Controlling the flow – digitally
Emmanuel Bernard, application engineer at Qualiflow, looks at the cost advantages of digital mass flow controllers
As integrated circuits (ICs) increase in complexity and manufacturing equipment becomes more expensive, costs associated with taking equipment off line for any length of time are growing dramatically.
Stopping production is a tremendous financial strain. When a problem appears with a mass flow controller (MFC), a solution must be found in a minimum of time and very often this solution is to change the MFC. Using digital MFCs, as opposed to analog MFCs, can provide a great benefit by allowing semiconductor manufacturers to replace malfunctioning MFCs immediately while keeping a reduced number of spare MFCs in stock.
Most effective means For many years, MFCs have served as the most effective means of precisely controlling gas flow in processes used in the semiconductor industry. They are an essential component of many process tools. Since the development of digital MFCs, the most significant advantage to an original equipment manufacturer or end-user is the potential for lowering the cost of maintaining an inventory of spare mass flow controllers. Because digital MFCs can be programmed to run various gases, the inventory can be significantly reduced. Where wafer manufacturers have traditionally needed to maintain an inventory calibrated for specific gases used in production, they can now maintain a smaller spares inventory pre-calibrated for each of the gases used in the fab facility. Original equipment manufacturers (OEMs) face the same problem. Often, the configuration of gas type and flow rate are not defined until the machine is almost ready to ship. Changing from customer specification for gas type and flow rate means manufacturing has to take the MFC out, recalibrate and leak check the system again. An MFC consists of a sensor tube, a bypass, a regulating valve and a feedback circuit (Fig. 1). Analog MFCs are calibrated to a dedicated gas and full-scale flow by the manufacturer. If the user wishes to select a different gas or full-scale flow, it is necessary to remove the MFC from the system in which it is installed and do a bench recalibration. A purge and leak check of the system is also necessary. This modification is time consuming, expensive and inconvenient. Analog MFCs must be operated as closely as possible to full-scale flow for optimal accuracy and linearity result. This is a limitation when the user wishes to use an MFC at a high and low flow setpoint. When the setpoint is set at the low end of a specific calibration, the accuracy and linearity are degraded considerably. Therefore, two analog MFCs with different full–scale flow are required in a gas panel to maintain the accuracy (one for the low flow and another for the high flow). This, naturally, requires larger stocking levels and expense to cover full spares support. Analog MFCs require an excessively large spares inventory to cover the various needs of a wafer fab and equipment manufacturer. The user must have as many spare MFCs in stock as MFCs in use at any given time. Fortunately, the development of digital MFCs has overcome many of the limitations of the analog versions.
Digital MFCs open up new era With digital MFCs, a new era is entered. Among other enhancements, the marriage of digital control to mass flow technology provides greater flexibility, better communication between the MFC and control system, and better optimisation of performance. Digital MFCs can be calibrated at many points throughout their operating range, resulting in accuracy and linearity specifications that are the function of a setpoint rather than full-scale for most of the operating range. Users want to run MFCs at widely varying flow rates to accommodate a process while keeping the number of MFCs in a gas panel to a minimum. With digital MFCs, different curves can be stored in a single MFC. With this capability, a single MFC can be used for a multitude of gases and a wide variety of full-scale flow. It can be used effectively for the entire calibrated flow range, hence the greatly reduced inventory of spares, as it is not necessary to stock MFCS dedicated to specific gases.
Limitations of multicalibration Some limitations appear when calibrating an MFC with several gases. These limitations are both mechanical (bypass and valve design) and physical (gas properties). Certain rules can be applied so that a multicalibrated (multicurved) MFC works, despite these limitations: • Two calibration curves within one MFC cannot have full-scale flows, equivalent N2, too wide apart. The highest calibration curve (in full-scale flow N2 equivalent) must be a maximum of three times larger than the smallest calibration curve (in full-scale flow N2 equivalent) to keep the benefit of accuracy. It is better for multicalibration to have different gases with the same full-scale flow (N2 equivalent) rather than one gas with different full-scale flow. • Gases containing fluorine (SF6 Sulphur hexafluoride, WF6 Tungsten hexafluoride, C2F6 Perfluorethane [Freon-116], C4F8 Octofluorocyclobutane [Freon-C318] etc) need to be worked out differently because of a much higher diffusivity. • All calibration curves on one MFC should have similar operating pressures. • Inert gases can be stored in the same MFC. • Unless metal seals are used, corrosive gases should not be run with non-corrosive gases in the same MFC.
Digisoft advantages Qualiflow's digital MFC multicalibration allows up to 10 curves to be stored inside the MFC (typically five curves, but it depends on the gases and their associated flows). Using the Qualiflow interface Digisoft, the user is able to shift from one calibration curve to another and select the adequate flow and gas, on site, without recalibration. Digisoft also allows the user to create new calibration curves from one existing curve. For example, in any process, the user can shift from a 1000 sccm full-scale MFC to a 500 sccm full-scale MFC when changing some parameters on the process. Instead of using a spare MFC and stopping the equipment, purging the system, changing the MFC etc, Digisoft allows the creation of a new calibration curve in a few seconds, without recalibration.
Cost reductions In a wafer fab that is typically processing all the steps of chip manufacturing (epitaxy, chemical and physical vapour deposition, diffusion, etching etc), each service associated with each step will try to minimise its stock of spare MFCs. The following example illustrates the impact of multicalibrated digital MFCs in reducing the cost-base for the fab facility.
Example of savings In processes such as chemical vapour deposition, for example (deposition of Si3N4, SiO2), if we consider a small cluster of equipment (three CVD machines), the service may need the gases and their associated flow as listed in Table 1. In order to change a malfunctioning MFC immediately, and therefore to avoid keeping one of the tools down too long, the user must maintain a stock of MFCs large enough to allow replacement of any of the MFCs present in the three tools. Using analog or single curve digital spare MFCs, the user has to stock 29 MFCs (see Tables 2-4). With multicalibrated MFCs, in respect to the rules mentioned previously, the user can reduce this number from 29 to only eight spare MFCs (Tables 5–6 and Fig. 5). Digisoft is a convenient solution for dealing with these MFCs. The user can change the gas calibration of the spare digital MFC to any other gas on site, without recalibration.
Summary The advent of digital MFCs and multicalibration is resulting in significant cost savings to the semiconductor industry. These savings mean that prospective users of this evolution in technology are giving serious consideration to this form of calibration.