Patent Publication Number: US-8530867-B1

Title: Electron generation and delivery system for contamination sensitive emitters

Description:
CLAIM OF PRIORITY 
     This application is a continuation of and claims the priority benefit of U.S. patent application Ser. No. 12/561,969 to Mehran Nasser-Ghodsi entitled “ELECTRON GENERATION AND DELIVERY SYSTEM FOR CONTAMINATION SENSITIVE EMITTERS” filed Sep. 17, 2009, the entire contents of which are incorporated herein by reference. U.S. patent application Ser. No. 12/561,969 claims the priority benefit of provisional application No. 61/099,870 to Mehran Nasser-Ghodsi entitled “ELECTRON GENERATION AND DELIVERY SYSTEM FOR CONTAMINATION SENSITIVE EMITTERS” filed Sep. 24, 2008, the entire contents of which are incorporated herein by reference. This application claims the priority benefit of provisional application No. 61/099,870 to Mehran Nasser-Ghodsi entitled “ELECTRON GENERATION AND DELIVERY SYSTEM FOR CONTAMINATION SENSITIVE EMITTERS” filed Sep. 24, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates electron generation system and more particularly to a cold field emitter (CFE) system with improved performance. 
     BACKGROUND OF THE INVENTION 
     There has been an increasing need for the electron/ion beam equipment that can be operated at low voltages. The cold field emitter (CFE) provides a stable electron beam and has a long lifetime for conventional electron microscopy and electron beam lithography. Cold field-emission cathode units produce electron beams with higher current density and with lower energy spread than thermionic cathodes. 
     A cold field emitter for producing an electron beam includes at least one cold cathode unit. Each of the cold cathode units includes an emitter cone having an emitter tip and a gate spaced apart from the emitter tip for extracting electrons from the emitter tip in a propagation direction upon application of a positive dc voltage on the gate with respect to the emitter tip. Each of the cold cathode units also includes a lens electrode disposed further in the propagation direction from the emitter tip than the gate for focusing the extracted electrons in the propagation direction. The emitter tip may be a single crystal tungsten tip. Single crystal hafnium carbides (HfC) and other metal carbides (TiC, NbC, etc) are used as alternative to W for the use as an electron emitter. 
     Conventional electron generation systems using cold field emitters with W or HfC tips are based on the operation of the tips until states of instability are arrived at. These instabilities, which are typically caused by surface contamination, result in increased current ultimately causing tip failure. Therefore once the beam current starts to demonstrate instabilities, the tip is flashed with a long settling time and the process is refreshed. 
     The current operating modes of the conventional electron generation system result in extended down time between tip flash to allow for beam stabilization. In addition, to reduce the rate of contamination adsorption on tip surface, ultra high vacuum (UHV) is required. 
     It is within this context that embodiments of the present invention arise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
         FIG. 1A  is a schematic diagram of an electron generation system according to an embodiment of the present invention. 
         FIG. 1B  is a block diagram of the system of  FIG. 1A . 
         FIG. 2  is a flow diagram illustrating a method for cleaning the emitter tip of a cold field emitter according to a preferred embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
     Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. 
     Embodiments of the present invention is based on standard electron beam (e-beam) gun designs with an additional beam defining/current measurement aperture included in the optical column and beam deflection on to the aperture as part of the operation of the electron gun. Flashing of the emitter is initiated at preset periods of time before beam current drift has started. This allows the emitter tip to operate in vacuum environments with less stringent vacuum requirements than UHV. 
       FIG. 1A  is a schematic diagram of an electron beam system  100  according to an embodiment of the present invention. The electron generation system  100  is preferably a cold field emitter (CFE)  102 , which includes at least one cold field-emission cathode unit, for producing an electron beam  104 . An exemplary field emitter  102  is a vacuum cone-type field emitter. The cold cathode unit  102  emits an electron beam  104  from the tip  103 . Electrons from the emitter tip are focused on to a sample  101  by an electron optical column having one or more electrodes  114 ,  116 . The electron generation system  100  also includes an electron deflector  106 A located between the emitter tip  103  and an electrode  114  having a beam-defining aperture  115 . 
     The deflector  106 A deflects the electron beam  104  from the emitter  102  away from a beam path  105  through the aperture  115  and onto an electron detector  108 , such as a Faraday cup. 
     By way of example deflector  106 A may be an electrostatic deflector having a pair of electrodes with a gap between them. The gap may be aligned with the beam defining aperture  115 . Alternatively, the deflector  106 A may be a magnetic deflector having one or more deflection coils that produce magnetic fields that deflect the electron beam  104  onto the electron collector  108 . The electron collector  108  can be located either before or after beam defining aperture (BDA)  115 . 
     According to an embodiment of the invention, the electron collector  108  may be electrically connected to a current meter  110  which is connected to a controller  112 . The deflector  106 A and emitter  102  may operate in response to electrical signals from the controller  112 . The outputs from the controller  112  that provide control signals to the emitter  102  and deflectors may be coupled to the electron collector  108  in a closed feedback loop. The controller  112  may be configured, e.g., by appropriate circuitry or programming to control the flashing of the emitter tip  103  based on regular measurements of the beam current with the Faraday cup  108 . The system  100  may optionally include an additional electron deflector  106 B which may be located between the electrode  114  and the sample  101 . The additional deflector  106 B may be an electrostatic or magnetic deflector. The additional deflector  106 B may be used to deflect the electron beam  104  to sweep the electron beam over the sample  101 . 
     By way of example, as shown in the block diagram of  FIG. 1B , the controller  112  may be a self-contained microcontroller. Alternatively, the controller  112  may be a general purpose computer configured to include a central processor unit (CPU)  122 , memory  124  (e.g., RAM, DRAM, ROM, and the like) and well-known support circuits  128  such as power supplies  121 , input/output (I/O) functions  123 , clock  126 , cache  134 , and the like, coupled to a control system bus  130 . The memory  124  may contain instructions that the CPU  122  executes to facilitate the performance of the system  100 . The instructions in the memory  124  may be in the form of the program code  125 . The code  125  may control the flashing of the electron emitter  102  as described below with respect to  FIG. 2 . 
     The code  125  may conform to any one of a number of different programming languages such as Assembly, C++, JAVA or a number of other languages. The controller  112  may also include an optional mass storage device,  132 , e.g., CD-ROM hard disk and/or removable storage, flash memory, and the like, which may be coupled to the control system bus  130 . The controller  112  may optionally include a user interface  127 , such as a keyboard, mouse, or light pen, coupled to the CPU  122  to provide for the receipt of inputs from an operator (not shown). The controller  112  may also optionally include a display unit  129  to provide information to the operator in the form of graphical displays and/or alphanumeric characters under control of the processor unit  122 . The display unit  129  may be, e.g., a cathode ray tube (CRT) or flat screen monitor. 
     The controller  112  may receive information, i.e., electron beam current value, from the current meter  110  through the I/O functions  123  in response to data and program code instructions stored and retrieved by the memory  124 . Depending on the configuration or selection of controller  112 , the emitter  102  may interface with the I/O functions via conditioning circuits. The conditioning circuits may be implemented in hardware or software form, e.g., within code  125 . It is noted that in some embodiments, the current meter  110  may be a separate unit that is connected to signal input of the controller. Alternatively, the current meter may be an integral part of the controller  112 . In either case, for the purposes of the present application, the current meter  110  may be said to be connected to the controller. 
       FIG. 2  is a flow diagram illustrating a method  200  for regularly flashing the emitter tip to clean the contaminants. As shown in  FIG. 2 , an electron beam is generated from the emitter tip  103  of the field emitter  102  as indicated at  202 . After waiting for a predetermined interval, as indicated at  204 , the electron deflector  106 A may deflect the electron beam  104  in to the electron collector  108  as indicated at  206 . In some embodiments, the electron beam  104  may be swept over the sample in a regularly repeated pattern having a characteristic cycle time. By way of example, the controller  112  may be configured, e.g., by suitable programming, to cause the additional deflector  106 B to sweep the electron beam  104  over the sample  101  repeatedly in a predetermined pattern. The controller  112  may be further configured to cause the electron deflector  106 A to deflect the electron beam  104  away from beam path  105  and onto the electron collector  108  at an end of a cycle of the predetermined pattern. For example, the controller  112  may sweep the electron beam over the sample  101  in a raster pattern and the electron beam  104  may be measured at end of each raster trace, e.g., every 15 ms to 35 ms. 
     The beam current collected by the electron collector  108  is measured, e.g., by a current meter  110 , as indicated at  208 . At  210  the measured beam current value may be compared to a threshold value. Based on the beam current value sent from the current meter, the controller  112  may control flashing of the emitter tip to clean contaminants as indicated at  212 , which closes the feedback loop. By way of example, the emitter tip  103  may be flashed at elevated temperature of about 2100K to 2400K, e.g., by providing sufficient electrical current to the field emitter  102 . The method  200  allows to flashing the emitter tip regularly, thereby the electron generation system can operate at lower vacuum. The predetermined interval and threshold current may be empirically determined such that the flash heating in  212  is sufficient to remove contaminants built up on the emitter tip  103  during the predetermined interval when the emitter tip  103  is operated in an environment at a pressure of between 10 −6  torr and 10 −7  torr. 
     Regularly monitoring electron beam current with electron collector allows for regular tracking the performance of the electron emitter so that the emitter tip may be flash heated as needed. By monitoring the beam current at regular intervals and regularly flash heating the emitter tip, the tip can be operated at high vacuum, e.g., at environmental pressure between 10 −6  torr and 10 −8  torr as opposed to ultra-high vacuum (UHV), which typically requires pressures of 10 −9  torr or less. 
     An electron beam system that can operate at a lower level of vacuum can be used with simpler and less expensive vacuum systems. This allows electron beam systems that use field emitter tips to be incorporated into high volume vacuum processing environments, such as semiconductor processing systems. 
     While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.”