PROCESS FOR MANUFACTURING COBALT SILICIDE CoSi2

Abstract

The invention relates to a method for manufacturing a cobalt silicide CoSi.sub.2 layer including the steps of: Providing a substrate comprising a silicon layer; Depositing a cobalt Co layer onto the substrate; Annealing the stack by a nanosecond laser comprising at least one laser pulse with a duration between 50 nanoseconds and 20 microseconds and an energy density selected so as to form the cobalt silicide CoSi.sub.2 layer in the solid state.

Claims

1. A method for manufacturing a cobalt silicide CoSi.sub.2 layer, the method comprising: providing a substrate comprising a single crystal silicon layer; depositing a cobalt Co layer onto the substrate, the thickness of the Co layer being between 0.5 nm and 10 nm; annealing the stack by a nanosecond laser comprising at least one laser pulse with a duration between 50 nanoseconds and 20 microseconds and an energy density selected so as to form the layer of cobalt silicide CoSi.sub.2 in the solid state, after the annealing the stack by the nanosecond laser, rapid thermal annealing RTA the stack.

2. The method according to claim 1, further comprising depositing a layer protecting Co and/or Si against oxidation onto the Co layer before the annealing.

3. The method according to claim 1, wherein laser annealing is performed by a nanosecond laser operating in step and repeat mode.

4. The method according to claim 1, wherein the duration of the laser pulse is between 0.1 microsecond and 1 microsecond.

5. The method according to claim 1, wherein the laser annealing is performed in the form of a single laser pulse.

6. The method according to claim 1, wherein laser annealing is performed in the form of a plurality of laser pulses.

7. The method according to claim 5, wherein each laser pulse of said plurality of laser pulses is emitted with a same energy density.

8. The method according to claim 5, wherein the first laser pulse of said plurality of laser pulses has a higher energy density than the subsequent pulses, the energy density of the first pulse being selected close to but strictly lower than the energy density causing melting of cobalt silicide CoSi.sub.2.

9. The method according to claim 1, wherein a thickness of the Co layer is greater than or equal to 0.5 nm and strictly lower than 5 nm.

10. The method according to claim 1, wherein the wavelength of the laser is between 150 nm and 900 nm.

11. The method according to claim 1, wherein the silicon Si layer is cleaned, prior to the deposition of the cobalt Co layer.

12. The method according to claim 1, wherein the laser annealing performed on a hot platen for maintaining the plate at a temperature ranging from 25 to 500 C.

13. The method according to claim 1, wherein the thickness of the Co layer is equal to 3 nm, said Co layer being deposited onto an SOI substrate the Si layer of which has a thickness of 33 nm and the buried insulating layer BOX has a thickness of 20 nm, the laser pulse duration being 160 ns and the energy density of the laser pulse being between 0.6 J/cm.sup.2 and 0.775 J/cm.sup.2.

14. An electronic device, comprising at least one zone made of CoSi.sub.2 obtained by the method according to claim 1.

15. The method according to claim 2, wherein the layer protecting Co and/or Si against oxidation is a TiN layer.

16. The method according to claim 9, wherein the thickness of the Co layer is greater than or equal to 0.5 nm and lower than or equal to 4 nm.

17. The method according to claim 16, wherein the thickness of the Co layer is greater than or equal to 1 nm and lower than or equal to 3.5 nm.

18. The method according to claim 1, wherein the wavelength of the laser is between 250 nm and 550 nm.

19. The electronic device according to claim 14, wherein the electronic device is a transistor.

20. The electronic device according to claim 14, wherein the at least one zone is a drain, a source or a gate contact.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0037] Further characteristics and advantages of the invention will be clearly apparent from the description thereof given hereinbelow, by way of indicating and in no way limited purposes, with reference to the appended figures, of which:

[0038] FIG. 1 represents, in the form of a flow chart, the different steps of the method of the invention,

[0039] FIG. 2, FIG. 3, FIG. 4, FIG. 5 and FIG. 10 represent the different steps of the method of FIG. 1,

[0040] FIG. 6 shows an image of a Si/CoSi.sub.2/TiN stack obtained by the method of the state of the art and two images of two other Si/CoSi.sub.2/TiN stacks obtained by the method according to the invention,

[0041] FIG. 7 represents the course of sheet resistance as a function of laser energy density for three different numbers of laser pulses,

[0042] FIG. 8 shows the simulated temperature course within a stack used in the method of the invention for several energy densities,

[0043] FIG. 9 shows the course of the critical superconductivity temperature as a function of the number of laser pulses, with and without additional RTA annealing.

[0044] For greater clarity, identical or similar elements are identified by identical reference signs throughout the figures.

DETAILED DESCRIPTION OF THE INVENTION

[0045] FIG. 1 represents the flow chart illustrating the different steps of the manufacturing method 100 according to the invention.

[0046] As shown in FIG. 2, the method 100 starts with an optional step 101 of cleaning 4 a silicon layer 3. The silicon layer 3 may, for example, be a single crystal silicon layer belonging to a substrate of the Silicon-On-Insulator (SOI) type including a lower silicon region 1 having thereabove a buried insulating layer 2 commonly designated by those skilled in the art as a BOX, for example formed of silicon dioxide. Above this buried insulating layer is situated the silicon layer 3. This cleaning aims at removing the chemical or native oxide initially present on the surface of the silicon layer 3. Cleaning can be carried out in one or two steps using one of the following techniques: wet process with a dilute HF hydrofluoric acid solutionabrasion by argon Ar plasmaSiCoNi method. It will be noted that the method of the invention is not limited to a silicon layer present in an SOI substrate and can be applied to any type of Si layer, for example a fully single crystal Si substrate. Si onto which Co is deposited can be single crystal, polycrystalline or amorphous. Any stack is contemplatable under this Si layer. It would also be possible to make a CoSi.sub.2 contact on another material, for example Ge, GeSn or SiGe, present under the silicon.

[0047] The method 100 according to the invention continues with a step 102 (FIG. 3) of depositing a cobalt layer 5 onto the silicon layer 3, immediately after the latter has been cleaned. This deposition is made, for example, by Physical Vapour Deposition (PVD). The thickness of the Co layer is between 0.5 and 50 nm and preferably between 0.5 nm and 10 nm and more preferably between 1 and 10 nm. Advantageously, the thickness of the Co layer is greater than or equal to 0.5 nm and strictly lower than 5 nm, and preferably greater than or equal to 0.5 nm and lower than or equal to 4 nm, and even more preferably greater than or equal to 1 nm and lower than or equal to 3.5 nm. According to this embodiment, the Co layer is directly deposited onto the Si layer. According to another embodiment, there could also be an oxide or nitride layer between the Si layer and the Co layer (in this case, the oxide or nitride layer preferably has a maximum thickness of 2 mm so that CoSi.sub.2 can be formed).

[0048] In order to prevent oxidation of the Co layer 5 and the underlying Si layer 3, the method 100 according to the invention includes a step 103 (FIG. 4) of depositing an oxidation protective layer 6. This protective layer 6 may, for example, be made of TiN deposited by PVD. The material of the protective layer is selected not only to protect the Co and Si layers 5 and 3 from oxidation but also to absorb the laser radiation which will subsequently be used to heat the Co layer.

[0049] The method 100 according to the invention continues with a laser annealing step 104 (FIG. 5). This step 104 is made by means of a pulsed laser emitting at a frequency between 0.1 and 1000 Hz, preferably between 1 and 10 Hz and advantageously between 3 and 6 Hz, for example here 4 Hz (i.e. a pulse shot every 250 ms), one or more laser pulses Pi (i varying from 1 to n, with n a strictly positive integer) through the stack formed by the Si/Co/TiN layers. Each pulse Pi has a duration between 50 nanoseconds and 20 microseconds and preferably between 0.1 microsecond and 1 microsecond, for example 160 ns. The wavelength of the laser is between 150 nm and 900 nm and preferably between 250 nm and 550 nm; in other words, the laser, which is monochromatic by nature, has a beam with a wavelength preferably in the ultraviolet (for example, at 293, 308 or 355 nm) range but may also be selected in the blue or green (for example 532 nm) range, or even the red (for example 633 nm) range. This step 104 of laser annealing will make it possible to react Co with Si so as to obtain a CoSi.sub.2 layer in the solid state during this step 104. It is essential to note that the energy density of the laser pulse or pulses applied is selected so that the Co, Si or any Co silicide (for example CoSi.sub.2 materials never turn to the liquid state and remain in the solid state. It is understood that the laser used by the method of the invention operates in step and repeat mode, making it possible to shoot a single laser pulse or a train of controlled laser pulses onto a zone without any risk of overlap.

[0050] As will be seen later, it is possible to obtain the expected result with a single laser pulse, but also with a plurality of laser pulses, the pulse duration and energy density having to be selected to obtain CoSi.sub.2 only in the solid state.

[0051] According to a first embodiment, during step 104 one or more laser pulses are applied at a constant energy density below the melting threshold of Co and all silicides CoSi.sub.x, especially CoSi.sub.2. This first embodiment is illustrated for a sample made from a 3 nm thick Co layer deposited onto an SOI substrate (with a 20 nm BOX layer and a Si layer above 13 nm). A 10 nm TiN layer is deposited onto the Co layer. Each laser pulse used has a wavelength of 308 nm and a pulse duration of 160 ns. FIG. 6 illustrates the result obtained by the method of the invention compared with the method of prior art with two RTA steps: for this purpose, FIG. 6 shows an image 11 of a first Si/CoSi.sub.2/TiN stack obtained by the method of prior art, an image 12 of a second Si/CoSi.sub.2/TiN stack obtained by the method according to the invention using a single laser pulse at an energy density of 0.7 J/cm.sup.2 and an image 13 of a third Si/CoSi.sub.2/TiN stack obtained by the method of the invention using 100 identical pulses at an energy density of 0.7 J/cm.sup.2. The Si/CoSi.sub.2 interface of image 11 is less planar than the interfaces obtained by the method according to the invention (images 12 and 13). The planar Si/CoSi.sub.2 interface according to the invention is especially advantageous for obtaining the Josephson effect in JoFET transistors.

[0052] FIG. 7 shows the course of the sheet resistance Rsheet as a function of the energy density applied and the number of pulses applied to the previously described stack (Co layer with a thickness of 3 nm deposited onto a 33 nm SOI substrate and a 10 nm TiN layer deposited onto the Co layer). Curve C1 illustrates the course of Rsheet as a function of energy density when applying 1 pulse across the stack, curve C2 when applying 10 pulses across the stack and curve C3 when applying 100 pulses across the stack.

[0053] Each of these curves C1, C2 and C3 passes through a minimum corresponding to a CoS.sub.2 phase having very good crystal quality (with a good interface with Si). As illustrated, when the number of pulses is increased, the energy density to reach the minimum of R.sub.s decreases: indeed, an energy density in the order of 775 mJ/cm.sup.2 is required to reach a minimum resistance Rs with a single pulse, whereas a density in the order of 725 mJ/cm.sup.2 is used to reach a minimum resistance with 10 pulses and the minimum resistance Rs is reached at a density of 700 mJ/cm.sup.2 for 100 pulses. This minimum resistance R.sub.s additionally decreases with the number of pulses: thus, it is observed that the minimum sheet resistance for 100 pulses is lower than the minimum sheet resistance for 10 pulses, which is itself lower than the minimum sheet resistance for 1 pulse. Lower sheet resistance results in less resistive CoSi.sub.2 contacts. Increasing the number of pulses at constant energy density additionally makes it possible to increase the critical superconductivity temperature Tc: this phenomenon is illustrated in FIG. 9, which shows on curve C4 the course of the critical temperature Tc as a function of the number of pulses: there is therefore a shift from a critical temperature in the order of 0.6K for 100 pulses to a critical temperature in the order of 0.9K for 300 pulses.

[0054] An alternative to increasing the number of pulses to reduce sheet resistance may be to increase the pulse duration. Insofar as the pulse shot frequency is about 4 Hz, it can be assumed that there will be no heat build-up between each shot and that the stack returns to room temperature between each shot. Thus, as a first approximation, a pulse lasting one microsecond is equivalent to ten pulses lasting 0.1 microsecond shot at 4 Hz.

[0055] Of course, it is suitable not to exceed a certain energy density coupled with the number of pulses and the pulse duration for the CoSi.sub.2 not to turn to the liquid phase. Various experiments have shown that the solid phase of CoSi.sub.2 is effectively formed for low Rs values (i.e. in the phase of decreasing Rs values observed on curves C1, C2 and C3) before reaching the liquid phase (i.e. melting of CoSi.sub.2) during the phase of rising Rs values as the energy density increases. Profile results obtained by EDS-TEM spectroscopy (Energy Dispersive SpectroscopyTransmission Electron Microscopy) have shown that the stoichiometry revealed in the samples obtained from laser annealing with 1 pulse, 10 pulses and 100 pulses is indeed that of CoSi.sub.2. Furthermore, the laser annealing machine is equipped with another laser, referred to as secondary laser, tilted at 45 to the heating laser, for in-situ characterisation, enabling the course of the surface reflectivity during the laser pulse to be monitored. It has been noticed that a sharp increase in the reflectivity of the material occurs for a pulse from 775-800 mJ/cm.sup.2, evidencing the transition to the liquid state. Additionally, this tool enables the laser strategy to be defined quickly without having to resort to more cumbersome characterisation techniques (such as TEM microscopy or X-ray diffractometry): it can especially be used to determine energy densities that must not be exceeded. In practice, the secondary laser sends a pulse before, during and after the annealing pulse sent by the primary laser: if a liquid phase is reached, a sudden change in reflectivity is observed. Finally, 1D simulations of the stack provided have been performed. FIG. 8 shows the temperature course within the stack for several energies. The maximum temperature recorded (in the order of 1220 C.) is below the melting temperature of all silicides CoSi.sub.x (in the order of 1350 C.). These simulations provide a non-destructive approach for anticipating energy densities to be applied to obtain CoSi.sub.2 in the solid state.

[0056] According to a second embodiment, during step 104 several laser pulses are applied, at least the first of which has a different and higher energy density than the subsequent ones, while remaining below the melting threshold of Co and all the silicides CoSi.sub.x, especially CoSi.sub.2. This second strategy therefore consists in adapting the energy density as silicidising progresses. Indeed, as mentioned above, it has been noticed that applying more pulses reduces the energy density required to form CoSi.sub.2 while reducing the sheet resistance R.sub.s. One solution may therefore be to apply a first pulse close to melting of CoSi.sub.2, and then one or more pulses at a slightly lower density to reduce the sheet resistance R.sub.s. The first pulse close to melting enables the cobalt to be partially transformed into CoSi.sub.2 in the solid phase over a given thickness. The energy density of the laser pulses, especially the first one, can be determined beforehand by tests made on samples using the secondary laser in order to achieve an energy density close to melting without reaching the liquid phase. From this new stack, one or more further laser pulses are made until the desired sheet resistance Rs is reached, by reducing the total number of pulses relative to the first embodiment (constant energy density) to achieve the same sheet resistance value Rs.

[0057] It will be noted that it is optionally possible, prior to the laser annealing step 104, to place the stack obtained at the end of step 103 on a hot platen at a predetermined temperature, for example 400 C., to wait for the stack to reach the predetermined temperature, and then to apply the laser pulse or pulses while the stack is on the hot platen at the predetermined temperature. The use of such a plate makes it possible, in particular, to lengthen the annealing time at the desired temperature.

[0058] The method 100 according to the invention can additionally include a step 105 illustrated in FIG. 10 consisting in applying a rapid thermal annealing of the RTA type as an extension of the pulsed laser annealing. This rapid RTA annealing is made, for example, by conduction using heating blocks in proximity to the stack, it being understood that other types of RTA could be implemented (microwave or UV source, for example). The addition of this RTA step makes it possible to significantly improve the critical superconductivity temperature Tc. As illustrated in FIG. 9 showing the course C5 of the critical temperature Tc as a function of the number of pulses in the presence of RTA annealing at 850 C. for 1 sec, it is possible to reach a superconductivity critical temperature of 1.3 K, i.e. a gain ranging from 44% to 100% relative to critical temperatures of 0.6 to 0.9 K of the curve C4 without RTA.

[0059] It will be noted that obtaining satisfactory superconductivity for the cobalt silicide obtained by the method of the invention is greatly improved by starting with single crystal silicon and a thin cobalt layer (i.e. lower than or equal to 10 nm, or even strictly lower than 5 nm) as well as by virtue of the finishing RTA step (i.e. after the laser annealing step).

[0060] Of course, once the CoSi.sub.2 layer has been made, the method of the invention can be continued with removing the TiN oxidation protective layer.