Lithographic apparatus

A vacuum operated lithographic apparatus includes a vacuum housing for providing a vacuum environment to a support constructed to support a patterning device, or a substrate table, or a projection system, or any combination thereof. An interior of the vacuum housing includes a plurality of transport circuits for transporting fluids and/or electrical signals for use in a first process mode for lithographic processing. At least one of the transport circuits is adapted to transport energy towards the interior of the vacuum housing to stimulate outgassing in the vacuum housing for use in a second process mode for bringing the lithographic apparatus into a vacuum operating condition.

FIELD

The present invention relates to a lithographic apparatus, and in particular to a vacuum operated lithographic apparatus.

BACKGROUND

In certain classes of lithographic apparatus, especially in lithographic apparatus operative in the EUV (extreme ultraviolet) area of the electromagnetic spectrum, a desire exists to carry out the lithographic process in vacuum conditions to optimize and/or enable process conditions for the lithographic process. To this end, large parts of the apparatus are contained in a vacuum housing that is subject to vacuum pressure. In this housing, the lithographic process is carried out, specifically, the substrate is irradiated in a vacuum environment. While a substantial amount of equipment is often needed to carry out this lithographic process, specifically, at least one of a support constructed to support a patterning device, the patterning device being capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto a target portion of the substrate, in a vacuum environment, a desire exists to keep the parts exposed to vacuum pressure levels to a minimum. Indeed, the larger the exposed vacuum surface area is, the larger is risk of entrapping contaminants, such as water particles or hydrocarbons. In particular, water has a tendency to stick to surfaces and is often difficult to evacuate. The presence of such contaminants may cause serious shortening of performance and service lifetime of machine parts, and optical elements in particular.

However, the nature of these apparatus is such that it is difficult to reduce the effective area of surfaces exposed to the vacuum environment because of numerous other constraints. For example, because of image resolution requirements, a variety of telemetry and sensor elements are present that cannot easily be reduced or prepared for vacuum conditions. Also, in vacuum lithographic environments, various parts of the machine may be subject to different vacuum regimes, where it is also possible that a controlled leakage of service gases, for example, for creating a optimal lithographic process conditions, be present.

All these factors may contribute to serious challenges for bringing the lithographic apparatus into a vacuum operating condition. One of the main challenges is to bring the down-time of such machines to a minimum, in order to reduce costs. A conventional approach to achieve this is to heat the vacuum housing of the machine in which the lithographic apparatus is housed, in particular, the substrate table and/or the projection system. Furthermore, in order to minimize vacuum exposed areas, the tendency is to position heating elements outside the housing and to “bake-out” the vacuum exposed parts in the preparation process of bringing the lithographic apparatus into vacuum operating condition.

SUMMARY

It is desirable to provide a vacuum operated lithographic apparatus in which downtime problems may be further minimized and in which an effective area of vacuum exposed parts of the lithographic apparatus is kept to a minimum.

In an embodiment, there is provided a vacuum operated lithographic apparatus. The apparatus includes a support constructed to support a patterning device. The patterning device is capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam. The apparatus also includes a substrate table constructed to hold a substrate, a projection system configured to project the patterned radiation beam onto a target portion of the substrate, and a vacuum housing for providing a vacuum environment to the support, or the substrate table, or the projection system, or any combination thereof. An interior of the vacuum housing includes a plurality of transport circuits for transporting fluids and/or electrical signals for use in a first process mode for lithographic processing. At least one of the transport circuits is adapted to transport energy towards the interior of the vacuum housing to stimulate outgassing in the vacuum housing for use in a second process mode for bringing the lithographic apparatus into a vacuum operating condition.

The invention provides an elegant solution to the “bake-out” problem by using mainly already present transport circuits in an alternative way to transport energy towards the interior of the vacuum housing.

In an embodiment, there is provided a method of preparing a vacuum lithographic apparatus for vacuum process conditions. The apparatus includes a vacuum housing for providing a vacuum environment to a substrate table and/or to a projection system. The method includes transporting energy via a transport circuit towards an interior of the vacuum housing to stimulate outgassing in the vacuum housing during a process mode for bringing the vacuum housing into a vacuum operating condition.

In an embodiment, there is provided a lithographic apparatus vacuum housing for providing a vacuum environment to a component within a lithographic apparatus. The vacuum housing includes a plurality of transport circuits for transporting fluids and/or electrical signals for use in a first process mode for lithographic processing. At least one of the transport circuits is adapted to transport energy towards an interior of the vacuum housing for use in a second process mode for bringing the lithographic apparatus into a vacuum operating condition.

DETAILED DESCRIPTION

As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask). Alternatively, the apparatus may be of a transmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may also be of a type in which at least a portion of the substrate may be covered by a fluid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. An immersion fluid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in fluid, but rather only means that fluid is located between the projection system and the substrate during exposure.

The depicted apparatus may be used in at least one of the following example modes:

FIG. 2shows a first embodiment of the invention in which a vacuum lithographic apparatus1is operated. In a first process mode, the vacuum lithographic apparatus1is active in a normal lithographic process mode, where a lithographic process is carried out in vacuum operating conditions. In this respect, the term vacuum refers to the normal vacuum pressure ranges that are used, which are pressures at least considerably lower than atmospheric pressure. In these vacuum conditions, at least the partial pressures of contaminants should be kept to an absolute minimum; for example, an indicative pressure for water is less than 10−5Pa, and for hydrocarbons is even as little as less than 10−7Pa. For providing such a vacuum environment, a vacuum housing2is provided and at least the support (not illustrated inFIG. 2), or the substrate table WT, or the projection system PS, or any combination thereof, are kept in vacuum conditions. In this respect, it is noted that subsections of this vacuum environment may be subject to other vacuum conditions, or may be altogether sealed off from the vacuum environment provided by housing2. In this normal lithographic process mode illustrated inFIG. 2, a variety of subsystems are kept stable, in particular, thermally stable, for providing optimum process conditions. To this end, the projection optical system PS is provided with a coolant circuit5, and also the substrate table WT, in particular, the stage6carrying the substrate table, is cooled by another coolant circuit7. Generally, these circuits are separated since the amount of thermal energy generated in these various sections may differ. For example, the amount of energy generated in the stage6is often larger than the amount of energy generated in the projection optical system, in particular, in the mirrors present therein. Also, the stability requirements of various subsections may differ, thereby leading to different heat exchange units8,9having differing heat exchanging characteristics. Furthermore, to provide a barrier against temperature fluctuations from the outside, the entire housing2is cooled, or more precisely, kept thermally stable by thermostabilizing fluids for stabilizing the temperature inside the housing2. To this end, the fluids are transported in a circuit10that is provided near the wall of the housing2, usually on the outside thereof.

According to the invention as depicted inFIG. 3, in a second process mode for bringing the vacuum housing in a vacuum operating condition, instead of removing thermal energy from the lithographic system, thermal energy is transported into the interior of the vacuum housing to stimulate outgassing in the vacuum housing. Especially by using existing transport circuits5,7,10, by reversing the energy flow to transport thermal energy towards the interior of the housing2, the temperature of the lithographic system1may be elevated for providing bake-out conditions, while the vacuum environment is kept substantially free from additional heating elements for heating the interior of the vacuum housing2.

In particular, a bake-out temperature lies in the range of 40-150° C., depending on particular details of the lithographic system1. To provide such temperature elevation, heaters11are connected to the thermostabilizing circuits5,7,10as a temporary way to heat; alternatively, they may form an integral part of the circuits5,7,10. In the illustrated example, the stage thermostabilizing circuit7and the circuit10near the wall of the housing2are connected. However, it will be understood that other combinations and connections are possible and fall within the scope of the invention. Normally, in the first process mode depicted inFIG. 2, a number of heat exchanging elements8,9are connected to the circuit. Depending on particular details, however, in the second mode, these heat exchanging elements are better shut off from the circuit, since these are designed to provide minimal fluctuations of less than 0.5 mK/5 min, especially the heat exchangers9for the projection system PS are very sensitive. The circuit used in the second process mode may seriously damage such exchange elements8,9. To this end, in the circuit, valve members12may be provided to shut off the flow towards circuit parts that are otherwise used in the first process mode. Furthermore, valve members12may be arranged to shut off the heat exchanging unit8from the circuit in the second process mode or to partly shut the circuit in the second process mode for selectively supplying energy towards the interior of the vacuum housing2. Thus, in particular embodiments, the valves12may be arranged to combine or split a plurality of thermostabilizing circuits as illustrated schematically by dotted lines13inFIG. 2.

Furthermore, preferably, the heater units11are controllable to elevate the temperature in the housing in a predefined way. If the temperature is elevated too drastically, this may damage the fragile components of the projection system PS by uncontrollable thermal deformation. While conventionally, for preparing the lithographic apparatus1for process conditions, this bake-out process may take as much as four days, by the arrangement according to the inventive embodiment ofFIG. 2, it is feasible that the bake-out time may be reduced to as little as ten hours.

In a further embodiment of the invention depicted inFIG. 4, other transport circuits are adapted as well to transport energy towards the interior of the housing in order to provide accelerated bake-out for bringing the vacuum housing2in a vacuum operating condition. In particular, these transport circuits include an electrical circuit for generating heat to bring the vacuum housing in vacuum operating condition at a predetermined second temperature elevated with respect to the working temperature. For example, in one embodiment, the circuit includes cabling elements14, which may be partly freely suspended in the vacuum housing2. These cabling elements14may (re)absorb contaminants quickly, even after a bake-out preparatory process when some cabling is (re) mounted in the vacuum housing. Because after some preparation, the lithographic system1as a whole is preferably not resubject to elevated temperatures, for example, since some magnetic parts (not illustrated) may loose their magnetic properties or other damage may be done due to elevated temperatures, a problem exists of preparing the cables14for vacuum conditions. To this end, additional resistive elements15may be included in the circuit, in particular, in the cabling elements14, which may be heated electrically. It is noted that these elements may be included in existing wiring if the presence thereof may be accounted for, or do not affect the electrical characteristics of the circuit when operative in the normal lithographic processing mode. For example, signal circuits, which only account for very limited electrical currents in the normal processing mode, may be adapted to function as heating circuits, for example, by incorporating an additional power supply and a load wire in the signal circuit. For example, in the cabling parts, a resistive load wire15may be present that is heated by a high frequency voltage supply16. Other configurations include a double wire in the cabling or a single wire that uses structural parts for return currents. In still another configuration, the shielding of cable parts may be used as a conductor as well. In this embodiment also, the power supply may be controllable to elevate the temperature in the housing in a predefined way. Further the electrical circuit may include switches (not illustrated) for redefining the electrical current in the circuit in the first and second process modes. Such switches may be arranged to partly shut the electrical circuit in the second process mode for selectively supplying energy towards the interior of the vacuum housing. Also, the switches are arranged to combine or split a plurality of electric circuits. In certain embodiments, only the cabling elements14will be heated for bringing the lithographic apparatus1in process conditions. Since the heating is done local by thermoelectric elements15, other sensitive parts of the system1need not be removed, which may substantially reduce down-time. Furthermore, the local heating may speed up the outgassing process of non-metal parts, in particular, plastic parts of the cable14, so that down-time may be further reduced, and less pumping may be needed to evacuate contaminants from vacuum housing.

Another embodiment of the invention uses local heating elements17that are integrated in lightweight materials, for example, a carbon fiber enforced plate18used in the wafer stage6or other moving parts of the lithographic apparatus or epoxy materials that include such a circuit. Such electrical heating circuits may be used to assist the bake-out and to accelerate the process for bringing the lithographic apparatus in process conditions.

In still another embodiment of the invention depicted inFIG. 4, other forms of energy than thermal energy are used to transport energy towards the interior of the vacuum housing. In particular, in one embodiment, a gas supply circuit19is provided that is used for providing dry gas to supply kinetic energy to absorbents. In a preferential embodiment, the gas supply circuit19is operative in the first process mode for stabilizing the temperature inside the housing at a predetermined working temperature, in particular, the gas supply circuit19functions as a backfill gas supply circuit20for stabilizing a backside of substrate to be patterned in a lithographic process. Thus, in the second mode, for bringing the apparatus in vacuum operating conditions, the backfill gas circuit20is used to supply dry air or dry nitrogen having a partial pressure for water of less then 10−5Pa. It is noted that this backfill gas circuit20is movable within the housing2, which may provide additional benefits for defining the flow of contaminants towards a pump21. In the (near) molecular flow regime, such dry gas may be used to provide a better definition to the exit path of contaminants towards the pump21, which may be otherwise trapped to other surfaces present in the vacuum housing. In this respect, while bringing the apparatus to vacuum pressure, instead of only vacuum pumping, a dry gas24is also supplied, which may be used to provide a “rinsing” of the system1in a particular pumping phase of the preparation process mode. After the system1has been rinsed for a while, to remove further contaminants25, the dry gas supply26may be stopped, after which the system may be brought more easily to the desired vacuum pressure level.

Likewise, as in the embodiments explained above, in the gas supply embodiment, the gas circuit19may include valve members for redefining the flow of the circuit in the first and second process modes.