Product
Review
SIMS can also be used to measure isotope ratios for geochemical applications. "SIMS offers in situ analysis rather than bulk analysis, providing spatial information," says Davis, contrasting the surface technique with traditional methods such as isotope ratio MS. "It could really have a profound effect on geochemistry, especially as more methods are developed, appropriate isotopes identified, and precision improved," says Todd. Basic researchers are also working on ways to use SIMS to study biological samples. "The big race [among research groups] is to find targeted compounds in living cells," says Todd; for example, locating where a toxin or pharmaceutical binds within a cell. Broadly stated, SIMS is a mass spectrometric technique for detecting charged particles emitted from the surface of a sample. Energetic particles are fired at a sample, generally a solid, under high vacuum, which leads to a stream of secondary atomic and molecular ions, along with neutral species, being sputtered from the surface of the sample. The charged particles are collected and focused by the secondary-ion optics and fed into a mass analyzer. Positive and negative ion currents can be measured. Generally, two types of instruments are used for SIMS analysis. Dynamic SIMS instruments rapidly erode the surface with an intense beam of ions and are the traditional workhorse system for depth profiling and inorganic trace analybillion levels. "SIMS also has a unique sis. Static SIMS, on the other hand, is a capability for measuring an analyte as a function of depth," says Todd. By destruc- less destructive technique used primarily for analyzing organic molecules on the tively eroding away the surface, analysts surface monolayer. Because static SIMS can routinely probe down to micrometer uses a lower flux of primary ions, fewer depths with depth resolutions better than secondary ions are available for analysis 100 A. With these attributes, SIMS has become by MS. Efficient collection of ions using time-of-flight MS (TOFMS), a developa routine technique for detecting and mapment largely attributed to Alfred Benningping species on and below the surface of hoven's group at the University of Miinsemiconductors. It has also found a role in the computer industry and the broader ma- ster in Germany, has significantly interials science community, examining prob- creased the popularity of static SIMS over the last decade. lems such as finding grain boundaries, diagnosing metal fatigue, characterizing the Imaging SIMS is a refinement of the surface of computer disks, or detecting SIMS technique, in which a finely focused trace distributions of elements. In addition, primary ion beam is moved in a raster SIMS is being used for the analysis of poly- pattern across the sample surface to cremers, plastics, and coatings. ate a microscope-quality chemical picture
BEYOND THE
SURFACE WITH SIMS
Secondary ion MS represents a small market with some key applications It is expensive to buy and, at times, difficult to run, but secondary ion MS (SIMS) has earned a place as an essential surface analysis technique. "SIMS is, above all, a method for analysis of trace elements, detecting all the elements to some degree," says SIMS researcher Peter Todd (Oak Ridge National Laboratory). "It is an in situ measurement that looks at both light and heavy species," says Andrew Davis, regional sales manager for Cameca Instruments. Although the inorganic applications of SIMS have become almost routine, the technique is now carving a niche for organic surface analysis. SIMS also offers extremely good spatial resolution—on the order of 1000 A or less. Detection limits for elemental analysis range from part-per-million to part-per-
Analytical Chemistry News & Features, November 1, 1996 6 8 3 A
Product
Review
Table 1. Representative SIMS instruments Product Company
| IMS 6f Cameca Instruments 204 Spring Hill Road Trumbull, CT 06611-1356 203-459-0623
I T O F S I M S IV Cameca Instruments 204 Spring Hill Road Trumbull, CT 06611-1356 203-459-0623
I IMS 1270 Cameca Instruments 204 Spring Hill Road Trumbull, CT 06611-1356 203-459-0623
I NANOSIMS 50 Cameca Instruments 204 Spring Hill Road Trumbull, CT 06611-1356 203-459-0623
URL ore-mail Price
[email protected] Information not available
[email protected] Information not available
[email protected] Information not available
camecaf lo@aol .com Information not available
Experiments
Dynamic SIMS (standard), static SIMS, 3-D imaging SIMS, ion microprobe, isotopic analysis Cs» standard Computer-controlled sample stage with 20-mm xy axis movement (± 1 urn), 70x microscope. UHV (< 5 x 10~10 mbar) design
Static SIMS (with maximum 20-kHz rate charge compensation)
Dynamic SIMS, static SIMS, 3-D imaging SIMS, isotopic analysis, ion microprobe, mineral dating Cs* standard Computer-controlled sample stage with 20-mm xy axis movement (+ 1 urn), 70x microscope, UHV (< 5 x 10 - 1 0 mbar) design
Analysis and imaging of ultrafine features, isotope measurements, some depth profiling Cs + microbeam Fast-entry load lock, cornputer-controlled sample stage (± 1 urn), optical microscope with CCD camera, vacuum baking system
Magnetic sector (585-mm radius), stigmatic mass-filtered ion imaging 1-280 (10kV) 6000 (full transmission)
Double-focusing magnetic sector, local plane with mass dynamic range of 25x Up to 595 (10 keV) 600 (full transmission)
MicroChannel plate, fluorescent screen, electron multiplier, Faraday cup Sun UNIX workstation, company-supplied software; LabView for all other funclions Duoplasmatron source, scanning-ion image display, digital-image processor, primary-beam mass filter, FAB. secondary electron imaging, CCD camera, charge auto compensation electron gun for insulator analysis, resistive anode encoder ion imaging detector, multicollection detector system with 5 adjustable collectors Higher sensitivity than the IMS 6f. fast isotope analysis in multicollection mode (five channels)
Four moveable electron multipliers and moveable Faraday cup SUN SPARCstation with company-supplied software
Ion source Vacuum chamber features
Mass analyzer
Mass range (amu) Mass resolving power Detector(s) _ ^ _ _ _ _ Hardware/Software
Options
Special features
Reader service number
684 A
Up to 4 ion guns Horizontal sample mounting, 100- x 100-mm motion, sampie at ground potential, sampie motion: 360 azimuthal rotation, 90-mm x and 150mm y translation, and tilt; 10x and 200x microscope with color CCD camera: HeNe spot laser Double-focusing magnetic sec- TOF reflectron tor (120-mm radius), stigmatic mass-filtered ion imaging 1-280 (10kV), 1-560 (5kV) > 10000 800 (full transmission) 7000 FWHM (29 amu) > 25,000 (10% definition) > 10000 FWHM (> 200 amu) MicroChannel plate, fluoresSingle channel plate-to-glass cent screen, electron multipli- scintillator coupled to photoer. Faraday cup multiplier Sun UNIX workstation, comLicensed software written in C pany-supplied application soft- and C++ requiring WINDOWS, ware including Chain-Task. 4 MB. and 16-MB Pentium LabView for isotopic analysis high-end CPU Duoplasmatron source, scan- Secondary electron imaging ning-ion image display, digital- option, gas flooding of samimage processor, primarypies, detection post-accleration beam mass filter, FAB, secup to 20 keV, non-resonant ondary electron imaging, CCD laser postionization of sputcamera. charge auto compen- tered neutrals, cold finger. sation electron gun for insula- MALDI, custom prep-chambers tor analysis, resistive anode sample manipulator options: encoder ion imaging detector, automation, 8-in wafer hanUHV preparation chamber, dling, heater, cooler fast airlock, auto-duo control, rotation stage Shallow-depth profiling, lamiSample electrically ground to nated magnet for fast-peak zero; 5-axis, horizontal, switching, continuously ground-potential sample adjustable primary and secmounting; flexible sample ondary HV. flexible samplechamber with many ports and chamber design for alternative connectivity to other techpumping and attachments niques; liquid metal ion gun design for TOF applications; rapid dual beam ultrashallow profiling; burst-mode liquidmetal ion gun imaging; high mass resolution TOF database 401 402
Analytical Chemistry News & Features, November 1, 1996
403
Duoplasmatron source, charge autocompensation electron gun for insulator analysis, secondary electron imaging, total secondary ion current monitor, multiple-sample storage chamber
High lateral resolution down to 50 nm, simultaneous detection of up to five images, elemental analysis in multicollection mode (four channels)
404
PHI 6650 Physical Electronics 6509 Flying Cloud Drive Eden Prairie, MN 55344 612-828-6100
I PHI TRIFT II Physical Electronics 6509 Flying Cloud Drive Eden Prairie. MN 55344 612-828-6100
I PHI 5700 Physical Electronics 6509 Flying Cloud Drive Eden Prairie, MN 55344 612-828-6100
I S930 SIMS Kratos Analytical 535 East Crescent Ave. Ramsey, NJ 07446 201-825-7500
I ESCALAB 220J-XL VG Scientific West Peabody Office Park 83 Pine St. West Peabody, MA 01960 888-275-2902 www.phi.com www.kratos.com www.vgscientific.com $425,000-$800,000 ~ $500,000 $850,000 (imaging and mono XPS and TOF SIMS) Static SIMS, dynamic SIMS, Dynamic, option for static and Static SIMS, XPS XPS, monoXPS, scanning imaging SIMS, FAB Auger electron spectroscopy
www.phi.com $600,000-$900,000
www.phi.com $750,000-$1,200,000
Dynamic SIMS, 3-D ion imaging, static SIMS option
Static SIMS. TOF-SIMS depth profiling, pulsed secondary electron imaging
Options of O2, Cs+, and Ga+ 60-mm platen for single or multiple samples; x. y, z, tilt & motorized rotation, option of optical microscope
Ga+ standard 50-mm x 50-mm multiplesample platen with rectilinear stage, 200-mm diameter wafer-stage option with x, y, and rotation
Argon 60-mm platen for single or multiple samples; x, y. z. tilt and motorized rotation, option of optical microscope
Duoplasmatron Manual or automated x,y,z sample stage; fast sample entry
Ga+ Four- or 5-axis manipulation with optional automation, rapid sample entry
Quadrupole with sector energy analyzer
Stigmatic imaging. 3 electrostatic sector TOF analyzer
Quadrupole with sector energy analyzer
Quadrupole
TOF
1-340 Unit mass resolution Channeltron
~ > 10,000 1-340, option of 1-511 > 9,000 FWHM (28Si) Unit mass resolution > 15,000 FWHM (> 200 amu) Dual microchannel plates Channeltron (> ± 10 kV postacceleration)
1^350. option of 1-511. 2-1023 Information not available Electron multiplier
HP Pentium PC with licensed complete data acquisition and reduction software Automated eight sample mount stage, vacuum transfer vessel, heating and cooling, scanning electron microscopy electron gun, ion-induced secondary electron detector
HP Pentium PC with licensed complete data acquisition and reduction software MALDI, 115 ln liquid-metal ion gun, cold stage, cold sample transfer, 200-mm stage. O2 flood and Cs ion guns
HP Pentium PC with licensed complete data acquisition and reduction software Automated 5 axis stage, vacuum transfer vessel, heating and cooling, scanning electron microscopy and 100-nm scanning Auger electron gun, ion-induced secondary electron detector
High current, low voltage. micrometer probe Cs and 0 2 duoplasmatron ion guns, grounded sample lowextraction field analysis of insulating samples
Compound spectral library search software, high mass resolution retrospective imaging and depth-profile software, grounded sample charge neutralization with 200-kHz high-extraction field secondary electron and SIMS analysis. 115ln liquid-metal ion gun increases organic ion yield by 3 to 10-fold
Integrated multitechnique instrument-control electronics and software, ultrahigh vacuum sample-processing chamber options, multistrip detector XPS for chemicalstate imaging
405
406
407
> 6000 > 1000 at 100 amu Channel plate
Sun workstation, VISION soft- Pentium PC, OS/2 operatware/WINDOWS-based sysing system, VG Eclipse tern data system Liquid metal-ion guns, inertgas ion guns
Auger electron spectroscopy, inert gas ion guns, 8-in. sample handling
45° electric-sector energy pre-filter provides more control
XPS and TOF SIMS analysis without moving the sample
~^08
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Analytical Chemistry News & Features, November 1, 1996 6 8 5 A
Product
Review
ments] are pretty reliable," says Ron Fleming, a senior scientist and contract R&D manager with Charles Evans AssociCompany I lonwerks FEI Company ates, a contract laboratory in Redwood 2472 Bolsover, Suite 255 7451 NW Evergreen Parkway City, CA, that has helped develop SIMS Houston, TX 77005 Hillsoboro, OR 97124 techniques and now routinely analyzes 713-522-9880 503-640-7509 samples for the semiconductor industry. URL or e-mail www.ionwerks.com
[email protected] "We hire Ph.D.s for their background and General product Manufactures SIMS components. SIMSmap III, a high lateral-resolufor their wider experience with the world, description including TDC-1 four-channel tion SIMS instrument for elemental which is good for times when you hit time-to-digital converter with fast analysis, is an add-on to FEI's something new," says Fleming. However, router, position-sensitive detector focused-ion beam workstation; he says that he has observed technicians with resolution of 500 u.m and quadrupole MS: liquid-metal ion with no college training successfully run a rate of up to 20-MHz, reflectron guns SIMS instrument. "They get good at it by TOFMS, and integrating hisdoing it over and over." togram memory Reader service 410 411 Routine maintenance is key to mainnumber taining the instrument, Fleming says. The Charles Evans scientists regularly replace apertures and clean up the material sputinformation," says Todd. Producing second- tered onto the insulators inside the MS. of the surface. Fast atom bombardment Good housekeeping is also important to ary ions is a complex process that can be MS, which bombards the surface with eliminate "memory effects" from materiaffected by the nature of the elements or neutral beams, is a variation of SIMS and als left from previous samples. In fact, molecules, matrix effects, and whether the has spawned its own lines of instruments, surface is clean or oxidized. As a rule, con- with this type of maintenance most instruwhich will not be explored here. ments can continue to produce good data centration data require appropriate referTable 1 lists representative features for a long time. The Charles Evans folks ence standards, such as ion implanted stanof nine instruments that can be configstill run samples on a Cameca Instrument dards, as well as careful operation by the ured with various options, depending on with serial number 19 (out of several hunanalyst, says Todd. the application. In addition, two compadred built). Todd says that he replaces an nies that provide SIMS accessories are Laser-induced post ionization of neutral electron multiplier detector in his quadrulisted in Table 2. FEI is a well-regarded species flying off the surface from a SIMS pole mass analyzer only once a year. "The manufacturer of liquid metal ion guns experiment offers one way to avoid matrix vacuum pumps are usually the biggest and other products; lonwerks is a newly effects and improve quantification, says maintenance expense," he says. formed company suppling major SIMS Davis. In addition, SIMS instrumentation components. works well with the matrix-assisted laser desorption/ionization (MALDI) technique. Better features "This is an attachment that doesn't have a Because the market for SIMS is a small one Advantages and disadvantages lot of compromises," Davis states. with sales less than other techniques such SIMS complements many of the other Samples that are insulators or poor con- as XPS, instrument designs have evolved popular surface techniques, according to ductors pose another problem by building slowly. "Instruments today have more usethe experts. In fact, VG Scientific's ful features [than the older versions]," says ESCALAB 220i is a combined static SIMS up an unwanted charge on the surface caused by the primary ion bombardment. Fleming, citing advances such as improved and X-ray photoelectron spectroscopy vacuum systems, new charge compensa(XPS) instrument, Kratos Analytical offers This charging phenomenon reduces or a SIMS option on its AXIS 165 system for even eliminates the spectral signal. A com- tion techniques, and better imaging. XPS and Auger spectroscopy (for more on mon technique to neutralize positive The feature that most distinguishes charges is to introduce low-energy elecAXIS and XPS instruments, see Anal. the instruments listed in Table 1 from Chem., 1995, 67, 675A-78A), and Physi- trons from an electron flood gun. This tech- each other is the choice of mass analyzer. nique requires alternately pulsing electrons Each has certain strengths and weakcal Electronics sells the 5700 for SIMS/ and positive ions from the primary source XPS or SIMS/XPS/Auger combinations. nesses. "Quadrupoles are relatively inexon and off. "Unfortunately, these methods According to David Surman, sales manpensive and easy to use," says Todd, but don't always work well for the detection of ager for Kratos, static SIMS provides from a practical standpoint, they don't chemical structure information that corre- negative secondary ions, such as 0~ emithave very high transmission of the sected from insulators," says Todd, adding that ondary ions. This is typically not a problates "quite well" with XPS. "I've conusing a negative primary ion beam can verted people from an XPS/Auger to an lem in dynamic SIMS, says Todd, because XPS/SIMS combination," he reports. John sometimes solve the problem. "generally you have more signal than you Hammond, product director for Physical Given all these obstacles, SIMS is typi- know what to do with." However, for static Electronics, agrees and says that XPS ofSIMS, which has a significantly lower ioncally not a method for novices. "Training fers quantitative data based on "first prinbeam flux hitting the surface, transmisand an understanding of what factors ciples", whereas SIMS is better at provid- need to be carefully controlled in the insion can be a severe limitation. Todd uses ing elemental and molecular information. a triple quadrupole tandem MS arrangestrument are of extreme importance," SIMS, however, has its own limitations. says Hammond. ment for static SIMS work, but he says "SIMS provides excellent distributional that it took a "ton of work" to get the However, SIMS does have a role as a information, but relatively poor quantitative routinely run method. "[SIMS instrutransmission high enough to run the exTable 2. SIMS components
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Analytical Chemistry News & Features, November 1, 1996
LEGAL NOTICE periment "Because secondary ion spectra from tissue can be very complex, we know of no better way [than tandem MS] to identify the ions we detect and map," states Todd. The triple quadrupole arrangement allows his group to run MS/MS rapidly and easily, which is essential for MS/MS imaging. Magnetic sectors have higher transmissions and mass ranges, as well as better mass resolution, than quadrupole analyzers. Detection limits are about a factor of 10 lower than quadrupoles, says Fleming, who adds that "The semiconductor industry is always looking for lower detection limits." Operating at high extraction potentials maximizes the sensitivity of sector instruments, but sacrifices depth resolution; new designs can operate at lower extraction potentials to improve resolution. Sector instruments are also well suited for isotopic ratios and sorting out matrix interferences and, because they have good stigmatic imaging of surface ions, they offer good lateral resolution for ion microscopy, says Hammond. However, the sector instruments are more expensive to purchase. TOF mass analyzers, which boast the best transmission efficiencies, are the ideal systems for static SIMS experiments. They work well for samples that require charge compensation and have high mass ranges that can aid in collecting data on organic constituents. However, they suffer from a low duty cycle. "Most of the time with TOF you are detecting ions and not bombarding the sample," says Todd, which limits and slows depth-profiling experiments. Primary ion guns are another variable in the SIMS experiment. Cesium ion guns are the traditional standard—relatively inexpensive, with moderate spatial resolution in the 1- to 10-um range, according to Todd. Duoplasmatron guns extract ions from a plasma formed from an inert gas such as Ar or from 0 2 . The newest entry in this arena are the liquid metal guns designed for static SIMS experiments, typically producing 69 Ga+. These guns generate lower primary and secondary ion currents and require some skill to operate, but offer resolutions on the order of 0.1 um or better, says Todd. Finally, the secondary ion optics play a role by determining the sensitivity of the instrument. "There are many ways to go wrong in running SIMS," says Todd, "but the reward for making measurements correctly can be the solution to a serious analytical problem." Alan Newman
U.S. POSTAL SERVICE STATEMENT OF OWNERSHIP, MANAGEMENT, AND CIRCULATION (Required by 39 U.S.C. 3685) 1. 2. 3. 4. 5. 6. 7.
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