Gold-gold flip chip bonding processes for RF, optoelectronic, high temperature and power devices

Most of the solder flip chip interconnections require flux for oxide removal to achieve a sufficient bonding. The contamination due to the residues of the flux after reflow soldering could influence the functionality, especially for RF and optoelectronic devices, and should therefore be avoided. An advantage of soldering processes is their low reflow temperature (e.g. eutectic PbSn 183°C, SnAg 221°C ) and the self alignment effect. A fluxless solder process with proven reliability, self alignment effect but a reflow temperature above 280°C is AuSn. The use of gold-gold flip chip bonding is an alternative technology for RF, optoelectronic, high temperature and power applications. The advantages of the gold-gold process are the fluxless bonding, the controlled shape of the interconnections after bonding, the excellent heat conductivity (297 W/mK), the use for high temperature devices like temperature sensors and the possibility of sub-micron alignment in combination with precise flip chip bonding equipment. Gold bumping which is suitable for prototyping, small and medium volume production can be applied by stud bumping on single chips or wafers. Electroplating on wafer level can be used for high volume processes. The bonding process chosen is thermocompression. Temperatures in the range of 250 to 300°C and forces up to 300 MPa result in a deformation of the bump on the pad with a material flow along the bonding surface and the formation of a solid metal contact. This paper summarizes the pros and cons of thermocompression bonding based on stud bumping and electroplating. A main difference of course is the bump shape. Gold stud bumps have a relatively soft tip and a cone shape with a harder basis. In contrast electroplated bumps are cylindrical or rectangular. The aspect ration between bump height and diameter becomes important for the thermocompression bonding of electroplated gold bumps. Experiments were performed on gold electroplated silicon test chips and substrates with bump diameters in the range of 15 to 250 µm. The bonding temperature was reduced down to 100°C and forces up to 300 MPa were applied. This parameter window is not suitable for the reliable formation of gold-gold interfaces but points out very clearly the influence of the bump shape on the adhesion after bonding. The results will be presented in detail based on shear testing, cross sectioning and DC electrical measurements. The temperature range of the thermocompression bonding from 250 to 300°C for some devices exceeds (e.g. Teflon based organic RF boards) the maximum applicable temperature. The thermosonic flip chip bonding can then be used. Ultrasonic energy will be applied during bonding as an additional parameter besides temperature and force. A temperature decrease down to 125°C for gold-gold bonding is possible. Thermosonic bonding experiments were performed with gold stud bumped silicon chips on silicon substrates and FR-4 boards. The assemblies were characterized by shear testing and cross sectioning. Finally the thermosonic bonding results will be compared to the thermocompression bonding pointing out the broad parameter window for the use of gold-gold flip chip interconnections