A Novel Technique to Re-construct 3D Void in Passivated Metal Interconnects
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A Novel Technique to Re-construct 3D Void in Passivated Metal Interconnects Cher Ming Tan, Zhenghao Gan, Guan Zhang, Krishnamachar Prasad and Dao Hua Zhang Division of Microelectronics, School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798
ABSTRACT In the present work, a novel method is proposed to re-construct voids in passivated metal interconnections. In this method, the conventional SEM and EBIC systems are assembled and utilized without much modification. In principle, a constant current is applied to the metal interconnections while an electron beam is scanning and impinging upon the surface of the sample. The voltage at the terminals is monitored simultaneously during electron beam scanning. Resistance change, and hence voltage perturbation are expected when the electron beam approaches the defective area, caused by uneven electron beam heating (EBH) and heat transmission. Information on defects or voids is thus obtained by analyzing the voltage alteration. Finite element simulation showed that the recorded voltage perturbation is not dependent of the length of the interconnect, but a linear function of the void volume. Thus, the method is essentially useful as the metal length has increased tremendously in copper technology. In addition, it can provide the void size and depth, with the possibility to reconstruct the entire void shape in 3D.
INTROCUCTION Continuous shrinkage of integrated circuits results in a continuing increase in current density in the metal interconnection lines and in a corresponding increase in the potential electromigration (EM) and stress-migration (SM) hazard. These failures are highly related to the generation and propagation of voids and/or cracks in the metal interconnections. Presently, the voids are found using manual inspection under SEM. As metal line becomes long and narrow, such a method becomes too tedious and inefficient. Many methods are proposed and patented, but they have their shortcomings. A detailed summary and comparison of the various methods used on the voids detection was given elsewhere [1,2]. While some are effective in the detection, the smallest void detectable is too large to be applied where the metal line width can go below 0.25 µm [3]. Some can provide good detectivity, but it may cause damage or defects to the scanned sample [4,5]. Hence, there is a need to detect and re-construct very small voids (as small as 50 nm) non-destructively as we enter into deep submicron era. In the present method, the conventional electron beam is utilized as the detection probe due to its small and controllable beam diameter. Hence, high spatial resolution is achievable. Electron beam has sufficient energy to penetrate through the passivation layer and heat the covered metal line. Its energy is also not too high to cause damage or defects in the testing structure during examination. In this technique, the electron beam is also a heating source to the specimen, causes signal perturbation. This signal variation is used in
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