局部溫度控制下電遷移對覆晶封裝中UBM消耗及其失效機制研究

Abstract

隨著電子元件的構裝密度不斷提高以及不斷朝向高功能、微型化發展，覆晶封裝（Flip-Chip package）技術已成為半導體工業中高階電子元件，如CPU、GPU ( 繪圖晶片 )的主流封裝技術。近年來覆晶銲點的直徑以及間距 ( Pitch ) 不斷縮小，導致通過覆晶銲點的電流密度持續攀升，而高電流密度所引發之電遷移現象將使得作為晶片與基板連接線路的銲點成為相對脆弱的接點，進而影響覆晶元件的可靠度。因此，電遷移效應已成為高階電子元件可靠度的重要研究內容之一。 電遷移效應中「電流密度」與「溫度」兩大因素，對覆晶封裝的失效機制而言一直是相當重要並難以分開討論。許多研究此兩大因素的電遷移文獻中，大多以固定電流密度，改變不同環境溫度來進行研究，鮮少能在固定晶片溫度下，進行不同電流密度間的探討。本研究透過良好的溫度控制系統，以強制對流的方式解決了因高電流密度所造成的大量焦耳熱並成功將晶片端溫度予以控制，試圖將「熱遷移」效應從高電流密度加速測試中降至最低。本研究所使用之覆晶銲點結構為Al導線/Ti(1K)/Cu(2K)/Cu(8μm)/Sn2.6Ag/Cu/Cu導線，所施加之電流密度分別為3.5×104A/cm2、4.5×104A/cm2、5.5×104A/cm2，並將晶片端溫度控制於50℃。針對加裝與未加裝溫度控制系統的覆晶元件實驗結果比較發現，在恆溫控制下不僅能延長覆晶銲點之壽命 ( 超過1500小時 )，更發現與過去「高溫」通電加速測試中截然不同的界面形貌，本論文也試圖對此一特殊界面形貌的分佈、形狀與形成原因加以探討、解釋。

With the requirement for decreasing packaging size and increasing quantity for interconnect efficient, flip chip package has become a predominant technology for CPU and GPU…The miniaturization of bump pitch always accompany the rise of interconnect current density, and it will made the solder joint of the device a weakly spot, which makes the subsequent electromigration phenomenon cannot be ignored. Current density and temperature are two key effects in flip chip electromigration study, and it is very important and hard to separate to discuss. In the past few years, many studies were carried out to investigate the reliability of solder joints under the condition of the same current density but varied ambient temperature. However, there were few studies can successfully investigate the condition of fixed chip temperature but varied current density. In this research, a temperature control system, employing forced fluid convection, was adopted to separate thermomigration and electromigration effect in flip chip solder joints. With this temperature control system, the die temperature can be precisely fixed at 50 ℃ under different current density ( 3.5×104 A/cm2、4.5×104 A/cm2、5.5×104 A/cm2). The configuration of flip chip solder joint in this research is Ti(1K)/Cu(2K)/Cu(8μm)/Sn2.6Ag/Cu. And the experimental results showed that temperature control system could prolong the life of solder joints even under extra high current density ( over 1860h ). Results also indicate an unique “ sawtooth-type ” Cu UBM morphology was retained at the cathode/chip or substrate side during high current stressing compared with the past accelerated tests. This thesis will discuss the morphology and shape of this unique “ sawtooth-type ” Cu UBM morphology, and give some possible reasons for the formation of this unique morphology.