질량 센서를 위한 실리콘 질화막 앵커로 고정된 실리콘 마이크로 빔 공진기의 기계적 특성 연구

Abstract

실리콘 기반의 Nano/Microelectro Mechanical System(N/MEMS) 공진기는 높은 정확성 및 감도를 가지기 때문에 다양한 센서 응용 분야에서 집중적으로 연구되고 있다. 일반적으로 N/MEMS 공진기는 질량이 증가됨에 따라 공진 주파수가 이동하는 특성을 이용하여 흡착 물질을 검출하게 된다. 따라서 N/MEMS 공진기의 질량 감도는 품질계수, 유효 질량, 활성 면적 및 분배 계수에 의존하나, 구조적 관점에서 보면 품질계수의 증가를 위해서는 영률과 밀도 그리고 공진기의 구조가 중요하다. 본 연구는 질량 검출을 기반으로 하는 바이오 및 가스 센서를 연구하기 위하여 MEMS 공진기의 제작 및 특성을 평가하였다. 먼저 낮은 에너지 손실 구조를 가지는 고성능의 MEMS 공진기를 제작하기 위하여 silicon on insulator(SOI) 기판을 이용하여 실리콘 질화막 앵커에 의해 실리콘 마이크로 빔이 감싸인 구조를 가지는 이중으로 고정된 MEMS 공진기를 고안하였다. 실리콘 마이크로 빔은 CMOS 공정 기술 및 표면 미세 가공 기술을 이용하여 상부 실리콘 층에 형성하였으며, 특히 실리콘 마이크로 빔의 양단은 실리콘 질화막 앵커에 의해 감싸인 구조를 가지도록 제작하였다. 실리콘 질화막 앵커는 습식 식각 공정에서 실리콘 빔의 길이의 변화를 최소화하였으며, 기판과 실리콘 빔 사이의 고정력을 높였다. 기존의 N/MEMS 공진기는 빔이 앵커와 직접적으로 연결되는 브리지 구조로 언더컷 영역을 가지게 되며, 진동시 빔의 준 1차원 모드와 연결된 판의 2차원 모드의 커플링이 발생한다. 이러한 커플링 모드는 품질 계수를 감소시킬 뿐만 아니라 비선형 특성을 증가시키게 된다. 반면 실리콘 질화막 앵커를 가지는 MEMS 공진기는 진동시 앵커부에서 진동의 크기를 제한하도록 지지하기 때문에 이로 인해 단일 모드의 진동을 나타내며, 그에 따라 광학적 측정에서 단일 반사 피크를 나타내어 3×104의 높은 품질 계수를 가지는 것을 확인하였다. 또한 추가적인 직류 전압 및 압력에 대한 공진 특성의 변화를 측정하였으며, 직류 전압은 하드닝과 소프트닝을 유발하며, 특히 소프트닝의 경우 진동자의 비선형 특성이 증가되어 품질 계수가 지속적으로 감소되었다. 또한 압력이 증가됨에 따라 가스 입자로부터 발생하는 점성 감쇠로 인해 공진 주파수의 품질 계수가 급격히 감소되는 것을 확인 하였다. 실리콘 질화막 앵커의 효과를 체계적으로 분석하기 위해서 언더컷 영역에서 실리콘 질화막 앵커가 실리콘 빔을 감싸는 비율을 13~36%로 조정하여 MEMS 공진기를 제작하였다. 품질 계수는 실리콘 질화막 앵커가 감싸는 것에 큰 영향을 받았으며, 감싸는 비율이 높아질수록 증가하여 선폭 = 2 ㎛, 길이 = 20 ㎛, 두께가 100 nm인 공진기의 경우 실리콘 질화막 앵커가 감싸는 비율이 21%에서 3×104의 높은 품질 계수를 가지는 결과를 얻었다. 또한 제작 공정에서 발생하는 실리콘 빔 표면의 스트레스를 줄이기 위하여 제작 공정을 수정하였으며, 특히 실리콘 빔 표면을 식각 공정의 플라즈마나 인산으로부터 보호하기 위하여 8 nm의 얇은 실리콘 질화막 희생층을 사용하여 공진기를 제작하였다. 또한 실리콘 마이크로 빔 MEMS 공진기들의 질량 검출 응용 가능성을 확인하기 위하여 연속적으로 Ti/Au 박막을 이용하여 각각 실리콘 빔의 중심과 가장자리에서 질량이 증가되는 경우의 공진 특성을 측정하였다. 측정된 결과에 따르면, 중앙부인 경우 검출 한계는 2.16 ag/Hz(길이 = 16.5 ㎛)에서 7.74 ag/Hz(길이 = 24.6 ㎛)를 얻었으며, 가장자리의 경우 1.58 ag/Hz(길이 = 13.9 ㎛) 에서 5.38 ag/Hz(길이 = 30.5 ㎛)의 검출 한계를 얻었다. 따라서 중앙의 경우가 가장 자리에 비해 3.2배 높은 질량 분해능을 가지며, 질량비(증가된 질량/ 빔의 질량)에 대해 짧고 가벼운 실리콘 빔이 넓은 측정 범위를 가지는 것을 확인 하였다. 또한 질량비에 따른 검출 특성에서 좌굴에 의해 검출 감도가 반전되는 것을 확인하였다. 검출 감도는 중심 및 가장자리 모두에서 좌굴이 발생하기 전까지 질량비가 증가함에 따라 점차 감소하였으나, 좌굴이 발생한 이후로 중앙의 경우 질량비 35%이상에서 검출 감도의 요동이 발생하였으며, 가장자리의 경우 질량비 70% 이상에서는 정지 마찰이 발생하여 공진 주파수의 이동을 측정하기 어려웠다. 그러나 실험 결과에 따르면 실리콘 질화막 앵커는 추가적인 장력을 제공하여 효과적으로 좌굴 발생을 감소시키는 것을 확인 하였다. 마지막으로 바이오 및 가스 센서로서의 응용을 위해 실리콘 마이크로 빔 표면에 3-aminopropyltrimethoxysilane(APTMS)를 이용하여 표면 처리 공정을 연구하였다. 바이오센서의 경우 표면 처리된 실리콘 빔에 금 나노 입자를 흡착시켜 공진 주파수의 이동을 측정하였으며, 공진 주파수는 각각 APTMS 및 금 나노 입자가 흡착됨에 따라 316 kHz 및 910 kHz가 이동하였다. 측정 결과 선폭이 2 ㎛이며, 길이가 10 ㎛ 및 두께가 100 nm인 공진기의 검출 한계는 1.05 ag/Hz를 가지는 것을 확인 하였다. 또한 가스 센서의 경우 가스 흡착을 위해 산화 그래핀을 실리콘 마이크로 빔 위에 부착 시켰으며, 아세톤 가스를 사용하여 흡착시간에 따른 공진 주파수의 이동을 측정하였으며, 공진 주파수는 흡착시간에 따라 3.643 MHz에서 3.567 MHz로 지속적으로 낮아지는 것을 확인 하였다.|Silicon-based Micro/Nano Electromechanical Systems (M/NEMS) resonators are one of the most intensively researched fields for various sensor applications because of their high resolution and accuracy. The M/NEMS sensors employ the characteristics of resonators such that the mass loading on the system induces a shift of its resonant frequency. The related mass sensitivity for the M/NEMS resonator depends on the quality factor (Q-factor), the effective mass, the active area and the partition coefficient. From a structural point of view, however, the main factors that increase the Q-factor are Young’s modulus, the material density and the geometry of the resonator. In this study, for the research of gravimetric sensing-based bio- and gas- sensor, we studied fabrication and resonant property of the MEMS resonator. In order for the high performance MEMS resonator of low energy dissipation, we investigated the new structure of MEMS resonators having a doubly-clamped Si beam structure surrounded by silicon nitride (SiN) anchors on an SOI wafer. Silicon microbeam was fabricated on the top Si layer by using compatible CMOS process and surface micromachining techniques. The purpose of silicon nitride anchor is to minimize the change in length of the silicon beam in a wet etching process step and to tighten the Si beam on the substrate strongly. Conventional M/NEMS resonators have the freestanding beams directly connected to anchors with a undercut beneath the plate and coupling of oscillating modes can occur such that the quasi 1-dimensional mode of the beam couples with the 2-dimensional mode from the plate above the undercut. This coupling reduces Q-factors and induces the growth of non-linear modes. On the other hand, the silicon microbeam of our MEMS resonator has both ends surrounded by SiN anchors that played the role of pivots so that the minimum point of oscillation occurred at the point of the anchors, and the resulting motion appeared as a single normal mode of oscillation. The non-coupled oscillation mode resulted in a single reflection peak in the optical measurement and an extremely high Q-factor(3×104) relative to conventional microbeams. We also measured the effects of the additional DC bias and ambient pressure. The Q-factor of the resonance frequency significantly was suppressed by DC bias applied to the substrate due to the softening and also decreased with increasing the ambient pressure because the gas particle produced viscous damping. In order to systematically analyze the effect of the SiN anchor, we controlled the coverage ratio(13~36%) of the SiN layer at the undercut areas of the Si microbeam structure. The quality factor (Q) was strongly affected by the coverage ratio of the SiN anchors: it increased with the coverage ratio. The MEMS resonator of width x length x thickness being 2 ㎛ x 20 ㎛ x 10 nm showed the highest Q-factor 3×104 at the coverage ratio of 21%. In addition, we improved the fabrication process to reduce the Si microbeam surface stress generated during fabrication process steps. 8 nm thin sacrificial SiN layer was used to protect of the Si microbeam surface which was exposed by plasma and phosphate in etching process steps. In order to verify the performance of our MEMS resonators in mass sensing applications, we measured the resonance frequency shift caused by the sequential mass loading of Ti/Au thin films. The measurements were carried out at the different location in the microbeam: center and edge. The limit of detection (LOD) was obtained from –2.16 ag/Hz(Length = 16.5 ㎛) to –7.74 ag/Hz(Length = 24.6 ㎛) at the center position, and from –1.58 ag/Hz(Length = 13.9 ㎛) to –5.38 ag/Hz(Length = 30.5 ㎛) at the edge position. We found that the sensitivity of mass detection in the case of the center position was displayed higher mass resolution about 3.2 times better than the edge position. Shorter and lighter Si microbeam has a advantage of a wider dynamic range with regard to the mass ratio (mass of the load / the mass of the beam). The transition point of the mass resolution by the buckling was observed: before the buckling occurs, the mass sensitivity gradually decreased with increasing mass ratio, but after buckling, the fluctuation occurred at the 35% mass ratio. We found, however, that the SiN anchor effectively reduced this buckling phenomenon because the Si microbeam received additional tension generated by the SiN anchors. Finally, in order to demonstrate the feasibility of our MEMS in the bio sensor and the gas sensor application, we developed surface modification process on the Si microbeam surface by using APTMS. First, in the case of biosensor, we measured the resonance frequency shift related to the mass loading of Au nanoparticles through covalent linkage. The measured resonant frequency shift was 316 kHz and 910 kHz for APTMS and Au nanoparticles, respectively. From the mass sensing results, the obtained LOD of the devices(Width = 2 ㎛, Length = 10 ㎛, TSi = 100 nm) was 1.05 ag/Hz. Secondly, in the case of gas sensor, we attached the graphene oxide onto the Si microbeam for gas adsorption. And then we measured resonance frequency shift relative to adsorption time by using acetone gas. The resonance frequency continuously was moved toward the lower frequency(from 3.643 MHz to 3.567 MHz) as an increasing adsorption time.; Silicon-based Micro/Nano Electromechanical Systems (M/NEMS) resonators are one of the most intensively researched fields for various sensor applications because of their high resolution and accuracy. The M/NEMS sensors employ the characteristics of resonators such that the mass loading on the system induces a shift of its resonant frequency. The related mass sensitivity for the M/NEMS resonator depends on the quality factor (Q-factor), the effective mass, the active area and the partition coefficient. From a structural point of view, however, the main factors that increase the Q-factor are Young’s modulus, the material density and the geometry of the resonator. In this study, for the research of gravimetric sensing-based bio- and gas- sensor, we studied fabrication and resonant property of the MEMS resonator. In order for the high performance MEMS resonator of low energy dissipation, we investigated the new structure of MEMS resonators having a doubly-clamped Si beam structure surrounded by silicon nitride (SiN) anchors on an SOI wafer. Silicon microbeam was fabricated on the top Si layer by using compatible CMOS process and surface micromachining techniques. The purpose of silicon nitride anchor is to minimize the change in length of the silicon beam in a wet etching process step and to tighten the Si beam on the substrate strongly. Conventional M/NEMS resonators have the freestanding beams directly connected to anchors with a undercut beneath the plate and coupling of oscillating modes can occur such that the quasi 1-dimensional mode of the beam couples with the 2-dimensional mode from the plate above the undercut. This coupling reduces Q-factors and induces the growth of non-linear modes. On the other hand, the silicon microbeam of our MEMS resonator has both ends surrounded by SiN anchors that played the role of pivots so that the minimum point of oscillation occurred at the point of the anchors, and the resulting motion appeared as a single normal mode of oscillation. The non-coupled oscillation mode resulted in a single reflection peak in the optical measurement and an extremely high Q-factor(3×104) relative to conventional microbeams. We also measured the effects of the additional DC bias and ambient pressure. The Q-factor of the resonance frequency significantly was suppressed by DC bias applied to the substrate due to the softening and also decreased with increasing the ambient pressure because the gas particle produced viscous damping. In order to systematically analyze the effect of the SiN anchor, we controlled the coverage ratio(13~36%) of the SiN layer at the undercut areas of the Si microbeam structure. The quality factor (Q) was strongly affected by the coverage ratio of the SiN anchors: it increased with the coverage ratio. The MEMS resonator of width x length x thickness being 2 ㎛ x 20 ㎛ x 10 nm showed the highest Q-factor 3×104 at the coverage ratio of 21%. In addition, we improved the fabrication process to reduce the Si microbeam surface stress generated during fabrication process steps. 8 nm thin sacrificial SiN layer was used to protect of the Si microbeam surface which was exposed by plasma and phosphate in etching process steps. In order to verify the performance of our MEMS resonators in mass sensing applications, we measured the resonance frequency shift caused by the sequential mass loading of Ti/Au thin films. The measurements were carried out at the different location in the microbeam: center and edge. The limit of detection (LOD) was obtained from –2.16 ag/Hz(Length = 16.5 ㎛) to –7.74 ag/Hz(Length = 24.6 ㎛) at the center position, and from –1.58 ag/Hz(Length = 13.9 ㎛) to –5.38 ag/Hz(Length = 30.5 ㎛) at the edge position. We found that the sensitivity of mass detection in the case of the center position was displayed higher mass resolution about 3.2 times better than the edge position. Shorter and lighter Si microbeam has a advantage of a wider dynamic range with regard to the mass ratio (mass of the load / the mass of the beam). The transition point of the mass resolution by the buckling was observed: before the buckling occurs, the mass sensitivity gradually decreased with increasing mass ratio, but after buckling, the fluctuation occurred at the 35% mass ratio. We found, however, that the SiN anchor effectively reduced this buckling phenomenon because the Si microbeam received additional tension generated by the SiN anchors. Finally, in order to demonstrate the feasibility of our MEMS in the bio sensor and the gas sensor application, we developed surface modification process on the Si microbeam surface by using APTMS. First, in the case of biosensor, we measured the resonance frequency shift related to the mass loading of Au nanoparticles through covalent linkage. The measured resonant frequency shift was 316 kHz and 910 kHz for APTMS and Au nanoparticles, respectively. From the mass sensing results, the obtained LOD of the devices(Width = 2 ㎛, Length = 10 ㎛, TSi = 100 nm) was 1.05 ag/Hz. Secondly, in the case of gas sensor, we attached the graphene oxide onto the Si microbeam for gas adsorption. And then we measured resonance frequency shift relative to adsorption time by using acetone gas. The resonance frequency continuously was moved toward the lower frequency(from 3.643 MHz to 3.567 MHz) as an increasing adsorption time.