Contains an introduction, reports on seven research projects and a list of publications.Defense Advanced Research Projects Agency/U.S. Army Research Office Contract DAAL03-88-K-0108National Science Foundation Grant ECS 89-21728U.S. Army Research Office Contract DAAL03-87-K-0126U.S. Navy - Naval Research Laboratory/Micrion Agreement M08774SEMATEC

Antoniadis, Dimitri A.

Chiang, Tony P.

Chu, Alex

Chu, Larry

Chu, William

Chung, James E.

Della Ratta, Anthony D.

Etchin, Sergey

Hartney, Mark A.

Kazior, Thomas E.

Lezec, Henri J.

Liao, Kenneth S.

Mahoney, Leonard J.

Melngailis, John

Musil, Christian R.

Ro, Jaesang

Shaver, D. C.

Shimase, Akira

Smith, Henry I.

Sosonkina, Jane

Thompson, Carl V.

Vignaud, Dominique

Xu, Xin

DSpace@MIT

Chapter 6. Focused Ion Beam MicrofabricationChapter 6. Focused Ion Beam MicrofabricationAcademic and Research StaffDr. John Melngailis, Professor Dimitri A. Antoniadis, Professor CarlEtchin, Mark I. ShepardV. Thompson, Dr. Xin Xu, SergeyVisiting Scientists and Research AffiliatesDr. Dominique Vignaud, Akira Shimase, Jane SosonkinaGraduate StudentsTony P. Chiang, William Chu, Anthony D. Della Ratta,Jaesang RoHenri J. Lezec, Kenneth S. Liao, Christian R. Musil,Technical and Support StaffDonna R. Martinez6.1 IntroductionThe focused ion beam research program at MITcan be divided into two areas. The first, served bya 150 kV system, is the high energy regime andincludes implantation and lithography. Thesecond is the lower energy regime and is aimed atdeveloping repair processes for masks and inte-grated circuits.The high energy system includes automated pat-terning capability over wafers up to six inches indiameter. Alignment of the focused ion beamwriting to within +0.1/m 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 havebeen carried 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 fabricated at RaytheonResearch Laboratory. The ion species available forimplantation include the principal dopants of GaAsand of Si. The minimum beam diameter availableis on the order of 0.1 jm at an ion current of 20pA. In many of the implantation projects wherethe minimum diameter is not needed, a higherbeam current can be used.In the lower energy regime our work has mainlyfocused on the development of ion induceddeposition. This technique uses a local ambient ofa precursor gas, usually organometallic or metalhalide, to permit deposition to be carried out withminimum linewidth of 0.1 /m. The local patterneddeposition complements material removal by ionmilling and is used to add missing absorber mate-rial in the repair of photomasks and x-ray lithog-raphy masks or to rewire local connections inintegrated circuits. Our efforts have focused ongold and platinum deposition, on understandingthe fundamentals of the process, and on deposi-tion at non-normal incidence. Mainly Ga ions inthe range of 10-50 kV are used.Our work in this area will be greatly enhanced bythe recent donation of an FEI 500D focused ionbeam system to MIT. This system is more conven-ient and flexible and compliments our existing,largely home-built machine. The FEI machinedelivers Ga+ ions at energies of 5 to 25 kV andbeam diameters down to 0.05 im.6.2 Tunable Gunn Diodes-HighFrequency Performance andApplicationsSponsorDefense Advanced Research Projects Agency/U.S. Army Research OfficeContract DAAL03-88- K-0108Chapter 6. Focused Ion Beam MicrofabricationProject StaffHenri J. Lezec, Christian R. Musil, Leonard J.Mahoney,1 Dr. Alex Chu, 2 Larry Chu, 3 ProfessorDimitri A. Antoniadis, Dr. John MelngailisGunn diodes have been fabricated by focused-ion-beam implanting Si in the GaAs surface areabetween two contacts. This planar fabrication iseasily incorporated into GaAs microwave inte-grated circuits. To achieve tunability the focusedion beam is programmed to implant a gradient ofdoping in the direction of current flow. Thismeans that the frequency of oscillation of theGunn diode depends on the bias resulting, ineffect, in a voltage controlled oscillator (VCO).The maximum frequency of oscillation observed sofar is 42 GHz with a tuning range of 20 GHz. Theperformance of two of these devices is shown infigure 1. These VCOs are expected to havenumerous applications in high performance GaAsmonolithic microwave integrated circuits(MMICs). The most immediate application is abuilt-in test where a swept frequency Gunn diodeis coupled to the input, and a planar diodedetector is coupled to the output of the deviceunder test. This permits the RF performance to betested by means of D.C. probes.50 1 I I ' I1 1-3 umrnsingle ramp30Frequency -.(GHz)20-118 um I-10 -double rampo 0 I I I 6 7 8 9 10 11Bias Voltage (Volts)Figure 1. Measured fretwo Gunn diodes. Thedoping profiles, whichimplanted dose of1.5 x 1013 ions/cm 2.!quency versus bias voltage forinserts show schematically theare generated by ramping theSi ions from 0.5 to6.3 Simulation of TunableGunn Diodes and MESFETswith Doping GradientsSponsorDefense Advanced Research Projects Agency/U.S. Army Research OfficeContract DAAL03-88-K-0108Project StaffChristian R. Musil, Henri J. Lezec, ProfessorDimitri A. Antoniadis, Dr. John MelngailisThe tunable Gunn diodes and MESFETs we havefabricated with focused ion beam gradients havecomplex carrier dynamics. The properties of thesedevices have been simulated on the Cray computerusing a non-stationary hydrodynamic solution ofthe Boltzmann transport equation. In theMESFETs this permits visualization of the effectsof doping gradients and the formation of stationaryGunn domains. In the case of the tunable Gunndiodes the bias-dependent domain propagationlength, which in effect produces the tunability, issimulated. As shown in figure 2, a non-equilibrium carrier concentration is associated withthe propagating Gunn domain. Each domaintravels up the implanted carrier concentration gra-dient until a new domain is nucleated at the lowcarrier concentration end.6.4 Limited Lateral Straggle ofFocused-Ion-Beam ImplantsSponsorDefense Advanced Research Projects Agency/U.S. Army Research OfficeContract DAAL03-88-K-0108Project StaffDr. Dominique Vignaud, Kenneth S. Liao, ChristianR. Musil, Akira Shimase, Professor James Chung,Sergey Etchin, Professor Dimitri A. Antoniadis, Dr.John MelngailisThe minimum lateral dimensions of focused ionbeam implanted doping profiles is ultimately1 MIT Lincoln Laboratories.2 Mitre Corporation, Bedford, Massachusetts.3 M/A Com Corporation, Burlington, Massachusetts.48 RLE Progress Report Number 134Chapter 6. Focused Ion Beam MicrofabricationFigure 2. Simulated electron concentration profiles versus lateral position at successive time intervals duringdomain propagation. Left: bias = 2 V, frequency = 40 GHz, At = 6 psec. Right: bias = 6 V, frequency = 25 GHz, At= 12 psec.limited by the lateral straggle of the ions as theypenetrate into the crystal lattice. This is partic-ularly true for light ions. One way to minimize thelateral straggle is to exploit channeling byimplanting along a crystal axis. To verify this wehave implanted Be in GaAs and B in Si bothon-axis and off-axis. The focused ion beam wasused to implant gratings of various periods. Theextent of overlap between the grating lines wasmeasured electrically. In GaAs two types of exper-iments were carried out. In the first a grating wasimplanted with lines parallel to the direction ofcurrent flow in an n-type mesa structure situatedbetween two contacts. Since the implant was notannealed, insulating stripes were produced. As theperiod of the grating decreased the conductionwas cut off. The results for 230 keV Be ions showthat the lines begin to overlap at a period of 0.3 to0.4 /m, but there is still a modulation of the con-ductivity down to below 0.1 pm for on-axisimplants. In the second experiment, gratings wereimplanted in semiinsulating GaAs between twop-type contacts with the grating lines perpendic-ular to the current flow. These implants wererapid-thermal-annealed, and the conductivitysharply increased as the period decreased and thelines began to touch. Various doses were usedbetween 3 x 1012 and 1 x 1014 ions/cm 2, and aclear difference was observed between on-axisand off-axis implants. The standard deviation ofthe lateral spread of the carriers was deduced froma one-dimensional model. The minimum half-width of an implanted line was found to be 140nm in on-axis material. In off-axis material thewidth was 50 to 90 nm larger depending on dose.In all cases the width is smaller than the 450 nmstraggle predicted by Monte-Carlo simulations inamorphous material. Similar structures have alsobeen made by implants in Si, but results are notyet available.6.5 Focused Ion BeamImplantation of GaAs MMICsand Transistor OptimizationSponsorNational Science FoundationGrant ECS 89-21728Project StaffSergey Etchin, Dr. John Melngailis in collaborationwith Dr. Thomas Kazior 4Some GaAs monolithic microwave integrated cir-cuits (MMICs) require as many as five implanta-tion steps in order to produce highly optimizedtransistors for various functions. Yet the areaimplanted in each step is small. The focused ionbeam provides a potentially cost effective means ofcarrying out all five implants in a single fabricationstep, thereby eliminating a large number of con-4 Raytheon Corporation, Lexington, Massachusetts.Chapter 6. Focused Ion Beam Microfabricationventional steps: resist spinning, lithography,development, blanket implantation, and resiststripping, each repeated five times. Device pat-terns have been transferred from Raytheon to MITand aligned local implants of MMICs have beencarried out to verify that the performance of thefocused ion beam implanted devices is equivalentto the the conventionally fabricated devices.In addition, the capability of the focused ion beamto implant devices side by side each with a dif-ferent dose or at a different ion energy has beenused to optimize the performance of buriedp-layer, n-channel MESFETs. In one experiment,for example, devices were implanted with Be ionsat energies of 70, 90, and 110 keV and doses from4 x 1011 to 1.5 x 1012 in order to find the optimumset of parameters. In another experiment, the Si-channel implant was also carried out with thefocused ion beam, and the dose was varied fromdevice to device. These successful experimentsdemonstrate the utility of the focused ion beam fordevice optimization. Similar optimization by con-ventional broad beam implantation would requiremany more wafers and would introduce compli-cations due to wafer-to-wafer variations.6.6 Ion Induced Deposition ofGold, Models andMicrostructureSponsorU.S. Army Research OfficeContract DAAL03-87-K-01 26U.S. Navy - Naval Research Laboratory/MicrionAgreement M08774Project StaffJaesang Ro, Professor Carl V. Thompson, Dr. JohnMelngailisWhen a local gas ambient of, for example, anorganometallic gas is created on a substrate, inci-dent energetic ions can cause the adsorbed gasmolecules to dissociate, creating a metallic deposit.With a focused ion beam, this technique can beused to write patterns of minimum dimensionsdown to 0.1 pm. The technique is being usedcommercially for the local repair of integrated cir-cuits. First, the ion beam is used to mill viasthrough the passivation, and then gas is intro-duced locally with a capillary gas feed and a metal"jumper" is connected between the vias. Deposi-tion of metals, particularly high Z metals such asAu and W, is also being developed for the repair ofx-ray lithography masks.We have studied the microstructure of the goldfilm and the correlation between its microstructureand deposition conditions. Some carbon is alwaysincorporated into the gold films. Three types ofmicrostructures have been identified: (1) granulesof gold in a carbon background produced at highion current densities at room temperature, (2)columns of gold in a carbon background producedat lower current densities or at higher current den-sities on substrates at 1000 C, and (3)polycrystaline gold with minimal carbon contentproduced at lower current densities in substrates at1000 C. For x-ray mask repair the granular orcolumnar structures are not desirable.The deposition yield was also measured for all ofthe noble gas ions at energies of 50 and 100 keV.The deposition yield varied from 3 to 29 and washighest for the highest mass ion, Xe. A model ofthe process based on the Monte Carlo calculationof collision cascades fits the data. The collisioncascades that terminate on the surface transferenergy to the adsorbed monolayer of organ-ometallic gas molecules and cause dissociation.6.7 Focused Ion BeamLithography for X-Ray MaskMakingSponsorSEMATECHProject StaffTony P. Chiang, Sergey Etchin, W. Chu, ProfessorHenry I. Smith, Dr. John Melngailis, in collab-oration with Mark A. Hartney5 and D.C. Shaver5One of the main challenges in the application ofx-ray lithography is the making of the mask. Theabsorber material on the thin mask membrane mustbe fabricated of high Z material (e.g., Au or W)with high aspect ratio, (e.g., 0.25 to 0.5 pm thickat minimum dimensions down to 0.1 pm). Twotechniques for writing the original pattern of themask in resist are electron beam lithography andfocused ion beam lithography. While electronbeams have been extensively developed andapplied, ion beams have been used to write pat-terns down to 0.015 pm minimum linewidth andshow no proximity effects even for resists over50 RLE Progress Report Number 1345 MIT Lincoln Laboratory.high Z material. The resist exposure times for thetwo techniques are comparable. Previously wehave fabricated x-ray masks at MIT with minimumlinewidth of 0.05 pm. The goal of this program isto fabricate x-ray masks for use at the Center forX-Ray Lithography at the University of Wisconsin.Masks will be made initially by exposing PMMAand plating up the gold absorber. An alternateprocess which may be preferable for the making ofx-ray masks with a tungsten absorber is focusedion beam exposure followed by resist silylation.We have demonstrated this silylation process incollaboration with Lincoln Laboratory and haveachieved minimum dimensions of 0.08 pm in 1 pmthick resist. The writing in this process is as fastas the fastest e-beam. In addition, the resist usedis compatible with reactive ion etching. Thus, wewill pursue making x-ray masks using reactive ionetching to define the high aspect ratio patterns intungsten.6.8 Focused IonRepair of X-RayBeams for theMasksSponsorU.S. Navy - Naval Research Laboratory/MicrionAgreement M08774Project StaffDr. Xin Xu, Anthony D. DellaSosonkina, Dr. John MelngailisRatta, JaneX-ray lithography is being developed for the fabri-cation of integrated circuits with minimum dimen-sions of 0.25 mm and below. The masks neededfor this lithography will have over 109 features.Thus it is unlikely that they will be fabricated freeof defects. The focused ion beam is the logicaltool for the local repair of mask defects and isbeing developed for this purpose. Local ionmilling can be used to remove unwanted absorberand ion induced deposition can be used to addmissing absorber. The challenge is that thesestructures must be refabricated with high aspectratios often in the proximity of existing high aspectratio structures. Thus, ion milling and ion induceddepositions at non-normal incidence is unavoid-able. We have measured the ion induced deposi-tion of gold as a function of the angle ofincidence. The precursor gas used was dimethyl-gold hexafluoro-acetylacetonate. The depositionyield was found to rise rapidly as the angle of inci-dence approaches a grazing angle. In addition, atangles above about 50' (0' is normal, 90' isgrazing) the deposited structure takes on a ripply,irregular aspect which develops into an almostperiodic array of protrusions at grazing incidence.Chapter 6. Focused Ion Beam MicrofabricationWe are also measuring the milling yield as functionof the angle of incidence. Both of these aspects ofthe repair process need to be taken into account ifhigh aspect ratio mask structures are to be accu-rately refabricated.6.9 PublicationsChu, A., L. Chu, W. Macropoulos, K. Khair, R.Patel, M. Cordaro, H.J. Lezec, J. Melngailis,and L.J. Mahoney. "Performance and Applica-tions of Novel Oscillators Utilizing Focused IonBeam Implanted Gunn-Effect Devices." Paperpresented at the 1991 IEEE MTT-S Interna-tional Microwave Symposium, Boston, June11-14, 1991. Published in IEEE MTT-S Digest1179 (1991).Dubner, A.D., A. Wagner, J. Melngailis, and C.V.Thompson. "The Role of the Ion/Solid Inter-action in Ion Beam Induced Deposition ofGold." J. Appl. Phys. 70: 665 (1991).Ehrlich, D.J., and J. Melngailis. "Fast Room-Temperature Growth of Si02 Films byMolecular-Layer Dosing." Appl. Phys. Lett. 58:2675-2677 (1991).Hartney, M.A., D.C. Shaver, M.I. Shepard, J.Melngailis, V. Medvedev, and W.P. Robinson."Surface Imaging of Focused Ion BeamExposed Resists." Presented at the 35th Inter-national Symposium on Electron, Ion andPhoton Beams, Seattle May 28-June 1, 1991;J. Vac. Sci. Technol. B 9: 3432 (1991).Hartney, M.A., D.C. Shaver, M.I. Shepard, J.S.Huh, and J. Melngailis. "Silylation of FocusedIon Beam Exposed Resists." Appl. Phys. Lett.59: 485-487 (1991).Huh, J.S., M.I. Shepard, and J."Focused Ion Beam Lithography."Technol B 9: 173 (1991).Melngailis.J. Vac. Sci.Kunz, R., D.J. Ehrlich, J. Melngailis, and M.W.Horn. "Selective Area Growth of Metal OxideFilms Induced by Patterned Excimer LaserSurface Photolysis." Presented MRS Meeting,Boston, December 1991. MRS SymposiumProceedings. Forthcoming.Lattes, A.L., S.C. Munroe, M.M. Seaver, J.EMurguia, and J. Melngailis. "Improved Drift inTwo-Phase, Long-Channel, Shallow-Buried-Channel CCDs with Longitudinally NonuniformStorage-Gate Implants." Submitted to IEEETrans. Electron Devices.Chapter 6. Focused Ion Beam MicrofabricationLezec, H.J., C.R. Musil, J. Melngailis, L.J.Mahoney, and J. Woodhouse. "Dose-RateEffects in Focused-Ion-Beam Implantation ofSi into GaAs." Presented at the US/JapanTechnology Seminar on FIB, Portland, Oregon.J. Vac. Sci. Technol. B 9: 2709 (1991).Melngailis, J. "Focused Ion Beam Induced Depos-ition - A Review." SPIE Proc. 1465 (1991).Melngailis, J. "Ion Lithography and Focused IonBeam Implantation." In Handbook of VLSILithography. Eds. W.B. Glendinning and J.N.Helbert. Noyes Publications, 1991.Melngailis, J., P.G. Blauner, A.D. Dubner, J.S. Ro,T. Tao, and C.V. Thompson. "Focused IonBeam Induced Deposition." Proceedings of theSecond International Symposium on ProcessPhysics and Modeling in Semiconductors 91 -4:653 (ECS 1991).Murguia, J.E., M.I. Shepard, J. Melngailis, A.L.Lattes, and S.C. Munroe. "Increase in SiliconCCD Speed with Focused Ion Beam ImplantedChannels." Presented at the US/Japan Tech-nology Seminar on FIB, Portland, Oregon. J.Vac. Sci. Technol. B 9: 2714 (1991).Tao, T., W. Wilkinson, and J. Melngailis."Focused Ion Beam Induced Deposition ofPlatinum for Repair Processes." J. Vac. Sci.Technol. B 9: 162 (1991).ThesesMurguia, J.E. The Application of Focused IonBeam Implantation to the Design and Fabri-cation of MOS Devices. Ph.D. diss. Dept. ofElectr. Eng. and Comput. Sci., MIT, 1991.Ro, J.S. Microstructure and Mechanism of GoldFilms Grown by Ion Induced Deposition. Ph.D.diss. Dept. of Mater. Sci. and Eng., MIT, 1991.52 RLE Progress Report Number 134

Focused Ion Beam Microfabrication

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Focused Ion Beam Microfabrication

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