When the rays constructively interfere the pathlength difference is equivalent to an integer number of wavelengths, represented by nλ.

Copper thin films were grown by thermal evaporation on Si (100) substrates at 10E-5 Torr following the procedure of Longiaru, Krastev, and Tobin. 1,2 In situ resistance measurements were taken, and θ-2θ Bragg diffraction was used to examine the epitaxy normal to the surface. The films ranged from 65 nm to 212 nm with HF etching times between 2 and 10 minutes. The x-ray diffraction results failed to show epitaxy for these copper films.

Abstract

Laura CarpenterAdvisor: Dr. Dennis Kuhl

Thin Film Growth

Copper Thin Film10 min. HF etch, 212 nm

X-Ray Diffraction

Tel-X-Ometer X-Ray Apparatus

•X-Ray source shines on sample•Geiger Muller tube placed 2θ from incident beam•Sample moves by θ, while GM tube moves by 2θ in the CCW direction•Scanning parameters: angle increment and time per increment

The Bragg Angle•Lattice planes are spaced by d •θ, the angle between the beam and the lattice plane•Pathlength difference of reflected rays: 2dsinθ.

X-Ray Scans

Au (111) Film, 150 nm

Conclusions•Bulk copper - proportion of (111) peak to (200) peak is 1.7:1. The tabulated ratio for powdered copper is 2.17:1.4 Thus, the bulk copper is fairly polycrystalline.•Au (111) - (111) peak indicates epitaxy with the (111) orientation.•Copper thin films - no Cu (200) peaks and thus no signs of epitaxy. The XRD results indicate that the films are closer to the atomic structure of powders. •The low bulk conductivities and mean free paths are likely a result of the disorder of the films. Obstructions are shortening the electron’s path. •It is supposed that a low HF concentration and poor vaccuum conditions contributed to the lack of epitaxy.

Effective Conductivity versus Thickness2 min. HF etch, 66 nm

•Apparatus: Denton Thermal Evaporator•99.9999% copper wire (0.25 mm diameter) •Si (100) substrate prep: 15 min. cleaning in ultrasonic bath with acetone and methanol, 2% HF etch•Four-wire dc measurements •Quartz crystal thickness monitor

3.1 nm

σ0 14.9 (μΩ-m)-1

t0 37.2 nm

References[1] Minsu Longiaru, E. T. Krastev, and R. G. Tobin, “Epitaxy above 10-5 Torr: A student’s introduction to thin film growth and characterization,” J. Vac. Sci. Technol. 14 (5), 2875, (1996).[2] E. T. Krastev, L. D. Voice, and R. G. Tobin, “Surface morphology and electric conductivity of epitaxial Cu (100) films grown on H-terminated Si (100),” J. Appl. Phys. 79 (9), 6865, (1996).[3] NDT Resource Center, “Conductivity and Resistivity Values for Copper & Alloys,” http://www.ndt- ed.org/GeneralResources/MaterialProperties/ET/Conductivity_Copper.pdf.[4] J. Yang, C. Wang, K. Tao, and Y. Fan, J. Vac. Sci. Technol. A 13, 481, (1995).

L (length of film), W (width of film), G (conductivity), t (thickness), σ0 (bulk conductivity), (mean free path length), to (thickness when film is continuous)

EpitaxyEpitaxy means that the deposited crystal takes on the orientation of the crystal substrate. A highly oriented epitaxial copper film grown on Si (100) can be identified by the presence of a strong Cu (200) peak in x-ray diffraction. It is important to know the epitaxial nature of the copper films in order to do surface science. Copper is a good candidate for surface adsorbate studies which was the motivation of this project.

The Fuchs-Sondheimer Model takes into account the surface scattering that is present during growth. The bulk conductivities are low compared to standards, 58 (μΩ-m)-1. 3 The mean free path is lower than that reported by Krastev, Voice, and Tobin, 15 nm. 2

Effective Conductivity

Result

The bulk copper and Au (111) were scanned as standards.

Investigating the Epitaxial Nature of Copper Thin Films

Bulk Copper