Patent Application
20110242520
1. Field of the Invention
The present invention relates to optical properties measurement methods, exposure methods, and device manufacturing methods, and more particularly to an optical properties measurement method in which optical properties of an optical system that generates a pattern image on a predetermined plane is measured, an exposure method exposed in which exposure is performed taking into consideration optical properties measured by the optical properties measurement method, and a device manufacturing method using the exposure method.
2. Description of the Background Art
Semiconductor devices (integrated circuits) are becoming highly integrated year by year, and with this, a higher resolution is becoming required in a projection exposure apparatus such as a stepper, which is a manufacturing apparatus of semiconductor devices and the like. Further, it is also important to improve the overlay accuracy with respect to a pattern which is already formed on an object subject to exposure from the next layer onward. As a premise for this, it becomes necessary to measure and evaluate the optical properties (including an image-forming characteristic) of the projection optical system accurately, and to improve (including the case of improvement by adjustment) the optical properties of the projection optical system based on the evaluation results.
In order to obtain the optical properties of the projection optical system, such as for example, astigmatism, as a premise, the optimum focus position (best focus position) with respect to two measurement directions which are orthogonal to each other has to be precisely measured at an evaluation point (measurement point) within an image plane.
As an example of a measurement method of the best focus position of the projection optical system, a method disclosed in, for example, U.S. Patent Application Publication No. 2004/0179190 is known. In this method, exposure is performed using a reticle on which a predetermined pattern (for example, a dense line pattern (line-and-space pattern) and the like) is formed as a test pattern, and the test pattern is transferred onto a test wafer at a plurality of positions in an optical axis direction of the projection optical system. A resist image (an image of the transferred pattern) which is obtained by developing the test wafer is picked up, for example, by an image-forming alignment sensor and the like equipped in the exposure apparatus, and the best focus position is obtained, based on a relation between a contrast value (for example, dispersion of the brightness value of a pixel) of the image of the test pattern obtained from the imaging data and a position of the wafer in the optical axis direction of the projection optical system. Therefore, this method is also referred to as a contrast focus method.
However, it has recently become clear that it is difficult to obtain astigmatism of the projection optical system equipped in the current exposure apparatus, precisely at the required level, using the contrast focus method. It is conceivable that the cause is due to sensitivity which is generated in a non-measurement direction orthogonal to the measurement direction when the length of the pattern image in the non-measurement direction becomes shorter along with defocus, in addition to the sensitivity of the contrast value of the image with respect to the measurement direction (a sequence direction of the crowd line) which decreases when the dense line pattern transferred onto the test wafer is not resolved under the exposure condition of the device manufacturing process due to finer patterns in recent years.
According to a first aspect, there is provided a first optical properties measurement method to measure optical properties of an optical system which generates an image of a pattern placed on a first plane on a second plane, the method comprising: sequentially transferring a pattern used for measurement whose measurement direction is in a predetermined direction on an object via the optical system while changing a position of the object placed on a side of the second plane of the optical system in an optical axis direction of the optical system, and generating a plurality of divided areas including an image of the pattern used for measurement on the object; imaging a predetermined number of divided areas of the plurality of divided areas on the object, and extracting, of the image of the pattern used for measurement generated in each of the predetermined number of divided areas that have been imaged, imaging data related to at least a part of an image whose both ends in a non-measurement direction intersecting the measurement direction is excluded; and computing an evaluation amount in the measurement direction related to a brightness value of each pixel in each of the predetermined number of divided areas using the extracted imaging data, and obtaining the optical properties of the optical system based on the evaluation amount of each of the plurality of divided areas that has been computed.
According to a second aspect, there is provided a second optical properties measurement method to measure optical properties of an optical system which generates an image of a pattern placed on a first plane on a second plane, the method comprising: sequentially transferring a pattern used for measurement whose measurement direction is in a predetermined direction on a plurality of areas on an object via the optical system and generating an image of the pattern used for measurement in each of the plurality of areas, while changing a position of the object placed on a side of the second plane of the optical system in an optical axis direction of the optical system; performing a trim exposure to each of the plurality of areas to remove both ends in the non-measurement direction of the image of the pattern used for measurement that is generated; imaging a predetermined number of divided areas among a plurality of divided areas on an object including each of image of the pattern used for measurement which has both sides removed in the non-measurement direction; and processing imaging data obtained by the imaging, and computing an evaluation amount in the measurement direction related to a brightness value of each pixel for each of the predetermined number of divided areas which have been imaged, and also obtaining optical properties of the optical system, based on the evaluation amount of each of the predetermined number of divided areas which have been computed.
According to a third aspect, there is provided An exposure method, comprising: measuring optical properties of an optical system using one of the first and second optical properties measurement method; and adjusting at least one of the optical properties of the optical system and a position of the object in the optical axis direction of the optical system and exposing an object by generating a predetermined pattern image on a predetermined plane via the projection optical system, taking into consideration measurement results of the optical properties.
According to a fourth aspect, there is provided device manufacturing method, including exposing an object by the exposure method described above; and developing the object which has been exposed.
In the accompanying drawings;
An embodiment of the present invention will be described below, with reference to
Exposure apparatus 100 is equipped with an illumination system IOP, a reticle stage RST holding a reticle R, a projection unit PU which projects an image of a pattern formed on reticle R on wafer W to which a photosensitive agent (a photoresist) is applied, a wafer stage WST which holds a wafer W, and moves in a two dimensional plane (in an XY plane), a drive system 22 which drives wafer stage WST, and a control system for these parts. The control system is mainly configured of main controller 28 composed of a microcomputer (or a workstation) that performs overall control of the entire apparatus.
Illumination system IOP includes a light source consisting, for example, of an ArF excimer laser (output wavelength 193 nm) (or a KrF excimer laser (output wavelength 248 nm) and the like), an illumination system housing connected to the light source via a light-transmitting optical system, and an illumination optical system inside of the illumination system housing. The illumination optical system includes an illuminance uniformity optical system which includes an optical integrator and the like, a beam splitter, a relay lens, a variable ND filter, a reticle blind and the like (none of which are shown), as is disclosed in, for example, U.S. Patent Application Publication No. 2003/0025890 description and the like. The illumination optical system shapes a laser beam output from the light source, and illuminates a slit-shaped illumination area extending narrowly in the X-axis direction (the orthogonal direction of the page surface in
Reticle stage RST is placed below illumination system IOP in
Projection unit PU is disposed below reticle stage RST in
As an example, the projection magnification of projection optical system PL is to be ¼. Therefore, when reticle R is illuminated by illumination light IL with uniform illumination as is previously described, a pattern of reticle R within the illumination area is reduced by projection optical system PL, and is projected on wafer W to which a photoresist is applied, and a reduced image of the pattern is formed on a part of an area subject to exposure (a shot area) on wafer W. At this point, projection optical system PL forms the reduced image in a part (that is to say, a rectangular shaped area which is an exposure area, and is conjugate with the illumination area in projection optical system PL) of its field. Incidentally, while the image-forming characteristic correction controller previously described moved at least one optical element (lens elements) of projection optical system PL so as to adjust the optical properties of projection optical system PL, or in other words, to adjust the image-forming state of the pattern image on wafer W, alternatively, or in combination with such movement, for example, at least one of changing the optical properties (e.g. center wavelength, spectral wavelength and the like) of illumination light IL by the control of the light source and driving wafer W in the Z-axis direction (and in a tilt direction with respect to the XY plane) parallel with optical axis AXp of projection optical system PL can be performed.
Wafer stage WST is driven by drive system 22 including a linear motor and the like, and is equipped with an XY stage 20 and a wafer table 18 on which XY stage 20 is mounted. On wafer table 18, wafer W is held by vacuum suction or the like by a wafer holder (not shown). Wafer table 18 finely drives the wafer holder holding wafer W in the Z-axis direction and the tilt direction with respect to the XY plane, and is also referred to as a Z tilt stage. On the upper surface of wafer table 18, a movable mirror (or a reflection surface which has been mirror processed) 24 is provided, and on movable mirror 24, a laser beam (a measurement beam) is irradiated from laser interferometer 26, and positional information in the XY plane and rotational information (including yawing (θz rotation which is rotation around the Z-axis), pitching (θx rotation which is rotation around the X-axis), and rolling (θy rotation which is rotation around the Y-axis)) of wafer table 18 are measured, based on the reflection light from movable mirror 24.
Measurement values of laser interferometer 26 are supplied to main controller 28, and based on the measurement values of laser interferometer 26, main controller 28 controls the position (including the θz rotation) of wafer table 18 in the XY plane by controlling XY stage 20 of wafer stage WST via drive system 22.
Further, the position of the wafer W surface in the Z-axis direction and the amount of inclination are measured by a focus sensor AFS consisting of a multiple point focal position detection system of an oblique incidence method that has a light-transmitting system 50a and light receiving system 50b as the one disclosed in, for example, U.S. Pat. No. 5,448,332 and the like. Measurement values of focus sensor AFS are also supplied to main controller 28.
Further, on wafer table 18, a fiducial plate FP is fixed whose surface is set to be the same height as the height of the surface of wafer W. On the surface of fiducial plate FP, at least a pair of first fiducial marks which is detected along with a pair of reticle alignment marks by a reticle alignment system to be described later on, a second fiducial mark (the first and second fiducial marks are not shown) used in the so-called baseline measurement of alignment system AS which is described below, and the like are formed.
In the embodiment, on a side surface of barrel 40 of projection unit PU, an alignment system AS is provided which detects an alignment mark formed on wafer W. As alignment system AS, as an example, an FIA (Field Image Alignment) system is used which is a type of an alignment sensor by an image processing method that measures a mark position by illuminating a mark using a broadband (a wide band wavelength range) light such as a halogen lamp and performing image processing of the mark image. The resolution limit of alignment system AS is larger (the resolution is low) than the resolution limit of projection optical system PL.
Detection signal DS of alignment system AS is supplied to an alignment controller 16, and alignment controller 16 performs an AD conversion of detection signal DS, computes the waveform signal that has been digitized, and detects the mark position. The results are supplied to main controller 28 from alignment controller 16.
Furthermore, in exposure apparatus 100 of the embodiment, although it is omitted in the drawings, above reticle stage RST, a pair of reticle alignment detection systems consisting of a TTR (Through The Reticle) alignment system which uses light of an exposure wavelength is arranged as disclosed in, for example, U.S. Pat. No. 5,646,413 and the like, and detection signals of the reticle alignment system are supplied to main controller 28 via alignment controller 16.
Next, an example of a reticle used to measure the optical properties of the projection optical system in exposure apparatus 100 will be described.
Each of patterns MPn (n=1-5) which are used for measurement include four types of line-and-space patterns (hereinafter also described as “L/patterns”) LSVn, LSHn, LSRn, and LSLn that are shown enlarged in
In the embodiment, in four square areas (27 μm×27 μm) surrounded by solid lines and dotted lines into which aperture pattern APn is divided as shown in
Further, on both sides in the X-axis direction of pattern area PA which passes through the reticle center previously described, a pair of reticle alignment marks RM1 and RM2 are formed (refer to
Next, a measurement method of optical properties of projection optical system PL in exposure apparatus 100 of the embodiment will be described, according to a flow chart of
First of all, in step 402 of
In the following step 404, predetermined preparatory operations such as alignment of reticle RT with respect to projection optical system PL are performed. To be concrete, reticle stage RST and wafer stage WST (XY stage 20) are moved based on measurement values of laser interferometers 14 and 26, respectively, so that the pair of first fiducial marks (not shown) on fiducial plate FP previously described and the pair of reticle alignment marks RM1 and RM2 of reticle RT are detected by the reticle alignment system (not shown) previously described. And, based on detection results of the reticle alignment system previously described, a position (including rotation) of reticle stage RST in the XY plane is adjusted. This sets rectangular area IAR′ of reticle RT within the illumination area previously described, and the entire surface will be irradiated with illumination light IL. Further, in the embodiment, a position where a projected image (a pattern image) of pattern MPn used for measurement is generated via projection optical system PL within its field (especially the exposure area) becomes an evaluation point within the exposure area of projection optical system PL where optical properties (for example, focus position) of projection optical system PL should be measured. In the embodiment, a total of five evaluation points are set, which are the center and the four corners of the exposure area previously described. Incidentally, while the evaluation points which are actually set may be more, such as, for example, around 9×9=81 points, in this case, for the sake of convenience of the drawings and the explanation, the evaluation points are the five points described above. However, the number of evaluation points can be of any number, and one evaluation point is also acceptable.
When the predetermined preparatory operations are completed in the manner described above, the processing moves to the following step 406, in which a target value of an exposure energy amount is set to an optimum value. The optimum value of the exposure energy amount is obtained beforehand by an experiment, simulation, or the like, and as an example, an energy amount which is around 60 to 70% of the energy amount that can resolve a L/S pattern having a minimum line width on the reticle used to manufacture a device is to be the optimum value quantity.
In the following step 408, a count value i of a first counter is initialized (i←1). In the embodiment, count value i is also used (refer to
Accordingly, in the embodiment, M times of exposure (in this example, M=15) to sequentially transfer pattern MPn (n=1 to 5) used for measurement on wafer WT will be performed, while varying a position (a focus position) of wafer WT in an optical axis direction (the Z-axis direction) of projection optical system PL. In the embodiment, a projection area on wafer WT of aperture pattern APn by projection optical system PL is referred to as a measurement pattern area, and in the measurement pattern area, a projection image of pattern MPn used for measurement is generated, and by each exposure, aperture pattern APn is transferred on wafer WT and a divided area which includes a transferred image of pattern MPn used for measurement is formed. Therefore, at areas (hereinafter referred to as “evaluation point corresponding areas”) DB1 to DB5 (refer to
Now, although the description lacks in sequence, each of the evaluation point corresponding areas DBn on wafer WT to which pattern MPn used for measurement will be transferred by the exposure described later on will be described using
Referring back to
This allows an image of aperture pattern APn including pattern MPn used for measurement to be transferred onto divided area DA1 of each of the evaluation point corresponding areas DBn on wafer WT, respectively, as shown in
Referring back to
In this case, because step pitch SP is equal to or less than the size in the Y-axis direction of the projection image of aperture patterns APn on wafer WT, there are no frame lines formed by a part of an image of aperture patterns APn or areas which are not exposed at a border section of divided area DA1 and divided area DA2 of each evaluation point corresponding area DBn.
Hereinafter, until judgment in step 416 is affirmed, or in other words, until the target value of the focus position of wafer WT set then is judged to be ZM, a loop processing (including judgment) of steps 416→418→410 is repeated. This allows aperture pattern APn including pattern MPn used for measurement to be transferred onto divided areas DAi (i=3−iM) of each of the evaluation point corresponding areas DBn on wafer WT, respectively. However, also in this case, for the same reasons as is previously described, there are no frame lines or areas which are not exposed at a border section between adjacent divided areas.
On the other hand, when exposure of divided area DAM (DA15 in this example) of each of the evaluation point corresponding areas DBn has been completed, and judgment in step 416 described is affirmed, the processing moves to step 420. At the stage when judgment in step 416 is affirmed, in each of the evaluation point corresponding areas DBn on wafer WT, M (M=15 in the example) transferred images (latent images) of pattern MPn used for measurement are formed whose exposure condition (in the example, focus position) is different as shown in
Referring back to
After carriage of wafer WT to the coater developer described above, the processing proceeds to step 422 and waits for development of wafer WT to be completed. During the waiting time in step 422, development of wafer WT is performed by the coater developer. By the development being completed, on wafer WT, resist images of evaluation point corresponding areas DBn (n=1 to 5) as shown in
While
In the waiting state in step 422 described above, when it has been confirmed that the development of wafer WT has been completed by a notice from a control system of the coater developer (not shown), the processing moves to step 424 where instructions are given to a wafer loader (not shown), and after when wafer WT is loaded again onto wafer table 18 as in step 402 previously described, the processing moves to a subroutine (hereinafter also referred to as an “optical properties measurement routine”) in step 426 where the optical properties of the projection optical system is computed.
In this optical properties measurement routine, first of all, in step 502 of
In the next step 504, a resist image of evaluation point corresponding area DBn (in this case, DB1) on wafer WT is picked up using alignment system AS, and the imaging data are taken in. Alignment system AS splits the resist image into pixel units of an imaging device (CCD and the like) that the system has, and supplies the grayscale of the resist image corresponding to each pixel to main controller 28, for example, as an 8-bit digital data (pixel data). In other words, the imaging data are configured of a plurality of pixel data. In this case, when the gray level of the resist image becomes higher (becomes closer to black), the value of pixel data is to increase. Incidentally, because the size of evaluation point corresponding area DBn is 101.25 μm (the Y-axis direction)×27 μm (the X-axis direction) and the entire area is set in a detection area of alignment system AS in the embodiment, it becomes possible to pick up the images of M divided areas DAi simultaneously (collectively) for each evaluation point corresponding area.
In the next step 506, the imaging data of the resist image formed on evaluation point corresponding area DBn (in this case, DB1) from alignment system AS are arranged, and an imaging data file is made.
In the next step 508, image processing on the imaging data is performed and an outer periphery of evaluation point corresponding area DBn (in this case, DB1) is detected. This outer periphery detection, as an example, can be performed in the following manner.
In other words, based on the imaging data obtained by the imaging, with a straight line portion configuring an outer frame consisting of an outline of evaluation point corresponding area DBn serving as an area subject to detection, by scanning a window area of a predetermined size in a direction which is substantially orthogonal to the straight line portion in the area subject to detection, a position of the straight line section subject to detection is detected, based on the pixel data within the window area during the scanning. In this case, because the outer frame section has pixel data whose pixel values (pixel values) are obviously different from the pixel values of other sections, the position of the straight line portion (a part of the outer frame) subject to detection is detected without fail, for example, based on a variation of the pixel data within the window area corresponding to a variation by one pixel each of the position of the window area in the scanning direction. In this case, the scanning direction is preferably in a direction heading from the inner side of the outer frame to the outer side. This is because when a peak of the pixel value corresponding to the pixel data within the window area previously described is obtained at first, the position coincides with the position of the outer frame without fail, which allows a more secure outer frame detection to be performed.
Such a detection of the straight line portion is performed on each of the four sides configuring the outer frame consisting of the outline of evaluation point corresponding area DBn. Detection of this outer frame is disclosed in detail, for example, in U.S. Patent Application Publication No. 2004/0179190 and the like.
In the next step 510, by dividing the outer frame of evaluation point corresponding area DBn detected above, or in other words, dividing the inside of the frame line of the rectangle into M equal parts (e.g. 15 equal parts) in the Y-axis direction, divided areas DA1 to DAM (DA15) are obtained. In other words, (positional information of) each divided area is obtained, with the outer frame serving as a reference.
In the next step 512, a detection area is set for each measurement mark area regarding each divided area DAi (i=1 to M). To be concrete, main controller 28 sets detection areas DVi, DHi, DRi, and DLi (refer to
Determination of detection area DVi will be described, with an image (a resist image) LS″Vn of L/S pattern LSVn formed in divided area DAi serving as an example. L/S pattern LSVn in pattern MPn corresponding to resist image LS″Vn is a multi-bar pattern which has eight line patterns arranged in a measurement direction (the X-axis direction), as shown in
In this case, from the viewpoint of detection sensitivity and the like, it is preferable to obtain the area of resist image LS″Vn as a quantity corresponding to the expanse in the measurement direction. However, as it can be seen from the two-dimensional data shown in
Therefore, in the embodiment, detection area DVi is set with respect to resist image LS″Vn, as shown in
To the other resist images LS″Hn, LS″Rn, and LS″Ln corresponding to L/S patterns LSHn, LSRn, and LSLn in pattern MPn as well, detection areas DHi, DRi, and DLi are set as shown in
In the following step 513, regarding the four resist images LS″Vn, LS″Hn, LS″Rn, and LS″Ln within each divided area DAi (i=1 to M), respectively, the area within each of the detection areas DVi, DHi, DRi, and DLi is computed. For example, area Cni of resist images LS″Vn, LS″Hn, LS″Rn, and LS″Ln can be obtained by Cni=Σkθ(xk−xth). In this case, xk is a detection signal (brightness) of the kth pixel within the detection area, xth is a threshold value (threshold brightness), and θ(x) is a step function. In other words, area Cni is equal to a number of pixels of brightness xk that exceeds threshold brightness xth of the pixels within the detection area. Incidentally, threshold brightness xth is appropriately decided, according to the required measurement accuracy, the detection sensitivity and the like. Area Cni which has been obtained of the resist images is stored for each type (V, H, R, and L) of the four resist images and for each divided area DAi (i) in a storage device (not shown).
In the following step 514, the best focus position for each of the measurement directions at evaluation point corresponding area DBn (an nth evaluation point) is obtained, using detection area Cni of the four resist images LS″Vn, LS″Hn, LS″Rn, and LS″Ln stored in the storage device (not shown). In this case, for each of the resist images LS″Vn, LS″Hn, LS″Rn, and LS″Ln, main controller 28 plots detection area Cni with respect to focus position Zi as shown in
As is obvious from
Main controller 28 obtains a best focus position Zbest using focus curve c, from its peak center. In this case, the peak center is defined, for example, as a center of two focus positions corresponding to intersecting points of a focus curve and a predetermined slice level.
Main controller 28 performs a computation of the best focus position Zbest described above for all of the four resist images LS″Vn, LS″Hn, LS″Rn, and LS″Ln. This allows the best focus position Zbest to be obtained for each of the four measurement directions. And, main controller 28 computes an average of the best focus position Zbest for each of the four measurement directions as a best focus position in evaluation point corresponding area DBn (the nth evaluation point).
In the following step 516, a judgment is made of whether or not the processing has been completed for all of the evaluation point corresponding areas DB1 to DB5, referring to count value n previously described. In this case, because only the processing of evaluation point corresponding area DB1 has been completed, the decision made in this step 516 is negative, therefore, after the processing moves to step 518 where count value n is incremented by 1 (n←n+1), the processing returns to step 502 in which wafer WT is set to a position where evaluation point corresponding area DB2 can be detected with alignment system AS.
Then, the processing (including judgment) of steps 504 to 514 described above is performed again, and then, the best focus position is obtained for evaluation point corresponding area DB2, as in the case of evaluation point corresponding area DB1.
Then, when computing the best focus position for evaluation point corresponding area DB2 has been completed, in step 516, the judgment is made of whether processing of all of the evaluation point corresponding areas DB1 to DB5 has been completed or not, and the judgment in this case is negative. Hereinafter, until the judgment in step 516 is affirmed, the processing (including judgment) of steps 502 to 518 described above is repeated. This allows the best focus position to be obtained for each of the other evaluation point corresponding areas DB3 to DB5, as in the case of evaluation point corresponding area DB1 previously described.
When the computation of the best focus position for all of the evaluation point corresponding areas DB1 to DB5 on wafer WT, or in other words, computation of the best focus position is performed at each of the evaluation points previously described that serve as projection positions of the five patterns MP1 to MP5 used for measurement within the exposure area of projection optical system PL, judgment in step 516 is affirmed. While optical properties measurement routine can be completed here, in the embodiment, the processing moves to step 520 where other optical properties are computed based on the best focus position data obtained above.
For example, in this step 520, curvature of image plane of projection optical system PL is computed based on the data of the best focus position at evaluation point corresponding areas DB1 to DB5, as an example. Further, the depth of focus at each evaluation point in the exposure area previously described can be obtained.
In the embodiment, for the sake of simplicity in the description, while the best focus position at evaluation point corresponding area DBn (the nth evaluation point) was obtained based on an average of the best focus position Zbest in each of the four measurement directions at each evaluation point corresponding area (a position corresponding to each evaluation point), as well as this, astigmatism at each evaluation position can be obtained from the best focus position obtained from a pair of L/S patterns whose periodic direction is orthogonal to each other. Furthermore, for each evaluation point within the exposure area of projection optical system PL, based on the astigmatism computed in the manner described above, it is also possible to obtain uniformity within the astigmatism plane, for example, by performing an approximation processing by the least-squares method, as well as to obtain a total focus difference from the uniformity within the astigmatism plane and curvature of image plane.
Then, the optical properties data of projection optical system PL obtained in the manner described above is stored in the storage device (not shown), as well as is shown on a screen of a display device (not shown). This completes the processing of step 520 in
Next, an exposure operation by exposure apparatus 100 of the embodiment in the case of manufacturing a device will be described.
As a premise, information on the best focus position decided in the manner described above, as well as information on astigmatism (and curvature of image plane) is to be input into main controller 28 via an input-output device (not shown).
For example, in the case information on the astigmatism (and curvature of image plane) is input, prior to exposure, main controller 28 gives instructions to the image-forming characteristic correction controller (not shown) based on the optical properties data, and corrects the image forming characteristic of projection optical system PL as much as possible so that the astigmatism (and curvature of image plane) is corrected, for example, by changing a position (including the distance between other optical elements) or a tilt of at least one optical element (in the embodiment, a lens element) of projection optical system PL. In this case, the optical element whose position or tilt is changed is not limited to a lens element, and depending on the configuration of the optical system, for example, the optical element can be a catoptric element such as a concave mirror and the like, or an aberration correcting plate which corrects the aberration (such as distortion, spherical aberration and the like), especially correcting the non-rotational symmetrical component. Further, as a correction method of the image forming characteristic method of projection optical system PL, for example, a method of slightly shifting the center wavelength of illumination light IL or a method of changing a refractive index in a part of projection optical system PL can be employed singularly, or by combining the methods with the movement of the optical element.
Then, by main controller 28, reticle R on which a predetermined circuit pattern (device pattern) that is subject to transfer is formed is loaded on reticle stage RST using a reticle loader (not shown), and wafer W is loaded similarly on wafer table 18 using a wafer loader (not shown).
Next, by main controller 28, preparatory operations such as reticle alignment, baseline measurement of alignment system AS and the like are performed in a predetermined procedure using reticle alignment system (not shown), fiducial plate FP on wafer table 18, alignment system AS and the like, and following the operations, wafer alignment is performed, for example, by an EGA (Enhanced Global Alignment) method and the like. In this case, reticle alignment, and baseline measurement of alignment system ALG are disclosed in, for example, U.S. Pat. No. 5,646,413 and the like, and EGA that follows is disclosed in, for example, U.S. Pat. No. 4,780,617 and the like. Incidentally, reticle alignment can be performed, using an aerial image measuring instrument (not shown) provided on wafer stage WST, instead of the reticle alignment system.
When the wafer alignment described above is completed, main controller 28 controls each section of exposure apparatus 100, repeatedly performs scanning exposure of the shot areas on wafer W and a stepping operation between shots, and sequentially transfers the pattern of reticle R on all the shot areas subject to exposure on wafer W.
During the scanning exposure described above, based on the positional information in the Z-axis direction of wafer W detected by focus sensor AFS, main controller 28 drives wafer table 18 via drive system 22 in the Z-axis direction and the tilt direction so that the surface of wafer W (shot areas) is set within the depth of focus in the exposure area of projection optical system PL after the optical properties correction previously described, and performs focus leveling control of wafer W. In the embodiment, prior to the exposure operation of wafer W, the image plane of projection optical system PL is computed based on the best focus position at each evaluation point previously described, and based on results of the computation, optical calibration (for example, adjustment of the tilt angle of a plane parallel plate placed in light receiving system 50b) of focus sensor AFS is performed. As well as this, for example, focus operation (and leveling operation) can be performed, taking into consideration an offset corresponding to a deviation of the image plane computed earlier and a detection reference of focus sensor AFS.
As discussed above, according to the optical properties measurement method related to the embodiment, pattern MPn used for measurement formed on reticle RT is transferred sequentially on wafer WT via projection optical system PL, while changing the position of wafer WT used for testing placed on the image surface side of projection optical system PL in the optical axis direction of projection optical system PL, and a plurality of number of divided areas including the image of pattern MPn used for measurement is generated on wafer WT used for testing. Then, of the plurality of number of divided areas on wafer WT, a predetermined number of divided areas is imaged, and imaging data of images LS″Vn, LS″Hn, LS″Rn, and LS″Ln (of L/S patterns LSVn, LSHn, LSRn, and LSLn) of pattern MPn used for measurement generated in each of the predetermined number of divided areas whose images are picked up are extracted, and then, as an evaluation amount in the measurement direction related to brightness value of each pixel in each divided area, an amount corresponding to the expanse of images LS″Vn, LS″Hn, LS″Rn, and LS″Ln of each L/S pattern in the corresponding measurement direction is obtained, and the optical properties of projection optical system PL is obtained, based on the amount corresponding to the expanse which has been obtained. This makes it possible to obtain the optical properties of projection optical system PL with good precision.
Further, according to the optical properties measurement method related to the embodiment, to each of images LS″Vn, LS″Hn, LS″Rn, and LS″Ln of pattern MPn transferred on wafer WT used for testing, at least a part of the image excluding both ends in a corresponding non-measurement direction is detected, and the area of the detected image (at least a part of images LS″Vn, LS″Hn, LS″Rn, and LS″Ln) is obtained as a quantity corresponding to the expanse in the measurement direction. By this, the optical properties of projection optical system PL obtained from the quantity corresponding to the expanse do not have sensitivity to the non-measurement direction; therefore, it becomes possible to precisely obtain the optical properties with respect to the measurement direction. Further, in order to make such treatment easy, the plurality of multi-bar patterns extending in the non-measurement direction arranged in the measurement direction will be used as the pattern used for measurement.
Further, according to the optical properties measurement method related to the embodiment, because the area of a part of images LS″Vn, LS″Hn, LS″Rn, and LS″Ln that has been detected is obtained as an amount corresponding to the expanse in the measurement direction, it becomes possible to perform measurement using a microscope whose resolution is lower than a SEM, such as, for example, a measurement device as in alignment system AS and the like of exposure apparatus 100. By this arrangement, a severe focusing as in the case of using a SEM becomes unnecessary, which allows the measurement time to be reduced. For example, even in the case described above where each of the evaluation point corresponding areas DBn is not imaged simultaneously, and is imaged for each divided area DAi, the measurement time per point can be reduced. Further, measurement becomes possible regardless of the type (such as a line and space (an isolated line, a dense line), contact hole, size, and an arrangement direction) of the pattern image, and moreover, regardless of the illumination condition at the time of generation of the projection image (pattern image) of pattern MPn used for measurement.
Further, in the embodiment, because quantity corresponding to the expanse in the measurement direction described above is detected, it is not necessary to place a pattern (e.g. a reference pattern for comparison, a mark pattern for positioning and the like) besides pattern MPn used for measurement within pattern area PA of reticle RT. Further, the pattern used for measurement can be made smaller in comparison with the conventional method (CD/focus method, SMP focus measurement method and the like) of measuring dimensions. Therefore, the number of evaluation points can be increased, and distance between the evaluation points can be narrowed. As a result, measuring accuracy of the optical properties and reproducibility of the measurement result can be improved.
Further, according to exposure apparatus 100, the pattern formed on reticle R is transferred on wafer W via projection optical system PL, after an operation related to adjustment of the image forming state of the pattern image projected on wafer W via projection optical system PL, such as for example, adjustment of the image forming characteristic by moving the optical element of projection optical system PL, or calibration of the focus sensor AFS has been performed, so that an optimum transfer can be performed taking into consideration the optical properties of projection optical system PL measured with good precision by the optical properties measurement method previously described.
Therefore, according to the exposure method related to the embodiment, optical properties of projection optical system PL is measured with high precision using the optical properties measurement method described above, and a pattern image is generated with high precision within the exposure area of projection optical system PL taking into consideration the measurement results of the optical properties, and a highly precise exposure (pattern transfer) is realized.
Incidentally, a pattern which is the line having a linewidth of 0.8 μm (linewidth 0.2 μm is a conversion value on the wafer) configuring each of the L/S patterns of pattern MPn used for measurement in the embodiment above that is further divided, such as for example, a pattern which is configured of three lines and two spaces which is the line further divided into five, can be employed as a pattern used for measurement. In the case of this pattern, the width of each line and each space becomes 40 nm on a wafer. It is more preferable when such a pattern is used, because “the change of the area of the pattern previously described” with respect to focus change becomes large, and the best focus position can be detected with high sensitivity. In this case, the line width (or pitch) of a plurality of lines configuring each line pattern of L/S pattern is a conversion value on the wafer, and is set smaller than the limit of resolution of a measurement device (optical system) such as alignment system AS previously described. Incidentally, in fine dense lines including such L/S pattern and the like which is smaller than the limit of resolution, it is desirable to reduce the exposure amount so as to avoid a pattern slant. It is said that pattern slant, here, is easily generated when an aspect ratio which is the ratio of the photoresist film thickness with respect to the pattern dimension is three or more.
Further, in the embodiment above, while the case has been described where four kinds of L/S patterns (multi-bar patterns) placed in aperture pattern APn as pattern MPn used for measurement on reticle RT are used, as the pattern used for measurement, the pattern can be a pattern whose number or kind is only one, or instead of, or in combination with the L/S pattern, isolation lines and the like can be used.
Further, while the entirety was imaged simultaneously for each evaluation point corresponding area in the embodiment above, for example, one evaluation point corresponding area can be divided into a plurality of numbers and imaged a plurality of times. On this imaging, for example, the imaging can be performed by setting the entire evaluation point corresponding area within the detection area of alignment system AS and then imaging a plurality of the evaluation point corresponding areas at a different timing, or the imaging can be performed by setting a plurality of sections of the evaluation point corresponding area sequentially within the detection area of alignment system AS. Furthermore, while the plurality of divided areas configuring one evaluation point corresponding area DBn were formed adjacent to each other, for example, a part of at least one of the divided areas) the plurality of divided areas can be formed separated by a distance longer than the distance corresponding to the size of the detection area of alignment system AS previously described. Further, while the plurality of divided areas was arranged in a line for every evaluation point corresponding area in the embodiment above, a position in a direction (the X-axis direction) orthogonal to the arrangement direction (the Y-axis direction) can be made to be partially different in the plurality of divided areas, and for example, in the case such as when the length of the evaluation point corresponding area exceeds the detection area of alignment system AS in the arrangement direction (the Y-axis direction) the divided areas can be placed in a plurality of lines (two-dimensionally) in each of the evaluation point corresponding areas. That is to say, the placement (layout) of the plurality of divided areas can be decided according to the size of the detection area of alignment system AS so that the whole evaluation point corresponding area can be imaged simultaneously for each of the evaluation point corresponding areas. On deciding the placement, it is desirable to decide the step pitch in the X-axis direction so that there are no frame lines or unexposed sections also in the border of adjacent divided areas in the direction (the X-axis direction) orthogonal to the placement direction. Incidentally, while pattern MPn used for measurement was transferred onto wafer. WT by static exposure in the embodiment, scanning exposure can be employed instead of the static exposure, and in this case, dynamic optical properties can be obtained. Further, exposure apparatus 100 of the embodiment can be of a liquid immersion type, and by transferring the image of pattern MPn used for measurement via the projection optical system and liquid, optical properties of the projection optical system including the liquid can be measured.
Incidentally, in the embodiment above, the case has been described where the area (the number of pixels previously described) within the detection area of the image of the measurement mark was detected, and the best focus position and the like of the projection optical system was obtained based on the detection results. However, as well as this, the embodiment described above can be suitably applied to the case where contrast values for every measurement mark area (or each divided area DAi) are detected instead of the area described above, and the best focus position and the like of the projection optical system is obtained based on the detection results, as is disclosed in, for example, U.S. Patent Application Publication No. 2008/0208499 and the like. In this case as well, astigmatism in each evaluation point can be obtained more securely from the best focus position which is each obtained by a set of an L/S pattern whose periodic directions are orthogonal. In this case, as the contrast value for every measurement mark area (or each divided area DAi), dispersion or standard deviation of the brightness value of each pixel of every measurement mark area (or each divided area DAi), or other statistics including deviation with respect to a predetermined reference value of the brightness value of each pixel in each measurement mark area (or each divided area) can be used. Besides this, a statistic of some kind on brightness of each pixel can be employed as contrast information, such as, for example, information on the brightness value of each pixel in each measurement mark area (or each divided area) which does not include the deviation described above, such as, for example, a total sum value or an averaged value of the brightness of each pixel in an area of a predetermined area (predetermined number of pixels) including the pattern image used for measurement, among the measurement mark areas (or divided areas). The point is, when the area (number of pixels and the like) of the imaging area used to compute contrast information in each measurement mark area (or, each divided area) is made to be constant, any statistic related to the brightness value of each pixel can be used. Further, for example, in the case of setting an area of the imaging area so that the area includes the pattern image used for measurement and is also set around the same level or smaller than the area of the measurement mark area (or divided area), step pitch SP of wafer WT at the time of transferring the pattern used for measurement can be made larger than the size of the projection image (corresponding to the measurement pattern area previously described) in the Y-axis direction of each aperture pattern APn on wafer WT.
Further, in the embodiment described above, while the images formed in each of the divided areas on the wafer were all imaged, not all of the images have to be picked up. For example, the divided areas can be imaged alternately.
Incidentally, in the embodiment above, for example, the subject of the imaging can be a latent image formed on the resist on exposure, or an image (etched image) obtained by etching the wafer on which the image above has been formed and developed. Further, the photosensitive layer on which an image on an object such as a wafer is formed is not limited to a photoresist, and can be, for example, an optical recording layer, a photomagnetic recording layer and the like, as long as an image (a latent image and a manifest image) is formed by an irradiation of light (energy).
The exposure apparatus of this modified example is configured in the same manner as the exposure apparatus of the embodiment described above. Accordingly, measurement of the optical properties of the projection optical system is basically performed in a procedure similar to the embodiment previously described. However, in this modified example, in step 410 of
Since other processing is similar to the embodiment described above, a detailed description will be omitted.
According to the optical properties measurement method related to the modified example described so far, besides being able to obtain an equivalent effect as in the embodiment previously described, it becomes possible to suppress a focus error (an error included in the computation results of the best focus position) depending on a particular position on the wafer, a focus error caused by dust and the like.
Incidentally, in the modified example, while the case has been described where the exposure condition changed on transferring the pattern used for measurement was the position (focus position) of wafer WT in the optical direction of projection optical system PL, as well as this, the exposure condition described above can include exposure amount (dose) as well as the focus position. In this case, prior to deciding the best focus position, before the decision of the best focus position, it is necessary to decide the optimum dose, for example, by selecting a focus curve (for example, curve c) having an ideal chevron shape from a plurality of focus curves for each dose in
Further, in the modified example described above, while the image of pattern MPn used for measurement was transferred onto each divided area by scanning exposure, static exposure can be used instead of scanning exposure, and in this case as well, the step pitch is to be set in a similar manner.
Incidentally, in the embodiment described above, while the image formed in each divided area on the wafer was picked using the alignment system of the exposure apparatus, besides the exposure apparatus, for example, an optical inspection equipment can also be used.
To confirm the efficiency of the present invention, example 1 related to an aerial image computation (simulation) that the inventor et al., performed will be described here.
In the example, as the exposure condition serving as a premise, an exposure wavelength of 193 nm, projection lens NA=1.30, and a cross-pole illumination condition of azimuth polarization were used. This illumination condition was set with an aperture stop plate whose outer diameter σ=0.95, inner diameter σ=0.75 and has four apertures (angle of view, or in other words, central angle of 35 degrees) which are placed at an angle interval of 90 degrees, as shown in
Under such preconditions, an aerial image intensity distribution of various types of marks used for measurement is obtained changing the focus, and in the case the image intensity is smaller than the slice level at the time of image shape evaluation, assuming that a resist having a thickness proportional to the image intensity difference remains, a total sum (corresponding to the volume of the residual resist) of the thickness of the residual resist at each point inside the mark area was computed. And, using the computation results, focus dependence of the residual resist volume that accompanies the focus change is obtained, and based on the results, when a value of the residual resist volume that becomes maximum is to be 1, focus positions on both the + and − sides of focus positions corresponding to the maximum value 1 whose relative values are 0.8 are obtained, and a point corresponding to the midpoint of such focus positions is decided as the best focus position.
Further, under the conditions described above, as a result of obtaining a focus position where contrast of an image of a vertical L/S pattern (ratio of the width of the line section and the space section is 1:1) having a linewidth of 45 nm becomes maximum, the focus position was computed to be +10.5 nm. Incidentally, the vertical L/S pattern refers to an L/S pattern whose periodic direction is in the X-axis direction.
In the example, in a half tone reticle having a transmissivity of 6%, 4 marks MM1, MM2, MM3, and MM4 were used which are vertical L/S patterns (mark) having a linewidth of 45 nm and a length of 6 μm in a light-transmitting section, and whose number of lines are 33 (width, 2.925 μm), 22 (width, 1.935 μm), 16 (width, 1.935 μm), and 11 (width, 0.945 μm) as shown in
As a comparative example, as shown in
In this manner, astigmatism amount whose dependence on exposure amount and on the number of marks is extremely small and the offset (deviation) between the dense line image and the focus position where the image contrast becomes maximum is extremely small becomes possible.
Next, as example 2, a case will be described when double exposure (trim exposure) is performed to remove information on both ends in the non-measurement direction of a dense line mark (a mark consisting of a L/S pattern).
In this example, as shown in
And, as shown in
Incidentally, in the embodiment and the modified example described above, while the pattern image on the wafer was detected using a measurement device (alignment system AS, an inspection equipment 2000) of an imaging method, the measurement device is not limited to a device whose light receiving element (sensor) is an imaging device such as the CCD, and for example, can include a line sensor and the like.
In this case, the line sensor can be one-dimensional. Further, in the embodiment described above, while positional information of wafer stage WST was measured using an interferometer system (26), as well as this, for example, an encoder system can be used which irradiates a measurement beam on a scale (diffraction grating) provided on one of an upper surface of wafer stage WST and outside of wafer stage WST, and includes a head receiving a reflected light (diffraction light) provided on the other of the upper surface of wafer stage WST and outside of wafer stage WST. In this case, the system is preferably a hybrid system that is equipped with both an interferometer system and an encoder system, and it is desirable to perform calibration of the measurement results of the encoder system, using the measurement results of the interferometer system. Further, the interferometer system and the encoder system can be switched, or both of the systems can be used when performing position control of the wafer stage.
Further, in the embodiment and the like previously described, while the best focus position, curvature of image plane, and astigmatism were obtained as optical properties of the projection optical system, the optical properties are not limited to these, and other aberrations can also be obtained. Furthermore, the exposure apparatus of the embodiment described above is not limited to the exposure apparatus used for producing semiconductor devices, and can also be an exposure apparatus used for manufacturing other devices such as, for example, a display (liquid crystal display), an imaging device (a CCD), a thin film magnetic head, a micromachine, a DNA chip and the like.
Incidentally, in the embodiment above, while a transmissive type mask (reticle), which is a transmissive substrate on which a predetermined light shielding pattern (or a phase pattern or a light attenuation pattern) is formed, is used, instead of this mask, as is disclosed in, for example, U.S. Pat. No. 6,778,257, an electron mask (which is also called a variable shaped mask, and includes, for example, a DMD (Digital Micromirror Device) that is a type of a non-emission type image display device (spatial light modulator) or the like) on which a light-transmitting pattern, a reflection pattern, or an emission pattern is formed according to electronic data of the pattern that is to be exposed can also be used. Further, the projection optical system is not limited to a dioptric system, and can also be a catadioptric system or a catoptric system, and is also not limited to a reduction system, and can be an equal magnifying system or a magnifying system. Furthermore, the projection image by the projection optical system can be either an inverted image or an upright image. Further, as disclosed in, PCT International Publication No. 2001/035168, the embodiment described above can also be applied to an exposure apparatus (a lithography system) which forms a device pattern on wafer W by forming interference fringes on wafer W. Moreover, as disclosed in, for example, U.S. Pat. No. 6,611,316, the embodiment described above can also be applied to an exposure apparatus that synthesizes two reticle patterns via a projection optical system on a wafer, and almost simultaneously performs double exposure of one shot area on the wafer by one scanning exposure. The point is that the embodiment above can be applied as long as the exposure apparatus exposes an object by generating a pattern image used for measurement within the exposure area of the optical system.
Incidentally, in the embodiment described above, the sensitive object (substrate) subject to exposure on which an energy beam (illumination light IL and the like) is irradiated is not limited to a wafer, and may be another object such as a glass plate, a ceramic substrate, a mask blank or the like, and its shape is not limited to a round shape, and can also be a rectangle.
Incidentally, the disclosures of all publications, the PCT International Publications, the U.S. patent application Publications and the U.S. patents that are cited in the description so far related to exposure apparatuses and the like are each incorporated herein by reference.
Semiconductor devices are manufactured going through the steps; a step where the function/performance design of the wafer is performed, a step where a reticle based on the design step is manufactured, a step where a wafer is manufactured using silicon materials, a lithography step where the pattern of the reticle is transferred onto the wafer by the exposure apparatus in the embodiment previously described, a device assembly step (including processes such as a dicing process, a bonding process, and a packaging process), inspection steps and the like. In this case, in the lithography step, because the device pattern is formed on the wafer by executing the exposure method previously described using the exposure apparatus in each of the embodiments previously described, a highly integrated device can be produced with good productivity.
While the above-described embodiment of the present invention is the presently preferred embodiment thereof, those skilled in the art of lithography systems will readily recognize that numerous additions, modifications, and substitutions may be made to the above-described embodiment without departing from the spirit and scope thereof. It is intended that all such modifications, additions, and substitutions fall within the scope of the present invention, which is best defined by the claims appended below.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-261636 | Nov 2009 | JP | national |
This non-provisional application claims the benefit of Provisional Application No. 61/272,954 filed Nov. 23, 2009, the disclosure of which is hereby incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61272954 | Nov 2009 | US |
