Using Neutron Reflectivity for Insight on the Origin of Delamination

In the microelectronics industry, the fabrication of integrated circuits (ICs) often involves multilayer structures with stack materials (dielectrics and metals) on a silicon substrate. Obviously, delamination of a particular interface is a serious problem in manufacture, partly due to the loss of the wafer itself but mainly because of equipment downtime due to contamination.

It is mainly for this reason that the semiconductor industry seeks solutions to minimize delamination risks well in advance of full-scale IC manufacture.

The Challenge

This study aims to develop a method that can anticipate, for a given interface, the risk of delamination well before the IC manufacture. Predicting the risk of delamination both qualitatively and quantitatively would be desirable.

In order for this to be possible, the mechanism behind the delamination should be identified and characterized fully so that one or several of these characteristics can be related to the risk of delamination. This will enable a procedure for a prediction of the delamination risk to be implemented before full-scale IC manufacture, allowing it to be mitigated.

The Results

The procedure was carried out at the Institut Laue-Langevin (ILL). Neutron reflectivity, which is widely employed in the microelectronics industry, was used to characterize the interface between amorphous carbon (a-C) and SiO2. Two different deposition conditions that were used to grow the SiO2 layer on top of a-C were tested.

When attempting to detach layers using adhesive tape, one particular condition showed a qualitative difference in terms of adhesion. In order to unambiguously determine the chemical composition of the structure as a function of depth, the NR measurements were complemented with XRR. Samples that measured 50x50x0.75 mm3 were cut from two different 300 mm silicon wafers.

XRR measurements were performed using a commercial X-ray bench at the nanocharacterization platform (PFNC) at the Minatec Campus in Grenoble whilst the NR measurements were performed at ILL on the D17 reflectometer. XRR showed the same result for both samples which means that it was unable to detect any difference between the characterized samples aside from slight differences in the density of the SiO2 layer and in roughness.

The NR results however, showed the presence of an extra layer between the SiO2 and the a-C layers. Due to the presence of hydrogen, the intermediate layer (IL) has a lower neutron Nb than the SiO2 and a-C layers. Moreover, the IL main characteristics are difference for the two samples with regards to H-concentration and thickness. The existence of a microstructure associated with cracking at the interface is indicated by the appearance of a non-specular signal in the NR data of the sample with the weakest interface.

Conclusion

Combined XRR and NR characterization of the a-C/SiO2 interface demonstrated that the weakness of this interface is related to hydrogen accumulating at the interface. The weakness is more important for an IL that is thinner and has a higher H-concentration. Thickness of the IL and H-concentration appear to correlate with a greater susceptibility of cracking in the vicinity of this interface.

This is consistent with the appearance of off-specular scattering. This discovery should enable the anticipation and quantification delamination risk. H-accumulation might take place in other interfaces of interest in microelectronics which would broaden the application of this technique.

The Technique

Reflectivity techniques are powerful tools for characterizing interfaces, surfaces and multi-layers. Reflectivity involves reflecting a well-collimated beam from a surface and then measuring the fraction as a function of wave vector transfer. The roughness, thickness and density of a material as a function of depth can then be accurately modeled from the X-ray reflectivity (XRR) characterization.

As a result of this, XRR is used widely in the microelectronics industry for quality assurance on the production line. However, this technique is unable to detect differences in layers having different risks of delamination. Neutron reflectivity (NR), the neutron counterpart of XRR, is complementary.

Neutrons are very sensitive to the presence of hydrogen as well as being highly penetrative. Light elements such as hydrogen are particularly challenging to detect with X-ray as they only measure the electron density and do not take the elemental composition into account. If hydrogen, widely used in deposition techniques used in the microelectronics industry, is involved in the delamination mechanisms, NR could be suitable.

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 The Platform for Advanced Characterisation Grenoble (PAC-G).

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