(Invited) Metallic Impurity Control in Silicon Processing
Authors: Polignano Maria , Borionetti Gabriella 
Published: July 2022

In this paper we review the performances of a few techniques for the analysis of metal contamination in silicon. A few case studies are discussed to compare the ability of these techniques in detecting contaminants with different properties as silicon impurities. Common contaminants and elements recently introduced in the fabrication process are considered.
TXRF (Total reflection X Ray Fluorescence), recombination lifetime measurement techniques, DLTS (Deep Level Transient Spectroscopy) and micro-photoluminescence analyses are compared. The results reported in this paper show that it is not possible to define a unique recipe that can be applied in all cases. Different approaches are required depending, on one hand, on the contaminant diffusivity and solubility, and, on the other hand, on the process step to be monitored.
The techniques for monitoring metal contamination can schematically be divided into two main groups, one consisting of the techniques that measure the chemical concentration at the silicon surface and the other of the techniques that measure the electrical activity of the contaminant in the silicon volume.
Mostly in case of metal contamination detection during wafer manufacturing, techniques which intentionally deposit thin films of polysilicon or thermal oxide and then perform chemical etching of them associated to ICP-MS analysis are also used, mostly addressed to detect fast diffusers like Cu and Ni [1], not always associated to clear electrical response. Indeed, these elements retain significant diffusivity even near room temperature, where the solid solubility is negligibly low, so they mostly segregate at the silicon surface or in getter sites, and for this reason they are hardly revealed in the silicon volume. However, these elements can be revealed by the recovery from deep depletion of MOS capacitors, because the segregated metals significantly increase the surface generation velocity.
Some medium-fast diffusers like iron diffuse through several hundred microns during an ordinary thermal treatment, and mostly remain in the solid solution in silicon [1]. A comparison was carried out between TXRF and carrier recombination lifetime measurement for their ability to reveal iron contamination after a cleaning process. To prepare the samples for carrier lifetime measurements, the samples had to be thermally treated by RTP (Rapid Thermal Process). This study showed that TXRF and recombination lifetime measurements have essentially the same sensitivity to iron contamination. Vice versa, the detection of slow diffusers is found to be critical, because a very low concentration per unit area (≈107cm-2) may result in a non-negligible volume concentration in the device region (≈1010cm-3). As a consequence, the sensitivity per unit area required for these elements is difficult to reach with surface techniques such as TXRF. Molybdenum and tungsten [1] are examples of this sort. These elements do not diffuse deep enough to be efficiently revealed by recombination lifetime measurements, but they are easily revealed in the silicon volume by DLTS, which is probably the best approach for these elements, though it requires a considerable sample preparation. On the other hand, these elements are very common contaminants in ion implantation or epitaxial processes, and are very harmful for devices sensitive to carrier recombination, such as image sensors. Therefore, a technique able to detect near-surface contamination with no or limited sample preparation would be needed. Band-to-band micro-photoluminescence proved to be a good candidate, though DLTS still has the best sensitivity.
Silicon contamination may also happen in the final part of the device process flow when the wafer frontside is protected by several dielectric layers and low-temperature thermal treatments only are allowed. In that case, contamination may come from the wafer backside only, but fast diffusers are still able to reach the wafer frontside and damage the device. In a study of palladium contamination from a contaminated chuck, TXRF proved not to be sensitive enough to palladium contamination. Palladium is best revealed by recombination lifetime measurements, such as the SPV (Surface PhotoVoltage) technique, but a high temperature treatment is required to activate it, so also in this case some sample preparation is required.
Finally, we recall that the device itself can be a very sensitive monitor of metal contamination [2], though of course methods that prevent the impact of contamination of the device are generally preferred. The analysis of the dark current in image sensors not only reveals the presence of metal contamination (see the enclosed figure), but also provides some hints about the contaminant element.

References

[1] K. Graff, Metal impurities in silicon devices fabrication, Berlin, Springer, 2000
[2] C. Claeys and E. Simoen, physica status solidi (a) 216, (2019)

Figure 1