Design, Fabrication, Optimization and Verification of FePt based Bit Patterned Media

s group has been supported by INSIC and recording industry for years and may be the only university group in US with the demonstrated and integrated capabilities to fabricate and investigate FePt based bit patterned media. We developed seedlayer/underlayer(s) and a low-substrate-temperature (350C) process to grow smooth continuous L10 FePt film on glass substrate with relatively large Hk (~ 80kOe). Recently we designed and demonstrated FePt/Fe exchange coupled composite (ECC) bit patterned media (BPM) with 30 nm dot size and 35 nm pitch using diblock copolymer lithography on 2.5 inch disk substrate [1]. We also proposed and experimentally demonstrated a new ECC media design concept, which increased the Gain Factor (Figure of Merit of ECC stack) up to 3.2 for the first time with total magnetic layer thickness less than 10 nm [2]. Furthermore, we have developed a Nano-Imprinting process and demonstrated the feasibility for fabricating FePt bit patterned media with dot size less than 20 nm on 1 cm x 1 cm Si substrate [3]. Switching field distribution of the FePt ECC BPM has been characterized by several methods through collaboration [1,2]. Several fabrication processing and stack design factors were identified, which may cause the switching field distribution. In this proposal, we will address key challenges listed by ASTC on the magnetic materials and properties for future bit patterned media by using the above-mentioned established research capabilities on the growth of continuous and smooth FePt ECC stack, fabrication processes of FePt ECC BPM and characterization methods for the switching field distribution. 1. To develop the continuous L10 FePt to reach large perpendicular Hk with minimum in-plane hysteresis and reduced roughness. This will be realized by selecting and exploring different seed and underlayer(s) by engineering its surface energy and interface energy; 2. To design, fabricate and demonstrate FePt ECC BPM media (both enclosed structure and graded structure) to reach the maximum Gain Factor (~ 4) of ECC media with ultra thin magnetic recording layer (< 10 nm); The targeted dot size and pitch size are 15 nm and 20 nm respectively for the first year. 3. To characterize and understand quantitatively the origins of the switching field distribution for FePt ECC BPM; 4. To pattern and demonstrate FePt ECC BPM with low switching field distribution (~ 5 10%) with targeted 15nm dot size and 20 nm pitch size; Proposal to ASTC J. P. Wang, MINT, University of Minnesota, April 29, 2011 2 Research description This proposal is focused on the BPMR topic No. 10: BPM Magnetic materials and properties for BPM. Also is related with the BPMR topic No. 4: BPM island fabrication strategies (etch, ion implant, alternative) and effects on magnetic properties. We will try to address several key challenges for future FePt type BPM media: 1) how to reduce the in-­‐plane component while keeping its high perpendicular Hk; 2) how to design, grow and pattern ECC type FePt BPM media (both graded and enclosed structures); 3) what are the real key factors for the switching field distribution; 4) how to reduce the switching field distribution for FePt BPM media; 5) how the patterning processes (both diblock copolymer mask and nanoimprinting processes) contribute to the in-­‐plane component and the switching field distribution; In this research, several ideas will be tried to grow high quality continuous L10 FePt film with large perpendicular (~100 kOe), minimum in-­‐plane hysteresis and a small roughness. Our very recent interesting data (figure 1c) showed a very promising effort along this line. ECC bit pattern media with both graded structure and enclosed structure will be fabricated and tested based on the optimized L10 FePt film. Several schemes for soft region will be explored including the temperature and composition control. Further optimization and design will be combined with micromagnetic simulation to achieve the maximum gain factor (~4) and meanwhile keep the thickness of the magnetic recording layer within 10 nm. To theoretically verify our design, we will carry out the simulation (LLG & OOMMF) and also collaborate with Prof. Randall Victora and Prof. Haowen Liu for a more complicated recording system level . We have successfully established and demonstrated two FePt BPM patterning processes (30 nm dot size and 35 nm pitch size). We will continue to optimize these processes to pattern FePt ECC dots (graded structure) with size around 15 nm and pitch size around 20 nm by end of the first year. We have experimentally demonstrated that an enclosed or so-­‐called core-­‐shell type FePt ECC media has several advantages over other designs including the large gain factor (~ 3.2) with ultra thin recording layer (<10 nm) and low dipole interaction between dots [2]. We admit that it is a great challenge to fabricate enclosed type FePt ECC media (thickness control of side wall). However, its great potential to reduce the dipole interaction between neighboring dots and lower the total magnetic layer thickness makes it very attractive to address the concern of the broadening switching field distribution caused by the dipole interaction and to increase the writing field gradient. We plan to demonstrate the feasibility of the enclosed ECC FePt BPM structure with 30 nm dot size and 35 nm pitch size. The fabricated FePt ECC BPM will be characterized to quantitatively understand the origins of the switching field distribution and its correlation with the design stack(s), thin film deposition parameters and patterning parameters. The target of this research is to demonstrate writable and thermally stable FePt type bit patterned media with low switching field distribution of less than 10% and high density with 15 nm dot size and 20 nm pitch size. Proposal to ASTC J. P. Wang, MINT, University of Minnesota, April 29, 2011 3 Current status and proposed research approaches Fabrication and optimization of FePt based composite continuous film. The quality of the continuous thin film is the key to prepare bit patterned media with low or no in-­‐plane component. To keep the switching field distribution (SFD) of the patterned media as small as possible, the continuous film must be super flat and uniform. Also to fabricate the exchange coupled composite films, the hard magnetic film has to be very thin as well. In our previous work, 5 nm FePt continuous film was fabricated on Si wafer with structure of CrRu(30 nm)/Pt(3 nm)/FePt(5 nm) using an 8-­‐target magnetron sputtering system with the base pressure as low as 4x10 torr. Good ordering and FePt (001) texture were achieved at 350 C substrate temperature. Fe layer was coated on top of the FePt after the sample was cooled down to room temperature. Fig. 1 (a)-­‐ (c) shows the TEM cross-­‐sectional images and XRD spectra of the composite continuous film with structure of CrRu(30 nm)/Pt(3 nm)/FePt(5 nm)/Fe(7.5 nm). Fig.1 (a) and (b) verify the continuality and flatness of FePt and Fe layer in both low and high magnification. The XRD spectra in (c) indicates the good ordering and (001) texture. Fig.1(d) shows the out-­‐of-­‐plane and in-­‐plane MH loops of a recently optimized MgO substrate/CrRu(30 nm)/Pt(3 nm)/FePt(5 nm) continuous film with ultra low in-­‐plane coercivity (almost zero); This film also shows huge perpendicular Hk (estimated around 80 kOe).