THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING

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1 THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING ADHESIVE-BASED SUBSTRATE MOUNTING METHOD FOR USE IN OXIDE MOLECULAR BEAM EPITAXY MATTHEW STEVEN RAHN Spring 2016 A thesis submitted in partial fulfillment of the requirements for a baccalaureate degree in Materials Science and Engineering with honors in Materials Science and Engineering Reviewed and approved* by the following: Roman Engel-Herbert Assistant Professor of Materials Science and Engineering Thesis Supervisor Robert Allen Kimel Associate Head of Materials Science and Engineering Assistant Professor of Materials Science and Engineering Honors Adviser * Signatures are on file in the Schreyer Honors College.

2 i ABSTRACT Molecular beam epitaxy is a nanoscale synthesis technique used to grow crystalline thin films with atomic layer precision. While molecular beam epitaxy gives precise control over the impinging flux of atoms being incorporated into each monolayer of a film, sequencing of these atomic fluxes provides the ability to build complex crystals with large unit cells from the bottom up. To successfully employ this growth technique, measuring and controlling atomic fluxes impinging on the growing surface is vital, however other growth parameters, such as substrate temperature and its variability, are key parameter as well, which need to be precisely determined and regulated to ensure a successful growth and high reproducibility. Although this seems trivial to accomplish at first sight, it has been proven difficult in a molecular beam epitaxy setup. This thesis details the design and testing of an alternate, adhesive-based strategy for mounting substrates for use in an oxide molecular beam epitaxy system dedicated to the growth of complex oxide thin films. Such a strategy is highly desired due to the drawbacks of the currently employed gravity mounting method, which can only mount one sample of fixed geometry, partially limiting growth and in-situ analysis techniques due to the difficulties of mechanically mounting a fragile, transparent single crystal substrate used to condense the impinging atom fluxes on the surface. The proposed adhesive-based system must provide increased thermal coupling between the substrate heater and the substrate mounted on the sample manipulator and reduce temperature variability across the substrate surface, while not outgassing any unwanted species at the high growth temperature and under ultra-high vacuum conditions employed in the growth. Several different adhesives were evaluated using thermogravimetric analysis with gas mass spectrometry to determine their suitability for the high temperature, ultra-high vacuum

3 ii growth environment. Substrates were radially mounted to a silicon wafer using an additively manufactured mounting bracket to test different adhesives. Once mounted, temperature gradients across the sample were measured using a pyrometer and contamination levels on the sample surface were analyzed using x-ray photoelectron spectroscopy. A substrate removal tool was developed for a quick, controlled removal of the mounted substrates after film growth. Contamination levels and temperatures from the current mounting method, a reusable tantalum face plate, were also evaluated and compared to the different adhesive mounting methods tested. The sample mounting strategy tested and developed in this thesis was found to offer an increased level of functionality, temperature control and a lesser degree of variation, while approaching the ease of use of the currently employed gravity mounting mechanism, directly impacting future growth results in the oxide molecular beam epitaxy research group at Penn State University.

4 iii TABLE OF CONTENTS LIST OF FIGURES... iii LIST OF TABLES... iv ACKNOWLEDGEMENTS... v Chapter 1 Introduction to Molecular Beam Epitaxy : Fundamentals of Molecular Beam Epitaxy : Challenges Specific to Oxide Molecular Beam Epitaxy : Substrate Mounting Methods : Research Motivation Chapter 2 Device Design and Experimental Procedures : Bracket and Substrate Removal Tool Design and Testing : Thermogravimetric and Residual Gas Analysis of Adhesives : Substrate Annealing and Temperature Measurements : Outgassing and Contamination Evaluation Chapter 3 Results: Assessing Adhesive and Mount Device Performance : Determining Suitability of Adhesives using Thermogravimetric Analysis : Temperature Comparisons and Gradient Mapping : Substrate Contamination from Outgassing and Redeposition Chapter 4 Conclusions : Future Work Appendix A Code for Automated Temperature Gradient Mapping BIBLIOGRAPHY... 43

5 iv LIST OF FIGURES Figure 1. Schematic of hybrid molecular beam epitaxy chamber using a metalorganic precursor in conjunction with conventional effusion cells. Figure adapted from reference [3] Figure 2. Schematic showing cross sections of substrates mounted using a tantalum gravity mount (left) and a solder mount (right). The arrows indicates the direction of flux from the effusion cells Figure 3. Graph of vapor pressure as a function of temperature for effusion cell materials (Ca, Sr), solder materials (Ga, In, Sn), and structural materials (Ta). Metalorganic precursors instead of effusion cells can be used to incorporate materials with low vapor pressures at high temperatures (Ti, V), such as TTIP and VTIP in hybrid molecular beam epitaxy. Data from reference [7] Figure 4. Designed bracket for substrate mounting and accompanying substrate removal tool. The substrate-wafer assembly as shown consists of a silicon wafer with mounted substrates, shown in black Figure 5. Schematics of FDM (left) compared to SLA (right) printing methods. In both figures, the printed part is shown in light blue Figure 6. Thermogravimetric data showing mass loss over time for all adhesives. A low mass loss at growth temperature is desired to avoid contaminating the growing film Figure 7. Residual gas analysis highlights of cured (light colors) compared to uncured (dark colors) carbon paste. Primary outgassing species include water (H2O+, OH+) and to a lesser extent, carbon dioxide (CO2+). Helium was used as a carrier gas Figure 8. Discrepancy between the residual gas analysis background and sample for carbon paste showing reaction of oxygen with elements of the paste to form other gaseous species above 750 C. The oxygen ion current produced by the sample drops below the oxygen ion current of the background, showing that oxygen is reacting Figure 9. Surface temperature of silicon substrates mounted on silicon with adhesives and a silicon wafer mounted using a tantalum face plate with increasing manipulator temperature.30 Figure 10. Temperature gradient mapping across two substrates mounted using zirconia paste at different manipulator temperatures. The two valleys in the graph are maps across each substrate, while the center peak shows the underlying wafer temperature Figure 11. XPS data showing the C1s peak indicating the presence of carbon on silicon substrates. Three substrates were evaluated: one mounted with carbon paste and annealed, one mounted using a tantalum face plate and annealed, and one exposed to vacuum and not annealed Figure 12. Lever mounting scheme for substrates consisting of a silicon wafer (black), adhesive (grey), and a lever (black) with substrates (blue) to produce a temperature gradient at a fixed manipulator temperature. A projected temperature profile is shown above. Additional

6 substrates could be fixed at points on the lever to grow an assay of films at different temperatures quickly v

7 vi LIST OF TABLES Table 1: Adhesive Properties and Specifications Table 2: Advantages and Drawbacks of Viable Mounting Methods... 37

8 vii ACKNOWLEDGEMENTS I would like to thank my research advisor, Dr. Roman Engel-Herbert, for his patience and assistance with this work. I would also like to thank Dr. Michael Hickner, for his support and assistance with rapid prototyping parts using additive manufacturing.

9 1 Chapter 1 Introduction to Molecular Beam Epitaxy 1.1: Fundamentals of Molecular Beam Epitaxy Molecular beam epitaxy (MBE) is an epitaxial growth technique carried out under ultrahigh vacuum conditions used to grow thin films and superlattices of a wide variety of materials. Films are grown through the interaction of several atomic or molecular species at the substrate surface, which are generated from heated effusion cells mounted radially around and pointing towards the substrate manipulator holding the substrate in the focal point of all effusion cells within the growth chamber. The substrate upon which the film is grown is heated to provide adequate energy for the diffusion of adsorbed species on the surface and formation of bonds with the underlying substrate. This bond formation allows the film to pick up the atomic registry of the underlying substrate, an epitaxial relationship between substrate and film is established. To avoid flux gradients across the sample, the sample manipulator is rotated to average over flux difference caused by the finite substrate size and geometric arrangement of substrate manipulator and effusion cells, creating a homogenous film. Typical rotation speed are between 1-10 rounds per minute, similar to typical growth rates of 1-10 monolayers per minute. A molecular beam epitaxy system is composed of several key elements that include tools for growth and in-situ characterization. A schematic of a hybrid molecular beam epitaxy system is shown in Figure 1. The main feature of a molecular beam epitaxy system is an array of

10 2 Knudsen effusion cells which point at the substrate. These cells contain a crucible composed of a material with thermal stability and low vapor pressure, such as boron nitride or refractory metals, filled with a charge of elements [1]. When heating the charge, the vapor pressure in the crucible increases, producing a molecular beam which deposits on the substrate surface. To control deposition from multiple effusion cells, mechanical shutters are used to block the cell when not in use. These shutters are composed of metals with low vapor pressures at elevated temperatures, such as tantalum or molybdenum. Aside from effusion cells supplying elements, an oxygen plasma source generating molecular and atomic oxygen, and - in the special setup of a hybrid molecular beam epitaxy system available at Penn State - gas injectors to supply metalorganic molecules containing elements directly bonded to oxygen can be used to supply oxygen in various states to form a film [2]. Typical oxygen sources include molecular oxygen in the gas phase released into the system by a gas inlet system or oxygen plasma generated by an RF plasma source mounted in a similar manner to an effusion cell. The substrate is mounted in front of a radiative heating element, which heats the substrate to the desired growth temperature. A heated substrate is required to provide sufficient energy for diffusion of impinging species at the substrate surface and allow layer by layer growth. Surface diffusion length, the distance an adsorbed molecule or atom can travel on a surface, increases with increasing temperature, allowing the molecule or atom to find energetically favorable sites on the surface to incorporate itself. If the substrate is not heated, the surface diffusion length of atoms will be too small to allow the formation of single monolayers, resulting in island growth on the film surface. Like the effusion cell shutters, the substrate holder is made of tantalum to avoid contamination at high growth temperatures.

11 3 Figure 1. Schematic of hybrid molecular beam epitaxy chamber using a metalorganic precursor in conjunction with conventional effusion cells. Figure adapted from reference [3]. Commonly employed in-situ analysis and metrology techniques in a molecular beam epitaxy system include reflection deposition rate analysis, high-energy electron diffraction (RHEED), residual gas analysis (RGA), and temperature analysis via an infrared pyrometer. A deposition rate monitor is used to measure the atomic fluxes supplied from the effusion cells and determine deposition rate on the substrate from these sources. The monitor consists of a piezoelectric crystal which is driven into oscillation, close to its resonant frequency by an applied AC voltage. As material emanating from the effusion cell is deposited on the crystal, the oscillating mass and thus the resonant frequency shifts, which can be directly related to the deposition rate. This method, in conjunction with RHEED oscillations, provides an in-situ metrology of deposition rate and film growth [3]. In RHEED, a monochromatic electron beam strikes the atomically smooth substrate surface at a glancing angle of θ < 2. This beam

12 4 produces a diffraction pattern indicative of the crystal structure of the top few monolayers of the substrate. This diffraction pattern is displayed using a phosphor screen mounted opposite of the electron gun. As this diffraction technique is extremely surface sensitive, the intensity of the diffraction pattern can be used to determine the number of monolayers grown based on the cyclic intensity oscillations of the RHEED pattern, a third independent measure of the film s growth rate. A maximum intensity in the RHEED diffraction spots occurs when the surface is atomically smooth, that is, when a single monolayer has been completed [3]. When a new monolayer is forming, the surface is not perfectly smooth. Small islands with thickness of around one atomic layer are formed. This increase in surface roughness at the atomic scale results in a slight decrease in the intensity of the diffraction pattern. RGA is a form of mass spectroscopy used to analyze the composition of background gases within the chamber. In RGA, gas species are ionized by an applied potential and their mass to charge ratio measured, enabling species identification. Several requirements exist for molecular beam epitaxy. Firstly, the mean free path of atoms, λ, originating from effusion cells within the chamber should be larger than the chamber size to avoid their collision in the gas phase and potential pre-reaction before reaching the growing surface: λ > 1 m The mean free path is a function of the pressure within the chamber, the molecular diameter, d0 of the species, and the Boltzmann constant, kb, thus: λ = 1 2πd 0 2 N ap chamber RT k B T = 2πd 2 0 P chamber

13 P chamber = torr 10 5 torr [1] 5 For a strontium atom of d0 = 448 pm at a temperature of 298 K, the pressure at which the mean free path is larger than the chamber diameter can be calculated, as seen above. Thus, the condition for inelastic mean free path of impinging atoms will be fulfilled if the chamber pressure is less than 10-5 torr and the gas molecules will collide with the chamber walls rather than colliding with each other. A second, more stringent pressure requirement must also be satisfied for the growth of high quality thin films. In order to avoid unintentional incorporation of species from the gas phase rather than the atoms and molecules supplied from the effusion cells, the deposition time of one monolayer of contaminant from background vapors present in the chamber must be several orders of magnitude smaller than the deposition time for one monolayer of effusion cell material: t background < 10 4 t cell To fulfil this condition, even lower pressures are required. Under high vacuum conditions with a background pressure of pbackground=10-6 torr, the deposition rate of contaminants is on the order of one monolayer of adsorbate per second, assuming a sticking coefficient of one [1]. The background pressure must therefore be lowered to ultra-high vacuum conditions ( torr), where the deposition rate of contaminants is 10-4 monolayers of adsorbate per second and thus low enough to critically impact film quality [1]. To attain and maintain ultra-high vacuum conditions within the molecular beam epitaxy system, a turbomolecular pump fitted with a backing scroll pump are used in conjunction with cryogenic panels within the body of the growth chamber in addition to a cryopump. Despite the growth chamber all molecular beam epitaxy

14 systems have a load lock and a buffer chamber to preserve the vacuum integrity of the growth 6 reactor, allowing to load and transfer substrates without breaking the vacuum in the growth chamber [3]. To load samples the load lock, a small chamber, is vented to atmosphere, samples are placed on a transfer system and the chamber is pumped down and baked to drive off all water vapor adsorbed to the inner walls of the load lock chamber until the ultra-high vacuum conditions are reached (ploadlock 10-8 torr). The gate valve separating the load lock from the buffer chamber (pbuffer 10-9 torr) is then opened to transfer the samples into the buffer chamber. After closing the gate valve to the load lock chamber, a second gate valve connecting the buffer chamber and the growth reactor is opened and the sample is mounted onto the sample manipulator for growth. 1.2: Challenges Specific to Oxide Molecular Beam Epitaxy Of particular interest in the oxide molecular beam epitaxy community is the growth and characterization of complex oxides possessing the perovskite structure with general formula ABO3 such as SrTiO3 and SrVO3. Perovskites in general are of interest due to their great span of physical, electrical, and magnetic properties, ranging from insulating to metallic, superconducting, ferromagnetic, ferroelectric, multiferroic and more [2]. Challenges in growing complex oxide films include managing the partial pressure of oxygen in the chamber. While high oxygen partial pressures are needed to ensure sufficient supply of oxygen to avoid the formation of oxygen vacancies, the flux stability of high temperature effusion cell to produce desired cation flux is detrimentally affected [9]. The high surface temperatures these films must be grown at are much higher compared to conventional III-V semiconductor materials such as GaAs. Complex

15 oxides typically have a high melting temperature due to strong ionic bonds in their crystal 7 structure. In order for sufficient diffusion to occur to allow layer by layer growth, a higher thermodynamic driving force is required, and thus a higher growth temperature. To mitigate and overcome these challenges, a hybrid molecular beam epitaxy approach is employed at Penn State. In hybrid molecular beam epitaxy, conventional high temperature effusion cells are replaced with metalorganic precursors such as titanium tetra iso-propoxide (TTIP) or vanadium oxo-tri-isopropoxide (VTIP) [2]. Metalorganic precursors are necessary as the vapor pressures for titanium and vanadium are low at typical effusion cell temperatures. 1.3: Substrate Mounting Methods The task of mounting samples within the molecular beam epitaxy chamber is not an easy one. In order to grow high quality films, the substrate surface temperature must be uniform and the mounting method must be able to withstand high temperatures without melting and dropping the sample, be compatible with an ultra-high vacuum environment, i.e. must not react or outgas to avoid even spurious contamination of the film during growth. Schematics of the two main mounting methods commonly employed in MBE are shown in Figure 2. The mounting methods commonly employed for the low temperature growth of compound semiconductors, such as GaAs, uses a solder based approach in which substrates are soldered using indium, gallium, or tin metal to a heated manipulator block made from molybdenum [4]. These solder metals have a low vapor pressure at growth temperatures typically employed (less than 600 C). The good metal contact between substrate and Mo manipulator block results ensures excellent temperature

16 uniformity across the substrate due to the high thermal conductivity of the solder. At growth 8 temperatures, the solder metal is a liquid, and binds the substrate to the manipulator block by surface tension forces. Figure 2. Schematic showing cross sections of substrates mounted using a tantalum gravity mount (left) and a solder mount (right). The arrows indicates the direction of flux from the effusion cells. At higher temperatures, such as those required for growth of complex oxides, this method does not work, primarily due to the high vapor pressures of these solder metals at temperatures greater than 600 C. Figure 3 shows the vapor pressure curves of commonly employed solder elements as a function of temperature. At temperatures in the growth range of C, the vapor pressures of these elements are high enough to produce a significant vapor pressure and thus sizeable flux density. For example, considering a growth temperature of 850 C, the flux produced from pure indium metal can be calculated: P In (T = 850 C) 10 3 torr P InN A J In = 2πM In RT = atoms cm 2 s

17 9 This flux can be compared to a typical strontium effusion cell flux. For one monolayer of growth every 10 seconds, the necessary flux must be: atoms 14 1 ML 10 cm 2 atoms 13 J Sr = 10 cm 2 s The indium flux generated is much higher than the intentional Sr flux supplied to the substrate. Even though In vapor will emanate from the sample and only a small fraction of the In flux generated from the solder bond will get incorporated into the growing film, nevertheless sizeable amounts of In can be expected to contaminate the sample. All pure solder metals employed in conventional molecular beam epitaxy of III-V semiconductors are unsuitable for use in oxide molecular beam epitaxy as their vapor pressures are too high at appropriate growth temperatures, resulting in contamination and doping of these semiconducting complex oxides. The lack of an effective adhesive to mount substrates poses a serious roadblock to the growth of electronic grade complex oxide thin films.

18 10 Figure 3. Graph of vapor pressure as a function of temperature for effusion cell materials (Ca, Sr), solder materials (Ga, In, Sn), and structural materials (Ta). Metalorganic precursors instead of effusion cells can be used to incorporate materials with low vapor pressures at high temperatures (Ti, V), such as TTIP and VTIP in hybrid molecular beam epitaxy. Data from reference [7]. For high temperature molecular beam epitaxy, a variety of mechanical mounting systems consisting of mainly tantalum clips and face plates of varying geometries are used to clamp or suspend the substrate in front of a radiative heating element to reach desired growth temperature [4]. These mechanical contact based mounting systems result in a less uniform temperature gradient across the sample and can induce strain in the substrate. Several other issues exist with contact based mounting systems. In the case of smaller substrates, the mounting mechanism can cast a shadow over the substrate, preventing the use of RHEED since the incident electron beam is blocked. Mechanical mounting systems also cover a portion of the substrate, limiting the area

19 11 upon which a film can be grown. Since a physical gap exists between the manipulator and the substrate, the thermocouple embedded in the manipulator does not provide an accurate representation of the substrate surface temperature, and a pyrometer must be used to evaluate the surface temperature instead. For transparent substrates, weak thermal coupling between the radiative heating element and the substrate limits the maximum growth temperature. Most of the energy of the electromagnetic radiation emanating from the heater element is in the IR and visible spectrum, which cannot be absorbed by the sample. In addition, transparent substrates may also give erroneous pyrometer readings, as radiation from the manipulator is transmitted through the substrate directly influencing the pyrometer measurements, leading to a systematically higher temperature read by the pyrometer than actual sample temperature. The existing tantalum gravity mount suspends a single substrate beneath the manipulator radiative heating element, contacting each corner of the square substrate. This method has several drawbacks. Substrates mounted must be of a specific size and shape, conforming to the geometry of the tantalum face plate. Current face plates in use are capable of mounting a single 1 cm 2 substrate. As the tantalum face plate lies within the path of the Knudsen cells which deposit material onto the sample, face plates accumulate films of their own over time consisting of deposited elements from many different growths. As the face plate itself is heated, these elements may redeposit and negatively affect the stoichiometry and structure of a grown film, despite the tantalum face plates being cleaned after a certain period of time. Finally, the tantalum face plates become warped through use and loading and unloading procedures, potentially resulting in non-uniform contact with the substrate from face plate to face plate. This lack of contact is hypothesized to result in temperature differences from face plate to face plate,

20 12 or across the surface of the mounted substrate due to variable heat transfer effects from the face plate to the substrate. 1.4: Research Motivation The motivation behind this research endeavor is twofold. It is necessary to better quantify and observe both the temperatures present at the substrate surface and the contamination that occurs when using a tantalum gravity mount to suspend substrates for growth. In addition to better understanding the limitations of the current mounting method, the viability of mounting samples using a range of high temperature, ultra-high vacuum suitable adhesives will be pursed as an alternative to the current mounting method. This new method is hypothesized to offer several advantages over the existing tantalum gravity mount, such as improved substrate surface temperature stability, reduced contamination, and additional methods of substrate temperature analysis. Another key motivation for using adhesive to mount substrates is cost. Substrates used for the growth of thin films such as LSAT, SrTiO3, or CaTiO3 are expensive and limited in size range, typically costing between $50-$400 for a single, 1 cm 2 substrate. The high cost of individual substrates limits the amount of growth experiment. Instead of using a single 1 cm 2 substrate for a single growth, it is desired to use smaller substrates, such as 5 by 5 mm square substrates, which cost much less. The use of these smaller substrates is not possible using the current tantalum gravity mount as does not allow to use RHEED due to shadowing. Also, switching between gravity mounts of different geometry will affect actual substrate temperature. Therefore applying the same growth recipes to a sample mount with different size will not be

21 possible, limiting the usefulness to develop growth recipes using smaller samples and 13 transferring them to larger samples. The goal of this thesis is to better understand the drawbacks of the existing mounting system and develop an improved mounting system to overcome these disadvantages while keeping the key advantage: ease of use. An ideal adhesive-based mounting system would be able to utilize smaller substrates for cost effectiveness and allow the application of in-situ characterization methods (RHEED), predictable and reproducible sample temperatures with good uniformity, efficient radiative heat coupling into the transparent substrate, virtual absence of potential contamination of the film from the adhesive, as well as a simple sample mount and adhesive preparation and application routines.

22 14 Chapter 2 Device Design and Experimental Procedures 2.1: Bracket and Substrate Removal Tool Design and Testing Several different adhesives were tested in this experiment. These adhesives include a silver paste, a carbon epoxy, a zirconia cement, and a carbon paste. Information about the properties, composition, and curing schedules of these adhesives can be seen in Table 1. One of the intrinsic drawbacks with mounting samples using adhesive is that it is difficult to remove and clean the samples for post-growth analysis techniques such as X-ray diffraction or electrical transport measurements. In order to overcome this challenge and render adhesive-based substrate mounting viable to growers demanding sufficient convenience, a substrate removal tool was created to facilitate controlled breakage of the substrate off of the wafer after growth. This tool acts as a wrench by exerting a torsional force on the substrate to break the adhesive bonding it to the wafer. The substrate removal tool can quickly remove mounted substrates while retaining only a small amount of adhesive applied to secure the substrate to the wafer. Due to the simple, mechanical nature of this tool, it is inevitable some leftover adhesive is retained, which is not desired as it can limit further analysis. However, this removed state is far more preferable than leaving the substrates mounted on the wafer for further analysis, making it much easier to handle. In order to facilitate and standardize the mounting of substrates to a silicon wafer for growth, a mounting bracket was designed and fabricated using additive manufacturing. Renders of the mounting bracket and removal tool are shown in Figure 4.

23 Table 1: Adhesive Properties and Specifications 15 Product Name Composition Cost Curing Schedule Thermal Conductivity Shelf Life Notes Solder Metals Pure In, Ga, or Sn Varies None. Heat to apply W/mK Infinite Easy to apply. Hard to remove without release layer. PELCO High Performance Silver Paste Silver flakes in inorganic silicate binder $ /50 g C 9.1 W/mK 6 months Unusable above 400 C due to high outgassing. Very expensive. Resbond 931 Graphite Adhesive 99% graphite with binder $95.50/ 1 pt C 8.65 W/mK 6 months Unsuitable due to high outgassing Resbond 904 Zirconia Adhesive ZrO 2 with binder $97.25/ 1 pt 4 RT 1.44 W/mK 6 months Sets very quickly, making application difficult PELCO High Temperature Carbon Paste Graphite flakes in inorganic silicate binder $54.50/ 50 g 2 RT, C, C 1 W/mK 6 months Life can be extended by adding water to paste. The mounting bracket and removal tool were designed and rapidly prototyped on several 3D printers using two different printing techniques: fused deposition modelling (FDM) and stereolithography (SLA) of a photopolymer resin. Fused deposition modelling and stereolithography are the two main techniques used in 3D printing of polymers [10]. These

24 16 techniques are considerably different in the materials they employ and the process upon which a part is created. Figure 4. Designed bracket for substrate mounting and accompanying substrate removal tool. The substrate-wafer assembly as shown consists of a silicon wafer with mounted substrates, shown in black. Both printing methods print layer by layer on a flat surface known as the build plate. In fused deposition modelling, polymer filament is heated and extruded through a nozzle, forming a part on the build plate. Common polymer filaments used in FDM printing include thermoplastics polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS). In contrast, stereolithography printing utilizes a reservoir of a liquid photosensitive polymer resin and an ultraviolet laser to selectively cure the resin on the build plate, creating a part. FDM printers are often faster, less expensive and easier to use than SLA printers, but have lower resolution than SLA printers. Figure 5 shows schematics of a SLA and a FDM printer setup highlighting differences between each technique.

25 17 Figure 5. Schematics of FDM (left) compared to SLA (right) printing methods. In both figures, the printed part is shown in light blue. Initially, the mounting bracket was printed using a MendelMax 2 FDM printer using PLA filament. During the experiments to fabricate the piece it became apparent that the FDM process was incapable of printing prototypes to the degree of precision and tolerance levels required by the mounting bracket. The resolution of parts printed using the FDM method was limited by two main factors. In the plane of the build plate, the resolution of the part is limited by the diameter of the nozzle used to extrude the filament and the precision of the stepper motors used to position the extruder head and build plate. Normal to the build plate, the resolution of the printer is limited by the layer height, which is user defined and limited by the z-axis control of the printer. The MendelMax 2 printer used was fitted with a 0.35 mm nozzle and parts were printed with a layer height of 0.1 mm. For substrates that are only 0.5 mm thick and 5 mm by 5 mm in size with very tight tolerances, a greater degree of precision was needed to produce a bracket to manipulate and fix substrates to the wafer in exact positions without harming the substrate and allow efficient removal. Furthermore, the materials printed using FDM, primarily PLA and

26 ABS, were not hard enough to grip the substrate tightly without deforming when removing it 18 using the removal tool. To address these issues, a Form1 SLA printer was used to produce final versions of the bracket and removal tool from a photopolymer resin. Compared to the MendelMax 2, the Form1 has fewer resolution limitations in the plane of the build plate, as a laser is used to selectively cure the polymer with a high degree of precision. It also has a greatly increased resolution in the direction normal to the build plate due to the advantages of the SLA technique. The Form1 has a minimum layer height of mm, rendering it suitable to produce features on the order of substrate feature size. One additional feature of using a photocurable polymer is that it can be hardened in an ultraviolet oven after printing to cure any uncured resin and further strengthen the part. Finished parts were hardened for 15 minutes in an ultraviolet oven at high power to obtain greater strength. The bracket allows the mounting of up to nine substrates on a three inch diameter silicon wafer, with one substrate mounted in the center and the other eight radially mounted. A silicon wafer was chosen as the base to mount substrates on for several reasons. Unpolished silicon wafers are inexpensive, excellent absorbers in the infrared and visible part of the electromagnetic spectrum. Si wafers are easy to oxidize. This outermost layer, which is exposed to contamination from the adhesive as well as the elements supplied from the effusion cells, can be easily etched away, rendering them suitable for reuse. Of particular importance is the absorbance of the silicon wafer. In order to be heated by the manipulator element, the silicon wafer must absorb well in the range of radiation emitted by the heater. This is necessary to reach high temperatures

27 19 on the affixed substrate surface. Compared to common substrates used in oxide molecular beam epitaxy, such as LaAlO3, NdGaO3, SrTiO3, DySCO3 and Al2O3, all having a fundamental band gaps > 3 ev making them optically transparent, silicon is a more ideal black body and thermally couples more effectively to the radiation field, resulting in increased heat transfer and ultimately higher substrate temperatures. Three inch diameter silicon wafers also fit easily in the existing molecular beam epitaxy system and require no additional hardware to suspend in the chamber for growth. One design consideration for the bracket was the temperature profile of a mounted three inch diameter silicon wafer mounted in the molecular beam epitaxy chamber. The resistive heating element heats only a fraction of the mounted wafer, thus, the substrates mounted on the wafer must be within this uniform, circular heated region. Based on temperature measurements across the wafer surface and manufacturer specifications, the usable area on the silicon wafer consists of a circular region 2 inches in diameter located at the center of the wafer. Samples were mounted by placing a silicon wafer in the mounting bracket, applying a spot of adhesive at a mount location, and then immediately after placing a substrate and applying pressure via the substrate edges to secure the substrate to the sample. This procedure was repeated until the desired number of substrates were secured to the wafer. After mounting, the adhesives were allowed to cure before the substrate-wafer assembly was placed in the molecular beam epitaxy chamber for analysis.

28 2.2: Thermogravimetric and Residual Gas Analysis of Adhesives 20 Adhesive samples were prepared following manufacturer instructions with respect to curing times and methods. Adhesives samples for thermogravimetric and residual gas analysis were prepared by coating aluminum foil with the adhesive, curing it, and then breaking the cured adhesive off of the foil. Samples were equilibrated at 30 C for one hour, then heated using a ramp rate of 10 C per minute to a maximum temperature of 1000 C. Thermogravimetric and residual gas analysis was performed using a TA Instruments TGA Q50 fitted with a Pfeiffer Vacuum mass spectrometer and a platinum crucible. Residual gas analysis was performed using helium as a carrier gas. Thermogravimetric and residual gas analysis are necessary to evaluate different aspects of the prospective adhesives. During thermogravimetric analysis, the mass of a sample as a function of temperature is measured, allowing determination of how much mass reacts or vaporizes at elevated temperatures. In a molecular beam epitaxy system, any species that outgasses considerably at elevated temperature can detrimentally affect the integrity of the vacuum and introducing contaminants into a growing film. As such, it is important to assess both the extent of adhesive outgassing and individual outgassing species. In particular, medium vapor pressure species are critical and need to be avoided, because they will have a sizeable contribution to the background pressure levels and will be hard to be removed using a system bake. To analyze the types of elemental species formed during outgassing, a residual gas analyzer was used. In residual gas analysis, gas species are ionized as a whole or in fragments, and then identified by their mass to charge ratio. Identification of adhesive outgassing products allows elimination of adhesives with strong outgassing behavior that would more severely

29 21 impact film growth. These species include carbon dioxide, which adsorbs well to oxide surfaces, and lower vapor pressure elements or ions, such as silver, that react easily and may be incorporated into the growing film. 2.3: Substrate Annealing and Temperature Measurements Only two out of the four adhesives tested using thermogravimetric and residual gas analysis proved suitable for testing at high temperature in the molecular beam epitaxy chamber. Samples consisting of 1 cm by 1 cm polished monocrystalline silicon substrates were mounted in the center of a 3 inch diameter unpolished silicon wafer using zirconia cement and carbon paste. These substrate-wafer assemblies were transferred in the molecular beam epitaxy chamber and exposed to simulated growth conditions to determine ultimate substrate surface temperature possible at maximum manipulator power and cause outgassing and potential redisposition of adhesive components on the mounted substrate surface. Temperature measurements of the substrate surface within the chamber were performed using an infrared pyrometer mounted within the chamber using the appropriate emissivity value for a polished silicon substrate. The infrared pyrometer used in this experiment was a Lumasense Technologies IMPAC IGA 140, which evaluates intensity of emitted light with a wavelength between 1.45 and 1.8 μm to determine the surface temperature. The spectral irradiance of a black body can be described by Planck s Law as a function of wavelength and temperature using the Plank constant (h), Boltzmann constant (kb) and speed of light (c):

30 E(λ, T) = 2hc2 λ 5 1 hc ek BT 1 22 As silicon is not an ideal black body, the Stefan-Boltzmann relation along with the Stefan-Boltzmann constant, σ, must be used in conjunction with the emissivity of silicon ε Si in the growth temperature range to determine the true Si temperature observed by the pyrometer: I(T) = ε Si σt 4 The growth temperature range spans from C. In this range, the emissivity of a polished silicon wafer is 0.65 [5]. Temperature measurements were also performed on a polished silicon substrate placed in a tantalum face plate that was contaminated with deposited materials from past growths during the same annealing conditions. These contaminated tantalum face plates are referred to as dirty face plates from this point forward. 2.4: Outgassing and Contamination Evaluation Several experiments were carried out in the molecular beam epitaxy chamber to determine level of contamination of the mounted substrates from the adhesive and the dirty face plate. The samples were heated from room temperature to a maximum temperature of 900 C for 5 minutes. A ramp rate of 20 C/min was used. Samples were removed from the growth chamber and immediately X-ray photoelectron spectroscopy (XPS) was performed using a Physical Electronics VersaProbe II. Different XPS scans were performed on different spots of the sample, including point scans near the substrate edge, towards the center, and at the center of the substrate. Elemental mapping was done to determine if contamination from the face plate or the adhesives was delocalized or occurred randomly across the sample surface.

31 X-ray photoelectron spectroscopy is a surface sensitive technique used to probe and 23 establish the elemental composition of the outermost surface located within the first 10 nm of a material [6]. X-ray photoelectron spectroscopy works by using an X-ray source to excite and eject electrons from the sample surface. The kinetic energies of these ejected electrons are measured and related to specific binding energies, allowing determination of elemental species present. A surface sensitive characterization technique is desired as contamination originating from adhesive outgassing in an ultra-high vacuum environment is expected to produce several monolayers of contamination on the surface. In order to evaluate the suitability of any adhesive for mounting substrates, the amount and composition of contamination species must be quantified. If considerable contamination from an adhesive exists, then that adhesive is not suitable for the epitaxial growth of thin films. Similar elemental analysis techniques, such as energy-dispersive x-ray spectroscopy, are not sensitive enough to discern the existence of monolayers on a surface. Even at low accelerating voltages, the probe electrons penetrate too far into the sample, resulting in much more characteristic X-rays from the underlying substrate than any surface features. The depth at which characteristic X-rays can originate from ranges from nm for accelerating voltages of 5-30 kev compared to the 10 nm sensitivity of X-ray photoelectron spectroscopy [6].

32 24 Chapter 3 Results: Assessing Adhesive and Mount Device Performance 3.1: Determining Suitability of Adhesives using Thermogravimetric Analysis Out of the four adhesives tested, only two proved to have the stability necessary for high temperature growth employed in oxide molecular beam epitaxy. These adhesives were the zirconia cement and the carbon paste. The carbon paste was unique in that it required a step curing process, with an initial cure at 100 C for two hours, then a final cure at 275 C for another two hours. During the thermogravimetric analysis of the adhesives, three carbon paste samples were evaluated: a room temperature cured sample referred to as the uncured sample, a sample cured at 100 C for two hours, and a sample fully cured as per the manufacturer s instructions. The uncured and partially cured carbon paste samples showed no statistical difference in both the thermogravimetric and residual gas analyses, as such, only the uncured sample data is shown.

33 25 Figure 6. Thermogravimetric data showing mass loss over time for all adhesives. A low mass loss at growth temperature is desired to avoid contaminating the growing film. Figure 6 shows the thermogravimetric analysis results for all adhesives. The fully cured carbon paste and the zirconia cement displayed the best thermal stability and lowest weight loss, retaining up to 97% of their original mass when heated to 1000 C. Both the carbon epoxy and the silver paste lost large amounts of their original mass due to outgassing at high temperatures to values of 80% and 55% at 1000 C for silver paste and carbon epoxy respectably. The silver paste is usable to 400 C, while the carbon epoxy is not suitable due to its poor thermal stability.

34 Both of these adhesives are not designed for ultra-high vacuum use and are typically used for 26 mounting samples for scanning electron microscopy or similar analysis techniques. Residual gas analysis for the zirconia cement, the silver paste, and the carbon epoxy showed that the primary species outgassed consisted of carbon dioxide and carbon in ionized form. Maximum ion currents registered relative to the ion current for carrier gases (He, 10-7 A) ranged from 10-9 A (carbon epoxy, CO2+) to A (silver paste, CO2+) and 10-9 A (zirconia cement, CO2+). Ion currents for carbon were similar for each adhesive but an order of magnitude lower. Figure 7. Residual gas analysis highlights of cured (light colors) compared to uncured (dark colors) carbon paste. Primary outgassing species include water (H2O+, OH+) and to a lesser extent, carbon dioxide (CO2+). Helium was used as a carrier gas.

35 Residual gas analysis of carbon paste showed several features of interest, as seen in 27 Figure 7. Primary outgassing species were water (18 - H2O+, 17 - OH+) and carbon dioxide (44 - CO2+). Considering that water outgassing events occurred at 200 C and 500 C, well above the evaporation point of water at 100 C, this water could be produced from decomposition reactions within the adhesive rather than simple evaporation of leftover water in the cured adhesive. Another possibility is that another gaseous species is evolving with the same or double charged mass to charge ratio as water species. Such a species has the same signal as water species, but could be completely different in composition. This would explain the two separate peaks for water at vastly different temperatures. Proof that the carbon paste was reacting with itself or other background gases present in the chamber at elevated temperature can be seen in the background concentration of oxygen compared to the experimental concentration of oxygen. At temperatures greater than 750 C, the oxygen ion current drops below the background oxygen ion current, suggesting that oxygen is reacting with the sample to form another product, such as carbon dioxide. This result is in line with the increased concentration of a mass to charge ratio corresponding to carbon dioxide which is seen in Figure 7 for the carbon paste. It is likely that carbon in the paste is reacting with background molecular oxygen to form carbon dioxide. This could be problematic for oxide molecular beam epitaxy, as oxygen incorporated in the film is provided gas form. Since the adhesive is of close proximity to the substrate surface, this proximity could allow the adhesive to react and starve a film of oxygen locally, affecting the stoichiometry of the resulting film. This effect can be seen in Figure 8. The ratio of ion currents of sample oxygen to background oxygen remains constant up to a certain temperature, then rapidly decreases as the oxygen within the chamber reacts to form carbon dioxide.

36 28 Figure 8. Discrepancy between the residual gas analysis background and sample for carbon paste showing reaction of oxygen with elements of the paste to form other gaseous species above 750 C. The oxygen ion current produced by the sample drops below the oxygen ion current of the background, showing that oxygen is reacting. 3.2: Temperature Comparisons and Gradient Mapping This section details the results from in-situ temperature measurements of substrates mounted using both the adhesive-based method and tantalum face plate from within the molecular beam epitaxy chamber. Data analysis of the temperature shows that mounted samples experience a lower surface temperature than the samples mounted in the face plate. This is

37 29 expected as substrates mounted using adhesive experience additional thermal insulation from the underlying wafer and paste compared to the gravity mount, where the sample is heated directly by the manipulator. The higher temperature of the silicon substrate mounted in the tantalum face plate largely stems from the excellent absorption behavior of silicon in the emission range of the manipulator element. For transparent and commonly used substrates mounted using this scheme, such as LaAlO3, this thermal coupling effect will not be as strong, resulting in a lower substrate temperature using the tantalum face plate. As such, the use of a backing silicon wafer to mount a transparent substrate using adhesive represents a way to improve both the thermal coupling between the substrate and manipulator and the temperature uniformity of the substrate surface. It is expected that for the case of a transparent substrate, the adhesive mounting scheme is superior since the absorption of the transparent substrate will be lower, resulting in a lower surface temperature than an identical adhesive mounted transparent substrate. Figure 9 illustrates the substrate surface temperatures as a function of manipulator temperature for different mounting techniques. The ultimate temperatures possible using different adhesives differ due to the different thermal conductivities of the adhesives, as the bond line was similar between all adhesives applied.

38 30 Figure 9. Surface temperature of silicon substrates mounted on silicon with adhesives and a silicon wafer mounted using a tantalum face plate with increasing manipulator temperature. Using the developed mounting bracket, temperature gradients across mounted samples were able mapped. Two substrates were mounted radially on a silicon wafer, a fixed angle apart. A step scan mode, detailed in Appendix A, was used to slowly rotate the substrate if the pyrometer beam passed across each substrate while recording the substrate temperature to enhance the lateral resolution across the sample. The sampling rate of the pyrometer is approximately 1.5 seconds per data point. During normal rotation of the substrate stage, the sample rotates too quickly to obtain many data points across the substrate, resulting in a single data point per substrate and coarse lateral resolution. By step rotating by 1, then pausing for 1.5

39 31 seconds to allow the pyrometer to record, then step rotating and pausing again and again across the length of the substrate, a high resolution temperature profile can be obtained. The resulting temperature profiles can be seen in Figure 10. Temperature gradient mapping across the surfaces of the mounted substrates indicates that the substrate surface temperatures do not vary across the mounted sample in the range of typical growth temperatures. This effect can be observed visually within the growth chamber at elevated temperature: at high temperatures, the underlying silicon wafer, adhesive, and mounted substrate are almost indistinguishable from one another. At low temperatures, a discrepancy exists between the temperature measured by the pyrometer and the temperature of the manipulator where the surface temperature measured by the pyrometer is higher than the manipulator temperature. This is unreasonable as the manipulator is the heating element, and should always have a higher temperature than the surface temperature at all times as the sample is being heated. This is attributable to the fact that the pyrometer is inaccurate at temperatures below 500 C. Figure 10. Temperature gradient mapping across two substrates mounted using zirconia paste at different manipulator temperatures. The two valleys in the graph are maps across each substrate, while the center peak shows the underlying wafer temperature.