The knee is most stable in two positions, just short of fully extended (not quite locked out) an fully flexed. As long as your technique is good, full squats are actually safer than changing direction of travel from a partially flexed position.
Here's a trick from the power gyms if you are having problems coming back up from a full squat. Reduce the weight and add a length of heavy chain to each side of the bar. The chain should just clear the floor when you are in the standing position. As you squat, the chain will coil on the floor reducing the weight slightly at the bottom and increasing it as you push upward. As you improve, wrap the chain around the bar one time so a bit less of the chain coils on the floor. Repeat wrapping the chain one more revolution around the bar every few workouts until the chain doesn't touch the floor even at the very bottom. Great way to even out quads and glutes that are strong at the top but weaker at the bottom of the squat.