During the fabrication process of semiconductor devices, any contaminant present could result in failure of the finished die. The latest innovations in methods of fabrication, such as increasing usage of SMIFs and in clean room technology, have reduced the number of contaminant particles to a minimum. Regardless of these precautions constant observation is required, and the process should be regularly monitored at every stage until the line is established. After establishing the line, regular monitoring needs to be performed to ensure that particulates are being controlled. An unexpected reduction in yield requires inspection at key stages in the process, to locate the cause of the problem.
Wafer Inspection Using Microscopy and X-Ray Analysis
During the fabrication process, wafers are inspected at all stages using microscopy. Growing demand for smaller semiconductor device structures have resulted in a gradual replacement of optical microscopy inspection with SEM inspection. One of the reasons is because several "killer defects" are at, or below, the resolution limit for optical microscopes. Multiple layer devices currently in use are highly topographic in nature, and the increased usage of polymeric materials has resulted in limited depth of field, and reduced contrast associated with optical microscopy, making defect identification and characterization very hard.
The use of SEM integrated with X-ray analysis to test particulates on semiconducting devices is a proven method. This combination offers an enhanced technique of identifying key features, when compared to optical inspection, along with the possibility of performing in situ analysis. Although X-ray analysis is suitable for characterizing metallic particles, it is not possible for inorganic compounds to be identified clearly, because the method is limited to elemental analysis. With organic particles there are extra limitations, as X-ray detectors are quite insensitive to light elements, below atomic number 11- sodium, so often only carbon is detected.
Raman analysis is a vibrational spectroscopy, and can probe the chemistry of the test sample. A Raman spectrum comprises bands matching the internal vibrational frequencies of molecules. These bands are unique for each material.
Raman spectra contain bands that provide comprehensive data about the chemical bonds of the sample being tested, and simple library searches can link these groups of bands to detect the material under study.
Similar to X-ray analysis, the method’s spatial resolution is in the micrometer-order, although scanning probe methods have been created to allow sub-micrometer testing. Raman spectroscopy not only provides chemical analysis of the sample, but also explores the structural properties of samples. The shape and width of Raman bands are affected by the crystalline quality, while their positions are sensitive to lattice strain.
The Structural and Chemical Analyzer
Renishaw were inspired to create its structural and chemical analyzer (SCA) for SEMs due to the increasing trend of using SEM for wafer inspection. The company combined SEM’s imaging capabilities with the chemical identification and structural data available from Raman spectroscopy, resulting in two powerful material characterization techniques, made possible in a single instrument.
The structural and chemical analyzer is completely configurable, supporting various laser excitation wavelengths for Raman and photoluminescence (PL) spectroscopies, and extra techniques such as cathodoluminescence (CL). It also offers connectivity to a number of spectrometer models. This flexibility allows the system to be tailored to meet definite analytical requirements, from a regular inspection system to a multiple-method research-grade analytical workstation.
As Raman spectroscopy uses optical wavelengths, it is effective regardless of the SEM operating environment, making the method appropriate for SEMs operating at low vacuum to ultra-high vacuum.
Figure 1 illustrates the UHV variant of the structural and chemical analyzer, fitted to a LEO-1450VP SEM. For Raman spectroscopy the electron beam has no role in the spectrum formation, allowing low accelerating voltages to be utilized to image the sample, avoiding charging and damage. This is crucial when observing particles, as they can be easily damaged by a high-energy electron beam, or become charged in relation to the substrate and repelled from it, rendering it waste to the analyst. After testing of the inorganic and organic species has been fulfilled, the beam voltage can be increased to characterize the remaining particles using X-ray analysis.
Figure 1. UHV-version of the structural and chemical analyser interface coupled to the LEO-1450VP that was used for this study
Particulates on a TTL device
The benefits of the SEM-SCA can be explained with the following example; samples from a batch of transistor transistor logic (TTL) integrated circuit (IC) wafers were intentionally contaminated to simulate defects causing poor yield, and typically be directed to device failure analysis before passivation.
The research aims to reveal how to discover the cause of the failures, and to see if these failures are associated to defects or contaminants, to characterize and establish their origin.
Figure 2a shows a number of micrometer-sized particles in the SEM image, these particles were analyzed using the SCA. The silicon substrate is a strong Raman scatterer, so a silicon spectrum was subtracted from the spectra of the particles to highlight the Raman bands emitting the particles. One particle generated a spectrum as seen in Figure 2b (red), which is determined as a PMMA spectrum.
The "green" spectrum is a trait of poly-tetra-fluoroethylene (PTFE), a very frequently used polymer - the particle’s morphology from SEM reveal that it could be a “shaving” from a worn bearing.
Previous examination of this sample using optical microscopy did not reveal the presence of the PMMA particle, as it is optically transparent. It could also not resolve the sub-micrometer morphological detail in the PTFE particle.
The "blue" spectrum reveals that the particle is an assortment of amorphous carbon, silicon carbide, and diamond. A small silicon band linked to the wafer is also present. It is possible that this particle is residue from a lapping process, and its presence indicates that the post-process cleaning regime may require re-evaluation.
During this sample examination, the traditional SEM-EDS combination failed to characterize any of the particles illustrated in Figure 2c. The O and Al signals start from the metallized tracks and the passivation layer. The two inorganic particles revealed mainly silicon in their spectra, from the substrate with a small carbon peak, and the SiC/diamond/C particle displayed only carbon and silicon peaks.
Figure 2. a) Secondary electron image of TTL device and particles, b) in situ Raman spectra of particles acquired using Renishaw’s SCA, and c) typical EDS spectrum of particles
The SCA-SEM-EDS combination offers a technique to characterize highly metallic, organic, inorganic particles on dies and wafers. It provides an exceptional solution to the needs of process monitoring and device failure analysis
Renishaw plc wishes to thank the Chemistry Division of the Naval Research Laboratory (Washington DC) for access to their equipment.
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This information has been sourced, reviewed and adapted from materials provided by Renishaw.
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