JP3178926B2 - Water quality evaluation method and apparatus - 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 measuring the amount of impurities in water, particularly impurities contained in ultrapure water, as the amount of evaporation residue.

[0002]

2. Description of the Related Art Conventionally, the quality of ultrapure water has been evaluated by its specific resistance, total organic carbon (TOC), fine particles, silica, and viable bacteria. Furthermore, the dissolved oxygen concentration has recently been added to the items, and is evaluated mainly by the six items in Table 1. At present, the water quality of ultrapure water, which is considered to be the highest purity, is shown in Table 1. All of these items are at levels below the detection sensitivity, and the evaluation methods (means) to date have evaluated the superiority of the water quality. I cannot do it.

[0003]

[Table 1]

Therefore, an LSI called a watermark method is used.
A new evaluation method has been proposed in line with the manufacturing method. In this method, ultrapure water is dropped on a silicon wafer, and stains remaining after evaporation and drying are referred to as watermarks, which are observed with a microscope to evaluate water quality based on the amount, color, shape, and the like. Normally, ultrapure water is used to remove impurities adhered to the wafer by treatment with a chemical solution or the like in a wafer cleaning step, and the wafer is dried after rinsing with ultrapure water.
Therefore, it is best that nothing remains on the wafer after evaporation and drying, and it is considered that the watermark method is a method of evaluating ultrapure water for LSI in accordance with the LSI manufacturing process. Attempts have also been made to quantitatively determine the amount of "stain" formed on the wafer surface by using a computer to perform image processing, but this has not yet reached practical use.

A well-known example of this watermark is disclosed in Chemical Engineering, January 1990, p.
There are 68.

[0006]

According to the above-mentioned conventional watermarking method, qualitative evaluation of the presence or absence and size of a mark is possible, but quantitative evaluation by weight cannot be performed. The object of the invention is to make this possible.

[0007]

The object of the present invention is to form a watermark on an electrode on the surface of a vibrator using a crystal weight sensor composed of a crystal vibrator and electrodes, and to determine the frequency of the crystal weight sensor before and after drying. It can be solved by measuring the change.

[0008]

Normally, a crystal resonator has a natural frequency. However, if a weight change occurs due to something adhering to the surface of the crystal resonator, the frequency changes. It is also known that the frequency decreases according to equation (1).

ΔF = −2.3 × 10 ⁶ F ² Δm / A (1) Here, ΔF is the amount of change in frequency (Hz), F is the natural frequency (MHz), and Δm is the weight of the deposit. Change (g) and A is the area of the electrode (cm ² ). Therefore, it is possible to measure the impurities contained in the sample water by weight by dropping a water drop on the electrode on the surface of the quartz weight sensor and measuring the change in the frequency after evaporation and before the drop. By the way, natural frequency is MHz, electrode area is 1cm
Assuming that ² , the change of the frequency of 1 Hz corresponds to 12 ng. Therefore, in the case of ultrapure water containing 10 μg / 1 impurity, about 1.2 ml is dropped and dried to obtain 1 Hz.
Can be measured as a change in the vibration frequency. As a publicly known example of the principle of the present invention, there is Bunseki February, 1989, and research is currently being conducted for the purpose of measuring the corrosion rate and deposition rate of metal by utilizing the characteristics of a quartz weight sensor. I have.

However, the condition (1) is a condition for uniform deposition on the electrode surface. In the case of non-uniform deposition, the condition (1) does not hold. When an evaporation residue is formed on a normal electrode surface, the residue remains at a certain point, so that the change in the frequency varies depending on the remaining position. Therefore, by making the electrode surface uneven or attaching a water-absorbing substance, it is always left in the same place, thereby suppressing the sensitivity dependency of the frequency change depending on the position.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an example of a water quality evaluation method using a quartz weight sensor according to the present invention. This evaluation device configuration includes a crystal unit 101, electrodes 102 and 103, an oscillator 104, a frequency measuring device 105, and a calculator 106. The operation of this apparatus is as follows. First, the fixed frequency of the crystal unit 101 in a state where nothing is attached to the surface of the electrode 103 is measured by the frequency
05. Thereafter, the sample water 1 is placed on the upper electrode 103 of the quartz oscillator 101 by using the sample tube 107.
08 (a known amount is good if possible) is added dropwise. further,
After the sample water 108 after the dripping is dried, the natural frequency of the quartz oscillator 101 is measured again by the frequency measuring device 105, and the weight is calculated by the calculator 106 from the change amount of the frequency before and after, and the sample water is calculated. Finally, the concentration of impurities in the water can be calculated from the amount of. Using this device,
A known concentration (Na) is placed on a quartz oscillator having a natural frequency of 6 MHz.
FIG. 5 shows the results of measuring the change in frequency before and after the addition of Cl) and a known amount of sample water. Here, the vertical axis represents the frequency change ΔF (Hz), and the horizontal axis represents the weight of NaCl contained in the sample water. As a result, a first-order relationship is established between ΔF and the weight of the attached matter, and it is understood that the amount of impurities in water can be measured by this method. However, the quartz weight sensor greatly varies in sensitivity to frequency change depending on the position of the deposit on the electrode surface. In the present results, the results are varied because no particular consideration is given to this point. Therefore, it is necessary that the deposits adhere uniformly or that the deposits always remain at the same point.

FIG. 2 shows an example of an electrode shape of a quartz weight sensor used in the water quality evaluation apparatus according to the present invention. Here, the present sensor 200 includes a quartz oscillator 201, a lower electrode 202, an upper electrode 203, and a wiring 204. The present sensor 200 has fine irregularities 205 on the surface of the upper electrode, and the sample water dropped by the irregularities 205 stays in the concave portions, so that the residue can be uniformly left on the upper electrode 203. In this embodiment, fine irregularities are provided, but as another method, a concave portion is provided at the center of the electrode so that the electrode can always remain at the same place. Conversely, even if a convex portion is provided at the center, the sample water is dried while adhering to the convex portion, so that finally, the residue can be left near the convex portion.

FIG. 3 also shows an example of the shape of a crystal weight sensor electrode used for water quality evaluation according to the present invention. Here, the sensor 300 includes a quartz oscillator 301, a lower electrode 302, an upper electrode 303, and wirings 305 and 306. In the present sensor 300, a water-absorbing substance 304 capable of absorbing moisture is provided on an upper electrode 303 thereof. When the sample water is dropped onto the water-absorbing substance 304, the sample water is uniformly sucked into the water-absorbing substance 304, and the impurities remaining after drying also uniformly remain in the water-absorbing substance 304.
The impurities in the water remain uniformly on the upper electrode 303. Examples of the water-absorbing substance include woven fabric and paper.

FIG. 4 shows an example of a water quality evaluation system using the present invention. This system includes a water quality evaluation device 400 and a generated water pipe 408 of an ultrapure water device. The water quality evaluation device further includes a crystal weight sensor 401, a sample water dropping system 4
02, a sample water drying system 403, an oscillator 404, a frequency measuring device 405, and a computing device 406. This system supplies ultrapure water flowing in the ultrapure water generation pipe 408 to the sample water dropping system 402 from the sample water introduction unit 409 of the water quality evaluation device 400. Here, a fixed amount of sample water is dropped on the electrode of the quartz weight sensor 401, and the sample water is dried by blowing clean air, if possible, from the sample water drying system 403 if possible. After drying, the natural frequency of the crystal weight sensor 401 is measured by the frequency measuring device 405, the weight is calculated by the calculator 406 from the variation from the frequency measurement result before dropping, and finally the weight is calculated from the amount of the sample water. Can calculate and monitor the concentration of impurities in water. Here, the example in which the sample water drying system 403 is dried by blowing high-temperature or normal-temperature air has been described, but other methods do not pose any problem. In the present embodiment, the monitoring of the generated water of the ultrapure water production apparatus has been described as an example, but the present invention can be applied to the measurement or monitoring of the impurity concentration in the generated liquid from another liquid processing apparatus. Further, in the water quality evaluation device, if a mechanism for removing the evaporation residue on the surface of the vibrator, for example, a mechanism for brushing the electrode surface or a mechanism for cleaning with ultrapure water or the like, can be used for a long time, Online measurement becomes possible.

[0016]

According to the present invention, the water quality can be measured or monitored by measuring the evaporation residue contained in not only ordinary pure water but also ultrapure water by its weight. In particular, as a method for evaluating the quality of ultrapure water for LSIs, residues remaining on wafers in LSI cleaning are problematic, and thus this evaluation method is considered to be very effective.

[Brief description of the drawings]

FIG. 1 shows an example of a water quality evaluation method using a quartz weight sensor according to the present invention.

FIG. 2 shows an example of the shape of a quartz weight sensor electrode used for water quality evaluation according to the present invention.

FIG. 3 shows another example of the shape of a quartz weight sensor electrode used for water quality evaluation according to the present invention.

FIG. 4 shows an example of a water quality evaluation system using the present invention.

FIG. 5 shows an example of an experimental result of the water quality evaluation method according to the present invention.

[Explanation of symbols]

101: quartz oscillator 102, 103 ...
Electrode 104 ... Oscillator 105 ... Frequency measuring device 106 ... Calculator

Continuation of the front page (72) Inventor Takayuki Matsumoto 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside the Energy Research Laboratory, Hitachi, Ltd. (56) References Chemical Engineering, issued January 1, 1990, volume 35, volume 35 No. 1, p. 68-72 Chemical Engineering, issued August 5, 1994, Vol. 58, No. 8, p. 650-651 (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 5/00-5/04 G01N 33/18 JICST file (JOIS)

Claims (5)

(57) [Claims] 1. A sample water is dropped on an electrode surface of a crystal weight sensor composed of a crystal oscillator having a natural frequency and an electrode, and is included in the sample water based on a change in the frequency of the crystal weight sensor before and after the dripping and drying. Water quality evaluation method characterized by measuring the amount of impurities or total evaporation residues. 2. A sample water is dropped on an electrode surface of a quartz weight sensor composed of a quartz oscillator having a fixed frequency and an electrode, and is included in the sample water based on a change in the frequency of the quartz weight sensor before and after dripping and drying. In the quartz weight sensor for water quality evaluation, which measures the amount of impurities or total evaporation residues, the electrodes are provided so that there is a correlation between the oscillation frequency and the weight of the deposit after evaporation and drying. A quartz weight sensor characterized by the following. 3. The quartz weight sensor according to claim 2,
A quartz weight sensor comprising a mechanism for drying sample water such that the oscillation frequency is correlated with the weight of the attached matter, wherein the quartz crystal weight sensor has irregularities in the center or on the entire surface of the electrode. 4. The quartz weight sensor according to claim 2,
As a mechanism for drying the sample water so that the oscillation frequency is correlated with the weight of the deposit, the electrode is composed of a water-absorbing substance, or a water-absorbing substance is installed on the electrode surface. Quartz weight sensor. 5. A quartz weight sensor comprising a quartz oscillator having a natural frequency and electrodes, an oscillator for oscillating the oscillator, and a measuring instrument for measuring the frequency, and an operation for calculating the weight from the measured value. It consists of a device, a dripping device that drops sample water on the quartz weight sensor, and a drying device that dries the sample water that has been dropped. A water quality evaluation device for measuring the amount of impurities or total evaporation residues.