Directed Self Requirements for S by fjhuangjun

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									 Directed Self-Assembly Requirements for Sub 20nm Lithography
                                  C. Michael Garner, Daniel Herr, and Christof Krautschik

Abstract: This paper summarizes a set of critical requirements that directed self-assembly materials
and methods must satisfy before this emerging technology warrants consideration as a potential
patterning solution for sub-20 nm technology. These requirements transcend specific material systems
and are intended to guide research, which identifies promising families of self-assembling materials that
enable the fabrication and extensibility of charge based technology to its ultimate limits. If one family of
smart resist materials is found that simultaneously meets these requirements, then directed self-assembly
may move from the realm of academic exercise to that of an enabling option. Also, provided in Appendix
I is an overview of the state-of-the-art in directed self assembly for one class of self-assembling
polymeric materials, diblock copolymers.

Introduction: Figure 1 reflects current industry consensus on lithography exposure tool potential
solutions for enabling top down patterning through the 16 nm DRAM half-pitch node, in 2019. It
suggests that EUV and innovative 193 nm optical immersion technologies will dominate between the 32
nm and the 22 nm technology nodes. At the 22 nm technology node, the industry recognizes a need to
consider innovative patterning options. This potential solutions roadmap suggests that research on such
innovative options must be launched by 2008 to impact this node‘s development insertion window, in
2011.
                                                          2007                2010                 2013                  2016                 2019
                                                                                                                                                                 Notes: EPL is a
                                             2005 2006            2008 2009            2011 2012             2014 2015            2017 2018          2020 2021   potential solution
            DRAM 1/2 Pitch                                65nm                  45nm                  32nm                 22nm               16nm
                                                                                                                                                                 for 65-nm ½ pitch
                                                                                                                                                                 in one geographical
                                                                                    DRAM Half-pitch
                                                                                                                                                                 region. RET and
                                                                                                                                                                 lithography friendly
       193 nm
  65   193 nm immersion with
       water

                                                                                                             Flash Half-pitch
                                                                                                                                                                 design rules will be
       193 nm immersion with water
                                                                                                                                                                 used with all optical
                                                                                                                                                                 lithography
                                                        Narrow
  45   193 nm immersion with other fluids               options
       EUV, ML2
                                                                                                                                                                 solutions, including
       EUV                                                                                                                                                       with immersion;
                                                                                i
  32
       193 nm immersion with other fluids and lens material
       Innovative 193 nm immersion with water
                                                                      Narrow
                                                                      options                                                                                    therefore, they are
       Imprint, ML2
                                                                                                                                                                 not explicitly noted.
       EUV
                                                                                                                                                                 RET—resolution
  22   Innovative 193 nm immersion
       Imprint, ML2, innovative technology
                                                                                            Narrow
                                                                                            options                                                              enhancement
                                                                                                                                                                 technology EUV—
                                                                                                                                                                 extreme ultraviolet
                                                                                                                                                                 EPL—electron
 16    Innovative technology                                                                                     Narrow
       Innovative EUV, imprint, ML2                                                                              options

                                                                                                                                                                 projection
                                                                                                                                                                 lithography ML2—
                 Research Required               Development Underway                  Qualification/Pre-Production               Continuous Improvement
                                                                                                                                                                 maskless
          This legend indicates the time during which research, development, and qualification/pre-production should be taking place for the solution.           lithography PEL—
                                                                                                                                                                 proximity electron
                                                                                                                                                                 lithography.
Figure 1. 2005 ITRS Lithography Exposure Tool Potential Solutions1a


While identified exposure tool technologies, augmented with RETs, may satisfy average resolution
specifications through the 32 nm node, alternate approaches are needed to achieve the projected
patterning dimensional and positioning tolerance requirements. Consider, for example, the recent
projected ITRS MPU gate CD, line edge roughness, and overlay control requirements, shown in Figure 2.
A key message from this table is that the lithography community foresees no known potential solutions



File: DSAR10 031606 DH.doc                                                                                                                                                           1
for achieving the projected 2007 MPU gate CD control requirements. By 2010, it expects to face a ‗red
brick wall‘, with respect to MPU gate CD, line edge roughness, and overlay control requirements.

                 Year of Production                     2005     2008     2011     2014    2017     2020
 MPU physical gate length (nm) [after etch]               32       23      16       11        8       6
 MPU gate in resist length (nm)                           53       38      27       19       13       9
 Resist meets requirements for gate
                                                          3.3     2.3      1.7      1.1     0.8      0.6
 resolution and gate CD control (nm, 3 sigma)
 Line width roughness:
                                                          2.6     1.8      1.3      0.9     0.6      0.5
 (nm, 3 sigma) <8% of CD
 Overlay (3 sigma) (nm)                                   15       10      7.1      5.1     3.6      2.5


Figure 2. Selected ITRS lithography variability control requirements1b

Significant targeted research progress must be made over the next four to five years for directed self-
assembly to become a viable sub 20nm patterning technology option. A fundamental question is ―how
can these intermolecular attractive and repulsive forces be harnessed to fabricate features, which are
aligned with top down generated patterns and make critical patterns at the required sizes, with low
defect densities, and with reasonable annealing times?‖

High Level Requirements: For the semiconductor industry to seriously consider this technology as
a viable potential solution for sub 20 nm lithography (ITRS 2016), several fundamental capabilities need
to be demonstrated by 2010. This decision point is based on the assumption that, prior to leading edge
production, it would take at least six years to develop the necessary infrastructure within the supply base
(chemical and CAD Tool) and semiconductor companies.

Key directed self-assembly research milestones include:
   1. Improved long range dimensional control [~1.2 nm 3 by 2010] and low frequency line edge
roughness (LER) [~1.5 nm 3 by 2010] that satisfy projected ITRS requirements;
   2. Improved resolution and linear density by at least a factor of two over that achieved by top down
lithography [~11 nm resolution by 2010];
   3. Demonstrated essential set of sub-lithographic isolated and periodic lines and circular openings;
  4. Fabricated features with multiple sizes and pitches* in the same layer in different regions of a chip;
  5. Characterized and quantified defects levels; developed foundational understanding of material and
process related defect mechanisms, and established guiding principles for ameliorating and managing
defects;
   6. Satisfied projected ITRS alignment and registration tolerances [Overlay of ~5.1 nm by 2010];
   7. Demonstrated a proof of concept for achieving projected ITRS throughput requirements, either via
single wafer or batch processing [Average net throughput of ~1 wafer per 1-2 minutes]. Can batch
processing achieve the required patterns at a rate of minutes per wafer?
   8. Demonstrated potential extensibility beyond the 10 nm ITRS technology generation;
   9. Satisfied projected ITRS etch and pattern transfer requirements for fabricating electronically useful
features;
 10. Demonstrated ease of integration and an enhanced process window.

*Note: 1) Pitch is effectively the period of repeated features, which corresponds to the sum of a line‘s
width and an adjacent space. 2) This paper considers self-assembling materials as smart resists, which
may offer enhanced dimensional control and sub-lithographic pattern formation. Additionally, but not
considered here, self-assembling materials may be designed to express electronically useful functionality
that can be integrated within the structure of a device.



File: DSAR10 031606 DH.doc                                                                                    2
Key challenges: There are several fundamental challenges that must be addressed for this technique
to be useful in extreme lithography and these were discussed earlier. As was shown above, self
assembly appears to be making progress in controlling line edge roughness and mechanisms may be
emerging to force alignment, but significant progress is needed in these and the other areas. By 2010,
directed self-assembly must:

1.      Demonstrate the feasibility of achieving projected 2017 low frequency Line Width Roughness
requirements of 1.1 nm 3 by 2013.

2.         Determine limits of sub-lithographic resolution extensibility.

3.      Be able to fabricate a basis set of essential features, shown in Figure 6. The biggest challenges
will be the controlled self assembly these features with 2X higher linear density than lithography and
having multiple sizes and pitches within a given mask layer. This corresponds to at least 11 nm dense
lines and spaces demonstrated by 2010. While achieving this 2X linear density for contacts should be
reasonable, this may be much more difficult for the isolated lines, contacts, and the periodic lines,
because new techniques will need to be developed to enable these capabilities.2




                 (a)                         (b)                 (c)                (d)

Figure 6. The most fundamental features needed are a) isolated lines, and c) contact openings and b)
periodic lines and d) periodic hexagonal arrays of contact openings.

In addition to this essential basis set of shapes, the following additional features are considered
desirable.2




     (a)                (b)                  (c)                  (d)             (e)

Figure 7. Additional features considered desirable: a) T-shapes, b) L-shapes, c) boxes, d) periodic
square arrays of contact openings, and e) dense lines with embedded jog and T structures.


4. Demonstrate an ability to form features with multiple sizes and pitches in different locations on the
same layer. The size of features and spaces between them is controlled by the molecular weight of the
different components of the block co-polymers and the relative volume occupied by the different
components used. Thus, new mechanisms may need to be developed to modulate the co-polymer size
controllably in different regions of the same layer.

5. Develop a foundational understanding of material and process related defectivity mechanisms and
establish strategies for ameliorating and managing defects. Characterization of defect levels and
defectivity mechanisms are needed.




File: DSAR10 031606 DH.doc                                                                             3
6. Demonstrate a path to satisfy projected alignment and registration requirements: The next major
research capability is to demonstrate mechanisms to align features from different layers, such as contacts
to or between lines as shown in Figure 8a. Directed self assembly would be most useful in fabrication of
transistors, memory cells, and the highest density interconnects that are present in the lower levels.
Thus, there would be a need to align to a second layer of interconnects to the ―contacts‖ in 8a, as shown
in 8b.




                        a)                                              b)

Figure 8. The alignment of self assembled contacts to pre-existing lines is illustrated in a) and it should
be noted that the self assembly of ―contacts will be in hexagonal patterns unless new processes constrain
it to other order. In b) a second interconnect layer is then being aligned to one set of the contacts in the
previous layer. These examples are only to illustrate the type of alignment that may be needed to
fabricate devices and interconnects with directed self assembly.


Furthermore, the alignment between contacts and lines in different levels needs to meet the ITRS goal of
~30% to achieve the required improvements in density and this may require development of new
structures and mechanisms. While the work of Ross and co-workers demonstrates that introducing
sharp structures can cause alignment of the ―contacts or dots‖, significant work must be done to
translate this to level-to-level registration.

7.      Demonstrate anneal times that are competitive with projected ITRS throughput requirements.
Since these processes are driven by molecular repulsion, thermodynamics, diffusion, and interfacial
energy, defects may be generated and the time to reach equilibrium may take considerable time. Also,
the time required for the directed self assembly to reach completion needs to be less than an hour and
preferably be on the order of minutes, so mechanisms to accelerate this need to be studied.

8.   Demonstrate sub-10 nm pattern formation. Can directed self assembly be extended to the
patterning of sub 10nm features, with a 20 nm pitch? For this technology to be of interest to industry, it
must be demonstrated that it can be extended multiple generations. Since the size of features is
controlled by the size of polymers in one of the constituents, decreasing the number of blocks will be
needed, but it is not clear how this affects defect density and reproducibility.
9.   Demonstrate competitive etch resistance, sufficient to satisfy projected pattern transfer
requirements.

10. Compatibility with Si processing and enable affordable extensibility of patterning charge based
technology to its ultimate limits.




File: DSAR10 031606 DH.doc                                                                                4
Please refer to Appendix II for the most recent quantitative set of projected directed self-assembly
research requirements.

Current research has been using di-block copolymers, but research may need to investigate use of more
complex tri-block co-polymers and other families of materials to achieve the control of structures and
properties required for this technology to be useful. Furthermore, as this research is progressing, models
of the mechanisms will need to be developed to accelerate progress.

Future families of materials may utilize more advanced means of self assembly than the hydrophilic/
hydrophobic block approach suggested here. For example, it is possible to synthesize objects with
asymmetric chemical reactivity that causes randomly dispersed arrays of such objects to spontaneously
assemble into predetermined, programmed sequences or arrangements. The process spontaneously
creates order out of chaos and can, in principle produce arbitrary shapes. In one demonstration, for
example, DNA was demonstrated to be a useful ―glue‖ for bringing nanoscale 3 and microscale4 building
blocks together with high specificity and functionality. DNA assemblies have been made that range in
complexity from simple aggregates4 to periodic crystals,5 lattices,6 nets,7 and even the letters of the
alphabet (i.e. D-N-A).8 Other molecules with high information content, such as RNA, 9 peptides,10 and
peptide-mimetics11 have been used for similar purposes. Macroscopic components have been shown to
undergo analogous types of assembly on an air-water interface if their shape and surface energies are
designed appropriately.12,13

The term ―programmed‖, or ―algorithmic‖ self assembly has been coined to describe these new types of
self assembling systems. Work in this field is barely ten years old, but it is developing a rich theoretical
basis.14-16 The state of this field and the length scale of objects that can be assembled in this way
suggest that it could play a very important role in many nanoscale electronics applications.

The key challenge is to devise designs for the chemical interactions that allow these systems to
spontaneously create order from chaos without the use of a physical template. We need to learn how to
incorporate the appropriate chemical reactivity into functional objects so that at equilibrium, they
spontaneously assemble into highly aperiodic structures. The principle is clear but the design of the
process is lacking. New equilibrium interactions based on cooperative hydrogen bonding must be
developed in order to demonstrate the full potential of this very powerful technique. Guiding principles
and positional/orientation/conformational control.

Finally, if this research is successful, the design community will need CAD tools to develop design rules
and alignment strategies to achieve the required dimensional and placement control. Furthermore, resist
suppliers and chemical companies will need to develop sustainable, benign, high performance material
formulations with required control of features, defects, cure time, and alignment.




File: DSAR10 031606 DH.doc                                                                                5
                                            Appendix I
Background: Industry concern for these potential show stoppers is driving research on alternate
approaches that exhibit the potential of extending top down methods to their ultimate limits. This paper
focuses on recent advances in directed self-assembly materials that impact these critical patterning
requirements, specifically dimensional and positional control. Within this context, phase segregating block
copolymers represent a promising evolutionary step forward in resist technology. These high information
content resist materials are designed to make use of additional nanoscale chemical processes that can be
tuned to enhance dimensional control.

The design of a directed self-assembly technology requires an understanding of three interdependent
factors: A material assembly instruction set, an internal assembly tool box, and a set of processes that
guide how a given material interacts with its local environment. For example, consider the directed
epitaxial self-assembly of block copolymers on lithographically defined nanopatterned substrates. 17 A
copolymer is any polymer that is built from more than one monomer type. The family of copolymers
made from two monomer types, A and B, can exist as one of three forms: Random, alternating or block.
A random copolymer is characterized by a random ordering of the A and B monomer units in the
polymeric chain, such as BBABAAABAABBBAB. Alternating copolymers exhibit a regular alternating
pattern of monomer units in the polymeric chain, such as ABABABABABAB. Block copolymers represent
an unusual class of polymers that are synthesized from two or more distinct homogeneous polymeric
blocks. For the case of a diblock copolymer, the structure might correspond to AAAAAAAAAAA-
BBBBBBBBB and be represented by the blue and red sections of the polymer chain shown in the last
frame of Figure 3.

Figure 3. Generic structure of a linear
AAAAAAAAAAA-BBBBBBBBB (Blue-Red) diblock
copolymer.




Each block is homogeneous and may exhibit similar or dissimilar, oil-like (blue) vs. water-like (red),
behaviors. A diblock copolymer contains information that determines its preferred mode of assembly into
a film. It contains information that codes for whether the blocks will remain miscible or tend to phase
segregate into sheets, lamellae, embedded rods or cylinders, or embedded spheres. The fundamental
mechanism that drives self assembly in this family of polymers is molecular attraction and repulsion, such
as that between oil and water. Each block also codes for the dimensions of the preferred phase
segregated structures, preferred orientation with respect to the local external
environment, and other useful macroscopic properties. For example, the volume fraction of one block
determines its morphology within the polymer matrix. A polymeric block that makes up a volume fraction
of 15% may exhibit a spherical morphology, while a corresponding block design to make up a volume
fraction of 30% may yield cylindrical structures. Additionally, the interaction energy between the blocks
controls the sharpness of the phase segregated interface. Hence, a diblock copolymer contains a
rudimentary internal instruction set that influences its internal assembly options.

Could the mechanisms that drive the phase segregation of these blocks extend lithographic technology by
enabling the controlled formation of sub 20nm patterns to extend lithography? If recent research
progress continues, the answer is a definite maybe. While the size of phase segregated structures tend
to scale with the size of the polymeric blocks, the driving force for phase segregation also diminishes.
The self assembling behavior of these thin film systems in the sub-10 nm regime has yet to be
demonstrated.
Progress to date:


File: DSAR10 031606 DH.doc                                                                                 6
1.      Dimensional control: Profs. Nealey‘s and DePablo‘s teams, at the University of Wisconsin at
Madison, recently compared the post etch LER of features formed via the phase segregation of a
polystyrene-PMMA block copolymer with that of lithographically patterned and etched features in PMMA.
The directed self-assembly process exhibited reduced post etch LER, compared with a lithographically
patterned and etched feature in PMMA. Quantitatively, the measured post etch line edge roughness of
~2.4 nm (3) in the diblock copolymeric resist is about half of that measured in PMMA, ~4.65 nm (3).
These early results are shown in Figure 4.




Figure 4. Post etched dense line/space structures in a) PMMA and in b) phase segregated PS-PMMA
diblock copolymers.18


A number of directed self-assembly methods get their directions from a pre-established pattern, which is
generated lithographically. This naturally leads to the question: How can self-assembly be better than the
lithography from which it originates? It is like wafer patterns being limited by the masks. P. Nealey‘s
team applied self assembling materials of rather uniform size over a lithographically patterned surface of
constant pitch, with features that varied in width. They report that the final feature size appeared to be
locked in by and reflect the size of the polymer more than that of the underlying lithography. In this case,
they argue that the size of the polymer in these phase segregating systems drives the long range
dimensions of features and tends to smooth out and reduce some of the variability in the underlying
lithographically defined template. They infer that dimensional tuning of the polymeric blocks and the
phase segregation process may enable enhanced long range dimensional control. Their results suggest
that this family of materials exhibit a self-healing attribute during the bake, i.e. anneal, step.

While this work is suggestive, it remains to be seen whether an optimally designed family of self-
assembling materials offers improved LER and long range dimensional control over that observed with
conventional resist systems, under all essential and competitive patterning situations.

2.       Directed sub-lithographic feature formation: At MIT, Prof. Ross‘s team has shown that sub-
lithographic features can be formed and ordered, via templated self-assembly, when the block copolymer
is constrained within a lithographically formed trench. An early example of directing the self-assembly of
features with sub-lithographic resolution is shown in Figure 5. Can we embed and control the position of
electronically useful materials within these features?

Her group also was able to force alignment of these sub-lithographic features to small patterned
alignment structures, such as the small wall defect in circled in Figure 5, with ~6 nm positional control
over a distance of ~1 micron. Thus, this may be a first step to establishing mechanisms to force the self
assembled structures to align to top down structures or structures generated by earlier process steps.



File: DSAR10 031606 DH.doc                                                                               7
Figure 5. Demonstration of sub-lithographic feature registration induced by a programmed ~20 nm side
wall defect.19


3. Essential shapes: Demonstrated a limited set of shapes required by the design community, such as:
a) Dense linear arrays of lines and spaces, b) dense L-shaped arrays of lines and spaces; c) dense
circular arrays of lines and spaces, and d) dense linear arrays of hexagonally packed spheres. 20 Directed
self-assembly has yet to demonstrate the formation of isolated structures. This topic warrants additional
focus.

4. Multiple sizes and pitches on the same level: This has yet to be demonstrated.

5. Defectivity: This topic has received limited research attention within the context of patterning and
warrants significant emphasis. However, the polymer physics community has a built a considerable
knowledgebase on defects within bulk polymeric materials.21

6. Alignment and Registration: Figure 5 suggests a proof of concept. This topic requires considerable
focus.

7. Throughput: Current anneal times range from minutes to days. There are reports that some
polymers anneal at ambient temperatures and their anneal times exhibit some tunability.22 This topic
requires significant emphasis.

8. Extensibility to 10 nm and beyond: Preliminary models suggest that directed self-assembly may be
extensible to the sub-10 nm regime. However, this topic warrants considerable research focus to gain a
more fundamental understanding of the thermodynamic limits of this technology. Shorter polymer chains
imply shorter phase segregation periods and a lower ordering temperature, which may require that the
process be quenched to lock features in place. However, decreasing the size of the polymer blocks also
reduces the driving force for pattern formation, i.e. phase segregation.

9. Etch resistance:    Early results suggest resist-like etch resistance.   However, this topic warrants
considerable focus.

10. Integration and process window:       Results are suggestive thus far. This topic warrants further
emphasis.




File: DSAR10 031606 DH.doc                                                                              8
Appendix II. Directed Self-Assembly Research Requirements for 2011-2012

                          Metric                                           Requirement

Defects and defect management strategies                                   <0.02 20 nm defects/cm2
Line Edge Roughness                                                                      ~2.1 nm 3 
Gate CD Control                                                                          ~1.7 nm 3 
Resolution                                                                                     11 nm
Essential shapes                                                      Dense and Isolated L/S, circles,
                                                                                   hexagonal arrays
Overlay and registration                                                              5.1-7.1 nm 3 
Throughput                                                                                  1 W/Min
Etch and pattern transfer                                                                     ~CARs
Placement and orientation                                                        Under development
Multiple Sizes-Pitches/Layer Overall Performance                                           2-3/layer
Other                                                                                   ESH Impact?


                                        Selected References


1) International Technology Roadmap for Semiconductors, Santa Clara, CA: Semiconductor Industry
Association - a) p. 470; b) pp. 700-701 [2005]
2) SRC Patterning TTAB, Consensus position on a basis set of essential and desired shapes, March 2006.
3) Mirkin, C., Programming the assembly of two- and three- dimensional architectures with DNA and
nanoscale inorganic building blocks. Inorg Chem., 2000. 39(11): p. 2258-72.
4) Valignat, M., et al., Reversible self-assembly and directed assembly of DNA-linked micrometer-sized
colloids. PNAS, 2005. 102(12): p. 4225-9.
5) Winfree, E., et al., Design and self assembly of two-dimensional DNA crystals. Nature, 1998.
394(6693): p. 539-544.
6) Paukstelis, P., et al., Crystal Structure of a continuous three-dimensional DNA lattice. Chem. Biol.,
2004: p. 1119-11126.
7) Li, H., et al., DNA templated self-assembly of protein and nanoparticle linear arrays. JACS, 2004.
126(2): p. 418-9.
8) Park, S., et al., Finite-size, fully addressable DNA tile lattices formed by hierarchical assembly
procedures. Angew Chem Int Ed Engl., 2006. 45(5): p. 735-9.
9) Chworos, A., et al., Building programmable jigsaw puzzles with RNA. Science, 2004. 306(5704): p.
2068-2072.
10) Kotch, F. and R. RT, Self-assembly of synthetic collagen triple helices. PNAS, 2006. Epub ahead of
print.
11) Percec, V., et al., Self-assembly of amphiphilic dendritic dipeptides into helical pores. Nature, 2004.
430(7001): p. 764-8.
12) Bowden, N., et al., Self-Assembly of Mesoscale Objects into Ordered Two-Dimensional Arrays.
Science, 1997. 276(5310): p. 233-5.
13) Rothemund, P., Using lateral capillary forces to compute by self-assembly. Proceedings of the
National Academy of Science, 2000. 97(3): p. 984-989.
14) Adleman, L., Toward a Mathematical Theory of Self Assembly. 1999, University of Southern
California: Los Angeles.
15) Adleman, L., et al. Running Time and Program Size for Self-assembled Squares. in 33rd ACM
Symposium on Theory of Computing. 2001.




File: DSAR10 031606 DH.doc                                                                               9
16) Olivier, P. and A. Weimerskirch. Algorithmic Self-Assembly of DNA Tiles and its Application to
Cryptanalysis. in Genetic and Evolutionary Computation Conference. 2002. New York.
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18) P. Nealey, Presentation, SRC Summer Study, Vail, CO: Semiconductor Research Corporation (June
2005)
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                                         Acknowledgements

The authors wish to thank the following people for their review, insightful feedback, and contributions to
this requirements document: Charles Black, Luigi Capodieci, Ralph Cavin, Juan DePablo, John Hartley,
Harry Levinson, Paul Nealey, Chris Ober, Mihri Ozkan, Caroline Ross, Thomas Russell, Frank Schellenberg,
Marc Ulrich, James Watkins, C. Grant Willson, H.-S. Philip Wong, Victor Zhirnov, and the SRC‘s Patterning
Thrust Technical Advisory Board.




File: DSAR10 031606 DH.doc                                                                              10

								
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