高频功率或信号转换用微型变压器
A MICROFABRICATED TRANSFORMER FOR HIGH-FREQUENCY POWER OR SIGNAL CONVERSION

一、引 言
现在对实现能在高频下工作的，完全把微型变压器集成在芯片或模块上有大量的需求。这种微型变压器的应用包括全集成DC/DC功率转换器或信号隔离器。实现由薄膜磁芯和线圈组成的微型变压器，表面加工技术是有效的。当工作频率推向MHz范围时，磁芯的涡流损耗是主要的功率损耗。为了保证器件有高的效率，必须有效地控制涡流损耗。典型的做法是，在制作高频功率转换用微型变压器时，通过反复溅射磁性材料层和绝缘层，垂直叠放磁芯，以使涡流损耗最小化。但是，制造工艺和成本完全取决于磁性薄膜的层数。另外，需要复杂的制作工艺步骤来得到高电感，低涡流损耗。而且当工作频率冲得太高，由趋肤效应增大的线圈电阻会产生高功耗，特别是负载大的时候。
本文介绍，采用简单UV（紫外线辐射）基光刻和电镀工艺（即LIGA工艺）来设计、制造作芯片上的新型平面微型变压器（1:1），并描述其性能参数。变压器中，初次级线圈绕在Ni/Fe（80%/20%）电镀坡莫合薄膜层周围。邻近放置，以使其耦合系数最大化。提出了用一种新的横向叠放磁芯结构，来减小涡流损耗和制造成本。变压器是在Si晶片上制造的，其尺寸约5mm×2mm×130mm。
二、设计和制造
A 设计
图1表示具有一种新的磁芯结构的平面微型变压器结构图。叠层磁芯带的方向与基片平行。因此，允许横向叠放磁芯。所以用简单的制造步骤就可能制造叠放磁芯带。由通道连接顶层和底层导体，这样，导体可以环绕在横向叠放磁芯周围。叠片磁芯带与导体之间有一绝缘层。在这个结构中，初级和次级线圈可以交错和均匀地排列在磁芯周围，彼此之间间隔很小。因为这样使漏磁最小化，采用这种线圈交织和磁芯结构可获得高的耦合系数。相反，对于传统的罐型磁芯结构，当磁通从顶部磁层到底磁层时，磁路垂直于基片。在该垂直方面的磁通会产生严重的涡流损耗。
溅射出多种非晶合金，用以构成垂直叠片磁芯。但是，这种溅射工艺不适合制作这种新的横向叠片磁芯结构。Ni/Fe坡莫合金已广泛用于磁性微型器件。特别是，采用光刻和电镀技术可以容易作成排列Ni/Fe（80%/20%）坡莫合金镀层图形。在本项工作中采用新开发的微加工技术，以实现横向叠片磁芯结构。每个叠片磁芯带的宽度决于磁性材料和频率。为避免涡流引起的高损耗，磁芯必须叠放得比趋肤深度薄，因为反方向的环流开始叠加并消失，这会使涡流损耗最小化。趋肤深度σ与材料的磁导率、电导率和工作频率的关系如下:
(1)
式中:ω——角频率;σ——电导率;μ——磁导率。Ni/Fe（80%/20%）坡莫合金镀层的电导率为，相对磁导率近800。确定在100kHz，1MHz和10MHz下，Ni/Fe坡莫合金的趋扶深度分别是24.5μm，7.9μm和2.5μm。Cu的电导率为。估计Cu的趋扶深度分别为:100kHz时，208μm，1MHz时65.6μm和10MHz时20.8μm。提高工作频率可以减小磁化电流。但是，如果减小导体的趋肤深度，线圈内的电阻就增大。在微型变压器的设计中，主要的设计要求来自于我们如何用新开发的微切削加工技术的在趋肤深度范围内实现磁芯带。如图1的磁芯结构，要制造宽度薄于2.5μm，高度厚于30μm的叠片磁芯带是不容易的。因此，在这个变压器中，我们选择了8mm宽的叠片磁芯带。两面叠片磁芯的厚度为30μm，这还可以增大到60μm以上。可以通过增大磁芯的垂直厚度来减小磁化电流。导线的宽度为50μm，这也可使导线内的涡流损耗最小化。因为该宽度小于1MHz时Cu的趋肤深度。

图1 带横向叠层磁芯变微型变压器的结构图
B 制 造
图2概述了微型变压器的制造步骤。制造工艺首先是，使2英寸的Si晶片氧化0.6μm以用作基片。在这个制造过程中，采用新LIGA工艺（用厚的AZ系列光刻胶），类LIGA工艺制造，以保证连续多层光刻和电镀。多次把光刻胶旋甩在基片上，以形成厚的多层结构。基于规则的紫外线光刻（UV），可以获得高的径厚比图形。光刻胶即可以作电镀模板，也可以在固化后作平面化的绝缘层。通过电子光来沉积电镀用的Ti/Cu/Ti种晶层。然后，把光刻胶旋甩在种晶层上。用多层光刻胶，可以获得所希望的厚度。光刻后，去掉顶部Ti种晶层，把底部Cu导线电镀在Cu种晶层上，如图2（a）所示，去掉剩余的光刻胶。然后，腐蚀掉种晶层，使导线绝缘。而后，在导线上旋甩上新的光刻胶。打开通道和焊点，用作电触点。然后使光刻胶固化作为绝缘材料，如图2(b）所示。然后在固化的光刻胶上沉种一新的种晶层。在种晶层上旋甩上光刻胶，并构图以形成电镀Ni/Fe坡莫合金叠层磁芯的模板。去掉光刻胶，再旋甩上。通过如图2（c）所示的图样电镀Cu。履盖新的光刻胶层，并把通道打开作电触点，然后固化光刻胶以形成绝缘材料。固化后，在第二层固化的光刻胶上沉积电镀顶层导体用种晶层。然后如图2（d）所示，电镀顶层导线（Cu）。图3是制造的变压器的显示微照片。刻变压器的尺寸为4.0μmx1.0μmx130μm，初次级线圈分别为32匝。

图2 制造步骤:（a)电镀底部导电层;(b)打开通道并固化光刻胶;(c)电镀磁芯和Cu通道;(d)固化光刻胶并电镀顶部导电层

图3 制造的微变压器的显微照片
三、实验结果与讨论
图4表表示线圈的电感和电阻测量结果。初次级线圈几乎有同样的自感和电阻频率特性。这是因为制造工艺可以得到两个对称图形的线圈。电感在1MHz开始下降，这可能主要是电镀坡莫合金的频率特性所致。当在初级线圈上加9MHz，0.5V正弦波输入电压时，在次级线圈上感生0.32V输出电压。输入输出波形如图5所示，当负载为50Ω时，其畸变几乎可以忽略。在初级线圈上加输入电压5V，次级加载50Ω负载时，测得次级线圈的输出电压。图6描绘出不同频率下的电压增益情况。从7MHz到11MHz范围内，得到的电压增益几乎恒定为-4dB。在此频率范围以下，由于线圈损耗在减小，增益随频率增大。而在此频率以上，因磁芯损耗急剧增大而使增益减小。

图4 测量得的线圈电感和电阻

图5 在9MHz微变压器初级（顶部）和次级（底部）的电压波形

图6 次级负载为50W时不同频率下的电压增益
互感系数和耦合系数k分别采用公式（2）、（3）计算，从增加和减少初次级线圈的连接可得到自感和分别是初次级线圈的自感。
（2）
（3）
耦合系数的频率特性如图7所示。由于环形磁芯的低漏磁通和Ni/Fe坡莫合金镀层的高磁导率，耦合系数在5MHz以下是平滑的，且高于0.9。

图7 耦合系数的频率特性
四、结 论
一个改进的横向叠片磁芯结构的新型变压器成功地在芯片上实现了。本课题中，通过新的微切削加工技术，采用并实施一种横向叠片磁芯新概念来减小涡流损耗。制造的变压器在7~11MHz范围以上，有-4dB的电压增益。初次级线圈至到5MHz都有高于0.9的耦合系数。因为微型变压器与基本变压器一样。具有良好的磁电特性，这种新型微变压器结构为完全集成DC/DC转换器和信号隔离器提供了一个新的机会。

Abstract-A new microtransformer(1:1) for high frequency power of signal conversion has been designed，fabricated and characterized in this work，An innovative laterally laminated-core structure has been adopted and implemented by using new micromachining techniques.Using the fabricated microtrand-former，a voltage gain of - dB has been achieved over the frequency range of 7-11 MHz.The coupling coefficient between the primary and secondary coils is higher than 0.9 up to 5 MHz.The transformer realized in this research functionally works as a basic transformer for power or signal conversion.
Index Terms - high frequency transformer， inductance， magnetic core， micromachining.Micr-otransformer，photolithography.
I.INTRODUCTION
There is a large demand for the realization of a fully integrated microtransformer on a chip or a module，which can be driven at a high frequency[1].Applications for such micro-transformers include fully integrated DC/DC power converters or signal isolators[2].Surface micromachining technology is effective in realixing the microtransformers，which are composed of thin film magnetic cores and coils[3].When operating frequency is pushed up to MHz region，eddy current loss in the magnetic core is a major power loss.Thus，in order to keep high efficiency，the eddy current loss has to be effectively controlled.Typically the magnetic cores of the microtransformers for high frequency power-conversion are vertically laminated by repeatedly sputtered magnetic material layer and insulation layer in order to minimize the eddy current loss[4]-[5].However，the fabrication process and costs greatly depend on the number of magnetic film layers. In addition，many complicated fabrication steps are needed to achieve high inductance and low eddy current loss.Furthermore，when the operating frequency is pushed very high，the increased coil resistance due to the skin effect causes high power loss，especially for large load[6].
In this paper，a new planar microtransformer(1:1) on a chip has been designed，fabricated and characterized by using a simple UV-based lithography and electroplating techniqued(i.e.，LIGA-like process).In the transformer，primary and secondary coils are adjacently positioned to maximize their coupling coefficient while they are wrapped around electroplated Ni/Fe (80%/20%) permalloy film.A newlaterally laminated core structure is proposed to reduce both the eddy current loss and fabrication cost.The transformer is fabricated on a Si wafer and its dimension is about 5 mm×2mm×130mm.
II.DESIGN AND FABRICATION
A.Design
The schematic diagram of a planar microtransformer with a new kind of core structure is chown in Fig.1.The directions of laminated magnetic core strips are parallel to the subtrate，thus allowing a laterally laminated core.Therefore it is possible to fabricate the laminated core strips with a single fabrication step.The top layer and the bottom layer conductors are connected by vias，and thue the conductors can be wound around the laterally laminated core.There is an insulation layer between the laminated magnetic core strips and the conductors.In this structure ths primary and secondary windings can be interleaved and distributed evenly around the core with little space between each other.Since this minimizes the flux leakage，a high coupling coefficient can be achieved with the interwoven coil and core structures.In contrast，for a conventional pot core stucture，the flux path is perpendicular to the sheet when flux gets from the top magnetic layer to the bottom magnetic layer.Flux in this perpendicular direction produces serious eddy current loss[4].
Numerous amorphous alloys have been sputtered to form a vertically laminated magnetic core[4]-[5]. However，the sputtering process is not suitable for this new kind of laterally laminated core structure.Ni/Fe Permalloy has been widely used for magnetic micro devices.Specifically，electroplated Ni/Fe(80%/20%) Permalloy can be easily patterned using photolithography and plating techniques.To realize a laterally laminated thin core structure.newly developed micromachining techniques are adopted in this work.The width of each laminated core strip depends on the magnetic material and frequency.To avoid high losses due to eddy current，the magnetic core must be laminated thinner than skin depth since oppositely directed circulating currents begin to overlap and cancel，thus minimizing eddy current loss. The shin depth d is related to material perme-ability，conductivity，and operating frequency as follows:
(1)
where ω is angular frequency，σ is conductivity，and μ is permeability.Electroplated Ni/Fe(80%/20%) Permalloy has a conductivity of and a relative permeability of approximately 800.The evaluated skin depth of Ni/Fe Permalloy is 24.5μm at 100 kHz，7.9μm at 1 MHz，and 2.5μm at 10 MHz respectively/The conductivity of Cu is .The evaluated skin depth of Cu is 208μm at 100 kHz，65.6μm at 1 MHz，and 20.8μm at 10MHz，respectively.Increased operating frequency can reduce magnetizing current.However，if skin depth of the conductor decreases，then the resistance in the coil increases.In the design of microtransformers，the main desing criteria come from how we can realize the core strips in the range of skin depth by using newly developed micromachining techniques.It is not casy to fabricate a laminated core strip which is thinner than 2.5 μm in width and thicer than 30μm in height for the core structure shown in Fig.1.Thus，we chose the width of laminated core strips to be 8 μm in this transformer.The thickness of the planar laminated core is 30 μm，which can be increased to more than 60μm.The magnetizing current can be reduced by increasing the vertical thickness of magnetic core.The width of conductor line is 50μm，which can also minimize the eddy current loss in the conductor line because it is less than the skin depth of Cu at 1 MHz.
B.Fabrication
Fabrication steps for the microtransformer ane summarized in Fig.2.The fabrication process starts with an oxidized(0.6um) 2-inch Si wafer as a substrate.A new LIGA-like technique using a thick photoresist(AZ series was adopted in this fabrication to insure a consecutive multilevel photolithography and electroplating .The photoresist was multi-spun on a substrate to form thick multilayers[7].A high aspect ratio pattern can be achieved based on regular UV lithography.The photoresist can serve both as an electroplating mold and a planarized insulation layer after hard curing.Seed layers (Ti/Cu/Ti) for electroplating were deposited by e-beam.Photoresist was then spun on the seed layer.Desired thickness was achieved with multilayers of photoresist.After photolithography，the top Ti seed layer was removed and the bottom Cu conductor lined were electroplated on the Cu seed layer，as shown in Fig.2.(a).The rest of photoresist was removed and then the seed layers were etched away to insulate the conductor lines.New photoresist layers were then spun onto conductors.The vias and pads were opened for electric contacts.Then the photoresist was hard cured to become insulation material，as shown in Fig.2.(b). A new seed layer was then deporited on the hard-cured photoresist.Photoresist was spun on the seed layer and patterned to form te molds for electroplating the laminated Ni/Fe Permalloy core.The photoresist was removed and spun again.The Cu was electroplated through teh patterns as shown in Fig.2.(c).New layers of photoresist were coated and vias were opened for electric contacts before hard curing the photoresist to form insulation material.A fter the hard curing， a seed layer for electroplating top layer conductor lines was deposited on the second hard-cured photoresist.Then the top layer conductor lines(Cu)were electroplated as shown in Fig2.(d).Fig.3 shows the microphotograph of a fabricated transformer whose size is 4.0mmx1.0mmx130m，where the primary and secondary coils have 32 turns resprctively.
III.EXPEREMENTAL RESULTS AND DISCUS-SION
The measured inductance and resistance of the coils are shown in Fig.4.The primary and secondary coils have almost the same frequency characteristics of self-inductance and resistance because the fabrication process can achieve symmetric patterns of the two coils.The inductance begins to decrease at 1 MHz.which would be mainly due to the frequency characteristics of electroplated Permalloy.When an input voltage of 0.5V at 9 MHz in a sine wave was applid to the primary coils，0.32 V of output voltage was induced at the secondary coils.Shown in Fig.5 are the input and output waveforms，which have almost negligible distortion when the load is 50 .with a load of 50 in the secondary，the output voltage of the secondary coil was measured by applying an input voltage of 0.5 V at the primary coil.For various frequencies，its voltage gain is plotted in Fig.6.The achieved voltage gain of-4 dB is nearly constant in the frequency range of 7 MHz to 11 MHz.Below this freuqency range the gain increases with frequency because the coil loss decreases.Whereas，above the frequency range，the gain decreases due to greatly increased core loss.
The mutual inductance Lm and the coupling coefficient K are calculated by using (2)and (3)respectively.L+and L.are self-inductance obtained from the additive and subtractive connection of the primary and the secondary windings.L1 and L2 are self-inductance of the primary and secondary windings respectively.
（2）
（3）
Frequency characteristics of the coupling coefficient are shown if Fig.7.The coupling coefficient is flat and higher than 0.9 Below 5 MHz due to the low leakage flux of the toroidal core structure and the high permeability of the electriplated Ni/Fe Permalloy.
IV. CONCLUSION
A new microtransformer with an innovative laterally laminated core structure has been successfully realized on a chip.In this work， a new concept of laterally laminated core has been adopted and implemented to reduce eddy current loss by using new micromachining techniques.From the fabricated microtransformer，a voltage gain of -4 dB has been achieved over the freuqncy range of 7-11MHz.The coupling coefficient between the primary and secondary coils is higher than 0.9 up to 5 MHz. Since the microtransformer has favorable magnetic and electrical characteristics as a basic transformer，this new microtransformer structure provides a new choice for fully integraged DC/DC converters and signal isolators.
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