In Situ Growth and Characterization of Ultrahard Thin FilmsE. BENGU,* C. COLLAZO-DAVILA, D. GROZEA, E. LANDREE, I. WIDLOW, M. GURUZ, AND L.D. MARKSDepartment of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208

KEY WORDS electron microscopy; thin film deposition; ultrahard thin films

ABSTRACT Results concerning the operation of a new ultrahigh vacuum (UHV) ion-beamassisted deposition system for in situ investigation of ultrahard thin films are reported. A molecularbeam epitaxy (MBE) chamber attached to a surface science system (SPEAR) has been redesigned fordeposition of cubic-boron nitride thin films. In situ thin film processing capability of the overallsystem is demonstrated in preliminary studies on deposition of boron nitride films on clean Si (001)substrates, combining thin film growth with electron microscopy and surface characterization, all insitu. Microsc. Res. Tech. 42:295–301, 1998. r 1998 Wiley-Liss, Inc.

INTRODUCTIONEvery substance interacts with the outside world

through its surface, and frequently surface propertiesdetermine the utility of a material. Coatings and thinfilms are one way to modify and enhance surfaceproperties, yielding a composite material made up ofthe thin film, interface, and substrate. Often, none ofthe three phases alone achieves the performance of thecomposite material, and thin films are involved inapplications as diverse as desktop computers and jetengines. Various techniques, including in situ transmis-sion electron microscopy (TEM), can be used to investi-gate the complex nature of thin film deposition. One offirst accurate descriptions of the nucleation and growthof thin films came from in situ experiments conductedusing TEMs fitted with metal deposition stages (Pash-ley, 1959, 1965; Pashley et al., 1964). These studies leadto the conclusion that deposition follows one of threemodes: (1) Volmer-Weber mode (VW), 3-D island growth.(2) Frank-van der Merwe mode (FM), layer by layergrowth. (3) Stranski-Krastanov mode (SK), layer plusisland mode, an intermediary path between VW modeand FM mode. Mathematical models of nucleation andgrowth of thin films have also been derived and testedusing in situ TEM experiments (Venables and Spiller,1982; Venables et al., 1984). However, as pointed out byPashley et al. (1964) the vacuum in a conventionalTEM, around 10-7–10-8 torr, is not acceptable for most insitu work on surfaces and thin films; at such vacuumlevels, there are only seconds to examine a cleansurface.

Structures and reactions on surfaces is another fieldwhere in situ TEM can be useful, for instance directobservation of the ordering of surface atomic steps,chemical reactions, and surface reconstructions(Doraiswamy et al., 1995; Grozea et al., 1997; Landreeet al., 1997). For example, the first reasonably accuratemodel for the Si (111) 7 x 7 structure was proposed byTakayanagi et al. (1985a,b) using transmission electrondiffraction (TED) data. In a sense, surface reconstruc-tions are a snapshot of the very early stages of nucle-ation and growth. Combining both diffraction and imag-ing, in situ TEM has been used to solve and refine anumber of metal-semiconductor surface reconstruc-tions (Collazo-Davila et al., 1997, 1998; Jayaram and

Marks, 1995; Marks and Plass, 1995; Plass and Marks,1995;). In situ UHV-TEM has also become a fundamen-tal tool in thin film science, to cite an instance with thework of Yagi et al. (1982, 1985) on the initial stages offilm growth on oxide, semiconductor, and metal sur-faces. An advantage TEM has over other surface analy-sis techniques is the ability to acquire information notjust from the top surface atoms but also from buriedlayers, demonstrated by Bengu et al. (1996) by imagingthe dimers in the third layer of Si (111)-7 x 7 surface.Marks and colleagues (1997) also give other applica-tions of TEM on buried interfaces.

We report the results from the preliminary experi-ments conducted using a new UHV ion-beam assisteddeposition system attached to a unique surface sciencesystem. The goal is to achieve complete in situ process-ing of the ultrahard thin films, starting from substratepreparation and characterization to film deposition andcharacterization.

SPEAR, SINBAD, AND MIBEThis section is a brief description of the equipment

used for in situ investigation of surfaces, interfaces, andthin films. The UHV analysis chamber, Sample Prepa-ration Evaluation Analysis and Reaction (SPEAR), isattached to a Hitachi UHV H-9000 microscope (Bon-evich and Marks, 1992; Collazo-Davila et al., 1995;Jayaram et al., 1995). The SPEAR unit is the centralmodule of the analysis system, and attached to it aretwo other units: SINBAD and MIBE. SINBAD, Stabiliz-ing Ion and Neutral Beam Assisted Deposition, allowsion-beam-assisted deposition of ultrahard thin films forthe investigation of nucleation and growth. The Molecu-lar and Ion Beam Epitaxy (MIBE) unit, under construc-tion, will be used for the study of coatings and multilay-ers grown by magnetron and ion-beam sources. Aschematic of the layout for SPEAR, SINBAD, MIBE,

*Correspondence to: Erman Bengu, Northwestern University, Department ofMaterials and Science, 2225 N. Campus Drive, MLSF, Room 2036, Evanston, IL60201. E-mail: Bengu@nwu.edu

Contract grant sponsor: National Science Foundation; Contract grant num-bers: DMR-9204117, DMR-92145505; Contract grant sponsor: Air Force Office ofScientific Research; Contract grant numbers: F49620–94–1-0164, F49620–92-J-0250.

Received 29 April 1998; accepted in revised form 6 May 1998

MICROSCOPY RESEARCH AND TECHNIQUE 42:295–301 (1998)

r 1998 WILEY-LISS, INC.

Fig. 1. a:. Photograph of the SPEAR, SINBAD, and Hitachi UHV H-9000 microscope. b: Schematic(top view) of the complete UHV system: SPEAR, SINBAD, MIBE, and the UHV-TEM.

and the UHV H-9000 microscope is given in Figure 1aand b. Figure 1a is a picture of SPEAR and SINBAD.Figure 1b is a schematic representation (top view) ofthe SPEAR, SINBAD, MIBE, and UHV H-9000 micro-scope.

SPEARThe SPEAR system includes four separate chambers,

as shown in Figure 1b. The sample is introduced from aload-lock chamber that can hold five samples at a time,without breaking the vacuum in any other chamber.The central component of SPEAR is the transfer cham-ber equipped with a carrier assembly that can shuttlesamples between various chambers. A separate storagemodule in the transfer chamber can hold up to eightsamples and four microscope cartridges. At any time,

one can transfer a sample through the transfer cham-ber to any part of SPEAR including the UHV H-9000microscope. In addition, an evaporation stage has beenadded to the transfer chamber consisting of five differ-ent metal boats. The analytical chamber plays a dualrole, namely preparation and chemical analysis ofsamples. For sample preparation, the analytical cham-ber houses a duoplasmatron microbeam ion-gun, anelectron gun for direct beam annealing, and a multi-purpose sample manipulation stage capable of 360° ofrotational freedom, d.c. biasing, resistive heating, andliquid nitrogen cooling. The duoplasmatron microbeamion-gun with three gas sources (xenon, oxygen, andargon) is used for cleaning of sample surfaces with aprobe size less than 5 µm. Chemical characterizationtools include an X-ray source (either Al K a or Mg K a),a field-emission electron gun with scanning ability, and

Fig. 2. a: Schematic (side view) of the MBE chamber on the SPEAR system before modifications.b: Schematic (side view) of the SINBAD chamber on the SPEAR system after modifications.

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Fig. 3. a: Photograph of the MIBE system. b: Schematic (top view) of the MIBE system displaying thedeposition attachments.

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a spherical capacitance electron energy analyzer (SCA)that can be used for either Auger electron spectroscopy(AES) or X-ray photoelectron spectroscopy (XPS). Imag-ing of the sample during preparation is via an electronmultiplier (Channeltron detector) and a video imagingsystem. Along with SEM capability, this provides pre-cise control over the area being milled.

SINBADInitially, the MBE chamber on the SPEAR system

had two effusion cells configured for thin film depositionof GaAs. This MBE unit was redesigned for the deposi-tion of cubic-boron nitride (c-BN) films using ion beam-assisted deposition techniques. Figure 2a and b is aschematical representation of the MBE and SINBADchambers. The new unit, SINBAD, is pumped by a 280l/s turbomolecular pump (Varian Vacuum, Lexington,MA) and a 220 l/s ion pump (Physical Electronics, EdenPrairie, MN), with a base pressure 1 x 10-9 Pa.

Just like the SPEAR system, SINBAD is designed tohandle and deposit on thin 3-mm TEM ready samples.The sample manipulation stage can be used for the d.c.biasing of the sample as well as resistive heating duringdeposition. The SINBAD unit is equipped with a singleposition electron-beam evaporator (Thermionics Inc.,Hayward CA), a 4 keV ion-gun (Perkin-Elmer Model04–300, Eden Prairie, MN), and a compact electron

cyclotron resonance (ECR) plasma source (AX-4300Astex Inc., Boston, MA). Both the ECR and the 4keVion-gun can be utilized as ion-sources for purposesother than deposition.

MIBEMIBE is designed to be a mobile unit such that the

deposition of films can be accomplished when thesystem is not attached to SPEAR. The deposited filmscan be transferred to SPEAR after MIBE is re-attachedthrough a bakeable load-lock chamber. Figure 3a is aphotograph of the MIBE system. There are two cham-bers, a load-lock chamber, and a process chamber.Figure 3b shows the layout of the deposition attach-ments and sources in the process chamber. Two d.c.Magnetron-sputtering sources (Innovac/ Korea VacuumTech Model MDC, Seoul, Korea) are mounted on the topof the process chamber. Two direct negative ion beamsources (SKION Corporation, Hoboken, NJ) are alsomounted internally to maintain a short working dis-tance. These sources can be used to create negative ionsof energies up to 300 eV with an energy spread of 6 5%(Ko and Kim, 1997). A 3 cm diameter Kaufmann gas ionsource (,50 eV) (Commonwealth Scientific Comp., Alex-andria, VA; low voltage ion source) is located at the topof the chamber. Once mounted on the stage, the samplecan be rotated to a maximum of 20 revolutions per

Fig. 4. Image from 2 x 1 reconstructed Si (001) surface. Inset: TED pattern from the surface showing 2domains of the 2 x 1 reconstruction.

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minute, d.c. biased, and heated by a Si3N4 element up to900°C.

RESULTSWe describe here results from ion-beam assisted

deposition experiments conducted using the SINBADsystem. In addition, results from the structural andchemical characterization of the same films conductedin SPEAR are also included.

The substrate material consisted of 3-mm disks ofp-type Si (001) (13.5–18.5 Ohm cm). Following conven-tional TEM ex situ sample preparation for silicon, thesample was dimpled and polished to approximately30–20 µm at the center. As the final ex situ step, thesample was chemically etched using a solution of 10%HF and 90% HNO3. The as-etched Si (001) substratewas further cleaned through cycles of ion milling with 1keV Ar1 ions and electron beam annealing in SPEARsystem. The substrate surface chemistry was character-ized using XPS, to ensure that the surface was free ofcontaminants such as oxygen and carbon. The last stepin substrate preparation was the structural character-ization of the substrate surface using the UHV H-9000microscope, which showed the Si (001)- 2 x 1 reconstruc-tion on the surface indicative of a clean surface, asshown in Figure 4.

Deposition was carried out using the SINBAD sys-tem, over the clean and reconstructed Si (001) substrateat room temperature (RT). During deposition, the sub-strate was biased at -45V (d.c.), and the ECR wasoperating at 200 W with 10 sccm of N2 (%99.999) flow.The total pressure in the chamber was kept around 5 x10-4 torr. The film was then chemically analyzed usingXPS in SPEAR, which showed boron and nitrogen onthe surface of Si (001), shown in Figure 5.

The nitrogen to boron ratio revealed that the film wasnitrogen rich (N / B 5 1.18). Using the Si 2p and Boron1s peaks, the thickness of the film was estimated to beapproximately 80 Angstroms. The film was then trans-ferred to the UHV H-9000 microscope for structuralcharacterization. Figure 6 is an off-zone high-resolutionelectron micrograph of the film on the Si (001) surfacetaken at 300 kV, and the inset is the correspondingdiffraction pattern. The film is continuous, nanocrystal-line, and randomly oriented as indicated by the struc-ture it displays in the micrograph and the rings visiblein the diffraction pattern.

DISCUSSIONThe results described here demonstrate the overall

performance of the SINBAD and the SPEAR systems indepositing thin BN films on Si (001) substrates. Thesesubstrates were prepared and characterized in theSPEAR unit under UHV conditions, and then trans-ferred to the SINBAD unit for deposition. After thedeposition process, the films were transferred back tothe SPEAR for chemical analysis. For the structuralcharacterization, the films were finally transferred tothe UHV H-9000 microscope. During the transfer be-tween various systems, the films were always in UHVconditions.

The most important aspect of these experiments wasto explore the utilization of SPEAR and SINBAD for the

in situ investigation of nucleation and growth of ultra-hard thin films. Film deposition in the SINBAD did noteffect the operation of SPEAR or the UHV H-9000,indicating more than one experiment can be conductedin parallel at the same time. However, several problemsmust be solved in order to attain completely indepen-dent operation of these systems. The ECR plasmasource on the SINBAD and the dual anode X-ray sourceon the SPEAR share the same cooling water linelimiting the availability of XPS during deposition foranother experiment.Another issue is the extra mechani-cal vibrations and noise (including electronic noise)generated by the SINBAD during deposition that caninterfere with the high-resolution operation of themicroscope. For example, an air-cooling fan for a turbopump on the SINBAD was found to be the major sourceof the vibrations generated in the system. This problemwas solved by water cooling the turbo pump. Remainingissues regarding the parallel operation of the SPEARand SINBAD are relatively minor, and will be dealtwith in the near future.

This modular arrangement of the side chambersallowing independent usage of each unit also necessi-tates the transfer of samples between different units,which can cause a different type of problem. The mosttime-consuming and crucial part of any experiment inthe SPEAR and SINBAD is the sample transfer be-tween different chambers. During sample transfer be-tween chambers, excessive damage can be induced tosamples due to rough handling. However, there areadvantages to have a modular design such that eachchamber can be brought up to air for maintenance andrepairs, and pumped back down independently of allothers.

ACKNOWLEDGMENTSWe acknowledge the support of the National Science

Foundation for grants DMR-9204117 and DMR-921-4505, and the support of the Air Force Office of Scien-

Fig. 5. XPS spectrum from the h-BN over the Si (001).

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tific Research for grants F49620–94–1-0164 andF49620–92-J-0250 in funding this work.

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