Abstract:
A photolithography tool includes an anode and a cathode composed of a first material and a second material. The second material has a lower work function than the first material. Electrons emitted from the cathode ionize a gas into a plasma that generates EUV light. The EUV light is focused on a mask to produce an image of a circuit pattern. The image is projected on a semiconductor wafer to produce a circuit.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to plasma generation for photolithography.  
         BACKGROUND  
         [0002]    Photolithography systems are used to produce circuit patterns on semiconductor wafers. To produce devices with smaller dimensions, the optical resolving power of the photolithography system needs to be increased. Because optical resolving power is proportional to the wavelength of light, a light source with a shorter wavelength provides better optical resolution capability. Extreme ultraviolet (EUV) light may be utilized in manufacturing microelectronic semiconductor devices with feature sizes less than 100 nm. EUV light may be obtained from a synchrotron or from a high energy plasma. The plasma may be generated by focusing a high energy laser beam onto a stream of inert gas, such as Xenon. The plasma may also be produced by using electron emission in which electrodes emit electrons to ionize a gas to form a plasma. To energize the plasma to generate sufficient EUV light, the electrodes may have to operate at high temperatures with high power flowing through the electrodes. This causes the electrodes to emit particles that contaminate other components in the lithography system, such as a condenser lens. EUV light generated from Xenon plasma has a main component with a wavelength of about 13 nm. FIG. 1 shows an example of an EUV lithography system. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0003]    [0003]FIGS. 1 and 2 show lithography systems.  
         [0004]    [0004]FIGS. 3 and 4 show EUV light sources. 
     
    
     DETAILED DESCRIPTION  
       [0005]    By using zirconiated tungsten electrodes to emit electrons to ionize a gas to form a plasma, EUV light may be produced with less debris, resulting in less contamination in the lithography system. Referring to FIG. 2, an EUV lithographic system  100  includes a chamber  120  for generating EUV light and a chamber  122  for using the EUV light to produce lithography patterns on a wafer  114 . Chamber  120  includes an EUV light source  102  that uses electron emissions to ionize a stream of gas (e.g., Xenon gas) to produce a plasma  104 . Plasma  104  emits EUV light  108  that is collected by condenser mirrors  106  and projected through a filter  124  positioned between chambers  120  and  122 . (For clarity of illustration, several mirrors and lenses are omitted in the figure.) The EUV light is focused onto a reflective mask  112  having enlarged circuit patterns. EUV light reflected from mask  112  is projected onto wafer  114  by a reduction camera  116  to generate circuit patterns on the wafer  114 . A vacuum pump  118  removes exhaust plasma gas from chamber  120 .  
         [0006]    Referring to FIG. 3, EUV light source  102  includes an electrode  126  that functions as a cathode, and an electrode  128  that functions as an anode. A cross-sectional view of light source  102  is shown in FIG. 4. The electrodes  126 ,  128  have a ring or tubular shape that defines a hollow or tube region  132  that allows a gas to pass through. Electrodes  126  and  128  are connected to a power supply  110  (FIG. 2), which supplies a high DC voltage in the range of 1 to 10 kilo-volts. As the gas  103  passes through the hollow region  132 , gas molecules are ionized by electrons emitted from electrode  126  to become a plasma  104 . A portion of plasma  104  extends beyond the hollow region defined by electrode  128 . Plasma  104  radiates light as the ionized gas molecules transition from the higher energy states back to the lower energy ground state. When Xenon is used to generate plasma  104 , the plasma emits light having strong line emissions with wavelengths between 13 to 14 nm.  
         [0007]    A dielectric material  130  separates electrodes  126  and  128 . Between dielectric  130  and electrode  128  is a tubular region  140  that forms a passage for gas  103  to pass through. A pipe  142  connects light source  102  to a container (not shown) having gas  103 .  
         [0008]    Electrode  126  includes a tungsten core  134  in the shape of a sleeve with a conical nozzle  138  at one end. The inside wall of the tungsten core  134  is coated with a layer  136  of zirconia (also known as zirconium dioxide, ZrO 2 ) to protect the tungsten sleeve from chemical and mechanical erosion by the plasma  104 . The zirconia layer  136  may be formed on the tungsten core  134  by either chemical or physical deposition methods. The thickness of the zirconia layer  136  may be in the range of 0.5 to 10 nm. Zirconia has a work function of about 2.5 to 2.6 eV, while tungsten has a work function of about 4.5 eV. Because zirconia has a lower work function than tungsten, it is easier for electrons to be emitted from a zirconia surface, reducing damage to the electrode. Zirconium dioxide is the stable form of zirconium oxide at room temperature; it is possible that zirconia dioxide may change to other forms of zirconium oxide at higher temperatures.  
         [0009]    Zirconia also has a stronger resistance to chemical and mechanical erosion. Electrode  126  operates under high temperature, high voltage, and high current conditions, so without protection of the zirconia layer  136 , electrode  126  wears down rapidly and has to be replaced often. Also, tungsten reacts with plasma gas at high temperature to form materials that become debris in chamber  120 . The debris contaminates other components (e.g., condenser mirrors  106 ) in chamber  120 . By adding the zirconia layer  136  on the tungsten core  134 , damage to electrode  126  is reduced, which results in less debris in chamber  120 , improving overall performance of system  100 .  
         [0010]    Materials other than zirconia may be used to coat the tungsten core  134 . Examples of such materials include cesium oxide (work function≈2.15 eV), rubidium oxide (work function≈2.2 eV), strontium oxide (work function≈2.6 eV), and barium oxide (work function≈2.7 eV).  
         [0011]    Electrode  128  may be tungsten, tantalum, or another refractory metal with a high melting point. Because plasma  104  is generated at a distance from electrode  128 , electrode  128  may or may not be coated with zirconia.  
         [0012]    An example of the electrodes  126  and  128  without the zirconia coating may be found in the EUV-Lamp manufactured by AIXUV GmbH, Aachen, Germany.  
         [0013]    Although some implementations have been described above, other embodiments are also within the scope of the following claims.  
         [0014]    For example, the shape of the electrode  126  may be different depending on the shape and position of plasma  104  that is required. The shape of the electrode  128  may be different. Electrode  126  may include a core that is composed of materials other than tungsten, such as tantalum. In applications that does not require the cathode to have strong mechanical strength or in applications where the cathode is supported by other structures, electrode  126  may also comprise a single material (e.g., zirconia) that has a low work function. Plasma  104  may be generated from other gases to produce light with different wavelengths.