Surface Analysis by Laser Ionization Applied to Polymeric Material

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31 Surface Analysis by Laser Ionization Applied to Polymeric Material S. M . Daiser and S. G . 1

MacKay * 2,

Physical Electronics Division, Perkin-Elmer Corporation, 6509 Flying Cloud Drive, Eden Prairie, M N 55344

Single-photon ionization surface analysis by laser ionization (SPI—SALI) coupled with a static primary ion beam or a soft laser beam for desorption is a promising new method for the characterization of polymer surfaces. SPI-SALI combines nonresonant photoionization of desorbed or sputtered neutral atoms and molecules with analysis by time-of-flight mass spectrometry in ultrahigh vacuum. Stimulated desorption is accomplished by an ion source (Cs , Ar , or Ga ), electron gun, or laser-induced desorption (CO or Nd:YAG). Single-photon ionization using 118-nm light is useful for identifying polymer material, such as polyethylene glycol, polystyrene, polydimethyl siloxane, and many others, by the ease of the monomer identification. +

+

+

2

. A N A L Y S I S OF SOLID SURFACES BY POSTIONIZATION TECHNIQUES has received m u c h attention over the past several years ( I ) . T h e s e techniques i n c l u d e surface analysis b y laser ionization ( S A L I ) ( 2 ) , sputter-initiated resonance ionization spectroscopy ( S I M S ) (3), a n d surface analysis b y laser ionization o f sputtered atoms ( S A R I S A ) (4).

T h e use o f postionization techniques for

surface analysis has received w i d e s p r e a d interest because o f the increased sensitivity p r o v i d e d relative to the more traditional surface analytical tech niques such as X - r a y photoelectron spectroscopy ( X P S ) . Postionization tech-

1

Present address: Berger and Partner, Arabella Str. 33, 8000 Munich, Germany. Present address: 3M Company, 3M Center, Building 201-2S-16, St. Paul, MN 55144. *Corresponding author.

2

0065-2393/93/0236-0727$06.00/0 © 1993 American Chemical Society

Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

728

STRUCTURE-PROPERTY RELATIONS IN POLYMERS

niques also provide more reliable quantification c o m p a r e d to secondary i o n mass spectrometry ( S I M S ) .


Postionization methods have b e e n used to study a material systems. W h e n a surface is b o m b a r d e d by ( F i g u r e 1), the y i e l d for secondary neutral emission magnitude higher than the y i e l d for secondary i o n

variety of interesting a primary ion beam is usually orders o f emission (5). L a s e r

ionization o f desorbed neutrals i n the gas phase above the sample surface is the basis for the S A L I postionization technique. D e c o u p l i n g the sputtering process f r o m the ionization process a n d i o n i z i n g the dominant neutral species above the sample surface avoids the variations i n ionization probabilities ("matrix effects") that often plague S I M S measurements. Laser-based photoionization techniques fall into two categories: nonresonant-enhanced ionization a n d resonant-enhanced ionization. T h e use o f n o n resonant-enhanced ionization ( S A L I ) is the most p r o m i s i n g m e t h o d w h e n quantification a n d general applicability to a variety o f material systems are considered.

Surface Analysis by Laser Ionization S A L I is a three-step process that involves a p r o b e b e a m , photoionization, a n d mass analysis. F i g u r e 2 depicts the S A L I process. U n d e r v a c u u m , the surface of interest is irradiated b y a p u l s e d p r o b e b e a m o f ions, electrons, or photons.

SAMPLE Figure 1. Sputter-induced formation of ions and neutral particles.


31.

DAISER AND MACKAY

729

Surface Analysis by Laser Ionization


Time - of - Flight (TOF) Mass Analyser

TOF Extraction Lens Photoionized Neutrals

Laser Beam

Ion Beam Sputtered Neutrals

manmm Figure 2. Technique description of the SALI process.

T h i s irradiation causes the desorption o f a small amount of surface material. T h e neutral species released i n this process are nonresonantly i o n i z e d b y a high-intensity p u l s e d ultraviolet laser passing i n close proximity to the sample surface. T h e resulting photoions are then accelerated, focused, a n d allowed to drift i n a field-free region for time-of-flight ( T O F ) mass analysis.

Desorption Beam. Surface removal for sampling involves the move ment o f atoms a n d molecules f r o m the top surface layer into the vapor phase. T h e fact that the ionization step is d e c o u p l e d f r o m the surface-removal step implies a great deal of flexibihty a n d control i n the type a n d conditions of the energetic b e a m chosen to stimulate desorption. F o r elemental analysis of


730


inorganic materials, typically a 5 0 - 1 0 0 - μ π ι A r o r C s , o r a submicrometer diameter G a b e a m at several kiloelectronvolts is used to enable submonolayer o r "static" analysis b y p u l s i n g the b e a m a n d k e e p i n g t h e total dose extremely l o w ( < 1 Χ 1 0 i o n s / c m ) . T h e small-spot G a b e a m is w e l l suited to quantitative c h e m i c a l m a p p i n g w i t h submicrometer spatial resolu tion. F o r b u l k polymers or t h i n p o l y m e r films, energetic electrons o r another laser b e a m sometimes result i n superior mass spectra relative to p u l s e d i o n - b e a m sputtering. E v e n t h e r m a l desorption can b e used to investigate the temperature dependence o f thermally sensitive samples. +

+

+


1 3

2

Nonresonant Photoionization.

+

T h e two forms o f nonresonant (and

therefore nonselective) photoionization that are used for S A L I are illustrated in Figure 3. F o r elemental analysis, a p o w e r f u l p u l s e d laser that delivers focused p o w e r sensitivities greater than 1 0 W / c m is used for m u l t i p h o t o n ioniza tion ( M P I ) . T y p i c a l l y all the species w i t h i n the laser focus v o l u m e are i o n i z e d without t h e n e e d f o r wavelength t u n i n g and regardless o f c h e m i c a l type. 1 0

2

Vacuum Level

Ionization Potential

Ground State

Non-Resonant Multi-Photon Ionization Excimer Laser KrF: E = 5.0eV ArF: E = 6.4eV

Non-Resonant Single Photon Ionization VUV Laser 118 nm: E = 10.5eV

Figure 3. SALI ionization process.



31.

DAISER AND MACKAY


731

Nonresonant photoionization yields the desired u n i f o r m i t y of detection p r o b ability essential for quantification. F o r molecular analysis, a "soft" (i.e., olet ( V U V ) hght w i t h wavelengths i n the range of 1 1 5 - 1 2 0 n m (10.5 eV). Photoionization at this wavelength is achieved b y single-photon ionization (SPI). Because relative photoionization cross-sections for molecules do not vary greatly i n this wavelength region, semiquantitative raw data result. Improvement i n quantification for b o t h photoionization methods is achieved w i t h calibration. S a m p l i n g the majority neutral channel means m u c h less stringent requirements for calibrants than for direct i o n p r o d u c t i o n f r o m surfaces b y energetic particles. Relaxed calibrant requirements are especially important for the analysis o f u n k n o w n b u l k p o l y m e r materials. A detailed description o f the S P I - S A L I process is given b y Pallix et al. (6).

Time-of-Flight Mass Spectrometric Analysis. T w o features of the T O F mass spectrometer make it an ideal choice for a p u l s e d ionization measurement. First, there is an inherent multiplex advantage i n that all masses arrive at the detector for each laser pulse. T h i s advantage is very valuable f r o m a sensitivity point o f v i e w w h e n unknowns may be present. T h e second m a i n feature is that T O F mass spectrometers have very h i g h effi ciencies; transmission frequently is i n the range of 0.3 to 0.7. T h e T O F mass spectrometer used i n this study incorporates a special electrostatic m i r r o r called a reflectron that acts as an energy-focusing device to achieve h i g h mass resolution (7). T h e reflectron works because the higher kinetic energy ions penetrate deeper into the reflecting electrostatic field a n d thus take longer to t u r n around. T h e shorter drift t i m e o f the faster particles is closely matched b y t u n i n g the instrument to the longer t u r n a r o u n d time required. T h e instrument is set so that fast a n d slow ions o f the same mass arrive almost simultaneously at the detector. A n o t h e r feature of the reflectron-based analyzer is that the reflectron electrostatically discriminate against secondary ions that are f o r m e d at the sample surface a n d thereby eliminate a potential source o f background. Secondary ions are f o r m e d i n a region o f higher potential and have higher kinetic energies than the photoions that are f o r m e d i n the laser focal v o l u m e above the surface. T h e reflecting g r i d is adjusted to an intermediate potential that reflects the desired photoions a n d does not reflect the secondary ions. A n o t h e r advantage o f the reflectron analyzer is that w i t h the laser off, T O F - S I M S measurements, w h i c h allow for mass resolutions > 10, 000, can be taken.

Modes of Analysis S A L I applies to two methods o f postionization: m u l t i p h o t o n and single-pho ton ionization ( M P I a n d SPI). E a c h m e t h o d can be u t i l i z e d i n one o f the


732



three modes o f analysis: survey analysis, d e p t h profiling, a n d c h e m i c a l m a p p i n g . B o t h the survey a n d m a p p i n g modes are used i n p o l y m e r material characterization. Survey spectra using the S P I m o d e are used p r i m a r i l y for m o n o m e r identification i n a n d modifications to p o l y m e r films as w e l l as b u l k p o l y m e r materials. F u r t h e r m o r e , S P I survey spectra can be used for the quantification of surface components i n inorganic and organic materials that have a typical parts per m i l l i o n detection l i m i t (8). T h e extremely h i g h sensitivity o f the S P I m o d e i n application to organic materials is one o f the most p r o m i s i n g features o f S A L I . I n certain cases, such as t h i n films o f polycyclic 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin a n d 7-methylguanin, attomole sensitivities ( < 1 0 ~ m o l or 1 0 ~ monolayer over 1 m m ) have b e e n f o u n d (9). 1 5

6

2

S A L I m a p p i n g is a sensitive a n d quantitative m e t h o d for characterization o f the spatial distribution o f elements i n b o t h organic a n d inorganic materials. S A L I m a p p i n g is currently l i m i t e d to elemental (rather than molecular) analysis because it is not practical to use S P I i n this m o d e . This hmitation is due to the l o w laser repetition rate ( 1 0 - 5 0 H z ) o f the N d : Y A G lasers used for S P I analysis a n d the fact that useful yields for S P I - S A L I are typically 1 0 ~ . A t this l o w duty cycle, only neutral species w i t h h i g h useful yields ( > 1 0 ~ ) can be detected w i t h reasonable sensitivity i n a practical time frame for a 128 X 128 map. 5

2

Polymer Applications Polymer Films. T h e first example is a t h i n film (a f e w monolayers) o f polyethylene glycol ( P E G ) that was dissolved i n methanol a n d cast onto a silicon wafer. T h e S P I - S A L I analysis u s e d i o n sputtering to desorb the p o l y m e r material. T h e p r i m a r y b e a m conditions were as follows: 7 - k V A r , Photoionization b y S P I was achieved using 118-nm light. T h e laser pulse w i d t h p r o d u c e d was ~ 8 ns, a n d the spectra were r e c o r d e d using 10-ns time-resolution steps. T h e laser repetition rate was 10 H z , w h i c h y i e l d e d an average analysis time o f 1 m i n . N o charge compensation was n e e d e d for this analysis. +

T h e dominant feature i n the S P I - S A L I spectrum o f the P E G film ( F i g u r e 4) is the m o n o m e r peak at m/z = 44 ( M = — C H = C H — Ο 2

I n addition to the strong m o n o m e r peak, there are peaks at m/z 132 that correspond to the d i m e r a n d trimer, respectively.

2

= 88 a n d

I n comparison, F i g u r e 5 shows the T O F - S I M S spectrum of an identical sample over the same mass range. U n l i k e the S A L I spectrum, the T O F - S I M S spectrum shows only a small m o n o m e r peak w i t h an intense M + H peak at a mass-to-charge ratio m/z — 45. T h i s result seems to indicate that the more characteristic m o n o m e r peak preferably sputters as a neutral species. T h e soft +



20

Figure 4. SPISALI

LI 60

80 MASS/CHARGE,

IIL—.JI.I.

2

m/e

J

Bfl

"2

2

M = 44

2

n

+

100

CCH - C H - 0)

120

., 132

«3*

140

spectrum of a thin polyethykne glycol (PEG) film on Si acquired with 500 pulses of 118-nm VUV postionization radiation, 5 E12 ions/cm Ar probe beam.

40

II* 44



gure 5. TOF-SIMS

20

60

80

2

Mass/Charge (m/z]

100

spectrum of a thin polyethylene glycol (PEG) film on Si acquired with 200, 000 pulses of Cs ~ ion source, 1.6 Ε10 ions /cm .

40



31.

DAISER AND MACKAY

735


ionization o f the S P I process is evident i n the h i g h intensities o f the d i m e r and t r i m e r ions relative to the m o n o m e r i o n i n the S A L I spectrum. T o complete the comparison, F i g u r e 6 displays the static S I M S spectrum o f the same p o l y m e r film taken o n a quadrupole S I M S instrument. T h e static S I M S spectrum shows similarities w i t h the T O F - S I M S spectrum i n that the intensity o f the m o n o m e r peak is also very weak a n d the fragmentation pattern is similar. T h e experimental conditions i n T O F - S I M S a n d static S I M S were similar to S P I - S A L I except that there was n o laser for postioniza tion (real secondary ions w e r e analyzed). I n the T O F - S I M S mode, the i o n g u n was p u l s e d w i t h a very short pulse w i d t h o f 1.0 ns, whereas i n the static S I M S , the i o n g u n was operated i n the dc m o d e w i t h an i o n dose o f 5 Χ 1 0 i o n s / c m . T h e fully interpreted static S I M S spectra, i n c l u d i n g individual peak assignments, c a n b e f o u n d i n Static SIMS Handbook of Polymer Analysis (10).

1 2

2

T h e ease o f m o n o m e r identification a n d the relatively clean spectrum i n S A L I are the result o f the soft ionization p r o d u c e d b y using S P I . T h e result o f this type o f ionization process is that there is fragmentation f r o m b o t h the desorption step a n d the ionization step. T h e contribution o f the ionization step i n many cases is m i n i m a l . T h e superiority o f S P I - S A L I over conventional techniques like T O F - S I M S or static S I M S i n terms o f fragmentation a n d ease o f m o n o m e r identification has already b e e n measured a n d c o n f i r m e d o n a variety o f different polymers, such as polystyrene (6), poly(tetrafluoroethylene) (Teflon) (11) , a n d poly (methyl methaerylate) ( P M M A ) (6) as w e l l as p o l y m e r blends (12).

Bulk Polymers. T h e first example is a 1-mm-thick film o f p o l y ( d i methylsiloxane) ( P D M S ; silicon rubber sheet) w i t h an average molecular weight between cross-links o f 5726. T h e S A L I experimental conditions were 7 - k V A r , 2 - μ Α d c current, 5 - μ 8 pulse w i d t h , a n d 5 X 1 0 ions/cm . +

1 2

2

Charge compensation for the P D M S sample was p e r f o r m e d using a p u l s e d neutralizer-extractor assembly i n w h i c h 12-eV electrons were used to flood the sample surface. F i g u r e 7 is the S P I - S A L I spectrum for a thick film o f P D M S . T h e dominant peak i n the low-mass range is again the m o n o m e r at m/z = 74 [ M = S i ( C H ) 0 ] . A second scan, taken to extend the mass range, is shown i n F i g u r e 8. T h e observed distribution is a series o f peaks at mass intervals o f 74 ( S i C H 0 ) w i t h a n average weight o f 355 mass units. T h e presence o f a lower molecular weight species o n the surface, w h i c h c o u l d be either a thermal degradation product or a non-cross-linked l o w molecular weight siloxane species, is indicated. F i g u r e 9 is an expanded region o f the higher mass range spectrum that includes the ( M ) + S i C H 0 peak at m/z = 355. T h e mass resolution, defined as Μ / Δ M , where Δ M is the f u l l w i d t h at half m a x i m u m , is approximately 1500 at mass 355. 3

2

3

4

3

+



10

10

15

20

31

SO

λί\ί\

27 60

Mass, amu

, Jvl|

40

43

45

βΟ

59 70

73 βΟ

+

Λ

2

Figure 6. Static SIMS spectrum of a thin polyethylene glycol (PEG) film on Si acquired using 4-kF Xe ions/cm .

τ


dc, 5 E12

90

89

100

tu

ζ

ο

H Ο

id M

ο

I

tMa

a

Ω Η

1Λ

Η

in


"

5

40

Figure 7. SPISALI

1

2

3

4

-

6

7

8

9

10

80 Mass/Charge (n/z)

ι il* 11, 11»!it

M

2

100

1

+

1-

CH3

(-Si-O-)n

CH3

Atii

120

11=74

spectrum of a 1-mm-thick poly (dimethylsiloxane) (PDMS) film acquired with 100 pulses of 118-nm VUV postionization radiation, 1 E12 ions/cm Ar probe beam.

60

H* 74


3

es*

g

ο

M

d


t

t

-

0

300

400

-74-

n=4 355

n=5 429

n=7 577

500 Mass/Charge (n/z)

n=6 503

1—

n=8 651

spectrum of PDMS.

600

(M)n + SiCH30 11=74

—ι

Figure 8. Extended mass range of the SPI-SALI

-iiililiLliilili.il I

1 --

2

fn=3 3 281

4

5

6

7

8

9

10

700


800

50

ο

δ

I

Ο

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C Ω

C/3

00

-^1


-·

348

^

35Θ

1

1

352

1

M = 355

1

1

1 3»

Μ/ΔΜ = 1480

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'Z α

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740


A n o t h e r example o f the S P I - S A L I analysis f o r b u l k p o l y m e r materials is a thick piece o f polystyrene. F i g u r e 10 shows the i o n - i n d u c e d S P I - S A L I spectrum o f the polystyrene sample. I o n b o m b a r d m e n t was p e r f o r m e d using 5-keV A r

+

at a total i o n dosage o f 7 Χ 1 0

1 1

i o n s / c m . T h e S A L I spectrum 2

again has an intense m o n o m e r peak at m/z

— 104 ( M = C H ) , u n l i k e 8

8

T O F - S I M S o r quadrupole static S I M S spectra o f the same material (13). O t h e r characteristic polystyrene peaks are observed i n the S A L I spectrum at


m/z

= 115 ( C H ) , 91 ( C H ) , a n d 78 ( C H ) . T h e dominance o f the 9

7

7

7

6

6

m o n o m e r peak is even more p r o n o u n c e d w h e n the i o n b e a m (used f o r desorption) is replaced b y a laser b e a m . F i g u r e 11 shows the laser-induced S P I - S A L I spectrum o f the same sample. T h i s spectrum shows m i n i m a l damage m/z

to the p o l y m e r . T h e p r e v a i l i n g peak is the m o n o m e r peak at

= 104 ( C H ) . F o r laser-induced desorption, the t h i r d harmonic o f a 8

8

standard N d : Y A G laser at 355 n m was focused onto the surface. T h e S A L I postionization m o d e u s i n g the 118-nm line was the same as i n the i o n i n d u c e d S A L I experiments. T h e r e is clear evidence that p h o t o n - i n d u c e d desorption, c o u p l e d w i t h subsequent p h o t o n postionization, further reduces the p o l y m e r sample damage caused b y i o n - i n d u c e d desorption S A L I . D a m age is r e d u c e d because, i n contrast to i o n sputtering, there is n o nuclear m o m e n t u m transfer involved. H o w e v e r , w h e n a laser is used as the p r o b e beam, S A L I is no longer a surface-specific technique.

Future Directions Recent progress i n o u r laboratory has i m p r o v e d the resolution to the T O F analyzer a n d s h o u l d increase the mass resolution achievable b y S P I - S A L I . F u r t h e r improvements c o u l d result f r o m using the ultrashort pulse m o d e o f the N d r Y A G laser, w h i c h c a n p r o d u c e laser pulse widths o f 2.5 ns. T h e increase i n mass resolution along w i t h the ease of analysis w i l l establish S P I - S A L I as a p o w e r f u l tool for surface analysis o f insulating materials. T h e analytical versatility o f this technique w i l l b e exploited i n studies using the various desorption methods ( i o n sputtering versus electron-stimulated d e sorption versus laser desorption). T h e ability to analyze the surface o f thick insulating materials w i t h m i n i m a l sample preparation c o u p l e d w i t h the ease o f m o n o m e r identification makes this an extremely attractive technique for p o l y m e r surface characterization. F u t u r e study w i l l involve the quantification aspects o f S P I - S A L I a p p l i e d to m o d i f i e d p o l y m e r surfaces a n d c o p o l y m e r systems.



40

80

80 Mass/Charge ( η / ζ )

2

+

Figure 10. Ion-induced SPI-SALI spectrum of bulk polystyrene acquired using 100 pulses of 118-nm VUV postionization radiation, 1 E12 ions/cm Ar probe beam.

20



3

100

150 Mass/Charge, m/z

Figure 11. Laser-induced SPISALI spectrum of bulk polystyrene acquired using 100 pulses of 118-nm VUV postionization radiation and 100 pulses of 355-nm VUV probe beam.

's


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DAISER AND MACKAY


743

Acknowledgments T h e authors thank D r . D a v i d G . W e l k i e o f the Physical E l e c t r o n i c s D i v i s i o n for his assistance i n the interpretation o f the data. T h e authors also acknowl edge the cooperation o f D r . C h r i s t o p h e r H . B e c k e r o f S R I International, w h o initiated the S A L I technology.


References 1. Gruen, D. M.; Pellen, M. T.; Calaway, W. F.; Young, C. E. Laser Post-Ionization Sputtered Neutral Mass Spectroscopy; Benninghoven, Α.; Huber, A. M.; Werner, H. W., Eds.; John Wiley and Sons: Chichester, England, 1988; p 789. 2. Becker, C. H.; Gillen, Κ. T. Anal. Chem. 1984, 56, 1671. 3. Parks, J. E.; Schmitt, H . W.; Hurst, G. S.; Fairbanks, W. M., Jr. Thin Films 1983, 8, 69. 4. Pellin, J. J.; Young, C. E.; Calaway, W. F.; Gruen, D. M . Surf. Sci. 1984, 144, 619. 5. Storms, Η. Α.; Brown, K. F.; Stein, J. D. Anal. Chem. 1977, 49, 2023. 6. Pallix, J. B.; Schüle, W.; Becker, C. H.; Huestis, D. L. Anal. Chem. 1989, 61, 807. 7. Mamyrin, Β. Α.; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. SOV. Phys. JETP (Engl. Transl.) 1973, 37, 45. 8. Becker, C. H.; Gillen, Κ. T. J. Vac. Sci. Technol., A 1985, 3(3), 1347. 9. Schüle, U.; Pallix, J. B.; Becker, C. H . J. Am. Chem. Soc. 1988, 110, 2323. 10. Carlson, Β. Α.; Katz, W.; Newman, J. G.; Van Ooji, W. J.; Moulder, J. F. Static SIMS Handbook of Polymer Analysis; Perkin-Elmer, Physical Electronics: Min neapolis, MN, 1991; p 38. 11. Schüle, W.; Pallix, J. B.; Becker, C. H . J. Vac. Sci. Technol., A 1988, 6(3), 936. 12. Pallix, J. B., Schüle, U.; Becker, C. H . Surface Analysis of Bulk Organic Polymers by Single-Photon Ionization; Benninghoven, Α.; Evans, C., Eds.; John Wiley and Sons: New York, 1990; p 207. 13. Carlson, Β. Α.; Katz, W.; Newman, J. G.; Van Ooji, W. J.; Moulder, J. F. Static SIMS Handbook of Polymer Analysis; Perkin-Elmer, Physical Electronics: Min neapolis, MN, 1991; p 26. RECEIVED for review July 15, 1991. ACCEPTED revised manuscript September 1, 1992.


Surface Analysis by Laser Ionization Applied to Polymeric Material

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