Patent Application
20050181584
This invention relates to a method of implanting ions in a substrate using an ion beam where instabilities in the ion beam may be present and to an ion implanter for use with such a method. This invention also relates to an ion source for generating an ion beam that can be switched off rapidly.
Ion implanters are well known and generally conform to a common design as follows. An ion source produces a mixed beam of ions from a pre-cursor gas or the like. Only ions of a particular species are usually required for implantation into a substrate, for example a particular dopant for implantation into a semiconductor wafer. The required ions are selected from the mixed ion beam using a mass-analysing magnet in association with a mass-resolving slit. Hence, an ion beam containing almost exclusively the required ion species emerges from the mass-resolving slit to be transported to the process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder.
Frequently, an ion beam used for implantation has a smaller cross-sectional area than the substrate to be implanted. In order to ensure ion implantation across the whole of the substrate, the ion beam and substrate are moved relative to one another such that the ion beam scans the entire substrate surface. This may be achieved by (a) deflecting the ion beam to scan across the substrate which is held in a fixed position, (b) mechanically moving the substrate whilst keeping the ion beam path fixed, or (c) deflecting the ion beam and moving the substrate.
Substrates are generally implanted serially one after another or as a batch at one time: for serial processing, relative motion between ion beam and substrate is effected such that the ion beam traces a raster pattern on the substrate surface by scanning to and fro across the substrate to form a series of parallel, equally-spaced scan lines; for batch processing, the substrates are held on spokes of a rotating wheel such that the ion beam scans across each substrate in a series of scan lines that form adjacent arcs.
To achieve uniform implantation, there must be adequate overlap between adjacent scan lines. Put another way, if the spacing between adjacent scan lines (with respect to the ion beam width profile) is too great, “striping” of the substrate will result with periodic bands of increased and decreased doping levels.
The precautions described above cannot be effected if the ion beam incident on the substrate is not itself uniform over time. Unfortunately, instabilities of the ion beam are inevitable and result from discharges in the ion source area for example. The effect of these instabilities is that there is a “glitch” in the ion beam in that the flux will usually drop significantly within a short period of time. The drop in ion beam flux leads to areas of the semiconductor wafer receiving a lower level of doping that may lead to the production of faulty semiconductor devices. More unusually, a rapid rise is seen in the ion beam flux. Again, this produces incorrect dosing that may lead to faulty devices.
The above problem is particularly severe for serial processing ion implanters that use mechanically scanned substrate holders, as will now be explained. To create the raster pattern, the substrate holder is moved in a reciprocating fashion and there is a limit to the maximum speed at which this can be done. To date this has been far lower than the scanning speeds that can be achieved with rotating batch substrate holders. Fast scan speeds require the ion beam to make many passes over the substrate to achieve a desired dosing: any instability in the beam during a single pass leads to a small residual dosing error due to dilution by the many subsequent passes. The adverse effects are far more severe in serial processing where the slow scan speeds result in fewer passes to achieve the same dosing.
The problem of ion beam instabilities has been addressed previously, see The Ion Beam Optics of a Single Wafer High Current Ion Implanter by White et al., Proceedings of the Eleventh International Conference on Ion Implantation Technology, North Holland (1997), pages 396-399. However, this disclosure is made in the context of high-current implantation using a ribbon beam (i.e. a beam with a width wider than the substrate such that scanning in effected in the direction perpendicular to the beam's width only rather than with two-dimensional, mechanical scanning). Upon detecting a beam instability during a scan, the ion beam is gated off for the rest of the scan. The scan is then repeated in the reverse direction and the ion beam gated off once more upon reaching the position corresponding to where the instability had been detected.
Hence, there is a demand for methods of addressing the problem of ion beam instability such that a uniform dosing of a substrate can be achieved, particularly for systems using an ion beam of a smaller size than the substrate and also for mechanically-scanned implantation.
According to a first aspect, the present invention resides in a method of implanting ions in a substrate using an ion beam having cross-sectional dimensions smaller than the substrate comprising the steps of: (a) establishing a stable ion beam with the substrate clear of the ion beam; (b) implanting the substrate by causing relative motion between the ion beam and the substrate such that the ion beam traverses the substrate along at least one path; (c) monitoring the ion beam for instabilities during step (b); (d) upon detecting an ion beam instability, switching off the ion beam as the relative motion continues to leave an unimplanted portion of the path; (e) recording an off position corresponding to the ion beam's position relative to the substrate when the ion beam is switched off in step (d); (f) establishing a stable ion beam once more; and (g) continuing to implant the substrate by causing relative motion between the ion beam and the substrate along the unimplanted portion of the path.
Extinguishing the ion beam upon detecting an instability is advantageous as it stops implantation and thus avoids creating an area of non-uniform implantation in the substrate.
Recording the off position is beneficial as it allows control of further implantation to ensure uniform dosing of the substrate. The off position may be recorded when an action is taken to switch off the ion beam (e.g. interrupting power to an ion source). If this is done, it is clearly advantageous for the ion beam to be switched off rapidly. Where there is a known latency in switching off the ion beam, the off position may be recorded as the position where the action is taken to switch off the ion beam plus the distance corresponding to this latency.
Alternatively, the ion beam flux may be monitored and the off position may be recorded when the ion beam flux is zero or drops below a threshold. Clearly, the phrase “recording an off position corresponding to the ion beam's position relative to the substrate when the ion beam is switched off” can be construed to cover these possibilities.
In addition, a profile of the ion beam may be taken to identify any changes in beam shape of movements in the centre of the beam. Any changes identified may be corrected by tuning the beam or by slightly altering the position of the beam as it follows the path.
The relative motion may form a series of scan lines that extend in parallel and the scan lines may, optionally, form a raster pattern.
The relative motion between ion beam and substrate is preferably controlled to ensure the same dosing as for the previously implanted portion of the path. For example, the same relative speed should be used if the ion beam has the same flux as before it was extinguished. If a difference in ion beam flux is determined, the relative speed may be adjusted to ensure the same dosing (i.e. the relative speed may be measured in response to an increase in ion beam flux).
According to one embodiment, step (f) comprises establishing a stable ion beam with the substrate clear of the ion beam prior to step (g); step (g) comprises causing relative motion between the ion beam and the substrate such that the ion beam travels along said path in a reverse direction, that is in an opposite direction as for step (b); and switching off the ion beam when the ion beam crosses the off position.
Restarting the ion beam clear of the substrate avoids non-uniformities in implanting as the ion beam settles to a stable flux. In addition, extinguishing the ion beam can be performed rapidly and so the drop in dosing concentration is abrupt. Moreover, the exact timing of switching the ion beam off as it reaches the off position can be adjusted to optimise overlap of any short tailing-off regions where the ion beam is extinguished. As the ion beam is scanned in the reverse direction, the overlap of the tailing-off regions complement each other to give the desired uniformity.
According to a second embodiment, step (g) further comprises switching the ion beam on at the off position prior to the ion beam traversing the unimplanted portion of said path in the forward direction, that is the same direction as for step (b). Preferably, step (g) comprises causing relative motion between the ion beam and substrate in the forward direction from a point along said path such that the ion beam is switched on upon crossing the off position. After starting the ion beam, there is a brief period where the ion beam flux increases to its stable value. This behaviour can be determined and the operation of the ion implanter adjusted to ensure the tailing-off region where the ion beam was extinguished complements the ramping-up region where the ion beam is restarted to give uniform dosing. The exact timing of when the relative speeds of ion beam and substrate can be adjusted to provide uniform dosing.
Where recovery is performed by scanning in the reverse direction, the method may further comprise repeating steps (c), (d) and (e) during step (g) such that, if a second beam instability is detected, a central portion of said path is not implanted; and continuing to implant the substrate once more by causing relative motion between the ion beam and the substrate such that the ion beam travels across the substrate along the central portion of said path. Preferably, the method comprises the steps of commencing the relative motion along said path outside of the central portion, switching the beam on when first crossing an off position and switching the beam off when crossing the other off position. As will be appreciated, this dosing may be performed in either direction.
From a second aspect, the present invention resides in a method of implanting ions in a substrate held in a substrate holder moveable bidirectionally along a first axis of translation, the method comprising the steps of: (a) establishing a stable ion beam having cross-sectional dimensions smaller than the substrate with the ion beam clear of the substrate in a start position adjacent the substrate along the first axis; (b) implanting the substrate by moving the substrate holder along the first axis such that the ion beam transverses the substrate along a first scan line and continues until clear of the substrate; (c) causing relative motion between the ion beam and the substrate holder along a second axis; (d) repeating steps (b) and (c) to implant a series of scan lines across the substrate; (e) monitoring the ion beam during implantation in step (b) and as repeated according to step (d); (f) upon detecting an ion beam instability, switching off the ion beam as the relative motion continues to leave an unimplanted portion of the scan line; (g) recording an off position corresponding to the position of the substrate holder when the ion beam is switched off in step (f); (h) establishing a stable ion beam once more; (i) completing implantation of the scan line by moving the substrate holder along the first axis so that the ion beam scans over the unimplanted portion of the scan line; and (j) completing implantation of the substrate by repeating steps (b) and (c) to complete the series of scan lines across the substrate.
Movement along the first axis may form a series of scan lines that extend in parallel and the scan lines may, optionally, form a raster pattern. The movement may be in one direction along the first axis or may be in both directions along the first axis.
Preferably step (c) comprises translating the substrate holder along a second axis of translation relative to a fixed ion beam, the first and second axes being perpendicular. Alternatively, the ion beam may be deflected along such a second axis.
From a third aspect, the present invention resides in an ion implanter controller for an ion implanter operable to generate an ion beam for implanting into a substrate, the controller comprising: ion beam switching means operable to cause the ion beam to switch on and off; scanning means operable to cause relative motion between the ion beam and the substrate such that the ion beam traverses the substrate along at least one path; ion beam monitoring means operable to receive a signal indicative of the ion beam flux and to detect instabilities in the ion beam therefrom during said relative motion; and indexing means operable to determine the position of the ion beam relative to the substrate during said relative motion; wherein the controller is arranged such that: the ion beam switching means is operable to cause the ion beam to switch off during the relative motion when the ion beam monitoring means detects an instability in the ion beam to leave an unimplanted portion of the path; the indexing means records an off position of the ion beam relative to the substrate when the ion beam is switched off; the ion beam switching means is operable to cause the ion beam to switch on once more; and the scanning means is operable to cause relative motion between the ion beam and the substrate such that the ion beam traverses the substrate along the unimplanted part of the path.
The ion implanter controller may be embodied in hardware or software form, i.e. parts of the controller may be implemented electronically or using software provided on a computer or the like. In fact, a part-hardware and part-software implementation could be followed where some parts are based on electronic components and others are based in software.
Movement along the first axis may form a series of scan lines that extend in parallel and the scan lines may, optionally, form a raster pattern. The movement may be in one direction along the first axis or may be in both directions along the first axis.
From a fourth aspect, the present invention resides in an ion implanter for implanting a substrate using an ion beam, including the controller described herein above.
From a fifth aspect, the present invention resides in an ion source for an ion implanter comprising: a cathode; an anode; biasing means for biasing the anode relative to the cathode; a first switch; and a first electrical path connecting anode to cathode via the biasing means and switch arranged in series; wherein the first switch is operable to make or break the first electrical path. This simple arrangement rapidly isolates the biasing means that otherwise biases the anode relative to the cathode. Hence, an ion beam may be rapidly extinguished when an instability is detected.
Optionally, the ion source further comprises a second conductor path connecting anode to cathode with at least a portion that extends in parallel across the biasing means, the portion comprising a second switch operable to make or break the second electrical path. Preferably, the first switch is operable in response to a first binary switching signal and the second switch is operable in response to a second binary switching signal that is the complement of the first switching signal. This allows a convenient way of switching the potential of the anode to be biased either relative to the cathode or at the same potential as the cathode. When a potential difference exists, an ion beam is produced: when no potential difference exists, there is no ion beam.
Preferably, the first switch and/or any second switch is a power semiconductor switch as this allows particularly rapid switching and hence particularly rapid extinction or creation of an ion beam.
The present invention also extends to an ion implanter including the ion source described herein above and to a method of switching such an ion source comprising the step of operating the first switch to break the first electrical path in response to detection of an instability in the ion beam generated by the ion source.
This method may be accompanied by the steps of maintaining or increasing the power supplied to the cathode. For example, the ion source may comprise an indirectly heated cathode and three power supplies: a filament supply (for the cathode's filament), a bias supply (for biasing within the indirectly heated cathode) and an arc supply (for biasing the anode relative to the cathode). Power supplied by the filament supply and the bias supply may be maintained, or may be increased to match the power of the arc supply prior to operating the first switch. This is to minimise any cooling in the ion source, and in the cathode in particular, when arc discharging ceases. Indirectly heated cathodes comprise a filament in front of an end cap. Increasing power supplied by the filament supply generates more electrons to be accelerated into the end cap, whilst increasing the power supplied by the bias supply increases the energy with which the electrons strike the end cap: in either case, the cathode enjoys greater heating from the electrons to compensate for the heating otherwise provided by the arcing.
Other preferred features of the invention are set forth in the appended claims.
Examples of the invention will now be described with reference to the accompanying drawings, in which:
a to 4d show an ion beam scanning scheme according to a first embodiment of the present invention for use during ion implantation where a glitch in the ion beam is detected;
a to 5d correspond to
a to 6d correspond to
This potential difference causes positively charged ions to be extracted from the ion source 22 into the flight tube 24. The flight tube 24 includes a mass-analysis arrangement comprising a mass-analysing magnet 28 and a mass-resolving slit 32. Upon entering the mass-analysis apparatus within the flight tube 24, the electrically charged ions are deflected by the magnetic field of the mass-analysis magnet 28. The radius and curvature of each ion's flight path is defined, through a constant magnetic field, by the mass/charge ratio of the individual ions.
The mass-resolving slit 32 ensures that only ions having a chosen mass/charge ratio emerge from the mass analysis arrangement. In fact, the ion source 22 and mass analysing magnet 28 are rotated through 90° when compared to the arrangement of
To maintain the ion beam current at an acceptable level, an ion extraction energy is set by a regulated high-tension power supply 26: the flight tube 24 is at a negative potential relative to the ion source 22 by virtue of this power supply 26. The ions are maintained at this energy throughout the flight tube 24 until they emerge from the tube 34. It is often desirable for the energy with which the ions impact the wafer 36 to be considerably lower than the extraction energy. In this case, a reverse bias voltage must be applied between the wafer 36 and the flight tube 24. The wafer holder 38 and beamstop 40 are contained within a process chamber 42 that is mounted relative to the flight tube 24 by insulating standoffs 44. Both the beamstop 40 and wafer holder 38 are connected to the flight tube 24 via a deceleration power supply 46. The beamstop 40 and wafer holder 38 are held at a common ground potential so that, to decelerate the positively-charged ions, the deceleration power supply 46 generates a negative potential with respect to the grounded wafer holder 38 and beamstop 40 at the flight tube 24.
In some situations, it is desirable to accelerate the ions prior to implantation in the wafer 36. This is most easily achieved by reversing the polarity of the power supply 46. In other situations, the ions are left to drift from flight tube 24 to wafer 36, i.e. without acceleration or deceleration. This can be achieved by providing a switched current path to a short out the power supply 46.
Turning now to
The indirectly heated cathode 52 comprises a filament 54 supplied by a filament power supply unit 56. The filament supply 56 provides sufficient current to cause thermionic emission of electrons from the filament 54. The indirectly heated cathode 52 also comprises a tube 58 enclosing the filament 54 that is connected across a bias power supply unit 60 such that the tube 58 is at a positive potential relative to the filament 54. This ensures that electrons emitted by the filament 54 are attracted and accelerated into the end-cap of the tube 58. The impacts of the electrons heat the end-cap of the tube 58 such that it emits electrons into the ion source chamber 48.
The chamber walls 50 are held at a positive potential relative to the tube 58 by virtue of their connection to an arc power supply unit 62. Accordingly, electrons emitted by the tube 58 are attracted to the chamber walls 50. In fact, the motion of the electrons emitted from the cathode 52 is constrained by creating a magnetic field across the ion source 22 using a pair of coils of an associated electromagnet (not shown). The magnetic field created is such that electrons emitted by the cathode 52 follow a spiral path towards the far end of the ion source chamber 48.
Located at this far end is a counter-cathode 64 also connected to the bias supply 60 so as to be at the same potential as the tube 58 of the indirectly heated cathode 52. Accordingly, electrons approaching the counter-cathode 64 are repelled such that they travel back along the spiral path in a reverse direction. This increases the chances of electrons interacting with the pre-cursor gas that fills the ion source chamber 48 thereby creating more ions that may be extracted through an aperture 66 provided in the chamber walls 50 to form the ion beam 23.
As described previously, the wafer holder 38 can be moved along the X and Y axes. Movement of the wafer holder 38 is controlled such that the fixed ion beam 23 scans across the wafer 36 according to the raster pattern 68 shown in
The ion beam 23 is scanned over the wafer to form a raster pattern of parallel, spaced scan lines 70. This is achieved by scanning the ion beam 23 forwards along the X-axis direction to form the first scan line 70 until the ion beam is clear of the wafer 36, moving the ion beam 23 up along the Y-axis direction as shown at 72, scanning the ion beam 23 backwards along the X-axis direction until clear of the wafer 36 once more, moving the ion beam 23 up along the Y-axis direction 72, and so on until the whole wafer 36 has seen the ion beam 23.
During scanning of the ion beam 23 across the wafer 36, the ion beam current is measured such that any glitches in ion beam flux can be detected. A detailed description of how the ion beam current is measured and the conditions that correspond to a glitch follows later. As scanning is performed by moving the wafer holder 38 in a controlled manner, the position of the ion beam 23 relative to the wafer 36 is known at any instant. Hence, the position of the ion beam 23 on the wafer 36 at the instant a glitch is detected or at the instant the ion beam 23 is turned off may be determined.
a shows the initial stages of a raster scan 68 formed during implantation. Seven complete scan lines 70 have been formed on the wafer 36. However, a glitch in the ion beam 23 is detected during the eighth scan line 74. The ion implanter 20 responds to detection of the glitch by extinguishing the ion beam 23 as rapidly as possible. Extinguishing the ion beam 23 results in the ion beam 23 switching off at the position shown in
Movement of the wafer holder 38 continues along the scan line when and after the ion beam 23 is extinguished such that the ion beam 23, were it still switched on, would follow the remainder of the current scan line in a forward direction to end beyond the far side of the wafer 36 at the position 79 (this movement is shown by the dashed line 78 in
In this position 79, the ion beam 23 is switched on once more and is monitored to detect when stability has been achieved. Upon confirmation of a stable ion beam 23, the wafer holder 38 is moved once more such that it follows the current scan line, but in the reverse direction as shown by the solid line 80.
The ion beam 23 is restarted once more at 83 and, upon confirmation of a stable ion beam 23, the remainder of the raster scan 68 is performed as shown in
It is inadvisable to restart the ion beam 23 when it will be incident upon the wafer 36 as this will dose further the wafer 36 at that point. In addition, it is inadvisable to restart the ion beam 23 when it will be incident upon the wafer holder 38 as this may produce contamination. This may be the case as the wafer holder 38 extends adjacent the wafer 36 along the X-axis and so a movement along the X-axis direction alone may not be enough to ensure the ion beam is clear of the wafer holder 58. Accordingly, after a scan line 70 has been followed with the ion beam 23 switched off following a glitch, the wafer holder 38 is moved in the Y-axis direction prior to restarting the ion beam 23 if it would otherwise strike the wafer holder 38. Once a stable ion beam 23 is obtained, the wafer holder 38 is moved back along the Y-axis direction and the next movement along a scan line 70 is performed.
An alternative method for recovering from a glitch in the ion beam 23 is shown in
In addition to extinguishing the ion beam 23, movement of the wafer holder 38 is stopped and then reversed such that, if the ion beam 23 were still switched on, it would follow the current scan line 74 but in the reverse direction to end up clear of the wafer 36 at 79. This movement is reflected in
Movement of the wafer holder 38 is started once more, with the ion beam 23 still switched off, such that the ion beam 23 would follow the current scan line 74 in the forwards direction as shown by the dashed line 86. When the “off” position 76 is reached, the ion beam 23 is switched on rapidly while movement of the wafer holder 38 continues to complete the current scan line 70. This is shown in
The method of
Of course, the possibility exists that a further beam instability may occur during a second pass 80;88 along a scan line 74 where a previous glitch is being repaired. Were this to happen in the method described with relation to
Clearly the situation is different for the method already described with respect to
b shows the start of the recovery operation where the ion beam 23 is switched on at 79 and, upon confirmation of a stable ion beam 23, the wafer holder 38 is moved such that implantation occurs along the current scan line 74 in the reverse direction as shown by 80. However, at the point 90 indicated in
The ion beam 23 is extinguished while translation of the wafer holder 38 continues such that, if the ion beam 23 were still switched on, it would follow the current scan line 70 along the reverse direction to reach the far side of the wafer 36 at 83 (the movement is shown by the dashed line 92). Movement of the wafer holder 38 is then reversed to follow the current scan line 70 in a forwards direction and continues along the entire length of the current scan line 70. During this movement, initially the ion beam 23 is switched off as shown by 94, the ion beam 23 is switched on when reaching the first “off” position 76 to form the line 96 and is then switched off upon reaching the second “off” position 90 to continue as dashed line 98.
Accordingly, the remaining central portion of the current scan line 70 is implanted thereby forming a complete scan line 70 with uniform implantation. As before, the remainder of the wafer 36 can be implanted using the standard raster pattern 68 as shown in
In order to determine when beam glitches occur, the ion beam current is monitored continuously by using a return current monitor. This arrangement will now be described with reference to
As mentioned previously, in usual operation the deceleration supply 46 generates a negative potential with respect to the grounded wafer holder 38 and beamstop 40 to decelerate positively-charged ions exiting the tube 34. In order for the deceleration power supply 46 to maintain a regulated voltage between the wafer holder 38/beamstop 40 and the flight tube 24, it is important to ensure that a forward current flows through the deceleration power supply 46 to compensate for the positively charged ions flowing between flight tube 24 and the wafer holder 38/beamstop 40. This is achieved by connecting a deceleration supply load resistance 122 in parallel with the power supply 46.
In order to provide cooling to assemblies in the beam line and ion source areas of the ion implanter 20, a closed circuit cooling water flow is required from a heat exchanger located at ground potential. The flow and return pipes must cross the post mass acceleration or deceleration voltage gaps. The water is slightly electrically conductive and part of the return current flow from the wafer 36 passes through these pipes. This represents a further effective load resistance in parallel with the deceleration power supply 46. Although the current through the water used to cool the wafer holder 38 (that is usually deionised) is typically negligible, the current return through the cooling pipes will not necessarily be negligible. For example, when high post-mass acceleration or deceleration voltages are employed, a cooling water current of several mA may arise. To take this into account,
The current flowing through the deceleration supply load resistance 122 will then be the sum of the forward current through deceleration power supply IDECEL and the net current IBEAM absorbed by both the wafer 36 and beamstop 40 minus a small cooling system water current.
The output of the beamstop 40 is monitored by a first current monitor 126 that generates a voltage signal representative of the beamstop current. This voltage signal is connected to one input of a comparator 128, as will be described below. The ion implanter 20 also contains a second current monitor 130 arranged in the path of the total current (the sum of the beam and deceleration currents) as it returns to the flight tube 24. The second monitor 130 also generates a voltage signal VTOTAL that indicates the total returning to the flight tube 24. In one embodiment, the signal VTOTAL may be measured directly without comparing it to the beamstop current.
Alternatively, the signal VTOTAL is fed to a second input of the comparator 128. Thus the comparator 128 generates an output VDIFF representative of the difference of the beamstop current IBEAMSTOP and the total current ITOTAL returned to the flight tube 24.
This arrangement is described in more detail in our U.S. Pat. No. 6,608,316 that is incorporated herein in its entirety by reference. Briefly, the voltage output of the current monitor 126 is connected to a differential amplifier that fulfils the function of the comparator 128. The total current from the wafer holder 38 and beamstop 40 passes through the deceleration power supply 46, deceleration supply load resistance 122 and any cooling systems 124. The total current ITOTAL is fed to a second current monitor 130 that operates in a similar manner to the first current monitor 126.
The advantages of monitoring the total current returning to the flight tube 24, instead of or as well as the beamstop 40 is that it is broadly indicative of the ion beam current at the point when it impacts the wafer holder 38/beamstop 40 assembly. Any arcing, for example, in the ion source 22 will manifest itself as a glitch in the ion beam 23. This in turn may be monitored by monitoring ITOTAL. At any time during the implantation cycle, a qualitative indication of the ion beam integrity may then be obtained as is required for the method of the present invention. In particular, the voltage signal which is an output of the current monitor 130 allows wide band stability monitoring of the ion beam 23.
The arrangement shown in
An alternative embodiment of an incident ion beam 23 current measurement arrangement is shown in
As shown in
Briefly, the potential difference between the wafer holder 38/beamstop 40 (normally held at ground potential) and the flight tube 24 is controlled by varying the resistance of a chain of FET's connected in series between the wafer holder 38/beamstop 40 (at ground potential) and the flight tube 24. This is done by measuring the voltage across the FET chain, with a potential divider buffering the voltage and comparing the voltage to a reference voltage (VREF) using a differential amplifier. The error signal (i.e. the amplified difference between the desired acceleration potential and the active deceleration potential) as measured by the potential divider is used to adjust the effective resistance of the FET chain.
The potential drop across the FET chain, VTOTAL, is indicative of the total current return to the flight tube 24. In one embodiment, this is fed through the comparator 128 which may be a differential amplifier. The other input to the comparator 128 is a voltage representative of the beamstop current. This is derived from the beamstop current monitor 126. The output of the comparator 128 is similar to that already described with reference to
The continuous measurement of the ion beam 23 current is used to determine whether or not a beam glitch has occurred. The continuous beam current is monitored for fast changes to indicate a beam glitch, rather than looking for slow changes. This is because slow changes in the ion beam current frequently occur and may be due to such mechanisms as residual gas neutralisation of the ion beam 23. A threshold value for the rate of change can be set and this is likely to be dictated by any particular ion implantation recipe.
Any event which does not meet the slow changing criteria is assumed to indicate instability of the change is above a certain size.
Quantifying changes in the ion beam current is performed using a comparison to an average ion beam current value. This average is obtained by taking a number of readings of the ion beam current once a stable ion beam 23 has been obtained, e.g. by using a rolling average of the total current obtained by measuring the total current ITOTAL with a time constant of 50 to 200 ms. Obviously, this method cannot be employed initially and so pre-set average values are used as initial starting conditions. With an average value determined, upper and lower thresholds may be used to test any variation in the ion beam current. The thresholds are measured relative to the average ion beam current and may be offset from that average by differing amounts. The offset may, for example, correspond to a drop of 50%. The thresholds are often specific to a particular implantation recipe. Either every single ion beam current measurement can be compared against the thresholds or a small number of consecutive measurements can themselves be averaged before comparison to the thresholds (e.g. measure ITOTAL with a short time constant of 1 ms). A further condition may be imposed that consecutive readings (e.g. ten) should exceed the thresholds before the ion beam is switched off.
As described previously, detection of an ion beam glitch leads to the ion beam 23 being switched off. This may be achieved in any number of ways, although it is clearly advantageous to achieve a rapid extinction of the ion beam 23. To date, an ion beam 23 has been extinguished by interrupting the power input to the arc power supply unit 62. However, it is relatively slow, taking in excess of 20 ms. An alternative method of extinguishing the ion beam 23 that is far quicker is now described.
The power semiconductor switches 134a,b are supplied with command signals derived from a common line indicated at 136 in
Reversing the signal on line 136 inverts the two switches 134a,b so that the first switch 134a is open and the second switch 134b is closed. This isolates the arc supply 62 to connect directly the chamber walls 50 to the tube 58 of the indirectly heated cathode 52. The resulting zero potential difference between anode 50 and cathode 52 causes immediate collapse of the plasma and immediate extinction of the ion beam 23.
The collapse of the plasma in this way will cause the ion source chamber 38 to cool down. Restarting the ion source 22 from cold will prolong the time for the ion beam 23 to settle to the previous steady flux value. This can be avoided by increasing the power delivered to the filament 54 or across the filament 54 and tube 58 using the bias power supply 60.
Reversing the signal on line 136 once more leads to rapid creation of an ion beam 23 because the two switches 134a,b are inverted such that the anode 50 is biased relative to the cathode 52 and ions are created by the ion source 22. This is helped by keeping the chamber 48 hot, as described above.
As will be appreciated by the skilled person, variations may be made to the embodiments described above without departing from the scope of the amended claims.
Examples of scanning schemes are presented in FIGS. 4 to 6, but these are merely examples and the present invention may be employed with other schemes. It will be readily apparent that the present invention may be adapted to any scheme where an ion beam 23 is scanned relative to a substrate along one or more pre-defined paths. The paths may be linear, arcuate or may follow any other shape. For example, a spiral scan may be used where the ion beam follows a spiral path around the wafer. If raster scans are used, the scan lines need not be parallel, for example the ion beam may follow a zig-zag pattern. Where movement along the path may be reciprocated, the method illustrated in
The present invention may also be used with different overall scanning schemes. For example, the present invention may be used with an interlaced series of raster scans 68, i.e. where only certain scan lines 70 are allowed on one pass, other missed scan lines 70 being implanted on the next pass. For example, the first pass may implant the first, fifth, ninth, . . . scan lines 70 of
The above embodiments of the present invention are all used in the context of serial processing of wafers 36 using raster scans 68. As mentioned previously, scanning may be achieved by (a) translating the wafer 36 relative to a fixed ion beam 23, (b) deflecting an ion beam 23 across a fixed wafer 36 or (c) using a hybrid method of translating the wafer 36 and deflecting the ion beam 23. In addition, the present invention may be used with batch processing of wafers 36 where an ion beam 23 scans over each wafer along a plurality of scan lines 70. For example, the invention may be used with a batch implanter comprising a spoked-wheel wafer holder (i.e. a plurality of wafers are held at the ends of a number of spokes extending from a central hub).
The method given above for determining the ion beam 23 current is merely one example of doing so. The ion beam 23 current may also be determined by monitoring the beam line power supplies (e.g. the pre-acceleration power supply, the lens voltage power supply, the deceleration power supply), monitoring the current flowing from the chuck to ground or by using a current clamp method. The current clamp method comprises placing a solenoid around a part of the ion beam path 23. Any change in ion beam current will cause a change in the current flowing through the solenoid. Thus, ion beam glitches can be detected by measuring the current flowing through the solenoid.
The arrangement shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 040045.9 | Jan 2004 | GB | national |
