Modeling Facet Formation for Non-Planar Thermal Oxidation of Silicon Victor Moroz Avant! Corporation, Fremont, USA
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Charles Dachs and Alex Schoonveld Philips Research, Leuven, Belgium
[email protected] [email protected]
Abstract A model for simulating 2D and 3D oxide shape for oxidation of non-planar silicon surfaces is presented. The model is based on an idea that the stress dependence of the reaction rate can be anisotropic with respect to the crystallographic orientation of silicon surface. It describes facet formation, observed in liner oxide at the corners of STI (Shallow Trench Isolation). It is also necessary to describe the shape of top STI corner which is critical in determining the MOSFET leakage.
1. Introduction The shape of the thermal oxide grown over a silicon trench is affecting device performance in several aspects. The shape of the silicon/oxide interface at the upper (convex) corner of STI is critical for the MOSFET source-to-drain leakage current. Sharp corners are known to increase the leakage due to the earlier channel inversion, in extreme cases even inducing a second hump on the Id(Vgs) curves. Rounding of the top STI corner by the thermal oxidation is routinely used to suppress the leakage.
The shape of the silicon/oxide interface at the lower (concave) corner of the trench affects electric field distribution there. Any nonuniformities in oxide thickness increase the field and degrade the performance of deep-trench capacitors in DRAM. A capability to simulate the exact shape of the thermal oxide and silicon/oxide interface is important for successful design and optimization of the process flow. Fortunately, TEM is very instrumental in monitoring shape of the 2D cross-sections of the semiconductor structures. However, there is no comparable measurement methodology developed for monitoring the 3D shape. 3D simulation based on the models, verified and calibrated by 2D measurements can shed light on the 3D effects and help analyse the devices with inherent 3D behaviour. Figures 1 and 2 show typical TEM pictures before and after the trench oxidation, respectively. Oxidation of intrinsic (100) silicon wafers with trenches, aligned along crystallographic orientation was performed for 20 minutes at 1100oC in dry oxygen, diluted by nitrogen to 0.1 atmosphere. Figure 3 shows a close-up view of the lower trench corner from Figure 2. A distinctive facet is observed at the silicon/oxide interface.
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Figure 3. Close-up view of the lower trench corner from Figure 2. Figure 1. TEM of silicon trench before oxidation. Now, let’s see if the facet observed in TEM pictures on Figures 2 and 3 can be simulated. Simulation is performed using 1D/2D/3D process simulator TaurusProcess [1].
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Figure 4. Simulation with stressindependent oxygen diffusivity and reaction rate.
Figure 2. TEM of silicon trench after oxidation.
Figure 4 shows the simulated oxide shape using stress-independent oxygen diffusivity and reaction rate. The scales on Figures 3 and 4 (as well as 5 and 7) are identical for an easier comparison. Contrary to the TEM, the simulated oxide thickness on Figure 4 is larger at the corner, which has orientation, close to (111), than at
the bottom (100) and side (110) of the trench. This is due to the fact that silicon surface with orientation (111) oxidizes faster than either (100) or (110) [2]. Figure 5 shows the oxide shape, simulated using the de facto standard model for the stress-dependent oxygen diffusivity and reaction rate [3].
2. Anisotropic stress dependence of the reaction rate The (111) facet, observed on the silicon/oxide interface in TEMs, suggests a strong anisotropic stress dependence of the oxidation reaction rate. Figure 6 depicts orientational dependence of the reaction rate for the stress-free silicon surface as a solid line. The dashed line shows the reaction rate reduced in half due to the stress, according to the isotropic model from [3]. The dotted line depicts the reaction, considerably suppressed at orientation (111).
Reaction Rate / (100) Reaction Rate
2 Stress-free Isotropic Anisotropic
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Figure 5. Simulation with conventional stress-dependent oxygen diffusivity and reaction rate. Volume expansion of the growing oxide generates high compressive stress at the concave corner. The stress suppresses both oxygen diffusivity and reaction rate. For this particular initial trench shape and oxidation conditions, the enhanced orientationdependent oxidation and stress suppression nearly compensate each other, resulting in a fairly uniform oxide thickness along the curved trench surface. Apparently, the conventional model does not come close to describing the observed facet formation.
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Figure 6. Orientational dependence of the oxidation reaction rate. The anisotropic stress dependence of the reaction rate, shown in Figure 6, leads to the simulated oxide shape, shown in Figure 7. The initial trench surface and oxide shape, obtained by the conventional model are also shown for reference. The simulated oxide shape now has a distinctive facet and resembles the TEM, shown in Figure 3.
Figure 7. Simulation of the lower trench corner with anisotropic model. The Figures 8 and 9 demonstrate that the anisotropic reaction rate helps to obtain the observed shape of the upper STI corner. The conventional model, depicted with the dashed lines, fails to accurately describe the corner shape.
Figure 9. Simulation of the upper trench corner with anisotropic model.
3. Conclusions TEMs exhibit geometrical features of the non-planar oxidation that can not be explained by the conventional oxidation models. The observations suggest a strong anisotropic stress dependence of the reaction rate. The presented model is able to describe the experimental observations. Atomistic simulations are necessary to reveal the microscopic mechanisms of the apparent anisotropic stress dependence and gain a better understanding of the observed phenomena.
4. References
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Figure 8. Close-up view of the upper trench corner from Figure 2.
[1] Taurus-Process Users Manual. Avant! Corporation, 2000. [2] B.E. Deal and A.S. Grove, “General relationship for the thermal oxidation of silicon”, J. Appl. Phys., 36(12), pp. 33703378, 1965. [3] D-B. Kao, “Two Dimensional Oxidation Effects in Silicon – Experiments and Theory”, Ph.D. thesis, Stanford University, June 1986.
