Gas Flow Monitor – Real Time, In Process Gas Flow Monitoring
All semiconductor fabs use specialized process gas delivery systems within their wafer processing tools. These systems utilize flow control devices (FCD), such as mass flow controllers (MFC) or flow control systems (FCS), to supply precise quantities of process gases to the process chamber, such that processes remain within their process windows during wafer processing.
As processes migrate to advanced technology nodes, the process window for gas flow control narrows. This creates a broad need for the capability to determine when an FCD is drifting or performing out of specification. The Pivotal Gas Flow Monitor (GFM) system has the capability to monitor FCDs during normal wafer processing, determining their actual flow rates to an accuracy of ±0.5%.
The GFM SolutionThe GFM system measures an induced rate of pressure drop within the gas stick upstream of the MFC over a known volume and temperature. The system consists of a GFM control/data collection computer installed outside of the gas box and takes advantage of existing and available pressure transducers and regulators commonly found inside the gas box. An optional wetted part can be included that replaces the existing regulator and pressure transducer for automated NIST-traceable gas flow measurements.
Real Time, In Situ MonitoringThe GFM system is the only system on the market designed for gas flow measurement during wafer processing. Each and every time an MFC turns on for wafer processing, GFM takes its measurement of the MFC’s performance within the first 3-5 seconds.
High Accuracy, High RepeatabilityGFM employs a rate of drop measurement (akin to the industry standard rate of rise measurement) thereby making it a “primary standard” measurement as defined by NIST. Furthermore, due to the accuracy of its volume, pressure and temperature measurements, GFM is able to guarantee each and every measurement within ±0.5% of actual flow.
Transparent to Process, No Qual RequiredUnlike downstream off-line volumes where the downstream pressure can perturb the performance of the MFC, the GFM measurement approach operates well within the specified upstream operating pressure range for both pressure insensitive (PI) and non PI MFCs. The GFM system is therefore transparent to wafer processing and does not require lengthy qualification processes to be implemented in production.
Monitor Average Flow, Monitor TransientsDue to its high sampling rate (50ms) the GFM system can provide critical insight into both average flow rate deviations from wafer to wafer and chamber to chamber, as well as MFC transient behaviors during flow change – critical information that is often incorrectly reported by the MFC’s flow feedback system.
Match Chambers, Improve YieldSmaller gas flow rates and shorter processing steps are making gas flow variations one of the largest sources of chamber to chamber matching / yield issues today. GFM provides a highly accurate window into these gas flow variations thereby allowing for immediate chamber matching and yield improvements.
Avoid Gas Flow Related ScrapGas flow drifts outside of MFC specifications often cause multi-million dollar scrap events due to the infrequency of gas flow calibration events. With GFM, a high accuracy flow calibration measurement is taken for each and every processing step, thereby minimizing wafer scrap to no more than 1 wafer.
Increase Tool AvailabilityWith 10-16 MFCs per chamber, fabs can spend anywhere from 2-4% of tool up time performing off-line MFC calibrations. With GFM, because the measurement is in situ, fabs can increase throughput by removing any off line MFC calibrations.
Troubleshoot MFCs Cost EffectivelyIn an effort to minimize tool downtime in production, MFCs are swapped out on a frequent basis. Anywhere from 30-50% of these swapped-out MFCs are deemed to be no-fault found (NFF) and should not have been removed in the first place. With GFM, tool engineers can immediately know if their MFCs are performing in or out of spec for immediate corrective action.
The GFM Solution
Real Time, In Situ Monitoring
High Accuracy, High Repeatability
Transparent to Process, No Qual Required
Monitor Average Flow, Monitor Transients
Match Chambers, Improve Yield
Avoid Gas Flow Related Scrap
Increase Tool Availability
Troubleshoot MFCs Cost Effectively