By Pete Singer

Increasingly complicated 3D structures such finFETs and 3D NAND require very high aspect ratio etches. This, in turn, calls for higher gas flow rates to improve selectivity and profile control. Higher gas flow rates also mean higher etch rates, which help throughput, and  higher rates of removal for etch byproducts.

“Gas flow rates are now approaching the limit of the turbopump,” said Dawn Stephenson, Business Development Manager – Chamber Solutions at Edwards Vacuum. “No longer is it only the process pressure that’s defining the size of the turbopump, it’s now also about how much gas you can put through the turbopump.”

Turbopumps operate by spinning rotors at very high rates of speed (Figure 1). These rotors propel gases and process byproducts down and out of the pump. The rotors are magnetically levitated (maglev) to reduce friction and increase rotor speed.

Figure 1. Spinning rotors propel gases and process byproducts out of the pump.

The challenge starts with processes that have high gas flow rates, over a thousand sccm, and lower chamber pressures, below 100 mTorr.  Such processes include chamber clean steps where high flows of oxygen-containing gases are used to remove and flush the process byproducts from inside the chamber, through Silicon via (TSV) in which SF₆is widely used at high gas flowrates for deep silicon reactive ion etch (RIE) and more recently, gaseous chemical oxide removal (COR) which typically uses HF and NH₃to remove oxide hard masks.

However, the challenge is intensified with the more general trend to higher aspect ratio etch across all technologies.

Stephenson said the maximum amount of gas you can put through a maglev turbo is determined by two things: the motor power and the rotor temperature. Both of these are affected adversely by the molecular weight of the gas. “The heavier the molecule, the lower the limit. For motor power, if the gas flow rate is increased, the load on the rotor is increased, and then you need more power. Eventually you reach a gas flow at which you exceed the amount of power you have to keep the rotor spinning and it will slow down,” she said.

The rotor temperature is an even bigger limiting factor. “As gas flow rates increase, the number of molecules hitting the rotor are increased. The amount of energy transferred into the rotors is also increased which elevates the temperature of the rotor. Because the rotor is suspended in a vacuum and because it’s levitated, it’s not very easy to remove that heat from the rotor because its primary thermal transfer is through radiation,” she explained.

Pumping heavier gases, particularly ones that have poor thermal conductivity, cause the rotor temperature to rise, leading to what is known as “rotor creep.”Rotor creep is material growth due to high temperature and centrifugal force (stress).  Rotor creep deformation over time narrows clearances between rotor and stator and can eventually lead to contact and catastrophic failure (Figure 2).

Figure 2. Edwards pumps have the highest benchmark for rotor creep life temperature in the industry, due to the use of a premium aluminum alloy as the base material for its mag-lev rotors, combined with a low stress design.

Where it gets even worse are in applications where the turbopump is externally heated to reduce byproduct deposition inside the pump. Such a heated pump will have a higher baseline rotor temperature and significantly lower allowable gas flowrates than an unheated one. This becomes a challenge particularly for the heated turbopumps on semiconductor etch and flat panel display processes using typical reactant gases such as HBr and SF₆.  “Those are very heavy gases with low thermal conductivity and the maximum limit of the turbopump is actually quite low,” Stephenson said.

The good news is that Edwards has been diligently working to overcome these challenges. “What we have done to maximize the amount of gas you can put into our turbopumps is to  ensure our rotors can withstand the highest possible temperature design limit for a 10 year creep lifetime.   We use a premium alloy for the base rotor material and then beyond that we have done a lot of work with our proprietary modeling techniques to design a very low stress rotor because the creep is due to two factors: the temperature and the centrifugal stress. Because of those two things combined, we’re able to achieve the highest benchmark for rotor creep life temperature in the industry,” she said.

Furthermore, the company has worked on thermal optimization of the turbopump platform. “That means putting in thermal isolation where needed to try to help keep the rotor and motor cool. At the same time, we also need to keep the gas path hot to stop byproducts from depositing. We have also released a high emissivity rotor coating that helps keep the rotor cool,” Stephenson said. A corrosion resistant, black ceramic rotor coating is used to maximize heat radiation, which helps keep the rotor cool and gives more headroom on gas flowrate before the creep life temperature is reached.

Edwards has also developed a unique real-time rotor temperature sensor: Direct, dynamic rotor temperature reporting eliminates over-conservative estimated max gas flow limits and allows pump operation at real maximum gas flow in real duty cycle while maintaining safety and lifetime reliability.

In summary, enabling higher flows at lower process pressures is becoming a critical capability for advanced Etch applications, and Edwards have addressed this need with several innovations, including optimized rotor design to minimize creep, high emissivity coating, and real time temperature monitoring.