Abstract:
A method for etching with a laser beam having a predetermined wavelength an area of a layer of a first material, said area being deposited at the surface of at least two second materials, includes: depositing a layer of a third material on the layer of the first material, the first and the third materials having a chemical affinity on application of the laser beam greater than the chemical affinity during said application between the first material and each of said at least two second materials; and applying the laser beam to an area of a free surface of the layer of third material vertically above the area of the layer of first material with a fluence of said laser beam causing the separation of said area.

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of laser etching, such as implemented in microelectronic component manufacturing methods. 
     BACKGROUND 
     Laser etching is a known method to remove a layer of material deposited at the surface of another material. By irradiating the layer of material to be removed, the electromagnetic energy heats up the surface thereof, after which the heat propagates all the way to the interface between the two materials where it is stored until a blister forms. The layer of material to be removed thus separates from the material supporting it. 
     The energy necessary for the separation characterized by the fluence received from the laser, the irradiation time, and the laser wavelength, or equivalently by the surface power, the irradiation time, and the laser wavelength, depends on the characteristics of the layer of material to be removed and on the characteristics of the material having this layer deposited thereon. The properties of the laser thus have to be adapted to each specific case. 
     Now, it is frequent for a layer of material to be removed to be deposited on two different materials. For example, on manufacturing of an organic transistor, the metal drain and source electrodes, usually made of gold, are deposited on a plastic substrate, after which an organic semiconductor layer having a thickness of some hundred nanometers is deposited and covers the assembly. At this stage of the manufacturing, the electrodes then need to be exposed. However, irradiating the semiconductor layer with a fluence and an irradiation time selected to separate the portion of semiconductor layer deposited on the plastic substrate also deteriorates the metal electrodes, or even separates them from the plastic substrate. 
     Thus, for example, to separate a 100-nanometer semiconductor layer deposited on a polyethylene naphthalate (PEN) substrate, a minimum fluence of 70 mJ/cm 2  is required for an irradiation of 30 nanoseconds with a laser at 245 nanometers. Now, such an irradiation is incompatible with gold drain and source electrodes deposited on the substrate since these electrodes are deteriorated as soon as the fluence is greater than 55 mJ/cm 2 . It is thus impossible to separate both the portion of semiconductor layer deposited on the plastic substrate and the portion of semiconductor layer deposited on the metal electrodes by means of one and the same irradiation applied to the entire organic semiconductor layer. Usually, this layer is thus removed by means of a chemical processing, which has the disadvantage of leaving residues. 
     SUMMARY OF THE INVENTION 
     The present invention aims at providing a laser etching method which decreases the laser energy necessary to separate a first material deposited on a second material, which in particular makes it possible to separate a material deposited on two different materials. 
     For this purpose, the present invention aims at a method for etching with a laser beam having a predetermined wavelength an area of a layer of a first material, said area being deposited at the surface of at least two second materials, the method comprising:
         depositing a layer of a third material on the layer of first material, the first and the third materials having a chemical affinity on application of the laser beam greater than the chemical affinity during said application between the first material and each of said at least two second materials; and   applying the laser beam to an area of a free surface of the layer of the third material vertically above the area of the layer of the first material with a fluence of said laser beam causing the separation of said area.       

     In other words, it was observed that by deliberately depositing a layer which bonds to the layer of first material more strongly than the layer of first material bonds to the second materials, the minimum energy necessary to separate the layer of the first material is lower. 
     As described previously, to separate a layer of semiconductor material of 100 nanometers deposited on a PEN layer by directly irradiating the layer of semiconductor material, it is necessary to provide a fluence of at least 70 mJ/cm 2  for an irradiation of 30 nanoseconds with a 248-nanometer laser. 
     As an example of the invention, by depositing on a layer of organic semiconductor material a layer of 30 nanometers of fluorinated polymer, the minimum necessary fluence falls to 50 mJ/cm 2 . 
     Generally, it could have been observed that there is a decrease of the minimum energy to be applied as soon as the bonding between the third material and the first material is greater than the bonding between the first material and the second materials. 
     According to an embodiment, the laser is an excimer laser. 
     According to an embodiment of the invention, the thickness of the layer of third material ranges between 1 nanometer and 1 micrometer. 
     According to an embodiment of the invention, the material and the thickness of the layer of third material is selected according to the fluence of the laser beam, to the nature of the first material, and to the thickness of the area of first material to be separated. 
     According to an embodiment of the invention, the first material is an organic semi-conductor material and the second materials respectively are a plastic material and a conductive material. 
     In particular, the organic semiconductor material is a fluorinated material, the conductive material is a metal or a conductive polymer, and the third material is a fluorinated polymer. 
     More specifically, the third material is CYTOP®, and/or the first material is TIPS, and/or the third material has an enthalpy for the bonding with the first material greater than 15 kJ·mol −1 . Further, the thickness of the layer of fluorinated polymer is substantially equal to 100 nanometers, the thickness of the layer of the first material is substantially equal to 100 nanometers, the thickness of the layer of conductive material is substantially equal to 30 nanometers, the fluence of the laser beam is lower than 50 mJ/cm 2 ; and the time of irradiation by the laser beam is substantially equal to 30 nanoseconds. 
     According to an embodiment of the invention, the third material has a greater absorption of the laser beam wavelength than the first material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on analysis of the following description, given as an example only, and made in relation with the appended drawings, where  FIGS. 1 to 5  are simplified cross-section views illustrating a laser etching method according to the invention applied to the removal of an organic semiconductor layer deposited both on a plastic substrate and on a metal electrode. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An application of a method according to the invention tending to remove a layer of organic semiconductor material present on top of and around a metal electrode deposited on a plastic substrate, as is for example the case during the manufacturing of an organic transistor where the drain and source electrodes require being exposed for the rest of the transistor forming, will be described. 
       FIG. 1  thus schematically illustrates in cross-section view a stack obtained during a method for manufacturing an organic transistor. This stack comprises a substrate  10  made of PEN (polyethylene naphthalate), having a drain or source electrode  12 , for example, made of gold, deposited thereon, the assembly being covered with a layer of organic semiconductor material  14 , for example, of the pentacene family, for example, TIPS (propylsilylethynyl pentacene), or amorphous polymers, for example, of TFB (dioctyl-fluorene-butylphenyl-diphenylamine) type. Layer  14  is thus deposited both on plastic substrate  10  and on metal electrode  12 . At this stage of the manufacturing, layer  14  requires being removed from an area  16  on top of and around electrode  12 . 
     To remove portion  16  of semiconductor material layer  14  by means of an excimer UV laser beam, a method according to the invention starts with the deposition on layer  14  of a layer of a polymer, for example, fluorinated,  18 , and especially a layer of CYTOP® of Asahi Glass Co. Ltd having a thickness lower than 1,000 nanometers ( FIG. 2 ). 
     Fluorinated polymer layer  18  absorbs UVs, and thus the radiation originating from the laser, and its bonding to semiconductor material layer  14  very strongly increases by the development of chemical bonds having a high bonding enthalpy, for example, a bonding enthalpy of energy greater than 15 kJ·mol −1  in the case of hydrogen bonds. Advantageously, semiconductor material  14  and/or the material of layer  18  is a fluorinated polymer. 
     Such a high bonding enthalpy especially results in a stronger bonding of layer  18  to layer  14  than that of layer  14  to electrode  12  and to substrate  10 . 
     The method then carries on with the irradiation of portion  16  of semiconductor layer  14  with a laser beam  20  according to a fluence and according to an irradiation time suited to the separation of layer  14  both of gold electrode  12  and of PEN substrate  10 , as discussed hereafter ( FIG. 3 ). A mask  21  is for example deposited on layer  18  without covering the portion of layer  18  corresponding to portion  16  to be removed and laser beam  20  is submitted to a full plate irradiation. 
     The incident electromagnetic energy is then stored at interface  22  between metal electrode  12  and semiconductor material layer  14  and at interface  24  between semiconductor material layer  14  and plastic substrate  10 , so that blisters  26 ,  28  form at interfaces  22  and  24  ( FIG. 4 ). 
     Once blisters  26 ,  28  have reached a critical size, the semiconductor material above them separates, which results in exposing metal electrode  12  without leaving residues of semiconductor material ( FIG. 5 ). Mask  21  and/or layer  18  are then removed. 
     For a semiconductor material layer  14  having a 100-nanometer thickness, a metal electrode  22  having a 30-nanometer thickness, and a laser having a wavelength of 245 nanometers generating a laser pulse of 30 nanoseconds:
         if the thickness of layer  18  ranges between 1 and 50 nanometers, a minimum fluence ranging between 30 and 40 mJ·cm 2  is necessary to separate semiconductor layer  14  both from metal electrode  12  and from plastic substrate  10 . No damage to metal electrode  12  can be observed;   if the thickness of layer  18  ranges between 50 and 100 nanometers, a minimum fluence ranging between 40 and 50 mJ·cm 2  is necessary to separate semiconductor layer  14  both from metal electrode  12  and from plastic substrate  10 . Here again, no damage to metal electrode  12  can be observed; and   if the thickness of layer  18  is greater than 100 nanometers, a minimum fluence ranging between 50 and 70 mJ·cm 2  is necessary to separate semiconductor layer  14  both from metal electrode  12  and from plastic substrate  10 . For fluences greater than 55 mJ·cm 2 , some damage to metal electrode  12  can be observed.       

     Accordingly, the thickness of layer  18  is selected to be lower than 100 nanometers and the laser beam is set for a fluence lower than 50 mJ·cm 2 . 
     This value of the fluence should be compared with the 70 mJ·cm 2  value which is the minimum fluence necessary to remove a portion of semiconductor material layer deposited on plastic substrate  10  in the absence of fluorinated polymer layer  18 . 
     It could thus be observed that the designer has at least two parameters relative to layer  18  to adjust the laser fluence to a value which does not damage metal electrode  12 , that is:
         the nature of the material of layer  18  which defines the “intensity” of the bonding between layer  18  and semiconductor layer  14 ; and   the thickness of layer  18  which also adjusts the intensity of a mechanism, in all likelihood, as mentioned hereabove, of vibratory nature, which helps the separation of layer  14 .       

     It has already been observed that for an equal bonding, the minimum fluence decreases as the absorption of the laser radiation by layer  18  increases. 
     Preferably, the material of layer  18  is selected to bond as strongly as possible to the layer of semiconductor material to obtain the highest possible difference between, on the one hand, the bonding of layer  18  and of semiconductor material layer  14 , and on the other hand, the bonding of layer  14  with the other materials on which it is deposited, since the fluence is adjusted by the thickness of layer  18 . Indeed, the more strongly layer  18  bonds to layer  14  and the more the minimum fluence necessary to separate layer  14  decreases. The range of usable fluences is thus higher. 
     Generally, a decrease in the energy necessary for the separation can be observed as soon as a stronger bonding layer of material is deposited on a layer to be removed, this layer being itself deposited on one or several materials. 
     For example, although the above-described example relate to an organic semiconductor layer, the invention also works with layers of organic insulator, such as for example a SiO 2 , TiO 2 , or Al 2 O 3  layer. Similarly, the invention also works with electrodes formed of another conductive material than gold, like for example electrodes made of Ni, Cu, or of conductive polymer (for example, PDOT). Similarly, the invention works with substrates made of other types of plastic than PEN, such as for example substrates made of PET (polyethylene-terephthalate) or of Kafton® (based on polyimide). 
     It should be noted that the above-described embodiment relates to the removal of a layer deposited on two materials. It should be understood that the invention also applies to the case of the removal of a layer deposited on a single material, for example, and without this being a limitation, to the case of a layer of organic semiconductor material deposited on a plastic substrate. Indeed, the simple fact of providing an appropriate layer  18  results in decreasing the minimum energy necessary for the separation, which may be advantageous, for example, in terms of laser equipment and of optics. 
     It should also be noted that the relations between the different involved bondings need not be established before the irradiation. Such relations may be materialized during the very application of the irradiation under the effect thereof, for example, due to a modification of the bonds between the different materials. Indeed, these relations should be verified at the time of the separation, and the fact for these relations not to be verified before application of the irradiation matters little. 
     Similarly, the nature of the material of the layer deposited on the layer to be removed and/or its thickness may be selected to obtain a slight difference, for example, due to manufacturing costs and constraints. In this case, the minimum energy necessary for the separation will be higher and will most likely result in a degradation of one of the materials. However, due to the sole decrease, even minute, of the energy to be brought, such a degradation will thus be minimized with respect to that observed in the state of the art.