Infoscience

Thesis

Encapsulation hermétique pour systèmes hydro- et thermo-sensibles

The quantity of MEMS manufacturers has considerably been increasing these last years. This doesn't mean that the introduction of such products on the market has become easier. A packaging operation is needed to obtain a usable product out of a microsystem. The packaging ensures protection but also interconnection with the outside. Too foten, during the conception of a microsystem, the engineer usually does not think about packaging method, which increases costs and time to obtain a product. This thesis is a conception tool to help those who need to realise a microsystem requiring protection against gas and vapours. Some microsystems are sensitive to humidity and temperature. Up to now, to obtain an hermetic package, the easiest solution was soft soldering of a cap on a base. This operation was often done in an oven, which may heat the microsystem to temperatures it can not withstand. In this thesis we describe methods which allow the design and manufacturing of hermetic packages together with a low sealing temperature. In a first phase, material permeability is defined, as well as the notion of package hermeticity and leak measurement methods. The conclusion is that the only materials which allow long-term hermeticity (up to 10 years) are ceramics, glasses and metals. Usual methods for hermetic sealing of packages are then described. Soft soldering by the mean of a laser diode, which is used in this thesis, allows to obtain an hermetic package, together with a low thermal budget. The advantage of the laser diode over another type of laser is its low power density, as well as the possibility to continuously control its power during operation. The power can be controlled during soldering depending on the measured temperature, for example by the mean of a pyrometer. In order to design a package, it is necessary to formalize the requirements in terms of functions that need to be fulfilled by the package. Functional analysis is used to formalize these requirements. This iterative method needs to be done during the conception of the microsystem, but can also be done when the design of the microsystem is already done. In order to reduce reconception phases, most of the functions need to be found at an early stage of the package design. Some methods which allow to find these functions are described. A thermal modelling method is also proposed. This method allows to find an appropriate model to each situation. The first step consists in finding orders of magnitude. The model is then refined by the mean of an electrical analogy or by the mean of numerical simulations when necessary. Several demonstrators have been realized during this thesis. The feasibility of an LTCC (Low Temperature Cofired Ceramic) package for a micromirrors array is demonstrated. The manufacturing of this package is based on screen-printing technology. LTCC packages are suitable for small to medium series of complex microsystems. For complex devices, standard ceramic packages are more expensive than LTCC-based ones, and may not provide the required flexibility. The proposed method is adapted to manufacturing of hermetic cells containing rubidium, for miniature atomic clocks. Sealing time and temperature are drastically reduced compared to anodic bonding, which reduces evaporation of the rubidium during the sealing operation. Some improvements are needed, mainly adhesion of the metallization and atmosphere control during the sealing. The thermal modelling method is illustrated in an example of laser curing of epoxy glue. In order to determine the behaviour of the system, dimensional analysis is used together with experimentation. Versatility of the laser diode is also shown in this project. Finally, some technical points related to the use of a laser diode for soft soldering are also described. Recommendations are given regarding wettability of the solder and reduction of the heating time. Notably, pre-tinning the surfaces with solder is found to be preferable to the use of preforms.

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