Characterization of Oxide Etching and Wafer Cleaning using Vapor-Phase Anhydrous HF and Ozone
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Mat. Res. Soc. Symp. Proc. Vol. 470 01997 Materials Research Society
II. EXPERIMENTAL Vapor Phase Cleaning Module The cleaning process is carried out in a STEAG-AST Vapor Phase Cleaning module (VPC) integrated in a state-of-the-art cluster tool also consisting of a STEAG-AST Rapid Thermal Oxidation module (RTO). These modules are described elsewhere [5]. All parts of the reactor are kept at a temperature of 40'C. The VPC chamber wall is heated by an integrated, temperature controlled water circuit, whereas the quartz-window is heated by a hot air blower. Anhydrous HF (AHF) (99.9%), contained in a nickel cylinder at 40'C, is fed at cylinder pressure through a heated gas line into a temperature controlled mass flow controller. The nitrogen bubbler, filled with methanol is kept at 20'C. All tubes in contact with either methanol or HF are heated to prevent condensation. Ozone at a concentration of about 5 % in oxygen is delivered from an ozone generator via a mass flow controller. The wafer is heated through the quartz window with linear IR-Lamps. The temperature of the wafer is measured by an integrated thermocouple (PinTC TM). The UV lamps are low pressure mercury vapor lamps with a total power of 120 Watt. They are alternating with the IR lamps and can be independently controlled in a process recipe. Caused by using high silica glass as lamp envelope, the maximum in radiation is at the 254 nm wavelength. Etching of thermally grown Silicon dioxide The etch rate of anhydrous HF is controlled by etching 100 nm of thermal oxide grown in a horizontal furnace on 150 mm diameter silicon wafers, boron doped in the range of 4 - 6 Qccm with (100) orientation. The etch rate is controlled by measuring the thickness using a Plasmos ellipsometer with autofocus function. The etching of Si0 2 using AHF in the VPC module can be divided into four steps, as described in Tab. I. Tab. I: Process conditions for silicon etching using AHF. Step 1 2 3 4
Pressure 200 mbar 50 mbar 50 mbar rough valve open
Temperature 40 0C 40 0C 40 0C 40'C
Gas Methanol/Nitrogen no AHF Nitrogen
Remark Wetting of Silicon Surface Pump down to Process Pressure Etching of Silicon Surface Desorption of Etch Products
Cleaning sequences using AHF and Ozone To improve the cleaning sequence ozone /UV steps are performed in the same reactor by introducing ozone at a pressure of 700 mbar and a temperature of 200'C after the desorption step. The ozone step is performed for 60 s at 700 mbar and at an ozone concentration of 5% in oxygen. During this ozone step the wafer is additionally illuminated with UV light.
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Electrical characterization
For electrical characterization of thin gate oxide layers, MOS-capacitors are formed on 4-6 Qcm, p-type (100) Si-wafers. The silicon wafers are cleaned using standard Sulfuric Peroxide Mixture (SPM). Afterwards the remaining thin chemical oxide is etched in the VPC module using different AHF/methanol sequences. Standard HF-dipped wafers are used for reference. After growth of a 4.0 nm thick dielectric film in dry oxyge
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