In the Laboratory
Chemical Vapor Deposition of Aluminum Oxide Thin Films Jason K. Vohs,* Amy Bentz, Krystal Eleamos, and John Poole Department of Chemistry, Saint Vincent College, Latrobe, Pennsylvania 15650 *
[email protected] Bradley D. Fahlman Department of Chemistry, Central Michigan University, Mount Pleasant, Michigan 48859
Chemical vapor deposition (CVD) is a widely used process for the preparation of thin film materials. Though the term applies to a number of different processes and operating conditions, atmospheric pressure CVD (APCVD) involves a volatile precursor, carried by an inert gas, entering a heated deposition chamber containing the substrate. Nucleation and growth of the thin film results from surface migration of chemisorbed species following thermolysis of the ancillary ligands. Depending on the nature of the co-reactants, a variety of vaporous byproducts may be formed, which are removed from the chamber by the carrier gas. Although CVD is a common technique in industrial and academic research laboratories, it is not commonly encountered in undergraduate instruction. Although this Journal has published two laboratory experiments describing the thermal decomposition of methane at very high temperatures (1), neither deals with the preparation of thin films. Furthermore, the specialized nature of the equipment required makes such investigations impractical or expensive for many smaller programs. Thin films of aluminum oxide (AlxOy) are of particular interest in materials science because of their utility in semiconductor devices. Although there are a number of common precursors for the CVD of AlxOy films, aluminum alkoxides are preferred (2). Whereas AlCl3 (3), AlMe3 (4), and Al(acac)3 (5) require high temperatures or the presence of reducing atmospheres, the controlled hydrolysis of aluminum alkoxides can take place at lower temperatures and in the absence of a reducing gas without increasing carbon incorporation in the resulting film. These factors not only make for a safer laboratory exercise, but also a more facile experimental setup. Dimethylaluminum isopropoxide, [Me2Al(i-OPr)]2, is the precursor used in this exercise. This article describes a laboratory module that was designed to introduce upper-level undergraduate inorganic chemistry students to materials chemistry. The module described herein utilizes low-temperature (∼200 °C) metal-organic chemical vapor deposition (MOCVD) in an easily constructed apparatus made from readily accessible parts. The exercise can serve as an instructor-led demonstration or an inquiry-based laboratory project where students seek to optimize the experiment by varying precursor temperature, flow rates, chamber temperature, and so forth. Furthermore, characterization may be accomplished simply by visual inspection (i.e., the students see a red or blue coating, a white granular powder, or nothing on the wafer), through either optical or electron microscopy, film thickness analysis, and so forth depending on the resources available in-house or through collaborations with other institutions and analytical laboratories. 1102
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Figure 1. Schematic of the CVD apparatus used to prepare thin films of aluminum oxide. The water and precursor vapors were introduced through separate tubes and allowed to come into contact only in the immediate vicinity of the substrate to prevent extensive gas-phase reactions and thus a granular coating. Heating tape was used as the heat source.
Experimental Section This experiment was designed for inclusion into a upperlevel level inorganic or materials course. It was introduced to a class of 16 students and a class of 12 students. It has subsequently been undergoing modification as an inquiry-based exercise. Not only does this module introduce the technique of chemical vapor deposition, but also introduces students to the handling of air and moisture sensitive reagents through the synthesis of the dimethylaluminum isopropoxide precursor (prepared from the addition of trimethylaluminum to aluminum isopropoxide). The synthesis of the precursor and the deposition can be completed over a 3-h laboratory period. The precursor may be prepared by the instructor ahead of time if desired, with the deposition occurring over approximately a 1-h period. A photograph and thorough description of the CVD system and preparation of reactants used in this module are available in the supporting information. A diagram of the apparatus highlighting the major components is shown in Figure 1. Flow meters and heating mantles may be used for both the water and precursor to control the rates of introduction into the chamber as well as the temperature. Deposition was allowed to
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Vol. 87 No. 10 October 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100391p Published on Web 08/02/2010
In the Laboratory
occur over a 30-60 min period depending on the flow rates and amount of precursor material.
Immediately after removing the substrate, students can determine whether a coating is present (Figure 2) and whether that coating appears granular or conformal in nature. A granular coating would be characterized as having more particulates across the surface, whereas a conformal coating would be a generally uniform, even coating, without areas where there is no coating. Further characterization, using either optical or scanning electron microscopy (SEM), elemental analysis (EDS), and film thickness can be performed if available, but are not critical for students to formulate claims on whether the deposition conditions were favorable or not. Hazards Some of the chemicals used in this experiment require care in handling and their use should be restricted to fume hoods: aluminum isopropoxide (toxic, irritant) and trimethylaluminum (highly flammable, toxic). Trimethylaluminum may be used neat or as a solution in hexanes with no apparent difference in results. Though still pyrophoric, the trimethylaluminum solution is less expensive and may be preferable for safety reasons for less experienced students. Portions of the glass apparatus will be hot, so care should be taken when handling. Gloves and eye protection should be worn at all times.
Figure 2. Photograph of a coated silicon wafer.
Results
Figure 3. Scanning electron micrographs showing an overview of the wafer fragment (top left), magnification (250) of the granular region (top right), magnification (250) of the blue-coated region (bottom left), and an image of the uncoated region (bottom right).
Representative student-generated SEM images and energydispersive X-ray spectra for aluminum oxide deposited onto a silicon wafer are shown in Figures 3 and 4, respectively. The scanning electron micrographs show a generally conformal coating with some areas being more granular. Moreover, three primary areas were observed with EDS elemental analysis; the granular region (Figure 4A) contains significant C in addition to Al and O; the blue-coated region (Figure 4B) contains substantially more Al and O relative to C; the uncoated region (Figure 4C) shows the presence of very little aluminum and oxygen. Additionally, film thickness analysis showed an average thickness of 70 nm.
Figure 4. Energy-dispersive X-ray spectra for the analysis of aluminum oxide deposited onto a silicon wafer. Shown are representative spectra for (A) the granular region, (B) the blue-coated region, and (C) the uncoated region of the wafer.
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Conclusions
Acknowledgment
The combination of chemical synthesis and chemical vapor deposition led to a successful undergraduate laboratory experiment. Student comments showed that they appreciated that they “made something with a potential application, not just something to put in the NMR”. In addition to working with new techniques (syringe techniques and the glovebag), the students completed the challenge of optimizing the deposition with a lot of interest. In particular, this experiment was done among others in a 6-week rotation of experiments. Pairs of students each took on the task of varying reaction conditions (flow rate, temperature, etc.) to see the effects. As each group was able to make claims regarding whether a certain temperature range or flow rate was effective, groups later in the rotation were able to take note and design their experiments accordingly. New questions were also generated during this process such as “Is the water vapor really necessary?” or “Does the placement angle of the wafer in the chamber have an effect?”, which future cohorts of students may be asked to examine. Characterization during the laboratory sessions was solely visual, with students seeing either colored regions on the substrate, a white granular coating, or nothing on the wafer. Further advanced characterization was completed through interdepartmental and off-site collaborations later in the term. These experiences where students were able to visit other facilities and perform the analyses were also rewarding for the students as they got to see and use equipment that is not routinely used by undergraduate students.
The Departments of Chemistry at both Saint Vincent College and Central Michigan University, and the Department of Biology at Central Michigan University are gratefully acknowledged for their continuing support and timely free access to instrumentation for both research and pedagogical usage. The Materials Science and Engineering Center at Penn State University is also gratefully acknowledged for allowing students of Saint Vincent College to tour and use their SEM facilities.
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Literature Cited 1. (a) Ahmed, W.; Sein, H.; Rajab, H.; Jackson, M. J. Chem. Educ. 2003, 80, 636–641. (b) Fahlman, B. D. J. Chem. Educ. 2002, 79, 203–206. 2. Blittersdorf, S.; Bahlawane, N.; Kohse-Hoinghaus, K.; Atakan, B.; Muller, J. Chem. Vap. Deposition 2003, 9, 194–198. 3. Lux, B.; Colombier, H.; Altena, H.; Sternberg, K. Thin Solid Films 1986, 138, 49–64. 4. Groner, M. D.; Fabreguette, F. H.; Elam, J. W.; George, S. M. Chem. Mater. 2004, 16, 639–645. 5. Ajayi, O. B.; Akanni, M. S.; Lambi, J. N.; Burrow, H. D.; Osasona, O.; Podor, B. P. Thin Solid Films 1986, 138, 91–95.
Supporting Information Available A photograph and description of the CVD system; preparation of reactants; notes for the student; postlab questions; notes for the instructor. This material is available via the Internet at http://pubs. acs.org.
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