US 3756847 A
Description (OCR text may contain errors)
METHOD FOR CONTROLLING THE COMPOSITION OF A DEPOSITED FILM Filed Nov. 4, 1971 FiG. 2
H //V|/E/VTO/?$ O I I Donald Leibowitz, I500 2000 2500 3000 3500 4000 DOI'OIII) M HOf/Illflfl MK) 8 Frederick J. Tums III AGE/V7 United States Patent Oflice Patented Sept. 4, 1973 US. Cl. 117-933 6 Claims ABSTRACT OF THE DISCLOSURE Deposition of a film of a compound on a substrate by electron beam evaporation of the compound is controlled by reestablishing substrate electrical potential. An electrical potential is applied to the substrate and is properly adjusted to attract or repel a predetermined portion of the negatively charged ions formed during evaporation of the compound.
BACKGOUND OF THE IINVENTION This invention relates to a method for depositing a film of a compound on a substrate and more particularly to a method for controlling the composition of a compound film during deposition on a substrate to affect the electrical and physical properties of the final compound film.
Apparatus and methods utilizing electron beams for evaporating a material for deposition on a substrate are well known in the art. The high temperature generated at the focal point of an electron beam permits rapid heating and evaporation of many materials including refractory materials and insulators, e.g. aluminum oxide, cerium oxide, and silicon dioxide. Evaporation of these materials permits their utilization upon deposition as thin films for optical filters (e.g., cerium oxide), insulating layers for thin film capacitors (e.g., silicon dioxide) or as passivating and radiation resistant films (e.g., aluminum oxide on metal oxide semiconductors). Success of such utilization, however, is often highly dependent on the quality of film deposited.
In order to obtain a high quality film, it has been found necessary to maintain the composition of evaporated material as close as possible to the stoichiometry of the source material. Methods that have been utilized to attain the desired stoichiometric relationship have included rapid evaporation techniques, such as flash evaporation and exploding wires to limit dissociation losses, and techniques for maintaining selected ambient conditions to permit recombination of any dissociated vapor stream constituents at the substrate. The latter method involves controlling gas composition, ambient pressure of the vacuum system, substrate temperature and/or evaporation rates. Although the foregoing prior art methods have met with some degree of success, there is still a need to control the stoichiometry of a deposited film.
SUMMARY OF THE INVENTION It has been discovered that a characteristic disadvantage of poor control of composition of film in prior art electron beam evaporation processes for depositing compound materials is caused by electrostatic charges that develop on the substrate during deposition of the film. These charges play a significant role in affecting the electrical and physical properties of the deposited film by changing the composition of the film. The present invention utlizes this discovery by applying a variable electrostatic potential to the substrate thereby permitting either increase or decrease of the potential to control the composition of the final film.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a schemaic section view of an apparatus that may be used for carrying out the method of the present invention;
FIG. 2 is a bottom plan view of a portion of the apparatus of FIG. 1 taken along the line 22 of FIG. 1; and
FIG. 3 is a graph of ultraviolet absorption spectra of two films deposited by the method of the invention.
DETAILED DESCRIPTION One embodiment of apparatus for practicing the present invention is shown in FIG. 1. An eighteen inch stainless-steel cylindrical vacuum chamber 10', closed at one end by an aluminum lid 12, is positioned on a stainlesssteel base collar 14. Internal evacuation of chamber 10 is accomplished by withdrawing the contained air or gasses from the chamber through a six inch pipe 16, located in the collar 14, that is in communication with the interior of the chamber and connected to a suitable vacuum pump (not shown). A substrate 18 is attached to a disk-shaped substrate holder 20 that is positioned in the upper portion of chamber 10. The relative position of the substrate on the substrate holder is shown in greater detail in FIG. 2. The substrate holder 20 is suspended from a vertical shaft 22 which has an insulator portion 24 to provide both thermal and electrical isolation of the substrate holder 20. The shaft 22 extends through the lid 12 and is directly connected to the armature of an electrical rotor 26 that is mounted on the top of the lid 12. Rotation of the substrate 18 by the rotor 26 permits greater uniformity of deposition when the substrate area is large.
A hemispherically-shaped stainless-steel heater 28 having a plurality of Nichrome heating coils 29 therein, is positioned above the substrate holder 20 to heat the substrate 18 prior to and during deposition. This heater is designed to provide uniform heat over a flat surface. The heater 28 is mounted to a heat shield 30 by ceramic insulators 32 and in turn the heat shield 30 is mounted to the lid 12 by other ceramic insulators 34.
An electron beam gun 36 is mounted on collar 14 with its electron beam 38 directed horizontally toward the center of chamber 10. Gun 36 is preferably one specifically designed for vaporization of materials in a vacuum sys tem, such as that manufactured by Denton Vacuum, Inc. and designated model number DEG-801. The electron beam 38 passes through an aperture 40 in a gun shield 42 and is deflected downwardly toward a water cooled copper hearth 44 by deflection plates 46 that are integral parts of the electron beam gun.
A compound 48 to be evaporated is placed in hearth 44 for vaporization by the electron beam 38. It is desirable to shield the substrate from the evaporated material prior to establishment of a constant evaporation rate and following deposition of the compound on the substrate. Therefore, a shutter 50, such as a Japanese fan shutter, is provided between the hearth 44 and the substrate 18.
Temperature of the substrate holder 20 may be monitored by a stainless-steel sheathed chromel-alumel thermocouple 52 in contact with the holder 20. Since it is desirable to control deposition thickness, a quartz crystal oscillator 54 also may be placed adjacent the substrate holder. Oscillator 54 should be heat bafiled and thermally insulated from the heater 28. As deposition on the substrate takes place, simultaneous deposition on the quartz crystal oscillator 54 also occurs thereby varying the output frequency of the oscillator. Therefore, by predetermining the correlation between deposit thickness and oscillator output frequency, the frequency can be monitored and deposit thickness continuously determined. Suitable gases to aid in controlling the composition of the deposited film may also be provided to an area surrounding the substrate through a tube 56 which enters the vacuum chamber through the collar 14 and projects upwardly to a point adjacent the substrate 18.
There are four methods of controlling the electrostatic potential of the substrate 18 utilizing the embodiment of FIG. 1. The particular method selected is determined by the respective positions of two switches 58 and 60. One method is set-up by connecting the substrate 18 to the negative terminal of a variable direct current voltage supply 62 and the positive terminal of the voltage supply to ground G. This configuration is accomplished by moving switch 58 to contact A and switch 60 to contact C. In this configuration, the potential difference between the substrate 18 and the electron gun 36 can be varied by adjusting the voltage level of voltage supply 62.
In a second configuration, switch 60 is moved to open contact D and switch 58 to contact B thereby connecting the substrate 18 directly to ground G. Although utilization of this configuration does not provide means for fine control of electrostatic potential, it does eliminate the build-up of a charge on the substrate.
A third configuration is set-up by moving switch 58 to contact A and switch 60 to contact E. In this configuration, the substrate 18 is isolated from ground and is connected to the negative terminal of voltage supply 62 and the positive terminal of supply 62 is connected to a grid 64 located between the hearth 44 and the substrate 18. Grid 64 is a highly transparent wire mesh that also may be heated to prevent excessive adherence of the vaporated material. One method of electrostatic potential control utilizing this configuration is accomplished by adjusting the voltage of supply 62. Another method of control is also possible by moving grid 64 vertically to various positions with respect to the substrate.
In an experiment performed with the foregoing apparatus utilizing an aluminum foil substrate 18 and chunks of a hot pressed aluminum oxide disk as the compound charge 48, the chamber 10 was evacuated to the mid 10* torr range and the substrate heated to 350 C. This temperature was maintained for one hour to allow adequate outgassing of the substrate. Thereafter, the substrate temperature was lowered to 250 C. for subsequent deposition. With the shutter 50 closed, the electron beam gun 36 was energized and the aluminum oxide charge was preheated. At this stage, the chamber pressure had risen to the low 10- torr range. The crystal oscillator frequency was then monitored to determine when the desired evaporation rate was achieved. Next, an oxygen leak was established through tube 56 to given the chamber a resultant pressure in the torr range. At this point, a DC. voltage potential of 22 volts was established between the substrate 18 and the grid 64 by the appropriate adjustment of voltage supply 62. The shutter 50 was then opened and the substrate 18 was coated to a predetermined thickness. When the crystal oscillator frequency output indicated that the desired thickness had been achieved, the shutter 50 was closed and the oxygen turned off. The apparatus was then allowed to cool to room temperature and the chamber was opened by back filling with nitrogen to break the seal. Later the preceeding experiment was repeated under the same conditions, only this time, utilizing a substrate-to-grid voltage potential of 8 volts.
The results of this experiment indicate that the optical density of the resultant aluminum oxide film can be varied from a dark brown coloration, when a relatively high resultant negative potential is applied to the substrate, to an essentially clear film at relatively low resultant potentials. Therefore, the variations in film shading may be correlated with resultant potential and the results used to predetermine the proper setting of voltage or grid position to meet any particular shading requirement.
FIG. 3 shows the optical density results for the foregoing experiment. Curve A was obtained at the resultant negative potential of -22 volts and Curve B at the re sultant negative potential of -8 volts. Visually, the film of Curve A appeared to be dark brown whereas the film of Curve B had a light brownish shading. Further experimentation confirmed these results and also established that an essentially clear film could be obtained at a 2.5 volts resultant potential.
The color variations noted in the aluminum oxide films prepared at various electrostatic potentials are the result of composition variations induced by the electrostatic charges. One possible mechanism for this result is that dissociated positive aluminum ions and negative oxygen ions are produced during electron beam bombardment. Divergent electrons from the beam negatively charge the substrate which thereby repels the oxygen ions. Proper recombination of the oxygen ions with the aluminum can only occur if the energy of the impacting ions exceeds the repelling potential of the substrate. Therefore, by adjusting substrate potential it is possible to repel or attract various portions of the available oxygen ions.
The foregoing process also is readily adaptable to the deposition of ferroelectric materials, i.e., niobates (lithium and potassium) and titanates (barium and lead). The ferroelectric properties of these materials are dependent upon the composition obtained at the substrate.
It therefore can be seen that the composition of a deposited film of a chemical compound can be changed by the removal of negative charge from the substrate and that the final composition can be controlled by proper adjustment of the substrate potential thereby also determining the resultant electrical and physical properties of the deposited film.
Although four methods of controlling electrostatic potential of a substrate have been described in the foregoing discussion, it is to be understood that there are many other variations in control method that may be used without departing from the scope of the present invention.
1. In a method of depositing a film of an ionisable inorganic compound onto a substrate by electron beam evaporation of a source compound containing the elements of the deposited compound, the improvement which comprises:
(a) applying a variable DC electrical potential between the electron source of said electron beam and said substrate with said substrate being negative with respect to said electron source, and
(b) varying said DC potential to control the composition of said inonganic compound as it is deposited on said substrate.
2. The method defined in claim 1 wherein said compound is aluminum oxide.
3. The method defined in claim 1 wherein said compound is a ferroelectric.
4. The method defined in claim 1 wherein said compound is aluminum oxide and said substrate is silicon. 5. In a method of depositing a film of an ionisable inorganic compound onto a substrate by electron beam evaporation of a source compound containing the elements of the deposited compound, said source compound being supported on a hearth, the improvement comprising:
(a) applying a variable DC electrical potential between said substrate and a grid placed b t h 5 6 substrate and said hearth with said substrate being References Cited negative with respect to said grid and, FOREIGN PATENTS (b) varying said DC potential to control the composition of said inorganic compound as it is deposited on said substrate. 5 D AR P E 1AM I16 6. The method defined in claim 5 wherein said grid is WILL M nmary xaml r adjustably positionable and adjustably positioning said NEWSOMEASSIStam Exammel' grid relative to said hearth and said substrate to control U S Cl XR the composition of said inorganic compound as it is deposited on said substram 10 117-106 R, 123 A, 169 R, 229; 11849.1
1,065,745 4/1967 Great Britain 11793.3