JPH08310804A - Method for purifying phosphoric acid and method for manufacturing semiconductor device - Google Patents

Abstract

(57) [Summary] [Purpose] A method for purifying phosphoric acid, which aims to remove radioactive elements contained in phosphoric acid. [Structure] The silicon simple substance activated by removing the oxide film by surface treatment was put into phosphoric acid until the radiation dose of the polonium adsorbed on the surface of the silicon simple substance was reduced to 0.001 cph / cm ² or less, and held. A method for purifying phosphoric acid is characterized by repeating the operation of taking out.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying phosphoric acid which removes polonium which is a radioactive element contained as an impurity.

Semiconductor integrated circuits such as ICs and LSIs are inter-layer
It is multi-layered using an insulating layer and provided in the interlayer insulating layer.
A three-dimensional circuit is formed by a via,
Silicon nitride (Si₃N_Four) Is used
And heated phosphoric acid (H ₃PO_Four) Is Si₃N_FourIs melt
Sumono: Silicon oxide (SiO 2₂ ) Does not dissolve
It is widely used as a selective etching solution.

[0003]

2. Description of the Related Art Since it is necessary to process a large amount of information at high speed,
The capacity of semiconductor integrated circuits has been increased, and LSIs and VLSIs have been developed.
Has been put into practical use, but this is due to the miniaturization of transistor unit elements, and the electrodes and lines that form the unit elements are becoming smaller and smaller, and the minimum line width of the line is sub-micron. Has reached.

As the miniaturization progresses and the distance between the pn junction, which acts as a transistor, and the electrode and the distance to the line come closer, soft error due to α rays becomes a problem.

A large number of chemicals are used in the formation of semiconductor devices, and the mixing of impurity elements has a significant effect on the characteristics of the device, so only purified special grade products are used in the manufacturing process. The problem with H ₃ PO ₄ is that a radioactive element is present in the H ₃ PO ₄ and this radioactive element adheres to the Si substrate and causes a soft error.

[0006]

As described above, chemicals used for forming semiconductor devices are refined as described above to use radioactive elements as well as special grades containing a small amount of impurities, and H ₃ PO ₄ is also used. No exception. Most of the liquid chemicals are purified by the distillation method.

However, there is a problem in that the distillation method cannot be applied to H ₃ PO ₄ . Here, H ₃ PO ₄ used for manufacturing semiconductor devices is manufactured by a dry method. That is, phosphorus ore is melted with coke and the like to produce phosphorus vapor, which is then condensed to produce yellow phosphorus (P ₄ ), which is burned in excess air to form phosphorus pentoxide (P ₂ O ₅ ). This is absorbed in water to make H ₃ PO ₄ .

However, since phosphorus ore contains the radioactive element polonium (Po), a small amount of Po is contained in H ₃ PO ₄ , which forms the pattern of the gate electrode of the MOS transistor. Although H ₃ PO ₄ is used, for example, because the cause of soft error Po with radioactive when this continues to irradiation of α rays attached to the vicinity of the gate electrode, the H ₃ PO ₄ It is necessary to remove a small amount of Po contained therein, and the practical application of this removal method is an issue.

[0009]

[Means for Solving the Problems] The above problems are related to surface treatment.
The Si simple substance activated by further removing the oxide film is
i Radiation dose of Po adsorbed on the surface of simple substance is 0.001cph / cm²To
H until decreasing₃PO _FourPut in, hold, take out
This can be achieved by repeating the treatment.

[0010]

[Action] is included as Po impurities in the H ₃ PO _4, although it was known that emits α-rays are adsorbed on Si wafer when using H ₃ PO ₄ etc. selective etching solution, No method has been known for removing Po contained in the H ₃ PO ₄ solution. However, a method of electrodepositing on silver (Ag) is known as one of the methods of detecting Po.

The present inventor has considered a method of removing Po from the H ₃ PO ₄ solution by utilizing the fact that Po is selectively adsorbed on Si. That is, when the Si wafer that has been subjected to selective etching using a heated H ₃ PO ₄ solution is washed and then measured with an α ray counter, the radioactivity due to Po is detected.

On the other hand, Si and SiO ₂ are insoluble in the H ₃ PO ₄ solution. In the present invention, high-purity Si is introduced into a heated H ₃ PO ₄ solution so that Po in the solution is adsorbed by Si. At this time, the surface of Si is treated with hydrofluoric acid (HF) or the like. Passive oxide film (Si
It is preferable to remove O _x ). That is, HF
When a high-purity Si wafer such as a Si wafer is immersed in an aqueous solution,
The passive oxide film (Si O _x ) existing on the surface is dissolved and activated. Then, after washing with water and drying, oxidation progresses again and the oxide film is covered, but even in that case, the shorter the elapsed time from the activation treatment, the more efficiently adsorption can be performed.

According to the present invention, activated pure Si as an adsorbent is put into an H ₃ PO ₄ solution as an adsorbent, held for a certain period of time to adsorb Po, taken out from the solution, washed and dried to obtain α rays. The operation of measuring with a counter is repeated until the α dose falls below the safe amount of 0.001 cph / cm ² .

[0014]

EXAMPLE The concentration of special grade H ₃ PO ₄ purchased for the manufacture of semiconductor devices is about 85-87%, but the α dose contained in each lot is very different. That is, the content of Po differs greatly from lot to lot.

The H ₃ PO ₄ refining apparatus used in the practice of the present invention is a vessel having a temperature controllable heater at the bottom of the vessel, an agitation rod and a quartz wafer mount inside. It was filled with concentrated H ₃ PO ₄ obtained in Example 1 and heated to 150 ° C.

On the other hand, high purity Si has a diameter of 4 inches.
Prior to the experiment, the i wafer was immersed in an HF solution to remove the passivation oxide film (Si O _x ) existing on the surface, washed thoroughly with water and dried.

Then, these 10 Si wafers were inserted into a wafer mount, heated for 100 minutes while stirring with a stirring rod, taken out with time and replaced with a new Si wafer, and the taken out Si wafer was thoroughly washed. After drying, the amount of α was measured with an α-ray counter.

FIG. 1 shows the results. When the first measurement value is used as a reference value, the second measurement is reduced to about 50% and the third measurement is about 5%.
%, Which is below the safety value of 0.001 cph / cm ² .

[0019]

The present invention is intended to remove Po contained in H ₃ PO ₄ by adsorbing it on Si, and by carrying out the present invention, Po can be removed to below the allowable value. Line disturbance can be suppressed.

[Brief description of drawings]

FIG. 1 is a relationship diagram between the number of times Si wafers are loaded and α dose.

Claims (3)

[Claims] 1. A method for purifying phosphoric acid, which comprises pouring silicon simple substance into phosphoric acid and adsorbing and removing polonium contained as an impurity in the phosphoric acid. 2. The silicon simple substance is activated by removing the oxide film on the surface of the simple substance by surface treatment, and the radiation dose of polonium adsorbed on the surface of the simple substance is 0.001 cph / c.
^{2. The} method for purifying phosphoric acid according to claim 1, wherein an operation of charging, retaining and taking out the activated silicon simple substance to the phosphoric acid solution is repeatedly performed until the activated silicon is reduced to m ² or less. 3. A method of manufacturing a semiconductor device, wherein phosphoric acid purified by the method according to claim 1 or 2 is used as an etching solution.