Design of Perpendicular Magnetic Anisotropy of Co/Pt and Co/Ni Superlattice Structure Ferro-coupled to Co2Fe6B2 Free Layer with High Thermal Stability

Abstract

차세대 메모리 중 spin transfer torque magnetic random access memory (STT-MRAM)은 비휘발성이며, 적은 전력소모, 빠른 스위칭 속도 등의 장점으로 주목받고 있다. p-STT-MRAM을 메모리로서 상용화하기 위해서는 thermal stability와 밀접한 관련이 있는 높은 retention time(>10년)을 충족 시켜야한다. 본 연구에서는 메모리 셀 사이즈 15nm×15nm 이하에서 p-STT-MRAM의 thermal stability가 77(tera-bit 메모리 셀에서 retention time 10년을 만족시키기 위해 요구되는 thermal stability 값)을 넘을 수 있는 PMA구조를 설계하였다. Thermal stability를 높이기 위한 구조로, Co2Fe6B2와 [Co/Pt]n, [Co/Ni]n multilayers가 각각 W bridge layer를 통해 커플링 되어 free layer로 작용하는 구조를 제작하여 진동 시료 자력계(VSM) 측정 장비를 이용하여 M(magnetic moment) - H(anisotropy field) 곡선을 얻었다. 이를 바탕으로 포화 자기 모멘트 값(Ms×t), 이방성 자기장(Hk)을 측정하였다. [Co/Pt]n과 [Co/Ni]n에서 모두 multi layer의 수가 증가 할수록 Ms×t는 선형적으로 증가하였고, Hk는 급격히 증가하다가 점점 증가량이 감소하였다. Ms×t의 경우 [Co/Ni]n에서 [Co/Pt]n에 비해 평균적으로 58.91μemu씩 큰 값을 가졌지만, Hk의 경우 [Co/Ni]n에서 평균적으로 5670Oe씩 작았다. 또, Ms×t와 Hk를 이용하여 thermal stability를 계산하였고, 그 결과 [Co/Pt]n, [Co/Ni]n 모두 n이 커질수록 thermal stability는 증가하였다. [Co/Pt]n을 이용한 구조에서는 큰 anisotropy energy로 인해 높은 thermal stability를 얻기에 유리하였다. 15nm×15nm 사이즈의 셀을 기준으로 하였을 때, [Co/Pt]4에서의 thermal stability는 92로 77보다 충분히 큰 값을 갖는다. [Co/Ni]n에서는 n=6일 때 thermal stability 77을 달성하였다. [Co/Pt]n의 경우 더 작은 층수에서도 [Co/Ni]n보다 더 큰 thermal stability를 얻을 수 있었다. 하지만, [Co/Ni]n은 damping constant가 작아 스위칭에 유리하다는 장점이 있다. 즉, 새롭게 제안된 두 가지 구조에서 모두 10년 retention time을 갖는 tera-bit 메모리를 위해 요구되는 thermal stability를 충분히 충족시킬 수 있음을 보였으며, thermal stability의 관점에서만 본다면 [Co/Pt]n을 이용한 구조의 특성이 더 우수함을 알 수 있었다.|Recently, spin transfer torque magnetic random access memory (STT-MRAM) which is the next-generation memory is attracting attention due to many advantages such as non-volatility, low power consumption (< 1pJ/bit), fast switching speed (~1 ns), and high retention time (>10 years). In addition, perpendicular STT-MRAM (p-STT-MRAM) has been suggested as a memory mapped storage-class-memory (SCM), having a similar write/read speed and 2 times higher latency compared to the DRAM. However, recent studies show that the conventional CoFeB/MgO based p-STT-MRAM cannot maintain the required thermal stability for retention time of 10 years below the cell size of 15nm×15nm. In this thesis, the bulk like perpendicular magnetic ansiotropy(PMA) structures were designed to exceed thermal stability() of 77(the required thermal stability to satisfy 10 years of retention time in tera-bit memory cells) at the p-STT-MRAM under memory cell size 15nm×15nm. The Co2Fe6B2 free layer was ferrocoupled through the W bride layer to [Co/Pt]n or [Co/Ni]n multilayers in order to enhance the thermal stability. The M(magnetic moment) - H(external field) curve was obtained using vibration sample magnetometer(VSM) measuring equipment. The saturation magnetic moment value(MS×t) and the anisotropic field(Hk) were measured. In both [Co/Pt]n and [Co/Ni]n, MS×t increased linearly as the number of multi layers increased. HK increased rapidly as the number of multilayers increased at the beginning, but the increment saturated when the number of multilayers reach a certain value. The saturation magnetic moment value of [Co/Ni]n was slightly larger than [Co/Pt]n by 58.91μemu on average, but Hk was very small in [Co/Ni]n by 5670Oe on average. As a result, the thermal stability increased as n increased in [Co/Pt]n and [Co/Ni]n multilayers. The large anisotropy energy in the structure using [Co/Pt]n was beneficial to achieve high thermal stability. At the cell size of 15nm×15nm, the thermal stability of [Co/Pt]4 multilayers is 92, which is more than enough for retention time of 10 years (>77). In the case of [Co/Ni]n multilayers, minimum of 6 multilayers are needed to achieved the thermal stability of 77. The thermal stability can be achieved with smaller number of multilayers using [Co/Pt]n. However, [Co/Ni]n has the low damping constant and is advantageous for switching. In other words, both of the newly proposed structures could satisfy the required thermal stability for tera-bit memory cell with 10 years retention time, and from the standpoint of thermal stability alone, the properties of the structure using [Co/Pt]n were better.; Recently, spin transfer torque magnetic random access memory (STT-MRAM) which is the next-generation memory is attracting attention due to many advantages such as non-volatility, low power consumption (< 1pJ/bit), fast switching speed (~1 ns), and high retention time (>10 years). In addition, perpendicular STT-MRAM (p-STT-MRAM) has been suggested as a memory mapped storage-class-memory (SCM), having a similar write/read speed and 2 times higher latency compared to the DRAM. However, recent studies show that the conventional CoFeB/MgO based p-STT-MRAM cannot maintain the required thermal stability for retention time of 10 years below the cell size of 15nm×15nm. In this thesis, the bulk like perpendicular magnetic ansiotropy(PMA) structures were designed to exceed thermal stability() of 77(the required thermal stability to satisfy 10 years of retention time in tera-bit memory cells) at the p-STT-MRAM under memory cell size 15nm×15nm. The Co2Fe6B2 free layer was ferrocoupled through the W bride layer to [Co/Pt]n or [Co/Ni]n multilayers in order to enhance the thermal stability. The M(magnetic moment) - H(external field) curve was obtained using vibration sample magnetometer(VSM) measuring equipment. The saturation magnetic moment value(MS×t) and the anisotropic field(Hk) were measured. In both [Co/Pt]n and [Co/Ni]n, MS×t increased linearly as the number of multi layers increased. HK increased rapidly as the number of multilayers increased at the beginning, but the increment saturated when the number of multilayers reach a certain value. The saturation magnetic moment value of [Co/Ni]n was slightly larger than [Co/Pt]n by 58.91μemu on average, but Hk was very small in [Co/Ni]n by 5670Oe on average. As a result, the thermal stability increased as n increased in [Co/Pt]n and [Co/Ni]n multilayers. The large anisotropy energy in the structure using [Co/Pt]n was beneficial to achieve high thermal stability. At the cell size of 15nm×15nm, the thermal stability of [Co/Pt]4 multilayers is 92, which is more than enough for retention time of 10 years (>77). In the case of [Co/Ni]n multilayers, minimum of 6 multilayers are needed to achieved the thermal stability of 77. The thermal stability can be achieved with smaller number of multilayers using [Co/Pt]n. However, [Co/Ni]n has the low damping constant and is advantageous for switching. In other words, both of the newly proposed structures could satisfy the required thermal stability for tera-bit memory cell with 10 years retention time, and from the standpoint of thermal stability alone, the properties of the structure using [Co/Pt]n were better.