- Email: [email protected]
Nitrogen doped silicon films heavily boron implanted for MOS structures: Simulation and characterization
Materials Science in Semiconductor Processing 13 (2010) 383–388
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Nitrogen doped silicon films heavily boron implanted for MOS structures: Simulation and characterization R. Mahamdi a,b,n, F. Mansour a, H. Bouridah c, P. Temple-Boyer d,e, E. Scheid d,e, L. Jalabert d,e,f a
LEMEAMED, Department of Electronics, University Mentouri Constantine, Algeria Department of Electronics, University Hadj Lakhdar Batna, Algeria Department of Electronics, University of Jijel, Algeria d CNRS-LAAS; 7 avenue du colonel Roche, F-31077 Toulouse, France e Universite´ de Toulouse; UPS, INSA, INP, ISAE; LAAS; F-31077 Toulouse, France f LIMMS-CNRS/IIS, The University of Tokyo, Tokyo, Japan b c
a r t i c l e i n f o
abstract
Available online 26 May 2011
In this work we study the boron diffusion and its activation into recrystallized nitrogen doped silicon thin films (NIDOS) and we also discuss the influence of the chemical interaction between boron and nitrogen in NIDOS films. These films are deposited by low pressure chemical vapor (LPCVD) for the development of a P þ polysilicon gate for MOS structures. The reduction of boron diffusion with increasing nitrogen content is observed by SIMS profiles. SUPREM IV software is used in order to estimate the boron diffusion coefficients in NIDOS films. FTIR analyses show the appearance of a B–N complex whose density strongly depends on the annealing treatment in terms of temperature and duration. It is deduced through resistivity measurements and SEM observation that the formation of B–N complexes tends to degrade the electrical properties of polysilicon thin layers through the decrease of both electrically active boron and polycrystalline grains growth. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Activation B–N bond Crystallinity Diffusion Resistivity
1. Introduction In CMOS technology, the strong diffusivity of boron atoms into polysilicon [1,2] or more recently metal gate [3] during annealing treatment is detrimental to the integrity of the gate oxide. High-k materials, as well as polysilicon/TiN gate stack are an example of emerging gate engineering using BF2 source (4.1015 cm 2, —20 keV) for implantation and Rapid Thermal Annealing (RTA)—‘‘spike’’ annealing. Low thermal annealing budget is required especially in case of low voltage applications, so that metal gates have shown better performance than polysilicon ones [4] even if nitridation can help in adjusting the
n Corresponding author at: LEMEAMED, Department of Electronics, University Mentouri Constantine, Algeria. E-mail address: [email protected] (R. Mahamdi).
1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.05.003
work function in the second case [5]. The problem of boron diffusion into amorphous, semi-crystalline, and polycrystalline thin films and its effect on the reliability of the gate oxide are still challenging nowadays with sputtered thin films (ZrN, TaN, y) or LPCVD deposition. Because the control of the nitrogen content into the SiN thin film has a consequence on the microstructure of the gate and its evolution during a thermal treatment, the study of the boron diffusivity into various nitrogen doped silicon (NIDOS) films can help in understanding the benefit limit of using nitrogen in CMOS gate engineering. Indeed, it is well known that small amount of nitrogen can be used to lower the boron diffusion into the gate oxide, this present study tends to evaluate quantitatively this lowering for slightly or heavily nitrogen doped layers. However, the interaction between nitrogen and boron atoms can also affect dramatically the CMOS electrical characteristics. In this work we focus on the study of the B–N
384
R. Mahamdi et al. / Materials Science in Semiconductor Processing 13 (2010) 383–388
complex effect on the boron ionization and on the crystalline growth of films, with an aim of understanding how the electrical characteristics of polycrystalline films are affected by nitrogen and boron co-doping.
1022
1021
Boron Dose (5.1015at/cm2) T = 850°C , t = 15 min
2. Experimental procedures
3. Results and discussion 3.1. Diffusion of boron in NIDOS films The diffusion phenomena of boron atoms in NIDOS films have been studied by SIMS. Fig. 1 gives the experimental SIMS profiles after annealing for various X ratios [6]. For the nitrogen ratio x=0%, the profiles superposition show clearly the boron diffusion reaches the oxide layer. On the contrary, even with the lowest nitrogen content (1%), the boron diffusion is clearly reduced, and the corresponding profile reaches no longer than the lower interface. 3.2. Simulation with SUPREM IV The boron concentration has been simulated using the SUPREM IV software. As the boron diffusion coefficients into NIDOS film are unknown. Thus, diffusion coefficient values have been determined in order to fit experimental and simulated profiles (Fig. 2). Fig. 3 shows the boron diffusion coefficient in NIDOS films as a function of the X ratio obtained by SUPREM IV software. Diffusion coefficient values decrease with increasing nitrogen ratio, confirming the diffusion decrease observed on the SIMS profiles. These values obtained by the software SUPREM IV are in good agreement with the results from literature [6,7]. 3.3. Electrical characterization This electrical characterization is the continuity of our recent works where it has been found that the films without nitrogen content (X ¼0%), highly implanted boron and annealed at temperatures lower than 1000 1C were electrically conductive. On the other hand, the NIDOS
1020 [B](/cm3)
The amorphous nitrogen doped silicon (NIDOS) films (thickness 200nm) were obtained by low pressure chemical vapor deposition (LPCVD) with low pressure (E200 mT). These films are deposited at low temperature (480 1C) and low pressure (200 mTorr) using disilane (Si2H6) associated with ammonia (NH3), on pre-oxidized (E120 nm SiO2) silicon substrate. The gas flow ratio NH3/Si2H6 is controlled in order to deposit NIDOS films with a [N]/[Si] ratio X ranging from 0% to 16%. This deposition process is followed by a boron implantation with a 5 1015 at/cm2 dose. These films then undergo annealing treatments in a traditional furnace at different temperatures and durations. Experimental secondary ion masse spectroscopy (SIMS) profiles are recorded using a CAMECA IMS4F ionic probe. This apparatus gives the boron concentration profile as a function of the depth in the sample. The electrical properties are obtained by the four probe method. The Fourier transform infrared spectrometry analysis (FTIR) was performed to identify the absorption spectrum of films. The crystalline structure of the polysilicon films is characterized by scanning electron microscopic (SEM) using an XL-30 microscope equipped with an energy dispersive X-ray (EDX) spectrometer with a Be window. The SEM images were taken using secondary electrons (SE) detector.
(X = 0%) (x = 1%) (X = 2%) (x = 16%)
1019
1018
1017
1016 0.00
0.05
0.10
0.15 0.20 Depth (μm)
0.25
0.30
Fig. 1. Experimental SIMS profiles after annealing (850 1C–15 min). films thermally treated under the same conditions remain very resistive [8,9]. The results illustrate the effect of the annealing temperature and the nitrogen ratio on the films resistivity (Fig. 4). The film resistivity decreases with the temperature and increases with the nitrogen ratio. These phenomena noticed in thin NIDOS films doped boron can be explained by the presence of the nitrogen, which prevents the boron atoms redistribution during the annealing by the formation of B–N complexes. This result may be related to the increase in the B–N pairs density causing both a decrease in boron electrically active and in layer crystalline growth, noticing that several works in literature [10,11] associate the polysilicon films resistance both to the grain size and to the impurity active density. Indeed, The B–N interaction is supported by several works in the literature. Spreading-resistance-profiling of boron-doped silicon channels with and without the presence of nitrogen resulted in 7% reduction in the active channel doping density [12]. The reduction of electrical activity of boron in the presence of nitrogen was explained using Ab initio pseudopotential calculations of the energy of formation of systems with negatively charged acceptors. A physico–chemical model suggested that upon interaction, boron and nitrogen atoms form a covalent bond and both atoms become trivalent. Because adjacent silicon atoms are four-valent, all electrons in the system are strongly bound, eliminating the electrical activity of the B–N pair and reducing the active doping density according to the number of B–N pairs. The B–N interaction has been also invoked by Ahmed et al. [13] in explaining reduction in the extracted C–V active doping density of boron-doped polysilicon gates in pMOS devices with oxynitride gate dielectrics.
3.4. Physico–chemical characterization The most significant range of infrared spectrum corresponds to the B–N bond as presented in Fig. 5. This bond assumes the formation of boron–nitrogen complexes in silicon. It is located at about 1530, 1400, 1350 and 1085 cm 1 [14–17]. An intense peak evidenced at about 1085 cm 1 corresponds to the B–N bond in its cubic phase (c-BN), the B–N bond in its hexagonal phase (h-BN) being located at other wave numbers [17–19]. We note a reduction in the peak h-BN intensity as well as an increase in the c-BN peak intensity with the temperature increase. These observations can be explained by the transformation of the h-BN to a c-BN at high temperatures [18,19]. This phenomenon involves an improvement of the electrical and structural films properties because the c-BN is known to be less resistive than the h-BN. These results are in agreement with the
R. Mahamdi et al. / Materials Science in Semiconductor Processing 13 (2010) 383–388
385
Fig. 2. Superposition of SIMS and simulated profiles by SUPREM IV (X ¼0%, 1%, 2% and 16%).
electrical characterization ones, showing a reduction in the values of the resistivity of films as the annealing temperature increases.
(dark areas) and polycrystalline (clear areas) phases given by [20]
a¼
Spoly Spoly ¼ Stotal Samorphous þ Spoly
ð1Þ
3.5. Structural characterization The SEM observation of NIDOS films requires their dipping during few seconds in a SECCO in order to reveal the polysilicon grains by attacking the amorphous grains boundaries. We observe clearly on Fig. 6, the influence of nitrogen atoms on the NIDOS films crystallization. Thanks to the SEM images, we can estimate the NIDOS films crystallinity by analyzing the ratio of the amorphous
where Samorphous and Spoly represent the amorphous and polycrystalline surfaces, respectively. Then, the polycrystalline fraction or crystallinity is given by [20]
w ¼ a3=2
ð2Þ
In Fig. 7 we represent the NIDOS films crystallinity variation according to their nitrogen content. We observe the reduction in the
386
R. Mahamdi et al. / Materials Science in Semiconductor Processing 13 (2010) 383–388
polycrystalline fraction for an increasing nitrogen ratio. This reduction reveals an increase in the size of the amorphous grain boundaries, which is responsible for the reduction in the electrical conductivity.
In this work, we study the high dose-implanted boron redistribution phenomena into nitrogen doped polysilicon films by SIMS profiles. The diffusion coefficients obtained by SUPREM IV software are in agreement with literature for similar experimental conditions and we have shown that the introduction of very weak nitrogen rate in the films (1%) is sufficient to induce an important boron diffusivity reduction. This explains the very experimental shorts diffusion depths observed, and proves that these films can be used for MOS technologies. We also study the various properties of boron and nitrogen co-doped polysilicon films according to the nitrogen ratio and the annealing parameters. The results showed that the films resistivity decreases with the nitrogen content increase and increases with the annealing temperature and duration. By SEM observations, we have evaluated the influence of the nitrogen doping level on the film crystallinity. Thus, we have deduced that the nitrogen content tends to inhibit the silicon crystallization. This can be explained by the interaction between boron and nitrogen atoms in
Suprem IV
Boron dose 5x1015 at/cm2
DB (cm2/s)
4. Conclusion
1E-14
1E-15 0
2
4
6
8 10 12 X = N/Si(%)
14
16
18
Fig. 3. Variation of boron diffusion coefficient versus nitrogen ratio.
100
<ρ> (Ω.cm)
T = 1000°C T = 1100°C T = 1200°C
10-2
T = 1000°C T = 1100°C T = 1200°C
10-1
10-2
X = 1%
10-3 60
120
180
X = 2%
10-3 60
240
120
t (min)
180 t (min)
100
<ρ> (Ω.cm)
<ρ> (Ω.cm)
10-1
T = 1000°C T = 1100°C T = 1200°C
10-1
10-2 X = 4% 60
120
180
240
t (min) Fig. 4. Resistivity evolution versus annealing duration at different temperatures, a) X ¼0%, b) X ¼1% and c) X ¼ 4%.
240
R. Mahamdi et al. / Materials Science in Semiconductor Processing 13 (2010) 383–388
t = 60 min., X = 1%
T = 1000°C T = 1100°C T = 1200°C
0.4
t = 240 min., X = 1%
T = 1000°C T = 1100°C T = 1200°C
0.4
387
Absorbance (u.a)
Absorbance (u.a)
c-BN 0.3
0.2 h-BN
h-BN
h-BN
c-BN 0.2 h-BN
0.1
0.0
h-BN
h-BN
0.0 1600 1500 1400 1300 1200 1100 1000 Wavenumber (cm-1)
900
1600 1500 1400 1300 1200 1100 1000 Wavenumber (cm-1)
900
Fig. 5. Infrared absorbance spectra of NIDOS films boron implanted.
Fig. 6. SEM image of Secco-etched polysilicon film co-doped boron–nitrogen after annealing (1100 1C/60 min), a) X ¼ 0%, b) X ¼ 1% and c) X¼ 2%.
100
T = 1100°C, t = 60 min. Boron dose 5x1015 cm-2
χ (%)
80
ensure a conducting behavior for the boron/nitrogen co-doped polysilicon films. In these conditions, heavily boron-doped NIDOS can be used as an intermediary thin layer between the polysilicon gate and the thermal gate oxide in order to improve the performances of CMOS technology.
60
References 40
20 0
1 X (%)
2
Fig. 7. Crystallinity variation versus nitrogen content (X¼ 0%, 1% and 2%).
order to form a complex B–N, responsible for the film conductivity reduction. The concentration of this complex depends strongly on the nitrogen ratio, requiring an optimization of the nitrogen doping ( o2%) in order to
[1] Pfiester JR, Baker FK, Mele TC, Tseng HH, Tobin PJ, Hayden JD, Miller JW, Gunderson LC, Parrillo LC. IEEE Trans Electron Devices 1990;37: 1842–51. [2] Yu B, Ju DG, Lee WC, Kepler N, King TJ, Hu C. IEEE Trans Electron Devices 1998;45:1253–61. [3] Kumar A, Solomon PM. IEEE Trans Electron Devices 2006;53: 1208–15. [4] Song SC, Zhang Z, Lee BH. IEEE Electron Device Lett 2005;26:366–8. [5] Gotoh Y, Tsuji H, Ishikawa J. J Vac Sci Technol B Microelectron Nanometer Struct 2003;21:1607–11. [6] Mahamdi R, Mansour F, Temple-Boyer P, Jalabert L, Study of redistribution and activation of boron implanted into nitrogen doped silicon thin films, In: Proceedings of the 4th International Conference: SETIT, Tunisia; March 25–29, 2007. [7] Nakayama S, Sakai T. J Electrochem Soc Phys 1997;144:4326–30. [8] Temple-Boyer P, Jalabert L, Scheid E, Fadel P, Rousset B. Thin Solid Films 2002;414:13–7. [9] Bouridah H, Mansour F, Mahamdi R, Temple-Boyer P. J Mater Sci 2005;40:1405–8.
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[10] Sciuto M, Papalino L, Gagliano C, Padalino C, Coccorese C, Mello D, Renna G, Franco G. Cryst Res Technol 2005;40:955–7. [11] Nakazawa K. J Appl Phys 1991;69:1703–6. [12] Aronowitz S, Zubkov V, Puchner H, Kimball J. J Vac Sci Technol B 2002;20:230–7. [13] Ahmed KZ, Kraus PA, Olsen C, Hung S, Nouri F. IEEE Electron Device Lett 2003;24:445–7. [14] Nakamura K, Sasaki T. J Solid State Chem 2004;177:542–6. [15] Chen L, Huang M, Luo T, Gu Y, Shi L, Yang Z, Qian Y. Mater Lett 2004;58 3634–3634.
[16] Wang RZ, Zhou H, Song XM, Wang B, Wang H, Yan H. J Cryst Growth 2006;291:18–21. [17] Li W, Gu G, Li Y, He Z, Feng W, Liu L, Zhao C, Zhao Y. Appl Surf Sci 2005;239:432–6. [18] Battiston GA, Berto D, Convertino A, Emiliani D, Figueras A, Gerbasi R, Viticoli S. Electrochim Acta 2005;50:4600–4. [19] Chan CY, Eyhusen S, Meng XM, Bello I, Lee ST, Ronning C, Hofsass H. Diamond Relat Mater 2006;15:55–60. [20] Bouridah H, Mansour F, Mahamdi R, Bounar N, Temple-Boyer P. Phys Status Solidi (a) 2007;204:2347–54.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Nitrogen doped silicon films heavily boron implanted for MOS structures: Simulation and characterization R. Mahamdi a,b,n, F. Mansour a, H. Bouridah c, P. Temple-Boyer d,e, E. Scheid d,e, L. Jalabert d,e,f a
LEMEAMED, Department of Electronics, University Mentouri Constantine, Algeria Department of Electronics, University Hadj Lakhdar Batna, Algeria Department of Electronics, University of Jijel, Algeria d CNRS-LAAS; 7 avenue du colonel Roche, F-31077 Toulouse, France e Universite´ de Toulouse; UPS, INSA, INP, ISAE; LAAS; F-31077 Toulouse, France f LIMMS-CNRS/IIS, The University of Tokyo, Tokyo, Japan b c
a r t i c l e i n f o
abstract
Available online 26 May 2011
In this work we study the boron diffusion and its activation into recrystallized nitrogen doped silicon thin films (NIDOS) and we also discuss the influence of the chemical interaction between boron and nitrogen in NIDOS films. These films are deposited by low pressure chemical vapor (LPCVD) for the development of a P þ polysilicon gate for MOS structures. The reduction of boron diffusion with increasing nitrogen content is observed by SIMS profiles. SUPREM IV software is used in order to estimate the boron diffusion coefficients in NIDOS films. FTIR analyses show the appearance of a B–N complex whose density strongly depends on the annealing treatment in terms of temperature and duration. It is deduced through resistivity measurements and SEM observation that the formation of B–N complexes tends to degrade the electrical properties of polysilicon thin layers through the decrease of both electrically active boron and polycrystalline grains growth. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Activation B–N bond Crystallinity Diffusion Resistivity
1. Introduction In CMOS technology, the strong diffusivity of boron atoms into polysilicon [1,2] or more recently metal gate [3] during annealing treatment is detrimental to the integrity of the gate oxide. High-k materials, as well as polysilicon/TiN gate stack are an example of emerging gate engineering using BF2 source (4.1015 cm 2, —20 keV) for implantation and Rapid Thermal Annealing (RTA)—‘‘spike’’ annealing. Low thermal annealing budget is required especially in case of low voltage applications, so that metal gates have shown better performance than polysilicon ones [4] even if nitridation can help in adjusting the
n Corresponding author at: LEMEAMED, Department of Electronics, University Mentouri Constantine, Algeria. E-mail address: [email protected] (R. Mahamdi).
1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.05.003
work function in the second case [5]. The problem of boron diffusion into amorphous, semi-crystalline, and polycrystalline thin films and its effect on the reliability of the gate oxide are still challenging nowadays with sputtered thin films (ZrN, TaN, y) or LPCVD deposition. Because the control of the nitrogen content into the SiN thin film has a consequence on the microstructure of the gate and its evolution during a thermal treatment, the study of the boron diffusivity into various nitrogen doped silicon (NIDOS) films can help in understanding the benefit limit of using nitrogen in CMOS gate engineering. Indeed, it is well known that small amount of nitrogen can be used to lower the boron diffusion into the gate oxide, this present study tends to evaluate quantitatively this lowering for slightly or heavily nitrogen doped layers. However, the interaction between nitrogen and boron atoms can also affect dramatically the CMOS electrical characteristics. In this work we focus on the study of the B–N
384
R. Mahamdi et al. / Materials Science in Semiconductor Processing 13 (2010) 383–388
complex effect on the boron ionization and on the crystalline growth of films, with an aim of understanding how the electrical characteristics of polycrystalline films are affected by nitrogen and boron co-doping.
1022
1021
Boron Dose (5.1015at/cm2) T = 850°C , t = 15 min
2. Experimental procedures
3. Results and discussion 3.1. Diffusion of boron in NIDOS films The diffusion phenomena of boron atoms in NIDOS films have been studied by SIMS. Fig. 1 gives the experimental SIMS profiles after annealing for various X ratios [6]. For the nitrogen ratio x=0%, the profiles superposition show clearly the boron diffusion reaches the oxide layer. On the contrary, even with the lowest nitrogen content (1%), the boron diffusion is clearly reduced, and the corresponding profile reaches no longer than the lower interface. 3.2. Simulation with SUPREM IV The boron concentration has been simulated using the SUPREM IV software. As the boron diffusion coefficients into NIDOS film are unknown. Thus, diffusion coefficient values have been determined in order to fit experimental and simulated profiles (Fig. 2). Fig. 3 shows the boron diffusion coefficient in NIDOS films as a function of the X ratio obtained by SUPREM IV software. Diffusion coefficient values decrease with increasing nitrogen ratio, confirming the diffusion decrease observed on the SIMS profiles. These values obtained by the software SUPREM IV are in good agreement with the results from literature [6,7]. 3.3. Electrical characterization This electrical characterization is the continuity of our recent works where it has been found that the films without nitrogen content (X ¼0%), highly implanted boron and annealed at temperatures lower than 1000 1C were electrically conductive. On the other hand, the NIDOS
1020 [B](/cm3)
The amorphous nitrogen doped silicon (NIDOS) films (thickness 200nm) were obtained by low pressure chemical vapor deposition (LPCVD) with low pressure (E200 mT). These films are deposited at low temperature (480 1C) and low pressure (200 mTorr) using disilane (Si2H6) associated with ammonia (NH3), on pre-oxidized (E120 nm SiO2) silicon substrate. The gas flow ratio NH3/Si2H6 is controlled in order to deposit NIDOS films with a [N]/[Si] ratio X ranging from 0% to 16%. This deposition process is followed by a boron implantation with a 5 1015 at/cm2 dose. These films then undergo annealing treatments in a traditional furnace at different temperatures and durations. Experimental secondary ion masse spectroscopy (SIMS) profiles are recorded using a CAMECA IMS4F ionic probe. This apparatus gives the boron concentration profile as a function of the depth in the sample. The electrical properties are obtained by the four probe method. The Fourier transform infrared spectrometry analysis (FTIR) was performed to identify the absorption spectrum of films. The crystalline structure of the polysilicon films is characterized by scanning electron microscopic (SEM) using an XL-30 microscope equipped with an energy dispersive X-ray (EDX) spectrometer with a Be window. The SEM images were taken using secondary electrons (SE) detector.
(X = 0%) (x = 1%) (X = 2%) (x = 16%)
1019
1018
1017
1016 0.00
0.05
0.10
0.15 0.20 Depth (μm)
0.25
0.30
Fig. 1. Experimental SIMS profiles after annealing (850 1C–15 min). films thermally treated under the same conditions remain very resistive [8,9]. The results illustrate the effect of the annealing temperature and the nitrogen ratio on the films resistivity (Fig. 4). The film resistivity decreases with the temperature and increases with the nitrogen ratio. These phenomena noticed in thin NIDOS films doped boron can be explained by the presence of the nitrogen, which prevents the boron atoms redistribution during the annealing by the formation of B–N complexes. This result may be related to the increase in the B–N pairs density causing both a decrease in boron electrically active and in layer crystalline growth, noticing that several works in literature [10,11] associate the polysilicon films resistance both to the grain size and to the impurity active density. Indeed, The B–N interaction is supported by several works in the literature. Spreading-resistance-profiling of boron-doped silicon channels with and without the presence of nitrogen resulted in 7% reduction in the active channel doping density [12]. The reduction of electrical activity of boron in the presence of nitrogen was explained using Ab initio pseudopotential calculations of the energy of formation of systems with negatively charged acceptors. A physico–chemical model suggested that upon interaction, boron and nitrogen atoms form a covalent bond and both atoms become trivalent. Because adjacent silicon atoms are four-valent, all electrons in the system are strongly bound, eliminating the electrical activity of the B–N pair and reducing the active doping density according to the number of B–N pairs. The B–N interaction has been also invoked by Ahmed et al. [13] in explaining reduction in the extracted C–V active doping density of boron-doped polysilicon gates in pMOS devices with oxynitride gate dielectrics.
3.4. Physico–chemical characterization The most significant range of infrared spectrum corresponds to the B–N bond as presented in Fig. 5. This bond assumes the formation of boron–nitrogen complexes in silicon. It is located at about 1530, 1400, 1350 and 1085 cm 1 [14–17]. An intense peak evidenced at about 1085 cm 1 corresponds to the B–N bond in its cubic phase (c-BN), the B–N bond in its hexagonal phase (h-BN) being located at other wave numbers [17–19]. We note a reduction in the peak h-BN intensity as well as an increase in the c-BN peak intensity with the temperature increase. These observations can be explained by the transformation of the h-BN to a c-BN at high temperatures [18,19]. This phenomenon involves an improvement of the electrical and structural films properties because the c-BN is known to be less resistive than the h-BN. These results are in agreement with the
R. Mahamdi et al. / Materials Science in Semiconductor Processing 13 (2010) 383–388
385
Fig. 2. Superposition of SIMS and simulated profiles by SUPREM IV (X ¼0%, 1%, 2% and 16%).
electrical characterization ones, showing a reduction in the values of the resistivity of films as the annealing temperature increases.
(dark areas) and polycrystalline (clear areas) phases given by [20]
a¼
Spoly Spoly ¼ Stotal Samorphous þ Spoly
ð1Þ
3.5. Structural characterization The SEM observation of NIDOS films requires their dipping during few seconds in a SECCO in order to reveal the polysilicon grains by attacking the amorphous grains boundaries. We observe clearly on Fig. 6, the influence of nitrogen atoms on the NIDOS films crystallization. Thanks to the SEM images, we can estimate the NIDOS films crystallinity by analyzing the ratio of the amorphous
where Samorphous and Spoly represent the amorphous and polycrystalline surfaces, respectively. Then, the polycrystalline fraction or crystallinity is given by [20]
w ¼ a3=2
ð2Þ
In Fig. 7 we represent the NIDOS films crystallinity variation according to their nitrogen content. We observe the reduction in the
386
R. Mahamdi et al. / Materials Science in Semiconductor Processing 13 (2010) 383–388
polycrystalline fraction for an increasing nitrogen ratio. This reduction reveals an increase in the size of the amorphous grain boundaries, which is responsible for the reduction in the electrical conductivity.
In this work, we study the high dose-implanted boron redistribution phenomena into nitrogen doped polysilicon films by SIMS profiles. The diffusion coefficients obtained by SUPREM IV software are in agreement with literature for similar experimental conditions and we have shown that the introduction of very weak nitrogen rate in the films (1%) is sufficient to induce an important boron diffusivity reduction. This explains the very experimental shorts diffusion depths observed, and proves that these films can be used for MOS technologies. We also study the various properties of boron and nitrogen co-doped polysilicon films according to the nitrogen ratio and the annealing parameters. The results showed that the films resistivity decreases with the nitrogen content increase and increases with the annealing temperature and duration. By SEM observations, we have evaluated the influence of the nitrogen doping level on the film crystallinity. Thus, we have deduced that the nitrogen content tends to inhibit the silicon crystallization. This can be explained by the interaction between boron and nitrogen atoms in
Suprem IV
Boron dose 5x1015 at/cm2
DB (cm2/s)
4. Conclusion
1E-14
1E-15 0
2
4
6
8 10 12 X = N/Si(%)
14
16
18
Fig. 3. Variation of boron diffusion coefficient versus nitrogen ratio.
100
<ρ> (Ω.cm)
T = 1000°C T = 1100°C T = 1200°C
10-2
T = 1000°C T = 1100°C T = 1200°C
10-1
10-2
X = 1%
10-3 60
120
180
X = 2%
10-3 60
240
120
t (min)
180 t (min)
100
<ρ> (Ω.cm)
<ρ> (Ω.cm)
10-1
T = 1000°C T = 1100°C T = 1200°C
10-1
10-2 X = 4% 60
120
180
240
t (min) Fig. 4. Resistivity evolution versus annealing duration at different temperatures, a) X ¼0%, b) X ¼1% and c) X ¼ 4%.
240
R. Mahamdi et al. / Materials Science in Semiconductor Processing 13 (2010) 383–388
t = 60 min., X = 1%
T = 1000°C T = 1100°C T = 1200°C
0.4
t = 240 min., X = 1%
T = 1000°C T = 1100°C T = 1200°C
0.4
387
Absorbance (u.a)
Absorbance (u.a)
c-BN 0.3
0.2 h-BN
h-BN
h-BN
c-BN 0.2 h-BN
0.1
0.0
h-BN
h-BN
0.0 1600 1500 1400 1300 1200 1100 1000 Wavenumber (cm-1)
900
1600 1500 1400 1300 1200 1100 1000 Wavenumber (cm-1)
900
Fig. 5. Infrared absorbance spectra of NIDOS films boron implanted.
Fig. 6. SEM image of Secco-etched polysilicon film co-doped boron–nitrogen after annealing (1100 1C/60 min), a) X ¼ 0%, b) X ¼ 1% and c) X¼ 2%.
100
T = 1100°C, t = 60 min. Boron dose 5x1015 cm-2
χ (%)
80
ensure a conducting behavior for the boron/nitrogen co-doped polysilicon films. In these conditions, heavily boron-doped NIDOS can be used as an intermediary thin layer between the polysilicon gate and the thermal gate oxide in order to improve the performances of CMOS technology.
60
References 40
20 0
1 X (%)
2
Fig. 7. Crystallinity variation versus nitrogen content (X¼ 0%, 1% and 2%).
order to form a complex B–N, responsible for the film conductivity reduction. The concentration of this complex depends strongly on the nitrogen ratio, requiring an optimization of the nitrogen doping ( o2%) in order to
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