Fabrication of thick silicon nitride blocks for integration of RF devices

Introduction: The enormous growth of wireless and portable applications has led to strong demands for high-performance monolithic low-cost passive components in RF and microwave integrated circuits (ICs). However, some traditional microwave passive components such as transmission lines and filters are difficult to integrate on the same chip with the RF and microwave circuits owing to the high substrate losses associated with standard low-resistivity CMOS-grade silicon substrates. As a result, most RF and microwave components are realised on special substrates such as AF45 glass [1, 2] or quartz [3]. Because of the need for monolithic integration with electronics, several techniques have been developed to allow the realisation of low-loss RF devices on standard silicon. These techniques include the use of dielectric layers such as polyimide [4] and benzocyclobutene [5], the use of polysilicon patterned ground shields [6], the use of silicon bulk-micromachining to remove the substrate locally under the RF components [7–9], and the use of surface-micromachined suspended metal structures at a distance of several tens of micrometres above the silicon surface [10, 11]. However, all these techniques impose restrictions on the device structures that can be realised or result in considerably higher losses than dedicated RF substrates. The best results are obtained with the freely suspended structures [7–11], but such free hanging structures are rather delicate, vulnerable to shocks and vibration, and difficult to package. Therefore, in this Letter we propose a new technique in which the silicon substrate is locally replaced by thick (tens of micrometres) blocks of silicon nitride, which has very good RF properties (tan d1⁄4 5–9 10 ). These silicon nitride blocks can be realised in a pre-CMOS process, i.e. before performing the regular CMOS process. The RF and microwave devices are realised on top of the silicon nitride islands by post-processing, i.e. after the CMOS process. Fig. 1 shows a schematic impression of a CMOS chip with integrated RF waveguide realised in this way. The only restriction is that the post-processing of the RF devices needs to be CMOS compatible, which is the case for most current surface micromachined devices. Low-stress, siliconrich silicon nitride [12, 13] (SiRN) is chosen in order to minimise curvature of the silicon wafer due to residual tensile stress.