Fluxless Die Attach by Activated Forming Gas

Eutectic god-tin (Au80Sn20) is widely used as the die-attach material for making radio-frequency (RF) and microwave devices. The metallic bonding is typically achieved by soldering using a gold-tin preform in a forming gas containing hydrogen (H2) and nitrogen (N2) to a peak temperature at 300 ̊C. The performance and reliability of the devices are strongly dependent on the quality of the die-attach layers. It is always expected to make the die-attach layers as free of voids as possible since the voids are poor thermal and electrical conductors and also are stress concentration centers. There are three major causes for void formation, which include poor solder wetting due to surface oxides, vapor out-gassing by flux decomposition, and gas entrapment from preform melting. It is known that the gas entrapment is more significant for larger dies and can be managed by gas evacuation before the melting of the solder. However, a good fluxless and oxide-free technology for metal die attach is still lacking. The organic fluxes used in conventional soldering not only induce void formation but also leave residues, which contaminate the dies and are corrosive. In addition to being costly and inconvenient to clean, the residues at the bonding interfaces of the die-attach layer are trapped, thus degrading interfacial bonding over time. The current study introduces a novel technology of using electron attachment (EA) to activate H2 diluted with N2 for fluxless die attach. EA is a new concept for ambient-pressure gas activation, which brings several advantages compared with plasma-based gas activation. Results obtained in this study demonstrate that solder oxides and organic contaminations on the surfaces to be bonded can be effectively removed by EA-activated forming gas, which leads to better solder wetting compared to flux-based process. The EA technology also shows a feasibility of reducing peak temperature of die attach from 300 ̊C to 290 ̊C, thus minimizing high-temperature induced damages. These advantages are believed to be attributed to the higher surface tension of the oxide-free molten solder compared with that of an organic flux, thus leading to a larger wetting force. As demonstrated in the study, the fluxless and oxide-free technology using EA has made it possible to achieve a high quality die attach with zero or near-zero (≤ 5%) voids. Introduction Radio-frequency (RF) and microwave devices are widely used in aerospace and defense industries for telecommunications. In these applications, a significant power at RF and microwave frequencies is generated and the devices are typically operated in some of the toughest and most demanding environments. Therefore, these devices have to be manufactured with proven reliability and performance. The core of the devices is made of semiconductor dies. Currently, gallium arsenide (GaAs) and silicon (Si) are most popularly used as the semiconductor dies in building RF and microwave devices 1-3 . When RF and microwave devices are in an operating state, heat will be generated from the active region of each die, which results in a rise in temperature. As the temperature of a die increases, its performance and reliability will both degrade. To promote heat dissipation, the overall device thermal resistance needs to be minimized. One contribution is from the semiconductor die itself. For this reason, the thickness of the dies used for RF and microwave devices is especially thin 4 . To prevent damage of the delicate dies with sensitive structures on their top surfaces, each die is mounted and fixed on a substrate or package base. This manufacturing step during assembly of devices is named as “die attach”. The performance and reliability of a RF and microwave device are strongly dependent on the quality of the die-attach layer. The die-attach layer needs to provide a good thermal dissipation path and electric contact between the die and the package base. It also has to withstand the thermal stresses when the device is subjected to power and temperature cycling during its operation. Because of the need for low thermal and electrical resistances in the die-attach layer, the dies used in RF and microwave devices are metallically bonded onto the package base by using a solder material to form a very thin bond layer. The backside of each die and its bond pad on the package base are typically terminated with a layer of gold (Au) as a thermal and electrical interface and a layer of nickel (Ni) underneath the gold as a diffusion barrier. Eutectic gold-tin (Au80Sn20) is widely used as the die-attach material for making RF and microwave devices due to its superior thermal and mechanical properties, good compatibility with the gold metallization surface, and a relatively high melting temperature (280 ̊C) to prevent thermal fatigue. For this type of die attach, a preform with a thickness as thin as 25μm 4, 5 is typically used and a general rule for preform size is 90% of the die metallization 6 . The bond formation is usually done at a peak temperature around 300 ̊C for one minute or less 7 . There is always a desire to reduce the peak temperature if possible to avoid problems such as die degradation by high-temperature exposure and die cracking due to large thermal stresses. Since the preform-based die-attach layer is so thin, its thermal and electrical resistances are determined not only by the solder material’s intrinsic properties but also by the quality of the formed joint. It is always expected to have a defect-free die attach for building RF and microwave devices. However, the defect-induced die-attach failures may still happen, which are generally localized at the interface of different layers, such as de-laminations, cracks, voids, and metal corrosions. One of the major reasons to cause defects is that the gold-tin preform is easily oxidized, forming a tin-rich oxide layer on its surface. An oxide layer at the bonding interface will adversely affect solder wetting. Defects such as solder de-wetting, delamination, and interfacial voids tend to form, which can largely increase the thermal and electrical resistances of the dieattach layer. A forming gas containing hydrogen (H2) and nitrogen (N2) is commonly applied during the preform-based die attach, but the forming gas only can help in preventing solder oxidation at the normal process temperature range up to 300 ̊C. The initial temperature for hydrogen to effectively reduce tin oxides is around 400 ̊C 8-11 . Therefore, to remove native oxides and help solder wetting, organic fluxes are still required, especially for making high power RF and microwave devices. However, the use of organic fluxes brings other chances of forming defects in the die attach layers. It leaves residues, which not only contaminate dies with unexpected ionic species but also induce corrosion when moisture is present. Therefore, a cleaning step has to be applied after the soldering, which is inconvenient and costly. Besides that, the cleaning cannot remove flux residues left at the interfaces of the die-attach layer, which can degrade the interfacial bonding and cause delamination. In addition, the flux vapor generated by flux decomposition always has a chance to remain in the molten solder or at the bonding interfaces, thus forming voids when the solder is solidified. The amount of flux needed is much less when using a preform compared with using a solder paste 12 , which is the major reason that preform-based die attach is typically used for high-reliability applications. However, the flux is not the only cause of forming voids when using a preform. This is because when two flat and rigid surfaces touch each other, the true contact area is typically less than 20% of the total interfacial area. Tiny gaps as thin as 0.1 μm at the interface is occupied by gas films 12 . When the preform is melted, the gas films are wrapped in the molten solder. Since gas diffusion through a molten solder is slow 13 , the entrapped gas forms voids. The larger the contact area, the greater is the gas entrapment. The voids are very poor thermal and electrical conductors and are stress concentration centers. Therefore, for building high-power RF and microwave devices, there is a high need to make the die attach layers as free of voids as possible. Typically, the acceptable void rate in eutectic die-attach bonds is less than 5% for total voids and less than 1 to 2% for each individual void 14 . To pursue a void-free die attach, a process named “pressure variation” or “vacuum release” has been developed 12, 15 . It applies a vacuum to evacuate gas films at the soldering interfaces before the solder is melted and then release the vacuum to ambient pressure of an inert gas to compress the remaining voids before the solder is solidified. However, the approach still needs an organic flux or formic acid vapor to clean native oxides on the preform surface, and the related issues of corrosive residues remain to be solved. Therefore, to pursue a defect-free solder die attach for high-power and high-reliability applications, there is a need to develop a fluxless and oxide-free process. The objective of this study was to meet the need by activating a reducing gas to efficiently remove oxides on the soldering surfaces in the normal die-attach temperature range, thus eliminating the use of organic fluxes. A reduction of peak temperature for die attach to minimize the potential of high-temperature induced damages was also evaluated.