WO2012025947A1

WO2012025947A1 - Device for generating plasma and for directing a flow of electrons towards a target

Info

Publication number: WO2012025947A1
Application number: PCT/IT2011/000301
Authority: WO
Inventors: Carlo Taliani; Libuse Nozarova
Original assignee: Organic Spintronics S.R.L.
Priority date: 2010-08-23
Filing date: 2011-08-23
Publication date: 2012-03-01
Also published as: IT1401417B1; ITBO20100525A1

Abstract

Device for generating plasma and for directing a flow of electrons towards a given target (3); the device (1) comprises a hollow cathode (5); an activation electrode (7); a substantially dielectric tubular element (21), which extends through a wall of a cathode (5); an annular anode (25) located around the tubular element (21); and an additional electrode (28), which is located in the area of an external end (21b) of the tubular element (21) and is designed to work initially as a an additional anode so as to accelerate the electrons coming from the cathode (5) and then, once this additional electrode (28) reaches the potential of the cathode (5), as a cathode so as to direct the electrons towards the target (3 ).

Description

"DEVICE FOR GENERATING PLASMA AND FOR DIRECTING A FLOW OF ELECTRONS TOWARDS A TARGET"

TECHNICAL FIELD

The present invention relates to a device for generating plasma, an apparatus that comprises this device and a method for applying a layer of a material on a support . BACKGROUND OF THE INVENTION

Pulsed electron flows are nowadays used for deposition of thin layers of specific materials on substrates. This type of technique is finding a particularly interesting application in the field of electronics for the production of microchips and photovoltaic cells.

Different experimental systems for generating pulsed electron flows for the production of thin layers are known. Nevertheless, according to our knowledge only two systems have been successful in finding an industrial application. These systems are based on a process named Channel Spark Ablation. In these systems the generation of the flows occurs through electron extraction from plasma generated in rarefied gas by applying potential difference.

Examples of known devices that use the Channel Spark Ablation process are described in patent application having the publication number WO2006/105955A2 and comprise a hollow cathode; an auxiliary cathode; a capillary of a substantially dielectric material that extends from the cathode; and an anode around the capillary. Usually, for the production of thin layers the electrons extracted from the plasma are accelerated from the cathode towards the anode against a target. Through the impact of the accelerated electron pack (impulse) on the target surface the energy of the pack (impulse) is transferred into the material of the target and causes its ablation, that is, an explosion of the surface in the form of plasma of the target material, defined also as "plume", that spreads in the direction of a substrate, where it is deposited.

The known devices suffer from function instability caused by modifications of the chemical-physical properties of the target during operation.

In this regard, it is important to note that the target constitutes the true anode for the state of the art devices. Its electric conductivity properties, the evaporation enthalpy etc. influence the behavior of the devices (interval of the working gas pressings, interval of the acceleration voltages, even the impediments of the discharge process) .

The functioning of the devices is influenced by the target especially through the limitation of the power of the discharge that takes place in the capillary. The high resistance of the devices or the difficulty of the ablation lower the discharge power significantly, therefore, the temperature and density of the plasma in the capillary. The diluted plasma cannot release the electron packs of intensity sufficient for the ablation and the process "dies" gradually.

All of this influences negatively also the velocity of deposition of the material on the substrate. Furthermore, it has been also observed that the capillaries get very easily contaminated by the target material. It should be noted, for example, that using a target from Cadmium Telluride (CdTe) it is necessary to substitute the capillary after ca. every 20 thousand discharges; using a Zinc Oxide (ZnO) target it is necessary to change the capillary after ca. every 60-100 thousand discharges .

Consequently, frequent maintenance interventions are necessary to change the entire capillary. This causes an increase of maintenance costs and a decrease of operating times of the device with resulting further reduction of total productivity.

The known devices also need a relatively high energy expenses, because it is necessary to enforce relatively high potential differences between the anode and the cathode .

The aim of the present invention is to provide a plasma generation device, an system and a method of application of a layer of a material on a surface that would allow to overcome, at least partially, the disadvantages of the know state of the art and at the same time would be easily and economically produced.

SUMMARY

According to the present invention a device for generating plasma, an apparatus and a method for applying a layer of a material on a support are provided as recited in the independent claims that follow, and, preferably, in any of the claims directly or indirectly dependent on the independent claims .

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described referring to the attached drawings that illustrate some non-limiting embodiments , where :

Figure 1 shows schematically and system and a device produced according to the present invention

Figures 2 and 3 show schematically and in section two different embodiments of the system in figure 1 Figure 4 is a schematic section with parts removed for clarity of a detail of the device of figures 1 to 3

Figures 5 and 6 are schematic sections with parts removed for clarity of different of the detail of figure 4. EMBODIMENTS OF THE INVENTION

In figure 1, with 1 is indicated as a whole an apparatus for the deposition of a given material. The apparatus 1 comprises a device 2 for the plasma generation (that is an at least partial ionization of a rarefied gas) and for directing the electron flow towards a target 3 which features (in particular is formed by) the determined material, so that at least a part of the determined material separates from the target 3 and deposits on support 4.

According to alternative embodiments the determined material can be formed by one sole homogeneous material or by two or more different materials.

Advantageously, the target 3 is connected to earth. In this way, the target 3 does not repel (and, on the contrary, attracts) the electron flow even when the electrons have already hit the target 3 itself. The device 2 comprises a hollow element 5 which is able to act from the cathode and features (delimits from the outside) an internal cavity 6; and an activation electrode 7, which contains (particularly, is made of) an electrically conductive material (particularly, metallic) . The activation electrode is placed inside the cavity 6 (delimited from the outside by the hollow element 5) . Particularly, the hollow element 5 contains (more specifically, is constituted by) an electrically conductive material (more specifically, a metallic material) .

In particular, by electrically conductive material is meant a material that has an electric resistivity (measured at 20 °C) lower than 10^"1 Q.m. Advantageously, the electrically conductive material has an electric resistivity (measured at 20 °C) lower than 10^"3 Q.m

According to some embodiments, the hollow element 5 contains (specifically, is made of) a material chosen from the group consisting of: stainless steel, Tungsten, Molybdenum, Chrome, Iron, Titanium. According to some embodiments, the activation electrode 7 contains (specifically, consist of) a material chosen from the group consisting of: stainless steel, Tungsten, Molybdenum, Chrome, Iron, Titanium.

According to the embodiment illustrated in figure 1, the activation electrode 7 extends through a wall 8 of the hollow element 5. Between the electrode 7 and the wall 8 there is an interposed ring 9 of a substantially electrically isolating material (specifically, ceramics) . Specifically, under substantially electrically isolating material we understand a material that has an electric resistivity (measured at 20 °C) higher than 10³ Q.m. Advantageously, the electrically non-conductive material has an electric resistivity (measured at 20 °C) higher than 10⁷ Q.m (more specifically, higher than 10⁹ Q.m. According to^' some embodiments, the substantially electrically isolating material is a dielectric material.

The device 2 comprises also a resistor 10, which connects the activation electrode 7 to earth and has a resistance of at least 100 Ohm, advantageously of at least 1 kOhm. In particular, the resistor has a resistance of ca. 20 kOhm.

According to further embodiments, in place of the resistor 10 there is a different electronic device which has equivalent function.

In the interior of the cavity 6 there is a rarefied gas. According to some embodiments, the cavity contains the rarefied gas under pressure inferior or equal to 10^"2 mbar (in particular, lower than 10^"3 mbar) . Advantageously, the rarefied gas contained inside the cavity 6 has a pressure lower than or equal to 10^"4 mbar. In particular, the rarefied gas contained inside the cavity 6 has a pressure higher than or equal to 10^"6 mbar (more particularly, higher than or equal to 10^"5 mbar) .

In this regard, it should be noted that the device 2 contains a gas supply device (known per se and not depicted) for the supply of anhydrous gas (non-limiting examples - oxygen, azote, argon, helium, xenon etc.) into the cavity 6 and a suction unit (known per se and not depicted) comprising a pump to rarefy the gas in the cavity 6 (in other words, to reduce the gas pressure inside the cavity 6) . The supply and suction units are connected to the hollow element 5 through a shaft 23.

The hollow element 5 is electrically connected to an activation assembly 11, which is able to diminish the electric potential of the hollow element 5 by at least 4 kV (in particular, starting from an electric potential substantially equal to zero) in less than 20 ns directing an impulse of electric charge of at least 0,16 mC towards the hollow element 5 itself. According to some embodiment, the electric impulse mentioned is smaller than or equal to 0,5 mC. Advantageously, the activation assembly 11 is able to diminish the electric potential of the hollow element by at least 5 kV (specifically, at least 6 kV) in less than 20 ns .

Advantageously, the activation assembly 11 is able to impose on the hollow element 5 a decrease in the electric potential from 8 kV to 25 kV in less than 15 ns, specifically in circa 10 ns .

Therefore, in use, the activation assembly imposes 11 a potential difference between the hollow element 5 and the activation electrode 7 according to the properties described above. As a consequence, the plasma (i.e. an at least partial ionization of the rarefied gas) is generated inside of the cavity 6.

Making a particular reference to the design shown in figure 1, the hollow element 5 is connected to earth. Thus, when no electron flow emission is performed, the hollow element is maintained at a substantially null potential and the risk of spontaneous discharges between the hollow element 5 and the activation electrode 7 is substantially erased.

In particular, a resistor 12 is connected between the hollow element 5 and earth. According to some embodiments, the resistor 12 has a resistance of at least 50 kOhm. Advantageously, the resistor 12 has a resistance of at least 100 kOhm, in particular of ca. 0,5 MOhm. According to some embodiments, the resistance is lower than 1 MOhm. According to further embodiments, instead of the resistor 12 another electronic device with equivalent function is used.

According to the embodiment illustrated in figure 1, the activation assembly comprises a thyratron 13 ; a capacitor 14, which has an armature connected to the anode 15 of the thyratron 13 and another armature connected to the hollow element 5; and a power supply 16 which has a positive electrode 17 connected electrically to the anode 15 and a negative electrode 18 connected to earth .

The thyratron 13 has also a cathode 19 which is connected to earth.

It should be noted that the capacitor 14 is electrically connected to earth (specifically, through the resistor 12) .

The activation assembly 11 comprises also a control unit 20 of the thyratron 13 - the control unit 20 is able to operate the thyratron 13 and is connected to earth. According to not shown embodiments, the activation assembly 11 contains a magnetic compressor of the electric pulse or a generator of electric impulses of high potential of the Blumlein type. Advantageously, the magnetic compressor (or the impulse generator) substitutes the thyratron 13 and its relative operating unit 20.

The device 2 also comprises an operator interface assembly (known per se and not depicted in the drawings) which allows the operator to regulate the functioning (e.g. the operation and/or modification of operating parametres) of the device 2. In particular, the operator interface assembly comprises a personal computer, a display, a keyboard and/or a tracking device (for example a mouse) . The operator interface unit is connected to the control unit 20.

According to other embodiments, instead of the capacitor 14 another electronic device of equivalent function is used.

The device 2 comprises also a tubular element 21 which features (in particular, is made of) a substantially electrically isolating material (in particular, glass) and is connected to the hollow element 5. The tubular element 21 has two open ends 21a and 21b and (making a particular reference to figures 4-7) an internal lumen 21c which puts into fluidic communication the cavity 6 with the outside. In particular, the tubular element 21 shows an opening 2 Id placed in correspondence with the open end 21b.

The tubular element extends at least partially inside of an outside chamber 24 (figure 1) , in which the target 3 and the surface element 4 are placed. The lumen 21c puts in fluidic communication the cavity 6 and the external chamber 24. The tubular element 21 and the related internal lumen 21c have respective substantially circular cross sections.

Inside the external chamber 24 there is a rarefied gas (according to some embodiments, anhydrous) . According to some embodiments, the cavity contains the rarefied gas at a pressure lower than or equal to 10^"2 mbar (in particular, lower than 10^"3 mbar) . Advantageously, the rarefied gas contained inside the external chamber 24 has a pressure lower than or equal to 10^"4 mbar. In particular, the rarefied gas contained inside the external chamber 24 has a pressure higher than or equal to 10^"6 mbar (more in particular, higher than or equal to 10^"5 mbar) . The gas is selected in a group consisting of: oxygen, azote, argon, helium, xenon and a combination thereof.

When the cavity 6 and the external chamber 24 are in the fluidic communication, the gas composition and the pressure inside of the cavity 6 and in the external chamber 24 are substantially identical.

According to the embodiment illustrated in figure 1, the tubular element 21 extends through the wall 22 (opposite to the wall 8) of the hollow element 5, partially inside of the cavity 6 and partially on the outside (specifically, inside the external chamber 24) .

According to specific embodiments, the tubular element 21 shows a length from 90 mm to 220 mm. the tubular element 21 has a diameter of ca. 4 mm to ca. 10 mm. The lumen 21c has a diameter of ca. 1,5 mm up to 8 mm. The external chamber 24 is built so as to be fluid- tight in respect to the external environment.

The device 2 also comprises an external element 25 which is placed externally to the hollow element 5 (specifically, in the external chamber 24) along the tubular element 21 (i.e. not in the area of an end of the tubular element 21) and serves as an anode. In particular, the external element 25 is placed in contact with the external surface of the tubular element 21.

In use, when the electrons formed inside of the cavity 6 enter the tubular element 21, the potential difference that was established with the external element 25 allows the electrons to be accelerated along the tubular element 21 towards the target 3. These electrons, while moving, hit other gas molecules and cause, therefore, the emission of secondary electrons that also are accelerated towards the target. The device 2 comprises also a potential maintenance unit 26, which is connected electrically to the external element 26 for the maintaining the electric potential of the external element 25 substantially equal to or higher than zero. In particular, the potential maintenance unit

26 maintains the electric potential of the external element 25 substantially grounded.

The external element 25 is shaped so as to be placed around the tubular element 21. In particular, the external element 25 has a hole through which the tubular element 21 extends. According to specific embodiments, the external element 25 has a ring-like form.

According to some embodiments, the hollow element 5 shows a substantially cylindrical form and a substantially circular cross section and, advantageously, is obtained through mounting of two drilled plates on a ring-like element, the plates then create the walls 8 and 22 respectively.

The activation electrode 7 has a mesh end (not shown) from metallic material and connected to earth through an HV electrical loop.

The device 2 comprises also a stabilization assembly

27 which comprises an external element 28 which is placed externally to the hollow element 5.

According to some embodiments (figures 4 and 5) , the external element 28 is placed at least partially in correspondence with the end 21b.

According to some embodiment (figures 4 and 6) , the external element 28 is placed at least partially beyond the end 21b in respect to the hollow element 5. In other words, the end 21b is placed between the external element

28 and the hollow element 5. In this way the efficiency of the device 1 is improved. This is probably due to the fact that the plasma and the electrons approach the external element 28 more easily.

Advantageously, the external element 28 is substantially symmetrical in respect to the direction of the electrons exiting from the tubular element 21. In particular, the external element 28 is substantially symmetrical in respect to a longitudinal axis of the tubular element 21. In this way, the efficiency of the device 1 is improved. This is probably due to the fact that there are only few disturbances that that influence negatively the directing of the plasma and the electrons towards the target 3.

The external element 28 comprises (in particular, is made of) a metallic material (electrically conductive) . Advantageously, this material has a melting point higher than 1300°C.

According to some embodiments, this material is chosen from a group consisting of: steel (particularly stainless) , Tungsten, Molybdenum, Chrome and a combination therreof .

According to some embodiments (figures 4 and 5) , the external element 28 is placed at least partially around the tubular element 21. In particular, in the embodiment of figure 5 the external element 28 is placed around the tubular element 21.

According to some embodiments (figure 4) , the external element 28 is placed at least partially around the opening 2Id.

According to the embodiment shown in figure 4, the external element is placed at least partially at the end of the tubular element 21 (in particular, between the end 21b and the target 3) . Advantageously, the external element 28 is in contact with the tubular element 21 (in particular, with the end 21b) . More precisely, the external element 28 is in fluid-tight contact with the tubular element 21 (in particular, with the end 21b) . When the external element 28 is in contact with the tubular element 21 the efficiency of the device 1 is improved. This is probably due to the fact that the electrons are not dispelled passing through the space between the external element 28 and the tubular element 21.

Advantageously, the external element 28 has a hole

29 passing through. The hole 29 is placed in correspondence with the opening 21d. An particular, the hole 29 is co-axial to the opening 2 Id.

In particular, the external element 28 (and more particularly, the hole 29) is formed in such a way that the entire opening 21d is exposed towards the outside (in particular, towards the external chamber 24) . In other words, the external element 28 (and more specifically, the hole 29) is formed in such a way so that it does not even partially block the opening 21d.

The external element 28 has a part 30 which is placed at the end of the tubular element 21, and a part 31, which extends from the first part towards the hollow element around the tubular element. Advantageously, the part 31 has a tubular shape. The part 31 has an internal lumen with a diameter of 0,1-0,2 mm, larger than the external diameter of the tubular element 21. This allows an easy positioning of the external element 28 at the end 21b. The parts 30 and 31 have respective thicknesses of T and T' from 0,1 up to 5 mm (specifically, from 0.5 up to 1 mm) .

The embodiment shown in figure 5 differs from the embodiment shown in figure 4 mainly because the external element 28 is placed in the area of the end 21b but not in contact with the end 21b itself. The external element 28 is, however¹, in contact with the tubular element 21.

The external element 28 of the embodiment shown in figure 6 is placed between the end 21b and the target 3. In this case, the external element 28 is distanced from the end 21b.

With particular reference to figure 1, the stabilization assembly 27 comprises also a capacitor 32 electrically connected to the external element 28 and to earth.

Advantageously, the capacitor 32 has a capacity lower than the capacity of the capacitor 14. In particular, the capacitor 14 has a at least twice as big a capacity with respect to capacity of the capacitor 32.

More specifically, the capacitor 32 has a capacity from 0,5 nF to 10 nF, advantageously form 1 nF to 5 nF.

According to specific embodiments, the capacitor 32 has a capacity of ca. 3 nF.

According to some embodiments, instead of the capacitor 32 other electronic device of the same function and possibly equal capacity is used.

The stabilization assembly comprises also a resistor

33, which connects the external element 28 to earth. In particular, the resistor 33 is placed in parallel to the capacitor 14.

According to some embodiments, the resistor 33 has a resistance of at least 100 KOhm and, advantageously, up to 100 MOhm. In particular, the resistor 33 has a resistance of ca. 3 up to ca.5 MOhm.

According to other embodiments, instead of the resistor 33 another electronic device with equivalent function and possibly same resistance is used. In use, when the activation assembly induces an electric impulse into the hollow element 5, the electric potential of the external element 25 rises to the earth potential in ca. 10-20 ns . The hollow element 5 is, for such short intervals in the "floating" condition. This brings the electric potential of the hollow element 5 to a very rapid decrease.

As a consequence, and arch ignites between an internal surface of the cavity 6 and the main electrode 7. The plasma of the arch expand on the inside of the tubular element 21 and ignites the discharge of the channeled spark which produces a flow of electrons of high energy.

The apparatus 1 end the device 2 (except for what concerns the stabilization assembly 27) present a structure and a functioning analogous to the system, and respectively, to the device described in the patent application PCT82010000644 by the same applicant.

However, differently from what occurs in the device in the patent application PCT82010000644 , it was experimentally observed that the external element 28 of the device 2 acts, at the beginning of the discharge, as a stabile anode and maintains surprisingly substantially unchanged its chemical-physical characteristics for relatively long times. In other words, the potential of the discharge is located between the hollow element 5 and the external element 28.

When the discharge occurs, electrons and plasma are produced, which spread through the tubular element 21 from the hollow element 5 to the external element 28. When the plasma and the electrons reach the external element 28, the capacitor 32 charges, surprisingly maintaining the high electric power of the plasma and, as a consequence, the elevated density and temperature of the plasma. When the capacitor 32 is completely charged and the external element 28 shows the same potential as the hollow element 5, the external element 28 acts as a cathode (with the target 3 that works as an anode) and being covered with dense plasma it creates easily packs of dense electrons and directs these electrons towards the target 3. The capacitor 32 releases its charge (electrons) through the current that passes from the external element 28 to the target 3, without dispersing energy to earth but returning it in order to create ablation.

Therefore, it was experimentally possible to observe that the efficiency of the device 2 and of the apparatus 1 of the present invention is surprisingly much higher than the efficiency of the known devices. In particular, with the apparatus 1 it is possible to obtain a deposition speed up to fifty times higher than the speed of attainable with the apparatus disclosed by patent application PCT8201000064 . Moreover, it is possible to work in conditions that allow saving of energy.

It should be noted that also the presence of the stabilization assembly 27 allows avoiding the contamination of the tubular element 21. In fact, it has been experimentally observed that using the device 2, particles of the target 3 do not deposit on the tubular element 21 but on the external element 28. The external element 28 is much easier and much cheaper to clean and/or change compared to the tubular element 21.

It is important to stress that to place the external element 28 in the area of the end 21b (as illustrated for example in figures 4 and 6) is particularly useful. In this way, it has been experimentally observed that it is possible to reduce the dispersion of the plasma between the opening 2 Id and the external element 28 thus increasing the complex efficiency of the device 2. This dispersion is further reduced when the external element is in contact with the end 21b (as illustrated for example in figure 4) .

Improvements, from this point of view, can be further obtained if the perimeter of the hole 29 and the perimeter of the opening 21d lie substantially on the same level (as illustrated for example in figure 4) .

The device 1 has proven itself particularly efficient also when the opening 21d exposed entirely to the outside (as illustrated for example in figures 4 and 6) .

Figures 2 and 3 illustrate alternative embodiments of the device 2 and apparatus 1. In figures 2 and 3 , the elements analogous to those represented in figure 1 have been indicated with the same reference numbers.

The devices 2 from figures 2 and 3 have an ampoule 32 (generally of glass), on the inside (figure 2) and on the outside (figure 3) of which there is the activation electrode 11 and the stabilization assembly 27 as described above. The capacity of the capacitors 32 and 14 are also in this case same as indicated with respect to the embodiment in figure 1.

In actual use, the hollow element 5 is maintained at a relatively high negative electric potential (that is, with negative charge) ; when an electric impulse is produced on the activation electrode 11 (for example by connecting this electrode to earth) a glow discharge is created, which generates a positive electric charge inside of the hollow element 5. The electric charge is then compensated by emission of electrons, which in turn are accelerated towards the first external element 25 inside of the tubular element 21. The electrons, while moving towards the outside, ionize further molecules, producing further electrons (called secondary electrons) .

The apparatuses 1 and devices 2 (with the exception of what concerns the stabilization assembly 27) from figures 2 and 3 have a structure and a function analogous to the systems described in the patent application PCTEP2006003107 by the same applicant and in the patent application with publication number US2005/0012441.

The stabilization assembly 27 of the embodiments of figures 2 and 3 has the structure, the function and advantages (in this case compared to the systems and devices described in the patent application PCTEP2006003107) analogous to what was described in reference to the embodiment from figure 1.

According to a further aspect of the present invention, an application method of a given material onto a support 4 is provided, the method comprises an emission phase, during which the device 2 as described above directs a flow of electrons towards the target 3 having the given material so as to remove at least a part of the given material from the target 3 and direct it towards the support 4. In particular, the method requires usage of the apparatus 1 as defined above.

Unless explicitly indicated otherwise, the contents of the references (articles, books, patent applications etc.) cited in this test herein entirely recalled, in particular, the mentioned references are herein incorporated by reference.

Claims

1. - Device for generating plasma and for directing a flow of electrons towards a target (3); the device comprising a hollow element (5), which has a cavity (6) and is designed to act as a cathode; an activation electrode (7); a substantially electrical insulating tubular element (21) , which is connected to the hollow element (5); a first external element (28), which is externally placed with regard to the hollow element (5) and externally placed with regard to the tubular element (21) ; and an activation assembly (11) , which is designed to impose a difference of potential between the hollow element (5) and the first external element (28) and between the hollow element (5) and the activation electrode (7) ;

the tubular element (21) having a first and a second end (21a, 21b) , which are opposite to each other, and an internal lumen (21c) , which provides fluidic communication between the cavity (6) and the outside;

the first end (21a) being placed in the area of the hollow element (5) ; the second end (21b) facing the outside;

the device (2) being characterised in that it comprises a stabilization assembly (27) , which comprises the first external element (28) ; and first capacitive means (32), which are electrically connected to the first external element (28) ; the first external element (28) being at least partially placed in correspondence to the second end (21b) of the tubular element (21) and/or beyond the second end (21b) with regard to the hollow element (5) .

2. - Device according to claim 1, wherein the first external element (28) is at least partially placed beyond the second end (21b) with regard to the hollow element (5); the first capacitive means (32) being electrically earthed.

3. - Device according to one of the preceding claims, and comprising second capacitive means (14) , which are electrically connected to the hollow element (5) and have a higher capacitance than the capacitance of the first capacitive means (32) .

4. - Device according to one of the preceding claims, wherein the first capacitive means (32) have a capacitance ranging from approximately 0.5 nF to approximately 10 nF; the second capacitive means (14) having a capacitance which is at least twice the capacitance of the first capacitive means (32) .

5.- Device according to one of the preceding claims, wherein the stabilization assembly (27) comprises resistive means (33) electrically earthing the first external element (28) ; in particular, the resistive means (33) are placed in parallel to the capacitive means (32) .

6.- Device according to claim 5, wherein the resistive means (33) have an electrical resistance at least equal to 10 KOhm.

7. - Device according to one of the preceding claims, wherein the first external element (28) comprises an electrical conductive material having a melting point of at least 1300°C.

8. - Device according to one of the preceding claims, wherein the first external element (28) is placed in contact with the second end (21b) .

9.- Device according to one of the preceding claims, wherein the tubular element (21) has an opening (2 Id) , which is placed in the area of the second end (21b) ; the first external element (28) has a through hole (29) placed at the opening (21d) .

10. - Device according to one of the preceding claims, wherein the tubular element (21) has an opening (2Id) ; the first external element (28) is shaped and placed so that the whole said opening (2Id) is exposed towards the outside.

11. - Device according to one of the preceding claims, wherein the first external element (28) has a first portion (30) , which is placed at the end of the tubular element (21) ; and a second portion (31) , which has a tubular shape and extends from the first portion (30) towards the hollow element (5) around the tubular element (21) .

12.- Device according to one of the preceding claims, and comprising a second external element (25), which is placed in a area between the first and the second end (21a, 21b) ; the activation assembly (11) is designed to impose a difference of potential between the hollow element (5) and the second external element (25) .

13. - Device according to one of the preceding claims, wherein the activation assembly (11) is electrically connected to the hollow element (5) and is designed to reduce the electric potential of the hollow element (5) itself by at least 4 kV in less than 20 ns; the activation electrode (7) is at least partially placed inside the cavity (6) of the hollow element (5); the tubular element (21) extends through a wall (22) of the hollow element (5) .

14.- Apparatus for applying a specific material on a support (4), the apparatus (1) comprising an external chamber (24), a target (3) having the specific material and placed in the external chamber (24) , and the support (4) placed in the external chamber (24) ;

the apparatus (1) being characterised in that it comprises a device (2) of the type claimed in one of the preceding claims; the device (2) being designed to direct the flow of electrons against the target (3) so that at least part of the specific material is removed from the target (3) and settles on the support (4); the second end (21b) of the tubular element (21) and the first external element (28) being placed inside the external chamber (24) ; the external chamber (24) being in fluidic communication with said cavity (6) through the tubular element (21) ; the first external element (28) being at least partially placed in the area of the second end (21b) of the tubular element (21) and/or between the second end (21b) and the target (3) .

15. - Apparatus according to claim 14, wherein the external chamber (24) and the cavity (6) contain a gas at a pressure lower than 10^"3 mbar, in particular lower than ICT⁴ mbar.

16. - Method for applying a specific material on a support (4) , the method comprising an emission step, during which a device (2) according to one of the claims from 1 to 13 directs a flow of electrons against a target (3) having the specific material, in order to remove at least part of the specific material from the target (3) and to direct it towards the support (4) .