Optimizing Reliability Testing of Power Semiconductor Devices and Modules

Optimizing Reliability Testing of Power Semiconductor Devices and Modules
application brief
Optimizing Reliability Testing of Power
Semiconductor Devices and Modules with
Keithley SMU Instruments and Switch Systems
Introduction
Typical Reliability Tests
To minimize early defect rates and to continuously improve
the overall reliability and lifetime of power semiconductors,
a variety of important tests are performed by both
manufacturers and end-use designers. Many of these tests
are outlined in JEDEC Standards such as JESD22-A108D
“Temperature, Bias, and Operating Life,” JESD22-A110D
“Highly Accelerated Temperature and Humidity Stress Test
(HAST),” or JESD236 “Reliability Qualification of Power
Amplifier Modules.” This application brief discusses methods
to optimize reliability testing of silicon and wide band gap
(WBG) power semiconductor devices, modules, and materials
by using Keithley SourceMeter® Source Measure Unit (SMU)
Instruments and Switch Systems (Figures 1 and 2).
Typical reliability tests involve stressing a batch or batches
of sample devices for hundreds or thousands of hours with
bias voltages that are greater than or equal to their normal
operating voltages while subjecting them to temperatures
that are well beyond normal operating conditions. During this
stress, a variety of key operating parameters are measured at
specific time intervals. Some of the more popular reliability
tests for power semiconductors are HTOL (High Temperature
Operating Life), ELFR (Early Life Failure Rate), HTFB (High
Temperature Forward Bias), HTRB (High Temperature Reverse
Bias), and HAST (Highly Accelerated Temperature & Humidity
Stress Test). These tests will either use a continuous bias
(Figure 3) or cycled bias (Figure 4). A continuous bias can be
a fixed voltage or a staircase ramp. A cycled bias will typically
vary the duty cycle and/or frequency of the bias voltage. In both
cases, key device parameters will be tested continuously or at
specific time intervals.
Reliability Testing Challenges
Figure 1. Keithley Series 2650A High Power SourceMeter
SMU Instruments.
Figure 2. Keithley Series 3700A and 707B Series Switch Systems.
Reliability testing of today’s WBG power semiconductors
presents several key challenges for engineers and test system
designers. Most importantly, since most of these devices are
being targeted for energy-efficiency applications, they have
much lower leakage and on-resistance specifications compared
to traditional silicon. The test instrumentation must therefore
be capable of providing the necessary accuracy, resolution, and
stability to meet the electrical requirements of these devices.
In addition, since WBG devices exhibit failure mechanisms
that are different from silicon, effective reliability testing per
JEDEC standards requires larger sample sizes and longer
stress durations to adequately predict important reliability
parameters. This requires test instrumentation that is capable
of supplying enough power to test many devices in parallel,
while maintaining the accuracy and resolution mentioned
above. Finally, the test instrumentation must be able to respond
to the high speed behaviors associated with these devices and
produce the masses of data associated with testing devices in
parallel. Each instrument in the system must be fast, and all
units must operate in a highly synchronized manner.
1
application brief
Optimizing Reliability Testing of Power Semiconductor Devices
and Modules with Keithley SMU Instruments and Switch Systems
Vd = 200V
Vd = 1320V
+5V steps every 8 hours
Vd = 50V
Vd = 1200V
±120V steps every 10 hours
Temp = 140°C
Temp = 130°C
Temp = 80°C
0
48
96
144
192
240
0
Hours
480
720
960
Hours
Figure 3. Continuous Bias Test. Vd=50V. Step temperature 15°C
every 48 hours from 80°C to 140°C. Step Vd 5V every 8 hours
from 50V to 200V. Collect I-V curve data before and after each
voltage step. Test 72 devices per batch.
Figure 4. Cycled Bias Test. Vd cycles between 1200V to 1320V
every 10 hours with a 50% duty cycle. Maintain temperature of
130°C for 1000 hours, and continuously measure Id. Test 240
devices per batch.
Keithley Solutions
works together with TSP technology to enable high-speed,
multi-channel, SMU-per-pin parallel testing or seamless
integration with Keithley’s Series 3700A or Series 700 Switch
Mainframes (Figure 6). The Keithley Series 3700A Mainframes
also have a temperature measurement option that can be
used for monitoring internal oven temperature at multiple
locations. Synchronization with external equipment such as
temperature chambers is enabled by the built-in digital I/O
capability. Because Series 2600B and 2650A instruments have
fully isolated channels that do not require a mainframe, they
can easily be reconfigured and re-deployed as reliability test
requirements evolve.
Keithley reliability test solutions based on the Series 2600B
and 2650A System SourceMeter SMU Instruments meet these
demanding requirements (Figure 5). Keithley’s Series 2600B
and 2650A are modular, independent, isolated SMUs that
provide up to 200W per channel with measurement resolutions
to sub-pA levels.
In addition, all Series 2600B and 2650A SMU Instruments
contain Keithley’s Test Script Processor (TSP®) and TSPLink® technology for ultra- high speed operation and parallel
test. Keithley’s TSP technology enables the instrument to
perform advanced tests without PC intervention through
the use of embedded test scripts. These scripts are complete
test subroutines executed from the instrument’s non-volatile
memory that can perform conditional branching, flow control,
advanced calculations, pass-fail testing, and more. On-board
memory buffers can store over 140,000 readings, and can
be queried while data is being written to them. For larger,
multi-channel applications, Keithley’s TSP-Link technology
Model 2657A: 180W, High Voltage
• Source and measure up to 3000V @
20mA or 1500V @ 120mA.
• Basic voltage source accuracy of 0.03%
with 5mV programming resolution.
• Basic current measure accuracy of
0.02%. 1fA resolution.
2
240
Solution Details
Reliability test algorithms are typically very specific to the
device under test (DUT) and its market requirements. In
this application brief, we are referring to the cycled bias test
example in Figure 4. This test cycles the drain voltage (Vd)
from 1200V to 1320V every 10 hours with a 50% duty cycle,
and maintains a temperature of 130°C for 1000 hours while
Model 2636B: Dual Channel,
Low Current
• Source and measure up to 200V @
100mA or 20V @ 1.5A per channel.
• Basic voltage source accuracy of 0.03%
with 5µV programming resolution.
• Basic current measure accuracy of
0.02%. 0.1fA resolution.
Model 2651A: 200W, High Current
• Source and measure up to 40V @ 5A
or 10V @ 20A
• Basic voltage source accuracy of 0.02%
with 5µV programming resolution.
• Basic current measure accuracy of
0.02%. 1pA resolution.
application brief
Model 3706A: High Speed
Switch System
• Up to 576 channels of 2-pole switching
• Multiplexer or matrix configurations
with 300V, 1A capability
• Built-in temperature
measurement option.
Model 707B: Low Current
Switch System
• Up to 576 matrix cross-points
• 1300V, 1A switching with <1pA offset • 200V, 2A switching with <100fA offset
Figure 6. Key specifications of Keithley switch systems.
continuously measuring the devices’ off-state leakage current (Id). A batch size of
240 is tested, so the most economical approach is to use a switch system to route
the power of one SMU instrument to all DUTs.
An example system block diagram is shown in Figure 7*. The Keithley Model 2657A
High Voltage SMU Instrument provides power to the drain terminal of all 240 DUTs,
which are connected in parallel. Maximum current available from the Model 2657A
at voltages of 1200V–1320V is 120mA, so there is plenty of capacity. In fact, the Model
2657A can supply power to multiple batches. Current limiting resistors are placed in
series with each DUT in case of device failure.
The Keithley Model 2635B SMU Instrument is used to measure the leakage current
(Id) of each device at the source terminal, and is connected to each device through
the Keithley Model 3706A Switch System with Model 3720 Switch Cards.
GPIB
PC
or LAN
2657A
TSP-Link
2635B
TSP-Link
3706A
I Limit
DUT #1
G
D
I Limit
DUT#240
G
S
D
S
3720 Cards
Ch.1
Low Leakage Diode
Ch.240
A low-leakage diode is also placed at the
source terminal in parallel with each
switch. When a given switch is open,
any leakage current from the DUT will
flow through this diode. When a given
switch is closed, the leakage current for
that DUT will flow through the Keithley
Model 2635B (since its output is 0V
when measuring current). The Model
3720 Switch Cards are configured for
single-pole switching, and each card has
a 60-channel capacity; therefore, four
switch cards are used (the Model 3706A
Switch System chassis has a total capacity
of six switch cards). The scanning speed
of the Model 3720 Switch Card is 120
channels per second.
For maximum system speed and
programming simplicity, the TSP-Link
intercommunication bus is used to
connect the three instruments together.
Only one connection is needed to the
PC controller, and this is through the
Model 2657A over GPIB or LAN (LXI).
The PC controller contains the main
test program that calls the TSP scripts
(subroutines) that reside inside the
Model 2657A’s non-volatile memory.
Keithley’s TSP technology enables
all instrument control and most data
management to be performed at the
instrument level, thus eliminating
the typical GPIB or LAN traffic delays
that slow system-level throughput in
instrument-based systems.
In this example, Keithley’s Series
2600B and 2650A System SourceMeter
SMU Instruments and Series 3700A
Switch Systems are used to meet the
accuracy, resolution, power, and speed
requirements of today’s semiconductor
reliability test applications.
Low Leakage Diode
Figure 7.
*This example is for illustrative purposes only. Full test system design and adherence to proper safety
guidelines and procedures are the responsibility of the designer and operator.
3
application brief
Optimizing Reliability Testing of Power Semiconductor Devices
and Modules with Keithley SMU Instruments and Switch Systems
Specifications are subject to change without notice. All Keithley trademarks and trade names are the property of Keithley Instruments, Inc.
All other trademarks and trade names are the property of their respective companies.
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© Copyright 2014 Keithley Instruments, Inc.
Printed in the U.S.A
No. 3253
3.24.14
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