Surface MS: probing real-world samples - Analytical Chemistry (ACS...
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Surface MS: Probing Real=WorldSamples w
Alfred Benninghoven, Birgit Hagenhoff, and Ewald Niehuis
Physikalisches lnstitut UniversitAt Munster Willhelm-Klemm-Str 10 D-W 4400 Munster, Germany
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I The demands of expanding technological fields often are a driving force for the development of analytical techniques. For example, because the microelectronic industry needed quantitative elemental information with high lateral resolution, it supported the development and application of Auger electron spectroscopy ( 1 , 2).Surface phenomena such as adhesion, friction, corrosion, adsorption, wettability, and bioeompatibility become increasingly important in such diverse technological areas as catalysis, microelectronics, clinical analysis, polymer development, and environmental sciences (3,4). Because these surface phenomena are governed by the molecular structure of the uppermost monolayers of a solid‘s surface, it is not enough to obtain elemental information. An analytical tool with high sensitivity and lateral resolution that can provide detailed molecular information about surface structures is required. It is also an indispensable prerequisite for the controlled modification of the “molecular architecture” of the surface. Because most materials used in technological applications are organic in origin, the following techniques are important in this regard: X-ray photoelectron spectroscopy (XPS),surface MS, and the compara630 A
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tively new technique of scanning tunneling microscopy (STM) and its related counterpart, atomic force microscopy (AFM). These methods must be evaluated with respect to t h e general analytical question, “What is the nature, concentration, and location of all atomic and molecular species present in the surface region?” To answer this question, the analyst must consider the unambiguous identification of unknown surface species (atoms as well as molecules
ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15,1993
or molecular clusters); the quantification of these surface species (i.e., information on the relative or absolute coverage by these species); and the location of these surface species (i.e., information on their lateral and depth distributions). The identification should be universal; it should be possible to identify all types of elements and molecular species and to distinguish between isotopes. Quantification should he possible, even for very low surface mncentrations, and locating species should be possiOW3-2700/93/W65-630A/$04.00/0
CJ1533 American Chemical society
ble with high lateral and depth resolution. In addition, all types of materials and all sample shapes should be accessible, including insulators. In XPS, information on the elemental composition of the uppermost atomic layers is obtained with sensitivities down to 0.1% of a monolayer. Important information on the chemical environment of the identified elements is supplied by the chemical shift (i.e., the influence of the chemical environment on the exact energy of the emitted photoelectrons). However, identification of unknown complex molecules by this chemical shift is not possible, and the achievable lateral resolution is limited to a few micrometers. STM and AFM allow lateral resolution in the sub-nanometer range so that single atoms can be probed. However, these techniques cannot be used to identify unknown surface species. The most important feature of surface MS, in addition to high sensitivity, is its ability to provide detailed molecular information and information at shallow depths. It allows, in principle, t h e identification a n d quantification of all elements, isotopes, and molecular species. Therefore, surface MS should be an excellent means for surface analysis, provided the following are addressed controlled desorption of atoms and molecular species, efficient ionization of these desorbed particles, and unambiguous identification of the generated ions by their chargelmass ratios. A considerable fraction of molecular surface species should survive these processes without fragmentation. In the past decade, static secondary ion MS (static SIMS) and laser secondary neutral MS (laser SNMS) have proved to be well suited for elemental and molecular applications, and they should be described in more detail. In SIMS, surface species are desorbed by keV particle bombardment (5).Ionization of these particles occurs during their desorption by intrinsic processes. In principle, SIMS is a destructive technique because it is based on the removal of particles originating from t h e uppermost monolayer of the surface. By tuning and focusing the primary ion current, a very controlled surface depletion in the submonolayer and in the multilayer range is possible for all types of materials. To get information on the original surface composition of radiation-sensitive molecular surfaces such as polymers, only a very small fraction up to 1%of the uppermost monolayer should be con-
sumed, a s in static SIMS. Such a controlled surface depletion cannot be achieved by laser excitation of the surface for a variety of reasons, including focus diameter, excitation depth, and reproducibility in laser desorption experiments. Therefore, in this REPORT, we concentrate on particle-induced desorption by static SIMS and laser SNMS. For sensitivity well i n the submonolayer range under static SIMS conditions, efficient use of all emitted secondary particles is required. Here, time-of-flight (TOF) mass analyzers are uniquely suited to offer extremely high transmission in combination with parallel detection of all masses. In addition to high sensitivity, TOF-SIMS offers unlimited mass range, high mass resolution, short overall flight times, and accurate mass determination-considerable advantages compared with the capabilities of SIMS instruments equipped with magnetic sector fields, quadrupoles, and ion cyclotron resonance analyzers. The combination of ion bombardment-induced particle desorption and TOFMS has made TOF-SIMS a most useful surface analytical technique. Recently, laser SNMS has shown considerable analytical potential. The modular design of a TOF instrument makes it verv flexible
Figure 1. Particle keV energy.
with respect to primary ion source, sample stage, or laser postionization, or in combination with other surface analytical techniques such as XPS or ...~ . Auger electron spectroscopy. Principles of sputtering and ion :.:’.,: . . . . formation SIMS and SNMS are based on the primary particle-induced emission (with energies on the order of several keV) of secondary particles characteristic of the chemical composition a n d s t r u c t u r e i n t h e uppermost monolayer (Figure 1). What is unexpected is that the technique can also be applied to the analysis of nonvolatile and thermally labile organic materials. The major portions of secondary particles a r e emitted as neutrals, whereas only a fraction of - 10-6-10-’ of the total are positively or negatively charged (secondary ions). At present, the desorption of secondary particles from elemental targets because of the interaction between the primary particle (mostly ions) and target atoms is fairly well understood (6, 7).However, an explanation of the emission of charged particles and, i n particular, t h e emission of molecular species is still being sought. Briefly, the penetration of the primary ion leads to the formation of a “collision cascade’’ in the target, con-
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ANALYTICAL CHEMISTRY, VOL. 65. NO 14, JULY 15,1993
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REPORT sisting of a cloud of target particles set in motion by t h e primary ion (primary recoil particles) or by target particles t h a t are already moving (secondary recoil particles). The dimension of the collision cascade depends on the energy and mass of the primary ion as well as the density and structure of the target material. For the bombardment of organic materials with 10-keV Xe', typical values are 3 nm for the diameter of the collision cascade, and 15 nm for the depth of the collision cascade, according ta model calculations (8). Most recoil particles have low energies, and only cascade particles from near-surface regions-with momenta directed toward the surface-can overcome t h e surface binding energy and thus leave the target (sputtered particles). SIMS and SNMS, therefore, are very surface-sensitive analytical techniques (information depth is < 3 monolay ers). The charge state of the sput tered particles depends largely o the chemical environment in the uppermost monolayer (matrix effect), which in general prevents the direct quantification of SIMS results. The flux of emitted neutrals is almost unaffected by the composition of the matrix, and SNMS can be used to obtain quantitative information about the surface composition if a n effective postionization mechanism can be provided (e.g., nonresonant or resonant laser postionization, which will be discussed later). Both secondary ions a n d secondary neutrals show characteristic energy and angular distribution. The maximum of the energy distribution for elements is - 5 eV, with a tail toward higher energies. For molecular species it resembles a Boltzmann distribution with a maximum of 2 eV. The angular distribution for atoms and neutrals follows a cosine law (5). Ions originating from an elemental matrix can be positively or negatively charged, depending on their electron configurations in the outermost shell. The highest secondary ion yields (i.e., number of secondary ions Xq emitted from a surface species M per number of primary ions) of molecular ions are achieved from monolayers on noble metal s u b strates. Typical secondary ions are Me", (M + HI+, (M - HT,(M + Sa)', and (M + Me)', where Me is a metal, M is a molecule, H is hydrogen, and S a is either Na or K (9, 10). From bulk materials and thick layers, typically (M t HI* and larger fragments can be obtained. To elucidate the fragmentation process, fragmenta-
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Figure 2. TOF-SIMS instrument with laser postionization and charge compensation capabilities.
tion rules that are !mown from electmn impact MS can be applied (a-, @cleavages; rearrangement processes). Principles of TOFMS Because t h e principles of TOFMS were recently reviewed in this JOURNAL ( I l l , we wish to concentrate only on t h e key features of TOFSIMS (Figure 2). Mass determination in TOFMS is achieved by meas u r i n g t h e flight time t of t h e desorbed secondary ions in a drift path of known length I after accelerating them in an extraction field to a common energy E of some keV. The relationship between energy a n d flight time is
1 E =-m I '/t 2
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This equation shows that the flight time is proportional to the square root of the mass of a secondary ion. For a n accurate determination of flight times, the s t a r t and arrival times of the secondary ion must be well defined. To meet these requirements, either the primary ion beam or the extraction field is pulsed and the arrival time of the secondary ion is determined by a fast detection system. In a typical cycle, 0.1-10 secondary ions are detected. A spectrum with a dynamic range of several orders of magnitude is obtained by the accumulation of a large
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number of cycles with high repetition rates (typically 5-20 kHz), depending on the mass of the largest ion of interest (12).Energy and angular focusing elements in the flight path of the secondary ion guarantee compensation for an energy and angular spread of the secondary ions caused by the desorption process. Because all secondary ions a r e detected quasi-simultaneously and t h e instrumental transmission is high, the achievable sensitivity of TOF analyzers is several orders of magnitude higher than that of quadrupoles or magnetic sector i n s t r u m e n t s in which mass scans are necessary for accumulating a complete spectrum. This high sensitivity makes TOFSIMS suitable for analysis of extremely small volumes and materials that yield small amounts of ions. The mass resolution in TOF-SIMS, which is well above 10,000, depends on t h e time-focusing properties of the analyzer, the pulse width of the primary beam, and the time resolution of the registration electronics (13).The strict and simple relationship between mass and flight time t h a t is valid over t h e whole mass range makes accurate mass determination straightforward. The mass range in TOF-SIMS increases with the recorded time range and can be extended without limitations. Therefore, the highest masses observed are . ., determined by desorption. .,.. . :. .
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Figure 3. Positive secondary ion spectrum (a) and mass-resolved ion images (b) of a monolayer of a solution of CsA and CsD.
Sample charging caused by primary ion bombardment a n d t h e emission of charged particles can be avoided by flooding the surface with low-energy electrons during the drift time of the secondary ions. A wellstabilized self-adjusting surface potential is then achieved in the positive and negative SIMS modes and facilitates the analysis of all types of insulating materials (14). The high sensitivity of TOF-SIMS and the small size of the collision cascade qualify the technique for surface and trace analysis with high lateral resolution. In an ion microprobe the primary beam is focused to a small spot and rastered across the surface. For every pixel of the digital raster a complete spectrum is acquired, resulting in maps for all masses of interest. A lateral resolution better than 100 nm can be obtained with a pulsed Ga liquid metal ion gun (LMIG) (15).In an ion microscope a large area on the sample is illuminated by the pulsed primary beam. Through stigmatic focusing of the secondary ion optics, a magnified secondary ion image is produced on the detector. Position and flight time of the secondary ions are determined by a channelplate detector and a resistive anode encoder. A lateral resolution of 1-3 w can be obtained (16). The majority of the emitted particles are neutrals, and MS of these particles beeomes possible only by postionization (17-19). The most efficient method of postionization is multiphoton ionization in either a resonant or a nonresonant mode with a pulsed laser beam. After desorption by the primary ion pulse, the laser beam is fired into the cloud of sputtered neutrals above the surface. Generated photoions (hundreds up to several thousand per pulse, depending on the primary beam current and pulse width) are extracted from t h e ionization volume by a pulsed extraction field. Because of the desorption process, the size of the ionization volume, and the timeresolving properties of the detector, mass resolution of only 3500 can be achieved. In an ion microprobe, laser SNMS can be combined with a high lateral resolution by using a Ga LMIG. Laser SNMS imaging is not possible in the microscope mode because of the lateral dispersion of the sputtered neutrals.
(a) CSDserves as the internal standard. The solution is 500 nglmL CsA and 400 nglmL CsD in ethanol; 1 NL of the solution (submonolayer preparation) is applied to 0.5 cm* 01 an etched Ag target. (b)The masresolved ion images am also on Ag. In all images, a linear thermal color a l e is used to represent dillerem ion intensities (yellow is high intensity: dark red is low intensity). The ten below each image refers to mass (m, in Da). t y p 01 secnndary ion. number of counts in the pixel with highest intensity. and total number of counts. The overall intensities am normalized to the pixel with highest intensity in the respective image. Field of view is 4W x 4W Nm2. SO 1 and soi 2 are the oils used for the matrix.
Analytical applications Static SIMS initially was achieved by using magnetic sector and quadrupole mass spectrometers (20).Applications were limited by mass
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REPORI range, mass resolution, and sensitivity of these instruments. Recent developments in TOFMS have resulted in substantial improvements in these features, and TOF-SIMS is applied in virtually all fields of science and technology where solid surfaces and their behavior are important. Over the past decade we have analyzed tens of thousands of samples covering a wide variety of materials related to surface phenomena that are important t o or essential for
many technologies. A close interaction between instrumental development and analytical problems introduced by industrial and academic partners has continuously opened new fields of application. From this material we have selected real-world samples that illustrate the type of information supplied by TOF-SIMS and laser SNMS. I d e n t i f i c a t i o n of biomolecules.
The ability of TOF-SIMS to identify and locate the smallest amounts of
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labile organic compounds is demonstrated in Figure 3. Figure 3a shows the positive secondary ion spectrum of a mixture of cyclosporin A (CsA), a n important immunosuppressive drug used in transplant surgery, and its physiologically ineffective derivative, cyclosporin D (CsD). Cyclosporines are cyclic peptides consisting of 11partially modified amino acid residues. CsD contains a -CH(CH,), group instead of the -CH2-CH, contained in CsA (21). In the spectrum CsA and CsD can be identified by their (Cs + Ag)+ peaks. Metal cationization is a common feature of almost all organic secondary ions and occurs for molecules prepared as a monolayer on noble metal substrates. The respective peak shapes mirror the isotopic distributions. Furthermore, (Cs + HI' and (Cs + Na)+ peaks can be detected. Although the surface coverage of CsA exceeds that of CsD, the (CsD + Ag)' peak is slightly more intense than the (CsA + Ag)' peak-an indication of the matrix effect that must be considered in SIMS experiments. Nevertheless, quantification of CsA can be achieved with an accuracy better than 15%by using CsD as an internal standard (i.e., in every CsA quantitation, the result of a single TOF-SIMS m e a s u r e m e n t a t worst has a deviation of 15%from the true value). Detection limits are below 50 ng/mL for CsA (22,23). F i g u r e 3b shows four m a s s resolved secondary ion images of CsA dissolved in a mixture of oils suitable for oral application to patients. AU images show the edge of a droplet of the oily solution deposited on Ag. The bare Ag is located in the left part, and the CsA covers t h e right part of each image. When the solution spreads, the oils used to form the matrix concentrate at the boundaries (lower images). This example shows that imaging of organic molecules is possible even in t h e higher mass range. Note, however, that for organic imaging the useful lateral resolution is limited to 1pm because of t h e generally low ion yields of organic molecules (12). Static SIMS requires high-transmission mass spectrometers. If, however, the analyte molecules are deposited in a liquid matrix, higher secondary ion currents can be generated and double-focusing magnetic sector field instruments can be used for their analysis. Liquid SIMS has been given the name fast-atom bombardment (241, with the emphasis on the relatively unimportant aspect of using neutral primary particles
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Flgure 4. Positive secondary ion spectrum of a monolayer of the perfluorinated polyether Krytox on a Ag substrate. Tne OllQOmer Sene8 are as lollows 1. (FRC,F,J': 2: (0,.CF, As)'. 3 (0"C,F, + Agr 4 (0, t Ag,' 5 I O. - C3F6 + AQ)': 6: 10. C,F. + Ag).. 7: (0, - CF, + Agr Lower panlon Shows
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634 A * ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15,1993
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rather t h a n on the key feature of preparing samples in liquid reservoirs where the samDle molecules are mobile. Characterization of Dolvmers. The following two exampies Focus on the application of TOF-SIMS for the characterization of polymer materials (10, 25-30~.The upper part of Figure 4 shows the positive secondary ion spectrum of a monolayer of the perfluorinated polyether known as Krytox (manufactured by W o n t ) on a Ag substrate. Intact oligomers
O.(F-(CF(CF,)-CF,-OkCF,-CF,) appear as Ag-cationized species (0.+ AgJ' ( p e a k 4 ) a t m a s s e s > 4000 Da. The peak distribution mirrors the molecular weight distribution of Krytox oligomers. The mass of the polymer repeat unit R can be determined from t h e mass distance between two oligome peaks. The exact mass of an oligome peak gives evidence of the mass 0. the two end groups of the polymer. In addition to the oligomer distri. bution, several series of fragment ions (peaks 1 and 5-7) can be observed. The series of peaks 5-7 in the top of Figure 4 are shown enhanced in the lower part of Figure 4. (The 1 in the upper spectrum indicates not only a single peak but also a complete series of fragments starting a t high intensities in the low mass range and decreasing with increasing mass.) Most of them are neutral fragments cationized by Ag. Only fragments of series 1 carry intrinsic charges and can also be detected from thick layers. The mass of the polymer end groups can be derived from a comparison of the exact masses of each fragment series. In general, monolayer preparation of dissolved polymers on noble metal substrates allows the determination, characterization, and identification of repeat units,end groups, oligomer distributions (Mp, M,, M w ) , additives, Contaminants, and the composition of copolymers. Because of the desorption process, t h e highest achievable mass is 15,000 Da. As discussed earlier, most secondary ions with masses above 300 Da appear as species cationized by substrate ions. From thick molecular films,such a s bulk polymers, only ions carrying intrinsic charges can be expected, and these normally have masses below 300 Da. Nevertheless, these so-called fingerprint ions are so characteristic of the analyzed material that extensive information on the chemical structure can be obtained. Figure 5 shows a positive second.
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ary ion spectrum of poly(hydr0xyethylmethacrylate) before and after treatment with gaseous propionylchloride. Before treatment, the most dominant peaks can be assigned to the polymer backbone (69 Da) and the side chain (45 Da). After treatm e n t , t h e 45-Da side chain is quenched and peaks characteristic of the new side chain appear at 29,57, and 101 Da, thus proving the success of the surface modification. The application of TOF-SIMS to the characterization of bulk polymers and thick polymer films allows determination of repeat units, end groups, and additives; monitoring of surface modifications; and identification and localization of contaminants.
An example of the identification and localization of contaminants is given in Figure 6, which shows massseparated ion images of two organic layers a and b prepared by the Langmuir-Blodgett (LB) technique (31). The LB layers consist of a n a m phiphilic poly(methacry1ate) (PMA), one monolayer of which was transferred to a Ag-coated slice of polycarbonate. The positive secondary ion spectrum, characteristic of both samples, indicates that contamination with silicon oil occurred (peaks at 73, 147, 207, 221, and 281 Dah Typical PMA fingerprint peaks are present at 69, 115, 143, and 185 Da. The mass-separated ion images give information about the origins 01
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rather than on the key feature of preparing samples in liquid reservoirs where the sample molecules are mobile. Characterization of polymers. The following two examples foeus on the application of TOF-SIMS for the characterization of polymer materials (IO,25-30). The upper p a r t of Figure 4 shows the positive secondary ion spectrum of a monolayer of the perfluorinated polyether known a s Krytox (manufactured by W o n t ) on a Ag substrate. Intact oligomers O,(F-(CF(CF,)-CF,-O)-CF,-CF,) appear as Ag-cationized species (0.+ Ag)' ( p e a k 4) a t m a s s e s > 4000 Da. The peak distribution mirrors the molecular weight distribution of Krytox oligomers. The mass of the polymer repeat unit R can be determined from t h e mass distance between two oligome peaks. The exact mass of an oligome peak gives evidence of the mass of the two end groups of the polymer. In addition to the oligomer distribution, several series of fragment ions (peaks 1 and 5-7) can be observed. The series of peaks 5-7 in the top of Figure 4 are shown enhanced in the lower part of Figure 4. (The 1 in the upper spectrum indicates not only a single peak but also a complete series of fragments starting a t high intensities in the low mass range and decreasing with increasing mass.) Most of them a r e neutral fragments cationized by Ag. Only fragments of series 1 carry intrinsic charges and can also be detected from thick layers. The mass of the polymer end groups can be derived from a comparison of the exact masses of each fragment series. In general, monolayer preparation of dissolved polymers on noble metal substrates allows the determination, characterization, and identification of repeat units,end groups, oligomer distributions (Mp, M,, M J , additives, contaminants, and the composition of copolymers. Because of the desorption process, t h e h i g h e s t achievable mass is 15,000 Da. As discussed earlier, most secondary ions with masses above 300 Da appear a s species cationized by substrate ions. From thick molecular films, such as bulk polymers, only ions carrying intrinsic charges can be expected, and these normally have masses below 300 Da. Nevertheless, these so-called fingerprin' ions are so Characteristic of the ana lyzed material that extensive information on the chemical structure can be obtained. Figure 5 shows a positive second-
ary ion spectrum of polflhydroxyethylmethacrylate) before and after treatment with gaseous propionylchloride. Before treatment, the most dominant peaks can be assigned to the polymer backbone (69 Da) and the side chain (45 Da). After treatment, t h e 45-Da s i d e c h a i n is quenched and peaks characteristic of the new side chain amear at 29. 57. and 101 Da, thus prosng the success of the surface modification. The application of TOF-SIMS to the characterization of bulk polymers and thick polymer films allows determination of repeat units, end groups, and additives; monitoring of surface modifications; and identification and localization of contaminants.
An example of the identification and localization of contaminants is given in Figure 6, which shows massseparated ion images of two organic layers a and b prepared by the Langmuir-Blodgett (LB) technique (31). The LB layers consist of a n a m phiphilic poly(methacry1ate)(PMA), one monolayer of which was transferred to a &-coated slice of Dolvcarbonate. TKe positive secon'da;y ion spectrum, characteristic of both samples, indicates t h a t contamination with silicon oil occurred (peaks a t 73, 147, 207, 221, and 281 Da). Typical PMA fingerprint peaks are present at 69, 115, 143, and 185 Da. The mass-separated ion images give information about the origins of
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Figure 5. Positive secondary ion fingerprint Spectrum of a bulk sample of poly(hydroxyethylmethacry1ate)before (upper) and after (lower) treatment with gaseous propionylchloride (CH,CH,COCI). ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15,1993
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Results from the analyses of two defects in car paint, samples a and b, are shown in Figure 8. In Figure Sa, the spectra of sample b (the defect itself and a spot remote to the defect) indicate that a perfluorinated polyether (typical fingerprint peaks at masses 12, 31, 50, 69 Da, etc.) is responsible for the defects. In Figure 8b, the images show the transition from the defect area to the unaffected paint of sample a and provide a closer look at the defect area in
sample b. The images clearly show the lateral distribution of fluorinecontaining species in the defect area a t 31, 69, and 169 Da, whereas the paint can be identified by peaks at masses 1, 29, and 47 Da. In contrast to sample a, where the defect resembles a flat crater, the contamination of sample b led to a bubblelike topographic structure. Perfluorinated polyethers are used to lubricate assembly line components and can contaminate the paint bath in the form
of small droplets, leading to the observed crater structures. Surface characterization by laser postionlzatlonof sputtered neutrals Although TOF-SIMS has been a sensitive, versatile surface analytical tool, its application often is limited to qualitative results because of the matrix effect. Given that sputtered neutrals are less affected by the matrix, laser postionization of these species promises information more related to the original surface concentrations. In laser SNMS the detection limits of elements are in the low part-per-million range for the uppermost monolayer (10' atoms/ cm?. The combination of laser SNMS with an ion microprobe allows quantitative elemental mapping with high sensitivity. Compared with scanning Auger microscopy, there are fewer problems caused by microtopography and edge effects. For example, the total neutral image and the mass-resolved images for various elements are shown in the top two rows in Figure 9 for a gold pattern on a GaAs wafer. The images were recorded with a %aenriched LMIG for the desorption and a high-power femtosecond exci. mer laser set at 248 nm for postionization. For every pixel a mass spectrum generated by only one shot was evaluated. Images for all elements were recorded in 23 min with a sample consumption of 4% of the uppermost monolayer. ' The extremely high yield can be seen by the fact that up to 730 photoions were detected with a pulse of only 3700 primary ions. The intensity ratio for Ga and As is close to the stoichiometric value, whereas in SIMS the sensitivities for Ga, As, and Au differ by several orders of magnitude. The "Ga signal in the area of the Au structure results from the "Ga ion bombardment. A few photoions in the pixel with highest intensity are sufficient to show the distribution of Ni atoms in the Au structure. The bottom row of images in Figure 9 shows the detail of a different spot of the same sample, with a lateral resolution of 200 nm. Laser SNMS is not restricted t o elemental analysis; it can also be applied to the characterization of molecular surfaces. Although high power density is most important for efficient ionization of atoms, photodissociation must be taken into account in multiphotoionization of molecules. For a n optimum yield of intact molecular ions and character-
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gure 7. Spectra of Si wafer treated with CHF, plasm: Upper nsgasve seconaary on spec1r.m Lower posilwe SecondaJ on swclmm 01 !ne same wafer aher adaillona LV ozone lnalmenl Peaks are as lo ow6 1 5eFe' 2 Sr, 3 ' O S C O 4 28SCNH2 5 %C,H; 6 "SC,cT,. 7 C,OK 8 C,N% 9 C.%
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istic fragments, it is necessary to adjust the power density, wavelength, and pulse width of the laser (35,36). For a particular group of compounds a considerable increase in the obtainable molecular ion yield was recently observed by applying femtosecond UV l a s e r pulses, v e r s u s nanosecond pulses of the same wavelength (37),which allowed imaging of corresponding molecular surfaces with a lateral resolution < 1 pm as shown in Figure 10 (38).Nevertheless, postionization of molecular species depends largely on the spectroscopy of the respective molecule, a n aspect t h a t must be considered in the development of laser concepts.
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Summary In addition to elemental information, which can be obtained by other surface analytical techniques such a s Auger electron spectroscopy or XPS, static SIMS supplies detailed molecular information on the composition of t h e uppermost monolayer of a solid. In combination with TOF analyzers i t provides extremely high sensitivities, high mass resolution, and a principally unlimited mass range. Because of the high sensitivity, the primary ion dose densities can be kept well below t h e static limit and TOF-SIMS can be viewed a s a n almost nondestructive technique. The achievable lateral resolution aa determined by the obtainable secondary ion yields is on the order of 0.1 pm for atoms and 1pm for organic materials. Most important for wide analytical application is the fact that all kinds of solid samples, single crystals, fibers, particles, and flat or rough surfaces can be a n a lyzed. This holds for all types of materials independent of their conduct i v i t i e s (e.g., m e t a l s , ceramics, polymers, and biomolecules). Although SIMS provides sensitive, qualitative information, the value of static SIMS results is limited by the lack of quantitation. Considerable progress has been made in laserSNMS;quantitative analysis, a t least for elements, is possible down to surface concentrations in the ppm range. Recently, for some groups of organic compounds, an efficient laser postionization has been achieved by optimization of laser power density, wavelength, and pulse width. Future developments in SIMS and SNMS will include improvements in instrumentation, sample preparation, and spectra interpretation. Progress in instrumentation can be expected on mass and lateral resolution, sample-handling devices, opti-
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