US 3184329 A
Description (OCR text may contain errors)
3,184,329 INSULATION Leslie L. Burns, Jr., Princeton, N.J., assiguor to Radio Corporation of America, a corporation of Delaware Filed Dec. 16, 1960, Ser. No. 76,288 7 Claims. (Cl. 117-106) This application deals with the problem of providing very thin, evaporated layers (films) of insulating material which are free of pin holes. While not restricted thereto, the solution to this problem is of considerable importance in the making of certain superconductor electrical circuits, and in other applications, such as the making of capacitors, for example.
There are a number of electrical circuit applications in which thin insulator films are useful. For example, in superconductor circuits it may be desired to provide very close coupling between a superconducting lead carrying a drive current and an element such as a sheet of superconductor material, a portion of which it is desired to drive from its superconducting to its normal state. A thin insulator film makes the close spacing possible. Capacitor technology is another area in which close spacing between metal layers may be desiredhere for the purpose of increasing capacitance. Thin insulating films make the close spacing possible.
Vacuum evaporation is one method for obtaining thin films. In this method, the surface on which it is desired to deposit the film is placed in a vacuum chamber. The insulator material to be deposited is placed in a basket or crucible, also in the vacuum chamber and is heated to a high temperature. The evaporated insulator material then deposits on the surface in the form of a film.
Unfortunately, vacuum evaporation has not proved to be an entirely satisfactory method of producing thin films. Films produced in this manner have very minute holes therein which hereafter are termed, pin holes. Since the film normally occupies the space between two layers of metal, one on which the film is deposited and the other deposited on the other surface of the fihn, the pin holes provide a path through which the deposited metal extends. The metal extending into the pin holes produces a short circuit between metal layers thereby greatly lessening or destroying the insulating property of the film.
There have been a number of theories to explain the pin holes and solutions based on these theories have been attempted. Perhaps the most accepted theory up to the present has been that very small particles are present on the surface on which the insulator layer is formed. These particles were thought to produce a shadow area which remained free of insulation when the evaporated insulator was deposited on the surface. This shadow area was thought to be the pin hole. Based on this theory, a number of solutions, none of which were successful, were attempted to overcome the problem. For example, in one method insulating material was vacuum evaporated simultaneously from two spaced baskets in an attempt to make the insulating material particles land on the surface at different angles and cover the shadow area. In another method, the basket containing the insulating material was moved to various locations in the vacuum chamber during the vacuum evaporation.
It is believed that the theory above and others which have been advanced are incorrect. Two new theories, either one of which can explain the presence of pin holes, are discussed in greater detail below. However, whether or not either one of these theories is correct, they have provided a clue toward the solution of this problem and,
3,i84,329 Patented May 18, 1965 in any case, it has been found that the method of producing films disclosed herein does solve the problem.
According to the present invention, a film of insulating material is applied in two separate steps. First, a thin layer of the material is evaporated onto the surface it is desired to cover. This evaporation takes place in a vacuum. The pressure is then raised. The pressure is then lowered to that of a vacuum again and a second layer of insulating material is applied over the first. It is found that the film produced in this manner is free of pin holes over a very large area.
The invention is described in greater detail below and is illustrated in the following drawing of which:
FIG. 1 is a cross-sectional view of the way in which a thin insulator film on a metal surface is believed to appear during the vacuum evaporation of the insulator film;
FIG. 2 is a cross-sectional view of the film of FIG. 1 at atmospheric pressure;
FIG. 3 is a cross-sectional view of an insulator film according to the present invention; and
FIG. 4 is a cross-sectional view of two vacuum deposited metal films with an insulator film according to the present invention between.
FIG. 1 shows a metal film 10 on which is deposited an insulator film 11. The film is deposited in conven tional manner, that is by vacuum evaporating the insulator material in the manner briefly discussed in the introductory portion of the specification. For superconductor applications, silicon monoxide (SiO) has been found to be especially suitable. The vacuum evaporation is preferably carried out at a rather low pressure such as 10 or 10*- millimeters of mercury. However, any pressure lower than lO millimeters of mercury is satisfactory. The silicon monoxide is located in a crucible within the vacuum chamber and is heated to a temperature of 1290 degrees C. The vacuum evaporation is continued until a desired thickness such as 1,500 Angstroms is reached. The thickness can be determined by timing the heating step or alternatively by allowing the silicon monoxide to simultaneously deposit on the desired substrate and a quartz crystal that is in an electrical oscillating circuit. As the silicon monoxide is deposited on the crystal, the frequency of oscillation is changed. By measuring the frequency change, the evaporation can be terminated when the desired thickness is reached.
It is believed that during the vacuum evaporation described above, very small bubbles of gas at reduced pres sure become included in the film. Two such inclusions are shown at 12 and 13 in FIG. 1. These inclusions occur in a random pattern. "It is also believed that when the ambient pressure is increased to atmospheric pressure, these small inclusions implode as is shown at 12 and 13 in FIG. 2. The implosion-s cause the pin holes which provide passages for subsequent layers of vacuum evaporated metal. Such a layer (not shown) would extend into the pin holes and short circuit with the metal layer 10.
The above theory explains why the previous methods proposed for solving the problem have been unsuccessful. It also explains why increasing the thickness of the layer is not entirely successful either. As a matter of fact, silicon monoxide layers of greater than 20,000 Angstroms have been evaporated in a single step without overcoming the problem of pin holes. Even with layers this thick, it is believed that implosions which still occur cause the undesired pin holes shown in FIG. 2.
According to the present invention, an insulating layer such as silicon monoxide is first formed by vacuum evaporation in the manner already described at a thickness one-half that of the desired thickness. For example, if it is desired to produce a layer 3,000 Angstroms thick, the
first step is to lay down a layer 1,500'Angstroms thick. randomly occurring electrostaticcharges build up on the Then the ambient pressure is increased, preferably to atmospheric pressure. This can be done by permitting air to enter the vacuum chamber. However, this introduces. 1 undesired impurities such as moisture. It is preferred that the ambient pressure he raised by admitting a dry inert than millimeters of mercury. Then a second layer of theinsulator material, that is, silicon monoxide is vacuum evaporated onto the first. The secondlayer also has the gas atlow pressure .as inclusions therein. However, these occur in a random pattern and this random pattern is difierent than the random pattern of the pin holes in the Thus, when the ambient pressure is raised again, implosions in the second layerproduce pin holes in the second layer which do not coincide withthe' pin holes in the first layer and no continuous path exists between the upper surface of the second insulator layer and the lower surface of the first insulator layer.
FIG. 3 shows an insulator film formed according to the presentinvention. The metal layer is shown at 14, the first insulator is shown at 15, and the second insulator layer is shownat 16. The first layer has pin holes 17 and 18.-therein and the second layer 16 has pin holes 19,
20 and 21 therein. The pin holes in the two l'ayer s 1 are not aligned and no path exists from upper surface 22 to lower surface 23'. If the two layers are deposited without raising the ambient pressure during the interval between charges are dissipated.
' insulator material being vacuum deposited. Charges also occur in the particles of the material being deposited. If
the static charges built upinthe insulator layer are the same as those of the insulatorparticles being deposited, these particlesare repelled "from the randomly spaced static charge centers and the result is the pin holes already described. According to this theory, when the ambient pressure is raised to atmospheric pressure; the electrostatic Accordingly, the second layer which is subsequently vacuum evaporated onto the first not repelled from the pin h-oleareas but is instead the deposition steps, the improved film does not result.- 7
Instead, there are pin holes which extend between Op-r posite surfaces, just as in FIG. 2. V
FIG. .4 illustrates a capacitor construction according to the present invention. Layers 25 and 26-are metal and may be tantalumpor copper, for example, or some other metal, andlayers 27 and 28' together comprise the insulat-ing film formed according to the present invention. The pin holes in each-of layers 27 and 28 are not shown.
In a practical capacitor, the combined thickness of layers 27 and 28 may be of the order of 500 Angstrom-s to 1,000 Angstroms. Prior to this invention, it was not possible, so far as Applicant is aware, to obtain large area'vacuum deposited insulator films for capacitors of smaller thickness than 20,000 Angstroms.
Silicon monoxide has been given as an example of sulating material which is useful in practicing the present invention. It is, of course, to be understood that the invention isnot limited to this specific material. Siliconmonoxide is especially suitable for superconducting applications as it does not craze at thelow temperatures '(several degrees Kelvin) involved. It is also comm-onlyused for capacitor applications. The process of making the film is exactly the same as described above. Other materials which are suitable in practicing the invention arecalcium fluoride, silicon vdioxide,aluminum oxide, beryllium oxide and magnesium fluoride. 7 j
In the discussion above, thetwo films which are laid.
down are both of the samematerial. 'It is also possible to use difiterent materials for the different layers as, for example, calcium fluoride for one layer and silicon monoxide for the other. It is also sometimes desirable to use more than two insulating films laid down iii-the manner described. Three or more films may be used, for example, when the total thickness of insulation desired is very:
small. When the individual'layers are very, very thin,
repelled from different randomly spaced static charge "centers;
It is to understood that the present invention does not depend on either theory discussed above. Both are simply a means to explain why .theap'resent invention has given the improved performance it does. The fact is that regardlessof the'theories, the present invention does opante and does operate successfully. Several hundred films according to the invention have already been fabricated up to the present time and no insulation Ltiailures have yet been detected. I
What-is claimed is: V v
1. A method of producing a film of insulating material on a surface comprising the steps of evaporating a first layer of an insulating material onto the surface, in
vacuum; raising the ambient pressure on the evaporated layer by admitting an inert gas; reducing the ambient pressure on theevaiporated layer to that of a vacuum; and
evaporating a second layer of the insulating material onto 2. A method as set forth in claim 1, in which said sec- 10nd layer is formed of the same material as said first ayer. I
3. A method of producing a film of insulating material on a sunface'comprising the steps of evaporating a first layer of an insulating material onto: the; surface, in vacuum; raising the. ambient pressure 'on'the evaporated layer to atmospheric pressure by admitting an inert gas; reducing the ambient pressure on the evaporated layer to that, of a vacuum; and evaporating a second layer of the insulating material onto the first. V
4. A method of producing a film of insulating material on a surface comprising the steps of evaporating -a first layer less than 10,000 Angstroms thick of an insulating material onto the surface, ]at a pressure of lessthan 10- millimeters of mercury; raising the ambient pressure on the evaporated layer to atmospheric pressure; reducing the ambient pressure on the evaporated .layer to a value less than 10 millimeters of mercury; and evaporating a second layer. less than 10,000 Angstroms thick of the insulating material onto the undisturbed. first layer.
5. A method of producing a film of insulating material on a surface comprising the steps .of evaporating a first layer of insulatingfmaterial on a surface, invacuum; admitting a-dry inert gas in order to raise the ambient pressure on the evaporated layer; reducing the ambient pressure on the evaporated layer to that of a vacuum; and evaporating a secondlayer of the insulating material onto thej first.
. 6. A method of producing a thin silicon monoxide film comprising the steps of:
' a surface comprising the steps of:
, evaporating a silicon monoxide film onto a substrate,
in vacuum; raising the ambient pressure on the film; reducing the ambient pressure on the film to that of a vacuum; and. g evaporating -a second film of silicon monoxide onto the first silicon monoxide 7. A method of producinga silicon monoxide film on evaporating a first film less than 5000 Angstroms thick of silicon monoxide onto the surface at a pressure of less than 10- millimeters of mercury; admitting a dry inert gas to raise'the ambient pressure enemas on the evaporated film to atmospheric pressure without introducing contaminants; reducing the ambient pressure on the evaporated film to a value less than 10 millimeters of mercury; and evaporating a second silicon monoxide film less than 5000 Angstrorns thick of insulating material onto the first silicon monoxide film.
References Cited by the Examiner UNITED STATES PATENTS 6 Kempter et a1 117-106 Bamard 2925.42 Burger et a1. 317242 Frank 117-106 West 317258 FOREIGN PATENTS Great Britain.
RICHARD D. NEVIUS, Primary Examiner.
S. BERNSTEIN, JOHN P. WILDMAN, Examiners.