Replication of an UV-NIL stamp using DLC coating
Ki-don Kim  *, Jun-ho Jeong, Altun Ali, Dong-il Lee, Dae-geun Choi, Eung-sug Lee
Research Center for Nanoscale Processes and Tools, Korea Institute of Machinery and Materials, 171 Jangdong, Yuseong-gu, Daejoen, South Korea
Available online 27 January 2007
Abstract
We fabricated a stamp for UV nanoimprint lithography using a diamond-like carbon (DLC) coating on polyvinyl alcohol (PVA), which is commonly used in water-soluble polymer replica stamps. The RF-PECVD method was used to deposit a DLC on the PVA replica. An optical adhesive was then used to affix a glass plate to theDLC-coated PVAreplica as a backup material, and thePVA replica was dissolved in water from the DLC layer stuck to the glass plate. The same topology was applied to successfully replicate the master stamp with sub- 1  lm features. The correlation between the dimensions of the master stamp’s features and the corresponding replicated features was excel- lent. When replicating stamps, the properties of DLC yield the benefits of both good mechanical properties and low surface energy.  2007 Elsevier B.V. All rights reserved.
Keywords:  UV nanoimprint lithography; Diamond-like carbon
1. Introduction
Nanoimprint lithography (NIL)   [1]   is one of the most promising new technologies for fabricating patterns with resolutions of less than 10 nm because it allows a higher throughput and lower cost than conventional photolithog- raphy. Ultraviolet (UV) NIL, which is performed at low pressures and room temperature, is particularly advanta- geous compared to thermal-type NIL. In UV-NIL, a nan- opatterned UV-transparent stamp is pressed onto a dispensed resin or spin-coated resin on a substrate, and then the stamp is exposed to UV light from above to cure the resin. After 10–20 s, the stamp is separated from the patterned layer on the substrate. As elevated temperature or increased pressure is not needed, this method is both suf- ficiently rapid and prevents stress on the stamp or sub- strate. Finally, an anisotropic etching is used to transfer the patterns onto the substrate.
Because UV light must penetrate the stamp, it is con- structed from quartz or glass, and the stamp is fabricated using reactive ion etching to achieve the desired nanoscale patterns. After the process of electron beam lithography, reactive ion etching is used to etch patterns onto the hard
mask layer of the quartz or glass substrate. This patterned hard mask layer is used to etch the substrate. The general procedure for nanoscale stamp fabrication as explained above is very expensive and time-consuming. Recently, sev- eral polymer materials have been applied in most types of  most soft lithography as alternative low-cost stamps, including polydimethylsiloxane (PDMS), polyurethane (PU), and amorphous fluoropolymer. These polymer rep- lica molds of quartz stamps have an obvious advantage in that they can preserve the expensive original master.
However, a one-time replicated stamp from a master stamp has an opposite surface topology to the master stamp; to attain the same stamp surface topology, a repli- cated stamp must be replicated yet a second time. When PDMS is used as the stamp material in both the first and second replications, it is difficult to separate the two stamps. Although other polymers can be used instead of  PDMS as the stamp material in the first replication, the ori- ginal features are modified by the double effect of polymer shrinking through polymer solidification. Even if these drawbacks could be overcome, transparent polymers such as PDMS are not appropriate materials for the second rep- lication. PDMS has lower mechanical properties (e.g., a low Young’s modulus of 1.8 MPa) than harder stamp materials used for imprinting, which has a negative effect on minimizing distortion and maximizing life cycle.
0167-9317/$ - see front matter    2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.042
* Corresponding author. Tel.: +82 42 868 7866; fax: +82 42 868 7123. E-mail address:  kdkim@kimm.re.kr (K.-d. Kim).
www.elsevier.com/locate/mee
 
When general organic polymers are used for stamps, they require an anti-sticking treatment, and coating a release layer onto these polymers is difficult because it is usually coated onto Si or SiO2. Organic fluoropolymer is useful, but has similar mechanical properties to PDMS [2]. Thus, was necessary to develop a new cost-effective stamp that did not require anti-sticking treatment and yet provided sufficient rigidity for nanopatterning.
We propose a replication process for nanoscale stamps using the UV-NIL process. The stamp’s first replication uses a well-known water-soluble polymer, polyvinyl alco- hol (PVA). A diamond-like carbon (DLC) is then coated onto the sacrificial PVA replica. Many studies have reported the application of PVA to nanoscale patterning. Spin-cast PVA films have been used to create templates for imprint lithography and molecular transfer lithography [3–6]. Replicated PVA templates have been used to fabri- cate a polymer nanonozzle array by dissolving the sacrifi- cial template [7].
Normally, the UV-NIL stamp is coated with an anti- adhesion layer such as an alkysiloxane self-assembled monolayer (SAM) to reduce the incidence of patterning failure due to adhesion between the stamp and the resist layer. However, reiterative imprinting reduces SAM dura- bility depending on process parameters such as the imprint pressure, resin properties, and aspect ratios of patterns. Therefore, an anti-adhesion layer must be periodically redeposited onto the stamp. DLC is well known to provide the benefits of lower levels of surface energy and friction and greater hardness. UV-curable resins and DLC layers have a contact angle of about 70, and these excellent prop- erties are of considerable importance in many diverse appli- cations. A stamp made from DLC does not require an anti- adhesion layer, and the DLC layer can be produced using a wide range of deposition methods such as ion beam depo- sition, magnetron sputtering, and ion beam sputtering. A DLC stamp fabricated using a focused-ion-beam chemical
vapor deposition (FIB-CVD) was previously applied to a thermal-type NIL   [8]. Multilayered 3-D UV-NIL stamps have also been fabricated using DLC coating on polymer patterns  [9].
2. Results and discussion
In this study, we applied a two-step replication to pro- duce an UV-NIL stamp.  Fig. 1  presents the schematics of  the replication process. In principle, the master and pat- terns are without limits. First, the master was coated with
Fig. 1. Schematics for the replication of a master stamp using diamond-like carbon (DLC) deposition and polyvinyl alcohol (PVA) molding: (a) surface treatment of the original stamp; (b) pouring PVA onto the original stamp; (c) DLC deposition; (d) pouring the optical adhesive; (e) placing the glass on the adhesive; and (f) dissolving PVA from the adhesive.
Fig. 2. Schematics for the radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) process.
 
an anti-adhesion layer for easy release, after which an aqueous solution of a water-soluble polymer was cast onto the master mold. After drying, the sacrificial template was peeled off and attached to a flat substrate (i.e., a silicon wafer). The DLC was deposited onto this sacrificial tem- plate, and then a thick glass plate was affixed to the DLC film for reinforcement using a transparent adhesive. After the adhesive was cured, the reinforced DLC stamp was released by dissolving the sacrificial template in water. Because the sacrificial template was composed of a water- soluble polymer, it therefore allowed fast, easy, and safe removal, unlike previous methods involving chemical etch- ing or heating. This process prevents challenges associated with shrinking, which commonly occur during the conven- tional replication process.
The experimental conditions can be summarized as fol- lows. A silicon master nanoscale stamp fabricated using
electron beam lithography followed by directional etching was treated by liquid phase deposition of trichloro (1H-, 1H-, 2H-perfluorooctyl) silane (97%; Aldrich, Milwaukee, WI, USA) for 10 min. The stamp was rinsed with ethanol and acetone following the anti-adhesion treatment. A self-assembled monolayer (SAM) composed of –CF3 formed with this treatment, decreasing surface energy and increasing the contact angle. Most sacrificial templates are composed of PVA, a water-soluble polymer. A 5% PVA (average MW 70,000; Aldrich) aqueous solution was poured over the Si master placed in a Petri dish, after which it was dried overnight at room temperature. After drying, a PVA sacrificial template with a thickness of about 500  lm was peeled off and cut into rectangular slabs while retaining a margin. It was then attached to a 4-inch silicon wafer. Prior to depositing the DLC, pattern-free regions were protected with cellophane adhesive tape. As shown
Fig. 3. Results of 1-lm scale pattern replication using a water-soluble polymer (PVA) and a diamond-like carbon (DLC) coating: (a) SEM image of Si master stamp; (b) AFM image of Si master stamp; (c and d) SEM images of PVA sacrificial template; (e and f) SEM images of replicated DLC stamp.
 
in Fig. 2, radio frequency plasma-enhanced chemical vapor was deposited at a frequency of 13.56 MHz to form the DLC layer on top of the PVA sacrificial template. The deposition used a 400-Volt bias voltage, and each 30-nm- thick deposition was conducted for 5 min. The UV trans- parency of a DLC-coated substrate depends highly on the thickness of the DLC. The 100-nm-thick DLC coated glass wafer had UV transparency values of 5–18% for wavelengths between 350 and 450 nm. However, DLC coatings less than 10 nm had an UV transparency of  approximately 80% with good anti-adhesion characteris- tics. To strengthen the DLC film, a DLC coating with a thickness of 30 nm was placed on the sacrificial template. Methane gas (CH4, 99.9999%) was used as a precursor. The base pressure ranged from 3 to 10 Torr, and working pressure was maintained at 10 m Torr. After the protective tape was removed, an UV-curable adhesive (Norland Opti-
cal Adhesive 65; Norland Products, Inc., New Brunswick, NJ, USA) was poured onto the DLC-coated PVA sacrifi- cial template. A glass slide was affixed, and it was exposed to UV. After adhesive curing, the DLC layer and glass slide were detached from the Si wafer by dissolving the sacrificial PVA template in hot water. PVA regions not coated with DLC were released very rapidly.
Figs. 3a and b present scanning electron microscopy (SEM) images of the Si master stamp and its atomic force microscope (AFM) image. The checkerboard features are 1  lm in width, 1  lm in height, and 150 nm in depth. The sacrificial PVA template was replicated from the master stamp (see Figs. 3c and d). PVA exhibits surfactant charac- teristics due to the presence of several hydroxyl groups. The solution easily moistened the hydrophobic surface and completely filled the master stamp without air entrap- ment.  Figs. 3e and f present the deposited DLC patterns
Fig. 4. Results of 500-nm scale pattern replication using a water-soluble polymer (PVA) and a diamond-like carbon (DLC) coating: (a and b) SEM image of Si master stamp; (c and d) SEM images of PVA sacrificial template; (e and f) SEM images of replicated DLC stamp.
 
released from the sacrificial PVA template. The 30-nm- thick DLC was perfectly deposited on the vertical wall of  the PVA template. After dissolving PVA, a replicated DLC stamp was obtained, which exhibited an excellent correlation with the Si master stamp. The proposed method was conducted on a line feature with a width of  500 nm. Figs. 4a and b present SEM images of the Si mas- ter stamp. It was perfectly replicated using PVA (see Figs. 4c and d).  Figs. 4e and f show that the DLC stamp is in excellent agreement with the Si master stamp.
3. Conclusion
We replicated a master stamp using a DLC coating on the sacrificial PVA template. PVA is a water-soluble poly- mer, which was dissolved in water to release the reinforced DLC stamp from the attached Si wafer. Features on the master stamp ranged from the microscale to nanoscale level. A DLC with a thickness of 30 nm was applied as a partition between the UV curable adhesive and the sacrifi- cial PVA template. The shape of stamp features was well correlated with the corresponding DLC replica. This approach could reduce the dimensional differences that occur during conventional two-times replication. Further-
more, this method could be used to create replicated stamps with sufficient dimensional stability, mechanical strength, and anti-adhesion characteristics for the UV- NIL process without requiring expensive lithography equipment and clean-room facilities.
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