For the development of small and low cost microbolometer, wafer level reliability characterization techniques of vacuum
packaged wafer are introduced. Amorphous silicon based microbolometer-type vacuum sensors fabricated in 8 inch
wafer are bonded with cap wafer by Au-Sn eutectic solder. Membrane deflection and integrated vacuum sensor
techniques are independently used to characterize the hermeticity in a wafer-level. For the packaged wafer with
membrane thickness below 100um, it is possible to determine the hermeticity as screening test by optical detection
technique. Integrated vacuum sensor having the same structure as bolometer pixel shows the vacuum level below
100mTorr. All steps from packaging process to fine hermeticity test are implemented in wafer level to prove the high
volume and low cost production.
We investigated silicon as a promising material for a IR transparent window platform of IR(Infrared Ray) sensors with
WLP(wafer level package), because silicon has advantages in price and CMOS process compatibility compared to Ge
window although Ge exhibits higher IR transmittance than Si. Having comparable transmittance to Ge is the key to use
silicon as a IR transparent window platform. We compared several types of AR coating films, SiN, SiO2, only ZnS, and
Ge/ZnS for finding the condition of maximizing transmittance of Si in the range of 8 ~12 um , LW-IR(Longwave IR).
Also we investigated changing of transmittance for LW-IR after thermal treatments in several ambient gases and several
temperatures.
Silicon is a promising material as an IR(Infrared Ray) transparent window platform for packaging MEMS( microelectro
mechanical systems), especially, IR sensors with WLP(wafer level package), because silicon has advantages in price and
CMOS process compatibility compared to Ge, although Ge exhibits higher IR transmittance than Si. This paper reports
on optimizing the thickness of Si window in the range of 8 ~ 12 um, LW-IR(Long wave IR). Two of important things
which have to be considered in window material of IR sensor are minimizing absorption of IR(maximizing transmittance)
and minimizing deformation due to the pressure differences between outside and inside of the package.
Because of trade-off between minimizing IR absorption and minimizing mechanical deformation, optimization of
thickness is important. Infrared absorbance of silicon was measured as varying thickness from 100 um to 700 um of the
Si window. Decreasing the thickness of silicon made the absorption smaller. Under 300 um, the difference of absorbance
with decreasing thickness becomes negligible. Degree of deformation according to the thickness of the Si window was
calculated by simulation varying pressure differences, and package area. Based on this analysis, we suggest the
optimized thickness of silicon window for WLP of LW-IR sensor.
For the development of small microbolometer for mobile applications, new pixel design to enhance fill factor by sharedanchor
structures is suggested and it can be possible to make a one anchor per unit pixel. Fill factor increases 10% more
than that of normal unshared-anchor design. Amorphous-silicon based microbolometer has been fabricated with 64x64
arrays of 25um pixel size to verify proposed design. Mechanical flatness of shared-anchor structure is enhanced.
Responsivity is enhanced from 1.08e+5 V/W to 1.23e+5 V/W due to the increase of fill-factor compared to unsharedanchor.
There are no mechanical, electrical and thermal crosstalk problems with adjacent pixels.
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