Direct Local Strain Measurement In Damascene Interconnects
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0990-B07-06
Direct Local Strain Measurement In Damascene Interconnects Moustafa Kasbari1,2, Christian Rivero2, Sylvain Blayac1, Florian Cacho3, Ola Bostrom2, and Roland Fortunier1 1 CMP-GC, EMSE, Gardanne, 13541, France 2 STMicroelectronics, Rousset, 13106, France 3 STMicroelectronics, Crolles, 38920, France ABSTRACT A new mechanical stress characterization method has been developed for Damascene copper interconnects. The micro strain gauge based on a rotating beam has been fabricated in situ on a standard industrial CMOS production line. Comparison of the beam deviation in the fabricated sensor with usual geometrical model is discussed. The saturation of the beam deviation leads to an analytical model which takes the stiffness of the anchoring points into account. This model gives a direct value of the local stress in the copper line for different annealing times. We show that this value is different from the one given by a curvature measurement method. The microstructure of the Damascene copper induces a higher stress level than full sheet deposition copper. The sensor was developed to be compatible within a CMOS process. It is suitable for in situ mechanical stress monitoring in Damascene lines and process optimization. INTRODUCTION The growing integration of modern circuits leads to yield and reliability issues associated with mechanical stress phenomena [1]. The monitoring of mechanical stress induced in the materials used for the fabrication of semiconductor devices become mandatory as the industry migrates to 300 mm wafers [2]. Conventional local strain measuring techniques [3] are not easy to implement on a production line. In previous work we proposed easy probing electrical test structures that can be directly integrated on the product wafer [4]. But they only give an indirect value of the strain in the materials because of the distance that separates the stress source from the sensor active area. MEMS-based stress sensors have already been described for the measurement of stress in polysilicon, and metallic thin films [5]. Nevertheless, works published previously [6] does not report on sensors for measuring strain in damascene interconnects. In this paper we propose a micro gauge constructed on product wafers along a standard copper Damascene process flow. The analytical model we developed describes the mechanical equilibrium of the MEMS and allows a direct extraction of the longitudinal stress component in the copper lines.
SENSOR DESIGN AND PROCESS CONSIDERATION The principle of rotating sensor has been shown in [7]. In this works it has been successfully used on patterned metal films. We propose here to integrate it on product wafers to evaluate strain in Damascene copper interconnects. This induces two constraints. First, the sensor has to be integrated in 100 µm width scribe lines. Secondly, it has to be fabricated with the standard materials and standard thickness along a Damascene process flow. The pointer is an in-plane micro mechanical system. As it is shown in figure 1 the structure is compos
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