Testing RF Circuits With True Non-Intrusive Built-In Sensors Louay Abdallah * , Haralampos-G. Stratigopoulos * , Salvador Mir * and Josep Altet † * TIMA Laboratory (CNRS-Grenoble INP-UJF), 46 Av. F´ elix Viallet, 38031 Grenoble, France † Department of Electronic Engineering, Universitat Polit` ecnica de Catalunya, 08034 Barcelona, Spain Abstract—We present a set of sensors that enable a built- in test in RF circuits. The key characteristic of these sensors is that they are non-intrusive, that is, they are not electrically connected to the RF circuit, and, thereby, they do not degrade its performances. In particular, the presence of spot defects is detected by a temperature sensor, whereas the performances of the RF circuit in the presence of process variations are implicitly predicted by process sensors, namely dummy circuits and process control monitors. We discuss the principle of operation of these sensors, their design, as well as the test strategy that we have implemented. The idea is demonstrated on an RF low noise amplifier using post-layout simulations. I. I NTRODUCTION Current and new generation electronic systems increasingly rely on RF circuits that enable wireless communication. The various manufacturing steps for such circuits may induce spot defects or large process variations that, in turn, may result in complete malfunction or in violation of a subset of specifications. Therefore, testing is necessary to verify that the specifications of every manufactured circuit are met and to guarantee that the functionality of the overall system is not jeopardized. However, testing the RF circuits of a system is a complex task which involves a very high cost. This is mainly due to the sophisticated automatic test equipment (ATE) of latest technology that is required, the long test times, the limited test access, etc. Various design-for-testability (DfT) and built-in test (BIT) techniques have been proposed to date with the aim to reduce test cost and to improve testability. Examples include loop- back test [1], oscillation-based test [2], tuning-knobs for self- calibration [3], and built-in sensors [1], [4], [5], [6]. However, these techniques rely on connecting auxiliary circuitry along the signal path of the circuit under test (CUT) and, thereby, may degrade the matching and the overall performance. The fact that they are intrusive and that they often require to redesign the circuit to meet the desired specifications makes RF circuit designers rather reluctant to employ them. In this paper, we propose sensors to be used in the context of RF BIT. The key characteristic of these sensors is that they are non-intrusive and transparent to the RF circuit since they do not tap into its signal path. Furthermore, they are generic (e.g. they can be used virtually for any RF circuit) and they provide a fast DC test response which lets us interface the CUT to basic ATE. Non-intrusive process sensors were recently proposed with the aim to predict implicitly the performances of an RF CUT [7]. They consist of basic analog stages (called “dummy circuits”) and process control monitors (PCMs) that are placed on the same substrate, in close proximity to the CUT. They operate on the basis that they are subjected to the same process variations as the CUT. As a result, the performances of the CUT and the sensor measurements are expected to be highly correlated. In this case, alternate test can be used to infer the performances from the sensor measurements [8]. However, a random spot defect occurring within the CUT cannot be detected by the process sensors. In this paper, we study a non-intrusive temperature sensor and we focus on its application for detecting defects within the CUT. The starting observation is that a defect will shift the power dissipated in the CUT which, in turn, will produce a temperature increase in the vicinity of the CUT that can be captured by the temperature sensor. The temperature sensor and the aforementioned process sensors offer a complete non- intrusive BIT solution for RF circuits to detect spot defects and to predict shifts in their performances. The rest of the paper is structured as follows. Next, we present our case study which is an RF low noise amplifier (LNA). In Section III, we discuss the substrate thermal cou- pling mechanism, we present the design of the temperature sensor, and we show how it should be laid out on the die. In Section IV, we discuss the test strategy using the temperature sensor to detect defects. In Section V, we revisit the design of the temperature sensor in view of the test strategy. For the purpose of completeness, in Section VI, we discuss the operation of the process sensors and we provide a brief overview of alternate test. In Section VII, we demonstrate the operation of the various sensors on the case study using post- layout simulations. Finally, Section VIII concludes the paper. II. CUT Our case study is an inductive source-degenerated cascode LNA used in the 802.11g standard receivers that operate in the 2.4 GHz ISM band. The biasing stage is formed by resistors R1 and R2 and transistor M3. The inductors Lg and Ls and transistor M1 are chosen to provide appropriate input matching at 50Ω. The gain stage is composed by transistors M1 and M2. M1 provides the high gain, whereas M2 isolates the input from the output and reduces the Miller effect. The parallel Ld-Cd tank resonates at 2.4GHz and the resistor Rd controls the gain at this frequency. III. TEMPERATURE SENSOR A. Thermal coupling basics The power dissipated by the CUT is a signature of its state and performances. The consequence of dissipating power is 978-3-9810801-8-6/DATE12/ c 2012 EDAA