JP3288321B2 - Semiconductor substrate cleaning method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for cleaning a semiconductor substrate contaminated with metal impurities.

[0002]

2. Description of the Related Art As the demand for higher reliability of semiconductor devices increases, the demand for cleaning the surface of silicon wafers has become more severe.

Conventionally, as a method of cleaning a silicon wafer before a gate oxide film is formed, wet cleaning based on so-called RCA cleaning has been the mainstream. The RCA cleaning includes removing organic substances using sulfuric acid-hydrogen peroxide solution (SPM), removing particles using ammonia-hydrogen peroxide solution (APM), removing metal impurities using hydrochloric acid-hydrogen peroxide solution (HPM), and natural oxidation using diluted hydrofluoric acid. It consists of film removal and cleaning with ultrapure water.

Hereinafter, a first conventional technique using RCA cleaning will be described with reference to the drawings.

FIGS. 2 and 3A are schematic cross-sectional views of a substrate to be cleaned. A natural oxide film 2 is formed on the surface of the silicon substrate 1. Organic substances 3 and particles 6 adhere to the surface of the natural oxide film 2,
Cu particles 5 and Fe particles 4 adhere to the surface of the silicon substrate 1 in a form embedded therein. These contaminants adhere to the silicon substrate due to various factors when processing or transporting the silicon substrate. The figure shows C as a metal impurity.
Although the u-particles 5 and the Fe-particles 4 are representatively shown, generally, metal impurities such as Ti, Co, and Pt may be attached.

The substrate is first cleaned by SPM to remove organic substances 3, and then particles 6 are removed by APM (FIG. 3B).

Next, metal impurities adhering to the surface of the native oxide film 2 are removed by HPM (FIG. 3C).

Next, the natural oxide film is removed using dilute hydrofluoric acid (FIG. 3D). Thereafter, final cleaning is performed with ultrapure water, and the cleaning of the substrate surface is completed.

However, this method has a problem that noble metal-based metal impurities such as Cu particles 5 remain on the silicon substrate as shown in FIG. 3D. This is because noble metals such as Cu particles 5 are present in a form adsorbed on the silicon substrate 1 and are difficult to remove with dilute hydrofluoric acid or the like.

[0010] Further, the above-mentioned noble metal tends to adhere again to the substrate even once removed. This is because the noble metal has a high oxidation-reduction potential and, when present as an ion in a diluted hydrofluoric acid solution, causes an electron exchange reaction to be adsorbed on the silicon substrate. The phenomenon of reverse adsorption of a noble metal onto a silicon substrate is described in, for example, Journal of Ele
ctrochemical Society 191 (10) (1994) 2834 H. Morinaga
et al.

In the above process, the natural oxide film made of diluted hydrofluoric acid is removed after the HPM cleaning. However, the order may be reversed. That is, after the natural oxide film is first removed with dilute hydrofluoric acid, HPM cleaning can be performed. However, even in this case, the problem that the noble metal-based particles remain as shown in FIG.

Next, a second conventional technique including a step of providing a sacrificial oxide film will be described with reference to FIG.

FIG. 4A is a schematic sectional view of a substrate to be cleaned. A natural oxide film 2 is formed on the surface of the silicon substrate 1. Organic substances 3 and particles 6 adhere to the surface of the natural oxide film 2, and Cu particles 5 and Fe particles 4 adhere to the surface of the silicon substrate 1 in a form embedded in the natural oxide film 2. As in the description of the first related art, Cu particles 5 and Fe particles 4 are representatively shown as metal impurities.

A heat treatment is applied to the substrate to grow a sacrificial oxide film 7 (FIG. 4B). At this time, the organic substance 3 is decomposed and removed by oxygen during the heat treatment. On the other hand, some metal impurities are easy to oxidize and some are hard to oxidize, and exhibit different behavior depending on the type of metal. For example, Fe or the like is oxidized and remains as an oxide in the sacrificial oxide film 7. Cu
Noble metals hardly turn into oxides and remain as non-oxides. Here, since Cu diffuses into the silicon substrate at a high speed, Cu enters the inside of the silicon substrate 1 as shown in FIG. 4B during the formation of the sacrificial oxide film.

Next, as shown in FIG. 4C, the sacrificial oxide film 7 is removed with diluted hydrofluoric acid. Thus, the Fe oxide and the like existing in the sacrificial oxide film 7 are removed.

However, this method has a problem in that heat treatment is applied at the stage of growing the sacrificial oxide film 7, so that particles such as Cu diffuse into the substrate and cannot be removed. Metals that diffuse rapidly to the silicon substrate include Cu and C
o, Ni, etc., and the same problem occurs when these metal impurities adhere to the substrate.

[0017]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and is intended to sufficiently remove noble metal-based particles and metals which are rapidly diffused into silicon, which were conventionally difficult to remove. An object of the present invention is to provide a cleaning method that can perform the cleaning.

[0018]

According to the present invention, a clean sky is provided.
By irradiating the surface of the semiconductor substrate with ultraviolet rays in a gaseous atmosphere in the presence of ozone or oxygen and at an ambient temperature of 5 to 40 ° C., the metal impurities attached to the semiconductor substrate are oxidized to form a metal oxide. And the process of
A method for cleaning a semiconductor substrate is provided, wherein the semiconductor substrate is subjected to a wet treatment, and at the same time, the metal oxide is removed.

Further, according to the present invention, there is provided a method for cleaning a semiconductor substrate having a natural oxide film formed on a surface thereof, comprising the steps of:
In囲気, in the presence of ozone or oxygen, and, at an ambient temperature of 5 to 40 ° C., by irradiating ultraviolet rays on the surface of the natural oxide film, oxide to metal oxide and metal impurities contained in the natural oxide film And a step of removing the natural oxide film by wet treatment and, at the same time, removing the metal oxide.

In each of the above-described cleaning methods, first, ultraviolet irradiation is performed in the presence of ozone or oxygen. By this irradiation, oxygen molecules in the processing atmosphere become atomic active oxygen. on the other hand,
A natural oxide film is formed on the surface of the substrate before and during the ultraviolet irradiation, and various metal impurities such as Cu and Fe exist on the surface and inside of the natural oxide film. The active oxygen easily diffuses in the natural oxide film and oxidizes these metal impurities. Since the oxidized metal impurities are easily removed by a chemical solution, they are easily removed at the same time as the removal of the natural oxide film in the next step.

Although the metal impurities are efficiently removed by the above-described operation, the present invention has an advantage that the redeposition of the once removed metal impurities can be effectively prevented. In removing the natural oxide film, an acid such as a hydrofluoric acid-based cleaning solution is usually used. However, in the related art, there is a problem that the noble metal causes reverse adsorption to the silicon substrate at this time. On the other hand, in the present invention, the noble metal has already been changed to the oxide at this stage, and thus the problem of reverse adsorption can be solved. This is because an oxide layer is formed on the surface of the noble metal, which prevents direct electron exchange with the silicon substrate.

As described above, according to the cleaning method of the present invention, metal impurities can be efficiently removed by a combination of ultraviolet irradiation and removal of a natural oxide film by wet treatment.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The semiconductor substrate in the present invention is
It refers to a silicon substrate, an SOI (Silicon On Insulator) substrate, or the like.

The object to be cleaned of the present invention is preferably a silicon substrate before a gate oxide film is formed. This is because cleaning at this stage requires a particularly high degree of cleanliness, so that the effects of the present invention are more remarkably exhibited. Note that a semiconductor substrate on which a metal film is formed is also a cleaning target of the present invention.

In the present invention, when at least a part of the metal impurities is adsorbed on the surface of the semiconductor substrate, the effect of the present invention is more remarkably exhibited. For example, in the case of a silicon substrate having a natural oxide film formed on the surface, metal impurities exist at various positions (FIG. 2). That is, there are a substance adhering to the surface of the natural oxide film, a substance existing inside the natural oxide film, and a substance adsorbing to the silicon substrate. In the present invention, the “metal impurities contained in the natural oxide film” refers to those in the above-mentioned forms, and among them, those in the forms are particularly difficult to remove. Since noble metal-based metal impurities are likely to exist in such a form, it has been particularly difficult to remove them with the prior art. On the other hand, in the present invention, the above-mentioned metal impurities become oxides upon irradiation with ultraviolet rays, and their adsorbing power to the silicon substrate is weakened and taken into the natural oxide film. Since this oxide is easily removed at the same time as the removal of the natural oxide film in the next step, efficient cleaning is performed.

In the present invention, when the metal impurity contains a noble metal, particularly when it contains copper or platinum, the effect of the present invention is more remarkably exhibited. This is because such a metal is likely to be present by adsorbing on the surface of the semiconductor substrate as described above, and the cleaning effect by ultraviolet irradiation becomes more remarkable.

In the present invention, the effect of the present invention is more remarkably exhibited even when the metal impurity contains Co or Ni. Such a metal is easily diffused in the semiconductor substrate by applying a heat treatment, and is effectively removed by the method of the present invention which does not require a heat treatment.

In the present invention, the ambient temperature when irradiating ultraviolet rays is preferably 5 to 40 ° C., more preferably 1 to 40 ° C.
0 to 30 ° C. By doing so, it is possible to prevent metal impurities from diffusing into the substrate during ultraviolet irradiation.

The ultraviolet light in the present invention is 172 to 240.
It preferably includes a wavelength region of nm. This is because, if such ultraviolet rays are used, oxygen and ozone can be effectively photoexcited and active oxygen can be suitably generated. It is desirable to irradiate ultraviolet rays over the entire wavelength range. However, as long as active oxygen can be generated, the light may include light having a wavelength in a part of the wavelength range.

The source of the ultraviolet light is not particularly limited, and a sufficient effect can be obtained even with a simple one such as a table lamp.

The irradiation time of the ultraviolet ray is preferably 12
Within 0 seconds, more preferably within 60 seconds. By doing so, the throughput can be improved. Note that the lower limit of the irradiation time is not particularly limited, and is appropriately set depending on the intensity of ultraviolet rays and the like.

In the present invention, the atmosphere in which the ultraviolet irradiation is performed contains oxygen or ozone. The concentration of oxygen or ozone is not particularly limited, but is usually 1 to 50 VOL-%
And

It is simple and desirable to use clean air as the UV irradiation atmosphere. Specifically, it is desirable to arrange the semiconductor substrate to be cleaned in a closed space filled with clean air and perform ultraviolet irradiation.

The wet treatment in the present invention is preferably performed using a cleaning solution containing hydrofluoric acid. By using such a cleaning solution, the natural oxide film can be suitably removed.
Examples of the cleaning liquid containing hydrofluoric acid include dilute hydrofluoric acid (DHF), a mixed liquid of hydrofluoric acid and ammonium fluoride (buffered hydrofluoric acid; BHF), a mixed liquid of hydrofluoric acid and hydrogen peroxide (FPM), and the like.

[0035]

(Embodiment) This embodiment is an example of cleaning the surface of a semiconductor substrate before a gate oxide film is grown. Hereinafter, the cleaning process will be described with reference to the drawings.

FIG. 1A shows a sectional structure of a substrate to be cleaned. A natural oxide film 2 is formed on the surface of the silicon substrate 1. Organic substances 3 and particles 6 adhere to the surface of the natural oxide film 2.
Cu particles 5 and Fe particles 4 adhere to the surface of the silicon substrate 1 in a form embedded therein. Although Cu particles 5 and Fe particles 4 are typically shown as metal impurities in the figure, other metal impurities such as Co and Pt are also attached.

This substrate was placed in a space filled with clean air and irradiated with ultraviolet rays. 172 as ultraviolet light
A UV lamp with a distribution of ２４０240 nm was used. The temperature was room temperature (about 25 ° C.), and the irradiation time was 30 seconds.

By this ultraviolet irradiation, oxygen molecules contained in the processing atmosphere become atomic active oxygen. Organic substances are decomposed by the active oxygen and removed from the substrate. Further, since this active oxygen easily diffuses in the natural oxide film on the substrate surface, it oxidizes metal impurities such as Cu and Fe. As a result, the noble metal-based particles such as Cu that have firmly adhered to the silicon substrate 1 are detached from the substrate surface by being oxidized, and are taken into the natural oxide film. This state is shown in FIG.
It is shown in (b).

Next, the native oxide film 2 was removed by BHF (FIG. 1C). In the first prior art described above, FIG.
As shown in (d), noble metal-based particles such as Cu remained on the silicon substrate 1, but according to the method of the present embodiment, these metals are oxidized by the UV irradiation in the previous step.
It is easily removed simultaneously with the removal of the natural oxide film. Thereafter, final cleaning was performed with ultrapure water to complete cleaning of the substrate surface.

(Comparative Example) RCA cleaning was performed on the same cleaning target that was cleaned in the example by the same procedure as that described in the first prior art (FIG. 3).

Table 1 shows the results of measuring the metal impurity concentration on the substrate surface before and after cleaning in order to compare the effects of the cleaning performed in the example and the comparative example. Metal impurity concentration is TXR
It was measured by an F (total reflection fluorescent X-ray) analyzer.

[0042]

[Table 1]

As described above, according to the present invention, since a step of irradiating an ultraviolet ray and a step of removing a natural oxide film by a wet treatment are performed, impurities such as noble metal impurities and silicon which have been difficult to remove in the past. It is possible to sufficiently remove metal impurities that rapidly diffuse into the semiconductor.

[Brief description of the drawings]

FIG. 1 is a process sectional view showing a cleaning method of the present invention.

FIG. 2 is a schematic sectional view of an object to be cleaned.

FIG. 3 is a process sectional view showing a conventional cleaning method.

FIG. 4 is a process sectional view showing a conventional cleaning method.

[Explanation of symbols]

 Reference Signs List 1 silicon substrate 2 natural oxide film 3 organic substance 4 Fe particle 5 Cu particle 6 particle 7 sacrificial oxide film 8 iron oxide 9 copper oxide

Continuation of front page (56) References JP-A-9-82679 (JP, A) JP-A-5-102115 (JP, A) JP-A-10-50646 (JP, A) JP-A-4-213824 (JP) , A) JP-A-5-182945 (JP, A)

Claims (8)

(57) [Claims] An ultraviolet ray is applied to the surface of a semiconductor substrate in a clean air atmosphere in the presence of ozone or oxygen and at an ambient temperature of 5 to 40 ° C. to remove metal impurities attached to the semiconductor substrate. Oxidizing to a metal oxide and wet processing the semiconductor substrate,
A method for cleaning a semiconductor substrate, wherein the metal oxide is removed at the same time. 2. A method for cleaning a semiconductor substrate having a native oxide film formed on a surface thereof, wherein the method is performed in a clean air atmosphere in the presence of ozone or oxygen, and at an ambient temperature of 5 to 40 ° C.
Irradiating the surface of the natural oxide film with ultraviolet light to oxidize metal impurities contained in the natural oxide film to form a metal oxide; and removing the natural oxide film by wet treatment. Removing the metal oxide at the same time. 3. The method for cleaning a semiconductor substrate according to claim 1, wherein at least a part of the metal impurity exists in a state of being adsorbed on a surface of the semiconductor substrate. 4. The method according to claim 1, wherein the metal impurities include a noble metal.
4. The method for cleaning a semiconductor substrate according to any one of items 1 to 3. 5. The method according to claim 4, wherein the noble metal is copper or platinum. 6. The method for cleaning a semiconductor substrate according to claim 1, wherein the metal impurities include Co or Ni. 7. The method for cleaning a semiconductor substrate according to claim 1, wherein the ultraviolet light has a wavelength range of 172 to 240 nm. 8. The method according to claim 1, wherein the wet processing is performed using a cleaning liquid containing hydrofluoric acid.