Iqbal Saraf

Mask undercut in deep silicon etch

Mask undercut in the time-multiplexed deep silicon etch process is becoming an increasingly significant issue as it is used to produce smaller critical dimension features. Models of the process must contain the necessary physics to reproduce the dependencies of mask undercut. We argue that the reason undercut develops is the dependence of the deposition step on ion flux. Our experiments of C4F8 (and CHF3 not shown) plasmas show that the film growth is dominantly ion-enhanced. This leads naturally to a mask undercut that increases in time. A more neutral flux dominant deposition step would result in reduced mask undercut.

Kinetics of the deposition step in time multiplexed deep silicon etches

The time multiplexed deep silicon etch (TMDSE) process is the etch process of choice to make MEMS devices and through wafer vias. It has been used to produce deep trenches and vias at reasonable throughputs. Significant issues remain for the TMDSE process as well as room for improvement even though it has been both experimentally studied and modeled by a wide variety of researchers. This is because it is a highly complex process. Aspect ratio dependencies, selectivity, and the ability to use photoresist masks (instead of SiO2) are examples of remaining issues. The presently obtainable etch rates do not indicate efficient use of the etchant species. In this article, the authors focus on the deposition step in the TMDSE process. While prior research has generally assumed that the deposition step can be adequately modeled as being controlled by a reactive sticking coefficient, they have experimentally examined the deposition step of the process and found that the film growth is dominantly ion-enhanced. The r...

The direct injection of liquid droplets into low pressure plasmas

A much greater number of useful precursors for plasma-enhanced chemical vapor deposition (PECVD) can be dispersed in high vapor pressure solvents than can be put into the vapor phase directly. In order to enable the use of such precursors, the authors investigated a method by which one can directly inject these liquids as microdroplets into low pressure PECVD environments. The solvent evaporates first leaving behind the desired precursor in the gas/plasma. The plasma dissociates the vapor and causes the deposition of a composite film (from precursor, solvent, and plasma gas). The authors made preliminary tests using Fe nanoparticles in hexane and were able to incorporate over 4% Fe in the resulting thin films. In addition, the authors simulated the process. The time required for a droplet to fully evaporate is a function of the background pressure, initial liquid temperature, droplet-vapor interactions, and initial droplet size. A typical evaporation time for a 50μm diameter droplet of hexane is ∼3s witho...

Progress report: Direct injection of liquids into low-pressure plasmas

While laboratory based plasmas are always in contact with solid surfaces (often vacuum chambers) they have historically been formed in gas environments. In more recent times, the use of plasmas has grown to include plasma contact with liquids including biological items. Inevitably the plasmas in contact with liquids had been at or near atmospheric pressures. This need not be the case. We have developed a novel method for injecting liquids directly into low-pressure discharges. As such, this technique opens new areas of possible industrial use for plasmas. For example, we have injected inorganic nano-particles into argon plasma by suspending them in hexane (or ethanol) as a high vapor pressure liquid carrier. As a result, we believe that metals, dielectrics, superconductors, aromatics, proteins, viruses, etc. could all potentially be injected into low-pressure plasma environments using this technique. The resulting films indicate the ability to synthesize nano-structured composites. Here we examine some of the basic phenomenon that are observed both experimentally and theoretically.