EFM - A Pernicious New Electric Field-Induced Damage Mechanism In Reticles
Gavin Rider
Microtome Precision Inc., 4805 Northpark Drive, Colorado Springs, CO 80918, USA
Abstract
Data are presented showing the development of a previously uncharacterised mode of damage that
degrades reticle CD and eventually results in the formation of a “bridge” between lines, damage that has
hitherto been attributed to multiple low level ESD events. The phenomenon is shown, through correlation of
the damage pattern on the reticle with the field-induced potentials, to occur below the voltage threshold for air
discharge ESD events. This new damage mechanism is given the acronym EFM – for Electric Field induced
Migration – to distinguish it from ESD. A comparison is given between ESD and EFM damage signatures and
the consequences for the safety of advanced photomasks are discussed.
Introduction
Many images have been published showing various forms of ESD damage in reticles, ranging from
catastrophic ablation of the chrome and erosion of line edges to more subtle effects such as loss of
antireflection coatings and gap bridging. Bridging is becoming the prevalent signature of ESD damage in
modern reticles with sub-micron feature sizes and this has previously been attributed to multiple low-energy
electrostatic discharges [1].
Discharges in air between two electrodes generally take place by a Townsend Avalanche process and follow
Paschen’s Law. At atmospheric pressure this means that as the spacing between two electrodes is reduced
the voltage required to produce a spark between them gradually decreases to a minimum then rapidly rises.
For air gaps between smooth spheres, as used in Paschen’s pioneering research, the voltage minimum is
around 350V at an electrode spacing of 7.5µm [2]. It had been predicted that reticles would be protected by
Paschen’s Law, becoming more resistant to ESD as reticle gap dimensions are reduced below the Paschen
curve minimum.
It has been shown that between electrodes other than smooth spheres, phenomena such as field emission
can modify the discharge characteristics across very small gaps [3]. It has recently been shown by
experimentation that this reduces the breakdown voltage on reticles with micron-sized gaps to around 150V
[4]. Such research into ESD damage criteria is prompting a reduction in the ITRS [5] of the recommended
maximum electric field strengths to which reticles should be exposed. In this there is an implied assurance
that fields below the prescribed level will pose an acceptable level of risk to advanced reticles, but this may
not be a sound premise. To check this, some data from previous ESD tests conducted using special test
reticles were reviewed in detail. Some interesting phenomena were discovered.
Field-induced ESD damage
The data were obtained from field-induced ESD experiments, so it was important first to fully understand the
nature of the interaction of reticles with electric fields. This was studied by finite element analysis and the
findings were described in a previous paper [6]. The interaction induces potential differences between reticle
features that increase in magnitude close to the guard ring. This concentration of the highest induced
potential gradients near the guard ring is thought to be responsible for the “ring of fire” damage pattern
depicted in figure 1, that has previously been attributed to conductive ESD (transfer of charge onto the reticle
through handling with ungrounded implements or by tribo-charging). The most appropriate explanation of this
damage signature was shown to be field induced ESD. The damage pattern would be the same with a
neutral, grounded reticle in the presence of a nearby charged object as with a charged reticle being placed
near a grounded surface. The field configuration around the reticle was shown to be the critical factor and this
must be controlled to minimise damage to the reticle.
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Figure 1.
A reticle inspection tool defect map showing the “ring of fire” ESD damage pattern.
Experiments designed to quantify the risk presented by electric fields around reticles have been conducted in
recent years with the aid of special test reticles. The reticle used in the study reported by Levit et al [7] was a
“Canary” ESD test reticle produced by DuPont Photomasks. In a calibration experiment accompanying that
work, an electrode representing a human finger was positioned near the reticle to simulate the charged hand
of an operator. The electrode was charged once to 9kV and afterwards the reticle was inspected for damage
by scanning the gaps between reticle features with an atomic force microscope. (It is important to appreciate
that tens of kilovolts can be generated on reticle pods by tribocharging, so this was a realistic test criterion).
The approximate field configuration within the reticle would have been as shown in figure 2a and the
expected ESD severity (an estimate of the potential differences that would be induced in different parts of the
reticle, based on the finite element analysis conducted previously) is represented by the colour map in
figure 2b. Green denotes light or no damage, yellow is moderate and red is severe damage.
Figure 2.
a) Simplified schematic of potentials induced in a reticle by a charge positioned above the centre of the
reticle. Field lines are red and contours of equal potential are blue. (Localised perturbations due to the
details of the reticle pattern are not shown).
b) The expected damage pattern in the Canary reticle.
Each quadrant of the reticle contains four rows (flocks) of isolated conductive features (birds) each being a
few mm2 in area with a 200µm long line (beak) pointing towards its neighbour at a separation of ~1.5µm.
These reticle structures are specifically designed to be sensitive to ESD damage so that inspection of the
reticle after a handling process will easily reveal if there would be any ESD hazard to normal reticles in the
same circumstances. The pattern of damage sustained by the reticle was very similar to that predicted and is
shown in figure 3. Each picture is an atomic force microscope image of the gap between the reticle features.
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During this test there was no electrical contact with the chrome pattern, indicating that the damage produced
was solely the result of potential differences induced by the field from the electrode above the reticle.
Figure 3
Damage pattern seen in a Canary reticle exposed to a high voltage (9kV) applied to an electrode
positioned above the reticle.
Experimental studies of low level discharges in reticles, thought to be responsible for such damage, have
been conducted by Montoya et al [8] and also Wallash and Levit [4], showing that the small spacing of
features on modern reticles causes air breakdown (sparking) at lower voltage than in previous reticle
generations. Since the power in a discharge is proportional to the square of the voltage, the power dissipated
in an ESD event on a reticle reduces rapidly with shrinking gap size. This means that when reticles of
different generations are exposed to the same electric fields, modern reticles suffer multiple low power
discharges while older reticles experience fewer but more powerful discharges. This has been given as the
explanation for the changing ESD damage signature in reticles and particularly gap bridging. However,
detailed inspection of the onset of the formation of the bridge between features seen in figure 3 shows that
this is probably not so – a completely different mechanism is involved.
A detailed consideration of the damage signatures
In the images shown in figure 3, sites close to the guard ring in the damaged flocks show a protrusion
bridging the gap completely. On site AR1 (at the guard ring) the bridge appears to have grown across the gap
and then been vaporised. Higher up in the flock the protrusion becomes progressively smaller until no
evidence of any damage is seen on the gaps near the centre of the reticle. This damage pattern matches the
predicted map of induced potentials, so figure 3 is a good representation of the type and severity of damage
that occurs as a function of the induced voltage. On close inspection two different types of damage
mechanism can be identified.
Air discharge
This results in damage to the top edges and upper surfaces of the metal lines as shown in figure 4, which is a
perspective AFM view corresponding to the plan view image shown in figure 3 from gap AR5 (Flock A, right
line, fifth gap up from the guard ring). The vertical scale in this image is magnified 10x for clarity. Here there
are two small bumps on the upper surface of the bird body that identify the landing sites of two air discharges
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from each corner of the bird beak on the other side of the gap. Similar marks are also seen at other sites on
the same reticle and in images published from other work [9]. At this point on the reticle the induced voltages
and electric field conditions seem to be ideal for generation of this type of damage without causing so much
destruction that the damage signature is lost.
Figure 4.
AFM perspective view of gap AR5
To explain the distinctive air discharge signatures seen in reticles and particularly to understand why at the
onset of sparking the discharges land on the upper surface as shown in figure 4, we need to consider the
field configuration between reticle features that have voltages induced on them by an external electric field.
The electric field (i.e. voltage gradient) around a conductor is dependent upon the conductor’s shape and the
distance “r” from its surface; the field between two parallel planes is constant, around a line it reduces as 1/r
and around a point it falls off as 1/r2. Therefore, between a line end and the edge of a second line as in the
Canary test reticle configuration there will be a highly non-uniform field as shown in figure 5. The tilted
configuration of the field around the reticle features is due to the interaction of the applied field with the guard
ring that surrounds the image area of the reticle [6].
Figure 5
a) Schematic of the voltage contours that would be induced between reticle features in the Canary reticle
experiment. The trajectory of a discharge launched from the corner of the bird beak would cross the
voltage contours at right angles, landing on the top surface of the adjacent bird body.
b) Perspective view corresponding to the AFM image in figure 4.
At the line corners the potential contours are closest together, meaning that the highest local field strengths
are located here. The local field at the conductor surface is hence much higher at these points than the
average field value calculated by simply dividing the voltage difference by the feature spacing. As the voltage
induced between the two features is gradually raised by an applied electric field, the critical field strength
needed to initiate a discharge is first reached at the line corners, such as at the end of the bird beak on the
Canary reticle. The field lines at these points radiate outwards and upwards away from the corners and
because of the tilted field configuration these field lines terminate on the upper surface of the opposite
electrode. This explains the particular form of damage seen just above the voltage threshold for air discharge,
namely small bumps on the upper surface of the reticle features.
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If the voltage induced across the gap on the reticle is only just sufficient for an air discharge to take place, the
transfer of charge will immediately reduce the voltage across the gap and quench the discharge. The power
dissipated will be small, leading to a low level of damage and a clear damage signature as shown in figure 4.
If the voltage induced between features is higher than this, the critical air breakdown field strength will be
reached at more places near the corners and along the line edges. Field lines with lower trajectories running
closer to the reticle surface will now be able to channel discharges between the features and these will
terminate on the opposite line edges. More power will be dissipated by these higher voltage discharges and
more damage will result. This is shown in figure 6b, where significant damage is seen to be localised in the
regions of field line concentration near the corners of the bird beak and along the opposite line edges.
a
Figure 6
b
a) Schematic plan view of the field configuration at the end of the bird beak, linking the line edge facing the
bird beak and the corners where the field lines are concentrated.
b) AFM image of a damaged Canary reticle showing a damage pattern that matches this field layout
Line Spreading
Critical dimension (CD) degradation through line spreading is seen on gaps experiencing a lower induced
voltage than that which causes air discharge as described above. This appears to be caused by electric field
induced migration of chrome from the lines onto the quartz surface, a process that is given the acronym EFM.
Two different modes of line spreading are seen, which take place at different rates and over different ranges
of the induced potentials between reticle features.
a)
Lowest induced potentials
On gaps experiencing the lowest induced potentials, spreading of the line edge is observed where the field
strength at the edge of the line is above a threshold value, which we can arbitrarily call Em since its value is
not known. This can be seen from the AFM images in figure 7, taken from three gaps in the same flock on a
Canary reticle where the induced potentials (hence the gap field strengths) are slightly different. The field
configuration would be approximately the same for all gaps, as shown in figure 7a, but the potential gradients
along the field lines across each gap would be different.
Figure 7
a) Field line schematic of the Canary reticle gaps
b), c) and d) are AFM images from gaps 18, 12 and 10 respectively in the same flock, corresponding to
increasing induced potentials from b to d.
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As the induced voltage across the gap is increased, the point at which a field line with strength Em intersects
the metal lines gradually moves away from the central axis of the gap, back along the bird beak and outwards
along the edge of the bird body. This is seen in images 7b) to 7d). The outward extent of the line spreading
seen on all the gaps appears to be the same and spreading is seen to be taking place on both sides of the
gap. The material migration mechanism thought to be responsible for this type of CD degradation is not
proportional to the field strength, otherwise there would be more migration of material directly between the
features than along the sides of the bird beak. The presence of the field has apparently enabled metal
migration from the line onto the quartz to take place at a rate that is determined by factors that are equivalent
for all the gaps. Such factors could include the reticle temperature and the time for which the electric field was
applied to the reticle.
b)
Higher induced potentials
Signs of growth of a protrusion are seen on the gap shown in figure 7d, where the induced voltage is the
highest of the gaps illustrated. As is shown by the AFM images of figure 3, the higher is the voltage across a
gap the further the protrusion grows during the exposure to the field. This is different from the mode of
damage seen at lower induced potentials. The AFM data can also be used to generate surface topography
plots as shown in figure 8. This representation clearly shows how the growth of the protrusion starts at the
base of the line and moves out further across the quartz with increasing induced voltage (inferred from the
position of the feature on the reticle).
Figure 8
a) Topography profiles of gaps from the centre of the reticle (left)
towards the edge, showing various stages of the build-up of the bridge
between lines as a function of the induced voltage between features.
b) Overlay of the profiles showing growth of the protrusion starting at
the base of the lines.
Both forms of line spreading are attributed to field-induced metal migration across the quartz surface.
Evidence for this is the presence of surface “beading” which is clearly seen in figure 7d and which is also
visible to varying degrees in other images (figure 9).
Figure 9
Images showing “beading” on the surface of the quartz accompanying line spreading, which is attributed to
metal migration
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Conclusions and consequences
Three different regimes for field-induced damage to reticles have been identified and the mechanism of the
damage is different in all three:
1) Air discharge (sparking) occurs when the highest induced potentials are present. Spark damage in
reticles can be avoided by keeping field-induced potentials below a safety threshold level. This type
of damage will be dependent upon the type of reticle pattern (e.g. line length) and the strength of the
electric field to which the reticle is exposed. Metallization layer masks with long lines will be most
susceptible to this type of damage.
2) Gap bridging by metal migration occurs when moderate induced potentials are present (overlapping
with but extending below the threshold for spark damage). This damage is cumulative and is greater
with higher induced voltages. Masks that do not contain long chrome lines and hence are not very
susceptible to sparking may nevertheless suffer from this form of damage. Those that are prone to
damage by sparking will also exhibit this form of damage on exposure to weaker electric fields.
3) CD degradation by field-induced metal migration occurs with the lowest induced potentials. The rate
of metal migration appears not to be dependent on either the induced voltage or the local field
strength (once the threshold needed to trigger migration is reached). Hence all types of mask pattern
exposed to low levels of electric field are likely to be susceptible to this type of damage.
In a study by Rudack et al [10], metallization-, gate- and via-layer production masks were subjected to
calibrated electric fields. The above forms of damage were all seen to different extents in the reticles,
indicating that the Canary reticle results are representative of ESD in real production reticles. The finding that
grounding of the guard ring increases the severity of field induced ESD as reported from field simulation in [6]
was also verified in the production reticles.
EFM will have significant consequences for advanced photomasks, since all the Resolution Extension
Technologies (RETs) employed within them will be at risk:
•
In phase shift masks even a transparent film deposited onto the surface between features will affect
the phase of the transmitted light. Thus, even if the migrating chrome becomes oxidised and hence
transparent, mask integrity could be compromised.
•
Non-printing optical-proximity-correction features on binary masks could broaden:- if the migrating
chrome causes too much light to be blocked the features themselves might become printable; if the
migrating chrome becomes oxidised and hence the features become increasingly transparent, this
would reduce the power of the OPC features and their effectiveness could degrade over time.
•
Reticle lines that are written with serifs to make the line-ends print correctly could be badly
degraded by line-shape changes of the kind seen here.
It is seen that the most critical and expensive masks in a mask set are likely to be the most susceptible to this
kind of degradation.
The seriousness of the degradation caused by EFM depends on how sensitive the wafer-level image is to this
kind of reticle damage. The printability of the same defects shown in figure 3 has been assessed using an
aerial image metrology system (AIMS) in a study by Rudack et al [11] and this showed that even the small
protrusion at gap AR20 of figure 3 produces a completely bridged aerial image. This demonstrates how CD
errors at mask level do not translate directly to wafer level CD error simply by the lithography demagnification
factor. The imaging process, when working at the limit of the lithography system’s resolution, amplifies errors
in mask CD resulting in a greater CD error on the wafer. This is referred to as the mask error enhancement
factor (MEEF), which with binary masks on current processes and line pitches is typically around 1.5 [12].
Reticle CD is taking up an increasing portion of the overall critical dimension budget for semiconductor
processing as device dimensions move below 100nm. The 2002 ITRS gives a CD control requirement of
3.7nm (3σ) for microprocessor gates and 12.2nm (3σ) for DRAMs at the 100nm node (2003). It is evident that
for a 4x reduction factor and MEEF of 1.5, mask CD needs to be controlled to better than 10nm for
microprocessor gates and 35nm for DRAMs, allowing for no other CD tolerances in the lithography process.
Line width- and shape-distortions as reported here, caused by reticles being exposed to electric fields, will
make this level of CD control a very challenging objective. For reference, the reticle gap reduction shown in
figure 7b, which represents the earliest observed stage of line spreading at the lowest induced voltage,
amounts to 30% of the gap width (450nm).
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Summary
It has been shown that reticles can be damaged by electric fields at levels below that required to cause ESD
damage by sparking. The damage mechanism has been identified as field induced metal migration and has
been given the acronym EFM to distinguish it from ESD. EFM is a cumulative process and even at its very
earliest stages of development represents an extreme hazard to reticle CD. The present study has identified
this phenomenon in chrome-on-glass (COG) reticles, but it may not be restricted to this medium. As a
consequence it is proposed that no amount of electric field exposure should be considered safe for an
advanced photomask.
References
[1]
“Investigating a new generation of ESD-induced reticle defects”, Wiley and Steinman, Micro
Magazine April 1999.
[2]
Naidu, M.S. and Kamaraju, V., High Voltage Engineering, 2nd ed., McGraw Hill, 1995,
ISBN 0-07-462286-2
[3]
R. M. Schaffert, Electrophotography, (Wiley and Sons: New York) 1975, pp. 516-517.
[4]
A. Wallash and L. Levit, “Electrical breakdown and ESD phenomena for devices with nanometer-tomicron gaps,” Proceedings of SPIE Vol. 4980, 2003, pp. 87-96.
[5]
International Technology Roadmap for Semiconductors, http://public.itrs.net
[6]
“Protection of Reticles Against Damage From Field-Induced Electrostatic Discharge” G Rider,
Semiconductor Manufacturing Magazine, September 2003.
[7]
“A study of the mechanisms for ESD breakdown in reticles”, Levit, Desai, Coates and Rudack,
SEMATECH ESD Symposium, October 2000
[8]
“A Study of the Mechanisms for ESD Damage to Reticles”, Montoya, Levit and Englisch, IEEE
Transactions on Electronics Packaging Manufacturing, vol 24.2 pp 78-85, April 2001.
[9]
“Diagnosing and Solving the Reticle ESD Problem”, Steinman, Cleanrooms East, Baltimore, 2002.
[10]
“Induced ESD Damage on Photomasks: A Reticle Evaluation”, Rudack, Pendley, Gagnon and Levit,
EOS/ESD Symposium, Las Vegas, September 2003.
[11]
“Mask Damage by Electrostatic Discharge: A Reticle Printability Evaluation”, Rudack, Levit and
Williams, Proceedings of SPIE – Optical Microlithography XV, vol 4691-2, pp 1340 –1347, 2002
[12]
“Mask Error Enhancement-Factor (MEEF) metrology using automated scripts in CATS”,
P.J.M. van Adrichem, F.A.J.M. Driessen, K. van Hasselt and H.-J. Brück, www.numeritech.com
Acknowledgements
The author gratefully thanks Andy Rudack of International Sematech, for supplying the AFM data; Larry Levit
of Ion Systems Inc. for conducting the Canary reticle tests and for valuable discussions; and Vector Fields
Ltd., of Oxford, UK for the use of their OPERA2d electrostatic field simulation software.
About the Author
Gavin Rider is Vice President of Technology and Development with Microtome Precision
Inc., of Colorado Springs, www.microtome.com. E-mail: [email protected]
He was previously Product Manager for Material Handling and Factory Integration with
ASM Lithography in Veldhoven, before which he spent 15 years working in the surface
analysis and plasma processing equipment industries, after graduating with a PhD in
Surface Physics from Southampton University, England in 1981.
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