Apparatus and method for inspection of masks for semiconductor lithography

Abstract

The present invention relates to an inspection apparatus (4) for a semiconductor lithography mask (8), comprising an imaging apparatus (9) for the imaging mask (8) and an image recording apparatus (10), wherein one or more correction bodies (21, 21', 21") demonstrating the dispersion behavior of at least one subrange of illumination radiation (6) for imaging are arranged in an optical path between the mask (8) and the image recording apparatus (10). The invention also relates to a method for taking into account longitudinal chromatic aberration in the inspection apparatus (4) for the mask (8), the method comprising the steps of: recording a specified number of images with different defocus positions (F1, F2, F3, F4, F5), and selecting a subset of the images and simulating the longitudinal chromatic aberration of the projection exposure apparatus.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 201810754164.1, filed on July 10, 2018, and entitled "Apparatus and method for inspection of masks for semiconductor lithography".

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to German Patent Application DE 10 2017 115 365.9, the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0004] The present invention relates to an apparatus for inspection of masks for semiconductor lithography and to a corresponding method. Masks of this type are typically used to image structures located on the mask onto a light-sensitive layer on a semiconductor substrate (so-called wafer) in a 1:4 scale reduction by means of a projection lens of a projection exposure apparatus (so-called scanner). This technology makes it possible to create very small structures on semiconductor substrates and enables large-scale integrated electronic circuits to be realized in this way. BACKGROUND

[0005] In order to increase the performance of the manufacturing process, it is advantageous if the masks are subjected to a checking or inspection process as early as possible at the end of the defined intervals which they contain before their use in the scanner or after removal of maintenance. For this purpose, inspection apparatuses for masks are typically used which image the masks in an enlarged manner by means of an arrangement like a microscope and make defects identifiable in this way. Measuring systems of this type are also typically referred to as "space image measuring systems" and are known commercially under the designations AIMS or WLCD. They are typically able to simulate certain conditions during the actual use of the masks in the scanner, in particular such as illumination settings and imaging settings; for example sigma shape, numerical aperture and polarization. However, different scanners have different so-called longitudinal chromatic aberrations which influence the behavior of the entire system depending on the respective manufacturer or the year of construction. Longitudinal chromatic aberration is understood to mean the phenomenon that, taking into account the chromatic behavior of the optical materials used, the imaging of the wafer by the projection lens lies in planes which slightly deviate from one another for deviating wavelengths, in other words the best focus position of the imaging can vary with the wavelength. Due to the limited spectral width of the electromagnetic radiation used (typically with a wavelength of approximately 193 nm, but also radiation with deviating wavelengths is used, for example EUV radiation with wavelengths in the range of 10 nm to 121 nm, in particular approximately 13.5 nm), the effects mentioned lead to an unignorable image unsharpness. The prior art to date has not disclosed any methods which can be used to simulate longitudinal chromatic aberration in an inspection apparatus for masks. SUMMARY

[0006] It is therefore the object of the present invention to specify a method and a device in which the longitudinal chromatic aberration of a scanner used for the measurement of a mask in a mask inspection device can be taken into account in an improved manner compared to the solutions known from the prior art.

[0007] This object is achieved by the device and method having the features appearing in the independent claims. The dependent claims relate to advantageous embodiments and developments of the invention.

[0008] The mask inspection device for semiconductor lithography according to the invention comprises an imaging device for imaging the mask and an image recording device. The imaging device is for example a microscope objective. The image recording device can be for example a CCD camera, a CMOS camera or a linear array camera with an associated optical system. According to the invention, one or more correction bodies, which exhibit a chromatic dispersion behavior for at least one sub-range of the imaging illumination radiation, are arranged in the optical path between the mask and the image recording device.

[0009] The correction bodies advantageously contain a medium which is dispersive in the wavelength range of interest, such as calcium fluoride or quartz glass.

[0010] Since the imaging device is typically configured as a light-transmitting microscope, the correction bodies are usually located in the optical path between the mask and the image recording device on the side of the mask facing away from the radiation source. In this case, the correction bodies can be arranged between the mask and the imaging device or between the imaging device and the image recording device. It is also possible for the correction bodies to be arranged between the radiation source and the illumination unit of the mask. Furthermore, the correction bodies can be arranged between the illumination unit of the mask and the mask itself. In other variants, the correction bodies can be arranged within the illumination unit of the mask or within the imaging device. It goes without saying that it is possible, in the case of the use of a plurality of correction bodies, for the correction bodies to be arranged at a plurality of locations as described above.

[0011] In this case, the correction bodies have a simulation effect due to their dispersive properties (the behavior of the same dispersive medium in the projection lens of the scanner). Due to the relatively small, thin optical elements in the microscope objective of the inspection device, the longitudinal chromatic aberration as already mentioned cannot be simulated without additional measures, and the mentioned correction bodies can therefore provide a remedy here. This is particularly the case because the radiation used in the mentioned projection lens covers a considerably longer optical path in the dispersive material of the optical elements of the projection lens compared to the inspection device.

[0012] Here, one advantage of the invention is that various longitudinal chromatic aberrations can be set, so that different longitudinal chromatic aberrations of a variety of projection systems can be taken into account.

[0013] In this case, the correction body's dispersive behavior need not necessarily be static, but can be configured to be dynamically selectable by external influences. In this regard, first of all, the correction body's dispersive behavior can be influenced, for example, by the action of mechanical stress, in a manner known per se. Likewise, the presence of an electric, magnetic or electromagnetic field in the correction body's region can also influence the correction body's dispersive properties in a desired manner. Furthermore, it is possible to influence the correction body's dispersive behavior by means of thermal stress, for example by means of heating elements or Peltier elements, or by means of the gas composition or pressure of the environment in which the correction body is arranged. Not to mention that it is conceivable to influence the correction body's dispersive behavior by means of a corresponding configuration of the device according to the application, utilizing all the effects mentioned in parallel or simultaneously.

[0014] Furthermore, there is the possibility of changing the influence of the correction body's dispersive material by altering the spatial orientation, position or shape of the correction element. In this regard, as an example, a correction body of variable thickness can be used. This can be realized, for example, by means of a correction body of wedge shape, which can be displaced laterally with respect to the optical path. Furthermore, it is also possible to use two wedges that slide on their respective wedge surfaces.

[0015] All the measures mentioned above have the following advantage in common: By means of an appropriate selection of the constitution of the correction body's material and / or by means of its appropriate thermal, mechanical or electrical driving, it is possible to simulate the conditions in a wide variety of scanners and thus to better calculate the behavior of the mask to be examined in the respective target system in advance.

[0016] In the following, a method according to the application is described, which can alternatively or additionally be used to take into account the longitudinal chromatic aberration in a mask examination device.

[0017] One possible advantage of the method is that the effect of the longitudinal chromatic aberration can be measured explicitly. This can be used to make further adaptations and / or optimizations. This can concern both lithography parameters such as exposure settings or etching processes and design specifications of the mask.

[0018] In a first step, a focus stack is first measured, that is to say a specified number of images with different defocus positions are recorded. These images can then be used to simulate the longitudinal chromatic aberration, which behaves essentially like a focus aberration as well. In this regard, as an example, it is possible to measure 5 planes at a distance of 100 nm with respect to the mask. Given the knowledge of the line width of the radiation used and the longitudinal chromatic aberration of the projection exposure apparatus, then those images can be selected from the individual images of the focus stack which are closest to the images that will correspond in the scanner. The longitudinal chromatic aberration of the scanner can then be simulated by means of appropriate interpolation and, if appropriate, weighting of the images.

[0019] The step size of the defocus can also be fixedly configured, for example, into a main step size and a sub-step size. In this regard, as an example, it is possible to select a main step size of 0.3-2 μm, in particular 1 μm, with respect to the mask, and a sub-step size of 10-150 nm, in particular 100 nm, with respect to the mask.

[0020] In other words, the main step size addresses the focus aberration in the scanner, which is process specified, that is, for example, from the case that the wafer is curved or the mask is not located at the best focus. These generally case-dependent defocus aberrations add up to form the known longitudinal chromatic aberration of the scanner in question.

[0021] The sub-step size makes it possible, in particular by interpolation, to further improve the accuracy of the assignment of the defocus of the measuring microscope to the defocus of the mask or wafer (sum of the scanner-specific longitudinal chromatic aberration and the case-dependent deviation from the position of the best focus in the scanner) in the scanner for each assumed case.

[0022] By means of the main step size, it is then possible to evaluate the process window of the structure (structure width versus defocus depending on the exposure dose), and / or the imaging behavior of defects. BRIEF DESCRIPTION OF DRAWINGS

[0023] Exemplary embodiments and variants of the application are explained in more detail below with reference to the accompanying drawings. In the drawings:

[0024] Figure 1 A schematic diagram of the longitudinal chromatic aberration of an optical lens element is shown;

[0025] Figure 2 A schematic diagram of the configuration of the correction body for the target influence of the dispersion behavior of the inspection device of the mask is shown;

[0026] Figure 3 A schematic diagram of the inspection device of the mask according to the application is shown, with a possible arrangement of the position of the correction body for the target setting of the specific longitudinal chromatic aberration;

[0027] Figure 4 A variant of mechanically influencing the correction body is shown;

[0028] Figure 5 Two variants of influencing the correction body by means of electromagnetic fields and electromagnetic waves are shown;

[0029] Fig. 6 shows two possible embodiments of setting the path length of the optical radiation within the correction body;

[0030] Figure 7 An embodiment of changing the spatial orientation and position of the correction body is shown;

[0031] Figure 8One variant of influencing a correction subject by the gas composition and the air pressure of the environment is shown;

[0032] Figure 9 Three variants of influencing a correction subject by a magnetic field are shown;

[0033] Figure 10 One embodiment of influencing a correction subject thermally is shown;

[0034] Figure 11 One variant of changing the spectral properties of the radiation source of the inspection device of the mask for the target setting of the longitudinal chromatic aberration is shown;

[0035] Figure 12 A schematic diagram of a method of simulating longitudinal chromatic aberration by recording focus stacks is shown. DETAILED DESCRIPTION

[0036] Figure 1 A schematic diagram of the longitudinal chromatic aberration of an optical lens element 1 due to chromatic aberration is shown. Due to the wavelength-dependent refractive index n of the optical lens element 1, the electromagnetic radiation 2 having a finite but essentially non-zero bandwidth is refracted to different degrees within the optical lens element 1, thus resulting in different focal planes Fl, F2, F3. As an example, the schematic diagram shows three wavelengths LI, L2, L3 of the electromagnetic radiation 2 with corresponding focal planes Fl, F2, F3, for which LI < L2 < L3 applies. This is referred to as longitudinal chromatic aberration due to the wavelength dependence of the refractive index n and thus the focal planes Fl, F2, F3. The object of the present invention is to integrate this actually undesired optical aberration in a targeted manner within the inspection device of the mask in order to be able to simulate various projection exposure apparatuses having a specific longitudinal chromatic aberration. In this case, according to the present invention, one or more correction subjects are used to influence the chromatic behavior.

[0037] Figure 2 The construction of one exemplary correction subject 21 for the targeted influencing of chromatic aberration is shown in a schematic diagram. In this case, the correction subject 21 comprises an optical dispersion medium 3, for example calcium fluoride or quartz glass, through which the optical axis OA of the inspection device 4 of the mask extends. Via dispersion control means 22, the dispersion properties of the dispersion medium 3 of the correction subject 21 are influenced in a targeted manner in order to produce a specified longitudinal chromatic aberration. In this case, the optical medium 3 can be influenced statically and dynamically. For the targeted control and adjustment of the longitudinal chromatic aberration, the dispersion control means 22 are controlled via an electronic control unit 23.

[0038] Figure 3A device schematic of a mask's inspection device 4 according to the application is shown with possible positions (P1, P2, P3, P4, P5, P6) of one or more correction bodies 21 for a target setting of a specified longitudinal chromatic aberration. The mask's inspection device 4 comprises a radiation source 5 for generating electromagnetic illumination radiation 6 along an optical axis OA of the mask's inspection device 4. In this case, electromagnetic illumination radiation 6 having a wavelength of 193 nm is conceivable as an example. The electromagnetic illumination radiation 6 is guided via an illumination unit 7 onto a mask 8 to be inspected. By means of an imaging device 9, the electromagnetic illumination radiation 6 is imaged onto a camera 10 and there detected, wherein the camera 10 can be configured as a CCD, CMOS or line array camera, for example. The recorded signals are then processed electronically in a data processing unit (not shown in the figure).

[0039] The possible positions P1, P2, P3, P4, P5, P6 of the one or more correction bodies 21 are in particular: position P1 between the radiation source 5 and the illumination unit 7, position P2 within the illumination unit 7, position P3 between the illumination unit 7 and the mask 8, position P4 between the mask 8 and the imaging device 9, position P5 within the imaging device 9, position P6 between the imaging device 9 and the camera 10.

[0040] It is also possible to arrange a plurality of correction bodies 21 for each position. It is also conceivable that the system comprises a plurality of correction bodies 21 integrated at different positions within the mask's inspection device 4, wherein it is possible to arrange a plurality of correction bodies 21 for each position.

[0041] Figure 4A variant of mechanically influencing the correction body 21 is shown. In this case, the correction body 21 is clamped via two actuator contact surfaces 11 extending parallel to the optical axis OA. By means of an actuator 24, via the actuator contact surfaces 11, a mechanical force can be introduced into the correction body 21 perpendicular to the optical axis OA. It is also possible for the actuator 24 to move in the arrow direction 27 parallel to the optical axis OA, respectively, so that the force vector 26 of the actuator 24 can act on the correction body 21 in opposite directions and in a spatially offset manner. By means of the actuator 24 as a mechanical influencing member, it is thus possible to introduce tensile and compressive forces, torques and any type of static and dynamic mechanical stress in the correction body 21, so that the refractive index n of the dispersive medium 3 and thus the dispersive behavior of the correction body 21 can be controlled. It is conceivable for the correction body 21 to be fixedly clamped in situ and for the force to be applied only via the actuator 24. A change in the geometry of the dispersive medium 3 of the correction body 21 can also be achieved by means of a compressive or tensile force, so that the optical path length L of the optical radiation in the correction body 21 can additionally be influenced in a targeted manner. As an example, it is conceivable to use electrical, hydraulic, pneumatic, thermal and magnetic actuators as actuators 24. Furthermore, it is possible to use piezoelectric actuators or acousto-optic modulators to mechanically modulate the refractive index n of the correction body 21. Furthermore, instead of the actuator 24, it is also possible to use dynamics, in particular in the case of static influencing.

[0042] Figure 5 Two variants of influencing the correction body 21 by means of electromagnetic fields and / or electromagnetic waves are shown. In this case, in variant A, an electric field 13 is applied in the dispersive medium 3 of the correction body 21 via two electrical contact pads 12. The electric field 13 is controlled statically and dynamically via a voltage generator 25. The acting electric field 13 influences the dispersive behavior of the correction body 21, so that different longitudinal chromatic aberrations can be produced in a targeted manner. As an example, the refractive index n of the dispersive medium 3 is modulated in a targeted manner via the electromagnetic field 13 by means of linear effects such as the Pockels effect or the Kerr effect or by means of nonlinear effects. An alternative embodiment is shown in variant B, in which electromagnetic radiation 14 is applied into the dispersive medium 3 in the correction body 21 by means of an emitter 15.

[0043] Fig. 6 shows two possible embodiments of setting the optical path length L in the correction body. In this case, Figure 6A A variant is shown in which the correction body 21' is in the form of an optical wedge 16. The optical path length L in the correction body 21' can be set by means of a displacement of the optical wedge 16 perpendicular to the optical axis OA. Figure 6BA correction body 21" in the form of an arrangement of two optical wedges 16 with similar functional principles is shown in the variant shown in the middle. In this case, the two optical wedges 16 can be displaced so that they slide on their respective wedge surfaces 17 and thus cause a change in the optical path length L. It is likewise conceivable that more than two optical wedges 16 can be used.

[0044] Figure 7 One embodiment of changing the spatial orientation and position of the correction body is shown. In this case, the optical lens elements LI, L2, L3, L4 are arranged along the optical axis OA within the correction body 21. The optical lens elements LI, L2, L3, L4 are mechanically positioned within the correction body 21 by means of the lens element fixture 19 and the lens element mount 20. It is possible to influence the chromatic behavior first via the insertion and withdrawal of the optical lens element L2, since the presence of different longitudinal chromatic aberrations depends on the presence of the optical lens element L2 within the correction body 21. In this case, the optical lens element L2 can be changed depending on the desired chromatic aberration, for example by means of a lens element turret. The optical lens element L3 and the optical lens element L4 can be rotated around their axes in a coordinate system identified in the x-y-z directions and are mechanically freely arranged three-dimensionally. In this case, the lens elements L3, L4 can take up different degrees of freedom by means of their respective mechanisms M3, M4. First, the lens elements L3, L4 can be displaced in the x-y-z directions at their respective lens element mounts 20 by means of the mechanisms M3, M4, respectively. In addition, each lens element L3, L4 can be rotated around the x-y and z axes. The lens elements L3, L4 can likewise be tilted with respect to one another, if different distances B' and B are selected, for example, by means of the mechanisms M3, M4. By means of the different settable spatial orientations and positions of the optical lens element L3 and the optical lens element L4, it is thus possible to set and simulate different longitudinal chromatic aberrations within the inspection device 4 of the mask.

[0045] Figure 8 One variant of influencing the correction body by means of the gas pressure and gas composition of the environment is shown. In this case, the dispersive medium 3 of the correction body is located within a controllable gas environment 30. Via a purging gas unit 31, it is possible to influence the pressure P via a pressure regulator 32 and the composition of the gas N via a gas composition regulator 33. As an example, the gas environment uses nitrogen, which can be enriched in the controlled environment by means of inert gas additives. Since the gas environment acts directly on the dispersive medium 3, it is possible to influence the chromatic behavior of the correction body in a targeted manner.

[0046] Figure 9Three variants of influencing the correction body 21 by a magnetic field are shown. In this case, in variant E, a magnetic field 41 can be imposed inside and outside the dispersive medium 3 of the correction body 21 by a permanent magnet 40. As a further variant F, a coil 42 can be wound around the correction body 21 and its dispersive medium 3 in order to establish a magnetic field 43. The coil 42 and the magnetic field 43 are influenced statically and dynamically via the voltage generator 25. It is likewise conceivable to have an electromagnet 44 as a variant G, which is controlled via the voltage generator 25 to generate a magnetic field 45.

[0047] Figure 10 One embodiment of influencing the correction body 21 by a thermal element 50 is shown. In this case, the dispersive medium 3 of the correction body 21 is influenced thermally by a heating element 50, for example a Peltier element, such that a temperature change occurs within the dispersive medium 3. Due to the temperature dependence of the reflectivity n, the dispersive behavior of the correction body 21 is influenced and thus a specified longitudinal chromatic aberration occurs.

[0048] Figure 11 One variant of influencing the spectral properties of the radiation source of the inspection apparatus 4 of the mask for the target setting change of the longitudinal chromatic aberration is shown. In this case, the electromagnetic illumination radiation 6 of the spectral source 60 can be influenced spectrally via a filter system 61. Spectral sources 60 which are adjustable in their spectral properties, for example spectrally adjustable LEDs, are conceivable, thus it is possible to introduce different longitudinal chromatic aberrations wavelength-selectively in the inspection apparatus 4 of the mask. Furthermore, a spectral source assembly 63 comprising various spectral sources 60, for example in the form of an array, is conceivable, wherein one spectral source 60 from the spectral source assembly 63 is selectively introduced into the beam path. Thus, it is possible to simulate different projection exposure apparatuses and their longitudinal chromatic aberrations within the inspection apparatus 4 of the mask.

[0049] Figure 12 The method of simulating a longitudinal chromatic aberration by recording a focus stack Fx is shown in a schematic view. This involves recording, for example, 5 planes (F1, F2, F3, F4, F5) in a distance A of 100 nm in a defocus position. For this purpose, the camera of the inspection apparatus of the mask and the imaging device can first be adapted variably per recording. The focusing of the main plane L3 on its corresponding focal plane F3 is shown by way of example. Knowing the linewidth of the radiation used and the longitudinal chromatic aberration of the projection exposure apparatus to be simulated, those images from the individual images of the focus stack Fx can then be selected which correspond to the longitudinal chromatic aberration to be simulated. The longitudinal chromatic aberration can then be simulated by appropriate interpolation and, if appropriate, weighting of the images.

[0050] List of reference signs

[0051] 1 optical lens element

[0052] 2 electromagnetic radiation

[0053] 3 dispersive medium

[0054] 4 inspection apparatus for a mask

[0055] 5 radiation source

[0056] 6 illuminating radiation

[0057] 7 illumination unit

[0058] 8 mask

[0059] 9 imaging apparatus

[0060] 10 camera

[0061] 11 actuator contact surface

[0062] 12 electrical contact pad

[0063] 13 electric field

[0064] 14 electromagnetic radiation

[0065] 15 emitter

[0066] 16 optical wedge

[0067] 17 wedge surface

[0068] 19 lens element holder

[0069] 20 lens element mount

[0070] 21 correction body

[0071] 22 dispersion control member

[0072] 23 control unit

[0073] 24 actuator

[0074] 25 voltage generator

[0075] 26 force vector

[0076] 27 direction parallel to optical axis

[0077] A distance of focal plane

[0078] OA optical axis

[0079] F1...F5 focal plane

[0080] L1...L3 wavelength

[0081] N gas composition

[0082] N refractive index

[0083] P pressure

[0084] Fx focal stack

[0085] P1...P6 possible positions of the correction body

[0086] 30 gaseous environment

[0087] 31 gas purifying unit

[0088] 32 pressure regulator

[0089] 33 gas composition regulator

[0090] 40 permanent magnet

[0091] 41 magnetic field of the permanent magnet

[0092] 42 coil

[0093] 43 magnetic field of the coil

[0094] 44 electromagnet

[0095] 45 magnetic field of the electromagnet

[0096] 50 heating element

[0097] 60 spectral source

[0098] 61 optical filter

[0099] 63 assembly of spectral sources

Claims

1. A method for taking into account longitudinal color difference in an inspection apparatus (4) of a mask (8), the method comprising the following steps: - Record a specified number of images with different defocus positions. - Select a subset of the image and simulate the longitudinal chromatic aberration of the projection exposure device. The defocusing step size is divided into a primary step size and a secondary step size, and The selected major step size relative to the mask is 0.3-2 μm.

2. The method according to claim 1, This includes interpolation and / or weighting of the selected image.

3. The method according to claim 2, The selected minor step size relative to the mask is 10-150 nm.

4. The method according to claim 3, The main step size is 1 μm.

5. The method according to claim 3, The secondary step size is 100 nm.

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