DE102012109093B4 - Method for controlling the performance of reactive sputtering processes - Google Patents

Abstract

Method for controlling the power in reactive sputtering processes, wherein a voltage between a target and a counter electrode is applied, the target by an impedance between the target and counter electrode receives an actual power and the flow rate of the sputtering process supplied reactive gas by means of a regulator in a control process with a control time wherein the control process comprises a switch-on mode, an operating mode and a switch-off mode, wherein the regulator (2) is acted upon by the actual power Pist absorbed by the target and a power setpoint Psoll is set as a reference variable on the controller (2) and the reactive gas flow F is set by the controller (2) such that the control deviation (Psoll - Pist) from the power setpoint Psoll and the actual value of the power Pist becomes minimal, characterized in that the energy supply (1) takes place with a constant voltage UT in the time range of the control time andIn the operating mode, with a change in the setpoint power Psoll of the target, the actual power Pist of the target is tracked in a time-delayed manner with the value of the setpoint power Psoll provided with a time-varying limitation.

Description

The invention relates to a method for controlling the power in reactive sputtering processes, wherein a voltage between a target and a counter electrode is applied, the target by an impedance between the target and counter electrode receives an actual power and the flow rate of the sputtering process supplied reactive gas by means of a regulator in a control process is controlled by a control time, wherein the control process comprises a power-on mode, an operation mode and a power down mode, wherein the controller with the power absorbed by the target actual power P _is applied as a controlled variable and a desired power value P _soll is set as a guidance variable to the controller and the reactive gas flow F is adjusted by the controller so that the control deviation (P _set - _{P) is intended} for performance target value P and the actual value of the power _P is minimal.

It is known for coating substrates to introduce them into a vacuum coating system in which material is dissolved out of a target by means of cathode sputtering, a so-called sputtering, which then deposits on the substrate. It is also possible to perform a reactive sputtering.

In reactive sputtering, a reactive gas is added to the process which reacts with the material of the target. This makes it possible to deposit not only a material layer of the material of the target but also a chemically modified material, namely the reaction product on the substrate. However, the reactive sputtering process is often very unstable in the transition zone between the metallic and reactive modes. In this area, the reactive sputtering process is often associated with a very pronounced and undesirable hysteresis. This is a rule problem, since there are two operating points above a certain oxygen partial pressure, which converge again only at high gas flows. A switch-on control in a sputter coating system is, for example, in DE 101 35 761 A1 disclosed.

• 1. Bei dem Plasma-Emissions-Monitoring (PEM), erfolgt eine Regelung des Reaktivgasflusses, um den Arbeitspunkt im Transition-Mode stabil zu halten, wobei Verschiedene Möglichkeiten genutzt werden, den Arbeitspunkt zu bestimmen. Der Nachteil besteht darin, dass die Regelung eine schnelle Reaktion des Gaseinlass und Bestimmung des Arbeitspunktes voraussetzt (Reaktionszeit von kleiner als 50 Millisekunden).
• 2. Bei einer Regelung der eingespeisten Leistung besteht der Nachteil darin, dass sie eine schnelle Reaktion des Generators und eine schnelle Bestimmung des Arbeitspunktes voraussetzt.
• 3. Es wird ein hohes Saugvermögen der Pumpe genutzt, um nichteineindeutige Kennlinienbereiche zu umgehen (S-Kurve ohne rückwärts gerichteten Teil). Der Nachteil besteht in einem hohen Hardwareaufwand.
• 4. Es kann auch eine spannungskonstanter Energieeinspeisung genutzt werden, um nichteineindeutige Kennlinienbereiche zu umgehen. Allerdings besteht hierbei das Problem darin, dass der Sputterprozess nicht konstant ist und somit Veränderungen im Sputterprozess Auswirkungen auf die Qualität der Beschichtung haben.

According to the prior art, the following measures are known to solve the problem:

• 1. In plasma emission monitoring (PEM), the reactive gas flow is controlled to keep the operating point stable in the transition mode, whereby various possibilities are used to determine the operating point. The disadvantage is that the control requires a rapid reaction of the gas inlet and determination of the operating point (reaction time of less than 50 milliseconds).
• 2. When controlling the power fed in, the disadvantage is that it requires a fast reaction of the generator and a quick determination of the operating point.
• 3. It is used a high pumping speed of the pump, so as not to bypass unambiguous characteristic ranges (S-curve without backward part). The disadvantage is a high amount of hardware.
• 4. It is also possible to use a voltage-constant energy supply in order to avoid non-unique characteristic ranges. However, the problem here is that the sputtering process is not constant and thus changes in the sputtering process affect the quality of the coating.

When regulating the reactive gas flow as a function of the target voltage, the regulator must work very fast to compensate for the instability described above. In 1 the control process according to the prior art is shown. It will be at the power supply 1 a target power value P set _is set, wherein a certain voltage U _T , which is applied between the target and counter electrode, and adjusts a current corresponding to the impedance R and the power setpoint P _soll . The current voltage value U _T is from the power supply 1 as a controlled variable to the controller 2 fed. On the regulator 2 is a voltage value U _to set as a reference variable. Via a flow rate regulator, not shown in detail, which is commonly referred to as MFC (= mass flow controller), then the reactive gas flow F is set so that the control deviation (U _soll - U _T) from the actual value of the voltage U _T and voltage setpoint U _{soll is} minimal ,

The regulator 2 is, as mentioned above, form very fast working, which brings a high technical effort with it.

It is therefore an object of the invention to make the control of the power in reactive sputtering processes so that the cost of producing the regulator while maintaining the stability of the sputtering process can be reduced.

The object is achieved by a method having the features of claim 1. The claims 2 to 12 indicate particular embodiments of the method.

The method of the aforementioned type is characterized in that the energy supply is provided in the time domain of control time constant voltage U _T, and that in the operating mode with a change of the target power P _to the target actual power P _is of the target to the value provided with a time-varying boundary the target power P _{target is} tracked with a time delay.

For carrying out the method, such a power supply circuit is expediently used, on which a desired voltage value can be set with which the level of the voltage provided by the power supply is determined.

This makes it possible to set the constant voltage at a defined height. However, in a further embodiment, this also makes it possible to track the voltage setpoint over the target service life. Thus, the coating quality can be kept constant over the target life.

One way of voltage tracking is to read out and set the voltage setpoint from a table in which voltage setpoints corresponding to the target life are stored.

Another possibility is that the voltage setpoint acts as a manipulated variable of a cascade control loop whose controlled variable is determined from an external measurement of a parameter of the deposited layer. That is, while the voltage setpoint is within the inventive solution, i. is constant, but is adjusted over longer periods of time, with the measurement of the layer parameters, the adjustment is made so that the layer parameters remain constant over the target life. Thus, the target erosion is corrected by the voltage tracking. Since this control loop is in addition to the control loop, which serves to carry out the method according to the invention, this is referred to here as a cascade control loop.

To safely avoid exceeding a power at the target can be provided that at the power supply, a power limit is adjustable.

It is advantageous to use the method according to the invention also during a switch-on process or during ignition of the plasma. In this case, the actual power of the target is incrementally increased in time as a function of reaching the voltage setpoint and a preset maximum power of the target at the switch-on time. Thus, an overload of the target can be avoided when switching.

To design the power available to the target in the switch-on or ignition process, in one embodiment of the method the controller is switched inactive and a fixed set reactive gas flow is set and kept constant during the switch-on or ignition process. This would immediately set a power corresponding to the reactive gas flow, which could lead to a load on the target. This is prevented by the second feature of this method design, namely the power limit value is evaluated by means of a first time-increasing ramp function. As a result, the power at the target can always go to the respectively set power limit due to the constantly set reactive gas flow and gradually increase with it.

Performance jumps on the target, which can lead to an overload can be avoided in a further process design that follows after switching on or igniting the plasma at a change in the power setpoint by gradually increasing the power limit, the actual power of the target a second ramp function. So as soon as the power would change very quickly as a result of the change in the power setpoint, the time slowly changing limitation of the power provided, whereby the output power to the target can change only slowly.

In order to ensure that the power dropping above the target does not exceed an allowable limit in any case, it can be provided that the limit value to be set is limited to a preset power maximum.

In a further embodiment of the method, it is provided that the controller has a "flow" mode, in which the controller keeps the reactive gas flow constant to a preset value independently of the control deviation.

Upon an intentional turn-off, the power setpoint set to the controller is decreased by a third time-dependent ramp function until a power setpoint corresponding to a reactive flow in "flow" mode is established. Subsequently, the controller is switched to the "flow" mode and the actual power of the target at the time of initiating the turn-off is reduced in time by evaluating the power limit with the continued third ramp function.

The invention as a whole provides a robust method to stabilize the reactive sputtering process at a constant transition mode performance.

The invention will be explained in more detail with reference to an embodiment. In the accompanying drawings shows

1 a schematic representation of the control method according to the prior art,

2 a schematic representation of the control method according to the invention

3 , the sequence of the method according to the invention when switching on according to claim 7 or 8,

4 the sequence of the method according to the invention in the operating mode according to claim 9,

5 the sequence of the method according to the invention in the operating mode with a limitation of the power setpoint according to claim 10 and

6 the sequence of the method according to the invention in an intended turn-off according to claim 12.

As in 2 is shown - starting from the well-known method described above, the constant-voltage power supply, which can also be done in conjunction with high pumping speed of the installed pump - a scheme of the power P _T delivered through the power supply via the position of the oxygen flow F. This regulation can be done much slower (reaction time> 1 second) than with the regulations according to the state of the art. In addition, the regulation of the power P _T is performed so that the power actually delivered to the process P _T by a ramp function 3 is limited. The procedure during operation is in the 4 and 5 shown. There is also a switch-on regime, as in 3 shown and a shutdown regime, as in 6 shown in which the power P _T via a ramp function 3 is driven up or down. The invention offers the additional possibility that the voltage setpoint U set _is tracked over the target service life. _{Is intended} for setting the voltage command value U are methods such as "Lookup Table" or cascade controller with an external measurement of a parameter of the deposited layer (also metrology) is available.

The individual process steps are explained for clarity in the drawings, with the rectangles with the left bevelled corner represent values or parameter inputs, the hexagonal fields comments and all other fields functions or functionalities.

The inventive design of the method, it is possible to use a robust method of stabilizing the operating point by the type of power supply in conjunction with a constant energy input.

Further embodiments can be seen in a regulation of the reactive gas flow F, so that the fed-in current is kept constant, wherein the power P _T can vary. A regulation of the reactive gas flow F so that its partial pressure is kept constant (measurement, for example by lambda probe or mass spectrometer) is possible.

Basically, two setpoints (power P _soll and voltage U _soll ) via apparatuses (power supply unit 1 and regulators 2 ) provided.

The problem with this is that in control mode "VOLTAGE" of the power supply unit 1 to achieve the power P _T provided to the target, the controller 2 responsible for. This works quickly and without time regime.

• – Einschalten (z.B. 0 auf 20kW) [ ]
• – Ausschalten (z.B. 20 auf 0kW) [ ]
• – Leistungsanpassung (z.B. 20 auf 60kW im Betrieb)

diese Leistung P_T nahezu ohne Zeitverzögerung an dem Target anliegt und dieses „stresst“. Ein abrupter Leistungsanstieg könnte dabei bis zum Aufschmelzen eines metallischen Targets führen. The consequence is that when changing the value of the target power P _soll , in three cases:

• - switch on (eg 0 to 20kW) [ ]
• - switch off (eg 20 to 0kW) [ ]
• - power adjustment (eg 20 to 60kW in operation)

this power P _T is applied to the target with almost no time delay and this "stresses". An abrupt increase in power could lead to the melting of a metallic target.

The invention solves this problem by successively increasing the power P _T as a "ramp". In the embodiment presented here, the increase and also reduction, ie the change of the power available to the target P _{T is performed} in substeps.

In this example, the maximum power change per sub-step is either about 10% of the previously provided to the target power P _T or it corresponds to a fixed value, for example, 1kW. The rate of power change, ie, the increase, is either determined by a fixed time for a substep, eg, 3s for 1 kW, or by a control or measurand from the sputtering process. This control variable, the actual value of the power _P, ie, the power that is currently actually consumed by the target be, as in 2 is shown. However, it can also be taken from a regulation of the reactive gas flow F, either so that the fed-in current is kept constant, wherein the power can vary or so that its partial pressure is kept constant.

With the target protection of this kind, the power supply unit can be installed 1 operate only in control mode "VOLTAGE".

The control mode "VOLTAGE", in turn, allows a control for the reactive sputtering, in particular SiO ₂ . There, it is possible to keep the operating point in the "transition" mode of hysteresis stable.

LIST OF REFERENCE NUMBERS

1

power supply

2

regulator

3

ramp function

P _should

 Power setpoint

U _T

target voltage

R

impedance

U _shall

 Voltage setpoint

F

Reactive gas flow

P _is

 actual power

P _T

 the power provided to the target

P _max

 power limit

Claims (12)

Method for controlling the power in reactive sputtering processes, wherein a voltage between a target and a counter electrode is applied, the target by an impedance between the target and counter electrode receives an actual power and the flow rate of the sputtering process supplied reactive gas by means of a regulator in a control process with a control period wherein the control process comprises a switch-on mode, an operating mode and a switch-off mode, the controller ( 2 ) With the power absorbed by the target actual power P _is applied as a controlled variable and a desired power value P _set (as a reference variable to the controller 2 ) and the reactive gas flow F through the regulator ( 2 ) is adjusted so that the control deviation (P _soll - P _is ) from power setpoint P _setpoint and actual value of the power P _is minimal, characterized in that the power supply ( 1 ) Are constant in the time domain of control time voltage U _T, and that in the operating mode with a change of the target power P _to the target the actual power P _is of the target which is provided with a time-varying boundary value of the target power P _soll is tracked delayed. Method according to claim 1, characterized in that at the power supply ( 1 ) a voltage setpoint U _{soll is} adjustable, with which the height of the energy supply ( 1 ) provided voltage U _{T is} determined. A method according to claim 2, characterized in that the voltage setpoint U _{soll is} tracked over the target life. A method according to claim 3, characterized in that the voltage command value V _set is read out from a table in which the target life corresponding to voltage command values are stored and adjusted. A method according to claim 3, characterized in that the voltage command value U _to the manipulated variable of a cascade loop, whose control variable is determined from an external measurement of a parameter of the deposited layer, acts. Method according to one of claims 1 to 5, characterized in that at the power supply ( 1 ) a power limit P _{max is} adjustable. Method according to one of claims 2 to 6, characterized in that at a switch-on or ignition of the target, the actual power of the target in response to reaching the voltage setpoint U _soll and a preset maximum power of the target at the switch-on is delayed in time gradually increased. Method according to claim 7, characterized in that the controller ( 2 ) is switched inactive and a fixed set reactive gas flow F is set and kept constant during the switch-on or ignition process and that the power limit P _max by means of a first time-increasing ramp function ( 3 ), Is evaluated. Method according to one of claims 1 to 6, characterized in that after switching on or igniting the plasma at a change of the power setpoint P _soll by a stepwise increase of the power limit, the actual power of the target follows a second ramp function. Method according to one of claims 2 to 6 and 9, characterized in that the limit value to be set is limited to a preset power maximum. Method according to one of claims 1 to 6, characterized in that the controller ( 2 ) has a "flow" mode in which the controller ( 2 ) The reactive gas flow F _will to a preset value regardless of the control deviation (P - P _is holding) constant. Method according to one of claims 1 to 6, characterized in that at an intended turn off the on the controller ( 2 ) set power value P _soll by means of a third time-dependent ramp function ( 3 ) is lowered until a power setpoint P _soll , which corresponds to a reactive gas flow in the "flow" mode, is reached Controller ( 2 ) is then switched to the "flow" mode and that the actual power of the target at the time of initiating the switch-off is reduced delayed by the power limit P _{max is} evaluated with the continued third ramp function.

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