Simon Johnson

Residual charge on the faulty floating gate CMOS transistor

A common defect that can occur in CMOS integrated circuits is a break in a signal track. The effect of this defect is strongly dependent on the amount of charge trapped on the isolated MOS transistor gate. Results of measurements on test structures are presented which reveal a range of values for the trapped charge. This causes the resulting fault to have a widely varying effect on circuit performance. A test strategy which guarantees detection of such faults is proposed.

Nonplanar photolithography with computer-generated holograms.

We outline a method for accomplishing photolithography on grossly nonplanar substrates. First we compute an approximation of the diffraction pattern that will produce the desired light-intensity distribution on the substrate to be patterned. This pattern is then digitized and converted into a format suitable for manufacture by a direct-write method. The resultant computer-generated hologram mask is then used in a custom alignment tool to expose the photoresist-coated substrate. The technique has many potential applications in the packaging of microelectronics and microelectromechanical systems.

Photolithographic patterning of bihelical tracks onto conical substrates

We demonstrate the direct photolithographic patterning of a grossly nonplanar substrate by creating 62-m helical tracks on a 22- mm-high cone. The projection of focused light onto the 3-D surface is achieved using a computer-generated hologram CGH suitably illumi- nated so as to create the required pattern on the photoresist-coated surface. The approach adopted forms the basis of a novel method for patterning nonplanar structures. We address the key challenges encoun- tered for the implementation of holographic photolithography in three dimensions, including mask design and manufacture, exposure compen- sation, mask alignment, and chemical processing. Control of linewidth and resolution over the nonplanar surface is critical. We describe the methods adopted and critically assess the structures created by this pro- cess. The bihelical cone is representative of a broadband, high- frequency coil-like structure, known in wireless communications as a log- periodic antenna.

Non-Planar Photolithography Using Digital Holograms

A photolithographic process using digital holograms enables the fine-pitch patterning of grossly non-planar substrates, with many potential applications in microelectronics packaging. Considerations of hologram design, fabrication and verification particular to the lithographic process are presented.