Patent Application
20120086925
The invention relates to a method for preventing contaminating gaseous substances from passing through an opening of a housing of an EUV lithography apparatus, wherein at least one optical element for guiding EUV radiation is arranged in the housing. The invention also relates to an EUV lithography apparatus for carrying out the method.
In the vacuum system of EUV lithography apparatuses, the optical systems, in particular beam shaping optical unit, illumination optical unit and projection optical unit, are in each case encapsulated in a housing in order that contaminating substances which can form outside the housings e.g. under the action of EUV radiation in exposure operation are kept away from the optical surfaces. At the transitions between the housings, openings for passage of the EUV radiation are provided, at which contaminating substances can enter into the housings, unless suitable countermeasures are implemented.
US 2006/0001958 A1 describes an EUV lithography apparatus wherein a first housing is provided, in which a projection optical unit for imaging a structure on a mask onto a light-sensitive substrate is accommodated, and also a second housing, in which the mask or the light-sensitive substrate is provided. There is a pressure difference between the first and the second housing, wherein the pressure in the first housing is at least a hundred times greater than the pressure in the second housing. By means of the pressure difference, the intention is to bring about a constant gas flow from the first housing into the second housing, in order in this way to avoid the ingress of contaminating substances in the opposite direction. In order to maintain the gas flow or gas curtain, however, a considerable amount of gas is required, which gas can be circulated only by means of pumps having high pumping capacity.
U.S. Pat. No. 6,198,792 has disclosed an EUV lithography apparatus wherein a chamber with a wafer is separated from a projection optical unit arranged in a housing by means of an opening. Situated in the chamber with the wafer there is a device for generating an inert gas flow or inert gas curtain over the surface of the wafer, in order to remove contaminating substances that are released from the wafer during EUV irradiation, by means of the contaminating substances being entrained by the inert gas flow. In this way, the intention is to be able to dispense with the provision of membrane filters at the opening.
US 2006/0268246 A1 has disclosed a lithography apparatus having a gas-purged opening extending between two different regions of the apparatus. A gas supply device supplies the opening with one or more gases selected from a group comprising hydrogen and argon, inter alia. The opening can be configured, in particular, as a tubular passage and a supplied gas flow can be directed counter to contaminating substances which outgas from a wafer.
EP 1 349 010 A1 describes a lithography apparatus wherein a controllable (rotatable) aperture for providing an opening through a barrier between two parts of the apparatus is provided in order to enable a radiation pulse to pass through from the first part of the apparatus into the second part of the apparatus. The controlled aperture closes the opening between the radiation pulses in order to minimize the gas flow between the two parts, and is synchronized with the radiation pulses in order that the latter can pass through the opening of the barrier. In some embodiments, an additional inlet is provided, through which a buffer gas can flow into an interspace between the parts of the apparatus.
An object of the invention is to improve a method and an EUV lithography apparatus of the type mentioned in the introduction such that the passage of contaminations through the opening of a housing can be prevented in a process-reliable manner and with little outlay.
According to one formulation, this object is achieved by a method of the type mentioned in the introduction, comprising: generating at least one gas flow which deflects the contaminating substances, and in particular is directed counter to the flow direction thereof, in the region of the opening, wherein the gas flow and the EUV radiation are generated in pulsed fashion and the pulse rate of the gas flow is defined in a manner dependent on the pulse rate of the contaminating substances released under the action of the EUV radiation, wherein both pulse rates are in particular equal in magnitude, and wherein, in the region of the opening, the gas pulses temporally overlap the pulses of the contaminating substances.
In EUV lithography apparatuses, the EUV radiation is typically generated in pulsed fashion since the high power densities required there cannot be maintained in continuous operation. The inventors have recognized that the release of contaminating substances into the gas phase for example from the photoresist of the wafer takes place under the action of the EUV radiation and thus in pulsed fashion, such that it is not necessary to permanently maintain the gas flow. Rather, the gas flow can likewise be generated in pulsed fashion. The quantity of gas required for this purpose is significantly lower than in the case of a continuously maintained gas flow. Accordingly, the pumping capacity of the pumps that remove the gas flow from the vacuum environment can be significantly reduced, which leads to a considerable reduction of costs.
In this case, the gas pulses can be generated in a delayed fashion with respect to the EUV pulses, wherein the delay time is chosen or set such that, in the region of the opening, the gas pulses temporally overlap the pulses of the contaminating substances. Since the contaminating substances need a certain time to move from the originating location of the contaminations—e.g. the photoresist or the EUV light source—into the region of the opening or to undergo transition to the gas phase under the influence of the EUV radiation, it is necessary to delay the gas pulses with respect to the EUV pulses such that a respective gas pulse in the region of the opening impinges on a respective pressure pulse of the contaminating substances in order to deflect said pressure pulse. In this case, the delay time can be varied, if appropriate, in a manner dependent on parameters of the EUV lithography apparatus which influence the duration of the time of flight of the contaminating substances as far as the opening. By way of example, the delay time can be set e.g. in a manner dependent on the materials used for the coating of the wafer. The dependence of the delay time on the parameters that influence the time of flight can be stored in tables, wherein said dependence was determined experimentally, for example. However, it is also possible to perform regulation of the delay time, e.g. by providing a contamination sensor (e.g. a pressure sensor) outside the housing in the vicinity of the opening. The pulsed gas flow is then activated as soon as the sensor detects the presence of the contaminating substances, e.g. when a threshold value of the measured pressure is exceeded.
In one variant, a pulse duration of the gas pulses is less than 5%, preferably less than 1%, in particular less than 0.5%, of the time interval between two successive pulses of the EUV radiation. As a result, the amount of gas required for deflecting the contaminating substances can be significantly reduced by comparison with a continuous gas flow. In particular, the pulse duration of the gas pulses can also be less than 5 times, preferably less than 3 times, more particularly less than twice, the time duration of the EUV pulses, since the time duration of the contamination pulses is of the order of magnitude of the time duration of the EUV pulses. The amount of gas which is released during an individual gas pulse can in this case be approximately a factor of five to ten greater than the total amount of contaminating substances generated by an EUV pulse.
In a further variant, the momentum of the gas particles contained in the gas flow is chosen to be greater than the momentum of the gaseous contaminating substances. If, in the EUV lithography apparatus, there prevail partial pressures of less than 10−6 mbar of the contaminating substances, on the one hand, and sufficiently high partial pressures of the gas flow, on the other hand, at which the respective gas molecules follow the physical law of laminar flow (Knudsen number, Λ/d<<0.1, Λ=average free path length of the contaminant in the gas flow, d=typical passage cross section or length), then the ingress of contamination molecules, which are typically long-chain molecules, e.g. hydrocarbons, having a high mass, can be effectively prevented by virtue of the generally lighter molecules of the gas flow being generated with high speed and at high pressure. In this way, in the event of a collision (or ideally a large number of collisions) between a gas molecule of the gas flow and a molecule of the contaminating substance, it can be ensured that the latter reverses its flow direction.
In a further variant, the pulsed gas flow is generated in at least one controllable gas valve. In this case, the gas valve can be driven electronically with a high pulse rate (in the range of a few μs), wherein the amount of gas in the gas flow can be set via the pulse duration or the duty ratio between open and closed states of the gas valve. In particular, variable pressure regulation can also be performed, e.g. by setting a control voltage for opening the valve in a suitable manner (in a temporally varying manner). In this case, the gas valve is designed in such a way that it generates a steep pressure pulse upon opening, such that the emerging gas has a high velocity component and hence a high momentum. The pressure difference between the gas reservoir and the pressure in the interior of the EUV lithography apparatus should be as large as possible, in order to generate a steep pressure pulse, that is to say that the gas reservoir connected to the valve should have a pressure of more than 4 bar, typically of 6 to 10 bar or higher. Such a valve can be realized e.g. as a piezo-valve such as is used e.g. for generating metal clusters which are employed e.g. in fundamental research. In this case, a focused laser beam is directed onto a metal lamina and the metal is locally evaporated. The plasma cloud arising as a result of the laser evaporation is then caused to effect clustering by the steep gas pulse of the piezo-valve, since the atomic collision is fostered by the steeply rising gas pulse edge and the formation of clusters of arbitrary size is thus made possible.
It is expedient to arrange the gas valve in the housing in order to keep the contaminating substances away from the housing using the countercurrent principle. However, if appropriate, a pulsed gas flow oriented transversely with respect to the flow direction of the contaminating substances could also be used, which is arranged outside the housing, e.g. as is described in U.S. Pat. No. 6,198,792, which is incorporated by reference in the content of this application. Moreover, if appropriate, gas valves can also be fitted outside the housing and be oriented toward the originating location of the contaminations.
In a further variant, the gas valve or the outlet opening thereof is arranged in a manner oriented toward the opening of the housing and the gas valve is arranged in a manner offset with respect to the opening. The contaminating substances that penetrate into the housing through the opening or an, in particular tubular, passage arranged, if appropriate, in the region of the opening are ideally exposed to a gas flow having a flow direction opposite to the contaminating substances. Since the opening in the housing is arranged in the region of the beam path of the EUV radiation, the gas valve or gas valves generally cannot be arranged directly at the opening, but rather are arranged in a manner offset with respect thereto, wherein the angle of the gas flow or the gas valve with respect to the opening should be chosen to be as small as possible. The geometry of the outlet opening of the gas valve can be chosen such that a desired geometry of the gas flow generated is established. In the case of a round outlet opening, the gas flow is generally conical; in the case of other forms of the outlet opening, e.g. in the case of an elongate rectangular geometry, a correspondingly shaped, flat gas flow is generated.
In particular, it is also possible for a plurality of gas valves to be arranged in a regular arrangement around the opening, in order to obtain metering of the gas flow generated that is as homogeneous as possible and thus a homogeneous pressure distribution into the opening. The number of gas valves is dependent, inter alia, on the size of the opening. A regular or symmetrical arrangement is understood to be an arrangement wherein the number N of gas valves is distributed at an angle of in each case approximately 360°/N along the circumference of the opening and oriented toward the latter. By way of example, it is possible to use four gas valves each arranged at an angle of 90° with respect to one another.
The gas flow contains at least one gas selected from the group comprising: hydrogen (H2), nitrogen (N2), deuterium (D2) and noble gases, in particular helium (He), argon (Ar) and xenon (Xe). These gases are inert gases, wherein the choice of a suitable gas depends, inter alia, on the mass of the contaminating substances. In the case of contaminating substances having large molecular masses, in general use is also made of gases that tend to have larger masses in the gas flow, in order to generate a greater momentum during the collision processes. In particular H2, N2, D2 and He have a low absorption for the EUV radiation, which has a favorable effect on the absorption if these gases are still present in the EUV lithography apparatus when a subsequent EUV pulse is generated.
In one variant, gases contained in the pulsed gas flow are pumped away (virtually completely) before a subsequent pulse of the EUV radiation, in order that absorption of the EUV radiation by the gases contained in the gas flow is prevented as completely as possible.
In a further variant, a static pressure within the housing is chosen to be at least 10 Pa greater than a static pressure outside the opening of the housing. Such a pressure difference suffices to enable a gas flow from the housing through the opening toward the outside, to prevent the ingress of contaminating substances which are not generated by the EUV radiation, even without the generation of the gas pulses. The pressure in the region of the opening, more particularly within a tubular body arranged there, is approximately 3 Pa in the exposure pauses, that is to say between two successive pulses, and can rise to as much as 20 Pa during EUV pulses.
In one variant, for deflecting electrically charged contaminating substances released under the action of the pulsed EUV radiation, an electromagnetic field, more particularly a homogeneous electric field, is generated in pulsed fashion, the pulse rate of which is defined in a manner dependent on the pulse rate of the contaminating substances, wherein both pulse rates are in particular equal in magnitude. The pulses of the electromagnetic field are generally delayed relative to the EUV pulses, wherein the delay time is chosen such that, in the region of the field, the field pulses temporally overlap the pulses of the contaminating substances. For deflection purposes, it is possible to employ electric and/or magnetic fields which are superposed e.g. in sections. A (pulsed) homogeneous electric field is favorable in this case since this can be generated in a particularly simple manner. In this case, the pulsed homogeneous field can be oriented, in particular, transversely with respect to the opening.
A further aspect of the invention is realized in an EUV lithography apparatus, comprising: a light source for generating EUV radiation, at least one housing with at least one optical element for guiding the EUV radiation, wherein the housing has at least one opening through which contaminating substances can pass, at least one gas generating device for generating a pulsed gas flow in the region of the opening, wherein the gas flow deflects the contaminating substances and is in particular directed counter to the flow direction thereof, and a control device for driving the gas generating device with a pulse rate, which is dependent on the pulse rate of the EUV radiation generated in pulsed fashion, wherein both pulse rates are in particular equal in magnitude, and wherein the control device drives the gas generating device in such a way that, in the region of the opening, the gas pulses temporally overlap the pulses of the contaminating substances.
As explained above, the pulsed gas flow can be used to alter the flow direction of the contaminating substances, in order to prevent the latter from entering into the housing or passing through the opening. In order to synchronize the generation of the gas flow with the EUV pulses, the control device can be connected to the EUV light source.
In one embodiment, the control device is designed for driving the gas generating device for delayed generation of the gas pulses relative to the EUV pulses, wherein a, in particular variable, delay time is chosen such that, in the region of the opening, the gas pulses temporally overlap the pulses of the contaminating substances.
A suitable delay time can be set by measuring or calculating the time until the contamination pulse and the gas flow have respectively reached the opening. The value thus determined is used in the control device in order to synchronize the gas flow with the contamination pulses.
In one embodiment, the control device is designed or programmed for driving the gas generating device for generating gas pulses having a pulse duration of less than 5%, preferably of less than 1%, more particularly of less than 0.5%, of a time period between two pulses of the EUV radiation. The time period between two EUV pulses is generally of the order of magnitude of microseconds, e.g. approximately 100 μs, while the individual EUV pulses generally having a pulse duration of approximately 100 ns. The time duration of the contamination pulses is of the same order of magnitude as the duration of the EUV pulses, e.g. 400 to 500 ns. Even given a duration of the gas pulses of only 0.5% of the time duration between the pulses, therefore, the contaminating substances can be virtually completely kept away from the housing.
In one embodiment, the gas generating device has at least one controllable gas valve. Since in general, although it is indeed possible, using the duration of the pulses, to set the amount of gas, it is not possible to set the achievable maximum gas pressure (typically between 3 and 6 bar) at the gas valve, it may be advantageous to use two or more gas valves in order to generate enough gas molecules with a high momentum such as arise directly after the respective gas valve is switched on. In the case where two or more gas valves are used, they can be opened and closed simultaneously, but they can also alternatively be switched with a short time delay, in order that the number of molecules in the gas flows which have a high momentum is distributed better over the duration of the contamination pulse.
In a further embodiment, the gas valve is arranged in the housing. This is the normal case, wherein, as already explained above, the gas valve is typically arranged in a manner oriented toward the opening and offset with respect to the opening outside the beam path of the EUV radiation. In a further embodiment, a plurality of gas valves are arranged in a in particular regular arrangement around the opening, in order to enable the gas flow to be metered as homogeneously as possible.
In one embodiment, the opening is formed at a tubular passage. The passage can be used to concentrate the contaminating substances at the opening within a spatially narrowly delimited region, such that, with the aid of the gas flows, the contaminating substances can be more easily prevented from passing through the opening.
In one development, the tubular passage has a length of more than 2 cm, preferably of more than 5 cm. This is favorable in order that the gas pulse can form a barrier in the tubular passage, as a result of which the contaminating substances can be suppressed or deflected as effectively as possible.
In a further embodiment, the EUV lithography apparatus additionally comprises a generating device for the pulsed generation of an electromagnetic field, in particular a homogeneous electric field, for deflecting electrically charged contaminating substances released under the action of the pulsed EUV radiation, wherein a pulse rate of the field is defined in a manner dependent on a pulse rate of the contaminating substances, and wherein both pulse rates are in particular equal in magnitude. In general, the field is switched on only after a delay time, which takes account of the propagation time of the contaminating substances until reaching the region in which the field is generated.
In one embodiment, the housing contains a projection optical unit for imaging a structure on a mask onto a light-sensitive substrate. The housing of the projection optical unit has a respective opening to the mask and to the substrate for passage of the EUV radiation. Contaminating substances can outgas from the light-sensitive substrate as a result of the EUV radiation, and the same applies to contaminants (“debris”) which are produced, if appropriate, during pulsed operation by the EUV light source itself and which can pass into the region of the mask.
In a further embodiment, the housing has an illumination optical unit for illuminating a structure on a mask. In this case, too, the opening of the housing to the mask or to the module with the EUV light source can be protected against the ingress of contaminations by one or more pulsed gas flows.
The housing with the beam shaping unit, in which the EUV light source is arranged, can also be protected against the ingress of contaminations in the manner described above. Alternatively, it is also possible, in the manner described above, to prevent contaminants generated by the EUV light source (e.g. when a plasma light source is used) from emerging from the housing.
Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can each be realized individually by themselves or as a plurality in any desired combination in a variant of the invention.
The exemplary embodiments are illustrated in the schematic drawing and are explained in the description below. In the figures:
a-d show schematic illustrations of a pulse sequence of the EUV radiation (a), of a contamination pulse (b), of the control voltage of the gas valves from
The radiation treated with regard to wavelength and spatial distribution in the beam shaping system 2 is transferred into the illumination system 3 via an opening 15 on the beam shaping system 2, said illumination system having—by way of example—a first and second reflective optical element 9, 10. The two reflective optical elements 9, 10 are embodied as facet mirrors for pupil shaping and direct the EUV radiation onto a mask 11 as a further reflective optical element, said mask having a structure that is imaged on a demagnified scale onto a wafer 12 by the projection system 4. For this purpose, a third and fourth reflective optical element 13, 14 are provided in the projection system 4. The reflective optical elements 9, 10, 11, 12, 13, 14 respectively have an optical surface 9a, 10a, 11a, 12a, 13a, 14a arranged in the beam path 6 of the EUV lithography apparatus 1. A respective opening 16a, 16b, 17a, 17b for entry/exit for the EUV radiation 6 is formed both at the housing 3a of the illumination system 3 and at the housing 4a of the projection system 4.
A description is given below by way of example, with reference to
The temporal profile when the contaminating substances 18 arise is illustrated below with reference to
In order to prevent the pressure pulse 24 of the contaminating substances 18 from being able to enter into the interior of the housing 4a through the opening 17b, the gas valves 20a, 20b are driven with a pulsed control voltage V, the temporal profile of which is illustrated in
Since, in the EUV lithography apparatus 1, the total pressure or the partial pressures of the contaminating substances 18 and of the gas flows 21a, 21b are chosen such that the latter generate a laminar flow through the opening 17b or the tube 19, the contaminating substances 18 can be kept back from the housing 4a by collisions between the respective gas particles. In this case, the mass mG and the velocity vG of the gas molecules of the gas flows 21a, 21b are chosen such that the momentum pG=mG vG thereof is greater than the momentum pK of the contaminating substances 18, which is composed of mass mK and velocity vK (pK=mK vK). In this way, the flow direction of the contaminating substances 18 can be reversed and the latter can thus be effectively prevented from being able to enter into the housing 4a. In this case, although providing a tube 19 at the opening 17b is favorable, it is not mandatory. However, if the tubular passage 19 is used, it should have a length L of typically more than 2 cm, more particularly of more than 5 cm, such that the gas flows 21a, 21b in the tubular passage 19 can form a barrier keeping the contaminating substances 18 away from the housing 4a as effectively as possible.
In this case, the gas type(s) chosen for the gas flows 21a, 21b and also the background pressure for the gas valves 20a, 20b—typically between 6 and 10 bar—should be adapted to the type or mass and velocity of the contaminating substances 18, such that the condition mG vG>mKVK is met as well as possible. By way of example, hydrogen, heavy water, nitrogen or noble gases such as He, Ne, Ar, Kr, Xe can be used as gases in the gas flows 21a, 21b. As can be discerned in
Typically, a pulse duration TG of the gas pulses 26 is chosen which is less than 5%, preferably less than 1%, more particularly less than 0.5%, of the time interval T1 between two successive pulses 23 of the EUV radiation. More particularly, the time duration TG of an individual gas pulse 26 can be at most five times, preferably at most three times, more particularly at most twice, the time duration of an individual EUV pulse 23.
Although the gases contained in the pulsed gas flow 21a, 21b are generally chosen such that they have only low absorption for EUV radiation, it is favorable if they are removed from the housing 4a and also from the region in front of the opening 17b of the housing 4a before a subsequent EUV pulse 23 is generated. For this purpose, a pump device 30 (cf.
In contrast to the illustration in
In general, a (static) pressure PIN within the housing 4a is chosen to be at least 10 Pa greater than a (static) pressure POUT outside the opening 17b of the housing 4a in order, even in the time periods in which no gas pulses are generated, to avoid ingress of contaminating substances into the housing 4a or to enable a continuous gas flow from the housing 4a through the opening 17b.
As can likewise be discerned in
As in the case of the gas pulses 26, the electric field 29 is also generated with a pulse rate 1/TEL defined in a manner dependent upon the pulse rate 1/T1 of the contaminating substances 18 or 18′, wherein both pulse rates are generally chosen to be equal in magnitude. Moreover, the field pulses are generally generated with a delay time relative to the EUV pulses 23 which corresponds to the time duration required by the contaminating substances 18′ from the wafer 12 to the field 29. Since the path of the contaminating substances 18′ from the wafer 12 to the field 29 is smaller than the path to the opening 17b, the delay time of the electric field pulses is typically somewhat shorter than the delay time TD of the gas pulses 26.
The method described above can be used not just in the case of the exit opening 17b of the housing 4a of the projection system 4, but that it can also be effected at the other openings 15, 16a, 16, 17a of the housings 2, 3, 4. In particular, in this case, the projection system 4 or the illumination system 3 can be protected not only against contaminating substances 18 outgassing from the wafer 12, but also contaminating substances which, depending on the used type of EUV light source 5, are generated by the latter, if appropriate. It is likewise possible to prevent contaminating substances from passing through at the opening 15 of the housing 2a of the beam shaping system 2, wherein, in this case, if appropriate, the suppression of contamination can also be effected in the opposite direction, appropriately, that is to say that gas valves are arranged outside the housing 2a in order to prevent contaminating substances generated by the EUV light source 5 from emerging from the beam shaping system 2.
In all cases described above, a pulsed “gas curtain” can be generated by the pulsed metering of gases, in order to achieve effective avoidance of contamination at the optical surfaces 9a to 14a of the optical elements 9 to 14 of the EUV lithography apparatus 1, without a large amount of gas being required for this purpose. Furthermore, the passage 19 shown in
More or fewer than two gas valves can also be provided at an opening 15, 16a, 16b, 17a, 17b depending on how great the amount of contaminating substances or the momentum thereof turns out to be and depending on the dimensioning of the opening 15, 16a, 17a, 17b. In general, the gas valves are distributed uniformly along the circumference of the opening 15, 16a, 17a, 17b, in order to ensure homogeneous pressure metering. Such a uniform arrangement can be achieved e.g. if a number N of gas valves are distributed at an angle of in each case approximately 360°/N along the circumference. By way of example, for this purpose it is possible to use four gas valves arranged in each case at an angle of 90° with respect to one another.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 016 319.0 | Apr 2009 | DE | national |
This is a Continuation of International Application PCT/2010/001908, with an international filing date of Mar. 26, 2010, which was published under PCT Article 21(2) in German, and the complete disclosure of which, including amendments, is incorporated into the present application by reference. The present application claims the priority of Apr. 6, 2009, of U.S. Provisional Application No. 61/166,975 and also claims the priority of German patent application DE 10 2009 016 319.0 to Apr. 6, 2009. The entire contents of both priority applications are likewise incorporated by reference in their entireties into this present application.
| Number | Date | Country | |
|---|---|---|---|
| 61166975 | Apr 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2010/001908 | Mar 2010 | US |
| Child | 13267663 | US |
