WO2008109875A1 - Pulsed laser atom probe and associated methods - Google Patents

Abstract

The present invention is directed generally toward atom probe specimen shape, atom probe control, data and associated systems and methods. Aspects of the invention are directed toward improving mass resolution by utilizing a pulsed laser system configured to reduce the laser spot size to 5 microns or less.

Description

PULSED LASER ATOM PROBE AND ASSOCIATED METHODS

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims the benefit of U.S. Provisional Patent

Application No. 60/893,802, filed March 8, 2007, entitled PULSED LASER ATOM PROBE AND ASSOCIATED METHODS which is fully incorporated herein by reference. BACKGROUND

[0002] An atom probe (e.g., atom probe microscope) is a device which allows specimens to be analyzed on an atomic level. For example, a typical atom probe includes a specimen mount, an electrode, and a detector. During analysis, a specimen is carried by the specimen mount and a positive electrical charge (e.g., a baseline voltage) is applied to the specimen. The detector is spaced apart from the specimen and is negatively charged. The electrode is located between the specimen and the detector, and is either grounded or negatively charged. A positive electrical pulse (above the baseline voltage) and/or a laser pulse (e.g., photonic energy) are intermittently applied to the specimen. Alternately, a negative pulse can be applied to the electrode.

[0003] Occasionally (e.g., one time in 100 pulses) a single atom is ionized near the tip of the specimen. The ionized atom(s) separate or "evaporate" (e.g., field evaporate) from the surface, pass though an aperture in the electrode, and impact the surface of the detector. The elemental identity of an ionized atom can be determined by measuring its time of flight between the surface of the specimen and the detector, which varies based on the mass/charge ratio of the ionized atom. The location of the ionized atom on the surface of the specimen can be determined by measuring the location of the atom's impact on the detector. Accordingly, as the specimen is evaporated, a three-dimensional map of the specimen's constituents can be constructed.

SUMMARY

[0004] The present invention is directed generally toward atom probe specimen shape, atom probe control, data and associated systems and methods. Aspects of the invention are directed toward improving mass resolution by utilizing a pulsed laser system configured to reduce the laser spot size to 5 microns or less. [0005] Other aspects of the invention are directed toward utilizing a pulsed laser system with wavelengths of less than 550 nm.

[0006] Other aspects of the invention are directed toward the relationship between mass resolution and the laser beam incidence angle as defined by the incident beam path with respect to the long axis of the specimen, the shank angle of the specimen and the thermal diffusivity of the specimen.

[0007] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This

Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a graph of the frequency distribution of local Si concentration in stainless steel as a function of PF_e in accordance with certain embodiments of the invention.

[0009] Figure 2 is a graph of the effect of laser spot size on the mass resolving power of aluminum in accordance with certain embodiments of the invention.

[0010] Figure 3 is a graph of the effect of laser wavelength on the mass resolving power of stainless steel in accordance with certain embodiments of the invention.

[0011] Figure 4 is a partially schematic illustration of a laser beam delivered at a large incidence angle with respect to the long axis of the specimen in accordance with certain embodiments of the invention.

[0012] Figure 5 is a partially schematic illustration of a laser beam delivered at a small incidence angle with respect to the apex of the specimen in accordance with certain embodiments of the invention. Laser spot zone includes more of the specimen tip and less of the specimen shank.

[0013] Figure 6 is a partially schematic illustration of laser spot size as a function of laser wavelength in accordance with certain embodiments of the invention.

[0014] Figure 7 is a partially schematic illustration of a specimen tip illustrating the shank angle and the tip radius in accordance with certain embodiments of the invention. [0015] Figure 8 is an estimated graph of Shank angle vs. Mass resolving power for (a) aluminum and (b) stainless steel in accordance with certain embodiments of the invention.

[0016] Figure 9 is a graph of Specimen tip radius versus Mass resolving power for aluminum and stainless steel in accordance with certain embodiments of the invention.

[0017] Figure 10 is a graph of Taper half angle versus (predicted) Mass resolving power at full-width-tenth-max and full-width-hundredth-max for 5 different laser spot sizes based on one heat flow model in accordance with certain embodiments of the invention.

DETAILED DESCRIPTION

[0018] In the following description, numerous specific details are provided in order to give a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well known structures, materials, or operations are not shown or described in order to avoid obscuring aspects of the invention.

[0019] References throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase "in one embodiment" or "in an embodiment" in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0020] Several specific details of the invention are set forth in the following description and in Figures 1-10 to provide a thorough understanding of certain embodiments of the invention. One skilled in the art, however, will understand that the present invention may have additional embodiments, and that other embodiments of the invention may be practiced without several of the specific features described below.

[0021] The performance of the pulsed-laser atom probe can be affected by both instrument and specimen factors. Experiments described in this disclosure were designed to identify these factors so as to provide direction for further instrument and specimen development. It was discovered that instrument performance including mass resolving power generally improves as the laser spot size and laser wavelength are decreased and as the specimen tip radius, specimen taper angle, and thermal diffusivity of the specimen material are increased. This implies that a laser atom probe consisting of a shorter wavelength laser beam impinging on a specimen at a smaller incidence angle (i.e. one approaching the line defined by the specimen tip extended) can yield higher resolution data than a laser atom probe with either a longer wavelength or a larger incidence angle.

[0022] Interest in pulsed-laser atom probe (PLAP) tomography has recently increased with the advent of stable, mode-locked laser sources. Typically the pulsed laser beam is introduced into the vacuum through a viewport and illuminates a sharp, needle-like specimen from the side, although other means such as fiberoptic coupling or an in-vacuum laser source are possible. A dc bias voltage of approximately +10 kV is applied to the specimen such that a field on the order of tens of volts per nanometer is generated at the specimen tip. The strong electric field, in combination with the pulsed laser, provides the excitation needed to overcome the potential barrier for ionization of the specimen material. An ion produced from the specimen surface is rapidly accelerated towards a detector where its position and time of arrival is encoded. The detected position of the ion is proportional to the original position of the atom in the specimen. The time difference between the laser pulse and the time at which the atom is detected is used to identify the atomic species. The ability of the microscope to discern one atomic species from another is referred to as the mass resolving power of the instrument.

[0023] Initial PLAP designs generally used nitrogen lasers with a wavelength of 337 nm, pulse widths greater than 300 ps, and pulse repetition rates below 0.1 kHz. It should be noted that laser spot sizes were not indicated in the literature, nor was the effect of different spot sizes investigated. Further the early designs did not utilize vibration isolated systems, hence it is postulated that the spot sizes were on the order of 25 microns. Without vibration isolation a small laser spot can easily drift off of the 50 to 200nm in diameter specimen tip.

[0024] Experiments performed throughout the 1980's with such lasers identified that field evaporation is assisted by heating of the specimen tip. Under normal PLAP conditions, specimen heating is limited to less than a few hundred Kelvin. It is commonly assumed that laser heating of the specimen is uniform as a result of the relatively fast thermal conduction across the narrow dimensions of the tip. Cooling after the laser pulse is mediated by thermal conduction through the specimen shank and, due to the small dimensions of the specimen, complete cooling is realized within approximately 100 ns.

[0025] Initial experiments performed by Kellogg and Tsong demonstrated that laser- assisted ionization could be used to expand the application space for atom probe to nonconductive materials. Despite these very exciting new applications, the development of PLAP stalled in the early 1990's for primarily two reasons: (1) commercially-available lasers were relatively unstable (e.g., pulse amplitude instability, transverse mode hopping), and (2) the laser-assisted ionization process was found to be very difficult to control. However, these problems have been overcome more recently with the advent of practical, stable, ultrafast laser sources, high speed control electronics, and precise beam-steering equipment. The ultrafast class of lasers utilize mode-locking laser cavities to achieve short pulses (< 15 ps) with high repetition rates, excellent pointing stability, excellent transverse mode stability, excellent beam quality (M² ~ 1), and extremely low background noise. [0026] These laser features, coupled with high-speed beam steering systems, as discussed in PCT Application No. US2004/026823, Attorney Docket No. 39245- 8109.WO00, filed August 19, 2004, entitled ATOM PROBE METHODS and PCT Application No. US2005/046842, Attorney Docket No. 39245-8111. WOOO, filed December 20, 2005, entitled LASER ATOM PROBES, and U.S. Provisional Patent Application No. 60/969,892, Attorney Docket No. 39245-8023.USOO, filed September 4, 2007, entitled METHODS AND APPARATUSES TO ALIGN ENERGY BEAM TO ATOM PROBE SPECIMEN which are fully incorporated herein by reference), allow the atom probe designer to create and control a diffraction-limited laser focus at the specimen apex and, in turn, fully realize the instrument performance benefits of a small heated volume.

[0027] In recent experiments specimens were prepared from both 316 stainless steel and aluminum wires. Stainless steel and aluminum were chosen in part because of their very different thermal properties. In addition, both intrinsic silicon and nickel- based superalloy specimens were analyzed in one experiment designed to measure the effect of laser pulse width. It is anticipated that other materials could yield similar results.

[0028] The metal samples were electropolished to an end radius of typically less than 50 nm using a solution of 10% perchloric acid diluted in acetic acid. The silicon specimens were prepared by a deep reactive-ion etch process and sharpened in a focused ion beam (FIB) instrument to a final tip radius on the order of lOOnm.

[0029] Instrument parameters such as wavelength and laser spot size were varied in these experiments. In all experiments reported here, the polarization of the laser is parallel to the specimen axis, although other polarizations are possible.

[0030] What follows are results obtained from experiments run under the indicated conditions with the indicated parameters. It should not be construed that the conditions and parameters indicated are the only ones that affect mass resolution.

Effective Pulse Fraction

[0031] The effective pulse fraction, PF _e, is defined by the ratio of the specimen voltage during the laser-assisted data acquisition, V, and the standing voltage required to field evaporate ions from the specimen at a prescribed rate of 10³ ions per second,

(1)

[0032] Effective pulse fraction is perhaps the most critical instrumentation parameter in a pulsed-laser atom probe experiment. Excessive laser pulse fraction can lead to overheating of the specimen tip and unwanted surface migration of atoms; insufficient laser pulse fraction can lead to problems with preferential evaporation and excessively high stresses caused by the increased specimen voltage.

[0033] Figure 1 illustrates the distribution of Si in 316 stainless steel as measured in pulsed-voltage versus pulsed-laser modes of operation. One may expect that the distribution of Si atoms should be uniform as observed in the pulsed-voltage experiment; any differences seen in the distribution of Si are most likely the result of surface migration and/or preferential evaporation. The data shown in Figure 1 are produced by measuring the local concentration of Si atoms in small cubic volume elements, or voxels, extracted from the reconstructed atom probe data. Each voxel was 1 nm³ in size. The variance of the local concentration values should follow a binomial distribution centered about the average Si concentration. The expected binomial distribution in the pulsed-voltage experiment as well as the pulsed-laser experiments for PF_e values of 0.1 to 0.4 are graphed . However, at high laser pulse fractions, the observed average composition decreases from -0.45% Si to -0.29% Si while, at the same time, there is an increasing frequency of voxels with greater than 1% Si. These data indicate that accurate composition and distribution measurements in stainless steel can be obtained at PF_e values below approximately 0.2 whereas, at higher pulse fractions, there is strong evidence of Si migration or clustering. This clustering can be attributed to surface migration.

[0034] In another example the laser spot size was controlled by first finding the optimal focus position on a given specimen tip at one spot size and then defocusing the beam by a known amount. Beam profile measurements were used to calibrate the minimum and defocused spot sizes. In this example spot sizes (as defined by the \le beam diameter) of 2.5 and 5 microns were used, but others are possible. The focusing lens should have a focal ratio (f/#) of less than 3 to produce the desired spot sizes. Further, if the lens is adjacent to the specimen (i.e. in vacuum) the position and orientation of the lens should be controllable within about a micron. Vacuum compatible nanopositioning stages should be utilized as well as vibration isolation systems.

[0035] PF_e was set to 0.1 at the beginning of each experiment and then held constant when changing laser spot size by using the following procedure: (1) with specimen voltage held constant, the laser pulse energy was decreased by -75% to prevent specimen rupture, (2) the spot size was then changed by moving the objective lens by a known amount, and (3) the laser pulse energy was increased until steady evaporation from the specimen tip was restored. This procedure ensures that the same DC bias voltage, and hence effective pulse fraction (assuming that the tip radius is essentially constant), is used for both laser spot sizes. As will be discussed later, specimen geometry can have a significant effect on mass resolving power and, therefore, must also be taken into consideration when designing the experiment. In these experiments, the effect of different specimen geometries were minimized by measuring the effect of spot size on each tip independently.

[0036] As the spot size decreased, mass resolving power was found to improve for the aluminum samples while there was no observable effect on stainless steel. Figure 2 reveals this effect for an aluminum sample where the mass resolving power improved from m/Δm of 409 ± 19 to 529 ± 33 at full width tenth-maximum (FW0.1M), and from 176 ± 4 to 300 ± 10 at full width hundredth-maximum (FW0.01M). [0037] Mass resolving power was measured on both aluminum and stainless steel samples using two discrete wavelengths: (1) the fundamental wavelength of the laser at 1064 nm, and (2) the second harmonic wavelength at 532 nm, but other wavelengths are feasible. Shorter wavelengths can be generated by utilizing higher harmonics (e.g., 266nm), different laser sources (e.g., helium-silver (224nm)) or other means.

[0038] In this example PF_e was maintained at 0.1 for both wavelengths.The spot size (diameter) of the laser, S, is known to increase with wavelength, λ, and therefore must be corrected for in the experiment design to ensure that changes in the spot size are not responsible for any observed changes in mass resolving power. The spot size is described by the following relationship for diffraction-limited optics:

S = 2λ^- D

(2) where/is the focal length of the objective lens and D is the diameter of the laser beam at the objective lens. Since the wavelengths cited previously differ by a factor of two (1064 and 532nm), the spot size of the 532 nm beam was doubled by defocusing the objective lens to eliminate this effect from the experiment design. Figure 6 illustrates the relationship between laser spot size and wavelength. Shorter wavelength lasers have inherently smaller spot sizes.

[0039] Figure 3 reveals the effect of laser wavelength on the mass resolving power for 316 stainless steel. Significant improvement in mass resolving power were obtained as the wavelength decreased; at 1064 nm, the mass resolving power of the Fe²⁺ peak at half maximum was 76.3±0.7 whereas, at 532 nm the mass resolving power was measured at 429±22. No significant change in mass resolving power was observed for the aluminum samples. When these results are compared to the spot size experiment two distinctly different behaviors became evident: smaller spot sizes improve the mass resolving power of aluminum but have no apparent effect on the stainless steel. On the other hand, shorter wavelengths improve the mass resolving power of stainless steel but seem to have no apparent effect on the aluminum. [0040] Mass resolving power for stainless steel and aluminum specimens can depend on various factors differently. Experimental data indicates that the mass resolution of aluminum can be improved by using a smaller laser spot size, however spot size did not seem to influence mass resolving power for stainless steel. On the other hand the mass resolving power of stainless steel can be improved by utilizing a shorter wavelength and forming the specimen with a larger tip radius, although neither of these factors had an effect on mass resolving power for aluminum. Stainless steel and aluminum have very different thermal properties so it seems reasonable that this difference could be used to explain the findings. At cryogenic temperatures similar to those present during the atom probe process (~30K) the thermal diffusivity of stainless steel is approximately 0.03 cm²/sec whereas the thermal diffusivity of aluminum is about 5 cm²/sec, more than 100 times greater.

[0041] Figure 4 illustrates the effect of laser incidence angle and the portion of the specimen tip that is illuminated. Figure 5 illustrates an incidence angle of about 20 degrees. It is apparent that smaller laser incidence angles with respect to the specimen axis can result in greater illumination of the tip. If one illuminated the tip at an incidence angle of zero (end-on) the entire tip would be illuminated. The data indicates that specimen tip geometry plays a strong role in determining the mass resolution performance of the laser atom probe. In some cases, instrument parameters such as spot size and wavelength can play an important role as well. For materials with large thermal conductivity and long electron diffusion depths, mass resolving power is improved by using larger specimen shank angles and smaller laser spot sizes. [0042] On the other hand, mass resolving power is improved by using large tip diameters and shorter wavelengths when analyzing materials with poor thermal conductivity and short electron diffusion depths. Figure 7 illustrates how specimen shank angle and tip radius are defined. Figure 8 estimates the effect of specimen shank angle on the mass resolving power for (a) aluminum and (b) stainless steel. The approximate limit of mass resolving power for the instrument at a given configuration was indicated. The shank angles were calculated based on the relationship V=F/(kr), where V is the voltage required to evaporate ions from a specimen tip, F is the field required to evaporate ions, r is the radius of the tip and k is a numerical constant (Miller et. al. 1996). Figure 9 illustrates the effect of specimen tip radius on the mass resolving power of aluminum and stainless steel. Two unique stainless steel specimens were analyzed. As is apparent from the data the effect of tip radius and mass resolving power for stainless steel is pronounced, while the effect of tip radius and mass resolving power for aluminum is not as great.

[0043] Figure 10 illustrates the predicted mass resolving power at full width tenth- max and full width hundredth-max for aluminum at taper half angles based on a heat flow model referenced in the paper Bunton, J., Olson, J., Lenz, D. & Kelly, T. Advances in Pulsed-Laser Atom Probe: Instrument and Specimen Design for Optimum Performance. Microscopy and Microanalysis, VoI 13 6, December 2007 (fully incorporated herein by reference). The model indicates that smaller laser spot sizes result in higher mass resolving power. Taper half angles are equal to the shank angle divided by two.

[0044] It should be noted that mirrors may be utilized to reflect or "steer" the beam of the laser, hence the laser itself may be positioned in or out of vacuum. In other embodiments fiber optic cable may be used to guide the laser beam (see e.g. Provisional Patent Application number 60/861,356, entitled ATOM PROBES USING PHOTONIC ENERGY AND ASSOCIATED METHODS filed November 27, 2006, re-filed as 60/989,191 on November 20, 2007, which is fully incorporated herein by reference). As such, the physical position of the laser source with respect to the specimen or other atom probe components is somewhat arbitrary. [0045] The following additional documents are fully incorporated herein by reference:

(a) Kellogg, G.L. (1981). Determining the field emitter temperature during laser irradiation in the pulsed laser atom probe. Journal of Applied Physics 52(8), 5320- 5328.

(b) Kellogg, G.L. & Tsong, T.T. (1980). Pulsed-laser atom-probe field-ion microscopy. Journal of Applied Physics 51(2), 1184-1192.

(c) Tsong, T.T., McLane, S.B. & Kinkus, TJ. (1982). Pulsed-laser time-of-flight atom-probe field ion microscope. Review of Scientific Instruments 53(9), 1442-1448. [0046] The teachings of the invention provided herein can be applied to other systems, not necessarily the system described herein. The elements and acts of the various embodiments described above can be combined to provide further embodiments.

[0047] All of the above patents and applications and other references, including any that may be listed in accompanying filing papers, are incorporated herein by reference. Aspects of the invention can be modified, if necessary, to employ the systems, functions, and concepts of the various references described above to provide yet further embodiments of the invention.

[0048] These and other changes can be made to the invention in light of the above Detailed Description. While the above description details certain embodiments of the invention and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the atom probe system may vary considerably in its implementation details, while still being encompassed by the invention disclosed herein. Particular terminology used when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. Accordingly, the actual scope of the invention encompasses not only the disclosed embodiments, but also all equivalent ways of practicing or implementing the invention.

[0049] From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the invention. For example, aspects of the invention described in the context of particular embodiments may be combined or eliminated in other embodiments. Furthermore, while advantages associated with certain embodiments of the invention have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the invention.

Claims

1. A method for increasing the mass resolution of an atom probe comprising:

(a) providing a constant electric field from a specimen tip to an electrode, wherein the magnitude of the electric field is less than that required to produce evaporation of ions from the tip; and

(b) providing a pulsed laser beam focused onto the tip to evaporate atoms on the tip, wherein the laser wavelength and spot size is selected based on a characteristic of the specimen.

2. The method of claim 1, wherein the laser wavelength is selected to be from approximately 200 nanometers to approximately 550 nanometers in wavelength.

3. The method of claim 1, wherein the laser is introduced at a small incidence angle with respect to the long axis of the specimen.

4. The method of Claim 2 wherein the spot size is less than 5 microns.

5. The method of Claim 3 wherein the spot size is less than 5 microns.

6. A method to improve the mass resolution of a laser atom probe wherein at least one of the wavelength and effective spot size of the incident laser beam is controlled based on at least one of the following parameters: thermal conductivity of a specimen, a diameter of the tip of the specimen, and a shank angle of the specimen tip.