Page 425 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN009J-427 July 6, 2001 20:25

510 Metalorganic Chemical Vapor Deposition

software simulations is by far more effective and efficient clude laser-induced fluorescence, differential mass spec-
(and less costly) than the old-fashioned “cut and try” trometry, and absorption spectroscopy. The measurement
method commonly used in the early “frontier” days of of surface species in a CVD environment is still problem-
MOCVD reactor development. atical, although reflectance-difference spectroscopy has
shown some promise. Additional complications arise due
to the lack of spatial uniformity in the gas phase inside
VI. FUTURE VISION a reactor and near the growing surface. Real-time three-
dimensional chemical mapping of the reactant species in-
The MOCVD process has been used for a wide variety side a CVD growth chamber is a daunting problem and
of III–V binary, ternary, quaternary, and pentanary semi- one that will not yield easily using conventional tech-
conductor films. It has also been used for the growth niques. Many more years of research and development
of oxides, superconductors, dielectrics, and the deposi- are required to realize a true “process control” system for
tion of metal films, including Cu interconnects on Si MOCVD. However, it is an area of continued activity and
integrated circuits. We can expect that the usage in all research results are being continually translated into com-
these areas will increase dramatically in the next few mercial products.
years. It is clear that the future development of MOCVD
will continue to rely on improvements in the purity of
precursors (both organometallics and hydrides). Further- VII. SUMMARY AND CONCLUSIONS
more, advances in the understanding of chemical reac-
tions, hydrodynamics, precursor kinetics, etc., should lead The growth of epitaxial films of the III–V compound semi-
to improved large-scale reactor designs capable of grow- conductors by MOCVD was patented in various forms
ing simultaneously on more than a dozen 6-in. diameter prior to 1965 and first reported in the scientific literature
substrates using a wide range of growth pressures. Fur- in 1968. In the late 1970s, MOCVD was shown to be
thermore, the efficiencies of scale in the production of a viable technology for the growth of high-performance
metal alkyls should permit the cost factor of precursors solar cells and sophisticated injection lasers. From this
to be reduced. MOCVD reactors with kilogram quanti- work, it was possible to predict that the MOCVD process
ties of metal alkyls are now common in production envi- would become an important element in the fabrication of a
ronments. Recently, an automatic filling system for metal wide variety of high-performance semiconductor devices.
alkyls has been developed that employs two 20-kg storage Because of the economics and flexibility of the process,
vessels and a dedicated piping system that will automati- the quality of the materials produced, and the scalability
cally fill the smaller “bubblers” that are placed on the indi- of the technology, it has come to dominate the epitax-
vidual MOCVD reactors. The current generation system ial growth of III–V semiconductors. Today, most optical
will monitor up to eight MOCVD vessels simultaneously memory and information recording systems, (e.g., CD-
and automatically fill them to maintain a constant precur- ROMs, DVD players, etc.) and optical communications
sor molar flow rate over an extended period of time. This systems employ QW injection lasers based upon MOCVD
greatly reduces the requirements to change bubblers, re- epitaxial films and high-performance visible LEDs rely
sulting in markedly increased reactor “up time,” run repro- almost exclusively on MOCVD materials technologies.
ducibility, and the more effective use of the metalorganic In addition, today, high-performance digital cellular com-
sources. munications rely on the performance of MOCVD-grown
One important aspect of MOCVD (and all other CVD heterojunction field-effect transistors and heterojunction
epitaxial growth processes, including CVD for Si) that bipolar transistors.
still remains to be developed is “real-time monitor- Future advances in precursor purity and manufacturing
ing and process control”—while some in situ moni- technology, real-time monitoring of chemical reactions,
toring techniques have been developed, most notably MOCVD reactor chamber design, computer-controlled
spectroscopic ellipsometry, spectrally resolved reflec- epitaxial growth systems, detailed chemical process mod-
tivity, reflectance anisotropy spectroscopy, multibeam els, and real-time process control will lead to improved
optical reflectance, and emissivity-corrected pyrometry. process efficiencies, reduced hazardous waste, and en-
These techniques permit some useful degree of “real- hanced device reproducibility, yield, and performance.
time monitoring” but the missing element—the “real- The future of MOCVD is certainly bright. We are on the
time control”—is still sorely needed. One important com- frontier of a great expansion of the abilities of MOCVD
ponent to this control loop is the monitoring of the to provide materials for products that improve and expand
gas phase and surface species. Techniques for determin- the human experience on earth, under the oceans, and in
ing gas-phase composition are well established, and in- space.