Characterizing the Microstructure of Metal Thin Films for IC Interconnect

Integrated circuit (IC) manufacturing requires conductive interconnections between the many active semiconductor regions from deposited metal thin films. The microstructure of these films has a direct impact on the consistency of the IC devices. Failures often occur frequently due to electromigration. Electromigration is the diffusional movement of metal ions. Since diffusion rates are a function of the microstructure, precise characterization of film microstructure is important to obtain highly reliable devices.

Limitations of Existing Solutions

X-ray diffraction (XRD) is one of the conventional methods used for thin film microstructure measurement. However, applying XRD to perform characterization has certain limitations:

  • XRD is time intensive especially when acquiring full crystal orientation data.
  • Grain size cannot be measured directly, but only infers size from diffraction peak broadening. There are many competing factors involved in broadening which must be distinguished.
  • XRD is insensitive to variations in grain sizes between approximately 100 nm to 10 μm, which is a common grain size range for metallic thin films.
  • XRD provides no information on the grain boundary character of the material, which strongly influences the diffusional properties and resistance to electromigration failure.

Transmission electron microscopy (TEM) is another microstructural characterization method, which also has its own drawbacks when used to study metal films:

  • TEM requires intensive and time consuming sample
    preparation prior to analysis.
  • TEM requires expert operators to accurately measure
    crystal orientation and grain boundary character.
  • TEM orientation measurements are generally collected
    manually, making data collection time consuming.
  • TEM provides a limited number of measurements,
    limiting the full description of the crystal orientation
    distribution and the grain boundary character.

Advantages of EBSD

Compared to TEM and XRD, electron backscatter diffraction (EBSD) can measure thin film microstructure in a rapid and automated manner. The benefits of EBSD are listed below:

  • Offers a detailed description of the microstructure
  • Offers data on crystal orientation, grain boundary character, grain size, local plastic strain, and phase distribution
  • Rapid data acquisition rates
  • Requires limited sample preparation for testing of metallic thin films
  • With the Hikari XP, it is possible to perform a comprehensive microstructural analysis within a minute
  • Spatial resolution below 50nm with the capability to analyze areas larger than 1cm x 1cm, thereby allowing large area analysis and high-resolution data collection to fulfill most of the requirements of microstructural analyses
  • Measures statistically significant data
  • More than one million individual orientation measurements can be acquired in less than 30 minutes at 650 measurements per second

Analysis of Metal Films

Aluminum has been traditionally been used for IC metallization. The material’s electromigration behavior and mean time to failure are strongly dependent on grain size, grain boundary behavior, and crystal orientation. EBSD maps were collected from an alloyed aluminum film (Figure 1) using the Hikari XP at 650 points per second. The sampling step size and mapping area were changed to acquire a range of total data collection times.

Orientation maps from aluminum film collected for a) 1 minute, b&c) 5 minutes, and d) 60 minutes

Figure 1. Orientation maps from aluminum film collected for a) 1 minute, b&c) 5 minutes, and d) 60 minutes

The results (Table 1) reveal that the film microstructure’s statistically representative sampling can be acquired in 60 seconds. The texture results estimated from the orientation data reveal a strong (111) preferred orientation. In earlier research, the (111) orientation was shown to increase the mean time to IC device failure subsequent to interconnect fabrication.

Table 1. Results from EBSD scans collected at 650 indexed points per second from aluminum thin film

Scan Time (min) Ave Grain Size (µm) (111) Texture
1 1 3.75 13.4
2 5 3.96 13.8
3 5 4.14 13.3
4 60 3.97 13.6

With the continuous miniaturization of IC devices, copper is more frequently used as the interconnection metal because of its higher conductivity. With aluminum, grain boundaries take on the role of diffusion paths assisting electromigration failure. Conversely, with copper, the diffusion rates of the coherent twin boundaries in the microstructure are lower than that of random high-angle grain boundaries.

Using film deposition and thermal processing, the fraction of twin boundaries is increased, which, in turn, aids in decreasing the unfavorable diffusional paths and improving mean time to failure. EBSD is ideal for measuring numerous grain boundaries and for classifying them as useful twin boundaries or detrimental random high-angle grain boundaries.

An EBSD grain map where twins are either excluded or included during the grain calculation is shown in Figure 2. The copper film’s grain size with twin boundaries excluded (1.4µm) indicates electromigration performance and service life better than the grain size measurement including twins (500nm).

EBSD is exceptional with its capability to measure grain size without twin boundaries. Therefore, it can be successfully used to enhance processing conditions and to optimize the service life of semiconductor devices.

Grain maps for copper film where twin boundaries are included (top) and excluded (bottom) from grain determination. Calculated grains are randomly colored to show size and morphology.

Figure 2. Grain maps for copper film where twin boundaries are included (top) and excluded (bottom) from grain determination. Calculated grains are randomly colored to show size and morphology.

Recommended EDAX Solution

The EDAX TEAM™ EBSD analysis system is recommended for this particular application. If elemental analysis data is required, the TEAM™ Pegasus system equipped with both EDS and EBSD detectors is more appropriate. For crystallography, the Hikari XP EBSD camera is advised, while for elemental analysis, the Octane SDD Series is recommended.

This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.

For more information on this source, please visit EDAX Inc.

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