| Summary: | 博士 === 國立中正大學 === 機械系 === 92 === Abstract
As semiconductor devices are reduced in scale, the dimension of interconnects shrinks to the sub-micron level. The resistance-capacitance (RC) time constant becomes a major part of total delay. In order to reduce the RC time constant, the chip with copper interconnects and low k materials were proposed IBM in 1997. However, copper easily oxidizes at elevated temperature; oxidation is thus a serious concern in thermosonic wire bonding of gold wire onto chips with copper interconnects.
Thermosonic wire bonding was conducted at 90℃-200℃ under air atmosphere. The bondability and bonding strength of Au/Cu bonds were far below the minimum requirements stated in industrial codes. The degradation was attributed the formation of copper oxide on copper pad. After heating at 90℃, 150℃ and 200℃ for 5min under air atmosphere, two kinds of copper oxides were identified based on the shifted binding energy. Cu(OH)2 forms mainly on the top surface of copper pads and the underlying layer consists mainly of CuO. Thus, protective measures such as deposition of a passivation layer or using shielding gas are inevitable for thermosonic wire bonding on chips with copper interconnects.
In this work, three schemes were proposed to achieve a 100% bondability and sufficient bonding strength for gold wire thermosonic bonding to copper pad. First, the argon shielding methods were applied to prevent copper pad from oxidizing during gold wire thermosonic bonding to copper pad. With argon shielding in the thermosonic wire bonding process, 100% gold wire attach on copper pad can be achieved at bonding temperature of 180℃ and above. The ball-shear force and wire-pull force far exceed the minimum requirements specified in the related industrial codes. In a suitable range of bonding parameters, increasing bonding parameters resulted in greater bonding strength. The reliability of high temperature storage (HTS) test for Au/Cu ball bonds was verified in this study. The bonding strength of Au/Cu ball bonds increases slightly with prolonged storage duration due to atomic diffusion between gold ball and copper pad during HTS test. As a whole, argon shielding is an effective way to ensure Au/Cu ball bond in the thermosonic wire bonding process applied for packaging of chips with copper interconnects.
Second, a thin titanium passivation layer was deposited onto Cu pad to improve the bondability and bonding strength. The thickness of titanium passivation layer is crucial to bondability and bonding strength. An appropriate thickness of 3.7nm titanium film is proposed in this work. 100% bondability and high bonding strength was achieved. A thicker titanium film results in poor bondability. The mechanisms for successful bonding lies in that only film with suitable thickness can be removed by an appropriate range of ultrasonic power during thermosonic bonding. The protective mechanism of titanium passivation layer was interpreted by the results of FEAES and ESCA analysis. Titanium dioxide (TiO2) formed during die saw and die mount process plays an important role in preventing the copper pad from oxidizing. The bonding strength did not degrade after prolonged storage at elevated temperature during HTS test, thus the reliability of HTS test for gold ball bonded on copper pad with titanium layer was verified.
The third proposal is to deposit a silver layer on the surface of copper pad as a bonding layer. Both ball-shear forces and wire-pull forces of gold wire bonding to copper pads with silver bonding layer are far higher than that minimum requirements stated in JEDEC standard and MIL specifications. Silver bonding layer reveals superior bonding strength on Au/Ag interface. The reliability of Au/Ag bonds after high temperature storage test is also validated. The bonding strength increases with increasing holding time. The wire-pull forces far exceed the industrial codes. Bondability and bonding strength for gold wire thermosonic bonding to chips with copper interconnect is significantly improved by the deposition of silver as a bonding layer on copper pads.
According to experimental results in this work, three protective methods not only prevents copper pads from oxidation during thermosonic wire bonding process but also could be applied to the electronic packaging industry for solving the challenges of thermosonic wire bonding process on chip with copper interconnects.
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