Goswami, A

Goswami, P

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(nflian J, Phys. 49, PJS-320 (1975)A. Gosw am i a n d  A mtt P . G o sw a m i**National Chemical Laboratory, Poona 411008 {Hccckml 30 November 1974)SbgOj film capacitors (Al/Sb^Og/Al) liave Ix^cn studied in details for tlieir di(4cctric l)ohaviom‘ at different temperatures (78-380"K) and frecjuoncios (10--10^ Hz). Hieleetrie constant was thickness dei)en- dent for tliiimer films but became independent (8.2) for thicker films (>  800 A). Loss factor (tan (J) (ab(3ut 0.003) at 1000 Hz was found to be indopendent of film thickness. C.apacitance, though dependent on temjjeratures and frequencies, becjame invariant with the fre­quency at low temperatures. Loss factor showing a pronounced minimum (tan ^min) ifi^  fhe frequency spectrum increased with tem ­peratures and tan shifted to the higher frequency region. Break- flown field strength Ibllowed the Forlani-Minnaja relation lliesci capacitors had high stability, and low TCC and loss factors. The a.c. behaviour of these capacitors especially tan and its variation have adequately been explained from a capacitor model proposed jireviously by us.1. I n t r o d u c t io nBieleetric films are extensively used in optical and electrical devices and also as film capacit-ois in microelectronic circuits. The latter applications, however, require amongsi. others high resistivity and dielectric field strength, low loss factors and TCC, and a near frequency invariant capacitance. Only a few oxide films, howevcT’, fulfil these requirements. A.C. properties of several films including some rare eai tli oxid(\s have recently b(H>n studied by us (Goswami & Goswami 1 9 7 3 ,  1974f«, 1074^).  The bulk antimony trioxide has been reported to  have high (dectrical resistivity as well as energy band ga]) (Korzh ct al 1909, Wood et al19 7 2 ) .  A detailed study has therefore been made on the dielectric behaviour of the cvaporattnl films. Studies on the optical properties of these films have already been reported by us (Goswami & Goswami 1974c).2 . E x p e r im e n t a lPure antiijiony trioxide powder (SboOg) white in colour (supplied by M/s Sujacheme Expojl, Moscow, USSR) was placed inside a conical silica boat which was heated by a tungsten filament and the powder was initially degassed in vacuo (10““*^ mm of Hg) for about half an hour at a low heat. The temperature of the*NCL ConiiTiumention No. 1796.**Prosent adclre.ss ; H. C. LiviBion, Electronic Corporation of India, Hyderabad-40,D ie le c tr ic  p r o p e rtie s  o f  a n t im o n y  tr io x id e  film s*318Antimony trioxide films 319boat was then raised and the deposition of the oxide films was made tlirou^h appropriate masks at room temperature on glass substrates in tlie manner des­cribed previously by us (Goswami Sc Goswami 1973, 19746). A set of samples of varying thickness were prepared in each evaporation and many such sots were made for measurements.Thick aluminium deposits were used both as base and as counter electrodes Ibr the fabrication of the capacitors comprising of AJ/Sh^Og/Al system. These were then aged at room temperature for several days and subjected to annealing by repealed heating (about 110°C) and cooling cycles in vacuo. This stabilisation Avas essential for reproducible measurements. Capacitance (G), loss factor (tan f^ ) were measured in vacuo at different frequencies (lO’-lO*^  Hz) and tempera­tures (78-380"K) using a Marconi universal bridge (model TF 2700) in the manner described previously by us (Goswami & Goswami 1973, 10746). The breakdown voltage (F/j) was also measured and the dielectric field strength (Fb) estimated. Multiple beam interferometry method was used for measuring the film thickness. Unless otherwise stated, the measurements wore generally carried out at room temperature at 1 kHz frequency. Dielectric constant (e) was evaluated from tile knowledge of capacitance, film thickness and the effective capacity area.l<"ig 1. Electron diffraction pattern of polycrystallino SbaOs film.In order to identify the nature of the dielectric films, the deposition was also made on polycrystallino sodium chloride tablets at room temperature. These were examined by the transmission electron diffraction method. Figme 1 shows it typical polycrystalline pattern of the deposits which conforms to the normal I'cc structure of the SbaO, films (a =  11.31).320 A* GoBwami and A m it P, Goswami3. R esults(i) Ageing and annealing effects :Al/SbgOs/Al films capacitors showed little ageing effect with time when kept in vacuo. But when subjected to cyclic heating and cooling in vaquo as men­tioned earlier capacitance decreased only slightly and attained stability within a short i>eriod. The loss factor was generally low (0.003) and did not significantly change on annealing.(ii) Effects of film thickness (d) :Variations of capacitance (as measured at room temperature) and dielectric constant with the film thickness are shown in figure 2, where G and e (log scale) are plotted against Ijd. I t is seen that although capacitance decreased with the increase of film thickness the proportionality law i.e., Col Ijd was valid for thicker films (say >  700 A) only and for thinner films this was not ti ue. ThisC A P A C IT A N C EFig. 2. Variation of C and c with 1/d.is more clearly seen from the dielectric constant graph where e increased from 5 to 8.2 for the film thicknesses ranging between 260-700 A and remained invariant for all thicker films. Tan 8 was low (<c 0.003) and remained indepen­dent of d. This behaviour was similar to those of zinc sulphide and praseo­dymium film capacitors reported earlier {loc. cit).(iii) Frequency and temperature effects on r and tan 8 :A typical variation of capacitance with the frequency ( f  =  o)l2n) for a film thickness about 1680 A at different temperatures is shown in figure 3. I t  is seen that capacitance decreased with the increase of the applied frequency and this variation was more at higher temperatures and lower frequencies. With the decrease of temperature C became reduced and eventually was independent ofAntimony trioxide films 321tho froquoucy near tlio liquid nitrogen temperature region. This irend was i)bsorV(?d for all film thicknesses (250-5000 A) studied by us. Tho curves show tho corresponding e at different temperatures and frequencies. I t is interesting to note that a t low temperatures e was about 8.05 for all frequencies compared to 8.2 at 1 kHz at room tomporature.Fjg. 3. VariftUon of C and c with frequency (/) at different temporaturos; d ^  168oA.0^Fig. 4. Variation of tan with frequency at different temperature; d 1580A.The influence of frequency and tompcratiu’e on tho loss factor is shown in figure 4. Tan d shows a minimum value as a function of frequency. This tan Smin occurred at a frequency fnun (or cj„iin) depending on tho temperature and other factors to be discussed later on. The position of fmtn a»nd hence tan dmin shifted9322 A. Goswami and Amit P. Goswamito the froquoncy region with the rise of temperature and vice versa» Both tan  ^and tan increased with the rise of temperature. The above features in the loss factor curves wore true for all thiokriosses of Sb20a film eapacitors.(iv) Breakdown voltage { V a n d  dielectric fi,dd strength {Fb) *.As expected the breakdown voltage increased from 2V to 9 V with the increase of film thickness from 200-3000 k  whereas the field strength decreased from 9.0 to 6.9X10® V/cm (table 1). When Fb was plotted against d (log-log scale) the graph (figure 5) showed two distinct linear regions. Thicker films (>  700 k)  showed a negative slope (0.5) thus obeying the Forlani-Minnaja law (Fb a  d*) whereas for the thinnei- films the curve was practically parallel to the d-axis.Tabic J. Variation of breakdown voltage (Fp) and dielectric field strength (Fb) witli film thicknessT h ick n ess (ri)AB r ea k d o w n  v o lta g e  (Vb) v o lt sD ie le c tr ic  field  s tr e n g th  {Fb) v o lt s /c m260 2 .5 9 .5 x 1 0 '520 5 .0 9 .6 X  1 0 '740 6.8 9 .4 x  10'850 7 .2 8.5X  10'1050 8.0 7 .5 x 1 0 '1300 9 .0 O-OXIO®Fif?. 6. Variation of Fb with thickness {d); slope =  - 0 .6.(v) TGC mid b.GIC :Two other parameters which are of great importance for practical applica- tious as capacitor films arc the temperature coefficient of capacitance (TCC) and the percentage variation of capacitance with frequency normalised a t anAntimony trioxide films 323arbitrary frequency (^(7/(7%). The variation of capacitance with temperature for different film thicknesses (900 to 3200 A) at 1 kHz is shown in figure 6. I t  ia seen that TOC is generally low (<  100 ppm/°K) for tliicker films and decreases further in the lower temperature region. Thinner films, however, have a slightly higher TOC (200-600 ppm/°K).Fip. 6 . Variation of TCC with d.The variation of capacitance at room temperature with the applied fie- (pioncy normalised at 1 kHz is shown in figure 7. I t is seen that below 1 kHz this is positive, and above this it is negative. Howt^ver, AC/C was small (<  0.8%) and more or less independent of film thickness.Fig. 7. Variation of AC/(7 with frequency.4. D is c u s s io nDielectric properties of thin films have been investigated by a number of workers, Recent studies on the a,c. behaviour of film capacitors at a w-ide range324 A. GoBwami and Amit P. Goswamiof frequoncicB and tomjxnatures including low tomporatures showed two charac­teristic foaturcH viz., (a) a loss peak in tan 6 vs. frequency curves at different tom7)cratuT’(^H and a thickness dependent dielectric constant (Nadkarni et al 1970, Goswami et al 19746) and (h) a pronounced loss minimum in tan d vs frequency (mi'vos at all temperatiires togothei- with a thickness independent dielectric constant except for very thin films (Goswami et al 1973, 1974a). Simmons et al (1970), assuming the presemee of two additional barrier* layers (Schottky barriers) at the two dielectric-electrodes interfaces, made predictions which agreed rasonably well with the exyrerirnental observations, such as loss peak, etc. in a-t}^pe of films. The dielt^etric beliaviour of the (b) type of film cajmeitors has recently been exfdained by us (Goswami H al 1973, 1974a) from a model comprising of a capacitor element (C) invariajit with frequency and temperature in parallel with a dielectric resistive element (R) and in series with a small lead resistaitco (r) where ?’ < <  ij?. The above model of ours predicts almost all th(  ^ basic char acteristics observed for zinc sulphide and ])raseodymium oxide film capacitors and the important conclusions derived from (un* theor y (Goswami Goswami 1973) arc summai’ised below for comparisons.(i) Loss factor (tan r^ ) at any frequency is related to G, and r by the eqiiatioristan -- (] loj)EC+oyr(J+(rloj)llKlSince r <  <  /»*,tan 6' ^  (] Ico)ECoyrCK  At low IWnpKUUiies the above reduces totan -- IjwECand at high frequenciestan =  corC.... (1 )... (2)... (3) ... (4)Thus the loss factor is directly or indii’cctly proportional to the frequency at flilTcrent frequency regions.(ii) TsindjnUi occin’ring at a particular frequency (oj f^iin) giv6n by the equation-  IKrECy. ... (5)Since E is related to the temperature by the equation E =r T^ ^^ exp ^EjkTy whereAE is the activation energy, E will decrease with the increase of temperature.Hence by tlu^  eq. (5) wjftin will shift to the higher frequency region.Antimony trioxide films 325(iii) Series capacitance {0$) »t any temperature and frequency is given by i'he equation+ ... (6)Noav at low temperatures and at high frequencies, H and ai^R^G- will be large and henceGs^--G. ... (7)Thus Gs be invariant with frequency.Comparing the a.(*. behaviour of SbgOg film capacitors it is seen tliat the s^ ai jation of capacitance’* with the frequency at different timiperatures is explained from eqs. (6) and (7). Eqs. (3), (4) and, (5) explain the different features of tan vs. frequency curve and also the significance of tan Smin- The shift in cojnin ^mtm with the iiicrease of the temperature is also explained. A1-high frequencies the rise of tan govonred by eq. (4) requires the lead resistance (r) to be independent of frequency and temperature. The estimated values of r fi'om tan ^ vs. frequency measurements at differnt temi)eratures have boon found to be very close to those obtained by actual measurementsBoth tan S and tan dmUi increase Avith the temperature due to the incieaso of (Muiductivdty of the dielectric layers at higher temperatures. TCC is, therefore, positm^. The low dissipation loss seems to be associated with the high dielectric i t^sistance (/?) of the deposited layers. The calculated value at room temi>eraturo obtained from eq. (3) and that estimated from wmin position from (^ q. (5) were found to be of the same order fl). The resistivity of SbgOrj films was,therefore, about ohm cm which was close to that of the bulk value(about 10^  ^ ohm cm) reported by Korzh et al  (19G9).The dependence of dielectric constant on d in the thin film region can be atti ibuted to the defects, chiefly voids, presoni- invariably in vapour phase deposits (loswami (Sr. GosAvami (1973). For thicker films these voids are effective^ filled up and hence e attained a constant value close to that of the bulk. I t  is also interesting to note that the Foriani-Minnaja relation holds good for thicker films only.Optical propoiioios of the Sb20a films have also been studied by us (Gowami c'l' Goswami (1974c). These films had high refractive indices (n pii 1.90 to 1.97) and practically .no absorption in the visible regions except for very thick films (;:r 10,000 A) at lower wavelengths (<  4000 A). The optical dielectric constant (-- iir 4) was nearly half the observed value (8.2) at 1 kHz at room tempera­ture (of. figure 2). Hence remaining half must have arisen from the contributions horn atomic (including ionic), dipole orientational or interfacial polarisations. As there was no peak in the tan S graph at low temperatures as well as at low fre­quencies, the contributions from the last two factors would hence bo absent.326 A. Goswami and Amit P. GoswamiSince the moasureraontH of 0  at liquid nitrogen tem perature region  would suppress the contribution of ionic polarisation to  e and also because there was no significant variation o f e measuroed at 1 kHz a t room tem perature from th e dielectric constant at low temperature, it m ay be concluded th a t th e rem aining h a lf w as th e con­tribution primarily from the atom ic polarisation.The above study of the a.c. behaviour o f  Sb^Oj films clearly shows th a t th e  m odel adopted by us for zinc sulphide and praseodym ium  oxide films is also  applicable in the present case. Further, because o f low  dissipation lossses ( <  0,003), low TCC ( < 1 0 0  ppm /°K ), high bi-eakdown field strength (7x10®  V/cm ) and praeticially fi-equoncy invariant capacitance, Sb203 films appear to  be good dielectric materials for several applications.R e v e b e n c e sPorlani V. & Mimiaja N. 1904 Phys. Slat. Solidi. 4, 311.Forlani F. & Mitinaja N. 1969 ./. rof. tin , Tachnol. 6, 518.Uoswami A. & Goswami Aniit I*. 1973 Thin Solid Films 16, 175.Goswami A. & Goswami Amit V. 1974a Thin Solid Films 20, 83.Goswami Amit P. & Goswami A. 19745 Indian J. Pure. Appl. Phys. 12, 26. Goswami A. & Goswami Amit P. 1974c Thm Solid Films 20, 833.Korzh P. D. &> Gulayova G. 1’. 1969 Fiz. tekh. Popuprov. 3, 782.Nttdkami G, 8 . &, Simmons J. G. 1970 -/. Appl. Phys. 41, 546.Simmons J. G., Narlkami G. S. Lanoasior M. C. 1970 J. Appl. Phys. 41, 538. Wood C., Van Polt B. k  Dwipht A. 1972 Phys. Slat. Solidi. b54, 701.

Dielectric Properties of Antimony Trioxide Films

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Dielectric Properties of Antimony Trioxide Films

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