News Column

Patent Issued for Illumination System of a Microlithographic Projection Exposure Apparatus

July 23, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- A patent by the inventors Deguenther, Markus (Aalen, DE); Major, Andras G. (Oberkochen, DE); Andresen, Anne Christine (Westensee, DE), filed on December 7, 2009, was published online on July 8, 2014, according to news reporting originating from Alexandria, Virginia, by VerticalNews correspondents.

Patent number 8773639 is assigned to Carl Zeiss SMT GmbH (Oberkochen, DE).

The following quote was obtained by the news editors from the background information supplied by the inventors: "Projection exposure apparatus making it possible to produce microstructures on semiconductor components are known from the prior art. These structures are conventionally produced by imaging a mask, with structures arranged on it, through a projection objective of the projection exposure apparatus onto a photosensitive layer, which is supported by a semiconductor component.

"WO 2005/026843 A2 discloses an illumination system including a multiple mirror arrangement, which is sometimes also referred to as multi-mirror array (MMA). Each micromirror of this arrangement can be tilted about two orthogonal tilt axes. This makes it possible to variably illuminate a system pupil plane of the illumination system without incurring a substantial loss of light. Since the intensity distribution in the system pupil plane translated into a angular distribution in a subsequent mask plane in which the mask is to be positioned, the position of the micromirrors determine the angular distribution of light in the mask plane.

"It is desirable for the micromirrors are tilted with extremely high positional precision and long-term stability. If there are inaccuracies in the alignment of the micromirrors, this causes a displacement of the light spots in the pupil plane, which can have an unfavorable effect on the imaging of the mask."

In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "In some embodiments, the disclosure provides an illumination system which makes it possible to stably maintain desired illumination angle distributions in a mask plane.

"In certain embodiments, the disclosure provides an illumination system of a microlithographic projection exposure apparatus, which includes a pupil surface and an arrangement of individually drivable beam deviating elements. Each beam deviating element is configured to direct light impinging thereon onto different positions on the pupil surface in response to a control signal applied to the beam deviating element. According to the disclosure an attenuation unit is provided which is configured to reduce the intensity of light, which is directed by any arbitrary beam deviating element onto the pupil surface, by more than 50%.

"The task of the attenuation unit is thus to attenuate light in the pupil surface that has been reflected from defective beam deviating elements. Because a defect of a beam deviating element usually results in light that is directed on locations in the pupil surfaces where it should not pass through, a reduction of the light intensity associated with this light improves the illumination angle distribution in the mask plane. This reduction can be complete, i.e. no such light at all reaches the pupil plane. This may be achieved, for example, by absorbing this light or by directing this light to a region outside the usable pupil surface where it is absorbed.

"The intensity reduction may be achieved by measures which relate to varying the direction and/or to reducing the intensity of the light rays striking any of the beam deviating elements, or by measures which relate to varying the direction and/or reducing the intensity of the light rays reflected from any of the beam deviating elements. In both cases it is ensured that the light rays assigned to a defective beam deviating element do not strike, or only partially strike the usable region of the pupil surface.

"If the intensity of the light rays reflected from any of the beam deviating elements shall be reduced, this may be accomplished by varying at least one tilt angle of the defective beam deviating element and/or reducing the reflectivity of the respective beam deviating element and/or arranging an element which at least partially absorbs or reflects the light rays associated with the defective beam deviating element.

"The measures taken in this respect may be reversible or irreversible.

"A reversible measure may be ended when a beam deviating element is working correctly again after a malfunction has occurred. Such a malfunction may occur by exceeding or falling below a predeterminable operating temperature for the beam deviating elements. During this period of time, individual beam deviating elements may sometimes not be drivable correctly.

"An irreversible measure will be carried out when a fault occurring on the beam deviating element is not correctable, for example when an electronic driver of the beam deviating element has failed.

"According to one embodiment, the attenuation unit is configured to reduce the intensity of light, which is directed by any arbitrary beam deviating element onto the pupil surface, by more than 75% (e.g., by more than 95%, by more than 99%).

"In one embodiment the attenuation unit contains a further arrangement of individually drivable beam deviating elements and imaging optics (51), which optically conjugate the two arrangements of beam deviating elements with one another. The second arrangement of individually drivable beam deviating elements may in particular be a micromirror array having tiltable micromirrors or a microlens array having microlenses which can be swivelled individually or have their refracting power varied individually, which act in the manner of digital switches.

"The beam deviating elements of the further arrangement may be configured such that they can be switched so that light striking a beam deviating element of the further arrangement either strikes a beam deviating element, associated by the optical conjugation, in the other arrangement, or does not strike the other arrangement at all.

"According to another embodiment, the attenuation unit includes a diaphragm unit which is configured to exclusively block light striking an individual beam deviating element. The diaphragm unit makes it possible to affect the optical transmission of the light path respectively formed individually between the light source, the respective beam deviating element and the pupil surface. The number and/or intensity of the light rays striking the respective beam deviating element can thus be temporarily and individually reduced by the diaphragm unit.

"According to another embodiment, the diaphragm unit includes a diaphragm and a diaphragm holder which is configured to receive the diaphragm. The diaphragm may in particular be provided as a support material with locally applied attenuating or blocking elements, in particular with a light-absorbent coating. The diaphragm may also be designed as a honeycombed arrangement of transmission openings for the individual light paths, in which case the transmission openings can be closed by insertable blocking elements. In some embodiments, the diaphragm is produced in the manner of a photo slide with a support made of a material which is at least almost fully optically transparent for the light from the light source, and can be provided with an optically opaque coating in those regions in which light passing through would otherwise strike the defective beam deviating elements.

"According to another embodiment, the diaphragm unit includes a switchable diaphragm which is arranged permanently in the beam path and has a transmissivity which can be varied locally by applying a control signal. Here, it is advantageous that the individual light paths can be affected without needing to remove or replace the diaphragm unit. Rather, the local transmissivity of the diaphragm instrument is influenced merely by applying a corresponding control signal. The switchable diaphragm may be include an LCD panel (liquid crystal element), in which liquid crystal regions arranged in a matrix can be driven individually and make it possible to locally attenuate or optionally fully prevent transmission of the light when driven correspondingly.

"According to another embodiment, the illumination system contains an arrangement of microlenses, each microlens being assigned to one beam deviating element, and the attenuation unit includes a device by which the optical properties of individual microlenses can be modified. The device can include an application instrument for applying a light-absorbent coating onto individual microlenses. The optical transmission of the microlenses is varied individually with such an attenuation unit, so that microlenses through which light would strike defective beam deviating elements are optically blocked.

"According to another embodiment, the attenuation unit is configured to modify the optical properties of any of the relevant beam deviating elements. Here, it is advantageous that no optical instruments such as diaphragms, lenses or mirrors need to be introduced into the light paths for those beam deviating elements which are functioning properly, so that any negative impact on the overall optical system can be kept very small. The light rays whose light path includes a defective beam deviating element, which cannot be deflected into the desired direction, should not strike the pupil surface in an uncontrolled fashion. In order to prevent this, the respectively associated beam deviating element has its optical properties modified so that only a fraction of the incident light rays, or even none of the incident light rays, are reflected or otherwise deviated; rather, they are at least partially or fully absorbed or are deviated so that they cannot reach the pupil surface.

"According to another embodiment, a light-absorbing layer can be applied onto any of the beam deviating element by the attenuation unit. This may in particular be done by applying a light-absorbing film such as a light-absorbing lacquer layer. The corresponding layer may be applied by a manipulator, which is assigned to the arrangement of beam deviating elements and carries an applicator which can be moved in particular at least along a principal extent direction of the arrangement of beam deviating elements as a function of a control signal. The applicator may be designed as an instrument for applying film sections or ink onto the respective beam deviating elements Ink may be applied contactlessly by spraying or by dropping the ink or, in the manner of a printing process, by applying the ink with a suitable printing pad.

"According to another embodiment, a light-deviating optical element can be applied onto the relevant beam deviating element by the attenuation unit. The light-deviating optical element may in particular be a prefabricated component with a suitable shape, for example a glass wedge, an auxiliary mirror, or (in the case of transmitting beam deviating elements) a scattering element. The light-deviating optical element may be formed from a shapeless, optionally curable compound onto the surface of the beam deviating element such that it is at least essentially geometrically stable after a certain time.

"According to another embodiment, the relevant beam deviating element can be destroyed by the attenuation unit. The destruction of the deviating element may in particular be induced by mechanical forces, electrical currents, chemical reactions or action of heat. A manipulator can be provided, which carries an applicator with the aid of which the selective destruction of individual beam deviating elements can be carried out. The destructive effect may be directed onto an optical surface of the beam deviating element or, in the case of position-variable beam deviating elements, onto their mechanical suspension.

"According to another embodiment, light can be directed by the attenuation unit onto any of the beam deviating elements so that a defective beam deviating element is destroyed by the light chemically or by action of heat. The light used for the destruction may be the light provided by the light source of the illumination system. The attenuation unit may also include its own light source for generating the light. This light source may have its power and wavelength adapted so as to destroy an optical surface or a reflective layer of the beam deviating element.

"According to another embodiment, the attenuation unit includes a mirror instrument which can be inserted into the beam path between the beam deviating elements and the pupil surface, and the beam deviating elements can be driven such that they can direct the light onto the inserted mirror instrument in such a way that the light reflected back by the mirror instrument exclusively impinges on the beam deviating element to be destroyed. An additional light source is not therefore required; rather, the light provided by the light source of the illumination system is used to destroy the defective beam deviating element.

"The mirror instrument may in particular be designed as a plane mirror, as a curved mirror or as an arrangement of individual mirrors. The mirror instrument can be translated or rotated into the beam path in a motorized fashion.

"According to another embodiment, the attenuation unit includes an actuation element, which can be arranged in the immediate vicinity of any of the beam deviating elements with the aid of a displacement device. The displacement device may be a manipulator, which has at least one rotational and/or at least one translational movement degree of freedom and can bring the actuation element from an idle position away from the beam deviating elements into an active position immediately next to the defective beam deviating element.

"The beam deviating elements can be designed as tiltable mirrors including a mirror support and a reflective coating. In the event of a malfunction, the actuation element at least partially destroys the reflective coating of any of the mirrors so that the reflectivity of the coating is reduced by at least 50%.

"According to another embodiment, a defective mirror can be tilted by direct mechanical action of the actuation element into a position in which the relevant mirror can no longer direct light into the pupil surface. Here, by exerting a force on the respective mirror, a suspension provided for aligning the mirror is permanently deformed or otherwise disabled so that the mirror can no longer deviate the incident light rays onto the pupil surface.

"As has been mentioned above, light coming from a defective beam deviating element may be prevented by the attenuation unit from striking the pupil surface. This may be achieved by configuring the attenuation unit as a diaphragm unit, which is arranged between the beam deviating elements and the pupil surface and which is configured to exclusively block light coming any of the beam deviating elements.

"The diaphragm unit may be arranged in or in the immediate vicinity of the pupil surface and may include a number of surface regions, the optical transmission of which is adjustable, as is the case particularly with an LCD panel. The number of adjustable surface regions corresponds at least to the number of beam deviating elements provided for deviating the light rays. The number of drivable surface regions can exceed the number of beam deviating elements by a multiple. If the illumination is nevertheless intended to take place at the position where a defective beam deviating element illuminates the pupil surface, then the corresponding surface region of the diaphragm unit will be reset to being transmissive in order to allow illumination of this position. The defective beam deviating element may then even re-contribute to the illumination of the pupil surface.

"A further aspect of the disclosure provides a method for operating a microlithographic projection exposure apparatus which includes an illumination system having a pupil surface and including an arrangement of individually drivable beam deviating elements, wherein each beam deviating element is configured to direct light impinging thereon onto different positions on the pupil surface in response to a control signal applied to the beam deviating element. According to the disclosure, the functional status of the beam deviating elements is determined constantly or at intervals in a step a). In particular, it is possible to check whether the beam deviating elements do deviate incident light rays to desired spots in the pupil surface. A subsequent step determines whether the functional status determined in step a) satisfies predetermined threshold criteria. The threshold criterion provided may, for example, be that the beam deviating element can or cannot direct the light rays to be deviated into a spot of predeterminable size, as a function of the respectively associated control signal. If the beam deviating element cannot be deviated suitably for this and compensation for the malfunction of the beam deviating element cannot be carried out in another way, it is established that the functional status of a beam deviating element does not satisfy the predetermined threshold criteria.

"In this case the intensity of light directed by this particular beam deviating element onto the pupil surface is attenuated by more than 50% using suitable measures, so that any perturbing effect of the misguided light is reduced.

"If the threshold criterion is not satisfied, light may be either entirely prevented from being directed by the beam deviating element onto the pupil surface, or light may be prevented from striking the relevant beam deviating element.

"If the at least one threshold criterion is not satisfied, light striking the relevant beam deviating element may be prevented from being transmitted or reflected by it. This may be done by affecting an optical surface or an optical layer of the defective beam deviating element. As an alternative or in addition, if the at least one threshold criterion is not satisfied, provision may be made to prevent light coming from the defective beam deviating element from striking the pupil surface.

"According to another aspect of the disclosure, an illumination system of a microlithographic projection exposure is provided includes a first optical system that is configured to produce a bundle of N individual input light beams, wherein N is a positive integer. The illumination system further includes an arrangement of N individually drivable beam deviating elements, wherein the arrangement is configured to produce from the bundle of N individual input light beams a bundle of N individual output light beams whose directions are variable by control signals applied to the beam deviating elements. The beam deviating elements may be arranged as a matrix, for example an m.times.n matrix having m columns and n lines. A beam extinction unit is configured to reduce the number of output light beams by k, with 1.ltoreq.k.ltoreq.N.

"In one embodiment the beam extinction unit is configured to reduce the number of output light beams by reducing the number of input light beams that impinge on the arrangement of beam deviating elements. This may be achieved by blocking light beams with the aid of a diaphragm, for example an LCD panel, on their way between the first optical system and the arrangement of beam deviating elements.

"In another embodiment, the beam extinction unit is configured to reduce the number of output light beams by (optionally irreversibly) disabling k beam deviating elements, for example by applying an absorbing layer on an optical surface of the k elements.

"In some embodiments, the disclosure provides an illumination system for monitoring multi-mirror arrangements in such a system which makes it possible to efficiently determine and check the deflection produced by the micromirrors or other beam deviating elements.

"In certain embodiments, the disclosure provides an illumination system including a pupil surface and an arrangement of individually drivable beam deviating elements. Each beam deviating element is configured to direct primary light impinging thereon onto different positions on the pupil surface in response to a control signal applied to the beam deviating element. According to this aspect of the disclosure, a first detector instrument is arranged outside a beam path of the primary light which eventually illuminates the mask. The first detector instrument is configured and arranged in such a way that primary light reflected by a single beam deflecting element alone can be directed onto the first detector instrument.

"The disclosure is based on the discovery that exact monitoring of the deflection produced by a beam deflection element (in the case of micromirrors this corresponds to the position (i.e. alignment) of the micromirrors) can be carried out exactly only if at least some of the deflected primary light is actually used for determining the alignment, since only in this case the relationship between incidence directions of primary light and directions of the deflected primary light can be determined. In order to ensure that only a small portion of the primary light is not available for illuminating the mask, the disclosure proposes to control only individual the beam deviating elements (in the case of micromirrors: to orient only individual micromirrors) periodically so that the incident primary light is directed onto the first detector instrument. In this way, it is possible to successively obtain a relationship of the primary light incidence directions and the deflection produced by the beam deviating elements.

"In order to avoid perturbations of the reflected primary light and to allow measurement during operation of the beam deviating elements, the first detector instrument is arranged outside the beam path of the reflected primary light. It is to be understood that the aforementioned relationship is determined for a configuration of a beam deviating element in which the deviated light does not contribute to the illumination of the mask. However, it is possible carry out measurements at different configurations of the beam deviating element (in the case or micromirrors: different alignments). Then a measurement curve may be determined which can be extrapolated/interpolated to the configurations of the beam deviating elements in which it directs primary light into the system pupil surface.

"Since measuring or monitoring individual beam deviating elements in an arrangement including several thousand to a few million such elements involves that the measurement of all elements or the cycle time for a repeated measurement is very long, this measurement or monitoring principle is suitable essentially for deviations which take place slowly, for example drift processes which occur for example owing to low-frequency perturbations in the frequency range

"In the case of micromirrors, the disclosure makes it possible to determine, based on a predetermined geometry of the micromirrors, the extent to which the incidence direction of the primary ray bundle or the geometry and arrangement of the micromirrors correspond to the given specifications. A corresponding evaluation may be carried out fully automatically by an in particular software-based control, regulating and evaluation unit.

"In order to determine high-frequency perturbations as well, for example with a frequency in the range of from 100 Hz to 1000 Hz, and to optimize the measurement and monitoring of the beam deviating elements based on the partially extracted primary light, a second measurement principle may be employed at the same time. This second measurement principle involves monitoring and determining the beam deviating effect produced by the beam deviating elements with the aid of a separate measurement illumination instrument, which includes a light source producing secondary light, which is distinct from the primary light, and a second detector instrument. The secondary light is directed onto the beam deviating elements. The second detector instrument is configured to detect the secondary light after it has been deviated by the beam deviating elements.

"Such a measurement may ideally be carried out continuously or at least with a very fast repetition frequency.

"The measurement or monitoring with the aid of the first detector instrument may furthermore be correlated with the results of the measurement or monitoring with the aid of the second detector instrument. In particular, the determined data may be used for mutual calibration. In this way, for example, the variation in the deviation of the primary light may be deduced from a pure position change of a micromirror without the need to carry out a measurement using primary light.

"By virtue of the high measurement frequency possible which may be obtained with the second measurement principle, high-frequency perturbations can be readily determined and compensated for by a suitable regulating unit.

"The incidence direction of the secondary light may differ from the incidence direction of the primary light in angle of incidence and/or azimuthal incidence direction. This allows simple arrangement of the light source producing secondary light and the second detector instrument. In particular the azimuthal incidence direction may differ by a rotation angle of more than 30.degree. (e.g., more than 60.degree., approximately 90.degree.) around the surface normal of the arrangement of beam deflection elements.

"Between the light source producing secondary light and the arrangement of beam deviating elements, or between this arrangement and the first and second detector instrument, an optical system may be provided which allows variable arrangement of the components, or which simplify the measurement or monitoring.

"This optical system may include at least one collimator, optionally including a perforated plate and a microlens array so that, for example, the perforated plate is arranged in the focal plane of the microlenses, so as to generate parallel primary light bundles which can be directed onto the individual beam deviating elements.

"The optical system which may be arranged between the beam deviating elements and the detector instruments, in particular the second detector instrument, may also serve to allow corresponding use of different types of sensors.

"Thus, a position sensor which is arranged in a focal plane of a converging lens, or an array of position sensors arranged next to one another which are arranged in focal planes of a lens array including converging lenses, may be used in order to convert the angle dependency of the reflected secondary light into a position on the position sensor or sensors.

"Additional optics may also be provided, which are designed for example as telecentric imaging optics or so-called relay optics, in order to make the arrangement of the detector instrument variable. This is also ensured, for example, when the optical system of the second detector instrument is configured so that the beam deviating elements are imaged onto the converging lenses, while satisfying the Scheimpflug condition.

"The sensors of the detector instruments may be either angle-resolving or position-resolving sensors."

URL and more information on this patent, see: Deguenther, Markus; Major, Andras G.; Andresen, Anne Christine. Illumination System of a Microlithographic Projection Exposure Apparatus. U.S. Patent Number 8773639, filed December 7, 2009, and published online on July 8, 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=8773639.PN.&OS=PN/8773639RS=PN/8773639

Keywords for this news article include: Electronics, Semiconductor, Carl Zeiss SMT GmbH.

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