Hi Rony,
You are dissipating 700 mW in order to convert 12 V to 5 V at 100 mA. We can use the Thermal information table for the DCY package to estimate that the junction temperature will be ~43.12 C higher than your ambient temperature (0.7 W x 61.6 C/W).
If you wish to reduce the power dissipated in the LM1117, you can generate an intermediate voltage rail between the 12 V and the LM1117 by using either DCDC switching regulator (most power efficient solution for the system) or another linear regulator. You could also place a power resistor in series before the LM1117 input capacitor. This resistor would reduce the input voltage local to the LM1117 by 100 mA x the resistance; however, you would want to make sure that the resistor is rated for the power that you would be dissipating in it.
Very Respectfully,
Ryan
