Energy influx measurements with an active thermal probe in plasma-technological processes
© Wiese et al.; licensee Springer. 2015
Received: 25 September 2014
Accepted: 4 February 2015
Published: 24 February 2015
Many plasma-technological applications are based on plasma wall interaction, which can be characterised by calorimetric probes to measure the energy influx from the plasma to the substrate surface. Passive probes are based on the principle of recording the temperature course during heating and cooling of the probe for calculating the energy influx. The disadvantages of these probes are that the energy influx has to be interrupted by switching off the energy source or by using suitable apertures and by the necessity of knowing the exact heat capacity of the probe.
A continuously operating active probe is, therefore, developed which does not need to be calibrated and which compensates the environmental effects as well as the heat conduction by the probe holder. By means of controlled electrical heating the probe is set to a given working temperature and then the energy supply supporting the fixed operating temperature is measured. The energy influx by the plasma is compensated by decreasing the heating power and is directly displayed in J/cm2s. Some practical measurements are presented. Even, if the probe is designed as double probe the directionality of the energy influx can be determined.
KeywordsSensors Thermal probe Energy influx Process control
A large variety of plasma-technological applications are based on plasma wall interaction which occurs via the generated plasma sheath. A good possibility for its characterisation is offered by calorimetric thermal probes for energy influx measurements from the plasma to the substrate, e.g. for the measurement of the deposited power.
Passive thermal probes are often used by several groups, based on the principle of recording the temperature temporal course at the heating and the cooling phase to calculate the energy influx [1,2]. A disadvantage of such passive probes is that the energy influx has to be interrupted by switching off the energy source or by using suitable apertures.
Active probes are also established, constructed as a planar substrate or as a rotationally-symmetric membrane, whereby the energy influx is determined by the measurement of a temperature gradient [3-5]. However, the temperature of these probes is not freely adjustable and its distribution along the surface is not constant. A calibration is necessary before the measurement which may constitute an additional error source.
A novel continuously operating active probe is presented which does not need to be calibrated and which compensates the environmental effects as well as the heat conduction by the probe holder. The change in the heat capacity of the probe by thin film deposition also does not influence the measured energy influx. With a double probe configuration it is even possible to measure the directionality of the energy influx.
Different methods are established for determination the energy flux at plasma-technological processes: for example by measuring the temperature difference generated on a substrate [1,6], by measuring the increasing temperature in the centre of a membrane being exposed to an energy flux and clamped and cooled by a guard ring [4,7], or by recording the temporal heating and cooling temperature course of a dummy substrate [5,8,9].
These measuring methods often produce problems by measurement errors, caused by undesirable heat transport processes that are difficult to control. This concerns for example the heat conduction of the probe and/or the change of the heat capacity by coating the probe. Furthermore, the probe has to be calibrated by an energy source of known intensity, which is associated with uncertainty again. For example, the frequently unknown environmental conditions during calibration usually differ and changing reflexion and emission properties of the thermal probe occur.
Ellmer et al. carried out measurements with a Gardon sensor and tried to minimize the errors by blackening the probe [4,7]. However, the calibration results deviated strongly from the expected value because of the heat conduction of the thermal probe.  Even through calibrating the passive thermal probe, errors of at least 20% were estimated [5,10,11].
Stahl et al. have calibrated passive probes by an electron beam to minimize the measurement error . Furthermore, Bornholdt et al. used a transient method for measurements with a passive thermal probe . In addition, Ellmer et al. investigated a method for calibrating their calorimetric sensor by charged particles emitted from the plasma .
Theoretical considerations and principle of measurement
The problems of calibration were the reason for investigating other methods to measure at a constant thermal balance where heat conduction should not influence the results and calibration is not required.
The following principle of the active thermal probe is applied: likewise as a “dummy substrate” the probe is used to determine the energy flux to a given area. The probe is placed in the region of the plasma where the energy influx needs to be measured.
At the beginning, the probe is heated until the temperature balance reaches the equilibrium temperature T equ without any plasma exposure.
The probe is conditioned by adjusting the temperature balance and determining the required energy input J h | cool .
Here, J Q is the energy influx at the probe, which is produced by the heat radiation of the source and J gain is the energy influx at the probe produced by the plasma process (without the heat radiation of the source).
There are no terms in equation (7) which depend on probe temperature, environmental temperature or the temperature of the probe holder, respectively.
Furthermore, the measured energy is independent of the heat capacity of the probe. That is an important fact because now the determination of the energy by calibrating a known radiation source is not necessary. Another advantage is the independence of the measurement on the heat conduction J con along the probe holder.
Because the measured energy influx is, in principle, the disturbing variable of the temperature balance, the probe becomes less sensitive due to higher heat loss through the holder and connection cable. The compensation of this disturbance is directly proportional to the efficiency of the disturbance, i.e. all other heat flows in equation (6) which influence the heat flux J h | heat . If the thermal radiation J rad of the probe or the heat conduction J con through the connecting cables is high, all other terms remain ineffective.
These conclusions can be found in the given balance of equation 6: The heating power J h | heat cannot be negative. Therefore, considering equation (5) the heating power must always be set higher than the energy flux to be measured. The sensitivity and the maximum of the measurable energy flux can be influenced by the geometry of the probe holder.
For accurate measurements of the energy influx all incoming and outgoing heating fluxes to the probe which are not involved in measurement value have to be assumed constant in time.
This requires specifically constructed probe holders. They contain one or more heated zones, which are set to a given constant operation temperature close to the probe temperature. Thus, the heat fluxes between holder and probe are zero or constant.
A similar configuration is used for the construction of the double probe. Two identical probes are bonded together with their back sides. If both probes are at the same temperature, the heating flux between them is zero. Hence, each probe detects only the incoming energy flux from one half-space. By rotation of the probe this can be used to obtain a “heating radar image” of the environment.
In particular, the ability for continuous measurement is a great advantage of the active probe. After reaching the temperature balance, any change of energy influx from plasma is registered and the temperature balance is restored again. However, setting the temperature balance level as well as the energy influx measurement occurs with a time delay. Note that only the change of the energy influx can be measured and be used as a control variable at the same time when used in process controlling.
Configuration of the probe and measurement procedure
A specially constructed PT100-cell heated by an electrical current, whose temperature can be measured and controlled by a supplied external power is exposed to the energy flux of the plasma. The supplied power and the temperature can be calculated from the potential difference and the current. The probe is connected through a shunt to the output of an amplifier which is controlled by a digital-analog converter. Customised software is installed to filter the readings, control the temperature and calculate the energy influx. For thermal isolation, a second heating is located between the sensor area and the holder, which works in the same way and is set to the same temperature as the sensor. This is necessary to ensure that during the measurement the energy fluxes between holder and probe are zero or constant in time. This is important for the validity of equation 7.
The procedure stores the value of power necessary to maintain the temperature balance level at set-point temperature without energy influx and controls the temperature balance level by analysing the value and the changing rate of the probe temperature. Also it calculates the difference between the power currently needed and the power needed without energy influx from plasma. It displays directly if measurement starts the incoming energy influx at the probe in mW.
Test measurements were obtained by a prototype of the active probe in a plasma chamber at a pressure of 0.04 Pa. The first test should show if the active thermal probe can provide reliable and repeatable data of the energy influx in a plasma environment. For that reason an ion beam is quite suitable because it is a very constant energy source without coating the probe.
The energy influx at the probe can be measured with an accuracy of approximately 1 mW/cm2, which has been confirmed by the fluctuations of the used prototype. Smaller changes in the energy influx are not measurable because of the stochastic fluctuations of the data.
The time needed to reach the temperature balance level of the probe was in the range of 30 s which may be for many applications, especially for controlling and regulating long-time plasma processes, an acceptable value. The time for reaching the equilibrium depends on the heat capacity of the sensor and the quality of the temperature control technique.
However, by the process control the reaction time of the probe can be much shorter. A dysfunction of the process can already be detected by small variations of the probe heating power. That means, the temporal gradient of the heating power can be an indicator for the process regulation. Typical dimensions of the probe (from 2 × 2 mm to 7 × 7 mm) allow for measurements with sufficient spatial resolutions.
Hence, the application of the thermal probe at different coating processes is possible. However, the coating material should be not changed during the measurement.
A quite interesting benefit of the probe is the application as a double probe. By means of such a probe setup the direction of the incoming energy influx can be detected. Two identical probes are bonded by their back sides. The practical experience has shown that all control cycles on the probe chips operate accurately. Obviously, the thermal resistance between both chips is sufficiently high for the thermal decoupling.
The observed behavior of the double probe indicates clearly that there is no heat flux from the back side to the measurement area on the front side of the probe.
Thus, the double probe can be used to obtain a “heating radar image” of the probe environment.
By means of the described prototype it could be verified that the principle of an active heated thermal probe is applicable for the determination of the energy influx in several plasma-technological processes. The principle is based on the decrease of the external input heating power at the probe, which is needed to compensate the incoming energy flux from the plasma. The attained sensitivity almost reaches the level of passive probes. Further optimisation in terms of sensitivity and time resolution should be made for broader applications. An increase in sensitivity while measuring lower energy influxes could be achieved by minimising the heat capacity of the probe and improving the temperature regulation. For special applications, a miniaturised version of the probe in the form of a microchip is conceivable, which would lead to a considerable decrease in the probe’s response time.
We like to thank the AIF for supporting the investigations in the frame of the ZIM-project VP2345701DF9 (E-impact).
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