WO2010058281A1

WO2010058281A1 - Method and apparatus for producing thin films on a substrate via a pulsed- electron deposition process

Info

Publication number: WO2010058281A1
Application number: PCT/IB2009/007528
Authority: WO
Inventors: Stefano Rampino; Edmondo Gilioli; Francesco Bissoli; Francesco Pattini; Massimo Mazzer; Claudio Ferrari
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 2008-11-21
Filing date: 2009-11-20
Publication date: 2010-05-27
Also published as: IT1391801B1; ITMI20082090A1

Abstract

A method for producing thin films on a substrate via a pulsed- electron deposition process comprises sending a pulsed- electron beam (3) onto a target material (5), set in the proximity of a substrate (7), in such a way as to cause ablation of the target material and emission from the target material of vapour phase particles that deposit on the substrate to form a layer (8) or film; the discharge current of the beam (3) is measured for controlling the parameters of the deposition process and optimizing, in particular, the rate of growth of the layer (8). The measurement of the discharge current is based upon the use of an induction coil (27), which does not affect the primary circuit (15) for beam discharge; in particular, a coil (27) is used wound according to a particular geometry, referred to as "Rogowski coil".

Description

METHOD AND APPARATUS FOR PRODUCING THIN FILMS ON A SUBSTRATE VIA A PULSED-ELECTRON DEPOSITION PROCESS

TECHNICAL FIELD The present invention relates to a method and an apparatus for producing thin films on a substrate via a pulsed-electron deposition process, thanks to which it is possible to optimize the parameters of the deposition process and, in particular, the rate of growth of the layers .

BACKGROUND ART

As is known, the pulsed-electron deposition (PED) technique is a technique of a physical type for producing thin layers (with a thickness of between a few tenths and a few tens of microns) of conductive and dielectric materials, used in particular for producing functional devices in the sector of electronics, magnetism, sensors, generation and transport of energy and in general in nanotechnology.

Said technique is based upon the generation of a pulsed beam of high-energy electrons (indicatively, an energy of 1 to 25 keV) , and the subsequent collimation of the latter towards a multi-elemental target material having a given stoichiometry.

The generation of the beam occurs by extraction of electrons from a plasma created on the walls of a hollow metal cathode. The negative charges of the plasma are accelerated by applying a potential difference (typically 1 to 25 kV) between the hollow cathode and an anode . The flow of electrons in acceleration towards the anode is channelled by means of a particular dielectric tube (made, for example, of glass, quartz, alumina, etc.), which is concentric with the anode itself. Applied at the tip of the dielectric tube is the target material, appropriately supported, which is to be deposited in the form of thin layer. The electrons strike the target material and interact wich the atoms present, causing rapid evaporation of the elements from the surface of the target material. The vapours move away in a direction basically perpendicular to the surface of the target material to form a flow of evaporation particles (referred to also as "plume") the dimensions, rate, and density of which depend upon the parameters of acceleration and collimation of the electron beam, as well as upon the nature of the target material. The interposition of a material functioning as substrate on the path of the vapour plume causes condensation of the vapours on the surface of the substrate and consequent formation of the thin layer of target material .

The electron discharge along the dielectric tube occurs in a pulsed way (with a pulse duration of the order of 100 ns) , provided by a trigger circuit with variable frequency (from 1 to 10 Hz) . Given the pulsed nature of the electron-beam discharge process, the interaction between the electrons and the target material occurs exclusively within a small surface layer of the target material, generating a localized and violent heating of the target material . Since the immediate evaporation of the target material as a result said heating occurs far from thermodynamic equilibrium, the transfer of the elements into the vapour phase occurs with complete preservation of the stoichiometry of the target material. This peculiar phenomenon of evaporation out of thermodynamic equilibrium is useful for preserving the stoichiometry of the target materials in which low-concentration dopants are present, or in ternary or quaternary systems the transition of state of which at thermodynamic equilibrium gives rise to incongruent melting. Examples of materials the transition of state of which gives rise to incongruent melting are superconductors of the second type, such as REBa₂CUsO₇ (RE = element of the rare earths) , and ternary and quaternary semiconductors of the Cu (In, Ga) (Se₇S)₂ type.

For the energy exchange between the electron beam and the target material to give rise to the phenomenon of evaporation out of thermodynamic equilibrium (or "ablation"), to obtain the best stoichiometric transfer possible it is necessary for the heating transient of the surface of the target material to be as high as possible. If the heating of the target material- does not occur in a violent and immediate way, it is possible to encounter phenomena of transitions of state at thermodynamic equilibrium, which, as has already been said, can lead to the decomposition of particular target materials and to an erroneous stoichiometric transfer in vapour phase and consequently on the layer being deposited on the substrate.

Even though the general principles of the pulsed-electron deposition (PED) technique are currently well known, the methods and equipment currently available for its technological and industrial applications still present margins for improvement, above all in terms of control of the parameters of growth of the deposited layers and of efficiency and rate of deposition.

DISCLOSURE OF INVENTION

An aim of the present invention is to provide a method and an apparatus for producing thin films on a substrate via a pulsed-electron deposition process that, in a relatively simple, inexpensive, and reliable way, enables a high level of efficiency and an optimal control of the parameters of growth of the layers and, in particular, of the rate of growth of the layers .

The present invention hence regards a method and an apparatus for producing thin films on a substrate via a pulsed-electron deposition process as defined in essential terms in the annexed Claim 1 and Claim 9, respectively, and, as regards their auxiliary features, in the dependent claims. Basically, in accordance with the invention, the deposition process is controlled on the basis of measurements of the discharge current of the pulsed-electron beam. In particular, the measurement of the discharge current is based upon the use of an induction coil, which does not attect the primary circuit for electron-beam discharge. In particular, a coil is used wound according to a particular geometry, referred to as "Rogσwski coil".

The method and apparatus of the invention enable a complete and accurate control of the deposition process, enabling in particular setting of the parameters for optimizing and maximizing the rate of growth of the layers, at the same time preventing a heating with decomposition of the target material and consequent change of the stoichiometry of the deposited layer with respect to the target material.

In fact, in order for the ablation process (practically instantaneous evaporation out of thermodynamic equilibrium) of a target material to be optimized, it is necessary to maximize the variation in time of the temperature of the surface of the target material struck by the pulsed-electron beam.

This variation is given by the following expression: d^T Q - ^IV/S - ^{IV (Eq- 1)} dt ^~ [Cp(D+D_T)]^~ [Cp(D+D_T)]^~ [SCp(D+D₇)]

where :

- Q is the minimum density of power absorbed by the target material for generating the ablation process, and is equal to the product of the (intensity of) discharge current I of the electron beam and the acceleration voltage V (potential difference applied between the cathode and the anode for accelerating the beam) divided by the cross section of the beam S;

- C is the heat capacity of the target material; - p is the density of the target material;

- {D + D_τ) is the thickness of the area of the target material where the electron-matter interaction occurs; in particular, D is the electron-absorption length of the target material (also referred to as "electron range"), whilst D_τ i»s the thermal- diffusion length.

D varies with the acceleration voltage according to the law D-V² for voltages of between 10 and 100 kv, whilst D_τ =2 (CW)*, where τ is the pulse time of the electron discharge (-100 ns) , whilst a is the thermal diffusivity of the target material.

In general, the value of D_T is approximately 6 μm for the metal target materials, whilst it is approximately 0.6 μm for the dielectric ones; the value of D is instead generally 1.4 μm for acceleration voltages of 20 kV. If D > D_T (as occurs in dielectric target materials), the dependence D-V² controls the denominator of Eq. 1. The variation of surface temperature, and hence the effectiveness of ablation, is markedly dependent upon the applied voltage.

It has been shown (M. Strikovski et al . , Appl . Phys . Lett., 82, 853-855, 2003) that for a given target material and for a given geometry of the electronic source there exists a precise evolution of dT/dt as a function of V. In particular, for dielectric target materials, there is a considerable increase in the number of the electrons extracted, hence of J, as V increases. This evolution is respected up to a certain voltage value, referred to as V_max, beyond which the discharge current remains constant, or in other words, goes into saturation. Above V_max the energy transferred to the material by the electron pulse becomes directly proportional to V, whilst the variation dT/dt tends to decrease since the dependence D-V² starts to dominate in the denominator of Eq. 1. This means that beyond V_max the energy yielded by the beam does not remain confined only on the surface, causing ablation, but the electrons, interacting with the material at greater depths, cause a more internal heating thereof that can lead to melting and decomposition of the target material.

It hence becomes of fundamental_' importance to be able to measure the variation of the discharge current as a function of the acceleration voltage, in such a way that the saturation of the current will indicate the point at which V_max is reached and at which there is the maximum effectiveness of ablation of the dielectric material without diffused heating of the target material. Maximum effectiveness of the ablation process likewise means maximum rate of ablation, and hence maximum rate of deposition.

Consequently, the measurement of the discharge current of the beam is fundamental for optimizing the ablation process of dielectric target materials and the deposition rate of layers obtained with said materials.

Also for metal target materials, even though the situation is in part different, the measurement of the discharge current is fundamental for optimizing the deposition process. For metal target materials, in fact, at the denominator of Eq. 1 we have D_τ > D; hence the dependence of the denominator upon the square of the voltage becomes negligible. For this reason the quantity dT/dt increases as V increases throughout the working range of the pulsed-electron technique (0 to 25 keV) . Notwithstanding this, it remains of fundamental importance to measure the discharge current, since dT/dt and hence the deposition rate depend upon the amplitude of the discharge current, which in turn depends upon geometrical parameters (for example, the distance between the end of the dielectric tube and the target material and between the target material and the support) and operative parameters (voltage, pressure in the hollow cathode and in the deposition chamber, pulse frequency, etc.). In accordance with the present invention, the discharge current is hence measured for verifying that the parameters of the deposition process present reference values (preliminarily- determined, for example, in an experimental way) that maximize the deposition rate without altering the stoichiometry of the layer being formed with respect to that of the target material. In the case of deviations from the reference values, the parameters of the deposition process are modified as a function of the measurement of discharge current. In particular, the discharge current is measured in order to implement the deposition process maximizing the discharge current I₁ intervening and varying all the parameters involved in the process, thus being able to maximize dT/dt and the rate of deposition of the layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will emerge clearly from the ensuing description of a non-limiting example of embodiment thereof, with reference to the figures of the annexed drawings, wherein:

- Figure 1 is a schematic view of an apparatus for producing thin films on a substrate via a pulsed-electron deposition process in accordance with the invention;

- Figure 2 is a of fundamental importance view of a component of the apparatus of Figure 1, in particular an induction coil; and

- Figures 3 and 4 are schematic representations of respective integrator circuits used in the apparatus of Figure 1.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to Figure 1, an apparatus 1 for producing thin films on a substrate via a pulsed-electron deposition process comprises an emission unit 2 for emitting a pulsed-electron beam 3, a target-carrier 4 on which, in use, is set a target material 5 that is to be deposited in the form of film or thin layer, and a support element 6 for supporting a substrate 7 on which a layer 8 obtained with the target material 5 is to be deposited.

The unit 2 is in itself substantially known and comprises a substantially tubular hollow metal cathode 11 and an anode 12, for example with cylindrical symmetry, both arranged about an axis A. It is understood that the cathode 11 and the anode 12, as likewise the unit 2 as a whole, can be obtained with geometries different from those described and illustrated purely by way of example herein.

The unit 2 further comprises a dielectric tubular element 13 for emission of the beam 3, constituted, for example, by a tube of glass, quartz, alumina, etc., which extends substantially along the axis A from the cathode 11 towards the target-carrier 4 and through which the beam 3 is sent onto the target material 5. The tubular element 13 is arranged concentric and radially internal to the anode 12 and extends beyond the anode 12 as far as in the proximity of the target- carrier 4.

The unit 2 also comprises a primary circuit 15 for generation of the beam 3 provided with a trigger 16 with variable frequency (for example in the range 1 to 10 Hz) for discharging electrons along the tubular element 13 in a pulsed way (for example with a pulse duration in the region of -100 ns) .

The cathode 11 and the anode 12 are connected to respective terminals 17, 18 to which there can be applied a potential difference (for example in the range 5 to 20 kV) defining an acceleration voltage of the beam 3.

Set at a top end 19 of the tubular element 13, within a deposition chamber 20 at a controlled pressure, is the target- carrier 4, comprising, for example, a copper rotary drum, on which the target material 5 is fixed. The target-carrier 4 is mobile (in the case in point rotatable) so as to homogeneize the incidence of the beam 3 over the entire surface of the target material 5.

,5

Set in the proximity of the target-carrier 4 is the element 6 for supporting the substrate 7 , in a position such that the substrate 7 will intercept in use the evaporation flow 21, the so-called "plume", generated by the interaction of the beam 3

10 with the target material 5 and constituted by particles of the target material 5 in the vapour phase. The element 6 can be provided with movement systems for displacing the substrate 7 with respect to the evaporation flow 21 and/or with systems for heating the substrate 7.

15

The apparatus 1 further comprises a detection device 25 for detecting the discharge current of the pulsed-electron beam 3 sent on the target material 5, and a control unit 26 for controlling the parameters of the deposition process, 0 preferably operatively connected to the detection device 25 for varying the parameters as a function of the measurement of discharge current.

With reference also to Figure 2, the detection device 25 5 comprises an induction coil 27, set substantially coaxial around the tubular element 13, and a potential-metering device 28 connected to the opposite terminals 29 of the coil 27 for generating a signal representing the discharge current. 0 Preferably, the coil 27 is set between the anode 12 and the end 19 of the tubular element 13, around an end portion 30 of the tubular element 13 set inside the deposition chamber 20.

In particular, the coil 27 is a Rogowski coil, having the 5 conformation shown in detail in Figure 2. The coil 27 comprises a conductor 33 (for example, a wire made of conductive material) wound in a helix to form a plurality N of adjacent turns 34 of radius ri constituting a winding 35 of a substantially toroidal shape, with an internal radius r₂, which is set around a central axis, which, in the case in point, coincides with the axis A (given that the coil 27 is coaxial to the tubular element 13) . The conductor 33 comprises a substantially annular portion 36 that is set within the turns 34 around the axis A and traverses the turns 34 centrally. The winding 35 has an interruption 37 defined between two terminal turns 34a, 34b that face one another but are not directly connected, are arranged at respective ends 38a, 38b of the winding 35 and are separated by the interruption 37. The conductor 33 has an input terminal 29a and an output terminal 29a that are set both at one and the same end 38a of the winding 35 (on one and the same side of the interruption 37) and define the terminal 29 of the coil 27.

The detection device 25 is able to convert the signal representing the discharge current coming from the potential- metering device 28 into a value (measurement) of current.

The detection device 25 comprises an integrator circuit 40 (indicated in Figure 1 only schematically) connected to the coil 27 for generating a signal directly proportional to the discharge current, instead of to a variation in time of the discharge current, as will be clarified in what follows.

Operation of the apparatus 1 that implements the method in accordance with the invention is described in what follows.

Once the parameters (both the geometrical ones and the operative ones) of the apparatus 1 with which to carry out the deposition process have been set (for example, the applied voltage, the cross section of the beam, the gas pressure in the deposition chamber and/or in the cathode, the pulse frequency, the distance between the end of the tubular element and the target material, the distance between the target material and the substrate, etc.), the emission unit 2 generates the pulsed-electron beam 3 and sends it onto the target material 5 through the tubular element 13.

The beam 3 impinges upon the target material 5, and the electron/matter interaction determines the process of ablation of the target material 5, which evaporates to form the evaporation flow 21. The evaporated target material 5 then deposits on the substrate 7 to form the layer or film 8.

The variation in time of the discharge current of the beam 3 , which is a pulsed electronic current (with duration of -100 ns) that traverses the tubular element 13, generates a variation of magnetic flux inside the coil 27, and consequently a variable potential difference, Uχ(t), at the terminals 29 the coil 27 according to the law:

where fi^ is the magnetic permeability in a vacuum.

The induced potential Ui is hence proportional to the variation of the discharge current I in time. To obtain a signal that is directly proportional to the discharge current I₁ and not to its variation in time, the integrator circuit 40 set downstream of the coil 27 is used. The integrator circuit 40 can be an active or passive integrator circuit. Figures 3 and 4 show purely by way of example a passive integrator circuit 40 and an active integrator circuit 40, respectively.

The passive integrator circuit 40 shown in Figure 3 comprises two branches 41 in parallel connected to the terminals- 29 of the coil 27 and one of which carries a resistor 42; a capacitor 43 of pre-set capacitance C is set between the two branches 41; the voltage U₁ induced at the terminals 29 of the coil 27 charge the capacitor 43, across which the voltage U₂ that provides the voltage value directly proportional to the discharge current I is measured.

The active integrator circuit 40 shown in Figure 4 includes, instead, an operational amplifier 44, which is connected to a resistor 45 and to a capacitor 46.

The resistor 45 is set between a connection to the terminals 29 of the coil 27 (on which there is U₁) and the operational amplifier 44; the capacitor 46 is set in series to the resistor 45 and in parallel to the operational amplifier 44, which has a connection to ground. The active integrator circuit 40 is able to integrate the signal U₁ and transform it into a signal U_∑, depending upon the variation of the discharge current J:

Once the discharge current I has been measured, it is possible to intervene, if necessary, on the parameters of the deposition process so as to maximize the deposition rate.

In particular, the parameters of the deposition process are varied, as a function of the measurement of discharge current, to maximize the discharge current and the rate of deposition of the layer.

As already mentioned, the parameters of the deposition process include geometrical and/or operative parameters of the apparatus 1 used for the deposition process, such as, for example, applied voltage, cross section of the beam, gas pressure in the deposition chamber and/or in the cathode, pulse frequency, distance between the end of the tubular element and the target material, distance between the target material and the substrate, etc.

Finally, it is understood that modifications and variations may be made to what has been described and .illustrated herein, without thereby departing from the scope of the invention as defined in the annexed claims .

Claims

C LA X M S

1. A method for producing thin films on a substrate, comprising a pulsed-electron deposition process in which a pulsed- electron beam (3) is sent onto a target material (5) to be deposited, arranged in the proximity of a substrate (7), in such a way as to cause ablation of the target material and emission from the target material of vapour phase particles that deposit on the substrate to form a layer or film; the method being characterized by comprising: a step of detecting the discharge current of the pulsed-electron beam (3) sent onto the target material by means of an induction coil (27) set around the beam (3) for generating upon passage of the beam (3) a signal representing the discharge current; and a step of controlling the parameters of the deposition process as a function of the measurement of discharge current.

2. The method according to Claim 1, wherein the parameters of the deposition process are modified for maximizing the discharge current and the rate of deposition of the layer.

3. The method according to Claim 1 or Claim 2 , wherein the pulsed-electron beam (3) is generated by means of an emission unit (2) having a cathode (11) and an anode (12); and the coil (27) is set between the anode (12) and the target material (5) .

4. The method according to any one of the preceding claims , wherein the coil (27) is set inside a deposition chamber (20) that houses the target material (5) and the substrate (7) .

5. The method according to any one of the preceding claims , wherein the coil (27) is a Rogowski coil.

6. The method according to any one of the preceding claims, wherein the step of detecting the discharge current includes the steps of measuring a potential difference at opposite terminals (29) of the coil (27), and calculating the discharge current on the basis of the potential difference measured at the terminals (29) of the coil (27).

7. The method according to any one of the preceding claims, wherein at the terminals (29) of the coil (27) a variable potential difference is measured, which is proportional to the variation of the discharge current in time, and the method comprises a step of integrating the variable potential difference to obtain a signal directly proportional to the discharge current.

8. The method according to Claim 7, wherein the integration is carried out via an active or passive integrator circuit (40) .

9. An apparatus (1) for producing thin films on a substrate via a pulsed-electron deposition process, comprising an emission unit (2) for emitting a pulsed-electron beam (3), a target- carrier (4) on which, in use, is set a target material (5) to be deposited, a support element (6) for supporting a substrate (7) and arranged in the proximity of the target-carrier (4) , and a tubular element (13) for emission of the beam (3), which extends substantially along an axis (A) and through which the beam is sent towards the target material (5) ; the apparatus being characterized by comprising a detection device (25) for detecting the discharge current of the pulsed-electron beam

(3) sent onto the target material, comprising an induction coil (27) set around the beam emission tubular element (13) for generating upon passage of the beam a signal representing the discharge current; and control means (26) for controlling the parameters of the deposition process so as to vary the parameters as a function of the measurement of discharge current .

10. The apparatus according to Claim 9, wherein the coil (27) _{i r}

- 16 -

is set between an anode (12) of the emission unit (2) and the target-carrier (4) , around an end portion (30) of the tubular element (13) .

11. The apparatus according to Claim 9 or 10, comprising a deposition chamber (20); the coil (27) being housed inside the deposition chamber (20) .

12. The apparatus according to any one of Claims 9 to 11, wherein the coil (27) is a Rogowski coil.

13. The apparatus according to any one of Claims 9 to 12, wherein the detection device (25) comprises a potential metering device (28) connected to opposite terminals (29) of the coil (27) .

14. The apparatus according to any one of Claims 9 to 13, wherein the detection device (25) comprises an integrator circuit (40) connected to the coil (27) for generating a signal directly proportional to the discharge current.

15. The apparatus according to Claim 14, wherein the integrator circuit (40) is an active or passive integrator circuit.