硅谐振器的晶圆级封装技术

Recently, a family of low frequency silicon resonators with AlN piezoelectric activation has been developed. These devices need to be operated under vacuum. The main motivation for the packaging of resonator under vacuum is to increase the Q factor, and  to guarantee the long-term stability of the resonance frequency. In order to minimize the size of packaged resonators, wafer level  packaging (WLP) has been chosen.

Electronic devices of all kinds need timing references. Currently, the main technology for clock products is quartz.  Miniaturization of quartz has been driven by the mobile phone industry. Today 32kHz quartz resonator are found in  packages as small as 2x1.25x0.6mm3  (Epson Toyocom FC-12M) while 30MHz or above resonators can be found in  1.6x1.25x0.3mm3  packages (TEW Tokyo DenpaTAS1612C/D). Since the early 1980s, companies have been trying  to replace quartz with silicon MEMS-based oscillators. Until recently however, the low temperature stability of  silicon and the need for expensive ceramic or metal vacuum packaging prevented MEMS from becoming a serious  alternative to quartz. SiTime claims the smallest integrated silicon resonator with electrostatic actuation packaged at  the wafer level. The resonator cavity is sealed at high temperature (1000°C) and under high vacuum during the  processing of the resonator itself (not a backend post-processing) . An alternative – piezoelectric through AlN -  actuation solution has been proposed in recent years. Devices covering a large frequency range from 20kHz to  36MHz were demonstrated. One of the major advantages of piezoelectric devices over electrostatic devices is that  the impedance levels are much closer to those achievable with quartz and would hence allow similar drive circuits to  be used.

The solution uses a capping Pyrex wafer sealed by anodic bonding. This bonding method requires an extremely  smooth and residue free surface of the wafer. The thickness of silicon oxide on rear side of the wafer shall not  exceed 500 nm. The bonding process is conducted at 350°C, -1200V, 3 mbar during 30 min. The performances of  resonators were not altered during this sealing process. Thin (300 microns) Pyrex substrates have been hermetically  bonded with device SOI wafers (thickness 100+390 microns).  

A reflow process was developed minimizing the size and amount of voids (thought to be Kirkendall voids),  controlling the dendrite height and the amount of eutectic formed. The reflow process developed involves oxide  reduction, annealing and AuSn (80/20) eutectic formation steps. A tacking process is involved before the vacuum  sealing process to use the advantage of accurate alignment possible in a flipchip machine. Tacking is done using a  tacking media which can be evaporated and degassed before the vacuum sealing step. The vacuum sealing is done at  a peak temperature of 320ºC and the process time is optimized to have less consumption of the UBM. Pressure is  needed during the sealing process to have uniform wetting throughout the ring due to the high surface tension of the  AuSn eutectic. Fig.4b shows a cross section view after sealing process (UBM/Au/AuSn/UBM). In this version, the  electrical contact with the resonator is established via metal lines crossing the sealing ring (Fig4c). The electrical  signal and metal sealing ring are isolated by a passivation SiON layer.

Fig3

All types of resonators are built from (100)-oriented Silicon on Insulator substrates. the vibrating structure consists  mainly of single crystal silicon that is oxidized on both sides (top and buried oxide), and that is topped by AlN and  its electrodes. Polycrystalline piezoelectric (002) AlN films are deposited by magnetron sputtering on Pt (111)  forming a piezoelectric activation for the SOI structure. All thin films are patterned by standard photolithography  and dry etching. The Si beams are created by deep reactive ion etching (DRIE) of the Si device layer from the top  side, followed by the DRIE of the Si handle from the backside. Fig.3 shows a top view of 1MHz extensional and  100kHz flexural AlN/Si resonators with sealing rings.