Page 253 - Sami Franssila Introduction to Microfabrication
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232 Introduction to Microfabrication





















                                         (a)                        (b)
            Figure 23.12 Cavity formation by etching of sacrificial oxide (gray) and (a) deposition sealing of a lithographically
            defined, plasma-etched, vertical access hole and (b) sealing of a horizontal-access hole defined by film deposition: very
            little deposition takes place inside the cavity when the access channel is long and narrow

            material. Generally, however, sealing is done at a wafer
            level.


            23.5 SEALING OF CAVITIES
            Cavities are closed structures with a controlled atmo-
            sphere inside. Absolute pressure sensor is a simple
            example: the cavity holds the reference pressure. In
            resonating structures, such as accelerometres and gyro-
            scopes, squeeze-film damping requires cavity pressure
            to be reduced from atmospheric pressure. This can be
            done in a bonding process or in a deposition process.
              CVD processes with conformal deposition are well
            suited for cavity sealing, but conformality also means           (a)
            that a film will deposit on the inner walls of the cavity.
            CVD processes with high surface mobility of adatoms
            and long mean free paths are best candidates for sealing.
            Schematic CVD sealing is shown in Figure 23.12 and
            SEM micrographs are shown in Figure 23.13.
              In order to reduce the influence of the sealing film on
            the structural films, the sealing film should be as thin as
            possible. This is often best achieved with horizontal-
            access holes rather than with plasma-etched vertical
            holes. Horizontal-access hole minimum dimension is
            determined by film thickness, which can be made small
            easily compared to lithographically determined plasma-
            etched access holes.
              If ultimate vacuum is needed inside the cavity,
            evaporation is the method of choice. Contrary to                 (b)
            CVD sealing, no (potentially) harmful gases will be  Figure 23.13 Cavity sealing by CVD: plasma-etched,
            incorporated into the cavity. Owing to the directional  chevron-shaped access holes are closed by LPCVD nitride
            nature of evaporation, horizontal-access holes have to  deposition. Reproduced from Chen, J. & K.D. Wise (1997),
            be used.                                     by permission of IEEE
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