Nondestructive in-cell conductivity metrology for memory semiconductors using near-field THz spectroscopy

We have developed a non-destructive, in-line wafer metrology technique using terahertz (THz) waves that overcomes conventional limitations in the handling of electrical properties in the thin film processes. By combining a near-field microprobe with THz time-domain spectroscopy (TDS) and establishing an accurate alignment system, we have achieved sub-10 μm resolution. It has been integrated as an in-line metrology system in a memory production line and demonstrated on device wafers. Our signal correction method effectively suppresses signal variations in the lower part of the multi-stack enabling the non-invasive measurement of metal film resistivity through THz TDS, which is used as the gate metal or various contacts. This was utilized to non-destructively measure resistance in the cell, where the actual transistors are formed during the memory semiconductor integration process. Currently, the sheet resistance (Rs) is measured at the observation site (OS) with a 4-point probe (4PP), but as the distance from the OS increases, the resistance value changes and the representativeness of the actual cell decreases. We found that after the tungsten contact deposition process of vertical NAND flash, the agreement with the Rs of the OS drops to R² 0.42 in the cell area away from the OS. On the other hand, using THz transmittance, we were able to measure Rs in the cell without contact, and found that the agreement with Rs of 4PP at the same location was R² 0.97. In the case of the TiN metal gate of a DRAM cell transistor, it is not possible to measure the electrical properties with 4PP after the etching process and thermal process where the actual structure is formed, but it is possible with THz TDS. The Rs^THz obtained from the THz transmittance shows that the resistivity changes significantly after the etching and thermal process from the initial metal deposition state.