Ion implantation and subsequent dopant activation are two of the most important fabrication steps for VLSI and ULSI circuits. The need for on-line monitoring of those steps is imperative due to the tighter tolerances being imposed by the reduction in geometries and the increase of wafer dies and wafer diameters. Knowledge of the distribution of the implanted ions with depth, both before and after activation, is crucial in the control of device performance. The aim of this project was to develop a novel approach to obtain fast and accurate implant profiles, suitable for on-line monitoring. In the new approach, the Pattern Etch Transfer (PET) technique is used to create a number of rectangular trenches of increasing depth into silicon, by use of a single photolithography step. The trenches are first patterned into photoresist by variation of the exposure time for each trench. Reactive Ion Etching (RIE) is then used to transfer those trenches into silicon, by a simultaneous etch of photoresist and the Si substrate. Implant damage, dopant profiles and carrier profiles can then be obtained from measurements performed on the flat silicon surface at the bottom of each trench. Experiments were performed to obtain better depth resolution (75 A), minimize the induced-damage levels and achieve high accuracy and repeatability. In the PET technique measurements are performed at neighbouring trenches, therefore the resulting depth profile includes an error associated with the implant variation across the wafer. The effect of the implant profile variation across the wafer was seperately assessed by use of a second method, the two-dimensional Reactive Ion Etching technique. This method is not suitable for fast monitoring since RIE is used to successively strip thin layers of Si (75 A) across the whole wafer. Etch rate variations across the wafer were measured and minimized in order to accurately determine the depth from the initial Si surface. At each depth a two-dimensional sheet resistance mapping was obtained. Two-dimensional implant profiles across the wafer can thus be acquired and 3a limits of the carrier concentration values at each depth can be obtained. Therefore, the limiting curves that are evaluated from the two-dimensional RIE technique define the upper and lower margins for the implant profile curve obtained by the PET technique. The PET technique provides a platform for the application of most of the existing implant characterization techniques. Preliminary experiments revealed the suitability of the stripping methods for monitoring of sheet resistance, optical constants and thermal-waves with depth. Differential Hall measurements were used as the benchmark test for the assessment of carrier concentration profiles obtained by the four-point probe measurements in combination with the PET structure and the two-dimension RIE technique. Damage profiles were acquired by use of ellipsometry and thermalwaves inside the PET trenches. Secondary Ion Mass Spectroscopy and SUPREM IV simulation results were used as the benchmark for dopant profiles. Shallow implants for the source/drain junctions of the 1.2tm CMOS process and high dose BF2double implants were monitored. Theoretical predictions were used to explain the resulting dopant, damage and carrier concentration profiles. The thesis also contains a critical review of ion implantation theory, problems and the most important existing monitoring and profiling techniques.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:669269 |
| Date | January 1992 |
| Creators | Porfiris, Nikolaos E. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11272 |
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