Abrasive grains, evaluation methods for them and wafer manufacturing methods

Abstract

Abrasive grain evaluation procedure, which includes the following: Preparing a predetermined quantity of abrasive grains as an abrasive grain sample group; Measuring the grain diameter of individual abrasive grains in the abrasive grain sample group and counting the number of abrasive grains in the abrasive grain sample group; Defining abrasive grains with a grain diameter equal to or smaller than a predetermined diameter criterion (R2) that is smaller than the average grain diameter (R1) of the abrasive grain sample group as small grains, and counting the number of small grains; Calculating a small grain fraction, which is the numerical proportion of small grains in the abrasive grain sample group; and Determine whether the small grain fraction is equal to or less than a predetermined threshold, whereby if the small grain fraction is equal to or less than the threshold, it is determined that the abrasive grain sample group can be used when cutting an ingot by a wire saw, whereas if the small grain fraction is greater than the threshold, it is determined that the abrasive grain sample group cannot be used in cutting.

Description

TECHNICAL FIELD

[0001] The present invention relates to abrasive grains and an evaluation method therefor, and more particularly to abrasive grains used in a slicing step using a wire saw with loose grains and an evaluation method therefor. The present invention also relates to a wafer manufacturing method including a step of slicing an ingot using a slurry containing such abrasive grains. TECHNICAL BACKGROUND

[0002] A dicing step is one of the manufacturing processes for a silicon wafer, a semiconductor device material. A wire saw is used in the dicing step, and a number of silicon wafers, each having a specified thickness, are produced by dicing a silicon single-crystal ingot grown, for example, by a CZ method using a wire saw with loose grains.

[0003] Regarding cutting using the wire saw with loose grains, for example, JP 2013 - 091 141 A describes that a slurry obtained by mixing abrasive grains with a roundness of 0.900 or greater and a coolant is used. Furthermore, JP 2004 - 255 534 A describes that in a method for reusing a slurry by recovering effective abrasive grains from a used slurry, removing worn abrasive grains and adding new abrasive grains in an amount corresponding to the removed abrasive grains, the average roundness of the new abrasive grains is set to a range of 0.855 to 0.875, the weight fraction of the new abrasive grains is set to approximately 20% and the average roundness of the abrasive grains contained in the reused slurry is set to a range of 0.870 to 0.885.

[0004] Furthermore, JP 2011-218516 A describes that an additional abrasive grain group is prepared, and a predetermined amount of the additional abrasive grain group is added to and mixed with an abrasive grain group used in cutting processing, and its sorting processing is performed on the mixed abrasive grain group so that the grain size becomes equal to or larger than a first abrasive grain diameter and equal to or smaller than a second abrasive grain diameter. According to this method, the mixed abrasive grain group can be used to perform a polishing step whose quality is equivalent to that of the new abrasive grain group, while reducing the amount of abrasive grains that are discarded. Furthermore, JP 2000-061843 A describes a slurry tank capable of uniformly dispersing abrasive grains in a coolant without aggregation.When this slurry tank is used, a slurry without aggregations is supplied to a wire saw to stabilize a dicing work, thereby obtaining a wafer with excellent quality stability.

[0005] Furthermore, JP 2005 - 039 174 A describes abrasive grains and a method for separating a semiconductor ingot using the abrasive grains. The size of the abrasive grains is determined by a volume ratio. SUMMARY OF THE INVENTION [Problem to be solved by the invention]

[0006] When cutting an ingot using the wire saw, abrasive grains are examined with respect to predetermined quality evaluation elements, such as grain size distribution or roundness thereof, to stabilize a machining quality, and only the abrasive grains that meet a certain quality criterion are used.

[0007] However, even when conventional abrasive grains that meet the quality criteria are used to perform dicing, the flatness of a wafer is sometimes good and sometimes poor. Accordingly, wafer processing quality is not stabilized satisfactorily.

[0008] The object of the present invention is therefore to provide abrasive grains capable of improving the flatness of a wafer dicing by a wire saw with loose grains and stabilizing processing quality, and an evaluation method thereof. Another object of the present invention is to provide a wafer manufacturing method including a step of dicing a wafer using a slurry containing such abrasive grains. [Means of solving the problem]

[0009] To solve the above problem, an abrasive grain evaluation method according to the present invention includes: preparing a predetermined amount of abrasive grains as an abrasive grain sample group; measuring the grain diameters of individual abrasive grains in the abrasive grain sample group and counting the number of abrasive grains in the abrasive grain sample group; defining abrasive grains having a grain diameter equal to or smaller than a predetermined diameter criterion, which is smaller than the average grain diameter of the abrasive grain sample group, as small grains and counting the number of small grains; calculating a small grain ratio, which is the numerical ratio occupied by the small grains in the abrasive grain sample group;and determining whether or not the small grain fraction is equal to or less than a predetermined threshold value, wherein if the small grain fraction is equal to or less than the threshold value, it is determined that the abrasive grain sample group can be used in severing an ingot by a wire saw, while if the small grain fraction is greater than the threshold value, it is determined that the abrasive grain sample group cannot be used in severing.;

[0010] According to the present invention, it is possible to eliminate causes of deterioration of the flatness of a wafer by judging the proportion of small grains contained in the abrasive grain sample group by the number proportion, thereby making it possible to improve and stabilize a wafer dicing quality.

[0011] The abrasive grain evaluation method according to the present invention preferably includes: measuring the grain size distribution based on the volume of the abrasive grain sample group from the grain diameters of the individual abrasive grains in the abrasive grain sample group; calculating the average grain diameter of the abrasive grain sample group from the measured grain size distribution; and defining the abrasive grains distributed in an area equal to or smaller than a grain diameter criterion smaller than the average grain diameter as the small grains. Further, the grain diameter criterion is preferably 1 / 2 of the average grain diameter. Accordingly, the small grains can be clearly and easily defined.

[0012] In the present invention, the threshold value is preferably 50%. When the small grain content of the abrasive grains is equal to or less than 50%, it is possible to produce a wafer with satisfactory flatness and stabilize the wafer processing quality.

[0013] By prohibiting the use of abrasive grains with a small grain content greater than the threshold value, wafer flatness and wafer processing quality can be improved.

[0014] According to the present invention, it is possible to manufacture a wafer having satisfactory flatness by dicing an ingot using a wire saw and to stabilize the wafer processing quality.

[0015] A wafer manufacturing method according to the present invention includes an inspection step of inspecting abrasive grains for quality; and a dicing step of dicing an ingot using a loose-grain wire saw with a slurry containing the abrasive grains that have passed the inspection. The inspection step includes a small-grain content inspection step of inspecting the small-grain content of the abrasive grains.The small grain fraction examination step includes: preparing a predetermined amount of abrasive grains as an abrasive grain sample group; measuring the grain diameters of individual abrasive grains in the abrasive grain sample group and counting the number of abrasive grains in the abrasive grain sample group; defining abrasive grains with a grain diameter equal to or smaller than a predetermined diameter criterion, which is smaller than the average grain diameter of the abrasive grain sample group, as small grains and counting the number of small grains; calculating a small grain fraction, which is the numerical proportion occupied by the small grains in the abrasive grain sample group; and determining whether the small grain fraction is equal to or smaller than a predetermined threshold value. According to the present invention, it is possible to manufacture a wafer with satisfactory flatness and stabilize wafer processing quality.

[0016] Furthermore, the wafer manufacturing method may include: measuring the grain size distribution based on the volume of the abrasive grain sample group from the grain diameters of the individual abrasive grains in the abrasive grain sample group; calculating the average grain diameter of the abrasive grain sample group from the measured grain size distribution; and defining the abrasive grains distributed in a region equal to or smaller than a grain diameter criterion smaller than the average grain diameter as the small grains. Further, the grain diameter criterion is preferably 1 / 2 of the average grain diameter. Furthermore, in the present invention, the threshold value is preferably 50%. [Advantageous effects of the invention]

[0017] According to the present invention, it is possible to provide abrasive grains capable of improving the flatness of a wafer dicing by a wire saw with loose grains and stabilizing processing quality, and an evaluation method thereof. Furthermore, according to the present invention, it is possible to provide a wafer manufacturing method including a step of dicing a wafer using a slurry containing such abrasive grains. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flowchart for explaining a silicon wafer manufacturing process; Fig. 2 is a perspective view schematically illustrating the configuration of the wire saw with loose grains; Fig. 3 is a flowchart schematically illustrating the abrasive grain inspection step; Fig. 4A and Fig. 4B are each a grain size distribution graph for explaining a small grain fraction calculation method; Fig. Figure 5 is a graph illustrating the volume-based grain size distributions of abrasive grain sample groups A and B; Fig. Figure 6 is a graph illustrating the number-based grain size distributions of the abrasive grain sample groups A and B; and Fig. Figure 7 is a graph (box-whisker diagram) illustrating the GBIR values ​​of the wafers subjected to dicing using the abrasive grain sample groups C1 to C5. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[0019] Fig. 1 is a flowchart for explaining a silicon wafer manufacturing process.

[0020] As in Fig. As illustrated in Figure 1, a silicon wafer is manufactured by sequentially performing an outer peripheral grinding step S11, a dicing step S12, a lapping step S13, an etching step S14, a double-side polishing step (rough polishing) S15, a single-side polishing step (mirror surface polishing) S16, a cleaning step S17, and the like. Of the above steps, the dicing step S12 is a step of dicing a silicon single-crystal ingot using a loose-grain wire saw, thereby producing a number of silicon wafers each having a fixed thickness.

[0021] Fig. Figure 2 is a perspective view schematically illustrating the configuration of the wire saw with loose grains.

[0022] As in Fig. 2, a loose-grain wire saw 1 is a so-called multi-wire saw and includes three parallel main rollers 10A, 10B, and 10C, wire lines 11R arranged at a fixed pitch between the main rollers 10A and 10B and formed by spirally winding a wire 11 over the three rollers 10A, 10B, and 10C, slurry nozzles 20A, 20B for supplying a slurry to the wire lines 11R, and a lifting device 18 for a silicon single-crystal ingot 2. A series of annular notches are formed at a fixed pitch on the outer peripheral surface of each of the main rollers 10A, 10B, and 10C, and the wire lines 11R are formed by winding the wire 11 in multiple turns with the wire 11 fitted into the annular notches. The wire lines 11R run reciprocally between the main rollers 10A and 10B by rotating the main rollers 10A, 10B and 10C.Ingot 2 is pressed against wire lines 11R and is thereby cut off.

[0023] One drum 12A is a wire supply drum around which a new wire is wound, and the other drum 12B is a wire take-up drum for winding a used wire. The wire 11 is fed from one drum 12A, passes through several guide rollers 13, is repeatedly wound over the main rollers 10A, 10B, and 10C in this order, passes through the several guide rollers 13 again, and is wound by the other drum 12B.

[0024] A tension adjustment mechanism 14A is provided in the movement path of the wire 11 between one drum 12A and the main roller 10A, and a tension adjustment mechanism 14B is provided in the movement path of the wire 11 between the other drum 12B and the main roller 10C. The tension adjustment mechanisms 14A and 14B each consist of a dancer roller 15 around which the wire 11 is wound, a dancer arm 16 that rotatably supports the dancer roller 15, and an actuator 17 that supports the dancer arm 16, and each imparts an appropriate tension to the wire 11 by utilizing the driving force of the actuator 17.

[0025] While the wire 11 reciprocally travels in one direction from one drum 12A to the other drum 12B or in the opposite direction, it is possible to continue to supply the new wire gradually by making the feed amount of the wire 11 from the drum 12A greater than the feed amount of the wire 11 from the drum 12B. The drums 12A and 12B are driven by a torque motor (not shown).

[0026] During cutting, a slurry is supplied from the slurry nozzles 20A and 20B to the wire lines 11R. The slurry is supplied from a slurry tank (not shown) through piping to the slurry nozzles 20A and 20B. The slurry nozzles 20A and 20B each have a row of nozzle holes positioned above the wire 11 constituting the wire lines 11R, and the slurry discharged from the nozzle holes flows downward like a curtain onto the running wire 11 constituting the wire lines 11R. The slurry adhering to the wire 11 reaches the cutting point of the ingot together with the running wire 11, and the ingot 2 is cut by the wire 11 to which the slurry adheres.

[0027] The slurry is obtained by dispersing abrasive grains in a water-soluble or oil-based coolant. The material of the abrasive grains is preferably SiC, Al2O3, or the like, and the coolant is preferably a glycol-based coolant. To ensure wafer processing quality in the dicing step S12, the abrasive grains must meet a certain quality criterion. Accordingly, the abrasive grains are examined for quality before actual use in the dicing step S12.

[0028] Fig. Figure 3 is a flowchart schematically illustrating the abrasive grain testing step.

[0029] As in Fig. As illustrated in Figure 3, in the abrasive grain inspection step, first, a predetermined amount of abrasive grains extracted as an abrasive grain sample group from a batch of abrasive grains is prepared (step S21), and the prepared abrasive grains are inspected for quality (step S22). Quality evaluation items may include "grain size distribution," "roundness," "small grain content," and the like. If the result of the quality inspection of the abrasive grains shows that the values ​​of all quality evaluation items meet their corresponding criteria, it is determined that the abrasive grains subjected to the inspection are usable in the separation step (Y in steps S23 and S24), whereas if the value of at least one quality evaluation item does not meet its corresponding criterion, it is determined that the abrasive grains are unusable (N in steps S23 and S25).

[0030] The "small grain ratio," one of the quality assessment elements, is an index indicating the proportion (numerical fraction) of small grains (fine grains) contained in the abrasive grain sample group, and therefore has not been used as a quantitative index to date. Small grains do not contribute to ingot cutting and, more importantly, hinder the cutting performance of medium-sized abrasive grains that contribute to ingot cutting. Therefore, if the proportion of small grains contained in the abrasive grain sample group is large, the flatness of the wafer after dicing may deteriorate. However, if the small grain ratio is reduced to a certain level or below, the flatness of the wafer after dicing can be made satisfactory, thereby stabilizing the wafer processing quality.

[0031] In the present embodiment, the grain diameter of the small grains is preferably equal to or less than 1 / 2 of the average grain diameter in a grain size distribution based on the volume of the abrasive grain sample group. By monitoring that the grain diameter of abrasive grains falls within this range, the quality of the abrasive grains can be stabilized, and wafer processing quality can be improved accordingly. Furthermore, when the grain diameter criterion for determining the small grain is set equal to or less than 1 / 2 of the average grain diameter, the definition of the small grain is clear, making it easy to set the grain diameter criterion based on a grain size distribution, facilitating identification of the small grains that have an influence on wafer processing quality.

[0032] The small grain content of the abrasive grains is preferably 50% or less. When the small grain content is 50% or less, the overall flatness of the wafer after dicing can be made satisfactory, enabling the stabilization of the wafer processing quality. However, when the small grain content exceeds 50%, the flatness of the wafer is deteriorated, resulting in the wafer failing to meet a predetermined quality criterion.

[0033] Fig. 4A and Fig. 4B are each a grain size distribution graph for explaining a small grain fraction calculation method.

[0034] When calculating the small grain fraction, the grain diameters of individual abrasive grains in the abrasive grain sample group are first measured and a grain size distribution D1 is calculated based on the volume of the entire abrasive grain sample group, as shown in Fig. 4A illustrates, calculated. A method for measuring the grain diameters of individual abrasive grains is not particularly limited. For example, the abrasive grains are photographed one by one while causing a liquid containing the abrasive grains to flow in a flow path between thin glass cells, followed by image processing, whereby the grain diameters of the individual abrasive grains can be measured.

[0035] Then, the average grain diameter R1 of the volume-based grain size distribution D1 of the abrasive grain sample group is calculated, and the grain diameter equal to 1 / 2 of the average grain diameter is defined as the abrasive grain diameter criterion R2 (= 0.5R1) for the small grains. Accordingly, the small grains are defined as abrasive grains with a grain diameter of the grain diameter criterion R2 = R1 / 2 or smaller.

[0036] Then, on the basis of a grain size distribution D2 based on the number of abrasive grain sample group, as shown in Fig. 4B illustrates, the number of small grains having a diameter equal to or smaller than 1 / 2 of the average grain diameter R1 is counted, and the small grain ratio, which is the numerical proportion of the small grains occupied in the abrasive grain group, is calculated.

[0037] If the small grain fraction is equal to or less than a predetermined threshold, it is determined that the extracted abrasive grain sample group is usable in the dicing step. On the other hand, if the small grain fraction is greater than the threshold, it is determined that the extracted abrasive grain sample group is unusable in the dicing step. The threshold for the small grain fraction is preferably 50% or less. If the small grain fraction exceeds 50%, the flatness indices of the silicon wafer, such as GBIR (Global Backside Ideal Focal Plane Range), warpage, and nanotopography, deteriorate, resulting in the wafer failing to meet a predetermined quality criterion.

[0038] As described above, an abrasive grain evaluation method according to the present embodiment includes measuring the volume-based abrasive grain distribution D1 of the entire abrasive grain sample group, counting the number of small grains distributed in an area that are equal to or smaller than 1 / 2 of the average grain diameter R1 of the grain size distribution D1, determining whether the small grain ratio indicating the numerical proportion occupied by small grains in the abrasive grain sample group is 50% or less, and dividing the ingot using a slurry containing the abrasive grains extracted as the abrasive grain sample group when a confirmatory result (50% or less) is obtained. Accordingly, it is possible to improve the flatness of the wafer cut by the wire saw with loose grains and stabilize the wafer processing quality.

[0039] Although the preferred embodiment of the present invention has been described, the present invention is not limited to the above embodiment and various modifications can be made within the scope of the present invention and all such modifications are included in the present invention.

[0040] For example, in the above embodiment, the value of 1 / 2 of the average grain diameter of the volume-based grain size distribution of the abrasive grain sample group is set as the grain diameter criterion, but the grain diameter criterion is not limited to 1 / 2 of this value. Furthermore, the value of 1 / 2 of the average grain diameter of the number-based size distribution of the abrasive grain sample group is set as the grain diameter criterion.

[0041] Furthermore, although the silicon single crystal ingot is subjected to dicing in the above embodiment, the present embodiment is not limited thereto, and an ingot of any material may be subjected to dicing. [Examples]

[0042] Two abrasive grain sample groups, A and B, extracted from different batches, were prepared, and the volume-based and number-based grain size distributions of each of the groups A and B were measured. In measuring the grain size distribution, the diameters of each sample abrasive grain were measured using a particle size / shape analyzer FPIA-3000 manufactured by Sysmex Corporation.

[0043] Fig. Figure 5 is a graph illustrating the volume-based grain size distributions of the abrasive grain sample groups A and B. The horizontal axis represents the abrasive grain size (µm) and the vertical axis represents the frequency (volume fraction). As can be seen from the graph of Fig. As can be seen from Figure 5, there is almost no difference between the volume-based grain size distributions of the abrasive grain sample groups A and B.

[0044] Fig. Figure 6 is a graph illustrating the number-based grain size distributions of the abrasive grain sample groups A and B. The horizontal axis represents the abrasive grain size (µm) and the vertical axis represents the frequency (number fraction). As can be seen from the graph of Fig. 6, the abrasive grain sample group B contains more finer grains than the abrasive grain sample group A. Furthermore, the presence of the small grains, which can be seen in the volume-based grain size distribution ( Fig. 5) can hardly be confirmed in the number-based abrasive grain distribution ( Fig. 6) can be easily confirmed. The small grain fraction of abrasive grain sample group A was 10%, and the small grain fraction of abrasive grain sample group B was 70%.

[0045] Next, five abrasive grain sample groups C1 to C5 with different small grain fractions were prepared. The small grain fractions of the abrasive grain sample groups C1 to C5 were 10%, 30%, 50%, 70%, and 90%, respectively. Then, the abrasive grain sample groups C1 to C5 were used to cut a silicon single-crystal ingot using a wire saw. Afterward, the GBIR, which is the flatness index of the resulting wafer, was measured for each of the sample groups. The GBIR is a global flatness index using the back side of the wafer as a reference and is defined by the difference between the maximum and minimum thickness values ​​measured with the back side as a reference.

[0046] Fig.Figure 7 is a graph (box-whisker diagram) illustrating the GBIR values ​​of wafers subjected to dicing using abrasive grain sample groups C1 to C5. The horizontal axis represents the small grain content (%), and the vertical axis represents the GBIR (standard value). As can be seen from the graph, the GBIR values ​​of abrasive grain sample group C1 with a small grain content of 10% and abrasive grain sample group C2 with a small grain content of 30% are small, while the GBIR value of abrasive grain sample group C3 with a small grain content of 50% becomes slightly larger. The GBIR values ​​of abrasive grain sample group C4 with a small grain content of 70% and abrasive grain sample group C5 with a small grain content of 90% become even larger. Accordingly, it is understood that the abrasive grain group with a small grain content of 50% or less can achieve satisfactory wafer flatness. [List of reference symbols] 1 wire saw with loose grains 2 silicon single crystal ingot 10A, 10B, 10C Main roles 11 wire 12A, 12B drums 11R wire lines 13 Leadership role 14A, 14B Voltage adjustment mechanisms 15 Dancer role 16 Dancer arm 17 Actuator 18 Lifting device 20A and 20B slurry nozzles D1 Grain size distribution of the abrasive grain sample group (volume-based) D2 Grain size distribution of the abrasive grain sample group (number-based) R1 average grain diameter of the grain size distribution D1 R2 grain diameter criterion (R1 / 2) S11 External peripheral loops S12 Cutting S13 Lapping S14 Etching S15 double-sided polishing S16 single-sided polishing S17 Cleaning

Claims

[1] Abrasive grain assessment procedure, which includes: Preparing a predetermined amount of abrasive grains as an abrasive grain sample group; Measuring the grain diameters of individual abrasive grains in the abrasive grain sample group and counting the number of abrasive grains in the abrasive grain sample group; Defining abrasive grains having a grain diameter equal to or smaller than a predetermined diameter criterion (R2) that is smaller than the average grain diameter (R1) of the abrasive grain sample group as small grains, and counting the number of small grains; Calculating a small grain fraction, which is the numerical fraction of small grains in the abrasive grain sample group; and Determining whether or not the small grain fraction is equal to or less than a predetermined threshold value, wherein when the small grain fraction is equal to or less than the threshold value, it is determined that the abrasive grain sample group can be used in severing an ingot by a wire saw, while when the small grain fraction is greater than the threshold value, it is determined that the abrasive grain sample group cannot be used in severing. [2] The abrasive grain evaluation method according to claim 1, comprising: Measuring the grain size distribution (D1, D2) based on the volume of the abrasive grain sample group from the grain diameters of the individual abrasive grains in the abrasive grain sample group; Calculating the average grain diameter (R1) of the abrasive grain sample group from the measured grain size distribution; and Defining the abrasive grains distributed in an area that are equal to or smaller than a grain diameter criterion (R2) that is smaller than the average grain diameter (R1) as the small grains. [3] The abrasive grain evaluation method according to claim 1 or 2, wherein the grain diameter criterion (R2) is 1 / 2 of the average grain diameter (R1). [4] The abrasive grain evaluation method according to any one of claims 1 to 3, wherein the threshold value is 50%. [5] Wafer manufacturing process comprising: an examination step of examining abrasive grains with regard to quality; and a cutting step of cutting an ingot using a wire saw with loose grains with a slurry containing the abrasive grains that have passed the test, wherein the examination step includes a small-grain fraction examination step of examining the small-grain fraction of the abrasive grains, The small grain fraction testing step includes: Preparing a predetermined amount of abrasive grains as an abrasive grain sample group; Measuring the grain diameters of individual abrasive grains in the abrasive grain sample group and counting the number of abrasive grains in the abrasive grain sample group; Defining abrasive grains having a grain diameter equal to or smaller than a predetermined diameter criterion (R2) that is smaller than the average grain diameter (R1) of the abrasive grain sample group as small grains and counting the number of small grains; calculating a small grain proportion, which is the numerical proportion of the small grains in the abrasive grain sample group; and Determining whether the small grain fraction is equal to or less than a predetermined threshold. [6] A wafer manufacturing method according to claim 5, comprising: Measuring the grain size distribution (D1, D2) based on the volume of the abrasive grain sample group from the grain diameters of the individual abrasive grains in the abrasive grain sample group; Calculating the average grain diameter (R1) of the abrasive grain sample group from the measured grain size distribution; and Defining the abrasive grains distributed in an area that are equal to or smaller than a grain diameter criterion (R2) that is smaller than the average grain diameter (R1) as the small grains. [7] A wafer manufacturing method according to claim 5 or 6, wherein the grain diameter criterion (R2) is 1 / 2 of the average grain diameter (R1). [8] A wafer manufacturing method according to any one of claims 5 to 7, wherein the threshold value is 50%.

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