To avoid over doing it in the freezer, I often use ice packs (meant for keeping picnic food cold or for treating bruises or aches). I set the ice pack under a saucer or jar lid, add the ant, and then cover it with another jar lid (in my case, one with a clear lens). I watch the insect to see when it stills, then remove the lid and start taking pictures.
When I’m done with my pics, I turn the ant out onto the patio, where it revives in a minute or two, and resumes it’s business.