(Digital Presentation) A Novel Method For Molybdenum Etching Using a Combination of Surface Oxidation By Ozone-Gas-Bake and Wet Selective Removal for Future Semiconductor Devices

1. Introduction

Alternative metals are being researched to mitigate the resistivity of the metal wiring at 3 nm technology node and beyond. A performance index or figure of merit (FOM) calculated by multiplying the resistivity and the electron mean free path of the bulk metal has been defined to allow the selection of candidate metals to reduce the resistance of fine wiring. Mo (6.0) is a good candidate compared to the conventional metals such as Cu (6.7) and W (8.2) [1-3]. Commodity wet chemistries, e.g. SC-1 or mixed acids like, Phosphoric:Acetic:Nitric-acid (PAN) are not suitable for wet recess etch of Mo since they increase its surface roughness too much. The model for the increase in roughness has been reported to be due to the presence of grain boundaries on the Mo surface, where preferential "oxidation-dissolution" reactions occur simultaneously in the chemical solution [4,5]. To minimize the roughness, a new cyclic process of dry oxidation and wet etching is developed so that oxidation and etching reactions are separated. In the oxidation step, a homogeneous oxide layer is formed by baking in ozone (O₃) gas, then it is selectively etched by subsequent wet chemistry which doesn't contain any oxidant [6]. In this paper, we report the development of this molybdenum recess process with a higher baking temperature to increase the recess depths, targeting application of buried power rail in logic and word line recess in 3D NAND memory.

2. Experimental

Sample: 50 nm Mo films were deposited by Plasma Vapor Deposition on a 100 nm SiO₂ film. After metal deposition, the wafers were annealed for 20 minutes at 420°C in pure N₂ gas.

Etching process: oxidation process is performed in an O₃ gas chamber on a single wafer cleaning tool. Bake plate in the O₃ gas chamber was heated to 180-290°C. O₃ concentration was 100 g/m³ and gas flow rate was 18 L/min. The subsequent wet etch process was performed in wet process chamber on the same tool. 29% NH₄OH was mixed with ultrapure water in a 1:8 or 1:100 volume ratio at RT or 70°C.

Characterization: the recess amounts of metallic Mo and MoOx thicknesses were measured by sheet resistance measurements and mass measurements of 300 mm wafers. Roughness was measured by Atomic Force Microscope, XSEM was used to validate the above. The polycrystallinity of MoOx was analyzed by Grazing Incidence X-ray Diffraction.

3. Results/Discussion

Mo thickness decreases as the number of oxidation and etching cycles increases, and the recess amount per cycle was 6 nm at 290°C (Fig. 1). The recess amount per 3 cycles increased as bake temperature increases (Fig. 2). Rq increased from 0.4 nm to 2.8 nm at bake temperatures ≥ 250°C (Fig. 3). XSEM showed that the cause of the surface roughness is residual MoOx (Fig. 4). XRD showed that the MoOx changes significantly at bake temperatures ≥ 250°C since other than the metallic crystalline Mo peak at 2θ = 40° were detected, which suggests that polycrystalline MoOx is formed at high temperature (Fig. 5). The formation of a polycrystalline MoOx could be the cause of the surface roughness because these MoOx are tough to dissolve and therefore the surface would not be recessed evenly. On the other hand, the residual MoOx was etched by NH₄OH in a 1:8 ratio at 70°C and Rq was reduced to 0.38 nm after a cyclic process using a 290°C bake with O₃ (Fig. 3, 4).

4. Conclusion

A novel etching method using a combination of "ozone-gas-bake" and "wet selective removal" was developed to overcome the roughness increase during etching of Mo observed with commodity wet chemistries. This method demonstrates good controllability of Mo etching amount by changing the number of cycles without roughening the surface. The surface roughness increased at bake temperatures ≥ 250°C due to the formation of polycrystalline MoOx that could not be removed with the conventional NH₄OH solution, but this was mitigated by increasing the temperature of the subsequent NH₄OH oxide dissolution step. This new process is expected to be used to recess Mo in logic and memory devices for next generation applications.

References

[1] D. Gall, J. Appl. Phys. 127, 050901. 2020.

[2] D. Gall, J. Appl. Phys. 119, 085101. 2016.

[3] C. Adelmann, et al., Proc. IEEE IITC 2018, pp.: 154-156

[4] A. Zambova, et al., J. Electrochem. Soc., 139, 9, 2470. 1992.

[5] A. Pacco, et al., Solid State Phenomena, 2021, 314, pp. 295-301.

[6] A.Pacco, et al., IITC 2021.

Figure 1