LU87880A1 - METHOD FOR DEPOSITING A THIN FILM BY PHOTO-ABLATION - Google Patents

Description

The invention relates to a method for depositing a thin layer of at least two bodies on a substrate or multilayers of different composition by photo-ablation obtained by irradiating targets. respective bodies by pulsed laser beams.

The photo-ablation technique using a pulsed laser is used in the manufacture of semiconductors to produce metallic, semiconductor or insulating deposits on any substrate. The method is implemented in a vacuum enclosure in which a target of the material to be deposited and a substrate which is to receive this deposit are placed opposite one another. A pulsed beam from a power laser is directed and focused on the target. Under well-defined conditions, this results in an emission of particles forming a plume. Part of the plume particles is then deposited on the substrate.

It is difficult to master this method and always obtain a layer according to required specifications, because it is necessary to optimize a large number of parameters which depend on the nature of the material to be deposited, on the characteristics of the laser beam (intensity, length wave, spatial and temporal profile), the temperature of the substrate and the configuration of the elements contained in the enclosure. In addition, some of these parameters vary over time, such as for example the surface condition of the target after prolonged irradiation. Very variable quality results are thus obtained and a high percentage of samples which do not meet the specifications and must be rejected.

Hereinafter the term "body" means any material in the form of a solid target, whether it is a simple element or a compound body. Thus, for example, we want to deposit on a substrate on the one hand silicon and on the other hand a metal. If it is already very difficult to ensure a reproducible and satisfactory result when depositing a single body, a combined deposit of two or more bodies is even more complicated if one does not want to exceed an unbearable rejection rate .

Such a deposit consisting of two bodies has been made so far using a rotating target. The target, in this case, is a circular disc, a sector of which is constituted by the second body. The disc is rotated about an axis perpendicular to its surface and passing through its center, and the laser beam is directed eccentrically on this surface. It therefore sometimes strikes the first body and at times the second. The composition of the layer deposited on the substrate therefore depends on the relative size of the two sectors on the target, the speed of rotation of the latter and the nature of the two bodies. In this configuration, the deposits obtained consist of a regular stack of different layers, but in no case of a homogeneous film of well defined composition.

The aim of the present invention is to propose a method making it possible to deposit a thin layer of at least two bodies on a substrate using a photo-ablation obtained by irradiating with pulsed laser beams targets made up of the respective bodies. , this method ensuring a very reproducible result and therefore a very low rejection rate.

To achieve this objective, the invention consists in using as many laser beams as there are target bodies, the power of these beams being individually controllable as a function of a real-time spectral analysis of the plume. ablation between the targets and the substrate.

With regard to preferred features of this method, reference is made to the dependent claims. The invention will be described below in more detail using a preferred embodiment and the accompanying drawing, the single figure of which schematically shows this method.

In this figure, we can see a substrate 1 which must receive the deposit of a thin layer 2 on one of its main faces. This face is opposite two targets 3 and 4 each consisting of one of the two bodies of which the layer 2 must be composed. The two targets and the substrate are contained in an enclosure 5, which is constantly evacuated using a pumping system (not shown).

Each of the targets is struck by the beam 6 and 7 respectively of a laser 8 and 9 respectively passing through a window 10 and 11 in the wall of the enclosure 5. The laser is a power laser, for example of the Nd-YAG type . It is capable of creating pulses lasting a few tens of nanoseconds at a rate of 30 kHz. By optical means not shown, the beam is focused on the surface of the corresponding target in order to reach energy densities 8 2 of approximately 10 Watt / cm. The material ejected from the target immediately melts and vaporizes. The vaporized atoms or molecules are excited or ionized. The plasma recombination light is visible. This light is analyzed by a spectral analyzer 12 whose optical field covers the plume evaporated in front of the targets 3 and 4. Preferably, an analyzer is used making it possible to carry out a spatial and temporal analysis of the plume. It provides significant signals from this analysis to a control device 13. The latter optimizes in real time the parameters of the two laser beams in order to bring the plume of atoms or molecules evaporated closer to the optimal criteria for the desired deposit. This optimization concerns first of all the power of the two generators 8 and 9. Then, the duration, the cadence and the phase ratio of the pulses can be varied by taking into account the differences in the flight time of the two bodies. Finally, one can act on attenuators 14 and 15 which are inserted in the path of the two lasers.

It is also useful to insert a shutter 16 and 17 in the path of each beam in order to be able to measure the effect of each laser individually for the purpose of calibrating the system.

In the context of the invention, it is also useful to provide mechanical devices for driving the two targets 3 and 4 in a reciprocating or rotational movement for a better distribution of the ablation products. their surfaces. Likewise, it is useful to rotate the substrate, which makes layer 2 uniform.

In the context of the invention, it is also possible to use only one laser generator and to extract two beams therefrom by a frequency doubler or another beam splitter.

The representation of the system in the figure being only very schematic, it is clear that it is better to carry out the analysis of the plume in a plane which does not coincide with that of the two beams, therefore for example in a direction perpendicular to the plane drawing.

The method is also not limited to the combined deposit of two bodies, that is to say to two targets such as targets 3 and 4, but it can be extended to a deposit made up of several bodies.

Thanks to the method according to the invention, the deposition conditions are continuously optimized throughout the duration of the operation and can therefore even take account of transient disturbing phenomena. It is also possible with the described method to voluntarily modify the composition of the deposit during the operation.

Claims (3)

1. Method for depositing a thin layer of at least two bodies on a substrate, using photo-ablation obtained by irradiating targets of the respective bodies with pulsed laser beams, characterized in that provides as many laser beams as there are target bodies, the power of these beams being individually controllable according to a spectral analysis of the ablation plume being established between the targets and the substrate. 2. Method according to claim 1, characterized in that one controls, in addition, the instants of the pulses of the lasers (8, 9) in consideration of the flight times of the bodies between the respective target and the substrate. 3. Method according to one of claims 1 to 2, characterized in that the spectral analysis of the ablation plume being established between the targets and the substrate is a spatial and / or temporal analysis in real time.