WO2010000732A1

WO2010000732A1 - Method and apparatus combining secondary ion mass spectrometry and scanning probe microscopy in one single instrument

Info

Publication number: WO2010000732A1
Application number: PCT/EP2009/058172
Authority: WO
Inventors: Henri-Noël MIGEON; Tom Wirtz
Original assignee: Centre De Recherche Public Gabriel Lippmann
Priority date: 2008-07-01
Filing date: 2009-06-30
Publication date: 2010-01-07

Abstract

The present invention relates to an analytical instrument,and a method for analysing a sample, comprising Secondary Ion Mass Spectrometry (SIMS) means and surface imaging microscopy means.

Description

METHOD AND APPARATUS COMBINING SECONDARY ION MASS SPECTROMETRY AND SCANNING PROBE MICROSCOPY IN ONE SINGLE

INSTRUMENT

Field of the invention

[0001] The present invention relates to an instrument, and to an analytical method, combining Secondary Ion Mass Spectrometry (SIMS) and Scanning Probe Microscopy (SPM) . The invention also relates to an analytical method carried out with said instrument.

Prior art and related technical background

[0002] Among sample analysis techniques, Secondary Ion Mass Spectrometry (SIMS) is a well-established and widely used technique.

[0003] Owing to its excellent sensitivity, its high dynamic range and its good depth resolution, SIMS is an extremely powerful technique for analyzing samples. [0004] Today, SIMS is widely used for analysis of trace elements in solid materials, especially semiconductors and thin films. Other emerging fields of application for SIMS are nanotechnologies, biology and medicine in particular.

[0005] The SIMS technique is based on the detection of secondary ions emitted by a sample exposed to a primary ion bombardment produced by a focussed ion or cluster ion beam, typically Cs⁺, Ga⁺, Ar⁺, O^~ or O₂ ⁺, Cδo⁺ or Bi_n ⁺. The ion emission produced by the sample following the primary ion bombardment, also called sputtering phenomenon, which is a characteristic of the material itself, is then analysed by mass spectrometry, for example by magnetic sectors, time of flight analysers or quadrupole filters. Modern magnetic sector spectrometers provide transmissions in the range of 50% at high mass resolution (M/ΔM= 5000). SIMS instruments allow producing 3D chemical mappings with excellent lateral and depth resolutions, for example 50 nm lateral resolution on a Cameca NanoSIMS 50 instrument and 1 nm range depth resolution on a Cameca SC-Ultra instrument. [0006] Considerable efforts are currently spent to further improve the spatial resolution of SIMS instruments. On the one hand, the development of new ion sources with an increased brightness allows producing smaller spot sizes and thus increased lateral resolutions. On the other hand, new ion optics permitting lower impact energies of the primary ions reduce the dimensions of the collision cascades in the sample and hereby improve the depth resolution.

[0007] However, SIMS has a number of important drawbacks .

[0008] SIMS analyses are only quantified with difficulty. This drawback is known as the "matrix effect", which is a quantification problem due to the fact that the ionisation probability of atoms and molecules emitted from the sample strongly depends on the sample composition. In the case of metallic samples or for semi-conductors for instance, the ionization probability for positive and negative ions exponentially depends on the electron work function of the sample ("electron tunnelling model") . Furthermore, the surface polarization model postulates that surfaces can be microscopically heterogeneous and that the enhancement of both positive and negative ion yields is controlled by the local "photoelectron threshold" i.e. energy difference between the highest occupied electronic level in the surface and the local vacuum level. As a consequence, the intensity of the measured signals also greatly depends on the composition of the analysed sample .

[0009] Sample surfaces, presenting an initial roughness, lead to an uncertainty on the depth scale. Moreover, this roughness changes during the ion bombardment as the local sputtering yields depend on parameters such as the local angle of incidence of the ion beam and the crystal orientation. In addition, the situation is worsened if the sample is constituted of different materials due to preferential sputtering phenomena. Hence, the obtained 3D resolution is often much worse than what could be expected considering the instrumental performances.

[0010] Taking into account that there are no means to check whether the step size of the scanning beam is calibrated correctly, it may happen that part of the sample inside the crater is not sputtered away. In this case, spikes form inside the crater at regular intervals. As a consequence, the depth resolution is deteriorated. [0011] Among surface analysis techniques, Scanning Probe Microscopy (SPM) is a well established and widely used technique. This technique includes for example Atomic Force Microscopy (AFM) , Kelvin Probe Force Microscopy (KPFM) , Scanning Tunnelling Microscopy (STM) , Electrostatic Force Microscopy (EFM) and Magnetic Force Microscopy (MFM) .

[0012] In AFM mode, for instance, topographical mappings of the surface can be performed making use of the electrostatic forces. In STM mode, for instance, so- called voltage-ampere mappings can be performed on conductive samples. In MFM mode, magnetic signals are recorded making use of the magnetic forces between the sample and the tip. In EFM mode, mappings of relative strength and direction of electric polarization can be performed. Finally, in KPFM mode, height variations on the sample surface are measured. Simultaneously, the electrostatic forces between sample and tip are used to measure the contact potential differences (CPD) . Once, the cantilever' s work function is known, it is possible to determine the sample's work function from the measured CPD.

[0013] SPM analysis presents the main drawback of giving no information regarding element identification. [0014] Currently, there is no analytical method, and no instrument, enabling to give the spatial distribution of the species composition of a sample surface .

Aims of the invention

[0015] The present invention aims to provide an analytical instrument and an analytical method which do not have the drawbacks of the prior art.

[0016] The present invention aims to provide an analytical instrument and an analytical method which enable to correct SIMS artefacts and SIMS images. [0017] The present invention aims to provide an analytical instrument and an analytical method in which the volume of sputtered atoms or molecules is determined more accurately.

[0018] The present invention aims to provide an analytical instrument and an analytical method allowing to accurately calibrate the depth scale of the SIMS analysis .

[0019] The present invention aims to provide an analytical instrument and an analytical method in which the sample is not contaminated by air contact. [0020] The present invention aims to provide an analytical instrument and an analytical method which are time saving.

Summary of the invention

[0021] The present invention relates to an analytical instrument comprising, under Ultra High Vacuum

(UHV) , Secondary Ion Mass Spectrometry (SIMS) means and surface imaging microscopy means.

[0022] According to particular embodiments, the analytical instrument comprises one or a combination of one, or several, of any of the following characteristics: the SIMS means comprise mass spectrometry means selected from the group consisting of a magnetic sector mass spectrometer, a Time-of-Flight mass spectrometer and a quadrupole mass spectrometer,

- the SIMS means can be operated in the dynamic or static mode ,

- the surface imaging microscopy means comprise Scanning Probe Microscopy (SPM) means selected from the group consisting of Atomic Force Microscopy (AFM) means, Scanning Tunnelling Microscopy (STM) means, Kelvin Probe Force Microscopy (KPFM) means, Electrostatic Force Microscopy (EFM) means, and Magnetic Force Microscopy

(MFM) means,

- the analytical instrument comprises a sample holder, SIMS positioning means and/or SPM positioning means and/or sample holder positioning means,

- the Secondary Ion Mass Spectrometry (SIMS) means and the surface imaging microscopy means are both located in the same analytical chamber.

[0023] The present invention relates also to an analytical chamber for an analytical instrument, said analytical chamber being under Ultra High Vacuum (UHV) and comprising Secondary Ion Mass Spectrometry (SIMS) means and surface imaging microscopy means. [0024] According to a particular embodiment, the analytical chamber comprises a sample holder, SIMS positioning means and/or SPM positioning means and/or sample holder positioning means.

[0025] The present invention relates also to a method to analyse under Ultra High Vacuum (UHV) a sample comprising the step of: a) providing a sample to be analysed, b) positioning in the vicinity of SPM means a portion of the surface of said sample, c) analysing said surface portion using the SPM means, to provide topographical data and work function data regarding said surface portion of said sample, d) positioning in the vicinity of SIMS means said surface portion analysed by the SPM means, e) analysing said surface portion analysed by the SPM means in step c) , using the SIMS means, to provide chemical data regarding said portion of surface, f) positioning in the vicinity of the SPM means said surface portion analysed by the SIMS means in step e), g) analysing said surface portion analysed by SIMS means in step e) , using the SPM means, to provide topographical data and work function data regarding said portion of surface, h) combining the chemical data, the topographical data and work function data to provide a 3D representation of the spatial distribution of the species composing said surface portion of said sample. [0026] According to particular embodiments, the method comprises one or a combination of one, or several, of any of the following characteristics: - steps d) to g) are repeated several times to record a SIMS depth profile of a portion of the surface of the sample,

-the positioning of the surface portion analysed by the SPM and SIMS means, is performed by using as a position reference the sample edge, or the edge of the crater created by SIMS sputtering, or at least one mark on the sample, or at least one mark on the sample holder,

- the SPM means are selected from the group consisting of Atomic Force Microscopy (AFM) means, Scanning Tunnelling Microscopy (STM) means, Kelvin Probe Force Microscopy

(KPFM) means, Electrostatic Force Microscopy (EFM) means and Magnetic Force Microscopy (MFM) means.

[0027] The present invention relates also to a computer program comprising a software code adapted to perform the method according to the invention, to a computer program comprising a software code adapted to process and combine topographical, work function and chemical data obtained by using the method according to the invention, and to computer program comprising a software code adapted to control the analytical instrument according to the invention, so that the analytical instrument performs the method according to the invention.

Brief description of the drawings

[0028] Figure 1 is a schematic representation of a SIMS analysis.

[0029] Figure 2 is a schematic representation of a SPM analysis.

[0030] Figure 3 is a schematic representation of the analytical instrument according to the present invention in the SPM analysis position. [0031] Figure 4 is a schematic representation of the analytical instrument according to the present invention in SIMS analysis position.

[0032] Figure 5 represents an example of a 3D- reconstruction of a PS-PMMA blend sample using SIMS software .

[0033] Figure 6 represents an example of a 3D mapping of the PS-PMMA blend sample using SPM software. [0034] Figure 7 represents a schematic diagram of a 3D SIMS analysis of a sample composed of three different chemical elements and having a surface roughness. In figure 7a), the 3D mapping is not accurate as the software will assume a flat surface and will thus place all atoms sputtered during the first raster scan in a first flat atomic layer of the sample. In figure 7b, the chemical information from SIMS is combined with the topographical information from SPM, resulting in an accurate 3D reconstruction of the analysed zone. [0035] Figure 8 represents two KPFM mappings of the same solar cell module. Figure 8a shows the mapping of the solar cell module, which was exposed to air, before argon-sputter cleaning (contaminated sample) . Figure 8b shows the mapping of the same area on the solar cell module after argon-sputter cleaning.

[0036] Figure 9 shows a 2D-reconstruction of the PS-PMMA blend after 33 minutes of SIMS sputtering using AFM software. The sputtering dose is around 3,53xlO¹⁵ atoms/cm².

[0037] Figure 10 shows a 3D-reconstruction of the PS-PMMA blend after 132 minutes of SIMS sputtering using AFM software. In this case, the sputtering dose is around 1,4IxIO¹⁶ atoms/cm².

[0038] Figure 11 represents the work function shift (Δφ) with respect to the caesium concentration C_cs for an aluminium (Al) substrate, the sample being bombarded with Ga⁺ ions in conjunction with different doses of Cs⁰ evaporated on top of the sample. [0039] Figures 12 represents the work function shift (Δφ) with respect to the caesium concentration C_cs for a nickel (Ni) substrate, the sample being bombarded with Ga⁺ ions in conjunction with different doses of Cs evaporated on top of the sample.

[0040] Figures 13 represents the work function shift (Δφ) with respect to the caesium concentration C_cs for silicon (Si) substrate, the sample being bombarded with Ga⁺ ions in conjunction with different doses of Cs⁰ evaporated on top of the sample.

[0041] Figure 14 represents the ionization probability (β) of sputtered Cs ions with respect to the work function shift (Δφ) for an aluminium (Al) substrate bombarded with Ga⁺ ions in conjunction with different doses of Cs⁰ evaporated on top of the sample. [0042] Figure 15 represents the ionization probability (β) of sputtered Cs ions with respect to the work function shift (Δφ) for a nickel (Ni) substrate bombarded with Ga⁺ ions in conjunction with different doses of Cs⁰ evaporated on top of the sample. [0043] Figure 16 represents the ionization probability (β) of sputtered Cs ions with respect to the work function shift (Δφ) for a silicon (Si) substrate bombarded with Ga⁺ ions in conjunction with different doses of Cs⁰ evaporated on top of the sample.

Detailed description of the invention

[0044] The originality of the present invention is to combine SIMS means and SPM means in one single instrument to perform a combined Secondary Ion Mass Spectrometry (SIMS) and Scanning Probe Microscopy (SPM) analysis .

[0045] Complementary in-situ SPM mappings of the sample surface roughness allow correcting SIMS images with respect to the SIMS artefacts. A careful image overlay procedure ensures exact alignment of the resulting SPM images with the portion of the sample analyzed by SIMS. As a consequence, the total eroded depth, created by the SIMS sputtering, can be determined as a function of the lateral position within the sputter crater, thus allowing an independent depth scale calibration on each pixel of the imaged area. Combining the mass spectral data processed in this way with the topography information from the SPM images, it is possible to reconstruct the true spatial distribution of the species composing the sample within the investigated sample volume. Figure 7, for example, shows a 3D SIMS analysis of a sample composed of three different chemical elements and having a surface roughness. In traditional SIMS analysis (figure 7a) the 3D mapping is not accurate as the software will assume a flat sample surface and will thus place all atoms sputtered during the first raster-scan in a first atomic layer of the sample. [0046] A combination of one or several SIMS mappings and a number of successively recorded SPM mappings of the analysed zone allows for an accurate chemical mapping to be obtained. In fact, a 3D reconstruction of a single combined chemical mapping at a particular sputtering time can be performed by using a software for overlaying the 2D SIMS reconstruction and the corresponding 3D topographical data obtained from AFM. An edge filter may be used on the SPM data for denoting the 3D coordinates of the crater edge created by SIMS sputtering. [0047] As the sputtering intensity depends on the incidence angle of the ion beam, resulting in a bright crater edge in the SIMS mapping, this bright edge can be used as a position reference to realign the AFM and SIMS mappings. In other embodiments, this position reference may be any suitable mark on the sample to be analysed or on the sample holder.

[0048] Thereafter, having reconstructed the various 2D planes using the topographical data from SPM and the 2D chemical data from the corresponding SIMS mapping, a 3D reconstruction can be achieved taking into account the sputtering time in between the recording of the different images. The accuracy is increased by acquiring several successive SIMS and SPM images in order to take into account an evolution of the topography during the sputtering. In all cases an accurate 3D mapping including physical and chemical properties is possible .

[0049] Kelvin Probe Force Microscopy (KPFM) mappings revealing work function shifts on the sample surface allow a correction of further SIMS artefacts. As the ionization probability is linked to the work function, a laterally inhomogeneous work function leads to artefacts in the SIMS 2D and 3D mappings. These can be corrected on a pixel by pixel basis by using the complementary KPFM images. In case the postulates of the surface polarization model are shown to be correct, the local work function differences could also be corrected by using KPFM and consequently more accurate measurements are possible.

[0050] Being analysed in the same instrument, the sample is not contaminated by air contact in between SIMS and SPM analysis mode. A transfer of the sample at air between SIMS and SPM instrument would lead to the contamination of the sample and consequently to non- usable results.

[0051] Moreover, knowing the exact position of the reference position, for example the crater formed during SIMS sputtering, allowing to combine the SIMS data and SPM data, it is much easier to perform SPM imaging of the same area. In fact, if a separate standalone SPM is used, the search for the reference position, for example the crater created by SIMS sputtering, is technically not feasible .

[0052] Furthermore, the combination of SIMS means and SPM means in one single instrument helps to save time. No samples need to be transferred from one standalone instrument to another one.

[0053] The SIMS means are any suitable means. It may comprise, for example, an ion beam, focussed ion or cluster ion beam, an ion extraction lens, and positioning means .

[0054] The SPM means are any suitable means. It may be for example Atomic Force Microscopy (AFM) , Kelvin Probe Force Microscopy (KPFM) means, Scanning Tunnelling Microscopy (STM) means, Electron Force Microscopy (EFM) means or Magnetic Force Microscopy (MFM) means. [0055] The SPM means may comprise for example a SPM head comprising a cantilever, comprising a tip, and SPM head movement means.

[0056] Preferably, the SIMS means and the SPM means are arranged in the same analytical chamber of the analytical instrument.

[0057] The analytical instrument, comprising the SIMS means and the SPM means, comprises Ultra High Vacuum (UHV) means, and may further comprises mass spectrometry means, sample handling means, computer means and computer program means. Preferably, the computer program means comprises a computer-readable medium having computer executable instructions adapted to cause the computer means to perform the method according to the present invention and/or to control the analytical instrument, or analytical chamber, according to the present invention. [0058] The analytical instrument may also further comprise, for example, a docking station or chamber, able to fit a transfer vessel for transferring the sample to be analysed, between the instrument and said transfer vessel .

[0059] With regard to the SPM specifications, the maximum scan range is preferably set to 100 μm. The SPM lateral resolutions are better than 20 nm. The SPM depth resolutions are better than 0.5 nm. For KPFM mode, the resolution is better than 0.5 eV in energy. [0060] In order to reduce the noise and to increase the bandwidth, preamplifiers may be included as close as possible to the deflection sensor. Preferably, the larger bandwidth (DC-3MHz) is used. It allows, for instance, measuring higher flexural modes as well as higher torsional modes by optical beam deflection AFM. [0061] As represented in figure 1, in Secondary Ion Mass Spectrometry (SIMS) analysis, following the primary ion bombardment 1, the secondary particles 2 (atoms, molecules, or ions) emitted (or sputtered) from the sample 3 to be analysed, placed on a sample holder 4, are analysed by mass spectrometry.

[0062] As represented in figure 2, in Scanning Probe Microscopy (SPM), the cantilever 6, comprising the tip 5 at one end and held by the tip holder 7 at the other end, is brought in close vicinity of the surface of the sample 3, said sample 3 being placed on the sample holder 4. [0063] The AFM means allow recording the interaction forces in between the tip 5 and the sample 3 to generate topographical data of the scanned area. While scanning the sample 3, the cantilever 6 is preferably frequency modulated. The frequency should be close to its first resonant frequency.

[0064] While using KPFM means, the cantilever 6 is exited at the second flexural mode by the application of an AC-bias voltage. By analysing the deflection signal using the Lock-In technique, compensation by a DC-bias voltage of the electrostatic interaction is provided and the absolute work function of the sample surface can be acquired. Therefore, an adequate cantilever calibration needs to be performed. If voltage modulation is used in this second mode, variations in contact potential differences (CPD) of the order of a few electron Volts (eV) are also detectable.

[0065] The STM means allow recording the volt- ampere characteristics (VAC) of tip-surface tunnelling contact. The STM means allow, for example, getting additional mappings of the electronic structure of conductive samples such as metals and semiconductors. [0066] The MFM means allow recording the magnetic forces in between the tip 5 and the sample 3. The MFM means allow for example getting additional mappings of the magnetic structure of a sample 3 (e.g. magnetic bits written on a hard disk) .

[0067] The EFM means allow recoding the electrostatic forces between the tip and the sample 3. The EFM means may, for example, be used for drawing additional mappings of the relative strengths and directions of electrical polarisations.

[0068] According to the invention, the sections of the analytical chamber 12 shown in figures 3 and 4 are under Ultra High Vacuum (UHV) . In order to obtain sensitive reproducible results, the materials should be free of contamination, hence the use of UHV. The UHV may be created by any suitable means, for example by turbomolecular pumps and ion getter pumps.

[0069] In a first preferred embodiment of the analytical instrument, or of the analytical chamber 12, the SIMS means and the SPM means may both stay at fixed positions whereas the sample holder 4 may be moved so that the sample 3 can be mapped by SIMS means and SPM means .

[0070] In a second preferred embodiment of the analytical instrument, or of the analytical chamber 12, the SIMS means may stay at a fixed position whereas the SPM means may be moved in front of the sample holder 4 for performing the SPM mapping in SPM mode. In this case, the sample holder 4 is not moved.

[0071] In a third preferred embodiment of the analytical instrument, or of the analytical chamber 12, the SIMS means may stay at a fixed position whereas the SPM means and the sample holder 4 may be movable. In this case, the SPM means may be moved in close vicinity of the sample holder 4, or vice-versa, so that the sample 3 can be mapped by SPM means (figure 3) . Furthermore, the sample holder 4 may be moved so that the sample 3 can be mapped by SIMS means (figure 4) .

[0072] In order to guarantee that the sample 3 is positioned correctly so that the SIMS and SPM mappings are performed at the same position, high precision SIMS positioning means and/or SPM positioning means and/or sample holder positioning means are used. These means may be operated and/or controlled by a computer program running any suitable code, or may be operated manually. [0073] Regarding figures 3 and 4, where the SIMS means stays at a fixed position whereas the SPM means and the sample holder 4 are movable, the sample holder 4 is positioned in the analytical chamber 12 using the sample holder positioning means, comprising for example a high precision motorized stage comprising an X-Y stage 14 and a Z-stage 13. Preferably, the high-precision movable stage has nanometer precision (figure 3) .

[0074] The SPM head, comprising the SPM tip 5, the cantilever 6, the SPM tip holder 7, and the scan unit 8, is fixed on a movable lift 9.

[0075] The sample holder 4 is moved away from the secondary ion extraction lens 10. The SPM head is moved in front of said lens 10, then the sample holder 4 is moved in close vicinity of the SPM cantilever 6. [0076] During SPM analysis mode, either the SPM head tip 5 scans the sample 3 on the sample holder 4, or the sample holder 4 scans the SPM tip 5. The sample holder 4 as well as the SPM head are held at mass potential .

[0077] In preparation to SIMS analysis mode, the SPM head is moved away from the ion extraction lens 10 using the SPM lift 9. Next, the sample holder 4 is moved in place with respect to the lens 10 using the Z-stage 13 of the sample holder positioning means. In order to avoid any resolution deteriorating vibrations, the sample holder 4 is preferably held in position using fixing means, preferably pressure means.

[0078] During SIMS analysis mode, in all the embodiments, the sample holder 4 is held at a non-zero voltage potential. If necessary, a shielding plate 11 may be used to shield off the sample holder 4 from any other instrumentation . [0079] All movements of the SPM lift 9 and sample holder lift are performed by any suitable positioning means, preferably at 50 nanometer precision. [0080] To correctly combine the SIMS data obtained from a single 2D mapping with the data obtained from the corresponding SPM mapping, a reference position is selected on the sample 3, or on the sample holder 4 in order to allow the positioning of sample 3 in respect with the SPM and SIMS means, and in order to be able to overlay both mappings with the highest precision. This reference position may be, for example, the sample edge, the edge of the crater created by SIMS sputtering, or any suitable mark on the sample 3, or on the sample holder 4. [0081] According to the present invention, the analyses are preferably carried out as follows. At first, in SPM analysis mode, the sample surface is analysed and mapped, for example as described in the European Patent Application EP 0551 814 Al. The word "mapping" should be understood as a complete scan of a determined surface area, either in SIMS or in SPM mode. Next, in SIMS analysis mode, at least one SIMS mapping is recorded during sputtering. While rastering an ion or cluster ion beam over a predefined area of the sample 3 and synchronizing the detection electronics to this primary ion beam rastering, a SIMS mapping is constructed on a pixel by pixel basis from the secondary ions emitted from the analysed region of the sample. The number of SIMS mappings recorded depends on the sample to analyse, on the depth of the analysis required, and on the change in roughness at the crater bottom. After a given depth has been reached, between for example 10 nm and 10 μm, the sputtered area and the area surrounding this sputtered area is analysed using SPM to record the topology and work function of the sputtered area. SIMS analysis and subsequently SPM analysis are repeated until a full SIMS 3D profile over the target depth is recorded, the final step consisting in the analysing by SPM of the sample's sputtered area as well as the area surrounding this area. [0082] The elemental 3D reconstruction of the sputtered volume is thereafter performed by combining the SIMS data and SPM data. The SIMS mappings are recalibrated both regarding the spatial distribution (using topographic information from the SPM analyses performed before and after the considered SIMS mappings) and the secondary ion intensities (using the work function mappings from the SPM analyses performed before and after the considered SIMS mappings) . A linear extrapolation between the two SPM mappings performed before and after the SIMS mappings is used for these reconstruction purposes. Using a reference point, the different reconstructed mappings are aligned. [0083] Both, the reconstruction of the 2D mappings and the full 3D reconstruction of the volume sputtered away, are performed using a computer program comprising a code suitable for processing data obtained by using the instrument .

[0084] The process and the instrument according to the present invention may be used for the analysis of any organic and inorganic material, for example material used or produced in the semiconductor industry, but also in all industries using coatings or surface.

Examples

Example 1: Blend samples of different materials. [0085] The sputtering of a phase separated thin film composed of PS-PMMA blend using Cs⁺ ions on a stand alone SIMS instrument followed by AFM analysis (tapping mode) on a standalone SPM instrument, was used as a model system to study the impact of AFM topography data on traditional 2D and 3D SIMS reconstruction. In this study, the SIMS and AFM images have been recorded on two separate standalone instruments. Furthermore, the sample has been transferred at air between both instruments. [0086] Such PS-PMMA blend thin films have been proposed for antireflection coatings whereas PS-b-PMMA diblock co-polymers have potential applications as patterned surface templates in the fabrication of high- density magnetic storage media. This combination of copolymers is known to produce thin films suitable for nanotemplating applications.

[0087] The traditional SIMS 2D mapping is shown in figure 5, the SIMS sputtering being performed with a dose of 3,53xlO¹⁵ atoms/cm². The reconstruction software assumes a flat surface. The 3D reconstruction is composed of a large number of 2D chemical mappings.

[0088] The AFM 2D mapping (top view) of the same sample after SIMS sputtering is shown in figure 9. The surface topography inside the crater is not flat. [0089] Combining the topographical data obtained from AFM and the chemical data obtained from SIMS allows correcting SIMS 2D and 3D reconstructions.

[0090] A 2D reconstruction of a single mapping after a particular sputtering time can be performed by using a software for overlaying the 2D SIMS reconstruction and the corresponding 3D topographical data obtained from AFM. An edge filter may be used on the AFM data for denoting the 3D coordinates of the crater edge created by SIMS sputtering.

[0091] After having reconstructed the various 2D planes using the topographical data from AFM and the 2D chemical data from the corresponding SIMS mapping, a 3D reconstruction can be achieved taking into account the sputtering time in between the recording of the different images .

[0092] As such phase separated blend samples often group several materials presenting different sputtering yields, preferential sputtering takes place. [0093] Knowing the sputtering yields of the given materials from measurement on standards, the related sputtering speeds can be used to check that the final 3D reconstruction is correct.

[0094] Whereas the PS hills (in light gray) in the first crater (of squared form) shown in figure 9 have the same shape than the PS hills shown at the outside of that crater, the shape of the PS hills inside a second crater (see figure 10) has changed with respect of the hill shape at the outside of the crater. It can be thus concluded that SIMS sputtering also modifies the shape of the PS hills with respect to sputtering time. Combining both SIMS and SPM in one single instrument allows taking these effects into account at reconstruction level by performing SPM analyses at different crater depths.

Example 2: Organic sample.

[0095] A solar cell "CIGSe" sample, having been exposed to air, was analysed using UHV-KPFM before and after cleaning using an Ar⁺ sputtering beam. [0096] Figure 8a shows the mapping of the sample work function recorded by KPFM of the contaminated sample exposed to air. The KPFM mapping shows little differences in the sample's work function (contact potential difference, CPD: -200 mV to -100 mV) .

[0097] Figure 8b shows the mapping of the sample work function recorded by KPFM of the same sample after sputter-cleaning for 5 minutes using a 500 eV Ar⁺ beam. Looking at this KPFM mapping, the differences in the work function are larger (contact potential difference, CPD: -10OmV to 400 mV) and more realistic. As a consequence, it can be said that the effect of air has a large influence on the sample's work function due to the adsorption of contaminants.

[0098] Combining SIMS means and SPM means in one single instrument presents the advantage of having a sample not exposed to air between subsequent SIMS and SPM analyses. As a consequence, more realistic results of the local work function can be obtained.

Example 3: Effect of the work function on ionization probabilities of sputtered atoms and molecules. [0099] The influence of Cs surface concentration on sample work function was studied by sputtering a number of samples using a Ga⁺ primary ion beam in conjunction with the evaporation of Cs⁰ on the sample's surface. The energy distributions of the secondary Cs⁺ ions were recorded for different surface concentrations of Cs in order to determine the work function shifts. [0100] Figures 11 to 13 represent the work function shift (Δφ) with respect to the Cs concentration on the sample surface of Aluminium (Al), Nickel (Ni) and Silicon (Si) respectively.

[0101] The SIMS analysis of a sample using reactive species (such as Cs) changes the work function of the sample and consequently also changes the ionization probability and the useful yield, UY of the analysed species (figures 14 to 16: ionization probability versus Δφ) .

[0102] Taking into account that samples are inhomogeneous and rough, the surface concentration of Cs is inhomogeneous over the sample surface. As a consequence, the secondary ion signals coming from different pixels can vary while the concentration of the sample element "M" stays the same. Being able to locally determine the work function of a sample using KPFM in a combined SIMS-SPM instrument is a means for correcting for these SIMS artefacts.

Claims

1. An analytical instrument comprising, under Ultra High Vacuum (UHV) , Secondary Ion Mass Spectrometry (SIMS) means and surface imaging microscopy means .

2. The instrument according to claim 1, wherein the SIMS means comprise mass spectrometry means selected from the group consisting of a magnetic sector mass spectrometer, a Time-of-Flight mass spectrometer and a quadrupole mass spectrometer.

3. The instrument according to claim 1 or claim 2, wherein the SIMS means can be operated in the dynamic or static mode.

4. The instrument according to any of the preceding claims, wherein the surface imaging microscopy means comprise Scanning Probe Microscopy (SPM) means selected from the group consisting of Atomic Force Microscopy (AFM) means, Scanning Tunnelling Microscopy

(STM) means, Kelvin Probe Force Microscopy (KPFM) means, Electrostatic Force Microscopy (EFM) means, and Magnetic Force Microscopy (MFM) means.

5. The instrument according to any of the preceding claims, further comprising a sample holder (4), SIMS positioning means and/or SPM positioning means and/or sample holder positioning means.

6. The instrument according to any of the preceding claims, wherein the Secondary Ion Mass Spectrometry (SIMS) means and the surface imaging microscopy means are both located in the same analytical chamber (12) .

7. An analytical chamber (12) for an analytical instrument, said analytical chamber (12) being under Ultra High Vacuum (UHV) and comprising Secondary Ion Mass Spectrometry (SIMS) means and surface imaging microscopy means.

8. The analytical chamber (12) according to claim 7, further comprising a sample holder 4, SIMS positioning means and/or SPM positioning means and/or sample holder positioning means.

9. A method to analyse under Ultra High Vacuum (UHV) a sample comprising the steps of: a) providing a sample (3) to be analysed, b) positioning in the vicinity of SPM means a portion of the surface of said sample (3) , c) analysing said surface portion using the SPM means, to provide topographical data and work function data regarding said surface portion of said sample (3) , d) positioning in the vicinity of SIMS means said surface portion analysed by the SPM means, e) analysing said surface portion analysed by the SPM means in step c) , using the SIMS means, to provide chemical data regarding said portion of surface, f) positioning in the vicinity of the SPM means said surface portion analysed by the SIMS means in step e), g) analysing said surface portion analysed by SIMS means in step e) , using the SPM means, to provide topographical data and work function data regarding said portion of surface, h) combining said chemical data, said topographical data and work function data to provide a 3D representation of the spatial distribution of the species composing said surface portion of said sample (3) .

10. The method according to claim 9, wherein steps d) to g) are repeated several times to record a SIMS depth profile of a portion of the surface of the sample (3) .

11. The method according to any of the claims 9 to 10, wherein the positioning of the surface portion analysed by the SPM and SIMS means, is performed by using as a position reference the sample edge, or the edge of the crater created by SIMS sputtering, or at least one mark on the sample, or at least one mark on the sample holder.

12. The method according to any of the claims 9 to 11, wherein the SPM means are selected from the group consisting of Atomic Force Microscopy (AFM) means, Scanning Tunnelling Microscopy (STM) means, Kelvin Probe Force Microscopy (KPFM) means, Electrostatic Force Microscopy (EFM) means and Magnetic Force Microscopy

(MFM) means.

13. A computer program comprising a software code adapted to perform the method according to any of the claims 9 to 12.

14. A computer program comprising a software code adapted to process and combine topographical, work function and chemical data obtained by using the method according to any of the claims 9 to 12.

15. A computer program comprising a software code adapted to control the analytical instrument according to any of the claims 1 to 6, so that said analytical instrument performs the method according to any of the claims 9 to 12.