Barrier Effect on Electroplated Cu Films
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ABSTRACT The effect of various barrier materials on the microstructure of electroplated Copper films was investigated. Analysis of the Cu was performed at the as-deposited, room temperature stabilized, and annealed states. It shows that the barrier material can have a dramatic effect on the properties of electroplated Cu. INTRODUCTION Previous studies [1-4] have shown that electroplated Cu undergoes self-annealing after plating, in which the electrical resistivity and its non-uniformity will change as a function of time. These changes can be attributed to Cu grain consolidation as the Cu recrystallizes, where the grains grow from the initial -0.1 pm to several microns. It's also been reported that the Cu seed microstructure can affect the room temperature stabilized electroplated Cu microstructure [5]. In this paper, we report our studies on how barrier materials affect the subsequent electroplated Cu sheet resistance, non-uniformity, stress, and crystal orientation. The study involved both crystalline and amorphous barriers. An additional anneal of the film was carried out to determine if the Cu changes further after room-temperature "stabilization". Problems, such as stress changes, may arise in subsequent wafer processing if the electroplated Cu film is not completely stabilized. EXPERIMENT Barrier materials of Ta, TaN, TaN/Ta stack, Ti/TiN stack, and TaSiN were deposited onto blanket thermal oxide substrates. Barrier thickness was 100A with the exception of Ti/TiN, where a total of 300A was deposited, and an additional sample of Ta,, where 250A was deposited. The TaN/Ta stack was comprised of 70A TaN followed by 30A Ta. All samples were deposited in the same Physical Vapor Deposition (PVD) system, with the exception of Ti/TiN. The Cu seed was deposited in-situ (i.e., no vacuum break) with the barrier for all samples. All wafers were electroplated at the same time to eliminate any additional variations of the electroplater or plating solution. Sheet resistance, non-uniformity, stress, and crystal orientation were measured over time for each of the electroplated Cu films. Sheet resistance was measured using a 4-point probe. Stress was measured using an FSM stress gauge. After the films reached steady-state sheet resistance, they were annealed in an anneal oven at 450'C for 30 minutes in N2 ambient and then reanalyzed. X-ray analysis of the barriers was also done in an attempt to make correlations to the Cu data. Supplemental Transmission Electron Microscopy (TEM) images of the barrier/Cu interface were collected for select post-annealed samples.
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Mat. Res. Soc. Symp. Proc. Vol. 564 © 1999 Materials Research Society
RESULTS AND DISCUSSION The sheet resistance change over time followed the same trend for all barrier materials. There was approximately a 20% decrease in the sheet resistance from as-deposited to room temperature stabilized. Figure 1 is a composite graph for all the barrier materials. The sheet resistance offset of the Ti/TiN sample may be explained by a thickness variation from the other sample
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