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Laser mass spectroscopy
Specnochimica ACI, Vol. 39B, No. 12, pp. 1513-1516, 1984. Pmtted in Great Britain.
0
0584-8547/84 so3.00 + .oo 1964. Pergomoa Prma Ltd.
Laser mass spectroscopy H. VAN DOVEREN Philips Research Laboratories, Eindhoven, The Netherlands (Received 26 August 1983)
Abstract-Laser Mass Spectroscopy (LMS) is an analytical technique for (micro) trace analysis of all elements in practically all kind of materials. In this report the experimental set-up and the mechanism of ionisation by laser impact are briefly described. Two examples of preliminary nature are given to illustrate some features of LMS.
INTRODUCTION IN MASS Spectroscopy (MS) the Spark Source MS (SSMS) technique is one of the older techniques for inorganic chemical analysis [l]. It is used for the determination of (trace) elements in bulkmaterials and thick films. A more recently developed technique is Laser MS (LMS). In short LMS has a number of advantages above SSMS: (1) the ability to analyse insulators directly, (2) a decreased information depth (from about 200 to 50nm), (3) an increased lateral resolution (from about 100 to 20pm). LMS can be used for (ultra)trace analysis and micro-analysis of all elements in relatively small volumes near the surface of any material and any sample modification.
EXPERIMENTAL The experimental LMS configuration, and some specifications of the laser and mass analyser are given in Fig. 1. Both the sample as well as the analyser are contained in an ultra-high-vacuum system. Only a part of the ions produced by laser impact enters the mass analyser. The ion beam leaving the electrostatic section of the analyser (Fig. 1) is splitted up in two equal parts. Correlation of the measured charge and the intensity of all element-lines on the photoplate is used in quantitative LMS.
THE LASER-SAMPLEINTERACTION The ions in LMS are produced by laser impact on the sample material. Some determining laser parameters for the ion production are the wavelength, irradiance (W cmm2), pulse frequency, laser pulse form and its duration. Determining material parameters are at least the
MATTAIXH-HERZOG
LASER SPECIFICATIONS : Q-swttched : 1060 nm wavelength : 10” W/cm?shot ~rrodlance beamdnmeter ! 2Opm
lasermode
ANALYZER mass range resolution at 50 m/e tmnsmisslon
SPECIFICATIONS 1 - 230 m/e
:
3000 lo-7
m/e
Fig. 1. Experimental configuration of the Laser Mass Spectroscopic technique. Some speciikations of the laser and the mass analyser are included. [l] Maw Spectrometric
Analysis
ofSolids, Ed. A. J. AHEARN.Ekvier 1513
Publishing Company, Amsterdam (1966).
1514
H.
VAN DOVEREN
density, thermal diffusivity, heat capacity and light absorbance. The interaction can range from thermal desorption and electron emission to plasma formation. Our Q-switched laser involves the production of a plasma, schematically shown in Fig. 2. The time dependent laser energy is partly absorbed by the sample. The penetration depth of the light is for most materials about 10 to 20 nm. Then, the absorbed photons heat the material for some volume. This volume is determined for at least by the previous mentioned parameters. A part of the volume reaches very quickly its boiling point. At this temperature evaporation starts at a large scale. A cloud of vapor atoms leaves the target surface. These vapor atoms are heated further by the still incident laser beam. A further fragmentation of the cloud of vapor atoms into a cloud of individual electrons, ions and photons occurs. Some of the energy stored in this plasma reradiates towards the surface and induces additional evaporation. EXAMPLES OF LMS Examples of analytical work with LMS can be found at various places in the literature [2,3]. Therefore this extended abstract will not give analytical results to show and emphasize again the applicability of LMS in spectrochemistry. Besides the analytical work with LMS, we also want to tackle the experimental possibilities and quantitative difficulties of LMS. Thereby, knowledge of the mechanisms of the ion production process is essential. In the next we will show two examples of research on LMS. Both examples must be seen as work in progress rather than well worked out and understood phenomena. At first we illustrate the effect of structure of the sample on the ion yield. For that purpose we made samples of gold on nickel and gold on glass. The thickness of the goldlayer is varied from 50 nm up to 5 pm. The nickel substrate exhibits good heat conduction and absorbs 1060nm laser light, both in contrast to the glass substrate. The effects on the ion yield, measured with the coulometer, as a function of substrate material, the gold layer thickness and incident laser energy are illustrated in Fig. 3. For the gold on nickel case only one curve (50 nm Au) is given. Thicker gold layers resemble this dashed curve. For gold on glass the ion yield is a complex function of gold layer thickness and laser energy. Only for thick layers the curves approaches to the curve of gold on nickel. The results obtained with LMS from these rather exotic samples emphasize the possible difficulties that may rise with LMS for at least the analysis of (thin) layer structures. The second example deals with the ionisation efficiency
photons
Fig. 2. Schematic diagram of the laser-sample interaction. In this Figure E,,,, means the laser k-radiance (W cmeZ); a,_,, (A)is the penetration depth of the laser light determined 8.0. parameters by is the heat penetration depth determined a.o. parameters by the material the wavelength; dheat(p, cP, KC) density, heat capacity and thermal diffusivity.
PI
D. KOVALEV,G. A.MAKSIMOV,A. I. SUCHKOVand N. V. LARIN,Int. J. Mass Spectrom. Ion Phys. 27,101 (1978). (b) R. J. CONZEMIUSand J. M. CAPELLEN,Int. J. Mass Spectrom. Ion Phys. 34, 197 (1980). c31 J. A. J. JANSENand A. W. WITMER, Spectrochim. Acto 37B,483(1982). (a) I.
1515
Laser mass spectroscopy 15 1000A
t
Au onglass
5
15
10
20
Laser energy (mJ)
Fig. 3. The ion yield after laser impact on gold on nickel and gold on glass substrates as a function of gold layer thickness. The dashed curve represents the results from the gold on nickel system.
of an element. We define this parameter as the ratio between the number of ions detected by the coulometer and the photoplate detector and the number of eroded atoms per laser shot. This ratio is not identical with the elemental sensitivity factor [l]. The number of eroded atoms is determined by calculation whereby it is assumed that each element absorbs a fraction of the available laser energy. This fraction divided by the heat of vaporization per atom yields the number of eroded atoms. After some calculation we get the following semiempirical formula for the ratio between the ionisation efficiency of a pure element qe vs the efficiency of a reference element, in our case iron ylFe:
In this formula Q represents the total measured charge (K), CL the fraction of 1 + ions, R the optical reflectance, AH,,, the heat of vaporization (J/atm.). Notabene, this formula only holds for pure elements and single laser pulses. Figure 4 reveals the relative ionisation efficiencies of various elements. Only copper and selenium exhibit large scattering. These data can be correlated in some way, which we will not work out here, with the plasma production time.
Al . lC
t
Rh.,In
N’ .
--r_*___----_e_-_ 51 Fe . . Sn Zn
T,, Pt 00 W
Fig. 4. The relative ionisation efficiencies,as defined in the text, of various elements. Iron is the used reference element.
1516
H.
0
0584-8547/84 so3.00 + .oo 1964. Pergomoa Prma Ltd.
Laser mass spectroscopy H. VAN DOVEREN Philips Research Laboratories, Eindhoven, The Netherlands (Received 26 August 1983)
Abstract-Laser Mass Spectroscopy (LMS) is an analytical technique for (micro) trace analysis of all elements in practically all kind of materials. In this report the experimental set-up and the mechanism of ionisation by laser impact are briefly described. Two examples of preliminary nature are given to illustrate some features of LMS.
INTRODUCTION IN MASS Spectroscopy (MS) the Spark Source MS (SSMS) technique is one of the older techniques for inorganic chemical analysis [l]. It is used for the determination of (trace) elements in bulkmaterials and thick films. A more recently developed technique is Laser MS (LMS). In short LMS has a number of advantages above SSMS: (1) the ability to analyse insulators directly, (2) a decreased information depth (from about 200 to 50nm), (3) an increased lateral resolution (from about 100 to 20pm). LMS can be used for (ultra)trace analysis and micro-analysis of all elements in relatively small volumes near the surface of any material and any sample modification.
EXPERIMENTAL The experimental LMS configuration, and some specifications of the laser and mass analyser are given in Fig. 1. Both the sample as well as the analyser are contained in an ultra-high-vacuum system. Only a part of the ions produced by laser impact enters the mass analyser. The ion beam leaving the electrostatic section of the analyser (Fig. 1) is splitted up in two equal parts. Correlation of the measured charge and the intensity of all element-lines on the photoplate is used in quantitative LMS.
THE LASER-SAMPLEINTERACTION The ions in LMS are produced by laser impact on the sample material. Some determining laser parameters for the ion production are the wavelength, irradiance (W cmm2), pulse frequency, laser pulse form and its duration. Determining material parameters are at least the
MATTAIXH-HERZOG
LASER SPECIFICATIONS : Q-swttched : 1060 nm wavelength : 10” W/cm?shot ~rrodlance beamdnmeter ! 2Opm
lasermode
ANALYZER mass range resolution at 50 m/e tmnsmisslon
SPECIFICATIONS 1 - 230 m/e
:
3000 lo-7
m/e
Fig. 1. Experimental configuration of the Laser Mass Spectroscopic technique. Some speciikations of the laser and the mass analyser are included. [l] Maw Spectrometric
Analysis
ofSolids, Ed. A. J. AHEARN.Ekvier 1513
Publishing Company, Amsterdam (1966).
1514
H.
VAN DOVEREN
density, thermal diffusivity, heat capacity and light absorbance. The interaction can range from thermal desorption and electron emission to plasma formation. Our Q-switched laser involves the production of a plasma, schematically shown in Fig. 2. The time dependent laser energy is partly absorbed by the sample. The penetration depth of the light is for most materials about 10 to 20 nm. Then, the absorbed photons heat the material for some volume. This volume is determined for at least by the previous mentioned parameters. A part of the volume reaches very quickly its boiling point. At this temperature evaporation starts at a large scale. A cloud of vapor atoms leaves the target surface. These vapor atoms are heated further by the still incident laser beam. A further fragmentation of the cloud of vapor atoms into a cloud of individual electrons, ions and photons occurs. Some of the energy stored in this plasma reradiates towards the surface and induces additional evaporation. EXAMPLES OF LMS Examples of analytical work with LMS can be found at various places in the literature [2,3]. Therefore this extended abstract will not give analytical results to show and emphasize again the applicability of LMS in spectrochemistry. Besides the analytical work with LMS, we also want to tackle the experimental possibilities and quantitative difficulties of LMS. Thereby, knowledge of the mechanisms of the ion production process is essential. In the next we will show two examples of research on LMS. Both examples must be seen as work in progress rather than well worked out and understood phenomena. At first we illustrate the effect of structure of the sample on the ion yield. For that purpose we made samples of gold on nickel and gold on glass. The thickness of the goldlayer is varied from 50 nm up to 5 pm. The nickel substrate exhibits good heat conduction and absorbs 1060nm laser light, both in contrast to the glass substrate. The effects on the ion yield, measured with the coulometer, as a function of substrate material, the gold layer thickness and incident laser energy are illustrated in Fig. 3. For the gold on nickel case only one curve (50 nm Au) is given. Thicker gold layers resemble this dashed curve. For gold on glass the ion yield is a complex function of gold layer thickness and laser energy. Only for thick layers the curves approaches to the curve of gold on nickel. The results obtained with LMS from these rather exotic samples emphasize the possible difficulties that may rise with LMS for at least the analysis of (thin) layer structures. The second example deals with the ionisation efficiency
photons
Fig. 2. Schematic diagram of the laser-sample interaction. In this Figure E,,,, means the laser k-radiance (W cmeZ); a,_,, (A)is the penetration depth of the laser light determined 8.0. parameters by is the heat penetration depth determined a.o. parameters by the material the wavelength; dheat(p, cP, KC) density, heat capacity and thermal diffusivity.
PI
D. KOVALEV,G. A.MAKSIMOV,A. I. SUCHKOVand N. V. LARIN,Int. J. Mass Spectrom. Ion Phys. 27,101 (1978). (b) R. J. CONZEMIUSand J. M. CAPELLEN,Int. J. Mass Spectrom. Ion Phys. 34, 197 (1980). c31 J. A. J. JANSENand A. W. WITMER, Spectrochim. Acto 37B,483(1982). (a) I.
1515
Laser mass spectroscopy 15 1000A
t
Au onglass
5
15
10
20
Laser energy (mJ)
Fig. 3. The ion yield after laser impact on gold on nickel and gold on glass substrates as a function of gold layer thickness. The dashed curve represents the results from the gold on nickel system.
of an element. We define this parameter as the ratio between the number of ions detected by the coulometer and the photoplate detector and the number of eroded atoms per laser shot. This ratio is not identical with the elemental sensitivity factor [l]. The number of eroded atoms is determined by calculation whereby it is assumed that each element absorbs a fraction of the available laser energy. This fraction divided by the heat of vaporization per atom yields the number of eroded atoms. After some calculation we get the following semiempirical formula for the ratio between the ionisation efficiency of a pure element qe vs the efficiency of a reference element, in our case iron ylFe:
In this formula Q represents the total measured charge (K), CL the fraction of 1 + ions, R the optical reflectance, AH,,, the heat of vaporization (J/atm.). Notabene, this formula only holds for pure elements and single laser pulses. Figure 4 reveals the relative ionisation efficiencies of various elements. Only copper and selenium exhibit large scattering. These data can be correlated in some way, which we will not work out here, with the plasma production time.
Al . lC
t
Rh.,In
N’ .
--r_*___----_e_-_ 51 Fe . . Sn Zn
T,, Pt 00 W
Fig. 4. The relative ionisation efficiencies,as defined in the text, of various elements. Iron is the used reference element.
1516
H.