Method of manufacturing a semiconductor device by electroless metallisation

Abstract: By using an electroless metallization bath to which a stabilizer is added which suppresses the cathodic partial reaction, pyramid-shaped bumps (53) can be grown on the bond pads of semiconductor devices without lateral overgrowth of the coating layer 3. The angle of inclination α is a function of the concentration of the stabilizer.

Description: The invention relates to a method of manufacturing a semiconductor device comprising bond pads which, after an activation treatment, are provided with metal bumps in an electroless metallization bath containing metal ions and a reducing agent. Wet-chemical processes play an important role in the manufacture of electronic components, both in the removal (etching) and in the deposition of materials (plating). When semiconductor devices (ICs and LSIs) are mounted on a circuit, the bond pads of the semiconductor devices are provided with projecting metal contacts (so-called bumps), after which said bumps are connected to the circuit by bonding, welding, soldering or thermocompression (so-called flip chip principle). By TAB (tape automated bonding) ICs can be provided on a flexible tape comprising circuits. Such a method is known from European Patent Application EP-A 308971. A semiconductor device comprising aluminium bond pads is first subjected to an activation treatment with an aqueous palladium-salt solution and then provided with nickel bumps. For this purpose, an electroless nickel bath is used. Such electroless metallization processes are, in principle, isotropic, i.e. the deposition rate of the metal is equal in all directions. In the case of substrates comprising a thin coating layer in which apertures are formed which define the bond pads and which are to be metallized, lateral overgrowth of the coating layer takes place in the metallization process as soon as the metal layer has completely filled the aperture in the coating layer. In the case of semiconductor devices, said coating layer (also termed passivating layer) consists mostly of SiO₂ or Si₃N₄. The metal bumps project from the coating layer and, as a result of lateral overgrowth, partly cover said coating layer. As a result of the increasing miniaturization the risk of a short-circuit between adjacent bumps increases. One of the objects of the invention is to suppress and even preclude the above-mentioned lateral overgrowth of the coating layer. According to the invention, this object is achieved by a method as described in the opening paragraph, which method is characterized in that a stabilizer suppressing the reduction of the metal ions is added in a concentration effective to lead to anisotropic metallisation so that the bumps are provided in the form of a pyramid having flat faces. Electroless metallization baths contain metal ions and a reducing agent. During the metallization process, reduction of the metal ions to metal (the so-called cathodic partial reaction) and, simultaneously, oxidation of the reducing agent take place. The stabilizer which is added in accordance with the invention has the property that the reduction of the metal ions is suppressed. The presence of such a stabilizer leads to anisotropic metallization of the bond pads, as a result of which pyramid-shaped bumps having flat faces are formed, while lateral overgrowth of the coating layer is precluded. It is assumed that said anisotropic metallization is caused by differences in mass transport of the stabilizer. The edges of the bond pad to be metallized are subject to a larger mass transport than the centre of the pad. By adjusting the concentration of the stabilizer it becomes possible to poison the edges of the bond pad, so that metallization is precluded at the edges, but continues in the centre of the pad. Continuous metallization leads to a further lateral diffusion of the stabilizer towards the centre of the growing bond pad, as a result of which the growth of the surface area decreases as a function of time. The result is a metal pyramid having flat faces. Faces are to be understood to mean herein the oblique faces and the upper face. Lateral overgrowth of the coating layer does not take place. An embodiment of the method according to the invention is characterized in that a lead salt is used as the stabilizer. Said lead salt must be dissolvable in the electroless metallization bath. Suitable lead salts are, for example, lead acetate and lead nitrate. Another suitable embodiment of the method according to the invention is characterized in that a cadmium salt is used as the stabilizer. Suitable cadmium salts are, for example, cadmium acetate and cadmium nitrate. An embodiment of the method according to the invention is characterized in that the concentration of the stabilizer is 0.1-1.5 mg per litre. Higher concentrations lead to a complete discontinuation of the metallization process. Lower concentrations lead to isotropic growth of the bond pads and, hence, to lateral overgrowth of the coating layer. A suitable electroless metallization bath is, for example, a nickel bath comprising a nickel salt, succinic acid and hypophosphite in water. Other metals which can be electrolessly deposited are, for example, copper and gold. If necessary, the metal bumps can be provided with a thin coating of gold, tin, copper or solder. Said materials are more ductile than electroless nickel and enhance the soldering process with the circuit to be connected. The invention will be explained in greater detail by means of exemplary embodiments and with reference to the drawings, in which Fig. 1 diagrammatically shows a part of a cross-section of a semiconductor device having bumps obtained by the method according to the state of the art, Fig. 2 diagrammatically shows a part of a cross-section of a semiconductor device comprising bumps obtained by the method according to the invention, Fig. 3 diagrammatically shows a part of a cross-section of a semiconductor device comprising bumps obtained by the method according to the invention, and Fig. 4 diagrammatically shows a part of a cross-section of a semiconductor device comprising bumps on aluminium bond pads, said bumps being obtained by the method according to the invention. Exemplary embodiment 1. (not according to the invention) Fig. 1 diagrammatically shows a part of a cross-section of a semiconductor device, in which reference numeral 1 denotes a n⁺ silicon substrate on which a 0.7 µm thick SiO₂ layer 3 is provided in a customary manner (for example CVD or spin-on-glass). Recesses having dimensions of 100 x 100 µm are lithographically formed in said layer 3. The silicon surface is activated by palladium nuclei by means of an electrochemical exchange reaction in a diluted PdCl₂/HF solution. The activation bath contains per litre 5 mg of PdCl₂, 175 µl of concentrated HCl, 1% of HF and is used at a temperature of 70°C for 60 seconds. After rinsing with water, the silicon substrate is immersed in an acid electroless nickel bath having the following composition: 0.07 mol/l of nickel sulphate 0.01 mol/l of nickel acetate 0.1 mol/l of succinic acid 0.1 mol/l of H₃PO₂. Using ammonium hydroxide, the pH-value is adjusted to 4.5. The temperature of the bath is 90°C. Under these conditions, the deposition rate is 20 µm per hour. The residence time of the silicon substrate in the bath is 20 minutes, so that the nickel layer formed (bump 51) has a thickness of approximately 7 µm. Due to the absence of a stabilizer in the electroless nickel bath, the degree of lateral overgrowth on the SiO₂-layer 3 is comparable to the layer thickness, which is a result of the isotropic character of the metallization process. Exemplary embodiment 2. Exemplary embodiment 1 is repeated, with this difference that the electroless nickel bath also contains 0.5 mg/l of lead acetate as the stabilizer. In this case, the nickel bump formed has the shape of a truncated pyramid 52 (Fig. 2) the faces of which are flat. Lateral overgrowth of the SiO₂ coating layer 3 does not take place. Exemplary embodiment 3. Exemplary embodiment 1 is repeated, with this difference that 1.5 mg/l of lead acetate is added to the electroless nickel bath. The nickel bump formed has the shape of a truncated pyramid 53 (Fig. 3) having flat faces and the angle of inclination α enclosed by the oblique faces and the silicon substrate 1 being smaller than the angle obtained in accordance with exemplary embodiment 2. If the metallization process is continued ( i.e. in this case longer than 20 minutes) a pyramid without truncation will be formed (see dotted line 7 in Fig. 3), after which metallization stops. Lateral overgrowth does not take place. Exemplary embodiment 4. Exemplary embodiment 1 is repeated with a lead-acetate concentration of 2 mg/l in the electroless nickel bath. In this case, no nickel deposition at all takes place. This is also the case when the lead-acetate concentrations exceed 2 mg/l. At said concentrations, the surface to be metallized is completely poisoned. Exemplary embodiment 5. Fig. 4 diagrammatically shows a part of a cross-section of a semiconductor device. Said device comprises a silicon substrate 11, a sputtered aluminium bond pad 15 having a thickness of 0.5 µm and dimensions of 100 x 100 µm. A 0.7 µm thick SiO₂ layer 13 is present on the substrate 11, in which SiO₂ layer an aperture is lithographically formed at the location of the aluminium bond pad 15. The aluminium surface is activated with a zincate solution as described in United States Patent Specification US 4,205,099. The natural oxide skin of the aluminium is removed and replaced by a very thin zinc film (not shown). Subsequently, the zinc is substituted by the nobler nickel in an electroless nickel bath according to exemplary embodiment 1. Lead acetate is added to said nickel bath in a quantity of 1 mg/l. After 20 minutes a nickel bump 54 has formed which has the shape of a truncated pyramid with flat faces and which does not exhibit lateral overgrowth of the SiO₂ coating layer 13.

Claims: A method of manufacturing a semiconductor device comprising bond pads which, after an activation treatment, are provided with metal bumps in an electroless metallization bath containing metal ions and a reducing agent, characterized in that a stabilizer suppressing the reduction of the metal ions is added in a concentration effective to lead to anisotropic metallisation so that the bumps are provided in the form of a pyramid having flat faces. A method as claimed in Claim 1, characterized in that a lead salt is used as the stabilizer. A method as claimed in Claim 1, characterized in that a cadmium salt is used as the stabilizer. A method as claimed in Claim 2 or 3, characterized in that the concentration of the stabilizer is 0.1-1.5 mg per litre. A method as claimed in Claim 1, 2, 3 or 4, characterized in that a nickel bath is used as the electroless metallization bath.

Inventors: VAN DER PUTTEN ANDREAS M T P, VAN DER PUTTEN, ANDREAS M.T.P.
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: H01L21
IPC codes: C23C18, H01L21