Fine pitch copper wire bonding process optimization with 33µm size ball bond

Gold wire bonding is widely used as the electrical connection or interconnection between the semiconductor component (die) and the package. Bond pads pitch on the device chips are reducing continuously as device technology is moving towards nano-IC technologies. Gold wire is not suitable in the case where the bond pad pitch is less than 50μm because of gold´s lower intensity and stiffness. Recently, copper wire becomes more and more popular in industry due to the lower in price compare to gold wire and also better electrical performance. Copper wire can also be suitable for fine pitch wire bonding due to its properties. The challenge of Cu wire bonding is obtaining its process window without compromising on it quality. This paper specifically discuss the critical copper wire bonding parameters for 50μm fine pitch bond pads on stress sensor chip with aluminum pad of 1.5μm thickness using 4N copper wire of 0.7mils from Heraeus. The three main critical parameters discussed in this work are ultrasonic power, time and force. The wire bonding process parameters are optimized to achieve ball size of 33μm and ball height of 8-10μm with reasonable Al remnant under wire bond ball. To avoid the copper oxidation in the sparking process, the copper-kit is used to provide the forming gas shielding with a flow rate of 0.6-0.8l/min during sparking process. Destructive tests such as ball shear and wire pull test are used to check the bonding quality. Failure modes are analysed using high power optical microscope. Focus Ion Bean (FIB) and Scanning Electronic Microscope (SEM) are used for microsectioning on the first bond location of the samples to analyze aluminum remnant and aluminum splash.