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Fig. 2 displays the process of fabrication of the novel mold insert. The sequence between structuring the micro-cavities and forming the electrically conducting micro lines (doping) can be exchanged to suit the design requirements of the mold insert.
Fig. 2. Fabrication processes of heat-generable mold insert
3. Doping characteristics and performance of silicon-based electrically conducting lines
The implantation energy and dose are the main parameters in the ion implantation process. Distribution of the concentration of implanted phosphorus ions in the direction of wafer thickness are controlled by adjusting these two parameters， which determines the characteristics of the silicon-based conducting lines. Fig. 3 shows the results of the secondary ion mass spectroscopy (SIMS) analysis of two cases of doping with different implantation energies. The maximum of the concentration of phosphorus ions shifts to a deeper part of the silicon wafer as the implantation energy is increased. Fig. 4 presents the effect of the implantation dose on the resistance of the conducting line. A greater implantation dose yields a lower resistance.
Since the working temperature of a mold insert in the injection molding of micro-structures generally varies cyclically between room temperature and 170 temperature， the property stabilities of the silicon- based conducting lines created by doping phosphorus ions demand attention. Fig. 5 plots the variations of resistance with time at constant temperature and pressure. The resistance of the silicon-based conducting line slightly increases with time at a given temperature， as plotted in Fig. 5(a), which effect can be neglected in the common injection molding of micro-structures with a cycle time that does not exceed one minute. Similarly, the resistance of the conducting line remains stable with time at a constant pressure (Fig. 5(b)). Fig. 6 reveals the effects of temperature and pressure on the resistance. The resistance of the silicon-based conducting line declines as the temperature rises (Fig. 6(a)), and his maximum variation of the resistance in the temperature range of a general injection molding process is about 30% of the initial value. In comparison, the effect of the pressure variation on the resistance is weak, as shown in Fig. 6(b). Fig. 7 plots the resistance of the silicon-based conducting line under cyclic variations of temperature and pressure. Evidently, the resistance characteristic of the conducting line does not change with the cyclic variation of temperature or pressure, which fact demonstrates that the novel mold insert proposed herein suffices for use in continuous injection molding.
In real injection molding, the conducting lines embedded in the silicon mold insert are charged with an initial voltage that slightly exceeds 50 V, which is the breakdown voltage of the silicon-based conducting line developed here. Then, the power supply is immediately shifted to the constant-current-control model (with a current of 1.6 A) to reach stable power when ‘breakdown’ occurs Fig. 8 plots the effect of the original resistance of a doped conducting line on the magnitude of the stable power at breakdown. The achievable power falls as the original resistance increases. In the design of the heat-generable mold insert presented herein, the data plotted in Figs. 8 and 4 are important for determining the number and the layout of the silicon-based conducting lines in the mold insert, as well as the conditions for ion implantation.
Fig. 3. Distribution of concentration of phosphorus ions under the surface of the doped silicon wafer in the depth direction, obtained by SIMS analysis.
Fig. 4. Effects of implanted dose on the resistance of the silicon-based conductingline.
Fig. 5. Variations of the resistance of the silicon-based conducting line with time at(a) 170 °C， 0 MPa (b) 20 °C， 12.5 MPa.
Fig. 6. Effects of (a) temperature, and (b) pressure on the resistance of the silicon-basedconducting line.