Studying Mixtures of the Novel Gases and Photoresist Masked Silicon-Based Dielectric Etching

The July issue of PROCESSNEWS published a report from the Cornell Nanoscale Facility (CNF) related to a photoresist masked silicon-based dielectric etching comparing unique gas chemistries in a modified Plasmalab System 100 ICP380 in the CNF cleanroom.

This system features a gas ring around the wafer electrode (Figure 1) plus a new gas pod with 12 gas lines including 6 fluorocarbon gases that can be directed to the top of the ICP source or to the gas ring. Figure 2 shows the PLC and software that have been updated for gas control.

Gas ring around electrode

Figure 1. Gas ring around electrode

Software page for gas control

Figure 2. Software page for gas control

Fluorocarbon Gases

Although gas rings have been utilized in ICP systems for methane (CMT, InP, etching etc.) and silane (ICP-CVD), this is the first time where fluorocarbon gases have been introduced via a gas ring for dielectric etching. The method is usually preferred as overcracking of the fluorocarbon is brought down by lower influence of the inductively coupled power. Overcracking reduces the amount of polymer and increases the free fluorine, resulting in lower selectivity.

So far at OIPT, silicon dioxide (SiO,) materials have been etched through mixtures based on CHF3 or C4F8 gases. The other aspect of the collaborative work with CNF was to examine new gases such as octafluorocyclopentene (C5F8), hexafluorobutadiene (C4F6), and difluoromethane (CH2F2). These gases offer a high C:F ratio and hence are preferred for the formation of polymer in plasma for selective SiO2 etching.

Moreover, in the case of (C4F6), and C5F8 gases, unsaturated carbon bonds in the molecules offer a different polymer chemistry which is assumed to be favourable to dielectric etching.

Another benefit is that the unsaturation leads to low global warming potentials or GWPs (shown in the table below) since saturated CH,F, has lower GWPs than CHF3 or C4F8. However, these gases have certain disadvantages and they include flammability, mild toxicity, and higher cost when compared to CHF or C4F8. In the case of C5F8, the cost is exorbitant.

Table 1. Global warming potentials for fluorocarbon gases compared to CO2.

Gas GWP
CO2 1
CHF3 9100
C4F3 6000
CH2F2 2100
C4F6 0.1

Helium Dilution

Vincent Genova from CNF has analyzed mixtures of the new gases with O2. Part of the cooperative deal with CNF is, when appropriate an annual visit will be made by Colin Welch, Principal Etch Applications Engineer at Oxford Instruments, to do some process development.

During the 2013 visit, excellent results were reported for thermal SiO2 etching through the use of gas ring and high helium dilution of the new gases. Helium dilution is a feasible option as it can provide a cleaner process and also achieve steep profiles and high selectivities. Tapered profiles and poor cleanliness were the main issues of selective SiO2 etching. Additionally, the high dilution reduces consumption cost of the new gas.

TEOS Delivery Module

Oxford Instruments’ TEOS delivery module enables TEOS-based PECVD in PlasmaPro 133 and PlasmaPro 100 process tools. The TEOS module has applications in intermetal dielectrics for narrow pitch metal wiring designs. Key benefits include high quality, conformal deposition of SiO2 dielectric layers, photonics, and other structures; control of deposition directionality and hence degree of step coverage by controlling oxygen radicals; and control of film stress by pulsed high/low frequency power mixing.

Oxford Instruments also offers a range of system upgrades that add new functionality and also extend the life of systems.

Conclusion

The mixtures of novel gases provide a high C:F ratio and hence are preferred for polymer formation in plasma, which is important for selective SiO2 etching.

This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.

For more information on this source, please visit Oxford Instruments Plasma Technology.

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