High-Aspect-Ratio CMOS add-on modules for RF passive components

Commercial wireless communication technologies stemmed mostly from the research done through and after the Second World War as outlined in Chapter 1. Earlier systems were intended for military applications, hence had very high performance and were very expensive and bulky. Later, with the dawn of commercial microwave applications, the systems began to shrink in size and cost. This was possible due to technological developments in the fabrication of integrated circuits. These developments allowed for the integration of more and more active components into one chip. However passive components could not catch up with the trend due to integration problems. Passive components were very big and, when fabricated on a substrate, suffered from parasitic effects, as well as substrate and metal loss. To overcome these issues, researchers developed several technological solutions, some of which are explained in chapter 2, to improve the integration of passive components. Some key points of these technologies are compatibility with core processes such as CMOS, and cost and complexity of fabrication. This work focuses on a technological process intended to reduce metal loss in passive devices. The process is based on the implementation of a thick top metal layer that will reduce the metal loss. However just increasing metal thickness would not be very effective in high frequencies due to skin effect, which practically renders the core of a metal useless in terms of current carrying capability. Therefore rather than just increasing the thickness the process focuses on increasing aspect ratio (height/width). In this way it is possible to utilize side walls of metal tracks for carrying current and have a much lower resistance for the same metal width. The process is tested with spiral inductors fabricated on several different substrates. The experimental results are presented in Chapter 3. Both experimental and theoretical results (simulations) verify that the idea of using side walls for carrying current is actually viable. The process helps increasing quality factors of inductors on both low-loss (HRS) and lossy (LRS) substrates. Performance improvement is more significant on LRS, since high aspect ratio process also reduces device footprint, hence substrate loss. In addition to evaluating its electromagnetic performance, thermal characteristics of the process are also investigated. This became a necessity after experimenting on suspended inductors with thin metallization under high input power. As outlined in chapter 4 under high-power conditions suspended inductors may heat up to 400 oC. This was expected due to lack of thermal dissipation paths for a suspended inductor. What was unexpected was the role of the underlying membrane, which acted as a thermal shunt and averaged the temperature between windings preventing even higher local temperatures. After modeling the structure through simulation it was seen that majority of the power was dissipated through metal leads. Hence increasing the thickness of metal with high aspect ratio processing would help in alleviating the problem. Experiments done on suspended inductors with thick metallization indeed showed that the problem was significantly reduced. Final part of the work investigates the effect of high aspect ratio process on integrated coplanar waveguides. Therefore several CPWs were fabricated on HRS and glass substrates and their performance were measured. The results given in Chapter 5 indicate lower loss due to high aspect ratio metallization. However metal loss is not the only problem that sets back transmission line integration. The chip area consumption of transmission lines is also a big issue that prevents integration. In order to reduce transmission line size another high aspect ratio process was introduced. This process uses DRIE to etch trenches in silicon and electroplating to fill them with copper. Resulting structures are high aspect ratio CPWs that are embedded in the silicon substrate. They have considerably higher effective permittivities which results in ~80% shorter transmission lines for the same electrical length. In conclusion, high aspect ratio metal processing helps in integration of passive devices such as spiral inductors and CPW lines by reducing their loss and size. Since it is a simple and cost effective process that is compatible with core processes, it is expected to be widely applicable.