CAIBE (Oxford Ion Mill)
This is a Materials Research Corporation RIE-51 parallel plate, 13.56 MHz system used for etching with fluorine-containing gases (CF4, SF6, and CHF3). The system is used primarily for etching of Si, SiO2, and Si3N4 films. Metals such as tungsten may also be etched. Tool features include: six inch diameter water cooled cathode/substrate platform, pyrex cylinder for plasma confinement and gas flow control, adjustable cathode-anode spacing, fixed DC bias or RF power control and a HeNe laser etch monitor with chart recorder. It is turbo pumped and has no loadlock.
CF4/O2 and SF6/O2 will etch Si, SiO2 and Si3N4 readily since free fluorine is readily liberated in the plasma. The oxygen (up to 40%) initially enhances the fluorine concentration resulting in a higher etch rate. The oxygen also minimizes polymer formation in CF4/O2. Too much Oxygen will compete for fluorine available, suppressing the etch rate. Argon can be added to increase the physical component of etching. The highest etch rates are achieved with SF6 due to the ease of liberating fluorine compared with CF4. The relative etch rate decreases as one goes from Si to Si3N4 to SiO2. CF4/H2 and CHF3 can be used to selectivity etch SiO2 over Si and resist due to increased polymer formation from the presence of hydrogen. This polymer layer is thicker on Si and resist than on SiO2. The trade-off is selectivity versus sidewall profile as the polymer will result in a tapered wall profile. Also, the polymer can be difficult to remove after etching.
The etches have good selectivity to many metals and semiconductors such as Ni, Al, Cr, Ti, GaAs, InP, and GaN. The system generally produces anisotropic etch profiles unless one goes into a purely chemical fluorine etch mode with higher pressure SF6 processes. The system also has a strong loading effect so that larger substrates and open areas will require more feed gas and higher pressure to compensate. As a result, individual processes need to be characterized.
- Etch gases include: CF4, CHF3, SF6, Ar, O2, H2
- Low 1 E -6 ultimate chamber pressure
- 13.56 Mhz excitation frequency
- Manual gas control
- Automatic pressure control
- Manual RF tuning network
- Timer circuit for stopping the plasma
- HeNe laser monitoring for etch stop
- Sample size limited to approximately 4 inches
- Masking materials include: Ni, photoresist (limited to low bias/power), Cr, Al