Device for plasma generation

The invention relates to a device for generating plasma in a vacuum chamber by means of electromagnetic alternating fields, wherein at least one rod-shaped conductor protrudes inside a tube of insulating material in the vacuum chamber and the inner diameter of the insulating tube is greater than the diameter of the conductor, wherein the insulating tube is held at least at one end in a wall of the vacuum chamber and sealed relative to this on its outer surface and the conductor is connected at least at one end to a source for generating the alternating electromagnetic fields.

A known device for producing plasma (DE 195 03 205) makes it possible to produce plasmas for surface treatments and coating technology in a limited operating range (process range, gas pressure, microwave power). The known device consists essentially of a cylindrical glass tube installed in a vacuum process chamber and a metallically conductive tube located therein, wherein atmospheric pressure prevails in the interior of the glass tube. Microwave power is introduced on both sides by two feeds and two metallic coaxial cables, consisting of inner conductor and outer conductor, through the walls of the vacuum processing chamber. The missing outer conductor of the coaxial line within the vacuum processing chamber is replaced by a plasma discharge which is ignited and maintained under sufficient firing conditions (gas pressure) by the microwave power, the microwave power being able to exit the two metallic coaxial lines and through the glass tube into the vacuum processing chamber. The plasma surrounds the cylindrical glass tube from the outside and forms, together with the inner conductor, a coaxial line with a very high damping lining. For fixed, both sides fed microwave power, the gas pressure of the vacuum processing chamber can be adjusted so that the plasma evidently burns evenly along the device where missing within the vacuum process chamber of the outer conductor of the coaxial line.

Also known is a device for the local generation of a plasma in a treatment chamber by means of microwave excitation (DE 41 36 297), which is divided into an outer and an inner part by a flange which can be installed in a wall or the wall itself, wherein a microwave generating device is arranged on the outer part, whose microwaves are guided via a microwave coupling device to the inner part, the microwave coupling device having an outer guide waveguide made of insulating material passing through the flange, in which an inner conductor runs from metal, wherein the microwaves are coupled by the microwave generating device in the inner conductor.

Finally, a device has been proposed for generating plasma in a vacuum chamber by means of electromagnetic alternating fields (DE 197 22 272.2), wherein a rod-shaped conductor is guided within a tube of insulating material through the vacuum chamber and the inner diameter of the insulating tube is greater than the diameter of the conductor, wherein the insulating tube is held at both ends in walls of the vacuum chamber and sealed against the walls on its outer surface and the conductor is connected at both ends to a respective first source for generating the electromagnetic alternating fields, wherein the rod-shaped conductor is in each case enclosed in the region of both wall penetrations in the direction of its central part to a distance from a piece of pipe of electrically conductive material, wherein the two pipe sections are arranged concentrically to the insulating tube and in each case the annular cylindrical, Intermediate spaces formed by the insulating tube and the respective pipe piece are connected to a second source for generating an electromagnetic alternating field.

The present invention has for its object to provide a device of the type in question, which is suitable to avoid fluctuations in the deposited layer, which are based on the occurrence of threshold fields in the microwaves. The device should also be inexpensive to manufacture, in particular, the installation of expensive disposable lines for the protection of microwave transmitters of reflected power should be avoided; d. H. the antenna should be designed so that it still has the property to be able to radiate high-frequency waves in addition to the radiation of each antenna own radiation characteristics, without the application (z. B. the plasma) reflected power again.

This object is achieved in that the rod-shaped conductor is formed in the region of his extending into the vacuum chamber in the lot as a helix, the winding length of this lot corresponds to a wavelength λ at a pitch angle of 10 ° <α <15 °.

In a preferred embodiment, the rod-shaped conductors are guided inside a tube of insulating material through the vacuum chamber, wherein the inner diameter of the insulating tube is greater than the diameter of the conductors and the insulating tube is held at both ends in walls of the vacuum chamber and sealed against the walls on its outer surface and the conductors are each connected to a separate source for generating alternating electromagnetic fields, wherein the rod-shaped conductors are formed as a helix in the region of their extending into the vacuum chamber into the parts and the winding lengths of these two parts approximately a wavelength λ and an angle of 10 ° <α <15 °.

In an alternative embodiment, rod-shaped conductors are guided inside tubes of insulating material through the vacuum chamber, wherein the inner diameters of the insulating tubes are each larger than the diameters of the conductors, and the insulating tubes are each held in walls of the vacuum chamber at one end and sealed against the walls on their outer surfaces, wherein each conductor is connected at one end to a source for generating electromagnetic alternating fields, wherein the rod-shaped conductors are formed as a helix in the region of their extending into the vacuum chamber into the parts and the winding lengths of these two parts approximately a wavelength λ and a pitch angle of 10 ° <α <15 °.

The invention allows a variety of execution options; three of them are shown purely schematically in the appended drawings, showing:<dl id="dl0001"><dt>Fig. 1</dt><dd>the section through a device with a helix and a vacuum chamber side closed insulating tube,</dd><dt>Fig. 2</dt><dd>the single representation of a typical helix,</dd><dt>Fig. 2a</dt><dd>the winding of a coil,</dd><dt>Fig. 3</dt><dd>the representation of the plasma cloud in an operation of the apparatus of FIG. 1 in T₁ Fashion,</dd><dt>Fig. 3a</dt><dd>the representation of the plasma cloud in an operation of the apparatus of FIG. 1 in T₀ Fashion,</dd><dt>Fig. 4</dt><dd>the representation of the plasma cloud in one embodiment with two mutually oppositely arranged coils in each closed insulating tubes and</dd><dt>Fig. 5</dt><dd>the representation of the plasma cloud in a device similar to that of FIG. 4, but with a common, both coil encompassing insulating.</dd></dl>

The device shown in Fig. 1 relates to a helical antenna which in the T₁ mode is operated, wherein circularly polarized microwave radiation along the central axis of the free end of the helix is ​​emitted and ignites a plasma discharge and maintains.

Microwave power is supplied by means of a rectangular waveguide 10 from a microwave generator, which is transmitted by means of impedance matching elements 11,12 on a coaxial line 4. The helix of the antenna 2 is connected to the inner conductor of the coaxial line 4 and converts the transverse electromagnetic waves of the coaxial line into circularly polarized waves and radiates them through a vacuum-tight, but microwave-permeable tube 5 into the vacuum chamber 3. The metallically conductive wall 6 of the vacuum chamber 3 serves as a reflector shield for backward waves.

Several devices of the type described may emit microwave power into one and the same plasma processing chamber (Figure 4), with the individual antennas being microwave-decoupled, provided opposing devices have opposite helix directions (helicity).

More specifically, if the device has right-handed helicity on one side, the other device must be left-handed helicity. Under ideal conditions left-circular polarized microwave radiation can not be absorbed by a clockwise helical antenna (helical antenna) and vice versa. Circularly polarized microwave radiation changes its polarization direction when reflected by metallically conductive planar surfaces.

Each two opposing devices according to the invention can be arranged in a common, vacuum-tight and microwave-permeable tube, as shown in FIG. 5, the two devices being of opposite helicity.

The microwave power radiated forwardly from the devices of the present invention produces plasma discharges that, for microwave wave-mechanical vision, is an absorbing dielectric in which microwaves experience high attenuation. To attenuate the attenuation and thus change the geometric shape of the plasma (especially: to stretch in the longitudinal direction), it may be advantageous to create a plasma-free connection channel between the two devices. With a suitable choice of the operating parameters, a closed plasma column can be generated between in each case two oppositely arranged devices according to the invention, which is largely free from standing wave field patterns.

The helically shaped rod antenna (also referred to as helix) used for the radiation of microwaves is designed to be in the so-called T₁-mode is operated. Fig. 2 shows a helical antenna with about 4 turns, which is supplied via a coaxial cable with microwave power. The radiation characteristic of a helix depends firstly on the ratio of the coil diameter D to the wavelength λ₀ the applied microwaves and secondly from the winding pitch angle α. Depending on the choice of these parameters, the helix antenna radiates in extreme cases in two different modes, which show largely complementary radiation characteristics, as indicated in FIG. 3.

T

<b>1</b>

Fashion

:

winding length <maths id="math0001" num=""><math display="inline"><mrow><mtext>L = C / cos (α)</mtext></mrow></math><img file="EP0961528A2_D0001.tif" /></maths> corresponds to approximately one wavelength λ₀ and 10 ° &lt;α &lt;15 °.

In this operating state, the helical antenna with a pronounced main maximum and small secondary maxima radiates concentrically to the helical central axis from the free end of the helix (endfire). In addition, the radiation in helix antennas with at least 4 turns largely circular polarization whose direction of rotation is determined by the helicity of the antenna. Helix antennas of this type are called Kraus coil (monofilar, axial mode T₁R₁) designated. More about this in Antennas, 2^nd Edition, John D. Kraus, McGraw-Hill Book Company, Chapter 7. The possible antenna gain depends on the geometry of the helix, such as coil diameter, coil pitch angle and overall length and can be up to 15 dB, but is almost independent of the diameter of the tubular, metallically conductive material and its specific electrical resistance (see. D. T. Emerson, National Radio Astronomy Observatory, Antenna Compendium Volume 4, pp 64-68, 1995, published by AARL).

T

<b>0</b>

Fashion

:

winding length <maths id="math0002" num=""><math display="inline"><mrow><mtext>L = C / cos (α)</mtext></mrow></math><img file="EP0961528A2_D0002.tif" /></maths> is considerably smaller than a wavelength λ₀ and 10 ° &lt;α &lt;15 °.

In this operating state, the helix radiates with an intensity distribution whose maximum runs approximately perpendicular to the central axis of the helix, ie radially, similar to a straight rod antenna. This operating state is the basis for the device according to published patent application DE 41 36 297 or patent DE 195 03 205. The T₀ Mode is of no interest to the subject of the invention.

T

2

, T

3

, .... Fashion:

winding length <maths id="math0003" num=""><math display="inline"><mrow><mtext>L = C / cos (α)</mtext></mrow></math><img file="EP0961528A2_D0003.tif" /></maths> is considerably larger than a wavelength λ₀ and 10 ° &lt;α &lt;15 °.

LIST OF REFERENCE NUMBERS

<dl id="dl0002" compact="compact"><dt>2</dt><dd>Wendel, Helix</dd><dt>3</dt><dd>vacuum chamber</dd><dt>4</dt><dd>rod-shaped conductor</dd><dt>5</dt><dd>insulating</dd><dt>6</dt><dd>chamber wall</dd><dt>8th</dt><dd>source</dd><dt>9</dt><dd>source</dd><dt>10</dt><dd>Rectangular waveguide</dd><dt>11</dt><dd>Impedance matching element</dd><dt>12</dt><dd>Impedance matching element</dd><dt>13</dt><dd>Wendel, Helix</dd><dt>14</dt><dd>insulating</dd><dt>15</dt><dd>rod-shaped conductor</dd><dt>16</dt><dd>insulating</dd></dl>