Apparatus and method for high spectral and temporal resolution glow discharge spectrometry

Abstract

The invention relates to a glow discharge spectrometry apparatus (100) comprising a glow discharge lamp (1) and an optical emission spectrometer adapted to receive a light beam (20) emitted by a glow discharge plasma (2). According to the invention, the optical emission spectrometer comprises a dispersive optical component (7) and a ladder grating (8) arranged and configured to form a two-dimensional spectrum (26) of the light beam, the two-dimensional spectrum (26) being dispersed in a plurality of diffraction orders (P1, …Pj,…PT), the plurality of diffraction orders (P1, …Pj,…PT) extending along a first direction (X) and each diffraction order (P1, …Pj,…PT) extending spectrally along a second direction (Y) transverse to the first direction (X), and a pixel-matrix CMOS detector (10) arranged and configured to acquire the two-dimensional spectrum (26) as a function of time. Figure for the abbreviation: Fig. 1

Description

Title of the invention: Apparatus and method for high spectral and temporal resolution glow discharge spectrometry. Technical field

[0001] The present invention relates to devices and methods for analyzing solid samples by glow discharge spectrometry. Previous technique

[0002] Glow discharge spectrometry (GDS) is an analytical technique that allows the measurement of the elemental and / or molecular chemical composition of homogeneous or multilayered solid samples. The measurement can be performed at the core or with depth resolution. Furthermore, a distinction is made between glow discharge devices or sources that allow the analysis of solid samples by mass spectrometry (GD-MS for glow discharge mass spectrometry) or by optical emission spectrometry (GD-OES for glow discharge optical emission spectrometry).

[0003] The principle of glow discharge spectrometry consists of eroding a sample surface with plasma, then exciting and / or ionizing the eroded chemical species and detecting the ionized species by mass spectrometry, or respectively the excited species by optical emission spectrometry, to deduce the composition of the sample. A gas, called plasma gas, is injected into the vacuum chamber of a discharge lamp, and electrical power is applied between the lamp's electrodes to generate a plasma. The plasma gas is generally an inert gas such as argon, neon, krypton, or helium. The plasma gas can also be formed from a mixture of gases, for example, a mixture of argon and another gas such as oxygen, hydrogen, nitrogen, or helium. The surface of the sample placed in the lamp's vacuum chamber is thus exposed to an ablation plasma.This plasma ensures both the erosion of the solid material to be analyzed and the excitation and / or ionization of the eroded species in the gas phase. A mass spectrometer or an optical emission spectrometer, coupled to the vacuum chamber, allows for the analysis of the chemical species present in the plasma. Depending on the erosion time, GDS enables quantitative analysis of certain samples with depth resolution, thus providing a compositional profile of the analyzed sample.

[0004] GDS is relatively simple to use and has varied applications. It allows the analysis of trace, minor, and major elements in metallic or non-metallic solid samples. Glow discharge spectrometry (GDS) allows the analysis of the chemical composition of Solid materials at their core (bulk) but also as a function of depth (depth profile). The ability to obtain depth profiles is clearly what distinguishes SDL from other elemental analysis techniques such as spark spectrometry, laser-induced breakdown spectroscopy (LIBS), or X-ray fluorescence.

[0005] The optical setups used in devices marketed in SDL (or GDS) are generally based on the use of a polychromator in Paschen-Runge configuration associated with photomultiplier (PM) detectors to acquire the signal at several wavelengths and / or a series of CCD detectors arranged to each acquire a part of the spectrum.

[0006] However, the signal intensity in SDL is much lower than in LIBS. To perform profile measurements with high spatial depth resolution in SDL, it is necessary to use PM detectors that provide both high sensitivity and a high acquisition speed. However, PM detectors are fixed and do not allow acquisition over the entire spectrum, but only at a few predetermined wavelengths.

[0007] For applications of measuring the profile of thin film samples, it is desirable to have an SDL device that has both high temporal resolution and allows the acquisition of a greater number of wavelengths than a polychromator SDL device equipped with PMs.

[0008] Some samples are of unknown composition. Determining the wavelengths at which to position the PMs then requires a preliminary study. It is therefore desirable to acquire optical emission spectra over the widest possible spectral range and with high spectral resolution, to allow the detection of all the chemical species that make up the sample, without knowing them in advance. Description of the invention

[0009] For this purpose, the invention relates to a luminescent discharge spectrometry apparatus comprising a luminescent discharge lamp capable of forming a luminescent discharge plasma and an optical emission spectrometer capable of receiving a part of a light beam emitted by the luminescent discharge plasma.

[0010] According to the invention, the optical emission spectrometer comprises a dispersive optical component and a ladder grating, the dispersive optical component and the ladder grating being arranged and configured to form a two-dimensional spectrum of the light beam, the two-dimensional spectrum being dispersed in a plurality of diffraction orders (PI, .. .Pj,.. .PT), the plurality of diffraction orders (PI, .. .Pj,.. .PT) extending along a first direction (X) and each diffraction order fraction (PI, ...Pj,...PT) extending spectrally along a second direction (Y) transverse to the first direction (X) and a pixel matrix CMOS detector arranged and configured to acquire the two-dimensional spectrum as a function of time.

[0011] According to a particular and advantageous aspect, the CMOS detector is capable of acquiring at least 20 images per second, for example 30, 50 or even 100 images per second.

[0012] In one embodiment, the CMOS detector comprises N lines of M pixels, where N is greater than or equal to 512 and M is greater than or equal to 512, for example 1024x1024 pixels or preferably 2048x2048 pixels.

[0013] According to another particular and advantageous aspect, each diffraction order (PI, .. .Pj,.. .PT) extends over a line of the CMOS detector.

[0014] Advantageously, the glow discharge spectrometry apparatus includes a data processing system configured to process the signals from the CMOS detector by macropixels, each macropixel comprising at least 2x2 adjacent pixels of the CMOS detector.

[0015] In one embodiment, the glow discharge spectrometry apparatus includes an optical coupling system between the glow discharge lamp and an input of the optical emission spectrometer.

[0016] According to a particular and advantageous aspect, the dispersive optical component comprises a prism.

[0017] According to a particular embodiment, the glow discharge spectrometry apparatus comprises a monochromator or a polychromator suitable for receiving another part of the light beam emitted by the glow discharge plasma, the monochromator, respectively the polychromator, comprising a diffraction grating and a photomultiplier detector, respectively several photomultiplier detectors, each photomultiplier detector being adapted to detect an optical emission at a determined wavelength.

[0018] Optionally, the glow discharge spectrometry apparatus includes a differential interferometer for measuring the etching depth of an erosion crater in a sample exposed to glow discharge plasma.

[0019] The invention also relates to a glow discharge spectrometry method comprising the following steps: forming a glow discharge plasma; receiving a portion of a light beam emitted by the glow discharge plasma at an input of an optical emission spectrometer; spectrally dispersing the portion of the light beam on a ladder grating and a dispersive optical component to form a two-dimensional spectrum, the two-dimensional spectrum being dispersed in a plurality of diffraction orders (PI, ..., Pj, ..., PT), the plurality of diffraction orders (PI, ..., Pj, ..., PT) extending along a first direction (X) and each diffraction order (PI, ..., Pj, ..., PT) extending spectrally along a direction transverse to the first direction; and acquire the two-dimensional spectrum on a time-dependent pixel matrix CMOS detector.

[0020] Of course, the various features, variants, and embodiments of the invention can be combined with one another in various ways, provided they are not incompatible or mutually exclusive. Brief description of the drawings

[0021] In addition, various other features of the invention become apparent from the attached description made with reference to the drawings which illustrate non-limiting embodiments of the invention and where:

[0022] [Fig-1] is a schematic view of a light discharge spectrometry apparatus nescente according to the invention,

[0023] [Fig.2] is a schematic view of a two-dimensional spectrum on a CMOS detector,

[0024] [Fig.3] is a schematic view of an optical system for simultaneous coupling to a scaling spectrometer and to a polychromator equipped with photomultipliers according to a particular embodiment.

[0025] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references. Detailed description

[0026] In [Fig. 1], a luminescent discharge lamp 1 configured to form a luminescent discharge plasma 2 is schematically represented. In a manner known in SDL, the luminescent discharge plasma is used to erode the surface of a sample that one wishes to analyze.

[0027] The luminescent discharge plasma 2 generates optical emission. A portion of the light beam 20 emitted by the plasma 2 is collected, for example via an optical fiber 3. The optical fiber 3 guides the emitted light beam 20 towards the input 4 of an optical emission spectrometer.

[0028] In the case where an optical fiber 3 is used to carry the light beam 20 to the inlet 4 of the spectrometer, this inlet 4 is formed by the core of the optical fiber. Alternatively, the inlet 4 of the spectrometer comprises an elongated slit.

[0029] The optical emission spectrometer is here a ladder spectrometer combined with a 10-pixel CMOS detector. The ladder spectrometer comprises a combination of a dispersive optical element and a ladder grating mounted to spectrally disperse the light beam along two transverse directions. Advantageously, the dispersive optical element is a prism mounted transversely to the ladder grating. Alternatively, the dispersive optical element comprises another diffraction grating.

[0030] Unlike conventional systems, the ladder spectrometer is not associated with a CCD image detector, but with a CMOS image detector. For example, the CMOS image detector includes a CMOS camera. CMOS camera technology allows for the acquisition of 2D spectra at a much higher speed than a CCD camera with the same number of pixels. In practice, acquiring an image on a 2048x2048 pixel CCD detector takes approximately 4 seconds, while acquiring an image on a CMOS detector with the same number of pixels is virtually instantaneous, on the order of 20 ms. However, CMOS detectors have a much lower sensitivity than CCD detectors, which are themselves much less sensitive than photomultiplier (PM) detectors. Furthermore, CMOS detectors have a much narrower dynamic range than PM or CCD detectors.Finally, CMOS detectors have a high cost compared to CCD detectors.

[0031] In the example illustrated in [Fig. 1], the ladder spectrometer comprises a first mirror 5, a second mirror 6, a prism 7, a ladder grating 8, and a final mirror 9. More precisely, the ladder spectrometer is mounted in a tetrahedral configuration. The first mirror 5 and the second mirror 6 receive the light beam 20 from the inlet 4 and form a collimated beam directed towards the prism 7. In a known manner, the prism 7 spectrally disperses the light beam along a direction transverse to the prism edge. The light beam dispersed once by the prism is incident on the ladder grating 8. The ladder grating 8 is a grating comprising a set of parallel lines. The ladder grating 8 receives the light beam dispersed once by the prism 7. The ladder grating 8 is oriented so that the direction of the grating lines is perpendicular to the edge of the prism 7.The 8-scale grating thus diffracts the light beam in a direction transverse to that of the prism. The 8-scale grating separates the light beam into numerous diffraction orders PB .. .Pj,.. .PT where T is an integer between 2 and 100, for example T is equal to 30.

[0032] The 8-scale grating is a metallic grating that operates by reflection. Advantageously, the 8-scale grating is a blazed grating mounted in autocollimation. In this configuration, the angle of incidence on the 8-scale grating is equal to the diffraction angle. In this way, the beam diffracted by the 8-scale grating passes back through prism 7 to be dispersed a second time. This double pass through prism 7 allows for greater spectral dispersion in each diffraction order.

[0033] The last mirror 9 receives the spectrally dispersed beam in two transverse directions and forms a spectral image on the CMOS detector 10. The spectral image is here called a two-dimensional spectrum 26.

[0034] The combination of reflection and diffraction makes it possible to extract practically The full intensity of the light beam. The ladder spectrometer has very high luminosity, in other words, little loss.

[0035] The ladder spectrometer is for example based on an Aryelle 200 spectrometer from LTB Lasertechnik Berlin GmbH, in which the CCD detector is replaced by a CMOS detector, for example a detector from the Prime BSI range from Teledyne.

[0036] Figure 2 schematically illustrates a two-dimensional spectrum 26 received on the CMOS detector. The CMOS detector is generally square in shape. The CMOS detector comprises, for example, 2048 rows and 2048 columns of pixels. For example, each pixel has an elementary size of 6.5 pm x 6.5 pm. Alternatively, the received signals are accumulated on a macropixel, comprising, for example, 2x2 adjacent pixels in 2 rows and 2 columns. A macropixel covers an area of ​​approximately 13 pm x 13 pm. This pixel grouping analysis (or "binning") increases the sensitivity of the detected signal by a factor of 4, without degrading the resolution of the entire Echelle + CMOS assembly, because in the setup used, the limiting factor for resolution is not the pixel size but the width of the input slot. This pixel group operating mode is well suited to a light discharge source that exhibits low brightness.Such a source presents little risk of saturating the pixels of the CMOS detector, even in macropixel mode.

[0037] The two-dimensional spectrum 26 is dispersed by the scaling grating according to the diffraction orders Pb .. .Pj,.. .PT in the Y direction. The CMOS detector is oriented so that each diffraction order extends over a line of pixels. It can be seen in [Fig. 2] that the distance between the orders is not constant due to prism dispersion. The lowest diffraction orders Pb and P2 are separated by a distance smaller than the distance between the highest diffraction orders PT1 and PT2.

[0038] In each diffraction order, for example Pj, the one-dimensional spectrum is dispersed as a function of wavelength along the X direction between a low wavelength L and a high wavelength LR. The ladder spectrometer combined with the CMOS detector thus makes it possible to measure the entire optical emission spectrum simultaneously and rapidly.

[0039] The diffraction efficiency of the scaling spectrometer varies with wavelength in each diffraction order. A signal processing system combines the different orders detected on the CMOS detector to reconstruct a complete one-dimensional spectrum of the light beam as a function of wavelength. The data stream generated by the CMOS detector requires a specific processing system and / or algorithm to extract a complete spectrum as a function of time.

[0040] The rapid acquisition of the CMOS detector makes it possible to acquire a bidirectional spectrum dimensionally accurate with a temporal resolution of a few milliseconds, for example, 10 ms or 20 ms. This acquisition frequency allows for temporal analysis of SDL signals corresponding to a depth resolution for very thin films, with thicknesses of nanometers or a few nanometers. Etching a thin film using glow discharge plasma can be very fast. In some thin-film analysis applications, a thin film can be etched in a few milliseconds.

[0041] Currently available CMOS detectors allow the acquisition of two-dimensional spectral images at an acquisition rate of at least 20 frames per second, for example 30, 50 or even 100 frames per second. The images are transferred to a hard drive for further processing.

[0042] The ladder spectrometer offers numerous advantages. It is compact and easy to couple to a glow discharge lamp. It has no moving parts, making it very robust. The ladder spectrometer is generally factory-calibrated, for example, using a mercury vapor lamp emitting light lines at very precise wavelengths. It allows for obtaining, with each detected spectral image, a continuous spectrum over a very wide spectral range, for example, from 203 nm to 800 nm. The optical resolution is not constant with a ladder spectrometer: for example, a spectral resolution roughly comparable to that of the polychromator is obtained in the GD, i.e., approximately 20–25 pm around 220 nm and approximately 50 pm in the red. Absorption by the optical fiber 3 and the prism 7 limits transmission in the UV.To obtain results in the UV range, it is necessary to use a UV-transparent optical fiber or to directly couple the spectrometer to the discharge lamp. The detection of a continuous spectrum over a wide spectral range allows for the detection of elements not expected to be present in the sample being analyzed.

[0043] Thanks to its compact size, the ladder spectrometer combined with a CMOS detector can easily be installed on an existing glow discharge spectrometry device.

[0044] The glow discharge spectrometry apparatus based on a ladder spectrometer and a CMOS detector finds numerous applications in the analysis of the profile of thin-film samples. For example, the present disclosure can be applied to the analysis of hard drives formed from stacks of several thin films, for example, photovoltaic layers, electrolytic deposits, or CVD or PVD deposits.

[0045] Figure 3 shows an example of an optical coupling system between a glow discharge lamp 2 and, on the one hand, the inlet 14 of a polychromator 15 and, on the other hand, the inlet 4 of the scaling spectrometer equipped with a CMOS detector. A lens 11 collimates the light beam 22 emitted by the glow discharge plasma in the Discharge lamp 1. A mirror 12 is placed on one half of the collimated light beam to deflect this half of the light beam 21 towards the input of a polychromator. The other half of the light beam 20 propagates to another lens 13 which focuses this other half of the light beam onto the input 4 of the ladder spectrometer equipped with a CMOS detector. Although the light flux is divided in two on each detection system, this optical coupling offers numerous advantages.

[0046] Such an optical coupling system makes it possible to simultaneously detect and track predetermined emission lines via the polychromator 15 equipped with photomultiplier detectors, each with a slit, and, on the other hand, the complete spectrum detected via the CMOS detector 10 of the lattice spectrometer. In particular, the polychromator 15 makes it possible to track emission lines in the ultraviolet, which are more difficult to detect on the lattice spectrometer due to the presence of the prism 7. A polychromator also makes it possible to spectrally resolve complex emission lines. Advantageously, the electronic acquisition systems of the polychromator and the lattice spectrometer combined with the CMOS detector are compatible. For example, the polychromator equipped with photomultiplier detectors makes it possible to acquire emission lines at a frequency between 1 Hz and 100 Hz.The data acquired by the two types of detectors, PM and CMOS respectively, are transferred to a computer processing system.

[0047] This results in a hybrid glow discharge spectrometry device, i.e., with two types of detectors: PMs at the output of the polychromator and a CMOS imaging detector at the output of the scaling spectrometer. Combining these two detection systems with the same glow discharge lamp allows for synchronized data acquisition and leverages the advantages of each detection system. Advantageously, the acquisition speed of the PMs is adjusted to be identical to that of the CMOS detector. Both the PMs and CMOS detectors are fast, thus enabling high temporal resolution. The polychromator, for example in a Pashen-Runge configuration, equipped with PMs, allows detection up to the UV, to track emission lines at a precisely determined wavelength. High-voltage control of each PM prevents saturation of the PM detectors.PM detectors thus benefit from a very high dynamic range that can adapt to any intensity of the detected signals. However, since the number of PMs is limited and the position of each PM is fixed, the polychromator can only track certain preselected spectral lines. The CMOS detector has a more limited dynamic range than PMs, but it allows the acquisition of a complete spectrum over a continuous and relatively wide spectral range, for example, from 200 nm to 800 nm. In this way, the CMOS detector allows for the detection of... certain emission lines that were not predicted in advance. Furthermore, the CMOS detector allows the detection of several lines associated with the same chemical element, making it possible to detect certain elements present in trace amounts in the analyzed sample.

[0048] The hybrid glow discharge spectrometry apparatus of the present disclosure finds new applications for the analysis of numerous solid materials and compounds. The polychromator makes it possible to track lines in the ultraviolet that are more difficult to detect using the ladder spectrometer and CMOS detector. For example, the polychromator makes it possible to resolve the optical emission lines of hydrogen and deuterium located around 120 nm, which are measured either sequentially or by means of two slits placed in two different diffraction orders. The ladder spectrometer combined with the CMOS detector does not allow access to the far UV.

[0049] The glow discharge spectrometry apparatus of the present disclosure is compatible with the use of a differential interferometer type interferometric system to measure the depth of the erosion crater in the sample during its exposure to the etching plasma, as described in patent FR1453997.

[0050] Of course, various other modifications can be made to the invention within the scope of the annexed claims.

Claims

Demands

1. A glow discharge spectrometry apparatus (100) comprising a glow discharge lamp (1) capable of forming a glow discharge plasma (2) and an optical emission spectrometer arranged to receive a portion of a light beam (20) emitted by the glow discharge plasma, characterized in that the optical emission spectrometer comprises a dispersive optical component (7) and a ladder grating (8), the dispersive optical component (7) and the ladder grating (8) being arranged and configured to form a two-dimensional spectrum (26) of the light beam, the two-dimensional spectrum (26) being dispersed in a plurality of diffraction orders (Pb .. .Pj,.. .PT), the plurality of diffraction orders (PB .. .Pj,.. .PT) extending along a first direction (X) and each diffraction order (PB .. .Pj,.. .PT) extending spectrally along a second direction (Y) transverse to the first direction (X) and a pixel matrix CMOS detector (10) arranged and configured to acquire the two-dimensional spectrum (26) as a function of time, the apparatus comprising a monochromator or a polychromator (15) arranged to receive another part of the light beam (21) emitted by the luminescent discharge plasma, the monochromator, respectively the polychromator (15), comprising a diffraction grating and a photomultiplier detector, respectively several photomultiplier detectors, each photomultiplier detector being adapted to detect optical emission at a determined wavelength.

2. A light discharge spectrometry apparatus (100) according to claim 1 in which the CMOS detector is capable of acquiring at least 20 images per second.

3. Luminescent discharge spectrometry apparatus (100) according to claim 1 or 2 wherein the CMOS detector comprises N lines of M pixels, where N is greater than or equal to 512 and M is greater than or equal to 512.

4. Luminescent discharge spectrometry apparatus (100) according to any one of claims 1 to 3 wherein each diffraction order (Pb .. .Pj ,.. .PT) extends over a line of the CMOS detector.

5. A glow discharge spectrometry apparatus (100) according to any one of claims 1 to 4 comprising a data processing system configured to process the signals from the CMOS detector by ma- macropixels, each macropixel comprising at least 2x2 adjacent pixels of the CMOS detector.

6. Light discharge spectrometry apparatus (100) according to any one of claims 1 to 5 comprising an optical coupling system between the light discharge lamp (1) and an input (4) of the optical emission spectrometer.

7. A glow discharge spectrometry apparatus (100) according to any one of claims 1 to 6 in which the dispersive optical component (7) comprises a prism.

8. A glow discharge spectrometry apparatus (100) according to any one of claims 1 to 7 comprising a differential interferometer for measuring the etching depth of an erosion crater in a sample exposed to glow discharge plasma (2).

9. A method for glow discharge spectrometry comprising the following steps: - forming a glow discharge plasma; - receiving a portion of a light beam (20) emitted by the glow discharge plasma onto an inlet (4) of an optical emission spectrometer; - spectrally dispersing the portion of the light beam onto a ladder grating (8) and a dispersive optical component (7) to form a two-dimensional spectrum, the two-dimensional spectrum being dispersed into a plurality of diffraction orders (Pb .. .Pj,.. .PT), the plurality of diffraction orders (Pb .. .Pj,.. .PT) extending along a first direction (X) and each diffraction order (Pb .. .Pj,.. .PT) extending spectrally along a direction transverse to the first direction; and - acquire the two-dimensional spectrum on a time-dependent pixel matrix CMOS detector; - receive another part of the light beam emitted by the luminescent discharge plasma on a monochromator or polychromator, equipped with a photomultiplier detector, or respectively with several photomultiplier detectors; - detect via each photomultiplier detector an optical emission at a determined wavelength.

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