The requirements for highly specialized photosensitive materials for nanotechnology and Micro-Electro-Mechanical Systems (MEMS) applications are being driven by the rapid growth of consumer products incorporating these devices. These high volume consumer devices including accelerometers for air-bag sensors, biomedical sensors, optical switches and ink jet print heads. These applications all require ultra-thick photosensitive materials with highly controllable lithographic properties. For ink-jet print head applications, the lithography requirements include the formation of high aspect ratio structures with a negative (re-entrant) profile for nozzle formation. In order to form the required nozzle geometry for high resolution ink-jet printers, photosensitive materials need to be capable of providing up to 10 degree negative profiles at a film thickness of up to 25 microns. For consistent print dot size it is necessary to maintain excellent control and repeatability of the sidewall angle of the nozzle. Since this material remains on the substrate as a permanent part of the ink-jet print head, the mechanical and adhesive properties of the material are as important as the lithographic properties. This paper investigates modifications to an existing MicroChem epoxy-based SU8-4000 thick photoresist to generate highly re-entrant sidewall angles for next-generation high resolution ink-jet nozzle formation. Multiple versions of SU8-4000 with different levels of dye tuned for the exposure wavelength are exposed using a 1X lithography system optimized for thick photoresist processing. This stepper uses a combination of low numerical aperture, broadband exposure and large focus offsets for optimal processing of thick photosensitive materials. Basic photoresist characterization techniques in conjunction with cross sectional SEM analysis are used to establish lithographic capabilities for nozzle formation.
Polymers with high viscosity, like SU8 and BCB, play a dominant role in MEMS application. Their behavior in a well defined etching plasma environment in a RIE mode was investigated. The 40.68 MHz driven bottom electrode generates higher etch rates combined with much lower bias voltages by a factor of ten or a higher efficiency of the plasma with lower damaging of the probe material. The goal was to obtain a well-defined process for the removal and structuring of SU8 and BCB using fluorine/oxygen chemistry, defined using variables like electron density and collision rate. The plasma parameters are measured and varied using a production proven technology called SEERS (Self Excited Electron Resonance Spectroscopy). Depending on application and on Polymer several metals are possible (e.g., gold, aluminum). The characteristic of SU8 and BCB was examined in the case of patterning by dry etching in a CF<sub>4</sub>/O<sub>2</sub> chemistry. Etch profile and etch rate correlate surprisingly well with plasma parameters like electron density and electron collision rate, thus allowing to define to adjust etch structure in situ with the help of plasma parameters.
Repairing phase shift structures on phase shift masks (PSMs) presents formidable challenges. Requirements for PSM repair go far beyond those needed for conventional chrome masks, where pinholes are made opaque and pindots are removed from the surface with what are essentially two-dimensional processes. Defects in the phase material, such as inclusions, chips, or excess material must be repaired to leave optically correct, three-dimensional structures. Unavailability of equipment and methods to handle phase shift defects is one of the greatest barriers to the routine commercial acceptance of phase shift mask technology.
In this paper we will demonstrate multiple exposure (or "vote-taking") lithography methods for eliminating the effect of PSM defects on the wafer. Two (or more) exposures are made for a single wafer lithography level using multiple masks. The probability of a random defect occurring in the same location on separate masks is virtually nil. Therefore, dark defects - the consequence of defective phase structures - caused by any single exposure are overprinted by the other exposure(s).
This technique also can provide relief from other potential error sources unique to PSMs. Wafer CD uniformity can be improved. Dimensional variations caused by transmission differences between phase and non-phase features will be averaged out by reversing the phase sense among the masks. PSM layout flexibility is enhanced. Normally, abrupt phase transitions (between 0° and 180°) within a bright region will print a dark band. By eliminating this "defect" through complementary exposures, such phase transitions can be used to help meet the boundary conditions imposed by a PSM layout.