Copper film containing tungsten nitride for improving thermal stability, electrical conductivity and electric leakage properties and a manufacturing method for the copper film

Abstract

A copper film containing tungsten nitride is manufactured by co-sputtering method under an Ar/N2 atmosphere and has a composition in ratio of tungsten nitride contained in the copper layer in atomic ratios of more than 97.5% in copper, 0.5 to 1.5% in tungsten and of less than 2.0% in nitrogen. By adding the tungsten nitride, the copper film has improvements in thermal stability, good electrical conductivity and low electrical leakage current. Moreover, the copper film attached on a silicon substrate will generate a self-passivated silicon compound layer to serve as a diffusion barrier layer between the copper film and the silicon substrate during annealing.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a copper film, and more particularly to a copper film that contains insoluble tungsten nitride in saturated situation to improve thermal stability, electrical conductivity, and electric leakage of the copper film by magnetron sputtering. A manufacturing method for the copper film is also disclosed in the present invention.

2. Description of Related Art

Because cupric materials such as copper and cupric alloy have excellence in each of electrical conductivity, thermal conductivity, and mechanical properties at room temperature, these cupric materials are commonly used in the semiconductor field. However, these cupric materials have poor mechanical properties at high temperatures and thus are used at low operational temperatures, whereby the cupric materials can not be used efficiently and temperature limitation restricts further applications of these cupric materials.

Additionally, the cupric materials substitute aluminum to construct conductive layers in semiconducting elements because of their excellent electrical conductivity, high resistance capability for electromigration, long durability and good stability. Therefore, the cupric materials have more utilization such as forming copper films in semiconducting elements. However, the copper films still have some drawbacks such as forming an oxidation membrane, reacting with silicon substrate at low temperature or having poor attachment thereby further incurring damages in the semiconducting elements.

If the copper film is added with other metal elements, electrical conductivity of the copper film is reduced and hardness of the copper film is increased.

Therefore, some references disclose adding insoluble elements (such as carbon) or pure metals into the copper film instead of the foregoing metal elements and this may overcome the drawbacks. These references are:

• • (a) P. Chu, C. H. Chung, P. Y Lee, J. M. Rigsbee, and J. Y Wang, “Microstructure and Properties of Cu—C Pseudoalloy Films Prepared by Sputter Deposition”, Metallurgical and Materials Transactions A, 29A, pp. 647-658, (1998);
  • (b) J. P. Chu and T. N. Lin, “Deposition, Microstructure and Properties of Sputtered Copper Film Containing Insoluble Molybdenum” in Journal of Applied Physics, 85,6462-6469(1999);
  • (c) C. H. Lin, J. P. Chu, T. Mahalingam, T. N. Lin and S. F. Wang, 2003/06, “Thermal Stability of Sputtered Copper Films Containing Dilute Insoluble Tungsten: A Thermal Annealing Study” in Journal of Materials Research, Vol 18, No. 6, P. 1429-1434 (2003);
  • (d) S. L. Zhang, J. M. E. Harper and F. M. D'Heurle, “High Conductivity Copper-boron Alloys Obtained by Low Temperature Annealing”, in Journal of Electronic Materials, 30, L1, (2001); and
  • (e) Taiwan Patent application No. 88113088 (certification No. 152100), Republic of China, “Sputtered Copper Films Containing Tantalum for Improving Electrical Conductivity, Thermal Stability and Hardness Properties and Method for Making the Same”.

However, the copper films with an additional materials made of the insoluble elements in the foregoing references have suboptimal properties. Moreover, the diffusion rates of copper atoms are high.

The present invention has arisen to improve the copper films and a manufacturing method is also disclosed.

SUMMARY OF THE INVENTION

A first main objective of the present invention is to provide a copper film that contains dilute tungsten nitride, whereby the copper film has fine crystallites in microstructure, excellent electrical conductivity, low electric leakage current, and good thermal stability at high temperatures.

A second main objective of the present invention is to provide a manufacturing method that particularly forms the copper film containing tungsten nitride.

To achieve the foregoing first main objective, the copper film contains tungsten nitride adapted to attach on a silicon substrate and comprises:

• • a copper layer containing tungsten nitride in atomic ratios of more than 97.5% in copper, 0.5 to 1.5% in tungsten and of less than 2.0% in nitrogen;
  • wherein, all the atomic ratios are on a basis of total atoms in the copper film.

To achieve the foregoing second main objective, the copper film having the tungsten nitride is made by the method having acts of:

• • preparing a vacuum sputtering system having a pressure within 1×10⁻²−1×10⁻³ torr at 1.4˜3.7 W/m² of sputtering power;
  • introducing argon and nitrogen into the vacuum sputtering system to create an Ar/N₂ atmosphere;
  • co-sputtering the copper target and the tungsten target in the Ar/N₂ atmosphere to form the copper film containing the tungsten nitride on a silicon substrate.

By using the nitrides in the copper film, nitrogen atoms efficiently fill into boundaries of the copper crystallites to make the copper crystallites fine in microstructure. Moreover, nitrogen in the copper film is precipitated at the boundaries of the copper crystallites and reacts with the silicon substrate to compose a self-passivated silicon compound barrier layer) to reduce diffusion of the copper atoms.

Further benefits and advantages of the present invention will become apparent after a careful reading of the detailed description with appropriate reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an arrangement of a copper target and a tungsten target used in a co-sputtering process;

FIG. 2 shows top views of a preferred embodiment of the copper target and the tungsten target in an area Cu/W ratio of 11.0%;

FIG. 3 is a secondary ion mass spectroscopy (SIMS) line graph showing relationships between variations of nitrogen contents and thickness of the copper film before annealing and after annealing at 530° C.;

FIG. 4(A) is a secondary ion mass spectroscopy (SIMS) line graph showing relationships between nitrogen contents and diffusion depths in a WN_x-containing copper film annealing at 200° C. and in an etched substrate that is etched with nitric acid to remove the copper film;

FIG. 4(B) is a secondary ion mass spectroscopy (SIMS) line graph showing relationships between nitrogen contents and diffusion depths in a WN_x-containing copper film annealing at 400° C. and in an etched substrate that is etched with nitric acid to remove the copper film;

FIG. 4(C) is a secondary ion mass spectroscopy (SIMS) line graph showing relationships between nitrogen contents and diffusion depths in a WN_x-containing copper film annealing at 530° C. and in an etched substrate that is etched with nitric acid to remove the copper film;

FIGS. 5(A) and 5(B) show peak diagrams from X-ray photoelectron spectroscopy (XPS) to respectively indicate existence of silicon oxide and nitrogen oxide by examining the substrates and the WN_x-containing copper films with different etching duration to verify the existence of self-passivated silicon compound barrier made of silicon oxide nitride (sinoite), Si₂N₂O, Cu₂WO₄ and Cu₃Si;

FIG. 6 is a line graph showing XRD patterns for copper films on Si substrates after annealing for 1 hour: (a) pure Cu, at 400° C.; (b) Cu(W), at 530° C.; (c) Cu(WN_x), at 530° C.; and (d) Cu(WN_x), at 580° C.;

FIG. 7 is a line graph showing XRD patterns for self-passivated silicon compound barrier layer on Si substrates after annealing at 530° C.;

FIG. 8 comprises cross-sectional focused ion beam (FIB) micrography photos showing the structure of (a) WN_x-containing copper film on the substrate annealing at 530° C. for 5 min duration; (b) WN_x-containing copper film on the substrate annealing at 530° C. for 1 hour duration; (c) W-containing copper film on the substrate annealing at 530° C. for 1 hour duration

FIG. 9 is a transmission electron microscope (TEM) photo of the WN_x-containing copper film annealing at 530° C. for 1 hour duration; and

FIG. 10 is a line graph of electrical leakage current test of (a) pure copper film; (b) W-containing copper film; and (C) WN_x-containing copper film after 400° C. for 1 hour duration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A copper film containing tungsten nitride in accordance with the present invention is adapted to be formed on a silicon substrate and comprises a copper layer in form of a supersaturated solid solution and tungsten nitride present inside the copper layer that is in structure of nano-crystallite. Composition of a copper film containing the tungsten nitride has atomic ratios of more than 97.5% in copper, 0.5 to 1.5% in tungsten, and less than 2.0% in nitrogen. All atomic ratios above are on a basis of total atoms in the copper film.

A method for manufacturing the copper film containing tungsten nitride on the silicon substrate in the present invention comprises acts of:

• • preparing a vacuum sputtering system having a pressure within 1×10²−1×10⁻3 torr and 1.4-3.7 W/m²W sputtering power;
  • introducing argon and nitrogen into the vacuum sputtering system to create an Ar/N₂ atmosphere;
  • optionally, adjusting a non-overlapping area between a copper target and a tungsten target in the Ar/N₂ atmosphere; and
  • co-sputtering the copper target and the tungsten target in the Ar/N₂ atmosphere to form the copper film containing the tungsten nitride on a silicon substrate under room temperature to 100° C.

Moreover, the method further comprises an annealing act after the co-sputtering act and the annealing act comprises step of annealing the copper film containing tungsten nitride at an annealing temperature ranging from 200 to 650° C. for one hour duration at a heating rate within 4 to 6° C./minute under pressure within 1×10⁻⁶−1×10⁻⁷ torr until self-passivated silicon compound barrier is generated between the copper film and the silicon substrate.

In the following description about the present invention, the term of “coating” is substantially the same with the term “film”. Therefore, the copper coating and the copper film are the same. The term of “insoluble” in the present invention particularly means that elements (such as tungsten nitride) can not be dissolved in copper material.

A Direct current (DC) Magnetron Sputtering Deposition system is particularly used in the method of the present. The DC magnetron sputter deposition system enables the making of insoluble tungsten nitride to synthesize in atom-by-atom growth to form a supersaturated solid solution. Therefore, synthesized materials in this system are not limited by phase-equilibrium and particularly have non-equilibrium property and nano-scale microstructure to increase thermal stability and mechanical strength of the copper film.

To achieve the copper film containing a certain ratio of the tungsten nitride, a manufacturing method for the copper film is developed and comprises the acts of:

• • preparing a vacuum sputtering system having a pressure less than 7×10⁻⁷ torr;
  • introducing argon and nitrogen into the vacuum sputtering system to create an Ar/N₂ atmosphere;
  • adjusting a non-overlapping area between a copper target and a tungsten target in the vacuum sputtering system; and
  • co-sputtering the copper target and the tungsten target in the Ar/N₂ atmosphere to form the copper film containing tungsten nitride.

In this manufacturing method, the non-overlapping area is adjusted to reveal parts of the tungsten target. The tungsten target is attached to the copper target to achieve a combined target. With reference to FIGS. 1 and 2, the combined target is adapted to be located under a substrate (not shown) with a 20 cm distance to the substrate and has multiple round holes defined through the copper target to reveal the tungsten target. When the copper target and the tungsten target are co-sputtered by high-speed particles, the targets are struck by the high-speed particles to sputter to the substrate. The substrate is preferably a silicon chip. To make the copper film evenly coated on the substrate, the substrate is rotated at a constant rotating speed during sputtering.

By adjusting the non-overlapping area of the tungsten target, contents of tungsten, nitrogen and copper can be regularized. Table 1 shows a desired content composition in the copper films, wherein non-overlapping area is preferably in 11% area ratio of the tungsten target to the copper target. Other important parameters in the DC magnetron sputter deposition system are shown in Table 2. film.

TABLE 1
Cu(WNx)
Target area fraction (W)	Tungsten (at %)	Nitrogen (at %)
11.0%	1.5	<2.0

TABLE 2
Items of parameters             Parameter
System base pressure            Below 7 × 10−7 torr
Argon/Nitrogen working pressure 1 × 10−2 torr
Sputtering power density        1.4-3.7 W
Temperature of the substrate    Room temperature to 100° C.
Targets (purity)                Oxygen-free pure copper (99.9%)
                                Tungsten (99.5%)
Distance between the targets    The substrate is located over the
and the substrate               targets at a 20 cm distance
Sputtering speed                4.8 nm/min

In the following analyses and experiment, the copper films containing tungsten nitride are experimented.

<Qualitative Analyses and Quantity Analyses of the Tungsten and Nitrogen Elements in the Copper Coating and Thermal-Stability Tests of the Copper Coating>

Quantities of nitrogen were detected by a secondary ion mass spectroscopy (SIMS) and shown in Table 1 and FIG. 3. According to FIG. 3, nitrogen existed in the copper coating before annealing and diffused to interfaces between the copper coating and the silicon substrate after annealing. The copper coatings annealed at different temperatures were then etched with nitric acid and removed from the substrate to reveal the interfaces between the copper coatings and the substrates. The interfaces were examined by the secondary ion mass spectroscopy (SIMS) and, as shown in FIGS. 4(A) to 4(C), presented the existence of the self-passivated silicon compound layer at the interfaces. In other words, the nitrogen atoms transferred close to the silicon substrate to react with the silicon to perform the silicon compound after annealing. Therefore, the nitrogen atoms were supposed to diffuse at high temperature. FIGS. 5(A) and 5(B) further verify the structure of the self-passivated layer is silicon oxide nitride (sinoite), Si₂N₂O. In comparison with lines of pure copper coating and copper coatings containing tungsten nitride in FIG. 6, tungsten nitride was supposed to dissolve in the copper coating to exhibit generation of undesirable copper silicide (Cu₃Si) that was generated at more than 580° C. FIG. 7 indicated that the _yself-passivated silicon compound barrier consists Si₂N₂O, Cu₂WO₄ and Cu₃Si.

Therefore, the copper coating containing the tungsten nitride has excellent thermal stability at high temperature and does not easily react with silicon to form the undesirable copper silicide.

<Microstructures and Mechanical Tests for the Copper Coating Containing Tungsten Nitride>

In those embodiments, the microstructure of the copper coating containing tungsten nitride was refined. Moreover, tungsten nitride was solid-dissolved in the copper coating in a supersaturated situation to increase thermal stability of the copper coating. Influence of the tungsten nitride in the copper coating was justified by the focused ion beam (FIB) micrographs in FIGS. 8(a) to 8(d), wherein the copper coating containing tungsten nitride did not have obvious self-passivated silicon compound barrier layer generated after annealing at 530° C. for 5 min duration as shown in FIG. 8(a). However, after the copper coating was annealed at 530° C. for one hour duration, the self-passivated silicon compound barrier layer with 170 nm thickness was generated at the interfaces between the silicon substrate and the copper coating (see FIG. 8(b)). This result was also accordant with that the nitrogen atom diffused to the interfaces and further entered the silicon substrate as shown in FIGS. 3 and 4. The W-containing copper coating having no nitrogen had undesirable cupric silicide generated at the interfaces. Obviously, adding tungsten nitride into the copper coating improved the microstructure of the copper coating, and this result proves the tungsten nitride had solid-dissolved in the copper coating. By generating the self-passivated silicon compound barrier buffer to serve as the barrier, copper silicide was inhibited and crystallites in the copper coating had grown and coalesced to form columnar structures, which corresponds to FIG. 9. In FIG. 9, the self-passivated layer formed a 170 nm thickness barrier layer after annealing at 530° C. for one hour duration and the copper coating was still performed in columnar structures. FIG. 10 shows a result of electric leakage tests of different copper coatings after annealing at 400° C. According to FIG. 10, the copper coating containing tungsten had lower electrical leakage current than the one of pure copper coating and the copper coating containing tungsten nitride had much lower electrical leakage than the one of pure copper coating. This result proves that copper diffusion to the silicon substrate had been reduced as the same as shown in the former experiments. Table 3 shows the result of electrical resistivities of different copper coatings before and after annealing at 530° C.

	TABLE 3
			After annealing
	Electrical Resistivity		at 530° C. for 1 hr
	(μΩ-cm)	Before annealing	530° C.
	Cu(W)	9.1 ± 0.02	14.9 ± 0.15
	Cu(WNx)	17.7 ± 0.03	2.7 ± 0.01
	Pure Cu	3.96 ± 0.01	20.1 ± 1.47

According to Table 3, the WN_x-containing copper coatings had high resistivity of 17.7 μΩ-cm before annealing and had low resistivity of 2.7 μΩ-cm after 530° C. annealing because nitrogen atoms precipitated to the interfaces between the copper coating and the substrate and coating defects were reduced. Therefore, the WN_x-containing copper coatings in the present invention has much lower resistivity in comparison with other coatings. According to the above observation, the forming mechanism of the interfaces comprises the following theories.

Influence of residual oxygen to the interfaces during annealing is shown in a Cu/Ta/Si report. This influence also happened in the present invention. When the Cu(WNx)/Si was heated to 530° C. in vacuum annealing, WNx decomposed into W and N atoms. Because W-O (1.7) has an electronegativity higher than ones of other compositions such as Si—O (1.7)Cu—O(1.6)Si—N(1.2) and Cu—Si(0.1). Therefore, the W atoms capture the residual oxygen to react to W—O and further to react with Cu atoms when proportion of Cu/W increases, especially approximates 100%. However, CuO and native oxide (SiO_x) have free energy values lower than standards so that the CuO and native oxide are unstable at high-temperature annealing and barely form.

Moreover, N and Si atoms react to SiN_x at interfaces and further react with the residual oxygen to compose Si₂N₂O in the annealing environment. In the present invention, the interfaces are continuous and composed of Si₂N₂O and Cu₂WO₄ both have high electronegativities. Therefore, the interfaces forms before reaction of Cu and Si and enable to inhibit this reaction to further barricade Cu₃Si to extend into the Cu(WN_x) coating. This result is justified by examining X-ray diffration in the interfaces and proved by the low resistance and low leakage current evidently. Chemical reaction of the interfaces are shown in the following formulas:

In summary, the WN_x-containing copper film (coating) in the present invention has the following advantages and inventive steps.

1. The copper coating containing tungsten nitride is manufactured by the co-sputtering method in a vacuum system. In this system, the co-sputtering is particularly carried in an argon/nitrogen mixed atmosphere to manufacture the copper film containing tungsten nitride. Contents of the copper films are controlled by adjusting the sputtering ratios of the tungsten target and copper target.

2. The self-passivated silicon compound barrier layer is generated between the copper film and the silicon substrate after annealing to serve a diffusion barrier layer for isolating copper and silicon or gas/mixture.

3. Because the silicon compound layer generates in spontaneity to perform the diffusion barrier layer when the copper coating is annealed, no further procedure such as chemical vapor deposition is needed to manufacture the diffusion barrier layer. Therefore, manufacturing processes of the semiconducting elements are simplified and manufacturing cost is reduced.

4. To combine the copper and the metal nitride insoluble with the copper, nitrogen is introduced into the co-sputtering procedure to manufacture the copper film containing the tungsten nitride. After annealing, the nitrogen is precipitated to react with the silicon substrate to perform the silicon compound layer that has excellent thermal stability, low resistivity and low electric leakage current. Therefore, the copper coating has excellent conductivity and is greatly improved in mechanical properties to overcome the disadvantages of the conventional copper film.

Although the invention has been explained in relation to its preferred embodiment, many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.

Claims

1. A copper film containing tungsten nitride for improving thermal stability, electrical conductivity and electrical leakage current properties, the copper film adapted to attach on a silicon substrate and comprising:

a copper layer in form of a supersaturated solid solution and having a composition in atomic ratios of more than 97.5% in copper, 0.5 to 1.5% in tungsten and of less than 2.0% in nitrogen;

all the atomic ratios are on a basis of total atoms in the copper film.

2. The copper film as claimed in claim 1, wherein a self-passivated silicon compound barrier layer is generated between the silicon substrate and the copper film after annealing of the copper film.

3. A manufacturing method for forming a copper film containing tungsten nitride as claimed in claim 1, wherein the manufacturing method 14 comprising acts of:

preparing a vacuum sputtering system having a pressure within 1×10−2−1×10−3 torr and 1.4-3.7 W/m2W Sputtering power;

introducing argon and nitrogen into the vacuum sputtering system to create an Ar/N2 atmosphere;

adjusting sputtering ratios of a copper target and a tungsten target in the Ar/N2 atmosphere; and

co-sputtering the copper target and the tungsten target in the Ar/N2 atmosphere to form the copper film containing the tungsten nitride on a silicon substrate.

4. The method as claimed in claim 3, wherein the co-sputtering act is carried out under room temperature to 100° C.

5. The method as claimed in claim 2, wherein the method further comprising an annealing act after the co-sputtering act and the annealing act comprises step of:

annealing the copper film containing tungsten nitride at an annealing temperature ranging from 200 to 650° C. at a heating rate within 4 to 6° C. per minute under pressure within 1×10−6−1×10−7 torr for a duration until self-passivated silicon compound barrier layer is generated between the copper film and the silicon substrate to against copper silicide fomation and making the copper film containing tungsten nitride have a low electrical resistance and low leakage current.