DE102013114093A1 - Plasma source and method for generating a plasma - Google Patents

Abstract

A plasma source (1) comprises a planar coil (2) with a plurality of arms (10a, 10b, 10c) arranged in parallel, which extend from a common center (12) to a respective peripheral high-frequency power supply point (3a, 3b, 3c), and high frequency generating means (16a, 16b, 16c) arranged to provide high frequency power at the high frequency power feeding points (3a, 3b, 3c) such that phase differences between the arms (10a, 10b, 10c ) are provided, which sets the center (12) of the planar coil (2) at least approximately at ground potential.

Description

The invention relates to a plasma source and a method for generating a plasma.

Technological background

Inductively coupled gas discharges are used for plasma-stimulated processes in a range of high technologies. In inductive coupling, a high frequency current through a coil generates an electric field in a volume of gas that initiates and maintains discharge at a suitable pressure. Widely used are arrangements of a planar coil, which is separated by an electrically insulating, vacuum-tight plate, coupling plate, from the gas volume in which the discharge takes place. Such an arrangement is in US 4948458 (A) disclosed. The discharge provides neutral and charged species for the plasma-stimulated processes.

The planar coil, which may also be referred to as an antenna, may be annular or of one or else as in FIG 1 shown several spiral arms 10a . 10b . 10c consist of a common center 12 to a common periphery 14 run. Such devices are for example in US 5711850 (A) or in DE 19548657 C2 disclosed. The individual spirals can thereby orbit the center by less than or more than 360 °. The periphery 14 the spiral is nominally at ground potential and the output of the high frequency (HF) generator 16 will be with the center 12 connected.

It is further described that the RF feed from a common generator on the periphery ( EP 0710055 B1 ) or at points between the periphery and the center ( US 6475335 B1 ) or from two separate generators in the center and at the periphery, with one intervening point of each spiral arm lying on earth ( US 6475335 B1 ). All variants of the RF feed are aimed at improving the uniformity of the results of plasma-stimulated processes and to avoid an excessive RF potential of the antenna.

If a high-frequency current, for example in the center, is fed in via a capacitive matching network, which is required to adapt the impedance of planar coil including plasma to the output resistance of the HF generator, then the required electric field required for the discharge arises in the gas volume below the coupling plate. However, the center of the antenna receives a high RF voltage to ground because the inductance of the planar coil and the capacitance of the matching network form a resonant circuit. The RF potential of the planar coil then also drives a capacitive current across the plasma boundary layers and the plasma volume to ground. Due to the rectifier effect of the surface layer, this current component causes the high-frequency voltage between the coil and the plasma volume to be superimposed by a DC voltage which is particularly high in the center. As a result, ions are accelerated from the plasma volume in the direction of the coupling plate. Their energy is typically so high that material of the coupling plate is sputtered or atomized and significantly disturbs the process of plasma processing of a substrate, in particular by particle loading. This is to be prevented by the choice of RF feed points described above.

A coil having a plurality of parallel arms is already advantageous with respect to the reduction of the RF potential compared to a coil of only one spiral, because the resulting pulsating DC voltage with the same efficiency for the input of electrical energy into the plasma is lower. A disadvantage, however, is the higher inhomogeneity of the energy coupling as a result of the current distribution over several arms. On the one hand, this distinguishes the geometry of the spiral arms in the geometry of the electric field in the discharge space and on the other hand, it is practically difficult to ensure an equal current flow through all the arms. These disadvantages are to be avoided by the invention.

Described as a measure against an excessive RF potential of the antenna is, for example, that the high frequency current is fed to one end and 180 ° out of phase to another end of the antenna ( DE 19900179 C1 ). It is also described that the previously grounded side receives a higher potential through a suitable impedance between ground and antenna connection ( US Pat. No. 6,204,604 B1 ). The goals of all these measures are to reduce the plasma potential and consequently the minimum energy of the ions striking a substrate or coupling plate to be processed and to achieve a uniform distribution of the plasma properties.

The change in the potential distribution along the antenna arms but has a deterioration of the ignition behavior of the discharge result, as this requires a certain proportion of capacitive coupling. The potential distribution is fixed by the mechanical structure and can e.g. can not be adapted to the respective plasma-simulated machining process to be carried out without further measures. These shortcomings are also to be eliminated by the invention described below.

Summary of the invention

The invention has for its object to improve the generation of a plasma. This object is achieved with a plasma source according to claim 1 or a method for producing a plasma according to claim 8.

The plasma source according to the invention comprises a planar coil having a plurality of arms arranged in parallel in parallel and extending from a common center to a respective peripheral high-frequency power supply point, and a high-frequency generating device configured to provide high-frequency power to the high-frequency power supply. Feed points, so that phase differences between the arms are provided, which put the center of the planar coil at least approximately to ground potential.

The invention proposes to divide the entire RF power at a provided for the maintenance of a gas discharge coil or antenna and supply the shares at several particularly suitable feed points. For this purpose, for example. the RF power provided by multiple RF generators. Preferably, but not necessarily, the frequency of the RF power supplied to the feed points is the same. There are phase differences between the RF voltages provided by the individual RF generators, their values e.g. can be adjusted by means of phase shifters in order to achieve an optimal result of the plasma process. In addition, the amplitudes of the voltages generated by the individual RF generators can be chosen so that the result of the plasma process is further improved. The time profiles of the phase and / or amplitude values can be regulated or controlled during the course of the process.

The advantages of this arrangement and the method based thereon are that there are additional process control parameters that significantly affect the plasma and its spatial distribution. For example, the potential difference between plasma volume and reactor wall is reduced at critical points. As a result, for example, an undesired material removal by high-energy ions (sputtering) can be reduced.

Particularly advantageous is the described new type of RF feed in coils or antennas, which consist of several electrically parallel arms. For antennas with multiple arms, it is possible to divert different RF currents into the different arms. The RF currents into the individual arms can be independent in frequency, amplitude and phase. Frequency, amplitude and phase are then available to adapt the gas discharge to the respective process.

Advantageously, only the arms are connected to the common center. Thus, the potential of the center is free in an idle state, so that the potential on the fed RF power, or RF currents or applied RF voltages, is adjustable, preferably at or approximately at ground potential.

The number of arms can be between two and six, preferably three. The number of arms can be used to precisely set the field distribution. With three arms, the effort and the result achieved are particularly balanced.

In a preferred embodiment, it is provided that the phase difference between the RF voltages at a first and at a second arm is 120 ° and between the RF voltages at the first arm and at a third arm is 240 °. This symmetrical structure achieves good electromagnetic fields and thus plasmas.

The high-frequency generating device can have n high-frequency generators, where n corresponds to the number of arms. Alternatively, a high-frequency generator with a power divider with n outputs can be provided.

It is also possible that at least n-1 phase shifter are provided, where n corresponds to the number of arms. By means of the phase shifters, the phase differences between the arms or between the individual RF powers or RF voltages can be adjusted easily and precisely.

Advantageously, a sensor for measuring current and / or voltage at the center of the coil is provided and the measured variable or the measured variables are provided for controlling the amplitude and / or phase of the high-frequency power or the RF voltages. The sensor allows a closed loop to generate the plasma so that it can react very quickly to changes.

The phase differences between the high-frequency voltages at the individual arms can also be changed programmatically during a process.

At the same frequency, the amplitudes of the high frequency voltages can be controlled so that even with impedance differences between the individual arms of the center is approximately or exactly at ground potential.

At the same frequency, the amplitudes of the high frequency voltages can be controlled so that the potential of the center can be set to ground to a setpoint.

With multiple generators, the frequency common to the generators can be controlled to minimize the mismatch of the antenna to the generator output resistors.

The phase-shifted high-frequency voltages can be generated by phase-shifted distribution of the output voltage of an HF generator.

At the peripheral ends of the antenna, high frequency voltages of the same frequency can be conducted from the generator outputs via the phase shifters whose phase differences are adjusted so that the center of the antenna is approximately at ground potential.

At the peripheral ends of the antenna, high frequency voltages of the same frequency can be conducted from power divider outputs via phase shifters and matching networks whose amplitudes and phase differences are adjusted so that the center of the antenna is approximately at ground potential.

• – Aufteilen einer für die Aufrechterhaltung einer Gasentladung erforderlichen Gesamt-HF-Leistung in Teil-HF-Leistungen, wobei die Anzahl der Teil-HF-Leistungen der Anzahl der Arme entspricht; und
• – Erzeugen eines rotierenden elektromagnetischen Feldes durch an den Hochfrequenz-Leistungs-Einspeisepunkten mit Phasendifferenz eingespeisten Teil-HF-Leistungen, derart, dass das Zentrum der Spule annähernd auf Massepotential liegt.

The inventive method for producing a plasma with a coil having a plurality of arms arranged in parallel, which extend from a common center to a respective peripheral high-frequency power supply point, comprises the following steps:

• Dividing a total RF power required to maintain a gas discharge into partial RF powers, the number of partial RF powers corresponding to the number of arms; and
• Generating a rotating electromagnetic field by partial RF powers fed in at the phase-difference high frequency power feed points, such that the center of the coil is approximately at ground potential.

The same advantages and modifications apply as described above.

Frequency and / or amplitude or, in the case of a common frequency, the phase and / or amplitude of each partial RF power are regulated particularly advantageously. Thus, many parameters are available for control or regulation of the plasma generation process.

The ratio of capacitive to inductive coupling can be controlled by changing the phase relationship between the sub-RF powers. Thus, the process may also target different phases, such as ignition, where a minimum of capacitive coupling is desired, while it is rather undesirable during a plasma process.

Brief description of the figures

The invention will be explained in more detail with reference to an embodiment and associated drawings. The figures show:

1 a schematic representation of a known planar coil for a plasma source;

2 a schematic circuit diagram of a plasma source according to the invention;

3 a side view of a plasma source according to the invention; and

4a / 4b Representations of electromagnetic fields associated with the coil 2 be generated.

Detailed description of the invention

2 shows an embodiment of a plasma source 1 with a coil 2 , The coil or antenna shown here by way of example 2 is planar and consists of three arms arranged in parallel 10a . 10b and 10c , The three arms 10a . 10b and 10c are geometrically identical and rotated against each other by 120 ° each. They have the form of spiral arms, each covering an angular range of about 180 °. Two of the three arms 10a . 10b and 10c each have a common coverage area of about 60 ° to 80 °.

The three arms 10a . 10b and 10c go from a common center 12 out. This center 12 has except the three named arms 10a . 10b . 10c no further connections and thus has a floating potential, its value over the three arms 10a . 10b and 10c is adjustable.

The poor 10a . 10b and 10c each ends at their peripheral ends in high frequency injection points 3a . 3b respectively. 3c , At the entry points 3a . 3b and 3c is in each case an HF generator 16a . 16b respectively. 16c connected. The HF generators feed RF power at the feed points 3a . 3b and 3c on, either as an RF current or as an applied RF voltage.

The total RF power required to maintain a gas discharge is supplied by a controller (not shown), for example, in FIG Partial RF power split, then each from the generators 16a . 16b and 16c to be provided. The RF currents and / or RF voltages have at least one controllable phase difference. Advantageously, the phase differences between all three RF currents, RF voltages or between the arms 10a . 10b and 10c greater than 0 °. In the event that the phase differences between the RF currents in each of two of the three arms 10a . 10b and 10c each 120 °, is the common center 12 at least approximately at ground potential.

In addition to the phase difference may also be the amplitudes or the frequencies and amplitudes of all or individual at the RF feed points 3a . 3b and 3c be made available RF currents or RF voltages controllable.

For clarity, other possible components, such. As matching networks for adjusting the amplitude and / or frequency of the injected RF power or at least two phase shifters, for generating the phase differences, not shown. Furthermore, it is possible to use a single high frequency generator and a power divider with one input and three outputs. In addition, three synchronized high-frequency generators can be used whose output impedances can be adapted to the impedances of the respective antenna arms.

3 shows a side schematic representation of the plasma source 1 with the planar coil 2 , The planar coil 2 is over a dielectric plate 18 which is part of an RF-tight housing in which the plasma volume 20 is produced. Between the plasma volume 20 and the coupling plate 18 is a plasma sand layer 22 educated. The housing further comprises one or more reactor walls at ground potential 24 ,

Now, as described above, if the phase differences between the RF currents in each of two of the three arms are each 120 °, flows from the center 12 the planar coil 2 at no time a stream to the mass. This means that at this point no capacitive HF current through the coupling plate 18 , the plasma sand layer 22 , the plasma volume 20 to the reactor wall at ground potential 24 can flow. Thus, the potential difference between the coupling plate 18 and the plasma volume 20 minimized. An additional connection of the common center 12 with a ground potential is then no longer necessary. Rather, the potential between the center 12 and the ground potential monitored by a sensor and used to control the process.

It is, however, to be expected that for various reasons in the practical realization of the impedances of the arms 10a . 10b and 10c are different. This leads in the prior art to an uneven distribution of current to the antenna arms and thus to a spatial inhomogeneous discharge and that the power from the center 12 the coil 2 to the mass does not disappear. These practically unavoidable deviations and their effects on the results of plasma processes can now be minimized according to the invention in that the phase differences or the amplitude values or both are at the same time adjusted to the present impedance relationships such that the potential of the center 12 minimized and the inhomogeneity of the discharge is reduced. In such cases, a small current can flow to ground via a sensor at the center.

A surprising effect, which has been found in the simulation of the electromagnetic field in such an arrangement, is that the electromagnetic field in the discharge zone rotates at the excitation frequency. In the case of three identical antenna arms 10a . 10b . 10c , are fed to the same RF currents with 120 ° phase difference, the electromagnetic field in the 4a and 4b shown. The right representation of 4b corresponds to a later date than the left representation of 4a and shows that the electric field is rotating in time. The center remains almost at zero potential.

It is clear that rotating the field will also cause design-related and assembly-related local inhomogeneities, thus improving the azimuthal uniformity of plasma processes. For example, in etching processes the uniformity of etch rate, profile and selectivity, in coating processes of growth rate, refractive index and edge coverage. Homogenization has hitherto been achieved for example by additional, externally generated magnetic rotating fields or by mechanical rotation of the substrate table. In contrast, the solution according to the invention is characterized by greater flexibility in the adaptation to the various plasma processes and by less effort.

Another positive effect of the invention results from the fact that the distribution of the electromagnetic field, the ratio between inductive and capacitive coupling can be selectively influenced. While a minimum of capacitive coupling z. B. is required for the autonomous ignition of the discharge, it is rather undesirable during a plasma process. In addition to the sputtering effects already mentioned, the interaction of capacitive and inductive coupling is also a source of plasma instabilities. According to the invention can now by changing the phase relationship between the feed currents, the field distribution z .B. be influenced so that there is a high ratio at the time of ignition and a low ratio of capacitive to inductive coupling during the process.

The three effects described-minimization of the potential difference from coil to plasma volume, rotation of the electromagnetic field and change in the ratio of capacitive to inductive coupling-occur in a modified form compared to the exemplary case described also in arrangements with more than three arms. Possible applications of the invention are therefore not limited to antennas with three arms. Other influences such. As the change in the distribution of trapped particles in the plasma are conceivable and allow a reduction in the particle load of the processed substrate in the affected processes.

LIST OF REFERENCE NUMBERS

1

 plasma source

2

 Kitchen sink

3a, 3b, 3c

 Eispeisepunkt

10a, 10b, 10c

 poor

12

 center

16a, 16b, 16c

 RF generator

18

 coupling plate

20

 plasma volume

22

 Plasma sheath

24

 reactor wall

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 4948458 A [0002]
• US 5711850 A [0003]
• DE 19548657 C2 [0003]
• EP 0710055 B1 [0004]
• US 6475335 B1 [0004, 0004]
• DE 19900179 C1 [0007]
• US 6,204,604 B1 [0007]

Claims (10)

Plasma source comprising a planar coil ( 2 ) with several arms arranged in parallel ( 10a . 10b . 10c ) based on a common center ( 12 ) to a respective peripheral high frequency power feed point ( 3a . 3b . 3c ), and a high-frequency generating device ( 16a . 16b . 16c ) adapted to provide radio frequency power at the radio frequency power feed points ( 3a . 3b . 3c ), so that phase differences between the arms ( 10a . 10b . 10c ), which are the center ( 12 ) of the planar coil ( 2 ) at least approximately at ground potential. A plasma source according to claim 1, wherein the common center ( 12 ) only the arms ( 10a . 10b . 10c ) are connected. Plasma source according to claim 1 or 2, wherein the number of arms ( 10a . 10b . 10c ) is three. A plasma generator according to claim 3, wherein the phase difference between the RF voltages on a first arm ( 10a ) and on a second arm ( 10b ) 120 ° and between the RF voltages on the first arm ( 10a ) and on a third arm ( 10c ) Is 240 °. Plasma source according to one of claims 1 to 4, wherein the high frequency generating means n high frequency generators ( 16a . 16b . 16c ), where n is the number of arms ( 10a . 10b . 10c ) corresponds. Plasma source according to one of claims 1 to 5, wherein at least n-1 phase shifter are provided, where n is the number of arms ( 10a . 10b . 10c ) corresponds. Plasma source according to one of claims 1 to 6, wherein a sensor for measuring current and / or voltage at the center ( 12 ) of the coil ( 2 ) is provided and wherein the measured variable for controlling the amplitude and / or phase of the high-frequency power is provided. Method for producing a plasma ( 20 ) with a coil ( 2 ) with several arms arranged in parallel ( 10a . 10b . 10c ) based on a common center ( 12 ) to a respective peripheral high frequency power feed point ( 3a . 3b . 3c ), comprising the steps of: - splitting a total RF power required to maintain a gas discharge into partial RF powers, the number of partial RF powers being the number of arms ( 10a . 10b . 10c ) corresponds; and - generating an electromagnetic field by at the high frequency power supply points ( 3a . 3b . 3c ) with phase difference injected partial RF power such that the center ( 12 ) of the coil ( 2 ) is approximately at ground potential. The method of claim 8, wherein the frequency, phase and / or amplitude of each partial RF power is controlled. The method of claim 8 or 9, wherein the ratio of capacitive to inductive coupling is controlled by changing the phase relationship between the sub-RF powers.

Images