US20050087268A1

US20050087268A1 - Copper film containing tungsten carbide for improving electrical conductivity, thermal stability and hardness properties and a manufacturing method for the copper film

Info

Publication number: US20050087268A1
Application number: US10/969,482
Authority: US
Inventors: Jinn Chu; Yung-Yen Hsieh
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-10-24
Filing date: 2004-10-20
Publication date: 2005-04-28
Also published as: TW200514861A

Abstract

A copper film containing tungsten carbide is adapted to be formed on a substrate and contains a copper layer having tungsten carbide in atomic ratios of 0.4 to 12.2% in tungsten and of 0.7 to 7.4% in carbon. To achieve the copper film, a manufacturing method has the acts of: adjusting a non-overlapping area between a copper target and a tungsten carbide target; co-sputtering the copper target and the tungsten carbide target to form the copper film containing tungsten carbide; and optionally annealing the copper film containing tungsten carbide to change the microstructure of the copper film. By sputtering the tungsten carbide with copper, the achieved copper film has excellent electrical conductivity, thermal stability at high temperatures and hardness properties.

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 tungsten carbide to improve electrical conductivity, thermal stability and hardness properties. 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 excellent 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 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 oxidation membrane that reduces electrical conductivity or having poor attachment to the semiconducting elements, which make the copper film incomplete in use. 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 metal elements and this may overcome the drawbacks. These references are:
J. 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);
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. 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, o. 6, P. 1429-1434;
Zhang S L, Harper J M E and D'Heurle F M, “High Conductivity Copper-boron Alloys Obtained by Low Temperature Annealing”, in Journal of Electronic Materials, 30, L1, (2001); and
Invention Patent application No 88113088 (certification No. 152100), “Forming Copper Film Containing Tantalum by Sputter Method to Increase Hardness and Electrical Conductivity of the Copper Film”.
However, the copper films with an additional layer made of the insoluble elements in the foregoing references have rough crystallites because boundaries between crystallites are empty. Moreover the diffusion rates of copper atoms are high.
The present invention has arisen to mitigate or obviate the disadvantages of the conventional copper films and conventional manufacturing methods for forming the copper films.

SUMMARY OF THE INVENTION

A first main objective of the present invention is to provide a copper film that contains tungsten carbide whereby the copper film has fine crystallites, excellent electrical conductivity, high hardness, and good stability at high temperature.
A second main objective of the present invention is to provide a manufacturing method that particularly forms the copper film containing tungsten carbide.
To achieve the foregoing first main objective, the copper film contains tungsten carbide adapted to be formed on a substrate and comprises:
a copper layer containing tungsten carbide in atomic ratios of 0.4 to 12.2% in tungsten and of 0.7 to 7.4% in carbon, the atomic rations are on a basis of total atoms in the copper film.
To achieve the foregoing second main objective, the copper film having the tungsten carbide layer is made by the method having acts of:
adjusting a non-overlapping area between a copper target and a tungsten carbide target; and
co-sputtering the copper target and the tungsten carbide target to form the copper film containing tungsten carbide, wherein sputtering power is 100W and sputtering pressure is 1×10⁻²to 10×10⁻³;
by adjusting the non-overlapping area between the copper target and the tungsten carbide target, ratios of tungsten carbide in the copper film are regularized.
In this present invention, adding the tungsten carbide that is insoluble to copper makes the copper film have excellent electrical conductivity, thermal stability and hardness properties at high temperature. Moreover, by adjusting the non-overlapping area of the copper target and the tungsten carbide target, tungsten carbide is conveniently adjusted in different ratios to achieve various embodiments of the copper films.
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 carbide target used in a co-sputtering process;
FIG. 2 shows top views of four embodiments of the copper target and the tungsten carbide target in different configurations;
FIG. 3(a) is a line graph showing a relationship between the contents of the tungsten and carbon in the copper film and area ratios of the tungsten carbide target;
FIG. 3(b) is a line graph showing a relationship between the deposition speeds of the copper film and the area ratios of the tungsten carbide target;
FIG. 4 shows four peak diagrams from X-ray photoelectron spectroscopy to indicate molecular structures of tungsten carbide in the copper film; FIG. 5 is a line graph showing a relationship between the lattice parameter of the copper film at different annealing temperatures and the area ratios of the tungsten carbide target;
FIGS. 6(a) to 6(d) are SEM photos in cross-sectional views of four copper films having different contents of tungsten carbide, wherein the four copper films respectively represent (a) pure copper film, (b) Cu(W_1.0C_1.5), (c) Cu(W_2.1C_2.1) and (d) Cu(W_12.2C_7.4);
FIGS. 7(a) to 7(b) are SEM photos in planar views of two copper films having different contents of tungsten carbide, wherein the two copper films are respectively represented as (a) pure copper film and (b) Cu(W_12.2C_7.4);
FIGS. 8(a) to 8(b) are two SEM photos in cross-sectional views of a pure copper film and a copper film of Cu(W_12.2C_7.4) both are annealed at different temperatures for a one-hour duration, wherein (a) represents the copper films annealed at 200° C. and (b) represents the copper films annealed at 400° C.;
FIGS. 9(a) to 9(b) are two SEM photos in planar views of a pure copper film and a copper film of Cu(W_12.2C_7.4) both are annealed at different temperatures for a one-hour duration, wherein (a) represents the copper films annealed at 530° C. and (b) represents the copper films annealed at 650° C.;
FIG. 10 is a line graph showing a relationship between Knoop ultra-microhardness (HK) of the copper films at different annealing temperatures and area ratios of the tungsten carbide target;
FIG. 11 is a line graph showing a relationship between resistances of the copper films at different annealing temperatures and area ratios of the tungsten carbide target;
FIG. 12 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_0.4C_0.7), at 400° C.; and (c) Cu(W_0.4C_0.7), at 530° C.; and
FIGS. 13(a) to (c) are cross-sectional FIB micrographs taken from copper films on Si substrates after annealing for 1 hour: (a) pure Cu, at 400° C. (b) Cu(W_0.4C_0.7) at 400° C., and (c) Cu(W_0.4C_0.7) at 530° C., wherein arrows in (c) indicate formations of Cu₄Si at interfaces of the copper film and the Si substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A copper film containing tungsten carbide in accordance with the present invention is adapted to be formed on a substrate and comprises a copper layer in the form of a supersaturated solid solution and tungsten carbide present inside the copper layer that is in structure of nano-crystallite. In the copper film, tungsten carbide is represented in atomic ratios of 0.4 to 12.2% in tungsten and of 0.7 to 7.4% in carbon.
In the copper film, carbon atoms of the tungsten carbide fill in boundaries between metallic materials to make the crystallites minimized. Moreover, tungsten carbide has a high melting point to efficiently reduce lattice diffusion of copper atoms. Therefore, the tungsten carbide is better than the conventional insoluble elements (such as carbon) or pure metal elements.
To achieve the copper film containing a certain ratio of tungsten carbide, a manufacturing method for the copper film is developed and comprises the acts of: adjusting a non-overlapping area between a copper target and a tungsten carbide target; co-sputtering the copper target and the tungsten carbide target to form the copper film containing tungsten carbide, wherein sputtering power is 100W and sputtering pressure is 1×10⁻²to 10×10⁻³; and optionally annealing the copper film containing tungsten carbide to change the microstructure of the copper film.
In this manufacturing method, the non-overlapping area is adjusted to reveal parts of the tungsten carbide target. With reference to FIGS. 1 and 2, a copper target (10) is adapted to be located under a substrate (not shown) with a 12 cm distance to the substrate and has multiple round holes (12) defined through the copper target (10). A tungsten carbide target (20) is attached under the copper target (10) and partially revealed via the round holes (12). When the copper target (10) and the tungsten carbide target (20) are co-sputtered by high-speed particles, the targets (10, 20) are struck by the high-speed particles to sputter to the substrate. Preferably, the substrate is a glass substrate and the co-sputtering process is a radio frequency (RF) magnetron sputter deposition system. In the RF magnetron sputter deposition system, sputtering pressure is reduced to 1×10⁻²to 10×10⁻³first. Then, pure argon is introduced into the system and accelerated with 100W sputtering power to become the high-speed particles to strike the targets (10, 20). When co-sputtering, the substrate is rotated at a constant rotating speed. In FIG. 2, four preferred configurations of the targets are shown and correspondingly converted into fraction ratios of WC in Table 1. Other important parameters in the RF magnetron sputter deposition system are shown in Table 2.

TABLE 1

Target area fraction of WC Tungsten (atom %) Carbon (atom %)

4.0% 0.4 0.7

21.3% 1.0 1.5

29.3% 2.1 2.1

49.9% 12.2 7.4

TABLE 2


Items of parameters	parameter

System base pressure	Below 5 × 10⁻⁷torr
Argon working pressure	1 × 10⁻²
RF power	100 W
Temperature of the	Room temperature to 100° C.
substrate
Targets	Oxygen-free pure copper (99.9%)/WC(99.5%)
Distance between the	The substrate is located over the targets at a 12
targets and the substrate	cm distance

The RF magnetron sputter deposition system enables the making of insoluble elements or compounds to synthesize in atom-by-atom growth to form 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.
Moreover, the copper film is further annealed at different annealing temperatures to change the microstructure of the copper to improve hardness and thermal stability of the copper film. Preferably, the copper film containing tungsten carbide is annealed at an annealing pressure of 1×10⁻⁶to 1×10⁻⁷torr, with a heating speed of 4 to 6° C. per min, at an annealing temperature ranging from 200 to 650° C. for a one-hour duration.
<Qualitative and Quantity Analyses of Tungsten and Carbon in the Copper Layer>
The quantities of tungsten and carbon were detected by an electron probe micro-analyser (EPMA) and respectively shown in FIGS. 3(a) and 3(b) that correspond to Table 1. With reference to FIG. 3(a), the relationship between the non-overlapping area of the targets and the quantity of tungsten carbide in the copper film is shown in a positive ratio. FIG. 3(b) indicates that increasingly adding tungsten carbide decreases deposition of the copper film. To clarify that the tungsten carbide exists in the copper film in the form of solid liquid, the copper film was examined with X-ray photoelectron spectroscopy (XPS) and result is shown in FIG. 4. According to FIG. 4, tungsten and carbon are bound with each other to confirm tungsten carbide that exists in a supersaturated solid solution of Cu. FIG. 5 shows that lattice parameter was increased with increment of the tungsten carbide. However, the lattice parameter was usually reduced with decrement of annealing temperature. Microstructures of the copper film are shown in FIGS. 6(a) to 6(d), when the content of tungsten carbide was increased, crystallization of the copper film became finer and finer from 100 nm (pure copper) to 10 nm (W_12.2C_7.4). The copper film was further observed under SEM photos as shown in FIG. 7, wherein the copper film was constructed by nano-crystallites. Obviously, the presence of tungsten carbide reduces the depositing speed of the copper film, increases the lattice parameters, improves microstructures of the copper film, and exists in the solid solution of copper.
<Mechanical Properties Test for the Copper Film Containing Tungsten Carbide>
Thermal stability at high temperature and the hardness of the copper film are the major subjects in this test. With reference to FIGS. 8 and 9, microstructure photos were taken of the copper films annealing at different annealing temperature in a one-hour duration. In FIG. 8(a), crystallites started to grow in the copper film at 200° C. annealing temperature and the copper film still has a fine microstructure. In FIG. 8 (b), the crystallites have grown and coalesced to form columnar structures. However, the columnar structures still had fine texture in comparison with grains in pure copper films. In FIG. 9(a), the crystallites apparently become twinned structures at 530° C. annealing temperature. However the twinned structures still have a smaller size than the grains in pure copper films. In FIG. 9(b), although the crystallites still have twinned structures at 650° C. annealing temperature, tungsten carbide has precipitated from the solid solution of copper. According to the above observations on the microstructure of the copper film, the copper film containing tungsten carbide has thermal stability at high temperature. The thermal stability of the copper film is demonstrated in the following paragraph when the copper film is combined with a silicon substrate.
Thermal stability of the copper films containing different ratios of tungsten carbide is evaluated in use of depositing copper films deposited on Si substrates and results are shown in XRD patterns in FIG. 12. The silicide is formed at a temperature as low as 200° C. for pure Cu and a noticeable Bragg peak for Cu₄Si is seen in pure Cu after annealing at 400° C. (FIG. 12 a). In contrast, Cu(W_0.4C_0.7) film is thermally stable with Si and no apparent copper silicide (Cu₄Si) formation is found up to 530° C. annealing for one-hour duration. Cross-sectional FIB images of these copper films (FIG. 13) further confirm good thermal stability of copper films. Many pinholes and numerous twin crystallite are observed in pure Cu annealed at 400° C. (FIG. 13 b), as a result of recrystallization. Conversely, the Cu(W_0.4C_0.7) film remains dense and no apparent pinholes are observed after thermal annealing at 400° C. (FIG. 13 b). Copper silicide appears to form at the Cu/Si interface in Cu(W_0.4C_0.7) film after annealing at 530° C. (FIG. 13 c) and consistent with the XRD result shown in FIG. 12 c. Addition of dilute tungsten carbide is thus shown beneficial for the thermal stability improvement in copper film.
With regard to ultra-microhardness of the copper film, as shown in FIG. 10, the copper film annealed at 650° C. for one hour had an ultra-microhardness that is 4 times greater than one of the pure copper film. Moreover, even when the copper film was not annealed or annealed at low temperature, the ultra-microhardness of the copper film containing tungsten carbide is still 3 times greater than the one of the pure copper film. Therefore, the mechanical property of the copper film is increased because of the presence of tungsten carbide no matter whether the copper film is annealed or not. The resistance of the copper film containing tungsten carbide is shown in FIG. 11; the resistance of the copper film is high before annealing. After annealing, the resistance of the copper film greatly reduce, particularly in two samples of Cu(W_0.4C_0.7) and Cu (W_1.0C_1.5). The resistances of these two samples are close to one of pure copper film. The reason for the decrement of the resistance in the copper film is that annealing processes and content of the tungsten carbide change the microstructure of the copper film. Therefore, the copper film containing tungsten carbide in proper proportion and treated by the proper annealing processes has thermal stability at high temperature and low resistance (i.e. high electrical conductivity).
The invention has been described in detail with particular reference to certain preferred embodiments. However, variations and modifications can be effected within the spirit and scope of the invention.
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 carbide for improving electrical conductivity, thermal stability and hardness properties, the copper film comprising:

a copper layer in a form of a supersaturated solid solution; and

tungsten carbide present inside the copper layer in nano-crystallite.

2. The copper film as claimed in claim 1, wherein tungsten carbide is represented as atomic ratios of 0.4 to 12.2% in tungsten and of 0.7 to 7.4% in carbon, the atomic rations are on a basis of total atoms in the copper film.

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

adjusting a non-overlapping area between a copper target and a tungsten carbide target; and

co-sputtering the copper target and the tungsten carbide target to form the copper film containing tungsten carbide, wherein sputtering power is 100W and sputtering pressure is 1×10⁻²to 10×10⁻³torr;

by adjusting the non-overlapping area of the tungsten carbide target, ratios of tungsten carbide in the copper film are regularized.

4. The method as claimed in claim 3, wherein the sputtering temperature in the sputtering act has a range from 25° C. to 100° C.

5. The method as claimed in claim 3, wherein ratios of the non-overlapping area of the tungsten carbide target to the copper target are selectively preferable 4.0%, 21.3%, 29.3% and 49.9 to obtain the copper film containing tungsten carbide in different ratios.

6. The method as claimed in claim 3, wherein the method further comprising an annealing act after the sputtering act and the annealing acts is of:

annealing the copper film containing tungsten carbide at an annealing pressure of 1×10⁻⁶to 1×10⁻⁷torr, a heating speed of 4 to 6° C. per min, at an annealing temperature ranging from 200 to 650° C. for one hour duration.

7. The method as claimed in claim 2, wherein an ultra-microhardness of the copper film containing tungsten carbide increases with increment of the tungsten carbide and maximally is 4 times greater than an ultra-microhardness of a pure copper film.

8. The method as claimed in claim 6, wherein an ultra-microhardness of the copper film containing tungsten carbide after annealing is 3 times greater than an ultra-microhardness of a pure copper film.

9. The method as claimed in claim 4, wherein an ultra-microhardness of the copper film containing tungsten carbide after annealing at 650° C. for one-hour duration is 4 times greater than an ultra-microhardness of a pure copper film.