Abstract:
A superconductor-based modulator includes a superconductor coupled to a cold reservoir to receive extreme ultra-violet (EUV) light beams. The light beams are modulated by altering transmission and reflection properties of the superconductor. Magnetic field, temperature, or a combination of both, may be used to control the superconducting properties of the superconductor. The modulator may perform temporal, spatial, and grey-scale modulations. The surface of the superconductor may be patterned with heat or infrared radiation to facilitate light focusing.

Description:
BACKGROUND  
       [0001]     1. Field  
         [0002]     Modulating an extreme ultraviolet (EUV) light.  
         [0003]     2. Background  
         [0004]     The extreme ultraviolet (EUV) light commonly refers to the region of spectrum having a wavelength of approximately 13.5 nm. One application of the EUV is in photolithography where reduced feature size (the critical dimension) of a circuit is desired. The EUV may be used for patterning; that is, creating a circuit design by projecting the EUV light on a wafer covered by a mask. The mask blocks the EUV light from entering the covered area. Thus, selective areas of the wafer may be etched according to the circuit design.  
         [0005]     As the EUV is highly reflective, conventional system including lens and quartz cannot be used to direct the projection of the EUV light. Thus, EUV mirrors are generally used. Presently, EUV mirrors are made of machined flat materials, usually diamond turned, which are coated with a material with a high index of refraction at 13.5 nm. Ruthenium is commonly used as a coating material, as are stacks of materials that have alternating high and low refractive indices in the EUV region of the spectrum. Examples are MoRu and Be multilayers, which has been disclosed by, for example, J. F. Seely et al. in the article “High-Efficiency MoRu—Be Multilayer-Coated Gratings Operating near Normal Incidence in the 11.1-12.0 nm Wavelength Range,” published by Applied Optics, vol. 40, No. 31, pp. 5565-5574. The EUV mirrors are typically used at grazing incidence to enhance the reflection coefficient.  
         [0006]     In photolithography, the EUV may be used not only to pattern a wafer with a mask, but also to write the mask. EUV light beams may be modulated such that selective portions of the beams may be projected onto a wafer while the rest of the beams may be directed away from the wafer. As such, the projected beams form a mask pattern on the wafer. Due to the short wavelength, EUV light beams cannot be easily modulated. Conventional mechanical shutters typically have slow modulation speed and therefore do not achieve good performance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     Embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.  
         [0008]      FIG. 1  shows an embodiment of a modulator using electromagnets to control the superconductivity of a superconductor layer on top of the modulator;  
         [0009]      FIG. 2  is a diagram showing a characteristic curve for a typical type II superconductor as a function of magnetic field and temperature;  
         [0010]      FIG. 3  shows a top view and a side view of another embodiment of a modulator having an embedded microheater array for controlling the superconductivity of the superconductor layer;  
         [0011]      FIG. 4  shows modulating EUV light beams across the surface of the modulator of  FIG. 3 ;  
         [0012]      FIG. 5  shows yet another embodiment of a modulator patterned by infrared radiation to form a zone plate; and  
         [0013]      FIG. 6  shows a mask-writing system using a superconductor-based modulator to pattern a mask.  
     
    
     DETAILED DESCRIPTION  
       [0014]      FIG. 1  shows an embodiment of a superconductor-based modulator system  10  using electromagnets to control the modulation of light. System  10  includes a superconductor-based modulator  11  having a layer of a type II superconductor in the form of a bulk slab or a thin superconducting film  12  coated on a machined mirror surface. The mirror surface may be flat or shaped suitably for focusing light, e.g., curved or concave. A cold reservoir  13  is thermally coupled to superconducting film  12  to maintain the temperature of the superconductor in a superconducting temperature range. Cold reservoir  13  may be a bulk slab of any thermal conductive material, e.g., copper, or a container of refrigerant, e.g., liquid nitrogen. System  10  may also include an extreme ultraviolet (EUV) light source  17  for generating EUV light beams. The EUV light beams may be projected onto the surface of superconducting film  12  at any angle, without being blocked by the electromagnets. In the embodiment as shown, the incidence angle is 80° with respect to the surface normal. Although the EUV is described in the following discussion, it is understood that modulator  11  may be used to modulate any light beams having a similar wavelength as the EUV, e.g., the X ray.  
         [0015]     System  10  may include a switching device to control or switch the superconductivity of superconducting film  12 . The switching device may be one or more pairs of electromagnets (e.g., coils  15 ) to generate a magnetic field (B-field). Coils  15  may be positioned, without blocking the incident EUV light, in close proximity (e.g., a few centimeters) to the upper and lower sides of modulator  11  to increase magnetic coupling between the magnets and modulator  11 . Alternatively, system  10  may include one or more coils positioned below modulator  11  without any coil above the modulator to avoid blocking the incident EUV light. A ferrite core  19  may be inserted into each coil  15  to increase the concentration of the magnetic field. Each coil  15  is electrically coupled to a current controlling device to control the amount of current passing through coil  15  which determines the magnetic field generated by the coil.  
         [0016]     In one embodiment, superconducting film  12  may be a high temperature type II superconductor, e.g., Yba 2 Cu 3 O 7-x  (hereinafter “YBCO”). Type II superconductors have the property of excluding magnetic field lines below a certain threshold. Type II superconductors are highly reflective in a superconducting mode, but become transmissive in a non-superconducting mode in a strong magnetic field or high temperature. To prepare the YBCO, the following reaction or a similar reaction may be used in a simple laboratory: 
 
0.5Y 2 O 3 +2BaCo 3 +3CuO=Yba 2 Cu 3 O 6.5 +2CO 2  
 
         [0017]      FIG. 2  is a diagram showing a characteristic curve  20  for a typical type II superconductor (e.g., YBCO) as a function of magnetic field and temperature. Curve  20  defines a superconducting mode  22  and a non-superconducting mode  23 . Bc represents the critical (or maximum) magnetic field that the superconductor is able to withstand at absolute zero temperature (0° Kelvin) before becoming non-superconducting. Similarly, Tc represents the critical (or maximum) temperature at zero magnetic field that the superconductor is able to withstand before becoming non-superconducting. The superconductivity of a superconductor is “quenched” when the superconductor becomes non-superconducting. Curve  20  also indicates that, as temperature increases, the Bc required for quenching the superconductivity decreases.  
         [0018]     In superconducting mode  22 , the superconductor reflects impinging light like a mirror. In non-superconducting mode  23 , the superconductor allows light to enter and pass though. Thus, the reflected light beams may be turned on and off as the superconductivity of superconducting film  12  changes.  
         [0019]     Referring back to  FIG. 1 , in operation, the amount of current passing through coils  15  determines the strength of the magnetic field received by modulator  11 . By changing the amount of current and hence the magnetic field, the reflectivity of superconducting film  12  changes to direct the incident EUV light into two different directions. Superconducting film  12  either reflects the EUV light like a mirror when the magnetic field is lower than a critical magnetic field, or allows the light to pass through when the magnetic field is stronger than the critical magnetic field.  
         [0020]     Modulator  11  may modulate incident light beams temporally, spatially, or a combination of both. Temporal modulation may be achieved by a temporal variation of the current flowing through coils  15 . Spatial modulation may be performed by using a plurality of coils  15  positioned across the surface of modulator  11 . Each coil  15  may be provided with an individually adjustable amount of current to vary the magnetic field distributed across the modulator surface. The plurality of coils  15  may be positioned on the upper, lower, or both sides of modulator  11 . Thus, the EUV light may be modulated by the entire surface area of modulator  11  to form a modulated spatial pattern.  
         [0021]     Modulator  11  may perform grey-scale modulation. When the magnetic field is high but before the superconductivity is quenched, superconducting film  12  may have reduced reflectivity to reflect a reduced amount of a light beam. Thus, the spatial pattern created by the modulated light may have some bright portions and some dim portions. In photolithography, the thickness of a mask may be controlled by the brightness of the reflected beams.  
         [0022]     In alternative embodiments as discussed below, the modulation of the EUV light may be controlled by temperature or a combination of magnetic filed and temperature. That is, the switching device may be implemented with elements other than coils  15  and may generate heat instead of magnetic fields. It is understood that these alternative embodiments have the same capability as modulator  11  with respect to temporal, spatial, and grey-scale modulations.  
         [0023]      FIG. 3  shows another embodiment of a superconductor-based modulator  31  that uses temperature to control the modulation of the EUV. Top view  301  and side view  305  show modulator  31  as viewed from the top and from the side, respectively. Compared to modulator  11 , modulator  31  has additional heating elements that serve to switch or control the superconductivity of superconducting film  12 . Below superconducting film  12  is an array of microheaters  35  for heating local parts of the film. In the embodiment as shown, each microheater  35  is formed by a winding strip of heat conductive material, e.g., metal, which includes heat resistive tracks for generating heat as current flows through. Alternative heating elements having other shapes or based on different heating principles may also be used. Microheaters  35  may be coupled to a current controlling device for controlling the amount of current passing through the microheaters. The amount of current determines the amount of heat generated. The current passing through each microheater  35  may be individually controlled to adjust the temperature distribution across the surface of modulator  31 . Microheaters  35  when heated above a critical temperature are able to quench the superconductivity of superconducting film  12 . The portion of superconducting film  12  heated above the critical temperature cannot reflect light but instead transmits light. By selectively heating parts of all of microheaters  35 , the EUV light beams may be modulated by the entire surface of superconducting film  12  as shown in  FIG. 4 .  
         [0024]     Modulator  31  may alternatively use heating elements arranged in the form of gratings, holograms, zone plates, or other suitable arrangements, to allow diffractive focusing or beam manipulation of the EUV light. The heating elements may be embedded under the superconducting film  12 .  
         [0025]      FIG. 5  shows an alternative embodiment in which superconducting film  12  is heated by an infrared radiation (IR) source  52  projecting the IR thereon. The IR heats up superconducting film  12 , raising the temperature above the superconducting transition and thus making a low-reflectivity region of the superconducting material. The IR may create a heated pattern on the surface of superconducting film  12  in the form of gratings, holograms, zone plates, or other suitable shapes, to create similar effects as those created by the equivalent embedded heating elements.  
         [0026]     In the embodiment as shown in  FIG. 5 , the IR patterns the surface of modulator  51  to form a zone plate  53 . Dark area of zone plate  53  represents the heated region of superconducting film  12  which does not reflect light, and the light region represents the portion of the film capable of reflecting light. EUV light beams reflected by zone plate  53  may create a more focused projection than without the zone plate. The same zone plate  53  may be similarly created by embedded heating elements.  
         [0027]     In an alternative embodiment, modulators  31  or  51  may be placed in a magnetic field, using a combination of magnetic field and temperature to change the superconductivity of superconducting film  12 . The magnetic field may be generated by coils  15  as shown in  FIG. 1 , or other suitable means. Parts of superconducting film  12  may be heated by embedded heating elements or IR patterns. As indicated in  FIG. 2 , the critical magnetic field decreases as temperature increases. Thus, less magnetic field is required to quench the superconductivity of superconducting film  12  when the temperature is slightly raised.  
         [0028]      FIG. 6  is a mask-writing system  60  including an EUV source  61 , a superconductor-based modulator  62 , and a wafer  63 . Modulator  62  may be any of modulator  11 ,  31 ,  51  as described above, or any modulating device that modulates light by controlling the superconductivity of a superconductor disposed thereon. Modulator  62  receives EUV light beams projected from EUV source  61  and reflects portions of the light beams onto wafer  63  to write an optical mask pattern thereon. The optical mask has a similar effect on wafer  63  as a physical mask of photo-resist does in conventional photolithography. The wafer  63  area receiving the reflected light may be etched away. The wafer  63  area not exposed to the reflected light remains inactive. Some of the reflected light beams may be weaker than others, as a result of grey-scale modulation as mentioned above. Weak reflected light beams may produce a shallower etch than stronger light beams. Thus, a circuit pattern may be formed by the reflection of modulator  62 .  
         [0029]     In the foregoing specification, specific embodiments have been described. It will, however, be evident that various modifications and changes can be made thereto without departing from the broader spirit and scope of the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.