Abstract:
The present invention relates to an aqueous composition and a process for etching of copper and copper alloys. The aqueous composition comprises Fe 3+  ions, an acid and a N-alkoxylated polyamide. The aqueous composition is particularly useful for making of fine structures in the manufacture of printed circuit boards, IC substrates and the like.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to aqueous compositions for etching of copper and copper alloys and a process for etching of copper and copper alloys in the manufacture of printed circuit boards, IC substrates, copperised semiconductor wafers and the like. 
       BACKGROUND OF THE INVENTION 
       [0002]    Circuit formation by etching of copper or copper alloy layers is a standard manufacturing step in production of printed circuit boards, IC substrates and related devices. 
         [0003]    A negative pattern of the circuit is formed by a) applying an etch resist, e.g., a polymeric dry film resist or a metal resist on a layer of copper, b) etching away those portions of copper not covered by the etch resist and c) remove the etch resist from the remaining copper circuit. 
         [0004]    Etching solutions applied for this task are selected from different types of compositions such as mixtures of an oxidizing agent and an acid. Two main types of etching solutions are based on an acid such as sulphuric acid or hydrochloric acid and contain as the oxidizing agent either hydrogen peroxide or Fe 3+  ions added as FeCl 3 . Such etching solutions are disclosed in C. F. Coombs, Jr., “Printed Circuits Handbook”, 5 th  Ed. 2001, Chapter 33.4.3, pages 33.14 to 33.15 and Chapter 33.4.5, pages 33.17. 
         [0005]    The ongoing miniaturization of circuits in terms of line width/interline-space values and thickness of the copper layers to be etched does not allow to use conventional etching solutions such as the ones described above. 
         [0006]    The disadvantage of known etching solutions is even more present if the copper tracks are manufactured by a semi additive process (SAP). Here the bare dielectric substrate is first coated with a seed layer serving as an electrically conductive layer. The seed layer comprises for example copper deposited by electroless plating. Next, a patterned resist layer is formed on the seed layer and a thicker, second copper layer is deposited by electroplating into the openings of the patterned resist layer onto the seed layer. The patterned resist layer is stripped and the seed layer in between copper tracks deposited by electroplating needs to be removed by a differential etch step. The seed layer deposited by electroless plating has a finer grain structure than the second copper layer deposited by electroplating. The different grain structures can lead to a different etching behaviour of the individual copper layers. 
         [0007]    A similar situation is present when copper tracks are manufactured by a modified semiadditive process (m-SAP) wherein a thick, second copper layer is deposited in the openings of the patterned resist layer onto a first thin layer of copper. The first copper layer is manufactured, e.g. by thinning a copper clad attached to the dielectric substrate. Again, both first and second copper layer have a different grain structure. 
         [0008]    The etching solution applied for the differential etching step should only remove the first copper layer in between the copper tracks while not attacking the sidewalls and the top of the copper tracks deposited by electroplating and the underlying first copper layer or copper seed layer. 
         [0009]    Etching solutions based on sulfuric acid and hydrogen peroxide lead to an undesired undercutting of the first copper layer during etching ( FIG. 1 ) which results in an insufficient adhesion of the copper layer on the dielectric substrate. 
         [0010]    Etching solutions based on sulfuric acid and Fe 3+  ions typically show an etching behaviour as shown in  FIG. 2 . The broader base of the etched copper line can lead to circuit shorts which are not acceptable. 
       OBJECTIVE OF THE PRESENT INVENTION 
       [0011]    Therefore, it is the first objective of the present invention to provide an aqueous composition for etching of copper and copper alloys which results in the formation of rectangular shaped copper features after etching. 
         [0012]    It is a second objective of the present invention to remove Cu 2+  ions from said aqueous composition. 
       SUMMARY OF THE INVENTION 
       [0013]    The first objective is solved by an aqueous composition for etching of copper and copper alloys, the composition comprising Fe 3+  ions, at least one acid and at least one etching additive selected from the group consisting of N-alkoxylated polyamides obtained by 
         [0000]    a) N-alkoxylation of a polyamide with one or more compounds of formula (I), 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R 1  is hydrogen or a hydrocarbon residue with 1 to 6 carbon atoms and R 2  is hydrogen or a hydrocarbon residue with 1 to 6 carbon atoms and/or
 
b) polymerization of a N-alkoxylated lactam.
 
         [0014]    A rectangular line shape of etched copper or copper alloy structures is achieved when applying the aqueous composition of the present invention. Thereby, the risk of circuit shorts between individual copper or copper alloy structures is minimized. Furthermore, a sufficient adhesion between the etched copper or copper alloy structure and the underlying dielectric substrate is achieved. 
         [0015]    The second objective of the present invention is solved by a process for etching of copper and copper alloys comprising, in this order, the steps of
       a. providing a substrate having a copper or copper alloy surface,   b. contacting said substrate with the aqueous composition according to the present invention in a first tank wherein during contacting copper is oxidized to Cu 2+  ions and the Cu 2+  ions are disclosed in the aqueous composition according to the present invention,   c. transferring a portion of the aqueous composition according to the present invention after contacting with the substrate to a second tank wherein said second tank comprises an anode and a cathode and   d. reducing said Cu 2+  ions to copper by applying a current between said anode and said cathode.       
 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]      FIG. 1  shows two copper tracks obtained by etching with an aqueous composition consisting of sulfuric acid and hydrogen peroxide (comparative example). 
           [0021]      FIG. 2  shows a copper track obtained by etching with an aqueous composition consisting of sulfuric acid and Fe 3+  ions (comparative example). 
           [0022]      FIG. 3  shows two copper tracks obtained by etching with an aqueous composition according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    A layer of copper or a copper alloy is etched with an aqueous composition for etching of copper and copper alloys, the composition comprising Fe 3+  ions, at least one acid and at least one etching additive selected from the group consisting of N-alkoxylated polyamides, obtained by a) N-alkoxylation of a polyamide with one or more compounds of formula (I), 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R 1  is hydrogen or a hydrocarbon residue with 1 to 6 carbon atoms and R 2  is hydrogen or a hydrocarbon residue with 1 to 6 carbon atoms
 
and/or b) obtained by polymerization of a N-alkoxylated lactam.
 
         [0024]    Preferably, R1 and R2 in compounds according to formula (I) are preferably hydrogen and methyl. Most preferably R1 and R2 are hydrogen. 
         [0025]    The N-alkoxylated lactam is selected from compounds according to formula (II): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R3 is hydrogen or methyl,
 
A is a hydrocarbon residue,
 
n is a whole number from 2 to 10, more preferably from 2 to 5,
 
and m is a whole number from 1 to 50, more preferably from 1 to 12.
 
         [0026]    The hydrocarbon residue A is preferably a —CH 2 — group. 
         [0027]    Preferably, the N-alkoxylated lactam according to formula (II) is selected from the group consisting of ethoxylated beta-lactam, hexaethoxylated gamma-butyrolactam, octaethoxylated delta-valerolactam, pentapropoxylated delta-valerolactam, hexaethoxylated epsilon-caprolactam and dodecaethoxylated epsilon-caprolactam. 
         [0028]    The at least one etching additive obtained by N-alkoxylation of a polyamide with one or more compounds of formula (I) is a N-alkoxylated polyamide. Such polymers can be for example obtained by swelling the polyamine in an alkyleneoxide compound according to formula (I) at a temperature in the range of 10 to 40° C. followed by addition of further alkyleneoxide compound according to formula (I) at a temperature in the range of 100 to 160° C. A detailed description of the synthesis method is disclosed in DE 913 957. 
         [0029]    N-alkoxylated polyamides are obtainable by a ring-opening polymerization reaction of monomers according to formula (II) at for example 260° C. in an inert atmosphere of nitrogen for about 4 to 5 hours. 
         [0030]    The concentration of the at least one etching additive in the aqueous composition ranges from 0.001 to 10 g/l, more preferred from 0:005 to 5 g/l and most preferred from 0.01 to 1 g/l. 
         [0031]    The source for Fe 3+  ions is selected from water soluble salts of Fe 3+  ions. The most preferred source of Fe 3+  ions is Fe 2 (SO 4 ) 3 . 
         [0032]    The concentration of Fe 3+  ions ranges from 1 to 100 g/l, more preferred from 1 to 50 g/l and most preferred from 5 to 40 g/l. 
         [0033]    The at least one acid is selected from the group comprising mineral acids such as sulfuric acid, and organic acids such as methanesulfonic acid. The most preferred acids are sulfuric acid and methanesulfonic acid. 
         [0034]    The concentration of the acid (or all acids together in case more than one acid is added) ranges from 10 to 400 g/l, more preferably from 30 to 300 g/l and most preferably from 50 to 200 g/l. 
         [0035]    Optionally, the aqueous composition further contains one or more of Fe 2+  ions, surfactants, a corrosion inhibitor selected from triazoles, benzotriazoles, imidazoles, benzimidazole, tetrazoles and isocyanates, organic sulfur compounds such as thiourea and sulfosalicylic acid and polyalkylene compounds such as polyethylene glycol, polypropylene glycol, copolymers of polyethylene glycol and polypropylene glycol and derivates thereof. 
         [0036]    The substrates are preferably treated by spraying the aqueous composition according to the invention onto the substrates. The aqueous composition can be sprayed in a vertical mode or horizontal mode, depending on the equipment desired. Alternatively, the substrates can be immersed into the aqueous composition by dipping. 
         [0037]    The temperature of the aqueous composition according to the present invention during use ranges from 10 to 60° C., more preferably from 20 to 50° C. and most preferably from 30 to 45° C. The contact time depends on the copper thickness to be etched and is in the range from 10 to 360 s, more preferably from 20 to 200 s and most preferably from 30 to 90 s. 
         [0038]    During use the aqueous composition is enriched in Cu 2+  ions. At the same time Fe 3+  ions are reduced to Fe 2+  ions. Cu 2+  ions have a negative impact on the performance of the aqueous composition. Cu 2+  ions increase the density and viscosity of the etching solution and thereby negatively effect the etch behaviour. The etch rate and side wall etching performance is for example decreased. Furthermore, precipitation of Cu 2+  ion complexes can occur. Such precipitates pose a mechanical danger to the equipment and the surfaces coming into contact with the aqueous composition. 
         [0039]    In processes according to prior art some or all of the added Cu 2+  ions are removed by bleeding (removing) an adequate amount of etching solution and adding (feeding) fresh etching solution to the remaining solution. 
         [0040]    One particular method to remove Cu 2+  ions from an aqueous solution is to electrolytically reduce Cu 2+  ions to metallic copper. 
         [0041]    In principle, a second tank equipped with an anode and a cathode and a rectifier is required for electrolysis. 
         [0042]    Portions of the aqueous composition according to the present invention are transferred from a first tank where or from which the etching of copper or a copper alloy layer is performed to a second tank equipped for electrolysis. During electrolysis Cu 2+  ions are cathodically reduced to metallic copper and at the same time Fe 2+  ions are oxidised anodically to Fe 3+  ions. 
         [0043]    The metallic copper can be collected and recycled. Without an electrolytic regeneration cell the oxidizing agent (Fe 3+  ions) would need to be continuously added to the etching solution. By the application of the above described regeneration the spent Fe 3+  ions are regenerated at the anodes (Fe 2+  is oxidised to Fe 3+ ) and thereby no adding (feeding) of the oxidising agent during use of the etching solution is required. 
         [0044]    Halogenide ions are not preferred in the aqueous composition when used in the process according to the present invention because they are oxidized during the electrolysis reaction and thereby elemental dihalogene molecules are formed. 
         [0045]    A method and an apparatus useful for this process are disclosed in US 2006/0175204 A1. This method involves feeding of the etching solution into an electrolysis cell being hermetically sealed or having an anode hood, the electrolysis cell comprising a cathode, an inert anode, means for removing the electrolytically deposited copper from the cathode and means for collecting the removed copper and applying a potential to the removed copper, wherein the electrolysis cell does not have an ion exchange membrane or a diaphragm. 
         [0046]    Another application of the aqueous composition for etching of copper and copper alloys according to the present invention is cleaning of copper or copper alloy surfaces prior to further process steps such as deposition of a solder- or bondable surface finish. Such a cleaning process is also referred to as an etch cleaning process because in order to clean the copper or copper alloy surface from e.g. copper oxide phases and/or organic residues, a portion of said copper or copper alloy surface is etched away and thereby a “fresh” copper or copper alloy surface is exposed and thereby suited for further process steps. 
         [0047]    Still another application of the aqueous composition for etching of copper and copper alloys according to the present invention is a (microscopic) roughening of the copper or copper alloy surface for improvement of the adhesion of a resist material such as a photo resist or a solder mask. 
       EXAMPLES 
       [0048]    The invention will now be illustrated by reference to the following non-limiting examples. 
         [0049]    A substrate containing a dielectric layer, a seed layer obtained by electroless plating of copper and a patterned second layer of copper obtained by electroplating was used throughout all examples. 
         [0050]    The seed layer not covered by the patterned second layer of copper was etched away by spraying different aqueous compositions with an adjusted etch rate of 2 μm/min onto the substrates. The contact time was adjusted to provide the required copper removal. The samples were rinsed with water and the dry film resist was stripped. 
         [0051]    The obtained line shapes of the etched copper tracks were investigated from cross-sections by optical microscope after depositing a nickel layer onto the copper tracks which serves as a protection layer during preparation of the cross-sections. 
       Example 1 (Comparative) 
       [0052]    An aqueous composition consisting of 20 g/l H 2 O 2  and 90 g/l sulfuric acid was applied as etching solution. 
         [0053]    The resulting line shape of copper tracks (track width: 8 μm, track height: 10 μm) is shown in  FIG. 1 . The copper tracks show a severe undercutting of the track base (i.e. the seed layer obtained by electroless plating of copper). Hence, the copper track has not the required rectangular shape and no sufficient adhesion to the underlying dielectric substrate. 
       Example 2 (Comparative) 
       [0054]    An aqueous composition consisting of 5 g/l Fe 2 (SO 4 ) 3  and 90 g/l sulfuric acid was applied as etching solution. 
         [0055]    The resulting line shape of the copper tracks (track width: 20 μm, track height: 10 μm) is shown in  FIG. 2 . The copper track shows a strong trapezoidal shape. Hence, the copper track does not provide the desired rectangular shape and poses the danger of circuit shorts or cross-talk between individual tracks. 
       Example 3 
       [0056]    An aqueous composition consisting of 15 g/l Fe 2 (SO 4 ) 3 , 90 g/l sulfuric acid and 0.1 g/l of a N-ethoxylated Polyamide 6, obtained by a ring-opening polymerization of ∈-caprolactam-N-ethoxylate was applied as etching solution. 
         [0057]    The resulting line shape of the copper tracks (track width: 8 μm, track height: 10 μm) is shown in  FIG. 3 . The copper tracks have the desired rectangular line shape, have a sufficient adhesion to the underlying dielectric substrate and pose no risk for circuit shorts.