The Chemical Mechanical Polishing (CMP) process is a critical piece of technology enabling the continued reduction in feature size in semiconductors, with the concomitant increase in feature density. CMP in turn is a major contributor to continuing increases in microprocessor speed and power with continuing reductions in cost to performance measures, often referred to as Moore's Law. Continued reduction of feature sizes on semiconductor chips has necessitated the recent introduction of low dielectric constant materials as the insulator between connectors. These low dielectric constant (low k) materials are generally fragile and much more prone to stress induced damage than the traditional silicon dioxide material. This stress induced damage can be severe enough to destroy the insulating capability in the vicinity of the damage event, leading to a short circuit and failure of the semiconductor device. Thus, a collaborative effort seeks to obtain detailed understanding of the high shear rheology of CMP slurries and both temporary and permanent changes to the particles in the polishing slurry, including both particle size distribution changes and the structure of any new particle agglomerates produced. Three objectives will test the hypothesis that shear thickening of concentrated slurries can cause solid like behavior during polishing, which can lead to defect formation when polishing dielectric wafer surfaces, particularly the newly introduced low dielectric constant insulating materials. The objectives are:
1. Integrate simultaneous structure and rheology studies using rheo-optic measurements on CMP slurries at process relevant shear rates in the presence and absence of shear thickening.
2. Quantify temporary and permanent changes to the particle sizes in the slurry using field flow fractionation (FFF) separation as the shear thickening of CMP slurries may be caused by a very small fraction of oversized particles.
3. Correlate shear response and particle size to polishing conditions causing surface defect formation on dielectric wafer surfaces.
Intellectual Merit:
Evaluate whether shear thickening is a reasonable hypothesis for defect formation in chemical mechanical polishing. Proving or disproving the hypothesis has immediate implications for slurry design, manufacture and use to enable future development of semiconductors with smaller feature sizes. Also, this work will extend the basic understanding of shear thickening in high solids colloidal dispersions to dispersions composed of particles with their own fractal structure.
Broader Impact:
The three pronged approach proposed here will provide important insights on the potential and limitations of the colloidal nature of CMP. This will enhance the ability of slurry producers and semiconductor manufacturers to optimize both the formulations and the processing parameters, thereby enabling rapid progress in future semiconductor production. The research will impact other industries that utilize large area silicon technology, e.g., flat panel displays and solar cells and tie into the Colorado School of Mines, NSF Materials Research Science and Engineering Center on renewable energy. In addition, this research will help educate graduate and undergraduate students in the fields of rheology and particle characterization as they apply to problems of real industrial interest. A senior industrial scientist will spend time teaching in the academic environment while students will spend months in residence in the industrial setting.
Intellectual Merit The Chemical Mechanical Polishing (CMP) process is a critical piece of technology enabling the continued reduction in feature size in semiconductors with applications from computers to smartphones. This work serves as the first comprehensive study of CMP slurry shear thickening (i.e., viscosity increasing under flow) under high shear rates (>10,000 s−1). Shear rates of this magnitude are challenging to measure and as a result, little to no published data on the high shear rheological behavior of CMP slurries existed before this project. Additionally, this project developed a methodology for the synchronized measurement of rheological behavior (i.e., viscosity) while polishing a semiconductor wafer, the first of its kind (a technique termed rheo-polishing). The newly established rheo-polishing technique allows for shear thickening of a CMP slurry to be directly connected to surface scratching during a polishing event; making this project the first documented study linking shear thickening to surface defects. This work also implemented in situ small-angle light scattering during rheological characterization (rheo-SALS) to monitor agglomerate formation during shear thickening. Real-time rheo-SALS images indicated the formation of micrometer scale structures (about ten times larger than the individual particles in the slurry) that are directly correlated with the discontinuous and irreversible shear thickening behavior of the fumed silica slurries. This work is the first in situ observation of micrometer scale agglomerate formation within the CMP slurry while under shear. Overall, the project will help inform the semiconductor industry on how to better formulate CMP slurries and better design polishing processes in order to prevent shear thickening and limit shear-induced agglomeration; potentially saving the industry billions of dollars in annual lost production due to CMP-induced defects. Broader Impact The project provided important insights on the potential and limitations of CMP. This will enhance the ability of slurry producers and semiconductor manufacturers to optimize both the formulations and the processing parameters, thereby enabling rapid progress in future semiconductor production. The research will impact other industries that utilize large area silicon technology, e.g., flat panel displays and solar cells. In addition, this research educated two graduate students (one earned a PhD in 2013), four undergraduate students (including 3 U.S. military veterans), and three middle or high school science/math teachers in the fields of rheology and particle characterization as they apply to problems of real industrial interest. A senior industrial scientist worked closely with the students and professors on the project while two students spent months in residence in the industrial setting. The undergraduate students and teachers studied corn starch and water shear thickening, commonly seen on YouTube and called "walking on water". These researchers identified the ratio of corn starch to water necessary to "walk on water" and published on journal article on the topic. Shear thickening was also brought into the engineering classroom through a teaching technique called YouTube Fridays. YouTube Fridays has been shown to engage students and may improve learning of engineering concepts such as heat transfer.
