Ultrananocrystalline diamond films with optimized dielectric properties for advanced RF MEMS capacitive switches
An efficient deposition process is provided for fabricating reliable RF MEMS capacitive switches with multilayer ultrananocrystalline (UNCD) films for more rapid recovery, charging and discharging that is effective for more than a billion cycles of operation. Significantly, the deposition process is compatible for integration with CMOS electronics and thereby can provide monolithically integrated RF MEMS capacitive switches for use with CMOS electronic devices, such as for insertion into phase array antennas for radars and other RF communication systems.
1. A process for fabricating a radio frequency (RF) microelectromechanical switch (MEMS) capacitive switch, comprising the steps of:
providing a substrate;
placing a bottom electrode on the substrate;
depositing layers of ultrananocrystalline diamond (UNCD) thin film on the bottom electrode by microwave chemical vapor deposition (CVD) with a microwave CVD plasma gas comprising argon (Ar), methane (CH 4 ) and hydrogen (H 2 ) to form multilayered UNCD dielectric films including
depositing a bottom UNCD layer on the bottom electron with H 2 flow for rapid nucleation of the UNCD and forming a substantially continuous dielectric film;
depositing an intermediate UNCD layer on the bottom UNCD layer with a higher H 2 flow and forming a higher resistivity film with a hydrogen enriched grain boundary;
depositing an upper UNCD layer on the intermediate UNCD layer with a lower H 2 flow than for the intermediate UNCD layer for dense nucleation of the UNCD to fill in the gaps or pinholes between grains and forming a substantially continuous layer;
positioning a moveable metallic membrane above the multilayered UNCD dielectric film;
positioning a top electrode above the membrane; and
forming a long life RF MEMS capacitive switch with the multilayered UNCD dielectric film, membrane, electrodes and substrate so that the RF MEMS capacitive switch is operable for more than 100 billion cycles.
2. A process for fabricating a RF MEMS capacitive switch in accordance with claim 1 wherein:
the layers are deposited with 400 sccm Ar and 1-2 sccm CH 4 ;
the bottom layer is deposited with 5-8 sccm H 2 ;
the intermediate layer is deposited with 14-16 sccm H 2 ; and
the upper layer is deposited with 5-8 sccm H 2 .
3. A process for fabricating a RF MEMS capacitive switch in accordance with claim 1 wherein the layers are deposited at a temperature ranging from 400-500° C.
4. A process for fabricating a RF MEMS capacitive switch in accordance with claim 1 where in the H 2 concentration is increased 4-5% by volume in the microwave CVD plasma for the intermediate layer and decreased by 1-2% by volume in the microwave CVD plasma for the upper layer.
5. A process for fabricating a RF MEMS capacitive switch in accordance with claim 1 wherein:
the dielectric film comprises an ultra thin UNCD dielectric film ranging from 200-300 nm;
discharging and leaking charges accumulated in the dielectric film and recovering the RF MEMS capacitive switch within at least 80 μsec; and
substantially preventing failure of the RF MEMS capacitive switch due to dielectric charging of the film.
6. A process for fabricating a RF MEMS capacitive switch in accordance with claim 1 including substantially eliminating stiction and tribo-charging problems when the RF MEMS capacitive switch is closed and the membrane contacts the dielectric film.
7. A process for fabricating a RF MEMS capacitive switch in accordance with claim 1 wherein:
the membrane comprises molybdenum (Mo);
the bottom electrode comprises tungsten (W) or a stack comprising chromium (chrome) (Cr), tungsten and chromium;
the bottom electrode is positioned on a substrate selected from the group consisting of silicon on sapphire (SOS) and a silicon wafer; and
the RF MEMS capacitive switch is monolithically integrated with a complementary metal-oxide-semiconductor (CMOS) electronic device.