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Activation of Silicon Implanted with Phosphorus Atoms by Heat Treatment at Low Temperature

We report activation of silicon implanted with phosphorus atoms by heat treatment at 350C. The sheet resistance was reduced to 950 Ω/sq heat treatment for 24 h for samples implanted at 10 keV with a concentration of 5×10 cm. Compared with the sheet resistance obtained by laser annealing, ratios of conductance increase were estimated to be 0.21. It was close to a recrystallization ratio of 0.20 obtained by analysis of optical reflectivity spectra. In contrast, the sheet resistance decreased only to 2200 Ω/sq for samples implanted at 70 keV with 5×10 cm. Ratios of conductance increase were 0.06. It was lower than that in the case of 10-keV implantation. On the other hand, the recrystallization ratio for samples implanted at 70 keV was 0.25, which was comparable to that in the case of 10-keV implantation. Introduction Activation of silicon implanted with phosphorus atoms is an essential process technology for fabrication of field effect transistors,bipolar transistors and thin film transistor (TFT). Furnace and flash lamp heating have been used for the purpose. In general, a high heating temperature above 800C has been used for activation and recrystallization of the disordered region caused by ion implantation [1]. On the other hand, a processing temperature lower than 400C has been required in order to fabricate TFTs over a large glass substrates with a low heat proof. A simple furnace heating lower than 400C has been already applied for TFT fabrication. It has been also believed that the activation ratio is low about 2% and the recrystallization ratio is also low in the case of the low temperature heat treatment. On the other hand we have reported that the activation ratio depends on a condition of ion implantation, and that there can be a condition for achieving an activation ratio much higher than 2% in the case of 300C-heat treatment of polycrystalline silicon [2]. In this paper, we report behaviors of activation and recrystallization of silicon implanted with phosphorus atoms by heat treatment at different temperature in the case of phosphorus ion implantation to the single crystalline silicon. We especially concentrate investigation of heat treatment of the samples at 350C. We discuss the ratios of activation and recrystallization as functions of phosphorus energy and its concentration. Experimental The ion implantation of phosphorus atoms was conducted for p-type silicon substrates with a resistivity of 3~10 Ω cm. Acceleration energies were 10 and 70 keV. Concentrations were 5x10, 1x10, and 2x10 cm. The samples implanted at 10 and 70 keV with 1x10-cm phosphorus atoms were heated at 300, 400, 500, 600 and 800C for 1h by a furnace. Heat treatment was also carried out at 350C from 15min to 24h for samples implanted at 10 and 70 keV with concentrations of 5x10, 1x10 and 2x10 cm. The sheet resistance was measured using a four-point-probe measurement system. Optical reflectivity spectra were measured between 250 and 1100 nm by a conventional spectrometer. The optical reflectivity spectra were analyzed by a numerical calculation program, which was constructed with the optical interference effect for a structure of air/seven Si layers/Si-substrate [3]. The optical reflectivity at the sample surface depends on the complex refractive indexes of Si. Using the effective dielectric model, the complex refractive index f n with a crystalline volume ratio, X, was determined by combination of the crystalline refractive index c n with amorphous refractive index a n [4] as, ( ) 1 f c a n X n X n = + − (1). The values of the parameters of thickness and the crystalline volume ratio were changed for each seven doped layer for calculation of the reflectivity. Most possible in-depth distribution of the crystalline volume ratio was obtained by fitting calculated reflectivity spectra to the experimental ones. Results and discussion Figure 1 shows the sheet resistance as a function of the heating temperature of samples implanted at 10 and 70 keV with 1×10cm phosphorus atoms. The figure 1 also represents the sheet resistance for samples as implanted and laser annealed using 940-nm semiconductor laser at 70 kW/cm for 2.6 ms by arrows. We believe the samples were completely activated by the semiconductor laser annealing [5]. The sheet resistance decreased as the heating temperature increased for samples implanted at 10 and 70 keV. It decreased to 135 Ω/sq at a heating temperature above 500C, which was the same of that annealed by the semiconductor laser for samples implanted at 10 keV. On the other hand, it decreased to 80 Ω/sq at a heating temperature of 800C, which was higher than 70 Ω/sq obtained by semiconductor laser annealing for samples implanted at 70 keV. The high sheet resistance of 80 Ω/sq suggests that the activation ratio was lower than 1 for samples implanted at 70 keV even in the case of 800C-heat treatment. We define the ratio of conductance increase C by sheet resistance obtained by laser annealing Rlaser divided by that obtained by heat treatment with a furnace R as, C = Rlaser/R (2). Figure 2 shows the C value as a function of the heating temperature for samples implanted at 10 Heating temperature [°C] 107 106 105 104 103 102 10 S he et R es is ta nc e [Ω /s q] 300 500 400 700 600 800 10kev 70keV 70keV 10keV