JP3207393B2 - High-purity tantalum material, tantalum target using the same, thin film and semiconductor device formed using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-purity tantalum material, a tantalum target using the same, a thin film formed using the same, and a semiconductor device.

[0002]

2. Description of the Related Art At present, a tantalum oxide (Ta ₂ O ₅ ) thin film is being studied as a material for a thin film capacitor of a VLSI instead of silicon oxide (SiO ₂ ). Ta ₂ O ₅ is S
Since the dielectric constant is about six times that of iO ₂ , the capacitor area can be reduced. However, Ta ₂ O ₅
Has not been used because it has a larger leakage current than SiO ₂ , or its effective relative dielectric constant decreases when it is made thinner.

The Ta ₂ O ₅ thin film is formed by a reactive sputtering method, a CVD method, or the like. In the case of the reactive sputtering method, a film is formed by sputtering in a mixed gas of argon and oxygen using a tantalum target. Is done.

On the other hand, refractory metal silicides such as molybdenum (Mo) and tungsten (W) have been used as electrode materials for VLSI. Tantalum (Ta) silicide is being studied as the next electrode material. There are several methods for forming a tantalum silicide film.However, when a tantalum film is formed on polycrystalline silicon and then silicon and tantalum react to form tantalum silicide in a self-aligned manner, a pure tantalum target is used. Is used.

In general, the following impurities in a metal material used for a VLSI have an adverse effect on an element, and therefore, high purity is required.

A. Alkali metals such as Na and K (deterioration of interface characteristics) b. Radioactive elements such as U and Th (soft errors) c. Heavy metals such as Fe and Cr (interfacial bonding problems) By the way, tantalum targets that are currently manufactured industrially are obtained by dissolving tantalum purified by an electrolytic method or the like to form a tantalum ingot, which is processed into a target.

However, since it contains a large amount of the above-mentioned elements, it cannot be used for LSI. Since even a very small amount of these elements adversely affects the characteristics of the device, it is necessary to further purify tantalum and produce a tantalum target using the same.

[0008]

The tantalum produced by the conventional technique has a high impurity concentration and cannot be used as an LSI material. Accordingly, an object of the present invention is to provide a high-purity tantalum material usable for a semiconductor device, a tantalum target using the same, a thin film formed using the same, and a semiconductor device.

[0009]

According to the first aspect of the present invention, there is provided a high-purity tantalum material having an oxygen content of 50 ppm or less,
The content of each element of nickel and chromium is 0.05 ppm or less, which is obtained by melting and used as a target .

A tantalum target according to a fourth aspect of the present invention is characterized by using the high-purity tantalum material of the present invention.

The thin film according to the invention of claim 6 of the present invention is characterized in that it is formed by using the tantalum target of the present invention.

A semiconductor device according to a tenth aspect of the present invention is characterized in that the thin film according to the present invention has at least a part thereof.

In response to an increase in the degree of integration of LSIs and miniaturization of elements, signal delay due to an increase in electric resistance has become a problem. Against this background, the next electrode material is
Low electrical resistance is required.

Incidentally, oxygen in the refractory metal silicide film increases the electric resistance. In particular, in recent years, contamination during the film formation process has become extremely small, and the impurities in the target have been directly reflected on the impurity concentration in the film.

Therefore, we have examined in detail the relationship between the oxygen concentration in the tantalum target and the specific resistance of the reactive tantalum target film.

First, a 0.1 μm tantalum film was formed on polycrystalline silicon, and lamp annealing was performed at 1000 ° C. to react tantalum and silicon to form a tantalum silicide film.
The oxygen concentration of the tantalum target is 30 ppm and 50 ppm, respectively.
ppm, 100 ppm, 250 ppm, and 400 ppm. Other impurities have almost the same concentration.

FIG. 1 shows the relationship between the specific resistance of the tantalum silicide film thus formed and the oxygen concentration. As is clear from this result, when oxygen is contained in an amount of 100 ppm or more, the specific resistance increases as the oxygen concentration increases.

As described above, in order to keep the specific resistance of the reactive tantalum silicide film low, the oxygen concentration in the tantalum target must be 50 ppm or less.

On the other hand, when Ta ₂ O ₅ is used as a storage capacitor material instead of SiO ₂ , the biggest problem is that
The point is that the leak current is large. Recently, it has been found that the leakage current is related to the impurity concentration in the target. In particular, when the film thickness becomes very thin, the influence of the trace impurities becomes remarkable.

Therefore, in order to investigate the influence of heavy metal impurities on the leakage current, Ta ₂ O was subjected to reactive sputtering using three types of targets having different manufacturing processes.
_Five thin films were prepared. Table 1 shows the concentrations of iron, nickel, chromium and other elements.

[0021]

[Table 1] The concentrations of the elements other than those shown in Table 1 above are substantially the same for A, B, and C. In addition, all the film thicknesses were about 15 nm.

FIG. 2 shows the relationship between the electric field and the leak current density of each film.

Ta ₂ O ₅ formed using the target A having the lowest concentration of iron, nickel, and chromium has a very low leak current as compared with those using the targets B and C.
Reduction of heavy metal elements is effective for suppressing leakage current, and the respective concentrations need to be 0.05 ppm or less.

As described above, in the tantalum target for VLSI, it is important to reduce sodium, potassium and uranium and thorium, but the concentration of oxygen and heavy metal elements must be reduced.

The high-purity tantalum target satisfying the specifications of the present invention can be manufactured by the following process.

The high-purity tantalum target as described above can be obtained from a high-purity tantalum material produced by combining an iodide decomposition method and electron beam melting.

This iodide decomposition method is a kind of chemical transport method and is a method used for refining active metals such as tantalum, titanium, zirconium and hafnium.
Purification is performed using the reactions of the following formulas (1) and (2).

Ta + 5 / 2I ₂ → TaI ₅ (300 to 700 ° C.) (1) TaI ₅ → Ta + 5 / 2I ₂ (800 to 1500 ° C.) (2) That is, tantalum is iodine and 300 to 700 TaI5 is produced up to a temperature of ° C (Equation (1)). In addition, TaI ₅ is 80
It has the property of decomposing into tantalum and iodine at a high temperature of 0 to 1500 ° C (formula (2)).

FIG. 3 shows an example of an apparatus for producing high-purity tantalum by the iodide decomposition method. In the figure, reference numeral 1 denotes a reaction vessel containing tantalum 4 and iodine 5 as raw materials. Numeral 2 denotes a filament, and a power source 6 is connected through connectors 7a and 7b.
And heated to a temperature of 800 to 1500 ° C. by energizing heating. The entire reaction vessel is placed in a thermostat 3, and 300
Maintained at ~ 700 ° C. In this temperature range, as described above, tantalum and iodine react to generate TaI ₅ by the reaction of the formula (1). TaI ₅ is decomposed on the filament into iodine and tantalum according to the formula (2), and tantalum is deposited on the filament, and the iodine reacts again with tantalum as a raw material to carry tantalum onto the filament. At this time, impurities in the raw material tantalum have lower reactivity with iodine than tantalum, and thus remain in the raw material. In principle, only pure tantalum is carried on the filament. High-purity tantalum by the iodide decomposition method is purified on such a principle. The vapor pressure of various metal iodides greatly depends on the temperature, and the purification temperature of tantalum iodide (300 to 700
C), the vapor pressure of sodium (Na), potassium (K), uranium (U), thorium (Th), iron (Fe), and chromium (Cr) iodides is very low, and the purification effect is higher than this.

On the other hand, the electron beam melting method is a method of decomposing impurities using a difference in vapor pressure. In particular, sodium, potassium and the like having a high vapor pressure have a high refining effect. The titanium purified by the above-mentioned iodide decomposition method is further purified by electron beam melting. Since the melting is performed in a high vacuum of 5 × 10 −5 mbar or less, contamination by oxygen and nitrogen is small, and a high-purity tantalum ingot can be produced.

This ingot is finished into a tantalum target of any shape by forging and machining.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Commercially available tantalum and iodine are placed as raw materials in a Hastelloy reaction vessel shown in FIG.
It was placed in a thermostat heated to 0 ° C. A tantalum filament having a diameter of 2.0 mm was heated to about 1000 ° C. by direct current heating to deposit tantalum on the filament. After 105 hours, the filament grew to a diameter of 25 mm.

The high-purity tantalum thus produced is
Electron beam melting was performed in a vacuum of 1 × 10 ⁻⁵ mbar to further purify. Thereafter, the target was finished by forging and machining.

Table 2 shows the analytical values after the raw material, the iodide decomposition method, and the electron beam melting.

[0035]

[Table 2] As shown in this table, the content of each element can be greatly reduced by combining the iodide decomposition method and the electron beam melting.

Next, using this target, a 0.1 μm tantalum thin film was formed on polycrystalline silicon by a sputtering method, and lamp annealing was performed at 1000 ° C. to produce a tantalum silicide film. When the specific resistance of the film was measured by a four-terminal method, it was 35.2 μΩ · cm.

A Ta ₂ O ₅ film was formed by reactive sputtering using the above-mentioned target, and an electric field was applied to measure the leak current.
The leakage current density was 10 ⁻² A · cm ⁻⁷ .

[0038]

According to the present invention, a higher purity tantalum material can be obtained by electron beam melting tantalum by an iodide decomposition method, thereby obtaining a higher purity tantalum target.

Further, by using the tantalum target of the present invention, a thin film and a semiconductor device having a low electric resistance and effective for suppressing a leak current can be obtained.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a relationship between a specific resistance of a reactive tantalum silicide film and an oxygen concentration in a tantalum target.

FIG. 2 is a characteristic diagram showing electric field strength dependence of leakage current of a Ta ₂ O ₅ thin film.

FIG. 3 is a schematic diagram of a production apparatus for an iodide decomposition method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reaction container 2 Filament 3 Thermostat 4 Tantalum 5 Iodine 6 Power supply 7a, 7b Connector

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-166276 (JP, A) JP-A-62-297463 (JP, A) JP-A-63-238265 (JP, A) JP-A 63-238265 227771 (JP, A) SUZUKI "developm ent of refractory metals and silicid es targets, and the ir chraraceristic s", Tungsten and Ot her Refractory Met als for VLSI (Appli cations ▲ II ▼, Proce edings of the 1986 W orkshop held Novem ber 12-14,1986 ), P. 339-345 (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 14/34 C22C 27/02 H01L 21/203

Claims (10)

(57) [Claims] 1. A high-purity tantalum material characterized by having an oxygen content of 50 ppm or less and an iron, nickel or chromium content of 0.05 ppm or less, obtained by melting and used as a target . 2. The high-purity tantalum material according to claim 1, wherein the content of each element of sodium and potassium is less than 0.1 ppm. 3. The high-purity tantalum material according to claim 1, wherein the nitrogen content is 10 ppm or less. 4. The method according to claim 1, wherein
Characterized by using a high-purity tantalum material of
Tal target. 5. The tantalum target according to claim 4, wherein the aluminum content is less than 0.1 ppm . 6. A thin film formed by using the tantalum target according to claim 4 . 7. Tantalum , tantalum silicide or acid
7. The thin film according to claim 6, comprising at least one of tantalum fluoride. 8. claims, characterized in that a tantalum oxide formed by reactive sputtering using a tantalum target according to claim 4 or claim 5 7
The thin film as described. Of 9. forth in claim 4 or claim 5 after forming a thin film by using a tantalum target, according to claim 7, characterized in that the silicide film obtained is reacted with silicon Thin film. 10. A semiconductor device comprising the thin film according to claim 6 in at least a part thereof.