Prepositional phrases are groups of words that start with a preposition and end with a noun or pronoun (the object of the preposition).
For example:
* in the house (preposition: in, object: house)
* on the table (preposition: on, object: table)
* with a smile (preposition: with, object: smile)
"Became", however, is a verb, which shows action or a state of being. It's not a preposition.
Let me know if you'd like to see examples of prepositional phrases in sentences.