Method for controlling energy density for reliable pulsed laser deposition of thin films P. C. Dowden, Z. Bi, and Q. X. Jia Citation: Review of Scientific Instruments 85, 025111 (2014); doi: 10.1063/1.4865716 View online: http://dx.doi.org/10.1063/1.4865716 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Increased grain boundary critical current density J c gb by Pr-doping in pulsed laser–deposited Y1−x Pr x BCO thin films J. Appl. Phys. 110, 113905 (2011); 10.1063/1.3664773 Nanostructure evolution of Y Ba 2 Cu 3 O x thin films grown by pulsed-laser glancing-angle deposition J. Vac. Sci. Technol. B 24, 1230 (2006); 10.1116/1.2194945 Improvement of critical current density and thermally assisted individual vortex depinning in pulsed-laserdeposited Y Ba 2 Cu 3 O 7 − δ thin films on Sr Ti O 3 (100) substrate with surface modification by Ag nanodots J. Appl. Phys. 97, 10B107 (2005); 10.1063/1.1851877 A novel approach for doping impurity in thin film insitu by dual-beam pulsed-laser deposition Rev. Sci. Instrum. 69, 3659 (1998); 10.1063/1.1149168 In situ plume-emission monitoring during pulsed-laser deposition of YBa2Cu3O7−δ and yttria-stabilized zirconia thin films Rev. Sci. Instrum. 68, 170 (1997); 10.1063/1.1147803
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 025111 (2014)
Method for controlling energy density for reliable pulsed laser deposition of thin films P. C. Dowden,a) Z. Bi, and Q. X. Jiaa) Center for Integrated Nanotechnologies, Division of Materials Physics and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
(Received 23 October 2013; accepted 1 February 2014; published online 26 February 2014) We have established a methodology to stabilize the laser energy density on a target surface in pulsed laser deposition of thin films. To control the focused laser spot on a target, we have imaged a defined aperture in the beamline (so called image-focus) instead of focusing the beam on a target based on a simple “lens-focus.” To control the laser energy density on a target, we have introduced a continuously variable attenuator between the output of the laser and the imaged aperture to manipulate the energy to a desired level by running the laser in a “constant voltage” mode to eliminate changes in the lasers’ beam dimensions. This methodology leads to much better controllability/reproducibility for reliable pulsed laser deposition of high performance electronic thin films. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865716] INTRODUCTION
Since the first report of using pulsed laser deposition (PLD) to grow high-temperature superconducting YBa2 CuO3 O7 films in 1987,1 PLD has been extensively investigated as one of the most powerful techniques to grow high-performance metal-oxide films with specific electrical, optical, magnetic, thermal, and/or superconducting properties. A typical PLD setup can be divided into three blocks: (i) a high-power laser which delivers high enough energy to ablate the target materials, (ii) optical mirrors and a lens to focus the laser beam on the target, and (iii) a vacuum chamber to house the target(s) and the substrate(s) where the deposition takes place.2 The most important advantages of PLD for the growth of films over other deposition techniques include (i) good stoichiometric preservation; (ii) high deposition rate; (iii) operable in a wide range of gas pressure during deposition; and (iv) high crystallinity films resulting from extended surface diffusion of adatoms in-between the pulses at a given substrate temperature. It, however, should be pointed out that some of the advantages listed above can be achieved only under certain circumstances. For example, the stoichiometry in a subsequent film will be the same as that of the target only when the fluence (defined as energy per unit area and expressed in J/cm2 ) is above a certain minimum threshold. To grow complex materials or multiple component compounds by PLD, it is important that the focused laser spot on the target has uniform energy density distribution, constant spot size, and stable intensity. This is particularly vital to grow superlattices of compound materials where not only is the chemical composition critical but also the individual layer thickness needs to be controlled accurately. Any change of these parameters will affect the properties of the film and inhibit the ability to control the layer thickness in unit cell accuracy. Even though there are many extensive reviews to discuss the processing-structure-property relationship of differa) Electronic addresses: [email protected] and [email protected]
0034-6748/2014/85(2)/025111/4/$30.00
ent materials deposited by PLD,3–7 there are very limited reports to describe the deposition setup2, 8 in order to obtain reproducible and controllable results. The scarcity of such reports may be due to the belief that the growth of thin films by PLD is a straightforward deposition technique. The fact is that it can be a very demanding task to reproducibly grow films with controllable and reproducible properties if the deposition setup is not well established. For instance, the properties of the films deposited by one group may not be repeated by another although the same processing parameters (such as laser energy, substrate temperature, gas pressure, pulse rate, and distance between the substrate and the target) and materials (such as the target and the substrate) are used. In this article, we report a methodology of pulsed laser deposition of thin films. The apparatus allows operators to accurately control the imaged laser spot on the target surface and to easily stabilize the laser energy. Such a deposition system enables us to reproducibly grow high-performance complex metal-oxide films with desired physical properties. APPROACHES
Control of laser energy density is a very complicated issue since gas-discharge lasers such as excimer lasers (ArF, KrF, and XeCl) commonly used for PLD have complex energy distribution. The laser energy density of such lasers varies spatially from the edge to the center of the beam spot. Furthermore, the gas in an excimer laser is excited by a high voltage discharge that generates photons for the lasing process. The beam size and shape are related to the freshness of the gas and to the drive voltage across the electrodes in the laser. It takes less drive voltage to achieve a desired energy for fresh laser gas than for old gas. Old gas must be driven to higher voltages to achieve a similar energy as for fresh gas. As a consequence, only a small area of the electrodes will produce a discharge at a lower voltage, resulting in smaller beam dimensions. At higher voltage, a larger area of the electrodes is involved, leading to an output with larger dimensions. These things should be considered when
85, 025111-1
© 2014 AIP Publishing LLC
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025111-2
Dowden, Bi, and Jia
calibrating the laser energy density because the focused spot size is related to the size of the beam incident on the focusing lens. This is particularly tricky for the commercially available excimer lasers operating the laser in a “constant energy” mode in which an operator simply enters a desired energy and the software that controls the laser determines the voltage required to achieve the output energy. The software associated with these lasers makes adjustments so that the voltage is driven higher to achieve the desired energy as the gas ages. This is particularly important since the gas used in an excimer laser is usually only good for a relatively short lifetime, depending on amount of laser usage. As a consequence of above, the laser energy density or fluence of the focused spot on a target can vary from deposition to deposition. It is important to realize that the dimensions of the focused spot size on a target change as a function of beam dimensions generated by the laser. This is significant in the context of stabilizing laser energy density or fluence. Since the fluence is critical to grow thin films with desired physical properties, it is essential to hold both the energy and spot size constant during the deposition. It should be also pointed out that there is a secondary effect of the focused spot size. The shape of the ejected plume is related to both the laser fluence and the size of the focused spot. It is clear that maintaining stable ablation conditions can be a very challenging task if both the spot size and the fluence are changing in an uncontrolled manner. It should be further noted that there exists a non-uniform energy distribution across the rectangular output beam of an excimer laser: a high energy around the central area but a low energy on either side of the central region as shown in Fig. 1, where the beam profile was captured using a Spiricon Beam profiler system with Beamgage software. Such an energy distribution leads to the following challenges. First, it is nearly impossible to accurately evaluate the energy density since the area of the focused spot is not well defined. Second, the energy density (or fluence) over the focused spot area is not uniform, which can cause composition variation of ablated materials due to the spatial distribution of the fluence on the target. This is especially true if a target is composed of two or more cations. To address these challenges, we have established a
FIG. 1. Image of laser beam and laser beam profile right at the output (or the laser aperture) of the KrF excimer laser operated at 22 kV. The energy density at the center of the laser aperture is defined as 100.
Rev. Sci. Instrum. 85, 025111 (2014)
FIG. 2. Schematic drawing of laser beam pathways for both image and lens focuses. SM is a switchable mirror which is used to steer the beam into deposition chamber #1 or deposition chamber #2.
methodology for reliable pulsed laser deposition of thin films. The following describes our experimental details. To control the focused laser spot on a target, we have imaged a defined aperture (or image-aperture) in the beamline (so called image focus) instead of focusing the beam based on a pure “lens focus.” The relationship between the image distance (si , the distance from the lens to the image of the laser beam on the target), the object distance (so , the distance from the lens to the aperture), and the focal length (f) of the lens can be described by 1 1 1 = + . f si so
(1)
It should be noted that both “image focus” and “lens focus” have been used in many research groups where the PLD systems are home-made or purchased from commercial vendors such as Neocera Incorporated. Figure 2 shows the schematic diagrams of both an image focus (Deposition chamber #1) and a lens-focus (Deposition chamber #2) setup. The laser beam is steered into Chamber #1 simply by moving switchable mirror SM away. In a specific setup where the geometrical constraints have to be considered, more mirrors are required to steer the beam. In our design, we have selected an image-aperture size of 20 mm × 10 mm (smaller than the output size of the beam with a dimension of 25 mm × 15 mm) to image onto the target. Slightly smaller image-aperture size allows us to capture the central region of the beam with the most uniform energy distribution (shown in Fig. 1). Importantly, this image-aperture in the beamline can produce a de-magnified image of the central portion of the laser beam on the target. The de-magnified image ratio is defined by m = −si /so , where so is the object distance and si is the image distance as defined in Eq. (1). This imaging technique results in a very uniform rectangular spot on the target. It is noted that we chose the size of the aperture to fully capture the
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FIG. 3. Photographs of laser burn spots: (a) image focus and (b) lens focus. The output energy of the laser is 770 mJ/pulse in both cases. The output energy after the image-aperture is 235 mJ/pulse (3.5 J/cm2 ).
“centroid” of the output from this particular laser. One can use the aperture dimension as an easy “knob” to adjust the image area on a target while leaving the beamline fixed at a known de-magnification ratio. As shown in Figure 3(a) image focus leads to a much cleaner and well defined spot on the target surface. Importantly, there are no stray and low intensity regions around the edge. In comparison, the spot on the target surface with pure lens focus is shown in Figure 3(b). The energy distribution across the imaged spot is much more uniform when compared to a simple lens focus. It should be noted that de-magnification ratio is limited by the minimum distance from the laser entrance window and the target surface. In other words, this distance can be used to roughly estimate where to start in the equation to calculate the object distance. Furthermore, there is a practical limit to the image distance without going to extremely large diameter optics due to the beam divergence and scattering by air. To stabilize the laser energy density on a target, we need to address the change of the beam size resulting from the degradation of the laser gas and corresponding change in discharge voltage when operating the laser in constant energy mode. This can be done by running the laser in a “constant voltage” mode instead of “constant energy” mode. After studying the beam shapes at different discharge voltages, we have found that the output beam is at its maximum dimensions (25 mm × 15 mm) at a drive voltage of 22 kV (Lambda Physik LPX300). According to Lambda Physik, the laser has no known ill effects from running a full rated voltage. At this voltage the laser is producing far more energy per pulse than required for effective growth of a wide range of materials. Importantly, the beam size is maximized and doesn’t change. We would like to point out that another benefit of running the laser at higher voltage is that the laser gas (the active medium) is in a state of gain saturation, meaning that the volume of gas is excited to its maximum which leads to far more stable shotto-shot performance. All that is necessary now is to introduce a continuously variable attenuator with a high enough damage threshold between the output of the laser and the imaged aperture (as shown in Fig. 2) to limit the energy to a desired level. There are many ways to accomplish the attenuation. In our setup, a commercial Lambda Physik attenuator is chosen since it is continuously variable and repeatable with a reasonably high damage threshold. By measuring the pulse energy
Rev. Sci. Instrum. 85, 025111 (2014)
FIG. 4. Relationship between the fluence on the target and the energy output from the attenuator. Inset shows the relationship between the transmittance and the dial position of the attenuator.
through the imaged-aperture and knowing the exact area of the imaged spot on a target, we can now accurately adjust and control the fluence on the target. Figure 4 shows the relationship between the energy densities (or fluence) and the attenuator output (mJ/pulse), where the attenuator output is simply controlled by the attenuator dial position as shown in the inset of Fig. 4. As can be seen from Figure 4, the energy measured at the output of image-aperture can be continuously and accurately varied between 20 mJ/pulse and 270 mJ/pulse, which translate to an energy density on the imaged spot between 0.3 J/cm2 and 4.25 J/cm2 , respectively. EXAMPLES
Using the setup described above, we deposited superconducting YBa2 Cu3 O7-δ (YBCO) films on (100) SrTiO3 at different energy densities simply by dialing the attenuator to different positions. The target was a stoichiometric YBa2 Cu3 Ox pellet commercially available from MTI Corp. (Richmond, California, USA). The laser (KrF, λ = 248 nm) operated at a repetition rate of 10 Hz at a constant voltage of 22 kV. All depositions were done at a temperature of 790 ◦ C, an oxygen pressure of 200 mTorr, and a distance of 5.5 cm from the target to the substrate. The YBCO films were cooled to room temperature in an oxygen pressure of 250 Torr without any further thermal treatment. Figure 5 shows the superconducting transition temperature and the transition width of the films deposited at
FIG. 5. Superconducting transition temperature (Tc) and transition width (Tc) of YBCO films as a function of laser fluence. All films were deposited under the same conditions except for the fluence.
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different fluences. At a fluence lower than 1.5 J/cm2 , the superconducting film showed lower transition temperature and broad transition width. As discussed above, the stoichiometry in the deposited film can be different from that of the target if the fluence is below a threshold. Both the transition temperature and the transition width of the superconducting films reach their desired values when the fluence is above 1.5 J/cm2 . This example clearly shows that the fluence plays an important role in the deposition of high performance YBCO films. Using our methodology, one can easily optimize the laser fluence for the growth of a wide range of materials with desired properties.
SUMMARY
In conclusion, we have developed a methodology to stabilize laser energy density on a target in pulsed laser deposition of films. By driving the laser near its maximum voltage level, we have better control of beam size and can maintain a stable energy output. By introducing a variable energy attenuator between the aperture of an excimer laser and the imageaperture, using a well-defined image-aperture to capture the central region of the output beam of the laser where the spatial distribution of energy is most uniform, and imaging as well as de-magnifying that aperture on the target surface, we can accurately control the focused laser spot on the target and importantly stabilize the laser energy density or fluence. This methodology leads to much better controllability and repro-
Rev. Sci. Instrum. 85, 025111 (2014)
ducibility of pulsed laser deposition of films and is applicable to a broad range of applications in laser ablation of a variety of materials. ACKNOWLEDGMENTS
This work was performed at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility and under the auspices of the Department of Energy, Office of Basic Energy Sciences. It was also partially supported by the NNSA’s Laboratory Directed Research and Development Program. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under Contract No. DE-AC52-06NA25396. 1 D.
Dijkkamp, T. Venkatesan, X. D. Wu, S. A. Shaheen, N. Jisrawi, Y. H. Min-Lee, W. L. McLean, and M. Croft, Appl. Phys. Lett. 51, 619 (1987). 2 Pulsed Laser Deposition of Thin Films, edited by D. B. Chrisey and G. K. Hubler (John Wiley and Sons, New York, 1994). 3 D. H. Lowndes, D. B. Geohegan, A. A. Puretzky, D. P. Norton, and C. M. Rouleau, Science 273, 898 (1996). 4 M. D. Shirk and P. A. Molian, J. Laser Appl. 10, 18 (1998). 5 T. J. Jackson and S. B. Palmer, J. Phys. D: Appl. Phys. 27, 1581 (1994). 6 M. N. R. Ashfold, F. Claeyssens, G. M. Fuge, and S. J. Henley, Chem. Soc. Rev. 33, 23 (2004). 7 H. M. Christen and G. Eres, J. Phys.: Condens. Matter 20, 264005 (2008). 8 J. A. Greer and M. D. Tabat, J. Vac. Sci. Technol., A 13, 1175 (1995).
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 025111 (2014)
Method for controlling energy density for reliable pulsed laser deposition of thin films P. C. Dowden,a) Z. Bi, and Q. X. Jiaa) Center for Integrated Nanotechnologies, Division of Materials Physics and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
(Received 23 October 2013; accepted 1 February 2014; published online 26 February 2014) We have established a methodology to stabilize the laser energy density on a target surface in pulsed laser deposition of thin films. To control the focused laser spot on a target, we have imaged a defined aperture in the beamline (so called image-focus) instead of focusing the beam on a target based on a simple “lens-focus.” To control the laser energy density on a target, we have introduced a continuously variable attenuator between the output of the laser and the imaged aperture to manipulate the energy to a desired level by running the laser in a “constant voltage” mode to eliminate changes in the lasers’ beam dimensions. This methodology leads to much better controllability/reproducibility for reliable pulsed laser deposition of high performance electronic thin films. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865716] INTRODUCTION
Since the first report of using pulsed laser deposition (PLD) to grow high-temperature superconducting YBa2 CuO3 O7 films in 1987,1 PLD has been extensively investigated as one of the most powerful techniques to grow high-performance metal-oxide films with specific electrical, optical, magnetic, thermal, and/or superconducting properties. A typical PLD setup can be divided into three blocks: (i) a high-power laser which delivers high enough energy to ablate the target materials, (ii) optical mirrors and a lens to focus the laser beam on the target, and (iii) a vacuum chamber to house the target(s) and the substrate(s) where the deposition takes place.2 The most important advantages of PLD for the growth of films over other deposition techniques include (i) good stoichiometric preservation; (ii) high deposition rate; (iii) operable in a wide range of gas pressure during deposition; and (iv) high crystallinity films resulting from extended surface diffusion of adatoms in-between the pulses at a given substrate temperature. It, however, should be pointed out that some of the advantages listed above can be achieved only under certain circumstances. For example, the stoichiometry in a subsequent film will be the same as that of the target only when the fluence (defined as energy per unit area and expressed in J/cm2 ) is above a certain minimum threshold. To grow complex materials or multiple component compounds by PLD, it is important that the focused laser spot on the target has uniform energy density distribution, constant spot size, and stable intensity. This is particularly vital to grow superlattices of compound materials where not only is the chemical composition critical but also the individual layer thickness needs to be controlled accurately. Any change of these parameters will affect the properties of the film and inhibit the ability to control the layer thickness in unit cell accuracy. Even though there are many extensive reviews to discuss the processing-structure-property relationship of differa) Electronic addresses: [email protected] and [email protected]
0034-6748/2014/85(2)/025111/4/$30.00
ent materials deposited by PLD,3–7 there are very limited reports to describe the deposition setup2, 8 in order to obtain reproducible and controllable results. The scarcity of such reports may be due to the belief that the growth of thin films by PLD is a straightforward deposition technique. The fact is that it can be a very demanding task to reproducibly grow films with controllable and reproducible properties if the deposition setup is not well established. For instance, the properties of the films deposited by one group may not be repeated by another although the same processing parameters (such as laser energy, substrate temperature, gas pressure, pulse rate, and distance between the substrate and the target) and materials (such as the target and the substrate) are used. In this article, we report a methodology of pulsed laser deposition of thin films. The apparatus allows operators to accurately control the imaged laser spot on the target surface and to easily stabilize the laser energy. Such a deposition system enables us to reproducibly grow high-performance complex metal-oxide films with desired physical properties. APPROACHES
Control of laser energy density is a very complicated issue since gas-discharge lasers such as excimer lasers (ArF, KrF, and XeCl) commonly used for PLD have complex energy distribution. The laser energy density of such lasers varies spatially from the edge to the center of the beam spot. Furthermore, the gas in an excimer laser is excited by a high voltage discharge that generates photons for the lasing process. The beam size and shape are related to the freshness of the gas and to the drive voltage across the electrodes in the laser. It takes less drive voltage to achieve a desired energy for fresh laser gas than for old gas. Old gas must be driven to higher voltages to achieve a similar energy as for fresh gas. As a consequence, only a small area of the electrodes will produce a discharge at a lower voltage, resulting in smaller beam dimensions. At higher voltage, a larger area of the electrodes is involved, leading to an output with larger dimensions. These things should be considered when
85, 025111-1
© 2014 AIP Publishing LLC
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025111-2
Dowden, Bi, and Jia
calibrating the laser energy density because the focused spot size is related to the size of the beam incident on the focusing lens. This is particularly tricky for the commercially available excimer lasers operating the laser in a “constant energy” mode in which an operator simply enters a desired energy and the software that controls the laser determines the voltage required to achieve the output energy. The software associated with these lasers makes adjustments so that the voltage is driven higher to achieve the desired energy as the gas ages. This is particularly important since the gas used in an excimer laser is usually only good for a relatively short lifetime, depending on amount of laser usage. As a consequence of above, the laser energy density or fluence of the focused spot on a target can vary from deposition to deposition. It is important to realize that the dimensions of the focused spot size on a target change as a function of beam dimensions generated by the laser. This is significant in the context of stabilizing laser energy density or fluence. Since the fluence is critical to grow thin films with desired physical properties, it is essential to hold both the energy and spot size constant during the deposition. It should be also pointed out that there is a secondary effect of the focused spot size. The shape of the ejected plume is related to both the laser fluence and the size of the focused spot. It is clear that maintaining stable ablation conditions can be a very challenging task if both the spot size and the fluence are changing in an uncontrolled manner. It should be further noted that there exists a non-uniform energy distribution across the rectangular output beam of an excimer laser: a high energy around the central area but a low energy on either side of the central region as shown in Fig. 1, where the beam profile was captured using a Spiricon Beam profiler system with Beamgage software. Such an energy distribution leads to the following challenges. First, it is nearly impossible to accurately evaluate the energy density since the area of the focused spot is not well defined. Second, the energy density (or fluence) over the focused spot area is not uniform, which can cause composition variation of ablated materials due to the spatial distribution of the fluence on the target. This is especially true if a target is composed of two or more cations. To address these challenges, we have established a
FIG. 1. Image of laser beam and laser beam profile right at the output (or the laser aperture) of the KrF excimer laser operated at 22 kV. The energy density at the center of the laser aperture is defined as 100.
Rev. Sci. Instrum. 85, 025111 (2014)
FIG. 2. Schematic drawing of laser beam pathways for both image and lens focuses. SM is a switchable mirror which is used to steer the beam into deposition chamber #1 or deposition chamber #2.
methodology for reliable pulsed laser deposition of thin films. The following describes our experimental details. To control the focused laser spot on a target, we have imaged a defined aperture (or image-aperture) in the beamline (so called image focus) instead of focusing the beam based on a pure “lens focus.” The relationship between the image distance (si , the distance from the lens to the image of the laser beam on the target), the object distance (so , the distance from the lens to the aperture), and the focal length (f) of the lens can be described by 1 1 1 = + . f si so
(1)
It should be noted that both “image focus” and “lens focus” have been used in many research groups where the PLD systems are home-made or purchased from commercial vendors such as Neocera Incorporated. Figure 2 shows the schematic diagrams of both an image focus (Deposition chamber #1) and a lens-focus (Deposition chamber #2) setup. The laser beam is steered into Chamber #1 simply by moving switchable mirror SM away. In a specific setup where the geometrical constraints have to be considered, more mirrors are required to steer the beam. In our design, we have selected an image-aperture size of 20 mm × 10 mm (smaller than the output size of the beam with a dimension of 25 mm × 15 mm) to image onto the target. Slightly smaller image-aperture size allows us to capture the central region of the beam with the most uniform energy distribution (shown in Fig. 1). Importantly, this image-aperture in the beamline can produce a de-magnified image of the central portion of the laser beam on the target. The de-magnified image ratio is defined by m = −si /so , where so is the object distance and si is the image distance as defined in Eq. (1). This imaging technique results in a very uniform rectangular spot on the target. It is noted that we chose the size of the aperture to fully capture the
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FIG. 3. Photographs of laser burn spots: (a) image focus and (b) lens focus. The output energy of the laser is 770 mJ/pulse in both cases. The output energy after the image-aperture is 235 mJ/pulse (3.5 J/cm2 ).
“centroid” of the output from this particular laser. One can use the aperture dimension as an easy “knob” to adjust the image area on a target while leaving the beamline fixed at a known de-magnification ratio. As shown in Figure 3(a) image focus leads to a much cleaner and well defined spot on the target surface. Importantly, there are no stray and low intensity regions around the edge. In comparison, the spot on the target surface with pure lens focus is shown in Figure 3(b). The energy distribution across the imaged spot is much more uniform when compared to a simple lens focus. It should be noted that de-magnification ratio is limited by the minimum distance from the laser entrance window and the target surface. In other words, this distance can be used to roughly estimate where to start in the equation to calculate the object distance. Furthermore, there is a practical limit to the image distance without going to extremely large diameter optics due to the beam divergence and scattering by air. To stabilize the laser energy density on a target, we need to address the change of the beam size resulting from the degradation of the laser gas and corresponding change in discharge voltage when operating the laser in constant energy mode. This can be done by running the laser in a “constant voltage” mode instead of “constant energy” mode. After studying the beam shapes at different discharge voltages, we have found that the output beam is at its maximum dimensions (25 mm × 15 mm) at a drive voltage of 22 kV (Lambda Physik LPX300). According to Lambda Physik, the laser has no known ill effects from running a full rated voltage. At this voltage the laser is producing far more energy per pulse than required for effective growth of a wide range of materials. Importantly, the beam size is maximized and doesn’t change. We would like to point out that another benefit of running the laser at higher voltage is that the laser gas (the active medium) is in a state of gain saturation, meaning that the volume of gas is excited to its maximum which leads to far more stable shotto-shot performance. All that is necessary now is to introduce a continuously variable attenuator with a high enough damage threshold between the output of the laser and the imaged aperture (as shown in Fig. 2) to limit the energy to a desired level. There are many ways to accomplish the attenuation. In our setup, a commercial Lambda Physik attenuator is chosen since it is continuously variable and repeatable with a reasonably high damage threshold. By measuring the pulse energy
Rev. Sci. Instrum. 85, 025111 (2014)
FIG. 4. Relationship between the fluence on the target and the energy output from the attenuator. Inset shows the relationship between the transmittance and the dial position of the attenuator.
through the imaged-aperture and knowing the exact area of the imaged spot on a target, we can now accurately adjust and control the fluence on the target. Figure 4 shows the relationship between the energy densities (or fluence) and the attenuator output (mJ/pulse), where the attenuator output is simply controlled by the attenuator dial position as shown in the inset of Fig. 4. As can be seen from Figure 4, the energy measured at the output of image-aperture can be continuously and accurately varied between 20 mJ/pulse and 270 mJ/pulse, which translate to an energy density on the imaged spot between 0.3 J/cm2 and 4.25 J/cm2 , respectively. EXAMPLES
Using the setup described above, we deposited superconducting YBa2 Cu3 O7-δ (YBCO) films on (100) SrTiO3 at different energy densities simply by dialing the attenuator to different positions. The target was a stoichiometric YBa2 Cu3 Ox pellet commercially available from MTI Corp. (Richmond, California, USA). The laser (KrF, λ = 248 nm) operated at a repetition rate of 10 Hz at a constant voltage of 22 kV. All depositions were done at a temperature of 790 ◦ C, an oxygen pressure of 200 mTorr, and a distance of 5.5 cm from the target to the substrate. The YBCO films were cooled to room temperature in an oxygen pressure of 250 Torr without any further thermal treatment. Figure 5 shows the superconducting transition temperature and the transition width of the films deposited at
FIG. 5. Superconducting transition temperature (Tc) and transition width (Tc) of YBCO films as a function of laser fluence. All films were deposited under the same conditions except for the fluence.
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Dowden, Bi, and Jia
different fluences. At a fluence lower than 1.5 J/cm2 , the superconducting film showed lower transition temperature and broad transition width. As discussed above, the stoichiometry in the deposited film can be different from that of the target if the fluence is below a threshold. Both the transition temperature and the transition width of the superconducting films reach their desired values when the fluence is above 1.5 J/cm2 . This example clearly shows that the fluence plays an important role in the deposition of high performance YBCO films. Using our methodology, one can easily optimize the laser fluence for the growth of a wide range of materials with desired properties.
SUMMARY
In conclusion, we have developed a methodology to stabilize laser energy density on a target in pulsed laser deposition of films. By driving the laser near its maximum voltage level, we have better control of beam size and can maintain a stable energy output. By introducing a variable energy attenuator between the aperture of an excimer laser and the imageaperture, using a well-defined image-aperture to capture the central region of the output beam of the laser where the spatial distribution of energy is most uniform, and imaging as well as de-magnifying that aperture on the target surface, we can accurately control the focused laser spot on the target and importantly stabilize the laser energy density or fluence. This methodology leads to much better controllability and repro-
Rev. Sci. Instrum. 85, 025111 (2014)
ducibility of pulsed laser deposition of films and is applicable to a broad range of applications in laser ablation of a variety of materials. ACKNOWLEDGMENTS
This work was performed at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility and under the auspices of the Department of Energy, Office of Basic Energy Sciences. It was also partially supported by the NNSA’s Laboratory Directed Research and Development Program. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under Contract No. DE-AC52-06NA25396. 1 D.
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