6.1.4 Oxidation with LOCOS

When performing LOCOS steps for thermal oxidation growth, a bird's beak effect is commonplace. As the oxide grows, the nitride mask, which is meant to block the oxide from growing everywhere, is slightly bent due to stress caused by the oxide pushing the nitride as it grows.

Figure 6.4: Geometry of the bird's beak occurrence during LOCOS processing. $H_{bb}$ and $L_{bb}$ describe the maximum height and length of the nitride after oxidation, respectively.

The geometry of the bird's beak, depicted in Figure 6.4 depends on several physical factors [190]:

The thickness of the nitride mask plays an important role in determining the size of the LOCOS bird's beak. From a simple physical point of view, the force required to lift up a beam by a certain distance is proportional to the third power of the beam thickness. Even though the oxidation mechanism is more complex, the basic concept can still be applied. It is evident that thicker nitride layers are less prone to bending due to their increased stiffness, which leads to a shorter bird's beak.

The thickness of the pad oxide also has an influence on the length of the bird's beak, but does not seem to affect the height in a meaningful way. The effects of the nitride and pad oxide thicknesses on the bird's beak geometry, when 600nm of oxide is grown thermally at 1000 $^{\textrm {o}}$C in a vapor environment, is shown in Figure 6.5. Based on this data, a topographical simulation can be performed, as shown in Figure 6.6.

Figure 6.5: Bird's beak length and height dependences on nitride and pad oxide thicknesses from[190]. The field oxide is simulated to grow at $1000^{\textrm {o}}$C for a thickness of approximately 600nm.

The length of the birds beak depends on the silicon crystal orientation, mainly due to the difference in the amount of silicon available for bonding at the (111) surface compared to the (100) surface. The ratio of $L_{bb}$ to the oxide thickness decreases as the field oxide thickness increases until $L_{bb}$ reaches a saturation length.

When thermal oxidation is performed at high temperatures, less nitride lifting, and therefore a shorter and lower bird's beak is seen. This is due to the increased oxidation rate compared to the diffusion rate. Since the diffusion of oxidant under the nitride layer is the main reason for the bird's beak phenomenon, less diffusion exposure results in decreased bird's beak effects.

Figure 6.6: Thermal oxidation with the bird's beak effect. The field oxide is simulated to grow on (100) silicon at 1000 $^{\textrm {o}}$C in a wet environment for 2 hours, resulting in a field oxide thickness of approximately 600nm. The oxide thickness is 15nm and the nitride thickness is (a)-(b) 200nm and (c)-(d) 100nm.

(a) Initial geometry ( $t_{Si_3N_4}=200nm$).	(b) Final geometry ( $t_{Si_3N_4}=200nm$).
(c) Initial geometry ( $t_{Si_3N_4}=100nm$).	(d) Final geometry ( $t_{Si_3N_4}=100nm$).

Figure 6.6 depicts a topographical simulation of a nitride mask lifting as thermal oxidation progresses. The geometry of the lifting nitride is taken from the results in Figure 6.5 for a 200nm nitride mask layer and a 15nm of pad oxide. The topography moves as a results of silicon oxidation at 1000 $^{\textrm {o}}$C for 2 hours, resulting in an expected oxide thickness of approximately 600nm.