EP1718135B1

EP1718135B1 - Plasma generator

Info

Publication number: EP1718135B1
Application number: EP05425273A
Authority: EP
Inventors: Paolo Veronesi; Cristina Leonelli; Marco Garuti
Original assignee: Alter SRL
Current assignee: Alter SRL
Priority date: 2005-04-29
Filing date: 2005-04-29
Publication date: 2008-01-02
Anticipated expiration: 2025-04-29
Also published as: EP1718135A1; DE602005004124D1; DE602005004124T2; ATE383063T1

Abstract

A plasma generator (1) comprising a main duct (10) for a gas to be ionised (20), a waveguide (50) for electromagnetic radiation (30), and at least one supporting element (60) on which the waveguide (50) and main duct (10) are mounted; a first portion (51) of the waveguide (50) extends transversely of the supporting element (60) in a first region (61a) facing a first surface (61) of the supporting element. A second portion (52) of the waveguide (50) intersects the duct (10) for interaction between the gas (20) and the radiation (30) and for plasma generation (40); the duct (10) expends transversely of the supporting element (60) in the first region (61a).

Description

The present invention relates to a plasma generator; in particular, the plasma generator in accordance with the invention is applicable to an apparatus suitable for carrying out working operations on predetermined materials, surface working operations for example.
It is known that plasma generators are able to generate a ionised gas (or plasma) starting from a non-ionised gas, such as argon (Ar), nitrogen (N₂), hydrogen (H₂), oxygen (O₂), methane (CH₄), silane (SiH₄), etc., causing an electromagnetic radiation of predetermined frequency to strike thereon, which radiation can either be a radio-frequency (RF) radiation, or fall within the microwave range.
In the case of use of microwaves, the system is provided with a microwave generator associated with a waveguide whose task is to address the radiation.
The plasma generator further comprises a duct connected to a tank in which the gas to be ionised is contained; this gas is introduced into a suitable cavity and then impinged on by said radiation to obtain the plasma.
A structure such the one briefly described above can be used in apparatus for surface treatment of materials, said apparatus being provided with a vacuum chamber in which the workpiece is positioned and maintained under suitable conditions for the treatment process.
Typically, the waveguide and duct for the ionised gas will flow into said vacuum chamber, thus allowing the gas susceptible of ionisation and the microwaves to freely propagated in the chamber itself.
However, in this case ionisation is of very poor efficiency, due to the fact that the ionisation region is not well defined and the gas-radiation interaction has reduced chances of taking place correctly.
Other known solutions contemplate intersection of the gas duct and waveguide so that the ionisation region is more confined and the likelihood that an interaction may occur between the radiation and the gas is greatly increased (see for instance US-A1-2002/0050323 ).
However, in all apparatus of known type it is possible to notice that the duct into which the gas to be ionised flows and the waveguide reach the treatment chamber at different side surfaces of this chamber, thus increasing the overall bulkiness of the apparatus.
In other words, due to the fact that the duct and waveguide are oriented in directions that are very different from each other (typically, a vertical direction and a horizontal direction, when the device is under use conditions), an important increase in the overall sizes of the apparatus is involved, which will bring about clear complications both for transport and installation of the apparatus itself within a factory.
Further problems in the generators of the known art refer to the fact that, at the intersection between the gas duct and waveguide a high heat amount is generated, due to the interaction between the microwaves and the gas to be ionised.
At such an intersection however, it is also necessary to position suitable sealing elements or seals to ensure a hermetically tight insulation between the inside of the waveguide (typically at ambient pressure) and the region facing the outlet of the gas duct (that on the contrary is at low pressure, as it is a working region of the workpiece to be treated).
The heat generated in this region therefore adversely affects the sealing elements to a great extent, causing quick decay of same and consequently making it necessary to carry out frequent servicing and replacement operations.
It is an aim of the present invention to provide a plasma generator having a reduced overall bulkiness.
Another aim of the invention is to make available a plasma generator enabling the distance between the workpiece and the ionisation region to be reduced.
A further aim of the invention is to provide a plasma generator in which the sealing elements are not deteriorated to an important degree by heat resulting from interaction between the gas and the incident electromagnetic radiation.
The foregoing and further aims are substantially achieved by a plasma generator having the features set out in the appended claims.
Further features and advantages will be best understood from the detailed description of a preferred but not exclusive embodiment of the invention. This description is taken hereinafter with reference to the accompanying drawings, given by way of non-limiting example, in which:

Fig. 1 diagrammatically shows an apparatus for surface treatment of materials in which a plasma generator in accordance with the invention is employed;
Fig. 2 is a section taken along line II-II of the generator shown in Fig. 1;
Fig. 3 is a detail seen in Fig. 2;
Fig. 4 is a section taken along line IV-IV of the detail seen in Fig. 3;
Fig. 5 is an exploded view of some parts of the detail seen in Fig. 3.

With reference to the drawings, a plasma generator in accordance with the present invention has been generally identified by reference numeral 1.
Generator 1 (Figs. 1 and 2) first of all comprises a main duct 10 to convey a gas to be ionised 20; in fact, generally generator 1 has the function of causing a predetermined electromagnetic radiation 30 to strike on a predetermined gas 20 so as to obtain a plasma (or ionised gas) 40 that is generated exactly due to the fact that, following interaction between the gas and microwaves, some gas atoms (or molecules) lose at least one of their electrons thus becoming positive ions.
The gas 20 flowing in the main duct 10 can for example be argon (Ar), nitrogen (N₂), hydrogen (H₂), oxygen (O₂), methane (CH₄), silane SiH₄), etc.
The main duct 10 can be made of quartz, sapphire or alumina, for example; generally, as clarified in the following, the main duct 10 is made of a material permeable to the electromagnetic radiation 30, so that this radiation can suitably interact with the gas 20.
The main duct 10 has an inlet 11 and an outlet 12; a tank 13 to hold the gas 20 can be connected to the inlet 11.
Control means 201 can be advantageously interposed between the inlet 11 of the main duct 10 and the tank 13, to control and/or measure the gas flow 20 to be ionised.
In the diagrammatic representation in Fig. 1 a single tank 13 is present; obviously, should use of other process gases be also required, other tanks will be provided possibly connected to the main duct 10 through a suitable header.
Generator 1 further comprises a waveguide 50 to guide the electromagnetic radiation 30 on the gas flowing in the main duct 10; this electromagnetic radiation 30 preferably consists of microwaves and can have, in particular for industrial use, a frequency included between 915MHz (L-band) and 3300 MHz (S-band) and be equal to 2450 MHz, for example.
In a first aspect of the invention, generator 1 further comprises a supporting element 60 on which the main duct 10 and waveguide 50 are mounted.
The supporting element 60 has a first surface 61, preferably substantially planar, facing a first space region 61a.
The supporting element 60 further has a second surface 62, preferably substantially planar, which is opposite to the first surface 61 and faces a second space region 62a.
Advantageously, the supporting element 60 has a platelike conformation and, as clarified in the following, defines a flat flange.
In more detail, the waveguide 50 is provided with a first portion 51 having a transverse longitudinal extension with respect to the supporting element 60 and extending therefrom into the first space region 61a.
Practically, the supporting element 60 can have at least one through hole 63 into which an axial end of the first portion 51 of the waveguide 50 is inserted.
At the opposite axial end, the first waveguide portion 51 is connected to a microwave generator 54. The waveguide 50 is further provided with a second portion 52 associated with, and preferably contiguous to, the first portion 51 to receive the radiation 30 propagating therefrom.
The second portion 52 intersects the main duct 10 to guide this radiation 30 on the gas to be ionised 20.
The main duct 10 has a transverse longitudinal extension with respect to the supporting element 60; said supporting element 60 in particular can have a through hole 64 into which the main duct 10 is inserted. Preferably, the main duct 10 extends at least partly from the supporting element 60 into the first space region 61a. In particular, the main duct portion 10 extending into the first region 61a is the portion terminating with the inlet 11 of the main duct 10 itself.
In this manner, the microwave generator 54 and gas tank 13 are on the same side with respect to the supporting element 60. Preferably, the first waveguide portion 51 is perpendicular to the planar extension of the supporting element 60. Preferably, the main duct 10 is perpendicular to the planar extension of the supporting element 60.
The second waveguide portion 52 is in the second space region 62a; preferably, the second waveguide portion 52 is parallel to the planar extension of the supporting element 60.
The second waveguide portion 52 can be connected to the first portion 51 through a main connecting portion 55; said connecting portion 55 in particular connects an outlet 51b of the first portion 51 with an inlet 52a of the second portion.
Practically, the first and second waveguide portions 51, 52 (connected to each other by the main connecting portion 55) define a substantially L-shaped structure inside which the electromagnetic radiation 30 (i.e. the microwaves) can propagate.
In order to enable the main duct 10 to intersect the second waveguide portion 52, said main duct 10 partly extends in the second space region 62a as well; practically, the main duct 10 crosses the supporting element 60 right through, passing through said through hole 64. Consequently, also the insertion between the main duct 10 and second waveguide portion 52 is in the second space region 62a.
Preferably the waveguide 54 further has a third portion 53 connected downstream of the second portion 52 to receive the radiation propagating into the second portion 52 itself.
To connect an output 52b of the second portion 52 with an input 53a of the third portion 53 the waveguide may comprise an auxiliary connecting portion 56.
The third portion 53 has a transverse longitudinal extension, and preferably a perpendicular extension, with respect to the supporting element 60; in particular the third portion 53 extends from the supporting element 60 into the first space region 61a. In fact, the supporting element 60 can have a through hole 65 into which an axial end of the third portion 53 is inserted.
At the opposite axial end, adjusting means 70 is preferably mounted for adjustment of the electromagnetic field defined by the electromagnetic radiation 30 present in the waveguide 50. This adjusting means 70 can consist of a translatable short-circuit, for example.
Through the adjusting means 70 the interaction efficiency between the microwaves 30 and the gas 20 can be maximised at the intersection between the waveguide 50 (second portion 52) and main duct 10.
In the light of the above, it is apparent that the first, second and third portions 51, 52, 53 of the waveguide 50 define a U-shaped structure through which the microwaves 30 propagate from the microwave generator 54, at the intersection with the main duct 10 into which the gas to be ionised 20 flows, until reaching the translatable short-circuit 70.
Advantageously, under use conditions, generator 1 is disposed in such a manner that the supporting element 60 has a planar extension that is substantially parallel to the ground (horizontal extension), while the main duct 10 and first and third portions 51, 53 of the waveguide 50 substantially extend in a vertical direction.
As above mentioned, generator 1 can be employed in an apparatus 80 for surface treatment of materials. This apparatus 80, in addition to generator 1 comprises a vacuum chamber 81 in which the workpiece 82 is to be positioned; the supporting element 60 of generator 1 in particular defines a closing wall for said vacuum chamber 81.
Practically, the supporting element 60 defines a flange 60a on which the main duct 10 and waveguide 50 are mounted and by which the vacuum chamber 81 is hermetically closed. In more detail, the second portion 52 of the wave guide 50 is located within the vacuum chamber 81, while the first and third portions 51, 53 extend externally of the vacuum chamber 81 itself. The vacuum chamber therefore is located in the second space region 62a.
The apparatus 80 can further comprise a support 83 for the workpiece 82 to position said workpiece to a location facing the outlet 12 of the main duct 10 from which the ionised gas 40 comes out.
The support 83 is preferably made of metal material.
In order to obtain the desired atmosphere within chamber 81, the latter is connected to suitable vacuum-creating means 85 such as a pump.
The connecting opening 86 between the vacuum chamber 81 and vacuum-creating means 85 is at a lower height than the height to which the workpiece 82 is maintained by means of the support 83.
Generally, the workpiece 82 is maintained to an intermediate height between the connecting opening 86 and the outlet 12 of the main duct 10.
As above stated, generator 1 comprises a main duct 10 into which the gas 20 to be ionised flows, and a waveguide 50 into which the microwaves 30 are conveyed.
The main duct 10 and waveguide 50 are intersected in such a manner that the radiation 30 and gas 20 can interact for generating the plasma 40.
In a further aspect of the invention, generator 1 is further provided with spacer means 100 that is at least partly radially external to the main duct 10 and positioned at the outlet 12 of the main duct 10 itself (Figs. 2-5).
The outlet 12 of the main duct 10 is the axially farthest portion of the main duct 10 from which the already ionised gas 40 comes out.
Generator 1 further comprises a sealing element 110 coupled with the spacer means 100 for sealingly insulating the region facing the outlet 12 of the main duct 10 from at least one region 200 radially external to said main duct 10. Preferably, the radially external region 200 comprises the waveguide 50 and/or a heat-exchange chamber 121 to be described in the following. The sealing element 110 can be a seal of the type currently known as "O-ring", for example.
When generator 1 is used in an apparatus 80 for surface treatment of materials, the region facing the outlet 12 of the main duct 10 is within the vacuum chamber 81 in which the workpiece is impinged on by the plasma, while the inside of the waveguide 50 is at ambient pressure.
The sealing element 110 therefore keeps the inside and outside of the vacuum chamber 81 insulated from each other and in particular the region facing the outlet 12 of the main duct 10 (the inside) from the waveguide 50 (the outside).
The spacer means 100 has the task of keeping the sealing element 110 separated from the surface of the main duct 10; this surface increases its temperature to a very high extent, as a result of the heat developed during interaction between the gas 20 and microwaves 30, and there would be the risk of the sealing ring 110 being damaged, should it be in direct contact with the main duct 10. The spacer means 100 exactly aims at avoiding occurrence of this decay.
The spacer means 100 is preferably positioned at the outlet 12 of the main duct 10. In more detail, the spacer means 100 may comprise an annular body 101 radially external to the main duct 10, at the outlet 12 of said duct. In other words, the peripheral extension of the annular body 101 surrounds the main duct 10.
The spacer means 100 may further comprise an annular connecting element 102 to connect the outlet 12 of the main duct 10 to an axial end 101a of said annular body 101.
The sealing element 110 is preferably coupled with the radially external surface 101b of the annular body 101, so that it is maintained to a predetermined radial distance from the outlet 12 of the main duct 10 and therefore limits damages due to the high temperature of this portion of the main duct 10.
The annular body 101 and annular connecting element 102 can be made of one piece construction with the surface defining the main duct 10; for instance, the end portion of this main duct 10 can be turned over outwardly, so as to obtain the above described spacer means 100.
Alternatively, the annular body 101 and annular connecting element 102 can be mutually fastened through welding, in the same manner as the annular connecting means 102 can be welded to the outlet 12 of the main duct 10.
To further reduce the effect of the heat generated by interaction between the gas 20 and microwaves 30, generator 1 may further comprise a heat dissipation unit 120, that is active on the main duct 10 and/or the sealing element 110.
More particularly, the heat dissipation unit 120 is active on the region radially interposed between the outlet 12 of the main duct 10 and the sealing element 110 to cause the heat generated close to the outlet 12 of the main duct 10 not to affect the sealing element 110.
In more detail, the dissipation unit 120 comprises a heat-exchange chamber 121 interposed between the main duct 10, and in particular the outlet 12 of said duct, and the sealing element 110; in this heat-exchange chamber 121 flowing of a cooling fluid (air, for example) is caused, to avoid the sealing element 110 to be overheated too much.
Advantageously, the heat-exchange chamber 121 is defined by the end portion of the main duct 10 (i.e. the outlet 12 of the main duct 10), the annular connecting element 102 and the annular body 101 associated therewith.
The dissipation unit 120 may further comprise a flow deflector 122 positioned at least partly in the heat-exchange chamber 121 to define a path P of said cooling fluid in this chamber.
Deflector 122 can have a substantially annular structure, radially interposed between the end portion of the main duct 10 and the annular body 101.
Deflector 122 has a longitudinal extension substantially parallel to the longitudinal extension of the main duct 10 and the annular element 101; deflector 122 longitudinally extends in the heat-exchange chamber 121 until a predetermined (non-zero) distance from the lower surface S of the chamber itself; in the embodiment shown in Fig. 3, this lower surface is defined by the annular connecting element 102.
In this way, deflector 122 in co-operation with the surface defining the heat-exchange chamber 121, defines a path P for the cooling fluid. To enable introduction of the cooling fluid, generator 1 may comprise a feeding duct 130 having an inlet portion 131 and an annular portion 132. The inlet portion 131 is preferably straight and enables communication with the outside; in particular, a tubular body 135 is provided to be in fluid communication with the inlet portion 131 and the region external to said vacuum chamber 81.
To this aim, the vacuum chamber 81 is provided with a through hole 66 equipped with suitable seals, said tubular body 135 being inserted in said through hole.
The tubular body 135 at its end opposite to the one in engagement with the feeding duct 130, can be connected to a device for delivery of air under pressure (not shown), said air under pressure being the above mentioned cooling fluid.
The annular portion 132 of the feeding duct 130 is radially external to the main duct 10 and is at a location longitudinally offset relative to the outlet 12 of the main duct 10.
The annular portion 132 of the feeding duct 130 is in fluid communication both with the inlet portion 131 to receive the cooling fluid, and the heat-exchange chamber 121, to transfer said cooling fluid to the latter.
In the preferred embodiment, the heat-exchange chamber 121 has an annular opening 121a preferably at an opposite position relative to said annular connecting element 102; the annular portion 132 of the feeding duct 130 can be in continuous fluid communication with the heat-exchange chamber 121 along the circumferential extension of the latter.
A surface of the annular portion 132 of the feeding duct 130 can be defined by a wall 122a of said flow deflector 122.
The annular portion 132 of the feeding duct 130 is further in fluid communication with an auxiliary annular cavity 140, within which said sealing element 110 is positioned.
Generator 1 can further be provided with an outlet duct 133, through which the fluid introduced into the heat-exchange chamber 121 can come out, and in particular can be caused to flow into the waveguide 50.
The outlet duct 133 is preferably radially external to, and in particular substantially coaxial with the main duct 10. The outlet duct 133 has an inlet 133a to receive the cooling fluid from the heat-exchange chamber 121, and an outlet 133b to convey said fluid into the waveguide 50.
Practically, the cooling fluid, through the inlet portion 131 of the feeding duct 130 is introduced into the annular portion 132 and therefrom into the heat-exchange chamber 121; within said chamber, the fluid follows the path P defined by the flow deflector 122 and absorbs the heat generated within the main duct 10. Finally, through the outlet duct 133, the fluid is directed to the inside of the waveguide 50 where the stored heat can be dissipated without creating damages or malfunctions.
It is to be noted that the feeding duct 130, outlet duct 133 and auxiliary annular cavity 140 can be at least partly defined by a single main body 141 mounted around the main duct 10 at the outlet 12 of the latter. Also formed in said main body 141 can be a further annular passage 142 for an auxiliary cooling circuit preferably consisting of water; a radially external wall of said annular passage 142 can be defined by a closing ring 143. Generator 1 may further comprise a closing body 144, coupled with the outlet 12 of the main duct 10 and the main body 141.
The closing body 144 has an opening 144a facing the outlet 12 of the main duct 10. The closing body 144 further has one or more teeth 144b extending longitudinally from the peripheral wall 144c of the closing body 144 itself. Said teeth 144b are in abutment against an annular spacer 145 that in turn supports the sealing element 110. Also provided is an annular plate 146 interposed between the closing body 144 and main body 141.
It is to be noted that the inventive aspects highlighted above (conformation of the waveguide 50 and presence of the spacer means 100) can be employed both separately and in combination with each other, so as to obtain a generator offering optimal performance.
The invention achieves important advantages.
First of all, the generator in accordance with the invention has a reduced overall bulkiness, due to the very practical arrangement of the different elements of which it is made up.
In addition, the integrity of the sealing element used to insulate the inside of the waveguide and the region facing the outlet of the duct from which the plasma comes out is protected in an optimal manner.
A further advantage resides in that in the apparatus for surface working operations in which the generator of the invention can be utilised the distance between the workpiece and the outlet of the main duct can be greatly reduced, so that interaction between the plasma and the material to be treated is made particularly efficient and reliable.

Claims

A plasma generator comprising:
- a main duct (10) to convey a gas to be ionised (20);

- a waveguide (50) to guide an electromagnetic radiation (30) on the gas conveyed in said main duct (10);

- at least one supporting element (60) on which said waveguide (50) and main duct (10) are mounted, said supporting element (60) having a first surface (61) facing a first space region (61a), and a second surface (62) opposite to said first surface (61) and facing a second space region (62a),
said main duct (10) having a longitudinal extension transverse to said supporting element (60) and extending at least partly in said first region (61a)
characterised in that said waveguide comprises:
- a first portion (51) having a longitudinal extension transverse to said supporting element (60) and extending at least partly in said first region (61a);

- a second portion (52) intersected by said main duct (10) and associated with said first portion (51) to receive the radiation (30) propagating in said first portion (51) and guide it on the gas conveyed in said main duct (10) and obtain a corresponding plasma (40) .
A generator as claimed in claim 1, characterised in that said waveguide (50) further comprises a third portion (53) connected downstream of said second portion (52) to receive the radiation (30) propagating in said second portion, said third portion (53) having a longitudinal extension transverse to said supporting element (60).
A generator as claimed in claim 2, characterised in that said third portion (53) extends from said supporting element (60) into said first region (61a).
A generator as claimed in anyone of the preceding claims, characterised in that said main duct (10) partly extends in said second region (62a).
A generator as claimed in claim 4, characterised in that the intersection between said main duct (10) and second waveguide portion (52) is in the second region (62a).
A generator as claimed in anyone of the preceding claims, characterised in that said second waveguide portion (52) is positioned in said second region (62a).
A generator as claimed in anyone of the preceding claims, characterised in that said first portion (51) is substantially perpendicular to said supporting element (60).
A generator as claimed in anyone of the preceding claims, characterised in that said second portion (52) is substantially parallel to said supporting element (60), said waveguide (50) being also preferably provided with a main connecting portion (54) for connection between an outlet (51b) of said first portion (51) and an inlet (52a) of said second portion (52).
A generator as claimed in anyone of the preceding claims, characterised in that said main duct (10) is substantially perpendicular to said supporting element (60).
A generator as claimed in anyone of claims 2 to 9, characterised in that said third portion (53) is substantially perpendicular to said supporting element (60), said waveguide (50) being also preferably provided with an auxiliary connecting portion (55) for connection between an outlet (52b) of said second portion (52) and an inlet (53a) of said third portion (53).
A generator as claimed in anyone of the preceding claims, characterised in that said electromagnetic radiation (30) consists of microwaves.
A generator as claimed in anyone of the preceding claims, characterised in that it further comprises means (70) for adjusting the electromagnetic field defined by said electromagnetic radiation (30), said adjusting means (70) being preferably connected downstream of said third portion (53) and being in particular provided with a translatable short-circuit.
A generator as claimed in anyone of the preceding claims, characterised in that it further comprises:
- spacer means (100) at least partly radially external to said main duct (10);

- a sealing element (110) coupled with said spacer means (100), for a sealed insulation between the region facing the outlet (12) of said main duct (10) and at least one region (200) radially external to said main duct (10).
A generator as claimed in claim 13, characterised in that it further comprises a heat-dissipation unit (120) active on said main duct (10) and/or said sealing element (110).
A generator as claimed in claim 14, characterised in that said dissipation unit (120) comprises at least one heat-exchange chamber (121) interposed between said main duct (10) and sealing element (110).
A generator as claimed in claim 15, characterised in that said dissipation unit (120) further comprises at least one flow deflector (122) positioned at least partly in said heat-exchange chamber (121) to define a path P of a cooling fluid therein.
A generator as claimed in anyone of claims 13 to 16, characterised in that said radially external region (200) comprises said waveguide (50) and/or said heat-exchange chamber (121).
A generator as claimed in anyone of claims 13 to 17, characterised in that said spacer means (100) comprises:
- an annular body (101) radially external to said main duct (10) at the outlet (12) of the main duct itself;

- an annular connecting element (102) to connect the outlet (12) of said main duct (10) to an axial end (101a) of said annular body (101).
A generator as claimed in claim 18, characterised in that said sealing element (110) is coupled with a radially external surface (101b) of said annular body (101).
A generator as claimed in claim 18 or 19, characterised in that said annular body (101), annular connecting element (102) and outer surface of said main duct (10) define said heat-exchange chamber (121).
An apparatus for surface working of materials through plasma, said apparatus (80) comprising:
- a vacuum chamber (81) to house a material to be worked;

- a plasma generator (1) as claimed in anyone of claims 1 to 20, said supporting element (60) defining a closing wall of said vacuum chamber (81).
An apparatus as claimed in claim 21, characterised in that the second portion (52) of said waveguide (50) is positioned within said vacuum chamber (81), said first and third portions (51, 53) being preferably positioned externally of said vacuum chamber (81).
An apparatus as claimed in claim 21 or 22, characterised in that it further comprises a support (83) for said material to be worked (82) to position the latter at a location facing the outlet (12) of said main duct (10).
An apparatus as claimed in anyone of claims 21 to 23, characterised in that said sealing element (110) defines a hermetically tight insulation between the inside and the outside of said vacuum chamber (81).