Abstract:
An energy contamination detection apparatus includes a membrane and a charge collection plate disposed at a distance from the membrane. The membrane is configured to receive an ion beam and allow a portion of the ion beam having energy levels above a desired energy level to pass therethrough toward the charge collection plate and absorb or reflect portions of the ion beam having energy levels at or below the desired energy level. A voltage source is electrically coupled to the charge collection plate for providing a bias voltage to the charge collection plate. A detection circuit is coupled to the charge collection plate and is configured to detect energy contamination based on an amount of charge collected on the charge collection plate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relate to the field of ion implantation. More particularly, the present invention relates to in-situ energy contamination detection in an ion implanter. 
     2. Discussion of Related Art 
     Ion implantation is a process used to dope ions into a work piece. One type of ion implantation is used to implant impurity ions during the manufacture of semiconductor substrates to obtain desired electrical device characteristics. Typically, arsenic or phosphorus may be doped to form n-type regions in the substrate and boron, gallium or indium is doped to create p-type regions in the substrate. 
     An exemplary high current ion implanter tool  100  is generally shown in  FIG. 1  and includes an ion source chamber  102 , and a series of beam line components that direct the ion beam to a wafer or substrate. These components are housed in a vacuum environment and configured to provide ion dose levels with high or low energy implantation based on the desired implant profile. In particular, implanter  100  includes an ion source chamber  102  to generate ions of a desired species. The chamber has an associated heated filament powered by power supply  101  to ionize feed gas introduced into the chamber  102  to form charged ions and electrons (plasma). The heating element may be, for example, an indirectly heated cathode. 
     Different feed gases are supplied to the source chamber to generate ions having particular dopant characteristics. The ions are extracted from source chamber  102  via a standard three (3) extraction electrode configuration used to create a desired electric field to focus ion beam  95  extracted from source chamber  102 . Beam  95  passes through a mass analyzer chamber  106  having a magnet which functions to pass only ions having the desired charge-to-mass ratio to a resolving aperture. In particular, the analyzer magnet includes a curved path where beam  95  is exposed to the applied magnetic field which causes ions having the undesired charge-to-mass ratio to be deflected away from the beam path. Deceleration stage  108  (also referred to as a deceleration lens) includes a plurality of electrodes (e.g. three) with a defined aperture and is configured to output the ion beam  95 . A magnet analyzer  110  is positioned downstream of deceleration stage  108  and is configured to deflect the ion beam  95  into a ribbon beam having parallel trajectories. 
     A magnetic field may be used to adjust the deflection of the ions via a magnetic coil. The ribbon beam is targeted toward a work piece which is attached to a support or platen  114 . An additional deceleration stage  112  may also be utilized which is disposed between collimator magnet chamber  110  and support  114 . Deceleration stage  112  (also referred to as a deceleration lens) is positioned close to a target substrate on platen  114  and may include a plurality of electrodes (e.g. three) to implant the ions into the target substrate at a desired energy level. Because the ions lose energy when they collide with electrons and nuclei in the substrate, they come to rest at a desired depth within the substrate based on the acceleration energy. The ion beam may be distributed over the target substrate by beam scanning, by substrate movement using platen  114 , or by a combination of beam scanning and substrate movement. 
     Deceleration of the ions by one or more stages  112  may be required when forming devices with shallower junction depths, but at high current levels. A deceleration stage  112  is positioned reasonably close to the target substrate to reduce the distance the beam must travel at low energy where the efficiency of transporting the beam is poor. However, ions directed at a substrate may lose their charge in a charge exchange reaction with residual gas along the beam line. These ions, commonly referred to as “neutrals”, are unaffected by one or more of the deceleration stages  112  and impact the target substrate at a higher energy level. This higher energy level causes the ions to implant deeper in the target substrate than desired and is Energy Contamination (EC). In other words, EC occurs when a fraction of the ion beam that is at a higher energy level for a given implant recipe reaches the target substrate. This is particularly problematic when forming, for example, a gate metal implant, where avoiding contamination of the oxide beneath this gate is important due to the fragility of the oxide layer. 
     Currently, attempts have been made to suppress and or deflect ions at higher energy levels than desired from reaching the target substrate to avoid EC through the use of high energy filters disposed downstream of the deceleration stage. However, a drawback associated with these filters is that a decelerated, low energy ion beam is very difficult to transport even over small distances because it is subject to large space charge blow-up. Thus, transporting the beam through an energy filter will not only attenuate the high energy neutrals, but will also attenuate the desired ions and prevent them from reaching the target substrate with a desired energy and at a desired concentration. Also, only a limited amount of current may be transported through such a filter, often with significant degradation of beam parallelism. 
     Other known techniques for limiting EC include the use of an electrostatic or magnetic bend disposed between the deceleration stage and the analyzer magnet, increased gas pumping to limit the neutralization of beam ions by residual gas, an aperture and liner design to prevent neutrals formed by collisions with the structures inside the implanter from reaching the workpiece, and limiting the voltage allowed when running deceleration to reduce the implanted depth of the contaminant ions. 
     In U.S. Pat. No. 7,250,617 entitled “Ion Beam Neutral Detection” assigned to the assignee of the present invention, a system and method for measuring the current of secondary electrons emitted due to the impact of energetic neutral particles is disclosed. This system measures a current of the ion beam at the collector plate wherein different portions of the current are measured depending on a bias voltage provided to the chamber. However, this requires the measuring of the various components of the beam current including the lower energy ions as well as the desired energy level ions and subtracting these measurements to determine the EC. This method of quantifying the high energy neutrals is thus dependent on the subtraction of two relatively large numbers and the error in the result is compounded by the arithmetic operation with a resulting loss in accuracy. Furthermore, this method depends on measuring the secondary electrons emitted from a surface when an ion or neutral atom impinged on that surface. However, secondary electron yields are very sensitive to surface cleanliness and can vary unpredictably. Accordingly, an improved EC detection system and method which provides a more direct measurement of the high energy ions associated with EC is desirable. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention are directed to an energy contamination detection device. In an exemplary embodiment, the detection device includes a membrane having a thickness selected to block ions of an ion beam having a desired energy level and allow particles of the beam having energy levels above the desired energy level to pass therethrough. A housing including a support frame is used to support the membrane. A charge collection plate is disposed a distance from the membrane. The charge collection plate is configured to receive the particles of the ion beam having energy levels above the desired energy level. A voltage source is electrically coupled to the charge collection plate for providing a bias voltage to the charge collection plate. A detection circuit is coupled to the charge collection plate and is configured to detect energy contamination based on an amount of charge collected on the charge collection plate from the received particles of the ion beam having energy levels above the desired energy level. 
     In an exemplary energy contamination detection method, an ion beam is received by a membrane positioned within a process chamber of an ion implanter. A portion of the ion beam that is above a desired energy level passes through the membrane. A portion of the ion beam that is below the desired energy level is prevented from passing through the membrane. A bias voltage may be applied to a collection plate. The collection plate is spaced apart from the membrane and configured to receive the portion of the beam that is above the desired energy level. The current from the collection plate is measured and the energy contamination is detected if the current is above a threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a conventional ion implanter tool. 
         FIG. 2  is a block diagram of an exemplary EC detection system in accordance with the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
       FIG. 2  is a block diagram of an exemplary energy contamination (EC) detector  200 , which may be disposed on or near platen  114  of an ion implanter  100 . In particular, the detector  200  may replace a target substrate in situ to obtain the desired implant processing parameters. Once the desired parameters are achieved, the detector  200  is removed from or near the platen and replaced by the target substrate. EC detector  200  includes a membrane  202  supported by a housing  204  including a support frame  206 . The housing  204  defines an interior chamber  226 . A charge collection plate  208  is separated from membrane  202  by insulators  210  defining one or more pumping holes  212  therebetween. Charge collection plate  208  is coupled to a direct current (DC) bias circuit  214  and to a detection circuit or device  216 . 
     Support frame  206  of housing  204  is configured to support membrane  202 . When EC detection device  200  is positioned on or near platen  114 , membrane  202  receives ion beam  250  comprised of ions and neutrals. Support frame  206  may include a grid  222  for providing a backing to membrane  202 , and clamps  224  or other mounting device may be used to releasably secure membrane  202  to grid  222 . Membrane  202  receives the ion beam  250  and only allows the portion of the beam having high energy particles (EC) to emerge on the downstream side of membrane  202 . These particles may emerge as ions or neutrals that release secondary electrons which provides a detectable signal at collection plate  208  as described below. The energy particles that form beam  250  that are not part of the EC particles (i.e. the ions at or below the desired lower energy level for shallow implantation) are prevented from emerging from surface  220  and are essentially “filtered” out by membrane  202 . Thus, only the high energy contaminating particles emerge from the surface  220  of membrane  202  and can be measured directly without any numerical subtraction. 
     As mentioned above, prior attempts at EC detection required the measuring of the various components of the beam current including, for example, the total beam and the desired energy level ions and then inferring the EC by subtracting these two quantities. In contrast, in the present disclosure the role of the subtraction operation is substituted by the physical filtering of the EC particles in the membrane. 
     The thickness of membrane  202  may be selected based on an energy of the desired ion beam  250  for a specific implant recipe for fabricating devices on a substrate or wafer. For example, membrane  202  may have a thickness of approximately 30 nm for detecting EC in a semiconductor fabrication process using an ion beam having an energy of approximately 300 keV, and a thickness of approximately 15 nm for detecting EC for a recipe using a 150 keV process. Membrane  202  may be a diamond-like carbon (DLC) foil such as those available from Micromatter of Vancouver, British Columbia, Canada. Membranes  202  may be fabricated from other similar materials and have a thickness between approximately 10 nm and approximately 600 nm. However, membranes  202  having other thicknesses may also be implemented. 
     Charge collection plate  208  is fabricated from a conductive material and is electrically separated from support frame  206  by insulating walls  210 . This enables a DC bias to be applied to collection plate  208  without being applied to membrane  202 . Insulating sidewalls  210  may be formed from any substantially non-conductive material. One or more holes  212  enables interior chamber  226  of EC detection device  200  to be pumped with gas for equilibrating the pressure or the gas concentration within the chamber. Alternatively, holes  212  may be used to create a vacuum within chamber  226 . Holes  212  also enable the gas composition and pressure in the interior chamber  226  of EC detection device  200  to provide a consistent proportional electron gain. For very small amounts of EC, a higher DC bias may be applied to operate in an avalanche or Geiger counter mode and count individual high energy ions. 
     Detection circuit or device  216  may be any circuit or device configured to detect a voltage or current on charge collection plate  208 . In one embodiment, detection device  216  is a nanoammeter such as, for example, a Model 285 nanoammeter available from Monroe Electronics of Lyndonville, N.Y. 
     In operation, a membrane  202  is loaded onto housing  204  of EC detection device  200 . As described above, the thickness of membrane  202  is selected based on a desired ion beam energy in a recipe for fabricating devices on a target substrate. The EC device  200  is positioned on platen  114  of ion implanter  100  in the position in which a target substrate is to be positioned during the fabrication process. The EC detector  200  and in particular housing  204 , has a sufficient size to receive at least a portion of incident ion beam  250 . Obviously, the size of detector  200  is such that it fits within the confines of a processing chamber of ion implanter  100 . Once EC device  200  is positioned on platen  114 , an ion bean  250  is incident on membrane  202 . 
     Membrane  202  absorbs or deflects ions having an appropriate energy for the semiconductor fabrication recipe. Only the EC particles, which may be ions or neutrals, emerge from the downstream surface  220  of membrane  202  toward charge collection plate  208 . Some of these high energy particles will be ionized as their remaining kinetic energy is greater than the electron binding energy. These ions are collected via collection plate  208  by applying a relatively small DC bias to the collection plate with DC bias circuit  214 . For example, ions may be collected by providing a negative DC bias (e.g., 10-50V) to charge collection plate  208 . Alternatively, as the EC particles emerge from the downstream surface  220  of membrane  202 , they may be negatively charged ions or secondary electrons which are generated by the neutrals. These negatively charged particles may be collected by applying a positive DC bias to charge collection plate  208 . Larger DC biases may be applied to induce electron multiplication in the residual gas in order to increase the collected current above a threshold level associated with these secondary electrons. 
     As charge accumulates on charge collection plate  208 , a measurable signal (e.g., voltage or current) develops on collection plate  208 . For example, a typical high current ion beam has a current density of approximately 100 μA/cm 2 , and a typical maximum EC requirement for a semiconductor wafer is approximately 0.1%. Accordingly, the detection device  216  of EC detection device  200  having a membrane  202  with an area of approximately 1 cm 2  may measure a current of approximately 100nA, which identifies the presence of EC contamination. Consequently, an operator may adjust the settings of ion implanter  100 , replace membrane  202 , and have ion implanter  100  transmit another ion beam towards membrane  202  to test for energy contamination. 
     If a current or voltage above a threshold is not detected by detection device  216 , then the ion implanter  100  is properly calibrated for fabricating devices on the semiconductor wafer according to the recipe for which the EC detection was performed. Accordingly, the ion implanter  100  may then be used to fabricate devices on semiconductor wafers with a lower risk of energy contamination. 
     While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.