Process for structuring polymer films

A process for structuring polymer films, such as printed circuit boards and film boards, uses a plasma, which is formed in a container by the excitation of gas mixtures by microwaves, in which there is a control of the surface temperature of the polymer films during structuring by the plasma. By adjusting the parameters it is possible to operate just below a material-damaging temperature limit for polymer films, so that structures or microshapes are carefully plasma-eroded in polymer films. At high surface temperatures and with a dense plasma, structures or microshapes are rapidly eroded by the plasma, in which the structures or microshapes are homogeneously plasma-eroded with a uniform distribution of the surface temperature and gas flow. The surface temperature is maintained below the material-damaging temperature by controlling the power density of the microwave energy, the level of energy provided to a heater in the container, and the pressure of gas in the container in accordance with a time pattern algorithm using a computer.

The invention is in the field of processing technology under plasma action 
and relates to a process for structuring e.g. insulating layers in printed 
circuit boards and film circuit boards or polymer films. 
BACKGROUND OF THE INVENTION 
Nowadays films made from polymer materials are used in numerous technical 
fields. One such field making particularly high demands with respect to 
the miniaturization of functions, structures, weight and thickness of said 
films is printed circuit board technology. Printed circuit boards are 
single-layer or multilayer printed circuits comprising a combination of 
flat current paths, blind holes and interfacial connections (connections 
of different layers of printed circuits) and insulating layers. In film 
circuit boards current paths are often structured in thin, electrically 
conductive foils or films and electrically insulating films are used as 
insulating layers or dielectrics. 
A known, economic and universally usable production method for small 
structures in polymer films is plasma etching. Plasma etching has the 
advantage that a large number of holes or hole structures can be 
simultaneously produced on large surface areas. 
As prior art, reference is made to the process for the plasma etching of 
insulating films described in U.S. Pat. No. 4,720,322. Use is made of a 
plate reactor, which is equipped with pairs of parallel, plate-like 
electrodes and which excites a gas mixture by means of a.c. voltages in 
the range 40 kHz to 13.56 MHz (preferably 100 kHz) to a plasma. The 
reactor can be evacuated and for producing the plasma a mixture of oxygen 
(O.sub.2) and carbon tetrafluoride (CF.sub.4) is excited under vacuum by 
an electric a.c. field with frequencies in the range 40 kHz to 13.56 MHz. 
The ions formed in the plasma are accelerated towards the etching material 
by a superimposed, electric accelerating voltage, where they produce blind 
holes. In a specific example adopting this procedure 50 to 100 .mu.m deep 
blind holes are produced after etching for 4 hours in epoxy-aramide 
material. 
European patent 144,943 describes another process for the plasma etching of 
polyimide films, which also uses a plate reactor and which aims at a 
particularly high underetching or undercutting of the etching material. 
The etching material consists of polyimide films copper clad on both sides 
(in each case 35 .mu.m copper, 25 .mu.m acrylic adhesive and polyimide, Du 
Pont LF9111). Through-etching of these films takes place in accordance 
with masks in the form of openings in the copper coatings at frequencies 
of 13.56 MHz over a period of 70 minutes. 
A disadvantage of the aforementioned prior art is its low etching rate. 
Admittedly small structures can be simultaneously produced, but this takes 
a long time. In addition, the thin films to be etched have a very limited 
thermal capacity and are consequently very rapidly heated during etching. 
To avoid overheating and therefore a destruction of the films, the 
coupled-in power must be kept low, but this leads to a reduction in the 
plasma density and further reduces the etching rate. 
Another disadvantage of the cited prior art is the low uniformity of the 
etching rate in such plate reactors, i.e. the holes are etched through 
faster in the central areas than in the marginal areas. In order to be 
able to reliably etch through all the holes of a film, the holes in the 
central areas of the films are over-etched. This limited uniformity is 
reinforced by the fact that the etching material is not homogeneously 
heated, which is in particular always the case if the film to be etched 
has to be brought into thermal contact with other components, such as 
plate electrodes or holders. 
It would be desirable to further extend and refine the variety of 
structures in polymer films, e.g. in insulating layers of printed circuit 
boards and film circuit boards and to make such structures rapidly, 
uniformly and in material-protecting manner. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process for structuring 
polymer films, which permits a rapid, homogeneous and material-protecting 
production of such structures. 
The invention provides a process, which is carried out in a plasma reactor 
with numerous process parameter control possibilities. This control 
permits an optimization of the most important process parameters. The 
process is optimized as a function of the surface temperature of polymer 
films, such as e.g. printed circuit boards and film circuit boards. 
Structures are plasma-eroded at temperatures below a critical, 
material-damaging limiting temperature with respect to the polymer films. 
In order to achieve a high removal rate, the structures are made at very 
high temperatures and with a very high plasma density. In addition, a very 
uniform temperature distribution on the surface of the polymer films and a 
very homogeneous gas flow, together with a movement of the polymer films 
in the plasma performed during structuring, lead to a uniform, high 
erosion rate over the entire surface extension thereof. 
Thus, according to the invention, the surface temperature of the polymer 
film and its distribution is measured using, e.g., pyrometers and the 
process parameters of the plasma reactor are then optimized in such a way 
that working occurs at surface temperatures just below a critical 
temperature and with a very dense plasma. A very dense plasma is obtained 
in the case of optimum, homogeneous gas mixtures in a specific flow and 
pressure range, with high power densities of a microwave transmitter and 
at high excitation frequencies. 
Thus, due to the critical, material damaging limiting temperature, the 
temperature of the polymer films is a dominant process parameter. However, 
it is also important from other respects. In this connection it is less 
the chemical reaction on the surface of the material to be removed, which 
is naturally also dependent on the temperature, than the desorption rate 
of the reaction products, which very rapidly rises with the temperature. 
With rising temperature the reaction kinetics can be better arranged, so 
that high erosion rates are obtained. Advantageously the surface 
temperature is chosen in such a way that on the one hand there is no 
thermal damage or destruction with respect to the polymer films and on the 
other a high erosion rate is obtained. This optimum limiting temperature 
varies for the different plastics materials used for the production of 
polymer films. Finally, it is also possible to optimize the temperature 
pattern. Thin polymer films with a very low thermal capacity are rapidly 
heated to high temperatures during structuring, whereas e.g. thicker 
printed circuit boards which have a high copper percentage, only heat 
slowly during structuring and therefore require a longer erosion. 
It is known that the plasma density for a given microwave power increases 
with rising excitation frequency until all the particles are ionized and 
pass into a saturation state. This saturation level is dependent on the 
gas pressure present. With high gas pressures of approximately 1 hPa the 
free path length of the particles is so small that on the one hand the 
acceleration of the ionized particles in the electric a.c. field, caused 
by the high impact probability at high pressures, is very small, but on 
the other hand the recombination probability also rises and consequently 
the plasma density falls. At low pressures below 0.1 hPa the free path 
length of the particles is sufficiently high to obtain an adequately high 
acceleration without any interfering intermediate impacts and also the 
recombination probability is much lower. However, the availability of 
particles to be ionized is greatly reduced, so that with pressures of 0.1 
hPa and lower the plasma density falls again. In the intermediate range of 
said parameter field there is a zone of optimum high plasma density with 
high erosion rate and optimum shielding of the electric high frequency 
field, so that the polymer films are rapidly plasma-eroded and protected 
against overheating. 
Another important process parameter is the gas flow, i.e. the availability 
of new reactive particles per time unit. When the gas flow is too low, the 
reactive particles of the plasma are rapidly consumed and the plasma is 
enriched with reaction products, so that the plasma density and therefore 
the erosion rate drop. This high concentration of reaction products in the 
plasma even leads to the redeposition thereof on the polymer films, which 
naturally causes a drop in the erosion rate. With a too high gas flow the 
residence time of the reactive particles in the chamber is too short for 
effective erosion, which once again leads to a reduction in the erosion 
rate. Thus, with regards to the gas pressure and gas flow an optimum 
parameter range can be obtained where, apart from an optimum high erosion 
rate, there is a careful structuring of the polymer films without any 
overheating. 
The microwave frequency is another process parameter. According to the 
invention, microwave excitation takes place in the GHz frequency range, 
which has the advantage of high plasma densities and shielding action of 
the plasma compared with microwave radiation. At frequencies of 2.45 GHz 
the microwave energy is better absorbed by the gas mixture and 
consequently only a small part is coupled into the polymer films, so that 
the temperature thereof cannot rise to supercritical values. For plasma 
excitation purposes it is naturally also possible to use microwaves with a 
frequency lower than 2.45 GHz, e.g. frequencies in the kHz or MHz range. 
However, then the plasma is less dense and consequently the erosion rate 
is much lower and the shielding action of the plasma relative to microwave 
radiation is reduced, so that microwave radiation is increasingly absorbed 
on the polymer films and leads to the overheating thereof. 
In the invention microwaves are produced by magnetrons as microwave 
transmitters with a frequency of 2.45 GHz, which is approximately two 
orders of magnitude above those of known processes. With gas pressures of 
0.1 to 1 hPa and gas flows of a few 100 ml/min, during the structuring of 
polymer films erosion rates of 2 to 3 .mu.m/min are obtained, which 
represents an acceleration by a factor of 3 to 4 compared with known 
processes. The speed gain makes it possible to produce complicated 
structures in relatively short times of 10 to 20 minutes. The most varied 
plastics materials can be structured, the above-mentioned, fundamental 
considerations applying to all these materials. The absolute level of the 
erosion rate is naturally dependent on the type of material, its chemical 
structure and degree of crosslinking. This novel process is called plasma 
erosion and the numerous differently shaped structures are called 
microshapes. This process is fully usable for the production of printed 
circuit boards and film circuit boards and is compatible with known, 
proven circuit board technology methods.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 schematically shows the structure of part of a first embodiment of a 
plasma reactor for the process according to the invention. Its chamber or 
container 2 can be evacuated by means of one or more suction connections 3 
and pumps (see the arrows). Gas mixtures can be introduced in regulated 
manner by means of inlet connections 4 and flow regulators 5 (see the 
arrows). Advantageously the gas flows are in the range 50 to 2000 ml/min. 
The chamber pressure is measured by means of one or more pressure 
recorders 10. Advantageously the operating pressure is between 0.1 and 1 
hPa. For plasma erosion use is made of gas mixtures consisting of gases 
such as O.sub.2, CF.sub.4, SF.sub.6 and C.sub.2 F.sub.6, which have proved 
satisfactory in plasma etching. Advantageous gas mixtures have an O.sub.2 
excess with approximately 3 to 25% CF.sub.4. The optimum CF.sub.4 
proportion is in the range 6 to 16%. The container 2 can be opened by a 
door 9 closable in vacuum-tight manner, so as to bring the polymer films 
to a loading basket 6, e.g. by clamping. Naturally operation is also 
possible where the container 2 does not have to be opened for loading 
purposes. Thus, it is also possible to continuously process polymer films 
7 from a reel, being led continuously into the containers 2 via tight 
passages and traverse the same guided on guidance means See, for example, 
U.S. Ser. No. 08/556,921, commonly owned. The microwave transmitter 8, 
e.g. a magnetron, emits microwaves with a fixed, high excitation frequency 
of 2.45 GHz. As can be gathered from the prior art, excitations are 
naturally also possible with lower frequencies of 13.56 MHz. However, with 
a view to obtaining rapid structuring high excitation frequencies above 
13.56 MHz are preferred. The number of possible microwave frequencies is 
legally limited, so that it is appropriate to use 2.45 GHz. The power 
density is adjustable and for rapid plasma erosion is more than 10 
Watt/liter. The temperature and temperature distribution can be locally 
determined by means of a pyrometer 11. For insulating layers such as 
polyimide films the surface temperature should not exceed 200.degree. C. 
In the plasma erosion process according to the invention it is advantageous 
to proceed in such a way that the polymer films 7 are brought into the 
loading basket 6, the container 2 is evacuated to a basic pressure below 
the operating pressure, a gas mixture is introduced into the containers 2, 
the gas flow and chamber pressure are stabilized, the microwave 
transmitter 8 is then switched on and the transmitting power adjusted, so 
that a plasma is ignited in the gas mixture. Everywhere in the container 2 
the plasma can be excited by microwaves. Due to the fact that gas mixtures 
are introduced and pumped out continuously, the reactive particles of the 
plasma strike the insulating layers and erode the same to microshapes. The 
erosion rate is kept high and homogeneous by temperature control. At the 
end of this structuring, e.g. at the end of a specific erosion time, the 
microwave transmitter 8 is switched off, the gas supply stopped and the 
container 2 ventilated in order to remove the polymer films. 
Thus, the most important plasma erosion process parameters are the gas 
pressure, gas flow, polymer film surface temperature, gas type and gas 
mixture composition and finally the microwave transmitter power. All these 
parameters are measurable and regulatable, can be controlled by a computer 
and the plasma reactor 1 can consequently be controlled in a fully 
automatic manner. Such a control also leads to an optimization of the 
process parameters or a determination of optimum parameter sets. In the 
process according to the invention the parameters are optimized with 
respect to the surface temperature. The surface temperature is controlled 
during the structuring by the plasma and by a corresponding parameter 
setting is kept just below the material-damaging limiting temperature for 
polymer films, so as to carefully erode microshapes. 
FIG. 2 shows such an optimization of the process parameters and in a 
diagram is shown a zone of high erosion rates on an insulating layer as a 
function of the parameters gas flow F in ml/min and gas pressure D in hPa. 
The other parameters such as the surface temperature, gas mixture 
composition and microwave power are kept constant in optimum manner. A 
rapid plasma erosion requires a dense plasma with a very high surface 
temperature, the existence range of such a dense plasma being determinable 
in a parameter field P on the basis of apparatus and physical boundary 
conditions with respect to the gas flow F and the gas pressure D. These 
boundary conditions are maximum gas flows and pressures and minimum gas 
flows and pressures. For gas flows F smaller than 50 ml/min there is a 
marked drop in the erosion rate, because insufficient reactive particles 
of the plasma are present in the container 2. However, with gas flows F 
higher than 2000 ml/min the residence time of the reactive particles in 
the container 2 is too short (it is necessary to pump out more strongly in 
order to maintain the gas pressure D), so that the erosion rate drops 
sharply. For gas pressures D below 0.1 hPa the heat transfer by 
non-absorbed microwaves in the insulating layers is so high that they can 
be destroyed by overheating. For gas pressures D higher than 1hPa the 
recombination probability of the ions and radicals in the plasma is so 
high that the erosion rate drops drastically. The parameter field P 
defines an optimum range, which only differs slightly with variable 
microwave power. Thus, by microwave power control it is possible to set an 
optimum high surface temperature, without having to change the other 
parameters. 
FIG. 3 shows microshapes in polymer films produced by the process according 
to the invention in a three-layer film circuit board. The latter comprises 
a 50 .mu.m thick polyimide film 13 on both sides of which are laminated 
thin copper foils and having a surface area of 250.times.250 mm.sup.2 as 
an insulating layer. Such a process for making small holes in such film 
circuit boards has been described in U.S. Pat. No. 5,436,062 by the same 
inventor. In preceding working stages, e.g. by wet chemical etching, 5% of 
the copper foil faces have been exposed down to the polyimide film 14 and 
form a mask for a plasma erosion. A typical gas mixture for plasma erosion 
consisting of 92% O.sub.2 and 8% CF.sub.4 is excited to a plasma at a gas 
flow of 500 ml/min and a gas pressure of 0.4hPa, by 2.45 GHz microwaves 
and a power density of 18 Watt/liter, the film circuit board surface 
temperature being below 200.degree. C. Thus, within 23 minutes 
through-holes 15 are eroded in the polyimide film, i.e. at an erosion rate 
of 2.2 .mu.m/min. It is also possible to structure numerous other and in 
part complicated microshapes. Thus, apart from the through-holes 15, it is 
possible to shape single sloping through-holes 15.2, double-sloping 
through-holes 15.3, through windows 15.1, blind holes 16 and through 
grooves 17. 
The erosion rate of 2.2 .mu.m/min is at least a factor 3 higher than the 
etching rates of the plasma etching processes described in U.S. Pat. No. 
4,720,322 and European patent 144,943. In order to be able to compare the 
rates of the different eroded and etched materials of these processes, 
material-specific conversions were made. Thus, the epoxy-aramide material 
of the US patent can be etched roughly 30% faster than polyimide giving a 
polyimide-related rate of approximately 0.13 to 0.26 .mu.m/min. The 
copper-clad polyimide films (Du Pont LF9111) of the European patent has 
acrylic adhesives which can be etched 300% faster, so that here there is a 
polyimide-related rate of 0.5 to 0.6 .mu.m/min. 
FIG. 4 schematically shows the structure of part of a second embodiment of 
a plasma reactor for the process according to the invention. This plasma 
reactor 1' is substantially identical to that according to FIG. 1, so that 
hereinafter only the additions provided by the second embodiment will be 
described. They relate to obtaining very uniform surface temperature and 
gas flow distributions. The arrangements and movements of the polymer 
films in the containers are as uniform as possible, so as in this way to 
be able to erode homogeneous microshapes. 
The temperature control concept is broadened and intensified. It is no 
longer a question of reaching high temperatures for rapid plasma erosion, 
but instead very uniform temperature profiles are produced on the polymer 
films 7. For this purpose the plasma reactor 1' contains further devices 
such as a heater 12, baffle plates 12.2 and distributor plates 12.1, as 
well as a rotary loading basket 6 with special clamping or fixing devices. 
The heater 12 is used for minimizing heat emission in the direction of cold 
chamber areas, particularly in the direction of the inner walls of the 
chamber 2. In this way the chamber walls are heated to a temperature which 
is as close as possible to the critical temperature. This heating can take 
place in that the plasma reactor 1' is placed in an oven, or in that the 
outer walls of the container 2 are heated by means of strip heaters, or in 
that the inner walls of the container 2 are heated by means of a heater 
12. The heater 12 can be made from a metal such as aluminium and polished 
in order to obtain as emission protection an optimum homogeneous 
distribution of the heat emission in the container. The heater 12 can also 
be passively worked and consists solely of small, thin aluminium plates, 
which shield the cold chamber areas against the polymer films. Thus, 
during plasma erosion, it can be heated rapidly by trapping microwave 
beams or by contact with hot particles and can consequently prevent an 
excessive temperature gradient between the polymer films and the chamber 
walls. 
The gas flow distribution is also an important factor. Thus, the erosion 
rate is significantly dependent on the availability of reactive particles 
in the plasma. It is also highly dependent on how rapidly the reaction 
products are removed. Thus, importance is attached to the arrangement and 
construction of the gas inlets, the construction of the pumping-out 
openings and the arrangement of the polymer films 7 with respect to the 
flow directions of the gas mixture in the plasma reactor 1'. p 
Advantageously the gas mixture is mixed upstream of the inlet and 
uniformly introduced by means of inlet connections 4 distributed uniformly 
over the walls of the container 2. The aim of the inlet means is to 
provide an equal quantity of reactive particles per time unit over the 
polymer films. To carry this out in an optimum manner, corrections are 
made by means of adjustable nozzles, e.g. through the flow regulators 5, 
so that different gas mixture quantities pass through different openings 
to different points of the container, so as in this way to produce an 
optimum, homogeneous gas mixture in the entire erosion zone on the polymer 
films 7 to be eroded. The inlet nozzles suffer from the disadvantage that 
in the container 2 frequently lobar gas flows occur, which is prejudicial 
to an optimum gas propagation. This disadvantage is obviated by fitting 
baffle plates 12.2. The baffle plates 12.2 are e.g. integrated into the 
heater 12 provided with openings and installed at the outlet of the inlet 
connections 4, so that the gas mixture flowing out of the same is whirled 
up thereon. 
In the same way as the nature of the gas inlets, the position and 
arrangement of the pumping-out openings are critical. The suction 
connection 3 projecting into the container 2 leads to a local, excess 
suction action there, which must be avoided. Advantageously, for this 
purpose a distributor plate 12.1 in the form of an aluminium plate having 
holes is fitted above the suction connection 3, so that after flowing 
through the container the gas mixture is homogeneously sucked off through 
these holes. In the embodiment according to FIG. 4 the distributor plate 
12.1 is integrated into the heater 12. A homogeneous sucking off can e.g. 
be brought about by holes arranged with varying density and size on the 
distributor plate 12.1, so that with increasing distance from the suction 
connection 3 the holes become larger and denser, so as in this way to 
compensate for the decreasing suction power. 
A further homogenization of the temperature distribution is brought about 
by rotating the polymer films 7 in the container. It has proved 
advantageous for the polymer films 7 to be eroded to be loaded parallel to 
the gas flow direction and to be rotated in this orientation with respect 
to the gas flow by means of a drive 14 about the longitudinal axis of the 
loading basket 6. In this orientation parallel to the gas flow the gas 
mixture flows longitudinally around the surfaces to be eroded. Within the 
largely closed heater 12 a directed flow is consequently formed with a 
limited, or minimum resistance and extends from the baffle walls 12.2 to 
the distributor plates 12.1. This flow can be varied by the spacing of the 
polymer films 7 in the loading basket 6. Advantageously the polymer films 
7 are not too closely juxtaposed and their spacings should not be below 10 
mm. A spacing of 20 mm is optimum for the gas flows and pressures. If the 
polymer films 7 are too far apart, then there is a decrease in the filling 
level and throughput of the plasma reactor 1'. 
Finally, the temperature distribution is optimized through the manner of 
clamping or fixing the polymer films 7 in the loading baskets 6. In order 
to avoid the occurrence of heat flows by direct contact between bodies at 
different temperatures, the fastenings for the polymer films 7 are made 
from particularly poor heat conducting materials and the contact points of 
said fastenings 18 are made particularly small. Advantageously the polymer 
films 7 are fixed by means of high-grade steel spring clips with pointed 
clipping faces in the loading baskets 6. 
Taking account of all the above measures a plasma erosion inhomogeneity of 
below .+-.5% can be obtained. This should be compared with conventional 
plasma reactors such as plate reactors, where inhomogeneities of 30% have 
to be accepted. 
FIG. 5 shows a graph of a temperature profile for rapid plasma erosion. The 
influence of the surface temperature on the erosion rates in the 
insulating layers is plotted over the erosion time and it is shown that 
the duration of plasma erosion of the polymer films can be shortened by 
.DELTA.t.sub.g by switching in a preheating phase .DELTA.t.sub.h. Two 
curves K.sub.1 and K.sub.2 are plotted. Curve K.sub.1 gives the surface 
temperature during plasma erosion without a preheating phase and curve 
K.sub.2 the surface temperature during plasma erosion following a prior 
preheating phase. The preheating phase .DELTA.t.sub.h commences at a time 
t.sub.o and ends at a time t.sub.o ' and at this time the surface 
temperature of curve K.sub.1 has risen to just below the harmful critical 
temperature T.sub.k. It is then controlled and kept in advantageous, 
constant manner close to said critical temperature T.sub.k. Plasma erosion 
starts for both curves K.sub.1, K.sub.2 at time t.sub.o ' and for curve 
K.sub.1 ends at time t.sub.1 and for curve K.sub.2 at time t.sub.1 '. FIG. 
5 shows that the surface temperature of the not preheated curve K.sub.1 
rises during plasma erosion and only after approximately 1/3 of the 
erosion time reaches the critical temperature T.sub.k. As a result of the 
relatively cold surface temperature in the first 1/3 of the plasma 
erosion, the erosion time is increased compared with the preheated curve 
K.sub.2. The preheating phase .DELTA.t.sub.h is approximately 10% of a 
typical plasma erosion time and the time gain .DELTA.t.sub.g roughly 
corresponds to the time required by the surface temperature of the not 
preheated curve K.sub.1 to approach the critical temperature T.sub.k. 
From the process standpoint the preheating phase is brought about by 
igniting a non-eroding plasma, e.g. nitrogen (N.sub.2) or argon (Ar) with 
a high microwave power. The heat transfer from such a plasma, under 
similar conditions, is even higher than that from the eroding gas mixture 
O.sub.2 /CF.sub.4, so that the preheated curve K.sub.2 rises more rapidly 
towards the limiting temperature T.sub.k than the non-preheated curve 
K.sub.1. For example, when using N.sub.2, a flow of 1000 ml/min and a 
pressure of 0.2 hPa, a power density of 20 Watt/liter is transferred and 
such a preheating phase .DELTA.t.sub.h lasts 3 minutes. 
FIG. 6 is a flow chart of the parameter optimization during plasma erosion. 
According to this flow chart the surface temperature is optimized. The 
surface temperature of polymer films is continuously measured by means of 
a pyrometer 11. Said data are processed by means of a signal integrator S, 
e.g. in a computer PC. They can be represented in graph form or stored on 
media and can be further processed in a selectable manner. Such integrated 
data, which e.g. represent the type-specific temperature behavior of a 
polymer film, e.g. a printed circuit board or film circuit board and 
reproduce the different thermal capacity of said boards, are controlled by 
an algorithm A with respect to their time pattern, continuity and relative 
distance from an upper limiting temperature. Differences between the 
desired value S and actual values I of the surface temperature and 
temperature profile can be regulated. This can e.g. take place by the 
action of a temperature regulator, such as a PID controller PID. If e.g. 
the temperature rises above the critical limit, then by means of the PID 
controller the power of the microwave transmitter 8 can be reduced. If 
e.g. a local heating is measured in the marginal areas (by absorbed 
microwave radiation), then by means of the PID controller the gas pressure 
D can be increased. According to FIG. 6 important parameters such as the 
gas pressure D, gas flow F, power of the microwave transmitter 8, speed U 
of the rotating loading basket and the heating power of the heater 12 are 
regulatable (cf. FIG. 4). The plasma reactor can consequently be fully 
automatically controlled with optimum parameters. 
FIG. 7 is a flow diagram of the storage and provision of optimized 
parameter sets for plasma erosion. According to this flow diagram for 
obtaining a maximum erosion rate use is made of interlinked sets of 
parameters such as excitation frequency, gas pressure, gas flow, residence 
time, surface temperature, container temperature, erosion time, 
arrangement and movement of the polymer films as independent sets for 
different, maximized erosion tasks for plasma erosion control. From the 
quantity of existing sets the in each case desired set for the erosion 
task is made available for loading. This flow diagram is consequently an 
extension of that of FIG. 6. Whereas in the flow diagram of FIG. 6 
dependent parameters are monitored and regulated as a function of the 
surface temperature, in the flow diagram of FIG. 7 there is a monitoring 
of complete parameters sets. Therefore the plasma reactor can be fully 
automatically controlled with optimum parameter sets. The data for 
algorithms A of the computer PC are kept in stock in library form in 
pockets or magazines 19 and when necessary are loaded into the memory 20, 
where they are virtually buffer-stored. Each of the selected algorithms A 
now operates with complete parameter sets with which it controls the 
plasma reactor 1 as a function of desired values S and actual values I. 
The algorithm A can operate interactively, i.e. it has the expert 
knowledge with respect to the plasma reactor, such as e.g. the optimum gas 
flows and critical temperature limits for materials to be eroded and can 
communicate with an operator. New parameter sets are stored in the 
magazine 19.