Pre-Diffusion Cleans

Pre-diffusion cleans are critical steps within the manufacturing process. They occur several times within the manufacturing sequence and thus a significant amount of equipment capacity (throughput) is required for these cleans. They are performed immediately prior to wafers being introduced into a diffusion furnace. Diffusion creates the doped well areas of a gate structure (see Figure 1). A pre-deposition or implant step is performed to first deposit dopants into the substrate material. These dopants, generally phosphorous or boron, accumulate at the wafer surface. The wafer surface is then cleaned in the pre-diffusion process to remove any contaminants, particularly particles and metals. It is critical to prepare a very clean surface at this point because the wafers next undergo a diffusion step. In this step, a furnace is used to redistribute the dopants within the wafer and properly diffuse them into the substrate material. Any surface contaminants are likely to be driven into the wafer as well, causing unpredictable electrical properties within wafer devices.

There are many instances where photoresist patterns remain on wafers during pre-deposition and implant. In these cases the pre-diffusion clean requires a significant amount of organic removal. At times, etch uniformity is also critical for exposed areas.

Historically, pre-diffusion cleans referred strictly to the process performed immediately before the diffusion process. More recently, pre-diffusion cleans have referred generically to any process preceding a furnace operation. In either case the same concerns exist. Particle and metallic contamination must be minimized and etch uniformity optimized while maintaining high throughput.

CFM Technologies, Inc., 150 Oaklands Blvd., Exton, PA 19341. Tel: 610-280-8300; Fax: 610-280-8309.