Method of cleaning a semiconductor wafer

Abstract: A method of cleaning a semiconductor silicon wafer, which can suppress and reduce adhesion of particles to the surface of the wafer comprises the steps of cleaning the semiconductor silicon wafer by using hydrofluoric acid aqueous solution containing a surfactant, and there-after rinsing the semiconductor silicon wafer by using pure water containing ozone.

Description: The present invention relates to an improvement of a method for cleaning a semiconductor silicon wafer, which may be hereinafter referred to simply as a wafer, by using hydrofluoric acid. Generally, hydrofluoric acid is used to remove an oxide film on the surface of the wafer, e.g., a thermal oxidation film, a CVD oxide film, a natural oxide film and the like. Because a treatment using hydrofluoric acid makes the surface of the wafer hydrophobic, it raises a problem of easily adhering particles to the surface of the wafer. Reduction of particles adhered to the surface of the wafer is required because such adhesion of particles brings reduction of yield of devices produced. A method for cleaning the surface of a wafer by rinsing with pure water containing ozone after a cleaning which uses hydrofluoric acid has been suggested as a method for suppressing adhesion of particles to the surface of the wafer after a treatment using hydrofluoric acid, in Japanese Patent Application No. 62-198127. Under the present conditions, such a proposed method is not enough to suppress or reduce adhesion of particles to the surface of the wafer after the treatment using hydrofluoric acid. In the paper "Ultra clean surface preparation using ozonized ultrapure water", International Conference on solid state Devices and Materials, August 1992, p. 193-195, it is proposed that the wafer can be cleaned by following an RCA type cleaning with a step of rinsing in ozonized ultrapure water. This removes unwanted surfactant from the water surface. In EP-A-0,438,727 it is proposed that either an organic compound capable of forming inclusion compounds and ring molecules, or an acid which does not oxidize the surface and has a pKa of less than 3.14, or both, be added to the hydrofluoric acid. The present inventors have extensively studied the above mentioned problems so as to achieve the present invention which can effectively and sufficiently suppress and reduce adhesion of particles to the surface of the semiconductor silicon wafer. In accordance with this invention, the method of cleaning a semiconductor silicon wafer comprises the steps of: cleaning the semiconductor silicon wafer by using aqueous hydrofluoric acid solution containing no less than 0.01 % by weight of a surfactant thereby forming a film of surfactant on the surface of the wafer to supress the adhesion of particles to the surface of the wafer, and thereafter, rinsing the semiconductor silicon wafer by using pure water containing a concentration of ozone not less than 0.5 ppm and not more than the normal solubility thereof thereby removing the surfactant film. When a treatment using hydrofluoric acid is carried out on the surface of the wafer, because the surface of the wafer becomes hydrophobic, particles come to easily adhere to the surface of the wafer. In order to reduce the adhesion of particles to the surface of the wafer, it is effective to keep the surface of the wafer hydrophilic. In the present invention, a film of a surfactant is formed on the surface of the wafer to make the surface hydrophilic by adding the surfactant in a hydrofluoric acid aqueous solution. Thereafter, the surfactant film formed on the surface of the wafer is removed by using pure water including ozone for rinsing the wafer which was treated by hydrofluoric acid, and a natural oxide film is formed on the water surface. Accordingly, the wafer surface can be kept hydrophilic, and it is possible to enhance the advantageous effect of preventing adhesion of particles to the wafer surface. The concentration of ozone to pure water less than the lowest limit value gives substantially no advantageous effect of reducing adhesion of particles. Preferably, the amounts of the surfactant contained in the hydrofluoric acid aqueous solution are not more than 5.00% by weight. The amounts of the added surfactant less than the lowest limit value give substantially no advantageous effect of reducing adhesion of particles, and those more than 5.00% by weight give no further improved effect thereof and are not economical. The type of the surfactant is not limited in particular. Every one of non-ionic, anionic, or cationic surfactants can give a similar effect. The concentration of hydrofluoric acid in the HF aqueous solution is not limited particularly so long as the cleaning by using the HF aqueous solution gives advantageous effects. Although the concentration of hydrofluoric acid is preferably in the range of 0.1-10% by weight, another concentration out of the range may be also used. Time for cleaning by hydrofluoric acid is preferably about 1-15 minutes which depends on the thickness of the oxide film on the surface of the wafer. Time for rinsing by water containing ozone is preferably about 1-10 minutes. More preferably, the time for rinsing is about 1-5 minutes. Particular examples in accordance with this invention will now be described and contrasted with the prior art, with reference to the accompanying drawings which are for purpose of illustration only and are not intended as a definition of the limits of the present invention, and wherein; FIG. 1 is a graph illustrating the measurement results of adhered particles to the surface of the wafer after cleanings according to the Example 1 and the Comparison Examples 1-3, FIG. 2 is a graph illustrating the measurement results of adhered particles to the surface of the wafer after cleanings according to the Example 2 and the Comparison Examples 1, 2 and 4, and FIG. 3 is a graph illustrating the measurement results of adhered particles to the surface of the wafer after cleanings according to the Example 3 and the Comparison Examples 1, 2 and 5. In the description the number of particles is expressed in number of particles on a wafer having a diameter of 200 mm (/200mm). Example 1: Experimental Condition: Sample wafer: 20 silicon wafers; p-type; crystal orientation of <100>; a diameter of 200 mm; having a CVD oxide film with a thickness of 300 nm; Cleaning by hydrofluoric acid (HF): concentration of hydrofluoric acid of 5% by weight; aqueous solution temperature of 25°C; cleaning time of 10 minutes; a non-ionic surfactant [Trade name: NCW-601A, produced by Wako Pure Chemical Industries, Ltd.] added of 0.5% by weight; Rinsing by pure water containing ozone (O 3 ): supply of 2 litres per minute; pure water temperature of 25°C; rinsing time of 3 minutes; ozone concentration of 2 ppm; Vessel for hydrofluoric acid cleaning: 320 x 30 x 280 (mm) Vessel for rinsing: 320 x 30 x 200 (mm) Twenty sample wafers were cleaned by using hydrofluoric acid under the above-described conditions and thereafter rinsed by pure water containing ozone. Number of particles (number/200 mm) each of which has a diameter not less than 0.11 µm on these wafers which had been cleaned by using hydrofluoric acid, was measured by a laser particle counter LS-6000 produced by Hitachi Electronics Engineering Co., Ltd. The result of the measurement is shown in FIG. 1. The average measured number of particles was 24.1/200 mm. Comparison Example 1: Other twenty sample wafers were cleaned and rinsed under conditions similar to that of the Example 1 except that no surfactant was added in the hydrofluoric acid aqueous solution and that the pure water included no ozone. Number of particles on these wafers were similarly measured. The result of the measurement is shown in FIGS. 1, 2 and 3. The average measured number of particles in this case was 1361/200 mm. Comparison Example 2: Other twenty sample wafers were cleaned and rinsed under conditions similar to that of the Example 1 except that no surfactant was added in the hydrofluoric acid aqueous solution. Number of particles on these wafers were similarly measured. The result of the measurement is shown in FIGS. 1, 2 and 3. The average measured number of particles in this case was 1172/200 mm. Comparison Example 3: Other twenty sample wafers were cleaned and rinsed under conditions similar to that of the Example 1 except that the pure water included no ozone. Number of particles on these wafers were similarly measured. The result of the measurement is shown in FIG. 1. The average measured number of particles in this case was 112.4/200 mm. Example 2: Other twenty sample wafers were cleaned by using hydrofluoric acid and thereafter rinsed by pure water containing ozone, under conditions similar to that of the Example 1 except that 0.1% by weight of an anionic surfactant [Trade name: Unidine DS-101, produced by DAIKIN INDUSTRIES, LTD.] was added instead of a non-ionic surfactant. Number of particles was similarly measured. The result of the measurement is shown in FIG. 2. The average measured number of particles was 29/200 mm. Comparison Example 4: Other twenty sample wafers were cleaned and rinsed under conditions similar to that of the Example 2 except that the pure water included no ozone. Number of particles on these wafers were similarly measured. The result of the measurement is shown in FIG. 2. The average measured number of particles in this case was 215/200 mm. Example 3: Other twenty sample wafers were cleaned by using hydrofluoric acid and thereafter rinsed by pure water containing ozone, under conditions similar to that of the Example 1 except that 0.1% by weight of a cationic surfactant [Trade name: Unidine DS-202, produced by DAIKIN INDUSTRIES, LTD.] was added instead of a non-ionic surfactant. Number of particles was similarly measured. The result of the measurement is shown in FIG. 3. The average measured number of particles was 33/200 mm. Comparison Example 5: Other twenty sample wafers were cleaned and rinsed under conditions similar to that of the Example 3 except that the pure water included no ozone. Number of particles on these wafers were similarly measured. The result of the measurement is shown in FIG. 3. The average measured number of particles in this case was 326/200 mm. It is clearly observed from the results of FIGS. 1-3 that only one step of adding a surfactant in a hydrofluoric acid aqueous solution can give the effect of suppressing adhesion of particles to the wafer surface to a certain extent, as indicated in Comparison Examples 3, 4 and 5. However, the only one step is not sufficient because less levels of particle contamination are preferable. It is confirmed that by containing ozone in the pure water in addition to the state of the Comparison Examples 3, 4 or 5, the number of measured particles is further decreased, so that the levels of particle contamination are extremely lowered. In the above-described embodiments, although only 5% by weight of concentration of hydrofluoric acid is illustrated, the concentration of hydrofluoric acid is not limited particularly so long as the cleaning by using hydrofluoric acid aqueous solution gives advantageous effects. In general, a concentration of hydrofluoric acid in the range of 0.1-10% by weight is sufficient, and another concentration out of the range can be also used. In the embodiments, only p-type of sample wafers are used. However, it is confirmed that also n-type of wafers give advantageous effects similarly. The time for cleaning by hydrofluoric acid is preferably about 1-15 minutes which depend on the thickness of the oxide film on the surface of the wafer. The concentration of the ozone is preferably not less than 0.5 ppm. The concentration up to the solubility thereof can be used, and the upper limit thereof is about 40 ppm at a solution temperature of at least 25°C. Rinsing time by water containing ozone is preferably about 1-10 minutes. More preferably, the time for rinsing is about 1-5 minutes. As described above, according to the method for cleaning a wafer of the present invention, it is possible to extremely reduce adhesion of particles to the surface of the wafer and therefore to solve the problem of a lowering of yield.

Claims: A method of cleaning a semiconductor silicon wafer comprising the steps of: cleaning the semiconductor silicon wafer by using aqueous hydrofluoric acid solution containing no less than 0.01 % by weight of a surfactant thereby forming a film of surfactant on the surface of the wafer to suppress the adhesion of particles to the surface of the wafer, and thereafter, rinsing the semiconductor silicon wafer by using pure water containing a concentration of ozone not less than 0.5 ppm and not more than the normal solubility thereof thereby removing the surfactant film. A method as claimed in claim 1, wherein the amounts of the surfactant contained in the hydrofluoric acid aqueous solution are not more than 5.00% by weight. A method as claimed in any preceding claim, wherein the concentration of hydrofluoric acid in the aqueous solution is in the range of 0.1 - 10% by weight. A method as claimed in any preceding claim, wherein the cleaning by using hydrofluoric acid aqueous solution is carried out for one to fifteen minutes. A method as claimed in any preceding claim, wherein the rinsing by using pure water containing ozone is carried out for one to ten minutes.

Inventors: NAKANO MASAMI, TAKAMATSU HIROYUKI, UCHIYAMA ISAO, NAKANO, MASAMI, TAKAMATSU, HIROYUKI, UCHIYAMA, ISAO, Nakano, Masami, Shin-Etsu Handotai Co., Ltd., Takamatsu, Hiroyuki, Shin-Etsu Handotai Co., Ltd., Uchiyama, Isao, Shin-Etsu Handotai Co., Ltd.
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1995
Main classification: H01L21
IPC codes: C11D7, B08B3, C11D3, H01L21