Aluminum Nitride Chip Carrier for Micro-electro-mechanical Sensor Applications
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Aluminum Nitride Chip Carrier for Micro-electro-mechanical Sensor Applications T. F. Marinis and J. W. Soucy Draper Laboratory 555 Technology Square Cambridge, MA 02139-3563, U.S.A. ABSTRACT A commercially fabricated aluminum nitride chip carrier was evaluated for packaging various types of MEMS inertial sensors. They were successfully assembled and vacuum-sealed within AlN chip carriers and their pressures have remained stable for over one year. Aging tests were conducted under electrical bias at 85°C and 85 %RH. The leakage currents were not as stable as those measured in alumina chip carriers and post test inspection of the AlN parts revealed etching of the ceramic between conductors. INTRODUCTION Navigation quality, MEMS inertial sensors have traditionally been packaged in leadless ceramic chip carriers for use in high-G applications. These commercial chip carriers are comprised of high alumina composition ceramic, with co-fired tungsten metal signal traces. Exposed surfaces of the tungsten metallization are plated with one micron of nickel and gold. The MEMS sensor is attached to the chip carrier floor by brazing with a gold-tin eutectic alloy, which melts at 280°C. Aluminum wire bonds are used to make electrical connections between the chip carrier and sensor. Depending on the type of sensor, the assembly is sealed under vacuum or inert atmosphere, by brazing on a kovar cover with gold-tin eutectic alloy. A top view and a cross section, of this assembly, are shown in Figure (1). The most serious deficiency of this construction is the mismatch in thermal expansion coefficients between the MEMS chip and chip carrier. When the assembly cools after brazing, a significant shear stress arises within the bond.
Figure 1. A gyroscope assembly is shown from the top in (a) and in cross section in (b).
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An estimate of this stress can be obtained from a balance on the forces, which are needed to insure continuity of the structure. Referring to Figure (2), at the braze temperature, the length of interface between the chip and ceramic carrier is L0 . If unattached, their lengths would decrease to L1f and L2f respectively, when the temperature was lowered by ∆ T . This component free
length is given by Lif = L0 (1 + α i ∆ T ) , where α i is the applicable coefficient of thermal expansion.
The braze transmits stresses between both surfaces, which constrain their lengths to a value of Lc , that is given by L1f w1 t1 E1 + L2f w2 t 2 E 2 , w1 t1 E1 + w2 t 2 E 2 where wi and E i are the width and elastic modulus, respectively, of the chip and carrier. The magnitude of the constraining force, Fc , required is Lc
=
Fc
=
(L
f 1
)
− Lc w1 t1 E1 , Lc
so the shear stress, τ , transmitted through the braze joint is τ = Fc w1 Lc . For a typical MEMS package configuration, the estimated value of τ is 22 MPa, when computed using the properties of glass and Al2O3 that are given in Table (1). The shear strength of gold-tin braze is only 185 MPa, so it is expected that a creep mechanism would allow the stress to r
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