To pattern a semiconductor wafer to form integrated circuits thereon, it is generally necessary to use photolithographic techniques. Photolithography requires the use of a light source and a photoresist that is sensitive to the light source. In conventional positive photolithographic patterning, a photoresist is applied to the semiconductor surface and then exposed to a light source through a patterned reticle. The reticle allows light from the light source to strike the photoresist only where the pattern is to be formed, thus exposing the resist corresponding to the pattern. The wafer is then immersed in an organic-based wash to develop and remove the exposed resist.
As integrated circuits have become more complex, they have also become smaller. To reproduce patterns that are smaller, optics having higher resolution are required. As resolution becomes more defined, there is a corresponding loss in depth of focus. Thus, a positive pattern process reaches a point where the photoresist is too thick to be fully developed all the way through to the base layer, leaving an incomplete pattern for further processing.
To overcome the problem of a loss of depth of focus for a gain of resolution, negative resist processes have been devised. One such negative resist method is diffusion-enhanced silylating resist (DESIRE). The DESIRE process uses a photoactive compound (PAC) which is mixed into the photoresist to form barriers to a reactive ion etch. The sections of the photoresist which are covered through the DESIRE process act as an etch stop, while the nonprotected areas may be etched to allow further processing.
The DESIRE process is advantageous because it does not suffer from the limitations of photo-optics. The DESIRE process does not require exposure of the photoresist beyond a depth of a few thousand Angstroms, therefore, high resolution optics with a low depth of focus produce acceptable patterns. However, the DESIRE process is a negative resist process, and is therefore subject to the limitations of a negative process. Such limitations include sensitivity to dirt particles on the reticle and the necessity to expose large surface areas. Thus, there is a need for a method and apparatus to provide a positive resist technique that does not suffer from limitations of photo-optics.