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Tungsten silicide contacts to polycrystalline silicon and silicon–germanium alloys
Materials Science and Engineering B 114–115 (2004) 223–227
Tungsten silicide contacts to polycrystalline silicon and silicon–germanium alloys G. Srinivasan∗ , M.F. Bain, S. Bhattacharyya, P. Baine, B.M. Armstrong, H.S. Gamble, D.W. McNeill Northern Ireland Semiconductor Research Centre, School of Electrical and Electronic Engineering, Queen’s University, Ashby Building, Stranmillis Road, Belfast BT9 5AH, Ireland, UK
Abstract Silicon–germanium alloy layers will be employed in the source–drain engineering of future MOS transistors. The use of this technology offers advantages in reducing series resistance and decreasing junction depth resulting in reduction in punch-through and SCE problems. The contact resistance of metal or metal silicides to the raised source–drain material is a serious issue at sub-micron dimensions and must be minimised. In this work, tungsten silicide produced by chemical vapour deposition has been investigated as a contact metallization scheme to both boron and phosphorus doped polycrystalline Si1−x Gex , with 0 ≤ x ≤ 0.3. Cross bridge Kelvin resistor (CKBR) structures were fabricated incorporating CVD WSi2 and polycrystalline SiGe. Tungsten silicide contacts to control polysilicon CKBR structures have been shown to be of high quality with specific contact resistance ρc values 3 × 10−7 ohm cm2 and 6 × 10−7 ohm cm2 obtained to boron and phosphorus implanted samples respectively. The SiGe CKBR structures show that the inclusion of Ge yields a reduction in ρc for both dopant types. The boron doped SiGe exhibits a reduction in ρc from 3 × 10−7 to 5 × 10−8 ohm cm2 as Ge fraction is increased from 0 to 0.3. The reduction in ρc has been shown to be due to (i) the lowering of the tungsten silicide Schottky barrier height to p-type SiGe resulting from the energy band gap reduction, and (ii) increased activation of the implanted boron with increased Ge fraction. The phosphorus implanted samples show less sensitivity of ρc to Ge fraction with a lowest value in this work of 3 × 10−7 ohm cm2 for a Ge fraction of 0.3. The reduction in specific contact resistance to the phosphorus implanted samples has been shown to be due to increased dopant activation alone. © 2004 Elsevier B.V. All rights reserved. Keywords: Tungsten silicide; Polycrystalline; SiGe alloy; Contact resistance
1. Introduction Future MOS transistors will employ novel device architectures. At present a key issue is the source–drain engineering of MOSFETs which requires critical control over the junction depth. Raised source–drain (or recess-gate) MOSFETs offer advantages in decreasing junction depth resulting in reduction in punch-through and SCE problems [1]. Polycrystalline silicon or silicon–germanium can be deposited for the raised source–drain. SiGe is preferred due to its lower resistance and its favourable electronic energy band structure. At deep submicron dimensions, the contact resistance to the source and drain regions can become very large and must be minimised. ∗
Corresponding author. E-mail address: [email protected] (G. Srinivasan).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.07.019
The contact resistance of any material is strongly related to the Schottky barrier height of the metal to silicon or in this case SiGe. In this particular study, tungsten silicide (WSix ) contacts have been employed as a metallic layer with a work function greater 4.6 eV, highly favourable for the formation of a Schottky barrier height φb of value matching near midband gap of the silicon. The tungsten silicide contact was deposited directly by LPCVD. The main emphasis of the paper relates to the SiGe alloy. The energy band structure is strongly related to the germanium content and the purpose of this paper is to experimentally determine the influence of germanium content [2] on the resultant specific contact resistance. A UHV compatible cluster tool has been employed for the LPCVD of the polycrystalline SiGe layers. The reliable measurement of specific contact resistance is difficult and requires a custom designed test structure. In this work, a
224
G. Srinivasan et al. / Materials Science and Engineering B 114–115 (2004) 223–227
cross bridge Kelvin resistor structure has been employed. Minor misalignment of the contact windows in this structure can lead to inaccurate extraction of the specific contact resistance ρc . Thus, considerable care has been taken to ensure reliable results. SEM microscopy has been employed to measure any misalignment and the one-dimensional model employed by Ono [3] has been used in the extraction of ρc.
2. Experimental The raised source and drain regions of modern MOSFETs employ heavily doped polycrystalline SiGe. In order to measure the specific contact resistance to these layers, cross bridge Kelvin resistors have been designed. The structure is shown in Fig. 1. It consists of one L-shaped region formed in the doped polycrystalline SiGe to create two arms of the bridge resistor. There is a centrally located square contact which yields the contact resistance measurements, and the overlying metal forms the other two arms of the bridge structure. Current is injected via two opposite arms of the resistor and induced voltage is measured across the other two. This is shown in Fig. 1 along with critical dimensions. The contact window size employed was varied from 4 to 12 m. The width of the resistor arms was also varied from 6 to 40 m to yield data for graphical presentation which aids in the analysis of results. A simple one-dimensional transmission line model was used to extract specific contact resistance from the measured Kelvin resistance data [3]. The equation used in this extraction is given below. √ √ ρc + ρc ρs coth (L ρs /ρc )L1 + L21 ρs /2 √ √ +L2 ρc ρs / sinh (L ρs /ρc ) RK = (1) (L1 + L2 + L)W The terms ρc and ρs are the specific contact resistance of the metal silicide and the sheet resistance of the polycrystalline SiGe, L1 and L2 the alignment margins of the contact window, L and W the dimensions of the contact window and in this case are equal.
Approximately, 250 nm of polycrystalline SiGe was deposited on oxidised silicon substrates. The layers were deposited in an LPCVD chamber which forms part of a UHV compatible cluster tool. The deposition temperature was 800 ◦ C. Silane gas was employed at a flow rate of 100 sccm in all depositions and germane gas was added with values ranging from 0 to 15 sccm to yield a germanium content in the resultant films from 0 to 30%. Each film had a uniform germanium content which was verified using Raman spectroscopy. The polycrystalline SiGe layer was coated with a thin layer of LPCVD oxide and then ion implanted with either boron (energy = 30 keV and dose = 1×1016 cm−2 ) or with phosphorus (energy = 30 keV and dose = 1 × 1016 cm−2 ). The oxide thickness for the boron and phosphorus implant samples were approximately 45 and 30 nm, respectively. The polycrystalline SiGe was patterned by RIE to provide the first L-shaped arms of the resistor structure. A layer of silicon dioxide was deposited at 720 ◦ C using a TEOS precursor to provide electrical isolation. A deposition time of 75 min was used to yield a thickness of 500 nm. Rapid thermal annealing at 900 ◦ C for 1 min was employed to densify the oxide and activate the implanted dopants. It is important to note that with grain boundary diffusion this thermal treatment is sufficient to redistribute the boron and phosphorus implant profiles yielding a uniform concentration throughout the polycrystalline SiGe layer. Following contact window formation, the exposed SiGe was hydrogen passivated and the tungsten disilicide was deposited by LPCVD. This was done in a Varian 4101 cold walled single wafer processing system [4]. The deposition temperature was 370 ◦ C and the reactant species were Ar (550 sccm), WF6 (3 sccm) and SiH4 (200 sccm). The process pressure was 300 mTorr yielding a deposition rate of 0.9 nm s−1 . The process was controlled to ensure the silicide layer was silicon rich yielding WSi2.6 . This composition of layer is more stable during subsequent processing. The silicide was annealed at 700 ◦ C for 1 h to improve the grain structure and yield a sheet resistance of 4 ohm square−1 . The final metallisation layer was sputter deposited aluminium. The aluminium and underlying tungsten silicide were patterned using wet etching and RIE
Fig. 1. Schematic of the cross bridge Kelvin resistor test structure.
G. Srinivasan et al. / Materials Science and Engineering B 114–115 (2004) 223–227
Fig. 2. Measured and calculated values for Kelvin resistance for several CBKR test structures manufactured in polycrystalline silicon.
to yield the second L-shaped arms of the resistor. A final anneal was carried out in forming gas at 450 ◦ C for 30 min.
3. Results The resistance of various cross bridge structures was measured covering the full range of contact window sizes and alignment margins. Fig. 2 shows the measured Kelvin resistance data for the WSix contact scheme applied to boron implanted polysilicon (0% Ge). The solid lines represent the experimentally measured values. The Kelvin resistance decreases as expected with increasing contact area but increases with increasing semiconductor arm width D for all contact window sizes. This is due to the increased contribution to contact resistance of the alignment margin L1 . The dashed lines are the theoretical fit to this data using Eq. (1) with a chosen, but fixed value of ρc . The dimensions were measured using SEM and Van der Pauw structures yielded the sheet resistance of the polysilicon layer. Best-fit to all the data was then achieved with and a value for specific contact resistance ρc of 3 × 10−7 ohm cm2 . The measurement technique and the fitting equation have therefore proved sound and this polysilicon sample was then used as a reference for further measurements carried out on polycrystalline SiGe samples. Similar measurements were undertaken on polycrystalline SiGe structures with percentage Ge ranging from 10 to 30%. All contact window sizes were employed in determining specific contact resistance but for simplicity the results on 4 m square contacts only are shown in Fig. 3. It can be seen that for all semiconductor arm widths the Kelvin resistance reduced as germanium fraction increased. Eq. (1) was employed in analysis of the measured data to obtain best-fit value of specific contact resistance for each germanium percentage over the full range of contact sizes. These values, plotted in Fig. 4 decrease as germanium content is increased. The value of ρc is observed to decrease by a factor of six from
225
Fig. 3. Measured Kelvin resistance values from CBKR structures with contact size 4 m × 4 m on boron doped polycrystalline SiGe samples.
3 × 10−7 ohm cm2 for polysilicon to 5 × 10−8 ohm cm2 for polycrystalline SiGe with 30% germanium content. The dependence of ρc on Ge content is most sensitive for Ge content up to 20%. Cross bridge resistors were also manufactured with phosphorus implanted polycrystalline silicon and SiGe. Specific contact resistance was measured in the same way and these values are also plotted in Fig. 4. A less pronounced decrease in ρc is also observed as germanium content increases. ρc decreases from 6 × 10−7 ohm cm2 for polysilicon to 2 × 10−7 ohm cm2 for polycrystalline SiGe with 30% germanium content. In this case the sensitivity of ρc to Ge content increases for Ge content greater than 20%.
4. Discussion The specific contact resistance is strongly related to the doping concentration of the semiconductor material and the work function of the contact material [3,5–8]. This relationship is normally described by the Eq. (2). k qφb ρc = (2) cFE exp qA∗ T E0 where tunnelling probability is given by: E00 E0 = E00 coth kT
(3)
and the characteristic tunnelling energy E00 is defined as: E00 =
qh √ 4π N/m∗ ε
(4)
where k is Boltzmann’s constant, A∗ Richardson’s constant, N the carrier concentration, q the electronic charge, h Plank’s constant, m∗ the effective mass of tunnelling electrons and ε the dielectric constant of the semiconductor. The quantity cFE is a dimensionless constant associated with the tunnelling of the carriers with the characteristic tunnelling energy E00 and in this work was taken as 0.42 for contacts to n-type
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Fig. 4. Experimental values of specific contact resistance for boron and phosphorus doped polycrystalline SiGe films with a range Ge content.
SiGe and 0.36 for contacts to p-type SiGe [8]. The values of ρc for the polysilicon contacts agree well with the theoretical predictions of these equations using a barrier height for WSix of 0.55 eV and doping levels of 1 × 1020 cm−3 and 7 × 1019 cm−3 for the boron and phosphorus implanted samples respectively. The bandgap of SiGe is known to reduce with germanium content [9]. The resultant impact of the germanium on ρc is mainly through a reduction in the barrier height value φb. Since the reduction in bandgap is mainly due to a significant change in the valence band energy with little change in the conduction band energy, greatest change in contact resistance will be seen in the value of ρc to the boron implanted sample. This is observed experimentally but the results show a decrease in ρc for both types of doped SiGe so bandgap reduction alone is not responsible for the observed results. Eq. (2) has also been employed assuming a work function for WSix of 4.6 eV to calculate the specific contact resistance of WSix to boron doped SiGe for various dopant concentrations. The barrier height has been modified to account for the reduction in band gap energy. The values for SiGe band gap are given in Table 1 and the reduction has in all cases been attributed as a shift in the valence band energy and hence as a direct reduction in barrier height φb. The calculated values are shown in Fig. 5 for boron doping ranging from 9 ×
1019 cm−3 to 4 × 1020 cm−3 . The experimental values have also been included in Fig. 5 as open squares. It can be seen that the values of ρc determined experimentally can only be explained if the effective doping concentration in the samples increases with germanium content from 1 × 1020 cm−3 when germanium content is zero to 2 × 1020 cm−3 for germanium content in excess of 20%. This suggests that the implanted boron in the polysilicon was not completely activated by the 900 ◦ C 1 min anneal, but greater activation was achieved for the SiGe samples. It is also interesting to note that a change in boron concentration from 1 × 1020 cm−3 to 2 × 1020 cm−3 causes a greater modulation in ρc than 50% Ge. A decrease in polycrystalline SiGe sheet resistance has also been observed with increasing Ge content as measured with Van der Pauw structures. This is shown in Fig. 6 for both the boron and phosphorus implanted samples. Similar reduction in sheet resistance for boron doped SiGe has been observed by other workers [10–12]. From these findings, it
Table 1 Variation of bandgap in silicon–germanium with germanium content Germanium content (%)
Bandgap in unstrained polycrystalline SiGe (eV)
0 10 20 25 30
1.170 1.110 1.080 1.065 1.055
Fig. 5. Calculated and measured specific contact resistance in boron doped polycrystalline SiGe films.
G. Srinivasan et al. / Materials Science and Engineering B 114–115 (2004) 223–227
Fig. 6. Measured sheet resistivity of doped polycrystalline SiGe films with a range Ge content.
is inferred that dopant activation of boron in the SiGe samples is enhanced with increasing Ge content and that hole mobility in SiGe increases with Ge content. The phosphorus doped SiGe samples show a less pronounced drop in ρc with increasing germanium content, Fig. 4. In this case the WSix barrier height to the SiGe remains relatively fixed as the electron affinity of SiGe is insensitive to germanium fraction. The reduction in ρc is therefore due to increased activation of the implanted phosphorus in the SiGe. This is supported by the data of Fig. 6 which shows a reduction in the sheet resistance of the phosphorus implanted SiGe with increasing Ge content. Other published work reports increased carrier concentration and mobility in phosphorus implanted SiGe only for germanium content less than 30% [12]. The work in this paper is confined to this Ge content range.
5. Conclusions The specific contact resistance of WSix contacts to polycrystalline SiGe has been investigated. The WSix has been shown to provide good quality contacts with a near mid-band gap barrier height to polysilicon. Specific contact resistance values ρc of 3 × 10−7 ohm cm2 and 6 × 10−7 ohm cm2 have
227
been achieved to boron and phosphorus doped polysilicon, respectively. The inclusion of germanium to form SiGe layers has been shown to yield reduced specific contact resistance for both boron and phosphorus doped samples. The inclusion of greater than 20% germanium in boron implanted polycrystalline SiGe resulted in a very low value for ρc of 5 × 10−8 ohm cm2 . The reduction in ρc has been shown to be due to both a reduction in Schottky barrier height of the WSix to SiGe and to the increased activation of the boron implanted in the SiGe. The phosphorus implanted samples show less sensitivity to Ge content, reaching a lowest value for ρc in this work of 3 × 10−7 ohm cm2 . The reduction in ρc for phosphorus implanted samples has been shown to be due to increased activation of the dopant alone.
Acknowledgements This work was undertaken within the UK SiGeC HBT research programme. The financial support of the Engineering and Physical Sciences Research Council, UK, is gratefully acknowledged.
References [1] C.S. Jeoung, Oh Hyuckjae, C. Hoon, S. Takeshi, K. Hiroyuki, K. Mitsumasa, Appl. Surface Sci. 224 (2004) 260. [2] R. Braunstein, A.R. Moore, F. Herman, Phys. Rev. 109 (1958) 695. [3] M. Ono, A. Nishiyana, A. Toriumi, Solid State Electron. 46 (2002) 1345. [4] M.F. Bain, B.M. Arsmtrong, H.S. Gamble, Vacuum 64 (2000) 227. [5] F.A. Padovani, R. Stratton, Solid State Electron. 9 (1966) 695. [6] A.Y.C. Yu, Solid State Electron. 13 (1970) 239. [7] C.Y. Fang, Y.K. Fang, Solid State Electron. 14 (1971) 541. [8] K. Varahramyan, E.J. Verret, Solid State Electron. 39 (1996) 1601. [9] R. People, Phys. Rev. B 32 (1985) 1405. [10] H.J. Huang, K.M. Chen, C.Y. Chang, T.Y. Huang, L.P. Chen, G.W. Huang, Electrochem. Solid State Lett. 3 (2000) 569. [11] S.B. David, C. Ming, W. Albert, K.C. Saraswat, Appl. Phys. Lett. 66 (1995) 195. [12] T.J. King, J.P. Mcvittie, K.C. Saraswat, J.R. Pfiester, IEEE Trans. Electron Devices 41 (1994) 228.
Tungsten silicide contacts to polycrystalline silicon and silicon–germanium alloys G. Srinivasan∗ , M.F. Bain, S. Bhattacharyya, P. Baine, B.M. Armstrong, H.S. Gamble, D.W. McNeill Northern Ireland Semiconductor Research Centre, School of Electrical and Electronic Engineering, Queen’s University, Ashby Building, Stranmillis Road, Belfast BT9 5AH, Ireland, UK
Abstract Silicon–germanium alloy layers will be employed in the source–drain engineering of future MOS transistors. The use of this technology offers advantages in reducing series resistance and decreasing junction depth resulting in reduction in punch-through and SCE problems. The contact resistance of metal or metal silicides to the raised source–drain material is a serious issue at sub-micron dimensions and must be minimised. In this work, tungsten silicide produced by chemical vapour deposition has been investigated as a contact metallization scheme to both boron and phosphorus doped polycrystalline Si1−x Gex , with 0 ≤ x ≤ 0.3. Cross bridge Kelvin resistor (CKBR) structures were fabricated incorporating CVD WSi2 and polycrystalline SiGe. Tungsten silicide contacts to control polysilicon CKBR structures have been shown to be of high quality with specific contact resistance ρc values 3 × 10−7 ohm cm2 and 6 × 10−7 ohm cm2 obtained to boron and phosphorus implanted samples respectively. The SiGe CKBR structures show that the inclusion of Ge yields a reduction in ρc for both dopant types. The boron doped SiGe exhibits a reduction in ρc from 3 × 10−7 to 5 × 10−8 ohm cm2 as Ge fraction is increased from 0 to 0.3. The reduction in ρc has been shown to be due to (i) the lowering of the tungsten silicide Schottky barrier height to p-type SiGe resulting from the energy band gap reduction, and (ii) increased activation of the implanted boron with increased Ge fraction. The phosphorus implanted samples show less sensitivity of ρc to Ge fraction with a lowest value in this work of 3 × 10−7 ohm cm2 for a Ge fraction of 0.3. The reduction in specific contact resistance to the phosphorus implanted samples has been shown to be due to increased dopant activation alone. © 2004 Elsevier B.V. All rights reserved. Keywords: Tungsten silicide; Polycrystalline; SiGe alloy; Contact resistance
1. Introduction Future MOS transistors will employ novel device architectures. At present a key issue is the source–drain engineering of MOSFETs which requires critical control over the junction depth. Raised source–drain (or recess-gate) MOSFETs offer advantages in decreasing junction depth resulting in reduction in punch-through and SCE problems [1]. Polycrystalline silicon or silicon–germanium can be deposited for the raised source–drain. SiGe is preferred due to its lower resistance and its favourable electronic energy band structure. At deep submicron dimensions, the contact resistance to the source and drain regions can become very large and must be minimised. ∗
Corresponding author. E-mail address: [email protected] (G. Srinivasan).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.07.019
The contact resistance of any material is strongly related to the Schottky barrier height of the metal to silicon or in this case SiGe. In this particular study, tungsten silicide (WSix ) contacts have been employed as a metallic layer with a work function greater 4.6 eV, highly favourable for the formation of a Schottky barrier height φb of value matching near midband gap of the silicon. The tungsten silicide contact was deposited directly by LPCVD. The main emphasis of the paper relates to the SiGe alloy. The energy band structure is strongly related to the germanium content and the purpose of this paper is to experimentally determine the influence of germanium content [2] on the resultant specific contact resistance. A UHV compatible cluster tool has been employed for the LPCVD of the polycrystalline SiGe layers. The reliable measurement of specific contact resistance is difficult and requires a custom designed test structure. In this work, a
224
G. Srinivasan et al. / Materials Science and Engineering B 114–115 (2004) 223–227
cross bridge Kelvin resistor structure has been employed. Minor misalignment of the contact windows in this structure can lead to inaccurate extraction of the specific contact resistance ρc . Thus, considerable care has been taken to ensure reliable results. SEM microscopy has been employed to measure any misalignment and the one-dimensional model employed by Ono [3] has been used in the extraction of ρc.
2. Experimental The raised source and drain regions of modern MOSFETs employ heavily doped polycrystalline SiGe. In order to measure the specific contact resistance to these layers, cross bridge Kelvin resistors have been designed. The structure is shown in Fig. 1. It consists of one L-shaped region formed in the doped polycrystalline SiGe to create two arms of the bridge resistor. There is a centrally located square contact which yields the contact resistance measurements, and the overlying metal forms the other two arms of the bridge structure. Current is injected via two opposite arms of the resistor and induced voltage is measured across the other two. This is shown in Fig. 1 along with critical dimensions. The contact window size employed was varied from 4 to 12 m. The width of the resistor arms was also varied from 6 to 40 m to yield data for graphical presentation which aids in the analysis of results. A simple one-dimensional transmission line model was used to extract specific contact resistance from the measured Kelvin resistance data [3]. The equation used in this extraction is given below. √ √ ρc + ρc ρs coth (L ρs /ρc )L1 + L21 ρs /2 √ √ +L2 ρc ρs / sinh (L ρs /ρc ) RK = (1) (L1 + L2 + L)W The terms ρc and ρs are the specific contact resistance of the metal silicide and the sheet resistance of the polycrystalline SiGe, L1 and L2 the alignment margins of the contact window, L and W the dimensions of the contact window and in this case are equal.
Approximately, 250 nm of polycrystalline SiGe was deposited on oxidised silicon substrates. The layers were deposited in an LPCVD chamber which forms part of a UHV compatible cluster tool. The deposition temperature was 800 ◦ C. Silane gas was employed at a flow rate of 100 sccm in all depositions and germane gas was added with values ranging from 0 to 15 sccm to yield a germanium content in the resultant films from 0 to 30%. Each film had a uniform germanium content which was verified using Raman spectroscopy. The polycrystalline SiGe layer was coated with a thin layer of LPCVD oxide and then ion implanted with either boron (energy = 30 keV and dose = 1×1016 cm−2 ) or with phosphorus (energy = 30 keV and dose = 1 × 1016 cm−2 ). The oxide thickness for the boron and phosphorus implant samples were approximately 45 and 30 nm, respectively. The polycrystalline SiGe was patterned by RIE to provide the first L-shaped arms of the resistor structure. A layer of silicon dioxide was deposited at 720 ◦ C using a TEOS precursor to provide electrical isolation. A deposition time of 75 min was used to yield a thickness of 500 nm. Rapid thermal annealing at 900 ◦ C for 1 min was employed to densify the oxide and activate the implanted dopants. It is important to note that with grain boundary diffusion this thermal treatment is sufficient to redistribute the boron and phosphorus implant profiles yielding a uniform concentration throughout the polycrystalline SiGe layer. Following contact window formation, the exposed SiGe was hydrogen passivated and the tungsten disilicide was deposited by LPCVD. This was done in a Varian 4101 cold walled single wafer processing system [4]. The deposition temperature was 370 ◦ C and the reactant species were Ar (550 sccm), WF6 (3 sccm) and SiH4 (200 sccm). The process pressure was 300 mTorr yielding a deposition rate of 0.9 nm s−1 . The process was controlled to ensure the silicide layer was silicon rich yielding WSi2.6 . This composition of layer is more stable during subsequent processing. The silicide was annealed at 700 ◦ C for 1 h to improve the grain structure and yield a sheet resistance of 4 ohm square−1 . The final metallisation layer was sputter deposited aluminium. The aluminium and underlying tungsten silicide were patterned using wet etching and RIE
Fig. 1. Schematic of the cross bridge Kelvin resistor test structure.
G. Srinivasan et al. / Materials Science and Engineering B 114–115 (2004) 223–227
Fig. 2. Measured and calculated values for Kelvin resistance for several CBKR test structures manufactured in polycrystalline silicon.
to yield the second L-shaped arms of the resistor. A final anneal was carried out in forming gas at 450 ◦ C for 30 min.
3. Results The resistance of various cross bridge structures was measured covering the full range of contact window sizes and alignment margins. Fig. 2 shows the measured Kelvin resistance data for the WSix contact scheme applied to boron implanted polysilicon (0% Ge). The solid lines represent the experimentally measured values. The Kelvin resistance decreases as expected with increasing contact area but increases with increasing semiconductor arm width D for all contact window sizes. This is due to the increased contribution to contact resistance of the alignment margin L1 . The dashed lines are the theoretical fit to this data using Eq. (1) with a chosen, but fixed value of ρc . The dimensions were measured using SEM and Van der Pauw structures yielded the sheet resistance of the polysilicon layer. Best-fit to all the data was then achieved with and a value for specific contact resistance ρc of 3 × 10−7 ohm cm2 . The measurement technique and the fitting equation have therefore proved sound and this polysilicon sample was then used as a reference for further measurements carried out on polycrystalline SiGe samples. Similar measurements were undertaken on polycrystalline SiGe structures with percentage Ge ranging from 10 to 30%. All contact window sizes were employed in determining specific contact resistance but for simplicity the results on 4 m square contacts only are shown in Fig. 3. It can be seen that for all semiconductor arm widths the Kelvin resistance reduced as germanium fraction increased. Eq. (1) was employed in analysis of the measured data to obtain best-fit value of specific contact resistance for each germanium percentage over the full range of contact sizes. These values, plotted in Fig. 4 decrease as germanium content is increased. The value of ρc is observed to decrease by a factor of six from
225
Fig. 3. Measured Kelvin resistance values from CBKR structures with contact size 4 m × 4 m on boron doped polycrystalline SiGe samples.
3 × 10−7 ohm cm2 for polysilicon to 5 × 10−8 ohm cm2 for polycrystalline SiGe with 30% germanium content. The dependence of ρc on Ge content is most sensitive for Ge content up to 20%. Cross bridge resistors were also manufactured with phosphorus implanted polycrystalline silicon and SiGe. Specific contact resistance was measured in the same way and these values are also plotted in Fig. 4. A less pronounced decrease in ρc is also observed as germanium content increases. ρc decreases from 6 × 10−7 ohm cm2 for polysilicon to 2 × 10−7 ohm cm2 for polycrystalline SiGe with 30% germanium content. In this case the sensitivity of ρc to Ge content increases for Ge content greater than 20%.
4. Discussion The specific contact resistance is strongly related to the doping concentration of the semiconductor material and the work function of the contact material [3,5–8]. This relationship is normally described by the Eq. (2). k qφb ρc = (2) cFE exp qA∗ T E0 where tunnelling probability is given by: E00 E0 = E00 coth kT
(3)
and the characteristic tunnelling energy E00 is defined as: E00 =
qh √ 4π N/m∗ ε
(4)
where k is Boltzmann’s constant, A∗ Richardson’s constant, N the carrier concentration, q the electronic charge, h Plank’s constant, m∗ the effective mass of tunnelling electrons and ε the dielectric constant of the semiconductor. The quantity cFE is a dimensionless constant associated with the tunnelling of the carriers with the characteristic tunnelling energy E00 and in this work was taken as 0.42 for contacts to n-type
226
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Fig. 4. Experimental values of specific contact resistance for boron and phosphorus doped polycrystalline SiGe films with a range Ge content.
SiGe and 0.36 for contacts to p-type SiGe [8]. The values of ρc for the polysilicon contacts agree well with the theoretical predictions of these equations using a barrier height for WSix of 0.55 eV and doping levels of 1 × 1020 cm−3 and 7 × 1019 cm−3 for the boron and phosphorus implanted samples respectively. The bandgap of SiGe is known to reduce with germanium content [9]. The resultant impact of the germanium on ρc is mainly through a reduction in the barrier height value φb. Since the reduction in bandgap is mainly due to a significant change in the valence band energy with little change in the conduction band energy, greatest change in contact resistance will be seen in the value of ρc to the boron implanted sample. This is observed experimentally but the results show a decrease in ρc for both types of doped SiGe so bandgap reduction alone is not responsible for the observed results. Eq. (2) has also been employed assuming a work function for WSix of 4.6 eV to calculate the specific contact resistance of WSix to boron doped SiGe for various dopant concentrations. The barrier height has been modified to account for the reduction in band gap energy. The values for SiGe band gap are given in Table 1 and the reduction has in all cases been attributed as a shift in the valence band energy and hence as a direct reduction in barrier height φb. The calculated values are shown in Fig. 5 for boron doping ranging from 9 ×
1019 cm−3 to 4 × 1020 cm−3 . The experimental values have also been included in Fig. 5 as open squares. It can be seen that the values of ρc determined experimentally can only be explained if the effective doping concentration in the samples increases with germanium content from 1 × 1020 cm−3 when germanium content is zero to 2 × 1020 cm−3 for germanium content in excess of 20%. This suggests that the implanted boron in the polysilicon was not completely activated by the 900 ◦ C 1 min anneal, but greater activation was achieved for the SiGe samples. It is also interesting to note that a change in boron concentration from 1 × 1020 cm−3 to 2 × 1020 cm−3 causes a greater modulation in ρc than 50% Ge. A decrease in polycrystalline SiGe sheet resistance has also been observed with increasing Ge content as measured with Van der Pauw structures. This is shown in Fig. 6 for both the boron and phosphorus implanted samples. Similar reduction in sheet resistance for boron doped SiGe has been observed by other workers [10–12]. From these findings, it
Table 1 Variation of bandgap in silicon–germanium with germanium content Germanium content (%)
Bandgap in unstrained polycrystalline SiGe (eV)
0 10 20 25 30
1.170 1.110 1.080 1.065 1.055
Fig. 5. Calculated and measured specific contact resistance in boron doped polycrystalline SiGe films.
G. Srinivasan et al. / Materials Science and Engineering B 114–115 (2004) 223–227
Fig. 6. Measured sheet resistivity of doped polycrystalline SiGe films with a range Ge content.
is inferred that dopant activation of boron in the SiGe samples is enhanced with increasing Ge content and that hole mobility in SiGe increases with Ge content. The phosphorus doped SiGe samples show a less pronounced drop in ρc with increasing germanium content, Fig. 4. In this case the WSix barrier height to the SiGe remains relatively fixed as the electron affinity of SiGe is insensitive to germanium fraction. The reduction in ρc is therefore due to increased activation of the implanted phosphorus in the SiGe. This is supported by the data of Fig. 6 which shows a reduction in the sheet resistance of the phosphorus implanted SiGe with increasing Ge content. Other published work reports increased carrier concentration and mobility in phosphorus implanted SiGe only for germanium content less than 30% [12]. The work in this paper is confined to this Ge content range.
5. Conclusions The specific contact resistance of WSix contacts to polycrystalline SiGe has been investigated. The WSix has been shown to provide good quality contacts with a near mid-band gap barrier height to polysilicon. Specific contact resistance values ρc of 3 × 10−7 ohm cm2 and 6 × 10−7 ohm cm2 have
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been achieved to boron and phosphorus doped polysilicon, respectively. The inclusion of germanium to form SiGe layers has been shown to yield reduced specific contact resistance for both boron and phosphorus doped samples. The inclusion of greater than 20% germanium in boron implanted polycrystalline SiGe resulted in a very low value for ρc of 5 × 10−8 ohm cm2 . The reduction in ρc has been shown to be due to both a reduction in Schottky barrier height of the WSix to SiGe and to the increased activation of the boron implanted in the SiGe. The phosphorus implanted samples show less sensitivity to Ge content, reaching a lowest value for ρc in this work of 3 × 10−7 ohm cm2 . The reduction in ρc for phosphorus implanted samples has been shown to be due to increased activation of the dopant alone.
Acknowledgements This work was undertaken within the UK SiGeC HBT research programme. The financial support of the Engineering and Physical Sciences Research Council, UK, is gratefully acknowledged.
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