Patent Application
20080099696
This invention relates to shaped apertures in an ion implanter that may act to clip an ion beam and so adversely affect uniformity of an implant. In particular, the present invention finds application in ion implanters that employ scanning of a substrate to be implanted relative to the ion beam such that the ion beam traces a raster pattern over the substrate.
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 precursor gas or the like. Only ions of a particular species are usually required for implantation in a substrate, for example a particular dopant for implantation in 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 a process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder.
Ion beams often have approximately circular cross-sectional profiles and are much smaller than the substrate to be implanted. In order to implant the entire surface of the substrate, the ion beam and substrate must be 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 that is held in a fixed position, (b) mechanically moving the substrate whilst keeping the ion beam path fixed or (c) a combination of deflecting the ion beam and moving the substrate.
Our U.S. Pat. No. 6,956,223 describes an ion implanter of the general design described above. While some steering of the ion beam is possible, the ion implanter is operated such that ion beam follows a fixed path during implantation. Instead, a substrate is held in a substrate holder that is scanned along two orthogonal axes to cause the ion beam to trace over the wafer following a raster pattern like that illustrated in
The substrate is moved continuously in a single direction (the fast-scan direction) to complete a first scan line. The substrate is then stepped up a short distance orthogonally (in the slow-scan direction), and a second line is then scanned. This combination of reciprocating scan lines and indexed stepwise movement results in scanning of the whole surface of the substrate through the ion beam. The pitch of the scan lines is chosen to be less than the height of the ion beam, such that successive scan lines overlap. The pitch is carefully chosen with reference to the ion beam profile to ensure uniformity of implant: typical profiles see most of the ion beam current at the centre of the ion beam, and the current tails away towards the edges of the ion beam. An ideal profile would be a Gaussian, although such profiles are rarely seen in practice. Overlapping adjacent scan lines may be used to ensure uniform implants due to the smoothly varying profile.
Further improvements may be made to improve the uniformity of implants made using such raster scans. For example, multiple passes over the substrate may be made and interlacing may be effected (e.g. make a first pass implanting the first, fifth, ninth, etc. scan lines, then make a second pass implanting the second, sixth, tenth, etc. scan lines, then make a third pass, etc.). Also, the wafer may be rotated between passes, for example four passes are made with a 90° twist of the substrate between each pass in a quad implant. Our U.S. patent application Ser. No. 11/527,594 provides more details of such scanning techniques.
It has been realised that the use of overlapping scan lines to ensure uniformity of implant is particularly prone to a problem. Specifically, the success of this technique relies on a smoothly varying profile to the ion beam in the direction transverse to the fast-scan speed across the substrate. Preferably, the variation is a Gaussian variation. However, ion implanters often employ rectangular apertures along the ion beam's path. It has been appreciated that, should the ion beam clip straight edges of the aperture, it will lose its smooth variation at those clipped edges. In particular, this is severe where the ion beam is clipped by an edge extending in the same direction as the fast scan direction as this leads to a sharp edge on the ion beam that is effectively drawn along the substrate. Thus, a hard edge is formed on the scan lines across the substrate where they overlap, leading to periodic sharp jumps in dose level as you travel across the substrate in the slow scan direction (i.e. as you traverse across the scan lines). This is true irrespective of whether the substrate is scanned or the ion beam is scanned. The effect is described in more detail below with reference to
Against the above background, and from a first aspect, the present invention provides an ion implanter comprising: an ion source arranged to generate an ion beam; ion beam optics arranged to guide the ion beam along an ion beam path; a substrate scanner arranged to scan a substrate relative to the ion beam in a scanning direction substantially transverse to the ion beam path such that the ion beam forms a series of scan lines across the substrate; and an aperture plate having provided therein an aperture defined by internal edges of the aperture plate, the aperture being positioned on the ion beam path upstream of the substrate scanner, and wherein the aperture is defined in part by an edge extending generally in the scanning direction provided with at least one inwardly-facing projection.
The present application may find application in an ion implanter that uses deflection of the ion beam to effect scanning in the scanning direction. Such scanning is generally performed after the ion beam has cleared the final aperture on the ion beam path, i.e. the ion beam follows a fixed path through the apertures and then is scanned. Nonetheless, if the ion beam is clipped upstream by an aperture, the resulting ion beam profile will have a harder edge where it was clipped that may lead to a loss of uniformity in an implant. Accordingly, it is still useful to use an aperture plate as shaped above.
However, the present invention is primarily intended for use in ion implanters that use mechanical scanning of the substrate relative to a fixed ion beam. The problem of ion beam clipping is often worse in such implanters because the final apertures tend to be positioned closer to the substrate than for scanned-beam implanters, meaning that any angular variation in the ion beam has less chance to smooth any hard edges imposed by clipping. Hence, preferably the substrate scanner is arranged to scan the substrate repeatedly through the ion beam in the scanning direction substantially transverse to the ion beam path such that the ion beam forms a series of scan lines across the substrate.
Provision of the inwardly facing projection is advantageous as it addresses the problems of a sharp edge being formed if the ion beam clips that edge. This is because the inwardly facing projection must provide an edge that extends transversely to the scanning direction. Hence, a single edge extending along the scanning direction is avoided. For example, the projection may simply be a tooth that sees a step introduced to the edge that extends in the scanning direction. This alleviates the problem in that there will then be two sharp edges introduced to the ion beam that average as the ion beam is traced across the substrate (i.e. the substrate sees dosing contributions from both edges). As an improvement, the projection may not be a tooth, but may comprise a series of steps.
Clearly, it is better to present a smoothly varying projection such that contributions to the ion beam's edge may be made at many positions transverse to the scanning direction. For example, the projection may be arcuate or v-shaped, thereby leading to a better smoothed edge to the ion beam should it clip that edge. Another contemplated shape is for the projection to have sinuous edges. For example, the projection may be generally v-shaped, but have sides that are each s-shaped (i.e. in the shape of an “s” or the mirror image of an “s”). These sides may have the shape of the side of a Gaussian peak, preferably with the peak extending in the fast-scan direction. Put another way, if the projection is provided on a top or bottom edge, the sides may be shaped like the side of a Gaussian peak lying on its side. Both sides may have such a shape such that the projection is symmetrical and adopts the shape of an onion dome, or at least the tapering top half of an onion dome.
Preferably, the projection is provided centrally on the edge. This is advantageous as the projection is more likely to act on the centreline of the ion beam where the current will be greater.
Rather than the edge comprising a single projection, it may comprise a plurality of inwardly-facing projections. These projections may all be alike, or they may differ. For example, the edge may comprise a plurality of like onion domes or the edge may comprise a plurality of teeth that extend inwardly to different depths.
Generally, an aperture will be defined by two edges that extend generally in the scanning direction. If so, both edges are preferably provided with at least one inwardly-facing projection. Optionally, the edges are mirror images.
From a second aspect, the present invention resides in a method of improving uniformity in an implant made by an ion implanter comprising an ion source arranged to generate an ion beam, ion beam optics arranged to guide the ion beam along an ion beam path, a substrate scanner arranged to scan a substrate relative to the ion beam in a scanning direction substantially transverse to the ion beam path such that the ion beam forms a series of scan lines across the substrate, and an aperture plate having provided therein an aperture defined by internal edges of the aperture plate, the aperture being positioned on the ion beam path upstream of the substrate scanner, the method comprising providing the aperture plate with an edge that partly defines the aperture, and that extends generally in the scanning direction but that is provided with at least a portion that extends in a direction other than the scanning direction.
From this aspect of the invention, other shapes to the aperture edge are contemplated when trying to address the problem of uniformity in an implant where the ion beam may clip the aperture edge. For example, circular, ovoid, diamond shaped or hexagonal shaped apertures may be used to address uniformity. While not being as effective as an inwardly-facing projection acting on the centreline of an ion beam where the current is highest, these other shapes nonetheless act to smooth the edge if the ion beam is clipped. The method may also comprise providing the edge with the portion that extends over 25%, 50%, 75% or 90% of the length of the edge. Optionally, the portion may be positioned centrally.
Other preferred features are defined by the appended claims.
Embodiments of the present invention, along with aspects of the prior art, will now be described with reference to the accompanying drawings, of which:
a shows a conventional aperture plate with an ion beam passing through the aperture without being clipped;
b is an ion beam profile taken along line III-III of
a shows a conventional aperture plate with an ion beam passing through the aperture such that its top and bottom are clipped;
b is an ion beam profile taken along line IV-IV of
a shows schematically an ion beam being scanned across a substrate;
b is an ion beam profile taken along line V-V of
a shows schematically an ion beam being scanned across a substrate twice to form two overlapping scan lines;
b shows the dose received by the substrate of
a shows schematically an ion beam being scanned across a substrate twice to form two separated scan lines;
b shows the dose received by the substrate of
The ion implanter 10 contains an ion source 14 for generating an ion beam of a desired species that is located within a vacuum chamber 15 evacuated by pump 24. The ion source 14 generally comprises an arc chamber 16 containing a cathode 20 located at one end thereof. The ion source 14 may be operated such that an anode is provided by the walls 18 of the arc chamber 16. The cathode 20 is heated sufficiently to generate thermal electrons.
Thermal electrons emitted by the cathode 20 are attracted to the anode, the adjacent chamber walls 18 in this case. The thermal electrons ionise gas molecules as they traverse the arc chamber 16, thereby forming a plasma and generating the desired ions.
The path followed by the thermal electrons may be controlled to prevent the electrons merely following the shortest path to the chamber walls 18. A magnet assembly 46 provides a magnetic field extending through the arc chamber 16 such that thermal electrons follow a spiral path along the length of the arc chamber 16 towards a counter-cathode 44 located at the opposite end of the arc chamber 16.
A gas feed 22 fills the arc chamber 16 with the species to be implanted or with a precursor gas species. The arc chamber 16 is held at a reduced pressure within the vacuum chamber 15. The thermal electrons travelling through the arc chamber 16 ionise the gas molecules present in the arc chamber 16 and may also crack molecules. The ions (that may comprise a mixture of ions) created in the plasma will also contain trace amounts of contaminant ions (e.g. generated from the material of the chamber walls 18).
Ions from within the arc chamber 16 are extracted through an exit aperture 28 provided in a front plate of the arc chamber 16 using a negatively-biased (relative to ground) extraction electrode 26. A potential difference is applied between the ion source 14 and the following mass analysis stage 30 by a power supply 21 to accelerate extracted ions, the ion source 14 and mass analysis stage 30 being electrically isolated from each other by an insulator (not shown). The mixture of extracted ions are then passed through the mass analysis stage 30 so that they pass around a curved path under the influence of a magnetic field. The radius of curvature travelled by any ion is determined by its mass, charge state and energy, and the magnetic field is controlled so that, for a set beam energy, only those ions with a desired mass to charge ratio and energy exit along a path coincident with the mass-resolving slit 32. The emergent ion beam is then transported to the process chamber 40 where the target is located, i.e. the substrate 12 to be implanted or a beam stop 38 when there is no substrate 12 in the target position. In other modes, the beam may also be accelerated or decelerated using a lens assembly 49 positioned between the mass analysis stage 30 and the substrate position.
The substrate 12 is mounted on a substrate holder 36, substrates 12 being successively transferred to and from the substrate holder 36, for example through a load lock (not shown).
The ion implanter 10 operates under the management of a controller, such as a suitably programmed computer 50. The controller 50 controls scanning of the wafer 12 through the ion beam 34 to effect desired scanning patterns such as raster patterns like that shown in
a shows an aperture plate 52 that may be placed on the ion beam path 34. For example, the aperture plate 52 may correspond to one of the electrodes in the lens assembly 49 that is used to accelerate or decelerate ions in the ion beam 34 before reaching the substrate 12. The aperture plate 52 has a conventional rectangular aperture 54 with top and bottom edges 56 and 58 that extend in the fast scan (x axis) direction.
a shows the ion beam 34 passing comfortably through the aperture 54 provided in the aperture plate 52 such that there is clearance between the top edge 56 and the bottom edge 58 of the aperture 54.
a also indicates axes that are used to define the geometry within the ion implanter 10. The ion beam 34 is taken to define the z axis, the y axis is defined as the vertical and the x axis is defined as the horizontal. In these embodiments, raster scans are described that see the ion beam 34 trace a series of scan lines horizontally across the substrate 12, i.e. the x axis defines the fast scan direction and the y axis defines the slow scan direction where the substrate 12 is stepped between successive scan lines.
b shows a profile 60 of the ion beam intensity (i.e. current) taken along a vertical line through the centre of the ion beam 34 at a position immediately downstream of the aperture plate 52. This line is indicated as III-III in
a and 4b are like
a shows an ion beam 34 being scanned across a substrate 12 along a first scan line 66. This is effected by scanning the ion beam 34 across a stationary substrate 12 or by moving the substrate 12 relative to a fixed ion beam 34.
a shows the substrate 12 after the top-hat ion beam 34 has performed two scan lines 66 and 70 of a raster pattern.
a and 7b correspond to
As will be appreciated, if the two scan lines 66 and 70 can be made to abut perfectly leaving no gap and with no overlap, perfect uniformity may be achieved. However, this is impossible to achieve, meaning that there will always be some overlap or separation leading to striping of the substrate 12.
It will also be understood that the problems of sharp edges 64 caused by the ion beam 34 being clipped by the aperture 54 will also lead to a loss of uniformity in implants. This is because, as explained above, the smoothly varying profile of an unclipped ion beam 34 is used to achieve uniform dosing by overlapping adjacent scan lines. Loss of the smoothly varying tails destroys the compensating effect otherwise provided by the overlapping scan lines.
In normal use, the ion beam 34 is intended to pass through the aperture 54a without being clipped as shown by the solid hashed cross-section at 34. However, should the ion beam 34 increase in size, as indicated by the dashed cross-section at 34′, the teeth 57a and 59a provided on the top edge 56a and bottom edge 58a respectively may clip the ion beam 34′. Any single profile taken vertically through the ion beam 34′ at any x-axis position will still display sharp edges like those shown at 64 of
While the above embodiments are effective in addressing problems in uniformity of implant due to clipped ion beams 34′, there remains some residual loss of uniformity due to the stepped nature of the projections. Hence, it is preferred to use projections having sides that extend at an angle to the scanning direction so as to provide a continuous range of the depth of the edges 56 and 58 into the aperture 54.
As before, any single profile taken vertically through the ion beam 34, at any x-axis position will still display sharp edges like those shown at 64 of
As per the stepped arrangement of
Yet another arrangement is shown in
The skilled person will appreciate that changes may be made to the above-described embodiment without departing from the scope of the present invention defined by the appended claims.
For example, the number of projections may be varied. The shape of the projections may also be varied, provided the resulting shape still serves to project inwardly. Where multiple projections are used on an edge, these projections need not share a common shape. The depth to which the projections protrude inwardly may be varied according to need. A balance is to be struck between deeper projections having a greater smearing effect and the increased likelihood of deeper projections clipping the ion beam 34.
Although described with respect to linear raster scans like that shown in
