Strain measurement of a channel between Si/Ge stressors in a tri-gate field effect transistor utilizing moiré fringes in scanning transmission microscope images

Abstract

We measure the strain of a channel between Si/Ge stressors in a tri-gate p-channel metal-oxide semiconductor device, known as a fin field-effect transistor (FinFET), by utilizing moiré fringes in scanning transmission electron microscopy (STEM). These fringes reveal a pseudomagnified Si lattice, resulting from undersampling of the crystalline lattice with the nodes of the scanning grid of STEM. A practical device sample is prepared using a focused ion-beam instrument. The sample lamella is cut along the X direction to allow observation of the strained channel between Si/Ge stressors. The measurement of channel strain in a FinFET is not easy, since the channel is sandwiched between top and bottom layers of gate electrodes and insulators. For the strain measurement, we use the moiré fringes of the Si[220] lattice. These moiré fringes extract only the targeted lattice and act as a real spatial frequency filter. Other fringes with different directions and/or spacings are thereby filtered out. The strain along the channel between the Si/Ge stressors is measured to be -0.9%, with the whole procedure taking less than 5 min, including data acquisition time, using a dedicated program. As the fringe contrast is weak owing to disturbances by the gate and insulator layers, a microscope is equipped with an aberration corrector in the probe-forming system to enhance the contrast. The proposed method offers a high-throughput strain measurement, since it is performed in the image acquisition mode, and is easily incorporated into the standard workflow for critical dimension measurements.