FR2843801A1 - Emission spectrometer solid sample composition determination having luminous discharge source creating surface atom/ion plasma and optical collection unit having mechanism limiting collection volume along common focus axis. - Google Patents

Abstract

The emission spectrometer has a luminous discharge source creating a plasma of atoms (4) and ions from the surface (5). An optical component (11) collects the luminous radiation. A mechanism measures the distance between the sample external layer (1a) and the exposed surface (6). There is a mechanism (18) limiting the plasma in a limited collection volume (19) on the optical axis of common focus.

Description

The present invention relates to a spectrometry method and a

  glow discharge source emission spectrometer for determining the composition of a

solid sample.

  Glow discharge spectroscopy (GDS) emission spectrometry is a

  fast and efficient technique for studying solid samples.

  The solid sample to be analyzed constitutes "the cathode" of a

  light discharge system and is subjected to ion bombardment.

  The elements torn from the solid sample by this bombardment then penetrate into the plasma, the support gas of which is typically argon. They are then the seat of excitations resulting from collisions with highly energetic electrons or with metastable argon atoms. The excitation of these elements results in the emission of detectable radiation. These emitted radiations are then collected, dispersed and then analyzed spectroscopically in order to quantitatively determine the

  elementary composition of the sample.

  The crater which arises from the erosion of the surface of the solid sample under exposure to plasma has a depth which changes over time. These time-dependent measurements can therefore be linked to a transverse (in-depth) analysis of the

  chemical composition of the solid sample.

  Nevertheless, the determination of the chemical elements contained in the material removed by the plasma presupposes that one can draw up such a transverse analysis, a sufficient spatial resolution to probe homogeneous layers in depth. Obtaining such a resolution in depth therefore requires optimizing the discharge parameters (power, partial pressure of the support gas, applied voltage, etc.) and results in the appearance of a crater having a some

surface flatness.

  The performance of current glow discharge source emission spectrometers allows the study of thin films under conditions close to layer-by-layer analysis. In depth resolutions of the order of a nanometer have been recently reported [SHIMIZU K. et al .; Surf. Interface. Anal. 31

(2001) 869-873).

  However, if at such depth resolutions, the crater 1 formed has perfect flatness at the center 2 of the eroded area, the edges 3 of the crater are more hollowed out (Figure 1). These differences in depth between the edges 3 of the crater and the central zone 2 can reach ratios 10 close to 3/2. As a result, elements from

  layers deeper than that of the eroded zone in center 2 contribute to the radiation emitted by the plasma. These elements therefore constitute an undesirable and disturbing source of radiation for the spectroscopic analysis of the elements of the layer studied, i.e. from the eroded zone to the center 2.

  The objective of the present invention is to provide a glow discharge source emission spectrometer and a measurement method, simple in their design and in their operating mode allowing the determination of the composition of a solid sample in depth and in real time with great depth resolution while solving the problem of

  undesirable contribution of elements from the edges of the crater.

  To this end, the invention relates to a glow discharge source emission spectrometer for the measurement of a solid sample comprising: - means for creating a plasma of atoms and ions torn from an exposed surface of the sample to be analyzed, said plasma having a section D, a first optical component collecting the light radiation emitted by the atoms and ions of the plasma, and defining an optical collection axis with the entry slit of an optical system analysis, - a second optical component placed at said input slot receiving the light radiation, - means for measuring the distance between the outer layer of the exposed surface of the sample and a reference surface, said outer layer having a planar eroded part in the center, a control unit controlling receiving spectroscopic analysis of said light radiations and measuring distance, and producing signals, and

  a unit for processing the signals produced by the control unit.

  According to the invention, the emission spectrometer with a glow discharge source comprises means for delimiting in the plasma a collection volume limited transversely of the light radiations emitted by atoms and ions, said means being placed on the optical axis. collection at a home

  common of the first and second optical components.

  In different possible embodiments, the present

  The invention also relates to the characteristics which will emerge during the following description and which should be considered in isolation or in all their combinations.

  technically possible: - the first optical component combines the means to delimit a collection volume limited transversely with the plasma, - the first optical component combines the means to delimit a collection volume limited transversely and the plane containing the surface of the sample, and said means combine the first and second optical components, - the first and second optical components are lenses, - the first and second optical components are mirrors, - the means for delimiting a collection volume limited transversely have a diameter d which is less than the section D 35 of the plasma, - the means for delimiting a collection volume limited transversely comprises a field diaphragm, - the diameter of the diaphragm is equal to the diameter of said flat part at the center of the eroded area, means for measuring the distance between the outer layer of the exposed surface of the sample on and the reference surface comprise a laser interferometer, - said means for creating a plasma of atoms and ions

  comprise an anode having a tubular end and the reference surface 10 is a planar area of said anode.

  The invention also relates to a spectrometric method for determining the composition of a solid sample in which a plasma of atoms and ions torn from an exposed area of the sample is formed. According to the invention, (a) means for delimiting in said plasma a transversely limited volume of collection of light radiations emitted by atoms and ions are placed at a common focus of a first optical component collecting said emitted radiations and a second optical component placed at the input of an optical analysis system, said first component combining said means to delimit a

  volume limited transversely with plasma.

  (b) the radiation emitted in said collection volume is analyzed with said optical analysis system, (c) the distance between the exposed area and a fixed reference surface is measured to determine the depth of the exposed area,

  (d) a correlation is made between the analysis of the radiation emitted and the depth measured.

  In different possible embodiments, the present

  The invention also relates to the characteristics which will emerge during the following description and which should be considered in isolation or in all their technically possible combinations:

  steps (b), (c) and (d) are carried out in real time continuously or periodically, before proceeding with the formation of the plasma, a step of self-calibration of the fixed reference surface is carried out, the means for delimiting in the plasma a transversely limited volume of collection of light radiation comprises a field diaphragm, - the first and second optical components are lenses, - the first and second optical components are mirrors. The invention will be described in more detail with reference to the accompanying drawings in which: - Figure 1 is a schematic representation of a crater profile obtained in a solid sample for optimum depth resolution; FIG. 2 is a schematic representation of a glow discharge source emission spectrometer for measuring a sample, according to a first embodiment of the invention; - Figure 3 is a schematic representation of a glow discharge source emission spectrometer for measuring a sample, according to a second embodiment of the invention. According to Figure 2, the emission spectrometer with glow discharge source comprises means for creating a plasma 4 of atoms and ions torn from an exposed surface 5 of the sample 6 to be analyzed and an optical system d analysis 7, the

  two being controlled by a control unit 8.

  The means for creating a plasma 4 of atoms and ions comprise an insulating ceramic joint 9 receiving an anode having a typically tubular end 10, hereinafter called "tubular anode". Other end forms of anodes are also possible, for example, ovodes. The anode 10 defines a plasma chamber just above the sample 6 to be analyzed, of which said insulating joint 9 is the support. The solid sample then constitutes the cathode of the means for creating a plasma 4 of atoms and ions. The upper part of the tubular anode 10 is closed by a first optical component 11. This optical component 11 placed 5 between the means for creating a plasma 4 of atoms and ions and the optical analysis system 7 collects the radiation. light emitted by the atoms and ions of the plasma 4. It defines an optical collection axis 12 with the entry slit of said optical analysis system 7. A second optical component 13 is placed at said entry slit and receives light radiation collected by the

first optical component 11.

  Typically, the atoms of the sample 6 to be analyzed are torn off by the plasma 4 created. These atoms enter plasma 4 and are then the site of collisions with highly energetic electrons or with argon (Ar) atoms.

  metastable. Following these collisions, the atoms of the sample are excited and they de-excite by optical emission, thus producing luminescence, i.e. the emission of photons of given wavelengths. The intensity of these light radiations is measured by the optical analysis system 7.

  The first and second optical components (11, 13) are, for example, lenses or mirrors. In one embodiment (FIG. 3), a first mirror 11 placed at 45 opposite the plasma receives the light radiation emitted by the atoms and the ions of the plasma 4 and sends them on a second mirror 13 which

  focuses said radiation on the entry slit of an optical analysis system 7. In the following description, we will only consider lenses but we understand that the spectrometer of the invention is unchanged if we replace the 30 lenses by mirrors.

  The erosion of the surface 5 of the sample 6 under the effect of the plasma 4 gives rise to a crater 1 in the said sample. The depth of this crater 1 increases with the time of erosion, ie with the time of exposure of the surface to the plasma 4. Means 35 therefore make it possible to measure the distance between the outer layer la of the exposed surface of l sample 6 and an area of

reference 14.

  The depth of crater 1 is measured in a mode of

  preferred embodiment, in real time, using a laser interferometer. This interferometer produces two laser beams 15, 16 from a single source. A first laser beam 16 is sent directly to the surface of the crater 1, ie to the outer layer la of the exposed surface of the sample and the second laser beam 15 (the reference beam) is sent to a determined reference surface 14.

  The laser beams reflected by these surfaces are then recombined to form an interference diagram which is used to measure the distance separating the outer layer from said

  exposed surface 1a and reference surface 14.

  The reference surface 14 is advantageously a flat area of the tubular anode 10 used to create a plasma 4 of atoms

and ions.

  The determination of said distance can be carried out in real time by means of a processing unit 17. This processing unit 17 receives the signals produced by the control unit 8 which manages the emission spectrometer with glow discharge source and the laser interferometer. The processing unit 17

  is in a preferred embodiment a computer.

  The analysis of the sample is carried out after optimization of the depth resolution so that the said outer layer presents it with an eroded part which is perfectly flat at the center 2 and with the exposure time of the edges 3 which widen further.

(Figure 1).

  As explained above, the elements contained in the material sampled by the plasma 4 therefore schematically come from two different regions of the sample. The first elements come from the edges 3 of the crater 1 while

  second elements come from the flat central part 2.

  The first elements come from layers of the sample 6 deeper than those of the second elements. It follows that these first elements will emit radiation which will disturb the measurement of the elementary chemical composition of the layer studied, i.e. that constituting the part

hovers in the center 2.

  However, these first elements are found

  mainly at the periphery of the plasma 4 of atoms and ions. The object of the invention is therefore to collect essentially the radiation emitted inside the plasma 4 in order to preferentially collect information originating from the plane part at the center 2 of the crater 1.

  According to the invention, the emission spectrometer with source at

  glow discharge therefore comprises means 18 for delimiting in the plasma 4 a collection volume 19 limited transversely of the light radiations emitted by the atoms 15 and the ions.

  By "limited collection volume"

  transversely ", the volume defined in the plasma by said means 18 and which is limited, in its spatial extension, on either side of a main axis of said plasma 4, i.e. transversely to this axis.

  These means are placed on the optical collection axis 12 at a

  common focus of the first 11 and second optical lenses 13.

  The first lens 11 combines said means 18 to define a collection volume with the plasma 4. Advantageously, these means 18 make it possible to define a limited collection volume 19

  transversely inside the plasma 4 which does not contain the atoms and ions located at the periphery of said plasma 4. The means 18 for delimiting a collection volume therefore have a diameter d which is less than the section D of the plasma, with D equals 30 to the section of the tubular anode 10.

  The means 18 for delimiting a collection volume comprise for example a field diaphragm. In a preferred embodiment, the first optical lens 11 combines the means 18 for defining a collection volume and the plane 35 containing the surface of the sample 6, and said means combines the first 11 and the second 13 optical lenses. The diameter of the diaphragm is then advantageously taken equal to the diameter of said flat part at the center 2 of the eroded area. By directly diaphragming the central part 2 of the eroded zone and therefore the layer whose elemental composition we are trying to analyze, the contribution of the radiations emitted by the elements

  coming from the edges of the crater is very noticeably lessened.

  The invention also relates to a spectrometric method for determining the composition of a solid sample 6 in which a plasma 4 of atoms and ions torn from an exposed area of the sample 6 is formed. The plasma 4 is created in a plasma chamber formed by an anode having a typically tubular end 10 carried by a

insulating seal 9 in ceramic.

  According to the invention, means 18 for delimiting in the

  plasma a collection volume 19 limited transversely of the light radiations emitted by the atoms and the ions are placed at a common focus of a first optical lens 11 collecting said emitted radiations and of a second optical lens 20 placed at the entrance of '' an optical analysis system 7.

  Said first lens 11 combines said means 18 to delimit a volume limited transversely with the plasma 4 or with the plane containing the surface 5 of the sample 6. In this latter embodiment, the diameter of the diaphragm is advantageously taken equal to the diameter from said flat part to

center 2 of the eroded area.

  The radiation emitted in said collection volume 19 is collected and analyzed with said optical analysis system 7. In order to produce a profile of the chemical composition of the solid sample as a function of the depth, the distance between the zone exposed to the plasma 4 and a determined reference surface 14 is measured. The reference surface 14 is advantageously a zone plane of the tubular anode 10 used to create the plasma of atoms and ions. Additionally, this flat area 14 is located on the anode 10 out of reach of the plasma 4

  and therefore cannot be affected by it.

  The depth of crater 1 is measured in a preferred embodiment, in real time, using a laser interferometer. This determination of the depth of the crater 1 can be carried out continuously or periodically. Finally, a correlation is made between the analysis of the radiation emitted and the depth measured by means of a processing unit 17 which, for example

is a computer.

  As the anode 10 can vary, the initial distance separating the reference surface 14 from the surface 5 of the untreated sample is not determined over time once and for all. Before the formation of the plasma 4, a self-calibration step of the reference surface 14 is therefore carried out. This step is carried out when the sample is placed on the insulating seal 9 made of ceramic. A diffraction diagram is then produced for a crater of zero depth, i.e. a sample not yet exposed to a plasma. By using standard software such as quantum to operate the spectrometer 7, one can take advantage of the time required to purge the discharge chamber (typically of

  60 s) to perform this measurement.

Claims (16)

  1. Emission spectrometer with glow discharge source for measuring a solid sample comprising: - means for creating a plasma (4) of atoms and ions torn from an exposed surface (5) of the sample (6) to be analyzed, said plasma (4) having a section D, - a first optical component (11) collecting the light radiation emitted by the atoms and ions of the plasma (4), 10 and defining an optical axis (12) collecting with the entry slit of an optical analysis system (7), - a second optical component (13) placed at said entry slit receiving the light radiation, - means for measuring the distance between the layer external 15 (la) of the exposed surface of the sample (6) and a reference surface (14), said external layer (la) having a plane eroded part in the center, - a control unit (8) controlling receiving spectroscopic analysis of said light radiations and distance measurement, and produced isolating signals, a processing unit (17) for the signals produced by the control unit (8), characterized in that the emission spectrometer comprises means (18) for delimiting in the plasma (4) a collection volume (19) limited transversely of the light radiations emitted by atoms and ions, said means (18) being placed on the optical axis (12) for collection at a common focus of the first   (11) and second (13) optical components.   2. Emission spectrometer according to claim 1, characterized in that the first optical component (11) combines the means (18) to delimit a limited collection volume   transversely with the plasma (4).   3. Emission spectrometer according to claim 1, characterized in that the first optical component (11) combines the means (18) for delimiting a collection volume limited transversely and the plane containing the surface (5) of the sample (6), and said means combine the first (11) and the second (13) optical components.   4. Emission spectrometer according to any one of   Claims 1 to 3, characterized in that the first (11) and the   second (13) optical components are lenses.   5. Emission spectrometer according to any one of   Claims 1 to 3, characterized in that the first (11) and the second (13) optical components are mirrors.   6. Emission spectrometer according to any one of   Claims I to 5, characterized in that the means (18) for delimiting a collection volume (19) limited transversely have   a diameter d which is less than the section D of the plasma (4).   7. Emission spectrometer according to any one of   Claims 1 to 6, characterized in that the means (18) for delimiting a collection volume (19) comprise a diaphragm field.   8. Emission spectrometer according to claims 3 to 5 and 20 7, characterized in that the diameter of the diaphragm is equal to   diameter of said flat part at the center of the eroded area.   9. Emission spectrometer according to any one of   Claims I to 8, characterized in that the means for measuring the distance between the external layer (la) of the exposed surface of the sample (6) and the reference surface (14)   include a laser interferometer.   10. Emission spectrometer according to claim 9, characterized in that said means (18) for creating a plasma of atoms and ions comprises an anode having a tubular end (10) and the reference surface (14) is a flat area of said anode.   11. Spectrometric method for determining the composition of a solid sample in which a plasma (4) of atoms and ions torn from an exposed area of the sample (6) is formed, characterized in that , (a) means (18) for delimiting in said plasma (4) a collection volume (19) limited transversely of the light radiations emitted by the atoms and the ions 5 are placed at a common focus of a first component   optical (11) collecting said emitted radiation and a second optical component (13) placed at the entrance of an optical analysis system (7), said first optical component (11) combining said means (18) to delimit 10 a volume limited transversely with the plasma (4).   (b) the radiation emitted in said collection volume (19) is analyzed with said optical analysis system (7), (c) the distance between the exposed area (la) and a fixed reference surface (14) is measured to determine the depth of the exposed area, (d) a correlation is made between the radiation analysis emitted and the depth measured.   12. Method according to claim 11 characterized in that   steps (b), (c) and (d) are carried out in real time continuously or periodically.   13. Method according to claims 11 or 12, characterized   in that one proceeds before the formation of the plasma (4) to a stage   autocalibration of the fixed reference surface (14).   14. Method according to any one of claims 11 to 25 13, characterized in that the means (18) for delimiting in the   plasma a transversely limited volume of collection of   light radiation includes a field diaphragm.   15. Method according to any one of claims 11 to   14, characterized in that the first (11) and the second (13) optical components are lenses.   16. Method according to any one of claims 11 to   14, characterized in that the first (11) and the second (13)   optical components are mirrors.