1. Field of the Invention
The invention relates to an ion optical system that extracts and forms an ion beam which can be used for ion implantation processes, particularly in the low energy range 100 eV-4 keV. The invention enables a broad energy range of the transported ion beam and also enables the extraction of molecular ions as well as more conventional monomer ion beams using a simple triode extraction structure. Novel features are incorporated into the invention that enable beam formation and variable focusing of ion beams over a very broad range of beam current, ion mass and source brightness, while being compatible with many commercial beam line implantation platforms.
2. Description of the Prior Art
—Ion Implantation Process
The ion implantation process relies on ionizing gaseous or vaporized solid feedstock material in an ion source and extracting either positive or negative ions from the source through an extraction aperture using electric fields. The beam is then mass analyzed, transported and implanted to target semiconductor wafer.
—Ion Source and Extraction
In traditional implanter ion sources, arc discharge or RF excitation is typically used to form a dense plasma, which is a mix of thermal electrons, fast ionizing electrons, and ions.
Typically the suppressor and the ground electrode are a movable unit in order to change the gap between the extraction aperture plate and the suppression electrode. This is required as the ion beam final energy, which is set by the source potential, is varied and the electric field in the extraction gap has to be adjusted accordingly in order to maintain the same extraction conditions for the ion beam. This relation stems from the fact that the extracted current density depends on the extraction electric field through Child's law:
Where j is the maximum extractable current density of the ion beam, Q and M are the charge state and the mass number of the ion and U [kV] and d [cm] are the applied voltage and gap between the ion source body/extraction aperture plate and the suppression electrode, respectively. Child's law gives the space charge limit for the extractable current density from the ion source.
y2=y1 cos(α1), (2)
Where y1 and y2 are the beam half-widths at the entrance and exit field boundaries, respectively, and α1 is the magnet sector angle. If the sector angle is smaller than 90 degrees, the beam leaves the magnet converging. At a 90 degree sector angle the beam has a focal point at the magnet exit, and with a sector angle larger than 90 degrees the beam has a focal point inside the magnet and leaves the magnet diverging.
The requirement set for the extraction optics will be the ability to form a beam that has small enough divergence and beam size in the dispersive plane to match the acceptance of the analyzer magnet. In the non-dispersive plane, the beam focusing can be accomplished by the curvature of the electrodes, but additionally the analyzer magnet can have some focusing properties either through pole rotation or pole face indexing.
—Space Charge Forces
It can be problematic to achieve a desired beam focusing in the non-dispersive plane if the space charge of the beam is varying significantly between different operation modes of the extraction system. The space charge of the beam depends on beam energy and current. The transverse space charge force FSPC,SLIT acting on the envelope of the ion beam can be written for a slit beam in a following form:
In equation (3), e is the elementary charge, J is the beam current per unit length of the slot, ε0 is the permittivity of free space and v is the directed velocity of the particle along the beam direction. For round beam the same equation can be written in form:
where q is the total charge of the ion, I is the beam current and r0 is the beam envelope radius.
The space charge forces described in equations (3) and (4) are transverse forces with respect to the beam direction, which will blow up the beam as it drifts in the beam transport system. This has implications for the extraction of the ions from the ion source. Ideally, the extraction optics should be designed so that the resulting electric fields will compensate the transverse space charge force and form an approximately parallel, or only slightly diverging, beam in the dispersive plane, while focusing or containing the beam envelope in the non-dispersive plane.
In typical ion implanters atomic ion species are used to form the implanted beams of boron, arsine and phosphorus. The extracted current densities can be in the range of a few mA/cm2 and higher. This sets boundary conditions for the design of the extraction optics in the existing implanters. Typically slit extraction is used with slit sizes of a few mm in width (dispersive plane) and 20-40 mm in height (non-dispersive plane). The extraction gap between the aperture plate and the suppression electrode typically varies from a few mm to a few tens of mm when the beam energy is in the range used in implanters, which is from a few hundred eV to 80 keV.
Traditional triode extraction systems with thin ion extraction aperture plates have been proven to work acceptably for high current density extraction systems when using atomic or small molecular species ion beams. The development of cluster ion beams (for example, B18Hx+, B10Hx+, C7Hx+) for next generation implanter technology, however, has exposed the inadequacy of traditional extraction optics for this application. For low current density beam extraction, the thin plate optics setup is poorly matched, especially at higher energies. Extracted B18Hx+ current densities are typically between 0.5 and about 1 mA/cm2, which is quite low compared to many plasma ion sources used in ion implantation. In order to extract the desired ion currents the extraction slot has a larger area (for example, 10 cm2 or more), which creates a sizable punch-through of the extraction electric field into the ion source. To achieve a matched extraction condition, the extraction gap has to be very large to reduce the effect of this punch-through. Especially at high extraction voltages >10 kV, the beam will cross over strongly and hit the suppression and ground electrodes. The strong cross over also leads to high beam divergence which increases beam losses in mass analyzer magnet and in the following beam line due to beam vignetting, i.e., beam intersection with beam line apertures.
To overcome these issues a new type of triode extraction system, a Cluster Ion Beam Extraction System, has been developed for broad energy range cluster ion beam extraction applications while still being applicable to atomic and molecular ion species as well. The extraction aperture plate contours are set to minimize the beam cross over and at the same time shield the source from excess extraction electric fields thus allowing smaller values of the extraction gap. In addition, a novel focusing feature is integrated into these new optics which allows the beam to be either focused or de-focused in the non-dispersive plane by using a bipolar bias voltage of only a few kV over a broad range of beam energy. This is a superior solution to a stand-alone electrostatic lens solution, for example an einzel lens, which would require tens of kV of bias voltage in order to be able to focus an energetic beam.
These and other advantages are described in the following specification and attached drawing wherein:
a illustrates a transverse electric field Ex and space charge field ESPC plotted as a function of beam velocity.
b is an experimental comparison between traditional Pierce-type extraction geometry and the Cluster Ion Beam Extraction System.
a illustrate modeled transverse electric field components Ey at two different y-heights for the geometry shown in
The suppression and ground electrodes are typically moved along the beam direction. This allows a proper electric field value to be achieved when the ion beam energy and extraction voltages or the extracted ion current density are changing.
The extraction system of the invention herein described was designed to match 4 to 80 keV (0.2 to 4 keV boron equivalent energy) B18Hx+ beams with 0.5 to 0.7 mA/cm2 current density and a maximum allowed extraction gap of about 100 mm.
A dispersive and non-dispersive plane cross section of the invention is shown in
The prominent features of the extraction aperture plate are the flat middle section around the extraction slot, the 90 degree included angle and the thick profile of the extraction aperture electrode. Referring to
The 90 degree included angle creates a deep channel to shield the excess electric field while at the same time enabling the electric field to have optimum profile across the ion beam, thus minimizing beam divergence and producing a brighter beam. The included angle should be matched to the space charge of the beam so that the force created by the transverse electric field components match or only slightly exceed the intrinsic transverse space charge force of the beam.
The front plate, puller and ground inserts have a radius of curvature in vertical YZ-plane to optimize the vertical focal length. In the presented extraction system the radius of curvature of the front plate is 1000 mm.
The flat section adjacent to the extraction aperture is identical for cases 3 and 4. In case 3 the extraction trench has a uniform angle throughout the thickness of the plate, whereas in case 4 the angle is similar to case 3 up to halfway through the thickness of the plate after which the angle increases. The electric fields generated by each 4 geometries were modeled using Lorentz EM electromagnetic solver and the transverse component Ex is plotted in
As an example 2 variations of a traditional extraction electrode design and 2 variations of the new optics were modeled using Lorentz-EM and are presented.
a plots the resulting transverse electric field and the space charge generated electric field ESPC, which is given by dividing equation 3 by elementary charge e:
In order to form a parallel beam, Ex and ESPC have to be approximately equal in strength and opposite in sign throughout the acceleration of the ion. As can be seen from
For the new Cluster Ion Beam Extraction System, Ex starts at very similar strength as the space charge field and follows in general the same trend throughout the acceleration. In this specific example the 90 degree included angle geometry creates slightly high Ex in intermediate ion beam velocity. This is often desirable as the slight excess in Ex will focus down the beam in dispersive plane and thus help form a smaller beam entering the analyzer magnet. This effect can be also toned down by making a larger included angle cut to the extraction channel. Looking at the Ex values in these 2 cases it is clear that the flat edge adjacent to the extraction slit helps to minimize the critical over-focusing in the beginning, and maintains a good balance between Ex and ESPC through the rest of the beam acceleration, which will result in less diverging beam that is easier to transport than the beam created by a traditional Pierce-type geometry.
Another significant difference between the traditional Pierce-geometry and the new optics can also be seen from the above example. The extraction gap that is needed to accommodate high energy beams is significantly smaller in case of the new geometry. In the traditional Pierce-geometry where the extraction gap is overly large the beam will have more time to blow up and strike the suppression and ground inserts. This effect is only made worse by the larger divergence introduced by this type of traditional geometry. The required axial movement of the suppression and ground electrodes is also reduced as well as the space requirement.
Two of the geometries that were presented in the example of
As can be seen from
The size and shape of the extraction slot can vary greatly in the new optics. The features described in
The aperture plate is thinner overall and the flat sections adjacent to the extraction slot are smaller. In the dispersive plane the optics features are similar to the case presented in
The channel shape provides electric field distribution which will focus the beam sufficiently in the non-dispersive plane. The suppression and ground electrodes are also without curvature. This type of smaller extraction slot is better suited for plasma ion sources, where a large aperture is undesirable as dense plasma can blow-out of the source and form a plasma bridge between the source and suppression potential very easily.
A flat middle section around the extraction slot is maintained to reduce beam divergence. As the front plate is thinner than in the geometries presented above due to smaller extraction slot size the flat part can be uniform all around the slot.
Electrostatic Ion Optical Lens Integrated into the Cluster Ion Beam Extraction Aperture Plate
At different beam energies and beam currents the focal length of the triode system described here can vary significantly due to varying space charge effects of the beam. At the dispersive (XZ) plane this variation is controlled by changing the extraction gap and suppression voltage. In the non-dispersive (YZ) plane these adjustments are not effective due to the height of the beam. This is a problem when transporting the beam long distances (through an analyzer magnet) to a beam line with limited acceptance. To better control the beam optics without adding additional electrodes or bulky magnetic lens elements a simple solution for controlling the y-focusing is presented here.
By biasing the top and bottom section positively with respect to the front plate a transverse electric field component which will focus the extracted ion beam in the non-dispersive plane is formed. If a negative bias voltage is added to the lens elements this will increase the focal length of the triode and act as a defocusing lens. Bi-polar voltage supply with modest ±2 kV voltage range is sufficient for the lens to work effectively at all energies, currents and ion species used in ion implantation. The bias voltage has minimal effect on the beam in dispersive plane even when bias voltage is applied, and when no bias is present the lens extraction aperture plate functions identically to the standard plate shown in
Beam Emittance
The split lens of
In order to describe the effects of the electrostatic lens on the beam we give a description of beam emittance. Ion beam emittance is the most important parameter describing ion beam quality and ion optical properties. It is defined as the volume that the ion beam particles occupy in the six dimensional phase space (x, px, y, py, z, pz), where x, y and z are the space coordinates of the beam particles and px, py and pz are the corresponding linear momenta of the particles along the space coordinate axis.
Usually the longitudinal emittance projection along the beam axis is of no interest and only the two transverse emittance planes (x, px) and (y, py) are considered. In
In
Let's consider the linear momentum of the ion along x axis. It can be written as
The gradient x′ can be written in terms of the divergence angle αx:
Usually Vx is much smaller than Vz and x′≈αx. In this case the beam emittance is defined as the area that the particles occupy in the (x,x′) and (y,y′) planes. The emittance pattern is usually an ellipse with half axis A and B. The emittance value is then given by the area of the ellipse
εx,y=πAB[mm−mrad] (8)
The emittance ellipse orientation indicates if the beam is divergent, convergent, parallel or focused. In
In defining the transverse emittance as the area the beam occupies in (x,x′) and (y,y′) plane we have neglected the effect of ion beam velocity along the beam axis, vz. If vz increases, beam divergence and thus the emittance will decrease. This effect is eliminated by using normalized emittance εn, which is given by:
εn=βγε (9)
where
is the ratio of the beam axial velocity and the speed of light and
A widely used emittance definition is the root mean square, or RMS, emittance. It is given by:
Equation (10) is often multiplied by 4 when measured laboratory emittance values are reported, as this gives an emittance value that corresponds well to the area of ellipse fitted into measured data.
a shows the effect of applied lens element voltage on the vertical electric field component Ey, which is the field responsible for focusing and de-focusing of the ion beam in the vertical plane.
The higher the negative Ey value is, the more the beam is focused in the vertical plane.
Emittance Ellipse Orientations
Shown in
A positive bias on the lens elements decreases the beam vertical height, whereas a negative bias makes the beam taller. This illustrates how it is possible to tune the beam vertical size using the vertical lens integrated into the Cluster Ion Beam Extraction System.
The vertical tuning of the beam will also benefit implant operations where the beam current is varied based on the dose requirement of each individual implant. The variation in the beam current on wafer can be as large as 2 orders of magnitude, in which case the space charge effects and thus beam focal lengths will vary significantly. In dispersive plane the extraction gap and suppression voltage can be used to match the beam horizontally. In non-dispersive plane the fixed curvature of the extraction aperture plate and the suppression/ground inserts that are typically used in ion implanter optics will be well matched to only certain energy/beam current range. The integrated electrostatic lens will broaden this range considerably and will allow matching of beam profiles in the non-dispersive plane throughout the energy—and current range of commercial implanter systems.
This application claims the priority to and the benefit of U.S. Provisional Patent Application No. 60/939,505, filed on May 22, 2007, hereby incorporated by reference.
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