Deposition Apparatus

Abstract

A magnetron sputtering apparatus for depositing material onto a substrate, comprises: a chamber comprising a substrate support and a target; a plasma production device configured to produce a plasma within the chamber suitable for sputtering material from the target onto the substrate; and a thermally conductive grid comprising a plurality of cells. Each cell comprises an aperture and the ratio of the height of the cells to the width of the apertures is less than 1.0. The grid is disposed between the substrate support and the target and is substantially parallel to the target. The upper surface of the substrate support is positioned at a distance of 75 mm or less from the lower surface of the target.

Claims

1. A magnetron sputtering apparatus for depositing material onto a substrate, comprising: a chamber comprising a substrate support and a target; a plasma production device configured to produce a plasma within the chamber suitable for sputtering material from the target onto the substrate; and a thermally conductive grid comprising a plurality of cells, in which each cell comprises an aperture, and wherein a ratio of a height of the cells to a width of the apertures is less than 1.0, and wherein the grid is disposed between the substrate support and the target and substantially parallel to the target, and an upper surface of the substrate support is positioned at a distance of 75 mm or less from a lower surface of the target.

2. An apparatus according to claim 1, wherein the aspect ratio is in the range 0.1 to 0.8.

3. An apparatus according to claim 1, wherein the height of each cell is 10 mm or less.

4. An apparatus according to claim 1, wherein the upper surface of the substrate support is positioned at a distance in the range 40 to 75 mm from the lower surface of the target.

5. An apparatus according to claim 1, wherein a shape of each of the apertures is substantially hexagonal.

6. An apparatus according to claim 1, wherein the grid is electrically conductive and grounded.

7. An apparatus according to claim 1, wherein the material is Mo, W, Ta, Ti, Pt, Cr, Ru or Al.

8. An apparatus according to claim 1, wherein the substrate support is RF biased.

9. A method for depositing material onto a substrate by magnetron sputtering comprising: providing the magnetron sputtering apparatus according to claim 1; supporting the substrate on the substrate support; providing a plasma so that the material is sputtered from the target onto the substrate; and wherein the sputtered material passes through the apertures of the grid before reaching the substrate.

10. A method according to claim 9, wherein the aspect ratio is in the range 0.1 to 0.8.

11. A method according to claim 9, wherein the height of each cell is 10 mm or less.

12. A method according to claim 9, wherein the upper surface of the substrate support is positioned at a distance in the range 40 to 75 mm from the lower surface of the target.

13. A method according to claim 9, wherein a shape of each of the apertures is substantially hexagonal.

14. A method according to claim 9, wherein the grid is electrically conductive and grounded.

15. A method according to claim 9, wherein the material is Mo, W, Ta, Ti, Pt, Cr, Ru or Al.

16. A method according to claim 9, wherein the substrate support is RF biased.

17. A substrate comprising a layer of material thereon, in which the material is deposited by a method according to claim 9 and the deposited layer of material has a within wafer stress value of less than 180 MPa.

18. A device comprising the substrate according to claim 17.

19. Use of the apparatus according to claim 1 for depositing a material layer on a substrate, in which the deposited material has a within wafer stress value of less than 180 MPa.

20. A kit for retrofitting an existing magnetron sputtering apparatus in order to provide a retrofitted magnetron sputtering apparatus according to claim 1, the kit comprising a connection means permitting the grid to be connected to one or more portions of the existing magnetron sputtering apparatus to locate the grid in place between the target and the substrate support.

Description

DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of metal deposition apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which:

[0045] FIG. 1 is a schematic view of a DC magnetron sputtering system showing major and minor erosion zones.

[0046] FIG. 2 is a schematic view of a DC magnetron sputtering system including a grid of the present invention.

[0047] FIG. 3 is a schematic view of a grid of the invention with hexagonal shaped apertures.

[0048] FIG. 4 shows within wafer stress range and deposition rate as a function of grid aspect ratio.

[0049] FIG. 5A shows a 3D stress map and FIG. 5B shows a single line scan of film stress across as a function of wafer diameter.

[0050] FIG. 6A shows mean stress and FIG. 6B shows within wafer stress range as a function of argon flow.

[0051] FIG. 7A shows mean stress and FIG. 7B shows within wafer stress range as a function of platen RF bias.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0052] FIG. 1 shows a typical DC magnetron rotating sputtering system, depicted generally at 20. The apparatus 20 comprises a chamber 22, in which the interior of the chamber 22 houses a platen (or substrate support) 24 on which a workpiece 26 such as a semiconductor wafer may be loaded. The apparatus further comprises a magnetron 28 and a target 30.

[0053] Stress in thin films arising as a result of the deposition method has two main componentsthermal stress and intrinsic stress. For materials with a high melting point, such as metals including Mo, Ti and W, the variation in thermal stress across a wafer can generally be ignored as the wafer temperature is too low to induce any variation in thermally activate growth processes in the film structure. However, sputter deposition is an energetic process and can generate variable stresses in thin films due to ballistically activated changes in the film structure.

[0054] Sputtering thin films involves ejecting material from a target 30 onto a substrate 26, such as a silicon wafer. Intrinsic stresses are generated in the thin layers during the deposition process, and these intrinsic stresses are closely related to the energy and flux of particles generated in the plasma arriving at the wafer. These intrinsic stresses are largely controlled by the geometry of the magnetron pole piece. Simple and reliable models have been established to estimate the relationship between the magnetic fields and the deposited film thickness. However, the same cannot be said for the relationship between thin film stress and the magnetron design. It is believed that this is because intrinsic stresses in thin films are closely related to the microstructure evolution and growth process for any particular film. Any theory relating to thin film stress must also take into account a multitude of processes occurring at the atomic scale, for example interfacial effects between the substrate and metal, crystal orientation, grain-boundary formation and growth, defect formation and mobility. As a result, a conventional magnetron apparatus can be designed to deliver a highly uniform film in terms of thickness and resistivity, but designing a magnetron apparatus to optimise stress uniformity is very difficult.

[0055] The plasma profile for a rotating magnetron sputter system optimised for thickness is not uniform across the target. In fact, the plasma is concentrated near the cathode and beneath the region of highest magnetic field, which is swept over the sputter target. FIG. 1 depicts this problem in a typical rotating magnetron system. The apparatus 20 shows the major erosion zones 32 and the minor erosion zone 34 of the target 30 (i.e. the cathode) as dictated by the outer magnets. The wafer 26 sits inside the main erosion zone 32. Beneath the magnet is a high density plasma, such as Ar and metal ions. A proportion of these ions escape the negative glow of the plasma and move towards the wafer platen, causing the generation of compressive stress preferentially at the edge of the wafer.

[0056] FIG. 2 shows a first embodiment of a deposition apparatus of the invention, depicted generally at 36. The embodiment shown in FIG. 2 is in fact a commercially available magnetron sputtering apparatus which has been retrofitted to produce apparatus in accordance with the present invention. The apparatus 36 is a conventional deposition chamber fitted with a grid 38 of the invention. The grid comprises a plurality of cells 44, in which each cell has an aperture 40 and the area between the cells is shown by 42. In this embodiment of the invention, the grid 38 is grounded and positioned approximately half way between the target 30 and the substrate support 24. Without wishing to be bound by theory, it is believed that the grounded grid filters out high energy species generated at the major erosion zone 32 and thermalizes the plasma. The insert in FIG. 3 shows the width (w) and height (h) of the cell aperture. The dotted line shows the axis of rotation of the magnetron 28. In this embodiment of the invention, the upper surface of the substrate support 24 is positioned at a distance of about 50 mm from the lower surface of the target 30, and the upper surface of the grid 38 is positioned at a distance of about 18 mm from the lower surface of the target 30. Therefore, the grid 38 sits approximately half way between the target 30 and the substrate support 24.

[0057] It has been found that the within wafer stress of thin films, such as for example thin metal films of Mo, W, Ta, Ti, Pt, Cr, Ru and Al, which have been deposited by magnetron sputtering, can be decreased dramatically by the addition of a grid 38 placed between the target 30 and the wafer 26. Without wishing to be bound by theory, it is believed that the grid 38 modifies the deposition flux and the energy of the impinging species by selectively reducing the portion of the high energy species generated in the outer erosion profile reaching the wafer and by thermalizing the contribution from the rest of the plasma. In this way the within wafer stress uniformity can be advantageously reduced to less than about 180 MPa. This in turn leads to higher mechanical reliability of the films and higher performance of the device incorporating the film. In fact, it has been found that low within wafer stress uniformity can advantageously be maintained largely independently of material and deposition parameters.

[0058] FIG. 3 shows a grid 38 according to the present invention. In an embodiment of the invention, the grid 38 comprises an array of uniform cells 44 with substantially hexagonally shaped apertures 40. The grid is formed of a suitable thermally conductive material, such as titanium, stainless steel, copper, aluminium, or any combination thereof.

[0059] Known collimating filters are widely used in directional sputtering apparatus to improve the step coverage of small diameter, deep vias. Collimating filters are used to impart a high degree of directionality to the impinging species.

[0060] Wafer stress measurements were made using an industry standard instrument, TOHO Flexus-3300 Stress Gauge. The within wafer stress is a range value of stress, in which the stress is the intrinsic stress of the deposited film.

[0061] FIG. 4 shows how within wafer stress range and deposition rate are affected by the aspect ratio of the grid 38. FIG. 4A shows the within wafer stress range versus aspect ratio and FIG. 4B shows deposition rate versus aspect ratio for a typical Mo film. In this embodiment, the apertures of the grid have a substantially hexagonal shape, and the upper surface of the grid is positioned at approximately 18 mm from the lower surface of the target. The upper surface of the substrate support is positioned at a distance of around 50 mm from the lower surface of the target.

[0062] It can be seen that for a grid with a high aspect ratio and greater than 1:1, the stress range is very high and the deposition rate collapses. By increasing the aspect ratio, the directionality of the incoming species is increased. However, this regime does not help to solve the stress uniformity issue. In fact, it tends to exaggerate the effect of the source and increase the stress range.

[0063] As the aspect ratio of the grid 38 is reduced to less than 1:1, it has been found that the deposition rate begins to improve and, surprisingly, the within wafer stress range of the film is dramatically reduced. Without wishing to be bound by theory, it is believed that these effects are due to the removal of high energy species from the plasma onto the grid. In essence, it is believed that the low aspect ratio grid used in the apparatus of the present invention provides some thermalizing or spreading effect on the plasma, without imparting a high degree of directionality to the deposition species. By judicious optimisation of the grid aspect ratio, it is possible to minimise the impact of the major erosion zone 32 on the wafer stress and achieve excellent within wafer stress ranges with minimal impact on other film properties, such as thickness uniformity and density. The combination of a low aspect ratio grid and a short throw apparatus provides much higher deposition rates than are commonly achieved with a typical collimated sputtering apparatus. In particular, the deposition rate using the apparatus according to the present invention is reduced only by 50% compared to that for the conventional apparatus without a grid.

[0064] FIG. 5 shows a stress map and a cross wafer stress profile for a typical Mo electrode film deposited using the optimised grid geometry. FIG. 5A shows a 3D stress map and FIG. 5B shows a single line scan showing the stress across the wafer diameter for a 300 nm Mo film deposited at 6 kW, 200 C. and 50 sccm Ar on a 200 mm wafer. These results show a reduction of within wafer stress range from about 400 MPa when using the conventional hardware without a grid, to less than about 100 MPa when using the apparatus of the present invention. Excellent results have also been demonstrated on a 250 nm Ti film as shown in Table 1. These results show that the improvement in within wafer stress uniformity appears to be generally independent of the deposited material, film thickness or deposition properties, such as deposition power, pressure or absolute film stress.

TABLE-US-00001 TABLE 1 Deposition Power Stress Mean WIW Stress Range (kW) (MPa) (MPa) 2 51.32 113 4 131.9 175 6 168 79

[0065] FIG. 6 shows that the average stress for a Mo film can be adjusted by changing the gas flow without impacting on the stress range of the films. FIG. 6A shows mean stress versus Ar flow and FIG. 6B shows within wafer stress range versus Ar Flow for a 220 nm and 440 nm Mo film deposited at 6 kW, 200 C. on a 200 mm Si wafer.

[0066] Without wishing to be bound by theory, it is believed that the increased deposition pressure as a result of the increased Ar flow leads to an overall increase in flux of reflected neutral Ar ions. These neutral Ar ions with an increased flux are incorporated into the growing metal film, resulting in an increase in average tensile stress of the film. Therefore, increased Ar flow leads to an increase in average tensile stress of the film. In the apparatus of the present invention, the target-to-wafer distance is typically much less than the mean free path of the sputtered atoms, so small variations in the gas flow do not make much difference to the atom trajectories. By judicious selection of the grid geometry of the present invention, a low within wafer stress range is advantageously maintained in the deposited film, even as the gas flow is changed. Therefore, the average tensile stress of the deposited film can advantageously be tuned by varying the gas flow, while the within wafer stress range is maintained at a relatively constant low level.

[0067] The grid 38 of the invention is compatible with an RF biased substrate support. By varying the RF bias to the substrate support, the absolute stress of the films can advantageously be tuned. FIG. 7A shows mean stress versus platen RF bias and FIG. 7B shows within wafer stress range versus platen RF bias for a 220 nm Mo film deposited at 6 kW, 200 C., with 50 sccm Ar on a 200 mm Si wafer. FIG. 7 shows how film stress for a 220 nm Mo film varies linearly with the addition of platen RF power from 50 to 350 MPa, but importantly that the within wafer stress range remains relatively constant at about 100 MPa across the whole stress range. Without wishing to be bound by theory, it is believed that the amount of ion bombardment taking place can be controlled by the application of an accelerating RF bias to the substrate support, while the ion distribution generated at the target in response to the rotating magnetron is controlled by the aspect ratio of the grid which flattens out the flux as seen at the wafer. As a result, the absolute stress of the deposited film can advantageously be tuned by varying the RF bias to the substrate support, while the within wafer stress range is maintained at a relatively constant low level.