Method for operating an optical apparatus, and optical apparatus

Abstract

A method for operating an optical apparatus (100A, 100B, 200), having a structural element (201) which is arranged in a residual gas atmosphere (RGA) of the apparatus and which is formed at least partly from an element material subjected to a chemical reduction process and/or an etching process with a plasma component (PK) present in the residual gas atmosphere includes: feeding (S2) a gas component (GK) that at least partly suppresses the reduction process depending on a detected suppression extent (UM) for a suppression of the etching process and/or reduction process by the suppressing gas component in the residual gas atmosphere; and detecting (S1) the suppression extent with a sensor unit (208) arranged in the residual gas atmosphere. The sensor unit includes a sensor material section (211) composed of a sensor material and exhibiting a sensor section property that is measurable under the influence of the suppressing gas component.

Claims

1. Method for operating an optical apparatus, comprising: arranging a structural element in a residual gas atmosphere of the optical apparatus, which is formed at least partly from an element material which is subjected to a chemical reduction process and/or an etching process with a plasma component present in the residual gas atmosphere; feeding a suppressing gas component that at least partly suppresses the reduction process and/or the etching process depending on a detected suppression extent for suppressing the reduction process and/or the etching process by the suppressing gas component in the residual gas atmosphere; detecting the suppression extent with a sensor unit arranged in the residual gas atmosphere, wherein the detecting is based on a sensor section property of a sensor material of a sensor material section of the sensor unit that is measurable under influence of the suppressing gas component; and effecting a material removal or a material supplementation such that the sensor material is etched, reduced or oxidized under the influence of the plasma component and the suppressing gas component in the sensor material section.

2. Method according to claim 1, wherein the sensor section property is an electrical conductivity of the sensor material section, a thickness of the sensor material section on a substrate, an optical reflection and/or a transmission coefficient and/or a mass of the sensor material section.

3. Method according to claim 1, further comprising: measuring the sensor section property; and determining a substance amount to be fed of the suppressing gas component in the residual gas atmosphere depending on the measured sensor section property for detecting the suppression extent.

4. Method according to claim 1, wherein the suppression extent is detected as a ratio of proportions of the plasma component and the suppressing gas component in the residual gas atmosphere.

5. Method according to claim 1, further comprising: measuring the sensor section property; mapping the measured sensor property with a) a mapping specification, b) a reaction model for the influence of the suppressing gas component and/or the plasma component on the sensor material and/or the element material, c) a heuristic model for the influence of the suppressing gas component and/or the plasma component on the sensor material and/or the element material and/or d) a table onto the suppression extent.

6. Method according to claim 1, wherein feeding the suppressing gas component comprises determining that the detected suppression extent lies within a predefined range during operation of the optical apparatus.

7. Method according to claim 6, wherein the predefined range is selected to at least partly prevent an etching process of the element material reduced on account of the reduction process.

8. Method according to claim 1, wherein the optical apparatus comprises at least one optical element having a mirror surface, wherein the suppressing gas component is an oxidant, and wherein feeding the suppressing gas component comprises depositing element material oxidized by the suppressing gas component and previously reduced by the plasma component to an extent smaller than a predefined extent.

9. Method according to claim 1, wherein the optical apparatus is a lithography apparatus which comprises an illumination device for generating ultraviolet (UV) light, a reticle device for a photomask with lithographic information, and a projection device for imaging the lithographic information onto a wafer, and wherein the sensor unit is arranged in the projection device.

10. Method according to claim 1, wherein the optical apparatus is a mask inspection apparatus which comprises an illumination device for generating UV light, a reticle device for a photomask with lithographic information, and a projection device for imaging and for inspecting the lithographic information, and wherein the sensor unit is arranged in the projection device.

11. Method according to claim 1, wherein the structural element is an optical element, a mirror body, a mirror coating, a reticle mount and/or a carrying element.

12. Method according to claim 1, wherein the element material is selected from the group consisting of: an oxide material, a glass material, a ULE material, a zinc oxide, amorphous or crystalline silicon and a silicon carbide.

13. Method according to claim 1, wherein the plasma component comprises hydrogen.

14. Method according to claim 1, wherein the gas component that suppresses the etching and/or the reduction process comprises oxygen and/or water.

15. Optical apparatus comprising: a housing closing off a residual gas atmosphere; a structural element which is arranged in the residual gas atmosphere and which is formed at least partly from an element material which is subjected to a chemical reduction process and/or an etching process with a plasma component present in the residual gas atmosphere; a feed unit arranged to feed a suppressing gas component that at least partly suppresses the reduction process and/or the etching process into the residual gas atmosphere; and a sensor unit arranged in the residual gas atmosphere and comprising a sensor material section composed of a sensor material, wherein the sensor material section has a measurable sensor section property that varies under influence of the suppressing gas component, and wherein the sensor material is configured to be etched, reduced or oxidized under the influence of the plasma component and the suppressing gas component in the sensor material section.

16. Optical apparatus according to claim 15, further comprising a control unit configured to control the feed unit.

17. Optical apparatus according to claim 15, wherein the sensor unit comprises a variable-resistance nanowire fabricated from the sensor material as the sensor material section and a measuring unit for measuring an electrical resistance of the nanowire as the sensor section property, wherein the sensor material is configured to be oxidized by the suppressing gas component to form a sensor material oxide.

18. Optical apparatus according to claim 17, wherein the nanowire is configured such that the sensor material oxide is stripped from the sensor material.

19. Optical apparatus according to claim 15, wherein the sensor unit comprises a variable-resistance thin layer fabricated from the sensor material and arranged on a substrate as the sensor material section and a measuring unit for measuring an electrical resistance of the thin layer as the sensor section property, wherein the sensor material is configured to be oxidized by the suppressing gas component to form a sensor material oxide.

20. Optical apparatus according to claim 15, wherein the sensor unit comprises a film fabricated from the sensor material as the sensor material section and a measuring unit for measuring an optical property as the sensor section property.

21. Optical apparatus according to claim 20, wherein the sensor material is configured to be oxidized by the suppressing gas component to form a sensor material oxide.

22. Optical apparatus according to claim 20, wherein the sensor material is configured to be reduced under the influence of the plasma component.

23. Optical apparatus according to claim 20, wherein the sensor unit is configured to determine ellipsometric parameters for a surface of the film as the sensor section property and/or to measure an optical transmission property of the film as the sensor section property.

24. Optical apparatus according to claim 15, wherein the sensor unit comprises a quartz crystal microbalance for measuring a sensor material removal from or supplementation to the sensor section under the influence of the suppressing gas component and/or the plasma component on the sensor material.

25. Optical apparatus according to claim 24, wherein the quartz crystal microbalance comprises a ruthenium, palladium, platinum or rhodium layer coated with silicon oxide.

26. Optical apparatus according to claim 24, wherein the residual gas atmosphere has a pressure of between 10.sup.−4 and 5 mbar.

27. Optical apparatus according to claim 15, wherein a partial pressure of the suppressing gas component is between 10.sup.−10 and 10.sup.−7 mbar.

28. Optical apparatus according to claim 15, further comprising a projection device configured to image lithographic information with extreme ultraviolet (EUV) radiation.

29. Optical apparatus comprising: a housing closing off a residual gas atmosphere; a structural element which is arranged in the residual gas atmosphere and which is formed at least partly from an element material which is subjected to a chemical reduction process and/or an etching process with a plasma component present in the residual gas atmosphere; a feed unit arranged to feed a suppressing gas component that at least partly suppresses the reduction process and/or the etching process into the residual gas atmosphere; and a sensor unit arranged in the residual gas atmosphere and comprising a sensor material section composed of a sensor material, wherein the sensor material section has a measurable sensor section property that varies under influence of the suppressing gas component, wherein the sensor unit comprises a variable-resistance nanowire fabricated from the element material as the sensor material as the sensor material section and a measuring unit arranged to measure an electrical resistance of the nanowire as the sensor section property, wherein the sensor material is configured to be reduced under the influence of the plasma component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A shows a schematic illustration of one embodiment of an EUV lithography apparatus;

(2) FIG. 1B shows a schematic illustration of one embodiment of a DUV lithography apparatus;

(3) FIG. 2 shows a schematic illustration of a further embodiment of a lithography apparatus for elucidating a method for operating same;

(4) FIG. 3 shows a diagram for elucidating an etching process of silicon with a hydrogen plasma depending on an oxygen-hydrogen ratio in a residual gas atmosphere;

(5) FIG. 4 shows a diagram for elucidating contaminations by silicon in a lithography apparatus depending on an oxygen addition in a residual gas atmosphere;

(6) FIGS. 5 and 6 show schematic flow diagrams with method steps for elucidating two exemplary variants of a method for operating a lithography apparatus in accordance with FIG. 2;

(7) FIGS. 7A-7F show schematic illustrations of embodiments of measuring units in which different sensor materials are exposed to the plasma components for use in a lithography apparatus in accordance with FIG. 2, specifically a nanowire in plan view (FIG. 7A), a nanowire in perspective view (FIG. 7B), nanowires in cross section (FIGS. 7C, 7D and 7E), and a decomposed nanowire in plan view (FIG. 7F);

(8) FIGS. 8, 9A, 9B, and 10-12 show schematic illustrations of further embodiments of measuring units in which different sensor materials are exposed to the plasma components for use in a lithography apparatus in accordance with FIG. 2, specifically a plurality of nanowires (FIG. 8), silicon dioxide films (FIGS. 9A and 9B, and films measured through reflection (FIG. 10), transmission (FIG. 11) and changes in mass (FIG. 12).

DETAILED DESCRIPTION

(9) Analogous elements or elements having a mutually analogous function have been provided with the same reference signs in the figures, unless indicated to the contrary. In so far as a reference sign has a plurality of reference lines in the present case, this means that the corresponding element is present multiply. Reference sign lines pointing to concealed details are illustrated in a dashed manner. It should also be noted that the illustrations in the figures are not necessarily true to scale.

(10) A system referred to hereinafter as a lithography apparatus should in particular also be understood to also encompass mask inspection systems. Furthermore, consideration is given to a lithography apparatus as simply an example of a general optical apparatus.

(11) FIG. 1A shows a schematic view of an EUV lithography apparatus 100A, which comprises a beam-shaping and illumination system 102 and a projection system 104. In this case, EUV stands for “extreme ultraviolet” and refers to a wavelength of the working light of between 0.1 nm and 30 nm. The beam shaping and illumination system 102 and the projection system 104 are respectively provided in a vacuum housing (not shown), each vacuum housing being evacuated with the aid of an evacuation device (not shown). In embodiments, the vacuum housings can also communicate with one another. The vacuum housings are surrounded by a machine room (not shown), in which drive devices for mechanically moving or adjusting the optical elements are provided. Furthermore, electronic controllers and the like can also be provided in said machine room.

(12) The EUV lithography apparatus 100A comprises an EUV light source 106A. A plasma source (or a synchrotron), which emits radiation 108A in the EUV range (extreme ultraviolet range), that is to say for example in the wavelength range of 5 nm to 20 nm, can for example be provided as the EUV light source 106A. In the beam shaping and illumination system 102, the EUV radiation 108A is focused and the desired operating wavelength is filtered out from the EUV radiation 108A. The EUV radiation 108A generated by the EUV light source 106A has a relatively low transmissivity through air, for which reason the beam guiding spaces in the beam shaping and illumination system 102 and in the projection system 104 are evacuated to the greatest possible extent. All that remains is a residual gas atmosphere RGA.

(13) The beam shaping and illumination system 102 illustrated in FIG. 1A has five mirrors 110, 112, 114, 116, 118. After passing through the beam shaping and illumination system 102, the EUV radiation 108A is guided onto a photomask (called a reticle) 120 in a reticle device 103. The photomask 120 is likewise formed as a reflective optical element and can be arranged outside the systems 102, 104. Furthermore, the EUV radiation 108A can be directed onto the photomask 120 by a mirror 122. The photomask 120 has a structure with lithographic information which is imaged onto a wafer 124 or the like in a reduced fashion with the projection system 104 using the EUV light.

(14) The projection system 104 (also referred to as projection lens or projection device) has six mirrors M1 to M6 for imaging the photomask 120 onto the wafer 124. In this case, individual mirrors M1 to M6 of the projection system 104 can be arranged symmetrically in relation to an optical axis 126 of the projection system 104. The number of mirrors M1 to M6 of the EUV lithography apparatus 100A is not restricted to the number illustrated. A greater or lesser number of mirrors M1 to M6 can also be provided. Furthermore, the mirrors M1 to M6 are generally curved on their front face for beam shaping.

(15) FIG. 1B shows a schematic view of a DUV lithography apparatus 100B, which comprises a beam shaping and illumination system 102 and a projection system 104. In this case, DUV stands for “deep ultraviolet” and refers to a wavelength of the working light of between 30 nm and 250 nm. As has already been described with reference to FIG. 1A, the beam shaping and illumination system 102 and the projection system 104 can be arranged in a vacuum housing and/or surrounded by a machine room with corresponding drive devices.

(16) The DUV lithography apparatus 100B has a DUV light source 106B. By way of example, an ArF excimer laser that emits radiation 108B in the DUV range at 193 nm, for example, can be provided as the DUV light source 106B.

(17) The beam shaping and illumination system 102 illustrated in FIG. 1B guides the DUV radiation 108B onto a photomask 120. The photomask 120 arranged in the reticle device 103 is embodied as a transmissive optical element and can be arranged outside the systems 102, 104. The photomask 120 in turn has a structure with lithographic information which is imaged onto a wafer 124 or the like in a reduced fashion with the projection system 104.

(18) The projection system 104 has a plurality of lens elements 128 and/or mirrors 130 for imaging the photomask 120 onto the wafer 124. In this case, individual lens elements 128 and/or mirrors 130 of the projection system 104 can be arranged symmetrically in relation to an optical axis 126 of the projection system 104. The number of lens elements 128 and mirrors 130 of the DUV lithography apparatus 100B is not restricted to the number illustrated. A greater or lesser number of lens elements 128 and/or mirrors 130 can also be provided. Furthermore, the mirrors 130 are generally curved on their front face for beam shaping.

(19) An air gap between the last lens element 128 and the wafer 124 can be replaced by a liquid medium 132 which has a refractive index of >1. The liquid medium 132 can be high-purity water, for example. Such a construction is also referred to as immersion lithography and has an increased photolithographic resolution. The medium 132 can also be referred to as an immersion liquid.

(20) Gas plasmas are present within the residual gas atmosphere RGA on account of the high-energy UV light, in particular in EUV lithography or mask inspection apparatuses. Hereinafter, by way of example, hydrogen plasma is considered as plasma component in the residual gas atmosphere RGA. The component parts present in the respective lithography or mask inspection apparatus are fabricated partly from materials which are attacked under the influence of the respective plasma component, such as hydrogen plasma. It may be mentioned that a hydrogen-assisted etching process can commence. As a result, volatile material constituents can enter the residual gas atmosphere RGA and precipitate at undesired locations.

(21) One difficulty is, in particular, that silicon as a constituent of mirror or lens element bodies or other structural elements within the lithography or mask inspection apparatus can be incipiently etched under the influence of the hydrogen plasma. The silicon can then precipitate on optical surfaces, e.g. as oxide. On mirror bodies or else optical surfaces, such silicon contaminants can then have undesired influences on the optical properties. By way of example, investigations by the applicant have revealed that the transmission properties of the projection optical unit are impaired by silicon etching processes within lithographic apparatuses and contamination with materials comprising silicon.

(22) That is countered with the aid of the feed of a gas that suppresses this etching process, for example oxygen as gas component. A description is given below of how the metering of the gas component that suppresses the etching process can be carried out expediently on the basis of the example of oxygen gas. Therefore, the following text discusses, in particular, silicon etching and deposition within the residual gas atmosphere RGA with reference to an element material comprising silicon. The etching or chemical processes described can be present analogously at other materials or substances in the lithography or mask inspection apparatus.

(23) The applicant has gained the insight that within the residual gas atmosphere of a lithographic apparatus, a silicon etching process occurs with the occurrence of a precursor, namely a silicon dioxide precursor. Silicon dioxide of this type is also already present, for example, in the above- and below-described mirror mounts or coatings of quartz glass or ULE materials. Corresponding silicon dioxide is firstly reduced to elemental silicon by a hydrogen plasma, said silicon reacting to form silanes SiH.sub.4. Water furthermore arises during the reduction process, said water likewise being present in the residual gas atmosphere. Silanes can be broken up under the influence of a catalyst, for example ruthenium, that is likewise present in the residual gas atmosphere, such that silicon and/or, on account of the water and/or oxygen constituents present, silicon dioxide deposit(s). Such deposited layers comprising silicon are considered to be contamination and are regarded in particular as disturbances that adversely influence the optical surfaces.

(24) The corrosive or caustic influence of the hydrogen plasma on component parts containing silicon dioxide as element material can be reduced by the addition of oxygen, water or other oxidants. Therefore, reference is also made to oxygen or water as suppressing gas component. It has been found that oxygen, in particular, shifts the equilibrium of the reduction reaction between silicon dioxide and radical hydrogen from the hydrogen plasma towards the oxidic phase. However, it has likewise been found that the addition of oxygen can have further adverse effects on surfaces of component parts, on account of its oxidation effect.

(25) The reactions possible during a silicon etching process in a residual gas atmosphere of a lithography or mask inspection apparatus are summarized in the following section:

(26) As a result of the influence of the EUV radiation, molecular hydrogen present in the residual gas atmosphere, for example as a purge gas residue, is ionized and converted into plasma components.
H.sub.2+γ.sub.EUV.fwdarw.Hi.sup.+,H*.

(27) Firstly, native silicon dioxide is reduced by the hydrogen plasma components. The silicon dioxide as element material of a component part in the lithography or mask inspection apparatus is subjected to a chemical reduction process in this respect:
SiO.sub.2+2Hi.sup.+,H*.fwdarw.2H.sub.2O(g)+Si(s).

(28) It is furthermore plausible for the hydrogen plasma to convert the formed silicon and/or silicon from a different source into gaseous silanes:
Si(s)+2Hi.sup.+,H*.fwdarw.SiH.sub.4(g).

(29) Reference may be made here to a chemical etching process of the silicon. Silanes can be decomposed under the influence of ruthenium, such that silicon is once again deposited elsewhere, for example on mirror surfaces:
SiH.sub.4(g).fwdarw.Si—Ru+2H.sub.2.

(30) The silicon emerging from the reaction is subsequently oxidized by water or oxygen in the residual gas atmosphere to form silicon dioxide, which remains as contamination on the ruthenium:
Si—Ru+2H.sub.2O/O.sub.2.fwdarw.SiO.sub.2.

(31) The SiO.sub.2 formed and deposited may once again be subjected to a chemical etching process by reduction with the hydrogen plasma constituents, as a result of which silanes again form in the gas phase. The etching rate of thin SiO.sub.2 layers on Ru is lower than in the case of SiO.sub.2 volume bodies, as a result of which a permanent contamination can be established.

(32) In order then to limit the silicon etching process or the reduction process of silicon dioxide by hydrogen plasma with the aid of addition of oxygen into the residual gas atmosphere, a suitable metering is desired. This is carried out with the aid of the operating method for a lithography apparatus illustrated in FIG. 2, said operating method being explained below on the basis of said lithography apparatus. In this case, as elucidated in FIGS. 5 and 6 on the basis of exemplary flow diagrams, the oxygen feed of a suppressing gas component is controlled in a suitable manner.

(33) FIG. 2, then, shows one exemplary embodiment of a further lithography apparatus 200. The lithography apparatus 200 illustrated can be understood for example as part of the projection system 104 as illustrated in FIG. 1A of the EUV lithography apparatus (100a). The lithography apparatus 200 comprises a housing 209, which, through suitable vacuum pumps, provides an evacuated atmosphere for the structural elements present within the housing 209. A vacuum having a pressure of between 10.sup.−4 mbar and 2 mbar is preferably generated within the housing 209.

(34) On account of measures not explained any further here, in particular hydrogen as purge gas passes into the residual gas atmosphere RGA that prevails within the housing 209. FIG. 2 indicates that the residual gas atmosphere RGA can comprise plasma components PK, hydrogen plasma being considered here. The hydrogen plasma, represented as small circles, arises as a result of the influence of the EUV radiation used for imaging the lithographic structures, with hydrogen as purge gas, as was explained above.

(35) Furthermore, the lithography apparatus comprises by way of example a structural element designated by 201. The structural element 201 can be e.g. a mirror body fabricated from a glass ceramic or a ULE material. In this respect, a structural element body 210 and a mirror coating 202 are present. EUV mirrors usually comprise multi-layered coatings 202 comprising alternating layers of high and low refractive index layer materials. By way of example, individual layers formed from beryllium, ruthenium, lanthanum, molybdenum and in particular also silicon are used. The hydrogen plasma PK also serves, inter alia, to protect such layers 202 against corrosion. The EUV mirror as structural element 201 mentioned by way of example has, in particular, a surface composed of silicon dioxide 203. This means that silicon dioxide is present in or on the component part 201. Silicon dioxide can be present in particular in mixtures or as native silicon dioxide.

(36) As described above, the silicon dioxide 203 or the layer is under the influence of the plasma component, that is to say the hydrogen plasma PK. This can lead to the etching process indicated above, such that silicon or silicon dioxide adversely deposits elsewhere within the residual gas atmosphere. In order to counteract that, a gas component GK that reduces or decreases this reduction process is introduced into the residual gas atmosphere. This is indicated by the triangles GK. This involves in particular oxygen as gas component GK that decreases the reduction process, “decreasing gas component” for short. To that end, an opening 206 is provided in the housing 209, through which opening oxygen GK from a gas reservoir 205 passes via a controllable valve unit 204 into the interior of the housing 209, that is to say the residual gas atmosphere RGA.

(37) Since it has been found that the addition of oxygen shifts the above-described reduction reaction of the native or present silicon dioxide in the direction of the oxide phase, the reduction process driven by the hydrogen plasma PK can be at least partly prevented, with the result that less chemical etching of silicon subsequently takes place within the residual gas atmosphere.

(38) In order to suitably meter the oxygen feed, a sensor unit 208 is provided within the residual gas atmosphere RGA, said sensor unit supplying a sensor signal SS to a control unit 207. The control unit 207 then controls the valve 204 via a control signal CT depending on the control signal SS. In this case, the sensor unit 208 has a sensor material section 211 that is exposed to the residual gas atmosphere RGA. That is to say that the sensor material section 211 is under the influence of the hydrogen plasma PK and of the oxygen GK fed in.

(39) According to investigations by the applicant, the effect of the oxygen GK, as gas component GK that decreases the reduction process, is dependent on the proportion of the oxygen and/or the hydrogen in the residual gas atmosphere. In this respect, a suppression extent UM is defined, which can be determined by measuring the gas constituents within the residual gas atmosphere. Suppression extent is understood to mean, by way of example, a quantitative description of the etching process, on the one hand, and of the reduction process, on the other hand. One possible choice for the suppression extent UM is the ratio of the amount of oxygen and hydrogen in the residual gas atmosphere RGA or the reduction rate of the sensor material section.

(40) Since such a determination of the gas constituents in the gas phase of the residual gas atmosphere RGA is practically impossible using conventional measures, for example by using a residual gas analyser such as e.g. a mass spectrometer, a suitable sensor unit 208 is introduced into the residual gas atmosphere. The sensor unit 208 enables such a determination of the suppression extent.

(41) As is indicated in FIG. 5, during the operation of the lithography apparatus 200, firstly a detection of the suppression extent is carried out in step S1, that is to say that, by way of example, a ratio of oxygen to hydrogen in the residual gas atmosphere RGA is detected on the basis of the measurement results from the sensor unit 208 in the manner of the sensor signals SS.

(42) Afterwards, in step S2, the oxygen is fed in such that process parameters are established which are expedient for operation and suppression of the reduction process of the silicon dioxide. That is to say that the proportion of oxygen to hydrogen can be considered as the suppression extent UM.

(43) The control unit 207, using the closed-loop or open-loop control of the valve 204, feeds an amount of oxygen GK to the residual gas atmosphere RGA such that the suppression extent UM lies within a specific range.

(44) FIG. 3 reproduces an illustration created from investigations by the applicant, which compares the silicon etching rate against the oxygen-hydrogen ratio. The ratio of oxygen O.sub.2 to hydrogen ions H.sup.+ in arbitrary units is indicated on the x-axis. A qualitative silicon etching rate likewise in arbitrary units is represented on the y-axis. The curve SE shows how the silicon etching rate changes as a function of the O.sub.2/H.sup.+ ratio. It can be stated that the O.sub.2/H.sup.+ ratio represents a suppression extent UM for the silicon etching or the reduction process—present during the etching process—by the hydrogen plasma.

(45) A high etching rate is present in a range ER of a low O.sub.2/H.sup.+ ratio, while a very low silicon etching rate is present in a range of a high O.sub.2/H.sup.+ ratio. The transition between these two ranges, namely an etching range or etching regime ER and the suppression range or suppression regime SR, is designated by TR and has a steep progression. That is to say that in the transition range TR the alteration of the suppression extent UM or of the O.sub.2/H.sup.+ ratio leads to an increase in the silicon etching.

(46) It is rather desired to leave as few foreign gases, e.g. oxygen, as possible in the UHV region. This is because the oxidative effect of the oxygen fed into the residual gas atmosphere RGA can have further effects on the substances present in the residual gas atmosphere RGA. In particular, a layer oxidation of an underlying silicon layer or barrier, e.g. of the optical coating of the mirrors, can arise, which can likewise impair the transmissive properties of the optical unit. This is taken into account in the metering of the oxygen GK.

(47) The oxygen is then fed (step S2) at least such that the suppression extent UM as O.sub.2/H.sup.+ ratio lies in a value range UMB in the suppression regime SR. This preferably takes account of the fact that there is a degree of uncertainty about the suppression extent determined or, in the example in FIG. 3, the ratio of oxygen to hydrogen proportions in the residual gas atmosphere RGA. The suppression extent UM is set by the addition of the oxygen into the residual gas atmosphere such that modulo an uncertainty or tolerance of a measurement of the suppression extent UM by the sensor unit 208 the value of the suppression extent UM lies within the UMB range. It can be stated that the oxygen feed in the step S2 (cf. FIG. 5) is regulated such that the suppression extent always remains in the suppression regime SR. The feeding of the gas component GK is regulated in such a way that the smallest possible value is achieved and that the suppression regime SR continues to be just upheld.

(48) The applicant carried out further investigations regarding the influence of the silicon contamination in a residual gas atmosphere RGA of a lithography apparatus such as is illustrated schematically in FIG. 2, for example. This involved investigating how the proportion of the oxygen in the residual gas atmosphere, measured in O.sub.2 partial pressure, influences the transmission properties of a mirror surface. FIG. 4 reproduces a corresponding diagram. The x-axis indicates the oxygen partial pressure logarithmically in arbitrary units (a.u.) within the residual gas atmosphere RGA with the presence of a hydrogen plasma PK. A transmission loss of a mirror surface of an EUV mirror is indicated on the y-axis. In the investigation, transmission loss is understood to mean the relative loss of light throughput through the entire optical unit relative to the initial value.

(49) In this case, FIG. 4 shows the influence of the silicon etching process and thus of the influence present on account of the precursor reaction, that is to say the reduction of silicon dioxide to silicon by the hydrogen plasma, on the transmissivity of EUV mirrors. The curve SC shows closed squares as measurement points for the silicon etching process. The curve corresponds approximately to the curve SE illustrated in FIG. 3. The curve SO formed from closed circles shows the influence of the oxidative effect of the oxygen within the residual gas atmosphere with regard to the layer oxidation. As indicated above, an increased addition of oxygen can lead to a layer oxidation of the optical coating, which results in a transmission loss that is considered to be disadvantageous. In this respect, by way of example, the O.sub.2 partial pressure is set as an alternative suppression extent UM such that the suppression extent UM still lies within a tolerable range for the formation of layer oxide SO, that is to say below a predefined threshold for the curve SO. FIG. 4 indicates such a range UMB. It has been found that the minimum of the combined transmission loss lies on the left in the UMB, that is to say at the smallest possible O.sub.2 concentrations which, however, are still just sufficient for the suppression of the etching process.

(50) In order to detect or to determine the suppression extent UM, it is proposed to monitor the influence of the gases present in the residual gas atmosphere RGA, such as, in particular, the hydrogen plasma PK, but also the oxygen GK fed in, with the aid of a sensor unit 208. A direct measurement in the gas phase may be prevented on account of the structural circumstances at the respective component part 201. In this respect, the sensor unit is positioned elsewhere within the residual gas atmosphere RGA, where an identical or similar plasma exposure is preferably present.

(51) FIG. 6 schematically shows the possible method steps for operating the lithography apparatus 200 with a sensor unit 208, wherein the suppression extent UM is monitored and the oxygen feed is set accordingly. Reference may also be made to a control loop, although the influence of the fed oxygen GK (cf. FIG. 2) on the reductive effect of the hydrogen or of the hydrogen plasma PK takes effect both at the surfaces 203 of the component part 201 and on the sensor material section 211. The control unit 207 controls the valve 204 via control signals CT in such a way that the suppression extent UM lies in the suppression regime SR (cf. FIG. 3) and also below a predefined threshold for the influence of the layer oxide production (cf. FIG. 4). To that end, in a method step S12, the sensor section property is measured preferably regularly over the course of operation of the lithography apparatus 200.

(52) Possible sensor unit configurations and sensor section properties are elucidated in the following FIGS. 7 to 12. From the sensor section property, or the changing sensor section properties, such as, for example, an electrical resistance, the suppression extent UM is deduced by the control unit 207. To that end, by way of example, a calibration curve can be present or a suitable specification for mapping the measured sensor section value onto a suppression extent can be used. By way of example, if the sensor section 211 has a specific resistance, this is mapped onto a suppression extent value. The oxygen feed is then set in such a way that the resistance value measured by the sensor unit 208 lies in a predefined range. This ensures that the metering of the oxygen feed will decrease the contamination on account of a silicon etching process within the residual gas atmosphere RGA.

(53) The following FIGS. 7 to 12 show sensor units in which different sensor materials within the residual gas atmosphere are exposed to the plasma components, such as the hydrogen and/or the gas that suppresses the reduction process, such as oxygen. From the measurement and investigation of the changing sensor material properties, the suppression extent is then determined through suitable mapping.

(54) FIG. 7A shows a first embodiment for a sensor unit 700 such as can be used as sensor unit 208 for example in the lithography apparatus in accordance with FIG. 2. The same applies to the modified sensor units in the subsequent figures as described below.

(55) The sensor unit 700 comprises a nanowire 701 formed from silicon dioxide, said nanowire being coupled to a resistance measurement instrument 703 via contacts 702. The resistance measuring instrument 703 supplies in particular the present resistance of the nanowire 701 to an evaluation unit 704. It is furthermore indicated that the plasma component PK, namely the hydrogen in the lithography apparatus, acts on the silicon dioxide nanowire 701. Corresponding nanowires 701 can be produced in various ways and have in particular diameters of between 10 and 500 nm. They can have for example lengths of between 10 nm and 10 to 100 μm. By way of example, silicon dioxide nanowires are obtained by oxidation of silicon structures.

(56) On account of the influence of the hydrogen plasma PK, the outer surface of the nanowires is reduced to silicon. This is indicated in FIG. 7B, in which a cross section of the nanowire 701 is illustrated and comprises an inner core composed of the silicon dioxide material 705 and a sheath 706 composed of reduced silicon. As a result of the chemical reduction process of the hydrogen at the surface of the nanowire 701, the conductivity or the resistance of the nanowire 701 changes depending on the reactivity of the hydrogen radicals present in the plasma PK. FIG. 7B reveals how the thickness T of the outer cladding 706, which has a lower resistance than the inner silicon dioxide 705, can vary. An electrical measurement in the sensor unit 700, for example a radiofrequency or else direct-current measurement or the respective current-voltage characteristic curve, is used to measure the influence of the plasma PK and the fed oxygen GK in the residual gas atmosphere can be deduced from the resulting resistance or conductance. That is to say that the resistance value can be used as a measure of the suppression extent UM.

(57) In one variant of the embodiment for a sensor unit 700, a silicon nanowire can be used as a sensor material section. This is indicated in FIGS. 7C to 7F. Proceeding from a cross section of a silicon nanowire having the diameter D as illustrated in FIG. 7C, silicon is present for example in amorphous form 708 in the nanowire 707. Under the influence of the oxygen that is introduced as a decreasing gas component GK, into the residual gas atmosphere, the surface of the nanowire 707 oxidizes, as is shown in FIG. 7D, to form silicon dioxide 709. This gives rise to a core-cladding structure comprising an inner silicon region 708 and the outer silicon dioxide surface 709. The altered mechanical and chemical structure of the nanowire 707 results in a variable electrical property, such as, in particular, a changing resistance.

(58) It is possible for the nanowire to be oxidized in such a way that the oxide layer becomes detached from the core, such that, as is shown in FIG. 7E, the nanowire is decreased with respect to its diameter D′. If the nanowire 707 oxidizes further as a result of the oxidation or etching process, the nanowire 707 can completely break or be decomposed in the etching region CR. This is illustrated in FIG. 7F. In the etching region CR, the thickness of the nanowire D2′ is thinner than in the preceding figures. If the nanowire 707 breaks, this results in a resistance value of infinite magnitude, which can be detected by the resistance measuring unit 703.

(59) The setting of the original thickness D of a nanowire 707 makes it possible to detect for example the effect or the influence of the oxygen fed to the interior of the lithography apparatus. In an embodiment of the sensor unit such as is shown in FIG. 8, a plurality of nanowires of different diameters can be provided and their electrical properties can be measured and monitored. Therefore, a further embodiment of a sensor unit 800 is illustrated in FIG. 8. Four nanowires 801, 802, 803, 804 are provided, which in each case are contacted via contacts 805 and are monitored by a respective resistance measuring unit 806. The resistance measuring units 806 supply measurement signals to an evaluation unit 807, which in turn supplies a sensor signal SS, for example to the control unit 204 (cf. FIG. 2). It is possible then to use for example the first silicon nanowire with a diameter of 7 nm, the second with 14 nm, the third with 21 nm, and the fourth with 28 nm. With the aid of the measured resistance values or else current-voltage characteristic curves for the respective nanowires 801, 802, 803 and 804, upon respective perforation of the nanowire 801, 802, 803, 804, the oxygen influence on the etching process can be detected in a stepped manner. In particular, in such a circuit it is also possible to combine the sensor variants 7B and 7E in order to enable an improved detection of the suppression extent UM.

(60) A further embodiment of a sensor unit 900 is illustrated in FIG. 9A. The sensor unit 900 comprises a silicon dioxide film or a layer 901 applied on a substrate 902. The substrate 902 can contain ruthenium, for example. The silicon dioxide film 901 is electrically contacted via contacts 903, such that a resistance value can be detected with the aid of a resistance measuring instrument 904. Once again the resistance, the conductance or a current-voltage characteristic curve that is measured with the current and/or resistance measuring instrument 904 provides information about the effect of the hydrogen or oxygen within the residual gas atmosphere.

(61) The evaluation unit 905 supplies a sensor signal that is fed to the control unit 207 (see FIG. 2).

(62) At the surface of the silicon dioxide film 901, a reduction to silicon can take place, in particular, in a manner similar to that at the component parts exposed to corrosion as a result of the hydrogen plasma. This is indicated in FIG. 9B. This reveals the ruthenium substrate 902 with the silicon dioxide layer 901 as sensor section material. The starting point is a thickness W of the silicon dioxide layer 901. Under the influence of the hydrogen radicals of the hydrogen plasma that have a reducing effect, silicon dioxide is reduced at the surface, such that solid silicon 906 is added to the surface. The electrical property that changes as a result, namely the resistance value and/or a conductance or a current-voltage characteristic curve, can be considered as a measure of the effect of the hydrogen concentration in the residual gas atmosphere. FIG. 9B on the right reveals how, in particular, the thickness W′ of the silicon dioxide 901 has decreased as a result of the reduction.

(63) Alongside electrical properties of the sensor material used, it is also possible for optical properties of the sensor material to be measured and tested. A further embodiment of a sensor unit is arranged schematically in FIG. 10. The sensor unit 1000 comprises a substrate 1002, which is formed for example from ruthenium and on which a thin material layer 1001 of a few nm to μm is arranged. The material layer 1001 is formed from silicon or silicon dioxide, for example. The sensor unit 1000 furthermore comprises a measuring arrangement comprising an optical emitter 1004 and a receiver 1005, which respectively receive control signals CT from the evaluation unit 1006 and emit them. The optical emitting unit 1004 radiates incident light I onto the surface 1003 of the material layer 1007. Reflected light R is received by the receiving unit 1005.

(64) It is possible, for example, for the measuring arrangement to be configured to carry out ellipsometric investigations. This involves measuring a change in a polarization state of the light I upon the reflection on the surface 1003 of the sensor material layer 1001. Since the surface 1003 changes on account of the hydrogen plasma effect and/or the oxidation effect of the oxygen fed in, a suppression extent UM is able to be derived from the change in the optical properties, for example the ellipsometric parameters ψ and δ. The evaluation unit 1006, likewise illustrated in FIG. 10, supplies a sensor signal SS.

(65) Besides the reflective measurement of the changing sensor material properties, it is also possible to carry out a transmissive measurement. To that end, yet another embodiment of a sensor unit is illustrated in FIG. 11. The sensor unit 1100 comprises a sensor material film 1101, which comprises silicon or silicon oxide, for example. Once again a measuring arrangement comprising an optical emitter 1104 and a receiver 1105 is provided, which emit and/or receive control signals CT and perform a transmission measurement through the thin film 1101, having a thickness of between 5 and 20 nm, for example. The evaluation unit 1106 once again supplies a sensor signal SS for further processing.

(66) Yet another embodiment of a sensor unit that is suitable for use in a lithography or mask inspection apparatus is illustrated in FIG. 12.

(67) The sensor unit 1200 comprises a quartz crystal microbalance 1201. In the case of the corresponding quartz crystal microbalance 1201, a piezoelectric element such as, for example, a quartz crystal as mass part is caused to oscillate. A resonant frequency is established in accordance with the respective configuration with regard to the dimensioning and mass of the oscillating element. As a result of a change in the mass of the oscillating mass part, the resonant frequency changes as well, which can in turn be detected by an evaluation unit 1204.

(68) A sensor material section, for example in the manner of a substrate layer arrangement, is then applied to the quartz crystal microbalance 1201. In this respect, in the example in FIG. 12, a substrate layer of ruthenium 1202 and a silicon or silicon oxide layer 1203 are applied to the quartz crystal microbalance 1201.

(69) As a result of the reaction with the hydrogen plasma present in the residual gas atmosphere, the mass of the film or of the layer 1203 changes on account of the etching processes and/or the chemical reactions. This results in a change in the resonant frequency of the quartz crystal microbalance 1201, which is in turn detected. In this respect, a change in mass of the sensor section can be detected precisely in an electronic manner. In this case, the change in mass may be brought about in particular by the reduction of silicon oxide to silicon, but also by an oxidation of silicon or an etching of silicon dioxide.

(70) The sensor units illustrated in FIGS. 7 to 12 are suitable in particular for arrangement in the projection device of a lithography apparatus. The proposed measuring or sensor arrangements and units comprising the possible sensor materials allow the suppressing gas component to be determined, without the need to carry out measurements in the gas phase. This is particularly expedient since the structural space within lithography or mask inspection apparatuses is usually limited. The dimensions of the sensors or the areas of the sensor sections are small enough that outgassing from the sensor material sections used can be disregarded.

(71) The proposed sensor units allow detection of the precursor of an etching process of silicon material within lithography or mask inspection apparatus component parts, which leads to a suitable metering of oxygen into the residual gas atmosphere. On account of an adjustment or a calibration of the sensor section properties detected with the aid of the sensor units to a suppression extent UM, the suppression extent can be set suitably in order at least partly to prevent the contamination within the lithography or mask inspection apparatus on account of undesired etching processes.

(72) Although the invention has been explained in greater detail on the basis of exemplary embodiments, it is modifiable in diverse ways. Although the function has been explained on the basis of the example of hydrogen plasmas and oxygen gas components, the contamination of other substances within the residual gas atmosphere as a result of etching processes and/or processes exhibiting reduction precursors can also be prevented. By way of example, contamination with phosphorus or tin constituents is also possible. Although an explanation has been given on the basis of the example of element materials comprising silicon, the corrosion of silicon layers in mirror surfaces is also decreased by the proposed methods and devices. Controlling the feed of suppressing gas components depending on mechanisms of action—detected via sensor units—of the harmful plasmas (in particular hydrogen plasma) in the manner of the suppression extent enables improved operation of the lithography or mask inspection apparatus. Although consideration is given to the example of the projection optical unit of a lithography apparatus in the description, the method can also be used in other vacuum regions or evacuated regions of a lithography or mask inspection apparatus. This applies in particular to the illumination optical unit, the reticle device or in the case of lithography apparatuses in the region of the wafer plane.

(73) It is advantageous, in particular, that a direct residual gas analysis required in the gas phase does not have to be carried out. Said analysis yields only indirect indications, however, such as a partial pressure distribution of H.sub.2, O.sub.2 and H.sub.2O, and moreover requires a reaction model (exhibiting errors) in order that the partial pressure distribution at other locations of the lithography apparatus and also the interaction there between the detected H.sub.2O, O.sub.2, H.sub.2 constituents and, in particular, silicon-containing material (e.g. reduction and etching processes) can be described.

(74) The proposed sensor units allow a direct measurement of the effect or effectiveness of the etching process, and a detailed reaction model is not required. With the aid of the sensor units, it is possible to implement a direct and local measurement variable in particular for the adjustment of oxygen/water metering. By contrast, conventional RGA measurement techniques normally only yield values for the gas composition in the residual gas atmosphere at a specific location of the lithography apparatus. Such measurements of the gas composition are therefore less well suited as measurement variables for the control of local chemical reactions. The proposed measures enable, with the aid of a small, structural-space-saving and comparatively expedient sensor unit, the targeted metering of the suppressing gas component, without having to evaluate the detailed reaction kinetics of the multi-component gas phase and the interaction thereof with the EUV radiation and the element material.

LIST OF REFERENCE SIGNS

(75) 100A EUV lithography apparatus 100B DUV lithography apparatus 102 Beam shaping and illumination system 103 Reticle device 104 Projection system 106A EUV light source 106B DUV light source 108A EUV radiation 108B DUV radiation 110 Mirror 112 Mirror 114 Mirror 116 Mirror 118 Mirror 120 Photomask/reticle 122 Mirror 124 Wafer 126 Optical axis 128 Lens element 130 Mirror 132 Medium 200 Lithography apparatus 201 Structural element 202 Coating 203 Reduction layer 204 Valve unit 205 Gas reservoir 206 Feed opening 207 Control unit 208 Sensor unit 209 Housing 210 Structural element body 211 Sensor material section 700 Sensor unit 701 Nanowire 702 Contact 703 Resistance measuring instrument 704 Evaluation unit 705 Material oxide 706 Reduced material oxide 707 Nanowire 708 Nanowire material 709 Nanowire material oxide 800 Sensor unit 801-804 Nanowire 806 Resistance measuring instrument 807 Evaluation unit 900 Sensor unit 901 Sensor material layer 902 Substrate 903 Contact 904 Resistance measuring instrument 905 Evaluation unit 906 Sensor material oxide layer 1000 Sensor unit 1001 Sensor material film 1002 Substrate 1003 Film surface 1004 Emitting unit 1005 Receiving unit 1006 Evaluation unit 1100 Sensor unit 1101 Sensor material film 1104 Emitting unit 1105 Receiving unit 1106 Evaluation unit 1200 Sensor unit 1201 Quartz crystal microbalance 1202 Substrate 1203 Sensor material layer 1204 Evaluation unit CR Etching region CT Control signal D, D1′, D″ Diameter ER Etching region GK Gas component I Incident light M1 Mirror M2 Mirror M3 Mirror M4 Mirror M5 Mirror M6 Mirror PK Plasma component R Reflected light RGA Residual gas atmosphere SC Silicon contamination SE Etching rate SO Layer oxide SR Suppression range SS Sensor signal S1 Detecting the suppression extent S2 Feeding the suppressing gas component S11 Arranging the sensor unit S12 Measuring the sensor section property S13 Determining the suppression extent T Transmitted light TR Transition range UM Suppression extent UMB Suppression extent range W, W′ Thickness