Patent Application
20250174452
The techniques disclosed herein relate to a residual gas analyzer, especially for analyzing a residual gas in a microlithography projection exposure apparatus. The disclosed techniques also relate to a projection exposure apparatus having a residual gas analyzer and to a method of residual gas analysis.
Microlithography projection exposure apparatuses are utilized for the production of integrated circuits with particularly small structures. A mask (i.e., a reticle) exposed to very short-wave deep ultraviolet or extreme ultraviolet radiation (DUV or EUV radiation, referred to herein collectively as “very short-wave UV radiation”) is imaged onto a lithography object in order to transfer the mask structure to the lithography object.
The projection exposure apparatus includes multiple mirrors from which the radiation is reflected. The mirrors have a precisely defined shape and are precisely positioned such that the imaging of the mask onto the lithography object is of sufficient quality.
Because of the low transmittance of all gases for very short-wave UV radiation, the optical elements of such a projection exposure apparatus are operated in a vacuum environment. Interaction of the very short-wave UV radiation with the residual gas in the vacuum environment in the course of operation of the projection exposure apparatus gives rise to ionic, free-radical or neutral (excited) constituents of the residual gas. Knowledge of the composition of this gas or plasma atmosphere is important for the operation of the projection exposure apparatus. If the composition of the residual gas specified in the design of the apparatus is not observed, this can result in degradation effects on the mirrors of the projection exposure apparatus.
Inter alia, the degradation effects are based on the fact that some of the residual gas present in the apparatus is firstly ionized and secondly provided with a certain amount of kinetic energy. When such particles are incident on the surface of an optical element, degradation effects set in as a result of the combination of mechanical impact and great reactivity. State-of-the-art residual gas analyzers are not capable of energy-resolved and mass-resolved detection of such ionized constituents of the residual gas.
It is an object of the disclosed techniques to present a residual gas analyzer, a projection exposure apparatus, and a method of residual gas analysis that avoid these disadvantages.
The disclosed techniques relate to a residual gas analyzer, especially for analyzing a residual gas in a microlithography projection exposure apparatus. The residual gas analyzer includes a mass spectrometer and admission equipment for admission of ionized constituents of the residual gas into the mass spectrometer from a vacuum environment. The admission equipment includes an ion decelerator. The ion decelerator has an adjustable deceleration voltage.
By virtue of the ions being exposed to a deceleration voltage prior to the transfer into the mass spectrometer, the ions can be subjected to a selection according to kinetic energy. Only ions whose kinetic energy is sufficient to overcome the deceleration voltage reach the mass spectrometer and contribute to the analysis result.
The ion decelerator can include a deceleration path that is arranged between an inlet opening and an outlet opening of the ion decelerator. An electric potential that differs from the electric potential at the outlet end can be applied to the inlet end of the deceleration path so that the ions are decelerated by the potential difference. The potential difference corresponds to the deceleration voltage. The deceleration voltage can be set by modifying the potential difference.
The electric potential at the inlet end may have a fixed value. In particular, the electric potential at the inlet end may correspond to the electric potential of a housing of the vacuum environment, i.e., of the housing from which the residual gas is taken. The electric potential at the outlet end of the deceleration path may be adjustable in order to modify the deceleration voltage and hence the deceleration force acting on the ions. The electric potential at the outlet end may be positive in relation to the potential at the inlet end such that positively charged ions are repelled.
An intermediate electric potential may be formed between the inlet end and the outlet end of the deceleration path. The intermediate electric potential can be negative in relation to the potential at the inlet end such that free electrons are filtered out and prevented from passing the ion decelerator.
In one embodiment, the potentials of the ion decelerator are formed by grids, to which a voltage is applied. The ion decelerator may have a longitudinal direction that corresponds to the direction of movement of the ions. The grids can be aligned transversely to the longitudinal direction such that the ions pass through the grids on their way between the inlet end and the outlet end. The voltage applied to at least one of the grids may be modified in order to set the deceleration voltage. The potential of the first grid at the inlet end of the deceleration path preferably remains unchanged.
The ion decelerator can be traversed only by those ions whose kinetic energy is high enough not to be completely consumed by the deceleration voltage. The ionized constituents emerge from the ion decelerator at a speed that corresponds to the excess of kinetic energy.
It is advantageous for the mass spectrometry analysis if the ionized constituents have a defined state on entry into the mass spectrometer. The admission equipment of the residual gas analyzer may include an ion transfer device arranged between the ion decelerator and the mass spectrometer. The ion transfer device may be designed to guide the ionized constituents onward to the mass spectrometer without said ionized constituents being neutralized as a result of contact with component parts of the residual gas analyzer.
The ion transfer device may be configured such that the ions are guided through the ion transfer device in the longitudinal direction. The longitudinal direction of the ion transfer device may correspond to the longitudinal direction of the ion decelerator.
The ion transfer device may include a pole arrangement composed of multiple electric poles. An alternating electric field may be applied to the poles, in order to steer the ionized constituents onto a path in the longitudinal direction of the ion transfer device. Each pole may be arranged between two adjacent poles of opposite electric potential. An RF voltage may be applied to the poles, i.e., an AC voltage in the radiofrequency range. In this way, the pole arrangement may act as ion optics, via which the ionized constituents of the residual gas are guided from an inlet end to an outlet end of the ion transfer device.
The pole arrangement may take the form of a hexapole. The hexapole may extend in the longitudinal direction of the ion transfer device and include six electric poles that are distributed over the circumference of the ion transfer device. The ion transfer device may include a single hexapole that extends over the entire length of the ion transfer device. Also possible are multiple hexapoles arranged in succession in the longitudinal direction.
A DC bias matched to the electric potential at the outlet end of the ion decelerator may be applied to the pole arrangement. In this way, the ionized constituents maintain their preferred axial direction when passing through the ion transfer device. In addition to, or in an alternative, the ion transfer device may also be provided with an ion funnel, i.e., equipment for focusing an ion beam using a number of successively arranged ring electrodes of decreasing internal diameter.
There may be free gas exchange between the interior of the ion transfer device and the vacuum environment from which the residual gas is taken, and so the residual gas atmosphere in the ion transfer device is similar to that of the vacuum environment. In that case and on account of collisions with other constituents, especially hydrogen constituents of the residual gas, the ionized constituents of the residual gas are decelerated during their passage through the ion transfer device. Should the path to be traveled within the ion transfer device be sufficiently long, the ionized constituents are decelerated to such an extent that all excess kinetic energy still present upon departure from the ion decelerator is consumed, and the ionized constituents are reduced to their thermal energy. This is advantageous because the ionized constituents thus have a defined state on entry into the mass spectrometer. It is advantageous if the ion transfer device extends over a distance of at least 20 cm, preferably at least 50 cm, further preferably at least 80 cm.
The mass spectrometer may also take the form of a time-of-flight mass analyzer (TOF analyzer), especially of a time-of-flight mass analyzer with an orthogonal acceleration stage. The mass spectrometer may encompass a measurement range of 1 to 500 daltons, preferably 1 to 1000 daltons, such that complete mass spectra can be recorded over this mass range. The recording frequency of the mass spectrometer may be greater than 10 kHz. An improvement in the signal-to-noise ratio can be achieved by adding up a plurality of the spectra recorded with the mass spectrometer. The time resolution of the overall system emerges from the frequency with which the spectra are transferred to the evaluation. This frequency may be between 0.1 Hz and 100 Hz. This is a distinct improvement over existing systems in which several minutes are required for recording of a single spectrum over the measurement range of 1 to 200 daltons.
The residual gas analyzer may include a filter device (or filter) in order to filter out ionized constituents having a particular mass/charge ratio. The filter device can filter ionized constituents still present upon emergence from the ion decelerator out of the ion stream, such that they cannot make any contribution to the mass spectrometry analysis. In particular, constituents that are ionized with the filter device and are present in high density in the ion stream may be filtered out in order to thus increase the dynamic range of the mass spectrometer. Constituents that are frequently present in high concentration in the residual gas are, for example, hydrogen (H2) or nitrogen (N2).
The filter device may take the form of a quadrupole. By applying a suitable alternating field, the quadrupole can be set such that specific constituents of the residual gas are filtered out and consequently make no contribution to the mass spectrometry analysis. The filter device may be arranged between the ion transfer device and the mass spectrometer. The filter device may be connected to the mass spectrometer to form a common component.
A switchable ion source may be arranged between the ion decelerator and the mass spectrometer. In a first switching state, the switchable ion source may act as ion optics, via which the ion stream is guided to the mass spectrometer. The ion source may include an ionization unit (or ionizer) which is designed to ionize neutral constituents of the residual gas. The ionization unit may be inactive in the first switching state of the ion source, and so the ion stream coming from the ion decelerator can pass through the ion source without being influenced by the ionization unit. The ionization unit may be active in a second switching state. In this switching state, the residual gas analyzer may be utilized to analyze neutral constituents of the residual gas that have arrived at the switchable ion source from the vacuum environment via free gas exchange. The switchable ion source may be arranged between the ion transfer device and the mass spectrometer. The switching time for the switching between the first switching state and the second switching state may be less than 1 s, and so a change between the two modes of operation of the residual gas analyzer is possible at short notice at any time.
The result of the mass spectrometry analysis is independent of the excess kinetic energy of the ionized constituents upon departure from the ion decelerator. The result of the mass spectrometry analysis corresponds in each case to the sum of ions whose kinetic energy on entry into the ion decelerator was above a certain threshold value.
Performing the mass analysis multiple times with different values for the deceleration voltage of the ion decelerator allows the ionized constituents to be analyzed for different threshold values of the initial kinetic energy. Subtracting the summed values allows determination of a mass spectrum associated with a specific interval of kinetic energy.
The residual gas analyzer may include a control unit (or controller) designed to control the component parts of the residual gas analyzer. The control unit may direct control commands, which set the deceleration voltage of the ion decelerator to different values, to the ion decelerator. The control unit may be designed to store a mass spectrum ascertained with the mass analyzer in a memory unit (or memory) such that assignment between the measured mass spectrum and the corresponding deceleration voltage of the ion decelerator is assured. The control unit may further be designed to form the difference between a first mass distribution ascertained at a first deceleration voltage and a second mass distribution ascertained at a second, higher deceleration voltage in order to obtain a mass distribution of those ions whose kinetic energy at the inlet of the ion decelerator was sufficient to overcome the first deceleration voltage and whose kinetic energy at the inlet of the ion decelerator was not sufficient to overcome the second deceleration voltage.
It was found that the kinetic energy of the ions relevant to the degradation effects is between 1 eV and 50 eV. Ions with a kinetic energy of between 5 eV and 20 eV are of particular interest. The deceleration voltage of the ion decelerator may be set such that the spectra recorded at the voltages yields differentiation within this energy range. For example, the deceleration voltage may be set to at least three values between 5 V and 20 V, preferably to at least five values and further preferably to at least ten values. A mass spectrometry analysis can be performed at each of the values for the deceleration voltage.
The disclosed techniques also relate to a microlithography projection exposure apparatus equipped with such a residual gas analyzer. The projection exposure apparatus may include a radiation source that emits very short-wave UV radiation. Very short-wave UV radiation includes DUV radiation in the deep ultraviolet spectral range with wavelengths between 100 nm and 300 nm and EUV radiation in the extreme ultraviolet spectral range with wavelengths between 5 nm and 100 nm, especially with wavelengths between 5 nm and 30 nm. The projection exposure apparatus may include an exposure system and a projection lens, wherein the exposure system directs the very short-wave UV radiation onto an object field in an object plane, and wherein the projection lens images the object field into an image plane. The projection exposure apparatus may include a plurality of optical elements with which the very short-wave UV radiation is directed along an exposure beam path to the image plane. The optical elements may take the form of reflective optical elements.
The optical component parts of the projection exposure apparatus may be enclosed in a vacuum housing. The projection exposure apparatus may include a vacuum pump connected to the vacuum housing, which is designed to generate a high vacuum in the vacuum housing. The pressure of the high vacuum may, for example, be between 10−7 and 10−8 mbar.
An inner housing may be formed within the vacuum housing. The inner housing may wholly or partly enclose sections of the exposure beam path and/or one or more optical elements, such that a micro-environment is formed within the vacuum housing. The inner housing need not be impervious in a macroscopic sense. Under high vacuum conditions in the vacuum housing, an inner housing which is not impervious in a macroscopic sense may be sufficient to create a distinctly different atmosphere within the micro-environment than in the rest of the vacuum housing.
The inner housing may be provided with a connection for a purge gas, such that an atmosphere different from the rest of the vacuum housing can be applied in the micro-environment. The pressure in the micro-environment may be in the fine vacuum region, for example between 10−1 mbar and 10−3 mbar. The purge gas may be hydrogen (H2), which serves to remove contamination from the micro-environment and especially from the mirror surfaces present therein. The atmosphere in the inner housing may be formed by the residual gas which is examined by the residual gas analyzer according to the disclosed techniques.
An inlet opening for the residual gas analyzer may be arranged in the inner housing. In particular, the inlet end of the ion decelerator may be connected to the inner housing of the projection exposure apparatus such that the residual gas from the micro-environment can enter the ion decelerator. The mass spectrometer may be arranged outside the vacuum housing. Should the residual gas analyzer include a switchable ion source, the latter may also be arranged outside of the vacuum housing. The path taken by the ions between the micro-environment and the mass spectrometer may extend within a vacuum tube. A vacuum tube refers to a housing wall that separates an interior enclosed by the housing wall from the atmosphere in the vacuum housing. The term “vacuum tube” does not imply any restriction to a particular structure or shape of the housing wall. Free mass transfer may exist between the interior of the vacuum tube and the micro-environment, such that the atmosphere in the vacuum tube corresponds essentially to the atmosphere in the micro-environment.
The predominant portion of the distance between the wall of the inner housing and the wall of the vacuum housing may be occupied by the ion transfer device. For example, the ion transfer device may extend over at least 50% of this distance, preferably over at least 70% and further preferably over at least 90%. In that case, a sufficient amount of distance is available so that the ions that have passed through the ion decelerator can be thermalized by collisions with constituents of the residual gas. The deceleration path of the ion decelerator preferably makes up less than 20% of the distance between the inlet opening for the residual gas analyzer and the inlet opening for the mass spectrometer, further preferably less than 10% and further preferably less than 5%.
The disclosed techniques also relate to a method of residual gas analysis, especially for a residual gas in a microlithography projection exposure apparatus. In the method, ionized constituents of the residual gas are guided from a vacuum environment into a mass spectrometer. An ion decelerator is arranged between the vacuum environment and the mass spectrometer in order to decelerate the ionized constituents using an adjustable deceleration voltage. Mass spectra are recorded for different values of the deceleration voltage.
This disclosure encompasses developments of the method with features that are described in the context of the residual gas analyzer according to the disclosed techniques. The disclosure encompasses developments of the residual gas analyzer with features that are described in the context of the method according to the disclosed techniques.
The disclosed techniques are described by way of example hereinafter using advantageous embodiments, with reference to the appended drawings. In the figures:
The exposure system 10 includes an exposure radiation source 14 which emits electromagnetic radiation in the EUV range, i.e., with a wavelength of between 5 nm and 30 nm in particular. The EUV radiation emanating from the exposure radiation source 14 is first focused into an intermediate focus plane 16 with a collector 15.
A deflecting mirror 17 deflects the EUV radiation onto a first facet mirror 18. The first facet mirror 18 is followed by a second facet mirror 19, with which the individual facets of the first facet mirror 18 are imaged into the object field 13.
With the aid of the projection lens 22, the object field 13 is imaged into an image plane 21 using a plurality of mirrors 20. Arranged in the object field 13 is a mask (also called a reticle) which is imaged onto a light-sensitive layer of a wafer arranged in the image plane 21.
The various mirrors of the projection exposure apparatus at which the illumination radiation is reflected take the form of EUV mirrors. The EUV mirrors have been provided with highly reflective coatings. These may be multilayer coatings, especially multilayer coatings having alternating layers of molybdenum and silicon.
The component parts of the projection exposure apparatus are arranged in a vacuum housing 23. The interior of the vacuum housing 23, in operation of the projection exposure apparatus, is put under high vacuum with a vacuum pump (not shown) at a pressure of, for example, 10−8 mbar. Formed in the interior of the vacuum housing 23 is an inner housing 25 that surrounds the exposure beam path.
The inner housing 25 is not impervious in the macroscopic sense with respect to the remainder of the interior of the vacuum housing 23. For example, gaps that may be formed between the inner housing 25 and optical elements 17, 18, 19, 20, at which the exposure beam path is reflected, permit mechanical adjustment of the optical elements. Nevertheless, the inner housing 25, under the high-vacuum conditions that exist in the vacuum housing 23, enables an atmosphere in the interior of the inner housing 25 which is distinctly different from the atmosphere in the rest of the vacuum housing 23.
The projection exposure apparatus includes a purge gas connection (not shown in
Interaction between the EUV radiation and the hydrogen creates a plasma, forming ionic plasma species (H+) or free-radical plasma species (H), inter alia. The hydrogen plasma has the effect that contamination is removed from surfaces of the optical elements arranged in the inner housing 25. This gives rise to compounds, for example, of hydrogen and carbon or of hydrogen and nitrogen that are distributed in the residual gas atmosphere in the interior of the inner housing 25.
The residual gas analyzer 24 according to the disclosed techniques has the purpose of obtaining information about the composition of the residual gas in the interior of the inner housing 25. Ionic species contained in the residual gas atmosphere are of particular interest. Going beyond the type of species, the residual gas analyzer according to the disclosed techniques allows ascertainment of the distribution of kinetic energy in the species.
The residual gas analyzer 24 includes a vacuum tube 26 that extends outward from the interior of the inner housing 25 through the wall of the vacuum housing 23. The length of the vacuum tube 26 is on the order of 80 cm. The vacuum tube 26 has an inlet opening arranged in the inner housing 25, such that exchange of gas takes place between the residual gas atmosphere in the inner housing 25 and the interior of the vacuum tube 26. The vacuum tube 26 is sealed with respect to the interior of the vacuum housing 23.
The ionic species are guided through the vacuum tube 26 to a mass spectrometer 27 arranged outside the vacuum housing 23, where they are analyzed for their mass-to-charge ratio.
According to
The ion decelerator 29 is arranged adjacent to the inlet opening of the vacuum tube 26, and so ionic species that pass into the vacuum tube 26 go in high number into the region of influence of the ion decelerator 29, without being neutralized beforehand by contact with other component parts of the residual gas analyzer 24.
The ion decelerator 29, according to
The ion decelerator 29 has a length of a few centimeters, and so a considerable distance within the vacuum tube 26 still has to be covered before the composition of the ionic species can be examined by the mass spectrometer 27. The ion species are guided onward through the vacuum tube 26 by the ion transfer device 34. The ion transfer device 34 includes a hexapole 35 that extends in the direction of movement of the ions. The hexapole 35 is formed by six poles distributed uniformly over the circumference of the ion transfer device 34, which are alternately at a positive or negative potential. In addition to an AC voltage applied to the hexapole 35, it is also possible to apply a DC bias, the magnitude of which is such that the ionic species retain their preferential direction. The hexapole 35 acts as ion optics, with which the ionic species are concentrated to an ion beam that moves along the central axis of the hexapole 35.
Since uncharged species can pass through the ion decelerator 29 unhindered, there is an atmosphere with a high proportion of hydrogen in the region of the ion transfer device 34, similarly to that in the inner housing 25. Collisions with the hydrogen constituents result in deceleration of the ionic species, such that the kinetic energy of the ionic species corresponds essentially to thermal movement at the outlet from the ion transfer device 34. This means that the ionic species at the outlet from the ion transfer device 34 have a defined state, which creates a favorable starting point for the subsequent mass spectrometry analysis.
In the exemplary embodiment as per
The ion source 28 further includes an ionization unit 36 which is designed to ionize neutral constituents of the residual gas. In the first switching state of the ion source 28, the ionization unit 36 is inactive, such that the ions coming from the ion transfer device 34 can pass through the ion source 28 unaffected by ionization unit 36.
The ionization unit 36 is active in a second switching state. In this switching state, the residual gas analyzer 24 is utilized in order to analyze neutral constituents of the residual gas that have arrived at the switchable ion source 28 from the inner housing 25 via free gas exchange. The ion decelerator 29 and the ion transfer device 34 play no role when the ion source 28 is in the second switching state. The switching time for the switching between the first switching state and the second switching state is less than 1 s, such that a change between the two modes of operation of the residual gas analyzer is possible at short notice at any time.
The mass spectrometer 27 may also take the form of a time-of-flight mass analyzer (TOF analyzer), especially of a time-of-flight mass analyzer with an orthogonal acceleration stage. The mass spectrometer 27 may have a measurement range of 1 to 500 daltons, such that complete mass spectra can be recorded over this mass range. The recording frequency of the mass spectrometer may be greater than 10 kHz. Addition of the spectra can achieve a distinct improvement in the signal-to-noise ratio.
The time resolution of the overall system emerges from the frequency with which the spectra are transferred to the evaluation. This frequency may be between 0.1 Hz and 100 Hz. This is a distinct improvement over existing systems in which several minutes are required for recording of a single spectrum over the measurement range of 1 to 200 daltons.
The inlet of the mass spectrometer 27 is formed by a quadrupole 39. The quadrupole 39 can filter out ions that enter the mass spectrometer 27 in high density, in order thus to increase the dynamic range of the mass spectrometer. In particular, it is possible to specifically filter individual constituents of the residual gas that have a particular mass-to-charge ratio, for example the hydrogen (H2) or nitrogen (N2) constituents that are frequently present with a high partial pressure. By applying a suitable AC field, the quadrupole 39 can be set such that particular constituents of the residual gas are filtered out and consequently make no contribution to the mass spectrometry analysis.
The residual gas analyzer 24 includes a control unit (or controller) 40 that controls the interplay of the component parts of the residual gas analyzer 24. The control unit 40 transmits control commands to the ion decelerator 29 in order to set the ion decelerator 29 to a specific deceleration voltage. The control unit 40 stores the mass spectra obtained with the mass spectrometer 27 in such a way that there is assignment between the mass spectra and the respective associated deceleration voltage.
The analysis can be performed in such a way that, in a first step, a voltage of 0 eV is applied to the third grid 32 of the ion decelerator 29 such that the ionic species can pass out of the inner housing 25 and through the ion decelerator 29 in unimpeded fashion, independently of their kinetic energy. The associated mass spectrum captures all ionic species, independently of the magnitude of their kinetic energy on entry into the ion decelerator 29. A positive voltage of 1 V can be applied to the third grid 32 in a subsequent step, and so the ion decelerator 29 is passed only by those ionic species whose kinetic energy on entry into the ion decelerator 29 was so high that the deceleration voltage was overcome. The difference from the two mass spectra corresponds to the number of ions with a kinetic energy of between 0 eV and 1 eV. This method can be performed up to 50 eV with appropriate gradation, and so an analysis of the composition of the ionic species is obtained in 50 different classes, with each class corresponding to a specific range of the kinetic energy on entry into the ion decelerator 29. The region of 5 eV and 20 eV is of particular interest.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2022 207 285.5 | Jul 2022 | DE | national |
This is a Continuation of International Application PCT/EP2023/067054, which has an international filing date of Jun. 22, 2023, and the disclosure of which is incorporated in its entirety into the present Continuation by reference. This Continuation also claims foreign priority under 35 U.S.C. § 119 (a)-(d) to and also incorporates by reference, in its entirety, German Patent Applications DE 10 2022 207 285.5 filed on Jul. 18, 2022.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2023/067054 | Jun 2023 | WO |
| Child | 19027800 | US |
