Contains an introduction, reports on x research projects and a list of publications.Defense Advanced Research Projects Agency/U.S. Army Research Office Grant DAAL-03-92-G-0217National Science Foundation Grant ECS 89-21728Defense Advanced Research Projects Agency/U.S. Army Research Office (ASSERT Program) Grant DAAL03-92-G-0305Semiconductor Research CorporationNational Science Foundation Grant DMR 92-02633U.S. Army Research Office Grant DAAL03-90-G-0223U.S. Navy - Naval Research Laboratory/Micrion Contract M0877

Antoniadis, Dimitri A.

Armstrong, Mark A.

Chiang, Tony P.

Chu, Alex

Cobb, M. H.

Della Ratta, Anthony D.

Etchin, Sergey

Haynes, Tony E.

Kazior, Thomas E.

Lezec, Henri J.

Mahoney, Leonard J.

Melngailis, John

Musil, Christian R.

Sosonkina, Jane

Thompson, Carl V.

Vignaud, Dominique

Xu, Xin

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Chapter 4. Focused Ion Beam MicrofabricationChapter 4. Focused Ion Beam MicrofabricationAcademic and Research StaffDr. John Melngailis, Professor Dimitri A. Antoniadis, Professor Carl V. Thompson, Dr. Xin Xu, Sergey EtchinVisiting Scientists and Research AffiliatesDr. Dominique Vignaud, Jane Sosonkina, Professor Guang-Sup ChoGraduate StudentsMark A. Armstrong, Tony P. Chiang, Anthony D. Della Ratta, Henri J. Lezec, Christian R. MusilTechnical and Support StaffDonna R. Martinez4.1 IntroductionThe focused ion beam research program at MIThas been mainly aimed at developing new applica-tions. Our work can be divided into two areas: Thefirst, served by a 150 kV system, is the high energyregime and includes implantation and lithography.The second is the lower energy regime and isaimed at developing repair processes for masksand integrated circuits. It is served by twomachines, a 50 kV system mounted on a homebuilt UHV chamber and an FEI 500D system whichwas donated by the FEI Company to MIT last year.This system produces Ga+ ions between 5 and 25keV at minimum beam diameters of 0.05 tm.The high energy system includes automated pat-terning capability over wafers up to six inches indiameter. Alignment of the focused ion beamwriting to within -±0.1im of existing features on awafer has been demonstrated. Software has beendeveloped which permits patterns to be transferredfrom the layout system used in the MicrosystemsTechnologies Laboratory to the focused ion beammachine. Accordingly, this permits flexible, mixedfabrication where standard steps have been carriedout in the integrated circuits laboratory at MIT, andspecial implantation or lithography steps have beencarried out on the focused ion beam system.Similar mixed fabrication has also been carried outon Si wafers partly fabricated at Ford Aerospaceand on GaAs wafers at Raytheon Research Labora-tory. The ion species available for implantationinclude the principal dopants of GaAs and Si. Theminimum beam diameter available is on the orderof 0.1 m at an ion current of 20 pA. In many ofthe implantation projects where the minimum diam-eter is not needed, a higher beam current can beused.In the lower energy regime our work has mainlyfocused on the development of ion induced deposi-tion. This technique uses a local ambient of a pre-cursor gas, usually organometallic or metal halide,to permit deposition to be carried out with minimumlinewidth of 0.1 mm. The local patterned depositioncomplements material removal by ion milling and isused to add missing absorber material in the repairof photomasks and x-ray lithography masks or torewire local connections in integrated circuits. Ourefforts have focused on (1) gold, tungsten, andcopper deposition, (2) understanding the fundamen-tals of the process, and (3) deposition at non-normal incidence. Ga ions in the range of 10-50kV are mainly used.4.2 Tunable Gunn Diodes -Optimization for MicrowaveApplicationsSponsorDefense Advanced Research Projects Agency/U.S. Army Research OfficeGrant DAAL-03-92-G-0217Chapter 4. Focused Ion Beam MicrofabricationProject StaffChristian R. Musil, Henri J. Lezec, Sergey Etchin,Leonard J. Mahoney,' Alex Chu,2 Professor DimitriA. Antoniadis, Dr. John MelngailisTunable Gunn diodes are two-terminal voltage con-trolled oscillators (VCOs). They are fabricated byusing a focused ion beam to implant a gradient ofdoping in the direction of current flow. The bestperformance we have achieved so far is tunabilityover 20 GHz with a maximum frequency of 42 GHz.To optimize the performance, we have recentlyimplanted a large array of devices (over 3000)varying the dose, geometry, and gradient as para-meters. An immediate application of these simpleVCOs is the built-in-test for monolithic microwaveintegrated circuits (MMICs). A tunable Gunn diodeis coupled to the input, and a diode detector iscoupled to the output. By switching these two ele-ments into the circuit and applying a ramp biassignal to the Gunn diode, the circuit is tested byvarying the frequency of the input signal over itsentire range of operation.4.3 Dose Rate Effects in theImplantation of Si in GaAsSponsorDefense Advanced Research Projects Agency/U.S. Army Research OfficeGrant DAAL03-92-G-0217Project StaffChristian R. Musil, Tony E. Haynes,3 Sergey Etchin,Professor Dimitri A. Antoniadis, Dr. John MelngailisThe instantaneous current density of a focused ionbeam is of order 0.1 to 1 A/cm 2. In normal, broadbeam implantation the current density is 4 to 6orders of magnitude lower. We have shown thatthis high instantaneous current density can lead toincreased lattice damage in some cases of focusedion beam implantation. One way to mitigate theseeffects is to scan the beam over the sample so asto present a lower average current density. Wehave found surprising effects of a large variations inlattice damage as a function of the scan rate of thefocused ion beam. At a given point on the sub-strate the scan schedule produces a pulsed implantof a selected dwell time which is repeated aftersome period. We have measured the damageusing Rutherford backscattering as well as electricalproperties. At a given dwell time, for example, theamount of damage decreases for repetition periodsup to times of order seconds. This implies that,contrary to one's intuition, after an ion (Si) has pen-etrated into the solid (GaAs), it does not come intoequilibrium even after one second. In general, thelattice would be expected to be back in equilibriumin times of order nanoseconds.4.4 Limited Lateral Straggle ofFocused-Ion-Beam ImplantsSponsorDefense Advanced Research Projects Agency/U.S. Army Research OfficeGrant DAAL03-92-G-0217Project StaffDr. Dominique Vignaud, Christian R. Musil, SergeyEtchin, Professor Dimitri A. Antoniadis, Dr. JohnMelngailisThe lateral straggle of ions as they penetrate into asubstrate limits the minimum lateral dimensions ofimplant profiles that can be defined in a substrate.This potential spreading of the profile has to beconsidered to fabricate devices at ultrasmall dimen-sions or confined carrier structures for quantumeffect studies. The lateral distribution of focused-ion-beam implanted Si and Be atoms has beenstudied by measuring the electrical resistivity ingrating structures. The gratings which were ori-ented perpendicular to the direction of the currentflow were implanted with silicon and beryllium at280 and 260 keV respectively. They wereimplanted into semi-insulating materials cut on- andoff-axis, and then repaid-thermal-annealed. Thelateral straggle was found to be less than 100 nmfor Si and equal to 190 nm for the Be implants.The standard deviation of the lateral distributionwas found to increase with the dose. This is attri-buted to a concentration-dependent diffusion whichresults in an anomalously high diffusion coefficient.Comparison of the experimental parameters of theimplanted distribution with values found in standardtables or calculated by a Monte-Carlo TRIM code1 MIT Lincoln Laboratories.2 MITRE Corporation, Bedford, Massachusetts.3 Oak Ridge National Laboratory.32 RLE Progress Report Number 135seems to indicate that all simulations overestimatelateral straggle at the expense of penetration depth.4.5 Focused Ion Beam Implantation inGaAs for Transistor OptimizationSponsorNational Science FoundationGrant ECS 89-21728Project StaffT.E. Kazior,4 M.H. Cobb,4 Sergey Etchin, Dr. JohnMelngailisFocused ion beam implantation is a useful tech-nique for optimizing channel implants in integratedcircuits. The fact that the implants are done in amaskless, direct-write fashion permits the dose, ionenergy, and ion species to be varied from device todevice on the same wafer. Wafer to wafer vari-ations do not obscure interpretation of the data. Todemonstrate this, we have implanted the Be buriedp-layer beneath the active Si-implanted channel in aGaAs MESFET. The Be and Si implant conditionswere varied over a range of energies and doses.The optimum performance is expected for implantconditions which yielded the steepest gradient inthe tail of the electron concentration profile withoutproducing conducting holes. All implants were per-formed through a 50 nm SiNx film and rapid thermalannealed at 9000C for 10 seconds. The transistorgates were 0.25 im long with a T-shaped profile.Within the range of Be implant conditions explored,the gradient in the tail of the electron distributionwas strongly dependent on the dose, but did notvary systematically with changes in energy. Theexperiments carried out on a single wafer indicatethat a large process window exists for the Beimplant parameters and this results in an improved10 GHz noise performance. This type of optimiza-tion with conventional broad beam implants wouldrequire an unacceptable number of wafers andwould be complicated by wafer to wafer variations.These results also show that focused ion beamimplantation could be useful as a production tech-nique if a given integrated circuit required a largevariety of implants, each one over a limited area.In conventional fabrication, each implant requires acomplete sequence of steps (resist spinning, expo-sure, development, broad beam implantation, andresist stripping). These steps are eliminated by theChapter 4. Focused Ion Beam Microfabricationfocused ion beam implant and, in some circum-stances, may justify the cost in implantation time.4.6 Confined Carrier Distributions inIll-V Semiconductors Created byFocused Ion BeamsSponsorDefense Advanced Research Projects Agency/U.S. Army Research Office (ASSERT Program)Grant DAAL03-92-G-0305Project StaffMark A. Armstrong, Professor Dimitri A. Antoniadis,Dr. John MelngailisOne way to obtain confined carrier distributions forquantum effect studies is to use focused ion beamimplantation damage. In particular, the insulatingregion produced by the damage can be used tocarve up the two-dimensional electron gas pro-duced by modulation doping. One of the effectswhich may limit the resolution of this technique isthe depletion of carriers around the implantedregion. We are attempting to calculate the effect ofthis depletion by using analogies to the widelystudied and modeled Si/SiO 2 interface in MOSstructures. The width of the depletion will also bemeasured by writing different geometries to createplanar capacitors and transistors in GaAs/AIGaAsmesas.4.7 Focused Ion Beam Lithography forX-ray Mask MakingSponsorSemiconductor Research CorporationProject StaffTony P. Chiang, Sergey Etchin, Dr. John MelngailisMaking an x-ray mask is a challenging task. Theabsorber material on the thin mask membrane mustbe fabricated of high Z material (e.g., Au or W) withhigh aspect ratio, (e.g., 0.25 to 0.5 pm thick atminimum dimensions down to 0.1 [m). Two tech-niques for writing the original pattern of the mask inresist are electron beam lithography and focusedion beam lithography. While electron beams havebeen extensively developed and applied, ion beams4 Raytheon Research Laboratory, Lexington, Massachusetts.Chapter 4. Focused Ion Beam Microfabricationhave been used to write patterns down to 0.015 tmminimum linewidth and show no proximity effectseven for resists over high Z material. The resistexposure times for the two techniques are compa-rable. Previously, we had fabricated x-ray masks atMIT with minimum linewidth of 0.05 m in absorber0.2 llm thick. One goal of this program is to fabri-cate x-ray masks for use at the Center for X-rayLithography at the University of Wisconsin. Theabsorber material must be 0.5 im thick because ofthe higher energy of the x-ray photons used. Thusthe resist (PMMA) thickness in this case has to beof order 0.6 m, and the lithography is more chal-lenging to accomplish. The masks are fabricatedby exposing the PMMA over a thin gold platingbase, writing the pattern with either Be- + or Si- ionsat 260 keV, developing the resist, and plating upthe gold absorber features. We have determinedthe optimum ion exposure dose, developing condi-tions and plating conditions. In test structures wehave succeeded in producing gold features with 0.1tm lines 0.5 im high. In the Prometrix test patternwhich was successfully delivered to the Universityof Wisconsin the minimum lines written were 0.2im wide.4.8 Focused Ion Beam InducedDeposition of CopperSponsorsNational Science FoundationGrant DMR 92-02633U.S. Army Research OfficeGrant DAAL03-90-G-0223Project StaffAnthony D. 'Della Ratta, Professor Carl V.Thompson, Dr. John MelngailisFocused ion induced deposition is used in therepair of integrated circuits and masks. For therepair of circuits, a low resistivity of the depositedmaterial is desirable. In the deposition process alocal ambient of an organometallic gas is formed onthe surface where the ion beam is incident. Theincident ions cause the adsorbed molecules to bedecomposed, leaving the metal, and usually somecarbon, deposited on the surface. Typically theresistivity of the film is in the 100-1000 iQcmrange, compared to 1-10 iQcm for pure metals.We have achieved the first focused ion beam depo-sition of copper from a novel organometallic pre-cursor gas, Cu (hfac) TMVS. Ga ions at 35 keVwere used. Resistivity and yield have been meas-ured as a function of temperature and average ioncurrent density. Submicron copper lines depositedat room temperature from this precursor exhibitresistivities as low as 70 tQcm; a sharp drop inthese values is noted for deposition at 700C, anddeposition on a substrate heated above about1000C yields resistivities near those of pure bulkcopper. Composition analysis by Auger ElectronSpectroscopy shows the high temperature deposi-tion to be nearly pure copper. Deposition yields of25 copper atoms per incident Ga+ ion have beenobtained on both silicon and silicon dioxide sub-strates, with growth rates of up to 13 A per secondat an average ion current density of 200 tA/cm 2The microstructure of the film will be examined byscanning electron microscopy (SEM) and trans-mission electron microscopy (TEM). The precursorshows special promise for the deposition of lowresistivity submicrometer interconnects onintegrated circuits.4.9 Focused Ion Beam InducedDeposition and Ion Milling as aFunction of Angle of Ion IncidenceSponsorsU.S. Army Research OfficeGrant DAAL93-90-G-0223U.S. Navy - Naval Research Laboratory/MicrionContract M08774Project StaffDr. Xin Xu, Anthony D. Della Ratta, JaneSosonkina, Dr. John MelngailisIn the repair of integrated circuits, x-ray masksfocused ion beam induced deposition and ionmilling often have to be performed over quitenonplanar topography. Thus, the milling and thedeposition as a function of the angle of ion inci-dence are important. The milling yield of Si, SiO 2 ,Au, and W versus angle of incidence using 25 keVGa+ ions has been measured. In qualitative agree-ment with simulations, the yield rises with angle andthen falls as grazing incidence is approached.Deposition yield versus angle was measured usingdimethylgold hexafluoro-acetylacetonate and W(CO)6 as the precursor gases. The measurementswere carried out using cylindrical quartz fibers30-50 im in diameter which automatically provide arange of angles. Rippling of the deposited materialis observed at angles of incidence greater than 500.34 RLE Progress Report Number 1354.10 PublicationsChu, A., H.M. Cronson, J.F. Devine, S. Soares,M.N. Solomon, H.J. Lezec, and C.R. Musil. "RFBuilt-In Test and Enabling Technologies forIntegrated Diagnostics." Paper presented at theIEEE Systems Readiness and Automatic TestingConference, Dayton, Ohio, September 21-24,1992.Ehrlich, D.J., R.R. Kunz, M.A. Hartney, M.W. Horn,and J. Melngailis. "New Photoresist Processesat UV Wavelengths Less Than 200 nm." InIrradiation of Polymeric Materials. Eds. E.Reichmanis, O'Donnel, and Frank. ACS Sym-posium Series, vol. 527. Forthcoming.Kunz, R., D.J. Ehrlich, J. Melngailis, and M.W.Horn. "Selective Area Growth of Metal OxideFilms Induced by Patterned Excimer LaserSurface Photolysis." Proc. Mat. Res. Soc.Symp. 236: 105 (1992).Lattes, A.L., S.C. Munroe, M.M. Seaver, J.E.Murguia, and J. Melngailis. "Improved Drift inTwo-Phase, Long-Channel, Shallow-Buried-Channel CCDs with Longitudinally NonuniformStorage-Gate Implants." IEEE Trans. ElectronDevices 39: 1772 (1992).Chapter 4. Focused Ion Beam MicrofabricationMelngailis, J. "Focused Ion Beam Lithography."Invited plenary paper at the International Confer-ence on the Ion Beam Modification of Materials,Heidelberg, Germany September 1-11, 1992.Nucl. Instrum. Methods Phys. Res. Forth-coming.Vignaud, D., S. Etchin, K.S. Liao, C.R. Musil, D.A.Antoniadis, and J. Melngailis. "Lateral Straggleof Focused-Ion-Beam Implanted Be in GaAs."Appl. Phys. Lett. 60: 2267 (1992).Vignaud, D., C.R. Musil, S. Etchin, D.A. Antoniadis,and J. Melngailis. "Lateral Straggle of Si andBe Focused-lon-Beam-Implanted in GaAs." J.Vac. Sci. Technol. Forthcoming.Xu, X., A.D. Della Ratta, J. Sosonkina, and J.Melngailis. "Focused Ion Beam Induced Depo-sition and Ion Milling as a Function of Angle ofIncidence." Paper presented at the InternationalSymposium on Electron, Ion, and PhotonBeams, Orlando, Florida, May 26-31, 1992. J.Vac. Sci. Technol. B 10: 2675 (1992).ThesisLezec, H.J. Tunable-Frequency Gunn Diodes Fab-ricated by Focused Ion-Beam Implantation.Ph.D. diss., Dept. of Electr. Eng. and Comput.Sci., MIT, 1992.36 RLE Progress Report Number 135

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