退火孿晶銀鈀合金線之熱穩定性及電遷移現象研究

Abstract

因應封裝打線接合使用金線與銅線的缺點，銀合金線的性質與金線相似，但不會因此不會有像銅線會損傷晶片亦或是接合強度不佳的問題發生，本論文將探討對二元銀鈀合金線材之熱穩定性、打線接合與鋁墊之界面反應以及其電遷移性，針對高頻積體電路(IC)產品的低電阻率要求，經合金設計之改良並改善抽線及退火製程開發出具有大量退火孿晶之二元銀鈀合金線材及三元銀金鈀線材。含大量退火孿晶與傳統製程之二元Ag-Pd合金線材比較，在經由600ºC時效後，退火孿晶之銀合金線材其晶粒幾乎保持不變，晶粒結構維持高熱穩定性，可導致在放電結球時形成更小的熱影響區，材料內部之退火孿晶可以提高抗拉強度及延伸率，亦影響時效的時間變化，這是有利於打線製程及接合封裝可靠度的雙重優點。 其後再觀察不同合金比例之二元合金線材與鋁墊之界面反應，高溫儲存試驗之溫度100ºC至200ºC，高溫儲存時間從0小時到1000小時，樣品進行接合強度測試及界面反應觀察，發現銀合金線材與鋁墊的接合非常緊密，添加元素的多寡也會影響接合強度及界面反應，而過厚的介金屬化合物(IMC)會造成接點的脆化及大量孔洞的產生，實驗發現在二元銀鈀合金線材在200 ºC、1000小時之熱時效作用下，其接合強度仍維持一定的強度值，介金屬化合物層的生成可分為兩階段式成長，第一階段為銀合金線材在打線過程中先與鋁墊發生的界面反應，迅速產生介金屬化合物層，第二階段為銀合金線材與鋁墊間發生擴散反應而形成富鈀層(Pd-rich layer)，鈀的擴散係數較慢，因此在此階段介金屬化合物層生長較慢。 研究中發現線材的導電性好壞及平均使用壽命長短可以經由增加亦或是減少金(Au)或鈀(Pd)元素的含量來達成，退火孿晶之Ag-Pd線材其平均使用壽命皆高於退火孿晶之Ag-8Au-3Pd線材，其原因在於銀具有高導電性及導熱性，在通電時可降低焦耳熱及溫度的影響，延緩電遷移效應，退火孿晶之二元Ag-4Pd合金線材之電阻率約為2.5μΩ•cm，傳統製程之二元Ag-4Pd合金線材之電阻率約為3.7μΩ•cm，接近傳統金線（約3.5μΩ•cm），略高於鍍鈀銅線(1.8μΩ•cm)，低於三元Ag-8Au-3Pd之線材(約5μΩ•cm) ，退火孿晶之二元銀鈀合金線材已經成為三元Ag-8Au-3Pd之線材之替代材料，滿足高可靠度、低電阻及低成本，在電流密度為1.23x105 A/cm2下，與傳統二元合金線材相比，在通電情況時，晶粒成長較為緩慢，具有大量退火孿晶之二元銀合金線材其抗拉強度和延伸率也比傳統的二元銀合金線材高，因此，證實退火孿晶可提高抗電遷移性。

In response to the shortcomings of electronic package wire bonding with gold wire and copper wire and the requirement of low resistivity in high-frequency IC products, Ag alloy wires, which have physical properties similar to those of Au wire, are being developed. Unlike Cu wire, which has a high hardness, Ag alloy wire will not damage an IC chip or result in poor bonding strength. Improvements to the alloy design and the drawing and annealing processes have allowed the development of binary Ag-Pd alloy wire and ternary Ag-Au-Pd alloy wire containing a large number of annealing twins. In contrast to the apparent grain growth in a conventional Ag-4Pd wire during aging at 600ºC, the grains of this annealing-twinned Ag alloy wire remain almost unchanged. The high thermal stability of the grain structure leads to a smaller heat-affected zone near the free air ball of this twinned wire. The annealing twins in this material also confer the merits of increased tensile strength and elongation with aging time, which increase the reliability of wire-bonded packages. The bonding interface of the Ag-alloy wires and Al pad containing various Pd elements was also studied. Thermal aging temperatures of 100ºC to 200ºC and aging times of 0hr to 1,000hr were used for bond strength testing, and interface reactions were observed. The bonding interface of the Ag alloy wire and Al pad was quite complete. The amounts of the added elements also affected the bonding strength and interfacial reaction. The thickness of the intermetallic compounds (IMC) caused embrittlement and produced large amounts of contact holes due to the decrease in the contact strength. After 1,000 hours of thermal aging at 200ºC, bonding strength was still very high. The growth of the IMC layer was observed to occur in two stages. The first stage of the reaction takes place with the aluminum pad for the silver alloy wire, quickly generating an IMC layer. In the second stage, a Pd-rich layer forms between the silver wire and the aluminum pad. Since the diffusion coefficient of palladium is slow, the IMC layer grows slowly in this stage. In this study, the conductivity and lifetime during current stressing were increased by reducing the addition of Au or Pd. The annealing-twinned Ag-Pd wire has a much higher lifetime than annealing-twinned Ag-8Au-3Pd because silver has high electrical and thermal conductivity. As a result of these characteristics, the Joule heating and temperature during current stressing are lowered, thereby retarding the electromigration. The electrical resistivity of this Ag-4Pd bonding wire, manufactured with a conventional method, is 3.7 μΩ．cm, which is close to the values of traditional 3N Au wire (3.5μΩ．cm), Pd coated Cu wire (1.8μΩ．cm), and Ag-8Au-3Pd ternary alloy wire (5μΩ．cm). The electrical resistivity of the annealing twinned Ag-4Pd wire is 3.5μΩ．cm. An annealing twinned Ag-4Pu wire has been produced as an alternate material for a previously developed Ag-8Au-3Pd ternary alloy wire to meet requirements for high reliability, low electrical resistivity and low cost. Under electrical stressing with a current density of 1.23x105 A/cm2 for various times, the grains in this annealing twinned wire grow much more slowly than the grains in the conventional Ag-4Pd wire. The breaking load and elongation of this annealing twinned Ag-4Pd wire are also higher than those of conventional wire. Furthermore, the annealing twins increase the durability to electromigration of this Ag-4Pd wire under electrical stressing with various current densities.