GB2574220A

GB2574220A - Thermal and electron-beam PVD deposition of metals

Info

Publication number: GB2574220A
Application number: GB1808811.2A
Authority: GB
Inventors: Huggins Jonathan; Spain Elliot
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2018-05-30
Filing date: 2018-05-30
Publication date: 2019-12-04
Also published as: GB201808811D0

Abstract

A method of depositing a metal (which may be silver) by physical vapour deposition (PVD) comprising the steps of; i) adding a solid material to a metal source and ii) heating the metal source provided in a crucible to evaporate the metal atoms from the source, wherein the solid material has a higher melting point than the metal. The solid material is preferably added in the form of a powder having a particle diameter of 10 nm to 10 mm. More preferably the powder comprises a metal powder selected from tantalum powder, molybdenum powder or tungsten powder. A second aspect is directed towards the use of a solid material (which may be tantalum powder) in a PVD method of depositing a metal (which may be silver), in order to provide boiling nucleation sites during evaporation of the metal source – this may reduce defect levels in the metal surface product that are attributable to spitting effects due to superheating.

Description

THERMAL AND ELECTRON-BEAM PVD DEPOSITION OF METALS

FIELD OF INVENTION [0001] This invention relates to a method of depositing a metal (e.g., silver) by evaporationbased physical vapour deposition (PVD) with improved reliability, yield and cycle time. In a further aspect, the present invention relates to the use of a solid material additive in an evaporation-based physical vapour deposition method of a metal to provide boiling nucleation sites during the evaporation of the metal source.

BACKGROUND OF THE INVENTION [0002] Evaporation-based PVD methods are widely applied in the manufacturing of semiconductors or other electronic components (e.g., capacitors) to deposit thin layers of metal or metal alloy, which may serve as functional layers, electrical contacts or conductive pathways, for example. In such methods, a metal source placed in a crucible is heated, (i.e., by resistive heating, inductive heating or electron beams (e-beam evaporation)) in a vacuum chamber until the metal melts and atoms thereof are evaporated from the metal source. The evaporated metal atoms travel through the vacuum chamber and are deposited on a substrate provided within the vacuum chamber.

[0003] A problem frequently observed in metal deposition by evaporation is the formation of defects due to spitting, wherein metal vapour bubbles formed on the crucible surface explode and forcibly eject molten material into the coating surface. Such defects are observed as typically spherical nodules on the coating film with a diameter of about 0.5 pm to 10 pm. Upon impact on the surface, the temperature of the spherical particles (i.e., the spit) typically exceeds the softening or melting point of the underlying polymer layers. When melting through the coated layer and the substrate (a process referred to herein as “spit melting”), the spit causes crater-like dents or even holes on the surface below the nodules, which potentially results in malfunctions (e.g. leakage or shorts) in the final device and reduced device yields.

[0004] It has been shown that residual carbon contaminations present in the metal source (i.e., slug) or the crucible liner due to the use of lubricants and solvents significantly contribute to spitting defects, and also cause electron back scattering in electron-beam evaporation. In evaporation-based PVD systems, the most common method to reduce defects due to carbon contaminants is to reduce the deposition rate and/or the power applied to the source. This, however, increases cycle time and decreases throughput.

[0005] The formation of nodule defects in gold (Au) deposition has been studied in “Electron Radiation as an Indicator of Gold Nodule Defect during E-beam Evaporation K. Cheng, International Conference on Compound Semiconductor Manufacturing Technology (CS Mantech) May 16-19, 2011. Herein, it is disclosed that tantalum (Ta) may be used as an additive (e.g., in solution so as to avoid solid phase electron back scattering in the e-beam process) to getter carbon in gold slugs and thereby reduce spitting effects.

[0006] Tantalum is widely known as wetting material and is conventionally applied in crucible liners (see e.g., JP 56 116 873 A and US 3,617,348 A) or in heating elements (see e.g., US 2,413,606 A, EP 1 987 171 A2 and EP 3 245 313 A1). Thus, as is disclosed in US 8,373,427 B2, using Ta in the metal source may induce wetting of the crucible by the molten metal and cause the molten metal to wick out of the liner, which potentially results in cracks in the crucible, (i.e., due to differential thermal contraction between crucible material and the molten metal). This can also cause a power loss to the evaporator deposition system, tool alarms and interruptions to production. To this end, US 8,373,427 B2 discloses the implementation of a system which monitors and detects impurities in the metal slug, to enable preventive replacement or decontamination of the metal slug before producing excessively defective metal layers.

[0007] However, in view of the above, there still exists a need to provide a method of depositing a metal (such as silver, for example) by evaporation-based PVD, which reduces defects in the deposited layer, exhibits a high reliability, and simultaneously enables deposition at high yields, shortened cycle times and reduced downtime after source filling.

SUMMARY OF THE INVENTION [0008] The present invention seeks to solve these problems in the thermal and electronbeam deposition of metals with the claims as defined herein. Further advantages of the present invention will be further explained in detail in the section below.

[0009] The present invention is based on the finding that a solid material having a higher melting point than the metal added to the crucible containing the metal source may behave similarly to boiling beads in that it provides boiling nucleation sites which reduce spitting effects due to superheating.

[0010] In one aspect, the present invention therefore generally relates to a method of depositing a metal by evaporation-based physical vapour deposition, comprising: adding a solid material to a metal source, and heating the metal source provided in a crucible to evaporate metal atoms from the source; wherein the solid material comprises a material having a higher melting point than the metal.

[0011] In another aspect, the present invention relates to the use of a solid material in a physical vapour deposition method of a metal to provide boiling nucleation sites during the evaporation of the metal source.

[0012] With the above embodiments, defects observed in metal layers and devices comprising said layers may be reduced without decreasing the deposition rate, which results in shortened cycle time and improved process throughput.

[0013] Preferred embodiments of the method of depositing a metal by evaporation-based PVD and the use of a powder according to the present invention, as well as other aspects of the present invention are described in the following description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS [0014] Fig. 1A is a schematic illustration of a system for evaporation-based PVD.

[0015] Fig. 1B illustrates a system for electron-beam PVD.

[0016] Fig. 2 is a SEM image of Ag nodules formed on a spit event on a polymeric substrate layer.

[0017] Fig. 3A shows an image of an Ag film deposited at 1 A/s using conventional thermal deposition.

[0018] Fig. 3B shows an image of an Ag film deposited at 1 A/s by thermal deposition with addition of tantalum powder.

DETAILED DESCRIPTION OF THE INVENTION [0019] Fora more complete understanding of the present invention, reference is now made to the following description of the illustrative embodiments thereof:

[0020] In a first embodiment, the present invention generally relates to a method of depositing a metal by physical vapour deposition (PVD), comprising: adding a solid material to a metal source, and heating the metal source provided in a crucible to evaporate metal atoms from the source; wherein the solid material has a higher melting point than the metal. [0021] The PVD process as such is not particularly limited. Fig. 1A schematically illustrates a system for evaporation-based PVD. Herein, the target material (5) (i.e., the metal source) is heated in a vacuum deposition chamber (1), wherein a pressure of typically less than 10' ⁵ Torr is maintained by use of a vacuum pump (7) equipped with an exhaust outlet (8). The metal source (5) is placed in a crucible (4) and heated up to its melting point and above by a heater (6) until metal atoms are evaporated from the metal source (5), travel through the vacuum chamber (1) and are deposited on one or more substrates (2) held in place with a substrate holder (3), which is usually dome-shaped. The heating may be brought about by resistive heating, inductive heating or by an electron beam (e-beam), for example.

[0022] Electron beams can be generated by thermionic emission, field electron emission or the anodic arc method, for example. Fig. 1B schematically illustrates a configuration of an electron beam PVD system comprising water cooling lines (9) and a shield (6a), wherein a charged filament (6b) gives off an electron beam (6f) under high vacuum, which is accelerated by an accelerating grid (6c) and bent by a magnet (6e) to direct the electron beam at the ingot location. Deflection plates (6d) may be used to raster scan the beam (6f) across the target material surface. Typically, the crucible (4) is equipped with an optional crucible liner (4a) (made of graphite, tungsten, molybdenum, aluminum oxide, boron nitride, copper, molybdenum, tantalum, vitreous carbon or intermetallic compositions, for example). [0023] In the method of the present invention, a solid material is added to the metal source, preferably in form of a powder having an average particle diameter of from 10 nm to 10 mm. The solid material is preferably added before or during the step of filling the crucible with the metal source.

[0024] The solid material essentially exhibits a higher melting point than the metal in order to prevent melting of the solid material particles and to enable them to form boiling nucleation sites during evaporation of the metal source under the vacuum conditions employed in the PVD system.

[0025] In preferred embodiments, the solid material has a melting point of at least 2000°C. [0026] In a further preferred embodiment, the solid state density of the solid material (under room temperature conditions) is higher than the liquid density of the metal to be deposited at the melting point of the metal. Such a configuration advantageously ensures that the solid material particles are submerged within the metal source melt, which further enhances the boiling bead function of the powder and reduces high-energy electron back scattering effects due to the presence of solid particles floating on the surface of the metal source melt. The solid state density of solid materials at room temperature and the liquid density of metals at their melting point represent physical characteristics which may be derived from literature values.

[0027] While the concept of the present invention is not limited to specific combinations of metals and solid materials as long as the above-identified conditions are met and the solid material does not react with the metal (e.g. by forming intermetallic species etc.), the method is particularly preferred for the deposition of silver (Ag), which shows a spitting behaviour similar to gold, but allows a wider range of solid materials to be effectively used as additives due to the comparatively low liquid density at the melting point (9.32 g/cm³).

[0028] In preferred embodiments, the solid material can be any metal powder from the group known as refractory metal. As specific examples, tantalum (Ta) powder, molybdenum (Mo) powder or tungsten (W) powder may be mentioned.

[0029] In an especially preferred embodiment, the metal powder is tantalum powder as tantalum combines a relatively high solid state density at room temperature (about 16,7 g/cm³) with a high melting point (about 3017°C). Moreover, it has been found that tantalum surprisingly reduces the burn-in period required before the actual deposition process. Usually, if the metal source is initially heated too rapidly, liquid droplets tend to be ejected from the metal source, and applying higher levels of power results in more spitting of the material. Therefore, materials prone to spitting normally require a slow ramping procedure while the shutter is closed to slowly increase the temperature of the melt, or the power has to be reduced overall during deposition to decrease or eliminate spitting. By adding tantalum powder to the metal source, the pre-heating period may be shortened and/or a relatively high power may be applied without significant increase in spitting, and the cycle time (i.e. the average time between the start of production of one unit and the start of production of the next unit) may be remarkably reduced.

[0030] The amount of solid material added to the metal source and the ratio may be suitably selected by the skilled artisan.

[0031] In a second embodiment, the present invention relates to the use of a solid material in a physical vapour deposition method of a metal to provide boiling nucleation sites during the evaporation of the metal source.

[0032] Overall, it will be understood that the preferred features of the first embodiment may be employed in and combined with the second embodiment in any combination, except for combinations where at least some of the features are mutually exclusive.

EXAMPLES [0033] As a reference example, a 200 nm thick silver layer has been deposited by an electron beam-PVD method at a deposition rate of 1 A/s on a polymeric substrate without adding a powder to the Ag crucible.

[0034] Fig. 3A is a photograph of the resulting Ag film surface, illustrating the typical defect level when e-beam deposition of silver is performed without spitting control.

[0035] Fig. 2 represents a SEM image of the same Ag film surface and shows a close-up view of the spherically shaped spit defects on top of the Ag film. It can be seen in Fig. 2 that craters are formed due to spherical silver spit melting the underlying polymer layers. In a metal-insulator-metal (MIM) arrangement (as is used in capacitors, for example) the observed phenomenon could lead to a hard short-circuit between cathode and anode. [0036] In order to demonstrate the effect of the present invention, a 200 nm thick Ag layer has been deposited by an electron beam-PVD method according to the reference example at a deposition rate of 1 A/s, with the exception that tantalum powder has been added during source filling. Fig. 3B shows a photograph of the resulting Ag film surface. The comparison of Figures 3A and 3B demonstrates that the addition of tantalum powder significantly reduces the defect level.

[0037] Accordingly, it is demonstrated that the method and use of the present invention reduces the level of defects without the need for reducing the deposition rate.

[0038] Once given the above disclosure, many other features, modifications, and improvements will become apparent to the skilled artisan.

REFERENCE NUMERALS

1: vacuum deposition chamber

2: substrates

3: substrate holder

4: crucible

4a: optional crucible liner

5: target material

6: heater

6a: shield

6b: filament

6c: accelerating grid

6d: deflection plates

6e: magnet

6f: electron beam

7: vacuum pump

8: exhaust outlet

9: cooling lines

Claims

1. A method of depositing a metal by physical vapour deposition, comprising:

adding a solid material to a metal source, and heating the metal source provided in a crucible to evaporate metal atoms from the source;

wherein the solid material has a higher melting point than the metal.

2. The method according to claim 1, wherein the solid material is added in form of a powder having an average particle diameter of from 10 nm to 10 mm.

3. The method according to claim 2, wherein the powder comprises a metal powder selected from any of a tantalum powder, molybdenum powder or tungsten powder.

4. The method according to claim 3, wherein the metal powder is tantalum powder.

5. The method according to any of claims 1 to 4, wherein the solid state density of the solid material is higher than the liquid density of the metal at the metal melting point.

6. The method according to any of claims 1 to 5, wherein the solid material has a melting point of at least 2000°C.

7. The method according to any of claims 1 to 6, wherein the deposited metal is silver.

8. The method according to any of claims 1 to 7, wherein the method is an electron beam evaporation method.

9. Use of a solid material in a physical vapour deposition method of a metal to provide boiling nucleation sites during the evaporation of the metal source.

10. Use of a solid material according to claim 9, wherein the solid material has a higher melting point than the metal.

11. Use of a solid material according to any of claims 9 or 10, wherein the solid state density of the solid material is higher than the liquid density of the metal at the metal melting point.

12. Use of a solid material according to any of claims 9 to 11, wherein the solid material is added in the form of a powder having an average particle diameter of from 10 nm to 10 mm.

13. Use of a solid material according to claim 12, wherein the powder is tantalum powder.

14. Use of a solid material according to any of claims 9 to 13, wherein the deposited metal is silver.