AU737850B2

AU737850B2 - Method for analyzing a solid sample's constituents, device for preparing a gaseous mixture, use of a laser emitter in said device, analysis method using the device and use of an ICP or AAS analyser

Info

Publication number: AU737850B2
Application number: AU74231/98A
Authority: AU
Inventors: Wilfried Vogel
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-06-03
Filing date: 1998-06-03
Publication date: 2001-08-30
Anticipated expiration: 2018-06-03
Also published as: WO1998055848A1; AU737850C; AU7423198A; EP0986744A1

Description

1 Method for analyzing a solid sample's constituents, device for preparing a gaseous mixture, use of a Laser emitter in said device, analysis method using the device and use of an ICP or AAS analyzer The technical problem of controlling the quality of materials during production of certain constituents, e.g. the production of semiconductors, becomes more and more important, so that there are constant efforts of research to find new methods and devices permitting to improve, to simplify or to accelerate this control. The aim of the present invention is to propose improvements in this field.

Known instruments for analyzing the elemental composition of a sample are analyzers based on ICP (Inductively Coupled Plasma) and AAS (Atomic Absorption Spectroscopy). In the case of ICP-OE(Optical Emission) one uses for measurement the optical light emission from the sample, whilst in the ICP-MS (Mass Spectrometer) the ionized particles from the sample are directed into a mass spectrometer. In the case of AAS one measures the absorption of light at a characteristic wavelength of the element that one wants to analyze in the sample.

These instruments permit to analyze samples, which can be conditioned in a liquid form. The liquid to be analyzed is nebulized into an argon stream, and the aerosol thus produced injected into the plasma of the ICP.

However, if the sample material can not or only with difficulties be conditioned in a liquid form, then the use of the ICP presents difficulties.

2 There are several approaches for analyzing solid samples by ICP: one dissolves the sample in a solvent and measures the liquid thus obtained. For practical reasons, the dilution has to be at least 100 fold. The solution risks to be contaminated. Certain solids are difficult to be dissolved. The dissolution process may be very time consuming and may require very pure acids.

one grinds the solid material and produces an aerosol with the carrier gas. This method encounters practical problems: in certain cases in the grinding process and in other cases in the production of a usable aerosol.

if the solid material is an electrical conductor, one can extract particles with electrical spark impacts, by CSN method (Conductive Solids Nebulizer), but the electrical conductivity is compulsory. Also, the extracted material recondenses in particles of about 1 micron diameter, and in the case of certain metals, these particles tend to agglomerate.

one "shoots" onto a surface of the sample with a Laser in order to extract particles by "ablation" (Laser Ablation, LA). The drawback of this method is the fact that the particles obtained have very variable dimensions: vapors, fine droplets and solid remaining particles. But, now the sensitivity of the ICP is inversely proportional to the size of the analyzed particles. The Laser ablation is used for microanalysis. This method provides only qualitative analysis. Further, in this method, the energy of the Laser pulse has to be absorbed by the analyzed material. It is therefore necessary to adapt the wavelength of the Laser to the material, which 3 material varies. In order to be efficient with about any material, one selects very short wavelengths in the UV, which are more costly to produce. Furthermore, it is quite difficult with this method to produce during several minutes an aerosol composition which is representative of the sample: this makes the method' inappropriate for sequentially measuring instruments, which are slow.

finally, a very small sample can be introduced on a support(for example in graphite) directly into the plasma of the ICP. Thus the sample is vaporized, ionized and excited. The sample needs to be very small, and the obtained signal is transient.

The aim of the present invention is to permit more accurate, faster and easier analysis of solid samples than has been possible unto now.

To reach this aim, the present invention comprises several different objects.

It relates to a method for analyzing the constituents of a solid sample, wherein one passes a flow of carrier gas in a zone close to a portion of the surface of said sample, one provokes in said zone the formation of a short lived plasma of the carrier gas so as to evaporate, by means of the energy contained in the plasma, some material from the said portion of the surface, which condenses in the carrier gas, and one submits the mixture of carrier gas and particles from the sample to an ICP-MS, ICP-OE or AAS analyzer.

The invention also relates to a device for preparing a gaseous mixture comprising a carrier gas and particles 4 extracted from a solid sample, including a mixing chambre provided with conduits for feeding with carrier gas and evacuation of the gaseous mixture, an emitter of laser pulses with an optical system which directs the pulses into the said chamber and focuses them, means for positioning the sample relative to the chamber and means for adjustment. In this device the means for positioning are arranged in such a way that the solid sample can be fixed with a portion of its surface directed towards the interior of the chamber, and the characteristics of the laser emitter and the means for adjustment are arranged in such a way that the power concentration of each laser pulse creates a plasma of the carrier gas at a predetermined location above the said sample surface, and that the plasma acts during its lifetime onto the said surface to evaporate material, and that the carrier gas gets charged by the said particles.

According to an embodiment of the device, the mixing chamber has an opening that is sealed by the said surface of the sample, set about perpendicular to the axis of the laser beam.

The invention also relates to the use of a laser emitter of the Nd-YAG type having a wavelength of 1064 nm, pulsed at Hz and an energy of 100 to 300 mJ per pulse, in a device according to the invention.

The invention relates also to a method for analyzing materials captured in solid samples, wherein one produces a gaseous mixture by means of the device according to the invention, and introduces said mixture into an ICP-OE, ICP-MS or AAS analyzer. It relates also to an ICP or AAS analyzer 5 using a carrier gas flow of approximately 0.8 1/min. to carry out quantitative analysis by using this method.

Finally, the invention relates to a method analyzing the light emitted by the excited sample vapors when using the device according to the invention, wherein one captures the light and sends it to an optical spectrometer for analysis according to the "Optical Emission" method.

We describe now, by way of examples, various embodiments of the invention, referring to the attached drawing whose single figure, is a schematic view of a group of devices, accomplishing the analysis of a solid sample.

This group of devices makes up an apparatus with its most important elements, the laser emitter 9, the table 3 for preparing the gaseous mixture, with the mixing chambre 14, and an ICP analyzer It is known that the ICP (Inductively Coupled Plasma) analyzers are devices in which appropriate electromagnetic fields ionize the gas and indirectly evaporate and ionize an aerosol which passes in the active zone of the apparatus, with specific means provided to: either decompose the optical radiation emitted by the excited atoms contained in the aerosol, and to analyze this spectrum of radiation (ICP-OE: Optical Emission), or to direct the atoms themselves, whilst they are ionized, into a mass spectrometer (ICP-MS) which measures the distribution of ions as a function of their mass.

A preferred analyzer 15 is of the MS type. As a matter of fact, the elements 9 and 3 permit to prepare a gaseous mixture -which is an aerosol with homogeneous and very small particle wich 6 size, which is particularly favorable for a precise analysis with that type of instrument.

For preparing the said aerosol one uses a laser emitter 9 with the following characteristics: Pulsed laser with preferably a repetition rate of Hz, and an energy of 100 300 mJ per pulse.

Wavelength of radiation in the infrared, typically 1064 nm.

Duration of pulse 5 10 nanoseconds.

The beam 10 of the laser emitter is parallel, with a well defined direction, travelling through a focussing optics 11, with a focal length of 100 200 mm, preferably 150 mm.

Means to adjust exactly the various parameters of the described equipment are provided.

The use of a laser emitter of the known type Nd-YAG is preferred.

Remains to describe the table 3. One of its faces is flat and acts as supporting surface for the solid sample 1 to be analyzed. The sample 1 is fixed against this flat surface by the sample clamp 2. A removable, bell-shaped cover 4 is clamped to the periphery of the supporting surface, sealed with a gasket. This cover 4 eliminates the risk of exposure to untimely laser pulses and guarantees the tightness of the system against leaks of the carrier gas.

The mixing chambre 14 is built within the table 3. Its general shape is spherical with a volume of a few -cubiccentimeters. It has a window under the face of the sample 7 1 which is pressed against the table by the sample clamp 2.

The chamber 14 is connected through a funnel 13 to a straight conduit, which is closed at it's end opposite to the chamber by a partition. This partition is transparent to the radiation of the Laser 9. In the carrier gas feeding conduit are mounted the optical system 11 and the means to adjust. A tubing connects it to the gas flow controller 8. This controller receives permanently, through feeding tube 7, carrier gas at a pressure of several bars. Most often argon is used as carrier gas. The gas flow is regulated by the controller 8 in order to provide a constant flow. It is the ICP analyzer which requires a very constant flow of e.g. 0.8 1/min. One can add some oxygen or eventually air into the argon, in order to eliminate possible effect that oxides in the sample material may induce onto the function of the analyzer.

In certain cases it is advantageous to humidify either the carrier gas before its entry into the device, or to humidify the aerosol before its entry into the ICP analyzer, with a humidifying device 9, as shown in the figure.

The device can contain a device 18 to dampen the acoustic chocks caused by the plasma pulses, mounted between the mixing chamber 14 and the analyzer 15, preferably in the conduit 6 as shown in the figure, for not disturbing the ICP.

One understands that the carrier gas coming from the conduit 5 and passing through the funnel 13 sweeps the chamber 14 and the exposed surface of the sample 1, and is then evacuated laterally through the conduit 6 towards the analyzer. It is important to avoid any leak, particularly in the region of the sample 1. However, during an analysis, the 4kparatus stabilizes, because the interior of the bell-shaped 8 cover 4 reaches rapidly the same pressure as the one in the chamber 14.

The device works as follows. After the sample has been put in place and the carrier gas flow been established, one starts to shoot pulses with the Laser 9. The power of the pulses and the position of the lens 11 are adjusted as a function of the sample characteristics in such a way that for each pulse the specific power [W/cm 2 of the focalised beam exceeds the breakdown threshold and generates a plasma, and this at a predetermined place, in front of and very close to the sample surface. This plasma 12 has a life time in the order of a microsecond. Its diameter is a few millimeters, e.

g. 4 mm, and the adjustments are such that during this "argon spark" a zone of 1 to 1.5 mm diameter on the sample surface is attacked and a layer of about 5 nm of sample material is evaporated. This material is mixed to the argon flow and moved to conduit 6 and the ICP analyzer. At 20 Hz repetition rate of these "sparks", a crater is formed in the sample surface and is growing in depth at a speed of 0.1 pm/s. The values given above are typical for metal samples like Fe and Cu. For other materials they can be greater or smaller.

The shape of the plasma and thus the rate of sample evaporation stay quite constant until the crater depth has reached a few tenths of a millimeter. Therefore the aerosol production is also constant over several minutes.

This constant flow of aerosol entering the analyzer permits to make precise quantitative measurements. In the case of an ICP-OE, one has enough time to do a sequential analysis. In the case of ICP-MS, it is also advantageous to be We to measure during one minute at least.

9 Further, one finds that with the indicated parameters, i.

e. a carrier gas flow of 0.5 to 1 1/min., volume of chamber 14 of a few cubiccentimeters, life time of sparks 1 ps, repetition rate 10 to 20 Hz, the sweeping is sufficient to limit the deposit of material on the wall of the chamber to a low extent. The material of each "spark" is evacuated before the next spark, and the particles of the aerosol are sufficiently small (less than 0.1 pm) and are well mixed with the carrier gas (concentration of sample in aerosol in the order of 0.1 to 1 mg/liter) to give very reliable ICP measurements. By reducing the repetition rate from e. g. 20 to 4 Hz, one reduces proportionally the density of the aerosol, which can be very advantageous for measurement with ICP-MS., The sample is practically evaporated, and with the indicated repetition rates there is no formation of a liquid phase. One can call this an atomization by argon sparks.

In the case where one would be tempted to reduce dramatically the repetition rate of the laser, because the sample surface could melt or because the sample could crack due to excessive heat at one place, then one could foresee in the table 3 means for moving the plasma position relative to the sample, e.g. by a small cyclic movement of the optical system 11 or a rotation of the sample holder in its plane.

As we have indicated above, one can also vary slightly the composition of the carrier gas, e.g. by injecting some air or oxygen.

With a good design of the spark chamber one can make it very easy (smooth) to flush, and one reduces thus the so called memory effect, i.e. an influence of the preceding 10 analysis onto the following analysis. The chamber 14 represents about 3/4 of a complete sphere.

The adjustment of the parameters is an important step for preparing the analysis. The long focal distance of lens 11 is an advantage in this respect. The place, where the threshold of plasma formation is reached depends on the power of the pulses and the position of the lens. If, starting from an adjustment giving a given place, one would like to diminish the power of the Laser pulses, then the lens must be moved away from the sample surface. The threshold for plasma formation, i.e. the place where the necessary specific power is reached, will be situated at a location where the beam is more concentrated. For a higher power, one has to diminish the distance between lens and sample, otherwise the plasma will be formed too far from the sample surface and will be less efficient.

The described method permits a simple and rapid analysis by ICP-MS of all solid materials, be they electrically conducting or not: one can at last profit of the excellent sensitivity of ICP-MS for analyzing solids and not only liquids.

The described method permits also the simple and rapid analysis by ICP-OE of all solid materials, be they electrically conducting or not.

The device according to the present invention can also be used for analyzing the light emitted by the excited vapors of the sample, inside the chamber 14 by transmitting this light by means of an optical fiber 16, with one end emerging into 11 the chamber 14 and the other end into an optical spectrometer 17 (method LIBS: Laser Induced Breakdown Spectroscopy).

Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification, they are to be interpreted as specifying the presence of the stated features, integers, steps or components referred to, but not to preclude the presence or addition of one or more other feature, integer, step, component or group thereof.

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Claims

1. Method for analyzing the constituents of a solid sample, characterized by passing a carrier gas flow through a zone close to a portion of the surface of this sample, provoking in this zone the formation of a short-lived plasma of the carrier gas, for evaporating, by means of the energy contained in said plasma, some material of the said portion of the surface, which condenses then in the carrier gas, and submitting the mixture of carrier gas and particles originating from the sample to an ICP-MS, ICP-OE or AAS analyzer.

2. Device for preparing a gaseous mixture comprised of a carrier gas and particles detached from a solid sample, comprising a mixing chamber with a conduit for feeding the carrier gas and a conduit for removing the gaseous mixture, a laser pulse emitter with an optical system which directs the pulses into the said chamber and focuses them, means for positioning the sample relative to the chamber and means of adjustment, characterized in that the means for positioning are arranged in such a way that the solid sample can be fixed with a portion of its surface towards the interior of the chamber and in that the characteristics of the laser emitter and the means of adjustment are such that upon shooting, the beam concentration at a predetermined place in front of the said surface, reaches such a value that a plasma of the carrier gas is formed, and that this plasma acts during its life time onto the said surface and gets loaded with said particles by evaporation.

3. Device according to claim 2, characterized in that the mixing chamber has an opening, sealed by the said surface of 13 the sample which is about perpendicular to the axis of the laser beam.

4. Device according to claim 2, characterized in that the laser emits pulses with a wavelength in the infra red, in particular of 1064 nm.

Device according to one of claims 2 to characterized in that the optical system has a focal length in the order of 150 mm.

6. Device according to one of claims 2 to characterized in that the shape of the mixing chamber is about spherical and has a volume of a few cubiccentimeters.

7. Device according to claim 6, characterized in that the means for positioning the sample comprise a sample clamp in a bell-shaped cover with an uninterrupted rim which tightly seals against a portion of the table, when said sample is set in place, said portion being part of the chamber wall, equalizing the pressure inside the chamber with the pressure inside the bell-shaped cover occurring, as soon as the gas flow is established.

8. Device according to any of claims 2 to 7, characterized in that it comprises means for providing a relative displacement between the zone of plasma formation and said portion of surface of the sample.

9. Device according to any of claims 2 to 8, characterized in that it comprises a additional damping device (18) mounted between the mixing chamber and the analyzer OS7 OF C)/ 14 for deadening the acoustic chocks produced by the plasma pulses.

Use of a laser emitter of the Nd-YAG type with a wavelength of 1064 nm, pulsing at 20 Hz with an energy of 100 to 300 mJ per pulse, in a device as claimed 2.

11. Method of analyzing material extracted from solid samples, characterized in that one produces a gaseous mixture by means of the device according to claim 2 or one of claims 3 to 9, and one introduces this mixture into an ICP-MS, ICP-OE or AAS analyzer.

12. Method of analyzing the light emitted by the excited sample vapors, when using a device according to claim 2, characterized in that one captures the light and sends it into an optical spectrometer for analyzing it by the "optical emission" method.

13. Use of an ICP or AAS analyzer designed for a carrier gas flow of about 0.8 i/min., forperforming quantitative analysis by the method according to claim 11.