Technical Insight
A novel approach to metal lift-off for GaAs ICs
The ability to manipulate all thermal, mechanical and chemical variables is crucial in developing a high-yield lift-off process, according to Mehran Janani, James Thietje and colleagues.
Sputtering or evaporation of metal over patterned resist followed by metal lift-off have traditionally been used to define interconnects in GaAs wafer processing. In the lift-off process, a sacrificial material such as photoresist is first deposited and patterned on the substrate. The metal is then deposited on top and the sacrificial material is subsequently removed through exposure to solvent soaking and spraying, leaving behind only the material deposited directly on the substrate. These processes are useful for patterning materials that cannot be etched without affecting underlying materials on the substrate. Today, most metal lift-off processes are done in wet benches with megasonics, spray systems or dry systems using CO2. These methods include acetone or solvent soak, ultrasonic immersions and other recent methods using pressurized liquid CO2 through an aerosol spray, supercritical CO2 and dry CO2 (Burtch et al. 2001; Ober et al. 2002).
Current trends in lift-offFor acetone or solvent soak, the selection of a particular solvent or mixture of solvents is dependent upon the process and desired results. The metal-deposited wafers are immersed in a bath of solvent which helps in lifting/removing unwanted metal and resist through its reaction with the organic resist material, causing it to swell. One of the drawbacks of this approach is that the solvent must be either frequently replenished or constantly filtered to prevent re-deposition of metal on the substrate. Furthermore, agitation is necessary to completely lift the metal.
An ultrasonic cleaning system is often used to help the lift-off process by dislodging the unwanted metal. Other methods include a hot solvent soak followed by a rinse, and single-wafer processing with heated pressurized solvent spray. Here, each wafer is processed without the need to soak or immerse in a batch. The control parameters include solvent spray pressures of up to 1500 psi, solvent temperature, spin speed and process time. In addition to low defect density control on the wafer, high throughput, low cost of consumables and low cost of ownership are paramount to ensure successful integration of any lift-off process in high-volume manufacturing.
Lift-off by R-PHASTPlanar Semiconductor, in collaboration with GaAs IC manufacturer Skyworks Solutions, has studied ways to improve the lift-off process while addressing these needs. Planar Semiconductor has developed a vertical non-contact front and backside single-wafer cleaning process that creates discrete droplets of fluid by pulsing the spray using a special pump. This new approach is called rapid-pulse harmonic spray technology (R-PHAST).
The fluid droplets in each pulse interact with the wafer, which rotates slowly in a vertical orientation, to produce a hybrid of laminar and turbulent flow types. Rapid pulsing controls the fluid-mechanical interactions of jets and droplets with any adsorbed contaminants. As a result the chemical concentration gradient at the wafer/liquid interface is always positioned to favor desorption of contaminants. The moderate application of a pressurized solvent allows for a blend of mechanical and chemical energy for lift-off applications. Large particles are removed at the point of impact of the pulse spray due to the generation of turbulent flow. In the laminar flow regime where the boundary layer is less than 1 µm, the solvent is responsible for dispersing and rinsing small particles and labile layer removal.
Compared with the use of other technologies that use fluids at super critical pressures aided with co-solvents, dry CO2/liquid approaches and jet sprays, the rapid pulse approach manipulates all of the essential thermal, mechanical and chemical ingredients for effective cleaning, thereby offering a simple, elegant and cost-effective solution.
Figure 1 shows an RPC-100 metal lift-off tool, a GaAs wafer ready for lift-off and the vertical pulse chamber. Additional chambers can be added to increase throughput. The wafer is held from the edges during processing, and DI water or desired chemistry can be sprayed on the backside in the final pulse rinse step to keep the wafer backside clean. It is a dry-in, dry-out system.
In a typical process sequence, a single wafer is delivered to the R-PHAST chamber where the heated solvent is pulsed, followed by a DI pulse rinse to remove photoresist residue and metal stringers. The lifted metal can be reclaimed via a special module underneath the system. The wafer is then dried using a vertical spin or N2-assisted process with no handling. Process parameters such as fluid pulse pressure, droplet size, wafer rotation speed and solvent temperature can all be varied, and are key factors that allow R-PHAST to be used in a wide variety of front-end and back-end cleaning and lift-off applications.
Current trends in lift-offFor acetone or solvent soak, the selection of a particular solvent or mixture of solvents is dependent upon the process and desired results. The metal-deposited wafers are immersed in a bath of solvent which helps in lifting/removing unwanted metal and resist through its reaction with the organic resist material, causing it to swell. One of the drawbacks of this approach is that the solvent must be either frequently replenished or constantly filtered to prevent re-deposition of metal on the substrate. Furthermore, agitation is necessary to completely lift the metal.
An ultrasonic cleaning system is often used to help the lift-off process by dislodging the unwanted metal. Other methods include a hot solvent soak followed by a rinse, and single-wafer processing with heated pressurized solvent spray. Here, each wafer is processed without the need to soak or immerse in a batch. The control parameters include solvent spray pressures of up to 1500 psi, solvent temperature, spin speed and process time. In addition to low defect density control on the wafer, high throughput, low cost of consumables and low cost of ownership are paramount to ensure successful integration of any lift-off process in high-volume manufacturing.
Lift-off by R-PHASTPlanar Semiconductor, in collaboration with GaAs IC manufacturer Skyworks Solutions, has studied ways to improve the lift-off process while addressing these needs. Planar Semiconductor has developed a vertical non-contact front and backside single-wafer cleaning process that creates discrete droplets of fluid by pulsing the spray using a special pump. This new approach is called rapid-pulse harmonic spray technology (R-PHAST).
The fluid droplets in each pulse interact with the wafer, which rotates slowly in a vertical orientation, to produce a hybrid of laminar and turbulent flow types. Rapid pulsing controls the fluid-mechanical interactions of jets and droplets with any adsorbed contaminants. As a result the chemical concentration gradient at the wafer/liquid interface is always positioned to favor desorption of contaminants. The moderate application of a pressurized solvent allows for a blend of mechanical and chemical energy for lift-off applications. Large particles are removed at the point of impact of the pulse spray due to the generation of turbulent flow. In the laminar flow regime where the boundary layer is less than 1 µm, the solvent is responsible for dispersing and rinsing small particles and labile layer removal.
Compared with the use of other technologies that use fluids at super critical pressures aided with co-solvents, dry CO2/liquid approaches and jet sprays, the rapid pulse approach manipulates all of the essential thermal, mechanical and chemical ingredients for effective cleaning, thereby offering a simple, elegant and cost-effective solution.
Figure 1 shows an RPC-100 metal lift-off tool, a GaAs wafer ready for lift-off and the vertical pulse chamber. Additional chambers can be added to increase throughput. The wafer is held from the edges during processing, and DI water or desired chemistry can be sprayed on the backside in the final pulse rinse step to keep the wafer backside clean. It is a dry-in, dry-out system.
In a typical process sequence, a single wafer is delivered to the R-PHAST chamber where the heated solvent is pulsed, followed by a DI pulse rinse to remove photoresist residue and metal stringers. The lifted metal can be reclaimed via a special module underneath the system. The wafer is then dried using a vertical spin or N2-assisted process with no handling. Process parameters such as fluid pulse pressure, droplet size, wafer rotation speed and solvent temperature can all be varied, and are key factors that allow R-PHAST to be used in a wide variety of front-end and back-end cleaning and lift-off applications.
