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Steven M. GEORGE
Office Phone: 303 492-3398
E-mail: Steven.George@colorado.edu
FAX: 303 492 5894
Lab: Ekeley S166, S166A, S170
Lab Phone: 303 492 8516, 303 492 6992, 303 492 7173
Group Website: George Lab
Ph.D.: University of California at Berkeley, 1983
Postdoctoral Fellow: California Institute of Technology, 1983-84
Atomic Layer Deposition, Thin Film Engineering, Nanostructure Properties and Thin Film Technological Applications
A. Introduction
Miniaturization to the nanometer scale has been one of the most important trends in science and technology over the past ten years. The chemistry to fabricate nanolayers, the engineering for nanocomposite design and the physics of nanostructure properties have created many exciting opportunities for research. These new interdisciplinary areas in nanoscience and nanotechnology supersede the more traditional disciplines and demand new paradigms for collaboration.
Our research is focusing on the fabrication, design and properties of ultrathin films and nanostructures. We are developing new surface chemistries for thin film growth, measuring thin film nanostructures and characterizing thin film properties. This research is relevant to many technological areas such as semiconductor processing, gas sensors and MEMS. Our research bridges many disciplines and we have collaborators in the Departments of Chemistry, Chemical Engineering, Mechanical Engineering and Physics on campus and many others at universities, industries and national laboratories off campus.
Many of our surface chemistry and thin film growth investigations utilize atomic layer deposition (ALD) techniques [1]. ALD is based on sequential, self-limiting surface reactions as illustrated in Figure 1. This unique growth technique can provide atomic layer control and allow conformal films to be deposited on very high aspect ratio structures. ALD methods and applications have developed rapidly over the last few years. In particular, ALD is currently on the semiconductor road map for high-k gate oxides and diffusion barriers for backend interconnects.
Figure 1: ALD based on sequential, self-limiting surface reaction
B. Surface Chemistry of Atomic Layer Deposition
Our surface chemistry investigations are examining the atomic layer deposition
(ALD) of thin films to fabricate ultrathin and conformal films.
To achieve ALD, we are developing binary reactions sequence chemistry
[1]. For example,
for Al2O3 deposition, the binary reaction:
2Al(CH3)3 + 3H2O --> Al2O3 + 6CH4
can be split into the following two surface half-reactions [2]:
A) AlOH* + Al(CH3)3 --> AlOAl(CH3)2* + CH4
B) AlCH3* + H2O --> AlOH* + CH4
where the asterisks denote the surface species.
By applying these surface reactions repetitively in an ABAB... sequence,
Al2O3 ALD is achieved with a growth rate of 1.1 Å per AB cycle [3].
This approach is general and can be applied to many important binary materials
such as SiO2 [4] and Si3N4 [5]. We have also extended the ALD method to
deposit single-element metal films. For example, the binary reaction for
tungsten deposition:
WF6 + Si2H6 --> W + 2SiHF3 + 2H2
can be split into
separate WF6 and Si2H6 half reactions to obtain W ALD [6]. Film growth
during Al2O3 and W ALD can be recorded using a variety of techniques including
the quartz crystal microbalance (QCM). QCM results for Al2O3 and W ALD
are shown in Figure 2.
Figure 2: QCM results for Al2O3 and W ALD
C. Current Research Projects
The research group is working on a number of topics including surface chemistry, thin film growth, nanostructure properties and various technological applications. The current projects are described below:
1. Fabrication and Properties of Nanocomposites
Nanocomposites are a new frontier in materials science because composites can have very different properties than their constituents. We are focusing on nanolaminate composite materials. Nanolaminates show unique physical properties when the nanolayer thickness is less than the characteristic length scale that defines the physical property. For example, thermal conductivity is reduced when the nanolayer thickness is less than the mean free path of the phonon that transfers the heat. Likewise, hardness is increased when the nanolayer thickness is less than the dislocation length for the slip plane motion that characterizes the response of the material to stress.
Our nanolaminate work has recently focused on Al2O3/ZnO [7,8] and Al2O3/ W nanolaminates. These are interesting nanolaminates for a variety of reasons. Electrically, Al2O3 is an insulator and ZnO and W are both conductors. Structurally, Al2O3 is amorphous and ZnO and W are both nanocrystalline. By depositing these nanolaminates with various compositional ratios and different nanolayer thicknesses, the electrical and structural properties of the film can be tuned over a wide range. The Al2O3/W nanolaminate may also be useful as a high temperature thermal barrier coating to protect turbine engine blades. The Al2O3/W nanolaminates may also be very useful as X-Ray Bragg mirrors in the hard X-Ray region. A cross-sectional transmission electron micrograph (TEM) of one of the Al2O3/W nanolaminates is shown in Figure 3
Figure 3: Cross-sectional transmission electron micrograph of one of the Al2O3/W nanolaminates
2. Depositing Ultrathin, Conformal Coatings on Particles
Particles have many technological applications and are used to make a variety of composite materials. Coatings on particles can be used to change the surface properties while retaining the bulk properties. Coatings can also be used to functionalize, protect, strengthen or modify the properties of the particle. Despite the importance of coating particles, very few methods exist to deposit conformal thin films on particles.
Our previous work has developed ALD techniques to coat particles with ultrathin and conformal films. Recent investigations have studied Al2O3 [9] and SiO2 [10] deposition on BN particles, BN deposition on ZrO2 particles [11], and SiO2 on BaTiO3 particles. We have also demonstrated Al2O3 deposition on low-density polyethylene particles. Many of these studies are conducted using Fourier Transform Infrared (FTIR) spectroscopy because the particles have a very high surface area that facilitates these FTIR investigations. A transmission electron micrograph (TEM) of a Al2O3 ALD coating on a BN particle is shown in Figure 4.
Figure 4: Transmission electron micrograph of a Al2O3 ALD coating on a BN particle
3. Optimizing and Understanding Metal Oxide SemiconductorGas Sensors
An interesting class of gas sensors is based on the resistivity of metal oxide semiconductor thin films. Gases adsorb onto the surface of these metal oxide semiconductor thin films and alter the film resistivity. The surface adsorption perturbs the underlying charge carriers in the film or the electron conduction between individual crystalline grains in the film. However, the exact mechanism of the effect of surface adsorption is not clearly known. Optimization of these metal oxide semiconductor gas sensors must await additional understanding.
Our gas sensor work has focused on ZnO films. We have grown ZnO films using ALD techniques and simultaneously measured their resistivity [12]. Sensitivity is extremely high for ultrathin ZnO films. The ZnO film resistivity also varies dramatically with adsorbed surface species. ZnO films terminated with Zn-CH3* species have a much higher sensitivity that ZnO films terminated with Zn-OH* species. Resistivity results for ZnO films during ZnO ALD with diethyl zinc (DEZ) and H2O are shown in Figure 5. Future research will explore the effect gas adsorption and attempt to understand and optimize the sensor sensitivity.
Figure 5: Resistivity results for ZnO films
4. Metal Atomic Layer Deposition using Hydrogen Radicals
Many binary oxide and nitride materials can be deposited using ALD techniques. In contrast, the deposition of single-element metals is less compatible with the binary reaction sequence chemistry that defines the ALD process. To develop metal ALD procedures, a binary sequence can be defined using hydrogen radicals as one of the two reactants. The hydrogen radicals serve to strip the ligand from the metal precursor according to the general overall reaction scheme: MXn + nH --> M + nHX.
Our research is developing hydrogen radical plasma sources and optimizing hydrogen radical transport for efficient metal ALD. Hydrogen recombination to form H2 is a significant hydrogen radical loss mechanism. Figure 6 shows the rapid reduction of hydrogen radicals with transmission through a tube composed of pyrex, quartz, aluminum or stainless steel. Based on these results, a reactor for metal ALD has been constructed to minimize hydrogen radical recombination. This reactor has been employed recently to deposit various metals such as Ti using TiCl4 + 4H --> Ti + 4HCl.
Figure 6: Reduction of hydrogen radicals
5. Growth of High-k Dielectrics to Replace SiO2 in Capacitors and Gates
SiO2 has been the primary insulator in silicon microelectronics for both capacitors and gate oxides. Miniaturization has progressively reduced the SiO2 thicknesses to the limit where electron tunneling through the insulator is a significant problem. Higher dielectric constant materials are needed to increase capacitance density and reduce electron tunneling. These high-k dielectrics are preferably amorphous to avoid grain boundary current leakage.
HfO2 and ZrO2 and their associated silicates and aluminates have emerged as leading high-k dielectric candidates. Our recent research has focused on Al2O3 ALD, HfO2 ALD and HfO2-silicate ALD. The current-voltage curves for Al2O3 ALD films versus thickness on n-Si(100) are shown in Figure 7 [13]. ALD processing yields excellent pin-hole free insulating films. However, a primary concern is the interfacial oxide that forms between the high-k dielectric and silicon. This interfacial oxide must be minimized to benefit from the high-k dielectric.
Figure 7: ALD films versus thickness on n-Si(100)
6. Deposition of Inorganic Films on Polymers
Polymers are pervasive because of their flexibility, light weight and low cost. Many polymer properties could be enhanced by the addition of an inorganic coating on their surface. This inorganic film could serve as a gas diffusion barrier for various packaging applications. The inorganic layer could also serve to protect the underlying polymer and give the polymer higher strength. Unfortunately, inorganic layers are difficult to deposit on polymers because the deposition of inorganics is usually performed at temperatures above the melting temperature of the polymer.
Our recent work has shown that Al2O3 ALD using Al(CH3)3 and H2O can deposit an Al2O3 film on polymers at fairly low temperatures below the polymer melting temperature. This Al2O3 coating serves as a gas diffusion barrier for the polymer. The Al2O3 film may also act as an adhesion layer for the deposition of other materials. These Al2O3 adhesion layers may be useful to facilitate the deposition of diffusion barriers on low-k dielectrics for backend interconnects. Figure 8 shows quartz crystal microbalance (QCM) and X-Ray fluorescence results for Al2O3 ALD on the low-k SiLK polymer [14].
Figure 8: Quartz crystal microbalance and X-Ray fluorescence results
7. Reliability Enhancement for Micro Electromechanical Systems (MEMS) Devices
Micro electromechanical systems (MEMS) devices are developing rapidly for a variety of applications. MEMS devices are fabricated using silicon micro-machining. The resultant MEMS devices suffer reliability problems because of mechanical wear between the moving parts and because of stiction from attractive surface forces and static charge buildup. Many of these reliability problems could be solved by applying the appropriate thin film coating to the MEMS device.
ALD is ideal for the deposition of conformal films on the high aspect ratio structures encountered in MEMS devices. Our research has demonstrated that Al2O3 ALD yields a hard and protective coating that reduces abrasive wear in MEMS devices. Nanolaminates can also provide films with enhanced hardness to minimize further mechanical wear. In addition, Al2O3/ZnO alloys have been shown to provide a variable resistivity film that can dissipate static charge and minimize stiction. The wide range of resistivities achieved by Al2O3/ZnO alloys is shown in Figure 9 [15].
Figure 9: Resistivities achieved by Al2O3/ZnO alloys on four-point and Mercury probes
8. Fabrication of Superlattices for Extended Ultraviolet (EUV) and X-Ray Mirrors
Mirrors in the extended ultraviolet (EUV) and X-Ray regions are superlattices that reflect by Bragg diffraction. An illustration of a superlattice X-Ray mirror is shown in Figure 10. High reflectivities of ~70% in the EUV at ~12 nm can be obtained using Mo/Si superlattices. These EUV mirrors are important for next generation EUV lithography. Current Mo/Si superlattices fabricated using ion beam spattering suffer from high defect densities. Lower defect density superlattices will be necessary for EUV lithography
Figure 10: Superlattice X-Ray mirror
Superlattices can be fabricated using ALD techniques. These superlattices may produce well-defined multilayers that challenge the ion beam spattering fabrication methods. We are currently setting up to deposit Mo/Si superlattices using ALD. After demonstrating the fabrication of these Mo/Si superlattices, the structural properties will be evaluated prior to EUV reflectivity measurements.
D. Equipment and Techniques Employed in Research Group
The research group is well equipped to study surface chemistry, thin film growth and thin film properties. All of the techniques and equipment described below is available within the research group.
1. FTIR Spectroscopy
We monitor the surface species during ALD using vacuum chamber designed for in situ Fourier Transform Infrared(FTIR) spectroscopy studies. High surface area particles provide sufficient surface sensitivity for transmission FTIR experiments. Vibrational spectroscopy reveals the gain and loss of surface species during the two surface half-reactions. The vibrational spectrum of the deposited material also grows with AB reaction cycles. We have recently studied Al2O3 and SiO2 ALD on BN particles [9,10], BN ALD on ZrO2 particles [11], Al2O3 ALD on low density polyethylene particles and SiO2 ALD on BaTiO3 particles.
2. Quartz Crystal Microbalance and Surface Profilometry
To monitor ALD growth in our viscous flow reactors, we employ in situ quartz crystal microbalance (QCM) measurements. The QCM has exceptional mass sensitivity and the mass changes for each half-reaction are able to unravel the stoichiometry of the surface reactions. The growth of the film is also determined by the linear mass increase versus the AB reaction cycles. The ALD film growth is also determined using ex situ surface profilometry. The profilometer obtains the film thickness by measuring the step height between the deposited film and an area that was masked using tape or photoresist.
3. Auger Electron Spectroscopy and Spectroscopic Ellipsometry
Film nucleation and growth mechanisms during ALD are evaluated in another vacuum chamber equipped for Auger electron spectroscopy studies [16]. These Auger studies have illustrated the importance of film nucleation during the initial stages of W ALD on oxide surfaces. Film thickness and film refractive indices can also be measured using spectroscopic ellipsometry. These spectroscopic ellipsometry measurements are also useful to study interfacial layers such as the interfacial SiO2 layer between high k dielectrics and silicon substrates.
4. Atomic Force Microscopy and Electrical Characterization
The film topography and surface roughness can be obtained using and ex situ atomic force microscope (AFM). The group has a scanning probe microscope equipped for AFM, conducting-AFM, and scanning thermal microscopy. The current-voltage (IV) and capacitance-voltage (CV) properties of insulating films can be characterized using a Hg-probe. This Hg probe has been used recently to study the Fowler-Nordheim tunneling behavior of Al2O3 ALD films that may have application for high k capacitors and gates [13].
5. Thin Film Conductivity
Thin film conductivity can also be measured using an ex situ 4-point probe. In addition, we have recently developed a new in situ 4-point probe to measure film conductivity during ALD [12]. This new 4-point probe can monitor ALD growth during the sequential reactant exposures. In addition, this in situ probe is being used to understand and optimize metal oxide semiconductor gas sensors.
6. X-ray Diffraction
The research group also recently received funding for an x-ray diffraction instrument. The new apparatus will be used for x-ray reflectivity (XRR) and x-ray diffraction (XRD) of thin films and nanolaminates. XRR will be useful to evaluate film thickness, film density, interfacial roughness and will be especially valuable in characterizing superlattices. XRD is important for structural characterization and crystalline alignment.
Selected Publications
1. S.M. George, A.W. Ott and J.W. Klaus, "Surface Chemistry for Atomic Layer Growth", J. Phys. Chem. 100, 13121-13131 (1996).
2. A.C. Dillon, A.W. Ott, S.M. George, and J.D. Way, "Surface Chemistry of Al2O3 Deposition Using Al(CH3)3 and H2O in a Binary Reaction Sequence", Surf. Sci. 322, 230-242 (1995).
3. A.W. Ott, J.W. Klaus, J.M. Johnson and S.M. George, "Al2O3 Thin Film Growth on Si(100) Using Binary Reaction Sequence Chemistry", Thin Solid Films 292, 135-144 (1997).
4. J.W. Klaus, A.W. Ott, J.M. Johnson and S.M. George, "Atomic Layer Controlled Growth of SiO2 Films Using Binary Reaction Sequence Chemistry", Appl. Phys. Lett. 70, 1092-1094 (1997).
5. J.W. Klaus, A.W. Ott, A.C. Dillon and S.M. George, "Atomic Layer Controlled Growth of Si3N4 Films Using Sequential Surface Reactions", Surf. Sci. 418, L14-L19 (1998).
6. J.W. Klaus, S.J. Ferro and S.M. George, "Atomic Layer Deposition of Tungsten Using Sequential Surface Chemistry with a Sacrificial Stripping Reaction", Thin Solid Films 360, 145-153 (2000).
7. J.W. Elam, Z.A. Sechrist and S.M. George, "ZnO/Al2O3 Nanolaminates Fabricated by Atomic Layer Deposition: Growth and Surface Roughness Measurements", Thin Solid Films 414, 43-55 (2002).
8. J.M. Jensen, A.B. Oelkers, R. Toivola, D.C. Johnson, J.W. Elam and S.M. George, "X-ray Reflectivity Characterization of ZnO/Al2O3 Multilayers Prepared Using Atomic Layer Deposition", Chem. Mater. 14, 2276-2282 (2002).
9. J.D. Ferguson, A.W. Weimer and S.M. George, "Atomic Layer Deposition of Ultrathin and Conformal Al2O3 Films on BN Particles", Thin Solid Films 371, 95-104 (2000).
10. J.D. Ferguson, A.W. Weimer and S.M. George, "Atomic Layer Deposition of SiO2 Films on BN Particles Using Sequential Surface Reactions", Chem. Mater. 12, 3472-3480 (2000).
11. J.D. Ferguson, A.W. Weimer and S.M. George, "Atomic Layer Deposition of Boron Nitride Using Sequential Exposures of BCl3 and NH3", Thin Solid Films 413, 16-24 (2002).
12. M. Schuisky, J.W. Elam and S.M. George, "In situ Resistivity Measurements during the Atomic Layer Deposition of ZnO and W Thin Films", Appl. Phys. Lett. 81, 180-183 (2002).
13. M.D. Groner, J.W. Elam, F.H. Fabreguette and S.M. George, "Electrical Characterization of Thin Al2O3 Films Grown by Atomic Layer Deposition on Silicon and Various Metal Substrates", Thin Solid Films 413, 186-197 (2002).
14. J.W. Elam, C.A. Wilson, M. Schuisky, Z.A. Sechrist and S.M. George, "Improved Nucleation of TiN ALD Films on SiLK Low-k Polymer Dielectric Using an Al2O3 Adhesion Layer", submitted to J. Vac. Sci. Technol. B.
15. J.W. Elam, D. Routkevitch and S.M. George, "Properties of ZnO/Al2O3 Films Deposited Using Atomic Layer Deposition Techniques", submitted to J. Electrochem. Soc.
16. J.W. Elam, C.E. Nelson, R.K. Grubbs and S.M. George, "Nucleation and Growth During Tungsten Atomic Layer Deposition on SiO2 Surfaces", Thin Solid Films 386, 41-52 (2001).
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