No abstract

Manufacturing and integration of MEMS devices by wafer bonding often lead to problems generated by thermal properties of materials. These include alignment shifts, substrate warping and thin film stress. By limiting the thermal processing temperatures, thermal expansion differences between materials can be minimized in order to achieve stress-free, aligned substrates without warpage. Achieving wafer level bonding at low temperature employs a little magic and requires new technology development. The cornerstone of low temperature bonding is plasma activation. The plasma is chosen to compliment existing interface conditions and can result in conductive or insulating interfaces. A wide range of materials including semiconductors, glasses, quartz and even plastics respond favorably to plasma activated bonding. The annealing temperatures required to create permanent bonds are typically ranging from room temperature to 400°C for process times ranging from 15-30 min and up to 2-3 h. This new technique enables integration of various materials combinations coming from different production lines.

Oil/water emulsions can be produced through phase inversion, by adding water to a reverse water/oil microemulsion. According to small angle neutron scattering experiments and visual observations performed during phase inversion, the stages of this process are as follows: (i) upon water addition, the microemulsion gives way to a highly swollen lamellar phase; (ii) the transient lamellar phase breaks up to yield an array of droplets; (iii) the droplets loses the correlations of the lamellar phase. This emulsion is already present less than one minute after the initial addition of water, and it reaches the final size distribution in one hour. The final population of oil droplets is homogenous with a mean diameter below 100 nm.

Low temperature wafer bonding is a powerful technique for MEMS/MOEMS devices fabrication and packaging. Among the low temperature processes adhesive bonding focuses a high technological interest. Adhesive wafer bonding is a bonding approach using an intermediate layer for bonding (e.g. glass, polymers, resists, polyimides). The main advantages of this method are: surface planarization, encapsulation of structures on the wafer surface, particle compensation and decrease of annealing temperature after bonding. This paper presents results on adhesive bonding using spin-on glass and Benzocyclobutene (BCB) from Dow Chemicals. The advantages of using adhesive bonding for MEMS applications will be illustrated by presenting a technology of fabricating GaAs-on-Si substrates (up to 150 mm diameter) and results on BCB bonding of Si wafers (200 mm diameter).

Plasma activated wafer bonding generated a high interest in last decade due to the important process temperature reduction. With the main advantage of bringing some applications at industrial degree of feasibility. An example of process which benefits from this new process is silicon fusion bonding: by using plasma activation the bond process temperature and time are reduced at values which make wafer bonding compatible with industrial manufacturing requirements. A newly developed process allowing bonding of two substrates in the plasma activation chamber is foreseen as a very interesting approach for numerous applications, especially in the field of engineered substrates fabrication.