A new type of microscope (DDM) has been applied to submicron process development at the Fairchild Research Center. This high resolution video microscope produces an image which is the superposition of a dielectric discontinuity (phase contrast) and an absorptive optical image. With this instrument a Sparrow's resolution of 0.08 micrometers has been achieved at magnifications from 1150 times to 18,000 times. VIA and contact clearing have been observed from 0.1 micrometers to 1.4 micrometers at aspect ratios of up to 3:1. CD measurements have been made on both latent images and developed images and the results used to optimize the exposure energy for an I-line stepper. The DDM has also been used to visualize defects which are not visible with conventional microscopy. Both metallic and dielectric contaminant films have been detected and a submicron dielectric sidewall has been visualized on an advanced interconnect system. Material deposited during development using the MIMMI process has been observed. A simplified phase contrast transition theory is presented and applied to the observations.
Depth of focus is known to decrease with short wavelengths of light or high numerical aperture lenses. With the advent of short wavelength ultraviolet sources (DUV - 248 nm, 213 nm & 193 nm), and high numerical aperture lenses (NA > .40), the depth of focus for imaging sub-micron features in single layer photoresist becomes an area of concern. Surface imaging resist (SIR), or top surface imaging (TSI) has been proposed as the solution to the diminishing depth of focus for lithography of single layer photoresists. The most commonly known form this technology has taken is the silylation of organic photoresists following exposure to an appropriate illuminating source. This technology has drawbacks in that the resulting silylated resist is not too much different chemically from many of the inorganic substrates that one wishes to pattern. This limits its applicability in use with fluorine containing plasmas. It also creates a film that is difficult to remove after the pattern transfer is accomplished. A developmental type of surface imaging system is currently being evaluated for its applicability as a photolithographical resist. MIMMITM (micro imaging with metal mask integration) uses an ultrathin metallic layer over conventionally applied and exposed photoresist. The film is shallowly developed and metallization is electrolessly applied to the remaining pattern. The metal pattern is further dry developed to complete its structure for image transfer. The metallized pattern is impervious to all commonly used plasma etch environments and is easily stripped in dilute acid and common organic solvents.