Eutectic bonding, also referred to as eutectic soldering, describes a wafer bonding technique with an intermediate metal layer that can produce a eutectic system. Those eutectic metals are alloys that transform directly from solid to liquid state, or vice versa from liquid to solid state, at a specific composition and temperature without passing a two-phase equilibrium, i.e. liquid and solid state. The fact that the eutectic temperature can be much lower than the melting temperature of the two or more pure elements can be important in eutectic bonding.
Eutectic alloys are deposited by sputtering, dual source evaporation or electroplating. They can also be formed by diffusion reactions of pure materials and subsequently melting of the eutectic composition.
Eutectic bonding to transfer epitaxial materials such as GaAs-AlGaAs onto Si substrates with a high degree of success, for the general purpose of optoelectronics integration with Si electronics as well as to overcome fundamental issues such as lattice mismatch in hetero-epitaxy, was developed and reported by Venkatasubramanian et al. in 1992. The device performance validation of such eutectic metal bonded GaAs-AlGaAs materials for solar cells were further reported by Venkatasubramanian et al. in 1994. Eutectic bonding is able to produce hermetically sealed packages and electrical interconnection within a single process (compare ultrasonic images). In addition this procedure is conducting at low processing temperatures, low resultant stress induced in final assembly, high bonding strength, large fabrication yield and a good reliability. Those attributes are dependent on the coefficient of thermal expansion between the substrates.
The most important parameters for eutectic bonding are: bonding temperature bonding duration tool pressure
Overview
Eutectic bonding is based on the ability of silicon (Si) to alloy with numerous metals and form a eutectic system. The most established eutectic formations are Si with gold (Au) or with aluminium (Al). This bonding procedure is most commonly used for Si or glass wafers that are coated with an Au/Al film and partly with adhesive layer (compare with following image).
The Si-Au couple has the advantages of an exceptionally low eutectic temperature, an already widespread use in die bonding and the compatibility with Al interconnects. Additionally, often used eutectic alloys for wafer bonding in semiconductor fabrication are shown in the table. Choosing the correct alloy is determined by the processing temperature and compatibility of the materials used.
Further, the bonding has less restrictions, concerning substrate roughness and planarity than direct bonding. Compared to anodic bonding, no high voltages are required that can be detrimental to electrostatic MEMS. Additionally, the eutectic bonding procedure promotes a better out-gassing and hermeticity than bonding with organic intermediate layers. Compared to glass frit bonding, the advantage sticks out that the reduction of seal ring geometries, an increase of the hermeticity levels and a shrinking of device size is possible. The geometry of eutectic seals is characterized by a thickness of 1 - 5 µm and a wideness of > 50 µm. The use of eutectic alloy brings the advantage of providing electrical conduction and interfacing with redistribution layers.
The temperature of the eutectic bonding procedure is dependent on the used material. The bonding happens at a specific weight-% and temperature, e.g. 370 °C at 2.85 wt-% Si for Au intermediate layer (compare to phase diagram).
The procedure of eutectic bonding is divided into following steps: Substrate processing Conditioning prior to bonding (e.g. oxide removal) Bonding process (Temperature, Mechanical Pressure for a few minutes) Cooling process
Procedural steps
Pre-treatment
The surface preparation is the most important step to accomplish a successful eutectic bonding. This bonding procedure is due to oxide presence on the silicon substrates very limited based on the poor wettability of Au on the oxide layer. This leads to a poor adhesion of the eutectic bond. The oxide on the silicon surface acts as a diffusion barrier. The surface preparation's main task is to facilitate the deposition of the eutectic metal by oxide removal or adhesion layer deposition.
To remove existing native oxide layers wet chemical etching (HF clean), dry chemical etching or chemical vapor deposition (CVD) with different types of crystals can be used. Also some applications require a surface pre-treatment using dry oxide removal processes, e.g. H2 plasma and CF4 plasma.