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Title:
A METHOD OF GENERATING A HIGH QUALITY HOLE OR RECESS OR WELL IN A SUBSTRATE
Document Type and Number:
WIPO Patent Application WO/2013/029713
Kind Code:
A1
Abstract:
The present invention relates to a method of generating a hole or well in an electrically insulating or semiconducting substrate, and to a hole or well in a substrate generated by this method. The invention also relates to an array of holes or wells in a substrate generated by the method.

Inventors:
SCHMIDT CHRISTIAN (CH)
DITTMANN LEANDER (CH)
CHAIZE ADRIEN (CH)
HOYER SVEND (CH)
Application Number:
EP2012/002939
Publication Date:
March 07, 2013
Filing Date:
July 12, 2012
Export Citation:
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Assignee:
ASAHI GLASS CO LTD (JP)
SCHMIDT CHRISTIAN (CH)
DITTMANN LEANDER (CH)
CHAIZE ADRIEN (CH)
HOYER SVEND (CH)
International Classes:
B26F1/28; B23K26/38; B23K26/40; H01L21/00; H03K3/537
Domestic Patent References:
WO2010063462A12010-06-10
WO2011038788A12011-04-07
WO2005097439A22005-10-20
WO2009059786A12009-05-14
WO2011038788A12011-04-07
WO2010063462A12010-06-10
WO2005097433A12005-10-20
Foreign References:
EP1905532A12008-04-02
US20040140299A12004-07-22
Attorney, Agent or Firm:
BOEHMERT & BOEHMERT (ENGELHARD, MarkusHollerallee 32, Bremen, 28209, DE)
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Claims:
Claims

1. A method of generating a hole or recess or well in a substrate, said method comprising the steps:

a) providing a substrate which is electrically insulating or semiconducting at room temperature, and placing it between at least two electrodes connected to a user-controlled voltage supply,

b) melting substrate material in a region of said substrate where said hole or recess or well is to be generated by heating said region using a laser beam, preferably a focussed laser beam and, optionally, an AC voltage applied to said substrate via said electrodes,

c) applying, by means of said user-controlled voltage supply, and said electrodes, a voltage of a user-defined magnitude across said region of said substrate, said voltage being sufficient to give rise to an increase in electrical current through said substrate or said region, thereby applying a defined amount of electrical energy to said substrate, and dissipating it from said substrate, wherein said substrate is sandwiched between a first and second modulation layer, said first and second modulation layer being located on opposite sides of said substrate, said first modulation layer being located on a side of said substrate which, in step b), is exposed to said laser beam, preferably said focussed laser beam, said first modulation layer allowing to define the size, preferably the cross-section, of said region where said hole or recess or well is to be generated, allowing to avoid surface contamination and/or redeposition of molten substrate material, and allowing to shield substrate surface from heat,

and

said second modulation layer being located on a side opposite of said side on which said first modulation layer is located, said second modulation layer allowing to avoid surface contamination and/or redeposition of molten substrate material and allowing to shield substrate surface from heat.

2. The method according to claim 1 , wherein said first and second modulation layer are made of a material which has a transition between a solid and a liquid state, or a solid and a gaseous state, that occurs within a temperature range from room temperature to the melting temperature of the substrate , or that occurs at a defined amount of energy applied in step b) by using said focussed laser beam.

3. The method according to any of claims 1-2, wherein said first and second modulation layer are made of the same material or a different material.

4. The method according to any of claims 1-3, wherein said first and second modulation layer are, independently at each occurrence, made of a material selected from the group comprising polymers, in particular polydimethylsiloxane (PDMS), polyimide (PI), polyethyleneterephtha- late (PET), ethylene propylene rubber, inorganic silicone rubber, PVC with synthetic acrylic adhesive.

5. The method according to any of claims 1-4, wherein said first and second modulation layer, independently at each occurrence, have a thickness in the range of from 1 μιη to 1 mm, preferably 10 μιη to 500 μπι, more preferably 15 μηι to 300 μηι, more preferably 20 μπι to 200 μπι, even more preferably 20 μπι to 100 μηι.

6. The method according to any of the foregoing claims, wherein said first modulation layer has a modulating region which is adjacent to and in contact with said substrate region in which substrate region said hole or recess or well is to be generated, wherein step b) is performed such that said modulation layer is molten and/or evaporated in said modulating region, thereby allowing access of said focussed laser beam to said substrate region,

and wherein during step c), said modulation layer is partially or entirely displaced in said modulating region, through the dissipation of said electrical energy.

7. The method according to any of the foregoing claims, wherein said first and second modulation layer are attached to said substrate in such a manner that they cover said substrate in step a) on opposite sides.

8. The method according to any of the foregoing claims, wherein the first modulation layer is used to create during step b) and or c) a mask on said substrate, which mask exposes only the region(s) of the substrate where a hole or recess or well is to be generated.

9. The method according to any of the foregoing claims wherein the first and/or second modulation layer is used to define or modulate or restrict the surface area of the substrate which is exposed to the laser beam, preferably the focussed laser beam in step b) and/or to the heat of the AC voltage in step b) and/or to the heat of the dissipating in step c).

10. The method according to any of the foregoing claims, wherein the first and second modulation layer are used to improve the flatness/surface quality of a region surrounding the hole or recess or well introduced, by absorbing material ejected during step c) into the modulation layer(s) and thus avoiding redeposition of said ejected material directly onto the substrate surface, wherein preferably, said absorbed material is subsequently removed together with the modulation layer, after generation of said hole, recess or well in the substrate.

1 1. The method according to any of the foregoing claims, wherein step b) is performed by directly heating the modulation layer through absorption of laser radiation of a wavelength of said laser beam, preferably said focussed laser beam, absorbed by the modulation layer.

12. The method according to any of the foregoing claims, wherein steps a) - c) are performed once, such that a first hole, recess or well is generated in a first substrate region, thereafter the substrate is moved by a defined distance relative to the electrodes and laser beam, and steps b) - c) are performed a second time such that a second structure is generated in a second substrate region.

13. The method according to claim 12, wherein steps b) - c) are performed n times, such that an array of n structures is generated in said substrate, n being an integer > 1.

14. The method according to any of the foregoing claims, wherein said electrical energy applied in step c) is sufficient to generate a through hole or through channel in said substrate.

15. A hole or recess or well or an array of holes, recesses or wells, produced by the method according to any of claims 1-12, or 13-14, respectively.

Description:
A method of generating a high quality hole or recess or well in a substrate

The present invention relates to a method of generating a hole or well in an electrically insulating or semiconducting substrate, and to a hole or well in a substrate generated by this method. The invention also relates to an array of holes or wells in a substrate generated by the method.

WO 2005/097439 and WO2009/059786 disclose methods of generating structures in substrates using the application of voltages to a substrate. The holes of these earlier applications, when present in arrays, typically require large distances (typically > 1 mm) between adjacent holes to avoid flashovers during production of arrays without insulating layers. WO 2011/038788 and WO 2010/063462 disclose methods of generating structures in substrates using the application of voltages to a substrate which allow the fabrication of arrays of holes with small pitch (typically < 1 mm). These methods are sometimes hampered by a destruction of the substrate surface due to the laser spot required for positioning the process location, or by a contamination of the surface due to incomplete ejection of material or redeposition on the surface around the holes generated.

Accordingly there is a need in the art to provide for improved methods allowing the generation of high quality holes and dense arrays thereof in substrates as well as a significant improvement of surface quality and controllability of hole parameters.

It was therefore an object of the present invention to provide for a method for producing holes with a better controllability of the shape of the holes and the surface quality at and around the holes. It was also an object of the present invention to provide for such a method which is easy to perform and which can be adapted to mass production of such perforated substrates.

The objects of the present invention are solved by a method of generating a hole or recess or well in a substrate, said method comprising the steps: a) providing a substrate which is electrically insulating or semiconducting at room temperature, and placing it between at least two electrodes connected to a user-controlled voltage supply,

b) melting substrate material in a region of said substrate where said hole or recess or well is to be generated by heating said region using a laser beam, preferably a focussed laser beam and, optionally, an AC voltage applied to said substrate via said electrodes,

c) applying, by means of said user-controlled voltage supply, and said electrodes, a voltage of a user-defined magnitude, typically a DC voltage, across said region of said substrate, said voltage being sufficient to give rise to an increase in electrical current through said substrate or said region, thereby applying a defined amount of electrical energy said substrate and dissipating it from said substrate, wherein said substrate is sandwiched between a first and second modulation layer, said first and second modulation layer being located on opposite sides of said substrate, said first modulation layer being located on a side of said substrate which, in step b), is exposed to said laser beam, preferably said focussed laser beam, said first modulation layer allowing to define the size, preferably the cross-section, of said region where said hole or recess or well is to be generated, allowing to avoid surface contamination and/or redeposition of molten substrate material, allowing to shield substrate surface from heat, and

said second modulation layer being located on a side opposite of said side on which said first modulation layer is located, said second modulation layer allowing to avoid surface contamination and/or redeposition of molten substrate material and to shield substrate surface from heat.

In one embodiment, said first modulation layer shields the substrate surface from heat which is due to the laser and/or to an AC arc, generated if an AC voltage is applied, and/or to a DC arc, generated by the application of a DC voltage (in step c). In one embodiment, said second modulation layer shields the substrate surface from heat which is due to an AC arc, generated if an AC voltage is applied, and/or to a DC arc, generated by the application of a DC voltage applied in step c).

In some embodiments, a focussed laser beam is used on both sides of the substrates. In this case, also the second modulation layer shields the substrate surface from heat which is due to the laser beam irradiated on this side of the substrate. In one embodiment said first and second modulation layer are made of a material which has a transition between a solid and a liquid state, or a solid and a gaseous state, that occurs within a temperature range from room temperature to the melting temperature of the substrate , or that occurs at a defined amount of energy applied in step b) by using said focussed laser beam.

In one embodiment said laser beam, preferably said focussed laser beam is irradiated onto the side where the first modulation layer is located. In another embodiment, two laser beams, preferably focussed laser beams are irradiated on either side of the substrate.

In one embodiment said first and second modulation layer are made of the same material or a different material.

In one embodiment said first and second modulation layer are, independently at each occurrence, made of a material selected from the group comprising polymers, in particular polydi- methylsiloxane (PDMS), polyimide (PI), polyethylene terephthalate (PET), ethylene propylene rubber, inorganic silicone rubber, PVC with synthetic acrylic adhesive.

In one embodiment said first and second modulation layer, independently at each occurrence, have a thickness in the range of from 1 μηι to 1 mm, preferably 10 μηι to 500 μηι, more preferably 15 μπι to 300 μηι, more preferably 20 μηι to 200 μπι, even more preferably 20 μπι to 100 μιη.

In one embodiment said first modulation layer has a modulating region which is adjacent to and in contact with said substrate region in which substrate region said hole or recess or well is to be generated, wherein step b) is performed such that said modulation layer is molten and/or evaporated in said modulating region, thereby allowing access of said focussed laser beam to said substrate region,

and wherein during step c), said modulation layer is partially or entirely displaced in said modulating region, through the dissipation of said electrical energy.

In one embodiment said first and second modulation layer are attached to said substrate in such a manner that they cover said substrate in step a) on opposite sides. In one embodiment, the laser beam is a focussed laser beam, in another embodiment an unfo- cussed laser beam.

In one embodiment the first modulation layer is used to create during step b) and/or c) a mask on said substrate, which mask exposes only the region(s) of the substrate where a hole or recess or well is to be generated.

In one embodiment, the first and/or second modulation layer is used to define or modulate or restrict the surface area of the substrate which is exposed to the laser beam, preferably the focussed laser beam in step b) and/or to the heat of the AC voltage in step b) and/or to the heat of the dissipating in step c).

In one embodiment the first and second modulation layer are used to improve the flatness/surface quality of a region surrounding the hole or recess or well introduced, by absorbing material ejected during step c) into the modulation layer(s) and thus avoiding redeposition of said ejected material directly onto the substrate surface, wherein preferably, said absorbed material is subsequently removed together with the modulation layer, after generation of said hole, recess or well in the substrate.

In one embodiment step b) is performed by directly heating the modulation layer through absorption of laser radiation of a wavelength of said laser beam, preferably said focussed laser beam, absorbed by the modulation layer.

In one embodiment steps a) - c) are performed once, such that a first hole, recess or well is generated in a first substrate region, thereafter the substrate is moved by a defined distance relative to the electrodes and laser beam, and steps b) - c) are performed a second time such that a second structure is generated in a second substrate region.

In one embodiment steps b) - c) are performed n times, such that an array of n structures is generated in said substrate, n being an integer > 1.

In one embodiment said electrical energy applied in step c) is sufficient to generate a through hole or through channel in said substrate. The objects of the present invention are also solved by a hole or recess or well or an array of holes, recesses or wells, produced by the method according to the present invention.

The term "modulation layer" as used herein, is meant to refer to a layer that provides for a mask which defines the size, preferably the cross-section, of the region where a hole or recess or well is to be generated. In a preferred embodiment, the first modulation layer acts as a mask which is generated in-situ during the performance of the method, preferably during step b) and/or c), on the substrate which mask exposes only the region(s) of the substrate where a hole or recess or well is to be generated. Before the method according to the present invention is performed, there is no mask and the modulation layer has a closed surface. When step b) and/or c) is performed, a mask is generated from said modulation layer. Typically, the first modulation layer is located on the side of the substrate, where the heat is applied in step b). The first modulation layer also allows to avoid surface contamination and/or redeposition of molten substrate material. Moreover, in one embodiment, the second modulation layer is located on the opposite side of the substrate, i.e. distant from the side where the heat is applied in step b). The second modulation layer allows to avoid surface contamination and/or redeposition of molten substrate material. Without wishing to be bound by any theory, the inventors believe that the function of the modulation layer(s), in particular the second modulation layer is that material that is ejected from the substrate cannot come to rest and stick to the substrate surface but is deposited on the layer or incorporated therein. Alternatively, the presence of the modulation layer(s) changes the way that the material is being ejected from the substrate such that no filaments are formed that would stick to the entrance of the hole after the hole has been generated. All this ejected substrate material can be removed together with the modulation layer when the modulation layer(s) is (are) removed after the generation of hole, to provide for a clean substrate surface that has hitherto no been achieved.

In one embodiment, the user-defined magnitude of voltage is in a range of 10V to 10 6 V, pref- erably from 1(TV to 3x1 (TV, more preferably from 10 V to 30x10 V and most preferably from 2xl0 3 V to 15xl0 3 V.

In one embodiment, said impedance of said voltage supply is an impedance > 1 Ohm, preferably > 10 kOhm, more preferably > 100k Ohm, and, even more preferably > 1 MOhm.

In one embodiment, said impedance is in the range from 1 Ohm to 1 GOhm, wherein, pref- erably, said impedance is variable within said range during performance of said method.

In one embodiment, said electrically insulating or semiconducting substrate is made of a material selected from a group comprising, silicon-based substrates, such as glass, quartz, silicon nitride, silicon oxide, semiconducting materials such as elemental silicon, including doped silicon and crystalline silicon, germanium, compound semiconductors, such as gallium arsenide, indium phosphide, as well as aluminium based crystalline materials such as alumina, spinel, sapphire, as well as ceramics such as zirconia.

In one embodiment, said substrate is electrically insulating at room temperature and is preferably made of a substrate material selected from glass, quartz, diamond, alumina, sapphire, aluminium nitride, zirconia, ceramics, and spinel, more preferably quartz, glass and ceramics as e.g. low temperature co-fired ceramics (LTCC), wherein, preferably, said substrate has an electrical resistivity > 10 9 Ohm cm at room temperature.

In one embodiment, said substrate is electrically semiconducting at room temperature and is preferably made of a substrate material selected from elemental silicon, including doped silicon and crystalline silicon, germanium, compound semiconductors such as gallium arsenide and indium phosphide. Preferably, said substrate has an electrical resistivity < 10 9 Ohm cm at room temperature.

In one embodiment, step b) is performed using a laser, preferably having a wavelength in a wavelength range which is at least partially absorbed by said substrate materials and/or said modulation layer.

The present inventors have surprisingly found that the quality of holes, recesses or wells generated in substrates can be astonishingly improved by using two modulation layers located on opposite sides of the substrate.

The process by which holes are introduced into substrates are effectively already described in WO 2005/0974339 and WO 2009/059786. The application of a voltage to the substrate leads to a controlled dielectric breakdown across the substrate, which, in turn, leads to an ejection of material from the substrate. According to the present invention, the use of modulation layers allows to improve the quality of the holes thus generated by such control dielectric break- down. Typically, the means for heating the substrate and the modulation layer(s) is a laser. In one embodiment, the laser heats the modulation layer. The selection of a specific laser depends on the substrate and modulation layer material. Examples are CC^-lasers having a wavelength of 10.6 μπι or 9.3 μπι. Other preferred lasers are lasers having a wavelength in the range of from 800 nm to 1300 nm. It should be noted that the laser wavelength is also chosen such that it is absorbed by the modulation layer and heats it, and/or it is absorbed by the substrate which heats the modulation layer. This allows a heating of the modulation layer(s). Absorption may be 100% or less, i.e. substantially all of fractions of the incident radiation is absorbed by the substrate or the modulation layer(s) or both. As used herein, the term "is at least partially absorbed" is meant to refer to any scenario wherein the substrate and/or the modulation layer(s) absorb 0.1% to 100% of the incident radiation. As used herein, the term "modulation layer" refers to a layer that is attached to the substrate, preferably in a side-by-side- manner, such that the modulation layer(s) and the substrate are adjacent and opposite to each other. In this arrangement the region in which a hole is to be generated in the substrate is also sometimes referred to as a "substrate region", and the corresponding region in the modulation layer lying opposite such substrate region is also referred to as a "modulating region". Effectively, such modulating region lies on top of (for the first modulation layer) or underneath (for the second modulation layer) said substrate region. In one embodiment, the modulating region and the substrate region are of approximately the same size, preferably the same cross-section. In another embodiment, the modulating region is 5% or more, such as 10%, 15%, 20%, 25%, 30%, 100%, larger in cross-section than the substrate region.

An embodiment of a basic setup used for generating a structure in a substrate is shown in Figure 1. The substrate to be processed is placed between two electrodes and a volume of material of said substrate is molten by a heat source. This heat source can be e.g. a laser or high frequency source capable of directly heating (causing dielectric losses in the substrate) or indirectly heating (by the electric arc) the substrate or a combination of such heat sources. The molten material is removed by applying a voltage across the substrate using the two electrodes connected to a voltage source and placed at a distance from and on opposite sides of the substrate.

If the heat source is a laser, its wavelength must be adapted to the substrate properties as e.g. absorption, transmittance, reflectance, thickness. In one embodiment said laser emits light which, upon irradiation on said substrate, is capable of penetrating said substrate material and which light, upon irradiation on said substrate, is not being fully absorbed or reflected at a surface of said substrate, but penetrates also in substrate regions underneath said surface. In another embodiment, said laser emits light which, upon irradiation on said substrate, is fully or nearly fully (>90%) absorbed at a substrate surface, and wherein said volume of material is heated by internal heat conduction. In either case more radiation is absorbed on the surface compared to regions underneath said surface (Lambert-Beer's law). With respect to the laser it is specifically the focal spot size, the power and application time that determines a sufficient through heating of the material and the size of the hole formed. Due to the wavelength (which is adapted to the material to be processed), the beam quality of the laser and the properties of the focussing optics, the achievable minimal spot size of the laser beam is limited. If the spot size has a larger diameter than the hole to be drilled, more material on the substrate surface is heated which is not or not fully removed upon voltage application. This causes an excess heat on the substrate surface leading to reduced quality as e.g. surface damage and residual tensions inside the substrate. Furthermore, ejected material may be re-deposited on the substrate surface around the holes which causes considerable surface contamination and introduces high residual tensions.

In accordance with the embodiments of the present invention a modulation layer, preferably a laser beam modulation layer is attached to two sides of the substrate surface in order to modulate and restrict laser access to the substrate surface so as to reduce excess heat and surface damage and to improve the quality of the surface surrounding the hole. Material properties and thickness of the modulation layer(s) depend on substrate material properties, thickness, laser application time and laser power.

In one embodiment the modulation layer modulates or restricts the amount of laser light accessing the substrate surface in such a way that a defined volume of modulation layer is locally removed from the substrate so as to expose a defined region of the substrate surface to the laser light, said region having a defined size, preferably a defined cross-section. This local removal of the modulation layer is preferably achieved by the energy applied during hole drilling process itself, e.g.by the laser beam. Assuming a nearly Gaussian beam profile the intensity of the laser light and therefore the energy density irradiated onto the material is highest in the center of the beam. It is therefore advantageous to use materials for the modulation layer with a phase transition between solid and liquid/gaseous state or more generally a transformation/decomposition threshold at a certain temperature or in general at a certain en- ergy or energy density. When the laser beam is applied to the modulation layer the phase transition or decomposition will be reached in the center of the laser spot thus leading to a local material removal at this position but not at the circumference or outer regions of the laser spot where energy is insufficient to overcome the transition threshold (Fig. 2). This local removal of material creates an opening in the modulation layer which is smaller than the laser spot size and therefore restricts laser access to the substrate material underneath. Therefore the irradiated (and subsequently molten) region on the substrate surface is reduced, resulting in less or no excess heat and therefore less surface damage and residual tensions which allows e.g. for a smaller pitch.

If the modulation layer material does not obey a sharp transition or decomposition or is in general not thermally stable upon laser irradiation the amount of removed modulation layer material and thus the size of irradiated substrate surface can be tuned by adapting the thickness of the modulation layer to the duration and power of the laser pulse. For a given duration of the laser pulse more energy is dissipated and, consequently, the material removal rate is higher where the laser intensity is higher (typically in the center of the spot). The application of a laser pulse onto the modulation layer material therefore creates a recess in the modulation layer which resembles the intensity profile of the laser beam. Upon increasing the thickness of the modulation layer the size of the opening created in the modulation layer and thus the size of the exposed substrate surface region can be decreased (Fig. 3) leading to less excess heat and surface damage.

A modulation layer each is attached to each side of the substrate. Such a sandwich structure, i.e. a layer on each side is preferred to avoid surface contamination due to ejected material that may be re-deposited in terms of e.g. filaments or spheres on both substrate surfaces around the holes. Without wishing to be bound by any theory, the inventors believe that, when a modulation layer is used, the ejected material cannot come to rest and stick to the substrate surface but is deposited on the layer or merges with it, or the presence of the modulation layer changes the way material is being ejected (due to altered geometry) so that no filaments are formed that stick to the hole entrance after the hole is drilled. Upon removing the modulation layer from the substrate after hole drilling all ejected material is removed together with the layer providing for a clean contamination-free substrate surface.

Material properties and/or thickness of the modulation layer(s) may be different on either side. For instance, the thickness can be reduced on the side where no laser beam is applied since there is no need for a restriction or modulation of the laser beam and the layer primarily serves to avoid material re-deposition and shielding/modulating heat from an electric arc formed during step b) or c). The material properties may differ on both sides because the opening in the modulation layer on the side where no laser is applied must be created by a DC high voltage discharge which opens the hole in the substrate or optionally by an AC high frequency arc forming between the electrodes. In both cases the material properties of then modulation layer must allow for its local removal at the position of the hole which is mainly achieved by thermal effects of the electric arc (either AC or DC or both).

The modulation layer can be attached to the substrate material in various ways, e.g. using a tape that sticks by itself like Scotch® Magic Tape , by adhesion due to a thin film of liquid as e.g. water or ethanol between substrate and layer, or by spin coating or spraying a layer of material on the substrate surface, or by electrostatic forces between substrate and layer, or every other technique that ensures a homogeneous attachment of the layer to the substrate without distortions like enclosed air bubbles or thickness variations that counteract a reliable hole drilling process.

It is advantageous to use modulation layer material or methods to attach them to the substrate that allow for easy detachment after holes were drilled. The layer can be detached e.g. mechanically by peeling it off or scratching, thermal dissociation as e.g. burning or ashing, or plasma ashing, chemical dissolution.

Materials that have been used are e.g. Scotch® Magic Tape , selfsealing insulation tape (3M), PDMS, wafer dicing tape, various polymers like PET foil, polyimide, resins, and composite systems. For 170 μπι thick glass substrates a modulation layer thickness of 20 μηι to 100 μιη was successfully used without the need of altering the process parameters to drill holes. For thicker modulation layers it was necessary to increase the laser application time or laser power in order to supply enough energy to locally remove the modulation layer.

In the following reference is made to the figures which are given as exemplary embodiments to illustrate, not to limit the present invention.

Figure 1 shows an example of a basic experimental setup. The substrate S with a modulation layer L attached on both sides was placed between two electrodes E (electrode-substrate gap distances 0.1 - 5 mm). The electrodes were connected to a high voltage DC power supply consisting of a capacitor C which was charged by a high voltage source (Rj being the DC impedance of the actual voltage source). For thin substrates an additional parasitic capacitance C s could form across the substrate. The process relevant source impedance was defined by reactive or resistive elements placed in the circuit (not shown) which may be e.g. a resistor or an inductor or a combination thereof, also optionally including a capacitance. The substrate is locally heated by focussed laser radiation and optionally as additional heat source a high frequency high voltage (not shown) is applied to the electrodes. Upon melting of the substrate material a high DC voltage from the HV DC power supply was applied between the electrodes driving the molten material out of the substrate. To provide immediately the full operating voltage to the electrodes the capacitor C was charged and, upon reaching the voltage application time, connected to the electrodes via a fast switch such as a triggered spark gap.

Figure 2 illustrates a Gaussian intensity profile I(r) of a laser beam with a spot size D, defined as I(D/2)/I(0) = 1/e" where n equals 1 or 2. When this beam is directed onto a modulation layer for a given time period an amount of energy is dissipated in the modulation layer which is proportional to the intensity of the laser beam. In regions of radial distances r < t from the center of the beam where intensity is larger than a certain threshold intensity I t this energy is sufficient to overcome e.g. the evaporation temperature or a decomposition threshold so that the modulation layer material will be removed. The orifice of diameter d = 2t created in that way in the layer material allows only a fraction of the laser beam to pass through and access the substrate material underneath the modulation layer. The surface area of the substrate which is shined on by the laser beam and consequently affected by laser heating is therefore restricted to diameter of d.

Figure 3 If the material properties of the layer do not allow for a sharp transition from solid/liquid to evaporation/decomposition as described in Fig. 2 but instead the material of the modulation layer is gradually removed over the entire area of the applied laser spot the diameter d of the orifice formed in the modulation layer can be defined by the thickness of the modulation layer. (A) The laser beam applied to the modulation layer of thickness h 0 creates a recess in the layer but no opening to the substrate underneath is created. (B) A reduced thickness hi of the modulation layer allows the created recess (same laser conditions) to approach the substrate underneath resulting in an orifice of diameter di in the modulation layer. (C) Re- ducing the layer thickness further to h 2 leads to a bigger orifice of diameter d 2 and a larger substrate surface area is exposed to the laser irradiation.

Figure 4 shows SEM images of holes drilled in 170 μπι thick glass, viewed from the side where the laser beam was applied. (A) Holes drilled without using a first or any modulation layer. Since the laser beam was larger than the holes drilled the surface around the holes is affected by the laser beam forming a molten surface layer or a crater. Ejected material is deposited around the holes. (B) shows the modulation layer, here a 100 um thick adhesive PET foil, still attached to the substrate after holes were drilled. The diameter of the recess in the modulation layer decreases from top to bottom (comp. to Fig. 3) thus exposing only a small surface area of the substrate to the laser beam. (C) The same array of holes as shown in (B) after the modulation layer was removed. The holes and surface around are clean and without deformations.

Figure 5 shows SEM images of holes drilled in 170 μπι thick glass, viewed from the side where no laser beam was applied. (A) Holes drilled without using a second modulation layer, but with a first modulation layer on the opposite side (which is not shown). The side without modulation layer is shown. The area around the holes is molten and deformed by the heat created by the high frequency arc and the DC discharge arc for hole opening. (B) shows the result of an experiment with first and second modulation layer. The layer shown is the second modulation layer located on the side which is not exposed to the laser beam, here a 40 um thick adhesive PET foil, which is still attached to the substrate after holes were drilled. (C) The same array of holes as in (B) after removing the modulation layer. The holes and surface around are clean and without deformations.

Figure 6 shows SEM images of holes drilled in 170 μηι thick glass using a modulation layer of 40 μπι thick self-sticking Scotch® Magic Tape on either side. Images were taken after the layer was removed. (A) shows the side where the laser was applied. (B) shows the ground side where no laser was applied.

Figure 7 shows the effect of different thicknesses of the first modulation layer. A non- adhesive PET foil was attached on both sides of the substrate using a thin film of water between the substrate and the modulation layer. Holes were drilled in 170 μιη thick glass substrate. Images were taken after layer was removed. (A) A 40 μπι thick PET foil was attached to the side where the laser was applied. There is no re-deposited material left between the holes but there is still a crater shape because the orifice created in the modulation layer and therefore the substrate surface area exposed to laser light was larger than the hole drilled. (B) Using an 80 μηι thick PET foil on the side where the laser was applied results in straighter holes without a crater shape at the entrance. The bright spot in the center of the holes is the metallic holder plate of the SEM on which the sample was placed to take images. This view through the holes also reveals a cylindrical hole shape and a clean inner surface.

Figure 8 shows the difference of the material properties of the first modulation layer on the side where the laser was applied. Images were taken with the modulation layer still attached to the substrate. (A) Modulation layer was a 40 μπι thick adhesive PET foil. A considerable volume of layer material has been removed resulting in a conical shape of the recess created in the modulation layer. Due to conical shape the substrate surface exposed is depends on the thickness of the modulation layer as illustrated in Fig. 3. (B) Modulation layer was a 40 μηι thick polyimide (PI) foil. Process conditions were the same as in (A). Thermal stability of PI is significantly higher compared to PET. E.g. the glass transition temperature of PI is around 400 °C (PET 70 °C), and it does not obey a melting point (melting temperature of PET is about 250 °C) but decomposes/vaporizes at temperatures significantly higher than the glass transition temperature. Due to the higher thermal stability the conditions are closer to those illustrated in Fig. 2 resulting in a recess which is smaller and more cylindrical as compared to (A).

The features of the present invention disclosed in the specification, the claims and/or in the accompanying drawings, may, both separately and in any combination thereof, be material for realizing the invention in various forms thereof.