In the semiconductor industry, accurate and efficient measurement of epitaxial (Epi) layer thickness is paramount. The Fourier transform infrared (FTIR) technique is often used to measure the thickness of those layers to help ensure quality.
The Importance of Epitaxial Layers Thickness
The US. Department of Labor, Occupational Safety and Health Administration defines epitaxy as “the process of growing thin films of crystals, in which the substrate determines the crystallinity and orientation of the grown layer. A variety of epitaxial growth techniques are used in III-IV display and device production” – often for the fabrication of integrated circuits (IC).
The thickness of these layers is a critical factor in the fabrication process, affecting the isolation of highly conducting wells and etch times. Epi layers serve to isolate current carrying features, which reduces substrate noise, cross-talking, and latch-up. This is particularly significant as feature size is reduced. Therefore, a quick and accurate determination of Epi layer thickness is vital in wafer production.
Epitaxial layers are typically high-resistivity layers, transparent to infrared radiation. This makes them ideal for infrared observation.
What is FTIR?
Fourier transform infrared (FTIR) is a preferred method of infrared spectroscopy. When IR radiation is passed through a sample, some radiation is absorbed by the sample and some passes through (is transmitted). The resulting signal at the detector is a spectrum representing a molecular ‘fingerprint’ of the sample. The usefulness of infrared spectroscopy arises because different chemical structures (molecules) produce different spectral fingerprints.
FTIR spectroscopy offers a vast array of analytical opportunities in academic, analytical, QA/QC and forensic labs. Deeply ingrained in everything from simple compound identification to process and regulatory monitoring, FTIR covers a wide range of chemical applications, especially for polymers and organic compounds. You can learn more about FTIR on our FTIR Academy pages.
The Roles of FTIR Spectrometers in Quality Control
FTIR spectrometers are used to determine Epi layer thickness in an efficient and non-destructive manner, offering multiple benefits. The FTIR technique measures the reflectance of an incident beam of light after it hits the surface of the wafer, providing a precise thickness measurement. FTIR is quick, reproducible, lends itself to automation, and is operator independent.
FTIR is uniquely suited to the analysis of silicon and other electronics components. Semiconductor spectroscopy tools utilizing FTIR can analyze wafers for the carbon or oxygen content, the epitaxial coating thickness (as mentioned), and more. Special designs permit handling of production wafers or smaller discs with various data collection patterns enabled. Some tools for semiconductor research include the ability to analyze wafers up to 300 mm in diameter; various analysis patterns; C, O, and EPI analysis methods (quantitative tools).
Three Methods to Measure Epi Layer Thickness using FTIR
There are three methods of Epi layer thickness measurement using FTIR:
- Interferogram Subtraction
- Constant Angle Reflection Interference Spectroscopy (CARIS)
- Cepstrum
Interferogram subtraction is the most straightforward method, where the distance between the primary and secondary interferograms is observed. It offers the fastest measurement due to minimal calculations performed on the signal. However, it has limitations when the Epi layer becomes thin, and it does not correct for phase error.
CARIS, on the other hand, observes the interference maxima and minima in the spectral domain. It has been widely used to measure the thickness of photoresist and dielectrics applied to wafers. The method allows for theoretical curves to be generated and compared with actual observed data, albeit with a few limitations such as larger error magnitudes with decreasing thickness and difficulties in determining extrema in spectra of multiple layers.
The third method, Cepstrum, employs a second Fourier transform to yield data that is displayed as intensity versus thickness. It offers several advantages including phase correction after data collection, use of the entire spectral data domain, compensation for the reference, and enhanced sensitivity for fractional-area buried layers. It is the recommended approach due to its precision and accuracy.
With any of these methods, the success of these FTIR measurements heavily depends on the choice of a proper reference. A stable FTIR in a well controlled environment contributes to accuracy and precision.
Summary
FTIR serves as a fast, precise method for measuring Epi layer thickness in many of today’s IC structures. As the industry moves towards thinner Epi layer structures for new IC designs, different infrared methods like Cepstrum calculations prove to be invaluable tools.
For additional details about the three methods of interpretation, the relationship between them, and the advantages of each approach, including optical schematics and observations, read the application note FTIR measurement of epitaxial film thickness applications.
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