RF Characterisation and Process Control for Passive Integration Components

Abstract

Passive integration technologies gain more and more interest for cellular applications due to the increasing demand for integration of functionality for cost, performance and size reasons [1][2][3][4]. One of the examples is the 6" PASSI4 process of NXP Semiconductors [2]. This process consists of high ohmic Si substrates with high quality RF components such as Metal-Insulator-Metal (MIM) capacitors and thick metal inductors. The process also includes a thick backside metal and through wafer interconnect (TWI) via holes for low inductive grounding. In addition, a high density decoupling capacitor and a resistor are available. The process results in an RF component (a sub-module) which is utilized in a stacked power amplifier (PA)/front end module (FEM) which also includes active control and RF power technologies [4].One of the main difficulties with such a passive technology is the characterization and process control. In an active process, the performance of the circuits or sub circuits during pre-testing is closely related to the end product performance. However, the relation between the performance of the sub-modules and the performance of the final module is not straightforward at all. In this paper we will show how we have solved this issue for the process release of PASSI4. Characterisation methodThe method we have adapted consists of several characterization and simulations steps.First, the components of PASSI4 will be characterized. This will be done by measuring the relevant DC and RF parameters. During process release we have used a specific mask set which contains test structures for all the components (MIM cap, through wafer interconnect via holes and inductors) in a ground-signal-ground (GSG) test structure configuration. This enables the full wafer mapping of all relevant RF and DC parameters for the components. The 6" wafers contain 75 reticle fields which can be automatically measured using a Cascade 12000 semi-automatic prober. These measurements enable comparison between the relevant DC and RF parameters for each component.The next step is measuring the PCM (Process Control Monitor) parameters. These are placed at 5 positions across the wafer. PCM measurements are always DC (or low frequency) and contain capacitance values for MIM caps, resistances for TWI vias and resistances for all metal layers used. In principle, the PCM parameters can directly be correlated to the DC parameters of the component test structures. When a correlation between the DC and RF parameters is made, the correlation between the RF parameters and the PCM parameters can also be determined.After the wafer is fully characterized, the wafer is sawn and the PASSI4 sub-modules for the final module are taken from 5 positions near the PCM fields. The final modules are assembled and characterized using load-pull RF measurements [5].Once all measurements are ready, the correlation between the final module and the sub-component behavior is determined using simulations. In these simulations, the values for the relevant parameters of...