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Nano-contact for small power consumption in phase change memory
Sumio Hosaka1*, Hui Zhang, Miftahul Huda1, You Yin1
1
Faculty of Science and Technology, Gunma University
1-5-1 Tenjin, Kiryu 376-8515 Japan*e-mail: hosaka@el.gunma-u.ac.jp
ABSTRACT
We have studied new concept phase change memory (PCM) with nano-contacts to reduce power consumption. Nanocontact has been achieved by using PS-PDMS block copolymer self-assembly. Using PDMS dots formed on large
contact hole, nano-contacts were made of SiOx by exposing O2-plasma in O2-RIE. In self-assembly experiments andphase change simulation, we can obtain following conclusion. 1) We could predict that the power of nano-contact
PCM is estimated to a tenth consumption power of conventional PCM in switching. 2) Our experiments demonstrated
the reduction as same as this estimation. 3) We found out edge effect on current density, which rapidly increased atedge of nano-contact.
Key words: Nano-contact, Phase Change, PCM, GeSbTe, Self-assembly, Block coplymer etc.

1. INTRODUCTION
We have studied small power consumption in phase change memory(PCM). In order to achieve small power consumption, there are somesolutions such as selection of phase change material, heater structure,phase change memory structure, etc. So far, we have experiences thatdirect heating is very important and suitable to reduce power
consumption of PCM rather than indirect heating. In addition of the direct
heating, we more reduce it by decreasing phase change material volume[1]. Such a scaling-down depends on fabrication of contact hole, so that it
also depends on lithographic method. The minimum contact hole may be
50-200 nm in diameter even with advanced lithography. We need otherlithographic method to make small contact holes. Considering vertical
type phase change memory, it is very difficult to reduce the diameter ofcontact hole with EB or optical lithography. The purpose of this study is Fig. 1 Our final goal and conventional
to develop new concept PCM structure with nano-contact [2, 3] as shown PCM in reset current for contact holearea.
in Fig. 1.
In order to reduce the contact hole, we use nano-contact between thephase change material and heater instead of decrease of phase change material volume. Our idea is that insulate
nanodos are inserted to the interface between phase change material layer and heater. We use self-assembly withblock copolymer, PS-PDMS (polystyrene-poly dimethile siloxane) as a fabrication method of the insulate nanodots. In
this paper, we describe the possibility to form the nanocontacts using PS-PDMS self-assembly [3-6] and to reduce
switching power of PCM.
2. PHASE CHANGE MEMORY with NANO-CONTACTS
In order to reduce a power consumption of PCM, there aresome methods such as selection of phase change materials, thedevice structure, etc. Basically, as we have to supply small
power to device and heat up local phase change area tomelting point in reset mode, it is necessary to design resistanceof partial or all phase change area high as shown in Fig. 2. In
this study, we treat with the structure with vertical type PCM
with phase change material and heater in cascade connection

Fig. 2 Method of reduction of a heating power inphase change memory in its electric property.

(Fig. 3) as aconventionalPCM.
Andthe maximum
heatingposition
isaroundacenter of theheater. As it Fig. 3 Cross sectional view ofis a little far the basic conventional verticalfromthe PCM structure.
phase change
Fig. 4 Scheme of proposed nano-contact PCM with smallmaterial, the conventional PCM has a insulated dots on interface between GST and TiN heater.
disadvantage that large consumption power isrequired in reset process. In order to solve the problem,
we utilize nano-contacts in interface between phasechange material (Ge2Sb2Te5) and TiN heater using PSPDMS block copolymer self-assembly as shown in Fig. 4[2]. By inserting nano-contact structure, the resistance of
the interface and local phase change material, which is
O2-doped by the self-assembly process as described later,
becomes larger than that of heater (Fig. 5). Consequently,
we expect to heat up the interface and the phase changematerial part directly to contribute reduction of the power
consumption. In this study, as the interface size is 90 nm
in diameter, the self-assembled dot diameter is required to Fig. 5 Equivalent resistance circuits of conventional
PCM and nano-contact PCM.
be about 10 nm.
3. EXPERIMENTS
We have designed the proposed PCM and its
fabrication process, especially in the self-assembly.
As described above, new PCM has nano-contacts inthe interface. For nano-contacts, there are somekind patterns of sphere, cylinder, lamellar, hole, etc.
in self-assembly as shown in Fig. 6 [2]. As the dot
etc. may be made of SiOx in final stage, we predict Fig. 6 Various shapes self-assembled by PS-PDMS blockthat SiOx edge effect appears as edge current copolymer. FF is fill factor.
density is enhanced as described latter. In theexperiments, we used PDMS spheres. Especially, we describe
self-assembly for nano-contact in this section as following.
In the self-assembly, we have to select spherical shape and thediameter of < 20 nm. These requirements are decided bytheoretical and experimental microphase separation of block
copolymer of PS-PDMS. We studied fine dot size and pitch
regarding the molecular weight (MW) of PS-PDMS based ontheoretical relationship between the product of χN (χ: FloryHuggins parameter and N: total number of segments) and thePDMS concentration ratio of block copolymer for total MW [7]. 4
kinds of PS-PDMS MW were selected as shown in Fig. 7. Using
the self-assembly process including annealing and reactive ionetchings (RIEs) (Fig. 8), we obtained very fine dots and pitches ofself-assembled PDMS dots as shown in Fig. 9. The figure shows achange of average dots pitch and dot size of 12-33 nm and 6-22
nm in using of PS-PDMS MWs of 7000-1500 to 30000-7500g/mol, respectively. The dot size and pitch are determined by PSand PDMS molecular segment N as shown in Fig. 10. The self-Fig. 7 Configuration of BCP self-assembled
nanostructures and the PS-PDMS moleculeweights used in the experiments.

Fig. 8 Process flow ofFig. 9 SEM images of self-assembled PDMS dots using its MWs of 13500-4000,
self-assembly with PS11700-2900 and 7000-1500 g/mol.
PDMS.
assembled pitch and dot diameter decrease in a proportion to two thirds order of the N value. The value agrees wellwith theoretical result. On the other hand, we notice the self-assembly process. In final stage (Fig. 8), we did the RIEof phase change layer exposed with O2 plasma. Consequently, the phase change surface in the interface was changedto oxide-doped phase change layer with several nm in depth.
4. RESULTS AND DISCUSSION [2, 3]
With our proposed device, the power reduction was studied bysimulation using FEM (finite element method). We used the PCM
structure as shown in Fig. 4. Figure 11 shows estimated current
density, electrical potential and temperature of the nano-contact
PCM and temperature of conventional PCM when we applied acurrent of 100μA (10ns) between top and bottom electrodes [3]. In
the simulation, the nano-contact PCM and conventional PCM
resistances changed about 104 to 107 Ω and 103 to 106 Ω,
respectively, in reset process. When applying the current, current
density increases local hole area with a diameter of 10 nm.
Especially, we can observe edge effect that current density increase Fig.
10 Dot size and pitch for theat the edge of the dots rather than that with no nano-contact. The production of χN.
electric field is enhanced in small holes of the nano-contact.
Consequently, electric power concentrates to nano-contact and the O2-doped GST layer. The temperature increases toover the melting point in the nano-contact device. However, conventional PCM’s temperature increases a little to 320
K, which is too insufficient for melting of GST. Figure 12 shows estimated reset currents for top contact radius in bothnano-contact PCM and conventional PCM [3]. As the current in nano-contact PCM reduces to a tenth of that inconventional PCM so that we canreduce the power of nano-contact
PCM to a tenth magnitude ofconventional PCM. In experimental
results, we could demonstrate that thenano-contact phase change device
achieve small power consumption
reduction as same as the simulation
described above [3].
4. CONCLUSION
We have studied new concept
phase change memory (PCM) withnano-contacts to reduce power
consumption. Nano-contact has beenachieved by using PS-PDMS block
copolymer self-assembly. Using
PDMS dots formed on large contact
hole, nano-contacts were made of
SiOx by exposing O2-plasma in O2-Fig. 11 Simulated results of current density, electric potential and heatedtemperature of nano-contact PCM, and heated temperature of conventionalPCM.

RIE. As self-assembly experiments and phase change
simulation, we obtain following information.
1) We predict that the power of nano-contact PCM isestimated to a tenth consumption power ofconventional PCM. (Our experiments demonstrated
the reduction as same as this estimation.)
2) We found out edge effect on current density, which
rapidly increased at edge of nano-contact. By edge
effect, we could select sphere structure from various
shapes such as sphere, cylinder, lamellar, etc.
3) As the dot size of 10 nm in diameter, we have toselect PS-PDMS with a MW of 11700-2900 g/mol.

Fig. 12 Estimated reset current for top contact holeradius.

ACKOWLEADGMENT
This work was financially supported by the New Energy and Industrial Technology Development Organization(NEDO) under the “Development of nanobit technology for ultrahigh density magnetic recording (Green IT)” project,
Grant-in-Aid for Young Scientists and Grant-in-Aid for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology of Japan (No. 24686042, No. 21710135, No. 24360003), and JST Sentan Grant
Number H24sentan181-25.
REFERENCES[1] You Yin, Hayato Sone and Sumio Hosaka､ Jpn. J. Appl. Phys. 45 (2006) 6177-6181.
[2] W. I. Park, B. K. You, B. H. Mun, H. K. Seo, J. Y. Lee, S. Hosaka, Y. Yin, C. Ross, K. J. Lee, Y. S. Jung、ACS
NANO, 7 (2013) 2651-2658.
[3] You Yin, Sumio Hosaka, Woon Ik Park, Yeon Sik Jung, Keon Jae Lee, Byoung Kuk You, Yang Liu, and Qi Yu,Appl. Phys. Lett. 103 (2013). 033116 1～5.
[4] M. Sakurai, Y. Kamata, H. Hieda, Toshiba Rev. 57 (2002) 52-55.
[5] I. Bita, J. Yang, Y. Jung, C. Ross, E. Thomas, K. Berggren., Science. 321 (2008) 939-943.
[6] S. Hosaka, T. Akahane, M Huda, T. Tamura, Y. Yin, N. Kihara, Y. Kamata, A. Kitsutsu,, Microelectronic
Engineering, 89 (2011) 2571-2575.
[7] M. Huda, J. Liu, Y. Yin, and S. Hosaka, Jpn. J. Appl. Phys., 51 (2012) 06FF10 (1-5).