Device for producing oxidic thin films

Details Device for producing oxidic thin films Device for producing oxidic thin films

1998-12-07

2001-09-25

Knode, Marian C. (Department: 1763)

Coating apparatus

With means to apply electrical and/or radiant energy to work...

Radiant heating

C118S715000, C118S728000, C118S729000, C118S730000

Reexamination Certificate

active

06294025

ABSTRACT:

BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to an apparatus for producing thin oxide coatings including a vacuum chamber, an oxygen chamber with an opening and a rotary substrate holder.
Thin oxide coatings are modern working materials of electronics and electrical technology. They are chemical compounds of usually several metallic elements with oxygen. Depending on the composition, they may provide high-temperature superconductors, ferrous electricals, magnetoresistive coatings or ferromagnetic materials.
Examples of items which use thin coatings of high-temperature superconductors are magnetic field sensors (SQUIDs) with a corresponding transformer, antennas for nuclear resonance tomography and nuclear resonance analysis, filters and antennas in the microwave range for mobile radio and satellite communications, current limiters in energy technology, coated tapes as conductors for magnetic coils, and elements for the transmission of energy, and so on.
Ferroelectric thin oxide coatings are used, for example, for nonvolatile read-write memory (RAM) for the permanent tuning of microwave filters, for magnetoresistive coatings as read heads for computer hard disks, for ferromagnetic thin coatings generally for magnetic storage media, and for microwave components.
For most applications, large-area thin coatings are required. But components of large area can also be produced simultaneously in great numbers, so that an economic advantage results. An industrial production process must therefore be capable of coating large areas in uniform, repeatable quality.
For small and medium areas up to about 75 mm (about 3 inches) in diameter, there are a number of suitable methods and apparatus. The most important are cathode sputtering, laser ablation, chemical deposition from organometallic compounds (MOCVD), and co-evaporation of the metals in an oxygen atmosphere. Of the latter, the basic principle of thermal evaporation shall first be described in connection with the schematic drawing in FIG.
2
.
In this case, the metallic elements are evaporated individually in a vacuum apparatus, while the rates of deposition are controlled so that the desired composition of the thin coating is achieved. Oxygen is fed to the thin layer growing on the substrate, simultaneously with the metal atoms, so that the oxide is formed. If the distance between the evaporation sources and the substrate is sufficient, the coating rate becomes homogeneous even on large areas. Sources are electron beam evaporators, Knudsen cells, and vapor depositing crucibles directly heated by the passage of electrical current through them.
Heated evaporation crucibles have the advantages that their evaporation rates can be stably controlled and that they are not sensitive to the oxygen always present in the apparatus. Also, the small dimensions of directly heated evaporation crucibles are a great advantage, since they permit arranging the sources especially close to one another. As a result, three metals, for example, are evaporated virtually from the same point. Then, the directional characteristic of the sources can lead to low gradients of the composition, the result being a sufficiently uniform quality of the coating, even over a large surface area.
Since the coating is performed as a rule at elevated temperature (300 to 800° C.), the substrate is in this case situated in a small, vertically positioned tubular furnace which permits indirect heating by thermal radiation. The oxygen is fed into this furnace. The oxygen pressure in the furnace is about 20 times higher than in the rest of the chamber, since the latter is constantly being pumped out. This permits sufficient oxidation of the substrate and nevertheless rectilinear propagation of the metal atoms without great scatter.
The basic version described here, however, is not suitable for large areas, since there is a limit to which the tubular furnace can be enlarged. If its diameter is increased, the flight distance of the metal atoms through the denser oxygen gas in front of the substrate also increases. Therefore the metal atoms undergo such great scattering that the rate of deposition becomes dependent upon pressure and can no longer be regulated with sufficient precision. In practice only areas up to 3×3 cm
2
are produced, and rejects for poor quality result.
Therefore, in practice, a rotating arrangement became known, in which the zone of deposition and the zone of oxidation are separated from one another. This turntable principle is sketched in FIG.
3
. The substrate or substrates are disposed on a revolving table which again is heated by thermal radiation. The shaft of the turntable is brought out through a rotary lead-through and driven by a motor in the open air.
On the bottom of the turntable there is screwed a plate on which the substrates lie. The plate holds the substrates (wafers) only by the edge and leaves the rest of the wafer surface free so that they can be coated from underneath. Due to rotation at about 300 to 600 rpm, the wafer passes on, after being coated with about 1 atom thickness, into a pocket into which oxygen is fed. There, this atom layer is oxidized and the desired compound forms.
The oxygen pocket should be as ideally sealed as possible, so as to assure the greatest possible pressure difference with respect to the other parts. Since no sealing materials can be used on account of the high temperatures, and wear on a material gasket would greatly contaminate the growing coating, a controlled resistance to flow is used instead of a gasket, which is formed by the gap between the rotating plate and the margin of the stationary oxygen pocket.
The resistance to flow increases with the length of the gap and is inversely proportional to the square of its width. It is therefore especially important to make the gap as narrow as possible. There are limits imposed by mechanical considerations on the narrowness of the gap, which becomes more critical at greater diameters. Therefore, a turntable with central shaft permits the coating only of areas of up to 10 cm in diameter.
This limitation is therefore based on the fact that the central shaft is also greatly heated by thermal conductivity and is lengthened and distorted by thermal expansion. The result of elongation is that the gap narrows according to temperature. This can be compensated for, in principle, for a desired temperature by setting the gap too wide at room temperature. This procedure, however, is bothersome and is only marginally successful. One unavoidable difficulty, however, is that the shaft at the same time becomes distorted. As a result, the turntable is no longer precisely parallel to the oxygen pocket and begins to rub. The rubbing forms detritus which cakes together at high temperatures and deposits itself. The rubbing thus becomes rapidly more intense, until the turntable comes completely to a halt. The greater the diameter of the plate is, the smaller are the distortions of the shaft that will suffice to start this process. Therefore, turntables which are mounted on a central shaft are practical only up to about 10 cm diameter.
It is the purpose of the invention to create an apparatus for the production of thin oxide coatings in which the difficulties mentioned are avoided and vapor deposition surfaces of 20 cm and more are possible.
This purpose is accomplished with an apparatus for producing thin oxide coatings in which, for rotary arrangement of a substrate holder, a rotary mounting which engages a circumferential portion of the substrate holder is provided.
An apparatus according to the invention for the production of thin oxide coatings accordingly contains a vacuum chamber with an oxygen chamber with an opening, and a rotary substrate holder covering the opening. For the rotary arrangement of the substrate holder, a revolving mounting is provided which engages a circumferential area of the substrate holder. Thus the entire surface of the substrate holder is free to accommodate one substrate or several substrates and is not disturbed by the driven shaf

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