For an overview of how QSense QCM-D can be used in molecule - surface interaction analysis and optimization of CMP-related processes, please refer to our previous application note.
This case study shows how QSense® QCM-D (Quartz Crystal Microbalance with Dissipation monitoring technology) can be used to study slurry additive and abrasive interaction with surface materials relevant to the electronics industry.
The example, which takes you through the overall objective of the study, the experimental layout, the measurement execution and the analysis of the results, demonstrates how QSense analysis is used to extract information about the sample – surface interaction at a nanoscale level to support slurry development.
Figure 1. Schematic illustration of CMP tools
Understanding the ‘C’ of CMP
The effectiveness of a slurry largely depends on the nanoscale interactions of its active chemicals with the surface material which needs to be polished. Understanding of the surface-molecule interactions is crucial for the customization of slurry for high performance with regards to planarization and material removal, as well as development of efficient CMP protocols.
QSense QCM-D can be used to characterize nanoscale surface-molecule interactions involved in a CMP process, and customization of CMP slurries and protocols. The technology monitors surface-molecular interaction in real-time with nano-level sensitivity via two parameters, frequency (∆f) and energy dissipation (∆D) at multiple harmonics. The frequency relates to mass changes at the surface and the dissipation relates to the softness or viscoelasticity of the surface-adhering layer.
Case example
Analyzing the interaction of CMP slurry additives and an abrasive with a ceramic surface
In this case study, the user wanted to characterize the surface interaction of a certain abrasive, and assess how this interaction was affected by the presence of different additives. The user also wanted to analyze how mixtures of the different slurry components interacted with the surface material of interest, and if the components as well as the mixtures could be removed by rinsing.
The questions were addressed using QSense QCM-D analysis of surface interaction and removal of the two different additives, the abrasive, and mixtures thereof.
Experimental details
QSensors coated with a material of interest, a ceramic, were prepared prior to measurement by 20 minutes UV-Ozone exposure followed by vigorous N2 dust cleaning.
The following samples were used in the study:
- Additive A
- Additive B
- Abrasive P (pH adjusted to 5.51)
- Mixture A + B
- Mixture A + B + P (pH adjusted to 5.53)
Figure 2. QCM-D analysis of additive interaction with a ceramic surface. Representative plot of the time resolved mass showing the surface uptake of Additive A, Additive B, and the mixture A + B, as well as removal in the rinse step.
Five QCM-D experiments were performed in triplicates, each including the following three steps (sample sequence outlined in Figs. 2 and 3):
- Baselines of the bare sensors in deionized (DI) water were collected for 10 min.
- Sample solutions were flown over the sensor surfaces (20 min) and real-time surface interactions were monitored by recording the frequency (∆f) and dissipation (∆D) changes.
- The surfaces were rinsed with DI water for 30 min.
Figure 3. QCM-D analysis of abrasive and additive + abrasive interaction with the ceramic surface. Representative plot of the time resolved mass showing the surface uptake and removal of the Abrasive P and the mixture A + B + P.
All experiments were performed at 25 °C, and the solution flow rate was 40 μLmin-1.
Experimental results
The collected raw data, ∆f and ∆D, were analyzed with QSense software to extract the quantified mass, Figs. 2 and 3.
Additives A, B and mixture A+B
Looking at the mass uptake of the additives A, B and their mixture, Fig. 2, it is noted that the interaction of these components show very different behavior.
The insertion of additive A results in the characteristic signature of a bulk shift, i.e. a shift that is reversible upon rinse. The reversible shift indicates that the interactions between additive A and the ceramic surface is insignificant, and that no additive remains at the surface after rinse.
Additive B, on the other hand, shows a large mass uptake upon interaction with the surface, Fig. 2. In the rinse step, some of this mass is removed.
The mixture of the two additives A+B also led to irreversible mass adsorption, Fig. 2, similar to the behavior of additive B. The magnitude of the uptake, both before and after rinse, is however larger for the mixture A+B than for the mass uptake of additive B.
Abrasive P and mixture A+B+P
Looking at the quantified mass for the abrasive-surface interaction, Fig. 3, the result indicates that the interaction between the abrasive P and the ceramic surface is insignificant.
The abrasive mixture, A+B+P forms a rigid layer at the surface, as indicated by near zero dissipation value (data not shown). The irreversible adsorption to the surface is, however insignificant, and the molecules are removed upon rinse with DI water, Fig. 3.
Discussion and Conclusion
The two additives, A and B, show significantly different behavior when exposed to the ceramic surface.
The introduction of additive A predominantly showed a bulk shift, and after rinse, no additive remained at the surface analyzed. Additive B, on the other hand, followed a typical Langmuir adsorption profile and rapid desorption kinetics upon rinse with DI water. This rapid desorption together with the behavior of the dissipation value (not shown) indicate that additive B adsorbs and forms a layer that strongly interacts with the surface. On top of this strongly adhering layer, there are additional molecules that are loosely bound, and which are removed in the rinse step, leaving only the mass of the strongly interacting molecules after the rinse.
When the additives A and B are combined into a mixture, something interesting happens. The QSense analysis reveals that the surface interaction dynamics of the mixture is similar to that of additive B, but more importantly, the analysis also reveals that the magnitude of the mass uptake is much larger than what would have been expected if the two additives would have interacted with the surface in the same way as when they were exposed to the surface as individual components, i.e. the mass uptake of the A+B mixture was larger than the sum of the individual mass uptake of additives A and B. The analysis hence reveals that the two components are interacting with each other when combined into a mixture, and that the complexes formed result in an overall thicker layer at the ceramic surface than either of additives A or B alone.
The mixture of the abrasive with the two additives resulted in formation of a very thin and rigid layer on the ceramic surface, which was then removed by DI water during the rinse step. Both of the mixtures, A+B, and A+B+P, were pH adjusted.
Lowering the pH, e.g. increasing the proton concentration could lead to protonation of specific groups within the studied compounds. Protonation or charge re-distribution will most likely cause an alteration of the electrostatic interaction between the compounds and the surface.
Concluding remarks
The user in this study wanted to analyze the interaction between a ceramic surface and different slurry additives, abrasive particles, and combination thereof, as well as how easy they were removed from the surface by rinsing with DI water. In the example presented here, QSense QCM-D was used to assess the affinity between a set of different slurry components and the ceramic surface. QSense QCM-D analysis revealed how different slurry additives, additive components, and mixtures thereof interact with the target surface. The time-resolved information provided detailed insight into the surface – molecule interactions. This new detailed insight could be of value when exploring and optimizing CMP slurries, processes, and protocols.
Acknowledgement
We thank our industrial partner for the slurry samples, guidance, and feedback on the questions of issue.
References
- This case study was made in joint collaboration between Nanoscience Instruments and Biolin Scientific
- Information on QSense QCM-D technology: Quartz Crystal Microbalance (QCM) – Nanoscience Instruments