Kromka, A

Krátká, N.

Neykova, N.

Rezek, B.

Institute of Mathematics AS CR, v. v. i.

Acta Universitatis Carolinae. Mathematica et PhysicaN. Krátká; N. Neykova; A Kromka; B. RezekEtching Effects of Low Temperature Hydrogen plasma on encapsulateddiamond transistorsActa Universitatis Carolinae. Mathematica et Physica, Vol. 53 (2012), No. 2, 97--103Persistent URL: http://dml.cz/dmlcz/143704Terms of use:© Univerzita Karlova v Praze, 2012Institute of Mathematics of the Academy of Sciences of the Czech Republic provides access todigitized documents strictly for personal use. Each copy of any part of this document mustcontain these Terms of use.This paper has been digitized, optimized for electronic delivery andstamped with digital signature within the project DML-CZ: The CzechDigital Mathematics Library http://project.dml.cz972012 acta universitatis carolinae – mathematica et physica vol. 53, no. 2eTChIng effeCTS of loW TemperaTure  hydrogen plaSma on enCapSulaTed  dIamond TranSISTorSM. KráTKá, N. NEyKoVA, A. KroMKA, B. rEzEKPrahaReceived June 14, 2012Revised October 24, 2012We study etching effects of low temperature hydrogen plasma treatment (200–300 ◦C) intwo different microwave plasma reactors (linear and focused plasma) on diamond solution-gated field-effect transistors with various polymers for encapsulation (MA-P, OFPR, SU8).Three-dimensional transistor microstructures (20 µm) are grown from nanocrystallineH-terminated intrinsic diamond by MW-CVD on Si/SiO2 substrates. We observe increasedetching rates of the polymers in dependence on temperature of hydrogenation (from 3 to400 nm/min). We show that low temperature hydrogenation in linear plasma system isdue to larger distance (7 cm) between the high-density plasma region and the sample sur-face less aggressive on encapsulation and 3D transistors based on surface conductivity ofH-terminated diamond are fully operational after this treatment.1. I n t r o d u c t i o nDiamond exhibits unique combination of electrical, optical and mechanical prop-erties with chemical and biocompatible properties [Tang et al., 1995, Kalbacova etal., 2007] and it is a perspective material for bioelectronic and electrochemical ap-plications. Diamond films can be employed as biosensors such as capacitive sensors[Abouzar et al., 2008] or solution-gated field-effect transistors (SGFETs) [Rezek etal., 2010, Dankerl et al., 2009]. These biosensors are often encapsulated by differentInstitute of Physics ASCR, Czech RepublicCharles University in Prague, Faculty of Mathematics and Physics, Czech RepublicCzech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, CzechRepublicWe acknowledge a financial support by P108/12/0996 (GAČR), P108/12/G108 (GAČR), doctoralproject 202/09/H041 and SVV-2012-265304. This work occurred in frame of the LNSM infrastructure.Technical support of Vlastimil Jurka, Jitka Libertínová, Oleg Babchenko and Zdenka Poláčková is grate-fully appreciated. A. Fejfar is kindly acknowledged for software development package.Key words and phrases. encapsulated diamond transistors, hydrogen plasmaE-mail address: kratka@fzu.czmath_12_2.indd   97 1.3.2013   9:46:1598types of photoresists which are relatively easy to prepare and remove. Silicon nitridecan represent other way of encapsulation [Sakata et al., 2004] but its preparation ismore complicated.Common diamond field-effect transistors with gate immersed in solution based onhydrogen terminated diamond surface conductivity are prepared by diverse techno-logical steps including lithography, lift-off, selective oxidation, encapsulation, depo-sition of contacts etc. [Rezek et al., 2010, Krátká et al., 2012]. This is complicatedfor renewal of device because standard recycling process includes cleaning in pi-ranha solution, acid boiling, using aqua regia for removing gold contacts, oxygenplasma treatment etc. This process destroys C–H surface termination. Therefore,H-terminated diamond SGFETs must be prepared from the beginning after the sensorhas been used because for instance adsorption of proteins causes permanent shift ofthe characteristics in spite of rinsing [Rezek et al., 2010]. A direct growth of diamondstructures via patterning of nucleation/seeding layer can represent an alternative so-lution [Kromka et al., 2009, Kozak et al., 2010, Babchenko et al., 2012].Therefore, we employ here nanocrystalline diamond (NCD) films to fabricate di-rectly grown SGFET microchannels. We study if such complete encapsulated devicescan be recycled by low temperature hydrogenation (LTH) process in order to cleanand at the same time preserve active area of H-terminated diamond surface. More-over, we study effects of low temperature hydrogen plasma treatment (200–300 ◦C)in two different microwave plasma reactors (linear and focused plasma) on diamondtransistor characteristics and on etching of organic resins encapsulation.2. E x p e r i m e n t a l m e t h o d sDirectly grown NCD FET channels were prepared by initial patterning of diamondnanoparticle nucleation film on Si/SiO2 substrates using photolithographic processingwith two polymer layers and by subsequent reactive ion etching through photolitho-graphic mask [Babchenko et al., 2010]. Afterwards, growth of NCD microstruc-tures was performed in a microwave plasma CVD system (AIXTRON P6) from amethane/hydrogen gas mixture. Process parameters were as follows: microwavepower 2.5 kW, 1 % methane in hydrogen, total gas pressure 50 mbar, substrate tem-perature 800 ◦C, and total growth time 3 h [Babchenko et al., 2010]. The surfaces ofdiamond structures were hydrogenated in pure H plasma at 600 ◦C for 10 min.Source and drain contacts were prepared by thermal evaporation (10 nm of Ti and50 nm of Au) followed by lift-off technique. The area between contacts was coveredwith positive photoresist OFPR (thickness 2.5–4 µm) or ma40 (thickness 3–4 µm) orwith negative photoresist SU8 (thickness 10 µm) and deep UV curing of the resists forhardening and biocompatibility was applied. An opening of 60 × 60 µm2 in resist wasmade in the active area of the device [Rezek et al., 2010]. Thickness of the polymerencapsulation was measured by profilometer before and after each LTH experiment.Polymer etching rates were calculated from the thickness difference and process time.math_12_2.indd   98 1.3.2013   9:46:1699Figure 1. Diamond microwave CVD reactors: (a) focused plasma re-actor, (b) linear antenna reactor.Commonly used relatively high substrate temperatures (T ≥ 600 ◦C) during dia-mond growth and hydrogenation can lead to partial or complete damage of the metalelectrodes or other electronic parts [Neykova et al., 2009]. Thus, 3D transistors in-cluding the encapsulation and gold contacts were exposed to hydrogen plasma us-ing low temperature in two microwave plasma reactors (focused plasma reactor andlinear plasma reactor). Figure 1 shows schematic drawing of the two fundamen-tally different setups and conditions of focused and linear antenna MW plasma reac-tors. The parameters of LTH in focused plasma were as follows: microwave power800–1500 W, vacuum pressure 20–30 mbar, hydrogen flow 300 sccm, temperature300 ◦C (the lowest achievable), processing time 20 min. As for LTH in linear plasma,the parameters were as follows: microwave power 1000 W, vacuum pressure 0.3 mbar,hydrogen flow 100 sccm, processing time 20 min. Temperature was varied in therange of 200 ◦C and 300 ◦C with the accuracy of ±10 ◦C.3. R e s u l t s a n d d i s c u s s i o nFigure 2 shows different etching rates for different types of encapsulation afterLTH process. The etching rates of each encapsulation material increase (from 3 to400 nm/min) in dependence on temperature of hydrogenation (200 ◦C–300 ◦C). Errorbars of ±10 ◦C along the X axis show the inaccuracy of the temperature measurementduring the process. All data are related to linear plasma except for SU8 3010 at300 ◦C (“star”) which has been obtained after focused plasma LTH. This etching rateis bottom estimate because all employed photoresists were completely removed infocused plasma at 300 ◦C which you can see in optical images of active area of thetransistors (Figure 3). As for gold contacts, their quality appears good even after LTHin focused plasma.math_12_2.indd   99 1.3.2013   9:46:17100Figure 2. Etching rates for photoresists (OFPR, ma40 and SU8 3010)as a function of temperature.LTH in linear plasma is less aggressive on encapsulation than focused plasma mostlikely due to larger distance between the high density plasma region and the samplesurface [Kromka et al., 2012, Neykova et al., 2012]. Based on the data of LTH inlinear plasma, photoresist SU8 is the most stable of employed photoresists and it canbe used up to 300 ◦C. Photoresist OFPR is removed during LTH process in linearplasma at temperature about 260 ◦C and its quality is good up to this temperature.Photoresist ma40 appears as the least stable, having bad quality (cracks and bubbles)already at 200 ◦C.Figure 4 shows that channel currents after LTH are reduced compared to the cur-rent after standard hydrogenation at 600 ◦C in focused plasma. Yet 3D diamond tran-sistors are fully operational after LTH at 200 ◦C in linear plasma reactor even afterrepeated LTH process. This is in agreement with the study of hydrogen termina-tion at 200 ◦C performed in linear antenna plasma system which is efficient to inducehydrogen-terminated conductive surfaces and an increase in substrate temperature upto 400 ◦C results in an increase in surface conductivity [Neykova et al., 2012]. WhenLTH was repeated on the same device we observed fluctuations of threshold volt-age Ut within about 150 mV. Transconductances (slope of Ids(Ug)) calculated at Idsin the linear region of transfer characteristics are 10.6 nS, 12.9 nS and 12.2 nS atUds = −0.6 V, Ids = −2.0 nA for transfer characteristics after first, second and thirdLTH process, respectively. Thus transconductance remained similar. This indicatesthat the fluctuations are related to the variation of gate potential but not to the equilib-rium of surface conductive layer. This is unlike the case when proteins were adsorbedon the surface [Rezek et al., 2010]. On the other hand, it is in a good agreement withmath_12_2.indd   100 1.3.2013   9:46:18101Figure 3. Optical microscope images of active area of the transistorsencapsulated with photoresist before and after low T hydrogenationprocess in linear and focused plasma.Figure 4. (a) Initial transistor characteristic. (b) Transistor characteris-tics after repeated LTH process in linear plasma at 200 ◦C. All transfercharacteristics were measured in HEPES at Uds = −0.6 V.the fact that transistors characteristics were measured in buffer solution which shouldmaintain stable surface conditions. Origin of these Ut fluctuations is still unknown.Fluctuations after standard hydrogen termination (600 ◦C) as well as after LTH infocused plasma can not be investigated because covering resists are removed duringthese processes and thus transistor characteristics cannot be measured.math_12_2.indd   101 1.3.2013   9:46:231024. C o n c l u s i o nWe showed that low temperature hydrogenation at linear plasma system is moresuitable due to larger distance between the high-density plasma region and the sam-ple surface. We observed that low temperature hydrogen termination in linear antennaplasma is less aggressive on encapsulation than focused plasma. It kept the similarelectronic quality (yet reduced thickness) of covering encapsulation and gold con-tacts. However, the currents measured after this treatment are lower compared to thefocused plasma. When LTH was repeated on the same device we observed fluctua-tions of threshold voltage Ut within about 150 mV. Origin of these Ut fluctuations isstill unknown. Yet, diamond SGFETs are fully operational after this treatment. There-fore, 3D diamond transistors including the encapsulation and gold contacts can be re-cycled by low temperature hydrogenation in linear antenna reactor within 30 minuteswithout any other processing.ReferencesAbouzar M. H., Poghossian A., Razavi A., Besmehn A., Bijnens N., Williams O. A., Haenen K.,Wagner P., SchöningM. J.: Penicillin detection with a nanocrystalline diamond-based field-effectsensor, phys. stat. sol. (a) 205, 2141–2145, 2008.BabchenkoO., Izak T., Ukraintsev E., HruškaK., Rezek B., KromkaA.: Towards surface-friendlytreatment of seeding layer and selected-area diamond growth, phys. stat. sol. (b) 247, 3026–3029,2010.Babchenko O., Verveniotis E., Hruška K., Ledinský M., Kromka A., Rezek B.: Direct growth ofsub-micron diamond structures, Vacuum 86, 693–695, 2012.Dankerl M., Eick S., Hofmann B., Hauf M., Ingebrandt S., Offenhäusser A., Stutzmann M.,Garrido J.A.: Diamond transistor array for extracellular recording from electrogenic cells, Adv.Funct. Mater. 19, 2915–2923, 2009.Kalbacova M., Kalbac M., Dunsch L., Kromka A., Vaněček M., Rezek B., Hempel U. and KmochS.: The effect of SWCNT and nano-diamond films on human osteoblast cells, phys. stat. sol. (b)244, 4356–4359, 2007.Kozak H., Kromka A., Babchenko O., Rezek B.: Directly grown nanocrystalline diamond field-effect transistor microstructures, Sensor Lett. 8, 482–487, 2010.KrátkáM., Kromka A., Ukraintsev E., LedinskýM., Brož A., KalbacovaM., Rezek B.: Functionof thin film nanocrystalline diamond-protein SGFET independent of grain size, Sens. Actuators B166–167, 239–245, 2012.Kromka A., Babchenko O., Rezek B., Ledinský M., Hruška K., Potměšil J., Vaněček M.: Sim-plified procedure for patterned growth of nanocrystalline diamond microstructures, Thin SolidFilms 518, 343–347, 2009.Kromka A., Babchenko O., Izak T., Hruska K., Rezek B.: Linear antenna microwave plasma CVDdeposition of diamond films over large areas, Vacuum 86, 776–779, 2012.Neykova N., Babchenko O., Kozak H., Kromka A., Hruska K., VanecekM.: in 15th InternationalConference on Applied Physics of Condensed Matter, Slovak Republic, p. 256–261, 2009.Neykova N., Kozak H., Ledinsky M., Kromka A.: Novel plasma treatment in linear antenna mi-crowave PECVD system, Vacuum 86, 603–607, 2012.r e f e n c e smath_12_2.indd   102 1.3.2013   9:46:23103Rezek B., Krátká M., Kromka A., Kalbacova M.: Effects of protein inter-layers on cell–diamondFET characteristics, Biosens. Bioelectron. 26, 1307, 2010.Sakata T., Kamahori, M., MiyaharaY.: Immobilization of oligonucleotide probes on Si3N4 surfaceand its application to genetic field effect transistor, Materials Science and Engineering: C 24,827–832, 2004.Tang L., Tsai C., Gerberich W., Kruckeberg L., Kania D.: Biocompatibility of chemical vapour-deposited diamond, Biomaterials 16, 483–488, 1995.math_12_2.indd   103 1.3.2013   9:46:24

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Etching Effects of Low Temperature Hydrogen plasma on encapsulated diamond transistors

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Etching Effects of Low Temperature Hydrogen plasma on encapsulated diamond transistors

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