- Email: [email protected]
On temperature coefficient of resistance of boron-doped SiGe films deposited by sputtering
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 10 (2007) 143–149
On temperature coefficient of resistance of boron-doped SiGe films deposited by sputtering Emil V. Jelenkovica,, Milan M. Jevticb, K.Y. Tonga, G.K.H. Pangc, W.Y. Cheungd, Shrawan K. Jhaa a
Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China b Institute of Physics, Beograd, 11080 Zemun, Pregrevica 118, Serbia c Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, PR China d Department of Electronic Engineering, The Chinese University of Hong Kong, PR China Available online 24 October 2007
Abstract Silicon–germanium films, doped with boron, were deposited on oxidised silicon substrates by RF magnetron sputtering. The post-deposition dopant activation and film crystallisation was done by annealing in the temperature range from 580 to 900 1C. The structural changes in the silicon–germanium films caused by the presence of boron and annealing were investigated by high-resolution transmission electron microscopy. The temperature coefficient of resistance (TCR) was characterised in the temperature range from room temperature to 210 1C and correlated to the nano-structure of the films. The TCR values were explained by the contribution of different scattering mechanisms and confirmed by low-frequency noise measurement. Very low values of TCR can be obtained by selecting appropriate boron content and post-deposition annealing conditions. r 2007 Elsevier Ltd. All rights reserved. Keywords: SiGe; Boron; Thermal coefficient of resistance; TCR; Sputtering; TEM; Low-frequency noise
1. Introduction Recently, polycrystalline silicon–germanium films have been investigated from different aspects in the field of microelectronics. Some of the examples are applications of polycrystalline SiGe films for source/ drain regions in thin-film transistors [1,2], in thermoelectric structures [3,4], as emitters in bipolar transistor [5] or in thin-film resistors [6,7]. The integration of passive components with active elements on the same chip is very important Corresponding author.
E-mail address: [email protected] (E.V. Jelenkovic). 1369-8001/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2007.08.001
in mixed signal circuits. In cases where temperature stability is requested, implementation of thin-film resistors with a low-temperature coefficient of resistance (TCR) is required. Different materials are used for the fabrication of such thin-film resistors: polycrystalline silicon [8], silicon–germanium [6,7] or ruthenium oxide [9]. Polycrystalline SiGe is especially attractive because it offers a low thermal budget. Usually, if polycrystalline semiconductor films are used, low TCR is achieved by heavy ion implantation into the polycrystalline film [6–8]. Alternatively, the TCR of doped poly-Si or polySiGe may be electrically trimmed, but this approach requires a particular circuit design [7]. Another
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approach is to create a double-layer resistor in which one layer has large grain size while the other has small grain size [10]. In the mentioned approaches, two processes are included, CVD (LPCVD or RTCVD) and ion implantation. Deposition of such films in a single step should be cost effective. Such an alternative process can be sputtering, which is at the same time an environmentally friendly process. The suitability of sputtered-doped poly-SiGe films was demonstrated in thermoelectric devices [3] and in the formation of p–n junctions [2,11]. However, sputtered-doped SiGe films were not studied from the TCR point of view. In this paper, techniques of controlling the TCR values by structural modification via boron incorporation in sputtered SiGe films are reported. 2. Experiment Thin SiGe films were deposited on oxidised silicon wafers by the RF sputtering system (ANELVA-model SPF-332H). The sputtering target was a boron-doped (5 mO cm) silicon slice. The composition of the films was controlled by placing germanium and boron pieces on the silicon target. The background pressure before sputtering was about 3 105 Pa, while the sputtering was performed in argon plasma at 0.3 Pa. The samples were not intentionally heated during the deposition. The samples were placed on the rotating holder at about 10 cm above the target. Dopant (boron) activation and film crystallisation were carried out in the openend quartz tube in nitrogen ambient, in the temperature range from 580 to 900 1C. The structural properties were analysed by crosssectional transmission microscopy (TEM). The samples were glued and mechanically grinded to 10 mm thickness from both sides. The samples were then ion milled using 4 kV Ar+ ions. TEM studies were carried out on a JEOL JEM-2010 microscope operated at 200 kV. The composition of the films was determined by Rutherford backscattering (RBS) using 2 MeV He+. The electrical properties and TCR of SiGe films were characterised in Kelvin resistor configuration. The resistors were patterned using the lift-off technique. For the purpose of TCR and noise measurement, they were bonded in 24 pin packages. The noise measurement was carried out at room temperature from 10 Hz to 10 kHz. The noise set-up was based on a low noise nanovolt preamplifier (Keithley 103A) and dynamic spectrum analyser (HP3562A).
3. Results 3.1. Material and structural properties of SiGe films The composition of studied films was determined by RBS. Since RBS could not detect boron, its presence was obtained after fitting to the experimental results. Two compositions, labelled in this work as middle boron (MB) and high boron (HB), were studied. The respective atomic ratios of Si:Ge:Ar:B are found to be 0.61:0.37:0.02:0.01 and 0.61:0.28:0.02:0.09. The obtained boron content corresponds to concentrations of about 5 1020 and 4 1021 cm3. The presence of argon is common in sputtered films, and its amount depends on the sputtering condition [12]. Also, co-sputtering of boron with Si and Ge increases the level of incorporated argon in the growing film, which can be degassed during high-temperature annealing processes [12]. The as-deposited films were amorphous as recorded by XRD and TEM, but results are not presented. This was observed in our other investigations about SiGe films [11]. During the annealing process, the SiGe layers start to crystallise. With the boron incorporation into SiGe film, a significant change in crystallisation happens, as illustrated in cross-sectional TEM images (Fig. 1). In Fig. 1, MB and HB samples were annealed in nitrogen at 9001C for 30 min. In samples with boron concentration of about 5 1020 cm3 in SiGe films, the grain size is in the range 12–15 nm, while with 4 1021 cm3 boron concentration the grain size in reduced to less than 7 nm. In order to compare the effect of grain structure on the TCR value, the average grain size for different MB and HB samples was estimated from bright-field images. The dependence of the average grain size on the annealing temperature is illustrated in Fig. 2. The grain size of MB samples annealed at 580 and 800 1C is similar, while with higher temperatures it grows. For HB samples the grain size remains about the same for the annealing temperature range. The results in Figs. 1 and 2 will be later qualitatively correlated to the electrical properties of the studied films. 3.2. Electrical properties and temperature coefficient of resistance The guideline for the selection of boron concentration and annealing parameters (time and temperature) was to obtain a film resistivity of
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16 14
MB SiGe HB SiGe
Grain size (nm)
12 10 8 6 4 2 0 550
600
650 700 750 800 850 Annealing temprature (°C)
900
950
Fig. 2. Dependence of average grain size of boron-doped SiGe films on annealing temperature. The grain size was obtained from bright-field TEM images.
Fig. 1. (a) Cross-sectional image of SiGe film with middle boron concentration (about 5 1020 cm3) annealed at 900 1C for 30 min. (b) Cross-sectional image of SiGe film with high boron concentration (about 4 1021 cm3) annealed at 900 1C for 30 min.
for flicker noise analysis. The given data of resistivity, mobility and carrier concentration of SiGe films were shown to be suitable for thermoelectric devices [3]. The TCR measurement was performed from room temperature up to 220 1C. The upper limit is selected in order to assess the applicability of the investigated films in high-temperature (about 200 1C) electronic devices. The extracted TCR values in relation to the annealing temperature of the films are given in Fig. 3 for MB and HB samples. The TCR data were normalised by the resistance at 0 1C. The MB samples show a tendency of linear increase in TCR values starting from 580 1C annealing temperature to 900 1C. This linearity should be taken with the background that the annealing time at 580 1C was 50 h, while for the others it was only 30 min. The values are within the range of 7200 ppm/1C, a desirable level for resistors with low TCR values [10]. With a suitable annealing temperature selection, the TCR of MB samples can be tuned almost to zero. For the same annealing temperatures, the HB samples show a negative value of 200740 ppm/1C. 3.3. Noise measurement
673 mO cm. This value was chosen for thin-film transistor applications [2]. Typical Hall mobility and dopant concentration are given in Table 1, together with the TCR values for two annealing temperatures (580 and 900 1C), which will be used
It is known that flicker noise of thin-film resistors can be sensitive to the film structure and scattering mechanisms [9]. Accordingly, we have measured the noise in the MB and HB samples for two boundary annealing temperatures of 580 and 900 1C.
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Table 1 Electrical properties of MB and HB thin films Sample
Mobility (cm2/Vs)
Dopant concentration (cm3)
TCR (ppm/1C), Tann ¼ 580 1C
MB HB
6.5 3
5 1020 4 1021
310 170
200 100
10-15
0 Si (f)/I2, (1/Hz)
TCR (pmm/ °C)
10-14
SiGe (B) MB HB
-100 -200 -300 -400 550
10-16
1/f
10-17
600
650 700 750 800 850 Annealing temperature (°C)
900
950
10-18 101
102 103 Frequency, (Hz)
Fig. 3. TCR dependence of boron-doped SiGe films on annealing temperature.
104
10-13
4. Discussion In Fig. 1, the interplanar spacing of the SiGe [1 1 1] plane is given for the grains close to the interface SiGe/SiO2. The error in interplanar spacing determination is estimated to be about
MB
C
The samples were formed as SiGe thin-film resistors with a length of 546 mm, a width of 21 mm, and a thickness of 110 nm (MB) and 85 nm (HB). The results for the normalised power noise densities, Si(f)/I2, in frequency range from 10 Hz to 10 kHz at room temperature, are shown in Fig. 4a. Flicker or 1/f noise, Si1/f(f) ¼ CI2/f, is extracted from the experimental results by a fitting procedure. The fitting method is based on the assumption that all noise sources in resistor are independent and thus the total power noise density is the sum of those for thermal, 1/f and g–r noise sources. The full lines in Fig. 4a represent the fitting results. The flicker noise constant C ¼ fSi1/f/I2 is shown in Fig. 4b for the MB and HB samples and two annealing temperatures. The change of C for the annealing temperatures from 580 to 900 1C is small in the case of HB samples, while it is of one order of magnitude in the case of MB samples. This is in correlation with the results for grain size and TCR (Figs. 2 and 3).
10-14 HB
10-15 550
600
650
700
750
800
850
900
950
Annealing temperature, ( °C)
Fig. 4. (a) Normalised power noise densities vs. frequency for the samples with MB and HB sputtered SiGe films (’—MB 580 1C/ 50 h, &—HB 580 1C/50 h, m—MB 900 1C/0.5 h, n—HB 900 1C/ 0.5 h, full lines—fitting curves). (b) Noise constant C at two annealing temperatures for MB and HB samples (the lines are for guiding the eyes).
70.002 nm. The measurement was calibrated against the interplanar spacing of the [1 1 1] plane of silicon substrate. However, the measurement across the thickness of the film showed that there is a spread in the lattice constant of the grains (not shown). This deviation of the lattice constant may suggest germanium (silicon) segregation or
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impact of boron on SiGe lattice constant. The reduction of lattice constant in heavily doped epitaxial [13] and polycrystalline SiGe [14] has been reported. The boron concentration at which this effect significantly happens is in the range above 1 1021 cm3 [13]. Under such doping levels boron can be placed in the substitutional, interstitial position or even form clusters [13–15]. However, based on our TEM analysis it is not possible for us to confirm this effect of boron on the lattice constant. Neither could clustering of boron [15] be confirmed, though it can be speculated that it could be one of the reasons that inhibit grain growth. The observed behaviour of TCR can be rather well correlated to the structural properties, illustrated in Figs. 1 and 2, by speculating the roles of different scattering mechanisms of electrical carriers in the films. There are two possible scattering mechanisms of carriers inside a grain: scattering by ionised impurities and phonons. It is well known that TCR is positive for phonon scattering, and negative for impurity scattering. Also, TCR is negative for the conduction process across grain boundaries [16]. In heavily doped (HB) SiGe alloys, scattering by ionised impurities prevails over phonon scattering [17]. Therefore, TCR for conduction processes inside a grain and across the grain boundaries are both negative, which agrees with the negative TCR observed at all annealing temperatures for HB samples. For MB films, at lower annealing temperatures with smaller grains, TCR is dominated by the conduction process across grain boundaries, thus resulting in negative TCR. As the grain grows at higher annealing temperatures, contribution of scattering within the grain starts to dominate. Also, a higher annealing temperature reduces defects inside a grain leading to further importance of phonon scattering in determining the mobility as compared with impurity scattering. In MB films, phonon scattering inside a grain is probably becoming the dominant mechanism when the annealing temperature rises, thus contributing to produce more positive values of TCR. While there is significant segregation at grain boundaries for phosphorus and arsenic, boron seldom segregates to grain boundaries [18]. Therefore, there is no need to include segregation of boron dopants in our discussion of resistivity results. This is supported by data about boron solubility in silicon–germanium alloys [19]. By the quenching technique, Slack and Hussain [19] reported incorporation of 2.5 1021 cm3 boron
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atoms in Si0.7Ge0.3 alloy. The same reference suggested that SiGe alloys could be doped by the combination of ion implantation and laser annealing up to 1 1022 cm3. Therefore, it is reasonable that our technique can prepare similar highly doped SiGe films without boron segregation at the grain boundaries. The variation of TCR with annealing temperature is a combination of the effects of annealing on the grain size and defect densities. Higher annealing temperature usually reduces defects caused by the dangling bonds and strained bonds. However, we do not think that we are able to derive theoretically in the present paper the measured linear variation of TCR with annealing temperature, since many parameters related to the defects (such as trap density and energy distribution) have not been collected experimentally. We will consider this in our future work. For the heavily doped samples studied (5 1020 and 4 1021 cm3) and for the temperature range measured (room temperature to 220 1C), the carrier concentration p remains unchanged, equal to the activated dopant concentration at all temperatures from basic semiconductor calculations. Also, there is no boron segregation at the grain boundaries, which would alter the carrier concentration in the grains. Since resistivity r ¼ 1/(qpm), the temperature dependence of the mobility m can be represented by that of the resistivity. The flicker noise constant C ¼ fSi1/f/I2 is shown in Fig. 4b for MB and HB samples and two annealing temperatures. The change of C for the annealing temperatures from 580 to 900 1C is small in the case of HB samples, while it is of one order of magnitude in the case of MB samples. This is in correlation with the results for grain size and TCR (Figs. 2 and 3). As we can see in Fig. 4b, the flicker noise constant C has values ranging from 6 1015 (HB) to 6 1014 (MB, 580 1C) that is in agreement with those in the literature [20]. Using Hooge’s empirical relation Si1/f/I2 ¼ a/fN [21], which assumes carrier mobility fluctuation as the source of flicker noise, we can express C as C ¼ a/N, where a is the noise parameter that can have values between 107 and 102 and N is the effective total number of carriers in the sample. Using a ¼ aHE2 103 (the most used value for metal films) [20] and our results for C we have found the effective number of carriers N and hence the effective concentration of carriers: NHB(580 1C)E3 1021 cm3, NHB(900 1C)E
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4 1021 cm3, NMB(580 1C)E2.7 1020 cm3 and NMB(900 1C)E3.3 1021 cm3. The values obtained for NHB in the HB samples are close to the estimated boron concentration (4 1021 cm3) under the condition that all boron atoms are ionised. This confirms that the HB samples have a behaviour similar to metal films with negative TCR. The value for NMB (900 1C) obtained using aE2 103 is approximately one order of magnitude larger than the estimated boron concentration (5 1020 cm3) in the MB samples. This means that a in the case of MB (9001C) must be lower than aH. In the frame of the mobility fluctuation model the noise parameter a can be expressed as a ¼ (m/mlatt)2aH , where m is the mobility due to all scattering mechanisms and mlatt is the mobility due to lattice scattering only. Thus, the lower a is possible if new scattering mechanisms or new 1/f noise sources are present in the MB samples annealed at 900 1C. On the other hand, the results of measurements of Hall hole mobility and concentration in samples annealed at 580 1C (MB: mH ¼ 6.5 cm2/Vs, pH ¼ 1 1020 cm3; HB: mH ¼ 3 cm2/Vs, pH ¼ 3.9 1020 cm3) show that the Hall mobility has values lower than that for lattice scattering in polycrystalline SiGe (mlatt ¼ 260 cm2/Vs [22]) and that all boron atoms are not ionised. Using these results for hole concentrations and the values of C for samples annealed at 580 1C, we have obtained the noise parameter aMB(HB) values of 1.25 106 (MB) and 2.8 107 (HB). Different values of a in MB and HB samples are a consequence of different microstructure properties and scattering conditions in these samples. In this way, the noise results confirm the assumptions relating to the role of different scattering mechanisms involved in the explanation of behaviour of TCR in MB and HB films. 5. Conclusion The relation between the temperature coefficient of resistance and structural properties of borondoped SiGe films deposited by sputtering was investigated. The structure of the films and their TCR values were affected by both boron concentration and post-deposition annealing. The films with 5 1020 and 4 1021 cm3 boron concentration show potential for application in resistors with low TCR value. These films have nano-size grains, from about 2 to 15 nm. The grain size, dopant activation, carrier transport mechanisms and lowfrequency noise were qualitatively correlated to explain the TCR behaviour. It was shown that an
appropriate boron concentration selection together with post-deposition annealing can give a SiGe film with a TCR value close to zero, in the operation temperature from room temperature to 220 1C. Acknowledgement We gratefully acknowledge the contribution and collaboration of the late Prof. S.P. Wong.
References [1] Peng DZ, Chang TC, Liu CF, Yeh PH, Liu PT, Chang CY. A novel SiGe raised souce/drain polycrystalline silicon thinfilm transistor with improved on-current and larger breakdown voltage. Jpn J Appl Phys 2003;42(3):1164–7. [2] Tong KY, Jelenkovic EV, Liu W, Wang SG. Polysilicon thin film transistors using sputtered HfO2 gate dielectric and SiGe source/drain. Semicond Sci Technol 2007;22:574–6. [3] Tajima K, Shin W, Murayama N, Itoh T, Izu N, Matusubara I. Preparation of phosphorous-doped Si0.8Ge0.2 thermoelectric thin film using RF sputtering with induction coil. J Ceramic Soc Jpn 2005;113(8):558–61. [4] Wijngaards DDL, Wolffenbuttel RF. Thermo-electric characterization of APCVD polySi0.7Ge0.3 for IC-compatible fabrication of integrated lateral Peltier elements. IEEE Trans Electron Devices 2005;52(5):1014–25. [5] Kunz VD, de Groot CH, Hall S, Ashburn P. Polycrystalline silicon–germanium emitters for gain control with application to SiGe HBTs. IEEE Trans Electron Devices 2003;50(6): 1480–6. [6] Chen KM, Huang GW, Chiu DY, Huang HJ, Chang CY. Analysis of low-frequency noise in boron-doped polycrystalline silicon–germanium resistors. Appl Phys Lett 2002; 81(14):2578–80. [7] Babcoc JA, Francis P, Bashir R, Kabir AE, Schroder DK, Klee MSL, et al. Precision electrical trimming of very low TCR poly-SiGe resistors. IEEE Trans Electron Devices 2000;21(60):283–5. [8] Kato K, Ono T. Change in temperature coefficient of resistance of heavily doped polysilicon resistors caused by electrical trimming. Jpn J Appl Phys 1996;35(8): 4209–15. [9] Jevtic MM, Jelenkovic EV, Tong KY, Pang GKH. Noise and structural properties of reactively sputtered RuO2 thin films. Thin Solid Films 2006;496:214–20. [10] Shimamoto H, Ohnishi K, Shiba T, Hashimoto T, Kikuchi T, Tamaki Y. Proposal and experimental study of a highprecision polycrystalline-silicon film resistor with a quasidouble-layer structure. Electron Commun Jpn 2004; 87(7):9–16. [11] Jelenkovic EV, Tong KY, Cheung WY, Wong SP, Shi BR, Pang GKH. SiGe–Si junctions with boron-doped SiGe films deposited by co-sputtering. Solid State Electron 2006; 50:199–204. [12] Sun Z, Tong KY, Lee WB. Properties of furnace crystallized polysilicon films prepared by r.f. sputtering. Thin Solid Films 1996;228(1–2):224–8.
ARTICLE IN PRESS E.V. Jelenkovic et al. / Materials Science in Semiconductor Processing 10 (2007) 143–149 [13] Moriya A, Sakubara M, Matsuura T, Murota J. Doping and electrical characteristics of in situ heavily B-doped Si1xGex films epitaxially grown using ultra clean LPCVD. Thin Solid Films 1999;343–344:541–4. [14] Hirai M, Iwashita R, Arikado T, Suematsu H, Jiang W, Yatsui K, et al. Room temperature preparation of activated and crystallized p-type Si1xGex thin film on glass substrate by intense, pulsed, ion beam evaporation. J Vac Sci Technol A 2004;22(6):2398–401. [15] Mizushima I, Watanabe M, Murakohi A, Hotta M, Kashiwagi M, Yoshiki M. Hole generation by icosahedral B12 in high-dose boron as-implanted silicon. Appl Phys Lett 1993;63(3):373–5. [16] Tong KY, Jelenkovic EV, Cheung WY, Wong SP. Temperature dependence of resistance in reactively sputtered RuO2 thin films. J Mater Sci Lett 2001;20:699–700. [17] Yonenaga I, Li WJ, Akashi T, Ayuzawa T, Goto T. Temperature dependence of electron and hole mobility in
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heavily impurity-doped SiGe single crystals. J Appl Phys 2005;98. 063702-1–063702-4. Mandurah MM, Saraswat KC, Helms CR, Kamins TI. Dopant segregation in polycrystalline silicon. J Appl Phys 1981;51(11):5755–63. Slack GA, Hussain MA. The maximum possible conversion of silicon–germanium thermoelectric generators. J Appl Phys 1991;70(5):2694–718. Chen KM, Huang GW, Chin DY, Huang HJ, Chang CY. Analysis of low-frequency noise in boron-doped polycrystalline silicon–germanium resistors. Appl Phys Lett 2002; 81(14):2578–80. Hooge FN, Kleinpenning TGM, Vandamme LKJ. Experimental studies on 1/f noise. Rep Prog Phys 1981;44: 479–532. Chen XY, Salm C, Hooge FN, Woerlee PH. 1/f noise in polycrystalline SiGe analyzed in terms of mobility fluctuations. Solid-State Electron 1999;43:1715–24.
Materials Science in Semiconductor Processing 10 (2007) 143–149
On temperature coefficient of resistance of boron-doped SiGe films deposited by sputtering Emil V. Jelenkovica,, Milan M. Jevticb, K.Y. Tonga, G.K.H. Pangc, W.Y. Cheungd, Shrawan K. Jhaa a
Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China b Institute of Physics, Beograd, 11080 Zemun, Pregrevica 118, Serbia c Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, PR China d Department of Electronic Engineering, The Chinese University of Hong Kong, PR China Available online 24 October 2007
Abstract Silicon–germanium films, doped with boron, were deposited on oxidised silicon substrates by RF magnetron sputtering. The post-deposition dopant activation and film crystallisation was done by annealing in the temperature range from 580 to 900 1C. The structural changes in the silicon–germanium films caused by the presence of boron and annealing were investigated by high-resolution transmission electron microscopy. The temperature coefficient of resistance (TCR) was characterised in the temperature range from room temperature to 210 1C and correlated to the nano-structure of the films. The TCR values were explained by the contribution of different scattering mechanisms and confirmed by low-frequency noise measurement. Very low values of TCR can be obtained by selecting appropriate boron content and post-deposition annealing conditions. r 2007 Elsevier Ltd. All rights reserved. Keywords: SiGe; Boron; Thermal coefficient of resistance; TCR; Sputtering; TEM; Low-frequency noise
1. Introduction Recently, polycrystalline silicon–germanium films have been investigated from different aspects in the field of microelectronics. Some of the examples are applications of polycrystalline SiGe films for source/ drain regions in thin-film transistors [1,2], in thermoelectric structures [3,4], as emitters in bipolar transistor [5] or in thin-film resistors [6,7]. The integration of passive components with active elements on the same chip is very important Corresponding author.
E-mail address: [email protected] (E.V. Jelenkovic). 1369-8001/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2007.08.001
in mixed signal circuits. In cases where temperature stability is requested, implementation of thin-film resistors with a low-temperature coefficient of resistance (TCR) is required. Different materials are used for the fabrication of such thin-film resistors: polycrystalline silicon [8], silicon–germanium [6,7] or ruthenium oxide [9]. Polycrystalline SiGe is especially attractive because it offers a low thermal budget. Usually, if polycrystalline semiconductor films are used, low TCR is achieved by heavy ion implantation into the polycrystalline film [6–8]. Alternatively, the TCR of doped poly-Si or polySiGe may be electrically trimmed, but this approach requires a particular circuit design [7]. Another
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approach is to create a double-layer resistor in which one layer has large grain size while the other has small grain size [10]. In the mentioned approaches, two processes are included, CVD (LPCVD or RTCVD) and ion implantation. Deposition of such films in a single step should be cost effective. Such an alternative process can be sputtering, which is at the same time an environmentally friendly process. The suitability of sputtered-doped poly-SiGe films was demonstrated in thermoelectric devices [3] and in the formation of p–n junctions [2,11]. However, sputtered-doped SiGe films were not studied from the TCR point of view. In this paper, techniques of controlling the TCR values by structural modification via boron incorporation in sputtered SiGe films are reported. 2. Experiment Thin SiGe films were deposited on oxidised silicon wafers by the RF sputtering system (ANELVA-model SPF-332H). The sputtering target was a boron-doped (5 mO cm) silicon slice. The composition of the films was controlled by placing germanium and boron pieces on the silicon target. The background pressure before sputtering was about 3 105 Pa, while the sputtering was performed in argon plasma at 0.3 Pa. The samples were not intentionally heated during the deposition. The samples were placed on the rotating holder at about 10 cm above the target. Dopant (boron) activation and film crystallisation were carried out in the openend quartz tube in nitrogen ambient, in the temperature range from 580 to 900 1C. The structural properties were analysed by crosssectional transmission microscopy (TEM). The samples were glued and mechanically grinded to 10 mm thickness from both sides. The samples were then ion milled using 4 kV Ar+ ions. TEM studies were carried out on a JEOL JEM-2010 microscope operated at 200 kV. The composition of the films was determined by Rutherford backscattering (RBS) using 2 MeV He+. The electrical properties and TCR of SiGe films were characterised in Kelvin resistor configuration. The resistors were patterned using the lift-off technique. For the purpose of TCR and noise measurement, they were bonded in 24 pin packages. The noise measurement was carried out at room temperature from 10 Hz to 10 kHz. The noise set-up was based on a low noise nanovolt preamplifier (Keithley 103A) and dynamic spectrum analyser (HP3562A).
3. Results 3.1. Material and structural properties of SiGe films The composition of studied films was determined by RBS. Since RBS could not detect boron, its presence was obtained after fitting to the experimental results. Two compositions, labelled in this work as middle boron (MB) and high boron (HB), were studied. The respective atomic ratios of Si:Ge:Ar:B are found to be 0.61:0.37:0.02:0.01 and 0.61:0.28:0.02:0.09. The obtained boron content corresponds to concentrations of about 5 1020 and 4 1021 cm3. The presence of argon is common in sputtered films, and its amount depends on the sputtering condition [12]. Also, co-sputtering of boron with Si and Ge increases the level of incorporated argon in the growing film, which can be degassed during high-temperature annealing processes [12]. The as-deposited films were amorphous as recorded by XRD and TEM, but results are not presented. This was observed in our other investigations about SiGe films [11]. During the annealing process, the SiGe layers start to crystallise. With the boron incorporation into SiGe film, a significant change in crystallisation happens, as illustrated in cross-sectional TEM images (Fig. 1). In Fig. 1, MB and HB samples were annealed in nitrogen at 9001C for 30 min. In samples with boron concentration of about 5 1020 cm3 in SiGe films, the grain size is in the range 12–15 nm, while with 4 1021 cm3 boron concentration the grain size in reduced to less than 7 nm. In order to compare the effect of grain structure on the TCR value, the average grain size for different MB and HB samples was estimated from bright-field images. The dependence of the average grain size on the annealing temperature is illustrated in Fig. 2. The grain size of MB samples annealed at 580 and 800 1C is similar, while with higher temperatures it grows. For HB samples the grain size remains about the same for the annealing temperature range. The results in Figs. 1 and 2 will be later qualitatively correlated to the electrical properties of the studied films. 3.2. Electrical properties and temperature coefficient of resistance The guideline for the selection of boron concentration and annealing parameters (time and temperature) was to obtain a film resistivity of
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16 14
MB SiGe HB SiGe
Grain size (nm)
12 10 8 6 4 2 0 550
600
650 700 750 800 850 Annealing temprature (°C)
900
950
Fig. 2. Dependence of average grain size of boron-doped SiGe films on annealing temperature. The grain size was obtained from bright-field TEM images.
Fig. 1. (a) Cross-sectional image of SiGe film with middle boron concentration (about 5 1020 cm3) annealed at 900 1C for 30 min. (b) Cross-sectional image of SiGe film with high boron concentration (about 4 1021 cm3) annealed at 900 1C for 30 min.
for flicker noise analysis. The given data of resistivity, mobility and carrier concentration of SiGe films were shown to be suitable for thermoelectric devices [3]. The TCR measurement was performed from room temperature up to 220 1C. The upper limit is selected in order to assess the applicability of the investigated films in high-temperature (about 200 1C) electronic devices. The extracted TCR values in relation to the annealing temperature of the films are given in Fig. 3 for MB and HB samples. The TCR data were normalised by the resistance at 0 1C. The MB samples show a tendency of linear increase in TCR values starting from 580 1C annealing temperature to 900 1C. This linearity should be taken with the background that the annealing time at 580 1C was 50 h, while for the others it was only 30 min. The values are within the range of 7200 ppm/1C, a desirable level for resistors with low TCR values [10]. With a suitable annealing temperature selection, the TCR of MB samples can be tuned almost to zero. For the same annealing temperatures, the HB samples show a negative value of 200740 ppm/1C. 3.3. Noise measurement
673 mO cm. This value was chosen for thin-film transistor applications [2]. Typical Hall mobility and dopant concentration are given in Table 1, together with the TCR values for two annealing temperatures (580 and 900 1C), which will be used
It is known that flicker noise of thin-film resistors can be sensitive to the film structure and scattering mechanisms [9]. Accordingly, we have measured the noise in the MB and HB samples for two boundary annealing temperatures of 580 and 900 1C.
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Table 1 Electrical properties of MB and HB thin films Sample
Mobility (cm2/Vs)
Dopant concentration (cm3)
TCR (ppm/1C), Tann ¼ 580 1C
MB HB
6.5 3
5 1020 4 1021
310 170
200 100
10-15
0 Si (f)/I2, (1/Hz)
TCR (pmm/ °C)
10-14
SiGe (B) MB HB
-100 -200 -300 -400 550
10-16
1/f
10-17
600
650 700 750 800 850 Annealing temperature (°C)
900
950
10-18 101
102 103 Frequency, (Hz)
Fig. 3. TCR dependence of boron-doped SiGe films on annealing temperature.
104
10-13
4. Discussion In Fig. 1, the interplanar spacing of the SiGe [1 1 1] plane is given for the grains close to the interface SiGe/SiO2. The error in interplanar spacing determination is estimated to be about
MB
C
The samples were formed as SiGe thin-film resistors with a length of 546 mm, a width of 21 mm, and a thickness of 110 nm (MB) and 85 nm (HB). The results for the normalised power noise densities, Si(f)/I2, in frequency range from 10 Hz to 10 kHz at room temperature, are shown in Fig. 4a. Flicker or 1/f noise, Si1/f(f) ¼ CI2/f, is extracted from the experimental results by a fitting procedure. The fitting method is based on the assumption that all noise sources in resistor are independent and thus the total power noise density is the sum of those for thermal, 1/f and g–r noise sources. The full lines in Fig. 4a represent the fitting results. The flicker noise constant C ¼ fSi1/f/I2 is shown in Fig. 4b for the MB and HB samples and two annealing temperatures. The change of C for the annealing temperatures from 580 to 900 1C is small in the case of HB samples, while it is of one order of magnitude in the case of MB samples. This is in correlation with the results for grain size and TCR (Figs. 2 and 3).
10-14 HB
10-15 550
600
650
700
750
800
850
900
950
Annealing temperature, ( °C)
Fig. 4. (a) Normalised power noise densities vs. frequency for the samples with MB and HB sputtered SiGe films (’—MB 580 1C/ 50 h, &—HB 580 1C/50 h, m—MB 900 1C/0.5 h, n—HB 900 1C/ 0.5 h, full lines—fitting curves). (b) Noise constant C at two annealing temperatures for MB and HB samples (the lines are for guiding the eyes).
70.002 nm. The measurement was calibrated against the interplanar spacing of the [1 1 1] plane of silicon substrate. However, the measurement across the thickness of the film showed that there is a spread in the lattice constant of the grains (not shown). This deviation of the lattice constant may suggest germanium (silicon) segregation or
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impact of boron on SiGe lattice constant. The reduction of lattice constant in heavily doped epitaxial [13] and polycrystalline SiGe [14] has been reported. The boron concentration at which this effect significantly happens is in the range above 1 1021 cm3 [13]. Under such doping levels boron can be placed in the substitutional, interstitial position or even form clusters [13–15]. However, based on our TEM analysis it is not possible for us to confirm this effect of boron on the lattice constant. Neither could clustering of boron [15] be confirmed, though it can be speculated that it could be one of the reasons that inhibit grain growth. The observed behaviour of TCR can be rather well correlated to the structural properties, illustrated in Figs. 1 and 2, by speculating the roles of different scattering mechanisms of electrical carriers in the films. There are two possible scattering mechanisms of carriers inside a grain: scattering by ionised impurities and phonons. It is well known that TCR is positive for phonon scattering, and negative for impurity scattering. Also, TCR is negative for the conduction process across grain boundaries [16]. In heavily doped (HB) SiGe alloys, scattering by ionised impurities prevails over phonon scattering [17]. Therefore, TCR for conduction processes inside a grain and across the grain boundaries are both negative, which agrees with the negative TCR observed at all annealing temperatures for HB samples. For MB films, at lower annealing temperatures with smaller grains, TCR is dominated by the conduction process across grain boundaries, thus resulting in negative TCR. As the grain grows at higher annealing temperatures, contribution of scattering within the grain starts to dominate. Also, a higher annealing temperature reduces defects inside a grain leading to further importance of phonon scattering in determining the mobility as compared with impurity scattering. In MB films, phonon scattering inside a grain is probably becoming the dominant mechanism when the annealing temperature rises, thus contributing to produce more positive values of TCR. While there is significant segregation at grain boundaries for phosphorus and arsenic, boron seldom segregates to grain boundaries [18]. Therefore, there is no need to include segregation of boron dopants in our discussion of resistivity results. This is supported by data about boron solubility in silicon–germanium alloys [19]. By the quenching technique, Slack and Hussain [19] reported incorporation of 2.5 1021 cm3 boron
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atoms in Si0.7Ge0.3 alloy. The same reference suggested that SiGe alloys could be doped by the combination of ion implantation and laser annealing up to 1 1022 cm3. Therefore, it is reasonable that our technique can prepare similar highly doped SiGe films without boron segregation at the grain boundaries. The variation of TCR with annealing temperature is a combination of the effects of annealing on the grain size and defect densities. Higher annealing temperature usually reduces defects caused by the dangling bonds and strained bonds. However, we do not think that we are able to derive theoretically in the present paper the measured linear variation of TCR with annealing temperature, since many parameters related to the defects (such as trap density and energy distribution) have not been collected experimentally. We will consider this in our future work. For the heavily doped samples studied (5 1020 and 4 1021 cm3) and for the temperature range measured (room temperature to 220 1C), the carrier concentration p remains unchanged, equal to the activated dopant concentration at all temperatures from basic semiconductor calculations. Also, there is no boron segregation at the grain boundaries, which would alter the carrier concentration in the grains. Since resistivity r ¼ 1/(qpm), the temperature dependence of the mobility m can be represented by that of the resistivity. The flicker noise constant C ¼ fSi1/f/I2 is shown in Fig. 4b for MB and HB samples and two annealing temperatures. The change of C for the annealing temperatures from 580 to 900 1C is small in the case of HB samples, while it is of one order of magnitude in the case of MB samples. This is in correlation with the results for grain size and TCR (Figs. 2 and 3). As we can see in Fig. 4b, the flicker noise constant C has values ranging from 6 1015 (HB) to 6 1014 (MB, 580 1C) that is in agreement with those in the literature [20]. Using Hooge’s empirical relation Si1/f/I2 ¼ a/fN [21], which assumes carrier mobility fluctuation as the source of flicker noise, we can express C as C ¼ a/N, where a is the noise parameter that can have values between 107 and 102 and N is the effective total number of carriers in the sample. Using a ¼ aHE2 103 (the most used value for metal films) [20] and our results for C we have found the effective number of carriers N and hence the effective concentration of carriers: NHB(580 1C)E3 1021 cm3, NHB(900 1C)E
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4 1021 cm3, NMB(580 1C)E2.7 1020 cm3 and NMB(900 1C)E3.3 1021 cm3. The values obtained for NHB in the HB samples are close to the estimated boron concentration (4 1021 cm3) under the condition that all boron atoms are ionised. This confirms that the HB samples have a behaviour similar to metal films with negative TCR. The value for NMB (900 1C) obtained using aE2 103 is approximately one order of magnitude larger than the estimated boron concentration (5 1020 cm3) in the MB samples. This means that a in the case of MB (9001C) must be lower than aH. In the frame of the mobility fluctuation model the noise parameter a can be expressed as a ¼ (m/mlatt)2aH , where m is the mobility due to all scattering mechanisms and mlatt is the mobility due to lattice scattering only. Thus, the lower a is possible if new scattering mechanisms or new 1/f noise sources are present in the MB samples annealed at 900 1C. On the other hand, the results of measurements of Hall hole mobility and concentration in samples annealed at 580 1C (MB: mH ¼ 6.5 cm2/Vs, pH ¼ 1 1020 cm3; HB: mH ¼ 3 cm2/Vs, pH ¼ 3.9 1020 cm3) show that the Hall mobility has values lower than that for lattice scattering in polycrystalline SiGe (mlatt ¼ 260 cm2/Vs [22]) and that all boron atoms are not ionised. Using these results for hole concentrations and the values of C for samples annealed at 580 1C, we have obtained the noise parameter aMB(HB) values of 1.25 106 (MB) and 2.8 107 (HB). Different values of a in MB and HB samples are a consequence of different microstructure properties and scattering conditions in these samples. In this way, the noise results confirm the assumptions relating to the role of different scattering mechanisms involved in the explanation of behaviour of TCR in MB and HB films. 5. Conclusion The relation between the temperature coefficient of resistance and structural properties of borondoped SiGe films deposited by sputtering was investigated. The structure of the films and their TCR values were affected by both boron concentration and post-deposition annealing. The films with 5 1020 and 4 1021 cm3 boron concentration show potential for application in resistors with low TCR value. These films have nano-size grains, from about 2 to 15 nm. The grain size, dopant activation, carrier transport mechanisms and lowfrequency noise were qualitatively correlated to explain the TCR behaviour. It was shown that an
appropriate boron concentration selection together with post-deposition annealing can give a SiGe film with a TCR value close to zero, in the operation temperature from room temperature to 220 1C. Acknowledgement We gratefully acknowledge the contribution and collaboration of the late Prof. S.P. Wong.
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