Abstract:
A detector for extreme ultraviolet (EUV) energy uses incidence reflectance of the EUV beam off the detector to both capture a small but controllable fraction of the EUV energy and to redirect most of the energy to its target. In one embodiment, a reflective coating of material on a sensor surface is used. In another embodiment, a multi-layer reflector on a sensor is used. A method of making the multi-layer reflector/sensor is also described.

Description:
BACKGROUND 
     1. Technical Field 
     An embodiment of the invention relates generally to the detection of electromagnetic energy, and in particular relates to a detector to detect extreme ultraviolet radiation. 
     2. Description of the Related Art 
     Photo-lithography processes are used to create the very small features that make up integrated circuits, by projecting high-density patterns of electromagnetic radiation onto a wafer during manufacture. Higher density integrated circuits require smaller feature sizes. However, a limiting factor in how small the features can be produced is the wavelength of the radiation used to project the pattern. Current photo-lithography techniques may use radiation in the vacuum ultraviolet (VUV) range, with a wavelength approximately in the 100–200 nanometer (nm) range, but significant increases in feature density may require the use of extreme ultraviolet (EUV) radiation, which may have a wavelength approximately in the 10–14 nm range. However, EUV radiation is highly absorbed by most materials, so EUV-based lithography may require different techniques than are used with longer wavelengths of radiation. 
     Controlling the amount of energy projected during the lithography operation is important, and requires determining the amount of energy in the EUV beam. Diode based sensors may be used to measure EUV intensity. Unfortunately, directing a controlled portion of the beam to a diode sensor with a beam splitter, which works well with longer wavelengths of radiation, is impractical with the high-energy EUV radiation. Conventional forms of detecting the electromagnetic energy off-axis in the lithography tool have proven to produce significant errors, since the actual dose within the path must be inferred, and controlling the percentage of energy off-axis is difficult. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings: 
         FIG. 1  shows a cross section of a grazing-incidence detector comprising a sensor coated with a reflective material, according to one embodiment of the invention. 
         FIG. 2  shows a chart of the amount of reflectivity of a ruthenium layer based on surface roughness, according to one embodiment of the invention. 
         FIG. 3  shows a flow chart of a process for fabricating a detector, according to one embodiment of the invention. 
         FIG. 4  shows a cross section of a detector comprising a multi-layer reflector, according to one embodiment of the invention. 
         FIG. 5  shows a schematic of the effects of a multi-layer reflector, according to one embodiment of the invention. 
         FIG. 6  shows an example graph of reflectivity vs. the number of bilayers, according to one embodiment of the invention. 
         FIG. 7  shows a flow chart of a method of fabricating a detector, according to one embodiment of the invention. 
         FIGS. 8A through 8I  show a cross section of elements of a detector during fabrication, according to one embodiment of the invention. 
         FIG. 9  shows portions of an EUV lithography system, according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known structures and techniques have not been shown in detail in order not to obscure an understanding of this description. 
     References to “one embodiment”, “an embodiment”, “example embodiment”, “various embodiments”, etc., indicate that the embodiment(s) of the invention so described may include a particular feature, structure, or characteristic, but not every embodiment necessarily includes the particular feature, structure, or characteristic. Further, repeated use of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may. 
     Some of the drawings show physical devices. The drawings are not drawn to scale, and the relative dimensions shown in the drawings should not be interpreted as a limitation on the relative dimensions of physical devices. Any references to “up”, “down”, “above”, “below”, or similar directional terms, refer to the orientation as shown in the drawings, and not necessarily to the orientation of an actual physical device with respect to gravity. 
     Various embodiments of the invention pertain to an EUV detector with a reflective structure disposed on a sensor. The reflective structure permits a small portion of incident EUV energy to be captured and detected by the sensor, while reflecting a substantial portion of the energy to the intended target. One embodiment uses for the reflective structure a layer of material that is reflective at the EUV wavelengths at a grazing incidence angle. Another embodiment uses a multi-layer reflector as the reflective structure. 
     Coated Detector 
       FIG. 1  shows a cross section of a detector comprising a sensor coated with a reflective material, according to one embodiment of the invention. The detector  100  may comprise a substrate  110 , a sensor  120 , and a reflective structure in the form of a reflective layer  130 . In one embodiment reflective layer  130  comprises a material that reflects a substantial amount of EUV radiation striking the surface of reflective layer  130  at a shallow angle (e.g., an angle of incidence of less than 20 degrees), while absorbing a sufficient portion of the EUV radiation to be detectable by the sensor  120 . 
     In one embodiment, sensor  120  may comprise a pyroelectric sensor that detects thermally induced distortion of a crystal lattice at or near the surface of the sensor  120 , and is sensitive to rates of change in temperature. Commercially available pyroelectric sensors may be sensitive to wavelengths from infrared through x-ray, may be able to resolve pulses of less than 1 nanosecond (ns), and may have threshold sensitivities of approximately 0.2 micro joules. In a particular embodiment the sensor  120  is comprised of lithium tantalate and is 1–2 millimeters (mm) across, but other embodiments may use other materials and/or sizes. 
     The substrate  110  may comprise any suitable substance that provides physical support for the sensor  120  and the reflective layer  130 , and may also provide electrical connections (not shown) to the sensor  120  so that the amount of energy detected by sensor  120  may be converted into usable electrical signals. 
     In one embodiment the reflective layer  130  has a thickness in the range of approximately 100–200 nanometers (nm), but other embodiments may have other thicknesses. While in one embodiment reflective layer  130  comprises ruthenium, in an alternate embodiment reflective layer  130  may comprise other materials (e.g., gold). 
     In one embodiment substrate  110  and reflective layer  130  are circular with a diameter of approximately 3 inches, while sensor  120  is circular with a diameter of approximately 1–2 mm, but other embodiments may have other sizes and shapes. (Note: the figures show the various elements in cross section, so that the overall shape of those elements cannot be determined from the figures.) 
       FIG. 1  also shows a ray of EUV radiation  140  striking reflective layer  130  at an angle of incidence θ 1 , and reflecting off reflective layer  130  as ray  143  at an equal angle of reflection θ 2 , from where the ray  143  may travel to a target area, such as a focusing reflector. The focusing reflector may redirect the ray through a patterning mask to a wafer for lithographic patterning. A portion  145  of the ray may penetrate into the reflective layer  130 , where it may be partially or fully absorbed to create the thermal effects that permit detection. In one embodiment, angle of incidence θ 1  is a grazing angle of approximately 2 degrees, but other grazing angles may also be used. 
     Although a single ray  140  is shown striking the reflective layer  130  at a single point, this is a simplification that is intended to illustrate the effects of the reflective layer on incoming radiation. In actual usage, many parallel rays  140  may strike throughout a larger portion of the surface area of the reflective layer  130 , including areas not over the sensor  120 . 
     The surface roughness of the reflective layer  130  may have a significant effect on the amount of light reflected to the target area. Because surface roughness may cause any single point on the surface of reflective layer  130  to vary from the overall plane of that surface, the angle of incidence and angle of reflection may vary from θ 1  and θ 2  at that point, causing reflected ray  143  to be scattered and miss the target area. 
       FIG. 2  shows a chart of the amount of reflectivity of a ruthenium layer based on surface roughness, according to one embodiment of the invention. The surface roughness may be measured in statistical units of vertical variation, e.g., in this case expressed in nanometers root-mean-squared (nm rms). Surface roughness of the untreated reflective layer  130  (as the layer exists after being deposited but before further treatment) may be improved through various means (e.g., chemical mechanical polishing, magneto-rheological polishing, ion milling, etc.) One embodiment has a surface roughness of between approximately 0.2 nm rms and approximately 2.0 nm rms, but other embodiments may have other values. 
     In the chart of  FIG. 2 , the EUV radiation has a wavelength of 13.5 mn, and the angle of incidence is approximately 15 degrees. Other values for these parameters may produce somewhat different reflectivity. 
     Incoming radiation that is not reflected or scattered may penetrate into the reflective layer  130 , where it may be absorbed and converted into thermal energy that is conducted into sensor  120 , where the energy is detected. In one embodiment, approximately 16 percent of the incident radiation penetrates into the reflective layer  130  in this manner, but other embodiments may have other percentages. 
       FIG. 3  shows a flow chart of a process for fabricating a detector, according to one embodiment of the invention. In flow chart  300 , at  310  a pyroelectric sensor is provided. At  320 , a reflective layer is deposited on the sensor. In one embodiment, this deposition is accomplished through sputtering, but other embodiments may use other techniques, such as physical vapor deposition (PVD). In the illustrated example the reflective layer comprises ruthenium, but other embodiments may comprise other materials (e.g., gold, etc.) 
     At  330  the surface of the reflective layer is planarized to provide a flat reflective surface with very little surface roughness. In one embodiment, this may be accomplished through chemical mechanical polishing (CMP), but other embodiments may use other techniques (e.g., magneto-rheological polishing, ion milling, etc.). In one embodiment, the surface is planarized to a surface roughness of less than 1.0 nm rms, but other embodiments may have a surface roughness outside this range. 
     At  340  the combined sensor and reflective layer are packaged to provide a suitable structure for mounting and protecting them. At  350  electrical connections are made so that the sensor may be electrically coupled to suitable circuitry for converting the sensor output to usable electrical signals. 
     Multi-Layer Reflector and Sensor 
       FIG. 4  shows a cross section of a detector comprising with a multi-layer reflector, according to one embodiment of the invention. In the illustrated detector  400 , substrate  410  provides a base for the remaining materials. Substrate  410  may also comprise other details not shown, such as electrical connections between the remaining layers and other circuitry. In one embodiment the substrate  410  comprises silicon, but other embodiments may use other materials such as ultra low expansion glass, etc. The illustrated embodiment shows a hole through substrate  410  in which sensor  420  is inserted, but other embodiments may use other techniques, for example, fabricating a sensor on and/or in the substrate. 
     Sensor  420  may comprise material that reacts to electromagnetic radiation in a way that permits the generation of an electrical signal representing the amount of electromagnetic radiation received. In one embodiment sensor  420  comprises a pyroelectric material (e.g., lithium tantalate, lithium niobate, strontium barium niobate, lead zirconate titanate, etc.), but other embodiments may use other types of sensors (e.g., a diode sensor, etc.) 
     Insulating layer  430  may comprise a material that is essentially electrically non-conductive to provide electrical insulation between the substrate  410  and control layer  440 , and is also optically transmissive (at least at the EUV wavelengths) to convey EUV radiation to sensor  420  from above. In one embodiment, insulating layer  430  comprises silicon dioxide (SiO 2 ), but other embodiments may comprise other materials, such as silicon nitride (Si 3 N 4 ), etc. In one embodiment insulating layer  430  has a thickness of between approximately 50 and approximately 100 nm, but other embodiments may use other thicknesses. 
     Control layer  440  may be used to controllably reduce the amount of EUV radiation that reaches the sensor  420 , by restricting the area through which the EUV radiation may pass. In one embodiment, control layer  440  is comprised of chromium (Cr) to limit the radiation reaching the detector (at normal incidence, reflection would be minimal). In one embodiment, the control layer  440  has a thickness between approximately 190 and approximately 200 nm. Other embodiments may use other materials and/or other thicknesses. 
     Control layer  440  may contain a control hole  450  through which a predetermined percent of the electromagnetic radiation received from above may pass through to insulating layer  430 . This technique may be used to keep the expected levels of EUV radiation received by the sensor  420  within the linear region of the sensor and above the noise threshold of the sensor. The insulating layer  430  may absorb some of the EUV radiation that passes through the control hole  450 , which may be accounted for in determining the proper size of the control hole  450 . In one embodiment, a single hole per sensor is used, but other embodiments may use multiple holes per sensor. In various embodiments, the control hole  450  may be cylindrically shaped with a diameter of between approximately 40 nm and approximately 1 mm, but other embodiments may have holes of other shapes and sizes. 
     In some embodiments, control hole  450  is filled with a filler material to filter out unwanted wavelengths of radiation. In a particular embodiment, the hole is filled with Zirconium (Zr) to filter out infrared, visible and ultra-violet radiation. After filling, the filler material may be planarized to create a smooth surface at the top of the control hole  450 . In one embodiment the control layer  440  and filler material may be planarized in the same operation to present a uniform flat smooth surface for both the control layer material and filler material. 
     A multi-layer (ML) reflector may be disposed on the control layer. An ML reflector has alternating layers of two different materials with different refractive indices, so that the interface between any two adjacent layers will reflect a portion of incident radiation and allow another portion to pass through the interface. The layers may be spaced so that the reflected radiation from one interface will be substantially in-phase with reflected radiation from the adjacent interface. In the illustrated embodiment alternating layers of a first material  460  and a second material  470  are disposed directly above the control layer  440  to form an ML reflector. In one embodiment, the two materials are comprised of molybdenum (Mo) and silicon (Si), but other embodiments may use other combinations of materials, such as Mo and beryllium (Be). 
     In  FIG. 4  the layers of first material  460  are shown with a different thickness than the layers of second material  470 , but that is only for clarity of illustration. The thickness of each layer may be chosen so that electromagnetic radiation reflected from each layer will be in phase with electromagnetic radiation reflected from higher layers. Thus the thickness of each layer may be determined by the wavelength of electromagnetic radiation being used, the angle of incidence, and the refractive index of the materials being used. In one embodiment the thickness of each layer is between approximately 5 nm and approximately 10 nm, but other embodiments may use other thicknesses. 
     The number of bilayers (where a bilayer is a layer of the second material directly above and in contact with a layer of the first material) may be selected to achieve the desired percentage of electromagnetic radiation that is to be reflected and/or the percentage that is to reach the control layer. In one embodiment the number of bilayers is between approximately 30 and approximately 60, but other numbers of bilayers may also be used. In a particular embodiment the number of bilayers is approximately 40. 
       FIG. 5  shows a schematic of the effects of a multi-layer reflector, according to one embodiment of the invention. In the illustrated example, each bilayer (numbered  1  through n, and with a thickness d) has a first material A (with a thickness d A ) and a second material B (with a thickness d B ). The illustrated embodiment has an equal number of layers of material A and material B, but other embodiments may not. The illustrated embodiment shows two different types of layers, but other embodiments may have three or more different types of layers. 
       FIG. 6  shows an example graph of reflectivity vs. the number of bilayers, according to one embodiment of the invention. The graph is for an embodiment using EUV with a wavelength of approximately 13.5 nm, with bilayers of Mo and Si. As can be seen, beyond a certain number of bilayers (in this example approximately 60), additional bilayers do not contribute significantly to reflectivity, but might reduce the amount of energy reaching the detector. 
       FIG. 7  shows a flow chart of a method of fabricating a detector, according to one embodiment of the invention.  FIGS. 8A through 8I  show a cross section of elements of a detector during fabrication, according to one embodiment of the invention. The following description discusses elements of  FIG. 7  (labeled  7   xx ) and elements of  FIG. 8A through 8I  (labeled  8   xx ) together. However, it is understood that the embodiment of  FIG. 7  and the embodiment of  FIGS. 8A through 8I  may also be practiced separately. 
     The process may begin with a substrate  810 . At  710  a hole is created in the substrate and a sensor  815  is mounted in the hole. The hole may be created by various techniques, such as machining, laser drilling, etc. Various types of sensors may be used, such as diode sensors, pyroelectric sensors, etc. Alternately, a sensor may be fabricated on the substrate as a part of the fabrication process. At  720  an insulating layer  820  is placed on the substrate  810 . The insulating layer  820  may be comprised of various materials (e.g., silicon dioxide, silicon nitride, etc.) In one embodiment, chemical vapor deposition (CVD) is used to create the insulating layer, but other techniques may also be used (e.g., sputtering, thermally growing an insulating layer, etc.). 
     At  725  a control layer  830  is deposited on the insulating layer  820 . The control layer may be comprised of material that prevents transmission of radiation to the sensor. One embodiment uses chromium for the control layer  830 , but other embodiments may use other materials. Various techniques may be used to deposit the control layer  830 , e.g., physical vapor deposition (PVD), sputtering, etc. At  730  a control hole  835  is created in the control layer. The control hole  835  may be sized to control the percentage of radiation striking the control layer that is passed through to the insulating layer  820  and ultimately to the sensor  815 . In one embodiment the control hole is formed through lithographic patterning, but other embodiments may use other techniques (e.g., laser micromachining, e-beam writer, etc.). Although a single control hole  835  is shown, other embodiments may have multiple control holes above the sensor. 
     At  735  the control hole  835  is filled with a filler material  840 . One embodiment uses Zirconium as a filler material, but other embodiments may use other materials. In the illustrated embodiment, the filler material  840  is deposited on the control layer  830  through any of various techniques (e.g., sputtering), filling the control hole  835  and coating the surface of the control layer  830 , and the excess filler material  840  is then removed at  740  so that the filler material  840  remains only in the control hole  835 . At  740  the surface is planarized to create a smooth, planar surface upon which the remaining layers may be placed. In one embodiment, CMP is used to both remove the excess filler material  840  and to planarize the surface. Other techniques may also be used, for example magneto-rheological polishing, ion milling, etc. 
     A multi-layer reflector may then be fabricated by depositing alternating layers of a first reflective material  850  (as indicated at  745 ) and a second reflective material  860  (as indicated at  750 ). At  755  this process is repeated as many times as necessary to get the required number of alternating layers in the multi-layer reflector. The materials may be chosen with different refractive indices at the chosen wavelength, so that the interface between each pair of adjacent layers will reflect a first known portion of incident light and allow a second known portion of the incident light to pass through into the next layer. In one embodiment the two materials  850 ,  860  are molybdenum and silicon, but other embodiments may use other pairs of materials (e.g., Mo and Be, etc.). In one embodiment the two materials  850 ,  860  are deposited using magnatron sputtering, but other embodiments may use other techniques (e.g., ion beam coater, etc.). 
     In one embodiment the final layer is planarized at  760  to provide a smooth planar surface for the multi-layer reflector, but other embodiments may skip this operation. At  765  electrical connections  805  are made to the sensor  815  so that the final detector may be placed into service. 
       FIG. 9  shows portions of an EUV lithography system, according to one embodiment of the invention. System  900  may include a source  910  of EUV radiation, a reflective apparatus  920  coupled to a sensor  925 , a focusing reflector  930 , a mask  940 , and a target device  950 . In the illustrated system, the source  910  provides EUV radiation traveling in a parallel beam and striking reflective apparatus  920  at an angle. The angles shown in  FIG. 9  are for clarity of illustration and may not be the angles used in an actual system. 
     In one embodiment reflective apparatus  920  comprises a layer of material (e.g., ruthenium, gold, etc.) that reflects a major portion of the incident EUV radiation to focusing reflector  930 , while absorbing a smaller portion of the incident EUV radiation and converting the absorbed EUV radiation to thermal energy that may be detected by sensor  925 , thus permitting the strength of the incident EUV radiation to be measured. In a particular embodiment the angle of incidence between the EUV radiation and the layer of material is less than approximately 20 degrees, but other embodiments may have other angles. 
     In another embodiment reflective apparatus  920  comprises a multi-layer reflector that reflects a major portion of the incident EUV radiation to focusing reflector  930 , while permitting a smaller portion of the incident EUV radiation to pass through to sensor  925 , thus permitting the strength of the incident EUV radiation to be measured. In a particular embodiment the angle of incidence between the EUV radiation and the multi-layer reflector is greater than approximately 45 degrees, but other embodiments may have other angles 
     Focusing reflector  930  may be used to reflect and focus the EUV radiation received from reflective apparatus  920  in a path that will take the EUV radiation to a target area. The focusing reflector  930  may have a curved and very smooth surface so as to focus the EUV radiation accurately at the surface of target  950 . In one embodiment target  950  is a wafer being fabricated to create integrated circuits. Reflective mask  940  may be used to create the pattern being focused on the target  950 . In one embodiment the reflective mask  940  comprises a pattern of material that is essentially non-reflective to EUV radiation, disposed on the surface of a material that is essentially reflective to EUV radiation. In another embodiment a reflective material may be disposed on a non-reflective surface. In either embodiment the pattern of material on the mask  940  determines the pattern of radiation that reaches the target  950 . In some embodiments the pattern of EUV radiation that reaches the target  950  is a reduced-size version of the pattern on mask  940 . 
     Although the illustrated embodiment shows the elements of system  900  in a particular order, other embodiments may have the elements arranged in a different order (e.g., focusing reflector  930  might be disposed in the optical path between source  910  and reflector  920 , focusing reflector  930  might be disposed in the optical path between mask  940  and target  950 , etc.) 
     The foregoing description is intended to be illustrative and not limiting. Variations will occur to those of skill in the art. Those variations are intended to be included in the various embodiments of the invention, which are limited only by the spirit and scope of the appended claims.