DE102022200539A1 - Optical system for projection lithography - Google Patents

Abstract

An optical system for projection lithography has a light source for illuminating light (3), which can be tuned within a tuning bandwidth, for illuminating an object field (4) in which an object (10) to be imaged can be arranged. Imaging optics (7) are used to image the object field (4) in an image field (8), in which a substrate (11) can be arranged, along an imaging beam path. The optical system has a plurality of lenses (21 to 23, 28, 29, 31 to 39) with base bodies made of exactly one lens material. Spaces (45) between optical components of the imaging optics (7) that are adjacent in the imaging beam path are filled with a gas. The light source is signal-connected to a tuning controller (43) for specifying an actual illumination wavelength within the tuning bandwidth. The tuning controller (43) has a signal connection to a pressure sensor (46) for measuring an actual pressure of the gas in at least one of the intermediate spaces (45). The tuning control (43) is designed as a control device in such a way that it tracks the actual illumination wavelength as a function of the measured actual pressure of the gas. The result is an optical system whose operating costs are reduced without the imaging quality of the imaging optics suffering as a result.

Description

The invention relates to an optical system for projection lithography. The invention also relates to a projection exposure system with such an optical system, a method for producing a micro- or nanostructured component, and a micro- or nanostructured component produced using such a method.

An optical system of the type mentioned is known from U.S. 9,235,136 . Other optical systems of the type mentioned are known from DE 103 04 599 A1 and from the EP 1 855 160 A2 .

A certain type of such optical systems is optimized for use with a light source designed as a mercury vapor lamp. Imaging optics of such a known optical system uses helium as the flushing gas.

It is an object of the present invention to further develop an optical system of the type mentioned at the outset in such a way that its operating costs are reduced without the imaging quality of the imaging optics suffering as a result.

According to the invention, this object is achieved by an optical system having the features specified in claim 1 .

According to the invention, it was recognized that when imaging optics are used in which the base bodies of all lenses are made of exactly one lens material, imaging errors caused by pressure fluctuations in an intermediate space gas with a corresponding dispersion can be compensated for by appropriate tracking an actual illumination wavelength can be achieved with sufficient compensation quality. A space gas with a correspondingly higher dispersion can then be used, so that in particular helium can be dispensed with as the space gas.

A proportionality factor of a Lorentz-Lorenz approximation for a pressure dependency of the refractive index (dn/dp) can be greater than 50 ppm, can be greater than 100 ppm, can be greater than 150 ppm, can be greater than 200 ppm and in particular can be greater than 250ppm. In particular, this proportionality factor can be greater than that of helium, which is 31 ppm. These figures apply to a pressure sensitivity of the refractive index at 22°C, 950 mbar and at a light wavelength of 365 nm.

The space gas can be a purge gas.

Nitrogen as space gas according to claim 2 is inexpensive. For nitrogen, the aforementioned proportionality factor of the pressure dependence of the refractive index is 266 ppm.

A laser light source according to claim 3 has turned out to be particularly suitable for the tunable light source for the illuminating light.

This applies in particular to a solid-state laser according to claim 4, with which high illumination wavelength stability can be achieved.

An Nd-based laser according to claim 5 can be provided with a high output power in the range of more than 100 W with sufficient spectral quality.

This applies in particular to an Nd-YAG laser according to claim 6, which can have output powers in the range of several kW.

By trebling the frequency according to claim 7, when using a fundamental wavelength of the laser in the range of 1 µm, an illumination wavelength in the range of 350 nm can be used, so that designs of imaging optics can be used, which are particularly suitable for the i-line of a mercury vapor based light source were designed.

An optical system according to claim 8 enables the object field to be illuminated in a targeted manner, in particular with a predetermined distribution of intensity and/or angle of illumination over the object field.

The advantages of a projection exposure system according to claim 9, a production method according to claim 10 and a microstructured or nanostructured component according to claim 11 correspond to those which have already been explained above with reference to the imaging optics according to the invention.

In particular, a semiconductor component, for example a memory chip, can be produced.

• 1 schematisch eine Projektionsbelichtungsanlage für die Mikrolithographie einschließlich eines optischen Systems, und
• 2 eine abbildende Optik als Teil des optischen Systems der Projektionsbelichtungsanlage zur Abbildung eines Objektfeldes in ein Bildfeld.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawing. In this show:

• 1 schematically a projection exposure system for microlithography including an optical system, and
• 2 imaging optics as part of the optical system of the projection exposure system for imaging an object field in an image field.

A projection exposure system 1 for microlithography has a light source 2 for illumination light or imaging light 3. The light source 2 is a DUV or UV light source that emits light in a wavelength range, for example between 193 nm and 400 nm, especially in the range between 300 nm and 400 nm. These and also other wavelengths that can be used in microlithography (e.g. DUV, deep ultraviolet) and for which suitable laser light sources and/or LED light sources are available (e.g. 365 nm, 248 nm, 193 nm, 157 nm, 129 nm, 109 nm) are possible for the illumination light 3 guided in the projection exposure system 1. A beam path of the illumination light 3 is in the 1 shown very schematically.

The light source 2 is a laser, for example. The laser is a solid-state laser. The solid-state laser is a neodymium-based laser. The neodymium-based laser is an Nd:YAG laser. A fundamental wavelength of the Nd-YAG laser is frequency-tripled to generate the illumination light 3 . The illuminating light 3 has a useful wavelength of 355 nm.

Illumination optics 6 are used to guide the illuminating light 3 from the light source 2 to an object field 4 in an object plane 5. With a projection optics or imaging optics 7, the object field 4 is imaged in an image field 8 in an image plane 9 with a predetermined, possibly anamorphic reduction scale .

To simplify the description of the projection exposure system 1 and the various designs of the projection optics 7, a Cartesian xyz coordinate system is given in the drawing, from which the respective positional relationship of the components shown in the figures results. In the 1 the x-direction runs perpendicularly to the plane of the drawing and into it. The y-direction is to the left and the z-direction is up.

The object field 4 and the image field 8 are rectangular. Alternatively, it is also possible to construct the object field 4 and image field 8 in a curved or curved manner, that is to say in particular in the form of a partial ring. The object field 4 and the image field 8 have an xy aspect ratio greater than 1. The xy aspect ratio can also be 1, particularly in the case of a wafer stepper. The object field 4 therefore has a longer object field dimension in the x-direction and a shorter object field dimension in the y-direction. These object field dimensions run along the field coordinates x and y.

Together with the light source 2 and the illumination optics 6, the imaging optics 7 forms an optical system of the projection exposure system 1. For the projection optics 7, in FIG 2 illustrated embodiment are used, which is explained in more detail below.

The image field 8 has an x extent of 26 mm, for example, and a y extent of 10 mm, for example. The y extent can also be greater than 10 mm. Smaller y-extensions are also possible, for example a y-extension of 2 mm.

The image plane 9 is arranged parallel to the object plane 5 in the projection optics 7 . In this case, a section of a mask 10 coinciding with the object field 4, which is also referred to as a reticle, is imaged. The reticle 10 is supported by a reticle holder 10a. The reticle holder 10a can be displaced by a reticle displacement driver 10b. The reticle 10 can be a reticle that transmits the illumination light 3 or, as in FIGS 1 and 2 indicated to be a reflective reticle. A substrate material of the reticle 10 can be quartz glass.

The projection optics 7 image the surface of a substrate 11 in the form of a wafer, which is carried by a substrate holder 12 . The substrate holder 12 is displaced by a wafer or substrate displacement drive 12a.

In the 1 1 is shown schematically between the reticle 10 and the projection optics 7 a bundle of rays 13 of the illumination light 3 entering the latter and between the projection optics 7 and the substrate 11 a bundle of rays 14 of the illumination light 3 emerging from the projection optics 7 . An image field-side numerical aperture (NA) of the projection optics 7 is in the 1 not reproduced to scale.

The projection exposure apparatus 1 is of the scanner type. Both the reticle 10 and the substrate 11 are scanned in the y-direction during operation of the projection exposure apparatus 1 . A stepper type of the projection exposure apparatus 1 is also possible, in which a gradual displacement of the substrate 11 in particular in the y-direction takes place between individual exposures of the substrate 11 . These shifts can be synchronized to each other by appropriate control of the displacement drives 10b and 12a.

2 shows the optical design of a first embodiment of the projection optics 7. Components corresponding to those described above with reference to FIG 1 have already been explained bear the same reference numbers and will not be discussed again in detail.

Is shown in the 2 the beam path of three individual beams, those of two in the 2 object field points spaced apart from one another in the y-direction. Main rays 19 are shown, which run through the center of a pupil in a pupil plane of the projection optics 7, as well as an upper and a lower coma ray 20, i.e. rays that go through the upper or lower edge of the pupil, of these two object field points. the 2 1 shows an imaging beam path of the imaging optics 7 by means of these individual beams 19, 20. The projection optics 7 are catadioptric. Alternatively, the projection optics 7 can also be designed as a purely dioptric system, in particular as a purely lens objective. Figures through 5 show examples of this DE 103 04 599 A1 .

In the imaging beam path of the useful light 3, the object field 4 is initially followed by three lenses 21, 22 and 23. Each of the three lenses has a displacement manipulator 24, 25, 26 for displacing the lens along the z-axis, ie along the optical axis 2. Depending on the design of the projection optics 7, these manipulators can also be omitted. A displacement of the reticle 10 along the z-axis is also possible with the object displacement drive 10b. A displacement of the substrate 11 along the z-axis is also possible with the substrate displacement drive 12a.

A plane-parallel plate 27 is arranged downstream of the lens 23 in the imaging beam path of the useful light 3 and is arranged downstream of the object plane 6 in the region of a first pupil plane of the projection optics 7 . Two further lenses 28 , 29 are arranged downstream of the plane-parallel plate 27 in the imaging beam path of the useful light 4 . The lens 28 is in turn connected to a z-displacement manipulator 30 . Depending on the design of the projection optics 7, this manipulator can also be omitted.

In the projection optics 7, the lens 29 is followed by a mirror group with two mirrors M1, M2, which are numbered according to their impingement in the order of the imaging beam path. The two mirrors M1 and M2 are concave mirrors.

In the beam path between the last lens 29 in front of the mirror group and the mirror M1 is an intermediate image plane of the projection optics 7. In the beam path between the two mirrors M1 and M2 there is another pupil plane of the projection optics 7. In the beam path of the useful light 3 after the mirror M2 is another one Intermediate image plane of the projection optics 7 arranged.

Further lenses 31, 32, 33, 34, 35, 36, 37, 38 and 39 of the projection optics 7 are arranged in the imaging beam path of the useful light 3 after this further intermediate image plane. A further pupil plane 40 of the projection optics 11 is arranged in the beam path between the lenses 36 and 37 . There, a diaphragm 40a ensures that the pupil is delimited at the edge.

The lenses 31 and 36 are in turn connected to a z-displacement manipulator 41, 42, respectively. Depending on the design of the projection optics 7, these manipulators can also be omitted.

The last lens 39 in front of the image field 8 is an immersion lens. A liquid layer, for example a water layer, is arranged between an exit surface of this last lens 39 and the wafer 11 . Depending on the design of the projection optics 7, such an immersion lens can also be omitted.

A numerical aperture of the projection optics 7 on the image field side is 1.35. The magnification is β = -0.25. The projection optics 7 are thus reduced in size by a factor of 4. Depending on the design of the projection optics 7, there can also be a smaller numerical aperture on the image field side, in particular a numerical aperture on the image side smaller than 1. Depending on the design of the projection optics 7, a different imaging scale can also be used.

Optical design data of the projection optics 7 are known from U.S. 9,235,136 .

In practice, the projection optics 7 can be accommodated in the projection exposure system 1 in such a way that the z-axis runs vertically. In this case, the image plane 9 is generally arranged below the object plane 5 .

The base bodies of all lenses 21 to 23, 28, 29 and 31 to 39 of the imaging optics 7 are made of exactly one lens material, so that the base bodies of all these lenses are made of the same lens material. This lens material can be SiO ₂ as in the exemplary embodiment U.S. 9,235,136 or another material, for example a crown glass, a flint glass or a crystal material such as Cal act cium fluoride or sapphire. The type of material for the lens base bodies is specified in particular as a function of the wavelength of the illumination light used.

The other optical components of the imaging optics 7, which are passed through by the illumination light 3 in the beam path between the object field 4 and the image field 8, for example the plane-parallel plate 27, are made of the same material as the lenses with regard to their basic bodies.

The light source 2 can be tuned within a tuning bandwidth, so that a useful wavelength of the illumination light 3, ie an actual illumination wavelength, can be preset in an adjustable manner. To tune the light source 2, a wavelength manipulation device can be used, which is basically known from FIG U.S. 9,235,136 .

The tuning bandwidth of the light source 2 can be in the range of 1 nm. Depending on the pressure range used for the gas in the intermediate spaces 45 and the material used for the optical components of the projection optics 7, it is decided which tuning bandwidth of the light source 2 is required. This tuning bandwidth can also be less than 1 nm and can be, for example, in the range of 500 μm, 300 μm or else in an even smaller bandwidth range. The tuning bandwidth is regularly greater than 25 pm.

In principle, the tuning bandwidth can also be even smaller, but is regularly greater than 1 pm.

Alternatively, an excimer laser, for example an XeF laser, can also be used as the light source.

Optical surfaces of the optical components of the imaging optics 7 through which the illumination light 3 passes each have an antireflection coating to optimize transmission of these optical components. The mirrors M1, M2 each have a coating that is highly reflective for the illumination wavelength of the illumination light 3.

The light source 2 has a signal connection with a tuning controller 43 for specifying the actual illumination length within the tuning bandwidth.

The imaging optics 7 is housed in an optics tube 44, the inner, the optical components facing boundary wall in the 2 is indicated by dashed lines. In this case, a minimum necessary limitation is indicated by dashed lines so that no illuminating light is cut off. In practice, the optics tube 44 is generally more generously delimited towards the outside. In each case in the beam path of the imaging optics 7, adjacent optical components delimit interspaces 45 together with the respective inner wall section of the optics tube 44. These interspaces 45 are each filled with a gas. This gas can be a purge gas. In the imaging optics 7, all spaces 45 between the respective adjacent optical components 21 to 23, 27 to 29, M1, M2, 31 to 39 in the imaging beam path are filled with the same gas. The gas is nitrogen. Alternatively, another gas is also possible. The gas can also be ambient air.

The tuning controller 43 is in signal communication with a pressure sensor 46 for measuring an actual pressure of the gas in at least one of the intermediate spaces 45 . Such a pressure sensor 46 is shown schematically in the space between the mirror M1 and the lens 31. FIG. Another such pressure sensor 46 is shown schematically in space 45 between lenses 35 and 36 . The respective pressure sensor 46 can be arranged in the respective space outside the beam path, as in the example of the pressure sensor 46 in the space between the lenses 22 and 23 in FIG 2 implied. Depending on the design of the imaging optics 7 and the optics tube 44, the respective space can also communicate in a pressure-equalizing manner with a space section within the optics tube 44 in which the pressure sensor 46 associated with this space is arranged.

The tuning controller 43 is designed as a control device in such a way that it tracks the actual illumination wavelength of the light source 2 as a function of the actual pressure measured via the at least one pressure sensor 46 . A refractive index change in the gas caused by an actual pressure change and thus a change in a refractive index difference when passing through a respective optical surface of one of the optical components of the imaging optics 7 is compensated for by a corresponding change in the illumination wavelength using the tuning control, so that a Imaging performance of the imaging optics 7 remains constant regardless of fluctuations in the actual pressure.

A tracking of the actual lighting wavelength depending on the measured actual pressure by the tuning control can be done using a previously specified calibration table, where the respective actual pressure is assigned an actual lighting wavelength to be controlled, which is for the respective The optimal imaging result of the imaging optics 7 is generated with the actual pressure.

When the wave function of the image is broken down via the beam path of the imaging optics 7 into a series development of Zernike polynomials, the contributions of the Zernike polynomials Z2/3, Z4 and Z9 that dominate with regard to imaging errors can be corrected. The result of the correction or the compensation via the regulated tuning control can be that these error contributions are reduced by more than 10%, by more than 20% or by more than 50%.

The projection exposure system 1 is used as follows to produce a microstructured or nanostructured component: First, the reflection mask 10 or the reticle and the substrate or the wafer 11 are provided. A structure on the reticle 10 is then projected onto a light-sensitive layer of the wafer 11 using the projection exposure system 1 . By developing the light-sensitive layer, a microstructure or nanostructure is then produced on the wafer 11 and thus the microstructured component. In particular, a microchip, for example a memory chip, can be produced.

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US9235136 [0002, 0040, 0042, 0044]
• DE 10304599 A1 [0002, 0031]
• EP 1855160 A2 [0002]

Claims (11)

Optical system for projection lithography - with a light source (2) for illuminating light (3) that can be tuned within a tuning bandwidth for illuminating an object field (4) in which an object (10) to be imaged can be arranged, - with imaging optics (7) for imaging the object field (4) in an image field (8), in which a substrate (11) can be arranged, along an imaging beam path, - with optical components (21 to 23, 27 to 29, M1, M2, 21 to 39), having a plurality of lenses (21 to 23, 28, 29, 31 to 39), wherein the base body of all lenses (21 to 23, 28, 29, 31 to 39) are made of exactly one lens material, - wherein intermediate spaces (45) between adjacent optical components (21 to 23, 27 to 29, M1, M2, 21 to 39) of the imaging optics (7) in the imaging beam path are filled with a gas, - wherein the light source (2) is signal-connected to a tuning controller (43) for specifying an actual illumination wavelength within the tuning bandwidth, - wherein the tuning control (43) is in signal connection with a pressure sensor (46) for measuring an actual pressure of the gas in at least one of the intermediate spaces (45), the tuning control (43) being designed as a control device in such a way that it tracks the actual illumination wavelength depending on the measured actual pressure of the gas. Optical system after claim 1 , characterized in that the gas is nitrogen. Optical system after claim 1 or 2 , characterized in that the light source (2) is a laser. Optical system after claim 3 , characterized in that the laser is a solid-state laser. Optical system after claim 4 , characterized in that the solid-state laser is an Nd-based laser. Optical system after claim 4 , characterized in that the Nd-based laser is an Nd:YAG laser. Optical system according to one of claims 3 until 6 , characterized in that the laser is designed in such a way that the illumination wavelength is a frequency-tripled fundamental of the laser. Optical system according to one of Claims 1 until 7 , characterized by an illumination optics (6) for guiding the illumination light (3) from the light source (2) towards the object field (4). Projection exposure system for projection lithography with an optical system according to one of Claims 1 until 8th . Method for producing a structured component with the following method steps: - providing a reticle (10) and a wafer (11), - projecting a structure on the reticle (10) onto a light-sensitive layer of the wafer (11) using the projection exposure system claim 9 - Generating a microstructure and/or nanostructure on the wafer (11). Structured component, manufactured using a process according to claim 10 .

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