Pregate Oxidation Treatment using Radio Frequency Activated Nitrogen in a Rapid Thermal Reactor

Abstract

Changes in integrated circuit technology generations are marked by reductions in the minimum feature size of the semiconductor devices. Current state-of-the-art production involves feature size from 0.35 to 0.25 m. One particularly important parameter for scaled metal oxide semiconductor field effect transistors (MOSFETs) is the thickness of the gate dielectrics, which also has to be suitably reduced to maintain proper operation of the devices. Currently, thickness in the range of 6 to 10 nm is used. As the devices are scaled below 0.25 m, it becomes advantageous to change the doping of the polysilicon gate electrode from n-type to p-type for the p-channel (PMOS) devices and to use even thinner gate dielectrics. However, boron, the most conventional and suitable p-type dopant, has a well-known tendency to penetrate thin oxides of silicon at elevated temperature. This penetration degrades the oxide quality and causes threshold voltage instability, while the controlled formation of a uniform thin dielectric consisting of only a few tenths of monolayers of atoms or molecules greatly challenges the capability of the oxidation process. The present work aims to tackle these two issues: the controlled formations of uniform thin dielectrics and the reduction or elimination of boron penetration through the resulting films.Experimental Phosphorus doped, 1-10 ⍀ cm, (100) oriented 100 mm silicon wafers were used as the starting substrates. Prior to being loaded into the rapid thermal reactor, the wafers were first cleaned for 10 min in a 10:1 H 2 SO 4 :H 2 O 2 solution at 120ЊC and then dipped for 1 min in a 100:1 HF solution to remove the chemical oxide. The atmospheric pressure reactor, a schematic diagram of which is shown in Fig. 1, consists of a small rectangular quartz wafer chamber with a tiny inlet for nitrogen at one end and an open wafer loading port at the opposite end. A proprietary SiC-coated graphite heater lines the inside of the chamber and a set of radio frequency (RF) coils is on the outside. Induction heating is accomplished at 13.56 MHz with the process temperature controlled by using the readings fed back from a single wavelength pyrometer "looking" at the back (rough) side of the wafer. Typically, the nitrogen flow rate was set at 6 L/min. After having been treated in the reactor, the wafers were loaded into a conventional horizontal furnace for dry oxidation at 850ЊC. Some of the wafers were diced and dipped in a 6:1 buffered HF oxide etch (BOE) solution for etch rate studies.The thickness of the dielectric after the dry oxidation was measured using a constant angle ellipsometer, while fixing the refractive index at the thermal oxide value of 1.45. The amount and the distribution of nitrogen incorporated in the dielectrics were analyzed using X-ray photoelectron spectroscopy (XPS). ResultsThe dependence of the average dielectric thickness on dry oxidation time after various rapid thermal treatments is shown in Fig. 2. For a given oxidation time, a range of thickness values were measured. The largest va...