Method for cleaning a semiconductor wafer

Abstract

A method for cleaning a semiconductor wafer, comprising: Placing a semiconductor wafer into a hydrofluoric acid tank filled with hydrofluoric acid to immerse the semiconductor wafer in the hydrofluoric acid; Pulling the semiconductor wafer out of the hydrofluoric acid tank; and then placing the semiconductor wafer into an ozone water tank filled with ozone water to immerse the semiconductor wafer in the ozone water for cleaning; wherein the semiconductor wafer is introduced into the ozone water tank at an introduction rate of 20,000 mm / min or more after a lower end of the semiconductor wafer comes into contact with the ozone water, and the semiconductor wafer is completely immersed in the ozone water until an upper end of the semiconductor wafer is located 50 mm or more away from a liquid surface of the ozone water.

Description

Method for cleaning a semiconductor wafer TECHNICAL FIELD

[0001] The present invention relates to a method for cleaning a semiconductor wafer. RELATED PRIOR ART

[0002] Currently, immersion cleaning using hydrofluoric acid (HF) and ozone water is a common method for batch cleaning of semiconductor wafers. In this process, an oxide film on a wafer is removed using HF cleaning, while contaminants such as particles are simultaneously removed. The wafer is then oxidized by cleaning with ozone water to form a native oxide film and remove residual contaminants such as particles.

[0003] It is known that in this process, the rate at which a semiconductor wafer is pulled up (pulled out) from a cleaning tank filled with a cleaning solution (pull-up rate) is more important than the rate at which the semiconductor wafer is immersed (inserted) into the cleaning tank (immersion rate) (e.g., Patent Document 1). Furthermore, Patent Document 2 discloses a method for washing a silicon wafer, which describes the immersion rate of the silicon wafer in ozone water.

[0004] To reduce contaminants such as particles adhering to a semiconductor wafer, the pull-up rate from a hydrofluoric acid tank is typically reduced. LIST OF REFERENCES PATENT LITERATURE Patent Document 1: Unexamined Japanese Patent Application Publication No. JP H09 - 283 483 A Patent document 2: WO 2016 / 203 681 A1 SUMMARY OF THE INVENTIONTECHNICAL PROBLEM

[0005] When a native oxide film is removed from a semiconductor wafer by hydrofluoric acid cleaning, the semiconductor wafer is brought into a state where a bare surface (hydrophobic surface) is exposed. In the step of cleaning (oxidizing) such a wafer with an exposed bare surface using an ozone water tank in a conventional process, the immersion rate (introduction rate) into the ozone water tank is generally set to just about 10,000 mm / min without any modification during the process.

[0006] Meanwhile, contaminants such as particulate matter are removed simultaneously during oxide film formation in the cleaning step using an ozone water tank. However, it has been found that the slow immersion rate is problematic because particles are hardly removed due to the slow relative velocities between the ozone water and the semiconductor wafer, leaving particles and similar contaminants behind.

[0007] Furthermore, the direction in which a solution of ozone water flows is generally from lower to higher positions. Therefore, detached contaminants such as particles are likely to stagnate near a liquid surface in the cleaning tank. Thus, a further problem has been identified that detached particles may reattach to a point on a wafer where oxidation has not fully progressed if the immersion rate is slow.

[0008] The present invention was developed with the above problems in mind. An object of the present invention is to provide a method for cleaning a semiconductor wafer capable of preventing re-adherence of contaminants such as particles of remaining and removed contaminants in a process in which a semiconductor wafer is cleaned by immersion in hydrofluoric acid and then cleaned by immersion in ozone water. SOLUTION TO THE PROBLEM

[0009] To achieve the object, the present invention provides a method for cleaning a semiconductor wafer, comprising: Placing a semiconductor wafer into a hydrofluoric acid tank filled with hydrofluoric acid to immerse the semiconductor wafer in the hydrofluoric acid; Pulling the semiconductor wafer out of the hydrofluoric acid tank; and then placing the semiconductor wafer into an ozone water tank filled with ozone water to immerse the semiconductor wafer in the ozone water for cleaning; wherein the semiconductor wafer is introduced into the ozone water tank at an introduction rate of 20,000 mm / min or more after a lower end of the semiconductor wafer has come into contact with the ozone water, and the semiconductor wafer is completely immersed in the ozone water until an upper end of the semiconductor wafer is located 50 mm or more away from a liquid surface of the ozone water.

[0010] Such a method for cleaning a semiconductor wafer is capable of preventing contaminants, such as particles from remaining and removed contaminants, from reattaching. Furthermore, the conveyance rate is consequently increased, thereby improving throughput.

[0011] In addition, in this case, the semiconductor wafer is preferably pulled out of the hydrofluoric acid tank at a pull-out rate of 1000 mm / min or less.

[0012] By reducing the withdrawal rate from the hydrofluoric acid tank in this way, it becomes possible to further reduce contaminants such as particles.

[0013] Further, the semiconductor wafer is introduced into the ozone water tank at the introduction rate of 20,000 mm / min or more after the lower end of the semiconductor wafer comes into contact with the ozone water until an upper end of the semiconductor wafer is located 50 mm or more away from a liquid surface of the ozone water.

[0014] Placing a semiconductor wafer in the ozone water tank as described above is preferred because it makes it possible to reliably prevent the adhesion of contaminants such as particles stagnating near a liquid surface in the ozone water tank.

[0015] In addition, a silicon wafer is preferably cleaned as a semiconductor wafer.

[0016] The cleaning method according to the invention is particularly effective when a silicon wafer is cleaned. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0017] The method for cleaning a semiconductor wafer according to the present invention makes it possible to prevent the re-adhesion of contaminants, such as particles of remaining and removed contaminants, in a cleaning process in which a semiconductor wafer is immersed in hydrofluoric acid to remove a native oxide film and then immersed in ozone water to form a native oxide film. Furthermore, the method according to the present invention consequently also increases the conveying rate, thus improving throughput. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a cleaning flowchart illustrating an exemplary method for cleaning a semiconductor wafer of the present invention. Fig. 2 shows schematic drawings illustrating a step of placing a semiconductor wafer into an ozone water tank. Fig. 3 is a graph illustrating a relationship between the introduction rates of a silicon wafer into an ozone water tank and the particle measurement results in Examples 1 to 6 and Comparative Examples 1 to 4. DESCRIPTION OF EMBODIMENTS

[0018] In a method as described above, in which a semiconductor wafer is immersed in a hydrofluoric acid tank to remove a native oxide film and then immersed in an ozone water tank for oxidation to form a native oxide film, the conventional cleaning process is associated with the problem that contaminants such as particles remain and removed contaminants re-adhere.

[0019] Thereafter, the present inventors diligently conducted studies to solve the above problems. As a result, the inventors discovered that when a semiconductor wafer is placed into an ozone water tank at a predetermined insertion rate or faster, the remaining contaminants such as particles and the re-adhesion of removed contaminants can be prevented. Thus, the inventors completed the present invention.

[0020] Specifically, the present invention provides a method for cleaning a semiconductor wafer, comprising: Placing a semiconductor wafer into a hydrofluoric acid tank filled with hydrofluoric acid to immerse the semiconductor wafer in the hydrofluoric acid; Pulling the semiconductor wafer out of the hydrofluoric acid tank; and then placing the semiconductor wafer into an ozone water tank filled with ozone water to immerse the semiconductor wafer in the ozone water for cleaning; wherein the semiconductor wafer is introduced into the ozone water tank at an introduction rate of 20,000 mm / min or more after a lower end of the semiconductor wafer has come into contact with the ozone water, and the semiconductor wafer is completely immersed in the ozone water until an upper end of the semiconductor wafer is located 50 mm or more away from a liquid surface of the ozone water.

[0021] Note that a "loading rate" in the present invention refers to a relative speed between a semiconductor wafer and a cleaning tank. More specifically, the loading rate includes all of: (i) a rate of lowering a semiconductor wafer when the semiconductor wafer is lowered and loaded to a predetermined position in a cleaning tank; (ii) a rate of raising a cleaning tank when the cleaning tank is raised to load a semiconductor wafer at a predetermined position in the cleaning tank; (iii) a rate obtained by adding a rate of lowering a semiconductor wafer and a rate of raising a cleaning tank when the semiconductor wafer is lowered and the cleaning tank is raised simultaneously; and the like. Hereinafter, "pulling rate" similarly also refers to a relative speed between a semiconductor wafer and a cleaning tank.

[0022] The method according to the invention for cleaning a semiconductor wafer is described in detail below. Fig. 1 shows a cleaning flow chart illustrating an exemplary method according to the invention for cleaning a semiconductor wafer.

[0023] In the present invention, the semiconductor wafer as a target to be cleaned is not particularly limited, and a silicon wafer can be used.

[0024] Before cleaning with hydrofluoric acid, the semiconductor wafer can be subjected to cleaning with ammonia and hydrogen peroxide water (SC1 cleaning) and rinsing with pure water ( Fig. 1(A), (B)).

[0025] After that, the semiconductor wafer is placed in a hydrofluoric acid tank filled with hydrofluoric acid to immerse the semiconductor wafer in hydrofluoric acid, so that the semiconductor wafer is cleaned with hydrofluoric acid ( Fig. 1(C)). This hydrofluoric acid cleaning removes a native oxide film formed on the semiconductor wafer. The concentration and temperature of the hydrofluoric acid are not limited, but the concentration is preferably 0.3 to 3.0%, and the temperature is preferably 10 to 30°C.

[0026] The semiconductor wafer is preferably pulled out of the hydrofluoric acid tank at a pull-out rate of 1000 mm / min or less. By pulling the semiconductor wafer out of the hydrofluoric acid tank at a relatively low rate as described above, contaminants such as particles can be further reduced. Note that the lower limit of the pull-out rate is not particularly limited and can be a rate exceeding 0 mm / min.

[0027] After that, the semiconductor wafer is placed in an ozone water tank filled with ozone water to immerse the semiconductor wafer in ozone water for cleaning ( Fig. 1(D)).

[0028] Fig. Fig. 2 shows schematic drawings illustrating the step of placing the semiconductor wafer into the ozone water tank. The present invention is characterized in that, when a semiconductor wafer 1 is placed into an ozone water tank 3 filled with ozone water 2, this placing rate is 20,000 mm / min or more, at least after a lower end of the semiconductor wafer 1 comes into contact with the ozone water 2 ( Fig. 2(A)) until the semiconductor wafer 1 is completely immersed in the ozone water 2 ( Fig. 2(B)). Meanwhile, the upper limit of the insertion rate is not subject to any special restrictions, and the device limit is usually 50,000 mm / min.

[0029] The surface of the semiconductor wafer 1, from which a native oxide film has been removed by cleaning in the hydrofluoric acid tank, is a bare surface (hydrophobic surface). Thus, contaminants such as particles are likely to adhere to the surface, and a large amount of contaminants adhere to the semiconductor wafer 1.

[0030] In the ozone water tank cleaning (reoxidation treatment) after the hydrofluoric acid tank cleaning, when the semiconductor wafer 1 is immersed in the ozone water tank at a low rate (approximately 10,000 mm / min) as in a conventional technique, contaminants such as particles are also removed simultaneously when an oxide film is formed; however, the relative speeds between the ozone water and the semiconductor wafer are so slow that particles are hardly detached, and the particles and similar contaminants remain.

[0031] Furthermore, since an ozone water cleaning solution flows in a bottom-to-top direction, detached contaminants such as particles are likely to stagnate near the liquid surface in the ozone water tank. If the immersion rate is slow, detached particles will reattach to a point on a wafer where oxidation has not fully progressed.

[0032] In contrast, as in the present invention, when the loading rate of the semiconductor wafer 1 is set to as fast as 20,000 mm / min or more, particles are efficiently detached from the semiconductor wafer 1. However, since the semiconductor wafer 1 is quickly lowered into the ozone water tank 3, fewer particles re-adhere near the liquid surface (tank interface), so a high-quality wafer is obtained. Furthermore, the conveyance rate is also increased, so throughput is improved.

[0033] Furthermore, in the present invention, the semiconductor wafer 1 is introduced into the ozone water tank 3 at the introduction rate of 20,000 mm / min or more after the lower end of the semiconductor wafer 1 comes into contact with the ozone water 2 ( Fig. 2(A)) until an upper end of the semiconductor wafer 1 is located 50 mm or more from a liquid surface of the ozone water 2 ( Fig. 2(C)). In this way, it is preferably possible to securely prevent the contaminants such as particles stagnating near the liquid surface of the ozone water tank 3 from adhering. Meanwhile, the upper end of the semiconductor wafer 1 can be Fig. 2(C) in this process be arranged 200 mm or less away from the liquid surface of the ozone water 2.

[0034] The semiconductor wafer 1, which has been placed in the ozone water tank 3 as described above, is immersed in the ozone water 2 while being held by a wafer holder 4 ( Fig. 2(D)). The concentration of the ozone water is preferably 1 ppm or more, the temperature of the ozone water is preferably 10 to 30 °C, and a preferred purification time is 60 to 300 seconds.

[0035] Thereafter, the semiconductor wafer 1 is pulled out of the ozone water tank 3. In this case, the pull-out rate is not particularly limited and can be 1000 mm / min or less. After cleaning with ozone water, treatment such as drying ( Fig. 1(E)) shall be carried out accordingly.

[0036] Note that increasing the semiconductor wafer loading rate in tanks other than the ozone water tank is intended to suppress re-adhesion of particles and the like and increase throughput without causing undesirable effects. Therefore, it is desirable to set high loading rates in all tanks, for example, as high as 20,000 mm / min or more. EXAMPLE

[0037] Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to these examples. (Examples 1 to 6, Comparative Examples 1 to 4)

[0038] Silicon wafers each with a diameter of 300 mm were used and cleaned by immersion, that is, cleaning with ammonia hydrogen peroxide water, rinsing with pure water, cleaning with hydrofluoric acid, cleaning with ozone water, in this order, and then drying. In this process, the rates shown in Table 1 were selected as the loading rates for the silicon wafers into the ozone water tank after the lower end of each silicon wafer came into contact with ozone water until the upper end of the silicon wafer was positioned 50 mm away from the liquid surface of the ozone water. The loading rates into the tanks other than the ozone water tank were 15,000 mm / min, and the withdrawal rates from all tanks were 1,000 mm / min.

[0039] After drying, the number of particles with a diameter of 16 nm or more was counted using SP5, manufactured by KLA-Tencor Corporation. The number of measured particles is shown in Table 1. Fig. Figure 3 illustrates a relationship between the silicon wafer introduction rates into the ozone water tank and the particle measurement results. [Table 1] Comparison example 1 Comparison example 2 Comparison example 3 Comparison example 4 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Insertion rate (mm / min) 1000 3000 7500 15.000 20.000 25.000 30.000 35.000 40.000 50.000 Number of particles 360 223 111 76 52 42 37 40 35 30

[0040] As shown in Table 1 and Fig. As shown in Figure 3, the higher the immersion rate in the ozone water tank, the smaller the number of particles on the silicon wafers. It was found that the number of particles on the wafers converges and remains essentially constant when the insertion rates are set to 20,000 mm / min (Example 1) or higher. In other words, Examples 1 to 6 achieved improved particle counts on the wafers and improved wafer quality.

[0041] Meanwhile, larger numbers of particles were observed in Comparative Examples 1 to 4.

Claims

[1] A method for cleaning a semiconductor wafer, comprising: Placing a semiconductor wafer into a hydrofluoric acid tank filled with hydrofluoric acid to immerse the semiconductor wafer in the hydrofluoric acid; Pulling the semiconductor wafer out of the hydrofluoric acid tank; and then placing the semiconductor wafer into an ozone water tank filled with ozone water to immerse the semiconductor wafer in the ozone water for cleaning; wherein the semiconductor wafer is introduced into the ozone water tank at an introduction rate of 20,000 mm / min or more after a lower end of the semiconductor wafer has come into contact with the ozone water, and the semiconductor wafer is completely immersed in the ozone water until an upper end of the semiconductor wafer is located 50 mm or more away from a liquid surface of the ozone water. [2] The method for cleaning a semiconductor wafer according to claim 1, wherein the semiconductor wafer is pulled out from the hydrofluoric acid tank at a pull-out rate of 1000 mm / min or less. [3] A method for cleaning a semiconductor wafer according to claim 1 or 2, wherein a silicon wafer is cleaned as the semiconductor wafer.

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