Plasma-deposited silicon nitride films with low hydrogen content for amorphous silicon thin-film transistors application

Sensors and Actuators A, 37-38 (1993) 333-336

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Plasma-deposited silicon nitride films with low hydrogen content for amorphous silicon thin-film transistors application J Campmany, J L Andtijar, A Camllas, J Clfre and E Bertran Departament de Fmca Aplrcada I Electronrca, Umuersttat de Barcelona, Autnguda Diagonal 647, EO8028 Barcelona (Spatn)

Abstract A comparative study of the vlbratlonal properties of PECVD amorphous sihcon mtnde films obtamed from SIH, + NH3 and SIH, + N2 precursor gas mixtures has been performed by FT-IR transmission spectroscopy The bonded hydrogen, calculated from the absorption spectra, shows important quantitative and qualitative dflerences dependmg on the precursor gas muttures used The hydrogen content of near-stoicluometm films obtamed from SIH, + N, mixture 1s 10 ties lower than that of films prepared from !QH, + NH, mixture In addition, hydrogen 1s mainly bonded to mtrogen atoms III films from SIH, + NH,, whereas it IS mainly bonded to sibcon atoms m films from SIH,+N, These low-hydrogenated s&con mtnde films, obtarned from tmxtures contammg N,, have been apphed as insulator layers m the preparation of amorphous &con thm-film transistors (a-% Tl%) The TFTs were of normal staggered type composed of the structure Al/a-SIN H/a-Q H grown on MCr source and dram electrodes deposited on glass substrates TFTs with a 0 2 pm thick a-!+ H layer and 10 pm channel length have on-off current ratios of 5 x lO“, electron field-effect mobihhes of about 1 5 cm2/Vs and threshold voltages around 5 V

Introduction Slhcon mtnde thm films (a&N, H) can be obtained m a low-substrate-temperature process by plasmaenhanced chemical vapour deposition (PECVD) from SlH, + NH, and BH, + N2 gas rmxtures These films are amorphous and theu x = N/S1 ratio can easily be modified by changmg the proportion of precursor gases Films of a&N, H have been widely used m the semiconductor industry as passive layers More interesting IS their application as active layers m the fabrication of thm-film devices The BH4 + NH3 gas nuxture 1s commonly used, owmg to the low r f power needed because of the low bonding energy of the NH, molecule as compared with that of N,, the stab&y of the plasma durmg iilm deposltlon and the relatively high growth rate of a&N, H films Nevertheless, m spite of all these advantages, s&con mtrlde films grown by PECVD using NH, present a high concentration of hydrogen due to the particular chemistry of the SIH, + NH, r f plasma [ 1,2] This incorporation of hydrogen has been related to a high concentration of electronic defects [3] and, therefore, has detnmental effects on the performance of electronic devices In particular, the performance and rehablhty of amorphous slhcon thin-film transistors (a-S1 H TFTs) are cntlcally dependent on the a-S1 H/ a-&N, H mterface However, it 1s expected that the use of BH, + N2 gas rmxtures could yield low-hydrogenated interfaces because of the reduction m the hydrogen content of a-&N, H layers [4]

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In this work, we first present a study carried out on two series of srhcon mtnde fihns obtamed by PECVD from BH, + NH, and SlH, + N, gas nuxtures For both series, we compare the hydrogen content determined by FT-IR spectroscopy as a fun&on of rutrogen composltlon measured by XPS Once the low hydrogen content of a-&N, films obtained with N, 1sestabhshed, we have applied them as the insulator layer m the preparation of normal staggered (top gate) a-S1 H TFTs The electromc characterlstlcs of the obtained TFTs are also studied

Experimental We have obtained slhcon mtnde thm films m an r f plasma reactor ( 13 56 MHz) described elsewhere [ 51 The reactor mcorporates a phase-modulated elhpsometer that allows the m sztu optical characterlzahon of grown films, working m two modes spectroscopic (1 54 5 eV) and real-tune (one data point every 8 ms) [6] The films prepared from SIH, + NH3 gas rmxture were deposited on c-S1 substrates at 300 “C, 30 Pa deposition pressure, 62 5 mW/cm2 r f power density and a total gas-flow ratio of 10 seem The films prepared from SlH, + N2 were deposited at 350 “C substrate temperature, 30 Pa deposltlon pressure, 200 mW/cm* power dens@ and a total gas-flow ratio of 100 seem The substrates were placed vertically on a grounded electrode and a shutter protected them durmg the swltchmg-on and plasma stablhzatlon penod The FT-IR analysis was carned out with a DA3 Bomem working m transnusslon

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Cbmmg

Fig

1 Cross-sectional

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glass

view of the a-S1 H TFT structure

mode under vacuum m the wavenumber range 400 to 4000 cm-’ with a resolution of 4 cm-’ XPS measurements were performed m a Perkm-Elmer 5500, measurmg the 2p core level for SI and 1s core level for N Top-gate TFTs (Fig 1) were prepared through a simple three-step fabrlcatlon process conslstmg of (1) deposltlon of the source and dram electrodes through a metal mask on Corning glass by conventional thermal evaporation of NICr, (2) deposltlon of the a-S1 H and a-SIN, layers by PECVD, (3) deposltlon of gate electrodes by thermal evaporation of Al through a patterned metalhc mask Thermal evaporations were carried out m a vacuum chamber from where the samples were transferred to or from the PECVD reactor The a-S1 H layer was optoelectronic quahty undoped (EC - EF = 0 7 eV), 200 nm thick, deposited under the followmg condltlons 12 5 mW/cm’ r f power density, 30 seem pure SlH, flow rate, 350 “C substrate temperature, 30 Pa pressure The a-SIN, layer was deposited from a SIH, + N, gas mixture with a stlane fraction [BH,]/([SlH,] + [NJ) = 0 3%, 350 “C substrate temperature, 100 seem total gas-flow ratlo, 30 Pa pressure, and 200 mW/cm* r f power density The film obtained was also 200 nm thick, transparent and dlelectrlc (p > 1OL4R cm) urlth electrical transport based on a Poole-Frenkel mechamsm [ 71 The averaged length and width of the channel b&t between electrodes, measured by optlcal microscopy, were 12 pm and 190 p, respectlvely The gate/source-dram overlap had an area of 0 025 mm2 Electrical measurements at room temperature of the TFTs (transfer and output charactenstlcs) were made with a Kelthley 617 electrometer controlled by a computer

Results and discussion In order to compare the hydrogen content of silicon nitrides mth snmlar x ratios, obtained from both precursor gas nuxtures, we had to use N&H, gas-flow ratios 10 tunes higher than those of NHJGH,, for the r f density power used [8]

The mam absorption bands detected by m-IR spectroscopy are centred at wavenumbers around 840 cm-‘, 21OOcm-’ and 334Ocm-‘, which are attnbuted to stretchmg vibration modes of Sl-N, SI-H and N-H bonds, respectively [ 81 A band around 1180 cn- ’ 1salso detected, correspondmg to the bending mode of the N-H bond In addition, a small band centred around 1550 cm-’ and attnbuted to the stretching mode of N-H, bond has been identified m &con nitrides obtamed from BH, + NH3 gas mixture The samples obtamed from N2 + SIH, gas mixtures do not show this peak We have determmed the concentration of hydrogen bonded to silicon from the area of the SI-H peak at 2100 cm-’ and usmg a calibration constant AS,_H= 1 4 x 10” cnm2 [9] The concentration of hydrogen bonded to nitrogen was obtained from the area of the N-H peak (3350 cm-‘), with A,_, = 6 4 x 10zocmw2 [9] Figures 2 and 3 show the evolution of these concentrations of bonded hydrogen versus composition measured by XPS, using the standard sensltlvlty factors In Fig 2 we show the concentration of the bonded hydrogen m the case of a SIH, + NH, gas mxture The total concentration mcreases drastically for x above 0 7 due to the increase of hydrogen bonded to nitrogen, while the hydrogen bonded to s&on decreases This 1s m accordance wth the chemrcal reactlons m the plasma described m refs 2 and 3, which lead to a richly hydrogenated film In addition, this sudden increase m hydrogen concentration 1s accompamed by a change m

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08

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Rg 2 Concentration of bonded hydrogen determmed by FT-IR of a-SIN, films obtamed from a SIH, + NH, r f plasma at 62 5 mW/cm* r f power dens&y, 30 Pa pressure and 300 “C substrate temperature N/S1 ratio m the flms was determmed by XPS

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N2+S1H4 a-SiN,

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Fig 3 Concentration of bonded hydrogen determmed by FT-IR of a-SIN, films obtamed from a SIH, + N2 r f plasma at 200 mW/cn? r f power density, 30 Pa pressure and 350 “C substrate temperature N/S1 ratio m the films was determined by XPS

Fig 4 Transfer charactenstax length at 12 V dram voltage

of a TFT with IO pm channel

150L TFT a-SIN

optical properties, the extmctlon coefficient becommg practically zero [8] In contrast, m Fig 3 we show that films obtamed from an N2 + BH, gas rmxture do not present an increase m the concentration of hydrogen bonded to nitrogen when the nitrogen content m the film mcreases This result 1s a consequence of chemical reactions m the plasma described m ref 4, which @ve a low-hydrogenated film The total concentration of hydrogen m more mtrogenated fihns 1s 10 times higher for a mixture contalmng NH, than for a rmxture containing N2 In addition, the hydrogen m films from NH, + SlH, 1s mainly bonded to N, whereas m films from N, + SiH, it is mainly bonded to Si Once we have established the differences m hydrogen content for each precursor gas rmxture, we have prepared TFT devices \nth a low-hydrogenated amorphous &con mtnde layer m order to study any unprovement m their performance Elhpsometnc measurements carned out durmg the deposltlon process pomt to an abrupt interface between a-S1 H and a-SIN, layers [lo] Figure 4 shows the transfer charactenstics of a TFT wth a channel 10 pm long and 196 pm mde, where the dram current Id 1s plotted against the gate voltage VBat a fixed dram-source voltage V, of 12 V At V, below 3 V, Z, 1s around 1 nA For higher gate voltages Id Increases more than four orders of magmtude, reaching a value near 40 pA at I’, = 20 V Figure 5 shows the output charactenstlcs (Z, versus V,) for dfferent gate

Rg 5 Output charactenstlcs of a TFT length at different gate voltages

on a-SI

H :

with 12 pm channel

voltages, corresponding to a TFT havmg a channel of 12 w length and 180 pm \ndth In order to evaluate the effective electron field-effect mobhty of the TFTs, the equation correspondmg to an ideal MOSFET m saturation regnne was used [ 1l] Zdx (W2WlI~)PLFE(~g

- KY

(1)

W being the undth of the channel, L the length, l, the &electnc pernuttmty, d, the a-&N, layer thickness, pFE the field-effect mobity, VBthe gate potential and V, the gate threshold voltage Figure 6 shows an example of calculation of pF(FE by plotting the square-root of Z, m saturation agamst Vg The saturation condition was assured by connectmg the dram and the gate electrodes

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We have applied these results to prepare normal staggered a-S1 TFTs wth a low hydrogen content in the amorphous slhcon/amorphous silicon nitride mterface The TFTs have an acceptable on-off current ratlo ( w5 x 104) and a high field-effect mob&y (~1 5cm2V/s)

Acknowledgements

This work was supported by the CICYT of Spain under contract MAT 0955/90 We thank the SCIT of the Umversltat de Barcelona for the XPS and FT-IR measurements We also thank Professor J L Morenza for encouragement Rg 6 Determmatlon of the field-effectmohllty prfi from ltnear dependence of (Id)‘I2vs V, at V, = V,, correspondmg to a TFT wth a channel 12pm long and 200pm wide The pFEcalculated using eqn (1) was 1 47 cm*/Vs

the same potential From the slope of the straight line m Fig 6, the geometnc parameters of the TFT (W=200pm, L = 12pm, d,=O2pn) and the relative ddectnc pernuttlvlty calculated from capacitance measurements (en = 7 9 * 0 2), a pFE value of 1 5 cm’/V s 1s obtamed Average values of pFE and V, of the prepared TFTs are about I 5 cm’/V s and 5 V, respectively The obtained TFTs have higher mob&y than the usual pFE values reported ( x 0 5- 1 cm’/V s), generally correspondmg to mverted structures with an a-SIN, film prepared from NH, + SlH, gas mixture We attnbute these high pFE values to the low hydrogen content m the a-SIN, layer, along wtth the low hydrogen content m the a-S1 H layer (40/o), resultmg m a low-hydrogenated msulator/semlconductor Interface, which has been reported as an important condltlon for mprovmg PFE [31 to

We have synthesized &con mtrlde thm tims from SIH, + NH, and SIH, + N2 precursor gas mixtures mth comparable composltlons The total concentration of bonded hydrogen m the films with high mtrogen content 1s about 10 times hgher for those obtamed from SIH, + NH, than for those from SlH, + N, In addltlon, the hydrogen m the high-mtrogen films from SlH, + N, 1s mainly bonded to s&con, whereas the hydrogen m the high-mtrogen films obtained from SlH, + NH3 IS mainly bonded to mtrogen

References

1 D L Smrth, Plasma deposltlon of SIN, H, process chermstry vs film propertles, Symp Charact PECVD Proc, MRS Fall Meet, Boston, MA, USA, Nou 27, 1989, paper I-22 2 D L Smith, A S Alrmonda, C C Chen, S E Ready and B Wacker, Mechamsm of %N,H, deposUon from NH,SIH, plasma, J Electrochem Sot , 137 (1990) 614-623 3 K Hlranaka, T Ytslnmura and T Yamaguciu, Effects of the deposItion sequence on amorphous srhcon thm-film transistars. JD~I J ADDI Phvs. 28 (1989) 2197-2200 4 D i &mth, A’S Ahmdnda~~d fi J Prelsslg, Mechamsm of %N,H,, deposItIon from N,-SIH, plasma, J Vat Scr Technol, 88 (1990) 551-557 5 J L AnduJar, E Bertran, A Camllas, J Esteve, J Andreu and J L Morenza, Real time controlled r f reactor for deposition of a-S1 H tlun films, Vacuum, 39 (1989) 795-798 6 A Camllas, E Bertran, J L Andqar and J L Morenz.a, In suu optical characterlzatlons for r f plasma deposited a-% H thm films, Vacuum, 39 (1989) 785-787 7 E Bertran, J M Lopez-Vdlegas, J L Andujar, J Campmany and J R Morante, Optical and electrical propertIes of aSl, N, H films prepared by r f plasma using N2 + SIH, gas nuxtures, J Non-Cryst Solids, 137 h 138 (1991) 895-989 8 J Camumanv. E Bertran, J L Andular, A Camllas, J M Lopez-%lleg& and J R Morante, Comparative study of the optical and vlbratlonal properties of a-&N, H films prepared from SIH,-N, and SIH,-NH, gas mixtures by r f plasma, Symp Amorph Sd~c Tech, MRS Sprmg Meet, San FranCISCO,CA, USA, Apr 27, 1992, pp 643-648 A Monmoto, S Oozora, M Kumeda and T Shmuzu, Annealmg behavior of hydrogenated amorphous sihcon-mtrogen alloy films prepared by sputtering, Phys Status Sohd (6), 63 (1983) 715-720 J L AnduJar, E Bertran, A Camllas, J Campmany and J Cifre, Amorphous silicon thm film transistors with iugh electron field effect mobdlty, Symp Amorph &kc Tech , MRS Sprmg Meet, San Francuco, CA, USA, Apr 27, 1992, pp 1007-1012 S M Sze, m Sernrconductor Deuces, Physzcs and Technology, Wiley, New York, 1985, Ch 5, p 206