Lithographic apparatus and device manufacturing method

A gas knife configured to dry a surface in an immersion lithographic apparatus is optimized to remove liquid by ensuring that a pressure gradient is built up in the liquid film on the surface being dried.

The present invention relates to a lithographic apparatus and a method for manufacturing a device.

It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.

However, submersing the substrate or substrate and substrate table in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in PCT patent application WO 99/49504, hereby incorporated in its entirety by reference. As illustrated inFIGS. 2 and 3, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side.FIG. 2shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration ofFIG. 2the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated inFIG. 3in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.

There are many circumstances in immersion lithography in which one or more surfaces are covered in immersion liquid. Many of these surfaces must then be cleared of immersion liquid further down the manufacturing process.

Accordingly, it is desirable to provide an immersion apparatus with an effective means to remove immersion liquid from a surface.

According to an aspect of the invention, there is provided a lithographic projection apparatus arranged to project a pattern from a patterning device onto a substrate through a liquid, the apparatus comprising a gas knife configured to provide gas onto a surface, an extractor adjacent the gas knife to remove gas, liquid, or both, and a flow regulator configured to control a gas flow rate out of the gas knife to be within 20% of a gas flow rate into the extractor.

According to an aspect of the invention, there is provided a lithographic projection apparatus arranged to project a pattern from a patterning device onto a substrate through a liquid, the apparatus comprising a gas knife configured to provide gas to a surface, wherein the gas knife comprises an outlet to exit gas, the outlet having a width of between 10 and 50 μm and a length of between 100 and 500 μm.

According to an aspect of the invention, there is provided a lithographic projection apparatus arranged to project a pattern from a patterning device onto a substrate through a liquid, the apparatus comprising a gas knife configured to provide gas to a surface to be dried at an angle to the surface of between 70° and 85°.

According to an aspect of the invention, there is provided a lithographic projection apparatus arranged to project a pattern from a patterning device onto a substrate through a liquid, the apparatus comprising a gas knife configured to provide gas to a surface to be dried, a first extractor, and a second extractor, the first and second extractors being on opposite sides of the gas knife and configured to remove gas, liquid, or both, from the surface.

According to an aspect of the invention, there is provided a lithographic projection apparatus arranged to project a pattern from a patterning device onto a substrate through a liquid, the apparatus comprising a gas knife configured to remove a liquid from a surface, the gas knife arranged such that passage of the liquid is blocked by a formation of a pressure gradient in the liquid.

According to an aspect of the invention, there is provided a device manufacturing method comprising projecting through an immersion liquid a patterned beam of radiation onto a substrate, wherein immersion liquid is removed from a surface by a flow of gas from a gas knife to an extractor positioned adjacent the gas knife, the gas flow rate out of the gas knife being within 20% of the gas flow rate into the extractor.

According to an aspect of the invention, there is provided a device manufacturing method comprising projecting through a liquid a patterned beam of radiation onto a substrate, wherein liquid is removed from a surface by a flow of gas out of an outlet of a gas knife which outlet has a width of between 10 and 50 μm and a length of between 100 and 500 μm.

According to an aspect of the invention, there is provided a device manufacturing method comprising projecting through a liquid a patterned beam of radiation onto a substrate, wherein liquid is removed from a surface by a flow of gas impinging on the surface at an angle of between 70° and 85°.

According to an aspect of the invention, there is provided a device manufacturing method comprising projecting through a liquid a patterned beam of radiation onto a substrate, wherein liquid is removed from a surface by formation of a pressure gradient in the liquid by gas.

According to an aspect of the invention, there is provided a device manufacturing method comprising projecting through a liquid a patterned beam of radiation onto a substrate, wherein passage of liquid is blocked by formation of a pressure gradient in the liquid by gas.

5. DETAILED DESCRIPTION

FIG. 1schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation).

a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

A further immersion lithography solution with a localized liquid supply system is shown inFIG. 4. Liquid is supplied by two groove inlets IN on either side of the projection system PL and is removed by a plurality of discrete outlets OUT arranged radially outwardly of the inlets IN. The inlets IN and OUT can be arranged in a plate with a hole in its center and through which the projection beam is projected. Liquid is supplied by one groove inlet IN on one side of the projection system PL and removed by a plurality of discrete outlets OUT on the other side of the projection system PL, causing a flow of a thin film of liquid between the projection system PL and the substrate W. The choice of which combination of inlet IN and outlets OUT to use can depend on the direction of movement of the substrate W (the other combination of inlet IN and outlets OUT being inactive).

Another solution which has been proposed is to provide the liquid supply system with a liquid confinement structure which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. The liquid confinement structure is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). In an embodiment, a seal is formed between the liquid confinement structure and the surface of the substrate. The seal may be a contactless seal such as a gas seal. Such a system is disclosed in United States patent application publication US 2004-0207824 and European patent application publication EP-A-1,420,298, each hereby incorporated in its entirety by reference, and illustrated inFIG. 5.

As shown inFIG. 5, a liquid supply system is used to supply liquid to the space between the projection system and the substrate. The reservoir10forms a contactless seal to the substrate around the image field of the projection system so that liquid is confined to fill a space between the substrate surface and a final element of the projection system. The reservoir is formed by a liquid confinement structure12positioned below and surrounding the final element of the projection system PL. Liquid is brought into the space below the projection system and within the liquid confinement structure12. The liquid confinement structure12extends a little above the final element of the projection system and the liquid level rises above the final element so that a buffer of liquid is provided. The liquid confinement structure12has an inner periphery that at the upper end, in an embodiment, closely conforms to the shape of the projection system or the final element thereof and may, e.g., be round. At the bottom, the inner periphery closely conforms to the shape of the image field, e.g., rectangular though this need not be the case.

The liquid is confined in the reservoir by a gas seal16between the bottom of the liquid confinement structure12and the surface of the substrate W. The gas seal is formed by gas, e.g. air, synthetic air, N2or an inert gas, provided under pressure via inlet15to the gap between liquid confinement structure12and substrate and extracted via outlet14. The overpressure on the gas inlet15, vacuum level on the outlet14and geometry of the gap are arranged so that there is a high-velocity gas flow inwards that confines the liquid.

There are several instances in an immersion lithographic apparatus where drying of a surface previously covered in immersion liquid is needed. For example, after imaging of a substrate, it is advantageous to completely dry the substrate. A gas knife can be used to dry previously wet surfaces.

Several of the ways of containing immersion liquid in a lithographic projection apparatus between the projection system PL and the substrate W are related to the so-called localized area solution in which a surface of the substrate which is, in plan, smaller than the total surface of the substrate is wetted with immersion liquid. A liquid confinement system is used to contain the immersion liquid to only the localized area. A difficulty with an embodiment of such a localized solution is that during scanning of the substrate under the projection system a seal is formed between the liquid confinement system and the substrate W which is contactless. One way of ensuring that immersion liquid does not escape from the liquid confinement system and thereby contaminate other parts of the apparatus is to provide a gas knife around the periphery of the liquid confinement system to dry the surface of the substrate of any residual immersion liquid which has not been removed or contained by other components of the liquid confinement system. Indeed, a gas knife may be formed as part of a liquid confinement system, such as in a liquid confinement structure which surrounds the space containing the immersion liquid and forms a seal to the substrate.

In a liquid confinement structure12illustrated inFIG. 6, an extractor31extracts liquid from the localized area to the left hand side of the Figure through gauze30. The extractor31may extract both liquid and gas or only liquid. A recess32is provided radially outwardly of the extractor31and a gas knife33is provided radially outwardly of the recess32. The gas knife forms a jet of gas34which is used to dry the surface of the substrate W. In a similar embodiment illustrated inFIG. 7, a modification of the recess32is made such that a passage40exists which is open to a gas source, for example the atmosphere, such that a flow of gas from the passage40radially outwardly to a passage50which is connected to a low pressure source is created. The gas knives disclosed can be used for general drying applications and in particular for gas knives which may form part of a liquid confinement structure12.

A gas knife typically works on the principle of inducing a shear force on any residual liquid on a surface through a gas flow moving over the surface of the residual liquid. This requires an extremely high flow of gas out of the gas knife and also requires a narrow gap between the substrate W and the outlet of the gas knife (i.e. in the case of a liquid confinement structure12, between the bottom surface of the liquid confinement structure12and the top surface of the substrate W).

By adjusting the configuration of the gas knife, a zone of increased pressure can be created and a liquid can be prevented from passing that zone. The zone is created by having a “curtain” of gas aimed at the surface. This forms a high pressure liquid barrier and a pressure gradient can be formed in the liquid and it is the formation of this pressure gradient rather than the presence of a drag force at the liquid surface which is effective to dry the surface by keeping the liquid to one side of the gas knife during movement of the substrate W or other surface underneath the gas knife i.e. the pressure gradient is the dominant force. A pressure gradient can be formed at gas speeds of between 50 m/s and 200 m/s. A controller is provided to regulate the gas flows and the height of the gas knife. A shear force mechanism can be arranged to remove the liquid.

By removing liquid by the formation of a pressure gradient, relatively little gas may be used compared to when the surface is dried by shear forces and a relatively large distance between the outlet of the gas knife and the surface being dried may be possible.

The gas knife will be described below in relation to drying of a substrate W, in particular to the use of the gas knife in a liquid confinement structure. However, the gas knife can be used for any other application of drying a surface, perhaps for drying the top surface of the substrate table WT which may also have immersion liquid on it from time to time or any other component or indeed may be used for drying a substrate or another component at a position in the immersion lithographic apparatus perhaps other than under the projection system PL.

FIG. 8illustrates schematically a gas knife generally labeled33and a recess or extractor generally labeled32. The gas knife comprises an outlet310in the form of a nozzle. Several variables are illustrated inFIG. 8. These are the width K of the nozzle outlet310of the gas knife (clearly the gas knife nozzle310is formed as a slit which has a length and a width, the slit extending into and out of the page inFIG. 8). An angle A of the nozzle of the gas knife is illustrated which is the angle which the nozzle makes to a line perpendicular to the surface through which the nozzle exits and perpendicular to the surface W to be dried. When the angle A is zero the gas knife nozzle is pointing directly down. Tb is the nozzle length and G is the distance between the nozzle outlet and the surface W (in the embodiment where the gas knife is part of a liquid confinement structure, this is the so-called ‘ride height’ of the liquid confinement structure). A distance Li is the distance between the gas knife nozzle outlet310and the extractor32, V is the velocity of the surface W relative to the gas knife33, Qs is the gas flow through the gas knife33, and Qe is the gas flow through the extractor32.

As can be seen, the gas knife33comprises a chamber320. Gas enters the chamber320and the size of the chamber relative to the outlet dampens any possible pressure fluctuations before the gas exits through nozzle310. A number of discrete inlets can be provided into the chamber320for the introduction of gas into the chamber320.

The extractor32has a chamber410similar to that of chamber320of the gas knife33. The extractor32also has an inlet420which provides a passage between the bottom surface of the gas knife and the chamber410. However the narrow passage with inlet420can be omitted so that there is no narrowing between the chamber410and the inlet420in the bottom surface; the gap G can provide enough resistance for flow equalization.

The forces which act on liquid on the surface W are drag forces which occur due to a shear force at the gas/liquid interface, pressure forces which are forces due to local pressure gradient, virtual mass or inertia forces which are forces needed to accelerate a given volume of liquid, and general body forces. The last two components are related to movement of the surface whereas the first two components are determined by the gas knife design.

During optimization of the gas knife, the influence of evaporation of liquid (e.g., water) should be accounted for. Evaporation depends on a number of factors including temperature, humidity of gas, and gas velocity (which depends on nozzle design). All may play an important role. Liquid evaporation is an undesirable phenomenon because it can lead to the cooling of the surface. Such cooling may deleteriously influence other things, most notably the substrate W surface shape, the temperature of components in the apparatus, and/or the temperature of the immersion liquid (thereby changing the index of refraction of the immersion liquid). Evaporation may be reduced or minimized by supplying gas with a relative high humidity to the gas knife33. The pressure drop over the length of the outlet or nozzle310of the gas knife33should also be controlled to avoid cooling. In an embodiment using water and air, the pressure drop should not be greater than 0.2 bar otherwise the gas humidity will be decreased too much. Thus, the gas knife performance should be optimized for liquid removal as well as reducing or minimizing the pressure drop in the nozzle310.

A local pressure build-up between the surface to be dried, the gas knife and the extractor of 0.05 bar gauge pressure or higher is sufficient for a flow regime of liquid being removed to be driven by a pressure gradient in the liquid rather than by shear forces. A local pressure build-up of 0.1 or even 0.2 bar gauge may be provided. If this can be achieved, in an embodiment using water, a residual liquid layer thickness of much less than 1 μm is possible (simulations and/or practical tests have shown a residual water layer thickness of between 200 and 400 nm is possible). There may be a negative relative pressure under the exhaust area which exerts an attractive force on the surface W towards the gas knife33. This will need to be compensated for in the control dynamics of the apparatus if problems with imaging focus or of crashes is to be avoided and in any case the negative relative pressure should be reduced or minimized.

The table below shows the effect of various parameters of the gas knife on the relative pressure under the extractor (dPe) and on the pressure drop over the nozzle length (dPn).

Escape of gas from the gas knife33out from under the assembly can be deleterious to the performance of interferometers because the interferometer beams may pass through gas that is badly conditioned. One way to substantially prevent this is to arrange for the exhaust flow Qe to be within about 20%, 10% or 5% of the gas knife flow Qs. The variables in the table marked with * are effective in this regard. A further advantage is that liquid can be collected from the other side of the gas knife33from the extractor32in this way. A disadvantage of this embodiment is that a deep under pressure is needed for this so that forces exerted on the substrate W and substrate table WT are high and may lead to deformations of the substrate W and any sensor on the substrate table WT.

A further exhaust34may be provided on the other side of the gas knife33to the extractor32. This is illustrated in dashed lines inFIG. 8. The design of the further exhaust34may be the same as that of the extractor32. This may reduce the relative under pressure in the exhaust region and may therefore be desirable. The provision of two exhausts does not lead to loss of performance. In this embodiment the gas flow moves in two directions (radially inwardly and outwardly) from the gas knife. However, as it is the pressure gradient in the liquid and not the gas flow over the liquid which is applying the removing force to the liquid, the removal of liquid is substantially not deleteriously affected.

The smaller the gap size G the more efficient extraction may be. This is because as the gap size G increases, a larger flow of gas and a thinner nozzle is needed to achieve the required pressure build-up but this is at the expense of an increased pressure drop over the nozzle length. However, decreasing the nozzle length leads to a narrowing of the nozzle and can lead to too high a negative pressure under the extractor. In an embodiment, the width of the nozzle may be 10 to 50 μm or between 25 and 35 μm with a length Tb of between 100 and 500 μm. In order to have a well defined gas flow out of the nozzle, a ratio of nozzle length Tb to nozzle width K between 3 to 1 and 20 to 1 may be provided. If the gap G is too large, the gas flow may diverge and the speed of the gas flow may be too low. A gap size G of between 50 and 300 μm may be suitable. An advantage of a pressure gradient flow regime over a shear flow regime is that height (i.e. gap size G) sensitivity is less.

The surface W should be moved in direction V illustrated inFIG. 8such that a point on the substrate passes first under the extractor32before passing under the gas knife33.

In order to reach a given gas speed (for example between 50 and 200 m/s) the flow rate out of the nozzle is proportional to the nozzle width K and the length of the gas knife (in the direction perpendicular to the plane ofFIGS. 6 and 7). The flow rate out of the gas knife should typically be between 25 and 250 liters per minute. This means that the gas flow rate out of the gas knife should be between 75 and 750 liters per minute per meter length of the gas knife (with a nozzle width K of between 10 and 50 μm).

In an embodiment, the gas knife may be between 0.2 and 8 mm or 1 and 8 mm away from the extractor (distance Li). This range allows the establishment of a pressure zone to block the passage of liquid as well as shear action from gas passing over the surface of the liquid to move the liquid towards the extractor. If the distance Li is too short, the shear force can be hard to generate and the liquid is not moved towards the extractor.

In an embodiment, it is advantageous to angle the nozzle away from the vertical position in either direction (i.e. to arrange for the angle A to not be equal to zero). This helps in stabilizing the pressure zone which is formed by the gas knife33. It is also possible to angle the gas exiting the nozzle310by balancing Qs and Qe accordingly and this achieves the same advantages. If Qs and Qe are equal, or very similar, the flow of gas out of nozzle310does not deviate much from vertical. In an embodiment, a range of angle A is 5 to 20°. This means that it is arranged for gas to be blown onto the surface W at an angle of between 70 and 85° to the surface. If the angle of the gas exiting the nozzle is controlled by varying Qe and Qs, Qe is between 0.4 to 0.45 Qs. The angle can also be varied by changing the length Li of the lands between the nozzle and the inlet to the extractor32and the length Lo between the nozzle and the inlet to the further extractor34or the end of the assembly. Arranging for Li to be between 0.5 and 0.8 Lo should achieve an exit angle of between 5 and 20° off vertical.

In European patent application publication EP 1420300 and United States patent application publication US 2004-0136494, each hereby incorporated in their entirety by reference, the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two tables for supporting a substrate. Leveling measurements are carried out with a table at a first position, without immersion liquid, and exposure is carried out with a table at a second position, where immersion liquid is present. Alternatively, the apparatus has only one table.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive and reflective optical components.

One or more embodiments of the invention may be applied to any immersion lithography apparatus, in particular, but not exclusively, those types mentioned above and whether the immersion liquid is provided in the form of a bath or only on a localized surface area of the substrate. A liquid supply system as contemplated herein should be broadly construed. In certain embodiments, it may be a mechanism or combination of structures that provides a liquid to a space between the projection system and the substrate and/or substrate table. It may comprise a combination of one or more structures, one or more liquid inlets, one or more gas inlets, one or more gas outlets, and/or one or more liquid outlets that provide liquid to the space. In an embodiment, a surface of the space may be a portion of the substrate and/or substrate table, or a surface of the space may completely cover a surface of the substrate and/or substrate table, or the space may envelop the substrate and/or substrate table. The liquid supply system may optionally further include one or more elements to control the position, quantity, quality, shape, flow rate or any other features of the liquid.