Method and Apparatus for Evaporating Materials in a Vacuum Coating Plant

The invention relates to a method and apparatus of evaporating materials in a vacuum evaporator by bombarding the material to be evaporated with electrons from a low-voltage arc discharge.

For the evaporation by means of an electron beam, thre are known so-called electron beam evaporators with a hog cathode. In such evaporators, the power density required for the evaporation is obtained on the material to be evaporated by accelerating a relatively low electronic current (for example, of 1 A) with a relatively high voltage (for example, 10 kV), This produces a beam having a small cross section which, frequently, is directed onto the evaporative material through a magnetic field, along an arcuate path. Such an electron beam entails only a small activation (i.e. ionization, excitation, dissociation) of the residual gas and the vapor in the evaporator. This is due to several reasons: (1) the low current intensity; (2) with high energy levels, the effective cross sections are small; (3) higher residual gas pressures cannot be permitted, since they cause flash-overs in the acceleration path and sputtering of the hot cathode. However, to obtain layers of higher quality, an activation of the vapor of the evaporative material or of the residual gas in the evaporation space would be very desirable in many vapor deposition processes.

Further known are electron beam evaporators with a hollow cathode. Distinction is made between cold and hot hollow cathodes (see, for example, C. T. Wan, D. L. Chambers and D. C. Carmichael, The Journal of Vacuum Science and Technology, Vol 8, No. 8, p. VM 99, 1971). Unlike normal electron beam evaporators, these two kinds can be operated at pressures above 10.sup.-4 mb; the desired current-voltage characteristic, however, is furnished only by the hot hollow cathode. It delivers high currents at a low operating voltage (for example, 100 V, 100 A) and thus ensures an effective activation of the residual gas and the vapor. Hot hollow cathodes, however, are hard to ignite and, therefore, require an additional ignition device. Moreover, at the same time, the cathode material is sputtered and deposited on the substrates to be coated, as contamination.

The problem underlying the invention is to avoid the disadvantages of the prior art methods and to provide a simple electron beam evaporation method which can be carried out at relatively high pressures and ensures, at the same time, that the residual gas is highly activated and the sputtering of the cathode material is reduced or the still sputtered cathode material is prevented from reaching the substrates.

The inventive method of evaporating material in a vacuum evaporator by bombarding it with electrons from a low-voltage are discharge sustained between a hot cathode placed in a chamber which is separated from the evaporation space and communicates therewith through an aperture, and an anode, while, during the operation, continuously introducing a gas into the hot cathode chamber and keeping the evaporation space, by pumping, under a vacuum sufficient for the provided coating process, is characterized in that

(a) the material to be evaporated and placed in the evaporation space is itself connected as anode;

(b) in the evaporation space, the discharge constricted by the mentioned aperture is directed to the anode, in addition, through a magnetic field and concentrated to a beam, in order to obtain on the surface of the anode a power density satisfactory for the vaporization.

During the low-voltage arc discharge, a positive space charge layer is formed in front of the cathode, which, as a virtual anode, accelerates the electrons. To prevent the electrons from losing their energy by collisions with the gas atoms, a lower pressure than that in the hot cathode chamber is maintained along the following discharge path in the evaporation chamber. This differential pressure is obtained by continuously introducing gas into the hot cathode chamber and pumping gas out of the evaporation space. Then, in spite of the long distance they have to cover in the evaporation space, the electrons give off the greatest part of their energy to the evaporative material connected as anode.

Due to the inventive provisions, it is made possible to reduce the cross-sectional area of the focal spot of the electron beam on the material to be evaporated to such an extent that the required power density is easily obtained. By providing an adequate magnetic field extending parallel to the beamed discharge, the electron beam in the evaporation chamber, despite its high current intensity, maintains a cross section of the order of magnitude of the mentioned opening between the hot cathode chamber and the evaporation space. To obtain a satisfactory high rate of evaporation even for metals which are difficult to vaporize, a power density on the anode surface of more than 10 kW per cm.sup.2 is advisable.

It is further useful to introduce the gas into the hot cathode chamber continuously in an amount such as to obtain the mean free path of the gas molecules in this chamber in the order of magnitude of the cathode fall distance of the low-voltage arc dishcarge. For the discharge, arc voltages lower than 200 volt are advisable, but only with currents of at least 30 A. It is further advisable to electrically insulate the partition wall comprising the aperture between the hot cathode chamber and the evaporation space and, while carrying out the inventive method, to allow it to assume a floating potential. The material to be evaporated may be kept either at a positive potential or at the ground potential, in which case the ground potential or a negative potential is applied to the cathode. Also advantageous is an operation in which both the cathode and the material to be evaporated are kept at a positive potential relative to the ground. To the substrates to be coated, a negative potential relative to the anode may be applied, and, in addition, they may temporarily (in particular, intermittently) be connected as the cathode of an independent gas discharge. The voltage between the anode and the substrates may be varied while the process is carried out. For example, it is possible to start with a high voltage, to obtain a satisfactory adhesive strength of the layer, which is due to the high sputtering rate connected thereto, and to continue the process at a low voltage, to ensure a uniform growth of the layer.

In one and the same evaporation chamber, a plurality of anodes of different evaporative materials may be provided, to which the electron beam is alternately directed, for example, by means of suitable magnetic fields. Also, a plurality of anodes in the form of evaporation sources may be provided in the same evaporation space and associated with respective hot cathodes accomodated in separate chambers, so that the vapor deposition from these evaporation sources may take place simultaneously or sequentially and under uniform or different electrical conditions (power, power density). A simultaneous vapor deposition from two evaporation sources may be instrumental for producing alloys.

The mentioned various possibilities of connection result in different directions of motion of the ions and, thereby, to variations in the vapor distribution within the evaporation space.

The only FIGURE in the application is a cross-sectional view of an apparatus constructed in accordance with the invention.

In the following, the inventive method and a device suitable for carrying out the method are described in more detail with reference to the accompanying drawing. The drawing shows a vacuum evaporation chamber 17 with an evacuation connection 18, which communicates, through an aperture 6, with a hot-cathode chamber 13. A hot cathode 3 is accomodated in this chamber and supplied from a current supply source 1. On the bottom of evaporation chamber 17, a crucible 20 for the material 19 to be evaporated is provided which can be cooled by means of coolant channels 21 and coolant outlet and inlet lines 11. A support 8 in the form of a cylindrical sleeve for receiving the substrates to be coated is provided in the evaporation chamber. The hot-cathode chamber further comprises a gas inlet connection 2 and a coolant channel 14 which serves particularly for cooling the partition wall between the hot-cathode chamber and the evaporation chamber during operation. Magnetic field coils 5 are provided on the outsides of bottom part 10 and cover part 16 of the evaporation chamber, to generate in the evaporation space a magnetic field approximately parallel to the discharge axis 7.

To carry out the inventive method, the substrates to be coated are secured to the side of support 8 facing the evaporation source at 20, the evaporative material 19 is placed in the crucible 20, the evaporation chamber 17 is closed and evacuated, and a suitable gas is introduced into the hot-cathode chamber through line 2. Suitable gases are, primarily, rare gases, if a reaction with the substrates is to be prevented, or the corresponding reactive gases, if such a reaction is desired, for example, to produce layers of metallic compounds. Nitrogen or a gaseous nitrogen compound may be introduced for example to produce nitride layers, a gas containing a gaseous carbon compound may be used to produce carbide layers, a gaseous boron compound to produce boride layers, or oxygen or an oxygen compound containing gas to produce oxide layers. In a process of reactive vapor deposition and to spare the hot cathode 4, however, it is also possible to introduce into the hot-cathode chamber 13 an inert gas and, in order to obtain the required reaction in the evaporation space, to separately introduce into this space the desired gas through another line, so that during the vapor deposition, a reactive residual-gas atmosphere is predominant whose pressures may be adjusted by continuous pumping to a value which is optimal for the process.

To produce titanium nitride layers, metallic titanium in lumps was placed in crucible 20, the apparatus was evacuated to 10.sup.-4 mb, and nitrogen was introduced into evaporation chamber 17 through line 2, hot-cathode chamber 13, and aperture 6, and continuously pumped out therefrom through evacuation connection 18 and in such an amount as to maintain an N.sub.2 presssure of 5.times.10.sup.-2 mb in the hot-cathode chamber and a residual N.sub.2 gas pressure of 5.2.times.10.sup.-3 millibar in the evaporation chamber. Then, the grounded hot cathode was heated with 1.5 kW and, thereupon, a voltage of +70 volt was applied to the anode. Next, by briefly applying the anode voltage to the partition wall 15 between hot-cathode chamber 13 and evaporation chamber 17, the low-voltage arc was ignited (the voltage indications above, and all in the following relate to the differential voltage relative to the container to which the ground potential was applied). Thereby, a current of 116 A was produced flowing through the hot cathode. The current flowing through the anode was 131 A. The difference of 15 A between the two currents indicates the current circulating through the substrates and the container. By the electron current flowing through crucible 19 as anode, the titanium received therein was melted and evaporated at a rate of about 0.3 g per minute. Due to the effect of the residual gas which was strongly ionized by the low-voltage arc discharge between the hot cathode and the anode, a hard titanium nitride layer of extreme adhesive strength and gold-yellow color has been obtained on the substrates secured to support 8. Such layers are useful, for example, as coatings for articles which are exposed to strong wear.

In the following, further examples are given indicating the used voltage, currents, and residual gas atmosphere during the evaporation of titanium. In the examples, a power of about 5 to 8 kW is delivered on an anode surface of about 0.2 cm.sup.2 (half width of the beam about 0.5 cm).

(2) Gas: argon, 5.times.10.sup.-3 mb

Cathode: 0 V; 133 amp

Anode: +37 V; 150 amp

Container+substrates: 0 V; 17 amp; 629 w

(3) Gas: argon, 2.5.times.10.sup.-3 mb

Cathode: -40 V; 152 amp

Anode: 0 V; 117 amp

Container+substrates: 0 V; 35 amp; 0 w

(4) Gas: argon, 2.5.times.10.sup.-3 mb

Cathode: +30 V; 116 amp

Anode: +80 V; 144 amp

Container+substrates: 0 V; 28 amp; 2240 w

(5) Gas: argon, 5.times.10.sup.-3 mb

Cathode: -40 V; 152 amp

Anode: 0 V; 117 amp

Container: 0 V; 30 amp; 0 w

Substrates: -200 V; 5 amp; 1000 w

(6) Gas: N.sub.2, 5.times.10.sup.-3 mb

Cathode: 0 V; 116 amp

Anode: +70 V; 131 amp

Container+substrates: 0 V; 15 amp; 1050 w

While depositing pure metal layers, as already mentioned, the operation is carried out with an inert residual gas atmosphere. Hard, resistant metal compounds of nitrides, carbides, borides, or oxides, on the contrary, may be obtained by a reactive vapor deposition. For this purpose, in particular, the transition elements of the fourth to eighth group of the periodic system, thus titanium to nickel, zirconium to palladium, and hafnium to platinum, are to be considered.