Imager Helps Standardize Process Transfers

Ruth A. Mendonsa

In the advanced materials processing arena, the use of low-pressure gas discharges is on the increase in applications such as the creation of submicron microchip architectures and thin diamond film deposition. Plasma conditions and processes are sensitive to the size and shape of an apparatus and so difficulties arise from process transfer between systems -- even similar systems.

Comparing results between different labs can present a problem for researchers, making testing theoretical models difficult. Oriel Instruments' Insta-Spec V intensified charge-coupled device (ICCD) is on the job helping to overcome the difficulties.

Standard plasma

The process transfer problems led to "standard plasma," a concept developed at US Gaseous Electronics Conferences. A parallel-plate, radio frequency reactor design, now known as the GEC reference reactor, provides discharges analogous to those used in plasma processing for microelectronics and other applications.

One such reference reactor is at Queen's University in Belfast, Northern Ireland, where a group using hydrogen as the carrier gas is studying plasma behavior close to the driving electrodes, the so-called sheath region where energy is coupled into the plasma. Researchers accelerate electrons toward the plasma in the electric fields of the sheath. The electrons can then reach energies high enough to excite gas atoms to produce regions of bright emission generally associated with the sheath edge. Optical excitation studies derived from emission measurements can provide information about the electric field structure and sheath dynamics.

The standard driving frequency of the electrode voltage is 13.56 MHz, a cycle time of 73 ns. To time-resolve the light emission during the cycle, a vertical 30-mm-wide strip of the central plasma was imaged onto an InstaSpec V fast-gated (2.5-ns) ICCD. The InstaSpec's speed, sensitivity, accuracy, size and ease of use made it ideal for this application.

Filters selected emission from specific transitions of the excited gas atoms. Spatial dependence of the Balmer-alpha emission (656.3) from the hydrogen plasma at 20- and 40-ns time intervals during the radio frequency cycle were studied. An obvious difference in the spatial emission at different times during the cycle resulted, indicating that electron heating is taking place as the sheath expands and contracts.

This is the type of information that helps test theoretical models, thus improving the use of plasmas in industry. Reference cells have been operating for several years at US institutions and universities.