Pressure & Residual Gas Control In Plasma!
Harry Grover, MeiVac Inc. and Nir GurArye of Ricor

Precise pressure and residual gas control are critical to quality, yield and profitability. The purpose of this paper is to address basic concepts and provide unique solutions to common vacuum and process engineering problems. 

• Maximum open, pump-down, conductance through a variable conductance valve
• Linear throttling curves provide improved process pressure control
• Residual pressure control with species sensitive differential pumping

Plasma requires a partial vacuum to support ionization of process gases. At the same time, high vacuum pumps must operate at a vacuum lower than plasma processing. To achieve differential pressure we use variable conductance valves. In fact size, shape, actuation geometry and temperature can offer distinct process advantages; if properly implemented or production limitations if ignored.

Upstream pressure control is defined as: varying process gas flow to maintain a constant pressure against a fixed pumping orifice. Operation would include varying (reducing) process gas flow to accommodate pressure rises from leaks, long term out-gassing or gas bursts.  Downstream is a reverse technique.  Constant process gas flow is maintained, while valve conductance varies pumping speed via a valve mounted servomotor driver and commercially available vacuum equipment pressure controller.

The ratio of process gas to contamination, in any form, will impact product quality, reproducibility, yield, throughput, and profitability. Upstream control seems to aggravate the problem; not alleviate it. When there is a gas burst or variation in gas load (substrate to substrate or during the deposition of a single film), process gas flow is reduced driving the contamination ratio in the wrong direction. Fixed pumping speed further complicates the problem by slowing recovery response. Downstream control responds with an increased pump rate while maintaining undisturbed process gas flow and constant pressure. Contamination ratio excursions are minimized not aggravated. By increasing pumping speed, for the duration of the event, recovery times are minimized.

Linear throttling curve, at plasma process pressures the mean-free-path of the process gas may be greater than the cross-sectional depth of the control vane. Under these conditions gas will tend to bounce through a Venetian blind throttle valve; just as sunlight would bounce through a mirrored window blind. The unfortunate result of this bounce-through is a non-uniform throttling curve. Most of the functional effect is within the last few degrees of actuation; essentially a nearly flat response transitioning quickly to an almost vertical throttling curve near closure. Control and resolution are compromised. To counteract these gas dynamics; Pie shaped vanes are counter-rotated during actuation. Gas bounce is vane face to vane face and backward with an average conductance (Q) of the valve being similar to it's optical density. A throttling curve for a pie shaped vane valve is linear from maximum pump speed to minimum conductance. 

A MAJOR class of contamination is condensables.  e.g. moisture, process solvents, etc. Two general High-vacuum pumping schemes are employed in processes today, cryogenic and non-cryogenic pumping.  e.g. Turbo pumps. Cryopumps have a significant water pumping advantage with high rates on their top stage. However, during plasma process this water pumping capability is restricted along with the throttled process gas. Turbos and other non-cryogenic pumps have limited water-pumping speeds, however, they offer process specific capabilities that may not be available to cryo-pumped systems. 

A solution lies in the throttle valve itself.  Cryo-cooled (~100 Kelvin) throttle valve vanes provide downstream pressure control, while differentially pumping condensable contamination at a high rate. Located below the high vacuum valve, it does not have to be cycled to atmosphere upon chamber vent. In some cases the upgrade is as simple as a valve exchange and attachment to an existing refrigeration system. Two configurations of the Cryocooled Water-Pumping Throttle Valve are available. CVQ-"C" is designed for thermal attachment to the top stage of a cryo-pump. The valve is automatically regenerated along with the cryo-pump.  Turbo pumps and other non-cryogenic pumps, benefit from a similar configuration "CVQ-T" with the addition of a cold head as part of the valve.

Tests included injecting large quantities of water vapor above CVQ throttle valves. The goal of these tests is to show pressure control and pumping capabilities far greater than one would assume in a standard plasma process system. Prior to opening the injection orifice, pressures above and below the cryo cooled throttle valve are on the order of 5 x 10^-7 Torr. When the injection orifice is opened a minimum of 4000 sccm of water vapor flows into the test chamber.  Pressure below the valve increased to only 1 x 10^-6. Above the valve pressure increased to 2-4 x 10^-3. Tests continued for >75 minutes. The same test without cryo cooling was not too surprising. Pressure below the "closed" throttle valve exceeded 1 Torr in < 1.5 seconds, the turbo obviously shut down in a rapid manner.  Pressure above the valve went off-scale in the same period of time

MeiVac CVQ-T Cryocooled Water Pumping Throttle Valves use a Ricor's Coolstar^™ cold heads. Using the Gifford-McMahon refrigeration cycle allows use of standard helium compressors. Servo Motor control allows rapid cool down by running the cold head at boost speed. Once at the desired temperature, the cold head idles down and provides precise species-range specific valve temperature control. Smaller valve models may use the self-contained Ricor Stirling refrigeration cycle MicroStar^™ cold head. No compressor or lines are required for this head.

 Ambient temperature Vari-Q throttle valves are available from 100mm to 35" id in ASA, ISO, JIS and CF^™ flanges. CVQ-C valves are available in ISO 200, 250 and ASA 6,10 sizes. CVQ-T valves are in development in ISO 160, 200, 250 and ASA 6, and 10 sizes. Custom and future standard sizes for ISO 100 through 500 are under consideration.

Conclusion, Advancing technology puts more stringent demands upon the interaction of process, hardware and control systems. By combining unique plasma pressure control and residual gas differential pumping into a single unit an economical solution is at hand. Quality, yield and throughput are improved with minimal cost by replacing/upgrading an existing component without major system changes and using existing control systems. Economical, ease of installation, proven over decades, efficient, yield and throughput enhancements are features of this value added design.

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