Why Higher Resistivity Wafers?
Normal silicon wafer substrate resistivity ranges for CMOS technologies have typically spanned from a low of about 5 mohm-cm on heavily doped epi substrates to a high of around 30 ohm-cm on polished wafers. Although heavily doped substrates have proven useful for protection against latch-up, digital CMOS device design and performance has not been strongly coupled directly to substrate resistivity. This is changing in the emerging area of CMOS integration of radio frequency transceiver devices operating in the GHz frequency range.
Wireless chip designs can benefit significantly from higher substrate resistivity levels. Improvements in the performance of passive components, such as inductors, and substrate electrical isolation between the integrated digital, RF (radio frequency), and analog components are possible with higher resistivity silicon substrates Substrate resistivities greater than 40 ohm-cm are required now and in some cases resistivities in excess of 1000 ohm-cm will be needed.
Ultra-High Resistivity Crystal Growth
Growth of CZ crystals to 100 ohm-cm with acceptable resistivity gradients is easily achieved using existing growth processes. The amount of dopant added to the crystal is simply reduced in order to target the higher resistivity range. The rest of the crystal growth process parameters, as well as final wafer product characteristics, remain unchanged.
Growth of CZ crystals to the 1000 ohm-cm range presents some additional challenges. Because the amount of background dopant has to be significantly reduced, additional emphasis must be placed on the control of dopants, such as boron and phosphorous, introduced from the raw materials and components used in the crystal puller. These materials and components include the polysilicon source, the quartz crucible, and the graphite heater. In addition, the extremely low dopant level in the melt makes control of dopant mass transfer to, and then through, the boundary layer at the melt-solid interface important for achieving acceptable radial resistivity variation. By employing high purity puller components, and by optimizing dopant flow in the melt, MEMC R&D has successfully developed the capability to grow CZ crystals with maximum resistivities into the 1000 ohm-cm range using existing 200/300mm crystal pulling equipment. Accurate and reproducible measurements of these ultra-high resistivities is also a challenge that must be addressed but will not be discussed in detail here.