Patent Application
20080062385
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 schematically depicts a more detailed view of a first embodiment of the polarization measurement assembly of
b depicts a graph showing a typical result of a polarization measurement with the polarization measurement assembly of
a schematically depicts a more detailed view of a second embodiment of a polarization measurement assembly of
b depicts a graph showing a typical result of a polarization measurement with the polarization measurement assembly of
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 supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern 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 mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables 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 mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in
The depicted apparatus could be used in at least one of the following modes:
1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of an exposure field limits the size of the target portion C imaged in a single static exposure.
2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C. (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.
3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.
Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.
In lithography, a higher resolution may be obtained if the numerical aperture (NA) of the projection system PS increases. However, increasing the NA of the projection system typically results in having large angles of incidence of an image forming diffraction orders at wafer level. This will result in strong contrast degradation when an electric field vector of diffracted light beams is not aligned perpendicular to a plane of incidence. To check whether the polarization of the light employed is correct, a polarization measurement assembly is used
Any polarization state of an illumination beam can be effectively decomposed into a component parallel to a main orientation of patterning features on a patterning device, e.g. a mask MA, and one component perpendicular to aforementioned orientation. Generally the first polarization component as mentioned is referred to as so-called “S-polarization” while the latter component generally is referred to as “P-polarization”. The two components are orthogonal to each other. In view of aforementioned discussion related to the alignment of the electric field with respect to the plane of incidence, S-polarized illumination of a patterning device may be optimized.
When two S-polarized light beams interfere, the interference will be more constructive, as is well known to persons skilled in the art. Similarly, when two P-polarized light beams interfere, the interference will be less efficient, with a lower image contrast as a result.
Embodiments of the invention are based on the insight that the different interference properties of S-polarized light and P-polarized light can be used to monitor polarization properties of an image forming light beam at the substrate level.
The detector 43 comprises an image sensor 45 in front of which a slit 46 is positioned. The slit 46 enables localized image intensity sampling. The projection optics 42 is arranged to project an image, i.e. a so-called aerial image, on the slit 46. The aerial image geometry is highly dependent on the geometry of the polarization sensitive feature 44 and the polarization state of the light at substrate level. The geometry of the slit 46 determines what part of the aerial image is measured, i.e. integrated, by the image sensor 45.
The image sensor 45 may be connected with a processor 47 and a memory 48. The processor 47 may use information obtained with the image sensor 45 to perform operations. The information may be obtained from the image sensor 45 directly or retrieved from the memory 48 in which the information obtained with the image sensor 45 may be stored.
a schematically depicts a more detailed view of an example of a polarization measurement assembly as depicted in
b depicts a graph showing a result of a polarization measurement with the polarization measurement assembly of
First, in step 61, the detector 43 is positioned at a first position with respect to the polarization sensitive feature 44. A suitable first position may be a position at which the intensity of the image at the center of the image sensor 45 corresponds to either a maximum or a minimum value of the s-polarization intensity. Consider the case that aforementioned dimension ratio of pitch d and slit width Dslit is present, i.e. d:Dslit=2:3. In that case either two peaks and one valley are sampled, as shown in
Now two different paths may be taken, denoted by conditional step 73 in
Subsequently, in step 77, the contrast value as determined is stored in memory 48 as a reference contrast value CVref. The polarization measurement assembly is now ready for monitoring. As the number of measurements now equals 1, N is raised by 1 in step 79.
Alternatively, if aforementioned measurements in steps 63 and 69 were not first measurements, i.e. N>0, the method proceeds to step 81. In this case, in step 81, again a contrast value is determined by calculation using the same formula as mentioned earlier, i.e. formula 1. Subsequently, however, in step 83, a polarization purity is established by comparing the CV as determined in step 81 with the reference CVref as stored in the memory in step 77. If further measurements are desired, steps 61 until 71 may again be performed. To keep track of the number of measurements, N may again be raised, in step 85, by 1.
The method as depicted in
Consider the following values for I1,s, I2,s, I1,p and I2,p measured at t=0 by using the method as described with reference to
Aforementioned values are stored in memory 48 as reference values. After some time, with the polarization measurement assembly of
a schematically depicts a more detailed view of a second variant of a polarization measurement assembly as depicted in
b depicts a graph showing a typical result of a polarization measurement with the polarization measurement assembly of
First, in step 91, the detector 43 is positioned at a suitable position with respect to the polarization sensitive feature 44. After positioning of the detector 43, in step 93, an intensity of the image of the polarization sensitive feature 44 is measured with respect to S-polarized light. Based on this measurement, in step 95, an integrated intensity value S is determined. Subsequently, in step 97, an intensity of the image of the polarization sensitive feature 44 is measured. However, in this step the measurement is performed with respect to P-polarized light. Based on this measurement, in step 99, an integrated intensity value P is determined. Subsequently, in step 101, a difference D between S and P may be calculated. Finally, in step 103, the determined integrated intensity values S and P, and if calculated, difference D are stored in memory 48 as Sref, Pref and Dref respectively.
Subsequently, at a moment somewhat later in time, in step 105, the intensity of the image of the polarization sensitive feature 44 is measured with respect to one of the types of radiation, e.g. S-polarized light. In this case, based on the measurement in step 105, in step 107, an integrated intensity value S1 is determined. Subsequently, in step 109, a difference Ds is calculated between the integrated intensity value S1, as determined in step 107, and the integrated intensity reference value Sref as stored in the memory in step 103. Finally, in step 111, a polarization purity change is calculated by using Ds as calculated in step 109 and Dref as stored in the memory in step 103. In order to keep track of developments, steps 105-111 can be repeated, as denoted by the dashed arrow in
It will be understood that other measurement methods may be employed. For example, instead of measuring S-polarized light, P-polarized light may be measured in step 105. In that case, an integrated intensity value P1 is determined in step 107. In yet another alternative, first the intensity of the image of polarization with respect to P-polarized light is measured and then the image of polarization with respect to S-polarized light, i.e. steps 93-95 and steps 97-99 are swapped.
Consider the following values for S1 and P1, measured at t=0 by using the method as described with reference to
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, 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.
