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Detecting Buried Defects in Silicon with Photoluminescence Imaging

X-ray diffraction imaging, commonly known as X-ray topography, is a non-destructive technique used to characterize strain and visualize defects, imperfections and distortions in crystals. With the use of synchrotron radiation, the range of applications of this technique is much wider. It can be applied to study under reasonable conditions weak distortions, nucleation defects and movement in crystals and thin films, dynamics of piezoelectric domains, phase transitions and in-situ crystal growth.

All with enhanced spatial resolution and fast acquisition times. In this study, STMicroelectronics engineers applied it (among other comparative methods) to validate the use of an innovative technique in their industrial processes. PL imaging is a high-resolution, quick and non-destructive emerging method in the semiconductor industry. It allows several industrial applications including in-line product control of product wafers, process optimization and others.

X-ray diffraction topography was performed at beamline BM05 at the European Synchrotron ESRF member of the PAC-G (Platform for Advanced Characterisation-Grenoble, initiative supported by the French National Research Agency in the framework of the "Investissements d’avenir’’ program ANR-10-AIRT-05 网上手机网投游戏 IRT Nanoelec).

Introduction

网上手机网投游戏In CMOS manufacturing, sub-micrometer scale defects like stacking faults and dislocations can be produced by process steps and stress induced by patterns. The formation and curing of those defects are very complex and special care is needed during the development of new technology or the introduction of new pattern design.

Indeed, if located in the active device, those defects will most likely cause short circuits or leakage, which are detrimental to the device in numerous ways*. Identifying, locating, and quantifying these defects is vital, but also resource and time-intensive.

网上手机网投游戏When dealing with buried defects that are not captured by classical optical defectivity techniques, this becomes even more challenging. In fact, only off-line lab characterization methods can show the defects under the surface. Those are either fully destructive and need a lot of preparation and skills or require synchrotron radiation.

网上手机网投游戏This trade-off is well known in the semiconductor industry: off-line lab equipment provides better analytical power but it is slower. In-line metrology equipment have faster throughputs but limited capabilities. As devices are becoming more complex and with shorten manufacturing schedules, the pressure for having higher resolution and detection capabilities in-line increases.

2. PL Imaging Technique

网上手机网投游戏 has the property to emit light when excess carriers are produced by external excitation, which is known as luminescence. When excess carriers are initiated by photo-excitation, the corresponding luminescence is then called photoluminescence (PL).

网上手机网投游戏In a usual PL experiment, the light emitted by the sample is the consequence of a number of relaxation processes, depending on the experiment conditions (e.g., sample temperature, excitation light,), and is characteristic of the material properties (e.g., structural defects in the lattice, bandgap energy, or disorder due to variations of the chemical composition). Figure 1 illustrates these phenomena.

网上手机网投游戏PL is a widely employed method for the characterization of the electronic and optical properties of semiconductor materials, including establishing crystalline quality. This work aimed to demonstrate the applicability of PL imaging technique to detect buried defects in silicon non-destructively, in a fast and convenient manner.

Simple principle diagram of silicon PL measurement using En-Vision equipment. Step 1: excess carriers are being generated using a light source. Step 2: excess carriers are recombining through different processes including band-to-band and defect-band PL. Step 3: PL is collected using a camera and recorded as an image.

Figure 1. Simple principle diagram of silicon PL measurement using En-Vision equipment. Step 1: excess carriers are being generated using a light source. Step 2: excess carriers are recombining through different processes including band-to-band and defect-band PL. Step 3: PL is collected using a camera and recorded as an image.

3. Assessment of the PL Imaging Technique for the Detection of Buried Defects in Silicon

In order to demonstrate the ability to identify buried dislocations in silicon by utilizing the defect-band PL imaging method, STMicroelectronics engineers have benchmarked it against several techniques. More specifically, researches have compared the results of PL imaging to selective etching of silicon, Transmission Electron Microscopy-TEM, Synchrotron X-Ray Topography, and PL spectroscopy characterizations. The results and discussion were published in R. Duru et al., "Photoluminescence Imaging for Buried Defects Detection in Silicon: Assessment and Use-Cases," in IEEE Transactions on Semiconductor Manufacturing, vol. 32, no. 1, pp. 23-30, Feb. 2019, doi: 10.1109/TSM.2018.2871967.

网上手机网投游戏All these techniques have the ability to provide information on the crystal quality of a material to some extent. TEM, for instance, is a very widely used technique and allows seeing defects with very high resolution. Its main drawbacks are that it is destructive, and each lamella provides information on very small areas, furthermore sample preparation is quite complicated for the visualization of buried defects.  All the comparative techniques showed good correlation to PL imaging results. Below, we focus on the correlation of PL results with synchrotron X-ray topography measurements.

B. Correlation to X-Ray Topography

网上手机网投游戏Synchrotron X-ray topography measurements were performed at the ESRF, which has a long history of working with industry and is running an ambitious industrial program. More recently, in the frame of the IRT Nanoelec, several characterization techniques were made available as services to semiconductor companies through the Platform for Advanced Characterisation Grenoble (PAC-G). This service platform, promotes access to large-scale facilities such as synchrotrons and neutron sources for industrial applications.

One of PAC-G’s most used techniques by the semiconductors industry is Bragg diffraction imaging, commonly known as X-ray Topography. It is an extremely powerful method that is utilized for monitoring crystal quality and visualizing defects in many different crystalline materials.

X-ray Topography is one type of X-ray imaging, making use of diffraction contrast rather than absorption contrast, which is typically employed in radiography. Diffraction topographic images record the intensity profile of a beam of X-rays diffracted by a crystal.

网上手机网投游戏This intensity mapping reflects the distribution of scattering power inside the crystal, and so the irregularities in a non-ideal crystal lattice. It is then viable to identify a misalignment or a local distortion of a set of crystalline diffracting planes, contrasting with perfect crystal diffraction.

More in particular dislocations can be seen and show a typical “signature”. For this study, a quantitative version of monochromatic X-ray Bragg diffraction imaging, known as Rocking Curve Imaging (RCI) was used.

Principle of the Rocking Curve Imaging technique.

Figure 5. Principle of the Rocking Curve Imaging technique.

网上手机网投游戏This method (Fig. 5) supplies a high-resolution spatial mapping of the sample where an X-ray diffraction rocking curve is recorded by every single pixel of the 2D detector. A very bright X-ray source is required for the RCI version of X-ray Topography and very powerful instruments are needed to record and process the data. More information can be found here ()

X-ray topography data showed the presence of multiple defects in the bulk silicon crystal. The analysis was performed using the Bragg diffraction peak parameters, full width at half maximum (FWHM), namely integrated intensity, and peak position, for which we can define criteria specific to the signature of dislocation14 (Fig. 6).

Some of the defects were identified as being dislocations according to these criteria, but a number of other defects were also present and could be associated with inclusions or precipitates coming from the substrate (boron or oxygen impurities).

Description of the signature of dislocation as detected by the X-Ray topography RCI technique on our sample (2 neighbour dislocations are visible here). The observed signature does not translate the real physical size of the dislocation but rather the constraint field induced by the dislocation.

Figure 6. Description of the signature of dislocation as detected by the X-Ray topography RCI technique on our sample (2 neighbor dislocations are visible here). The observed signature does not translate the real physical size of the dislocation but rather the constraint field induced by the dislocation.

Overall, the density of dislocations has been assessed at ∼3E4 to ∼5E4 cm-2. In the same sample, the density of defects detected by defect-band PL imaging was ∼6.1E4 cm-2. The two quantities look to be in fair agreement, showing the correlation between the two methods.

PL imaging (a) is compared to X-ray Topography (b) for the detection of dislocations in a P-implanted silicon wafer. For X-ray topography, only FWHM data are presented. The dislocation density found by both techniques is in the same order of magnitude.

Figure 7. PL imaging (a) is compared to X-ray Topography (b) for the detection of dislocations in a P-implanted silicon wafer. For X-ray topography, only FWHM data are presented. The dislocation density found by both techniques is in the same order of magnitude.

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4. Conclusion

In this article, we show one industrial application of synchrotron , a technique in the service portfolio of the PAC-G. Tests were performed at the ESRF on beamline BM05.

网上手机网投游戏STMicroelectronics researchers demonstrated the introduction of a new optical method based on PL imaging that was found to be useful to detect buried dislocations in silicon. The examination of this innovative approach was based on comparative methods that are usually involved in crystal defect detection in silicon, namely: TEM cross section, selective etching, spectroscopic PL and synchrotron X-ray topography.

The PL imaging method was found to be compliant with in-line industrial metrology standards: reliable, non-destructive, quick, and high resolution.

网上手机网投游戏Synchrotron X-ray topography can be used in several industrial applications where crystals are involved. It allows the visualization of defects (dislocations, twins, domain walls, inclusions, impurity distribution, …) present in the crystal volume. Applications include process and product development in R&D, yield enhancement in semiconductor manufacturing and benchmarking lab equipment.

This article illustrates a successful case where a very powerful, reliable and non-destructive technique developed in a large-scale facility can be used as a benchmark to develop and validate other techniques suitable for in-line measurements.

References and Further Reading

网上手机网投游戏R. Duru et al., "Photoluminescence Imaging for Buried Defects Detection in Silicon: Assessment and Use-Cases," in IEEE Transactions on Semiconductor Manufacturing, vol. 32, no. 1, pp. 23-30, Feb. 2019, doi: 10.1109/TSM.2018.2871967.

Acknowledgments

Produced from materials originally authored by Romain Duru , Delphine Le Cunff , Maxime Cannac, Isabella Mica from STMicroelectronics; José Baruchel, Thu-Nhi Tran-Thi, from European Synchrotron Radiation Facility (ESRF); and Georges Brémond from INSA-Lyon.

网上手机网投游戏This information has been sourced, reviewed and adapted from materials provided by The Platform for Advanced Characterisation Grenoble (PAC-G).

For more information on this source, please visit

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