METHOD AND DEVICE FOR MEASURING ACTUATORS IN A PROJECTION EXPOSURE APPARATUS FOR SEMICONDUCTOR LITHOGRAPHY

Abstract

A method for measuring an actuator in a projection exposure apparatus for semiconductor lithography, comprises: driving and deflecting a first actuator with a constant control signal; deflecting a further actuator by way of the mechanical coupling; and determining the capacitance of the further actuator, which was deflected by way of the coupling. A projection exposure apparatus for semiconductor lithography comprises a control device and a measuring device, wherein the measuring device is configured to determine the capacitance of at least one actuator in the projection exposure apparatus.

Claims

1. A method of measuring an actuator in a semiconductor lithography projection exposure apparatus, the apparatus comprising a first actuator and a second actuator mechanically coupled with the first actuator, the method comprising: using a control signal to drive and deflect a first actuator in the apparatus and deflecting a second actuator in the apparatus via a mechanical coupling between the first and second actuators; and determining a capacitance of the second actuator.

2. The method of claim 1, wherein the apparatus comprises a projection lens, and the first and second actuators are in the projection lens.

3. The method of claim 1, wherein at least one of the first and second actuators comprises a piezoelectric actuator.

4. The method of claim 1, wherein at least one of the first and second actuators comprises an electrorestrictive actuator.

5. The method of claim 1, wherein the first and second actuators are connected to an optical element.

6. The method of claim 1, wherein the first and second actuators are connected to a mirror.

7. The method of claim 1, wherein the deflection of the first actuator comprises more than 40% of an overall deflection of the first actuator.

8. The method of claim 1, further comprising using an AC voltage at a constant frequency to detect the capacitance of the second actuator.

9. The method of claim 1, wherein the capacitance of the second actuator is measured with a resolution of at most 10.sup.−4 of the typical capacitance of the actuators.

10. The method of claim 1, further comprising using a constant temperature to detect the capacitance of the second actuator.

11. The method of claim 1, further comprising calibrating the deflection of the first and second actuators by comparing the determined capacitance value of the second actuator with setpoint capacitance values determined when the first and second actuators are put into operation.

12. The method of claim 11, wherein the projection exposure apparatus comprises a projection lens, and the first and second actuators are in the projection lens.

13. The method of claim 12, wherein the first and second actuators are connected to an optical element.

14. The method of claim 12, wherein the first and second actuators are connected to a mirror.

15. An apparatus, comprising: a first actuator; a second actuator mechanically coupled with the first actuator; a control device; and a measuring device configured to determine a capacitance of the second actuator, wherein the apparatus is a semiconductor lithography projection exposure apparatus.

16. The apparatus of claim 15, further comprising an optical element, wherein the first and second actuators are connected to the optical element.

17. The apparatus of claim 15, further comprising a mirror, wherein the first and second actuators are connected to the mirror.

18. The apparatus of claim 15, wherein the control device comprises an AC voltage source and/or a DC voltage source.

19. The apparatus of claim 15, wherein the measuring device comprises a sensor configured to detect a temperature when detecting the capacitance of the second actuator.

20. The apparatus of claim 15, wherein the control device is configured to drive the second actuator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Exemplary embodiments and variants are explained in more detail below with reference to the drawing, in which:

[0022] FIG. 1 shows a basic illustration of a DUV projection exposure apparatus;

[0023] FIG. 2 shows a basic illustration of an EUV projection exposure apparatus;

[0024] FIGS. 3A-3C show prior art actuator arrangements for deformable mirrors;

[0025] FIGS. 4A-4C show schematic illustrations for elucidating a principle; and

[0026] FIG. 5 shows a flowchart for a measuring method.

DETAILED DESCRIPTION

[0027] FIG. 1 illustrates an exemplary DUV projection exposure apparatus 21, in which the disclosure can be used. The projection exposure apparatus 21 serves for the exposure of structures on a substrate which is coated with photosensitive materials, and which generally consists predominantly of silicon and is referred to as a wafer 22, for the production of semiconductor components, such as computer chips.

[0028] The projection exposure apparatus 21 in this case substantially comprises an illumination device 23, a reticle holder 24 for receiving and exactly positioning a mask provided with a structure, a so-called reticle 25, by which the later structures on the wafer 22 are determined, a wafer holder 26 for holding, moving and exactly positioning the wafer 22 and an imaging device, to be specific a projection lens 27, with a plurality of optical elements 28, which are held by way of mounts 29 in a lens housing 30 of the projection lens 27.

[0029] The basic functional principle in this case provides for the structures introduced into the reticle 25 to be imaged on the wafer 22, the imaging generally reducing the scale.

[0030] The illumination device 23 provides a projection beam 31 in the form of electromagnetic radiation, which is used for the imaging of the reticle 25 on the wafer 22, the wavelength range of the radiation lying between 100 nm and 300 nm, for example. A laser, a plasma source or the like can be used as the source of this radiation. Optical elements in the illumination device 23 are used to shape the radiation in such a way that, when it is incident on the reticle 25, the projection beam 31 has the desired properties with regard to diameter, polarization, form of the wavefront and the like.

[0031] An image of the reticle 25 is produced by the projection beam 31 and transferred from the projection lens 27 onto the wafer 22 in an appropriately reduced form, as already explained above. In this case, the reticle 25 and the wafer 22 can be moved synchronously, so that regions of the reticle 25 are imaged onto corresponding regions of the wafer 22 virtually continuously during a so-called scanning process. The projection lens 27 has a multiplicity of individual refractive, diffractive and/or reflective optical elements 28, such as for example lens elements, mirrors, prisms, terminating plates and the like, wherein the optical elements 28 can be actuated for example using one or more of the actuator arrangements described here.

[0032] FIG. 2 shows by way of example the basic construction of a microlithographic EUV projection exposure apparatus 1 in which the disclosure can likewise find application. An illumination system of the projection exposure apparatus 1 has, in addition to a light source 3, an illumination optical unit 4 for the illumination of an object field 5 in an object plane 6. EUV radiation 14 in the form of optical used radiation generated by the light source 3 is aligned using a collector, which is integrated in the light source 3, in such a way that it passes through an intermediate focus in the region of an intermediate focal plane 15 before it is incident on a field facet mirror 2. Downstream of the field facet mirror 2, the EUV radiation 14 is reflected by a pupil facet mirror 16. With the aid of the pupil facet mirror 16 and an optical assembly 17 having mirrors 18, 19 and 20, field facets of the field facet mirror 2 are imaged into the object field 5.

[0033] A reticle 7 arranged in the object field 5 and held by a schematically illustrated reticle holder 8 is illuminated. A merely schematically illustrated projection lens 9 serves for imaging the object field 5 into an image field 10 in an image plane 11. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 12, which is arranged in the region of the image field 10 in the image plane 11 and held by a likewise partly represented wafer holder 13. The light source 3 can emit used radiation for example in a wavelength range of between 1 nm and 120 nm.

[0034] FIGS. 3A to 3C show various actuator arrangements, known from the prior art, for optical elements embodied as deformable mirrors 32, as are used in one of the apparatuses illustrated above.

[0035] FIG. 3A shows an actuator arrangement in which actuators 37 are arranged between the back side 35 of the deformable mirror 32 and a frame 36. Each actuator 37 is connected to a control device 40. If, as illustrated in FIG. 3A, the actuators 37 are driven and deflected from their neutral position, there is a deformation of the mirror body 33 and hence of the mirror surface 34 that is relevant to the imaging properties.

[0036] FIG. 3B shows a further actuator arrangement in which actuators 38 are arranged at the back side 35 of the deformable mirror 32. The actuators 38 deform the mirror body 33, and hence the mirror surface 34 that is relevant to the imaging properties, by way of a deflection carried out perpendicular to the back side 35 of the mirror body 33. In contrast with the actuators 37 illustrated in FIG. 3A, the deformation of the mirror body 33 is caused by the transverse contraction of the actuator 38 in this case. Like in FIG. 3A, the mirror 32 is illustrated in a deformed state. Usually, the actuators 38 are connected to the mirror back side 35 with pretension, i.e., in an already deflected state, as a result of which the actuators 38 can act in two directions. As described in FIG. 3A, each actuator 38 is connected to a control device 40.

[0037] FIG. 3C shows a further alternative actuator arrangement, in which the actuators 38 are embodied as part of an actuator main body 39. The actuator main body 39 comprises the same material as the actuator 38 and the actuators 38 are formed by arranging electrodes (not illustrated) in the actuator main body 39 such that the actuator main body 39 is only deflected in the region of the electrodes. The actuators 38 need not be individually attached to the mirror back side 35 of the mirror 32; instead, a group of actuators 38 can be attached in one component part to the mirror back side 35. The individual actuators 38 are mechanically coupled to one another by way of the actuator main body 39. The basic functionality of the deformation of the mirror body 33 and of the mirror surface 34 is as described in FIG. 3B. Likewise, each actuator 38 is connected to a control device 40.

[0038] FIGS. 4A to 4C each show an equivalent diagram, which should elucidate the mechanical coupling of the actuators 38 among themselves and to the mirror 32 (FIGS. 4A and 4B) and the measuring principle (FIG. 4C).

[0039] Here, FIG. 4A shows a plan view of the back side 35 of a mirror body 33 with an actuator main body 39, the latter comprising nine actuators 38.x. The actuators 38.x are all mechanically coupled to one another by way of the actuator main body 39, the mechanical coupling of the actuators among themselves being illustrated schematically by the springs 41 and the coupling of the actuators 38.x to the mirror body 33 being illustrated schematically by the springs 42. The actuator 38.2, illustrated using dashed lines in FIG. 4A, in the centre of the actuator main body 39 is driven by a control device 40 during the measurement according to the disclosure and deflected with a constant voltage.

[0040] is FIG. 4B shows a section through the mirror 32 along the line IVb in FIG. 4A. The three actuators 38.1, 38.2, 38.3 are clearly visible in the sectional illustration, the actuator 38.2 being connected to a control device 40 but not yet driven; i.e., the deflection is zero. Sensors 44, which are used to measure the temperature at the site of the actuators 38.x, are likewise clearly visible in FIG. 4B.

[0041] FIG. 4C, in turn, shows a section through the mirror 32, the actuator 38.2 being driven by the control device 40 with a constant voltage. The actuator 38.2 is deflected perpendicular to the back side 35 of the mirror body 33 and simultaneously contracts parallel to the back side 35 of the mirror body 33. The mirror body 33 is deformed as a result of the coupling between the actuator 38.2 and the mirror body 33. The actuators 38.1, 38.3 are mechanically coupled to the actuator 38.2 via the actuator main body 39 and via the mirror body 33, leading to stress and hence a deformation in the actuators 38.1, 38.3 when the actuator 38.2 is deflected. This leads to a change in capacitance in the case of an electrostrictive or piezoelectric actuator 38.1, 38.3, which can be measured. To this end, the two actuators 38.1 and 38.3, which act as sensors during the measurement, are connected to a measuring device 43. The latter detects the change in capacitance of the actuators 38.1, 38.3 between an unloaded state, as illustrated in FIG. 4B, and a state loaded by stress caused by the deflection of actuator 38.2, as illustrated in FIG. 4C. FIG. 4C illustrates that every actuator 38.1, 38.2, 38.3 is connected to the s control device 40 and the measuring device 43 such that sequential measuring of all actuators 38.1, 38.2, 38.3 is possible.

[0042] Naturally, the measuring principle illustrated on the basis of FIGS. 4A to 4C can be applied to very different actuator arrangements in addition to the arrangements shown here in exemplary fashion.

[0043] The theoretical background to this measuring process is briefly summarized below:

[0044] Equation (1), below, describes the relationship between the strain S of the actuator 38 and the electric field E and the stress G. The material parameter M represents the coupling between mechanical strain and electric field E. The inverse of Young's modulus of the material is represented by s:

S=M*E.sup.2s*σ  (1)

[0045] The electric field arises from (2):

E=U/d.sub.layer   (2),

[0046] where d.sub.layer denotes the layer thickness between two electrodes of the actuator 38 and U is the voltage.

[0047] Stress σ is calculated by (3) and is formed from the applied force F per unit area A.

σ=F/A   (3)

[0048] In a manner comparable to linear piezoelectric materials, electrostrictive materials also have an inverse electrostrictive effect ε, which is described in (4):

D=ε*E+2*M*E*σ  (4)

[0049] Consequently, an electric displacement field D arises, which depends on the electric field E and stress σ.

[0050] The capacitance C of an electrostrictive actuator can be calculated using (5) from the displacement field D and the applied voltage U. Here, C denotes the capacitance, A denotes the active area of the actuator, N denotes the number of layers and d.sub.layer denotes the thickness of the layers.

C=(D*A*N)/U   (5)

[0051] From (4) and (5), the capacitance of the actuator arises as

C=(ε*A*N)/d.sub.layer+(2*M*a*N)/d.sub.layer   (6).

[0052] Here, a constant bias voltage is applied in the case of the electrostrictive actuators 38.1, 38.3 shown here and the capacitance is determined at a constant frequency. Expediently, the measurement is carried out at a constant temperature in order to exclude the influence of a change in temperature on the capacitance measurement. The capacitance measurement can also be applied in the case of piezoactive actuators, wherein the stress, as a result of the piezoelectric effect, can also be determined by a voltage measurement in this type of actuator.

[0053] FIG. 5 shows a flowchart for a method according to the disclosure for measuring an actuator in a projection exposure apparatus, wherein two actuators are mechanically coupled to one another.

[0054] In a first method step 51, the first actuator 38.2 is driven and deflected using a constant control signal and a further actuator 38.1, 38.3 is deflected as a result of the mechanical coupling 41.

[0055] In a second method step 52, the capacitance of the further actuator 38.1, 38.3, deflected as a result of the coupling, is determined.

LIST OF REFERENCE SIGNS

[0056] 1 Projection exposure apparatus (EUV)

[0057] 2 Field facet mirror (EUV)

[0058] 3 Light source (EUV)

[0059] 4 Illumination optical unit (EUV)

[0060] 5 Object field (EUV)

[0061] 6 Object plane (EUV)

[0062] 7 Reticle (EUV)

[0063] 8 Reticle holder (EUV)

[0064] 9 Projection lens (EUV)

[0065] 10 Image field (EUV)

[0066] 11 Image plane (EUV)

[0067] 12 Wafer (EUV)

[0068] 13 Wafer holder (EUV)

[0069] 14 EUV radiation (EUV)

[0070] 15 Intermediate field focal plane (EUV)

[0071] 16 Pupil facet mirror (EUV)

[0072] 17 Assembly (EUV)

[0073] 18 Mirror (EUV)

[0074] 19 Mirror (EUV)

[0075] 20 Mirror (EUV)

[0076] 21 Projection exposure apparatus (DUV)

[0077] 22 Wafer (DUV)

[0078] 23 Illumination optical unit (DUV)

[0079] 24 Reticle holder (DUV)

[0080] 25 Reticle (DUV)

[0081] 26 Wafer holder (DUV)

[0082] 27 Projection lens (DUV)

[0083] 28 Optical element (DUV)

[0084] 29 Mounts (DUV)

[0085] 30 Lens housing (DUV)

[0086] 31 Projection beam (DUV)

[0087] 32 Mirror

[0088] 33 Mirror body

[0089] 34 Mirror surface

[0090] 35 Mirror back side

[0091] 36 Frame

[0092] 37 Actuator—controller output perpendicular to the mirror back side

[0093] 38.1-38.3 Actuator—controller output parallel to the mirror back side

[0094] 39 Actuator main body

[0095] 40 Control device

[0096] 41 Spring (mechanical coupling in the actuator main body)

[0097] 42 Spring (mechanical coupling with the mirror body)

[0098] 43 Measuring device

[0099] 44 Sensor

[0100] 51 Method step 1

[0101] 52 Method step 2