WO1997030186A1

WO1997030186A1 - Method and device for control of plasma enhanced vacuum coating processes

Info

Publication number: WO1997030186A1
Application number: PCT/DE1997/000265
Authority: WO
Inventors: Bert Scheffel; Klaus Goedicke; Christoph Metzner; Volker Kirchhoff
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1996-02-14
Filing date: 1997-02-07
Publication date: 1997-08-21
Also published as: DE19605316C1

Abstract

In the known methods for controlling vacuum coating processes, measurable variables such as plasma emission and coating rate are measured in the plasma or vapor in order to control the vacuum coating process using process parameters of the vaporization or sputtering. This subjects the sensors to high levels of stress, which adversely affects their durability. Using the force of the electrical discharge as a measurable variable, the plasma enhanced vacuum coating process is controlled by measuring the force of the electrical discharge on the coating source. The coating source is arranged in the vacuum chamber with the aid of force transducers (3), and the signal emanating from the force transducers can be used to control, for example, the electrical energy to heat the vaporizing material, the beam deflection or the distribution of the electrical discharge to several coating sources, without the sensors coming in contact with the plasma or the vapor. Plasma enhanced vacuum coating processes, particularly for coating tools and sheet substrates with functional coatings, can be controlled with this method and device.

Description

Process and device for controlling plasma-assisted vacuum coating processes

The invention relates to a method for controlling plasma-assisted vacuum coating processes by atomizing or evaporating

Coating material in which an electrical discharge for generating a plasma is maintained and in which the cathode of the electrical discharge is used as the coating source, and a device for carrying out the method. In particular, components and tools as well as substrates, e.g. Mirrors and glass panes, coated with functional layers.

A large number of vacuum coating processes are known in which plasmas generated by electrical discharges are used. Cathode sputtering processes are vacuum coating processes in which a glow discharge or magnetron discharge is ignited and material is atomized from the cathode by the bombardment of ions from the plasma onto the cathode. Furthermore, plasmas are used in vacuum coating processes to ionize and excite the steam generated by other processes. These vacuum coating processes include in particular the processes of plasma-activated evaporation.

In these plasma-assisted vacuum coating processes, the properties of the deposited layers depend to a large extent on the state of the plasma generated by the electrical discharge. For example, plasmas with a high charge density result in a high proportion of the vapor being ionized or excited. This results in a flow of high-energy particles to the substrate, which leads to the required layer properties, in particular to very dense layers. The use of plasmas in reactive vacuum coating processes increases the reactivity between vapor particles and reactive gas particles considerably. This leads to a change in the vapor density distribution in the space between the location of the vapor generation and the location of the vapor separation. This leads to a change in the layer thickness profile on the substrate. These influences of the plasma on the layer formation are often desirable, but sometimes an undesirable side effect.

In any case, it is important to know the plasma parameters, eg ion density, energy of the ions, electron density, with their influences on the layer formation monitor and keep constant via control processes or change them according to a coating technology.

It is generally known to use the parameters of the electrical discharge for the regulation of plasma-assisted vacuum coating processes by using these electrical parameters of the energy coupling, e.g. the electrical power to measure and keep constant. The current intensity or the voltage of the discharge can also be used as a measurement variable for a control. The disadvantage of these methods is that the total charge flow, which is formed from electrons and ions, is measured and used for regulation. The proportion and the energy of the ions cannot be regulated, although the properties of the deposited layer depend to a large extent on the proportion and the energy of these ions. Furthermore, when using a plurality of cathodes, the effects of the electrical discharge on the individual cathodes and on the steam flow to the substrate cannot be separated.

It is also known to determine the plasma parameters on the substrate. For example, the floating potential of an electrically insulated, conductive substrate is determined by voltage measurement, or the current to the substrate is evaluated after application of a voltage to the substrate (Schade, K. et al .; plasma technology: application in electronics, Verlag Technik Berlin, 1990, p 96ff). These methods have the disadvantage that they cannot be used with electrically insulating substrates or electrically insulating layers, since no voltage can be measured with electrically insulating substrates or layers.

A method with probe measurement is also known in which a small electrical probe of defined geometry is arranged in the plasma (Janzen, G .; Plasma Technology - Fundamentals, Applications, Diagnostics, Heidelberg: Hüttig, 1992, pp. 255ff). The current-voltage characteristic is evaluated and the plasma parameters are calculated. It is disadvantageous that the probe in the plasma is subject to a high vapor and temperature load, that the measurement results are falsified and that, in particular in coating processes with a high coating rate, only a short operating time of the probes is achieved. A method is also known in which the emission of the plasma, in particular a defined line of the plasma emission, is used to characterize the electrical discharge and to regulate the plasma-assisted vacuum coating process (DD 239 810 A1). A light guide cable is guided in the vicinity of the plasma and largely protected from coating with a collimator. The light is fed through a filter to an amplifier, for example a secondary electron multiplier, which drives a control loop. The disadvantage of this method is that only a small area of the plasma is used to determine the plasma parameters, which is used to extrapolate to the entire plasma area. In addition, this control method must be set for each coating material and also during the

Vacuum coating process are often calibrated, so that the implementation of this method is very expensive.

It is also known to use electrically insulating layers by means of plasma-assisted vacuum coating processes using medium-frequency pulsed plasmas in the

Apply frequency range from 1 kHz to 100 kHz (DD 252 205 A1). The disadvantage of these methods with pulsed plasmas is that no methods with sufficient accuracy for measuring or controlling such vacuum coating processes are available due to the complicated pulse shapes of the electrical quantities, which are dependent on numerous parameters.

Methods for plasma activation in electron beam evaporation are also known (DE 43 36 680 A1, DE 43 36 681 A1). Arc discharges are operated on at least one electron beam evaporator connected as a cathode. The disadvantage of these methods is that the electron beam and the arc burning in the vapor influence each other.

The invention has for its object to provide a method and a device for controlling plasma-assisted vacuum coating processes by atomizing or evaporating the coating material with direct current plasma or pulsed plasma in order to produce layers with defined layer properties. The method is intended to enable the regulation of the proportion of ions in the vapor space of the plasma-assisted vacuum coating process. It should be independent of the electrical conductivity of the layers and / or the substrates produced and in a simple manner be feasible over long periods of time. The device should be simple in terms of equipment.

The object is achieved according to the features of patent claim 1. Further advantageous embodiments are described in claims 2 to 6. The device for performing the method is described in claims 7 and 8. The essence of the invention is that a method has been found to use the known force of ions of a plasma during ion bombardment on a cathode to utilize a signal to be obtained therefrom for process control. This knowledge makes it possible to record the force of the ions of the plasma during ion bombardment on the coating source and to process it further as a signal. The coating source is an atomization source, preferably of the magnetron type, or an evaporator, preferably an evaporator heated by an electron beam. This force of the ions results from the potential difference between the cathode surface and the plasma, through which the ions receive a correspondingly high energy, which is expressed in a high ion pulse depending on the ion mass. In addition to the potential difference, this force on the cathode is determined by the number of ions accelerated to the cathode per unit of time.

An advantage of the invention is that by determining this force acting on the cathode, a measured variable is determined from which a signal is obtained with which it is possible to use a control circuit known per se to determine the effect of the electrical discharge on the flow of particles to To regulate the substrate in a defined manner and thus to influence the properties of the layer applied to the substrate. Further advantages of the invention are that by averaging the signal, plasma-assisted vacuum coating processes with pulsed plasmas with any pulse shape of the electrical parameters are controlled in such a way that coated substrates with the required layer properties can be produced. When using at least two coating sources acting as a cathode, this process can separate the effect of the discharge for each individual cathode and use the signals from the individual cathodes to regulate the entire vacuum coating process, for example by superimposing a direct voltage on the electrical discharge on average is equally distributed over the two cathodes. This is particularly advantageous if the individual cathodes have different properties - for example different magnetizations or different sizes - have, and thereby, for example, an AC voltage deviates from the sinusoidal shape or is distributed unevenly.

The arrangement of the sensors for determining the force on the cathode, on the side facing away from the plasma, eliminates faults caused by coating or thermal

Effects on the sensors avoided. That the procedure is independent of the

Coating with electrically conductive or electrically insulating materials and / or

Substrates feasible.

This increases the reliability and possible operating time of the device, so that it can be operated with long-term stability and safely.

To carry out the method, each cathode with load cells as sensors is arranged in the vacuum chamber. The load cells are preferably elastic elements with applied strain gauges. All supply lines are connected in such a way that no force bypass occurs or the resulting errors are negligible. If the cathode surface is arranged vertically during sputtering, ie if the preferred direction of ion bombardment on the cathode is horizontal, the force of the electrical discharge on the cathode is measured directly in the horizontal direction. In all other cases, for example if the preferred direction of ion bombardment is perpendicular to the cathode, this force is superimposed on the weight of the cathode and during evaporation, for example electron beam evaporation, the recoil force of the vaporized particles. In addition, the weight of the cathode changes over time due to atomization or evaporation. Consequently, the weight force must be separated from the force of the electrical discharge by measurement. The recoil force is usually negligible, or it is separated from the force of the electrical discharge together with the weight. This metrological separation is achieved according to the invention in that a parameter which determines the force of the electrical discharge which the ions exert on the cathode is changed in size so quickly that the rate of change of the force of the electrical discharge is large compared to the rate of change of the Weight of the cathode. This can be done, for example, by briefly switching the electrical discharge off and on again, or by briefly changing the electrical parameters of the electrical discharge (eg current, electrical power). The change in force of the total force during the switch-on or switch-off phase or change phase is then a measure of the force of the electrical discharge on the cathode. The device for carrying out the method has the advantage that existing devices can also be retrofitted without great effort, since the coating sources are no longer directly connected to the vacuum chamber, but instead force measuring cells are arranged between the coating source and the vacuum chamber. Furthermore, a conventional control loop is necessary, which in the simplest case consists of a filter and a controller with which the signal is processed and the corresponding parameters for controlling the vacuum coating process are controlled.

The invention is explained in more detail using two exemplary embodiments. The associated drawings show:

1 shows a section through a device for carrying out the method with two

Magnetron sputtering sources Figure 2: a section through a device for performing the method with an electron beam evaporator

Fig. 3: a diagram of the force, the current of the electrical discharge and the

Electron beam power, as a function of time, in the plasma-activated

Electron beam evaporation,

In Fig. 1, two magnetron sputtering sources 2 are arranged vertically on the side walls of a vacuum chamber 1 by means of two load cells 3, which are elastic elements with applied strain gauges. A substrate 4 to be coated on both sides is arranged between the two magnetrons 2. The magnetrons 2 are connected to the outputs of an AC voltage source 5 and are thereby alternately connected as a cathode or anode of the electrical discharge with a frequency of 50 kHz and a peak voltage Us. Furthermore, a DC voltage source 6 is connected to the magnetrons 2 in such a way that the AC voltage is superimposed on the DC voltage in such a way that the mean value over a period of the AC voltage can be shifted to values from -V ₂ Us to + V ₂ Us. If the electrical discharge burns between the magnetrons 2, they are switched on

Magnetrons 2 horizontally acting forces measured with the load cells 3. The signals obtained in this way are fed to a known control circuit in that the signals are each smoothed by a filter 7. The output signals of the filters 7 are then fed to a subtraction element 8, in which the two smoothed signals are subtracted from one another. The output signal of the subtractor 8, the Differential signal, is fed to a controller 9. This influences the superimposed DC voltage until the deviation of the difference signal from zero becomes minimal.

In FIG. 2, an electron gun 10 of the axial type is arranged on one side of a vacuum chamber 1, the electron beam 11 of which acts on an evaporator 12 filled with titanium. The evaporator 12 is fastened horizontally in the vacuum chamber 1 by means of load cells 3, consisting of elastic elements with applied strain gauges. A substrate 13 to be coated is arranged above the evaporator 12. The outputs of a direct current source 14 for arc power supply to maintain the electrical discharge are connected to the vacuum chamber 1 and the evaporator 12, which is connected as a cathode, in such a way that the electrical discharge burns. The electron gun 10 operates at a constant voltage and variable power. First, a certain power of the electron gun 10 and a fixed value of the current lar _{C of} the electrical discharge are set so that sufficient titanium evaporates. The control procedure using force measurement is carried out cyclically in the phases of changing the substrate.

3 shows how the force of the electrical discharge is now determined. The electron beam power Pe _b , the current intensity of the electrical discharge l _arc and the force F as the sum of the signals from the load cell 3 are shown as a function over time t. Evaporation takes place with the electron beam power P, *,. At time t- the force F- is measured, which is composed of the force of the electrical discharge, the recoil force of the evaporating titanium and the weight of the molten titanium and the evaporator 12. First, the current strength L ^ is switched from I- to l ₂ = 0 without delay. The force F _{2 is} measured at time t ₂ (t ₂ > tι). At time t ₃ , immediately before the discharge is switched on again to the current \ -, F _{3 is} measured. Then the current 1 ^ is switched back to the value l <without delay. The force F4 is measured at the time td (t ₄ > t3). This force F4 is composed again of the force of the electrical discharge, the recoil force, the weight of the molten titanium and the evaporator 12.

The signals determined before and after the process of reducing the current intensity I _arc from the measured forces are smoothed by a filter 7. The output signals of the filter 7 are then fed to a controller 9. The force of the electrical discharge is determined in the controller 9 from the difference between the forces Fi and F ₂ measured at the times t ₂ and t ₂ . The controller 9 compares this determined force with predetermined target values. The force of the electric discharge from the difference between the time t ₃ and t ₄ measured forces F ₃ and F ₄ is determined for control purposes.

If the force of the electrical discharge deviates from a predefined setpoint value, the size of the electron beam power Pet

Electron gun 10 changed so that the deviation of the force of the electrical discharge from the predetermined target value is minimal.

Claims

1 Method for controlling plasma-assisted vacuum coating processes in which an electrical discharge is maintained to generate a plasma and in which at least one cathode is used as the electrical discharge

Coating source, which is an evaporator, preferably an electron beam evaporator, or a sputtering source is used, and at least one process parameter is adapted to the coating conditions by means of a control loop, characterized in that the force acting on the coating source connected as a cathode by the electrical discharge acts on at least one The position is determined by measuring the force acting on the fastening of the coating source, that this measured variable obtained is recorded as a signal, and that this signal is fed as a controlled variable to the control loop which regulates the process parameters.

2 The method according to claim 1, characterized in that in the case of an atomization source arranged differently from the vertical position in the vacuum chamber or a horizontally arranged electron beam evaporator, the force acting on the cathode by the electrical discharge is determined in such a way that the acting on the coating source

Forces are measured as a sum, that the force of the electrical discharge is separated by measurement from the sum of the forces by changing at least one process parameter determining this force so quickly that the rate of change of this force is large compared to the rate of change of the weight force, and that the difference between the sum of the forces before the change and the sum of the forces after the change determines the force of the electrical discharge acting on the cathode.

3 The method according to claim 2, characterized in that the change of a process parameter determining the electrical discharge, in particular current strength and electrical power, takes place in such a way that the electrical discharge is switched off and on again or is briefly changed. Method according to Claims 1 to 3, characterized in that when operating at least two atomization sources with a pulsed alternating voltage with the signals obtained from the force of the electrical discharge, the electrical parameters of the pulsed alternating voltage are changed via a control circuit so that the forces of the electrical discharge are applied the single ones

Atomization sources can be regulated to specified target values.

Method according to Claims 1 to 4, characterized in that when two atomization sources are used with the signals obtained from the force of the electrical discharge, the time average of the pulse-shaped

AC voltage is changed over a period by changing the duty cycle or by superimposing a DC voltage such that the same average force of the electrical discharge acts on both atomization sources.

A method according to claim 1 to 3, characterized in that the

Using an electron beam evaporator with the signal obtained from the electrical discharge, the regulation of the power of the energy supply to the coating material is carried out in such a way that the force of the electrical discharge is regulated to a predetermined desired value.

Device for carrying out the method according to claim 1, consisting of a vacuum chamber in which at least one atomization source or at least one electron beam evaporator is arranged as the coating source, at least one coating source being connected as the cathode of an electrical discharge, and in that the atomization source in any position

Vacuum chamber is arranged, a gas inlet and a control circuit for controlling the process parameters, characterized in that the coating source connected as a cathode (2) is fastened in the vacuum chamber (1) with at least one load cell (3) and that the load cell (3) with the Control circuit is electrically connected.

Device according to claim 7, characterized in that the load cell (3) is an elastic element with an applied strain gauge.