Laboratory Experiment pubs.acs.org/jchemeduc
Simple Chemical Vapor Deposition Experiment Henrik Pedersen* Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden S Supporting Information *
ABSTRACT: Chemical vapor deposition (CVD) is a process commonly used for the synthesis of thin films for several important technological applications, for example, microelectronics, hard coatings, and smart windows. Unfortunately, the complexity and prohibitive cost of CVD equipment makes it seldom available for undergraduate chemistry students. Here, a simple CVD experiment designed to give hands-on experience with this techniqueusing common chemical laboratory equipmentis outlined. The experiment is suitable for an upperlevel or graduate course on inorganic chemistry, materials chemistry or materials science. In the experiment, crystalline thin films of titanium nitride (TiN) are deposited using titanium tetrachloride, hydrogen, and nitrogen gas in an experimental setup based on a tube furnace and common safety flasks. Typically, crystalline TiN films with some incorporation of TiO2 are deposited in this experiment. The experiment has been used in the teaching of both master and doctorial students. KEYWORDS: Upper-Division Undergraduate, Laboratory Instructions, Inorganic Chemistry, Hands-On Learning/Manipulatives, Laboratory Equipment/Apparatus, Materials Science hin films are layers of material with thicknesses ranging from a few atomic layers to several micrometers; for reference, a human hair is approximately 75 μm thick.1 Applications range from antireflecting coatings for eyeglasses to low friction coatings in automobile engines. Precision metal parts are typically machined by cutting tools that are coated with hard, wear resistant thin films. Also, replacement parts for the human bodyfor example, hip replacement jointsare often coated with a thin film to make them more biocompatible. Furthermore, in the field of microelectronics, structures comprised of stacks of thin films of various materials with different electrical properties can be deposited with great precision. To coat an object (i.e., substrate) with a thin film, it is preferable to start from atoms or molecules in a vapor phase. It is common that vapor-based thin film synthesis methods are classified as physical vapor deposition (PVD) or chemical vapor deposition (CVD) depending on whether the film deposition process relies on the principles of physics or chemistry, respectively. For PVD methods, a thin film condenses on a substrate from an atomic vapor created by ejecting atoms from a solid piece of material (i.e., target). This is achieved by applying substantial amounts of energy to the target (or cathode), often in the form of a plasma or an electric discharge.2 With CVD methods, the film is grown via a series of chemical reactions between gaseous molecules, denoted precursors, both in the gas phase and on the substrate surface. The precursor molecules containing the atoms which comprise the film are, with very few exceptions, diluted in a carrier gas that occupies a substantial part of the gas volume in the processanalogous to most chemical experiments where the
T
© XXXX American Chemical Society and Division of Chemical Education, Inc.
reactants are diluted in some solvent. The carrier gas in CVD is most often hydrogen, nitrogen, argon, or mixtures of these. Note that gas phase chemical reactions are not a component of all CVD processes; a form of CVD named atomic layer deposition (ALD) uses only surface chemical reactions to grow thin films with high precision.3 The majority of CVD processes are thermally activated by applying process temperatures typically in the range of 200−2000 °C. There are also CVD processes that use a plasma to activate the gas phase chemistry. The free electrons in a plasma open up new reaction pathways by electron impact collisions. These processes are referred to as plasma-enhanced CVD (PECVD) or, alternatively, plasma assisted CVD (PACVD).4 The gas phase chemistry can also be activated by photons from a laser; this process is referred to as laser-enhanced CVD (LECVD) or photo-assisted CVD.5 Interested readers are advised to consult, for example, the recent book edited by Jones and Hitchman6 for an up to date overview of the CVD field. CVD experiments on alumina7 and carbon nanotubes8 have previously been published in this Journal. The chemistry governing the mechanisms behind CVD (and also for the scientific field of thin films) is often neglected or considered to be beyond the scope of undergraduate chemistry education due to the high cost of complicated equipment requisite for CVD based thin film synthesis. At some universities where CVD equipment exists, the tools are not available for teaching purposes due to the highly specialized use of the equipment, for example, CVD tools used for semiconductor materials. Furthermore, most modern CVD tools are
A
dx.doi.org/10.1021/ed500183k | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
controlled via a computer and have a variety of safety features built in; course instruction primarily entails students sitting at a computer controlling gas flows without gaining insight into the actual thin film process. In this paper, a very simple CVD experiment is presented that is suitable for a high-level course on materials chemistry, solid state chemistry, or inorganic chemistry. Students first construct a CVD tool and then use it to synthesize thin films of titanium nitride (TiN). The goal of the experiment is not to produce high quality films but rather to give the students a basic understanding of CVD systems and CVD chemistry. The CVD system is set up using equipment commonly found in a teaching lab.
■
EXPERIMENTAL OVERVIEW The CVD system is constructed using a tube furnace; the author uses a 20 mm diameter tube. The diameter is not crucial as long as it allows convenient loading and unloading of samples. For simplicity, the CVD system operates at atmospheric pressure; thus, there is no need for a vacuum pump. During the CVD process, thin films of titanium nitride (TiN) will be deposited from titanium tetrachloride (TiCl4) in a nitrogen (N2)−hydrogen (H2) carrier gas mixture at 1000 °C via the overall reaction TiCl4(g) +
Figure 1. Schematic of the setup of the CVD system.
N2/H2 gas through the bubblers containing a drying agent, such as P2O5(s), to condition the gas delivery system for approximately 15 min prior to starting the deposition and by purging the tube furnace by flowing nitrogen during heating. A film is deposited for 30 min by the CVD process and then characterized using X-ray diffraction (XRD) in Bragg− Brentano mode (θ/2θ) or FTIR (Fourier transform infrared spectroscopy). Film thickness is estimated by weighing the sample or by scanning electron microscopy (SEM) depending on availability.
1 N2(g) + 2H 2(g) → TiN(s) + 4HCl(g) 2
■
Gaseous HCl is formed in the reaction; thus, the exhaust gases are lead through a concentrated NaOH (aq) solution to neutralize at least a part of the formed HCl(g). TiCl4 is a liquid at room temperature; therefore, a technique commonly referred to as bubbling is used to transform it into a gaseous state and then transport it into the gas mixture. Bubbling is as simple as it sounds. A gas is bubbled through the liquid in a closed container. Above the liquid, there will be a mixture of the bubbling gas and vapor of the liquid and this mixture is transported out from the container by the gas flow. In this experiment, TiCl4 bubblers are constructed using safety flasks with fritted tips. The precursor in this CVD process is nitrogen gas. The N2 molecule is well known for its stability; only a minor part of the molecule in the gas mixture (within the reactor) will be active in the CVD process and used to form the TiN thin film. As seen from the overall reaction, hydrogen is needed not to form the film but to form the byproduct HCl. Therefore, a mixture of hydrogen and nitrogen is used as the carrier gas. Two TiCl4 bubblers, one on each gas line, are used to monitor the flows of N2 and H2. This setup allows the N2/H2 ratio to be adjusted with some precision by controlling the bubble rates through the safety flasks. The CVD system (Figure 1) is constructed using rubber hoses as gas lines and rubber corks that are drilled and fitted with glass pipes to connect the rubber hoses on the inlet and outlet of the furnace tube. Any material that can tolerate the deposition temperature and chemistry can be used as a substrate; single crystalline silicon wafers are often available and well suited. The substrate is placed on a crucible and inserted into the middle of the furnace tube by pushing it into the tube by a steel rod. Because TiCl4 reacts rapidly with moisture to form TiO2
HAZARDS As in most CVD processes, there are dangerous gases used in this experiment. If N2 and H2 are not available via a central gas system in the lab, care must be taken when handling gas cylinders with gas under high pressure. Hydrogen gas is flammable and explosive, and TiCl4 forms HCl(g) upon contact with moisture in the air. Thus, great care must be taken to prevent the gas mixture (from the gas delivery system) from leaking out into the lab. Also, the gas mixture that leaves the NaOH(aq) scrubber must be regarded as dangerous, even though the NaOH(aq) solution neutralizes some of the HCl(g) formed in the CVD reaction. Finally, the furnace temperature is very highapproximately 1000 °Cand presents a significant risk for burns if great care is not taken, especially when the crucible is pushed out of the hot furnace after the deposition. Note that the steel rod used to push out the crucible gets very hot and that the crucible itself is also quite hot.
■
RESULTS AND DISCUSSION This experiment is suitable for two students but can be performed by a single student as well. If the experiment is performed as a single experiment where the students setup the equipment, deposit the TiN film, and dismantle the equipment, approximately 4 h are required. The students can also run a small project using this experiment where they vary, for example, the temperature and bubbling rate to study differences in the deposited films. This experiment has been run for two years as a small project with master students in a materials chemistry course and has also recently been included as part of the curriculum in a course on CVD for doctoral students studying materials science. The CVD setup has also formed the basis for a bachelor thesis. All students have found that the simplicity of the CVD setup makes the experiment easy to understand. In particular, the concept of bubblers has been clarified when performing the experiment. For further background information on CVD, the fairly recent and very
TiCl4(g) + 2H 2O(g) → TiO2 (s) + 4HCl(g)
it is of great importance that the CVD system is as dry as possible before starting the process. This is achieved by flowing B
dx.doi.org/10.1021/ed500183k | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
CVD process is then discussed and the film characterized, typically by XRD, by the students together with the instructor.
extensive review by Choy9 has been distributed to the students to serve as an overview without focusing too much on details. The most common problem that students encounter with the experiment is the formation of white films, which is a clear indication that TiO2 instead of TiN has been formed. This highlights the importance of drying the furnace and conditioning the gas delivery system prior to the deposition to prevent moisture from being present during the CVD experiment. A typical XRD diffractogram of films deposited in this experiment (Figure 2) also reflects the problem with reactions from moisture.
■
ASSOCIATED CONTENT
S Supporting Information *
Student handout and notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Michaela Repfenning is gratefully acknowledged for her useful comments and valuable input in the development of the CVD experiment. Sankara Pillay and Per-Olov Käll are gratefully acknowledged for critically reading the manuscript.
■
REFERENCES
(1) Ohring, M. Materials Science of Thin Films, 2nd ed.; Elsevier: Singapore, 2006; p xix. (2) Martin, P. M. Handbook of Deposition Technologies for Films and Coatings, 3rd ed.; Elsevier: Amsterdam, 2010. (3) George, S. M. Atomic layer deposition: an overview. Chem. Rev. 2010, 110, 111. (4) Hess, W.; Graves, D. B. Plasma-assisted CVD. In Chemical Vapor Deposition: Principles and Applications; Hitchman, M. L., Jensen, K. F., Eds.; Academic Press: San Diego, CA, 1993; pp 385−435. (5) Irvine, S. J. C.; Lamb, D. Photo-assisted CVD. In Chemical Vapour Deposition: Precursors, Processes and Applications; Jones, A. C., Hitchman, M. L., Eds.; Royal Society of Chemistry: Cambridge, U. K., 2009; pp 477−493. (6) Jones, A. C.; Hitchman, M. L., Eds. Chemical Vapour Deposition: Precursors, Processes and Applications; Royal Society of Chemistry: Cambridge, U. K., 2009. (7) Vohs, J. K.; Bentz, A.; Elamos, K.; Poole, J.; Fahlman, B. D. Chemical Vapor Deposition of Aluminum Oxide Thin Films. J. Chem. Educ. 2010, 87, 1102. (8) Fahlman, B. D. Chemical Vapor Deposition of Carbon Nanotubes: An Experiment in Materials Chemistry. J. Chem. Educ. 2002, 79, 203. (9) Choy, K. L. Chemical vapour deposition of coatings. Prog. Mater. Sci. 2003, 48, 57.
Figure 2. Typical XRD diffractogram of a TiN film deposited by a student in this CVD experiment. Diffractogram collected by the student with assistance from the author. Weak peaks from contaminations of rutile TiO2 can be observed.
Typically, strong peaks related to the (111), (200), and (220) planes of TiN and weak peaks related to the (110), (101), (111), and (211) planes of rutile TiO2 are evident in the XRD diffractogram. Depending on the course curriculum and availability of equipment, scanning electron microscopy (SEM) can also be used to characterize the deposited films. With SEM, the surface topography, film thickness, and microstructure of the film (Figure 3) can be studied. The latter requires that the samples are cleaved and the film cross section is studied. The purpose of this experiment is to give the students hands on experience of CVD and a basic understanding of the process and chemistry used to make thin films. Typically the students make a film on a substrate and show to the instructor. The
Figure 3. Cross-sectional SEM micrograph of an approximately 20 μm thick TiN film deposited by a student using the described CVD experiment. C
dx.doi.org/10.1021/ed500183k | J. Chem. Educ. XXXX, XXX, XXX−XXX
