MEMS – BiCMOS Monolithic Integration

Mehmet Kaynak1, Matthias Wietstruck1, Canan Baristiran Kaynak1,

Alexander Goritz1, Selin Tolunay1, Bernd Tillack1,2

1IHP, Frankfurt (Oder), 15236, Germany, kaynak@ihp-microelectronics.com 2Technische Universität Berlin, HFT4, Einsteinufer 25, 10587, Berlin, Germany

Abstract

This paper presents various RF-MEMS technologies and devices integrated to a standard BiCMOS process. SiGe BiCMOS technology acts as the baseline process for the integration of modules increasing the functionality and by this way enlarging the spectrum for embedded system applications. Back-end-off-line (BEOL) integrated RF-MEMS switches for mm-wave applications, through silicon vias (TSVs) for RF-grounding and 3D integration and a fully BiCMOS integrated microchannel technology for microfluidic applications are discussed. This paper covers the aspects of both technology challenges and performances figures.

1. Introduction The “More Than Moore” approach is targeting on diversification by combining different technologies based on a reasonable scaling level. There has been a substantial progress of the modular extension of CMOS/BiCMOS processes in last decades. On the other side, increase of the fmax of HBT transistors in SiGe BiCMOS technologies has paved the way for single chip mm-wave systems, including the integrated on-chip antennas. RF-MEMS switches and tuners have become an emerging technology for reconfigurable systems. Moreover, the combination of the RF-MEMS technologies with the CMOS technologies makes them more powerful by the means of high voltage generation and the digital control. On the other side, the TSV technologies are now widely used for the applications of power amplifiers in order to provide better ground connections to the chip. Moreover, TSVs are considered as the key technologies for the chip stacking and 3D integration. In addition to RF-MEMS switch and TSVs, the rapid development in CMOS integrated circuits and bio-technology enables the applied researches in micro-biochips. Advances in microfluidic based chip technology promise to integrate and miniaturize multiple lab processes into a single hand-held device for the applications of lab-on a chip (LoC) and micro-total analysis systems (µTAS). This paper presents the integration aspects and performance figures of three additional modules to a standard BiCMOS process. Firstly, an RF-MEMS switch for mm-wave applications is discussed. Secondly, the TSV technology embedded into BiCMOS process is described. Lastly, the microchannel technology integrated in a BiCMOS process is given. All three technology modules are detailed with device and circuit examples.

2. MEMS-BiCMOS Integration Under this section, three main technology modules, namely RF-MEMS switch, TSVs and microfluidic channels are given. The technology integration and the performance parameters of the modules are also provided in this section.

2.1 RF-MEMS Switch

For RF systems, multi-band and wide-band operation are absolutely mandatory to meet today`s requirements. Therefore, RF-MEMS switches are considered as the key components to fulfill these requirements. The implementation of RFMEMS into high frequency microsystems has largely realized by hybrid integration due to the conflicting technological requirements of RFMEMS and active electronic components. As we move well into the millimeter-wave range, however, the monolithic integration of RFMEMS with high-speed electronics becomes more and more attractive. The IHP RFMEMS integration combines RFMEMS functionality with the performance and circuit complexity of a SiGe BiCMOS process. It basically relies on layers and processes already available in the baseline technology. The three lower metallization layers in BEOL are used to implement the monolithically integrated RFMEMS switches. The capacitive switch is built between the Metal 2 (M2) and Metal 3 (M3) layers, while high-voltage electrodes are formed using Metal 1 (M1). The schematic cross-section and an SEM view of the fabricated device are given in Fig. 1 [1].

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Fig. 1 Schematic cross-section of the RFMEMS capacitive switch (left) and SEM view of the device (right) [1]. The high-frequency switch performance is characterized on wafer up to 110 GHz. The capacitive switch can be described with an equivalent circuit consisting of a series-resonant LC combination. The series inductance can be conveniently used to tune the isolation behavior (switch on the down-state) for best performance in a particular frequency band. This is shown in Fig. 2, which compares the isolation and insertion loss of switches optimized for 30, 50, 80, and 100 GHz operation. The switches provide an insertion loss better than 0.3 dB and isolation in excess of 20 dB for all the frequency bands [2]. A 20 stage charge pump circuit is also realized to create the on-chip excitation pulse (Fig. 2). The charge pump together with the internal ring oscillator can provide output voltages up to 50 V [3].

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Fig. 2 Performance figures of different RF-MEMS switches (left) and on-chip high voltage generator (right) [2, 3]. One of the main challenges for RF-MEMS technologies is the packaging requirements which limit the long-term reliability of this unique technology. To meet the requirements, a low cost and high yield wafer-to-wafer packaging method is strongly required and has been developed together with Fraunhofer IZM, Berlin. Fig. 3 (left) shows a packaged RF-MEMS device with a 50 µm thick cap, while Fig. 3 (right) shows a part of a wafer with many RF-MEMS devices packaged by using the developed wafer-to-wafer packaging technology.

Fig. 3 SEM picture of an packaged single RF-MEMS switch (left) and view of a part of a BiCMOS wafer with many packaged RF-MEMS devices (right).

2.2 Through Silicon Vias (TSVs) There is a growing interest on the integration of through-silicon vias (TSV) in CMOS or BiCMOS technologies. The integration of TSVs into a high-performance BiCMOS technology enables the realization of systems with better performance, increased functionality and smaller size. Recently, the potential of TSVs has been demonstrated for different applications e.g. grounding of mm-wave power amplifier and 3D-chip stacking, as shown in Fig. 4 [4].

Fig. 4 Main application areas using TSV; power amplifier grounding and 3D-chip stacking [4]. For grounding of silicon chips using TSVs instead of wire bonding is a promising solution to minimize interconnection parasitics and prevent from costly interconnection area which helps to decrease the system size and can significantly improve circuit performance. For 3D-chip stacking, the integration of TSVs is a promising solution to realize smaller-sized and highly integrated 3D-systems. BiCMOS embedded TSVs are integrated into IHP’s high performance SG25H1/H3 BiCMOS technologies [5]. After finalizing the Front-End-of-Line, deep annular trenches with a depth of 75 µm are etched into the silicon substrate using the Bosch process. A SiO2 layer is deposited at the trench sidewalls to isolate the TSVs from the medium-resistive silicon substrate (ρ= ~50 Ω*cm). To achieve low-resistive electrical connections from wafer front- to backside, trenches are filled with CVD-tungsten. Then, TSVs are connected to the BEOL via the first metallization layer (Fig. 5). After finalizing the BiCMOS fabrication including the TSV-module, a carrier-wafer is applied from the front side and the wafer is thinned down to 75 µm using a grinding process followed by a dry etch release. Finally, a thin aluminum layer with a thickness of ~1 µm is deposited as backside metallization (BSM) layer (Fig. 5) [4].

Fig. 5 BiCMOS embedded TSVs with first metallization layers and BiCMOS chip with TSV and BSM [4]. For electrical characterization, one-port test structures with a TSV diameter of 25 µm have been fabricated. The S-parameter measurements are done up to 50 GHz and open/short de-embedding is performed to remove the effect of the measurement pads. Inductance values are extracted in Fig. 6. The measured inductance of 24-28 pH above 5 GHz shows a very good agreement with simulated values with 27 pH [4].

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2.3 Microfluidics

Microfluidic technologies use mostly low-cost polymer based materials. However, silicon based microfluidic systems are getting more attractive since the integration of microfluidic systems with CMOS or BiCMOS would be advantageous over any heterogeneous integration because it provides the shortest access to the sensor and the capability of detecting very small change of electrical signals. Furthermore, components such as microfluidic switches, valves, mixers, micro-heaters and micro-pumps can be implemented on the silicon substrate using standard BiCMOS processes. Fig. 7 illustrates the concept of the microfluidic integration with BiCMOS. The entire microchannel has been manufactured using two wafers referred as BiCMOS and bottom wafers. BiCMOS wafer consists of the sensor with read-out circuitry and the BiCMOS channel. The bottom wafer has two depths of channels connecting to each other and to the BiCMOS channel. Fig. 7 also shows the cross section of the channels with bonding interface after fabrication [5].

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Fig.7 Concept of BiCMOS integrated microfluidic system (left) and cross section of fabricated wafer stack (right) [5]. The demonstration of the BiCMOS integrated microfluidic channel is done by using the 120 GHz dielectric sensor. The sensor includes the 120 GHz VCO, the transducer and the detector, fabricated in 0.25 µm BiCMOS process. The output of the VCO is filtered by the transducer and applied to the detector. The change of the dielectric constant of the fluid in the channel shifts the band-pass frequency characteristic of the transducer. Hence, the complex dielectric constant change inside the channel can be detected with a high sensitivity. Fig. 8 shows the die photo with integrated microfluidic channels for air and water filled cases [5].

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Fig. 8 The test chip during measurements. The channel has air inside (left) and water with fluorescence markers (right) [5].

Fig. 9 (left) shows the simulated filtering characteristics of the transducer for the cases of air, water and ethanol using the complex dielectric constants given in [7]. Dashed line shows the generated frequency from the VCO and 12 dB, 10 dB, 7.5 dB attenuation values have been simulated for the cases of air, water, ethanol, respectively [6]. Fig. 9 (right) shows the measured detector output voltages for the three different cases. The change of the detector output is 60 mV for water and 160 mV for ethanol with respect to the reference measurement with air. These results agree with the simulations in Fig. 9 (left) since the detector output voltage is related to the attenuation level [6].

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Fig. 9 Simulated results of the transducer (left) and measured results of the detector output for air, water and ethanol (right) [6].

3. Conclusion

The presented results show that the modular extension of BiCMOS process can open the way for highly integrated and multifunctional smart systems. The combination of these technologies with a high performance SiGe BiCMOS can significantly increase the functionality of the modules by adding the RF electronics for communication and digital electronics for control and processing purposes.

4. References [1] M. Kaynak, et al., IEDM Technical Digest, p. 797, 2009. [2] M. Kaynak, et al., IMS Technical Digest, 2012. [3] M. Kaynak, et al., SiRF Technical Digest, p. 177, 2013. [4] M. Wietstruck, et al., SiRF Technical Digest, 2014. [5] C. B. Kaynak, et al., BioWireless Technical Digest, 2014. [6] M. Kaynak, et al., IMS Technical Digest, 2014.