In many cases, wet or dry-etch processes can be used to transfer a pattern via a subtractive technique into an underlying dielectric or metal layer. However, in many other cases, this is better achieved by using a process known as metal lift off; this latter process is an additive technique. The choice to use Metal Lift-Off (MLO) may be driven due to sensitivity of the substrate to etch processes, or potentially also as a cost control. In the MLO process, a resist pattern is imaged on the target substrate via conventional lithography. This imaging step may take place in a bilayer-like (BL) process wherein the resist lies atop a fast-developing poly-imide layer. Alternately, it may take place in a single layer-like (SL) process wherein the resist alone lies atop the substrate. After imaging the resist, it is desirable to have a post-develop resist in order to have re-entrant profiles in the SL case or a t-top in the BL case. A metal layer is then deposited on the imaged resist layer. The undercut profiles enhance the ability of the resist (and its poly-imide under-layer, if present) to lift off during the subsequent step. In this last step, an organic solvent capable of dissolving the resist is used the “lift” the resist as well as the metal on top, leaving behind only deposited metal from those locations where no resist had been applied prior to the metal deposition step.
There are several key features of this process, chiefly that the MLO process is less expensive than the dry etch process as much less capital investment is needed. The resultant wall angles are much more vertical than that in the wet etch process, and the SL approach is even cheaper in both materials and wafer throughput than the BL approach. In order to make either the SL or BL approach work, it is necessary to use negative tone photoresists as the positive tone resists have too low a set of thermal properties, and will lose their pattern definition during the metal deposition step. However, as a result the negative tone resists being cross-linked have much better thermal properties and will better withstand the heat of metal deposition. Another great feature of negative tone resists is that unlike positive tone resists, they have the needed re-entrant profiles for defect-free metal lift-off. Failing to have sufficiently re-entrant profiles will result in certain types of metal defects that come from the positively-sloped profiles of positive tone resists.
As we have seen, metal lift-off has many advantages, however this process also has drawbacks .The difficulties associated with MLO processes stem from the way in which metal deposits onto the photoresist. For example, metal can deposit so tightly over the resist that during the lift off step, solvent cannot reach the resist to dissolve it, resulting in undesired metal residue. Another problem is a type of defect sometimes referred to as “ears.” In this scenario, the metal deposits to the side of the resist walls and can remain after the lift off step; this metal will project upward, roughly resembling the shape of an ear. If it falls over in any subsequent step, highly undesired electrical shorts may result within the circuit, rendering it unusable. The best prevention for this occurrence is to employ a resist with highly re-entrant profiles.