Interconnection circuitry and bonded terminal pad structures in microelectronic packages and devices are most commonly fabricated by additively pattern plating or vapor deposition. This is done through a mask with the required wiring and pad metallurgy disposed on top of a blanket base layer. Subsequently the exposed layer which is not below the pad metallurgy and the wiring metallurgy is subtractively etched. In this process, situations can occur wherein the top or second layer or stack of layers contain a metal that might be more vulnerable to attack by the etchants used to etch the base layer. An example of this is Cr/Cu/Co/Au bonding and terminal metallurgy which is described in U.S. Pat. No. 4,985,310 which is assigned to the assignee of the present invention, the teaching of which is incorporated herein by reference. The metallurgy described therein can be fabricated by starting with a Cr/Cu base blanket layer and pattern plating Co/Au thereover and then etching the Cr/Cu base layer.
In this metallization, the Co in the second stack will be attached by the etchant used to etch the Cu in the first stack. This is because Co is more reactive than Cu, in particular in aqueous red-ox reactions. This situation is not restricted to this particular application only. Other etching techniques in which there is a concern are the chemical-mechanical polishing of multi-layer metal stacks with incompatible combinations of metals to achieve planar metal-dielectric structures; and patterning of metal layers to form mask plates.
In the broadest sense the methods described herein are methods of etching a less reactive material in the presence of a more reactive material. The degree of reactiveness is defined on the basis of aqueous red-ox reactions.
Therefore, it is the object of the present invention to provide methods for etching a less reactive material in the presence of a more reactive material, using a non-aqueous etching medium.
The methods described herein are directed to an etching process which overcomes the disadvantages of etching in aqueous media described above. For example, FIGS. 1-4 show an example of a process generally described above wherein on a substrate 2 is disposed a layer 4 of a less reactive material on which is disposed a patterned lift off layer 6 having a pattern hole 8. Over the light off layer 6 is disposed a layer of the more reactive metal 10 of which 10' is disposed in pattern opening 8 of lift off layer 6. Layer 6 is removed as shown in FIG. 2 to leave the pattern 10' of the more reactive material. In order to remove that part of less reactive material beyond the periphery of the reactive material 10', 10' is coated with a resist material 12 as shown in FIG. 3. The structure of FIG. 3 is then exposed to an etchant which etches away that part of the less reactive material beyond the periphery of pattern 10' leaving the structure as shown in FIG. 4 with a patterned reactive material 4' below the pattern 10' of the more reactive material. The resist material 12 is removed leaving the structure shown in FIG. 5.
According to the present invention, the structure shown in FIG. 2 uses the pattern of more reactive material 10' as a mask for the etching of the layer 4 of the less reactive material to directly go from the structure of FIG. 2 to the structure of FIG. 5, thereby, avoiding the need to passivate the more reactive material layer 10' with a resist 12 as shown in FIG. 3. This is achieved by appropriately choosing the chemistry of the etchant system to selectively etch the less reactive material in the presence of the more reactive material 10.
In the chemistry of the method, the less reactive material has a lower and higher valence state and the more reactive material has an oxidation state of energy greater than the lower oxidation state of the less reactive material. Chemical environments are provided containing cations of the higher oxidation state of the less reactive material which react with the zero oxidation state of the less reactive material to produce cations of the lower oxidation state of the less reactive material. The chemical environment also contains an agent capable of complexing the lower oxidation state cations of the less reactive material, such that the concentration of the lower oxidation state does not reach an equilibrium condition, thereby permitting continued dissolution of the less reactive material.
U.S. Pat. No. 3,677,950 described a chemical etching solution particularly adapted to selectively dissolving copper from laminated printed wiring boards in the presence of metal selected from the group consisting of tin, lead, nickel, gold and alloys thereof. There is no mentioned of etching copper in the presence of a highly reactive metal such as cobalt. The method uses high concentration of ammonia which is difficult and hazardous to use in a manufacturing environment.
U.S. Pat. No. 3,717,520 describes an aqueous etchant for stripping Cu and Ni from ferrous metal and plastic substrates.
U.S. Pat. No. 4,543,153 describes a process for anisotropically etching Cu through a Ni-Au mask in an aqueous solution in a reactor by providing an electrical connection between the reactor wall and the Cu to maintain them at the same potential.
U.S. Pat. No. 4,632,727 describes an aqueous nitric acid based etchant for copper with a solder based etch resist.
U.S. Pat. No. 4,767,661 describes nitric acid based aqueous anisotropic etchants with selectivity to different copper crystal planes.
U.S. Pat. No. 4,913,768 describes substrates covered with a layer of Ni, Co, Mn and Ni/Fe or Ni/Co followed by electroless Cu and pattern plating. After plating both the electroless Cu and under layer Ni/Co are removed by etching with a single solution without any selectivity between materials.
U.S. Pat. No. 4,952,275 describes a non-aqueous solution to selectively etch Cu but not Ni, Ti, Cr which contains dimethyl sulfoxide and a halo carbon compound such as ethyl chloroacetate and carbon tetrachloride which are hazardous and difficult to use in a manufacturing environment. In contradistinction, the etch methods described herein are environmentally safe and compatible with manufacturing needs, and provide selectivity to etch the material that is normally less reactive in an aqueous medium.
IBM Technical Disclosure Bulletin, Vol. 33, No. 2, July 1990, p. 258-9 describes a subtractive etch technique for selectively removing Cu in a wet etch where certain regions are protected from the Cu wet etch by regions of Cr.
IBM Technical Disclosure Bulletin, Vol. 27, No. 5, October 1986 p. 2197 describes dry etching below an etch resistant mask of Co/Cu where the Co is between a W underlayer and Cu.
None of the above referenced claims teach a method wherein a less reactive metal is wet etched selectively in the presence of a more reactive material.
These and other objects, features and advantages will be apparent from the following description of the preferred embodiments, claims and the figures appended thereto.