Surface MS: Probing Real-World Samples - ACS Publications

Surface MS: Probing Real-World Samples REPORT Alfred Benninghoven, Birgit Hagenhoff, and Ewald Niehuis Physikalisches Institut Universitàt Munster Willhelm-Klemm-Str. 10 D-W-4400 Munster, Germany

The demands of expanding technological fields often are a driving force for t h e development of a n a l y t i c a l 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 biocompatibility 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 e l e m e n t a l information. An analytical tool with high sensitivity and lateral resolution t h a t 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 m a t e r i a l s used in technological applications are organic in origin, the following techniques are important in this regard: X - r a y photoelectron spectroscopy (XPS), surface MS, and the compara-

tively new t e c h n i q u e of s c a n n i n g tunneling microscopy (STM) and its related counterpart, atomic force mic r o s c o p y (AFM). T h e s e m e t h o d s m u s t be evaluated with respect to the 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 u n k n o w n surface species (atoms as well as molecules

630 A • 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 be possible, even for very low surface concentrations, and l o c a t i n g s p e c i e s s h o u l d be p o s s i 0003-2700/93/0365-630A/$04.00/0 © 1993 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, t h e s e t e c h niques 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, the identification and quantification of all elements, isotopes, and molecular species. Therefore, surface MS should be an excell e n t m e a n s for surface a n a l y s i s , provided the following are addressed: controlled desorption of atoms and molecular species, efficient ionization of these desorbed particles, and unambiguous identification of t h e generated ions by their charge/mass 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 n e u t r a l 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 o r i g i n a t i n g from t h e u p p e r m o s t monolayer of the surface. By tuning and focusing the p r i m a r y 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, as in s t a t i c 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 s e n s i t i v i t y well in the s u b monolayer 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 sensitivi t y , T O F - S I M S offers u n l i m i t e d m a s s range, high m a s s resolution, short overall flight times, and accurate mass determination—considerable advantages compared with the c a p a b i l i t i e s of SIMS i n s t r u m e n t s equipped with magnetic sector fields, quadrupoles, and ion cyclotron resonance analyzers. The combination of ion bombardm e n t - i n d u c e d p a r t i c l e desorption and TOFMS has made TOF-SIMS a most useful surface analytical techn i q u e . Recently, l a s e r SNMS h a s shown considerable analytical potential. The modular design of a TOF i n s t r u m e n t m a k e s it very flexible

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 in t h e u p p e r m o s t monolayer (Figure 1). What is unexpected is that the technique can also be applied to the analysis of nonvolatile and thermally labile organic mat e r i a l s . The major portions of secondary particles are emitted as neutrals, whereas only a fraction of - Î O ^ - I O " 1 of t h e t o t a l 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 p a r t i c l e s a n d , in p a r t i c u l a r , 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-

Figure 1. Particle emission from a surface after excitation with primary ions of keV energy. ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993 • 631 A

REPORT sisting of a cloud of target particles set in motion by t h e p r i m a r y ion (primary recoil particles) or by target particles t h a t are already moving (secondary recoil particles). The di mension of the collision cascade de pends 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 ma terials with 10-keV Xe + , typical val ues are 3 nm for the diameter of the collision cascade, and 15 nm for the depth of the collision cascade, ac cording to model calculations (8). Most recoil particles have low en ergies, and only cascade particles from n e a r - s u r f a c e r e g i o n s — w i t h momenta directed toward t h e sur face— can overcome t h e s u r f a c e binding energy and t h u s leave the t a r g e t (sputtered particles). SIMS and SNMS, therefore, are very sur face-sensitive analytical techniques (information depth is < 3 monolay ers). The charge state of the sput tered particles depends largely on the chemical environment in the up permost monolayer (matrix effect), which in general prevents the direct quantification of SIMS results. The flux of emitted neutrals is al most unaffected by the composition of the matrix, and SNMS can be used to obtain q u a n t i t a t i v e information about the surface composition if an effective postionization mechanism can be provided (e.g., nonresonant or resonant laser postionization, which will be discussed later). Both second ary ions and secondary neutrals show characteristic energy and an gular distribution. The maximum of the energy distribution for elements is ~ 5 eV, with a tail toward higher energies. For molecular species it re sembles a Boltzmann distribution with a maximum of - 2 eV. The an g u l a r d i s t r i b u t i o n for a t o m s a n d neutrals follows a cosine law (5). Ions originating from an elemental m a t r i x can be positively or nega tively charged, depending on their electron configurations in the outer most shell. The highest secondary ion yields (i.e., number of secondary ions X* emitted from a surface spe cies M per number of primary ions) of molecular ions are achieved from m o n o l a y e r s on noble m e t a l s u b strates. Typical secondary ions are Me*, (M + H) + , (M - ΗΓ, (M + Sa) + , and (M + Me) + , where Me is a metal, M is a molecule, H is hydrogen, and Sa is either Na or Κ (9, 10). From bulk materials and thick layers, typ ically (M ± H)* and larger fragments can be obtained. To e l u c i d a t e t h e fragmentation process, fragmenta

Figure 2. TOF-SIMS instrument with laser postionization and charge compensation capabilities.

tion rules that are known from electron impact MS can be applied (a-, β-cleavages; rearrangement processes). Principles of T O F M S

Because t h e principles of TOFMS were recently reviewed in this JOUR NAL (22), we wish to c o n c e n t r a t e only on t h e key f e a t u r e s of T O F SIMS (Figure 2). Mass d e t e r m i n a tion in TOFMS is achieved by mea s u r i n g t h e f l i g h t t i m e t of t h e desorbed secondary ions in a drift path of known length I after acceler ating them in an extraction field to a common energy Ε of some keV. The relationship between energy and flight time is E=

2

-ml2/t2

This equation shows t h a t the flight time is proportional to the s q u a r e root of the mass of a secondary ion. For an a c c u r a t e d e t e r m i n a t i o n of flight t i m e s , the s t a r t and a r r i v a l times of the secondary ion must be well defined. To meet these require ments, 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 sys tem. In a typical cycle, 0.1-10 sec ondary ions are detected. A spectrum with a dynamic range of several orders of magnitude is ob tained by the accumulation of a large


number of cycles with high repetition rates (typically 5 - 2 0 kHz), depend ing on the mass of the largest ion of interest (12). Energy and angular fo cusing elements in the flight path of the secondary ion g u a r a n t e e com pensation for an energy and angular spread of the secondary ions caused by the desorption process. Because all s e c o n d a r y i o n s a r e d e t e c t e d q u a s i - s i m u l t a n e o u s l y and t h e in strumental transmission is high, the achievable sensitivity of TOF a n a lyzers is several orders of magnitude higher t h a n t h a t of quadrupoles or m a g n e t i c 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 m a k e s T O F SIMS s u i t a b l e for a n a l y s i s of ex tremely small volumes and materials that yield small amounts of ions. The mass resolution in TOF-SIMS, which is well above 10,000, depends on the time-focusing properties of the analyzer, the pulse width of the primary beam, and the time resolu tion of the registration electronics (13). The strict and simple relation ship between mass and flight time t h a t is valid over the whole m a s s range makes accurate mass determi n a t i o n straightforward. The m a s s range in TOF-SIMS increases with the recorded time range and can be extended without limitations. There fore, the highest masses observed are determined by desorption.

Counts (χ 102)/channel

(a)

(b)

m: 107 4247

Ag+ 3.787 x10 7

m: 1309 143

(CsA + Ag)5+ 2.840x10

m: 726 348

(Sol 1 + Ag)e + 1.107 x10

m: 992 595

(Sol2+Ag)*6 2.603 x10

Figure 3. Positive secondary ion spectrum (a) and mass-resolved ion images (b) of a monolayer of a solution of CsA and CsD. (a) CsD serves as the internal standard. The solution is 500 ng/mL CsA and 400 ng/mL CsD in ethanol; 1 μ ι of the solution (submonolayer preparation) is applied to - 0.5 cm 2 of an etched Ag target, (b) The mass-resolved ion images are also on Ag. In all images, a linear thermal color scale is used to represent different ion intensities (yellow is high intensity; dark red is low intensity). The text below each image refers to mass (m, in Da), type of secondary ion, number of counts in the pixel with highest intensity, and total number of counts. The overall intensities are normalized to the pixel with highest intensity in the respective image. Field of view is 400 χ 400 μιτι2. Sol 1 and sol 2 are the oils used for the matrix.

S a m p l e c h a r g i n g caused by pri m a r y ion b o m b a r d m e n t 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 po tential is then achieved in the posi tive and negative SIMS modes and facilitates the analysis of all types of insulating materials (14). The high sensitivity of TOF-SIMS and t h e small size of the collision c a s c a d e qualify t h e t e c h n i q u e 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 r a s t e r a complete s p e c t r u m is ac q u i r e d , r e s u l t i n g in m a p s for all masses of interest. A lateral resolu tion better t h a n 100 nm can be ob tained with a pulsed Ga liquid metal ion gun (LMIG) (15). In an ion micro scope a large area on the sample is illuminated by t h e pulsed p r i m a r y 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 re sistive anode encoder. A lateral reso lution of 1-3 μπι can be obtained (16). The majority of the emitted parti cles are n e u t r a l s , and MS of these particles becomes possible only by postionization (17-19). The most effi c i e n t m e t h o d of p o s t i o n i z a t i o n is multiphoton ionization in either a r e s o n a n t or a n o n r e s o n a n t mode with a pulsed laser beam. After desorption by the p r i m a r y ion pulse, the laser beam is fired into the cloud of sputtered neutrals above the sur face. Generated photoions (hundreds up to several thousand per pulse, de pending on the primary beam cur rent and pulse width) are extracted from t h e i o n i z a t i o n v o l u m e by a pulsed extraction field. Because of t h e 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 l a t e r a l r e s o l u t i o n by u s i n g a Ga LMIG. Laser SNMS imaging is not possible in the microscope mode be cause of the lateral dispersion of the sputtered neutrals. Analytical applications Static SIMS initially was achieved by using magnetic sector and quadrupole mass spectrometers (20). Ap p l i c a t i o n s w e r e l i m i t e d by m a s s

ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993 • 633 A

REPORT range, mass resolution, and sensitiv ity of these instruments. Recent de velopments 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 ana lyzed tens of thousands of samples covering a wide variety of materials related to surface phenomena t h a t a r e i m p o r t a n t to or e s s e n t i a l for

many technologies. A close interac tion between instrumental develop ment and analytical problems intro duced by i n d u s t r i a l a n d academic p a r t n e r s h a s continuously opened new fields of application. From this material we have selected real-world samples t h a t illustrate the type of information supplied by TOF-SIMS and laser SNMS. Identification of biomolecules. The ability of TOF-SIMS to identify and locate the smallest amounts of

Figure 4. Positive secondary ion spectrum of a monolayer of the perfluorinated polyether Krytox on a Ag substrate. The oligomer series are as follows: 1 : (FRC3F6)*; 2: (On + CF2 + Ag)*; 3: (On - C2F4 + Ag)*; 4: (0„ + Ag)*; 5: ( 0 „ - C3F6 + Ag)*; 6: (On + C2F4 + Ag)+; 7: ( 0 „ - CF2 + Ag)*. Lower portion shows mass area 10,500-11,000 Da magnified six times. 634 A • ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993

labile organic compounds is demon strated in Figure 3. Figure 3a shows the positive secondary ion spectrum of a mixture of cyclosporin A (CsA), an i m p o r t a n t i m m u n o s u p p r e s s i v e drug used in transplant surgery, and its physiologically ineffective deriva tive, cyclosporin D (CsD). Cyclospo r i n s are cyclic peptides consisting of 11 partially modified amino acid res idues. CsD c o n t a i n s a - C H ( C H 3 ) 2 group instead of the - C H 2 - C H 3 con tained in CsA (21). In the spectrum CsA and CsD can be i d e n t i f i e d by t h e i r (Cs + Ag) + peaks. Metal cationization is a com mon feature of a l m o s t all organic secondary ions and occurs for mole cules prepared as a monolayer on no ble metal substrates. The respective peak shapes mirror the isotopic dis tributions. F u r t h e r m o r e , (Cs + H) + a n d (Cs + N a ) + p e a k s can be d e tected. Although the surface cover age of CsA exceeds t h a t of CsD, the (CsD + Ag) + peak is slightly more in tense than the (CsA + Ag) + peak—an indication of the matrix effect t h a t must be considered in SIMS experi ments. Nevertheless, quantification of CsA can be achieved with an accu racy better t h a n 15% by using CsD as an internal standard (i.e., in every CsA quantitation, the result of a sin gle T O F - S I M S m e a s u r e m e n t a t worst h a s 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 s h o w s f o u r m a s s resolved s e c o n d a r y ion i m a g e s of CsA dissolved in a m i x t u r e of oils suitable for oral application to pa tients. All images show the edge of a droplet of the oily solution deposited on Ag. The bare Ag is located in the left p a r t , a n d t h e CsA covers t h e right part of each image. When the solution s p r e a d s , t h e oils used to form the matrix concentrate at the boundaries (lower images). This ex ample shows that imaging of organic molecules is possible even in t h e higher mass range. Note, however, t h a t for organic imaging the useful lateral resolution is limited to - 1 μπι b e c a u s e of t h e g e n e r a l l y low ion yields of organic molecules (12). Static SIMS requires h i g h - t r a n s mission mass spectrometers. If, how ever, the analyte molecules are de posited in a liquid m a t r i x , h i g h e r secondary ion currents can be gener ated and double-focusing magnetic sector field instruments can be used for their analysis. Liquid SIMS has been given the name fast-atom bom bardment (24), with the emphasis on the relatively unimportant aspect of using neutral primary particles

r a t h e r t h a n on t h e key feature of preparing samples in liquid r e s e r voirs where the sample molecules are mobile. C h a r a c t e r i z a t i o n of p o l y m e r s . The following two examples focus on the application of TOF-SIMS for the characterization of polymer materials (10, 25-30). The u p p e r p a r t of Figure 4 shows the positive secondary ion spectrum of a monolayer of the perfluorinated polyether known as Krytox (manufactured by DuPont) on a Ag substrate. Intact oligomers 0„(F-(CF(CF3)-CF2-0)-CF2-CF3) appear as Ag-cationized species (0„ + A g ) + ( p e a k 4) a t m a s s e s > 4000 Da. The peak d i s t r i b u t i o n mirrors the molecular weight distrib u t i o n of K r y t o x o l i g o m e r s . T h e mass of the polymer repeat u n i t R can be d e t e r m i n e d from t h e m a s s d i s t a n c e b e t w e e n two o l i g o m e r peaks. The exact mass of an oligomer 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 a n d 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 at high i n t e n s i t i e s in t h e low mass range and decreasing with increasing mass.) Most of t h e m 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 p r e p a r a t i o n of dissolved polymers on noble metal substrates allows the determination, characterization, and identification of repeat units, end groups, oligomer d i s t r i b u t i o n s (Mp, M n , M w ), a d d i tives, contaminants, and the composition of copolymers. Because of the desorption process, the highest achievable mass is - 15,000 Da. As discussed earlier, most secondary ions with masses above 300 Da appear as species cationized by subs t r a t e ions. From thick molecular films, such as bulk polymers, only ions carrying intrinsic charges can be expected, a n d t h e s e n o r m a l l y have masses below 300 Da. Nevertheless, these so-called fingerprint ions are so characteristic of the analyzed material t h a t extensive information on the chemical structure can be obtained. Figure 5 shows a positive second-

ary ion spectrum of poly(hydroxyethy l m e t h a c r y l a t e ) before a n d after t r e a t m e n t 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 t r e a t m e n t , t h e 4 5 - D a s i d e c h a i n 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 a n d 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 Langm u i r - B l o d g e t t (LB) technique (31). T h e LB l a y e r s c o n s i s t of a n a m phiphilic poly(methacrylate) (PMA), one monolayer of which was t r a n s ferred to a Ag-coated slice of polycarbonate. The positive secondary 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 m a s s - s e p a r a t e d ion images give information about the origins of

Figure 5. Positive secondary ion fingerprint spectrum of a bulk sample of poly(hydroxyethylmethacrylate) before (upper) and after (lower) treatment with gaseous propionylchlohde (CH 3 CH 2 COCI). ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993 • 635 A

Results from the analyses of two defects in car paint, samples a and b, are shown in Figure 8. In Figure 8a, the spectra of sample b (the defect itself and a spot remote to the defect) indicate that a perfluorinated polyether (typical fingerprint peaks a t masses 12, 31, 50, 69 Da, etc.) is responsible for the defects. In Figure 8b, the images show the t r a n s i t i o n from the defect a r e a to t h e unaffected paint of sample a and provide a closer look at t h e defect a r e a in

sample b. The images clearly show the lateral distribution of fluorinecontaining species in the defect area at 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

Figure 7. Spectra of Si wafer treated with CHF 3 plasma. Upper: negative secondary ion spectrum. Lower: positive secondary ion spectrum of the same wafer after additional UV/ozone treatment. Peaks are as follows: 1: 56 Fe*; 2: 28SiJ; 3: 28 SiCO*; 4: 28SiCNHJ; 5: 29SiC2H3; 6: 28 SiC 2 H:; 7: C 3 0Hî; 8: C3NH£ 9: C4HS .

of small droplets, leading to the observed crater structures. Surface characterization by laser postionization of 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 t h a t sputtered neutrals are less affected by the matrix, l a s e r p o s t i o n i z a t i o n of t h e s e species promises information more related to the original surface concentrations. In laser SNMS the detection limits of elements are in the low p a r t - p e r - m i l l i o n range for the u p p e r m o s t monolayer (10 9 a t o m s / cm 2 ). The combination of laser SNMS with an ion microprobe allows quantitative elemental mapping with high sensitivity. Compared with s c a n n i n g Auger microscopy, t h e r e 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 t h e top two rows in Figure 9 for a gold p a t t e r n on a GaAs wafer. The images were recorded with a e 9 G a enriched LMIG for t h e desorption and a high-power femtosecond excim e r l a s e r set at 248 nm for postionization. For every pixel a m a s s 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 m a g n i t u d e . The 6 9 Ga signal in the area of the Au structure results from t h e 6 9 Ga ion b o m b a r d m e n t . 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 r e s t r i c t e d to 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 mole c u l e s . F o r a n o p t i m u m yield of intact molecular ions and character-




(a)

(b)

Figure 8. Positive secondary ion spectra (a) and mass-resolved images (b) of two defects in car paint. (a) Spectra are taken in and out of the defect in sample b. (b) Field of view for sample a is 400 χ 400 μηι2; for sample b, 120 χ 120 μπι2. 638 A • ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993

REPORT istic fragments, it is necessary to ad j u s t the power density, wavelength, and pulse width of the laser (35, 36). For a particular group of compounds a considerable increase in t h e ob tainable molecular ion yield was re cently observed by applying femto s e c o n d UV l a s e r p u l s e s , v e r s u s nanosecond pulses of the same wave length (37), which allowed imaging of corresponding molecular surfaces with a lateral resolution < 1 μπι as shown in Figure 10 (38). Neverthe less, postionization of molecular spe cies depends largely on the spectros copy of the respective molecule, an aspect t h a t m u s t be considered in the development of laser concepts.

Summary In addition to elemental information, which can be obtained by other sur face analytical techniques such as Auger electron spectroscopy or XPS, static SIMS supplies detailed molec ular information on the composition of t h e u p p e r m o s t m o n o l a y e r of a solid. In combination with TOF ana l y z e r s it provides e x t r e m e l y high sensitivities, high mass resolution, and a principally unlimited mass range. Because of the high sensitiv ity, the p r i m a r y ion dose densities can be k e p t well below t h e s t a t i c limit and TOF-SIMS can be viewed as a n almost n o n d e s t r u c t i v e tech nique. The achievable lateral resolu tion as determined by the obtainable secondary ion yields is on the order of 0.1 μπι for atoms and 1 μπι for or ganic materials. Most important for wide a n a l y t i c a l a p p l i c a t i o n is t h e fact t h a t 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 ma terials independent of their conduc t i v i t i e s (e.g., m e t a l s , c e r a m i c s , polymers, and biomolecules). Although SIMS provides sensitive, qualitative information, the value of static SIMS results is limited by the lack of q u a n t i t a t i o n . Considerable p r o g r e s s h a s been m a d e in l a s e r SNMS; quantitative analysis, at 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 prepara tion, and spectra i n t e r p r e t a t i o n . Progress in instrumentation can be expected on mass and lateral resolu tion, sample-handling devices, opti-

a.

Say Goodbye to Either/Or Electrochemistry

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M b.

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Figure 9. Laser SNMS images of a Au structure on GaAs. Field of view for top two rows is 40 χ 40 μηι 2 ; for bottom row, 1 4 x 1 4 μιτι2.

ples. Finally, considering the unique surface analytical features of molec ular surface MS, much more funda mental research on the most impor t a n t p r o c e s s of m o l e c u l a r ion formation is required. We thank K. Meyer and M. Terhorst for support and discussions, and J. Lub of Philips Research Laboratories (Eindhoven, The Netherlands) for providing the poly(hydroxyethylmethacry!ate). m: 104.05

Repeat unit

187

5.129 χ 10 5

Figure 10. Laser SNMS image of polystyrene dots on a silicon surface. 2

Field of view is 25 χ 25 μηι . Spectra acquisition conditions are the same as in Figure 9, with the laser power density reduced by a factor of 5. (Details of the experiment are found in Ref. 38.)

m i z a t i o n of l a s e r p o s t i o n i z a t i o n equipment, and on-line combination with other techniques such as XPS, S T M , or A F M . N e w m e t h o d s for cleaning, sectioning, a n d s t a i n i n g must be developed before the tech nique can be used for biological sam

References (1) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science; Cambridge University Press: Cambridge, 1986. (2) Czanderna, A. W.; Hercules, D. M. Ion Spectroscopies for Surface Analysis; Plenum Press: New York, 1991. (3) Fuchs, H.; Ohst, H.; Prass, W. Adv. Mater. 1991, 3, 10. (4) Wegner, G. Adv. Mater. 1991, 3, 8. (5) Benninghoven, Α.; Rudenauer, F. G.; Werner, H. W. Secondary Ion Mass Spec trometry; John Wiley & Sons: New York, 1987. (6) Sigmund, P. Phys. Rev. 1969, 184, 383; 187, 768. (7) Winograd, N.; Garrison, B. J. In Ion Spectroscopies for Surface Analysis; Czanderna, A. W.; Hercules, D. M., Eds.; Cambridge University Press: Cambridge, 1986; pp. 45-142. (8) Whitlow, H. J.; H a u t a l a , M.;

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O.J. Phys. D.Appl. Phys. 1992, 25, 818. Sundqvist, B.U.R. Int. J. Mass Spectrom. (32) G r a s s e r b a u e r , M; W e r n e r , Ion Proc. 1987, 78, 329. H. W Analysis of Microelectronic Materials (9) H o l t k a m p , D.; Kempken, M.; and Devices; John Wiley & Sons: Chich Klusener, P.; Benninghoven, A.J. Vac. ester, England, 1991. Sci. Technol. 1987, A5, 2912. (33) Knoth, J.; Schwenke, H.; Weisbrod, (10) van Leyen, D.; Hagenhoff, B.; NieU. Spectrochim. Acta 1989, 44B, 477. huis, E.; Benninghoven, Α.; Bletsos, I. V.; Hercules, D. M.J. Vac. Sci. Technol. (34) Kondo, H.; Ryuta, J.; Morita, E.; 1989, A 7, 1790. Yoshimi, T.; Shimanuki, Y.Jpn. J. Appl. Phys. 1992, 31, Lll. (11) Cotter, R. J. Anal. Chem. 1992, 64, (35) Hrubowchak, D. M.; Erwin, M. H.; 1027 A. Winograd, N. Anal. Chem. 1991, 63, 225. (12) Schwieters, J.; Cramer, H-G.; Heller, T.; Jurgens, U.; Niehuis, E.; (36) Terhorst, M.; Kampwerth, G.; Nie Zehnpfenning, J. F.; Benninghoven, A. huis, E.; Benninghoven, A.J. Vac. Sci / Vac. Sci. Technol. 1991, A9, 2864. Technol. 1992, AW, 3210. (37) Mollers, R.( Terhorst, M.; Niehuis, (13) Niehuis, E. In Secondary Ion Mass Spectrometry: Proceedings of the Eighth Inter E.; Benninghoven, A. Org. Mass Spectrom. 1992, 27, 1393. national Conference (SIMS VIII); Benning hoven, Α.; Janssen, K.T.F.; Tumpner, J.; (38) Terhorst, M.; Môllers, R.; Niehuis, Werner, H. W., Eds.; John Wiley & E.; Benninghoven, A. Surf. Interface Anal. Sons: Chichester, England, 1992; p. 269. 1992, 18, 824. (14) Hagenhoff, B.; van Leyen, D.; Nie huis, E.; Benninghoven, A.J. Vac. Sci. Technol. 1989, A 7, 3056. (15) Schwieters, J.; Cramer, H-G.; Jur gens, U.; Niehuis, E.; Rulle, H.; Heller, T.; Zehnpfenning, J.; Benninghoven, A. In Secondary Ion Mass Spectrometry: Pro ceedings of the Eighth International Confer ence (SIMS VIII); Benninghoven, Α.; Janssen, K.T.F.; Tumpner, J.; Werner, H. W., Eds.; John Wiley & Sons: Chich ester, England, 1992; p. 497. (16) Lindley, P. M.; Chakel, J. Α.; Odom, R. W. In Secondary Ion Mass Spectrometry, Proceedings of the Eighth International Con ference (SIMS VIII); Benninghoven, Α.; Alfred Benninghoven (left) is professor of Janssen, K.T.F.; Tumpner, J.; Werner, physics and director of the Physikalisches H. W., Eds.; John Wiley & Sons: Chich Institut. His main research interests are ester, England, 1992; p. 219. (17) Becker, C. H.; Gillen, Κ. Τ. Anal. fundamental, analytical, and instrumenChem. 1984, 56, 1671. tal aspects of the interaction between ions (18) Winograd, N.; Baxter, J. P.; Kimock, and solid surfaces. He pioneered static F. M. Chem. Phys. Lett. 1982, 8, 581. SIMS and its application to molecular (19) Winograd, N. Anal. Chem. 1993, 65, 622 A. surfaces and, in 1975, initiated the bian(20) Benninghoven, A. Surf. Sci. 1971, 28, nual international SIMS conferences. 541. Birgit Hagenhoff (right) studied physics (21) Lensmeyer, G. L.; Wiebe, D. Α.; and medical sciences at the University of Carlson, H.; de Vos, D. J. Clin. Chem. 1990,56(1), 119. Munster. In 1985 she joined Benningho(22) Hagenhoff, B.; Kock, R.; Deimel, M.; ven's research group. Her research activiBenninghoven, Α.; Bauch, H-J. In Sec ondary Ion Mass Spectrometry: Proceedings ties concentrate on application-oriented aspects of static SIMS in physics, chemisof the Eighth International Conference (SIMS VIII); Benninghoven, Α.; Janssen, try, biology, medicine, and related fields. K.T.F.; Tumpner, J.; Werner, H. W., She will be awarded her Ph.D. this year Eds.; John Wiley & Sons: Chichester, for her work on SIMS of molecular surEngland, 1992; p. 831. face structures. (23) Meyer, K.; Hagenhoff, B.; Deimel, M.; Benninghoven, Α.; Bauch, H-J. Org. Mass Spectrom. 1992,27, 1148. (24) B a r b e r , M.; Bordoli, R. S.; Sedgewick, R. D.; Tyler, A. N. Anal. Chem. 1982, 54, 645 A. (25) Briggs, D. Surf. Interface Anal. 1986, 9, 391. (26) Briggs, D.; Hearn M. J. In Ion Forma tion from Organic Solids: Proceedings of the Fourth International Conference (IFOSIV); Benninghoven, Α., Ed.; John Wiley & Sons: Chichester, England, 1989; p. 37. (27) Briggs, D.; Brown, Α.; Vickerman, J. C. Handbook of Static Secondary Ion Mass Spectrometry; John Wiley & Sons: Chich Ewald Niehuis studied physics at the Uniester, England, 1989. versity of Munster. He joined Benningho(28) Lub, J.; van Vroonhoven, F.C.B.M.; ven's research group in 1979 and received van Leyen, D.; Benninghoven, A. his Ph.D. in 1988 for his work on the de/ Polym. Science 1989, B27, 2071. (29) Lub, J.; van der Wei, H. Org. Mass velopment and application of high-resoluSpectrom. 1990, 25, 588. tion TOF spectrometers for static SIMS. (30) Hagenhoff, B.; Benninghoven, Α.; His research interests focus on instrumenBarthel, H.; Zoller, W. Anal. Chem. 1991, tal aspects of high-performance analysis of 63, 2466. sputtered material, especially TOF spec(31) Hagenhoff, B.; Deimel, M.; Benning trometry. hoven, Α.; Siegmund, H-U.; Holtkamp,


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