JP2002223020A - Fluorine molecular laser apparatus and fluorine exposure apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fluorine molecular laser apparatus and a fluorine exposure apparatus that can estimate a spectrum characteristic with high accuracy by simple means and can control it within an allowable range. SOLUTION: The fluorine molecular laser apparatus produces main discharge 26 in a laser chamber 2 in which a laser gas containing fluorine is packed to generate fluorine molecular laser light 11. The gas pressure P of the laser gas in the laser chamber 2 is detected and a pressure range (Pm-PM) of the gas pressure P is set, in such a way that the spectrum characteristic is within the allowable range based on the relationship between the spectrum characteristic of the fluorine molecular laser light 11 and the gas pressure P. The gas pressure P is controlled within the pressure range (Pm-PM).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluorine molecular laser apparatus and a fluorine exposure apparatus for performing exposure using a molecular fluorine laser beam oscillated from the molecular fluorine laser apparatus.

[0002]

2. Description of the Related Art FIG. 12 shows a basic configuration of an excimer exposure apparatus for exposing a wafer or the like using excimer laser light oscillated from an excimer laser apparatus. In FIG. 12, an excimer exposure apparatus includes an excimer laser beam 111.
An excimer laser device 101 for oscillating the laser beam, and an exposing machine 125 for exposing the wafer 137 using the oscillated excimer laser beam 111 are provided.

An excimer laser apparatus 101 has a laser chamber 102 in which a laser gas is sealed at a predetermined pressure. A high-voltage power supply 11 is provided between a pair of discharge electrodes 104 and 105 disposed opposite to each other inside the laser chamber 102.
By applying a high voltage from 3, the laser light 111 oscillates in a pulsed manner. The oscillated laser light 111 passes through the window 109 at the rear of the laser chamber 102, is expanded by the prisms 122, 122, the wavelength is narrowed by the grating 123, and a part of the laser light is emitted from the front mirror 106. At this time, the grating 123 is called a wavelength narrowing element. As the wavelength narrowing element, for example, an etalon may be used instead of the grating 123. The emitted laser light 111 enters the exposure device 125. Then, the wafer 137 is exposed by projecting a mask pattern called a reticle 135 onto the wafer 137 via a reduction projection lens 136.
114 is a mirror.

At this time, a part of the laser light 111 is reflected downward in FIG. 12 by a beam splitter 112 disposed in front of the laser chamber 102 and enters a monitor module 116 for measuring the spectral characteristics of the laser light 111. . In the monitor module 116, the laser light 11
Power detector 115 for detecting pulse energy of 1
And a monitor etalon 117 for detecting the center wavelength and the spectrum width of the laser beam 111. The monitor etalon 117 generates interference fringes 118 corresponding to the center wavelength and the spectrum width of the laser beam 111. The laser controller 131 measures the position of the interference fringe 118 with the position sensor 119, and detects a spectral characteristic including a center wavelength and a spectral width based on the measured position. Then, the angle of the grating 123 with respect to the laser beam 111 is controlled so that this spectral characteristic falls within the allowable range required from the exposure device 125.

[0005]

However, the prior art has the following problems. That is, with the recent miniaturization of semiconductors, there has been known a fluorine exposure apparatus that performs exposure using a fluorine molecule laser device that oscillates a fluorine molecule laser beam having a shorter wavelength than the excimer laser beam 111 as a light source. However, in a fluorine molecular laser device, when the band is narrowed by using the grating 123, the etalon, or the like, the pulse energy of the oscillating laser light becomes very small, and it is difficult to obtain sufficient pulse energy for exposure.

Further, the spectrum width required by an exposure machine of a fluorine exposure apparatus becomes very narrow, for example, 0.1 to 1 pm. Furthermore, the required stability of the center wavelength is also extremely high in accuracy. However, when the grating 123, the etalon, or the like is used for narrowing the wavelength band, the spectral width and the center wavelength greatly change only by slightly changing the angle. As a result, it is difficult to control the angle of the grating 123 with respect to the laser beam 111 as in the related art, and to suppress the spectral characteristics to the required range. Further, to calculate the spectral characteristics based on the interference fringes 118 generated by the monitor etalon 117, a complicated calculation process is required.
In some cases, the control of the spectral characteristics cannot be made in time.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has a fluorine molecular laser apparatus and a fluorine exposure apparatus capable of accurately estimating spectral characteristics by simple means and keeping the spectral characteristics within an allowable range. It is intended to provide a device.

[0008]

In order to achieve the above object, the present invention mainly provides a laser chamber containing fluorine-containing laser gas by applying a high voltage between discharge electrodes. In a fluorine molecular laser device that causes discharge and oscillates a fluorine molecular laser beam, a detector that detects the value of a spectral parameter that changes the spectral characteristics of the fluorine molecular laser beam, such that the spectral characteristics are within a predetermined allowable range And a laser controller for setting an allowable range of the spectral parameter. The laser controller controls the spectral parameter to fall within a predetermined allowable range, thereby performing control for keeping the spectral characteristic within the allowable range.

In a fluorine laser device, it is known that the spectral characteristics of laser light have a close relationship with various spectral parameters, and when the spectral parameters change, the spectral characteristics also change.
Therefore, by defining the range of the spectral parameter such that the spectral characteristic falls within the required allowable range and controlling the fluorine laser apparatus so that the spectral parameter falls within this range, the spectral characteristic can be within the allowable range. is there. According to such a configuration, a complicated device for detecting the spectral characteristic is not required, and the spectral characteristic can be surely kept within an allowable range. Further, by setting this spectral characteristic to at least one of the center wavelength, the spectral width, and the spectral purity of the fluorine molecular laser beam, a laser beam suitable for exposure can be obtained.

Further, according to the present invention, the spectral parameters are gas pressure, gas temperature, fluorine concentration,
It is at least one of the amount of impurities in the laser chamber, the high voltage, and the number of pulse oscillations. The gas pressure of the laser gas, the gas temperature, the fluorine concentration, the amount of impurities in the laser chamber, the high voltage, and the number of pulse oscillations are all closely related to the spectral characteristics. Therefore, by controlling at least one of these, the spectral characteristics can be efficiently brought within the allowable range.

Further, according to the present invention, the spectral parameter includes a gas pressure of a laser gas, and the laser controller sets the spectral characteristic to a predetermined allowable range based on a relationship between the spectral characteristic of the fluorine molecular laser beam and the gas pressure. A certain gas pressure range is set, and control is performed so that the detected gas pressure falls within the pressure range. Among the spectral parameters, the gas pressure has a particularly close relation to the spectral characteristics. Therefore, by controlling the gas pressure, the spectral characteristics can be efficiently kept within the allowable range.

Further, according to the present invention, a laser controller comprises:
Estimate the concentration of fluorine in the laser chamber, correct the relationship between the spectral characteristics of the fluorine molecular laser light and the spectral parameters based on the estimated concentration of fluorine, spectral parameters for the spectral characteristics to be within a predetermined allowable range Has changed the allowable range. The relationship between the spectral characteristics of the fluorine molecular laser light and the spectral parameters changes depending on the fluorine concentration. Therefore, the relationship between the spectral characteristics and the spectral parameters can be accurately obtained by correcting the relationship between the two based on the fluorine concentration and changing the allowable range in which the spectral characteristics are within the allowable range. Therefore, by setting the spectral parameter to fall within the corrected allowable range, it is possible to more surely bring the spectral characteristics into the allowable range.

Further, the present invention is directed to a fluorine molecular laser device that oscillates a fluorine molecular laser beam by causing a main discharge in a laser chamber filled with a laser gas containing fluorine, wherein the spectral characteristics of the fluorine molecule laser beam are changed. A detector for detecting a spectral parameter of a laser gas, and a laser controller for setting an allowable range of the spectral parameter such that the spectral characteristic is within a predetermined allowable range based on a relationship between the spectral characteristic of the fluorine molecular laser beam and the spectral parameter. The laser controller determines that the spectrum characteristic is abnormal when the detected spectrum parameter is out of the allowable range.

Since there is a close relationship between the spectral parameters and the spectral characteristics, it is possible to reliably determine an abnormality in the spectral characteristics based on the spectral parameters. Therefore, it is not necessary to separately provide a device for detecting spectral characteristics, and it is possible to prevent exposure from being performed by laser light having abnormal spectral characteristics, thereby preventing exposure from becoming defective. Further, the gas pressure of the laser gas, the gas temperature, the fluorine concentration, the amount of impurities in the laser chamber, the high voltage, and the number of pulse oscillations are all closely related to the spectral characteristics. Therefore, by controlling at least one of them, the abnormality of the spectrum characteristic can be accurately determined. In particular, since the gas pressure is most closely related to the spectral characteristics, the determination of the abnormality in the spectral characteristics based on this is more accurate.

Further, the present invention provides a fluorine molecular laser device for generating a main molecular discharge in a laser chamber filled with a laser gas containing fluorine and oscillating a molecular fluorine laser beam, and irradiating the object to be exposed with the molecular fluorine laser beam. In a fluorine exposure apparatus having an exposure machine for performing exposure, such as changing the spectral characteristics of fluorine molecular laser light, such as a detector that detects the spectral parameters of the laser gas, such that the spectral characteristics are within a predetermined allowable range, A laser controller that sets a permissible range of the spectral parameter, based on the permissible range of the spectral parameter and the spectral parameter, estimates a spectral characteristic of the fluorine molecular laser light, and adjusts the exposure machine based on the estimated spectral characteristic. I do.

According to such a configuration, the spectral characteristics are estimated based on the spectral parameters. By estimating based on closely related spectral parameters, spectral characteristics can be accurately estimated. Then, based on the spectral characteristics, the exposure device can correct, for example, the position of the reduction projection lens. Thereby, the system of the exposure machine is optimized, and the resolution of exposure is improved. In addition, the gas pressure of the laser gas,
Gas temperature, fluorine concentration, impurity amount in laser chamber,
By estimating the spectral characteristics based on at least one of the high voltage and the number of pulse oscillations, the estimation becomes accurate. Further, estimating the spectral characteristics based on the gas pressure makes the estimation more accurate.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment according to the present invention will be described in detail with reference to the drawings. First, a first embodiment will be described. FIG. 1 shows a configuration diagram of a fluorine exposure apparatus according to the present embodiment. The fluorine exposure apparatus includes a fluorine molecule laser device 1 that oscillates a fluorine molecule laser beam 11,
An exposure device 25 for exposing the wafer 37 using the fluorine molecular laser beam 11 is provided. In the following description, the fluorine molecular laser device is referred to as a fluorine laser device 1, and the fluorine molecular laser beam oscillated from the fluorine molecule laser device is referred to as a laser beam 11. In FIG. 1, a fluorine laser device 1 includes a laser chamber 2 in which a laser gas as a laser medium is sealed.
It has. The laser gas contains fluorine (F2) and neon (Ne) as a buffer gas in a predetermined composition. Inside the laser chamber 2, a pair of discharge electrodes 4 and 5 are arranged to face each other. The discharge electrodes 4 and 5
It is connected to a high voltage power supply 13 via a discharge circuit (not shown). The high-voltage power supply 13 applies a high voltage V to the discharge electrodes 4 and 5 based on an instruction from the laser controller 31 that is electrically connected. As a result, a main discharge 26 is generated between the discharge electrodes 4 and 5 to excite the laser gas and oscillate the laser light 11.

A front window 7 and a rear window 9 for transmitting a laser beam 11 are respectively provided at the front and rear portions of the laser chamber 2. A front slit 16 and a rear slit 17 each having an opening having a predetermined width are arranged in front (rightward in FIG. 1) and rear of the laser chamber 2, respectively. A front mirror 6 that partially transmits the laser beam 11 is provided in front of the front slit 16. Further, two dispersion prisms 18 and 18 are disposed behind the rear slit 17 and the dispersion prism 1
A rear mirror 8 that totally reflects the laser beam 11 is disposed behind the rear mirrors 8 and 18. The laser light 11 oscillated inside the laser chamber 2 passes through the windows 7, 9 and the dispersing prisms 18, 18, is reflected between the rear mirror 8 and the front mirror 6, and is amplified while being reciprocated. Is transmitted through the front mirror 6 and taken out.

In front of the laser chamber 2, a laser beam 1
A beam splitter 12 for sampling a part of the beam splitter 1 is provided. The laser beam 11 transmitted through the beam splitter 12 enters an exposure device 25 such as a stepper. The intensity distribution of the laser beam 11 is made uniform by an optical system (not shown) inside the exposure device 25, and the laser beam 11 is irradiated on a mask called a reticle 35. The laser beam 11 that has passed through the pattern provided on the reticle 35 is irradiated on the wafer 37 by the reduction projection lens 36 to perform exposure. Further, the laser beam 11 reflected upward by the beam splitter 12 in FIG.
A enters the power detector 15. Power detector 15
Outputs an electric signal corresponding to the pulse energy to the laser controller 31. The laser controller 31 detects the pulse energy of the laser beam 11 and the number of pulses of the laser beam 11 oscillating based on the electric signal. And so that the pulse energy is constant,
The value of the high voltage V is instructed to the high voltage power supply 13, and this is called power lock control. At this time, the laser controller 3
1 is always high voltage V in order to perform main discharge 26 stably.
Must be equal to or higher than the lower limit value Vm and equal to or lower than the upper limit voltage VM.

The laser chamber 2 has an internal gas pressure P
Is connected, and outputs an electric signal corresponding to the gas pressure P to the laser controller 31 which is electrically connected. The laser controller 31
Based on this electric signal, the gas pressure P inside the laser chamber 2 can be detected. Further, an exhaust pipe 16 and an injection pipe 17 are connected to the laser chamber 2. An exhaust valve 27 and an exhaust pump 19 are connected to an exhaust pipe 16 for exhausting the laser gas to the outside of the laser chamber 2, and the exhaust valve 27 opens and closes based on an instruction signal from a laser controller 31. A fluorine treatment device (not shown) is connected to the exhaust side of the exhaust pump 19 to remove fluorine from the exhaust gas.

An injection pipe 17 for injecting a laser gas into the laser chamber 2 has a fluorine gas cylinder 20 filled with a diluted fluorine gas (hereinafter referred to as fluorine gas) obtained by diluting fluorine with a buffer gas such as neon, and a neon gas, for example. And a buffer gas cylinder 21 filled with the buffer gas. The fluorine gas cylinder 20 and the buffer gas cylinder 21 are injected into the laser chamber 2 by opening and closing the fluorine gas valve 22 and the buffer gas valve 23 that open and close according to an instruction signal from the laser controller 31.

The oscillation wavelength of the laser light 11 will be described with reference to FIG. In FIG. 2, the horizontal axis represents the wavelength λ of the laser beam 11 and the vertical axis represents the intensity E of the laser beam 11. In the following description, the laser light 1 emitted from the fluorine laser device 1 will be described.
The center wavelength λc, the spectral width Δλ, and the spectral purity F are referred to as the spectral characteristics of the laser light 11. The spectral purity F is, as shown in FIG. 2, a wavelength width F of a hatched region in which, for example, 95% of the energy of the laser beam 11 falls.
Is shown. The smaller this value is, the less the component farther from the center wavelength λc is mixed in the wavelength of the laser light 11, which is suitable for exposure. The spectral purity F is
Depending on the requirements of the exposure device 25, what percentage of the energy falls is different, for example, about 90 to 99.5% is general. Although not shown, the spectral width Δλ represents the full width at half maximum (FWHM) of the wavelength, and the smaller the value, the more suitable for exposure.

As shown in FIG. 2, the laser light 11 oscillated inside the laser chamber 2 includes a strong line light 11A having a long wavelength (a central wavelength of 157.63 nm) and a weak line light 11B (a short wavelength). And a center wavelength of 157.52 nm). Strong line light 11A and weak line light 1
Since the wavelength is different from 1B, there is a difference in the refraction angle when the light enters and exits the dispersion prisms 18,18. Therefore, the strong line light 11A and the weak line light 11B
While passing through the dispersing prisms 18, 18, the optical path slightly shifts. As a result, the dispersion prism 18,
The strong line light 11A that has passed through the
The light exits from the front mirror 8 through the opening 17. On the other hand, the optical path of the weak line light 11B is shifted while passing through the two dispersing prisms 18 and 18, and is blocked by the front slit 16 or the rear slit 17 and does not oscillate. In the fluorine laser device 1, the wavelength of the laser light 11 is narrowed by oscillating only the strong line light 11A in this manner.

When the fluorine laser device 1 is used as a light source of the exposure device 25, the exposure device 25 needs to keep the spectral characteristics of the laser beam 11 within a predetermined allowable range in order to perform exposure well. However, in the fluorine laser device 1, there is a close relationship between the gas pressure P in the laser chamber 2 and the spectral characteristics of the laser light 11, and when the gas pressure P fluctuates, the spectral characteristics fluctuate sensitively. It has been known. In this embodiment, this gas pressure P is used as a spectrum parameter to control the spectrum characteristic so that the spectrum characteristic is controlled within a predetermined allowable range.

FIG. 3 is a graph showing the gas pressure P on the horizontal axis, the center wavelength λc, the spectral width Δλ, and the spectral purity F on the vertical axis. As described above, when the gas pressure P increases, the center wavelength λc increases, and the spectral width and the spectral purity F increase (deteriorate). The vertical axis in FIG. 3 shows the allowable range λ required by the exposure device 25 for the center wavelength λc, the spectral width Δλ, and the spectral purity F.
1 to λ2, Δλ1 to Δλ2, and F1 to F2 are shown, respectively. As shown in FIG. 3, for example, from the relationship between the gas pressure P and the center wavelength λc, the allowable range λ1 to λ
2, the allowable pressure ranges P1 and P2 of the gas pressure P are determined. Similarly, the allowable range Δλ1 to Δλ of the spectrum width Δλ
The allowable pressure range P3 to P4 for λ2 is equal to the allowable pressure range P5 to F5 for the allowable range F1 to F2 of the spectral purity F.
P6 is determined respectively.

These allowable pressure ranges P1 to P2, P3 to
A pressure range that satisfies all of P4, P5, and P6 is referred to as a pressure range Pm to PM. In the example of FIG. 3, the lower limit of the pressure range is Pm = P3 and the upper limit is PM = P6. That is, when the gas pressure P always falls within the pressure range Pm to PM, all of the center wavelength λc, the spectral width Δλ, and the spectral purity F fall within the allowable range. Therefore, the laser controller 31 controls the gas pressure P to
The center wavelength λc, the spectral width Δλ, and the spectral purity F can all be within the allowable range.
It should be noted that the pressure range Pm to PM is not limited to satisfying the allowable range for all of the center wavelength λc, the spectral width Δλ, and the spectral purity F. For example, the exposure device 25 has a center wavelength λc and a spectrum width Δλ.
If only the allowable range is specified for the pressure ranges Pm-P
M may be determined. Further, the present invention is not limited to the spectrum characteristics illustrated, and the pressure range Pm to PM may be determined for other wavelength parameters.

Furthermore, the lower limit value Pm of the pressure range is not taken into account the lower limit values P3 and P5 of the allowable pressure range of the spectral width Δλ and the spectral purity F, and the lower limit value P1 and P2 of the central wavelength λc is not considered. P1 may be the lower limit Pm of the pressure range. That is, as for the spectral width Δλ and the spectral purity F, the smaller the value, the better the exposure. Therefore, in response to a request from the exposure machine 25,
Since the lower limits of the spectral width Δλ and the spectral purity F may not be included, the allowable pressure range determined from this may not be included in the pressure range Pm to PM.

Hereinafter, a procedure for keeping the gas pressure P within the pressure range Pm to PM will be specifically described. First, before oscillation of the laser beam 11, the laser controller 31
Exhausts the inside of the laser chamber 2 and injects a laser gas composed of a fluorine gas and a buffer gas with a predetermined composition. The replacement of all the laser gases in the laser chamber 2 in this manner is called total gas exchange, and the gas composition immediately after the total gas exchange is called the initial composition. At this time, by setting the gas pressure P inside the laser chamber 2 after the entire gas exchange within the pressure range Pm to PM, the spectrum characteristics are always kept within the allowable range when the laser oscillation is started immediately after the entire gas exchange. It is possible.

Next, control during laser oscillation will be described. FIG. 4 is a flowchart illustrating an example of a procedure for controlling the gas pressure P according to the first embodiment. FIG.
, When laser oscillation starts (step S
1) The laser controller 31 constantly monitors the gas pressure P based on the electric signal of the pressure measuring device 24. Then, the gas pressure P is increased to a pressure Pm slightly higher than the lower limit value Pm.
1 and a pressure PM1 slightly lower than the upper limit value PM (step S2). At this time, Pm <Pm1 <PM1
<PM. When the gas pressure P is Pm1 or more and PM
If the value is 1 or less, the process returns to S2.

In S2, the gas pressure P is changed to Pm1
If not more than PM1, the gas pressure P is changed to the pressure PM.
If the gas pressure P is higher than the pressure PM1 as compared with 1, the exhaust starts (step S4). Then, evacuation is performed until the pressure becomes the pressure PM1 (step S
6) Stop the exhaust (step S7), and step S2
Return to If the gas pressure P is smaller than the pressure PM1 in Step S3, P <Pm1 is satisfied, so that predetermined amounts of fluorine gas and buffer gas are injected (Step S8). The injection is performed until the pressure reaches the pressure Pm1 (step S9), the injection is stopped (step S10), and the process returns to step S2. That is, when the gas pressure P approaches the upper limit value PM or the lower limit value Pm, the laser gas is evacuated or newly injected, and the gas pressure P is constantly increased.
Performs laser oscillation so that the pressure falls within the pressure range Pm to PM. Thereby, the spectral characteristics of the laser beam 11 are always within the allowable range, and the exposure can be suitably performed.

FIG. 5 is a flowchart showing another example of the procedure for controlling the gas pressure P according to the first embodiment. In FIG. 5, when the laser oscillation is started (step S21), the laser controller 31 constantly monitors the gas pressure P based on the electric signal of the pressure measuring device 24. Then, the gas pressure P is compared with the pressure range Pm to PM (Step S22), and the gas pressure P is changed to the pressure range Pm to PM.
If it is outside the PM, the shutter 30 is instructed to perform a closing operation (step S43), and the laser beam 11 is cut off. Then, an abnormal wavelength signal is output to the exposing device 25 to notify that the spectrum characteristic is out of the allowable range (step S4).
4) Stop laser oscillation (step S46).

Thereafter, the laser controller 31 partially exchanges the gas in the laser chamber 2 by partially exhausting the laser gas in the laser chamber 2 and injecting the fluorine gas and the buffer gas into the laser chamber 2. (Step S47). That is, the change in the gas pressure P is considered to be caused by impurities or the like generated in the laser gas.
For example, about one-third of the laser gas inside the laser chamber 2 is replaced and refreshed. At this time, the gas pressure P in the laser chamber 2 after the replacement is set to the pressure range Pm.
By setting it in the range of ~ PM, the spectral characteristics when laser oscillation is started fall within an allowable range.

Then, laser oscillation is started (step S).
49) High voltage V when power lock control is performed
Is compared with the upper limit voltage VM (step S51).
If the high voltage V is equal to or higher than the upper limit, it is determined that the amount of laser gas exchange is insufficient, and the process returns to step S47 to partially exchange the laser gas again. If the high voltage V is lower than the upper limit voltage VM in step S51, a normal wavelength signal indicating that the spectrum characteristic is within the allowable range is output to the exposure device 25 (step S52), and the shutter 30 is opened (step S52). (Step S53) Return to step S22. Alternatively, in steps S47 to S51, all the laser gases may be exhausted, and all the laser gases may be replaced. Also in this case, the gas pressure P inside the laser chamber 2 after all the gas exchanges is performed.
Within the pressure range Pm to PM, the spectrum characteristic falls within the allowable range when laser oscillation is started.

In step S22, the gas pressure P
Is within the pressure range Pm-PM, the high-voltage power supply 13
Is compared with the upper limit voltage VM (step S23). If the high voltage V is lower than the upper limit voltage VM,
It returns to step S22. If the high voltage V is equal to or higher than the upper limit voltage VM in step S23, the fluorine gas is injected by a predetermined amount ΔP1 (step S24). At this time, if the gas pressure P becomes the upper limit PM of the pressure range (step S26) or if the injection amount becomes the predetermined amount ΔP1 (step S27), the injection is stopped (step S2).
8) The high voltage V is compared with the upper limit voltage VM (step S30).

In step S30, the high voltage V is
If less than M, the process returns to step S22. If the high voltage V is equal to or higher than the upper limit voltage VM in step S30, the exhaust is started (step S31). At this time, while the gas pressure P is always equal to or higher than the lower limit Pm of the pressure range (Step S32), the exhaust is performed until the exhaust amount reaches the predetermined amount ΔP2 (Step S34), and the exhaust is stopped (Step S35). . That is, the gas pressure P is always equal to or higher than the lower limit Pm of the pressure range.

Then, a fluorine gas and a buffer gas are injected with a predetermined composition (step S36). At this time, while ensuring that the gas pressure P is always equal to or lower than the upper limit PM of the pressure range (step S38), the injection is performed until the injection amount of the fluorine gas reaches the predetermined amount ΔP3 and the injection amount of the buffer gas reaches the predetermined amount ΔP4. (Step S39), the injection is stopped (Step S41). Then, the high voltage V is compared with the upper limit voltage VM (step S42). If the high voltage V is lower than the upper limit voltage VM, the process returns to step S22.
Is greater than or equal to the upper limit voltage, the process proceeds to S43.

In the above flow chart, the predetermined amounts ΔP1 to ΔP4 for injecting and exhausting gas may be determined in advance, but may vary depending on the conditions inside the laser chamber 2 such as the number of pulses of the pulsed laser beam 11 and the like. It is even better to change it. For example, in steps S31 to S41, fluorine and a buffer gas are injected and exhausted while the gas pressure P does not always exceed the pressure range Pm to PM. Then, the gas composition in the laser chamber 2 after evacuation is made as close as possible to the initial composition before laser oscillation. For this purpose, for example, the total (ΔP3 + ΔP4) of the injection amounts of the fluorine gas and the buffer gas may be almost always equal to the predetermined amount ΔP2 to be exhausted. Moreover,
For example, at the time of exhaust, the gas pressure P becomes
If it becomes less than m, the total injection amount should be adjusted to the exhaust amount up to that point.

In steps S26, S32, and S38, instead of performing injection or exhaust to the upper limit value PM or the lower limit value Pm of the pressure range, the pressure P lower than the upper limit value PM is not increased.
Gas may be injected up to M1 or exhausted up to a pressure Pm1 higher than the lower limit Pm. Further, in step S22, the gas pressure P is adjusted to the pressure range P
rather than comparing with m-PM, a narrower pressure range Pm
You may make it compare with 1-PM1. in this way,
Rather than performing control after the gas pressure P deviates from the pressure range Pm to PM, by taking measures early, it is possible to more reliably prevent the spectral characteristics from deviating from the allowable range.

As described above, according to the first embodiment, each item of the required spectral characteristics (the center wavelength λ
The allowable pressure range is set for each of c, the spectral width Δλ, and the spectral purity F). A pressure range Pm to PM satisfying all of these pressure tolerance ranges
Is set so that the gas pressure P always falls within the pressure range Pm to PM. The spectral characteristics of the fluorine laser device 1 are highly dependent on the gas pressure P, and the gas pressure P
The change of the spectral characteristic with respect to the change of is predictable. Therefore, by keeping the gas pressure P within this pressure range Pm to PM, the spectral characteristics of the laser light 11 can be kept within the required allowable range. This allows
Since the exposure can be performed with the laser beam 11 having a spectral characteristic within an allowable range, the exposure is always performed satisfactorily, and the defective rate is reduced.

In particular, since the gas pressure P rarely changes suddenly, by monitoring the gas pressure P, it is possible to detect in advance that the spectrum characteristic is likely to be out of the allowable range. Therefore, for example, as shown in the flowchart of FIG. 4, it is possible to prevent the spectral characteristics from deviating from the allowable range by taking measures early and returning the gas pressure P to the pressure range Pm to PM.

Further, as shown in the flowchart of FIG. 5, by monitoring the gas pressure P in the laser chamber 2, it is possible to detect that the spectral characteristic has deviated from a predetermined allowable range. Accordingly, the wavelength abnormality signal can be notified to the exposing device 25 to stop the exposure, so that the laser beam 11 whose spectral characteristic is out of the allowable range is not subjected to improper exposure.
Exposure failure can be prevented. Further, a detection device for detecting the spectral characteristics is not required, and a complicated calculation required for obtaining the spectral characteristics is not required. When the high voltage V increases during laser oscillation, the laser gas is partially replaced while the gas pressure P is kept within the pressure range Pm to PM. As a result, the laser gas degraded by impurities or the like is partially refreshed, so that the high voltage V can be reduced without stopping the laser beam 11, and the oscillation can be continued for a longer time. Moreover, since the gas pressure P is within the pressure range Pm to PM, the spectral characteristics do not deviate from the allowable range. Therefore, it is possible to lengthen the period until all the gas is exchanged while performing the exposure satisfactorily, thereby improving the exposure efficiency.

In the above flowcharts, the pressure range Pm to PM is always constant, but the present invention is not limited to this. For example, when the allowable range of the spectral characteristics required by the exposure device 25 changes, the exposure device 2
5 notifies the laser controller 31 of the change in the allowable range of the spectrum characteristic. Laser controller 31
The pressure range Pm to PM may be corrected based on the request for the allowable range of the spectral characteristics sent from the exposure device 25 by communication.

Next, a second embodiment will be described. FIG. 6 is a configuration diagram of a fluorine exposure apparatus according to the second embodiment. In FIG. 6, between the fluorine gas valve 22 and the buffer gas valve 23 and the fluorine gas cylinder 20 and the buffer gas cylinder 21 of the injection pipe 17, a fluorine flow control device 32 and a buffer gas flow control device 33 are provided.
Each is interposed. An exhaust flow control device 34 is also interposed between the exhaust valve 27 of the exhaust pipe 16 and the exhaust pump 19. Each flow control device 32-34
Is, by an instruction signal from the laser controller 31,
The flow rate of the gas flowing through the pipes 16 and 17 can be changed.

For example, when the allowable range of the spectral characteristics required from the exposure device 25 is very narrow, the pressure range Pm to PM is also very narrow, and the gas pressure P should be kept as small as possible. Is required. FIG. 7 shows the flow chart of step S24 in the flowchart shown in FIG. 5 using the fluorine laser device 1 according to the second embodiment.
An example in which the fluorine gas injection in steps S28 to S28 is performed without changing the gas pressure P is shown in a flowchart. In FIG. 7, the laser controller 31 starts exhausting simultaneously with starting injection of fluorine gas (step S61). At this time, the laser controller 31
Outputs a signal to the fluorine flow controller 32 and the exhaust flow controller 34 in advance so that the fluorine injection flow rate and the exhaust flow rate are substantially the same.

Then, the laser controller 31 sets the gas pressure P to a pressure range Pm1 to Pm1 which is narrower than the pressure range Pm to PM.
Compare with M1 (step S62). If the gas pressure P is within the pressure range Pm1 to PM1, the injection amount and exhaust amount of the fluorine gas are compared with the predetermined amount ΔP5 (step S6).
3) When the injection amount and the exhaust amount of the fluorine gas reach the predetermined amount ΔP5, the injection and the exhaust of the fluorine gas are stopped (step S64). If the gas pressure P is not in the pressure range Pm1 to PM1 in step S66, the gas pressure P is compared with the pressure PM1 (step S66). If the gas pressure P is lower than the pressure PM1, the gas pressure P
It can be determined that it is lower than m1. Therefore, a signal is output to the fluorine flow control device 32 and the exhaust flow control device 34,
Increase the fluorine injection flow rate, reduce the exhaust flow rate,
Increase the gas pressure P. On the other hand, if the gas pressure P is higher than the pressure PM1 in step S66, the fluorine injection flow rate is decreased, the exhaust flow rate is increased, and the gas pressure P is decreased. That is, when the gas pressure P is in the pressure range Pm ~
When the gas is likely to deviate from the PM, the injection flow rate and the exhaust flow rate of the gas are increased or decreased so that the injection and exhaust are always performed at the same flow rate.

By replacing steps S61 to S68 with steps S24 to S28, it is possible to inject fluorine gas without changing the gas pressure P. The same applies to the injection of the fluorine gas and the buffer gas in steps S31 to S41. That is, when the gas pressure P is high, the injection flow rates of the fluorine gas and the buffer gas are reduced, and the exhaust flow rate is increased to keep the gas pressure in the pressure range Pm to PM. Gas pressure P
If the gas flow rate is low, the injection flow rate is increased and the exhaust flow rate is decreased. In this way, the gas is injected and the exhaust is performed at the same time, and the injection amount per hour and the exhaust amount are made substantially equal to each other, so that the gas pressure P is hardly changed and the gas pressure P is kept within the narrow pressure range Pm to PM. It is possible to fit in. As a result, the fluctuation of the spectral characteristics becomes smaller, and it is possible to meet the requirement of a narrower allowable range.

Next, a third embodiment will be described. In the first and second embodiments, the upper limit PM and the lower limit Pm of the pressure range are always constant, but in the third embodiment, the upper limit PM and the lower limit Pm are corrected based on the fluorine concentration in the laser chamber 2. I have.

FIG. 8 is a graph showing the relationship between the gas pressure P and the center wavelength λc with the horizontal axis indicating the gas pressure P and the vertical axis indicating the center wavelength λc. As shown in FIG. 8, when the fluorine concentration in the laser chamber 2 changes from the concentration B1 to the concentration B2, the relation between the gas pressure P and the center wavelength λc also changes. As a result, the pressure allowable ranges P1 and P2 with respect to the allowable ranges λ1 and λ2 of the center wavelength λc become the pressure allowable ranges P1n and P1n.
It changes to P2n. Similarly, the allowable pressure range also changes with respect to the spectral width Δλ and the spectral purity F, so that the lower limit Pm and the upper limit PM of the pressure range also change. Third
In the embodiment, the pressure range Pm
~ PM is corrected.

At this time, the fluorine concentration is determined by the initial composition of the laser gas, the injection amount of fluorine ΔP1, the injection amount of fluorine and buffer gas ΔP2, the exhaustion amount ΔP3, and step S47.
Can be estimated from a part of the exchanged laser gas and the like. In order to obtain the fluorine concentration with higher accuracy, it is preferable to correct the fluorine concentration based on the number of pulses of the laser beam 11 that has oscillated after all the gas exchanges.
This is because fluorine is consumed by the pulse oscillation and the fluorine concentration decreases. Alternatively, laser chamber 2
In addition, a fluorine concentration measuring device 39 may be additionally provided. As the fluorine concentration measuring instrument 39, an infrared gas analyzer (FTI)
R: Fourier Transform InfRared spectroscopy) is preferable.

For example, in the flow chart shown in FIG. 5, in step S22, the gas pressure P is changed to the pressure range Pm to Pm.
Before comparing with PM, the fluorine concentration at that time is estimated in advance, and the pressure range Pm to PM is corrected based on this fluorine concentration. Before comparing the gas pressure P with the upper limit PM or the lower limit Pm of the pressure range in steps S26, S32, and S38, the fluorine concentration at that time is estimated in advance,
The lower limit value Pm and the upper limit value PM of the pressure range are corrected based on the fluorine concentration.

As described above, according to the third embodiment, the pressure range Pm to PM is corrected by the fluorine concentration in the laser chamber 2. The relationship between the gas pressure P and the spectral characteristics changes depending on the fluorine concentration in the laser chamber 2. Therefore, by correcting the pressure ranges PM to Pm based on the fluorine concentration, the pressure ranges Pm to PM with respect to the allowable range of the spectral characteristics can be obtained more accurately. By keeping the gas pressure P within the corrected pressure range Pm to PM based on this, it is possible to reliably keep the spectral characteristics of the laser beam 11 within an allowable range.

Next, a fourth embodiment will be described. The fluorine exposure apparatus according to the fourth embodiment is, for example, the same as that shown in FIG. 1 or FIG. As shown in FIGS. 1 and 6,
The laser controller 31 and the exposure device 25 are connected by a communication line. As described in each of the above embodiments, the laser controller 31 constantly monitors the gas pressure P based on the electric signal of the pressure measuring device 24. By comparing the gas pressure P with the gas pressure P-spectral characteristic graph shown in FIG. 3, the laser controller 31 can know the current value of each spectral characteristic. For example, in FIG. 3, assuming that the current gas pressure P is Ps, the respective spectral characteristics are the center wavelength λs, the spectral width Δλs, and the purity Fs.
The laser controller 31 transmits these spectral characteristics (λs, Δλs, Fs) to the exposure device 25.

The exposing device 25 finely adjusts the positional relationship of the reduction projection lens 36 and the pressure of the atmosphere gas inside the exposure device 25 where the reduction projection lens 36 is installed, based on the received spectral characteristics. Thereby, the system of the exposure machine is optimized according to the spectral characteristics, so that exposure with higher resolution can be performed. Alternatively, only the gas pressure P is transmitted from the laser controller 31 to the exposure device 25, and a controller (not shown) inside the exposure device 25 performs an operation based on the gas pressure P,
The spectral characteristics may be calculated. Further, the laser controller 31 may double as the controller of the exposure machine 25.

As described above, according to the fourth embodiment, the fluorine laser apparatus 1 sends the spectrum characteristic or the gas pressure P for calculating the spectrum characteristic to the exposure unit 25.
Is to be notified. Thus, the exposure device 25 finely adjusts the position and the like of the internal reduction projection lens 36 in accordance with the spectral characteristics of the laser light 11, thereby enabling exposure with higher resolution.

Next, a fifth embodiment will be described. In the above description, the gas pressure P has been described as an example of the spectral parameter of the laser gas that changes the spectral characteristics of the laser beam 11. this is,
This is because the gas pressure P has the greatest influence on the spectral characteristics, and by controlling this, the spectral characteristics can be most suitably controlled. However, as parameters having an influence on the spectral characteristics, in addition to the gas pressure P, for example, gas temperature, fluorine concentration, impurity amount in the laser chamber 2,
There is a high voltage V applied between the discharge electrodes 4 and 5, or the number of pulse oscillations that have been performed by the laser so far. Hereinafter, these are referred to as spectral parameters. In the fifth embodiment, a technique will be described in which these spectral parameters are controlled so that spectral characteristics fall within a predetermined allowable range.

FIG. 9 is an explanatory view of an excimer laser device 1 according to the fifth embodiment. The laser chamber 2 has a temperature measuring device 38 for measuring the gas temperature of the laser gas, and a fluorine concentration measuring device 39 for measuring the fluorine concentration in the laser gas.
And an impurity measuring device 40 for measuring the amount of impurities in the laser chamber 2 are connected. These measuring instruments 38-
Reference numerals 40 are respectively electrically connected to the laser controller 31 and transmit signals to the laser controller 31. This allows the laser controller 31 to know the gas temperature, the fluorine concentration inside the laser chamber 2, and the amount of impurities inside the laser chamber 2, in addition to the gas pressure P of the laser gas.

Here, as the impurity measuring device 40, for example, in order to measure the amount of impurities that are fine particles,
A particle counter is suitable, and an infrared gas analyzer is suitable for measuring the amount of gaseous impurity gas. Further, as described above, the laser controller 31
Based on the signal of the electrically connected power detector 15,
The number of pulses can be detected. Further, the laser controller 31 can know the high voltage V applied between the discharge electrodes 4 and 5 based on a command value instructed to the high voltage power supply 13 by itself.

Hereinafter, a technique for estimating the spectral characteristics based on the relationship between the spectral parameters and the spectral characteristics and controlling the spectral parameters so that the spectral characteristics are within the allowable range will be described. Examples of spectral parameters deeply related to spectral characteristics are, for example, spectral parameters C and D. A spectral parameter C;
Between the center wavelength λc and the spectral width Δλ, FIG.
The relationship as shown in FIG.
And the center wavelength λc and the spectrum width Δλ have a relationship as shown in FIG. For example, FIG.
As shown in, the allowable ranges C1 and C2 of the spectral parameters C are
The allowable ranges C3 and C4 of the spectral parameters C are determined with respect to the allowable range of the spectral width Δλ. Therefore, the range of the spectrum parameter C that satisfies all the allowable ranges of the center wavelength λc and the spectrum width Δλ is C1 to C4. Similarly, in FIG. 11, the range of the spectrum parameter D is D1 to D4.

The laser controller 31 can estimate the spectral characteristics from the values of these spectral parameters C and D. However, for example, as described in the third embodiment, when the fluorine concentration changes, the spectral parameter D may change when the spectral parameter C changes so that the relationship between the gas pressure P and the spectral characteristics changes. is there. Therefore, the laser controller 31 stores the spectral characteristics for the values of the spectral parameters C and D, for example, as a table.
Then, based on this table, the spectral characteristics are estimated from the detected values of the spectral parameters C and D. If the spectral characteristics are likely to deviate from the predetermined allowable range, at least one of the spectral parameters C and D is controlled to return the spectral characteristics to within the allowable range. In addition, by transmitting the current spectral characteristics to the exposure device 25, the exposure device 25
The position and the like of the internal reduction projection lens 36 are finely adjusted to perform exposure with higher resolution.

Further, when the spectrum characteristic deviates from the predetermined allowable range, a wavelength abnormality signal is notified to the exposing device 25 and exposure is stopped, similarly to the flow chart shown in FIG. Therefore, the exposure can be stopped by notifying the exposing device 25 of the wavelength abnormality signal, so that the laser beam 11 whose spectral characteristic is out of the allowable range is not improperly exposed, thereby preventing exposure failure. it can. Although FIGS. 10 and 11 illustrate that the center wavelength λc and the spectral width Δλ both increase as the spectral parameters C and D increase, the present invention is not limited to this, and may decrease.

In the above embodiments, the buffer gas is neon, and the laser gas contains fluorine and neon. However, the present invention is not limited to this. For example, the buffer gas may be helium, or the buffer gas may be a mixed gas of neon and helium.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a fluorine exposure apparatus according to a first embodiment.

FIG. 2 is an explanatory diagram of an oscillation wavelength of laser light.

FIG. 3 is a graph showing a relationship between gas pressure and each spectrum characteristic.

FIG. 4 is a flowchart showing a gas pressure control procedure.

FIG. 5 is a flowchart showing another example of the gas pressure control procedure.

FIG. 6 is a configuration diagram of a fluorine exposure apparatus according to a second embodiment.

FIG. 7 is a flowchart showing a gas pressure control procedure according to the second embodiment.

FIG. 8 is a graph showing a relationship between a gas pressure and a center wavelength according to the third embodiment.

FIG. 9 is a configuration diagram of a fluorine exposure apparatus according to a fifth embodiment.

FIG. 10 is a graph showing an example of the relationship between spectral parameters and spectral characteristics.

FIG. 11 is a graph showing an example of the relationship between spectral parameters and spectral characteristics.

FIG. 12 is a configuration diagram of an exposure apparatus according to a conventional technique.

[Explanation of symbols]

1: excimer laser device, 2: laser chamber, 4: discharge electrode, 5: discharge electrode, 6: front mirror, 7: front window, 8: rear mirror, 9: rear window, 11: laser beam, 12: beam splitter, 13:
High voltage power supply, 15: power detector, 16: exhaust pipe, 1
7: Injection pipe, 18: Dispersion prism, 19: Exhaust pump, 20: Fluorine gas cylinder, 21: Buffer gas cylinder, 22: Fluorine gas valve, 23: Buffer gas valve, 24: Pressure measuring instrument, 25: Exposure machine, 26: Main discharge ,
27: exhaust valve, 30: shutter, 31: laser controller, 32: fluorine flow controller, 33: buffer gas flow controller, 34: exhaust flow controller, 35: reticle, 36: reduction projection lens, 37: wafer, 38:
Temperature measuring device, 39: fluorine concentration measuring device, 40: impurity measuring device.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H097 BA02 BB02 CA13 LA10 5F046 CA03 5F071 AA04 DD04 HH01 HH02 JJ10

Claims (12)

[Claims] 1. A main discharge (26) is generated by applying a high voltage (V) between discharge electrodes (4, 5) in a laser chamber (2) in which a laser gas containing fluorine is sealed, thereby generating a fluorine molecular laser. A fluorine molecular laser device that oscillates light (11), a detector (24) that detects the value of a spectral parameter that changes the spectral characteristics of the fluorine molecular laser light (11), and the spectral characteristics are within a predetermined allowable range. A laser controller (31) for setting an allowable range of the spectral parameter, and the laser controller (31) controls the spectral parameter to fall within a predetermined allowable range, thereby setting the spectral characteristic within the allowable range. A fluorine molecular laser device characterized by performing control to fit. 2. The fluorine molecular laser device according to claim 1, wherein the spectral characteristics include a center wavelength (λc), a spectral width (Δλ), and a spectral purity of the molecular fluorine laser beam (11).
(F) At least one of the fluorine molecular laser device. 3. The fluorine molecular laser device according to claim 1, wherein the spectral parameter is a gas pressure of a laser gas.
(P), a gas temperature, a fluorine concentration, an impurity amount in the laser chamber (2), a high voltage (V), and the number of pulse oscillations. 4. The fluorine molecular laser device according to claim 3, wherein the spectral parameter is a gas pressure of a laser gas.
(P), the laser controller (31) based on the relationship between the spectral characteristics of the fluorine molecular laser light (11) and the gas pressure (P), the gas pressure such that the spectral characteristics are within a predetermined allowable range
A fluorine molecular laser device, wherein a pressure range (Pm-PM) of (P) is set, and control is performed such that the detected gas pressure (P) falls within the pressure range (Pm-PM). 5. The fluorine molecular laser device according to claim 1, wherein the laser controller estimates a concentration of fluorine in the laser chamber (2), and based on the estimated concentration of fluorine. Laser molecule (1
A fluorine molecular laser device, wherein the relationship between the spectral characteristics and the spectral parameters of 1) is corrected, and the allowable range of the spectral parameters is changed so that the spectral characteristics fall within a predetermined allowable range. 6. A fluorine molecular laser device which generates a main discharge (26) in a laser chamber (2) filled with a laser gas containing fluorine and oscillates a molecular fluorine laser beam (11). A detector (24) that detects the value of a spectral parameter that changes the spectral characteristics of the fluorine molecular laser light (11), and the spectral characteristics are within a predetermined allowable range based on a relationship between the spectral characteristics and the spectral parameters. A laser controller (31) for setting an allowable range of such a spectral parameter.If the detected spectral parameter is out of the allowable range, the laser controller (31) determines that the spectral characteristic is abnormal. A fluorine molecular laser device characterized by the above-mentioned. 7. The fluorine molecular laser device according to claim 6, wherein the spectral parameter is a gas pressure of a laser gas.
(P), a gas temperature, a fluorine concentration, an impurity amount in the laser chamber (2), a high voltage (V), and the number of pulse oscillations. 8. The fluorine molecular laser device according to claim 7, wherein the spectral parameter is a gas pressure of a laser gas.
(P), the laser controller (31) based on the relationship between the spectral characteristics of the fluorine molecular laser light (11) and the gas pressure (P), the gas pressure such that the spectral characteristics are within a predetermined allowable range
(P) The pressure range (Pm-PM) is set, and if the gas pressure (P) is out of the pressure range (Pm-PM), it is determined that the spectrum characteristics are abnormal. Molecular fluorine laser device. 9. A fluorine molecular laser device (1) for generating a main discharge (26) in a laser chamber (2) filled with a laser gas containing fluorine to oscillate a molecular fluorine laser beam (11); A fluorine exposure apparatus having an exposure device (25) for irradiating the object with (11) to perform exposure, wherein a detector for detecting a value of a spectrum parameter for changing a spectrum characteristic of a fluorine molecular laser beam (11) is provided. (24), a laser controller (31) for setting an allowable range of the spectral parameter such that the spectral characteristics are within a predetermined allowable range, based on the spectral parameter and the allowable range of the spectral parameter, a fluorine molecule A fluorine exposure apparatus, comprising: estimating a spectral characteristic of a laser beam (11); and adjusting an exposure unit (25) based on the estimated spectral characteristic. 10. The fluorine exposure apparatus according to claim 9, wherein the spectral parameter is a gas pressure of a laser gas.
(F) a fluorine exposure apparatus characterized by at least one of (P), a gas temperature, a fluorine concentration, an impurity amount in a laser chamber (2), a high voltage (V), and a pulse oscillation number. 11. The fluorine exposure apparatus according to claim 10, wherein the spectral parameter is a gas pressure of a laser gas.
A fluorine exposure apparatus comprising (P). 12. The fluorine molecular laser device according to claim 1, wherein the spectral parameter is a gas pressure of a laser gas.
(P), the gas temperature, the amount of impurities in the laser chamber (2), the high voltage (V), and the number of pulse oscillations. The laser controller controls the fluorine in the laser chamber (2). Is estimated, and based on the estimated fluorine concentration, the molecular fluorine laser light (1
A fluorine molecular laser device, wherein the relationship between the spectral characteristics and the spectral parameters of 1) is corrected, and the allowable range of the spectral parameters is changed so that the spectral characteristics fall within a predetermined allowable range.