Endpoint detection using contrast gas

Abstract

The present invention includes a method of repairing a defect on a lithography mask comprising: (a.) directing a particle beam at the defect to induce a localized etching action on the defect; (b.) monitoring the etching action using backscattered particles and / or secondary particles and / or any other free space signal generated by the etching action to detect a transition point from the localized etching action on the defect to a localized etching action on an element of the mask below the defect; and (c.) providing at least one contrast gas to increase contrast in detecting the transition point.

Description

[Technical Field]

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

[0002] As a result of the steady increase in integration density in microelectronics, lithography masks (hereafter often simply referred to as "masks") are required to image ever-smaller features into the photoresist layer of the wafer. To meet these requirements, exposure wavelengths are shifting to shorter and shorter wavelengths. Currently, argon fluoride (ArF) excimer lasers are primarily used for exposure; these lasers emit light at a wavelength of 193 nm. Intensive research is being conducted on light sources emitting in the extreme ultraviolet (EUV) wavelength range (10 nm–15 nm) and corresponding EUV masks. The resolution of wafer exposure processes is being improved by the simultaneous development of several variants of the traditional binary lithography mask. Examples include phase or phase-shift masks and masks for multiple exposures.

[0003] Due to the ever-decreasing dimensions of structural elements, lithography masks cannot always be manufactured without printable or visible defects on the wafer. Because the manufacturing costs of masks are high, defective masks are always repaired when possible.

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

[0005] Dark defects are areas where absorber and / or phase-shifting material is present but where such material should not be present. These defects are repaired by removing the excess material, preferably using a localized etching action.

[0006] In contrast, bright defects are defects on a mask that, upon exposure in a wafer stepper or wafer scanner, have a greater transmittance than an identical defect-free reference spot. In a 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 the absorber or phase-shifting material.

[0007] A method for removing dark defects is to use an electron beam (exposure) that is directed directly at the defect to be repaired. The use of an electron beam specifically allows for precise steering and positioning of the beam at the defect. Together with a precursor gas, also called a process gas, that may be present in the atmosphere of the mask to be repaired or adsorbed on the mask itself, the incident electron beam can induce a reaction similar to a local etching action. This induced local etching action can remove portions of excess material (at the defect) from the mask, so as to create or restore the desired absorbent and / or phase-shifting properties of the lithography mask.

[0008] Alternatively, the precursor gas used can be chosen such that it can induce a deposition process upon exposure to the beam, so that additional material can be deposited on the bright defects in order to locally reduce the transmission of the mask and / or improve the phase shift properties.

[0009] The mask to be repaired generally has a multi-layer structure, typically consisting of at least two superimposed materials, where the upper material (the material facing the electron beam) can act as an absorber material, a phase-shift material, or a defect material, and the lower material can act as a substrate or carrier material for the lithographic mask to be repaired (or as the material of the element below the defect).

[0010] In the case of the interaction of the electron beam or other particle beam used for etching or deposition with the precursor gas or the material of the defect, there may be backscattering of electrons or particles. For example, the backscattered electrons may be detected in parallel with the etching and / or deposition process, resulting in a backscattered electron signal (e.g., an EsB signal; EsB: energy-selective backscattering). Additionally or alternatively, the interaction process between the particle beam and the precursor gas or the material of the defect may generate secondary particles, such as electrons. For example, the secondary electrons may generate a secondary electron signal (SE signal), which may also be detectable in parallel with the etching or deposition process. The progress of the repair operation can be monitored by detecting the particles or signals generated by them during the etching and / or deposition process.

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

[0012] Monitoring the etching process by detecting backscattered and / or secondary particles formed (on the material being etched) during the etching process can provide a kind of real-time image of the etching process. Thus, the changing contrast of the particle beam can determine the transition point of the etching process between materials. 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 precise determination of the endpoint (the transition point of the etching process from the material of the defect to the material of the element below the defect) is not possible.

[0013] Despite this problem, various techniques are known to obtain accurate results:

[0014] U.S. Patent Application Publication No. 2004 / 0121069 discloses a method for repairing phase-shift photomasks with a charged particle beam system, where topographic data from a scanning electron microscope is used as a surrogate for endpoint detection. The topographic data can be used to adjust the dose of the charged particle beam for every point within the defect environment based on the elevation and surface slope at that particular point.

[0015] U.S. Patent No. 6,593,040 B2 discloses a method and apparatus for the repair of phase-shift defects in a photomask. This involves scanning the photomask and performing a three-dimensional analysis of the defects using an AFM (atomic force microscope). 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 provide greater accuracy in the repair process, a FIB specimen is fabricated and analyzed in three dimensions.

[0016] However, these techniques are time consuming and complicated, and the etch rate is not always accurately predictable, so despite the effort and complexity, they do not always produce optimal results.

[0017] Therefore, the problem to be addressed is to further improve the etching action on defects. Summary of the Invention

[0018] The above objectives are achieved, at least in part, by the various aspects of the present invention described below.

[0019] This application claims priority from German patent application DE 102020216518.1, which is incorporated herein by reference.

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

[0021] The inventors of the present invention have recognized that the detection of transition points can be significantly improved by supplying a contrast gas (in the atmosphere surrounding the mask to be repaired). This can be particularly useful in situations where the signal used to detect the transition point (backscattered particles, secondary particles, and / or other free-space signals generated by the etching process; in principle, any other type of signal suitable for detecting transition points is also contemplated; for simplicity, we will always refer to free-space signals hereinafter) changes only barely detectably or is undetectable at the transition point. In particular, in such situations, a contrast gas that affects the signal generation on the material of the defect and on the material of the element below the defect to different degrees can contribute to a particularly high relative increase in contrast. Specifically, it has been found that this effect can be obtained to a significant degree without significantly interfering with the etching process. Therefore, the endpoint of the etching process can be reliably confirmed without the need for any repetitive methods or particularly complex measurement devices.

[0022] For example, in the context of EsB endpoint detection, it is desirable to use a monochrome gray scale difference of at least 10 levels, e.g., a total of 256 levels, to ensure accurate endpoint determination. However, in principle, different required monochrome gray scale differences are also possible, for example, depending on the detection device system used (which may include both hardware and software components). If the number of possible monochrome gray scales varies, correspondingly modified monochrome gray scale differences other than 10 are also conceivable to enable endpoint detection. The monochrome gray scale difference may relate here to the ratio of the signal intensity of backscattered electrons resulting from the removal of the material at the defect to the signal intensity generated when the particle beam strikes the material below the defect. However, endpoint detection is not limited to EsB endpoint detection as described herein and may also be performed using different mechanisms that result in the generation of backscattered and / or secondary electrons, for example, to enable accurate detection of the transition point from the processing (e.g., removal) of a first material to a second material, as described in general terms herein. The above monochrome tone difference can also be used to make corresponding endpoint determinations in processes other than the EsB endpoint detection described herein, and the 10-tone monochrome tone difference in the case of 256 possible monochrome tones should be considered merely as an exemplary 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. The contrast gas can then be selected, for example, in a material-dependent and / or application-specific manner. This allows for a much more accurate and reliable determination of the endpoint of the etching process, and thus a more accurate repair of defects in lithography masks, without having to accept adverse throughput losses or adverse effects of the etching process itself.

[0024] The particles of the particle beam may be, for example, electrons, protons, ions, atoms, molecules, photons, and the like.

[0025] For example, the contrast gas can be selected so that the contrast gas has a higher (at least time-averaged) adsorption rate and / or residence time in the material of the element under the defect (hereinafter sometimes referred to as the mask material) than the adsorption rate or residence time of the contrast gas on the material of the defect (the defect material). This can be achieved by the desired requirement that the contrast gas preferentially and / or more quickly adsorbs and / or resides longer on the material of the element under the defect. There can be various reasons for the preferential absorption of the contrast gas in the mask material. For example, the contrast gas can exhibit a longer residence time on the mask material than on the defect material due to physisorption. Similarly, or alternatively, the contrast gas can have a longer residence time on the mask material than on the defect material due to chemisorption.

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

[0027] The contrast gas used can generally be selected to have a lower affinity for the material of the defect than for the material of the element below the defect. This allows, firstly, for a clearer relative contrast increase, since the preferential adsorption of the contrast gas on the element below the defect will correspondingly affect the signal generated there for the detection of the transition point more than on the material of the defect. Secondly, this can also minimize the disturbance of the etching action, since the particle beam will only hit the contrast gas more if the local etching action on the defect has already ended.

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

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

[0030] The contrast gas can also be selected to affect particle backscattering and / or secondary particle generation and / or other free-space signals generated by etching on the defect material to a different extent than the underlying material. For example, the characteristics of the contrast gas can be such that its presence results in different characteristics of detectable backscattered particles and / or secondary particles and / or other free-space signals compared to the mask material and / or defect material. The presence and / or adsorption of the contrast gas on the defect material and / or mask material can affect the natural characteristics of the defect material and / or mask material with respect to backscattered particles and / or secondary particles and / or other free-space signals, such that the characteristics leading to the detection of these particles can vary depending on the contrast gas used. For example, a contrast gas adsorbed to the surface of the mask material can attenuate backscattered particles and / or secondary particles and / or other free-space beam signals emanating from the mask material.

[0031] It is also possible to select the contrast gas such that incidence of the particle beam on the contrast gas results in 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 any adverse effects of the contrast gas on the duration and quality of the etching action. The contrast gas, whether inert or not, can likewise be any potentially reactive gas that has little or no significant effect on the success of the etching process.

[0033] The contrast gas can be supplied in at least two separate periods. Thus, the contrast gas is not supplied only once (at a high dose), but is replenished at intervals (at a low dose). It is also possible to supply the contrast gas at multiple intervals during the etching process (chopping), for example, to respond to dynamic changes during the etching process. It is possible to ensure that a sufficient concentration of contrast gas is always present, but that excessive doses of contrast gas are avoided. The latter can also be advantageous to avoid adverse effects on the etching process as a result of the presence of contrast gas.

[0034] Chopping can also be represented, for example, by two or more characteristic periods. First, this can be a period during which gas can flow in. Second, it can be a subsequent period during which no gas flows in. This can be represented, for example, as the open time of a valve connected to a reservoir of precursor gas (or contrast gas) through which this gas can reach the reaction site, and the time during which the valve remains closed. Typical time ratios between open and closed valves can be 1:10 (e.g., the valve opens for 1 second and closes for 1 second), 1:30, or 1:60, although different ratios are also possible in principle.

[0035] The contrast gas can be supplied only after the etching action has begun, preferably just before the expected transition point from etching on the defect to etching on the mask element below the defect, thereby reducing interference of the etching action by the contrast gas.

[0036] It is also possible for the local etching action to occur in the absence of contrast gas. It is further conceivable that contrast gas is supplied only after a predetermined expected etching progress has been reached. Nevertheless, monitoring of the etching action 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 also possible to perform only individual of the latter method steps (e.g., to start monitoring the etching action only after contrast gas has been supplied).

[0037] The 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 etching progress at which the etching action transitions from etching the defect to etching the features below the defect. The etching action and / or etching progress can be monitored under operator supervision (e.g., as visual endpoint detection) or in a fully automated manner.

[0038] Before the etching action is initiated, for example, a lookup table may be calibrated. For example, the lookup table may be used to preset the etching progress, e.g., 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 may be supplied. The predetermined etching progress may be ascertained, for example, using a lookup table, specifically for the etching parameters used (e.g., beam parameters, precursor gas, material to be etched, etc.). Instead of or in addition to calibrating the lookup table, for example, a lookup table may be retrieved from memory that relates to etching parameters that correspond to, or at least approximate, the etching parameters of the etching action to be initiated at that time. Such a lookup table may also be used as described herein. Because the etching process in this case may be essentially linear, the use of a predetermined expected etching progress may enable accurate estimation of the etching progress, particularly in the case of homogeneous defect compositions (e.g., equal etching progress may be achieved within the same time period).

[0039] To induce the etching process, a precursor gas for the etching process can be supplied to the etching atmosphere, which interacts with the incident particle beam and ultimately triggers the etching reaction and removal of the defective material. The process can proceed in a time sequence such that the contrast gas is supplied only after the precursor gas is supplied. This can also contribute to further reducing the interference of the contrast gas with the etching process. In this way, for example, the defective material can be preferably covered with the precursor gas. However, it is also possible to simultaneously supply these two gases into the etching atmosphere. If appropriate, it is also conceivable to supply the contrast gas into the etching atmosphere before the precursor gas.

[0040] The precursor gas may affect particle backscattering and / or secondary particle and / or other free space signal generation on the material of the defect and / or on the material of the underlying element.

[0041] The contrast gas can be selected to displace the precursor gas on the material of the element below the defect material, preferably more strongly than on the material of the defect. This ensures that sufficient adsorption of the contrast gas on the mask material is always possible, thus ensuring early recognition of the transition point between etching of the defect material and etching of the material of the element below. At the same time, the lower displacement of the precursor gas on the defect material can also minimize interference with the etching process.

[0042] Useful contrast gases may include one or more oxidants, such as O2, O3, HO, HO2, NO2, NO, NO2, HNO3, and / or other oxygen-containing gases. Similarly, one or more halides, such as Cl2, HCl, XeF2, HF, I2, HI, Br2, HBr, NOCl, NF3, PCl3, PF3, and / or other halogen-containing gases, may also be used. Cl2 is considered a preferred contrast gas because it only slightly interferes with the local etching process and lowers the work function (thereby producing a high SE signal). Similarly, useful contrast gases may include reducing gases, such as H2, NH3, CH4, HS, HS, HSSe, HSte, 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 a plasma (preferably a remote plasma generated separately from the sample) may also be used. It is also possible to use noble gases (e.g., He, Ne, Ar, Kr, Xe). A further option is the use of surfactants (e.g., alkyl hydroxides, aliphatic carboxylic acids, mercaptoalkanes, alkylamines, alkyl sulfates, alkyl phosphates, alkyl phosphonates, and aromatic and other organic compounds can be used instead of alkyl compounds). It should be noted that the above-mentioned contrast gases can also 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 type. 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), triruthenium dodecacarbonyl (Ru3(CO)). 12 , iron pentacarbonyl Fe(CO)5, and / or other compounds of this type can also be used. 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 type can also be used. Also, one or more (metal, transition element, main group) halides, such as WF6, WCl6, TiCl6, BCl3, SiCl4, and / or other compounds of this type can also be used. Also, one or more (metal, transition element, main group) complexes, such as copper bis(hexafluoroacetylacetonate) Cu(C5F6HO2)2, dimethyl gold trifluoroacetylacetonate Me2Au(C5F3H4O2), and / or other compounds of this type can also be used. Furthermore, CO, CO2, aliphatic or aromatic hydrocarbons, components of vacuum pump oil, volatile organic compounds, and / or other compounds of this type can also be used. It should be noted that the listed precursor gases can also be used as contrast gases.

[0044] It will be appreciated by those skilled in the art that the above list is not exhaustive and that any desired combination of possible selected contrast and precursor gases, other than those selected herein merely by way of example, is possible.

[0045] In a preferred embodiment, there is a combination of contrast gases that have opposing effects on the ESB / SE signal (or different signals used) relative to the effect of the precursor gas, where the effect relates to the material that is etched and the material that is not etched. In this case, for example, the adsorbed precursor gas can lower the work function of the material (higher SE signal), while the contrast gas can increase the work function (lower SE signal), or vice versa.

[0046] It should be noted that rather than supplying contrast gas (e.g., after the etching process has begun), the contrast gas may already be present (at a low concentration), and then its concentration may simply be increased in a prescribed manner (e.g., after the etching process has begun but before its expected end point).

[0047] After the transition point of the etching action is detected, the etching action can be stopped, for example, by stopping the particle beam, to prevent undesired etching of the mask material underneath the defect material.

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

[0049] Repair of defects on a lithography mask can also be performed by an apparatus that can include (a.) means for directing a particle beam at the defect. The apparatus can further include (b.) 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 an element of the mask below the defect can be detected. Finally, the apparatus can include (c.) means for supplying at least one contrast gas so that 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 methods.

[0051] An apparatus for repairing defects in a lithography mask may also be configured to include the above-described computer program and to cause the apparatus 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 invention are described with reference to the following drawings: [Brief explanation of the drawings]

[0053] [Figure 1a] FIG. 10 shows an example of endpoint detection in the absence of contrast gas. [Figure 1b] FIG. 10 shows an example of endpoint detection in the absence of contrast gas. [Figure 2a] FIG. 1 shows an example of endpoint detection using contrast gas. [Figure 2b] FIG. 1 shows an example of endpoint detection using contrast gas. [Figure 3a] FIG. 10 is a diagram showing an example of the adsorption characteristics of a contrast gas. [Figure 3b] FIG. 10 is a diagram showing an example of the adsorption characteristics of a contrast gas. [Figure 4a] FIG. 10 is a diagram showing an example of the adsorption characteristics of a contrast gas and a precursor gas. [Figure 4b] FIG. 10 is a diagram showing an example of the adsorption characteristics of a contrast gas and a precursor gas. [Figure 5A] FIG. 10 illustrates signal progression at the transition point during local etching action in the absence and presence of contrast gas. [Figure 5B] FIG. 10 illustrates signal progression at the transition point during local etching action in the absence and presence of contrast gas. DETAILED DESCRIPTION OF THE INVENTION

[0054] The following description of embodiments of the present invention will be primarily directed 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 modifying and / or repairing other objects used in the field of microelectronics, such as structured wafer surfaces or microchip surfaces. For example, defects generally present on surfaces or surface elements can be repaired. Therefore, even if the following description refers to applications for processing mask surfaces for clarity and ease of understanding, those skilled in the art will have in mind other possible uses of the disclosed teachings.

[0055] It should also be noted that, in the following, only individual embodiments of the present invention may be described in more detail. However, those skilled in the art will recognize that the features and modification options described in connection with these embodiments may be further modified and / or combined with each other in other combinations or subcombinations without departing from the scope of the present invention. Also, individual features or subfeatures may be omitted if they are not necessary for achieving the intended results. Therefore, in order to avoid unnecessary repetition, reference is made to the descriptions and explanations in the preceding paragraphs, which also remain valid for the following detailed description.

[0056] Figure 1a shows a schematic diagram of a conventional endpoint detection method using an etching action induced by a beam of charged particles, as used in the repair of lithography masks. A beam of particles 1, e.g., electrons, can be guided onto a first material 2, although other charged particles can also be used. This first material 2 may have or be a dark defect D. This can result in undesirable absorption characteristics or an undesirable phase shift at the defect site, such as in the case of light transmission employed in the semiconductor industry for wafer fabrication. 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, although other material configurations are also possible. For example, the first material 2 can be locally bonded to a layer formed by the second material 3.

[0057] To remove the defects D in the desired manner, the surrounding, typically enclosed, atmosphere can be supplied with a precursor gas (not shown here) that can interact with the incident beam of charged particles 1 to produce a local etching effect at the location of the incident particle beam. The incident particle beam can then be systematically guided over the defect area by interaction with magnetic and / or electric fields and / or other control methods, resulting in corresponding removal of the defects D. As a result of the interaction of the incident beam 1 of charged particles 1, backscattered particles 4a and / or secondary particles 4b and / or other free-space beams 4c are obtained. (Even if the examples described below are limited to backscattered particles and / or secondary particles, other types of particles / beams that allow for determinations regarding the progress of the etching effect can be advantageously used as well.) These particles or beams provide an option for monitoring the etching effect. Because the first material 2 and the second material 3 typically have different compositions (e.g., with respect to their respective atomic numbers), there can be a change in the signal 5 detected from the backscattered particles 6 and / or secondary particles 7 and / or free-space beam. The change in the detected signal allows determining that the defect material D has been completely removed and that 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 directly impinges on the substrate material 3, without any subsequent local interaction with the first material 2. This results in a change in the detectable signal 5, such that the signals from the backscattered particles and / or secondary particles are altered compared to the scenario shown in Figure 1a. For example, the signal from the backscattered particles can be increased. Alternatively or additionally, the signal caused by the secondary particles can be attenuated.

[0059] Known problems with the lithography mask repair method shown in FIGS. 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, for example, defect D, consisting of first material 2, is completely removed, is possible only with very limited accuracy. As a result, the particle beam-induced etching process may also unintentionally remove part of 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 properties with charged particles 1.

[0060] This problem and this limitation have been recognised by the applicant and are optimised by the present invention in that a contrast gas can be supplied to the etching process so that the point of material transition from the first material 2 to the second material 3 during etching can be known with greater precision.

[0061] FIG. 2a shows an etching process that can be used to repair a lithography mask. In addition to the method according to FIGS. 1a and 1b, a contrast gas 8 can be supplied to the etching process. This contrast gas 8 can be selected here so that it preferentially adsorbs 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 to a lesser extent with the supplied contrast gas 8. Therefore, the signal intensities 6 and 7 detectable during the etching process on the first material 2 can initially be similar to the example described in FIG. 1a.

[0062] FIG. 2b illustrates a scenario for complete removal of defect D. In this scenario, second material 3 can be exposed to a supplied contrast gas 8, which can be selected to preferentially adsorb to second material 3, so that particle beam 1 does not directly impinge on second material 3 but instead impinges on gas particles of the contrast gas adsorbed on second material 3. The contrast gas 8 can have different properties than second material 3 with respect to the generation of backscattered particles 6 and / or secondary particles 7, or can at least alter the properties of second material 3 in this regard. This can increase the contrast between the signals from backscattered particles and / or secondary particles resulting from the interaction of particle beam 1 with first material 2 or at location 9 of contrast gas 8 adsorbed on the second material. By way of example, FIG. 2b illustrates an increase in the signal from backscattered particles 6 and a decrease in the signal from secondary particles 7. However, this is merely an example. In each case, it is possible to detect only one of these signals and / or other free space signals, with possible variations in signal strength in either direction.

[0063] In a preferred embodiment, the induction of the local etching action may be performed in the absence of a contrast gas.

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

[0065] In some embodiments, calibration does not necessarily have to be performed before every etching operation, since the look-up table may be stored in a storage medium and may also be based on previously recorded data or operating parameters. Based on the calibrated and / or stored look-up tables, it is possible to preset, for example, the expected etching progress over time with or without contrast gas.

[0066] Regardless of this, the contrast gas 8 can be supplied, for example, only when the etching progress has already progressed to a predetermined degree. The predetermined degree can be determined, for example, by a look-up table. Supplying the contrast gas only during the course of the etching process (e.g., near the end) can minimize any disruptive effects of the contrast gas 8 on local etching. These can be manifested, for example, in changes in etch rate and / or etch selectivity in the presence of the contrast gas compared to the absence of the contrast gas, which can potentially lead to inaccurate predictions of etching progress and / or reduced etch quality.

[0067] The etching action may be monitored only after the contrast gas has been delivered, in which case the respective sensors, programs, etc. may only be active after or during the delivery of the contrast gas.

[0068] An example of the adsorption characteristics of a contrast gas 8 is shown in FIGS. 3a and 3b. The contrast gas 8 can be selected to have a high affinity for adsorption on the second material 3 and a lower adsorption on the first material 2. The selected contrast gas 8 can therefore produce an artificial relative increase in the contrast of, for example, backscattered and / or secondary particle signals monitored during the etching process at the transition point of the etching process on the first material 2 to the second material 3. This can enable more accurate endpoint detection during repair operations on lithography masks. Although not shown, it is of course possible for a precursor gas to be present in the atmosphere above the first material 2 and / or the second material 3. This precursor gas can also adsorb to the surfaces of the first material 2 and / or the second material 3, in which case the absorption characteristics may vary. In these cases, the contrast gas 8 can also be selected to have a high affinity for adsorption on the second material 3 and a lower adsorption on the first material 2. Thus, the selected contrast gas 8 can contribute to an "artificial" relative increase in contrast even in the presence of the precursor gas 10.

[0069] 4a and 4b show an example of the absorption characteristics of a contrast gas 8 and an additional precursor gas 10. FIG. 4a shows the case where a first material 2 is exposed to both a contrast gas 8 and a precursor gas 10. The contrast gas 8 can be selected to adsorb to the first material 2 to a lesser extent than the precursor gas 10, e.g., to have a lower affinity for the first material 2 than the precursor gas 10. This can contribute to a reduced degree of influence of the contrast gas 8 on the etching process of the first material 2.

[0070] 4b shows a situation in which the second material 3 is exposed to a precursor gas 10 and a 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. Thus, the precursor gas 8 can adsorb to a greater extent on the second material 3 than on the first material 2. Alternatively, or in addition, 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 can arise in which there is initially more adsorption of the precursor gas 10 on the surface of the first material 2 (FIG. 4a), with at least partial replacement of the precursor gas 10 from the second material 3 by the contrast gas 8 at the transition point of the etching action to the second material 3.

[0071] Alternatively or additionally, the contrast gas 8 and precursor gas 10 can be selected such that the contrast gas 8 is more adsorbed by the second material 3 than the precursor gas 10. This still allows for at least partial replacement of the precursor gas 10 by the second material 3 at the point where the etching action transitions 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 less than on the first material 2 (although higher coverage is also possible, in which case the etching process tends to be more desirable to maintain high coverage of the first material 2 by the precursor gas 10). Higher contrast in the observable signal 5 (e.g., with respect to the EsB signal and / or the SE signal) during the etching action 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] It is also conceivable that the precursor gas 10 does not significantly adsorb onto either the first material 2 or the second material 3, but instead is present, for example, only in the atmosphere surrounding the two materials. It may be sufficient if the selected contrast gas 8 has a higher absorption rate (e.g., averaged over time) and / or a longer residence time on the second material 3 than on the first material 2. Absorption may be the result of processes such as physisorption and / or chemisorption and / or other processes that result in 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 and / or SE signals compared to the first material 2. This can result in a stronger or weaker EsB signal being generated by the contrast gas 8 adsorbed on the surface of the second material 3 compared to the second material 3. Furthermore, a stronger or weaker SE signal can be generated by the contrast gas 8 adsorbed on the surface of the second material 3 compared to the second material 3. Finally, alternatively or additionally, the contrast gas 8 adsorbed on the surface of the second material 3 can attenuate the EsB and / or SE signals emanating from the second material 3.

[0075] It is also possible that the contrast gas itself does not significantly adsorb, resulting in, on average, a change in the occupancy of the first or second material by the precursor gas.

[0076] 5A and 5B show possible effects of determining whether the local etching action on the first material 2 has already transitioned to an etching action on the second material 3 below the first material 2 in the absence of contrast gas 8 (FIG. 5A) and in the presence of contrast gas 8 (FIG. 5B).

[0077] FIG. 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, versus etching time (e.g., time). Reference numeral 2 indicates that the detectable signal is associated with a local etching event on the first material 2 prior to a transition point 12 between the etching event and the first material 2 and the second material 3. As can be seen from FIG. 5A, this transition point can be associated with a signal change 11. In this example, the signal change 11 includes a signal drop. However, it should be noted that this is merely an example, and a signal increase at transition point 12 is also possible. Transition point 12 can be considered here as occurring when signal change 11 exceeds a predetermined critical threshold, i.e., Δsignal > threshold. In FIG. 5A, the threshold is less than or equal to the noise in the detected signal. Therefore, the contrast is low. This can occur particularly when the signal change relative to the expected value is considered relatively low or equal to the expected noise level.

[0078] 5B is the same configuration as FIG. 5A, except that it illustrates, by way of example, the effect on the detectable signal when a contrast gas 8 is provided to the local etching action. This contrast gas 8, in this case, results in a more pronounced signal change 11 in the detectable signal (a drop in this example) at transition point 12 than, for example, shown in FIG. 5A. This allows for a more accurate determination of transition point 12 and therefore a more precise endpoint detection of the local etching action. Note that the presence of contrast gas 8 can also lead to an increase in the detectable signal at transition point 8.

Claims

1. 1. A method for repairing defects in a lithography mask, comprising: a. directing a particle beam at the defect to induce a localized etching action on the defect; b) monitoring the local etching action using backscattered particles and / or secondary particles and / or any other free space signal generated by the local etching action to detect a transition point from the local etching action on the defect to local etching action on an element of the lithographic mask below the defect; c) providing at least one contrast gas to increase contrast in the detection of said transition point.

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

3. A method as described in claim 1 or 2, wherein the degree of effect of the contrast gas on the material of the defect on backscattering of the particles and / or generation of secondary particles and / or on the other free space signals generated by the local etching action is different from the degree of effect on the material of the element below the defect.

4. The method according to any one of claims 1 to 3, wherein the incidence of the particle beam on 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 of any one of claims 1 to 5, wherein the contrast gas is supplied for at least two separate periods.

7. 7. The method of claim 1, wherein the contrast gas is supplied only after the local etching action has begun and just before an expected transition point from the local etching action on the defect to the local etching action on the element of the mask below the defect.

8. inducing the localized etching action in the absence of the contrast gas; The method of any one of claims 1 to 7, further comprising supplying the contrast gas only after a predetermined expected degree of etching has occurred.

9. The method of any one of claims 1 to 8, comprising providing a precursor gas for said localized etching action.

10. The method of claim 9 , wherein the contrast gas is provided after the precursor gas is provided.

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

12. 12. The method of claim 9, further comprising selecting the contrast gas such that the contrast gas displaces the precursor gas on the material of the element below the defect to a greater extent than on the material of the defect.

13. A computer program comprising instructions which, when executed, cause a computer to perform the method of any one of claims 1 to 12.

14. 1. An apparatus for repairing defects on a lithography mask, comprising: a. means for directing a particle beam at said defect to induce an etching action on said defect; b. means for monitoring said etching using backscattered particles and / or secondary particles and / or any other free space signal generated by said etching to detect a transition point from said etching on said defect to etching on an element of said lithographic mask below said defect; and c) means for supplying at least one contrast gas to increase contrast in the detection of said transition point.

15. Apparatus for repairing defects in lithographic material, comprising the computer program of claim 13.

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