Deposit and etchback approach for ultrathin Al₂O₃ films with low pinhole density using atomic layer deposition and atomic layer etching

­Ultrathin Al₂O₃ atomic layer deposition (ALD) films with low pinhole density were fabricated using a deposit and etchback approach. This strategy was able to avoid the pinholes that originated during nonuniform nucleation of Al₂O₃ ALD films. In this method, an Al₂O₃ ALD film was deposited to a thickness greater than the desired thickness to reduce the number of pinholes and form a more continuous Al₂O₃ ALD film. Subsequently, the Al₂O₃ ALD film was etched back to a smaller thickness using thermal Al₂O₃ atomic layer etching (ALE). The number of pinholes in the resulting Al₂O₃ ALD film was measured by the percentage yield of metal-insulator-metal (MIM) capacitors based on an Ag/Al₂O₃/Al structure that did not have an electrical short. The device yield was improved using the deposition and etchback approach. For example, using device areas of 0.01 mm², Al₂O₃ ALD films that were grown to 5 nm in the MIM capacitor gave a yield of 30%–40%. In contrast, Al₂O₃ ALD films that were grown to 24 nm and then etched back to 5 nm to form the MIM capacitor provided a yield of 65%–75%. This increase in yield of approximately 100% indicates that the deposit and etchback approach can improve the yield of MIM devices based on ultrathin Al₂O₃ ALD films. Although this method has been previously suggested to improve the quality of ultrathin films, this report is believed to be the first demonstrated application of using the deposit and etchback approach for device fabrication. Additional experiments revealed that a portion of the yield improvement can be attributed to the fluorination of the Al₂O₃ ALD films that produced a volume expansion when forming AlF₃. This expansion may produce a compressive stress that helps to close the pinholes. The dielectric constant was also measured for Al₂O₃ ALD films versus Al₂O₃ film thickness. The dielectric constant was the same for as-deposited Al₂O₃ ALD films and Al₂O₃ ALD films that were first grown to 24 nm and then etched back to smaller thicknesses. This agreement indicates that the dielectric constant can be understood in terms of a series capacitor model and that Al₂O₃ ALE does not affect the electrical properties of the Al₂O₃ films.