JP4810633B2 - Mask resist stripping method and apparatus using plasma asher - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for stripping a mask resist by a plasma asher, by which a resist on a surface and an end face of a mask can be stripped and cleaned without giving damages to a chromium pattern on the mask surface. <P>SOLUTION: The apparatus for stripping a mask resist by a plasma asher comprises: a vacuum chamber provided successively to a quartz bell jar that generates helicon wave plasma; a high frequency oscillator that applies a high frequency voltage to the quartz bell jar; an electromagnetic coil disposed around the quartz bell jar to generate a magnetic field for controlling the helicon wave plasma; an inlet provided in the vacuum chamber to introduce oxygen gas and steam gas; a stage disposed in a lower part of the vacuum chamber to mount a mask; and vacuum pump that evacuates inside the quartz bell jar and the vacuum chamber. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to a mask resist stripping method using a plasma asher that can strip and clean a resist on a mask surface and an end face without damaging a chromium pattern on the mask surface.

  As the resist cleaning of the photomask, the resist on the surface and the end face of the mask is peeled off by a method such as SPM cleaning using sulfuric acid, hydrogen peroxide and water, or ashing using oxygen plasma.

As described in Patent Document 1, an invention that efficiently realizes plasma doping with a high degree of vacuum by generating a plasma having a high density is also disclosed.
Japanese Patent Laid-Open No. 2002-170782

  However, in the SPM cleaning, since sulfate groups remain on the mask surface, when irradiated with an argon fluoride laser excimer light having a wavelength of 193 nm with a photomask exposure apparatus, it reacts with fine components in the atmosphere and becomes cloudy on the mask surface. May occur, and the circuit pattern may not be resolved accurately.

  In addition, ashing with oxygen plasma also damages the chromium pattern at the portion where the resist is peeled off, and degrades the reflectance of the mask surface. As for SPM cleaning, if the number of times the mask is cleaned increases, there is a high possibility that the chromium pattern is oxidized to cause reflectance fluctuation.

  Therefore, an object of the present invention is to provide a mask resist stripping method using a plasma asher that can strip and clean the resist on the mask surface and the end face without damaging the chromium pattern on the mask surface. .

  In order to solve the above problems, the present invention provides a vacuum chamber 4 in which a quartz bell jar 2 for generating a helicon wave plasma is connected, a high frequency oscillator 10 for applying a high frequency voltage to the quartz bell jar 2, and a helicon wave plasma. An electromagnetic coil 3 arranged around the quartz bell jar 2 for generating a magnetic field to be controlled, an oxygen introduction port 5 for introducing oxygen gas provided in the vacuum chamber 4, and the vacuum chamber 4. A water vapor inlet 6 for introducing a water vapor gas, a stage 8 for placing a mask 9 provided under the vacuum chamber 4, and a pick-up pin for floating and holding the mask 9 from the stage 8 A mask holder 8a having a mechanism 8b and a gate valve 7a are opened and closed to place or remove the mask 9 on the stage 8. And a vacuum pump 12 for evacuating the quartz bell jar 2 and the vacuum chamber 4 to clean the resist adhering to the surface and end face of the mask 9 by helicon wave plasma. It was set as the structure of the mask resist peeling apparatus 1 by a plasma asher.

  In the present invention, by performing ashing with plasma using oxygen and water vapor, the resist on the mask surface and the end face can be efficiently removed, and the reflectance fluctuation value on the mask surface can be suppressed to a detection limit or less.

  An object of the present invention is to remove and clean the resist on the mask surface and the end face without damaging the chromium pattern on the mask surface. A high-frequency oscillator for applying a high-frequency voltage, an electromagnetic coil arranged around the quartz bell jar to generate a magnetic field for controlling the helicon wave plasma, and oxygen introduction for introducing oxygen gas provided in the vacuum chamber A mouth, a water vapor introducing port for introducing a water vapor gas provided in the vacuum chamber, a stage for placing a mask provided in a lower portion of the vacuum chamber, and a mask for floating and holding the mask A mask holder with a pickup pin mechanism and a gate valve that opens and closes A mask insertion port for placing and removing a mask on the stage, and a vacuum pump for evacuating the quartz bell jar and the vacuum chamber, and cleaning the resist adhering to the mask surface and end face by helicon wave plasma. It was realized by a mask resist stripping device using the characteristic plasma asher.

  Below, based on an accompanying drawing, the mask resist peeling method by the plasma asher which is this invention is demonstrated in detail.

  FIG. 1 is a perspective view in which the front surface of a resist stripping apparatus in a mask resist stripping method using a plasma asher according to the present invention is partly cut, and FIG. 2 is a resist stripping apparatus in a mask resist stripping method using a plasma asher according to the present invention. FIG.

  The resist stripping apparatus 1 includes a quartz bell jar 2, an electromagnetic coil 3, a vacuum chamber 4, an oxygen inlet 5, a water vapor inlet 6, a mask inlet 7, a stage 8, a high frequency oscillator 10, a matching box 11, a vacuum pump 12, and the like. This is an apparatus for cleaning the resist adhering to the surface and end face of the mask 9.

  The quartz bell jar 2 is a plasma generator. A cylindrical container having a diameter of 100 mm, the upper surface is closed in a spherical shape, and the lower surface is connected to the center of the upper surface of the vacuum chamber 4 so as to be sealed.

  The electromagnetic coil 3 is an annular electromagnet installed so as to surround the quartz bell jar 2. By supplying a current from the DC power supply 3a, a magnetic field parallel to the axial direction is generated in the plasma source.

  The vacuum chamber 4 is a cylindrical room for diffusing plasma. It is made of aluminum, and permanent magnets 4a are arranged to generate a surface magnetic field so that energy is not lost even if electrons come into contact with the wall surface.

  The oxygen inlet 5 is a nozzle that supplies oxygen gas into the vacuum chamber 4. By installing on the upper part of the vacuum chamber 4 close to the plasma generating part, a large amount of oxygen plasma can be generated.

  The water vapor inlet 6 is a nozzle that supplies water vapor gas into the vacuum chamber 4. By installing in the lower part of the vacuum chamber 4 close to the mask 9, damage to the chromium pattern can be suppressed.

  The mask insertion port 7 is an inlet / outlet for entering the mask 9 into the vacuum chamber 4 from the outside or taking the mask 9 out of the vacuum chamber 4 and is opened and closed by a gate valve 7a.

  The stage 8 is a table for placing a mask 9 provided at the lower part of the vacuum chamber 4. The mask holder 8a for fixing the mask 9 has a pickup pin mechanism 8b for holding the mask 9 by floating 1 to 2 mm from the stage 8.

  The mask 9 is formed by forming a pattern with chromium on a transparent quartz plate, and is used when a circuit pattern is transferred to a wafer. The resist adhering after the treatment is peeled off, but it is necessary to protect the chromium pattern from being damaged.

  The high frequency oscillator 10 is an RF power source for generating plasma. A radio wave having a power supply frequency of 13.56 MHz is wirelessly blown to the antenna 10 a mounted around the quartz bell jar 2.

  The matching box 11 is an impedance converter for supplying RF power to the plasma. When transmitting electromagnetic wave energy into the quartz bell jar 2, adjustment is made so that the electromagnetic wave energy can be transmitted with maximum efficiency.

  The vacuum pump 12 is a device that decompresses the quartz bell jar 2 and the vacuum chamber 4. A pressure adjustment valve 12b is provided in a vacuum pipe 12a that connects the lower surface of the vacuum chamber 4 and the vacuum pump 12 to adjust the degree of vacuum.

  FIG. 3 is a flowchart showing the flow of the mask resist stripping method using the plasma asher according to the present invention.

  Mask resist peeling method 1a includes mask placement 13, gate valve closing 13a, evacuation 13b, gas introduction 13c, helicon wave plasma generation 13d, resist peeling 13e, plasma stop 13f, vacuum release 13g, gate valve opening 13h, and mask. It consists of procedures such as take-out 13i.

  In the initial state, the mask insertion slot 7 is open and the mask 9 is not placed on the stage 8. Further, no gas is supplied from the oxygen inlet 5 and the water vapor inlet 6, the vacuum pump 12 is stopped, and the DC power source 3a of the electromagnetic coil 3 and the power source of the high-frequency oscillator 10 are stopped.

  The mask placement 13 places the mask 9 into the vacuum chamber 4 from the mask insertion port 7 and places it on the stage 8. The back surface of the mask 9 is floated and fixed from the stage 8 by a pin-shaped mask holder 8a so that the back surface of the mask 9 is not damaged or contaminated.

  The gate valve closing 13a closes the gate valve 7a of the mask insertion port 7, blocks the passage of gas or the like between the vacuum chamber 4 and the outside, and seals the quartz bell jar 2 and the vacuum chamber 4.

  The evacuation 13b operates the vacuum pump 12 and adjusts the pressure adjustment valve 14b to reduce the pressure in the quartz bell jar 2 and the vacuum chamber 4 to a low pressure state close to vacuum.

  The gas introduction 13 c supplies oxygen gas from the oxygen introduction port 5 into the vacuum chamber 4 and supplies water vapor gas from the water vapor introduction port 6 to the vacuum chamber 4. In addition, it inject | pours adjusting the ratio of oxygen gas and water vapor | steam gas.

  The helicon wave plasma generator 13d generates a magnetic field in the quartz bell jar 2 by supplying current from the DC power source 3a to the electromagnetic coil 3, and generates plasma in the quartz bell jar 2 by supplying RF power from the high frequency oscillator 10 to the antenna. Let

  In general, electromagnetic waves below the cutoff frequency cannot propagate through the plasma, but when a magnetic field is present, charged particles rotate around the magnetic field lines, so the components that cross the magnetic field lines are limited, and electromagnetic waves below the cutoff frequency are also in the plasma. Will be able to invade. This electromagnetic wave is a clockwise circularly polarized wave and is called a helicon wave.

  In the longitudinal wave of the helicon wave, when the wave phase velocity and the electron thermal velocity are close to each other, the interaction increases and a phenomenon called Landau attenuation occurs where energy is efficiently given from the wave to the electron without causing a collision. A high-density plasma is generated by waves.

  The resist stripping 13e decomposes and removes the resist adhering to the surface and end face of the mask 9 by oxygen plasma. At the chromium oxide interface, the oxygen plasma prevents the oxygen plasma from sputtering chromium and deteriorating the reflectance.

  If the plasma potential, electron density, or electron temperature is not uniform, the electron current and ion current flowing into the substrate such as the wafer or mask 9 will not be uniform, and a large amount of charge will accumulate on the top of the oxide film to charge up. Will occur.

  While generating uniform and high-density plasma by helicon wave plasma and optimizing the distance between the plasma generating portion and the mask 9, damage to the mask 9 substrate material such as chromium oxide and chromium film quality is suppressed. A high ashing rate can be ensured. The distance between the quartz bell jar 2 which is a plasma generating part and the mask 9 is adjusted between 300 and 500 mm.

  Moreover, in order to implement | achieve low-temperature processing, the plasma damage of the surface of the mask 9 can also be prevented by switching generation | occurrence | production or a stop of plasma, switching a high state or a low state, or controlling according to a condition.

  When the resist is peeled off from the mask 9, the plasma stop 13f stops the power generation of the high-frequency oscillator 10 and the electromagnetic coil 3 to stop the generation of plasma. The vacuum release 13g adjusts the pressure adjusting valve 14b to return the pressure in the quartz bell jar 2 and the vacuum chamber 4 to atmospheric pressure.

  The gate valve opening 13h opens the gate valve 7a of the mask inlet 7 when the pressure inside and outside the vacuum chamber 4 becomes equal. The mask take-out 13 i takes out the cleaned mask 9 from the vacuum chamber 4 through the mask insertion opening 7.

  The present invention is a single-wafer type, and when one mask 9 is cleaned, it is taken out, and the next mask 9 is loaded and cleaned. In order to maintain a uniform gas distribution and perform high-speed processing to suppress damage, a single wafer type is preferable to a barrel type that can process a large number of sheets.

  FIG. 4 is a diagram showing conditions for evaluating the resist stripping rate by the mask resist stripping method using the plasma asher according to the present invention, and FIG. 5 shows the resist stripping rate by the mask resist stripping method using the plasma asher according to the present invention. It is a table | surface which shows the result evaluated.

  In the mask resist stripping method 1a, the resist stripping rate is measured using a stylus type step thickness measuring device. Cr blank is used as the mask 9, and 13 measurement positions 14 are set on the mask 9 as shown in FIG.

  As the conditions, the resist was NEB22A, the RF power source was 2000 W, the pressure was 30 mTorr, the flow rate of oxygen gas was 100 sccm, the flow rate of water vapor gas was 65 sccm, the set temperature of the stage was 120 ° C., and the processing time was 1 minute.

  For the 13 measurement positions 14 on the mask 9, the initial film thickness before processing, the film thickness after processing for 1 minute, and the difference between them, that is, the peeling rate film thickness that is the film peeled in 1 minute. As shown in FIG.

  The average rate of the entire measurement position 14 was 708.1 / min, the average rate at the center of the mask 9 was 805 / min, and the average rate at the outer periphery of the mask 9 was 650 / min.

  The uniformity of the peeling rate film thickness was ± 25.1%, but is desirably within the target value of ± 10%.

  FIG. 6 is a diagram showing conditions for evaluating the reflectance variation by the mask resist stripping method using the plasma asher according to the present invention. FIG. 7 shows the reflectance variation by the mask resist stripping method using the plasma asher according to the present invention. It is a table | surface which shows the result evaluated.

  In the mask resist peeling method 1a, the reflectance variation is measured using a reflectance measuring device. A cleaned Cr blank is used as the mask 9, and three measurement positions 14a are set on the mask 9, as shown in FIG.

  As conditions, the RF power source was 2000 W, the pressure was 30 mTorr, the oxygen gas flow rate was 100 sccm, the water vapor gas flow rate was 65 sccm, the set temperature of the stage was 120 ° C., and the processing time was 60 minutes.

  The reflectance fluctuation amount measured by changing the wavelength of the light source at three measurement positions 14a on the mask 9 is shown in the upper part of FIG.

  The average variation when the wavelength is 488 nm is −0.240%, the average variation when the wavelength is 365 nm is −0.448%, and the average variation when the wavelength is 248 nm is −0. It was 165%.

  Compared with the case of cleaning only with conventional oxygen plasma. For the Cr blank, the reflectance fluctuation amount measured after the oxygen asher 15 is shown in the lower part of FIG. The processing time was 30 minutes.

  In the case of the oxygen asher 15, the amount of change in reflectivity at any wavelength is very large, whereas in the case of the mask resist peeling method 1a of the present invention, the amount of change in reflectivity is considerably small at any wavelength.

  In the present invention, even when dry ashing is performed by oxygen plasma, it is understood that the amount of change in reflectivity is almost eliminated by introducing the water vapor gas, and the chromium on the mask 9 is not damaged.

  FIG. 8 is a table comparing the resist stripping rate and the reflectance variation by the mask resist stripping method using the plasma asher according to the present invention.

  The conditions of the mask resist peeling method 1a were as follows: the RF power source was 2000 W, the pressure was 30 mTorr, the oxygen gas flow rate was 100 sccm, the water vapor gas flow rate was 65 sccm, the set temperature of the stage was 120 ° C., and the processing time was 60 minutes.

  The conditions for the conventional oxygen asher 15 were as follows: the RF power source was 2000 W, the pressure was 30 mTorr, the oxygen gas flow rate was 165 sccm, the water vapor gas flow rate was 0 sccm, the stage set temperature was 120 ° C., and the treatment time was 30 minutes.

  The resist strip rate was 708.1 kg / min in the case of the mask resist stripping method 1a and 573.1 kg / min in the case of the oxygen asher 15, and the mask resist stripping method 1a was superior.

  With respect to the reflectance fluctuation rate, in the case of the mask resist peeling method 1a, the wavelength is -0.240% at 488 nm, the wavelength is -0.448% at 365 nm, and the wavelength is -0.165% at 248 nm. There is no. Incidentally, it sufficiently satisfies 2.0% or less, which is the standard for the reflectance variation rate.

  On the other hand, in the case of the oxygen asher 15, the wavelength is 44.17% at 488 nm, the wavelength is 39.66% at 365 nm, and the wavelength is 248 nm at 19.84%. It has deteriorated.

  FIG. 9 is a graph showing the gas ratio dependency of the mask resist stripping method using the plasma asher according to the present invention. The resist stripping amount 16 (left vertical axis) which is the depth at which the resist is stripped by changing the gas ratio and the substrate temperature 16a (right vertical axis) of the mask 9 and the like are shown.

  As conditions, the RF power source is 2000 W, the pressure is 30 mTorr, the stage set temperature is 120 ° C., the oxygen gas flow rates are 100, 120, 140, and 165 sccm, the water vapor gas flow rates are 65, 45, 25, and 0 sccm, The input and output current of the electromagnetic coil was 40 A, and the processing time was 120 seconds.

  In the mask resist peeling method 1a, the substrate temperature 16a is almost unchanged even when the gas ratio is changed, and is 130 to 150 ° C.

  Regarding the resist stripping amount 16, when oxygen is 100 and water vapor is 65, about 2600 liters, when oxygen is 120 and water vapor is 45, about 3300 liters, when oxygen is 140 and water vapor is 25, about 3700 liters, oxygen is 165 When the water vapor is 0, it is about 2900 kg.

  When the flow rate of oxygen gas is 140 sccm and the flow rate of water vapor gas is 25 sccm, that is, when the ratio of the flow rate of water vapor gas is 15.1% with respect to the total flow rate, the resist stripping amount 16 is the best.

  FIG. 10 is a graph showing the dependence of the ashing rate on the degree of vacuum due to the low-pressure region difference in the mask resist peeling method using the plasma asher according to the present invention. As a condition, the overall flow rate was 165 sccm / min, and the RF power source was 2000 W.

  In FIG. 10, solid curve lines 17, 17 a, and 17 b indicate the resist stripping rate (left vertical axis), and dotted line curves 17 c, 17 d, and 17 e indicate the reflectance fluctuation (right vertical axis). Reference numerals 17 and 17c denote a case where the pressure is 10 mTorr, reference numerals 17a and 17d denote a case where the pressure is 30 mTorr, and reference numerals 17b and 17e denote a case where the pressure is 40 mTorr.

  In the mask resist peeling method 1a, the resist peeling rate is better when the pressure is 30 mTorr than when the pressure is 40 mTorr, and when the pressure is 10 mTorr than when the pressure is 30 mTorr.

  This is because, in the low pressure region, the mean free path, which is the flight distance from one collision of a gas molecule to another gas molecule, and the next collision becomes longer, so the resist stripping rate is improved. .

  Further, when the water vapor gas partial pressure is high, the resist stripping rate is suppressed, but when the water vapor gas partial pressure is lowered and the oxygen gas partial pressure is raised, the resist stripping rate is increased. The partial pressure of water vapor gas means a pressure of only water vapor with respect to the whole pressure of oxygen and water vapor.

  On the other hand, when the oxygen gas partial pressure increases, the damage to the chromium on the mask 9 increases and the reflectance fluctuates. Note that, when the pressure is low, the mean free path becomes long, and the reflectance fluctuation is likely to occur.

  In the case of 40 mTorr, it can be seen that the reflectance fluctuation starts when the water vapor gas partial pressure becomes about 5% or less, and in the case of 30 mTorr, the reflectance fluctuation starts when the water vapor gas partial pressure becomes about 10% or less, In the case of 10 mTorr, it can be seen that the reflectance fluctuation starts when the water vapor gas partial pressure is about 15% or less.

  That is, as the pressure increases, the reflectance fluctuation decreases, but the resist stripping rate also decreases. If the water vapor gas partial pressure is 15% or more even when the pressure is 10 mTorr, the resist can be peeled while suppressing the change in the reflectance of chromium oxide on the surface of the mask 9. Even if ashing is performed at high speed and overashing is performed, the reflectance does not fluctuate.

  As described above, the mask resist stripping method using the plasma asher according to the present invention can efficiently strip the resist on the mask surface and the end face by performing ashing with plasma using oxygen and water vapor, The reflectance fluctuation value can be suppressed below the detection limit.

It is a flowchart which shows the flow of the mask resist peeling method by the plasma asher which is this invention. It is the perspective view which partially cut the front of the resist peeling apparatus in the mask resist peeling method by the plasma asher which is this invention. It is sectional drawing of the resist peeling apparatus in the mask resist peeling method by the plasma asher which is this invention. It is the figure which showed the conditions which evaluate the resist stripping rate by the mask resist stripping method by the plasma asher which is this invention. It is a table | surface which shows the result of having evaluated the resist stripping rate by the mask resist stripping method by the plasma asher which is this invention. It is the figure which showed the conditions which evaluate the reflectance fluctuation | variation by the mask resist peeling method by the plasma asher which is this invention. It is a table | surface which shows the result of having evaluated the reflectance fluctuation | variation by the mask resist peeling method by the plasma asher which is this invention. It is the table | surface which compared the resist stripping rate by the mask resist stripping method by the plasma asher which is this invention, and a reflectance fluctuation | variation. It is the graph which showed the gas ratio dependence of the mask resist peeling method by the plasma asher which is this invention. It is the graph which showed the dependence of the ashing rate by the vacuum degree by the low voltage | pressure area | region difference of the mask resist peeling method by the plasma asher which is this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Resist peeling apparatus 1a Resist peeling method 2 Quartz bell jar 3 Electromagnetic coil 3a DC power supply 4 Vacuum chamber 4a Permanent magnet 5 Oxygen inlet 6 Water vapor inlet 7 Mask inlet 7a Gate valve 8 Stage 8a Mask holder 8b Pickup pin mechanism 9 Mask 10 High-frequency oscillator 10a Antenna 11 Matching box 12 Vacuum pump 12a Vacuum piping 12b Pressure adjustment valve 13 Mask placement 13a Gate valve close 13b Vacuum 13c Gas introduction 13d Helicon wave plasma generation 13e Resist stripping 13f Plasma stop 13g Vacuum release 13h Gate valve open 13i Mask removal 14 Measurement position 14a Measurement position 15 Oxygen asher 16 Resist stripping amount 16a Substrate temperature 17 Resist stripping rate at 10 mTorr 1 Change in reflectance at the reflectance variation 17e 40 mTorr in reflectance variation 17d 30 mTorr in the resist stripping rate 17c 10 mTorr in the resist stripping rate 17b 40 mTorr at a 30 mTorr

Claims (4)

A vacuum chamber provided with a quartz bell jar for generating helicon wave plasma, a high frequency oscillator for applying a high frequency voltage to the quartz bell jar, and an electromagnetic wave disposed around the quartz bell jar for generating a magnetic field for controlling the helicon wave plasma. A coil, an oxygen inlet for introducing oxygen gas provided in the upper part of the vacuum chamber near the quartz bell jar, and a water vapor inlet for introducing water vapor gas provided in the lower part of the vacuum chamber near the mask A stage for placing a mask provided in a lower part of the vacuum chamber; a mask holder having a pickup pin mechanism for floating and holding the mask from the stage; and a gate valve that opens and closes the mask to the stage. A mask slot to be placed on and removed from the quartz bell jar Fine vacuum chamber consists of a vacuum pump to a vacuum,
A plasma asher characterized in that a large amount of helicon wave plasma is generated on the surface and end face of the mask , the etching rate is controlled by water vapor at the chrome interface, and the resist adhering to the surface and end face of the mask is cleaned by the helicon wave plasma. Mask resist stripping device. After installing the oxygen inlet at the top of the vacuum chamber near the quartz bell jar and the water vapor inlet at the bottom of the vacuum chamber near the mask, place the mask on the stage in the vacuum chamber and close the mask inlet Using a vacuum pump, the quartz bell jar and the vacuum chamber are reduced in pressure, oxygen gas is introduced from the oxygen inlet, and water vapor is introduced from the steam inlet, while a current is passed through the electromagnetic coil to generate a magnetic field in the quartz bell jar. In the generated state, a high frequency voltage is applied to the quartz bell jar with a high frequency oscillator to generate a large amount of helicon wave plasma on the surface and end face of the mask, while the etching rate is controlled by water vapor at the chrome interface, and the surface of the mask and Mask resist by plasma asher characterized by cleaning resist adhering to end face Isolation Method. The ashing rate is increased by lowering the pressure in the vacuum chamber to 30 mTorr or less, preferably 10 mTorr, and the water vapor partial pressure ratio is set to 15% or more to suppress the change in the reflectance of chromium on the mask surface. A method for removing a mask resist using a plasma asher according to claim 2 . 4. The mask resist stripping method using plasma asher according to claim 2, wherein the distance between the quartz bell jar and the mask is set to 300 to 500 mm, and damage to chromium on the mask surface is suppressed.

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