Patent Grant
8059266
1. Field of the Invention
This invention relates to a lithographic projection apparatus and systems and methods for measuring stray light in a lithographic projection apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs), and other devices involving fine structures.
In some lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the substrate. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a semiconductor wafer). The lithographic apparatus comprises an illumination system to illuminate the mask and a projection system (also referred to as a projection lens) to transfer the mask's pattern, via imaging, onto a layer of radiation-sensitive material (photo-resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Instead of a mask, in some lithographic apparatus, the patterning device can be a patterning array that comprises one or more arrays of individually controllable elements. Sometimes, the pattern can be changed more efficiently in a maskless system compared to a mask-based system. These types of apparatus are referred to as Optical Maskless Lithographic (OML) apparatus.
Known lithographic apparatus include so-called steppers or step-and-repeat apparatus, and so-called scanners or step-and-scan apparatus. In a stepper each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and the wafer is moved by a predetermined amount to a next position for a subsequent exposure. In a scanner, each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction, and next the wafer is moved to a next position for a subsequent exposure.
In order to achieve optimum performance in a mask-based or OML apparatus, monitoring the effect of stray light is important.
Stray light is measured on lithography systems using two primary methods: the Stray light At Multiple Object Size (SAMOS) test and the Kirk test.
The SAMOS test is a radiometric measurement that positions an image-plane detector (i.e., a detector placed in the wafer plane) within the image of a dark box surrounded by a bright field. The detector aperture is usually smaller than the image of the dark box. The light falling on the detector is an indication of amount of stray light present in the system. Details about the SAMOS test are described in U.S. Pat. No. 6,862,076 B2, which is incorporated herein by reference in its entirety.
The Kirk test is a lithographic measurement involving printing dark boxes (surrounded by bright fields, similar to the SAMOS test) into a photo-resist at increasing radiation or dose levels. The dose at which a dark box is not imaged into the photo-resist gives a measure of the stray light present in the system. In some embodiments, the Kirk test is implemented in positive photo-resist with multiple dark box sizes. Intensity is increased until the boxes of interest disappear. So, this test is also referred to as the “disappearing box test.” The Kirk test is not limited to positive photo-resist. For example, when a negative photo-resist is used, intensity is increased until boxes of interest appear.
Both the SAMOS test and the Kirk test are valuable tools for measuring the performance of lithography apparatus in the presence of stray light, but SAMOS is often used for its relative simplicity, because no wafer processing is involved. However, either of the SAMOS test and the Kirk test alone does not provide an accurate measure of stray light in a lithographic apparatus aiming to achieve a very high contrast and a desired critical dimension of features. Additionally, SAMOS test is not a reliable method to measure stray light, where an imaging field of the lithography apparatus is small for each step of exposure, for example, in an OML apparatus.
What is needed is a versatile system and method that embody the desirable features of the radiometric SAMOS test and the lithographic Kirk test to achieve accurate measurement of stray light in various lithographic apparatus having various configurations and imaging field sizes.
The present invention monitors the performance of a lithographic apparatus by measuring stray light. Embodiments of the present invention enables accurate measurement of stray light by virtually extending imaging fields of mask-based or maskless lithographic apparatus. Accuracy of stray light measurement is improved substantially by embodiments of the present invention, compared to accuracy achievable in a conventional radiometric SAMOS test method or a conventional lithographic Kirk test method.
According to an aspect of the invention there is provided a lithographic projection apparatus comprising an illumination system configured to produce a beam of radiation, a test pattern having a dark area surrounded by a substantially larger bright area that receives the beam, a detector having an aperture in an image plane, a positioning mechanism configured to position the aperture of the detector aligned to an image of the dark area and an image of the bright area, a moving mechanism configured to move the aperture of the detector across the image plane during a dark area measurement and a bright area measurement, a transducer coupled to the detector that measures radiation intensity during the dark area measurement and the bright area measurement, a processor coupled to the transducer that compares integrated signals from the dark area measurement and the bright area measurement to generate a final result that indicates effects of stray light in the lithographic apparatus, and an output indicator that communicates the results of the stray light measurement.
According to a further aspect of the invention there is a method for measuring stray light in a lithographic apparatus, wherein the method comprises the steps of: providing a test pattern configured to receive a beam of radiation, wherein the test pattern comprises a dark area surrounded by a substantially larger bright area; providing a detector having an aperture in an image plane where a projected image of the test pattern is created; positioning the aperture of the detector at a first location aligned to an image of the dark area; moving the aperture of the detector synchronously with or across the image of the dark area; measuring radiation intensity detected during the movement synchronously with or across the image of the dark area to yield a first integrated signal representing a result of the dark area measurement; positioning the aperture of the detector at a second location aligned to an image of the bright area; moving the aperture of the detector synchronously with or across the image of the bright area; measuring radiation intensity detected during the movement synchronously with or across the image of the bright area to yield a second integrated signal representing a result of the bright area measurement; comparing the first integrated signal and the second integrated signal to yield a final result that indicates effects of stray light in a lithographic apparatus; and indicating the final result as an output to facilitate monitoring and evaluation of performance of the lithographic apparatus based on the final result.
Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. For example, it is possible to vary reference positions of the test pattern and/or the aperture of the detector to measure position dependency of stray light across a complete imaging field.
The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.
One or more embodiments of the present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers can indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number can identify the drawing in which the reference number first appears.
As discussed above, measurement of stray light is important to evaluate overall performance of a lithographic apparatus. Stray light measurement data is often supplied to support the system specification of a lithographic apparatus. In general, presence of stray light degrades image contrast, adversely affects overall performance of a lithographic apparatus. Persons skilled in the art will appreciate that a performance of a lithographic apparatus may be evaluated by a critical dimension of feature that can be projected on a substrate consistently with a desired contrast and uniformity. Periodic monitoring of stray light facilitates in evaluating the consistency of performance of a lithographic apparatus. If stray light measurement indicates a degradation of performance over time or an abrupt degradation of performance, then components in the optical path may be inspected for possible damage or contamination, and may be replaced, repaired, or cleaned accordingly to restore better performance of the lithographic apparatus. Additionally, stray light measurement data can be an useful calibration tool to adjust operational parameters (such as radiation dose level) of a lithographic apparatus.
This specification discloses one or more embodiments that incorporate the features of the present invention involving stray light measurement with improved accuracy. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto.
The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described can include a particular feature, structure, or characteristic, but every embodiment cannot necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.
Embodiments of the invention can be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention can also be implemented as instructions stored on a machine-readable medium, which can be read and executed by one or more processors. A machine-readable medium can include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium can include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.
In the following sections,
Mask-Based Lithographic Apparatus
The mask-based lithographic apparatus of
The illumination system may include various types of optical components, such as refractive, reflective, and diffractive types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.
The support structure supports, i.e., bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern MP includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. In the context of a mask-based embodiment of the present invention, a “patterning device” is a reticle with a test pattern comprising an isolated dark area surrounded by a much larger bright area.
The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, and catadioptric optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.
As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask).
The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.
Referring to
The illumination system IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam at mask level. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil IPU of the illumination system can be adjusted. In addition, the illumination system IL may comprise various other components, such as an integrator IN and a condenser CO. The illumination system may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section at mask level.
The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device in accordance with a pattern MP. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W.
The projection system has a pupil PPU conjugate to the illumination system pupil IPU, where portions of radiation emanating from the intensity distribution at the illumination system pupil IPU and traversing a mask pattern without being affected by diffraction at a mask pattern create an image of the intensity distribution at the illumination system pupil IPU.
With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in
The depicted apparatus could be used in at least one of the following modes:
1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.
2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.
3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.
Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.
Maskless Lithographic Apparatus
The substrate table WT is constructed to support a substrate (e.g., a resist-coated substrate) W and connected to a positioner PW configured to accurately position the substrate in accordance with certain parameters.
The projection system (e.g., a refractive projection lens system) PS is configured to project the beam of radiation modulated by the one or more arrays of individually controllable elements onto a target portion C (e.g., comprising one or more dies) of the substrate W. The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein can be considered as synonymous with the more general term “projection system.”
The illumination system can include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.
The patterning device PD is one or more arrays of individually controllable elements that modulate the beam. In the context of a maskless embodiment of the present invention, a “patterning device” is one or more arrays of individually controllable elements that can project an image of a test pattern comprising an isolated dark area surrounded by a much larger bright area.
An array of individually controllable elements is referred to as a Spatial Light Modulator or SLM in the subsequent description. There may be more than one SLMs in a lithographic apparatus. For example, in an embodiment of an OML apparatus, 14 SLMs are used in a two-row configuration. In general, the position of a SLM will be fixed relative to the projection system PS. However, it can instead be connected to a positioner configured to accurately position the SLM in accordance with certain parameters.
Patterning devices whose pattern is programmable with the aid of electronic means (e.g., a computer), such as patterning devices comprising a plurality of programmable elements (e.g., all the devices mentioned in the previous sentence except for the reticle), are collectively referred to herein as “contrast devices.” The patterning device comprises at least 10, at least 100, at least 1,000, at least 10,000, at least 100,000, at least 1,000,000, or at least 10,000,000 programmable elements.
The lithographic apparatus can comprise one or more contrast devices. For example, it can have a plurality of SLMs, each element of which is controlled independently of each other. In such an arrangement, some or all of the SLMs can have at least one of a common illumination system (or part of an illumination system), a common support structure for the SLMs, and/or a common projection system (or part of the projection system).
In one example, the substrate W is a wafer, for instance a semiconductor wafer. The substrate referred to herein can be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool, and/or an inspection tool. In one example, a resist layer is provided on the substrate.
The projection system can image the pattern on the SLMs, such that the pattern is coherently formed on the substrate. Alternatively, the projection system can image secondary sources for which the elements of the SLMs act as shutters. In this respect, the projection system can comprise an array of focusing elements such as a micro lens array (known as an MLA) or a Fresnel lens array to form the secondary sources and to image spots onto the substrate. The array of focusing elements (e.g., MLA) comprises at least 10 focus elements, at least 100 focus elements, at least 1,000 focus elements, at least 10,000 focus elements, at least 100,000 focus elements, or at least 1,000,000 focus elements.
The number of elements in SLMs in the patterning device is equal to or greater than the number of focusing elements in the array of focusing elements. One or more (e.g., 1,000 or more, the majority, or each) of the focusing elements in the array of focusing elements can be optically associated with one or more of the individually controllable SLM elements, with 2 or more, 3 or more, 5 or more, 10 or more, 20 or more, 25 or more, 35 or more, or 50 or more of the individually controllable SLM elements.
The MLA can be movable (e.g., with the use of one or more actuators) at least in the direction to and away from the substrate. Being able to move the MLA to and away from the substrate allows, e.g., for focus adjustment without having to move the substrate.
As herein depicted in
The lithographic apparatus can also be of a type wherein at least a portion of the substrate can be covered by an “immersion liquid” having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the patterning device and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.
Referring again to
The source and the lithographic apparatus can be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source can be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, can be referred to as a radiation system.
The illuminator IL, can comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components, such as an integrator IN and a condenser CO. The illuminator can be used to condition the radiation beam to have a desired uniformity and intensity distribution in its cross-section. The illuminator IL, or an additional component associated with it, can also be arranged to divide the radiation beam into a plurality of sub-beams that can, for example, each be associated with one or a plurality of the individually controllable SLM elements. A two-dimensional diffraction grating can, for example, be used to divide the radiation beam into sub-beams. In the present description, the terms “beam of radiation” and “radiation beam” encompass, but are not limited to, the situation in which the beam is comprised of a plurality of such sub-beams of radiation.
The radiation beam B is incident on the patterning device PD (e.g., one or more SLMs) and is modulated by the patterning device. Having been reflected by the patterning device PD, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, capacitive sensor, or the like), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Where used, the positioning means for the SLM can be used to correct accurately the position of the patterning device PD with respect to the path of the beam B, e.g., during a scan.
In one example, movement of the substrate table WT is realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in
As shown in
The depicted apparatus in
1. In step mode, the SLMs and the substrate are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one go (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.
2. In scan mode, the SLMs and the substrate are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate relative to the SLMs can be determined by the (de-) magnification and image reversal or image mirroring characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.
3. In pulse mode, the SLMs are kept essentially stationary and the entire pattern is projected onto a target portion C of the substrate W using a pulsed radiation source. The substrate table WT is moved with an essentially constant speed such that the beam B is caused to scan a line across the substrate W. The pattern on the SLMs is updated as required between pulses of the radiation system and the pulses are timed such that successive target portions C are exposed at the required locations on the substrate W. Consequently, the beam B can scan across the substrate W to expose the complete pattern for a strip of the substrate. The process is repeated until the complete substrate W has been exposed line by line.
4. Continuous scan mode is essentially the same as pulse mode except that the substrate W is scanned relative to the modulated beam of radiation B at a substantially constant speed and the pattern on the SLMs is updated as the beam B scans across the substrate W and exposes it. A substantially constant radiation source or a pulsed radiation source, synchronized to the updating of the pattern on the SLMs, can be used.
5. In pixel grid imaging mode, which can be performed using the lithographic apparatus of
Combinations and/or variations on the above described modes of use or entirely different modes of use can also be employed.
In lithography, a pattern is exposed on a layer of resist on the substrate. The resist is then developed. Subsequently, additional processing steps are performed on the substrate. The effect of these subsequent processing steps on each portion of the substrate depends on the exposure of the resist. In particular, the processes are tuned such that portions of the substrate that receive a radiation dose above a given dose threshold respond differently to portions of the substrate that receive a radiation dose below the dose threshold. For example, in an etching process, areas of the substrate that receive a radiation dose above the threshold are protected from etching by a layer of developed resist. However, in the post-exposure development, the portions of the resist that receive a radiation dose below the threshold are removed and therefore those areas are not protected from etching. Accordingly, a desired pattern can be etched. In particular, the SLM elements in the patterning device are set such that the radiation that is transmitted to an area on the substrate within a pattern feature is at a sufficiently high intensity that the area receives a dose of radiation above the dose threshold during the exposure. The remaining areas on the substrate receive a radiation dose below the dose threshold by setting the corresponding individually controllable SLM elements to provide a zero or significantly lower radiation intensity.
As shown in
Referring back to
In one example, a positioning device (not shown) for pupil plane array of individually programmable elements 100 can be used to accurately correct the position of pupil plane array of individually programmable elements with respect to the path of beam B, e.g., during a scan. In one embodiment in which reflective device PD also comprises a array of individually programmable elements (“object plane array of individually programmable elements”), a second positioning device (not shown) can be used to accurately correct the position of object plane array of individually programmable elements PD with respect to the path of beam B.
In another example, movement of substrate table WT is realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in
Although specific reference may have been made above to the use of embodiments of the invention in the context of mask-based or maskless optical lithography, it will be appreciated that the invention may be used in other applications. For example, embodiments of the present invention are equally applicable to mask-based or maskless lithographic apparatus, adapted to support immersion lithography.
Principles of the Conventional Kirk Test and SAMOS Test
As discussed above, embodiments of the present invention measure stray light with high accuracy in lithographic systems, such as the various example lithographic apparatus described with respect to
In a lithographic apparatus, every bright point in an imaging field scatters light across a projected image due to various reasons, such as, the effect of roughness on optical surfaces, index variation in optical path, internal reflections from optical elements, external reflections from outside objects, etc. This scattered light is called stray light, and it has an effect on image contrast obtained by a lithographic apparatus. The intensity of this stray light falls off rapidly with distance. This intensity distribution over distance is described by an Intensity Scatter Function (ISF), where isotropy (i.e., ISF intensity varies only by radial distance and not angle) and stationarity (i.e., one ISF describes the stray light from any bright point, regardless of its position within the image field) are typically assumed for the purpose of analysis and understanding.
Contribution to stray light from a single bright point is very low in intensity at a measurable (microns or millimeters) distance from a projected center of the bright point in typical modern lithographic apparatus, but the integrated stray light at any one dark point in a field filled with many bright points (i.e., a bright field in a lithographic system) can be significant enough to affect imaging performance.
Since the intensity of stray light contributed from a single bright point is very low compared to a transmitted beam of radiation, the ISF may be difficult to measure directly. The intensity is so low in the dark areas, and so high in the bright area, that most detection mechanisms do not have adequate dynamic range for an accurate direct measurement. So lithographic Kirk test is used as an indirect method to integrate ISFs.
The lithographic Kirk test effectively duplicates convolution integration of ISFs from many bright points by imaging a small dark box in a large bright field. A bright field can be thought of as a collection of a very large number of individual bright points. The light in the center of the dark box (relative to the light in the bright field) is the stray light integrated from the many bright points.
Assuming stationarity, the lithographic Kirk test may be simulated in a processor by convolving the measured stray light ISF with a function representing the image of an isolated dark box (e.g., dark box 510) in a large bright field (e.g., bright field 520).
Reducing the upper bound by using a significantly smaller bright field will reduce the accuracy of the result of the test, not capturing all of the stray light that may be present in an actual full-field lithographic exposure. In order to simulate a comprehensive measurement of stray light, the size of the bright field is required to be increased by simulating a full-field scanning exposure. Embodiments of the present invention are capable of simulating a full-field scanning exposure by moving an aperture of a detector in an image plane in a predetermined manner, as in case of a conventional SAMOS test, which is a radiometric test.
The conventional SAMOS test has some similarities to the lithographic Kirk test in that it detects light scattering in from a large bright field into a small dark box. In the SAMOS test, the test pattern, (i.e., the image of the small dark box), does not move. An aperture of a detector is positioned sequentially in the bright field, and within the dark box. A conventional SAMOS test performs only static measurements in the dark and the bright area. No continuous or step-wise scanning of the detector is performed.
In a SAMOS test, when the aperture of the detector is positioned in the center of the dark box, light intensity in the center of the dark box relative to light intensity in the bright field is detected by the detector, and this ratio is a measure of stray light. Typical dark box sizes used in the SAMOS test are 33 μm and 108 μm. The size of the detector typically used is 28 μm. So, the scatter distance (as shown in
In the SAMOS test, the size of the imaging field is determined by the size of an imaging slit in a lithographic apparatus. The imaging field of a mask-based lithographic apparatus is usually large enough to obtain useful stray light measurement data. For example, in a particular reticle-based lithographic apparatus, an imaging field may be about 3-5 mm in length and about 26 mm in width on the image plane. Slit sizes may vary to about 8 mm in length for laser-based systems, and may be up to about 14 mm in length for lamp-based systems. However, the accuracy of stray light measurement increases in mask-based lithographic apparatus when the imaging field is “virtually” extended. This virtual extension of imaging field is not possible in the conventional SAMOS test. As discussed further below, embodiments of the present invention enables virtual extension of imaging field by scanning a detector.
In an OML apparatus, the imaging field for a single exposure is far smaller than conventional mask-based lithography apparatus. For this reason, the established SAMOS test, as currently implemented for larger imaging field mask-based lithographic apparatus, results in greatly reduced signal levels, and therefore, greatly degraded accuracy in stray light measurement in OML apparatus.
In an OML apparatus, an imaging field defined by a footprint of a SLM of the patterning device is typically only about 40 μm in length along a direction of scanning. A 40 μm imaging field dimension is too small to adequately surround a 33 μm dark box. In order to “virtually” increase the size of the imaging field in an OML apparatus, a full-field scanning exposure is simulated by embodiments of the present invention by performing a stepped exposure with suitably small step size.
It is to be noted that though the virtual enlargement of the imaging field is particularly useful for OML apparatus, where a conventional SAMOS test can not be applied, the virtual enlargement feature is useful to increase the accuracy of stray light measurement in a reticle-based lithographic apparatus as well.
System Embodiments of Radiometric Kirk Test
As discussed above, measuring stray light in an isolated dark part of an image is greatly aided when the dark part of the image is surrounded by a large bright field. A large bright field surrounding an isolated dark area gives increased signal levels on detector apertures and photo-resist. Thus, the radiometric Kirk test of the present invention involves a bright field reticle with at least one isolated dark area within the bright field.
The radiometric Kirk test includes at least two continuous or stepped scans of an aperture of a detector in an image plane of a lithographic system. Thus, it is also referred to as a “scanning SAMOS test”. During the first continuous or stepped scan, referred here as the “dark area measurement” or the “dark field measurement” step, the aperture of the detector is positioned such that at least at one point during the dark area measurement, the aperture of the detector is centered within an image of the dark area. During another continuous or stepped scan, referred here as the “bright area measurement” or the “bright field measurement” step, the aperture of the detector is positioned within the image of the surrounding bright field. The integrated (or averaged) detector signal from the dark area scan and the bright area scan are correspondingly computed. The ratio of the former and later signal is a measure of stray light present in the lithographic apparatus. Additionally, persons skilled in the art will appreciate that by shifting the aperture of the detector and dark area of the test pattern at SLM or reticle level, position dependency of the effect of stray light may be determined.
The test configuration includes a test pattern 905, an illumination system 915, a detector 930, a positioning mechanism 940, a moving mechanism 950, a transducer 960, a processor 970, and an output indicator 980. There may be additional components in the test configuration not shown in
The Illumination system 915 is configured to produce a beam of radiation B. This may be the illumination system IL shown in
The test pattern 905 receives the beam. The test pattern 905 is included on a physical reticle for a reticle-based system, or is created by one or more SLM arrays in case of an OML apparatus. The test pattern 905 comprises a dark area surrounded by a bright area, wherein the bright area is substantially larger than the dark area.
The configuration also includes the detector 930 that receives the beam patterned by the test pattern 905. The detector 930 has an aperture in an image plane where a projection system PS (as shown in
The positioning mechanism 940 included in the configuration is coupled to detector 930, and is configured to position the aperture of the detector 930 at a first and a second location within the image of the test pattern 905, the first location being aligned to an image of the dark area, and the second location being aligned to an image of the bright area at a distance from the image of the dark area. In the embodiments where the image of the test pattern 905 is moved synchronously with the detector aperture, positioning system 940 may include a first positioning system coupled to the detector 930 (this coupling is shown by a solid arrow in
The configuration also includes moving mechanism 950 coupled to the detector 930 that is configured to move the aperture of the detector 930 in a predetermined manner across the image plane. Note that positioning mechanism 940 and moving mechanism 950 may be combined in a single system, or they may be separate functional blocks that communicate with each other (as shown by a dotted bidirectional arrow in
The transducer 960 is coupled to the detector 930. The detector 930 detects radiation intensity during the dark area scan and the bright area scan, and the transducer 960 converts the detected intensity into measurable signals. In some embodiments, measured intensity data is actually a ratio of a transmissive intensity detected by a ratio sensor (RS) detector in image plane (the signal from which is referred to as an RS signal), and intensity detected by an intensity monitor (also referred to as an “energy sensor” (ES), the signal from which is referred to as an ES signal) located within the illuminator 915, that detects intensity before the beam is received by the test pattern 905. The RS signal varies with its position within the image and the radiation pulse energy. The ES signal only varies with the radiation pulse energy. Thus, the recorded ratio of the two signals is only a function of the RS detector position within the image plane. In this manner, the accuracy of the intensity measurements is not degraded by noise in the radiation pulse energy. Note that detector 930 and transducer 960 may be two separate functional units, or may be combined together.
The processor 970 coupled to the transducer receives measured signals from the transducer 960, and integrates the measured signals to yield a first integrated signal representing a result of the dark area scan, and a second integrated signal representing a result of the bright area scan. The processor 970 further compares the first integrated signal and the second integrated signal to generate a final result that indicates quantity and/or effects of stray light in the lithographic apparatus. A signal representing the final result is sent to the output indicator 980.
Note that processor 970 may be a separate functional unit, or combined with transducer 960 in some embodiments. Additionally, the processor 970 may communicate with the positioning mechanism 940 alone, or both the positioning mechanism 940 and the moving mechanism 950, in order to fine-tune alignment and movement of the detector (and the test pattern, where applicable) during the dark area scan and the bright area scan. This possibility is shown by two bidirectional arrows coupling the processor 970 with the positioning mechanism 940 and the moving mechanism 950.
The output indicator 980 is be a module coupled to the processor 970, that receives processed final result signal 971, and generates an output 981 based on the signal 971. The output 981 is communicated to a manual operator of the lithographic apparatus, or to an automated system, for the purpose of monitoring and evaluating the performance of the apparatus. The output indicator 980 may be located within the test configuration, or may be located away from the lithographic apparatus. For example, the output 981 may be transmitted to an instrumentation computer (not shown) having a graphic user interface that displays the output. Correctional measures can be taken if a degradation of performance is indicated by the output indicator 980. For example, a lens in the optical path may be cleaned or replaced or repaired to reduce effects of stray light enhancing bright spots. In some embodiments, operational parameters (such as, a correct radiation dose, or a correct alignment of a patterning device, etc.) of the lithographic apparatus may be controlled by a control module (not shown), or may be manually adjusted, based on the output 981 generated by output indicator 980.
In one embodiment of the radiometric Kirk test, during a dark area scan in a reticle-based lithographic apparatus, aperture 1110 is centered within image 1010 with the help of a positioning mechanism (as shown in
It is to be noted that though in
In one embodiment, a dimension 1270 of image 1210 of the dark area of a test pattern is bigger than the SLM length 1260 along the scanning direction, as shown in
In another embodiment, a dimension 1370 of the image 1310 of the dark area of a test pattern is smaller than the SLM length 1260 along the scanning direction, as shown in
Integrated signals from the dark area scan and the bright area scan are compared to compute stray light metrics.
Methods for Performing Radiometric Kirk Test
Flowchart 1500 begins with step 1510, in which a test pattern is provided with an isolated dark area surrounded by a large bright area. The test pattern may be provided on a reticle or by other pattern projection means, such as the patterning devices used in an OML apparatus. The test pattern receives a beam of radiation and patterns the beam to create a projected image on an image plane.
In the next step 1515, a detector is placed on the image plane with its aperture positioned in alignment with the image of the dark area. In case of a synchronous scanning, the aperture of the detector may be positioned centered within the image of the dark area. This step is a precursor for a dark area measurement.
In the next step 1520, the aperture is moved in a predetermined manner so that the aperture either crosses a stationary image of the dark box, or moves in a synchronous manner with a scanning image of the dark box.
In the next step 1525, a radiation intensity is measured corresponding to each position of the aperture of the detector during the dark area measurement.
In the next step 1530, measured intensity values are integrated (or averaged) to yield a first integrated signal representing the dark area measurement.
In next step 1535, a detector is placed on the image plane with its aperture positioned in alignment with the image of the bright area. In case of a synchronous scanning, the aperture of the detector may be positioned at a position within the image of the bright area, such that the relative position of the image of the dark area and the aperture do not change during subsequent scanning. This step is a precursor for a bright area measurement.
In the next step 1540, the aperture is moved in a predetermined manner so that the aperture either moves across a stationary image of the bright field, or moves in a synchronous manner with a scanning image of the bright field.
In the next step 1545, a radiation intensity is measured corresponding to each position of the aperture of the detector during the bright area measurement.
In the next step 1550, measured intensity values are integrated (or averaged) to yield a second integrated signal representing the bright area measurement.
In step 1555, one or more stray light metrics are computed by comparing the first and the second integrated signals representing the dark area measurement and the bright area measurement, respectively.
In step 1560, an output is communicated based on the computed results of step 1555 indicating the amount and effects of stray light in the lithographic apparatus. The output is used to monitor performance of the lithographic apparatus. If required, components, configurations, and operational parameters of the lithographic apparatus may be corrected or adjusted. For example, a lens may be cleaned or replaced, or a dose level of the beam of radiation may be adjusted, so that negative effects of stray light are countered, and better contrast is obtained in lithographic imaging.
While
Flowchart 1600 starts with step 1610, where an aperture of a detector is positioned in line with an intended portion of an image of a test pattern. In case of a dark area measurement, the initial location is aligned to an image of a dark box of a test pattern. In case of a bright area measurement, the initial location is aligned to an image of a bright field of the test pattern.
In the next step 1615, a radiation source (e.g., a laser) is turned on. For example, source SO in
In step 1620, the aperture moves to an exposure location.
In step 1625, the exposure location is exposed statically. The exposure can last for a duration of a single pulse, or a predetermined number of pulses (e.g., 100 pulses) of the radiation source. Using a plurality of pulses at a single exposure location averages out noise in radiation energy, and ensures more or less uniform radiation energy imparted at every exposure location along a direction of scanning.
In step 1630, a radiation intensity is measured. Measured intensity data is stored for a subsequent integration operation in a processor. In step 1635, the radiation source is turned off.
As indicated by the loop 1640, the process goes back to step 1610 to check (and if necessary, adjust) alignment of the aperture with respect to a desired test pattern, and eventually the aperture is moved to a next exposure location. Then, the steps of the flowchart 1600 repeat until a complete scan is performed.
Though not explicitly shown in flowchart 1600, it is implied that after all completion of both the dark area measurement and the bright area measurement, flowchart 1600 follows integration steps similar to steps 1630 and 1650 in flowchart 1600, and proceeds to a computing and indicating steps similar to steps 1655 and 1660 of flowchart 1600.
The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.
While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.
This application claims the benefit of U.S. Provisional Application No. 60/976,111, filed Sep. 28, 2008, which is incorporated by reference herein in its entirety.
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