US3405251A

US3405251A - Vacuum evaporation source

Info

Publication number: US3405251A
Application number: US553981A
Authority: US
Inventors: Reuben S Spriggs; Arthur J Learn; Marinoff Charles
Original assignee: TRW Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1966-05-31
Filing date: 1966-05-31
Publication date: 1968-10-08
Anticipated expiration: 1985-10-08

Description

1968 R. s. SPRIGGS ET AL 3,405,251

VACUUM EVAPORATION SOURCE Filed May 31/ 1966 Rueben S. Spri'ggs,

Arthur J. Learn,

Charles Morinoff, INVENTORS.

AGENT.

United States Patent O-"ice Ohio Filed May 31, 1966, Ser. No. 553,981 9 Claims. (Cl. 219--275) This invention relates generally to improvements in the art of vapor deposition, and more particularly to a vacuum evaporation source characterized by its improved efliciency and its ability to consistently deposit films with a high degree of thickness uniformity.

Some of the factors which are important in the design of a vacuum evaporation source are its temperature uniformity, temperature range, fiux density, and spatial distribution of evaporant. Other considerations are the ability to reproduce conditions of evaporation with a given source and freedom from chemical reaction or alloying between source and evaporant.

Additional problems arise when powder compounds are evaporated. Often such evaporation from metal strips or crucibles suffers from ejection of powder particles, particularly if the compound sublimes or dissociates. These eruptions are caused by released gases which have sufficient force to carry particles of the materials to the substrate. The particles, even when very small, may have adverse effects on electronic or optical properties of films. Methods generally used to circumvent the effects of particle ejection involve the use of a large substrate-to-source distance or a low source temperature. However, these techniques suffer from increased evaporation time and require a large charge of evaporant material. Another problem prevalent in the evaporation of compounds is the inadequate control of temperature. This problem is largely due to poor thermal conductivity of polycrystalline powders.

In accordance with the invention, a vacuum evaporation source comprises a block of thermally conducting material having a top surface thereof honeycombed with a multiplicity of uniformly spaced cylindrical cavities which serve as containers for the evaporant powder material. The depth of the cavities is less than the thickness of the block.

The lower portion of the block below the powder containing cavities is provided with a second group of cavities, which preferably extend into the block from side surfaces thereof and at right angles to the powder containing cavities. Each cavity of the second group houses an electrical heater. Means are provided for connecting the electrical heaters in parallel and to a pair of external leads for connection to a source of electrical power. The electrical heaters serve to heat the block and the powder evaporant material to a uniform temperature sufiicient to vaporize the powder evaporant.

The dimensions of the powder containing cavities and the minimum wall thickness of block material between adjacent cavities are designed such as to promote high evaporation rates and uniform film deposits without incurring particle ejection.

In the drawing:

FIG. 1 is an enlarged perspective view of one form of a vacuum evaporation source according to the invention;

FIG. 2 is a sectional view taken along line 22 of FIG. 1;

FIG. 3 is an enlarged fragmentary section of a portion of the source shown in FIG. 1; and

FIG. 4 is an enlarged fragmentary plan view of a modified form of vacuum evaporation source in which the evaporant powder containing cavities are arranged in a staggered array.

Referring now to FIGS. 1 and 2, there is shown a Patented Oct. 8, 1968 vacuum evaporation source 10 comprising a solid block 12 of thermally conducting heat refractory material, such as molybdenum or the like. The block material should be one with relatively low vapor pressure at the operating temperature. It should also be a material that does not alloy or chemically react readily with the evaporant material. The block 12 has upper and lower

major surfaces

14 and 16 respectively of great extent relative to its thickness dimension. As shown, the major cross section of the block 12 is a square, for applications where it is desired to deposit an evaporated film of square configuration. For applications where a film deposit of circular configuration is desired, the block 12 may have a circular shape. Thus, the block can be shaped to conform to the shape of the substrate or area to be coated.

The upper surface 14 of the block 12 is honeycombed With a multiplicity of substantially uniformly spaced cylindrical cavities 18 which extend to a depth approximately one-third the thickness dimension of the block 12. The cavities 18 are filled with a desired powder evaporant material 20, such as cadmium sulfide or silicon monoxide, for example. For the sake of clarity, the powder material 20 is shown in several of the cavities 18 only.

The block 12 typically may be 1 /2 inch on a side by A inch thick. The block -12 may contain 144 cavities 18 arranged in 12 parallel rows of 12 cavities each, with the cavities aligned in 12 parallel columns, as shown in FIG. 1. Each of the cavities 18 may be inch in diameter and /8 inch deep, with a center-to-center spacing horizontally and vertically of /8 inch, thereby providing a minimum wall thickness of inch between cavities.

Alternatively, the cavities 18 may be arranged in staggered fashion, vas in the block 12a of FIG. 4. In this embodiment, the cavities have the same spacing in the horizontal and diagonal directions, but a larger cavity spacing occurs in the vertical direction, whereas in the embodiment of FIG. 1 the cavities have the same spacing in the horizontal and vertical directions but a larger spacing in the diagonal directions. The embodiment of FIG. 4 provides a higher density of cavities. That is, the same number of cavities can be accommodated in a smaller block area, with the same minimum wall spacing between cavities being maintained.

The lower portion of the block 12 is provided with a second plurality of cavities 22 extending from the sides of the block 12 thereinto in directions transverse, or at right angles, to the evaporant containing cavities 18. Each of the second plurality of cavities 22 contains an electrical heater coil 24 for heating the block 12 and thereby heating the powder evaporant material 20 to the vaporization temperature of the material 20. For a block 12 of the size described above, the heater coils 24 may be arranged six in each of two opposing sides of the block 12 and twelve in each of the other two opposing sides. Typical dimensions for each heater cavity 22 may be 7 inch diameter by inch deep. The entire length of the heater coils 24 are disposed within the cavities 22 for minimizing radiation losses from the heater coils 24 to the environment external to the cavities 22. The heater coils 24 are coated with insulation material to prevent short circuiting to the block 12. I

A preferred arrangement for supplying electrical power to the heater coils 24 will now be described. A strip or bus bar 26 of tantalum is spot welded to the sides of the block 12 at regions just above the openings of the heater cavities 22. One leg 28 of each heater coil 24 is spot welded to the tantalum bus bar 26, which in effect electrically connects all of these one heater legs 28 in parallel and to the block 12. The tantalum bus bar 26 is needed to provide a readily weldable surface for the tungsten heater coils 24, which do not weld readily to the molybdenum block 12.

A sheet 30 of insulating material, such as mica, is dis posed on the lower surface 16 of the block 12. The insulating sheet 30 has a central opening 32 of sufficient diameter to clear a threaded bolt 34 that is screwed into the block 12 through its lower surface 16.

A conducting plate 38, which may be made of steel, is disposed on the bottom surface of the insulating sheet 30, the latter insulating sheet 30 serving to insulate the conducting plate 38 from the block 12. The conducting plate 3 8 has a central opening 40 of larger diameter than the threaded bolt 34. The clearance space between the conducting plate 38 and the bolt 34 serves to electrically insulate the conducting plate 38 from the bolt 34 and the block 12.

The block 12, the insulating sheet 30 and the conducting plate 38 are sandwiched together by means of a nut 42 threaded onto the external end of the bolt 34, with an insulating washer 44 between the nut 42 and conducting plate 38 serving to further insulate the conducting plate 38 from the bolt 34 and block 12.

A first electrical lead-in wire 46 is conductively connected to the bolt 34, as by means of a second nut 48 threaded onto the bolt 34 to hold the wire 46 tight between the two

nuts

48 and 42. The first lead-in wire 46 is thus conductively connected through the bolt 34, the block 12, and the bus bar 26, to the first heater legs 28 of the heater coils 24. The other heater legs 50 are connected in parallel to the conducting plate 38, as by spot welding. A second lead 'in wire 52 conductively connected to the conducting plate 38 serves as an external connection to theother heater legs 50.

The heater coils 24 may be energized by connecting the lead-in

wires

46 and 52 to the secondary winding of a 6.3 volt filament transformer. The primary voltage may be varied somewhat to vary the power supplied to the heater coils 24 within desired limits. Typical heater coil current is in the neighborhood of 600 milliamperes for each coil.

In order to achieve an optimum design for the honeycombed evaporation source, it has been found that several dimensional parameters must be considered. The physical dimensions of the evaporant powder containing cavities 18 and the spacings between adjacent cavities will affect such characteristics as the evaporation rate, the thermal efiiciency the maximum achievable thickness of film deposit, and the thickness uniformity of the film deposit.

One of the factors that must be considered is the depth of the cavities 18. Referring to FIG. 3, the depth D of the cavities 1'8 affects the amount of powder that can be evaporated and also the heat distribution within the volume of the powder. For example, if the cavities 18 are too shallow, the amount of powder material that can be evaporated is limited, which places limitations on the thickness of the film deposit and the film deposition rate. If the cavities 18 are too deep, temperature gradients will be set up along the length of the powder evaporant within the cavities 18.

Temperature gradients will arise from several effects. Because of the relatively large mass of block material in the lower portion of the block 12, that is, below the cavities, the heat conductance will be higher in the lower regions of the block 12, and the temperature will be higher there than in the regions that divide the cavities 18.

The powder within each cavity 18 receives its heat from the bottom and side wall surfaces of the cavity 18. The heating effect of the bottom surface of the cavity 18 will first be considered. If the cavity 18 is too deep, there will be an excessively long heat path from the bottom to the top surface of the powder. Since most powder compounds from which thin film deposits are usually made are relatively poor thermal conductors, a relatively long heat path through the powder material will result in large temperature gradients along the powder material such that the upper levels of the powder material Will be cooler than the lower depths of the powder material.

If the temperature gradients are extreme, it will result in particle ejection or spitting. That is, faced with the conditions of high temperature gradients, the operator would tend to raise the heat input to heat the upper levels of the powder. Raising the heat input lowers the efiiciency of the vapor deposition source. More importantly, however, as a result of raising theheat input, the lower levels of the powder would experience much higher temperatures than necessary for evaporation. These high temperatures would'result in high evolution of gases which would carry with them some of the cooler unvaporized powder particles and eject the latter onto the deposition surface. Such particle ejection is called spitting.

The powder material also receives its heat from the side wall surfaces of the cavity 18. A deep cavity would result in a long heat path along the length of the cavity wall. Thus the cavity wall surface near the cavity opening would be cooler than the cavity wall surface near the bottom of the cavity, and the powder material would likewise experience a temperature gradient from this effect.

An alternative to raising the heat input to compensate for high temperature gradients resulting from the last mentioned effect is to space the cavities 18 farther apart to reduce the thermal resistance of the block material separating the cavities. However, the spacing w of the cavities 18 should not be made too large, as this would unduly reduce the volume of the powder material per unit surface area of block material and would unduly reduce the maximum achievable thickness of film deposit.

In accordance with one aspect of the invention, it has been determined that to achieve practical deposition rates and film thicknesses, the volume of the cavities per unit surface area of block material encompassing the cavities, should be no less than .005 cubic inch per square inch. It has also been found that the minimum wall thickness w between adjacent cavities should be at least /5 the depth of the cavities in order to minimize the aforementioned temperature gradients and eliminate particle ejection.

Another factor to be considered in the design of a honeycombed vapor deposition source is the diameter d of the cavity. As mentioned previously, most useful powder compounds have a low thermal conductivity. Hence, temperature gradients can be experienced along the radial vextent of the cylindrical volume of powder. Thus, if the diameter of the cavity 18 is too high, the central portions of the powder will be substantially cooler than the peripheral portions nearest the cavity wall surfaces. As a result, evaporation may occur only from a thin annulus in the immediate vicinity of the cavity walls. All other factors remaining fixed, the useful thermal penetration depth, or the thickness of the aforementioned annulus will depend principally on the thermal conductivity of the particular powder used. The higher the thermal conductivity the greater the useful thermal penetration depth.

The absence or presence and extent of the aforemenw tioned annulus for a single cavity can be observed by employing a pinhole camera technique of the kind disclosed in an article by L. D. Preuss and C. E. Alt, entitled The Evaporation Mode of Certain Vacuum Metal Distillation Source Configuration, 1957 Fourth National Symposium on Vacuum Technology Transactions, American Vacuum Society, Inc., pages 47-72, published by the Symposium Publications Division, Pergammon Press, New York. Briefly, the technique described in the aboveidentified article consists in disposing an aperture plate or pinhole between a vapor deposition source and a target plate or substratefThe deposit produced on the substrate 'is an image of the distillation intensity of all points along the source within the confines of the solid angle subtended by the aperture. The image will consist of a thin metal film of condensed evaporant 'with a varying profile on the substrate. The elevation of this profile will correspond to the evaporation rates at corresponding points on the source.

In accordance with the foregoing technique, cadmium sulfide powder was loaded into one of the cavities arranged as shown in FIG. 1. The other cavities were filled with aluminum oxide powder. The aluminum oxide has a lower vapor pressure, and therefore did not evaporate. Like cadmium sulfide, it is a thermal insulator. This maintained the evaporation rate and temperature distribution for the cadmium sulfide filled cavity near the conditions which would normally prevail if all the cavities were filled with cadmium sulfide.

The pinhole Was .040 inch in diameter and positioned one inch directly above the cadmium sulfide filled hole. The substrate was a polished aluminum surface 3 /2 inches above the source. The aluminum substrate was cooled to near liquid nitrogen temperature to insure collection of all the cadmium sulfide evaporant. Three different cavity geometries, all using a square block 1 /2 inches on a side, were tested. The table below lists the results for the three cavity geometries. In the table, the evaporation, the evaporation data are for a source temperature of 770 C.

Source Source Source A B G Total number of cavities 144 36 86 Diameter d of cavity (inch) 9'32 Zz /32 Depth D of cavity (inch) }e 940 Minimum wall thickness 10 between cavities (inch). %2 le }{!2 Thickness of annulus (inch) 02 003 003 Evaporation rate (augstroms/secnd).. 37. 5 12. 6 7. 7

where S is the total surface area of a single cavity, V is the volume of the cavity,

r is the radius of the cavity, and

D is the depth of the cavity.

It can be seen, in comparing Source A with Source B, that they have the same depth, but Source A has the smaller radius, and the higher S/ V ratio.

Another factor that contributed to the differences in deposition rate was the wall thickness w to cavity depth D ratio. While this ratio was the same for Source A and Source B, Source C had a smaller ratio due to its larger depth.

A closer examination of the above expression for S/ V will show that the radius of a cavity has a greater effect on the S/ V ratio than the depth. Further investigation of cavity geometries has revealed that in order to achieve.

a minimum tolerable S V ratio for effective thermal heating of the powder within a single cavity, the radius of the cavity should be no greater than 4; inch. Thus the diameter d of the cavity should be no greater than /1 inch.

According to the teachings of this invention, therefore, a honeycomb structure should be designed to fall Within the following dimensional criteria; (1) The volume of cavities per unit surface area of the block encompassing the cavities should be no less than .005 cubic inch per square inch. (2) The minimum .wall thickness between adjacent cavities should be at least 6 the depth of the cavities. (3) The diameter of each cavity should be no greater than 4 inch.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A vacuum evaporation source, comprising:

a block of thermally conducting material provided with a multiplicity of uniformly spaced cylindrical cavities extending from a surface of the block to a depth less than the thickness thereof;

the minimum wall thickness of said block material between adjacent cavities being at least the depth of said cavities;

the volume of cavities per unit surface area of the block encompasing the cavities being no less than .005 cubic inch per square inch; and

the diameter of each cavity being no greater than A inch.

2. The invention according to claim 1, and further including means for heating said block to the vaporization temperature of a desired evaporant material placed within said cavities.

3. The invention according to claim 1, and further including means forming a second multiplicity of cavities within said block and extending at right angles to said cylindrical cavities;

a heater means within each cavity of said second multiplicity for heating said block to the vaporization temperature of a desired evaporant material placed within said cylindrical cavities.

4. The invention according to claim 1, wherein said cavities are arranged in mutually perpendicular rows and columns.

5. The invention according to claim 1, wherein said cavities are arranged in rows, with the cavities of alternate rows being staggered relative to the cavities in the other rows.

6. A vacuum evaporation source, comprising:

a block of thermally conducting material provided with a first multiplicity of uniformly spaced cylindrical cavities extending from a surface of the block to a depth less than the thickness thereof;

means forming a second multiplicity of cavities extending transversely to said cylindrical cavities;

and heater means within each cavity of said second multiplicity for heating said block to the vaporation temperature of a desired evaporant material placed within said cylindrical cavities.

7. The invention according to claim 6, wherein said second multiplicity of cavities extend at right angles to said cylindrical cavities.

8. The invention according to claim 6, wherein said block is made from a metal having a thermal conductivity at least as high as that for molybdenum.

9. The invention according to claim 6, and further in cluding vaporizable powder material disposed within said first multiplicity of cavities;

and said block being made of heat refractory material that is chemically inert with respect to said powder material.

References Cited UNITED STATES PATENTS 2,100,045 11/1937 Alexander 118--49 X 2,447,789 8/1948 Barr 219422 2,482,329 9/1949 Dimmick 11849 2,902,574 9/1959 Gudmundsen et a1. 1l8-50.1 3,244,557 4/1966 Chiou et a1 117l07 X RICHARD M. WOOD, Primary Examiner. c. L. ALBRITTON, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,405,251 October 8, 1968 Reuben S. Spriggs et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 20, "insure" should read ensure line 45, in the equation, "5" should read S Column 6, line 29, "a heater" should read and heater Signed and sealed this 3rd day of March 1970.

(SEAL) Auem Edward M. Fletcher, J1. WILLIAM E. SCHUYLER, IR.

Aflesting Officer Commissioner of Patents

Claims

1. A VACUUM EVAPORATION SOURCE, COMPRISING: A BLOCK OF THERMALLY CONDUCTING MATERIAL PROVIDED WITH A MULTIPLICITY OF UNIFORMLY SPACED CYLINDRICAL CAVITIES EXTENDING FROM A SURFACE OF THE BLOCK TO A DEPTH LESS THAN THE THICKNESS THEREOF; THE MINIMUM WALL THICKNESS OF SAID BLOCK MATERIAL BETWEEN ADJACENT CAVITIES BEING AT LEAST 1/5 THE DEPTH OF SAID CAVITIES; THE VOLUME OF CAVITIES PER UNIT SURFACE AREA OF THE BLOCK ENCOMPASSING THE CAVITIES BEING NO LESS THAN .005 CUBIC INCH PER SQUARE INCH; AND THE DIAMETER OF EACH CAVITY BEING NO GREATER THAN 1/4 INCH.