Oxygen-based plasma has been traditionally used for ashing patterned photoresist on low dielectric SiCOH (low-k) thin films. During the ashing process, energetic plasma species remove carbon (or CH3 group) from the exposed regions of the low-k film causing an increase in the dielectric constant of the film. The modified low-k film is also susceptible to water absorption which leads to higher dielectric constant and a degraded performance of the patterned device structure (such as collapsing and loss of critical dimensions) [1,2]. Moreover, higher porosity films will be required in future to achieve even lower dielectric constant and these films will be even more susceptible to plasma damage. Therefore, there was a need for new ashing chemistries to reduce the level of damage to the porous low-k film. A systematic study was performed with mixtures of O2 and selected molecules such as the standard CO2, CH4 or newly thought ashing chemistries. The ashing performance of the selected gas mixtures was compared with pure O2 for a similar thickness of resist removal. In order to highlight the chemistry effects of gas mixtures, the ashing was performed in similar conditions (i.e. RF power, temperature and pressure) as used for pure O2using a CCP RIE chamber.

 

A parametric study completed with pure O2 indicates that an increase in RF power and pressure generally leads to an increase in low-k damage. However, flow rate change with constant pressure did not show significant modification in damage characteristics. Damage to low-k film is improved by using gas mixture of the new chemistries and O­2. The Hg-probe dielectric measurements reveal a least increase in dielectric constant with this mixture. Dilute Hydrofluoric (HF) acid tests also reveal the higher etch resistance of low-k films ashed with this mixture. Auger/XPS depth profile metrology is used to obtain elemental profiles of the damaged low-k films. In addition, residual gas analyzer data is being reviewed to understand better the etch gas chemistry to correlate to the damaging behavior of selected chemistries.

 

 

 

 

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