Method for operating a discharge plasma source, as well as discharge plasma source and optical metrology system

Abstract

The invention relates to a method for operating a pulsed discharge plasma source, as well as a discharge plasma source and an optical metrology system. In the method, the discharge plasma source emits a plurality of light pulses, wherein a pulse rate set during this emission of light pulses is varied over time during the operation of the discharge plasma source.

Description

BACKGROUND OF THE INVENTION Area of ​​the invention

[0001] The invention relates to a method for operating a discharge plasma source, as well as a discharge plasma source and an optical metrology system. The metrology system can, in particular, be a mask inspection system for characterizing a mask for microlithography. State of the art

[0002] Microlithography is used to manufacture microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a projection exposure system, which includes an illumination unit and a projection lens. The image of a mask (= reticulum) illuminated by the illumination unit is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens. This transfers the mask structure onto the photosensitive coating of the substrate.

[0003] In the lithography process, unwanted defects on the mask have a particularly detrimental effect, as they can be reproduced with each exposure step. To minimize mask defects and to achieve successful mask repair, a direct analysis of the imaging effect at potential defect locations is therefore desirable. Consequently, there is a fundamental need to measure and qualify the mask quickly and easily, ideally under the same conditions as those actually present in the projection exposure system.

[0004] It is known to use a mask inspection system to capture and evaluate an aerial image of a section of the mask. To capture the aerial image, the structures to be measured on the mask are illuminated with a lighting optic, and the light emanating from the mask is projected onto a detector unit via a magnifying imaging optic and detected. Various approaches exist for emulating the projection exposure system in mask inspection (i.e., measuring the mask under conditions analogous to those of the projection exposure system, if possible).

[0005] Preferably, the same wavelength used in the projection exposure unit during the lithography process is used in the mask inspection system. The light source is typically an EUV plasma source, for example, a discharge-produced plasma (DPP) or laser-produced plasma (LPP) source. The EUV plasma source is typically pulsed, meaning it emits EUV light pulses at a specific repetition rate (hereinafter referred to as the "pulse rate").

[0006] In a discharge plasma source (DPP source), the plasma burns within a so-called bore. During operation, this bore wears down, for example, due to material sputtering. This, in turn, leads to a decrease in the brightness of the discharge plasma source, so the bore must be replaced after a certain time or number of pulses. Since replacing the bore is relatively time-consuming and typically takes several hours, this results in an increase in downtime and operating costs.

[0007] For the state of the art, reference is made only to EP 2 187 711 A2 as an example. SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide a method for operating a pulsed discharge plasma source, as well as a discharge plasma source and an optical metrology system, which enable a significant reduction in downtime and cost savings while at least partially avoiding the problems described above.

[0009] This problem is solved according to the features of the dependent patent claims.

[0010] According to one aspect, the invention relates to a method for operating a discharge plasma source which emits a plurality of light pulses.

[0011] The method is characterized by the fact that a pulse rate set during this emission of light pulses is varied over time during the operation of the discharge plasma source.

[0012] The invention is based in particular on the concept of actively controlling or varying the pulse rate over time to adjust the emitted light output such that, on the one hand, the light transmission required for measurement operation or for the (measurement) throughput specified is achieved, but on the other hand, this light output is not unnecessarily high. According to the invention, by reducing the emitted light output to just barely achieve the throughput required by the specification, the overall service life of the bore used is extended.

[0013] The temporarily reduced pulse rate according to the invention also has the advantage of reducing the thermal load acting on the bore, thereby further reducing sputtering effects and additionally reducing the degradation of the bore.

[0014] According to the present invention, the potential overall service life of the bore can be extended by a factor of 1.5 to 3. This results in significant savings in time and costs for bore replacement without negatively impacting the achievement of the measurement throughput required by the specification, while also reducing resource consumption – both in terms of bore material and the power consumption of the discharge plasma source.

[0015] The temporal variation of the pulse rate according to the invention, as described in more detail below, deliberately entails increased measurement and control engineering effort in order to achieve the advantages described above, particularly the longer service life and / or reduction of downtime and operating costs of the discharge plasma source. Accepting this increased effort also applies to any necessary adjustments to other components of the discharge plasma source or to an optical metrology system incorporating this discharge plasma source (such as a mask inspection system), especially adjustments to the operation of a high-speed shutter that may be present in such a mask inspection system (and described in more detail below).

[0016] Furthermore, the invention deliberately accepts that the stability of the discharge plasma source may be impaired when varying the pulse rate. However, the invention advantageously includes the optional additional adjustment of one or more further operating parameters (e.g., gas pressure and voltage) of the discharge plasma source, thereby ensuring the required stability of the discharge plasma source during operation.

[0017] According to one embodiment, this temporal variation of the pulse rate is set depending on the pulse energy of the light pulses.

[0018] According to one embodiment, the pulse energy or the average light power is repeatedly measured during operation of the discharge plasma source, with the adjustment of the pulse rate based on the measured pulse energy or the measured average light power taking place in a feedback control.

[0019] According to one embodiment, the pulse rate is set using a pre-calibration in which the dependence of the light power or pulse energy on the number of emitted light pulses is determined for the operation of a discharge plasma source with a constant pulse rate.

[0020] According to one embodiment, the temporal variation of the pulse rate is set based on a model for the pulse energy profile as a function of the number of emitted light pulses.

[0021] According to one embodiment, the temporal variation of the pulse rate is adjusted such that a function describing the temporal dependence of the pulse rate is continuous. This avoids undesirable abrupt changes in the plasma shape and thermal stress.

[0022] According to one embodiment, at least one further operating parameter of the discharge plasma source, in particular the gas pressure or the voltage, is varied over time during operation of the discharge plasma source.

[0023] According to one embodiment, the temporal variation of the pulse rate is further adjusted depending on at least one parameter of an optical (metrology) system comprising the discharge plasma source, in particular a numerical aperture (NA) or a fill level of the illumination pupil.

[0024] The invention further relates to a discharge plasma source which emits a multitude of light pulses during operation, with a control device which varies a pulse rate set during this emission of light pulses over time during the operation of the discharge plasma source.

[0025] The invention further relates to an optical metrology system designed for a working wavelength of less than 30 nm and comprising a discharge plasma source according to the invention. The metrology system can, in particular, be a mask inspection system for characterizing a mask for microlithography.

[0026] Further embodiments of the invention can be found in the description and the dependent claims.

[0027] The invention is explained in more detail below with reference to an embodiment shown in the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] They show: Fig. 1 a diagram showing the dependence of a measured light power on the bore usage time (normalized to the nominal bore lifetime) for the operation of a discharge plasma source with a constant pulse rate; Fig. 2 a diagram in which, for the operation of a discharge plasma source with a constant pulse rate according to Fig. 1 a model function for the light output as a function of the bore usage time (normalized to the nominal bore lifetime) is shown; Fig. 3 a diagram showing the course or variation of a pulse rate optimized according to the invention during the operation of a discharge plasma source as a function of the number of pulses (normalized to the nominal bore lifetime); Fig. 4 a diagram showing the course of the light power with optimization of the pulse rate according to the invention. Fig. 3 as a function of the bore usage time (normalized to the nominal bore lifetime) in the operation of a discharge plasma source; Fig. 5 a diagram showing a linear progression of the pulse rate as a function of the number of pulses (normalized to the nominal bore lifetime) during the operation of a discharge plasma source; Fig. 6 a diagram in which the linear course of the pulse rate according to Fig. 5 shows the course of the light output during the operation of a discharge plasma source as a function of the bore usage time (normalized to the nominal bore lifetime); Fig. 7 a schematic representation of the basic possible structure of a mask inspection system as an exemplary application of the present invention; Fig. 8 a schematic representation of a possible arrangement for realizing the method according to the invention in a mask inspection system; Fig. 9 a schematic representation of a control loop existing according to an embodiment of the invention; and Fig. 10 a schematic representation to illustrate an exemplary scenario in the operation of a discharge plasma source according to the invention. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS

[0029] As in Fig. Figure 7 is shown only schematically. A mask inspection system 700 comprises a lighting system 710 and a projection lens 720, wherein light from a (in Fig. 7 light source (not shown) enters the illumination system 710 and strikes a mask 730 arranged in the object plane of the projection lens 720, and wherein the illuminated area of ​​the mask 730 is imaged via the projection lens 720 onto a camera with a sensor arrangement 740. In order to predict the imaging result obtained with the mask 730 when carrying out the lithography process in a projection exposure system, a measurement of a value for the mask 730 in the mask inspection system 700 is carried out in accordance with Fig. 7 intensity distribution obtained with the sensor arrangement 740.

[0030] According to the invention, the emitted light power is adjusted by varying the pulse rate over time, and in particular by actively controlling or regulating it, so that on the one hand the light transmission required for the measurement operation or for the throughput required according to the specification is achieved, but on the other hand this light power is not unnecessarily high.

[0031] The mean emitted luminous power P(n) after emission of a number of pulses n is given by P(n)=E(n)⋅f(n)

[0032] Here, f(n) denotes the pulse repetition rate after emission of a number of pulses n, and E(n) denotes the pulse energy after emission of a number of pulses n.

[0033] The pulse energy E(n) varies during operation and typically decreases over time.

[0034] By choosing f(n) as the inverse function of E(n), a given luminous power P can be calculated. target to be achieved: f(n)=Ptarget⋅E−1(n)

[0035] E is involved -1 (n) the inverse function of E(n), i.e., it holds that: E(n)⋅E−1(n)=1

[0036] E(n) or E -1 (n) can be determined or estimated in different ways according to the invention. According to one possible embodiment, the pulse energy (or the mean light power) during operation of the discharge plasma source can be measured either continuously or at specific time intervals (e.g., at intervals of 1 second, 1 minute, or 15 minutes). The measured values ​​obtained in this way can be described below with reference to Fig. 8 explained in more detail, and can again be used as an input signal for a feedback control in a control loop. Fig. Figure 9 shows a possible embodiment of a suitable control loop, where "910" denotes the controller, "920" the optical system comprising the discharge plasma source, and "930" the measurement of the mean light power (or pulse energy). The pulse rate can be controlled as a function of time or as a function of the number of pulses, based on the pulse energy or mean light power determined as described above, such that the light power remains essentially constant over the bore operating time of the discharge plasma source.

[0037] The diagrams of Fig. 3 and Fig. Figure 4 shows exemplary corresponding curves for a (normalized) optimal pulse rate as a function of the number of pulses (normalized to the nominal bore lifetime ( Fig. 3)) or for the (normalized) luminous efficacy as a function of the bore service life (normalized to the nominal bore service life ( Fig. 4)).

[0038] As in Fig. As indicated by the dashed lines in Figure 3, in practice there are limits to the pulse rate f(n) within which stable source operation is still possible. A corresponding maximum pulse rate in the range of F can only be given as an example; this should not be exceeded. max ≈ (2-3) kHz and a corresponding minimum pulse rate in the range of F that cannot be undercut min ≈ (0.5-1) kHz. In principle, the range of possible pulse rates can, depending on the light source (e.g., with LPP light sources using laser-generated plasma for EUV lithography), extend downwards to a minimum pulse rate of F. min ≈ 1 Hz and upwards to a maximum pulse rate of F max can be extended to approximately 100 kHz.

[0039] Depending on the specific design of the optical system comprising the discharge plasma source according to the invention, in addition to the temporal variation of the pulse rate according to the invention, the corresponding adjustment of one or more further components is required. Fig. Figure 8 shows an exemplary structure in a schematic representation, in comparison to Fig. 7 analogous or essentially functionally identical components are designated with reference digits increased by "100".

[0040] With "801" in Fig. 8 is a light source designed as a discharge plasma source, from which the generated EUV light enters the illumination device 810 via a collector mirror 805. “860” denotes a high-speed shutter (HSS) arranged in the optical beam path between the collector mirror 805 and the illumination device 810, which, in a manner known per se, comprises a rotating disk provided with one or more holes. Blocking or transmitting individual light pulses is achieved by changing the time at which the light source 801 emits light pulses from one light pulse to the next, while the rotational speed of the HSS 860 is substantially constant. The temporal variation of the pulse rate according to the invention then necessitates consideration and adjustment of the corresponding boundary conditions of the HSS 860 (in particular, minimum and maximum rotational speed as well as the acceleration of the rotational speed change).

[0041] In the structure according to Fig. In the invention, a portion of the light emitted by the collector mirror 805 is directed via a coupling element 812 (e.g., designed as a mirror) to an energy sensor 815 (e.g., a photodiode). The measured values ​​determined by this energy sensor 815, corresponding to the respective pulse energy, are used as an input signal for a controller 850 in a feedback control loop to regulate the speed of the HSS 860. For this purpose, the actual speed of the motor can be determined, for example, using a magnetic or optical pulse generator, and the motor voltage and / or motor current can be regulated according to the control deviation. The invention is not limited to the use of a coupling element 812, so that the measurement of the pulse energy can also be implemented in another suitable manner (e.g., by using an energy sensor located directly in the beam path).

[0042] The invention is not limited to the measurement of pulse energy or average light output during operation of the discharge plasma source as described above. In a further embodiment, the pulse energy or average light output can also be measured in advance as part of a preliminary calibration for a test bore operating at a constant pulse rate. Assuming that other bores used in the actual operation of the discharge plasma source exhibit corresponding behavior, the calibration curve obtained in this way can be numerically inverted to determine E -1 to obtain (n) as the inverse function of E(n).

[0043] In another possible embodiment, the determination of the function E(n), which describes the dependence of the pulse energy on the number of pulses, can also be model-based. Here, a linear function can be used as an example, as follows: E(b)=E0−αn

[0044] Fig. 5 and Fig. Figure 6 shows exemplary diagrams for a corresponding embodiment, wherein in Fig. 5 a corresponding (linear) course of the (normalized) pulse rate as a function of the number of pulses (normalized to the nominal bore lifetime) and in Fig. Figure 6 shows the corresponding curve of the (normalized) light output during operation of the discharge plasma source as a function of the bore usage time normalized to the nominal bore lifetime. This is represented in Fig. 6. The dashed line represents the target luminous flux. In the present example, the effective lifetime (i.e., the period during which the luminous flux P is greater than the specified luminous flux P) is... target ) approximately 1.5 times larger than in the constant pulse rate scenario according to Fig. 1 and Fig. 2.

[0045] In further embodiments, instead of a direct measurement or estimation of the function E(n), an indirect determination can also be carried out by measuring system parameters of the discharge plasma source (e.g. the power consumption or source head temperature), which correlate with the pulse energy or the function E(n).

[0046] In further embodiments, the function E(n) can also be determined using a machine learning method. For this purpose, a model can be trained with measurement data of the pulse energy or average light power from several bores at different pulse outputs and different operating parameters (e.g., pulse rate, voltage, pressure). This allows the machine learning algorithm to determine the optimal operating parameters, in particular the pulse rate, depending on the respective bore state.

[0047] This also applies in the event of any deviations from the actual course of E(n) or E. -1(n) if sufficient measuring light is available from the model or the approximation, the specified light output P is preferably used. target A safety factor s larger than the actual required light output is chosen. This safety factor s can only be, for example, in the range of 1.2 to 1.5.

[0048] In embodiments of the invention, the pulse rate can additionally be varied on a shorter time scale and, for example, temporarily reduced during the acquisition of dark images.

[0049] Fig.Figure 10 shows a schematic representation illustrating an exemplary scenario in the operation of a discharge plasma source according to the invention. Typical exposure times can range from 100 to 500 ms. The limiting factor here is the camera's frame rate, which can be, for example, a few Hz. In an exemplary scenario, the camera can capture images at a frame rate of 2 Hz, corresponding to an exposure time of 500 ms. In this case, it is sufficient for the light source to be switched on for only 100 ms per image (and switched off for the remaining 400 ms). Thus, light pulses are not generated at all during the remaining 400 ms (i.e., not merely blocked by the shutter). An image sequence can, for example, consist of four bright frames and one dark frame, each with the same frame rate, whereby the light source can be switched off during the dark frame.

[0050] Preferably, the temporal variation of the pulse rate according to the invention is adjusted such that a function describing the temporal dependence of the pulse rate is continuous. Furthermore, a maximum value for the temporal change (corresponding to the first derivative of the function describing the time dependence of the pulse rate) can be specified. In this way, undesirable abrupt changes in the plasma shape and the thermal load can be avoided.

[0051] Even though the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments are apparent to the person skilled in the art, for example, through the combination and / or exchange of features of individual embodiments. Accordingly, it is understood to the person skilled in the art that such variations and alternative embodiments are included in the present invention, and that the scope of the invention is limited only to the extent of the appended claims and their equivalents. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] EP 2 187 711 A2

[0007]

Claims

[1] Method for operating a discharge plasma source which emits a multitude of light pulses, characterized by , that a pulse rate set during this emission of light pulses is varied over time during the operation of the discharge plasma source. [2] Method according to claim 1, characterized by , that this temporal variation of the pulse rate is set depending on the pulse energy of the light pulses. [3] Method according to claim 2, characterized by , that the pulse energy or the mean light power is repeatedly measured during operation of the discharge plasma source, with the adjustment of the pulse rate based on the measured pulse energy or the measured mean light power taking place in a feedback control. [4] Method according to claim 2, characterized by, that the pulse rate is set using a pre-calibration in which the dependence of the light power or pulse energy on the number of emitted light pulses is determined for the operation of a discharge plasma source with a constant pulse rate. [5] Method according to claim 2, characterized by , that the temporal variation of the pulse rate is set based on a model for the course of the pulse energy as a function of the number of light pulses emitted in each case. [6] Method according to any one of the preceding claims, characterized by , that the temporal variation of the pulse rate is adjusted such that a function describing the temporal dependence of the pulse rate is continuous. [7] Method according to any one of the preceding claims, characterized by, that during the operation of the discharge plasma source at least one further operating parameter of the discharge plasma source, in particular the gas pressure or the voltage, is varied over time. [8] Method according to any one of the preceding claims, characterized by , that the temporal variation of the pulse rate is further adjusted depending on at least one system parameter of an optical metrology system comprising the discharge plasma source, in particular a numerical aperture (NA) or a fill level of the illumination pupil. [9] A discharge plasma source which emits a large number of light pulses during operation, with a control device which varies a pulse rate set during this emission of light pulses over time during the operation of the discharge plasma source. [10] Discharge plasma source according to claim 9, characterized bythat it is configured to execute a method according to any one of claims 1 to 8. [11] Optical metrology system designed for a working wavelength of less than 30 nm, characterized by that this has a discharge plasma source according to claim 10. [12] Optical metrology system according to claim 11, characterized by that this is a mask inspection system for characterizing a mask for microlithography. [13] Optical metrology system according to claim 11 or 12, characterized by , that the temporal variation of the pulse rate is further adjusted depending on at least one system parameter of the metrology system, in particular a numerical aperture (NA) or a fill level of the illumination pupil.

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