Blob Defect Prevention in 193nm Topcoat

Transcription

Blob Defect Prevention in 193nm Topcoat-free Immersion Lithography
Deyan Wang, Jinrong Liu, Doris Kang, Cong Liu, Tom Estelle, Cheng-Bai Xu, George Barclay and
Peter Trefonas
The Dow Chemical Company
Dow Electronic Materials
455 Forest Street, Marlborough, Massachusetts, USA, 01752
1. Introduction
In 193nm immersion lithography, immersion top coat was the first proposed technique for preventing the leaching of
photoresist (resist) components, such as photoacid generator (PAG) and quencher base, into the immersion fluid (DI
water). In this approach, the top coat is coated onto a resist film in a separate step including coating the top coat film and
baking the film. This approach certainly adds extra cost to the device manufacturing and incurs reduced throughput as
compared to the dry lithography process.
The embedded barrier layer (EBL) technology1-5 developed at Dow Electronic Materials has been demonstrated to be a
revolutionary approach, in which a suitable EBL material is formulated into an existing resist, and in a spin coating
process the EBL material comes to the resist surface to forms a leaching barrier in situ. This approach has now been
widely accepted and implemented in the integrated circuit manufacturing industry for replacing the conventional
immersion top coat process.
In addition to being an excellent leaching barrier, EBL materials, in general, result in a resist surface with a high receding
angle for water. This property makes the EBL approach more desirable in topcoat free immersion lithography, since it
allow for the latest scanners to perform at their maximum scan speed without generating watermark defects.
For immersion lithography, the most important issue for mass production is defectivity control. This is true for both top
coat and topcoat free approaches. In the top coat approach, the formulation optimization for both top coat and resists
was extensively involved for this technique finally to reach an acceptable defectivity level for mass production of
semiconductor devices.
As a later developed technology, the EBL approach has gone through a series of research and development stages
particularly in material innovation to reach the same low defectivity level as that of an immersion top coat process. After
achieving the target of low defectivity in lithography, the challenges left to the EBL approach were to solve high
defectivity in bulk exposed and bulk unexposed regions, which became prominent in both bright field and dark field
lithographic applications. To solve the high defectivity issues, a thorough understanding of the blob defect formation
mechanism was imperative. In this paper, the defect formation mechanism in both bulk exposed and unexposed regions
is proposed, and this proposed mechanism is applicable not only to the EBL approach but also to the immersion top coat
approach in general.
2. High Blob Defectivity Issues with Earlier Generation EBL Materials
In earlier stages of EBL development, EBL materials were designed primarily for reaching a high receding angle while
functioning as an effective leaching barrier to DI water for PAG and other resist components. These properties were
achieved without much effort because of the inherent hydrophobic nature of the EBL materials. Efforts were then
focused on achieving the same low defectivity levels as the top coat process in lithography patterning. This was
eventually achieved by ensuring developer solubility of the EBL materials in the unexposed regions. 2 The blob defects in
bulk exposed and unexposed regions on a processed wafer, however, remained a prominent issue. Shown in Figure 1 is a
comparison of wafer maps showing defectivity between top coat and EBL with EPIC™2096 Photoresist (Dow Electronic
Materials).
Advances in Resist Materials and Processing Technology XXIX, edited by
Mark H. Somervell, Thomas I. Wallow, Proc. of SPIE Vol. 8325,83252G
© 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.916818
Proc. of SPIE Vol. 8325 83252G-1
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/03/2013 Terms of Use: http://spiedl.org/terms
Fig. 1. Defect wafer map comparison for a checkerboard defectivity test. Exposure dose is the dose for printing 50nm, 1:1 line space
patterns. Film thickness is 1200Å. The EBL material used is an EBL that does not change its hydrophobicity during a
development/rinsing step. Its loading is 3% relative to the total solid content of the resist. The top coat is OCTM2000 (Dow Electronic
Materials) with its POR process condition (coating at 1500 rpm, and bake at 90°C for 60 seconds)
Each dot in the wafer map represents an actual defect regardless of its nature. Although some of the defects are attributed
to particles or defects from the BARC (bottom antireflective coating) onto which the resist was coated, a majority of the
defects are classified as satellite defects. As suggested by their name, satellite defects are a type of defect in which a larger
mass is normally at the center and a few smaller pieces randomly reside around the center mass, as shown in Figure 2 6,7
Figure 2. SEM Images of typical satellite (blob) defects. The scale bar is 1µm.
When contact angles were determined on the EBL resist surface for a processed wafer, we found that the EBL containing
resist remains more hydrophobic on its surface than its counterpart processed with a top coat. To be specific, the static
contact angle of the EBL resist was 79° and its receding angle was 49.5°, whereas the static and receding angles of the
same resist but processed with the top coat were 58° and 20°, respectively. We rationalize the higher hydrophobicity of
the processed EBL resist surface as follows:
Although the EBL material tested was designed to have a certain developer solubility, the developer (an aqueous solution
of 0.26N TMAH) removes only the outermost surface of the resist film where the EBL concentration is essentially 100%.
As illustrated in Figure 3, which shows the EBL concentration profile across the depth of a resist film, immediately under
the EBL enriched surface, the EBL concentration drops rapidly to zero in the range of a few nanometers. It is not difficult
to imagine that after the EBL enriched surface is removed in the development process, a new resist surface is created from
the sharp gradient region where the EBL polymer and the resist polymer coexist. Because of the hydrophobic nature of
the EBL, the new surface is colored with the hydrophobicity of the EBL.
12
Ratio
8
4
0
0
20
40
60
80
100
120
140
160
180
200
nm
Figure 3. SIMS spectra of an EBL containing resist created by monitoring signal intensities corresponding to the characteristic mass
ratios of the EBL polymer.
Regardless of the defect formation mechanism, the high level of blob defects seems closely related to the new,
hydrophobic EBL resist surface. The question then becomes, how does the high hydrophobicity correspond to the high
level of blob defects?
3. Exploring Blob Defect Formation Mechanism of EBL Resists
There is a popular theory in this industry for the formation of blob defects on a processed wafer. Based on this theory, the
blob defects are due to the aggregation of the resist components during alkaline developer development and DI water
rinse steps, and the aggregated resist components then fall back onto the wafer surface forming the blobs. This
mechanism is also referred to as pH shock or reduced zeta potential of the resist components in the developing/rinsing
transition stage.7 This theory provides a good guideline for the selection or design of resist materials including resist
polymers, PAGs, quencher bases and other additives. Experimentally, people do see that a resist formulation with poor
developer soluble components often results in high defectivity, particularly blobs.
Is this the mechanism that governs blob defect formation for hydrophobic EBL resists? Our meticulous studies lead us to
the belief that these satellite defects are, in fact, water stains.
On a hydrophobic resist surface, water droplets bead up, and their shapes or contact angles depend on the
hydrophobicity of the surface. This can be measured not only by droplet static contact angle (θs) but also by its receding
angle (θr). The latter is more sensitive to the interaction strength between a liquid and a solid surface, and therefore is a
very important parameter in characterizing the behaviour of water droplets on a surface, particularly in a dynamic
condition such as spinning of a wafer.
Under a dynamic condition, i.e., wafer spinning, water droplets on a resist surface start to deform, skewing towards the
wafer edge, and then followed by one of two actions as follows depending on the different situations:
Situation 1. Resist surface is hydrophobic (high θs and high θr). In this situation, water droplets slide outwards when
centrifugal force exceeds the interaction strength between the droplet and the wafer surface, otherwise staying on the
wafer surface and possibly becoming dried during spinning. This is often called poor dynamic wetting.
Situation 2. Resist surface is hydrophilic (low θs and low θr). This corresponds to a strong water-resist interaction. In this
situation, instead of sliding, the droplets spread outwards to become coated on the resist surface and are coated evenly
and dried during the wafer spinning.
To elaborate the first situation, let’s look at the force analysis diagram of a droplet on a spinning wafer surface in Figure 4.
mω2R
Өa
Өr
~2r σ
~2r σ
Figure 4. A force analysis diagram of a water droplet on a spinning wafer surface. The direction as pointed by the centrifugal force,
mω2R, is towards the wafer edge. θr and θa are receding and advancing angles, respectively.
Ignoring all forces perpendicular to the wafer surface including gravity and normal force, the forces of interest are
centrifugal force, mω2R, with m being the mass of the droplet, R the distance from the mass center of the droplet to the
wafer center and ω the angular velocity of the spinning wafer, and two forces related to the interaction between the
droplet and the resist surface, which two forces are each, in magnitude, approximately 2rσ, where r is the radius of the
circular droplet base on the resist surface, and σ is the surface tension of water at the processing temperature, normally
21°C.
The minimum centrifugal force required for a droplet starting to slide is given by the following equation:
mω2R ≈ 2r σ (CosӨr - CosӨa)
≈ 2r σ CosӨr when Өa is ~ 90°
As seen, for the first approximation, the interaction strength between a droplet and a resist surface is proportional to
Cosθr. That is, the lower the receding angle is the stronger will be the interaction between the droplet and the resist
surface.
Using the above equation, we can determine the critical size of the droplets. The critical size is defined as the smallest size
at which a droplet will slide. When a droplet is smaller than the critical size, it will not slide since the centrifugal force is
weaker than the droplet-resist interaction. At a given spin speed, for example 2000 rpm, and a given droplet location, for
example 10cm away from the wafer center, and when the receding angle is 70° and advancing angle is 90°, the critical size
(radius) of droplets is calculated to be 73µm. This means that with the aforementioned conditions, all the droplets whose
sizes are smaller than 73 µm are not spun away but stay on the resist surface during the post rinsing spin.
DI water, once in contact with a resist surface during post development rinsing, is no longer pristine. All resist
components including polymer, PAG, quencher base and photoacid can all be leachable into DI water to a certain extent.
Therefore, when the droplets dry, the leached substances in them become concentrated and then deposit on the small area
where the droplets resided before becoming dried, and form blobs (satellite defects). This is for the aforementioned
situation 1.
For situation 2, since water is coated evenly onto a resist surface and soon dries during the post rinsing spin, trace
amounts of the leached substances in the coated water is evenly distributed on the resist surface, and thus leaves no
visible blobs.
Going back to the checkerboard defectivity of a hydrophobic EBL resist, the bulk unexposed regions are the sections of
the original resist surface, whereas the exposed regions are the sections of the BARC surface which in general is more
hydrophilic than the EBL resist surface owing to the enriched hydroxy groups on the BARC surface. Therefore, the above
arguments can well rationalize the high blob defectivity level on the bulk unexposed regions. Then what causes the high
blob defectivity in the bulk exposed regions?
The exposed regions are shallow wells whose depth is the thickness of the resist film, generally in the range of 1000Å.
Carefully examining the exposed regions, one can easily recognize that the areas inside the shallow well including the
partially deprotected resist sidewalls and the hydroxy group enriched BARC surface are all hydrophilic, whereas the
unexposed areas that abut the wells are all hydrophobic. During the post rinse spin, residual water is easily trapped in
the hydrophilic wells since the energy is less favorable for water to advance from a hydrophilic region into a hydrophobic
region. Thus, once the residual water dries in the wells, i.e., the exposed regions, the solutes in it will deposit on the areas
where water residues used to reside, and form blob defects. Shown in Figure 5 is a wafer map for the bulk exposed
region defectivity. The distribution of the defects in the wells depends on the amount of water trapped in the wells as
illustrated on the right hand side of the wafer map.
More water in well
Less water in well
Figure 5. A wafer map for the blob defects in the bulk exposed areas from the checkerboard defect test as aforementioned. The exposed
and developed BARC surface becomes more hydrophilic with a surface contact angle in the range of 50° to 55° for the BARC used in this
test.
Based on the above discussions, the high blob defectivity in both bulk exposed and unexposed regions is attributed to the
high hydrophobicity of the EBL resist surface.
Thus, the task for solving high blob defectivity is essentially to reduce the hydrophobicity of a resist surface to its
maximum extent in the wet processing, i.e., in post development rising and spin drying steps.
4. Developer Switchable EBL Materials and Low Blob Defectivity
EBL materials that can perform the change from hydrophobic to hydrophilic in a time frame of seconds during aqueous
alkaline development has been demonstrated to be an effective means for reducing the blob defect count and overall
defectivity. The developer switchable EBL materials are able to provide a resist surface with a high receding angle as
needed during the exposure scanning, and to reach a desired hydrophilicity in the wet processing to ensure an excellent
dynamic wetting of the resist surface. The latter is achieved by cleaving off the hydrophobic alkyl or fluoro-alkyl groups
and forming hydroxy, carboxy and the like on the EBL polymers.
Shown in Table 1 are static and dynamic contact angles of a bulk unexposed surface of EPIC™2096 Photoresist (Dow
Electronic Materials) containing a developer switchable EBL before and after development using 0.26N aqueous TMAH
developer.
Өs
Өr
Өa
Өt
Post Coat
82.8
73.7
88.2
16.7
Post Develop
44.8
22.2
64.7
33.3
Table 1. Static and dynamic contact angles of resist EPIC™2096 Photoresist with a developer switchable EBL at 3% loading relative to
total solids of the resist. The development time is 20 seconds with single puddle development method followed by DI water rinse and
spin dry. The symbols on the top row stands for (from left to right) static, receding, advancing and tilting (sliding) angles.
As seen in the table, an excellent switch from hydrophobic to hydrophilic is achieved with this EBL material. A greater
than 50° drop in receding angle was demonstrated, indicating a resist surface having excellent dynamic wetting, and as a
direct result of this, a low defectivity level in both bulk exposed and unexposed regions was achieved. Shown in Figure 6
is the defectivity wafer map comparison between a developer switchable EBL and a top coat for EPIC™2096 Photoresist.
Figure 6. Wafer map comparison for blob defects in a checkerboard defect test. TC is OCTM2000 top coat (Dow Electronic Materials), and
a developer switchable EBL. The test was performed with same test conditions as used for Figure 1.
5. Additional Test Results and Discussions
During the course of EBL material development for both developer switchable and non-switchable materials, a group of
EBL polymers was synthesized and tested for blob defectivity in both bulk exposed and bulk unexposed regions. These
EBL materials are with different hydrophobicity/hydrophilicity as measured by their receding angles after development
with 0.26N TMAH developer. As discussed in the previous section, a higher receding angle corresponds to a weaker
water-resist interaction, and a lower one to a stronger interaction. The tested EBL materials are listed in Table 2 along
with their post develop receding angles.
EBL
EBL 1
EBL 2
EBL 3
EBL 4
EBL 5
EBL 6
EBL 7
EBL 8
EBL 9
EBL 10
Post Develop Өr
51.5
67.3
69.4
30.2
29.7
47.1
40.2
16.0
22.2
23.1
Table 2. Different EBL materials and their post development receding angles (θr)
The blob defectivity of these EBL materials was tested with the aforementioned checkerboard test using the exposure
dose for printing 50nm, 1:1 line space patterns. Although the tests were conducted in different test sets and over a period
of time, a very interesting relationship between the post develop receding angle and the blob defect count in the bulk
unexposed regions was observed, and is illustrated in Figure 7.
Avg Unexposed CB Defects
30000
25000
20000
15000
10000
5000
0
10
20
30
40
50
60
70
80
Postdevelop Receding Angle, deg
Figure 7. Relationship between blob defect count in the bulk unexposed regions and the post develop receding angles of the EBL resist
surfaces
As seen, in the receding angle range from approximately 30 to approximately 70°, the number of blob defects increases
with a decrease in receding angle. As discussed earlier, a lower receding angle corresponds to a stronger interaction
between water and the resist surface. More water droplets are retained on the resist surface at a lower receding angle due
to the increased critical size (larger sized droplets are retained). In this receding angle range, the droplets having a size
larger than the critical size were spun off the resist surface (by sliding) during the post rinsing spin. This receding angle
range can also be considered a poor dynamic wetting range under the dynamic condition or spin speed used in the post
rinsing spin.
With a further decrease in receding angle, a dramatic change takes place. In the receding angle range lower than 23°, the
number of blob defects drops to an insignificant level. The strong water/resist interaction corresponding to this low
receding angle range is strong enough to anchor the droplet onto the resist surface without sliding, and the centrifugal
force from the wafer spinning spreads the droplets to cover the entire wafer surface. This low receding angle range
corresponds to a good dynamic wetting under the dynamic condition (spin speed) in the process. In this experiment,
however, we did not catch the break point that lies between the dynamic wetting range and the dynamic non-wetting
range, but it is between 23 and 30°. This breaking point, however, depends on the dynamic condition in the process, i.e.,
the post rinsing spin speed.
6. Acknowledgment
The authors are grateful to the support from the Semiconductor Technologies business division of Dow Electronic
Materials particularly to the Application Engineering group led by Dr. Sheri Ablaza. The acknowledgment is extended to
those who were involved in the EBL material development including Drs. Tony Zampini, Joon Seok and Mingqi Li, and
Ms. Janet Wu. Some contact angle measurement work performed by Mr. Paul Baranowski and Ms. Sue McNamara is also
acknowledged.
7. References
1.
2.
3.
4.
5.
Deyan Wang, US patent US 7968268
Wang, Deyan; Xu, Chengbai; Caporale, Stefan; Trefonas, Peter. Creating 193 nm immersion resists with
embedded top barriers. Solid State Technology (2007), 50(9), 50-51, 53, 55.
Wang, Deyan; Caporale, Stefan; Andes, Cecily; Cheon, Kap-Soo; Xu, Cheng Bai; Trefonas, Peter; Barclay, George.
Design consideration for immersion 193: embedded barrier layer and pattern collapse margin. Journal of
Photopolymer Science and Technology (2007), 20(5), 687-696.
Steven Wu, Aroma Tseng, Bill Lin, Chun Chi Yu, Bo-Jou Lu, Wen-Shiang Liao, Deyan Wang, Vaishali Vohra,
Cheng Bai Xu, Stefan Caporale, and George Barclay, Non-topcoat Resist Design for Immersion Process at 32-nm
Node, SPIE, Vol. 6993, 692307.
US patent applications: Deyan Wang and et al, US20100173245 A1, US20090130592 A1, US20090123869 A1,
US20090117489 A1, US20080193872 A1.
6.
7.
Yayei Wei; Non-lensing defects and defect reduction for 193i; SPIE Micro/Nano Lithography & Fabrication; 10
February 2008, SPIE Newsroom. DOI: 10.1117/2.1200801.0976; http://spie.org/x19495.xml?ArticleID=x19495
Yayi Wei; David Back; Immersion lithography: topcoat and resist processes; SPIE Micro/Nano Lithography &
Fabrication; 27 September 2007, SPIE Newsroom. DOI: 10.1117/2.1200709.0825;
http://spie.org/x16909.xml?ArticleID=x16909

Blob Defect Prevention in 193nm Topcoat

Transcription

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