JP2001297978A - Projection exposure method, projection exposure apparatus, projection exposure mask, and semiconductor device manufacturing method using the method - Google Patents

Abstract

(57) [Abstract] (with correction) [PROBLEMS] An exposure method and apparatus for facilitating short-wavelength photolithography by facilitating replacement of an inert gas for keeping an oxygen concentration in a peripheral portion of a mask below an allowable value, And a pellicle-equipped mask for use therein. A mask with a pellicle is housed in a sealed case and the inside of the case is replaced with nitrogen. On the other hand, in the projection exposure apparatus, the light source 1, the various optical systems 4, 5, 6, and 7, the mask stage area 107, and the wafer stage area 109
Are configured to be individually purged. Further, a mask preparation chamber 104 is provided, in which the mask 102 stored in the mask case 103 is transported, and a nitrogen purge is performed. When the oxygen concentration falls below a predetermined value, the mask is taken out and placed on the mask stage 9 to start exposure. Thus, the mask can be moved in and out of the exposure optical system purged with nitrogen without damaging the pellicle provided on the mask.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a projection exposure method and an exposure apparatus used for pattern transfer of a semiconductor integrated circuit device (LSI) and the like, and more particularly to a mask handling when a nitrogen purge is required for an exposure optical system. It is about effective technology.

[0002]

2. Description of the Related Art In the manufacture of a semiconductor integrated circuit device (LSI), a lithography technique is used as a method for forming a fine pattern on a semiconductor wafer. As the lithography technique, an optical projection exposure method of transferring a pattern formed on a photomask onto a semiconductor wafer via a reduction projection optical system has become mainstream. Such a projection exposure apparatus and method are described, for example, in "Innovation in ULSI Lithography Technology" (Science Forum Publishing,
(November 1994, pp. 30-32) and JP-A-04-3
No. 29623.

As shown in FIG. 5, an optical path 2 for guiding light 1 emitted from a light source, a dufuser 3, an illumination stop 4, illumination optical systems (condenser lenses) 5 to 7, a mask stage 9, a projection optical system 11, This is a method using a projection exposure apparatus including a wafer stage 13 and the like. The mask 8 is placed on the mask stage 9 and the wafer 12 is placed on the wafer stage 13, respectively, and the pattern on the mask is transferred onto the wafer. The mask 8 is appropriately replaced depending on the type of the pattern. The mask 8 is a mask stage driving unit 1.
0, the wafer 12 is moved to the wafer stage driving means 14
Are respectively moved or positioned by the control system 16.
The pattern is transferred onto the entire surface of the wafer by stepping or synchronous scanning under the instruction of (1). In particular, the wafer position is measured by the laser interferometer 15. Further, the input of data necessary for the exposure by the operator and the grasp of the exposure state are performed through the interface 17.

On the mask, a transparent thin film called a pellicle for preventing dust and foreign matter from adhering to the pattern surface is provided.
It is provided at a position separated from the pattern surface by a predetermined distance. Normally, as shown in FIG. 6, a pellicle frame 64 having a shape surrounding a pattern region 65 is attached to the surface of the mask substrate 61 where the pattern 62 is formed, and the pellicle 63 is attached to the pellicle frame. This prevents dust and minute foreign matter from adhering to the mask pattern area.
Since the pattern surface and the pellicle are separated from each other by the thickness of the pellicle frame 64, even if minute foreign matter adheres to the pellicle surface, the foreign matter is present together with the pattern because it is at a defocused position from the image forming position on the pattern surface. It will not be transcribed. When a device pattern as shown in FIG. 7 is formed by the above-described exposure method, for example, the size of the gate pattern 71 is an important factor that determines the performance of the semiconductor device. You.

The resolution R in such a projection exposure method is
Is generally represented by R = k × λ / NA. Here k
Is a constant depending on the resist material or process, λ is the wavelength of the illumination light, and NA is the numerical aperture of the projection exposure lens. As can be seen from this relational expression, as the pattern becomes finer, a projection exposure technique using a shorter wavelength light source is required.

At present, i-line (λ = 365 nm) of a mercury lamp or a KrF excimer laser (λ = 24
LSI is manufactured by a projection exposure apparatus using 8 nm). In order to realize further miniaturization, a light source having a shorter wavelength is required, and an ArF excimer laser (λ = 193 nm) or an F2 excimer laser (λ = 157)
nm) is being considered.

[0007]

In order to increase the resolution of optical lithography, exposure light is irradiated with an ArF excimer laser or F2.
When the wavelength is reduced to an excimer laser, these exposure lights are significantly attenuated by the presence of oxygen contained in the atmosphere. In particular, when an F2 excimer laser is used, the allowable oxygen concentration in the atmosphere is set to 10 ppm or less. Therefore,
In all optical paths from the light source through the illumination optical system, the mask, and the projection optical system to the wafer surface, the oxygen concentration is 10 p.
pm or less.

Therefore, there has been proposed a method of individually controlling the environment of a light source and various optical systems in a state close to sealing. For example, by replacing the atmosphere with nitrogen, the oxygen concentration can be reduced to an allowable value or less. Also, the region of the wafer stage on which the semiconductor wafer is mounted can be shielded from the outside air as a sealed space, and the oxygen concentration can be kept below an allowable value. For example, as in an electron beam lithography apparatus, a wafer to be exposed may be once placed in a preliminary chamber to be in a vacuum state, and the area of the wafer stage may be evacuated. Alternatively, the gas may be replaced with an inert gas such as nitrogen without maintaining the vacuum state.

[0009] However, in the conventional exposure technique, a method of replacing the area including the mask stage with nitrogen has not been established. Usually, the mask is housed in a case that can be mounted on the exposure apparatus, but this state is the same environment as in the general atmosphere. After mounting the mask on the mask stage, it is conceivable to perform nitrogen replacement similarly to the wafer, but it is extremely difficult to replace the atmosphere between the mask and the pellicle. Therefore, it is conceivable that the atmosphere between the mask and the pellicle is replaced with nitrogen in advance and sealed.

[0010] However, if the area around the mask is first evacuated lightly to replace with nitrogen, the pellicle will be destroyed due to the pressure difference between the two sides of the pellicle. A method of making a small hole in the pellicle frame to eliminate the pressure difference is also conceivable. However, in this case, when the inside of the pellicle frame is replaced with nitrogen, fluid resistance is large and it is difficult to sufficiently remove oxygen. Increasing the size of the hole in the pellicle frame decreases the fluid resistance, but conversely loses the pellicle's original function of preventing foreign matter from adhering to the mask pattern surface coming into contact with outside air. For the above reasons, there is a problem that it is difficult to replace the inert gas for keeping the oxygen concentration in the peripheral portion of the mask below the allowable value in the photolithography in which the wavelength is shortened.

An object of the present invention is to facilitate the replacement of an inert gas to keep the oxygen concentration at the periphery of a mask below an allowable value,
An object of the present invention is to provide an exposure method for realizing photolithography with a shorter wavelength. It is another object of the present invention to provide a projection exposure apparatus that realizes the above method. Yet another object of the present invention is to provide a mask for realizing the method.

[0012]

In order to achieve the above object, according to the present invention, the mask is housed in a sealed case at the time of manufacturing the mask, and the inside of the case is replaced with nitrogen. A pellicle is provided on the mask, and the space between the mask and the pellicle is also purged with nitrogen and sealed. This mask case is
The lid can be opened by a jig dedicated to the mask or a mask transport jig mounted on the exposure apparatus.

On the other hand, the projection exposure apparatus has a structure in which a light source, various optical systems, a mask stage area, and a wafer stage area can be individually purged. The mask is
In a state where the mask stage is housed in the sealed case, it is conveyed to an adjacent preliminary chamber via a valve and a region where the mask stage is mounted. In the preparatory chamber, the evacuation and the introduction of nitrogen are repeated a predetermined number of times, and sufficient nitrogen substitution is performed. When the oxygen concentration falls below a predetermined amount, open the valve between the preliminary chamber and the area where the mask stage is mounted, open the mask storage case, and place the mask on the mask stage with a dedicated transport system. I tried to attach it. By this means, while the space between the mask and the pellicle is filled with the inert gas, the oxygen concentration in the peripheral portion of the mask can be reduced to a predetermined value or less without damaging the pellicle.

As in the case of the mask, the wafer is placed on the wafer stage after oxygen in the atmosphere is sufficiently removed in a preliminary chamber adjacent to the area where the wafer stage is mounted. The periphery of the wafer may be either vacuum or replaced with an inert gas, and the oxygen concentration may be set to a predetermined value or less.

As another form of the mask, the pellicle may be made of a rigid glass member having a predetermined thickness. In this case, even if the peripheral portion of the mask is directly evacuated to produce a pressure difference between the front and back of the pellicle, the pellicle is not damaged.

As described above, according to the present invention, a step of preparing a mask on which a pattern is formed in a mask case in which the internal environment is maintained under constant conditions, and a step of preparing the pattern on a wafer substrate A measuring step of measuring an environment surrounding a predetermined position of the exposure apparatus on which the mask is mounted to transfer the mask, an environment control step of controlling the environment to predetermined environmental conditions, and A mask transporting step of taking out the mask from the mask case and placing the mask at the predetermined position, illuminating the mask placed at the predetermined position with light of a predetermined wavelength, and forming the pattern formed on the mask; And a projection exposure step of transferring onto the wafer substrate via a projection optical system.

Further, according to the present invention, a light source for emitting light of a predetermined wavelength, illumination optical means for controlling the light emitted from the light source to predetermined exposure conditions, and a mask for mounting a mask on which a pattern is drawn A projection exposure apparatus comprising: a stage; a wafer stage on which the wafer is mounted; and projection optical means for projecting a pattern drawn on the mask onto the wafer, wherein at least a region surrounding the mask stage and the wafer Environmental control means for independently controlling the area surrounding the stage to a predetermined environmental condition, and a mask housed in a mask case in which the internal environment is maintained at a constant condition is prepared at a position adjacent to the mask stage. Mask preparation means for removing the mask from the mask case while the area surrounding the mask stage is under the predetermined environmental conditions. Put out to provide a projection exposure apparatus comprising; and a mask conveying means for mounting to the mask stage.

Further, in the present invention, the predetermined environmental condition is a condition of an atmosphere surrounding the mask, wherein at least the oxygen concentration is included in the control target and the oxygen concentration is 10 ppm.
It was configured as follows.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments.

(Embodiment 1) FIG. 1 is a diagram showing the configuration of an apparatus for implementing the present invention. In the figure, FIG.
Portions in common with are omitted and shown with the same numbers as those shown in FIG. The optical path for guiding the light 1 emitted from the F2 excimer laser (not shown), the light source aperture 4, the illumination optical systems 5 to 7, and the projection lens 11 are individually filled with nitrogen to have an oxygen concentration of 10 ppm or less. Is controlled.

The mask mounting chamber 107 is filled with nitrogen at atmospheric pressure, and its environment is controlled independently of other optical system members. In these regions, in addition to the oxygen concentration, the concentration of water (H ₂ O) is not more than a predetermined value (here, 30 ppm).
Below). These environmental conditions are constantly monitored by the monitor 108. Exposure light 1
Illuminates the mask 8 via the light source aperture unit 4 and the condenser lenses 5 to 7, and the pattern drawn on the mask 8 is transferred onto the wafer 12 by the image forming action of the projection lens 11.

Prior to mounting the mask 8 on the mask stage 9, the mask 8 is first prepared in a mask preparation chamber 104 adjacent to the mask mounting chamber 107 with the valve 101 in a state 102 of being housed in a dedicated sealing case 103. Is done. Although the pellicle is attached to the mask in the same manner as the conventional mask, the pellicle is omitted in FIG. 1 and only the mask substrate is shown. Although the inside of the mask preparation room 104 is initially filled with the normal atmosphere, the mask storage case 1
When 03 is carried in, vacuum purging and nitrogen filling are repeatedly performed by the nitrogen purging means 105 to perform nitrogen substitution. Then, the oxygen concentration is set to 1 by the oxygen concentration monitor 106.
The valve 101 is opened when it is determined that it has become 0 ppm or less.

At the same time, as shown in FIG. 2, the lid 110 of the mask storage case 103 is opened in accordance with an instruction from the exposure apparatus control system, and the mask transport system member 111 on the exposure apparatus side moves the arrow 11
2, the mask is received in the state 102 in the mask case 103, and the mask is transported onto the mask stage. By the above steps, the oxygen concentration of the atmosphere between the mask and the pellicle was kept at 10 ppm or less, and the mask could be mounted on the mask stage without damaging the pellicle.

On the other hand, the semiconductor wafer 12 is mounted on a wafer stage 13 in an exposure chamber 109 which can individually control the environment. Prior to the mounting, similarly to a normal electron beam lithography apparatus, first, oxygen in the atmosphere is sufficiently removed by evacuation in an adjacent preliminary chamber (not shown) via a valve and an exposure chamber. Thereafter, when nitrogen was filled and the pressure reached the same level as that of the exposure chamber, the valve was opened, and the wafer was transferred onto the wafer stage by a dedicated transfer jig. The interior of the exposure chamber may be replaced with an inert gas such as nitrogen or vacuum.
The oxygen concentration may be lower than a predetermined value.

After positioning the mask and the wafer at predetermined positions by a usual method, the mask pattern was transferred onto the wafer. FIG. 3 shows the flow of the transfer. Step 20 to Step 2
At 5, a mask is prepared and placed on a mask stage. When pattern transfer is started on a new wafer with a mask already placed, the step of preparing this mask is omitted. In step 26, the oxygen concentration in the mask mounting chamber was checked again.

On the other hand, the wafer to be the substrate to be exposed was transported to the exposure chamber through steps 30 to 32. Since the exposure chamber where the wafer stage is placed is filled with nitrogen, the wafer is first put into the exposure preliminary chamber under a normal atmospheric environment, and then the preliminary chamber is replaced with nitrogen, and the oxygen concentration becomes 10 ppm or less. After confirming the above, the wafer was transferred from the preliminary chamber to the exposure chamber.

Steps 27 and 28 are steps for transferring a pattern to a wafer as in the conventional case. That is, the mask 8 is moved or positioned by the mask stage driving means 10 and the wafer 12 is moved or positioned by the wafer stage driving means 14, and the pattern is transferred to the entire surface of the wafer by stepping or synchronous scanning based on an instruction from the control system 16. In particular, the position of the wafer is accurately measured by the laser interferometer 15. Further, data input required by the operator for exposure and grasp of the exposure state are performed via the interface 17.

When the pattern transfer to one wafer is completed, the wafer is discharged, and the next prepared wafer is conveyed to a wafer stage. Step 30-
Reference numeral 32 may handle wafers one by one or a plurality of wafers. When handling wafers one by one, pattern transfer in step 28 and steps 30 to 3
2 so that the wafers can be prepared in parallel.

As described above, in this embodiment, the oxygen concentration in the entire exposure optical system is set to the allowable value of 10 pp without damaging the pellicle which is transparent and thin with respect to the exposure light.
m or less, and the pattern could be transferred with the mask being exchangeable. Therefore, the F2 laser (wavelength =
With the exposure apparatus using 157 nm as a light source, a fine gate pattern 71 and a dense wiring pattern 72 of the order of 0.1 μm as shown in FIG. 7 could be formed with high accuracy.

(Embodiment 2) FIG. 4 shows a mask according to another embodiment of the present invention. The mask substrate 41, the chrome pattern 42 on the mask, and the pellicle frame 44 are the same as the conventional mask (substrate 61, chrome pattern 62, pellicle frame 64) shown in FIG. 6, but the pellicle 43 is conventionally a thin film. On the other hand, in the present invention, the same glass member as the mask substrate was used. It is sufficiently rigid compared to conventional pellicles and does not deform with a slight pressure difference. The region surrounded by the mask substrate, the pellicle frame, and the pellicle is sufficiently purged with nitrogen and hermetically sealed. However, since the glass member is inserted between the mask substrate and the projection lens, spherical aberration occurs. Therefore, a configuration is adopted in which a projection lens for correcting this spherical aberration is mounted.

This mask was placed on the mask stage 9 of the exposure apparatus shown in FIG. 1, and pattern exposure was performed in the same manner as in Example 1. Mask preparation room 1 because the pellicle is rigid
It is not always necessary to seal the mask case when performing a nitrogen purge in the chamber 04. That is, the F2 laser (wavelength = 157 n) is used without the condition of sealing the mask case.
With an exposure apparatus using m) as a light source, a pattern of the order of 0.1 μm as shown in FIG. 7 could be formed with high accuracy.

(Embodiment 3) An embodiment in which a fine gate pattern is formed by overlapping and exposing a plurality of mask patterns will be described. FIGS. 8A and 8B are diagrams showing a part of two mask patterns used for forming a gate pattern. FIG. 8A shows the first mask,
A light-shielding pattern 82 is formed in the transparent area 81 by using a normal mask. On the other hand, FIG. 8B shows a second mask, which is a phase shift mask in which an opening pattern 84 is formed in a light shielding region 83. Opening pattern 84
The numerical value described in indicates the phase of the transmitted light,
Light transmitted through the adjacent openings has a phase difference of 180 degrees.

In this embodiment, a predetermined portion of the mask substrate is dug as shown in FIG. 9 in order to obtain the phase difference of 180 degrees. In the figure, reference numeral 93 denotes a phase shift mask section. That is, in the mask including the mask substrate 91, the light-shielding film 92, and the openings 94, 95, 96, and 97, the mask substrate in the openings 94 and 96 was removed by etching so as to cause a phase difference of 180 degrees. This depth is
Assuming that the refractive index of the mask substrate is n, λ / ｛2 (n−
1) I Here, λ is the exposure wavelength, and in this embodiment, λ is 157 nm.

The mask pattern was transferred onto the wafer in the same manner as in Example 1. The difference from the first embodiment is that two types of patterns are superposed and exposed on the wafer. FIG. 10 shows the flow of pattern transfer. The preparation of the wafer is the same as in the first embodiment.
2 was performed. The first mask shown in FIG.
1 and 112, and pattern exposure was performed on the wafer in steps 113 to 115. After exposing the pattern to all the prepared wafers, in step 116, the first mask was returned to the storage case while keeping the state of the nitrogen purge. The wafer was evacuated for the next exposure.

Next, a second mask shown in FIG. 8B is prepared in Steps 117 and 118, and Steps 119 to 118 are performed.
Pattern exposure was performed on all the wafers evacuated at 21. The order of pattern exposure may be reversed.

FIG. 11A shows the pattern positional relationship when the two masks are overlapped. In the figure, the dashed line corresponds to FIG.
This is a pattern of the phase shift mask shown in (b), and adjacent opening patterns are surely separated by extremely thin dark lines. Therefore, after the development process, as shown in FIG. 11B, the portion sandwiched between the phase shift patterns could be formed as a very thin gate pattern 122 having a thickness of 50 nm.

As described above, a pattern can be formed on the same wafer by replacing the mask while maintaining the environment around the mask. Thereafter, a semiconductor device having a fine gate was manufactured through a normal semiconductor manufacturing process.

In this embodiment, the illumination condition when the mask shown in FIG. 8A is used is σ (coherence factor) = 0.6, and the mask shown in FIG. 8B is used. Was set to σ = 0.4. The illumination condition is not limited to this, and various values of σ can be selected according to the mask pattern.

Various modified illumination conditions such as an annular shape and a quadrupole can be adopted. As the mask, a conventional binary mask and various phase shift masks can be used.

In any of the embodiments, when storing the mask in the mask case, the oxygen concentration between the mask and the pellicle must be 10 ppm or less.
In the description of the embodiment, an example of nitrogen purge is mainly shown, but it is also effective to substitute an inert gas such as argon, helium, or neon instead of nitrogen. In particular, in the case of helium, since the change in the refractive index with respect to the temperature change is sufficiently small, there is a feature that the position measurement of the mask or the wafer can be accurately performed using the interference light. These inert gases may be circulated and used. If necessary, a deoxidizing material or a member for monitoring the oxygen concentration may be provided inside the pellicle frame to maintain the oxygen concentration at a predetermined value or less.

[0041]

According to the present invention, it is possible to move the mask into and out of the exposure optical system purged with nitrogen without damaging the pellicle provided on the mask. Therefore, in a state where the oxygen concentration is sufficiently reduced, the exposure wavelength is 157.
Pattern exposure can be performed using short-wavelength exposure light of about nm or less. As a result, it is possible to manufacture a fine and highly accurate pattern transfer utilizing the effect of the short wavelength and manufacture a highly integrated semiconductor device.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an example of a projection exposure apparatus embodying the present invention.

FIG. 2 is a view showing a specific example of a mask and a mask case according to the present invention.

FIG. 3 is a view showing a flow of an exposure method of the present invention.

FIG. 4 is a diagram showing another specific example of the mask according to the present invention.

FIG. 5 is a diagram showing a configuration of a conventional exposure apparatus.

FIG. 6 is a diagram showing a configuration of a conventional mask.

FIG. 7 is a cross-sectional view of a semiconductor device.

8A is a diagram illustrating a normal mask, and FIG. 8B is a diagram illustrating a phase shift mask.

FIG. 9 is a schematic sectional view showing a phase shift unit.

FIG. 10 is a diagram showing a flow of pattern transfer in a third embodiment.

FIG. 11A shows a case where two masks of FIGS. 8A and 8B are overlapped, and FIG. 11B is a view showing a formed gate pattern.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Illumination light, 2 ... Optical path, 3 ... Dufuser, 4 ... Illumination stop, 5-7 ... Illumination optical system (condenser lens), 8 ... Mask, 9 ... Mask stage, 10 ... Mask stage driving means, 11 ... Reduction projection Lens, 12 ... Wafer, 13 ...
Wafer stage, 14 ... wafer stage driving means,
15 laser interferometer, 16 control system, 17 interface, 41 mask substrate, 42 chrome pattern, 4
3: Glass pellicle, 44: Pellicle frame, 63: Conventional pellicle, 64: Conventional pellicle frame, 81: Transparent area, 82, 83: Light shielding area, 84: Opening pattern, 91
... Mask substrate, 92 ... Light shielding film, 93 ... Phase shift section, 9
4 to 97: opening, 101: valve, 103: mask case, 104: mask preparation chamber, 105: nitrogen purge means, 106: oxygen concentration monitor, 107: mask mounting chamber,
108 monitor, 109 exposure chamber, 110 mask case lid, 122 gate pattern.

Continued on the front page (72) Inventor Toshihiko Tanaka 1-280 Higashi-Koigabo, Kokubunji-shi, Tokyo F-term in Central Research Laboratory, Hitachi, Ltd. DC05

Claims (5)

[Claims] 1. A step of preparing a mask on which a pattern is formed in a mask case in which the internal environment is maintained under a constant condition, and mounting the mask to transfer the pattern onto a wafer substrate. A measuring step of measuring an environment surrounding a predetermined position of the exposure apparatus to be placed; an environment controlling step of controlling the environment to predetermined environmental conditions; and taking out the mask from the mask case under the predetermined environmental conditions. A mask transporting step of placing the wafer at the predetermined position, illuminating the mask placed at the predetermined position with light of a predetermined wavelength, and projecting the pattern formed on the mask through the projection optical system to the wafer substrate. A projection exposure step of transferring onto the top. 2. The method according to claim 1, wherein the predetermined environmental condition is an atmosphere condition surrounding the mask, wherein at least an oxygen concentration is included in a control target, and the oxygen concentration is 10 ppm or less. The projection exposure method as described in the above. 3. A light source for emitting light of a predetermined wavelength, illumination optical means for controlling the light emitted from the light source to predetermined exposure conditions, a mask stage on which a mask on which a pattern is drawn is mounted, and the wafer In a projection exposure apparatus including a wafer stage to be mounted and projection optical means for projecting a pattern drawn on the mask onto the wafer, at least a region surrounding the mask stage and a region surrounding the wafer stage Environmental control means for independently controlling to predetermined environmental conditions, and mask preparation means for preparing a mask housed in a mask case in which the internal environment is maintained under constant conditions at a position adjacent to the mask stage. Taking out the mask from the mask case while the area surrounding the mask stage is under the predetermined environmental condition; Projection exposure apparatus characterized by comprising; and a mask conveying means for placing the di. 4. A mask substrate having a surface on which a pattern is drawn, and a glass member having a predetermined thickness and rigidity provided so as to cover the mask substrate surface,
A projection exposure mask, wherein the space between the mask substrate and the glass member is filled with nitrogen and sealed. 5. A method for manufacturing a semiconductor device, comprising manufacturing a semiconductor device using the projection exposure method according to claim 1.