JP2004056125A - Reflective projection optical system with discrete actuator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a system which operates in an EUV wavelength region and controls one or more elements of adaptive optics to optimize optical performance of the system and makes aberration minimum and a measuring system suitable for controlling elements of adaptive optics. <P>SOLUTION: The optical system includes a system for measuring an aberration and varies the shape of a reflecting mirror according to the results of the measurement to correct thermal distortion. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-precision optical system using a reflective optical element, and more particularly, to a high-precision lithographic exposure system and method, and a system for reducing aberrations using an adaptive reflective optical element. And methods comprising measuring and controlling aberrations of the optical system.
[0002]
[Prior art]
In many manufacturing processes and scientific processing processes, it is desired to observe and expose at a wavelength outside the visible region using an extremely accurate and aberration-free optical system. For example, an exposure step of exposing a circuit element having a determined line width at a determined position is always required. These circuit elements are included in tens to hundreds of millions in one chip. Each circuit element is very small and is located close to each other, and is generally referred to as a highly integrated circuit element. With such a circuit element, the signal propagation time can be shortened or noise can be reduced. Further, there is an advantage of higher functionality, and in some cases, there is an effect of reducing the manufacturing cost. Such a situation is a factor that increases the pattern area having a smaller line width. This narrow linewidth pattern is created by exposing the resist. Therefore, the resolution and aberration of the exposure apparatus must be within a well-defined budget, and the budget is usually about one tenth of the minimum line width.
[0003]
Although the resolution of the projection optical system is a function of the wavelength of the energy used for exposure, it is possible to achieve an exposure resolution equal to or less than the exposure wavelength by using means such as a phase shift mask. However, in order to resolve extremely small line width patterns, it is necessary to correspondingly shorten the exposure wavelength. The use of X-rays for lithographic exposure is known, but has not been practically used. The reason is that since the size cannot be reduced by X-rays, in order to make a mask used for exposure, it is necessary to make a mask having the same line width as the finally required minimum line width. The use of an optical exposure method or an electron beam projection system allows the image to be reduced, and the reticle can have a pattern larger than the projected image.
[0004]
However, comparing the two techniques, reticles for electron beam projection systems are much more expensive than optical reticles, and more importantly, more exposure is required for electron beam exposure because it exposes the entire integrated circuit. It is necessary. This is because the exposure field at the chip is limited by the exposure field of the electron beam projection system. Accordingly, there is continued interest in optical exposure systems, and there is a trend towards using shorter wavelengths, such as EUV.
[0005]
The wavelength range of EUV light is considered to be from 12 nm to 14 nm, but in particular, the bandwidth is within 1 nm centering on 13 nm. At such wavelengths, the imaging and lens materials that are transparent in the visible light region are also opaque. Thus, the only systems being developed are reflective systems. Such a reflection system is more complicated than a lens system. The reason is that interference between the illumination system of the reticle and the projection system that irradiates the target with the projection pattern must be avoided. This is generally associated with the fact that if the number of optical elements is increased to reduce aberrations and the entire system is made a sufficiently well-corrected system, the increase in the number of elements tends to cause mutual interference. In order to ensure a high production yield in the above environment, it is necessary to not only ensure that the projection optical system has high stability but also to measure and adjust the performance of the system frequently.
[0006]
Wavefront aberration measurement techniques are well known, and can thereby accurately characterize the characteristics of a projection optical system and optical elements. However, actually making such measurements is complicated and difficult. For example, measuring properties during exposure without interfering with the exposure itself is not possible on-axis or within the exposure field. This may cast shadows or occupy the focal plane of the system where the target is located. If the measurement is performed between exposures, the characteristic during the exposure is not measured, and the lithographic projection optical system during the exposure is not characterized. However, at present, it is actually used in anticipation of errors. Optical performance generally degrades away from the optical axis of the system. As a practical matter, projecting a given image requires the use of an optical field that is sufficiently accurate overall, has a high resolution, and is aberration free. Therefore, for this purpose, measurement methods that cannot directly measure the imaging performance of optical elements and optical systems, and unpredictable measurements are excluded. However, the actual measurement system is extremely complicated, and it is not easy to measure aberration correction.
[0007]
[Problems to be solved by the invention]
Active optics are well known, but have not been used in such systems to date. The active optical system has a function of changing the overall shape or local shape of the optical element in order to change the characteristics of the optical element. John W. Hardy, "Active Optics: A New Technology for the Control of the Light" (Pro. Of the IEEE, Vol. 66, No. 6, 1978) provides an overview of this technology and provides an overview of this invention. is there. In particular, the arrangement of a mechanism for giving local deformation to a reflecting mirror as an optical element to correct, for example, fluctuations in the atmosphere is a reference. However, there are many problems with mechanical actuators (described in the above-mentioned documents) that cause deformation of a part of an optical element. For example, there are problems with stability, problems with hysteresis, and the like. When the amount of deformation of the optical element is a fraction of the ultrashort wavelength, there are some which are not preferable for control.
[0008]
Accordingly, the present invention provides a system operating in the EUV wavelength range that controls one or more adaptive optics (adaptive optics) elements to optimize system optical performance and minimize aberrations. It is aimed at.
[0009]
Another object of the present invention is to provide a measurement system suitable for controlling the above-mentioned adaptive optical element.
[0010]
[Means for Solving the Problems]
First, the gist of the present invention will be described.
[0011]
The present invention provides a projection optical system having one or a plurality of deformable reflecting mirrors used for EUV light in a wavelength range of 1 to 50 nm, and a projection optical system in which a device for operating stably is incorporated. is there. That is, an apparatus is provided which prepares a deformable optical element and continuously deforms the optical element on a surface. At this time, forces for deforming the reflecting mirror are applied at a plurality of discrete places. Thereby, the aberration of the optical system operating in the EUV wavelength region is corrected as predetermined. Aberration is measured relative to the design shape. For example, pressure is applied directly to adjacent local areas of the reflective element by a plurality of air bellows arranged behind the deformable optical element. Another method provides a temperature gradient to an optical element having a coefficient of thermal expansion by means of a plurality of thermal actuators arranged behind the deformable optical element. By controlling each of the plurality of actuators in combination, the characteristics of the optical element are corrected so as to have predetermined characteristics.
[0012]
In addition, there are measurement systems that measure aberrations to create this correction data, and control means that controls an array of actuators that deform the deformable mirror.
[0013]
Hereinafter, the present invention will be described in detail.
[0014]
Means for solving the problems described above in the present invention are:
A projection optical system for EUV light lithography having a deformable reflector and projecting a pattern on a mask onto a wafer, comprising: a measurement system for detecting aberration of the projection optical system; and a deformable reflector. Is a projection optical system having an array of actuators for deforming the actuator and control means for controlling the actuators according to the output of the measurement system.
[0015]
A projection optical system using light in the EUV region is configured by a reflecting mirror, but the reflecting mirror has large absorption. Means of the present invention measures deformation of a reflecting mirror due to heat generated by absorption, and aberration accompanying the deformation, and corrects the aberration by deforming the reflecting mirror based on the measurement result. Therefore, aberration due to heat absorption is sufficiently corrected.
[0016]
At this time, a pouch using a pressure of a fluid, particularly air, a mechanical actuator using a piezo element using an electrostrictive effect, or an actuator using thermal expansion by applying heat is used as an actuator. . Such an actuator is operationally stable, has no hysteresis, and can easily control the amount of deformation of the reflecting mirror.
[0017]
Further, at this time, the measurement system measures the aberration by arranging a light source and a sensor on the mask surface and the wafer surface of the projection optical system so as not to block the exposure light. Therefore, aberration can be measured even during exposure, and high-definition imaging characteristics can always be maintained.
[0018]
Further, the present invention is a projection exposure method,
A projection exposure method for projecting a pattern on a mask onto a wafer using light in the EUV region, wherein a projection optical system is configured by a deformable reflecting mirror, aberration of the projection optical system is measured, and a measurement result is obtained. This is a projection exposure method in which the amount of deformation of the reflecting mirror for correcting aberration is calculated based on the aberration, and the reflecting mirror is deformed based on the calculation result.
At this time, the aberration is measured by arranging the measurement light source and the measurement sensor on the mask surface and the wafer surface so as not to block the exposure light beam.
[0019]
Therefore, the aberration can be measured even during the exposure, the aberration can be corrected in real time, and the stable high-definition imaging performance can always be maintained.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to the figures, and in particular to FIG. 1 shows a reflector system according to the invention. All the optical elements of this system are reflective and are suitable for projecting EUV light. That is, the exemplary projection optics are suitable for projecting and imaging the pattern beam produced by reticle 150 onto a target such as resist coated wafer 160. It should be further noted that this projection optics is quite complex, has six reflectors, the optical path bends between each element, and is essentially an off-axis projection optics. And can cause significant aberrations. (See, eg, US Pat. No. 5,815,310 to Williamson).
[0021]
According to the present invention, it is preferable to use an adaptive optical element as the element of the projection optical system of FIG. Further, any similar system may be used as long as it is all a reflection system and projects an image using EUV light. However, it is necessary to measure system aberrations at least periodically (eg, once a week). In this way, it is necessary to adjust the adaptive optical element so as to correct the aberration so as to be within an allowable range. The measuring system according to the invention is incorporated as part of the projection optics. The wavelength range in the best mode of the present invention is the EUV range, and the wavelength range of EUV radiation is 12 nm to 14 nm. In particular, it is 13.5 nm ± 0.1 nm. At this time, the allowable amount of deformation of the reflecting mirror is about 0.3 nm to 0.5 nm when the projection optical system is composed of 4 to 6 projection optical systems.
[0022]
As the measurement system light source 105, a light source having a wavelength different from that of the exposure light (which is possible because the projection system is a reflection system, which is possible) is disposed slightly off-axis from the exposure light flux. The position is in the plane of the reticle 150. The measurement light 107 has a different axis from the exposure light, and the target / wafer is generally conjugate to the reticle. Therefore, the output measurement light 107 is located at a position different from the position of the wafer to be exposed. Is output. Therefore, placing the sensor of the measurement system at the output position and measuring the aberration during exposure does not cause a serious problem. The position of this sensor may be slightly shifted in the optical axis direction when the projection system has a curvature of field. The expression "measurement system light source" in the present invention is a concept that actually emits light, an aperture illuminated by another illumination source, and a highly reflective portion illuminated by another illumination source. As a sensor of the measurement system, for example, a point diffraction interferometer can be used. In this regard, the following documents are helpful.
G. FIG. E. FIG. Sommagren et al .: "Sub-nanometer Interferometer for Aspheric Mirror Fabrication"; 9th Int. Conf. on Production Engineering, Osaka, Japan; 8 / 30-9 / 1 1999; UCRL-JC-134763.
G. FIG. E. FIG. Somemargren: "Phase Shifting Diffraction Interferometer for Measuring Extreme Ultraviolet Optics", OSA TOPS on Extreme UltraLite96. 108-112
G. FIG. E. FIG. Somemargren et al: "100-picometer intermetry for EUVL"; Emerging Lithographic Technology VI; Proceedings of SPIE, Vol. 4688; 316
− 328
As described above, the measurement can be performed during the exposure, but may be performed between the exposures, when exchanging the wafers, or at the time of the alignment, if necessary. Further, it may be another type of sensor. Regarding the position of the sensor, an arm 106 that expands and contracts can be provided to change the position of the light source and the sensor. In this way, the characteristics of the projection optical system on the exposure light beam axis can be confirmed, and the characteristics can be calibrated on the axis and off the axis. Providing a plurality of such measurement systems around the projection beam axis or moving one system to multiple points for measurement and joining the measurement results at those points will make the characteristics of the projection optical system more accurate. I can understand.
[0023]
Regarding the processing of the measurement results, the calculation 125 is empirically derived or interpolated, since the measurement is slightly off axis of the exposure beam. The control is in the form of a look-up table (LUT), and the performance is adjusted or corrected based on the measurement result so that the optimum performance is obtained.
[0024]
The aberrations of the system are determined based on the aberrations measured by the off-axis measurement system and then designed, for example, by calculation 125, from an empirically determined look-up table (LUT) 120 to the optics of the system. An appropriate correction amount is determined and sent to the control means 115. Although these details are not important to the invention, they are suitable mechanical devices for changing the shape of the adaptive optical element 130.
[0025]
As described above, the exposure state is completely or substantially maintained even during the measurement.
[0026]
The basic details of the optical element 130 will be described with reference to FIG. FIG. 2 illustrates a cross section of the optical element. Although the surface 210 of the reflector is depicted as concave, it can be of any shape. It may be flat or convex. Similarly, the mirror surface 210 is deformable, and the deformation characteristics of the mirror depend on the type and location of the actuator 230. This is described in connection with the preferred embodiment described below. The operation of the actuator 230 is supported by a body or substrate on which the optical element 210 is mounted. Other related devices are known and preferred characteristics have been studied in a wide range, but the present invention seeks potentially high spatial frequency operation (to the extent that 10th order correction is possible). On the other hand, it is required to avoid sudden changes on the surface of the optical element, reduce the cost per operation, achieve high stability, and at the same time, reduce the holding power. Such characteristics realize high-precision, high-resolution, and aberration-free functions that control a specific small area of a reflector with a deformation amount that is a fraction of a short wavelength and adjust it at a practical cost. Needed to do so. Needless to say, there is a need for a device that does not have hysteresis or mechanical memory that causes an error in the adjustment determined empirically. To achieve this characteristic, a plurality of discrete actuators 230 are mounted on the back of the optical element. FIG. 3 shows a top view and FIG. 4 shows a side view.
[0027]
FIG. 5 is a sectional view showing an example of the configuration of the adaptive optical element of the present invention. Here, the optical element according to the invention is a relatively thin reflector 310 (not shown in scale), so that it can be deformed by a relatively small force. A thicker, stiffer substrate 320 is placed on the back of the reflector. The end of the reflecting mirror is attached to the substrate member. The attachment can be performed by, for example, bonding, bolting, and integrated manufacturing. In this embodiment, the actuator that deforms the reflector 310 is a bladder 330 placed between the reflector and the substrate. The bladder changes the force applied to the reflector by the introduction and exhaust of air therein.
[0028]
The size of the air bladder is arbitrary, but it is preferably very small, and it is preferable that the air bladders are arranged continuously on the surface for correcting the shape of the optical element. There is a small duct 340 and a valve 350, which are provided as pressure inlets to the substrate according to the size of the air bag. Preferably, these are connected to a pressurizing device or an exhaust device so that the internal pressure of each air bag can be adjusted. The air bag is a sealed bag, and is made of rubber or another elastic film having flexibility. When the preferable shape of the optical element is determined by measuring the aberration of the system as described above, the internal pressure is maintained at a predetermined pressure by a pressure controller and sealing of the inlet.
[0029]
In actual operation, it is necessary to consider errors in manufacturing, thermal deformation, deformation during mounting, and the like, but the deformation of the reflecting mirror is mapped using the measurement system as described above. The measuring means at this time is, for example, a Fizeau interferometer other than those described above. In addition, there are calculation algorithms for calculating an appropriate correction amount and calculating the pressure of each air bag for correcting distortion.
[0030]
A positive pressure can be applied by using an air bag, but vacuum evacuation and negative pressure can also be used. If it is difficult to combine negative pressure and positive / negative pressure, the reflecting mirror should be deformed in advance (ie, more concave or more convex than the preferred shape), and the necessary control can be performed only with positive pressure. There is also a way to do it. Further, the pressure of the air bag may be slightly higher than the maximum operating pressure.
[0031]
As another form, FIG. 6 shows a preferable form of the air bag when a negative pressure is used in the air bag. In this case, the bladder is partially broken, and the partial break is controlled to adjust the force applied to the reflector. As shown in FIG. 6, the bladder is adhesively attached to the reflector and the substrate, such as a rigid ring or a vertical reinforcement 370 made of a thicker bladder. Are arranged to increase the strength around the vertical direction of the air bag. When the pressure inside the bladder rises, the bladder is destroyed and the junction between the reflector and the substrate is pulled inward.
[0032]
Yet another embodiment is shown in FIG. The air bladder 330 is formed in a ring shape or a donut shape (a car tire may be considered), and a tension spring 360 is disposed at the center thereof. In this case, the internal pressure of the bladder in effect exerts forces on the springs together, thereby improving control and possibly allowing the reflector to deform without the need for negative pressure on the bladder. . In other words, in the arrangement of FIG. 7, the deformation of the reflecting mirror can be performed both inside and outside with only a positive pressure.
[0033]
Another preferred embodiment of the present invention is illustrated in FIG. In this embodiment, the optical element according to the present invention is a reflecting mirror 410, which is of a thickness that provides a small deformation, which is favorable against deformation due to thermal expansion. The flow of heat is given by a thermal actuator, which is a resistor for heating and a Peltier element for cooling. The details of these thermal actuators are not important to the invention.
[0034]
Regardless of the nature of the thermal actuator 430, the thermal actuator 430 is preferably disposed between the reflector 410 and the heat sink 420 on the back side of the reflector 410. By adopting such an arrangement, the thermal actuator 430 can control the flow of heat into and out of the reflecting mirror, and can control the thermal expansion. The plurality of kinematic mounts 450 preferably comprises three (only one is depicted in FIG. 4) and are disposed every 120 degrees around the reflector. This mounting allows the reflector to be held without transmitting stress. The dynamic mount is also preferably located near the back of the mirror. By doing so, the front surface and the reflecting surface (regardless of concave, flat, or convex) of the reflecting mirror can move more than the back surface by heating and expanding the reflecting mirror. This leaves no stress inside. That is because most of the reflectors are forward of the dynamic mounting. Of course, this point can be changed depending on the design, and the movement of the reflecting surface of the reflecting mirror can be adjusted to match the selection of the thermal actuator 430 or the thermal resolution.
[0035]
Preferably, the thermal actuators 430 are arranged in a planar array and are equidistant from each other to form the vertices of an equilateral triangle, as shown in FIG. When viewed from the triangle formed by the actuator, the back surface of the reflecting mirror is inclined. If necessary, feedback can be provided by placing the thermal sensors in a similar array plane or at the center of gravity of the triangle. However, in general, such feedback does not seem necessary. This is because it is possible to measure the aberration of the system composed of the reflecting mirror and provide feedback based on the result. Further, unless the illumination light has extremely high intensity, energy incident on the reflecting mirror hardly causes local heating of the incident portion of the optical element. This is because the reflector has a high reflectivity. The thermal expansion of the reflector is also stabilized from heat capacity and heat sink efficiency. From such a viewpoint, it is preferable that the temperature be well controlled by arranging the heat sink, whereby the reproducibility of the correction is secured and the thermal stability of the entire projection optical system is established. The arrangement of the thermal actuator is inherently stable, and in combination with feedback from a temperature sensor or / and aberration measurement, corrections and adjustments need to be made only occasionally.
[0036]
Thus, the present invention provides an actuator system for driving adaptive optics, wherein the system has no sliding members and the system is capable of moving the surface of the mirror to within a tenth of a very short wavelength of light. It can be deformed with good precision and reproducibility. It should be noted that the system of the present invention can be easily applied to an optical element including a refractive optical system. Channels for correcting the surface of the optical element can be formed easily and at low cost, and by disposing them close to each other, deformation having a high spatial frequency can be corrected.
[0037]
【The invention's effect】
By using the projection optical system and the exposure method of the present invention, even in the exposure in the EUV region, the aberration caused by the deformation of the reflecting mirror due to heat can be easily reduced to an allowable range. The apparatus for this can be implemented without any problem in terms of price and practically without any problem.
[Brief description of the drawings]
FIG. 1 shows a preferred form of the overall system when the present invention is applied to a reflective system.
FIG. 2 is a schematic view of a cross section of a preferred reflecting mirror incorporated in the present invention.
FIG. 3 is a schematic plan view showing an arrangement of actuators arranged discretely according to the present invention.
FIG. 4 is a schematic side view showing an arrangement of actuators arranged discretely according to the present invention.
FIG. 5 is an embodiment of a reflector and an actuator according to the present invention, wherein the actuator is a bladder and operates by air pressure.
FIG. 6 is an air bag with a reinforcing material, which is an actuator according to the present invention, which adjusts the force by partially destroying (a part of the number).
FIG. 7 is an actuator according to the present invention, which is a bladder with a spring, and adjusts the force only by the positive pressure of air. .
FIG. 8 is another alternative embodiment of a reflector and actuator according to the present invention, wherein the actuator uses a thermal actuator.
[Explanation of symbols]
Reference numeral 105: Measurement system light source 106: Measurement light source or sensor holding / moving arm 107: Measurement light 130: Optical element 150: Reticle 160: Wafer 210: Optical element 230 ... Actuator 310 ... Reflecting mirror 340 ... Duct 350 ... Valve 360 ... Tension spring 410 ... Reflecting mirror 420 ... Heat sink 430 ... Thermal actuator

Claims (13)

A projection optical system for EUV light lithography having a deformable reflector and projecting a pattern on a mask onto a wafer,
A measurement system for measuring the aberration of the projection optical system,
An array of actuators for deforming the deformable reflector;
Control means for controlling the actuator according to the output of the measurement system,
A projection optical system comprising: The projection optical system according to claim 1, wherein
A projection optical system, wherein the actuator is a flexible pouch driven by the pressure of a fluid. The projection optical system according to claim 2, wherein
The pouch is ring-shaped,
A projection optical system, wherein a spring is disposed in the ring perpendicular to the reflecting mirror. 4. The projection optical system according to claim 2, wherein the fluid is air and the pouch is an air bladder.
A projection optical system, characterized in that: The projection optical system according to claim 1, wherein
The actuator is a thermal actuator;
A projection optical system, characterized in that: The projection optical system according to claim 5, wherein
A projection optical system, wherein a thermal sensor is arranged corresponding to the thermal actuator. The projection optical system according to claim 5, wherein:
The thermal actuator is
A projection optical system comprising an electric resistor for heating and a Peltier element for cooling. The projection optical system according to claim 1, wherein:
The measurement system comprises:
A measurement light source located in the plane of the mask and outside the area to be exposed and illuminated,
Having a sensor located in the plane of the wafer and outside the area to be exposed,
The projection optical system, wherein the light source position and the sensor position are in a conjugate relationship in the projection optical system. 9. The projection optical system according to claim 8, wherein
A projection optical system, wherein the light sources are arranged at a plurality of different positions in the mask plane. The projection optical system according to claim 8, wherein
A projection optical system, wherein the light source is held and moved by an expanding and contracting arm. The projection optical system according to claim 9, wherein:
A projection optical system, wherein an aberration of the projection optical system at a position where the pattern is projected is obtained based on measurement data at a plurality of measurement positions. A projection exposure method for projecting a pattern on a mask onto a wafer using light in an EUV region,
The projection optical system is composed of a deformable reflecting mirror,
Measuring the aberration of the projection optical system,
Calculate the deformation amount of the reflecting mirror to correct the aberration based on the measurement result,
A projection exposure method, wherein the reflecting mirror is deformed based on a calculation result. A projection exposure method according to claim 12,
Place the measurement light source outside the exposure illumination area in the mask plane,
Place the measurement sensor outside the exposure area in the wafer plane,
A projection exposure method, wherein the aberration is measured by setting the measurement light source and the measurement sensor at conjugate positions of a projection optical system.

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