JPH01319965A - Substrate adsorption device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is for flattening a semiconductor wafer for manufacturing semiconductor devices (LSI, VLSI, etc.) or a substrate such as a glass plate for manufacturing liquid crystal devices. The present invention relates to a device for suctioning and fixing.

[Conventional technology]

Conventionally, in apparatuses for processing this type of substrate, such as projection type exposure apparatuses and laser repair apparatuses, vacuum suction holders have been used that vacuum suction the substrate to flatten and straighten it within a predetermined plane. Particularly in this type of manufacturing equipment, it is necessary to planarize the substrate with high precision. In the case of a projection exposure device (stepper), the circuit pattern on the reticle is printed at the same magnification, 11
A projection lens is provided for forming and projecting an image onto the substrate surface at a magnification of, for example, 5 or 1/10. These projection lenses need to secure a wide projection area while also achieving high resolution of 1 μm or less, especially in the case of reduced projection lenses. Some of the emerging projection lenses have a depth of focus of only about ±1 .mu.m within a 15.times.15 square field, and as a result, more precise focusing techniques are also required.

On the other hand, since there is only a depth of focus of ±1 μm at the front of each 15×15a+a area to be exposed, the entire surface of one area to be exposed on the substrate must be precisely aligned with the best imaging plane of the projection lens. .

However, the surface of a wafer or glass plate has warps and unevenness of several micrometers locally and several tens of micrometers across the entire surface, making it difficult to expose patterns with good resolution characteristics. .

Therefore, as an example, it has been considered to flatten and straighten the wafer W using a wafer holder (vacuum chuck) l as shown in FIGS. 3(A) and 3(B).

This wafer holder 1 is provided at the top of a wafer stage of a stepper so as to face a projection lens, and moves two-dimensionally (stepping, etc.) under the projection lens together with the wafer stage.

FIG. 3(A) is a plan view of the wafer holder 1, and the third
Figure (B) is a sectional view taken along the line C-3 in Figure 3 (A). The wafer holder 1 is made of a metal or ceramic material that is sufficiently thicker than the wafer W in the shape of a disk, and the shape of the mounting surface is circular with a diameter slightly smaller than the diameter of the wafer W. do. A circular opening 1a is formed in the center of the wafer holder l, through which a wafer delivery lifting mechanism 2 for placing and removing a wafer W passes when moving up and down. Further, on the mounting surface of the wafer holder l, a plurality of concentric annular convex portions 10 are formed in a rim shape at a constant pitch in the radial direction from the center of the holder 1 in a radial direction. Here, an annular convex portion 10a located on the outermost side of the mounting surface
The radius is set to be slightly smaller than the radius from the center of the wafer W to the linear notch (orientation flat) OF. In addition, the width (radial dimension) of the upper end surface of each annular convex portion 10 is made as small as possible, and the surface defined by each upper end surface that has been subjected to grinding and lapping is the standard for flattening. It becomes a flat surface. The innermost annular convex portion 10g is formed around the opening 1a, and this convex portion 10g and the convex portion 10a reduce the atmospheric pressure (atmospheric pressure).
This prevents leaks. Furthermore, each recess (annular) 11 between each annular convex part 10 has an intake hole IC for vacuum suction.
are arranged in the radial direction, and each intake hole IC is connected to the holder-1.
It communicates with a sleeve-shaped hole 1b extending radially inside. By connecting this hole 1b to a vacuum source and reducing the pressure, the space defined by the back surface of the wafer W and the annular concave portions 11 becomes negative pressure, and the back surface of the wafer W is surrounded by a plurality of annular convex portions 10. It is flattened and corrected following the upper end surface.

[Problems to be solved by the invention]

According to this wafer holder 1, the concentric annular convex portion 1
If the radial pitch of 0 is large, that is, the radial width of the recesses 11 is large, the deformation (deflection amount) of the wafer W when each recess 11 is depressurized will be smaller than the depth of focus determined by the stepper. It has become impossible to ignore. In order to prevent this, the pitch of the annular convex part 10 should be made small (the concave part 1
1), but this necessarily means increasing the ratio of the contact area (contact ratio) to the total area of the back surface of the wafer W.

Generally, wafers are often supplied to the equipment with minute dust particles on both sides of the wafer, and the wafer is held by suction with the dust sandwiched between the mounting surface (convex portion 10) of the wafer holder 71 and the back surface of the wafer. In this case, the flatness of the top surface of the wafer deteriorates locally in an area corresponding to the size of the dust. Also, depending on the hardness of the dust, in the worst case, the convex part 10 of the placement surface
This may also cause damage to the upper end surface of the .

Therefore, in order to suppress the deformation during suction of the wafer (substrate), the concentric type in which the pitch of the convex parts 10 is made small and arranged densely has a fatal drawback against the influence of dust. I can say it.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, an object of the present invention is to provide a suction device having a structure that reduces deformation (deflection, warpage, etc.) of a substrate during suction and at the same time keeps the contact ratio low.

[Means for solving problems]

In order to solve the above-mentioned problems, the present invention has a mounting surface of a holder with a closed shape (ring-shaped or isolated shape) surrounding a plurality of partial regions on the back surface of the substrate in order to depressurize the plurality of partial regions on the back surface of the substrate. ) forming a plurality of first convex portions, further comprising a pressure reducing means for reducing the pressure in an inner concave portion surrounded by the first convex portions to a first pressure lower than atmospheric pressure; A pressure regulating means is provided to set the recess to a pressure tFi between the first pressure and the atmospheric pressure, and at least when the substrate starts to be attracted, almost the entire back surface of the substrate is reduced in pressure by the pressure reducing means and the pressure regulating means. It was configured to do so.

[For production]

In the present invention, a plurality of localized locations on the back surface of a substrate such as a wafer are vacuum-adsorbed with a first pressure to ensure reliable fixation, and portions other than the plurality of localized regions to be vacuum-adsorbed are Since the pressure is adjusted to between the atmospheric pressure and the atmospheric pressure, the deformation (deflection) of the substrate can be reduced overall even if the area in which the pressure adjustment works is widened.

Here, the differences between what is shown in FIG. 3 and what is in accordance with the spirit of the present invention will be explained with reference to FIGS. 4, 5, and 6. Figure 6 shows the annular convex portion 10 with a partial pitch shown in Figure 3.
This is a schematic exaggerated illustration of the suction state in the holder 1 equipped with
By reducing the pressure, the wafer W (about 3.5.6.8 inches in diameter) is corrected to a nearly uniform plane, although there is locally an extremely small amount of deflection Δδ1.

However, in this case, since the contact ratio between the upper end surface (reference surface) of the convex portion 10 and the back surface of the wafer becomes considerably large, the probability that dust particles will be caught between the upper end surface (reference surface) of the convex portion 10 and the back surface of the wafer increases accordingly. If the size of the dust is about several micrometers, large irregularities may occur locally on the back surface of the wafer, and the depth of focus may exceed the depth of focus of the projection lens.

Therefore, as shown in the partial cross section of the holder 1 shown in Fig. 4,
The pitch of the annular convex portions is set at unequal intervals, and the concave portion 11a to be vacuum-adsorbed is surrounded by convex portions 10a, 10b with a narrow pitch, and convex portions 10c, 10d. (between) are kept at a relatively wide pitch.If the recesses 11a and 11b are depressurized (about -300 to -760+mHg) through the intake hole IC, the convex parts on both sides of the recess and the Since the back surface of the wafer is firmly pressed, the wafer W may warp upward by Δδ2 at the wide recess 11b. Furthermore, if there is a slight gap partially between the convex portion fob, 10c and the back surface of the wafer, the concave portion 11a may
Vacuum pressure will also leak to b. This leak progresses gradually over time, and eventually the pressure in the recess 11b is also reduced to a considerably low pressure. As a result, a portion of the wafer facing the recess 11b also bends down to a large extent. However, if the concave portion 11b is constructed to be open to atmospheric pressure, no pressure reduction will occur due to leakage from the concave portion 11b, and the concave portion 11b will be fixed in a warped state by Δδ8 as shown in FIG.

Therefore, in the present invention, when the wafer is sucked, the pressure in the recess 11b, which is brought into a nearly sealed state, is adjusted to a negative pressure slightly lower than atmospheric pressure, thereby reducing the wafer deflection shown in FIG. , the amount of deflection Δδ shown in FIG. 5 is reduced. The pressure is adjusted by the recess 11b.
This is carried out by means of positive or negative pressure regulation via holes le formed in the.

Here, in the amount of deflection shown in FIGS. 4, 5, and 6, by setting the reduced pressure in the recess 11b to an appropriate value, Δ
δ2>Δδ, ≧Δδ, and the amount of deflection Δδ can be kept sufficiently small for practical purposes with respect to various quantities such as the depth of focus determined by the projection optical system of the stepper.

Moreover, since the contact ratio of the convex portions 10a to 10d can be reduced, the probability of dust being caught becomes considerably small.

〔Example〕

1A and 1B show the structure of a suction device (wafer holder) according to a first embodiment of the present invention, and the same members as in FIG. 3 are given the same reference numerals. Here, FIG. 1(A) is a plan view of the holder 1, and FIG. 1(B) is the C of FIG. 1(A).
-1 arrow sectional view.

A rim-shaped annular convex portion 10a is formed on the outermost periphery of the mounting surface of the wafer holder 1, and a pitch of 12 (in practice, 2 to
An annular convex portion 10b is formed with 5 to 6 convex portions. Two annular protrusions 10e and 10f are also formed around the central opening 1a of the holder 1 at a pitch lt. moreover,
There is also a pitch of 2! approximately in the center between the annular convex portions 10b and 10e. Two annular protrusions 10c and 10d are formed concentrically.

In this case, the pitch between the annular protrusions 10b and 10c and the pitch between the annular protrusions 10d and 10e are approximately 2. are equal. In this embodiment, it is set to about I-z/l+≦1/3. Therefore, an annular concave portion 1]b sandwiched between annular convex portions Job and loc, an annular convex portion 10d and lO
The radial width (2
1) is quite wide. Therefore, in this embodiment, the two annular recesses 1.1b and lid having a width of 21 are replaced with recesses 1.1a and 11c that generate the original vacuum suction force.
, 11e so that the same adsorption force does not work, as shown in Figure 1 (B).
As shown, the sleeve-shaped hole 1d and the inside of the recess 11b, li
d through the hole 1e formed in the recess 11b, li.
The pressure inside d was set to be lower than the atmospheric pressure, but higher than the vacuum pressure drawn through the hole 1b. Therefore, in the case of this embodiment, the width l defined by the annular convex portions 10a and 10b! The width defined by the annular protrusions 10e and 10f on the inside of the annular suction surface! ! annular suction surface and annular convex portions 10c, 10d
Width defined by 1. Both the inner and outer sides of the annular suction surface are energized by weak negative pressure.

In this embodiment, three annular vacuum suction surfaces are provided, but any number may be provided. Also, l in the radial direction of the annular suction surface
It is not necessary that all three 1i (pitch I!, □) be equal. An annular recess 11b that further reduces the pressure to a weaker negative pressure;
The widths (pitch 2.) of the lids do not have to be equal to each other, and can be determined in an appropriate dimensional relationship.

By the way, in the holder 1 shown in FIG. 1, the outer circumference of the wafer W slightly overhangs the annular convex portion 10a, and due to the way the wafer is deformed as shown in FIG. 4, the outer circumference of the wafer warps upward. It is expected that. This amount of warpage is
It can be roughly determined from the material mechanics model 2, and in particular, it can be predominantly determined by the arrangement dimensions of the vacuum suction surface and the convex portion inside the mounting surface. Therefore, the annular convex portion 10 is
Arrangement of a, 10 b, 10 c, 10 d, etc., pitch 7! +, it, etc., and the annular convex portion 10
It is desirable that each upper end portion of a to 10f is a flat surface with a slight width.

Now, FIG. 7 is a block diagram showing an example of pressure regulating means used together with the wafer holder 1 shown in FIG. 1. The suction/exhaust source 30 can generate a vacuum pressure of about 300 mmHH as well as pressurized gas. Vacuum pressure or pressurized gas passes through the vibrator 31 and is connected to the hole 1b of the holder l. The pressure il1 moderator 32 is the intake and exhaust S
Input vacuum pressure from aO to hole 1 via vibrator 33.
d to a predetermined negative pressure (however, to the hole 1b via the vibrator 31,
(higher than the applied vacuum pressure and lower than atmospheric pressure). The magnitude of the negative pressure supplied to the vibrator 32 is adjusted in response to commands from the control system 34.

The control system 34 also controls the suction/exhaust source 30, switching between supplying vacuum pressure and pressurized gas, and controlling the timing of the supply. When the wafer W is taken out from the holder 1, the pressurized gas is supplied to each of the recesses 11a, llb,
This is to prevent the llc and lid from continuing to be held at negative pressure.

For this reason, the pressure regulator 32 also has a function of supplying pressurized gas to the vibrator 33 in response to commands from the control system 34. The method shown in FIG.
b, lid are forcibly maintained at -like negative pressure, and as shown in Fig. 1, the pitch is 11 and ! , is 1:
If the pressure is about 3, the recess 11b is used, and the lid pressure is reduced by the recess 11aS.
It has been confirmed through experiments that a sufficient effect can be obtained at about 1,720 to 1/30 of the reduced pressure of Ilc and lie.

Now, FIG. 8 shows another modification of the pressure regulating means, in which the vacuum pressure in the recesses 11a, llc, and lid (approximately -
(approximately 300 g) passes through a small partial gap between the back surface of the wafer W and the upper end surface of each annular convex portion and enters the concave portion 11b.
, lid leak phenomenon (actually undesirable phenomenon)
The purpose is to actively utilize this to naturally reduce the pressure in the recess 11b and lid. FIG. 8 shows a partial cross section of the holder 1, in which an orifice 40 is provided between the hole 1d and the atmosphere, the flow rate of which can be semi-fixedly adjusted.

The orifice 40 is rotatably provided with a cylindrical member 40a having a needle hole connecting the hole 1d and the atmosphere, and by appropriately rotating the cylindrical member 40a, the flow rate passing through the needle hole can be adjusted.

When the wafer W is placed and vacuum pressure is supplied to the recess 11a,
Leakage occurs between the upper end surface of the annular convex portion 10b (or 10c) and the back surface of the wafer, and the concave portions 11b (or 10d),
The spaces within the intake hole 1e and the hole 1d also begin to be depressurized. However, since the hole d slightly leaks to the atmospheric pressure through the needle hole of the orifice 40, the recess 11b (or 11d) does not reach the high vacuum pressure as the recess 11a, but stabilizes at a certain pressure state. Here orifice 4
If the flow rate of the needle hole 0 is restricted, the recess 11b (l
The pressure in id) will reach a considerably high vacuum pressure (a value close to that of the recess 11a), and unless the flow rate is throttled, it will reach only a pressure slightly lower than atmospheric pressure.

Therefore, in the case of the pressure 1j1 node means configured as shown in FIG. , the pressure shifts to higher than the pressure in the recess 11a and lower than atmospheric pressure.

Incidentally, the orifice 40 shown in FIG. 8 may be used by adjusting the flow rate manually or electrically as appropriate.

By the way, ideally, it is desirable that a certain predetermined difference is stably generated between the vacuum pressure in the recesses 11a, llc, and 11e and the pressure in the recesses 11b and 11d. Therefore, it is conceivable to slightly modify the pressure adjusting means shown in FIG. 7. For example, first, a pipe is pulled out from a part of the vibrator 31 or from one of the recesses 11a111c and lie of the holder 1, and a first pressure sensor is provided there to monitor the back pressure during vacuum suction. The pressure regulator 32 is composed of a variable flow rate needle valve or the like, and a pipe is drawn out from a part of the vibrator 33, and a second pressure sensor is provided there. It is also possible to calculate the difference between the pressures monitored by the first pressure sensor and the second pressure sensor, and adjust the flow rate using the needle valve so that the difference becomes a predetermined value. Alternatively, the differential pressure between the pipe 33 and the vibrator 31 is directly determined using a differential pressure sensor, and the needle valve is adjusted so that the differential pressure becomes a predetermined value (which may be different at the start of adsorption and after the completion of adsorption). May be controlled.

In addition, when using a pressure sensor, a differential pressure sensor, etc. in this way, or when using the configuration shown in FIG. It goes without saying that temporal changes can be adjusted as appropriate.

Next, an adsorption device according to a second embodiment of the present invention will be described with reference to FIGS. 2(A) and 2(B).

FIG. 2(A) is a plan view of the wafer holder 1, and FIG. 2(B) is a sectional view taken along the line C-2 in FIG. 2(A). As shown in FIG. 2(B), in this embodiment, the holder 1 is composed of a placing part IA having an i3!1 surface and a bottom part IB provided integrally with the placing part IA on the bottom side. An annular convex portion 10a is formed on the outer periphery of the placement surface of the placement portion IA, as in the case of FIG. Also,
An annular convex portion 10f is formed. A large annular concave portion 11 sandwiched between these two annular convex portions 10a and 10f
A plurality of minute circular convex portions 10g are arranged two-dimensionally at a substantially constant pitch within.

The diameter of these circular convex portions 10g is approximately 2 to several circles, and the respective upper end surfaces of the circular convex portions 10g and the respective upper end surfaces of the annular convex portions 10a and 10f are ground, lapped, etc. so as to form a reference plane. Further, the arrangement pitch of each circular convex portion Log is set to about several meters to about 100 meters. Furthermore, in this example, FIG. 2(A)
As shown, when looking at the three minute circular protrusions 10g adjacent to each other, they are arranged so that they are almost at the vertices of an equilateral triangle.

Now, a hole 1e is further provided in the large annular recess 11, and this hole 1e is an opening on the back surface (joint surface with the bottom part IB) of the placement part IA, as shown in FIG. 2(B). It is connected to an annular recess 1f formed near the periphery of the portion 1a.

Furthermore, on the back surface of the placement part IA, a large annular concave portion 1g is formed concentrically in the area where each minute circular convex portion Log exists, and this concave portion 1g is formed at the center of each minute circular convex portion Log. It is connected to the intake hole IC.

The annular recess 1g is not connected to the inner annular recess 1f.

On the other hand, the bottom part IB has a hole 1 connected to the annular recess 1g.
A hole 1d connected to the annular recess 1f is formed, and the bottom part IB and the mounting part IA are brought into close contact and integrated as shown in FIG. 2(B) to prevent leakage.

Further, the holes 1b and Id are connected to an intake/exhaust source 30 and a pressure regulator 32, respectively, as shown in FIG. 7, for example.

In such a structure, approximately -300 mmHg is applied to the hole 1b.
When the above vacuum pressure is supplied, the vacuum pressure is supplied to all the holes 1c through the recess 1g. Therefore, the back surface of the wafer W is suctioned and fixed to the upper end surfaces (actually, minute ring shapes) of all the minute circular convex portions Log. Then, when negative pressure is supplied to the recess 1f through the hole 1d, the entire recess 11 on the mounting surface is depressurized through the hole 1e.

According to this embodiment, since the diameter of the minute circular convex portion Log can be made small, it is possible to keep the contact rate with the back surface of the wafer considerably small. Also, in the holder 1 shown in FIGS. 2(A) and 2(B), the pressure control for vacuum suction and the pressure control for weakly reducing the pressure on surfaces other than the suction surface are explained in the first embodiment. It goes without saying that the various methods described above can be applied as they are.

Furthermore, in this embodiment, a so-called suction cup type, in which each minute circular convex part 10g independently generates a local adsorption force, was explained, but the shape of each suction cup may be square (2 to several squares) instead of circular. You can.

As described above, in each embodiment of the present invention, by taking advantage of the structural features,
Dust particles caught between the wafer back surface and the mounting surface can be removed statically (without mechanical movement). This will be briefly explained. In this case, it is assumed that the holes 1b and ld of the holder 1 are connected to a forced pressurizing means as shown in FIG.

First, the wafer W is held on the elevating mechanism 2 with the mounting surface of the wafer delivery elevating mechanism 2 protruding above the mounting surface of the holder 1 . Although not shown, a vacuum suction section is formed on the upper surface of the lifting mechanism 2. Then, while lowering the lifting mechanism 2, a high vacuum pressure (approximately -300
mat(g) is supplied, and a relatively high vacuum pressure is simultaneously supplied to the hole 1d. When the wafer W comes into contact with the mounting surface,
Hole 1b.

Or, since the back pressure 1d suddenly decreases, this change is detected and the suction of the lifting mechanism 2 is interrupted. Thereafter, the elevating mechanism 2 descends to the position shown in FIG. 1(B) or FIG. 2(B) and stops.

Now, in this state, the upper end surfaces of the annular convex portions 10a to 10f,
Or suppose that dust is caught between the upper end surface of the minute circular convex portion 10g and the back surface of the wafer.

Next, the vacuum pressure being supplied to either one of the holes 1b and 1d is switched to pressurization. A leak (airflow) then occurs around the trapped dust particles, and the dust particles receive a force toward the side where the vacuum pressure is applied, allowing them to move. Therefore, the dust particles are removed from the upper end surfaces of the convex portions 10a to 10f and 10g toward the concave portions. Therefore, by alternately repeating the vacuum pressure and pressurization applied to the holes 1b and 1d a plurality of times, the dust particles adhering to the back surface of the wafer can be removed to the recessed portion of the mounting surface. In this case, a part of the back surface of the wafer must be attracted.

After this operation, it is sufficient to maintain a predetermined pressure state as described above. Therefore, in consideration of carrying out such dust removal, it is desirable to make the width of the contact surface of each convex portion 10a to 1Of, Log as small as possible, but considering the ease of manufacturing the holder 1, it is preferable to Ass seems to be the limit.

Note that the height of the convex part (or concave part) on the mounting surface of holder 1 is 0.
．． 1 to 0.5 m (and its planar shape can be freely set. For example, in the holder shape shown in Fig. 1, the annular convex portion 10d formed in the center and the IOC are omitted, and The wide annular recess between the convex portions 10b and 108 has a structure in which minute dots (circular or rectangular shapes of about 0.5 to 2!ll11) are regularly arranged at a predetermined pitch, and the upper end surface of each of the minute dots is may be used as the reference plane. In this case as well, the same effect can be obtained by slightly reducing the pressure in the recess between the annular protrusions 10b and 10e.

Further, the substrate to be fixed by suction may be a rectangular glass plate, and in this case, the shape of the mounting surface of the holder may be made to match the outer shape of the glass plate.

〔Effect of the invention〕

As described above, according to the present invention, when a substrate is suctioned and fixed,
Since the contact ratio between the back surface of the substrate and the mounting surface can be significantly reduced, the influence of dust can be removed, and the lack of straightening force for the substrate due to the small contact ratio can be sufficiently compensated for, thereby reducing the amount of deflection of the substrate, especially the wafer. can be kept small enough for practical use.

[Brief explanation of the drawing]

1(A) and (B) are a plan view and a sectional view showing the structure of an adsorption device according to a first embodiment of the present invention, and FIG. 2(A) and (
B) is a plan view and a sectional view showing the structure of the adsorption device according to the second embodiment, and FIGS. 4, 5, and 6 are partial cross-sectional views schematically exaggerating the states of adsorption between the conventional holder and the holder according to the present invention, and FIG. 7 is a forced pressure adjustment means. FIG. 8 is a partial sectional view showing the structure of a holder incorporating pressure regulating means using an orifice. [Description of symbols of main parts] 1... Wafer holder, lb, ld... Sleeve-shaped hole, 1c... Intake hole, 1e... Hole, 10a, 10b, 10c, 10d, 10e. 10f...Annular convex portion, Log...Minute circular convex portion, 11, lla, llb, llc, lid, 11e...
Annular recess, 30... Suction/exhaust source, 32... Pressure adjustment means, 40... Orifice

Claims (2)

[Claims] (1) It has a mounting surface with a shape equal to or smaller than the outer shape of the thin substrate to be flattened, and the mounting surface has a plurality of convex portions that come into contact with a part of the back surface of the substrate. In the apparatus for adsorbing and fixing the back surface of the substrate in alignment with a reference plane defined by the tip surfaces of the plurality of convex portions by reducing pressure in a concave portion formed and surrounded by the convex portions, is formed with a plurality of first convex portions in a closed shape surrounding the partial regions in order to reduce the pressure in the plurality of partial regions on the back surface of the substrate; surrounded by the first convex portions. pressure reducing means for reducing the pressure in the inner recess to a first pressure lower than atmospheric pressure;
1. A substrate adsorption device characterized by comprising: a pressure adjusting means for adjusting the pressure to a pressure between the pressure of the above and the atmospheric pressure. (2) The total area defined by the recesses outside the first protrusion is set larger than the total area defined by the inner recess surrounded by the first protrusion. 2. A device according to claim 1, characterized in that: