Are all lift-off resist negative photoresists?
There are definitely positive photo resists, but there tends to be little incentive to use them. Usually you use a negative mask with a negative resist to create a positive part. The pattern to be made tends to be a relatively small area of the photo resist on the mask and thus it takes less time for the lithography machine to produce a negative mask which can be a bottle neck in an R&amp;D setting. Also Positive resists tend to be perminate as they have chemically inter-reacted to stay in place not be dissolved later (like during lift-off), thus another reason positive resists are not used frequently.