Patent Application
20080148875
The present invention relates to a method of inspection usable, for example, in the manufacture of devices by a lithographic technique and to a method of manufacturing devices using a lithographic technique. The present invention is also applicable to any automated system that scans a surface to be worked on in a two-dimensional plane, such as a metalworking tool or an etching system.
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 integrated circuits (ICs). In that instance, 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 IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which 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. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, one or more parameters of the patterned substrate are typically measured, for example the overlay error between successive layers formed in or on the substrate. There are various techniques for making measurements of the microscopic structures formed in a lithographic process, including the use of a scanning electron microscope and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and one or more properties of the scattered or reflected beam are measured. By comparing one or more properties of the beam before and after it has been reflected or scattered by the substrate, one or more properties of the substrate may be determined. This may be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with a known substrate property. Two main types of scatterometer are known. A spectroscopic scatterometer directs a broadband radiation beam onto the substrate and measures the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. An angularly resolved scatterometer uses a monochromatic radiation beam and measures the intensity of the scattered radiation as a function of angle. An ellipsometer measures polarization state.
Several types of measurement are typically made of the substrate using a scatterometer. A first measurement is that of the alignment of the substrate with respect to a reference outside the substrate. A second type of measurement is an overlay measurement which measures overlay between different layers on the substrate. A further measurement includes the reconstruction of the profile of a target with a known shape in order to determine discrepancies in shape formation of a structure on the substrate surface. These and other measurements require the use of a target on the substrate surface.
Generally, a plurality of targets are used in measurements on the substrate in order to be able to obtain an average or a trend over the plurality of targets. Each of the targets may, for example, comprise a grating with the bars of the grating oriented in different directions in order to obtain alignment or overlay or profile data in a number of degrees of freedom.
An example of a substrate W containing a series of targets 200 is shown in
In order to move between the targets, the substrate is typically moved below a static sensor system. There are two drive systems that move the substrate—a first drive system moves the substrate in the X direction and a second drive system moves the substrate in the Y direction. In a further embodiment, the sensor system is moved in a first direction (e.g. along the X-axis) and the substrate is moved in the perpendicular axis (the Y-axis). In either embodiment, when moving from target to target in a meander fashion as shown in
By using only one of the drive systems at a time, only 50% of the power available to reposition the target is used. In other words, by having a dormant drive system during any of the movements 100, the power available from that dormant drive system is not being put to use, thus making the overall system not 100% efficient. Assuming the drive systems in both the X and Y directions are equivalent, a full 50% of the potential power is left unused per movement 100.
Furthermore, each measurement target is typically made up of a plurality of smaller targets, between which the drive systems also cause the measurement beam to travel. The path traveled between the smaller (sub-) targets is known as a microstep. By having the sub-targets also arranged in a lattice, this may not make optimum use of available substrate “real estate”, as there may be a large amount of useless space between these sub-targets.
It is desirable, for example, to provide a system that enables the use of as much of the power of the drive system(s) as possible when moving from one target to the next on a substrate.
According to an aspect of the invention, there is provided a method of measuring a property of a substrate, in a system that comprises at least two drive systems configured to drive the substrate in substantially perpendicular drive directions, the substrate comprising at least two measurement targets for consecutive measurement, the method comprising:
positioning the substrate such that a most economical path between consecutive measurement targets is at an acute angle with respect to at least one of the substantially perpendicular drive directions of the at least two drive systems; and
driving the substrate using both drive systems simultaneously such that their net movement moves the substrate along the angle.
There may alternatively be only one measurement target and the most economical path is from a starting point to the one measurement target. By “substantially perpendicular”, it should be understood that the drive directions may be at 90 degrees to each other, or they may be at any non-zero angle that allows movement of the substrate in any direction such that the measurement targets on the substrate may be visited in turn by a measurement radiation beam.
According to another aspect of the invention, there is provided a loading apparatus for loading a substrate onto a substrate table for subsequent measurement, the substrate table being drivable by at least two drive systems with substantially perpendicular drive directions, the loading apparatus comprising:
a detector configured to detect a marker on the substrate indicating the X-Y axes of the substrate, the X-Y axes being determined by the relative position of measurement targets on the substrate; and
a positioning mechanism configured to position the substrate on the substrate table such that the X-Y axes of the substrate are at an acute angle with respect to at least one of the substantially perpendicular axes of the at least two drive systems.
According to yet another aspect of the invention, there is provided an inspection apparatus, lithographic apparatus or lithographic cell configured to measure a property of a substrate comprising:
a substrate table configured to hold a substrate;
a first drive system configured to drive the substrate table in a first direction;
a second drive system configured to drive the substrate table in a direction substantially perpendicular to the first direction; and
a loader configured to load a substrate onto the substrate table such that a most economical (e.g., shortest) direction of travel between at least a first measurement target and second measurement target on the substrate is at an acute angle with respect to at least one of the drive directions of the first and second drive systems.
According to a further aspect of the invention, there is provided a substrate for use in an inspection apparatus configured to measure a property of the substrate (e.g., in order to determine the accuracy of printing on the substrate), the substrate comprising a measurement target configured to redirect a measurement radiation beam, the measurement target comprising a plurality of sub-targets, the sub-targets being substantially circular and being arranged such that the sub-targets are packed as closely as possible to each other within the measurement target. The sub-targets are, of course, not limited to a square or circle shape. They can be any shape that can be packed closely while containing sufficient space for radiation redirecting structures. Possible shapes may be based on rectangles and hexagons, for instance.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:
a depicts a lithographic apparatus;
b depicts a lithographic cell or cluster;
a schematically depicts a lithographic apparatus. The apparatus comprises:
an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or EUV radiation);
a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters;
a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and
a projection system (e.g. a refractive projection lens system) PL configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.
The illumination system may 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 support structure 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 can use mechanical, vacuum, electrostatic or other 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 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.
The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.
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 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 be of a type having two (dual stage) or more substrate tables (and/or two or more support structures). In such “multiple stage” machines the additional tables and/or support structures may be used in parallel, or preparatory steps may be carried out on one or more tables and/or support structures while one or more other tables and/or support structures are being used for exposure.
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. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask 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 to
The illuminator IL may 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 may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.
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. Having traversed the patterning device MA, the radiation beam B passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder, 2-D 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 support structure 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 support structure 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 support structure MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PL. 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 support structure 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.
As shown in
In order that the substrate that is exposed by the lithographic apparatus is exposed correctly and consistently, it is desirable to inspect an exposed substrate to measure one or more properties such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. If an error is detected, an adjustment may be made to an exposure of one or more subsequent substrates, especially if the inspection can be done soon and fast enough that another substrate of the same batch is still to be exposed. Also, an already exposed substrate may be stripped and reworked—to improve yield—or discarded—thereby avoiding performing an exposure on a substrate that is known to be faulty. In a case where only some target portions of a substrate are faulty, a further exposure may be performed only on those target portions which are good. Another possibility is to adapt a setting of a subsequent process step to compensate for the error, e.g. the time of a trim etch step can be adjusted to compensate for substrate-to-substrate CD variation resulting from the lithographic process step.
An inspection apparatus is used to determine one or more properties of a substrate, and in particular, how one or more properties of different substrates or different layers of the same substrate vary from layer to layer and/or across a substrate. The inspection apparatus may be integrated into the lithographic apparatus LA or the lithocell LC or may be a stand-alone device. To enable most rapid measurements, it is desirable that the inspection apparatus measure one or more properties in the exposed resist layer immediately after the exposure. However, the latent image in the resist has a very low contrast—there is only a very small difference in refractive index between the part of the resist which has been exposed to radiation and that which has not—and not all inspection apparatus have sufficient sensitivity to make useful measurements of the latent image. Therefore measurements may be taken after the post-exposure bake step (PEB) which is customarily the first step carried out on an exposed substrate and increases the contrast between exposed and unexposed parts of the resist. At this stage, the image in the resist may be referred to as semi-latent. It is also possible to make measurements of the developed resist image—at which point either the exposed or unexposed parts of the resist have been removed—or after a pattern transfer step such as etching. The latter possibility limits the possibility for rework of a faulty substrate but may still provide useful information, e.g. for the purpose of process control.
Another scatterometer SM2 that may be used with an embodiment of the invention is shown in
A reference beam is often used, for example, to measure the intensity of the incident radiation. To do this, when the radiation beam is incident on the partially reflective surface 16 part of it is transmitted through the surface as a reference beam towards a reference mirror 14. The reference beam is then projected onto a different part of the same detector 18.
One or more interference filters 13 are available to select a wavelength of interest in the range of, say, 405-790 nm or even lower, such as 200-300 nm. The interference filter(s) may be tunable rather than comprising a set of different filters. A grating could be used instead of or in addition to one or more interference filters.
The detector 18 may measure the intensity of scattered radiation at a single wavelength (or narrow wavelength range), the intensity separately at multiple wavelengths or the intensity integrated over a wavelength range. Further, the detector may separately measure the intensity of transverse magnetic—(TM) and transverse electric—(TE) polarized radiation and/or the phase difference between the transverse magnetic- and transverse electric-polarized radiation.
Using a broadband radiation source 2 (i.e. one with a wide range of radiation frequencies or wavelengths—and therefore of colors) is possible, which gives a large etendue, allowing the mixing of multiple wavelengths. The plurality of wavelengths in the broadband desirably each has a bandwidth of δλ and a spacing of at least 2δλ (i.e. twice the wavelength bandwidth). Several “sources” of radiation may be different portions of an extended radiation source which have been split using, e.g., fiber bundles. In this way, angle resolved scatter spectra may be measured at multiple wavelengths in parallel. A 3-D spectrum (wavelength and two different angles) may be measured, which contains more information than a 2-D spectrum. This allows more information to be measured which increases metrology process robustness. This is described in more detail in U.S. patent application publication no. US 2006-0066855, which document is hereby incorporated in its entirety by reference.
The target on substrate W may be a grating which is printed such that after development, the bars are formed of solid resist lines. The bars may alternatively be etched into the substrate. The target pattern is chosen to be sensitive to a parameter of interest, such as focus, dose, overlay, chromatic aberration in the lithographic projection apparatus, etc., such that variation in the relevant parameter will manifest as variation in the printed target. For example, the target pattern may be sensitive to chromatic aberration in the lithographic projection apparatus, particularly the projection system PL, and illumination symmetry and the presence of such aberration will manifest itself in a variation in the printed target pattern. Accordingly, the scatterometry data of the printed target pattern is used to reconstruct the target pattern. The parameters of the target pattern, such as line width and shape, may be input to the reconstruction process, performed by a processing unit, from knowledge of the printing step and/or other scatterometry processes.
As discussed above, when scatterometry is used to measure a substrate surface, several targets (or at least sub-targets of a single target) are generally used. These targets will have different orientations, positions or relative sizes so that variations in the target characteristics can be measured and a complete picture of various parameters can be determined as described briefly above. The substrate is moved underneath, for example, an alignment beam using two drive systems. The first drive system will drive the substrate in the X direction and the second drive system will drive the substrate in a (substantially perpendicular) Y direction. The targets on the surface of the substrate are generally arranged in a grid aligned with the X- and Y-axes. In order to travel between the targets, therefore, one drive system operates at a time. As shown in
The way that the drive systems are optimized (i.e. both used to their maximum capability rather than sitting idle while the other works) in an embodiment of the present invention is by turning the axis of the substrate by, for example, 45°. (The actual angle will have other parameters affecting it, as discussed below.). Specifically, the axis of the substrate W will be turned by 45° with respect to the drive axes of the drive systems PW if both drive systems have the same power output. This way, to move in the Y-direction, both drive systems will be working simultaneously. If both use their maximum power output, the speed of the movement of the substrate will be significantly greater than if only a single drive system is putting in the power and doing the work.
In fact, the angle of rotation of the axis of the substrate will vary depending on the relative power of both drive systems. If one drive system is more powerful than the other, to optimize the use of their power, the angle will be greater away from the direction of the stronger drive system so that its greater power compensates for the drive system with the lesser power. In this way, a weaker system may also be compensated for. As a specific example, if the drive system on the Y-axis is twice as powerful as the drive system on the X-axis, the substrate axis would be turned to, say, 22.5° from the X-axis (ignoring other parameters that affect the angle such as the mass of the substrate being moved as discussed below) so that both X- and Y-axis drive systems operating at maximum power would move the substrate in a straight line on the 45° axis between the X- and Y-axes.
A loader RO′ such as shown in
The power ratings of the drive systems are measured by the maximum velocity and maximum acceleration of the X and Y drive systems. The drive systems also have a finite settling time (because of, for example, the momentum of the substrate, which is dependent on its mass), which is a combination of deceleration time and time spent regaining stability. If the X and Y drive systems have different power ratings or require different settling times, the angle of rotation of the axis of the substrate W can be accordingly adapted. The best angle may be found by simulating or measuring the total travel time for a typical meander pattern for a number of different angles, and picking that angle which gives the shortest total travel time.
Referring to
Similarly, when each target 200 comprises multiple sub-targets 210, 220 that are visited separately as shown in
According to an example, there may be 36 targets 200 on a substrate surface. Assuming a meander pattern provides the most economical, i.e. shortest, path, a known design requires approximately 25 seconds per substrate. Using a standard drive system, each step takes approximately 0.5 seconds, for a total travel time of 17.5 seconds (with 35 steps between the 36 targets).
With the axes rotated 45° relative to the substrate and a lattice of targets, double the power is available (from double the number of drive systems). The travel speed will increase by a factor of √{square root over (2)}, so the total stepping time is reduced to 12.4 seconds. The gain of 5 seconds (17.5 minus 12.4) represents a 20% increase in substrate throughput compared to the standard drive system.
Further, the detector or sensor that is receiving the measurement beam once it has been redirected (e.g., reflected, refracted, scattered, etc.) from the measurement target can be rotated at the same angle as the substrate so that the redirected beam does not need to undergo any adjustment before being detected.
An embodiment of the invention may be applied to sub-targets 210, 220, 230 within a measurement target. The substrate is moved beneath a measurement beam in such a way that sub-targets within a target as shown in
Each sub-target itself comprises a grid which is scanned with parallel sweeps in a specific direction. In the standard geometries shown in
As mentioned above, the axis of the sub-targets can be rotated (e.g., by rotating the sub-targets, applying the sub-targets in a rotated manner, having the drive systems arranged in a rotated manner, or any combination thereof) in order to use both drive systems to their maximum capacity.
The sub-targets do not have to be square as illustrated in
The honeycomb lattice of the sub-targets works particularly well at a crossing of two scribelanes as shown in
In Table 1, calculated throughput is compared with a nominal microstep time of 50 ms with an arrangement as shown in
Advantages of the above include: throughput of the substrates is increased and/or scribelane real estate is decreased. The extra available real estate may be used to add more measurement targets on the substrate, or to increase the target size to improve measurement accuracy and quality. The saved time traveling from target to target and sub-target to sub-target may be reinvested in other time-critical tasks, such as signal measurement, signal acquisition or to improve substrate throughput.
With regard to throughput, it is assumed that a microstep 110 that aligns with the direction of the sub-target grid does not need the full microstep time of 50 ms, but can be completed in 49.5 ms. This is because any residual motion of the substrate will be in the direction of the sub-target grid pattern and would not be visible in a sensor output.
Furthermore, it is assumed that a 40% increase in speed of the movement of the substrate, such as can be achieved by rotating the axis of the substrate relative, translates into a reduction in microstep time of about 1 ms. The reason this is a relatively small gain lies in the fact that most of the 50 ms micro step time is spent on settling.
The scribelane is an area that is useful for varying types of targets, markers and test structures. In Table 1 above, the real estate is measured by linear stretch of the scribelane used.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin film magnetic heads, metalworking, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may 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. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.
While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein.
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.
