KR20120130030A - Apparatus of single crystal growth control and method of the same - Google Patents

Abstract

PURPOSE: A method and a device for controlling the growth of single crystal are provided to obtain high quality single crystal by keeping a constant growth which is in the range of a standard growth. CONSTITUTION: Raw material is in an inner crucible(100). A weight measurement part(300) measures the weight of the inner crucible. A susceptor(200) supports the inner crucible. The susceptor is connected to the weight measurement part. An outer crucible(120) surrounds the inside crucible.

Description

Single Crystal Growth Rate Control Device and Method {APPARATUS OF SINGLE CRYSTAL GROWTH CONTROL AND METHOD OF THE SAME}

The present disclosure relates to a single crystal growth rate control apparatus and method.

In general, the importance of the material in the electrical, electronics industry and mechanical parts field is very high, which is an important factor in determining the characteristics and performance index of the actual final component.

Si, which is used as a representative semiconductor device material, is vulnerable to temperatures of more than 100 degrees Celsius, causing frequent malfunctions and failures, and thus requires various cooling devices. As Si shows such physical limitations, broadband semiconductor materials such as SiC, GaN, AlN, and ZnO are in the spotlight as next-generation semiconductor device materials.

Here, compared to GaN, AlN and ZnO, SiC is excellent in thermal stability and excellent in oxidation resistance. In addition, SiC has an excellent thermal conductivity of about 4.6W / Cm ℃, has the advantage that can be produced as a large diameter substrate of 2 inches or more in diameter. In particular, SiC single crystal growth technology is most stably secured in reality, and industrial production technology is at the forefront as a substrate.

However, in order to grow from the stoichiometric solution, the SiC single crystal requires a pressure of 100,000 atm or more and a temperature of 3,200 ° C. or more which is thermodynamically similar to that of diamond. Therefore, single crystals are mainly manufactured by industrially using PVT (Physical Vapor Transport) method, that is, seed growth growth sublimation method, which mainly uses sublimation.

Since the seed crystal growth sublimation proceeds in a closed manner at high temperature, it is difficult to observe the inside during growth. Therefore, it is necessary to know exactly what is going on in real time. Therefore, many studies have been conducted to grow high quality SiC single crystals more accurately by predicting the growth state of SiC single crystals that cannot be observed inside.

Embodiments can grow high quality single crystals.

Single crystal growth rate control apparatus according to the embodiment, the inner crucible for accommodating the raw material; And a weight measuring unit for measuring the weight of the inner crucible.

Single crystal growth rate control method according to the embodiment, the step of measuring the weight change per unit time of the raw material; Deriving a growth rate per unit time of a single crystal using the weight change amount per unit time; And adjusting the growth rate by comparing the growth rate with the reference growth rate.

The single crystal growth rate adjusting apparatus according to the embodiment includes a weight measuring unit. The amount of reduction of the silicon carbide raw material may be measured in real time through the weight measuring unit. The growth rate of the silicon carbide single crystal may be calculated using the measured amount of reduction of the raw material. It is possible to derive more accurate growth rate through the growth rate measured and calculated in real time rather than the expected growth rate referring to the existing database. Therefore, it is possible to provide a high quality single crystal by maintaining a constant growth rate included in the range of the reference growth rate through the measured growth rate.

In the single crystal growth rate control method according to the embodiment, the growth rate can be adjusted by measuring the single crystal growth rate in real time.

1 is a cross-sectional view of a single crystal growth rate control apparatus according to the embodiment.
2 is a flowchart illustrating a method of controlling single crystal growth rate according to an embodiment.
3 is a first cross-sectional view for describing a single crystal growth rate adjusting method according to an embodiment.
4 is a second cross-sectional view for describing a single crystal growth rate adjusting method according to an embodiment.
5 is a graph showing a temperature gradient of an upper portion of the outer crucible and a lower portion of the outer crucible according to the position of the heat generating induction part.

In the description of embodiments, each layer, region, pattern, or structure may be “on” or “under” the substrate, each layer, region, pad, or pattern. Substrate formed in ”includes all formed directly or through another layer. Criteria for the top / bottom or bottom / bottom of each layer will be described with reference to the drawings.

The thickness or the size of each layer (film), region, pattern or structure in the drawings may be modified for clarity and convenience of explanation, and thus does not entirely reflect the actual size.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Referring to Figure 1, the single crystal growth rate control apparatus according to the embodiment will be described in detail. 1 is a cross-sectional view of a single crystal growth rate control apparatus according to the embodiment.

1, the single crystal growth rate adjusting apparatus according to the present embodiment includes an inner crucible 100, an external crucible 120, an upper lid 140, a seed crystal holder 160, a focusing tube 180, and a susceptor. 200, the weight measuring unit 300, the reflecting unit 400, the first temperature sensor 420, the second temperature sensor 430, the heat insulating material 500, the quartz tube 550, the heat generating induction unit 710, The controller 720, a pressure controller 730, and a gas flow controller 740 are included.

The inner crucible 100 may accommodate the raw material 130. The raw material 130 may include silicon and carbon. More specifically, the raw material 130 may include a silicon carbide compound. The inner crucible 100 may contain silicon carbide powder (SiC powder) or polycarbosilane (polycarbosilane).

The inner crucible 100 may have a cylindrical shape to accommodate the raw material 130.

The inner crucible 100 may include a material having a melting point higher than the sublimation temperature of silicon carbide.

For example, the inner crucible 100 may be made of graphite.

In addition, the internal crucible 100 may be coated with a material having a melting point higher than the sublimation temperature of silicon carbide. Here, the material to be applied on the graphite material, it is preferable to use a material that is chemically inert to silicon and hydrogen at the temperature at which the silicon carbide single crystal 190 is grown. For example, metal carbide or metal nitride may be used. In particular, a mixture comprising at least two or more of Ta, Hf, Nb, Zr, W and V and a carbide comprising carbon may be applied. In addition, a mixture comprising at least two or more of Ta, Hf, Nb, Zr, W and V and a nitride comprising nitrogen may be applied.

Subsequently, the outer crucible 120 may be positioned surrounding the inner crucible 100. The outer crucible 120 may seal the inner crucible 100 so that the raw material 130 accommodated in the inner crucible 100 may grow into the single crystal 190. The outer crucible 120 is spaced apart from the inner crucible 100. The outer crucible 120 may be positioned at a predetermined interval G from the inner crucible 100. That is, the outer crucible 120 may be positioned not to be in close contact with the inner crucible 100. This is because when the outer crucible 120 comes in contact with any part of the inner crucible 100, it interferes with measuring the weight of the inner crucible 100 only. That is, the interval G may not affect the weight measuring unit 300 that needs to measure a minute amount. Therefore, the weight of only the inner crucible 100 can be measured.

The outer crucible 120 may include the same material as the inner crucible 100 described above.

 An upper cover 140 may be positioned on an upper portion of the outer crucible 120. The upper cover 140 may seal the outer crucible 120. The upper cover 140 may be sealed to allow a reaction to occur in the crucible 100.

The upper cover 140 may include graphite. However, the embodiment is not limited thereto, and the upper cover 140 may include a material having a melting point higher than the sublimation temperature of silicon carbide.

The seed crystal holder 160 is positioned at the lower end of the upper cover 140. The seed crystal holder 160 may fix the seed crystal 170. The seed crystal holder 160 may include high density graphite.

The seed crystal 170 is attached to the seed crystal holder 160. The seed crystal 170 may be attached to the seed crystal holder 160, thereby preventing the grown single crystal 190 from growing to the upper cover 140. However, the embodiment is not limited thereto, and the seed crystal 170 may be directly attached to the upper cover 140.

The focusing tube 180 is located inside the outer crucible 120. The focusing tube 180 may be located at a portion where the single crystal 190 grows. The focusing tube 180 may narrow the movement path of the sublimated silicon carbide gas to focus diffusion of the sublimed silicon carbide into the seed crystal 170. Through this, the growth rate of the single crystal 190 may be increased.

Subsequently, the susceptor 200 may be located at a lower end of the inner crucible 100. The susceptor 200 supports the inner crucible 100.

The susceptor 200 may support the inner crucible 100 and connect the inner crucible 100 and the weight measuring unit 300.

The susceptor 200 may include a support part 210 supporting the internal crucible 100 and a connection part 220 connected to the weight measuring part 300.

The support 210 may be positioned to support a lower end of the inner crucible 100.

The connection part 220 may connect the internal crucible 100 and the weight measuring part 300. The connection part 220 may have a shape extending in a lower portion of the inner crucible 100.

Subsequently, the weight measuring unit 300 may be located under the inner crucible 100. The weight measuring unit 300 may be connected to the internal crucible 100 by the connection unit 220 of the susceptor 200.

The weight measuring unit 300 may support the inner crucible 100 and the susceptor 200.

The weight measuring unit 300 measures the weight of the inner crucible 100. That is, the weight of the entire inner crucible 100 including the weight of the raw material 130 accommodated in the inner crucible 100 may be measured. Specifically, the weight of the raw material 130, the inner crucible 100 and the susceptor 200 can be measured. Therefore, by subtracting the weight of the inner crucible 100 and the susceptor 200 from the weight measured by the weight measuring unit 300, it is possible to know the weight of the raw material 130 only. That is, the reduction amount of the raw material 130 can be known through the weight measuring unit 300.

The weight measuring unit 300 may be a load cell 320. Since the reduced amount of the raw material 130 is a small amount of 1 g to 50 g per hour, the load cell 320 having high resolution may be used for precise measurement.

The load cell 320 refers to a sensor capable of measuring the magnitude of the load or force acting on the structure or a specific mechanism. The load cell 320 has a magnetic, capacitive and strain gauge method.

In general, the load cell 320 uses a strain gauge type load cell 320. Strain gage is a measure of the amount of deformation (strain) of an elastic body when a load is applied to a metal elastic body by using a change in electrical resistance. It is a way to use.

That is, the load cell 320 is deformed, such as being compressed or stretched upon receiving the weight. When the deformation measuring device detects the electric signal as an electrical signal and converts the digital signal into a digital signal by the computer device, the weight can be represented numerically. It is to measure the weight.

A plurality of load cells 320 may be installed at the lower end of the susceptor 200 supporting the internal crucible 100.

Measurement data measured by the load cell 320 may be recognized by the control panel 310.

Silicon carbide single crystal 190 is difficult to observe the interior during growth because it proceeds in a closed type at a high temperature. Therefore, it is necessary to know exactly what is going on in real time. In the present exemplary embodiment, the amount of reduction of the silicon carbide raw material 130 may be measured in real time through the weight measuring unit 300. The growth rate of the silicon carbide single crystal 190 may be calculated using the measured reduction amount of the raw material 130. Therefore, the high quality single crystal 190 can be provided by maintaining a constant growth rate through the measured growth rate of the single crystal 190.

Subsequently, the reflector 400 may be located under the inner crucible 100. The reflector 400 may be located between the inner crucible 100 and the weight measuring unit 300. The reflector 400 may be located at the connection part 220 of the susceptor 200.

The reflector 400 may be used to measure the temperature of the lower portion of the inner crucible 100. The reflector 400 may reflect light from the lower portion of the inner crucible 100. Specifically, the reflector 400 may reflect the light to a position that does not affect the weight measuring unit 300.

The reflected light may be incident to the first temperature sensor 420. The first temperature sensor 420 may measure the temperature of the internal crucible 100.

The first temperature sensor 420 may include, for example, an optical pyrometer. The inside of the inner crucible 100 cannot directly insert a thermometer at a high temperature of more than 2000 ℃. Therefore, the temperature can be measured using the optical thermometer. In addition, since the reflector 400 reflects light, the first temperature sensor 420 may measure the temperature at a position that does not affect the weight measuring unit 300 positioned below the inner crucible 100. Can be.

In general, the first temperature sensor 420 may be located under the inner crucible 100 to measure temperature. However, in the present embodiment, since the weight measuring unit 300 is positioned below the inner crucible 100, this may interfere with the first temperature sensor 420. Therefore, the light may be reflected to the reflector 400 for a more accurate temperature measurement. As a result, the first temperature sensor 420 may be able to measure the temperature even if the first temperature sensor 420 is not directly located below the inner crucible 100. However, the embodiment is not limited thereto, and the weight measuring unit 300 may be installed so as not to interfere with the first temperature sensor 420 so that the reflecting unit 400 may be omitted.

Subsequently, the second temperature sensor 430 may be positioned above the external crucible 120. Therefore, the second temperature sensor 430 may measure the temperature of the upper portion of the external crucible 120. Thereby, the growth rate of the single crystal 190 can be measured.

Subsequently, the heat insulator 500 surrounds the outer crucible 120. The insulation 500 maintains the temperature of the external crucible 120 at a crystal growth temperature. Since the heat insulating material 500 has a very high crystal growth temperature of silicon carbide, graphite felt may be used. Specifically, the heat insulating material 500 may be a graphite felt made of a cylindrical shape of a predetermined thickness by compressing the graphite fiber. In addition, the heat insulating material 500 may be formed of a plurality of layers to surround the outer crucible 120.

The quartz tube 550 is located on the outer circumferential surface of the outer crucible 120. The quartz tube 550 is fitted to the outer circumferential surface of the outer crucible 120. The quartz tube 550 may block heat transferred from the heat generating induction part 710 into the single crystal growth apparatus. The quartz tube 550 may be a hollow tube. Cooling water may be circulated in the internal space of the quartz tube 550. Therefore, the quartz tube 550 can more accurately control the growth rate, growth size, and the like of the single crystal 190.

The heat generation induction part 710 is located outside the quartz tube 550.

The heat generation induction part 710 may be, for example, a high frequency induction coil. The external crucible 120 and the internal crucible 100 may be heated by flowing a high frequency current through the high frequency induction coil. That is, the raw material 130 accommodated in the inner crucible 100 may be heated to a desired temperature.

The central region inductively heated by the heat generating induction part 710 is formed at a position lower than the center of the outer crucible 120. Therefore, a temperature gradient having different heating temperature regions is formed on the upper and lower portions of the external crucible 120. That is, a hot zone HZ, which is a central portion of the heat generation induction part 710, is formed at a relatively low position from the center of the external crucible 120, so that the lower portion of the external crucible 120 is bounded by the hot zone HZ. The temperature of is formed higher than the temperature of the top of the outer crucible (120). Since the inner crucible 100 containing the raw material 130 is positioned below the outer crucible 120, the temperature of the inner crucible 100 is high. In addition, the temperature is formed high in the outer direction at the inner center of the inner crucible 100. Due to such a temperature gradient, the sublimation of the silicon carbide raw material 130 occurs, and the sublimed silicon carbide gas moves to the surface of the seed crystal 170 having a relatively low temperature. As a result, the silicon carbide gas is recrystallized to grow into the single crystal 190.

The controller 720 is connected to the heat generation induction part 710. The controller 720 may move the heat generation induction part 710 up and down. The controller 720 may move the position of the hot zone HZ by moving the heat generation induction part 710 up and down. That is, the controller 720 may adjust the temperature gradient of the upper portion of the external crucible 120 and the lower portion of the external crucible 120. As a result, the growth rate of the single crystal 190 can be controlled.

The pressure regulator 730 is connected to the growth rate regulator. The pressure regulator 730 may include a pump 734 and a valve 732. The pressure controller 730 may adjust the process pressure in the growth rate control device.

Operation of the pump 734 may maintain a vacuum inside the growth rate control device.

The valve 732 may lower the process pressure by removing the argon gas in the growth rate controller. When the process pressure becomes too high, the process pressure can be adjusted through the valve 732.

The gas flow controller 740 is connected to the growth rate controller. The gas flow controller 740 may measure and adjust the flow of gas. The gas flow controller 740 may be a gas flow controller (MFC).

In addition, the gas flow controller 740 may maintain the process pressure by argon gas is injected in a constant vacuum state. At this time, the process pressure serves to remove impurities when the silicon single crystal 190 is grown. Through this, the silicon single crystal 190 having high purity and excellent characteristics may be grown.

Hereinafter, the single crystal growth rate adjusting method will be described in detail with reference to FIGS. 2 to 5.

2 is a flowchart illustrating a method of controlling single crystal growth rate according to an embodiment. 3 is a first cross-sectional view for describing a single crystal growth rate adjusting method according to an embodiment. 4 is a second cross-sectional view for describing a single crystal growth rate adjusting method according to an embodiment. 5 is a graph showing a temperature gradient of an upper portion of the outer crucible and a lower portion of the outer crucible according to the position of the heat generating induction part.

2 to 4, the single crystal growth rate adjusting method according to the embodiment includes measuring a weight change amount (ST100), deriving a growth rate (ST200), and adjusting the growth rate (ST300).

In the step of measuring the weight change amount (ST100), the weight change amount per unit time of the raw material 130 accommodated in the inner crucible 100 may be measured. The step ST100 of measuring the weight change amount may be performed through the weight measuring unit 300 connected to the internal crucible 100. The weight measuring unit 300 may determine the amount of reduction of the raw material 130 by measuring the weight of the internal crucible 100 in real time.

Subsequently, the step of deriving the growth rate (ST200) may be derived using the weight change amount per unit time. Specifically, the step of deriving the growth rate (ST200) can be calculated through the following Equation 1.

<Equation 1> G = ΔW / (π x r2 x ρ)

Here, G is the growth rate per unit time, ΔW is the amount of change in weight per unit time, r is the radius of the single crystal 190 and ρ is the density of the single crystal 190. The growth rate may be calculated by putting the weight change amount measured in the step ST100 of measuring the weight change amount into ΔW of Equation 1.

Through this, it is possible to derive a more accurate growth rate with the growth rate measured and calculated in real time, rather than the expected growth rate referring to the existing database.

Subsequently, the growth rate is adjusted (ST300). In the adjusting of the growth rate (ST300), the growth rate calculated from the derivation of the growth rate (ST200) and the reference growth rate can be compared. The reference growth rate is approximately 0.1 mm / h to 1.0 mm / h. Therefore, when the growth rate is within the reference growth rate range, the conditions can be maintained without changing the conditions. However, when the growth rate is higher or lower than the reference growth rate, it can be adjusted within the reference growth rate range. This process is automatically controlled to keep the growth rate constant.

By maintaining the growth rate constant, defects in the single crystal 190 can be reduced, and the doping concentration can be made uniform, thereby making a high quality single crystal 190 possible.

In the controlling of the growth rate (ST300), temperature gradient control, pressure control, and gas flow control may be performed.

3 to 5, a method of controlling the growth rate by controlling the temperature gradient will be described in detail.

In the controlling of the growth rate (ST300), it may include adjusting a temperature gradient. 3 and 4, the growth rate may be controlled by adjusting the position of the heat generating induction part 710 outside the quartz tube 550. That is, the temperature gradient may be adjusted by adjusting the position of the heat generating induction part 710.

The temperature gradient refers to the temperature difference between the upper portion of the outer crucible 120 and the lower portion of the outer crucible 120. That is, a temperature gradient having a different heating temperature region is formed between the single crystal 190 positioned above the external crucible 120 and the raw material 130 positioned below the external crucible 120. In general, an upper portion of the outer crucible 120 forms a heating temperature region of 2000 ° C. or higher, and a lower portion of the outer crucible 120 forms a heating temperature region higher than an upper portion of the outer crucible 120. This is because the hot zone HZ, which is the center of the heat generating induction part 710, is formed under the outer crucible 120, that is, the inner crucible 100 is located.

Here, the growth temperature of the single crystal 190 growing on the top of the outer crucible 120 should be constant. This is to prevent defects inside the single crystal 190. Therefore, the step of adjusting the temperature gradient may be made by changing the temperature of the lower portion of the external crucible 120.

Information about the growth rate calculated through the derivation of the growth rate (ST200) is transmitted to the control unit 720, the control unit 720 may adjust the temperature gradient by adjusting the position of the heat generating induction unit 710.

In the derivation of the growth rate (ST200), when the growth rate is lower than the reference growth rate, the growth rate may be increased by increasing the temperature gradient.

3 and 5, when the position of the heat generating induction part 710 is lowered, the temperature gradient L2 between the temperature t of the single crystal 190 and the temperature T2 of the internal crucible 100 is known. It becomes larger than the temperature gradient L1. As a result, the growth rate of the single crystal 190 may be increased.

In the derivation of the growth rate (ST200), when the growth rate is higher than the reference growth rate, the growth rate may be lowered by lowering the temperature gradient.

Referring to FIG. 4, when the position of the heat generating induction part 710 is increased, the temperature gradient L3 between the temperature t of the single crystal 190 and the temperature T3 of the internal crucible 100 is an existing temperature gradient ( It becomes smaller than L1). As a result, the growth rate of the single crystal 190 may be lowered.

Hereinafter, a method of controlling the growth rate through the pressure control will be described in detail with reference to FIGS. 3 and 4.

First, the growth rate may be adjusted using the pressure controller 730.

Specifically, in the step of deriving the growth rate (ST200), when the growth rate is lower than the reference growth rate, the growth rate can be increased by lowering the pressure inside the growth rate control apparatus. In other words, by reducing the amount of argon gas that maintains the vacuum, the growth rate of the single crystal 190 can be made faster.

The pressure regulator 730 includes a valve 732 for adjusting the amount of gas in the growth rate regulator. Therefore, in order to lower the pressure, the valve 732 may be opened to degas the growth rate control device.

Similarly, in the step of deriving the growth rate (ST200), when the growth rate is higher than the reference growth rate, the pressure may be increased to lower the growth rate. That is, the growth rate of the single crystal 190 can be slowed by maintaining the vacuum state.

In order to increase the pressure, the valve 732 may be closed to maintain the gas in the growth rate regulator. In addition, the pressure may be increased by introducing gas into the growth rate controller.

Next, a method of controlling the growth rate through the gas flow control will be described in detail.

The growth rate may be adjusted using the gas flow controller 740 located outside the growth rate control device. The gas flow controller 740 may be, for example, a gas flow control device. In this case, the gas flow may be adjusted by inputting a value into the gas flow rate control device.

Specifically, in the step of deriving the growth rate (ST200), when the growth rate is higher than the reference growth rate, the growth rate may be lowered by slowing the gas flow.

Similarly, when the growth rate is lower than the reference growth rate, the gas flow may be increased to increase the growth rate.

As described above, the growth rate control through the temperature gradient control, the pressure control, and the gas flow control has been described, but the embodiment is not limited thereto. Thus, the growth rate can be controlled by changing other conditions affecting the single crystal growth rate.

The features, structures, effects and the like described in the foregoing embodiments are included in at least one embodiment of the present invention and are not necessarily limited to one embodiment. In addition, the features, structures, effects, and the like illustrated in the embodiments may be combined or modified with respect to other embodiments by those skilled in the art to which the embodiments belong. Therefore, it should be understood that the present invention is not limited to these combinations and modifications.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is clearly understood that the same is by way of illustration and example only and is not to be construed as limiting the scope of the present invention. It can be seen that various modifications and applications are possible. For example, each component specifically shown in the embodiments may be modified. It is to be understood that the present invention may be embodied in many other specific forms without departing from the spirit or essential characteristics thereof.

Claims (13)

An internal crucible to accommodate the raw material; And
Single crystal growth rate control apparatus comprising a weight measuring unit for measuring the weight of the inner crucible. The method of claim 1,
And a susceptor for supporting the inner crucible. The method of claim 2,
Single crystal growth rate control device is connected to the weight measuring unit and the susceptor. The method of claim 3,
Further comprising an outer crucible for receiving the inner crucible,
The inner crucible and the outer crucible are single crystal growth rate control device spaced apart from each other. 5. The method of claim 4,
One side of the outer crucible single crystal growth rate control device including a heat generation induction portion for applying heat. The method of claim 5,
And a control unit connected to the exothermic induction part and controlling the exothermic induction part. The method of claim 1,
Single crystal growth rate control device including a reflector between the inner crucible and the weight measuring unit. The method of claim 7, wherein
And a temperature sensor for measuring the temperature by the incident light reflected from the reflector. Measuring a weight change amount per unit time of the raw material;
Deriving a growth rate per unit time of a single crystal using the weight change amount per unit time; And
And controlling the growth rate by comparing the growth rate and the reference growth rate. 10. The method of claim 9,
The step of measuring the weight change amount per unit time, single crystal growth rate control method made by measuring the weight of the internal crucible containing the raw material. The method of claim 10,
Deriving the growth rate per unit time is a single crystal growth rate control method represented by the following formula (1).
Equation 1 G = ΔW / (πx r ² xρ)
Where G is the growth rate per unit time, ΔW is the weight change per unit time, r is the radius of the single crystal and ρ is the single crystal density. The method of claim 11,
The controlling the growth rate, the single crystal growth rate control method comprising the step of adjusting the temperature gradient of the single crystal and the raw material. The method of claim 11,
The controlling the growth rate, the single crystal growth rate control method for adjusting the internal pressure.

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