A suite of processes have been devised to address the prerequisites of today's high performance deep silicon etch applications in MEMS, NEMS and advanced packaging: smoother etch sidewalls, higher aspect ratios, higher etch rates, and high selectivities to standard mask materials, such as photoresist and silicon oxide.
PlasmaPro Estrelas100 Tool
The core processes on the PlasmaPro Estrelas100 tool from Oxford Instruments are built upon the gas-chopping deep silicon etch process utilizing an SF6 – C4F8 chemistry or the ‘Bosch process,’ where the equipment chops or alternates between short deposition and etch steps to obtain higher etch rates with more vertical profiles and optimized uniformity than traditional silicon etch processes. The alternating nature of the process results in a characteristic sidewall roughness called scalloping.
Figure 1. The Oxford Instruments PlasmaPro Estrelas100 tool
The aforementioned prerequisites come with their own associated system requirements, which can be addressed by the PlasmaPro Estrelas100. Higher rates demand more fluorine radicals, resulting in higher plasma densities in the new Estrelas source with corresponding higher RF powers and gas flows. Improved substrate cooling is necessary for higher etch rates because etching is exothermic.
Higher selectivities demand a move to an improved three-step process cycle, needing fast gas, power and pressure control. The three-step cycle consists of a deposition step, a low pressure step, with bias to the preferential removal of the passivation from the base of a feature, and a higher pressure isotropic silicon etch step.
High aspect ratios leverage improved etch product removal and excellent control of profile by multi-stage processes. More rapid changeover between process steps is essential for smooth sidewalls in order to reduce scallop size, while maintaining short gas residence times and etch rates by means of improved pumping and optimized chamber design.
Gas-Chopping deep silicon etch processes
Figure 2. High aspect ratio (30:1) at 7 µm/min
In the Applications Laboratory, the following gas-chopping deep silicon etch processes have been devised to illustrate the versatility and cutting edge performance of the PlasmaPro Estrelas100 tool, with less than ±3% uniformity typical on a 200 mm substrate:
- A high rate process capable of > 25 µm/min, and selectivities exceeding 250:1 to photoresist for microfluidic and through wafer applications with controlled scallop size (<250 nm)
- A mid-rate process capable of > 6 µm/min for SOI applications, including capacitive MEMS sensors
- A mid-rate process capable of > 10 µm/min for general MEMS applications (100 nm scallops)
- A high aspect ratio process with > 70:1 capability for reduced parasitics in capacitive devices like capacitor arrays and resonators
- A smooth sidewall process with sub-20 nm sidewall scallops for optical and micro-molding applications with a three-step gas-chopping cycle time of <2s
Figure 3. Deep silicon via etch at 16 µm/min
Although commercial production is also considered, the system is principally designed for the academic and industrial research and development market with the versatility to address multiple process requirements. For that purpose, it is possible to configure the system with mechanical or electrostatic clamping and with the option to carry out cryogenic deep silicon etch and mixed gas (continuously passivating) processes in the same chamber.
These processes are constantly passivated to protect the sidewalls, while maintaining the etching from clear utilizing accelerated ions. Cryogenic deep silicon etch processes provide smoother etch side walls as well as the unique combination of good selectivity, minimal undercut, and high etch rate for nano-scale etching and vertical or tapered sidewalls.
Figure 4. MEMs ring gyroscope example
Smooth sidewalls and nano-scale capability with vertical and tapered profiles can also be obtained using mixed gas silicon etch processes without the requirement for cryogenic cooling, but with lower aspect ratio capability and reduced mask selectivity.
Figure 5. Smooth sidewall process at 4 µm/min
The PlasmaPro Estrelas100 achieves a 5x improvement in switching speed, a 5x improvement in selectivity, and a 5x improvement in rate when compared to other systems. Nevertheless, systems like the PlasmaPro Cobra100 continue to provide a solid deep silicon etch process suite for applications that do not emphasize on selectivity, aspect ratio, and ultimate rate.
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.