Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Physica E 32 (2006) 547–549 Self-aligned doping profiles in nanoscale silicon structures Jouni Ahopelto à , Mika Prunnila, Eeva Pursula VTT Information Technology, PO Box 1208, FIN-02044 VTT, Finland Available online 10 February 2006 Abstract We propose and demonstrate a method for self-aligning control of doping profiles in nanoscale silicon devices. The method is based on different segregation behaviour of n-type and p-type dopants during thermal oxidation. The simulations show that in nanowires with compensated impurity concentrations the type of conductivity can be changed from p-type to n-type. We use the method to realize a lateral field effect device in silicon showing pn-diode-like characteristics at 300 K. r 2006 Elsevier B.V. All rights reserved. PACS: 64.75.+g; 73.63.Àb Keywords: Segregation; Semiconductor doping; Diode; Silicon on insulator The dimensions of semiconductor devices are continu- ously decreasing, reaching sub-50 nm regime in many applications [1–3]. The small dimensions set stringent requirements for the fabrication processes and in nm-scale structures the alignment is becoming an issue. In silicon technology a pn-junction forms a basic building block in the device fabrication. In the fabrication of CMOS circuits, even with gate lengths well below 100 nm, pn-junctions are self-aligned, because the gate electrode (and spacers) are used as an implantation mask. In nm-scale device structures in which this kind of masked implantation process cannot be used, the alignment-related issues may become unsurmountable. Here we report on a self-aligning doping method which relies on the difference in the segregation of impurities in silicon during thermal oxida- tion. The method allows to control the doping profile, e.g., along wires with sub-50 nm diameter. We demonstrate the method by fabricating a unipolar lateral field effect device with characteristics resembling those of a pn-diode. The Si–SiO 2 segregation coefficient of impurities can be given as m ¼ C Si =C SiO ¼ a e ÀD=kT where C SiO (C Si ) is the equilibrium impurity concentration in SiO 2 (Si) [4].Ifmo1 the impurity segregates into the oxide during thermal oxidation while if m41 the oxide repels the impurity. The values for a and D are a BðPÞ ¼ 1126 (30), D BðPÞ ¼ 0:91 eVð$0:0eVÞ for boron (phosphorus) [5]. Thus, during thermal oxidation of silicon the growing oxide takes up p-type boron while the n-type phosphorus tends to pile up in silicon. This effect can be used to change locally the net doping concentration. Fig. 1 shows results of Silvaco [5] process simulation. In Fig. 1(a) is shown the cross section of an originally 100 nm wide and 180 nm high silicon wire after 120 min dry oxidation at 950 1C. The doping concentrations before the oxidation were P 0 ¼ 7  10 17 cm À3 for boron and N 0 ¼ 1  10 17 cm À3 for phosphorus, leading to effective p-type concentration of 6  10 17 cm À3 . Fig. 1(b) shows the doping profiles before and after the oxidation. During thermal oxidation boron atoms segregate into the forming silicon dioxide leading to decreas e in boron concentration in silicon. On the other hand, the phosphorus concentration increases and the conduction type of silicon is converted to n-type with the average effective carrier concentration of 1  10 16 cm À3 . The effect is the stronger the smaller the remaining silicon volume is compared to the volume of the formed oxide. In case there are two bodies with different surface to volume ratios attached to each other, the oxidat ion- induced change in the effective doping varies from one structure to another. For example, if a narrow wire is attached to larger bulky leads the doping of the wire can be ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.12.148 à Corresponding author. Tel.: +358 20 722 6644; fax: +358 20 722 7012. E-mail address: jouni.ahopelto@vtt.fi (J. Ahopelto). tuned, as demonstrated in Fig. 1, while in the large leads the oxidation has a minor effect on the doping. Thus, by fabricating a compensated lead–wire–lead structure and exposing this to an oxidizing ambient we can obtain a self- aligned pnp structure. We demonstrate the feasibility of this self-aligning method by fabricating so called self-switching devices [6] in silicon. The device is essentially a lateral double gate FET connected as a diode. A top-view SEM image of such a device is shown in Fig. 2(a). Depending on the biasing condition, i.e., whether the source or the drain is at higher potential, the potential across the trenches defining the channel either opens or closes it. The devices were processed on a 100 mm diameter silicon on insulator (SOI) wafer with a 180 nm thick SOI film and a 400 nm thick buried oxide (BOX) layer. The whole wafer was implanted with boron to a dose of 2:4  10 13 cm À2 . Half of the wafer was then masked and an extra compensating implantation with phosphorus to a dose of 3:0  10 12 cm À2 was performed. These implantations produce roughly the doping concentrations P 0 $7  10 17 cm À3 for boron and N 0 $1  10 17 cm À3 for phosphorus in the SOI film. The large-scale mesas for the devices were defined by UV- lithography and dry etching. The devices were patterned by electron beam lithography and dry etching using a 30 nm thick layer of thermal oxide as hard mask. Prior to etching of the SOI film the oxide was patterned with PMMA mask in CHF 3 /CF 4 plasma. The trenches in the SOI film were etched using the oxide mask and Cl 2 /He plasma. The etching selectivity between silicon dioxide and silicon is high and the process can be used to create narrow trenches with vertical walls in silicon. After dry etching the wafer was oxidized in dry ambient at 950 1C for 55 min. This process results in formation of a 30 nm thick oxide and ARTICLE IN PRESS Fig. 1. Simulation by Silvaco process simulator. (a) 100 nm wide and 180 nm thick Si channel on SOI substrate after oxidizing at 950 1C for 120 min. Prior to the oxidation the concentrations of phosphorus and boron were N 0 ¼ 1  10 17 cm À3 and P 0 ¼ 7  10 17 cm À3 in the Si channel, respectively (zero elsewhere). (b) Concentrations along the line AB. During the oxidation the phosphorus concentration has exceeded the boron concentration. Fig. 2. (a) SEM image of silicon nanodiode G3/21 with P 0 $7  10 17 cm À3 and N 0 $1  10 17 cm À3 . The trench is defined by e-beam lithography and plasma etching. The SEM image is taken prior to the oxidation, simulated in Fig. 1. The channel is 50 nm wide, 150 nm thick and 530 nm long. (b) Room temperature I–V curves of G3/21 and a similar device fabricated with N 0 $0. The channel dimensions of the devices are the same. The finite initial n-type doping N 0 $1  10 17 cm À3 enables the diode type of operation due to the segregation-induced compensation enhancement in the channel. The n-type doping has negligible effect on the source and the drain, which remain strongly p-type. J. Ahopelto et al. / Physica E 32 (2006) 547–549548 controls the tuning of the doping profile in the device. The final thickness of the SOI film is 150 nm, the width of the channel 50 nm and the width of the trenches 40 nm. I–V curves measured from two devices with similar dimensions but the other without compensating implanta- tion are shown in Fig. 2(b). The uncompensated device shows nonlinear behaviour but the I–V curve is symmetric and not diode-like. The compensated device, on the other hand, has the desired diode like-behaviour. Essential condition for the diode operation is that the voltage drop between the source and the drain occurs along the wire so that the effective potential difference builds up between the wire and the drain. This condition is fulfilled for the compensated device where the conduction type of the channel has changed from p-type to n-type, and conse- quently, the device has a self-aligned pnp doping profile from drain to channel to source. The quantitative doping concentrations along the channel cannot be extracted from the two-dimensional simulation, because the simulation does not include the diffusion of dopants from the source and drain into the narrow channel. However, qualitatively the behaviour is as expected. In summary, we have proposed and demonstrated a method for self-aligning control of doping profiles in nanoscale silicon structures. The method is based on the different segregation of p-type and n-type dopants during thermal oxidation of silicon. We have applied the method in fabrication of lateral field effect devices and show that the doping profiles can be controlled in devices with sub- 50 nm dimensions. In principle, the method can be applied in local tuning of dopin g in arbitrary-shaped silicon devices as long as the different parts of the device have different surface to volume ratios. The authors want to thank M. Markkanen for assisting in the device fabrication. This work has been partially funded by European Commission (NEAR IST-2001- 32300) and the Academy of Finland. References [1] L. Chang, Y K. Choi, D. Ha, P. Ranade, S. Xiong, J. Bokor, et al., Proc. IEEE 91 (2003) 1860. [2] H. Ishikuro, T. Fujii, T. Saraya, G. Hashiguchi, T. Hiramoto, T. Ikoma, Appl. Phys. Lett. 68 (1996) 3585. [3] Y. Takahashi, A. Fujiwara, K. Yamazaki, H. Namatsu, K. Kurihara, K. Murase, Appl. Phys. Lett. 76 (2000) 637. [4] G. Charitat, A. Martinez, J. Appl. Phys. 55 (1984) 2869. [5] www.silvaco.com [6] A.M. Song, M. Missous, P. Omling, A.R. Peaker, L. Samuelson, W. Seifert, Appl. Phys. Lett. 83 (2003) 1881. ARTICLE IN PRESS J. Ahopelto et al. / Physica E 32 (2006) 547–549 549 . 547–549 Self-aligned doping profiles in nanoscale silicon structures Jouni Ahopelto à , Mika Prunnila, Eeva Pursula VTT Information Technology, PO Box 1208, FIN-02044. VTT, Finland Available online 10 February 2006 Abstract We propose and demonstrate a method for self-aligning control of doping profiles in nanoscale silicon