A major demand imposed on evaporators for vacuum film depositing is to obviate wastage of the evaporant failing to deposit on the substrate by ensuring its revaporization and to provide film thickness uniformity within a wide range of materials deposited on substantial surface areas of substrates.
Films can be deposited more uniformly by spacing the substrate at a considerable distance from the evaporator, whereas for reducing losses of the evaporant it is possible to recover the vaporized evaporant material failing to fall on the substrate.
By way of example, there is known a device for vacuum deposition of metal films on a substrate, which comprises a crucible with a substrate spaced therefrom both surrounded by a chamber serving to collect the evaporant failing to impinge onto the substrate. On terminating the process of vacuum plating, the evaporant deposited on the walls of this chamber is removed therefrom to be recharged to the crucible for revaporization. However, inherent in this device is a disadvantage of non-uniform thickness of the film deposited, because the distance from the crucible to the substrate is confined by the dimensions of the chamber. In addition, the evaporant material scraped off the walls of the chamber for reuse is contaminated by foreign impurities. Among the sources of such impurities are both the material of the chamber mixing with deposited evaporant during chamber walls cleaning, and residual gases of the vacuum chamber tending to condense on the walls of the chamber. The condensed residual gases are of considerable size since the substrate is rather remote from the crucible. Another disadvantage resides in much labour required for cleaning the chamber.
The process of cleaning the chamber can be made less labour-consuming by revaporization of the material condensed on the walls of the evaporant collecting chamber, which is offered by an apparatus for vacuum depositing disclosed in Japanese Application No. 57-134555, Int. Cl. C 23 C 13/00, 13/08, published Aug. 19, 1982. This apparatus comprises arranged in vacuum a vapor source in the form of a crucible having arranged thereabove a vapor collecting chamber which encloses the flow of vaporant traveling toward the substrate. This collecting chamber can be heated to the temperature of vaporant in the crucible or to a temperature sufficiently high for revaporizing the material deposited on the walls of the collecting chamber. Therefore, the evaporant material which does not impinge on the substrate tends to condense on the walls of the collecting chamber and thanks to high temperature of these walls is revaporized therefrom.
This apparatus also fails to ensure high purity of the film being deposited due to excessive liberation of gas from the collecting chamber because of the high temperature to which it is heated and its large size determined by the distance from the crucible to the substrate. For the same reason, the substrate is likewise heated to an intolerably high temperature by the heat radiating from the collecting chamber. Also, considerable losses of evaporant take place due to escape thereof through clearances between the crucible, collecting chamber and substrate.
Wastage of the evaporant material can be minimized, when the temperature of the collecting chamber is lowered. An attending advantage is associated with less pronounced overheating of the substrate. These advantages are materialized in a liquid phase regeneration device for vacuum deposition of films (cf., Japanese Application No. 57-155368, Int. Cl. C 23 C 13/00, 13/08, published Sept. 25, 1982) which comprises a crucible containing an evaporant with a collecting chamber arranged thereabove around the flow of vapor moving toward the substrate. The lower edge of the chamber occupies the upper part of crucible interior. During film deposition the vapor collecting chamber is heated to a temperature not below the melting point of the evaporant, whereas the liquid phase of the evaporant condensed in the collecting chamber trickles down its walls back to the crucible. However, such a liquid phase regeneration again fails to ensure low losses for a wide range of evaporants, since most of the evaporants used for vacuum film deposition have a vapor pressure at their melting point inducing vigorous evaporation. In view of the aforedescribed, the vapor still tends to escape through clearances between the crucible, collecting chamber and substrate. In addition, the high temperature of the collecting chamber determined by the temperature of the liquid phase of the evaporant causes outgassing and unwanted heating of the substrate.
Losses of the evaporant can be further minimized by eliminating clearances between the crucible and vapor collecting chamber, as well as by a more drastic reduction of the temperature of the vapor collecting chamber during film deposition process to a temperature substantially below the melting point of the material being deposited. The evaporant condensed on the walls of the collecting chamber can be caused to trickle down by periodically increasing their temperature to above the melting point of the evaporant. The attending advantageous effects include reduced gas liberation from the walls of the vapor collecting chamber and lowered temperature of the substrate. These advantages are materialized in an apparatus for vacuum deposition of metal films dislosed in U.S. Pat. No. 4,125,086 Int. Cl. C 23 C 13/08 published Nov. 14, 1978. This apparatus comprises an alumina crucible to contain an evaporant material in which there is immersed a tungsten tube having a hole in its side wall for the passage of vapor. Arranged opposite this hole is another hole for the passage of a portion of vapor and deposition on the substrate. The rest of the vapor is condensed on the wall of the crucible adjacent the hole. The crucible wall in this case serves as a means for directing the vapor flow. The device comprises separate heaters for the crucible and the tube. The temperature of the tube is maintained at a sufficiently high level to ensure vaporization and escape of the evaporant material from the hole. The temperature of the crucible is periodically raised for the evaporant condensed on its walls to trickle down, enter the tube, and vaporize.
However, during deposition of such metals as gold the tungsten components are wetted thereby to form a mechanical bond therewith, which due to different thermal expansion coefficients of gold and tungsten results in damage of the tungsten tube after several heating cycles. In addition, gold tends to leak through tungsten seals to render the apparatus inoperable.
Damage of the tube and leaks of the evaporant through the seals can be prevented by fabricating the apparatus from a material not wetted by the evaporant. One example is an evaporator for vacuum depositing films on substrates taught in U.S. Pat. No. 4,412,508, Int. Cl. C 23 C 13/08 published Nov. 1, 1983. The evaporator comprises a cylindrical graphite housing having a partition separating the housing into two parallel chambers. One such chamber, particularly the collimation chamber, functions as a means for guiding the flow of evaporant being deposited on the substrate and has in its side wall an aperture for impinging the flow of evaporant onto the substrate. The other, evaporation chamber, is provided with a coaxial graphite tube the side wall of which also has an aperture for the passage of vapor in line with the aperture of the collimation chamber, this line being perpendicular to the axis of the housing. The lower end of the collimation chamber forms a crucible communicating by a passage for conveying the liquid evaporant material to the evaporation chamber in which the liquid material ascends along a tungsten wick, because an electric current is applied to the tube for maintaining therein a high temperature. Having passed the aperture in the tube the vapor enters the collimation chamber wherein part of the vapor flows through the collimation aperture to impinge on the substrate, while the remainder of the vapor is condensed on the walls of the collimation chamber. This chamber is periodically heated by a separate heater to ensure that the liquid metal flows to the wick.
When used for depositing gold, this device features a long service life because it is fabricated from a material not wettable by gold, particularly graphite.
During film deposition the conditions for vapor emission from a small (about 0.5 mm) aperture in the tube at the normally practised rates of deposition make the flow dense or close to dense. Therefore, the vapor flow intensity distribution varies, depending on the conditions of emission, according to a higher cosine power, rather than according to the cosine of the angle relative to the centerline of the aperture. Therefore, the film thus deposited may have very non-uniform thickness; still more so, the larger is the surface area of the substrate.
Furthermore, since most of the materials used for film deposition tend to sufficiently vigorously evaporate at the temperature of melting point, then raising the temperature of the collimation chamber above the melting point for even a short period of time is liable to result in increased losses of the evaporant due to the lack of directional emission of vapor through the collimation aperture.
From various reference sources there are known seventy one evaporant materials widely practised in film deposition (cf., e.g., "Handbook of Thin Film Technology" edited by Leon I. Maissel and Reinhard Glans, published by the McGraw-Hill Book Company, New York, 1970, ppl1-37, 1-38, 1-66, 1-68). Of them only twenty three have at the melting point a pressure of vapor not above 10.sup.-2 Torr. It has to be noted (see p. 1-36) that the materials evaporated intensively already at 10.sup.-2 Torr, which means that losses of other forty eight materials for film deposition will be considerable. Also, such materials as chromium, arsenic, some oxides and tellurides feature a vapor pressure at the melting point amounting to tens and hundreds Torr, or even higher, thus making such materials inapplicable for liquid regeneration.
It has also to be taken into account that, as evidence in the above cited U.S. patents, the optimum diameter of the tube in the evaporation chamber is 1.56 mm. At larger tube diameters the heat applied to the tube results in excessive temperature of the crucible and collimation chamber to eventually lead to extra losses of the evaporant. Conversely, the tube of such a diameter can hold but negligible amounts of the evaporant material sufficient only for producing a film of small thickness, after which the material occupying the collimation chamber must be transformed to liquid to thereby ensure its successive delivery to the tube. It follows then that the application of heat to the collimation chamber to raise its temperature to above the melting point of the evaporant must be more frequent, practically after each deposition cycle. When a film of more substantial thickness is to be deposited, the temperature of the collimation chamber needs to be continuously maintained above the evaporant melting point throughout the film deposition process. For example, if the pressure in the collimation chamber is 10.sup.-2 Torr, losses due to non-directed emission through the collimation aperture of the evaporant material vaporized in the collimation chamber may be as high as 70%.
In addition, to ensure a stable vapor deposition rate, the end of the wick enclosed by the tube must be immersed into the evaporant occupying the entire bottom portion of the crucible, which make the inventory of the evaporant material quite considerable. Therefore, the minimum initial quantity of evaporant charged amounts to tens of grams, which may entail unjustifiable expenses because such an amount of precious and costly materials by far exceeds the quantity to be eventually evaporated, whereas the continuous presence of the evaporant material in the crucible and in the by-pass conduit is necessitated exclusively by the normal functioning of the apparatus.
The aforedescribed apparatus has two separate heaters to accurately maintain the temperature of the evaporation and collimation chambers. Otherwise, for example, in the case of a failure to accurately maintain the temperature of the collimation chamber, losses of the material occupying this chamber in the course of its retrieval for eventual reuse by making it flow to the evaporation chamber may grow considerably. The requirement of providing two heaters and, consequently, two temperature control systems, renders the aforedescribed apparatus overcomplicated in operation.