OTS (July 2006)-- Nitride etch using the MAGELLAN^® immersion cleaning system has demonstrated unrivaled selectivity and uniformity. In selectivity tests, the system has repeatedly removed up to 3,500Å of nitride with no measurable oxide loss. Uniformity within wafers averages 1.7 percent at one sigma for 300 mm wafers.

Added particles per run is nearly zero for 90nm and greater particles. This excellent performance can be attributed to a number of design features of the MAGELLAN system.

Selectivity
It is well known that selectivity for nitride over oxide is a function of the dissolved silica levels in the bath. Higher levels provide greater selectivity. Historically, higher silica levels have also been associated with higher particle levels. The user was therefore faced with a trade-off between selectivity and particle counts. The MAGELLAN system utilizes an advanced high-flow filtration system that maintains low particulate contamination even at high silicon concentrations (figure 1). The MAGELLAN system’s surface tension gradient (STG®) rinse and dry technology also contributes to its low particulate performance.

Figure 1. Standard hot phosphoric process requires tradeoff between achieving high selectivity or low particles. Advanced tool design provides a process to etch nitride with no measurable oxide etch and very low particle adders.

Uniformity
The system’s excellent within-wafer uniformity can be attributed to thermal uniformity within the bath, and rinse-tank technology that allows rapid termination of the etch process. Both are a function of extensive design engineering which concentrated on the chemical flow and circulation within the bath. Feed and bleed exchange of the bath fluids allows rapid, precisely-controlled refreshing of the bath chemistry between runs and reduces the need for complete path changeouts. High-flow spray and quick dump rinsing hardware in the rinse tank provide rapid termination of the etch process. These features and capabilities combine to provide excellent control, repeatability and uniformity (figure 2).

Figure 2: Advanced tool design provides excellent etch control, repeatability, and uniformity.

Run-to-Run Control
Consistency between runs is determined primarily by careful temperature control. MAGELLAN systems use a closed-loop control system based on manipulating the boiling point of the bath solution. The boiling point depends on the concentration of solutes in the solution. A thermocouple monitors the temperature of the heated bath. The natural tendency of the boiling point is to increase slowly as water evaporates and the solution becomes more concentrated. As the temperature rises, the control system dilutes the bath with de-ionized water until the boiling temperature (and bath temperature) returns to the set point.