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By: Jon W. Erickson, Ph.D., Nova Crystals, San Jose
The majority of Fourier-transform infrared (FT-IR) applications used in semiconductor technology have been developed in the past decade. Prior to this time FT-IR made its greatest impact in the research laboratory. The technique is powerful but somewhat non-specific and thus it has delivered the most value in situations with relatively well-defined boundary conditions. Many such situations do exist in semiconductor technology, since the materials and processes are under exceedingly stringent control:
- Wafer or substrate quality and uniformity.
- Process monitoring and quality control (Si or compound semiconductors)
- Product reliability and failure analysis.
Material purity is often in excess of 99.999%; with tightly controlled processes, many feature dimensions are known within nanometers. Within these boundary conditions, FT-IR can be a powerful and capable technique. Non-destructive measurements provide the least expensive means to monitor and control processes. FT-IR can be very precise with reproducibility better than 0.5%, and sensitivity to dopants at levels of 10 ppm or sub-monolayer films. Excellent spectral resolution (of 5-to-10 μm) and non-destructive measurement of 300 mm wafers make this a tool of value for the years to come.
Features:
- Familiar absorption peaks of infrared (IR) spectroscopy provide guide to sample composition.
- Interference fringes often present (with sinusoidal variations dependent on layer thickness).
- Electron transport from carriers gives rise to absorption in the far-IR to infer dopant levels.
- Detailed model building (including or stripping interference fringes from the interferogram) can simulate virtually the entire spectrum, as illustrated for a thin film of silicon nitride.
Compound semiconductors have many features that can be easily monitored with FT-IR, such as epilayers that result in interference fringes. The interference fringes for seven different epilayer structures on InP substrates are shown below. In some cases, the amplitude remains nearly constant across the spectrum and in others it gradually changes.
Due to its sensitivity to structure and composition, FT-IR can be used to monitor oxidation and corrosion so as to obtain activation energies for degradation processes in accelerated aging studies.
FT-IR finds and easily characterizes defects (that can affect device performance parameters) on or near wafer features.
CONCLUSION
The sensitivity of FT-IR, particularly FT-IR microscopy, can be greatly improved by isolating the sample and beam path from fluctuations in background. During a long FT-IR scan, the movement of human beings within a laboratory can result in the detection of fluctuations in low levels of gases. The full sensitivity of the FT-IR tool will be utilized if a continuous purge of the instrument and sample area with dry nitrogen gas is performed.
Literature in recent years will show that FT-IR has been applied to dozens of different semiconductors in microelectronics and photonics, as well as numerous processing steps. Modeling software is especially helpful in data reduction, and is best performed with a detailed knowledge of the refractive index and extinction ratio for the materials involved over the spectral range of interest. Sophisticated modeling software packages are becoming available to assist in this task.
FT-IR has emerged from its traditional roles in biological and chemical research to become a cost-effective and powerful tool in semiconductor technology.
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