In this paper focused ion beam (FIB) milling (sputtering) is demonstrated for the fabrication of brass microinjection moulding inserts which have been previously conventionally milled. It is found that FIB milling of the α phase of the material results in much smoother final surfaces than the β phase. An annealing procedure for minimizing the effects of differential sputtering has also been performed. Further with the help of Scanning Electron Microscopy (SEM) and White Light Interferometry (WLI) measurements the FIB milling yield for 70-30 cartridge brass is determined and analysed. Finally, FIB milling of 5µm square trenches with a flat bottom surface is demonstrated

Ratchev, Svetan

Segal, Joel

Vladov, Nikola

Nottingham ePrints

Vladov, Nikola and Ratchev, Svetan and Segal, Joel (2011) Focused ion beam milling of brass for microinjection mould fabrication. In: International Conference on Micromanufacturing (ICOMM 2011), 7-10 March 2011, Tokyo, Japan. Access from the University of Nottingham repository: http://eprints.nottingham.ac.uk/29543/1/ICOMM-2011_paper_Nikola_Vladov.pdfCopyright and reuse: The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions.· Copyright and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners.· To the extent reasonable and practicable the material made available in Nottingham ePrints has been checked for eligibility before being made available.· Copies of full items can be used for personal research or study, educational, or not-for-profit purposes without prior permission or charge provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way.· Quotations or similar reproductions must be sufficiently acknowledged.Please see our full end user licence at: http://eprints.nottingham.ac.uk/end_user_agreement.pdf A note on versions: The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the repository url above for details on accessing the published version and note that access may require a subscription.For more information, please contact eprints@nottingham.ac.ukx corresponding authorABSTRACTIn this paper focused ion beam (FIB) milling (sputtering)is demonstrated for the fabrication of brass microinjectionmoulding inserts which have been previously conventionallymilled. It is found that FIB milling of the Į phase of thematerial results in much smoother final surfaces than the ȕphase. An annealing procedure for minimizing the effects ofdifferential sputtering has also been performed. Further withthe help of Scanning Electron Microscopy (SEM) and WhiteLight Interferometry (WLI) measurements the FIB millingyield for 70-30 cartridge brass is determined and analysed.Finally, FIB milling of 5µm square trenches with a flat bottomsurface is demonstrated.INTRODUCTIONClassical manufacturing techniques for hard materialinjection moulding inserts such as milling are currentlyrestricted to approximately 25µm [1] and involve certain riskof expensive cutting tools breakage. Traditionally employedmanufacturing methods for micro-inserts are the differentLIGA based technologies, micro electro discharge machining(EDM), a variety of lithography (etching) techniques and laserablation systems [2]. A disadvantage of these methods is thatnot all of them are capable of producing real 3D structures andthey are applicable only to specific materials in different sizeranges from 0.1µm to 100µm. An alternative proposedmethod is to take advantage of the Focused Ion Beam (FIB)instrument, traditionally used in materials science for TEMspecimen preparation, failure analysis and mask repair in thesemiconductor industry. FIB milling is possible for virtuallyany material and geometry and covers a wide size range from50nm up to 25µm restricted by longer machining time.Therefore in this study classical milling (cutting) is combinedwith FIB milling (sputtering) to produce microinjectionmoulding inserts. Brass is chosen for this application as itsmechanical properties meet the requirements ofmicroinjection moulding processes and at the same time itshardness is lower than the commonly used steels hencedecreasing the possibility of cutting tool breakage. Because ofits excellent machinability improved by the addition of Pb themultiphase Į-ȕ brass alloy CZ121 is the primary choice forinitial experiments. However, FIB milling of the materialresults in unacceptably rough final surfaces due to differentsputtering rates for different grain orientations, preferentialsputtering of elements with lower binding energy and coneformation.The variation of the sputtering rate with the grainorientation is explained with the so called channelling effect.Along certain crystallographic directions the bombarding ionscan penetrate deeper into the crystal leading to a reducedsputtering rate. This way different grains are milled withdifferent speeds causing an uneven surface. Preferentialsputtering of some elements of the alloy over others can beexplained either by the larger probability of the lighter atomsbeing hit by an incoming ion or by the different surfacebinding energy for different elements. In both cases thesurface layers of the machined target are enriched to theelement with lower sputtering rates [3]. Cone formation wasfirst reported in the 1950s and although since then manyauthors tried to explain the phenomenon there is no availablesatisfactory explanation to date. Cones can form from ridges,jags and other asperities related to the grain boundaries ormicroscopic impurities in the crystal structure includingimplanted ions from the ion beam itself. The motive power forcone growing can be preferential sputtering caused by theimpurities or induced stress and amorphization of the crystalstructure on the surface followed by complicated diffusionprocesses [4-8].Unfortunately, existing methods of smoothing themachined surface in copper device editing [9] that rely onincreasing the sputtering yield ratio between the copper linesand the underlying isolators are not applicable in the case ofmicromould insert fabrication.The objectives of this study are to find a new process foravoiding differential sputtering, to isolate the cone formationand to investigate the FIB milling yields for brass. To achievethese objectives the following techniques are employed:energy-dispersive X-ray spectroscopy (EDX) to identify the Įand ȕ phases of the alloy; scanning electron microscopy(SEM) for visual inspection; heat treatment for control of thegrain sizes of the material; White Light Interferometry (WLI)to confirm the SEM measurements and determine the FIBmilling yield.EXPERIMENTAL PROCEDURES AND RESULTSA. MATERIAL SELECTION AND SAMPLE PREPARATIONThe initially chosen brass alloy CZ121M because of itsexcellent machinability is a mixture of two phases: alpha (Į)and beta (ȕ). The Į phase contains up to 35% Zn and hasface-centred cubic crystal structure whilst the ȕ phase is inbody-centred cubic structure and contains up to 50% Zn. Bothphases are balanced with Cu and few percent of Pb whichFocused Ion Beam Milling of Brass for Microinjection MouldFabricationICOMM2011No. 1Nikola Vladov1, Svetan Ratchev2, and Joel Segal31Nikola Vladov; Precision Manufacturing Centre, University of Nottingham; e-mail: epxnv2@nottingham.ac.ukx2Svetan Ratchev; Precision Manufacturing Centre, University of Nottingham; e-mail: svetan.ratchev@nottingham.ac.uk3Joel Segal; Precision Manufacturing Centre, University of Nottingham; e-mail: joel.segal@nottingham.ac.ukimproves the mechanical properties. Due to the differentstructure, different FIB milling behaviour can be expected. Inorder to discriminate between the phases EDX measurementsfrom selected areas of the sample are performed. In Fig. 1, aSEM micrograph of the alloy CZ121 after mechanicalpolishing, it can be seen that the ȕ phase is porous whereas theĮ is solid and free of macro defects. The Pb is found tosegregate in separated grains and it can be expected that at themicroscale it does not contribute to better machinability of thealloy. In Fig. 2, two pockets milled under identical beamparameters but on different phases of the alloy, Į on the leftand ȕ on the right, are captured at 30 deg tilt. Cones are clearlydistinguished on the surface of the porous ȕ phase whilst theonly defects on the Į phase are the asperities due todifferential sputtering. Although the Zn and the Cu havesimilar atomic weight their surface binding energy is in theratio of approximately three to one so depletion of Cu due topreferential sputtering can be expected in the near- surfacelayer [10]. However, it is hard to judge how this influences thequality of the FIB milled surfaces.Consequentially, taking in to account the segregation of thePb and the milling properties of the two phases of the material,further experiments are done with Pb free single Į phase 70-30cartridge brass instead of the CZ121 alloy.The defects caused by differential sputtering on the Į phaseare minimized by modelling the microstructure of the alloy.Following heat treatment procedures from the literature [11]the as supplied 70-30 cartridge brass is kept at 900 ºC for 45min and then air cooled. Using standard ASTM tables thegrain size of the annealed alloy is determined to be 213µmwhich is about two orders greater than the untreated material.This allows FIB milling to take place with very highprobability on a single grain resulting in smooth final surfaces.B. FOCUSED ION BEAM MILLINGTo prove the concept trenches with target size 5x5µm areFIB milled on arbitrarily chosen grains from the surface ofheat treated and mechanically polished 70-30 cartridge brass.All the work is carried out on a commercially available SEM/FIB CrossBeam workstation (NVision 40: Carl Zeiss SMT)utilizing a beam of 30KeV Ga+ions. Scanning is performed inraster mode with three different beam currents of 0.3nA,0.7nA and 1.5nA corresponding to spot size 38nA, 59nA and107nA respectively. Varying the magnification the pixeloverlap is kept at 160% and the dwell time is fixed to 53µs.Exposure time is set to 60s for all the trenches and theincidence angle is 0 deg. In Fig. 3 a SEMmicrograph of one ofthe series of the sputtered trenches is shown. Under theseprocessing parameters with this material the cones do not formand the asperities due to differential sputtering are notobserved. The only defects that can be noticed are the smallwavelike patterns on the bottom of the trenches finalsurfaces which need further investigation. The quality of themachined surfaces appears not be influenced by the magnitudeof the beam current. Quantitative roughness measurements areprevented by the non standard wavelike patterns. Singlemeasurements using the analyzing functions of the SEM showthe features to be within 50nm of the aimed size and again notdepending on the beam current. The error is less than 1% andshould be considered more like a tool calibration problem thanreproduction incapability. Fig. 3 shows slight distortioncaused by the tilt correction function (the sample is at 30degrelative to the beam).C. MILLING YIELD MEASURMENTSFig. 1: SEM micrograph of the alloy CZ121 after mechanicalpolishingFig. 2: SEMmicrographs at 30 deg tilt of two pockets milled in thealloy CZ121 under identical conditions but in different phasesPbĮĮĮȕȕȕȕPb5µm 1µmFig. 3: SEM micrographs at 30 deg tilt of three trenches milledwith different currents in a single grain of the 70-30 cartridgebrass1µmĮ ȕ1.5nA 0.3nA0.7nAA precondition for accurate fabrication of the micromouldinsert is knowledge of the FIB milling yield Y of the heattreated 70-30 cartridge brass. Measurements are done in cubicmicrometres per nanocoulomb (ȝm3/nC) using the followingequation:tIhAY uu (1)where A and h are the area and the depth of the milled featurein ȝm, I is the beam current in nA and t is the exposure time inseconds. The depth of the trenches from Fig.3 is primarilydetermined using the SEM and confirmed independently byWLI measurements in VSI mode performed on a VEECONPFlex instrument. A 3D plot of the features and typicalprofile of the middle trench are presented in Fig. 4. Due to thepoor optical resolution of the interferometer the overall shapeis not well captured. However, the resolution in the verticaldirection is quoted to be better than 0.15nm; the results aresummarized in Table 1. The yield increases with the highervalues of the current which at first glance seems to be incontradiction of other published works. To analyze thisproblem a model proposed by Santamore et al. [12] is appliedwhich gives the dependence of the yield Y on the localincidence angle ș:JvYc )tan()(TT uv (2)where v is the beam linear speed and J is the current density.7DQș can be approximated as the ratio between the depth ofthe trench achieved after single pass (loop) of the beam hsandthe beam spot size d. Although J follows Gaussian distributionradial to the centre point in order to simplify the calculationsin the present paper it is considered to be uniform across thewhole radius. It must be noted that change of current is alwaysfollowed by change of the spot size and since the dwell timeand pixel overlap are fixed the scan speed also changesinevitably. In Table 1 the required calculation data togetherwith the results are presented. The values obtained for thetheoretical yield are normalized to 0.776 so that easycomparison to the experimental values can be done. In spite ofthe approximations the calculated yield follows exactly thesame trend as the measured. However, the model does not takein to account the redepostion effects which will becomedominant below a certain value of the scan speed and thecontrary situation will be observed- decrease of the yield withincrease of the current.Since the above results are for an arbitrary chosen grainbefore proceeding to actual machining further experiments torelate the milling yield to the crystal orientation are needed.This can be done using an Electron Back Scattered Diffraction(EBSD) detector attached to the CrossBeam instrument.Nevertheless, Fig. 5 represents a uniformly milled pattern atthe boundary between two grains which shows that in certainorientations the milling rate is kept constant.CONCLUSIONS AND FURTHER DEVELOPMENT1. It is found that within the investigated process parameterscones appear only on the ȕ phase brasses hence Į phasebrasses are more suitable for FIB milling.2. It is demonstrated that the grain size of 70-30 cartridgebrass can be controlled by heat treatment in such a way thateffectively FIB milling is performed in a single crystalstructure.3. The quality of the FIB milled surfaces is not dependanton the beam current within the investigated processparameters.4. It is shown how the milling rate for 70-30 cartridge brasscan be determined so that accurate fabrication of themicromould insert is possible.Table 1:Calculated and measured milling yield for differentbeam currents, including the model related datacurrent I [nA] 0.3 0.7 1.5spot size d [µm] 38 59 107current density J [nC/µm2] 66 64 61beam speed v [µm/s] 452 693 1285depth h [µm] 0.354 0.949 0.560number of beam loops 27 63 214single loop depth hs[nm] 13.19 15.06 13.04local angle ș [deg] 19 14 7calculated yeld Yc [arb. unit] 0.490 0.571 0.776measured yeld Y [ȝm3/nC] 0.491 0.565 0.776Fig.4: a) a 3D plot of the milled features and b) typicalprofile of the middle trench obtained by WLI1µmb)Fig.5: Channelling contrast FIB image of patternmilled at the boundary of two grainsa)1µm5. The added lead in brass alloys segregates in separategrains thereafter it is assumed that it does not improve themachinability of the material in microscale.6. Further experiments with included EBSD analysis areneeded to relate the milling yield to the crystal orientation.ACKNOWLEDGMENTSThe authors wish to acknowledge the NottinghamInnovative Manufacturing Research Centre (NIMRC) and theEngineering and Physical Sciences Research Council(EPSRC) for their financial support of the work.REFERENCES[1] G. L. Benavides, L. F. Bieg, M. P. Saavedra, and E. A. Bryce, Highaspect ratio meso-scale parts enabled by wire micro-EDM,Microsystem Technologies, vol. 8, no. 6, 2002, pp. 395-401.[2] J. Giboz, T. Copponnex, and P. Mele, Microinjection molding ofthermoplastic polymers: a review, Journal of Micromechanics andMicroengineering, vol. 17, no. 6, 2007, pp. R96-R109.[3] M. Nastasi, J. Mayer, and J. Hirvonen, Ion- solid interactions:fundamentals and applications, Editor: 1996, pp.229-235.[4] G. K. Wehner, Cone formation as a result of whisker growth on ionbombarded metal-surfaces, Journal of Vacuum Science & Technologya-Vacuum Surfaces and Films, vol. 3, no. 4, 1985, pp. 1821-1835.[5] R. S. Robinson, and S. M. Rossnagel, Ion-beam-induced topographyand surface-diffusion, Journal of Vacuum Science & Technology, vol.21, no. 3, 1982, pp. 790-797.[6] G. Carter, B. Navinãek, and J. Whitton, "Heavy ion sputtering inducedsurface topography development," Sputtering by ParticleBombardment II, 1983, pp. 231-269.[7] N. G. Shang, X. L. Ma, C. P. Liu, I. Bello, and S. T. Lee, Arrays of Sicones prepared by ion beams: growth mechanisms, Physica StatusSolidi a-Applications and Materials Science, vol. 207, no. 2, 2010, pp.309-315.[8] L. B. Begrambekov, A. M. Zakharov, and V. G. Telkovsky,Peculiarities and mechanism of the cone growth under ionbombardment, Nuclear Instruments & Methods in Physics ResearchSection B-Beam Interactions with Materials and Atoms, vol. 115, no.1-4, 1996, pp. 456-460.[9] B.W. Kempshall, S. M. Schwarz, B. I. Prenitzer, L. A. Giannuzzi, R. B.Irwin, and F. A. Stevie, Ion channeling effects on the focused ion beammilling of Cu, Journal of Vacuum Science & Technology B, vol. 19,no. 3, 2001, pp. 749-754.[10] M. Szymonski, Sputtering of Cu and Zn atoms from elemental andalloy targets, Applied Physics, vol. 23, no. 1, 1980, pp. 89-92.[11] R. S. French, Grain growth and recrystallization of 70-30 cartridgebrass, Transactions of the American Institute of Mining andMetallurgical Engineers, vol. 156, 1944, pp. 195-210.[12] D. Santamore, K. Edinger, J. Orloff, and J. Melngailis, Focused ionbeam sputter yield change as a function of scan speed, Journal ofVacuum Science & Technology B, vol. 15, no. 6, 1997, pp. 2346-2349.

Focused ion beam milling of brass for microinjection mould fabrication

Abstract

Similar works

Full text

Available Versions

Nottingham ePrints