, PTFE, PMDS, PP, PE, PPS, PMMA, and the like. Fillers, on the other hand, are typically of a very wide spectrum, from organic dye pigments to carbon compounds.

In Figure 2, a SEM (Scanning Electron Microscope) image is shown of the surface of a simple untreated composite with a polymer matrix PP and one filler, graphite in this case. The surface is comparatively smooth, without any filler particles to be seen.

The surface of the untreated composite is comparatively smooth. This may be seen from Figure 3, in which the roughness of the untreated composite is shown, as measured by the method of tracing a needle probe over the surface of the specimen. As may be noted from the picture, the average roughness is under lμm, which is quite below the typical dimension of the fillers - around lOμm in our case .

If the untreated composite from Figure 2 is exposed to laser light, selective etching of the surface takes place. In Figure 4 a SEM image is shown of the surface of the treated composite, which was subjected to laser ablation, namely to a stream of laser light with 40mJ/mm² energy. The source of the laser stream used for the ablation was an excimer laser with a wavelength of 308nm (Lambda Physik 105E) . It may be noted from Figure 4 that no polymer can be observed on the surface, only filler particles sticking out of the surface.

The change in the surface roughness may be seen in Figure 5, showing the roughness of the treated composite, analyzed with the method of tracing a needle over the surface of the specimen. As may be noted from the picture, the average roughness is about 5μm, which is of the same order of magnitude as the dimensions of typical fillers, namely around lOμm. In addition, an increased wettability of the surface is observed, which is indicative of an increase in the surface energy brought about by the ablation, and also of activation.

By comparing Figures 2 and 3 to Figures 4 and 5, respectively, it may be seen that the morphology and the composition of the surface of the composite were essentially altered by subjecting it to laser ablation. Prior to the treatment the surface was covered by a layer of polymer, whereas after being treated with laser ablation, there is no polymer to be found on the surface, only filler particles. The roughness of the composite is also changed accordingly. The said change is caused by the polymer matrix and the fillers having different physical and chemical properties. The polymer always has a lower melting point than the fillers, having also an essentially higher vapor pressure than the fillers. When the surface of the composite is exposed to pulse laser light with a wavelength of 308nm, the surface warms up locally. Given that the penetration depth of laser light of said wavelength is exceptionally small, the beam energy is mostly dispersed in the surface layer. Both the polymer and the fillers in the surface layer get very hot. Given that the vapor pressure of the polymer is extremely high at elevated temperatures, the polymer evaporates from the surface, while the fillers remain virtually unharmed, as their vapor pressure is still low at these temperatures. The properties of the selective etching of the composite may be further enhanced by adding reactive gases which increase the high local concentrations of particles, thereby promoting better and faster decomposition of the evaporated surface areas and additionally activating the surface. Such reactive gases are typically oxygen, nitrogen, water vapors, nitrogen oxides as well as mixtures of said gases with inert gases, preferably argon. That way, the material that was etched off is also prevented from being re-deposited on the surface, and the surface remains perfectly clean after ablation. At normal atmospheric pressure, the polymer vapors expelled from the surface of the composite could be bound again to the surface of the composite, thus slowing down laser ablation etching. The added gas or gas mixture must be preferably below 150 Pa.

The device for carrying out such processes is schematically shown in Figure 6. The ^■ device is composed of a vacuum container 4, wherein composite materials 11 are treated. The vacuum container is evacuated with one or more vacuum pump(s) 7, separated from the system by a valve 8 and a recombination system 9 enabling the recombination of reactive radicals and the removal of unwanted substances from the system. Air is introduced into the reactor system via a valve 10. Prior to treatment, the composite 11 is placed on a support 12, which is movable so that any part of the surface of the composite may be reached by the stream of particles. The primary source of the stream of energy particles is a laser 5 of a convenient wavelength, powered via an optical laser system 6. An additional source of energy particles for selective etching, primarily for increasing the particle density near the surface, is represented by a low-energy ion cannon 17 powered via the system 18. In exceptional cases an electron cannon 19 may also be utilized, powered via the system 20, particularly when additional heating of a given part is required. There is also a combined option, in that an additional ion source may be mounted in the position of the electron cannon, so that homogeneous etching may be achieved when using the ion cannon 17. After placing the composite 11 on the support 12, the vacuum chamber 4 is evacuated in order to achieve a convenient pressure (typically below 1 Pa) , and as a primary stage the surface is treated with a pulse stream of energy photons from the source 5. The duration of the treatment or, respectively, the number of laser pulses, their duration and energy (intensity) primarily depend on the type of composite 11. Since the penetration depth of the energy particles from all the sources is small, only the surface of the composite 11 is modified by the treatment, leaving the properties of the lower layers thereof largely unchanged. By releasing additional reactive gas or gas mixture from containers 13, 14 via stop valves 15 and inlet valves 16, the treatment conditions as well as additional surface activation may be modified, so that the desired properties of the composite being etched may be achieved. Reactive gases 13 are typically oxygen, water vapor, nitrogen oxide, nitrogen, mixtures thereof, or combinations thereof with inert gases, typically with argon 14. By moving the support 12 or even the stream of particles 17, any part of the surface of the composite may be treated, so that with the device shown in Figure 6 any specific area on the composite 11 may be traced out and its surface properties modified. As for how precisely the said area will be delimited, it depends solely on the width of the beam of energy particles, preferably an ablating laser beam colminated with quartz lenses.

Claims

Patent Claims

1. A method for selective etching of composite materials with a polymer matrix, characterized in that the composite material is subjected to a pulse stream of energy photons from a laser source, wherein the ablating photon stream may be a laser of an arbitrary wavelength, preferably with a wavelength at which the penetration depth of the photon stream is small compared to the thickness of the composite, more preferably of the same order of magnitude as the thickness of the fillers in the composite, and there are two mutually or continuously combined beams of ions and/or electrons.

2. Method for treating a composite according to Claim 1, characterized in that the vacuum- reactor container is evacuated to a pressure lower than IPa, and the entire surface or a part thereof is subjected to a stream of particles .

3. Method for treating a composite according to Claims 1 and 2, characterized in that, in order to increase the density of particles and the efficiency of the selective etching, a reactive gas or a mixture of gases is introduced into the vacuum-reactor container, the combined pressure thereof not exceeding 150Pa.

4. Method for treating a composite according to Claims 1, 2 and 3, characterized in that the reactive atmosphere is oxygen, nitrogen, water vapor, nitrogen oxide, or a mixture thereof.

5. Method for treating a composite according to Claim 4, characterized in that the reactive atmosphere is a mixture of one or more reactive gases with inert gases, preferably argon.

6. Method for treating a composite according to Claims 1 to 5, characterized in that the reactive atmosphere is in a plasma state.

7. Device for selective etching of composite materials, characterized in that it consists of a reactor-vacuum container (4), evacuated with one or more vacuum pumps (7), a system for dosing gases, and three combined sources of energy particles, as well as a recombination system (9) for the reaction products.

8. Device according to Claim 7, characterized in that the sources of energy particles are a photon, an ion, or an electron source.