Projection exposure system with temperature control device

Abstract

Projection exposure system (1,101) for semiconductor lithography with an optical assembly (40), wherein the optical assembly (40) comprises a deformable optical element (41) and at least one actuator (46.1,46.2,46.3) for deforming the optical element (41), wherein the optical assembly (40) comprises a temperature control device (48) for temperature control of the actuator (46.1,46.2,46.3), characterized in that the temperature control device (48) is self-regulating and comprises an NTC material.

Description

The invention relates to a projection exposure system for semiconductor lithography with a temperature control device. Such systems are used to create extremely fine structures, particularly on semiconductor devices or other microstructured components. The operating principle of these systems is based on the creation of ultra-fine structures down to the nanometer range by means of a generally reduced-size imaging of structures onto a mask, known as a reticle, on a wafer coated with photosensitive material. The minimum dimensions of the generated structures depend directly on the wavelength of the light used. Recently, light sources with emission wavelengths in the nanometer range, for example between 1 nm and 120 nm, particularly in the 13.5 nm range, have been increasingly used. This wavelength range is also referred to as the EUV range. The microstructured components are manufactured not only using EUV systems but also with the established DUV systems operating at wavelengths between 100 nm and 400 nm, particularly 193 nm. With the introduction of the EUV range and the resulting ability to produce even smaller structures, the demands on the optical correction of DUV systems operating at 193 nm have increased further. Additionally, with each new generation of projection exposure systems, regardless of wavelength, throughput is increased to improve efficiency, which typically leads to greater thermal stress and consequently to an increase in thermally induced imaging errors.To correct imaging errors, manipulators can be used, among other methods, to change the position and orientation of the optical elements or to influence the imaging properties of the optical elements, particularly mirrors, by deforming their optical surfaces. An optical surface, in this context, is understood to be a surface that, during the normal operation of the associated system, is exposed to useful radiation, i.e., radiation used for imaging and exposure. The associated actuators can be arranged in a matrix-like fashion, particularly on the side of the optical element facing away from the optical surface, i.e., on its back. They can exert their deformation effect, for example, by stretching or compressing their surface connected to the optical element or by changing the length perpendicular to this surface. In the latter case, a backplate is usually present as a counter-support.During operation, however, the actuators are exposed to varying temperatures both temporally and locally. This affects the geometry of the actuators themselves due to thermal expansion, and also their controllability due to the temperature dependence of the physical effects used, such as electrostrictive, piezoelectric, or magnetostrictive effects. Consequently, a travel distance of the actuators must be reserved solely to compensate for these thermal effects, leaving less available for the intended purpose of influencing the optical surface to correct optical aberrations. Several approaches to addressing the aforementioned problem are known from the prior art. For example, US publication US 2004 / 0 036 940 A1 describes a projection exposure system with an optical element deformable by means of an actuator, which includes a temperature control device for the actuator. Comparable concepts can be found in Japanese publication JP 2015-50 353 A and European patent EP 1 712 955 B1. The object of the present invention is to provide a projection exposure system with actuators for influencing optical elements used in the system, which exhibits increased robustness against thermal influences on the controllability of the actuators. This problem is solved by a projection exposure system with features of the independent claim. The dependent claims relate to advantageous further developments and variants of the invention. A projection exposure system according to the invention for semiconductor lithography comprises an optical assembly, wherein the optical assembly includes a deformable optical element and at least one actuator for deforming the optical element. The optical assembly includes a temperature control device for temperature control of the actuator, which is designed to be self-regulating according to the invention and comprises an NTC material. In this context, "self-regulating" means that no external control is necessary. Self-regulation can be achieved, for example, through the material properties of certain materials in a resistance heater. In an electric resistance heater, a current-carrying conductor heats up due to its ohmic resistance. The heat output of a current-carrying conductor is determined by: Thus, the heating power is proportional to the electrical resistance R. If the electrical resistance decreases with temperature, the heating power also decreases. Materials exhibiting this behavior are called NTC – Negative Temperature Coefficient materials. Examples of such materials are oxides of manganese, nickel, cobalt, iron, copper, or titanium. The temperature dependence of the resistance can be described by the following equation: Here, RT describes the resistance at temperature T, RR the resistance at a reference temperature TR, and B the thermistor constant. By varying the thermistor constant, the resistance can be selected for a target temperature, such as 40° Celsius. The resistance heater can, for example, be designed as a meandering conductor in the inactive part of an actuator. By selecting the constant and power, a self-regulating temperature control device can be implemented. At the predetermined target temperature, the heating power corresponds to the heat loss through radiation and conduction, so the temperature does not rise further. In the areas of the actuator where the temperature is lower than the target temperature, the resistance of the resistance heater material is also higher, and the actuator is heated more intensely in these areas than in areas that have already reached the target temperature. In particular, the optical element can be a mirror. In this case, the problem arises that the temperature gradients of the mirror body, formed by the locally varying application of useful light to the optical surface, are transferred to the actuator. In the case of electrostrictive or piezoelectric actuators, this can lead to a varying electrostrictive effect and differential geometric changes. According to the invention, the temperature control device can compensate for the temperature gradients in the actuator and thus effectively counteract the problem described. The same or similar effects due to a temperature increase as described above are also observed in other types of solid-state actuators, such as magnetostrictive actuators, shape memory actuators, magnetic shape memory actuators, and active polymers, especially electroactive polymers. In an advantageous embodiment of the invention, the optical assembly can comprise several individual actuators. The actuators can be controlled individually, thereby providing two or more degrees of freedom for deformation of the mirror. This allows the deformation of the optical surface, which is used to image a structure onto a wafer and is caused by heating of the mirror, to be advantageously compensated by the plurality of actuators. In particular, the temperature control device can include a heating element for heating the actuator. Suitable heating of the actuator results in a spatially homogeneous temperature distribution within the actuator. The thermal influence on the electrostrictive, piezoelectric, or magnetostrictive effect and the change in geometry is therefore constant across the actuator and can advantageously be compensated for by a single temperature-dependent correction factor in a controller for regulating the actuator's displacement. The heating element can be connected to the actuator. In particular, the heating element can be designed as part of the actuator. The heating element can be embedded within the actuator. The actuator can comprise electrostrictive, piezoelectric, or magnetostrictive material, with only a portion of the material being activated by electrodes. If multiple electrodes are arranged within a layer of material, several actuators can be formed within that layer. The heating element can be located in the non-activated material of the respective actuator, which may be situated on the side of the activated area of ​​the actuator facing away from the optical surface. Thus, each actuator can be assigned a heating element. The heating element can, for example, be designed as a resistance heater. Alternatively, the heating element can also be based on infrared radiation. Alternatively or additionally, the temperature control device can also include means for cooling the actuator. Cooling the actuator has the same effect as heating it, namely to bring the actuator to a constant temperature or a homogeneous temperature distribution. Cooling or, more generally, temperature control can be achieved by the temperature control device including fluid channels for heating and / or cooling the actuator. In particular, the fluid channels can be arranged within the actuator. Similar to the heating elements in the embodiments described above, the fluid channels can be located in the inactive part of the actuator. This has the advantage that the actuator can be qualified separately as an independent system before being connected to the optical element, such as a mirror. Furthermore, the fluid channels can be arranged within the optical element. The fluid channels can be positioned on the back side of the optical element, which is designed as a mirror, thus advantageously avoiding deformation of the optical surface caused by the pressure of the cooling fluid or the manufacturing of the fluid channels. Combined cooling of the mirror and the actuator is also conceivable, in which both the actuator and the mirror are cooled, thereby advantageously reducing the heat output acting on the actuator. Furthermore, the temperature control device can include a sensor, which may be designed as a temperature sensor. Multiple sensors can also be distributed across the actuator, allowing for the detection of a temperature gradient. In the case of multiple actuators, each actuator can include one or more sensors. In particular, the temperature control device can include a control unit, which can be designed as an external controller or integrated into the respective heating element. The advantage of integrating the control unit into the heating element is that it minimizes the actuator's connections to a frame, thereby reducing the transmission of mechanical excitations and the complexity of the design. The control unit can then control the heating elements in such a way that the actuator, or all actuators, can be maintained at a constant temperature. Exemplary embodiments and variants of the invention are explained in more detail below with reference to the drawings. Fig. 1 shows the basic structure of a DUV projection exposure system in which the invention can be implemented, Fig. 2 the basic structure of an EUV projection exposure system in which the invention can be implemented, Fig. 3 the basic structure of an optical assembly with an actuator matrix, Fig. 4a-c a basic representation of possible arrangements of actuators on a mirror, Fig. 5 a diagram illustrating an exemplary temperature distribution across an actuator matrix, Fig. 6a-d a schematic representation of various arrangements of temperature control devices, Fig. 7a, b a diagram and a schematic representation illustrating a self-regulating temperature control device, and Fig. 8a, b a further embodiment of a temperature control device. Figure 1 shows an exemplary projection exposure system 1 in which the invention can be applied. The projection exposure system 1 serves to expose structures onto a substrate coated with photosensitive materials, which generally consists predominantly of silicon and is referred to as a wafer 2, for the production of semiconductor devices, such as computer chips. The projection exposure system 1 essentially comprises a lighting device 3 for illuminating an object field 8 in an object plane 9, a reticule holder 6 for receiving and precisely positioning a mask provided with a structure and arranged in the object plane 9, a so-called reticule 7, by which the subsequent structures on the wafer 2 are determined, a wafer holder 10 for holding, moving and precisely positioning this wafer 2 and an imaging device, namely a projection optic 13, with several optical elements 14, which are held in a lens housing 16 of the projection optic 13 via mounts 15. The basic operating principle is that the structures introduced into the reticle 7 are mapped onto the wafer 2; the mapping is usually performed in a reduced size. A light source 4 of the illumination device 3 provides a projection beam 17 in the form of electromagnetic radiation, which is required for imaging the reticulum 7 arranged in the object plane 9 onto the wafer 2 arranged in an image plane 12 within the area of ​​an image field 11. This radiation is particularly in a wavelength range between 100 nm and 300 nm. A laser, a plasma source, or the like can be used as the source 4 for this radiation. In an illumination optic 5 of the illumination device 3, the radiation is shaped by optical elements 18 such that, upon striking the reticulum 7 arranged in the object plane 9, the projection beam 17 illuminates the object field 8 with the desired properties with respect to diameter, polarization, wavefront shape, and the like. An image of the reticulum 7 is generated via the projection beam 17 and, as explained above, is reduced in size by the projection optics 13 and transferred to the wafer 2 arranged in the image plane 12. The reticulum 7 and the wafer 2 can be moved synchronously, so that areas of the reticulum 7 are mapped onto corresponding areas of the wafer 2 practically continuously during a so-called scan process. The projection optics 13 comprises a multitude of individual refractive, diffractive, and / or reflective optical elements 14, such as lenses, mirrors, prisms, end plates, and the like, whereby these optical elements 14 can be actuated, for example, by one or more actuator arrangements not shown separately in the figure. Fig. 2 shows an example of the basic structure of an EUV projection exposure system 101 for microlithography, in which the invention can also be applied. The structure of the projection exposure system 101 and the principle of imaging a structure on a reticle 107 arranged in the object plane 109 on a reticle holder 106 onto a wafer 102 arranged in the image field 111 is comparable to the structure and procedure described in Fig. 1. Identical components are designated with a reference numeral increased by 100 compared to Fig. 1; thus, the reference numerals in Fig. 2 begin with 101. In contrast to a transmitted light system as described in Fig. 1, due to the short wavelength of the EUV radiation 117 used in the range of 1 nm to 120 nm, in particular 13.5 nm, only optical elements 114, 118 designed as mirrors can be used for imaging or illumination in an EUV projection exposure system 101. The lighting device 103 of the projection exposure system 101 comprises, in addition to a light source 104, an illumination optic 105 for illuminating the object field 108 in an object plane 109. The EUV radiation 117 generated by the light source 104 as useful optical radiation is directed by a collector integrated in the light source 104 such that it passes through an intermediate focus in the region of an intermediate focus plane 119 before striking a field facet mirror 120. After the field facet mirror 120, the EUV radiation 117 is reflected by a pupil facet mirror 121. With the aid of the pupil facet mirror 121 and an optical assembly 122 with mirrors 118, the field facets of the field facet mirror 120 are imaged into the object field 108. The construction of the subsequent projection optics 113 with the lens housing 116 does not differ in principle from that shown in Fig., except for the use of mirrors 114.The structure described in section 1 is therefore not described further. Fig. 3 shows the basic structure of a prior art optical assembly 40, comprising an optical element designed as a mirror 41 and an actuator matrix 45. The mirror 41 is, for example, part of the projection optics 9 described in Fig. 1. The actuator matrix 45 comprises several matrix-like actuators 46, which are arranged on the back side 42 of the mirror, i.e., on the side of the mirror 41 opposite the optical surface. By deflecting the actuators 46, the back side 42 of the mirror is deformed, which, due to the stiffness of the mirror 41, also deforms the optical surface of the mirror 41. This deformation of the optical surface changes the imaging properties of the mirror 41, thereby preventing or compensating for imaging errors of the projection optics. Fig. 4a shows a prior art optical assembly 40 comprising a mirror 41 and actuators 46.1, depicted in an undeformed state. The actuators 46.1 are arranged on the back side 42 of the mirror, which corresponds to the side of the mirror 41 opposite the optical surface 43. The actuators 46.1 are connected at one end to the back side 42 of the mirror and at the other end to a frame 44. The frame 44 is very rigid and designed to absorb the reaction forces of the actuator 46.1 during deformation of the mirror 41 without itself undergoing significant deformation. The mirror 41 is thus deformed in its longitudinal direction by the expansion of the actuator 46.1. In contrast, Fig. 4b shows an arrangement of the optical assembly 40 in which the actuators 46.2 are connected to the back surface 42 of the mirror 41 only at one end, and the optical surface 43 is deformed. In this case, the deformation of the mirror 41, and thus of the optical surface 43, is caused by the transverse contraction of the actuator 46.2, i.e., the change in the cross-section of the actuator 46.2 as a result of a deflection in the longitudinal direction. The surface of the actuator 46.2 connected to the back surface 42 of the mirror changes, resulting in a bulge forming on the opposite optical surface 43 in the example shown. The individual actuators 46.2 can also be configured as part of an actuator layer 47 by means of a suitable arrangement of electrodes (not shown). This has the advantage that the actuators can be connected as a whole to the mirror 41 as an actuator matrix, as described in Fig. 3.Furthermore, the power supply and the ground line are also integrated in the actuator layer 47, which means that all actuators 46.2 can only be connected at one point to a control unit which is also not shown. Fig. 4c shows a combination of the arrangements described in Figs. 4a and 4b, known from the prior art. The actuators 46.3 are arranged between the back surface of the mirror 41 and a backplate 63. The backplate 63 is designed such that it has a stiffness of a similar order of magnitude to that of the mirror 41, i.e., the ratio of the stiffnesses of the backplate 63 and the mirror 41 lies between one and five. With this arrangement, the actuators 46.3 deform the mirror 41 both due to the longitudinal effect, i.e., due to a change in shape in the longitudinal direction of the actuator 46.3, and due to the transverse effect, i.e., due to the lateral contraction of the actuator 46.3. The ratio of the longitudinal and the transverse effect depends, among other things, on the ratio of the stiffnesses of the mirror 41 and the backplate 63. This ratio also influences the width and extent of the deformation of the mirror. Fig. 5 shows a diagram schematically illustrating a typical spatial temperature profile across an actuator matrix in the x-direction, i.e., essentially parallel to the optical working surface 43. The actuators are represented by the dashed lines, and the area between the dashed lines corresponds to the x-direction of an actuator. In this example, the temperature varies across the actuator matrix from approximately 30° Celsius to 37° Celsius. The significant temperature variations from actuator to actuator are clearly visible. Even within a single actuator, the temperature T can vary by more than one Kelvin, as can be seen in the figure.The variations shown lead to different thermal expansion for different actuators, and as shown, also within a single actuator, resulting in locally varying electrostrictive, piezoelectric, or magnetostrictive effects that influence the controllability of the affected actuators. It is therefore desirable to counteract the effect indicated in Fig. 5. Figs. 6a to 6d show various embodiments of the invention in which the temperature differences within an actuator 46.2 and between them are minimized by means of temperature control devices 48 with heating elements 54. Fig. 6a shows an optical assembly 40 with a temperature control device 48, which includes an external control unit 49. The mirror 41 is provided with several actuators 46.2, which are formed in an actuator layer 47, as shown in Fig. 4b, wherein each actuator 46.2 comprises a sensor designed as a temperature sensor 52 and a heating element 54. The signal generated by the temperature sensor 52 is transmitted to the control unit 50 of the external control unit 49 via a signal line 53. The control unit 50 determines the heating power required to reach the set temperature, and the power supply 51, also arranged in the control unit 49, controls the heating element 54 via a supply line 55. In contrast, Fig. 6b shows an embodiment of the optical assembly 40 in which, instead of an external control unit, each actuator 46.2 is assigned a control unit 50, the structure of the actuators 46.2 in the actuator layer 47 being identical to that in Fig. 6a. The control unit 50, together with the heating element 54 and the temperature sensor 52, is directly connected to the actuator 46.2. This advantageously minimizes the number of connections required between the so-called fixed world, i.e., the base frame of the projection exposure system, and the associated components, such as the control unit. The actuators 46.2 arranged on the mirror 41 are now only connected to the power supply 51 via a supply line 55, and the signal lines are no longer necessary due to the integration of the control unit 50, sensor 52, and heating element 54.The power supply 51 of all heating elements 54 is ensured by a connection of the heating elements 54 to each other, starting from the heating element 54 connected to the power supply 51. Fig. 6c shows a section through an actuator 46.2 comprising a heating element 54. The actuator 46.2 includes two electrodes 56, such that the area of ​​the actuator layer 47 between the electrodes acts as the active area 57, and the inactive area 58 is designed to connect the actuators 46.2 to each other and to the mirror 41. In this embodiment, the heating element 54 is located in the inactive area 58. The heating element 54 is designed as a resistance heater, so that the same technology used for inserting the electrodes into the actuator layer 47 can be applied to manufacture the heating element 54. Fig. 6d shows a cross-sectional view through the heating element 54 shown in Fig. 6c, where the heating element 54 is designed in a meandering shape. The meandering shape increases the conductor length per unit area, so that a higher heating power per unit area can be set. The variant shown in Fig. 4c with a back plate can also be heated by the heating elements shown in Figs. 6a to d. Figures 7a and 7b describe the operation of a further embodiment of a temperature control device, wherein this temperature control device is self-regulating, i.e. no external control is necessary. Fig. 7a shows a diagram plotting the resistance against temperature for various thermistor constants B. The thermistor constant B describes the resistance curve of a negative temperature coefficient (NTC) thermistor as a function of temperature. A key characteristic of a negative temperature coefficient thermistor is its conductivity, meaning it conducts electricity better at high temperatures than at low temperatures. By selecting a suitable thermistor constant, a heating element, as shown in Fig. 6d, can be designed as a self-regulating temperature control device. Fig. 7b shows the basic operating principle of an actuator 46.2 on a mirror 41 with a thermistor 54.1 as a heating element. The sections of the thermistor 54.1 shown as resistors 59 represent the temperature-dependent, and potentially varying, resistances of the thermistor 54.1. In the hatched area of ​​the actuator 46.1, where the target temperature is reached, the resistance of the thermistor is low, so that only the power loss through heat radiation and / or conduction is compensated. In the unhatched area, where the temperature is below the target temperature, the resistance of the thermistor 54.1 is higher, so that this area heats up to the target temperature. This ensures that the actuator 46.2 is always kept at a constant and uniform temperature. The meandering arrangement of the thermistor 54.1 shown in the figure...1 can also be formed rotated by 90° to the direction shown. Fig. 8a and Fig. 8b show further embodiments of the temperature control device 48, in which a fluid channel 61 is used. Fig. 8a shows an embodiment in which fluid channels 61 are formed in the inactive area 58 of the actuator 46.2 in the actuator layer 47 on the side of the actuator 46.2 facing away from the connection to the mirror 41, as a means of cooling the actuator 46.2. A temperature control fluid can flow through the fluid channels 61 as required. This allows the actuator 46.2 to be cooled to a set temperature. Fig. 8b shows a variant in which fluid channels 61 are formed in the mirror 41. The fluid channels 61 are located a few millimeters from the back surface 42 of the mirror 41, so that the optical surface 43 of the mirror is not deformed by the pressure of the temperature control fluid. The fluid channels 61 prevent the actuators 46.2 from being heated by the mirror 41 and dissipate the heat generated in the actuator 46.2. Alternatively, the temperature control device 48 can also be designed such that all the heat absorbed by the mirror 41 through the absorption of useful light is dissipated. Fig. 8c shows the arrangement depicted in Fig. 4c, wherein fluid channels 61 are arranged in the backplate 63, allowing the heat generated by the actuators 46.3 and from the heating of the mirror 41 to be transferred to the backplate 63. In principle, the fluid channels 61 can also be supplied with warm water or steam and thus, as in the embodiments described in Figs. 6a to 6d and 7a and 7b, maintain the actuator 46.3 at a constant temperature. Reference symbol list 1 DUV - Projection exposure system 2 Wafer 3 Lighting device 4 Light source 5 Lighting optics 6 Reticle holder 7 Reticle 8 Object field 9 Object plane 10 Wafer holder 11 Image field 12 Image plane 13 Projection optics 14 Optical elements (projection optics) 15 Mounts 16 Lens housing 17 Projection beam 18 Optical elements (lighting device) 40 Optical assembly 41 Mirror 42 Mirror back 43 Optical working surface 44 Frame 45 Actuator matrix 46.1, 46.2, 46.3 Actuator 47 Actuator layer 48 Temperature control device 49 Control unit 50 Regulation unit 51 Power supply 52 Temperature sensor 53 Signal line 54, 54.1 Heating element 55 Supply line 56 Electrode 57 Active area 58 Inactive area 59 Resistor 61 Fluid channel 63 Backplate 101 EUV - Projection exposure system 102 Wafer 103 Illumination device 104 Light source 105 Illumination optics 106 Reticle holder 107 Reticle 108 Object field 109 Object plane 110 Wafer holder 111 Image field 112 Image plane 113 Projection optics 114 Optical elements (projection optics) 115 Mounts 116 Lens housing 117 Projection beam 118 Optical elements (illumination device) 119 Intermediate focus 120 Field facet mirror 121 Pupil facet mirror 122 Optical assembly L Length of an actuator T Temperature.

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