薄膜制备与表面分析.pptVIP

概论 ?? 真空技术原理 真空的获得 * 概论 ?? 真空技术原理 真空的获得 Pump Down Typically follows a well-defined sequence according to the types of pumps on the vacuum system For UHV systems, typically requires a few hours to reach a medium vacuum after a vent to air Bake Out Heat the chamber to temperatures between 100oC and 200oC for 1 - 2 days. Rapidly remove adsorbed gases from the chamber walls at high temperatures in order to lower the outgassing rates at room temperature. Generally it takes another day to cool and recover base pressure. * 概论 ?? 真空技术原理 真空的测量 Heat filament with a constant current. Measure filament temperature with thermocouple. Gas molecules collide with and cool the filament. Voltage increases to keep filament at constant current. Atm to 10-4 Torr Fast, simple, inexpensive. Thermocouple Gauge * 概论 ?? 真空技术原理 真空的测量 Two identical heated filaments; one sealed at HV, one exposed to system. Current flows through Wheatstone bridge circuit. Pressure difference indicated by meter (non-linear). Atm to 10-4 torr. Simple, reliable, inexpensive. Pirani Gauge * 概论 ?? 真空技术原理 真空的测量 Heated filament produces electrons via thermionic emission. Electrons are accelerated towards anode grid. Many electrons pass through the grid and create positive ions from collisions with gas molecules. Ions are accelerated to collector wire. Measure the current between anode and collector. Operate at 10-4 to 10-11 Torr Sensitive, high accuracy, widely used. Ion Gauge (Bayard-Alpert) * 概论 ?? 真空技术原理 真空的测量 Quadrupole mass spectrometer - RGA (residual gas analyzer) 10-4 to 10-14 torr Total pressure mode integrates all ion intensities Partial pressure mode indicates residual vacuum composition Highly accurate, precise Complex, expensive. Mass Spectrometer * 概论 ?? 真空技术原理 真空的测量 Typical UHV mass spectrum of background gases after bakeout. H2O CO2 N2 H2 Leak rates Flanges Virtual leaks Outgassing rates Contaminants Materials Samples Pumping Rates Type of pumps Type of gases being pumped * 概论 ?? 真空技术