Transition Metal Carbides as Diffusion Barriers in Copper Metallization for ULSI Technology

• Main Authors: Hao-Yi Tsai, 蔡豪益
• Other Authors: S. J. Wang
• Format: Others
• Language: zh-TW
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/72303185571863420578

博士 === 國立成功大學 === 電機工程學系 === 89 === Cu is now being used in ULSI metallization below 0.18 µm as a replacement for Al due to its higher conductivity and higher resistance to electromigration as compared to Al or Al alloy. However, Cu is liable to diffuse into Si and SiO2 and then reacts with Si to form Cu3Si compounds, resulting in the degradation of device performance and reliability at low temperature. In order to successfully integrate Cu metallization into ICs, a diffusion barrier layer used to prevent the undesired interdiffusion or reaction between Cu and the adjoining material is necessary. In this dissertation, transition metal carbides as diffusion barriers in copper metallization for ULSI technology were investigated. Tantalum carbide (TaCx) films deposited by a sputtering process with a TaC target as diffusion barriers against Cu diffusion were investigated for the first time. The thermal stability of Cu/TaCx/n-Si and Cu/TaCx/p+n contact systems as a function of annealing temperature were reported and analyzed. The deposited TaCx, having an X-ray amorphous structure and a low resistivity of around , were characterized by sheet resistance measurement, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), secondary ion mass spectroscopy (SIMS) and diode leakage current measurement. From XRD and SEM analysis, it was found that 600Å-TaCx in Cu/TaCx/Si structure can effectively prevent Cu penetration up to for 30 min, while more sensitive diode leakage measurement of Cu/TaCx/p+n structure indicates that the failure temperature is around . The failure of TaCx layer was found mainly due to the diffusion of Cu along the localized defects of the TaCx barrier layer into underlying silicon. This has caused the formation of copper silicides and high junction leakage currents. The second diffusion barrier metal we have investigated is tungsten carbide (WCx). It is found the as-deposited WCx film has a nanocrystalline structure and a low electrical resistivity of around . Film characterization reveals that the WCx film was able to preserve the integrity of the Cu(2000Å)/WCx(600Å)/n-Si structure without formation of Cu3Si phase up to 600 annealing for 30 min. In addition, leakage current measurements on the Cu(2000Å)/WCx(600Å)/p+n-Si diode structure did not show deterioration of electrical characteristic up to 550 annealing. As the thickness of WCx barrier was reduced to 150Å, it is found that the WCx film can retain the integrity of diodes up to 500 without increasing diode leakage current. The failure of WCx film after high temperature annealing is attributed to the Cu diffusion into Si substrate through grain boundaries or local defects of WCx barrier layer, in which some local defects might arise from the formation of W5Si3. The third barrier metal we have studied is titanium carbide (TiCx), and the physical and electrical properties as well as thermal stability of the barrier metal as diffusion barriers for Cu metallization were investigated and presented. With thermal annealing in N2 ambient for 30 min, the unpatterned Cu(2000Å)/TiCx(600Å)/n-Si structure was found being metallurgically stable up to a temperature in between 600~650 without formation of Cu3Si phase, while measuring the reverse leakage current of the patterned Cu(2000Å)/TiCx(600Å)/p+n-Si diode structures showed that the TiCx barrier layer was capable of withstanding thermal annealing up to 500 . The failure of TiCx layer after high temperature annealing was found mainly attributed to the diffusion of Cu along localized defects or grain boundaries of the TiCx barrier layer into Si substrate, which caused the high junction leakage currents for the patterned structure and formation of Cu3Si for the unpatterned structure. Finally, we studied the effect of nitrogen doping on the barrier properties of sputter-deposited tantalum carbide (Ta-C) films. With increasing nitrogen concentration, it was found that the resistivity of the barrier layer increases, while the growth rate decreases. In addition, the use of an optimum N2/Ar flow rate ratio of 2/24 during sputtering allows one to achieve tantalum carbon nitride (Ta-C-N) films with the highest thermal stability. According to I-V measurements on reverse-biased Cu/barrier/p+n diodes, the 600Å-thick Ta-C-N barrier layer is seen effective in preventing Cu from reaching the Si substrate after 600 annealing in N2 for 30 min, which is about 100 higher as compared to the case without nitrogen incorporation. The failure of thermal annealed Ta-C-N film was attributed to the Cu diffusion through the local defects or grain boundaries of the layer into Si substrate, which results in a drastically increase in the diode leakage current.