END POINT DETERMINATION BY MEANS OF CONTRAST GAS

Abstract

The present invention encompasses a method of repairing a defect on a lithography mask, comprising the following steps: (a.) directing a particle beam onto the defect to induce a local etching operation on the defect; (b.) monitoring the etching operation using backscattered particles and/or secondary particles and/or another free-space signal generated by the etching operation, in order to detect a transition from the local etching operation on the defect to a local etching operation on an element of the mask beneath the defect, and (c.) feeding in at least one contrast gas in order to increase contrast in the detection of the transition.

Claims

1. A method of repairing a defect of a lithography mask, comprising: a. directing a particle beam onto the defect to induce a local etching operation on the defect, b. monitoring the etching operation using backscattered particles and/or secondary particles and/or any other free-space signal generated by the etching operation, in order to detect a transition from the local etching operation on the defect to a local etching operation on an element of the mask beneath the defect, and c. feeding in at least one contrast gas in order to increase contrast in the detection of the transition.

2. The method of claim 1, further including selection of the contrast gas such that an adsorption rate and/or dwell time of the contrast gas on the material of the element beneath the defect is higher than an adsorption rate or dwell time of the contrast gas on a material of the defect.

3. The method of claim 1, wherein the degree to which the contrast gas influences the backscatter of particles and/or generation of secondary particles and/or the other free-space signal generated by the etching operation on a material of the defect is different from that on a material of the element beneath.

4. The method of claim 1, wherein incidence of the particle beam on the contrast gas results in backscatter of particles and/or generation of secondary particles.

5. The method of claim 1, wherein the contrast gas is an inert gas.

6. The method of claim 1, wherein the contrast gas is fed in in at least two separate intervals.

7. The method of claim 1, wherein the contrast gas is fed in after the etching operation has commenced, preferably only shortly before the expected transition from the etching operation on the defect to the etching operation on the element of the mask beneath the defect.

8. The method of claim 1, further comprising: inducing the local etching operation in absence of the contrast gas; feeding in the contrast gas only after a predetermined expected progression of etching has been attained; wherein the etching operation is monitored only after the contrast gas has been fed in.

9. The method of claim 1, comprising: feeding in a precursor gas for the etching operation.

10. The method of claim 9, wherein the contrast gas is fed in after the precursor gas has been fed in.

11. The method of claim 9, wherein the precursor gas influences the backscatter of particles and/or generation of secondary particles on a material of the defect and/or on a material of the element beneath.

12. The method of claim 9, further including selection of the contrast gas in such a way that it displaces the precursor gas on a material of the element beneath, preferably to a greater extent than on a material of the defect.

13. A computer program with instructions which, when executed, cause a computer to perform the method of claim 1.

14. An apparatus for repairing a defect on a lithography mask, comprising: a. means of directing a particle beam onto the defect to induce an etching operation on the defect, b. means of monitoring the etching operation using backscattered particles and/or secondary particles and/or another free-space signal generated by the etching operation, in order to detect a transition from the etching operation on the defect to an etching operation on an element of the mask beneath the defect, c. means of feeding in at least one contrast gas in order to increase contrast in the detection of the transition.

15. An apparatus for repairing a defect in a lithography material, comprising the computer program of claim 13.

16. The apparatus of claim 15, wherein the computer program further comprises instructions which, when executed, cause the computer to perform: selection of the contrast gas such that an adsorption rate and/or dwell time of the contrast gas on the material of the element beneath the defect is higher than an adsorption rate or dwell time of the contrast gas on a material of the defect.

17. The apparatus of claim 15, wherein the computer program further comprises instructions which, when executed, cause the computer to perform the method such that the degree to which the contrast gas influences the backscatter of particles and/or generation of secondary particles and/or the other free-space signal generated by the etching operation on a material of the defect is different from that on a material of the element beneath.

18. The apparatus of claim 15, wherein the computer program further comprises instructions which, when executed, cause the computer to perform the method such that incidence of the particle beam on the contrast gas results in backscatter of particles and/or generation of secondary particles.

19. The apparatus of claim 15, wherein the computer program further comprises instructions which, when executed, cause the computer to perform the method such that the contrast gas is an inert gas.

20. The apparatus of claim 15, wherein the computer program further comprises instructions which, when executed, cause the computer to perform the method such that the contrast gas is fed in in at least two separate intervals.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0053] The following detailed description describes possible embodiments of the invention, with reference being made to the following figures:

[0054] FIGS. 1A-1B example of endpointing in the absence of a contrast gas;

[0055] FIGS. 2A-2B example of endpointing using a contrast gas;

[0056] FIGS. 3A-3B example of adsorption characteristics of a contrast gas;

[0057] FIGS. 4A-4B example of adsorption characteristics of a contrast gas and a precursor gas;

[0058] FIGS. 5A-5B illustrative diagram of the signal progression at a transition during a local etching operation in the absence and presence of a contrast gas.

DETAILED DESCRIPTION

[0059] There follows a description of embodiments of the present invention, primarily with reference to the repair of a lithography mask, especially masks for microlithography. However, the invention is not limited thereto and it may also be used for other kinds of mask processing, or more generally for surface treatment in general, for example of other objects used in the field of microelectronics, for example for modification and/or repair of structured wafer surfaces or of surfaces of microchips, etc. For example, it is possible to repair a defect generally assigned to a surface or above an element of a surface. Even if reference is therefore made hereinafter to the application of processing a mask surface, in order to keep the description clear and more easily understandable, the person skilled in the art will keep the other possible uses of the teaching disclosed in mind.

[0060] It is also pointed out that only individual embodiments of the invention are described in more detail hereinafter. However, a person skilled in the art will appreciate that the features and modification options described in association with these embodiments can also be modified even further and/or can be combined with one another in other combinations or sub-combination without this leading away from the scope of the present invention. Moreover, individual features or sub-features can also be omitted provided that they are dispensable in respect of achieving the intended result. In order to avoid unnecessary repetition, reference is therefore made to the remarks and explanations in the preceding sections, which also retain their validity for the detailed description which now follows below.

[0061] FIG. 1A shows a schematic of a conventional method of endpointing using an etching operation induced by a beam of charged particles, as used for repair of lithography masks. A beam of particles 1, for example electrons, although other charged particles may also be used, may be guided here onto a first material 2. This first material 2 may have or be a dark defect D. This may be associated with the consequence of creation of unwanted absorption characteristics or an unwanted phase shift at the site of the defect for transmitting light, as employed, for example, for the production of wafers in the semiconductor industry. It is therefore the aim of a repair method to correspondingly remove this excess material. The first material 2 may be applied here to a second material 3, with the second material 3 functioning as substrate or mask. Both materials may take the form of material layers, although other material arrangements are also possible. For example, the first material 2 may be in a locally bound arrangement atop a layer formed by the second material 3.

[0062] In order to remove the defect D in a desired manner, the surrounding, typically enclosed atmosphere may be supplied with a precursor gas (not shown here), which, interacting with the incident beam of charged particles 1, may lead to a local etching operation at the site of the incident particle beam. The incident beam of particles may be guided here systematically over the defect region by interaction with magnetic and/or electrical fields and/or another control method, which results in corresponding removal of the defect D. As a consequence of the interaction of the incident beam of charged particles 1, it is possible to obtain backscattered particles 4a and/or secondary particles 4b and/or another free-space beam 4c (even if the working example discussed hereinafter is limited to backscattered and/or secondary particles, any other type of particles/beams that permits conclusion as to the progress of the etching operation is advantageously utilizable analogously). These particles or this beam offer(s) the option of monitoring the etching operation. Since the first material 2 and the second material 3 may typically differ in their composition (for example with regard to their atomic number), there may be a change in the signal 5 detected from backscattered particles 6 and/or secondary particles 7 and/or the free-space beam. A change in the signal detected can enable the conclusion that the defect material D has been removed completely and the incident beam of charged particles is now interacting with the second material 3.

[0063] The scenario in which the defect D consisting of the first material 2 has been removed completely is shown by FIG. 1B. In this case, the charged beam 1 can directly hit the substrate material 3 and then no longer have any local interaction with the first material 2. This can lead to a change in the detectable signal 5 in such a way that the signals from backscattered particles and/or secondary particles are altered compared to the scenario shown in FIG. 1A. For example, the signal of backscattered particles may be increased. Alternatively or additionally, the signal resulting from secondary particles may be attenuated.

[0064] A known problem with the repair method on a lithography mask shown in FIGS. 1A and 1B arises particularly when the detectable signals, at the transition from the first to the second material, do not change or change in a manner which is undetectable or can be detected only with difficulty. In that case, the monitoring of the etching operation is possible only with difficulty. Precise determination of the endpoint, i.e. of the juncture at which the defect D, consisting, for example, of the first material 2, has been removed completely is thus possible only with very limited accuracy. The consequence of this could be that particle beam-induced etching operation also inadvertently removes parts of the second material 3 and, consequently, the absorption characteristics and/or phase shift characteristics of the mask are affected. This can occur especially when the two materials 2 and 3 have very similar interaction characteristics with the beam of charged particles 1.

[0065] This problem and this limitation have been recognized by the applicant and optimized in that, in accordance with the invention, the etching operation can be supplied with a contrast gas in order to be able to see the material transition during the etching of the first material 2 to the second material 3 with higher precision.

[0066] FIG. 2A shows an etching operation as usable for repair of a lithography mask. In addition to the method according to FIGS. 1A and 1B, the etching operation may be supplied with a contrast gas 8. This contrast gas 8 may be selected here such that it is adsorbed preferentially onto the second material 3. The particle beam 1, when it hits the defect D consisting of the first material 2, interacts primarily with the first material 2 and only to a lesser extent with the supplied contrast gas 8. The detectable signal intensities 6 and 7 during the etching operation on the first material 2 may thus at first be analogous to the working example described in FIG. 1A.

[0067] FIG. 2B shows the scenario in the case of complete removal of the defect D. Since, in this scenario, the second material 3 can be exposed to the contrast gas 8 supplied, and the contrast gas 8 can preferably be selected such that it is adsorbed preferentially onto the second material 3, the particle beam 1 does not directly hit the second material 3, but rather hits the gas particles of the contrast gas 8 adsorbed on the second material 3. The contrast gas 8 may have characteristics different from the second material 3 with regard to the production of backscattered particles 6 and or secondary particles 7, or at least change the characteristics of the second material 3 in this regard. This can lead to elevated contrast between the signals from backscattered and/or secondary particles that arise as a result of interaction of the particle beam 1 with the first material 2 or as a result of interaction with at the site 9 of the contrast gas 8 adsorbed on the second material 3. By way of example, FIG. 2B illustrates that the signal of backscattered particles 6 is increased, while the signal of secondary particles 7 is reduced. However, this is merely by way of example. In each case, it is also possible to detect only one of these signals and/or another free-space signal, and variances in the signal strength in either direction are conceivable.

[0068] In a preferred embodiment, induction of the local etching operation may be undertaken in the absence of the contrast gas.

[0069] Independently thereof, calibration of a lookup table may be envisaged. In a lookup table, parameters such as etch rate, etch time, number of cycles etc. may be associated with parameters of the particle beam 1 (e.g. power, acceleration voltage, particle type, etc.) and/or of the first material 2 and/or of the second material 3 and/or of the precursor gas and/or of the contrast gas. On this basis, for a particular etching operation, it may be made possible to predict the juncture of transition of the etching operation from the first material 2 to the second material 3 for various beams or etch parameters. What may be envisaged here is calibration of the lookup table both in the presence of the contrast gas and in the absence of the contrast gas.

[0070] In some embodiments, the calibration does not necessarily take place before every etching operation. This is because it may likewise be the case that the lookup table is stored in a storage medium and is based on historically recorded data or works parameters. On the basis of the calibrated lookup table and/or a stored lookup table, it is possible, for example, to predetermine the progression of etching to be expected over time with or without contrast gas.

[0071] Regardless of this, a contrast gas 8 can, for example, be supplied only when the etching progression has already advanced to a predetermined magnitude. The predetermined magnitude can be ascertained, for example, by use of a lookup table. The supply of the contrast gas only in the course of the etching process (for example toward the end thereof) may minimize any disruptive effects of the contrast gas 8 on the local etching operation. These may be manifested, for example, in a change in the etch rate and/or etch selectivity in the presence of the contrast gas compared to the absence of the contrast gas, which can possibly lead to incorrect predictions with regard to the progression of etching and/or reduction in the etch quality.

[0072] It is also possible that the etching operation is monitored only after the contrast gas has been fed in. In that case, the respective sensors, programs etc. must be active only after or on supply of the contrast gas.

[0073] One example of adsorption characteristics of a contrast gas 8 is shown in FIGS. 3A and 3B. The contrast gas 8 may be chosen here such that it has an elevated affinity for adsorption on the second material 3 and shows only lower adsorption on the first material 2. Thus, the contrast gas 8 chosen can lead to an “artificial” relative increase in contrast of the signal at the transition of the etching operation on the first material 2 to the second material 3, for example in the signal of backscattered and/or secondary particles monitored during the etching operation. This can enable more precise endpointing during the repair operation on a lithography mask. Although it is not shown, it is of course also possible for precursor gas to be present in the atmosphere above the first material 2 and/or the second material 3. This can likewise be adsorbed on the surface of the first material 2 and/or of the second material 3, in which case the absorption characteristics may vary. In these cases too, the contrast gas 8 may be chosen such that it has an elevated affinity for adsorption on the second material 3 and shows only lower adsorption on the first material 2. It is thus possible for the chosen contrast gas 8 to contribute to an “artificial” relative increase in contrast, even if precursor gas 10 is present.

[0074] FIGS. 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 in which the first material 2 is exposed both to the contrast gas 8 and to the precursor gas 10. The contrast gas 8 may be selected such that it is adsorbed onto the first material 2 to a lesser degree than the precursor gas 10, for example such that it has a lower affinity for the first material 2 than the precursor gas 10. This can contribute to a lesser degree of influence by the contrast gas 8 on the etching process on the first material 2.

[0075] FIG. 4B shows a situation in which the second material 3 is exposed to the precursor gas 10 and the contrast gas 8. The contrast gas 8 may be selected such that it has a higher affinity for the second material 3 than for the first material 2. It can thus be adsorbed to a higher degree onto the second material 3 than onto the first material 2. Alternatively or additionally, the precursor gas 10 may be selected such that it has a higher affinity for the first material 2 than for the second material 3. The overall situation may arise that there is at first greater adsorption of the precursor gas 10 on the surface of the first material 2 (FIG. 4A) and at least partial displacement of the precursor gas 10 from the second material 3 by the contrast gas 8 at the transition of the etching operation to the second material 3.

[0076] Alternatively or additionally, the contrast gas 8 and the precursor gas 10 may be chosen such that the contrast gas 8 is more significantly adsorbed onto the second material 3 compared to the precursor gas 10. In this way too, at the transition of the etching operation to the second material 3, there may be at least partial displacement of the precursor gas 10 by the second material 3.

[0077] The ratio of coverage of the surface of the second material 3 by a precursor gas 10 relative to a contrast gas 8 may be smaller than on the first material 2 (higher coverage is also conceivable, in which case it tends to be more desirable for the etching process to keep the coverage of the first material 2 with the precursor gas 10 high). Higher contrast (for example with regard to the EsB and/or SE signal) of the signal 5 observable during the etching operation may arise 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.

[0078] A likewise conceivable case is that in which the precursor gas 10 is not adsorbed significantly onto the first material 2 or onto the second material 3, but is instead to be found, for example, only in the atmosphere surrounding the two materials. It may be sufficient for a chosen contrast gas 8 to have a higher absorption rate (for example an average over time) and/or a longer dwell time on the second material 3 than on the first material 2. The absorption may result from processes such as physisorption and/or chemisorption and/or another process that results in adsorption.

[0079] More particularly, a chosen contrast gas 8 adsorbed on the surface of the second material 3 may generate a different contrast in the EsB signal and/or in the SE signal compared to the first material 2. This may result from 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. In addition, 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. Ultimately, it is alternatively or additionally possible for the contrast gas 8 adsorbed on the surface of the second material 3 to attenuate the EsB and/or SE signal emanating from the second material 3.

[0080] It is likewise conceivable that the contrast gas itself is not significantly adsorbed, but leads on average to altered occupation of the first or second material with the precursor gas.

[0081] FIGS. 5A and 5B show the possible effect of determining whether a local etching operation on the first material 2 has already transitioned to an etching operation on the second material 3 beneath the first material 2, in the absence of a contrast gas 8 (FIG. 5A) and in the presence of a contrast gas 8 (FIG. 5B).

[0082] FIG. 5A shows a possible detectable signal composed of backscatter particles and/or secondary particles or another free-space signal generated by the etching operation, plotted against a number of etching operations (e.g. time). Reference numeral 2 in this connection indicates that the detectable signal is associated with a local etching operation on the first material 2 before the transition 12 of the etching operation from the first material 2 to the second material 3. As apparent from FIG. 5A, this may be associated with a change in signal 11. In the present example, the change in signal 11 comprises a decrease in the signal. However, it is pointed out that this should be understood merely by way of example, and an increase in the signal at the transition 12 is also possible. A transition 12 may be assumed here when the change in signal 11 exceeds a predetermined critical threshold value, i.e. when: Δsignal>threshold value. In FIG. 5A, the threshold value is smaller or comparable to the noise in the signal detected. There is therefore low contrast. This may occur especially when the changes in signal to the expected are regarded as small relative to the expected noise level or comparable thereto.

[0083] FIG. 5B is of identical construction to FIG. 5A, except that it shows, by way of example, the effect on the detectable signal when the local etching operation is supplied with a contrast gas 8. This leads, in the present case, to a more marked change in signal 11 in the detectable signal (in this example a decrease in signal) at the transition 12 than shown, for example, in FIG. 5A. This enables more precise determination of the transition 12 and hence more exact endpointing of a local etching operation. It is pointed out that the presence of a contrast gas 8 can also lead to an increase in the detectable signal at the transition 8.