Toward Selective Ultra-High-Vacuum Atomic Layer Deposition of

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Toward Selective Ultra-High-Vacuum Atomic Layer Deposition of Metal Oxides on Si(100) Don Dick,† Joshua B. Ballard,§ R. C. Longo,‡ John N. Randall,§ Kyeongjae Cho,‡ and Yves J. Chabal*,‡ †

Physics Department, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States Materials Science and Engineering Department, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States § Zyvex Laboratories, LLC, 1321 North Plano Road, Richardson, Texas 75081, United States ‡

S Supporting Information *

ABSTRACT: The selectivity of clean Si(100)-(2 × 1) surfaces fully reacted with H2O- and hydrogen-passivated Si(100)-(2 × 1) surfaces is investigated for atomic layer deposition (ALD) of TiO2, Al2O3, and HfO2 using TiCl4, TMA, or TDMA-Hf precursors with H2O, respectively, in an ultra-high-vacuum (UHV) environment. The initial reaction probability is estimated by determining the minimum exposure necessary for complete reaction of the metal precursors on both H2O-reacted and Hpassivated Si(100)-(2 × 1) surfaces and examining the first full cycle of the ALD process for each oxide. Under these UHV conditions, the first cycle selectivity is 17:1 for TiO2, 37:1 for Al2O3, and only 4:3 for HfO2. Additionally, TMA is found to react with approximately half of the Si-H sites in addition to all the Si-OH sites, while TiCl4 and TDMA-Hf gases are found to react principally with the surface −OH on H2Oreacted Si(100) surfaces with no reaction with the −H sites.

I. INTRODUCTION Reducing the dimensions of semiconductor devices to the nanoscale has required increased understanding of the reaction dynamics involved in the creation of such structures.1,2 The role of selective area atomic layer deposition (ALD), for instance, has made some notable contributions to the scaling down of devices. One technique that enables selective area ALD is hydrogen depassivation lithography1,3 (HDL), which uses a scanning tunneling microscope (STM) to remove hydrogen in selected regions of a hydrogen-passivated Si(100) surface. These regions are more chemically reactive and can be covered by thin films while the remaining H-terminated regions remain uncovered. Recently, Longo et al.2 have examined the selective growth of metal oxides (TiO2, Al2O3, and HfO2), grown by (ALD) on oxidized and hydrogen-passivated Si(100) surfaces. They have observed, using typical ALD growth conditions of >106 L per half cycle, a selectivity of 5:2 for Al2O3, 4:3 for HfO2, and no deposition on the hydrogen-passivated Si(100) surface for TiO2 (i.e., selectivity > 15:1). Further, the selectivity did not depend on the H2O exposure, only to the metal precursor being used. Since ALD conditions involve effective pressures (fluxes) that are orders of magnitude above pressures required by the sticking coefficient, it is important to determine what the initial selectivity is when the total dosages are reduced to a minimum amount necessary to react a H2O-passivated surface. In this work, using the same precursors as was used in the work by Longo et al.,2 we investigate the first cycle growth of these © XXXX American Chemical Society

oxides by ALD in an ultra-high-vacuum environment (UHVALD) on both atomically clean surfaces fully reacted with H2O (i.e., covered by 50% Si-H and 50% Si-OH) and hydrogenpassivated Si(100)-(2 × 1) surfaces. The metal oxides under investigation, TiO2, Al2O3, and HfO2, are grown by exposing the surface titanium tetrachloride (TiCl4), trimethyl aluminum (TMA), or tetrakis dimethylamido hafnium (TDMA-Hf) as the source of metal, respectively, and H2O (as the oxygen source). In order to have a meaningful comparison (i.e., identical pretreatment of surfaces) for either the H2O-reacted or the Hterminated Si(100) surfaces, the first pulse is always a H2O pulse of sufficient length to fully react the clean Si(100)-(2 × 1) surfaces. The first cycle metal oxide growth on atomically clean Si(100) is monitored by in situ Fourier transform infrared (FT-IR) spectrometry and modeled by density functional theory (DFT) calculations of infrared modes. Thus, the determination of the minimum dosage required to have full saturation of the precursors is obtained for each half cycle of ALD growth on each surface. A precise comparison of the reaction level on hydrogen-passivated Si(100)-(2 × 1) surfaces can then be made using the same exposure conditions necessary to fully react the H2O-reacted surfaces, i.e., after both H-/OH-terminated and Received: August 11, 2016 Revised: September 29, 2016

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DOI: 10.1021/acs.jpcc.6b08130 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Differential infrared absorbance spectra of (a) 50 L H2O exposure to Si(100) referenced to clean Si(100), (b−e) increasing exposures of TiCl4 [cumulative doses: (b) 50 L, (c) 120 L, (d) 200 L, (e) 260 L] referenced to (a).

reoxidized in the same manner as described in our previous work on digermane.4 The chemically oxidized Si(100) sample is then inserted into the UHV chamber, described below, and annealed to 773 K for 6 h in order to thoroughly degas the sample holder. After cooling the whole chamber including the sample holder to room temperature, the sample is briefly (15:1) selectivity. For TMA, however, they noted that, even though I

DOI: 10.1021/acs.jpcc.6b08130 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Table 1. Low and High Pressure Selectivity Ratios for Three Metal Precursors and Their Reaction Barriers Calculated by Longo et al.2 reaction barrier2 metal precursor

ALD selectivity ratio

UHV-ALD selectivity ratio

H2O/Si(100) (eV)

H/Si(100) (eV)

TMA TiCl4 TDMA-Hf

5:2 >15:1 4:3

37:1 17:1 4:3

0.05 0.26 0.72

1.36 1.50 1.45

full reaction of the surface to the metal precursor by monitoring the changes in Si-H and Si-OH concentrations. In the cases of TiCl4 and TDMA-Hf, the metal precursor reacted primarily with the Si-OH groups on the water-passivated surface while leaving the Si-H portion of the surface unreacted. However, TMA reacted with all of the Si-OH groups and, contrary to what was theoretically expected,2,24 approximately half of the Si-H portion of the surface on the H2O-Si(100) surface. In order to determine the selectivity, this same exposure was applied to a hydrogenpassivated Si(100) surface and the amount of surface modification was determined by monitoring changes in Si-H concentration. On comparing the modification of the H2Opassivated Si(100) surface to the modification of the hydrogenpassivated Si(100) surface, we determined the selectivity to be 37:1 for TMA, 17:1 for TiCl4, and no overall selectivity for TDMA-Hf. These results are consistent with the reaction barriers calculated by Longo et al.2 for both hydrogen-passivated Si(100) and to H2O-reacted Si(100), which confirms that selectivity (change in reaction barrier) is greater for TMA than TiCl4 and that both TMA and TiCl4 are much larger than TDMA-Hf.

there was a large difference in reaction barriers, the exothermic nature of its reaction with the H/Si(100) surface led to a low selectivity (5:2). In the case of TDMA-Hf, the small difference in reaction barriers accounted for a low selectivity (4:3) when doses much higher than the minimum doses to react with Si-OH are used. In contrast, the present work explores the lowest doses necessary for reaction, very far from the high exposures used in standard ALD, which can influence the selectivity ratio, such as increasing the selectivity ratio by decreasing the relative reaction with the H/Si(100) surface. As can been seen in Table 1, there was no significant change in TiCl4 and TDMA-Hf selectivity and TMA’s selectivity improved dramatically. In the case of TiCl4, no change in selectivity was to be expected because the exposure under higher pressure ALD exposure conditions to H/Si(100) showed no detectable growth.2 Under low dose conditions, one would expect there to also be no detectable reaction of TiCl4 with H/Si(100). However, there was a very small (∼3%) reaction for the first 210 L exposure and no observable reaction for the next 70 L exposure (see Figure 7b). This initial reaction may be due to TiCl4 reaction with surface defects such as dangling bonds and step edges, after which continued exposure would yield no further reaction. For TDMA-Hf, the differences in reaction barrier were not enough to modify the selectivity; the minimum dosage required to fully react with the H2O/Si(100) surface was enough to react with a large amount (∼39%) of the H/Si(100) surface. In contrast, the reaction of TMA with H/Si(100) was quite different. Even though the calculated reaction barrier was nearly as high as TiCl4, the reaction with H/Si(100)2 at high TMA pressures was significant conditions, compared to the reaction at low pressure and low doses (only ∼2% ML). Physically, at high pressures, gaseous TMA molecules can agglomerate23 enough to influence the reaction barrier when reacting with H/Si(100), due to a relay mechanism effect. This cooperative effect effectively lowers the reaction barrier, which leads to a much lower selectivity at higher pressures than it does for the low pressure doses used in the present work. The high selectivity of TMA under low exposure conditions has warranted further investigation in order to evaluate the potential of continued Al2O3 growth. We show, in the Supporting Information, that the next H2O deposition on the TMA reacted H2O/Si(100) surface requires only 7260 L in order to saturate the surface, thus completing the full ALD cycle under UHV conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08130. Calculated infrared modes for the H/Si(100) and H2O/ Si(100) surfaces reacted with TiCl4, TMA, and TDMAHf, suggested products for Si-H reaction with TMA after TMA exposure to H2O/Si(100), and H2O depostion to the TMA reacted H2O/Si(100) surface (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-972-883-5751. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Zyvex Laboratories and the National Science Foundation (Grant CHE-1300180). REFERENCES

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V. CONCLUSIONS The selectivity of the first UHV-ALD cycle of a metal precursor (TiCl4, TMA, and TDMA-Hf) paired with H2O on atomically clean and hydrogen-passivated Si(100) surfaces was investigated using a combination of FT-IR measurements and density functional theory infrared calculations. For each metal precursor, the atomically clean Si(100) surfaces were first passivated with H2O and then exposed to the metal precursor in increasing amounts in order to find the minimum required dose to achieve J

DOI: 10.1021/acs.jpcc.6b08130 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b08130 J. Phys. Chem. C XXXX, XXX, XXX−XXX