|Publication number||US3634647 A|
|Publication date||Jan 11, 1972|
|Filing date||Jul 14, 1967|
|Priority date||Jul 14, 1967|
|Publication number||US 3634647 A, US 3634647A, US-A-3634647, US3634647 A, US3634647A|
|Inventors||Dale Ernest Brock Jr|
|Original Assignee||Dale Ernest Brock Jr|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (8), Non-Patent Citations (1), Referenced by (14), Classifications (11)|
|External Links: USPTO, USPTO Assignment, Espacenet|
1 man on  Inventor Ernest Brock Dale, Jr. 3,303,319 2/1967 Steigerwald 219/121 2344 Bellehaven Road, Manhatten, Kans. 3,321,278 5/1967 Theuerer.... 1 17/106 66502 3,420,978 1/1969 Webb 219/121 1 1 pp 653,462 OTHER REFERENCES  Filed July 14, 1967 Patented Jan 11 1972 Evaporation Techniques and Properties oflnSb by E, B.
Dale, pp. 348- 353 Primary Examiner-J. V. Truhe  EVAPORATION 0F MULTICOMPONENT ALLOYS Assistant Examiner-G. R. Peterson 18 Claims, 12 Drawing Figs. Attorneys-Giles C. Clegg, Jr. and John 0. Graham  U.S.Cl ..2l9/l2l EB,
118/495, 75/135 B 51 Int. Cl B23k 9/00 A STRACT' A echmque evapmmg alloy by continually feeding a small evaporating area with liquid alloy through a capillary from a sealed reservoir of the 121 igg 'i g liquid. The evaporating area is at a temperature higher than that of the liquid. and the material at this area will be rich in R f d the less volatile element but the evaporant W111 be stolchiom ct-  e erences I e ric. A crucible is sown which is self-heated by current passing  FieldofSearch 219/121,
UNITED STATES PATENTS along its length, with shaping of the cross-sectional area of the 3,367,394 2/l968 Rode! 75/135 crucible providing higher temperatures for the evaporating 2,423,729 7947 Ruhle 219/121 area, and for the upper end of the reservoir, to prevent deposi- 9 1 10959 f 1 tion inside the crucible of nonstoichiometric material from 3,005,859 10/1961 Candldus 219/121 vapor above the i Also, [he evaporating area may be f d 3,121,062 2/1964 Gould 148/1.6 by a thin Solid rod f the alloy instead f by a liquid Source PATENTEU JAN I I 1972 SHEET 1 0F 2 INVENTOR 5. Erock Da/e ATTOR NEY PATENTEUJANI 1 I972 30-63464.
SHEET 2 [1F 2 Fig. 7-A
. Brook 00/9 ATTORNEY IN VENTOR EVAPORATION OF MULTICOMPONENT ALLOYS In the manufacture of thin film electronic devices the preferred technique for forming the necessary metal films is by vapor deposition, ordinarily by evaporation. This technique produces the most continuous, uniform, and controllable films available on a production basis. However, the material to be deposited is often an alloy or mixture of metals, rather than elemental, and this has presented difficulties due to the fact that the components of the alloy have difi'erent vapor pressures at a given temperature and thus evaporate at different rates. The result of this fact is the deposition of a film which is of a composition differing from that of the evaporant charge, or of a layered structure. One attempt to circumvent the problem is to evaporate all of a very small charge of the alloy, producing a film which is stoichiometric on the average, but the film is still stratified. Usually the film is then annealed to attempt to reunite the strata into a stoichiometric alloy. A more successful method of solving the fractional distillation problem is by the simultaneous but separate evaporation of the components, but this method requires critically close control of the temperatures of the sources, such control being virtually impossible to attain in production. These and other attempts have not been effective in producing satisfactory stoichiometric deposits of alloys, especially of semiconductor materials such as lnSb where the quality of the deposited thin film can be measured in terms of electronic mobility or like characteristics. A review of the state of the art in this regard was contained in my paper, co-authored with G. Seneca] and D. Huebner, in the 1963 Transactions of the Tenth National Vacuum Symposium, American Vacuum Society, pp. 348-353, entitled Evaporation Techniques and Properties of InSb Films.
It is therefore the principal object of this invention to provide an improved technique and apparatus for depositing films of multicomponent alloys or mixtures by evaporation.
In accordance with the preferred embodiment of the invention, evaporation of a multicomponent alloy is provided using a crucible which is in the form of a sealed reservoir for containing a charge of the alloy in a liquid state and a capillarylike outlet for delivering a small quantity of the liquid to an evaporating surface which is maintained at a temperature higher than the melt. The reservoir is sealed, after inserting the charge of alloy to avoid loss of vapor which would change the composition of the melt, and also the upper part of the crucible is maintained at a temperature equal to or higher than that of the melt to avoid deposition of nonstoichiometric material on the inside walls since this would also change the composition of the melt. At the evaporating surface, the small quantity of material reaches a steady state rich in the less volatile component and is continuously replenished with stoichiometric material through the capillary, the result being that the vapor and thus the deposit are stoichiometric. The crucible may be self-heated by passing electric current through it, or may be externally heated. The evaporating surface may be heated by electric current or by an electron or ion beam or other beam of energy. In an alternative embodiment, the material to be evaporated is supplied in solid form by feeding a thin rod or wire of the material into an evaporating area heated by an electron or ion beam or the like. A small bead of liquid is thus formed on the end of the rod, from which evaporation takes place. The liquid will be rich in the less volatile element, but the evaporant will be stoichiometric as before. The rod must be shielded from the beam except at its end.
The novel features believed characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as further objects and advantages thereof, will best be understood by reference to the following detailed description of particular embodiments, when read in conjunction with the accompanying drawings, wherein:
FIG. 1 is a schematic representation of evaporating apparatus in accordance with the principles of this invention;
FIG. 2 is an elevation view of a crucible assembly in accordance with one embodiment of the invention;
FIG. 2a is a sectional view of the crucible of FIG. 2, taken along the line 2a-2a in FIG. 2;
FIG. 3 is an elevation view of a crucible assembly in accordance with another embodiment of the invention similar to FIG. 2;
FIG. 4 is an elevation view, partly in section, of another embodiment of the invention;
FIG. 5 is an elevation view in section of a crucible for performing the evaporation technique according to this invention, this embodiment being similar to FIG. 4;
FIG. 6 is an enlarged detailed view, partly in section, of the end of the capillary in embodiments of the invention using a crucible not wetted by the liquid;
FIGS. 7a and 7b are enlarged detailed views in section of the end of the capillary in other embodiments of the invention wherein materials are used for the capillary not wetted by the liquid; and
FIGS. 8a -8c are schematic representations in the form of elevation views of embodiments of the invention utilizing a solid source of the alloy to be evaporated.
With reference to FIG. 1, apparatus for deposition for thin films of multicomponent alloys from the liquid state is illustrated in schematic form. The apparatus basically comprises a crucible 10 which is an enclosed volume except for a small capillary tube 11 depending therefrom. Contained within the crucible 10 is a charge or melt 12 of the alloy to be evaporated, the charge being maintained in the liquid state by a suitable heater 13 which may take various forms as described below. The upper portion of the crucible 10 may be separately heated to a higher temperature by a heater 14 for the purposes described below. The liquid alloy flows through the capillary 11 onto an evaporation surface 15 which may comprise a heated metal strip or the like. The evaporation surface 15 is maintained at temperature higher than that of the melt 12, a temperature suitable for causing evaporation of the alloy as it leaves the end of the capillary 11. A substrate 16 onto which a thin film of the alloy is to be deposited is positioned to receive the evaporated material. A shutter 17 may be interposed between the evaporating surface 15 and the substrate 16 to prevent deposition of the material until a steady state condition is reached.
It is important that the crucible 10 be sealed so that it is gastight except for the capillary through which the liquid flows onto the evaporating surface 15. Of course the crucible 10 would be opened for loading a charge of the alloy to be evaporated and then sealed for the deposition operation. It is required that the crucible be gastight because if vapor escapes from the crucible 10 during the evaporation, the composition of the liquid 12 will change because the escaping vapor has a composition different from that of the liquid 12, the vapor being rich in the more volatile component. If vapor escaped, the extra amount of the more volatile component or components that must be supplied to make up for the vapor lost must come from the melt 12, thereby depleting the melt of these components to some extent. A second reason for having the crucible 10 closed is that, if the crucible is poorly wetted by the liquid, the liquid can be forced out through the capillary 11 by the pressure of the vapor in the upper part of the crucible, it being appropriate to increase the temperature of the crucible to provide a high vapor pressure.
During the evaporation operation, the crucible 10 is maintained above the melting point of the alloy by means of the external heater l3 and 14 as schematically illustrated, or by passing electric current through the crucible proper as described below. The crucible must be maintained at very nearly a uniform temperature throughout, or preferably the upper part of the crucible 10 must be at a higher temperature than the lower part surrounding the melt 12. The criticality of the allowable temperature differential along the crucible is due to the requirement that an insignificant amount of alloy of a composition different from that of the original charge of alloy in the crucible be deposited on the inside walls of the crucible 10. The vapor above the alloy melt.l2 will be at equilibrium with the liquid if the temperature of the crucible is uniform. Any liquid deposited on the inside walls of the crucible 10 above the normal liquid level will therefore be of the same composition as the charge 12, and hence will not cause the composition of the molten alloy flowing through the capillary to change. However, if the upper part of the crucible 10, above the level of the liquid 12, is at a lower temperature than the portion below the liquid level, any liquid deposited on the inside walls of the crucible will tend toward a composition such that it is in equilibrium with the vapor in the crucible at the temperature of this upper portion. In such a situation, the melt could be slightly depleted of one of the components. Strictly speaking, no equilibrium is achieved while a temperature difference along the crucible exists, but there would be tendency toward depositing a liquid of a different composition and thus depleting the melt. To avoid these problems the temperature of the crucible is maintained uniform, or the temperature of the upper portion of the crucible is maintained higher than that of the lower portion so that there will be no deposition on the upper portion. The requirement of temperature uniformity is not stringent in the usual case, when the composition of the vapor above the liquid changes rather slowly with the temperature; however, if there is a temperature variation it is advantageous to have the upper portion of the crucible at a higher temperature than the lower portion. it is for this reason that the heater 13 and 14 is illustrated as being split apart in two separate components, one surrounding the lower portion of the crucible and the other surrounding the upper portion. These two parts of the heater coil could be separately controlled, or of course the coils could be merely wound closer together in the portion 14.
With the deposition apparatus of FIG. 1, the thin film deposited on the substrate 16 will be a multicomponent alloy having the same composition as that of the original charge of alloy which provides the melt l2. Ordinarily this material would be a stoichiometric material such as the alloy indium antimonide or others as suggested below. The principle of the operation of the apparatus is that the stoichiometric material is supplied to the evaporating area at the same rate as which the material is evaporated so that the small quantity of material at the evaporating surface 15 reaches a steady state such that the evaporant is also stoichiometric. This steady state is not exactly realizable in practice, but it is very closely approximated by supplying the evaporating area 15 with fresh stoichiometric material through the capillary 11 connected to the much larger reservoir of liquid 12. A simplified analysis will reveal that deposition of stoichiometric material will be quite closely approximated.
The analysis is based on the assumption l that the volume of liquid on the evaporating surface remains constant as evaporation takes place, material being supplied to the evaporating surface from the reservoir at the same rate that it is lost from the surface by evaporation. It is further assumed, in order to simplify the analysis for the purpose of illustration, (2) that the two components have equal molar volume in the liquid state, and (3) that the ratio of vapor pressures of the components remains constant as the temperature changes, although the vapor pressures themselves may change. Assumptioris (2) and (3) are not exactly fulfilled in practice, but they are in no way necessary conditions to the establishment of steady state.
Under these assumptions, the analysis shows that during the approach of the composition of the vapor to steady state, the composition of the liquid on the evaporating surface is given y 1o lB)( 1O 1) where N and N represent the concentrations of component l and 2 in the charge in units of atoms/cm; N,, and N represent the concentrations of components 1 and 2 on the evaporation surface after steady state has been established; N and N represent the concentrations of components l and 2 at a time t after the beginning of the evaporation; v is the velocity of flow through the capillary; A is the cross-sectional area of the capillary; and V is the volume of material on the evaporating surface.
In a general multicomponent system, the approach to equilibrium is given, under the same assumptions, by the solution to a set of equations, one for each component, of the form V(dN,,. /dt =1v vA-B,,.N,,- where the subscript k denotes the component, 8,, is a multiplier such that B N is the evaporation rate of the kth component from the evaporating surface, and the other symbols have the same meaning as before. For a two-component system, these equations give the result above if the ratio 8 /8 is assumed to be constant. This is equivalent to assumption (3) above.
Assumption (l) is necessary to the maintenance of steady state once it is established, but it is not necessary to the establishment of steady state. For example, the volume V may increase or decrease with time during the approach to steady state, approaching a constant value as steady state is approached. This assumption is physically realizable in the steady state because, for any reasonable value of the flow rate through the capillary, there exists a rate of heat input to the evaporating surface and an area of evaporating bead such that the material will evaporate at the same rate that it flows onto the evaporating surface. If the power input to the evaporating surface is higher than this, the rate of evaporation is faster than the feed rate resulting after a time in a diminution of the size of the droplet and consequent diminution in area and hence of the evaporation rate. After a time the evaporation rate therefore approaches a constant value equal to the rate of flow onto the evaporating surface. Conversely, a slight decrease in the rate of power input results in an increase in the size of the evaporating droplet so that this evaporation rate eventually approaches the rate of flow. Another somewhat idealized analysis shows that the problem of diffusion currents, superimposed on the flow by virtue of a concentration gradient along the capillary, becomes less and less important as vL/D becomes larger and larger compared to unity. The diffusion current, which would result in contamination of the material in the reservoir by an excess of one component, and in a change in the ratio of the net flow rates of the components, can be made negligible by physically realizable values of the quantity vL/D. For this analysis, the current densities J, and J in atoms/cm. /sec. of the first and second components, respectively, through the capillary may be expressed:
where v is the linear velocity of the liquid through the capillary 11 connected to the much larger reservoir of liquid 12, L is the vertical length of the capillary from the lower end of the crucible 10 to the evaporating area 15, 8,, and B are defined by the equations B,,N,,,=N, v and B ,N ,=N v, D is the diffusion coefficient, and the other symbols have the same meaning as above. The current densities flowing into the capillary due to flow alone, disregarding diffusion, are vN and vN so the difference between these values and, respectively, J, and J represents the rate of diffusion of the components out of, or iback into, the melt 12 per unit area of the capillary 11. These jdifferences which are preferably very close to zero, approach lzero as vL/D becomeslarge compared to unity. This condition -is favored by a very narrow capillary and high evaporation rate.
With reference now to FIG. 2 of the drawings, one embodiment of a crucible for evaporation of multicomponent alloys according to the principles described above will be discussed. In this embodiment a crucible 20 is provided wherein the reservoir, capillary and evaporating surface are all continuous or integral portions of the same piece of metal. The crucible 20 is fabricated from a length of welded or seamless tubing, or from a thin metal sheet, and by appropriate shaping of this metal a capillary 11 is provided at the lower end of a reservoir 22, while electrodes 23 and 24 are fashioned at opposite ends of the assembly for application of electric current thereto for heating. The capillary 21 is formed by seam welding or overlapping spot welding of the metal along lines 25. Likewise. the upper end of the assembly is sealed, after introducing a charge to provide a melt 26, by seam welding the metal together and also narrowing or drawing out the metal of the tubing at an area 27. This provides a cross-sectional area 27 smaller than that occupied by the walls of the reservoir 22 so that current flowing vertically through the crucible 20 will heat the upper end of the crucible more than the body of the crucible. The cross-sectional area of the area 27 is also smaller than that of the electrodes 23 and 24 along with their associated lead-in conductors. At the lower end of the crucible 20, an evaporating surface 28 is provided just below the lower end of the capillary 21, and this evaporating surface is heated to a higher temperature than that of the reservoir 22 due to the fact that the metal is cut away so that this area has a smaller cross-sectional area than that of the crucible walls or the electrodes 23 and 24. Inaddition, in the embodiment of FIG. 2, separate control of the temperature of the evaporating surface 28 is provided by current from a separate source passing between a pair of electrodes 29 and 30. It is noted that current flowing through electrodes 23 and 24 heats the reservoir 22 and the capillary 21 as well as the evaporating surface 28, the amount of metal at the evaporating surface being smaller in cross section than that of the crucible; hence the evaporating surface 28 runs at a higher temperature. Current from the second source connected across the electrodes 29 and 30 heats principally the evaporating surface 28, giving a degree of independent control of the temperature of the evaporating surface.
In FIG. 2a is seen an enlarged sectional view of the portion of the crucible assembly around the end of the capillary and evaporating surface. While not to scale in the drawings, the shape and size of the bead of liquid at the evaporating surface should be noted. The volume of liquid here on the evaporating surface 28 should be small because the approach to steadystate goes exponentially as l/V where V is this volume. The asymptotic expression for the fractional deviation from steady-state is proportional to exp-Rt/V, Rt being the total flow from the reservoir (and hence the total amount evaporated) in the time t. The evaporating material is con fined to a narrow bead along the lower edge of the capillary plus a very small wetted region beyond the mouth of the capillary. The size of the wetted region is automatically limited if the flow rate is not too high, it growing until it reaches an area such that evaporation takes place as fast as material flows out, then it grows no further. This regulatory aspect is enhanced by a positive temperature gradient from the capillary mouth toward the edges of the hot surface.
With reference now to FIG. 3, a two electrode design for the crucible of FIG. 2 is illustrated. This embodiment is similar in all respects to the four electrode design of FIG. 2 except that the auxiliary electrodes 29 and 30 are absent. The temperature of the evaporating surface 28 is controlled solely by the current through the electrodes 23 and 24, with the temperature of the evaporating surface in relation to that of the crucb ble 22 being fixed by the size of the constriction through which the current must flow. It is noted that the cross-sectional area of the evaporating surface 28 is less than that of the electrodes 23 and 24 and of the walls of the reservoir 22.
The crucible of FIGS. 1, 2 or 3 would of course be mounted in an evacuable chamber such as a bell jar arrangement and would be positioned to direct the evaporant toward a substrate as in FIG. I, ordinarily a shutter arrangement being used. The crucibles 20 may be composed of metal such as tantalum, a material which is wetted by indium antimonide or gallium arsenide or other alloys to be evaporated. This material also exhibits substantial resistance to the flow of electrical current whereby heating may be provided at a fairly reasonable current level. Of course, as set forth below, the crucible 20 may be formed of other material which may not be wetted by the alloy which is to be evaporated, in which case other provision is made for delivering the molten alloy through the capillary.
In place of the self-heated crucible of FIGS. 2 and 3, externally heated crucibles may be used wherein heating is provided by thermal conduction, radiation, or by electron bombardment. Heaters of this type are commercially available and will not be described in detail, although in FIG. 1 heaters 13 and 14 are schematically illustrated as coils, and in FIG. 4 a coil 31 is depicted schematically as illustrative of a heater employing thermal conduction or radiation principles. FIG. 4 also illustrates another alternative feature of the crucible of the invention, this embodiment employing a crucible 32 composed of a material wetted by the alloy 33 to be evaporated. The upper end of the crucible 32 is sealed by a threaded stopper or cap 34 or may be sealed by a cap employing a tapered friction fit. In the crucible of FIG. 4, the wall thickness can be greater than is practical with crucibles heated by flow of electric current; thus, the crucible can be cast, machined, or sintered. The thick walls in an embodiment as in FIG. 4 facilitates maintaining a uniform temperature along the crucible due to the thermal inertia of the larger body. At the lower end of the crucible 32 a capillary 35 is provided as before, with the liquid spilling out from this capillary at an evaporating surface on a metal strip 37 which may be separate from that of the crucible 32. This strip 37 may be heated by electric current flowing along its length; thus, separate control of the temperature of the evaporating surface 36 may be provided. Alternatively, the evaporating surface 37 may be heated by electron bombardment wherein a beam of electrons is directed or focused onto the immediate vicinity of the evaporating surface by means of a commercially available electron gun mounted within the evacuated chamber.
With reference now to FIG. 5, a crucible very similar to that of FIG. 4 is illustrated utilizing an evaporating surface 40 which is situated such that liquid from a capillary 41 flows up onto it from underneath. The evaporating surface 40 may be the central part of a strip heater arrangement wherein current is passed between a pair of electrodes 42 and 43. The reservoir and sealing stopper, as well as the heater (not shown), are the same as in FIG. 4. A radiator 44 surrounding the capillary just below the evaporating surface functions to prevent the capillary from reaching an excessive temperature which it may tend to do because of its proximity to the high-temperature evaporating surface. Instead of being heated by electric current, the evaporating surface may, as mentioned above, be heated by an electron beam directed thereon.
It is sometimes necessary or expedient to make the crucible from a material which is not wetted by the alloy to be evaporated. For example, the alloy to be evaporated may be Nichrome, the trade designation for a nickel-chromium alloy, and the crucible constructed of alumina. Usually crucibles of this type must be externally heated, rather than by electric current. Otherwise the comments above about the necessity for sealing the crucible, for maintaining the temperature uniform and generally the required shape of the crucible apply equally to crucibles that are not wetted by the molten charge. The principal distinction is that if the crucible is not wetted by the charge, liquid will flow into and through the capillary only under pressure. This necessitates quite close control of the pressure within the reservoir, an excess pressure causing liquid to flow out through the capillary at too high a rate, this occurring as soon as the pressure becomes greater than that necessary to force the liquid through the narrowest path of the capillary.
In order to partially alleviate the stringent requirement of the control of the pressure within the crucible when it is constructed of a material not by the liquid, a concept as illustrated in FIG. 6 may be utilized. In this embodiment a capillary 45 composed of a material such as alumina not wetted by the liquid 46 has inserted therein a wire or narrow strip 47 which is composed of a different material such as tantalum which is wetted by the liquid. This wire 47 functions to conduct the liquid through the capillary and out onto the evaporating surface. The remainder of the assembly would be generally as seen in FIG. 4. Alternatively, the capillary 45 might be lined with a tube composed of a material wetted by the liquid.
Also, instead of relying upon increased vapor pressure within the crucible to force out the liquid this may be accomplished by gradually decreasing the volume which the liquid can occupy in the reservoir, as by a piston pushing against the liquid, for example. Thus, positive displacement would be utilized.
Another technique for overcoming the difiiculty which results from the use of a crucible not wetted by the liquid comprises forming the narrowest part of the crucible as an orifice at the extreme tip of a capillary made of a material either wetted by the liquid or not wetted by the liquid. With reference to FIGS. 7a and 7b, this technique is illustrated wherein a detailed enlarged view of the end of the capillary is shown in an embodiment otherwise similar to that of FIGS. 4 or 5. in FIG. 7a a capillary 48 composed of a material not wetted by the liquid is shown terminating in an orifice 49 at which the liquid 50 is constrained so that evaporation takes place from just inside the orifice at the liquid surface, in this case it not being necessary for the liquid to flow through the orifice. This can be accomplished by heating the tip of the capillary 48, or the orifice 49, by an electric heater, or by bombarding the liquid surface itself with an electron beam as before. In like manner, as illustrated in FIG. 7b, the capillary 51, composed of a material not wetted by the liquid, may be terminated in an orifice 52 composed of a material which is wetted by the liquid, in which case the liquid will flow through the orifice onto its outer surface where evaporation will take place. In the embodiment of FIG. 7b, heating can be accomplished by electron or ion bombardment, by passing current through the evaporating surface, or by radiation from an external heater.
With a crucible such as described above, particularly the embodiment of FIG. 2 composed of tantalum, lnSb may be evaporated with a reservoir temperature of 700-900 C. and with an evaporating surface temperature of 9001,200 C. Bismuth-antimony may be deposited with this crucible using temperatures of 500700 C. for the reservoir and 700-l, 000 C. for the evaporating surface. Germanium-antimony (0.1 percent Sb) may be used at l,000-1,200 C. and l,200 C., respectively, or germanium-indium at similar temperatures. Also with this crucible mixtures or alloys of silver with copper, beryllium, tin, strontium, thallium or manganese may be deposited. Also combinations of more than two of these metals may be used.
Using a crucible of the FIG. 2 design composed of molybdenum, any of the material just mentioned could be deposited, and in addition chromium, nickel iron, platinum or others, including combinations could be deposited.
It is noted that a tantalum crucible is specified for some of the materials in the preceding paragraphs, while a molybdenum boat is specified for others. This is because some of the metals wet tantalum and others molybdenum. Most metals are wetted by other metals, pertinent exceptions being lead, which does not wet Ta or Mo, and Cu which wets Ta and Mo poorly. Thus these factors must be considered in selecting the crucible. For nonmetallic crucibles, it will be noted that most ceramics are not wetted by liquid metals, although there are exceptions to this such as indium-wetting quartz or glass. Thus the crucible designs noted above for nonwetting combinations refer primarily to nonmetallic crucibles.
It will be noted that the technique of the invention is applicable to almost any mixture. Exceptions would be (a) mixtures in which one or more components begin to evaporate i.e., have a significant vapor pressure) at a temperature lower than the melting point of one or more other components and (b) mixtures containing one or more constituents that are very reactive when molten, the latter including many of the materials that melt above about l,500 C., the difiiculty being that such materials react with or dissolve the container. Molten sil icon dissolves all of the metals to some extent, and also reduces or dissolves quartz (silicon dioxide) and other oxides. Silicon could be evaporated from a beryllium oxide crucible, but the resulting films would tend to have silicon monoxide contamination due, perhaps, to oxygen dissolved out of the crucible.
Examples of materials unsuitable due to (a) in the preceding paragraph are mercury, selenium, sulfur, cesium (high vapor pressure) mixed with germanium, gold silver, nickel, etc. Exceptions to this case are such materials as 1:] cadmium with sulfur which forms a compound that is stable when molten, for example.
While evaporating the materials mentioned above or others, the temperature of the upper part of the reservoir must be few degrees hotter than that of the surface of the liquid, commonly a temperature difference of less than perhaps 50 C., preferably about 10 C. Ideally the temperatures should be equal, but this temperature differential is an expedient way to avoid the control problems inherent in attempting to keep the temperatures the same. Also, the temperature of the liquid should all be the same, ideally, but of course in a practical embodiment the tip of the capillary is somewhat hotter than the remainder because it is adjacent the evaporating surface and the two are connected by the evaporating liquid. However, since the shape of the crucible is such that the temperature gradient is steep, this introduces no departure from the intended conditions.
The capillaries in the embodiments described above, particularly FIG. 2, may be of dimensions of from about 5 to 16 mils in inside diameter and from 1 to 7 mm. long. Preferably the diameter is above 10 mils to prevent clogging. These dimensions given are not to be construed as critical, but merely as examples. The principal requirement in the capillary is that it be long enough and the linear flow velocity through it high enough so that the diffusion of material from the hot surface back to the reservoir be negligible at the desired rate of flow.
It will be noted that one of the basic principles involved in the evaporating techniques described above is that of performing the actual evaporation from a quite small quantity of the multicomponent alloy but yet supplying new material to the evaporating area at a rate such as to maintain a steady state wherein the vapor has the same composition as the original alloy. Whereas the crucibles and techniques thus far described utilized evaporation from a charge initially in the liquid state, the same principal may be implemented by apparatus wherein the charge is in the solid state. To this end, evaporation is accomplished from the end of a solid rod, wire, strip or tube which is composed of the alloy or mixture which is to be evaporated. Heat to cause evaporation may be applied by bombardment with a beam of electrons or ions, or a com centrated beam of radiation. With reference now to FIG. 8a 8c, it will be seen that three examples of this embodiment are illustrated wherein the charge in solid form is fed mechanically into the path of the concentrated beam of heating energy at the same rate the material is evaporated, with evaporation taking place at a molten bead at the end of the charge. In FIG. 8a, a rod or wire 60 of the material to be evaporated is mechanically fed up through a small cylinder or tube 61 while a beam 62 of ions, electrons or radiation is generally focused on the end of the tube to create a molten bead 63 from which the alloy is evaporated. The quantity of material in the bead 63 is limited so that the principles described above apply and the deposited film on the substrate very closely approximates the composition of the rod 60. The tube 61 functions as a shield so that the beam 62 need not be precisely focused and may be broader than the desired molten area. The shield or tube 61 may be cooled by fins or by a heat sink, and the shield may be in direct mechanical contact with the stock or rod 60 to facilitate movement of the stock within the shield in which case it may be preferable that the beam 62 be virtually horizontal so that the upper edge of the tube 61 limits the area of heating. The stock or rod 60 is fed upwardly at a rate corresponding to the evaporation of material therefrom. The diameter of the rod would be in the range of from a few mils up to perhaps as large as 100 mils.
In another embodiment as seen in FIG. 8b the stock 60 is likewise fed upwardly through a tube or shield 61, but the heating source is supplied from a circular filament 64 which functions as an electron source. With the rod 60 and/or the shield 61 suitably biased with respect to the filament 64, a stream of electrons will bombard the upper end of the rod 60 and create a molten bead as before. Evaporation will take place from the melt as described previously, material being deposited upon a substrate after passing through the open center of the circular filament.
instead of using the shield 61 as illustrated in FIGS. 8a and 8b the rod 60 may be suspended along, fed upwardly as before, with a beam 65 of electrons, ions or other radiation, being directed onto the upper end of the rod through a mask 66 which limits the extent of the molten area by limiting or concentrating the beam. The rod 60 would be mechanically fed into the point upon which the beam impinges at a rate corresponding to removal of material by evaporation. It will be noted of coursethat while the rods are shownbeing fed upwardly, the reverse could be used, i.e., the rod fed downwardly with evaporation taking place at the bottom.
Some of the restrictions on the use of the technique, such as for mixtures wherein one component evaporates at a temperature lower than the melting point of the other, do not apply here when a solid rod is used. This technique is useful for any mixture that can be made into a solid rod of uniform composition.
While this invention has been described with reference to particular embodiments, it is of course understood that this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this application. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.
1. A crucible assembly for depositing a mixture of alloy of at least two components by evaporation comprising a sealed reservoir for containing a charge of such material less than the volume of said reservoir, a capillary tube outlet from the reservoir located below the level of the liquid charge, an evaporating surface positioned at said outlet to receive a small quantity of the liquid charge and heating means for heating the sealed reservoir to maintain the charge in a liquid state and to maintain the temperature of the sealed reservoir above the level of the liquid charge at a temperature at least as great as the temperature of the portion of the sealed reservoir surrounding the liquid charge and for heating the evaporating surface to maintain the temperature of the evaporating surface higher than that of the liquid charge to produce evaporation of the components thereof, said evaporating surface having an area sufficiently large that the exposed area of the evaporating liquid is great enough to make the evaporation rate equal to the feed rate of the liquid charge through the capillary tube, the length L of the capillary tube being sufficiently large with respect to the diameter D of the capillary tube that the ratio vL/D where v is the linear velocity of the liquid through the capillary tube, is sufficiently large with respect to unity to prevent uncontrolled mixing of the evaporating liquid and the liquid charge and restrict contamination of the liquid charge by diffusion of one compound from the evaporating liquid to the liquid charge.
2. A crucible assembly according to claim 1 wherein the heating means includes a pair of electrodes positioned at opposite ends of the reservoir whereby electric current may be passed through the reservoir.
3. A crucible assembly according to claim 2 wherein the crucible assembly includes a depending portion integral therewith for providing said evaporating surface.
4. A crucible assembly according to claim 3 wherein the depending portion is of reduced cross section to increase heating thereof due to said electric current.
5. A crucible assembly according to claim 3 wherein the depending portion includes a second pair of electrodes whereby electric current may be passed therethrough for control of heating of the evaporating surface independently of the remainder of the crucible assembly.
6. A crucible assembly according to claim 4 wherein the upper end of the sealed reservoir is of reduced cross section to provide heating of the reservoir above the level of the liquid charge to temperature slightly greater than that of the portion of the reservoir surrounding the liquid charge.
7. A crucible assembly according to claim 5 wherein the upper end of the crucible sealed reservoir is of reduced cross section to provide heating of the reservoir above the level of the liquid charge to a temperature slightly greater than that of the portion of the reservoir surrounding the liquid charge.
8. A crucible assembly according to claim 1 wherein the capillary-type outlet is composed of material wetted by the charge whereby the liquid flows out by capillary attraction.
9. A crucible assembly according to claim 1 wherein the crucible is composed of a material which is not wetted by the liquid charge and wherein an elongated member composed of a material wetted by the liquid charge is included in said capillarylike outlet.
10. A crucible assembly according to claim 1 wherein the crucible is composed of a material which is not wetted by the liquid charge and wherein the outlet at the smallest diameter is composed of a material wetted by the liquid charge.
11. A crucible assembly according to claim 1 wherein said evaporating surface is heated by a beam of energy directed thereon.
12. A crucible assembly according to claim 11 wherein the crucible is composed of a material which is not wetted by the liquid charge and wherein the outlet is of smallest diameter at its outermost extremity, with the beam of energy heating the evaporating surface such that evaporation takes place from the liquid before the liquid traverses the outlet.
13. A crucible assembly according to claim 1 wherein flow of liquid through said outlet is aided by vapor pressure in the reservoir above the liquid charge.
14. A crucible assembly according to claim 1 wherein flow of liquid through said outlet is aided by positive reduction of the volume of the crucible to reduce the volume available for containing the liquid charge.
15. A crucible assembly according to claim 1 wherein the means for heating includes means for passing electric current through the reservoir and the upper end of the reservoir is of reduced cross section to provide the function of maintaining the temperature of the reservoir above the level of the liquid charge at a slightly greater temperature.
16. A method of depositing a film of a multicomponent material which is an alloy or mixture comprising the steps of feeding a narrow elongated column of said material into a small evaporating area, directing a concentrated beam of energy onto said evaporating area to heat said material in said area to a temperature above that necessary to cause evaporation of all of the components thereof, shielding the remainder of the column of said material from said beam, the elongated column being fed into said evaporating area at a rate corresponding to evaporation of material from the evaporating area.
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|U.S. Classification||219/121.15, 118/726|
|International Classification||C23C14/24, H05B3/00, C23C14/26|
|Cooperative Classification||C23C14/26, C23C14/246, H05B3/0014|
|European Classification||C23C14/26, H05B3/00B, C23C14/24B|