GB2303379A

GB2303379A - Thin film forming using laser and magnetic field

Info

Publication number: GB2303379A
Application number: GB9610953A
Authority: GB
Inventors: Mutsumi Tsuda; Koichi Ono; Kenyu Haruta; Taisuke Furukawa; Takaaki Kawahara
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1992-11-30
Filing date: 1993-11-29
Publication date: 1997-02-19
Anticipated expiration: 2013-11-29
Also published as: GB9610953D0; GB2300426A; GB2303379B; GB2300000A; GB2300001A; GB9611007D0; GB2300426B; GB9611008D0; GB2300001B; GB9610969D0

Description

is Thin Film Forming Apparatus Using Laser 2303379

This is a divisional application to Application No. 9324498.6 published as GB2272912. The entirety of that specification as filed is incorporated herein by reference.

The present invention relates to a thin film forming apparatus using laser and, more specifically, to a film forming apparatus using laser used for forming thin film having functions and to form thin films having large areas.

Fig. 6, is a conventional thin film forming apparatus using laser disclosed, for example in, Japanese Patent Laying-Open No. 4-45263 which apparatus includes a chamber 1, a substrate 2, a substrate holder 3, a heater 4, a raw material target 5, a nozzle 6, an inlet window 7, a condenser lens 9, a laser unit 10, a turntable 11, an XY stage 12, a control apparatus 13, a motor 14, a plume 15 and an evacuating apparatus 17.

The operation will be described. Laser beam 16 emitted from laser unit 10 is condensed by condenser lens 9, passes through laser inlet window 7 of chamber 1, and irradiate raw material target 5 placed on turntable 11 in chamber 1. At this time, the turntable 11 can be rotated by means of motor 14. This is to make uniform laser irradiation by rotating raw material target 5 so as to 9 prevent local generation of craters caused by sputtering of the same portion of raw material target 5.

At the portion of target 5 which is irradiated with the laser beam, plasma is generated abruptly, and in the process of cooling of the plasma in several ten ns, there are generated isolated excited atoms and ions. These groups of excited atoms and ions have the lives of at least several microseconds, which are emitted in this space to form a plume 15 which is like a candle flame. Meanwhile, a substrate 2 is placed fixed on a substrate holder 3 opposing to raw material target 5, and the excited atoms and ions in the plume 15 reach substrate 2 and are deposited thereon, forming a thin film.

In substrate holder 3, a heater 4 for heating the substrate is provided, so as to enable post annealing in which the film deposited at a low temperature is annealed at a temperature higher than the temperature for crystallization to provide a thin film of superior quality, and allowing as-deposition in which the substrate itself is held at a temperature higher than the temperature for crystallization at the time of deposition so as to form crystallized thin film at the site. In the as-deposition method, sometimes an active oxygen atmosphere is used as well. For example, as shown in the figure, a nozzle 6 for supplying gas including oxygen is provided so that the atmosphere around the substrate 2 is made an oxygen atmosphere in forming a high temperature superconductive thin film, whereby generation of oxide on substrate 2 is promoted.

In view of enlargement of the area of thin film formation, substrate holder 3 is mounted on XY stage 12, so that the position of forming the thin film can be moved. First, a control signal corresponding to an oscillation pulse of laser unit 10 is transmitted to XY stage 12 through control apparatus 13. The XY stage 12 is driven based on the control signal, and moves the position of forming the thin film on the substrate 2 at every laser pulse. Consequently, a uniform thin film can be formed on a wide area. In the conventional example, when XY stage 12 is not driven, the area of thin film formation is limited to 10mm x 10mm (with the variation of film thickness distribution of 10%), and when the XY stage is driven, the area can be expanded to 35mm x 35mm.

However, in the semiconductor industry, formation of a uniform thin film over a wafer of 6 to 8 inches in diameter has been desired, and conventional thin film forming apparatuses using laser could not meet such demand.

Fig. 7 shows another prior art example disclosed, for example, in Japanese Patent Laying-Open No. 4-114904.

Referring to the figure, 18 denotes an oxygen ion source, 19 denotes oxygen gas and 20 denotes oxygen ion beam. The process for forming a thin film in this example is the same as that of the above described prior art example. In such a thin film forming apparatus using laser, laser beam in the form of very short pulses of ten to about several ten ns is directed to the target, and the target material in the form of atoms, molecules or clusters are supplied onto the substrate only at the time of irradiation, so as to form a thin film. The excimer laser having extremely short pulse width and high energy has such advantage that (a) it allows generation of a large amount of target raw material to be deposited on the substrate so that the rate of thin film growth can be much increased, and that (b) a thin film of which composition is not very much changed from that of the raw material target can be obtained. However, the excimer laser may degrade the quality of the film due to insufficient crystallization. In order to promote crystallization of the raw material in the form of atoms, molecules or clusters deposited on substrate 2, heating of substrate 2 by a heater provided in substrate holder 3 so as to keep the substrate at a temperature higher than the temperature for crystallization has been proposed. However, if the substrate is kept at a high temperature during thin film formation, it may induce degradation of the substrate or undesirable reaction, which is inconvenient for the functional thin film from electronic or mechanic point of view. Therefore, in this prior art example, in order to reduce problems accompanying heating of the substrate, oxygen gas 19 is introduced to ion source 18 when raw material target 5 is irradiated with laser beam 16 so that substrate 2 is irradiated with the generated oxygen ion beam 20, whereby oxygen is supplied to the thin film and the temperature of crystal growth is lowered by the oxygen bombardment. Consequently, in this known example, a Y,Ba2CU307-, oxide superconductive thin film can be formed at the substrate temperature of 6001C.

However, the conventional thin film forming apparatus using laser has the following problems.

First, since the area of film formation which can be formed by one plume is limited, it has been impossible to form uniform thin film over a large area such as over a wafer having 6 to 8 inches in diameter required in the semiconductor industry.

In addition, there have been the problem of degradation of the substrate derived from high temperature of film formation and lower quality of the thin film caused by undesirable side reaction induced. In addition, when the film quality is to be improved by using active ion seeds, there has been possible damage of the substrate caused by ion beams, and therefore it has been difficult to improve the quality of the film.

Further, film formation parameters such as intensity of condensed beam incident on the target, condition for laser oscillation, position of the target, pressure for film formation and so on have been set initially and thereafter these parameters are not controlled. Therefore, delicate control of the film quality such as change in composition or orientation of the film which depends on composition of the surface of the target or on sudden change in energy of the particles incident on the substrate could not be done.

Various aspects of the present invention address one or more of these problems, and in combination may provide a thin film forming apparatus using laser which allows formation of a thin film having high quality and large area with uniform film quality distribution, without damaging the substrate.

One object of the present invention is to provide a thin forming apparatus using laser allowing delicate control of film quality, by controlling various conditions during the process for forming the thin film using laser.

One thin film forming apparatus using laser in is accordance with the present invention includes, as basic components, a chamber having evacuating means, a target placed in the chamber, laser beam irradiating means for directing laser beams to the target, and substrate holding means holding a substrate on which a substance included in a plume generated from the target by laser beam irradiation is deposited.

According to an aspect of the present invention, the thin film forming apparatus using laser includes means for applying magnetic field at least to the vicinity of the target.

In the thin film forming apparatus using laser, since a magnetic field is applied at least to the vicinity of the target, movement of charged particles such as ions and electrons in the plume are influenced by the magnetic field, so that the charged particles drift in the direction of the magnetic lines of force. Therefore, by applying a magnetic field such that the magnetic lines of force do not pass the surface of the substrate, the amount of ions in the plume incident on the substrate can be controlled and suppressed. Meanwhile, non-charged particles such as neutral atoms and clusters in the plume are not influenced by the magnetic field so that these particles reach the substrate and are deposited thereon.

The present invention provides a thin film forming apparatus using laser, comprising:

a chamber having evacuating means; a target placed in said chamber; laser beam irradiating means for irradiating said target with a laser beam; substrate holding means for holding a substrate on which a material included in a plume generated from said target by laser beam irradiation is deposited; and means for applying a magnetic field at least to the vicinity of said target.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Fig. 1 is a cross sectional view showing a schematic structure of the thin film forming apparatus using laser in accordance with a first embodiment of the present invention; Fig. 2 is a cross sectional view showing a schematic structure of the thin film forming apparatus using laser in accordance with a second embodiment of the present invention.

Fig. 3 is a cross sectional view showing a schematic structure of the thin film forming apparatus using laser in accordance with a third embodiment of the present invention.

9 - Fig. 4 is an illustration showing the manner of movement of charged particles in the plume near the surface of the target, in the think film forming apparatus using laser in accordance with third embodiment of the present invention..

Fig. 5 is a cross sectional view showing a schematic structure of the thin film forming apparatus using laser in accordance with a fourth embodiment of the present invention.

Fig. 6 is a cross sectional view showing a schematic structure of a conventional thin film forming apparatus using laser disclosed in Japanese Patent Laying-Open No. 4-452263.

Fig. 7 is a cross sectional view showing a schematic structure of another conventional thin film forming apparatus using laser disclosed in Japanese Patent Laying-Open No. 4-114904.

is A first embodiment of this invention will be described with reference to Fig. 1 In the figure, the same reference characters as in Figs 6 and 7 denote the same or corresponding portions.

The operation is as follows. Laser beam 16 emitted from laser unit 10 is condensed by condenser lens 9 and passed through laser inlet window 7 of chamber 1 to be incident on raw material target 5 placed on turntable in chamber 1. At the portion of raw material target which is irradiated with laser beam, a plasma of the target material is generated abruptly at the time of laser irradiation, and in the process of cooling the plasma in several ten ns, isolated excited atoms, molecules and ions are generated. These groups of excited atoms, molecules and ions have lives of several microseconds, and they are emitted in the space to form a plume 15 which is like a flame of a candle.

Meanwhile, substrate 2 is placed fixed on substrate holder 3, opposing to raw material target 5. Excited atoms, molecules and ions and the target material in the form of clusters in which these atoms and ions and the like are combined in plume 15 reach the substrate 2, and 11 are deposited and crystallized to form a thin film, in the similar manner as in the prior art.

Here, power is supplied to an electromagnetic coil 309 from a power source 311 for the electromagnetic coil and a magnetic field having magnetic lines of force 310 and the intensity of up to lkG is applied to and near raw material target 5, movement of charge particles such as ions and electrons in plume 15 are influenced by the magnetic field and these charged particles drift in the direction of the magnetic lines of force 310. Therefore, when the magnetic field is formed such that the magnetic lines of force 310 are parallel to the surface of target and not passing through the surface of substrate 2, ions in plume 15 do not reach substrate 2, suppressing impingement and deposition of ions on the surface of substrate 2. Meanwhile, non-charged particles such as neutral atoms, molecules and clusters in plume 15 are not influenced by the magnetic field so that they reach and are deposited on the surface of substrate 2 as in the prior art. Therefore, a thin film of high quality, in which the problem of degradation of the substrate and degraded functions of the thin film caused by undesirable side reaction derived from impingement and deposition of ions are eliminated, can be obtained.

At the initial stage of irradiation of raw material target 5 with laser beam 16, that is, at the initial stage of deposition and thin film formation of target material on substrate surface 2, preferable effects can be often obtained with respect to the quality of the completed film if ions in plume 15 reach the surface of the substrate 2. This is because the ions excite, in non-equilibrium, the state of electrons of impurity particles deposited on the surface of substrate 2 for atoms and molecules constituting the impurity thin film formed on the surface, which leads to removal and separation of the impurity particles or impurity thin film from the surface of substrate 2. Thus clear substrate surface is exposed at the initial stage of film formation on the surface of substrate 2, using the target material, and initial nucleus of the thin film of the target material can be generated at a pure state on the surface of the substrate 2 without impurities. Consequently, crystal of the thin film of the target material grows regularly, enabling formation of a thin film of high quality in a clean condition, relatively free from the problem of the impurities at the interface. Therefore, in such a case, in the initial stage of irradiation of raw material target 5 with laser beam 16, that is, in the initial stage of deposition and thin film formation of target material on the surface of substrate 2, the switch of power source 311 for supplying power to electromagnetic coil 309 should be turned off so that there is not a magnetic field generated. In the later step of film formation, power source 311 is turned on to supply power to electromagnetic coil 309, whereby a magnetic field having the magnetic lines of force 310 and having the intensity of up to about lkG is applied to and near target 5, suppressing impingement of ions in plume 15 to substrate 2.

In order to suppress impingement and deposition of ions on the surface of substrate 2 by preventing ions in plume 15 from reaching substrate 2, conventionally, a metal mesh is placed parallel to the surface of substrate 2 in the vicinity of substrate 2 and a negative potential is applied to the metal mesh, so that ions are trapped by the mesh. However, in that case, metal impurities are emitted due to mutual function of the mesh and the ions in the plume 15. Passage of non-charge particles such as neutral atoms and molecules as well as clusters in plume 15 is suppressed by the mesh, causing metal contamination of the thin film and lower rate of thin film deposition. By contrast, in the present embodiment employing magnetic field, such problems of metal contamination and lower rate of thin film deposition can be solved.

A heater 4 for heating the substrate is provided in substrate holder 3, and therefore, post annealing in which - 14 a film deposited at a low temperature is annealed at a temperature higher than the crystallizing temperature to form a thin film of superior quality, and as-deposition i which substrate itself is kept at a temperature higher than the temperature for crystallization at the time of deposition so that a crystallized thin film is formed at the site can be carried out. In the as-deposition method, active oxygen atmosphere is used as well, and, a nozzle 6 for supplying gas 19 containing oxygen during formation of an oxide thin film is provided as shown in the figure for example, so that oxygen atmosphere is provided near substrate 2 so as to promote generation of oxide on substrate 2, as in the prior art example.

A.second embodiment of the present invention will be described with reference to Fig. 2. In the figure, the same reference characters as in Fig. 1 denote the same or corresponding portions.

This embodiment is adapted such that a magnetic field having magnetic lines of force 310 and intensity of up to about 1kG or higher caused by electromagnetic coil 309 extend vertical to the surface of raw material target 5 and expanding toward the surface of substrate 2. Movement of charged particles such as ions and electrons in plume 15 are influenced by the magnetic field formed at target and substrate 2 as well as in the vicinity thereof, so that the charged particles drift in the direction of the magnetic lines of force 310. Therefore, if the magnetic lines of force 310 is formed vertical to the surface of target 5 and expanding toward the surface of substrate 2, ion flux in plume 15 decreased toward substrate 2, suppressing impingement and deposition of ions on the surface of substrate 2. Meanwhile, non-charged particles such as neutral atoms, molecules and clusters and the like in plume 15 are not influenced by the magnetic field, so that they impinge and are deposited on the surface of substrate 2 as in the prior art. Therefore, a thin film of high quality can be formed in which degradation of the substrate and degradation of thin film function caused by undesirable side reaction derived from impingementdeposition of ions can be eliminated.

In the embodiments described above, magnetic field is applied by using electromagnetic coils. However, a permanent magnet may be used instead of the electromagnetic coil, to provide the same effect. If a permanent magnet is used, power supply for supplying power necessary for electromagnetic coils can be dispensed of, making compact the whole apparatus. However, the effects obtained by turning on/off the magnetic field so as to control movement of charged particles such as ions and electrons in the plume cannot be expected.

A third embodiment of the present invention will be described with reference to Figs - 3 and 4 Fig. 3 is a schematic diagram showing the thin film forming apparatus using laser in accordance with one embodiment of the present invention, and Fig - 4 is an illustration showing movement of charged particles in the plume near the surface of the target. In these figures, the same reference characters as in the prior art example of Figs. 6 and 7 denote the same or corresponding portions, and description thereof will not be repeated.

Referring to Figs. 3 and 4, the apparatus of this embodiment includes a ring-shaped permanent magnet provided along target 5, which generates magnetic field parallel to the surface of target 5, near the surface of target 5.

The operation is as follows. When the target surface is irradiated with laser beam 16, a high density plasma is generated locally at the irradiated portion, generating a plume 15 which is like a flame of a candle toward substrate 2. Excited neutral particles in the form of atoms, molecules or clusters constituting the target as well as electrons and ions exist in the plume. Since there is a magnetic field parallel to the target surface, near the target surface, electrons and ions in the plume emitted from the target move spirally along the magnetic lines of force as shown in Fig. 4 At this time, radius of rotation of the electrons and ions at this time are determined by the speed of the particles when they exit the target and by the intensity of the applied magnetic field. Thus, by selecting an appropriate intensity of the magnetic field, the loci of the electrons and ions emitted from the target can be bent significantly so as to prevent these charged particles from reaching the surface of the substrate. Meanwhile, neutral particles in the plume are transmitted to the substrate without influenced by the magnetic field, so that they are adhered and deposited on the surface of the substrate, forming a thin film. Thus, damage to the substrate caused by electrons and ions reaching the substrate can be prevented.

In the conventional apparatus, in order to Prevent charged particles in the plume from reaching the substrate, front surface of the substrate is covered by a grid electrode, and electric field is put in the plume so as to repel electrons and ions to the target so that the electrons and ions can be prevented from reaching the substrate. However, in this method, the electrode is directly in contact with the plume so that the surface of the electrode is sputtered by ion impingement. Therefore, there is a problem of contamination of the substrate by - 18 magnetic therefore the material of the electrode. In this embodiment, the movement of charged particles is controlled by a magnetic field and the plume does not touch any metal such as an electrode. Therefore, this problem of contamination can be prevented.

Though a ring-shaped permanent magnet is provided around the target, what is important is to generate a field parallel to the surface of the target, and various shapes and methods of providing the permanent magnet are possible. The same effect can be expected when a magnetic coil is used.

As described above, in accordance with this embodiment, since a magnetic field parallel to the surface of the target is generated, charged particles in the plume generated by laser beam irradiation of the target surface have their loci bent, so that these particles are prevented from reaching the substrate. Thus a thin film can be formed over the substrate surface by using excited neutral particles in the plume only, which allows formation of a thin film of high quality without any damage.

A fourth embodiment of this invention will be described with reference to Fig. 5. Fig. 5 is a schematic diagram showing the thin film forming apparatus using laser in accordance with one embodiment of the present invention. In the figure, the same reference characters as in the prior art example of Figs. 6 and

7 denote the same or corresponding portions, and description thereof will not be repeated.

Referring to Fig. 5, the apparatus of this embodiment includes a set of magnetic coils 316 and 317 provided around a vacuum chamber 1, which are respectively connected to independent coil power supplies 318 and 319 feeding current.

The operation is as follows. When the target surface is irradiated with laser beam 16, a high density plasma is generated locally at the irradiated portion, generating a plume 15, which is like a flame of a candle toward substrate 2. Excited neutral particles in the form of atoms, molecules or clusters as well as electrons and ions constituting the target exist in the plume. These particles reach the substrate surface and are adhered and deposited on the surface to form a thin film. The direction of scattering of the particles emitted from the target surface tends to be approximately vertical to the target surface. Therefore, the extent of the plume reaching the substrate surface is very small compared with the area of the substrate having the diameter of about 6 to 8 inches used in the semiconductor industry. Therefore, in this embodiment, to a set of magnetic coils juxtaposed with an appropriate space, currents of opposite directions are applied by two coil power supplies connected thereto. Consequently, a cusp magnetic field having such a distribution of magnetic lines of force as shown in Fig. 5, in the space between the target and the substrate by these magnetic coils. The electrons and ions in the plume emitted from the target move spirally along the magnetic lines of force, so as to expand along the magnetic lines of force. Thus the plume mentioned above can be expanded in the radial direction by the function of the cusp magnetic field, and therefore a thin film can be formed uniform in the radial direction of a substrate having larger diameter, by a single plume.

Though the cusp magnetic field is generated by providing a set of magnetic coils around the vacuum container in this embodiment, what is important is to generate a cusp magnetic field in the space between the target and the substrate. Therefore, similar cusp magnetic field can be generated by providing a magnetic coil or a permanent magnet in the vacuum container to provide the same effect.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims

CLAIMS 1. A thin film forming apparatus using laser, comprising: a chamber

having evacuating means; 5 a target placed in said chamber; laser beam irradiating means for irradiating said target with a laser beam; substrate holding means for holding a substrate on which a material included in a plume generated from said target by laser beam irradiation is deposited; and means for applying a magnetic field at least to the vicinity of said target.
2. The thin film forming apparatus using laser according to claim 1, wherein said means for applying a magnetic field includes means for applying a magnetic field parallel to that surface of said target which is irradiated with the laser beam, in a space between said target and the substrate held by said substrate holding means.
3. An apparatus according to claim 1 or claim 2 wherein said magnetic field is arranged to reduce the proportion charged particles in the generated plume reaching the substrate.
4. An apparatus according to claim 1 wherein said - 23 - magnetic field is arranged to expand the flux of charged particles in the plume in a radial direction.
5. An apparatus according to claim 4 wherein the magnetic f ield is applied perpendicular to the irradiated surface of the target, the field lines expanding in the direction toward the substrate.
6. An apparatus according to claim 4 wherein the magnetic field is a cusp magnetic field applied in the space between the target and the substrate.
7. The thin film formation method of claim 5 or 6 wherein application of said magnetic field is inhibited during an initial stage of thin film formation.

2 LI -

7. An apparatus according to any preceding claim wherein said means for applying a magnetic field comprise an electromagnet.

8. An apparatus according to claim 7 further comprising means for switching off the electromagnet at an initial stage of deposition.

9. A method of film formation by depositing material onto a substrate in a laser sputtering device comprising the steps of irradiating a target with a laser beam and applying a magnetic field to the laser sputtering device, at least in the vicinity of said target at a later stage of film formation.

- 24 Amendments to the claims have been filed as follows 1. A thin film forming apparatus using laser, comprising:

chamber having evacuating means; target placed in said chamber; laser beam irradiating means for irradiating said target with a laser beam; substrate holding means for holding a substrate on which a material included in a plume generated from said target by laser beam irradiation is deposited; and means for applying a magnetic field at least to the vicinity of said target, so as to reduce substantially the proportion of charged particles in the generated plume reaching the substrate.

2. The thin film forming apparatus using laser according to claim 1, wherein said means for applying a magnetic field includes means for applying a magnetic field parallel to that surface of said target which is irradiated with the laser beam, in a space between said target and the substrate held by said substrate holding means.

3. An apparatus according to any preceding claim wherein said means for applying a magnetic field comprise an electromagnet.

4. An apparatus according to claim 3 further comprising means operable to switch of f the electromagnet at an initial stage of deposition.

5. A method of thin film formation by depositing material onto a substrate in a laser sputtering device comprising the steps of:

providing a chamber having evacuating means; placing a target in said chamber; irradiating said target with a laser beam; placing in said chamber a substrate on which a material included in a plume generated from said target by laser beam irradiation is deposited; and applying a magnetic field at least to the vicinity of said target, so as to reduce substantially the proportion of charged particles in the generated plume reaching the substrate.

6. The thin film formation method of claim 5 wherein said magnetic field is applied parallel to that surface of said target which is irradiated with the laser beam, in a space between said target and the substrate.