Method for the cooling of targets as well as cooling device for targets

The rear surface of an sputtering target (1) is actively cooled exclusively in regions spaced from breakthrough-prone regions. The cooling takes place in regions along the erosion profile outside of the regions of greatest erosion rate. For this purpose a cooling device for the target (1) has channels (7) through which a coolant flows. These channels (7) extend outside the regions (4) of strongest erosion of the target (1). The cooling channels (7) have on the side facing the back surface of the target, one foil-like closure wall (10). The cooling channels (7) extend in a cooling plate (6) and are separated from each other by webs (8). The webs (8) extend in the region of strongest erosion (4) of the target (1) in order to mechanically support these regions (4). Therewith it is possible to utilize expensive target material to the limit of its removal, without however risking a breakthrough of the target due to the erosion in an area where coolant will leak.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates to a method for the cooling of targets in 
sputtering sources as well as a cooling device for targets of sputtering 
sources with a target and with through which flow cooling agents. 
When sputtering targets with sputtering sources, in particular when using 
so-called high rate sources such as magnetron sputtering sources, the 
problem is encountered of recognizing in time when the target has been 
consumed. If the plasma process is not interrupted in time, the target 
erodes through whereby an opening is created which exposes the rearward 
constructional elements of the magnetron source facing the plasma. This 
leads to a joint erosion of construction material which means as a rule an 
unacceptable contamination of the high purity layer to be generated. A 
breakthrough would have even more serious consequences for the cooling 
device whereby cooling medium penetrates into the process chamber. This 
leads to a destruction of the substrates as well as to complete failure of 
the installation and concomitant high expenses. 
It is known that in the case of cathode sputtering installations the 
so-called targets must be cooled to achieve optimum operating capacity 
during sputtering of the targets. This cooling can be direct or indirect 
in that with the former the target can be brought into direct contact with 
the coolant liquid, e.g. water, or in the case of the latter, indirectly 
via an intermediate wall. The heat to be carried off can consequently 
either be carried off through heat transmission directly to the cooling 
medium in the case of direct cooling or through heat conduction through a 
wall in the case of indirect cooling. 
It is further known, that with direct cooling the target mounting must be 
constructed especially carefully due to the danger of leaks. 
With indirect cooling the cooling medium flows through cooling channels 
disposed in the interior of a back plate. This cooling method is without 
problems in terms of leakage, however it is significantly less effective 
with respect to cooling effect than direct cooling. 
To circumvent the disadvantages of direct cooling and those of indirect 
cooling as well, it has also become known to implement the back plate, as 
in the case of direct cooling, with channels that are open toward the rear 
surface of the target and to close off these channels by a flexible foil, 
preferably comprising a metal, toward the outside in a liquid-tight 
manner. In operation the liquid pressure in the cooling channels causes 
the flexible metal foil to be pressed against the rear surface of the 
target and in this manner, heat transmission is significantly improved 
through the increase of heat conduction. See: J. Vac. Sci. Technol. A2(3), 
July-Sept. 1984 "Cathode cooling apparatus for a planar magnetron 
sputtering system" by M. R. Lake and G. L. Harding as well as "Planar 
Magnetron Zerstaubungsquellen" by Dipl.-Ing. Urs Wegmann, Balzers 
Fachbericht, BB 800 014 DD 8102. 
In the Balzers report it is shown what the so-called removal profiles of 
targets generated by erosion look like. In the center of the erosion zone 
of the target a V-form depression originates. The target material, in 
other words, is not eroded uniformly. For the purpose of utilization of 
the expensive target materials however, one goes to the limit without 
risking a breakthrough of the target due to the erosion against the back 
plate. 
SUMMARY OF THE INVENTION 
The present invention has as its purpose the creation of a method for the 
optimum economical utilization of targets and reduction of the risk of 
having to prepare for serious breakthroughs of the target toward the 
cooling medium through erosion, in particular with disturbances of 
operation. 
Accordingly, an objection of the present invention is to provide a method 
for cooling targets which act as sputtering sources, wherein the rear 
surface of the target is cooled exclusively in areas where breakthrough is 
least likely, following an erosion profile in the target. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which the 
preferred embodiments of the invention are illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is the cross section of a target 1, in which an erosion profile 2 is 
drawn which results during operation in the final phase of utilization of 
the target. In the region of the largest material removal, so-called 
breakthrough zones 4 result, in the front surface of the target where the 
danger of water intrusion exists, if for example a disturbance causes the 
target to be completely sputtered through. 
Below the target 1 is a cooling plate 6 with cooling means channels 7 which 
are located outside of the region of the breakthrough zones 4. These 
regions are supported and sealed off by solid material webs 8 of the 
cooling plate 6 in such a way that with a potential breakthrough of the 
target 1 in the region of the breakthrough zones no immediate contact with 
the cooling water takes place. In this implementation the cooling means 
channels 7 are provided with channel closure diaphragms 10 toward the back 
surface of the target 1, which, since they are located outside the 
possible breakthrough zones 4 are not exposed to any direct sputtering. 
Therewith a breakthrough into the cooling means can be prevented. The 
cooling channels are implemented corresponding to the erosion profiles of 
the target 1, for example, angularly. It is also possible to place into 
the cooling means channels 7 metal tubes with flexible walls or pipes for 
conducting the cooling medium. 
Applying the cooling means channels 7 outside the breakthrough zones 4 
consequently decreases the risk, in particular in the event of 
disturbances which bring about a breakthrough into the cooling medium, 
wherein the potentially slightly lesser cooling effect due to this 
arrangement is practically not apparent. The cooling process can be 
controlled by means of the cooling means, thus can also be instationary. 
The solid material webs 8 prevent an excessive deflection of a channel 
closure diaphragm since their free span due to the intermediate web 
decreases correspondingly. Furthermore, these webs 8 reinforce the cooling 
plate 6 wherein they additionally represent a protection of the easily 
damaged thin channel closure diaphragm 10. 
In FIG. 2 a direct cooling of the target is provided wherein one cooling 
plate 14 is provided with open cooling means channels 15. For sealing 
purposes, as is apparent, sealing cords 16 or the like are used between 
the target and the plate 14. Here too these channels are disposed outside 
the breakthrough zones at least to decrease, in the event of disturbance, 
the danger of a water intrusion. 
When sputtering the target down to the solid part of the construction on 
the target back surface, the possibility still remains that the substrate 
becomes contaminated through the construction material. While this 
contamination bears no resemblance to that which would have been generated 
upon a cooling means intrusion, it is nevertheless desirable to exclude 
even the slightest possibility for contamination. A solution to this 
problem is achieved by the use of construction material which is tolerant 
to the process in the region of the target breakthrough to be expected as 
well as through timely switching-off of the process. 
Construction material tolerant to the process is a material which does not 
impair, to an unacceptable degree, the layer quality in the event of 
contamination. Consequently this is a question of the careful selection of 
material for the region of the construction exposed in the event of target 
breakthrough. In many cases a coating of this exposed region is sufficient 
whereby greater degrees of freedom are made possible in the selection of 
the material for the mechanical construction as well as cost-effective 
solutions. 
In the semiconductor industry for the coating of silicon wafers for example 
frequently metal alloys of aluminum with a few percentages of silicon and 
copper are used. The target to be used then comprises this alloy having a 
high degree of purity. A good choice for the construction material in the 
anticipated target breakthrough region is, in this case, copper having a 
high degree of purity. This material allows a favorable signal evaluation 
without influencing the process negatively since copper is a functional 
component of the layer. For signal evaluation, optical and/or electrical 
parameters can be used. As optical parameter the color of the plasma 
discharge can be monitored. In the stated example the plasma above the 
target of the above stated alloy with dominant aluminum fraction, when no 
target breakthrough is as yet present, has a blue color. If now a 
breakthrough occurs, some copper of the construction is exposed and 
activated through the plasma. The consequence is that the color of the 
plasma discharge changes from blue to green. This color change can be 
detected readily with known methods of optoelectronics. The resulting 
signal can be used to trigger further activities, as for example to switch 
off the process to be able to carry out a target change. A further 
possibility of signal detection consists in evaluating the electrical 
discharge conditions which change upon breakthrough. This can take place 
through simple measurements of changes in the current-voltage 
characteristic of the process. This is possible because the discharge 
voltage is a function of the material. A magnetron discharge with a target 
of the previously stated alloy at an operating power of 10 kW operates for 
example with a discharge voltage of 550 volts. Upon a target breakthrough 
the copper influences the plasma discharge and will lower the discharge 
voltage to approximately 500 volts. This process can be readily detected 
and, as already discussed in the case of the optical process, can be 
evaluated. 
Within the scope of the present implementation therefore, for example, the 
entire width of the two solid material webs 8 can amount to one fifth of 
the total width of the target 1. However, it is possible to fall below 
this value but it should not be less than approximately 10%. These limit 
values correspond to the uncooled areas of the target in the region of the 
strongest erosion. 
The described construction decreases the danger of water intrusion into the 
vacuum during sputtering of a target in the event of a disturbance. 
Possible cooling means intrusions upon eroding through of the target are 
avoided and thereby the operating safety of the device with simultaneous 
increase of the capacity is drastically improved. 
The present invention consequently solves the problem of an unacceptable 
cooling means intrusion and increases the reliability of the operating 
behavior significantly. 
All individual parts and individual features as well as their permutations, 
combinations and variations described in the specification and/or 
represented in the Figs. are inventive and specifically for n individual 
parts and individual features with the values n=1 to n-&gt; infinity. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.