Electron Holography as a Tool for Dopant Profile Characterization of Semiconductor Devices
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Electron Holography as a Tool for Dopant Profile Characterization of Semiconductor Devices Takao Matsumoto and Masanari Kouguchi Advanced Measurement and Analysis Center, Central Research Laboratory, Hitachi, Ltd., 1-280, Higashi-Koigakubo, Kokubunji-shi, Tokyo 185-8601, Japan ABSTRACT We used electron holography to analyze the dopant profile in a MOS transistor. The overall performance of the holography electron microscope at our laboratory has been confirmed by recording a maximum of 16,000 numbers of electron interference fringes on a conventional electron microscope film. For thin film specimen preparation, we have developed an FIB system with a modified beam scanning scheme in which a high-frequency analog modulation signal is added to the digital signal of the beam deflector. This has enabled us to smooth the residual surface roughness presumably caused by the glitch-noise of D/A converter and we proved that a nearly atomically smooth surface was obtained as estimated with an AFM and TEM. We used this optimized system to characterize the dopant profile in MOS transistors, and comparisons with the calculated profiles from device simulator have proved that they are in good agreement. INTRODUCTION Electron holography [1] is a promising tool for distinguishing the dopant profile in a MOS transistor [2-4] especially for devices using light dopant atoms such as borons. This is because it is the electrostatic potential of the specimen itself that is to be measured in electron holography rather than the density of the dopant atoms. Therefore, it is possible to describe the dopant profile as far as a detectable electrostatic potential distribution is formed by active dopant atoms in a semiconductor even if their densities are too thin to be detected. Future applications are promising, as the dimension of the device will be decreased, the high spatial resolution of the method compared with other possible dopant profiling techniques would be advantageous. In order for the method to become a reliable characterization tool, however, several problems must be overcome. For example, it is necessary to prepare a thin film specimen of uniform thickness from a bulk wafer. Secondly, the sensitivity of electron holography itself must be improved as much as possible. Also, electron beam induced charging of the sample must be eliminated or precisely controlled [5]. In the present study, we have used an FIB system with a modified beam scanning scheme in which a high-frequency analog modulation signal is added to the digital signal of the beam deflector. EXPERIMENTAL DETAILS A 300-kV cold-field emission microscope (Hitachi HF-3000) was used during the present study. It was equipped with an electron biprism between the objective lens and the first intermediate lens. Holograms were recorded on conventional electron microscope films
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(Kodak 4489), and developed with Kodak D-19 developer as the manufacturer recommended. Thin film specimens were prepared with a focused ion beam apparatus (Hitachi FB-2000A) equipped with a
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