Characterization of two-dimensional electrostatic potential profiles in deep submicron MOSFET devices



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Over many years, the direct observation of dopant profiles in semiconductor devices has been of great interest to researchers and engineers in the semiconductor industry. Recently, off-axis electron holography has been shown to be a promising technique for determining dopant profiles and solving problems in semiconductor device technology. This dissertation describes the application of electron holography to dopant distributions in deep submicron devices and compares the outcome with results obtained by other techniques. For this study, complementary metal oxide (CMOS) field effect transistors (FET’s) were obtained from a commercial manufacturer. The dopant profiles within these MOSFET’s were investigated using secondary ion mass spectrometry (SIMS), process simulations, dopant-selective etching, and electron holography. The simulated results were found to be in agreement with the direct measurements of the dopant profiles. In the first study, the dopant distributions in NMOS and PMOS devices, both as-implanted, and following annealing, were investigated using process simulation software, such as TSUPREM-4 Technology Computer-Aided Design (TCAD), the UT-Marlowe program, and the Stopping Range of Ions in Matter (SRIM). The results were corroborated using SIMS data. In the second study, dopant-selective etching methods were studied for both NMOS and PMOS devices. Solutions of HF (49%), HNO3 (70%) and CH3COOH (99%) in various ratios were used to etch the doped regions of specimen already thinned for transmission electron microscopy (TEM). The etching rate for PMOS was slower than for NMOS; profiles were obtained corresponding to contours of concentration equal between 1 × 1016 cm-3 and 5 × 1021 cm-3. In the third study, two-dimensional dopant profiles have been investigated using electron holography in the TEM. Three different specimen preparation techniques using a focused ion beam tool (FIB) were developed, and the ion beam damage in the FIB was investigated. Electron holograms were obtained using two different instruments, reconstructed, and used to determine the voltage profiles within the devices. The final phase images show that the dopant distribution determined by holography can be compared quantitatively with the results obtained using other techniques.