Breakdown Behavior of TMR Head in ESD Transients - Lirmm

Breakdown Behavior of TMR Head in ESD Transients
Zhao-Yu Teng, Marshall Mo, William Li, Min-Bing Wong and Sidney Chou
SAE Magnetics (HK) Ltd., Dongguan City, Guangdong Province, People’s Republic of China
Tel.: (86) 769-2810033 ext 6834, fax: (86) 769-2811524, e-mail: William_Li@sae.com.hk, ZY_Teng@sae.com.hk
Abstract –HBM and D-CDM breakdown testing were preformed on the latest TMR heads of different
resistance. Pspice simulations were conducted for individual TMR heads based on their actual failure voltage at
HBM and D-CDM, in an attempt to investigate damage current through TMR barrier, as well as voltage across
TMR barrier. Results show that in short transient test –-HBM and DCDM, damage current threshold (through
TMR barrier) is inversely proportional to TMR resistance, while damaging current density threshold σh is a
constant in each transient model.
Introduction
Ultra high density recording over 100Gbit/in 2
requires a highly sensitive read head. Because of its
much higher ∆R/R ratio, the TMR head is superior to
the GMR head, though it still has operating frequency
and noise issues due to its high resistance. This paper
investigates the ESD property of prototype TMR
heads for the next generation product featured with a
very thin Al-O insulator barrier ~0.9nm.
TMR stack tested in this research consists of spinvalue
tunneling sensor with PtMn antiferromagnetic
material and synthetic pinned layers. Resistance area
product (RA) is ~2.13Ohm*µm 2 . Figure1 below is a
250k X super SEM photo of the TMR head.
Figure 1. 250k X Supper SEM photo of TMR head, ABS view
To evaluate breakdown voltage threshold of prototype
TMR, two commonly accepted ESD test models in
MR industry, HBM and DCDM, were used in this
study.
Experimental
6 groups of TMR head with different MR heights
were evaluated. The MR/barrier height was controlled
by lapping process at row bar level. Herein MR/barrier
height size is expressed by arbitrary unit: 1, 2, 3, 4, 6
and 8. The TMR head’s resistance(MRR) is inversely
proportional to the MRH as shown in figure 2 -- MRR
distribution Vs MRH.
Figure 2. MRR Vs MRH
TMR heads were deposited on 20A Si and 20A DLC
film in vacuum process. All the TMR heads were built
into HGA with Hutchinson long tail suspension for
ESD testing.

HBM test was performed on Orxy700H tester where
QST parameters were recorded automatically after
each discharge. Bias current of QST testing was
0.4mA, with +/-150Oe magnetic field sweep. Starting
from 2.0V, the votage applied to the head was raised
every 0.5V, untill the head broke down. The HBM
ESD test circuit consists of a 100pF capacitor and a
1.5kOhm resistor. HBM transient current has a rising
time (10% Imax to 90% Imax) of 2nS~20nS (affected
by the parasitic test board capacitance), and decays
with a time constant RC of ~150ns;
DCDM ESD test was performed on ISI2002 tester,
QST parameters were recorded automatically after
each discharge. Bias current of QST test was 0.4mA,
with +/-150Oe magnetic field sweep. The voltage
applied to the head was raised from 0.2V, with a
coarse step of 0.2V and a fine step of 0.1V when it
reached 0.8V, untill the head broke dowm. Contrary
to HBM, DCDM transient has a very short duration to
simulate “metal to metal” contact discharge, and
actual current through MR sensor is much less than
the total discharge current in DCDM module. Its
waveform is related to trace design of HGA
suspension. More specifically in this evaluation,
DCDM has a rising time (10% Imax to 90% Imax) of
~370ps and PW50 ~2.5ns.
A Tektronix CT-6 current transformer and a 2GHz
LeCory 960M digital oscilloscope were used to
measure the discharge currents in HBM and DCDM
ESD tests.
Results & Discussion
The tests prove that most of TMR’s QST amplitude
showed the same decline trend with resistance after
ESD discharge. Dr. Wallash and Baril have arrived at
similar conclusions in their researches [1] and [2].
This paper focuses on the study of resistance
degradation caused by ESD discharge. Often, there is
a knee area (indicated with arrow) before breakdown
in the diagram of MRR vs ESD charge voltage, as
shown in figure 3.
Figure 3. TMR head breakdown in HBM and DCDM ESD test
When R(Resistence) drops more than 3% of it initial
value, the degradation trend of R becomes
irreversible. The lagging R drop in the “knee area” is
considered as “pin hole enlargment” after ESD
discharging. Once R drop reaches 10% after ESD
discharge, TMR head’s QST amplitude will drop more
than 20% dramatically.
The MRR drop during ESD charge is considered as
short path/ pin hole formation inside barrier, so MRR
is the total resistance of RTMR paralell-connected with
RShort which is caused by pinhole. R=
(RShort*RTMR)/(RShort+RTMR). In theory, if we normalize
resistance of TMR head without pinhole R=RTMR=1,
then MR ratio is expressed as ∆R/R=∆RTMR. After
ESD charge, R=RShort/(RShort+1), the MR ratio is
expressed as ∆R/R=R*∆RTMR/[1+∆RTMR*(1-R)].
Therefor TMR resistance(MRR) should be roughly
proportional to MR ratio(∆R/R) when MR ratio is
only several percent in the 0.4mA and 150Oe QST
testing. The ESD transient testing also proved this
point, two typical curves are shown in figure 4.

Figure4. TMR MRR Vs MR ratio—∆R/R (normalized) in HBM
and DCDM ESD test
Therefore, two criteria were adopted for TMR
breakdown: 3% R drop and 10% R drop, which
correponds to 6% amplitude drop and 20% amplitude
drop respectively according to the linear relationship
between MRR and MR ratio.
Since barrier thickness was only close to 0.9nm, and
the breakdown position after ESD discharge was
uncertain inside the TMR barrier, supper SEM failed
to give any clear evidence of 10% R drop sample after
ESD short transient charge.
Below are the results of two ESD test models —HBM
and DCDM.
A) HBM testing
Experimental HBM test result indicates that HBM
threshold voltage is inversely proportional to the MRR
of TMR, as shown in figure 5 for 3% R drop criterion
and for 10% R drop criterion.
Figure 5. HBM failure threshold Vs MRR --R 3% drop and R
10% drop criteria
Circuit simulation proposed by Van Roozendaal [3]
and Verhaege [4], as shown in figure 6, was used to
analyze the damaging current through the TMR
barrier during HBM discharge. The simulation results
show a good agreement with experimental results
measured with a CT-6 and an oscilloscope LeCroy
960M, as illustrated in figure 7.
The test result also indicates that, for the parasitic test
board capacitance in HBM test circuit and high
resistance of TMR head, rising time of HBM transient
becomes longer, which agrees with Van Roozendaal
[3] and Verhaege [4]’s findings.
Figure 6. HBM simulation circuit, containing the HBM capacitor
C1, the HBM resistor R3, a shunt capacitance C4 across R3, the
series inductance L1 of the discharge path, the test board
capacitance C2 and the resistive load R1 --MRR.
Figure 7. Waveform tested and simulated for 10V HBM, HGA
MRR=294.7ohm, MRR=50.9ohm.
Circuit simulation for breakdown current (Ih) through
TMR barrier shows that breakdown current Ih (3% R
drop) of TMR is inversely proportional to MRR
(figure 8). The trend is very clear—a power function
Ih= 677.36*R -1 , However, the breakdown voltage Vh
across the barrier has no clear correlation with MRR.
Some high MRR heads showed higher breakdown
voltage threshold Vh than low MRR ones.

Both breakdown current Ih (mA) and breakdown
voltage Vh (Vh=Ih*R) are ploted in figure 8. For
300Ohm MRR target, breakdown current is ~2.26mA,
voltage threshold Vh is ~0.68V in HBM testing.
Figure 8. Current threshold It and voltage threshold Vt of HBM
B). DCDM testing
The DCDM testing of MR head has been well
discussed in Baril and Cheung’s papers [5]. It is
believed that the current through TMR barrier is only
a part of the current measured using a CT-6 in ISI
DCDM tool. Pspice simulation is needed to obtain the
current through TMR barrier.
As shown in figure 9, a detailed model for an HGA
used in this testing is given. The discharge current in
DCDM is determined by transmission line model of
HGA suspension circuit.
Figure 9. Simulation circuit for HGA DCDM testing. The HGA
circuit part conatins the transmission line elements: Llx, Clx, Rlx
(left trace), Lrx, Crx, Rrx (right trace) and TMR resistance Rmr;
the discharge circuit part contains the resistive load Rd, the
parasitic elements: Cd (effective shunt capacitance), Ld1 and Ld2
(effective series inductances).
Experimental DCDM test result also indicates that
DCDM threshold is inversely proportional to the MRR
of TMR. Figure 10 illustrates this relationship.
Figure 10. DCDM failure threshold Vs MRR--R 3% drop and R
10% drop criteria
The DCDM circuit as mentioned above (figure 9) was
also used to analyze the current through TMR barrier
during DCDM charge. The simulation results indicate
a good agreement with the test results measured with a
CT-6 and an oscilloscope LeCroy 960M, as shown
figure11.
Figure11. DCDM waveform tested and simulated @1V DCDM,
HGA MRR=320ohm and short --MRR=3.8 ohm
The total DCDM current and current through MR of a
320 ohm HGA are illustrated in figure 12. The current
through TMR barrier is only about one third of the
total current measured using a CT-6 in DCDM tool.

Figure 12. DCDM current comparison-- total and through MR ,
320Ohm HGA @1V DCDM
Circuit simulation for the current through TMR
barrier shows that breakdown current Ih (3% R drop)
of TMR is inversely proportional to MRR. The trend
is very clear—a power function Ih=755.21*R -1 , and
again, there is no clear correlation between breakdown
voltage Vh across barrier and MRR. Some high MRR
heads also showed higher breakdown threshold Vh.
Both breakdown current Ih (mA) and breakdown
voltage Vh (Vh=Ih*MRR) are ploted in figure13. For
300ohm MRR target, the breakdown current is
~2.52mA, breakdown voltage Vh is ~0.76V in
DCDM.
Figure13 Current threshold It and voltage threshold Vt of DCDM-
-R 3% drop criteria
Figure 14 shows breakdown current Ih (mA) of both
HBM and DCDM. The breakdown current Ih of
DCDM is slightly higher than that of HBM.
Figure14. Current threshold Ih Vs MRR---- HBM and DCDM
Summary and Conclusions
Circuit simulation proves that, for both HBM and
DCDM, breakdown current Ih through barrier is
roughly inversely proportional to MRR, while
breakdown voltage Vh across barrier has no clear
dependence on MRR (MRH), the value of most heads
ranges from 0.6V to 1.0V. Considering the Al-O
barrier’s thickness is close to 0.9nm, the breakdown E
field is 6.7E8V/m--1.1E9V/m.
The scattering of breakdown threshold Vh is affected
by the quality of TMR barrier—pinhole distribution
and surface condition of pole area after slider
processes. Vh (Vh=Ih*MRR) of some high MRR
heads is higher than normal value, it may result from
the pinhole distribution in TMR barrier, high MRR
heads have a much smaller barrier area which should
have less pinhole probability in wafer process.
Based on the test results, it is concluded that the
equation of the damaging current threshold Ih can be
expressed in the form:
K K * MRW * MRH
Ih ≈ =
(mA)
MRR RA
K---Constant related with ESD transient pulse width
(Ohm*mA).
MRW ---MR sensor/barrier width (µm).
MRH---MR sensor/barrier height (µm).
RA---Resistance (barrier) area product (Ohm*µm 2 ).
The damaging current density threshold σh can be
expressed in the form:

Ih Ih K
σ h = =
≈ (mA/µm
S MRW * MRH RA
2 )
In the two ESD short transient tests--- HBM and
DCDM, the K is 677.36mA*µm 2 and 755.21mA*µm 2
respectively, RA is 2.13Ohm*µm 2 , therefore the
current density threshold σh is 318.0mA/µm 2 and
354.6mA/µm 2 . Once the damaging current density σ
reaches the threshold σh, TMR head will break down
with MRR drop. The slight threshold difference
between HBM and DCDM is considered related with
the duration of ESD transient, it needs further studies.
Acknowledgement
The authors would extend sincere thanks to Mr. Quick
Qi, Mr. Kunihiro Ueda, Mr. Nozomu Hachisuka
(TDK), Mr. Kazuhiro Barada (TDK), Mr. Takeo
Kagami (TDK), Mr. YouGui Wang, Ms. YuPin Qiu,
Mr. XueBin Shu, Mr. KePeng Sun, Mr. ZhongHua
Yang, Dr. Ge Yan, Dr. Lydia Baril (Maxtor) and Mr.
Tai Ching Lee for their kind support and helpful
discussions. Also a “thank you” to Mr. JianFeng
Zheng for his supper SEM photos.
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