Patent Grant
6911067
The present invention relates to the field of electroless plating, in particular to solution compositions and a method for electroless formation of alkali-metal-free coatings on the basis of metals, such as cobalt and nickel and composition of these metals with tungsten and phosphorus, which have high resistance to oxidation. Such coatings may find application in semiconductor manufacturing where properties of deposited films and controllability of the composition and physical and chemical characteristics of the deposited films may be critically important.
Copper is increasingly replacing aluminum in interconnects fabrication in ultra-large-scale (ULSI) microelectronic devices. Nevertheless, this technology faces a few problems such as metal corrosion, weak adhesion, high chemical reactivity, and considerable diffusion of copper in silicon. One of the recent approaches to successfully address these issues is the formation of barrier/capping layer by electroless deposition. Thin films of Co(W,P) and Ni(Re,P) prepared by electroless deposition have already been shown to have potential application as barrier/capping layers on copper interconnects. These films provide significantly lower resistivity than other barriers and the formation of very thin, selective, and conformal deposition can be achieved through the electroless deposition.
Several related deposition chemistries shown in Table 1 have been developed and published recently for depositing phosphorous-containing cobalt or nickel-based amorphous barriers.
λYosi Shacham-Diamand, Y. Sverdlov, N. Petrov: “Electroless Deposition of Thin-Film Cobalt-Tungsten-Phosphorus Layers Using Tungsten Phosphoric Acid (H3[P(W3O10)4]) for ULSI and MEMS Applications” Journal of The Electrochemical Society 148 (3), C162-C167 (2001).
1κA. Kohn, M. Eizenberg, Y. Shacham-Diamand, Y. Sverdlov: “Characterization of electroless deposited Co (W, P) thin films for encapsulation of copper metallization” Materials Science and Engineering A302, 18-25 (2001).
2κA. Kohn, M. Eizenberg, Y. Shacham-Diamand, B. Israel, Y. Sverdlov: “Evaluation of electroless deposited Co (W, P) thin films as diffusion barriers for copper metallization” Microelectronic Engineering 55, 297-303 (2001).
3κY. Shacham-Diamand, Y. Sverdlov: “Electrochemically deposited thin film alloys for ULSI and MEMS applications” Microelectronic Engineering 50, 525-531 (2000).
4κYosi Shacham-Diamand, Barak Israel, Yelena Sverdlov: “The electrical and material properties of MOS capacitors with electrolessly deposited integrated copper gate” Microelectronic Engineering 55, 313-322 (2001).
πYosi Shacham-Diamand, Sergey Lopatin: “Integrated electroless metallization for ULSI” Electrochimica Acta 44, 3639-3649 (1999).
θY. Segawa, H. Horikoshi, H. Ohtorii, K. Tai, N. Komai, S. Sato, S. Takahashi, Y. Ohoka, Z. Yasuda, M. Ishihara, A. Yoshio, T. Nogami: “Manufacturing-ready Selectivity of CoWP Capping on Damascene Copper Interconnects” (2001)
A common disadvantage of all known compositions and processes mentioned in Table 1 is that films deposited from the solutions of the aforementioned compounds contains alkali-metal i.e., of Na and K in various alkali metals in concentrations significantly exceeding 2×10−4 atomic % (2 ppm). It is well known, however, that high concentrations of Na and K, which have high mobility, is unacceptable for functional layers of semiconductor wafers used in the manufacture of semiconductor devices. More specifically, the detrimental effect of alkali metals is primarily related to their easy penetration into silicon dioxide and microelectronic components.
Other drawbacks of some of the known solution compositions and processes listed in Table 1 are the following: an increased amount of highly-volatile, contaminating, and toxic components in an electroless deposition solution; relatively noticeable toxicity of some compositions; insufficient anti-corrosive properties of the deposited films; increased amount of ions of precipitation metals with a high degree of oxidation; and non-optimal concentrations of complexing agents required for obtaining deposited films with desired properties.
It is an object of the invention to provide an alkali-metal-free solution for electroless deposition. Another object is to form smooth coating films which are free of alkali-metal components. A further object is to provide aforementioned coating films suitable for formation of barrier/capping layers on semiconductor substrates. Another object is to provide a method for forming alkali-metal-free coating films and for manufacturing IC devices at a reduced cost. It is another object to reduce the amount of highly-volatile, contaminating, and toxic components in an electroless deposition solution. It is a further object to provide the aforementioned solution with reduced toxicity. Still another object is improve anti-corrosive properties of the deposited films. Another object is to minimize the amount of ions of precipitation metals with a high degree of oxidation. A further object is to exclude or minimize the use of solutions, which have a tendency to the formation of gels and various other colloidal aggregates that may impair properties of deposited metal films. Still another object of the invention is to use complexing agents in optimal concentrations which improve quality of the deposited films.
An electroless deposition solution of the invention for forming an alkali-metal-free coating on a substrate comprises a first-metal ion source for producing first-metal ions, a pH adjuster in the form of a hydroxide for adjusting the pH of the solution, a reducing agent, which reduces the first-metal ions into the first metal on the substrate, a complexing agent for keeping the first-metal ions in the solution, and a source of ions of a second element for generation of second-metal ions that improve the corrosion resistance of the aforementioned coating.
The method of the invention consists of the following steps: preparing hydroxides of a metal such as Ni and Co by means of a complexing reaction, in which solutions of hydroxides of Ni and Co are obtained by displacing hydroxyl ions OH− beyond the external boundary of ligands of mono- or polydental complexants; preparing a complex composition based on a tungsten oxide WO3 or a phosphorous tungstic acid, such as H3[P(W3O10)4], as well as on the use of tungsten compounds for improving anti-corrosive properties of the deposited films; mixing the aforementioned solutions of salts of Co, Ni, or W and maintaining a temperature of the mixed solution within the range of 20° C. to 100° C.; and carrying out deposition from the obtained mixed solution.
The deposited films may include Co0.9W0.02P0.08, Co0.9P0.1, Co0.96W0.04B0.001, Co0.96W0.0436, B0.004, C0.9Mo0.02P0.08, or other compounds suitable, e.g., for the formation of barrier layers for copper interconnects in integrated circuits of semiconductor devices. In some embodiments, the film deposited from the deposition solution described herein may include a cobalt tungsten phosphorous alloy film having a phosphorous content of approximately 2% to approximately 14% and a tungsten content of approximately 0.5% to approximately 5%.
According to the invention, electroless plating is carried out in special electroless deposition apparatus disclosed in our earlier U.S. patent application Ser. No. 10/103,015 filed on Mar. 22, 2002. The process is performed by conducting autocatalytic oxidation-reduction reactions on the surface of a semiconductor substrate for deposition of pure metals, such as nickel, cobalt, tungsten, molybdenum, as well as of their accompanying elements such as phosphorus, and/or boron.
Given below is a description of the alkali-free electroless-deposition solution of the present invention. This solution contains no ammonia, and is suitable to deposit an alkali-metal-free layer on various substrates such as noble metals, noble metal activated metals as well as on nickel, cobalt, or copper.
More specifically, the alkali-metal-free deposition solution of the invention may consist of the following components: (i) a metal ion source which can be practically any soluble cobalt (II) salt; (ii) a quaternary ammonium hydroxide to adjust the pH of the solution; (iii) a reducing agent, which reduces the metal ions in the solution into metals layer on the substrate surface; (iv) one or more complexing agents, which keep the metal ions in the solution; (v) a secondary-element source, which improves the corrosion resistance of the layer; and (vi) buffering agent if needed.
Each of the components listed above will be further considered in more detail.
If necessary, other non-essential components can also be added to the bath in order to change properties of the deposited film, rate of deposition, solution stability, and to improve resistance to corrosion. Some of these auxiliary components and their functions are the following:
Tsuda and Ishii (U.S. Pat. No. 4,636,255) showed that the addition of N,N,N′-hydroxyethyleneethylenediamine triacetic acid in circa 4-12 mmol/l concentration could significantly increase the content of phosphorus in a nickel-phosphorous (NiP) deposit.
The applicants have also found that the addition of any inorganic phosphorous oxocompounds which contain phosphorus in oxidation states of III or V can significantly change the content of phosphorus in the deposited film in order to provide desirable properties, such as reduced stress, improved resistance to diffusion, and improved crystallinity of the film structure. Examples of these additional compounds are the following: phosphates, pyrophosphates, and tungsten phosphoric acid. For example, by using a bath containing 71.5 g/l citric acid monohydrate, 21 ml/l 50 wt. % hypophosphorous acid, 23 g/l cobalt (II) sulfate heptohydrate, 7.2 g/l tungsten (VI) oxide, 31 g/l cobalt (II) sulfate heptahydrate, 7.2 g/l tungsten (VI) oxide, 31 g/l boric acid, as well as an appropriate amount of TMAH to adjust the aqueous solution pH to 9-10.2, one can obtain a CoWP film having phosphorous content of about 10 atomic %. When citric acid is replaced with pyrophosphoric acid as a complexing agent in a 61 g/l concentration, the phosphorous concentration of the film changes from 10 atomic % to 2 atomic %.
For capping/passivation layer on copper or as a barrier layer for copper one requires a CoWP thickness of 50-300 Angstrom. Thicker film adversely affects the line resistance while thinner CoWP layer may not be enough for the film to function as a passivation or a barrier layer. Furthermore, the solution should provide a continuous, smooth film and the COWP layer should not contain any pinholes, since these sites can be preferential sites for copper diffusion.
In order to achieve a smoother deposit without using additives the mole ratio of citrate to cobalt should be more than 4 and preferably more than 5 and the pH should be above 9.2 and preferably around 10. The mole ratio of cobalt plus tungsten to hypophosphite should be between 0.4 and 0.90, preferably between 0.45 and 0.85 when tungsten (VI) oxide is used as the source of tungsten. When tungsten phosphoric acid used as the tungsten source the cobalt plus tungsten to hypophosphite ratio should be between 1.2 and 2.6, preferably around 1.68. Further improvement in surface smoothness can be achieved by adding polypropylene glycol to the solution in 0.01-0.1 g/l into the solution. While polypropylene glycols with an average molecular weight of up to 10,000 were tested and all of them exhibited improvement on the film quality, the preferred molecular weight was found to be from 400 to 1000 Mw.
Having described the components of the alkali-metal-free electroless deposition solution of the invention, let us consider the steps of the method of the invention based on the use of the aforementioned solution.
The method of the invention comprises three steps, which are described below in more detail. All these steps occur simultaneously.
Hydroxides of a bivalent cobalt [Co(OH)2, Ni(OH)2] are slightly-dissociated bases and therefore they are poorly soluble in water. In a general form, a reaction of hydroxides with water can be represented as follows:
Solubility of these compounds in water is much lower than 0.01%. Therefore, it has been known to those skilled in the art to prepare aqueous solutions from salts of the aforementioned metals, such as CoSO4 and CoCl2, rather from their hydroxides. However, the aforementioned salts leads to undesired increase in the contents of anions, such as SO42−, Cal−, NO3−, etc., which impair the properties of the deposited films, in particular, resistance of the metal films to corrosion.
Step 1
The authors have found that the aforementioned problems can be solved by dissolving metal hydroxides in the solutions of complexing agents, in which solutions of hydroxides of Ni and Co are obtained by displacing hydroxyl ions OH− beyond the external boundary of ligands of mono- or polydental complexants
where EDTA is ethylenediaminetetraacetic acid. Cobalt and nickel hydrides are known to be unstable in acidic solutions. Therefore the use of complexing agents as their acids can accelerate dissolving.
Reactions (3) and (4) comprise the first step in the process of the invention and determine the aforementioned autocatalytic process of deposition of metals and phosphorus into films.
As has been mentioned above, one of the problems associated with selection of components of the working media for electroless deposition is that a tungsten oxide, which has to be used in the process, is practically insoluble in water and acids and therefore cannot be converted directly into an acid, i.e., via a direct reaction with water. However, tungsten trioxides may be converted to soluble tungstate ions, if they are dissolved in highly alkaline solution. This particular property of trioxides was used by the applicants for achieving one of the objects of the invention. The compounds used by applicants for these purposes comprised alkylammonium hydroxides, such as tetramethylammonium hydroxide (CH4)4NOH (hereinafter referred to as TMAH), tetraethylammonium hydroxide (C2H5)4NOH (hereinafter referred to as TEAOH), tetrabutylammonium hydroxide (C4H9)4NOH (hereinafter referred to as TBAOH), tetrapropylammonium hydroxide (hereinafter referred to as TPA), methyltriethylammonium hydroxide (CH4)(C2H5)3NOH (hereinafter referred to as MTEOH), ethyltrimethylammonium hydroxide (CH4)3(C2H5)3NOH (hereinafter referred to as ETMOH), benzyltrimethylammonium hydroxide (C6H5)CH2(CH4)3NOH (hereinafter referred to as Triton B), phenyltrimethylammonium hydroxide, methyltripropylammonium hydroxide, and a compound that includes a molecular chain of butyl radicals, such as (C4M9—(CH4H7)n—C4H9).4NOH, which is also known as tetrabutylammonium hydroxide. In addition, the electroless deposition solution described herein may include any compound of formula R1R2R3R4NOH, where R1, R2, R3, R4 may be the same or different and may be represented by alkyl, aryl, or alkylaryl groups. In general, alkyl groups may be represented by the formula C2H2n+1. As such, exemplary aryl and alkylaryl groups which may be used for the deposition solution described herein may be selected from benzyl and benzylalkyl of C6H5 and C6H5—CnH2n+1, respectively.
Step 2
The second step of the process consists of preparing a complex composition based on a tungsten oxide WO3, phosphorous tungstic acid, such as H3[P(W3O10)4], or tungstic acid, as well as on the use of tungsten compounds with other degrees of oxidation. The presence of tungsten significantly improves anti-corrosive properties of the deposited films. However, the invention excludes the use of alkali-metal salts of tungstic acid, such as Na2WO4, since these salts are easily hydrolysable with the formation of Na2WO4.2H2O and are easily soluble in water. This is because the presence of sodium in the deposition solution to some extent limits formation of metal films of high purity required for use in semiconductor industry.
As has been mentioned above, one of the problems associated with selection of components of the working media for electroless deposition is that a tungsten oxide, which has to be used in the process, is practically insoluble in water and acids and therefore cannot be converted directly into an acid, i.e., via a direct reaction with water. However, tungsten trioxides may be converted to soluble tungstate ions, if they are dissolved in highly alkaline solution. This particular property of trioxides was used by the applicants for achieving one of the objects of the invention. The compounds used by applicants for these purposes comprised alkylammonium hydroxides, such as tetramethylammonium hydroxide (CH4)4NOH (hereinafter referred to as TMAH), tetraethylammonium hydroxide (C2H5)4NOH (hereinafter referred to as TEAOH), tetrabutylammonium hydroxide (C4H9)4NOH (hereinafter referred to as TBAOH), tetrapropylammonium hydroxide (hereinafter referred to as TPA), methyltriethylammonium hydroxide (CH4)(C2H5)3NOH (hereinafter referred to as MTEOH), ethyltrimethylammonium hydroxide (CH4)3(C2H5)NOH (hereinafter referred to as ETMOH), benzyltrimethylammonium hydroxide (C6H5)CH2(CH4)3NOH (hereinafter referred to as Triton B), phenyltrimethylammonium hydroxide, methyltripropylammonium hydroxide, and a compound that includes a molecular chain of butyl radicals, such as tetrabutylammonium hydroxide (C4M9—(CH4H7)n—C4H9).4NOH, which is also known as tetrabutylammonium hydroxide.
The use of TMAH is less desirable in view of its high volatility and toxicity.
It is more preferable to use ethyl-, propyl-, and butylammonium hydroxides which are less volatile and toxic.
In the aforementioned compounds, alkyl radicals should have optimal mobility required for maintaining pH of the medium. The applicants have found that such compounds as TBAOH, TEAOH, and TPA may satisfy the requirement of radical mobility, and at the same time do not create obstacles for formation of water-soluble complexes with tungsten trioxides. Heavier alkyls, beginning from pentyls, decrease solubility of the complexes in water. The applicants assume that this phenomenon is associated with electron-density screening which is higher in alkyls of larger dimensions.
Step 3
In the third step, for deposition of coating films, the aforementioned solutions of salts of Co, Ni, or W are mixed and maintained under a temperature within the range of 20° C. to 100° C. The deposited films may include, e.g., Co0.9W0.02P0.08, Co0.9P0.1, Co0.96W0.04B0.001, Co0.96W0.0436, B0.004, C0.9Mo0.03P0.08 or other compounds suitable, e.g., for the formation of barrier layers for copper interconnects in integrated circuits of semiconductor devices.
The invention will be further described with reference to Practical Examples. In the following examples, the content of elements in the coating films was obtained by means of an ion microprobe known as SIMS (Secondary Ion Mass Spectrometry technique), in which a high energy primary ion beam is directed at an area of the sample whose composition is to be determined. The values obtained by the SIMS method will be given in atomic percents.
Five deposition solutions, each having a volume of 1 liter, were prepared by mixing the following components with an increase in the content of each component: 50 g to 100 g of citric acid monohydrate (C6O7H8xH2O) with 10 g difference between the subsequent solutions; 15 ml to 27 ml of a 50 wt. % hypophosphorous acid (H3PO2) with 3 ml difference between the subsequent hypophosphorous acids; 18 g to 26 g of cobalt sulfate heptahydrate (CoSO4x7H2O) with 2 g difference between subsequent cobalt sulfate heptahydrates; 24 g to 36 g of boric acid (H3BO3 with 3 g difference between the subsequent boric acids; 11 g to 16 g of tungsten (VI) oxide (WO3) with 1.5 g difference between the subsequent; and an appropriate amount of TMAH for each solution required to reach an appropriate alkaline pH. The deposition was performed at a bath temperature of 75° C. The deposition rates were within the range of 180 to 220 Angstrom/min. The composition of the obtained coating film was determined with the use of SIMS showed that the film contained 5-6 atomic % phosphorous, 7.0-7.5 atomic % tungsten, and cobalt as balance. Furthermore, the results of the SIMS analysis showed that the content of Na and K did not exceed 2×10−4 atomic % (2 ppm).
Analysis showed that films deposited from the electroless deposition solution prepared in Practical Example 1 had high anti-corrosive properties.
Five deposition solutions, each having a volume of 1 liter, were prepared by mixing the following components with an increase in the content of each component: 50 g to 90 g of citric acid monohydrate (C6O7H8xH2O) with 10 g difference between the subsequent solutions; 15 ml to 27 ml of a 50 wt. % hypophosphorous acid (H3PO2) with 3 ml difference between the subsequent hypophosphorous acids; 18 g to 26 g of cobalt sulfate heptahydrate (CoSO4x7H2O) with 2 g difference between subsequent cobalt sulfate heptahydrates; 24 g to 36 g of boric acid (H3BO3 with 3 g difference between the subsequent boric acids; 11 g to 16 g of tungsten (VI) oxide (WO3) with 1.5 g difference between the subsequent; and an appropriate amount of TBAOH for each solution required to reach an appropriate alkaline pH of 9.3 to 9.7. The deposition was performed at a bath temperature of 75° C. The deposition rates were within the range of 220 to 260 Angstrom/min. The composition of the obtained coating film was determined with the use of SIMS showed that the film contained 6.5 to 7.5 atomic % phosphorous, 3.5 to 4.0 atomic % tungsten, and cobalt as balance. Furthermore, the results of the SIMS analysis showed that the content of Na and K did not exceed 2×10−4 atomic % (2 ppm).
It can also be seen that the electroless deposition solution prepared in Practical Example 2 possessed lower toxicity than a majority of the known deposition solutions.
Five deposition solutions, each having a volume of 1 liter, were prepared by mixing the following components with an increase in the content of each component: 50 g to 90 g of citric acid monohydrate (C6O7H8xH2O) with 10 g difference between the subsequent solutions; 15 ml to 27 ml of a 50 wt. % hypophosphorous acid (H3PO2) with 3 ml difference between the subsequent hypophosphorous acids; 18 g to 26 g of cobalt sulfate heptahydrate (CoSO4x7H2O) with 2 g difference between subsequent cobalt sulfate heptahydrates; 24 g to 36 g of boric acid (H3BO3 with 3 g difference between the subsequent boric acids; 11 g to 16 g of tungsten (VI) oxide (WO3) with 1.5 g difference between the subsequent; and an appropriate amount of TEAOH for each solution required to reach an appropriate alkaline pH of 9.3 to 9.7. The deposition was performed at a bath temperature of 75° C. The rates of deposition were within the range of 80 to 140 Angstrom/min. The composition of the obtained coating film was determined with the use of SIMS showed that the film contained 9.5 to 10.0 atomic % phosphorous, 0.5 to 1.0 atomic % tungsten, and cobalt as balance. Furthermore, the results of the SIMS analysis showed that the content of Na and K did not exceed 2×10−4 atomic % (2 ppm).
Analysis showed that, along with a reduced toxicity of the solution and high anti-corrosive properties of the deposited films, the deposited films has a very low concentration of metals prone to oxidation.
Five deposition solutions, each having a volume of 1 liter, were prepared by mixing the following components with an increase in the content of each component: 60 g to 100 g of citric acid monohydrate (C6O7H8xH2O) with 10 g difference between the subsequent solutions; 30 ml to 42 ml of a 50 wt. % hypophosphorous acid (H3PO2) with 3 ml difference between the subsequent hypophosphorous acids; 16 g to 24 g of cobalt sulfate heptahydrate (CoSO4x7H2O) with 2 g difference between subsequent cobalt sulfate heptahydrates; 9.5 g to 14.5 g of tungsten (VI) oxide (WO3) with 1.5 g difference between the subsequent; and an appropriate amount of TPA for each solution required to reach an appropriate alkaline pH of 10.1 to 10.5. The deposition was performed for each solution at three different bath temperatures of 55° C., 65° C., and 75° C. The rates of deposition were within the range of 90 to 260 Angstrom/min. The composition of the obtained coating film was determined with the use of SIMS showed that the film contained 6.5 to 7.5 atomic % phosphorous, 3.5 to 4.0 atomic % tungsten, and cobalt as balance. Furthermore, the results of the SIMS analysis showed that the content of Na and K did not exceed 2×10−4 atomic % (2 ppm).
Improved properties of the obtained films showed that complexing agents had optimal concentrations in the deposition solution.
Thus it has been shown that the invention provides an alkali-metal-free solution for electroless deposition, makes it possible to reduce the amount of highly-volatile, contaminating, and toxic components in an electroless deposition solution, provides aforementioned solutions with reduced toxicity, improves anti-corrosive properties of the deposited films, minimizes the amount of ions of precipitation metals with a high degree of oxidation, excludes or minimizes the use of solutions, which have a tendency to the formation of gels and various other colloidal aggregates that may impair properties of deposited metal films, makes it possible to use complexing agents in optimal concentrations which improve quality of the deposited films, allows to form smooth coating films which are free of alkali-metal components, provides aforementioned coating films suitable for formation of barrier/capping layers on semiconductor substrates, and provides a method for forming alkali-metal-free coating films and for manufacturing IC devices at a reduced cost.
The invention has been shown and described with reference to specific embodiments, which should be construed only as examples and do not limit the scope of practical applications of the invention. Therefore any changes and modifications in technological processes, components and their concentrations in the solutions are possible, provided these changes and modifications do not depart from the scope of the patent claims.
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| Number | Date | Country | |
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