TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The polyethylene terephthalate resin includes first polyethylene terephthalate resin having intrinsic viscosity of 0.55-0.65 dl/g and second polyethylene terephthalate resin having intrinsic viscosity of 0.75-0.85 dl/g. The master batch chip comprises 70-95 wt.% first polyethylene terephthalate resin and 5-30 wt.% second polyethylene terephthalate resin. The inherent viscosity of the solid state polymerized master batch chip is 0.7-0.8 dl/g. The total thickness of the biaxially stretched film is 50-300 mu m. The polyester film has intrinsic viscosity of 0.65-0.75 dl/g; deviation ( Delta SD) of MD (machine direction) elongation along the width direction before PCT (pressure cooker test) treatment satisfies the following formula: Delta SD is equal to 0-50; heat shrinkage ( Delta HS) along MD direction after processing at 150 degrees C for 30 minutes satisfies the following formula: Delta HS is equal to 0-2%; where average MD elongation is equal to average value based on the cutting elongation of 10 samples; S2 is equal to longitudinal length of the film measured at 150 degrees C for 30 minutes, S1 is equal longitudinal length of the treated film; delta HS is equal to (HS2-HS)1)/HS1x 100; elongation retention rate (S) and deviation ( Delta SDFPCT) of elongation retention rate processing at 121 degrees C for 75 hours under RH 100% satisfies the following formula: S is equal to 60-90% and Delta SDFPCT is equal to 0-50; where S is equal to S2/S1x 100, S is equal to elongation rate of MD after PCT treatment, S2 is equal to elongation rate of MD direction measured after processing at 121 degrees C for 75 hours under RH 100%, and S1 is equal to elongation of MD direction measured at 121 degrees C before processing; Delta (SDFPCT) is equal to value calculated according to Delta (SDFPCT) equal to square root of (summation(average MD elongation retention rate-calculated MD elongation retention rate)2/10) using MD elongation retention rate and its average. The sheet further comprises a metal deposition layer arranged on one or both sides of the polyester film. Preferred Method: In the step (a), mixing is carried out at 240-300 degrees C under a rotational speed of 150-250 revolutions/minute. The first polyethylene terephthalate resin and the second polyethylene terephthalate resin are introduced into the twin-screw in powder form by pulverizing. In the step (c), melt extrusion is carried out at 295-330 degrees C. EXAMPLE - Solid state polymerized master batch chip was melted at 290 degrees C, extruded at 22 degrees C to obtain undrawn sheet, and stretched 3 times along MD direction at 88 degrees C. Then cooled at 22 degrees C, heat treated at 225 degrees C by stretching 3.7 times along the transverse direction at 130 degrees C, and relaxed at 200 degrees C to obtain the polyester film having a thickness of 188 mu m.