Aluminum Reflow Sputtering
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MRS BULLETIN/NOVEMBER 1995
Recently, a simple sputter-filling process with only an increase of substrate temperature and using a conventional DC magnetron sputtering system has been developed for Al reflow sputtering with various underlayer films. There are two types of filling processes in Al reflow sputtering technology. One is a high-temperature sputtering in which Al is deposited on substrates at elevated temperatures and via/contact holes are filled with Al during sputtering.5'7"9'17'18 The other is a cold sputter deposition and a subsequent reflow process without breaking vacuum, resulting in via/contact filling.10'11'1920 An advanced two-step sputtering technology,21"24 which is a combination of cold sputter-deposition and subsequent high-temperature sputtering, has been developed to fill via/ contact holes with a high aspect ratio. It has been reported that vias with an aspect ratio above four can be filled with Al-0.5 wt% Cu at a substrate temperature lower than 450°C using the two-step sputtering technology.21 Very recently, varying process gas pressures during reflow and hightemperature sputtering have been introduced as another way to improve the Al reflow process. Pressure during sputter deposition that was lower than that of conventional sputtering, which was achieved using a hollow cathode in a magnetron sputtering system, caused a drastic reduction in reflow temperature.25 On the other hand, high pressure during the reflow process after cold deposition helps via/contact filling with high aspect ratio.26"29 Contact holes with an aspect ratio above four were filled with Al-Cu at the high pressure of 60 MPa and a substrate temperature of 400°C. To reduce the reflow temperature using a conventional sputtering system without any bias voltage and other additional attachments, the use of an Al-GeCu alloy, which has a lower eutectic
temperature than that of conventional Al alloys, has been investigated. We will introduce our recent development using Al-Ge-Cu. Experimental Al-x wt% Ge-0.5 wt% Cu alloys were sputtered using a DC magnetron sputtering system without any substrate bias voltage. The base pressure was 1 X 1CT9 Torr, and a partial pressure of H2O and O2 in Ar atmosphere was less than 1 X 1CT9 Torr. In order to investigate underlayer effects on filling characteristics, three different types of underlayers were prepared: a polysilicon film deposited by low-pressure chemical vapor deposition (LPCVD), and molybdenum silicide (MoSi24) and titanium tungsten (TiW), which were deposited by sputtering. The reflow temperature of sputtered Al-x wt% Ge-0.5 wt% Cu alloy films were measured as a function of Ge weight percent. The melting points of the bulk Al-x wt% Ge-0.5 wt% Cu alloy were also measured. The electromigration characteristics for conventional Al-x wt% Ge-0.5 wt% Cu lines, whose length and width were 100 ^m and 2 /xm, respectively, were investigated as a parameter of Ge concentration. All Al-x wt% Ge-0.5 wt% Cu wirings, which were reflowed at different individual reflow temperatures depending on
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