EP2559056B1 - Method and device for measuring glow discharge spectrometry in pulsed mode - Google Patents

Description

The present invention relates to a method and a device for measuring pulsed-mode glow discharge spectrometry. Glow discharge spectrometry is used for the quantitative analysis of the elemental chemical composition of solid samples or thin film stacks, this analysis can be solved in depth.

In a glow discharge spectrometer, a sample to be analyzed is exposed to an etching plasma that performs surface ablation. In addition, plasma provides, through various physicochemical mechanisms, the excitation and ionization of eroded species. The tracking of the species present in the plasma, by an optical spectrometer for the excited species and / or by a mass spectrometer for the ionised species makes it possible to obtain the profile of the chemical composition of a sample according to the depth of the sample. erosion with submicron resolution.

Initially limited to materials and conductive layers due to the use of direct current (DC) sources, glow discharge spectrometry now allows the analysis of semiconductor materials and insulators through the use of radio sources Frequency (RF).

Glow Discharge Spectrometer (SDL or GDS) spectrometers are known. An SDL device generally comprises a mechanical device called "lamp" in which is placed a sample to be analyzed, the body of the lamp being connected to an optical spectrometer and / or mass. The figure 1 schematically represents a cross-sectional view of a discharge lamp according to the state of the art. The discharge lamp 1 comprises an anode tube 3 inside a vacuum chamber 2. A sample 4 placed in the lamp facing one end of the anode tube 3 forms the second electrode of the device. A pumping system 7 makes it possible to carry out a primary vacuum in the lamp, then a gas 8, said carrier gas (generally argon) is introduced under low pressure. An electric generator 6 makes it possible to apply an electric field to the electrodes of the lamp and to generate a plasma 9 consisting of electrons 11, neutral atoms in a ground state or excited state 12 and ionized species 13, the remaining plasma 9 confined inside the anodic tube 3. By ion bombardment, the plasma 9 erodes the surface of the sample facing the end of the anode tube so as to form on the surface of the sample a crater whose diameter is close the diameter of the anode tube. The ionized species present in the plasma 9 are measured by a mass spectrometer and / or the species excited by an optical spectrometer. More particularly, a mass spectrometer includes a mass analyzer which separates the ions according to their mass-to-charge ratio (m / z), where m represents the atomic mass and z the electric charge of an ionized species. A glow discharge spectrometer thus enables the analysis of materials and thin layers. However, the erosion rate of SDL sources being high (of the order of 2 to 100 nm per second), it is necessary to have spectrometers for rapid acquisition and providing multi-elemental information. This can be achieved by using a multichannel optical spectrometer and / or an extremely fast flight time mass spectrometer. The combination of an optical spectrometer and a mass spectrometer is also envisaged and has been carried out in experimental assemblies.

In an RF glow discharge spectrometer, an RF generator supplies the electric power to the discharge lamp, for example by means of an RF applicator 5 in contact with the sample 4. The RF generator has an output impedance of 50 ohms . The generator must in principle always be connected to an electrical circuit having an impedance adapted to the output impedance of the generator, that is to say 50 ohms. An impedance matching device placed between the electric generator and the discharge lamp makes it possible to match the output impedance of the generator to the impedance of the electrical system formed by the discharge lamp, the plasma and the sample. However, the impedance of the electrical system varies depending on the plasma conditions as well as the nature of the sample.

In a non-pulsed RF glow discharge spectrometer, the impedance matching device is slaved to an impedance mismatch measurement system, based for example on a measurement of the reflected power. The enslaved impedance matching system optimizes the transfer of power to the plasma by minimizing the reflected power.

An impedance matching device generally comprises electrical components of variable capacitance and / or variable inductance for adjusting the impedance of the device. Since the power supplied by the generator is quite high (from a few Watts to more than a hundred Watts) the variable impedance components are generally electromechanical components such as variable capacitors or variable inductance coils which are compatible with the power delivered over a wide range of impedance variation. The figure 2 schematically represents an embodiment of known impedance matching system 17 comprising an inductor 17a and two variable capacitors 17b, 17c. A mechanical control makes it possible to modify the value of the impedance of a component (capacitance or impedance) so as to modify the real part (ReΩ) and the imaginary part (ImΩ) of the impedance of the tuning device. The known variable capacitors are, for example, plate capacitors whose distance is mechanically variable. A known variable impedance coil is for example a coil whose electrical contact point varies so as to change the number of turns used. Impedance matching devices are modeled in the literature by complex notations (real and imaginary values) and it is necessary to control two parameters to minimize the reflected power. The impedance match can be manually performed by an operator before the start of the SDL measurements or motorized so as to slave the position of the electromechanical components to a measurement of the power reflected by the sample and / or the current-voltage phase shift. .

In a non-pulsed RF glow discharge spectrometer, a controlled impedance matching device thus makes it possible to minimize the reflected power and to bring the current-voltage phase shift closer to 0 degrees at start-up and during spectrometric measurements. However, the impedance agreement is necessarily slow because of the slowness of the measurement system of a signal representative of the impedance mismatch and because of the slowness of the electromechanical tuning device. impedance. The response time to obtain an impedance match is of the order of 0.5 to 10 seconds.

An impedance matching device may optionally be coupled to a frequency deviation device which allows the frequency of the generator to be changed and the impedance mismatch to be modified. A frequency deviation device has a fast response time, of the order of 0.1s, however, it allows only one electrical parameter to be changed and does not always allow on its own to minimize the reflected power completely.

Another way to compensate for impedance mismatch is to increase the power provided by the RF generator. However, the additional power delivered dissipates, especially in the form of thermal energy that can induce heat stress in the sample. The presence of a cooling circuit in contact with the sample is not always sufficient to reduce the thermal heating induced on the sample, even for optimized power, particularly in the case of fragile materials or multilayer samples, for which heat stress can be harmful.

In recent years, the major advance in glow discharge spectrometry has been achieved by the introduction of pulsed RF sources. A pulsed RF source makes it possible, by optimizing the duty cycle of the pulses, to independently control the instantaneous power, responsible for the erosion of the material and the obtaining of the analytical signals, and the average power supplied to the sample which is responsible for its thermal heating.

In optical glow discharge spectrometry, the main benefit of using a pulsed RF source lies in the minimization of induced thermal stresses, especially for fragile materials.

In glow discharge mass spectrometry, the use of a pulsed RF source offers remarkable additional advantages because the ionization mechanisms of the species present in the plasma vary during the period of the RF source. The figure 3A schematically represents the power supplied P _f by the RF generator to generate an electric pulse 20 for a duration τ ₁ . The figure 3B schematically represents a measurement obtained by mass spectrometry just before the start of the electric pulse, during the pulse and after stopping this electrical pulse. The mass spectrometry signal can be analyzed in different time zones, respectively "prepeak" 31, "plateau" 32 and "afterglow" 33 offering original and rich analytical combinations of information not only for fragile materials but for any type of materials and stacks of thin layers. On the figure 3B the two curves respectively represented in solid lines and in dashed lines correspond to the monitoring by a mass analyzer of two different elements, for example the carrier gas for the solid line curve and an element originating from the sample for the dashed curve.

More specifically, the ionic signals generally appear more intense in the "afterglow" zone 33 after the extinction of a plasma pulse. The publication of N. Tuccito et al. (Rapid Comm, Mass Spectrom 2009, 23: 549-556 ) indicates that the temporal distribution of the mass spectrometry signal maxima is specific to each element. This publication also demonstrates that we can not only optimize the measurement of each element with a time-of-flight mass spectrometer but also analyze ionized molecular fragments, which makes it possible to discriminate polymers of similar elemental composition but of different molecular structure. . The publication of L.Lobo et al. (A comparison of non-pulsed radiofrequency and pulsed radiofrequency orthogonal glow discharge time-of-flight mass spectrometry for analytical purposes, J. Anal. At. Spectrom., 2009, 24, 1373-1381. ) has shown that it is possible to obtain signal / background ratios in luminescent discharge mass spectrometry and therefore much higher sensitivities in pulsed mode than those obtained in continuous (non-pulsed) mode. In addition, the publication Lobo et al. highlights that a precise selection of the time interval of integration in pulsed mode makes it possible to optimize the performances in term of ionic separation and of precision and reproducibility of the measurements of ratios of isotopes.

It is now very decisive to be able to perform measurements in simultaneous or near simultaneous mass spectrometry (as in time of flight devices) in pulsed mode.

However, in the case of a multilayer sample for example, the impedance of the material changes as a function of the erosion depth. However, impedance matching systems have a very high response time and the measurement systems of impedance mismatch are provided for continuous signals. The enslaved impedance matching devices existing up to now do not work satisfactorily in pulsed mode because they generally cause an erratic movement of the electromechanical components of the tuning box and do not minimize the power reflected at startup or when changing diapers. The solution to avoid these erratic movements of the electromechanical components of the tuning box and thus erratic impedance changes is generally to inhibit the control system of the tuning box. The operator wishing to optimize the measurements must then proceed with a series of trial and error, by presetting the impedance matching device at fixed positions, so as to minimize the reflected power at start-up, and then compensating for small deviations. by an increase in incident power during erosion of the sample. This method of trial and error can be destructive for the sample, which is sometimes only available in one copy. In addition, the increase in the power applied necessarily induces heat stress in the sample, while one of the purposes of the use of the pulsed mode is precisely to reduce the induced heat stress. The document Valledor R et al: "Direct chemical in-depth profile analysis and thickness quantification of nanometer multilayers using pulsed-rf-GD-TOFMS. Analytical and Bioanalytical Chemistry, Springer, vol. 396, No. 8, January 16, 2010 (2010-01-16), pages 2881-2887, XP019798714, ISSN: 1618-2650 discloses a device and method for measuring by glow discharge mass spectrometry of a solid sample using a pulsed RF electric field in which an impedance match is optimized on a silicon wafer prior to measuring the sample. There is no such thing as an impedance matching device or an impedance mismatch measurement system for real-time impedance matching servocontrol with a response time of less than 0.5s and capable of transmitting electrical power up to 200 W. There is no system of impedance matching and impedance mismatch measurement compatible with pulsed RF generator operation until now. .

The object of the present invention is to remedy these drawbacks and to improve a pulsed mode mass spectrometry measurement method and apparatus. The object of the invention is in particular to optimize the coupling of electrical power with a pulsed discharge mass spectrometer operating in pulsed mode while reducing the induced heat stress, in particular for multilayer samples.

• a) application d'un champ électrique RF pulsé aux bornes des électrodes d'une lampe à décharge luminescente en présence d'un gaz porteur et d'un échantillon à analyser, ladite lampe étant couplée électriquement à un dispositif d'accord d'impédance ayant une impédance électrique Ω variable, le dispositif d'accord d'impédance étant constitué de composants électromécaniques, de manière à générer un plasma de décharge luminescente en mode pulsé, la durée d'une impulsion électrique étant égale à τ₁, la fréquence de répétition des impulsions étant égale à F₁ comprise entre 0.1 kHz et 20 kHz et le rapport cyclique d'une impulsion étant égal à τ₁ X F₁ ;
• b) mesure par spectrométrie de masse d'au moins un signal représentatif d'une espèce ionisée ayant un rapport m/z prédéterminé, ladite mesure étant effectuée à une fréquence d'acquisition F₂ supérieure à 1/τ₁ ;
• c) mesure d'un signal représentatif du désaccord d'impédance ΔΩ entre le générateur de champ électrique RF pulsé et les électrodes de la lampe à décharge pendant au moins une partie des impulsions plasma au moyen d'un système d'acquisition numérique rapide de mesure synchronisé avec lesdites impulsions, ledit système d'acquisition rapide ayant une fréquence d'acquisition F₃ supérieure à 1/τ₁,
• d) détermination d'une variation d'impédance dΩ à appliquer au dispositif d'accord d'impédance en fonction de la mesure d'un signal représentatif du désaccord d'impédance ΔΩ ;
• e) modification de l'impédance Ω du dispositif d'accord d'impédance, les composants électromécaniques étant aptes à faire varier de manière continue sur un cycle de plusieurs impulsions ladite impédance électrique Ω variable en fonction de la valeur de dΩ déterminée à l'étape d) ;
• f) répétition des étapes c) à e) de manière à minimiser le désaccord d'impédance ΔΩ.

The present invention more particularly relates to a method for measuring a solid sample by pulsed mode glow discharge spectrometry comprising the following steps:

• a) applying a pulsed RF electric field across the electrodes of a glow discharge lamp in the presence of a carrier gas and a sample to be analyzed, said lamp being electrically coupled to an impedance matching device having a variable electrical impedance Ω, the impedance matching device consisting of electromechanical components, so as to generate a pulsed-mode glow discharge plasma, the duration of an electrical pulse being equal to τ ₁ , the frequency of repetition of the pulses being equal to F ₁ between 0.1 kHz and 20 kHz and the duty cycle of a pulse being equal to τ ₁ XF ₁ ;
• b) measuring by mass spectrometry of at least one signal representative of an ionized species having a predetermined m / z ratio, said measurement being performed at an acquisition frequency F ₂ greater than 1 / τ ₁ ;
• c) measuring a signal representative of the impedance mismatch ΔΩ between the pulsed RF electric field generator and the discharge lamp electrodes during at least a portion of the plasma pulses by means of a fast digital acquisition system of measurement synchronized with said pulses, said fast acquisition system having an acquisition frequency F ₃ greater than 1 / τ ₁ ,
• d) determining an impedance variation dΩ to be applied to the impedance matching device as a function of the measurement of a signal representative of the impedance mismatch ΔΩ;
• e) modifying the impedance Ω impedance matching device, the electromechanical components being able to vary continuously over a cycle of several pulses said variable electrical impedance Ω as a function of the value of dΩ determined at the step d);
• f) repeating steps c) to e) so as to minimize the impedance mismatch ΔΩ.

• la mesure d'un signal représentatif du désaccord d'impédance ΔΩ comprend une mesure de la puissance électrique réfléchie et/ou une mesure du déphasage courant-tension ;
• les variations de la partie réelle Re(Ω) et de la partie imaginaire lm(Ω) de l'impédance Ω dudit dispositif d'accord sont obtenues par modification des valeurs des impédances d'au moins deux composants du dispositif d'accord ;
• excursion de la fréquence RF du générateur de manière à minimiser le désaccord d'impédance ΔΩ.

In various aspects, the method of the invention further comprises one or more of the following steps:

• measuring a signal representative of the impedance mismatch ΔΩ comprises a measurement of the reflected electrical power and / or a measurement of the current-voltage phase shift;
• the variations of the real part Re (Ω) and the imaginary part lm (Ω) of the impedance Ω of said tuning device are obtained by modifying the values of the impedances of at least two components of the tuning device;
• RF frequency deviation of the generator so as to minimize the impedance mismatch ΔΩ.

According to a preferred embodiment of the method of the invention, the duty ratio of the pulses τ ₁ × F ₁ is between 5% and 50%.

• un générateur de champ électrique RF utilisable en mode pulsé apte à générer un champ électrique RF comprenant des impulsions électriques de durée τ₁ et de fréquence de répétition F₁ comprise entre 0.1 kHz et 20 kHz,
• une lampe à décharge comprenant des électrodes, des moyens de pompage et des moyens d'introduction d'un gaz porteur, ladite lampe à décharge étant apte à recevoir un échantillon solide à analyser et apte à générer un plasma de décharge luminescente,
• un spectromètre de masse relié à ladite lampe à décharge et apte à mesurer au moins un signal représentatif d'une espèce ionisée du plasma présentant un rapport m/z prédéterminé, à une fréquence d'acquisition F₂ supérieure à 1/τ₁ et
• un dispositif d'accord d'impédance relié électriquement d'une part au générateur de champ électrique RF pulsé et d'autre part aux électrodes de la lampe à décharge, ledit dispositif d'accord étant apte à transférer la puissance électrique fournie par le générateur RF en mode pulsé vers la lampe à décharge et ledit dispositif d'accord ayant une impédance électrique Ω variable.

The present invention also relates to a discharge spectrometry device, for measuring a solid sample, comprising:

• an electric field generator RF usable in pulsed mode suitable for generating an electric field RF comprising electrical pulses of duration τ ₁ and repetition frequency F ₁ of between 0.1 kHz and 20 kHz,
• a discharge lamp comprising electrodes, pumping means and means for introducing a carrier gas, said discharge lamp being able to receive a solid sample to be analyzed and capable of generating a glow discharge plasma,
• a mass spectrometer connected to said discharge lamp and capable of measuring at least one signal representative of an ionized species of the plasma having a predetermined m / z ratio, at an acquisition frequency F ₂ greater than 1 / τ ₁ and
• an impedance matching device electrically connected on the one hand to the pulsed RF electric field generator and on the other hand to the electrodes of the discharge lamp, said tuning device being able to transfer the electric power supplied by the generator RF in pulsed mode to the discharge lamp and said tuning device having a variable electrical impedance Ω.

According to the invention, the glow discharge spectrometry device comprises a fast digital measurement system able to measure a signal representative of the impedance mismatch ΔΩ between the generator and the discharge lamp, said measurement system comprising an acquisition system fast, synchronized with the plasma pulses, having an acquisition frequency F ₃ greater than or equal to 1 / τ ₁ and being able to provide the impedance matching device with a signal representative of the impedance mismatch ΔΩ for at least one part of said pulses.

According to the invention, the tuning device adapts the impedance Ω according to the measurement representative of the impedance mismatch, so as to minimize the impedance mismatch ΔΩ in a continuous manner. In the invention, the electromechanical components are able to vary continuously over a cycle of several pulses said variable electrical impedance Ω.

• le dispositif d'accord d'impédance comprend au moins deux composants électromécaniques à capacité(s) variable(s) et/ou à inductance(s) variable(s) aptes à modifier la partie réelle Re(Ω) et la partie imaginaire lm(Ω) de l'impédance Ω dudit dispositif d'accord ;
• le dispositif de spectrométrie comprend en outre un dispositif d'excursion de fréquence apte à faire varier la fréquence RF du générateur et asservi à la mesure du désaccord d'impédance ΔΩ ;
• le système de mesure du désaccord d'impédance comprend une mesure de la puissance électrique réfléchie et/ou une mesure du déphasage courant/tension ;
• le spectromètre de masse est un spectromètre à temps de vol, ou un spectromètre quadripolaire ou un spectromètre à secteur magnétique ou un spectromètre de masse à transformée de Fourier.

According to various aspects of the spectrometry device of the invention:

• the impedance matching device comprises at least two electromechanical components with variable capacity (s) and / or variable inductance (s) able to modify the real part Re (Ω) and the imaginary part lm (Ω) the impedance Ω of said tuning device;
• the spectrometry device further comprises a frequency deviation device adapted to vary the RF frequency of the generator and slaved to the measurement of the impedance mismatch ΔΩ;
• the impedance mismatch measuring system comprises a measurement of the reflected electrical power and / or a phase shift measurement current / voltage;
• the mass spectrometer is a time-of-flight spectrometer, or a quadrupole spectrometer or a magnetic sector spectrometer or a Fourier transform mass spectrometer.

The invention will find a particularly advantageous application in the glow discharge mass spectrometry operating in pulsed mode.

The present invention also relates to the features which will emerge in the course of the description which follows and which will have to be considered individually or in all their technically possible combinations.

• la figure 1 représente schématiquement une vue en coupe d'une lampe à décharge luminescente selon l'état antérieur de la technique ;
• la figure 2 représente schématiquement un circuit électrique de couplage entre un générateur électrique, un système d'accord d'impédance et une lampe à décharge selon l'art antérieur ;
• la figure 3A représente schématiquement une impulsion appliquée par un générateur en mode pulsé en fonction du temps ; la figure 3B représente schématiquement deux signaux temporels obtenus par spectrométrie de masse pour deux éléments distincts et indique les trois zones de mesure respectivement « prepeak », « plateau » et « afterglow » ;
• la figure 4A représente schématiquement une série d'impulsions électriques de durée τ₁ et de fréquence de répétition F₁ et la figure 4B représente schématiquement une série d'acquisition numériques correspondant aux différentes zones de mesure par spectromètre de masse ;
• la figure 5 représente schématiquement un système électrique de couplage entre un générateur électrique, une lampe à décharge, un système d'accord d'impédance et un système d'asservissement de l'impédance et/ou d'excursion de fréquence selon un mode de réalisation de l'invention ;
• la figure 6 représente une mesure temporelle d'intensité de la puissance électrique appliquée pendant une série d'impulsions électriques, ainsi qu'une mesure numérique rapide d'un signal représentatif de la puissance réfléchie, ainsi que des signaux de spectrométrie optique.

This description, given by way of non-limiting example, will better understand how the invention can be made with reference to the accompanying drawings in which:

• the figure 1 schematically shows a sectional view of a glow discharge lamp according to the prior art;
• the figure 2 schematically represents an electrical coupling circuit between an electric generator, an impedance matching system and a discharge lamp according to the prior art;
• the figure 3A schematically represents a pulse applied by a pulsed mode generator as a function of time; the figure 3B schematically represents two time signals obtained by mass spectrometry for two distinct elements and indicates the three measurement zones respectively "prepeak", "plateau" and "afterglow";
• the Figure 4A schematically represents a series of electrical pulses of duration τ ₁ and repetition frequency F ₁ and the Figure 4B schematically represents a series of digital acquisitions corresponding to the different measurement zones by mass spectrometer;
• the figure 5 schematically represents an electrical coupling system between an electric generator, a discharge lamp, an impedance matching system and an impedance and / or frequency deviation control system according to an embodiment of the invention. invention;
• the figure 6 represents a time measurement of the intensity of the electric power applied during a series of electrical pulses, as well as a fast digital measurement of a signal representative of the reflected power, as well as optical spectrometry signals.

We now detail the structure and operation of a pulse-mode RF glow discharge spectrometry apparatus according to one embodiment of the invention.

The figure 5 schematically represents a glow discharge spectrometry apparatus which comprises an electric generator 6, a tuning device impedance 17, a discharge lamp 1 and an impedance mismatch measuring system 18.

The discharge lamp 1 is a conventional lamp such as for example the discharge lamp detailed in connection with the figure 1 . The discharge lamp 1 comprises a tubular electrode 3. A sample 4 to be analyzed forms the second electrode. An RF applicator transmits the power delivered by the generator to the discharge lamp through the sample.

The electric generator 6 is an RF generator that can operate in continuous mode or in pulsed mode. The electric generator 6 delivers a maximum RF power of 150 W. The RF frequency of the generator is generally the normalized frequency of 13.56 MHz. However, there are also RF generators operating at other RF frequencies and compatible with the operating principle detailed below.

The Figure 4A schematically represents the electric power P _R supplied by the RF generator in pulsed mode. The generator 6 delivers pulses of duration τ ₁ and repetition frequency F ₁ (the variations due to the RF frequency are not represented on the figure 4 , because the RF frequency is extremely high compared to the pulse repetition frequency and the pulse duration). In pulsed mode, the repetition frequency F ₁ of the pulses can be set to a value generally between 0.1 kHz to 20 kHz and the duty cycle of the pulses τ ₁ × F ₁ can be set to a value typically between 5% and 50 %. The duration of a pulse is generally between a few microseconds and a few milliseconds. The lower the duty cycle, the lower the risk of heating the sample. The Figure 4B schematically represents the acquisition sequences by mass spectrometry in pulsed mode. Digital acquisitions are made at a frequency F ₂ equal to 1 / τ ₂ much greater than the frequency 1 / τ ₁ so as to acquire enough spectra respectively on the "prepeak", "plateau" and "afterglow" zones of each period. of the RF source. An acquisition sequence by the detector of the mass spectrometer extends over a duration T ₂ greater than the duration τ ₁ of a pulse of the RF generator. As illustrated on the Figure 4B , an acquisition sequence of the mass spectrometer starts a little before the electrical pulse so as to acquire the baseline of the mass spectra before the start of the pulse (zone 21), then continues at the beginning of the pulse (zone 22 "prepeak"), during the pulse (zone 23 "plateau") and finally ends after the end of the pulse so as to acquire spectra (zone 24 "afterglow"). At each acquisition, the mass analyzer makes it possible to obtain simultaneously or almost simultaneously the intensity of the signals as a function of the m / z ratio, which makes it possible to deduce from them a multi-elemental and / or molecular chemical analysis of the sample resolved in depth.

By construction, the RF generator has an output impedance of 50 ohms. The generator is connected to an electrical circuit whose impedance must in principle always be adapted to the output impedance of the generator, that is to say 50 ohms, to optimize the transfer of electrical power between the generator and the plasma . The impedance of the load connected to the generator is formed by the impedances placed in series (or in parallel according to the electric circuit) respectively of the discharge lamp 1, the plasma 9, the sample 4 and the tuning device. However, as detailed above, this impedance varies depending on the plasma conditions as well as the nature of the sample. In practice, the impedance of the discharge lamp 1 varies little while the impedance of the sample 4 varies during the measurement. Table I shows the impedances measured experimentally for different types of samples. On the one hand, it can be observed that the impedance of a sample in a glow discharge lamp is essentially capacitive in nature and, on the other hand, that the value of the impedance varies considerably depending on whether the sample is conducting, semiconductor or insulation. In addition, for a multilayer sample, the impedance of the sample varies during measurement of GD-MS as a function of the plasma-exposed layer. Table I: complex impedances of different materials Material Complex impedance 1261 steel 22Ω-j427Ω Painted steel 25Ω-j281Ω Semiconductor 53Ω-j338Ω Metallic glass 48Ω-j535Ω Thick ceramics 40Ω-j500Ω Glass 33Ω, 522Ω Aluminum plate 46Ω-j314Ω

The figure 5 schematically represents the electrical circuit connecting the pulsed RF electric field generator 6 to the glow discharge lamp 1. The glow discharge spectrometry device uses a conventional impedance matching device 17 placed between the generator 6 and the system formed by the discharge lamp 1 and the sample 4. The impedance matching device 17 comprises for example an inductor 17a and two capacitors 17b, 17c with variable capacitances (C _T , C _L ), respectively a capacitor 17b in series and a capacitor 17c in parallel. The impedance matching box has a variable impedance Ω as a function of the respective capacitance values (C _L , C _T ) of the capacitors 17b, 17c and the inductance of the coil 17a. In a first example, the capacity of a capacitor is variable mechanically, for example by decreasing the distance between the plates of a capacitor (vacuum capacitor for example) or by changing the surface between plates (finned capacitor for example). In a second example, a component of the impedance matching system is replaced by two components: for example the variable capacitor 17b is replaced by two capacitors in parallel, a capacitor of large capacity and a capacitor of small capacity. The motorization of the small capacity allows a fast response, while the large capacity in parallel allows an adaptation to the strong variations of impedance with a longer response time. In another exemplary embodiment, the impedance matching system comprises two inductance coils mechanically variable by modifying the electrical contact point of the circuit and therefore the number of turns used for each coil. Variable capacitors allow for continuous impedance variation while variable impedance systems have incremental impedance variations. These systems are robust and support high electrical powers (several tens or even hundreds of watts). However, the impedance variation is controlled by a mechanical movement which remains slow even when it is motorized.

The innovative part of the device represented in figure 5 resides in the impedance mismatch measuring device 18 and in the slaving of the impedance matching system 17 to this measuring device 18 during the application of a pulsed RF electric field.

In the non-pulsed RF apparatus, the impedance matching system 17 is continuously slaved to an analog measurement representative of the impedance mismatch, such as, for example, a measurement of the reflected power, and / or a phase shift measurement. current-voltage. The impedance of the components of the tuning system 17 is modified by a mechanical movement which is relatively slow compared to the duration of pulsed pulses and compared to the repetition frequency of the pulses (from 10 Hz to 20 kHz).

However, when the generator 6 operates in pulsed mode, a conventional continuous servo system is incompatible with the pulsed mode operation.

In prior mass spectrometry apparatus operating in pulsed mode, the slaving between the impedance matching system and the analog impedance mismatch measurement system is disabled to avoid the erratic impedance of the transmission box. 'agreement.

The device of the invention comprises a device 18 connected to the impedance matching device 17. According to the preferred embodiment of the invention, there is used a device 18 comprising a fast digital system for measuring a signal representative of the disagreement d impedance ΔΩ. According to an exemplary embodiment, measuring the intensity of the reflected electrical power P _r and / or the current-voltage phase-shift at a high rate, the measurement time of the reflected power or phase-shift current being very much lower than the duration of the shortest pulses. These control signals are measured in a manner synchronized with the plasma pulses so as to take into account only the signals measured when the plasma is on. The acquisition system of a measurement representative of impedance mismatch (reflected power and / or current-voltage phase shift) is represented symbolically on the figure 5 by the link 19a between the output of the impedance matching device 17 and the input of the system 18. One or more values representative of the impedance mismatch ΔΩ are thus acquired at a high acquisition frequency for each electrical pulse c that is for each plasma pulse.

A calculator is used to determine how much to vary the real part (ReΩ) and the imaginary part (ImΩ) of the impedance matching device, to minimize the impedance mismatch or to minimize the reflected power according to an algorithm predetermined servocontrol. A preliminary calibration thus makes it possible to determine which movement (s) to apply to the electromechanical components in order to modify their respective impedances according to the determined value. The servo control algorithm of the computer can be based on a function proportional to the impedance mismatch measured ΔΩ, to correct the errors observed, and / or on a differential function as a function of the rate of change of ΔΩ, so as to anticipate variations in impedance mismatch.

The slaving between the measuring device 18 and the tuning device is represented symbolically by the link 19b which makes it possible to act on the value of the capacitors 17b, 17c as a function of the measurement, for example of the reflected power. The feedback loop formed by the two links 19a and 19b makes it possible, for example, to minimize the reflected power P _r , and thus to obtain the impedance match between a pulsed mode RF generator 6 and its charge constituted by the light source. discharge, plasma and sample.

Optionally, the measuring device can also be used to act by frequency deviation on the generator 6 via the link 19c so as to minimize a measurement representative of the impedance mismatch. The frequency deviation changes the nominative RF frequency of 13.56 MHz by +/- 300 kHz.

The device of the invention thus makes it possible to act on an impedance matching device coupled to a pulsed mode RF generator, although this impedance matching device has an extremely slow response time compared to the durations of the devices. pulses as well as the time interval between two successive pulses.

The figure 6 represents a series of plasma pulses as a function of time, as well as the measurements of incident and reflected power. Curves I ₁ and I ₂ represent optical spectrometry analysis signals, which have maxima during the plasma pulse. The curve P _f represents a measure of the power supplied by the RF generator, in other words the incident power. The curve P _r represents a measure of the reflected power. The ordinate scale is in arbitrary units. The incident power measurements P _f and the reflected power P _r between two successive pulses are filtered. Only the power measurements taken during the pulses are kept. Reflected power and / or phase-to-voltage phase shifts allow control of the reflected power and also allows the reflected power to be minimized through feedback to the impedance matching system which slaves capacitor values and / or or variable inductances. The impedance matching of the impedance matching device is not effective during the pulse where the measurement is made, due to the response times of the mechanical movements to adjust the impedances of the device. agreement. In the invention, the impedance change is performed continuously on a cycle of several pulses. In the case where the impedance matching box comprises mechanically variable capacitors, the capacitances (17b, 17c) are continuously varied, smoothing the impedance variations. Depending on the repetition rate and pulse duty cycle on the one hand and the response time of the tuning system on the other hand, the impedance change can occur several pulses after the measurement of the disagreement. From one impulse to the next, a reduction in the reflected power P _f enslaved as a function of time to the evolution of the impedance of the discharge lamp and of the sample is thereby obtained step by step. It is not therefore a real time enslavement. The impedance adaptation in a continuous way corresponds well to the analyzed materials, because even in the case where the interfaces are clear, one passes progressively from one layer to another.

The method and device described herein, outside of the invention as claimed, nevertheless allow impedance matching in pulsed mode under conditions where power transfer is optimized. The optimization of the power transfer and in particular the minimization of the reflected power make it possible to protect the sample from dissipation of energy in the form of heat. This optimization also protects the generator because the power reflected towards the electric generator may damage it.

The digital impedance mismatch and impedance matching system can be operated in either continuous or pulsed mode. This device allows impedance matching at the start of the measurement and during a measurement, in particular at each interface of a multi-layer sample.

The extraction frequency of the mass spectrometer is of the order of 30 kHz, that is to say, much higher than the repetition frequency of the pulses, so as to extract a profile comprising enough points for each pulse. The Mass spectrometry measurements are averaged over a predetermined number of source periods following the depth resolution required to form a series of mass spectra of the sample. The evolution of the signal of one or more ionic species as a function of time makes it possible to construct the profile of the sample analyzed.

This produces a mass spectrometry apparatus operating in extremely powerful pulsed RF mode.

The discharge lamp may optionally be coupled to an optical spectrometer for optical emission measurements.

The method and the device of the invention make it possible to optimize the impedance matching in pulsed mode, although the impedance matching system can remain based on components (capacitor (s) and / or inductance (s) variable) whose impedance variation is controlled by a slow mechanical movement.

The method and apparatus of the invention allow for pulsed-mode glow discharge mass spectrometry analysis under conditions where the impedance matching of the plasma is optimized according to a measurement taken only during the pulses. which allows the optimal transfer of the power to the plasma in pulsed mode without increasing the power supplied.

The method and apparatus of the invention avoids a test on a sample to optimize the impedance matching start conditions, which limits the loss of samples, particularly in the case of small sample size to be analyzed. or fragile sample.

The method and the device of the invention allow the analysis of fragile samples without inducing adverse thermal stress and allow accurate analysis of multilayer samples, without drifting agreement conditions during layer changes. The method of the invention thus makes it possible to obtain measurements having a better accuracy, a better depth resolution and / or a faster speed, over a wide range of impedance matching, compared to a non-pulsed RF mode method. enslaved impedance and also compared to a pulsed RF mode without impedance servo.

The method and the device of the invention not only make it possible to improve the analytical performance of a GD-MS apparatus, but also to effectively protect the RF generator by effectively minimizing the power reflected back to the generator, capable of damage the electrical generator.

Claims (10)

1. A method for the measurement of a solid sample by pulsed glow discharge spectrometry, comprising the following steps:

   a) applying a pulsed RF electric field at the terminals of the electrodes (3) of a glow discharge lamp (1) in the presence of a carrier gas and a sample (4) to be analysed, said lamp being electrically coupled to an impedance matching device (17) having a variable electric impedance Ω, the impedance matching device (17) being consisted of electromechanical components, so as to generate a pulsed glow discharge plasma (9), the duration of an electric pulse being equal to τ₁ the pulse repetition frequency being equal to F₁ comprised between 0.1 kHz and 20 kHz and the cyclic ratio of a pulse being equal to τ₁ x F₁;

   b) measuring by mass spectrometry at least one signal representative of an ionised species having a predetermined m/z ratio, said measurement being carried out at an acquisition frequency F₂ higher than 1/τ₁,

   characterized by the steps:

   c) measuring a signal representative of the impedance mismatch ΔΩ between the pulsed RF electric field generator (6) and the electrodes of the discharge lamp during at least a part of the plasma pulses by means of a fast numerical measurement acquisition system synchronised with said pulses, said fast acquisition system having an acquisition frequency F₃ higher than 1/τ₁ ;

   d) determining an impedance variation dΩ to be applied to the impedance matching device as a function of the measurement of a signal representative of the impedance mismatch ΔΩ;

   e) modifying the impedance Ωof the impedance matching device, the electromechanical components being capable of varying continuously said variable electric impedance Ω over a cycle of several pulses as a function of the value of dΩ determined at step d);

   f) repeating steps c) to e) so as to minimize the impedance mismatch ΔΩ.
2. A method of measurement according to claim 1, characterized in that the measurement of a signal representative of the impedance mismatch ΔΩ comprises a measurement of the reflected electric power and/or a measurement of the current-voltage phase shift.
3. A method according to one of claims 1 to 2, characterized in that the variations of the real part Re(Ω) and the imaginary part Im(Ω) of the impedance Ω of said matching device are obtained by modifying the impedance values of at least two components (17a, 17b, 17c) of the matching device (17).
4. A method according to one of claims 1 to 3, characterized in that it further comprises a step of excursion of the RF frequency of the generator (6) so as to minimize the impedance mismatch ΔΩ.
5. A method according to one of claims 1 to 4, characterized in that the pulse cyclic ratio τ₁ x F₁ is comprised between 5 % and 50 %.
6. A glow discharge spectrometry device for the measurement of a solid sample, comprising:

   - a RF electric field generator (6) operable in pulsed mode, capable of generating a RF electric field comprising electric pulses (20) of duration τ₁ and of repetition frequency F₁ comprised between 0.1 kHz and 20 kHz;

   - a discharge lamp (1) comprising electrodes, pumping means (7) and means for introducing a carrier gas (8), said discharge lamp being capable of receiving a solid sample (4) to be analysed and of generating a glow discharge plasma (9);

   - a mass spectrometer (15) connected to said discharge lamp (1) and capable of measuring at least one signal representative of an ionised species of the plasma having a predetermined m/z ratio, at an acquisition frequency F₂ higher than 1/τ_1; and

   - an impedance matching device (17) electrically connected, on the one hand, to the pulsed RF electric field generator (6), and on the other hand, to the electrodes of the discharge lamp (1), said matching device (17) being capable of transferring the electric power provided by the pulsed RF generator (6) toward the discharge lamp (1) and said matching device (17) having a variable electric impedance Ω,

   characterized in that it comprises:

   - a fast numerical measurement system (18) capable of measuring a signal representative of the impedance mismatch ΔΩ between the generator (6) and the discharge lamp (1), said measurement system (18) comprising a fast acquisition system, synchronised with the plasma pulses, having an acquisition frequency F₃ higher than or equal to 1/τ₁ and being capable of providing the impedance matching device (17) with a signal representative of the impedance mismatch ΔΩ for at least a part of said pulses

   - and wherein the impedance matching device (17) is consisted of electromechanical components, said electromechanical components being capable of varying continuously said variable electric impedance Ω over a cycle of several pulses, so as to minimize the impedance mismatch ΔΩ.
7. A glow discharge spectrometry device according to claim 6, characterized in that the impedance matching device (17) comprises at least two variable capacitance and/or variable inductance electromagnetic components (17a, 17b, 17c), capable of modifying the real part Re(Ω) and the imaginary part Im(Ω) of the impedance Ω of said matching device (17).
8. A device according to one of claims 6 to 7, characterized in that it further comprises a frequency excursion device capable of varying the RF frequency of the generator and driven by the measurement of the impedance mismatch ΔΩ.
9. A device according to one of claims 6 to 8, characterized in that the impedance mismatch measurement system (18) comprises a measurement of the reflected electric power and/or a measurement of the current-voltage phase shift.
10. A device according to one of claims 6 to 9, characterized in that the mass spectrometer (15) is a time-of-flight spectrometer or a quadripolar spectrometer or a magnetic sector spectrometer or a Fourier transform mass spectrometer.

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