In this paper, we present a high yielding 20µm pitch CuSn electroplated microbump flip chip process. The 10µm diameter bumps are organized in an area array, consisting of 440 daisy chains of 1766 bumps each. The 2cm x 2cm flipchipped dies consist of about 1M bumps in total. The influence of processing materials like seed layer etchants and cleaning agents on the electrical performance of the daisy chains is discussed. Further Ti/Cu versus TiW/Cu seed layers for electroplating are compared. Finally inspection methods for tracing back electrically measured failures are screened.
IntroductionIn advanced 3D systems like fully hybrid CMOS imager systems, where detector and read out circuit are pixel wise interconnected, very dense bump interconnects are needed [1,2]. Each unconnected bump will show up in the image as a black dot. Also for field-programmable gate arrays (FPGA) a high performing area array interconnect scheme is applicable.In this paper full thickness, 725µm, Si dies were assembled face to face by means of CuSn microbumps. For the assembly, a transient liquid phase (TLP) CuSn thermo compression bonding was used. The bonding temperature is relatively low, just above the melting temperature of the solder. The intermetallic formed after bonding has a higher melting point than of the solder itself. This enables the stacking of more than 2 dies with more flexibility for future hybrid imagers or more in general for 3D integrated systems.For the above mentioned high-end applications, where there is no room for built-in bump redundancy, the bump yield should be very high. In [3] we showed 90% yielding peripheral arrays with 25µm diameter CuSn bumps on a 40µm pitch. In this paper we drastically scaled down to bump diameters of 10µm on a 20µm pitch. The 2cm x 2cm flipchipped stack consist of about 1M bumps in total, with bumps organized in an area array. To evaluate this high number of bump interconnects for failure detection and to optimize bump yield, we used very long daisy chains. In the next section the test vehicle is described.Starting from the choice of TiW/Cu as seed layer for electroplating, the influence of processing aspects such as choice of seed layer etchants and cleaning agents on the microbump performance are being discussed. From the electrical evaluation of the daisy chains yield and bump resistance are extracted.