1. 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 Carpenter
Advisor: Dr. Dennis Kuhl
Thin Film Growth
Copper Thin Film
10 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 Thickness
2 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
𝑙 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), 𝑙 (mean free path length), to (thickness when
film is continuous)
Epitaxy
Epitaxy 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