三維微小尺寸高頻濾波器之設計、模擬與製作

Abstract

無線傳輸是目前熱門的商品，在無線傳輸的線路中，濾波器則佔有重要的角色。而微小化是市場目前對此類型產品的要求。然而，傳統設計之濾波器由於操作頻段之限制無法無限制地縮小。本研究係結合slow-wave線路設計與微機電(MEMS)加工技術，製作出三維高頻微小化低通濾波器。並針對金屬厚度之變化來探討濾波器特性之改變。 首先，我們應用Slow-wave線路設計一組高頻微小化之濾波器，設計之規格為4 GHz Butterworth之低通濾波器。電波傳輸結構採共平面波導之結構。線路設計過程中分別利用interdigital線路及meander線路來取代電容及電感元件。微小化的濾波器之設計尺寸為 4.18 × 6.0 mm，與傳統型步接阻抗濾波器相比可有效減小90%的面積。使用高頻電磁波軟體 Sonnet的模擬結果顯示，濾波器之截斷頻率為4.1 GHz，且擁有良好的插入損失與阻抗匹配。此外，在本研究中，我們亦使用3D 高頻電磁波模擬軟體，Ansoft HFSS，來針對具有三維金屬結構之濾波器，探討其金屬厚度與濾波器特性之關係。 製程方面，我們使用面型微加工與微電鍍技術將不同金屬厚度之三維微小化濾波器製作於石英基材上。利用石英基材可有效降低基材之介電損失。製作的金屬厚度則由0.15 μm至20 μm厚。在製程當中，黏著層及電鍍起始層材料的選擇是非常重要的。選擇黏著材料時，必須考慮其黏著力及容易去除的特性，電鍍起始層則必須具有優良之傳導性。經由不同金屬材料之實驗測試，發現以鈦作為黏著層，銅作為電鍍起始層是較佳組合，並可成功地製作出微小化之濾波器。 量測時使用網路分析儀及微探針來量測微小化濾波器之散射常數。量測結果顯示，具有良好散射響應曲線之微小化高頻低通濾波器已被我們成功地研製，並且具有良好之低插入損耗(<-1.5 dB)。量測結果中也發現濾波器的特性在不同金屬厚度變化下有不同之量測結果。隨著金屬厚度之增加可得濾波功能更好之信號響應。同時意謂著濾波器之特性可利用厚度這個尺寸因素來加以改變。 對於三維微小化高頻濾波器之製作與特性而言，電鍍結構之厚度變化扮演決定性的角色。藉由設計不同金屬電鍍厚度之樣品及量測結果，我們成功地設計與製作出了具有潛力之微小化高頻濾波器。而且根據尺寸有效地縮小及良好濾波器效能等等特性，這使得我們有信心將我們的元件應用在微小化的微波積體電路。

Wireless products are popular with the public at present. In microwave circuits, filters play an important role. These products are required to be miniaturized. However, conventional designed filters cannot be limitless reduced due to the operation frequency. In this research, slow-wave lines and micromachined techniques are applied to fabricate 3D high frequency miniaturized lowpass filter. The characteristics of filters are simultaneously investigated with the increasing metal thickness. Firstly, slow-wave lines are applied to design a high frequency miniaturized lowpass filter. The designed filter is 4.18 × 6.0 mm in area. From the simulation results of Sonnet, the cut-off frequency of designed filter is set on 4.1 GHz, good insertion loss and impedance match are observed. In our research, Ansoft HFSS is the first time to be applied for the simulation of our designed filter, especially for the 3D metal structures. In fabrications, surface micromachining and micro-electroplating technologies are applied to fabricate 3D miniaturized filters with different metal thickness on quartz substrate. The selection of adhesion layer and seed layer is important that adhesion layers should provide good adhesive force and easily to be removed, and seed layers should have good conductivity. Per different metal test, it is found that composes of titanium and copper is a better choice. Therefore, our miniaturized lowpass filters are fabricated successfully. In measurement results, good scattering response curves of lowpass filters have been investigated and fabricated. It is observed that both power loss and 3 dB frequency are decreased with the increasing metal thickness. This means the cut-off frequency can be adjusted by controlling the thickness of interdigital and meander lines in our proposed lowpass filter, which implies to gain benefit from the thickening of conductor. In point of our fabrication and measured results, the variations of metal thickness play an important role to determine the characteristics of filters. Based on our electroplated samples and measured results, we have successfully investigated the miniaturized lowpass filters which are potential and popular at present. Our results show that this reduced-sized lowpass filter have good performance with low loss and compact size and is very promising for use in MIC/MMIC applications.