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Patent Issued for Arrangement for Actuating an Element in a Microlithographic Projection Exposure Apparatus

August 6, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventors Bleidistel, Sascha (Aalen, DE); Schoenhoff, Ulrich (Neu-Ulm, DE); Fischer, Juergen (Heidenheim, DE), filed on January 17, 2014, was published online on July 22, 2014.

The patent's assignee for patent number 8786826 is Carl Zeiss SMT GmbH (Oberkochen, DE).

News editors obtained the following quote from the background information supplied by the inventors: "The invention relates to arrangements for actuating an element in a microlithographic projection exposure apparatus.

"Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (=reticle) illuminated via the illumination device is in this case projected via the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.

"In a projection exposure apparatus designed for EUV (i.e. for electromagnetic radiation having a wavelength of less than 15 nm), for lack of light-transmissive materials being available, mirrors are used as optical components for the imaging process. The mirrors can be fixed on a carrying frame and can be designed to be at least partly manipulatable in order to enable the respective mirror to be moved for example in six degrees of freedom (i.e. with regard to displacements in the three spatial directions x, y and z and with regard to rotations R.sub.x, R.sub.y and R.sub.z about the corresponding axes). In this case, the position of the mirrors can be determined via position sensors fixed to a sensor frame.

"In a projection exposure apparatus designed for EUV, mirrors designed to be manipulatable are used both as actively deformable mirrors, in the case of which changes in the optical properties that occur e.g. during the operation of the projection exposure apparatus and resultant imaging aberrations, e.g. on account of thermal influences, can be compensated for by active deformation, and as non-actively deformable mirrors, in the case of which no targeted deformation is effected.

"The positional control of such mirrors serves, in conjunction with a suitable actuator system (e.g. with Lorentz actuators), to keep the mirrors in their position as stably as possible, such that a deviation of the mirror positions that is measured via the position sensors is as small as possible. One approach that is possible in principle for this purpose consists in increasing the controller gain and thus increasing the control bandwidth. In this case, however, the problem occurs in practice that the mirrors are not ideally rigid bodies, but rather each have specific natural frequencies of the mechanical structures (e.g. of a typical order of magnitude in the range of 2-3 kHz), wherein the corresponding natural frequency spectra for the dimensions of the mirrors and of the carrying and measuring structures, the dimensions increasing with increasing numerical apertures, are shifted further and further toward lower frequencies. This applies all the more to actively deformable mirrors, which have to be designed to be deformable and thus compliant in a targeted manner. An excitation of the natural frequencies via the actuators can have the effect, however, that on account of the relatively low damping in the control loop comparatively large amplitudes are detected by the respective position sensors, as a result of which the stability of the control loop can be jeopardized and active positional control can no longer be operated stably or can be operated only with low control quality.

"With regard to the prior art, reference is made for example to U.S. Pat. No. 6,842,277 B2, US 2007/0284502 A1 and the publication 'Benefits of over-actuation in motion systems', by M. G. E. Schneiders et al., Proceedings of the 2004 American Control Conference (ACC 2004), Boston (2004)."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "It is an object of the present invention to provide arrangements for actuating an element in a microlithographic projection exposure apparatus which enable active positional control of the element with higher control quality.

"This object is achieved in accordance with the features of the independent patent claims.

"In accordance with one aspect, the invention relates to an arrangement for actuating an element in a microlithographic projection exposure apparatus, comprising: a first number (n.sub.R) of degrees of freedom, wherein an adjustable force can be transmitted to the optical element in each of the degrees of freedom; and a second number (n.sub.A) of actuators, which are coupled to the optical element in each case via a mechanical coupling for the purpose of transmitting force to the optical element; wherein the second number (n.sub.A) is greater than the first number (n.sub.R); and wherein at least one of the actuators is arranged in a node of at least one natural vibration mode of the optical element.

"The invention is based on the concept, in particular, of performing, in an arrangement for actuating an element, in particular a mirror, an 'over-actuation' insofar as the number of actuators exceeds the number of degrees of freedom (that is to say for example for an actuation of the element or mirror in six degrees of freedom, at least seven actuators are used). This additional freedom with regard to the application of forces to the optical element, the additional freedom being obtained as a result of the surplus of actuators available (relative to the number of degrees of freedom) in comparison with an unambiguously statically determinate arrangement, can now be used further to the effect that the forces are applied to the optical element in such a way that the above-described excitation of natural vibrations of the mechanical structures is reduced or even completely masked out.

"A further advantage of this over-actuation is that a better distribution of the forces is made possible on account of the higher number of actuators.

"The additional freedom obtained as a result of the over-actuation according to the invention can be used firstly during the positioning of the actuators and secondly also during the driving of the actuators (i.e. the targeted configuration of the generated line of force). As far as the positioning of the actuators is concerned, in accordance with the abovementioned approach of the invention, at least one of the actuators is arranged in a node of a natural vibration mode, which has the consequence that the respective undesired natural vibration mode is not excited, independently of the excitation of the relevant actuator.

"In accordance with a further aspect, the invention relates to an arrangement for actuating an optical element in a microlithographic projection exposure apparatus, comprising a first number (n.sub.R) of degrees of freedom, wherein an adjustable force can be transmitted to the optical element in each of the degrees of freedom; and a second number (n.sub.A) of actuators, which are coupled to the optical element in each case via a mechanical coupling for the purpose of transmitting force to the optical element; wherein the second number (n.sub.A) is greater than the first number (n.sub.R); and wherein the actuators are arranged such that the actuation in the degrees of freedom is substantially orthogonal to at least one natural vibration mode of the optical element.

"In accordance with the above approach, the actuators are arranged such that the actuation in the degrees of freedom is substantially orthogonal to at least one natural vibration mode. Within the meaning of the present application, 'substantially' orthogonal should be understood such that the actuation of the natural vibration modes, which becomes visible in the transfer function of the open control loop as a weakly damped resonance, is reduced in terms of its magnitude compared with a non-over-actuation by at least 6 dB, in particular by at least 12 dB, more particularly by at least 20 dB.

"In accordance with one embodiment, the optical element is a mirror. Even if in the further embodiments the optical element is in each case a mirror of a projection exposure apparatus designed for EUV, the invention is not restricted thereto. In this regard, the invention can also be realized in conjunction with other optical elements, such as e.g. refractive or diffractive optical elements. In further embodiments, the invention can also be realized in a projection exposure apparatus designed for DUV (i.e. for wavelengths of less than 200 nm, in particular less than 160 nm).

"The mirror can be configured in particular in such a way that it is actively deformable in order to compensate for an undesirable disturbance in the projection exposure apparatus. Such a disturbance can be, for example, a thermal expansion on account of absorption of the radiation emitted by the (e.g. EUV) light source, and also imaging aberrations (caused by such thermal influences or in some other way).

"For actively deforming a deformable mirror, use is typically made of a comparatively high number of (deformation) actuators (e.g. of the order of magnitude of 10-100), wherein in addition the mirror is designed to be comparatively elastic in contrast to a non-actively deformable mirror. According to the invention, these (deformation) actuators, in particular, can be used for realizing the over-actuation described above. In accordance with this approach, therefore, the deformation actuators are used doubly insofar as firstly they are used for deforming the relevant mirror and secondly they are used for controlling the position of the mirror and in this case for generating the requisite forces in such a way that undesired natural vibration modes of the mirror are excited to a lesser extent or are not excited at all. In other words, therefore, the deformation actuators additionally perform the function of the positioning actuators (exclusively present in the case of a non-actively deformable mirror).

"In accordance with a further embodiment, the mirror can also be a non-actively deformable mirror.

"In accordance with one embodiment, the arrangement furthermore comprises a third number (n.sub.S) of sensor elements for determining the location and/or position of the optical element. In accordance with one embodiment, in this case the third number (n.sub.S) of sensor elements is greater than the first number (n.sub.R) of degrees of freedom.

"In accordance with this aspect of the invention, in particular in conjunction with a non-actively deformable mirror, therefore, a surplus n.sub.S (i.e. at least n.sub.R+1) of sensors can also be provided relative to the number of degrees of freedom (n.sub.R) that exist in the positioning of the optical element. This further concept according to the invention, which is equivalent to the above-described over-actuation in terms of control engineering, is also designated as 'over-sensing' hereinafter analogously to over-actuation. The additional freedom obtained as a result of the surplus of sensors can be used for choosing an arrangement of the sensors in such a way that specific natural frequencies or natural vibration modes are not even detected by the sensor system in the first place, with the consequence that the positional control cannot react to such natural frequencies. The concept of over-sensing has the further advantage that forces are still applied to the optical element or the mirror in a statically governed manner and inherent or undesired deformations of the optical element or mirror are thus avoided.

"The above-described concept of 'over-sensing' is also advantageous independently of the concept of 'over-actuation'.

"In accordance with a further aspect, therefore, the invention also relates to an arrangement for actuating an optical element in a microlithographic projection exposure apparatus, comprising: a first number (n.sub.R) of degrees of freedom, wherein an adjustable force can be transmitted to the optical element in each of the degrees of freedom; and a third number (n.sub.S) of the sensor elements for determining the location and/or position of the optical element; wherein the third number (n.sub.S) is greater than the first number (n.sub.R).

"The arrangement according to the invention can be designed, in particular, for actuating an optical element in a microlithographic projection exposure apparatus designed for EUV.

"The invention can furthermore be used both in the illumination device and in the projection lens of a microlithographic projection exposure apparatus.

"In accordance with one embodiment, the first number (n.sub.R) of degrees of freedom is at least three, in particular six.

"The invention furthermore relates to a microlithographic projection exposure apparatus comprising an arrangement having the features described above.

"In accordance with a further aspect, the invention relates to a method for actuating an optical element in a microlithographic projection exposure apparatus, wherein adjustable forces are transmitted to the optical element in a first number (n.sub.R) of degrees of freedom; wherein the force transmission is effected via a second number (n.sub.A) of actuators; wherein the second number (n.sub.A) is greater than the first number (n.sub.R); and wherein at least one of the actuators is arranged in a node of at least one natural vibration mode of the optical element.

"In accordance with a further aspect, the invention also relates to a method for actuating an optical element in a microlithographic projection exposure apparatus, wherein adjustable forces are transmitted to the optical element (100, 200) in a first number (n.sub.R) of degrees of freedom; wherein the force transmission is effected via a second number (n.sub.A) of actuators; wherein the second number (n.sub.A) is greater than the first number (n.sub.R); and wherein the actuation in the degrees of freedom is substantially orthogonal to at least one natural vibration mode of the optical element.

"In accordance with one embodiment, the optical element is actively deformed by the adjustable forces.

"In accordance with one embodiment, the position of the optical element is manipulated by the adjustable forces.

"In accordance with one embodiment, a third number (n.sub.S) of sensor elements are used to determine the location and/or position of the optical element. In this case, in particular, the third number (n.sub.S) can be greater than the first number (n.sub.R).

"The invention therefore also relates to a method for positioning and/or actively deforming an optical element in a microlithographic projection exposure apparatus, wherein a controllable force is transmitted to the optical element in a first number (n.sub.R) of degrees of freedom; and wherein a third number (n.sub.S) of sensor elements are used to determine the location and/or position of the optical element; wherein the third number (n.sub.S) is greater than the first number (n.sub.R).

"In this case, the method can respectively comprise in particular the following steps: determining at least one imaging aberration in the projection exposure apparatus; and positioning and/or actively deforming the optical element in such a way that the imaging aberration is at least partly compensated for.

"Further configurations of the invention can be gathered from the description and the dependent claims.

"The invention is explained in greater detail below on the basis of exemplary embodiments illustrated in the accompanying figures."

For additional information on this patent, see: Bleidistel, Sascha; Schoenhoff, Ulrich; Fischer, Juergen. Arrangement for Actuating an Element in a Microlithographic Projection Exposure Apparatus. U.S. Patent Number 8786826, filed January 17, 2014, and published online on July 22, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8786826.PN.&OS=PN/8786826RS=PN/8786826

Keywords for this news article include: Nanotechnology, Microlithography, Carl Zeiss SMT GmbH, Emerging Technologies.

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Source: Journal of Engineering


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