EP2076353A1

EP2076353A1 - Indirect pulsed laser machining method of transparent materials by bringing an absorbing layer on the backside of the material to be machined

Info

Publication number: EP2076353A1
Application number: EP07733865A
Authority: EP
Inventors: Béla HOPP; Tamás SMAUSZ KOLUMBÁN; Csaba Vass; Zsolt Bor
Original assignee: University of Szeged
Current assignee: University of Szeged
Priority date: 2006-05-26
Filing date: 2007-05-25
Publication date: 2009-07-08
Also published as: HU227254B1; WO2007138370A1; HUP0600443A2; HU0600443D0

Abstract

The present invention relates to a novel indirect pulsed laser machining process of a transparent material. The process according to the invention comprises the steps of bringing a transparent material (12), at least in a region (19) thereof to be machined, into coupling with an absorbing layer (18), said coupling ensuring intense heat exchange; directing a laser pulse of a laser light (20) emitted by a laser source (10) through the transparent material (12) into the absorbing layer (18); having the energy of the laser pulse absorbed within a portion of the absorbing layer (18) located in the vicinity of the transparent material's region (19) to be machined, and thereby heating up said absorbing portion of the absorbing layer (18) to boiling and removing mechanically material from the region (19) to be machined through a retroaction exerted on said region (19) by the boiling-away material of the absorbing layer (18), comprising (i) applying as the transparent material (12) such a material that is essentially fully transmissive at the wavelength of the laser light (20), (ii) making the absorbing layer (18) of such a material that is essentially fully absorbing at the wavelength of the laser light (20), (iii) setting the energy of the laser pulse so as to correspond to at least the transparent material's (12) threshold fluence value for etching, and (iv) forming the absorbing layer (18) with such a thickness that ensures boiling away of the absorbing layer (18) in its full thickness at the transparent material's region (19) to be machined as a consequence of absorbing the energy of the laser pulse by it.

Description

INDIRECT PULSED LASER MACHINING METHOD OF TRANSPARENT MATERIALS BY BRINGING AN ABSORBING LAYER ON THE BACKSIDE OF THE MATERIAL TO BE MACHINED

The present invention relates to a process for indirect laser machining, in particular for indirect pulsed laser micromachining of transparent materials, as well as workpieces made of such materials. Nowadays, several different machining techniques are available for the machining (surface etching, drilling, perforating) of transparent materials within the size ranges of nano- and micrometres, that is for the micromachining thereof. A class of such machining techniques is formed by laser machinings. When laser machining takes place, the workpiece made of transparent material(s) is exposed to a laser beam emitted by a laser source. The laser source can be either a continuous or a pulsed laser source, and the exposition itself can be of either a direct or an indirect type. When a direct laser machining is performed, the laser beam is guided directly onto that surface of a workpiece which is to be machined, wherein a definite surface portion of the workpiece is removed by the laser beam itself in a direct manner with no application of further auxiliary substances. When, however, an indirect laser machining is applied, the laser beam is not focused onto the surface portion of the workpiece which is to be machined - the laser beam performs its machining effect through the usage of an auxiliary substance. Here, and from now on, the term "transparent material/workpiece" refers to a material/workpiece that is capable of absorbing no laser light travelling therethrough at the wavelength of the laser light used for effecting the machining, or absorbs only up to a small extent (that is, preferably up to 10 percent, more preferably up to 5 percent thereof per centimetres).

The greatest disadvantages of direct machining techniques are the high prices of the lasers/laser systems of high power used to accomplish the techniques and the high qualification needed for their operation. Moreover, due to the large penetration depth of the laser light in machining, thickness of the work- piece's heated portion is large and hardly controllable that, in general, results in a large-scale destruction of the workpiece. Hence, direct laser machining tech- niques are inadequate for the preparation of fine structures (e.g. surface elements).

As opposed to this, indirect laser machining techniques require much cheaper lasers/laser systems and hence are more wide-spread in practice. One of the most important direct laser machining techniques is the technique of laser-induced backside wet etching (from now on LIBWE). In this technique a surface of a transparent workpiece to be etched is brought into direct contact with a liquid having high absorbance even in small thicknesses at the wavelength of the laser light used for the machining. Then to perform the etch- ing, the liquid in contact with the surface of the workpiece to be etched is irradiated through the workpiece by the laser light of a given wavelength. The laser light is absorbed within a thin layer of the liquid contacting the portion of the workpiece to be machined and simultaneously heats it up. As a consequence of the heating-up, a bubble of high pressure forms within the liquid layer. In parallel to this, the portion of the workpiece's surface to be etched in contact with the heated liquid layer is warmed up and softened, optionally even melted by said layer through heat transfer. The etching of the surface is performed by the dynamic effect exerted on the material of the workpiece in the region to be machined by the high pressure bubble(s) expanding in an abrupt manner by simply removing/dislodging a part of the softened material.

G. B. Patent No. 2,341 ,58OA discloses a LIBWE technique for precision machining of the surface of a transparent workpiece (for example of a fused silica plate) upon industrial circumstances, wherein machining is performed by laser pulses emitted sequentially one after the other within a set period of time by a pulsed laser source. As laser sources excimer or dye lasers emitting laser pulses in the ultraviolet (UV) range with a fluence of 0.01 to 100 J/cm²/pulse are used. Depending on the absorbance of the solution (pyrene/acetone solution) used for having the laser light absorbed and the fluence of the machining laser pulses (0.5 to 1.5 J/cm²/pulse) emitted by an excimer laser source (KrF), the etching rate ranges between 1 to 25 nm/pulse.

In a recent publication of Cs. Vass, T. Smausz and B. Hopp [Journal of Physics D: Applied Physics, Vol. 37, pp. 2449-2454 (2004)] micromachining of transparent materials (e.g. fused silica plates) performed by means of a LIBWE technique under laboratory conditions has been studied. As source of the machining laser light an ArF excimer laser operating at the wavelength of λ=193 nm is used, the duration of the laser pulses emitted is 20 ns, while their fluence ranges between 0.11 and 0.86 J/cm². The auxiliary substance used to have the laser light absorbed is a naphtalene/methyl methacrylate solution which is highly poisonous. According to the studies, the etching rate ranges between 4.7 and 49.5 nm/pulse at the applied fluence values.

When the LIBWE technique is applied, in general, solutions of organic halogenated benzene derivatives (such as C₆H₅F, C₆H₅CI₁ C₆H₄CI₂), that are harmful to both the environment and human health, and/or solutions of benzene, toluene, pyrene/acetone, naphthalene/methyl methacrylate, tetrachloromethane, methanol, dimethyl sulphoxide and further similar organic compounds are used as the liquid forming the auxiliary substance absorbing the laser light. Hence, the actual application of the LIBWE technique requires in practice a high degree of care, a leak-proof handling of the absorbing liquid and the adherence to proper safety specifications that makes together its industrial application highly expensive.

Due to the usage of a liquid phase medium that absorbs the laser light, to effect etching peculiar accessories, e.g. a vessel for storing the liquid absorbing medium and establishing a direct contact thereof with the surface of a workpiece to be machined are required which represents a further drawback of the LIBWE technique. Manufacturing of this vessel requires performing separate work processes and hence its usage generally also raises the costs of the industrial appli- cation of the LIBWE technique.

In addition, the etching rates obtainable by the LIBWE technique are of at most several tens of nanometres in magnitude which lags behind the etching rates obtainable in case of direct machining techniques.

U.S. Patent No. 6,990,285 B2 describes a micromachining method of a transparent workpiece (made of e.g. fused silica or crystalline sapphire), in particular, a laser-induced backside machining method of drilling one or more through holes in the workpiece through its entire thickness and an apparatus for - A -

accomplishing the method. The workpiece to be machined is at most several millimetres in thickness. To effect machining, preferably, a pulsed laser emitting ultrashort pulses, that is, of at most 100 fs in duration, at about a wavelength of λ=800 nm is applied. The fluence of the machining laser pulses is at most 0.16 J/cm²/pulse. The etching rate is 2.5 to 12.5 nm/pulse for a pulse energy falling between 7.5 and 10 μJ. To perform the method, simply water is used as the absorbing medium, the major function of which is to carry away the debris created in machining. When certain transparent materials are machined, the usage of water as an auxiliary substance serving as the absorbing medium is not ap- propriate, since its refractive index differs in a too great extent from that of the material to be etched, and hence a significant amount of refraction of light appears at the interface of the two substances. Therefore, in such cases instead of water preferably a water/methanol mixture or dimethyl-sulphide is used as the absorbing medium. A major drawback of said method is that the femtosecond Ia- ser source used for the machining is very expensive and its servicing requires fair skills.

In our studies the conclusion is drawn that bringing a transparent substance or a portion of a workpiece made of such a substance to be machined into close contact with a solid absorbing layer of suitable thickness, and then ir- radiating this layer with the laser light through the transparent workpiece, a desired etching pattern can be created within the portion of the workpiece to be machined with relatively high etching rates even when laser pulses of ns in duration are made use of.

In view of the above, the aim of the present invention is to provide a novel technique for the indirect micromachining of transparent substances/workpieces that eliminates or at least significantly decreases the above discussed drawbacks of the LIBWE technique. A yet further aim is to increase the efficiency of the laser-induced direct micromachining of transparent substances/workpieces.

In particular, the aim of the invention is to provide a novel technique of Ia- ser-induced backside etching of a transparent substance/workpiece wherein there is no need either for the handling of auxiliary substances that are harmful to both the environment and human health or for supplementary devices that are essential from the aspect of making use of these substances. A further aim is to provide a method which can be performed at higher rates compared to the LIBWE technique, that is the extent of etching induced by a single laser pulse (i.e. the etching rate) is considerably higher than what can be obtained by the LIBWE technique if laser pulses of the same fluence are assumed as those used in the LIBWE technique. A yet further aim is, in particular, to develop an etching process that does not require the application of expensive femtosecond lasers/laser systems of high power.

The aim of providing a novel technique for micromachining of a transpar- ent substance/workpiece is achieved by the process of Claim 1. Preferred further embodiments of the process according to the invention are defined by Claims 2 to 18.

The invention is further discussed in detail with reference to the accompanying drawings, wherein - Figure 1 schematically illustrates a possible layout suitable for the accomplishment of the novel indirect laser-induced backside micromachin- ing/etching process according to the invention;

- Figure 2 is a diagrammatic representation of the etching rates obtained in machining of a quartz surface by the process according to the present inven- tion and by the already known LIBWE technique as a function of the fluence of the laser pulses exploited for the machining;

- Figure 3 shows the etching depths measured in micromachining of a work- piece made of fused silica performed by the process in accordance with the present invention as a function of the fluence of the laser pulses of 30 ns in duration emitted at the wavelength of λ=248 nm by a nanosecond KrF exci- mer laser source used for the micromachining;

- Figure 4 illustrates a possible application of the process according to the invention, in particular it shows a possible embodiment of a layout for producing an interferometric grating of a given period; and - Figures 5A and 5B show the atomic force microscopic (AFM) images of the interferometric gratings with a period of 288 and 560 nm, respectively, formed in the surface of a quartz plate in the arrangement of Figure 4. Here, and in what follows, the method provided for the indirect laser- induced micromachining of transparent materials/workpieces according to the present invention is referred to - keeping its basic features to be discussed later in mind - as laser-induced backside dry etching (LIBDE). Figure 1 shows a diagrammatic sketch of a possible layout suitable for accomplishing the LIBDE process according to the invention. A predetermined region 19 of an optically transparent workpiece 12 delimited by a first surface 13 and a second surface 14 lying opposite to the first surface 13 is to be machined through the LIBDE technique - optionally in the size range of nano- or microme- tres - by means of a laser light 20 emitted by a laser source 10 in an indirect manner, that is along with making use of an auxiliary substance. For this purpose, a surface of the workpiece 12 to be machined that lies farther from the laser source 10 in the laser light's 20 direction of travel, in this case i.e. the second surface 14 of the workpiece 12, is brought into close contact with a solid ab- sorbing layer 18 of a predetermined thickness. To insure guidability of the laser light 20, as well as its focusability which is also required in some cases, an optical imaging system 24 containing preferably at least an optical lens is arranged within the light path defined by the workpiece 12 and the laser source 10. To machine the region 19 of the workpiece 12, the laser light 20 leaving the laser source 10 and travelling along the light path is led through the imaging system 24. However, it is noted hereby that the application of an imaging system is not necessary.

If the formation of a given pattern (for instance a plurality of etch pits with a given surface distribution or an optical grating or more complicated structures, such as linear arrays of microlenses or couplers for sensor technology or through holes) is required in the region 19 by the machining of the workpiece 12, one ore more masking elements 22 of the required pattern can be arranged in the light path. In general, the masking element(s) 22 is/are positioned into a portion of the light path that falls between the optical imaging system 24 and the laser source 10. It should be noted that if an etching pattern extending to a greater portion of the surface 14 of the workpiece 12 that cannot be irradiated by a single laser beam emitted by the laser source 10, it is also required that the absorption area of the laser light 20 within the absorption layer 18 be displaceable. This can be achieved e.g. by a precision displacement of the workpiece 12 and the laser source 10 relative to one another or, optionally, through modifying the light path of the laser light 20 by the optical imaging system 24. The physical devices (such as a stage, a stepping motor and similar further elements) required to perform the precision displacement and/or the modification of the light path, as well as their operation and actuation principles are known by a person skilled in the art and hence are not discussed here.

The workpiece 12 can be made of any kind of solid phase material that is optically transparent at the operational wavelength of the laser source 10 used for its machining, softens (optionally melts) due to the effect of heat, however, requires not too high temperature thereto. The softening (melting) point of the workpiece's 12 material is preferably at most about 4000K. As the workpiece 12 is made of such a transparent material that is capable of absorbing at the wave- length of the machining laser light 20 only to a small extent, the LIBDE technique according to the invention is suitable for the machining of a relatively thick workpiece 12, too. If e.g. a laser light 20 falling into the visible spectral range, preferably with a wavelength of λ=500 nm is applied, even a quartz prism with a thickness of about a metre can be successfully surface machined by this tech- nique. If, however, the prism is to be subjected to a bulk machining, to form e.g. through holes, openings or slots of arbitrary shape therein extending between the second surface 14 and the first surface 14, the workpiece 12 is provided with a thickness of at most several hundred nanometres, preferably of 100 to 300 nm. As far as its material is concerned, the workpiece 12 can be made of any opti- cally transparent inorganic substance, such as e.g. fused silica or crystalline quartz, ordinary glass, calcium fluoride, magnesium fluoride, lithium fluoride, silicon carbide, alumina, sapphire, diamond, as well as optically transparent hardened plastics, e.g. polycarbonate, acrylic or vinyl resins and/or organic glasses. The workpiece 12 can be in any desired shape; it can be formed e.g. as a plate, a block, a prism or a tube with a given wall thickness.

The absorbing layer 18 is comprised of a material that absorbs the laser light 20 passing through the workpiece 12 with a negligible amount of absorption within the workpiece 12 to almost a full extent, but preferably to an extent of at least 95 to 99 percent within a relatively short thickness. The absorbing layer 18 is preferably made of metal. Metals are especially suitable for this purpose, as in their case the condition at issue is satisfied within almost the whole UV-visible light range. As the absorbing layer 18, silver and aluminium can be applied highly preferably in the process according to the invention. Generally speaking, any metal with a boiling point higher than the melting point of the workpiece 12 to be machined, but lower than at most 4500K can be used as the absorbing layer 18. However, it is not necessary that the absorbing layer 18 be made of a metal. For instance, thin films formed of carbon can be used expediently as the absorbing layer 18 in the process according to the invention as they have appropriate absorption capacities equally in the infrared, visible and UV ranges. In case of an absorbing layer of a material being different from metal, it is, however, essential that the absorption capacity of the material used be relatively large just at the wavelength of the machining laser light. If a laser light 20 falling into the UV range is used for the machining, a layer made of polymers (such as polycarbonate, polyethylene terephthalate, polymethyl methacrylate, polyhydroxy butyrate, polyimide and similar further polymers) as well as biopolymers can also be used expediently as the absorbing layer 18. The thickness of the absorbing layer 18 is chosen so as to absorb the energy of the incident laser light 20 essentially to a full extent within the absorbing layer 18. Nevertheless, the absorbing layer 18 must be thin enough to be removed by the laser irradiation in its full depth from the surface 14 of the workpiece 12 at the region 19 to be machined, and hence to ensure simultaneous etching of said region 19. Taking all these requirements into account, the thickness of the absorbing layer 18 is chosen to fall into the range of 50 to 150 nm, preferably into the range of 80 to 120 nm, and most preferably to be about 100 nm, depending on the energy of the machining laser light 20 and the actual material of the absorbing layer 18. In general, the absorbing layer 18 is prepared by physical or chemical vapour deposition (PVD, CVD), vacuum deposition, evaporation or sputtering techniques that also ensure the close contact of the workpiece 12 and the absorbing layer 18 in a natural way. To perform the LIBDE technique according to the invention, any pulsed laser that is capable of emitting pulses with a fluence of 0.01 to 10 J/cm² can be used as the laser source 10. The duration of the emitted laser pulses is preferably 10 to 100 ns, more preferably 20 to 50 ns. Clearly, the fluence and the dura- tion of the laser pulses constituting the machining laser light 20 depend on the material of the workpiece 12 to be machined and the material of the absorbing layer 18. It should be noted that pulses of too high fluence can result in a bulk destruction of the workpiece 12, while pulses of too low fluence lead to an etching of low efficiency. As the pulsed laser, preferably e.g. an ArF excimer laser (λ=193 nm), a KrCI excimer laser (λ=222 nm), a KrF excimer laser (λ=248 nm), a XeCI excimer laser (λ=308 nm), a XeF excimer laser (λ=351 nm), various dye lasers, Kr ion laser, Ar ion laser and Cu vapour laser can be used. The higher harmonics of YAG and YLF lasers, that are obtainable through suitable nonlinear optical elements, can also be applied efficiently. Moreover, the experi- ments have also revealed that the LIBDE technique can also be successfully carried out by a laser falling into the visible range, e.g. by a green-light (λ=500 nm) laser, which means that the machining of materials which are non-transparent in UV light (such as glass) becomes also possible by the LIBDE technique.

In what follows, the implementation of a LIBDE process according to the invention is discussed briefly. Before commencing the process, the surface 14 of the workpiece 12 to be machined (at least in the region 19) is brought into close contact with an absorbing layer 18 of suitable (preferably 50 to 150 nm) thickness that is made of an auxiliary substance capable of absorbing the energy of the laser light 20. To ensure close contact, the absorbing layer 18 is applied onto the surface 14 preferably by vacuum deposition. After this step, the absorbing layer 18 is irradiated through the optically transparent workpiece 12 by the laser light 20 used for the machining in conformity with the pattern to be etched into the surface 14. The structure to be etched is optionally provided in the form of a masking element 22 arranged in the laser light path. For focusing the laser light 20, if needed, the laser light 20 is also guided through the optical imaging system 24 before it strikes the workpiece 12. The energy of the laser pulses of appropriate fluence is absorbed in a thin layer of the irradiated portion of the absorbing layer 18 lying in the vicinity of the surface 14 of the workpiece 12. Said layer uses a part of the absorbed energy to heat its surroundings that causes a local heat-up and/or boil-away of the material of the absorbing layer 18, basically in its full thickness. Simultaneously, due to an intense heat transfer, the region 19 of the workpiece 12 in contact with the portion being involved in the absorption process also heats up and softens/melts or can even be vaporized to a depth that depends on the irradiation energy. The material of the absorbing layer 18 leaving the surface 14 at a high velocity exerts a recoil effect onto the molten material of the region 19. As a consequence, a part of the material of the region 19 is being ejected from the bulk of the workpiece 12 - this actually results in the etching. After ejection, the remaining portion of the workpiece 12 rapidly cools and solidifies. The etched pattern/structure owns sharp edges. It is noted here that the etching itself takes place only in the case when the boiling point of the material of the absorbing layer 18 exceeds the melting point of the material of the workpiece 12. It is also noted that the etching furthermore takes place only in the case when the fluence of laser pulses of the laser light 20 used for the machining reaches a certain threshold limit. Based on the experiments carried out by the inventors it is clear that the threshold fluence value of etching is typically 100 to 500 mJ/cm²/pulse for a LIBDE process according to the invention and this value depends on the characteristics (wavelength, pulse duration) of the applied laser source 10, the material of the workpiece 12 to be machined and/or the material of the absorbing layer 18. Finally, after the desired pattern/structure has been formed within the region 19 of the workpiece 12, the residue of the absorbing layer 18 is removed from the surface 14 by means of physical (e.g. grinding) or chemical (e.g. solving) techniques, if this is required in view of later use of the workpiece 12.

Figure 2 illustrates the enhanced efficiency of the LIBDE technique ac- cording to the present invention over the LIBWE technique. Here, the etching rates measured in machining of workpieces made of fused silica are plotted versus the fluence of the machining laser pulses. The measuring results plotted can be found in the scientific papers as listed below, the labels used as reference in Figure 2 have the following meanings:

- 'Vass [1]: naphthalene/MMA': ..Comparing study of subpicosecond and nanosecond wet etching of fused silica" by Cs. Vass, D. Sebδk and B. Hopp (Ap- plied Surface Science, in press, online accessibility from 27 October 2005), wherein the auxiliary substance is a naphthalene/methyl methacrylate solution;

- 'Nino [2]: toluene' and 'Nino [2]: pyrene/acetone': ,,Surface micro-fabrication of silica glass by excimer laser irradiation of organic solvent" by H. Nino, Y. Ya- sui, X. Ding, A. Narazaki, T. Sato, Y. Kawaguchi and A. Yabe {Journal of

Photochemistry and Photobiology A: Chemistry 158 (2003) 179-182), wherein the auxiliary substance is toluene and a pyrene/acetone solution, respectively;

- 'Bόhme [3]: pyrene/toluene': ,,The influence of the laser spot size and the pulse number on laser-induced backside wet etching" by R. Bohme and K.

Zimmer (Applied Surface Science 247 (2005) 256-261), wherein the auxiliary substance is a pyrene/toluene solution; and

- 'dry etching': data obtained by the LIBDE technique according to the present invention, wherein the auxiliary substance is metallic silver. It can be clearly seen from Figure 2 that the LIBDE technique elaborated by the inventors results in significantly larger etching rates for any fluence value. Moreover, it is also clear from Figure 2 that the threshold fluence value of etching for the LIBDE process is about half of the threshold fluence value of etching measured for the LIBWE technique. This further means that under the same ex- ternal conditions, the size of a surface portion that can be machined by the LIBDE technique according to the invention is twice as large as that of a surface machineable by the LIBWE technique. Hence, to achieve the same effect, a much simpler and cheaper arrangement becomes adequate.

In what follows, the process according to the invention and some potential applications thereof will be illustrated with the accompanying examples. Example 1

To illustrate the LIBDE technique according to the invention, as the work- pieces 12 plates made of fused silica and of 1 mm in thickness were subjected to machining in the layout of Figure 1. As the absorbing layer 18 a silver film of 100 nm in thickness was applied to the second surface 14 of the individual work- pieces 12 by vacuum deposition. As the laser source 10 a nanosecond KrF ex- cimer laser operating at the wavelength of λ=248 nm was used. For^' this wavelength the absorption coefficient of the silver film is 71 1/μm, which means a penetration depth of 14 nm from the aspect of the laser light 20. The regions 19 of surfaces 14 to be etched were spots of 0.0105 cm² in size. To focus the laser light 20, a single condensing lens with a focal length of 10 cm and made of quartz was also inserted into the light path. The silver films were irradiated through the silica plates. The fluence of the laser pulses was varied within the range of 90 to 4030 mJ/cm². The experiments showed that if the fluence of the laser light 20 used for the machining reached a lower threshold fluence value of etching (in the present experiment a value of about 192 mJ/cm² was obtained for this), the silver film was simply boiled away in the spot irradiated and simultaneous etching of the silica plate within the region 19 also took place, that is silica was removed from the region 19. Figure 3 shows the etching depth as a function of the fluence of the laser pulses used for the machining; in this case the etching depth actually corresponds to the etching rate (as a "single-shot" process is applied). As can be seen in Figure 3, above the threshold fluence value of etching the etching rate has got a linear dependence on the fluence of the machining laser pulse. Based on this relation, the etching depth can be unambiguously de- termined/controlled through adjusting the fluence of the machining laser pulses, which means that the LIBDE technique can be automated in a very simple and reliable manner when a large-scale application thereof is realized.

Example 2

The present technique is also suitable for the creation of fine submicro- metre structures. For illustration purposes, Figure 4 schematically shows a possible arrangement for the accomplishment of the etching of an interferometric grating by the LIBDE process according to the invention. Here, the optically transparent workpiece 12 delimited by the first and second surfaces 13, 14 is a quartz block of 1 mm in thickness, wherein an absorbing layer 18 of about 100 nm in thickness has been applied onto its surface 14 to be machined of metallic silver by vacuum deposition. To perform the etching, the fluences discussed in Example 1 were used. To create a desired interferometric grating 29 in the surface 14 of the workpiece 12, the laser light 20 emitted by the laser source (not shown) is splitted into two beams by a beam splitter 25 (provided as e.g. a semipermeable mirror) arranged in the light path. The two beams are then led into the absorbing layer 18 along different light paths of the same length, wherein the en- ergy of the two interfering beams is absorbed in conformity with the desired interferometric pattern (a series of extinction and amplification lines) if an appropriate phase difference between the two beams is present. Here, one of the two beams is transmitted through an optical delay element 27. As is clear to a person skilled in the relevant art, the optical delay element 27 can be realized, in its sim- plest form, by means of perpendicular mirrors 27b, 27a arranged in the light path of one of the beams. Guidance of said beam outside the optical delay element 27, as well as directing the other beam into the absorbing layer 18 are effected by further mirrors 28a, 28b arranged in the light paths of the beams.

Figures 5A and 5B are atomic force microscopic images of the inter- ferometric gratings 29 etched into the surface 14 of a workpiece 12 made of quartz by laser lights 20 of different wavelengths by applying the LIBDE process according to the invention. Figure 5A and 5B each shows an interferometric grating with a period of 288 nm and 560 nm, respectively.

Contrary to the presently known laser machining techniques (such as e.g. ablation, LIBWE), the process for machining of materials according to the invention is a so-called "single-shot" process. This means that each piece of a desired pattern/structure to be etched forms due to a single laser pulse. In the irradiated region the absorbing layer is removed together with the etched material, and hence an occurrent second pulse will be capable of exerting no further influence on the same region of the workpiece. The absorbing layer is vaporized, an in turn the etched material of the workpiece spatters away from the region concerned. Building back of material into said region is not possible. As a consequence of the single-shot nature of LIBDE, an accidental movement of the workpiece from pulse to pulse is not problematic, that is the "noise" does not interfere with the desired submicrometre structure. This is of high importance when application of the LIBDE technique in the field of nanotechnology is considered. Furthermore, if forming a certain etching pattern requires multiple-pulsed machining and coating of the surface under machining by vacuum deposition can be performed without the workpiece being removed from the arrangement, by arranging the workpiece within a suitably formed vacuum chamber and performing the indirect laser machining of the workpiece within said vacuum chamber, after etching the etched surface of the workpiece can be recoated by the absorbing layer and then additional laser etching steps and vacuum deposition steps can be performed in an alternating manner, said steps being repeated in any number until the desired pattern is achieved. In this way, even bulk multilevel patterns can be formed in the workpiece. The LIBDE process according to the invention allows indirect laser mi- cromachining of transparent materials/workpieces, formation of nanotechnologi- cal structures, production of linear arrays of microlenses (laser beam homo- genizers) and Fresnel-type lenses, as well as fabrication of heavy-duty optical gratings with high destruction threshold and couplers for sensor technology. Moreover, through holes, openings, slots can also be formed successfully in transparent workpieces of suitable thickness by the process according to the present invention.

Claims

1. Indirect pulsed laser machining process of a transparent material, characterized by the steps of bringing a transparent material, at least in a region (19) thereof to be ma- chined, into coupling with an absorbing layer (18), said coupling ensuring intense heat exchange; directing a laser pulse of a laser light (20) emitted by a laser source (10) through the transparent material into the absorbing layer (18); having the energy of the laser pulse absorbed within a portion of the ab- sorbing layer (18) located in the vicinity of the transparent material's region (19) to be machined, and thereby heating up said absorbing portion of the absorbing layer (18) to boiling and removing mechanically material from the region (19) to be machined through a retroaction exerted on said region (19) by the boiling-away material of the ab- sorbing layer (18), comprising applying as the transparent material such a material that is essentially fully transmissive at the wavelength of the laser light (20), making the absorbing layer (18) of such a material that is essentially fully absorbing at the wavelength of the laser light (20), setting the energy of the laser pulse so as to correspond to at least the transparent material's threshold fluence value for etching, and forming the absorbing layer (18) with such a thickness that ensures boiling away of the absorbing layer (18) in its full thickness at the transparent material's region (19) to be machined as a consequence of ab- sorbing the energy of the laser pulse by it.

2. The process according to Claim 1 , further comprising preparing a workpiece (12) of the transparent material before machining, said workpiece having a first surface (13) and a second surface lying (14) opposite to the first surface (13), and arranging said region (19) to be machined on that surface of the workpiece (12) obtained, through which the laser light (20) leaves the workpiece (12).

3. The process according to Claim 1 or 2, wherein said coupling ensuring heat exchange is provided by bringing the absorbing layer (18) and the transparent material into close surface contact with each other.

4. The process according to Claim 3, further comprising forming the sur- face contact through the application of the absorbing layer's (18) material by physical vapour deposition (PVD), chemical vapour deposition (CVD), vacuum deposition, evaporation or sputtering onto the transparent material.

5. The process according to any of Claims 1 to 4, wherein the laser pulse directed into the absorbing layer (18) is of 10 to 100 ns, more preferably of 20 to 50 ns in duration and has a fluence of 0.01 to 10 J/cm².

6. The process according to any of Claims 1 to 4, further comprising applying a pulsed nanosecond laser as the laser source (10), by which a laser pulse having a fluence of 0.01 to 10 J/cm² is generated.

7. The process according to Claim 6, wherein said laser source (10) is selected from the group consisting of ArF excimer laser (λ=193 nm), KrCI exci- mer laser (λ=222 nm), KrF excimer laser (λ=248 nm), XeCI excimer laser (λ=308 nm), XeF excimer laser (λ=351 nm), various dye lasers, Kr ion laser, Ar ion laser and Cu vapour laser.

8. The process according to Claim 6, wherein said laser source (10) is a YAG or YLF laser and higher harmonics thereof obtained by non-linear optical elements are used as the laser light (20).

9. The process according to any of Claims 1 to 8, wherein the absorbing layer (18) is formed with a thickness of 50 to 150 nm, preferably with a thickness of 80 to 120 nm, and even more preferably with a thickness of about 100 nm.

10. The process according to any of Claims 1 to 9, wherein the absorbing layer (18) is made of a metal or carbon or a polymer that have a boiling point higher than the transparent material's melting point.

11. The process according to Claim 10, wherein said absorbing layer (18) is made of aluminium or silver.

12. The process according to any of Claims 1 to 11 , wherein said transparent material is selected from the group consisting of fused silica, crystalline quartz, ordinary glass, calcium fluoride, magnesium fluoride, lithium fluoride, silicon carbide, alumina, sapphire, diamond, as well as hardened polycarbonate resins, acrylic resins, vinyl resins and organic glasses.

13. The process according to any of Claims 1 to 12, further comprising setting the extent of material removal in the region (19) to be machined by the energy of the laser pulse directed into the absorbing layer (18).

14. The process according to any of Claims 1 to 13, further comprising performing directing of the laser pulse into the absorbing layer (18) by means of an optical imaging system (24).

15. The process according to any of Claims 1 to 14, wherein (i) an etched structure in conformity with a desired pattern in the surface of the transparent material and/or (ii) a through hole in the bulk of the transparent material is formed through the material removal performed in said region (19) of the transparent material to be machined.

16. The process according to any of Claims 1 to 14, wherein a pattern falling into the nano- or micrometre size range is produced through the material removal performed in said region (19) of the transparent material to be machined.

17. The process according to Claim 15 or 16, further comprising facilitating formation of said pattern by arranging a masking element (22) in the path of the laser pulse.

18. The process according to any of Claims 1 to 17, further comprising removing the residual absorbing layer (18) from the surface of the transparent material by a physical or chemical exposure after the desired machining of the transparent material.