JP3239998B2 - Semiconductor substrate cleaning method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for cleaning a surface of a semiconductor substrate such as a silicon wafer.

[0002]

2. Description of the Related Art Metal impurities, fine particles having a particle diameter of 1 μm or less, organic substances, and the like adhere to the surface of a semiconductor substrate of this type during the manufacturing process, and processing damage is formed. As semiconductor devices become more highly integrated and more sophisticated, it is increasingly required that the surface of the semiconductor substrate is not contaminated with these metallic impurities, fine particles, and organic substances, and that there is no processing damage. Cleaning technology has become extremely important in the overall semiconductor device technology.

As a conventional method for cleaning a semiconductor substrate, an RCA cleaning method using an SC-1 solution of hydrogen peroxide and ammonium hydroxide and an SC-2 solution of hydrogen peroxide and dilute hydrochloric acid is known. In the RCA cleaning method, a semiconductor substrate is first immersed in an SC-1 solution, and fine particles and organic substances are removed from the substrate by the oxidizing and alkaline properties of the solution. That is, in this SC-1 solution, both oxidation and reduction reactions are performed simultaneously, and reduction by ammonia and oxidation by hydrogen peroxide compete in the same tank, and at the same time, fine particles and organic substances are etched by the ammonium hydroxide solution. By removing from the substrate surface. Also, mechanical minute damage caused by processing of the semiconductor substrate is removed. Next, the semiconductor substrate is immersed in a hydrofluoric acid aqueous solution to remove a natural oxide film on the substrate surface, and then the semiconductor substrate is immersed in an acidic solution of SC-2 solution, and alkali ions or metal impurities insoluble in SC-1 solution. Is removed. Therefore, the RCA cleaning re-cleans the substrate surface cleaned by the etching action of the ammonium hydroxide solution by cleaning with an acidic solution.

On the other hand, when a semiconductor substrate is cleaned, metal impurities in the cleaning solution are prevented from adhering to the substrate surface, and metal impurities once removed from the substrate surface are prevented from re-adhering,
A semiconductor substrate cleaning solution and a cleaning method for suppressing unnecessary growth of a natural oxide film have been disclosed (JP-A-7-94).
458). This cleaning solution contains ammonia from 0.0001 to
It is composed of an acidic solution such as hydrofluoric acid containing 0.001% by weight or 0.0005 to 0.01% by weight of ethylenediaminetetraacetic acid (EDTA). This cleaning liquid is maintained at a strong acid of about 1 in order to remove an oxide film. According to this cleaning method, when the semiconductor substrate is immersed in the cleaning liquid, the removal of the natural oxide film and the removal of metal impurities proceed simultaneously, and the metal element transferred into the cleaning liquid forms a complex to form a metal complex salt and is masked. Since the surface of the semiconductor substrate is negatively charged in the acidic solution, the metal complex salt also forms complex ions in the acidic solution and the surface of the metal complex 7 is negatively charged. Re-adhesion is prevented.

[0005]

However, the above RC
In the A-cleaning method, since two actions of oxidation and reduction occur in the same bath, first, metal impurities released from the substrate surface remain in the SC-1 solution, and are re-applied to the substrate surface by the surface potential. Secondly, even if an organic acid tries to form a metal complex by complexing metal ions in the SC-1 solution with an organic acid, the organic acid is decomposed by redox treatment in the SC-1 solution, The complexing action becomes extremely reduced. For this reason, the RCA cleaning method has a disadvantage that metal impurities are not sufficiently removed depending on the type of metal. Further, in the cleaning method disclosed in Japanese Patent Application Laid-Open No. 7-94458, the surface of the fine particles is slightly positively charged in a strong acid cleaning solution. However, there is a defect that the hetero-aggregation adheres to the surface of the substrate charged to the positive side from the surface and cannot be removed. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for cleaning a semiconductor substrate in which minute damage caused by processing of the semiconductor substrate, organic substances, metal impurities and fine particles adhering to the surface of the semiconductor substrate can be satisfactorily removed in a small number of steps.

[0006]

The invention according to claim 1 is
Step 11 of oxidizing and reducing a semiconductor substrate as shown in FIG.
And oxidizing the redox semiconductor substrate 12
And a step of reducing the oxidized semiconductor substrate, a step of rinsing the reduced semiconductor substrate, and a step of re-oxidizing the rinsed semiconductor substrate.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A cleaning method according to a preferred embodiment of the present invention comprises a step 11 of dipping a semiconductor substrate in a mixed solution of hydrogen peroxide and ammonium hydroxide as shown in FIG. Step 12 of immersing the semiconductor substrate immersed in the liquid in a dissolved ozone aqueous solution, nitric acid or hydrogen peroxide aqueous solution of one kind or an oxidizing solution in which two or more kinds are mixed.
And a step 13 of immersing the semiconductor substrate immersed in the oxidizing solution in a mixed solution of an organic acid or an organic acid salt containing a carboxyl group and hydrofluoric acid, and a step of immersing the semiconductor substrate immersed in the mixed solution in an organic acid containing a carboxyl group or A step 14 of immersing in a liquid containing an organic acid salt or a mixed liquid of an organic acid or an organic acid salt and hydrofluoric acid; and a step of immersing the semiconductor substrate immersed in the liquid containing the organic acid or the organic acid salt in a dissolved ozone aqueous solution, nitric acid or hydrogen peroxide. Immersing in any one kind of oxidizing solution of water or an oxidizing solution obtained by mixing two or more kinds of water.

[0008] In step 11, the semiconductor substrate is oxidized and reduced continuously in the same solution to effectively remove the micro-damage layer having a thickness of about several nanometers on the surface of the semiconductor substrate. In particular, when a semiconductor substrate is oxidized and reduced with a mixed solution of hydrogen peroxide and ammonium hydroxide, which is a solution corresponding to the SC-1 solution used in the RCA cleaning method, the reduction with ammonia and the oxidation with hydrogen peroxide are performed in the same bath. At the same time, fine particles and organic substances are removed from the substrate surface by the etching action of the ammonium hydroxide solution, and minute damage caused by processing the substrate is removed. However, during these solution treatments, redeposition of metal impurities and fine particles may occur. In the step 12, the density of the oxide film formed after the step 11 is further improved by a chemical oxidizing action, so that the next step 13
By dissolving this oxide film, metal impurities and fine particles are easily separated from the substrate surface.

In step 13, the metal impurities and fine particles taken into the oxide film in steps 11 and 12 are separated from the substrate surface by dissolving the oxide film. In particular, when a semiconductor substrate is immersed in a mixed solution containing hydrofluoric acid and an organic acid or an organic acid salt, the hydrofluoric acid dissolves the oxide film, and metal impurities and fine particles are separated from the substrate surface. To form a metal complex salt. The complex ion of this metal complex salt is a negative ion. In addition, both the surface of the fine particles and the surface of the substrate where the oxide film remains have negative charges due to the adsorption of organic acid ions. As a result, redeposition of metal impurities and fine particles on the substrate surface is prevented. By changing the type and concentration of the organic acid or organic acid salt, the metal complexing effect by the organic acid ion and the surface potential (zeta potential) of the metal complex salt can be controlled. That is, the ability of the organic acid ion to form a complex is chemically determined by the complex stability constant of the organic acid ion and the metal ion forming the complex. The larger this constant, the more complex ion formation is promoted. By forming a complex ion as described above, the charge of the metal ion changes from plus to minus. The concentration of hydrofluoric acid used in step 13 is 0.005 to 0.25% by weight. In particular, 0.005 to 0.10% by weight is preferable, and 0.05 to
0.10% by weight is more preferred. If the amount is less than 0.005% by weight, the natural oxide film on the surface of the semiconductor substrate is poorly peeled off. If the amount is more than 0.25% by weight, the solution becomes a strong acid and dissociation of the organic acid in the solution is suppressed. As the effect decreases, the surface potential of the fine particles becomes close to 0, and the oxide film on the substrate surface is completely removed, so that the fine particles adhere again to the substrate surface.

In step 14, metal impurities and fine particles remaining on the substrate surface that cannot be completely removed in step 13 are more efficiently removed. These metal impurities and fine particles are residues at the solid-liquid interface in step 13, are in an equilibrium state in the water film forming the solvent molecule layer on the substrate surface, and are not adsorbed on the substrate surface. The metal impurities form a metal complex salt by the organic acid ion, and the organic acid ion is adsorbed on the fine particles. As a result, similarly to the step 13, the metal complex salt and the fine particles negatively charged by the organic acid ion are easily separated from the substrate surface. In step 14, a liquid containing an organic acid or an organic acid salt having the same composition as in step 13 may be used,
The concentration or type of the organic acid or organic acid salt may be mutually different. The type and concentration of the organic acid or organic acid salt in the liquid used in Steps 13 and 14 are determined according to the type of metal impurity to be removed. The concentration of the organic acid or organic acid salt in the liquid in both steps is 0.0001% by weight or more. Preferably it is 0.003 to 10% by weight. If it is less than 0.0001% by weight, there is a problem that the complexing action of metal impurity ions released from the substrate surface is not sufficient. If a small amount of hydrofluoric acid is further added to the organic acid or the organic acid salt in step 14, the natural oxide film formed on the surface of the substrate is lightly etched, so that the fine particles and metal impurities on the natural oxide film become organic acid or organic acid. It can be easily transferred into a solution obtained by adding hydrofluoric acid to an acid salt. That is, the addition of hydrofluoric acid can remove not only the natural oxide film but also the metal impurities in the natural oxide film. In this case, the concentration of hydrofluoric acid is 0.1% by weight or less. Especially 0.0
It is preferably at most 1% by weight. When the content exceeds 0.1% by weight, the surface potential of the substrate in the liquid varies due to excessive etching of the natural oxide film on the surface, so that the fine particles and the metal may be re-attached.

In step 15, first, the effect of removing silicide-based metal, particularly Cu, which has contaminated the substrate surface and its vicinity is enhanced, and secondly, the organic acid or organic acid salt used in steps 13 and 14 is removed. The remaining components or organic substances adhering to the substrate surface are decomposed and removed, and thirdly, the cleaned substrate surface is chemically protected with an oxide film. Cu is directly dissolved and removed in a slightly acidic solution having a high oxidation potential. Also, by protecting the substrate surface with a chemical oxide film, the adhesion of fine particles at the solid-gas interface is reliably prevented. Examples of the oxidizing solution used in step 12 or step 15 include a dissolved ozone aqueous solution, nitric acid, or a hydrogen peroxide solution. Among them, the dissolved ozone aqueous solution is preferable because it has high purity, low concentration, high oxidizing power, and easy availability. The dissolved ozone aqueous solution preferably has an ozone concentration of 0.5 ppm or more. If it is less than 0.5 ppm, it is difficult to form a hydrophilic oxide film on the substrate surface, and the action of decomposing and removing organic acids and organic substances attached to the substrate surface is reduced. Since the solubility limit of ozone in pure water is about 25 ppm, the ozone concentration of the dissolved ozone aqueous solution is more preferably 2 to 25 ppm.

The organic acid or organic acid salt used in step 13 or step 14 includes oxalic acid, citric acid, succinic acid, ethylenediaminetetraacetic acid, tartaric acid, salicylic acid,
Formic acid, maleic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, benzoic acid,
One or more organic acids selected from the group consisting of acrylic acid, adipic acid, malonic acid, malic acid, glycolic acid, phthalic acid, terephthalic acid and fumaric acid, or salts thereof. The above-listed organic acids or organic acid salts have a function of complexing metal ions of impurities that contaminate the substrate.

[0013]

Next, examples of the present invention will be described together with comparative examples. <Example 1> An uncleaned silicon wafer having undergone a normal polishing step was subjected to a cleaning treatment under the following conditions. In step 11, the above silicon wafer was immersed in an SC-1 solution (H ₂ O: H ₂
O ₂ (30%): NH ₄ OH (29%) = 5: 1: 0.5 mixture)
And treated at 80 ° C. for 10 minutes. Then step 12
The silicon wafer has an ozone concentration of 5 ppm
For 10 minutes in a dissolved ozone aqueous solution at room temperature. Next, as a step 13, a solution in which 0.05% by weight of hydrofluoric acid is added to a solution in which 0.06% by weight of citric acid as an organic acid is mixed with pure water is prepared. Was immersed in the silicon wafer for 5 minutes. Next, as a step 14, a liquid in which 0.6% by weight of an organic acid, citric acid, was added to pure water was prepared, and the silicon wafer after the step 13 was rinsed with this room temperature liquid for 5 minutes. Finally, as a step 15, the rinsed silicon wafer was immersed in a dissolved ozone aqueous solution having an ozone concentration of 5 ppm at room temperature for 10 minutes.

Example 2 In step 13, oxalic acid was used as an organic acid in pure water instead of citric acid in an amount of 0.03.
The silicon wafer was cleaned by repeating the method of Example 1 except that the mixing was carried out in the same manner as in Step 14 except that 0.3% by weight of oxalic acid was added instead of citric acid as an organic acid to pure water in Step 14. did.

<Example 3> In step 13, 0.001% by weight of ethylenediaminetetraacetic acid was mixed instead of citric acid as an organic acid with pure water. The silicon wafer was cleaned by repeating the procedure of Example 1 except that 0.001% by weight of ethylenediaminetetraacetic acid was added instead of citric acid.

<Example 4> In step 13, 0.014% by weight of formic acid was mixed as an organic acid with pure water instead of citric acid. In step 14, citric acid was used as an organic acid with pure water. The silicon wafer was cleaned by repeating the method of Example 1 except that 0.14% by weight of formic acid was added instead of.

Example 5 In step 13, succinic acid was used as an organic acid in pure water instead of citric acid in an amount of 0.03.
5% by weight and in step 14, 0.35 succinic acid was used instead of citric acid as an organic acid in pure water.
The silicon wafer was cleaned by repeating the method of Example 1 except that it was added by weight%.

EXAMPLE 6 In step 13, salicylic acid was used as an organic acid in pure water in place of citric acid in an amount of 0.06.
4% by weight and in step 14, 0.4 g of salicylic acid was used instead of citric acid as an organic acid in pure water.
The silicon wafer was cleaned by repeating the method of Example 1 except that it was added by weight%.

<Example 7> In step 13, maleic acid was added to pure water in place of citric acid as an organic acid in an amount of 0.0
35% by weight and in step 14, maleic acid was used instead of citric acid as an organic acid in pure water in an amount of 0.1%.
The method of Example 1 was repeated, except that 35% by weight was added, to clean the silicon wafer.

<Comparative Example> A conventional cleaning method using an SC-1 solution was employed as a cleaning method of a comparative example. That is, an uncleaned silicon wafer that has been subjected to a normal polishing step as in Example 1 is replaced with an SC-1 solution (H ₂ O: H ₂ O ₂ (30%): NH ₄ OH (29%) =
(A mixed solution of 5: 1: 0.5) and treated at 80 ° C. for 10 minutes, and then the silicon wafer was rinsed with ultrapure water for 5 minutes. <Comparison Test and Evaluation> (a) Number of Remaining Fine Particles The number of fine particles having a particle size of 0.12 μm or more remaining on the silicon wafer surface after cleaning in each of Examples 1 to 7 and Comparative Example was determined by particles. By counting with a counter, the number of fine particles remaining on the wafer was calculated. The result is shown in FIG. As is clear from FIG.
The number of fine particles remaining on the wafers cleaned by the methods of Examples 1, 2, 3, 4, 5, 6, and 7 was 11, 12, respectively.
6, 21, 33, 48, 20, and 22 were small.
On the other hand, the number of fine particles remaining on the wafer cleaned by the method of the comparative example was extremely large at 420. From this, it was found that the cleaning methods of Examples 1 to 7 clean fine particles better than the cleaning methods of Comparative Examples.

(B) Concentration of Metal Impurities Five kinds of metal impurity concentrations of Al, Fe, Ni, Cu and Zn on the silicon wafer surface after cleaning in Examples 1 to 7 and Comparative Example were measured. The metal impurity concentration is determined by dropping a mixed acid of hydrofluoric acid and nitric acid at the center of the cleaned silicon wafer, circulating the droplets so that the droplets pass over the entire surface of the wafer, and collecting the droplets. By atomic absorption spectrometry. Table 1 shows the results. In Table 1, “<1 × 10 ⁹ ” means less than 1 × 10 ^{9 and} below the detection limit.

[0022]

[Table 1]

As is clear from Table 1, the wafers of Examples 1 to 7 were the same as those of the conventional comparative example except that the Ni concentration and the Cu concentration were equal to each other. For all kinds of metals, Examples 1 to 7 showed better cleaning effects than Comparative Examples.

[0024]

As described above, according to the cleaning method of the present invention, the semiconductor substrate is subjected to a chemical reaction in the order of oxidation-reduction, oxidation, reduction, rinsing, and oxidation, whereby minute damage caused by processing of the semiconductor substrate is obtained. In addition, organic substances, metal impurities, and fine particles adhering to the surface of the semiconductor substrate can be satisfactorily removed in a small number of steps.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cleaning step according to an embodiment of the present invention.

FIG. 2 is a diagram showing the number of fine particles remaining on the silicon wafer surface after cleaning in Examples 1 to 7 and Comparative Example.

[Explanation of symbols]

 Reference Signs List 11 Redox process of semiconductor substrate 12 Oxidation process of semiconductor substrate 13 Reduction process of semiconductor substrate 14 Rinsing process of semiconductor substrate 15 Oxidation process of semiconductor substrate

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Ryoko Takada 1-5-1, Otemachi, Chiyoda-ku, Tokyo Mitsubishi Materials Corporation Silicon Research Center (56) References JP-A-10-209100 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01L 21/304

Claims (8)

(57) [Claims] A step of oxidizing and reducing a semiconductor substrate (11);
Oxidizing the redox semiconductor substrate (12), reducing the oxidized semiconductor substrate (13), rinsing the reduced semiconductor substrate (14), and re-rinsing the rinsed semiconductor substrate. A method of cleaning a semiconductor substrate, comprising a step (15) of oxidizing. 2. The method according to claim 2, wherein the oxidation of the semiconductor substrate is carried out by a dissolved ozone aqueous solution,
One type of oxidizing solution of nitric acid or hydrogen peroxide or 2
The cleaning method according to claim 1, wherein the cleaning method is performed by immersing in an oxidizing solution in which a plurality of types are mixed. 3. The cleaning method according to claim 1, wherein the reduction of the semiconductor substrate is performed by immersing the substrate in a mixed solution of an organic acid or an organic acid salt containing a carboxyl group and hydrofluoric acid. 4. The cleaning method according to claim 1, wherein the rinsing of the semiconductor substrate is performed by immersing the semiconductor substrate in a liquid containing an organic acid or an organic acid salt containing a carboxyl group or a mixed liquid of an organic acid or an organic acid salt and hydrofluoric acid. . 5. The cleaning method according to claim 3, wherein the concentration of hydrofluoric acid is 0.005 to 0.25% by weight. 6. The cleaning method according to claim 4, wherein the concentration of hydrofluoric acid is 0.01% by weight or less. 7. An organic acid or an organic acid salt having a concentration of 0.0
5. The cleaning method according to claim 3, wherein the amount is 001% by weight or more. 8. An organic acid or an organic acid salt comprising oxalic acid, citric acid, succinic acid, ethylenediaminetetraacetic acid, tartaric acid,
Salicylic acid, formic acid, maleic acid, acetic acid, propionic acid,
Butyric, valeric, caproic, enanthic, caprylic,
4. An organic acid selected from the group consisting of benzoic acid, acrylic acid, adipic acid, malonic acid, malic acid, glycolic acid, phthalic acid, terephthalic acid and fumaric acid, or a salt thereof. Or the washing method of 4.