Can I create a disk in memory for backups?
Of course, you can take something like Starwinds RAM disk to create the local drive based on the RAM. I haven't heard such configuration but it should work. As for the future needs, I would recommend you to get another server for the backup purposes. https://www.starwindsoftware.com/high-performance-ram-disk-emulator
Yes, you could use tmpfs like this:
mkdir /mnt/rd
mount -t tmpfs -o size=20g tmpfs /mnt/rd