Electric resistance heating devices – Heating devices – Vaporizer
Reexamination Certificate
2000-09-15
2001-07-31
Paik, Sang (Department: 3742)
Electric resistance heating devices
Heating devices
Vaporizer
C392S396000
Reexamination Certificate
active
06269221
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to vaporizers to convert a liquid feed to a vapor feed for chemical vapor deposition apparatus, and relates in particular to a vaporizer section suitable for a vapor feed material for depositing a highly dielectric or ferroelectric thin film, such as barium or strontium titanate.
2. Description of the Related Art
In recent years, there has been a quantum jump in circuit density of integrated circuit devices produced by the semiconductor industries, and intense development activities are underway in anticipation of giga-bit order DRAMs replacing the prevailing mega-bit order DRAMs of today. Dielectric thin film materials used to make high capacitance devices necessary for producing DRAMS have, in the past, included silicon oxide or silicon nitride films of dielectric constant less than 10, and tantalum pentaoxide (Ta
2
O
5
) films of dielectric constant less than 20. However, newer materials such as barium titanate (BaTiO
3
), strontium titanate (SrTiO
3
) or barium-strontium titanate (BST), having dielectric contestants of about 300, appear to be more promising. Promising also are even higher dielectric materials such as lead-zinc-titanate (PZT), lead-lithium-zinc-titanate (PLZT) and Y
1
.
Of the various methods of making such thin films, prospects are particularly bright for the chemical vapor deposition (CVD) process, and in this case, it is necessary that a gaseous feed must ultimately be supplied in a steady gas stream to a substrate disposed in the deposition chamber. The gaseous feed is produced by heating a liquid mixture of liquefied materials such as Ba(DPM)
2
or Sr(DPM)
2
(which is solid at normal temperature) and some organic solvent such as THF (Tetrahydrofuran) for stabilization of the vaporization characteristics. Some known examples of vaporizer sections include, for example, those that atomize the liquid feed through spray nozzles or ultrasonic transducers, and deliver the atomized mist to a high temperature zone to convert the mist to a gaseous feed.
However, it is extremely difficult to produce thermodynamically stable vapors of such highly dielectric and ferroelectric materials mentioned above. This is because, for these materials, {circle around (1)} the vaporization and decomposition temperatures are close; {circle around (2)} the vaporization temperature for the liquid feed material is different from that for the organic solvent; {circle around (3)} the vapor pressures are very low; and {circle around (4)} the materials are vulnerable to read with a small amount of oxygen, vapor water, etc.
For example, in a liquid feed obtained by dissolving Ba(DPM)
2
or Sr(DPM)
2
in THF, the solvent exists as a liquid in region (a) in
FIG. 34
, and the feed material exists as a liquid or solid in region (a+c). In region (b), the feed is totally a vapor. Therefore, when the liquid feed in region (a) is heated to be converted into a vapor feed and passes through the region (c), only the solvent vaporizes, leaving the solute components in the liquid feed to precipitate out. This results in blocking of the gas passages or quality degradation due to changes in composition. For this reason, it is considered important to heat the liquid feed to its high temperature vaporization region as rapidly as possible.
Furthermore, depending on the film material or film deposition conditions, it is sometimes necessary to supply the feed vapor at a minute rate to the deposition chamber. If the process of vaporization is not conducted smoothly and the gaseous feed delivery to the deposition chamber becomes unstable, deposition process will be seriously affected. Therefore, it is important to be able to control the vaporization of the gaseous feed down to very low flow rates.
In the conventional technologies for atomizing the feed liquid based on spray nozzles, it is difficult to control atomization at low flow rates of liquid feed, because of the high pressures used to atomize the liquid. In the ultrasonic atomization techniques, it is difficult to find transducer materials which can withstand the high temperatures used for vaporization. Additionally, it is desirable to carry out the liquid-to-vapor conversion process physically near the deposition chamber so as to minimize the distance of transport. However, the above-mentioned apparatus is usually designed to atomize first and then to vaporize so that it is difficult to make the apparatus small. Also, both techniques require fairly large facilities for atomization and spray purposes, and it is unavoidable that stagnant regions of liquid feed are created within the apparatus, and degradation of the liquid feed and difficulty in controlling the flow rates of gaseous feed are experienced in the current technologies.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a compact vaporizer section to be used in a chemical vapor deposition apparatus for depositing highly dielectric or ferroelectric materials. Because the thermodynamic behavior of such materials is complex, the vaporizer is designed to preserve delicate properties of the feed material during the process of converting a liquid feed to a vapor feed. The invention also provides a chemical vapor deposition apparatus that prevents blockages caused by premature precipitation of solute materials and allows effective cleaning of the apparatus.
The object has been achieved in a vaporization apparatus for converting a liquid feed to a vapor feed comprising: a feed tank for storing the liquid feed; feed delivery means for transporting the liquid feed by way of a feed delivery path; a vaporizer section disposed in the delivery path comprising a high temperature heat exchanger having a capillary tube for transporting the liquid feed and a heat source for externally heating the capillary tube; and a vaporization prevention section disposed upstream of the vaporizer section for preventing heating effects of the vaporizer section to the liquid feed within the vaporization prevention section.
According to the apparatus presented, a high surface-to-volume ratio of the capillary tube enables the apparatus to carTy out the necessary heat transfer to vaporize the liquid feed instantly and uniformly so that the liquid feed is not exposed to conditions which would be conducive to decomposition or degradation. The basic objective of a steady delivery of a minute amount of gaseous feed can be achieved by varying the diameter and the length of the tube to adjust the conductance of the delivery path so that the dwell time of the liquid feed in the vaporizer section is appropriate. Further, by providing the vaporization prevention section, premature degradation or partial loss of the solvent material in the liquid feed, caused by the heating effects of the vaporizer section, can be prevented so that a gaseous feed of a constant and uniform composition can be delivered to the substrate. Additional merits are that the construction of the vaporizer section is simple and the manufacturing cost is low, the device itself is less affected by plugging, maintenance and repair can be canied out easily, and the vaporization prevention section is easily attached to the apparatus.
The high temperature heat exchanger may have a double-wall structure comprised of an inner capillary tube and an outer jacket. The outer tube (jacket) is provided with a thermal medium passage for maintaining a surface temperature of the inner capillary tube constant by circulating a thermal medium supplied from a thermal medium tank maintained at a constant temperature. Steady supply of high quality feed vapor is thus assured.
The capillary tube in the high temperature heat exchanger is a variable output electrical heater whose output power may be controlled by sensor signals, thereby providing sensitive temperature control to enable efficient vaporization.
The capillary tube in the high temperature heat exchanger may have an inner diameter of not more than 3 mm. Such a size is most effective in providing rapid h
Abe Masahito
Araki Yuji
Horie Kuniaki
Kuriyama Fumio
Murakami Takeshi
Ebara Corporation
Paik Sang
Wenderoth , Lind & Ponack, L.L.P.
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