利用PDMA制造的MEMS直立平面線圈式感應(yīng)器的發(fā)展現(xiàn)狀通信類外文翻譯、中英文翻譯

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 直立平面線圈式 感應(yīng)器的發(fā)展現(xiàn)狀 摘要 這篇文章主要介紹了直立平面線圈感應(yīng)器 的發(fā)展現(xiàn)狀。這種平面線圈感應(yīng)器是首先將平面線圈制作在硅基底上，然后再利用一種新型的三維空間的標尺將其自行裝配到豎直位置（ PDMA）?，F(xiàn)在已經(jīng)制作除了不同維數(shù)的感應(yīng)器，并通過了檢測。通過對感應(yīng)器經(jīng) PDMA 過程前后的 S 參數(shù)的測量、比較，論證了減小基底可以增強感應(yīng)器的性能，同時也論證了增大基底可以為直立平面線圈節(jié)省空間。 關(guān)鍵詞： MEMS，平面線圈感應(yīng)器，品質(zhì)因素， PDMA，諧振頻率 1 簡介 隨著的無線通信系統(tǒng)的完善，高性能（足夠好的品質(zhì)因素和自共振頻率）的芯片感應(yīng)器是必需的。然而這種傳統(tǒng)的平面線圈感應(yīng)器存在著基底 損失和寄生現(xiàn)象，因為傳統(tǒng)感應(yīng)器的平面線圈是不通過薄的絕緣層而直接附在基底上的。近年來，為改進平面線圈的這種性能已經(jīng)做了很多努力。用高傳導(dǎo)率的金屬材料和厚金屬層來減小線圈的電阻。同時，用不同的金屬來減少基底的損失和寄生現(xiàn)象，包括除去感應(yīng)器下面的基底，用一層厚的聚酰亞胺將感應(yīng)器與基底隔離開等等。目前，利用一種金屬犧牲模式（ SMM）實現(xiàn)了感應(yīng)器漂浮在基底之上的過程。用三維激光平板印刷術(shù)或表面顯微機械加工技術(shù)實現(xiàn)的三維螺線管感應(yīng)器已經(jīng)被成功論證。這篇文章闡述了用一種新型的三維自組過程 PDMA 制作的直立平面線圈感 應(yīng)器的發(fā)展現(xiàn)狀。實驗結(jié)果顯示這種直立平面線圈感應(yīng)器要比傳統(tǒng)的水平感應(yīng)器有較少的基底損失和寄生現(xiàn)象，因此它可以達到高的品質(zhì)因素和自共振頻率。直立感應(yīng)器的另外一個主要優(yōu)點是它的底部占用面積幾乎為零，因此占用了非常少的基底空間。 2 塑性變形磁性裝配（ PDMA） 圖 1 為塑性變形磁性組裝過程的示意圖 PDMA 是實現(xiàn)直立平面線圈感應(yīng)器的關(guān)鍵技術(shù)。這種裝配的詳細過程將在其他刊物上介紹，下面我們主要通過一種用懸梁的例子來介紹 PDMA。注意在固定端附近的區(qū)域我們故意將其制作的很柔軟。首先，用刻蝕技術(shù)將粘在懸梁頂面的的一些 磁性物質(zhì)從基底的底層除去（圖 1a）。然后，應(yīng)用磁場，磁性材料塊被磁化，而且懸梁將通過磁轉(zhuǎn)矩旋離基底產(chǎn)生磁性材料塊（圖 1b）。如果這個旋轉(zhuǎn)度設(shè)計得當，懸梁將會在柔性區(qū)域產(chǎn)生塑性變形。并且懸梁將在基底的上方保持一個固定的夾角 （圖 1c）。如果柔性區(qū)域用柔性金屬（例如金），那么在裝配結(jié)構(gòu)與基底之間將很容易形成電路，這樣適用于射頻的應(yīng)用。當豎直結(jié)構(gòu)裝配完成后，這種結(jié)構(gòu)可以進一步加強，如果必要的話，磁性材料也可以除去。如果這個柔性區(qū)域是球形的，那么整個結(jié)構(gòu)就可以平行的裝配在一個基底上。 3 設(shè)計與制作 直立平面線圈感應(yīng)器 的核心結(jié)構(gòu)與傳統(tǒng)的由兩層金屬層和一層絕緣層組成的結(jié)構(gòu)是一樣的。通常我們用高傳導(dǎo)率的金屬材料和低損耗的絕緣材料。另外一個必要條件是：感應(yīng)器的結(jié)構(gòu)應(yīng)該使得 PDMA 實現(xiàn)起來比較容易。 這種直立平面線圈感應(yīng)器是利用一個帶有三個有 150 m厚瀝青的試驗襯墊的共面波導(dǎo)端口（ CPW）的結(jié)構(gòu)。這三個檢測襯墊也作為基底上直立感應(yīng)器的錨。 3.1 選材 金作為底部核心材料（線圈）。金是一種高傳導(dǎo)率的柔性材料，它是 PDMA實現(xiàn)過程中一種理想的塑性變形材料。銅是精心挑選出來的一種頂部核心材料（架橋）。銅也是一種高傳導(dǎo)率的金屬，它的 處理與這個結(jié)構(gòu)中底部核心材料金的處理方法一致。二氧化硅和氮化物是硅集成電路中理想的絕緣材料。但是它們不適合于直立平面線圈感應(yīng)器，因為它們的內(nèi)應(yīng)力太強，并且與金屬表面的結(jié)合力差。 CYTOP（ 氟化塑料光纖 ）非晶態(tài)聚合物是經(jīng)比較挑選出來的用于直立感應(yīng)器中的隔離物質(zhì)。 CYTOP 的電特性與 Polyimide（聚酰亞胺）和 Telfon（ 鐵氟龍 ）的相類似。但它有比較好的與金屬表面的粘附性和化學穩(wěn)定性。為了實現(xiàn)PDMA，將 Permalloy（鐵鎳合金）電鍍于金和銅結(jié)構(gòu)的表面。 Permalloy 層用來為 PDMA 提供必要的磁力 和感應(yīng)器結(jié)構(gòu)在豎直位置是足夠的硬度。 圖 2 所示為直立平面線圈感應(yīng)器的 PDMA 裝配過程和制作過程示意圖。 3.2 制作過程 整個制作和裝配過程如圖 2 所示。這里的基底是表面涂有 0.6 m 厚氮化物的硅片。制作步驟如下： （ a） 沉積 一層 的 厚為 0.5 m 的二氧化硅 作為 PDMA 的犧牲層。 （ b） 在基底上沉積一層 0.5 m 厚的金（下面帶有一層犧牲層），刻蝕成感應(yīng)器的底部核心材料（線圈）。 （ c） 在金上面濺射一層厚為 2.5 m 的 CYTOP 薄膜，作為隔離物。 （ d） 沉積一層 1.5 m 厚的銅作為感應(yīng)器頂部核心材料（架橋）。 （ e） 在金和銅的表面電鍍一層 5 m 厚的 Permalloy。 （ f） 刻蝕氧化犧牲層，并將感應(yīng)器結(jié)構(gòu)從基底釋放。 （ g） 通過 PDMA 將感應(yīng)器的整個結(jié)構(gòu)裝配到豎直的位置。 4 實驗檢測和測量結(jié)果 直立平面線圈感應(yīng)器制作中的 S11 參數(shù)通過一個 851的計算機分析器從 50 MHz 測量到 4GHz。 S11第一次測量是在 PDMA 過程之前，當感應(yīng)器還在硅基底上時。接下來感應(yīng)器被裝配到豎直位置后， S11被重復(fù)測量。感應(yīng)器的 S11參數(shù)與在耐熱玻璃基底上設(shè)計的結(jié)構(gòu)是一樣的， S11都要被測量以用來對照比較。 圖 3a 所示為平面線圈感應(yīng)器經(jīng)過 PDMA 前和后對 S11模擬和測量結(jié)果的史密斯圖。 B 為在玻璃基底上設(shè)計制作的一個同樣的感應(yīng)器的 S11 的模擬和測量結(jié)果的史