K. Overhage, Q. Tao, G. M. Jursich, C. G. Takoudis Advanced Materials Research Laboratory

1. K. Overhage, Q. Tao, G. M. Jursich, C. G. Takoudis Advanced Materials Research Laboratory University of Illinois at Chicago Atomic Layer Deposition of TiO2 on Silicon and Copper Substrates:Investigation of the Initial Growth

2. Acknowledgements REU 2010 at UIC, sponsored by the National Science Foundation and the Department of Defense EEC-NSF Grant # 0755115 CMMI-NSF Grant # 1016002

3. What is ALD?The Atomic Layer Deposition (ALD) process is used to deposit thin films layer by layer until a desired thickness is achieved. Photo from Barrier Layers Technology by Prof. Yosi Shacham-Diamand, Tel-Aviv University, 2000 Introduce one precursor, purge, then the other precursor, purge and repeat many times in the gas phase to deposit films on a substrate Useful because ALD can deposit very thin films with uniform, conformal coverage The focus of this study is deposition of TiO2

4. STEP 4 Substrate with active sites Purge Source A (TDEAT) STEP 1 Source B (H2O) Chemisorption of source A and saturation mechanism. STEP 3 STEP 2 Chemical reaction between source A and source B and saturation mechanism Purge Reaction Mechanism of typical ALD cycle ALD is a surface-saturation reaction that deposits each monolayer of film, allowing for precise thickness control.

5. Example application of ALDAn example application of an ALD process is the construction of the copper barrier layer in a chip. Diagram from http://www.tms.org/pubs/journals/JOM/9903/Frear-9903.fig.5.lg.gif • The copper barrier layer prevents Cu from reacting with other chip materials, particularly silicon

6. Objectives • Study TiO2 deposition on silicon and copper with different surface chemistries, with the goal of achieving selective deposition • Temperature-independent window • Early growth / nucleation period • Late growth / constant growth region • Findings can be used in future work to further promote selective deposition of TiO2 on Silicon

7. SubstratesDeposition was performed on substrates with different surface chemistries. • Silicon with native oxide (approximately 1.5 nm-thick) • Silicon with reduced oxide (less than 1 nm-thick, 2% HF etching treatment) • Copper with native oxide (approximately 2 nm-thick) ALD is surface reaction driven – therefore, the surface chemistry of the substrate is critical. Careful preparation steps were taken to properly prepare the substrates.

8. SE TheorySpectral Ellipsometry – measures film thickness • Optical test, measures film thickness • Light source shines on film, detector measures reflected light • Computer models calculate thickness based on reflective index of material

9. Temperature-independent window Silicon with native oxide: Slope 1.2 A / cycle TiO2 deposition on silicon is independent of temperature between 150 and 200 °C.

10. Late GrowthDeposition from 50 to 150 cycles on silicon with native oxide Once the early growth phase is complete, TiO2 deposition proceeds at 1.3 Å / cycle. This is in agreement with current literature values.

11. Early GrowthDeposition from 0 to 50 cycles on the two kinds of silicon surfaces Silicon with < 1 nm oxide Silicon with 1.5 nm native oxide Silicon with reduced oxide: Growth rate 1.0 Å / cycle Silicon with native oxide: Growth rate 1.2 Å / cycle Here we see a negligible nucleation time on both substrate surfaces. Growth rates are equal to the slope of the best fit line.

12. XPS TheoryX-ray Photoelectron Spectroscopy – used to analyze film composition • X-rays penetrate sample surface, knocking out core electrons of the film atoms • Detector records energy signal from electrons emitted • Each element has signature peak pattern Stronger signal = XPS detects more atoms Intensity (Counts) Binding Energy (eV) Sample Spectrum

13. Early GrowthXPS results – TiO2 signal on silicon substrate 4.2 nm 2.5 nm 2.3 nm 0.8 nm The TiO2 signal gets stronger as the number of cycles increases, indicating growth of the TiO2 film on the silicon substrate.

14. CopperXPS results – TiO2 signal on copper substrate 4.2 nm (Si) Thickness can’t be determined by SE, should be less than 2 monolayers (<0.3 nm) 0.3 nm The TiO2 signal is weak, but present after 15 cycles and it does not increase by 20 cycles. The effective nucleation time of TiO2 on copper is about 15 cycles.

15. Discussion • No nucleation period on silicon • Considerably delayed formation of TiO2 on copper • Selective deposition is achieved at the conditions used in this study • Nucleation period enables selective growth, for thicknesses up to 2.5 nm - could satisfy the requirement for copper barrier application1 1International Technology Roadmap for Semiconductors (Semiconductor Industry Association, San Jose, CA, 2001).

16. Future Work • Before I leave … • SEM (scanning electron microscopy) will be applied to probe the early TiO2 film nucleation on both silicon and copper substrates from 5 to 30 cycles of ALD • Later work … • Other surface treatments are still in progress to promote the growing selectivity, such as complete removal of native oxide without immediate reoxidation

17. Conclusions • TiO2 nucleation time on silicon substrate is negligible, and the initial growth rate is 1.0 to 1.2 Å / cycle, depending on surface chemistry • Temperature-independent window for TiO2 deposition on silicon is 150 to 200 °C • Nucleation time on copper substrate is found to be ~ 15 - 20 cycles • The potential to achieve greater selective deposition of TiO2 with further research appears to be high Questions?