Preheat oven to 250°F, and position oven rack in the center.
Rinse and thoroughly dry the sweet potatoes. Using a mandolin or very sharp knife, carefully slice potatoes into 1/8-inch thick pieces. (Be sure to go really thin to ensure a crispy chip when baked.)
Toss potato slices with olive oil, rosemary and salt. Arrange potatoes in a single layer on two baking sheets. Roast in oven until potatoes are crisp and golden brown, about 2 hours. Some potatoes may not be as crisp as others, so let crisps stand for 30 minutes before serving.