ON THE SPATIAL-RESOLUTION OF 2-DIMENSIONAL DOPING PROFILES AS MEASURED USING SECONDARY-ION MASS-SPECTROMETRY TOMOGRAPHY

Knowledge of the lateral or spatial distribution of doping impurities is important for accurate process and device simulations of submicrome ter silicon devices, since the edge effects of the electric fields can no longer be neglected. A newly developed technique, which utilizes t he high sensitivity and high depth resolution of secondary ion mass sp ectrometry (SIMS), is capable of measuring two-dimensional (2D, i.e., in the depth and the lateral direction) doping distributions. The tech nique is based on a series of one-dimensional (1D) SIMS depth profiles obtained at different directions through a sample. The individual SIM S depth profiles are then recombined to generate a 2D doping profile u sing the expectation maximization algorithm, which was originally used in human body computer-aided tomography. The SIMS tomography techniqu e gives a doping distribution as a function of position. The positiona l accuracy or spatial resolution of the technique needs to be fully un derstood in order to properly use the technique. During a reconstructi on process, the area of interest is divided into small volume elements or voxels, and the doping concentration for each voxel is estimated. Therefore, the size of the voxels determines the spatial resolution of the reconstructed profiles. The smaller the voxels are, the better th e spatial resolution is. However, the size of the reconstructed area c annot be made infinitely small, due to the limited number of SIMS meas urements. Since each SIMS measurement provides only one linear indepen dent equation, the total number of equations (or the total number of t he SIMS measurements) should be equal to the number of voxels (or unkn owns). Thus, the size of the voxels, or equivalently, the number of SI MS measurements available imposes a practical limit on the spatial res olution. In addition, since the 1D SIMS depth profiles are used as the input for the profile reconstruction, the depth resolution of the 1D SIMS measurements dictates the spatial resolution of the reconstructed profiles. Furthermore, the depth resolution of the ID SIMS measuremen ts is also affected by the sample skew, which is the measure of the la ck of parallelism of the dopant line with respect to the beveled surfa ce. And finally, the alignment of the 1D SIMS depth profiles relative to each other contributes to the positional uncertainty of the reconst ructed distributions. A 20 keV boron implant through a 1.2 mum wide wi ndow into germanium preamorphized silicon has been reconstructed recen tly. The spatial resolution of the reconstructed dopant profile is eva luated to be 40 nm. It is found that the spatial resolution is predomi nantly determined by the depth resolution of 1D SIMS profiles, and the microtopography of the SIMS craters is the major cause of the poor de pth resolution. It is expected that the spatial resolution of the 2D d oping tomography can be greatly improved through the use of a simplifi ed sample structure, which will reduce the SIMS profiling depth and pr ovide better depth resolution for the 1D SIMS measurements.