Semiconductor wafer Processing

July 5, 2014
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Now that a high level of purity has been attained (99% eleven nines), the atomic structure of the silicon must be dealt with. A process known as Crystal Growing transforms polycrystalline silicon into samples with a singular crystal orientation, known as ingots. The Polysilicon is mechanically broken into 1 to 3 inch chunks and undergoes stringent surface etching and cleaning in a cleanroom environment. These chunks are then packed into quartz crucibles for meltdown (at 1420oC) in a CZ furnace. A monocrystalline Silicon seed is installed into a seed shaft in the upper chamber of the furnace. Slowly, the seed is lowered so that it dips approximately 2mm into the Silicon melt. Next, the seed is slowly retracted from the surface allowing the melt to solidify at the boundary. As the seed pulls the Silicon from the melt, both the crucible and the seed are rotated in opposite directions to allow for an almost round crystal to form. CZ furnaces also must be very stable and isolated from vibrations. Once the proper crystal diameter is achieved, the seed lift is increased. This, along with the heat transfer from heater elements will control the diameter of the crystal. (Images from Wacker - How To Make Silicon)

During the growth process, the crucible slowly dissolves Oxygen into the melt that is incorporated into the final crystal in typical concentrations of around 25ppma. Intentional additions of dopants control the resistivity distribution of the final crystal. In addition to pull speed and heat transfer at the solid-liquid interface, heat dissipation during crystal cooling strongly determines microscopic defect characteristics in the final crystal. For modern CZ pulling systems, those variables can be accurately predicted by numerical simulations which allow designing the geometrical and thermal configuration of the CZ puller to the desired outcome of the crystals. Once the growth process is complete, the crystal is cooled inside the furnace for up to 7 hours. This gradual cooling allows the crystal lattice to stabilize and makes handling easier before transport to the next operation. For some applications, it is important to have even lower concentrations of impurity atoms (eg. Oxygen) than what can be achieved by CZ crystal growth. In this case, Float Zone Crystal Growth is used. In this process the end of a long polysilicon rod is locally melted and brought in contact with a monocrystalline Silicon seed. The melted zone slowly migrates through the poly rod leaving behind a final uniform crystal.

Ingots coming from crystal growing are slightly over-sized in diameter and typically not round. Hence, a machine employing a grindwheel shapes the ingot to the precision needed for wafer diameter control. Other grinding wheels are then used to carve a characteristic notch or a flat in order to define the proper orientation of the future wafer versus a particular crystallographic axis.

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