GATE OXIDE INTEGRITY STUDY

Abstract

The effect of rapid thermal annealing (RTA) on the integrity of 120 Å gate oxides was investigated. Oxidation was performed in a conventional furnace at 850°C using a dry-wet-dry process. RTA was performed at 900, 1000 and 1100°C for 60 s in a nitrogen ambient. The ramp-up was 60°C/s and the cool-down rate was 40°C/s. There were four groups of samples - the control sample, wafers subjected to RTA immediately after gate oxidation (oxide RTA), RTA after polysilicon deposition (poly RTA) and wafers that received both RTA treatments (double RTA).

Constant current charge-to-breakdown (Qbd) measurements were used to assess the reliability of the gate oxide. The control sample had a Qbd of 51.3 C/cm2. The 900°C oxide, poly and double RTA samples had a Qbd of 50.8, 47.6 and 52.6 C/cm2 respectively. RTA at l 000°C improved the oxide reliability, with the oxide and double RTA samples having a Qbd of 57.5 and 54.7 C/cm2 respectively. This improvement was due to due to stress relaxation by a pseudo-polymorphic transformation of the oxide structure and a vertical expansion of the oxide network that relieved the built-in stress caused by oxide growth at low temperature. This strengthened the oxide network and reduced the number of strained bonds that could be broken during electrical stressing. Qbd was reduced drastically when RTA was performed at 1100°C. The oxide, poly and double RTA samples had a Qbd of 20.9, 26.9 and 20.3 C/cm2.

Capacitance-voltage (C-V) measurements showed that the degradation of the 110 0°C samples when subjected to Fowler-Nordheim stress was different from those of the control, 900 and 1000°C samples. In the former, the damage during the wear out phase was manifested mainly as charges trapped in the bulk oxide. The C-V curves of the latter group exhibited stretch-out phenomena indicative of the presence

of interface traps. Hysteresis loops observed after FN stress showed that slow states were generated during the high field stress. Conductance measurements showed that fast and slow states were present. The interface state density of the 1100°C samples was in the low range of 1011 cm-2eV-1 after an injected charge density of 10 C/cm2.This was approximately an order of magnitude lower than the control, 900 and 1000°C samples.

Viscous flow was proposed as the mechanism that led to the drastic reduction in Qbd of the 1100°C samples. The anneal time of 60 s and the reduced viscosity at 1100°C allowed the oxide to flow and attain its free volume. This phenomenon was not possible for the control, 900 and 1000°C samples. Sudden cooling at the end of the 60 s soak step 'froze-in' the atoms, resulting in broken and strained bonds in the oxide. These weakened sites increased the susceptibility of the oxide to the generation of neutral traps and defects by hydrogenous species and holes released from the anode region by energetic electrons. Charge trapping and clc trapping led to erosion of the oxide network and conductive paths emerged. Breakdown occurred the moment a conductive path linked the anode to the cathode.