Endpoint determination using contrast gas

Abstract

This invention provides a method, apparatus, and computer program for repairing defects in lithography masks using a particle beam. [Solution] The present invention provides a method for repairing a defect on a lithography mask, comprising: (a) directing a particle beam to the defect to induce a local etching action on the defect; (b) monitoring the etching action using backscattered particles and / or secondary particles and / or any other free-space signals generated by the etching action to detect a transition point from the local etching action on the defect to a local etching action on the mask element below the defect; and (c) supplying at least one contrast gas to increase the contrast in detecting the transition point.

Description

Technical Field

[0001] The present invention relates to a method, an apparatus, and a computer program for repairing defects in a lithography mask using a particle beam.

Background Art

[0002] As a result of the steady improvement in integration density in microelectronics, lithography masks (hereinafter often simply referred to as "masks") increasingly need to image ever smaller structural elements into the photoresist layer of a wafer. To meet such requirements, the exposure wavelength is increasingly shifting to shorter wavelengths. Currently, mainly argon fluoride (ArF) excimer lasers are used for exposure, and these lasers emit light with a wavelength of 193 nm. Intensive research is being conducted on light sources that emit in the extreme ultraviolet (EUV) wavelength range (10 nm to 15 nm) and corresponding EUV masks. The resolution of the wafer exposure process has been improved by the simultaneous development of multiple variants of conventional binary lithography masks. Examples thereof include phase masks or phase shift masks and masks for multiple exposures.

[0003] Due to the increasingly miniaturized dimensions of structural elements, lithography masks cannot always be manufactured without printable or visible defects on the wafer. Since the manufacturing cost of masks is high, defective masks are always repaired if possible.

[0004] Two important groups of defects in lithography masks are, firstly, dark defects and, secondly, bright defects.

[0005] Dark defects are locations where absorber material and / or phase shift material is present but where such material should not be present. These defects are repaired by removing the extra material, preferably using a local etching action.

[0006] In contrast, bright defects are defects on a mask that have a higher transmittance than the same defect-free reference area during exposure in a wafer stepper or wafer scanner. In the mask repair process, such bright defects can be eliminated by depositing a material with appropriate optical properties. Ideally, the optical properties of the material used for repair should correspond to those of an absorbent or phase-shifting material.

[0007] One method for removing dark defects is to use an electron beam directed directly at the defect to be repaired (exposure). Specifically, the use of an electron beam allows for precise steering and positioning of the beam to the defect. The incident electron beam, along with a precursor gas (also called a process gas) that may be present in the atmosphere of the mask being repaired or adsorbed onto the mask itself, can induce a reaction similar to local etching. This induced local etching can remove excess material (from the defect) from the mask, allowing for the creation or restoration of desirable absorbent and / or phase-shift properties for the lithography mask.

[0008] Alternatively, the precursor gas used can be selected to induce a deposition process during beam exposure. As a result, additional material can be deposited on light defects to locally reduce the transmittance of the mask and / or to improve the phase shift characteristics.

[0009] The mask to be repaired generally has a multilayer structure consisting of at least two materials that are typically superimposed. Here, the upper material (the material facing the electron beam) can serve as an absorbent material, a phase-shifting material, or a defect material, while the lower material can serve as the substrate or support material (or material for the element beneath the defect) of the lithography mask to be repaired.

[0010] In the case of interaction between an electron beam or other particle beam used for etching or deposition with a precursor gas or defect material, backscattering of electrons or particles may occur. For example, backscattered electrons may be detected in parallel with the etching and / or deposition process, producing a backscattered electron signal (e.g., an EsB signal; EsB: Energy-selective backscattering). In addition to, or instead of, the process of interaction between the particle beam and the precursor gas or defect material may also generate secondary particles, such as electrons. For example, secondary electrons may produce a secondary electron signal (SE signal) that is similarly detectable in parallel with the etching or deposition process. By detecting the above particles or the signals they generate during the etching and / or deposition process, the progress of the repair operation can be monitored.

[0011] Specifically, accurate and precise detection of the transition point from the etching action on the defective material to the material of the element beneath the defect is crucial for the success of the repair operation. This transition point is also called endpoint detection. Accurate endpoint detection ultimately ensures that the mask to be repaired has the desired absorption and / or phase shift characteristics after the etching action is complete, and that, for example, the substrate material beneath the defective material is not eroded and / or removed by the etching action. Due to the high precision required for wafer structures in the semiconductor industry, similarly stringent requirements exist for the repair of lithography masks.

[0012] By monitoring the etching process by detecting backscattered and / or secondary particles formed (on the material being etched) during the etching process, a kind of real-time image of the etching process can be obtained. Therefore, the transition point of the etching process between materials can be determined by the changing contrast of the particle beam described above. However, in some cases, for example, when the materials present in the etching process are only slightly different (e.g., have similar atomic numbers), this contrast can be so weak that it is impossible to precisely determine the endpoint (the transition point of the etching process from the defective material to the material of the element below the defect).

[0013] Despite this problem, various methods are known for obtaining accurate results, such as the following:

[0014] U.S. Patent Application Publication 2004 / 0121069 discloses a method for repairing a phase-shift photomask using a charged particle beam system. In this case, topographic data from a scanning electron microscope is used as an alternative to endpoint detection. The topographic data can be used to adjust the irradiation dose of the charged particle beam for all points in the defect environment based on the elevation and surface gradient at that particular point.

[0015] U.S. Patent No. 6,593,040,B2 discloses a method and apparatus for correcting phase-shift defects in a photomask. This includes scanning the photomask and performing a three-dimensional analysis of the defects using an atomic force microscope (AFM). Based on the three-dimensional analysis, an etch map is created, and a focused ion beam (FIB) is controlled according to the etch map to remove the defects. To give the repair process higher accuracy, FIB test specimens are fabricated and analyzed in three dimensions.

[0016] However, these methods are time-consuming and complex. Furthermore, the etch rate is not always predictable, and therefore, despite the effort and complexity, they never consistently yield optimal results.

[0017] Therefore, the challenge to be addressed is to further improve the etching effect on defects. [Overview of the project]

[0018] The above objectives are achieved at least partially by the various embodiments of the present invention described below.

[0019] This application claims priority to German Patent Application DE 102020216518.1, which is incorporated herein by reference.

[0020] One embodiment may include a method for repairing defects on a lithography mask. In this method, (a) a particle beam can be directed to the defect to be repaired in order to induce a local etching action on the defect. The etching action can be monitored using backscattered particles and / or secondary particles and / or other free-space signals generated by the etching action in order to detect the transition point from the local etching action on the defect to the local etching action on the mask elements below the defect. Furthermore, (c) at least one contrast gas can be supplied to enhance the contrast in the detection of the transition point.

[0021] The inventors of this invention have found that the detection of transition points can be significantly improved by supplying a contrast gas (in the atmosphere surrounding the mask being repaired). This can be particularly useful in situations where the signals used for detecting transition points (backscattered particles, secondary particles, and / or other free-space signals generated by the etching action; in principle, all other types of signals that are generally suitable for detecting transition points are also conceivable; hereafter, for simplicity, we will always refer to free-space signals) change only to a barely detectable extent or become undetectable at the transition point. In particular, in such situations, a contrast gas that affects to different degrees the generation of signals on the material of the defect or on the material of the element beneath the defect can contribute to a relative increase in contrast, especially high. Specifically, it has been found that this effect can be obtained to a significant extent without significantly interfering with the etching action. Thus, the endpoint of the etching action can be reliably confirmed without the need for any iterative methods or particularly complex measuring devices.

[0022] For example, in the context of EsB endpoint detection, it is desirable to use at least 10 monochrome gradations, e.g., a total of 256 gradations, to ensure accurate determination of the endpoint. Here, in principle, it is also possible to obtain different required monochrome gradations depending on the detection device system used (which may include both hardware and software components). If the number of possible monochrome gradations changes, other monochrome gradations other than 10 may be considered, modified accordingly, so that endpoint detection can be performed. The monochrome gradation here may relate to the ratio of the signal intensity of backscattered electrons resulting when the defective material is removed to the signal intensity generated when the particle beam strikes the material beneath the defect. However, endpoint detection is not limited to EsB endpoint detection as described herein, and may also be performed using different mechanisms that cause backscattering and / or secondary electron generation so that the transition point from processing (e.g., removal) of the first material to the second material can be accurately detected, as described herein in general terms. The above monochrome gradation difference can also be used to determine the corresponding endpoint in processes other than EsB endpoint detection as described herein, and a monochrome gradation difference of 10 levels in the case of 256 possible monochrome gradations should be considered merely an illustrative guideline value.

[0023] Furthermore, endpoint detection can be improved by supplying a contrast gas, especially when the atomic numbers of the materials involved differ only slightly. In this case, the contrast gas can be selected in a material-dependent and / or application-specific manner. This allows for a much more accurate and reliable determination of the etching endpoint, and therefore more accurate repair of defects in the lithography mask, without having to accept unfavorable throughput losses or the adverse effects of the etching process itself.

[0024] The particles in a particle beam may be, for example, electrons, protons, ions, atoms, molecules, photons, etc.

[0025] For example, the contrast gas can be selected such that its adsorption rate and / or residence time (at least time-averaged) on the material of the element beneath the defect (hereinafter often referred to as the mask material) is higher than that of the contrast gas on the material of the defect (defect material). This can be done by the desirable requirement that the contrast gas adsorbs preferentially and / or more rapidly onto the material of the element beneath the defect, and / or resides for a longer period. There can be various reasons for the preferential absorption of the contrast gas on the mask material. For example, the contrast gas may exhibit a longer residence time on the mask material than on the defect material due to physicoadsorption. Similarly, and conversely, the contrast may have a longer residence time on the mask material than on the defect material due to chemiadsorption.

[0026] Due to preferential adsorption, a higher contrast with a greater influence on the resulting signal can be ensured by the contrast gas itself and / or by the stronger interaction of the contrast gas with a second material. For example, this can result in a stronger contrast of the mask material in EsB signals or SE signals (or other suitable signals). The contrast gas adsorbed on the surface of the mask can give a stronger or weaker EsB signal and / or a stronger or weaker SE signal compared to the defective material.

[0027] The contrast gas used can generally be selected to have a lower affinity for the defect material than for the material of the element beneath the defect. This allows for a clearer relative contrast increase, firstly, because the preferential adsorption of the contrast gas on the element beneath the defect affects the signal generated there for the detection of the transition point more significantly than on the defect material. Secondly, this can also minimize interference with the etching process, as the particle beam hits only the contrast gas more often if the local etching process on the defect is already complete.

[0028] Alternatively, or in addition to the above, the contrast gas can be selected to have a lower affinity (adsorption rate and / or residence time) for the defective material than the precursor gas used for the etching action. Alternatively, or in addition to the above, the contrast gas can be selected to have a higher affinity (adsorption rate and / or residence time) for the material of the element under the defect than the precursor gas used for the etching action.

[0029] Specifically, the contrast gas can be selected in this way based on material dependence and application.

[0030] Also, the contrast gas can be selected such that the degree to which it affects backscattering of particles and / or generation of secondary particles and / or other free space signals generated by the etching action on the defective material is different from the degree for the material of the underlying element. For example, the properties of the contrast gas can be such that its presence creates different properties with respect to backscattered particles and / or secondary particles, and / or other free space signals that can be detected by comparison with the mask material and / or the defective material. The presence and / or adsorption of the contrast gas on the defective material and / or the mask material can affect the natural properties of the defective material and / or the mask material with respect to backscattered particles and / or secondary particles and / or other free space signals such that the properties leading to the detection of these particles can vary depending on the contrast gas used. For example, a contrast gas adsorbed on the surface of the mask material can attenuate the signals of backscattered particles and / or secondary particles, and / or other free space beams emerging from the mask material.

[0031] It is also possible to select the contrast gas such that the incidence of the particle beam on the contrast gas causes further backscattering of the particles and / or generation of secondary particles or further other free space signals.

[0032] In one possible embodiment, the contrast gas may be an inert gas, such as a noble gas. This can help avoid the (adverse) effects of the contrast gas on the duration and quality of the etching action. The contrast gas may also be a gas with latent reactivity that has little to no effect on the success of the etching process, whether it is an inert gas or not.

[0033] The contrast gas can be supplied over at least two distinct periods. Therefore, the contrast gas is replenished at intervals (in low doses) rather than being supplied only once (in high doses). Furthermore, the contrast gas can be supplied at multiple intervals during the etching process (chopping). For example, it can be used to respond to dynamic changes during the etching process. While a sufficient concentration of contrast gas is always present, it can be ensured that overdoses of contrast gas are avoided. The latter is also advantageous in avoiding adverse effects on the etching process resulting from the presence of the contrast gas.

[0034] Chopping can also be represented by, for example, two or more characteristic periods. Firstly, this can be the period during which gas can flow in. Secondly, this can be the subsequent period during which gas does not flow in. This can be represented, for example, by the time a valve connected to a storage tank for a precursor gas (or contrast gas) is open, allowing this gas to reach the reaction site through it, and the time the valve remains closed. Typical time ratios for open to closed valves can be 1:10 (e.g., valve open for 1 second, closed for 1 second), 1:30, or 1:60, although it is also possible in principle to use different ratios.

[0035] The contrast gas can be supplied after the etching process has started, preferably only immediately before the expected transition point from etching on the defect to etching on the mask element below the defect. This reduces interference with the etching process by the contrast gas.

[0036] It is also possible to induce local etching in the absence of contrast gas. Furthermore, it is conceivable that contrast gas be supplied only after a predetermined expected degree of etching has been reached. Regardless, monitoring of the etching process may only begin after contrast gas has been supplied. Here, two or all of the latter three method steps may be performed. Alternatively, it is possible to perform only individual method steps from the latter method steps (for example, starting monitoring of the etching process only after contrast gas has been supplied).

[0037] A predetermined etching progress may relate to, for example, 25%, 50%, 75%, 90%, or any other degree of etching progress. 100% etching progress may be associated with the point at which the etching process transitions from etching the defect to etching the element beneath the defect. The etching process and / or etching progress can be monitored under operator supervision (e.g., as visual endpoint detection) or in a fully automated manner.

[0038] Prior to inducing the etching process, a lookup table may be calibrated, for example. The lookup table can be used to pre-set the etching progress, for example, as a function of time, as a function of the number of loops, etc. Once a predetermined expected etching progress is reached, a contrast gas can be supplied. The predetermined etching progress can be determined, for example, using a lookup table, specifically for the etching parameters used (beam parameters, precursor gas, material being etched, etc.). Alternatively, or in addition to, calibrating the lookup table, it is also possible to read a lookup table from memory that corresponds to, or at least relates to, the etching parameters of the etching process initiated at that point. Such a lookup table can be used as described herein. Since the etching process in this case can be essentially a linear process, the use of a predetermined expected etching progress allows for accurate estimation of the etching progress, especially in the case of homogeneous defect compositions (for example, equal etching progress can be achieved within the same period).

[0039] To induce etching, a precursor gas for etching can be supplied to the etching atmosphere. This precursor gas interacts with the incident particle beam, ultimately causing an etching reaction and removal of defective material. The process can proceed in a time series such that the contrast gas is supplied only after the precursor gas has been supplied. This can also contribute to further reducing interference with the etching process by the contrast gas. In this way, for example, defective material can be preferably coated with the precursor gas. Alternatively, it is equally possible to supply these two gases simultaneously into the etching atmosphere. Where appropriate, it is also conceivable to supply the contrast gas into the etching atmosphere before the precursor gas.

[0040] Precursor gases can influence the backscattering of particles and / or the generation of secondary particles and / or other free-space signals on the defect material and / or on the material of the element beneath it.

[0041] The contrast gas can be selected to replace the precursor gas on the material of the element below the defective material, preferably more strongly than on the defective material. This is particularly important to ensure that sufficient adsorption of the contrast gas on the mask material is always possible, and thus ensure early recognition of the transition point from etching the defective material to etching the material of the element below it. At the same time, a lower replacement of the precursor gas on the defective material can further minimize interference with the etching process.

[0042] Here, a useful contrast gas may be one or more oxidants, such as O2, O3, H2O, H2O2, N2O, NO, NO2, HNO3, and / or other oxygen-containing gases. Similarly, one or more halides, such as Cl2, HCl, XeF2, HF, I2, Hl, Br2, HBr, NOCl, NF3, PCl3, PF3, and / or other halogen-containing gases may be used. Cl2 can be considered a preferred contrast gas because it interferes only slightly with local etching and lowers the work function (thus producing a high SE signal). Similarly, useful contrast gases may include gases with reducing properties, such as H2, NH3, CH4, H2S, H2Se, H2Te, and other hydrogen-containing gases. Similarly, gaseous alkali metals (e.g., Li, Na, K, Rb, Cs) may be used as contrast gases, or components of the plasma (preferably a remote plasma generated separately from the sample) may be used. Furthermore, noble gases (e.g., He, Ne, Ar, Kr, Xe) can also be used. A further option is the use of surfactants (e.g., alkyl hydroxides, aliphatic carboxylic acids, mercaptoalkanes, alkylamines, alkyl sulfates, alkyl phosphates, alkylphosphonates, and aromatic compounds and other organic compounds can be used instead of alkyl compounds). It should also be noted that the above contrast gases can be used as precursor gases.

[0043] Useful precursor gases may be one or more (metal, transition element, main group) alkyls, such as cyclopentadienyl (Cp)- or methylcyclopentadienyl (MeCp)-trimethylplatinum (CpPtMe3 and / or MeCpPtMe3), tetramethyltin SnMe4, trimethylgallium GaMe3, ferrocene Cp2Fe, bisallylchromium Ar2Cr, dicyclopentadienylruthenium Ru(C5H5)2, and other compounds of this kind. Similarly, one or more (metal, transition element, main group) carbonyls, such as chromium hexacarbonyl Cr(CO)6, molybdenum hexacarbonyl Mo(CO)6, tungsten hexacarbonyl W(CO)6, dicobalt octacarbonyl Co2(CO)8, trilthenium dodecacarbonyl Ru3(CO) 12 It is also possible to use iron pentacarbonyl Fe(CO)5 and / or other compounds of this kind. Similarly, one or more (metal, transition element, main group) alkoxides, such as tetraethoxysilane Si(OC2H5)4, tetraisopropoxytitanium Ti(OC3H7)4 and other compounds of this kind, can be used. It is also possible to use one or more (metal, transition element, main group) halides, such as WF6, WCl6, TiCl6, BCl3, SiCl4 and / or other compounds of this kind. Similarly, it is also possible to use one or more (metal, transition element, main group) complexes, such as copper bis(hexafluoroacetylacetonate) Cu(C5F6HO2)2, dimethylgold trifluoroacetylacetonate Me2Au(C5F3H4O2) and / or other compounds of this kind. Furthermore, it is also possible to use CO, CO2, aliphatic or aromatic hydrocarbons, components of vacuum pump oil, volatile organic compounds and / or other compounds of this kind. It should also be noted that the listed precursor gases may be used as contrast gases.

[0044] Those skilled in the art will see that the above enumeration is not exhaustive, and that any desired combination of the selected contrast gases and precursor gases listed here merely as examples is also possible, including those not listed as selected.

[0045] In preferred embodiments, there is a combination of contrast gases that have opposing effects on the EsB / SE signal (or different signals used) with respect to the effect of the precursor gas. The effects here relate to the material being etched and the material not being etched. In this case, for example, the adsorbed precursor gas may lower the work function of the material (higher SE signal), while the contrast gas may increase the work function (lower SE signal), and vice versa.

[0046] Note that instead of supplying the contrast gas (for example, after the etching process has started), the contrast gas may already be present (at a low concentration), and its concentration may then be simply increased in the specified manner (for example, after the etching process has started but before its expected end).

[0047] After the etching transition point is detected, the etching process can be stopped to prevent undesirable etching of the mask material beneath the defective material. For example, this can be done by stopping the particle beam.

[0048] Similarly, the processes described herein can be implemented as a computer program. This may be a computer program that, when executed, has instructions causing a computer to perform a method having one or more steps of the methods described herein.

[0049] Repair of defects on a lithography mask can also be performed by an apparatus that includes means for directing a particle beam onto the defect. The apparatus may further include means for monitoring the etching action using backscattered particles and / or secondary particles and / or other free-space signals generated by the etching action so that a transition point from etching action on the defect to etching action on the mask elements below the defect can be detected. Finally, the apparatus may include means for supplying at least one contrast gas so that the contrast in detecting the transition point can be increased.

[0050] The apparatus may further include means configured to perform the steps described herein with respect to the method.

[0051] An apparatus for repairing defects in a lithography mask may also include the above-described computer program and be configured to perform one or more of the above-described method steps according to instructions in the program.

[0052] In the following detailed description, possible embodiments of the present invention will be described with reference to the following drawings. [Brief explanation of the drawing]

[0053] [Figure 1a] This figure shows an example of endpoint detection without contrast gas. [Figure 1b] This figure shows an example of endpoint detection without contrast gas. [Figure 2a] This figure shows an example of endpoint detection using contrast gas. [Figure 2b] This figure shows an example of endpoint detection using contrast gas. [Figure 3a] This figure shows an example of the adsorption characteristics of contrast gases. [Figure 3b] This figure shows an example of the adsorption characteristics of contrast gases. [Figure 4a] This figure shows an example of the adsorption characteristics of a contrast gas and a precursor gas. [Figure 4b] This figure shows an example of the adsorption characteristics of a contrast gas and a precursor gas. [Figure 5A] This figure shows the signal progression at the transition point during local etching in the absence and presence of contrast gas. [Figure 5B] This figure shows the signal progression at the transition point during local etching in the absence and presence of contrast gas. [Modes for carrying out the invention]

[0054] Embodiments of the present invention will be described below with reference primarily to the repair of lithography masks, particularly masks for microlithography. However, the present invention is not limited thereto and can be used for other types of mask processing, or more generally, for surface processing in general, such as for other things used in the field of microelectronics, such as for the modification and / or repair of structured wafer surfaces or microchip surfaces. For example, it is possible to repair defects that are generally attributable to a surface or surface element. Therefore, in order to make the description clearer and easier to understand, even when applications for processing mask surfaces are mentioned below, those skilled in the art will also keep in mind other possible uses of the disclosed teachings.

[0055] It should also be noted that the following may describe in more detail only individual embodiments of the present invention. However, those skilled in the art will see that the features and modification options described in relation to these embodiments can be further modified and / or combined with each other in other combinations or subcombinations, without departing from the scope of the present invention. Furthermore, individual features or sub-features can be omitted if they are not necessary for obtaining the intended results. Therefore, to avoid unnecessary repetition, refer to the descriptions and explanations in the previous section, which also remain valid for the detailed descriptions below.

[0056] Figure 1a shows a schematic diagram of a conventional endpoint detection method that uses etching induced by a beam of charged particles, such as that used for repairing lithography masks. Other charged particles can be used, but in this case, a beam of particle 1, for example, electrons, can be guided onto a first material 2. This first material 2 may have or may have dark defects D. This can result in undesirable absorption characteristics or undesirable phase shifts at the location of the defects to allow light to pass through, for example, as employed in wafer manufacturing in the semiconductor industry. Therefore, the objective of the repair method is to remove this excess material accordingly. Here, the first material 2 can be added to a second material 3, which functions as a substrate or mask. Both materials can take the form of material layers, but other material configurations are also possible. For example, the first material 2 can be locally bonded on top of a layer formed by the second material 3.

[0057] To remove defect D in a desirable manner, a precursor gas (not shown herein) can be supplied to the surrounding, typically enclosed atmosphere, which can interact with the incident beam of charged particle 1 to produce a local etching effect at the location of the incident particle beam. The incident particle beam can here be systematically guided over the defect region by interaction with a magnetic field and / or electric field and / or other control methods, resulting in the corresponding removal of defect D. As a result of the interaction of charged particle 1 with incident beam 1, backscattered particles 4a and / or secondary particles 4b and / or other free-space beams 4c are obtained (even if the embodiments described below are limited to backscattered particles and / or secondary particles, other types of particles / beams that allow determination regarding the progress of the etching effect are equally advantageously available). These particles or this beam provide options for monitoring the etching effect. Since the first material 2 and the second material 3 may typically differ in composition (e.g., with respect to their respective atomic numbers), there may be changes in the signal 5 detected from the backscattered particles 6 and / or secondary particles 7 and / or free-space beams. The detected change in signal allows for the determination that the defective material D has been completely removed and the incident charged particle beam is interacting with the second material 3.

[0058] Figure 1b shows a scenario in which the defect D, consisting of the first material 2, is completely removed. In this case, the charged beam 1 strikes the substrate material 3 directly, and there is no further local interaction with the first material 2. This results in a change in the detectable signal 5, such that the signals from backscattered particles and / or secondary particles are altered compared to the scenario shown in Figure 1a. For example, the signal from backscattered particles can be increased. Alternatively, or in addition to this, the signal produced by the second particle can be attenuated.

[0059] The known problems with the lithography mask repair method shown in Figures 1a and 1b arise particularly when the detectable signal at the transition point from the first material to the second material does not change, changes in an undetectable manner, or can only be detected with difficulty. In such cases, monitoring the etching process is only possible with difficulty. Therefore, precise determination of the endpoint, i.e., the point at which defect D consisting of the first material 2 has been completely removed, is only possible with very limited accuracy. As a result, the particle beam-induced etching process may also unintentionally remove a portion of the second material 3, thereby affecting the absorption and / or phase shift properties of the mask. This can occur particularly when the two materials 2 and 3 have very similar interaction characteristics with the charged particle 1.

[0060] This problem and its limitations are recognized by the applicant, and the present invention is optimized in that a contrast gas can be supplied to the etching process so that the material transition point from the first material 2 to the second material 3 can be known with greater precision during etching.

[0061] Figure 2a shows an etching action that can be used to repair a lithography mask. In addition to the methods shown in Figures 1a and 1b, a contrast gas 8 can be supplied to the etching action. This contrast gas 8 can be selected here to preferentially adsorb to the second material 3. When the particle beam 1 strikes a defect D made of the first material 2, it interacts primarily with the first material 2 and less with the supplied contrast gas 8. Therefore, the signal intensities 6 and 7 detectable during the etching action on the first material 2 may initially be similar to those in the embodiment described in Figure 1a.

[0062] Figure 2b shows a scenario for the complete removal of defect D. In this scenario, the second material 3 can be exposed to the supplied contrast gas 8, and the contrast gas 8 can be selected to preferentially adsorb to the second material 3, so that the particle beam 1 does not directly hit the second material 3, but rather hits the gas particles of the contrast gas adsorbed on the second material 3. The contrast gas 8 can have different properties from the second material 3 with respect to the generation of backscattered particles 6 and / or secondary particles 7, or can change the properties of at least the second material 3 with respect to this. This can increase the contrast between the signals from backscattered particles and / or secondary particles that result from the interaction between the particle beam 1 and the first material 2, or from the interaction at location 9 of the contrast gas 8 adsorbed on the second material. As an example, Figure 2b shows an increase in the signal of backscattered particles 6 and a decrease in the signal of secondary particles 7. However, this is just one example. In each case, it is possible to detect only one of these signals and / or another free-space signal, and fluctuations in signal intensity are possible in either direction.

[0063] In a preferred embodiment, local etching can be induced in the absence of a contrast gas.

[0064] Independently of the above, lookup table calibration can be considered. In the lookup table, parameters such as etch rate, etch time, and number of cycles can be associated with the parameters of particle beam 1 (e.g., power, acceleration voltage, particle type, etc.) and / or the parameters of the first material and / or second material 3 and / or precursor gas and / or contrast gas. Based on this, it is possible to predict the timing of the transition of the etching action from the first material 2 to the second material 3 for a particular etching action for various beam or etch parameters. Herein lies the calibration of the lookup table for both the presence and absence of the contrast gas.

[0065] Depending on the embodiment, calibration may not necessarily be performed before all etching operations. This is also true if the lookup table is stored on a storage medium and is based on previously recorded data or working parameters. Based on the calibrated lookup table and / or stored lookup table, it is possible to pre-set, for example, the expected etching progress over time with or without contrast gas.

[0066] Nevertheless, contrast gas 8 can be supplied only when, for example, the etching progress has already reached a predetermined stage. This predetermined stage can be determined, for example, by a lookup table. Supplying contrast gas only during the etching process (for example, near its end) can minimize any interfering effects of contrast gas 8 on local etching. These may manifest, for example, in changes in the etch rate and / or etch selectivity in the presence of contrast gas compared to the absence of contrast gas, and these changes may in some cases lead to inaccurate predictions regarding the progress of etching and / or a decrease in etch quality.

[0067] The etching process may be monitored only after the contrast gas has been supplied. In that case, each sensor, program, etc., only needs to be operational after or during the supply of the contrast gas.

[0068] An example of the adsorption properties of contrast gas 8 is shown in Figures 3a and 3b. Here, contrast gas 8 can be selected to have a high affinity for adsorption on the second material 3 and exhibit lower adsorption on the first material 2. Therefore, the selected contrast gas 8 can produce an "artificial" relative increase in the signal contrast at the transition point of the etching operation on the first material 2 to the second material 3, for example, in the signals of backscattered particles and / or secondary particles being monitored during the etching operation. This can enable more accurate endpoint detection during repair operations on the lithography mask. Although not shown, it is also possible, of course, for a precursor gas to be present in the (upper) atmosphere of the first material 2 and / or the second material 3. This precursor gas can also adsorb to the surface of the first material 2 and / or the second material 3, in which case its absorption properties may vary. In these cases as well, contrast gas 8 can be selected to have a high affinity for adsorption on the second material 3 and exhibit lower adsorption on the first material 2. Therefore, the selected contrast gas 8 can contribute to an "artificial" relative increase in contrast, even in the presence of the precursor gas 10.

[0069] Figures 4a and 4b show examples of the absorption characteristics of contrast gas 8 and an additional precursor gas 10. Figure 4a shows the case where the first material 2 is exposed to both contrast gas 8 and precursor gas 10. The contrast gas 8 can be selected to have a lower affinity for the first material 2 than the precursor gas 10, for example, so that it adsorbs less to the first material 2 than the precursor gas 10. This can contribute to reducing the degree of influence of the contrast gas 8 on the etching process of the first material 2.

[0070] Figure 4b shows the situation in which the second material 3 is exposed to the precursor gas 10 and the contrast gas 8. The contrast gas 8 can be selected to have a higher affinity for the second material 3 than for the first material 2. Therefore, the precursor gas 8 can be adsorbed to a higher degree on the second material 3 than on the first material 2. Alternatively, or in addition to this, the precursor gas 10 can be selected to have a higher affinity for the first material 2 than for the second material 3. An overall situation may arise in which there is initially more adsorption of the precursor gas 10 on the surface of the first material 2 (Figure 4a), and at the point of transition of the etching process to the second material 3, there is at least partial replacement of the precursor gas 10 from the second material 3 by the contrast gas 8.

[0071] Alternatively, or in addition to the above, the contrast gas 8 and the precursor gas 10 can be selected such that the contrast gas 8 adsorbs more of the second material 3 compared to the precursor gas 10. In this way, at least partial replacement of the precursor gas 10 by the second material 3 may occur at the point of transition of the etching action to the second material 3.

[0072] The coverage ratio of the surface of the second material 3 by the precursor gas 10 to the contrast gas 8 may be lower than that on the first material 2 (higher coverage is also possible, in which case the etching process tends to more favorably maintain a higher coverage of the first material 2 by the precursor gas 10). Higher contrast of the signal 5 observable during the etching process (e.g., with respect to the EsB signal and / or SE signal) may result as a result of the contrast gas 8 itself and / or as a result of the interaction of the contrast gas 8 with the second material 3.

[0073] Similarly, consider the case where the precursor gas 10 does not significantly adsorb onto the first material 2 or the second material 3, but instead is only present, for example, in the atmosphere surrounding these two materials. It may suffice if the selected contrast gas 8 has a higher absorption rate (e.g., average over time) and / or a longer residence time on the second material 3 than on the first material 2. The absorption may be the result of processes such as physicoadsorption and / or chemiadsorption and / or other processes that cause adsorption.

[0074] More specifically, a selected contrast gas 8 adsorbed on the surface of the second material 3 can produce a different contrast in the EsB signal and / or SE signal compared to the first material 2. This may result from the generation of a stronger or weaker EsB signal compared to the second material 3 by the contrast gas 8 adsorbed on the surface of the second material 3. Furthermore, a stronger or weaker SE signal compared to the second material 3 may be generated by the contrast gas 8 adsorbed on the surface of the second material 3. Finally, instead of, or in addition to, the contrast gas 8 adsorbed on the surface of the second material 3 can attenuate the EsB signal and / or SE signal emitted from the second material 3.

[0075] It is also conceivable that the contrast gas itself is not significantly adsorbed, but rather causes an average change in the occupation of the first or second material by the precursor gas.

[0076] Figures 5A and 5B show possible methods for determining whether the local etching action on the first material 2 has already progressed to the etching action on the second material 3 beneath the first material 2, in the state where contrast gas 8 is absent (Figure 5A) and in the state where contrast gas 8 is present (Figure 5B).

[0077] Figure 5A shows a plot of a possible detectable signal, consisting of backscattered particles and / or secondary particles or other free-space signals generated by etching, against the number of etching operations (e.g., time). Here, reference numeral 2 indicates that the detectable signal is associated with local etching on the first material 2, prior to the transition point 12 of etching from the first material 2 to the second material 3. As can be seen from Figure 5A, this transition point can be associated with a change in the signal 11. In this embodiment, the change in the signal 11 includes a decrease in the signal. However, it should be noted that this is just one example, and an increase in the signal at the transition point 12 is also possible. The transition point 12 can be assumed here to be when the change in the signal 11 exceeds a predetermined critical threshold, i.e., when Δsignal > threshold. In Figure 5A, the threshold is smaller than or equal to the noise in the detected signal. Therefore, the contrast is low. This can occur, in particular, when the change in the signal relative to the expected value is considered to be relatively low or equal to the expected noise level.

[0078] Figure 5B shows the same configuration as Figure 5A, but differs in that it illustrates the effect on the detectable signal when contrast gas 8 is supplied to the local etching process. In this example, contrast gas 8 causes a more pronounced signal change 11 (a signal decrease in this example) in the detectable signal at the transition point 12 than shown in Figure 5A. This enables more accurate determination of the transition point 12 and, therefore, more precise endpoint detection of the local etching process. Note that the presence of contrast gas 8 can also lead to an increase in the detectable signal at the transition point 8.

Claims

1. A method for repairing defects in a lithography mask, a. Directing a particle beam onto the defect in order to induce a local etching effect on the defect, b. Monitoring the etching action using backscattered particles and / or secondary particles and / or any other free-space signals generated by the etching action in order to detect the transition point from the local etching action on the defect to the local etching action on the mask element below the defect, c. A method comprising supplying at least one contrast gas to increase the contrast in the detection of the transition point.

2. The method according to claim 1, further comprising selecting the contrast gas such that the adsorption rate and / or residence time of the contrast gas on the material of the element below the defect is higher than the adsorption rate or residence time of the contrast gas on the material of the defect.

3. The method according to claim 1 or 2, wherein the extent to which the contrast gas has an effect on the material of the defect due to the backscattering of the particles and / or the generation of secondary particles and / or the etching action of the other free-space signals is different from the extent to which it has an effect on the material of the element below.

4. The method according to any one of claims 1 to 3, wherein the incidence of the particle beam onto the contrast gas results in backscattering of particles and / or generation of secondary particles.

5. The method according to any one of claims 1 to 4, wherein the contrast gas is an inert gas.

6. The method according to any one of claims 1 to 5, wherein the contrast gas is supplied for at least two separate periods.

7. The method according to any one of claims 1 to 6, wherein the contrast gas is supplied after the etching action has started, preferably only immediately before the expected transition point from the etching action on the defect to the etching action on the element of the mask below the defect.

8. Inducing the local etching effect in the absence of the contrast gas, The method further includes supplying the contrast gas after a predetermined expected degree of etching has been reached. The method according to any one of claims 1 to 7, wherein the etching action is monitored only after the contrast gas has been supplied.

9. The method according to any one of claims 1 to 8, comprising supplying a precursor gas for the etching action.

10. The method according to claim 9, wherein the contrast is supplied after the precursor gas has been supplied.

11. The method according to claim 9 or 10, wherein the precursor gas affects the backscattering and / or generation of secondary particles of particles on and / or below the material of the element of the defect.

12. The method according to any one of claims 9 to 11, further comprising selecting the contrast gas such that the contrast gas replaces the precursor gas on the material of the element below, preferably more significantly than the precursor gas on the material of the defect.

13. A computer program having an instruction that, when executed, causes a computer to perform the method described in any one of claims 1 to 12.

14. A device for repairing defects on a lithography mask, a. Means for directing a particle beam onto the defect in order to induce a local etching effect on the defect, b. Means for monitoring the etching action using backscattered particles and / or secondary particles and / or any other free-space signals generated by the etching action in order to detect the transition point from the local etching action on the defect to the local etching action on the mask element below the defect, c. An apparatus comprising means for supplying at least one contrast gas to increase the contrast in the detection of the transition point.

15. An apparatus for repairing defects in lithography materials, comprising the computer program described in claim 13.

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