An Overview: Electron Beam-Physical Vapor Deposition Technology- Present and FutureApplications

Jogender Singh, Farhat Quli, Douglas E. Wolfe, J. T. Schriempf and Jason SinghThe Applied Research Laboratory,The Pennsylvania State University,

University Park, PA 16804.

Abstract:Metallic and ceramic coatings including chromium, molybdenum, titanium carbide (TIC),

and yttria partially stabilized zirconia (YSZ) coatings deposited by electron beam-physical vapordeposition (EB-PVD) will be presented along with their potential applications. Coatings are oftenapplied on a variety of materials to extend the life of components under severe thermal, corrosion,wear, and oxidation environments. Interest in replacing chromium electroplating has sparked theuse of EB-PVD technology for the repair of navy landing gear due to the high deposition rates ofthe EB-PVD process. The surface morphology, microstructure, and texturing of TiC filmsproduced by reactive ion beam-assisted, EB-PVD will also be discussed for wear resistantapplications in the cutting tool industry. YSZ has also been grown by EB-PVD resulting in acolumnar microstructure for thermal barrier applications (TBC). Significant improvements in thelife of turbine components are obtained over plasma sprayed YSZ due to the quality andmicrostructure of EB-PVD coatings. Scanning electron microscopy, X-ray diffraction, and electronprobe microanalysis (EPMA) was used to characterize the films. In summary, this paper gives abroad overview of the various ceramic-metallic coatings deposited by EB-PVD at the AppliedResearch Laboratory, The Pennsylvania State University.

IntroductionElectron beam-physical vapor deposition (EB-PVD) is a derivative of the electron beam meltingtechnique. [Records reveal the first U.S. patent was issued to M. Von Pirani in EB melting andrefining of materials in March 1907.] Perhaps the most consequential growth phase in EBtechnology began in the early 80s and is still in progress. This significant progress was driven bythree factors: (1) much improved vacuum generation technology, (2) significant advances incomputers, and (3) availability of high-quality EB guns. Since then, EB coating technology isconstantly gaining reputation and confidence in a wide range of applications including thermalbarrier ceramic coatings and wear-resistant hard coatings.

The role of coatings is significant in the advancement of technologies for applications in the optics,auto, and aerospace industries. A high percentage (75%) of aircraft engine components is coatedby metallic or ceramic coatings for the purpose of enhancing performance and reliability. Thus,there is a continuous effort to engineer surface properties to enhance the life of componentsunder severe environmental conditions where corrosion, high-temperature oxidation, and wear areconcerns. Similarly, multilayered ceramic and metallic films are used in the fabrication ofmicroelectronic components. Processes to control thin film properties are also extremely importantin the microelectronic industry.

Before proceeding, three questions need to be answered:1. What are coating processes?2. What is the uniqueness of the EB-PVD process?3. What are ideal applications for EB-PVD coatings?

Coating ProcessesIndustrial coating techniques (exclusive of painting and electroplating) can be broadly classifiedinto three groups: physical vapor deposition (PVD), chemical vapor deposition (CVD), and sprayprocesses. Each process can again be sub-classified based on the source of energy used for thedeposition of coatings as shown in Table 1. Each of these processes has advantages anddisadvantages. Chemical and physical conditions during the deposition reaction can stronglyaffect the composition, residual stresses, and microstructure (i.e., amorphous, polycrystalline,epitaxial, and textured) of the product. The effect of these conditions must be understood tocontrol the process. Coating thickness, desired properties (including microstructure, physical, andmechanical properties), and the application will determine the coating process to be used.

In the spray process, pre-alloyed powder is injected along with a carrier gas through a high-energy source. The coating material is transported in the form of molten or semi-molten dropletsand directly deposited onto the components. The principle of the process is to feed powder intothe plasma where the particles are rapidly heated to their melting point and accelerated to speedson the order of 300 m/sec. After a few milliseconds, the molten powder particles strike and flattenon the object surface and rapidly solidify. The adhesion of the particles to the substrate is mainly amechanical bond. The rate of coating deposition and the quality of the coatings (density) dependsupon the spray process selected (D-gun, HVOF, plasma-transferred arc processes, etc.), theprocessing parameters, and the coating and substrate materials used. A significant advantage ofthis process is the high deposition rate (100-1,000 pm/minute) and the fact that various metallicand oxide coatings can be applied. A disadvantage of the spray processes is the inability to obtainhomogenous, high-quality, and dense coatings.

Spray Deposition processesThermal sprayHigh-velocity-oxy-fuel (HVOF)Detonation gun (D-gun)

Chemical Vapor Deposition Processes (CVD)Low pressure CVDPlasma enhanced CVDPhotochemical and laser-CVD

Physical Vapor Deposition Processes (PVD)Thermal evaporationElectron beam evaporationS p u t t e r i n g

-Balanced and unbalanced magnetron sputtering-Direct current diode sputtering-Radio frequency sputtering-Triode-assisted PVD

Some of the difficulties of the spray process can be overcome by the chemical vapordeposition (CVD) process. The term CVD is defined as a process whereby a reactant gasmixture is passed in a high-temperature reactor to form a solid product in the form of a thin film atthe substrate surface. The CVD coating process takes place between temperatures of 800 and1200 0C. Various metallic and ceramic (oxides, carbides, and nitrides) coatings have beendeposited at rates of 5-10 pm/hour. The disadvantages of the CVD process are: they often requirehigh-deposition temperatures (>10000C); they produce chemical waste (such as acids) that isenvironmentally unacceptable; deposition rates are usually slow (40 u/minute) for high-qualitycoatings.

Some of the shortcomings of the CVD process can be addressed by the physical vapordeposition (PVD) process. The term PVD denotes those vacuum deposition processes where thecoating material is evaporated by various mechanisms (resistance heating, high-energy ionizedgas bombardment, or electron gun) under vacuum, and the vapor phase is transported to thesubstrate, forming a coating. PVD is a line-of-sight process in which atoms travel from the sourcematerial to the substrate in a straight path. The PVD coating process takes place betweentemperatures of 100-600 C. Sputtering is one of the most versatile PVD processes available forthin film preparation. Various metallic and ceramic (carbides and nitrides) coatings can be appliedby this process typically at a rate of a few um or less per hour. Unlike the CVD process, PVDprocesses are clean and pollution free. The main disadvantages of PVD processes (withexception of EB-PVD) are the low deposition rates (1-5 pm per hour) and the difficulty in applyingoxide coatings efficiently.

In spite of significant advancements in the various PVD processes (such as cathodic-arcand DC and RF unbalanced-magnetron sputtering), there are still drawbacks in coating quality.For example, the cathodic-arc PVD process produces liquid droplets or macro-particles of metals,1 um to 15 um in size, during the evaporation of the target material. These molten particles can beentrapped in the growing film, resulting in nonhomogeneity in the microstructure and detrimentalphysical properties. In sputtering processes, an applied electric field produces plasma of inert-gasions, which strike the cathode composed of the source material, and ejects atoms from it. Atomsfrom the source material are deposited on the surface of the substrate, where they can alsocombine with reactive gases to form ceramic coatings (e.g., TIN, NbN, etc.).

Ion implantation is not a deposition process, but rather a surface modification process inwhich appropriate atoms are embedded in a material through high-energy beams. High-energyions are produced in an accelerator and directed on the surface of the substrate. A high-energyion implanter (1-10 MeV) is required for deep penetration (-5 pm). Ionized particles enter into thesubstrate with kinetic energies four to five orders of magnitude greater than the binding energy ofthe solid by a collision mechanism. The principal application of ion implantation has been in theelectronic industries. The disadvantage of the ion implantation process is the limited depth ofpenetration into the substrate (< 5um).

Ion plating is a derivative of ion implantation in which the substrate is made the cathodeand the source material is thermally evaporated before being partially ionized and accelerated tothe substrate (cathode). By the proper selection of materials and gas, carbides, nitrides, andoxides can be deposited. Potential applications of ion plating are in tool industries and certainaircraft components. Disadvantages of the above mentioned PVD processes can be addressedby the EB-PVD.

The EB-PVD ProcessThe electron beam-physical vapor deposition (EB-PVD) process has overcome some of thedifficulties associated with the CVD, PVD, and metal spray processes. In the EB-PVD process,focused high-energy electron beams generated from electron guns are directed to melt andevaporate ingots, as well as to preheat the substrate inside the vacuum chamber (Figure 1).

Penn State’s industrial pilot Sciaky EB-PVD unit has six electron beam guns, four of which areused to evaporate the coating materials and two of which are used to preheat the substrate tofacilitate coating adhesion. Each gun has a 45-kW capacity. The chamber will accommodate up tothree ingots ranging in size from 49-68 mm in diameter and 450 mm long. The overall dimensionof the production unit is about 1 cubic meter. [The maximum diameter of the substrate (withvertical rotation) that can be accommodated is about 400 mm; and can be rotated at a speed of5.5 to 110 rpm with a maximum load of about 100-Kg.] The unit also has a horizontal sampleholder with a three-axis part manipulator: two rotary axes of 0-14 rpm and a 0-4,000 mm/mintranslation axis. It can carry samples weighing up to 20 kg. Since, EB-PVD is primarily a line-of-

sight process, therefore uniform coatings of complex parts (such as turbine blades) can beaccomplished by continuously rotating the part during the coating process.

Vertical RotaryDrive

46 kW/G

Vapor Stream(Thermal Energy 0.1 ev)

Load Lock

Part Manipulator0-14 rpm, 2 rotary0-1000mm/mln

translatlon axis

VacuumPumps

Chamber DimenslonsDepth: 91OmmWidth: 896mmHeight: 726mm

Four Independent 8cmIon Sources(100 - 1000 ev)

Maximum Substrate Dimenslonas/WeightHorizontally Fed Cylinder: 200mm dia. x 289mm/20kg THREE INGOT FEEDERSVertically Held Disc: 400mm dia. /100 kg 70mm dir. x 500mm long Ingots

0.16 to 15mm/min Feedrate

Figure 1 Schematic diagram of the ion beam assisted, EB-PVD unit.

Advantages of the EB-PVD ProcessThe EB-PVD process offers extensive possibilities for controlling variations in the structure andcomposition of condensed materials. For example, the coating compositions can be variedcontinuously, as in so-called Functional Gradient Coatings (FGC). Also, coatings comprised ofalternating layers of different compositions can be made. These multilayered coatings can beapplied on top of the FGC. The EB-PVD process offers many desirable characteristics such asrelatively high deposition rates (up to 150 urn/minute with an evaporation rate of app. 10-15 Kg/hour),dense coatings, controlled composition control and microstructure, low contamination, and highthermal efficiency. Coatings produced by the EB-PVD process usually have a good surface finishand a uniform microstructure. The microstructure and composition of the coating can be easilyaltered by manipulating the process parameters and ingot compositions. Thus, multilayeredceramic/metallic coatings can be readily formed and various metallic and ceramic coatings(oxides, carbides, and nitrides) can be deposited at relatively low temperatures. Even elementswith low vapor pressure such as molybdenum, tungsten, and carbon are readily evaporated bythis process.

The attachment of an ion beam source to the EB-PVD system offers additional benefitssuch as forming dense coatings with improved microstructure, interfaces, and adhesion. Inaddition, textured coatings, that are desirable in many applications, can be obtained. The state ofthe internal stresses can be changed (i.e., from tensile to compressive) by the forcible injection ofhigh-energy ion (100-1,000 eV). A high-energy ion beam (as a source of energy) is quite oftenused to clean the surface of the specimen inside the vacuum chamber prior to coating. Thecleaning enhances the bonding strength between the coating and the substrate.

Many coating materials are used both in the microelectronics and heavy manufacturingindustries. For example, metal oxides and nitrides are used in microelectronic industries as aninsulator, buffer layer, or diffusion barrier layer. The thickness of coatings for such applications is<1um. On the other hand, in the aerospace and auto industries, oxide coatings are used forenhancing the performance of components under severe environmental conditions such ascorrosion, oxidation, and wear. The coating thickness for such applications is typically > 10 um.Such coatings are often called thermal barrier coatings (TBCs).

Multilayered metallic or ceramic coatings are often applied on the components to achievedesired properties. Properties and performance of the coating also depends upon the coatingthickness. It has been well established that multilayered coatings with layer thickness cl pm offersuperior structural and physical properties due to refined microstructure in the coating.

In summary, the choice of deposition technique is determined by the application for thecoating, the desired coating properties, cost or production rate available from the process,temperature limitation of the substrate, uniformity or consistency of the process, and itscompatibility with subsequent processing steps. Chemical and physical conditions during thedeposition reaction can strongly affect the resultant microstructure of the coating (i.e., single-crystalline, polycrystalline, amorphous, epitaxial).

Applications of the EB-PVD ProcessThe versatility of the EB-PVD process is very wide and new varieties of coatings and materialscontinue to be developed. Some successful applications of the EB-PVD and ion beam-assistedEB-PVD processes are given below.

Local repair and alternative to hard chromium electroplating: Chromium (Cr) electroplating is well known to be an environmentally hazardous process. The Crplating process is normally used in areas where wear, corrosion, and oxidation (<6000C) arefactors in equipment performance. An environmentally friendly process, which provides the sameor improved wear, corrosion, and oxidation protection, is required. To achieve this goal, twochallenges must be met: (1) identify potential candidate materials for the replacement of Cr and(2) develop/identify a process for applying the candidate coating materials. The selection of thealternative coating processes will depend upon the application and requirements including surfacefinish, wear resistance properties, fatigue and thermal properties. Three coating processes(thermal spray processes, laser cladding, and physical vapor deposition) have been consideredas a replacement for the electroplating process. Dini [1], Oberlander and E. Lugscheider [2] andSchroeder and R. Unger [3] have given comprehensive reviews about various coating processesalong with their shortcomings and advantages.

Among the spray coating processes, HVOF has been considered as a potential candidatereplacing the electroplating process. The HVOF is a cost-effective process. The WC-Co alloy hasbeen identified as an alternative coating material replacing the hard chrome. There are mainlythree shortcomings in the HVOF applied WC-Co coatings on large components such as aircraft orhelicopters landing gears. First, HVOF applied WC-Co coatings have non-uniform microstructurescontaining un-melted powder particles and porosity (Figure 2). Non uniform distribution of un-melted particles and porosity in the coating limits the component life. Second, it is not economicaland it is difficult to repair the localized damage in WC-Co coated components. Third, the coatingproduced by HVOF will have a rough surface. The machining of the WC-Co coatings will beanother issue.

A unique characteristic of EB-PVD is that it meets the above mentioned challenges andcan be used to tailor coatings for specific applications. With the EB-PVD process, corrosion-resistant materials can be applied economically on the surface where they are most needed.Applications for these coatings range from brass lighting fixtures to landing gear and other

components of aircraft and helicopters. IBAD, EB-PVD process offers additional flexibility inrepairing localized damage in landing gears which cannot be done by any other processes

Figure 2. Low (a) high (b) magnification SEM micrographs of WC-Co coatings deposited by HVOFshowing non-uniform microstructure and porosity.

without affecting the physical and mechanical properties of the base material. The significantadvantage of using IBAD along with EB-PVD is enhancing metallurgical bonding of the coatingwith the substrate at a relatively low temperature. For instance, EB-PVD chromium coatings wereflaking or debonding from the landing gear when deposited below 280 0C (<550 OF). This wasperhaps due to poor metallurgical bonding and high residual stresses in the deposit. When, thesame Cr coating was applied on the landing gear along with argon ion bombardment (Table 2),the resulting coating had a dense microstructure with good metallurgical bonding between thebase material as well as the electroplated chromium (region b of Figure 38).

Figure 3A is a photograph of a helicopter landing gear showing localized surface damagethat needs to be repaired. The current repair process is the chemical stripping of the chromiumcoating followed by re-plating, baking, and machining of the entire component. Localizedrefurbishment of the landing gear (Figure 3b) was successfully demonstrated by applying Cr in theion beam assisted, EB-PVD chamber. The SEM micrograph shows that the EB-PVD Cr deposit isrelatively denser than the electroplated Cr which has spacings/voids between the grains (region aand b, respectively shown in Figure 38). Unlike thermal spray coatings, these deposits had adense microstructure with good metallurgical bonding with the base metal. In addition, microcracksealers are not required. The cost saving is expected to be more than 20-30% with improvedcomponent life and performance.

Figure 3. Photograph showing a helicopter landing gear having localized surface damage (A) andSEM micrograph of the refurbished region (B) showing good metallurgical bonding of the Crdeposited by EB-PVD (region a) on the electroplated Cr (region b) and base material (region c).

Table 2. Process parameters used in depositing Cr by Sciaky IBAD, EB-PVD.EB-voltage (kV) 17EB-Current (mAmp) 900Deposition rate (um/hour) 15Substrate temperature (0C) <280Deposition time (hours) 2Ion gun parameters: 250 volts, 75 mAmp, 10 sccm Argon gas

Thermal barrier Coatings (TBCs) for the turbine industryThere is a continuous thrust in improving the life and performance of turbine components undersevere environmental conditions including erosion, oxidation, and corrosion [4, 51. The life ofcomponents has been increased by applying TBCs either by plasma spray or EB-PVD processes.Each process has advantages and disadvantages (Table 3). For instance, the thermalconductivity of the plasma sprayed TBCs is about 60% lower than the bulk value. This is due to

Figure 4. SEM micrograph of the fractured TBC coated button showing columnar grains.

Table 3: Properties of TBCs produced by plasma spray and EB-PVD processes.Properties EB-PVD Plasma sprayed

Thermal Conductivity (W/mK) 1.5 0.8

Surface roughness (um) 1.0 10

Adhesive strength (MPa) 400 20-40

Young’s modulus (Gpa) 90 200Erosion Rate 1 7(Normalized to EB-PVD)Microstructure Columnar laminated

presence of porosity between the laminated grains. In contrast, the thermal conductivity of TBCsapplied by EB-PVD is higher than the plasma spray coatings. The higher thermal conductivity inthe EB-PVD TBCs is due to limited porosity present in the individual columnar grains. The porosityin TBCs is mainly present between the columnar grains (Figure 4). In spite of higher thermalconductivity, TBCs applied by EB-PVD is preferred over the plasma spray process due to smoothsurface finish, better strain tolerance, better erosion-resistance properties, and improvedmetallurgical bonding with the substrate. The typical process parameters used in producing TBCsare listed in Table 4.

Table 4. Process parameters used in depositing TBCs by Sciaky EB-PVD EB-voltage (kV) 18

EB-Current (Amp) 1.7Deposition rate (um/min) 3.4Substrate temperature (0C) <1000Deposition time (hours) 1

In spite of significant advancement in the EB-PVD coating technology, there are still challenges inproducing low thermal conductivity EB-PVD TBCs. Future thrusts are in producing a low-conductivity EB-PVD TBC by altering the microstructure and creating micro-porosity within thecolumnar grains. or in providing new materials with different compositions and functional gradientcoatings [6-11]. Past, present and future thrusts of TBCs are summarized in Figure 5.

Lamellar TBC

- Low Modulus/Strength

EB-PVD:Textured ColumnarStructure(001 growth direction)l Columnar Porosity

Laser Grooved

TBC + Alloyings

Gradient Coating

GradedCoating

Figure 5. Schematic diagram showing the evolution of past, present, and future microstructure ofTBCs.

MultilayeredColumnar TBCl Biasing Parts

l IBAD, EB-PVD(pulsed) -

Coatings for machining tools and forging die industriesMetallic borides, carbides, nitrides, and oxides have long been known to be very hard, wear-resistant materials. Applying these hard coatings to tool steels and tungsten carbide-cobaltcutting inserts can increase the tool life by several hundred percent (400-600%), reducing costsassociated tool procurement, set-up time, and machine down time. Wear-resistant coatings areoften characterized as having high melting temperatures as well as high hardness values.However, it is important to mention here that the coating performance depends on several factorsincluding: coating material, the deposition process and machining conditions. The depositionprocess and parameters often dictate the coating’s microstructure features (i.e., grain size, degreeof texture, density, etc) and thus its properties [12-16].

Nitride coatings are commercially applied by the PVD process (Table 1) whereas carbidecoatings (such as Tic, HfC, and ZrC) are applied by CVD. Since CVD is a high-temperatureprocess (800-1200 0C), many temperature-sensitive substrates cannot be coated by this method.EB-PVD has overcome some of the problems associated with CVD, including environmentalissues (waste of hazardous chemical gases), undesirable high-temperature reactions, and lowdeposition rate. Hard coatings such as TiN, TiC, HfC, ZrC, TiN, and TiB2 have been economically

produced by ion beam-assisted EB-PVD at relatively low temperatures.

The machining of titanium and nickel-based alloys is very difficult and challenging. Toimprove the efficiency and life of cutting tools, titanium nitride and titanium carbide coatings havebeen deposited by reactive ion beam assisted, electron beam physical vapor deposition (RIBA,EB-PVD). By evaporating titanium metal and mixing the vapor with an ionized argon and nitrogen(nitride) or acetylene (carbide) mixture, near stochiometric compounds with high Vickers’hardness values (TIN: 2500 VH 0.050 Tic: 3500 VH 0.050) were produced. These high hardness

values are attributed to the high degree of texturing. Tailoring the processing conditions in orderto achieve a specific orientation of the grains (and crystal structure), will increase the hardness aswell as the wear-resistance of the coating.

Figure 6 shows a scanning electron micrograph of the textured surface morphology of

titanium nitride grown at 0.1 um/min on 304 stainless steel at 650°C, -100 volt d.c. bias, andnitrogen ion bombardment. The high degree of surface texturing of the TIN is explained by thepreferred orientation of certain planes within the TIN unit cell. From pole figure analysis, the (111)planes show the strongest orientation at an angle 44 degrees from the normal to the surface. Thisorientation was strongly supported by the highly localized intensities and closely spaced contourlines of the pole figure. The (200) planes were oriented 27 degrees from the plane of the surface.This high degree of grain orientation resulted in the surface morphology being textured. Itappeared that the growth of the grains grew in the orientation as shown by the arrows in Figure 6.

Figure 6: SEM micrograph showing the surface morphology of TiN coating deposited on stainlesssteel substrate by IBAD, EB-PVD.

Several TiC coatings were deposited by RIBA, EB-PVD under identical processing

conditions with the exception of the level of ion bombardment (27 uA/cm2 - 163 uA/cm2). Thehardness values ranged from 1721 to 3504 VH 0.050. The general trend shows that the hardness

of the TiC coating increases with increasing levels of ion bombardment as shown in Figure 7. Thegenerally accepted bulk hardness value of TiC is 2200-2800 VHN. The TiC coating deposited

with 162 uA/cm2 (3504 VH O.050) showed over 25% improvement in the hardness value. This

increase in hardness can be attributed to a more dense film, texturing, and/or stress.

Comparing the hardness of the films to the titanium/carbon ratio, no general trends exist.This is quite surprising, yet very informative as it strongly suggests that the composition was notthe determining factor in the degree of hardness in the films and that the hardness values aredirectly related to the structure properties of the films such as degree of texturing and density.Although the coatings were deposited under identical conditions, compositional variations canoccur resulting from the equipment itself or due to sputtering of the growing film during deposition.

4000

3500

3000

2500

2000

1500 I I I I I I I

20 40 60 80 100 120 140 160 180Current density (u A/cm2)

Figure 7. Plot showing hardness increase in TiC coatings produced by IBAD, EB-PVD as afunction of ion beam current density.

Table 5: Typical process parameters used in depositing TiC and TiB2 coatings on WC-Co inserts

by Denton IBAD, EB-PVD.

EB Current (Amp) 0.12 - 0.17Deposition rate (A /min) 500Substrate temperature (0C) 200-650Deposition time (hours) 1Substrate bias (- volts) -100 to -630Ion beam process parameters current 500 m Amp, voltage 150 volts

To improve the cutting performance of WC-Co cutting tools during the machining oftitanium and nickel-based alloys, TiB2-x coatings were deposited by direct evaporation under

various processing conditions. The hardness of the coatings is shown in Figure 8. Directevaporation of TiB2 yielded a TiB2-x coating with a hardness of 2940 VH 0.050. The hardness of

the coating was increased to 3041 VH 0.050 yb argon ion beam assist deposition. The hardness

was still further increased by applying both a negative bias and bombarding the TiB2-x growing

film with argon ions during deposition. The resulting surface morphology shows a uniform, fine-grained surface with an average grain size less than 100 nanometers as shown in Figure 9.Typical process parameters for TIC and TiB2-x coatings are listed in Tables 5 and 6.

Table 6. Process parameters used in depositing by TiB2-x Sciaky EB-PVD

EB-voltage (kV) 18EB-Current (mAmp) 900Deposition rate (um/min) 0.07Substrate temperature (0C) <750Ingot diameter (mm) 49.5

1Sample2Number

3

Figure 8: Plot showing increase in hardness of TiB2-x as a function of processing parameters

(direct evaporation, negative bias and argon ion bombardments).

Figure 9. SEM micrographs showing fine grained TiB2 coatings deposited on Si substrate by

Sciaky EB-PVD at 750 0C.

Lastly, Figure 10 shows the fractured surface of a TiB2-x / TIC multilayer deposited on a

WC-Co cutting tool. The TIC (grown by RIBA, EB-PVD) serves as a bond coat as well as to

prevent chemical reactions between the TiB2-x and substrate material. The interface between the

TiC and TiB2-x looks very good. The TiC has columnar structure whereas the TiB2-x shows a

very fine-grained microstructure. Similarly, multilayered coatings composed of TiC/Cr3C2, havebeen successfully produced by the co-evaporation of the Ti, Cr, and graphite ingots in the EB-PVD chamber (Figure 1). The future thrust is in developing superhard multilayered coatingscomposed of AI2O3/TiC and of TiB2/Tic/Cr3C2, by the ion beam-assisted EB-PVD.

Figure 10: SEM micrograph of a fractured surface of the TiB2-x /Tic coating deposited on WC-Co

tool showing excellent interface between the coatings.

Films for the microelectronic industryIon bombardment can be an effective method of altering the microstructure of films of refractorymaterials at low substrate temperature. This can be important in electronic materials applicationsinvolving the use of refractory metals on low temperature substrates such as the metallization ofactive matrix liquid crystal displays (AMLCDs). In metallization applications refractory metals areoften used due to their thermal stability, chemical resistance, and electrical properties [17]. Onedifficulty with the use of these metals is that at low processing temperatures the structure iscolumnar, leading to step coverage problems [18]. Using EB-PVD with ion-assistance we havebeen able to suppress the formation of a columnar structure and fabricate defect-freeinterconnects. The process parameters used in depositing MO films are given in Table 7. Inaddition to modifying the microstructure, the stress states of the films can be varied from tensile tocompressive with ion bombardment.

Table 7: EBPVD conditions investigated.Beam Current (A)Deposition Rate (A/min.)Substrate temperature (0C)Ion beam voltage (volts)Ion beam current (mA)Substrate bias (-volts)

0.12 - 0.17500ambient - 6000.6000,150-3000,600

Deposition by low temperature evaporation and sputtering has been found to lead to lowdensity regions at steps due to a misalignment of the columnar grains growing from the substratesurface with the those growing from the step. These regions are extremely susceptible to attackby wet etchants which leads to occurance of line breakages during interconnect patterning. If thefilm is deposited under conditions favoring an isotropic rather than columnar morphology this typeof defect can be prevented.

Molybdenum films have been deposited using EB-PVD at high and low substratetemperature and with and without argon ion bombardment to determine what conditions are

suitable for suppression of the columnar microstructure and what films are suitable for use asinterconnects. The stress states of these films were also investigated to determine the effect oftemperature and bombardment on film stress. Figures 11-15 show the interconnect lines etchedfrom EB-PVD MO films deposited under various conditions.

Figure 11. Ambient temperature,No Ion-assist.

Figure 12. 600 0C, no Ion-assist.

re 13. Ambient temperature, Ion-assist

Figure 14. 400 0C, Ion-assist. Figure 15. 600 0C, Ion-assist.

Of these films, the ones showing the least amount of preferential etching were those depositedunder high bombardment and concurrent substrate heating as shown in Figures 14 and 15. Thefilm evaporated with no substrate heating or bombardment is particularly poor due to weakadhesion of the film to the substrate (Figure. 11). It appears that bombardment or temperaturealone is unable to sufficiently alter the microstructure of the MO film as shown by Figs. 12 and 13;however, the combination of the two is sufficient to suppress the formation of the columnarmicrostructure leading to step induced defects.

Figures 16 and 17 show the TEM micrographs of evaporated MO deposited at 600 0C withand without ion bombardment respectively. It is clear that the bombardment has changed themicrostructure from a fine-grained columnar morphology to a roughly “broken” isotropic structure.This is done through a combination of increasing surface mobility of the depositing atoms throughenergy transfer from the bombarding ions [19] and by causing subsurface lattice damage andatomic rearrangement [20]. Figure 18 shows the results of the bulk film x-ray stress analysis ofthese films and films sputtered under various RF powers. This shows that through ionbombardment the stress state can be varied from highly compressive to tensile.

Figure 16 X-sectional TEM image of600 0C, ion assist. sample shown in

Figure 15.600 0C, no ion-assist sample shown in

Figure 12.

SummaryElectron beam-physical vapor deposition offers high flexibility in depositing ceramic and metalliccoatings for a wide variety of applications ranging from microelectronic to turbine industry.Attachment of an ion source to the EB-PVD unit offers additional benefits in depositing coatings ata relatively lower temperature. Processing parameters control the microstructure, physical andmechanical properties, and residual stresses present within the coating. Chromium wassuccessfully deposited at a relatively low temperature (< 550 OF) on a damaged landing gear. Thehardness of TiC and TiB2-x was increased by 35% and 15 %, respectively by controlling the

process parameters and bombarding the growing film with ionized gas. Similarly, both residualstress and microstructure of the molybdenum films were altered by ion beam assisted depositionwhich suppressed preferential etching that is undesirable for interconnects applications.

AcknowledgeAuthors would like to express their sincere thanks to laboratory staff members (Dale D. Donner,Thomas Medill, Dennis Mcgregor, and Tad Rimmey) for their support during the course ofexperiments. Special thanks to Navy ManTech office for their financial support.

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