88, Conl-97]. A lot of factors interfere in fact in these complex processes, such as gas annealing durations, inducing thus great confusion in the spirit of engineers in charge of these processes! Without precise insights about the physical and technological mechanisms involved, the process of optimization is nothing but a succession of "try and test", and this approach may often result in a series of expensive and disappointing adventures. 6.3.2. FURNACE ANNEALS AND RAPID ANNEALS (RTA) It is therefore known that for the classic oven anneals, the main effect is connected to the maximum temperature during the process. A more recent method of anneal, called "Rapid Thermal Anneal" (RTA), in which the heating is obtained by the exposure of the wafer to powerful halogen lamps. The duration of the exposure is reduced to some seconds or tens of seconds, against minutes or tens of minutes in a classic oven. Recent results show an analogous correlation between the temperature of the rapid anneal RTA and the variation of the threshold voltage Vot [Flam-95b]. The figure 62 shows the VT for NMOS transistors in the central part of wafers annealed to different temperatures by using two methods: (i) Classical oven anneal in nitrogen during 30 minutes (ii) Rapid Thermal Anneal during 10 seconds As shows figure 62, a threshold of radiation sensitivity appears between 975°C and 1025°C for the RTA anneal. The effect of threshold appears also for the classic anneals oven, at "low" temperature, as it has been found previously. III-72 Furnace Anneal Temperature (C) Figure 62. VT after 1 Mrad(SiO2), according to the temperature anneal for a NMOS transistor [Flam-95b]. SOME THEORIES ABOUT THE PROCESS ENGINEERING EMPIRICAL CURVES 6.4.1. INTERPRETATION BY THE FLOW OF SIO2 Results show that the gate oxide hardening is a sensitive function of anneals temperatures. However, the comparison between the RTA and oven anneals shows that the temperature is not the unique parameter of this sensitivity. The duration is a second very influential para