HTCC APPROACH FOR PACKAGING MEMS ACCELEROMETER SENSORS AND ASSOCIATED

Problems associated with assembly and packaging of two accelerometer Micro Electro Mechanical Sensors (MEMS) and associated Read Out Integrated Circuit (ROIC) into a single compact package as per customer requirements were discussed, analysed and a High Temperature Co-fired Ceramic (HTCC) solution was suggested.


ISSN: 2320-5407
Int. J. Adv. Res. 6(7), 537-552 538 Requirement of integration of associated ROIC for specific sensor further complicates the packaging issues. Location of an ROIC as close as possible to sensor becomes mandatory to avoid parasitics that may affect the precision and accuracy of sensor adversely. This is also a physical necessity for the wirebonds which connect both the components and sensor to prevent shorting during device operation.
Some devices also require internal atmosphere control since moisture and particle contamination can be damaging. Getters (trap moisture and particles), lubricants, or anti-stiction agents may also be added to the package to prevent wear, degradation, or stiction [4]. Stiction, a combination of sticking and friction, is a common problem in MEMS sensors with moving structures. Stiction occurs when smooth, planar surfaces make contact and become permanently locked together by short-range atom forces. Since MEMS devices are so small, these moving parts have a very high area-to-mass ration making stiction likely.
Thus while designing a package for MEMS modules one has to look into all these requirements.
Commercial Pacakge Designs:-Analog Devices, Inc., Cambridge, MA, did much of the pioneering work in this field and enjoys a leadership position in the accelerometer area. But there is substantial competition [5].
Freescale also has a long history in motion sensors and launched 3-axis motion sensor products a few years back. OKI, MemSense, Hitachi, Kionix and STMicroelectronics also have offered 3-axis accelerometers.
Most of these devices are packaged in small QFN format. The smallest package is 5mm x 5mm x 1.2mm offered by Kionix, but competitors have designs that are only slightly larger [6,7]. Fig. 1 shows several QFN (Quad Flat pack with No leads) MEMS packages. Most of them use ceramic hermetic packages, at least for now. The simple QFN has reduced the cost over earlier ceramic cavity style packages that had accounted for more than half of the total component cost. MEMS device is often capped at wafer-level so that the motion zone is hermetically enclosed [8,9]. The device thus is protected from both particulate contamination and moisture. The bonded silicon caps are specially singulated so that they are smaller than the chip allowing bonding pad access. The capped MEMS device can now be handled more like any ordinary electronic chip. The capped device can subsequently be packaged either in plastic or ceramic. The capped device is attached along with ROIC and wire bonded to the ceramic cavity package. Finally fully hermetic sealing is achieved through sealing by ceramic or combo coverlid using AuSn preforms or low temperature glass preforms. The general flow is highlighted in Fig. 2. The present paper describes a design approcah to relaize a pacakge meeting the requirements of one of the customers. Customer has fabricated an XY Accelerometer, a Z-Accelerometer and an associated ROIC and wanted to integrate all these into a single hermetic package that can pass through MIL-STD specifications. The paper describes a design approach that can be physically achieved through HTCC (High Temperature Co-fired Ceranmic) Technology [10]. Table I and shown in Fig.3, Fig.4 and Fig.5 for ROIC, XY Sensor and Z-Sensor respectively :  The layout takes care of design criterea such as minimum die spacing allowed and alignment of bonding pads among ROIC and sensor chips.

Details of ROIC chip and sensor devices fabricated by the Customer at his MEMS Fab are given below in
Constraints for Customer Sensor Integration:-1. ROIC chip has a very high pinout of 92 which is uncomparable with sensors available in the market (maximum 32). 2. Bonding pads are unevenly distributed 40 on top 40 on left 12 on bottom. This makes it difficult to try any other die layouts such as die stack. 3. Individual dies are also quite largelarger than even sensor packages available commercially. 4. Also dies are to be located at specific coordinates to align for proper die to die wire bonding between ASIC and XY Sensor and ASIC and Z Sensor. This practically makes the MCM pad size 7.1 mm x 9.0 mm. Due to capping of Z-Sensor, package height gets decided by height of this sensor which is about 1.4 mm.

Top -5 pads
542 In view of this it is proposed that only solution for any of the above combinations will be keeping the dies side by side as shown in Fig.6 and there will not be much change in final package dimensions.

Wire bonding and Packaging diagrams:-
Based on the proposed 3-chip MCM Layout above, following tentative wire bonding and packaging diagrams were drawn.

Proposed wire bonding diagram for Customer Sensor MCM:-
Tentative wire bonding diagram for the 3-chip module is shown in Fig. 7.
543 Design guidelines for structural parameters and for vertical interconnects are described in Table II and Table III respectively.

Final Package Information:-
The final package dimensions are expected to be as shown in Fig. 11 while charecteristics are given in Table IV.
General package top view, bottom pads, seal ring on the top for hermetic sealing and rooting of interconnections in inner layers of HTCC are shown in Fig.12 and cross sectional views in Fig.13.  Fig. 14 and Fig. 15 show the Top view and side view of the stack while Fig. 16 shows how the sensor looks after completion of process except for partila dicing of top cap to expose the bonding pads. Sensor thickness will be equal to [die thickness + top wafer thickness + bottom wafer thickness] ~ 1.7 to 2.0 mm Package height after lid sealing will be approximately 4.0 mm As per the request of the customer, the proposed package was compared with commercially available packages for similar sensors as indicated in Table V. pin It was pointed out to the customer that in view of large pinc count of ROIC, it would be practically impossible to realize a module with size comparable to commercial packages.