METHOD FOR FORMING OHMIC CONTACTS, PARTICULARLY OF Ni(GeSn) TYPE IMPLEMENTING LASER ANNEALING

Abstract

The invention relates to a method for producing ohmic contacts, of type including a metal, a semiconductor and tin, including: a) forming a first layer (6), of an alloy of the semiconductor and of tin; b) then, on the first layer, forming a second layer (8), of said metal; c) laser annealing the first layer and the second layer at an energy density between 0.1 and 2 J/cm.sup.2.

Claims

1. Method for producing ohmic contacts, comprising a metal, a semiconductor and tin, including: a) forming a first layer, of an alloy of the semiconductor and of tin; b) then, on the first layer, forming a second layer, of said metal; c) laser annealing at least the first layer at an energy density between 0.1 and 2 J/cm.sup.2

2. Method according to claim 1, the semiconductor being silicon or germanium.

3. Method according to claim 1, the metal being selected from nickel (Ni), cobalt (Co), titanium (Ti), platinum (Pt), nickel-platinum (NiPt) alloy, nickel-cobalt (NiCo) alloy, tantalum (Ta), tantalum nitride (TaN), titanium nitride (TiN), aluminium (Al), zirconium (Zr), ytterbium (Yb).

4. Method according to claim 1, the energy density of the laser being between 0.44 J/cm.sup.2 and 0.7 J/cm.sup.2.

5. Method according to claim 1, further including, after step c), a step of rapid thermal annealing, preferably at low temperature, for example between 150° C. and 250° C.

6. Method according to claim 1, including, between steps a) and b), a step of amorphising the first layer.

7. Method according to claim 6, the amorphisation step being obtained by implantation of one or more atomic species, for example carbon (C), and/or germanium (Ge) and/or silicon (Si).

8. Method according to claim 1, further including, after step b) and before step c), forming a protective metal layer.

9. Method according to claim 8, the protective metal layer being made of titanium nitride (TiN) or tantalum nitride (TaN).

10. Method according to claim 1, the first layer, made of an alloy of the semiconductor and of tin, being obtained by epitaxial growth on a substrate of said semiconductor.

11. Method according to claim 1, the laser annealing of the first layer having an energy density between 0.1 and 0.8 J/cm.sup.2.

12. Method according to claim 1, step c) of laser annealing being performed with the aid of a pulse laser, in one or more pulses.

13. Method according to claim 12, the pulse laser having a wavelength lower than 450 nm and/or a pulse width at mid-height between 10 ns and 300 ns.

14. Method according to claim 1, further including a step of doping the first layer, and optionally the substrate of said semiconductor.

15. Method according to claim 1, including a preliminary step of determining or estimating the melting limit of the alloy of the semiconductor and of tin.

16. Method according to claim 1, the first layer, of an alloy of the semiconductor and of tin, and optionally said semiconductor substrate being produced on a silicon substrate.

17. Method according to claim 16, said silicon substrate forming part of an optical component or of a CMOS device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Examples of embodiment of the invention will now be described with reference to the appended drawings wherein:

[0025] FIGS. 1A and 1B show an example of method according to the invention.

[0026] FIG. 2 shows the results of a simulation of a laser treatment of a silicon/germanium/nickel/titanium nitride (Si/Ge/Ni/TiN) stack.

[0027] FIGS. 3A and 3B show the evolution of the sheet resistance Rsh for a Ni(GeSn) sample treated by conventional RTA type annealing (FIG. 3A) and by laser annealing (FIG. 3B).

[0028] FIGS. 4A and 4B show diffraction measurements for a Ni/GeSn system treated at various temperatures by conventional RTA type annealing (FIG. 4A) and at various energy densities by laser annealing (FIG. 4B).

[0029] FIGS. 5A and 5B show the morphological evolution of the Ni/GeSn system depending on the evolution of the temperature for a contact obtained by conventional RTA type annealing (FIG. 5A) and for a contact obtained by laser annealing (FIG. 5B).

[0030] FIGS. 6A and 6C show a variant of a method according to the invention, including laser annealing followed by a RTA treatment.

[0031] FIG. 6D shows the evolution of the Rsh resistance for a Ni(GeSn) sample treated by laser annealing, without and with post RTA type annealing at low temperature.

[0032] FIGS. 7A-7C show another variant of a method according to the invention, including pre-amorphisation by implantation before metallisation.

[0033] FIGS. 8A-8D also show another variant of a method according to the invention, including pre-amorphisation by implantation before metallisation and RTA type annealing at low temperature post laser annealing;

[0034] In the figures, similar or identical elements are designated by the same reference numbers.

DETAILED DESCRIPTION OF EMBODIMENTS

[0035] One example of embodiment of a method according to the invention is shown in FIGS. 1A and 1B, for producing a Ni(GeSn) ohmic contact.

[0036] A stack comprising a germanium-tin (GeSn) alloy layer 6 and a nickel (Ni) layer 8 is first produced on a substrate 2, for example made of silicon (FIG. 1A) this substrate 2 able to be for example made of solid silicon or a SOI (Silicon on Insulator) and able to form part of the component on which such a contact is produced.

[0037] The germanium-tin (GeSn) alloy layer 6, of thickness for example between 10 nm and 1 μm, may be produced by epitaxy on a germanium layer 4 (of thickness of a plurality of μm, for example between 2 μm and 10 μm, for example also 2.5 μm), itself supported by the silicon substrate 2. The tin concentration in the germanium is for example approximately 10%, more generally between 2% and 20% or also between 8% and 15% (particularly for optical applications, such as a GeSn laser).

[0038] The layer 6 and the layer 4 may be doped, for example by boron or phosphorus, for example also at a dose between 10.sup.18 and 10.sup.20 at/cm.sup.3, for example 10.sup.19 at/cm.sup.3.

[0039] The nickel (Ni) layer 8 may have a thickness of a plurality of nm or of a plurality of tens of nm, for example between 5 nm and 50 nm, for example also of 10 nm.

[0040] The assembly is topped with a metal layer 10, for example made of titanium nitride (TiN) or tantalum nitride (TaN). This metal layer 10 has a thickness of a plurality of nm or of a plurality of tens of nm, for example between 5 nm and 50 nm, for example 7 nm. This layer is a protective layer, but it also has the function of absorbing most of the laser beam and the heat deposited by the laser, heat that subsequently diffuses towards the stack, and particularly towards the nickel 8 and germanium-tin (GeSn) layers 6.

[0041] This assembly is subjected to annealing with the aid of a laser beam 12 (FIG. 1B), which makes it possible to reach the melting temperature of the alloy concerned, here GeSn. It may be a pulse laser. The energy density deposited is between 0.4 J/cm.sup.2 and 0.7 J/cm.sup.2, preferably between 0.48 and 0.66 J/cm.sup.2. This energy density is measured or estimated in the plane of the upper surface of the layer 10. For example, it may be measured or estimated by deflecting (at each pulse in the case of a pulse laser) a portion of the beam towards a photodiode.

[0042] The laser is for example a XeCl laser (308 nm), having pulses of 160 ns (FWHM), having an energy density between 0.08 and 0.66 J/cm.sup.2.

[0043] A single pulse may be implemented to deposit the surface energy density selected; alternatively, a plurality of pulses may be used, to deposit, in total, the same surface energy density as a single pulse or a surface energy density making it possible to obtain results identical or comparable to those obtained with a single pulse. Alternatively, it is possible to use a continuous laser, which irradiates the surface of the metal layer 10 for a duration making it possible to deposit the desired energy density.

[0044] The result of the annealing is the structure presented in FIG. 1B, including a nickel, germanium and tin (Ni(GeSn)) layer 8′ on top of the germanium-tin (GeSn) layer 6′, the latter being of same composition as the initial layer 6, but of lower thickness since a portion of the latter has been consumed.

[0045] According to a particular example of embodiment: [0046] the germanium-tin (GeSn) alloy layer 6 is produced by epitaxy on a germanium (Ge) layer 4, itself produced on a silicon (100) substrate 2, for example the epitaxy of the germanium-tin (GeSn) alloy layer 6 is produced in a RPCVD (Reduced Pressure Chemical Vapour Deposition) apparatus or in a MBE (Molecular Beam Epitaxy) type apparatus; [0047] then a step of cleaning and of deoxidising may be performed; for example, this layer is quenched for 1 minute in a bath of HF (hydrofluoric acid) diluted at 1% in water; it is subsequently rinsed with the aid of deionised water and dried with a nitrogen gun; [0048] a step of doping the layer 6 and the layer 4 may be performed, for example by boron or phosphorus, for example also at a dose between 10.sup.18-10.sup.20 at/cm.sup.3, for example 10.sup.19 at/cm.sup.3; [0049] a nickel (Ni) layer 8 is subsequently deposited, followed by an upper layer 10 of a protective material, preferably a metal, such as titanium nitride (TiN) or tantalum nitride (TaN) (of thickness for example of approximately 7 nm), the whole at ambient temperature, by PVD (Physical Vapour Deposition) or EBE (Electron Beam Evaporation) technique; [0050] for example a XeCl laser (308 nm) is used, having pulses of 160 ns (FWHM), with a deposited energy density between 0.08 and 0.66 J/cm.sup.2.

[0051] The properties of such a TiN/Ni/GeSn/Ge/Si stack subjected to laser treatment are studied below. Comparisons are made with a stack subjected to a conventional RTA method.

[0052] The melting limit of the semiconductor-tin (GeSn in the example given above) alloy, that it is desired to achieve within the scope of the method according to the invention, may not be known beforehand. This is the case, for example, for a new stack (particularly for a new material and/or a new thickness of one of the layers of the latter). Therefore, a determination or an estimation of this melting limit (or temperature), and of the energy density that makes it possible to reach this limit, may be performed beforehand, for example by digital simulations. The result obtained by simulation may be experimentally refined.

[0053] A method according to the invention may therefore include a preliminary step of determining or estimating the melting limit of the alloy of the semiconductor and of tin (that it is desired to reach by laser annealing), and the corresponding energy density.

[0054] Here it is given an example of digital simulation performed in order to evaluate the melting limit, for a stack including: [0055] a silicon (Si) substrate of thickness 750 μm; [0056] a germanium (Ge) layer of thickness 2.5 μm; [0057] a nickel (Ni) layer of thickness 10 nm; [0058] a titanium nitride (TiN) layer of thickness 7 nm.

[0059] Within the scope of this simulation, it is considered that this stack is subjected to laser annealing, for energy densities between 0.1 J/cm.sup.2 and 0.8 J/cm.sup.2 (the arrow 15 of FIG. 2 indicates the increasing direction of the energy density). These simulations indicate melting at around 0.4-0.45 J/cm.sup.2, limit at which the curves (see FIG. 2) present a plateau. At these values, it is substantially reached the melting temperature of germanium, and contacts may be obtained, the energy of the system being sufficient to make possible the diffusion of tin atoms and the growth of the intermetallic phase. According to these simulations, the energy density range that is of interest is therefore, in this example, substantially between 0.4 J/cm.sup.2 and 0.6 J/cm.sup.2, which may be experimentally refined. This type of simulation, presented above in the case of a germanium (Ge) layer, may be transposed to the germanium-tin (GeSn) layer; or even, the results above may be experimentally refined for an application to the germanium-tin (GeSn) alloy.

[0060] Laser annealing makes it possible, from a stack such as that of FIG. 1A, to produce Ni(GeSn) intermetallic components having very good electrical properties in order to form an ohmic contact whilst maintaining a very good thermal stability and by limiting, or even by eliminating, the segregation phenomenon of tin (Sn).

[0061] FIGS. 3A and 3B show the evolution of the sheet resistance Rsh for the Ni/GeSn system treated, on the one hand, by conventional RTA type annealing (FIG. 3A) and, on the other hand, by laser annealing (FIG. 3B).

[0062] It is noted that the evolution of this electrical parameter is substantially the same for the two samples: [0063] the sheet resistance value first remains stable, at approximately 28-30Ω/□, until a slight increase due to the growth of the nickel-rich phase, at approximately 250° C. for the samples treated by RTA (FIG. 3A) and at approximately 0.34 J/cm.sup.2 for the samples treated by laser (FIG. 3B); [0064] subsequently, the sheet resistance reduces to 350° C. for the examples treated by RTA (FIG. 3A) and to approximately 0.5 J/cm.sup.2 for the samples treated by laser (FIG. 3B); the reduction of the sheet resistance may be related to the growth of the Ni(GeSn) phase; [0065] finally, the sheet resistance increases from approximately 350° C. for the samples treated by RTA (FIG. 3A) and from approximately 0.54 J/cm.sup.2 for the samples treated by laser (FIG. 3B); the increase of the sheet resistance for the samples treated by RTA (FIG. 3A) may be related to the segregation of tin and the agglomeration of the Ni(GeSn) layer, for the samples treated by laser (FIG. 3B) this growth may be related to the degradation of the TiN layer. It should be noted that the value of the resistance reached is not the same for the samples treated by RTA and for the samples treated by laser.

[0066] The increase of the sheet resistance is greater for the samples treated by RTA than by laser: in other words, the RTA treatment loses a portion of the conduction properties of the sample that are advantageous for forming an electrical contact.

[0067] Concerning the laser treatment (FIG. 3B), an energy density greater than 0.7 J/cm.sup.2, in this example, degrades the stack; this is the reason for which the results presented stop at a value slightly lower than this limit. The results presented above relate to a particular example of stack, also the values, ranges and limit(s) indicated may vary for another example of stack.

[0068] The evolution illustrated in FIG. 3A is related to the segregation of Sn and its surface distribution, these 2 aspects considerably degrading the electrical properties. Laser annealing makes it possible to form a Ni(GeSn) intermetallic compound without segregation of Sn, which explains the more favourable evolution of the electrical properties (FIG. 3B). Laser annealing also makes it possible to obtain a low resistive contact on a broader range, in terms of energy density, than the RTA treatment, for which there is a single point of operation (above which tin undergoes a segregation).

[0069] This is confirmed by FIGS. 4A and 4B, which show analysis results in X diffraction (XRD), of type .sup.«θ-2θ», between 20° and 90°, by using a copper source in an X-ray diffractometer (of X'Pert Pro PANalytical type). This analysis makes it possible to monitor the crystalline evolution and the formation of phases in the sample. FIGS. 4A and 4B show the evolution of the intensity of the diffraction (along y-axis, arbitrary units) for a Ni/GeSn system treated by conventional RTA type annealing (FIG. 4A) and by laser annealing (FIG. 4B, on which the energy density, in J/cm.sup.2 is indicated for each curve, from 0.08 J/cm.sup.2 to 0.66 J/cm.sup.2).

[0070] Therefore, it is seen in these Figures, the evolution of the phases of the Ni/GeSn system in the two treatments. This evolution seems similar, since it is first seen (at low temperature in FIG. 4A and at low energy density in FIG. 4B) the presence of a Ni phase and, fairly rapidly that of a Ni.sub.S(GeSn).sub.3 nickel-rich phase. The latter is subsequently consumed during the growth of the Ni(GeSn) phase.

[0071] But, for the samples treated by RTA (FIG. 4A), the diffraction peaks related to the β-Sn phase and to the segregation and to the agglomeration of Sn are also observed from 350° C. This peak type is not observed for the samples treated by laser, even at the highest energy density tested (0.66 J/cm.sup.2). This segregation of Sn in the case of a RTA treatment poses a problem for producing a quality ohmic contact, not disrupting the operation of the device—particularly if it is of optical type—to which the contact is applied.

[0072] In addition to the agglomeration phenomenon, effects of slotting by grain joints considerably degrade the electrical properties due to the loss of continuity of the layers. Images (5 μm×5 μm) produced by Atomic Force Microscope (AFM), in Tapping mode, with Bruker FastScan type equipment, have made it possible to study the morphological evolution of the surface of the layer 10: [0073] depending on the evolution of the temperature (from 250° C. to 550° C.) for conventional RTA type annealing: FIG. 5A shows this evolution (and that of the roughness, RMS) for 6 samples, one sample of which as deposited (As-dep) and 5 samples, each treated at a temperature (mentioned in FIG. 5A) between 250° C. and 550° C.; [0074] and depending on the energy density (from 0.08 to 0.42 J/cm.sup.2) of the pulse laser for laser irradiation annealing: FIG. 5B shows this evolution (and that of the roughness, RMS) for 6 samples, each treated with a single laser pulse, of energy density (mentioned in FIG. 5B) between 0.08 J/cm.sup.2 and 0.42 J/cm.sup.2, for a concentration of 10% of Sn. Therefore, it is seen in these Figures, the evolution of the phases of the Ni/GeSn system in the two treatments.

[0075] Regardless of the system studied, at low temperature (T<300° C.) for the samples treated by RTA, and at low energy density (<0.32 J/cm.sup.2) for the samples treated by laser, a cross hatch type pattern of crystallographic orientation <110> is observed. This pattern results from the propagation of dislocations (along the planes (111)). The morphology of the surface is then directly related to the epitaxy of the GeSn layers.

[0076] This means that, for the sample as deposited (FIG. 5A, As-dep), the layers based on nickel (Ni) and of titanium nitride (TiN) construction do not modify the morphology of the original surface.

[0077] For the systems treated only by RTA (FIG. 5A), this morphology is preserved up to 300° C.; the formation of the nickel-rich phase does not modify the original morphology. As seen in FIG. 5A, the morphology of the surface changes radically at the temperature of 350° C., at which a high increase of the roughness is noted. These modifications are related to the formation of the Ni(GeSn) phase and to its agglomeration.

[0078] At temperature higher than 350° C., the roughness gradually increases due to the amplification of two phenomena: the agglomeration of the Ni(GeSn) phase and the segregation of tin (Sn) (highlighted in FIG. 4A). The high increase of the resistance R.sub.sh also results from these two phenomena.

[0079] For the systems treated by laser (FIG. 5B), the cross hatch type pattern is preserved at low energy density. Beyond 0.34 J/cm.sup.2, a modification of the surface morphology is noted: the roughness also increases, because the melting temperature of GeSn has been passed; in addition, folds are observed on this surface, which is due to stress variations in the TiN coating layer (by relaxation of this layer) during the melting of the underlying Ni/GeSn layer. The amplitude of the folds depends on the depth of the melted material: the latter increasing with the energy density, the amplitude of the folds also increases. It should be noted that, in the case of systems treated by laser, these folds generate a significant roughness, but that this does not impact the electrical properties; in addition, the diffraction measurements (FIG. 4B) show that there is no Sn segregation. The tip used to produce the images by AFM interacts firstly with the titanium nitride (TiN) layer, which is significantly modified due to the relaxation of the stresses (already mentioned above). But the Rsh values are not high: the conductivity is therefore good and the Ni(GeSn) layer is probably still continuous, even though the roughness of the titanium nitride (TiN) layer is considerable.

[0080] Furthermore, no trace of segregation of tin (Sn) is observed (surface islands have only been observed for the samples treated by RTA).

[0081] Consequently, laser annealing makes it possible to obtain a Ni(GeSn) phase, of low resistivity, making it possible to produce a contact on GeSn-based devices, with a reduced thermal budget nevertheless compatible with the formation of this phase. Very good electrical properties are obtained, without segregation of tin Sn at the surface. Given the results of the various examples discussed above (FIGS. 3A-5B), the formation of a very good ohmic contact is, for these examples, obtained for an energy density range deposited by the laser between 0.44 and 0.66 J/cm.sup.2.

[0082] A method according to the invention may be followed by annealing at low temperature, which makes it possible to improve the electrical properties of the ohmic contact obtained by laser annealing, without degrading the surface morphology thereof. Such a method is illustrated schematically in FIGS. 6A-6C. It includes, in this order:

a) epitaxially growing GeSn on a germanium substrate;
b) optionally surface treating with HF;
c) optionally doping the GeSn layer 6 and the germanium substrate 4;
d) depositing nickel (Ni) and titanium nitride (TiN) layers, to form the stack of FIG. 6A;
e) laser annealing (FIG. 6B);
f) rapid annealing at low temperature (FIG. 6C), for example between 150° C. and 250° C.
Steps a)-e) of this method are those already described above.
Step f) of annealing at low temperature makes it possible to reduce the resistivity of the contact obtained and to use a lower surface energy density, close to 0.44 J/cm.sup.2. Thus, FIG. 6D shows the evolution of the resistance Rsh for the Ni/GeSn system treated by laser annealing without (curve I.sub.6) and with (curve II.sub.6) post low temperature (200° C.) RTA annealing. It is seen in this Figure that the minimum of the curve II.sub.6 is lower than that of the curve I.sub.6; in addition, it appears at a lower energy density (approximately 0.44 J/cm.sup.2 instead of 0.5 J/cm.sup.2)

[0083] According to another aspect, pre-amorphisation by implantation of the GeSn layer is performed during the preparation of a stack in view of the implementation of a method according to the invention, during which it is estimated that the melting temperature of GeSn is probably reached (with amorphised Ge). This pre-amorphisation makes it possible to improve the thermal stability of the stack; it uses atomic species such as silicon (Si), germanium (Ge), carbon (C), alone or in combination. The implantation of atomic species of smaller size, such as carbon, which can be better inserted into the structure of Ni.sub.x(GeSn).sub.y, alloys and, thus, better block the diffusion and the segregation of tin (Sn) is preferred.

[0084] Then, after this pre-amorphisation, the metal layer 8 and the protective layer 10 are formed. Finally, the stack is treated by laser, in accordance with the invention.

[0085] Such a method is illustrated schematically in FIGS. 7A-7C. It includes:

a) —epitaxially growing GeSn 6 on a germanium substrate 4;
b) optionally surface treating with HF;
c) optionally doping the GeSn layer 6 and the germanium substrate 4;
d) pre-amorphisation by implantation 14 (FIG. 7A), the GeSn layer being transformed into an α-GeSn amorphous layer 6″;
e) depositing nickel (Ni) 8 and titanium nitride (TiN) 10 layers, to form the stack of FIG. 7B;
f) laser annealing 12 (FIG. 7C) in accordance with the present invention.
Steps a)-c), e), f) of this method are those already described above.

[0086] Alternatively, it is possible to perform: [0087] pre-amorphisation by implantation of the GeSn layer, as described above, in relation to FIGS. 7A-7C, by using species such as silicon (Si), germanium (Ge), carbon (C), alone or in combination; [0088] then, after this pre-amorphisation, the metal layer 8 and the protective layer 10 are formed; [0089] then, the stack is treated by laser, in accordance with the invention; [0090] this annealing is followed here by a RTA type treatment at low temperature, as described above, in relation to FIGS. 6A-6C.

[0091] Thus, the advantageous effects of the 2 treatments already disclosed above are accumulated (FIGS. 6A-6C, FIGS. 7A-7C).

[0092] Such a method is illustrated schematically in FIGS. 8A-8D. It includes:

a) —epitaxially growing GeSn 6 on a germanium substrate 4;
b) optionally surface treating with HF;
c) optionally doping the GeSn layer 6 and the germanium substrate 4;
d) pre-amorphisation by implantation 14 (FIG. 8A), the GeSn layer being transformed into an α-GeSn amorphous layer 6″;
e) depositing Ni 8 and TiN layers 10 (FIG. 8B);
f) laser annealing (FIG. 8C);
g) thermal annealing at low temperature (FIG. 8D).
These steps a)-g) are those already described above.

[0093] The invention has been described for producing a Ni(GeSn) contact, but other ohmic contacts, of type including a metal, a semiconductor and tin, may also be produced, the semiconductor material being for example silicon or germanium, whereas the metal is selected from nickel (Ni), cobalt (Co), titanium (Ti), platinum (Pt), nickel-platinum (NiPt) alloy, nickel-cobalt (NiCo) alloy, tantalum (Ta), tantalum nitride (TaN), titanium nitride (TiN), aluminium (Al), zirconium (Zr), ytterbium (Yb). For each type of contact, it can be determined the melting limit (or temperature) (see the method explained above), and/or the limits of the laser surface energy density range to be applied in order to obtain the desired properties. Analyses such as those presented above for producing a Ni(GeSn) contact may be performed in view of identifying the segregation, or not, of tin (Sn). The ohmic contacts obtained by the invention are adapted: [0094] to the production of optical components (for example for laser applications, or applications with sensors in the infrared spectrum); [0095] to the production of Source-drain type contacts of Ge(Sn) channel CMOS devices.

[0096] A contact obtained by a method according to the invention with a structure such as that illustrated in FIG. 1B, 6C, 7D or 8D, the silicon layer 2 forming part of the component on which the contact is produced.