1 K. Overhage, Q. Tao, G. M. Jursich, C. G. Takoudis Advanced Materials Research Laboratory University of Illinois at Chicago

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

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.  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 TiO 2 P hoto from Barrier Layers Technology by Prof. Yosi Shacham-Diamand, Tel-Aviv University, 2000

4 Substrate with active sites Chemisorption of source A and saturation mechanism. Purge Chemical reaction between source A and source B and saturation mechanism Purge Source B (H2O) Source A (TDEAT) STEP 1 STEP 2 STEP 3 STEP 4 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 ALD An example application of an ALD process is the construction of the copper barrier layer in a chip.  The copper barrier layer prevents Cu from reacting with other chip materials, particularly silicon Diagram from

6 Objectives Study TiO 2 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 TiO 2 on Silicon

7 Substrates Deposition 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.

 Optical test, measures film thickness  Light source shines on film, detector measures reflected light  Computer models calculate thickness based on reflective index of material SE Theory Spectral Ellipsometry – measures film thickness

9 Temperature-independent window TiO 2 deposition on silicon is independent of temperature between 150 and 200 °C. Silicon with native oxide: Slope 1.2 A / cycle

10 Late Growth Deposition from 50 to 150 cycles on silicon with native oxide Once the early growth phase is complete, TiO 2 deposition proceeds at 1.3 Å / cycle. This is in agreement with current literature values.

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

12 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 Intensity (Counts) Binding Energy (eV) Sample Spectrum Stronger signal = XPS detects more atoms XPS Theory X-ray Photoelectron Spectroscopy – used to analyze film composition

13 Early Growth XPS results – TiO 2 signal on silicon substrate The TiO 2 signal gets stronger as the number of cycles increases, indicating growth of the TiO 2 film on the silicon substrate. 4.2 nm 2.5 nm 2.3 nm 0.8 nm

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

15 Discussion No nucleation period on silicon Considerably delayed formation of TiO 2 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 application 1 1 International 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 TiO 2 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 TiO 2 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 TiO 2 deposition on silicon is 150 to 200 °C Nucleation time on copper substrate is found to be ~ cycles The potential to achieve greater selective deposition of TiO 2 with further research appears to be high Questions?