How to save premultiplied alpha photoshop

How can i save textures with photoshop to use in unity with premultiplied shader?

This is not working properly:

  1. Open image.
  2. ctrl+click on layer and make mask
  3. make black background
  4. merge image and background
  5. delete background with mask created in 2

What im doing wrong?

Texture packer is saving alpha in some other way which working good.

  1. Go to channel
  2. Create new channel (alpha)
  3. ctrl+Click your main layer
  4. go to channel
  5. select alpha channel
  6. ctrl+del with white color
  7. create black background layer
  8. save as psd
  9. in unity use premultiplied alpha shader and uncheck “alpha is transparency”