Abstract:
A method for cleaning optics in a chamber. The method can include introducing a first etchant into a chamber that encloses an optical component and a source of electromagnetic radiation that is suitable for lithography, ionizing the first etchant, and removing debris from a surface of the optical component.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of and claims priority to U.S. application Ser. No. 10/197,628, filed on Jul. 15, 2002, the contents of which are incorporated herein by reference. 
     This description relates to in-situ cleaning of light source collector optics. 
    
    
     BACKGROUND 
     Lithography is used in the fabrication of semiconductor devices. It is the technique by which the patterns that make up the circuitry of a chip are defined. In lithography, a light-sensitive material coats a wafer substrate, such as silicon, that is exposed to light at some wavelength to reproduce an image of the mask that is used to define each die on the wafer. The mask is usually referred to as a reticle in optical lithography, in that it is separated from the wafer by a series of objective lenses. When the wafer and mask are illuminated, the light-sensitive material, or photoresist, undergoes chemical reactions to produce a replicated pattern of the mask on the wafer. The patterns can then be manipulated in ways to transform the structure or properties of the wafer, which leads to the creation of various semiconductor devices and applications. 
     The source of the light used in lithography, for example, can be an excimer laser light sources at wavelengths of 248 nm, 193 nm or 157 nm. Extreme Ultraviolet (EUV) light sources, which produce light with a wavelength of approximately 13 nm, can also be used. 
     EUV light can be produced using a small, hot plasma which will efficiently radiate at a desired wavelength, for example 13.4 nm. The plasma is created in a vacuum chamber, typically by driving a pulsed electrical discharge through the target material, or by focusing a pulsed laser beam onto the target material. The light produced by the plasma is then collected by nearby mirrors and sent downstream to the rest of the lithography tool. 
     The hot plasma tends to erode any materials nearby, for example the electrodes in electric-discharge sources, or components of the gas delivery system in laser-produced plasmas. The eroded material may then coat the collector optics, resulting in a loss of reflectivity and reducing the amount of light available for lithography. 
     The collector optics may be replaced once they are coated with a given level of debris. Alternatively, the collector optics could be cleaned, for example, by temporarily removing them from the system for cleaning. The optical components may then be put back in the EUV system, and recalibrated and realigned. 
    
    
     
       DRAWINGS 
         FIG. 1  shows a lithography system; 
         FIG. 2  shows a light source chamber; 
         FIG. 3  shows a EUV chamber, modified for etching, including collector optics in a grazing incidence configuration; 
         FIGS. 4(   a )–( d ) show alternative embodiments of an EUV chamber, modified for etching, including collector optics in a grazing incidence configuration; 
         FIG. 5  shows a EUV chamber, modified for etching, including collector optics in a normal incidence configuration; 
         FIG. 6  is a flowchart for a method of removing debris from lithography optics; 
         FIG. 7  is a flowchart for a method of removing byproduct from EUV lithography optics; and 
         FIG. 8  is a flowchart for a method of removing byproduct from EUV lithography optics. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a lithography system  20 . The wafer with the light sensitive coating and the mask are placed in the lithography chamber  22 . The pressure in the lithography chamber  22  is reduced to a near vacuum environment by vacuum pumps  24 . A light source chamber  26 , which houses a light source, is connected to the lithography chamber  22 . The pressure in the light source chamber  26  is also reduced to a near vacuum environment by vacuum pumps  24 . The light source chamber  26  and lithography chamber  22  are separated by a valve  28  that can be used to isolate the chambers  22 ,  26 . This allows for different environments within each chamber. 
     The light source chamber  26  can be a EUV chamber, which houses a EUV light source. A power supply  30  is connected to the EUV chamber  26  to supply energy for creating a EUV photon emitting plasma, which provides EUV light for lithography. The light source chamber is evacuated by vacuum pumps  24 . 
       FIG. 2  shows the light source chamber  26  connected to the lithography chamber  22 . (The valve  28  is shown open.) Inside the light source chamber  26  is a light source  32  and collector optics  34  for collecting and directing the light for use in the lithography chamber  22 . As described above, debris may be deposited on the collector optics  34 . The collector optics  34  can be cleaned without removing them from the light source chamber  26 . 
     To clean the optics, the light source chamber  26  is first isolated from the lithography chamber  22  by closing the isolation valve  28 . The pressure in the light source chamber  26  is reduced using vacuum pumps  24  ( FIG. 1 ), which may be the same pumps as used during light production, or different pumps. An etchant, or chemical reagent used in etching, is supplied from an etchant tank  36  ( FIG. 1 ) to the light source chamber  26  through an etchant valve  38 . Etching involves electrically driving a chemical reaction between gaseous reagents introduced into the light source chamber  26  and a surface, such as the surfaces of the collector optics  34 . Here, for example, the etchant is ionized to form a plasma  40  by introducing electrical energy from a power supply. The power supply can be the light source power supply  30 , or a separate etching power supply  31 . ( FIG. 1 ). Alternatively, laser energy could also be used to drive the ionization. The ions in the plasma  40  react with the debris on the surface of the collector optics  34 , forming stable gaseous compounds. These stable compounds are then pumped away using the vacuum pumps  24  ( FIG. 1 ). Then the value  28  can be reopened and photolithography can proceed again. 
     The collector optics can be, for example, grazing incidence mirrors. In a grazing incidence collector configuration, nested shells of mirrors are placed between the source  32  and the isolation valve  28 . For example, each shell may be an ellipse of rotation, with the source at one focus of the ellipse, which is then re-imaged at the other focus of the ellipse. Parabolic shells produce a collimated beam of light. More complicated geometries of nested-shell mirrors are also possible. 
     Grazing-incidence mirrors can have a metallic surface, allowing the mirrors to be used as electrodes for the purposes of ionizing an etchant gas.  FIG. 3  shows an example of a single mirror-shell, electrically split into two halves,  42  and  44 .  FIG. 3  shows a cross-section of the two parts of the mirror-shell. The complete un-sectioned shell is an essentially closed three-dimensional elliptical surface that is split longitudinally to form the two halves  42 ,  44 . By holding the segments  42  and  44  at different voltages, the etchant can then be ionized to create the plasma  40  between the two mirrors  42 ,  44 . The electrical energy can be supplied, for example, at microwave or radio frequency (RF). 
       FIGS. 4(   a )– 4 ( d ) show other possible grazing incidence mirror configurations where the mirrors are nested (and are, as in  FIG. 3 , shown in section). The mirrors here can be cleaned in stages. For example, the back side  45  (side further from the plasma  40 ) of the inner mirror  46  and the front side  47  (side closer to plasma  40 ) of the outer mirror  48  can be cleaned by creating a plasma between those surfaces as shown in  FIG. 4(   a ). Here, the inner  46  and outer  48  mirrors are used as the electrodes. The front side of the inner mirrors  46  can be cleaned by creating a plasma  40  between the front surface  49  of the mirror  46  and a centerline  50  between the two inner mirrors, as shown in  FIG. 4(   b ). Here, the inner mirror  46  is used as an electrode, and another electrode  52  is placed at the centerline. Alternatively, the front sides  49  of the inner mirrors  46  can be cleaned by creating a plasma  40  between the two inner mirrors  46 , as shown in  FIG. 4(   c ). Here, the two inner mirrors  46  are used as electrodes. 
     Instead of etching in multiple stages, as described above, the mirrors can be simultaneously etched by ensuring that the various mirror components are at the appropriate phases of the alternating voltage. For example, in  FIG. 4   a , the lower segment of mirror  48  can be at a positive voltage while the lower segment of  46  is at a negative voltage, while at the same time the upper segment of  46  is at positive voltage and the upper segment of  48  is at a negative voltage. 
     A segmented mirror configuration can also be used. A surface view of a segmented mirror is shown in  FIG. 4(   d ). In this configuration, each of the mirrors is divided into multiple segments  54  along its length. Neighboring segments are held at opposite potentials to ionize the etchant. The segments can also be split along the lengthwise direction. Combinations of segmenting and nesting can also be used. 
     A normal-incidence mirror configuration, shown in  FIG. 5 , can also be used to direct the light into the lithography chamber  22 . Here, the mirror  56  is a section of a rotated ellipse, parabola, or more complicated shape, that surrounds the source  32 , causing the individual light rays to re-direct an angle close to 180 degrees. Nested and segmented configurations, similar to those described for grazing-incidence collectors, can also be used. Alternatively, the source itself  32  can be used as one of the electrodes, with the mirror  56  as the other electrode held at opposite voltage. 
     By introducing other metallic pieces into the chamber, many other mirror-electrode combinations are possible for both grazing-incidence and normal-incidence mirror configurations. 
     Mirrors in a grazing-incidence configuration require a smooth surface of certain EUV-reflective metals. Ruthenium, for example, can be used, as it is relatively tough and has a high grazing-incidence EUV reflectance. Normal-incidence mirrors require a multilayer coating, such as multiple layers of Si and Mo, to be reflective to EUV. A protective capping layer of another material can also be used with the multi-layer mirrors, for example, ruthenium or SiO 2 . 
     The debris deposited on the mirrors can be composed of the materials making up the plasma-facing components of the EUV source. For example, tungsten (W) can be used for such components, as it is relatively resistant to plasma erosion. Specifically, tungsten can be used in the electrodes of an electric-discharge source and in the heat shields surrounding the gas-delivery system of a laser-produced plasma. The debris deposited on the mirrors determines the etchant that can be used to clean the mirrors. 
     The etchant is chosen such that when ionized, the ions react with the debris to form a volatile substance, which is gaseous in the light source chamber, while not reacting with the mirror surfaces to form volatile substances. Volatile substances are those that assume a gaseous form in the environment inside the light source chamber. If the debris deposited on the mirrors is tungsten, a fluorine-containing gas can be used as an etchant. Ruthenium and SiO 2 , examples of materials that can be used in the mirrors, do not react with fluorine atoms to form volatile substances. Examples of fluorine-containing gases include SF 6 , F 2 , XeF 2  and NF 3 . Other fluorine-containing gases can be used as well. As  FIG. 6  shows, the etchant is introduced into the chamber (step  600 ). It is then ionized to form free fluorine (step  602 ). The free fluorine is generated through plasma electron collisions with the etchant gas. 
     The free fluorine atoms then react with the solid tungsten debris to form tungsten-fluorine compounds such as tungsten hexafluoride (step  604 ). Tungsten hexafluoride is a relatively volatile compound, with a boiling temperature of about 20 C at 1 atm, and thus will be a gas that can be pumped out of the chamber (step  606 ). 
     If a multi-layer material is used in the mirrors, debris should be removed from only the outer or capping layer. This requires a more precise etch. If the debris includes tungsten, a fluorocarbon can be used as the etchant, as shown in  FIG. 7 . Examples of possible fluorocarbon etchants include CF 4 , CHF 3 , and C 4 F 8 , though there are many other possibilities. Fluorocarbon etchants can also be used for cleaning solid metal mirrors. The fluorocarbon etchant is introduced (step  700 ) and ionized to form free fluorine atoms (step  702 ). 
     The free fluorine atoms (F) react with the tungsten debris (step  704 ). Thus the tungsten is removed from the mirror and can be pumped away (step  706 ). 
     As a side effect of using fluorocarbon etchants, fluorocarbon chains such as CF 2  and CF 3  can form with tungsten, leaving a residue on the surface of the mirrors. A second plasma can be created to remove this residue from the mirrors. Hydrogen or oxygen can be introduced as etchants for the second plasma (step  708 ). Free hydrogen or oxygen is generated through plasma electron collisions with the etchant gas. The free oxygen or hydrogen reacts with the fluorocarbon-tungsten compounds. Methane (CH 4 ) or carbon monoxide (CO) is formed along with a tungsten-flourine compound (such as tungsten hexaflouride) and both are pumped out of the chamber. 
     Generation of EUV light can also crack hydrocarbon chains. Thus in addition to tungsten, carbon debris can then be deposited on the optical components. A plasma created using an oxygen etchant can be used to remove the carbon debris. 
     If the debris deposition is not uniform across the mirror, there may be regions of exposed mirror without debris on top that can potentially react with the fluorine atoms. For example, if the mirror is made of ruthenium, the areas on the mirror surface not covered by debris can potentially react with the fluorine atoms to form ruthenium fluoride on the mirror surface. An additional plasma can be created to clean the ruthenium fluoride, as  FIG. 8  shows. Hydrogen reactive ion etching (RIE) at room temperature, where hydrogen is used as the etchant, can be used to remove the ruthenium fluoride from the mirror without reacting with the ruthenium on the mirror surface. Hydrogen is introduced into the chamber (step  800 ), is ionized (step  802 ) and reacts with ruthenium fluoride to form HF gas (step  804 ), which can be readily pumped away from the chamber (step  806 ), leaving a clean ruthenium surface. 
     If the debris deposited on the mirrors is a material other than tungsten, the etchant can be chosen such that when ionized, free ions in the plasma react with the debris to form gaseous compounds, which can be pumped away. 
     An alternative to reacting a plasma with debris on the mirror surfaces is to sputter debris off the mirror surfaces. In this case, a high-energy plasma is created in the light source chamber. The plasma ions strike the surface of the collector optics and dislodge particles of debris such as tungsten, aluminum, copper, and other impurities. The energy and composition of the ions are be tuned so as to minimize the damage to the mirror surface and maximize the damage to the debris. A sputtering plasma can be created with any of the mirror configurations described above. Also, sputtering may be used in conjunction with the other methods described above. 
     Although some implementations have been described above, other embodiments are also within the scope of the invention.