Using foil in the microwave to cover various parts of food to keep them from overcooking is also called "shielding".  It is important to follow a few simple rules when using shielding to practice properly and prevent damage to the microwave.

The amount of food should be substantially greater than the amount of foil used (80% food volume - no more than 20% foil volume.)  Keep foil smooth and away from the side of the microwave which can cause arcing