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WHITE PAPER
Kevin Lee, Sungmin Yang,
Sekyeong Lee, Rafael Lim and
Youngchul Choi
High-Voltage Design Engineering
Fairchild Semiconductor
Fourth-Generation Field Stop IGBT
with High-Performance and Enhanced
Latch-Up Immunity
When tasked with developing the fourth generation 650 V rated Field Stop (FS) Trench IGBTs, Fairchild’s engineers had a high
bar to overcome to develop a successor to the successful third generation IGBTs. To meet their design goals for achieving higher
performance without sacrificing reliability or ruggedness, the design team took some innovative approaches toward optimizing
the sub-micron width trench and mesa cell design of Fairchild’s FS technology. In doing so, they stretched the ‘ideal limit of
silicon’ and were able to achieve a remarkable 30% reduction in switching energy loss as a result. In spite of highly increased
channel density, their work resulted in very strong dynamic latch-up immunity, safely operating without failure under high-current
switching of more than 3000 A/cm 2 under severe test conditions. The following paper was presented at PCIM Europe 2015.
Insulated-gate bipolar transistor (IGBTs) are widely used
For Fairchild’s fourth-generation FS IGBTs, electron
in a variety of high-power applications, such as power
injection was enhanced with a very fine cell pitch design
supplies, motor drive inverters and electric vehicles due
and a new buffer structure that restricts the hole carrier
to their low conduction and switching energy losses.
injection. The result was remarkably better trade off
Requirements for more state-of-the-art power devices for
performance, as well as strong
power applications have triggered development efforts
latch-up immunity.
industry-wide to stretch the ideal limit of silicon. These
An innovative self-aligned
efforts focus on innovative silicon-based development,
contact process was applied
as well as wide band gap material development. The
to realize the narrow mesa or
theoretical silicon limit for IGBTs was investigated by
high-density cathode design of
Akio Nakagawa (1). In order to realize optimal silicon
the trench IGBT. This proved to
characteristics, various injection methods to enhance IGBT
be very effective in optimizing
structures – such as CSTBT, IEGT and narrow mesa IGBT
the critical dimension of active cell design for enhanced
(2-4) – were proposed. The technology for a new generation
on-state performance, as well as maximizing the latch-up
of Fairchild FS IGBTs presented in this paper takes some
current capability. In addition, multiple buffer layers were
of these methods into account and shows a much higher
adopted for the anode side of the IGBT to effectively
trade-off performance compared to other conventional
control the minority carrier injection during the on-state and
approaches to IGBTs. Furthermore, in spite of the higher
at the same time, completely block the electric field during
current density, the new FS IGBT showed stronger dynamic
the off-state (5). The enhanced switching ruggedness
latch-up immunity under inductive load and hard current
enables the next generation FS IGBTs to have a much
switching conditions.
higher channel density. The resulting improved trade-off,
In order to push IGBT silicon to the limit, extremely high
with lower conduction and switching energy loss, moves
electron injection efficiency from the MOS gate is required,
while the hole carrier injection needs to be restricted to the
level of contribution only for the conductivity modulation (1).
Fourth-generation
FS IGBTs enhance
electron injection
with a very fine
cell pitch design
the IGBT performance further towards the silicon limit.
New Generation FS Trench IGBT
sensitive to a process variation, such as photo
Regarding the IGBT’s cathode design, the trench
misalignment, and could result in significantly
gate structure allows for lower conduction losses
varying electrical characteristics. Also, in many high
compared to a planar structure. This enables higher
power field applications, multiple IGBTs are placed
channel density and removes the well known
in parallel to achieve very high-current operation.
Junction Field Effect Transistor (JFET) resistance
However, if the IGBTs in a parallel application have
which makes the trench structure a fascinating
significant threshold voltage or on-state voltage
solution for getting the device performance closer
mismatch, a very large current will flow to the IGBT
to the theoretical limit of an IGBT as it achieves
that has a relatively lower conduction value. So
such extremely high electron injection efficiency.
each IGBT should have as small a deviation as
In the anode design, the latest IGBT technology
possible to ensure safe parallel operation. Even
adopts an FS layer structure which increases the
a single IGBT device can be affected by process
blocking capability and thereby reduces the nec-
variation. Many active cells comprise the single
essary drift layer thickness. This reduced device
IGBT chip, so if each unit cell doesn’t have a
thickness further lowers the conduction and switch-
uniform doping profile, the current cannot flow
ing energy loss. The additional benefit of reduced
evenly inside the IGBT and will crowd to the several
thermal resistance allows for smaller chip sizes with
weak local unit cells and thereby compromise the
increased power density.
IGBT’s ruggedness.
For this new generation of IGBT development, we
In order to overcome the process variation
have focused on the optimization of cathode-
dependency of the electrical performances, we
and anode-side engineering to achieve state-
employed an innovative self-aligned contact
of-the-art device performance, which takes the
process along with several photo processes. These
industry another step toward achieving the ideal
processes address the issues of active structure
IGBT using silicon material. The vertical structures
patterning and ultimately eliminate the factors that
of the proposed IGBT are illustrated in Figure 1 for
cause photo misalignment. The vertical structure
the cathode and anode sides.
of an IGBT fabricated with the self-aligned contact
Figure 1 Proposed structure
High Density Cell Structure
process in this experiment is illustrated in
To realize the extremely high electron injection
Figure 1(c). By employing the self-aligned contact
efficiency from the cathode side of the IGBT, we
process, the high-density cell design with sub-
adopted a mesa width design with a cell pitch that
micron narrow mesa is realized. The fabricated
is half that of our previous generation IGBT. The
IGBTs in this experiment showed tight electrical
trench was designed in the submicron range.
performance distribution, such as on-state
Normally this fine cell pitch design could be
voltage and maximum saturation current level.
Fourth-Generation Field Stop IGBT Technology
2
May 2015
The higher density active pattern shown in
this figure is beneficial for extremely enhanced
electron injection from the cathode side and,
subsequently, lowers the conduction loss.
Unique Multiple Buffer Layer Structure
A new, multiple buffer structure was proposed in
2013 (5) to achieve higher device performance
and robust short circuit capability without
Figure 2 Static avalanche breakdown mode
oscillation of the FS trench IGBT. Generally, a
temperature coefficient over a current density of
conventional FS IGBT uses a single uniform buffer
90A/cm2, critical for parallel operations. Having
layer with 1~5e15cm doping concentration
–3
multiple buffer layers that are highly optimized
for both hole injection control and electric field
is also important for the proper electrical field
blocking efficiently. In this multiple FS layer
blocking under static avalanche breakdown mode.
experiment, a thin buffer layer with a much higher
The simulated electric field distributions comparing
doping concentration was also embedded for
conventional FS layers are illustrated in Figure 2(a)
better trade-off performance. The higher doping
and the measured breakdown voltage of about
concentration in the double buffer layer is even
720V with a very hard waveform is shown in
more effective for electric field blocking and hole
Figure 2(b). The results demonstrate that the
carrier injection control by the first FS layer (L1).
multiple buffer layers sufficiently block the electric
The lower doping concentration for the second
field in the off state.
buffer layer (L2) is preferred for forming a lightly
Turn-off switching characteristics
doped p-type collector for high-speed switching
High-speed turn-off switching was also achieved
performance without impacting the lifetime of the
through multiple buffer layers and gate capacitance
device. In addition, the device’s switching waveform
optimization without any lifetime control. The
can be effectively improved by varying the doping
switching characteristics of the proposed FS4
concentration and thickness of the double buffer
trench IGBT was measured under a inductive load
layers, which ensures proper carrier distribution
switching test circuit in Figure 3(a). The test
control during the ON/OFF switching operation.
conditions were Vcc=400V, Vge=15V, Jc=470A/cm2
with 6 Ω gate resistance. Compared to previous
Experiment and results
generation IGBTs, the FS4 trench IGBT showed a
Static characteristics
30-percent lower turn-off energy loss at the same
Since lifetime control was not used for the FS IGBT
on-state voltage drop. Additionally, the turn-off
development in this work, the on-state voltage
switching at
drop depends mainly on the current gain through
lower current
the anode engineering with multiple FS layers.
conditions is
Therefore, optimization of the anode condition is
very important
critical for the overall trade-off in performance. The
because the
switching performance is also directly influenced
operating
by the current gain of the device. The on-state
current
voltage drop of the proposed FS trench IGBT
level in the
is 1.65V at 470A/cm2 while showing a positive
Fourth-Generation Field Stop IGBT Technology
3
Figure 3 Turn-off switching waveforms
May 2015
actual application is generally lower than the
Latch up immunity
rated current. To achieve the desired high-speed
Regarding the design targets of the fourth-
switching even under lower current conditions,
generation IGBT, the channel density has been
carrier distribution control was used to control
increased while
Vce slope during the turn off transient. The Vce
the chip size has
waveform during low current switching has two
been reduced,
slopes. The first slope, reflective of the state before
dramatically
Figure 3 Turn-off switching waveforms
the electric field
increasing the
reaches the buffer
power density. This
layer, has been
has traditionally
improved by
been a concern
carrier distribution
because increased
optimization with
power density increases the possibility of latch-up.
anode and cathode
The FS4 trench IGBT technology showed strong
engineering. The
latch-up ruggedness in spite of the high-power
measured switching
density and with a smaller active size than previous
waveforms under low-
generation IGBT technology. The latch-up immunity
Figure 5 Static latch-up characteristics
current switching with Jc=95A/cm are compared
was evaluated under static and dynamic conditions,
in Figure 3(b) for the proposed and the previous
as shown in Figures 5 and 6 respectively. Figure 5
generation devices.
shows that the maximum static saturation current
Trade-off performance
is around 4000A/cm2 with no latch-up occurring. In
2
particular, for the dynamic latch-up characteristics
Trade-off performance has been compared
shown in Figure 6, the proposed FS IGBT shows a
between Fairchild’s third- and fourth- generation FS
very strong ruggedness. The device safely operates
trench IGBT in terms of conduction and switching
over a 3000A/cm2 current density without failure
energy loss in Figure 4. The fourth-generation
while under the severe hard switching conditions
FS IGBT shows drastically improved trade-off
(T=150°C, Rg=0Ω, Vge=±15V to induce very high
performance compared with previous generation
voltage slop (dv/dt) between collector and emitter).
IGBT technology – about 30-percent turn-off
The self-aligned process removes any possible local
energy loss (Eoff) reduction at the same on-state
weak points from the contact photo misalignment
voltage. The increased channel density with
so that the injected minority carrier can flow evenly
reduced gate capacitance and restricted minority
without crowding into any specific area.
carrier injection from anode side was instrumental
in the improved trade-off performance.
Figure 6 Dynamic latch-up characteristics
Figure 4 Trade-off performance
Fourth-Generation Field Stop IGBT Technology
4
May 2015
Conclusion
Reference
In summary, Fairchild’s fourth-generation
1. A. Nakagawa, “Theoretical Investigation
FS IGBT technology was successfully developed
of Silicon Limit Characteristics of IGBT”,
using the injection enhanced carrier profile
Proc. ISPSD’06, pp. 5-8, 2006
that was optimized in an effort to approach the
2. Z. Chen, “Next Generation 600V CSTBTTM
limits of IGBT silicon. This new generation of
with an Advanced Fine Pattern and a
FS IGBTs with a high-density cell structure and
Thin Wafer Process technologies”,
well-designed double buffer layer shows superior
Proc. ISPSD’12, pp. 25-28, 2012
device performance under static and dynamic
3. K. Matsushita “Low Gate Capacitance IEGT
states as well as strong latch-up ruggedness. We
with Trench Shield Emitter (IEGT-TSE)
have demonstrated that the self-aligned process
Realizing High Frequency Operation”,
is a very effective method for the embodiment of
Proc ISPSD’13, pp. 269-272, 2013
sub-micron trench and mesa active design, as
4. M. Sumitomo, J. Asai, H. Sakane,
well as for realizing strong latch-up immunity. For
K. Arakawa, Y. Higuchi, and M. Matsui,
future iterations of IGBT development, the mesa
“Low loss IGBT with Partially Narrow Mesa
width will be continously narrowed through the
Structure (PNM-IGBT)”, Proc. ISPSD’12,
self-aligned process. This will further maximize the
pp. 17-20, 2012
injection enhancement and accordingly the buffer
5. K. Lee, “Optimized buffer layer for the high
structure for the minority carrier injection control
performance and enhanced short circuit
will be optimized.
immunity of Trench IGBT”, Proc. PCIM’13,
pp. 351-356, 2013
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