Contact resistance measurement structures for high frequencies

Knowledge of the interfacial contact impedance offered by the device at its operating frequency range is crucial for accurate modelling and understanding of the device. In this article, a novel modified TLM test-structure has been devised to extract interfacial contact parameters at frequencies upto 4 GHz. S-parameter measurements performed on these test-structures were validated with the measurements on identical test-structures suited for conventional DC four point force-sense measurements. Based on these measurements the electrical model for the metal-to-PCM interface is derived for different contact impedances.

I. INTRODUCTION

Electrical contacts are an integral part of all electrical or microelectronic devices. During operation, through these contacts current enters and exits the functional layers in devices. Depending on the nature and working of these devices current may flow continuously or for short intervals through the contacts in a device. In a Phase Change Random Access Memory (PCRAM) cell, the functional layer is a thin film of chalcogenide alloy or Phase Change Material (PCM) integrated into the first metalization level of the integrated circuit [1]. The contacts for this memory cell are established between the PCM layer and the interconnect metalization. The principle of operation of these PCRAM cells is based on the switching of this embedded PCM layer between the amorphous (reset state) and the crystalline (set state) phase. This operation is achieved by using electrical pulses of different width and amplitude. The state of the memory cell is read by checking the phase of the PCM layer using again pulsed electrical resistance measurement [2]. Typically set, reset or read operation are achieved by different electrical pulses of nano-second duration [2] [3]. Phase change memory cells have been demonstrated for very fast material switching times down to 1 ns for both set and reset operation [4]. Memory cells with a read-time of 2 ns have been reported to allow up to 200 MHz data throughput [3]. Furthermore, research is progressing to improve the electrical switching speed of PCM which has been demonstrated to switch with pico-second laser pulses [5]. With these improved switching speeds, a PCRAM cell can operate at even higher data throughput rates. Electrical current flows through the memory cell only when it is accessed for either set, reset or read operation. Hence, during operation of the device, the electric potential at the contacts follows this fast changing signal. Hence, for complete understanding of the performance of these devices, the properties of the PCM and the contacts at its operating frequency range is crucial.