Depth Profiling of Materials and Coatings by Laser Ablation ICP-MS

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1 Depth Profiling of Materials and Coatings by Laser Ablation ICP-MS 031LSX+0301 March 2001 Benefits of Laser Ablation for ICP Spectrometry Laser sampling using modern UV laser ablation systems provides the analytical community with a beneficial sample introduction system tool for direct solid sampling. Some of these benefits are: Sample dissolution is not required Quantitative when suitable standards are available Compatible with all ICP-MS and ICP-OES systems Systems are mobile and require no complex optical alignment Easy to use and robust for everyday use without alignment Well-suited for many application types High precision translation and swift ablation method setup and programming for precise and accurate ablation cell positioning Instrumentation The LSX-200 Plus Laser Ablation System (Figure 1) provides the latest innovation in solid sampling technology. The system incorporates a Flat-Top beam profile as illustrated in Figure 2. One major advantage of the beam profile is that a truly uniform energy distribution with each laser pulse is directed onto the sample surface. This results in especially clean and coherent ablations that ensure excellent crater characteristics in the sample matrix. The ablation from the Flat-Top craters generates a fine, dense aerosol that is carried into the plasma. The amount of analytically useful aerosol is dependent upon several factors: crater diameter, pulse repetition rate and laser energy. The LSX- 200 Plus system offers full control of these parameters so that each application can be tailored to suit desired sensitivity and crater morphology. The LSX-200 Plus optical system is superior, offering magnification from X, controlled illumination and precise focus control. A direct viewing petrographic microscope system incorporating dual polarizers (optionally) for even the most optically demanding samples and applications. The CETAC LSX-200 Plus system is capable of performing many types of applications. A growing trend has been observed in studying the elemental composition of layers that make up many different types of coatings. Samples from the materials science industry, particularly thin-films and substrates are ideal for analysis by ICP-MS using laser sampling. Some of these advantages include: High spatial depth resolution µm Flat ablation craters Advanced depth profiling software Variable energy setting Variable pulse rate Petrographic microscope (optional) Digital image capturing Figure 1: CETAC LSX-200 Plus Laser Ablation System. 1

2 Figure 2: Flat-Top beam profile. The aerosol generated by pulsing the high-powered UV laser onto the sample surface is carried to the ICP (see Figure 3). The liberated aerosol is efficiently transferred to the ICP torch for atomization. The ablation chamber plays an important role in the stability of the analytical signal. Four different models of ablation chambers are available (Figure 4). Each cell is suited and tested for a particular application requirement. The LSX-200 Plus system does not incorporate a sample size limiting cassette holder. Rather, the system was designed to house samples as large as 10 cm x 4 cm x 4 cm. Cell volume can often determine and change the signal intensity or the amount of aerosol that actually is transported to the plasma for analysis. Therefore, adaptable sample holders are used to adjust cell volume without sacrificing ablation efficiency. A thin-section holder is used for optically thin-sections with dimensions as long as 55 mm and a standard width of 27 mm (as in an ion microprobe section). These types of samples can then be viewed with reflected or transmitted light by utilizing the high quality software controlled polarizers. Figure 4: Laser sampling chambers for the LSX-200 Plus system. Other desirable characteristics of the UV 266 nm LSX-200 Plus system for depth profiling analysis include: High coupling efficiency allowing ablation of all types of materials, whether transparent, opaque, translucent or colored Predictable ablation process generating a continuous flow of fine, dense particles Reduced localized heating, and hence, minimized splashing of ablated material that can degrade analytical precision Variable spot sizes from µm in diameter Switchable optics producing either a flat-top beam for shallow, high spatial resolution work or a Gaussian beam for dynamic, high energy drilling of UV transparent materials. In Flat-top mode, the energy density is uniform, independent of spot size. See Figure 5 below for an example of the near and field images of these profiles. Figure 5: Flat-beam sampling on the left and Gaussian sampling on the right. 2 Figure 3: 50 µm laser shots in steel coating.

3 Figure 6: DigiLaz TM software showing the sample tracing method for depth profiling. DigiLaz TM software Depth Profiling The LSX-200 Plus system is easily programmed for depth profiling analysis providing optimal sampling flexibility. The first of these methods is shown above in Figure 6 and is called the sample tracing mode of analysis. The ablation points for profiling the sample are simply keyed into the system by pointing and clicking directly on the sample image. In this mode of analysis, however, the sampling area is limited to about 2.8 x 2.0 mm in the field of view. This method is most useful for inspecting micro-features of the matrix where distances are short and the user is focusing on developing the applications in a very limited area of the sample. The alternative to sample tracing mode is shown in Figure 7. This method is typically used more often as it allows the user to navigate and setup the profiling program anywhere in the sample chamber. This method is called the manual drawing mode and is accessed by clicking on the method mode button. Figure 7: DigiLaz TM software showing the manual drawing method for depth profiling. Profiling Key A confocal micrograph showing a flat beam crater resulting from a single laser shot is shown in Figure 8. Note the clean walls of the crater and excellent flat bottom. This type of crater is ideal for experiments that require precise layer removal during elemental profiling. Figure 8: Single laser shot into a ceramic coating, 0.25 mj energy, 5.4 x 154 µm. 3

4 Figure 9: Gradient depth profiling illustrated in NIST 612 glass. The DigiLaz TM software provides an expanded depth profiling method which allows the user to create multiple craters inside of larger ones (see Figure 9). The term proto-crater is used describing the drilling. The goal in most applications is to remove contaminants or other layers prior to switching spot sizes, while using the same power density for all spot sizes. NIST 1387 Coating Standard 3.00E E+07 Intensity (cps) 2.00E E E+07 Au197 Ni58 Fe E E Time (secs) Figure 9: Depth profile of NIST 1387 coating standard plotted as signal intensity against profiling time. Elan 6000 ICP-MS/LSX-200: 0.5 mj 10 Hz, Crater size 30 µm using depth profile mode, z-stage = 10 µm/sec 1 µm Au 30 µm Ni Fe Substrate 4

5 Intensity (cps) Cr52 Fe57 Ni58 W Time (seconds) Figure 10: Depth profile of a coating on an Fe matrix. Elan 6000 ICP-MS/LSX-200: 0.75 mj 5 Hz, Crater size 100µm using depth profile mode, z-stage = 5 µm/sec Figure 10 shows a signal intensity profile for a diamond like coating where Fe is consistent through the matrix but its distribution increases as W signal disappears. These data can be plotted as log plots or, as before, as a function of depth using complimentary optical data or confocal microscope examination. Summary Laser sampling has been shown to be an effective analytical tool for depth profile analysis of materials and coatings. The technique is simplified by the DigiLaz TM software and allows the user maximum flexibility for adjusting both the laser parameters and the host ICP-MS. There is a continuing increase in the number of users that rely on depth profiling LA ICP-MS in many non-geological applications such as the analysis of materials, coatings, semiconductor parts, and thin-films. The CETAC LSX-200 Plus system is well-suited to satisfy these demanding requirements. 5

6 LSX-200 Plus Laser Ablation System Laser Frequency quadrupled, Q-switched Nd:YAG laser, 266 nm 6.0 mj/pulse laser energy, 1% RSD RMS, computer controlled Flat-Top laser beam energy profile Laser pulse width: < 4 ns Repetition rate: 1-20 Hz Optical Imaging control system Switching optic for gaussian and flat beam sampling Q-switch controlled laser frequency selection Built in laser energy measurement system 5X, UV achromatic objective lens (10x Optional) Variable spot sizes, digital aperture system: µm, um, calibration not required Viewing Optics and Video System Coaxial viewing and laser focus optics with a single lens (option) Computer-controlled, continuously adjustable parfocal video microscope from 80X-800X Motorized zoom video camera Fiber optic illuminators with two 150 W lamps Reflected and transmitted light illumination Manual and computer control focusing Image integration on host computer using high resolution video card Sampling System 52 mm diameter X 50 mm high sample cell (104 mm travel axis optional) Stepper motor control integration on host computer using high speed stepper control card Motorized XYZ translation stage, 52 mm travel axis 0.25 micrometer resolution, 1 micrometer steps 3-stage motor drivers with manual override Adaptable volume, Teflon and viton construction Automated purge, by-pass or to ICP gas control Oversize sample cell 10 x 4 x 4 cm (optional) Universal thin -section and microprobe standard sample holder Custom integrated software control for segmented line scans Flexible focus control, automated or manual Specifications subject to change without notice Ablation cell with removable quartz window for easy cleaning and maintenance Extendible sampling stage for easy sample installation Computer hardware and software Integrated laser system control software with ICP or ICP-MS host computer DigiLaz TM Software, Digital Operating System Windows TM NT based software On screen display of safety interlocks and laser status including laser firing, door open, coolant temp and flow Universal communication protocol for synchronization of ICP-MS and laser systems 7 laser ablation methods and scan control with point to point, traverse, raster and gradient profiling Method file management Spot size measurement Image capturing On-line help, service, maintenance databases Automated control of output energy indicator Single shot or automated repetition Computer controlled spot size Automated video zoom and sample motion control Superior depth profile software control <0.25 µm System Dimensions: Laser Module 45 x 59 x 60 cm Power supply 20 x 30 x 37 cm Weight: 55 kg Power requirement: VAC +/- 10% Class 1 enclosure with safety interlocks Independent programmable laser power supply module Closed loop distilled water cooling system Options Geo-option: includes binocular direct viewing with variable magnification, computer controlled polarizing light with thin section holder Sample Auto-focus Custom software integration of external programs GeoPro TM data handling and manipulation software for time resolved analysis Pentium computer system w/19 monitor CETAC Technologies 5600 South 42 nd Street Omaha, NE USA FAX 6