Category: Computers

Key Takeaways

1. Researchers developed a method to grow 120 alternating layers of silicon and silicon-germanium on 300mm wafers for advanced 3D DRAM, maintaining strain essential for device yield.

2. The silicon-germanium layers used 20% germanium, allowing for selective etching and enabling over 100 bilayers on production-sized wafers, which increases memory density.

3. Process modifications were made to ensure sharp interfaces and minimize layer mixing, using reduced-pressure CVD techniques and maintaining consistent germanium profiles.

4. Defect management was crucial, with techniques confirming that the wafer remained fully strained and free of threading dislocations, despite the multilayer structure’s thickness.

5. Uniformity in layer deposition was enhanced through active temperature control in the reactor, addressing challenges related to thickness variations and maintaining consistent interface quality.

Researchers at imec and Ghent University have come up with a new way to grow 120 alternate layers of silicon and silicon-germanium on 300mm wafers, which will help in making three-dimensional DRAM. Each stack includes about 65 nanometers of silicon and 10 nanometers of silicon-germanium, which has 20 percent germanium, repeated 120 times. This keeps the inner wafer fully strained, which is vital for the yield of the devices. Most of the misfit dislocations occur near the edge of the wafer, where the bevel allows for relaxation.

Silicon-Germanium Layers

To create these channels, they needed silicon-germanium layers that could be etched selectively, which is why they opted for a composition that includes 20 percent germanium. The findings from the team show that it’s possible to build over 100 bilayers on production-sized wafers, which leads to a higher memory density.

Process Adjustments

To make this happen, the team modified their process to keep the interfaces sharp and to minimize mixing between the layers, all while ensuring good throughput. They utilized reduced-pressure CVD in ASM Intrepid tools, growing silicon with silane at around 675 degrees Celsius and silicon-germanium with dichlorosilane and germane. Secondary-ion mass spectrometry was used to compare a normal stack to one that was kept hot for as long as it would take to create 60 additional bilayers. The germanium profiles were consistent, showing that there was hardly any mixing between silicon and silicon-germanium under these conditions.

Managing Defects

Handling defects was also a key issue. High-resolution X-ray diffraction and cross-sectional TEM indicated that the superlattice in the wafer remained fully strained, and no threading dislocations were observed there. Even though the total thickness of silicon-germanium is about 1.2 micrometers—much greater than the typical critical thickness for a single layer—the multilayer structure and clean growth kept it stable. The authors noted that strain relaxation occurs near the edge due to the bevel effect, and they suggested either lowering the germanium content or adding a small amount of carbon to reduce lattice mismatch. They also kept an eye on wafer bow and, when needed, applied a compressive nitride layer to the back after safeguarding the front side.

Uniformity Challenges

The team paid close attention to the uniformity challenges in layer deposition. The paper links changes in layer thickness and non-uniformity in thick stacks to temperature changes caused by unwanted buildup on the reactor’s quartz tube, which affects how the lamps heat the chamber. A newer tool with active temperature control for the tube decreased this drift, enhancing both side-to-side uniformity and layer consistency. In comparison, optimized single-layer runs had thickness variations of about 1.3 percent, while very thick cap structures raised that to around 1.8 percent, with the edge being the most sensitive area. The analysis indicates that interface thicknesses are just a few nanometers, with bottom-of-stack interfaces measuring about 2.6–2.9 nanometers, and sharper transitions higher up. This is consistent with reduced segregation and interdiffusion at the chosen temperature and chemistry. These microscopy findings are in line with x-ray satellite peaks that stayed well-resolved and vertically aligned with the substrate peak, indicating a coherent, strained superlattice.

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