JPH0845947A - Thermal treatment method of silicon substrate - Google Patents

Abstract

PURPOSE:To provide a thermal treatment method wherein the surface of a silicon substrate is lessened in number of crystal defects, and a silicon integrated device is enhanced in electrical properties. CONSTITUTION:A silicon substrate is thermally treated through such a manner that it is thermally treated at temperatures of 1000 to 1350 deg.C or preferably 1100 to 1350 deg.C for above 10 minutes or preferably above 30 minutes in an atmosphere of hydrogen gas, inert gas, or mixed gas of inert gas loaded with 1ppba to 1ppma of SiH4 gas, Si2H6 gas, or mixed gas of SiH4 gas and Si2H6 gas. By this setup, the silicon substrate lessened in number of crystal defects in its surface, possessed of an oxide film high in dielectric strength, and small in leakage current can be manufactured, and a silicon integrated device higher in reliability can be manufactured by the use of the above silicon substrate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for a silicon substrate which reduces crystal defects on the surface of the silicon substrate and improves the electrical characteristics of a silicon integrated device.

[0002]

2. Description of the Related Art With the miniaturization of silicon integrated devices,
Reduction of crystal defects on the surface of a silicon substrate, high breakdown voltage strength characteristics of silicon oxide film used for a gate oxide film of a MOS transistor (hereinafter simply referred to as breakdown voltage characteristics), and reverse bias leakage current characteristics of a pn junction of a MOS transistor (Hereinafter, it is simply abbreviated as a leak current characteristic).

To address this problem, Japanese Patent Laid-Open No. 51-13
Japanese Patent No. 4071 discloses a method of heat-treating a silicon single crystal at 350 ° C. to 1350 ° C. in an inert gas or reducing gas atmosphere. By the heat treatment method disclosed in the publication, a single crystal having excellent characteristics such as crystal defect removal, lifetime, leak current, and breakdown voltage can be obtained. Further, JP-A-62-123098 discloses that hydrogen gas or a mixed gas atmosphere in which hydrogen gas is diluted with nitrogen gas, an inert gas or these gases is used at 800 ° C. to 13 ° C.
Disclosed is a method of heat treatment at a temperature of up to 50 ° C. for 1 second to 48 hours. The heat treatment method disclosed in the publication makes it possible to more efficiently reduce the crystal defect density near the surface of the silicon substrate.

[0004]

However, the above-mentioned JP-A-51-134071 and JP-A-62-1230.
Even for a silicon substrate that has been subjected to the heat treatment disclosed in Japanese Patent Publication No. 98, etc., crystal defects may occur again when the heat treatment necessary for manufacturing a silicon integrated device is performed.

In addition, high integration of silicon integrated devices,
With the reduction in power consumption, higher breakdown voltage characteristics and less leakage current characteristics are required.

Therefore, the present invention is for producing a silicon substrate having a more efficient reduction of crystal defects on the surface of the silicon substrate, a higher breakdown voltage characteristic, and a lower leakage current characteristic, which were impossible in the prior art. A heat treatment method is provided.

[0007]

The present inventor has completed the present invention as a result of extensive studies on a novel method for heat treatment of a silicon substrate in order to achieve the above object.

That is, the object of the present invention is 1000 to 1350 ° C. in a hydrogen gas added with SiH ₄ gas of 1 ppba or more and 1 ppma or less, an inert gas or a mixed gas atmosphere of hydrogen gas and an inert gas, This is achieved by a heat treatment method for a silicon substrate, which is characterized by performing heat treatment for 10 minutes or more.

Another object of the present invention is to provide a hydrogen gas containing 1 ppba or more and 1 ppma or less of SiH ₄ gas, an inert gas or a mixed gas atmosphere of hydrogen gas and an inert gas at 1100 to 1350 ° C. It can also be achieved by a heat treatment method for a silicon substrate, which is characterized by performing heat treatment for 30 minutes or more.

Further, an object of the present invention is 1000 to 1350 ° C. in a hydrogen gas added with Si ₂ H ₆ gas of 1 ppba or more and 1 ppma or less, an inert gas or a mixed gas atmosphere of hydrogen gas and an inert gas. The heat treatment method for a silicon substrate is characterized in that the heat treatment is performed for 10 minutes or more.

Still further, an object of the present invention is 1 ppba.
The heat treatment is performed at 1100 to 1350 ° C. for 30 minutes or more in a hydrogen gas added with Si ₂ H ₆ gas of 1 ppma or less and an inert gas or a mixed gas atmosphere of a hydrogen gas and an inert gas. It is also achieved by a method of heat treating a silicon substrate.

Another object of the present invention is to add hydrogen gas, an inert gas or a mixture of hydrogen gas and an inert gas to which a mixed gas of SiH ₄ gas and Si ₂ H ₆ gas of 1 ppba or more and 1 ppma or less is added. 1000 to 1 under gas atmosphere
It can also be achieved by a heat treatment method for a silicon substrate, which is characterized by performing heat treatment at 350 ° C. for 10 minutes or more.

Further, an object of the present invention is to add hydrogen gas, an inert gas or a mixture of hydrogen gas and an inert gas to which a mixed gas of SiH ₄ gas and Si ₂ H ₆ gas of 1 ppba or more and 1 ppma or less is added. 1100 under gas atmosphere
It is also achieved by a heat treatment method for a silicon substrate, which is characterized by performing heat treatment at 1350 ° C. for 30 minutes or more.

[0014]

When the present inventor conducted a detailed investigation on the mechanism of reducing the crystal defects in the surface layer of the silicon substrate and improving the withstand voltage characteristic and the leak current characteristic by heat-treating the silicon substrate, the following mechanism was observed. Became clear.

First, the main cause of deterioration in breakdown voltage characteristics and leak current characteristics is crystal defects such as oxygen precipitates, dislocations and stacking faults existing in the surface layer of the silicon substrate. Therefore, reduction of crystal defects in the surface layer of the silicon substrate leads to improvement in breakdown voltage characteristics and leak current characteristics.

Next, a silicon substrate manufactured by the CZ method (Czochralski pulling method) is generally used for manufacturing a silicon semiconductor device. Since interstitial oxygen is supersaturated in the silicon substrate manufactured by the CZ method, oxygen precipitates are always present. The oxygen precipitates grow into larger oxygen precipitates or secondary defects such as dislocations and stacking faults are generated during cooling of the silicon crystal and subsequent heat treatment. Therefore, reduction of oxygen precipitates on the surface layer of the silicon substrate leads to reduction of crystal defects in the surface layer of the silicon substrate and improvement of withstand voltage characteristics and leak current characteristics.

The surface layer of the silicon substrate is modified by heat treatment by outwardly diffusing supersaturated interstitial oxygen existing in the surface layer of the silicon substrate, thereby forming a lattice around the oxygen precipitates existing in the surface layer. The oxygen concentration is lowered to dissolve oxygen precipitates. Therefore, the greater the outward diffusion amount of interstitial oxygen, the more oxygen precipitates are dissolved. The outward diffusion amount of interstitial oxygen increases as the oxygen concentration on the surface decreases. This is because the diffusion of interstitial oxygen occurs due to the difference in concentration between the surface of the silicon substrate and the inside, and the greater the difference in concentration, the greater the amount of interstitial oxygen that diffuses outward.

The oxygen concentration on the surface of the silicon substrate during the heat treatment is determined by the difference between the amount of oxygen supplied to the surface of the silicon substrate by outward diffusion and the amount of evaporation of oxygen into the gas phase. From this, in order to reduce the oxygen concentration on the surface of the silicon substrate, the evaporation of oxygen into the gas phase may be promoted.

Therefore, in the present invention, SiH ₄ gas, Si ₂ H ₆ gas or SiH ₄ gas and S are added in a hydrogen gas, an inert gas or a mixed gas atmosphere of hydrogen gas and an inert gas.
By adding a mixed gas with i ₂ H ₆ gas, the evaporation of oxygen into the vapor phase on the surface of the silicon substrate is promoted according to the following mechanism. That is, the SiH ₄ gas or Si ₂ H ₆ gas added to the atmospheric gas causes the reaction of oxygen on the surface of the silicon substrate according to the following equations (1) and (2).

SiH ₄ + O → SiO + 2H ₂ (1) Si ₂ H ₆ + 2O → 2SiO + 3H ₂ (2) Silicon is produced by the reaction of the above formula (1) and / or (2). Oxygen on the substrate surface can be efficiently evaporated as SiO. From this fact, the oxygen concentration on the surface of the silicon substrate can be reduced, and further, the outward diffusion amount of interstitial oxygen can be increased by the mechanism described above, and consequently the crystal defects of the surface layer of the silicon substrate can be reduced.

As a method of adding SiH ₄ or Si ₂ H ₆ to hydrogen gas and performing heat treatment, a method of manufacturing an epitaxial substrate is generally known. However,
SiH ₄ gas and / or Si ₂ H ₆ in the present invention
The upper limit of the amount of added gas is 1 ppma, and SiH ₄ gas and / or Si ₂ H ₆ gas causes the reaction of the above formula (1) and / or (2) on the surface of the silicon substrate. Epitaxial growth cannot occur on the substrate. Therefore, it can be said that the present invention is not a generally known method for manufacturing an epitaxial substrate.

Next, as the heat treatment method for a silicon substrate according to the present invention, the SiH ₄ gas and / or the Si ₂ H ₆ gas is in the range of 1 ppba or more and 1 ppma or less in a hydrogen gas and / or an inert gas atmosphere. And is usually 1000 to 1350 ° C., preferably 1100 to
Heat treatment is performed at 1350 ° C. for 10 minutes or longer, preferably 30 minutes or longer.

At the time of the heat treatment, the atmosphere gas that can be used in the present invention is, as described above, a group consisting of hydrogen gas, an inert gas, and a mixed gas of hydrogen gas and an inert gas having an arbitrary mixing composition ratio. One atmosphere gas selected from the above can be used. This is because the atmosphere gas of the present invention is superior in terms of shortening the heat treatment time and eliminating oxygen precipitates, as compared with the conventional oxidizing gas atmosphere. In the present invention, 1p
pba is one-millionth of a mole fraction, 1ppma is 100
It represents a mole fraction of 1 / 10,000. The inert gas that can be used in the present invention is helium, xenon, argon alone or a mixed gas thereof.

Further, as the above atmosphere gas, from the viewpoint of suppressing the increase in microflurness on the surface of the silicon substrate, SiH ₄ gas and / or SiH ₄ gas which is at least twice the total amount of oxygen and water contained in the atmosphere gas is used. It is desirable to use the atmospheric gas mixed with the additive gas of Si ₂ H ₆ gas. As a result, the oxygen and water can be evaporated in the form of SiO and H ₂ , so that the influence on the silicon substrate can be suppressed. From this, in the present invention, when the atmosphere gas contains oxygen and water, the SiH ₄ gas and / or Si ₂ H ₆ gas which is twice the total amount of the oxygen amount and the water amount is added. Considering that it is consumed, it is one of desirable modes to add an additive gas of SiH ₄ gas and / or Si ₂ H ₆ gas of 1 ppba or more and 1 ppma or less within the scope of the present invention. I can say.

Subsequently, as the additive gas added to the above-mentioned atmosphere gas, SiH ₄ gas, Si ₂ H ₆ gas and a mixed gas of SiH ₄ gas and Si ₂ H ₆ gas having an arbitrary mixing composition ratio are used. One kind of additive gas selected from the group consisting of can be used. The amount of the added gas added is usually 1 ppba or more and 1 ppb or more in the atmosphere gas.
It is added in the range of ma or less. The addition amount is 1p
When it is less than pba, the oxygen concentration on the surface of the silicon substrate cannot be sufficiently reduced by the reaction of the above formula (1) and / or (2), so that the crystal defects of silicon cannot be removed, and the breakdown voltage characteristic and the leak current characteristic Is insufficiently improved,
It is not preferable because it cannot be applied to a silicon integrated device that requires high reliability. The addition amount is 1 pp
If it exceeds ma, a part of the added gas causes epitaxial growth, which is not preferable.

The heat treatment condition is usually 10
00 to 1350 ° C, preferably 1100 to 1350 ° C
The heat treatment is performed for 10 minutes or longer, preferably 30 minutes or longer. When the heat treatment temperature is lower than 1000 ° C., a considerably long heat treatment time is required to remove crystal defects of silicon, which has a drawback that the production efficiency is deteriorated. Crystal defects cannot be removed, improvement in withstand voltage characteristics and leak current characteristics becomes insufficient, and it is not preferable that it cannot be applied to silicon integrated devices requiring high reliability. Further, when the heat treatment temperature exceeds 1350 ° C. Is close to the melting point (1414 ° C.) of silicon, which is not preferable because the shape of the substrate (wafer) is easily deformed. If the heat treatment time is less than 10 minutes, it is not preferable because reduction of crystal defects due to the heat treatment cannot be sufficiently achieved.

Although the silicon substrate used in the present invention is not particularly limited, for example, the mirror wafer is processed, and before the heat treatment, the silicon substrate is immersed in, for example, a 1% by weight dilute hydrofluoric acid aqueous solution for surface treatment. It is preferable to use the one in which the natural oxide film existing in the above is removed, and then rinsed with ultrapure water and then spin-dried.

[0028]

Embodiments of the present invention will be described below.

Inventive Examples 1-52 and Comparative Examples 53-65
Is the plane orientation (100) and the interstitial oxygen atom concentration is 1 × 10 ^18.
cm ^-3, a silicon substrate produced by the boron atom 1 × 10 ¹⁵ cm ^-3 CZ method doped diameter 150 mm.

In Examples 1 to 52 of the present invention, the silicon substrate was first immersed in a dilute aqueous solution of hydrofluoric acid to remove the natural oxide film existing on the surface, and then rinsed with ultrapure water and then spin dried. did. Immediately thereafter, the silicon substrate was inserted into a normal vertical furnace, and SiH ₄ gas, Si ₂ H ₆ gas or SiH ₄ gas of 1 ppba or more and 1 ppma or less was inserted according to the conditions shown in Tables 1 to 3 below. Gas and S
Heat treatment was performed at 1000 ° C. to 1350 ° C. for 10 minutes to 24 hours in a hydrogen gas added with a mixed gas of i ₂ H ₆ gas, an inert gas, or a mixed gas atmosphere of a hydrogen gas and an inert gas.

Next, also in Comparative Examples 53 to 65, heat treatment was performed according to the contents described below according to the conditions shown in Table 4 below (however, in Comparative Example 65,
No heat treatment was performed. ).

In Comparative Example 53, the heat treatment temperature was 900 ° C., and the other conditions were the same as in Invention Examples 4 and 26.

In Comparative Example 54, the heat treatment temperature was 900 ° C., and the other conditions were the same as in Invention Examples 20 and 42.

In Comparative Example 55, the heat treatment temperature is 1400 ° C.
Other conditions were the same as those of Examples 4 and 26 of the invention.

In Comparative Example 56, the heat treatment temperature was 1400 ° C.
Other conditions were the same as those of Examples 20 and 42 of the invention.

In Comparative Example 57, the heat treatment time was 5 minutes, and the other conditions were the same as in Invention Examples 1 to 6.

In Comparative Example 58, the heat treatment atmosphere was SiH ₄
Gas and Si ₂ H ₆ gas were not added, and the other conditions were the same as in Invention Examples 4 and 7 to 16. Further, this comparative example is disclosed in Japanese Patent Laid-Open No. 62-12309 among the prior arts.
These are the heat treatment conditions disclosed in No. 8.

In Comparative Example 59, the heat treatment atmosphere was SiH ₄
Gas and Si ₂ H ₆ gas were not added, and the other conditions were the same as in Invention Example 20.

In Comparative Example 60, the heat treatment atmosphere was SiH ₄
In the case of adding 0.1 ppba of gas, the other conditions were the same as in Invention Examples 4 and 7 to 10.

In Comparative Example 61, the heat treatment atmosphere was SiH ₄
Other conditions were the same as those of Inventive Examples 4 and 7 to 10 when the gas was added at 2 ppma.

In Comparative Example 62, the heat treatment atmosphere was SiH ₄
Other conditions were the same as those of Examples 11 to 13 of the present invention when 0.1 ppba of gas was added.

In Comparative Example 63, the heat treatment atmosphere was SiH ₄
Other conditions were the same as those of Examples 11 to 13 of the present invention when the gas was added at 2 ppma.

In Comparative Example 64, the heat treatment atmosphere was SiH ₄
Gas and Si ₂ H ₆ gas were added at 1 ppma, respectively, and other conditions were the same as those of Examples 14 and 15 of the present invention.

Next, the evaluation of the crystal defects in the silicon surface layer is performed by the oxidation-induced stacking fault (hereinafter,
The evaluation was performed simply by abbreviated as OSF). After completion of the heat treatments of Examples 1 to 52 of the present invention and Comparative Examples 53 to 64, the silicon substrate of Comparative Example 65 was used together with 1
The silicon oxide film formed on the surface was removed by immersing in a dilute aqueous solution of dilute hydrofluoric acid (wt%), rinsed with ultrapure water, and then spin dried. After that, as a heat treatment for OSF evaluation, the silicon substrate is heat-treated in a heat treatment furnace in a dry oxygen atmosphere at 800 ° C. for 3 hours, and then at 1100 ° C. as it is.
Up to 3 ° C / min at 1100 ° C.
Wet oxidation was carried out for a period of time and the product was taken out of the furnace. After the heat treatment for evaluation is completed, the oxide film is removed in a dilute hydrofluoric acid solution of 10% by weight, etching is performed in a light etching solution for defect observation for 10 minutes, and rinsed with ultrapure water. After performing, dry and OS with a differential interference microscope
The F density was measured. The liquid composition for light etching is disclosed on page 442 of the VLSI Process Data Handbook (Editor: Yoichi Akasaka, Masahiro Kashiwagi, Kazuo Maeda, Takeo Yoshimi Publishing; Science Forum Publishing Year: 1990). Obeyed.

The withstand voltage characteristics were evaluated by measuring the breakdown electric field of 150 MOS diodes having an area of 0.20 cm ² produced on the wafer surface. First, these wafers were immersed in a dilute hydrofluoric acid solution to remove the silicon oxide film formed on the surface, rinsed with ultrapure water, and then spin dried. The 25.0 nm thick silicon oxide film of the MOS diode was formed by oxidizing these spin-dried wafers in a dry oxygen atmosphere at 1000 ° C. The electrodes of the MOS diode were formed by depositing a phosphorus-doped polycrystalline silicon film by low pressure chemical vapor deposition. M
The dielectric breakdown electric field of the OS diode keeps the back surface of the wafer at zero potential, sweeps and applies a negative potential to the electrodes on the front surface, and
It was determined by measuring the potential when the current flowing through the S diode reached 100 mA / cm ² as the dielectric breakdown potential. For the breakdown voltage characteristic, the index obtained by dividing the average value of the dielectric breakdown potential of the MOS diode in the substrate surface by the silicon oxide film thickness is used as an “average breakdown electric field”. Is said to be excellent.

Further, the leakage current characteristics were evaluated by measuring the reverse leakage current of 150 pn junction diodes with a diameter of 3 mm produced on the wafer surface. First, immerse these wafers in dilute hydrofluoric acid solution,
The silicon oxide film formed on the surface was removed, further rinsed with ultrapure water, and then spin dried. Then 1
An oxide film of 100 nm is formed in a dry oxygen atmosphere at 000 ° C., and 1 × 10 ¹⁵ cm ⁻² of phosphorus is ion-implanted from the surface of the silicon substrate to the pn junction formation region only, and then the region other than the pn junction region is doped. 1 × 10 ¹² cm ^{−2 was} doped with boron by ion implantation. After that, the silicon substrate was heat-treated in nitrogen at 1000 ° C. for 30 minutes to remove the oxide film right above the pn junction region portion, and an Al lead electrode was formed. After that, the silicon substrate was heat-treated at 400 ° C. for 30 minutes in a hydrogen atmosphere to remove the back surface oxide film, and then an ohmic contact was formed with Au. As the leak current of the pn junction diode, the current flowing when the back surface of the wafer was kept at zero potential and 10 V was applied to the electrode on the front surface was measured. The leakage current characteristic is pn
The average value of the leakage current of the junction diode in the substrate surface is pn
"Average leak current density" is the value obtained by dividing by the junction area.
The smaller the average leak current, the better.

Furthermore, the presence or absence of epitaxial growth was evaluated by measuring the weight of the silicon substrate before and after the heat treatment and dividing the weight change per unit area by the density of silicon to calculate the epitaxial film thickness. When the calculated epitaxial film thickness was 10 nm or more, it was determined that epitaxial growth had occurred and was rejected.

Tables 1 to 4 show heat treatment conditions and evaluation results of Examples 1 to 52 of the present invention and Comparative Examples 53 to 65.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

[Table 4]

From Tables 1 to 4 above, in Examples 1 to 52 of the present invention, the defect density of the surface layer of the silicon substrate, the breakdown voltage characteristic (average breakdown electric field), and the leakage current characteristic (average leakage current density) are the same as those of Comparative Example 58. Compared with the prior art disclosed in Japanese Patent Laid-Open No. 62-123098, the characteristics are superior. Further, epitaxial growth did not occur on the surface of the silicon substrate.

Since the heat treatment temperature conditions of Comparative Examples 53 to 56 are out of the range of the present invention, the defect density, breakdown voltage characteristic and leak current characteristic are inferior to those of the conventional example shown in Comparative Example 58.

In Comparative Example 57, the condition of the heat treatment time is out of the range of the present invention, so that the defect density, the breakdown voltage characteristic, and the leak current characteristic are the same as those of the conventional example shown in Comparative Example 58.

In Comparative Examples 59, 60 and 62, since the amounts of SiH ₄ and Si ₂ H ₆ added in the heat treatment atmosphere were less than the range of the present invention, the defect density, breakdown voltage characteristic and leak current characteristic were the same as those of Comparative Example 58. It is inferior to the conventional example shown.

In Comparative Examples 61, 63 and 64, since the amounts of SiH ₄ and Si ₂ H ₆ added in the heat treatment atmosphere were larger than the range of the present invention, the defect density, the breakdown voltage characteristic and the leak current characteristic were compared with Comparative Example 58. Although it is superior to the conventional example shown in (1), epitaxial growth occurs on the surface of the silicon substrate.

[0058]

As described in detail above, according to the present invention, more efficient reduction of crystal defects on the surface of a silicon substrate, higher breakdown voltage characteristics, and lower leakage current characteristics, which were impossible in the prior art, are achieved. It becomes possible to manufacture a silicon substrate having the above-mentioned characteristics, and by using the silicon substrate, it becomes possible to manufacture a silicon integrated device having higher reliability, which is extremely advantageous in industry.

Claims (6)

[Claims] 1. A hydrogen gas to which SiH ₄ gas of 1 ppba or more and 1 ppma or less is added, an inert gas or a mixed gas atmosphere of hydrogen gas and an inert gas of 1000 to
A heat treatment method for a silicon substrate, which comprises performing heat treatment at 1350 ° C. for 10 minutes or more. 2. A hydrogen gas to which SiH ₄ gas of 1 ppba or more and 1 ppma or less is added, an inert gas, or 1100 to 1000 in a mixed gas atmosphere of a hydrogen gas and an inert gas.
A heat treatment method for a silicon substrate, which comprises performing heat treatment at 1350 ° C. for 30 minutes or more. 3. 1000 in a hydrogen gas added with Si ₂ H ₆ gas of 1 ppba or more and 1 ppma or less, an inert gas or a mixed gas atmosphere of a hydrogen gas and an inert gas.
A method for heat treating a silicon substrate, characterized in that the heat treatment is performed at ˜1350 ° C. for 10 minutes or more. 4. A hydrogen gas to which Si ₂ H ₆ gas of 1 ppba or more and 1 ppma or less is added, an inert gas, or 1100 in a mixed gas atmosphere of a hydrogen gas and an inert gas.
A method for heat treating a silicon substrate, characterized in that the heat treatment is performed at ˜1350 ° C. for 30 minutes or more. 5. A hydrogen gas added with a mixed gas of SiH ₄ gas of 1 ppba or more and 1 ppma or less and Si ₂ H ₆ gas, an inert gas, or a mixed gas atmosphere of a hydrogen gas and an inert gas of 1000 to 1000 A heat treatment method for a silicon substrate, which comprises performing heat treatment at 1350 ° C. for 10 minutes or more. 6. A hydrogen gas to which a mixed gas of SiH ₄ gas of 1 ppba or more and 1 ppma or less and Si ₂ H ₆ gas is added, an inert gas, or 1100 to 1000 in a mixed gas atmosphere of a hydrogen gas and an inert gas. A heat treatment method for a silicon substrate, which comprises performing heat treatment at 1350 ° C. for 30 minutes or more.