CONTACT BUMP STRUCTURE AND METHOD FOR FABRICATING CONTACT BUMPS

Abstract: The invention relates to a contact bump structure formed onto an aluminium contact pad area (3) on a silicon substrate (1) and a method of forming said contact bump structure. According to the invention, said contact bump structure comprises a tin bump (8) formed by means of an autocatalytic reaction on said contact pad area (3).

Description: Contact bump structure and method for fabricating contact bumps The invention relates to a contact bump structure according to the preamble of claim 1. The invention also concerns a method of forming a contact bump structure. Conventionally, contact bumps on electronic components are formed by means of autocatalytic processes from nickel with gold deposited thereon. The contact bump structure is later bonded, e.g., by tin solder bonding to a larger circuit substrate/board. Due to different thermal expansion rates of the bonded elements, the contact bumps are subjected to stresses frequently resulting in an unsatisfactory per- formance of the bond on the circuit substrate/board. Conventional soldering tech¬ niques are unfit for the high contact densities required today, since the solder amount applied to the intimately located bonding areas tends to spread excessively during the soldering process. It is an object of the present invention to overcome the drawbacks of the above- described techniques and to achieve an entirely novel type of contact bump struc¬ ture and method of forming such contact bumps. The goal of the invention is achieved by depositing the tin solder bumps by means of an autocatalytic process directly onto aluminium contact bump structures formed on a silicon substrate. More specifically, the contact bump structure according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the method according to the invention is characterized by what is stated in the characterizing part of claim 6. The invention offers significant benefits. One of the principal advantages of the contact bump structure according to the invention is that a metallurgical structure is attained which during the soldering process and under operating conditions does not form brittle intermetal compounds or such uncontrolled intermetallic alloying that is detrimental to the solder bond. The bond structure disclosed herein has an inherent ductility and toughness which are liable to even out stresses caused by the differential thermal expansion rates of the substrate and the bonded component. The invention also makes it possible to implement bump structures that are compatible with conventional surface mounting technology. Moreover, the invention facilitates a high component bonding density without any risk of short circuits. In the following, the invention will be examined in more detail by means of exemplifying embodiments with reference to the attached drawings, in which: Figure 1 shows a longitudinally sectioned side view of a contact bump structure according to the invention; Figure 2 shows the contact bump structure of Fig. 1 after post-treatment; Figure 3 shows a longitudinally sectioned side view of a process step suitable for forming an alternative embodiment of the contact bump structure according to the invention; Figure 4 shows the contact bump structure of Fig. 3 after post-treatment; Figure 5a shows a longitudinally sectioned side view of a third embodiment of the contact bump structure according to the invention; Figure 5b shows the contact bump structure of Fig. 5a after post-treatment; and Figure 6 shows a fourth alternative embodiment of the contact bump structure according to the invention. The goal of the method according to the invention is both to dispense with the catalyzing zincate bath which is conventionally applied prior to the nickel bath but tends to attack the aluminium layer and is critical in operation, and to accelerate the start of metal deposition in the nickel bath such that no additional etching can occur. For this purpose, now referring to Fig. 1 , onto an aluminium contact pad area 3 patterned on a semiconductor wafer 1 is deposited another layer of nickel 5 with a sufficient thickness to form also laterally about the contact bump an about 7 μm thick metallization acting as a corrosion protection for the aluminium layer 3. Layer 2 is a passivation layer conventionally used in semiconductor element structures. Layer 4 is a thin nickel layer deposited by a catalytic (metal-exchange reaction) process serving to replace the conventional zincate bath. The stresses between the nickel layer 5 deposited from the present process and the aluminium layer 3 remain substantially smaller than those occurring in a contact bump structure processed in a conventional nickel bath. On the nickel layer 5 is deposited a thin copper layer 6 whose sole function is to carry out the metal- exchange reaction with tin. As the tin layer 7 next deposited by the metal- exchange reaction cannot reach any substantial thickness, another tin layer 8 of greater thickness (15-20 μm) is deposited in an autocatalytic bath thereonto. In order to prevent the growth of tin whiskers, the contact bump is finally covered by a thin (0.3- 1.0 μm thick) lead layer 9 which is converted into an eutectic tin-lead alloy by a heat treatment. Figure 2 shows the contact bump of Fig. 1 after the heat treatment. Now referring to Fig. 3, instead of the thin copper layer 6, a thick resist layer 10 with vertical walls can be used for depositing a thin, high copper "post" 6 which can yield substantially during thermal expansion thus reducing the risk of bond fractures. The copper post 6 is easily made ductile by subjecting the contact bumps to heat treatment at 150 °C for about 15 minutes. An alternative approach to improve the ductility of the contact bump is to leave the resist in place after the copper deposition step, whereby the tin 7 from the metal-exchange reaction is only allowed to deposit on top the copper post 6 as shown in Fig. 4. After the removal of the resist, the autocatalytically deposited thick tin 8 forms a blob-like ball on the top of the contact bump, wherefrom, during the fusing step, it cannot wet the sides of the copper post 6, these being al¬ ready oxidized at this stage of the bump-forming process. The best result is attained using a tenacious photoresist which can also endure the stress of the autocatalytic tin bath. Then, the copper layer can be made maximally thin and the tin post grown high. Resultingly, the ductility required of the contact bonds will be provided solely by the yielding of the high tin post. As shown in Fig. 5a, when permitted by the desired contact density, onto the thin nickel layer 5 and the copper layer 6 can naturally be deposited a thick tin layer 8 that in a heat treatment is melted into a blob-like bump bordered by the underlying metallization as shown in Fig. 5b. Herein, the yield properties of the tin layer may change somewhat due to the required heat treatment. The contact bumps structures described above can be bonded to a substrate by means of a plurality of techniques well-known from the literature of the art. The most common bonding method is to use solder cream subjected to a fusing step as is conventional in the surface mounting technology. The limits of this method are dictated at high contact densities by the finest achievable raster of solder cream dots, which in the printing process typically is in the order of about 200 μm. Higher contact densities can be attained by using anisotropically conducting adhesives. The conductive component in these compounds is a metallic filler comprising nonmelting metallic particles, or preferably, melting solder alloy particles. Even higher still contact densities (with an interbump spacing smaller than 100 μm) have been attained by directly fusing the contact bump surfaces to the contact pad areas of the substrate, which are coated with a suitable metal, in an inert gas atmosphere or protected by a flux. When using a tin-lead alloy bump on a gold-clad contact area, this metallic bond combination has been processed with good results in a normal dry air atmosphere even without using any flux. The present method offers an interesting, novel possibility of processing metallic microbonds by using certain semimetallic elements (such as bismuth) for making the bond. In this process a suitable, typically tin or tin-lead alloy coated contact bump and/or substrate is coated electrolytically or chemically by a thin bismuth layer or similar semimetallic compound. In the bonding process the bumps and the substrate are superimposed and heated until the bismuth due to its lowest melting temperature of the materials is melted and forms a solid solution with tin without causing total melting of the tin bump. The metallic bond thus obtained is extremely tough and ductile. If an adhesive layer is applied over the bonding areas prior to the bonding process and the contact bumps of the component to be bonded are pressed against the substrate so as to displace the adhesive from the area to be contacted by the bumps, a solder bond can be formed that is also pro¬ tected by an adhesive. This process dispenses with the filler normally applied in the intermediate space remaining between the component and the substrate. A sim¬ ilar bonding metallurgy can also be achieved using a bismuth-filled anisotropically conducting adhesive. The above-described chemically deposited contact bump structure and its metallurgy complement the earlier discussed method according to the invention offering the benefit of chemically deposited metallization, a thickness variation smaller than ±1 μm which is difficult to achieve by electrolytic deposition processes. The multi-step contact bond formation process according to the invention permits a relatively large variation in the bonding temperatures used. As shown in Fig. 6, it is possible to simplify the disclosed process appreciably by arranging a metal- exchange reaction to occur directly with the aluminium layer 3. Then, the nickel and copper layers can be omitted and the contact bump is formed by a thin tin layer 7 formed in the metal-exchange reaction, an autocatalytically deposited tin layer 8 and a thin, protective lead layer 9. A proper composition for the tin layer 7 formed by the metal-exchange reaction is known from the chemical tin deposition processes used in the manufacture of aluminium pistons. The low mutual solubil¬ ity of the aluminium layer 3 and the tin layers 7, 8 in the bonds made and operated at substantially low temperatures does not impose any practical draw¬ backs in most of the bonding processes disclosed above. Typical process param¬ eters for a metal-exchange tin bath are: 44.8 g/1 Na 2 SnO,- 3H 2 O, 3-5 min process time at 80 °C, tin layer thickness 4-5 μm. While the metal-exchange reaction with tin seems to occur invariably, this process step does not appear to be crucial for the function of the invention. The method according to the invention is particularly intended for forming contact bumps on semiconductor chips patterned on a homogeneous silicon substrate. After processing, the semiconductor chips processed on the substrate are separated from each other and bonded to larger circuit boards by means of the above- described contact bumps. The autocatalytic deposition method referred to in the present text is also known as the electroless process in which metallic layers are deposited without using an external source of electrical potential. Next, an example of the implementation of the contact bump structure according to the invention as well as the method steps for their formation are described: 1. Purification of aluminium surface NaHCO 3 5.0 g/1 Na 5 P 3 O 10 5.0 g/1 Process conditions 22 °C, 60 s Water rinse (with deionized water) 22 °C, 60 s 2. Oxide etch-away HN0 3 5 % (with 5 % HF) Process conditions 22 °C, 15 s Water rinse (with deionized water) 22 °C, 60 s 3. Ni metal-exchange reaction (catalyzing step) NiSO 4 - 6H 2 0 13.1 g/1 H 2 NCH 2 COOH 15.0 g/1 Process conditions pH 8-9, 60 °C, 10 min Water rinse (with deionized water) 22 °C, 60 s 4. Autocatalytic deposition Ni:P bath NiS0 4 - 6H 2 0 13.1 g/1 H-NCHjCOOH 1 1.2 g/1 NaH 2 PO 2 H 2 O 31.7 g/1 Process conditions pH 8, 60 °C While the amount of oxygen dissolved in the bath must be kept rather low to start the bath, too low an oxygen content in the process may cause premature decomposition of the bath. 5. Autocatalytic Cu bath CuSO 4 - 5H 2 O 5.9 g/1 C 6 H 5 Na 3 0 7 - 2H 2 O 15.2 g/1 NaH 2 PO 2 - H 2 O 28.6 g/1 H 3 BO 3 30.9 g/1 NiSO 4 - 6H 2 O 0.9 g/1 NH 2 CSNH 2 < 0.2 mg/1 Process conditions pH 9.2, 65 °C Water rinse (with deionized water) 22 °C, 60 s 6. Catalyzing step with Sn Sn(BF 4 ) 2 (50 %) 87.6 g/1 HBF 4 52.6 g/1 NH 2 CSNH 2 152.2 g/1 Process conditions 80 °C, 20 s Water rinse (with deionized water) 7. Activation HCl 16 % Process conditions 22 °C, 30 s Water rinse (deionized water) 8. Autocatalytic Sn bath SnCl 2 - 2H 2 O (0.35M) 78.9 g/1 KOH 255.3 g/1 C 6 H 5 K 3 O 7 - H 2 O (0.40M) 129.7 g/1 Process conditions 70 °C Water rinse (with deionized water) 9. Lead deposition (for prevention of tin whisker growth) Pb(CH 3 COO) 2 - 3H 2 O 7.5 g/1 CHN 2 Na 4 O x - 4H 2 O 45.2 g/1 NaOH 18.4 g/1 Process conditions 60 °C Water rinse (deionized water) Drying Heat treatment at about 215 °C for forming an eutectic tin-lead layer onto the bump.

Claims: 10 Claims: 1. A contact bump structure formed onto an aluminium contact pad area (3) on a silicon substrate (1), characterized by - a tin bump (8) formed by means of an autocatalytic reaction onto said contact pad area (3). 2. A contact bump structure as defined in claim 1, characterized in that a lead layer (9) for preventing the formation of tin whiskers is formed onto the surface of said tin bump (8). 3. A contact bump structure as defined in claim 1 or 2, wherein at least one nickel layer (4, 5) is formed onto the surface of said aluminium contact pad area (3) by means of an autocatalytic reaction, characterized in that a copper layer (6) is deposited between said tin layer (8) and said nickel layer (4, 5) for the purpose of improved tin adherence. 4. A contact bump structure as defined in claim 3, characterized in that said copper layer (6) has a thickness approximately equal to that of said tin layer (8). 5. A contact bump structure as defined in claim 1, characterized in that the uppermost layer in said structure is a bismuth layer. 6. A method of forming contact bumps, in which method - contact bumps are formed by means of an autocatalytic reaction onto the surface of aluminium contact pad areas (3) on a silicon substrate (1), 11 characterized in that - said contact bumps are formed from tin (8). 7. A method as defined in claim 6, characterized in that onto the surface of said tin bump (8) is formed a lead layer (9) for preventing the formation of tin whiskers. 8. A method as defined in claim 6 or 7, wherein at least one nickel layer (4, 5) is formed onto the surface of said aluminium contact pad area (3) by an autocatalytic reaction, characterized in that, prior to the autocatalytic deposition of said tin layer (8), onto said nickel layer (4, 5) is deposited a copper layer (6) for the purpose of improved tin adherence. 9. A method as defined in claim 6, c haracterized in that said copper layer (6) is formed into a columnar shape by means of a patterned photoresist (10). 10. A method as defined in claim 6, characterized in that, prior to the autocatalytic deposition of said tin layer (8), a tin layer (7) is deposited by a metal-exchange reaction. 11. A method as defined in claim 6, characterized in that the contact bump is finally coated with a bismuth layer.

Inventors: AINTILA AHTI, AINTILA, AHTI
Applicants: AINTILA AHTI, PICOPAK OY, AINTILA, AHTI
Application year: 1997
Main classification: H01L21
IPC codes: H01L21, H01L23