MEMS for mobile communications: packaging systems for MEMS have very specific requirements, hermetic sealing thermal and mechanical stability.

The first article in this series discussed the potential microsystem applications for mobile communications with a key focus on radio frequency microelectromechanical systems (RF MEMS). The focus of this article is packaging, which is a key factor in determining the total cost and the overall size of the microsystems.

The performance of the MEM device depends on the package used. Therefore, the packaging requirements of MEM devices are very important, Micromachining can also create new capabilities for packaging technologies. For example, a plastic substrate fabricated with very precise three-dimensional (3-D) structures including conductors will be described.

Packaging for RF MEMS

Several issues related to wafer-level or zero-level packaging of RF MEM devices are important. The main technological drivers in wireless applications are small size, low cost and modularity of the packaged components. In particular, modularity will be emphasized in the future because one key issue is how to increase integration to have system-on-package solutions that contain MEMS and integrated circuits (ICs).

Wafer-level packaging challenges of RF MEMS are explained in this article using a MEM tunable capacitor as an example. A MEM capacitor usually consists of two parallel plates; one plate is fixed and the other is suspended using mechanical springs so that the bias voltage changes the gap between the plates.

Hermetic packaging

A hermetically sealed enclosure is a key requirement for RF MEMS packages. First, the wafer-level package must protect the MEM devices from the waste and liquids during the dicing of the wafer, because capillary forces can cause stiction of the mechanical structures. Even more important is the stability of the gas pressure inside the cavity during the component's operation. The composition of the gas inside the package also must be controlled. Experimental results have shown a connection between certain ambient conditions like humidity to the accumulation of surface charge in dielectric surfaces that may deteriorate component operation. (1)

Various technologies can create hermetic wafer-level packages, but they often utilize high processing temperatures. However, RF MEMS may contain metal structures that cannot withstand high temperatures. As a result, an additional constraint for RF MEMS packaging may be low processing temperatures of less than 180[degrees]C.

Thermal and mechanical effects

State-of-the-art micromachined devices are mostly made of polysilicon because it has superb mechanical properties and its technology is well known. (2) However, for RF devices such as the micromachined tunable capacitor, ohmic loss must be minimized and a high quality (Q-value) must be achieved. As a result, metals are often used.

For example, a Q-value of 62 at 1 GHz for a 2.11 pF capacitance value has been achieved with aluminum. (3) A polysilicon structure with an additional gold layer to increase conductivity has been reported to achieve a Q-value of 256 at 1 GHz for a 0.101 pF capacitance value. (4)

The use of metal in MEM devices has so far been very restricted. Two major problems occur when metal is used in micromechanical devices. First, MEM components are very sensitive to external stresses. Such stresses can be caused by the package or by the difference between the coefficient of thermal expansion (CTE) of the substrate and the metal device itself.

The second problem is warping of the micromechanical metal film on top of the glass wafer, which is due to the mismatch of the thermal expansion coefficients of the substrate and the metal film (Figure 1).

[FIGURE 1 OMITTED]

Several methods have been suggested to eliminate the stress dependence. Corrugations of the diaphragm and springs have been used. (5-7) Nevertheless, when considering long-term stability and the thermal drift of the mechanical structures, package-induced stress is one of the most important issues.

RF properties

RF design is a constant battle to minimize electrical losses. The electrical loss mechanisms in the RF devices are ohmic losses due to resistivity of conductors, dielectric losses due to the imaginary component of the dielectric constant, parasitic coupling and radiation losses. Consequently, the major challenges in RF packages are coupling and interconnects.

MEM devices are usually rather large, and, thus, the interconnect pitch is not as critical as it is in ICs. However, the need for hermetic packages increases the length of interconnects and, thus, increases ohmic losses and parasitic coupling. In addition, the need of impedance matching in interconnects to achieve minimal return loss is one of the greatest differences as compared to the low frequency systems.

Micromachining for RF Packaging

Micromachining enables a precise fabrication of 3-D shapes by etching the structures into bulk silicon or by creating them into thick photoresist by lithographic methods and deep x-ray exposure. (8) These structures can be transferred into high precision molds by electrochemical methods. The created molds can be a basis for different molding techniques to create precise plastic structures.

Micromachined toroidal inductor 

A novel high quality toroidal 3-D inductor is fabricated in the micromolded plastic structure in two parts, which are connected together at the wafer level at the end of the process (Figure 2). The plastic microstructure is first fabricated by using microreplication technology: (9)

[FIGURE 2 OMITTED]

* The microstructure is first etched into the surface of a silicon wafer using standard photolithography and isotropic and/or anisotropic plasma or wet etching.

* A thick nickel film is electroplated on top of the silicon structures.

* The nickel film is attached on a carrier and released from the silicon surface to form the mold for the plastic process.

The final plastic structure is then fabricated using casting, injection molding or hot embossing.

The metallization is fabricated on the plastic structure using standard photolithography and electroplating. Figure 3 shows the electroplated metanization on the plastic structure before the 3-D coil is assembled. The most critical step in fabricating the coil is aligning the two halves of the coil (Figure 4). This alignment is done by pressing the gold metal contacts together with high contact pressure at elevated temperature.

[FIGURE 3-4 OMITTED]

Toroidal coil structures fabricated using microreplication technology offer several advantages:

* Real 3-D structures can be integrated in a chip-scale package of an RF IC.

* A wafer-level process is feasible.

* Higher quality factors as compared to planar coils are possible.

* Interference with the surrounding circuits is low because most of the electromagnetic field is concentrated inside the torus.

* Inexpensive batch processes for the coils can be used.

This process is not limited to RF inductors.

Conclusion

Microsystems with RF and other functions will be one of the enabling technologies of future mobile communications products. A variety of new features are enabled by these technologies. Cost and size are the key drivers for future mobile products and the pervasive computing applications.

The specific requirements of the packaging solutions for microsystems are hermetic sealing, stability of the precisely controlled gas pressure in some applications, thermal and mechanical stability, and use of materials and designs to achieve high quality RF characteristics.

Micromachining can create new technical possibilities when combined with advanced packaging technologies. Three-dimensional inductors and transformers on a plastic substrate enable excellent RF characteristics. Integration of 3-D micromachining with low cost RF packages is a worthwhile challenge to create new high-performance, miniaturized devices at low cost.

In the long run, microsystems with different integrated functions, such as sensors and actuators, signal processing and wireless communication, are needed. One key challenge in creating such systems is to develop reliable, low cost and small size packages. System-in-package solutions are needed to combine different functions and different technologies.

Acknowledgments

The authors wish to acknowledge Kari Hjelt, Erkki Kuisma, Jukka Salminen and Hans-Otto Scheck of Nokia; Tomas Lindstrom, Mike Read, Simon Yhrberg and Ore Ohman of Amic AB, Uppsala. Sweden; and Christian Pisella, Stephane Renard and Marjorie Trzmiel of Tranic's Microsystems, Grenoble, France.

A version of this article was originally presented at 2001 SMTA International.

References

(1.) Cabuz, C. 1999. Dielectric related effects in micromachined electrostatic actuators. Proc. of the 1999 Conf. on Electrical Insulation and Dielectric Phenomena, pp. 327-332.

(2.) Howe, R. 1995. Recent advances in surface micromachining. Technical digest, The 13th Sensor Symposium, p. 1-8.

(3.) Young, D. and B. Boser. 1996. A micromachined variable capacitor for monolithic low-noise VCOs. Solid-State Sensors and Actuators Workshop.

(4.) Feng, Z., etal. 1999. Design and modelling of RF MEMS tunable capacitors using electro-thermal actuators. Microwave Symposium Digest, 1999 IEEE MTT-S Int'l.

(5.) Zou, Q. et al. 1997. A study on corrugated diaphragms for high-sensitivity structures. J. Micromech. Microeng. Vol. 7. pp. 310-315.

(6.) Nieminen, H. et al. 2001. European Patent Application 01660182.5-2203.

(7.) Dec, A. and K. Suyama. 1998. RF micromachined varactors with wide tuning range. IEEE MTT-S Digest, pp. 357-360.

(8.) Guckel, H. 1998. High.aspect-ratio micromachining via deep x-ray lithography. Proc. IEEE, vol. 86, pp. 1586-1593, August.

(9.) Amic AB. Uppsala, Sweden. www.amic.se

Vladimir Ermolov, Heikki Nieminen, Kjell Nybergh, Tapani Ryhanen and Samuli Silanto are all with Nokia Group, Finland; e-mail: [e-mail].