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Innovations Keep Flowing In the MFC Industry.

Mass flow controllers have improved dramatically over the years, thanks in part to advances in manufacturing and higher; level electronics.

Thermal sensor-based mass flow controllers (MFCs), which often called the workhorse of the semiconductor industry, are used to precisely measure and control the flow of a reactive gas into a vacuum or process chamber. These gases are needed for almost all semiconductor processes to form or remove the thin films used to create integrated circuits.

There are ultimately hundreds of steps that are required in order to make a single chip in a semiconductor process. As geometries are decreased, the layers of film get thinner. Therefore, the control of the gases becomes increasingly more critical, making mass flow technology one of the most important elements of the chamber's gas delivery system.

As a gas moves into the chamber of a thermal mass flowmeter, most flows through a bypass tube, while a smaller amount is routed through a capillary sensor tube. Inside this sensor tube, a heat flux is presented. As the gas continues to flow through the device, it carries some of the heat flux generated from upstream heating coils to downstream coils. The difference in heat changes the resistance between the two legs of a Wheatstone bridge circuit. This resistance change is measured and is proportional to the flowrate. The signal can then be amplified and processed through a digital display to give a direct readout of flow.

Although MFCs have been around since the 1960s, they have physically undergone relatively little change. What has changed, however, is that the performance of MFCs has improved through the optimization of the manufacturing process, the advent of electronics, and advances in processing powers, according to Kaveh Zarkar, VP/semiconductor products for EMCO Flow Systems, an Advanced Energy Co. in Longmont, Colo.

When it comes to MFCs, it seems like the sky is the limit on ways to improve performance, reduce size, and increase simplicity and user-friendliness. And due to their popularity and versatility, many companies in the MFC industry have enjoyed improved sales. Corte Swearingen, marketing manager at ColeParmer Instrument Co., Vernon Hills, Ill., says thermal MFCs are one of the fastest growing technologies in the flowmeter section of the company's catalog.

"We've seen a good 25% increase in sales over the past year. In fact, sales have increased 20 to 25% each year for the past 5 years," he says.

The following are just some of the trends currently surfacing on the MFC horizon.

Although it's not currently an industry-wide feature, users may soon notice that their MFCs come with a built-in memory capability to store multiple calibrations. Traditionally, a thermal mass flowmeter only calibrates to a specific gas. In order to use it with a different gas, it must be sent back to the manufacturer to be recalibrated. This takes time and costs money.

However, some thermal mass units are being designed that allow users to store multiple gas calibrations. "Users can switch between different gases without having to recalibrate" says Swearingen.

Swearingen also notes the increase in accuracy for mass flow technology. "Right now you can get a thermal mass flowmeter that's accurate to 1% full scale. Ten years ago you couldn't do that," he says.

Digital electronics have been integral in tightening accuracy levels. For instance, digital signal processing eliminates zero drift and span drift, and may eventually push the accuracies of thermal mass technology up around 0.5% full scale, says Swearingen.

To make MFCs more versatile,manufacturers are starting to include dry contact closures. "If your flowmeter reaches a certain set point, a dry contact will close and it will allow you to turn on and off alarms, pumps, or valves" says Swearingen. "Not everyone needs this, and it's not a standard feature, but it is something we're starting to see in the industry."

Self-diagnostic functions are also breaking into the MFC market. These features can be built directly into the flowmeter itself to optimize the response of the gas under actual flow conditions.

"The core and heart of an MFC is a sensor and control valve," says Zarkar. The ultimate desire is to make these devices as small and simple as possible to conserve space on the gas panel. Gas panels may have over 30 of these MFCs side-by-side, plus other components like regulators, pressure sensors, and displays.

Because it's often a challenge to fit everything in the valuable space of the tool panel, Zarkar explains that MFCs will continue to decrease in size.

"Another trend that will emerge shortly is in situ flow verification," says Bill Bintz, director of marketing/ materials delivery products for MKS Instruments, Andover, Mass. A mass flow verifier is a unit that sits on the gas panel near the other flow controllers, and it can be used to do an online check of the flow controller's performance.

"Usually, when something goes wrong in the process, the first suspect is the flow controller," says Bintz. "The flow controller has to be removed and tested off-line, and meanwhile, the system is sitting idle waiting for the controller to be replaced or repaired."

In most cases when MFCs are removed from the panel, they weren't the cause of the problem. Therefore, the tool was taken down and productivity was needlessly lost. Experts predict that due to the potential benefits, the use of these verifiers will greatly increase within the next few years.

"One of the advantages of an in situ mass flow verifier is that you can perform a check without removing your flow controller" says Bintz. "Not only can you minimize downtime of the process tool, but if your MFC drifts slightly, rather than waiting until your process goes bad, you can check it every so often and catch the problem before you actually start to lose product yield."

Pressure fluctuation-induced flow deviations can often be a shortcoming of thermal sensor-based MFCs. For instance, when the inlet pressure fluctuates on a system, there is often a resulting fluctuation in gas flow. To minimize this, many users will employ pressure regulators upstream on the inlet side of the MFC.

"There's additional cost associated with using a pressure regulator, and it takes up valuable space and adds more weight" says Bintz. Therefore, interest has increased during the past two to three years in products that are resistant to these upstream pressure fluctuations, such as pressure sensor-based MFCs.

"A number of initiatives are underway among the flow controller companies to address this issue," says Bintz.

Another shift in MFC technology is toward increased use of down- or surface-mount connections. Typically, a flow controller has valves, filters, pressure regulators, and pressure transducers stacked up before and after it--which are connected via VCR (a metal gasket face seal) or VCO (O-ring face seal) type of fittings--to deliver the total desired functionality of the assembly.

"With down mount, it's a modular building-block approach to putting a gas panel assembly together," says Bintz. Down mount eliminates the use of tubing to connect items and instead uses machine blocks.

"The final product has a much smaller footprint," adds Bintz. This is particularly beneficial for semiconductor manufacturing tools that handle 300-mm wafers--the largest new generation of wafer size. For these systems, footprint has become an even bigger issue than in the past.

The cost of ownership has also decreased for users of MFCs. "The use of more advanced flow control techniques, such as sonic nozzle control, as well as faster process stability, wider control range, and improved valve leak rate, have all had a direct effect on the cost of ownership," says Zarkar.

However, when contamination and cleanliness specifications are extremely important, users will have to pay more to ensure higher performance levels.

The use of metal seals minimizes permeation from the atmosphere and the outgassing and particle generation inherent to elastomer-sealed MFCs. MKS Instruments offers flow controllers that are ideal for such high-purity applications, and in addition to metal seals, are available with a solenoid proportioning control valve.

"The high-end products tend to have all-metal seals, as opposed to elastomer seals," says Bintz. "There is a price advantage for customers who don't need all-metal seals and the improved contamination performance levels that come with them."

The need for cleanliness has also led manufacturers, to address the fact that most gases that are used in semiconductor processes are reactive, and exposing them to molecules of other gases will generate unwanted particles.

"Particles are killer defects because they basically short out the chips," says Zarkar. Therefore, manufacturers will focus on this issue as they work to enhance their existing systems.

Some companies are deviating from traditional methods of measuring mass flow and looking for new technologies to push the envelope in the MFC industry.

"More stringent industry demand will require new and continuously improving generations of MFCs that are superior in performance and more immune to process variables" says Zarkar. "These devices will also be smaller and simpler in design for improved long-term reliability"

Advanced Energy is utilizing sonic flow technology, which produces a high-precision average mass flow rate without the problems of over or undershooting the desired set point. The technology also provides a consistent [+ or -] 0.5% of reading accuracy over a 100:1 range of gas flows.

Because changes continue to emerge on the horizon for MFCs, users can be assured that these products will keep improving as technology gets more sophisticated and the manufacturing process is perfected.

"People see MFCs as a standard way to measure gas flow because they're fairly independent of pressure and temperature fluctuations" says Swearingen. "A variable-area flow controller will wildly fluctuate from its calibrated accuracy based on temperature and pressure fluctuations, but there is more stability with the thermal mass technology."

Forecasting the Thermal Mass Flow
Sensor Market

                         ($ MILLION)     RATE (%)

2000         50,600         144.8          6.7
2001         54,600         154.8          6.9
2002         58,900         165.3          6.8
2003         63,300         176.4          6.7
2004         67,900         187.8          6.5
2005         73,000         200.6          6.8
2006         78,400         214.5          6.9

Source: Frost & Sullivan

Although the thermal mass flowmeter is a relatively new type of
flowmeter in the flow sensor market, it is expected to continue
experiencing steady gains worldwide.
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Title Annotation:mass flow controllers
Comment:Innovations Keep Flowing In the MFC Industry.(mass flow controllers)
Author:Lewis, Sharon R.
Publication:R & D
Geographic Code:1USA
Date:May 1, 2001
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