New MFCs mean higher yield

05/01/2013

Pete Singer,

Editor-in-Chief

In what might be a case of the cobbler's children finally getting new shoes, new algorithms and control technology ??? powered by advanced semiconductors, of course ??? are enabling "intelligent" real-time flow error detection in mass flow controllers (MFCs).

The accuracy and repeatability of MFCs ??? which control the amount of process gas flowing into etch and deposition chambers, for example ??? can have a very direct impact on yield: "A simple 1% increase in yield on an etch system can equate to up to $60,000 a day savings," notes Shaun Pewsey, Director of Microelectronics Strategic Accounts at Brooks Instrument. "Process gas stability has been identified by virtually every IDM as critical to meeting yield enhancement goals and initiatives. MFC accuracy is critical in maintaining the level of control required," he said (the remarks were made during a recent webcast produced by Solid State Technology).

Pewsey said the challenges are only getting more severe as the industry moves to ever more challenging devices, larger die sizes, and greater die complexity. Compounding the problem is the push for a higher mix of products in the fabs, particularly foundries. "We're seeing tools that are being run with multiple recipe types, in some cases pushing the tool beyond its original design requirements," Pewsey said.

The end result of this is a stronger focus on basic MFC performance attributes. Today, 1% accuracy is required for challenging applications and Pewsey believes we will soon see a requirement for 0.5% accuracy. Tighter flow repeatability is also required for chamber matching.

It's well known that the accuracy of MFCs can drift over time, in part due to the build-up of particles from process gas. The common practice for checking the accuracy of MFCs is to take the gas panel off-line and perform a flow check. This can easily take half a day, says Pewsey. And, of course, until this check is done, the drifting MFCs could have impacted hundreds of wafers.

To address this common problem, engineers at Brooks Instrument have developed the smarter MFC. The latest version, the GF135, uses Brooks' real-time rate-of-decay flow error detection technology to continually test for changes in the device's performance. Data can be used to improve accuracy at critical low-flow set points, set up alarm limits for critical performance parameters and monitor trends for predictive maintenance.

How does the new smart MFC work? In operation ??? while process gas is flowing into the chamber ??? Brooks figured out a way to momentarily close the valve in order to run a diagnostic test. "As the valve closes, we continue to deliver the required amount of gas to the process chamber as the internal pressure starts to decrease. As that happens, our control valve opens up, continuing to deliver the exact amount of flow required for the process," Pewsey explained.

After completing the measurement, the valve is reopened and the pressure transient compensation algorithm compensates for the initial pressure spike. A proprietary algorithm computes the flow based on the rate of pressure change and time and compares this against the baseline.

In short, the new technology identifies and corrects issues before they happen, preventing wasted time and wasted wafers, increasing uptime and yield.

Solid State Technology | Volume 56 | Issue 3 | May 2013