Semiconductor element and fabricating method thereof

[0002] 1. Field of the Invention

[0003] The present invention relates to a semiconductor element provided with a layer of intermetallic compound and a fabricating method thereof.

[0004] 2. Description of Related Art

[0005] In surface mount technology (SMT) of a semiconductor element, it is very important to implement soldering process with reliability. In the SMT, solder that has been largely used is eutectic alloy of tin (Sn) and lead (Pb) (Sn: Pb=63:37% by weight). In addition, so as to cope with finer pitches, ternary alloy of composition of Sn—Pb—Bi (8% by weight) has been used too.

[0006] The primary reason for employing alloys as solder material is that, not mentioning to a large strength and corrosion resistance, alloys have a lower melting temperature compared with those of component simple elements. The alloys can be roughly distinguished into three categories of solid solutions, eutectic compounds and intermetallic compounds.

[0007] The solders that are provided in JIS Z 3282 include, other than the aforementioned solders of compositions of Sn—Pb and Sn—Pb—Bi, solders of compositions of such as Bi—Sn, Sn—Pb—Ag, Sn—Ag, Sn—Sb, Pb—Ag and Pb—Ag—Sn.

[0008] Among these solders, lead based solders are gradually declining in use. That is, Pb has a plurality of radioisotopes. These isotopes are intermediate or final products of decay series of uranium (U) or thorium (Th), and accompany α-decay that emits He atoms. Accordingly, in the solders, α-rays inevitably appear. The α-rays, upon reaching a CMOS element for instance, cause soft error problems to occur. Further, Pb, upon flowing out into soil, might be eluted due to acid rain to have adverse affect on the environment. From these reasons, use of solders containing lead (Pb) is declining.

[0009] Instead of these Pb based solders, Sn based solders (Sn—Ag solder for instance) that do not contain Pb are considered promising.

[0010] A method for fabricating a semiconductor device in which semiconductor element is surface-mounted with Sn—Ag based solder will be explained with reference to FIGS. 19 to 23.

[0011] First, as shown in FIG. 19, after forming an aluminum electrode pad 22 on a semiconductor substrate 21 consisting of silicon, while leaving an opening at a center portion of the aluminum electrode pad 22, passivation is implemented to 20 form a passivation film 23. Then, as shown in FIG. 20, films of titanium (Ti) 24, nickel (Ni) 25 and palladium (Pd) 26 are sequentially stacked in this order to form a barrier metal layer 27.

[0012] Then, as shown in FIG. 21, resist 28 is coated on the barrier metal layer 27 and a portion above the aluminum electrode pad 22 is opened. Thereafter, in the opening a layer of Sn—Ag solder 29 is formed.

[0013] Next, as shown in FIG. 22, the resist 28 is removed and the barrier metal layer 27 is etched. Thereafter, as shown in FIG. 23, the layer 29 of Sn—Ag solder is reflowed to form a protruded electrode (bump) 30. Then, the semiconductor substrate 21 is diced to form semiconductor chips. The semiconductor chip is mounted on a wiring board (a printed circuit board) by use of flip-chip mounting method to produce a semiconductor device.

[0014] However, in general, such solders as Sn—Ag based ones that are free from Pb contain much Sn compared with the Sn—Pb solder. Accordingly, when left in high temperature service circumstances, the barrier metal layer deteriorates sooner. That is, since Sn in the solder tends to intrude and diffuse into the barrier metal layer (Ni layer), the barrier metal layer tends to deteriorate to cause lowering of reliability of a solder joint between the semiconductor element (chip) and the wiring board. This is a problem.

[0015] Further, there is a method in which Cu or Ni as the barrier metal is plated thicker to solve the aforementioned problems. However, in this method, number of step increases to cause problems.

[0016] Thus, so far, with solders that do not contain lead (Pb), highly reliable solder joint has not been formed.

[0017] The present invention has been carried out to solve these problems. The object of the present invention is to provide a semiconductor element of high reliability and a fabricating method thereof, in which solders that do not contain Pb are used as joining means.

[0018] A first aspect of the present invention is a semiconductor element. The semiconductor element comprises a semiconductor substrate, a wiring pad formed on the semiconductor substrate, a layer of barrier metal formed on the wiring pad, an intermetallic compound Ag₃Sn formed on the barrier metal layer, and a protruded electrode. The protruded electrode consists of a low-melting metal formed on the intermetallic compound Ag₃Sn.

[0019] In the first aspect, the intermetallic compound Ag₃Sn is formed in a layer or segregated in particles.

[0020] Further, in the present first aspect, the layer of barrier metal is selected from single layers and laminate layers of titanium (Ti), chromium (Cr), copper (Cu), nickel (Ni), palladium (Pd), gold (Au), tungsten (W), titanium nitride (TiN), tantalum (Ta), niobium (Nb), tantalum nitride (TaN), mixtures thereof and compounds thereof. In particular, due to excellent adherence with the intermetallic compound Ag₃Sn, a laminate layer stacked titanium (Ti) film, nickel (Ni) film and palladium (Pd) film in this order is preferably used.

[0021] The low-melting metal is selected from tin (Sn), silver (Ag), bismuth (Bi), zinc (Zn), indium (In), antimony (Sb), copper (Cu) and germanium (Ge), mixtures thereof and compounds thereof. To be specific, binary mixtures (alloys) such as eutectic Sn—Ag or the like can be cited, and ternary alloys such as Sn—Ag—Bi or the like also can be used.

[0022] A second aspect of the present invention is a fabricating method of a semiconductor element. The present method comprises the steps of forming a wiring pad on a semiconductor substrate, of forming a barrier metal layer on the wiring pad, of forming a layer of intermetallic compound Ag₃Sn on the barrier metal layer, and of forming a protruded electrode. The protruded electrode consists of low-melting metal formed on the layer of intermetallic compound Ag₃Sn.

[0023] In the present fabricating method of the semiconductor element, the layer of barrier metal is selected from single layers and laminate layers of titanium (Ti), chromium (Cr), copper (Cu), nickel (Ni), palladium (Pd), gold (Au), tungsten (W), titanium nitride (TiN), tantalum (Ta), niobium (Nb), tantalum nitride (TaN), mixtures thereof, and compounds thereof. In particular, due to excellent adherence with the intermetallic compound Ag₃Sn, a laminate layer stacked titanium (Ti) film, nickel (Ni) film, and palladium (Pd) film in this order is preferably used.

[0024] The low-melting metal is selected from tin (Sn), silver (Ag), bismuth (Bi), zinc (Zn), indium (In), antimony (Sb), copper (Cu) and germanium (Ge), mixtures thereof and compounds thereof. To be specific, binary mixtures (alloys) such as eutectic Sn—Ag or the like can be cited, and ternary alloys such as Sn—Ag—Bi or the like also can be used.

[0025] A third aspect of the present invention is a fabricating method of the semiconductor element, comprising the steps of forming a wiring pad on a semiconductor substrate, of forming a layer of barrier metal on the wiring pad, of forming a metallic layer containing silver (Ag) on the layer of barrier metal, of forming a layer of low-melting metal containing tin (Sn) on the metallic layer containing Ag and of melting the layer of low-melting metal containing Sn to form a protruded electrode, thereby forming an intermetallic compound Ag₃Sn in the neighborhood of an interface of the metallic layer containing Ag and the layer of low-melting metal containing Sn.

[0026] In the third aspect, the intermetallic compound Ag₃Sn is formed in a layer. That is, when solder containing Sn is used as low-melting metal, during the solder reflow procedures, a lower layer containing Ag is also melted. Accordingly, the layer of intermetallic compound Ag₃Sn is formed in the layer containing Ag, that is, between the solder layer and the barrier metal layer.

[0027] In addition, in the third aspect of the present invention, the intermetallic compound Ag₃Sn can be segregated in particles. By appropriately adjusting thickness of the layer containing Ag and reflow temperature, the intermetallic compound Ag₃Sn can be formed in a layer or in particles.

[0028] In the third aspect, the layer of barrier metal is selected from single layers and laminate layers of titanium (Ti), chromium (Cr), copper (Cu), nickel (Ni), palladium (Pd), gold (Au), tungsten (W), titanium nitride (TiN), tantalum (Ta), niobium (Nb), tantalum nitride (TaN), mixtures thereof and compounds thereof. In particular, due to excellent adherence with the intermetallic compound Ag₃Sn, a laminate layer stacked titanium (Ti) film, nickel (Ni) film, and palladium (Pd) film in this order is preferably used.

[0029] Further, the low-melting metal containing Sn is selected from simple Sn and mixtures of Sn and one or more kinds of metals selected from silver (Ag), bismuth (Bi), zinc (zn), indium (In), antimony (Sb), copper (Cu) and germanium (Ge). To be specific, binary mixtures (alloys) such as eutectic Sn—Ag or the like can be cited, and ternary alloys such as Sn—Ag—Bi or the like also can be used.

[0030] The protruded electrode in the present invention is formed by implementing plating of the low-melting metal, or by printing paste containing the low-melting metal. When the plating method is used, Ag plating and Sn plating can be separately implemented. Both of the Ag plating and Sn plating, when a plating solution of alkyl-sulfonic acid (salt) is employed, in addition to affecting less adversely on the environment, is compatible to each other during plating, too. Further, in the present invention, the protruded electrode is formed on the semiconductor element side, but can be formed on the wiring board side that is connected to a semiconductor element.

[0031] In general, after melting and cooling the Sn—Ag solder to solidify, in investigating texture thereof due to electron diffraction analysis, precipitate of Ag₃Sn is found to be inevitably contained divided finely and distributed in matrix. According to the third aspect of the present invention, between the barrier metal layer and the low-melting metal layer for instance Sn—Ag solder layer, an intermetallic compound AggSn is formed in a layer, or segregated in particles. Accordingly, the barrier metal layer and the low-melting metal layer are compatible to each other and excellent adherence can be obtained therebetween.

[0032] According to the present invention, Sn contained in the low-melting metal is suppressed from diffusing into the barrier metal layer, resulting in an improvement of reliability. The intermetallic compound Ag₃Sn being prevented from growing coarse in grain size thereof even after high temperature protracted tests and thermal cycle tests, does not deteriorate due to temperature. In addition, in particular, when Pd is used as the barrier metal, due to excellent adherence between Pd and intermetallic compound Ag₃Sn and Ag, the Ag layer or the layer containing Ag is not peeled off. Further, since the layer of intermetallic compound Ag₃Sn can be formed by use of sputtering method, a film thickness can be easily controlled. In addition to this, the Ag layer, being able to form by use of sputtering or plating, can be easily controlled in the thickness thereof.

[0033] The semiconductor element of the present invention is connected to a wiring board of an arbitrary shape through a protruded electrode (bump) consisting of low-melting metal and is sealed by resin or the like to form a semiconductor device (a package). The semiconductor device can be further connected to a motherboard.

[0034] FIG. 1 is a cross section for showing schematically a step of forming an electrode pad and a passivation film on a semiconductor wafer in the first embodiment of the present invention;

[0035] FIG. 2 is a cross section for showing schematically a step of forming a barrier metal layer in the first embodiment of the present invention;

[0036] FIG. 3 is a cross section for showing schematically a step of forming a layer of an intermetallic compound Ag₃Sn in the first embodiment of the present invention;

[0037] FIG. 4 is a cross section for showing schematically a step of forming a layer of low-melting metal at an opening of resist in the first embodiment of the present invention;

[0038] FIG. 5 is a cross section for showing schematically a step of removing resist to etch lower layers in the first embodiment of the present invention;

[0039] FIG. 6 is a cross section for showing schematically a step of forming a protruded electrode (bump) in the first embodiment of the present invention;

[0040] FIG. 7 is a cross section for showing schematically a step of forming an electrode pad and a passivation film on a semiconductor wafer in the fourth embodiment of the present invention;

[0041] FIG. 8 is a cross section for showing schematically a step of forming a barrier metal layer in the fourth embodiment of the present invention;

[0042] FIG. 9 is a cross section for showing schematically a step of forming a layer of Ag at an opening of resist in the fourth embodiment of the present invention;

[0043] FIG. 10 is a cross section for showing schematically a step of forming a layer of low-melting metal on a layer of Ag in the fourth embodiment of the present invention;

[0044] FIG. 11 is a cross section for showing schematically a step of removing resist to etch lower layers in the fourth embodiment of the present invention;

[0045] FIG. 12 is a cross section for showing schematically a step of forming a bump in the fourth embodiment of the present invention;

[0046] FIG. 13 is a cross section for showing schematically a step of forming, through formation of a bump, particles of an intermetallic compound Ag₃Sn segregated in the fifth embodiment of the present invention;

[0047] FIG. 14 is a cross section for showing schematically a step of sequentially forming an electrode pad, a passivation film and a barrier metal layer on a semiconductor wafer, and of thereon further forming a layer of an intermetallic compound Ag₃Sn, and a resist pattern in the sixth embodiment of the present invention;

[0048] FIG. 15 is a cross section for showing schematically a step of etching lower layers, and of further removing a resist pattern in the sixth embodiment of the present invention;

[0049] FIG. 16 is a cross section for showing schematically a step of screen-printing paste of low-melting metal in the sixth embodiment of the present invention;

[0050] FIG. 17 is a cross section for showing schematically a step of removing a printing mask to form a layer of low-melting metal in the sixth embodiment of the present invention;

[0051] FIG. 18 is a cross section for showing schematically a step of forming a bump in the sixth embodiment of the present invention;

[0052] FIG. 19 is a cross section for showing schematically a step of forming an electrode pad and a passivation film on a semiconductor wafer in an existing fabricating method of a semiconductor element;

[0053] FIG. 20 is a cross section for showing schematically a step of forming a barrier metal layer in an existing fabricating method of a semiconductor element;

[0054] FIG. 21 is a cross section for showing schematically a step of forming a layer of low-melting metal at an opening of resist in an existing fabricating method of a semiconductor element;

[0055] FIG. 22 is a cross section for showing schematically a step of removing resist and etching lower layers in an existing fabricating method of a semiconductor element;

[0056] FIG. 23 is a cross section for showing schematically a step of forming a bump in an existing fabricating method of a semiconductor element.

[0057] FIG. 24 is a cross sectional view of a semiconductor device comprising a semiconductor chip and a wiring board according to the present invention.

[0058] In the following, preferred embodiments of the present invention will be explained with reference to drawings.

[0059] Embodiment 1

[0060] As shown in FIG. 1, on an entire surface of a semiconductor wafer 1 (a diameter of 6 inches and a thickness of 625 μm) composed of silicon or the like, aluminum electrode pads 2 of 100 μm square are formed in lattice with a pitch of 350 μm. Thereafter, thereon, while leaving an opening at a center portion of the aluminum electrode pad 2, a passivation film 3 is formed.

[0061] Then, as shown in FIG. 2, on the entire surface of the semiconductor wafer 1, by use of sputtering or electron beam evaporation, a titanium film 4, a nickel film 5 and a palladium film 6 are sequentially stacked to form a barrier metal layer. As the barrier metal layer, each layer of chromium (Cr), copper (Cu), gold (Au), TiN, tungsten (W), tantalum (Ta), niobium (Nb) and tantalum nitride (TaN) may be combined to stack.

[0062] Next, as shown in FIG. 3, on the barrier metal layer, a layer 7 of an intermetallic compound Ag₃Sn is formed by use of sputtering or electron beam evaporation. The layer 7 of an intermetallic compound Ag₃Sn can be subsequently formed in the same chamber where the barrier metal layer has been formed. A thickness of the intermetallic compound Ag₃Sn layer 7 is 10 μm or less, being preferable to be in the range of 0.5 to 5 μm. When the thickness of the intermetallic compound Ag₃Sn layer 7 exceeds 10 μm, not only it takes a longer time to form the layer but a later step of etching becomes difficult. When the thickness is less than 0.5 μm, the layer does not sufficiently work as a barrier.

[0063] Then, as shown in FIG. 4, on the intermetallic compound Ag₃Sn layer 7, resist 8 is coated with a thickness of approximately 50 μm. Thereafter, a portion of the resist corresponding to above the aluminum electrode pad 2 is opened in a hole of a 100 μm square. In this opening, by plating, a layer 9 of low-melting metal is formed with a thickness of 50 μm.

[0064] As the low-melting metal, for instance eutectic Sn—Ag solder can be cited. In addition to this, one kind of metal selected from Sn, Ag, Bi, Zn, In, Sb, Cu, and Ge and mixtures or compounds of metals of two kinds or more selected therefrom can be used. In plating the low-melting metal as such, the semiconductor wafer thereon the resist 8 is formed, is immersed in a solution containing 40 g/litter of Sn, 0.4 g/litter of Ag, 100 g/litter of alkyl-sulfonic acid and additional agent consisting mainly of surfactant. In a bath of 20° C., with the barrier metal layer and the layer of an intermetallic compound Ag₃Sn as a cathode, and with a plate of Sn as an anode, under the condition of a current density of 1 A/dm², while gently stirring, the plating is carried out.

[0065] Next, as shown in FIG. 5, after the resist is removed (stripped) by use of acetone or known resist remover, the intermetallic compound Ag₃Sn layer 7, the palladium film 6 and the nickel film 5 that remained as a substrate electrode are etched by a solution of aqua regia. Further, the titanium film 4 is etched with a solution of ethylenediaminetetraacetic acid.

[0066] Then, the semiconductor wafer, after flux is coated thereon, is heated in an atmosphere of nitrogen, at a temperature of 250° C. for 30 sec. Thereby, as shown in FIG. 6, the low-melting metal undergoes solder reflow procedures to form a protruded electrode (bump) 10. Thereafter, though omitted in the figure, after electrical tests, the semiconductor wafer is diced to form semiconductor chips. The semiconductor chip is mounted by use of flip-chip connecting method as shown in FIG. 24. That is, the semiconductor chip, after mounting on a wiring board 50, goes through a nitrogen reflow furnace set at 250° C. to melt the bump consisting of low-melting metal. Here, as connecting pad 52 disposed on the wiring board 50, one kind of films of Cu, Ni, Au, Pd and laminate films thereof or films of mixtures thereof, or laminate films in which on a conductive metal one of the low-melting metals such as Sn, Ag, Bi, Zn, In, Sb, Cu and Ge and films of mixtures thereof is formed can be used.

[0067] Thus, the semiconductor chip and the wiring board are electrically connected to obtain a semiconductor device 54. Further, though not shown in FIG. 24, the semiconductor device 54 thus obtained is connected to a motherboard through connecting terminals disposed on the wiring board. In the surroundings of the semiconductor chip mounted on the wiring board, and between the wiring board and the semiconductor chip, sealing resins such as silicone resin, epoxy resin or acrylic resin can be filled to cure.

[0068] In the semiconductor element thus obtained in the first embodiment, between the barrier metal layer and the bump 10 consisting of low-melting metal, the layer 7 of an intermetallic compound Ag₃Sn is formed. Accordingly, the barrier metal layer and the bump 10 are well adhered. Due to the layer 7 of the intermetallic compound Ag₃Sn, Sn contained in the low-melting metal can be prevented from intruding and diffusing into the barrier metal layer (Ni layer).

[0069] Accordingly, the barrier metal layer is prevented from deteriorating with the resulting improvement of reliability. The intermetallic compound Ag₃Sn not growing coarser particles even after the high temperature protracted test or the thermal cycle test, does not tend to deteriorate due to the high temperatures. Furthermore, at the uppermost layer of the barrier metal layer, the palladium film 6 is disposed, resulting in excellent adherence with the layer of intermetallic compound Ag₃Sn. Furthermore, the layer of intermetallic compound Ag₃Sn being able to form by use of sputtering or the like, can be regulated of thickness with ease.

[0070] Reliability of the semiconductor devices produced according to embodiment 1 is evaluated due to thermal cycle test. The results are shown in the following. For a test sample, a silicon chip of 10 mm square thereon 900 pieces of bump are formed is mounted on a glass-resin wiring substrate. The thermal cycle test is implemented with a cycle of −65° C.×30 min˜25° C.×5 min˜150° C.×30 min.

[0071] As the result of the thermal cycle test, even after 3000 cycles, there is found no snapping at all at solder joints. The bumps have sufficient connection strength, do not show any deterioration in the strength or do not peel off at all. Further, there occurred no short-circuiting between the bumps. When maintained the wafer at 150° C. after bump formation, even after 5000 hours, bump strength (bump share strength) of 50 g/piece is maintained and does not deteriorate.

[0072] Embodiment 2

[0073] In a second embodiment, on the barrier metal layer, instead of the layer of an intermetallic compound of Ag₃Sn, a layer of Ag is formed by use of sputtering or electron beam evaporation. As the low-melting metal for forming a protruded electrode, simple Sn, or metals consisting mainly of Sn and mixed with Ag, Bi, Zn, In, Sb, Cu or Ge can be used. Except for the above, similarly with embodiment 1, a semiconductor element and a semiconductor device are produced.

[0074] In the present embodiment, in the reflow procedure of the low-melting metal, Ag reacts with Sn in the low-melting metal to form an intermetallic compound Ag₃Sn, in a layer of a thickness of 0.5 μm to 5 μm at a boundary of the barrier metal layer and the low-melting metal layer. In order to obtain a layer of the intermetallic compound Ag₃Sn, a thickness of the Ag layer, though depending on temperatures, is preferable to be 4 μm or more.

[0075] Thus, in the second embodiment, between the barrier metal layer and the protruded electrode (bump) consisting of low-melting metal the layer of an intermetallic compound, Ag₃Sn is formed. Accordingly, the barrier metal layer and the bump are well adhered. Due to the layer of the intermetallic compound Ag₃Sn, Sn contained in the low-melting metal is prevented from intruding and diffusing into the barrier metal layer (Ni layer), resulting in preventing deterioration of the barrier metal from occurring and in an improvement of reliability. Even after the high temperature protracted test and thermal cycle test, the intermetallic compound Ag₃Sn does not grow coarser in particle size, with the resulting likelihood of less deterioration due to the temperatures.

[0076] Further, in the present embodiment, with simple Sn as a low-melting metal, a layer of Sn is formed on the layer of Ag. Thereafter, by use of reflow procedures to the layer of Sn, a layer of the intermetallic compound Ag₃Sn is formed. Thereby, problems in implementing Sn—Ag solder plating can be solved.

[0077] That is, there are problems such that due to a large difference of deposition potentials between Sn and Ag, simultaneous deposition is difficult, abnormal growth occurs during plating, voids occur due to an additional agent, or depending on wettability of the plating solution the resist opening is plated with difficulty. Further, in general, in Sn—Ag solder, due to too much content of Sn, when left in service circumstances of high temperatures, the barrier metal layer deteriorates sooner. Further, in Sn—Ag eutectic solder, due to too small content of Ag of 3.5%, in plating, composition can be regulated with difficulty. However, by adopting a method in which after forming a layer of Sn on a layer of Ag, the Sn layer undergoes solder reflow procedures to form a layer of an intermetallic compound Ag₃Sn, all the aforementioned problems in plating with Sn—Ag solder can be solved.

[0078] Embodiment 3

[0079] In a third embodiment, on the barrier metal layer, instead of the intermetallic compound Ag₃Sn, a layer of Ag is plated on an entire surface of the wafer with a plating solution of alkyl-sulfonic acid (salt). As the low-melting metal for forming a protruded electrode, simple Sn, or metals consisting mainly of Sn and mixed with Ag, Bi, Zn, In, Sb, Cu or Ge can be used. Except for the above, similarly with embodiment 1, a semiconductor element and a semiconductor device are manufactured.

[0080] In the present embodiment, in the reflow procedure of the low-melting metal, Ag reacts with Sn in the low-melting metal to form an intermetallic compound Ag₃Sn in a layer of a thickness of 0.5 μm to 5 μm between the barrier metal layer and the low-melting metal layer. In order to form the intermetallic compound Ag₃Sn in a layer, the thickness of the Ag layer, though depending on temperatures, is preferable to be 4 μm or more.

[0081] Thus, in the third embodiment, between the barrier metal layer and the bump consisting of the low-melting metal the layer of an intermetallic compound Ag₃Sn is formed. Accordingly, the barrier metal layer and the bump are well adhered. Due to the layer of an intermetallic compound Ag₃Sn, Sn contained in the low-melting metal is prevented from diffusing into the barrier metal layer (Ni layer), resulting in prevention of deterioration of the barrier metal from occurring and in an improvement of reliability. The intermetallic compound Ag₃Sn even after the high temperature protracted test and thermal cycle test, does not grow coarser in particle size with the resulting likelihood of less deterioration due to the temperatures.

[0082] Further, in the present embodiment, with simple Sn as the low-melting metal, the layer of Sn is formed on the layer of Ag. Thereafter, by use of reflow procedure to the layer of Sn, a layer of an intermetallic compound Ag₃Sn is formed. Thereby, difficulties in implementing Sn—Ag solder plating can be solved.

[0083] Both the Ag layer and the low-melting metal (Sn) layer, being formed with a plating solution of alkyl sulfonic acid, affect less adversely on the surroundings and is compatible with each other in plating. Further, since the Ag plating precedes the Sn plating, the Ag deposits uniformly to result in easy regulation. A thickness of plated layer can be readily regulated with a simple means of varying a plating time.

[0084] Embodiment 4

[0085] As shown in FIG. 7, on an entire surface of a semiconductor wafer 1 (a diameter of 6 inches and a thickness of 625 μm) composed of silicon or the like, aluminum electrode pads 2 of 100 μm square are formed in lattice with a pitch of 350 μm. Thereafter, thereon, while leaving an opening at a center portion of the aluminum electrode pad 2, a passivation film 3 is formed.

[0086] Then, as shown in FIG. 8, on the entire surface of the semiconductor wafer 1, by use of sputtering or electron beam evaporation, a titanium film 4, a nickel film 5 and a palladium film 6 are sequentially stacked to form a barrier metal layer. As the barrier metal layer, each layer of chromium (Cr), copper (Cu), gold (Au), TiN, tungsten (W), tantalum (Ta), niobium (Nb) and tantalum nitride (TaN) may be combined to stack.

[0087] Then, as shown in FIG. 9, on the barrier metal layer, resist is coated with a thickness of approximately 50 μm. Thereafter, a portion above the aluminum electrode pad 2 is opened in a hole of 100 μm square. In the opening, a layer 11 of Ag is plated with a plating solution of alkyl sulfonic acid. A thickness of the Ag layer 11 is 10 μm or less, being preferable to be in the range of 0.5 to 5 μm. When the thickness of the Ag layer 11 exceeds 10 μm, it takes a long time to form, and in the case of the thickness being less than 0.5 μm, a barrier effect can not be fully exhibited. In order to form the intermetallic compound Ag₃Sn in a layer, though depending on temperatures, the thickness of the Ag layer 11 is preferable to be 4 μm or more.

[0088] Then, as shown in FIG. 10, on the Ag layer 11 formed in the opening, a layer 9 of low-melting metal is formed with a thickness of 50 μm.

[0089] As the low-melting metal, for instance eutectic Sn—Ag solder can be cited. In plating the low-melting metal as such, the semiconductor wafer thereon the resist 8 is formed, is immersed in a solution containing 40 g/litter of Sn, 0.4 g/litter of Ag, 100 g/litter of alkyl-sulfonic acid and additional agent consisting mainly of surfactant. In a bath held at 20° C., with the barrier metal layer and the Ag layer 11 as a cathode, and with a plate of Sn as an anode, under a current density condition of 1 A/dm², while gently stirring, the plating is carried out. As the low-melting metals, simple Sn, or metals consisting mainly of Sn and mixed with Ag, Bi, Zn, In, Sb, Cu, or Ge can be used.

[0090] Next, as shown in FIG. 11, after the resist 8 is removed by use of acetone and known resist remover, the palladium film 6 and the nickel film 5 that remained as the substrate electrode are etched by a solution of aqua regia. Further, the titanium film 4 is etched with a solution of ethylenediaminetetraacetic acid. In the present embodiment, since the Ag layer 11 is formed in the opening of the resist 8, there is no need of etching the Ag layer 11 to remove.

[0091] Next, as shown in FIG. 12, the semiconductor wafer, after coating flux thereon, is heated in an atmosphere of nitrogen at 250° C. for 30 sec to apply solder reflow procedures to form a protruded electrode (bump) 10. In the reflow procedures of the low-melting metal, Ag reacts with Sn in the low-melting metal to form a layer of an intermetallic compound Ag₃Sn, of a thickness of 0.5 μm to 5 μm between the barrier metal layer and the low-melting metal layer.

[0092] Thereafter, though not shown in the figure, standard electrical tests are carried out. Then, the semiconductor wafer is diced into semiconductor chips, followed by flip-chip mounting. In the step of flip-chip mounting, a semiconductor chip is mounted on a wiring board and is let go through a nitrogen reflow furnace to melt the bump consisting of the low-melting metal. Here, as the connecting pad formed on the wiring board, one kind selected from Cu, Ni, Au, and Pd films and laminated films thereof or films of mixtures thereof, or laminated films in which one of films of low-melting metals such as Sn, Pb, Ag, Bi, Zn, In, Sb, Cu and Ge, and mixtures thereof is formed on a conductive metal can be used.

[0093] Thus, the semiconductor chip and the wiring board are connected to form a semiconductor device. Further, thus obtained semiconductor device is connected to a motherboard through connecting terminals disposed on the wiring board. In the surroundings of the semiconductor chip that is mounted on the wiring board, and between the wiring board and the semiconductor chip, sealing resins such as silicone resin, epoxy resin or acrylic resin may be filled to cure.

[0094] In the fourth embodiment as such, between the barrier metal layer and the bump 10 consisting of the low-melting metal, the layer 7 of an intermetallic compound Ag₃Sn is formed. As a result of this, the barrier metal layer and the bump 10 are well adhered. The layer 7 of the intermetallic compound Ag₃Sn suppresses Sn contained in the low-melting metal diffusing into the barrier metal layer (Ni layer). Accordingly, the barrier metal layer is prevented from deteriorating, resulting in an improvement of reliability. The intermetallic compound Ag₃Sn, not growing coarse even after the high temperature protracted test and thermal cycle test, does not deteriorate due to the temperatures. Furthermore, since the Pd film 6 that well adheres with Ag is disposed at the uppermost layer of the barrier metal layer, the Ag layer 11 or the layer 7 of the intermetallic compound Ag₃Sn does not peel off.

[0095] Further, in the present embodiment, with simple Sn as a low-melting metal, a layer of Sn is formed on the layer of Ag. Thereafter, by solder reflow procedures to the layer of Sn, a layer of the intermetallic compound Ag₃Sn is formed. Thereby, problems in implementing Sn—Ag solder plating can be solved.

[0096] That is, there are problems such that due to a large difference of deposition potentials between Sn and Ag simultaneous deposition is difficult, abnormal growth occurs during plating, voids occur due to additional agent, or depending on wettability of the plating solution the resist opening is plated with difficulty. Further, in general, in Sn—Ag solder, due to too much content of Sn, when left in service circumstances of high temperatures, the barrier metal layer deteriorates sooner. Further, in Sn—Ag eutectic solder, due to too small content of Ag of 3.5%, in plating, composition can be regulated with difficulty. However, by adopting a method in which after forming a layer of Sn on a layer of Ag, the Sn layer undergoes reflow procedures to form a layer of an intermetallic compound Ag₃Sn, all the aforementioned problems in plating with Sn—Ag solders can be solved.

[0097] Both the Ag layer and the low-melting metal (Sn) layer, being formed with a plating solution of alkyl-sulfonic acid, affect less adversely on the surroundings and is compatible with each other in plating. Further, since the Ag plating precedes the Sn plating, the Ag deposits uniformly to result in easy regulation. A thickness of plated layer can be regulated with ease with a simple means such as varying a plating time.

[0098] Embodiment 5

[0099] In a fifth embodiment, the thickness of the Ag layer 11 is set at 1 μm. Except for this, a semiconductor element is manufactured similarly with embodiment 4.

[0100] In the present embodiment, as shown in FIG. 13, Ag reacts with Sn in the low-melting metal to form, in the neighborhood of an interface of the Ag layer 11 and the low-melting metal layer, particles 12 of the intermetallic compound Ag₃Sn segregated. To form the generated intermetallic compound Ag₃Sn in particles, though depending on temperatures, the Ag layer 11 is preferable to be formed in a thickness of 4 μm or less.

[0101] In the present embodiment, in the neighborhood of the interface of the Ag layer 11 and the bump 10 consisting of the low-melting metal the particles 12 of the intermetallic compound Ag₃Sn are formed segregated. Accordingly, the barrier metal and the bump 10 are adhered well. The particles 12 of the intermetallic compound Ag₃Sn suppress Sn contained in the low-melting metal diffusing into the barrier metal layer (Ni). As a result of this, the barrier metal layer can be prevented from deteriorating, resulting in an improvement of reliability.

[0102] Embodiment 6

[0103] As shown in FIG. 14, on an entire surface of a semiconductor wafer 1 (a diameter of 6 inches and a thickness of 625 μm) composed of silicon or the like, aluminum electrode pads 2 of 100 μm square are formed in lattice with a pitch of 350 μm. Thereafter, thereon, while leaving an opening at a center portion of the aluminum electrode pad 2, a passivation film 3 is formed.

[0104] Then, on the entire surface of the semiconductor wafer 1, by use of sputtering or electron beam evaporation, a titanium film 4, a nickel film 5 and a palladium film 6 are sequentially stacked to form a barrier metal layer. As the barrier metal layer, each layer of chromium (Cr), copper (Cu), gold (Au), TiN, tungsten (W), tantalum (Ta), niobium (Nb) and tantalum nitride (TaN) may be combined to stack.

[0105] Next, on the barrier metal layer, the layer 7 of the intermetallic compound Ag₃Sn is formed by sputtering or electron beam evaporation. The layer 7 of the intermetallic compound Ag₃Sn can be subsequently formed in the same chamber where the barrier metal layer has been formed. A thickness of the layer 7 of the intermetallic compound Ag₃Sn is 10 μm or less, being preferable to be in the range of 0.5 to 5 μm. When the thickness of the layer 7 of the intermetallic compound Ag₃Sn exceeds 10 μm, not only it takes a longer time to form the layer but also a later step of etching becomes difficult. When the thickness is less than 0.5 μm, the layer does not sufficiently work as a barrier. Next, on the layer 7 of the intermetallic compound Ag₃Sn, the resist is coated in a thickness of approximately 50 μm. Thereafter, so as to form a shape overlapping the portion of the aluminum electrode pad 2, a resist pattern 13 is formed with a glass mask or the like.

[0106] Then, as shown in FIG. 15, with a solution of aqua regia, the layer 7 of the intermetallic compound Ag₃Sn, the palladium film 6 and the nickel 5 are etched. Thereafter, the titanium film 4 is etched with a solution of ethylenediaminetetraacetic acid. Further, with acetone or known resist stripping solution, the resist pattern 13 is removed (stripped).

[0107] Then, as shown in FIG. 16, paste 14 of the low-melting metal is coated by screen-printing method. That is, a print mask 15 of a thickness of 60 μm and a 160 μm square is aligned on the wafer, the paste 14 containing low-melting metal is poured into the opening of the print mask 15, and the excess of the paste 14 is squeezed off by use of a squeegee. Thereafter, as shown in FIG. 17, the print mask 15 is pulled up or the wafer is pulled down, thereby the layer 9 of low-melting metal is formed on the layer 7 of the intermetallic compound Ag₃Sn. As the low-melting metal, eutectic Sn—Ag solder can be cited. One kind of metal selected from Sn, Ag, Bi, Zn, In, Sb, Cu and Ge or mixtures or compounds of two kinds or more of metals thereof can be used.

[0108] Then, the semiconductor wafer is heated at 250° C. for 30 sec in an atmosphere of nitrogen to form, as shown in FIG. 18, a protruded electrode (bump) 10 by solder reflow procedure of the low-melting metal. Thereafter, not showing in the figure, after standard electrical tests, the semiconductor wafer is diced to form semiconductor chips, followed by flip-chip mounting. In the flip-chip mounting, the semiconductor chip, after mounting on the wiring board, is let go through a nitrogen reflow furnace set at 250° C. to melt the bump 10 consisting of the low-melting metal. Thus, the semiconductor chip and the wiring board are electrically connected to form a semiconductor device. For the connecting pad, one kind selected from Cu, Ni, Au and Pd films and films of laminate or mixtures thereof, or laminate films in which a film of low-melting metal such as Sn, Ag, Bi, Zn, In, Sb, Cu or Ge and a film of mixtures thereof is formed on a conductive metal can be used. In the surroundings of the semiconductor chip mounted on the wiring board, and between the wiring board and the semiconductor chip, sealing resins such as silicone resin, epoxy resin or acrylic resin can be filled to cure. Further, thus obtained semiconductor device is connected to the motherboard through a connecting terminal of the wiring board.

[0109] In the present embodiment, between the barrier metal layer and the bump 10 consisting of the low-melting metal the layer of the intermetallic compound Ag₃Sn is formed. As a result of this, the barrier metal layer and the bump are adhered well. The layer of the intermetallic compound Ag₃Sn suppresses Sn contained in the low-melting metal diffusing into the barrier metal layer (Ni layer). Accordingly, the barrier metal layer is prevented from deteriorating, to result in an improvement of reliability. The intermetallic compound Ag₃Sn, not growing coarse even after the high temperature protracted test and thermal cycle test, does not deteriorate due to the temperatures. Furthermore, since the layer 7 of the intermetallic compound Ag₃Sn is formed by sputtering, the thickness thereof can be easily regulated. Still further, since the low-melting metal layer 9 is formed by screen-printing method, steps of-formation thereof can be simplified.

[0110] As obvious from the above explanation, according to the present invention, between the barrier metal layer and the protruded electrode consisting of low-melting metal, an intermetallic compound Ag₃Sn which is excellent in adherence to both the barrier metal layer and the protruded electrode, is formed. Accordingly, a semiconductor element of high reliability can be obtained.