微机电系统自聚焦压电声学换能器
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摘要
微机电系统(Micro-Electrical-Mechanical Systems,缩写为MEMS)是基于微制造技术,集微型机械结构、微传感器、微执行器以及微电子电路等于一体的微型器件和系统。伴随着集成电路制造技术的成熟发展,研究人员还采用了化学和机械工艺来制造微结构和微器件。对于传统的“机械学”来说,微机电系统技术不仅为之打开了“微尺寸”这一新领域的大门,也是真正实现机电一体化的开始。自从1988年第一个硅微静电驱动马达面世以来,微机电系统技术的研究开发日益得到国际上的普遍关注。
对于微机电系统的研究,通常分为两大类：微结构和微换能器。微结构通常包含微透镜、微喷嘴、微探针和微流体系统；而微换能器通常包含微传感器和微执行器。菲涅耳自聚焦压电声学换能器作为一种重要的微机电换能器,已经被南加州大学的Kim博士小组开发和制造出来。
本文广泛深入地讨论了微机电系统菲涅耳自聚焦压电声学换能器的基本原理,并设计和开发了分别适用于局部细胞剥落和高频超声成像与多普勒探测的微机电换能器系统。不仅如此,我们还创造性地提出了可控曲率半径曲面的制作和用于微流体系统的封装技术。下面,我就从这四个方面介绍本文所做的工作。
一.传统的细胞剥落方法,要么作用于一大片的细胞,不能实际应用于小样本或精确控制的治疗；要么需要人手工操作,费事耗力。我们设计制造的菲涅耳自聚焦压电声学换能器能够产生高度聚焦的声束,激发超声空化泡,实现了局部细胞剥落。该换能器将声波集中在半径只有60微米的小区域,其产生的压强峰值能达到近3个大气压,足以在高达12兆赫兹的高频处激发空化泡。空化泡在爆破过程中产生了极大的能量,使得聚焦区域中近200个细胞被打落下来,而该区域外的细胞则完好无损。这项技术为局部细胞剥落的实现提供了可能,并能够应用于小样品采样和局部生物医学治疗。
二.高频超声成像因其具有较高的空间分辨率,已成为重要的医疗手段。在超声成像系统中,换能器是最重要的一个部分。受传统的制造技术的限制,换能器难以工作在高频,使得成像的分辨率产生了瓶颈。本文开发制造的菲涅耳自聚焦压电声学换能器,使用了厚的锆钛酸铅(Lead Zirconate Titanate,缩写为PZT)作为衬底,它的工作基频为20兆赫兹。由于设计了具有空气反射腔的菲涅耳透镜,该换能器可以工作在其谐振模式。在换能器的三倍(60兆赫兹)和五倍频率(100兆赫兹)处,我们都得到了较强的声波信号。实验结果和理论分析都证明了该换能器能够被应用于高达100兆赫兹的高频超声成像和多普勒系统中。
三.我们在硅片上开发了一种全新的微制造技术,实现了可控曲率半径曲面的制造。具有三维任意形状的微结构具有非常广泛的应用,包括光学元件,针阵列,以及其他任何需要严格控制其结构的MEMS器件。传统的刻蚀方法对此无能为力,而常用的灰度掩膜法又极其昂贵。我们基于溅射理论,通过荫罩掩模的遮挡,在硅片上沉积了具有不同半径的凸球面结构的二氧化硅。深反应离子刻蚀(Deep Reactive Ion Etching,缩写为DRIE)因其极佳的方向性和选择性,被用来刻蚀二氧化硅和硅衬底,由此二氧化硅的凸球面结构就被转移到硅片上。我们将此具有不同球面半径的硅片作为模子,使用聚酰亚胺(Polyimide)压膜成型,聚酰亚胺就相应地具有了不同球面半径的凹结构。它可以被用来制造三维自聚焦压电声学换能器。
四.我们使用聚对二甲苯-C(Parylene-C)为微流体系统的封装提供了一个很好的解决方案。微流体系统通常包含微沟道和微水槽等微结构,需要将它们封装以便保护液体样品不受污染。在用Parylene-C封装以前,厚的光刻胶和蜡分别作为牺牲材料被用于微结构的填充物。并且旋转和沉积这两种方法都被分别用于在硅片上覆盖类似聚四氟乙烯(Polytetrafluoroethene,缩写为PTFE,商标名Teflon(?),俗称特富龙)的无定形聚合物,它的作用是使硅片表面呈现疏水状态,以便蜡的填充。实验结果证明,与光刻胶相比,蜡能够帮助提高Parylene-C薄膜的平整度,并且非常适于批量生产。在去掉蜡以后,Parylene-C被加热到120℃,使得该薄膜具有较大的张力从而得到更平整的表面。
本论文涉及了微机电系统中最重要的两个方面——微结构和微换能器。实现了对菲涅耳自聚焦压电声学换能器的继承、发展和应用,对三维任意曲率半径曲面制造方法的创新以及对微流体系统的封装技术的有益探索。
Micro-Electrical-Mechanical Systems (MEMS) are the integration of mechanical elements, sensors, actuators and electronics on a wafer, which are relied on micro-fabrications. People use well-established IC fabrication techniques with chemical and mechanical processes to construct micro structures and devices. At the microscopic level, MEMS bridge the gap between the "electrical/computer world" and the real physical world, which is a great-leap-forward development. Since the first electrostatic micro-motor demonstrated in 1988, MEMS have been increasingly developed in the worldwide.
The study on MEMS is divided into two parts:micro-structures and micro-transducers. Micro-structures include micro-lens, micro-nozzles, micro-probes and micro-fluidic systems, while micro-transducers always refer to mirco-sensors and micro-actuators. As an important MEMS transducer, self-focused piezoelectrical acoustic transducers with Fresnel lens have been developed by Dr. Kim's group at University of Southern Califonia. In this paper, we extensively explored two kinds of these transducers and applied them to two important applications alternatively. Moreover, two novel micro-facbrcation techniques were developed as well.
First, most of the techniques for cell detachments either randomly affect a large area of cells or require much human labor. We used focused acoustic beams to generate cavitation bubbles for the localized cell detachment. A self-focused piezoelectrical acoustic transducer was used to focus the acoustic waves at a spot of 60μm in radius, where the peak intensity of the pressure reached 3 atmospheres, strong enough to activate cavitation bubbles at a high frequency (12 MHz). Due to the high pressure and temperature produced by the cavitation bubbles, around 200 cells in the localized area were detached from the substrate while leaving adjacent cells intact. This technique demonstrated the potential applications of localized cell detachment for small size sample preparations and localized bio-medical therapies.
Second, high frequency ultrasond imaging has become an important medical diagnosis tool. We developed high-overtone self-focused acoustic transducers for high frequency ultrasonic imaging and Doppler. By using harmonic frequencies of a thick bulk Lead Zirconate Titanate (PZT) transducer with a novel air-reflector Fresnel lens, we obtained strong ultrasound signals at 60 MHz (3rd harmonic) and 100 MHz (5th harmonic). Both experimental and theoretical analysis demonstrated that the transducers could be applied to Doppler systems with frequencies up to 100 MHz.
Third, we explored a new microfabrication technique to form controllable MEMS curvertural structures on Silicon wafers. By using Silicon-dioxide sputter, photolithography, DRIE (Deep Reactive Iron Etching) and polyimide coating, we achieved the first cost-effective fabrication on common Silicon wafers without using gray masks which is very expensive. Arbitrary structures can be applied to, but are not limited to, optical elements, needle arrays, and any other MEMS devices which require controlled profiles. The invention and development of this method showed great promise to form microstructures with controllable 3D structures. Meanwhile, it could be a potential to fabricate the self-focused acoustic transducers on a curvatural structure with great focal effects.
Finally, we demonstrated a novel technique for covering microfluidic systems using Parylene-C. Microfluidic systems consisting of micro channels and reservoirs need to be covered to protect or isolate liquid samples from the environment. Thick photoresist and wax were employed as the sacrificial layers in the enclosed micro channels and reservoirs before Parylene-C sealing. And both spinning and depositing Teflon-like amorphous fluorocarbon polymer methods were applied. The results showed that the melted wax improved adherence on a flat and neat Parylene-C film cover and could greatly benefit the mass production. After removing the sacrificial layers, Parylene-C was heated to 120℃to change the residual stress of Parylene-C film to strongly tensile for a flatter surface.
对于微机电系统的研究,通常分为两大类：微结构和微换能器。微结构通常包含微透镜、微喷嘴、微探针和微流体系统；而微换能器通常包含微传感器和微执行器。菲涅耳自聚焦压电声学换能器作为一种重要的微机电换能器,已经被南加州大学的Kim博士小组开发和制造出来。
本文广泛深入地讨论了微机电系统菲涅耳自聚焦压电声学换能器的基本原理,并设计和开发了分别适用于局部细胞剥落和高频超声成像与多普勒探测的微机电换能器系统。不仅如此,我们还创造性地提出了可控曲率半径曲面的制作和用于微流体系统的封装技术。下面,我就从这四个方面介绍本文所做的工作。
一.传统的细胞剥落方法,要么作用于一大片的细胞,不能实际应用于小样本或精确控制的治疗；要么需要人手工操作,费事耗力。我们设计制造的菲涅耳自聚焦压电声学换能器能够产生高度聚焦的声束,激发超声空化泡,实现了局部细胞剥落。该换能器将声波集中在半径只有60微米的小区域,其产生的压强峰值能达到近3个大气压,足以在高达12兆赫兹的高频处激发空化泡。空化泡在爆破过程中产生了极大的能量,使得聚焦区域中近200个细胞被打落下来,而该区域外的细胞则完好无损。这项技术为局部细胞剥落的实现提供了可能,并能够应用于小样品采样和局部生物医学治疗。
二.高频超声成像因其具有较高的空间分辨率,已成为重要的医疗手段。在超声成像系统中,换能器是最重要的一个部分。受传统的制造技术的限制,换能器难以工作在高频,使得成像的分辨率产生了瓶颈。本文开发制造的菲涅耳自聚焦压电声学换能器,使用了厚的锆钛酸铅(Lead Zirconate Titanate,缩写为PZT)作为衬底,它的工作基频为20兆赫兹。由于设计了具有空气反射腔的菲涅耳透镜,该换能器可以工作在其谐振模式。在换能器的三倍(60兆赫兹)和五倍频率(100兆赫兹)处,我们都得到了较强的声波信号。实验结果和理论分析都证明了该换能器能够被应用于高达100兆赫兹的高频超声成像和多普勒系统中。
三.我们在硅片上开发了一种全新的微制造技术,实现了可控曲率半径曲面的制造。具有三维任意形状的微结构具有非常广泛的应用,包括光学元件,针阵列,以及其他任何需要严格控制其结构的MEMS器件。传统的刻蚀方法对此无能为力,而常用的灰度掩膜法又极其昂贵。我们基于溅射理论,通过荫罩掩模的遮挡,在硅片上沉积了具有不同半径的凸球面结构的二氧化硅。深反应离子刻蚀(Deep Reactive Ion Etching,缩写为DRIE)因其极佳的方向性和选择性,被用来刻蚀二氧化硅和硅衬底,由此二氧化硅的凸球面结构就被转移到硅片上。我们将此具有不同球面半径的硅片作为模子,使用聚酰亚胺(Polyimide)压膜成型,聚酰亚胺就相应地具有了不同球面半径的凹结构。它可以被用来制造三维自聚焦压电声学换能器。
四.我们使用聚对二甲苯-C(Parylene-C)为微流体系统的封装提供了一个很好的解决方案。微流体系统通常包含微沟道和微水槽等微结构,需要将它们封装以便保护液体样品不受污染。在用Parylene-C封装以前,厚的光刻胶和蜡分别作为牺牲材料被用于微结构的填充物。并且旋转和沉积这两种方法都被分别用于在硅片上覆盖类似聚四氟乙烯(Polytetrafluoroethene,缩写为PTFE,商标名Teflon(?),俗称特富龙)的无定形聚合物,它的作用是使硅片表面呈现疏水状态,以便蜡的填充。实验结果证明,与光刻胶相比,蜡能够帮助提高Parylene-C薄膜的平整度,并且非常适于批量生产。在去掉蜡以后,Parylene-C被加热到120℃,使得该薄膜具有较大的张力从而得到更平整的表面。
本论文涉及了微机电系统中最重要的两个方面——微结构和微换能器。实现了对菲涅耳自聚焦压电声学换能器的继承、发展和应用,对三维任意曲率半径曲面制造方法的创新以及对微流体系统的封装技术的有益探索。
Micro-Electrical-Mechanical Systems (MEMS) are the integration of mechanical elements, sensors, actuators and electronics on a wafer, which are relied on micro-fabrications. People use well-established IC fabrication techniques with chemical and mechanical processes to construct micro structures and devices. At the microscopic level, MEMS bridge the gap between the "electrical/computer world" and the real physical world, which is a great-leap-forward development. Since the first electrostatic micro-motor demonstrated in 1988, MEMS have been increasingly developed in the worldwide.
The study on MEMS is divided into two parts:micro-structures and micro-transducers. Micro-structures include micro-lens, micro-nozzles, micro-probes and micro-fluidic systems, while micro-transducers always refer to mirco-sensors and micro-actuators. As an important MEMS transducer, self-focused piezoelectrical acoustic transducers with Fresnel lens have been developed by Dr. Kim's group at University of Southern Califonia. In this paper, we extensively explored two kinds of these transducers and applied them to two important applications alternatively. Moreover, two novel micro-facbrcation techniques were developed as well.
First, most of the techniques for cell detachments either randomly affect a large area of cells or require much human labor. We used focused acoustic beams to generate cavitation bubbles for the localized cell detachment. A self-focused piezoelectrical acoustic transducer was used to focus the acoustic waves at a spot of 60μm in radius, where the peak intensity of the pressure reached 3 atmospheres, strong enough to activate cavitation bubbles at a high frequency (12 MHz). Due to the high pressure and temperature produced by the cavitation bubbles, around 200 cells in the localized area were detached from the substrate while leaving adjacent cells intact. This technique demonstrated the potential applications of localized cell detachment for small size sample preparations and localized bio-medical therapies.
Second, high frequency ultrasond imaging has become an important medical diagnosis tool. We developed high-overtone self-focused acoustic transducers for high frequency ultrasonic imaging and Doppler. By using harmonic frequencies of a thick bulk Lead Zirconate Titanate (PZT) transducer with a novel air-reflector Fresnel lens, we obtained strong ultrasound signals at 60 MHz (3rd harmonic) and 100 MHz (5th harmonic). Both experimental and theoretical analysis demonstrated that the transducers could be applied to Doppler systems with frequencies up to 100 MHz.
Third, we explored a new microfabrication technique to form controllable MEMS curvertural structures on Silicon wafers. By using Silicon-dioxide sputter, photolithography, DRIE (Deep Reactive Iron Etching) and polyimide coating, we achieved the first cost-effective fabrication on common Silicon wafers without using gray masks which is very expensive. Arbitrary structures can be applied to, but are not limited to, optical elements, needle arrays, and any other MEMS devices which require controlled profiles. The invention and development of this method showed great promise to form microstructures with controllable 3D structures. Meanwhile, it could be a potential to fabricate the self-focused acoustic transducers on a curvatural structure with great focal effects.
Finally, we demonstrated a novel technique for covering microfluidic systems using Parylene-C. Microfluidic systems consisting of micro channels and reservoirs need to be covered to protect or isolate liquid samples from the environment. Thick photoresist and wax were employed as the sacrificial layers in the enclosed micro channels and reservoirs before Parylene-C sealing. And both spinning and depositing Teflon-like amorphous fluorocarbon polymer methods were applied. The results showed that the melted wax improved adherence on a flat and neat Parylene-C film cover and could greatly benefit the mass production. After removing the sacrificial layers, Parylene-C was heated to 120℃to change the residual stress of Parylene-C film to strongly tensile for a flatter surface.
引文
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