METHOD FOR IMPROVING ADHESION OF A WETTABLE METALLIZATION MULTILAYER IN AN INTEGRATED ELECTRONIC DEVICE

Abstract

A process for forming silver-containing wettable material structures, wherein, on a metal layer containing aluminum, a zinc layer is deposited, the zinc layer reacting with the metal layer and creating a surface micro-roughness; the zinc layer is removed; and a wettable layer containing silver is deposited by vapor deposition. The wettable layer is formed by an adhesion layer, containing titanium or chromium; a barrier layer, containing nickel, on the adhesion layer; and a bonding layer, containing silver, on the barrier layer.

Claims

1. A process for forming silver-containing wettable material structures, comprising: forming a metal layer containing aluminum; depositing a zinc layer on the metal layer, the zinc layer reacting with the metal layer and creating a surface micro-roughness; removing the zinc layer; and depositing a wettable layer containing silver by vapor deposition.

2. The process according to claim 1, wherein depositing a wettable layer includes: forming an adhesion layer, containing titanium or chromium, on a front side of the metal layer; forming a barrier layer, containing nickel, on the adhesion layer; and forming a bonding layer, containing silver, on the barrier layer.

3. The process according to claim 1, wherein the metal layer is of pure aluminum or an alloy of aluminum and copper or an alloy of aluminum, silicon, copper.

4. The process according to claim 1, wherein the barrier layer is of nickel or a nickel-vanadium alloy.

5. The process according to claim 1, wherein depositing a zinc layer and removing the zinc layer are repeated.

6. The process according to claim 1, wherein removing the zinc layer is performed by stripping.

7. The process according to claim 1, further comprising, after forming a metal layer, etching a surface of the metal layer using strongly acidic or strongly basic compounds.

8. The process according to claim 1, further comprising defining the wettable layer to form a contact structure on a top side of a wafer of semiconductor material.

9. The process according to claim 1, wherein depositing a wettable layer is performed by sputtering.

10. An electronic device comprising: a die of semiconductor material having a front surface and a back surface; a wettable contact structure extending on the front surface and including: a metal layer containing aluminum, the metal layer having a surface micro-roughness; and a wettable layer containing silver, deposited by PVD; and a back contact metallization, extending on the back surface of the die.

11. The device according to claim 10, wherein the wettable layer includes: an adhesion layer, containing titanium or chromium, on a front side of the metal layer; a barrier layer, containing nickel, on the adhesion layer; and a bonding layer, containing silver, on the barrier layer.

12. The device according to claim 10, wherein the metal layer is of pure aluminum or an alloy of aluminum and copper or an alloy of aluminum, silicon, copper.

13. The device according to claim 10, wherein the barrier layer is of nickel or a nickel-vanadium alloy.

14. The device according to claim 10, wherein the surface micro-roughness has a Ra between 20 and 30 nm.

15. A method, comprising: forming a first metal layer on a substrate of an integrated circuit die; increasing a roughness of the metal layer by forming a second metal layer on the first metal layer, the second metal layer reacting with the first metal layer; removing the second metal layer; depositing a wettable layer over the first metal layer after removing the second metal layer; and forming an electrical interconnection structure on the wettable layer.

16. The method of claim 15, wherein the second metal layer includes zinc.

17. The method of claim 16, wherein the first metal layer includes aluminum.

18. The method of claim 15, wherein the wettable layer includes silver.

19. The method of claim 15, wherein depositing a wettable layer includes: forming an adhesion layer, containing titanium or chromium, on a front side of the first metal layer; forming a barrier layer, containing nickel, on the adhesion layer; and forming a bonding layer, containing silver, on the barrier layer.

20. The method of claim 19, wherein the barrier layer is of nickel or a nickel-vanadium alloy.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0026] For a better understanding of the present description, an embodiment thereof is now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:

[0027] FIG. 1 shows a cross-section of a power MOSFET device having an external-connection wettable layer;

[0028] FIG. 2 shows a cross-section of a stack of layers including a wettable layer of an integrated electronic device, on an enlarged scale;

[0029] FIGS. 3A-3D are cross-sections during successive manufacturing steps of a stack of wettable material, wherein FIG. 3D is on an enlarged scale with respect to FIGS. 3A-3C;

[0030] FIG. 4 is a flow chart of an embodiment of a manufacturing process of a stack of wettable material;

[0031] FIG. 5A is a 3D image taken with a Transmission Electron Microscopy (TEM) on a layer of the stack of FIG. 3D;

[0032] FIG. 5B is a cross-sectional image of a part of the stack of FIG. 3D, taken through a Transmission Electron Microscopy (TEM);

[0033] FIG. 6 shows an electronic device having a contact structure formed with the stack of FIG. 53D.

DETAILED DESCRIPTION

[0034] The following description refers to the arrangement shown; consequently, expressions such as above, below, upper, lower, right, left relate to the attached Figures and are not to be interpreted in a limiting manner.

[0035] The present disclosure relates to a method that allows an increase of the adhesion of a wettable metallization multilayer to an underlying layer, increasing the roughness of the metal layer, such as the metallization layer 15 of FIG. 2, using a surface zincation of the metal layer, which causes a corrosion of its surface; removal of the obtained zinc layer, which increases the obtained roughness; and a PVD deposition of the wettable layer. By virtue of this process, the adhesion between the wettable layer and the underlying layer is ensured.

[0036] In particular, this process allows to increase an average roughness Ra of about 8 nm to an average roughness of 20-30 nm. Furthermore, the obtained roughness is uniform throughout the surface of the metal layer, which is very important in semiconductor manufacturing processes, to obtain batches of devices with uniform chemical-physical characteristics, and therefore uniform electrical performances.

[0037] With reference to FIGS. 3A-4D and 4, a treatment process to obtain the desired roughness will now be described.

[0038] With reference to FIG. 3A, a metal layer 26 containing aluminum, for example of Al, AlCu and AlSiCu, is deposited on a substrate 25, for example a wafer of semiconductor material integrating conductive regions and insulating regions (not shown), a glass sheet or other starting support, step 40 of FIG. 4.

[0039] The thickness of the metal layer 26 depends on the specific application; it has an intrinsic roughness, for example corresponding to the pitch P between the aluminum grains, of about 200 nm.

[0040] Then, step 42 of FIG. 4, the surface of the metal layer 26 is cleaned using, for example, organic solvents.

[0041] In step 44 of FIG. 4, passivation residues of alumina (native aluminum oxide) are removed from the surface of the metal layer 26, e.g., through wet etching using liquids that are commercially available to this end.

[0042] This step may be omitted if the metal layer 26 does not have passivation residues of alumina, e.g., due to previous treatments and/or storage conditions.

[0043] In step 46 of FIG. 4, the metal layer 26 is etched using strongly acidic or strongly basic compounds to increase the roughness.

[0044] In this manner, as shown in FIG. 3A, the roughness, also linked to the size of the aluminum grains 27 forming the metal layer 26, may lead to have indentations between the grains.

[0045] A possible washing follows.

[0046] In step 48 of FIG. 4, a zincation process is performed. For example, the zincation process may occur by immersing the structure including the substrate 25 and the metal layer 26 in a bath containing zinc, commercially available for performing zincation in different device zones.

[0047] For example, the bath may comprise a solution of zinc oxide (50 kg/m.sup.3) and sodium hydroxide (250 kg/m.sup.3).

[0048] Zincation may occur without an electro-less (e-less), process.

[0049] A zinc layer, indicated by 28 in FIG. 3B, is thus formed. The zinc layer 28 is typically thin, for example 0.1-0.3 m, although this value is not critical.

[0050] As is known, the deposited zinc, which is deposited in polycrystalline form (represented in FIG. 3B by grains 29), etches the underlying aluminum grains 27, partially dissolving them, based on the crystallographic planes thereof, and causing an increase in the roughness of the metal layer 26, as shown in FIG. 3B.

[0051] This step may last 15-45 seconds or more, for example about 30 seconds.

[0052] In step 50 of FIG. 4, the zinc layer 28 is removed. Removal may occur by stripping, by washing with a suitable, commercially available solution for removing and selectively removing zinc layers.

[0053] The steps 48 and 50 in FIG. 4 may be repeated two or more times, in some embodiments.

[0054] At the end of the (possibly repeated) zincation step, the metal layer 26 has a high micro-roughness 30, as visible in FIG. 3C, which shows the structure formed by the substrate 25 and the metal layer 26 after stripping. As is noted, on the top surface of the metal layer 26, indicated by 26A, the micro-roughness 30 is due to the presence of irregularities with dimensions between 20 and 30 nm (Ra=20-30 nm).

[0055] In particular, the micro-roughness 30 on the top surface 26A of the metal layer 26 is added to the presence of grooves 31, due to the grains of the metal layer 26.

[0056] The micro-roughness 30 is also evident in FIGS. 5A and 5B which show images obtained with an electron microscopy of the top surface 26A of the metal layer 26.

[0057] At the end of the zincation and stripping step(s) 48, 50, zinc layer 28 is completely removed.

[0058] Subsequently, cleaning of the surface 26A of the metal layer is performed, step 52.

[0059] Then, step 54 of FIG. 4, the process proceeds with final steps for growing the wettable layer.

[0060] For example, see FIG. 3D, step 54 includes:

[0061] depositing, through PVD, an adhesion layer 33, for example of titanium Ti or

[0062] chromium Cr;

[0063] depositing, through PVD, a barrier layer 34, for example of a nickel-vanadium alloy NiV or nickel Ni alone; and

[0064] depositing, through PVD, a silver-Ag-containing layer 35 (bonding layer). For example, the silver layer may be a pure silver-Ag-layer.

[0065] PVD deposition may done by sputtering.

[0066] A stack 36 of wettable multilayer material is thus formed.

[0067] Final steps then follow for manufacturing an integrated device.

[0068] As is noted, in FIG. 3D, the high micro-roughness 30 of the metal layer 26 at the end of the zincation is also maintained on the top layers 33-35 of the stack 36.

[0069] Furthermore, the final steps typically also comprise definition of the stack 36, by known photolithographic and etching processes.

[0070] Alternatively, and depending on the device to be manufactured, the layers of the stack 36 may be defined separately, possibly using suitable masks.

[0071] In this manner, an electronic device 60, schematically shown in FIG. 6 and including a die 61 integrating for example a transistor 62, represented schematically, may be provided. The die 61 has a top surface 61A, with a contact structure 63 thereon obtained by defining the stack 36, after its definition, and a bottom surface 61B, having a metallization layer 64 thereon.

[0072] The stack 36 may therefore be used for bonding wires 65, as shown schematically in FIG. 6, or other connection structures.

[0073] The wettable layer (stack 36) on the surface 26A of the metal layer 26 containing aluminum has improved adhesion by virtue of the mechanical anchoring due to the micro-roughness 30, avoiding the detachment of the silver layer (bonding layer 35) during the bonding step of the wires 65.

[0074] The process does not damage the active layers of the device, unlike current zincation processes performed on the back, which use sulfuric acid.

[0075] It is also well integrable with currently used process flows and therefore has high reliability, without increasing manufacturing costs.

[0076] Finally, it is clear that modifications and variations may be made to the process and device described and illustrated here without thereby departing from the scope of the present description, as defined in the attached claims.

[0077] In one embodiment, a process for forming silver-containing wettable material structures, includes: forming a metal layer (26) containing aluminum; depositing a zinc layer (28) on the metal layer (26), the zinc layer reacting with the metal layer and creating a surface micro-roughness (30); removing the zinc layer (28); and depositing a wettable layer (36) containing silver by vapor deposition.

[0078] In one embodiment, depositing a wettable layer (36) includes: forming an adhesion layer (33), containing titanium or chromium, on a front side (26A) of the metal layer (26); forming a barrier layer (34), containing nickel, on the adhesion layer (33); and forming a bonding layer (35), containing silver, on the barrier layer (22).

[0079] In one embodiment, the metal layer (26) is of pure aluminum or an alloy of aluminum and copper or an alloy of aluminum, silicon, copper.

[0080] In one embodiment, the barrier layer (34) is of nickel or a nickel-vanadium alloy.

[0081] In one embodiment, depositing a zinc layer (28) and removing the zinc layer (28) are repeated.

[0082] In one embodiment, removing the zinc layer (28) is performed by stripping.

[0083] In one embodiment, the process further includes, after forming a metal layer (26), etching a surface (26A) of the metal layer using strongly acidic or strongly basic compounds.

[0084] In one embodiment, the process further includes defining the wettable layer (36) to form a contact structure on a top side (61A) of a wafer (61) of semiconductor material.

[0085] In one embodiment, depositing a wettable layer (36) is performed by sputtering.

[0086] In one embodiment, an electronic device (60) includes: a die (61) of semiconductor material having a front surface (61A) and a back surface (61B); a wettable contact structure (63) extending on the front surface (61A) and including: a metal layer (26) containing aluminum, the metal layer having a surface micro-roughness (30); and a wettable layer (36) containing silver, deposited by PVD; and a back contact metallization (64), extending on the back surface (61B) of the die (61).

[0087] In one embodiment, the wettable layer (36) includes: an adhesion layer (33), containing titanium or chromium, on a front side (26A) of the metal layer (26); a barrier layer (34), containing nickel, on the adhesion layer (33); and a bonding layer (35), containing silver, on the barrier layer (34).

[0088] In one embodiment, the metal layer (26) is of pure aluminum or an alloy of aluminum and copper or an alloy of aluminum, silicon, copper.

[0089] In one embodiment, the barrier layer (34) is of nickel or a nickel-vanadium alloy.

[0090] The surface micro-roughness (30) has a Ra between 20 and 30 nm.

[0091] These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.