利用PDMA制造的MEMS直立平面线圈式感应器的发展现状通信类外文翻译、中英文翻译

Development of MEMS Vertical Planar Coil Inductors Through Plastic Deformation Magnetic Assembly (PDMA) Jinghong Chen*, Jun Zou*, Chang Liu* and Sung-Mo (Steve) Kang* Agere Systems, Holmdel, NJ 07733 Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 Jack Baskin School of Engineering, University of California, Santa Cruz ABSTRACT This paper presents the results of the development of a vertical planar coil inductor. The planar coil inductor is first fabricated on silicon substrate and then assembled to the vertical position by using a novel 3-Dimensional bathscale self-assembly process (Plastic Deformation Magnetic Assembly (PDMA). Inductors of different dimensions are fabricated and tested. The S-parameters of the inductors before and after PDMA are measured and compared,demonstrating superior performance due to reduced substrate effects and also increased substrate space savings for the vertical planar coil inductors. Keywords: MEMS, planar coil inductor, quality factor,PDMA, resonance frequency. 1 INTRODUCTION With the development of integrated wireless communication systems, on-chip inductors with satisfactory performance (enough quality factor and selfresonant frequency) are required. However, the conventional planar coil inductor suffers from substrate losses and parasitics since it is directly fabricated onto conductive substrate over a very thin dielectric layer 1. In recent years, much effort has been made to improve the performance of planar coil inductors. Metal materials with higher conductivity or thicker metal layers are utilized to decrease the resistance of the coil. Meanwhile,different methods are proposed to reduce the substrate loss and parasitics, including removing the substrate underneath the inductor 2, applying a thick polymide layer to separate the inductor farther away from the substrate 3, etc. More recently, planar coil inductors levitated above the substrate are realized using a sacrificial metallic mode (SMM) process 4. 3-Dimensional solenoid on-chip inductors developed by using 3-D laser lithography or surface micromachining technology have also been demonstrated 5,6. This paper reports vertical planar coil inductors developed by using a novel 3-D self-assembly process-Plastic Deformation Magnetic Assembly (PDMA).Experimental results show that the vertical planar coil inductor suffers less substrate loss and parasitics than the conventional horizontal counterparts, and thus can achieve a higher quality (Q) factor and self-resonant frequency. Another major advantage of the vertical inductors is that they have almost zero footprints and thus occupy much smaller substrate space. 2 PLASTIC DEFORMATION MAGNETIC ASSEMBLY (PDMA) Fig. 1 A schematic illustration of a Plastic Deformation Magnetic Assembly process (PDMA) PDMA is the key technology to realize the vertical planar coil inductors. A detailed discussion of this assembly process will be presented in other publications. A brief introduction of PDMA is given below by using a cantilever beam as an example. Note that the region near the fixed end is intentionally made more flexible. First, a cantilever beam with a piece of magnetic material attached to its top surface is released from the substrate by etching away the sacrificial layer underneath (Fig. 1(a). Next, a magnetic field Hext is applied, the magnetic material piece is magnetized and the cantilever beam will be rotated off the substrate by the magnetic torque generated in the magnetic material piece (Fig. 1(b). If the structure is designed properly, this bending will create a plastic deformation in the flexible region. The cantilever beam will then be able to remain at a certain rest angle ( ) above the substrate even after Hext is removed (Figure 1(c). By using ductile metal (e.g. gold) in the flexible region, a good electrical connection between the assembled structure and the substrate can be easily achieved, which is suitable for RF applications. After the vertical assembly, the structures can be further strengthened and the magnetic material can be removed if necessary. If the magnetic field is applied globally, then all the structures on one substrate can be assembled in parallel. 3 DESIGN AND FABRICATION The core structure of the vertical planar coil inductor is identical to the conventional horizontal one, which consists of two metal layers and one dielectric layer between. As a general rule, high conductivity metal and low loss dielectric material should be used. In addition to this requirement, the structure of the inductor should facilitate the implementation of PDMA. The vertical planar coil inductor utilizes one-port coplanar waveguide (CPW) configuration with 3 test pads (Ground-Signal-Ground) with a pitch of 150m. The three test pads also serve as the anchor of the vertical inductors on the substrate. 3.1 Material Consideration Gold is used as the material for the bottom conductor (the coil). Gold is a ductile material with high conductivity. It is an ideal plastic deformation material for the implementation of PDMA. Copper is selected as the material of the top conductor (the bridge). Copper is also a high conductivity metal and its processing is compatible with the gold bottom conductor during the fabrication. Silicon oxide and nitride are good dielectric materials available in silicon IC process. However, they are not suitable for vertical planar coil inductors due to high internal stress and poor adhesion on metal surface. CYTOP amorphous flurocarbon polymer is selected as the dielectric spacer of the vertical inductors. The electrical properties of CYTOP film are similar to those of Polyimide and Telfon . However, it has a better adhesion on metal surfaces and chemical stability. In order to implement PDMA, Permalloy (NiFe) is electroplated onto the surface of the gold and copper structures. The Permalloy layer will provide the magnetic force necessary for PDMA and enough stiffness to the inductor structure in the vertica position. Fig. 2 A schematic illustration of the fabrication and PDMA assembly of vertical planar coil inductors. 3.2 Fabrication Process A brief illustration of the entire fabrication and assembly process is shown in Fig. 2. The substrate is a silicon wafer with a 0.6m-thick nitride layer on top. The fabrication is conducted in the following steps: (a) A 0.5m-thick silicon oxide layer is deposited and patterned to serve as the sacrificial layer for PDMA. (b) A 0.5m-thick gold layer is deposited onto the substrate (with sacrificial layer underneath) and patterned to make the bottom conductor (coil) of the inductor. (c) A 2.5m-thick CYTOP film is spun onto the gold layer and patterned to make the dielectric spacer. (d) A 1.5m-thick copper layer is deposited and patterned to make the upper conductor (bridge) of the inductor. (e) A 5m-thick Permalloy layer is electroplated onto the copper and gold surface. (f) The oxide sacrificial layer is etched and the inductor structure is released from the substrate. (g) The entire inductor structure is assembled into vertical position using PDMA. 4 TESTING AND MEASUREMENT RESULTS The S11 parameter of the fabricated vertical planar coil inductors is measured from 50 MHz to 4GHz using an 851 network analyzer. The S11 parameter is first measured while the inductors are on the silicon substrate before PDMA. Next, the inductors are assembled to the vertical position using PDMA and the S11 measurement is repeated. The S11 parameter of inductors with identical designs fabricated on Pyrex glass substrate is also measured for comparison. Fig. 3 (a) A Smith chart showing the simulated and measured S11 parameter results of the planar coil inductor shown in Fig. 3 before and after PDMA. (b) A Smith chart showing the simulated and measured S11 parameter results of an inductor with identical design fabricated on glass substrate. The inductance of both inductors is 4.5nH. Fig.3(a) shows the simulated and measured S11 parameter results of the planar coil inductor shown in Fig. 3. The test pads feeding the inductors on which probing occurs are de-embedded. Fig. 3(b) shows the simulated and measured S11 parameter results of an inductor with identical design inductor fabricated on Pyrex glass substrate. The test pads are not de-embedded since the inductor is on glass substrate and the de-embedment has little effect. The simulated data is obtained by using a compact circuit model for planar coil inductor presented in 1. The quality factor (Q) as a function of frequency extracted from both the simulated and the measured S11 parameter results is plotted in Fig. 5. When the planar coil inductor is on the silicon substrate, it has a peak Q factor of 3.5 and self-resonant frequency of 1GHz. When the inductor is in the vertical position after the PDMA assembly, the peak Q factor increases to 12 and the selfresonant frequency goes well above 4GHz, which are close to those of the inductor on glass substrate. For the planar coil inductors fabricated on silicon (before assembly), a large insulator capacitance and the substrate resistance dominate at higher frequencies, which leads to a self resonate frequency of about 1GHz. Once the inductors are assembled into the vertical position, the substrate loss and capacitance are effectively removed,which leads to the improvement in the inductor performance. Furthermore, fo r frequencies far below 1 GHz (where substrate effects are negligible), the glass and silicon inductor measurements correspond exactly, as expected. 5 DISCUSSIONS 1) In this work, oxide is used as the sacrificial material for PDMA. Sometimes, oxide is used as the dielectric for IC devices on the same substrate and thus cannot be removed. In this case, the oxide sacrificial layer can be substituted by other materials, e.g. photo resist. 2) In order to facilitate the measurement, the vertical planar coil inductors tested are not strengthened after PDMA.However, the inductor structure can be strengthened after PDMA by using different methods (e.g. Parylene coating) to achieve the necessary stiffness and robustness. 3) While the Q factor of vertical planar coil inductors can be improved by depositing thicker metal layers during the fabrication, experiments are also being conducted to explore various methods to thicken and strengthen the metal layers of the planar coil inductors while they are in the vertical position after the PDMA assembly. 6 CONCLUSIONS Vertical planar coil inductors have been achieved by using a novel 3-D assembly-Plastic Deformation Magnetic Assembly (PDMA). Vertical planar coil inductors offer two major advantages over conventional on-substrate ones: they occupy much smaller substrate space and suffer less substrate loss and parasitics. The proposed fabrication and assembly process of the vertical planar coil inductor is compatible with the standard IC fabrication process, and is therefore suitable for various RF ICs. REFERENCES 1 C. P. Yue and S. S. Wong, “Physical modeling of spiral inductors on silicon,” IEEE Trans. On Electron Devices, vol. 47, no. 3, March, 2000, pp. 560-568. 2 M. Ozgur, M. Zaghloul, and M Gaitan, “High Q backside Micromachined CMOS inductors,”ISCAS99. Proceedings of the 1999 IEEE International Symposium on Circuits and Systems,vol.2, 1999, pp.577-80. 3 B. K. Kim, B. K. Ko, and K. Lee, “Monolithic planar RF inductor and waveguide structures on silicon with performance comparable to those in GaAs MMIC,” IEDM Tech. Dig., 1995, pp. 717-720. 4 J. B. Yoon, C. H, Han, E. Yoon and C. K. Kim,“High-performance three-dimensional on-chip inductors fabricated by novel micromachining technology for RF MMIC,” IEEE MTT-S Digest,1999, pp 1523-26. 5 D. Young, V. Malba, J. Ou, A. Bernhardt, and B.Boser, “Monolithic high-performance threedimensional coil inductors for wireless communication applications”, IEDM Tech. Dig.,1997, pp. 67-70. 6 J. B. Yoon, B. K. Kim, C. H. Han, E. Yoon, K. Lee and C. K. Kim, “High-performance electroplated solenoid-type integrated inductor (SI2) for RF applications using simple 3D Surface micromachining technology,” IEDM Tech. Dig., 1998, pp.544-547. 利用 PDMA 制造的 MEMS 直立平面线圈式 感应器的发展现状 摘要 这篇文章主要介绍了直立平面线圈感应器 的发展现状。这种平面线圈感应器是首先将平面线圈制作在硅基底上，然后再利用一种新型的三维空间的标尺将其自行装配到竖直位置（ PDMA）。现在已经制作除了不同维数的感应器，并通过了检测。通过对感应器经 PDMA 过程前后的 S 参数的测量、比较，论证了减小基底可以增强感应器的性能，同时也论证了增大基底可以为直立平面线圈节省空间。 关键词： MEMS，平面线圈感应器，品质因素， PDMA，谐振频率 1 简介 随着的无线通信系统的完善，高性能（足够好的品质因素和自共振频率）的芯片感应器是必需的。然而这种传统的平面线圈感应器存在着基底 损失和寄生现象，因为传统感应器的平面线圈是不通过薄的绝缘层而直接附在基底上的。近年来，为改进平面线圈的这种性能已经做了很多努力。用高传导率的金属材料和厚金属层来减小线圈的电阻。同时，用不同的金属来减少基底的损失和寄生现象，包括除去感应器下面的基底，用一层厚的聚酰亚胺将感应器与基底隔离开等等。目前，利用一种金属牺牲模式（ SMM）实现了感应器漂浮在基底之上的过程。用三维激光平板印刷术或表面显微机械加工技术实现的三维螺线管感应器已经被成功论证。这篇文章阐述了用一种新型的三维自组过程 PDMA 制作的直立平面线圈感 应器的发展现状。实验结果显示这种直立平面线圈感应器要比传统的水平感应器有较少的基底损失和寄生现象，因此它可以达到高的品质因素和自共振频率。直立感应器的另外一个主要优点是它的底部占用面积几乎为零，因此占用了非常少的基底空间。 2 塑性变形磁性装配（ PDMA） 图 1 为塑性变形磁性组装过程的示意图 PDMA 是实现直立平面线圈感应器的关键技术。这种装配的详细过程将在其他刊物上介绍，下面我们主要通过一种用悬梁的例子来介绍 PDMA。注意在固定端附近的区域我们故意将其制作的很柔软。首先，用刻蚀技术将粘在悬梁顶面的的一些 磁性物质从基底的底层除去（图 1a）。然后，应用磁场，磁性材料块被磁化，而且悬梁将通过磁转矩旋离基底产生磁性材料块（图 1b）。如果这个旋转度设计得当，悬梁将会在柔性区域产生塑性变形。并且悬梁将在基底的上方保持一个固定的夹角 （图 1c）。如果柔性区域用柔性金属（例如金），那么在装配结构与基底之间将很容易形成电路，这样适用于射频的应用。当竖直结构装配完成后，这种结构可以进一步加强，如果必要的话，磁性材料也可以除去。如果这个柔性区域是球形的，那么整个结构就可以平行的装配在一个基底上。 3 设计与制作 直立平面线圈感应器 的核心结构与传统的由两层金属层和一层绝缘层组成的结构是一样的。通常我们用高传导率的金属材料和低损耗的绝缘材料。另外一个必要条件是：感应器的结构应该使得 PDMA 实现起来比较容易。 这种直立平面线圈感应器是利用一个带有三个有 150 m厚沥青的试验衬垫的共面波导端口（ CPW）的结构。这三个检测衬垫也作为基底上直立感应器的锚。 3.1 选材 金作为底部核心材料（线圈）。金是一种高传导率的柔性材料，它是 PDMA实现过程中一种理想的塑性变形材料。铜是精心挑选出来的一种顶部核心材料（架桥）。铜也是一种高传导率的金属，它的 处理与这个结构中底部核心材料金的处理方法一致。二氧化硅和氮化物是硅集成电路中理想的绝缘材料。但是它们不适合于直立平面线圈感应器，因为它们的内应力太强，并且与金属表面的结合力差。 CYTOP（ 氟化塑料光纤 ）非晶态聚合物是经比较挑选出来的用于直立感应器中的隔离物质。 CYTOP 的电特性与 Polyimide（聚酰亚胺）和 Telfon（ 铁氟龙 ）的相类似。但它有比较好的与金属表面的粘附性和化学稳定性。为了实现PDMA，将 Permalloy（铁镍合金）电镀于金和铜结构的表面。 Permalloy 层用来为 PDMA 提供必要的磁力 和感应器结构在竖直位置是足够的硬度。 图 2 所示为直立平面线圈感应器的 PDMA 装配过程和制作过程示意图。 3.2 制作过程 整个制作和装配过程如图 2 所示。这里的基底是表面涂有 0.6 m 厚氮化物的硅片。制作步骤如下： （ a） 沉积 一层 的 厚为 0.5 m 的二氧化硅 作为 PDMA 的牺牲层。 （ b） 在基底上沉积一层 0.5 m 厚的金（下面带有一层牺牲层），刻蚀成感应器的底部核心材料（线圈）。 （ c） 在金上面溅射一层厚为 2.5 m 的 CYTOP 薄膜，作为隔离物。 （ d） 沉积一层 1.5 m 厚的铜作为感应器顶部核心材料（架桥）。 （ e） 在金和铜的表面电镀一层 5 m 厚的 Permalloy。 （ f） 刻蚀氧化牺牲层，并将感应器结构从基底释放。 （ g） 通过 PDMA 将感应器的整个结构装配到竖直的位置。 4 实验检测和测量结果 直立平面线圈感应器制作中的 S11 参数通过一个 851的计算机分析器从 50 MHz 测量到 4GHz。 S11第一次测量是在 PDMA 过程之前，当感应器还在硅基底上时。接下来感应器被装配到竖直位置后， S11被重复测量。感应器的 S11参数与在耐热玻璃基底上设计的结构是一样的， S11都要被测量以用来对照比较。 图 3a 所示为平面线圈感应器经过 PDMA 前和后对 S11模拟和测量结果的史密斯图。 B 为在玻璃基底上设计制作的一个同样的感应器的 S11 的模拟和测量结果的史