Semicond. Sci. Technol. 11 (1996) 463–473. Printed in the UK
TOPICAL REVIEW
Thin gate oxide damage due to
plasma processing
H C Shin† and Chenming Hu
Department of Electrical Engineering and Computer Sciences, University of
California at Berkeley, Berkeley, CA 94720, USA
Received 9 May 1995, in final form 22 September 1995, accepted for publication
18 October 1995
Abstract. Plasma processes cause current to flow through the thin oxide and the
resultant plasma-induced damage can be simulated and modelled as damage
produced by constant-current (or voltage) electrical stress. Plasma processing
causes MOSFET parameter degradation, from which one can deduce the plasma
charging current. Since the scattering of post-damage device parameters is due to
a reproducible variation of stress current across the wafer, one can easily analyse
the effect of device geometry on damage by comparing test structures in the same
die rather than the averages over a wafer.
We have developed a quantitative model for thin oxide plasma charging
damage by examining the oxide thickness dependence of the charging current.
The model successfully predicts the oxide thickness dependence of plasma
charging. It is shown that plasma acting on a very thin oxide during processing
may be modelled essentially as a current source. Thus the damage will not be
greatly exacerbated as the oxide thickness is further reduced in the future.
Although annealing in forming gas can passivate the traps generated during
plasma etching, subsequent Fowler–Nordheim stressing causes more traps to be
generated in these devices than in devices that have not been through plasma
etching.
The protection diode should be forward biased during processing to safely
protect the gate oxide. In CMOS circuits, the drains of the driver circuit can
generally act as adequate protection diodes for the oxide regardless of N or P
substrate and the polarity of the plasma charging current.
The plasma stress current can be reduced by reducing the ion density, which is
unfortunately linked to the etch rate or directionality, or by reducing the electron
temperature. Maintaining a very uniform plasma over the surface of the wafer,
reducing the plasma charging current during the over-etch time and judicious use
of protection diode and antenna design rules will reduce plasma damage to an
acceptable level for ULSI production even for very thin gate oxides.
1. Introduction
Plasma processes are widely used in the manufacturing
of VLSI devices for etching of polysilicon, oxide and
metal films, oxide deposition, sputter pre-clean prior to
PVD (physical vapour deposition), photoresist stripping and
even ion implantation. During plasma processing, devices
fabricated on silicon wafers are usually directly exposed
to plasma. In the plasma ambient, ions and electrons
are generated in the discharge. The ions are accelerated
by the bias or self-bias voltage toward the surface of
the wafer. The accelerated ions either physically sputter
substrate materials or help chemical reactions to occur on
the surface. Since the ion flux mainly bombards the wafer
† Present address: Motorola Inc., Advanced Custom Technologies, Mesa,
AZ 85202, USA.
c 1996 IOP Publishing Ltd
0268-1242/96/040463+11$19.50 surface in the perpendicular direction, plasma etching can
be directional (anisotropic).
However, degradation of gate oxides in MOS devices
due to plasma processing has been observed and attributed
to electrical charging during the process [1–23]. In the
plasma ambient, ions and electrons are collected by metal
or polysilicon electrodes, which serve as ‘antennas’. A
steady-state voltage appears on the electrode due to charge
collection and the resulting electrical stress can destroy the
underlying gate oxide by oxide breakdown or weaken it
by causing charge trapping in the oxide as well as interface
trap generation at the SiO2 –Si interface. Since the damaged
oxide may cause IC yield loss or become more vulnerable
to hot-carrier induced degradation and time-dependent
dielectric breakdown (TDDB), plasma-induced gate oxide
degradation is a serious problem in VLSI technology.
Interface traps and charge trapping can be determined from
463
H C Shin and C Hu
Figure 1. Conduction current during a 13.56 MHz rf
plasma discharge [25]. There would be no or little damage
if the areas under the ion current and the electron current
are the same.
MOSFET characteristics such as the subthreshold swing,
threshold voltage or mobility. Quantitative monitoring
of the interface traps and charge trapping is important
since they are problems in the degradation of MOSFET
performance and reliability [24].
This review describes the quantitative characterization
of charging damage due to plasma processing. After
the plasma damage model is presented, the determination
of the oxide charging current using various MOSFET
characteristics is explained. Using this technique, the
effect of oxide thickness scaling on damage could be
characterized. Also, the effect of a protection diode for
wafer charging is discussed. Finally, we discuss several
ways of reducing the plasma damage.
2. Plasma damage model
During rf (radio frequency) plasma processes, the surface of
the wafer collects conduction current Jp from the plasma
which is composed of an ion current Ji and an electron
current Je according to the well-known Langmuir theory
[25]
e(vp (t) − Vg )
Jp = Ji + Je = 0.6eni uB − 14 ene ue exp −
kTe
(1)
is
the
ion
density,
u
=
where
e
is
electronic
charge,
n
i
B
√
eTe /M is the Bohm velocity, M is the ion mass, ue
is the electron mean thermal velocity, ne is the electron
density, vp (t) = V0 +VI sin(wt) is the self-generated plasma
potential typically with frequency of 13.56 MHz, Te is the
electron temperature and Vg is the gate (antenna) potential.
Figure 1 shows the ion current and electron current as a
function of time. The ion current Ji is almost constant with
time and is determined by the ion density ni and the Bohm
velocity uB . Since the plasma potential vp (t) is higher than
the gate potential Vg for most of the time, the electron
current flows only during the short periods when the plasma
potential is near its minimum.
If the ion and electron currents collected by an electrode
balance each other perfectly over the rf cycle, plasma
charging would not be a problem. Assuming a typical ion
current density, Ji , of 10 pA µm−2 [10], an oxide thickness
464
Figure 2. The calculated transient behaviour of gate
voltage on a 100 Å capacitor. The magnitude of the ripples
is exaggerated in this figure.
of 100 Å, an rf frequency, f , of 13.56 MHz and an antenna
to oxide ratio of 1000, the maximum gate voltage rise over
half of the rf period due to Ji is
Ji × 1000
10 pA µm−2 × 1000
=
2f Cox
2 × 13.56 MHz × 345 nF cm−2
= 0.11 V.
1Vg =
(2)
This voltage excursion is reversed by the following Je pulse
and the process starts again. The average Vg is close to zero
and 1Vg is too small to cause significant electrical stress
to the oxide. In equation (2), Cox represents the oxide
capacitance per unit area. Charging occurs when the ion
and electron currents do not balance each other through
each rf cycle [12]. After averaging equation (1) over one
cycle, the net average plasma charging current involves a
modified Bessel function but can be approximated as
eVg
hJp i = Ji − Je0 exp
(3)
kTe
where
Je0 = 14 ene ue
exp[−e(V0 − V1 )/kTe ]
√
2π(eV1 /kTe )
(4)
is the magnitude of the electron current at Vg = 0 V
[26, 27].
After the plasma is turned on, the gate voltage Vg
increases or decreases over many rf cycles, depending on
which component of the currents is larger, until a steadystate oxide voltage is reached when the Fowler–Nordheim
(FN) tunnelling current through the oxide balances the net
current collected by the antenna, hJp i. This tunnelling
current degrades the oxide. The tunnelling current is always
carried by electrons. For positive charging, when the ion
current is larger than the average electron current, injection
of electrons from the substrate occurs; and for negative
charging, gate electron injection occurs. Electrons in transit
in the oxide will gain kinetic energy by the electric field
under FN tunnelling conditions and may gain still more
energy as they arrive at the anode [28]. These ‘hot’
electrons are quite efficient in generating traps and interface
Thin gate oxide damage due to plasma processing
Figure 3. An equivalent circuit for oxide charging. At
steady state, the plasma current Ip is equal to the oxide
current IFN .
states. Assuming a net current density hJp i of 2 pA µm−2
and an antenna ratio of 1000, the time to reach the steady
state at Vg = 10 V is approximately
Cox Vg
345 nF cm−2 × 10 V
= 17.3µs
=
hJp i × 1000
2 pA µm−2 × 1000
(5)
which is about 200 rf cycles long but very short compared
with the typical plasma processing time of seconds or
minutes. At steady state, the gate voltage is basically
d.c. only with small 13.56 MHz ripples having amplitudes
similar to that calculated from equation (2) as shown in
figure 2. Therefore, the plasma charging can be modelled
as a d.c. stress to the oxides (see section 3.3 for a discussion
of the effect of a low-frequency rotating magnetic field.)
Since this current flows through the oxide as a Fowler–
Nordheim tunnelling current, the oxide voltage and hence
the current is determined by equating hJp i of equation (3)
and the FN current:
hJp i × exposed antenna area
= JFN
thin oxide area
2
Vg
BTox
=A
exp −
(6)
Tox
Vg
Tcharging =
Figure 4. Id –Vg plots of a 10 nm oxide MOSFET for
different antenna sizes after plasma ashing. Curve A is the
I –V of a control device. The Id –Vg plots of MOSFETs with
larger antennas show more degradation.
3.1. Effect of plasma charging on MOSFETs and
capacitors
oxide. Figure 4 shows the Id –Vg plot of MOSFETs with
different aluminium antenna sizes after plasma ashing of
photoresist without subsequent annealing. The photoresist
stripping was done in a barrel-type stripper under normal
conditions. The ashing gas was oxygen at 280 mTorr, the
gas flow was 50 sccm and the power was 400 W. Control
wafers receiving only wet processing were also fabricated.
The size of the gate oxide is 50 × 1.4 µm2 with identical layouts except for the size of the aluminium antenna
ranging from (80 µm)2 to (640 µm)2 . The Id –Vg for a
wet-processed control device is also shown as curve A. We
clearly observed the drift of threshold voltage Vt , which is
due to the generation of interface traps and oxide trapping.
Because a larger aluminium pad collects more charging current during ashing, the I –V curves of MOSFETs with larger
pads show a higher degree of degradation. One can easily
obtain threshold voltages of MOSFETs from figure 4. Since
each gate oxide is covered with a polysilicon gate during
plasma processing, the oxide damage through UV radiation
is negligible. Moreover, the observed antenna effect (the
dependence of damage on antenna size) also suggests that
ionizing radiation due to UV or x-rays in the oxide is not the
main source of damage. Rather, the charging mechanism
described in section 2 is the cause of damage.
The subthreshold swing (S) of plasma-damaged
MOSFETs can be used to extract the interface trap density
distribution. The subthreshold slope is given by
Cit + Cd
kT
log 1 +
S=
q
Cox
Any device parameter that is sensitive to interface traps
and oxide charge trapping may be used to monitor
the plasma charging current. This section presents a
quantitative method for characterizing plasma damage. The
methodology can be applied to any specific equipment and
process.
Plasma processing degrades the MOSFET characteristics simply through Fowler–Nordheim electrical stress of
where Cit = qDit and Cd are the capacitance due
to interface traps and the depletion layer respectively.
Figure 5, which compares the interface state distribution
for different areas of aluminium antenna also indicates that
devices with a larger aluminium pad size have a higher
Dit . We can only obtain a small energy range of the Dit
distribution for figure 5. When the Fermi level is near the
midgap, the subthreshold current level is comparable to the
where Tox is the oxide thickness and A = 20 µA V−2 , B =
250 MV cm−1 are known constants. Again, equation (6)
determines the oxide stress voltage, Vg .
Figure 3 shows a device during plasma processing and
its equivalent circuit for oxide charging. At steady state, the
plasma charging current Ip is equal to the oxide tunnelling
current IFN .
3. Determination of the plasma charging damage
and oxide stress current
465
H C Shin and C Hu
Figure 6. Noise power density spectra for MOSFET
saturation drain current.
Figure 5. Energy distribution of interface states after
plasma ashing for different aluminium antenna sizes.
instrument sensitivity limit. This leads to an uncertainty in
the extracted Dit . When the surface is near strong inversion
there is no simple relationship between swing and Dit .
Figure 6 shows the measured saturation draincurrent noise power as a function of frequency in the
plasma-damaged MOSFETs with different antenna areas.
Obviously, the larger the antenna size, the higher is the
drain noise power. This confirms that plasma charging
increases the density of interface traps, since the drain
noise arises from charging and discharging of the traps
through modulating both channel carrier number and
surface mobility [29]. The drain 1/f noise of MOSFETs
can be interpreted with a unified flicker noise model [29],
which considers both the channel carrier fluctuation and
the correlated surface mobility fluctuation. In the linear
region (Vd = 0.05 V), the carrier density is uniform along
the channel and is given by N = Cox (Vg − Vt )/q, and the
drain-current noise power can be expressed as [29]
SId (f ) =
kT Id2 1
(1 + αµN )2 Nt∗ (x, E).
γf W L N 2
(7)
Nt∗ is the relevant near-interface oxide trap density, N is
the area density of channel electrons, α ≈ 10−15 V s [30].
All the other parameters have their usual meanings. The
oxide trap density Nt∗ can be extracted from the measured
drain noise spectra. The extracted Nt∗ are 7.0 × 1016 ,
1.4 × 1017 , 5.0 × 1017 , and 9.25 × 1017 cm−3 eV−1 for
plasma-damaged devices with antenna areas of (80 µm)2 ,
(160 µm)2 , (320 µm)2 and (640 µm)2 respectively. These
Nt∗ are 1.2–15 times higher than the 6.04×1016 cm−3 eV−1
of the control devices.
Process-induced oxide charging can also be detected
sensitively using a differential pair circuit shown in
figure 7(a). This structure has the advantage over a singletransistor test structure of minimizing the variations of
MOSFET parameters due to factors other than antenna size
such as layout and location. The size of gate antenna A1
is fixed at (80 µm)2 whereas that of A2 is varied from
(80 µm)2 to (640 µm)2 . The degradation of input transistor
466
Figure 7. (a ) Circuit diagram of a differential pair. The size
of gate antenna A1 is (80 µm)2 , that of A2 is varied. The
size of the input transistor is W /Leff = 50/1.4. (b ) The input
offset voltage before and after ashing as a function of size
of antenna A2 .
M2 is larger than that of the M1 transistor, resulting in
threshold and transconductance mismatches. Therefore, the
input offset voltage of the circuit increases with larger size
of antenna A2 whereas that of the wet-processed control
circuit is almost independent of the size of A2 (figure 7(b)).
Thin gate oxide damage due to plasma processing
Figure 8. Shift in subthreshold swing (1S ), threshold
voltage (1Vt ) and oxide trap density near the interface
(1Nt∗ ) versus interface traps (1Dit ) for different antenna
sizes. 1Nt∗ was calculated from noise. The relationships
between all the parameters are linear.
Figure 8 shows the subthreshold swing (S), threshold
voltage (Vt ) and oxide trap density near interface (Nt∗ ) as
a function of interface trap generation (determined from
charge pump current) for devices with different antenna
area. Since all the MOSFET parameters are correlated
with each other through interface traps, any parameter can
be used in quantifying the plasma charging stress. In our
experience Vt shift as well as the differential pair mismatch
are found to be more convenient to use than the other
parameters as monitors of charging damage.
3.2. How to quantify the plasma antenna current
Since the plasma-induced damage is primarily due to
electrical charging, the damage to the gate oxide should
be identical to that produced by applying a constant current
to the gate electrode of a control unstressed device for a
duration equal to the process time. The current necessary to
reproduce the plasma-charging damage would correspond
to that collected by the antenna during plasma processing.
Control MOSFETs may be obtained by using minimum
conductor (polysilicon or metal or via, etc) size and
by perhaps using a protection diode where appropriate.
Control devices may also be obtained by using a known
benign process or equipment. Control MOSFETs were
probe-stressed by passing different levels of current through
the gate oxide for the same length as time as the plasma
ashing (or over-ashing) time. Before stressing, MOSFETs
on control wafers had identical I –V (curve A in figure 4)
independent of the antenna geometry and a location across
the wafer as expected. A series of control devices were
stressed with varying levels of current to generate a series
of reference curves, as shown in figure 9. Also shown in the
figure is the I –V curve of a MOSFET having a (160 µm)2
aluminium pad with photoresist ashed in plasma. This
curve lies between the reference curves of 500 pA and 1 nA
stressing, indicating that the (160 µm)2 Al pad (curve C in
figure 4) collected about 700 pA of oxide charging current
during the ashing process. By using the same method, the
plasma charging current for each MOSFET can be extracted
Figure 9. Id –Vg plots of control MOSFETs after different FN
current stressing under positive and negative gate stress
for 20 min. Plasma-ash-induced damage can be closely
reproduced with a constant-current stress.
with a high resolution. Using an antenna ratio of 100,
one can detect the plasma charge flux with a sensitivity of
1 mC cm−2 (of antenna area), sufficient for characterizing
a typical plasma process charging of 10–1000 mC cm−2 .
Even higher resolution may be obtained by using a larger
antenna ratio.
3.3. Spatial distribution of oxide charging current
across the wafer
Using the technique described in the previous section, the
oxide charging current distribution across the wafer can
be characterized for a given set-up. Magnetized plasma
etchers such as magnetically enhanced reactive ion etch
(MERIE) reactors have been introduced as alternatives to
conventional reactive ion etchers (RIE). To investigate the
charging distribution in a MERIE etcher, Al was etched
with a static magnetic field. Due to the static magnetic
field perpendicular to the electric field inside the sheath
region, secondary electrons emitted from the wafer surface
drift with a cycloid motion toward one side of the wafer,
enhancing the ionization process and increasing the plasma
density there. Figure 10(a) shows the current path model
for a static magnetic field MERIE. Current flows from the
dense plasma region into the antennas, down through the
oxide into the silicon substrate, and up into the weak plasma
regions.
For RIE etching, the wafer centre received more
charging than the wafer edge (figure 10(b)).
The
distribution of the stress current during etching results from
the radial distribution of the plasma density and etch rate
in this etcher. Since oxide stressing effectively occurs after
the Al has been etched into individual patterns (until then,
the Al is likely to be shorted to the substrate through one
defect), the devices near the wafer edge not only experience
a lower charging current because of the lower plasma
density but also spend little time in the plasma after the
Al patterns are separated due to the slower etch rate.
Figure 11(a) compares the statistical distributions of
the threshold voltage before and after photoresist ashing.
467
H C Shin and C Hu
Figure 10. A plasma charging current path model for RIE
and MERIE etchers.
Eighty devices were measured. Plasma ashing causes a
shift in the mean as well as a broader spread in MOSFET
threshold voltages. However, if we plot Vt after ashing for
four different dies individually (figure 11(b)), the spread
is not broader after ashing. The scattering of Vt values in
figure 11(a) is not random but due to the spatial variation
of the plasma stress across the wafer. Therefore, one can
analyse the effect of device or antenna geometry on oxide
damage more easily and accurately by comparing different
test structures in the same die rather than the average over
a wafer.
4. Oxide thickness scaling and charging damage
There is an urgent concern about the effect of plasma
processes on future thinner oxides. If today’s equipment
and processes already cause damage in 15 nm gate oxide,
would they not be totally unacceptable to future 8 nm
or 6 nm oxides? In this section, a quantitative model to
explain the oxide thickness dependence of plasma charging
is presented. With this model one can predict the effect
of plasma processes on future thinner oxides. The test
structures used are fabricated on n-type (100) silicon
substrate with 6.4 nm, 11.6 nm and 18.0 nm gate oxides.
Al was etched using a parallel plate system at 250 W for
100 s. The photoresist was removed by a wet process.
It is useful to understand why the charging current has a
thickness dependence. Figure 12 compares the intersections
(solutions) of equations (3) and (6) with experimentally
deduced current and voltage for two antennas. Two sets
of curves are shown. The first set is three curves of IFN
(equation (6)) for the three oxide thicknesses. The second
468
Figure 11. (a ) Statistical distribution of threshold voltages
before and after ashing. The standard deviation increased
after ashing. (b ) Vt plots of devices in four different dies
after ashing. The increase in standard deviation simply
reflects the systematic radial position dependence of Vt
shift.
set is two curves of hJp i (equation (3)) for two antenna
sizes.
hIp i (equation (3)) is proportional to the exposed
antenna area. Ji = 1.0 mA cm−2 can be calculated
from the plasma d.c. voltage and sheath thickness (2 mm).
Je0 = 60 µA cm−2 (or Ie0 = 8 nA) and Te = 4 eV (see
equation (3)) can be found from a plot of ln(Ii − Ip ) versus
Vg with Ip and Vg determined from the technique described
in the section 3.2. Since the gate antenna voltage is smaller
for a thinner oxide, the plasma charging current Ip is larger.
Figure 12 shows that the charging current in a very thin
oxide (Vg → 0 V) remains finite and approaches the ion
current (equation (3)). Therefore, for thin oxides, it is
better to think of the plasma as an imperfect current source
with the current somewhat modified by the gate electrode
voltage.
The Dit generation in 6.4 nm oxide was less than in
11.6 nm oxide. It is due to the well known greater tolerance
of thinner oxides to FN current stress and the fact that the
plasma stress current does not increase very much with
reducing oxide thickness. According to figure 12, an oxide
of zero thickness will be stressed with only 20% more
current than the 6.4 nm oxide in this etcher.
Thin gate oxide damage due to plasma processing
Figure 14. An equivalent circuit of gate oxide in parallel
with (a ) a forward-biased protection diode and (b ) a
reverse-biased protection diode.
Figure 12. Plots of ‘Langmuir probe’ I –V (equation (3))
and oxide Fowler–Nordheim I –V (equation (6)). The oxide
charging current Ip decreases with increasing oxide (gate)
voltage since the electron current component in
equation (3) is exponentially dependent on gate voltage Vg .
Intersections of the two sets of I –V curves determine the
oxide stress currents and voltages. Elongated aluminium
antennas with the same area but different peripheral
lengths were used. The area of Al antennas is 80 000 µm2 .
Figure 13. (a) The density of interface traps generated
during the Al etching is larger at the wafer centre. (b) The
interface traps can be completely passivated with forming
gas annealing. (c) The density of interface traps generated
by subsequent FN stressing after plasma etching and
forming gas anneal. Even though the traps were
passivated with annealing, subsequent stress can
regenerate 60% of the passivated traps.
Figure 13(a) shows the increase of interface trap density
after plasma etching. After the plasma-etched devices
were annealed in forming gas, the interface traps generated
during etching were completely passivated, presumably
through the creation of Si–H bonds (curve b).
To see whether or not the forming gas anneal provides
a permanent cure for the traps generated during the etching,
subsequent FN stressing was done to the annealed devices
and compared with control devices. Figure 13 also
compares the Dit of plasma-etched and annealed devices
(curve c) and wet-etched devices after subsequent FN
stressing. The devices were stressed by passing a constant
current of 430 µA cm−2 through the gate oxide for 60 s.
The Dit generated in control capacitors are independent of
the position of the Al antenna size, as expected. The extra
Dit in devices that had been plasma etched and annealed
over the Dit in control devices are quite evident—note the
similarity between curves a and c. The wafer position
dependence of the extra Dit suggests that they are related
to the traps which were previously generated during Al
etching and subsequently passivated by annealing. Even
though the interface states were passivated by forming gas
anneal (curve b), about 60% of the traps reappeared after
subsequent stress (curve c). Since the bonding between
hydrogen and silicon formed during the anneal is not strong,
with a 0.3 eV bonding energy, it can be broken during
the FN stress resulting in higher Dit . This mechanism is
probably the explanation for the reported deleterious effect
of metal etching on MOSFET hot-carrier lifetime [35].
6. Protection diode for wafer charging
5. Plasma etching antenna effect on oxide–silicon
interface reliability
Doubts have been raised about whether plasma etchinginduced interface trap generation can be cured by annealing.
If such damage is eliminated in the annealing that usually
follows metal etching, then perhaps it is of no practical
significance except for the oxide shorts which are believed
to be incurable.
To pursue this issue, we study the effect of a forming
gas anneal and the effect of subsequent electrical stress
on oxides that have been damaged by plasma etching.
Protection diodes have been used as a means of protection
against wafer charging [4]. The protection can be obtained
through forward-biased diodes or reverse-biased diodes
during processing. Since the forward-biased diode can
shunt a very large current density, the protection of oxides
by even a very small forward-biased diode is very effective
(figure 14(a)). However, the current through a reversebiased diode is a leakage current (under plasma illumination
and high temperature) (figure 14(b)), and the effectiveness
of protection depends on the relative magnitudes of plasma
charging current and diode current.
469
H C Shin and C Hu
Figure 15. Maximum voltage which can be sustained
before breakdown of the gate oxide in 10 min and
ground-gate junction breakdown. Oxides thinner than
11 nm are hardly protected by Zener protection diodes.
Figure 17. Cross sections of CMOS with gate protection
diodes connected: (a ) positive charging, (b ) negative
charging.
Figure 16. Reverse leakage current of the protection diode
under the same light intensity as in a plasma ashing
chamber. The current density of diodes shielded by metal
was 6 µA cm−2 .
If the protection diode is reverse biased and the diode
leakage current is lower than the plasma charging current,
the oxide may be stressed by the breakdown (Zener) voltage
of the protection diode, i.e. MOSFET gated diode, which
has the lowest breakdown voltage. Figure 15 shows the
grounded-gate drain breakdown voltage and gate oxide
breakdown voltage. The total plasma processing time was
assumed to be 10 min. Gated diode breakdown provides
little or no protection for oxide thinner than 11 nm.
The reverse diode current density in a plasma ashing
chamber, when not obstructed by a light-blocking metal,
is found to be 75 µA cm−2 (figure 16) which is about
23 times larger than the plasma charging current density of
3.3 µA cm−2 for this ashing condition. The reverse leakage
current of a 4 × 4 µm2 diode shielded by 80 × 80 µm2
aluminium was 6 µA cm−2 . Therefore the diode is effective
only when the antenna to diode area ratio is less than 23,
or less than 2 if the diode is shielded by metal from plasma
illumination. Introducing strong illumination onto the wafer
surface during processing can increase the effectiveness of
protection by a reverse diode but may not be practical.
As a rule of thumb, a simple small reverse protection
470
diode may be assumed to be non-conducting and ineffective
in protection against plasma charging damage for an oxide
thinner than 11 nm. On the other hand, a CMOS well
junction typically has sufficient area relative to antenna
size to be assumed to be conducting (see next paragraph).
These assumptions may not apply to specific equipment or
a particular test pattern. For example, a large capacitor
may collect more antenna current than can a reverse well
junction underneath.
Figure 17 shows examples of forward and reverse
diodes for protection in a CMOS circuit. Simple cross
sections of typical CMOS circuits fabricated in twin-tub
technology are shown in the figure. The output of a CMOS
driver is connected to the gate oxide being considered for
damage. Since the output signal is from the node which
is simultaneously connected to the drains of an NMOSFET
and a PMOSFET, drain-to-well junction diodes of the driver
circuit act as the protection diodes for the oxide. For each
polarity of charging, one of the protection diodes is forward
biased and adequate protection can be always obtained.
For example, if the polarity of charging is positive, the
drain-to-well junction of the PMOSFET is forward biased
(figure 17(a)). For P-substrate technology, the reverse
biased well-to-substrate diode is in series with the forward
biased p+ drain-to-well diode. The size of the well is
usually big enough such that the well-to-substrate junction
leakage current can drain away the antenna current. If the
well-to-substrate reverse diode leakage is smaller than the
current collected by the antenna, the oxide stress current is
still reduced by the well leakage current.
Thin gate oxide damage due to plasma processing
Figure 18. Oxide charging current distribution
perpendicular to the magnetic field during static-field
MERIE etching. The charging current increases with B -field
due to increasing plasma non-uniformity.
Figure 19. Oxide charging current distribution across the
wafer in a MERIE system with a rotating field. A rotating
magnetic field of 75 G was used during etching. The
plasma non-uniformity charging currents are larger for
lower pressures.
7. How to reduce plasma charging damage
Figure 18 shows the oxide charging current distribution
in the direction perpendicular to the static magnetic field
during poly etch in a MERIE etcher for three different
magnetic fields. In this experiment, the magnetic field
was either rotated at the normal 0.5 Hz or stationary.
With a high magnetic field, the local imbalance between
ion current and electron current is large, resulting in
more charging to oxide. The breakdown rate of oxides
also increases with magnetic field. Figure 19 shows the
charging current distribution for three different pressures
with rotating magnetic fields. The charging current for
lower pressure is higher, owing to a more non-uniform
plasma density. At the lower pressure electrons have a
longer mean free path and magnetic field confinement is
better, resulting in a higher plasma density gradient across
the wafer. Plasma equipment which generates sufficiently
uniform plasma across the wafer tends to reduce the
damage.
To investigate when the damage is done during plasma
Figure 20. Dit generation versus total etching time. Most of
the damage is done during over-etching. It took 41 s to
completely etch through the Al film. Before that, Dit
generation was negligible.
process, the Dit generated in the devices during plasma
etching of Al were measured after three different times
of etching (figure 20). The endpoint of the etching was
detected about 41 s after the plasma was on. Before
reaching the endpoint, the Dit generation rate is negligible,
probably because the stress current is shunted by some short
from Al to ground. Dit is generated only during the overetch time, i.e. after the Al pad is isolated, and Dit increases
with the over-etch time.
To reduce the damage, one can decrease the rf power of
the machine, or in the MERIE case decrease the magnetic
field during the over-etch time or, better, before reaching
the endpoint. Reliable and accurate endpoint detection is
important. If the plasma is more uniform across the wafer,
one can have a shorter over-etch time and further reduce
the plasma damage.
A dielectric layer can block the plasma charging, and
this concept may be actively used to reduce charging
damage. For example, during LDD (lightly doped drain)
spacer oxide etching, the gate oxide damage is negligible
if sufficient oxide remains over the top of polysilicon
after etching [37]. The top oxide can block the charges.
However, if the oxide remaining over the antenna has been
sufficiently thinned, damage is observed to the gate oxide.
The ‘hard mask’ oxide which is deposited over the antenna
before spacer oxide deposition reduces oxide damage or
increases tolerance for over-etching compared with the case
without a TEOS cap oxide.
Protection diodes are only effective on or after metal-1
etch. For antennas which cannot be protected by diodes (for
example, during poly etch), design rules for antenna area
and edge length to oxide area ratio should be established
accordingly such that the charging current and time product
should be less than 10 mC cm−2 so that the impact on
oxide yield and the latent damage on hot-carrier lifetime
and interface stability are tolerable.
8. Conclusions
Plasma-induced oxide damage can be modelled as damage
produced by electrical current or voltage stress. Plasma
471
H C Shin and C Hu
processing causes MOSFET parameter degradation, from
which one can deduce the plasma charging current. Since
the scattering of post-damage device parameters is due to
a reproducible variation of stress current across the wafer,
one can easily analyse the effect of device geometry on
damage by comparing the test structures in the same die
rather than the averages over a wafer.
For very thin oxides the plasma acts not as a fixed
voltage source but rather as an imperfect current source.
This is fortunate for future oxide scaling. The stress on the
oxide is equivalent to applying a nearly fixed electric field
which is weakly dependent on the gate oxide thickness.
The effect of plasma etching on silicon–oxide interface
reliability is also presented. The interface traps generated
by plasma processing can be passivated with a forming gas
anneal. However, subsequent FN stressing or hot-carrier
stress, and probably temperature-bias stress, causes more
damage in these devices than in the devices that have not
been through plasma etching. This is the likely cause of the
enhanced hot-carrier degradation in plasma-etched devices.
This latent damage may be used to deduce the plasma
stress current that the oxide has experienced, even if tests
could not be performed before the damage is passivated by
annealing.
The protection diode should be forward biased during
processing to safely protect the gate oxide. In CMOS
circuits, the drains of the driver circuit can generally act as
adequate protection diodes for the oxide regardless of the
substrate being N or P and the polarity of plasma charging
current.
The plasma stress current can be reduced by reducing
the ion density, which is unfortunately linked to etch rate
or directionality, or by reducing the electron temperature.
Maintaining a very uniform plasma over the surface of
the wafer, reducing the plasma charging current during
the over-etch time, judicious use of protection diodes and
the antenna design rule will reduce plasma damage to
acceptable level for ULSI production even for very thin
gate oxide.
Acknowledgments
This research is sponsored by SRC, Signetics, TI, Rockwell
International and AMD under MICRO program and
ISTO/SDIO administered by ONR under contract N0001492-J-1757.
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