A new CMOS voltage reference circuit consisting of two pairs of transistors is presented. One pair exhibits a threshold voltage difference with a negative temperature coefficient (0.49 mV/ C), while the other exhibits a positive temperature coefficient (+0.17 mV/ C). The circuit was robust to process variations and exhibited excellent temperature independence and stable output voltage. Aside from conductivity type and impurity concentrations of gate electrodes, transistors in the pairs were identical, meaning that the system was robust with respect to process fluctuations. Measurements of the voltage reference circuit without trimming adjustments revealed that it had excellent output voltage reproducibility of within 2%, low temperature coefficient of less than 80 ppm/ C, and low current consumption of 0.6 A.

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IntroductionShallow source/drain junctions are one of the key requirements for subquarter micron gate length p-channel MOSFETs without short channel effects. Fabricating shallow p+-n junctions by low energy BF2 implantation. especially using furnace annealing. requires preamorphous implantation of crystalline silicon to eliminate the boron channeling and the suppression of diffusion in heat cycles .In this paper we present a new preimplantation technique using fluorine to influence the electric characteristics of p-channel MOSFETs. Using heat cycling of 850oC for 10 min. we achieved a junction depth of 80 nm and sheet resistance of 400 Q. The boron profile after annealing is close to a bell shape, like the profile obtained by boron only implantation.Usual preamorphous implantation of Si and Ge do not use high energy implantation. Though channeling does not occur, the boron diffusion tail reaches the interstitial rich region generated by preamorphous implantation and this makes the junction deeper since boron diffusion is primarily caused by interstitials. High energy im lantation can suppress boron diffusion in the tail region (1)b). In this case, the diffusion tail remains in the vacancy rich region and the junction should be shallower than with low energy preamorphous implantation. Fluorine reduces the amount of interstitials contributing to diffusion since the fluorine keeps small clusters from thermally dissoluting (3). Using fluorine for preimplantation could lower the implantation energy because it would prevent diffusion in the tail region. Furthermore, lowering preimplantation energy increases the excess vacancy density in the high boron concentration region and increases diffusion there. Experimenton an n-type. , silicon substrate. The p+-n junctions are formed by fluorine preimplantation from 25 keV to 40 keV at a dose of 2x1015 cm-2 to 4x1015 cm-2 followed by BF2 implantation at 10 keV and 1015 cm-2. Furnace annealing is used for junction activation because it is easier to control temperature and time than in FtTA. Measurements and diffusion modelFigure 1 shows boron profiles for as-implanted samples with and without fluorine preimplantation at 25 keV and a dose of 4x1015 cm-2. There is no channeling tail in the fluorine preimplanted samples. Boron profiles after fumace annealing at 85@C for 10 min. are shown in Fig. 2. The junction depth of the fluorine preimplanted samples is less than 80 nm at a substrate concentration of 1017 cm-3 .Diffusion of high concentrations is enhanced while the diffusion of low concentrations is suppressed. Figure 3 shows the boron diffusion model for fluorine preimplantation. At high energies boron dausion should be the same as that for other atoms, or diffusion is caused by vacancies in the vacancy rich region. Decreasing the implantation energy of fluorine makes the vacancy rich region shallower and causes the interstitial rich region to reach the diffusion tail. Generation of interstitials contributing to the boron diffusion is suppressed by fluorine. Therefore, on...

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