Differentiating tools and techniques for liquid flow control
01/01/2006
Ed Korczynski, Senior Technical Editor, [email protected]
Semiconductor manufacturing has always used aqueous processes. Process control in an aqueous batch tool can often be achieved by controlling reaction temperature and time, assuming proper uniformity within the batch, typically without requiring control over “second- and third-order” parameters such as flow rate and response-time-to-setpoint.
Single-wafer aqueous processing enables additional control parameters, with a corresponding need for greater control over critical subsystems within the tool. Chemical mechanical planarization (CMP) and electrochemical plating (ECP) are used to structure fundamental circuit elements, and controlling these processes mandates precise control of liquid flows within process tools.
CMP process characteristics establish the requirements for liquid flow control in our industry for both historic and technical reasons. From a historical perspective, CMP was used in production for many years before ECP, so CMP provided the first clear technical targets to hit. From a technical perspective, CMP slurry is a nonequilibrious mixture of chemicals and particles that can agglomerate and block sensors or the flow itself, so if a physical flow control component can “handle” slurry, it can handle any other fluid.
Flow measurement technologies
Rotometer LFM. As the paddlewheel or “rotometer” (part a of the figure) liquid flow meter (LFM) rotates with flow, a sensor counts the number of turns as proportional to the flow. However, slurry particles tend to agglomerate on complex structures of the paddlewheel, leading to inaccuracies and even catastrophic clogging, so this technology has been replaced in all current-generation CMP tools.
Differential pressure (∆P) LFM. The difference in pressure between two volumes separated by a fixed orifice is proportional to the liquid flow through the system (part b of the figure). While this technology requires an orifice that could be clogged by slurry particles, proper orifice design can effectively suppress clogging. This technology must be properly calibrated to the fluid it must measure. NT International (now a subsidiary of Entegris) began working with the core pressure-transducer technology in 1992.
 Fundamentally diferent sensing technologies inside LFMs. |
Ultrasonic LFM. The speed of an ultrasonic wave propagating through a liquid varies with the flow of the liquid, so an ultrasonic source and sensor can measure flow through a tube (part c of the figure). Slurry particles and bubbles in the liquid tend to scatter ultrasonic waves and thus add noise to the signal, but this technology can account for multiphase fluid flow with clever modeling. Several companies, including Celerity, Malema, and FutureStar, offer LFMs based on ultrasonic technology.
Coriolis-effect LFM. The mass flow of a fluid through a vibrating tube induces a coriolis-effect transfer of momentum that alters the tube’s vibration (part d of the figure). This technique measures mass instead of volume, so it is immune to typical fluid variations. Brooks Instruments supplies LFMs that use this effect.
Vortex-shedding LFM. Flow measurement can also be based on sensing the vortices that are “shed” off the sides of a fixed “bluff” body that is positioned in the main liquid flow (part e of the figure). The bluff body can be a site for particle agglomeration so this technology is not ideal for slurry control. A vortex requires a certain minimal flow for shedding to occur, so very low flows cannot be sensed.
LFMs and LFCs
Control of liquid flow used to be accomplished with in-line pumps and LFMs. However, peristaltic pumps typically required a lot of maintenance, and new CMP tools have eliminated in-line pumps from tool designs. Similar to the approach used to control gas flows, today’s OEM tools use upstream line pressure (typically 75-120psi) with proportional control valves and LFMs for more reliable control.
An LFM, a control valve, and the necessary control logic circuitry can be integrated into a single liquid flow controller (LFC) box. LFCs are similar to mass flow controllers (MFC) in function and design, except that they control fluids instead of gases. NT International’s product was the first one available for semiconductor OEMs, and most CMP and ECP tools use these ∆P-LFC components. BOC Edwards supplies LFCs as part of its hardware systems for slurry/chemical blending and distribution.
With similar valves and PCB control-boards used by component suppliers, the main technical differences between LFCs lie in the core LFM technology, the communication-bus-interface to the host tool, and the overall physical integration. Today, there are many LFC components available for aqueous processing, with competition for both new OEM tool designs as well as for retrofits to older tools.