Patent Document

FIELD OF THE DISCLOSURE 
   The present disclosure relates generally to ion implantation and, more particularly, to techniques for ion beam current measurement using a scanning beam current transformer. 
   BACKGROUND OF THE DISCLOSURE 
   Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with high-energy ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels. A specification of the ion species, doses and energies is referred to as an ion implantation recipe. 
   In conventional ion implantation, ions are extracted from a plasma source and are typically filtered (e.g., for mass, charge, energy, etc.), accelerated and/or decelerated, and collimated through several electro-static/dynamic lenses before being directed to a substrate.  FIG. 1  depicts a conventional ion implanter system  100 . As is typical for most ion implanter systems, the system  100  is housed in a high-vacuum environment. The ion implanter system  100  may comprise an ion source  102  and a complex series of components through which an ion beam  10  passes. The series of components may include, for example, an extraction manipulator  104 , a filter magnet  106 , an acceleration or deceleration column  108 , an analyzer magnet  110 , a rotating mass slit  112 , a scanner  114 , and a corrector magnet  116 . Much like a series of optical lenses that manipulate a light beam, the ion implanter components may filter and focus the ion beam  10  before steering it towards a target wafer  120  (located in a wafer plane  12 ). 
   A number of measurement devices, such as a dose control Faraday cup  118 , a traveling Faraday cup  124 , and a setup Faraday cup  122 , may be used to monitor and control the ion beam conditions. Specifically, measurement of ion dose rate in the ion implantation system  100  may be accomplished using these one or more measurement devices. Because incident ion flux may be measured as an electrical current, the ion dose rate of the target wafer  120  may be calculated by dose count electronics (DCE) (not shown) by taking a measured electrical current and dividing by an aperture area of the one or more measurement devices. 
   In the design and operation of an ion implanter, ion dose uniformity and ion beam utilization are major concerns since they directly impact the productivity of the ion implanter. 
   To achieve a uniform distribution of dopants, an ion beam is typically moved across the surface of a target wafer during an implantation process.  FIG. 2A  shows a typical setup for continuous implantation with an ion beam. In an ion implanter system, e.g., a scanned beam implanter, an ion beam spot  202  may be swept horizontally (i.e., in the X direction) along a scan path  204  across the surface of a wafer  206 . A dose control Faraday cup  210  may be used to measure ion beam current. At the same time, the wafer  206  may be translated vertically along a path  208  (i.e., in the Y direction) through a process chamber. Thus, the ion beam spot  202  is scanned with respect to the wafer  206  in both the X and Y directions. The net effect of the movement of the ion beam spot  202  in the X and Y directions is a beam path  20 , as depicted in  FIG. 2B , that zigzags across the entire surface of the wafer  206  as well as its surrounding area. Since the ion beam spot  202  moves completely off the wafer  206  in each sweep, the total area covered by the ion beam spot  202  may be approximated with a box  22 , which may be substantially larger than the wafer  206 . 
   However, the traditional implantation method as illustrated in  FIGS. 2A and 2B  has a number of problems. For example, such a method often assumes that the ion beam spot  202  maintains the same profile and delivers the same dose at any location. Because conventional measurement devices, e.g., the dose control Faraday cup  118 , the traveling Faraday cup  124 , and the setup Faraday cup  122 , are either intercepting or situated at the side of the wafer  206 , the actual ion beam current, and therefore ion dose uniformity, at the wafer  206  may not be accurately measured or determined. 
   Furthermore, secondary electrons are typically produced upon energetic ion bombardment on these measurement devices. If secondary electrons are not suppressed or confined, most of these electrons may end up colliding with other components of the system  100 , which may cause sputtering or heating up of these components or may interfere with the accuracy of ion beam current measurements. Consequently, the accuracy of measuring implant dose is greatly affected by unconfined secondary electrons. 
   Additionally, in the traditional method, the ion beam spot  202 , in its scan path  204 , may go completely off the wafer edge in each sweep, which is known as a “full overscan.” Full overscans are deemed necessary to provide a uniform ion dose even at the edges of the wafer  206  and to allow real-time monitoring of ion beam conditions at measurement devices. If the spot size is small, the ion beam is off the wafer surface only briefly. However, if the spot size is large (e.g., greater than about a quarter of the wafer size), as is often the case for low-energy ion beams, the ion beam spot spends almost as much, if not more, time off the wafer as it does on the wafer. As a result, beam utilization becomes extremely low for a low-energy ion beam that is scanned fully off the wafer. 
   In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current ion implantation technologies. 
   SUMMARY OF THE DISCLOSURE 
   Techniques for ion beam current measurement using a transformer are disclosed. In one particular exemplary embodiment, the techniques may be realized as an apparatus for ion beam current measurement using a scanning beam current transformer. The apparatus may comprise a measurement device positioned adjacent a wafer and an ion dose control module coupled to the measurement device. The measurement device may comprise a transformer through which an ion beam passes onto the wafer. The ion dose control module may calculate ion beam current passing through the transformer and adjust dose based at least in part upon the calculated ion beam current. 
   In accordance with other aspects of this particular exemplary embodiment, the ion dose control module may comprise a current integrator to calculate the ion beam current passing through the transformer. 
   In accordance with further aspects of this particular exemplary embodiment, the ion dose control module may further control movement of the ion beam across the wafer according to a scan path, wherein the scan path permits the ion beam to sweep beyond an inner periphery of the transformer. 
   In accordance with additional aspects of this particular exemplary embodiment, the apparatus may further comprise a calibration coil, coupled to the measurement device, to provide simulated ion beam current for calibrating the ion dose control module. 
   In accordance with further aspects of this particular exemplary embodiment, the transformer may comprise a core with a coil wrapped around the core and a casing for the transformer, wherein the casing may includes electrically conductive, non-magnetic material. 
   In accordance with additional aspects of this particular exemplary embodiment, the transformer may be in the shape of an annular toroid, a rectangular toroid, or an elliptical toroid. 
   In accordance with further aspects of this particular exemplary embodiment, the ion beam current may be measured in real-time. 
   The present disclosure will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present disclosure is described below with reference to exemplary embodiments, it should be understood that the present disclosure is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein, and with respect to which the present disclosure may be of significant utility. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to facilitate a fuller understanding of the present disclosure, reference is now made to the accompanying drawings, in which like elements are referenced with like numerals. These drawings should not be construed as limiting the present disclosure, but are intended to be exemplary only. 
       FIG. 1  depicts a conventional ion implanter system. 
       FIGS. 2A-2B  depict a conventional setup for scanning a wafer with an ion beam. 
       FIGS. 3A-3B  depict an exemplary scanning beam current transformer configuration in accordance with an embodiment of the present disclosure. 
       FIG. 4A-4B  depict an exemplary graphical representation of induced magnetic flux (Φ) and induced electromotive force (e) for scanning ion beam current in accordance with an embodiment of the present disclosure. 
       FIG. 5  depicts an exemplary graphical oscillogram representation of primary and secondary current within a transformer in accordance with an embodiment of the present disclosure. 
       FIG. 6  depicts an exemplary scanning beam current transformer configuration in accordance with an embodiment of the present disclosure. 
       FIG. 7  depicts an exemplary scanning beam current transformer configuration in accordance with an embodiment of the present disclosure. 
       FIG. 8  depicts an exemplary dose control system for ion implantation in accordance with an embodiment of the present disclosure. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Embodiments of the present disclosure provide an ion implantation solution that improves ion beam current measurement and monitoring using a scanning beam current transformer for optimizing ion beam utilization while maintaining uniform ion dose. 
   Referring to  FIG. 3A , a top view of a current monitor  310  is shown in accordance with an embodiment of the present disclosure. In one embodiment, the current monitor  310  may include a scanning beam current monitor. The current monitor  310  may include a transformer  311  having a core  312  and a coil  314  wrapped around the core  312 . The core  312  may be in the shape of an annulus or a toroid and may be positioned within a transformer casing  316 . The transformer casing  316  may be formed of an electrically conductive, non-magnetic material, such as graphite or aluminum, and may be used as a shield or protective covering for the transformer  311 . Another role of the transformer casing  316  may be to ensure that induced magnetic flux links the minor turns of the coil  314  and not the large major turn. Additionally, eventual azimuthal currents induced in the transformer casing  316  by an axial component of the induced magnetic field (e.g., in the case of slight deviations from perpendicularity of the scanning beam on coil plane) may cancel the azimuthal components of the flux in the transformer casing  316 . In one embodiment, the core  312  may be fabricated of high magnetic permeability material, e.g., Vitrovac®, μmetal, or other similar material, and the coil  314  may be fabricated of a ferroelectric and/or conductive material, e.g., copper or other similar material. Other various materials may also be utilized. 
   The current monitor  310  may be connected to a current integrator  318  through wires of the coil  314 . Additionally, the current integrator  318  may be connected to a dose control system  700 , as depicted in  FIG. 7 . Alternatively, in another embodiment, the current integrator  318  may be connected to the dose control system  700  through a feedback loop to compensate for dose variations during ion implantation. 
   A calibration coil  320  may wrap around the current monitor  310 . In one embodiment, the calibration coil  320  may include a single turn and provide the current monitor  310  with a simulated beam current, which may be useful for calibrating the current monitor  310 . In another embodiment, the calibration coil  320  may include a predetermined number of turns for more reliable and accurate calibration. 
   Referring to  FIG. 3B , a side view of the current monitor  310  is shown in accordance with an embodiment of the present disclosure. In this embodiment, the transformer casing  316  may include an inner casing  316   a  and an outer casing  316   b . One or more fasteners  317  may hold the inner casing  316   a  and the outer casing  316   b  together to secure the transformer  311 , which may be fitted within the inner casing  316   a . In one embodiment, the inner casing  316   a  and the outer casing  316   b  may be formed of the same electrically conductive, non-magnetic material, e.g., graphite or aluminum. In another embodiment, the inner casing  316   a  and the outer casing  316   b  may be formed of different electrically conductive, non-magnetic materials, e.g., the inner casing  316   a  may be formed of graphite and the outer casing  316   b  may be formed of aluminum. Other various materials may also be utilized. Furthermore, in yet another embodiment, the transformer casing  316  may be symmetrically grounded. This may ensure a short path to ground as well as no generation of azimuthal currents when the ion beam spot  202  scans across the transformer casing  316 . 
   Referring back to  FIG. 3A , as the ion beam spot  202  is be swept horizontally (i.e., in the X direction) along the scan path  204  across the surface of the wafer  206 , the current monitor  310  may be used to measure the ion beam current at the wafer  206 . At each sweep along the scan path  204 , the ion beam spot  202  may cover a distance beyond the outer border of the current monitor  310 , as depicted in  FIG. 3A . This is particularly important so that the current integrator  318  (and other measurement electronics) may accurately measure the ion beam current through the center of the transformer  311  by sweeping over the inner edge of the transformer casing  316 . The basis for calculating ion beam current within the transformer  311  will be discussed in further detail below. 
   Charges in motion, such as electrical current, may create a magnetic field. For example, according to Biot-Savart law, magnetic field generated by a current element Idl may be expressed as:
 
 dB =[(μ I )/(4π)]·[( dl×r )/ r   3 ],
 
where dB represents the magnetic field induction, μ represents magnetic permeability of a medium, and r represents a displacement vector.
 
   For the geometry of the current monitor  310  (e.g., a toroidal coil, as depicted in  FIG. 4A , where a current (I p ) perpendicular on the coil plane having a direction entering the paper sheet), the magnetic field induction may have a direction shown on  FIG. 4A  and may be expressed as:
 
 B =(μ c   I   p )/(2 πr ),
 
where μ c  represents magnetic permeability of the core  312 . Thus, if current (I p ) varies with time, the induced magnetic field (B) may also be a function of time. Accordingly, the magnetic flux (Φ) through the core may be expressed as:
 
Φ( t )= B ( t ) A,  
 
where A represents cross-section area of the core  312 . This forms the basis for calculating ion beam current within the transformer  311 .
 
   According to Faraday&#39;s law, the temporal variation of magnetic flux may then induce an electromagnetic force (e):
 
 e=−N ( dΦ/dt ),
 
where N represents the number of windings of the coil  314 . Therefore, for a toroidal current transformer, e.g., a Rogowski coil, the electromotive force (e) may be expressed as:
 
| e |=[(μ c   NA )/(2 πr )]×[ dI   p   /dt].  
 
   When a pulsed primary current, I p , having, for example, a shape provided by a Heaviside function, passes through the aperture of a Rogowski coil, an induced secondary current I s  in the windings of the coil  314  may be expressed as:
 
 I   s ( t )=(1/ N )·exp[(− R/L ) t],  
 
where N represents number of windings and (R/L) represents the “droop” rate (the inverse of the time constant). Accordingly, integration of such secondary current, I s , may yield a true value of pulsed primary current, I p .
 
   However, in the case of DC currents, or more specifically for implanting systems for which constant ion beam current for a constant dose during implant may be required, the induced emf (e) may be zero. As a result, the value of I p  may not be readily inferred. For example, as depicted in  FIG. 4A , when a scan path  404   a  does not extend beyond an inner border periphery of the transformer  311 , in spite of a nonzero magnetic flux through the core of the coil, the induced emf (e) may be zero since, according to Ampere&#39;s law along a contour C (the mean circumference of the coil), 
                 ∮   C     ⁢     B   ⁢     ⅆ   1         =     ∑     μ   ⁢           ⁢     I   p           ,         
there is no variation in the magnetic field induction (B) and no variation (implicit) in the induced magnetic flux (Φ).
 
   However, as depicted in  FIG. 4B , when a scan path  404   b  extends beyond the inner border periphery of the transformer  311 , there may be a variation of primary current, I p , due to its increasing or decreasing cross-section as the beam sweeps across the inner border of the grounded housing containing the core  312 . As a result, a temporal variation in the magnetic flux (Φ) and consequently an emf (e) may be induced, as shown in  FIG. 4B . Accordingly, integration of the secondary current, I s , may yield a value of the ion beam current at the wafer  206 . The secondary current, I s , may be integrated and the ion beam current, I p , at the wafer  206  may be measured. Here, by extending the ion spot beam  202  beyond the outer periphery of the transformer  311 , the value of the magnetic field B at the transformer  311  and the value of the electrical current, I p , as shown in  FIG. 4B , may provide values for which integration will yield a value for ion beam current at the wafer  206 . 
   For a linear variation of an ion beam current as it sweeps over the inner border periphery of the transformer casing  316 , an induced secondary current, I s , may be expressed as:
 
0, for t&lt;0;
 
 I   s ( t )= I   p (μ c   Na   2 /2 r   0   R τ)·[1−exp(− tR/L )], for 0≦t&lt;t 0 ;
 
I p (μ c Na 2 /2r 0 Rτ)·[1−exp(−t 0 R/L)]·exp [−(t−t 0 )R/L], for t≧t 0 ;
 
where r 0  and a represent a mean major and a minor radii of a torus, respectively, R represents total resistance (coil+external) viewed by the secondary current, I s , τ represents a sweeping time across the inner border periphery, L represents the self-inductance of the coil  314 , and t 0  represents the instant when the ion beam  202  is no longer sensed by the core  312 .
 
   For example, as depicted in  FIG. 5 , such analytical predictions on a shape of the secondary current, I s , may be reproduced in experimental measurements. Therefore, an integration of the secondary current, I s , as well as a previous accurate calibration, may yield an accurate value of primary current, I p . 
   In one embodiment, for the particular case of a torus having a mean major radius r 0 =6.75 inches (large enough to encircle a standard 300 mm wafer), a minor radius a=0.25 inches, made of magnetic material having μ r =1.5×10 5 , theoretical predictions may give a relative magnetic permeability of the core μ c =˜1720 and an optimal number of coil turns N=˜150. Then, under the approximation of a uniform current density across the beam, the time dependency of the ion beam current as it passes the inner border of the transformer casing  316  may be expressed by:
 
 I   p ( t )=( I   p0 /2π)·{Arc Cos(1− v   s   t /ξ)−(1− v   s   t /ξ)·[(1−(1− vt /ξ) 2 ] 1/2 ],
 
where I p0  represents total ion beam current, ξ represents beam radius, and v s  represents scanning speed. For usual operating parameters in an ion implanter, e.g., ion beam current of ˜1 mA, an ion beam diameter of ˜5 cm, and a scanning speed of ˜1 mm/μs, the induced secondary current amplitude may be ˜15 μA. This value may be large enough to be measured (e.g., as a voltage drop on an external resistor), integrated, and further processed to obtain the accurate value of the total ion beam current I p0  at the wafer  206 .
 
   In the illustrated embodiments of the present disclosure, the current monitor  310  is shown with a ring-like (annular) toroidal shape since this geometry may ensure magnetic flux uniformity inside the core  312 , minimal transmit time, and improved signal-to-noise ratio. However, a current monitor having other shapes (e.g., elliptical, rectangular, etc.) and sizes may also be utilized, provided that these dimensional factors are taken into account in calculating self-inductance, magnetic flux losses, coil winding uniformity, etc. 
   Referring to  FIG. 6 , a top view of a current monitor  610  is shown in accordance with another embodiment of the present disclosure. Similar to  FIG. 3A , the current monitor  610  may include a transformer  611  having a core  612  and a coil  614  wrapped around the core  612 . The current monitor  510  may be connected to a current integrator  318  through wires of the coil  614 . The current integrator  318  may be connected to a dose control system  800 , as depicted in  FIG. 8 . A calibration coil  320  may wrap around the current monitor  610 . In one embodiment, the calibration coil  320  may include a single turn and provide the current monitor  610  with a simulated beam current, which may be useful for calibrating the current monitor  310 . In another embodiment, the calibration coil  620  may include a predetermined number of turns for more reliable and accurate calibration. 
   However, in this embodiment, unlike  FIG. 3A , the transformer  611  may have a rectangular shape and may be positioned within a transformer casing  616 , which may also be rectangular in shape. The transformer casing  616  may be formed of an electrically conductive, non-magnetic material, such as graphite or aluminum, and may be used as a shield or protective covering for the transformer  611 . Other various materials may also be utilized. Furthermore, in yet another embodiment, the transformer casing  616  may be symmetrically grounded. This may ensure a short path to ground as well as no generation of azimuthal currents when the ion beam scans across the transformer casing  616 . 
   One benefit with utilizing a rectangular-shaped transformer  611 , as depicted in  FIG. 6 , may include a reduced size of the current monitor. Having a smaller beam-to-core distance may increase the magnetic field induction B and, therefore (implicitly), increase the magnetic flux Φ since the magnetic field B is inversely proportional with the distance from the current. A drawback with a rectangular-shaped transformer  611 , however, may include losses associated with sharp corners of the core  612 . As a result, other embodiments may be provided to balance the size of the transformer  611  with the magnetic field produced. 
   For example, referring to  FIG. 7 , a top view of a current monitor  710  is shown in accordance with another embodiment of the present disclosure. Similar to  FIG. 6 , the current monitor  710  may include a transformer  711  having a core  712  and a coil  714  wrapped around the core  712 . The current monitor  710  may be connected to a current integrator  318  through wires of the coil  714 . The current integrator  318  may be connected to a dose control system  800 , as depicted in  FIG. 8 . A calibration coil  320  may wrap around the current monitor  710 . In one embodiment, the calibration coil  320  may include a single turn and provide the current monitor  710  with a simulated beam current, which may be useful for calibrating the current monitor  710 . In another embodiment, the calibration coil  320  may include a predetermined number of turns for more reliable and accurate calibration. 
   However, in this embodiment, unlike  FIGS. 3A and 6 , the transformer  711  may have an elliptical shape and may be positioned within a transformer casing  716 , which may also be elliptical in shape. The transformer casing  716  may be formed of an electrically conductive, non-magnetic material, such as graphite or aluminum, and may be used as a shield or protective covering for the transformer  711 . Other various materials may also be utilized. Furthermore, in yet another embodiment, the transformer casing  716  may be symmetrically grounded. This may ensure a short path to ground as well as no generation of azimuthal currents when the ion beam scans across the transformer casing  716 . 
   The current monitor  710  with the transformer  711  having an elliptical shape may provide a smaller beam-to-core distance as compared to the annular toroidal transformer  311  of  FIG. 3A  and a reduction in losses (e.g., from sharp corners) as compared to the rectangular-shaped transformer  611  of  FIG. 6 . 
     FIG. 8  depicts an exemplary dose control system  800  for ion implantation in accordance with an embodiment of the present disclosure. The system  800  may comprise a processor unit  802  (e.g., a dose controller) which may be a microprocessor, micro-controller, personal computer (PC), or any other processing device. The system  800  may also comprise a beam movement controller  804  that controls the movement of an ion beam in an ion implanter system  80  according to instructions received from the processor unit  802 . The system  800  may further comprise a measurement interface  806  through which the processor unit  802  may receive ion beam measurement data (e.g., beam current, dose and shape) from the ion implanter system  80 . The measurement interface  806  may include or be coupled to one or more measurement devices. The system  800  may be used to set up a 2-D velocity profile for beam movement, to control an ion implantation process based on the 2-D velocity profile, and to provide real-time, closed-loop adjustments to the 2-D velocity profile. Furthermore, the system  800  may provide dose control at the ion implanter system  80  based on the ion beam current measurements obtained from a current monitor, e.g., a scanning beam current monitor. 
   One advantage with utilizing embodiments of a current monitor in accordance with embodiments of the present disclosure may include increased accuracy in ion beam current measurements at a wafer. Because the current monitor is non-intercepting and measures ion beam current directly bombarding the wafer, accurate ion beam current measurements may be obtained. Another factor contributing to increased accuracy may include the fact that current-to-area ratio calculations are no longer necessary for current monitors of the present disclosure. Ion beam drift effects on dose and acceptance angle errors may also be eliminated to ultimately provide a more accurate ion beam measurement. Also, since the current monitor is non-intercepting, not only is accuracy optimized, but real-time ion beam current measurements may also be obtained. 
   Another advantage of the present disclosure is that a current monitor in accordance with embodiments of the present disclosure may be integrated with existing electronics. This may lead to reduced costs associated with implementing the current monitor with current systems not only to provide accurate ion beam measurements but also for dose compensation. 
   Furthermore, since a current monitor in accordance with embodiments of the present disclosure involves no moving parts, little or no maintenance may be required. Therefore, consistency and reliability of ion beam current measurements and dose compensation may be achieved with relative regularity. 
   Other advantages of the present disclosure may include an increase in ion beam utilization and availability of external calibration. These features may serve to reduce costs and improve measurements and calculations. 
   The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.

Technology Category: 5