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If your questions are not answered below, please click here and EPRI's SEMI F47 experts will be glad to field any questions that you have about our services, the SEMI F47 standard, or other related questions. 

 

Q:  What is a Voltage Sag?

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A:  A sag or dip, as defined by IEEE Standard 1159-1995, IEEE Recommended Practice for Monitoring Electric Power Quality, is a decrease in rms voltage or current at the power frequency for durations from 0.5 cycles to 1 minute, reported as the remaining voltage.  Typical values are between 0.1 pu and 0.9 pu.

Terminology used to describe the magnitude of a voltage sag is often confusing.  The recommended usage is "a sag to 20%", which means that the line voltage is reduced down to 20% of the normal value, not reduced by 20%.  Using the preposition "of" (as in "a sag of 20%", or "a 20% sag") is discouraged.  This preference is consistent with IEC practice, and with most disturbance analyzers which also report remaining voltage.  Just as an unspecified voltage designation is accepted to mean line-to-line potential, so also will an unspecified sag magnitude refers to the remaining voltage.  Where possible, specify the nominal, or base, voltage and the remaining voltage.
Voltage sags are usually associated with system faults but can also be caused by the switching of heavy loads or the starting of large motors.  Figure 1 shows a typical voltage sag that can be associated with a single line-to-ground (SLG) fault.  Also, a fault on a parallel feeder circuit will result in a voltage drop at the substation bus which affects all of the other feeders until the fault is cleared.  Typical fault clearing times range from three to thirty cycles depending on the fault current magnitude and the type of overcurrent detection and interruption.

Figure 1 - Instantaneous Voltage Sag Caused by a SLG Fault
 

Large load changes or motor starts can also cause voltage sags.  An induction motor will draw six to ten times its full load current while starting.  This lagging current then causes a voltage drop across the impedance of the system.  Should the current magnitude be large relative to the system available fault current, the resulting voltage sag may be significant.  Figure 2 illustrates the effect of a large motor being started.

Figure 2 - Temporary Voltage Sag Caused by Motor Starting
 

The term sag has been used in the power quality community for many years to describe a specific type of power quality disturbance known as a short duration voltage decrease.  Clearly, the notion is directly borrowed from the literal definition of the word sag.  The IEC definition for this phenomenon is dip.  The two terms are considered interchangeable, with sag being preferred in the United States power quality community. Previously, the duration of sag events has not been clearly defined.  Typical sag durations defined in some publications range from two milliseconds (about one-eighth of a cycle) to a couple of minutes.  Undervoltages that last less than one-half cycle cannot be characterized effectively as a change in the rms value of the fundamental frequency value.  Therefore, these events are considered transients [1].  Undervoltages that last longer than one minute can typically be controlled with voltage regulation equipment and may be associated with a wide variety of causes other than system faults.

Sag durations are subdivided here into three categories -- instantaneous, momentary and temporary -- coinciding with the three categories of interruptions and swells.  These durations are intended to correlate with typical protective device operation times as well as with duration divisions recommended by international technical organizations.  These three different definitions of sags are defined by their duration and shown in Table 1 below.

 
Table 1 - Categories and Typical Characteristics of Power System Electromagnetic Phenomena
Categories Typical Duration Typical Voltage Magnitude
2.0 Short Duration Variations    
    2.1 Instantaneous    
        2.1.1 Sag 0.5 - 30 cycles 0.1 - 0.9 pu
        2.1.2 Swell 0.5 - 30 cycles 1.1 - 1.8 pu
    2.2 Momentary    
        2.2.1 Interruption 0.5 cycles - 3 s <0.1 pu
        2.2.2 Sag 30 cycles - 3 s 0.1 - 0.9 pu
        2.2.3 Swell 30 cycles - 3 s 1.1 - 1.4 pu
    2.3 Temporary    
        2.3.1 Interruption 3 s - 1 min <0.1 pu
        2.3.2 Sag 3 s - 1 min 0.1 - 0.9 pu
        2.3.3 Swell 3 s - 1 min 1.1 - 1.2 pu
 

Q:  What is Semi F47?

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A:  The SEMI F47 "Specification for Semiconductor Processing Equipment Voltage Sag Immunity" is a standard that defines the threshold that a semiconductor tool must operate through without interruption (per SEMI F42).  It also provides a target for the facility and utility systems.  The recognizing semiconductor factories require high levels of power quality due to the sensitivity of equipment and process controls.  As semiconductor processing equipment is especially vulnerable to voltage sags, the SEMI F47 document defines the voltage sag ride-through capability required for semiconductor processing, metrology, and automated test equipment.

The requirements in this international standard were developed to satisfy semiconductor industry needs.  While more stringent than existing generic standards, this industry-specific specification is not in conflict with known generic equipment regulations from other regions or generic equipment standards from other organizations.  It is the intent of this standard to provide specifications for semiconductor processing equipment that will lead to improved selection criteria for sub-components and to improvements in equipment systems design.  While it is recognized that in certain extreme cases or for specific functions battery storage devices may be appropriate, it is not the intent of this standard to increase the size or use of battery storage devices provided with equipment.  The focus on improvements in equipment component and system design should lead to a reduction or elimination in the use of battery storage devices to achieve equipment reliability during voltage sag events.

The SEMI F47 document specifies the minimum voltage sag ride-through capability design requirements for equipment used in the semiconductor industry.  The expected equipment performance capability is shown graphically on a chart representing voltage sag duration and percent deviation of equipment nominal voltage.  The primary focus for this specification is semiconductor processing equipment including but not limited to the following tool types:

  • Etch equipment (Dry & Wet)
  • Film deposition equipment (CVD & PVD)
  • Thermal equipment
  • Surface prep and clean
  • Photolithography equipment (Stepper & Tracks)
  • Chemical Mechanical Polishing equipment
  • Ion Implant equipment
  • Metrology equipment
  • Automated test equipment

The actual SEMI F47 ride-through curve is shown below in Figure 3.

Figure 3 - The SEMI F47 Voltage Sag Ride-Through Curve
 

Magnitude and durations of voltage sags defined in SEMI F47 are shown in Table 2.

 
Table2 - SEMI F47 Defined Voltage Sag Points
  Magnitude Duration
SEMI F47 Test Points Percent Nominal Seconds 60Hz 50Hz
Single and Two Phase Sag Magnitude and Duration 50 0.05 3 2.5
50 0.2 12 10
70 0.2 12 10
70 0.5 30 25
80 0.5 30 25
80 1 60 50
 

Q:  What is Semi F42 and how is it different from Semi F47?

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A:  The best way to illustrate the differences between the two, is to compare the purposes of each document.  The purpose of SEMI F42 is to define a test method used to characterize the susceptibility of equipment used in the semiconductor industry.  SEMI F47 specifies the minimum voltage sag ride-through capability design requirements for equipment used in the semiconductor industry.

 

Q:  My tool has subsystems such as cryogenic pumps, chillers, vacuum pumps and alike systems. Are they considered part of my tool under SEMI F47?

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A:  The specification includes the equipment mainframe and all subsystems whose electrical power is directly affected by the operation of the equipment's EMO system.  If EMO of the mainframe is connected to the subsystem, then the subsystem is part of the semiconductor equipment.  In this case, the subsystem must be tested and pass SEMI F47 for compliance of the entire tool.

 

Q:  What is the criteria for being Semi F47 compliant?

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A:  The specification states that Semiconductor processing, metrology, and automated test equipment must be designed and built to conform to the voltage sag ride-through capability per the defined curve.  Equipment must continue to operate without interrupt (per SEMI F47 ) during conditions identified in the area above the defined line, as shown in Figure 4.  In the context of SEMI F47, interrupt means any assist or failure.  An assist is defined as an unplanned interruption that occurs during an equipment cycle where all three of the following conditions apply:

  • The interrupted equipment cycle is resumed through external intervention (e.g., by an operator or user, either human or host computer).
  • There is no replacement of a part, other than specified consumables.
  • There is no further variation from specification of equipment operation.
Furthermore, a failure is any unplanned interruption or variance from the specifications of equipment operation other than assists.  Although no variation in the tool's process is the goal, this standard addresses these issues as related to the equipment operation only.
 
Figure 4 - SEMI F47 Equipment Ride-Through Regions
 

Q:  Why not just install a UPS on the mains of a semiconductor tool?

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A:  The intent of SEMI F47 is to provide specifications for semiconductor processing equipment that will lead to improved selection criteria for subcomponents and improvements in equipment system design.  While it is recognized that in certain extreme causes or for specific functions battery storage devices may be appropriate, it is not the intent of this standard to increase the size or use of battery storage devices provided with equipment.  Focus on improvements in equipment component and system design should lead to a reduction or elimination in the use of battery storage devices to achieve equipment reliability during voltage sag events.  The reason for this is that battery-based power conditioners, such as UPSs, require maintenance.  The reliability of the UPS becomes dependent on the reliability of the maintenance program.

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