Deposition Method

Abstract

Pulsed DC reactive sputtering of a target deposits an additive-containing aluminium nitride film onto a metallic layer of a semiconductor substrate. The additive-containing aluminium nitride film contains an additive element selected from scandium, yttrium, titanium, chromium, magnesium and hafnium. Depositing the additive-containing aluminium nitride film includes introducing a gaseous mixture comprising nitrogen gas and an inert gas into the chamber at a flow rate, in which the flow rate of the gaseous mixture comprises a nitrogen gas flow rate, and in which the nitrogen gas flow rate is less than or equal to about 50% of the flow rate of the gaseous mixture and also is sufficient to fully poison the target.

Claims

1. A method of sputter depositing an additive-containing aluminium nitride film from a target, the method comprising the steps of: providing a semiconductor substrate having a metallic layer thereon in a chamber; and depositing the additive-containing aluminium nitride film onto the metallic layer by pulsed DC reactive sputtering of the target, wherein the additive-containing aluminium nitride film includes an additive element, and wherein the additive element includes at least one of scandium (Sc), yttrium (Y), titanium (Ti), chromium (Cr), magnesium (Mg) or hafnium (Hf); wherein the step of depositing the additive-containing aluminium nitride film comprises introducing a gaseous mixture comprising nitrogen gas and an inert gas into the chamber at a flow rate in sccm, in which the flow rate of the gaseous mixture in sccm comprises a nitrogen gas flow rate in sccm, and in which the nitrogen gas flow rate in sccm is less than or equal to about 50% of the flow rate of the gaseous mixture in sccm and also is sufficient to fully poison the target.

2. The method according to claim 1, wherein the additive element is scandium or yttrium.

3. The method according to claim 2, wherein the additive element is scandium.

4. The method according to claim 1, wherein the additive element, as an atomic percentage of aluminium and the additive element content of the additive-containing aluminium nitride film, is present in an amount in a range of 0.5 at. % to 40 at %.

5. The method according to claim 4, wherein the range is 20 at. % to 25 at. %.

6. The method according to claim 1, wherein the nitrogen gas flow rate in sccm is less than about 45% of the flow rate of the gaseous mixture in sccm.

7. The method according to claim 6, wherein the nitrogen gas flow rate in sccm is less than about 35% of the flow rate of the gaseous mixture in sccm.

8. The method according to claim 1, wherein the nitrogen gas flow rate in sccm is greater than about 15% of the flow rate of the gaseous mixture in sccm.

9. The method according to claim 8, wherein the nitrogen gas flow rate in sccm is ≥30%, of the flow rate of the gaseous mixture in sccm

10. The method according to claim 1, wherein the nitrogen gas flow rate in sccm used during the step of depositing the additive-containing aluminium nitride film is in a range of 25 to 250 sccm.

11. The method according to claim 10, wherein the range is 45 to 100 sccm.

12. The method according to claim 1, wherein the inert gas is argon, krypton, xenon or a mixture thereof.

13. The method according to claim 1, wherein the additive-containing aluminium nitride film has a thickness of about 2 μm or less.

14. The method according to claim 1, wherein the additive-containing aluminium nitride film has a thickness of about 0.2 μm or greater.

15. The method according to claim 1, wherein the step of depositing the additive-containing aluminium nitride film comprises applying an electrical bias power to the semiconductor substrate.

16. The method according to claim 1, wherein the semiconductor substrate is a silicon wafer.

17. The method according to claim 1, wherein the metallic layer is selected from: tungsten (W), molybdenum (Mo), aluminium (Al), platinum (Pt), and ruthenium (Ru).

18. The method according to claim 1, further comprising a step of etching the metallic layer prior to the step of depositing the additive-containing aluminium nitride film.

19. The method according to claim 1, further comprising the step of depositing a seed layer onto the metallic layer prior to the step of depositing the additive-containing aluminium nitride film so that the additive-containing aluminium nitride film is deposited onto the seed layer.

20. An additive-containing aluminium nitride film on a metallic layer of a semiconductor substrate produced using the method according to claim 1.

Description

DESCRIPTION OF THE DRAWINGS

[0038] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0039] FIG. 1 is an SEM image of an Al.sub.100-xSc.sub.xN film deposited on a silicon substrate (a) without a pre-etch; and (b) with a pre-etch;

[0040] FIG. 2 is an SEM image of an Al.sub.100-xSc.sub.xN film deposited on a silicon substrate (a) without a seed layer; (b) with a compressive AlScN seed layer; and (c) with a AlN seed layer;

[0041] FIG. 3 is an SEM image of an Al.sub.100-xSc.sub.xN film deposited where the percentage of nitrogen gas in the gaseous mixture was 100%;

[0042] FIG. 4 is an SEM image of an Al.sub.100-xSc.sub.xN film deposited where the percentage of nitrogen gas in the gaseous mixture was 83%;

[0043] FIG. 5 is an SEM image of an Al.sub.100-xSc.sub.xN film deposited where the percentage of nitrogen gas in the gaseous mixture was 67%;

[0044] FIG. 6 is an SEM image of an Al.sub.100-xSc.sub.xN film deposited where the percentage of nitrogen gas in the gaseous mixture was 36%; and

[0045] FIG. 7 shows plots of target voltage (at constant power) and defect density (per 100 μm.sup.2) as a function of percentage nitrogen gas in the gaseous mixture.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0046] For the avoidance of doubt, when reference is made to ‘open-ended’ terms such as ‘comprising’, ‘comprise’ and ‘comprises’, the invention is understood to relate also to embodiments in which the open-ended terms are replaced by ‘closed’ terms such as ‘consisting of’ and ‘consisting essentially of’.

[0047] The inventors have discovered an advantageous process for sputter depositing an additive-containing aluminium nitride film (e.g. Al.sub.100-xX.sub.xN) onto a metallic layer of a substrate. The method can help to reduce crystallite defects in the additive-containing aluminium nitride film. The methods of the present invention have particular application when the additive element (X) is present at high concentrations (e.g. x>12, preferably x≥15). The additive-containing aluminium nitride film contains an additive element. The additive element can be scandium (Sc), yttrium (Y), titanium (Ti), chromium (Cr), magnesium (Mg), or hafnium (Hf). The results presented below are in relation to aluminium scandium nitride (Al.sub.100-xSc.sub.xN) films. However, the method can be readily applied to any of the additive elements mentioned above, i.e. Sc, Y, Ti, Cr, Mg and Hf. For example, the method is also applicable to deposit aluminium yttrium nitride (Al.sub.100-xY.sub.xN) films.

[0048] General details concerning apparatus which can be used or readily adapted for use in the present invention are described in the applicant's European Patent applications EP2871259, EP3153603, the entire contents of which are hereby incorporated by reference.

[0049] A SPTS Sigma fxP AlN PVD tool, which is commercially available from SPTS Technologies Limited of Newport, South Wales, UK, was used to deposit the additive-containing aluminium nitride films of the following examples. The apparatus comprises a substrate support disposed in a chamber. During a deposition process, a semiconductor substrate having a metallic layer thereon is positioned on the substrate support. For example, the substrate can be a silicon wafer with a metallic coating thereon. The metallic layer has a metallic surface. The metallic layer can be an electrode structure, for example an electrode of a piezoelectric device, such as a bulk acoustic wave (BAW) device. By way of example only, the metallic layer can be made from tungsten (W), molybdenum (Mo), aluminium (Al), platinum (Pt) or ruthenium (Ru).

[0050] The apparatus further comprises a target disposed within the chamber. The target is a composite target formed from aluminium and the additive element. The use of multiple targets is possible but is likely to be less economically attractive. Pulsed DC sputtering comprises applying pulses of DC power to the target (cathode) during the deposition process.

[0051] The method includes introducing a gaseous mixture into the chamber at a flow rate, and subsequently using pulsed DC reactive sputtering to reactively sputter deposit material from the target onto the metallic surface of the metallic layer.

[0052] The gaseous mixture comprises nitrogen gas (N.sub.2) and an inert gas. The inert gas can be argon, krypton or xenon, or any mixture thereof. Typically, the inert gas is argon.

[0053] The inert gas acts as a co-sputtering non-reactive species and can improve plasma stability.

[0054] The proportion of N.sub.2 gas in the gaseous mixture can influence the deposition process. Without wishing to be bound by any theory or conjecture, it is believed that at a very low N.sub.2 gas concentration (or in the absence of N.sub.2 gas), the pulsed DC sputter process will run in a “metallic mode”. In the metallic mode, the target material will be sputtered with little or no reaction with N.sub.2 gas. As the proportion of N.sub.2 gas increases, some nitrogen is incorporated into the surface of the target, and the target begins to be poisoned. When the proportion of nitrogen in the gas mixture is sufficiently high, a nitride layer will form on the target, the target will be fully poisoned and the sputter process will operate in a “poisoned mode” (also referred to as a “compound mode” or “reactive mode”). This occurs when the surface of the target is fully converted to a nitride, and the target is fully poisoned.

[0055] The transition between the “metallic mode” and the “poisoned mode” occurs at a “transition point” (or “poison threshold percentage” of nitrogen gas), which corresponds to the point at which the proportion of nitrogen gas in the gas mixture is sufficiently high to fully poison the target (i.e. to cause the sputter deposition to operate in the “poisoned mode”). The poison threshold percentage for a particular set of process conditions can be determined by measuring the target voltage (at constant target power) as a function of nitrogen gas percentage, as illustrated in FIG. 7 (as discussed in more detail below).

[0056] The overwhelming received wisdom in the art is to operate pulsed DC reactive sputtering processes in a “poisoned mode” far from the transition point (i.e. using a very high percentage of N.sub.2 gas in the reactive gas mixture). For example, in one known process, the ratio of flow rates of N.sub.2 gas to Ar gas (in sccm) is 50:10 (i.e. 83% N.sub.2 gas).

[0057] However, contrary to this received wisdom in the art, the present inventors have unexpectedly found that using a N.sub.2 flow that is ≤50%, of the total flow rate of the gaseous mixture (whilst still operating in the “poisoned mode”) can reduce the defect density of an additive-containing aluminium nitride film that is deposited onto a metallic surface by pulsed DC reactive sputtering. These effects are particularly beneficial at high concentrations of additive element (e.g. >12 at. %, preferably ≥15 at. %), where defects are typically more prevalent.

[0058] In one example, Al.sub.100-xSc.sub.xN films (x=30) were deposited by pulsed DC reactive sputtering onto a textured tungsten (W) underlayer. The gaseous mixture comprised N.sub.2 gas and argon. The proportion of N.sub.2 gas in the gaseous mixture was varied whilst all other process parameters, such as chamber pressure and target power, were kept constant. Exemplary process parameters used in this example are shown in Table 1.

TABLE-US-00001 TABLE 1 Al.sub.100-xSc.sub.xN Pulse DC power (kW)  6 Pulse frequency (kHz) 100 Pulse width (ps)  4 Ar flow rate (sccm)  0-90 N2 flow rate (sccm)  50 Platen RF bias power (W) Adjusted for required stress Platen temperature (° C.) 200

[0059] FIGS. 3, 4, 5, and 6 show SEM images of Al.sub.70Sc.sub.30N films deposited where the percentage of N.sub.2 gas in the gaseous mixture was 100%, 83%, 67% and 36% respectively. The results are summarised in Table 2 below, and are shown graphically in FIG. 7.

TABLE-US-00002 TABLE 2 No. Defects per N2 % of total flow 100 μm.sup.2 002 FWHM (°) 100 >>100 1.58  83    >50 1.62  67    <30 1.57  36    <20 1.63

[0060] FIG. 7 includes a plot of target voltage as a function of the percentage N.sub.2 gas in the total gas flow. The transition point (i.e. poison threshold percentage) 72 occurred when the N.sub.2 gas flow accounted for about 25% of the total gas flow rate. The transition point 72 can be determined due to a steep change in the target voltage (at constant target power). In this example, when the N.sub.2 gas flow accounted for less than about 25% of the total gas flow, the apparatus was operating in a “metallic mode” 70. When the N.sub.2 gas flow accounted for more than about 25% of the total gas flow, the target was fully poisoned and the apparatus was operating in a “poisoned mode” 74.

[0061] Changing the amount of N.sub.2 gas (as a percentage of the total gas flow, in sccm) did not significantly affect the film texture. All Al.sub.100-xSc.sub.xN films exhibited good crystal orientation. However, unexpectedly, the defect density (i.e. no. of defects per 100 μm.sup.2 as determined by analysing SEM images of the film) decreased as the amount of N.sub.2 gas (as a percentage of the total gas flow, in sccm) was reduced.

[0062] Without wishing to be bound by any theory or conjecture, it is believed that at lower N.sub.2 gas proportions, there is increased film bombardment due to the relatively higher proportion of the inert gas (i.e. Ar gas in this example). The inert gas acts as a non-reactive co-sputtering element, and the relatively higher proportions of the inert gas has the effect of increasing the amount of ion bombardment on the substrate surface. Again, without being bound by any theory or conjecture, it is believed that the increased film bombardment can be critical in film nucleation, and can help provide the template for crystal growth with good orientation. Still without being bound by any theory or conjecture, it is additionally believed that during film growth, the increased film bombardment can transfer sufficient energy to the additive element (e.g. relatively heavy Sc atoms in this example) to overcome energy barriers and move the additive element to a more favourable position within the crystal structure. Consequently, the number of misaligned grains, which are observed as crystallite defects, is reduced.

[0063] Again, without being bound by any theory or conjecture, it is also believed that a nitrogen-rich environment promotes the formation of different phases within the deposited film, which have different growth mechanics compared to the bulk material. These different phases appear as misaligned grains. However, reducing the relative proportion of nitrogen gas in the gaseous mixture moves the equilibrium away from forming these different phases.

[0064] Methods of the present invention can therefore be used to deposit high quality additive-containing aluminium nitride films with a high additive element content, whilst reducing the defect density to acceptable levels. Due to the improved defect density, the piezoelectric properties of the films are improved. The additive-containing aluminium nitride films produced using the present method are highly suited for use as a piezoelectric layer in piezoelectric devices, such as in bulk acoustic wave (BAW) devices.