CN113939773A - Lithographic apparatus - Google Patents

Abstract

A protection device for an optical system of a lithographic apparatus, the optical system employing electromagnetic radiation having a predetermined wavelength, the protection device comprising a film that is transparent to the radiation of the predetermined wavelength and the film is positioned adjacent to a component of the optical system during use of the lithographic apparatus.

Description

Lithographic apparatus

Cross Reference to Related Applications

This application claims priority to european application 19179946.9 filed on 13.6.2019, the entire contents of which are incorporated herein by reference.

Technical Field

The present invention relates to a lithographic apparatus. The invention has particular, but not exclusive, application in connection with EUV lithographic apparatus and EUV lithographic tools.

Background

A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, to manufacture Integrated Circuits (ICs). The lithographic apparatus may, for example, project a pattern from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on the substrate.

The wavelength of radiation used by the lithographic apparatus to project a pattern onto a substrate determines the minimum size of features that can be formed on the substrate. Lithographic apparatus using EUV radiation (i.e. electromagnetic radiation having a wavelength in the range 4nm to 20 nm) may be used to form smaller features on a substrate than lithographic apparatus using DUV radiation (e.g. having a wavelength of 193 nm).

As the size of features to be formed by lithography decreases, the performance requirements of all aspects of the lithographic apparatus become increasingly stringent. Therefore, the accuracy and precision of sensors used to measure parameters of a lithographic apparatus during calibration or operation must be improved.

Disclosure of Invention

According to a first aspect of the invention, there is provided a protection device for an optical system of a lithographic apparatus, the optical system employing electromagnetic radiation having a predetermined wavelength, the protection device comprising a film that is transparent to the radiation of the predetermined wavelength and that is positioned adjacent to a component of the optical system during use of the lithographic apparatus.

According to a second aspect of the invention, there is provided a method of manufacturing a device using a lithographic apparatus, the method comprising:

providing a membrane to protect components of an optical system of a lithographic apparatus, the optical system using electromagnetic radiation to measure a parameter of the lithographic apparatus; and

measuring the parameter using the optical system while the electromagnetic radiation passes through the film.

Features of different aspects of the invention may be combined with features of other aspects of the invention.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which:

FIG. 1 is a schematic view of a lithographic system including a lithographic apparatus and a radiation source;

FIG. 2 is a schematic side view of an arrangement for protecting a sensor component of the lithographic apparatus of FIG. 1;

fig. 3 is a schematic view of a part of the arrangement in fig. 2 seen from below; and

fig. 4 is a schematic diagram of an arrangement for feeding a protective film.

Detailed Description

FIG. 1 is a schematic diagram of a lithography system. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate a beam B of Extreme Ultraviolet (EUV) radiation. The lithographic apparatus LA comprises: an illumination system IL; a support structure MT configured to support a patterning device MA; a projection system PS; and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident on the patterning device MA. The projection system is configured to project a radiation beam B (which has now been patterned by patterning device MA) onto a substrate W. The substrate W may include a preformed pattern. In this case, the lithographic apparatus aligns the patterned radiation beam B with a pattern that has been previously formed on the substrate W.

The source SO, the illumination system IL, and the projection system PS can all be constructed and arranged SO that they can be isolated from the external environment. A gas (e.g., hydrogen) at a pressure below atmospheric pressure may be provided in the radiation source SO. A vacuum may be provided in the illumination system IL and/or the projection system PS. A small amount of gas (e.g., hydrogen) at a pressure substantially below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

The radiation source SO shown in fig. 1 is of a type that may be referred to as a Laser Produced Plasma (LPP) source. Laser 1 (laser 1 may be, for example, CO)₂Laser) is arranged to deposit energy via a laser beam 2 into a fuel, such as tin provided from a fuel emitter 3(Sn). Although reference is made to tin in the following description, any suitable fuel may be used. The fuel may for example be in liquid form and may for example be a metal or an alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, for example in the form of droplets, along a trajectory towards the plasma formation zone 4. The laser beam 2 is incident on the tin at the plasma formation zone 4. A plasma 7 is generated at the plasma formation region 4 due to the deposition of laser energy onto the tin. During the excitation and recombination of ions of the plasma, radiation comprising EUV radiation is emitted from the plasma 7.

EUV radiation is collected and focused by a near-normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multilayer structure arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength (e.g. 13.5 nm)). The collector 5 may have an elliptical configuration with two elliptical foci. The first focus may be at the plasma formation region 4 and the second focus may be at the intermediate focus 6, as discussed below.

In other embodiments of a Laser Produced Plasma (LPP) source, the collector 5 may be a so-called grazing incidence collector configured to receive EUV radiation at a grazing incidence angle and focus the EUV radiation at an intermediate focus. The grazing incidence collector may, for example, be a nested collector comprising a plurality of grazing incidence reflectors. The grazing incidence reflectors may be arranged axially symmetrically about the optical axis O.

The radiation source SO may comprise one or more contaminant traps (not shown). For example, a contaminant trap may be located between the plasma formation region 4 and the radiation collector 5. The contaminant trap may be, for example, a rotating foil trap, or may be any other suitable form of contaminant trap.

The laser 1 may be separate from the radiation source SO. In this case, the laser beam 2 may be delivered from the laser 1 to the radiation source SO by means of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optical components. The laser 1 and the radiation source SO may together be regarded as a radiation system.

The radiation reflected by the radiation collector 5 forms a radiation beam B. The radiation beam B is focused at a point 6 to form an image of the plasma formation region 4, which serves as a virtual radiation source for the illumination system IL. The point 6 onto which the radiation beam B is focused can be referred to as an intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near the opening 8 in the enclosing structure 9 of the radiation source.

The radiation beam B from the radiation source SO enters an illumination system IL, which is configured to condition the radiation beam. The illumination system IL may comprise a facet field mirror device 10 and a facet pupil mirror device 11. The faceted field mirror device 10 and the faceted pupil mirror device 11 together provide a radiation beam B having a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident on the patterning device MA, which is held on the support structure MT. The patterning device MA (which may be a mask, for example) reflects and patterns the radiation beam B. The illumination system IL may comprise other mirrors or devices in addition to or instead of the facet field mirror device 10 and the facet pupil mirror device 11.

After reflection from the patterning device MA, the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 configured to project the radiation beam B onto a substrate W held by the substrate table WT. The mirrors 13, 14 forming the projection system may be configured as reflective lens elements. The projection system PS may apply a reduction factor to the radiation beam to form an image having features smaller than corresponding features on the patterning device MA. A reduction factor of 4 may be applied, for example. Although the projection system PS has two mirrors 13, 14 in fig. 1, the projection system may comprise any number of mirrors (e.g. six mirrors).

The lithographic apparatus can be used, for example, in a scanning mode, in which the support structure (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto the substrate W (i.e., dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the scaling and image reversal characteristics of the projection system PS. The patterned beam of radiation incident on the substrate W may comprise a band of radiation. The band of radiation may be referred to as an exposure slit. During a scanning exposure, the movement of the substrate table WT and the support structure MT may be such that the exposure slit travels over the exposure field of the substrate W.

The radiation source SO and/or the lithographic apparatus shown in FIG. 1 may include components that are not shown. For example, a spectral filter may be provided in the radiation source SO. The spectral filter may substantially transmit EUV radiation but substantially block other wavelengths of radiation, such as infrared radiation.

In other embodiments of the lithography system, the radiation source SO may take other forms. For example, in an alternative embodiment, the radiation source SO may comprise one or more free electron lasers. The one or more free electron lasers may be configured to emit EUV radiation, which may be provided to one or more lithographic apparatuses.

Lithographic apparatus have been described that use EUV radiation to expose a substrate to enable smaller features to be formed without having to use techniques such as double patterning and self-assembly. However, to reduce the size of the features that can be formed (or in other words, to increase resolution), merely reducing the wavelength of the exposure radiation is not sufficient; while other subsystems of the lithographic apparatus have to be upgraded. In particular, improved control of the lithographic apparatus requires an improved sensor system providing input to the control system.

For example, the lithographic apparatus may use an interferometric displacement measurement system to measure the position of the substrate table, and hence the position of a substrate held on the substrate table. The interferometric system may use reflectors (mirrors) mounted on the substrate table WT to reflect the coherent beam of electromagnetic radiation, so that the interferometric system forms the interference fringes with the reference beam. Movement of the substrate table WT changes the path length of the beam reflected from the mirrors and so causes the interference fringes to move in a manner which can be detected by the light sensors. Interferometric displacement measurement systems may also use fixed mirrors located near the substrate table. For example, to measure the displacement of the substrate table in the Z direction (which is parallel to the optical axis of the projection system), a fixed mirror may be provided adjacent to the projection system and facing the upper surface of the substrate table. While the electromagnetic radiation used by the interferometric displacement measuring system conveniently has a wavelength in the visible range, other wavelengths are possible.

The substrate table of the lithographic apparatus may also comprise components of other sensor systems using electromagnetic radiation in the visible and other wavelength ranges. For example, a reference target (often referred to as a fiducial) for the alignment system may be provided in the upper surface of the substrate table. The alignment system may use one or more beams of different wavelengths (colors). In addition, various sensor elements may be mounted in or on the substrate table to make measurements using the exposure radiation beam B or other radiation beams. Examples include: transmissive and reflective image sensors, aberration sensors, energy sensors.

The inventors have determined that a possible source of error in measurements obtained by a sensor system (e.g., an interferometric displacement measurement system) is contamination of components of the sensor system (e.g., mirrors). In an EUV lithographic apparatus, a substrate is exposed while in a vacuum or low pressure environment. This means that contaminants released in gaseous form from the photosensitive layer travel further from the substrate than they would travel in a lithographic apparatus in which the substrate is exposed to an environment at atmospheric pressure.

In particular, the inventors have determined that contaminants that accumulate on components of measurement systems that use electromagnetic radiation (e.g., light) can lead to long term drift effects as well as random errors. Contamination of components interacting with electromagnetic radiation (which may be referred to as optical components), particularly on surfaces on which the electromagnetic radiation is incident, is most likely to cause errors. The optical components may include mirrors, windows, gratings, lenses, fiducials. Errors may be caused by a variety of mechanisms, such as scattering or absorption of electromagnetic radiation, refraction, and/or a change in the length of the optical path.

Cleaning contaminated surfaces is an obvious solution to this problem. However, such cleaning requires the lithographic apparatus to be opened, which then requires that a lengthy process of evacuating and repairing the lithographic apparatus must be undertaken before the lithographic apparatus can be used again. When the lithographic apparatus is opened for other reasons, cleaning may be performed to remove such contamination, but this still increases downtime. A general maintenance schedule for a lithographic apparatus may not be suitable for removing contaminants from the optical components of the sensor system. Cleaning may not always be effective because only limited solvents and non-abrasive materials can be used, otherwise the optical components may be damaged. Therefore, it is desirable to prevent the optical member from being contaminated.

According to an embodiment of the invention, a transparent film is provided, which film is arranged adjacent to a surface of a component of a sensor system in a lithographic apparatus at risk of contamination. Example as shown in fig. 2, fig. 2 is a side view showing a protective film 100, the protective film 100 being disposed parallel to and close to the operating surface of the interference mirror 101. An interference mirror 101 is arranged adjacent the projection system PS, opposite the surface of the substrate table WT. The interferometric mirror 101 forms part of an interferometric displacement measuring system, and is used, inter alia, to measure changes in position of the substrate table WT in the Z direction and/or rotation about an axis parallel to the X and Y directions (commonly referred to as Rx and Ry). The measurement beam of the interferometer may be directed onto the interference mirrors by 45 ° mirrors (not shown) mounted on the edges of the substrate table WT.

Contaminants released in gaseous form from a photosensitive layer (e.g., resist) on the substrate W will be deposited on the film 100 rather than on the interference mirror protected by the film 100. Thus, the need to clean protected optical components is reduced or eliminated.

Fig. 3 is a view from below of the interference mirror 101, showing that the film 100 only covers some parts of the interference mirror 101. These protected portions of the interference mirror are the portions closest to the projection system PS. These parts are most contaminated because the gas released from the photosensitive layer increases when the projection beam B is incident on these parts. In other embodiments, the optic is entirely covered by a film. However, if the contamination of the component to be protected is not uniform, this may save cost and space in the lithographic apparatus to protect only those parts of the component that are subjected to the heaviest contamination load. If the membrane 100 covers only a portion of the part, differences in response of the sensor system between those parts of the part that are covered and those parts that are not covered may need to be corrected for by calibration.

The film of the present invention may be considered to have a similar function as a pellicle for protecting a reticle or mask. However, it is generally required that the pellicle be spaced from the protected reticle by a distance sufficient to ensure that any particulate contamination deposited on the pellicle is out of focus or not focused. This requirement does not apply to the present invention, since the inventive membrane may be used in a sensor system in which the measuring beam is not focused and/or contaminants are deposited in the form of a thin layer rather than particles.

In an embodiment of the invention, the membrane is replaced when it has been contaminated. Contaminant levels can be measured to determine when to replace the membrane or simply to replace the membrane according to a predetermined schedule (e.g., after a fixed period or number of exposures). In case the membrane is cheap and easy to replace, it is preferable to use a planning scheme, since no means for measuring contaminants need to be provided. The scheduled replacement may be triggered automatically or manually.

Fig. 4 depicts a mechanism for allowing the membrane to be automatically replaced without opening the vacuum chamber of the lithographic apparatus. The film is provided in the form of a long tape 100a disposed on a supply roller 102 and taken up by a take-up roller 103. Guides (e.g., rollers) 104 ensure that the film is properly positioned in front of the interference mirror 101. An actuator (e.g., a motor) 105 drives the supply roll and/or take-up roll according to a schedule to advance the film a predetermined distance so that a clean area of the film is in front of the component to be protected. An active brake may be provided to maintain a constant tension in the protective film.

A variety of materials can be suitably used as the film, for example, a plastic film such as PET (polyethylene terephthalate), BoPET (biaxially oriented polyethylene terephthalate), BoPP (biaxially oriented polypropylene), or BoN (biaxially oriented nylon).

The desired properties of the film are: the film is substantially transparent at least to the wavelength of the radiation used in the sensor system to which the component to be protected belongs. Desirably, the film has a transmittance of greater than 90%, more desirably greater than 95%. In some cases, depending on the type of sensor, it is desirable for the film to have a uniform optical thickness so as not to introduce optical path length variations. Desirably, the amount of change in optical thickness does not exceed 10%, more desirably does not exceed 1%. If the sensor system uses multi-wavelength or broadband radiation, it may be desirable for the film to have a uniform refractive index at the wavelength of interest. The thickness of the film may be in the range of 1 μm to 10 μm.

In an embodiment of the invention, the film is in the form of a coating applied on the surface of the component to be protected. The coating is selected to be more easily removed from the component than the contaminants. For example, the coating can be easily dissolved in ultrapure water. The coating may be a gel or aerogel. The coating may be in the form of a patch that is easily peeled from the surface to be protected.

Desirably, the membrane is vacuum compatible, e.g., with minimal gas release. In most cases, the radiation intensity used in the sensor system is not large, and thus heat resistance and physical robustness are not particularly important. However, additional robustness may be required if the membrane is close to or in the path of the projection beam at any point.

Desirably, the film is formed of a material having an affinity for a substance released in a gaseous form from the photosensitive layer. For example, the film may adsorb or absorb substances released in gaseous form from the photosensitive layer. For example, the film of material may react with substances released in gaseous form from the photosensitive layer to form other substances that either remain in gaseous form for removal from the lithographic apparatus or adhere to the material. For example, the membrane may catalyse a reaction which converts a substance released in gaseous form from the photosensitive layer into another substance which either remains in gaseous form for removal from the lithographic apparatus or adheres to the material.

The component to be protected may be any form of optical component, such as a mirror, window, grating, lens, reference in any sensor system using electromagnetic radiation.

Although specific reference may be made in this text to the embodiments of the invention in the context of a lithographic apparatus, the embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterning device). These devices may be referred to generally as lithographic tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength in the range 4nm to 20nm (e.g. in the range 13nm to 14 nm). The wavelength of the EUV radiation may be less than 10nm, for example in the range 4nm to 10nm, such as 6.7nm or 6.8 nm.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, Liquid Crystal Displays (LCDs), thin film magnetic heads, etc.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative, and not restrictive. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims (16)

1. A protection device for an optical system of a lithographic apparatus, the optical system employing electromagnetic radiation having a predetermined wavelength, the protection device comprising a film that is substantially transparent at least to radiation of the predetermined wavelength and that is positioned adjacent to a component of the optical system during use of the lithographic apparatus.

2. The protective device according to claim 1, which is formed of a material having an affinity for a substance released in a gaseous form from the photosensitive layer.

3. A protective device according to claim 1 or 2, wherein the membrane is formed from a plastics material.

4. A protection device according to claim 1, 2 or 3, wherein the membrane has an optical thickness variation in its portion traversed by the electromagnetic radiation, which is less than 5%, desirably less than 1%.

5. A protective device according to any one of claims 1 to 4 wherein the membrane is spaced from the component.

6. The protective device according to any one of claims 1 to 4, wherein the membrane is in contact with the component.

7. The protective device according to any one of the preceding claims, further comprising a membrane replacement mechanism for replacing a contaminated portion of the membrane with a clean membrane.

8. The protective device of claim 7, wherein the membrane is elongated and the membrane replacement mechanism comprises:

a supply roller for storing a clean film;

a take-up drum for storing the contaminated film; and

an actuator for advancing the film from the supply roller to the take-up roller.

9. The protective device according to claim 7 or 8, wherein the membrane replacement mechanism is configured to replace the portion of the membrane according to a predetermined schedule.

10. A protection device according to any one of the preceding claims, wherein the membrane is positioned between the component and a substrate table so as to protect the component from substances released in gaseous form from a photosensitive layer provided on a substrate held on the substrate table.

11. The protection device according to any one of the preceding claims, wherein the component is a reflector, in particular in an interferometric displacement measuring system.

12. A protection device according to any one of the preceding claims, wherein the component is a sensor.

13. A lithographic apparatus, comprising:

an optical system comprising a component; and

a protection device for protecting the component according to any one of the preceding claims.

14. The lithographic apparatus of claim 13, further comprising:

an illumination system configured to illuminate a patterning device with EUV radiation; and

a projection system configured to project radiation patterned by the patterning device onto a substrate.

15. A method of manufacturing a device using a lithographic apparatus, the method comprising:

providing a membrane to protect components of an optical system of a lithographic apparatus, the optical system using electromagnetic radiation to measure a parameter of the lithographic apparatus; and

measuring the parameter using the optical system while the electromagnetic radiation passes through the film.

16. The method of claim 15, further comprising: replacing the contaminated portion of the membrane with a clean membrane.

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