Method for producing a printed circuit board

Abstract

The invention describes a method for producing a printed circuit board having a metallic conductor structure. The method comprises providing a base substrate (101) which is formed as a film or plate, has a first substrate side (101a) and a second substrate side and at least partially consists of an electrically non-conductive organic polymer material and in which the first substrate side (101a) is covered with a covering metal layer (102), and removing the covering metal layer (102) in regions. In order remove the covering metal layer (102) in regions, a mask layer (103) is applied to the covering metal layer (102). This mask layer is then removed in regions. In the process, the first substrate side (101a) is divided into at least one first subregion (104), in which the first substrate side (101a) is covered only with the covering metal layer (102), and into at least one second subregion (105), in which the first substrate side (101a) is covered with the covering metal layer (102) and by the mask layer (103). The covering metal layer (102) in the at least one first subregion (104) is then removed. The invention proposes removing the mask layer (103) in the first subregion (104) by an etching treatment using an etching solution and implementing a phase transition or a chemical transformation in the mask layer (103) before the etching treatment.

Description

[0001] Process for printed circuit board manufacturing

[0002] The invention described below relates to a method for producing printed circuit boards.

[0003] BACKGROUND OF THE INVENTION

[0004] A printed circuit board (PCB) serves as a carrier for electronic components and ensures their electrical connection. Almost every electronic device contains one or more printed circuit boards.

[0005] Printed circuit boards always comprise a base substrate that is electrically non-conductive and has a structure of conductor tracks (short: conductor structure) on at least one side of the substrate for electrically contacting the electronic components. Base substrates for printed circuit boards are typically made of fiber-reinforced plastic, plastic film, or laminated paper. The conductor tracks are usually made of a metal such as copper.

[0006] In the simplest case, only one side of the base substrate has a conductor structure. For more complex circuits, however, more than one conductor layer is often required, which is when a multilayer printed circuit board (MLB) is needed. In these cases, for example, both sides of a base substrate can be provided with a conductor structure, or several base substrates, each with a conductor layer, can be combined to form an MLB. In particular, base substrates provided with a conductor structure on both sides can also form the basis for multilayer structures. The conductor tracks of the various conductor layers can be electrically connected to one another via vias. For this purpose, holes can be drilled into the base substrates, for example, and the drill hole walls can be metallized.

[0007] The formation of the conductor structures on a base substrate is carried out classically subtractively in a multi-stage photolithographic process using a photoresist (short: resist), whose solubility in a developer solution can be influenced by radiation, particularly UV radiation. In a typical procedure, a metal layer is formed on the base substrate and covered with a layer of photoresist. The photoresist layer can, for example, be laminated onto the metal layer. The photoresist layer is then exposed to the aforementioned radiation in an exposure step, with partial areas of the layer being protected from radiation exposure by an exposure mask.Depending on the photoresist and developer solution used, after the exposure step, either the exposed or unexposed portions of the photoresist layer are soluble in the developer solution and can be removed in a subsequent step. In this subsequent step, the development step, portions of the metal layer on the base substrate are exposed, which can be removed wet-chemically in a further subsequent step, an etching step. The remnants of the metal layer remaining after the subsequent complete removal of the resist form the desired conductor structure. If necessary, this can be reinforced in a deposition step—for example, by electroplating a suitable metal.

[0008] Due to manufacturing constraints, the conductive traces are located on the surface of a base substrate. This can be disadvantageous in the production of MLBs. If a surface of a base substrate equipped with conductive traces is pressed onto another base substrate, subsequent inspection and correction is often required due to deviations caused by the pressures and temperatures encountered during pressing. Conductive traces on the surface of base substrates are particularly exposed to such stresses. The smaller the spacing and dimensions of the conductive traces on the substrate, the greater the need for inspection and correction, for example, with regard to existing impedance and signal speed requirements.

[0009] A general disadvantage of traditional subtractive processes is that the resolution of the conductor structures produced is limited. Conductor tracks with widths of no less than 40 to 80 pm are common. Conductor tracks with widths in the low double-digit or even single-digit pm range are almost impossible to produce this way.

[0010] WO 2021 / 001167 A1 discloses a method for printed circuit board production in which a thin plastic film covered on at least one side with a cover metal layer is provided as the base substrate. This layer is removed in certain areas, followed by a plasma treatment, which simultaneously creates depressions in the film wherever the film is exposed. To form a conductor structure, the depressions are then filled with a filler metal. The residues of the cover metal layer, which serve as a mask during the plasma treatment, are then removed. Printed circuit boards produced in this way are characterized by the fact that the conductor tracks are not arranged on a surface but are embedded in the processed film, which can compensate for some of the disadvantages of conductor tracks produced using conventional subtractive processes.However, the subtractive etching of the metal layer formed on the base substrate, which is required in classical subtractive processes, is also necessary in a procedure according to WO 2021 / 001167 A1.

[0011] Such etching processes are problematic for environmental reasons. Commercially available photoresists are not recyclable. The resulting waste solutions contain organic compounds, making their environmentally friendly disposal complex and expensive. Furthermore, many processes require the removal of the formed resist in a separate process step after subtractive etching.

[0012] WO 2022 / 028852 A1 discloses a method for producing a printed circuit board with a metallic conductor structure. A base substrate, designed as a foil or plate, is provided, having a first substrate side and a second substrate side, which base substrate consists at least partially of an electrically non-conductive organic polymer material and in which the first substrate side is covered with a cover metal layer. The cover metal layer is removed in certain regions. For the partial removal of the cover metal layer, a mask layer made of titanium, zinc, titanium dioxide, titanium nitride, zinc oxide, and / or a polymer material is applied to the cover metal layer.This mask layer is removed in regions using a laser, so that the first substrate side is divided into at least a first partial region in which the first substrate side is covered only with the cover metal layer, and into at least a second partial region in which the first substrate side is covered with the cover metal layer and by the mask layer. The cover metal layer in the at least one first partial region is then removed using an etching solution. This procedure makes it possible to replace the classic photoresist process in printed circuit board production. Instead of the classic photoresists, the metal and / or the metal compound and / or the polymer material is used to form the mask layer, which have the advantage of being structurable using a laser ablation process. The metal or polymer material ablated, for example, in the context of the above step d.The ablated metal compound can be vacuumed, collected, and recycled, for example, to form additional mask layers. Thus, the described process enables environmentally friendly production of printed circuit boards in a closed-loop system.

[0013] However, this approach also has its drawbacks. A significant disadvantage in some cases is that, depending on the material chosen for the mask layer, the laser-assisted removal of the mask layer in sections can generate fumes and dust that require laborious removal.

[0014] DESCRIPTION OF THE INVENTION

[0015] The object of the present invention was to further improve the procedure for producing printed circuit boards known from WO 2022 / 028852 A1, in particular to avoid or at least reduce the problems described.

[0016] To solve this problem, the invention proposes the method having the features of claim 1. Further developments of the invention are the subject of subclaims. The wording of all claims is hereby incorporated by reference into this description.

[0017] The inventive method for producing a printed circuit board with a metallic conductor structure always comprises the immediately following steps a. and b.: a. Providing a base substrate in the form of a foil or plate with a first substrate side and a second substrate side, which base substrate consists at least partially of an electrically non-conductive organic polymer material and in which the first substrate side is covered with a cover metal layer. b. Removing the cover metal layer in certain regions.

[0018] The cover metal layer is removed in sections as follows: c. A mask layer is applied to the cover metal layer. d. The mask layer is removed in sections, so that the first substrate side is divided into at least one first partial region, in which the first substrate side is covered only with the cover metal layer, and into at least one second partial region, in which the first substrate side is covered with the cover metal layer and by the mask layer. e. The cover metal layer is removed in the at least one first partial region.

[0019] The method is particularly characterized by the following step: f. The mask layer is removed in the first partial area by an etching treatment using an etching solution and before the etching treatment to remove the mask layer, a phase transformation or a chemical transformation is brought about in the mask layer in the first partial area.

[0020] A phase transformation here means that the material of the mask layer in the partial area is completely or partially converted either from a crystalline or semi-crystalline state into an amorphous state or from an amorphous state into a crystalline or semi-crystalline state.

[0021] Steps a. to e. are already known from WO 2022 / 028852 A1, the content of which is hereby incorporated by reference. However, step e. is new, which has the advantage that no dust is generated during the removal of the mask layer. It is preferred that the phase transformation occurs without removing material from the mask layer. Instead of ablation, the present invention provides for said phase transformation or said chemical transformation in combination with said etching treatment. The mask layer can be removed wet-chemically using the etching solution.

[0022] Phase transformations can fundamentally be brought about in several ways. However, according to the invention, at least one laser is preferably used for this purpose. Accordingly, the method according to the invention is preferably characterized by at least one of the immediately following features a. to e.: a. The phase transformation is brought about by means of a laser. b. The laser is operated in pulsed or continuous mode. c. The laser has a wavelength in the range of 300 nm to 600 nm. d. The laser is operated at an energy that is insufficient to cause ablation of the mask layer. e. The laser is operated in pulsed mode with a pulse energy in the range of 0.1-100 pJ.

[0023] Short laser pulses, in particular, can lead to amorphization or recrystallization of regions, depending on the material chosen for the mask layer, for example, regions with diameters of approximately 1 pm. The laser should not be operated at excessive energy to avoid ablation of the mask layer. Suitable laser parameters can be optimized through experimentation if necessary. The parameters depend, for example, on the material of the mask layer.

[0024] In a preferred embodiment, for example, a polycrystalline mask layer can be formed, for example by depositing silicon using PECVD (plasma-enhanced chemical vapor deposition) and a subsequent heat treatment (for example, annealing at 850 °C for a period of, for example, one minute), and converted into an amorphous phase state using a laser. Suitable lasers for this purpose include a wavelength of 354 nm, 515 nm, or 532 nm, a pulse energy of, for example, 100 nJ, and a pulse duration of, for example, 100 ps.

[0025] Alternatively, the phase transformation can also be achieved by heat treatment. This applies in particular to the transition from amorphous to crystalline or semi-crystalline. Accordingly, the process according to the invention is preferably characterized by the immediately following feature a.: a. The phase transformation is achieved by heat treatment.

[0026] In a preferred embodiment, for example, an amorphous mask layer can be formed, for example by sputtering silicon, and converted into a polycrystalline phase state by heat treatment. During the heat treatment, the mask layer in the first partial region can be heated, for example, to a temperature in the range of 800°C to 1000°C. The method according to the invention is particularly preferably further characterized by at least one of the immediately following features a. to c.: a. The mask layer is formed from a mask material that is capable of reversible amorphization and crystallization, wherein the resistance of the mask material to the etching solution depends on the phase state of the mask material. b.The mask layer consists of an inorganic semiconductor or a compound of an inorganic semiconductor, in particular from the group consisting of silicon, SiO2, SiN, gallium, indium, and GaAs. c. The mask layer consists of a polymer material that can assume at least a semi-crystalline state and an amorphous state.

[0027] Particularly preferably, features a. and b. or a. and c. are implemented in combination.

[0028] The mask layer can also consist of a ceramic material, an oxide or a nitride.

[0029] The concept of the present invention is based on the realization that the etching rate at which the mask layer is removed during the etching treatment in the first partial region depends on the phase state of the respective material for most materials in question. In other words, the effectiveness of an etching solution can be influenced by a phase change in the material to be etched. For example, a partial region in which the mask layer is amorphous may be chemically less resistant to an etching solution than a crystalline or semi-crystalline partial region. The same is possible in reverse, depending on the material selection.

[0030] The same effect can also be achieved by a chemical transformation of the material to be etched.

[0031] In the context of the invention, comparatively high chemical resistance means that a material exhibits a lower tendency to dissolve in contact with an etching solution than a comparison material. A material is therefore more chemically resistant than another if it dissolves more slowly, i.e., at a lower etching rate, in contact with an etching solution under identical conditions (in particular, the same etching solution, the same temperature).

[0032] The method according to the invention is particularly preferably characterized by one of the immediately following features a. and b.: a. The mask layer is converted by the phase transformation in the first partial region into a phase state in which it has a lower chemical resistance to the etching solution than before the phase transformation. b. The mask layer is converted by the chemical transformation in the first partial region into a state in which it has a lower chemical resistance to the etching solution than before the chemical transformation.

[0033] In preferred embodiments, a mask material is selected that is capable of laser-induced reversible amorphization and crystallization. This concept is applicable to both inorganic and organic mask layers. Essentially, all that matters is that said phase transformation can occur and that different phase states lead to different etch rates. This, too, can be determined and optimized experimentally for individual materials.

[0034] A particularly preferred material is silicon, which can be converted from an amorphous to a crystalline or semi-crystalline state, as well as from a crystalline or semi-crystalline to an amorphous state.

[0035] The above-mentioned chemical transformation means that the material of the mask layer in the first partial region is completely or partially chemically transformed. Particularly preferably, the chemical transformation is an oxidation (e.g., Si → SiOx with x < 2). However, other chemical transformations are also possible, such as nitride formation (e.g., silicon → SiN).

[0036] Accordingly, in some preferred embodiments, the process according to the invention is characterized by at least one of the following additional features a. to c.: a. The chemical conversion is an oxidation. b. The chemical conversion occurs by exposing the mask layer to an oxidative atmosphere. c. The chemical conversion occurs by heating the mask layer. d. The chemical conversion is effected by means of a laser.

[0037] Preferably, the immediately above features a. and b. are implemented in combination. Particularly preferably, the immediately above features a. to c. are implemented in combination. In further preferred embodiments, features a. and b. and d. or features a. to d. are implemented in combination.

[0038] In possible further developments, the following features may be preferred: a. The mask layer is formed from a metallic or semi-metallic mask material whose oxide or nitride is less chemically resistant to the etching solution than the metallic or semi-metallic mask material. b. The mask layer consists of an inorganic semiconductor or a compound of an inorganic semiconductor, in particular from the group consisting of silicon, gallium, indium, and GaAs. The inorganic semiconductor can be doped or undoped.

[0039] It may be necessary to protect the mask layer outside the first sub-area in order to carry out the chemical conversion. However, it is preferable to also carry out the chemical conversion using a laser. For this purpose, the mask layer can be exposed to an oxidative atmosphere or an atmosphere containing another reactant, such as nitrogen or ammonia. A laser then supplies the activation energy required for the respective chemical conversion specifically in the first sub-area. Masking outside the first sub-area is then not mandatory.

[0040] The cover metal layer is also preferably removed by an etching treatment using an etching solution. In principle, two different etching solutions can be used to remove the mask layer in the first partial region and the cover metal layer, and / or the etching treatment can be performed in separate steps. However, it is particularly preferred that the etching treatment of the mask layer and the etching treatment of the cover metal layer be performed in one step using one and the same etching solution.

[0041] Accordingly, the method according to the invention is particularly preferably further characterized by at least one of the immediately following features a. and b.: a. The etching treatment using the etching solution removes the mask layer and the cover metal layer in the first partial region in one step. b. The etching solution is an acidic or alkaline solution for etching copper.

[0042] Commercially available etching solutions, particularly aqueous commercially available etching solutions, can be used within the scope of the invention. The core idea of ​​the invention is to vary the sensitivity of a material to an etching solution through targeted phase transformation or chemical conversion. This can, in principle, be achieved with any etching solution.

[0043] In preferred embodiments, potassium hydroxide solution or sodium hydroxide solution can be used as alkaline etching solutions.

[0044] In preferred embodiments, an aqueous solution comprising nitric acid and hydrogen fluoride or an aqueous solution comprising hydrochloric acid and hydrogen peroxide can be used as the acidic etching solution. The solution comprising nitric acid and hydrogen fluoride may be particularly preferred when etching silicon.

[0045] In a preferred embodiment of the invention, the method additionally comprises at least one of the immediately following features a. and b.: a. The cover metal layer is formed from copper. b. The etching solution is a solution for etching copper.

[0046] Those skilled in the art will understand that the copper does not necessarily have to be pure copper. If appropriate, the cover metal layer may also comprise, preferably in small quantities, portions of one or more other metals. The cover metal layer may therefore also consist of a copper alloy. As an alternative to copper or the copper alloy, nickel-chromium alloys may also be used as the cover metal. In this case, the etching solution is a solution for etching the nickel-chromium alloy. The metal and / or the metal compound and / or the polymer material from which the mask layer is formed must, in this case, be more chemically resistant to the etching solution than to the nickel-chromium alloy.

[0047] The etching solution is particularly preferably an etching solution based on copper chloride, sodium persulfate, ammonium persulfate, copper sulfate, and ferric chloride. A solution containing hydrochloric acid and hydrogen peroxide is also suitable. This applies particularly to cases where the cover metal layer is made of copper or a copper alloy.

[0048] In particular, these copper etching solutions can also etch mask layer materials that are specifically designed for them. This makes it possible to etch the mask layer and the cover metal layer with the same etching solution, even in a single step.

[0049] Regardless of its material composition, the cover metal layer should be closed before partial removal. It preferably has a thickness in the range of 10 nm to 10 pm, particularly preferably in the range of 20 nm to 6 pm.

[0050] To form the cover metal layer, it is possible to apply, in particular laminate or cover, a thin metal foil, in particular a thin copper foil, to the first substrate side as a cover metal layer. Alternatively, to provide the base substrate, the cover metal layer can also be formed on the first substrate side by means of physical vapor deposition (PVD) or chemical vapor deposition (CVD), by sputtering, or by a wet-chemical coating process.

[0051] Metallizations by physical and chemical vapor deposition as well as the production of metal layers by wet chemical coating processes or sputter deposition are state of the art and require no further explanation.

[0052] It may be preferred that an adhesion-promoting adhesion layer is applied before the formation of the cover metal layer or when the cover metal layer is applied to the first substrate side.

[0053] In a preferred embodiment of the invention, the method additionally comprises at least one of the immediately following additional features a. to d.: a. The mask layer has a thickness in the range of 5 nm to 10 pm or 10 nm to 10 pm. b. The mask layer is formed by physical or chemical vapor deposition. c. The mask layer is formed by sputtering. d. The mask layer is formed by a wet-chemical coating process.

[0054] Particularly preferably, feature a. is implemented in combination with one of steps b., c. or d.

[0055] Particularly preferably, the mask layer is formed with a thickness in the range from 5 nm to 1000 nm, more preferably from 5 nm to 500 nm, especially preferably from 5 nm to 250 nm. When using a polymer material, the layer can also have a thickness in the pm range, for example in the range from 2 to 10 pm. In a particularly preferred first variant of the method, the method is additionally distinguished by the immediately following feature a.: a. The mask layer is removed after the removal of the cover metal layer in the at least one second partial region, the cover metal layer remaining in the second partial region as a metallic conductor structure.

[0056] The partial removal of the cover metal layer using a masking step is thus completed. The cover metal layer remaining in the second partial area forms the metallic conductor structure. The production of the printed circuit board can then be continued in the conventional manner. For example, the conductor structure can be

[0057] For protection, they can be coated with a solder mask. Free contacts can be coated with a precious metal, such as gold, silver, or platinum.

[0058] In an alternative, particularly preferred second variant of the method, the method is additionally characterized by at least one of the immediately following features a. to d.: a. After removal of the cover metal layer in the at least one first partial region, the first substrate side is exposed to a plasma, with the aid of which the polymer material is removed in the at least one first partial region to form at least one depression. b. The at least one depression is filled with a filler metal. c. Complete removal of the cover metal layer and the mask layer in the at least one second partial region to obtain a metallic conductor structure in the at least one depression. d. If appropriate, planarization of the first substrate side with the filled at least one depression.

[0059] Particularly preferably, the immediately preceding steps a., b., and c. are carried out in combination with each other. Step d. is an optional step that can follow. In some embodiments, the complete removal of the

[0060] However, the cover metal layer and the mask layer can also be planarized according to step d. Step c. and step d. can therefore be identical in some embodiments.

[0061] In some preferred embodiments of this variant, step c. is carried out before step b., i.e. it takes place after the first substrate side has been exposed to the

[0062] Plasma first completely removes the cover metal layer and the mask layer in at least one second partial area. This results in a first substrate side with recesses that are free of the cover metal layer and the mask layer. The recesses are then filled with the filler material in a subsequent step, ideally followed by planarization according to step d.

[0063] According to this second variant of the method, the conductor structure is formed in the at least one recess and not in the second partial region. The result is a conductor structure that is recessed into the base substrate.

[0064] In particularly preferred embodiments, the complete removal of the mask layer does not occur as part of a separate removal step. Rather, it is preferred that the mask layer be removed during the plasma treatment according to feature a. of the described, particularly preferred second variant of the method. This is particularly efficient if the mask layer is formed from the polymer material. With a sufficiently long treatment time, the mask layer can be completely removed by means of the plasma. This eliminates an entire process step, which can significantly increase the efficiency of the process.

[0065] In a further development of the particularly preferred first and second variants of the method, the method is used to construct a multilayer printed circuit board. The conductor structure sunk into the base substrate and obtained according to the second variant, or the cover metal layer functioning as a conductor structure and remaining in the second partial region according to the first variant of the method, forms a first conductor structure in this multilayer printed circuit board, which can optionally be connected to further conductor structures in the printed circuit board. In this further development, the method additionally comprises at least one of the immediately following additional features a. to f.: a.Covering the first conductor structure with a layer made of an electrically insulating material which, in combination with the base substrate, has a bottom side in direct contact with the first conductor structure and a top side facing away from the first conductor structure and which consists at least partially of an electrically non-conductive organic polymer material. b. If not already present, forming a cover metal layer on the top side of the layer made of electrically insulating material, c. Removing the cover metal layer in regions, dividing the top side into at least a first partial region in which the top side is free of the cover metal layer, and at least a second partial region in which the top side is covered with the cover metal layer, d. Exposing the top side to a plasma, with the aid of which plasma the polymer material is removed in the at least one first partial region to form at least one depression, e.Filling the at least one recess with a filler metal and f. Completely removing the cover metal layer in the at least one second partial region to form a second conductor structure or a part of a second conductor structure.

[0066] Preferably, at least the immediately above features a. to c. are implemented in combination with one another. The three steps according to features d. to f. follow in combination in preferred embodiments.

[0067] However, it is also conceivable that the cover metal layer remaining in the second partial region of the upper side already forms a second metallic conductor structure or part of the metallic conductor structure, analogous to the particularly preferred first variant of the method described above. In this case, the immediately preceding steps d. to f. are not necessary.

[0068] Of course, the region-wise removal of the cover metal layer in the immediately preceding step c. can be carried out by subdividing the upper side of the layer of electrically insulating material in preferred embodiments in the same way as in the case of the region-wise removal of the cover metal layer on the first substrate side of the base substrate.

[0069] The layer of electrically insulating material can be a plastic film. In particular, the layer can also be designed like the base substrate provided in step a. of claim 1. It thus preferably consists at least partially of an electrically non-conductive organic polymer material. In particularly preferred embodiments, the layer of electrically insulating material and the base substrate are identical.

[0070] By repeated repeated steps a. to c., in particular steps a. to f., MLBs with essentially any number of layers can be constructed sequentially.

[0071] In a preferred development of the invention, the method comprises at least one of the immediately following features a. and b.: a. The base substrate and / or the layer of electrically insulating material has a thickness in the range from 10 μm to 3 mm, preferably in the range from 10 μm to 2 mm. b. The organic polymer material of the base substrate and / or the layer of electrically insulating material is a thermoplastic polymer material, preferably selected from the group consisting of polyimide, polyamide, Teflon, polyester, polyphenylene sulfide, polyoxymethylene, and polyether ketone.

[0072] Preferably, the immediately above features a. and b. are implemented in combination with one another.

[0073] Particularly preferably, the base substrate and the layer of electrically insulating material are each a film made of a polymer material, in particular one of the polymer materials mentioned. This applies in particular if the

[0074] The printed circuit board is formed in multiple layers. In the case of a single-layer printed circuit board, a comparatively thicker base substrate formed as a plate is selected in some preferred embodiments.

[0075] The method according to the invention, in particular the particularly preferred second variant of the method, is particularly preferably characterized by at least one of the immediately following additional features a. to c.: a. The base substrate and / or the layer of electrically insulating material comprise fillers, in particular dielectric fillers. b. The base substrate and / or the layer of electrically insulating material is a plastic film with the fillers. c. The fillers have an average particle size (d50) < 1 pm.

[0076] Preferably, the immediately above features a. and b., in particular also a. to c., are realized in combination with one another.

[0077] Optionally, the base substrate and / or the layer of electrically insulating material may comprise fillers, in particular dielectric fillers. For example, the base substrate and / or the layer of electrically insulating material may each be a film made of one of the aforementioned polymer materials, in which silicon dioxide particles are embedded.

[0078] Suitable dielectric fillers include metal or semi-metal oxides (in addition to silicon dioxide, especially aluminum oxide, zirconium oxide, or titanium oxide) and other ceramic fillers (especially silicon carbide, boron nitride, or boron carbide). Silicon can also be used if necessary.

[0079] The fillers are preferably particulate, in particular with an average particle size (d50) in the nano range (< 1 pm).

[0080] For easier handling, the base substrate can be applied to a carrier or an auxiliary substrate, for example made of glass or aluminum, for processing.

[0081] In a further preferred development of the invention, in particular the preferred second variant of the method described above, the method comprises at least one of the immediately following steps a. and b.: a. To provide the plasma, a process gas from the group consisting of O2, H2, N2, argon, helium, CF4, CsFs, CHF3, and mixtures of the aforementioned gases, such as O2 / CF4, is used. b. The plasma is applied at a temperature in the range from minus 15°C to 200°C, preferably in the range from minus 15°C to 80°C.

[0082] Preferably, the immediately above features a. and b. are implemented in combination with one another.

[0083] Particularly preferably, the process gas used in the present invention for providing the plasma comprises at least one of the reactive gases from the group comprising CF4, CsFs and CHF3.

[0084] Plasma etching is also state of the art. Plasma etching uses process gases that can convert the material to be etched into the gas phase. The gas enriched with the etched material is pumped out, and fresh process gas is added. This achieves continuous removal. Particularly preferred within the scope of the invention is an inductively coupled plasma (ICP plasma), for example, generated by an ICP generator with DC bias.

[0085] The process gases mentioned immediately above are particularly well suited for etching the above-mentioned preferred polymer materials.

[0086] In the present case, it is important that in the at least one first partial region of the first substrate side and / or in the at least one first partial region of the top side, the base substrate consisting of the polymer material and / or the layer of the electrically insulating material can come into direct contact with the plasma, while the at least one second partial region of the first substrate side and / or the at least one second partial region of the top side is covered with the respective cover metal layer. In general, metals are etched more slowly by a plasma than polymer materials, particularly when using the aforementioned process gases.As a result, the action of the plasma results in the formation of depressions exclusively in the area of ​​the at least one first partial area of ​​the first substrate side, while the cover metal layer and the mask layer (the latter at least temporarily) form a barrier that shields the respective at least one second partial area from the plasma. The surface of the base substrate and the surface of the layer of electrically insulating material can thus be patterned with depressions in a targeted and highly precise manner.

[0087] In particularly preferred embodiments, the plasma is used in an anisotropic etching process. Ideally, plasma ions are accelerated perpendicular to the surface of the substrate to be etched. The accelerated ions ensure physical sputter removal.

[0088] Reactive ion etching (RIE) and reactive ion beam etching (RIBE) are particularly suitable as anisotropic etching processes.

[0089] In a further preferred development of the invention, the method comprises one of the immediately following steps a. to c.: a. To fill the at least one depression in the first substrate side of the base substrate and / or in the top side of the layer made of electrically insulating material, the at least one depression is metallized in one step and the metallized at least one depression is filled with the filler metal in a subsequent step. b. The metallization of the at least one depression takes place by means of physical or chemical vapor deposition, in particular by sputtering the first substrate side, or by a wet-chemical process. c. The first substrate side and / or the top side is metallized over its entire area.

[0090] Preferably, the immediately above features a. and b., in particular a. to c., are realized in combination with one another.

[0091] During metallization, a thin layer of copper or a copper alloy is preferably formed. In the case of wet-chemical metallization, the metallization is carried out, for example, by depositing copper from a solution.

[0092] Filling with the filler metal is preferably carried out by electrochemical deposition. Particularly preferably, the filling is carried out using a so-called via-fill process, which allows the deposition to occur primarily in the at least one recess and, if appropriate, in bores or blind holes, while simultaneously minimizing unwanted deposition on the first substrate side and / or the top side, while reinforcing the cover metal layer and, if appropriate, the mask layer in the at least one second partial region.

[0093] A metallization layer applied over the entire surface enables electrical contact to be made with the first substrate side and / or the top side, for example to position a cathodic contact there for subsequent electrochemical deposition and to ensure that the entire substrate side can be coated.

[0094] In principle, all metals and alloys from which conductor track structures can be produced on printed circuit boards can be used as filler metal. However, it is particularly preferred that a. the filler metal used to fill the at least one recess is copper or a copper alloy.

[0095] In a further preferred development of the particularly preferred second variant of the method, the method comprises one of the immediately following steps a. or b.: a. The removal of the cover metal layer and / or the mask layer in the at least one second partial region of the substrate side and / or the top side is carried out by means of an etching step. b. The removal of the cover metal layer and / or the mask layer in the at least one second partial region is carried out by means of mechanical processing of the first substrate side and / or the top side.

[0096] The etching step, for example, is a classic etching step using a strong acid such as hydrochloric acid.

[0097] If the cover metal layer is removed mechanically, it can be removed, for example, by polishing and / or grinding. The preferred goal is to completely remove the cover metal layer in at least one second partial area. Only then is the formation of the conductor structure complete.

[0098] The complete removal of the cover metal layer in the at least one second partial region may preferably also comprise the removal of filler metal in the at least one first partial region and optionally also in the region of the at least one depression, at least to the extent that the filler metal projects beyond the edge or edges of the at least one depression.

[0099] Particularly advantageously, during the mechanical processing of the first substrate side, not only can the cover metal layer be removed but the first substrate side can also be planarized simultaneously. The preferred goal of planarization is to level the first substrate side such that it has no conductor tracks protruding from the surface. Instead, the conductor structure is preferably completely recessed into the at least one recess. In preferred embodiments, external conductor structures formed according to the method are coated with a solder resist for their protection. Free contacts can be coated with a precious metal, for example, gold, silver, or platinum.

[0100] According to the described process, printed circuit boards with the highest resolution in the pm range can be manufactured with less effort and lower production costs while at the same time achieving higher yields than the state of the art allows.

[0101] When manufacturing MLBs, especially in the described sequential design, it is advantageous that the conductor structures are recessed into the base substrate. The pressures acting on the conductor structures when pressing the base substrates and additional layers together are comparatively low, which has a positive effect on existing impedance and signal speed requirements. Another positive aspect in this regard is that plasma etching allows channels to be formed with extremely high accuracy.

[0102] BRIEF DESCRIPTION OF THE DRAWINGS

[0103] Further features, details and advantages of the invention emerge from the claims and the summary, both of which are incorporated by reference into the description, the following description of preferred embodiments of the invention and the drawings. Here, schematically illustrate

[0104] Figure 1 shows the sequence of a method according to the invention according to the above-described, particularly preferred second variant and

[0105] Figure 2 shows the sequence of a further embodiment of the method according to the invention.

[0106] Figure 3 shows the sequence of a further embodiment of the method according to the invention.

[0107] DESCRIPTION OF PREFERRED EMBODIMENTS

[0108] In a method according to Fig. 1, a base substrate 101 is provided in step A. In step B, this base substrate 101 is covered on its first substrate side 101a with a cover metal layer 102 made of copper. To partially remove the cover metal layer 102, a mask layer 103 made of polycrystalline silicon with phosphorus doping is applied to the cover metal layer 102 in step C. For this purpose, a 100 nm thick layer of silicon with phosphorus doping is deposited using PECVD and heat-treated at 850 °C for one minute. In step D, the mask layer 103 is first subjected to a polycrystalline -> amorphous phase transformation in the first partial regions 104 using a laser (354 nm, pulses with a duration of 100 ps and 100 nJ pulse energy) and then removed using an etching solution (HF / HNO3).In step E, the cover metal layer 102 is removed in the first partial regions 104, which are no longer covered by the mask layer 103, using another etching solution, for example, an ammonium persulfate solution. It should be emphasized at this point that steps D and E can also be combined if an etching solution suitable for removing the mask layer and the cover metal layer is used.

[0109] The substrate side 101a, originally completely covered by the cover metal layer 102, is now divided into the first partial regions 104, in which it is free of the cover metal layer 102, and the second partial regions 105, in which it is still covered by the cover metal layer 102 and the mask layer 103. In step F, a plasma is applied to the substrate side 101a. While the partial regions 105 are shielded from the plasma by the cover metal layer 102 and the mask layer 103, the plasma causes material removal in the partial regions 104 and subsequently the formation of the recesses 106. With sufficient exposure to the plasma, however, the mask layer can also be removed in this step. In step G, the recesses 106 are metallized by sputtering, followed by filling the recesses 106 by electrochemical deposition of a filler metal 108 in step H.Excess filler metal 108 is then mechanically removed in step I, together with the cover metal layer 102 and the mask layer 103 (if not already removed), in the partial areas 105. This forms the conductor structure 109, which is recessed in the recesses 106.

[0110] To form an MLB, in step J, a film made of an electrically insulating polymer material 110 is laminated directly onto the substrate side 101a with the conductor structure 109. In step K, its upper side 110a is covered with a cover metal layer 111, which is partially removed again in steps L, M, and N—analogous to steps C, D, and E—by applying a mask layer 112 and removing it in certain regions. The upper side 110a of the film 110, originally completely covered with the cover metal layer 111, is now divided into the first partial regions 113, in which it is free of the cover metal layer 111, and the second partial regions 114, in which it is still covered with the cover metal layer 111 and the mask layer 112. In step O, a plasma is applied to the upper side 110a of the film 110.While the partial regions 114 are shielded from the plasma by the cover metal layer 111 and the mask layer 112, the plasma causes material removal in the partial regions 113 and subsequently the formation of the recesses 115. Here, too, with sufficient plasma exposure, the mask layer could be removed simultaneously. In step P, the mask layer 112 and the cover metal layer 111 are removed using an etching solution—unlike during the processing of the base substrate 101. Furthermore, one of the formed recesses 115 is connected by a hole 116 to a recess 106 of the first conductor structure 109, which recess is already filled with the filler metal 108. In step Q, the recesses 115, including the hole 116, are metallized by sputtering, followed by filling the recesses 115 by electrochemical deposition of a filler metal 118 in step R.Excess filler metal 118 is then mechanically removed in step S, along with the cover metal layer 111 and the mask layer 112 in the partial regions 114. This forms the conductor structure 119, which is recessed into the recesses 115. In step T, a solder resist 120 is applied, followed by partial gold plating 121 of individual contacts of the conductor structure 119.

[0111] In a method according to Fig. 2, a base substrate 101 is provided, which is covered on its first substrate side 101a with the cover metal layer 102 made of copper and on its second substrate side 101b with the cover metal layer 107, also made of copper. In a step A, a mask layer 103 made of silicon with a thickness of 100 nm is applied to the cover metal layer 102 by sputtering. In a step B, the mask layer 103 is subjected to an amorphous -> crystalline phase transformation in at least a first partial region 104 by means of a laser (354 nm, pulses with a duration of 100 ps and 100 nJ pulse energy). The mask layer in the partial region 104 is then removed using an etching solution (HF / HNO3).The first substrate side 101a is then divided into at least one first partial region 104, in which the first substrate side 101a is covered only with the cover metal layer 102, and into at least one second partial region 105, in which the first substrate side 101a is covered with the cover metal layer 102 and by the mask layer. In a step C, the cover metal layer 102 is removed in the at least one region 104 using an etching solution. Finally, in step D, the mask layer 103 is removed. The cover metal layer 102 remaining in the second partial region 105 forms a metallic conductor structure.

[0112] In a method according to Fig. 3, the same base substrate 101 is provided as in the method according to Fig. 2. Analogous to Fig. 2, in step A, a 100 nm thick mask layer 103 made of polycrystalline silicon with phosphorus doping is applied, which is removed in regions in step B by laser-induced phase conversion (laser at 354 nm, pulses with a duration of 100 ps and 100 nJ pulse energy) and subsequent etching treatment, so that the first substrate side 101a comprises at least a first partial region 104 in which the first substrate side 101a is covered only with the cover metal layer 102. In step C, the cover metal layer 102 is then removed in the at least one region 104 using an etching solution. In step D, the substrate side 101a is subjected to a plasma which causes material removal in at least one partial region 104 and subsequently formation of the depressions 106.Subsequently, the cover metal layer 102 and the mask layer 103 are completely removed, whereby the mask layer can also be removed beforehand using plasma if necessary. In step E, the recesses 106 are metallized by sputtering (not shown), followed by filling the recesses 106 by electrochemical deposition of a filler metal 108. In step F, the substrate side 101a is planarized. Excess filler metal 108 is mechanically removed. This forms the conductor structure 109, which is recessed in the recesses 106.

Claims

PATENT CLAIMS 1. Method for producing a printed circuit board with a metallic conductor structure, comprising the steps of a. providing a base substrate (101) designed as a film or plate, having a first substrate side (101a) and a second substrate side, which base substrate consists at least partially of an electrically non-conductive organic polymer material and in which the first substrate side (101a) is covered with a cover metal layer (102), and b. removing the cover metal layer (102 in some areas), wherein for the area-wise removal of the cover metal layer (102) c. a mask layer (103) is applied to the cover metal layer (102), d.the mask layer (103) is removed in regions such that the first substrate side (101a) is divided into at least one first partial region (104), in which the first substrate side (101a) is covered only with the cover metal layer (102), and into at least one second partial region (105), in which the first substrate side (101a) is covered with the cover metal layer (102) and by the mask layer (103), and e. the cover metal layer (102) is removed in the at least one first partial region (104), characterized in that f. the mask layer (103) is removed in the first partial region (104) by an etching treatment using an etching solution, and before the etching treatment for removing the mask layer (103) in the first partial region (104), a phase transformation or a chemical transformation is brought about in the mask layer (103).

2. The method according to claim 1, having at least one of the following additional features: a. The phase transformation is effected by means of a laser. b. The laser is operated in pulsed or continuous mode. c. The laser has a wavelength in the range of 300 nm to 600 nm. d. The laser is operated at an energy that is insufficient to cause ablation of the mask layer (103). e. The laser is operated in pulsed mode with a pulse energy in the range of 0.1-100 pJ.

3. The method according to claim 1 or claim 2, having at least one of the following additional features: a. The mask layer (103) is formed from a mask material capable of reversible amorphization and crystallization, wherein the resistance of the mask material to the etching solution depends on the phase state of the mask material. b. The mask layer (103) consists of an inorganic semiconductor or a compound of an inorganic semiconductor, in particular from the group consisting of silicon, SiO2, SiN, gallium, indium, and GaAs. c. The mask layer (103) consists of a polymer material that can assume an at least partially crystalline state and an amorphous state.

4. Method according to one of the preceding claims with the following additional feature: a. The mask layer is converted by the phase transformation or the chemical transformation in the first partial region into a phase state or state in which it has a lower chemical resistance to the etching solution than before the phase transformation or the chemical transformation.

5. The method according to claim 1, with at least one of the following additional features: a. The chemical conversion is an oxidation. b. The chemical conversion occurs by exposing the mask layer to an oxidative atmosphere. c. The chemical conversion occurs by heating the mask layer. d. The chemical conversion is effected by means of a laser.

6. The method according to claim 5, comprising at least one of the following additional features: a. The mask layer (103) is formed from a metallic or semi-metallic mask material whose oxide or nitride is less chemically resistant to the etching solution than the metallic or semi-metallic mask material. b. The mask layer (103) consists of an inorganic semiconductor or a compound of an inorganic semiconductor, in particular from the group consisting of silicon, gallium, indium, and GaAs.

7. Method according to one of the preceding claims, with at least one of the following additional features: a. The etching treatment using the etching solution removes the mask layer (103) and the cover metal layer (102) in the first partial region (104) in one step. b. The etching solution is an acidic or alkaline solution for etching copper.

8. The method according to claim 1, comprising at least one of the following additional features: a. The cover metal layer is formed from copper. b. The etching solution is a solution for etching copper.

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