Ahmad,M.M.

Collins,D.J.

Gilderdale,D.

Leach,M.O.

Syms,R.R.A.

Young,I.R.

Crossref

Batch Fabrication of Micro-Coils for MR Spectroscopy on Silicon

Spiral - Imperial College Digital Repository

Batch Fabrication of Micro-Coils for MR Spectroscopy on SiliconR.R.A. Syms, M.M.Ahmad, I.R.Young, D.GilderdaleEEE Dept,Imperial collegeExhibition Road, London SW7 2AZ, UKEmail: r.syms@ic.ac.ukD.J.Collins, M.O.LeachRoyal Marsden NHS TrustDowns Road, Sutton, Surrey SM2 5PT, UKAbstract— A process for batch fabrication of low-costmicrocoils for magnetic resonance (MR) spectroscopy ofclinical biopsy samples is demonstrated. Conductors arefabricated on oxidized, intrinsic silicon substrates byelectroplating metals inside a deep photoresist mould.Through-wafer deep reactive ion etching (DRIE) is used todefine precise sample volumes and stencil cuts around each die,and dies are separated by snap-out. Single- and multiple-coil(Helmholtz-type) sensors are formed, by stacking parts onsilicon baseplates. MR imaging and spectroscopy are bothdemonstrated on model samples, with high SNR. I. INTRODUCTIONThere is increasing evidence that magnetic resonancespectroscopy (MRS) may be useful in evaluating cancerouslesions [1], and MR spectra have now been obtained from avariety of body organs including the brain [2, 3]. When abrain tumor is resected, a large number (> 12) of biopsysamples may be taken. Samples obtained with a biopsyneedle are often cylindrically shaped, and around 1 mm indiameter and 5 - 10 mm long. The samples are used todetermine the cause and form of the tumor and to locate itsboundaries. The components of interest in MRS (which isalmost invariably based on 1H spectroscopy) exist in tissuein 0.5 to 5 mmol quantities. To obtain data quickly, closecoupling is needed between the sample and the MRdetector. Small coils, which are expensive and requirecareful handling, are therefore used. One major potentialproblem is sterilization between patients. Ideally, the coilsshould therefore be both cheap and disposable.In MRS, the signal-to-noise ratio should be maximized,so that the advantages obtained from close coupling to thesignal are not lost [4]. Coil resistance and capacitive leakagemust be minimized, so a high Q-factor may be obtainedfrom a resonant detector. The most common operatingfrequency in clinical MRI is 63.8 MHz - the resonancefrequency of protons in a 1.5 T magnetic field. At thisfrequency, the skin depth δ = 1/√(πfµ0/ρ) is ≈ 8.3 µm forCu. Conductors thickness up to 2δ = 16 µm are thereforeused to minimize resistance. Methods of combining thickconductors with substrates that can be etched to providesampling troughs are therefore desirable.Planar MR detectors were originally fabricated byphotolithographic patterning by Peck et al. [5, 6]. Morerecently, coils have been combined with sample volumes onglass by Dechow et al. [7], implantable devices have beendeveloped by Renaud et al. [8], and integrated preamplifiershave been developed [9]. The highest Q-factors are obtainedon insulating substrates such as glass [10]. However, modestperformance can be obtained on oxidized intrinsic siliconusing MEMS technologies. For example, deep reactive ionetching (DRIE) is a method of near vertical etching thatoperates by a cyclic repetition of etching and passivationprocesses [11]. DRIE has a high rate (> 2 µm/min), and mayetch through a 500 µm thick wafer in under 4 hours. Inaddition to forming sample volumes, DRIE may also beused to segment a wafer, thus eliminating the dicing step,which is often dirty, and time-consuming.In this paper, we show that deep etching of silicon maybe used to combine structuring steps that segment the waferand define sample locations with electroplating steps thatform the coil. The fabrication process is described inSection II. Electrical characterization and magneticresonance imaging and spectroscopy are described inSection III, and conclusions are reached in Section IV. II. DESIGN AND FABRICATIONA. DesignMicr o - coils ar e fo r m ed as sh o w n in F ig u r e 1 a. H er e aw afer is f ir st p atter ned to d ef in e an electr o plated co n du cto r.S ing le- tu r n co n d ucto r s m ay b e f or m ed u s ing s ing le- lay er p atter n in g , an d mu lti-tu r n co n d ucto r s u s in g m ulti- lay er lith o g r ap h y .  Th e w af er is th en p atter n ed an d etch ed to d efin e a lo catin g tr o ug h f or th e s am ple an d a s ten cil cut th atp ass es ar o u n d th e co il, ap ar t f ro m s ho r t s ectio n s of co nn ectin g s pr u e. W h en th e s p r u e is br o k en , the co il is r eleas ed f r o m th e w af er w ith o u t th e need f o r dicin g o r majo r clean in g. Co mp o n en ts su ch as b ase- p lates car r yin g ad ditio nal cir cu itr y , as s ho w n in F igu r e 1 b , may als o b ef abr icated . Fig u re 1 c s h o w s h o w s in g le coils may b em o un ted o n a b as e- p late to f o r m a s amp le tr o u gh w ith aclos ed bas e. F ig ur e 1 d s h o ws h o w p ar ts m ay b e s tacked tof o rm a Helm h oltz co il. O n e co il co m p on en t is flip - ch ip b o nd ed to a bas eplate, w h ile a seco n d is s tacked o n to p o f thef irs t and w ire- b on d ed to com p lete th e co il w ind in g .2270-7803-9056-3/05/$20.00 © 2005 IEEE.a) b)Sample troughConductorContact Connecting sprueBond wireBaseplate Solder connection to trackingStackeddiesc) d)PassivationSampletroughConductor5 mmFigu r e 1. a ) Mic ro c o il; b) b a se p la te ; c ) c o il a s se m b ly; d ) H e lm ho ltz c o il a s se m b ly.B. FabricationF igu r e 2 s h o ws the f abr icatio n pr o cess , wh ich is b as ed en tir ely o n no n - mag n etic m ater ials . A 1 0 0 m m diam eter <1 00 > o rien ted , in tr ins ic (> 5 0 00 Ω cm) S i w af er is f ir s tth er m ally o x id ized to f o r m an iso latio n lay er ( s tep 1 ) . W eh ave u s ed 1 .5 µ m of S iO2 . Th e o xid e is then coated w ith an ad hes io n lay er o f 3 0 0 Å Ti an d a s eed layer o f 2 0 0 0 Å Cu b y RF s p u tter in g ( s tep 2 ). A th ick ( 2 2 µ m ) lay er o f AZ6 2 90 p h oto r esis t (Clarian t U K Ltd .) is th en s pin coated o v er th ew afer , an d p attern ed by U V lith og r ap hy u sin g th e con d u cto r m ask to f o r m a d eep m ou ld . th e co n du cto rs are f o rm ed by d epo s itin g a 1 4 µ m th ick Cu lay er u s in g an acid plating s o lu tio n ( F B, f r om Tech n ic I n c.) an d a 4 µ m th ick A u lay er u s in g a th io su lp hate/su lp h ite s olu tion ( ECF 6 0, f r om Metalo r ) ( s tep 3 ).Cu/AuSU-8OxideSi substrateAZ9260Ti/CuAZ 9260DRIE87654321Figu r e 2. Fa br ic a tio n of m ic r o - co ils b y d ee p e tc h ing o f s ilico n .Th e m o u ld is th en r em ov ed by a co m b ination o f w et an d d r y s tr ip p in g. A 2 0 µ m th ick lay er o f S U - 8 2 0 5 0 ( Ch estech ,U K ), a ru g g ed, epo x y - bas ed r es ist [ 1 2] , is th en d epo s ited b y s p in co atin g . Th is layer f or m s a m as k f o r d eep r eactiv e io n etch in g an d als o p r o tects th e coil d ur in g s u b seq u ent u s e ( s tep 4 ) . Th e co n tact pad s ar e p ro tected w ith a lay er o f A Z9 2 60 r esis t du r in g etch in g . Th e S iO 2  is etched u sin g a P las m aTech n o log y S ys tem 8 0 + p ar allel p late reactiv e ion etch er ,u s in g CHF 3 , O2  and A r g as es ( s tep 5 ). Th e w af er is th enetch ed us in g a S ur f ace Techn o lo gy S y stem s S in gle Cham b erMu ltip lex in du ctiv ely co u p led p las m a etcher , op er atin g av arian t o f the cyclic Bo s ch/S TS ad v anced s ilico n etch [ 11 ] .Th e w af er is etched r ig h t th r o u gh , at a rate of 2 µ m /min ( step 6 ) . D u al- f r equ en cy p las m a is u s ed to co m bin e hig h - rateetch in g w ith s to p- o n - ox id e. Res id u al o x ide is th en r em o ved f r om th e b ack - s ide o f th e w af er b y reactiv e ion etch in g in th eS y stem 80 +. Th e r es is t p ad s ar e th en r em o ved f ro m th eco ntacts u s ing a co m b in ation o f w et an d dr y s tr ip p in g .P r oto ty pe d evices w er e f ab ricated as s in gle- tir n coils w ith 1 0 0 µ m wid e co n d u cto r s an d 1 m m x 5 mm s amp letr ou g h s . Th e d ie s ize w as 3 m m x 1 1 mm . Fig u r e 3 a sh o w s p art o f a co mp leted w af er car r y in g b oth co il an d b as e- p lated ies . S nap - o ut d ie s epar atio n w as s u cces sf u lly d em on s tr ated w ith h igh y ield .  Sin gle coils wer e th en m o u n ted o n s m allP CBs , and p air s of co ils w er e s tack ed to f o r m H elm ho ltzco ils . In th e latter cas e, th e dies wer e alig ned m ech an ically,an d ep o xied to g eth er . Electr ical co n nectio n s wer e fo r m ed b y b u mp b o nd in g w ith u s ing in diu m so ld er. F ig u r e 3 b s ho w s ( lef t to r ig ht) i) a co il, ii) a co il m o un ted o n a s m all P CB, iii) a bas e- plate, iv ) a coil attach ed to a b as ep late, v) a Helm h oltzco il, and v i) a Helm h oltz co il mo u n ted o n a P CB.a ) b ) Figu r e 3. a ) D ie s b e f or e sn a p - ou t a nd b ) c o il c o m po n e n ts d u rin g as s e m bly  III. EVALUATIONA. Electrical measurementsQ - facto rs o f is o lated an d res o n an t coils w er e m eas ur ed w ith an A g ilen t E5 0 6 1 A n etwo r k an aly zer . S in g le co ils h ad an in d u ctan ce o f 9 n H , w h ich r o se to 1 1 nH af ter m ou n ting o n a s ilico n b as e- p late. H elm h o ltz coils h ad an in du ctance o f2 4 n H af ter w ir e- b o nd in g to a s m all P CB. Co il as sem b lies 228w ere co nf ig u red as p arallel r es on ato rs u sin g no n - m ag n etics u rf ace m o u n t capacitor s . Hig h Q- f acto r s w er e o b tain ed ,s h ow in g th at g o o d p er fo r m ance can s till be achiev ed d es piteth e u s e o f a s ilico n su b s trate an d s old ered inter con n ects .F igu r e 4a s h ow s th e f req u ency v ar iatio n of th eim ped an ce o f r es on an t d etecto r s b as ed o n a s ing le- tu r n co il,co nf ig u red f or o per atio n at 6 3 .8 MH z ( i.e., in a 1 .5 T field )an d 1 2 7 .6 MH z ( 3 T) . In th e f o r mer cas e, Q - f acto r is 1 5 an d th e s h u nt im ped ance at r es on an ce is 60 Ω. I n th e latter cas e,th e Q - f acto r h as r is en to 22 . F ig u r e 4 b sh o w s th e fr eq u en cy v ariation o f th e Q - f acto r , w h ich p eaks at ≈ 2 5 at aro u n d 2 0 0 MH z, s u gg es tin g th at go o d per f o rm an ce w o uld s till beo b tain ed in a 6 T f ield . Res o n ant H elm h o ltz d etector s h ad aQ - facto r o f 13 w hen con f ig ur ed fo r o per atio n at 6 3 .8 MH z.a ) 200150100500-150-100-50050100150200250Frequency (MHz)Impedance (Ω) For operation      at 1.5TFor operation      at 3 TRXRXb ) 3002502001501005000510152025Frequency (MHz)Q-factorFigure 4. Frequency variation of a) the impedance and b) the Q-factorofsingle turn resonators.B. MR experimentsMagnetic resonance experiments were performed using a1.5 T Siemens Magnetom Vision MRI system at the RoyalMarsden Hospital. The main body coil was used for RFtransmission, and the microcoils were used for signalreception. Additional PIN diode-switched detuning circuitrywas provided to protect the microcoil during transmission,as shown in Figure 5. Here the capacitor CT is used fortuning, the capacitor CM for matching to 50 Ω  and theinductor LD for detuning.LCPIN CTLD CMTo pre-ampFigure 5. Circuit for tuning, matching and protection.Microcoils are sensitive to both interior and exteriorsignal sources, and interior imaging was first demonstrated.The sample was a slice of a “Spenco” dermal pad, a vinylplastisol material from Spenco Healthcare International,Horsham, West Sussex, UK which has previously been usedas an internal signal source in fiducial marker coils [13].Figure 6a shows a coronal slice image of a Spencosample taken from a single-turn coil. The slice is ahorizontal cut through a sliver of Spenco measuring 3 x 4.5x 0.7 mm, placed upright in the sample trough. The imageswere obtained using a 28 x 28 mm field of view, a 500 µmslice thickness and four signal averages from a 35 ms spinecho sequence. Excellent SNR is clearly obtained. Figure 6bshows a similar image obtained from a Helmholtz coil; herethe image definition is rather better.Imaging of an exterior sample was then demonstrated,using a semi-cylindrical cod-liver oil capsule placed directlyon top of a single turn coil. The capsule diameter was ≈ 8mm. Figure 6c shows a transverse slice image. The majorityof the capsule is in view, but the signal clearly reduces withdistance from the coil plane, falling to around 10% of itspeak at ≈ 2.4 mm above the coil. This image was obtainedusing a 56 x 56 mm field of view, a 500 µm slice thicknessand two signal averages of a 35 ms spin echo sequence.a) SpencosampleImage planeCoil   b) Coil    c) CoilCod-liver-oilcapsuleImageplane  Figure 6. MR imaging of a), b) Spenco and c) cod liver oil.229Magnetic resonance spectroscopy of both types ofsample was then demonstrated. Figure 7a shows a MRspectrum obtained from the Spenco sample using a singleturn coil. The spectrum consists of a large, narrow peakwith two clearly resolved side peaks, which ischaracteristic of this material. The measured line-width ofthe main peak was 54 Hz.Figure 7b shows the spectrum of the cod-liver oilsample. The line-width of the lipid peak is 12 - 17 Hz.The signal to noise ratio was estimated from the peaksignal divided by the RMS value of the noise in theabsence of signal as ~ 900. The data shown have beenapodized to boost high frequencies; this procedure helpsto identify doublets, which can otherwise appear as asingle broad peak. Considerable structure may be seen inthe spectrum, and assignment of the peaks is in progress.a)-10-8-6-4-20246810-1000010002000300040005000Frequency deviation (ppm)Signal strength (a.u.)b)-10-8-6-4-20246810-1000010002000300040005000Frequency deviation (ppm)Signal strength (a.u.)Figure 7. MR spectra obtained from a) Spenco and b) cod-liver oil. IV. CONCLUSIONSDeep reactive ion etching and electroplating have beencombined in a wafer-scale process for batch fabrication ofmagnetic resonance detectors on oxidised silicon substrates.The conductors are formed first, by electroplating metalsinside a photoresist mould, and the die topography is thendefined by deep reactive ion etching. Detectors may bedetached from the wafer by a simple snap-out process andassembled into a variety of configurations, including singleturn and Helmholtz coils.Q-factors up to 15 have been obtained at 63.8 MHz,despite the use of a semiconducting substrate. PreliminaryMRI and MRS experiments have been performed, andexcellent SNR and line-width have been obtained. Furtherimprovements in signal obtained from optimised multi-turncoil geometries and from an increase in magnetic fieldstrength are expected. An obvious near-term target is toperform similar evaluations of tissue samples, and this workis in progress. Alternative needle coil geometries are nowbeing investigated, to allow tissue samples to be assessedwithout the need to remove them from a biopsy needle.ACKNOWLEDGMENTTh e A u tho r s ar e gr atefu l to D r J. S tag g fo r D RI E w or k .REFERENCES[1] C.E. Mountford, S. Doran, C.L. Lean, P. Russell, “Cancer pathologyin the year 2000” Biophys. Chem., vol 68, pp. 127-135, 1997.[2] S.J. Barton, F.A. Howe, A.M. Tomlins, S.A. Cudlip, J.K. Nicholson,B.A. Bell, J.R. Griffiths, “Comparison of in vivo 1H MRS of humanbrain tumours with 1H HR-MAS spectroscopy of intact biopsysamples in vitro” Magma, vol 8, pp. 121-8, 1999.[3] H. Poptani, J. Kaartinen, R.K. Gupta, M. Niemitz, Y. Hiltunen, R.A.Kauppinen, “Diagnostic assessment of brain tumours and non-neoplastic brain disorders in vivo using proton nuclear magneticresonance spectroscopy and artificial neural networks” J. Cancer Res.Clin. Oncol., vol 125, pp. 343-9, 1999.[4] D.J. Gilderdale, N.M. deSouza, G.A. Coutts, M.K. Chui, D.J.Larkman, A.D. Williams, “Design and use of internal receiver coilsfor magnetic resonance imaging” Brit. J. Radiol., vol 72, pp. 1141-51,1999.[5] T.L. Peck, R.L. Magin, J. Kruse, M. Feng, “NMR microspectroscopyon 100-µm planar RF coils fabricated on gallium arsenide substrates”IEEE T. Biomed. Eng., vol 41, pp. 706-9, 1994.[6] T.L. Peck, R.L. Magin, P.C. Lauterbur, "Design and analysis ofmicrocoils for NMR microscopy", J. Magn. Reson. Ser. vol. B108,pp. 114-124, 1995.[7] J. Dechow, A. Forchel, T. Lanz, A. Haase “Fabrication of NMR-microsensors for nanoliter sample volumes” Microelectr. Engng., vol.53, pp. 517-519, 2000.[8] L. Berry, L. Renaud, P.Kleimann, P. Morin, M. Armenean, H. Saint-Jalmes, “Development of implantable detection microcoils forminimally invasive NMR spectroscopy” Sensors and Actuators vol.A93, pp. 214-218, 2001.[9] J. Dechow, T.Lanz, M. Stumber, A.Forchel, A. Haase, “Preamplifiedplanar microcoil on GaAs substrates for microspectroscopy”, Rev.Sci. Instrum. vol. 74, pp. 4855-4857, 2003.[10] C. Massin, C. Boero, F. Vincent, J. Abenhaim, P.A. Besse, R.S.Popovic, “High-Q factor RF planar microcoils for micro-scale NMRspectroscopy” Sensors and Actuators vol. A97-98, pp. 280-288, 2002.[11] J.K. Bhardwaj, H. Ashraf, “Advanced silicon etching using highdensity plasmas”, Proc. SPIE vol. 2639, pp. 224-233, 1995.[12] E.H. Conradie, D.F. Moore, "SU-8 thick photoresist processing as afunctional material for MEMS applications", J. Micromech.Microeng.vol. 12, pp. 368-374, 2002.[13] G.A. Coutts, D.J. Gilderdale, K.M. Chui, L. Kasuboski, N.M.deSouza, “Integrated and interactive position tracking and imaging ofinterventional and internal devices using small fiducial receiver coils”Magn. Reson. Med. vol. 40, pp. 908–913, 1998.230

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Batch fabrication of micro-coils for MR spectroscopy on silicon

Batch fabrication of micro-coils for MR spectroscopy on silicon

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