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SEMI E136-1104 © SEMI 2004 3 adequate lifting devices for large weights such as long RF cables and generators themselves. 7.4 Pers ons perform ing tests should determi ne which method of w ater cooling of the resist or i…
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connect various parts of the RF power delivery system.
5.2.2 calorimeter — an RF power measurement
instrument using differential temperature and mass flow
rate to determine true heating power.
5.2.3 harmonic frequency — the harmonic frequencies
are defined as integer multiples of the fundamental
frequency. For example, the second harmonic of 13.56
MHz is 27.12 MHz.
5.2.4 RF generator — a component in the RF power
delivery system used to develop RF energy.
5.2.5 RF termination — a device for terminating RF
transmission systems, and converting RF electrical
energy into heat. RF terminations normally have values
that are the same as the characteristic impedance of the
transmission system.
5.2.6 RF load — another term used to describe an RF
termination.
6 Test Apparatus
6.1 Calorimetric Power Meter — This instrument is
designed to be used in conjunction with a high
efficiency water-cooled RF termination, rated for the
maximum power output of the RF generator. Typical
calorimetric power meters use internal thermocouple-
or thermistor-type temperature sensors, a precision
coolant flow meter, and processing circuits for storage
of calibration constants and coolant physical
characteristics. Choose instruments capable of
providing a power measurement accuracy of ± 1% after
calibration at a specific power level.
6.2 Water-Cooled RF Termination — This device
provides a low VSWR (i.e., high return loss)
termination for the testing of the RF generator. Use an
RF termination sized appropriately for the RF generator
under test. Finally, use an RF termination capable of
dissipating DC energy, without damage due to the
effects of electrolysis. The return loss of the RF
termination, including the interconnecting cable
between the RF generator and the termination shall be
no less than 30 dB. This return loss ensures maximum
power transfer between the RF generator and the
calorimeter.
NOTE 1: See Section 7 for safety-related recommendations
related to RF terminations.
6.3 Voltage and Current Meters — These meters are
used to measure the voltage and current presented to the
calorimeter load during the DC calibration of the
calorimeter. These instruments shall have a certificate
indicating NIST traceability. Choose meters capable of
voltage and current measurement accuracy of less than
0.2%.
6.4 Calibration Power Supply — This power supply is
used as a source of clean DC energy for the purposes of
calibrating the calorimetric power meter. Use a power
supply sized for the calibration to be performed and that
provides a very stable output at all levels.
6.5 Cables and Connectors — Various interconnecting
cables are required to perform the tests outlined in this
test method. These include low-loss cables to connect
the DC power source to the measuring instrument and
to the RF load, in addition to low-loss RF cables and
connectors to connect the RF generator output to the
load.
6.5.1 In order to keep cable losses to a minimum,
minimize the lengths of interconnecting cables.
6.5.2 In addition, compensate for the losses associated
with interconnecting cables used.
NOTE 2: For example, if 6 m (20 feet) of #10 AWG copper
wire is used to connect the DC supply to the calorimeter, this
wire will add approximately 0.04 of resistance between the
DC source and the 50 termination. Using a DC supply
voltage of 500 V terminated by 50 (i.e., 5 kW), this will
result in a loss of about 4 W in the copper wire.
6.5.3 Adjust the reading obtained by the calorimeter
according to the cable loss value.
NOTE 3: For example, suppose that an RF generator,
operating at 13.56 MHz and developing 3 kW is connected to
the calorimeter through a 6 m (20-foot) length of RG-217/U
coaxial cable. RG-217/U coaxial cable has a loss
specification of 0.41 dB per 30 m (100 feet) of cable at 13.56
MHz. Therefore, for 6 m (20 feet) of cable, the loss would be
approximately 0.41/5, or 0.082 dB. If the RF generator is
developing 3 kW, this means that approximately 2.944 kW
would appear at the calorimeter due to the cable loss.
7 Safety Precautions
NOTE 4: The tests described in this test method involve
using low-output power test instrumentation (typically less
than 10 mW).
7.1 Work should be conducted in accordance with
local safety requirements and test device
manufacturer’s recommended safety procedures.
7.2 The area immediately surrounding the test setup
should be kept free and clear of unnecessary equipment
and materials.
7.3 Testing personnel should ensure they have
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adequate lifting devices for large weights such as long
RF cables and generators themselves.
7.4 Persons performing tests should determine which
method of water cooling of the resistor in the RF
termination is utilized by the test device manufacturer
and should take steps to ensure safe use of the device.
7.4.1 Cooling-water flow rates should be appropriate to
the test device manufacturer’s recommendations for
cooling of the power load level being tested. Consult
the RF generator operating manual load specifications
for this information.
7.4.2 An RF termination using internal resistor cooling
only is highly recommended because such resistors (of
the tubular ceramic, thin film type) are usually cooled
by routing the coolant to the inside of the ceramic tube
and away from the resistive film, thereby eliminating
the possibility of electrolysis affecting the resistive
film.
7.4.3 In an alternative method, the coolant is routed on
both the inside and the outside of the resistor. While
this method provides for more efficient cooling, the
proximity of the coolant to the resistive film can
potentially result in damage to the resistive film caused
by electrolysis generated by the DC energy passing
through the fluid in the termination.
7.4.3.1 For this reason, if an RF termination using both
internal and external cooling is used, it is important to
periodically check the resistor for damage due to
electrolysis. This can be done easily by measuring the
resistance of the load resistor using a simple digital
multimeter.
7.4.3.2 With the RF termination disconnected from the
RF cable, place one probe on the center conductor of
the termination, and the other probe on the
termination’s outer conductor.
7.4.3.3 Measure the DC resistance of this path. If the
DC resistance is more than 53 , replace the resistor in
the termination.
7.5 In order to prevent the possibility of ground loops
between the RF generator and the calorimeter system,
be sure to operate the calorimeter and the RF generator
on the same AC supply.
8 Test Setup for RF Generator Power Output
Test
8.1 The test setup for the RF generator output power
test consists of the RF generator connected to the
calorimeter power meter through a length of coaxial
cable.
8.2 Prior to making any measurements, calibrate the
calorimeter power meter using a source of stable DC
energy.
9 Test Procedure for Determining RF
Generator Output Power
9.1 Calorimetric power meters provide excellent
reference standards for the measurement of RF energy
in that they are capable of measuring the true heating
power of a waveform, including any harmonic- or
nonharmonic-related frequency components. The
technique employed for this test is to first calibrate the
calorimetric power meter with DC energy, which can
be very precisely determined. Then, use two digital
multimeters (e.g., Fluke™ 87 or equivalent) to
determine the actual DC power applied to the
calorimeter. Finally, once the DC calibration has been
performed at a particular power level, connect the
output of the RF generator under test to the calorimeter
and measure the output power of the RF generator. The
basic steps involved in the use of this system are
outlined below.
9.2 Calibrate the Calorimeter — Calibrate the
calorimeter with the stable DC power supply, using the
two digital multimeters to read the voltage and current
applied to the calorimeter’s RF load.
9.3 Connections — Connect the DC power supply, the
digital multimeters, and the RF load according to
Figures 1 and 2. Make the connections between the
multimeters, the DC power supply, and the RF load
using high-quality low-loss cable and high-quality
corrosion-free cable terminations.
Figure 1
Calorimeter Calibration Configuration
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Figure 2
Calorimeter Calibration Connection Diagram
9.3.1 Use a calorimeter equipped with a coolant
system.
NOTE 5: Most calorimeter systems use either a tap-water
coolant system or a recirculating system of some type. Best
results may be obtained through the use of a self-contained
heat exchanger and coolant recirculating system.
9.3.2 Make all of the electrical and coolant connections
to the calorimeter.
9.3.3 Apply main power to the calorimeter system and
allow one hour for stabilization.
9.3.4 Load Differential Temperature — Most
calorimeter systems will yield optimum performance
when certain operating parameters relating to the
differential temperature across the RF load and the flow
rate are maintained. In most cases, a differential
temperature of 2°C represents a good beginning point.
The coolant flow rate may then be set to provide for
this differential temperature.
9.3.4.1 The following formula may be used to arrive at
the correct coolant flow rate.
Power = 69.32 ×
T × Flow Rate (1)
where:
Power is in W,
T is in C, and
Flow Rate is in L/min.
9.3.4.2 Solving for flow rate yields:
Flow Rate = (Power / 69.32) /
T (2)
9.3.5 Coolant Types — In most cases, the coolant used
in calorimetric power measurement systems is pure
distilled water. In this case, most modern calorimeters
will automatically correct for changes to the coolant
physical characteristics (e.g., specific heat, specific
gravity) as the coolant temperature changes.
9.3.5.1 If coolant other than pure water is used and the
calorimeter chosen will not automatically compensate
for changes to the coolant, adjust the power readings
obtained for the effects of the coolant changes. Consult
the calorimeter operating manual for specific details.
9.3.6 Calorimeter Calibration — Following the
calorimeter warm-up, the device is ready for
calibration.
9.3.6.1 With the calorimeter operating at the
appropriate flow rate, apply DC power to the RF load,
according to the voltage and current readings obtained
with the two digital multimeters.
9.3.6.2 Multiply the voltage reading by the current
reading to obtain the power applied to the load.
9.3.6.3 Allow 15 minutes for the calorimeter reading to
stabilize.
9.3.6.4 Using the procedure in the calorimeter
operating manual, set the calorimeter such that the
calorimeter reading is exactly the same as that obtained
using the voltage and current meters.
9.3.6.5 At this point, add any offsets that are associated
with interconnecting cable losses, as outlined in
Sections 6.5.2 and 6.5.3.
9.3.6.6 The calorimeter is now calibrated at this
specific power level.
NOTE 6: If a new power level is selected, repeat this process
in order to compensate for instrumentation linearity effects of
the calorimeter.
9.3.7 RF Generator Testing — With the calorimeter
calibrated at a particular power level, it is now possible
to test the RF generator power output at this level.
9.3.8 Connections — Connect the RF generator output
to the RF load according to Figure 3. Use cables and
connectors between the RF generator output and the RF
load of a high-quality, low-loss type. As outlined
above, some power will be lost in the cables between
the generator and the load. Add these losses as offsets
to the calorimeter reading, in order to obtain the total
power output of the generator. Base the losses upon
cable length, cable type, and operating frequency. See
SEMI E114 test method for details on cable loss
compensation.
Figure 3
RF Generator Output Power Test