Abstract
The main focus of this thesis is to integrate the laser interference lithography (LIL) process for our systems with reactive ions etch (RIE) to fabricate silicon molds. We cannot just pattern photoresist on silicon and then etch. Reflections from the silicon surface back into the resist will degrade the exposed pattern, and the thin photoresist pattern will not work as an etch mask for the deep silicon etch. Therefore we must apply additional layers of material (a thin film “stack”) to reduce reflections and to act as a secondary etch hard mask. We have selected the following materials for the stack. They are photoresist on BARC (Bottom Antireflection Coating), on Silicon Nitride (LPCVD furnace deposition) on Silicon. There are three major concerns that should be considered when optimizing the stacks. First, the stack must minimize reflectivity at wavelength of 325nm incident at about 45 degrees. Second, we need to make the stack “etch-friendly”. Any BARC etch will attack the photoresist – both the height, and from the sides. We would like to minimize the BARC thickness to minimize the length of this etch step. Second, we need to have the flexibility to choose a nitride hard mask thickness thick enough to allow for deep silicon etches (~400nm or greater). Third, the low reflectivity property of the film must be almost independent of the nitride thickness, so that the process will tolerate unwanted variations in the nitride thickness. We have found that the LPCVD Silicon Nitride deposition process is not repeatable; each batch comes back with a different thickness. The reflectivity must be insensitive to these variations so that the LIL exposures are high quality even when the nitride thickness is varying. The optimization shows that all three concerns are satisfied simultaneously by choosing a BARC thickness of 90 to 110nm, and a Silicon Nitride thickness of 50 to 80 nm. Since spin coating is a reliable method to carefully control thickness, the narrow thickness window (+/-10%) window of the BARC is easily achieved. The wide thickness window of the nitride, over +/-20% is smaller than the actual variations we have seen in the Silicon Nitride depositions we have observed, so it is also sufficient. A flexible process can give more parameters for designing wire grid polarizer (WGP).WGP is a polarizer with period structures. Controlling the linewidth in lithography process by changing the dose can be achieved. When the pitch is fixed in 230nm, we can control duty cycle from 0.3~0.48, in other words, the linewidth can vary from 76nm to 115 nm. Final goal is to make silicon mold for Nanoimprinting by using RIE process, this thesis successfully fabricate silicon molds with 350nm depth, pitch is 230nm and 90nm linewidth by using CHF3 and Cl2.