WO2010094605A1

WO2010094605A1 - Ion mass determination

Info

Publication number: WO2010094605A1
Application number: PCT/EP2010/051627
Authority: WO
Inventors: Eugen Stamate
Original assignee: Danmarks Tekniske Universitet
Priority date: 2009-02-20
Filing date: 2010-02-10
Publication date: 2010-08-26

Abstract

An apparatus (100) is described for determining the mass of ions, the apparatus configured to hold a plasma (101 ) having a plasma potential. The apparatus (100) comprises an electrode (102) having a surface extending in a surface plane and an insulator (104) interfacing with the electrode (102). An electric potential provider (106) is configured to provide an electric potential different than the potential of the plasma to the electrode (102), thereby forming a curved (110) potential distribution in the plasma surrounding the electrode (102). A magnetic field source (114, 116) is configured to provide a magnetic field (B) across at least part of the curved potential distribution in the plasma surrounding the electrode (102). An ion impact detector (108) is configured to detect impacts of ions arriving at the electrode (102), the detecting comprising detecting of locations of the impacts, and a processing unit (108) configured to interpret the detected impact locations in terms of the mass of the impacting ions.

Description

Ion mass determination

Technical field

The present invention relates to an arrangement and a method for determining the mass of ions.

Background

The semiconductor manufacturing industry relies on plasma processing for etching, deposition, implantation, passivation, ashing and other processes necessary for fabricating silicon wafer based electronic devices. Several primary factors significantly affect the conditions inside the chamber. Among these factors are component gas concentrations, chamber pressure, and surrounding electromagnetic fields. The manner in which these factors are controlled during the process determines the quality and uniformity of the plasma and thus the quality of the wafer produced. Thus, the precise tuning of plasma parameters in processing reactors requires a new generation of sensors that are process transparent, easy in maintenance, sensitive to wafer chamber conditions, accurate and reliable and that can provide direct information on more process parameters than, pressure, gas flows and plasma density.

The movement of electrical charges in vacuum can be controlled in a desired manner using electric and magnetic fields. Appropriate configurations can generate mass-to-charge ratio separation, a technique that defines what is known as a mass spectrometer. The design of a mass spectrometer has three essential modules: an ion source, which transforms the molecules in a sample into ionized fragments; a mass analyzer, which sorts the ions by their masses by applying electric and magnetic fields; and a detector that provides data for calculating the abundances of each ion fragment present.

Monitoring of the mass distribution between ion species in a plasma has been described in the prior art. For example by G. H. Kim, G. H. Rim and S. A. Nikiforov, in "Monitoring of ion mass composition in plasma immersion ion implantation", Surface and Coatings Technology, Volume 136, Issues 1-3, 2 February 2001 , Pages 255-260.

Here, the authors describe a moderate resolution ion mass analyzer to monitor the ion mass and charge state during plasma immersion ion implantation. Despite of its compactness and reduced cost the proposed system is yet very intrusive being based on a large extraction head that collects the ions through a small orifice.

Summary

In order to improve on prior art solutions there is provided, according to a first aspect, an apparatus for determining the mass of ions, the apparatus configured to hold a plasma having a plasma potential. The apparatus comprises an electrode having a surface extending in a surface plane and an insulator interfacing with the electrode. An electric potential provider is configured to provide an electric potential different than the potential of the plasma to the electrode, thereby forming a curved potential distribution in the plasma surrounding the electrode. A magnetic field source is configured to provide a magnetic field across at least part of the curved potential distribution in the plasma surrounding the electrode. An ion impact detector is configured to detect impacts of ions arriving at the electrode, the detecting comprising detecting of locations of the impacts, and a processing unit configured to interpret the detected impact locations in terms of the mass of the impacting ions.

Such an arrangement with a curved potential distribution, a plasma sheath lens, magnetized by a magnetic field source, is capable of mass separation in the sense that the trajectory of ions, accelerated by the combined influence of the magnetic field and the curved potential distribution in the lens, depend on the mass of the ions. Ions of different mass impact the electrode at different locations. This principle is utilized to provide an improved apparatus for determining the mass of ions, i.e. essentially a new type of ion mass spectrometer. An advantage of such a mass spectrometer is that it can be made very small in size, even a few cm, and considerably cheaper than prior art mass spectrometers, making it very usable, e.g., as plasma monitor in micro- and nano-electronic industry.

In other words, a new type of mass spectrometer based on mass-to-charge ratio separation of charged particles (positive or negative ions) using a magnetized plasma-sheath-lens (PSL) is provided. The novel mass spectrometer has advantages regarding price, size and distributed use in comparison with current technologies, making it a viable solution for diagnostics of various plasma reactors used in micro- and nano-electronic industry. It is also expected that the concept will give rise to a range of general applications linked to mass spectrometry and ion and electron optics. PSL relates to a complex potential distribution (electrostatic lens) that forms adjacent to a plasma-immersed biased, conductor-insulator interface. The lens bends the trajectories of ions or electrons accelerated up to a few hundred eV with almost 180 degrees in a distance of less than one centimeter, which can be compared to the tens of centimeters needed to achieve the same purpose for ions in vacuum. In addition, the magnetic field further twists the trajectories, which results in mass dependence being exhibited.

The electrode surface and the insulator may be in different planes, whereby the interface between the electrode and the insulator is outside the electrode surface plane. Such a configuration provides a wider extension of the plasma-sheath-lens structure that can exhibit modal focusing for ions entering the lens at locations not facing the electrode surface.

An interface may be a surface forming a common boundary of two bodies, spaces, or phases.

The insulator may be adapted to insulate a portion of the electrode. The insulator may insulate a portion of the electrode by covering a part of the surface of the electrode. Embodiments include those were a limited portion of the surface area of the electrode is covered by the insulator. In some embodiments below 80 % of the surface area of the electrode is covered by the insulator. In some embodiments below 50 % of the surface area of the electrode is covered by the insulator.

In some embodiments below 25 % of the surface area of the electrode is covered by the insulator.

In some embodiments below 10 % of the surface area of the electrode is covered by the insulator.

Alternatively, the electrode surface and the insulator may be in a common plane, whereby the interface between the electrode and the insulator is in the common plane. Such a configuration limits the development of modal focusing but yet exhibits the discrete focusing effect.

Embodiments include those where the magnetic field source is configured to provide the magnetic field in a direction essentially perpendicular to the electrode surface plane. By having the magnetic field essentially perpendicular to the electrode surface plane, symmetry is obtained that results in central passive surfaces that are essentially symmetrical. For example, in the case of a circular electrode, the central passive surface will be a circle at the center of the electrode, the radius of which will depend on the mass of the impacting ions. Such dependence will make the processing much easier than in a situation where the central passive surface is not symmetric.

Embodiments include also those where the magnetic field source is configured to provide the magnetic field concentrated in a region covering the interface between the electrode and the insulator. Due to the fact that the curvature of the potential distribution in the plasma is largest in the vicinity of the interface, most of the ions that impact on the electrode have their origin in this region. Hence there is less need for a magnetic field that covers all parts of the plasma sheath lens. With regard to the detecting of the ion impacts arriving at the electrode, embodiments include those where the electrode comprises a radial slit in the electrode surface. In such embodiments, the ion impact detector is configured to detect the ions arriving at the electrode through the slit or above the slit using a movable sensor (surface) biased at the same potential as the electrode.

Alternatively, embodiments include those where the electrode comprises concentric ion detectors interspaced by concentric insulators. The interspaces are chosen to be small enough as not to distort the plasma sheath lens structure.

Depending on the specific implementation of an ion mass determining apparatus as summarized above, the electrode surface may have essentially any shape. For example, circular as well as polygonal shapes may be used, the selection of which may depend on the specific implementation of the apparatus (i.e. type of ion impact detection, size of the whole system with respect to plasma volume etc).

The apparatus may in some embodiments be configured to allow access to ions from an ion source, such as an ion beam source, for mass determination. That is, mass determination of any ion source can be achieved, further emphasizing the versatility of the apparatus as summarized above.

In a second aspect, having similar effects and advantages, there is provided a method for determining the mass of ions, using a plasma having a plasma potential. The method comprises providing an electric potential different than the potential of the plasma to an electrode having a surface extending in a surface plane and interfacing with an insulator. Thereby, a curved potential distribution is formed in the plasma surrounding the electrode. A magnetic field is provided across at least part of the curved potential distribution in the plasma surrounding the electrode, and impacts of ions arriving at the electrode are detected, the detecting comprising detecting of locations of the impacts. The detected impact locations are then interpreted in terms of the mass of the impacting ions. As for the apparatus summarized above, depending on the electrode bias polarity with respect to the plasma potential, both positive and negative ions can be mass analyzed.

Similarly, use of an arrangement as summarized above is an aspect that provides effects and advantages as summarized above.

Brief description of the drawings

Embodiments will now be described with reference to the attached drawings, where: figure 1 schematically illustrates an apparatus for determining the mass of ions in a plasma, figure 2a schematically illustrates ion trajectories, figure 2b is a graph on which ion impact locations are plotted, figure 3 schematically illustrates a configuration of an electrode and an insulator, figure 4 schematically illustrates a configuration of an electrode and an insulator, figures 5a and 5b schematically illustrate a configuration of an electrode and an ion impact detector, figures 6a and 6b schematically illustrate a configuration of an electrode and an ion impact detector, figure 7 is a graph on which ion trajectories are plotted, figures 8a, 8b and 8c show detected ion impact locations on an electrode, figures 9 and 10 are diagrams showing detected current density resulting from ion impacts, figure 11 schematically illustrates an apparatus for determining the mass of ions that is arranged in a capacitive coupled plasma (CCP) reactor, and figures 12a and 12b schematically illustrate an apparatus for determining the mass of ions in an ion beam.

figures 13a and 13b illustrates difference in focusing for different electrode/insulator interfaces. Detailed description of embodiments

An apparatus 100 for determining the mass of ions in a plasma is schematically illustrated in figure 1. The apparatus 100, which is immersed in a plasma as schematically illustrated by ions 101 comprises an electrode 102 having a surface extending in two dimensions in a plane perpendicular to the plane of the drawing. The electrode is joined with an insulator 104, which has an extent in two dimensions in a plane parallel to the plane of the surface of the electrode 102. An electrode/insulator interface 103 is defined by a plane joining the electrode 102 and the insulator 104.

A voltage source 106 is connected to the electrode 102 and is configured to provide a voltage to the electrode 102 such that any desired electric potential difference is obtained between the electrode 102 and the plasma 101. In figure 1 , the potential difference between the electrode 102 and the plasma 101 has created a curved potential distribution in the plasma 101. The potential distribution is in figure 1 illustrated by a dashed line representing the cross- section of an equipotential surface 110 with the plane of the drawing. The entity enclosed by the boundary delineated by the dashed line 110 and the electrode 102 is a lens-formed sheath, a non neutral space charge, while the plasma 101 surrounds the sheath as a collection of positive and negative charges giving a zero net charge.

As figure 1 shows, the curvature of the equipotential surface 110 of the plasma- sheath-lens edge is strongest in the vicinity of the electrode/insulator interface 103. In the following, such a curved potential distribution will be denoted "plasma sheath lens", due to its effect of focusing ions entering the lens onto the electrode 102, as will be described in more detail below.

An ion impact detector 108 is schematically illustrated as being connected to the insulator. However, as will be described in more detail below, the detector 108 is arranged such that it is capable of detecting ions impacting the electrode 102, the detecting involving a spatial discrimination such that the impact locations of individual ions can be detected.

A magnetic field 112 is provided by one or more magnetic field sources 114, 116. As will be described in more detail below, a configuration where the magnetic field lines 112 are perpendicular to the surface plane of the electrode 102 is beneficial in that it enables simple interpretation of the detected ion impacts in terms of ion mass.

Turning now to figures 2a and 2b, in which a closer view of an electrode/insulator interface 203 is presented, trajectories of ions will be described. However, before describing results from a magnetized PSL, a setup without magnetic field will be described. Hence, similar to the arrangement in figure 1 , figure 2 schematically illustrates an electrically biased electrode 202 attached to an insulator 204, immersed in plasma 201. Figure 2 does not show any voltage provider or ion impact detector. The electrode 202 and the insulator 204 are circular in a plane perpendicular to the plane of the drawing and, as figure 2 shows, they both have a radius of 5 mm (in the x-direction) and the surface of the electrode 202 is a Z=O, No magnetic field is present in the setup shown in figure 2.

As a consequence of the bias voltage applied to the electrode 202, a curved potential distribution is obtained in the plasma 201. An equipotential surface 210, or edge of a plasma sheath lens, is shown in figure 2.

A number of ions 221 , 223, 225, 227 are illustrated being located at the edge of the plasma sheath lens. A first illustrative ion 221 and a second ion 223 are located at positions having z<0 and a third ion 225 and a fourth ion 227 are located at positions having z>0. Each ion 221 , 223, 225, 227 is affected by the curved potential distribution 210 such that respective resulting forces accelerate them, via respective trajectories 231 , 233, 235 and 237, to impact locations on the surface of the electrode 202. The first ion 221 impact at a radius smaller than that of the impact location of the second ion 223, which is smaller that the radius of the impact location of the third ion 225 and the fourth ion 227.

Figure 2b is a plot of a large number of ion impact locations on the electrode 202 in the setup of figure 2a. The distribution of impacts exhibits two focusing effects: discrete focusing and modal focusing. The discrete effect is a focalization effect associated with a PSL that consists of a discrete ion current distribution over the surface of a biased electrode where "discrete" refers to a zero current over a certain area and a very high current adjacent to it. The modal focusing effect is a focalization effect involving charge particles entering the PSL from the proximity of the insulated interface. The discrete and modal focusing effects can act and be controlled independently of each other. In figure 2b, the modal focusing effect shows up as a concentration of impacts around (x,y)=(0,0), and the discrete focusing effect shows up in the form of a sharp "edge" at a radial distance from (x,y)=(0,0) of 3 mm. Figure 2b also shows a so-called passive surface, being a very well delineated surface of zero ion current, i.e. no impacts, on the biased electrode surface at radial distances larger than about 3 mm.

Examples of test results where a magnetic field is involved, showing impact locations of a large number of ions, will be presented below in connection with figures 7 and 8.

Before illustrating such test results, a few alternative electrode/insulator configurations and impact detecting arrangements will be described with reference to figures 3 to 6.

Figure 3 is a side view schematically illustrating an electrode 302 and an insulator 304 joined at an interface 303, where the insulator 304 has a significantly larger extent than the electrode 302 in the z-direction, as compared with the arrangements in figures 1 and 2, where the electrode and insulator have essentially the same z-extent. A voltage provider 306 is schematically shown connected to the electrode 302 for providing a bias. Figure 3 also shows plasma 301 and an equipotential surface 310 that defines a plasma sheath lens and a magnetic field B, generated by magnetic field generators 305.

Figure 4 is a side view schematically illustrating an electrode 402 and an insulator 404 joined at an interface 403. Also in this example, the insulator 404 has a significantly larger extent than the electrode 402 in the z-direction.

However, in contrast to the previous examples, in figure 4 the electrode 402 and the insulator 404 have a common upper surface on which aiso an interface 403 between the electrode 402 and the insulator 404 is located. As in figure 3, a voltage provider 406 is schematically shown connected to the electrode 402 for providing a bias. Furthermore, as in figure 3, figure 4 also shows plasma 401 and an equipotential surface 410 that defines a plasma sheath lens and a magnetic field B, generated by magnetic field generators 405.

Figure 5a is a top view of an electrode 502 having a radial slit 530. Figure 5b is a cross-sectional view A-A of the electrode 502 and the slit 530. The electrode 502 is attached to an insulator 504 that is significantly larger in z-direction extent than the electrode 502. A detector 532 is arranged such that it may be displaced in a radial direction, as indicated by a dashed line denoted with r. The detector 532 is configured, e.g. in terms of choice of material, to detect ions impacting at the electrode. The array 532 is connected to an impact detector 508, similar to the situation described above in connection with figure 1 and by detecting impacts at different radii r, ion impact locations can be detected in the detector 508.

Figure 6a is a top view of an electrode 602 that is divided into a plurality of concentric detector rings 640 that are isolated electrically from each other by concentric insulator rings 642. Figure 6b is a cross-sectional view A-A of the electrode 602 and the rings 640, 642. The electrode 602 is attached to an insulator 604 that is significantly larger in z-direction extent than the electrode 602. Within the insulator 604 is a plurality of detector connectors 644 arranged. The connectors 644 are each individually connected to an impact detector 608, similar to the situation described above in connection with figure 1 and by detecting impacts in individual detection rings 640, ion impact locations can be detected in the detector 508.

Turning now to figures 7, 8a-c and figures 9 and 10, results will be presented from a setup of a PSL where a magnetic field is present. The setup is that of figure 2a with an added magnetic field along the z-axis. In figure 7, ion trajectories are plotted in a projection in the x,y-plane. Figures 8a-c show ion impact locations and figures 9 and 10 show corresponding current densities recorded at the electrode.

Figure 7 illustrates that, in the presence of a magnetic field parallel with the z- axis, the ion trajectories are further twisted in three dimensions. One trajectory 701 is emphasized in order to illustrate the twisting. This makes it difficult to illustrate their appearance and as a result of this, figure 7 shows the projection of ion trajectories on the two-dimensional electrode plane (i.e. the x,y-plane). As figure 7 shows, by applying a magnetic field B≠O, all trajectories will get twisted so that the maximum flying distance of the ions get shorter than in a case with no magnetic field (as in figure 2a). This is seen in figure 7 in that the trajectories exhibit a clear location where from there will be no incident ions entering the sheath with z<0. This location is giving the small peak in the recorded current density shown in figure 10, for instance at r=1 , The large peak at r=4 in figures 9 and 10 corresponds to the discrete focusing involving all ions from locations close to 225 and 227 in figure 2.

Figures 8a, 8b and 8c are plots of impact locations on an electrode surface 802 for all ions entering the sheath with z<0. An interface 803 with an electrode/insulator interface is indicated by reference numeral 803. The setup is similar to that of figure 2 with a magnetic field present parallel with the z-axis.

Figure 8a shows impact locations on the electrode surface for ions having a mass m=1 and for a magnetic field β_z=1000 G. Figure 8b shows impact iocations on the electrode surface for ions having a mass m=40 and for a magnetic field S_z=1000 G,

Figure 8c shows impact locations on the electrode surface for ions having a mass m=40 and for a magnetic field B₂=2000 G.

As in the case of no magnetic field, illustrated in figure 2b, the ion impact patterns of figures 8a-c exhibit the modal and discrete focusing effects as well as passive surfaces at radial distances close to the electrode/insulator interface 803.

In addition to these impact patterns, figures 8a-c also exhibit passive surfaces for ions entering the sheath with z<0 at smalier radial distances, i.e. at r<1.2 in figure 8a, r<0.1 in figure 8b and at rθ.2 in figure 8c. The ions entering the sheath with z>0 will exhibit the discrete focusing at larger r, seen as a passive surface for r>4.1 in figure 8a and r>3.75 in figure 8b, and then a rather uniform incidence for smaller r, thus just adding a background level to the peaks at smaller r, as is evident from impact density plots in figures 9 and 10. The larger mass of the ions in figure 8b compared to figure 8a, will influence ions entering the sheath with z>0, to impact the electrode closer to its center as a result of the discrete focusing effect. Both the discrete focusing effect and the modal focusing effect influence the impact pattern and can therefore be used to determine the mass of the ions.

Turning now to figures 11 and 12, apparatuses will be described where ion mass determination as discussed above is performed.

Figure 11 schematically illustrates an apparatus 1101 for determining the mass of ions that is arranged in an inductively coupled plasma (ICP) reactor 1150 containing a suitable gas. The reactor 1150 is grounded 1154 and configured with a vacuum pump 1152. A top electrode (antenna) 1160 covered with a dielectric material 1162 is connected to a radio frequency source 1164, A bottom electrode 1170 is supplied with high voltage from a high voltage source 1172. A wafer 1190 for chip manufacturing is arranged on the bottom electrode. Plasma is created by the top electrode 1160 acting on the gas in the reactor 1150, and by suitable control of the high voltage provider 1172, ions are implanted into the wafer 1190.

An apparatus 1101 for determining the mass of the ions in the plasma created by the top electrode 1160 is arranged at the reactor 1150. The apparatus 1101 comprises, as illustrated very schematically, a detecting part 1103 and a control part 1105. The detecting part 1103 may, e.g. correspond to any of the arrangements described above in connection with figures 1 , 3, 4, 5 and 6. The control part 1105 may comprise all or part of an ion impact detecting apparatus.

Turning now to figure 12, a magnetized plasma sheath lens structure will be described that is used as an independent mass spectrometer where the ions are injected in the sheath from an external source at a certain location and then detected as separate mass signals on the electrode.

Similar to the arrangement in figure 1 , figure 12a illustrates an apparatus 1200 for determining the mass of ions using a customized plasma-sheath-lens. The apparatus 1200 comprises a housing 1219 inside which plasma is present, as schematically illustrated by ions 1201. The apparatus 1200 comprises an electrode 1202 having a surface extending in two dimensions in a plane perpendicular to the plane of the drawing. The electrode is joined with an insulator 1204, which has an extent in two dimensions in a plane perpendicular to the plane of the surface of the electrode 1202 (cf. the arrangement illustrated in figure 3). An electrode/insulator interface 1203 is defined by a plane joining the electrode 1202 and the insulator 1204.

A device 1272 configured as a combined voltage source and ion impact detection device is connected to the electrode 1202 and is configured to provide a voltage to the electrode 1202 such that any desired electric potential difference is obtained between the electrode 1202 and the plasma 1201. In figure 12, the potential difference between the electrode 1202 and the plasma 1201 has created a curved potential distribution in the plasma 1201. The potential distribution at the edge of plasma-sheath-lens is in figure 12 illustrated by a dashed line representing the cross-section of an equipotential surface 1210 with the plane of the drawing. As figure 12a shows, the curvature of the equipotential surface 1210 is strongest in the vicinity of the electrode/insulator interface 1203 and defines a plasma sheath lens as discussed above, A magnetic field along the z-axis is provided by magnetic field sources 1211.

The housing 1219 is configured such that it allows access to ions from an external source, here in the form of a beam of ions 1217 from an ion beam source 1215, such as an ionization and acceleration chamber of an initial neutral gas or any type of ion source. As illustrated in figure 12b, and as discussed above in connection with figure 7, depending on the mass of the ions in the beam 1217, the paths of the ions will twist. In figure 12, three paths are indicated and denoted with M1 , M2 and M3, respectively.

Depending on the mass of the ions in the beam 1217, impacts may be detected on the surface of the electrode 1202 at varying radial distances, and by comparing the signals detected as shown in Figure 6 with and without the ion beam, the mass of the impacting ions can be determined.

Figure 13a-b illustrates differences in focusing for different electrode/insulator interfaces. Figure 13a shows a setup where a significant part of an electrode 1303 is covered by an insulator 1304. The electrode 1303 and the insulator 1304 is surrounded by a plasma 1301 , and a charge is supplied to the electrode 1303 generating an electrical field in the plasma 1301. An equipotential surface 1308 and three ions 1304, 1305 and 1306 is shown. As a result of the electrical field generated by the electrode 1303, an electrical force acts on the ions accelerating them along the direction of the electrical field, thereby attracting them to the electrode along the trajectories shown.

Dependent on their initial position and their mass, the ions 1305, 1306, 1307 will follow different trajectories. The two ions positioned above the electrode z>0 1305, 1306 will be influenced by the discrete focusing effect described above. In the start of their trajectory they will be influenced by an electrical field having a relative large component parallel with the electrode surface. As the ions 1304, 1305 moves closer to the electrode, the parallel component of the electrical field will decrease and the component of the electrical field perpendicular to the surface of the electrode will increase. Dependent on the mass of the ions resulting in different inertia, ions with a larger mass will be more influenced by the large parallel component of the electrical field in the start of their trajectory and will as a result impact the electrode closer to the centre than ions with a smaller mass. This will result in the discrete nature of the impact pattern described earlier, with almost zero ion impacts at distances above a particular distance to the centre of the electrode. Where the particular distance is dependent on the ion mass e.g. for ions with a large mass the particular distance is small and for ions with a small mass the particular distance is large. Thereby by measuring the particular distance a first measure of the ion mass of the ions in the plasma is provided.

The ion 1307 positioned below the electrode will as described earlier be influenced by the modal focusing effect, and as a result impact the electrode 1302 with a distance to the centre dependent on its mass. This will result in a central passive surface with almost zero ion impacts in the centre of the electrode, the passive surface having a radius dependent on the mass of the ions. Thereby by measuring the radius of the central passive surface, a second measure for the mass of the ions is provided.

Figure 13b shows a setup where only a limited part of the electrode 1302 is covered by an insulator 1303. The electrode 1303 and the insulator 1304 is surrounded by a plasma 1301 and a charge is supplied to the electrode 1303 generating an electrical field in the plasma 1301. An equipotential surface 1308 and three ions 1305, 1306 and 1307 is shown. As the electrical field above the electrode z>0 is the same as the electrical fieid in 13a, the forces acting on the ions above the electrode 1305, 1306 are the same. The discrete focusing effect is therefore also present for the electrode/insulator setup shown in figure 13b, and the ions 1305, 1306 follow the same trajectory as the corresponding ions shown in figure 13a. For the ion 1307 positioned below the electrode z<0 the module focusing effect is no longer present, as the electrical field below the electrode is different than the electrical field in figure 13a, causing the ion 1307 to impacts the electrode on its back side. As a result the module focusing effect can no longer be used to detect the mass of the ions in the plasma, only the discrete focusing effect. As both the module focusing effect and the discrete focusing effect is influenced by a magnetic field, an apparatus for determining the mass of ions according to the present invention, having an electrode only covered a small portion by an insulator, will still be able to detect the mass of ions as a result of the discrete focusing effect.

The charge sign of ions to be measured may depend on the sign of the applied bias on the electrode. For analyzing positive ions the applied bias may be negative with respect to plasma potential so that the plasma-sheath-lens is formed only by positive charges. For analyzing negative ions the applied bias may be positive, corresponding to a plasma-sheath-lens formed by electrons and negative ions.

Claims

1. An apparatus (100) for determining the mass of ions, the apparatus configured to hold a plasma (101) having a plasma potential, comprising:

- an electrode (102) having a surface extending in a surface plane, - an insulator (104) interfacing with the electrode,

- an electric potential provider (106) configured to provide an electric potential different than the potential of the plasma to the electrode, thereby forming a curved (110) potential distribution in the plasma surrounding the electrode, - a magnetic field source (114,116) configured to provide a magnetic field across at least part of the curved potential distribution in the plasma surrounding the electrode,

- an ion impact detector (108) configured to detect impacts of ions arriving at the electrode, the detecting comprising detecting of locations of the impacts, - a processing unit (108) configured to interpret the detected impact locations in terms of the mass of the impacting ions.

2. The apparatus of claim 1 , where a limited portion of the surface area of the electrode is covered by the insulator .

3. The apparatus of any of claims 1 to 2,, where the electrode surface and the insulator are in different planes, whereby the interface between the electrode and the insulator is outside the electrode surface plane.

4. The apparatus of any of claims 1 to 2, where the electrode surface and the insulator are in a common plane, whereby the interface between the electrode and the insulator is in the common plane.

5. The apparatus of any of claims 1 to 4, where the magnetic field source is configured to provide the magnetic field in a direction essentially perpendicular to the electrode surface plane.

6. The apparatus of any of claims 1 to 5, where the magnetic field source is configured to provide the magnetic field concentrated in a region covering the interface between the electrode and the insulator.

7. The apparatus of any of claims 1 to 6, where the electrode comprises a radial slit (530) in the electrode surface, and where the ion impact detector (508) is configured to detect the ions arriving at the electrode through the slit.

8. The apparatus of any of claims 1 to 6, where the electrode comprises concentric ion detectors (640) interspaced by concentric insulators (642).

9. The apparatus of any of claims 1 to 8, where the electrode surface is essentially circular.

10. The apparatus of any of claims 1 to 8, where the electrode surface is polygonal.

1 1 . The apparatus of any of claims 1 to 10, configured to allow access to ions (1217) from an ion source for mass determination.

12. The apparatus of claim 11 , configured to receive ions (1217) from an ion beam source (1215).

13. A method for determining the mass of ions, using a plasma having a plasma potential, comprising:

- providing an electric potential different than the potential of the plasma to an electrode having a surface extending in a surface plane and interfacing with an insulator, thereby forming a curved potential distribution in the plasma surrounding the electrode,

- providing a magnetic field across at least part of the curved potential distribution in the plasma surrounding the electrode, - detecting impacts of ions arriving at the electrode, the detecting comprising detecting of locations of the impacts, - interpret the detected impact locations in terms of the mass of the impacting ions.

14. Use of an apparatus as claimed in any of claims 1 to 12, for determining the mass of ions.